tunable, single-frequency, and multi-watt continuous-wave cr^2+:znse lasers

Tunable, Single-Frequency, andMulti-Watt Continuous-Wave

Cr2+:ZnSe Lasers

I. S. Moskalev, V. V. Fedorov and S. B. MirovUniv. of Alabama at Birmingham, Department of Physics, 310 Campbell Hall, 1300

University Blvd., Birmingham, AL 35294

Phone: (205)-934-5318, Fax: (205)-934-8042, E-mail: [email protected]

Abstract: We demonstrate CW rapidly-tunable (4.5 μm/s), high-power (150 mW), single-longitudinal-mode (120 MHz) single-crystallineCr2+:ZnSe laser; CW widely-tunable (2.12–2.77 μm), multi-watt (2 Wover 2.3–2.7 μm), polycrystalline Cr2+:ZnSe laser; CW multi-watt (6 W, at2.5 μm), highly efficient (48% real efficiency) polycrystalline Cr 2+:ZnSelaser; CW multi-watt (3 W, at 2.5 μm), highly efficient (41% real effi-ciency) ultra-compact polycrystalline Cr2+:ZnSe laser; and CW hot-pressedceramic Cr2+:ZnSe laser.

© 2008 Optical Society of America

OCIS codes: (140.3580) Lasers, solid-state; (140.3600) Lasers, tunable; (140.3070) Infraredand far-infrared lasers; (140.5680) Rare-earth and transition-metal solid-state lasers;

References and links1. L. D. DeLoach, R. H. Page, G. D. Wilke, S. A. Payne, and W. F. Krupke, “Transition metal-doped zinc chalco-

genides: spectroscopy and laser demonstration of a new class of gain media,” IEEE J. Quantum Electron. 32,885–895 (1996).

2. R. H. Page, K. I. Schaffers, L. D. DeLoach, G. D. Wilke, F. D. Patel, J. B. Tassano, S. A. Payne, W. F. Krupke,K. T. Chen, and A. Burger, “Cr2+-doped zinc chalcogenides as efficient, widely tunable mid-infrared lasers,”IEEE J. Quantum Electron. 33/4, 609–619 (1997).

3. S. B. Mirov, V. V. Fedorov, K. Graham, I. S. Moskalev, I. T. Sorokina, E. Sorokin, V. Gapontsev, D. Gapontsev,V. V. Badikov, and V. Panyutin, “Diode and fibre pumped Cr2+:ZnS mid-infrared external cavity and microchiplasers,” IEE Optoelectronics 150, 340–345 (2003).

4. S. B. Mirov, V. V. Fedorov, K. Graham, and I. S. Moskalev, “Erbium fiber laser-pumped continuous-wave mi-crochip Cr2+:ZnS and Cr2+:ZnSe lasers,” Opt. Lett. 27, 909–911 (2002).

5. I. T. Sorokina, E. Sorokin, S. B. Mirov, V. V. Fedorov, V. Badikov, V. Panyutin, and K. Schaffers, “Broadlytunable compact continuous-wave Cr2+:ZnS laser,” Opt. Lett. 27, 1040–1042 (2002).

6. I. T. Sorokina, E. Sorokin, S. B. Mirov, V. V. Fedorov, V. Badikov, V. Panyutin, A. DiLieto, and M. Tonelli,“Continuous-wave tunable Cr2+:ZnS laser,” Appl. Phys. B, Laser Opt. 74, 607–611 (2002).

7. U. Hommerich, X. Wu, V. R. Davis, S. B. Trivedi, K. Grasza, R. J. Chen, and S. Kutcher, “Demonstration ofroom temperature laser action at 2.5μm from Cr2+:Cd0.85Mn0.15Te,” Opt. Lett. 22, 1180–1182 (1997).

8. U. Hommerich, J. T. Seo, M. Turner, A. Bluett, S. B. Trivedi, H. Zong, S. Kutcher, C. C. Wang, and R. J. Chen,“Mid-infrared laser development based on transition metal doped cadmium manganese telluride,” J. Lumin. 87-89, 1143–1145 (2000).

9. J. Mckay, K. L. Schepler, and G. C. Catella, “Efficient grating tuned mid-infrared Cr2+:CdSe laser,” Opt. Lett.24, 1575–1577 (1999).

10. R. H. Page, J. A. Skidmore, K. I. Schaffers, R. J. Beach, S. A. Payne, and W. F. Krupke, “Demonstrations ofdiode-pumped and grating tuned ZnSe:Cr2+ lasers,” in OSA Trends Opt. Photonics, pp. 208–210 (1997).

