Supplemental Homework 6a 

(Voluntary, for those who would like to improve the 

homework performance, due November 27, 2017) 

Is it possible to design Cr:ZnSe radiation pumped CW RT Fe:ZnSe laser? 

We want to find the following: 1) Saturation intensity at pump and lasing wavelength

2) Effective length of the crystal

3) Optimal output coupler

4) Survival (photon) factor S for the passive cavity

5) Threshold power for a mode diameter of 100 m6) Pump power necessary for providing 30W of 4400nm output power

Assumptions 1) Lifetime at 300K 380 ns 

2) Quantum efficiency of fluorescence 380/55000=0.7% 

3) Thermal management issues are taken care of with the use of spinning ring/cavity architecture

(see attached article)

4) Fe:ZnSe is described by an ideal four level scheme

5) Wavelength of the pump beam 2800 nm 

6) Absorption cross section at room temperature at 2800 nm 0.9x10‐18 cm2 

7) Wavelength of laser beam 4400 nm 

8) Emission cross section at 4400 nm at RT 1.1x10‐18 cm2 

9) Fe:ZnSe index of refraction 2.4 

10) Pump beam waist (radius) 0.05 mm 

11) Wavelength of laser beam 4400 nm 

12) Coefficient of passive losses in the Fe:ZnSe crystal 0.5 cm‐1 

13) Fe concentration 1x1019 cm‐3 

140 W Cr:ZnSe laser system

IGOR MOSKALEV,1,* SERGEY MIROV,1,2 MIKE MIROV,1 SERGEYVASILYEV,1 VIKTOR SMOLSKI,1 ANDREY ZAKREVSKIY,1 AND VALENTINGAPONTSEV3

1IPG Photonics - Mid-Infrared Lasers, 1500 1st Ave N, Birmingham, AL 35203, USA2Center for Optical Sensors and Spectroscopies, University of Alabama at Birmingham, 1530 3rd AvenueSouth, Birmingham, AL 35294, USA3IPG Photonics Corporation, 50 Old Webster Rd, Oxford, MA 01540, USA*moskalev@ipgphotonics.com

Abstract: We report a significant breakthrough in the development of fiber-pumped high-powerCW laser systems based on Cr2+:ZnS and Cr2+:ZnSe gain media. We demonstrate output powerlevels of up to 140 W near 2500 nm, and 32 W at 2940 nm with corresponding optical efficien-cies of 62% and 29%. Our novel approach is based on rapid simultaneous scanning of thecollinear laser mode and pump beam across the Cr:ZnS/Se gain element which allows us to vir-tually eliminate thermal lensing effects and obtain unprecedented levels of output power withvery high optical-to-optical efficiency.

© 2016 Optical Society of America

OCIS codes: (140.3580) Lasers, solid-state; (140.5680) Rare earth and transition metal solid-state lasers; (140.3410)

Laser resonators.

References and links1. U. Demirbas and A. Sennaroglu, “Intracavity-pumped Cr2+:ZnSe laser with ultrabroadband tuning range between

1880 and 3100 nm,” Opt. Lett. 31, 2293–2295 (2006).2. E. Sorokin, I. T. Sorokina, M. S. Mirov, V. V. Fedorov, I. S. Moskalev, and S. B. Mirov, “Ultrabroad continuous-

wave tuning of ceramic Cr:ZnSe and Cr:ZnS lasers,” in Adv. Solid-State Photon. (Optical Society of America, 2010),paper AMC.

3. 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(6),885–895 (1996).

4. R. H. Page, K. I. Shaffers, 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 sinc chalcogenides as efficient, widely tunable mid-infrared lasers,” IEEE J.Quantum Electron. 33(4), 609–619 (1997).

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

6. T. J. Carrig, G. J. Wagner, A. Sennaroglu, J. Y. Jeong, and C. R. Pollock, “Mode-locked Cr2+:ZnSe laser,” Opt. Lett.25, 168–170 (2000).

7. E. Sorokin, I. T. Sorokina, and R. H. Page, “Room-temperature cw diode pumped Cr2+:ZnSe laser,” in OSA TrendsOpt. Photon. 46, 101–105 (2001).

8. T. J. Carrig, G. J. Wagner, W. J. Alford, and A. Zakel, “Chromium-doped chalcogenides lasers,” Proc. SPIE, 5460,74–82 (2004).

9. M. N. Cizmeciyan, H. Cankaya, A. Kurt, , and A. Sennaroglu, “Kerr lens mode-locked femtosecond Cr2+:ZnSelaser at 2420 nm,” Opt. Lett. 34, 3056–3058 (2009).

10. P. Moulton and E. Slobodchikov, “1-GW-peak-power, Cr:ZnSe laser,” in CLEO: Appl. Technol. 2010, paperPDPA10.

11. N. Coluccelli, M. Cassinerio, P. Laporta, and G. Galzerano, “100 kHz linewidth Cr2+:ZnSe ring laser tunable from2.12 to 2.58 µm,” Opt. Lett. 37(24), 5088–5090 (2012).

