Vol.:(0123456789) 1 3 Applied Physics B (2021) 127:54 https://doi.org/10.1007/s00340-021-07593-x REGULAR PAPER Passively Q‑switched Cr:LiCAF laser with a saturable Bragg refector Serdar Okuyucu 1  · Yusuf Ozturk 1  · Umit Demirbas 1,2 Received: 9 January 2021 / Accepted: 11 February 2021 / Published online: 19 March 2021 © The Author(s), under exclusive licence to Springer-Verlag GmbH, DE part of Springer Nature 2021 Abstract We present a low-cost multimode diode pumped passively Q-switched Cr:LiCAF laser operating near 800 nm. AlGaAs-based saturable Bragg refectors (SBRs) were used for passive Q-switching. The system is experimentally characterized in detail using four diferent output couplers and two SBRs with diferent modulation depths. Pulse widths in the 1.5–4 µs range at repetition rates between 18 and 40 kHz were achieved with average powers up to 125 mW. The experimentally obtained results are compared with basic Q-switching theory and further performance improvement in terms of pulse length shorten- ing and peak power scaling is elaborated. 1 Introduction Implementation of lasers with pulse widths in the nano- second–microsecond range is desired for a variety of applications including range fnding, nonlinear frequency conversion, material processing, and remote sensing [1]. Q-switching or gain switching techniques are usually employed in obtaining short, high-energy pulses with high peak powers. In Q-switching, intracavity loss of a laser reso- nator is modulated to produce pulsed output; whereas, gain- switched operation is based on pulsed pumping of the laser material with a pump pulse width in the order of the fuores- cence lifetime of the gain material. In Q-switched operation, resonator losses can be modulated by the use of active or passive modulation schemes [2, 3]. In the implementation of actively Q-switched lasers, acousto-optic modulators, elec- tro-optic modulators, or rotating mirrors with external driv- ing circuitry are typically used, but these systems sufer from higher cost, lack of compactness, and increased complexity. On the other hand, implementation of passively Q-switched lasers is more economical, simple, and practical where cav- ity losses are modulated by the use of saturable absorbers, such as dye-based absorbers, Cr 4+ :YAG (for 1μm lasers) [4], Cr 2+ :ZnSe (for 1.5μm and 2μm lasers) [5], or SESAM/SBR- based nonlinear mirrors (semiconductor saturable absorber mirror [6]/saturable Bragg refector [7]). However, in passive Q-switching, it is harder to provide independent control of parameters such as pulse shape, pulse energy, peak power, and pulse repetition rate. Ti:Sapphire [8], Alexandrite [9], and Cr:Colquiriite crys- tals (Cr:LiSAF [10], Cr:LiCAF [11], Cr +3 :LiSGaF [12], and Cr:LiSCAF [13]) are among the favorable solid-state laser gain media that could generate broadly tunable Q-switched output in the ∼ 700–1100 nm range and below (375–550 nm and 235–365 nm regions via frequency doubling/tripling [14–16]). In this work, we focus our attention on Cr:Colquiriites, which attracted a renewed interest over the last two dec- ades, due to the brightness improvement observed in LED (light-emitting diode) and laser diode pump sources. Strong electron–phonon coupling creates three intense and broad absorption bands in Cr:Colquiriite gain media that are cen- tered around ∼275 nm, ∼445 nm, and ∼640 nm [10, 17]. The 640 nm transition ( 4 A 2 to 4 T 2 excitation) has a full- width-half-maximum (FWHM) approaching 100 nm, and it is strong both for the E//a and E//c axis in the uniaxial Cr:Colquiriite crystals. This broad absorption band facili- tates direct-diode pumping of Cr:Colquiriites by low cost and well-developed AlGaAs- and AlGaInP-based diodes in the red spectral region. Additionally, at room temperature, Cr:Colquiriites have upper state fuorescence lifetimes (  f ) that are ∼20–50 times longer than that of Ti:Sapphire (67 μs in Cr:LiSAF, 175 μs in Cr:LiCAF, 3.2 μs in Ti:Sapphire [ 18]). Moreover, unlike Alexandrite and Ti:Sapphire * Serdar Okuyucu serdar.okuyucu@antalya.edu.tr 1 Laser Technology Laboratory, Department of Electrical and Electronics Engineering, Antalya Bilim University, 07190 Antalya, Turkey 2 Center for Free-Electron Laser Science, Deutsches Elektronen Synchrotron, Hamburg 22607, Germany