rus
Issue #3/2015
M.Belkin, V.Yakovlev
VCSEL Photonics – New optoelectronic method for RF signals processing. Part 1. Component Base
VCSEL Photonics – New optoelectronic method for RF signals processing. Part 1. Component Base
Acronym VCSEL Photonics is a dominative of Vertical-cavity surface-emitting laser (VCSEL). Devices on the VCSEL base have a number of vantage features thanks to which this discipline is being developed abroad at an intensive rate. What kind of VCSEL components are accessible today, what are their characteristics and capabilities? The answer you can find in this review written by two well known specialists, one of which is developing VCSEL Photonics many years and the other creates optoelectronic and radioelectronic systems on their base.
Теги: laser photonics radiophotonics vcsel vcsel photonics викселоника лазер поверхностно-излучающий лазер с вертикальным резонатором радиофотоника фотоника
I
n up-to-date understanding, photonics [1] means the broad field of knowledge, in which the various phenomena and systems connected with the optical radiation (photons) are studied. Its public rationality at the present day is approved by the European Union which acknowledged the photonics as one of five most efficient and ecological technologies providing the significant improvement of life quality [2]. Telecommunication fiber optical systems (TFOS), which have taken the main place in the world infrastructure of communication networks in the 21st century, refer to one of the most significant products of photonics development. The general principle of their functioning consists in the electrooptical transformation of information signal, transmission of modulated optical signal in fiber light guide and reverse optical-electric transformation to the modulating frequency range. In order to execute the operation of electrooptical transformation in TFOS, semiconductor laser emitters (SLE) are generally used; since the beginning of TFOS development SLE with edge emission and horizontal resonant cavity has been the only representative of SLE [3]. However, in early 90s of the last century in Japan SLE with different structure [4] was invented; its distinction consisted in the surface radiation from vertical micro-resonator. Afterwards, the laser of this type received the abbreviated name VCSEL, and due to the distinctive properties which will be considered below, at the present time so many developments of different photon devices based on VCSEL are known that we can speak about the separate area [5].
Following the aforementioned, in this review the potential of VCSEL application is considered in photonics and, particularly, in its new scientific-technical area "microwave photonics", which is deemed to be very prospective not only for the civil but for the military application as well [6]; existing component base of vicselonics is described; the most typical examples of modern developments of radio photon devices based on VCSEL, which are designated for the improvement of technical and economical parameters of radio-electronic and combined systems for telecommunication and radio-locating purposes, are given in the article.
Potential of VCSEL Application in Photonics and Radio Photonics Devices
As is well-known, in the second part of the last century at the junction of photonics and traditional electronics the new scientific and technical area occurred – optoelectronics, which has reached the stage of industrial maturity at the present time and continues to remain one of the most topical fields. It is proved by its continuous development and extraction of independent interdisciplinary fields, one of which is represented by the super-high frequency (SHF) optoelectronics[1], which has occurred as a result of the integration of optoelectronics and SHF radio electronics [7]. In the recent years, in the Russian scientific and technical periodical publications the term "SHF optoelectronics’ has been replaced by more general equivalent – "radio photonics’ [8].
The generic essence of the radio photon structural principle [9] of radio electronic equipment (REE) is illustrated with the help of Fig.1. In figure the input signal of SHF range is converted into the optical range via the electrooptical converter. Using the fiber optical or integral optical node and devices the modulated optical signal is processed in appropriate manner or simply transmitted to the distant point of equipment where the reverse optical-electric transformation into SHF range is executed. It follows from the TFOS structural principle described in the Introduction that besides the transmission in optical range the new requirement of direct processing of the optical radiation carrying the radio signal, which is performed with help of functional elements and radio frequency band nodes in the traditional REE, occurs. Transfer of processing operation to the optical band simplifies the general scheme and increases the key technical parameters of REE, for example, response time, operating frequency band, mass and dimension parameters, dynamic range, electromagnetic compatibility, authentication etc. [10]. Difference of this operation from the transmission operation consists in the fact that, as a rule, the significant laser radiation is not required for its implementation and it limits the application of more powerful VCSEL in TFOS by relatively short distribution lines or reverse channel with considerably lower amount of branches [11].
VCSEL use in the radio photon processing nodes grants the additional advantages based on its known merits in comparison with edge-emitting laser [5, 12].
diminutiveness (resonant cavity length is lower almost by 2 orders);
low threshold generation current (0.5–2 mA versus 10–15 mA);
low consumption power (by 5–10 times lower);
better efficiency of input to the fiber at the expense of spatial symmetry and relatively low divergence of output beam (10–12 ° versus 30–40 °);
considerably smaller temperature dependence of threshold current and energy characteristic;
simplicity of single-frequency generation provision;
relatively broad band of continuous wavelength tuning (5–7 nm);
simple capability of the formation of two-dimensional lattice on one substrate;
economy at the expense of the testing capability during the production process on plate.
Semiconductor technologies with the use of surface-emitting lasers offer the unique opportunities for the creation of not only compact but also quite efficient devices. From the very beginning, VCSEL development took place on the basis of two material systems [4]. The first one includes the lasers based on AlGaAs/GaAs operating in the first fiber transparency window of quartz light guide in the region 0.85 μm – so-called short-wave VCSEL. Devices using the lasers of this type have already found wide application in economical TFOSs for local data networks, computer optical interfaces of the type "active cable" and computer optical "mice" [12].
In lasers of the second type, so-called "long-wave" VCSEL, the materials based on InP are used; thanks to this fact they can function in the second (wavelength in the region 1.3 μm) and third (in the region 1.55 μm) fiber transparency windows, which are more prospective for the telecommunication systems, which also were named O, S, C and L spectral ranges respectively in ITU-T recommendations. Implementation of the lasers of this type proceeds comparatively slowly mainly due to the technological difficulties. Besides, the provision of reliable VCSEL functioning at high ambient temperatures required for REE is significant problem. Analysis of the various laser heterostructures shows that during the development of long-wave VCSEL the systems with the active region based on InAlGaAs are mainly used in the second fiber transparency window and systems based on InGaAsP are used in the third fiber transparency window [11]. Modern laser structures in long-wave range are constructed according to the planar-epitaxial technology with the use of two structures – completely epitaxial structure when the active region and mirrors are formed by means of the same materials and alloyed structure with individually made Bragg mirrors based on AlGaAs/GaAs and further alloying at certain temperatures and pressure with the heterostructure of active region [13]. The merit of the first method consists in the comparative simplicity of technological process but it has significant shortcoming – worse heat removal from the active region due to relatively low heat conductivity coefficient, which is applied in this case for the formation of multi-layer reflector of the fourth solid solution. This shortcoming is eliminated in the alloyed structure at the expense of the fact that the mirrors are formed by means of alternating layers AlGaAs/GaAs in the same manner as in short-wave VCSEL. However, this process technology increases the scope of operations and, therefore, production cost.
Nevertheless, specifically long-wave VCSEL with alloyed structure is considered to be the key component base of the equipment of modern and prospective local telecommunication networks [14] and nodes of radio photon REE processing [15]. State of the art and potential of VCSEL application with this structure in the radio photon REE nodes of SHF range are studied in detail in the paper [15] and, therefore, its review in the capacity of forming element of vicselonics component base is given below.
