Mid-IR Kerr lens mode locked laser with normal incidence mounting of polycrystalline TM:II-VI materials and method for controlling parameters of polycrystalline TM:II-VI Kerr lens mode locked laser

Abstract

A Kerr Mode Locked (KLM) laser is configured with a resonant cavity. The gain medium, selected from polycrystalline transition metal doped II-VI materials (TM:II-VI), is cut at a normal angle of incidence and mounted in the resonant cavity so as to induce the KLM laser to emit a pulsed laser beam at a fundamental wavelength. The pulses of the emitted laser beam at the fundamental wavelength each vary within a 1.8-8 micron (m) wavelength range, have a pulse duration equal to or longer than 30-35 femtosecond (fs) time range and an average output power within a mW to about 20 watts (W) power range. The disclosed resonant cavity is configured with a plurality of spaced apart reflectors, two of which flank and are spaced from the gain medium which is pumped to output a laser beam at a fundamental wavelength and its higher harmonic wavelengths. The gain medium is mounted on a translation mechanism operative to controllably displace the gain medium along a waist of the laser beam. The displacement of the gain medium causes redistribution of a laser power between a primary output at the fundamental wavelength and at least one secondary output at the higher harmonic wavelength.

Claims

1. A Kerr Lens Mode Locked (KLM) laser comprising: a resonant cavity; and a gain medium selected from polycrystalline transition metal doped II-VI materials (TM:II-VI), the gain medium being cut at a normal angle of incidence and mounted in the resonant cavity so as to induce Kerr-lens mode locking sufficient for the resonant cavity to emit a train of ultrashort pulses of a laser beam at a fundamental wavelength, wherein ultrashort pulses of the emitted laser beam at the fundamental wavelength, ranging from 1.8 m to 8 m, each have a pulse duration equal to at least 30 femtosecond (fs) and an average output power of at most 20 watts (W), the gain medium being configured with a plurality of microscopic single-crystal grains which are non-uniformly dimensioned and have, differently oriented crystallographic axes.

2. The KLM laser of claim 1, wherein the gain medium generates second, third and fourth harmonic wavelengths of the fundamental wavelength.

3. The KLM laser of claim 1, wherein the resonant cavity is planar.

4. The KLM laser of claim 1, wherein the gain medium includes TM doped binary and ternary materials.

5. The KLM laser of claim 4, wherein the materials include Cr2+:ZnSe, Cr2+:ZnS, Cr2+:CdSe, Cr2+:CdS, Cr2+:ZnTe, Cr2+:CdMnTe, Cr2+:CdZnTe, Cr2+:ZnSSe, Fe2+:ZnSe, Fe2+:ZnS, Fe2+:CdSe, Fe2+:CdS, Fe2+:ZnTe, Fe2+:CdMnTe, Fe2+:CdZnTe, Fe2+:ZnSSe.

6. The KLM laser of claim 1 further comprising a linearly polarized fiber laser pump source selected from an erbium or thulium doped single mode fiber and operative to emit a pump beam which is coupled into the gain medium at a pump wavelength different from the fundamental wavelength, wherein the laser and pump beams remain circular while propagating through the gain medium.

7. The KLM laser of claim 6, wherein the gain medium is configured to uniformly release heat in response to the coupled pump beam which generates various uniform, axially symmetric thermal-optical effects inside the pumped gain medium.

8. The KLM laser of claim 1, wherein the optical intensity inside the gain medium is increased by a factor of n if compared with a conventional Brewster mounting scheme.

9. The KLM laser of claim 1, wherein the gain medium is configured to substantially compensate for astigmatism of the resonant cavity.

10. The KLM laser of claim 6, wherein the resonant cavity includes at least two adjacent upstream and downstream dielectrically coated folded mirrors spaced apart along a path of the pump beam and each configured with a high reflectivity at the fundamental wavelength and high transmission at the pump wavelength, the gain medium being located between and spaced from the folded mirrors, the downstream folded mirror being configured to at least partially transmit the high harmonic wavelength.

11. The KLM laser of claim 10, wherein the resonant cavity further includes a partially transmitting at the fundamental wavelength output coupler, and at least one plane dichroic mirror upstream from the output coupler and configured with the high reflectivity at the fundamental wavelength, and at least one intermediary plate with high transmission at the fundamental and high harmonic wavelength.

