unctional capabilities of the devices are constantly expanded and it is connected with the continuous process of improvement of the electron components which were designed earlier and generation of new components. For the most part, it applies to the semiconductor components. Optoelectron pairs have significant role among them: source-detector or optocouplers operating in the first atmospheric transparency window (3–5 μm). Characteristic bands of the absorption of many industrial and natural gases (CH4, CO, CO2 and several other gases) are located in this spectrum range. The role of Ioffe Physical-Technical Institute in the creation of key component of such optocoupler – light emitting diode (LED) has been covered earlier in the review . However, that work was laconic and did not cover all aspects of the considered subject matter. In particular, such important peculiarity of the medium-wave IR emitters as their capability to generate negative luminescence was not mentioned. Also, the issues of application of similar LEDs in gas-analysis equipment designed on the basis of the results of radiation attenuation measurement upon radiation propagation through the analyzed gas mixture were not covered. Attempt to fill up this gap, cover the methods of performance enhancement of the optocouplers – LED-photodetector is made in the article. Also, the constructions of gas analyzers (GA) suggested by the institute scientists in different periods and afterwards, which were implemented in industrial GAs, are considered.
Works concerning the study of capabilities of use of the LED with medium-wave IR band (3–5 μm) in GAs based on the measurement of air transparency on designated wavelengths were commenced at the initiative of Professor Nasledov D. N. . They became natural continuation of the works related to the synthesizing and research of narrow-band compounds InSb and InAs in the Laboratory of Electron Semiconductors organized in Ioffe Physical-Technical Institute (PTI) in 1951. By the moment of commencement of "light emitting diode" works, the compounds А3В5 have proved to be good material which can be used for semiconductor lasers. Fundamental contribution to their design was made by the works of PTI; this team was awarded to the Lenin Prize in 1964. Works were devoted to the study of recombination radiation in GaAs. Thus, the headings which were typical for that time were given to the articles devoted to the GaAs-based lasers: "Hyperboloids’ of Professor Nasledov" , "Beam from Fairy-Tale" .
Expansion of works connected with the development of medium-wave LEDs was supported by the Academy of Sciences and industry organizations. In April 1972 Tuchkevich V. M., Academician, Director of PTI, received recognition letter for the operative solution of the issue connected with the design of the first samples of LEDs emitting on the wavelength of 4.2 μm and operating at room temperature, and for the justification of new prospective area in the creation of gas analyzers of the year. Significance of this event was so great that PTI and Special Design Engineering Bureau of Technical Cybernetics of Leningrad Polytechnic Institute (LPI) were in contention for the first place in the area of use of such LEDs in the legal field . Many PTI employees were involved in the study of methods of GA structural optimization. The main challenge in GA design became clear from the moment when the works were commenced – temperature instability of LED intensity against the background of insignificant changes of desired signal. Therefore, one of the first constructions of "light emitting diode" GAs (without mechanical chopper of radiation flux) provided the presence of reference channel  in the form of light guide for the correction of temperature instability.
