Development of Construction Principles of Variable Optoelectronic Attenuators
Occurrence of fiber optics, which is stipulated by the creation of coherent light sources, in particular, resulted in the necessity of development of measuring instruments capable to function similarly to the instruments operating within radio-frequency band. It is referred to the generators, attenuators, watt-meters and other devices intended for the operation with radiation within optical range.
Attenuators are used for the variation of light flux level before recording device , measurement of sensitivity of optoelectronic devices and improvement of detector operation. For example, detector transmission to linear characteristic sector . The following principles of light attenuation are observed: use of radiation beam divergence, photometric grids, diaphragms, rotating disk with the alteration of sectors with different transparency, scattering or absorbing media, combinations of lenses and diaphragms, multiple Fresnel reflection.
In this paper, the peculiarities of construction, functional capabilities of fiber optic attenuators produced by NRIEI "Kvarz" and methods of their improvement are described.
STRUCTURAL PECULIARITIES AND FUNCTIONAL CAPABILITIES OF FIBER OPTIC ATTENUATORS
When transiting to fiber optics, the objectives and principles of radiation attenuation remain the same; however, the structural design of attenuators becomes more complex due to the fact that in most cases the fiber couplers are present at their entry and exit. To be more definite, hereinafter the dampers mean elements, in which the radiation absorption takes place, and attenuator is the device, in which this element is placed with matching and mechanical units.
Fiber optic attenuators can be classified on the basis of different features.
One of such features consists in the character of radiation attenuation. There are models of attenuators which can reduce the radiation intensity by fixed value. The switch of discrete values of attenuation is implemented in some devices. Some attenuators are developed for continuous variation of intensity. In addition, there are modifications, in which step and smooth adjustments of light attenuation are combined.
Based on the intended purpose, attenuators can be built into more complex units or produced as independent items.
Based on the method of loss introduction, attenuators are divided into sequential and concurrent attenuators. In the first case, the sequential switch of several elements with different values of attenuation takes place. In the second case, there is simultaneous switching-on of several elements with different values of attenuation.
Attenuator control can be mechanical and programmable.
Development of fiber optic attenuators at NRIEI "Kvarz" started in the 80s of last century. When creating the first domestic fiber optic generators OG4–162 and OG4–163, the necessity of inclusion of attenuator with variable attenuation value into them occurred . Attenuation of optical radiation was performed at the expense of diffraction losses during the radiation transmission between the ends of optically connected light guides. Change of attenuation value within the range 0–25 dB was performed by the variation of distance L between the ends of light guides (Fig. 1). At the same time, during the adjustment of attenuation value the resolution was not worse than 0.1 dB, error was not less than 0.5 dB, and initial losses did not exceed 0.8 dB.
Structurally the attenuator consists of the guide, in which the heads with accurately centered light guides are located. Regulation of required value of attenuation is performed by the handle connected with displacement mechanism of one of the heads and indicating device (Fig. 2).
The best reproducibility of attenuation parameters can be achieved on the basis of operation with parallel beams and via the interruption of fiber optic path and installation of lens matching elements at its entry and exit. Microlenses of matching elements are adjusted relatively to fiber ends and each other by minimum value of initial introduced losses. The accuracy of installation of fiber end must not be worse than 0.5 µm in cross-section and 5 µm in longitudinal section.
Attenuation of optical radiation is achieved by the introduction of one or several discrete dampers (Fig. 3).
Attenuation at the radiation wavelength at the entry to channel n of reducers is equal to
n is the number of introduced discrete dampers,
ρi (λ) is the reflection from surface of ith absorber,
Li is the thickness of ith absorber.
The accurate value of attenuation is achieved by the introduction of variable damper, which changes radiation intensity through the variation of rotation angle of disk with applied absorber.
α is the disk rotation angle,
f (α) is the dependence of film thickness on rotation angle,
ρпер (α, λ) is the coefficient of decrease of radiation intensity for smooth damper determined by the reflection on surface with applied absorber.
Built-in attenuator with the method of loss introduction of sequential type is shown in Fig. 4. Five discrete dampers with fixed values of attenuation from 1 to 30 dB are located on the disk and upon its rotation they can be sequentially introduced into optical channel. The disk is connected with control knob by the train of gears. It is located between the entry and exit matching elements. Operating wavelengths are 0.85; 1.3; 1.55 µm.
