LASER SYSTEMS USING PHASE CONJUGATE FEEDBACK

Abstract: A laser system for emission of a highly coherent, possibly single mode, output light beam, comprising a first laser (30), such as an array of broad area lasers, for emission of a first high power light beam. An external cavity is formed between the laser and e.g. a phase conjugator (35) emitting a second light beam in response to the first incident light beam. A frequency selective element (37), such as an etalon, is positioned in the external cavity. The feedback from the external cavity forces the first laser to emit a stable and significantly improved spatially and temporally coherent high power output beam. Furthermore, in the external cavity a frequency doubler crystal (166) may be positioned for frequency doubling at least a part of the light beam in the cavity. The frequency doubler crystal may advantageously be positioned inside the etalon (221) where the laser beam has high intensity and high temporal coherence.

Description: LASER SYSTEMS USING PHASE CONJUGATE FEEDBACK FIELD OF THE INVENTION The present invention relates to an improvement of the coherence properties of laser systems operated far above threshold. The laser system may comprise laser arrays, such as semiconductor laser arrays. BACKGROUND OF THE INVENTION It is well known in the art to use lasers as sources of diffraction limited, temporally and spatially coherent', high energy light beams. Typically, it is desirable to use a single element semiconductor laser as a light source in laser systems because of its low cost, small size and ruggedness. However, the maximum output power of semiconductor lasers is typically 100 - 200 m . In "Influence of phase conjugate optical feedback on emission properties of visible low-power diode lasers" by S. Mailhot and N. MacCarthy in Canadian Journal of Physics, Vol. 71, 1993, a system to improve the coherence of a low-power diode laser is disclosed. The system comprises a phase conjugate mirror and an etalon, and is operated at a drive current slightly above threshold. It is a disadvantage of the disclosed system that it does not improve the coherence when the laser is operating well above threshold. It is another disadvantage that the system does not provide for emission of a single mode light beam. It is a further disadvantage of the disclosed system that the output power is only 1-2 mW limiting the usefulness of the system. If a higher output power is desired, the cross section of the gain medium of the semiconductor laser may be increased and/or several laser elements may be combined into an array. For example, a broad area laser is a linear array of high- gain regions from which laser beams may be emitted and separated by low-gain regions . A broad area laser may provide a laser beam with an output power of up to 2 Watts. Broad area lasers may themselves be combined into an array of broad area lasers, such as laser bars, providing a laser beam with an output power of 20 Watts. Typically, broad area lasers support multiple longitudinal and spatial modes, thus the system is dynamic and the mode structure is constantly changing. This is an important disadvantage of broad area lasers. It is a further disadvantage of broad area lasers that the output beam has low spatial and temporal coherence and it is non-diffraction limited. The lack of coherence decreases spectral purity, controllability, and the general usefulness of the output beam for traditional laser applications. It is known to improve the spatial and/or the temporal coherency of the output beam from a broad area laser by returning light back into the laser utilizing an external mirror, the laser and the mirror defining an external cavity. If the mirror is on-axis, a spatial filter (an aperture) can be positioned in the external cavity to prevent certain spatial modes from lasing so that the laser is forced to operate in a single spatial mode. However, it is a disadvantage that much of the available power is lost due to the spatial filtering. Selection of a high order spatial mode of a broad area laser may be obtained by arranging the mirror off-axis. However, such an arrangement is very sensitive to mirror misalignment which may be caused, e.g., by mechanical vibrations, temperature changes, etc. It is known to substitute the mirror with a phase conjugator. Needless to say, a mirror only retroreflects an incident beam that propagates along an axis that is perpendicular to the mirror back towards the beam emitter. A phase conjugator retroreflects an incident light beam propagating along an axis forming an angle within a wide range of angles with the phase con ugator back towards the beam emitter. Further, a phase conjugator adjusts to small changes m alignment m real time so that laser operation is not disturbed by low frequency vibrations and temperature changes. In US 5.430.748, a system is disclosed, comprising a broad area laser for emission of an output light beam with a coherency axis, a part of the output beam from the laser array being directed through external optics to a phase con ugator crystal having a conjugation axis, the coherency axis of the light beam being substantially aligned with the conjugation axis of the phase conjugator crystal. An external cavity is thereby formed between the laser and the phase conjugator and the output part of the light beam (the part of the light beam not directed to the phase con ugator) is a near diffraction limited output beam. It is another disadvantage of the disclosed system that temporal coherence of the emitted light beam is too low for various applications, such as frequency doubling, lnter- ferometπc sensors, etc. It is a further disadvantage of the disclosed system that the emitted light beam has a short coherence length due to presence of several longitudinal modes m the emitted light beam . It is still another disadvantage that the system disclosed does not provide for emission of a light beam with a single longitudinal mode. It is a further disadvantage of the system disclosed that it does not provide for any adjustments of the wavelength due to lack of any wavelength adjusting means. US 5.430.748 also discloses a system further including an external separate single-mode laser. In this system, a phase conjugator is utilized to guide the output beam of the single-mode laser into the broad area laser. The injected beam locks the frequency of the broad area laser and causes it to emit a single beam which is 1.34 times the diffraction limit . It is a disadvantage of this system that an external smgle- mode laser is needed to obtain single mode operation. It adds cost and complexity to the system and it is difficult to align the single-mode laser with the phase conjugator and the broad area laser. It is a another disadvantage of this system that tight temperature control of the single-mode laser and the broad area laser is required to prevent changes in output beam wavelength. SUMMARY OF THE INVENTION It is an object of the present invention to provide a laser system and a method for emission of a stable and highly spatially and temporally coherent high power light beam with a narrow band spectrum. It is another object of the present invention to provide a laser system and a method for emission of a light beam with a long coherence length. It is yet another object of the present invention to provide a laser system and a method for emission of a near diffraction limited light beam. It is still another object of the present invention to provide a laser system and a method for emission of a light beam of a single spatial mode. It is yet still another object of the present invention to provide a laser system and a method for emission of a light beam of a single longitudinal mode. It is yet still another object of the present invention to provide a laser system and a method for emission of a light beam of a single spatial mode and a single longitudinal mode. It is a further object of the present invention to provide a laser system for emission of a light beam which is not scanning with respect to the center frequency of the free running laser. It is a still further object of the present invention to provide a laser system and a method for emission of a light beam with an adjustable wavelength. It is a still further object of the present invention to provide a laser system without a need for temperature control of the laser. It is another object of the present invention to provide a laser system with low sensitivity to mechanical vibrations. It is yet another object of the present invention to eliminate light frequency chirps caused by modulation of current supplied to the laser. According to a first aspect of the invention, the above and other objects are fulfilled by a laser system comprising a laser having an external cavity with an adaptive light feedback device and a frequency selective element. Preferably, the laser system comprises a first laser for emission of a first high power light beam and an adaptive light feedback device for emission of a second light beam m response to light incident upon it and being positioned m relation to the first laser so that, during emission of the first light beam, the device is illuminated by a first part of the first light beam and the second light beam is injected into the first laser, the adaptive light feedback device and the first laser defining an external cavity there between. Further, a frequency selective element is positioned m the external cavity m the light path of the part of the first light beam, the frequency selective element and the adaptive light feedback device cooperating to select a wavelength range of the second light beam that is injected into the first laser whereby the laser system is controlled to emit a stable and highly spatially and temporally coherent output light beam with a narrow band spectrum. According to a second aspect of the invention, the above- mentioned and other objects are accomplished by a method of generating a coherent light beam, comprising the steps of: operating a first laser to emit a first high power light beam, forming an external cavity between an adaptive light feedback device and the first laser by illuminating the adaptive light feedback device by a part of the first light beam thereby causing emission of a second light beam from the adaptive light feedback device, and injection of the second light beam into the first laser, and selecting a wavelength range of the second light beam by positioning a frequency selective element in the external cavity m the light path of the part of the first light beam illuminating the adaptive light feedback device whereby the laser system is controlled to emit a stable and highly spatially and temporally coherent output light beam with a narrow band spectrum. The light beam with a narrow band spectrum is defined as an output light beam with an optical power spectrum m which the full width at half maximum (FWHM) of the best fit to the optical power spectrum of a Gaussian envelope is less than one longitudinal mode spacing of the solitary free running laser. The longitudinal mode spacing is given by c/ (2nL) where c is the speed of light and nL is the optical path length of the laser cavity, where n is the refractive index and L is the length of the laser cavity. In the present context, a