Laser Gyroscope Is 50 Years Old
Age of "iron" gyroscopes which started from the invention of Léon Foucault in 1851 (in reality, Johann Bohnenberger in 1817) gave to the humankind the keys to the revealing of outer space secrets and depths of the World Ocean, formed the preconditions for the development of navigation systems of new generation and control of different civil and military facilities. During cold war years "iron" gyroscopic technologies reached their peak from the height of which the capacity of military confrontation and deterrence of two world systems was evaluated .
By the middle of the 20th century academic science in the USSR and USA developed the theory of quantum molecular generators which was the basis of the devices of new generation – lasers. When they were designed, fantastic projects of laser hyperboloids, high-efficiency guidance systems, new technologies etc. were being developed and implemented in the military industrial establishments. Country ability to design and develop laser technologies showed its greatness and power not less than the possession of nuclear weapons and outer space. Starting from 1961, lasers of different types had been holding strong positions in optical laboratories. Occurrence of the first optical gyroscopes was predetermined.
Laser Gyroscopy Origins
Research activities in the area of optics of moving bodies, considering the physical phenomena in the cases when there is movement of the medium in which the light wave propagates, served as the backgrounds for the laser gyroscope creation. Practically all effects which are the basis of the physics of moving bodies were discovered during the research activities intended for the study of properties of "aethyr" – certain medium which was responsible for the light propagation according to the opinion of the majority of scientists at the end of the 19th century. Results of the experiments aimed at the study of properties of "aethyr" to considerable degree determined the creation of the special theory of relativity by A. Einstein which he set forth in the paper "On the Electrodynamics of Moving Bodies" in 1905 .
One of the first tests on the study of aethyr properties was the experiment set up by Georges Sagnac, French physicist, in 1913. During the study on the detection of "aethyr" drag by the rotary apparatus he discovered the "swirling optical effect" which allows measuring the object rotating velocity relative to the inertial reference point using the optical methods . During Sagnac experiment the interrelation between the value of interference pattern shift formed at the output of interferometer with closed optical loop (ring interferometer) by the counter-propagating light beams and its angular velocity was established. Later the experiments of A. Michelson and H. Gale demonstrated the capability of the Earth rotation velocity measurement using the ring interferometer with the perimeter of 1.9 km. In the navigation area before the gyroscope which did not have mechanical parts, big challenges were offered. Nevertheless, Sagnac effect remained unused for a long time, first of all, due to low sensitivity. In his experiment interferometer rotated with the velocity of 2.3 turns per second. With the interferometer area of 866 cm2 the interference pattern shift was only 0.04 band. Therefore, during many decades the optical gyroscope was not needed.
This situation has changed considerably with the beginning of quantum electronics development and creation of the first lasers. The fundamental discovery in the quantum electronics development was A. Einstein prediction of induced radiation phenomenon in 1916. For the first time the induced radiation was obtained in 1950 by E. Purcell and R. Pound, American physicists, during the experiments on the generation of population inversion of nuclear spin systems. In 1953-1954 N.G. Basov and A.M. Prokhorov (USSR) and independently of them C. Townes (USA) obtained the generation within the centimeter range on ammonia molecules. This is how the first quantum generator – maser (maser – microwave amplification by stimulated emission of radiation) was made. In 1955 Basov and Prokhorov offered three-level method of obtaining of population inversion of molecular levels. Operating three-level solid-state quantum amplifiers were created in 1957-1958 in the USA and USSR. For the obtained results N.G. Basov and A.M. Prokhorov and C. Townes were awarded to the Nobel Prize in Physics in 1964.
Further development of quantum electronics was guided to the transfer to the optical range. In 1958 A.M. Prokhorov and R. Dicke (USA) advanced the concept of open resonator which was the important factor for the creation of solid-state and gas optical quantum generators – lasers (laser – light amplification by stimulated emission of radiation). The first laser was created by T. Maiman (USA) in 1960 using the open resonator and crystal of synthetic ruby as the actuating medium (radiation wavelength was 0.7 µm). In 6 months A. Javan, V. Bennett and D. Herriott (USA) constructed the first gas laser based on the mixture of helium and neon.
Then, during the years of quantum-optical engineering origins the scientific world was constantly disturbed by many effects, discoveries and hypotheses. Information flows on the application of new amplifying media and wavelengths gave rise to the bold forecasts of the creation of superpower "hyperboloids", holographic television etc.
