Abstract
This paper describes an optical phase lock loop (OPLL) implemented as an ultraselective optical frequency comb line filter. The OPLL is based on a photonic integrated circuit (PIC) fabricated for the first time through a generic foundry approach. The PIC contains a distributed Bragg reflector (DBR) laser whose frequency and phase are stabilized by reference to an optical frequency comb generator. The OPLL output is a single-mode DBR laser line; other comb lines and noise at the output of the OPLL filter are attenuated by >58 dB below the peak power of the OPLL-filter output line. The OPLL bandwidth is up to 200 MHz, giving a filter quality factor greater than 1,000,000. The DBR laser can be tuned over 1 THz (8 nm), enabling different comb lines to be selected. Locking to a comb line with a frequency offset precisely selectable between 4 and 12 GHz is also possible. The coherence between the DBR laser and the comb lines is demonstrated by measurements of the heterodyne signal residual phase noise level, which is below −100 dBc/Hz at 5 kHz offset from the carrier. The OPLL-filter output can be up to 6 dB higher than the peak power of the comb line to be isolated by the filter. This optical gain is a unique characteristic which can significantly improve the SNR of communication or spectroscopy systems. This OPLL is envisaged to be used for high purity, tuneable microwave, millimetre-wave, and THz generation.
Highlights
M ICROWAVE Photonics is an interdisciplinary research field that looks into developing new, and optimising existing, ways of using photonic devices for generation, manipulation, distribution and detection of microwave, millimeter-wave and terahertz (THz) signals [1], [2]
This paper focuses on a laser locking technique which can be implemented to isolate a single line from an optical frequency comb generator (OFCG) with closely spaced (
The presented optical phase lock loop (OPLL) demonstrates a good example of a system where a foundry fabricated photonic integrated circuit (PIC) and commercial, offthe-shelf (COTS) electronics have been used to build an optoelectronic feedback loop capable of phase stabilising the semiconductor laser to the optical reference with offset frequency setting resolution of 1 Hz
Summary
M ICROWAVE Photonics is an interdisciplinary research field that looks into developing new, and optimising existing, ways of using photonic devices for generation, manipulation, distribution and detection of microwave, millimeter-wave (mm-wave) and terahertz (THz) signals [1], [2]. A narrow linewidth microwave source with a stable absolute frequency, the phase coherence between optical tones must be maintained This can be achieved through direct or external modulation of a single laser if a signal of a few tens of GHz is to be generated, the frequency limit being set by laser or external modulator bandwidth. While passive optical filters are available commercially and are widely researched, one could alternatively use an independent laser (slave laser) source that could be phase stabilized using optical injection locking (OIL) [9] or an optical phase lock loop (OPLL) [10] This approach enables frequency tracking and the provision of optical gain within the filter. The OPLL, as presented in this paper, would find applications in coherent communication [11], LIDAR and clock signal distribution
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