METHOD AND SYSTEM FOR MUTLILINE MIR-IR LASER

Abstract

A method of performing spatial separation of different wavelengths in a single laser cavity includes generating, from a pump radiation source, pump radiations in spatially separate channels and focusing the generated pump radiations in the spatially separate channels towards an active gain medium having amplification spectra. The method also includes emitting from the active gain medium, amplified radiations of the spatially separate channels, each channel of the spatially separate channels representing a corresponding wavelength and focusing the emitted amplified radiations of the spatially separated channels towards an aperture. The method further includes suppressing, at the aperture, an off-axis mode of the amplified radiations of the spatially separate channels, diffracting the amplified radiations of the spatially separate channels received through the aperture to provide diffracted radiations and returning a portion of the diffracted radiations back to the aperture, and collimating the diffracted radiations of the spatially separate channel.

Claims

1. A method of performing spatial separation of different wavelengths in a single laser cavity, the method comprising: generating, from a pump radiation source, pump radiations in spatially separate channels; focusing the generated pump radiations in the spatially separate channels towards an active gain medium having amplification spectra; emitting from the active gain medium, amplified radiations of the spatially separate channels, each channel of the spatially separate channels representing a corresponding wavelength; focusing the emitted amplified radiations of the spatially separated channels towards an aperture; suppressing, at the aperture, an off-axis mode of the amplified radiations of the spatially separate channels; diffracting, at a diffraction grating, the amplified radiations of the spatially separate channels received through the aperture to provide diffracted radiations and returning a portion of the diffracted radiations back to the aperture; and collimating the diffracted radiations of the spatially separate channel.

2. The method of claim 1 wherein the active gain medium comprises at least one of a chromium doped ZnS crystal or a chromium doped ZnSe crystal.

3. The method of claim 2 wherein the chromium doping varies across a width of the chromium doped ZnS crystal or the chromium doped ZnSe crystal.

4. The method of claim 1 further comprising: receiving, at the aperture, the portion of the diffracted radiations returned from the diffraction grating; and extracting radiation of main radiation modes from the diffracted radiations.

5. The method of claim 1 further comprising selecting, at the aperture, fundamental transverse modes of the amplified radiations of the spatially separate channels.

6. The method of claim 1 wherein the pump radiation source comprises an array of diode stripes or a fiber bunch.

7. The method of claim 1 wherein the diffraction grating comprises a Littrow mount grating.

8. The method of claim 1 wherein the active gain medium comprises a high reflectivity (HR) coating and an anti-reflection (AR) coating.

9. The method of claim 8 wherein the HR coating comprises a dichroic mirror.

10. A multiline laser comprising: a pump radiation source operable to provide a plurality pump radiations in spatially separate channels; a pump focusing system optically coupled to the pump radiation source; a resonant cavity defined by a high reflectivity (HR) mirror and a diffraction grating; an active gain medium optically coupled to the pump focusing system; an intracavity lens disposed in the resonant cavity between the active gain medium and the diffraction grating; an aperture disposed in the resonant cavity between the intracavity lens and the diffraction grating; and a collimating lens optically coupled to the diffraction grating.

11. The multiline laser of claim 10 wherein: the active gain medium is operable to produce a plurality of amplified radiations, each of the plurality of amplified radiations characterized by fundamental transverse modes and one or more off-axis modes; and the aperture is operable to select the fundamental transverse modes and suppress the one or more off-axis modes.

12. The multiline laser of claim 10 wherein the active gain medium comprises at least one of a chromium doped ZnS crystal or a chromium doped ZnSe crystal.

13. The multiline laser of claim 12 wherein the chromium doping varies across a width of the chromium doped ZnS crystal or the chromium doped ZnSe crystal.

14. The multiline laser of claim 10 wherein the pump radiation source comprises an array of diode stripes or a fiber bunch.

15. The multiline laser of claim 10 wherein the diffraction grating comprises a Littrow mount grating.

16. The multiline laser of claim 10 wherein the active gain medium comprises a high reflectivity (HR) coating and an anti-reflection (AR) coating.

17. The multiline laser of claim 16 wherein the HR coating comprises a dichroic mirror.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The herein described features of the present invention, as well as others which will become apparent, are attained and can be understood in more detail by reference to the following description and appended drawings, which form a part of this specification. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments of the invention and therefore not be considered limiting of its scope, for the invention may admit other equally effective embodiments.