laser is said to emit a high power output light beam when it is pumped with energy at a level substantially above the threshold level. The threshold level is the lowest possible energy level at which the laser can lase. It is well known that laser may be pumped with various types of energy, such as electrical energy, electromagnetic energy, such as light, etc, etc. For example, a semiconductor laser may be pumped with electrical energy by supplying a current to the laser. The semiconductor laser lases when the current supplied to it is greater than or equal to the threshold current of the laser and the laser is emitting a high power output light beam when the current supplied to it is substantially larger than the threshold current, such as 1.5 times the threshold current. The laser may comprise any suitable laser beam generation means, such as a gas laser, a semiconductor laser, a semiconductor laser array, a superlummescent laser diode, a dye laser, a Nd-YAG laser, an Argon ion laser, etc. Further, the laser may comprise any array of lasers of the above-mentioned types, such as a broad area laser or an array of broad area lasers, etc. The invention is particularly useful for lasers having a broad bandwidth gain medium, such as dye lasers, semiconductor lasers, titanium sapphire lasers, F-center lasers etc. The laser system may comprise two cavities associated with the first laser, an internal cavity of the first laser and the external cavity. In this case, the external cavity may operate to return a selected part of the light emitted by the first laser to the first laser so as to lock the laser output to the selected and returned part of the light. Alternatively, a first end mirror of the first laser may be removed so that the laser cavity is formed between the second end mirror of the first laser and the adaptive light feedback device . The adaptive light feedback device is a device that retrore- flects, for example by optical phase conjugation, light in response to light incident upon it. The adaptive light feedback device may comprise a phase conjugator wherein gratings are dynamically formed in response to light incident upon it. The phase conjugator may comprise a non-linear medium, such as a photorefractive crystal, such as a BaTiO^ crystal, etc, a semiconductor, a non-linear gas, such as a SBS cell (optical phase conjugation by Stimulated Bπlloum Scattering), e.g., containing a CS ^ gas, a liquid crystal, an organic polymer, etc. The non-linear medium may have a conjugation axis which is an axis along which the medium lacks inversion symmetry. The adaptive light feedback device may be self-pumped by the light beam to be retroreflected or pumped by one or more separate laser beams. The adaptive light feedback device may adapt to counteract various types of changes of or imposed on the system, thereby stabilizing the wavelength of the output beam against variations m ambient conditions, such as temperature, pressure, moisture, etc, wavelength changes m the first laser, changes m spatial modes emitted by the first laser, etc. For example, as previously mentioned, a BaTι0 3 crystal is capable of adjusting to small changes m alignment of the system m real time by dynamically changing the crystal gratings so that a beam transmitted along a changed direction is still retrore- fleeted whereby operation of the first laser is not disturbed by misalignment which may be caused, e.g., by mechanical vibrations, temperature changes, etc. Further, formation of retroreflecting gratings m the crystal is influenced by characteristics, such as wavelength range, number of spatial and/or longitudinal modes, etc, of the light that illuminates the crystal and it is an important advantage of the present invention that if the wavelength range and/or the number of spatial and/or longitudinal modes of the illuminating light is narrowed, the crystal seems to narrow corresponding characteristics of the second light beam even further whereby the coherence of the system is further enhanced. The frequency selective element is an element that either deflects, such as by reflection, refraction, scattering, diffraction, etc, light propagating along a first propagation axis and impinging upon the frequency selective element into light propagating along a second propagation axis forming an angle with the first propagation axis, the size of the angle being dependent upon the wavelength of the impinging light, or, transmits a selected part of the impinging light within a specific wavelength range while the remaining part of the impinging light is absorbed and/or deflected. The frequency selective element may comprise an interference filter, an absorbance filter, such as a semiconductor doped glass, etc, an etalon, a prism, a grating, such as a diffrac- tive optical element, such as a hologram, etc, etc. The operating characteristics of the frequency selective element either alone, such as for an etalon, or in combination with the adaptive light feedback device, such as for a grating, determines the wavelength range of the second light beam that is injected into the first laser. The frequency selective element is positioned in the external cavity in the light path of the part of the first light beam illuminating the adaptive light feedback device and it is adapted to select a specific wavelength range of the part of the first light beam illuminating the adaptive light feedback device. In response to the illuminating light, the adaptive light feedback device emits the second light beam having a wavelength range corresponding to the selected wavelength range so that the frequency selective element in cooperation with the light feedback device selects the wavelength range of the second light beam without reducing the total power of the output beam significantly. The etalon is a frequency filter and will only pass a limited number of frequencies that can interact with the adaptive light feedback device. Single spatial mode operation can be achieved if the orientation of the etalon is adjusted so that the wavelength for peak transmission matches the lasing wavelength of a spatial mode with high gain. In this case, the bandwidth of the output light may be less than 0.03 nm. It is an important advantage of the invention that selection of a small wavelength range of the light beam injected into the adaptive light feedback element causes the first laser to be stabilized so that a centre wavelength of the first light beam remains constant substantially independent of various operating parameters, such as operating temperature of the first laser, mechanical vibrations of the system, light modulation of the first laser, etc. Typically, the response time of the adaptive light feedback device is in the order of a few seconds and it is, therefore, capable of compensating for slow temperature changes in the external cavity and the first laser. Thus, in many applications, a laser system according to the present invention does not need any temperature control. Typically, the temperature drift of a semiconductor laser is 0.25 nm per °C . For example for the embodiment according to the present invention shown in Fig. 3, the temperature induced change of the wavelength of a semiconductor laser is reduced to 0.1 nm for a 20 °C temperature change. In optical communication system, light signals are generated by modulation of light emitted from a semiconductor laser by modulation of the current supplied to the laser. However, the wavelength of light emitted by a free running semiconductor laser varies as a function of the current supplied to the laser and thus, modulation of the supply current causes generation of light frequency chirp, typically corresponding to 1 nm wavelength chirp, which lowers the available capacity of communication channels of the communication system. It is an important advantage of the invention that light frequency chirp caused by modulation of current supplied to the first laser is substantially eliminated. It is another advantage of the invention that a laser system emitting a light beam of high optical brightness is provided, the optical brightness of a source is defined as the energy per unit area, per unit time, per unit solid angle, per unit frequency interval. The wavelength of the output light beam may be adjusted by corresponding adjustment of the wavelength range transmitted or deflected by the frequency selective element. Thus, a laser system with an adjustable wavelength may be provided. For example, a laser system with a first laser with a free- running centre wavelength equal to 800 nm may be adjusted +/- 3 nm. For example, when the frequency selective element comprises a grating, the laser system may comprise frequency adjustment means for selection of the frequency of the output light beam, the frequency adjustment means being adapted to adjust the angular tilt of the grating in relation to a propagation axis of the light beam illuminating the adaptive light feedback device. As the grating deflects light into propagation directions that depend on the wavelength of the light, the wavelength of light impinging on the adaptive light feedback device depends on the angular tilt of the grating m relation to the feedback element. The adjustment of the angular tilt of the grating does not result m a continuous tuning of frequencies, but m discrete frequency steps between different longitudinal modes, each of which belongs to the same spatial mode. The angular tilt of the grating may be controlled by a piezo element. Hence, the frequency can be automatically adjusted by the application of a voltage to the piezo element. Further, for a fixed position of the grating the wavelength may be continuously adjusted by varying the temperature of the first laser. By changing the temperature less than 1°, the wavelength may be adjusted over a wavelength range corresponding to one longitudinal mode spacing. It is presently preferred that the adaptive light feedback device is a phase conjugator. A phase conjugator retrore- flects an incident light beam, the phase of the retrore- fleeted light beam being reversed in relation to the incident light beam. Thus, positioning of the phase conjugator m relation to the remaining laser system and m particular in relation to the light beam incident upon it is not critical as the phase conjugator automatically returns incident light in the desired direction and in the desired phase. The phase conjugator may comprise a photorefractive crystal. It is further preferred that the phase conjugator comprise an anisotropic crystal that has a conjugation axis which is the axis along which the phase conjugator lacks inversion symmetry. It is even more preferred that the crystal is a BaTi0 crystal, e.g. doped with rhodium, that is used in a self- pumped configuration. This crystal is photorefractive and, typically, it responds in the visible and in the near infrared wavelength range comprising 800 nm. A polarization axis is the direction in which a light beam is linearly polarized. A coherency axis is the direction of maximal spatial coherence of a light beam. Both of these axis extend substantially perpendicular to the propagation axis of the light beam. The polarization axis or the coherency axis are aligned with a conjugation axis if, at the conjugator surface at which incident light impinges