It is not surprising that against this background many people did not notice the report on the capability of creation of the fundamentally new measuring instruments based on lasers – laser gyroscopes; this report was made by A.M. Prokhorov, future winner of the Nobel Prize, at the Physical Institute of the Academy of Sciences of the USSR at the end of 1962. But the group of young engineers of the Research Institute of Applied Physics (V. Kuryatov, E. Nasedkin, G. Koshkin) seriously considered the idea of creation of such devices. In addition, even 10 years before the occurrence of the first lasers in the Soviet Union, I.L. Bernstein carried out the experimental study of Sagnac effect within the radio-frequency band on the basis of the scheme which essentially complies with the modern architecture of fiber-optic gyroscopes construction . However, there were no preconditions for the transfer of this study to the optical range at that time. Nevertheless, the priority of I.L. Bernstein who anticipated the concept of fiber-optic gyroscope construction is recognized in Russia as well as in the USA.
In 1962 A. Rosenthal (USA) suggested  and V. Macek and D. Davis (USA) implemented the first He-Ne laser with ring resonator (ring laser) from which the development of laser gyroscopy has started (Fig. 1) . Mixture of He-Ne was used as the actuating medium; 4 discharge tubes were filled with this mixture. Together with reflecting mirrors this construction represented closed resonator with squared shape and side of approximately 1 m. Resonator operating frequency was selected for the reasons of obtaining of maximum amplification coefficient which was reached on the wavelength of 1152.3 nm. Laboratory model demonstrated the capability of measurement of angular displacements relative to the inertial space with quite high sensitivity.
It should be noted that in early 60s laser gyroscopy was winning its recognition under the conditions of divided industry of mechanical ("iron") gyroscopes production which had been formed for many decades. They completely complied with the ideology of platform inertial systems which dominated at that time. There were developed theory, necessary production base and particularly important time-proven traditions which relied upon the indisputable authorities. That is why, as a rule, timid attempts of enthusiasts to develop research activities in the area of laser gyroscopy were not successful and in a number of ministries they were not even commenced.
Laser gyroscopy was more "lucky" at the enterprises which did not have the pressing authority of traditional scientists but had the optical and electro-vacuum production base. Therefore, the most efficient LG development began at the Research Institute of Applied Physics (L.N. Kurbatov was the research supervisor of the Research institute of Applied Physics) and subsequently it obtained the widest scope at Polyus Research Institute (M.F. Stelmakh, General Director) where the batch production of LG was organized.
In the middle of 1963 the first model of LG in the USSR was assembled and (as Foucault’s pendulum) suspended to the room ceiling for the generation of angular displacements (dithering) and isolation from the building foundation which experienced the seismic loads. Despite a number of technological restrictions and inconveniences (limited lifetime of discharge tubes, infrared radiation band invisible for eyes, semiconductor radiation detector from the exotic materials – indium-stibium which requires the regular cooling by liquid nitrogen and compels to be under constant stress due to the construction fragility, use of powerful 300 W high-frequency pump oscillators which are hazardous for health etc.) the model functioned successfully! Many interesting characteristics and effects were detected on it which led to the new technical terminology: "capture", unidirectional emission, "dithering", diffraction non-reciprocity, quantum noises etc. .
In 6 months on the following model operating within the visible spectrum range it was possible to detect the Earth rotation, study in details the synchronization of counter-propagating waves, feel the influence of magnetic field, test different methods of linearization of LG output characteristic (mechanical rotation, Faraday effect, Fizeau effect, Zeeman effect etc.).
In 1965 the first applied candidate’s dissertation on LG was defended (V.N. Kuryatov) main provisions of which have not lost their topicality up to the present day. Activities in the area of laser gyroscopy received the following development in a number of organizations in Moscow and Kiev. At Polyus Research Institute the works were carried out in two areas, one area was under the supervision of V.N. Kuryatov and developed the creation of monoblock LG based on totally reflecting prisms (TRPs), the second area was under the supervision of B.V. Rybakov and worked on the creation of LG based on Zeeman effect. Astrophysica Scientific Production Association under the supervision of V.A. Zborovsky worked on the creation of monoblock LG based on mirrors with Faraday nonreciprocal element which became the prototype of the devices manufactured in batches by Kiev Arsenal Factory under the supervision of V.I. Buzanov.
