Tunable, Single-Frequency, and Multi-Watt Continuous-Wave Cr 2 + :ZnSe Lasers

: We demonstrate CW rapidly-tunable (4.5 μ m/s), high-power (150 mW), single-longitudinal-mode (120 MHz) single-crystalline Cr 2 + :ZnSe laser; CW widely-tunable (2.12–2.77 μ m), multi-watt (2 W over 2.3–2.7 μ m), polycrystalline Cr 2 + :ZnSe laser; CW multi-watt (6 W, at 2.5 μ m), highly efﬁcient (48% real efﬁciency) polycrystalline Cr 2 + :ZnSe laser; CW multi-watt (3 W, at 2.5 μ m), highly efﬁcient (41% real efﬁ-ciency) ultra-compact polycrystalline Cr 2 + :ZnSe laser; and CW hot-pressed ceramic Cr 2 + :ZnSe laser.


Introduction
The presence of strong fundamental and overtone vibrational absorption lines of organic molecules within the so-called "molecular fingerprint" spectral region of 2-20 μm gives rise to a large number of scientific and technological applications of the middle-infrared (mid-IR) laser sources.These applications include eye-safe laser radar and remote sensing of atmospheric constituents, eye-safe medical laser sources for non-invasive medical diagnostics, eye-safe efficient laser surgery, and numerous military applications.Consequently, high-power, narrowlinewidth, broadly-tunable mid-IR solid-state CW lasers operating over the entire molecular fingerprint spectral interval, are of great interest.
This demand has inspired the development of novel laser gain media for mid-IR laser sources, and after pioneering publications [1] and [2], room temperature mid-IR lasing has been reported for Cr 2+ :ZnS [3][4][5][6], Cr 2+ :ZnSe, Cr 2+ :Cd 1−x Mn x Te [7,8], Cr 2+ :CdSe [9] crystals.Recently, much attention of researchers was devoted to development of Cr 2+ :ZnSe lasers due to excellent lasing and optical properties of this material in the mid-IR spectral region.The regimes of laser operation of Cr 2+ :ZnSe were significantly extended after the first demonstrations of direct diode excitation [10,11]: continuous wave lasing with efficiency exceeding 60% [11,12] with power levels in excess of 1.8 W [13], gain switched lasing with output power up to 18.5 W [14], and a range of tunability over 1880-3100 nm [15] have been reported.Despite this significant progress, the optimization of Cr 2+ :ZnSe crystal fabrication technology and design of laser sources capable of generating multi-watt output powers are still to be demonstrated.The maximum reported output power of the chromium lasers in CW operation mode is 1.8 W [13] and is below the required level for many applications.On the other hand, although the tunability of Cr 2+ :ZnSe lasers was demonstrated in a significant number of research works [5,10,12,15], the single frequency laser operation was reported only once in [16].In that work, the laser was based on a dispersive cavity with a diffraction grating and two intra-cavity Fabry-Perot etalons, and operated in single-longitudinal-mode (SLM) regime, however, broadband wavelength tuning in the SLM regime was not documented.
In this work we describe a range of high-power CW laser systems based on single-, polycrystalline, and hot-pressed ceramic Cr 2+ :ZnSe gain media.We begin the paper with presentation of our single-longitudinal-mode (δ λ ≈ 120 MHz), high-power (150 mW) laser rapidly-tunable over 120 nm spectral range around 2.5 μm with the wavelength tuning speed of up to 4.5 μm/s.Then we turn our attention to the "power-scaling" of Cr 2+ :ZnSe lasers and for the first time we demonstrate true multi-watt output powers in pure CW regime of operation of Cr 2+ :ZnSe laser systems: (a) 6 W Kogelnik cavity-based, and (b) 3 W "mirror-less" ultra-compact polycrystalline Cr 2+ :ZnSe lasers (demonstrating 48% and 41% real optical efficiencies, respectively).This breakthrough in Cr 2+ :ZnSe power-scaling was made possible primarily due to improved technology of fabrication of high-quality, low-loss, uniformly-doped Cr 2+ :ZnSe gain media, first reported at the CLEO'07 conference [17] (the details of the high-quality Cr 2+ :ZnSe crystal fabrication are to be published in a separate paper elsewhere).We continue the topic of power-scaling of Cr 2+ :ZnSe lasers with presentation of our new 2-W Cr 2+ :ZnSe laser, "one knob"-tunable from 2.12 to 2.77 μm.Finally, we show a pure CW laser based on hot-pressed Cr 2+ :ZnSe ceramic.