[0026] FIG. 1 is a flow chart of an embodiment of a three-stage method for producing transitional method doped wafer according to the present invention that will be further diced into numerous microchip active elements.

[0027] FIG. 2 is a graph of room temperature absorption and emission spectra of Cr.sup.2+:ZnS (A) and Cr.sup.2+:ZnSe (C) crystals prepared according to the invention, measured at 300K, and plotted in cross-sectional units, and corresponding emission lifetime temperature dependences (B, D).

[0028] FIG. 3 is a graph of room temperature absorption and emission spectra of Cr.sup.2+:CdS (A) and Cr.sup.2+:CdSe (C) crystals prepared according to the invention, measured at 300K, and plotted in cross-sectional units, and corresponding emission lifetime temperature dependences (B, D).

[0029] FIG. 4 is a graph of saturation of ground state absorption in Cr.sup.2+:ZnS crystal. Solid curve is a result of calculation with Frantz-Nodvic equation.

[0030] FIG. 5 is a block-diagram of experimental nonselective hemispherical cavity used for Cr.sup.2+:ZnS gain switched lasing.

[0031] FIG. 6 is a graph of output-input energies of Cr.sup.2+:ZnS gain switched laser in hemispherical cavity with 10% output coupler. The measured slope efficiency is 9.5%.

[0032] FIG. 7 is a block-diagram of experimental selective hemispherical cavity with CaF.sub.2 prism dispersive element used for Cr.sup.2+:ZnS tunable gain switched lasing.

[0033] FIG. 8 is a graph of Cr.sup.2+:ZnS tuning curve with CaF.sub.2 prism selector. The tuning is limited by the coatings of available cavity optics. Currently tunability from 2050 to 2800 nm is achieved.

[0034] FIG. 9 is a block diagram of experimental set-up for Cr.sup.2+:ZnS CW lasing under Er fiber laser excitation in external hemispherical cavity.

[0035] FIG. 10 is a graph of output-input characteristics of the Cr.sup.2+:ZnS continuous wave laser in hemispherical cavity under 1.55 m Er-fiber laser pumping with different output couplers; (.circle-solid.) T=20%, and (.square-solid.) T=2% correspond to minimum threshold adjustment; (.box-tangle-solidup.) T=2%-adjustment to maximum output power.

[0036] FIG. 11 is a block diagram of Cr.sup.2+:ZnS and Cr.sup.2+:ZnSe gain switched microchip lasers with no mirrors deposited on the crystal facets.

[0037] FIG. 12 is a graph of output-input energies for gain switched ZnSe microchip laser with no mirrors deposited on the crystal facets. (.box-tangle-solidup. and .circle-solid. represent different excitation spots on the crystal).

[0038] FIG. 13 is a block diagram of experimental set-up for Cr.sup.2+:ZnS and Cr.sup.2+:ZnSe CW lasing under Er fiber laser excitation in microchip configuration.

[0039] FIG. 14 is a graph of output-input characteristics of the Cr.sup.2+:ZnS and Cr.sup.2+:ZnSe continuous wave microchip lasers under 1.55 m Er-fiber laser pumping.

[0040] FIG. 15 is a graph of output-input curve of the optimized Cr.sup.2+:ZnS continuous wave microchip laser under 1.55 m Er-fiber laser pumping.

[0041] FIG. 16 is a graph of the mode structure of the microchip lasers (A) and coupled cavity (B) microchip lasers (with external etalons) for Cr.sup.2+:ZnS and Cr.sup.2+:ZnSe crystals.

[0042] FIG. 17 is a block diagram of experimental set-up for microchip output beam divergence measurements combined with a graph of spatial distribution of the output radiation of Cr.sup.2+:ZnS (red) and Cr.sup.2+:ZnSe (green) lasers at a distance of L=330 mm from the output laser surfaces.

[0043] FIG. 18 is a block diagram of spatially dispersive cavity made from stand alone components for realization of flexible laser module easily reprogrammable from monochromatic to ultrabroadband and multiline regimes of operation.