It is not a secret that considerable scientific and practical results in the area of laser and fiber-optic gyroscopy were received in the military industrial establishments of leading states. Detailed information on the technological processes, tests and practical applications of LGs was and in many aspects has been staying confidential up to the present day. This process worsened due to "iron curtain" which separated NATO countries from Warsaw Pact countries. Under these conditions the international collaboration and scientific and technical cooperation of LG researchers and developers were practically eliminated. Therefore, despite the large amount of open publications which, first of all, reflected the issues of optical gyroscopes theory many essential details connected with the industrial development and application of new technologies turned out to be confidential and the period from the demonstration of the first laboratory models to the batch production turned out to be quite long.
In addition, the development history of one of the leading world firms in the area of laser gyroscopy – Honeywell – convincingly testifies of the fact that considerable commercial success can be achieved through the efficient development of military and civil markets or, in other words, through the common technological processes of dual application. Simultaneous development and implementation of large amount of devices and systems provide the sharp reduction in value which, in turn, leads to the marketing development. It can be illustrated through the performance results of Honeywell for the period from 1965 to 1994 (Fig. 2) .
As it is seen from Fig. 2, the first stage of research and development activities which took the considerable period from 1965 to 1979 could be performed only on the basis of the government financing which, obviously, continued up to the first deliveries of laser navigation equipment for the airplanes Boeing 757/767. Then, obtained results made it possible to develop the series of new LGs with the sequentially improved weigh-size characteristics during relatively short period of time and increase the amount of production units with the simultaneous reduction in its value. By the beginning of 1992, the ratio of civil products to military products was more than 10:1 with the reduction in unit value by 6.5 times from 1981 to 1992.
The development history of laser gyroscopy in the former Soviet Union looks differently at one of the leading enterprises – Central Design Bureau and Arsenal Factory (Fig. 3).
Here the development of prototypes and output of batch products were performed at all stages upon the systematic government financing of orders which almost did not provide the development of equipment samples for civil application. Plan nature of the production and tight control were aimed for the reduction of necessary research and development terms, improvement of tactical and technical characteristics of products, as it is seen from Fig. 3.
First Samples of Laser Gyroscopes
Soon after the first demonstration of laser gyroscope the development of its semi-industrial samples has begun. One of the first models was demonstrated by Lockheed Martin Company in the middle of 60s. Photo and construction of the offered device are shown in Fig. 4 .
LG was constructed according to the module scheme and included He-Ne laser with the wavelength of 1152.3 nm, triangular circuit with the sides of 7.62 mm formed by totally reflecting prisms, Faraday cell for the simulation of nonreciprocity and piezoelectric actuator on one of the prisms for the control and direction of perimeter. Resonator was made of aluminum and additionally equipped with the temperature sensor and heating set. Heaters maintained the constant temperature of 65 °C providing at the same time the constancy of resonator geometry. General assembly was put into the case equipped with thermal and magnetic screens for the maintenance of stable operating conditions. This is how the first demonstrated laser gyroscope sample looked like; it did not have rotating rotor.
By its characteristics LG turned out to be the most suitable for the usage in strapdown inertial systems (SIS) development of which started rapidly; it was caused by the occurrence of quick operating computer technology. The idea of installation of the sensors triad in the case appeared attractive for the creation of inertial measuring modules. Despite the number of shortcomings (tube lifetime was less than 1000 h, long readiness time, high energy consumption) developed gyroscopes were in demand. Particularly, their testing was carried out in NASA as well as in laboratories of naval and air forces of the USA.
Approximately during the same period Europe showed interest in laser gyroscope too. In 1967 in Farnborough, Great Britain, the first demonstration of laser gyroscope capabilities was performed. Sensor constructed on the basis of order of the Ministry of Defense of the United Kingdom was shown in all its beauty but for some reason the government did not take interest in it. As a result, the development activities on LG systems in Great Britain recommenced only in 10 years .
During this time in the USSR the development activities on LG were carried out by the employees of Polyus Research Institute in the most intensive manner. One of the first problems which the developers faced was the absence of high-quality optical mirrors. For that matter it was decided to use the totally reflecting prisms which by that time had loss of less than 0.01% and it insured the value of capture area of about 100 Hz. Carried out research activities made it possible to reach the record measurement accuracy of the absolute angular Earth rotation velocity of 8·10-4 °/hour by the monoblock prism LG by 1969 and develop the precision maritime navigation complex using these LGs in several years. The first sample of LG with totally reflecting prisms developed at Polyus Research Institute by V.N. Kuryatov is shown in Fig. 5.
