Pulsed lasers: Q-switched and Mode-locked techniques

Written by: Dr Baktiar Musa, Suziana Omar, Ir. Dr. Zulzilawati Jusoh and Norizan Ahmed

This article tries to explain about our research on Q-switched and mode-locked lasers. We begin with the definition of laser first, LASER is actually an acronym for Light Amplification by Stimulated Emission of Radiation. Historically, the first laser was realized in 1960 at Hughes Research Laboratories by Theodore H. Maiman following the theoretical work by Charles Hard Townes and Arthur Leonard Schawlow [1].

In order to understand how pulsed lasers work, we need to revisit our fundamental knowledge in physics. But explaining all those fundamentals can be tiresome, so here we just focused on differentiating pulsed laser and continuous wave (CW). CW refers to a laser that is continuously pumped and continuously emits light. The emission can occur in a single resonator mode or on multiple modes. An example of CW laser is CO₂, where initially the gas is ionized to the threshold level and then by using pulse width modulation (PWM), the laser output can be controlled. For comparison, CO₂ molecules readily lase at 10.6 µm, while neodymium-based crystals (like YAG or vanadate) produce wavelengths in the range between 1047 and 1064 nm. Each laser wavelength is associated with a linewidth, which depends on several factors: the gain bandwidth of the lasing medium and the design of the optical resonator [2]. On the other hand, a pulsed laser operates in such a way that all of its energy is dumped out in a single pulse which normally lasts from picoseconds to few nanoseconds. After that the laser output goes to zero. Again, the pulse appears at the output. This switching is done by Q switch.

Two commonly used techniques employed in producing pulsed lasers are Q-switching and mode-locking. A Q-switched laser is a laser to which the technique of active or passive Q switching is applied, so that it emits energetic pulses [3]. Typical applications of such lasers are material processing (e.g. cutting, drilling, laser marking), pumping nonlinear frequency conversion devices, range finding, and remote sensing. Q-switching technique allows the production of light pulses with extremely high (gigawatt) peak power, much higher than would be produced by the same laser if it were operating in a CW mode. Using mode-locking technique, the laser output will be pulses of light of extremely short duration, on the order of picoseconds (10⁻¹² s) or femtoseconds (10⁻¹⁵ s). Here, the laser resonator contains some kind of mode locking device – either an active element (an optical modulator) or a nonlinear passive element (a saturable absorber), which causes the formation of an ultrashort pulse circulating in the laser resonator [4]. In terms of repetition rates and pulsed durations, Q-switched lasers showed lower values compared to ones produced by using mode-locking technique.  Depending on the applications, sometimes the techniques are used together to produce pulsed lasers.

For generation of pulsed laser, a passive mode-lockers are preferred due to their simpler configuration and thus far, a variety type of saturable absorber (SA) have been proposed [3-6]. Our research focused on finding and exploring new materials that are suitable as saturable absorbers. Previously, carbon materials such as carbon nanotubes (CNTs) and graphene show promising performances as saturable absorber to achieve mode-locking in fiber lasers [5, 6]. It offers characteristics such as ultrafast recovery time and capable to achieve broadband operation. Recently, numerous novel 2D materials such as topological insulators [8,9], transition metal dichalcogenide (TMD), black phosphorus, MXene, bismuthene, metal-organic frame-works, and perovskite have demonstrated broad-band optical nonlinearities [7]. The properties of these saturable absorbers will be discussed in the next article.

1.     https://en.wikipedia.org/wiki/Laser

2.     https://www.photonics.com/Articles/Lasers_Understanding_the_Basics/a25161

3.     https://www.rp-photonics.com/q_switched_lasers.html

4.     https://www.rp-photonics.com/mode_locking.html

5.     Luo Z, Liu C, Huang Y, Wu D, Wu J, Xu H, Cai Z, Lin Z, Sun L and Weng J.  IEEE Journal of Selected Topics in Quantum Electronics 20 1-8 (2014)

6.     Bao Q, Zhang H, Wang Y, Ni Z, Yan Y, Shen Z X, Loh K P and Tang D Y.  Advanced Functional Materials 19 3077-83 (2009)

Li, L., Lv, R., Chen, Z. et al. Nanoscale Res Lett 14, 59 (2019)