11. M. Mond, E. Heumann, G. Huber, H. Kretschmann, S. Kuck, A. V. Podlipensky, V. G. Shcherbitsky, N. V.Kuleshov, V. I. Levchenko, and V. N. Yakimovich, “Continuous-wave diode pumped Cr2+:ZnSe and high powerlaser operation,” in OSA Trends Opt. Photonics, Adv. Solid State Lasers, Vol. 46, pp. 162–165 (2001).

12. G. J. Wagner, T. J. Carrig, R. H. Page, K. I. Schaffers, J. O. Ndap, X. Ma, and A. Burger, “Continuous-wavebroadly tunable Cr2+:ZnSe laser,” Opt. Lett. 24, 19–21 (1999).

(C) 2008 OSA 17 March 2008 / Vol. 16, No. 6 / OPTICS EXPRESS 4145#91244 - $15.00 USD Received 2 Jan 2008; revised 7 Mar 2008; accepted 9 Mar 2008; published 12 Mar 2008

13. G. J. Wagner and T. J. Carrig, “Power scaling of Cr2+:ZnSe lasers,” in OSA Trends Opt. Photonics, Adv. SolidState Lasers, Vol. 50, pp. 506–510 (2001).

14. T. J. Carrig, G. J. Wagner, W. J. Alford, and A. Zakel, “Chromium-doped chalcogenides lasers,” in Proc. SPIE,Vol. 5460 of Solid State Lasers and Amplifiers, pp. 74–82 (2004).

15. U. Demirbas and A. Sennaroglu, “Intracavity-pumped Cr2+:ZnSe laser with ultrabroadband tuning range be-tween 1880 and 3100 nm,” Opt. Lett. 31, 2293–2295 (2006).

16. G. J. Wagner, B. G. Tiemann, W. J. Alford, and T. J. Carrig, “Single-frequency Cr:ZnSe laser,” in AdvancedSolid-State Photonics on CD-ROM, p. WB12 (2004).

17. I. S. Moskalev, V. V. Fedorov, and S. B. Mirov, “CW single frequency tunable, CW multi-Watt polycrystalline,and CW hot-pressed ceramic Cr2+:ZnSe lasers,” in Technical Digest in CDROM, CLEO’07, p. CTuN6 (Balti-more, MD, 2007).

18. K. L. Schepler, R. D. Peterson, P. A. Berry, and J. B. McKay, “Thermal Effects in Cr2+:ZnSe Thin Disk Lasers,”IEEE J. Sel. Top. Quantum Electron. 11, 713–720 (2005).

19. S. Mirov, V. Fedorov, I. Moskalev, and D. Martyshkin, “Recent progress in transition metal doped II-VI mid-IRlasers,” J. Spec. Top. Quantum Electron. 13(3), 810–822 (2007).

20. A. Gallian, V. V. Fedorov, S. B. Mirov, V. V. Badikov, S. N. Galkin, E. F. Voronkin, and A. I. Lalayants, “Hot-pressed ceramic Cr2+:ZnSe gain-switched laser,” Opt. Express 14, 11,694–11,701 (2006).

21. S. B. Mirov and V. V. Fedorov, “Mid-IR microchip laser: ZnS:Cr2+ laser with saturable absorber material,” USPatent 6960486 (2005).

1. Introduction

The presence of strong fundamental and overtone vibrational absorption lines of organic mole-cules within the so-called “molecular fingerprint” spectral region of 2–20 μm gives rise to alarge number of scientific and technological applications of the middle-infrared (mid-IR) lasersources. These applications include eye-safe laser radar and remote sensing of atmosphericconstituents, eye-safe medical laser sources for non-invasive medical diagnostics, eye-safe ef-ficient laser surgery, and numerous military applications. Consequently, high-power, narrow-linewidth, broadly-tunable mid-IR solid-state CW lasers operating over the entire molecularfingerprint 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 reportedfor Cr2+:ZnS [3–6], Cr2+:ZnSe, Cr2+:Cd1−xMnxTe [7, 8], Cr2+:CdSe [9] crystals. Recently,much attention of researchers was devoted to development of Cr 2+:ZnSe lasers due to excel-lent lasing and optical properties of this material in the mid-IR spectral region. The regimes oflaser operation of Cr2+:ZnSe were significantly extended after the first demonstrations of directdiode excitation [10, 11]: continuous wave lasing with efficiency exceeding 60% [11, 12] withpower 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 signifi-cant progress, the optimization of Cr2+:ZnSe crystal fabrication technology and design of lasersources capable of generating multi-watt output powers are still to be demonstrated. The max-imum reported output power of the chromium lasers in CW operation mode is 1.8 W [13] andis below the required level for many applications. On the other hand, although the tunability ofCr2+: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 wasbased 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 tun-ing in the SLM regime was not documented.

