Abstract
We report a photonic integrated circuit implementation of an optical clock multiplier, or equivalently an optical frequency comb filter. The circuit comprises a novel topology of a ring-resonator-assisted asymmetrical Mach-Zehnder interferometer in a Sagnac loop, providing a reconfigurable comb filter with sub-GHz selectivity and low complexity. A proof-of-concept device is fabricated in a high-index-contrast stoichiometric silicon nitride (Si3N4/SiO2) waveguide, featuring low loss, small size, and large bandwidth. In the experiment, we show a very narrow passband for filters of this kind, i.e. a -3-dB bandwidth of 0.6 GHz and a -20-dB passband of 1.2 GHz at a frequency interval of 12.5 GHz. As an application example, this particular filter shape enables successful demonstrations of five-fold repetition rate multiplication of optical clock signals, i.e. from 2.5 Gpulses/s to 12.5 Gpulses/s and from 10 Gpulses/s to 50 Gpulses/s. This work addresses comb spectrum processing on an integrated platform, pointing towards a device-compact solution for optical clock multipliers (frequency comb filters) which have diverse applications ranging from photonic-based RF spectrum scanners and photonic radars to GHz-granularity WDM switches and LIDARs.
Highlights
Optical clock signals with the spectrum of a frequency comb have enabled a number of industrial applications, such as data carrier for fiber-optic communications and networks [1,2], optical coherence tomography in medical diagnosis [3], free-space communications in defense [4], light detection and ranging (LIDAR) for topography and airborne observatory [5], astrocombs [6,7], photocurrent mapping [8] and Raman spectroscopy [9] in material science
For LIDAR systems, the optical clock rate variability allows for optimum detection of objects at different distances or with different reflections [11]
Spatial light modulators are a straightforward approach to synthesize arbitrary filter shapes [20]. This approach requires a combination of free-space optical devices which typically have a spectral resolution about 10 GHz [21] and needs critical control of stability
Summary
Optical clock signals (periodically pulsed signals) with the spectrum of a frequency comb have enabled a number of industrial applications, such as data carrier for fiber-optic communications and networks [1,2], optical coherence tomography in medical diagnosis [3], free-space communications in defense [4], light detection and ranging (LIDAR) for topography and airborne observatory [5], astrocombs [6,7], photocurrent mapping [8] and Raman spectroscopy [9] in material science. PIC-based filters enjoy the general advantages inherent to the PIC technologies [22,23,24,25,26,27,28] such as small size, low weight, and low power consumption (small SWaP), ultimate stability and control precision, and strong potential for low-cost volume fabrication, and offer great design flexibility and the possibility to be incorporated in programmable signal processors [29,30] Such filters conventionally suffer from a tradeoff between the key performance metrics such as operation bandwidth, circuit complexity, insertion loss, and the chip size. This circuit topology provides a promising solution for optical comb filters that have significantly lower requirement for control precision than conventional ring resonator-based filters [33,34,35,36] and have a 2-orders-of-magnitude advantage in size when compared with tapped-delay-line filters [37,38]
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