But still, works connected with the new, or as it is commonly accepted to say, innovative trend expanded with insufficient speed. One of the reasons consisted in the fact that suggested variants of the LED constructions were based on the use of "bulk" materials – indium arsenide (for wavelengths of 3.8–4.2 μm)  and solid solution InGa1-xAsx (x < 0.06) (λ = 3.3–3.7 μm) [7, 8]). Therefore, they did not have many advantages of the heterostructures, which are similar to such advantages as controlled electrical constraint of recombination region of injected charge carriers, recombination radiation coupling through wide-band gap semiconductor layers ("windows"). Besides, quantum yield in the compensated indium arsenide suggested in the capacity of active region for the obtainment of radiation on the wavelength of 4.2 μm in  was extremely low. That is why, it is not surprising that development of studies found new lease of life only after the assimilation of epitaxial technology of structures growth on the basis of indium arsenide. It was developed under the supervision of Professor Rogachev A. A. in the Laboratory of Electron Semiconductors by the employees Stus N. M. and Talalakin G. N. . Only after five years they managed to improve LED characteristics considerably. Works carried out within the framework of long-term commercial agreement with the title "Research of Capabilities of Gas Analyzer Design on the Basis of Solid-State Radiation Source" concluded between the Ministry of Instrument-Making, Automation Facilities and Control Systems (All-Union Research Institute of Analytical Instrument Engineering, Kiev) and PTI preceded to the result. In 1983 uncooled LEDs based on the new solid solution InAsSb (P), which functioned on the wavelengths of more than 4.3 μm , were designed for the first time. In addition, InGaAs-based LEDs, which have become "traditional" already, also obtained hetero-epitaxial form. Also, it should be noted that epitaxial technology changed the concept of the "correct" wavelength of maximum LED radiation from indium arsenide – henceforth this maximum for undoped active regions from InAs was located in the region 3.3–3.4 μm (300 K) corresponding to its energy gap .
As opposed to the former technical solution , which suggested the application of reflectors for the formation of directed radiation beam, А.А.Rogachev offered to use one wonderful property of LEDs – their high brightness. As of today, the brightness achieved for the best samples of LEDs is equivalent to the brightness of the absolute black body heated up to 1250 K . Specifically brightness, but not integrated radiation power, became the key property which made it possible to create the prototype of high-sensitive methane analyzer with relatively small dimensions. Due to the small area of LED emission region, it was placed into the dispersive optical scheme with spherical mirror and plane diffraction grating. The spectral resolution of used grating was not worse than 0.02 μm and it turned out to be enough for the transmission measurement in the region of principal line of methane absorption (3.32–3.34 μm). Made prototype of GA (dimensions 350×150×200 mm) with PbSe photoresistor in the capacity of photodetector and LED based on p-InGaAs/n-InGaAs/n-InAs provided the detection of methane with the limit concentration in gas mixture of 0.002 mol% .
Construction which was offered by A.A.Rogachev got its further development in the multi-component analyzers. They made it possible to study and measure transmission spectrums of gases upon the scanning of spectrums with the help of monolithic LED arrays  and measure gas concentrations in wide spectral range using the set of discrete LEDs and concave grating .
Collapse of sectoral science which followed the dissolution of the USSR did not allow for the group of authors and invention applicant  to continue works and accomplish the industrial production of prototype. However, execution of works in PTI did not stop and it was possible thanks to the contacts with foreign specialists [15, 16]. Cooperation of PTI and Technical Research Center of Finland (VTT) made it possible to develop and design the miniaturized light emitting diode spectrometer. With the dimensions which are close to the size of matchbox, its optical resolution in the region of 3 μm in each of seven measurement channels was 60 nm. It was sufficient condition in order to carry out the qualitative analysis of complex mixtures . Example of such qualitative analysis based on the individual peculiarities of the absorption spectrums of different hydrocarbons can be found in .
In 1994 one of the best constructions of non-dispersive infrared GAs (Non Dispersional Irradium Radiation – NDIR)  was offered and subsequently it was used in commercially produced devices by the Russian Research Institute "Elektronstandart" [18, 19], LLC "Emi" and their subsidiary companies. In Fig. 1 the flow chart of one variant suggested in  is given: upon sequential switching of light emitting diodes 1 and 2 the system of four signals is formed (two simultaneous signals per each photodetector 3 and 4). Radiation of one LED (which is "calibrating" LED-2) does not propagate through the cell. In the paper  it is shown that there is parameter A, combination of signals from photodetectors – where the indices i, k refer to the numbers of elements
Parameter value depends only on the degree of LED radiation attenuation by the absorption of analyzed material (gas) placed into the cell. Therefore, the value of parameter A calculated by microprocessor does not depend on the variation of individual properties of photodetectors and LEDs which occur, for example, during the change of their own temperatures or environmental temperature. Similar scheme and method of the signal processing, which result in the obtainment of measurement data, are poorly influenced by the temperature parameter changes of LEDs and photodetectors. Such method of measurements can be applied to the scheme with large amount of elements, for example, (see Fig. 1) for two measurement LEDs (1, 7) and one "calibrating" LED (5).