on the conjugator, the axis lies in the plane formed by the conjugation axis and the beam propagation axis. The angle through which an axis would have to be rotated to provide for this alignment is a measure of misalignment. Substantial alignment indicates that misalignment is not greater than 20°. As previously mentioned, a BaTi0 3 crystal is capable of adjusting to small changes in alignment of the system in real time by dynamically changing the crystal gratings so that a beam transmitted along a changed direction is still retrore- flected whereby operation of the first laser is not disturbed by misalignment which may be caused, e.g., by mechanical vibrations, temperature changes, etc. Further, formation of retroreflecting gratings in the crystal is influenced by characteristics, such as wavelength range, number of spatial modes, etc, of the light that illuminates the crystal and it is an important advantage of the present invention that if the wavelength range and/or the number of spatial modes of the illuminating light is narrowed, the crystal seems to narrow corresponding characteristics of the second light beam even further whereby the system is further stabilized. It is a further advantage of utilization of a phase conjugating crystal that it operates to recover wavefronts of light incident upon it, i.e. if such wavefronts have been dis- torted, e.g., by optical components of the system. The original non-distorted wavefronts will be recovered by the gratings in the crystal so that the retroreflected light has non-distorted wavefronts. It is another advantage of utilization of a phase conjugating crystal that it retroreflects each beam incident upon it back to its source, i.e. when the first laser comprises an array of lasers light is injected into each laser of the array so that each laser is locked to injected light. Thus, throughout the present disclosure a retroreflected beam may be composed of a plurality of retroreflected beams, each of which may be generated in response to a beam emitted from a single laser in an array of lasers. When light emitted from the first laser is linearly polarized, the polarization axis is preferably substantially aligned with the conjugation axis whereby start of emission of the second or retroreflected light beam is facilitated. Thus, preferably, the laser system comprises means for' aligning the polarization axis of the first laser with the conjugation axis of the phase conjugator . The means for aligning the polarization axis with the conju- gation axis may comprise a waveplate. Preferably, the wave- plate is a half-waveplate or a zero-order half-waveplate in which the thickness of the plate is substantially equal to half the wavelength of light illuminating the waveplate plus any integer number of wavelengths hereof. Further, when light emitted from the first laser has a coherency axis, the phase conjugator is preferably positioned m relation to the first laser so that the laser coherency axis is substantially aligned with the conjugation axis thereby increasing the energy of the second light beam m relation to the first part of the first light beam. Alignment of the conjugation axis and the coherence axis provides maximum coupling between the pump beam that is formed along the conjugation axis and the beam to be retrore- flected. The laser system may further comprise a spatial filter positioned m the light path of the first part of the first light beam and preventing transmission of selected spatial modes towards the adaptive light feedback device. According to a preferred embodiment of the invention, the laser system may comprise a beam splitter positioned m the external cavity m the light path of the first light beam for transmission of the first part of the first light beam and for reflection of a second part of the first light beam, at least a part of the reflected light beam forming the output light beam. The laser system may further comprise a frequency conversion device, such as a frequency doubler, optical parametric oscillator, etc., for frequency conversion of at least part of the incident light beam so that the wavelength of the coherent light beam is selected to a desired wavelength. For example, the laser system may further comprise a fre- quency doubler for frequency doubling at least part of the incident light beam so that the wavelength of the coherent light beam is substantially equal to half the wavelength of the incident light beam. This method may lead to new fre- quency doubled light sources with wavelengths ranging from 1 nm - 50 μm, such as from 100 nm - 10 μm, such as from 100 nm - 3 μm, preferably such as from 100 nm - 500 nm, such as 300 nm - 550 nm. Throughout the present description, it is to be understood that the frequency doubler may be substituted by a crystal of an optical parametric oscillator for generation of any desired wavelength. Inside the external cavity the intensity of the light beam may be high, and positioning of the frequency doubler within the external cavity provides a high power frequency doubled output . In the external cavity the frequency doubler may be placed inside the frequency selective element, for example inside an etalon formed by reflecting surfaces, or the surfaces of the frequency doubler may be the frequency selective element itself. The intensity of the beam inside a frequency selective element, such as inside an etalon, is enhanced with a factor of l/(l-r) 2 , where r is the reflectivity of the reflecting surfaces, relative to the beam outside the etalon. The intensity inside an etalon with a reflectivity at each surface of 0.9 is thus amplified with a factor 100. The intensity of the frequency doubled light is proportional to square of the intensity of the light incident on the frequency doubler and typically only a few percent of the incident light is frequency doubled when the frequency doubler is positioned in the external cavity. By positioning the frequency doubler inside the etalon m the external cavity or by having the surfaces of the frequency doubler constituting the frequency selective element of the external cavity, the beam to be frequency doubled has a high intensity and is highly collimated whereby the efficiency of the frequency doubling is increased. Due to the high intensity, the stability, and high spatially and temporally coherence of the beam inside the external cavity, the invention also provides for the first laser to be a laser emitting a narrow band output light beam, which output light beam is to be frequency doubled. In conventional systems the frequency of the narrow band laser is frequency doubled using an external frequency doubler cavity comprising reflecting mirrors and a frequency doubler. In order to obtain high frequency doubled conversion efficiency m such systems, the length of the external frequency doubler cavity must be carefully controlled, typically using an electrical servo system, so as to achieve resonance for the light beam to be frequency doubled. By using an adaptive light feedback device, such as a phase conjugator, the frequency doubler cavity inside the external cavity is automatically tuned to resonance for the light beam to be frequency doubled. This eliminates the need for control of the length of the frequency doubler cavity m relation to the emitting wavelength of the first laser. The frequency doubler or the frequency doubler cavity may be also be positioned outside the external cavity and being pumped by the stable and highly spatially and temporally coherent output light beam. The laser system with the frequency doubler inside the external cavity may further com ¬ prise a beam splitter for transmission of light having a wavelength that is substantially equal to the wavelength of the first light beam, at least part of the transmitted light beam constituting the first part of the first light beam, and for reflection of light having a wavelength that is substantially equal to half the wavelength of the first light beam and being positioned m the external cavity m the light path of the first light beam downstream m relation to the fre- quency doubler whereby the frequency doubled output light beam is spatially separated from the other light beams of the system. The laser system may further comprise a second grating for deflecting a third light beam having a wavelength that is substantially equal to the wavelength of the first light beam a first angle in relation to the first light beam, at least part of the third light beam constituting the first part of the first light beam, and deflecting a fourth light beam having a wavelength that is substantially equal to half the wavelength of the first light beam a second angle in relation to the first light beam, and being positioned in the external cavity in the light path of the first light beam downstream in relation to the frequency doubler, at least part of the fourth light beam constituting the frequency doubled output light beam whereby the fourth light beam is spatially separated from the other light beams of the system. This other grating may also constitute the frequency selective element. The first light beam may be emitted from a first surface of the first laser while the output light beam may be emitted from a second surface of the first laser. The system may further comprise a second laser positioned in relation to the first laser in such a way that a gain medium of the second laser is illuminated by the output light beam whereby the second laser is pumped by the output light beam. The second laser may, in the case of the first laser lasing at a wavelength at substantially 810 nm, be a Nd:YAG laser, a Nd:YV0 4 laser, etc. By pumping these lasers with the highly stable coherent light beam a number of advantages is achieved. The output frequency of the first laser may be adjusted to a frequency that matches the optimum absorption peak in the laser rod of the second laser thereby increasing conversion efficiency of the system. Furthermore, the stable and highly spatially and temporally coherent, tuneable, and near diffraction limited output light beam of the first laser enables a tight focusing onto the laser rod thereby increas- ing conversion efficiency from electrical input power to laser output power. The system may further comprise a single mode fibre and a spatial filter that is positioned in the light path of the first part of the first light beam for transmission of a selected spatial mode towards the adaptive light feedback device, and means for coupling the output light beam into the single mode fibre. A single-mode fiber may have a core diameter of approximately 5 μm. The advantageous coupling efficiency obtained by such a system is caused by the output beam of the system being near diffraction limited and by the high brightness of the output light beam. The system may further be used in interferometric sensors and laser spectroscopy . Applications with interferometric sensors include measurements of length in Twyman-Green interferometers and flow velocity sensors based on laseranemometry . The invention will now be described by way of example and with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig 1. shows the farfield energy distribution of an output beam of a broad area laser, Fig. 2 shows schematically two types of embodiments according to the