Built-in attenuators can be integrated into fiber optic generators OG4–181 and OG4–182 as well . The channel with parallel radiation beam is formed in optical blocks of these generators; this channel contains the following components located sequentially: coupler, smooth attenuator, additional coupler, discrete attenuators with the attenuation of 20 and 40 dB introduced into the channel with the help of relay. Signal recording from the couplers using photodiodes allow controlling the radiation power at the entry and after smooth damper. In addition, the introduction of switch into the structure results in the capability to cut off the parallel beam completely.
All developed attenuators, except for the models built into the generators OG4–162 and OG4–163, are constructed on the basis of loss introduction into parallel beam between two matching units. The sequential and concurrent methods of loss introduction are used. The advantage of concurrent method, which is implemented in all programmable attenuators, includes the capability of obtainment of limit values of attenuation upon comparably low characteristics of attenuation of constituent elements. Among the disadvantages, the increase of initial losses should be noted; it occurs due to the increase of distance between matching elements.
The range of attenuators, which represent independent devices providing the different degree of attenuation, is developed at NRIEI "Kvarz". The attenuator with concurrent method of attenuation introduction should be marked out among them (Fig. 5); it consists of coaxially arranged dampers with the set values of attenuation. The device allows using any combination of dampers within the range 0–80 dB with the step 3–4 dB.
In the attenuator showed in Fig. 6, the combined method of loss introduction is implemented. Discrete elements with set attenuation values from 3 to 30 dB are installed on the disk and serially introduced into radiation beam. Sequentially with the disk where the discrete dampers are arranged, the disk of smooth damper is located; during its rotation the radiation absorption varies from 0 to 20 dB. These two disks are connected with control knobs by the train of gears. The limbs calibrated in the units dB are located on the front panel of attenuator.
Besides the disks, the plate, which has 3 positions (opened, closed and additional damper with attenuation of 20 or 30 dB) is introduced into radiation beam in the area of back panel using the electric motor. The total range of variation of attenuation values is from 0 to 80 dB. Specifically developed matching units are arranged in such manner that the radiation using microlenses is focused on the ends of outer couplers connected to attenuator. Attenuator modification, in which the standard sockets are arranged at entry and exit, is developed; these sockets are connected with matching units using internal standard couplers (Fig. 7).
The most technological method of damper production consists in the film deposition on transparent substrate. The main requirement specified in relation to the deposition material, is minimum dependence of attenuation coefficient on wavelength. This aspect is primarily met by the films of titanium and nickel. The films of nickel have the lowest selectivity; however, they are characterized by low adhesion to glass. Therefore, the dampers are made on the basis of titanium films.
In addition to absorbing coatings, specifically developed antireflection coatings (MgF2-Al2O3-ZrO2-ZnS) were used for the production of attenuators in order to decrease the reflection from lenses in matching elements and eliminate the interference in dampers (Fig. 8).
Use of piezoelectric motors is the prospective area for automation of attenuator operation in control and measuring equipment of fiber optic communication lines (FOCL). Vibration motors have a number of specific properties: high sensitivity – within the limits 0.9–0.1 µm, low time constant – about 0.1 ms, variation of angular velocity within wide range, instant fixation of rotor position at the expense of dry friction, miniature dimensions and simple structure. All these peculiarities allow creating electrically operated combined attenuator with smooth and stage adjustment of absorption of collimated radiation beam (Fig. 9).
Programmable attenuators are the most perfect devices, which are used for attenuation of optical radiation.
The first domestic programmable attenuator VM 1602 was intended for the attenuation of signals within optical range in the structure of module control and measuring instrumentation with interface bus VXI (Fig. 10). The range of variation of attenuation values is 0–60 dB with the error of its adjustment of 0.2 dB. Operating radiation wavelength corresponds to 1.3±0.05 µm. Attenuator contains optical block, micro-controller and interface VXI.
Attenuator control is performed using the user graphic interface developed on the basis of modern methods of visual design. Videowall for device interactive control executed in the form of autonomous application allows using the attenuator with OS Windows 2000/XP/7 (Fig. 11).
Microcontroller consists of the units for control of discrete and smooth dampers and units for signal generation in case of their actuation.
The programmable fiber optic attenuator OD1–28 (Fig. 12) developed for automated workplace OK6–13  is independent device with metrological parameters, and it is entered into the State Register of Measuring Devices of the Russian Federation.
Attenuator consists of the base block and three replaceable modules, each one of which is designed for the corresponding wavelength (Fig. 13).
Optical module with the actuators of dampers and control device is located in replaceable block (Fig. 14).
