Abstract:
The invention relates to devices for the quantum-optical amplification of modulated light, in particular in optical free-space communications systems. In the process a light beam ( 4 ) is conducted through a plurality of adjoining crystals ( 66 ), ( 68 ), ( 70 ), which are delimited from each other by means of polarization-selectively reflecting layers ( 104 ), ( 106 ). The light beam ( 4 ) is repeatedly reflected at the edge areas of the crystals into quarter-wave plates ( 86 ), ( 88 ), ( 90 ), ( 92 ), ( 94 ), and in the process its polarization is respectively rotated by 90 degrees.

Description:
FIELD OF THE INVENTION 
     The invention relates to devices for the quantum-optical amplification of modulated light, in particular in optical free- space communications systems. 
     BACKGROUND OF THE INVENTION 
     In the near future, the optical free-space communication between satellites, as well as between satellites and ground stations, will constitute an important supplement to the existing microwave technology, which also saves weight on board the satellite. So-called optical terminals consist of one or several telescopes, which limit the angular range of the field of view of an optical receiver in the direction toward a counter-station, and also provide a directional radiation of the signals to be transmitted. Several movable mirrors are furthermore provided, by means of which the alignment of the transmitting and receiving directions is performed. Besides the direct detection of the optical output of the transmitter of the counter-station constituting the transmission process, the coherent superimposition of the received light with the light of the same frequency of a local oscillator laser plays an important role since, besides great sensitivity to the signal to be detected, the insensitivity to interference by background radiation is important. 
     The diode lasers, which have reached a high degree of development because of their extensive application in fiber-optical communications, also represent an alternative, at least for simple systems operating with intensity modulation, and in addition also save space and weight. But in spite of their operation on only a single optical frequency, which has also been achieved here, they are generally not yet suitable for coherent transmission processes, except for complicated structures with large, additionally coupled-in resonators. One reason is the still too large spectral width of this radiated optical frequency. Although fiber-optical coherent transmission systems do operate with customary commercial diode lasers, because of the line guided transmission, detection takes place with a relatively large optical output. 
     But the background radiation, which is interferingly present in free-space transmissions, as well as the mostly very low power of the received signals, require an optical bandwidth of the unmodulated signal which is considerably narrower than the modulation bandwidth. This is a requirement which, together with small size and low weight, can be best met by diode laser-pumped solid-state lasers. Existing attempts to integrate the laser systems required for operation into a terminal for optical free- space communications have been described by Carlson et al., as well as by Marshalek et al. (R. T. Carlson et al., “Monolithic Glass Block Lasercom Terminal: Hardware Proof of Concept and Test Results”, SPIE, vol. 2381, Free-Space Laser Communications Technologies VlI, Feb. 7-8, 1995, San Jose, Calif., pp. 90 to 102; R. G. Marshalek et al., “Lightweight, High Data-Rate Laser Communications Terminal or Low Earth Orbit Satellite Constellations”, SPIE vol. 2381, Free-Space Laser Communications Technologies VlI, Feb. 7-8, 1995, San Jose, Calif., pp. 72-82). 
     Both groups of authors describe laser systems which are mechanically coupled to the optical devices of a terminal and which guide their light emissions into the optical device by means of collimated beams. However, diode lasers in accordance with the state of the art have been used here. 
     Diode laser-pumped solid-state lasers have a larger volume and lower efficiency and therefore generate a larger amount of waste heat than comparable diode lasers. The increased amount of heat produced in the vicinity of the optical system has been shown to be a risk for the undisturbed operation of the optical device. 
     The insufficient modulation capability of diode laser- pumped solid-state lasers represents a further problem. In contrast to diode lasers, the medium generating the optical amplification remains in an excited state for a relatively long time after the supply of pump energy. Furthermore, the resonator of such lasers is considerably larger than that of diode lasers. As a result, cut-off frequencies of approximately 100 kHz are typical for amplitude modulation, for example. The external modulation required for this is also quite difficult to perform, since a high optical power must be manipulated, which requires the use of electro-optical modulators which have low cut-off frequencies. 