invention. Fig. 2a shows schematically an embodiment where the entire output beam is phase conjugated and Fig. 2b shows schematically an embodiment where only part of the output beam is phase conjugated, Fig 3. shows schematically an embodiment according to the invention comprising an etalon and a phase conjugator in which the entire beam is phase conjugated, Fig. 4 shows the farfield energy distribution of light emitted by the embodiment shown m Fig. 3 and having an etalon with a finesse of 17, Fig. 5 shows the farfield energy distribution of light emitted by the embodiment shown m Fig. 3 and having an etalon with a finesse of 2.6, Fig 6. shows schematically an embodiment according to the invention comprising a phase conjugator, a frequency selective element, an etalon, and a spatial filter. Only part of the beam is phase conjugated, Fig. 7. shows the farfield energy distribution of light emitted by the embodiment shown m Fig. 6, Fig. 8 shows the wavelength spectrum of light emitted by the embodiment shown m Fig. 6, Fig. 9 shows the coherence degree of light emitted by the embodiment shown m Fig. 6, Fig. 10 shows the minimum sized focus spot of the light beam emitted by the embodiment shown m Fig. 6, Fig. 11 shows schematically an embodiment according to the present invention comprising a phase conjugator, a spatial filter, and a grating, Fig. 12 shows the wavelength spectrum of light emitted by the embodiment shown m Fig. 11, Fig. 13. shows the farfield energy distribution of light emitted by the embodiment shown m Fig. 11, Fig. 14 a)-e) shows the optical spectrum of light emitted by the embodiment shown m Fig. 11, when the grating is tilted, Fig. 15 shows the coherence degree of light emitted by the embodiment shown in Fig. 11, Fig. 16 shows an embodiment according to the invention for pumping a solid state laser, Fig. 17 shows schematically an embodiment according to the invention comprising a frequency doubler, an etalon, and a beamsplitter for deflecting the output beam, Fig. 18 shows schematically an embodiment according to the invention comprising a frequency doubler and a grating, Fig. 19 shows schematically an embodiment according to the invention in which the beam illuminating the phase conjugator is emitted from one surface of the laser while the output beam is emitted from another surface of the laser, Fig. 20 shows schematically an embodiment according to the invention comprising a grating and a non-linear etalon, Fig. 21 shows schematically an embodiment according to the invention wherein the frequency doubler is positioned inside the etalon, and Fig. 22 shows schematically another embodiment according to the invention comprising a phase conjugator, a grating, a frequency doubler and a second light feedback element. DETAILED DESCRIPTIONS OF THE DRAWINGS In Fig. 1, a broad area laser 10 and the farfield energy distribution 11, 12 of light emitted by the broad area laser are shown. The light emitted from a broad area laser 10 has different spatial coherence properties along its two main axes as seen in Fig. 1. A slice through the beam perpendicular to the broad area laser junction reveals a near-Gaussian mode 11 with a high spatial coherence across the width of the beam. A slice through the beam parallel to the junction of the broad area laser reveals a complicated spatial mode structure, known as a twin lobe structure 12 which has a limited spatial coherence across its width. Fig. 2 shows schematically two types of embodiments according to the invention. The embodiment 20 according to the present invention, a double lobe phase conjugate feedback configuration, shown m Fig. 2a comprises a laser array 21 forming an external cavity with a phase conjugator 22 wherein an etalon 23 and a focusing lens 24 are inserted, and is very stable due to the fact that the entire output beam from the laser is phase conjugated. The output 26 from this embodiment is generated by inserting a beamsplitter 27 m the external cavity. The farfield energy distribution of the light emitted from this embodiment is not diffraction limited and is still a twin lobe; however, this embodiment according to the present invention may operate m one single spatial mode. It is an advantage of the embodiment shown m Fig. 2a that light emitted from it is stable against temperature changes and that the coherence length of the emitted light is long. The embodiment 28 shown m Fig. 2b, a single lobe phase conjugate feedback configuration, comprises a laser array 21 forming an external cavity with a phase conjugator 22 wherein an etalon 23 and a focusing lens 24 are inserted, only one lobe of the described twin lobe structure is phase conju- gated. The feedback from the crystal to the laser array forces more energy into the lobe that is not being phase conjugated, the output 29. The coupling between the crystal and the laser is smaller compared with the configuration 20. It is an advantage of the embodiment shown m Fig. 2b that the light emitted from it is close to diffraction limited. Fig 3. shows schematically an embodiment according to the invention comprising an etalon and a phase conjugator m which the entire beam is phase conjugated. The laser array 30 used m the embodiment shown m Fig. 3, as well as m the embodiments according to the invention shown m Fig. 6 and Fig. 11, is a SDL-2432 broad area laser (BAL) which is a GaAlAs 10-stripe proton implanted gam guided laser, with a threshold of 0.29 Amps and a maximum output power of 0.5 Watts at a drive current of 0.9 Amps (3.2-I trι ). The las g wavelength at 20.0°C is 813.5 nm. The longitudinal mode spacing of the described BAL is 0.11 nm. When the BAL is freely running each longitudinal mode consists of a number of spatial modes. The mode spacing between two spatial modes is -0.02 nm. The BAL is operated at a current of 0.55 Amps (=2-1^) f nothing else is stated. At a drive current of 2 , I t the total output power is 0.19 Watt. When the BAL is freely running the spectrum has a Full Width Half Maximum (FWHM) of approximately 0.68 nm. At a drive current of 3'I th the FWHM of spectrum is 1.2 nm when the laser is freely running corresponding to approximately 10 longitu- dmal modes. The emitting junction is lxlOOμm. The light emitted from the BAL is collimated with a Thorlab C230TM-B lens 31 with an effective focal length of 4.5 mm and a numerical aperture of 0.55. 32 is a spherical singlet with a focal length of 76.2 mm. 33 is a cylindrical lens with a focal length of 150 mm. 34 is a spherical smglet with a focal length of 150 mm. All lenses has a broad band anti- reflection coating (R<1 percent) in order to minimize the loss of the external cavity. A coupling loss of approximately 20 percent is occurring between the BAL and the C230TM-B lens 31. The external cavity is terminated by a Rhodium doped (800 ppm) BaTι0 3 crystal 35 arranged in a self-pumped configuration. Both a 45-degree and a 0-degree cut crystal may be used with no significant differences in the obtained performance of this embodiment. The output of the BAL is polarized parallel to the junction of the BAL. To obtain the largest phase conjugate reflectivity both the coherency axis, the polarization and the crystal c-axis must be m the same plane. This requirement can only be satisfied if the beam polarization is rotated through 90 degree while keeping the coherency axis fixed in the plane of the conjugation axis. This is done by inserting a half wavelength waveplate 36 the path between the BAL and the phase conjugator. The waveplate is a zero-order λ/2 at 815 nm wavelength. Alternatively, the coherency axis may be rotated by use of a pair of mirrors. The entire light beam emitted from the BAL is directed towards the crystal and self-pumped four-wave-mix g takes place and returns the phase conjugated wave front towards the BAL. Due to the nature of the phase conjugated feedback mechanism of the BaTι0 3 m the double lobe feedback configuration self- induced frequency scanning of the BAL usually takes place, i.e. the centre frequency of the output beam scans a specific frequency range as a function of time. However, in order to avoid this self-induced frequency scanning of the BAL a frequency selective element 37 is inserted between the BAL and the phase conjugator as indicated in Fig. 3. The frequency selective element Fig. 3 is an etalon. Two different etalons are used the experiments. The first etalon has a thickness of 300 μm with a finesse of approximately 17. The free spectral range (FSR) is 350 GHz (or 0.75 nm at 815 nm) . The FWHM bandwidth of the etalon is 20 GHz (or 0.04 nm at 815 nm) . The second etalon has a FSR of 225 GHz and a finesse of 2.6. The FWHM bandwidth was 86 GHz (or 0.19 nm at 815 nm) . The purpose of the cylindrical lens 33 is to colli- mate the beam so as to obtain plane waves at the etalon. With the etalon with a finesse of 17 inserted the phase conjugate reflectivity of the phase conjugator ranges from 15% at a drive current of 2 , I down to 8% at higher drive current (3'I th ) • The feedback level measured at the beamsplit ¬ ter (BS) ranges from 4-8o at a drive current of less than 2*I th to 1.5-2°o at higher drive current. The amount of power that is fed back into the array is 4 mW and is independent of the drive current the range 2-I t h to 3*I t h (the total radiated power was 200 mW and 440 mW, respectively) . At a drive current of 3 # I h narrow band operation cannot be achieved and nearby spatial modes (sidebands) start to emerge, and the bandwidth is increased to 0.1 nm. However, it should be noted that the output spectrum is still a narrow band spectrum as previously defined. The output beam 39 of the embodiment shown m Fig. 3 is generated by the beamsplitter 38 in the external cavity. The etalon is a frequency filter which only passes a limited number of frequencies which then may interact with the phase conjugator. As the reflectivity of the phase conjugator increases the spectrum narrows down significantly. Single spatial mode operation can be achieved if the orientation of the etalon is adjusted so the wavelength for peak transmission match the las g wavelength of a spatial mode with high gain. In that case the bandwidth of the feedback locked spectrum may be less than the resolution of the spectrometer (0.03 nm) . In Fig. 4 the farfield energy distribution of light emitted from the embodiment shown m Fig. 3 is shown. Two cases are shown; l) Narrow band operation 40 and n) two spatial modes 41. The etalon with high finesse (a finesse of 17) is used. The bandwidth of the etalon is 0.04 nm and the mode spacing between two spatial modes are approx. 