Abstract
An Optical Phase Lock Loop (OPLL) is a feedback control system that allows the phase stabilization of a laser to a reference laser with absolute but adjustable frequency offset. Such phase and frequency locked optical oscillators are of great interest for sensing, spectroscopy, and optical communication applications, where coherent detection offers advantages of higher sensitivity and spectral efficiency than can be achieved with direct detection. As explained in this paper, the fundamental difficulty in realising an OPLL is related to the limitations on loop bandwidth and propagation delay as a function of laser linewidth. In particular, the relatively wide linewidth of semiconductor lasers requires short delay, which can only be achieved through shortening of the feedback path, which is greatly facilitated through photonic integration. This paper reviews the advances in the development of semiconductor laser-based OPLLs and describes how improvements in performance have been enabled by improvements in photonic integration technology. We also describe the first OPLL created using foundry fabricated photonic integrated circuits and off-the-shelf electronic components. Stable locking has been achieved for offset frequencies between 4 and 12 GHz with a heterodyne phase noise below –100 dBc/Hz at 10 kHz offset. This is the highest performance yet reported for a monolithically integrated OPLL and demonstrates the attractiveness of the foundry fabrication approach.
Highlights
R ESEARCH on optical phase lock loops started at an early stage of laser development when the use of lasers for optical communication applications began to be considered [1], [2]
A single mode laser is usually used as the optical reference tone, a sideband resulting from a modulated laser or a line from an optical frequency comb can be used as the reference tone for the optical phase lock loop (OPLL)
We provide an extensive overview of semiconductor laser-based OPLL systems developed over the years, with a particular focus on the effect the improvements in photonic integration have had on the progress in their development
Summary
R ESEARCH on optical phase lock loops started at an early stage of laser development when the use of lasers for optical communication applications began to be considered [1], [2]. The application of this technique is restricted by its homodyne nature and by a small locking range – typically up to a few hundred MHz – it is necessary to control the laser temperature with milli-Kelvin precision [7] This latter requirement is overcome in the OIPLL technique, in which wideband phase noise is suppressed, thanks to the injectionlocking mechanism, while the laser frequency drift and closeto-carrier phase noise are controlled through a phase-lock loop path. We present, for the first time, an OPLL realised using a generic foundry fabrication process and off-the-shelf microwave SMA-connectorised components This novel OPLL is based on a tuneable Distributed Bragg Reflector (DBR) laser, which can be phase stabilised to the reference laser with a frequency offset in the range 4 GHz to 12 GHz. The generated heterodyne phase noise is less than −100 dBc/ Hz at 10 kHz offset from carrier for a separation of 8 GHz between the lasers.
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