Vicselonics Component Base
Long-wave VCSEL with alloyed structure
Example of the modern transverse structure of long-wave VCSEL with alloyed construction [14] is represented in Fig. 2. Its main elements include: heterostructure containing doped n-layers based on InP and multi-quantum dimensional active region 2 based on InP/InAlGaAs, and two Bragg reflectors 1 and 3 based on GaAs/AlGaAs. These elements interfuse in the production process by the planes 4. The tunnel junction 5 is used for the optical and current restriction. The typical structure of VCSEL crystal with so-called intracavity location of electrical contacts is shown in Fig. 3. As it follows from figure, as opposed to the edge-emitting laser, the laser emission channel of VCSEL type is positioned vertically. In order to ensure the laser generation, the active region is located between two mirrors, which in this case are executed in the form of distributed Bragg reflectors (DBR), in the same manner as in traditional lasers. However, the cavity length of VCSEL is almost by two orders smaller and approximately corresponds to the operating wavelength and, therefore, for the formation of efficient laser generation it is required: (1) to increase the optical intensification in active region as much as possible; (2) to ensure high (close to 1) coefficient of mirror reflection. The first condition is implemented with the help of the quantum dimensional structure of active region using up to 10 nanolayers with the width of several nanometers based on quantum wells or quantum dots. For the implementation of the second condition the multi-layer structure of mirrors with the amount of layers of more than 30 is applied. Herewith, the total amount of epitaxial layers in the structure exceeds 100 and this fact creates significant difficulties during its physical simulation.
Experience in the development of radio photon devices allowed detecting the following additional advantages of long-wave VCSEL:
simple capability of significant improvement of dynamic characteristics at the expense of optical injection synchronization [16];
compatibility with silicon integral-optical technology [17].
For the purpose of the specific evaluation of the potential of considered VCSEL in the capacity of forming element of vicselonics component base, the basic achieved static and dynamic parameters of long-wave VCSEL with alloyed structure developed by the Laboratory of Physics of Nanostructures (LPN) of the Swiss Federal Institute of Technology in Lausanne (EPFL)[2] are described below.
Electrical and energy characteristics
Typical volt-ampere and watt-ampere characteristics of the single-frequency VCSEL [18, 19] are specified in Fig. 4. As is seen from figure, the following results are obtained at the room temperature: threshold current of 2 mA in О-range and less than 1 mA in С-range; maximum radiation power in continuous mode of 5–6 mW; power consumption in quasi-linear transformation mode of 20 mW in О-range and 8 mW in С-range. Besides, at the operating current of 9 mA the power of continuous radiation of 1.5 mW is reached at the temperature of 100 °C in О-range and 80 °C in С-range. Obtained results constantly improve, and at the present time the record value of radiation power in continuous single-frequency mode was reached for VCSEL of any type: 8 mW at the room temperature [20].
Small-signal frequency-modulating characteristics (FMC)
The typical FMCs at different displacement currents [21] are given in Fig. 5. As it follows from figure, the band of direct modulation by the level –3 dB exceeds 7 GHz at the displacement current of 10 mA. At the present time, the improvement of laser structure resulted in the increase of modulation band up to 11 GHz [22].
Noise characteristics
The typical characteristics of relative intensity noise (RIN) of the laser with О-range at the ambient temperature of 20 °C [23] are shown in Fig. 6. As it follows from figure, the values RIN decrease with the increase of displacement current and grow with the increase of modulation frequency and this fact corresponds to the known data. Particularly, at the room temperature the value RIN at the modulation frequency of 1.5 GHz is just –160 dB/Hz (minimum threshold of measurement unit) when the displacement current reaches 5 mA.
Linearity in the mode of large signal
As it is known, the most visual method for the evaluation of linearity properties of active device (semiconductor laser in this case) consists in the determination of so-called input intersection point (IIP) [24]. The advantage of this parameter includes the capability to compare different devices without regard to the modulating signal power Pi. The simplified evaluation of IIP of the third and fifth orders (IIP3 and IIP5) can be accomplished on the basis of the following formulas [25].
IIP3 = Pi + IMD3 / 2,
IIP5 = Pi + IMD5 / 4.
Here, IMD3 and IMD5 are intermodulation disturbances of the third and fifth orders respectively, which can be easily measured using the photodetector with the band in SHF range and radio-frequency spectrum analyzer.
Calculation results for IIP3 and IIP5 of VCSEL with O-range based on measurement data and aforementioned formulas [26] at the frequency domains of modulating signals in the region of 1 GHz and 6 GHz are specified in Table 1.
As it follows from table, the level of IIP3 and, thus, linearity of the studied VCSEL decreases with the growth of modulation frequency, and this fact agrees with the known experimental data [27]. Comparison with the analogous results given in other publications shows that the intersection point of the third order of long-wave VCSEL with alloyed structure and intracavity location of contacts, which is considered in this article, is higher approximately by 10 dB than in long-wave VCSEL with different structure [27] and located at the level of the best results for short-wave VCSEL with oxide aperture [28].
Spectral and tuning characteristics
Results of the measurement of side-mode suppression ratio (SMSR) in the whole range of operating currents of the laser with C-range [20] are given in Fig. 7. The spectral characteristics of VCSEL with C-range during the tuning by displacement current within the range of 3–13 mA with the step of 2 mA and during the tuning by temperature within the range of 25–50 °C with the step 5° [15] are given in Fig. 8а and 8b respectively. The results of measurement of laser spectral line width [20] are specified in Fig. 9. As is seen from figures, at the present time the following results are obtained: the laser operates in single-frequency mode with minimum suppression of side modes of more than 40 dB; the mean steepness of tuning of radiation wavelength by the temperature is 0.18 nm/°C, by the displacement current 0.3 nm/mA; the minimum width of generation line is about 4 MHz.
The specific properties of VCSEL described above allowed designing a number of structural modifications, which together with VCSEL make the component base of vicselonics, at the present time. Its main representatives will be reviewed below: VECSEL, MEMS-VCSEL, LICSEL, MIXSEL operating in О-, S- and С-ranges. The unified approach, which includes the consistent description of distinctive peculiarities, construction principles, schemes and structures achieved at the modern level of photonics scientific and technical development, is adopted in the account.
Continuous Wave Lasers: VECSEL, MEMS-VCSEL, LICSEL
The vertical external cavity surface-emitting laser, which is named VECSEL[3], represents the most prospective and developed element of vicselonics component base in which the functioning within the broad spectral range of semiconductor lasers and proven technology of external pumping and efficient heat removal of disk solid-state lasers are well combined. Its creation was predetermined by the further development of photon technologies, during which the restrictions of VCSEL mainly connected with insufficiently high output power (Fig. 4) and insufficiently narrow emission line (Fig. 9) have been detected. According to the generalized structure of VECSEL (Fig. 10), its principal elements include intensifying semiconductor crystal (ISC), which represents VCSEL with distant top mirror (in comparison with Fig. 2), and external spatial optical resonant cavity, which is formed by ISC Bragg reflector on one side and semitransparent spherical mirror on other side [29]. Such exit mirror ensures the focusing of intracavity beam on ISC surface and formation of diffraction-limited symmetric exit flux, which quality is not worse than in gas and solid-state lasers. It should be noted that for the operation of the actual laser of this type, additional elements which are not shown in Fig. 10 might be needed, for example: optical pumping source and cooling device of intensifying crystal which will cause the complication of its scheme.