In a number of cases, for example, upon the absence of the requirement of high accuracy of GA measurements, and/or if it is necessary to calibrate the reference zero right before the measurements, "calibrating" LED can be removed (in Fig. 1 it is equivalent to the absence of items 2 and 7). Such optical scheme includes only two photodetectors, one LED and spherical mirror . The scheme was implemented in portable GAs of carbon dioxide GIAM-302, which work well in practice, designed on the basis of the LED radiating on the wavelength of 4.2 μm and photodetector based on PbSe (photodetector was produced by Scientific Production Association "Analitpribor", Smolensk).
The next important stage in the development of GAs based on LED was triggered by the beginning of use of immersion conjugation of LED chips with optical elements, for example, with silicone lenses and optical fibers, and with use of "optical adhesive" consisting of chalcogenide glass with high refraction index (n = 2.4) [20–26]. Thanks to such conjugation and number of other improvements of the construction of LED chip (Fig. 2), it was possible to increase considerably (by 3–5 times) the coefficient of radiation coupling from semiconductor and design a number of efficient LEDs with the wavelength of 2 to 5.5 μm (Fig. 3) and narrow directional diagram [12, 24, 26]. Subsequently, use of immersion layers for the improvement of efficiency of medium-wave optoelectronic devices was repeated in many papers devoted to photodiodes  and optically excited LEDs based on А4В6 . And creation of the technology of batch production of immersion LEDs based on А3В5 made it possible to start using these emitters in many branches of industry. In medical capnographs MPR 6–03 "Triton" LEDs allowed measuring the patient breathing parameters  and in above-mentioned portable analyzers GIAM-302 – concentration of carbon dioxide. Construction of the LEDs which are small in size and coupled with concave reflectors was used in the generation of straight radiation beams. They are used in trace (with the length of optical path up to 100 m) GAs which are the part of safety control systems at oil refineries . Constructions of miniature optical-acoustic GAs , in which the entrance of convergent radiation beam into optical acoustic cell is accomplished through the window with low aperture, are made on the basis of immersion coupled LED chips with optical elements.
Instead of conclusion
From the moment of commencement of works in relation to the design of GAs based on IR light emitting diodes, constructions of light emitting diodes have been considerably changed and many innovations were introduced into the configuration of GA suggested by D.N.Nasledov and his colleagues. However, despite the successful cooperation for many years of PTI and its subsidiary organizations with GA producers, it should not be assumed that all scientific and technical problems connected with the design and use of medium-wave LEDs have been already solved and there are no grounds for unexpected surprises. Good prospects are open for the use of LEDs in optical acoustic GAs [26, 31, 32]; in optical schemes of spectrometers based on arrays and matrices produced according to the flip chip technology in which the interaction of elements is reduced ; upon the use of matched pairs LED-photodiode including optical fiber pairs  which allow implementing the minimum energy consumption of gas sensor in comparison with all existing components . Maybe, many people will think it is strange that for the efficient operation of GA under the conditions of higher temperatures reverse (non-conducting) voltage must be supplied to the LED [35–37] and for the obtainment of the efficiency coefficient exceeding 100% regular immersion LEDs based on narrow-band gap heterostructures А3В5 can be used . But these are the facts and dispute against them is not worth it; it is more important to learn how to use the peculiarities of medium-wave LEDs with maximum efficiency in order to move forward and surprise consumers with the new capabilities of optoelectronic equipment.
Author expresses gratitude to the workers of diode optocouplers group of the Laboratory of Infrared Optoelectronics of Ioffe Physical-Technical Institute of the Russian Academy of Sciences.