[0044] FIG. 19 is a block diagram of a chip scale integrated multiline TM:II-VI laser.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] In the preferred embodiment, the Cr.sup.2+:ZnS crystals are prepared by a three-stage method according to a flow chart depicted in FIG. 1. At the first stage, undoped single crystals are synthesized by a chemical transport reaction from gas phase using an iodine gas transport scheme, preferably in a quartz tube 20 mm in diameter and 200 mm in length placed in a two heating zone furnace. Powder obtained by a joint ignition of initial components serves as raw material. Temperatures in the zones of raw material and crystallization are approximately 1200 C. and 1100 C. respectively. 12 concentration is in the range of 2-5 mg/cm.sup.3. High optical quality unoriented ingots, preferably 2cm.sup.3, are cut and ground to slabs of 553 mm size.

[0046] At the second stage and third stages, introduction of chromium (or other transitional metal) into the crystalline host is performed by thermal diffusion (third stage) from a then film deposited, preferably, by the pulse laser deposition method (second stage). Plasma spluttering or other thin-film deposition methods could also be sued. Thermal annealing can be carried out in sealed ampoules under a pressure of, preferably, approximately 10.sup.5 torr and temperature of approximately 830 to approximately 1100 C. over 3 to 20 days. In some cases to provide more effective thermo-diffusion it was performed under simultaneous action of electric field of 1-30 kV/cm magnitude with positive terminal being applied to Cr film and negativeto the Ag electrode deposited on the opposite surface of the wafer. Polished samples of 1-3 mm thickness and up to 5 mm aperture can then be produced.

[0047] The room-temperature absorption and fluorescence spectra of the studied Cr.sup.2+:ZnS and Cr.sup.2+:ZnSe crystals are given in cross section units in FIGS. 2A and 2C, respectively. The absorption spectra were measured using a (Shimadzu UV-VIS-NIR-3101PC) spectrophotometer. The fluorescence spectra were measured using an (Acton Research ARC-300i) spectrometer and a liquid nitrogen cooled (EGG Judson J10D-M204-RO4M-60) InSb detector coupled to amplifier (Perry PA050). This InSb detector-amplifier combination featured a temporal resolution of 0.4 s. The fluorescence spectra were corrected with respect to the spectral sensitivity of the recording system using a tungsten halogen calibration lamp (Oriel 9-2050). As an excitation source we used CW Erbium doped fiber laser (IPG Photonics, ELD-2), modulated at 800 Hz. It is noteworthy that Cr.sup.2+:ZnSe crystals did not exhibit any polarization dependence of the absorption and the difference due to the polarization dependence of the absorption and fluorescence spectra for Cr:ZnS did not exceed 10% at room temperature. This allowed us to treat the studied crystal in the first approximation as optically isotropic.

[0048] The luminescence kinetics of the crystals were measured at 1950, 2100, 2400, and 2600 nm across a broad temperature range using D.sub.2 and H.sub.2 Raman-shifted Nd:YAG laser excitation at 1560 and 1907 nm. Within the 0.4 s accuracy of measurements there was no difference in the lifetime of luminescence for different wavelengths of excitation and registration. FIG. 2B shows that the emission lifetime drops only slightly for ZnS, i.e. from 5.7 to 4.3s, between 14 and 300 K and is practically unchanged for ZnSe (FIG. 2D). This shows that quenching is not important below 300 K.

[0049] The spontaneous-emission cross-sections .sub.em() (FIGS. 2A and 2C) were obtained from fluorescence intensity signal I() using the Fuchtbauer-Ladenburg equation:

[00001]$\begin{array}{cc}{}_{\mathrm{em}}\left(\right)=\frac{{}^5.Math.I_{}\left(\right).Math.A}{8.Math..Math..Math.n^2.Math.c.Math.I_{}\left(\right).Math.d.Math..Math.},& \left(1\right)\end{array}$

where A is the spontaneous emission probability from the upper laser level, and n is the index of refraction.