     External modulation of laser light can be performed at high cut-off frequencies in modulators in which the light is conducted in a waveguide, which permits a small mutual distance between the electrodes that provide the modulating voltage, and therefore permits a lower modulation voltage. Since, because of the strong increase of the optical intensity caused by the narrow cross section of the optical waveguide, this method only permits low optical output, the modulated optical signal must be post- amplified. Attempts to do this consist in the application of processes and devices which, in the meantime, have proven themselves in fiber-guided optical communications, for example by means of the post-amplification of the modulated optical signal with a fiber amplifier doped with erbium (T. Araki, M. Yajima. S. Nakamori, Y. Hisada, “Laser Transmitter Systems for High Data-Rate Optical Inter-Orbit Communications”, SPIE vol. 2381, Free-Space Laser Communications Technologies VII, Feb. 7-8, 1995, San Jose, Calif., pp. 264-272). 
     Besides diode laser-pumped solid-state lasers, appropriate traveling wave amplifiers are also used, wherewith, especially for the post-amplification of light, devices operating with lasers from the same technology are available, in particular for diode laser-pumped neodymium-YAG solid-state lasers, which are very useful for optical free-space communications because of their narrow spectral width. The light to be amplified is conducted into an amplifying crystal, in which the photons of the light beam, with a defined probability, meet atoms which are in an optically excited state, which is comparatively stable over time because of the special properties of the material. The relative stability of this state is interrupted by a photon having the same energy as the difference between the excited state and the lower laser level of the atom, wherewith the respective atom releases an additional photon with the same wavelength (i.e. the same energy) and phase. 
     The excited state of the atoms is caused by so-called pump light, which generally has a shorter wavelength than the light to be amplified and puts the atoms in an excited state corresponding to the energy of its photons, from which the latter spontaneously change into a relatively stable state, whose energy difference with the non-excited lower laser level corresponds to the energy of the photons of the light to be amplified. A high amplification of the light is achieved if, during the passage through the amplifying medium, the photons of the light to be amplified meet many excited atoms. The volume density of excited atoms therefore must be very high. However, since a certain portion of the excited atoms per unit of time spontaneously transits into the lower laser level because of a finite average lifetime of the excited state, and the photon emitted in the process is lost for the amplification of the light, it is necessary to continuously pump light with a high intensity into the medium, even when there is a lack of light to be amplified, in order to maintain the high volume density of excited atoms. At low input intensity such devices provide high amplification factors but, their efficiency is extremely low. On the other hand low amplification factors are observed when the light to be amplified already has a high intensity, i.e. if a large average rate of photons passes through the amplifying medium and the density of excited atoms is reduced because of a high rate of stimulated emissions of additional photons. 
     After a short average time each atom excited by the pump light transits into the lower laser level induced by a photon of the light to be amplified. With a comparatively long average lifetime of the excited atoms, there is a comparatively low probability of a spontaneous, and therefore useless, transit to the lower laser level, because of which the efficiency at high intensity and therefore low amplification is high. 
     In order to achieve a high amplification, along with a simultaneously high rate of stimulated transits into the lower laser level, it is necessary, despite the low density of excited atoms in the amplifying medium, to assure a large average number of additional photons generated by stimulated transits of excited atoms into the lower laser level. This is mostly achieved in that the light to be amplified is guided over as many paths as possible through the zone of an amplifying medium irradiated with pump light. With a respectively constant volume density of excited atoms, for each photon of the light to be amplified the probability to generate additional, stimulatedly emitted photons is multiplied by the number of paths through the gain medium. 
