Stabilization of wavelength of diode laser radiation. Dynamic characteristics of Peltier Elements
Semiconductor laser diodes found broad application in various technical and scientific areas due to a number of advantages in comparison with other sources of monochromatic radiation. Relative low cost and pumping simplicity, which have become practical breakthrough at one time, refer to the main advantages of these lasers in comparison with other emitters; we should add here the compact size of source construction. However, compared with other lasers the semiconductor laser diodes have low coherence of beam. For the last half a century, semiconductor lasers have covered long way, and their technology have had many modifications and improvements. Lasers can be divided into high-power diodes, which allow obtaining up to several tens of Watts of optical power , and precision diodes with the line width of 30 kHz to 2 MHz and power up to 1 W . The conditions for stabilization of parameters of the second group are considered in this article.
Application range of laser diodes of the second group is very broad. Thus, the methods of laser diode spectroscopy, which are broadly used in the studies of parameters of gas media (structure, concentration, temperature, pressure) use the whole number of existing semiconductors in order to cover maximum spectral range.
In accordance with the requirements, the needed type of semiconductor emitter is selected, for example, DFB, ECL or FP where:
• FP – laser diodes with Fabry-Perot resonator, which is formed by the end surfaces surrounding the heterogeneous junction. One of the surfaces practically completely reflects the radiation, and the other one is semitransparent and promotes its output. It has the best ratio of price-quality but at the same time it generates multimodal radiation;
• DFB – laser diodes with distributed feedback, resonators of which are represented by the modification of Fabry-Perot resonator with the addition of periodic spatial modulation structure. This structure influences on the radiation characteristics and conditions of radiation propagation. Advantage of these lasers in comparison with FP includes lower dependence of laser wavelength on injection current and temperature, high level of stability of single mode;
• ECL – external cavity lasers. Using this cavity it is possible to perform wavelength adjustment and use narrow-band filters (for example, Bragg grating) in the capacity of resonator mirror; it ensures the single mode.
Practically all above-listed parameters of laser diodes have one or another dependence on crystal temperature . Let us analyze its influence on the stability of radiation wavelength.
In any laser of semiconductor type, the radiation generation is invariably accompanied by unwanted heat-producing processes, such as nonradiative recombination, Auger effect or surface recombination. Even in high-efficiency radiators the major part of electric energy is transformed into the heat energy. Temperature growth results in the deterioration of laser basic characteristics, such as threshold current and output power, as well as spectral characteristics, and as a result it can cause the breakdown .
In the paper , the main reasons for displacement of laser diode wavelength are shown in case of variation of its temperature; it results in the function of the following type: λ ( T ) ~ ( 1 - αT ) –1, where α is the temperature coefficient of alteration of width of semiconductor band gap. In practice, the operating temperature range covers only small part of the curve, which is approximated by linear  and quadratic  dependences in experimental studies:
λ ( T ) = χ T 2 + β ( 0 ) T + λ0
where χ is nonlinearity coefficient having the dimensionality of nm/°C2, β ( 0 ) is coefficient of dependence of wavelength on temperature at Т = 0 °C nm/°C, λ0 is some nominal (certified) value of radiation wavelength, nm.
The average value of nonlinearity coefficient χ for different laser diodes varies around the value of –0.0002 nm/°C2, for coefficient β (0) such value is equal to 0.1 nm/°C. It is possible to calculate that at the temperature spread from 0 to 40 °C the variation of wavelength will be equal to 3 nm. This property can be used for the organization of deliberate adjustment of wavelength . The main restriction of temperature range from the side of high temperatures is the danger of damage of laser diode due to burning-out of optical coatings, which form the resonator. However, from both sides of the range the other restriction is in effect: alteration of transfer functions of the elements forming the circuit of temperature stabilization and causing the decrease of stability of temperature maintenance. Firstly, nonlinear characteristic of thermistor at high temperatures loses the steepness, which causes the temperature signal attenuation. Secondly, when increasing the module of temperature difference between the external radiator (body of laser module) and laser diode, the efficiency of heat removal through Peltier element is reduced, and this fact is understood as the decrease of amplification coefficient from the point of view of regulation circuit. Influence of both processes results in the fact that the circuit, which is optimally tuned up to the operation at room temperature, at the margins of the range demonstrates significantly lower stability of wavelength.