[0050] To derive the absorption cross-section magnitude from the absorption spectrum, one needs to know the Cr.sup.2+ concentration. Unfortunately, the absolute dopant concentration is neither uniform nor accurately known in the case of diffusion doping. We therefore used the reciprocity method for the broadband transition:

[00002]$\begin{array}{cc}{}_a\left(\right)={}_{\mathrm{em}}\left(\right).Math.\frac{g_2}{g_1}.Math.\exp \left(\frac{\mathrm{hc}/-E_{\mathrm{ZFL}}}{\mathrm{kT}}\right)& \left(2\right)\end{array}$

[0051] In conjunction with measured absorption spectra to calculate the absorption cross-section in FIG. 2A,C, making use of the known ground and upper level degeneracies g.sub.1=3 and g.sub.2=2, respectively. Here E.sub.ZFL is the energy of the zero phonon line of the corresponding transition, k is the Boltzmann constant, and T is the temperature. We also assumed that the Jahn-Teller splitting of both upper ands lower levels can be neglected, as it is less or comparable to kT at room temperature. Our value for the peak absorption cross-section of .sub.a=1.610.sup.18 cm.sup.2 at =1690 nm for Cr.sup.2+:ZnS agrees reasonably well with the value of .sub.a=1.010.sup.18 cm.sup.2 known in the prior art and obtained using the absorption coefficient and the known concentration of Cr.sup.2+.

[0052] Similar graphs of room temperature absorption and emission spectra of Cr.sup.2+:CdS (A) and Cr.sup.2+:CdSe (C) crystals prepared according to the invention, measured at 300K, and plotted in cross-sectional units, and corresponding emission lifetime temperature dependences (B, D) are displayed in FIG. 3.

[0053] One of the important potential applications of TM:II_-VI crystals is the passive Q-switching of the resonators of solid state lasers (e.g. Cr.sup.2+: ZnS crystals for passive Q-switching of Er:glass lasers). Experiments on saturation of Cr.sup.2+:ZnS absorption were performed under 1.56 m excitation. The radiation of a D.sub.2-Raman-shifted YAG:Nd laser with a pulse duration of 5 ns and pulse energy of up to 20 mJ and repetition rate of 10 Hz was used. Saturation experiments utilized a 2.5 mm thick Cr.sup.2+:ZnS crystal with initial transmission of T=0.43 at 1.56 m. The pump radiation was focused on the sample by a 26.5 cm lens and the dependence of the crystal transmission as a function of pumping energy density was measured by means of the sample Z-scanning. Spatial energy distributions of the pump radiation were determined by a standard knife-edge method. The effective radius of the pumping beam was measured at the 0.5 level of maximum pump intensity of radiation.

[0054] As one can see from FIG. 4, the active absorption changes more than 1.4 times under increasing of pump energy fluence from W=0.810.sup.18 to 6.710.sup.18 photon/cm.sup.2. Since the pump pulse duration (5 ns) is much shorter than the relaxation time of Cr.sup.2+: ZnS saturable absorber (4.5 s) the saturation behavior was analyzed in terms of energy fluence with a modified Frantz-Nodvik equation for a four level slow absorber. According to this equation the crystal transmission depends on pump energy fluence, W, and absorption cross section as follows:

[00003]$\begin{array}{cc}T=\frac{1}{z}.Math.\ln \left(1+T_0\left(e^z-I\right)\right),& \left(3\right)\end{array}$

where z=W.sub.ab, T.sub.o-initial crystal transition at W=0, and .sub.abs-absorption cross section (cm.sup.2). Equation (3) was solved numerically, and from the best fit to the experimental results (FIG. 4, solid line), the value of .sub.abs (=1.56 m) was estimated to be 0.710.sup.18 cm.sup.2. Taking into account the ratio of absorption at 1.56 m and in the maximum of absorption band (=1.7 m, see FIG. 2) the peak absorption cross section was determined to be 1.410.sup.18 cm.sup.2, which is in a very good agreement with the value of cross section estimated in the current study from spectroscopic measurements.