In C-range which is the most important for radio photonics (band of wavelengths of 1530–1565 nm), the intensifying semiconductor crystal is constructed on indium phosphide substrate with InGaAsP or InGaAlAs quantum dimensional active structure in the same manner as in VCSEL. However, when forming the Bragg reflector on the basis of these material structures, relatively weak contrast of refractive index results in the necessity to increase the amount of lattice layers with the relevant growth of optical signal losses. The best variant consists in the use of the mirror on the basis of alternations AlGaAs/GaAs or dielectric layers with the further alloying to intensifying structure [30, 31]. Also, the application of high-quality spatial optical resonant cavity, besides the evident opportunity to increase the power and significantly narrow the generation line, allows introducing the different optical elements inside, which create additional advantages. In particular, the introduction of Fabri-Perot filters ensures the implementation of single-frequency coherent operation mode.
At the present time, two basic variants of the construction of laser of this type are known: with electrical pumping (EP-VECSEL) and with optical pumping (OP-VECSEL). Basically, the first variant is more economic because it does not require the special pumping laser and it keeps the diminutiveness which is attributable to VCSEL. Examples of the schemes EP-VECSEL in hybrid (with spatial resonant cavity) [32] and in monolithic [33] execution forms are shown in Fig. 11 and 12 respectively. OP-VECSEL which provides principally greater output power also can be built on the basis of hybrid (with separate pumping source) and monolithic (with built-in pumping source) systems. The hybrid execution of OP-VECSEL is implemented in the form of two schemes [34]: linear (Fig. 13а) with single-frequency output signal and Т-shaped (Fig. 13b, pumping circuits of ISC1 and ISC2 are not shown) with two output signals of orthogonal polarization. Example of the implementation of OP-VECSEL scheme in monolithic execution [35] is shown in Fig. 14.
Having compared the schemes in Fig. 11–14, we can draw conclusion that their principal component is aforementioned ISC which is implemented as individual structural element in the hybrid scheme of VECSEL or is a part of monolithic structure. In both cases the necessary condition for its reliable operation in VECSEL which determines the longevity of all device, consists in the availability of efficient heat sink excluding the overheating of semiconductor structure due to the dissipated power up to tens of watts. This requirement gets considerably stringent for the long-wave VECSELs operating in radio photonics devices due to worse heat conductivity of compound semiconductors applied in C-range (see above). The alloyed structure of active region and mirrors with integrated diamond radiator installed on the copper bedding are used as the prospective solution of this problem here in the same manner as in VCSEL [36, 37]. Example of the transverse structure of alloyed ISC [38] is shown in Fig. 15. Significant difference in comparison with Fig. 2 consists in the introduction of the region providing the expansion of carriers flow towards the top electrode with ring shape between the quantum dimensional intensifying layer and top reflector, which is necessary for the improvement of laser quantum efficiency. In order to enhance the homogeneity of current distribution over the region, the tunnel junction (TJ), the crosswise dimensions of which correspond to the thickness of current expansion region, is used here in the same way as in VCSEL. The trajectories of injection current flowing between the top and bottom disk electrodes are indicated by dashed lines. The intermediate semitransparent (70%) Bragg reflector based on the layers AlGaAs/GaAs of n-type is designated for the compensation of absorption losses in the doped regions of ISC. The deposition of copper plating coating on the bottom electrode ensures the efficient heat removal with the thermal resistance within the limits of 13–25 K/W in the whole range of device operating temperatures.
Using the ISC described above, at the present time two prospective approaches for the construction of high-power VECSELs have formed; they can be applied in the capacity of driving generators of multi-element radio photon antenna array with SHF range and in the capacity of pumping source (1450–1480 nm) of fiber erbium and Raman amplifiers: use of thermoelectric microcooler (TEMC) [38] for the devices with electrical pumping (Fig. 16а) and water or air cooling [39] for the devices with optical pumping (Fig. 16b). When studying the schemes in Fig. 16, the electrical and energy characteristics were obtained which are given in Fig. 17a and 17b respectively. Thus, during the operation of long-wave VECSEL with electrical pumping in continuous single-frequency mode the output power of more than 100 mW can be obtained which exceeds the typical output power of VCSEL with the same spectral range by 40 times (see Fig. 4). Application of optical pumping in the laser with S- and C-ranges grants the opportunity to increase it up to 1–3 W at the temperature of heat sink up to 50 °C.
As is known, besides the output power, the quality of operation of the generator with any frequency range is characterized by the noise parameters which are typically determined [3] in the form of relative intensity noise[4] (RIN) and width of emission line by half level for lasers. Examples of RIN characteristics for long-wave OP-VECSEL with С-range and power of about 100 mW at the room temperature [40] are given in Fig. 18. As it follows from figure, the intensity noise of VECSEL is less than –160 dB/Hz near the carrier and reaches the threshold of measurement device (–170 dB/Hz) at the frequencies of more than 500 MHz which are considerably lower than the analogous parameter for VCSEL [15] and edge-emitting leaser as well [3]. The fundamental reason for it consists in greater ratio of photon lifetime in resonant cavity and carriers in ISC. As it has been mentioned already, the VECSEL advantage also consists in the principal narrowing of emission line because its width is inversely proportional to the length of resonant cavity [3]. Example of the spectral characteristics of OP-VECSEL with С-range and output power of 77 mW at the room temperature [41] is shown in Fig. 19. In figure the first insertion shows the result of the measurement using the optical spectrum analyzer in the narrow band near carrier; the left insertion – result of the measurement using auto-heterodyne methods with the help of radio technical spectrum analyzer. As it follows from figure, in long-wave OP-VECSEL it is possible to provide the single-mode operation with the suppression of side modes of more than 60 dB and emission line with the width of tens of kilohertz which is narrower than so-called Lorentz line (dashed curve in the left insertion). It should be noted that above mentioned data is significantly better than the results of the measurement of VCSEL spectral characteristics (see Fig. 7 and 9).
One more significant shortcoming of the VCSEL with continuous operation, which has been detected in the process of photon technology development, consists in insufficiently broad band of generation line tuning. Particularly, according to Fig. 8 at the simplest control of displacement current and temperature it does not exceed several nanometers, whereas for the efficient functioning of some radio photon devices (for example, beamformer) the tuning, at least, in the whole C-range (35 nm) is required. In order to solve this problem, at the present time two approaches are suggested using the regulation of physical or optical length of VCSEL resonant cavity, which is comparable with the radiation wavelength, as it has been mentioned already. The laser, in which the first approach is implemented, is called MEMS-VCSEL[5]; the most widespread name of the laser of the second approach is LICSEL[6]. These structures have been widely studied by the various university scientific groups over the last 10 years. In the course of the works, the large scope of results has been obtained and, therefore, only the most significant results are considered below.
The typical transverse structure of modern MEMS-VCSEL with С-range [42] is shown in Fig. 20. The general principle of its construction consists in the formation of top Bragg mirror (see Fig. 2) in the form of moving micro-electromechanical membrane. Example of the characteristic of MEMS-VCSEL tuning in Fig. 20 at the fixed displacement current and stabilized crystal temperature at the level of 20°С is shown in Fig. 21. As is seen from figure, the band of continuous tuning is equal to 102 nm, which is greater than the tuning band of standard VCSEL by 25 times (see Fig. 8). From the point of view of the operation in practical device, the obvious shortcoming of the considered structure of MEMS-VCSEL includes the dependence of power and, especially, radiation wavelength on the ambient temperature, which corresponds to the standard VCSEL (Fig. 8). In order to compensate the temperature sensitivity of wavelength, the specific structure of MEMS-VCSEL [43] is suggested, in which the top mirror is installed on the micro-cantilever with the length of about 100 μm (Fig. 22). According to the specified experimental studies, the minimum temperature sensitivity of radiation wavelength in C-range was equal to 0.0016 nm/°С. This result corresponds approximately to 100-fold sensitivity reduction in comparison with the standard VCSEL and allows considering that the described structure operates in athermal mode or, in other words, the availability of thermostating node, which is typical for semiconductor laser, with the energy consumption power of several watts is not required for it.