12. The KLM laser of claim 10, wherein the resonant cavity further includes a dispersion compensation element configured as a plane parallel plate or prism and operative to limit a dispersion, which is mounted at a Brewster angle.

13. The KLM laser of claim 10, wherein the resonant cavity further includes a Brewster mounted birefringent tuner.

14. The KLM laser of claim 10 further comprising a translation stage displacing the gain medium within the resonant cavity along a waist of the laser beam to controllably redistribute the average power of the laser beam among a primary output of the emitted laser beam at the fundamental wavelength and secondary outputs at respective second, third and fourth harmonic wavelengths.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other aspects, features and advantages of the disclosure will become more readily apparent from the following drawings, in which:

(2) FIG. 1 is one exemplary schematic of the known prior art KLM laser;

(3) FIG. 2 is emission spectra and autocorrelation traces for the KLM laser of FIG. 1.

(4) FIG. 3 is another exemplary schematic of the known art KLM laser;

(5) FIG. 3A is the enlarged detail of FIG. 3 illustrating beam propagation in a Brewster mounted gain medium;

(6) FIG. 4 is an optical schematic of one design of the inventive KLM resonator;

(7) FIG. 4A is the enlarged gain medium of FIG. 2;

(8) FIG. 4B is one of possible schematics of the disclosed KLM laser of FIG. 4;

(9) FIG. 5 is a measured laser emission spectrum of the disclosed KLM laser fitted with a theoretical curve for transform limited laser emission spectrum.

(10) FIG. 5A is an autocorrelation trace;

(11) FIG. 5B is the image of the output laser beam emitted by the disclosed KLM laser;

(12) FIG. 6 is an optical schematic of the optimized inventive KLM laser;

(13) FIG. 7 illustrates measured emission spectra of the disclosed KLM laser of FIG. 6 configured with the laser's output coupler having respective different reflectivity;

(14) FIG. 8 illustrates autocorrelation traces of the KLM laser of FIG. 6 corresponding to respective emission spectra of FIG. 7;

(15) FIG. 9 is an enlarged view of the polycrystalline transition metal (TM) TM:II-IV gain medium of the disclosed KLM laser;

(16) FIG. 10 is still another optical schematic of the inventive KLM laser;

(17) FIG. 11A are four images of the laser's output beam at respective fundamental, second, third and fourth harmonic wavelengths;

(18) FIG. 11B are spatial profiles of the output of the inventive KLM laser at a fundamental wavelength and second harmonic acquired by a pyrocamera;

(19) FIG. 11C is a waveform of the KLM laser pulse train acquired at the second harmonic wavelength.

(20) FIG. 12 is a further optical schematic of the inventive KLM laser provided with a means for controlling laser's parameters;

(21) FIG. 12A is the enlarged view of the gain medium of FIG. 12;

(22) FIGS. 13, 13A and 13B illustrates the controllable generation of 68 fs and 84 fs pulses by the inventive KLM laser; and

(23) FIG. 14 illustrates the controllable generation of 46 fs pulse by the inventive KLM laser.

SPECIFIC DESCRIPTION

(24) Reference will now be made in detail to embodiments of the invention. Wherever possible, same or similar numerals are used in the drawings and the description to refer to the same or like parts or steps common to the prior art and inventive configurations. The drawings are in simplified form and are not to precise scale. Unless specifically noted, it is intended that the words and phrases in the specification and claims be given the ordinary and accustomed meaning to those of ordinary skill in the diode and liber laser arts. The word couple and similar terms do not necessarily denote direct and immediate connections, but also include mechanical and optical connections through free space or intermediate elements.

(25) FIG. 4 illustrates the exemplary configuration of the disclosed KLM laser 50 having gain medium 4 mounted at normal incidence to the plane of beam propagation in the shown resonator. The remaining components are similar to those of FIG. 3 and include optical pump source 1 seeded by source 10 and emitting a pump beam (shown in green). The pump source 1 may be configured as a standard linearly polarized single transverse mode (SM) Er-doped fiber laser (EDFL). Alternatively, source 10 may be based on a thulium (Tm)-doped SM fiber laser.