12. S. Mirov, V. Fedorov, D. Martyshkin, M. M. I. Moskalev, and S. Vasilyev, “Progress in mid-IR lasers based on Crand Fe doped II-VI chalcogenides,” IEEE J. Sel. Topics Quantum Electron. 21, 1601719 (2015).

13. S. B. Mirov, V. V. Fedorov, I. S. Moskalev, and D. V. Martyshkin, “Recent progress in transition metal doped II-VImid-IR lasers,” IEEE J. Sel. Topics in Quantum Electron. 13, 810–822 (2007).

14. K. L. Schepler, R. D. Peterson, P. A. Berry, and J. B. McCay, “Thermal effects in Cr2+:ZnSe thin disk laser,” IEEEJ. Quantum Electron. 11(3), 713–720 (2005).

15. J. McKay, W. Roh, and K. Schepler, “Thermal lensing in Cr2+:ZnSe face-cooled disks,” in OSA Trends in Opticsand Photonics, vol. 83 of Advanced Solid-State Photonics, J. Zayhowski, ed. (2003), paper 220.

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#272061 Journal © 2016

http://dx.doi.org/10.1364/OE.24.021090 Received 27 Jul 2016; revised 19 Aug 2016; accepted 19 Aug 2016; published 1 Sep 2016

16. H. Krol, C. Grezes-Besset, L. Gallais, J. Natoli, and M. Commandre, in “Study of laser-induced damage at 2 micronson coated and uncoated ZnSe substrates,” Proc. SPIE 6403, 640316(2007).

17. S. McDaniel, D. Hobbs, B. MacLeod, E. Sabatino, P. Berry, K. Schepler, W. Mitchell, and G. Cook, “Cr:ZnSe laserincorporating anti-reflection microstructures exhibiting low-loss, damage-resistant lasing at near quantum limitefficiency,” in “Opt. Mater. Express”, 4, 2225–2232 (2014).

18. IPG Photonics, “Specifications sheets of Mid-IR lasers,”http://www.ipgphotonics.com/Mid_ir_lasers.htm.

19. S. Mirov, V. Fedorov, D. Martyshkin, I. Moskalev, M. Mirov, and V. Gapontsev, “Progress in Cr and Fe doped ZnSeand ZnS polycrystalline materials and lasers,” in Advanced Solid State Lasers, OSA Technical Digest, (OpticalSociety of America, 2014), Paper AM4A.

20. H. H. Li, “Refractive index of ZnS, ZnSe, ZnTe and its wavelength and temperature derivatives,” J. Phys. Chem.13(1), 103–150 (1984).

21. V. V. Fedorov, I. S. Moskalev, M. S. Mirov, S. B. Mirov, T. J. Wagner, M. J. Bohn, P. A. Berry, and K. L. Schepler,“Energy scaling of nanosecond gain-switched Cr2+:ZnSe lasers,” Proc. SPIE 7912, 79121E (2011).

22. S. Basu and R. L. Bayer, “Diode-pumped moving disk laser: a new configuration for high average power generation,”Opt. and Quant. Electron. 22, 833–837 (1990).

23. S. Basu, “Nd-YAG and Yb-YAG rotary disk lasers,” IEEE J. Selected Topics in Quant. Electron. 11, 626–630(2005).

24. S. Basu, “A 7.5-mJ, 21-ns, 7-kHz green rotary disk laser with diffraction limited beam quality,” in Solid State LasersXXV: Technology and Devices, Proc. SPIE, 9726-22 (2016).

25. “Gaussian beams and the knife-edge measurement,”http://massey.dur.ac.uk/resources/grad_skills/KnifeEdge.pdf.

26. S. Vasilyev, I. Moskalev, M. Mirov, S. Mirov, and V. Gapontsev, “Multi-watt mid-ir femtosecond polycrystallineCr2+:ZnS and Cr2+:ZnSe laser amplifiers with the spectrum spanning 2.0–2.6 µm,” Opt. Express 24, 1616–1623(2016).

1. Introduction

There is a rapidly growing demand for high-power Cr2+:ZnS and Cr2+:ZnSe lasers operatingin the mid-IR spectral range of 1900–3400 nm [1, 2] for numerous applications in the fields ofindustrial material processing (e.g. fast cutting, welding, marking of polymers near 2400 nmwavelength region, processing of glasses in the range of 2700–2950 nm), laser surgery near2940 nm, and science. These promising laser optical material were introduced to the scientificcommunity as early as 1996 by Lawrence Livermore National Laboratory [3, 4]. Since then,and noticeably in recent years, many Cr2+:ZnS and Cr2+:ZnSe laser systems have been demon-strated covering a broad range of regimes of operation and output parameters [5–11]. However,in order to be successful in the industrial applications, where high throughput is critical, mid-IRlaser systems must deliver tens and hundreds of Watts of output power with very high beamquality and overall system reliability.

