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.Index Terms-Optical phase locked loops, photonic integrated circuits, semiconductor laser, microwave photonics.

Abstract:We present a review of recent developments in THz coherent systems based on photonic local oscillators. We show that such techniques can enable the creation of highly coherent, thus highly sensitive, systems for frequencies ranging from 100 GHz to 5 THz, within an energy efficient integrated platform. We suggest that such systems could enable the THz spectrum to realize its full applications potential. To demonstrate how photonics-enabled THz systems can be realized, we review the performance of key components, show recent demonstrations of integrated platforms, and give examples of applications. References and links1. A. G. Davies and E. H. Linfield, eds., "Special Supplement: THz Technology," Electron. Lett. 46(26), (2010). 2. S. J. Savory, "Digital filters for coherent optical receivers," Opt. Express 16(2), 804-817 (2008). 3. H. Eisele, "480GHz oscillator with an InP Gunn device," Electron. Lett. 46(6), 422-423 (2010). 4. L. Moeller, J. Federici, and K. Su, "THz wireless communications: 2.5 Gb/s error-free transmission at 625 GHz using a narrow-bandwidth 1 mW THz source," XXXth URSI General Assembly and Scientific Symposium (2011). 5. C. C. Renaud, M. Robertson, D. Rogers, R. Firth, P. J. Cannard, R. Moore, and A. J. Seeds, "A high responsivity, broadband waveguide uni-travelling carrier photodiode," Proc. SPIE 61940, 61940C, 61940C-8 (2006

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We experimentally demonstrate photonic generation of a multichannel THz wireless signal at carrier frequency 200 GHz, with data rate up to 75 Gbps in QPSK modulation format, using an optical heterodyne technique and digital coherent detection. BER measurements were carried out for three subcarriers each modulated with 5 Gbaud QPSK or for two subcarriers modulated with 10 Gbaud QPSK, giving a total speed of 30 Gbps or 40 Gbps, respectively. The system evaluation was also performed with three subcarriers modulated with 12.5 Gbaud QPSK (75 Gbps total) without and with 40 km fibre transmission. The proposed system enhances the capacity of high-speed THz wireless transmission by using spectrally efficient modulated subcarriers spaced at the baud rate. This approach increases the overall transmission capacity and reduces the bandwidth requirement for electronic devices.

A monolithically integrated photonic source for tuneable mmwave signal generation has been fabricated. The source consists of 14 active components, i.e. semiconductor lasers, amplifiers and photodetectors, all integrated on a 3 mm 2 InP chip. Heterodyne signals in the range between 85 GHz and 120 GHz with up to -10 dBm output power have been successfully generated. By optically injection locking the integrated lasers to an external optical comb source, high-spectral-purity signals at frequencies >100 GHz have been generated, with phase noise spectral density below -90 dBc/Hz being achieved at offsets from the carrier greater than 10 kHz.

The achievable throughput using high symbol rate, high order QAM is investigated for the current generation of CMOS-based DAC/ADC. The optimum symbol rate and modulation format is found to be 80GBd DP-256QAM, with an 800Gb/s net data rate.

We present a review of the critical design aspects of monolithically integrated optical phase lock loops (OPLLs). OPLL design procedures and OPLL parameters are discussed. A technique to evaluate the gain of the closed loop operating system is introduced and experimentally validated for the first time. A dual-OPLL system, when synchronised to an optical frequency comb generator without any prior filtering of the comb lines, allows generation of high spectral purity signals at any desired frequency from several GHz up to THz range. Heterodyne phase locking was achieved at a continuously tuneable offset frequency between 2 and 6 GHz. Thanks to the photonic integration, small dimensions, and custom-made electronics, the propagation delay in the loop was less than 1.8 ns, allowing good phase noise performance with OPLLs based on lasers with linewidths less than a few MHz. The system demonstrates the potential for photonic integration to be applied in various microwave photonics applications where narrow-bandwidth tuneable optical filters with amplification functionality are required.

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.