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The Basics of Lasers
Published in Helmut H. Telle, Ángel González Ureña, Laser Spectroscopy and Laser Imaging, 2018
Helmut H. Telle, Ángel González Ureña
For passive mode locking, saturable absorbers are used—as in passive Q-switching; these include liquid organic dyes, doped crystals, and semiconductor devices. In particular, the latter exhibit very fast response times, of the order of 100fs. Essentially, the saturable absorber can modulate the resonator much faster than any (active) electronic devices, thus allowing for much shorter pulse durations than are achievable by active mode locking.
Advanced Beam Manipulation, Cooling, Damping and Stability
Published in Andrei Seryi, Unifying Physics of Accelerators, Lasers and Plasma, 2015
Mode locking is the technique for achieving short laser pulses. In this method a rapid modulator is installed in the laser cavity, one that can open for short moments exactly in sync with the pulse’s round-trip time around the cavity; see Fig. 10.13. Therefore, all the generated and amplified photons will be clustered only within those short moments when the modulator is open.
Basic Photon and Quantum Optics
Published in Daniel Malacara-Hernández, Brian J. Thompson, Fundamentals and Basic Optical Instruments, 2017
A major advance in the generation of ultrashort optical pulses has been the process of mode-locking. Another approach to generating short pulses is the process of pulse compression. Some other novel pulse-generation schemes have been recently developed. We will try to describe them briefly along with applications.
Mode-locked thulium doped fibre laser with copper thin film saturable absorber
Published in Journal of Modern Optics, 2019
A. R. Muhammad, M. T. Ahmad, R. Zakaria, P. P. Yupapin, S. W. Harun, M. Yasin
Figure 5 shows the output pulse train profile of the mode-locked TDFL at four different pump powers namely 387.4, 458.4, 529.4 and 600.4 mW. A stable pulse period of around ∼117.35 ns is maintained at various pump power. This pulse period indicates the repetition rate of the pulses. From the oscilloscope, a single pulse envelope is shown to have a pulse width of 55.7 ns. This is much broader than the actual pulse width due to the resolution limitation of the oscilloscope used in this work. However, the pulse width can be numerically determined using the time-bandwidth product (TBP) analysis. By assuming that the TBP is 0.315 for Sech2 pulse fitting, the minimum possible pulse width corresponding to 3-dB spectral bandwidth (0.648 nm) is about 14.80 ps. This figure matches with the existence of a few harmonics in the RF spectrum, based on the Fourier transform. Moreover, it indicates that only a few longitudinal modes are locked with no pulse distortion. Thus, the laser is expected to have a low timing jitter and an excellent mode-locking stability.
Ultra-short pulse generating in erbium-doped fiber laser cavity with 8-Hydroxyquinolino cadmium chloride hydrate (8-HQCdCl2H2O) saturable absorber
Published in Journal of Modern Optics, 2021
Mustafa Mohammed Najm, Hamzah Arof, Bilal Nizamani, Ahmed Shakir Al-Hiti, Pei Zhang, Moh Yasin, Sulaiman Wadi Harun
Mode-locking is a laser pulsing technique that widely contributes to the field of optical communications due to its ability to produce picosecond and femtosecond pulses [1]. The mode-locking technique consists of two different schemes: passive and active. The active scheme modulates the loss inside the laser cavity by a complex acoustic-optic modulator [2]. On the other hand, the passive scheme normally employs a thin-film saturable absorber (SA) fabricated by functional materials. Besides, it has characteristics of simplicity in the design, flexible configuration, and compactness [3].
Diode-pumped passively mode-locked Nd:GYSGG laser at 1061 nm with periodically poled LiNbO3 nonlinear mirror
Published in Journal of Modern Optics, 2020
Fangxin Cai, Luyang Tong, Ye Yuan, Yangjian Cai, Lina Zhao
Disordered crystals as laser gain media have attracted widespread attention in recent years, such as Nd:CNGG [1–4], Nd:CLNGG [5–7], Nd:CTGG [8] Nd:CaYAlO4 [9,10] and Nd:GYSGG [11–17]. Among them, Nd:GYSGG is obtained by introducing some Gd3+ instead of part of the Y3+ ion in Nd:YSGG. Nd:GYSGG crystal has a relatively high effective segregation, which is very important for growing high Nd3+ concentration crystals [11,12]. Nd:GYSGG has several advantages compared with others. Firstly, it has good thermal conductivity, thus it is suitable for high power laser operation. Secondly, the inhomogeneous broadening of the absorption and emission spectra of the crystal is beneficial for laser diode pumping and ultrashort pulse generation. Thirdly, due to the splitting of the spectral lines it is suitable for generating multi-wavelength pulses. In recent years, continuous wave (CW), passively dual-wavelength Q-switched, passively Q-switched mode-locked (QML), CW mode-locked (CWML) Nd:GYSGG laser using different types of saturable absorbers have been researched [12–18]. Zhang et al. reported the performance of CW and passively Q-switched pulse with 6.6 ns using Cr4+:YAG wafer as saturable absorber [12], and obtained a 3.1 ps mode-locked pulse with spectral bandwidth of 0.9 nm at wavelength of 1061.5 nm using a semiconductor saturable absorber mirror (SESAM) in 2011 [13]. Qi Song et al. demonstrated passively dual-wavelength Q-switched pulse with 115 ns using graphene oxide as the saturable absorber [14], dual-wavelength QML with pulse duration of 441 ps with graphene oxide [15] in 2015 and dual-wavelength Q-switched pulse of 254 ns with TiS2, MoS2, WS2/Sb2Te3 mixed nanosheets saturable absorber in 2018 [16]. Baomin Ma reported dual-wavelength passively Q-switched laser of 374 ns with gold nano-triangles in 2018 [17]. Besides SESAM and 2D saturable absorbers, nonlinear mirror (NLM) mode-locking is also a promising method to generate ultrashort pulse. The NLM mode-locking technique was proposed by Stankov and Jethwa in 1988 [19]. The NLM consists of a frequency doubling crystal and a dichroic mirror. The dichroic mirror is highly reflective at the second harmonic (SH) wave and partially reflective at the fundamental wave (FW). When the FW passes through the nonlinear crystal for the first time, it is converted to SH, then SH and FW are reflected by the dichroic mirror and traverse nonlinear crystal for the second time. If a specific phase difference between SH and FW is introduced, SH will be reversed to the FW. Due to the different reflectivity of the SH and FW, a nonlinear positive feedback is established. The nonlinear mirror acts like an effective saturable absorber, leading to passively mode-locking. Commonly there are two types of frequency doubling crystals used for NLM mode-locking. They are birefringence-phase-matching crystals or quasi-phase-matching crystals [20–26]. Especially PPLN which is based on quasi-phase-matching has higher effective nonlinear coefficients deff and no walk-off effect, thus it is a good candidate for NLM.