TECHNICAL NOTE

Performance of a Nd:YVO4 microchip laser with continuous-wave pumping at wavelengths between 741 and 825 nm

Qiu Mingxin, David J. Booth, Gregory W. Baxter, and Glenn C. Bowkett

We report the performance of a Nd:YVO4 laser, which uses cw Ti:sapphire pumping, for a range of pumping wavelengths, polarizations, and crystal temperatures.

Rare-earth ions such as Nd3+ have been found to lase well in a large group of crystal materials such as YAG, YLF, GSGG, BEL, CaWO4, and YVO4.1,2 Recently there has been considerable interest in laser action in materials with short absorption depths, as these can produce simple single-frequency lasers.3 The single longitudinal mode operation of a temperature-tuned Nd:YVO4 laser has been achieved in a crystal 500 μm thick by using AlGaAs laser diode excitation.4 Other authors have investigated the pumping efficiency of diode-pumped Nd:YVO4 lasers over a limited wave­length range by varying the temperature of the pumping laser diode.5 This note describes a more extensive investigation of the pumping efficiency and operation of the Nd:YVO4 laser at temperatures between 0 and 100°C when the much wider tuning range that is possible with a Ti:sapphire laser is used.

The laser consisted of a 1.1-at. % doped Nd:YVO4 crystal with a 3 mm × 3 mm cross-sectional area and 0.5 mm long with laser mirrors coated directly onto the crystal. The laser was end pumped through a mirror with a transmission that varied between 90% and 97% over the 740-825-nm wavelength range and had a reflectivity of 99.9% at 1064 nm. The output mirror reflectivity was 99% at 1064 nm. The Ti:sapphire pump laser was focused to a spot diame­ter of 200 μm at the entrance face of the crystal. The linewidth (FWHM) of the 1064-nm laser transi­tion was 257 GHz and the longitudinal mode spacing was 138 GHz. With this arrangement, up to three longitudinal modes can lase simultaneously, depend­

ing on the position of the resonator modes within the laser gain profile. If the crystal temperature is adjusted so that one mode is tuned to the center of the gain profile, then single longitudinal mode oscillation results.4 For the widely tuned pumping experi­ments reported in this note, the crystal was main­tained at room temperature. The laser output, as measured with an ANDO AQ-6310B optical spectrum analyzer (0.1-nm resolution), consisted of a strong laser mode, 0.18 nm wide, centered at 1064.0 nm, together with a small second longitudinal mode of less than 10% intensity of the strong mode. There was some evidence of a transverse mode structure.

Figure 1 shows the laser output power variation with a pump wavelength for a pump laser power of

Q. Mingxin is with the Shanghai Institute of Laser Technology, Shanghai, 200233, China. The other authors are with the Depart­ment of Applied Physics, Victoria University of Technology, Footscray,

ba 3011, Australia. Received 8 June 1992. 0003-6935/93/122085-02$05.00/0. © 1993 Optical Society of America.

Fig. 1. Laser output power variation with pump wavelength and an uncoated Nd:YVO4 absorption spectrum (0.2-nm resolution).

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Fig. 2. Output power variation with incident pump power at a pump wavelength of 808.72 nm.

approximately 200 mW at 1-GHz linewidth and polar­ization oriented parallel (Π) and perpendicular (σ) to the c axis of the YVO4 crystal. Substantially higher output powers and lower thresholds are obtained with the π orientation. The full widths of these output profiles are consequently narrower by a factor of approximately 3 for the σ orientation. The output power available when pumping the 4F7/2:4S3/2 bands at near 750 nm is, at best, approximately half that available for the same pump power in the 4F5/2:2H9/2 bands near 810 nm. The reduced output power in the shorter wavelength bands can be seen from Fig. 1 to be consistent with the reduced absorption of the pump radiation at these wavelengths.

The input-output curves for the two pump polariza­tions at a wavelength of 808.72 nm are shown in Fig. 2. The laser thresholds for the Π and σ orientations are 34 and 46 mW, respectively. The corresponding slope efficiencies (the ratio of output power to inci­dent power) are 41% and 32%. The output power that was obtained with a pump power of 200 mW (Π orientation) was 68 mW. This compares favorably with the performance obtained with this crystal under similar conditions by using laser diode pump­ing (SLD 303V), where the threshold, slope efficiency, and power output at the 200-mW pump level were measured as 54 mW, 33% and 46 mW, respectively.

Fig. 3. Variation of the output power (Π polarization) and the wavelength of the central peak with temperature.

Fig. 4. Output spectrum of the Nd:YVO4 laser at the indicated temperatures (Celsius).

The improved performance with the Ti:sapphire pumping is presumably due to the superior beam quality of the Ti:sapphire laser. The beam diver­gence of the microchip laser when operated at 68 mW was 14 mrad (full angle).

The laser crystal was mounted on a Peltier device, and this was used to investigate the performance of the laser as the temperature was varied from 0 to 100°C. Figure 3 shows the temperature variation of the output power for the π pump polarization and also the wavelength of the major peak in the output spectrum. The laser frequency varied at a rate of approximately -1.4 GHz/K. As the temperature was increased, the spectrum of the output changed as the laser gain profile moved with respect to the longitudinal modes of the cavity. Figure 4 illus­trates this behavior by showing the output spectrum for temperatures of 1.3, 45.4, and 100°C. The spac­ing of the two fines that are evident in the high-temperature spectrum corresponds to the longitudi­nal mode spacing of the laser resonator. In order to obtain a single longitudinal mode with this laser it is necessary to adjust carefully the crystal temperature to place a cavity mode at the center of the laser gain profile. A wider tuning range, together with single-mode operation, can be obtained by reducing the thickness of the crystal and thus increasing the longitudinal mode separation. This, however, will reduce the laser power because of the reduced absorp­tion of pump radiation. References 1. A. A. Kaminskii, Laser Crystals, Their Physics and Properties,

2nd ed. (Springer-Verlag, New York, 1990), Chap. 3, p. 99. 2. N. P. Barnes, M. E. Storm, P. L. Cross, and M. W. Skolaut,

"Efficiency of Nd laser materials with laser diode pumping," IEEE J. Quantum Electron. 26, 558-568 (1990).

3. G. J. Kintz and T. Baer, "Single-frequency operation in solid-state laser materials with short absorption depths," IEEE J. Quantum Electron. 26, 1457-1459 (1990).

4. T. Taira, A. Mukai, Y. Nozawa, and T. Kobayashi, "Single-mode oscillation of laser-diode-pumped Nd:YVO4 microchip lasers," Opt. Lett. 16, 1955-1957 (1991).

5. R. A. Fields, M. Birnbaum, and C. L. Fincher, "Highly efficient Nd:YVO4 diode-laser end-pumped laser," Appl. Phys. Lett. 51, 1885-1886(1987).

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