Abstract
Semiconductor mode-locked lasers (MLLs) are promising frequency comb sources for dense wavelength-division-multiplexing (DWDM) data communications. Practical data communication requires a frequency-stable comb source in a temperature-varying environment and a minimum tone spacing of 25 GHz to support high-speed DWDM transmissions. To the best of our knowledge, however, to date, there have been no demonstrations of comb sources that simultaneously offer a high repetition rate and stable mode spacing over an ultrawide temperature range. Here, we report a frequency comb source based on a quantum dot (QD) MLL that generates a frequency comb with stable mode spacing over an ultrabroad temperature range of 20–120°C. The two-section passively mode-locked InAs QD MLL comb source produces an ultra-stable fundamental repetition rate of 25.5 GHz (corresponding to a 25.5 GHz spacing between adjacent tones in the frequency domain) with a variation of 0.07 GHz in the tone spacing over the tested temperature range. By keeping the saturable absorber reversely biased at − 2 V , stable mode-locking over the whole temperature range can be achieved by tuning the current of the gain section only, providing easy control of the device. At an elevated temperature of 100°C, the device shows a 6 dB comb bandwidth of 4.81 nm and 31 tones with > 36 dB optical signal-to-noise ratio. The corresponding relative intensity noise, averaged between 0.5 GHz and 10 GHz, is − 146 dBc / Hz . Our results show the viability of the InAs QD MLLs as ultra-stable, uncooled frequency comb sources for low-cost, large-bandwidth, and low-energy-consumption optical data communications.
Highlights
Optical frequency combs consisting of spaced discrete optical frequency components have emerged as promising tools for a wide range of applications including metrology, optical communications, optical clock distribution/recovery, radioover-fiber signal generation, and optical sampling [1,2,3,4,5,6,7]
The effect of temperature on thermal expansion of the laser and the change of refractive index is well balanced, and the tone spacing remains ultra-stable with temperature, which is a desirable feature for optical communications
Given that the temperature-dependent operation of a passive two-section mode-locked laser (MLL) involves a mutual interplay of the gain section and the saturable absorber (SA), here, for the device presented, with the SA reversed biased at a constant voltage, stable mode-locking over an ultrabroad temperature range has been achieved by adjusting the electrical biasing conditions, which yields an added benefit of bias simplicity
Summary
Optical frequency combs consisting of spaced discrete optical frequency components have emerged as promising tools for a wide range of applications including metrology, optical communications, optical clock distribution/recovery, radioover-fiber signal generation, and optical sampling [1,2,3,4,5,6,7]. The recent development of quantum dot (QD) semiconductor material promises an ultrabroad gain bandwidth and ultrafast carrier dynamics [9], with many other important features including large gain and saturable absorber (SA) saturation energy ratio [10], low spontaneous emission rate [11], and the capability for monolithic integration with silicon substrates [12,13,14,15] These promising features have inspired much research in the development of highperformance QD MLLs [16,17,18,19,20,21] and their applications for multi Tbit/s communications [22,23,24,25,26]. The demonstrated performance suggests the developed QD MLL is a strong candidate for an ultra-stable, uncooled frequency comb light source that can be employed in a low-cost optical network system with high capacity and efficiency
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