(26) The normal incidence mounting of gain medium 4 is critical to achieving higher output laser powers and efficiency. In particular, polycrystalline antireflection coated gain element 4 is coupled between folded concave dielectric-coated mirrors 3 and 5. While gain element 4 is shown to be plane parallel, it can also be wedged. The mirror 3 is highly reflective at the laser wavelength and highly transmissive at the pump wavelength, whereas mirror 5 is configured with high reflectivity at the laser wavelength and optional high transmission at the pump wavelength. The resonator may have a variety of configurations. For example, FIG. 4B illustrates the resonator with more than two folding mirrors as shown. Regardless of the resonator configuration, eventually the laser beam impinges on output coupler (OC) 8 and is coupled out of the resonator as output beam 9 at the laser wavelength.

(27) It should be mentioned that the dispersion compensation can be implemented using specially optimized dispersive mirrors. A highly reflective mirror may be a viable alternative to output coupler 8. The resonator may include additional components for the laser wavelength tuning.

(28) Unlike the conventional resonator of the KLM laser of FIGS. 1 and 3, the inventive planar resonator has astigmatism. However, in a particular situation of TM:II-VI laser materials, the high uniformity of the pump and laser beams inside the gain element, as shown in FIG. 4B, allow the gain element to substantially compensate for astigmatism of the resonant cavity. The principle experiment, which is described below, confirms that statement. Furthermore, the astigmatism of the disclosed planar resonator can be kept even lower by careful choice of the radii of curvature of the folded concave mirrors and of the folding angles. If necessary, there are also a number of techniques for astigmatism compensation in a folded resonator without a Brewster optical element well known to one of ordinary skill in the laser arts, but all of them are simply optional and not necessary as in the known prior art devices.

(29) Accordingly, a well-pronounced nonlinear Kerr effect in TM:II-VI medium may allow for a significant compensation of the astigmatism of the resonator. Thus, the use of TM:II-VI gain elements at normal incidence allows for somewhat relaxed requirements to the compensation of astigmatism in the resonator of KLM laser 50.

(30) Main advantages of the normal incidence mounting of the gain element are (i) better management of the thermal optical effects in the gain element due to circularity of the pump and laser beams, (ii) significant increase of pump and laser intensity inside the gain element (if compared with the standard Brewster mounting), (iii) greater simplicity of use of the gain elements with large length and volume and hence high pump absorption. The normal incidence mounting could be also more favorable for Kerr-lens mode-locking.

(31) FIGS. 5-5B illustrate the results of KLM laser planar resonator 50 of FIG. 4 configured with standard mass-produced AR coated polycrystalline Cr:ZnS gain element 4. The KLM regime of the laser has been easily obtained. The KLM regime of the laser was confirmed by measurement of the laser emission spectrum of FIG. 5 and of the nonlinear autocorrelation function of FIG. 5A, as discussed herein below. The Fourier transform limited pulses with 84 fs pulse duration were confirmed by mathematical analysis of the experimental data, also discussed herein below. The transform-limited pulse is a theoretically shortest possible pulse determined as t{acute over ()}=, where t is a pulse duration and t{acute over ()}spectral width. As can be seen from the above, in order to generate the shortest possible light pulsed with a specific duration, a broad spectral bandwidth is required. As can be seen, the tested KLM TM:II-VI laser 50 was operative to output beam 9 at the fundamental/laser wavelength with an output power of about 1.3 W at 93 MHz repletion rate and pulse energy 14 nJ and featured transformed limited pulses and good beam quality, as shown in FIG. 5B. The obtained output power is believed to be uniquely high.

(32) FIG. 6 illustrates inventive KLM laser 50 based on Cr.sup.2+:ZnS gain element 4 and having the optimized resonator. HRdispersive high reflectors (GDD200 fs.sup.2), YAG2 mm thick Brewster mounted dispersion compensation plate, OCoutput coupler (|GDD|<150 fs.sup.2), MgF.sub.2optional 0.5 mm thick Brewster mounted birefringent tuner (Lyot filter), Lpump focusing lens. SHGsecondary outputs of the laser at second harmonic wavelength. The laser is pumped at 1567 nm by a linearly polarized radiation of Er-doped fiber laser (EDFL).

