Laser-diode-pumped neodymium-doped fibre laser with output power in excess of 1 Watt

Abstract

Fiber lasers are normally considered to be low-power devices and not competitive with diode-pumped Nd:YAG lasers. However, by employing special glass fiber technology and a novel inner-cladding geometry we demonstrate here a Nd³⁺-doped single-mode fiber laser emitting an output power of 1.07 W at a wavelength of 1.057 µm. The laser is pumped with a 3-W GaAlAs multistripe laser diode array and has several advantages over its similarly pumped Nd:YAG counterpart. For example, the broad absorption line of Nd³⁺ in glass obviates the need for selection and temperature stabilization of the pump diode wavelength. The waveguide nature of the fiber resonator makes mirror alignment simple, eliminates thermal focusing, and ensures perfect transverse-mode selection. The laser cavity is therefore substantially immune from environmental effects. In addition, fiber lasers are broadly tunable and, by use of a variety of well-established fiber techniques [1], can give high-power Q-switched output, femtosecond mode-locked pulses, and narrow-linewidth operation. The technique of cladding pumping [2,3] has been shown to allow pumping of fibers with a single-mode Nd³⁺-doped core by multimode diode arrays, thus allowing the fiber laser to exploit the increasingly available high-power diode pump sources. However, to optimally match the diode emission to the fiber requires careful choice of materials and design of the fiber cross-sectional geometry. We have utilized compound glass technology, we believe for the first time, to obtain both an optimal geometry and a high radiative cross-section. By this means we have obtained an overall laser conversion efficiency of ~30%, and a quantum efficiency of ~70%.

The fiber geometry is shown in Fig. 1 and comprises a heavily Nd³⁺-doped (3 wt.%) core of Schott F7 glass (N.A. = 0.13, diameter =5.3 µm) located centrally within a rectangular, highly multimode, undoped inner-cladding waveguide of Schott F2 glass into which the pump light is injected. This inner cladding is in turn clad with Schott LF8 to give a high numerical aperture (N.A. = 0.42) and a circular fiber cross-section. The inner cladding is designed to match the large diode diffraction angle and emitting area, whilst minimizing the core/inner cladding area ratio, thus optimizing the pump absorption in the core and minimizing the laser threshold.

Pump light at 808 nm from a 3-W SOL diode array was launched (inset Fig. 2) into the rectangular inner cladding of the laser fiber through an 808/1057-nm highly reflective dichroic mirror butted to the end-face, the other mirror being simply the cleaved fiber end-face giving 4% Fresnel reflection. The pump launch efficiency was ~65% and the optimum length of the laser was 1.8 m. The output characteristics of the fiber laser are shown in Fig. 2. With the pump diode laser operating at full power the fiber laser gave 1.07-W output power at a peak wave­length of 1.057 µm. The output spectrum is illustrated in Fig. 3 and shows a 3-dB bandwidth of ~8 nm, a spectrum that can be narrowed by employing, for example, fiber-grating feedback. The overall efficiency with respect to diode power is ~30% while the slope efficiency with respect to absorbed power is ~50%, which is close to the maximum attainable.

Owing to its simplicity and robustness, we believe that the high-power fiber laser reported here provides a competitor for diode-pumped Nd:YAG lasers in many uses, being potentially cheaper and widely tunable. Furthermore, fiber compatibility allows ready exploitation of the wide range of fiber devices available and permits integration into complex fiber optical circuits. As a first step in this direction, we have recently employed the laser as a pump source for a high-power Er/Yb fiber telecommunications amplifier operating at 1.55 µm, using an all-fiber tandem-pumping arrangement. [4]

1. P. Urquhart, "Review of rare earth doped fibre lasers and amplifiers," IEE Proc. Pt. J, 135, 385-406 (19xx).

2. E. Snitzer, H. Po, F. Hakimi, R. Tumminelli, B. C.McCollum, "Double­-clad, offset core Nd fiber laser," in 1988 Optical Fiber Communication Conference, Vol. l, OSA Technical Digest Series (Optical Society of America, Washington DC, 1988), Paper PDS.

3. H. - Po, E. Snitzer, R. Tumminelli, L. Zenteno, F.Hakimi, N.M. Cho, T. Haw, "Doubly clad high brightness Nd fiber laser-pumped by GaAlAs phased array," in 1989 Optical Fiber Communication Conference. Vol. 1, OSA Technical Digest Series (Optical Society of America, Washington DC, 1989).

4. J. D. Minelly, R. I. Laming, J.E. Townsend, W. L. Barnes,E. R. Taylor, K. P. Jedrzejewski, D.N. Payne, "High-­gain power amplifier tandem-pumped by a 3-W multistripe diode," in 1992 Optical Fiber Communication Conference, Vol. 5. OSA Technical Digest Series (Optical Society of America, Washington DC, (1992), paper TuG2.

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