Abstract:
A sub-nanosecond passively Q-switched microchip laser is disclosed. It combines an optically pumped, passively Q-switched, high-frequency, microchip laser producing short pulses with an optically end-pumped amplifier producing high small-signal gain while pumped at low power. The microchip laser for emitting pulsed laser radiation is a monolithic body comprising two reflective elements defining an optical resonator for laser radiation, a laser gain medium, e.g., Nd:YAG, and a saturable absorber medium, e.g., Cr 4+ :YAG placed inside said resonator. The optical amplifier stage for amplifying the laser radiation comprises an amplifying medium, e.g., Nd:YVO 4 . The microchip laser and the amplifier are optically end-pumped, preferably by high-brightness diodes. This entirely passive laser system directly produces μJ pulses at repetition rates of about 45 kHz.

Description:
FIELD OF THE INVENTION 
     This invention relates to pulsed lasers, to laser amplifiers and to methods for generating and amplifying pulsed laser radiation and, more particularly, to an entirely passive laser system both for the generation and the amplification of short pulses. 
     BACKGROUND OF THE INVENTION 
     Production of short pulses with high energy per pulse is usually achieved by a combination of one oscillator and one amplifier. The oscillator is traditionally a mode-locked laser producing very short pulses of typically less than 100 ps at high frequency of typically a few tens of MHz and with a low energy per pulse of a few nJ. To increase the pulse energy to several μJ, one uses an amplifier working at a lower repetition rate from a few kHz to a few hundreds of kHz, depending on the pumping configuration. These systems are complex and complicated to use because they require active modulation (acousto-optic or electro-optic modulators) with high-speed electronics for short-pulse production for the oscillator plus injection and synchronization of the pulses inside the amplifier. 
     Passively Q-switched lasers using Nd-doped crystals can produce high-peak-power pulses of several kW at a wavelength of 1064 nm. Depending on the experimental setup, the pulse width can vary from few tens of ns (A. Agnesi, S. Dell&#39;Acqua, E. Piccinini, G. Reali and G. Piccinno, “Efficient wavelength conversion with high power passively Q-switched diode-pumped neodymium laser”, IEEE, J. Q. E., Vol. 34, 1480-1484, 1998) to few hundreds of ps (J. J. Zayhowski, “Diode-pumped passively Q-switched picosecond microchip lasers”, Opt. Lett., Vol. 19, 1427-1429, 1994). For instance pulses of 19 ns and 108 μJ can be obtained at 25 kHz and 1064 nm from a diode-pumped Nd:YAG laser with a Cr 4+ :YAG saturable absorber crystal. The high peak power of these lasers allows efficient wavelength conversion into the ultra-violet (UV) range with optically nonlinear materials (A. Agnesi, S. Dell&#39;Acqua, E. Piccinini, G. Reali and G. Piccinno, “Efficient wavelength conversion with high power passively Q-switched diode-pumped neodymium laser”, IEEE, J. Q. E., Vol. 34, 1480-1484, 1998; J. J. Zayhowski, “Diode-pumped passively Q-switched picosecond microchip lasers”, Opt. Lett., Vol. 19, 1427-1429, 1994; J. J. Zaykowski, “UV generation with passively Q-switched microchip laser”, Opt. Lett., Vol. 21, 588-590, 1996). 
     To reduce the pulse width with the same material combination, one must combine the active medium and the saturable absorber in a short distance to reduce the cavity length to about 1 mm typically. A microchip laser combines the two materials in a monolithic crystal (J. J. Zaykowski, “Non linear frequency conversion with passively Q-switched microchip lasers”, CLEO 96, paper CWA6, 23 6-237, 1996); the energy is then smaller, e.g., 8 μJ at 1064 nm. The two materials, i.e., the laser material and the saturable absorber, can be contacted by a thermal bonding, or the saturable absorber can be grown by liquid phase epitaxy (LPE) directly on the laser material (B. Ferrand, B. Chambaz, M. Couchaud, “Liquid Phase Epitaxy: a versatile technique for the development of miniature optical components in single crystal dielectric media”, Optical Materials 11, 101, 1998). At the same time, in order to obtain sub-nanosecond pulses, the saturable absorber must be highly doped and therefore the repetition rate is lower (e.g., 6-8 kHz with Nd:YAG). The wavelengthconversion efficiency from infrared (IR) to UV is in the order of 4 %. A solution to simultaneously obtain short pulses and a high repetition rate is to combine a Nd:YVO 4  crystal, whose short fluorescence lifetime of Nd:YVO4 is well suited for a higher repetition rate, with a semiconductor-based saturable absorber in an anti-resonant Fabry-Perot structure (B. Braun, F. X. Kdarner, G. Zhang, M. Moser, U. Keller, “56 PS passively Q-switched diode-pumped microchip laser”, Opt. Lett., 22, 381-383, 1997). This structure is nevertheless complex to produce. 
