Ultra-efficient Ho_YAG laser end-pumped by a cladding-pumped

Ultra-efficient Ho:YAG laser end-pumped by a cladding-pumped Tm-doped silica

fiber laser

A. Abdolvand, D. Y. Shen, L. J. Cooper, R. B. Williams and W. A. Clarkson

Optoelectronics Research Centre, University of Southampton, Southampton, SO17 1BJ, United Kingdom

Tel. +44 23 80593136

Fax. +44 23 80593142

email: ama@orc.soton.ac.uk

Abstract: We report a Ho:YAG laser with 5.2W of TEMoo output at 2097nm at room-

temperature and with a slope efficiency with respect to incident pump power of 80%,

pumped by a cladding-pumped tunable Tm-doped silica fiber laser operating at 1905nm.

©2003 Optical Society of America

OCIS Codes: (140.3070) Infrared and far-infrared lasers; (140.3480) Lasers, diode-pumped; (140.5680)

Rare earth and transition metal solid-state lasers.

Introduction

High-power solid-state lasers operating in the eye-safe 2µm spectral region have numerous applications and provide an ideal starting point for nonlinear frequency conversion to the mid-infrared (~3-5µm) spectral region. For many of these applications, Ho:YAG is the preferred laser material due to a combination of a long fluorescence lifetime (~8ms), allowing the production of high Q-switched pulse energies, and good thermo-mechanical properties. Direct diode pumping of Ho:YAG by high-power near-infrared diodes requires that the crystal is co-doped with Tm leading to strong upconversion losses and hence reduced efficiency [1,2]. Recently, attention has focussed on using singly-doped Ho:YAG crystals and pumping ‘in-band’ with a diode-pumped Tm:YLF [3] or Tm:YVO4 laser. This approach has the advantages that upconversion losses are much lower and quantum defect heating in the Ho:YAG is greatly reduced, leading to very high lasing efficiencies. The upper limit on output power is, however, limited by thermal effects in the Tm-doped crystal laser. An alternative approach is to use a cladding-pumped Tm-doped fibre laser as the pump laser. Cladding-pumped fiber lasers benefit from a geometry that allows relatively simple thermal management and hence offer the potential for scaling to very high power levels. In recent work, we reported a Tm-doped silica fiber laser with an output power of 14W[4], and a Tm-doped fiber laser with multiwatt output power tunable from 1860nm-2090nm [5]. The combination of high output power, good beam quality and wide wavelength tunability suggest that cladding pumped fibers lasers would be ideal pump sources for many Tm and Ho-doped crystal lasers. Preliminary experiments on Tm-doped fiber laser pumping of a Ho:YAG laser yielded a maximum output power of ~0.5W and a slope efficiency with respect to incident pump power of ~37% [6]. Here we report a Ho:YAG laser, end-pumped by a cladding-pumped Tm-doped silica fiber laser, with >5W TEMoo output and a very high slope efficiency with respect to incident pump power of 80%.

Experiment

The Tm-doped fiber used in our experiments was fabricated in-house using the standard modified chemical vapour deposition and solution doping technique and had a 20µm diameter Tm-doped alumino-silicate core with 0.12NA, and a 200µm diameter pure silica inner-cladding. The latter was coated with a low refractive index UV-cured polymer outer-cladding resulting in a calculated numerical

aperture for the inner-cladding pump guide of 0.49. The fiber laser arrangement employed is shown in Fig.1. Pump light from beam-shaped diode-bars [7] was launched into opposite fiber ends using dichroic mirrors with high reflectivity at the pump wavelength (785-795nm) at 45º and high transmission at the lasing wavelength (1850-2100nm) to allow extraction of the fiber laser output. This arrangement allows the use of different lenses for collimating the fiber laser output and focussing the pump, and hence has the advantage that the resonator alignment and pump launching optics can be independently optimised.

S1: HR@790nm, HT@1850-2100nm

S2: HT@1850-2100nm

Fig. 1. Tunable cladding-pumped Tm-doped silica fiber laser.

Wavelength tuning was achieved by employing an extended cavity comprising an antireflection-coated Infrasil plano-convex collimating lens of focal length, 25mm, and a simple diffraction grating with 600 lines/mm in the Littrow configuration to provide wavelength selective feedback. The grating was blazed at wavelength of 1.9µm and had measured reflectivities of 90% (polarised perpendicular to the grooves) and 70% (polarised parallel to the grooves) at 2µm. The fiber-end nearest the grating was angle-cleaved to suppress broadband feedback from the uncoated face that might otherwise compete with the wavelength-dependent feedback provided by the grating and thus restrict the tuning range. The opposite end of the fiber was cleaved perpendicularly to provide the feedback necessary for laser oscillation. This end of the fiber also acted as the output coupler and, due to its high transmission (~96.5%), dominates over the feedback losses at the grating end of the laser. A longer fiber (~4.7m) than would be optimum was used to ensure that negligible pump light was transmitted by the fiber as a precaution against damage to opposing diodes. The threshold pump power incident on the pump focussing lens was measured as ~7.5W (~6W launched), and at the maximum available incident pump power of 55W (corresponding to 44W launched) the fiber laser produced a maximum output power 10.5W at 1921nm (see Fig.2). The lasing wavelength could be tuned, by a simply adjusting the diffraction grating angle, over 215nm from 1855 to 2070nm at multi-watt power levels, and over 150nm from ~1860 to 2010nm at output power levels in excess of 9W (see Fig.3). The bandwidth of the tunable laser output (FWHM) was ~1nm. From Fig.2 it can be seen that the a …… 此处隐藏：10118字，全部文档内容请下载后查看。喜欢就下载吧 ……