0.02 nm. Even the most closely spaced modes of the BAL has significantly different transmission loss when they pass the etalon and narrow band operation can therefore be achieved. However, as seen Fig. 4 multimode operation does also occur if the etalon is misaligned. In Fig. 5 the farfield energy distribution of light emitted from the embodiment shown Fig. 3 is shown for the case where the etalon with low finesse (a finesse of 2.6) is inserted the external cavity. When the high finesse etalon is replaced by the low finesse etalon more spatial modes are emerging. In Fig. 5 the farfield energy distribution 50 of light emitted from the embodiment shown m Fig. 3 is shown when the etalon with a low finesse is inserted the external cavity. Several spatial modes are identified. The band- width of the etalon is 0.19 nm and only different longitudinal modes (consisting of several spatial modes) has different transmission losses, hence, a cluster of spatial modes around one longitudinal mode are lasmg. The bandwidth of the spectrum is 0.15 nm. The feedback into the BAL is up to 5 percent measured front of the output facet of the BAL. The placement of an etalon m the external cavity causes narrow band operation. However, the farfield energy distribution is still far from the diffraction limit. For a 100-μm wide BAL at 815 nm wavelength, the diffraction limit is 0.55 degree. The diffraction limit being defined as FWHM of the lowest order BAL mode m the farfield (intensity profile) and given by 1.189λ/2x 0 , where λ is the wavelength and x 0 is the half-width of the BAL. When the laser is freely running the FWHM angular width of the farfield energy distribution pattern is 4 degrees (7.3 times the diffraction limit). Fig 6. shows schematically an embodiment according to the invention comprising a phase conjugator, a frequency selec- tive element, an etalon, and a spatial filter. Only part of the beam is phase conjugated. By insertion of a spatial filter m the embodiment according to the invention shown Fig. 3 the brightness of the light emitted from the BAL may be improved. Such an embodiment comprising a spatial filter is shown m Fig. 6. Compared to embodiment shown Fig. 3, two modifications are made. First, only one lobe of the described twin lobe structure the plane of farfield 62 is directed towards the phase conjugator. The opposite lobe is the output beam 63. Secondly, a spatial filter is included, allowing only one or few spatial modes to interact with the phase conjugator. The spatial filtering 61 is performed with two razor blades mounted on translation stages. The lens 31 collimates the output of the BAL. The lens 32 generates a pseudo farfield at a distance of 585 mm from the output facet of the BAL. At the plane of farfield approximately 1 mm is corresponding to 1 degree. The cylindrical lens 33 collimates the beam m the plane that contains the coherency axis. The spherical singlet 34 focuses the light at the phase conjugator . The highest brightness is obtained when both the spatial filter 61 and the frequency filter 37 are applied at the same time. Fig. 7. shows the farfield energy distribution of light emitted by the embodiment shown in Fig. 6. The FWHM angular width of the farfield energy distribution pattern is 0.75 degree at a drive current of 2- I t i (1-4 times the diffraction limit) and 0.92 degree at a drive current of 3*I t h (1.7 times the diffraction limit) . The best results are obtained with the razor blades of the spatial filter 61 positioned at 1.8 degree and 2.4 degree, respectively, as the case is for the embodiment comprising the etalon. In the s gle-lobe experiment 70o of the available energy after lens 33 is present m the output beam, and 50° of the total amount of radiated energy is contained m the output beam. However, as seen from the profile curve 71 in Fig. 7, the positive lobe contains more than 80c of the total radiated far field energy. Based on this profile it is estimated that 80° of the radiated energy can be contained the output beam provided that losses at lenses etc. are eliminated. The measured power of the output beam is 107 mW and 227 mW for a drive current of 2 , I t ι and 3 * I t , respectively. The amount of feedback measured at the beamsplitter is typically the range of 0.4-1° (highest at lower current). The phase conjugate reflectivity of the phase conjugator ranged from 12 to 15% for all drive currents. The amount of power that is fed back into the array is 0.5-1.4 mW (highest at lower current) . At 2-I t h the total radiated output power is 200 mW, which corresponds to an amplification of more than 21 dB (= 200 mW/1.4 mW) of the energy that is fed back into the array. Fig. 8 shows the wavelength spectrum of light emitted by the embodiment shown Fig. 6 for narrow band operation. Fig. 9 shows the coherence degree of light emitted by the embodiment shown Fig. 6. The coherence degree, V, is measured by a standard Michelson interferometer based on a beamsplitter and two mirrors. One mirror is fixed at a distance of 110 mm from the beamsplitter and the other mirror is mounted on a translation stage for variation of the difference between the path lengths the two arms. The interference pattern is observed on a photodiode array. The intensity the two arms is equal and the coherence degree is therefore given by: V= ( I max -I mιn ) / ( I md y+I, nln ) , where I max and I πun is the maximum and minimum intensity observed the interference pattern, respectively. The measured coherence degree 90 of light emitted from the embodiment shown m Fig. 6 (etalon+spatial filter) is shown m Fig. 9 as function of the path difference between tne two arms. In Fig. 9 is also shown the measured coherence degree for the case where the BAL used m the embodiments according to the invention is freely running 91. By comparing the coherence degree at 0.5, for the case when the laser is freely running and the case where feedback is applied, it is observed that the coherence length has increased by a factor of approx . 75. If the coherence length is defined as: L c = Δλ F wHM/λ , where Δλ™ H (0.68 nm at I=2 , I t ) is the bandwidth and λ is the las g wavelength (815 nm) , respectively, then L c equals 0.9 mm when the laser is freely running at a drive current of 2-I th . The phase conjugated feedback has increased the coherence length to 75*0.9 mm = 68 mm. Once the phase conjugate device is turned on and narrow band operation is obtained the output power becomes stable with respect to wavelength and power. The measured standard deviation for continuous operation over 3 hrs is 0.6% of the detected power and the measured standard deviation of the center wavelength is less than 0.01 nm (resolution limited). Fig. 10 shows the minimum sized focus spot of the light beam emitted by the embodiment shown Fig. 6. The output beam is 0.96x7.7 mm (smallest dimension parallel to the junction), measured at 1 /X points at 330 mm from the mirror 63. A beam expander made of two cylindrical lenses (one f=25 mm and one f=150 mm) expands the smallest dimension of the output beam to approximately 6 mm. The output beam is then focused with an achromat with a focal length of 80 mm. The smallest spot obtained is 23 μm x 14 μm. The minimum sized focus spot of the light beam emitted by the embodiment shown m Fig. 6 is measured with a beam scanner and the circular spot is shown Fig. 10 100. The total power of the output beam is 100 mW at a drive current of 2 • I tl the single lobe beam configuration. By focusing the output beam with an achromat with a focal length of 40 mm, the smallest spot obtained is 11.7 μm x 11.8 μm. Fig. 11 shows schematically an embodiment according to the present invention comprising a phase conjugator, a spatial filter, and a grating. The etalon may be replaced with a grating 110 as the frequency selective element and Fig. 11 this embodiment according to the invention is shown. All of the energy diffracted off the grating is collected with the lens 34 and directed towards the phase conjugator 35. Conse- quently the grating can not be regarded as a frequency bandpass filter as the case is for the etalon. The frequency selection is done collaboration between the grating and the phase conjugator. If no spatial filtering is applied, the feedback may cause the locked spectrum of the BAL to perform continuous scan cycles. For that reason only one lobe of the BAL is directed towards the grating. The grating is taken from a commercial spectrum analyzer and has 1200 lines/mm and a standard aluminium coating. The reflection coefficient for the extraordinary light is 63 % . A ruled grating with 1200 1/ιrtm and a blaze angle of 26° at 750 nm wavelength is also tested the embodiment shown in Fig. 11. The results are m general similar for both gratings. The best results are obtained with the razor blades of the spatial filter 61 positioned at 1.8 degree and 2.4 degree, respectively, as the case is for the embodiment comprising the etalon. Fig. 12. shows the wavelength spectrum of light emitted by the embodiment shown m Fig. 11 and as shown, narrow band operation is achieved. The wavelength spectrum shown Fig. 12 shows the wavelength spectrum both when the laser is locked 120 and when the laser s freely running 121. Fig. 13. shows the farfield energy distribution of light emitted by the embodiment shown in Fig. 11. The farfield energy distribution shown Fig. 13 shows the distribution when the laser is freely running at a drive current of 2'I t h 131. Also shown is the farfield energy distribution when feedback is applied and the laser is operated at a drive current of 2'I t h 132, and 3*I t h 133 respectively. The FWHM angular width of the farfield energy distribution pattern 132 is 0.84 degree (1.5 times the diffraction limit) for I=2 , I th - As seen from the curve 133, the main peak is still narrow as the drive current is increased to 3*I rh . However, additional spatial modes start to emerge and increases the bandwidth to 0.1 nm. The power measured the output beam is 110 mW and 220 mW for drive currents of 2-I t h and 3'Ith, respectively. When the array runs freely the total power after the cylindrical lens 33 is 150 mW and 320 mW for 2-I t h and 3'I t h, respectively. At 2'I t and 3'I th and when the phase conjugate feedback is applied, the total radiated energy from the array is 222 mW and 465 mW, respectively, i.e. 70% of the available energy after the lens 33 is contained the output beam, and 50% of the total radiated energy (before lens 31) is contained the output beam. The amount of phase conjugate feedback measured at the beamsplitter is typically 0.4 to 0.7 % (highest at lower drive current) . The amount of power that is fed back into the array is estimated to be 0.5-1.4 mW (highest at lower current) corresponding to an overall feedback level of 0.1 to 0.6 °o. Once the phase conjugator has started and narrow band operation is achieved, the locked spectrum may be frequency tuned by tilting of the grating. The frequency tuning is not continuous but discrete steps between the same spatial mode belonging to different longitudinal modes. For that reason the frequency may be adjusted to with one longitudinal modes spacing (0.11 nm) . However, by slight adjustment of the temperature (< 1 degree Celcius) of the BAL junction, the absolute frequency of the longitudinal modes will shift and consequently any wavelength withm a range of ± 3 nm around the centre wavelength can be obtained with this embodiment. In Fig. 14 an optical spectrum for I=2'I t ι, is shown. In Fig. 14a) the optical spectrum is shown when the laser array is freely running. Several longitudinal modes are present and the FWHM is 0.7 nm. In Figs. 14b) -e) the spectrum of the output beam at the same drive current is shown when the feedback is applied for different tilts of the grating. Once narrow band operation has been obtained the frequency is tuned by tilting the grating. The angular tilt of the grating is controlled by a piezo element. Between the recording of Figs. 14b) and 14e) the grating is tilted 0.43°, and the sensitivity is 12 nm/deg. A