Patent Application: US-201113309496-A

Abstract:
a method of fabrication of laser gain material and utilization of such media includes the steps of introducing a transitional metal , preferably cr 2 + thin film of controllable thickness on the zns crystal facets after crystal growth by means of pulse laser deposition or plasma sputtering , thermal annealing of the crystals for effective thermal diffusion of the dopant into the crystal volume with a temperature and exposition time providing the highest concentration of the dopant in the volume without degrading laser performance due to scattering and concentration quenching , and formation of a microchip laser either by means of direct deposition of mirrors on flat and parallel polished facets of a thin cr : zns wafer or by relying on the internal reflectance of such facets . multiple applications of the laser material are contemplated in the invention .

Description:
in the preferred embodiment , the cr 2 + : zns crystals are prepared by a three - stage method according to a flow chart depicted in fig1 . 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 . i 2 concentration is in the range of 2 - 5 mg / cm 3 . high optical quality unoriented ingots , preferably ø2 × cm 3 , are cut and ground to slabs of 5 × 5 × 3 mm size . 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 − 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 negative — to 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 . the room - temperature absorption and fluorescence spectra of the studied cr 2 + : zns and cr 2 + : znse crystals are given in cross section units in fig2 a 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 - r04m - 60 ) insb detector coupled to amplifier ( perry pa050 ). this insb detector - amplifier combination featured a temporal resolution of 0 . 4 μs . 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 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 . the luminescence kinetics of the crystals were measured at 1950 , 2100 , 2400 , and 2600 nm across a broad temperature range using d 2 and h 2 raman - shifted nd : yag laser excitation at 1560 and 1907 nm . within the 0 . 4 μs accuracy of measurements there was no difference in the lifetime of luminescence for different wavelengths of excitation and registration . fig2 b shows that the emission lifetime drops only slightly for zns , i . e . from 5 . 7 to 4 . 3 ˜ s , between 14 and 300 ° k and is practically unchanged for znse ( fig2 d ). this shows that quenching is not important below 300 ° k . the spontaneous - emission cross - sections σ em ( λ ) ( fig2 a and 2c ) were obtained from fluorescence intensity signal i ( λ ) using the fuchtbauer - ladenburg equation : σ em ⁡ ( λ ) ⁢ λ 5 ⁢ i λ ⁡ ( λ ) ⁢ a 8 ⁢ π ⁢ ⁢ n 2 ⁢ c ⁢ ∫ i λ ⁡ ( λ ) ⁢ ⅆ λ , ( 1 ) where a is the spontaneous emission probability from the upper laser level , and n is the index of refraction . to derive the absorption cross - section magnitude from the “ absorption spectrum , one needs to know the cr 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 : in conjunction with measured absorption spectra to calculate the absorption cross - section in fig2 a , c , making use of the known ground and upper level degeneracies g 1 = 3 and g 2 = 2 , respectively . here e 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 σ a = 1 . 6 × 10 − 18 cm 2 at λ = 1690 nm for cr 2 + : zns agrees reasonably well with the value of σ a = 1 . 0 × 10 − 18 cm 2 known in the prior art and obtained using the absorption coefficient and the known concentration of cr 2 + . similar graphs of room temperature absorption and emission spectra of cr 2 + : cds ( a ) and cr 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 fig3 . 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 2 + : zns crystals for passive q - switching of er : glass lasers ). experiments on saturation of cr 2 + : zns absorption were performed under 1 . 56 μm excitation . the radiation of a d 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 2 + : zns crystal with initial transmission of t = 0 . 43 at 1 . 56 μm . 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 . as one can see from fig4 , the active absorption changes more than 1 . 4 times under increasing of pump energy fluence from w = 0 . 8 × 10 18 to 6 . 7 × 10 18 photon / cm 2 . since the pump pulse duration ( 5 ns ) is much shorter than the relaxation time of cr 2 + : zns saturable absorber ( 4 . 5 μs ) 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 : t = 1 z ⁢ ln ⁡ ( 1 + t 0 ⁡ ( ⅇ z - 1 ) ) , ( 3 ) where z = w σ ab , t o crystal transition at w = 0 , and σ abs - absorption cross section ( cm 2 ). equation ( 3 ) was solved numerically , and from the best fit to the experimental results ( fig4 , solid line ), the value of σ abs ( λ = 1 . 56 μm ) was estimated to be 0 . 7 × 10 18 cm 2 . taking into account the ratio of absorption at 1 . 56 μm and in the maximum of absorption band ( λ = 1 . 7 μm , see fig2 ) the peak absorption cross section was determined to be 1 . 4 × 10 18 cm 2 , which is in a very good agreement with the value of cross section estimated in the current study from spectroscopic measurements . the cr 2 + concentration in the crystal was 3 . 5 × 10 18 cm − 3 . this satisfactory agreement of σ abs values determined from spectroscopic and saturation measurements indicates negligible excited state absorption losses for cr 2 + : zns at 1 . 56 μm and the wavelength of er : glass laser oscillation ( 1 . 54 μm ). hence , cr 2 + : zns crystals feature a relatively high cross section of absorption 0 . 7 × 10 − 18 cm 2 at 1 . 56 μm compared with 7 × 10 − 21 cm 2 for er : glass . this value is practically two times larger than 0 . 27 × 10 − 18 cm 2 cross section value for cr 2 + : znse known in the prior art and in conjunction with negligible excited state absorption losses reveal possible application of cr 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 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 2 + : zns lasing . in this coupled cavity configuration cr 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 2 + : zns one can expect that the effective time of depopulation of cr 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 . a block - diagram of experimental nonselective hemispherical cavity used for cr 2 + : zns gain switched lasing is depicted in fig5 . laser experiments were performed using the 1 . 5607 μm output from a d 2 raman cell pumped in the backscattering geometry by the 1 . 064 μm 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 μm 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 μm , or 20 - 30 % transmission in the spectral region 1 . 95 - 2 . 5 μm . both mirrors had their peak reflectivity at 2 . 