     It is therefore difficult to simultaneously produce sub-nanosecond short pulses, at frequencies of a few tens of kHz, with several micro-Joule per pulse in a simple and compact system. The solution consists in combining a compact oscillator producing short pulses at high frequency with an amplifier to increase the pulse energy. Amplifiers have been used in the past with pulsed microlasers. After amplification, pulses with 87 nJ (small-signal gain of 3.5) at 100 kHz have been produced using a 10-W diode bar as a pump (C. Larat, M. Schwarz, J. P. Pocholle, G. Feugnet, M. Papuchon, “High repetition rate solid-state laser for space communication”, SPIE, Vol. 2381, 256-263). A small-signal gain of 16 has been obtained with an 88-pass complex structure using two 20-W diode bars as a pump (J. J. Degnan, “Optimal design of passively Q-switched microlaser transmitters for satellite laser ranging”, Tenth International Workshop on Laser Ranging Instrumentation, Shanghai, China, Nov. 11-15, 1996). In these two examples, the amplification efficiency that can be defined as the ratio between the small-signal gain and the pump power is small because the transverse pumping has a low efficiency due to the poor overlap of the gain areas with the injected beam. Furthermore, these setups use Nd:YAG crystals not suited for high-frequency pulses (the fluorescence lifetime is 230 μs). 
     A combination of Nd ions in two different hosts, in a oscillator-amplifier system, has been performed in the past in continuous wave (cw) (H. Plaesmann, S. A. Ré, J. J. Alonis, D. L. Vecht, W. M. Grossmann, “Multipass diode-pumped solid-state optical amplifier”, Opt. Lett., 18, 1420-1422, 1993) or pulsed mode (C. Larat, M. Schwarz, J. P. Pocholle; G. Feugnet, M. Papuchon, “High repetition rate solid-state laser for space communication”, SPIE, Vol. 2381, 256-263). In these cases, the spectral distance between the emission lines of the two different material Nd:YAG and Nd:YVO 4  limits the small-signal gain to a value tower than the obtained when only Nd:YVO 4  is used in both the oscillator and the amplifier; it lies between from 5.5 cm −1  and 7.0 cm −1  (J. F. Bernard, E. Mc Cullough, A. J. Alcock, “High gain, diode-pumped Nd:YVO 4  slab amplifier”, Opt. Commun., Vol. 109, 109-114, 1994). 
     A number of amplification schemes using Nd ions in crystals have been studied, but often end up with complex multipass setups and with low efficiency due to transverse pumping. 
     End-pumped single-pass or double-pass amplification schemes based on guiding structures to increase the interaction length between the pump beam and the injected beam have been studied in the past: in planar guides (D. P. Shepherd, C. T. A. Brown, T. J. Warburton, D. C. Hanna and A. C. Tropper, “A diode-pumped, high gain, planar waveguide Nd:Y 3 Al 5 O 12  amplifier”, Appl. Phys. Left., 71, 876-878, 1997) or in double-cladding fibers (E. Rockat, K. Haroud, R. Dandliker, “High power Nd-doped fiber amplifier for coherent intersatellite links”, IEEE, JQE, 35, 1419-1423, 1999; I. Zawischa, K. Plaman, C. Fallnich, H. Welling, H. Zellner, A. Tunnermann, “All solid-state neodymium band single frequency master oscillator fiber power amplifier system emitting 5.5 W of radiation at 1064 nm”, Opt. Lett., 24, p. 469-471, 1999). These schemes are, however, not suited for high-peak-power pulses because unwanted nonlinear effects, such as the Raman effect, start to appear around 1 kW of peak power. 