In this work we describe a range of high-power CW laser systems based on single-, polycrys-talline, and hot-pressed ceramic Cr2+:ZnSe gain media. We begin the paper with presentationof our single-longitudinal-mode (δλ ≈ 120 MHz), high-power (150 mW) laser rapidly-tunableover 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 lasersystems: (a) 6 W Kogelnik cavity-based, and (b) 3 W “mirror-less” ultra-compact polycrys-talline Cr2+:ZnSe lasers (demonstrating 48% and 41% real optical efficiencies, respectively).This breakthrough in Cr2+:ZnSe power-scaling was made possible primarily due to improvedtechnology 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 crys-tal fabrication are to be published in a separate paper elsewhere). We continue the topic ofpower-scaling of Cr2+:ZnSe lasers with presentation of our new 2-W Cr2+:ZnSe laser, “oneknob”-tunable from 2.12 to 2.77 μm. Finally, we show a pure CW laser based on hot-pressedCr2+:ZnSe ceramic.

2. Single-frequency, rapidly-tunable Cr2+:ZnSe laser

The CW Cr2+:ZnSe SLM laser is based on a modified Kogelnik/Littman cavity shown schemat-ically in Fig. 1(A). The cavity consists of 25 mm radius of curvature (ROC) input mirror, 50mm ROC folding mirror (both of which are AR/HR coated at 1.56 μm/2.0-3.0 μm, respec-tively), a highly efficient (50% into the 1st order at 75 ◦ incident angle) gold-coated reflectivediffraction grating (600 mm−1 groove frequency), and a flat tuning mirror, mounted on a piezo-driven fast mirror shaker. The shaker allows for fast tilting of the mirror around an axis lyingin its reflecting surface with a repetition rate of up to 250 Hz. The single-crystalline, 1.5 mmlong Cr2+:ZnSe gain element, which is installed in the laser cavity at the Brewster angle forhorizontal polarization, is mounted on a thermo-electrically/air cooled (TEC) copper block forits thermal stabilization.

The fine spectral structure of the laser output radiation, acquired with a scanning Fabry-Perotinterferometer (FPI), is shown in Fig. 1(B). The measured FPI finesse is about 7.3 and its baseis 160 mm (while the SLM laser optical length is 100 mm), and thus the 120 MHz linewidth isan upper estimate limited by the interferometer spectral resolution.

The laser wavelength can be rapidly scanned around a desired central wavelength within2.45–2.55 μm spectral range with a scanning amplitude of up to 20 nm and scanning speed ofup to 4.5 μm/s . In order to investigate the tuning characteristics of the laser, its intensity wassimultaneously monitored on the detectors D1 and D2, as shown in Fig. 1(A). The detector D1

shows the laser output intensity, while the signal from the detector D 2 is proportional to the FPItransmission and thus provides a frequency scale as the laser wavelength is rapidly tuned. Typ-ical output curves obtained in this way are demonstrated in Fig. 2, where a rapid tuning around2489 nm is shown. One can see in the figure that a scanning speed of 2.85 μm/s is achievedwith the total scanning interval of 20 nm. Reducing the scanning interval to 10 nm allows for ascanning speed of up to 4.5 μm/s. As the laser wavelength passes through absorption spectrallines of the atmospheric water vapor, the laser intensity drops. This way, the intracavity watervapor spectroscopy is performed. The laser delivers 150 mW output power at 6 W pump in theSLM regime of operation, which corresponds to 2.5% real optical efficiency limited mainly byinadequate efficiency of the Littman grating.

