Semiconductor laser diode produces stabilized optical-frequency combs

Abstract

Over the past decades, the use of lasers in communications, signal processing, test andmeasurement systems, and spectroscopy has enabled many key advances owing to the wide range of useful characteristics inherent in coherent laser radiation. Further advances are expected from simultaneous deployment of multiple lasers spanning a range of wavelengths. This will allow generation of parallel optical channels, thus increasing the system’s measurement and processing capability. Additional functionality is achieved if each optical-frequency channel is phase-locked to the other channels, i.e., if the relative phase relations among all channels are well established, fixed, and not drifting over long times.1 To construct such a set of frequency combs, one can use a single continuous-wave laser, combined with modulation techniques such as amplitude or phase modulation to create sidebands.2 Alternatively, mode-locked lasers can generate a frequency comb with phase-coherent relationships between all wavelength components. The main drawback of a conventional mode-locked laser is that the frequency comb can drift because of both environmental and background quantum effects.3 We have developed an optical source that is capable of producing a set of widely spaced optical frequencies and suitable for a broad range of communication and signal-processing applications. Our approach is also self-referencing because the opticalfrequency comb is referenced to a secondary optical standard, such as a high-quality etalon. To achieve wide optical-channel spacing and high spectral purity, we engineer the optical cavity so that the optical-frequency comb has simultaneous wide channel spacing and a very narrow linewidth for each frequencyFigure 1. Optical-frequency-stabilized mode-locked laser. SOA: Semiconductor optical amplifier. DCF: Dispersion-compensating fiber. I: Isolator. IML: Intensity modulator. FPE: Fabry-Perot etalon. OPS: Optical-phase shifter. VOD: Variable optical delay. PBS: Polarization beam splitter. PC: Polarization controller. PM: Phase modulator. Cir: Circulator. PS: Phase shifter. PD: Photodetector. PID: Proportional-integral-derivative controller. PDH: PoundDrever-Hall.