     A high small-signal gain of 240 was achieved in an end-pumped double-pass bulk Nd:YLF amplifier, but it was used with a cw laser with an expensive diode-beam shaping optical setup (G. J. Friel, W. A. Clarkson, D. C. Hanna, “High gain Nd:YLF amplifier end-pumped by a beam shaped bread-stripe diode laser”, CLEO 96, paper CTUL 28, p. 144, 1996). 
     SUMMARY OF THE INVENTION 
     The object of this invention is to provide an entirely passive laser system both for the generation and amplification of short pulses. The oscillator shall directly produce μJ pulses at the required repetition rate and shall be amplified in a few passes only in a non-synchronized amplifier. 
     The uniqueness of our approach is to combine an optically pumped, passively Q-switched, high frequency, Nd:YAG microchip laser producing short pulses with an optically end-pumped Nd:YVO 4  amplifier producing high small-signal gain while pumped at low power. The use of the two materials in our system allows nevertheless to best use of their respective properties: 
     Nd:YAG/Cr 4+ :YAG microchip lasers are simpler and easier to manufacture than Nd:YVO 4  microchips because they use the same crystal (YAG) for the laser medium and the saturable absorber and can be produced in a collective fashion. In addition they produce shorter pulses except in the case of the semiconductor saturable absorber described in B. Bräun, F. X. Kartner, G. Zhang, M. Moser, U. Keller, “56 ps passively Q-switched diode-pumped microchip laser”, Opt. Lett., 22, 381-383, 1997. 
     Nd:YVO 4  is on the other hand well suited for amplification due to its high stimulated emission cross section. It is also better suited than Nd:YAG for higher repetition rate due to a shorter fluorescence lifetime (100 μs instead of 230 μs). 
     The laser system for emitting pulsed electromagnetic laser radiation according to the invention comprises: 
     a microchip laser for emitting pulsed laser radiation, said microchip laser comprising 
     two reflective elements defining an optical resonator for laser radiation, a laser gain medium placed inside said resonator and a saturable absorber medium placed inside said resonator for passively Q-switching said laser radiation, said reflective elements, said gain medium and said saturable absorber medium being rigidly and irreversibly bonded such as to form a monolithic body, and 
     a first pumping source for emitting first pumping radiation which impinges on said monolithic body and excites said gain medium to emit laser radiation; and 
     an optical amplifier stage for amplifying electromagnetic radiation, said amplifier stage comprising 
     an amplifying medium and 
     a second pumping source for emitting second pumping radiation which impinges on said amplifying medium and excites it to amplify electromagnetic radiation; 
     said microchip laser and said optical amplifier being mutually arranged such that pulsed laser radiation emitted by said microchip laser is amplified by said optical amplifier. 
