Quasi-cw diode-pumped Nd:GdVO4 laser passively Q-switched and mode-locked by Cr4+:YAG saturable absorber

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Abstract

We report on the passively Q-switched mode-locking of a quasi-cw (QCW) diode pumped Nd:GdVO4 laser with Cr4+:YAG saturable absorber. We show that by using the QCW pumping high quality Q-switched mode-locking can be easily obtained in the laser even with a simple two-mirror laser cavity configuration. Mode locked pulses with pulse width of about 1 ps at repetition rate variable from 500 MHz to 1 GHz has been obtained.

Introduction

The Nd3+-doped crystals due to their excellent lasing property and availability of direct laser diode pumping have been widely used for the diode pumped solid-state lasers. Apart from the traditional Nd:YAG and Nd:YVO4 crystals, recently a new type of Nd3+-doped vanadate crystal – Nd:GdVO4 – has also attracted great attention. The Nd:GdVO4 is an isomorph of Nd:YVO4 crystal, however, compared with the Nd:YVO4, it has even larger absorption cross-section and higher thermal conductivity along the 〈1 0 0〉 direction, which makes it promising even for the high power diode pumped solid-state lasers. Like Nd:YVO4 crystal, Nd:GdVO4 has a very large stimulated emission cross-section. A recent measurement showed that the stimulated emission cross-section of the Nd:GdVO4 is in the range of 14.76 × 10−19–16.4 × 10−19 cm2, which is higher than that of the Nd:YVO4 (12 × 10−19 cm2) [1]. In addition, Nd:GdVO4 crystals have a broad gain bandwidth of about 1.3 nm, which is broader than those of the Nd:YVO4 (0.96 nm) and Nd:YAG (0.45 nm). This property of the Nd:GdVO4 indicates that it has a potential to generate even narrower mode-locked pulses. Up to now, passive mode-locking of the diode-pumped Nd:GdVO4 lasers has been demonstrated by several groups. Sorokin et al. [2] firstly reported the passive mode locking of the Nd:GdVO4 laser. In their experiment, a mode-locked pulse width of 1.9 ps was achieved. Mode–locking of the laser with various solid-state saturable absorbers such as the saturable Bragg reflector (SBR) [3], semiconductor saturable absorber mirror (SESAM) [4], [5] and GaAs wafer [6] was also reported.

The Cr4+:YAG crystal is a well-known solid-state saturable absorber widely used for Q-switching the Nd3+-doped solid-state lasers [7], [8]. Cr4+:YAG has a large saturable absorption cross-section in the 0.9–1.2 μm spectral region, high damage threshold and excellent thermal and optical properties. Passively Q-switched operation of the Nd:GdVO4 lasers by using the Cr4+:YAG as saturable absorber has also been reported [9], [10]. A special property of the Cr4+:YAG saturable absorber is that apart from the ground state absorption, it also has strong excited state absorption (ESA). Different to the ground state absorption, which has a recovery time of a few microseconds, ESA has a much shorter recovery time that is in the sub-nanosecond range [11]. This property of the Cr4+:YAG saturable absorber makes that in the case of strong ground state absorption, Q-switched mode-locking could also appear in the passively Q-switched lasers. Q-switched mode locking of the diode-pumped Nd:YAG and Nd:YVO4 lasers with Cr4+:YAG saturable absorber has already been experimentally demonstrated [12], [13], [14]. Very recently, Zhang et al. [15] have also demonstrated the Q-switched mode-locking of a diode-pumped Nd:GdVO4 laser with Cr4+:YAG saturable absorber. As the ESA effect only takes place after that the ground state of the absorber is strongly saturated, for using the ESA to generate the passive mode locking, the laser beam intensity on the Cr4+:YAG must be sufficiently strong. Therefore, a common characteristic of previous lasers with the Cr4+:YAG to achieve the Q-switched mode-locking is that they have used a folded mirror cavity configuration, as with such a cavity configuration a relatively small beam spot on the Cr4+:YAG crystal and a good mode matching between the laser mode and the pump beam can be simultaneously achieved. However, this laser cavity configuration is complicated in alignment and also difficult to easily change the mode-locked pulse repetition rate. In this paper, we report on a quasi-cw (QCW) diode pumped, passively Q-switched and mode-locked Nd:GdVO4 laser with Cr4+:YAG saturable absorber. We demonstrate experimentally that through using the QCW laser diode pumping, high quality Q-switched mode-locking with the Cr4+:YAG crystal can be easily obtained even with a simple plane-parallel two-mirror laser cavity. By simply changing the cavity length, we further showed that the mode locked pulse repetition rate of our laser could be varied over a wide range from 500 MHz to 1 GHz. Mode-locked pulses with pulse width of about 1 ps have been obtained.

Section snippets

Experimental

A schematic diagram of the laser setup is shown in Fig. 1. A QCW fiber-coupled laser diode array (Jenoptik, JOLD-120-QPXF-2P iTEC) was used as the pump source. The center emission wavelength of the laser diode array is 808 nm at 25 °C. The diode radiation from the 0.6 mm fiber bundle was focused into the laser crystal with an optical re-imaging system (Coherent, OIA 1:1.8). The pump beam in the crystal has a beam size of about 340 μm in diameter. An a-cut 0.5 at.% Nd:GdVO4 crystal with a dimension

Conclusions

In conclusion, we have experimentally demonstrated passively Q-switched mode-locking in a QCW laser diode pumped Nd:GdVO4 laser with Cr4+:YAG saturable absorbers. We found that by using the QCW pumping, high quality Q-switched mode-locking can be easily obtained in the laser even with a simple two-mirror laser cavity configuration. Mode-locked pulses with pulse width of about 0.97 ps and variable repetition rate from 500 MHz to 1 GHz have been obtained. The advantage of using QCW pumping compared

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