Abstract
This paper describes the first foundry-based InP photonic integrated circuit (PIC) designed to work within a heterodyne optical phase locked loop (OPLL). The PIC and an external electronic circuit were used to phase-lock a single-line semiconductor laser diode to an incoming reference laser, with tuneable frequency offset from 4 GHz to 12 GHz. The PIC contains 33 active and passive components monolithically integrated on a single chip, fully demonstrating the capability of a generic foundry PIC fabrication model. The electronic part of the OPLL consists of commercially available RF components. This semi-packaged system stabilizes the phase and frequency of the integrated laser so that an absolute frequency, high-purity heterodyne signal can be generated when the OPLL is in operation, with phase noise lower than -100 dBc/Hz at 10 kHz offset from the carrier. This is the lowest phase noise level ever demonstrated by monolithically integrated OPLLs.
Highlights
Optical phase lock loops (OPLL) have been studied for decades resulting in numerous demonstrations of homodyne [1] and subsequently heterodyne [2] laser phase locking through the implementation of an electronic feedback loop
We have demonstrated the first OPLL based on generic foundry-fabricated photonic integrated circuit (PIC) and off-theshelf electronic commercial components creating potentially a simpler overall circuit assembly than previous such systems
The PIC, designed to offer distributed Bragg reflector (DBR) laser monolithically integrated with photodiode and low-loss optical interconnections, was successfully fabricated and demonstrated all the necessary performance to operate within an optical phase lock loop
Summary
Optical phase lock loops (OPLL) have been studied for decades resulting in numerous demonstrations of homodyne [1] and subsequently heterodyne [2] laser phase locking through the implementation of an electronic feedback loop. The development of semiconductor laser diodes [4] led to a more compact and tuneable solution, their wider linewidth and higher frequency instabilities [5] required a broadband phase lock loop circuit and short loop delay To alleviate these limitations, hybrid integrated OPLLs [6, 7] were demonstrated. More recent progress in both photonic and electronic integration has resulted in monolithically integrated OPLLs, with small dimensions, operating as homodyne [8, 9] or heterodyne [10, 11] systems The compactness of such phase-locked optical sources is of particular interest due to numerous applications in optical communication, spectroscopy and high-frequency, high-purity signal generation through heterodyning [12].
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