(33) The pump source 1 includes a standard linearly polarized Er-doped fiber laser (EDFL). In order to increase the laser output power, a 5 mm long polycrystalline Cr.sup.2+:ZnS gain element 4 with 11% low-signal transmission at 1567 nm pump wavelength is inserted between AR coated folded mirrors 3 and 5 at normal incidence on a water cooled copper heat sink. The lengths of the cavity legs were unequal with 3 to 5 ratio. Overall dispersion of the resonator at the maximum of laser emission (2300-2400 nm) was about 1400-1600 fs.sup.2. For the experiments on wavelength tuning of the KLM laser a 0.5 mm thick Brewster-mounted birefringent tuner 11 (single-plate Lyot filter) made of MgF.sub.2 was used. Additionally, a 2 mm thick Brewster mounted YAG plate 12 was placed next to tuner 11 in the leg defined between mirrors 6 and 13. The planar resonator further includes an additional leg defined between plain mirrors 15 and 14 immediately upstream from OC 8. Dispersion of the OCs is within 150 fs.sup.2 in 2200-2400 nm range. The outputs 16 at SHG are shown in blue, pump beam is in green, and laser beam is shown in red.

(34) The KLM regime of the laser with the optimized planar resonator has been obtained using the output couplers with 96, 90, 70, and 50% reflectivity. Most measurements were carried out at the pulse repetition rate of 94.5 MHz. However, KLM laser oscillations were obtained in a range of the pulse repetition rates (80-120 MHz). Results of the laser characterization are summarized in the following table.

(35) TABLE-US-00001 TABLE R.sub.OC, P.sub.out, , fs , .sub.C, P.sub.pump, 96 0.3 85 70 2380 3.4 90 0.6 46 120 2300 5.2 70 1.2 68 84 2332 6.7 50 2.0 67 82 2295 10.0 R.sub.OC - reflectivity of the output coupler, P.sub.out - average output laser power in KLM regime, - laser pulse duration (FWHM), - width of the laser emission spectrum (FWHM), .sub.C - laser emission peak, P.sub.pump - optimal pump power

(36) Emission spectra and autocorrelation traces of KLM laser 50 of FIG. 6 obtained for the OCs with different reflectivity are illustrated in FIGS. 7 and 8, respectively. The shape of the spectra and of the autocorrelation functions correspond to sech.sup.2 pulses for all four types of the OC. This allows the use of the time bandwidth product of 0.315 for estimation of the pulse duration. The small peak in the spectrum measured at R.sub.OC=90% (2525 nm) indicates a presence of Kelly sidebands in the laser emission. The opposite sideband is suppressed due to a leakage through high reflectors at the wavelengths below 2200 nm. The flat-top spectra measured at R.sub.OC=90 and 70% for the case when a second harmonic generation (SHG) was directly obtained in polycrystalline Cr.sup.2+:ZnS are explained below.

(37) Summarizing the above disclosed configurations of KLM laser 50 based on normally mounted polycrystalline II-VI materials and particularly polycrystalline Cr.sup.2+:ZnS laser, stable single-pulse fs laser oscillations are routinely obtained in a range of the pulse repetition rates 80-120 MHz with output power of fs laser of about 2 W, and shortest pulse duration about 46 fs. All of the above data is believed unprecedented for the II-VI gain medium. Furthermore, at several occasions, KLM laser 50 was operative to generate even more unique data with the output power of up to 20 W and the pulse duration as low as 30-35 picoseconds.

(38) The practical applications of femtosecond lasers often require the nonlinear frequency conversion (e.g. optical harmonic generation, sum and difference frequency generation, and optical parametric generation. For instance, the 1.1-1.5 m spectral range, which is of importance for multi-photon imaging, can be addressed using Ti:S fs laser combined with the optical parametric generator. The same spectral range can be addressed using SHG of TM:II-VI mid-IR fs laser operating in 2.2-3.0 m spectral range.