[0055] The Cr.sup.2+ concentration in the crystal was 3.510.sup.18 cm.sup.3. This satisfactory agreement of abs values determined from spectroscopic and saturation measurements indicates negligible excited state absorption losses for Cr.sup.2+:ZnS at 1.56 m and the wavelength of Er: glass laser oscillation (1.54 m). Hence, Cr.sup.2+: ZnS crystals feature a relatively high cross section of absorption 0.710.sup.18 cm.sup.2 at 1.56 m compared with 710.sup.21 cm.sup.2 for Er: glass. This value is practically two times larger than 0.2710.sup.18 cm.sup.2 cross section value for Cr.sup.2+:ZnSe known in the prior art and in conjunction with negligible excited state absorption losses reveal possible application of Cr.sup.2+:ZnS crystals as a promising saturable absorber for resonators of Er: glass lasers. In addition to this it is advantageous to utilize for solid state laser Q switching and mode-locking Cr.sup.2+:ZnS crystals with dichroic mirrors deposited on their faces. These mirrors are supposed to be transparent at the wavelength of solid state laser (e.g. Er-glass laser) oscillation and reflective in the region of Cr.sup.2+:ZnS lasing. In this coupled cavity configuration Cr.sup.2+:ZnS element will serve simultaneously as passive Q-switch or mode-locker, as a load for solid state laser, and as an active element. Due to stimulated processes in Cr.sup.2+:ZnS one can expect that the effective time of depopulation of Cr.sup.2+:ZnS excited levels will be much faster than for regular arrangement without coupled cavity. It will result in a shorter pulsed duration in a Q-switch regime and even possibility of mode-locked operation.

[0056] A block-diagram of experimental nonselective hemispherical cavity used for Cr.sup.2+:ZnS gain switched lasing is depicted in FIG. 5. Laser experiments were performed using the 1.5607 m output from a D.sub.2 Raman cell pumped in the backscattering geometry by the 1.064 m radiation of a Nd:YAG laser. An optical diode was placed before the Raman cell to prevent possible damage of Nd:YAG laser optics by amplified backscattered 1.06 m radiation. Pump pulses from the Raman cell had pulse duration of 5 ns at FWHM; output energy reached 100 mJ and was continuously attenuated by a combination of a half-wave plate and a Glan prism. Amplitude stability of the pump pulses was about 5%. The hemispherical cavity consisted of the input mirror deposited on the facet of the ZnS crystal and output mirror with 20 cm radius of curvature. Output mirrors had either 10-20% transmission in the spectral region 2.05-2.5 m, or 20-30% transmission in the spectral region 1.95-2.5 m. Both mirrors had their peak reflectivity at 2.360 m. Length of the cavity was 18.5 cm. Pump radiation was focused on the crystal with a 26.5 cm lens placed 22.5 cm before the crystal providing a good match for the pump caustics and the cavity mode size (200 m). Low doped samples (3-4 cm.sup.1 at 1.7 m) of 1.7 mm thickness were utilized. The second facet of the crystal was anti-reflection (AR) coated in the lasing region and was fully reflective at the wavelength of pumping, providing a double pass pumping scheme. A Ge filter was used to separate residual pump light from the Cr.sup.2+:ZnS laser 5 beam.

[0057] Room temperature laser operation was realized with a threshold of 170 J and slope efficiency of 9.5% with respect to the pump energy when output coupler R.sub.2.360 m=90% was utilized. The laser had an output linewidth of approximately 90 nm (FWHM), centered at 2.24 m and maximum output energy reached 100 J. A graph of output-input energies of Cr.sup.2+:ZnS gain switched laser in hemispherical cavity is depicted in FIG. 6. Further increase of the pump energy resulted in optical damage of the input mirror. The laser performance of the diffusion doped Cr.sup.2+:ZnS crystals is expected to be improved by optimization of crystal quality, doping technology and optimization of the output coupler.

[0058] With the R.sub.2.360 m=80% mirror laser operation was obtained with a threshold of 250 J. This allowed a Findlay-Clay calculation of the losses within the cavity.sup.29. With the crystal length of 1.7 mm and .sub.abs=0.810.sup.18 cm.sup.2 the losses in the cavity were calculated to be 14.7%. It is felt that this can also be improved by the optimization of the crystal preparation techniques.

[0059] In the wavelength tuning experiment, depicted in FIG. 7, a hemispherical cavity of the length 19.7 cm was utilized. Wavelength tuning was realized using a CaF.sub.2 Brewster prism as the dispersive element placed 5 cm from the output coupler. The focusing lens and crystal remained at the positions that were used in the nonselective cavity. The output coupler was the 20 cm, R.sub.2.360 m=90% mirror that was used in the nonselective cavity. This arrangement provided a nice match of the cavity waist and pump beam spot (200 m) in the crystal.