It should be noted that the wavelength of semiconductor laser has strong dependence not only on the temperature but also on the injection current . In this paper it is assumed that in the application ranges, which require maximum possible frequency stability for certain type of laser, continuous operation mode is used, in other words, the optical power is constant. This condition can be fulfilled by the combination of constant injection current (which is quite trivial task) and constant radiator temperature. The task of analysis of frequency and temperature stability under conditions of direct current modulation of optical power, for example, goes beyond the scope of this paper.
Also, there is reversed situation as well. Performing the process of temperature stabilization in fixed operating point at invariable injection current, the optical power will be constant. However, in case of displacement of operating point by the temperature the prior value of injection current will correspond to the new value of optical power . In this paper this matter is not considered because the maximum level of stability by wavelength, as a rule, is needed in the devices and methods operating not by the tracking of amplitude or intensity but by frequency. In such case, the displacement in the average level of power does not have influence on the operation of systems.
CONSTRUCTION OF HEAT SINK OF LASER DIODES IN BUTTERFLY BODY AND WAVELENGTH STABILITY
The basic layout of heat sink of laser diodes in Butterfly bodies or analogs is shown in Fig. 2. Due to poor heat conduction of semiconductor, the passive heat sink is not sufficient for the laser diodes with the power of more than 5 mW, strong temperature fluctuations deteriorate all radiation parameters. Therefore, often the radiating crystal is located on active heat-sinking element (Peltier element), which in turn is connected to radiator with sufficient heat capacity and area of heat sink surface.
Peltier element itself (or thermal electrical module) is represented by the set of thermocouples, changing the temperature difference on its operating surfaces depending on the direction and value of flowing current. The main parameters of the elements include operating current and voltage, maximum refrigeration capacity in Watts, which is equal to 55–60% of electrical power consumption, as a rule.
The elementary scheme of Peltier driver is shown in Fig. 3. Its operation is based on the presence of feedback by temperature in the system – crystal heating results in variation of the resistance of thermistor, which is built in close proximity from radiator and influences on the amplification stage of Peltier element. Through Peltier element the current is altered towards the side, which allows compensating the temperature fluctuations of crystal.
In order to analyze existing stability by wavelength, let us measure the wavelength on a real-time basis in relation to the laser diode, driver of which is arranged in accordance with the scheme (see Fig.3), using the wavelength meter Angstrom WSU-2 . The instrument is based on several Fizeau interferometers examined by photodiode arrays. The radiation is supplied to wedge prism dividing the flux into two components, which interfere in radiation detector plane. Resultant wavelength is the result of Fourier transformation from intensity distribution of interference pattern in photodetector plane. The margin of permissible error by the wavelength for this device upon short-term measurement is equal to 30 pm (by frequency – 4 mHz). In case of long-term operation, the margin can grow to 150 pm (depending on the periodicity of instrument calibrations), and this fact does not have impact on short-term measurements of wavelength stability. Operating spectral range of the instrument is from 1 300 to 1 650 nm.
As it is seen from the measurement results (Fig. 4), the drift of wavelength in 5 minutes of operation was more than 10 pm (1.3 GHz), and this value corresponds to the relative stability 6.6 · 10–6. This result is estimated as moderate result and it is not suitable for the application as reference channel of infrared Fourier spectrometer, for example .
Even during the visual analysis of obtained plot, the conclusion can be drawn that the observed noise has broad spectrum – from pronounced low-frequency component to conventionally high-frequency spikes, whereas the latter have noticeably greater contribution into spread of values.
These high-frequency noises can be result of various radioelectronic disturbances, which can be controlled in a classical way by the introduction of measuring bridge with differential amplifier. Modernized implementation of Peltier driver is shown in Fig. 5. Thermistor plays the role of lower arm of measuring bridge connected to one of the inputs of instrumental amplifier. In case when the temperature is constant, the voltage difference at amplifier inputs is equal to the constant value, and the amplifier sets the constant signal, which is required for Peltier operation under the conditions of balance between the processes of crystal self-heating during radiation and cooling. When the balance is shifted to one or another side, the temperature starts changing, the thermistor resistance is changed and the amplifier starts setting the signal, which is equal to the difference of input voltages multiplied by amplification coefficient. This signal gets at the input of power output Peltier stage on the basis of operational amplifiers, then it is repeatedly amplified and at the final stage it changes Peltier current. The second input of instrumental amplifier is connected with digital-to-analog converter of microcontroller and allows setting the operating temperature, which the system will try to maintain.