Linearization of LG Output Characteristic
It should be noted that from the first steps of laser gyroscopy development the issue of the methods of struggle against the capture zone or mutual synchronization of counter-propagating waves arouse. According to the generally accepted LG model it consists of two quasiautonomous generators exciting the counter-progressive waves, on their way in resonator the inhomogeneities which scatter the counter-propagating beams inevitably occur. Scattered radiation plays the role of synchronizer which tends to the approximation of counter-propagating waves frequencies. This effect is shown in the greatest manner in the area of low angular velocities where the synchronization of beams causes their strong bond. As a result, counter-progressive waves obtain the same frequency at which the difference frequency becomes equal to zero. The typical LG output characteristic is given in Fig. 6 .
Here the dead zone and nonlinearity areas are clearly observed. Thus first of all, the struggle against capture zone is aimed at the linearization of output characteristic.
Several variants of the problem solution were suggested and tested:
LG rotation with the constant angular velocity.
Use of nonreciprocal phase-shifting devices (NPSD) introduced into the resonator and based on Fizeau effect, Faraday effect etc.
Application of vibratory angular motion which is called dither.
Use of Zeeman splitting of counter-propagating waves frequencies in magnetic field.
Multi-frequency operating modes.
LG with natural NPSEs .
When using the dither device LG obtains the alternating-sign initial shift. As a result, the operating point is located within the capture zone during very short time intervals. At the early stages of laser gyroscopy development such method became the most popular. Construction of the mechanical dithering suspension was being actively developed and different methods for the removal of "shelves" in the output characteristic which are inherent to LG with dither and many other innovations were being suggested. As a result, the system with "carousel" drive became the most suitable when the gyroscope performs several turns in one direction and afterwards practically instantly (less than in 0.1 s) stops and repeats its motions in opposite direction. Despite the obvious advantages of dithering as the method of LG output characteristic linearization long-term operational experience allowed detecting a number of serious shortcomings:
Considerable tangential accelerations of LG construction elements which approximate to 30 g;
Excitation of associated mechanical oscillations;
Considerable level of random walk which does not make it possible to approximate the potential accuracy of LG;
Increasing of weigh-size characteristics.
Besides, introduction of the element which creates the similar "mechanical shivering" into LG construction causes the return to the age of "iron" gyroscopes depriving the optical gyroscopes of their main advantage – solid state. Use of nonreciprocal phase devices (NPD) appears to be prospective from this point of view.
One of the possible schemes of nonreciprocal phase-shifting device (NPSD), device for the initial frequency spacing, which sometimes is called Faraday cell, is shown in Fig. 7 . Here the quarter-wave plates 1 and 2 located on the ends of magnetooptic medium 2 play the role of polarization transformers turning the linearly polarized oscillation outside the megnatooptic medium into the oscillation polarized circularly inside of it. For the light wave progressing from left to right, as it is shown in Fig. 7, the plate 1 is the input plate which transforms the linearly polarized oscillation E into the circular oscillation rotating clockwise, for example. External magnetic field H generated with the help of solenoid or permanent magnet causes the change of refraction index n1,2 of isotropic medium 2, as it was shown above, which results in the additional phase increment Δφ. Passing the way with the length of l the light wave of circular polarization passes through the polarization transformer 3 restoring the initial linear polarization E. Counter-progressive wave is subjected to the analogous transformations. The difference consists in the fact that for it the transformer 3 serves as input transformer after which the linear polarization is transformed into the circular one but with counterclockwise rotation. As a result, counter-progressive wave at NPSD output obtains the phase shift opposite in sign (in the considered case negative −Δφ) and at the output keeps the initial linear polarization. Appearance of the classic Faraday cell is shown in Figure 8.
It is obvious that the application of NPSD based on Faraday effect increases the magnetic sensitivity of LG. For its reduction the differential NPSD is applied (Fig. 9) . It consists of two magnetooptic sections 3 divided by the half-wave plate 2 crystallographic axes of which are randomly oriented. Orientation of crystallographic axes of polarization transformers 1 is orthogonal. Half-wave plate plays the role of internal polarization transformer which turns the left-circular polarization in one of NPSD sections into the right-circular polarization in the other section. Quarter-wave plates 1 play the role of transformers of linearly polarized oscillations into the circular oscillations and inversely.
Simultaneously several areas were developed at Polyus Research Institute. V.N. Kuryatov supervised the group on the development of gyroscopes of KM series which use the dither device and simultaneously the areas with NPSD of different types, Zeeman splitting of LG (ZLG) counter-propagating waves based on the non-planar resonators were developed by the efforts of A.V. Melnikov, B.V. Rybakov and others.
To be continued.