Non-destructive testing using laser-generated ultrasound has a wide range of applications owing to its non-contact and broadband nature. However, there are applications that require a narrowband generation. Narrowband guided ultrasonic waves are attractive for long-range inspection of plate-like structures. Laser-generation of narrowband ultrasonic waves is possible using comb transduction techniques. Comb transduction enables constant wavelength excitation of ultrasonic waves which are relatively easy to post-process. This research work explores one such comb transduction technique known as a pattern source. Guided waves (Lamb waves) in the frequency range 1 - 10 MHz were generated. Effect of variation of the duty ratio of the pattern source on guided wave generation on a 0.6 mm thick aluminium plate is studied analytically and numerically and a mode selection method is discussed in this presentation.