Availability of high-power fiber lasers operating in the 1.5–1.9 µm spectral range and ourcore technology of manufacturing high-quality large size Cr2+:ZnS and Cr2+:ZnSe gain ele-ments have resulted in significant progress in the development of practical mid-IR laser systemsbased on these laser gain media [12]. However, due to the inherent properties of these opticalmaterials, the maximum output power with practically acceptable beam quality was limited toapproximately 10 W for 2.4–2.5 µm lasers and 2 W for 2.94 µm laser systems [13]. The mainlimiting factors for obtaining high output power in Cr2+:ZnS and Cr2+:ZnSe lasers based oneither conventional slab gain media or thin disk geometries are very strong thermal lensing ef-fects [14, 15] and relatively low optical damage threshold [16, 17]. To manage thermal effectsof thin disks one has to use small disk thickness which results in small optical density, thusmitigating efforts of power scaling.

The novel approach to significantly resolve these challenges, which we demonstrate in thiswork, consists of performing a simultaneous rapid infinite transverse scan of the coaxial lasermode and pump beam across the gain element along a closed or open path. Here we describeour proof of concept experiments and demonstrate previously unseen levels of output powersfrom Cr2+:ZnSe CW laser systems. We experimentally consider several laser configurations and

Vol. 24, No. 18 | 5 Sep 2016 | OPTICS EXPRESS 21091

resonator designs, including standalone master oscillators and power amplifier systems (MOPA).We also demonstrate various pump configurations based on single- and multi-pump sources.

2. Technology background and first proof of concept experiments

In recent years we have developed and delivered a broad range of high-power CW laser systemsbased on Cr2+:ZnS and Cr2+:ZnSe gain media, which include but are not limited to: widely-tunable single-frequency lasers with a maximum output power of up to 5 W and a tuning rangeof up to 550 nm; narrow-line widely tunable lasers with typical linewidths of <0.25 nm, tuningranges well over 1000 nm, and maximum output power reaching 10 W level; multiple modelsof fixed-wavelength lasers both in single-frequency and narrow-line configurations with multi-Watt output power [18]. To further boost the output power of these laser systems we exploredvarious configurations based on resonator designs with multiple gain elements and multistagepower amplifiers and obtained some promising preliminary results [19].

Unfortunately, due to relatively high thermo-optic coefficients in host ZnS and ZnSe materials[20], obtaining tens of Watts output power in conventional slab or rod gain element geometrieshave proved to be very difficult: at high levels of pump power thermal lensing rapidly leads tosignificant mode distortion as the pump power is increased, which results in rapid heat build-up in the active region, significant degradation of laser efficiency due to growing non-radiativerelaxation, unusable output beam quality, and eventually optical damage of the gain element.Based on numerous experiments and tests of our standard and experimental laser systems witha large variety of gain element optical densities and sizes we found that ∼ 10 W to ∼ 15 W ofabsorbed pump power per gain element is the upper acceptable limit for all practical purposes(the exact limit depends on pump wavelength, mid-IR laser type, output wavelength and tuningrange). These difficulties called for active exploration of alternative methods and laser designsfor power scaling of Cr2+:ZnS and Cr2+:ZnSe based laser systems.

Our novel approach to virtually eliminate thermal lensing effects in the Cr2+:ZnS andCr2+:ZnSe gain media is based on a simple observation that the strength of the thermal lensis determined by the total thermal energy absorbed by the volume of the gain element beingpumped. Indeed, if the heat energy absorbed by the active region of the gain element is suffi-ciently small, the local refractive index change is insignificant, and the thermal lens is so weakthat it does not have any noticeable effect on laser performance. On the other hand, the totalabsorbed thermal energy is directly proportional to the average power of the pump radiation.This observation is clearly supported by numerous experiments with QCW lasers operating atlow duty cycles as well as gain-switch lasers where we observe hundreds of Watts, kW or evenMW peak power levels with very high optical to optical efficiencies and high beam quality [21].

The proposed method to reduce heating of the active region of the Cr2+:ZnS or Cr2+:ZnSegain element is to perform a simultaneous continuous rapid scan of the collinear pump beam andlaser mode across the gain crystal with a sufficiently high speed to keep the instantly absorbedpump power below a certain level. In this case we expect to obtain highly-efficient CW lasingwith any level of output power while maintaining good beam quality.

In order to investigate the feasibility of this approach and test out proposition, we first con-ducted a simple experiment with the laser setup shown schematically in Fig. 1. In this lasersetup, a conventional slab Cr2+:ZnSe AR-coated gain element is mounted on a water-cooledcopper heatsink and we scan the collinear pump beam and laser mode with the help of an oc-tagon mirror.