(39) Efficiency of SHG in nonlinear materials is limited by dispersion (a difference in velocity of light propagation at a fundamental laser wavelength and half the fundamental (SH) wavelengths. Therefore, the energy transfer from the fundamental wavelength to the SH wavelength occurs at a limited length of the nonlinear material, so called coherence length (CL). In most materials CL is of the order of few tens of m resulting in weak SH generation efficiency. A number of techniques to overcome this limit have been developed during past decades. Traditional techniques are based on birefringence of some nonlinear crystals. More recent developments are based on engineering of the microscopic structure of the nonlinear material (quasi phase matching or QPM). Standard QPM crystals contain regular patterns, optimized for the most efficient nonlinear frequency conversion at the desired laser wavelength, e.g. they have limited bandwidth of the nonlinear frequency conversion. More sophisticated patterning allows for an increase of the bandwidth, which is accompanied by a decrease of the overall conversion efficiency.

(40) Polycrystalline TM:II-VI materials used here consist of microscopic single-crystal grains. The polycrystalline TM:II-VI samples used in the experiments have a grain size of the order of the coherence length of SHG process in middle IR wavelength range (3-6 m, depending on the wavelength and type of the material). Thus polycrystalline TM:II-VI materials can be patterned like standard QPM material. Unlike in the standard QPM material, the patterning is not perfect but randomized (there are dissimilarities in the grain size and in orientation of the crystallographic axes). This randomization of the patterning results in low nonlinear gain (if compared with standard QPM material). However the randomization allows for SHG in very broad spectral range. Thus, polycrystalline TM:II-VI materials have very large bandwidth of the nonlinear frequency conversion. Efficiency of the nonlinear frequency conversion strongly depends from the optical intensity (for instance, SHG efficiency is proportional to squared optical intensity). Therefore, relatively low nonlinear gain of polycrystalline TM:II-VI material can be compensated by a very high intensity of fs laser pulses. Described properties of polycrystalline TM:II-VI materials are of importance for nonlinear frequency conversion of fs laser emission.

(41) Referring specifically to FIGS. 9 and 10, KLM laser 50 is configured with polycrystalline TM:II-VI gain medium 4. The disclosed configuration of FIG. 10 is operative to simultaneously output the beams at respective second, third and fourth harmonic generation wavelengths shown respectively in yellow (SHG), green (THG) and blue (FHG) in FIG. 9, as well as sum- and difference-frequency wavelengths both shown in black (SFG and DFG).

(42) The laser output at a fundamental wavelength is implemented via partially transmitting OC 8. The laser outputs at respective second, third, and forth harmonics leave the resonator via mirror 20 after reflection at mirror 18. It is important to point out that all of the resonator's mirrors do not have to be specially designed to generate a high harmonic output. In the tested device, the mirrors transmission in SHG wavelengths range is about 50% and oscillates as a function of the wavelength. FIG. 11A illustrates snapshot of typical images of the outputs of 2 W KLM laser 50 at respective fundamental (A), second (B), third (C) and forth (D) harmonic wavelengths (2300, 1150, 770, 575 nm respectively) obtained by an IR sensitive card after placed behind mirror 20. FIG. 11B illustrates two images of the output beam at the fundamental frequency and second harmonic measured by pyrocamera placed behind the OC 8. FIG. 11C illustrates a graph related to a waveform of the KLM laser pulse train which is acquired at the SHG wavelength. Thus, a considerable fraction (up to 50%) of the mid-IR femtosecond laser emission is converted to the second harmonic and the amount of the SH power can be adjusted by control of the OC reflectivity.

(43) The output power of fs laser 50 of FIG. 10 at SHG wavelength was 30 mW after mirror 20. That allows obtaining 240 mW SHG power inside the resonator (one has to take into account 50% transmission of the mirrors 18, 20 and the fact that SHG occurs in two opposite directions). The obtained results reveal the following properties of polycrystalline TM:II-VI materials: Polycrystalline TM:II-VI materials are rather efficient nonlinear frequency converters of mid-IR fs pulses (e.g. approximately the same optical power at fundamental and at SHG wavelengths has been obtained during the proof-of-principle experiment). The phase-matching bandwidth of polycrystalline TM:II-VI materials is broad enough to allow for SHG of the whole fs laser emission spectrum. The phase-matching bandwidth of polycrystalline TM:II-VI materials is broad enough to allow for simultaneous SHG, THG, and FHG. Polycrystalline TM:II-VI materials inside the disclosed planar resonator can function as the fs laser gain medium as well as the nonlinear frequency converter. Thus the laser 50 may output multiple fs outputs at four different wavelengths due to the polycrystalline structure of gain medium 4 when the sizes of the microscopic single-crystal grains are of the order of the coherence length of SHG, THG and FHG processes.