More economic solution of the problem connected with the increase of tuning band of VCSEL emission wavelength is believed to be the introduction of transparent liquid-crystal layer, the refractive index of which varies under the action of external electric voltage VLC, inside the resonant cavity; it provides the opportunity to regulate the optical length of laser resonant cavity. Example of the transverse structure of long-wave LICSEL [44] is illustrated in Fig. 23. The main difference from the standard VCSEL in Fig. 2 consists in the introduction of the intracavity layer 6 based on the nematic liquid-crystal structure. The typical characteristics of LICSEL tuning in Fig. 23 at the fixed displacement current and stabilized crystal temperature at the level of 20°С is given in Fig. 24. As is seen from figure, the band of continuous tuning within the variation range VLC=0…2.4 V is equal to 34 nm, which is higher than the tuning band of the standard VCSEL (see Fig. 8) almost by 10 times and in the majority of practical cases it meets the requirements specified for radio photon devices.
It should be noted that the implementation of athermal MEMS-VCSEL or simpler functional analog, which is represented by LICSEL, provides the way for the efficient use of radio photon technologies in airborne radio instruments with the strict requirements for energy consumption. In this case, the temperature stabilization of emission spectrum with the power consumption of several watts, which is required during the operation of semiconductor laser of any type, can be replaced by practically non-consuming frequency stabilization operating in analogous manner as the radio technical scheme of frequency automatic tuning.
Lasers with Pulse Generation:
VECSEL-SESAM, MIXSEL
The development of vicselonics component base is connected not only with above described lasers of continuous generation but with pulse lasers as well. In this area, the significant distinctive feature of photon technologies is the capability of simple generation of the sequence of coherent ultra-short optical pulses using so-called lasers with passive mode synchronization (PMS). At the present time, the lasers with PMS found broad application in TFOS, spectroscopy, material science etc. The prospects of their use in radio photonics devices consists, for example, in the creation of multi-wave precision generators of optical pulses for beamformers and radio photon analog-to-digital converters which will be described in the second part of this review.
The PMS engineering has been developing for several decades already on the basis of solid-state and semiconductor lasers mainly in short-wave (in the region of 1 μm) spectral range. The scientific area received the new pulse with the occurrence of VECSEL combining the advantages of solid-state and semiconductor lasers, as it has been mentioned already. The traditional PMS scheme contains two fundamental elements: active laser structure and saturable absorber, in which the loss modulation in resonant cavity is performed. In the practical devices, this operation is typically carried out using the mirror on the basis of semiconductor saturable absorber called SESAM[7]. The structure of SESAM contains semiconductor DBR (distributed Bragg reflector), into which the quantum dimensional absorber layers are introduced. At the present time, two construction schemes of VECSEL with pulse generation are known [45, 46]: with separate ISC and SESAM (Fig. 25а) called VECSEL-SESAM and with integrated ISC and SESAM (Fig. 25b) named MIXSEL[8]. The optical pumping, which is introduced according to Fig. 16b, is used for the operation of both schemes. As a rule, the monolithic structure in Fig. 25b is grown by means of the molecular-beam epitaxy and it contains five sections: 1 – high-reflectivity AlAs/GaAs DBR; 2 – absorber based on the layer of self-organized InAs quantum dots implanted into the low-temperature layers of GaAs; 3 – intermediate AlGaAs/AlAs DPR preventing the pump signal absorption in absorber section; 4 – intensifying section with seven quantum-dimensional layers InGaAs; 5 – antireflection coating which provides the optimal distribution of intensification and absorption section fields.
The recent studies and developments of VECSEL-SESAM and MIXSEL were mainly executed in the areas of search for the methods of increase of mean emission power, decrease of optical pulse duration with the simultaneous shift of repetition frequencies to SHF range, which is the necessary condition for their use in the radio photonics devices. At the same time, in the scheme of Fig. 25 the effect, which occurred as a result of the application of epitaxial layers based on quantum wells (QW) and quantum dots (QD) in the structures of VECSEL (or more precisely, in ISC) and in SESAM, was studied. Obtained results at the repetition frequencies of 2 to 50 GHz [45, 47] are shown in the diagram in Fig. 26. Their analysis allows drawing the following conclusions. In the MIXSEL scheme the highest mean emission power (up to 6.4 W) is obtained but at the same time the pulse duration exceeds 20 ps which is not appropriate for the majority of radio photonics devices. Pulses with the duration of less than 200 fs at the mean power of about 20 mW are provided with the use of the laser with PMS of QW-layers in both elements. The parameters (pulse duration less than 1 ps, mean power of 100 mW…1 W), which are optimal for the application in radio photonics, can be achieved under the condition of implementation of both elements on the basis of QD-layers. Nevertheless, the further improvement of operation of the MIXSEL scheme allowed obtaining more prospective results [48] – pulse duration of 570 fs within the range of repetition frequencies 5–101 GHz at the mean power of 127 mW.
All above-listed experimental data is obtained for short-wave lasers, which are studied in the widest manner. As for now, the number of the publications devoted to long-wave MIXSEL is much smaller and the results described in them are considerably more meager. In the technological aspect, the alloyed structure is considered to be prospective in the same manner as for VCSEL (see Fig. 2, 3) and in ISC of high-power VECSEL with continuous generation (see Fig. 15). Example of the scheme of long-wave VECSEL-SESAM with Z-shape configuration using the optical pumping [49] is shown in Fig. 27а. For this VECSEL-SESAM the ISC (Fig. 27b), in which the active region grown on InP substrate with the use of metal-organic vapor-phase epitaxial device and DBR grown on GaAs substrate with the use of molecular-beam epitaxial device are alloyed, was specifically developed. During the experimental study, the ISC temperature was maintained at the level of 15 °C using the water cooling, and SESAM temperature – at the level of 21 °C using TEMC. During the measurements the following results were obtained: the pulse duration of 6.4 ps at the mean emission power of about 100 mW and pumping power of about 9 W. The repetition frequency determined on the basis of the total resonant cavity length of the scheme in Fig. 27a was 950 MHz. Higher repetition frequency was obtained during the experiments with MIXSEL with С-range [50], results of which are given in Fig. 28. In the scheme the ISC with hybrid metamorphic mirror, which was optimized for high emission power at the room temperature, and fast-response SESAM based on InGaAsN/GaAsN were used and this fact allowed avoiding the necessity of water cooling application. In Fig. 28, obtained autocorrelation function, which agrees with the secant-square pulse profile, is specified on the left side and corresponding Fourier spectrogram on which the spectrum width by half level is marked out – on the right side. In Fig. 28b, the radiofrequency spectrum with pulse sequence obtained at the exit of high-speed photodetector with the resolution RBW=30 kHz is given on the left side, and spectrum of the first harmonics obtained with the resolution RBW=30 Hz – on the right side. As it follows from the experimental data, the pulse duration is equal to 1.7 ps and corresponds with the signal total bandwidth of MIXSEL of 2.29 nm (about 300 GHz), the pulse repetition frequency is 2 GHz, generation line width, which characterizes the device noise properties, does not exceed 1 kHz.