Due to the relatively high intracavity total losses caused by double-reflections from machinedaluminum surfaces of the octagon and gold mirrors of the angular retroreflector, the laser hasrelatively low efficiency (which in fact mimics Cr2+:ZnSe laser performance near 2940 nm andother challenging spectral regions where the gain is relatively low). Nevertheless, this experi-ment verifies the premise of our approach and allows us to practically investigate its potential,

Vol. 24, No. 18 | 5 Sep 2016 | OPTICS EXPRESS 21092

Pump/Cavity LensDichroic Mirror

OC

End Mirror

Cr:ZnSe

Angular Reflector

Octagon Mirror

12

12

Pump Beam

Output

Fig. 1. Design of proof-of-concept laser resonator. The laser resonator is formed by theend mirror, cavity/pump lens, dichroic mirror, and output coupler (OC). The pump beamand laser mode are collinear and coupled through the dichroic mirror (AR@1908 nm,HR@2400–3000 nm). The spinning octagon mirror and fixed angular retro-reflector scanthe laser mode and the pump beam across the AR-coated Cr2+:ZnSe gain element at nor-mal incidence. Two octagon angular positions, corresponding to extreme locations of thelaser mode and pump beam in the gain element, are shown.

limitations, and requirements on the parameters of a practical laser system. The performancecomparison of this laser in pure CW regime of operation (scanner is OFF) and QCW mode(scanner is ON) is illustrated by the input-output characteristics shown in Fig. 2.

0 5 10 15 20 25 30 35 40 45 50 55 60Incident pump power, W

0

2

4

6

8

10

12

14

16

Peak

out

put p

ower

, W

QCWCW

0 2 4 6 8 10Time, ms

0

0.5

1

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al, a

.u.

Fig. 2. Input-Output characteristics of pure CW (scanner is OFF) and QCW (scanners isON) regimes of operation of the laser system shown in Fig. 1. In QCW regime of operationthe duty cycle is ∼9%, linear scan speed is ∼1 m/s. The graph insert shows QCW tracerecorded with a fast optical detector.

One can see from Fig. 2 that in pure CW regime of operation, corresponding to the conven-tional gain element slab geometry, the laser demonstrates a rapid power rollover just above 15 W

Vol. 24, No. 18 | 5 Sep 2016 | OPTICS EXPRESS 21093

of pump power. Further pump increase leads to a complete laser shutoff and eventually resultsin surface and internal optical damage of the gain element.

In sharp contrast, when the pump beam is rapidly scanned across the gain element (QCWregime in this case), the laser accepts much higher peak pump power and generates much higherinstant output power. Consequentially, if we manage to construct a system with an effectivelyinfinite and uninterrupted scan of the pump beam across the gain element we can expect toobtain very high pure CW powers limited only by the available pump sources.

3. Spinning ring gain element approach

In order to implement this idea and thoroughly investigate the limitations and capabilities ofthis approach we manufactured Cr2+:ZnSe and Cr2+:ZnS gain elements in the form of closedrings. The ring is installed on the shaft of a motor and is rotated at a high speed, thus providingan effectively infinite scan of the collinear laser mode and pump beam across the gain element.The test spinning ring system design is shown schematically in Fig. 3. This approach is notequivalent to, but to some extent similar to prior publications devoted to laser systems based onrotary disk gain elements [22–24].

In the framework of this investigation we have conducted a substantial set of experiments withvarious configurations of laser resonators utilizing the spinning ring gain element approach. Wehave tested the performance of standalone master oscillators in free-running mode with broad-band laser mirrors, tunable laser sources, and a number of systems with wavelength selectionoptical elements. We have also conducted a thorough investigation of spinning ring gain elementperformance in power amplifiers.

In this work we show only a selected subset of laser resonator designs and experimental re-sults which represent the most promising laser configurations suitable for practical applications.Figure 4 shows by far the simplest MOPA design based on spinning ring gain element technol-ogy with the highest optical efficiency. It is based on a very simple 2-mirror design of the masteroscillator, single-pass power amplifier, and a dual pump source configuration. Figure 5 showsa standalone master oscillator design for a high-power 2940 nm CW laser system intended forindustrial applications. Figure 6 shows a laser resonator design with dual pump sources.

It is noteworthy that all cavity configurations used off-the-shelf 50 mm FFL (Front FocalLength) CaF2 lenses with custom AR coatings spanning 1.5–3.1 µm. In some laser configura-tions the same lens is used for pump focusing and also as a laser resonator component, whichis very attractive for reasons of system design simplicity and cost-efficiency. The Tm-fiber laserpump source operating at central wavelength λ = 1908 nm emits a collimated laser beam witha low divergence and beam diameter of 2wp ≈ 1.2 mm. The pump lens with a focal lengthF = 50 mm focuses the pump laser beam and creates a beam waist whose diameter can be esti-mated as 2w0 ≈ 4Fλ/2wpπ ≈ 101 µm. Using the (ABCD)–matrix formalism, one can find thatin the simple single-lens master oscillator cavity the laser mode waist can be matched with thepump beam waist when the intracavity lens is located at approximately 62 mm from the outputfacet of the gain element (the lens to the OC distance is ∼ 150 mm). In practice, we set thelens at this theoretically calculated location and then gradually adjust its axial position to obtainthe lowest possible lasing threshold. Our experiments show that due to efficient suppression ofthermal lensing effects in the gain media, the minimum lasing threshold cavity adjustment isclose to optimal even at the highest pump power levels.

The more complex 4-lens laser resonators and the single-pass power amplifier are theoreti-cally consist of single-lens resonator “chains” and thus should not require any significant mod-ifications for keeping good mode matching conditions. In practice, however, some minor opti-mization of axial positions of all lenses is required to maximize optical efficiency.