[0060] The pump source was operating at 1.5607 m with the pulse energy of about 600 J and 5 ns pulse duration in a TEM.sub.00 mode. This pump energy was about three times larger than the threshold pump energy level. The Cr.sup.2+:ZnS laser output was directed through a CaF.sub.2 lens to a 0.3 m SpectraPro monochromator with a PbS detector for wavelength measurements. FIG. 8 demonstrates a continuous wavelength tuning that was realized over the 2.05-2.40 m spectral region.

[0061] The output of the chromium laser oscillation had a linewidth of approximately 30 nm (FWHM). The peak efficiency of the tunable output was centered at 2.25 m. The tuning limits were due to coatings of the cavity optics and not the emission spectrum of Cr.sup.2+:ZnS crystal. The use of proper broadband coatings could potentially increase the tuning range to 1.85-2.7 m.

[0062] The laser output linewidth could be further narrowed by means of a Littrow or Littman configured grating tuned cavity.

[0063] A block diagram of experimental set-up for Cr.sup.2+:ZnS CW lasing under Er fiber laser excitation in external hemispherical cavity is depicted in FIG. 9. Pump source was an Erbium Doped Fiber Laser (ELD-2, IPG Photonics). This laser delivers 2W of single mode CW non-polarized radiation at 1550 nm and was equipped with an optical isolator to prevent any possible feedback from the ZnS and ZnSe laser system. The fiber core was 5 m in diameter. For external non-selective resonator laser experiments, the hemispherical cavity consisted of the flat input mirror and output mirror with 20 cm radius. The input mirror crystal had 99.5% reflectivity in the spectral region from 2.2 to 2.5 m. The output mirrors had either 2-20% transmission in the spectral region 2.2-2.5 m, or 20-30% transmission in the spectral region 1.95-2.5 m. Both output mirrors had their peak reflectivity at 2.360 m. The antireflection coated chromium doped ZnS crystal with a thickness of 1.1 mm and an absorption coefficient of 5 cm.sup.1 at the pump wavelength was utilized. The crystal was mounted on an optical contact to the input flat dichoric mirror made from the YAG crystal for the sake of effective dissipation of heat. The pump radiation of the Er fiber laser was first collimated with a microscope objective in a parallel pencil of light having 1 mm in diameter, and then focused with a second 15 mm focal length objective into the crystal through the input mirror. The output laser parameters were different when the cavity was adjusted to minimum threshold and maximum output power. The output-input dependences for ZnS:Cr.sup.2+ continuous wave lasing under Er fiber pumping for two different output couplers and for different cavity adjustments to the minimum threshold and maximum output power are depicted in FIG. 10.

[0064] The minimum threshold values were measured to be 100 mW and 200 mW of absorbed pump power for output couplers with 2% and 20% transmission, respectively. An output power of 63 mW near 2370 nm at an absorbed pump power of 0.6W was demonstrated with an output coupler with 2% transmission for maximum output power adjustment. The maximum slope efficiency if with respect to the absorbed pump power was 18% in this experiment. The round trip passive losses L.sub.d in the cavity were estimated to be of 3.7% from the Findley-Clay analysis. The limiting slope efficiency of studied crystal was estimated to be 51% from a Caird analysis of inverse slope efficiency versus inverse output coupling using equation

[00004]$\begin{array}{cc}\frac{1}{}=\frac{1}{{}_o}.Math.\left(1+\frac{L_d}{T}\right),& \left(4\right)\end{array}$

where is the slope efficiency, .sub.o is the limiting slope efficiency, and T is the mirror transmission. This value is close to the quantum defect of 65% for the studied crystal.

[0065] A block diagram of Cr.sup.2+:ZnS and Cr.sup.2+:ZnSe gain switched microchip lasers with no mirrors deposited on the crystal facets is depicted in FIG. 11. Gain switched microchip laser experiments were performed with Cr.sup.2+ doped ZnSe and ZnS. The crystal used were 0.5-3 mm thick with polished but uncoated parallel faces and had coefficient of absorption of k6 cm.sup.1 at 1.77 m. Pumping was from the 1.56 m output of a D.sub.2 Raman shifted Nd:YAG operating at 10 Hz with a pulse duration of about 5 ns and 1.5 mm beam diameter. Output-input energies for pulsed ZnSe microchip lasing for different lasing spots are shown in FIG. 12. Threshold input energy was found to be 7 mJ. A maximum slope efficiency of 6.5% and maximum output power of 1 mJ were obtained for a microchip without mirrors, when positive feedback was only due to the Fresnel reflections. The spectral range of the free-running laser output was from 2270 to-2290 nm.