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[1] Foreign equivalent: Microwave photonics.
[2] www.epfl.ch
[3] Vertical External Cavity Surface-Emitting Laser
[4] Related Intensity Noise
[5] Micro Electro-Mechanical System-VCSEL
[6] Liquid Crystal Surface Emitting Laser
[7] Semiconductor Saturable Absorber Mirror
[8] Mode-locked Integrated External-Cavity Surface Emitting Laser
n up-to-date understanding, photonics [1] means the broad field of knowledge, in which the various phenomena and systems connected with the optical radiation (photons) are studied. Its public rationality at the present day is approved by the European Union which acknowledged the photonics as one of five most efficient and ecological technologies providing the significant improvement of life quality [2]. Telecommunication fiber optical systems (TFOS), which have taken the main place in the world infrastructure of communication networks in the 21st century, refer to one of the most significant products of photonics development. The general principle of their functioning consists in the electrooptical transformation of information signal, transmission of modulated optical signal in fiber light guide and reverse optical-electric transformation to the modulating frequency range. In order to execute the operation of electrooptical transformation in TFOS, semiconductor laser emitters (SLE) are generally used; since the beginning of TFOS development SLE with edge emission and horizontal resonant cavity has been the only representative of SLE [3]. However, in early 90s of the last century in Japan SLE with different structure [4] was invented; its distinction consisted in the surface radiation from vertical micro-resonator. Afterwards, the laser of this type received the abbreviated name VCSEL, and due to the distinctive properties which will be considered below, at the present time so many developments of different photon devices based on VCSEL are known that we can speak about the separate area [5].
Following the aforementioned, in this review the potential of VCSEL application is considered in photonics and, particularly, in its new scientific-technical area "microwave photonics", which is deemed to be very prospective not only for the civil but for the military application as well [6]; existing component base of vicselonics is described; the most typical examples of modern developments of radio photon devices based on VCSEL, which are designated for the improvement of technical and economical parameters of radio-electronic and combined systems for telecommunication and radio-locating purposes, are given in the article.
Potential of VCSEL Application in Photonics and Radio Photonics Devices
As is well-known, in the second part of the last century at the junction of photonics and traditional electronics the new scientific and technical area occurred – optoelectronics, which has reached the stage of industrial maturity at the present time and continues to remain one of the most topical fields. It is proved by its continuous development and extraction of independent interdisciplinary fields, one of which is represented by the super-high frequency (SHF) optoelectronics[1], which has occurred as a result of the integration of optoelectronics and SHF radio electronics [7]. In the recent years, in the Russian scientific and technical periodical publications the term "SHF optoelectronics’ has been replaced by more general equivalent – "radio photonics’ [8].
The generic essence of the radio photon structural principle [9] of radio electronic equipment (REE) is illustrated with the help of Fig.1. In figure the input signal of SHF range is converted into the optical range via the electrooptical converter. Using the fiber optical or integral optical node and devices the modulated optical signal is processed in appropriate manner or simply transmitted to the distant point of equipment where the reverse optical-electric transformation into SHF range is executed. It follows from the TFOS structural principle described in the Introduction that besides the transmission in optical range the new requirement of direct processing of the optical radiation carrying the radio signal, which is performed with help of functional elements and radio frequency band nodes in the traditional REE, occurs. Transfer of processing operation to the optical band simplifies the general scheme and increases the key technical parameters of REE, for example, response time, operating frequency band, mass and dimension parameters, dynamic range, electromagnetic compatibility, authentication etc. [10]. Difference of this operation from the transmission operation consists in the fact that, as a rule, the significant laser radiation is not required for its implementation and it limits the application of more powerful VCSEL in TFOS by relatively short distribution lines or reverse channel with considerably lower amount of branches [11].
VCSEL use in the radio photon processing nodes grants the additional advantages based on its known merits in comparison with edge-emitting laser [5, 12].
diminutiveness (resonant cavity length is lower almost by 2 orders);
low threshold generation current (0.5–2 mA versus 10–15 mA);
low consumption power (by 5–10 times lower);
better efficiency of input to the fiber at the expense of spatial symmetry and relatively low divergence of output beam (10–12 ° versus 30–40 °);
considerably smaller temperature dependence of threshold current and energy characteristic;
simplicity of single-frequency generation provision;
relatively broad band of continuous wavelength tuning (5–7 nm);
simple capability of the formation of two-dimensional lattice on one substrate;
economy at the expense of the testing capability during the production process on plate.
Semiconductor technologies with the use of surface-emitting lasers offer the unique opportunities for the creation of not only compact but also quite efficient devices. From the very beginning, VCSEL development took place on the basis of two material systems [4]. The first one includes the lasers based on AlGaAs/GaAs operating in the first fiber transparency window of quartz light guide in the region 0.85 μm – so-called short-wave VCSEL. Devices using the lasers of this type have already found wide application in economical TFOSs for local data networks, computer optical interfaces of the type "active cable" and computer optical "mice" [12].
In lasers of the second type, so-called "long-wave" VCSEL, the materials based on InP are used; thanks to this fact they can function in the second (wavelength in the region 1.3 μm) and third (in the region 1.55 μm) fiber transparency windows, which are more prospective for the telecommunication systems, which also were named O, S, C and L spectral ranges respectively in ITU-T recommendations. Implementation of the lasers of this type proceeds comparatively slowly mainly due to the technological difficulties. Besides, the provision of reliable VCSEL functioning at high ambient temperatures required for REE is significant problem. Analysis of the various laser heterostructures shows that during the development of long-wave VCSEL the systems with the active region based on InAlGaAs are mainly used in the second fiber transparency window and systems based on InGaAsP are used in the third fiber transparency window [11]. Modern laser structures in long-wave range are constructed according to the planar-epitaxial technology with the use of two structures – completely epitaxial structure when the active region and mirrors are formed by means of the same materials and alloyed structure with individually made Bragg mirrors based on AlGaAs/GaAs and further alloying at certain temperatures and pressure with the heterostructure of active region [13]. The merit of the first method consists in the comparative simplicity of technological process but it has significant shortcoming – worse heat removal from the active region due to relatively low heat conductivity coefficient, which is applied in this case for the formation of multi-layer reflector of the fourth solid solution. This shortcoming is eliminated in the alloyed structure at the expense of the fact that the mirrors are formed by means of alternating layers AlGaAs/GaAs in the same manner as in short-wave VCSEL. However, this process technology increases the scope of operations and, therefore, production cost.
Nevertheless, specifically long-wave VCSEL with alloyed structure is considered to be the key component base of the equipment of modern and prospective local telecommunication networks [14] and nodes of radio photon REE processing [15]. State of the art and potential of VCSEL application with this structure in the radio photon REE nodes of SHF range are studied in detail in the paper [15] and, therefore, its review in the capacity of forming element of vicselonics component base is given below.