Vol. 24, No. 18 | 5 Sep 2016 | OPTICS EXPRESS 21094

DC MotorCooling Flange

Cooling Flange

Spinning Ring

Beam Scan Trace

Axis of Rotation

Laser Beam Path

(A)

(B)

50

mm

20

mm

Beam Scan Trace

7mm

(C)

11.5

6-6

-1-2.5

0.5-0.5 0

Laser Beam PathCooling Air Jets

Fig. 3. Test design of the spinning ring gain element system: (A) Technical drawing(front and side views) of the Cr2+:ZnSe spinning ring gain element with key dimen-sions. The Cr dopant concentration is 5 × 1018 cm−3, gain element thickness is 7 mm.The ring gain element has been manufactured with the following specifications and tol-erances: (1) Scratch-Dig: 40/20; (2) Flatness: 1/6 wave at 632 nm; (3) Wedge: <10 arcsec; (4) Concentricity error between the external and internal circumferences: <0.05 mm;(5) AR coating specifications (measured per surface): R<0.2% @1.9 µm, R<0.8% @1.9–2.8 µm, R<0.5%@2.94 µm. (B) Expanded view of the test opto-mechanical system. Thespinning ring is mounted between 2 cooling flanges. Indium foil (not shown in the scheme)is used to provide improved thermal contact between the gain element ring and flanges.The spinning flanges are self-cooled in open air at moderate heat loads (up to 100 W oftotal incident pump power). The flanges are additionally cooled with 2 compressed air jetsdirected from one side. The ring is spinning at a nominal speed of ∼ 9500 RPM. (C) Sidebroken view if the test opto-mechanical system also showing additional cooling air jetsand numbering scheme of the cooling fins for thermal analysis. The fins of cooling flangesare numbered with integers, the grooves are numbered with fractions, numbers -0.5, 0,0.5 designate the locations at the Cr2+:ZnSe gain element where surface temperature wasmeasured. The pump radiation enters the spinning ring from direction of fin -6 (left in thisdrawing).

Vol. 24, No. 18 | 5 Sep 2016 | OPTICS EXPRESS 21095

Pump Fiber Collimator

Dichroic Mirror

Pump Lens

Input MirrorCavity LensOC (VBG)

Steering Mirror

Focusing Lens Output

Master Oscillator

Power Amplifier

Fig. 4. Schematic diagram of the test MOPA laser system based on the spinning ring gainelement approach (relative scale is preserved, optomechanical mounts are not shown). Thelaser system consists of a simple master oscillator and a single-pass power amplifier. Thesystem is pumped with two 100 W Tm-fiber lasers (IPG TLR-100-1908-WC models). Theoutput wavelength is generally determined by the spectral curve of the output coupler. Inthe range of 1950–2400 nm, narrow-linewidth radiation (δλ ≤ 0.25 nm) is obtained byspectral control with volumetric Bragg gratings (VBG).

Pump Fiber Collimator

Pump LensInput MirrorAR@1567-2700 nm, HR@2940-3200 nm

Selector MirrorAR@1567-2700 nm, HR@2940-3200 nm

OC 80% @ 2940AR@2000-2800 nm

Cavity LensOutput

Fig. 5. Schematic diagram of high-power 2940 nm laser system based on the spinning ringgain element laser technology. Wavelength selection is performed by the output coupler anda triple dichroic mirror set (input mirror and 2 intracavity mirrors) which force the laser tooscillate near 2940 nm. Due to relatively low gain of Cr2+:ZnS and Cr2+:ZnSe laser medianear 3 µm, strict wavelength control is essential to prevent the laser from free-runningoscillation near its gain maximum within the range of 2300–2500 nm.

Pump Fiber CollimatorDichroic Mirror

Pump/Cavity LensCavity Lens

Steering Mirror

OC

End Mirror

Output

Fig. 6. Schematic diagram of a laser resonator design based on the spinning ring gainelement laser technology with dual pump source. This laser system can also be used as thebasis for high power tunable lasers when the end mirror is replaced with diffraction gratingin the Littrow or Littman mount configurations. A tuning prism or Liot filters can also beused for wavelength control within a limited spectral range.

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4. Major experimental results

In this section we present our most significant results obtained with Cr2+:ZnSe laser systemsbased on the spinning ring gain element technology. We have tested the performance of theselaser systems in single master oscillator configurations, single-pass MOPA arrangements, dual-pump laser systems, and beam combined laser designs.

An important note must be made here regarding the selection of pump sources for Cr2+:ZnSand Cr2+:ZnSe laser gain media. In all our CW mid-IR laser systems we utilize collinear pump-ing approach (also known as “end pumping”). This approach provides the highest possible op-tical to optical efficiency when there is a good mode matching between the pump beam and thelaser mode. The latter is readily achievable when the pump source delivers high beam qualitywith M2 factor close to 1.0, which is why we choose high-power fiber lasers with a single-modedelivery fiber optics for pumping.