     The method for generating pulsed electromagnetic radiation according to the invention comprises the steps of: 
     generating first pumping radiation; 
     impinging said first pumping radiation on a monolithic body comprising two reflective elements defining an optical resonator, a laser gain medium placed inside said resonator and a saturable absorber medium placed inside said resonator; 
     exciting said laser gain medium to generate laser radiation, by impinging said first pumping radiation on said laser gain medium; 
     recirculating said laser radiation in an optical resonator; 
     passively Q-switching said laser radiation by passing it through said saturable absorber medium; 
     partially outcoupling said pulsed laser radiation out of said optical resonator; 
     generating second pumping radiation; 
     exciting an amplifying medium to amplify electromagnetic radiation, by impinging said second pumping radiation on said amplifying medium; and 
     amplifying said outcoupled pulsed laser radiation by passing it through said amplifying medium. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a first embodiment of the laser system according to the invention; 
     FIG. 2 is a schematic illustration of a microchip laser cavity used in a preferred embodiment of the laser system according to the invention; 
     FIG. 3 is a graphical representation of the pulse repetition rate versus the pump brightness; 
     FIG. 4 is a schematic illustration of an embodiment of the laser system according to the invention pumped by fiber-coupled diodes; 
     FIG. 5 is a schematic illustration of an embodiment of the laser system according to the invention for generating UV radiation; and 
     FIG. 6 is a schematic illustration of an embodiment of the laser system according to the invention with a wavelength-tunable output. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of the laser system according to the invention is schematically shown in FIG.  1 . The laser system comprises two main subsystems: 
     a microchip laser  1  (cf. J. J. Zayhowski, “Diode-pumped passively Q-switched picosecond microchip lasers”, Opt. Lett. 19, 1427-1429, 1994; L. Fulbert, J. Marty, B. Ferrand, E. Molva, “Passively Q-Switched monolithic microchip laser”, Proc. of Conference on Laser and Electro Optics 1995, Optical Society of America, paper CWC5, Washington D.C., 1995) optimized for a high repetition rate; and 
     an amplifier stage  2  (cf. F. Druon, F. Balembois, P. Georges and A. Brun, “High repetition rate 300 ps pulses ultraviolet source passively Q:switched microchip laser and a multipass amplifier”, Opt. Lett. 24, 499-501, 1999). 
     The microchip laser  1  is described in greater detail with reference to FIG.  2 . It essentially comprises a laser gain medium  111 , e.g., Nd:YAG, a saturable absorber medium  112 , e.g., Cr 4+ :YAG, and two reflective elements  113 ,  114  which define an optical resonator (the laser cavity) of length L, e.g., L=1.5 mm or less. The laser gain medium  111  and the saturable absorber medium  112  are either bonded together by diffusion bonding, or the saturable absorber is deposited on the laser medium by liquid phase epitaxy; the two media  111 ,  112  thus form one single solid body with a first and a second end face  115 ,  116 . The reflective elements  113 ,  114  are preferably multiple stacks of dielectric and/or semiconductor layers as known in the art, grown on the end faces  115 ,  116  of the solid body and optimized to have a desired reflectivity at a given wavelength. Preferably, the first end face  115  is coated with a highly reflective mirror and the second end face  116  is coated with a mirror reflectivity between 75% and 95% at the laser wavelength of 1064 nm. All elements  111 - 114  form together a small, monolithic body or “microchip”  11 . 
     The microchip laser  1  is optically pumped by first pumping radiation  13  emitted by a first pumping source  12  which is preferably a high-brightness diode, i.e., a diode with maximum power within a solid angle which is as small as possible. The first pumping radiation  13  (with a pumping wavelength, of, e.g., 808 nm) for optically exciting the gain medium  111  is focused by coupling optics  14  on the gain medium  111  and enters into the microchip  11  via the first end face  115 . Generated laser light  10  (with a laser wavelength of, e.g., 1064 nm) is coupled out of the microchip  11  via the second end face  116 . 
     In order to produce short pulses at high repetition rates, the cavity length is preferably reduced to L≦1.5 mm, producing a single-frequency, Gaussian-mode output beam  10 . The repetition rate is further increased by optimizing further parameters such as the concentration of the saturable absorber in the saturable-absorber medium  112 , the pumping-power density and/or the brightness of the first pumping source  12 . 
     The first pumping diode  12  and the coupling optics  14  must be carefully selected. The dependency of the pulse repetition rate (or frequency) PRF on the brightness of the first pumping-diode  12  is shown in FIG.  3 . The horizontal axis represents the ratio of the pumping power P p  to the M 2  figure of merit (cf. T. F. Johnston, Jr., “M 2  concept characterizes beam quality”, Laser Focus World, May 1990) of the pumping beam  13 , multiplied by a coefficient ζ which depends on the pumping-beam geometry (e.g., ζ=1 for a perfect circular beam and ζ={square root over (3)} for a beam emitted by a diode junction). The experimental results are plotted and compared to a calculation. Based on these results a high-brightness diode with a “selfoc” (grin lens) coupling arrangement to correct for some aberrations is used as the first pumping-light source  12 . In a first embodiment a 2-Watt high-brightness diode  12  with an emitting area of 100 μm×1 μm is used, in a second embodiment a 1-Watt diode  12  with an emitting areaof 50 μm×1 μm is used. Further scaling of the pumping power while maintaining brightness is possible. With a 2-Watt 100 gμm×1 μm diode  12  the microchip laser  1  emits, for instance, about 1 μJ at a repetition rate of 45 kHz. Turning again to FIG. 1, the amplifier stage  2  is described in the following. The amplifier  2  stage essentially comprises an amplifying medium  21  pumped by second pumping radiation  23  emitted by a second pumping source  22 . The amplifying medium  21  is preferably a Nd:YVO 4  crystal chosen for its large stimulated-emission cross section, its high absorption at 808 nm and its fluorescence lifetime well suited for repetition rates in the order of tens of kHz. 