     It is therefore possible to generate a comparatively high amplification factor in spite of low pump power. However, the devices in accordance with the state of the art are constructed of several elements requiring a lot of space and mass, which therefore only poorly satisfy space travel-specific requirements. Special developments also contain the risk of insufficient mechanical stabilities (T. J. Kane, E. A. P. Cheng, B. Nguyen, “Diode-Pumped ND:YAG Amplifier with 52 dB Gain”, SPIE vol. 2381, Free-Space Laser Communications Technologies VII, Feb. 7-8, 1995, San Jose, Calif., pp. 273-284; T. E. Olson, T. J. Kane, W. M. Grossmann, H. Plaessmann, “Multiple Diode-Pumped ND:YAG Optical Amplifiers at 1.96 μm and 1.32 μm”, Optical Letters, vol. 6, No. 5, May 1994, pp. 605-608). 
     An additional problem for space travel applications consists in that the diode lasers used for generating the pump light also have a limited lifetime. Accordingly it is necessary to keep several diode lasers in reserve for every diode laser-pumped solid-state laser and each diode laser-pumped optical amplifier in order to be able to replace broken-down ones. 
     But diode lasers provided in redundancy require optical devices which permit switching between the light beams emitted by the individual laser diodes. 
     OBJECT AND SUMMARY OF THE INVENTION 
     It is therefore the object of the invention described below to avoid the disadvantages of the prior art and to amplify light quantum-optically with high amplification and efficiency, employing multi-path solid-state amplifiers. 
     Usually, after passing an optical insulator, light from a diode-laser pumped solid-state laser operating at low optical power is coupled into a waveguide modulator in order to have its phase modulated with a broad-band information signal. Thereafter, the optical post-amplification of the modulated signal takes place, which is performed either in a diode laser-pumped amplifier or a doped optical fiber. The base element of a diode laser-pumped amplifier in accordance with the invention consists of a laser medium of appropriate geometry (hereinafter called a crystal), whose volume is irradiated with pump light by diode lasers along respectively two beams. The light to be amplified is coupled into the crystal in such a way that, before it leaves the crystal amplified, because of reflection at several reflecting layers it moves several times through the crystal along zones irradiated by pump light. The reflecting layers are either directly applied to lateral faces of the crystal or are located at sides facing away from the crystal on quarter-wave plates respectively arranged in the immediate vicinity of a face of the crystal. 
     A further development of this concept includes an amplifying medium consisting of two crystals separated by a polarization-selective layer, wherein the light to be amplified passes the zones of a crystal irradiated with pump light eight times, and those of the other crystal twice. In the process two quarter- wave plates with a mirror integrated on the side facing away from the crystal, as well as a mirror, are used. 
     Both amplifiers are supplied with pump light by diode lasers. The pump light can be conducted from several redundantly supplied diode lasers into a multimode optical fiber, whose outlet then leads via a beam-generating optical device into the crystal of an amplifier. 
     In a continuation of the described further development, additional crystals with respectively two correspondingly designed quarter-wave plates, as well as respectively one additional polarization-selective layer, are inserted between the crystals of the quantum-optical amplifier in accordance with the described further development, wherein pump light is supplied through the quarter-wave plates and their mirrors acting in a wavelength- selecting manner. Here, the number of the optically pumped zones through which the light to be amplified passes is respectively increased by 2. 
     This device has the advantage of assuring a high amplification because of a high number of passages of the light to be amplified through optically pumped zones. In addition the structure of the quantum-optical amplifier has a special mechanical ruggedness, while its mass and spatial extension are comparatively small. A further advantage in connection with four-level systems for quantum-optical amplification is the fact that the loss or reduction of optical pump power in one of the optically pumped zones does not cause any absorption by the medium of the light to be amplified. It is therefore possible to keep redundantly embodied pump light sources available at different locations of a crystal for generating an optically pumped zone, because of which the use of special, mechanically actuated optical switching devices for detecting different pump light sources for a single optically pumped zone can be omitted. 
    
    
     Further details, characteristics and advantages of the invention ensue not only from the claims and the characteristics to be taken from them, by themselves and/or in combination, but also from the following description of a preferred embodiment. 