There are generally 2 families of commercially available high power fiber lasers suitable forpumping Cr2+:ZnS and Cr2+:ZnSe laser systems: Er fiber lasers operating near 1.5 µm and Tmfiber lasers with output wavelengths near 1.9 µm. Currently, Tm fiber lasers deliver much higheroutput powers than Er fiber systems. Since the main goal of this work was to perform powerscaling of the mid-IR laser to the highest possible level, we chose to use Tm-fiber lasers as theoptical pump sources for this work.

It is also noteworthy that we have conducted preliminary power-scaling experiments withCr2+:ZnS spinning ring gain media and obtained very promising results comparable to the per-formance of spinning ring Cr2+:ZnSe lases. Unfortunately, the AR coating of Cr2+:ZnS spinningrings was not suitable for pumping at 1.9 µm and we decided to devote a separate publicationto this gain material in the future.

4.1. Master oscillator performance

Figure 7 shows general performance of the master oscillator configured for different outputwavelengths. The output wavelength of 2500 nm results from using broadband 50% output cou-pler and represents a free-running CW oscillation of the master oscillator based on spinningring Cr2+:ZnSe gain element. The output wavelength of 2300 nm is obtained with a volumetricBragg grating (VBG) manufactured by Optigrate, Inc. The output wavelength of 2940 nm is de-termined by a 4-mirror selective laser resonator shown in Fig. 5. The output spectra of the masteroscillators corresponding to these regimes of operation and laser resonator configurations areshown in Fig. 8, Fig. 9, and Fig. 10.

4.2. MOPA performance

The greatest advantage of rapidly scanning the laser mode/pump beam through gain media isthe almost complete elimination of thermal lensing effects. As a result, these laser systems be-have almost identically to pure theoretical laser devices where all thermal effects are generallyignored. This opens enormous opportunities for significant power scaling of these laser systems.One can see in the previous data that no thermal rollover is observed even at such unfavorableoutput wavelengths as 2.94 µm, where gain is very low and passive losses are very high (due,in particular, to intracavity water vapor absorption in un-sealed and un-purged cavities). Thisalso results in excellent performance of single-pass power amplifiers, as is demonstrated in theexperimental data shown in Fig. 11, where single-pass amplifier MOPA input-output character-istic at 2.3 µm is shown. Figure 12 demonstrates calculated gain as a function of input masteroscillator signal when the single-pass amplifier is pumped at full 117 W pump power.

Vol. 24, No. 18 | 5 Sep 2016 | OPTICS EXPRESS 21097

0 10 20 30 40 50 60 70 80 90 100 110 120Incident pump power, W

0

10

20

30

40

50

60

70

80

Out

put p

ower

, W

2500 nm (FR)2300 nm (VBG)2940 nm (MS) y = -1.65 + 0.65 *x

y = -1.11 + 0.59 * x

y = -0.71 + 0.31 * x

74

62

32

Fig. 7. Input-Output characteristics of the Cr2+:ZnSe master oscillator at different outputwavelengths. The laser demonstrates ∼ 63% absolute and ∼ 65% slope efficiencies near2.5 µm in free-running CW regime of operation with bradband 50% output coupler (FR);∼ 59% absolute and ∼ 62% slope efficiencies at 2.3 µm where the central wavelength isselected with 50% efficient VBG designed for 2300 nm (VBG); and ∼ 29% absolute and∼ 31% slope efficiencies near 2.94 µm, where the output spectrum is determined by the4-mirror intracavity selector (MS). The data labels show maximum output power values(the label values are rounded to whole Watts).

2400 2420 2440 2460 2480 2500 2520 2540 2560 2580 2600Wavelength, nm

0

0.2

0.4

0.6

0.8

1

Sign

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.u.

Fig. 8. Measured output spectrum of the master oscillator near central wavelength of2500 nm. The laser is operating in free-running CW regime with a broadband output cou-pler (OC) and the output wavelength is determined by the location of the gain maximumof Cr2+:ZnSe gain media. In this free-running mode, the laser shows approximately 40 nmlinewidth FWHM.

Vol. 24, No. 18 | 5 Sep 2016 | OPTICS EXPRESS 21098

2300 2300.5 2301 2301.5 2302 2302.5 2303 2303.5 2304Wavelength, nm

0

0.2

0.4

0.6

0.8

1

Sign

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Fig. 9. Measured output spectrum of the master oscillator at 2300 nm. The central wave-length is determined by a specific custom-made VBG output coupler with ∼ 50% reflectiv-ity. The VBG OC results in extremely narrow output linewidth of less than 0.25 nm (whichis actually the upper estimate determined by the resolution of available laser spectrum ana-lyzer, Bristol 721).

2880 2900 2920 2940 2960 2980 3000 3020Wavelength, nm

0

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al, a

.u.

Fig. 10. Measured output spectrum near 2.94 µm. The output spectrum is determined byintracavity 3-mirror selector and specific output coupler in the laser resonator design shownin Fig. 5. Due to relatively broad slopes of the cut-off edges of the selector mirrors andOC dielectric coatings, the laser demonstrates relatively broad linewidth of approximately30 nm FWHM.