Vicselonics Component Base
Long-wave VCSEL with alloyed structure
Example of the modern transverse structure of long-wave VCSEL with alloyed construction [14] is represented in Fig. 2. Its main elements include: heterostructure containing doped n-layers based on InP and multi-quantum dimensional active region 2 based on InP/InAlGaAs, and two Bragg reflectors 1 and 3 based on GaAs/AlGaAs. These elements interfuse in the production process by the planes 4. The tunnel junction 5 is used for the optical and current restriction. The typical structure of VCSEL crystal with so-called intracavity location of electrical contacts is shown in Fig. 3. As it follows from figure, as opposed to the edge-emitting laser, the laser emission channel of VCSEL type is positioned vertically. In order to ensure the laser generation, the active region is located between two mirrors, which in this case are executed in the form of distributed Bragg reflectors (DBR), in the same manner as in traditional lasers. However, the cavity length of VCSEL is almost by two orders smaller and approximately corresponds to the operating wavelength and, therefore, for the formation of efficient laser generation it is required: (1) to increase the optical intensification in active region as much as possible; (2) to ensure high (close to 1) coefficient of mirror reflection. The first condition is implemented with the help of the quantum dimensional structure of active region using up to 10 nanolayers with the width of several nanometers based on quantum wells or quantum dots. For the implementation of the second condition the multi-layer structure of mirrors with the amount of layers of more than 30 is applied. Herewith, the total amount of epitaxial layers in the structure exceeds 100 and this fact creates significant difficulties during its physical simulation.
Experience in the development of radio photon devices allowed detecting the following additional advantages of long-wave VCSEL:
simple capability of significant improvement of dynamic characteristics at the expense of optical injection synchronization [16];
compatibility with silicon integral-optical technology [17].
For the purpose of the specific evaluation of the potential of considered VCSEL in the capacity of forming element of vicselonics component base, the basic achieved static and dynamic parameters of long-wave VCSEL with alloyed structure developed by the Laboratory of Physics of Nanostructures (LPN) of the Swiss Federal Institute of Technology in Lausanne (EPFL)[2] are described below.
Electrical and energy characteristics
Typical volt-ampere and watt-ampere characteristics of the single-frequency VCSEL [18, 19] are specified in Fig. 4. As is seen from figure, the following results are obtained at the room temperature: threshold current of 2 mA in О-range and less than 1 mA in С-range; maximum radiation power in continuous mode of 5–6 mW; power consumption in quasi-linear transformation mode of 20 mW in О-range and 8 mW in С-range. Besides, at the operating current of 9 mA the power of continuous radiation of 1.5 mW is reached at the temperature of 100 °C in О-range and 80 °C in С-range. Obtained results constantly improve, and at the present time the record value of radiation power in continuous single-frequency mode was reached for VCSEL of any type: 8 mW at the room temperature [20].
Small-signal frequency-modulating characteristics (FMC)
The typical FMCs at different displacement currents [21] are given in Fig. 5. As it follows from figure, the band of direct modulation by the level –3 dB exceeds 7 GHz at the displacement current of 10 mA. At the present time, the improvement of laser structure resulted in the increase of modulation band up to 11 GHz [22].
Noise characteristics
The typical characteristics of relative intensity noise (RIN) of the laser with О-range at the ambient temperature of 20 °C [23] are shown in Fig. 6. As it follows from figure, the values RIN decrease with the increase of displacement current and grow with the increase of modulation frequency and this fact corresponds to the known data. Particularly, at the room temperature the value RIN at the modulation frequency of 1.5 GHz is just –160 dB/Hz (minimum threshold of measurement unit) when the displacement current reaches 5 mA.
Linearity in the mode of large signal
As it is known, the most visual method for the evaluation of linearity properties of active device (semiconductor laser in this case) consists in the determination of so-called input intersection point (IIP) [24]. The advantage of this parameter includes the capability to compare different devices without regard to the modulating signal power Pi. The simplified evaluation of IIP of the third and fifth orders (IIP3 and IIP5) can be accomplished on the basis of the following formulas [25].
IIP3 = Pi + IMD3 / 2,
IIP5 = Pi + IMD5 / 4.
Here, IMD3 and IMD5 are intermodulation disturbances of the third and fifth orders respectively, which can be easily measured using the photodetector with the band in SHF range and radio-frequency spectrum analyzer.
Calculation results for IIP3 and IIP5 of VCSEL with O-range based on measurement data and aforementioned formulas [26] at the frequency domains of modulating signals in the region of 1 GHz and 6 GHz are specified in Table 1.
As it follows from table, the level of IIP3 and, thus, linearity of the studied VCSEL decreases with the growth of modulation frequency, and this fact agrees with the known experimental data [27]. Comparison with the analogous results given in other publications shows that the intersection point of the third order of long-wave VCSEL with alloyed structure and intracavity location of contacts, which is considered in this article, is higher approximately by 10 dB than in long-wave VCSEL with different structure [27] and located at the level of the best results for short-wave VCSEL with oxide aperture [28].
Spectral and tuning characteristics
Results of the measurement of side-mode suppression ratio (SMSR) in the whole range of operating currents of the laser with C-range [20] are given in Fig. 7. The spectral characteristics of VCSEL with C-range during the tuning by displacement current within the range of 3–13 mA with the step of 2 mA and during the tuning by temperature within the range of 25–50 °C with the step 5° [15] are given in Fig. 8а and 8b respectively. The results of measurement of laser spectral line width [20] are specified in Fig. 9. As is seen from figures, at the present time the following results are obtained: the laser operates in single-frequency mode with minimum suppression of side modes of more than 40 dB; the mean steepness of tuning of radiation wavelength by the temperature is 0.18 nm/°C, by the displacement current 0.3 nm/mA; the minimum width of generation line is about 4 MHz.
The specific properties of VCSEL described above allowed designing a number of structural modifications, which together with VCSEL make the component base of vicselonics, at the present time. Its main representatives will be reviewed below: VECSEL, MEMS-VCSEL, LICSEL, MIXSEL operating in О-, S- and С-ranges. The unified approach, which includes the consistent description of distinctive peculiarities, construction principles, schemes and structures achieved at the modern level of photonics scientific and technical development, is adopted in the account.
Continuous Wave Lasers: VECSEL, MEMS-VCSEL, LICSEL
The vertical external cavity surface-emitting laser, which is named VECSEL[3], represents the most prospective and developed element of vicselonics component base in which the functioning within the broad spectral range of semiconductor lasers and proven technology of external pumping and efficient heat removal of disk solid-state lasers are well combined. Its creation was predetermined by the further development of photon technologies, during which the restrictions of VCSEL mainly connected with insufficiently high output power (Fig. 4) and insufficiently narrow emission line (Fig. 9) have been detected. According to the generalized structure of VECSEL (Fig. 10), its principal elements include intensifying semiconductor crystal (ISC), which represents VCSEL with distant top mirror (in comparison with Fig. 2), and external spatial optical resonant cavity, which is formed by ISC Bragg reflector on one side and semitransparent spherical mirror on other side [29]. Such exit mirror ensures the focusing of intracavity beam on ISC surface and formation of diffraction-limited symmetric exit flux, which quality is not worse than in gas and solid-state lasers. It should be noted that for the operation of the actual laser of this type, additional elements which are not shown in Fig. 10 might be needed, for example: optical pumping source and cooling device of intensifying crystal which will cause the complication of its scheme.
In C-range which is the most important for radio photonics (band of wavelengths of 1530–1565 nm), the intensifying semiconductor crystal is constructed on indium phosphide substrate with InGaAsP or InGaAlAs quantum dimensional active structure in the same manner as in VCSEL. However, when forming the Bragg reflector on the basis of these material structures, relatively weak contrast of refractive index results in the necessity to increase the amount of lattice layers with the relevant growth of optical signal losses. The best variant consists in the use of the mirror on the basis of alternations AlGaAs/GaAs or dielectric layers with the further alloying to intensifying structure [30, 31]. Also, the application of high-quality spatial optical resonant cavity, besides the evident opportunity to increase the power and significantly narrow the generation line, allows introducing the different optical elements inside, which create additional advantages. In particular, the introduction of Fabri-Perot filters ensures the implementation of single-frequency coherent operation mode.