Single-frequency, rapidly-tunable Cr 2+ :ZnSe laser
The CW Cr 2+ :ZnSe SLM laser is based on a modified Kogelnik/Littman cavity shown schematically in Fig. 1(A).The cavity consists of 25 mm radius of curvature (ROC) input mirror, 50 mm ROC folding mirror (both of which are AR/HR coated at 1.56 μm/2.0-3.0 μm, respectively), a highly efficient (50% into the 1st order at 75 • incident angle) gold-coated reflective diffraction grating (600 mm −1 groove frequency), and a flat tuning mirror, mounted on a piezodriven fast mirror shaker.The shaker allows for fast tilting of the mirror around an axis lying in its reflecting surface with a repetition rate of up to 250 Hz.The single-crystalline, 1.5 mm long Cr 2+ :ZnSe gain element, which is installed in the laser cavity at the Brewster angle for horizontal polarization, is mounted on a thermo-electrically/air cooled (TEC) copper block for its thermal stabilization.
The fine spectral structure of the laser output radiation, acquired with a scanning Fabry-Perot interferometer (FPI), is shown in Fig. 1(B).The measured FPI finesse is about 7.3 and its base is 160 mm (while the SLM laser optical length is 100 mm), and thus the 120 MHz linewidth is an upper estimate limited by the interferometer spectral resolution.
The laser wavelength can be rapidly scanned around a desired central wavelength within 2.45-2.55μm spectral range with a scanning amplitude of up to 20 nm and scanning speed of up to 4.5 μm/s .In order to investigate the tuning characteristics of the laser, its intensity was simultaneously monitored on the detectors D 1 and D 2 , as shown in Fig. 1(A).The detector D 1 shows the laser output intensity, while the signal from the detector D 2 is proportional to the FPI transmission and thus provides a frequency scale as the laser wavelength is rapidly tuned.Typical output curves obtained in this way are demonstrated in Fig. 2, where a rapid tuning around 2489 nm is shown.One can see in the figure that a scanning speed of 2.85 μm/s is achieved with the total scanning interval of 20 nm.Reducing the scanning interval to 10 nm allows for a scanning speed of up to 4.5 μm/s.As the laser wavelength passes through absorption spectral lines of the atmospheric water vapor, the laser intensity drops.This way, the intracavity water vapor spectroscopy is performed.The laser delivers 150 mW output power at 6 W pump in the SLM regime of operation, which corresponds to 2.5% real optical efficiency limited mainly by inadequate efficiency of the Littman grating.

Multi-Watt, highly-efficient Cr 2+ :ZnSe laser
Power-scaling of Cr 2+ :ZnSe lasers is generally a challenging task mainly due to high thermal lensing effects in this laser material [18].Consequently, the reported maximum output power of pure CW Cr 2+ :ZnSe lasers was limited to only 1.8 W [13].An attractive approach was demonstrated in [18] where a face-cooled disk design enabled power-scaling of Cr 2+ :ZnSe 1 mm and 0.5 mm disk laser outputs to 4.2 W in 10 kHz repetition rate gain-switched, and 1.4 W CW regimes, respectively.However, as we will demonstrate in this section, multi-watt output powers from Cr 2+ :ZnSe CW lasers can be obtained in the conventional slab geometry of the high-quality, uniformly-doped Cr 2+ :ZnSe gain element and with single-pass pumping, even without special arrangements for cooling of the active medium.Our high-power Cr 2+ :ZnSe laser is based on a 7 × 3 × 9 mm (9 mm is length) polycrystalline Cr 2+ :ZnSe gain element pumped by 1.56 μm Er-fiber laser through the 3 × 7 facets.The absorption coefficient of this crystal at 1.56 μm is about 3 cm −1 .The laser cavity is the standard non-dispersive Kogelnik cavity shown schematically in Fig. 1(A) by the dashed lines.The gain element is uncoated and thus is installed at the Brewster angle for the horizontal polarization (the plane of the cavity as shown in Fig. 1).