The measurements of the wavelength of modernized driver are repeatedly given (Fig. 6). Mutual subtraction of identical noises at the inputs of differential amplifier gave the noticeable result – decrease of spread to three hundred femtometers, in other words, by 3–12 times. The visual presentation of decrease of noise level can be obtained from the comparative Fourier analysis of the results of two measurements shown in Fig. 7. The operating point by temperature for laser diode is equal to room temperature (25 єС), and in this conditions the operation of Peltier element is the most efficient.
For the further increase of temperature stability as the main factor influencing on diode parameters, it is necessary to analyze the dynamic characteristics of used Peltier elements and peculiarities of formation of cooling circuit on its basis.
During the analysis the following models of laser diodes were used: – EP1310-ADF-B, manufactured by EBLANA PHOTONICS, Ireland. According to documented parameters, the maximum current through Peltier element is 1.8 A; – LDIH-DFB (CWDM)-1550-30P-T2-SM-FC/APC, manufactured by Laserscom, Belarus. Maximum current through Peltier element is 1.5 А; – LC96, manufactured by Oclaro, USA. Current through Peltier element is 1.8 A, Peltier voltage 3 V, refrigeration capacity – 3.3 W.
Control circuit, which is not optimized in accordance with the dynamic properties of Peltier element, will not allow fulfilling the characteristics set for certain laser diode. In order to determine which actions are needed for the enhancement of circuit efficiency, it is required to study qualitatively the transmission characteristics of thermoelectric circuit of thermistor – Peltier.
Let us introduce the concept of thermoelectric transmission characteristic describing response of the system, which is covered by feedback by the temperature, to the input signal with certain frequency. In order to determine thermoelectric characteristics of Peltier elements of various laser diodes, the following circuit was assembled (Fig. 8).
Supplied sinusoidal signal at Peltier input results in the periodic temperature variation in the system. Thermistor is connected to one of the inputs of instrumental amplifier. Variation of thermistor resistance has impact at the output signal of operational unit.
On the basis of comparison between the input signal and thermistor response, the amplitude-frequency characteristic (AFC) and phase-frequency characteristic (PFC) of the feedback circuit can be formed by the temperature (Fig. 9 and 10). Several conclusions can be drawn on the basis of analysis of obtained dependences. Firstly, as it was assumed, with the growth of frequency thermistor "does not keep pace" with temperature changes. It is seen from the decrease of amplitude of output signal fluctuations and gradually growing difference between the phases of output and input signals. Secondly, the dynamic characteristics of various batch-wise used Peltier elements are qualitatively similar and can be maintained by universal circuit. Nevertheless, in order to reach the highest characteristics the circuit must be additionally coordinated with AFC/PFC of the specific used product.
CHECK OF AFC OF PELTIER ELEMENT THROUGH WAVELENGTH MODULATION
Roll-off of amplitude-frequency characteristic of our system in the area of 0.5–1.0 Hz is stipulated by inertness of thermal elements contained in the circuit: thermistor, Peltier element and their temperature connection. In order to evaluate individual contribution of Peltier element into thermoelectric characteristic, we organized the following experiment.
The software-based control of laser driver was executed through UART interface, using which the sinusoidal signal altering the operating point of temperature of thermal stabilization system was given at controller digital-to-analog converter. Responding to the temperature difference the diode radiation turns out to be modulated.
Recording the spread of radiation frequency fluctuations, gradually increasing the oscillation frequency, we obtained AFC of optical response. As it is seen from the plot in Fig. 12, AFC of optical response is similar to AFC of thermoelectric feedback, and this fact proves the inertness of heat-sinking element one more time. The lower attenuation of optical response at the frequency increase can be explained by the fact that radiating crystal is more sensitive to temperature difference than the feedback of thermal stabilization system.
Results of the studies showed that in order to reach the highest stability of wavelength semiconductor laser radiation it is necessary to perform the optimization of thermal stabilization process.
In order to suppress the common-mode interference, the circuit of thermal stabilization must include the measuring bridge in combination with differential amplifier.
Dynamic characteristics of different batch-wise used Peltier elements are qualitatively similar and can be maintained by the universal circuit.
In order to reach the highest characteristics, the circuit must be additionally coordinated with AFC/PFC of the specific used product.