360 μm . 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 μm ). low doped samples ( 3 - 4 cm − 1 at 1 . 7 μm ) 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 2 + : zns laser beam . room temperature laser operation was realized with a threshold of 170 μj and slope efficiency of 9 . 5 % with respect to the pump energy when output coupler r 2 . 360 μm = 90 % was utilized . the laser had an output linewidth of approximately 90 nm ( fwhm ), centered at 2 . 24 μm and maximum output energy reached 100 μj . a graph of output - input energies of cr 2 + : zns gain switched laser in hemispherical cavity is depicted in fig6 . further increase of the pump energy resulted in optical damage of the input mirror . the laser performance of the diffusion doped cr 2 + : zns crystals is expected to be improved by optimization of crystal quality , doping technology and optimization of the output coupler . with the r 2 . 360 μm = 80 % mirror laser operation was obtained with a threshold of 250 μj . this allowed a findlay - clay calculation of the losses within the cavity 29 . with the crystal length of 1 . 7 mm and σ abs = 0 . 8 × 10 − 18 cm 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 . in the wavelength tuning experiment , depicted in fig7 , a hemispherical cavity of the length 19 . 7 cm was utilized . wavelength tuning was realized using a caf 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 2 . 360 μm = 90 % mirror that was used in the nonselective cavity . this arrangement provided a nice match of the cavity waist and pump beam spot (˜ 200 μm ) in the crystal . the pump source was operating at 1 . 5607 μm with the pulse energy of about 600 μj and 5 ns pulse duration in a tem 00 mode . this pump energy was about three times larger than the threshold pump energy level . the cr 2 + : zns laser output was directed through a caf 2 lens to a 0 . 3 m “ spectrapro ” monochromator with a pbs detector for wavelength measurements . fig8 demonstrates a continuous wavelength tuning that was realized over the 2 . 05 - 2 . 40 μm spectral region 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 μm . the tuning limits were due to coatings of the cavity optics and not the emission spectrum of cr 2 + : zns crystal . the use of proper broadband coatings could potentially increase the tuning range to 1 . 85 - 2 . 7 μm . the laser output linewidth could be further narrowed by means of a littrow or littman configured grating tuned cavity . a block diagram of experimental set - up for cr 2 + : zns cw lasing under er fiber laser excitation in external hemispherical cavity is depicted in fig9 . 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 μm 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 μm . the output mirrors had either 2 - 20 % transmission in the spectral region 2 . 2 - 2 . 5 μm , or 20 - 30 % transmission in the spectral region 1 . 95 - 2 . 5 μm . both output mirrors had their peak reflectivity at 2 . 360 μm . the antireflection coated chromium doped zns crystal with a thickness of 1 . 1 mm and an absorption coefficient of 5 cm − 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 than 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 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 fig1 . 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 . 6 w was demonstrated with an output coupler with 2 % transmission for maximum output power adjustment . the maximum slope efficiency “ η ” with respect to the absorbed pump power was 18 % in this experiment . the round trip passive losses “ l 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 1 η = 1 η 0 ⁢ ( 1 + l d t ) , ( 4 ) where η is the slope efficiency , η 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 . a block diagram of cr 2 + : zns and cr 2 + : znse gain switched microchip lasers with no mirrors deposited on the crystal facets is depicted in fig1 . gain switched microchip laser experiments were performed with cr 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 k ˜ 6 cm − 1 at 1 . 77 μm . pumping was from the 1 . 56 μm output of a d 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 fig1 . 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 . a block diagram of experimental set - up for cr 2 + : zns and cr 2 + : znse cw lasing under er fiber laser excitation in microchip configuration is displayed in fig1 . for microchip laser experiments both cr 2 + : zns and cr 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 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 . fig1 shows the output power of the cr 2 + : zns and cr 2 + : znse microchip laser plotted as a function of absorbed pump power . 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 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 2 + : zns microchip laser reached 150 mw as demonstrated in fig1 . 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 . 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 2 + : zns and cr 2 + : znse microchip lasers , respectively . the maximum output power of the cr 2 + : zns microchip laser was practically unchanged while it dropped for cr 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 . 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 2 + : znse laser consisted of more than 100 axial modes with a free spectral range δν = 0 . 8 cm − 1 . the typical output spectra of the microchip lasers are depicted in the “ a ” traces of fig1 . due to a smaller crystal thickness , the free spectral range of the cr 2 + : zns microchip laser was δν = 2 cm − 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 fig1 . in these experiments the number of axial modes decreased to 18 - 24 modes ( each line in fig1 b 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 . fig1 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 2 + : zns ( red ) and cr 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 2 + : zns laser . it is slightly less than that for cr 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 2 + : zns crystal (+ 46 × 10 − 6 k − 1 in zns vs .+ 70 × 10 − 6 k − 1 in znse ). 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 fig1 . 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 fig1 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 . 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 . fig1 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 2 + 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 . 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 . 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 . 1 . c . sitori , j . faist , f . capasso , d . l . sivco , a . l . hutchinson , a . y . cho , ieee photonics technology letters 9 , 294 - 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