To reduce the thermal lensing effects and obtain high output powers, the crystal is mounted between two thermally-connected copper cold plates (on the 9 × 7 mm surfaces) and the whole copper block is cooled with a 20 W air-cooled Peltier element.We found, however, that in general the gain element can be conductively without any active thermal stabilization, which will not lead to any negative effects on the laser performance characteristics.
Two 1.56 μm Er-fiber pump lasers were available for the Cr 2+ :ZnSe high-power laser experiments: (a) polarized 9 W Er-fiber laser; in this case about 7.5 W reached the gain element due to the loss caused by pump optics; (b) randomly-polarized 30 W Er-fiber laser.In the second case, due to high losses of the vertical polarization on the Cr 2+ :ZnSe crystal Brewster input facet, and additional losses on the pump delivering optics, only about 12.5 W could be used for pumping.The power input-output characteristics of the laser are shown in Fig. 3.The left graph shows the performance of the laser pumped by the polarized Er-fiber laser for several output couplers (OC).One can see that the highest slope and real efficiencies (≈ 50% and ≈ 42%, respectively) obtained with the high transmission OCs and the output power exceeds 3 W.When the Cr 2+ :ZnSe laser is pumped with the unpolarized fiber laser, the maximum usable pump is about 12.5 W and the Cr 2+ :ZnSe output power reaches a record 6 W output power, demonstrating 52% slope and 48% real optical efficiencies, as one can see in Fig. 3(B).In all these cases the laser operates in pure CW mode.

Multi-Watt highly-efficient ultra-compact Cr 2+ :ZnSe laser
The ultra-compact laser consists of only two elements: a flat input mirror (AR/HR-coated at 1.56 μm / 2.0-3.0 μm, respectively) and uncoated, rectangular, plane-parallel, 7 × 3 × 9 mm (9 mm is the length) uniformly-doped polycrystalline Cr 2+ :ZnSe gain element (this sample has exactly the same physical properties as the one used in the multi-Watt laser described in Section 3).The crystal is placed in close vicinity from the flat input mirror with a small (10-100 μm) adjustable air gap.The 7 × 3 mm input and output facets are perpendicular to the incident pump beam and the laser mode, which leads to some loss of the pump power as well as an intracavity losses at the lasing wavelengths due to Fresnel reflection on the input facet of the gain element.The flat input mirror and the input facet of the gain element form a low-finesse FPI, which allows to significantly reduce the intracavity losses by careful adjustment of the thin air gap between the input mirror and the crystal input facet.The cavity stability is achieved by the positive thermal lens in the gain element.The output facet of the gain element serves as a 18% (reflectivity) output coupler.The laser is pumped by the linearly-polarized 9 W Er-fiber laser.The 5 mm pump beam is focused into the gain element with a 70 mm pump lens.The output characteristics of this "mirrorless" ultra-compact laser are depicted in Fig. 4. One can see that the laser operates around ∼ 2.4 μm and delivers up to 3 W output power at about 7.5 W pump with 59% slope and 41% real optical efficiencies, respectively, and generates a broadband 20 nm output spectrum.The broadband spectrum indicates a highly multimode operation of the laser, and direct measurements of the laser beam divergence with the knifeedge method show the laser beam quality factor M 2 ≈ 2.2.The roll-off of the output power can be explained by a change of the thermal lens refractive power (and thus the stability conditions of the laser resonator) as the pump grows.A flat input-output characteristics and higher laser efficiency can be obtained by depositing the input mirror directly onto the input facet of the gain crystal, which is a task of our future experiments with the microchip Cr 2+ :ZnSe lasers of this type.