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 represents a quantum-optical amplifier consisting of a single crystal, 
     FIG. 2 represents a quantum-optical amplifier consisting of two crystals, 
     FIG. 3 represents a quantum-optical amplifier consisting of two crystals without a Faraday rotator, 
     FIG. 4 represents a quantum-optical amplifier consisting of three crystals, 
     FIG. 5 represents a quantum-optical amplifier consisting of three crystals without a Faraday rotator, 
     FIG. 6 represents a variant of a further quantum-optical amplifier consisting of two crystals. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a quantum-optical amplifier embodied in the form of a single crystal  2 . A light beam  4  passes through a polarizing beam splitter  6 . After the subsequent passage through a Faraday rotator  8 , the polarization of the light beam  4  is rotated by 45 degrees. Indicated by a symbol  12 , the light beam  4 , in a polarization parallel to the drawing plane, is guided through a lens  10 , to a point  16  of a polarization-selective layer  14  of the crystal  2 . In order to meet this condition for the layer  14  of the crystal  2  as well as for the polarizing beam splitter  6 , the polarizing beam splitter  6  should be imagined to be turned by 45 degrees in respect to the drawing plane. After passing the point  16 , the light beam  4  coupled into the crystal  2  propagates through the optically denser crystal  2  as the medium along a line  26  in the direction of a quarter-wave plate  18 , which is directly attached to the appropriate surfaces of the crystal  2 . The side of the quarter-wave plate  18  facing away from the crystal has been coated in such a way that light of the wavelength of the light beam  4  is reflected in the opposite direction as completely as possible, while a pump light beam  24  can pass through the quarter-wave plate  18  with the smallest possible losses. The pump light beam  24  generates an optical amplification of the light beam  4  by means of the optical excitation of a zone of the crystal  2  which overlaps as well as possible that of the light beam  4 . 
     Because of the double passage through the quarter-wave plate, the direction of polarization of the light beam  4  moving back in the crystal  2  is now orthogonal in respect to the direction of the polarization of the light beam  4  refracted into the crystal  2  at the point  16  of the surface  14  in the direction of the quarter-wave plate  18 . The light beam  4  is now propagated along the line  26  in the crystal  2  in the direction of the surface  14  acting reflectively in a polarization-selective manner in order to be reflected in the direction of a mirror  20  at the point  16 . The mirror  20  reflects the greatest possible portion of the light beam  4  being propagated toward it along a line  28  in the opposite direction. The greatest possible portion of a further pump light beam  22  is guided into the crystal  2  through the mirror  20 , which is embodied to be wave-selective, in order to optically pump the crystal  2  along the line  28 . As an alternative, the mirror  20  can be coated directly on to the crystal face. Following its reflection at the mirror  20 , the light beam  4 , whose polarization is unchanged, is propagated along the line  28  in the direction of the point  16  in order to be reflected a second time on the surface  14  and to be propagated along the line  26  in the direction of the quarter-wave plate  18 . The polarization direction of this light beam is now orthogonal to the light beam  4  refracted at the point  16  into the crystal. After again being reflected at the side of the quarter-wave plate  18  facing away from the crystal  2  and passage through it, the light beam  4  is again propagated in the direction of the point  16 , wherein its polarization is parallel to that of the light beam  4  entering the crystal at the beginning. For this reason, its reflection-free refraction in the direction of the lens  10  takes place at the point  16 , its passage through the Faraday rotator  8 , as well as the reflection of the light beam  4 , now polarized orthogonally with the incoming light beam, at the polarizing beam splitter  6  in the direction of a line  30 . The light beam  4  altogether passes six times through optically pumped zones of the crystal  2 . The light beam  4  passes four times through the respective optically pumped zone along the line  26 , wherein every propagation direction and respectively both polarizations, which are orthogonal in respect to each other, are used. Light radiation passes through the respective optically pumped zone along the line  28  in both oppositely directed propagation directions with the same linear polarization. 
