Sub-nanosecond pulse generation in a microchip Nd:La_0.15Gd_0.85VO₄ laser

Highlights

• A sub-nanosecond Nd:La_0.15Gd_0.85VO₄ microchip laser was built for the first time.

• The luminescence parameters of Nd:La_0.15Gd_0.85VO₄ for ⁴F_3/2→⁴I_J’/2 were listed.

• The system transferred single picosecond pulse into two pulse train form.

• The pulse shape for Nd:La_0.15Gd_0.85VO₄/Cr⁴⁺:YAG laser was rebuilt by model.

• The shortest pulse, highest peak power reached 880 ps, 3.5 kW, respectively.

Abstract

A sub-nanosecond microchip laser was constructed using a Nd:La_0.15Gd_0.85VO₄ with Cr⁴⁺:YAG saturable absorber. A single 880 ps pulse was recorded under an incident pump power of 4.91 W, corresponding to a repetition rate of 35 kHz. When the incident pump power exceeded 5 W, the system transferred a single picosecond pulse train into 0.9/1.1 ns two pulse train forms. The average output power increased from 108 mW to 130 mW, corresponding to a peak power decrease from 3.5 kW to ~1.8 kW, which might be utilized for laser pulse self-calibrations in time-of-flight distance measurements.

Introduction

Diode pumped passively Q-switched microchip lasers have highly compact structures, large pulse energy, and short pulse width, so they are well known as sub-nanosecond pulse sources for medical care, spectral analysis, and radar detection [1], [2], [3]. Sub-nanosecond microchip lasers contain saturable absorbers (SAs), such as Cr ion doped crystals (Cr⁴⁺:YAG and Cr²⁺:ZnS), gallium arsenide (GaAs), and semiconductor saturable absorber mirrors (SESAMs) [4], [5], [6], [7]. Cr⁴⁺:YAG is a typical partner with neodymium (Nd³⁺) or ytterbium (Yb³⁺) doped gain medium for generating ~1 μm high energy (μJ to mJ) and high peak power (kW to MW) pulses [4], [8], [9], [10], [11]. To date, many gain media such as garnet, tungstate, borate, and vanadate [8], [9], [10], [11], [12], [13], [14], [15], [16] have been reported for ~1 μm sub-nanosecond lasers.

Neodymium doped vanadate is commonly used as a gain medium in green microchip lasers. The large stimulated emission cross-sections enable vanadate microchip lasers to easily operate in continuous wave modes. But a barrier must be placed for sub-nanosecond laser oscillations. Researchers have explored several methods of constructing sub-nanosecond vanadate microchip lasers. Lightly doped and thick crystals combined with quasi-continuous wave pumping are one technique [9]. Highly doped thin crystals are another method [16], [17]. By changing the crystal orientation or host crystal field, smaller stimulated emission cross-sections can be obtained. This is the third method [18], [19], [20].

Compared to Nd:YAG crystals, Nd:GdVO₄ possesses excellent thermal conductivity along the 〈0 0 1〉 and 〈1 0 0〉 directions [21]. But stimulated emission cross-section of Nd:GdVO₄ at 1.06 μm is 2.7 times the value of Nd:YAG. In Nd:GdVO₄ crystals, gadolinium (Gd) can be slightly substituted by lanthanum (La) ions, which have the largest ion radii in rare earth ions. Notably inhomogeneous broadened spectra of Nd:La_xGd_1-xVO₄ crystals were reported [22], [23], [24], [25], [26]. The maximum stimulated emission cross-section of Nd:La_0.15Gd_0.85VO₄ is 1.3 times the value of Nd:YAG. This indicates that Nd:La_0.15Gd_0.85VO₄ can be candidate materials for microchip laser gain media operating at short pulse range. To date, there are no reports on sub-nanosecond micro-type Nd:La_xGd_1-xVO₄ crystal lasers.

In this paper, we reported a passively Q-switched sub-nanosecond Nd:La_0.15Gd_0.85VO₄/Cr⁴⁺:YAG microchip-type laser for the first time. The maximum stimulated emission cross-section of 3.62 × 10⁻¹⁹ cm² was much smaller than that of Nd:GdVO₄ crystals. Using lightly doped Nd:La_0.15Gd_0.85VO₄ and adjusting the incident pump power, we obtained 880 ps pulses with a pulse energy of 3.1 μJ, corresponding to a peak power of 3.5 kW. When the incident pump power exceeded 5 W, the system transferred from a single picosecond pulse train into 0.9/1.1 ns two pulse trains.