At the present time, two basic variants of the construction of laser of this type are known: with electrical pumping (EP-VECSEL) and with optical pumping (OP-VECSEL). Basically, the first variant is more economic because it does not require the special pumping laser and it keeps the diminutiveness which is attributable to VCSEL. Examples of the schemes EP-VECSEL in hybrid (with spatial resonant cavity) [32] and in monolithic [33] execution forms are shown in Fig. 11 and 12 respectively. OP-VECSEL which provides principally greater output power also can be built on the basis of hybrid (with separate pumping source) and monolithic (with built-in pumping source) systems. The hybrid execution of OP-VECSEL is implemented in the form of two schemes [34]: linear (Fig. 13а) with single-frequency output signal and Т-shaped (Fig. 13b, pumping circuits of ISC1 and ISC2 are not shown) with two output signals of orthogonal polarization. Example of the implementation of OP-VECSEL scheme in monolithic execution [35] is shown in Fig. 14.
Having compared the schemes in Fig. 11–14, we can draw conclusion that their principal component is aforementioned ISC which is implemented as individual structural element in the hybrid scheme of VECSEL or is a part of monolithic structure. In both cases the necessary condition for its reliable operation in VECSEL which determines the longevity of all device, consists in the availability of efficient heat sink excluding the overheating of semiconductor structure due to the dissipated power up to tens of watts. This requirement gets considerably stringent for the long-wave VECSELs operating in radio photonics devices due to worse heat conductivity of compound semiconductors applied in C-range (see above). The alloyed structure of active region and mirrors with integrated diamond radiator installed on the copper bedding are used as the prospective solution of this problem here in the same manner as in VCSEL [36, 37]. Example of the transverse structure of alloyed ISC [38] is shown in Fig. 15. Significant difference in comparison with Fig. 2 consists in the introduction of the region providing the expansion of carriers flow towards the top electrode with ring shape between the quantum dimensional intensifying layer and top reflector, which is necessary for the improvement of laser quantum efficiency. In order to enhance the homogeneity of current distribution over the region, the tunnel junction (TJ), the crosswise dimensions of which correspond to the thickness of current expansion region, is used here in the same way as in VCSEL. The trajectories of injection current flowing between the top and bottom disk electrodes are indicated by dashed lines. The intermediate semitransparent (70%) Bragg reflector based on the layers AlGaAs/GaAs of n-type is designated for the compensation of absorption losses in the doped regions of ISC. The deposition of copper plating coating on the bottom electrode ensures the efficient heat removal with the thermal resistance within the limits of 13–25 K/W in the whole range of device operating temperatures.
Using the ISC described above, at the present time two prospective approaches for the construction of high-power VECSELs have formed; they can be applied in the capacity of driving generators of multi-element radio photon antenna array with SHF range and in the capacity of pumping source (1450–1480 nm) of fiber erbium and Raman amplifiers: use of thermoelectric microcooler (TEMC) [38] for the devices with electrical pumping (Fig. 16а) and water or air cooling [39] for the devices with optical pumping (Fig. 16b). When studying the schemes in Fig. 16, the electrical and energy characteristics were obtained which are given in Fig. 17a and 17b respectively. Thus, during the operation of long-wave VECSEL with electrical pumping in continuous single-frequency mode the output power of more than 100 mW can be obtained which exceeds the typical output power of VCSEL with the same spectral range by 40 times (see Fig. 4). Application of optical pumping in the laser with S- and C-ranges grants the opportunity to increase it up to 1–3 W at the temperature of heat sink up to 50 °C.
As is known, besides the output power, the quality of operation of the generator with any frequency range is characterized by the noise parameters which are typically determined [3] in the form of relative intensity noise[4] (RIN) and width of emission line by half level for lasers. Examples of RIN characteristics for long-wave OP-VECSEL with С-range and power of about 100 mW at the room temperature [40] are given in Fig. 18. As it follows from figure, the intensity noise of VECSEL is less than –160 dB/Hz near the carrier and reaches the threshold of measurement device (–170 dB/Hz) at the frequencies of more than 500 MHz which are considerably lower than the analogous parameter for VCSEL [15] and edge-emitting leaser as well [3]. The fundamental reason for it consists in greater ratio of photon lifetime in resonant cavity and carriers in ISC. As it has been mentioned already, the VECSEL advantage also consists in the principal narrowing of emission line because its width is inversely proportional to the length of resonant cavity [3]. Example of the spectral characteristics of OP-VECSEL with С-range and output power of 77 mW at the room temperature [41] is shown in Fig. 19. In figure the first insertion shows the result of the measurement using the optical spectrum analyzer in the narrow band near carrier; the left insertion – result of the measurement using auto-heterodyne methods with the help of radio technical spectrum analyzer. As it follows from figure, in long-wave OP-VECSEL it is possible to provide the single-mode operation with the suppression of side modes of more than 60 dB and emission line with the width of tens of kilohertz which is narrower than so-called Lorentz line (dashed curve in the left insertion). It should be noted that above mentioned data is significantly better than the results of the measurement of VCSEL spectral characteristics (see Fig. 7 and 9).
One more significant shortcoming of the VCSEL with continuous operation, which has been detected in the process of photon technology development, consists in insufficiently broad band of generation line tuning. Particularly, according to Fig. 8 at the simplest control of displacement current and temperature it does not exceed several nanometers, whereas for the efficient functioning of some radio photon devices (for example, beamformer) the tuning, at least, in the whole C-range (35 nm) is required. In order to solve this problem, at the present time two approaches are suggested using the regulation of physical or optical length of VCSEL resonant cavity, which is comparable with the radiation wavelength, as it has been mentioned already. The laser, in which the first approach is implemented, is called MEMS-VCSEL[5]; the most widespread name of the laser of the second approach is LICSEL[6]. These structures have been widely studied by the various university scientific groups over the last 10 years. In the course of the works, the large scope of results has been obtained and, therefore, only the most significant results are considered below.
The typical transverse structure of modern MEMS-VCSEL with С-range [42] is shown in Fig. 20. The general principle of its construction consists in the formation of top Bragg mirror (see Fig. 2) in the form of moving micro-electromechanical membrane. Example of the characteristic of MEMS-VCSEL tuning in Fig. 20 at the fixed displacement current and stabilized crystal temperature at the level of 20°С is shown in Fig. 21. As is seen from figure, the band of continuous tuning is equal to 102 nm, which is greater than the tuning band of standard VCSEL by 25 times (see Fig. 8). From the point of view of the operation in practical device, the obvious shortcoming of the considered structure of MEMS-VCSEL includes the dependence of power and, especially, radiation wavelength on the ambient temperature, which corresponds to the standard VCSEL (Fig. 8). In order to compensate the temperature sensitivity of wavelength, the specific structure of MEMS-VCSEL [43] is suggested, in which the top mirror is installed on the micro-cantilever with the length of about 100 μm (Fig. 22). According to the specified experimental studies, the minimum temperature sensitivity of radiation wavelength in C-range was equal to 0.0016 nm/°С. This result corresponds approximately to 100-fold sensitivity reduction in comparison with the standard VCSEL and allows considering that the described structure operates in athermal mode or, in other words, the availability of thermostating node, which is typical for semiconductor laser, with the energy consumption power of several watts is not required for it.