     The pumping of the amplifying medium  22  is optimized to insure a good overlap of the second pumping  23  beam and the microchip-laser beam  10 . The selected scheme is an end-pump configuration, i.e., the propagation direction of the second pumping light  23  is essentially parallel to the optical axis of the amplifier  2 , and thus also essentially parallel to the propagation direction of the laser beam  10 . The pumping power has to be sufficiently high to allow enough energy storage in the amplifier  2  but not too high to avoid thermal effects. A thermal lens created by the focusing of the second pumping beam  23  would degrade the performance. Such thermal effects appear beyond a pumping power of about 2 W; this is the power used in the preferred embodiment of the laser system according to the invention. 
     The coupling of the second pumping beam  23  into the amplifying medium  21  is very important, because its quality defines the quality of an output beam  20  of the laser system. Best results are obtained using a coupling optical system  24  consisting of a combination of an aspherical lens, anamorphic prisms and an objective, as shown in FIG.  5 . 
     A back face  27  of the amplifier crystal  21  is preferably coated with a reflective coating  26 . The microchip-laser beam  10  is reflected from the coating  26  and thus passes twice through the amplifier crystal  21 . The performance relies on two optimizations: 
     Parallelism between the polarization of the microchip-laser beam  10  and the direction in which the stimulated-emission cross section of the amplifier crystal  21  is the largest. To insure this, a half-wave plate is inserted into the microchip beam  10  before it enters the amplifier stage  2  and is used to tune the polarization to the proper direction. Alternatively, the microchip with a given polarization could be rotated to position its polarization parallel to the one from the amplifier. 
     Parallelism between the polarization of the second pumping beam  23  and the direction of the largest absorption in the amplifier crystal  21 . 
     The output beam  10  of the microchip laser  1  is collimated by a collimating optical system  31 , and is focused into the amplifier crystal by a focusing optical system  32 . The same focusing optical system  32  can be used to collimate the output beam  20  of the amplifier  2 . The focused laser beam  10  enters and leaves the amplifier crystal  21  at a very small angle of incidence (e.g., of about 4°). Of course, other, preferably multi-pass, amplifier types can be used for the laser system according to the invention, e.g., a four-pass amplifier as disclosed in U.S. Pat. No. 5,268,787 (McIntyre). 
     With the preferred embodiment shown in FIG. 1, an output power of 450 mW at a repetition rate of 45 kHz (10 μJ per pulse) is achieved with a microchip-laser output of 45 mW and 45 kHz. With a higher microchip-laser power of 150 mW, an output power of 800 mW is achieved with the same excellent spatial and spectral beam quality, i.e., a Gaussian and single-longitudinal-mode beam  20 . 
     In another embodiment, replacing the Nd:YVO 4  amplifier crystal by a Nd:YAG crystal  21  allows to pump the amplifier stage  2  harder, beyond 2 W, and increase the output power of the laser system according to the invention both in average power and energy per pulse at a lower repetition rate. 