Vol. 24, No. 18 | 5 Sep 2016 | OPTICS EXPRESS 21099

10

25

41

57

72

87

100

114124

0 20 40 60 80 100 120 140 160 180 200 220 240Wavelength, nm

0

20

40

60

80

100

120

140Si

gnal

, a.u

.

y = 0.17 + 0.57 * x

MOPA 2300 nm

Fig. 11. Input-Output characteristic of MOPA system operating at 2300 nm. The laserdemonstrates ∼ 55% absolute and ∼ 57% slope efficiencies at 2.3 µm

15.9

6.7

4.43.5 2.9 2.5 2.3 2.1 2.0

0 10 20 30 40 50 60 70Master oscillator incident power, W

0

5

10

15

20

Gai

n

y = 52.8 * x-0.83

Fig. 12. Calculated gain of single-pass power amplifier as a function of incident masteroscillator power. The gain is calculated as the ratio of output power to the master oscillatorincident power. The amplifier incident pump power is constant and equals approximately117 W.

Vol. 24, No. 18 | 5 Sep 2016 | OPTICS EXPRESS 21100

4.3. High-power performance of Cr2+:ZnSe laser systems

In this section we present our best power scaling results wit Cr2+:ZnSe spinning ring lasergain media. We show that scanning the laser mode/pump beam across the gain media allowsfor obtaining unprecedented output powers in the 2–3 µm spectral region with Cr2+:ZnSe gainmaterial. Figure 13 shows the input-output characteristic of a 140 W laser system where theoutput beams of two simple master oscillators shown in Fig. 4 are polarization combined as wellas the more complex dual-pump laser resonator, shown in Fig. 6. Figure 14 demonstrates typicalpower stabilization curves after cold start of two laser systems: (1) dual-pump free-runninglaser operating near 2500 nm, and (2) 2.94 µm laser system. Figure 15 represents typical outputbeam profiles obtained with our power-scaling approach from the laser configurations discussedabove.

0 25 50 75 100 125 150 175 200 225 250Total incident pump power, W

0

20

40

60

80

100

120

140

160

Tot

al o

utpu

t pow

er, W

Dual Single-Lens Master OscillatorFour-Lens Laser Resonator

y = -2.95 + 0.59 * x

y = -3.14 + 0.63 * x131

140

Fig. 13. Performance of two beam-combined maser oscillators and 4-lens laser cavity withdual-pump source in single spinning ring laser system. The complete absence of thermalrollover demonstrates unsaturated pump power capability of these laser systems and indi-cates further power scaling potential for reaching kW levels of output power.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15Time, min

0

20

40

60

80

100

120

140

Out

put p

ower

, W

2500 nm2940 nm

Fig. 14. Typical power stabilization curves after cold start of high-power Cr2+:ZnSe lasersystems based on the spinning ring gain element approach. Power stabilization of 4-lensfree-running laser resonator operating near 2.5 µm and 2.94 µm master oscillators areshown.

Vol. 24, No. 18 | 5 Sep 2016 | OPTICS EXPRESS 21101

Fig. 15. Typical beam profiles of the spinning ring laser systems in various configurations.The beam profiles were acquired with Pyrocam III™ pyroelectric laser beam analyser lo-cated approximately 0.5 m from the laser output mirror. The camera was operated in CWlaser mode.

In order to perform a more accurate characterization of beam quality of Cr2+:ZnSe lasersystems based on spinning ring gain element technology we measured M2 factor of the leastefficient master oscillator operating near 2.95 µm. Due to relatively low optical to optical effi-ciency of this laser system the influence of residual thermal lensing effects on the output beamquality is more pronounced as compared to other output wavelengths within the gain profiles ofCr2+:ZnS and Cr2+:ZnSe laser systems.

The M2 measurements were performed using the Knife-Edge method (described in very de-tails in Ref. [25]) as follows. A small fraction of the output power was sampled with a CaF2

wedge and focused by a 100 mm FFL AR coated CaF2 lens. A razor blade was scanned acrossthe laser beam with a motorized linear stage moving at a constant speed of 0.05 mm/s. The trans-mitted signal was recorded as a function of time with the help of Ophir™ P3-A1 thermal sensor,Nova II™ power meter, and StarLab™ software. The beam radii were measured at multiplelocations along the beam axis. The measurement results are shown in Fig. 16.

0 25 50 75 100 125 150 175 200z, mm

0

1

2

3

4

w2 (z

), m

m2

Experimental data points

Fit: y = A0 * (1 + (x - A1)2 / A2), where:

A0 ~ 0.02 mm2, A1 ~ 15.94 mm, A2 ~ 160.09

Fig. 16. Measurement results of beam quality M2 of the 32 W master oscillator operatingnear 2.95 µm. The fitting parameter A0 ≈ w2

0 is the beam waist radius, the fitting parameterA1 is the offset of w0 from relative measurement origin, the fitting parameter A2 is theconfocal range. The value of M2 ≈ 1.77 is estimated from the fitting parameter A0 and A2

as: M2 ≈ πA0/(λA1/22 ), where λ ∼ 2.95 µm is the output wavelength.

