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Acoustically Pumped Sources of Coherent Light

#1043


Generators, Amplifiers and Converters of Light with Acoustic Pump

Tech Area / Field

  • PHY-OPL/Optics and Lasers/Physics

Status
8 Project completed

Registration date
11.07.1997

Completion date
24.10.2003

Senior Project Manager
Bugaev D V

Leading Institute
GNPO Polyus, Russia, Moscow

Supporting institutes

  • Institute of General Physics named after A.M. Prokhorov RAS, Russia, Moscow\nInstitute of Informatics Problems, Russia, Moscow

Collaborators

  • KATHO, Belgium, Kortrijk\nFriedrich-Schiller-Universität Jena, Germany, Jena\nUniversity of Twente, The Netherlands, Enschede\nKing's College London, UK, London

Project summary

The following considerations have been taken into account in deciding on the theme of the Project.
- Limited possibilities in financing and modern equipment do not allow competing with foreign laboratories in commonly accepted areas of investigations.
- The availability of skilled scientists in the area of ultrasonic light interaction.
- The availability of the native theory of cascading light conversion with ultrasonic light interaction.

The Project does not require significant investments but some support of ISTC is necessary.

The key idea of the Project is to investigate experimentally the possible maximal wavelength shift of a light wave with ultrasonic light interaction. As is known, there is the frequency shift from f to f+F in a usual Bragg cell, where f and F are frequencies of light and acoustic waves, respectively. Because f @ 5 1014 Hz, F @ 5 107 Hz, increasing the light frequency from f to f + F = f (f + F/f)= 1.0000001 f is very small. However, if the same scheme of conversion will be used recursively the light waves with frequencies f + 2F, f +3F...f + NF... can be obtained. A multistage or cascading conversion takes place in the case. For example, if N = 106 then/will be increased by ten percents. Needless to say, the conversion is not supposed to be obtained in a usual Bragg cell. A special optical device in a form of glass focon has been developed for these purposes. In any case, total time of the cascading conversion ttotal is more by N times than time of conversion t @ 0.1 ms in a usual Bragg cell. If N = 106 then ttotal = 100 ms. Until recently there were not optical mediums in which a light can propagate in such long time. Up-to-date technological achievements in fabricating glass for fibers with very small losses (less than 0.2 Db/Km) give such opportunity. Thus, the Project is trying to use the up-to-date technological achievements for the purposes different from primary ones.

The Project may be thought as the main step in the direction to radically new converters, amplifiers, and sources of coherent light in wide range of wavelengths (0.35 - 1.6 m) and output powers (10-3 - 103 W). A mechanical energy in form of elastic oscillations with frequency about 50 MHz is converted directly into the energy of coherent light. The devices use cheap sources of feeding based on usual HF transistors unlike the known parametric light converters and amplifiers based on nonlinear crystals that require cost feeding in form of power coherent light.

As is known, the present sources of the optical pump are powerful lasers operated in a pulse regime. They are large in size and weight, cost and short lived. That is the reason why amplifiers and converters based on nonlinear crystals are not wide spread. On the contrary, the sources of the pump with frequency about 50 MHz based on power high-frequency transistors are small in size and weight, cheap and long lived. Modulation of refractive index n in optical medium required for parametric conversion and amplification is performed in the glass on account of quite different physical effects than in nonlinear crystals. An acoustic wave can be used in the glass rather than the light wave that modulates n in a nonlinear crystal because of its nonlinearity.

Since the velocity of the acoustic wave in the glass is less than the velocity of light c/n in nonlinear crystals by the factor 20000 - 30000 the power of acoustic wave can be less by the same factor than the power of the light wave, all other things being equal.

An important point is the following peculiarities of the glass


- the glass has small dissipate losses not only for light waves but also for acoustic waves.
- the glass has high resistance to intensive light and acoustic waves
- the glass enables to perform light conversion and amplification in large volume, that enables to deal with large light and acoustic power.
- the glass is characterized by the simplicity of fabrication, used elements and material.

But all these quite right general considerations had been not confirmed by appropriate theoretical and experimental studies. Unfortunately, the classical theory of parametric interaction between light waves is not valid for the case because the theory is limited by so called 3-wave interaction when interaction the following 3 wave are considered: pump, input light wave, and idle wave. It is supposed that all other waves with various combinative frequencies can be neglected because they do not satisfy to the condition of space synchronism because of material dispersion in nonlinear crystals. This assumption is not valid in the case with very low-frequency pump because many combinative frequencies resides in a narrow frequency band and must be taken into account. Thus, instead of the theory of 3-wave interaction must be applied a theory of multi-wave interaction, that was developed in 1993-1996.

The first papers concerned these questions have been published in 1993. The further theoretical study of this problem has been performed in 1994-1996. In particular, "Theoretical study of interaction of light waves in a medium with the refractive index modulated at relatively low frequency" has bee performed with partial financial support of Sores International Science Foundation, Grants N MPF000 and MPF300.