Optical module represents the segment of fiber optic path with line interruption. Lens matching elements are installed at the entry and exit of interruption. Signal attenuation is performed within the range from 0 to 90 dB. Operating wavelengths correspond to 0.85, 1.31 and 1.55 µm. Introduction of seven discrete dampers with the attenuation values of 2, 3, 4, 8, 16, 30 and 32 dB and one damper with variable value from 0 to 3 dB is used for attenuation in optical channel. The dampers are located between the lens matching elements.
The error of adjustment of attenuator damping corresponds to ΔА = (0.2+0.013А) dB, where А is the nominal value of reproduced damping (dB). The initial losses are not more than 3 dB.
Introduction of discrete dampers into the optical channel is performed using the electromagnetic actuators, and control of variable damper is performed via the electric motor with reducer. Electromagnetic actuator with curtain is installed for complete cut off of optical channel.
Control of the system of electromagnetic actuators and electric motor is performed by microcontroller located in base block. The levels of controlling signals are generated in control devices located in the device replaceable modules. Attenuator works in manual and remote mode of control through the interfaces CUC, USB or RS 232.
Use of thin-film discrete dampers with high value of attenuation (16–32 dB) within wide spectral range (1300–1700 nm) causes the significant increase of spectrally-dependent component of attenuation error. Such error can reach units of dB at the boundaries of spectral range, and therefore in many cases it is not compensated by variable damper.
Direct increase of the value of variable damper by 5–7 dB results in the decrease of precision of attenuator dampening and occurrence of additional spectrally-dependent error introduced by the variable damper.
METHODS OF DECREASE OF ATTENUATOR DAMPENING ERROR
Use of complex structurally-technological methods for decrease of dampening error of discrete dampers does not always provide desired result. First of all, it is determined by the final precision of production of thin-film optical dampers. It should be noted that the error dispersion of coating sputtering reaches units and tens percent. Besides, the process of production of thin-film dampers is associated with high economic costs. In addition, the parameters of optical dampers depend on the ambient temperature.
The significant increase of metrological characteristics of fiber optic attenuators with discrete dampers can be performed at the expense of the use of algorithmic methods of correction of set dampening value, which are based on mathematical processing of information in relation to the actual parameters of discrete and variable-optical dampers.
Solution of the task of error correction of discrete dampers within wide spectral range was performed in two stages.
At the first stage using the method of standard gages, the individual calibration characteristics of dampers as the functions of the wavelength ai (λ) were obtained. The difference of values of calibration characteristic and standard gages was recorded in device memory in the form of array. The values of corrections at the wavelengths, which differ from calibration wavelengths, were obtained using the method of piecewise-linear approximation. This method was used for the purposes of decrease of the volume of internal memory of built-in computing instruments, reduction of the number of standard gages and calibration time.
However, the error correction of attenuator dampening using the array of corrections for all combinations of discrete dampers does not allow providing the required precision of attenuation adjustment.
Therefore, the method of optimization of combinations of discrete damper introduction was additionally developed on the basis of corrections stored in memory.
The method consists in the fact that during adjustment of set dampening value using built-in computing instruments of device such selection of introduction of discrete dampers is performed so that their total error for chosen wavelength would be lower than the value of variable damper. Thus, the following inequality is observed:
Ауст is the dampening set on front panel,
ai are the actual values of dampening of discrete dampers taking into account the corrections,
апл is the actual dampening of variable damper.
In case of perfectly precise discrete dampers, the value of dampening typed on the front panel (for example, 29 dB) can be implemented by the set of 16 dB + 8 dB + 4 dB discrete dampers and +1 dB smooth damper. However, the discrete dampers are made with certain error. Specific values of attenuation are entered into the device memory. The deviations of attenuation value of discrete dampers depending on wavelength are also included there. Thus, the optimal set for implementation of attenuation of 29 dB can be represented by totally different set, which is determined by computing instruments.
Use of such solution in fiber optic attenuator OD1–28 allowed decreasing the error level of attenuation device to 0.2 dB within wide spectral and dynamic ranges.
Modifications of the device OD1–28 include the attenuator rated at 3 wavelengths executed in one case (Fig. 15)  and attenuator rated at one wavelength (Fig. 16). The main parameters of abovementioned attenuators are given in Table.
Currently, the optimal nomenclature of fiber optic attenuators of different types and with different purposes, from special metrological systems to portable attenuators of tester type, is being determined at our institute. Their technical, functional and operational characteristics, which allow meeting the wide range of user requirements in various scientific and metrological organizations and industrial enterprises of our country, are being defined.