High-power, widely tunable Cr 2+ :ZnSe laser
This pure CW widely-tunable laser is based on a 7 × 3 × 9 mm (9 mm is the length), uniformlydoped polycrystalline Cr 2+ :ZnSe crystal (this sample has exactly the same physical properties as the one used in the multi-Watt laser described in Section 3), and an X-type Littrow grating cavity, shown schematically in Fig. 5(A).The 600 g/mm reflective diffraction grating is operating in the Littrow configuration and provides wavelength tuning over a spectral interval of 2.12-2.77μm.The grating efficiency exceeds 95% at 45 • incident angle, thus leading to a 5% intracavity loss.The grating is mounted on a computer controlled rotation stage and the laser wavelength tuning is performed with "one knob" over the entire tuning range: no additional alignments are ever necessary for obtaining reproducible results on the output wavelength and power.In order to obtain high output power, a 70% output coupler is used and the laser delivers more than 2 W output power at 12 W pump, depending on the operating wavelength.The laser output power as a function of output wavelength is shown in Fig. 5(B).The laser operates in a TEM 00 mode with the maximum linewidth of 2 nm.One can see in the figure that the laser output power is between 1.5 W and 2.1 W in the 2.3-2.7 μm wavelength range and decreases at other wavelengths following the gain curve of Cr 2+ :ZnSe and spherical mirror reflectivity profiles.(B) Output power vs output wavelength of the tunable Cr 2+ :ZnSe laser at 12 W pump power; the grating is turned with 0.5 • steps.

CW hot-pressed ceramic Cr 2+ :ZnSe laser
High-quality transparent ceramic laser gain media have a number of potential advantages over conventional solid-state gain elements: absence of internal stress typical for single crystals; negligible scattering losses common for polycrystalline media such as ZnSe; flexibility in the spatial distribution of the gain centers, which allows for efficient compensation of thermal lensing effects (which are very problematic for conventional Cr 2+ :ZnSe media).Therefore, fabrication of high-quality Cr 2+ :ZnSe ceramic gain media is of a great interest for development of advanced mid-IR laser systems.Recently, we demonstrated a gain-switched hot-pressed ceramic Cr 2+ :ZnSe laser in [19,20] (where one can find the details on fabrication of our ceramic Cr 2+ :ZnSe gain media).In this work we show our further advancements in this field and now we demonstrate a pure CW laser based on the same gain medium.The gain element, used in these experiments, is a Cr 2+ :ZnSe hot-pressed ceramic [19,20] slab with the sizes of 10 × 10 × 2 mm (10 mm is the length) which absorbs about 26% of 1.56 μm incident pump power (the absorption coefficient k ≈ 1.3 cm −1 ).The laser is based on the standard non-dispersive Kogelnik cavity shown schematically in Fig. 1(A) by the dashed lines.The gain element is uncoated and installed at the Brewster angle for the horizontal polarization.Due to low absorption of the pump radiation the heat load on the gain element is very small and thus it is cooled conductively with a simple copper block mount.
The output power versus absorbed pump power of the Cr 2+ :ZnSe ceramic laser for 2 different output couplers (90% and 95%) is shown in Fig. 6.One can see in the figure that the absorbed power optical efficiency is the same as the real efficiency of polycrystalline Cr 2+ :ZnSe lasers for the same output couplers (see Fig. 3), which indicates a high quality of the ceramic media and low scattering losses comparable to polycrystalline Cr 2+ :ZnSe gain media.

Discussion and conclusions
In this work we used two approaches for Cr 2+ :ZnSe thermal lens management.For the laser systems where high beam quality is required and relatively small output powers are sufficient, short gain elements are used, with the optical length comparable to the laser mode Rayleigh range (i.e., dn ≤ πw 2 0 /λ , where d is the physical length of the gain element, n is its refractive index, w 0 the mode waist radius, and λ is the lasing wavelength).When such a short gain element is installed into the laser mode waist, the influence of the thermal lens on the transverse mode structure is significantly reduced and single-transverse-mode of operation with low divergence is achievable.This approach was used for the single-longitudinal-mode laser described in Section 2. On the other hand, relatively long gain elements (where large gain volume is achievable) are more suitable for obtaining multi-Watt output powers.In this case the thermal lensing effects can be reduced by proper cooling of the laser crystal.Therefore, the second approach, which was used for thermal lens management of the high-power laser systems, consists of mounting the gain element slab between thermally connected and cooled copper heat sinks, as described in Section 3.