More economic solution of the problem connected with the increase of tuning band of VCSEL emission wavelength is believed to be the introduction of transparent liquid-crystal layer, the refractive index of which varies under the action of external electric voltage VLC, inside the resonant cavity; it provides the opportunity to regulate the optical length of laser resonant cavity. Example of the transverse structure of long-wave LICSEL [44] is illustrated in Fig. 23. The main difference from the standard VCSEL in Fig. 2 consists in the introduction of the intracavity layer 6 based on the nematic liquid-crystal structure. The typical characteristics of LICSEL tuning in Fig. 23 at the fixed displacement current and stabilized crystal temperature at the level of 20°С is given in Fig. 24. As is seen from figure, the band of continuous tuning within the variation range VLC=0…2.4 V is equal to 34 nm, which is higher than the tuning band of the standard VCSEL (see Fig. 8) almost by 10 times and in the majority of practical cases it meets the requirements specified for radio photon devices.
It should be noted that the implementation of athermal MEMS-VCSEL or simpler functional analog, which is represented by LICSEL, provides the way for the efficient use of radio photon technologies in airborne radio instruments with the strict requirements for energy consumption. In this case, the temperature stabilization of emission spectrum with the power consumption of several watts, which is required during the operation of semiconductor laser of any type, can be replaced by practically non-consuming frequency stabilization operating in analogous manner as the radio technical scheme of frequency automatic tuning.
Lasers with Pulse Generation:
VECSEL-SESAM, MIXSEL
The development of vicselonics component base is connected not only with above described lasers of continuous generation but with pulse lasers as well. In this area, the significant distinctive feature of photon technologies is the capability of simple generation of the sequence of coherent ultra-short optical pulses using so-called lasers with passive mode synchronization (PMS). At the present time, the lasers with PMS found broad application in TFOS, spectroscopy, material science etc. The prospects of their use in radio photonics devices consists, for example, in the creation of multi-wave precision generators of optical pulses for beamformers and radio photon analog-to-digital converters which will be described in the second part of this review.
The PMS engineering has been developing for several decades already on the basis of solid-state and semiconductor lasers mainly in short-wave (in the region of 1 μm) spectral range. The scientific area received the new pulse with the occurrence of VECSEL combining the advantages of solid-state and semiconductor lasers, as it has been mentioned already. The traditional PMS scheme contains two fundamental elements: active laser structure and saturable absorber, in which the loss modulation in resonant cavity is performed. In the practical devices, this operation is typically carried out using the mirror on the basis of semiconductor saturable absorber called SESAM[7]. The structure of SESAM contains semiconductor DBR (distributed Bragg reflector), into which the quantum dimensional absorber layers are introduced. At the present time, two construction schemes of VECSEL with pulse generation are known [45, 46]: with separate ISC and SESAM (Fig. 25а) called VECSEL-SESAM and with integrated ISC and SESAM (Fig. 25b) named MIXSEL[8]. The optical pumping, which is introduced according to Fig. 16b, is used for the operation of both schemes. As a rule, the monolithic structure in Fig. 25b is grown by means of the molecular-beam epitaxy and it contains five sections: 1 – high-reflectivity AlAs/GaAs DBR; 2 – absorber based on the layer of self-organized InAs quantum dots implanted into the low-temperature layers of GaAs; 3 – intermediate AlGaAs/AlAs DPR preventing the pump signal absorption in absorber section; 4 – intensifying section with seven quantum-dimensional layers InGaAs; 5 – antireflection coating which provides the optimal distribution of intensification and absorption section fields.
The recent studies and developments of VECSEL-SESAM and MIXSEL were mainly executed in the areas of search for the methods of increase of mean emission power, decrease of optical pulse duration with the simultaneous shift of repetition frequencies to SHF range, which is the necessary condition for their use in the radio photonics devices. At the same time, in the scheme of Fig. 25 the effect, which occurred as a result of the application of epitaxial layers based on quantum wells (QW) and quantum dots (QD) in the structures of VECSEL (or more precisely, in ISC) and in SESAM, was studied. Obtained results at the repetition frequencies of 2 to 50 GHz [45, 47] are shown in the diagram in Fig. 26. Their analysis allows drawing the following conclusions. In the MIXSEL scheme the highest mean emission power (up to 6.4 W) is obtained but at the same time the pulse duration exceeds 20 ps which is not appropriate for the majority of radio photonics devices. Pulses with the duration of less than 200 fs at the mean power of about 20 mW are provided with the use of the laser with PMS of QW-layers in both elements. The parameters (pulse duration less than 1 ps, mean power of 100 mW…1 W), which are optimal for the application in radio photonics, can be achieved under the condition of implementation of both elements on the basis of QD-layers. Nevertheless, the further improvement of operation of the MIXSEL scheme allowed obtaining more prospective results [48] – pulse duration of 570 fs within the range of repetition frequencies 5–101 GHz at the mean power of 127 mW.
All above-listed experimental data is obtained for short-wave lasers, which are studied in the widest manner. As for now, the number of the publications devoted to long-wave MIXSEL is much smaller and the results described in them are considerably more meager. In the technological aspect, the alloyed structure is considered to be prospective in the same manner as for VCSEL (see Fig. 2, 3) and in ISC of high-power VECSEL with continuous generation (see Fig. 15). Example of the scheme of long-wave VECSEL-SESAM with Z-shape configuration using the optical pumping [49] is shown in Fig. 27а. For this VECSEL-SESAM the ISC (Fig. 27b), in which the active region grown on InP substrate with the use of metal-organic vapor-phase epitaxial device and DBR grown on GaAs substrate with the use of molecular-beam epitaxial device are alloyed, was specifically developed. During the experimental study, the ISC temperature was maintained at the level of 15 °C using the water cooling, and SESAM temperature – at the level of 21 °C using TEMC. During the measurements the following results were obtained: the pulse duration of 6.4 ps at the mean emission power of about 100 mW and pumping power of about 9 W. The repetition frequency determined on the basis of the total resonant cavity length of the scheme in Fig. 27a was 950 MHz. Higher repetition frequency was obtained during the experiments with MIXSEL with С-range [50], results of which are given in Fig. 28. In the scheme the ISC with hybrid metamorphic mirror, which was optimized for high emission power at the room temperature, and fast-response SESAM based on InGaAsN/GaAsN were used and this fact allowed avoiding the necessity of water cooling application. In Fig. 28, obtained autocorrelation function, which agrees with the secant-square pulse profile, is specified on the left side and corresponding Fourier spectrogram on which the spectrum width by half level is marked out – on the right side. In Fig. 28b, the radiofrequency spectrum with pulse sequence obtained at the exit of high-speed photodetector with the resolution RBW=30 kHz is given on the left side, and spectrum of the first harmonics obtained with the resolution RBW=30 Hz – on the right side. As it follows from the experimental data, the pulse duration is equal to 1.7 ps and corresponds with the signal total bandwidth of MIXSEL of 2.29 nm (about 300 GHz), the pulse repetition frequency is 2 GHz, generation line width, which characterizes the device noise properties, does not exceed 1 kHz.
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[1] Foreign equivalent: Microwave photonics.
[2] www.epfl.ch
[3] Vertical External Cavity Surface-Emitting Laser
[4] Related Intensity Noise
[5] Micro Electro-Mechanical System-VCSEL
[6] Liquid Crystal Surface Emitting Laser
[7] Semiconductor Saturable Absorber Mirror
[8] Mode-locked Integrated External-Cavity Surface Emitting Laser
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