change of the tilt of the grating results in a change of the point of incidence and angle of incidence at the air-crystal interface. When the grating is tilted, the array will optimize the oscillating frequency for the best Bragg match to the existing gratings in the crystal and thereby again obtain high reflectivity from the phase conjugator. By tilting the grating the frequency is tuned by discrete steps corresponding to a longitudinal mode spacing of the array (0.11 nm) , however, two longitudinal modes can oscillate simultaneously since the energy from one longitudi ¬ nal mode is slowly transferred to the next mode as the grating is tilted. When the tilting of the grating stops the adaptive phase conjugator will adjust and optimize for highest reflectivity and narrow band operation will within seconds again be obtained. For fixed grating position the frequency can be tuned continuously over a range of 0.1 nm corresponding to one longitudinal mode spacing by changing the temperature of the junction of the array less than one degree Celsius. The gain band width of the GaAlAs array is much larger than 5 nm, so the limited wavelength range may be explained as follow: as the grating is tilted the beam is transversely shifted out of the regions with gratings inside the crystal and this leads to a reduced reflectivity and consequently a limited tuning range. The frequency can be automatically tuned by applying a saw- tooth modulation voltage to the piezo element controlling the angular tilt of the grating. At a drive current of up to 2*I th for the laser array and a modulation frequency for the piezo element of less than 0.2 Hz the phase locked spectrum scans smoothly back and forth in a single mode that transfers its energy to the next mode as described previously. During the scanning the reflectivity of the phase conjugator decreases 10-20% as compared to the case where the grating is fixed. If the modulation frequency is higher than 0.5 Hz the bandwidth of the phase locked spectrum will increase to more than 0.3 nm, and at 5-10 Hz the bandwidth is the same as the bandwidth when the arrays runs freely. If the modulation frequency is larger than approximately 0.2 Hz the reflectivity of the phase conjugator will continuously decrease towards zero as time elapse. The response time for the BaTi0 3 crystal is in the order of 1 second and only for very low modulation frequency (< 0.2 Hz) the crystal can respond to the changes of the incident beam and thereby maintain high phase conju- gate reflectivity. Fig. 15 shows the coherence degree of light emitted by the embodiment shown in Fig. 11. The measured coherence degree 140 is shown as a function of the path difference between the two arms. The measured coherence degree for the case where the laser is freely running 141 is also shown in Fig. 15. By comparing the coherence degree at 0.5, for the case where the laser is freely running and the case where feedback is applied, it is observed that the coherence length has in- creased by a factor approx. 45 to at least 16 mm for a drive current of 2'I t ι,. At a drive current of 3'I t h the coherence length is increased by a factor of 28. Once the phase conjugator has turned on and narrow band operation has been obtained, the output become very stable with respect to frequency and output power. The highest stability is achieved if the wavelength is shifted 1-2 nm towards the red with respect to the center wavelength λo of the multimode spectrum by tilting the grating. At a drive current of 2*I t h the power and wavelength of the output beam is recorded continuously over three hours of operation. The standard deviation of the detected power and wavelength is less than 0.7 % and 0.01 nm (resolution limited), respectively. During the forced tuning the farfield energy distribution remains almost unchanged with a maximum fluctuation of less than a few percent. Fig. 16 shows an embodiment according to the invention for pumping a solid state laser. Fig. 16 shows an embodiment of according to the invention with a laser array and a collimat- mg lens system 150. One lobe of the twin lobe structured output beam is reflected m the grating 151 and phase conjugated the phase conjugator 152 whereby more power is pumped to the opposite lobe forming the output beam 153. A focusing lens 154 is inserted m the light path of the output beam, focusing the light beam at the laser rod 155 of a solid state laser 156 with a mirror output 157 whereby pumping the external laser. Fig. 17 shows schematically an embodiment according to the invention comprising a frequency doubler, an etalon, and a beamsplitter for deflecting the output beam. Usually it is very difficult to obtain frequency doubling using the output from a laser array due to the very poor spatial and temporal coherence of the these lasers. However, as explained above the laser array 160 can achieve narrow band operation when a frequency selective element and spatial filter, taken together a spectral filter 161, is inserted m the light path of the laser array between the laser array 160 and the phase conjugator 163. The embodiment according to the invention shown Fig. 17 comprises a collimatmg lens system 164 and a waveplate 165 for rotation of the beam polarization. This embodiment further comprises frequency doubling means 166 for frequency doubling at least part of the first light beam. The frequency doubling means 166 comprises a nonlinear crystal which is inserted m the external cavity between the laser array 160 and the phase conjugator 163. The laser array will maintain the narrow band operation even when the non-lmeai frequency doubling crystal is inserted m the beam path since the crystal has a very low absorption at 800 nm. The infrared output beam of the laser array is converted to blue photons by the frequency doubling crystal 166. The frequency doubled blue laser beam at substantially 400 nm 168 is coupled out from the external cavity by a beamsplitter 167 which is transparent for the infrared light at 800 nm and 100 % reflecting for the blue light at 400 nm. Beside the frequency filter, spatial filtering may also be incorporated in order to achieve a better and more stable narrow band operation. Fig. 18 shows schematically an embodiment according to the invention comprising a frequency doubler and a grating. In Fig. 18 the beamsplitter and spectral filter of Fig. 17 is replaced with a diftractive grating 171. The diftractive grating is inserted in the light path of the frequency doubled beam and the first beam transmitted through the frequency doubler. The angle of reflection will be different for the two beams due to different wavelengths (800 and 400 nm) of the beams. Hereby, the frequency doubled beam 172 is directed in one angle forming the highly stable coherent output beam and the first beam is directed through a focusing lens to the phase conjugator, whereby maintaining the narrow band operation. This embodiment of the invention provides the possibility of tuning the frequency of the laser array by rotation of the grating and thereby also tuning the frequency of the blue output light beam. A very compact embodiment according to the invention is shown in Fig. 19. A collimatmg lens system 180 is inserted at both sides of the laser, so that it is necessary to have access to both sides of the laser. At one end of the laser system a conventional mirror 181 with R=100% is inserted at the plane of farfield 182 to reflect one lobe whereby pumping energy to the other lobe, which forms the output beam 183. At the other end of the laser at the plane of farfield 182 a spatial filter 184 is supplied followed by a waveplate 185 for rotation of the beam polarisation. The beam is then diffracted the grating 186 and phase conjugated in the phase conjugator 187. In this embodiment of the invention the laser array act as a gam medium (an mter-cavity amplifier) with a conjugating mirror at one end and a conventional mirror at the opposite end. The advantage of this embodiment of the invention is that the phase distortions made in the lasmg medium is eliminated from the output beam. The embodiments shown m Figs. 20 and 21 are similar to the embodiment shown m Fig. 18. Only is the frequency doubler placed inside the frequency selective element. In Fig. 20 the surfaces 191 of the nonlinear medium 192 is coated with a reflecting material (R > 70% for λ = 810 nm) , the coatings constituting the etalon. In Fig. 21 the frequency doubler 220 is placed inside an etalon 221 formed by two reflecting surfaces. The intensity of the beam inside a frequency selective element, such as inside an etalon, is enhanced w th a factor of l/(l-r) , where r is the reflectivity of the reflecting surfaces, relative to the beam outside the etalon. The intensity inside an etalon with a reflectivity at each surface of 0.9 is thus amplified with a factor 100. The intensity of the frequency doubled light is proportional to square of the intensity of the light incident on the frequency doubler and typically only a few percent of the incident light is frequency doubled when the frequency doubler is positioned m the external cavity. By positioning the frequency doubler inside the etalon m the external cavity or by having the surfaces of the frequency doubler constituting the frequency selective element of the external cavity, the beam to be frequency doubled has a high intensity and is highly collimated whereby the efficiency of the frequency doubling is increased. Due to the high intensity, the stability, and high spatially and temporally coherence of the beam inside the external cavity, the invention also provides for the first laser to be a laser emitting a narrow band output light beam, which output light beam is to be frequency doubled. In conventional systems the frequency of the narrow band laser is frequency doubled using an external frequency doubler cavity comprising reflecting mirrors and a frequency doubler. In order to obtain high frequency doubled conversion efficiency m such systems, the length of the external frequency doubler cavity must be carefully controlled, typically using an electrical servo system, so as to achieve resonance for the light beam to be frequency doubled. By using an adaptive light feedback device, such as a phase conjugator, the frequency doubler cavity inside the external cavity is automatically tuned to resonance for the light beam to be frequency doubled. This eliminates the need for control of the length of the frequency doubler cavity in relation to the emitting wavelength of the first laser. Fig. 22 shows schematically another embodiment wherein one of the lobes the far-field from the laser array is coupled to the external phase-conjugate feedback cavity as described m Fig. 11 and further comprising, a lens system 230, coupling optics 225, a frequency doubler 220, and a lens system 226. The other lobe m the far-field from the laser is coupled to an ordinary feedback cavity comprising coupling optics 227 and a light feedback device 228 such as a mirror or a grating. The purpose of the ordinary feedback cavity is to increase the intensity m the frequency doubling crystal so that efficient frequency conversion s obtained. The frequency doubled light 229 is coupled out by the grating 110 (or by a mirror 110 transparent at the frequency doubled frequency) .