[0066] A block diagram of experimental set-up for Cr.sup.2+:ZnS and Cr.sup.2+:ZnSe CW lasing under Er fiber laser excitation in microchip configuration is displayed in FIG. 13. For microchip laser experiments both Cr.sup.2+:ZnS and Cr.sup.2+:ZnSe crystals were studied. The crystals were polished flat and parallel (parallelism of 10) to 1.1 and 2.5 mm thickness, respectively. The mirrors were directly deposited on the parallel polished facets of a thin wafer of laser material. Input and output dichroic mirrors had 0.01 and 3.5% transmission over 2300-2500 nm spectral region, respectively, and their transmission at 1550 nm pumping wavelength was 75%. Two different pump arrangements were utilized. The first one was identical to the pump conditions for the Cr.sup.2+:ZnS CW lasing in hemispherical cavity when the pump radiation of the Er fiber laser was first collimated with a microscope objective in a parallel pencil of light having 1 mm in diameter, and then focused with a second 15 mm focal length objective into the crystal through the input mirror. The second pump arrangement was provided without any additional optics by means of the microchip laser mounting at a close (20 um) distance from the tip of the pump Er-fiber laser. In both cases the rather large value of the temperature derivative of the refraction index for ZnS and ZnSe crystals (5 times larger than for YAG crystal) played a constructive role by means of creating a strong positive lens and providing effective stabilization of the microchip cavity. FIG. 14 shows the output power of the Cr.sup.2+:ZnS and Cr.sup.2+:ZnSe microchip laser plotted as a function of absorbed pump power.

[0067] In a focused pump beam arrangement a laser threshold of 120 mW and a slope efficiency of 53% with respect to the absorbed pump power were realized for Cr.sup.2+:ZnS microchip laser. High, close to theoretical limit of 65%, slope efficiency of the microchip laser indicates a good quality of the used crystal. The maximum output power of optimized Cr.sup.2+:ZnS microchip laser reached 150 mW as demonstrated in FIG. 15.

[0068] In the case of ZnSe microchip lasing in a focused pump beam arrangement a laser threshold of 190 mW and a slope efficiency of 20% with respect to the absorbed pump power were demonstrated. The maximum output power reached 100 mW.

[0069] For the second pump arrangement, when the microchip lasers were directly coupled to the fiber tip laser thresholds of 150 mW and 240 mW and slope efficiencies of 36 and 14% with respect to the absorbed pump power were realized for Cr.sup.2+:ZnS and Cr.sup.2+:ZnSe microchip lasers, respectively. The maximum output power of the Cr.sup.2+:ZnS microchip laser was practically unchanged while it dropped for Cr.sup.2+:ZnSe by a factor of 1.6 in comparison to the focused pump arrangement. This can be explained by the excessive length and corresponding mismatch in the mode size and pump beam profile of the ZnSe microchip.

[0070] The output spectrum in free-running laser operation covered the spectrum range from 2280 to 2360 and from 2480 to 2590 for ZnS and ZnSe microchip lasers, respectively. At maximum pump power the output spectrum of the Cr.sup.2+:ZnSe laser consisted of more than 100 axial modes with a free spectral range v=0.8 cm.sup.1. The typical output spectra of the microchip lasers are depicted in the A traces of FIG. 16. Due to a smaller crystal thickness, the free spectral range of the Cr.sup.2+:ZnS microchip laser was v=2 cm.sup.1 and the output spectrum consisted of about 50 axial modes. We attempted to arrange mode control of the microchip lasers by means of a coupled cavity arrangement, with an additional external mirror. The coupled microchip and mirror produced the spectral structure shown in the B traces of FIG. 16. In these experiments the number of axial modes decreased to 18-24 modes (each line in FIG. 16B consists of 3 longitudinal modes) for both lasers. This can be further decreased to a single longitudinal mode oscillation in a double cavity configuration using a narrowband output coupler. This experiment demonstrates a feasibility of the microchip single longitudinal mode lasing using a selective output coupler in a combination with the external etalon.