     The light beam  4  passes through a greater number of optically pumped zones in a device in accordance with FIG.  2 . As in the device of FIG. 1, the light beam  4  first passes through the polarizing beam splitter  6 , the Faraday rotator  8  and the lens  10  in order to be beamed with the polarization represented by the symbol  12  into a first crystal  32 . After propagation along a line  38 , the light beam  4  reaches through a polarization-selectively reflecting layer  36  a further crystal  34  and is propagated therein along a line  40  in the direction of a quarter-wave plate  42 , which corresponds in its design to the quarter-wave plate  18  in FIG.  1 . The embodiment of a pump light source  44  also corresponds to that of the pump light source  24  in FIG.  1 . After the reflection at the quarter-wave plate  42  of the light beam  4 , which has been propagated along the lines  38  and  40  in the direction of the quarter-wave plate  42 , the light beam  4 , its polarization rotated by 90 degrees, is propagated in the opposite direction along the line  40  in order to be reflected at the polarization-selectively reflecting layer  36  in the direction of a further quarter-wave plate  48 , whose embodiment corresponds to that of the quarter-wave plate  18  in FIG. 1. A pump light source  50  also corresponds to all above described embodiments of pump light sources. Following the reflection at the quarter-wave plate  48 , the light beam  4  is propagated, its polarization again rotated by 90 degree, along the line  46  from the quarter-wave plate  48  to the polarization-selectively reflecting layer  36 , in order to pass through it into the crystal  32  and to be propagated along the line  52  in the direction of a mirror  54 . In its embodiment, the mirror  54  corresponds to the mirror  20  in FIG. 1, a pump light source  56  also corresponds to previous embodiments. Following reflection at the mirror  54 , the light beam passes in the opposite direction again through the crystals  32  and  34  without being reflected at the layer  36 . Following a further reflection at the quarter-wave plate  48 , the light beam  4  is propagated in the crystal  34  along the lines  46  and  40  in the direction toward the quarter-wave plate  42  wherein, because of the rotation of the polarization of the light beam  4  by 90 degrees during the reflection at the quarter-wave plate  48 , it is reflected at the layer  36 . A now following reflection of the light beam  4  at the quarter-wave plate  42  has the result that the light beam  4 , its polarization again rotated by 90 degrees, passes through the crystals  34  and  32  along the lines  40  and  38  without being reflected at the layer  36 . The separation of the light beam  4  leaving the crystal  32  takes place in the manner explained by means of FIG.  1 . In this embodiment, the light beam  4  passes a total of ten times through optically pumped zones. Respectively fourfold passages in both possible directions and polarizations take place along the line  40  as well as the line  46  inside the crystal  34 . A double passage in both possible directions and in the polarization represented by the symbol  12  takes place along the line  52  in the crystal  32 . 
     The replacement of the mirror  54  in the embodiment of FIG. 2 results in a further embodiment in accordance with FIG. 3, in which an optical insulator is not required for the separation of light beams  4  passing into and out of the device, which leads to considerable savings in mass and volume. 
     In the embodiment represented in FIG. 3, the light beam  4  radiates through a polarizing beam splitter  62  in the polarization represented by the symbol  58 , the lens  10  and the crystals  32  and  34  along the lines  38  and  40 , without being reflected at the polarization-selectively reflecting layer  36 . Thereafter reflection at the quarter-wave plate  42  takes place, because of which the light beam is propagated with a polarization rotated by 90 degrees in the opposite direction along the line  40  in order to be reflected in the direction toward the quarter-wave plate  48  at the polarization-selectively reflecting layer  36 . It is reflected at the quarter-wave plate  48  after propagation along the line  46  and is propagated with a polarization rotated by 90 degrees in the opposite direction along the line  46 . Without reflection at the layer  36 , the light beam  4  enters the crystal  32  in order to be propagated along the line  52  in the direction toward a quarter-wave plate  64 , which corresponds in its embodiment to the quarter-wave plates  42  and  48 . Following reflection at the quarter-wave plate  62 , the light beam  4  is propagated with a polarization rotated by  90  degrees in the opposite direction along the line  52 , in order to be reflected at the polarization-selectively reflecting layer  36  in the direction of the line  38 , on which it leaves the crystal  32  in order to be reflected, after passing the lens  10 , in the direction of the line  30  by means of the polarizing beam splitter  62 . The light beam  4  leaving the crystal  32  has a polarization represented by the symbol  60 . In this embodiment the light beam  4  passes six times through optically pumped zones in the two crystals  32  and  34 , the optically pumped zones located upstream of the quarter-wave plates  42 ,  48  and  64  along the lines  40 ,  46  and  52  are respectively passed in both possible directions in polarizations respectively orthogonal in respect to each other. 