Claims: CLAIMS 1. A laser system for emission of an output light beam, comprising a first laser for emission of a first high power light beam, an adaptive light feedback device for emission of a second light beam in response to light incident upon it and being positioned in relation to the first laser so that, during emission of the first light beam, the device is illuminated by a first part of the first light beam and the second light beam is injected into the first laser, the adaptive light feedback device and the first laser defining an external cavity there between, and a frequency selective element positioned in the external cavity in the light path of the first part of the first light beam, the frequency selective element and the adaptive light feedback device cooperating to select a wavelength range of the second light beam that is injected into the first laser whereby the laser system is controlled to emit a stable, and highly spatially and temporally coherent high power output light beam with a narrow band spectrum. 2. A system according to claim 1, wherein the first laser is adapted to support multiple spatial modes. 3. A system according to claim 1 or 2, wherein the first laser is adapted to emit a light beam having a polarization axis . 4. A system according to any of claims 1-3, wherein the first laser is adapted to emit a light beam having a coherency axis. 5. A system according to any of claims 1-4, wherein the first laser comprises a laser array. 6. A system according to claim 5, wherein the laser array comprises a semiconductor laser array. 7. A system according to any of the preceding claims, wherein the first laser comprises a dye laser. 8. A system according to any of the preceding claims, wherein the adaptive light feedback device comprises a phase conjugator. 9. A system according to claim 8, wherein the phase conjugator has a phase conjugation axis. 10. A system according to claim 9, wherein the phase conjugator comprises a BaTi0 3 crystal. 11. A system according to claim 9 or 10, wherein the first laser is adapted to emit a light beam having a polarization axis and further comprising means for aligning the polarization axis of the first laser with the conjugation axis of the phase conjugator at the surface of the conjugator to start emission of the second light beam. 12. A system according to claim 11, wherein the means for aligning the polarization axis with the conjugation axis comprises a waveplate. 13. A system according to any of claims 9-12, wherein the first laser is adapted to emit a light beam having a coherency axis and the phase conjugator is positioned in relation to the first laser so that the laser coherency axis is substantially aligned with the conjugation axis of the phase conjugator at the surface of the conjugator thereby increasing the energy of the second light beam in relation to the first part of the first light beam. 14. A system according to any of the preceding claims, wherein the frequency selective element comprises a first grating. 15. A system according to claim 14, further comprising frequency adjustment means for selection of the frequency of the output light beam. 16. A system according to claim 15, wherein the frequency adjustment means are adapted to adjust the angular tilt of the grating in relation to a propagation axis of the first part of the first light beam. 17. A system according to claim 15 or 16, wherein the frequency adjustment means are adapted to adjust the temperature of the first laser. 18. A system according to any of the preceding claims, wherein the frequency selective element comprises an etalon. 19. A system according to any of the preceding claims, wherein the frequency selective element comprises an interference filter. 20. A system according to any of the preceding claims, wherein the frequency selective element comprises a prism. 21. A system according to any of the preceding claims, further comprising a spatial filter positioned in the light path of the first part of the first light beam and preventing transmission of selected spatial modes towards the adaptive light feedback device. 22. A system according to any of the preceding claims, further comprising a beam splitter positioned in the external cavity m the light path of the first light beam for transmission of the first part of the first light beam and for reflection of a second part of the first light beam, at least a part of the reflected light beam forming the output light beam. 23a. A system according to any of the preceding claims, further comprising a frequency conversion device for frequency conversion at least part of the first light beam to a desired wavelength. 23b. A system according to claim 23a, wherein the frequency conversion device comprises a crystal of an optical parametric oscillator. 23. A system according to claim 23a, further comprising a frequency doubler for frequency doubling at least part of the first light beam so that the wavelength of the output light beam is substantially equal to half the wavelength of the first light beam. 24. A system according to claim 23, wherein the frequency doubler is positioned mside the frequency selective element. 25. A system according to claim 23 or 24, wherein the surfaces of the frequency doubler constitute the frequency selective element. 26. A system according to claim 23, wherein the first laser is a laser with a narrow band spectrum. 27. A system according to claim 23, wherein the frequency doubler is positioned outside the external cavity and m the light path of the output light beam. 28. A system according to claim 23, further comprising a beam splitter for transmission of light having a wavelength that is substantially equal to the wavelength of the first light beam, at least part of the transmitted light beam constituting the first part of the first light beam, and for reflection of light having a wavelength that is substantially equal to half the wavelength of the first light beam and being positioned in the external cavity in the light path of the first light beam downstream in relation to the frequency doubler whereby the frequency doubled output light beam is spatially separated from the other light beams of the system. 29. A system according to claim 23, further comprising a second grating for deflecting a third light beam having a wavelength that is substantially equal to the wavelength of the first light beam a first angle in relation to the first light beam, at least part of the third light beam constituting the first part of the first light beam, and deflecting a fourth light beam having a wavelength that is substantially equal to half the wavelength of the first light beam a second angle in relation to the first light beam, and being positioned in the external cavity in the light path of the first light beam downstream in relation to the frequency doubler, at least part of the fourth light beam constituting the frequency doubled output light beam whereby the fourth light beam is spatially separated from the other light beams of the system. 30. A system according to claim 29, wherein the second grating also constitutes the frequency selective element. 31. A system according to any of the preceding claims, wherein the first light beam is emitted from a first surface of the first laser and the output light beam is emitted from a second surface of the first laser. 32. A system according to any of the preceding claims, further comprising a second laser positioned in relation to the first laser in such a way that a gain medium of the second laser is illuminated by the output light beam whereby the second laser is pumped by the output light beam. 33. A system according to any of claims 1-31, further comprising a single mode fibre, a spatial filter being positioned in the light path of the first part of the first light beam for transmission of a selected spatial mode towards the adaptive light feedback device, and means for coupling the output light beam into the single mode fibre. 34. A system according to any of the preceding claims, for use in interferometric sensors. 35. A method of generating an output light beam, comprising the steps of: operating a first laser to emit a first light beam, forming an external cavity between an adaptive light feedback device and the first laser by illuminating the adaptive light feedback device by a first part of the first light beam thereby causing emission of a second light beam from the adaptive light feedback device, and injection of the second light beam into the first laser, and selecting a wavelength range of the second light beam by positioning a frequency selective element the external cavity the light path of the first part of the first light beam whereby the laser system is controlled to emit a stable and highly spatially and temporally coherent high power output light beam with a narrow band spectrum. 36. A method according to claim 35, wherein the first laser is adapted to emit a light beam having a polarization axis. 37. A method according to claim 35 or 36, wherein the first laser is adapted to emit a light beam having a coherency 38. A method according to any of claims 35-37, wherein the step of forming an external cavity comprises the step of phase conjugating light by utilizing a phase conjugator as the adaptive light feedback device. 39. A method according to claim 38, wherein the step of phase conjugating comprises the step of utilizing a phase conjugator with a phase conjugation axis. 40. A method according to claim 38 or 39, wherein the first laser is adapted to emit a light beam having a polarization axis and further comprising the step of aligning the polarization axis of the first laser with the conjugation axis of the phase conjugator at the surface of the conjugator facilitating start of emission of the second light beam. 41. A method according to any of claims 38-40, wherein the first laser is adapted to emit a light beam having a coherency axis and further comprising the step of aligning the coherency axis of the first laser with the conjugation axis of the phase conjugator at the surface of the conjugator thereby increasing the energy of the second light beam m relation to the first part of the first light beam. 42. A method according to any of claims 35-41, further comprising the step of selecting the wavelength of the ' output light beam by selecting with the frequency selective element a corresponding wavelength of the first part of the first light beam. 43. A method according to any of claims 35-42, further comprising the step of spatially filtering the first part of the first light beam so that transmission of selected spatial modes towards the adaptive light feedback device is prevented. 44. A method according to any of claims 35-43, further comprising the step of frequency doubling at least part of the first light beam so that the wavelength of the output light beam is substantially equal to half the wavelength of the first light beam. 45. A method according to claim 44, further comprising the step of positioning the frequency doubler inside the frequency selective element. 46. A method according to claim 44 or 45, wherein the surfaces of the frequency doubler is constituting the frequency selective element. 47. A method according to claim 44, wherein' the first laser is adapted to emit a high power light beam with a narrow band spectrum. 48. A method according to claim 44, further comprising the step of positioning the frequency doubler outside the external cavity in the light path of the output light beam. 49. A method according to claim 44, further comprising the step of positioning a beam splitter for transmission of light having a wavelength that is substantially equal to the wavelength of the first light beam, at least part of the transmitted light beam constituting the first part of the first light beam, and for reflection of light having a wavelength that is substantially equal to half the wavelength of the first light beam in the external cavity in the light path of the first light beam downstream in relation to the frequency doubler whereby the frequency doubled output , light beam is spatially separated from other light beams of the system. 50. A method according to claim 44, further comprising the step of positioning a second grating for deflecting a third light beam having a wavelength that is substantially equal to the wavelength of the first light beam a first angle in relation to the first light beam, at least part of the third light beam constituting the first part of the first light beam, and for deflecting a fourth light beam having a wavelength that is substantially equal to half the wavelength of the first light beam a second angle in relation to the first light beam in the external cavity in the light path of the first light beam downstream in relation to the frequency doubler, at least part of the fourth light beam constituting the output light beam whereby the output light beam is spatially separated from other light beams of the system. 51. A method according to any of claims 35-50, wherein the first laser is operated to emit the first light beam from a first surface of the first laser and the output light beam from a second surface of the first laser. AMENDED CLAIMS [received by the International Bureau on 20 November 1998 (20.11.98); original claims 1-51 replaced by new claims 1-58 (9 pages)] 1 . A laser system for emission of an output light beam, comprising a first laser for emission of a first high power light beam, an adaptive first light feedback device for emission of a second light beam in response to light incident upon it and being positioned in relation to the first laser so that, during emission of the first light beam, the device is illuminated by a first part of the first light beam and the second light beam is injected into the first laser, the adaptive first light feedback device and the first laser defining a first external cavity there between, and a frequency selective element positioned in the first external cavity in the light path of the first part of the first light beam, the frequency selective element and the adaptive first light feedback device cooperating to select a wavelength range of the second light beam that is injected into the first laser whereby the laser system is controlled tc emit a stable, and highly spatially and temporally coherent high power output light beam with a narrow band spectrum. 