In optical communications and other semiconductor-laser-based systems, testing the impact of laser linewidth at the source used to require purchase of several different lasers with varying linewidth parameters. But now test and measurement engineers can use a semiconductor laser with variable linewidth developed by researchers at Monash University (Clayton, Victoria, Australia). The linewidth variation does not impact the laser's output power, relative intensity noise, or modal structure, making the laser ideal for applications such as assessing the impact of dispersive fibers on system noise.
The phase of a narrow-linewidth laser is modulated to broaden its linewidth. The random phase modulation is digitally generated and applied using a modulator comprising two Mach-Zehnder interferometers: one to modulate an "in-phase" portion of the incoming light; the other to modulate a 90° shifted "quadrature" portion. The portions are recombined so any phase can be obtained by adjusting the ratio of the two portions. Unlike a single lithium niobate modulator whose phase modulation is limited by its drive voltage (requiring that it be reset periodically to add or subtract 2 radians when a voltage limit is reached), the complex modulator allows the phase to be driven up or down indefinitely without reset events. The linewidth is adjustable from the original narrow linewidth of the laser (in this case, 256 kHz) to 150 MHz (both full-width half-maximum values). The linewidth remains stable over several hours because it is generated digitally. Contact Arthur James Lowery at [email protected].