     A further increase of the number of passages of the light beam  4  through optically pumped zones can be achieved by means of a device in accordance with FIG.  4 . After passing through the polarizing beam splitter  6 , the Faraday rotator  8  and the lens  10 , the light beam  4 , in a polarization represented by the symbol  12 , passes first along a line  72 , a line  74  and a line  76  through a crystal  66 , a crystal  68  as well as a crystal  70 , without being reflected at a polarization-selectively reflecting layer  106  and a further polarization-selectively reflecting layer  104 . Following reflection at a quarter-wave plate  86 , the light beam  4  is propagated in a polarization rotated by 90 degrees in the opposite direction along the line  76 , in order to be reflected at the polarization-selectively reflecting layer  104  in the direction of a line  78 . Following reflection at a further quarter-wave plate  88 , the light beam  4  is propagated in the opposite direction, again with its polarization rotated by 90 degrees, in the direction of the line  78  as well as a line  80 , wherein a reflection-free transition from the crystal  70  to the crystal  68  takes place. Following reflection at a quarter-wave plate  90 , the light beam  4  is propagated in the opposite direction and with a polarization rotated by 90 degrees, along the line  80  in order to be reflected at the polarization-selectively reflecting layer  104  into the propagation path defined by the line  74 . Thereafter a further reflection at the polarization-selectively reflecting layer  106  in the direction of the line  82  takes place, after which the light beam  4  is reflected in the opposite direction at a further quarter-wave plate  94  and its polarization is again rotated by 90 degrees. Then the light beam  4  is propagated along a line  82  and a line  84  in the direction of a mirror  92 , at which it is reflected in the opposite direction without a rotation of its polarization. When the light beam  4  thereafter has passed once more the polarization-selectively reflecting layer  106  without being reflected, another reflection takes place at the quarter-wave plate  94 , because of which the light beam  4 , its polarization rotated by 90 degrees, is propagated along the line  82  in the direction of the polarization-selectively reflecting layer  106 , is now reflected at it and is propagated along the line  74  and, by another reflection at the polarization-selectively reflecting layer  104 , along the line  80  in the direction of the quarter-wave plate  90 . Because of its polarization being at the same time rotated by 90 degrees, the reflection of the light beam  4  at the quarter-wave plate  90  results in the reflection-free passage through the polarization-selectively reflecting layer  104  along the line  80  as well as the line  78 . Following reflection at the quarter-wave plate  88 , the light beam  4  is propagated along the line  78  in the direction of the polarization-selectively reflecting layer  104  in order to be reflected at it on the line  76  in the direction of the quarter-wave plate  86 . The reflection at the quarter-wave plate  86  causes a rotation of the polarization of the light beam  4  by a further 90 degrees. Therefore the light beam  4  then passes the polarization-selectively reflecting layer  104  as well as the polarization-selectively reflecting layer  106  along the line  76 , the line  74  as well as the line  72  without being reflected, in order to be separated, after passing through the lens  10 , by means of the Faraday rotator  8  and the polarizing beam splitter  6 . In this embodiment the light beam  4  passes through a total of  18  pumped zones. These are added together from respectively four passages through the optically pumped zones placed upstream of the four quarter-wave plates  86 ,  88 ,  90  and  94 , and from two passages through the optically pumped zone placed upstream of the mirror  92 . 
     FIG. 5 again represents an embodiment of the device, represented in its basic function in FIG. 4, which permits the omission of the high-mass and large-volume Faraday rotator  8 . 