The uniform Cr doping of the ZnSe media is critical for obtaining multi-Watt output powers.In our previous experiments [19] we worked with Cr 2+ :ZnSe crystals which were diffusiondoped from Cr gas phase.Those crystals demonstrate either insufficient Cr concentration and low pump absorption (making them non-suitable for high-power lasers) or suffer from very large Cr gradient in the vertical direction (perpendicular to the cavity optical axis).In the second case Cr ions are concentrated within a thin (200-300 μm) layer near one of the crystal surfaces.As a result, optimal Cr concentration layer is located close to the gain crystal surface and the laser mode experiences diffraction losses when a long crystal (longer than ≈ 0.5 cm) is used.Furthermore, the thermal lens is strongly non-symmetric due to high Cr gradient and is hardly manageable.These factors limited the laser output power that we could obtain before (up to 2.7 W) [19].The main recent improvement of the Cr 2+ :ZnSe crystal quality is the uniform Cr distribution which was obtained by diffusion-doping of ZnSe media from Cr metallic phase [21].The new crystals with uniform Cr distribution and high Cr concentration do not suffer from the disadvantages mentioned above and allow for obtaining of much higher output powers demonstrated in this work.
It is noteworthy that as a result of our numerous experiments with Cr 2+ :ZnSe gain media we found that there are no significant advantages of using single-crystalline over the polycrystalline ZnSe host material.However, from practical point of view, the technology for polycrystalline active elements is much cheaper, and high-quality undoped polycrystalline ZnSe material is widely available in various sizes for further thermal-diffusion Cr doping.For these reasons, all our experiments with high-power Cr 2+ :ZnSe lasers, where relatively long gain elements are required, were conducted with the polycrystalline Cr 2+ :ZnSe gain media.
The laser described in Section 6 is based on exactly the same laser cavity as the multiwatt polycrystalline Cr 2+ :ZnSe laser.However, in this case the gain element is made of a new Cr 2+ :ZnSe laser material: hot-pressed Cr 2+ :ZnSe ceramic gain media.It is noteworthy that the technology of hot-pressed Cr 2+ :ZnSe ceramic is in the very early stage of development [19].The used ceramic samples were fabricated with relatively low Cr concentration for spectroscopic characterization and optimization of the technological processes.For this reason in our current laser experiments with the Cr 2+ :ZnSe ceramic media we used the available ceramic gain element with significantly lower Cr doping level as compared to the Cr 2+ :ZnSe polycrystalline samples.This leads to much higher CW lasing threshold and much lower output power as compared to the polycrystalline Cr 2+ :ZnSe.
In conclusion, significant progress in technology of fabrication of high-quality uniformly doped Cr 2+ :ZnSe gain elements in combination with proper thermal management and cavity design enabled demonstration of multi-watt, tunable CW Cr 2+ :ZnSe lasers with record output characteristics.The Cr 2+ :ZnSe lasers came of age and can be considered as sources of choice for compact efficient, low-cost, reliable multi-watt lasers, broadly tunable over 1.9-3.1 μm spectral range.Further improvements in fabrication of thermally-diffusion-doped Cr 2+ :ZnSe polycrystalline gain media as well as hot-pressed ceramic gain elements will allow for building full range of high-power advanced laser systems operating in the mid-IR spectral range.
Fig. 3. (A) Input-output characteristics of the high-power Cr 2+ :ZnSe laser for several output couplers pumped by the 9 W polarized Er-fiber laser.(B) The input-output characteristics with unpolarized Er-fiber pump laser.