The last 3 papers with consideration of light conversion and amplification in concrete devices have been published in 1996. The most important of the papers are the following


- V.P. Torchigin. Propagation of Waves in Optical Waveguides with periodically varying Refraction Index. Laser Physics, 4, No.l, 168-177 (1994).
- V.P.Torchigin Amplification of light pulses in waveguides with periodically varying refractive index. Quantum Electronics 25(5), 484-485 (1995).
- V.P. Torchigin. Conversion of light in a focon with using an acoustic wave as a pump. Technical Physics, 66, no.4, 128-139 (1996).
- V.P. Torchigin. Interaction of acoustic and light acoustic cylindrical waves. Acoustical Physics, 42 no. 6, 853-859 (1996).
- V.P. Torchigin Light amplification in lightguides and resonators formed by acoustic wave. Technical Physics, 66, no.8, 107-123 (1996).

At the moment it is known no papers concerned this direction throughout the world besides the works of the authors of the Project.

The effect of increasing light frequency and light energy can be explained with various points of view. He most simple explanation is compression of light radiation in optical-open-dielectric-glass-ring resonators of travelling wave formed in a glass focon by the travelling acoustic wave propagated along the axis of the focon towards its taper part. The resonators are moving along the focon with the velocity of the acoustic wave about 6000 m/s, their diameter is decreasing and the light radiation stored in the resonators is being compressed. In result, the energy and frequency of the light radiation are increasing. Time of the compression is about 10 - 100 microseconds. It is shown that radioactive and dissipate losses in the glass resonators are small enough in this time interval.

The other explanation is based on the well-known effect of increasing light frequency at reflection of a light wave from the distributed Bragg reflector (DBR) formed by a travelling acoustic wave. The effect has been approved experimentally in the usual acousto-optic Bragg cell. Having reflected from the DBR, the light wave gains its frequency by F, where F is the frequency of the acoustic wave. If the effect is repeated by 106 times the frequency of the result output light wave can be increased by 106 F that is up to 1014 Hz. Of course, the all repetitions take place in the same device, in particular, in the glass focon.

Besides, the following additional approaches have been used to study processes of conversion of light in a medium with refractive index modulated in time.

Conversion of light in a waveguide with slowly varying cross-section along its axis when an acoustic wave propagates along the axis.

Conversion of light in curve moving waveguides formed by a travelling acoustic wave.

Propagation and transformation of light on the base of methods of geometric and wave optics.

The all approaches lead to the following result. At present, it is possible to create light converters and amplifiers with low-frequency pump on the base of currently available glasses with small losses.

The features of mechanism of conversion lead to the following properties. Light with any wavelengths for which dissipation in the glass is small enough can be converted. Both coherent and non-coherent light can be converted. Light with required spectrum can be obtained in result of conversion. Small dependence of parameters of the converters and amplifiers on temperature takes place. Small sizes, high efficiency, large output power per unit of volume of the glass, cheap AC feeding are the other features of the converters and amplifiers. As is known, electricity had been introduced in practical applications when electrical generators transformed mechanical energy into electrical one had been developed. The same history is possible with optical generators.

Scientists from the Institute POLUS developed military optical devices are recruited for the work. They are prominent experts in Russia in the field of research and development of acoustic-optic modulators, deflectors, correlators etc. as well as in the field of measurements of parameters of light and acoustic waves. Their efforts will be supported by experienced physicists of the internationally known Institute of General Physics headed by the winner of Nobel Prize academician A.M. Prokhorov.

There are all necessary materials, technology and instrument devices in Russia but the experimental study meets large difficulties because of lack of financing.

Expected Results

The experimental investigations are planned on the base of step by step approach. At the first stage, a usual Bragg cell will be modified to increase the light frequency shift by 1000 times in compare with the present up-to date shift. The next steps are design, fabrication and testing specimens in which the light frequency shifts are increased by 104 and 105 times, sequentially. The following problems will be solved in result of the investigations. Dissipative, radiative losses, as well as losses in input and output of light radiation will be determined in various ranges of wavelength. If the total losses occur less than 3 Db on increasing the light frequency by 2 times then a generator of coherent light without any additional light sources will be combined and studied.

In any case, the results obtained at any stage can have practical applications. The investigations open a new direction in direct conversion of mechanical energy presented by elastic oscillations of optical medium (glass) into coherent light. The optical devices based on studied phenomena can be used in various applications, in particular, in telecommunication, computers, television, medicine, scientific investigations and so on. To date, it is difficult to outline all possible areas in which the new approach to amplification, conversion, and generation of coherent light can be used.

Potential role of foreign collaborators is help in organization of experimental study (experience, advises about modern instrument devices) as well as the check of the results obtained at any stage of the study.

The experience of discussing the questions shows that they are not understood immediately by acoutooptics experts. The problems are understood much more by physicists. In our opinion, the Projects similar to the present one allow to carry out scientific investigations without costly equipment and technologies and to obtain new results that increase human knowledge and open new fields of applications.


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