Vol. 24, No. 18 | 5 Sep 2016 | OPTICS EXPRESS 21102

It is evident from the beam profiles shown in Fig. 15 as well as from the measured M2

value of 1.77 that some residual beam distortion caused by heat build-up in the gain element isstill present. An additional factor that contributes to non-ideal beam quality is the presence ofacoustic vibrations due to very simple mechanical design of the spinning ring opto-mechanicalsubsystem. Nevertheless, the presented results clearly demonstrate feasibility and a great poten-tial of the spinning ring gain element approach for significant power-scaling of the Cr2+:ZnSelaser systems. Further improvement of gain element cooling system and spinning drive mech-anism will result in significant improvement of output beam quality at very high output powerlevels.

4.4. Thermal performance

As it was mentioned earlier in the paper, the primary goal of this research project was to in-vestigate practical feasibility of the spinning ring gain elements for significant power scalingof Cr2+:ZnSe and Cr2+:ZnS laser systems via efficient thermal management of the active me-dia. Experimental analysis of thermal performance of the opto-mechanical system used in theseexperiments is of great importance for refining major parameters of the spinning gain elementand cooling arrangement. The latter is especially important within the framework of furtherpower-scaling of Cr2+:ZnSe and Cr2+:ZnS laser systems to kW output power levels.

In order to characterize heat built-up in the gain medium and performance of the coolingsubsystem we conducted direct measurements of the spinning ring gain element temperatureand cooling flanges. The experimental data of these thermal measurements are currently usedin development if a practical thermal model of the spinning gain element subjected to opticalexcitation at very high power levels. For direct temperature measurements of the spinning com-ponents we used a remote infrared thermometer (Cen-Tech™) equipped with a built-in point-ing laser beam. Preliminary calibration showed that one can expect an ±2◦C spot temperaturemeasurement accuracy. The thermal measurement results are demonstrated in Fig. 17. Thesemeasurements were performed for the dual-pump laser system operating near 2500 nm with atotal incident pump power of ∼ 226 W (estimated transmitted pump does not exceed 10 W).

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6Fin/Spot # (pump enters from left, fin -6)

20

25

30

35

40

45

50

55

60

Surf

ace

tem

pera

ture

, C

SimulationMeasured at full pumpMeasured at zero pump

Fig. 17. Temperature profiles of the cooling flanges and spinning ring gain element of dual-pump, 4-lens laser system. The x–axis indicates fin/spot numbers as shown in Fig. 3(C).The “Simulation” curve is given for illustrative purposes and corresponds to thermal profileobtained using Solidworks™ thermal simulation package. The measurements at zero pumplevel (green curve) show a gradual heating of the right cooling flange due to operation ofthe DC motor.

Vol. 24, No. 18 | 5 Sep 2016 | OPTICS EXPRESS 21103

5. Conclusions and future outlook

In this work we have demonstrated unprecedented output power levels of mid-IR laser systemsbased on Cr2+:ZnSe gain media. We have utilized a novel approach for this gain media basedon scanning the collinear pump beam and laser mode across the gain element to obtain outputpower levels of up to 140 W with real optical efficiencies reaching ∼ 62% near the gain maxi-mum (2.5 µm) and ∼ 29% at the key wavelength of 2.94 µm. The input-output characteristics ofvarious laser configurations based on our novel approach show significant unused pump powercapacity and indicate that we can potentially reach kW output power levels in the spectral rangeof 2000–3000 nm and beyond. These results clearly show that Cr2+:ZnS and Cr2+:ZnSe lasersystems have come of age and have became suitable for practical laser systems highly demandedfor material processing applications in the fields of medicine, industry, as well as cutting-edgeresearch.

Besides simple power scaling of CW mid-IR lases based on Cr2+:ZnS and Cr2+:ZnSe gainmedia there is a very promising possibility of significant power-scaling of mode-locked systemsbased on the same gain materials [26] using the spinning ring gain element approach. Thesimplest way to boost the output power of mode-locked Cr2+:ZnS and Cr2+:ZnSe lasers is toutilize a single-pass spinning ring power amplifier. According to Fig. 12, the output powers inexcess of 80 W should be readily available with the use of combination of 5–7 W Cr2+:ZnSfemtosecond seed (described in [26]) and a single-pass spinning ring amplifier pumped by 100–120 W radiation of Tm-fiber laser. A much more challenging task would be to obtain modelocking regime directly in master oscillators based on spinning ring gain elements. A detailedinvestigation of mode-locked performance of the spinning ring laser systems is a subject offuture studies.

One obvious engineering difficulty needed to be solved in order to produce a practical lasersystem is the removal of heat from a moving gain element. Our current efforts in this area, wherewe already have very promising experimental results, are subject to our future publications.

Acknowledgments

Sergey Mirov declares competing financial interests.

Vol. 24, No. 18 | 5 Sep 2016 | OPTICS EXPRESS 21104