     In a further embodiment, a different set of materials is used to generate an output beam in the so-called eye-safe spectral range of 1.54 μm (Ph. Thony, B. Ferrand, E. Molva, “1.55 μm passive Q-Switched microchip laser”, Advanced Solid State Laser, AWC 3-1, 327, 1998 ). This is achieved by using a passively Q-switched microchip laser  1  with a combination of an Er:glass or Yb:glass laser gain medium  111  with an LMA:Co 2+  (LaMgAl 11 O 19 :Co 2+ ) saturable absorber  112  (Er:Glass/LMA:Co 2+ , Yb:Glass/LMA:Co 2+ ) and an Er:glass or Yb:glass amplifier crystal  21 , both pumped by high-brightness diodes  12 ,  22  emitting at 980 nm. 
     In a still further embodiment, a different set of materials is used to produce an output beam at 1030 nm. This is achieved by using a passively Q-switched microchip laser  1  with a combination of an Yb:YAG laser gain medium  111  with a Cr 4+ :YAG saturable absorber  112  (Yb:YAG/Cr 4+ :YAG) and an Yb:YAG amplifier crystal  21 , both pumped by high-brightness diodes  12 ,  22  emitting in the range of 940-980 nm. Other possible laser-gain materials are Nd:YVO 4  or Nd:YLF. 
     FIG. 4 shows an embodiment of the laser system according to the invention where the first pumping diode  12  and the second pumping diode  22  are coupled by optical fibers  15 ,  25  to the monolithic body  11  and the amplifier crystal  21 , respectively. This embodiment has three advantages. Firstly, it removes the diode heat source from the laser head and puts it in a remote location, e.g., inside a power supply (not shown). Secondly, it reduces the size of the laser-head package, since no active element is located in the laser head. Thirdly, the beam emitted by a fibered diode is circular; it is easier and more simple to image the tip of the fiber into the laser medium than a diode junction. However, this embodiment has the disadvantage of being more expensive and potentially less stable. 
     With the high peak power and high repetition rate available at 1064 nm, wavelength conversion can be readily achieved in a very compact laser system according to the invention. Frequency doubling at 532 nm, tripling at 355 nm, quadrupling at 266 nm, and fifth harmonic at 213 nm can be performed. FIG. 5 shows an example in which in a laser beam  10  generated by a Nd:YAG microchip laser  1  is amplified by a Nd:YVO 4  crystal  21 . Thereafter, the second and third harmonic are generated by a set of optically nonlinear crystals  41 ,  42  (such as for instance KTP, BBO or LBO) in a single-pass configuration. In this example, the output beam  40  of the system lies in the UV spectral range. 
     In order to make the laser system more compact, the microchip-laser output beam  10  is folded by a high-reflectivity laser mirror  33  and a reflective prism  34  at 45° each. The coupling optics  24  for the second pumping beam  23  is more complex in this example than in FIGS. 1 and 4. The coupling optical system  24  consists of a combination of an aspherical lens  241 , anamorphic prisms  242 ,  243  and an objective  244 . In a preferred embodiment these components are an aspherical lens  241  with a focal length of f=8 mm, two anamorphic prisms  242 ,  243  with a magnification by a factor  4  and an objective  244  with a focal length of f=8 mm. In the same embodiment a half-wave plate (not shown) can be insertedbetween two optical components, e.g., between the aspherical lens and the anamorphic prisms, to reduce losses due to reflection on the anamorphic-prism faces. 
     In a further embodiment, by appropriately selecting different coatings for the mirrors  113 ,  114 ,  26  on both the microchip  11  and the amplifier crystal  21 , the 946-nm line of the Nd:YAG laser gain material  111  is favored. The output wavelength of the laser system is then changed from 1064 nm to 946 nm, which is a suitable wavelength to generate high power of blue light at 473 nm by frequency doubling. Frequency tripling and quadrupling techniques allow then to generate additional wavelengths at 315 nm and 236 nm 
     It is also possible to provide the laser system according to the invention with an optical parametric oscillator (OPO) cavity  5  (cf. J. E. Bjorkholm, “Efficient optical parametric oscillation using doubly and singly resonant cavities”, Appl. Phys. Lett., Vol. 13, No. 2, 1968), as shown in FIG.  6 . This embodiment has a tunable output, and finally a continuum of light can be generated by focusing the output beam  50  of the laser system into a fiber (not shown). 
     Numerous other embodiments may be envisaged, without departing from the spirit and scope of the invention.