A CW side-pumped Yb:S-FAP laser

 

Alex Dergachev, Yelena Isyanova, and Peter F. Moulton

Q-Peak, Inc., 135 South Road, Bedford, Massachusetts 01730

Tel.: (781) 275-9535, Fax: (781) 275-9726, E-mail: dergachev@qpeak.com

 

K. I. Schaffers

Lawrence Livermore National Laboratory, PO Box 5508, Livermore, California 94550

Abstract: The first demonstration of cw operation of a side-diode-pumped, 1047-nm Yb:S-FAP laser is reported. The output power achieved was 3 W, which represents, to our knowledge, the highest cw power yet reported from Yb:S-FAP.

Ó2002 Optical Society of America

OCIS codes: (140.3580) Lasers, solid-state (140.3480) Lasers, diode-pumped

1. Introduction

Yb-doped S-FAP has been recently shown to be a promising material for highly efficient diode-laser pumped pulsed 1-mm lasers [1, 2]. Both quasi-cw [2] and cw pumping [3] have been demonstrated to provide high slope efficiency with a longitudinal pumping configuration. Yet to date, the highest true cw power reported for Yb:S-FAP using a cw diode-end-pumped configuration was 130 mW [3]. Cw pumping represents an attractive means to operate the lasers in both cw and repetitive Q-switched modes, with variable repetition rates and/or pulse durations. Compared to four-level materials, limitations on laser designs for cw operation are more an issue for Yb:S-FAP due to its quasi-three level nature. In longitudinal-pumping configurations commonly used for Yb:S-FAP, the length of the material is defined not so much by the requirement to efficiently absorb the pump light but by the requirement to achieve a high population inversion. This leads to the need for pumping with high intensity close to the saturation level. We report here on the use of a side-pumped slab geometry, which allows us, to a great extent, to eliminate the connection between the absorption and gain length of the laser material and provides a means to scale the output power.

2. Experiment and results

      Highly efficient, multi-pass, side-pumped configurations have been successfully demonstrated for Nd:YLF [4], Nd:YVO4 [5], and Er:YLF [6] lasers. The design approach we used for the Yb:S-FAP gain module is similar to these lasers. In the gain module, a 2.2-cm long Yb:S-FAP crystal was transversely pumped by a pair of 1-cm long diode laser bars, as shown in Fig. 1. The diode-laser bars were coupled to the gain element through a single cylindrical lens attached directly to the each bar package. These lenses minimize the divergence of the pump light in the plane perpendicular to the linear emitter. The 43-W diode lasers were offset on opposite sides of the Yb:S-FAP crystal to create a sheet of gain in the crystal.

 

Fig.. 1. Schematic layout of the side-pumped Yb:S-FAP laser.  DL – Diode Lasers, OC – Output Coupler, HR – High Reflector, r - radius.

 

The diode laser wavelength and its spectral width, 900 nm and 3 nm, respectively, were chosen to correspond to these of the strong p-absorption line of Yb:S-FAP. The cylindrical lenses were expected to collimate the diode bar emission to provide a 0.3-mm-wide pump sheet in the slab. We measured the output power, spectral width and output beam size, and found that none of the above requirements was met. The output power of both the diodes at the specified temperature was 25 W. The beam width of one of the diodes was ~ 0.6 mm.

      We used a 1-% Yb-doped slab with the c-axis perpendicular to the “gain sheet”. The polarization of the pump beam was adjusted using a half-wave plate to make it parallel to the c-axis. The pump faces of the slab have segmented dielectric coatings (AR/HR) to allow double-pass absorption to generate a uniform deposition of pump power in the beam propagation direction. Heat was extracted from the laser medium in a unidirectional manner perpendicular to the plane of Fig. 1 by contacting water-cooled copper heat sinks on the top and bottom of the slab. This method of heat deposition and removal is particularly attractive for low thermal conductivity materials such as Yb:S-FAP. Due to the quasi-three level of the 1047-nm laser transition, it is important to minimize the length of unpumped regions of the slab, which increase laser threshold and absorption losses for the laser mode. In addition, maintaining a low crystal temperature is necessary to minimize re-absorption losses at wavelengths longer than the zero-phonon line.