3. Multi-Watt, highly-efficient Cr2+:ZnSe laser

Power-scaling of Cr2+:ZnSe lasers is generally a challenging task mainly due to high thermallensing effects in this laser material [18]. Consequently, the reported maximum output powerof pure CW Cr2+:ZnSe lasers was limited to only 1.8 W [13]. An attractive approach wasdemonstrated in [18] where a face-cooled disk design enabled power-scaling of Cr 2+:ZnSe1 mm and 0.5 mm disk laser outputs to 4.2 W in 10 kHz repetition rate gain-switched, and1.4 W CW regimes, respectively. However, as we will demonstrate in this section, multi-wattoutput powers from Cr2+:ZnSe CW lasers can be obtained in the conventional slab geometry


d3

d4

d1d2

Cr2+:ZnSer2

r1

Grating 600g/mm

D2

D1

Pump

FPI

Tuning mirror

BS

(A)

0 500 1000

Frequency, MHz

0.0

0.5

1.0

FPI

tran

smis

sion

(B)

Fig. 1. (A) Optical scheme of the SLM Cr2+:ZnSe laser based on modified Kogel-nik/Littman cavity. The standard Kogelnik cavity is shown by dashed lines. Cavity parame-ters: r1 = 25 mm, r2 = 50 mm, d1 = d2 = 25 mm, d3 = 30 mm, d4 = 20 mm. Measurementsystem components: D1 and D2 are optical detectors, BS is beamsplitter, FPI is Fabry-Perot interferometer. (B) The fine structure of the laser output spectrum obtained with anFPI demonstrating ≈ 120 MHz laser linewidth.

0 0.002 0.004 0.006 0.008 0.01

Time, s

0

0.01

0.02

0.03

0.04

Det

ecto

r si

gnal

, V

Δλ~20 nm

D1 signal

D2 signal

Fig. 2. Rapid scanning of the SLM laser wavelength.


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 Cr2+:ZnSe laser is based on a 7×3×9 mm (9 mm is length) polycrystallineCr2+:ZnSe gain element pumped by 1.56 μm Er-fiber laser through the 3× 7 facets. The ab-sorption coefficient of this crystal at 1.56 μm is about 3 cm−1. The laser cavity is the standardnon-dispersive Kogelnik cavity shown schematically in Fig. 1(A) by the dashed lines. The gainelement 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 mountedbetween two thermally-connected copper cold plates (on the 9×7 mm surfaces) and the wholecopper block is cooled with a 20 W air-cooled Peltier element. We found, however, that ingeneral the gain element can be cooled 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 exper-iments: (a) polarized 9 W Er-fiber laser; in this case about 7.5 W reached the gain element dueto the loss caused by pump optics; (b) randomly-polarized 30 W Er-fiber laser. In the secondcase, due to high losses of the vertical polarization on the Cr 2+:ZnSe crystal Brewster inputfacet, and additional losses on the pump delivering optics, only about 12.5 W could be used forpumping.

1 2 3 4 5 6 7 8Incident Pump power, W

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Out

put P

ower

, W

OC 35%, ηslope

~ 51%, ηreal

~ 42%

OC 50%, ηslope

~ 49%, ηreal

~ 42%

OC 90%, ηslope

~ 14%, ηreal

~ 13.5%

(A)

2 4 6 8 10 12Incident pump power, W

0

1

2

3

4

5

6

7O

utpu

t pow

er, W

OC 50%, ηslope

~52 %, ηreal

~48 %

(B)

Fig. 3. (A) Input-output characteristics of the high-power Cr2+:ZnSe laser for several out-put couplers pumped by the 9 W polarized Er-fiber laser. (B) The input-output characteris-tics with unpolarized Er-fiber pump laser.

The power input-output characteristics of the laser are shown in Fig. 3. The left graph showsthe 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%, respec-tively) obtained with the high transmission OCs and the output power exceeds 3 W. Whenthe Cr2+:ZnSe laser is pumped with the unpolarized fiber laser, the maximum usable pump isabout 12.5 W and the Cr2+:ZnSe output power reaches a record 6 W output power, demonstrat-ing 52% slope and 48% real optical efficiencies, as one can see in Fig. 3(B). In all these casesthe laser operates in pure CW mode.

