Power Scalable Continuous Wave Mid-IR (2.7 mm) Fiber Lasers

Compact and tunable, high efficiency mid-IR laser sources are critically needed for a variety of applications including: (i) surgical cuts enabled by the strong water absorption at 2.7 mm in tissue¹, (ii) mid-IR countermeasures, and (iii) monitoring of environmental and industrial gases such as NO, H₂S, and water vapor. The first two applications typically require Watts of mid-IR power and the latter requires a source that is tunable. The 2.7 mm transition in Er:ZBLAN fibers^2-5 is a promising candidate for the above applications due to its broad tunability (over 50 nm) and the demonstrated power scalability of fiber lasers, particularly with the use of double-clad designs.

In this paper, we report relatively high output powers and operating efficiencies from a continuous wave 2.7 mm double-clad Er:ZBLAN fiber laser, whose output powers should be scalable to the Watt power level with commercially available 780 nm diode pumps. In particular, we have demonstrated ~40 mW output powers using a 780 nm Ti:Al₂O₃ laser pump. We have also achieved diode-pumped operation, with ~10 mW output power levels at 2.7 mm using a readily available 980 nm pump. As such, this work represents the first report of the use of a double-clad fiber for a mid-IR fiber laser.

Figure 1 shows the basic experimental arrangement used in the present work. The choice of the 5.5 m long double clad fiber was based on our plans to replace the currently-used pump laser with high power diode pumps of relatively low beam quality. Results based on two different pump sources are discussed below.

In the first experiment, we used a 1 Watt 980 nm laser diode based on a tapered amplifier structure⁶. At the input end, an HR mirror (M₁) was butt-coupled to the fiber while the cleaved distal end was used as a 96% output coupler. The output power from the fiber laser was monitored by completely attenuating the 780 nm pump with a 2.7 mm (T=90%) transmitting filter. Figure 2 shows the 2.7 mm output power as a function of launched pump power. The low lasing threshold of 30 mW for a 96% output coupler, and the fact that no saturation of the output power is observed even at the highest pump powers used indicates that this 980 nm pumped laser can be further optimized to yield much higher output powers.

In the second experiment, we used a 780 nm Ti:Al₂O₃ pump and a simple cavity design comprising of the 4% Fresnel reflections at the two uncoated fiber ends. Note that for the data reported here, mirror M₁ (shown in Fig 1) was not used. A key feature of the work reported here is the use of a pump wavelength of 780 nm (compared to the 791 nm pump wavelength used previously for excitation of the ⁴I_11/2 upper laser level via the ⁴I_9/2 pathway); this choice of pump wavelength is demonstrated to be approximately 3 times more efficient than the use of a 791 nm pump in our laser design, as elaborated below.

Figure 3 shows the P_out vs. P_in curve for this fiber laser when pumped by 780 nm single transverse mode Ti:Sapphire pump radiation that is directly coupled to the fiber core (coupling efficiency ~50%) corresponding to its current use as a simple "single clad" fiber. The vertical axis in Fig 3 corresponds to mid-IR power output from both ends of the fiber laser, and lasing threshold corresponds to a gain of 5.85 dB/round-trip at a pump power of ~25 mW corresponding to a pump power density of ~400 KW/cm². Note that even at the highest pump power levels, there is no evidence of saturation of the output power from this 2.7 mm fiber laser. As such, in a follow-on experiment currently in progress, comparable gains should be attainable with the use of ~20 W of diode pump power (Optopower Corp.) coupled partially into the core and partially into the diode-pump confining inner cladding; for this experiment, output power levels of the order of a Watt are anticipated.

Figure 4 shows the "excitation spectrum" of such a fiber laser corresponding to a constant incident pump power of 600 mW. Note that in contrast to a previous report² on the output power of such a mid-IR fiber laser as a function of the pump wavelength, in our work 3 times greater output power was obtained with the use of the 780 nm excitation wavelength when compared to the use of the more traditional 791 nm pump wavelength^2-4, despite the higher ground state (⁴I_15/2 to ⁴I_9/2) absorption at 791 nm. We are studying this effect in detail, and we currently ascribe the observed behavior to reduction in ground state bleaching effects⁷, as well as to the reduction of deleterious effects caused by significantly lower ESA^5,8 (from the upper laser level ⁴I_11/2 to ⁴F_5/2) for 780 nm, and the wavelength dependence of the "beneficial" lower-level (⁴I_13/2) depleting ESA to the ⁴H_11/2 level^5,8.

References:

1. L. Esterowitz, R. Allen, Proc. SPIE 1048, 129 (1989)
2. R. Allen, L. Esterowitz, R.J. Ginther, Appl. Phys. Lett., 56, 1635 (1990)
3. M. Pollnau, Ch. Ghisler, G. Bunea, W. Luthy, and H.P. Weber, Appl. Phys. Lett, 66, 26 (1995)
4. M. Pollnau, Ch. Ghisler, W. Luthy, H.P. Weber, J. Schneider, and U.B. Unrau, Opt. Lett., 22, 612 (1997)
5. M. Pollnau, R. Spring, Ch. Ghisler, S. Wittwer, W. Luthy, H.P. Weber, IEEE J. Quant. Electron., 32, 657 (1996)
6. J.N. Walpole, E.S. Kintzer, S.R. Chinn, C.A. Wang, and L.J. Missaggia, Appl. Phys. Lett., 61, 740 (1992)
7. S. Bedo, M. Pollnau, W. Luthy, H.P. Weber, Opt. Commn., 116, 81 (1995)
8. T.J. Whitley, C.A. Miller, R. Wyatt, M.C. Brierly, D. Szebesta, Electron. Lett., 27, 1785 (1991)

List of figures:

Figure 1. Schematic of the Er:ZBLAN fiber laser

Figure 2. P_out vs P_in for 980 nm diode pump.

Figure 3. P_out vs P_in for 780 nm Ti:Sapphire pump.

Figure 4. Excitation spectrum for 2.7 mm laser.