(44) Dissimilarities in the grain size and in orientation of the crystallographic axes result in the patterning of the material, like in quasi phase matched (QPM) nonlinear converters. Unlike in the standard QPM material, the patterning is not regular but random. On the one hand, the nonlinear gain in randomly patterned material is very low. On the other hand, random patterning results in very large bandwidth of the nonlinear frequency conversion. Accordingly, low nonlinear gain of polycrystalline Cr.sup.2+:ZnS is compensated by a high peak intensity of fs laser pulses inside the resonator. In summary, the use of polycrystalline TM:II-VI materials with randomized QPM has following important features:

(45) (i) The use of polycrystalline TM:II-VI materials allows for nonlinear frequency conversion of the whole emission spectrum of the fs laser due to very large nonlinear bandwidth of the medium.

(46) (ii) The nonlinear frequency conversion in polycrystalline TM:II-VI materials may include SHG, sum frequency mixing between the laser emission at fundamental wavelength and its optical harmonics, sum and difference frequency mixing between the fs laser and other laser source (e.g. the pump laser), etc.

(47) (iii) Mounting of the polycrystalline TM:II-VI material at normal incidence allows to reduce the laser beam size inside the medium and, hence, increase the optical intensity and, hence, significantly increase the nonlinear conversion efficiency.

(48) (iv) Mounting of the polycrystalline TM:II-VI material inside the planar resonator of the fs laser allows for simultaneous generation of fs laser pulses at fundamental laser wavelength and at a number of secondary wavelengths (SHG, THG, FHG, SFG, DFG, etc.)

(49) (v) Mounting of the polycrystalline TM:II-VI medium at normal incidence inside the planar resonator of the KLM laser allows to increase the length of the gain element and, hence, increase the length of nonlinear interaction and, hence, further significantly increase the nonlinear conversion efficiency.

(50) (vi) Mounting of the polycrystalline TM:II-VI material inside the planar resonator of the KLM laser allows for precise control of fs laser parameters via the interplay between the Kerr nonlinearity and other nonlinearities in the material, as will be described below.

(51) (vii) Mounting of the polycrystalline TM:II-VI material inside the planar resonator of the KLM laser allows to maximize the nonlinear conversion efficiency as the optical power, which circulates inside the resonator, is always higher than the optical power outside the resonator. Furthermore, the optical power inside the planar resonator (and hence, the intensity of the laser beam in polycrystalline TM:II-VI material) can be precisely controlled by optimization of the reflectivity of the output coupler.

(52) (viii) The secondary output of polycrystalline TM:II-VI fs laser at SHG, THG, FHG, SFG, DFG wavelengths can be implemented via specially designed dichroic mirror with high reflectivity HR at fundamental laser wavelength and high transmission HT at secondary wavelengths.

(53) (ix) The secondary output of polycrystalline TM:II-VI fs laser at SHG, THG, FHG, SFG, DFG wavelengths can be implemented via specially designed dielectric coated plates with HT at fundamental laser wavelength and HR at secondary wavelengths. The plates can be mounted e.g. between the polycrystalline TM:II-VI optical element and the resonator mirrors.

(54) FIGS. 12 and 12A illustrate a schematic for controlling the parameters of polycrystalline TM:II-VI Kerr lens mode locked laser 50. Similar to the schematic of FIG. 4, the illustrated schematic is configured with optical pump source 1, pump beam focusing and shaping optics 2, folded concave dielectric coated mirror with high reflectivity at the laser wavelength and high transmission at pump wavelength 3, antireflection (AR) coated polycrystalline TM:II-VI gain element 4 mounted at normal incidence. The gain element 4 is mounted on a stage 30 of FIG. 12A, which allows for translation along the laser beam as shown by a two-arrow line. The KIM laser 50 further includes folded concave dielectric coated mirror with high reflectivity at laser wavelength (and optional high transmission at pump and/or SHG, THG, FHG, SFG, DFG wavelengths) 5. It also has a plane mirror 6 with high reflectivity at laser wavelength (dielectric or metal coated), optional polarization components and components for dispersion compensation, such as prism 7, configured as a plane parallel plate mounted in the laser resonator at Brewster's angle. The disclosed KLM laser further includes an output coupler 8 transmissive to output laser beam 9 at the fundamental wavelength, and secondary outputs 10 transmissive to SHG wave (and/or THG, FHG, SFG, DFG wavelengths). The path of the laser beam is shown by red color and the pump beam is shown by green color.