4. Multi-Watt highly-efficient ultra-compact Cr2+:ZnSe laser

The ultra-compact laser consists of only two elements: a flat input mirror (AR/HR-coated at1.56 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 inSection 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 theincident pump beam and the laser mode, which leads to some loss of the pump power as well asan intracavity losses at the lasing wavelengths due to Fresnel reflection on the input facet of thegain element. The flat input mirror and the input facet of the gain element form a low-finesseFPI, which allows to significantly reduce the intracavity losses by careful adjustment of the thinair gap between the input mirror and the crystal input facet. The cavity stability is achieved bythe positive thermal lens in the gain element. The output facet of the gain element serves as a18% (reflectivity) output coupler. The laser is pumped by the linearly-polarized 9 W Er-fiberlaser. 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. Onecan see that the laser operates around ∼ 2.4 μm and delivers up to 3 W output power at about7.5 W pump with 59% slope and 41% real optical efficiencies, respectively, and generatesa broadband 20 nm output spectrum. The broadband spectrum indicates a highly multimodeoperation of the laser, and direct measurements of the laser beam divergence with the knife-edge method show the laser beam quality factor M 2 ≈ 2.2. The roll-off of the output power canbe explained by a change of the thermal lens refractive power (and thus the stability conditionsof the laser resonator) as the pump grows. A flat input-output characteristics and higher laserefficiency can be obtained by depositing the input mirror directly onto the input facet of thegain crystal, which is a task of our future experiments with the microchip Cr 2+:ZnSe lasers ofthis type.

2 3 4 5 6 7 8Incident Pump power, W

0

1

2

3

Out

put P

ower

, W

ηslope

~ 59%, ηreal

~ 41%

(A)

2340 2360 2380Wavelength, nm

0

0.2

0.4

0.6

0.8

1

Inte

nsity

, a.u

.

λ~2.36 μmΔλ~20 nm

(B)

Fig. 4. (A) Output power vs pump power, and (B) Output spectrum of the CW ultra-compactCr2+:ZnSe laser.

5. High-power, widely tunable Cr2+:ZnSe laser

This pure CW widely-tunable laser is based on a 7×3×9 mm (9 mm is the length), uniformly-doped polycrystalline Cr2+:ZnSe crystal (this sample has exactly the same physical propertiesas the one used in the multi-Watt laser described in Section 3), and an X-type Littrow gratingcavity, shown schematically in Fig. 5(A). The 600 g/mm reflective diffraction grating is oper-ating in the Littrow configuration and provides wavelength tuning over a spectral interval of2.12–2.77 μm. 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 laserwavelength tuning is performed with “one knob” over the entire tuning range: no additionalalignments 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 deliversmore than 2 W output power at 12 W pump, depending on the operating wavelength. The laseroutput power as a function of output wavelength is shown in Fig. 5(B). The laser operates ina TEM00 mode with the maximum linewidth of 2 nm. One can see in the figure that the laseroutput power is between 1.5 W and 2.1 W in the 2.3–2.7 μm wavelength range and decreasesat other wavelengths following the gain curve of Cr 2+:ZnSe and spherical mirror reflectivityprofiles.

r2

r1

d2

d1

Cr2+:ZnSe

d3d4

Littrow Grating600 g/mm

Pump

~5% lossOC, R=70%

(A)

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8Wavelength, nm

0.0

0.5

1.0

1.5

2.0

Out

put P

ower

, W

(B)

Fig. 5. (A) Schematic diagram of the high-power, widely-tunable Cr2+:ZnSe laser. Thecavity parameters are: r1 = r2 = 50mm, d1 = d2 ≈ 24mm, d3 ≈ 150mm, d4 ≈ 100mm.(B) Output power vs output wavelength of the tunable Cr2+:ZnSe laser at 12 W pumppower; the grating is turned with 0.5◦ steps.

6. CW hot-pressed ceramic Cr2+:ZnSe laser

High-quality transparent ceramic laser gain media have a number of potential advantages overconventional solid-state gain elements: absence of internal stress typical for single crystals;negligible scattering losses common for polycrystalline media such as ZnSe; flexibility in thespatial distribution of the gain centers, which allows for efficient compensation of thermal lens-ing effects (which are very problematic for conventional Cr 2+:ZnSe media). Therefore, fab-rication of high-quality Cr2+:ZnSe ceramic gain media is of a great interest for developmentof advanced mid-IR laser systems. Recently, we demonstrated a gain-switched hot-pressed ce-ramic Cr2+:ZnSe laser in [19, 20] (where one can find the details on fabrication of our ceramicCr2+:ZnSe gain media). In this work we show our further advancements in this field and nowwe demonstrate a pure CW laser based on the same gain medium.