The Nd:YLF gain module design [3] uses external high reflectors to multi-pass the laser mode through the gain region. For the experiments reported here, we used a single-pass arrangement. A concave HR mirror and flat output coupler formed a near semi-concentric resonator.

 

Fig. 2. CW side-pumped Yb:S-FAP laser  output for different values of output coupling in multimode regime of operation.

 

Input-output data with various output couplers in cw operation are shown in Fig. 2. The threshold and slope efficiency were 34 W and 15%, respectively. The laser operated at the 1047-nm line with laser emission p-polarized. The output beam profile was diffraction limited in the vertical plane (normal to the plane of Fig. 1) and highly multimode (>10 times diffraction limited) in the horizontal plane. The maximum output power achieved was 3 W at a ~55 W of pump power.

Some of the reasons for lower than expected output powers can be related to the poor performance of the diode bars. Other reasons will be discussed.

We also conducted quasi-cw operation at 50%, 33% and 25% duty cycles. Lower duty cycles allowed operating at higher peak currents. (This is due to the fact that laser diode wavelengths depend on the heatsink temperatures and average current. Provided the heatsink temperature is fixed, the bar wavelength will be the same if the average current is kept at the same level for different duty cycles.)

Fig. 3. Output power for a cw and qcw side-pumped Yb:S-FAP laser with 10% output coupling,  multimode.

 

      The output power and slope efficiencies were substantially higher at quasi-cw operation: the maximum power and slope efficiency received were 10 W and 21%, respectively.

4. Conclusion and future work

In conclusion, we have demonstrated cw operation of a side-pumped  Yb:S-FAP laser with output power as high as 3 W. We generated quasi-cw (25% duty cycle) peak powers as high as 10 W. Efforts are underway to increase the output power by improved matching of the laser mode with the pumped volume as well as better matching of the diode laser wavelength and spectral linewidth to those of the laser material. We also plan to optimize the output coupling and resonator design. In the future, we would expect higher performance through the use of undoped end-caps bonded to the laser crystal, and perhaps through optimization of the Yb doping level.

 

5. References

[1]     S. A. Payne, L.D. DeLoach, L.K. Smith, W.L. Kway, J.B. Tassano, W.F. Krupke, B.H.T.Chai, G. Loutts “Ytterbium-doped apatite-structure crystals: a new class of laser materials”, J. Appl. Phys. 76, 497-503 (1994).

[2]     C.D. Marshall, L.K. Smith, R.J. Beach, M.A. Emanuel, K.I. Schaffers, J. Skidmore, S. A. Payne, B.H.T.Chai “Diode-pumped Ytterbium-doped Sr5(PO4)3F laser performance”, IEEE J. Quantum Electron. 32, 650-656  (1996).

[3]     L.A.W. Gloster, P. Cormont, A.M. Cox, T.A. King, B.H.T. Chai, “Diode-pumped Q-switched Yb:S-FAP laser”, Opt. Commun. 146, 177-180 (1998)

[4]     K. J. Snell, D. Lee, K. F. Wall, and P. F. Moulton, “Diode-pumped, high-power CW and modelocked Nd:YLF lasers”, OSA Trends in Optics and Photonics Vol. 34, Advanced Solid State Lasers, Hagop Injeyan, Ursula Keller, and Christopher Marshall, eds., (Optical Society of America, Washington, D.C., 2000), pp. 55-59.

[5]     K. J. Snell and D. Lee, “High average power, high repetition rate side-pumped Nd:YVO4 slab laser”, OSA Trends in Optics and Photonics Vol. 26, Advanced Solid State Lasers, Martin M. Fejer, Hagop Injeyan and Ursula Keller, eds., (Optical Society of America, Washington, D.C., 1999), pp. 295-297.

[6]     A. Yu. Dergachev, J. H. Flint, and P. F. Moulton, “1.8-W CW Er:YLF diode-pumped laser,” in Conference on Lasers and Electro-Optics, OSA Technical Digest (Optical Society of America, Washington, D.C., 2000), pp. 564-565.

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