Highly-efficient, widely-tunable, mid-IR Cr:ZnS and ... - OSA Publishing

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Highly-efficient, widely-tunable, mid-IR Cr:ZnS and Cr:ZnSe CW lasers pumped by 1685 nm InP laser diode Igor S. Moskalev1, Vladimir V. Fedorov1, and Sergey B. Mirov1,2 1

Center for Optical Sensors and Spectroscopies and the Department of Physics, University of Alabama at Birmingham, CH 310, 1300 University Blvd., Birmingham, AL 35294, USA 2

Photonics Innovations, Inc, 1500 1st Ave N Suite L108, Birmingham, AL 35203

Abstract: We demonstrate compact, highly-efficient, widely-tunable, CW Cr2+:ZnSe and Cr2+:ZnS lasers (35% and 44% slope efficiencies, 2200-2650 nm, and 2100-2650 nm tuning ranges, respectively) pumped by a single-emitter 1.5 W 1685 nm InP semiconductor laser diode. © 2010 Optical Society of America OCIS codes: (140.3580) Lasers, solid-state; (140.3600) Lasers, tunable; (140.3070) Infrared and far-infrared lasers; (140.5680) Rare-earth and transition-metal solid-state lasers;

1. Introduction Scientific and industrial applications (such as atmospheric sensing, eye-safe non-invasive medical laser diagnostics, eye-safe laser radar and remote sensing of atmospheric constituents, free-space optical communications, and numerous military applications) generate a growing interest in high power, tunable mid-infrared (mid-IR) solid-state lasers. Room-temperature mid-IR lasing has been reported for Cr2+:ZnS [1,2,3], Cr2+:ZnSe, Cr2+:Cd1-xMnxTe [4,5], and Cr2+:CdSe [6] crystals. Direct diode excitation [7,8], continuous wave (CW) lasing with efficiency exceeding 60% [8,9], high CW output power levels up to 1.8 W [10] with fiber-laser pumping, gain switched lasing with average output power of up to 18.5 W [11], exceptionally broad wavelength tunability over the 1880-3100 nm [12] spectral interval, and narrow-linewidth operation [13], have been reported. Despite the most challenging task of thermal lens management in Cr2+:ZnS and Cr2+:ZnSe gain media [14] we have recently demonstrated record levels of output power in polycrystalline Cr2+:ZnSe (up to 12 W) and Cr2+:ZnS (up to 10 W) lasers pumped by Tm-fiber and Er-fiber lasers, respectively [15, 16]. As it was demonstrated in those works, fiber lasers are probably the most suitable pump sources for high-power, widely-tunable, and singlefrequency Cr2+:ZnS and Cr2+:ZnSe mid-IR laser systems due to a number of their unique properties, in particular: practically unlimited output powers, exceptionally high beam quality (thus high brightness), output wavelengths located near the absorption maxima of the mid-IR gain materials, and portability. There is, however, a range of applications where compactness and low power consumption are strictly required properties of the mid-IR laser sources, while low- and mid-range output powers of up to a few hundred of mW are sufficient. Such applications in include (but not limited to) embedded mid-IR seed laser sources, compact sensor laser devices, low-power broadband and narrowband mid-IR laser devices. In this cases, low-power middle-infrared laser diodes find their applications as the pump sources of Cr2+:ZnS and Cr2+:ZnSe laser systems. In this work we demonstrate our recent results on development of compact, widely-tunable Cr2+:ZnS and 2+ Cr :ZnSe longitudinally-pumped by a single-emitter 1.5 W InP laser diode operating at 1685 nm. First, we investigate the performance of these lasers in non-dispersive cavities and demonstrate the output powers of 400 mW and 500 mW, and slope efficiencies of 35% and 44%, for Cr2+:ZnSe and Cr2+:ZnS lasers, respectively. Secondly, we perform an analysts of wavelength tunability of these laser sources and show the tunability ranges of 2200-2650 nm and 2100-2650 nm for Cr2+:ZnSe and Cr2+:ZnS lasers, respectively (limited only by available optical components). Finally, we discuss the properties of these laser systems specific to diode pumping and compare them to the fiberpumped sources. 2. Experimental setup and results The Cr2+:ZnS/ZnSe CW lasers are based on a simple linear cavity shown schematically in Fig. 1. The cavity consists of a flat dichroic input mirror, AR-coated for 1500-1950 nm pump radiation and HR-coated for 2100-2700 nm spectral range; AR-coated intracavity 20 mm CaF2 lens, and the output coupler, which can be either a flat mirror or a


Littrow-mounted diffraction grating. The polycrystalline 8.9×7.0×2.6 mm (L×W×H) Cr2+:ZnS/ZnSe gain elements are AR-coated for 2000-3000 nm spectral range and mounted on a TEC-cooled heat-sink in close vicinity to the input mirror. The gain elements are pumped along the 8.9 mm dimension with a linearly polarized, collimated , 1.5 W, 1685 nm, InP semiconductor diode laser module (model ALC-SE-MM-CL, AKELA Laser Corporation). The pump beam is focused into a gain element with AR-coated 15 mm pump lens.

Fig. 1 Schematic diagram of the Cr2+:ZnS and Cr2+:ZnSe diode-pumped lasers The total cavity length is 7 cm, and the cavity configuration is found experimentally to obtain highest output power and efficiency at the maximum available pump of 1.5 W. For wavelength tuning experiments the output coupler (OC) is replaced with a Littrow-mounted diffraction grating.

The input-output characteristics of the lasers are shown in Fig. 2. The figure shows the dependence of the measured output power versus the pump power incident onto the laser crystal. The lasers demonstrate maximum of 35% and 44% slope, and 24% and 31% real optical efficiencies for Cr2+:ZnSe and Cr2+:ZnS gain elements, respectively. The measurements were performed by aligning the laser cavity for the maximum output power at the maximum pump and then reducing the pump power.

Fig. 2. Output power vs incident pump power. The left graph shows the performance of the Cr2+:ZnSe laser, and the right graph corresponds to the performance of the Cr2+:ZnS laser system, respectively. The input-output characteristics are shown for 3 different output couplers (with transmissions of 8.6%, 27%, and 46%) in non-dispersive laser configuration. The highest slope and real optical efficiencies are obtained with the 27% OC.

The experimental results on the wavelength tunability of the Cr2+:ZnSe/ZnS laser systems is shown in Fig. 3. The Cr2+:ZnSe laser is tunable over 2200-2650 nm spectral range and shows the maximum output power of ~120 mW around 2470 nm. The Cr2+:ZnS laser is tunable over 2100-2650 nm spectral range and shows the maximum output power of ~320 mW around 2380 nm. The tuning ranges are mainly limited by the transmission curve of the input mirror (2100-2650 nm by design).


Fig. 3. Tuning curves of the Cr2+:ZnSe (red squares) and Cr2+:ZnS (blue circles). The tuning ranges are mainly limited by the transmission curve of the input mirror (2100-2650 nm by design).

4. Conclusions In conclusion, we have demonstrated Cr2+:ZnS and Cr2+:ZnSe tunable, mid-power, compact mid-IR laser systems pumped by a single-emitter InP semiconductor laser operating at 1685 nm. The Cr2+:ZnSe laser shows up to 370 mW output power at 2500 nm at 1.5 W of maximum available pump in non-dispersive cavity and is tunable over 2200-2650 nm spectral range. The Cr2+:ZnS laser shows up to 500 mW output power at 2400 nm at 1.5 W of maximum available pump in non-dispersive cavity and is tunable over 2100-2650 nm spectral range. 6. References 1. 2. 3. 4. 5. 6. 7.

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