2. A system according to claim 1 , wherein the first laser is adapted to support multiple spatial modes. 3. A system according to claim 1 or 2, wherein the first laser is adapted to emit a light beam having a polarization axis. 4. A system according to any of claims 1 -3, wherein the first laser is adapted to emit a light beam having a coherency axis. 5. A system according to any of claims 1 -4, wherein the first laser comprises a laser array. 6. A system according to claim 5, wherein the laser array comprises a semiconductor laser array. 7. A system according to any of the preceding claims, wherein the first laser comprises a dye laser. 8. A system according to any of the preceding claims, wherein the first laser comprises a gas laser. 9. A system according to any of the preceding claims, wherein the adaptive first light feedback device comprises a phase conjugator. 1 0. A system according to claim 9, wherein the phase conjugator has a phase conjugation axis. 1 1 . A system according to claim 1 0, wherein the phase conjugator comprises a BaTi0 3 crystal. 1 2. A system according to claim 10 or 1 1 , wherein the first laser is adapted to emit a light beam having a polarization axis and further comprising means for aligning the polarization axis of the first laser with the conjugation axis of the phase conjugator at the surface of the conjugator to start emission of the second light beam. 1 3. A system according to claim 1 2, wherein the means for aligning the polarization axis with the conjugation axis comprises a waveplate. 14. A system according to any of claims 10-1 3, wherein the first laser is adapted to emit a light beam having a coherency axis and the phase conjugator is positioned in relation to the first laser so that the laser coherency axis is substantially aligned with the conjugation axis of the phase conjugator at the surface of the conjugator thereby increasing the energy of the second light beam in relation to the first part of the first light beam. 1 5. A system according to any of the preceding claims, wherein the frequency selective element comprises a first grating. 1 6. A system according to claim 1 5, further comprising frequency adjustment means for selection of the frequency of the output light beam. 1 7. A system according to claim 1 6, wherein the frequency adjustment means are adapted to adjust the angular tilt of the grating in relation to a propagation axis of the first part of the first light beam. 1 8. A system according to any of the preceding claims, wherein the frequency selective element comprises an etalon. 1 9. A system according to any of the preceding claims, wherein the frequency selective element comprises an interference filter. 20. A system according to any of the preceding claims, wherein the frequency selective element comprises a prism. 21 . A system according to any of the preceding claims, further comprising a spatial filter positioned in the light path of the first part of the first light beam and preventing transmission of selected spatial modes towards the adaptive first light feedback device. 22. A system according to any of the preceding claims, further comprising a second light feedback device positioned in a path of a third part of the first light beam for emission of a fifth light beam in response to light incident upon it and being positioned in relation to the first laser so that, during emission of the first light beam, the device is illuminated by the third part of the first light beam and the fifth light beam is injected into the first laser, the second light feedback device and the first laser defining a second external cavity there between, whereby the intensity of the first part of the first light beam is increased. 23. A system according to claim 22, wherein the second light feedback device is an adaptive light feedback device. 24. A system according to any of the preceding claims, further comprising a beam splitter positioned in the first external cavity in the light path of the first light beam for transmission of the first part of the first light beam and for reflection of a second part of the first light beam, at least a part of the reflected light beam forming the output light beam. 25. A system according to claim 24, wherein the beam splitter is a grating. 26. A system according to any of the preceding claims, further comprising a frequency conversion device for frequency conversion at least part of the first light beam to a desired wavelength. 27. A system according to claim 26, wherein the frequency conversion device comprises an optical parametric oscillator. 28. A system according to claim 26, wherein the frequency conversion device comprises a non-linear optical material. 29. A system according to claim 26, wherein the frequency conversion device comprises a frequency doubler for frequency doubling at least part of the first light beam so that the wavelength of the output light beam is substantially equal to half the wavelength of the first light beam. 30. A system according to claim 28 or 29, wherein the frequency conversion device is positioned inside the frequency selective element. 31 . A system according to claim 26-30, wherein the surfaces of the frequency conversion device constitute the frequency selective element. 32. A system according to claim 26, wherein the first laser is a laser with a narrow band spectrum. 33. A system according to claim 26, wherein the frequency conversion device is positioned outside the first external cavity and in the light path of the output light beam. 34. A system according to any of claims 26-29, further comprising a beam splitter for transmission of light having a wavelength that is substantially equal to the wavelength of the first light beam, at least part of the transmitted light beam constituting the first part of the first light beam, and for reflection of light having a wavelength that is substantially equal to the wavelength of the frequency converted part of the first light beam and being positioned in the first external cavity in the light path of the first light beam downstream in relation to the frequency conversion device whereby the frequency converted output light beam is spatially separated from the other light beams of the system. 35. A system according to any of claims 26-29, further comprising a second grating for deflecting a third light beam having a wavelength that is substantially equal to the wavelength of the first light beam a first angle in relation to the first light beam, at least part of the third light beam constituting the first part of the first light beam, and deflecting a fourth light beam having a wavelength that is substantially equal to the wavelength of the frequency converted part of the first light beam a second angle in relation to the first light beam, and being positioned in the first external cavity in the light path of the first light beam downstream in relation to the frequency conversion device, at least part of the fourth light beam constituting the frequency converted output light beam whereby the fourth light beam is spatially separated from the other light beams of the system. 36. A system according to claim 35, wherein the second grating also constitutes the frequency selective element. 37. A system according to any of the preceding claims, wherein the first light beam is emitted from a first surface of the first laser and the output light beam is emitted from a second surface of the first laser. 38. A system according to any of the preceding claims, further comprising a second laser positioned in relation to the first laser in such a way that a gain medium of the second laser is illuminated by the output light beam whereby the second laser is pumped by the output light beam. 39. A system according to any of claims 1 -37, further comprising a single mode fibre, a spatial filter being positioned in the light path of the first part of the first light beam for transmission of a selected spatial mode towards the adaptive first light feedback device, and means for coupling the output light beam into the single mode fibre. 40. A system according to any of the preceding claims, for use in interferometric sensors. 41 . A method of generating an output light beam, comprising the steps of: operating a first laser to emit a first light beam, forming a first external cavity between an adaptive first light feedback device and the first laser by illuminating the adaptive light feedback device by a first part of the first light beam thereby causing emission of a second light beam from the adaptive light feedback device, and injection of the second light beam into the first laser, and selecting a wavelength range of the second light beam by positioning a frequency selective element in the first external cavity in the light path of the first part of the first light beam whereby the laser system is controlled to emit a stable and highly spatially and temporally coherent high power output light beam with a narrow band 5 spectrum. 42. A method according to claim 41 , wherein the first laser is adapted to emit a light beam having a polarization axis. 10 43. A method according to claim 41 or 42, wherein the first laser is adapted to emit a light beam having a coherency axis. 44. A method according to any of claims 41 -43, wherein the step of forming a first external cavity comprises the step of phase conjugating light by utilizing a phase 1 5 conjugator as the adaptive first light feedback device. 45. A method according to claim 44, wherein the step of phase conjugating comprises the step of utilizing a phase conjugator with a phase conjugation axis. 0 46. A method according to claim 44 or 45, wherein the first laser is adapted to emit a light beam having a polarization axis and further comprising the step of aligning the polarization axis of the first laser with the conjugation axis of the phase conjugator at the surface of the conjugator facilitating start of emission of the second light beam. 5 47. A method according to any of claims 44-46, wherein the first laser is adapted to emit a light beam having a coherency axis and further comprising the step of aligning the coherency axis of the first laser with the conjugation axis of the phase conjugator at the surface of the conjugator thereby increasing the energy of the second light beam in relation to the first part of the first light beam. 0 48. A method according to any of claims 41 -47, further comprising the step of selecting the wavelength of the output light beam by selecting with the frequency selective element a corresponding wavelength of the first part of the first light beam. 49. A method according to any of claims 41 -48, further comprising the step of spatially filtering the first part of the first light beam so that transmission of selected spatial modes towards the adaptive first light feedback device is prevented. 50. A method according to any of claims 41 -49, further comprising the step of frequency conversion at least part of the first light beam. 51 . A method according to claim 50, further comprising the step of positioning the frequency conversion device inside the frequency selective element. 52. A method according to claim 50 or 51 , wherein the surfaces of the frequency conversion device is constituting the frequency selective element. 53. A method according to claim 50, wherein the first laser is adapted to emit a high power light beam with a narrow band spectrum. 54. A method according to claim 50, further comprising the step of positioning the frequency conversion device outside the first external cavity in the light path of the output light beam. 55. A method according to claim 50, wherein the frequency conversion device is a frequency doubler so that the wavelength of the output light beam is substantially equal to half the wavelength of the first light beam. 56. A method according to claim 50, further comprising the step of positioning a beam splitter for transmission of light having a wavelength that is substantially equal to the wavelength of the first light beam, at least part of the transmitted light beam constituting the first part of the first light beam, and for reflection of light having a wavelength that is substantially equal to the wavelength of the frequency converted part of the first light beam in the first external cavity in the light path of the first light beam downstream in relation to the frequency conversion device whereby the frequency converted output light beam is spatially separated from other light beams of the system. 57. A method according to claim 50, further comprising the step of positioning a second grating for deflecting a third light beam having a wavelength that is substantially equal to the wavelength of the first light beam a first angle in relation to the first light beam, at least part of the third light beam constituting the first part of the first light beam, and for deflecting a fourth light beam having a wavelength that is substantially equal to the wavelength of the frequency converted part of the first light beam a second angle in relation to the first light beam in the first external cavity in the light path of the first light beam downstream in relation to the frequency conversion device, at least part of the fourth light beam constituting the output light beam whereby the output light beam is spatially separated from other light beams of the system. 58. A method according to any of claims 41 -57, wherein the first laser is operated to emit the first light beam from a first surface of the first laser and the output light beam from a second surface of the first laser.

Inventors: LOEBEL MARTIN, PETERSEN PAUL MICHAEL, LOEBEL, MARTIN, PETERSEN, PAUL, MICHAEL
Applicants: LOEBEL MARTIN, PETERSEN PAUL MICHAEL, TORSANA A S, LOEBEL, MARTIN, PETERSEN, PAUL, MICHAEL, TORSANA A/S
Application year: 1998
Main classification: H01S3
IPC codes: H01S3, H01S5