[0071] FIG. 17 displays a block diagram of experimental set-up for microchip output beam divergence measurements combined with a graph of spatial distribution of the output radiation of Cr.sup.2+:ZnS (red) and Cr.sup.2+:ZnSe (green) lasers at a distance of L=330 mm from output laser surfaces. As one can see, a 18 mrad FWHM of the intensity profile was measured for the Cr.sup.2+:ZnS laser. It is slightly less than that for Cr.sup.2+:ZnSe laser (25 mrad). Taking thermal effects, that are responsible for cavity stabilization, into account, the divergence difference may be explained by a lower dn/dT in Cr.sup.2+:ZnS crystal (+4610.sup.6 K.sup.1 in ZnS vs.+7010.sup.6 K.sup.1 in ZnSe).

[0072] The proposed approach of superbroadband/multiwavelength (SBML) system is based on spatial separation of different wavelengths in a single laser cavity. In that regard the teachings of U.S. Pat. Nos. 5,461,635 and 6,236,666 are incorporated herein by reference. The basic optical scheme of the laser transmitter is shown in FIG. 18.

[0073] The laser operates as follows. Emission from the spatially separated channels of the active medium passes through the intracavity lens into the off-axis mode suppression element, aperture A, which together with the spatially filtered pump radiation divides active zone of the gain waveguide into a number of channels and separates from the amplified emission of individual channel only part of it that is spread parallel to the resonator axis. This separated radiation is diffracted on the diffraction grating. The Littrow mount grating works as a retroreflector in the auto-collimating regime in the first order of diffraction and returns part of radiation back to the aperture. The off-axis mode suppression element, aperture, in turn extracts from the diffracted radiation only the radiation of the main laser modes. Secondary laser modes, which diverge from the optical axes, are expelled from the process of generation. Hence, the aperture should simultaneously select the fundamental transverse modes for all existing channels in the cavity. The radiation of the main laser modes, each with a distinct wavelength, is collimated by the focusing lens and directed back to the active medium. As FIG. 18 shows, the optical components of the cavity maintain distinct gain channels in the active zone of active element, reduce cross talk, suppress mode competition, and force each channel to lase at specific stabilized wavelength. This approach allows the construction of the laser that emits a plurality of narrow spectral lines that can be easily tailored to any pre-assigned spectral composition within the amplification spectrum of the gain medium. We believe that TM doped II-VI hosts and, specifically, chromium doped ZnS and ZnSe crystals featuring broad amplification spectra are ideal active media for superbroadband and multiline lasing.

[0074] There are different schemes that can provide single longitudinal mode operation of Ii-VI microchip laser coupled to external etalon cavity in combination with narrowband output coupler, fiber grating butt-coupling, external grating, hybridly coupled phase array demultiplexer, and waveguide grating mirror.

[0075] FIG. 19 displays further chip scale integration of multiline TM:II-VI laser. This integrated optical chip is made on II-VI substrate. The chip consists of several sections. The right section has multiple V-grooves etched in II-VI substrate and is provided for connection with fiber lasers or fiber coupled diode lasers. Central section consists of multiple waveguides (e.g. made by ion exchange or ridge technology) and provides delivery of the pump radiation to the active section. The active section consists of multiple II-VI waveguides doped with TM and can be further combined with dispersive element such as a tapered grating. Tapered grating, for example, can be provided by exposing active waveguides with UV interference pattern. Utilization of tapered grating provides an autocollimation regime of retroreflection for different wavelengths for each individual active waveguide giving rise to a multifrequency regime of oscillation. Due to electro-optic properties of II-VI materials it is possible to integrate Mach-Zehnder or electro-reflection internal modulator with the active section of the same waveguide (not shown on the Figure). Output multifrequency radiation can be coupled to an output fiber.

[0076] There are many other possible schemes of utilization of acousto-optic, electro-optic, photorefractive and birefringent properties of II-VI crystals in one integrated microchip system combining active medium, acousto- or electro-optic modulator, filter, other passive components of the cavity.

[0077] While our invention has been disclosed in various forms, this disclosure is not to be construed as limiting the invention solely to these forms, rather the invention is limited solely by the breadth of the claims appended hereto.