     In a polarization represented by the symbol  58 , after passage through the polarizing beam splitter  62  and the lens  10 , the light beam  4  first passes along the line  72 , the line  74  and the line  76  through the crystal  66 , the crystal  68  as well as the crystal  70 , without being reflected at the polarization-selectively reflective layer  106  and the polarization-selectively reflective layer  104 . Following reflection at the quarter-wave plate  86 , the light beam  4  is propagated, with its polarization rotated by 90 degrees, in the opposite direction along the line  76  in order to be reflected at the polarization-selectively reflecting layer  104  in the direction of the line  78 . After reflection at the quarter-wave plate  88 , the light wave  4  is propagated in the opposite direction, again with its polarization rotated by 90 degrees, in the direction of the line  78  as well as the line  80 , wherein a reflection-free transit from the crystal  70  to the crystal  68  takes place. Following reflection at the quarter-wave plate  90 , the light beam  4  is propagated in the opposite direction, with its polarization rotated by  90  degrees, along the line  80  in order to be reflected into the propagation path defined by the line  74  at the polarization-selectively reflecting layer  104 . Thereafter a further reflection at the polarization-selectively reflecting layer  106  in the direction of the line  82  takes place, after which the light beam  4  is reflected in the opposite direction at the quarter-wave plate  94  and its polarization is again rotated by 90 degrees. Then the light wave  4  is propagated along the line  82  and the line  84  in the direction of a further quarter-wave plate  110 , where its polarization is rotated by 90 degrees and it is reflected in the opposite direction along the line  84  in order to now be reflected at the polarization-selectively reflecting layer  106  in the direction of the line  72 . After leaving the crystal  66 , the light beam  4  passes through the lens  10  and is reflected at the polarized beam splitter  62  in the direction of the line  30 , since now the polarization of the light beam  4 , represented by the symbol  60 , extends orthogonally to the light beam  4  entering the crystal  66 . In the device represented in FIG. 5, a total of ten passages of the light beam  4  through optically pumped zones of the crystals  66 ,  68  as well as  70  takes place. These are added up from the respectively double passage of the light beam  4  through respectively one optically pumped zone upstream of the quarter-wave plates  86 ,  88 ,  90 ,  94  as well as  110 , each time in both directions and with polarizations orthogonally in respect to each other. 
     The device represented in FIG.  4  and FIG. 5 can be configured into amplifiers with any arbitrary number of passages of the light beam  4  through optically pumped zones of the crystals of such a quantum-optical amplifier by the respective addition of a further crystal  68 , a further polarization-selectively reflecting layer  104  and further quarter-wave plates  90  and  94  and pump light sources  96 ,  98 ,  100 ,  102 , and  108 . 
     If the crystals  2 ,  32 ,  34 ,  66 ,  68  as well as  70  mentioned in the above exemplary embodiments, whose material can be arbitrarily selected, are not made of optically isotropic, but optically uniaxial material, such as neodymium yttrium vanadate, for example, the optical axis of such crystals must be aligned perpendicularly in relation to the beams to be amplified. If the crystals have been cut in that way, it is possible in accordance with the principle of a Glan-Foucault prism, to achieve the polarization-selective reflection of beams to be amplified required in the arrangement of FIGS. 2 to  6  also be means of a simple, optically isotropic space. In addition, it is also possible to realize a Glan-Taylor prism by the different orientation of the optical axes of adjoining crystals. 
     The exterior shape of the crystals  32 ,  34 ,  66 ,  68  as well as  70  in FIGS. 2 to  5  is not limited to the shapes here represented. By way of example, FIG. 6 represents a variant of the amplifier of FIG. 3 in the form of a 45 degree polarizing beam splitter cube. It is again possible with this embodiment to design the polarization-selective layer  36  as an optically isotropic space in the manner of a Glan-Foucault prism or a Glan-Taylor prism, if optically uniaxial crystals are employed. The quarter-wave plate  64  can furthermore be replaced by a mirror in order to arrive at a structure analog to the one represented in FIG.  2 . In addition, structures analogous to those in FIGS. 4 and 5 can be realized by stringing together cubes made of crystals  160  and  161 .