The gain element, used in these experiments, is a Cr2+:ZnSe hot-pressed ceramic [19, 20]slab with the sizes of 10× 10× 2 mm (10 mm is the length) which absorbs about 26% of1.56 μm incident pump power (the absorption coefficient k ≈ 1.3 cm −1). The laser is based onthe standard non-dispersive Kogelnik cavity shown schematically in Fig. 1(A) by the dashedlines. The gain element is uncoated and installed at the Brewster angle for the horizontal polar-ization. Due to low absorption of the pump radiation the heat load on the gain element is verysmall 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 differentoutput couplers (90% and 95%) is shown in Fig. 6. One can see in the figure that the absorbedpower optical efficiency is the same as the real efficiency of polycrystalline Cr 2+:ZnSe lasersfor the same output couplers (see Fig. 3), which indicates a high quality of the ceramic mediaand low scattering losses comparable to polycrystalline Cr2+:ZnSe gain media.


0 0.5 1 1.5 2Absorbed Pump power, W

0

50

100

150

200

250

300

Out

put P

ower

, mW

OC 90%, ηabs

~14%

OC 95%, ηabs

~11%

Ceramic: Output Power vs Absorbed Pump

Fig. 6. Output power vs absorbed pump power of the hot-pressed ceramic Cr2+:ZnSe CWlaser.

7. Discussion and conclusions

In this work we used two approaches for Cr2+:ZnSe thermal lens management. For the lasersystems 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 Rayleighrange (i.e., dn ≤ πw2

0/λ , where d is the physical length of the gain element, n is its refractiveindex, w0 the mode waist radius, and λ is the lasing wavelength). When such a short gain ele-ment is installed into the laser mode waist, the influence of the thermal lens on the transversemode structure is significantly reduced and single-transverse-mode of operation with low diver-gence is achievable. This approach was used for the single-longitudinal-mode laser describedin Section 2. On the other hand, relatively long gain elements (where large gain volume isachievable) are more suitable for obtaining multi-Watt output powers. In this case the thermallensing effects can be reduced by proper cooling of the laser crystal. Therefore, the second ap-proach, which was used for thermal lens management of the high-power laser systems, consistsof 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 diffusion-doped from Cr gas phase. Those crystals demonstrate either insufficient Cr concentration andlow pump absorption (making them non-suitable for high-power lasers) or suffer from verylarge Cr gradient in the vertical direction (perpendicular to the cavity optical axis). In the secondcase 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 thelaser 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 hardlymanageable. These factors limited the laser output power that we could obtain before (up to2.7 W) [19]. The main recent improvement of the Cr 2+:ZnSe crystal quality is the uniform Crdistribution 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 sufferfrom the disadvantages mentioned above and allow for obtaining of much higher output powersdemonstrated in this work.

It is noteworthy that as a result of our numerous experiments with Cr 2+:ZnSe gain media wefound 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 polycrystallineactive elements is much cheaper, and high-quality undoped polycrystalline ZnSe material iswidely available in various sizes for further thermal-diffusion Cr doping. For these reasons, allour experiments with high-power Cr2+:ZnSe lasers, where relatively long gain elements arerequired, 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 multi-watt polycrystalline Cr2+:ZnSe laser. However, in this case the gain element is made of a newCr2+:ZnSe laser material: hot-pressed Cr2+:ZnSe ceramic gain media. It is noteworthy that thetechnology of hot-pressed Cr2+:ZnSe ceramic is in the very early stage of development [19].The used ceramic samples were fabricated with relatively low Cr concentration for spectro-scopic characterization and optimization of the technological processes. For this reason in ourcurrent laser experiments with the Cr2+:ZnSe ceramic media we used the available ceramicgain element with significantly lower Cr doping level as compared to the Cr 2+:ZnSe polycrys-talline samples. This leads to much higher CW lasing threshold and much lower output poweras compared to the polycrystalline Cr2+:ZnSe.

In conclusion, significant progress in technology of fabrication of high-quality uniformlydoped Cr2+:ZnSe gain elements in combination with proper thermal management and cavitydesign enabled demonstration of multi-watt, tunable CW Cr2+:ZnSe lasers with record outputcharacteristics. The Cr2+:ZnSe lasers came of age and can be considered as sources of choicefor compact efficient, low-cost, reliable multi-watt lasers, broadly tunable over 1.9–3.1 μmspectral range. Further improvements in fabrication of thermally-diffusion-doped Cr 2+:ZnSepolycrystalline gain media as well as hot-pressed ceramic gain elements will allow for buildingfull range of high-power advanced laser systems operating in the mid-IR spectral range.

8. Acknowledgments

We acknowledge support from the National Science Foundation Grants No. ECS-0424310,EPS-0447675, and BES-0521036.


tunable, single-frequency, and multi-watt continuous-wave cr^2+:znse lasers

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