(55) Kerr-lens mode locked lasers rely on the Kerr effect: a nonlinear optical effect occurring when intense light propagates in optical medium; it can be described as instantaneously occurring modification of the refractive index of the medium. The strength of the Kerr effect is proportional to the optical intensity: S.sub.KerrI. Therefore, the tight focusing of the laser beam in the gain medium is essential in KLM lasers. The required focusing is usually achieved by placing the gain medium between the two curved mirrors in the waist of the laser beam and by optimization of the distance between the curved mirrors. The waist of the laser beam is schematically shown in FIG. 12A. The optical intensity is proportional to the laser beam area and hence it reaches maximum in the waist and decreases with increase of the beam size.

(56) The experiments show that polycrystalline TM:II-VI medium is suitable material for KLM lasers. The experiments also show that polycrystalline TM:II-VI medium is rather an efficient SHG converter for mid-IR fs pulses as disclosed above. The strength of the SHG effect is proportional to the optical intensity squared: S.sub.SHGI.sup.2.

(57) Thus, two nonlinear effects simultaneously occur in polycrystalline TM:II-VI KLM laser: Kerr lensing (proportional to I) and SHG (proportional to I.sup.2). Different dependences of the Kerr lensing and of SHG on the optical intensity allows to vary the relative strengths of two nonlinear effects by translation of the polycrystalline TM:II-VI gain element along the waist of the laser beam Thus, the nonlinear action of polycrystalline TM:II-VI medium can be redistributed between those two nonlinear effects in a controllable manner.

(58) In particular, FIGS. 13, 13A and 13B illustrate the emission spectra, autocorrelation traces obtained for two locations (in red and blue) of gain medium 4 with respect to the curved mirrors 3 and 5 of FIG. 12. As can be seen, translation of the polycrystalline Cr:ZnS gain element resulted in significant variation of the fs laser parameters:pulse duration was reduced from 84 fs to 68 fs (the width of the emission spectrum has proportionally increased). Thus, proposed method allows for precise adjustment of fs laser parameters. Furthermore, translation of the polycrystalline Cr:ZnS gain element resulted in variation of fs laser output power at SHG wavelength (between 10 mW and 20 mW). The increase of the SHG output is exhibited by a distortion of the laser emission spectrum at fundamental wavelength (high SHG output corresponds to the flat-top emission peak as a significant fraction of the laser emission is converted to SHG). Referring to FIG. 14, the ability for precise control of the polycrystalline TM:II-VI KLM laser parameters results a 46 fs pulse duration, which as mentioned before has never been obtained in the prior art related to mid-IR TM:II-VI KLM lasers.

(59) The simultaneous presence of the Kerr effect and of strong enough SHG effect in the polycrystalline TM:II-VI materials has following important applications: (i) Translation of the polycrystalline TM:II-VI gain element, which is mounted inside the planar resonator of the KLM laser, along the waist of the laser beam allows for precise redistribution of the fs laser power between the primary output at fundamental wavelength and secondary output at SHG, THG, FHG, SFG, DFG wavelengths. (ii) Translation of the polycrystalline TM:II-VI gain element, which is mounted inside the planar resonator of the KLM laser, along the waist of the laser beam allows for precise control of fs laser parameters (pulse duration, width and shape of the emission spectrum). (iii) Simultaneous presence of the Kerr effect and of strong enough SHG effect in the polycrystalline TM:II-VI materials enables generation of shorter laser pulses (if compared with conventional Kerr-lens mode locked regime).

(60) A variety of changes of the disclosed structure may be made without departing from the spirit and essential characteristics thereof. Thus, it is intended that all matter contained in the above description should be interpreted as illustrative only and in a limiting sense, the scope of the disclosure being defined by the appended claims.