Monolithically Integrated Optical Phase Lock Loop for Microwave Photonics (original) (raw)

Monolithically integrated heterodyne optical phase-lock loop with RF XOR phase detector

Optics Express, 2011

We present results for an heterodyne optical phase-lock loop (OPLL), monolithically integrated on InP with external phase detector and loop filter, which phase locks the integrated laser to an external source, for offset frequencies tuneable between 0.6 GHz and 6.1 GHz. The integrated semiconductor laser emits at 1553 nm with 1.1 MHz linewidth, while the external laser has a linewidth less than 150 kHz. To achieve high quality phase locking with lasers of these linewidths, the loop delay has been made less than 1.8 ns. Monolithic integration reduces the optical path delay between the laser and photodiode to less than 20 ps. The electronic part of the OPLL was implemented using a custom-designed feedback circuit with a propagation delay of ~1 ns and an open-loop bandwidth greater than 1 GHz. The heterodyne signal between the locked slave laser and master laser has phase noise below 90 dBc/Hz for frequency offsets greater than 20 kHz and a phase error variance in 10 GHz bandwidth of 0.04 rad 2 .

Optical Phase Lock Loop as High-Quality Tuneable Filter for Optical Frequency Comb Line Selection

Journal of Lightwave Technology

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 OPLLfilter 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.

Hybrid Integrated Optical Phase-Lock Loops for Photonic Terahertz Sources

IEEE Journal of Selected Topics in Quantum Electronics, 2000

We present the first hybrid-integrated optical phaselock loop (OPLL) for use in high spectral purity photonic terahertz sources. We have achieved the necessary short loop delay to lock a 1-MHz linewidth slave laser by hybrid integration of the slave laser and photodetector on a silicon motherboard with silica optical waveguides and combining this with a custom-designed low-delay electronic loop filter circuit. The laser and photodetectors are InPbased and are flip chip bonded to silicon daughter boards, which are in turn attached to the motherboard. Delay between the slave laser and photodiode was approximately 50 ps. The heterodyne between slave and master sources has a linewidth of less than 1 kHz and achieved phase noise less than −80 dBc/Hz at an offset of 10 kHz. The slave laser can be offset from the master source by 2-7 GHz, using a microwave oscillator. This integrated OPLL circuit was used with an optical comb source and an injectionlocked laser comb filter to generate high spectral purity signals at frequencies up to 300 GHz with linewidths <1 kHz and powers of about −20 dBm, while the two integrated lasers could deliver a tunable heterodyne signal at frequencies up to 1.8 THz.

A Chip-Scale Heterodyne Optical Phase-Locked Loop with Low-Power Consumption

A chip-scale heterodyne optical phase-locked loop, consuming only 1.3 W of electrical power, with a maximum offset locking frequency of 17.4 GHz is demonstrated. The InP-based photonic integrated receiver circuit consumes only 166 mW. 1. Introduction and Design of the Heterodyne OPLL There has been significant effort for realizing highly-integrated chip-scale optical phase-locked loops (OPLLs) in the last decade along with the development in the photonic integration. Traditional free space optics creates loop delays in the order of tens of nanoseconds, which makes the loop bandwidth small. However, with the improvement in photonic integration, OPLLs can be realized with loop bandwidths in the order of hundreds of MHz [1] or even more than 1 GHz [2]. This makes OPLLs attractive and they can be used in a wide range of applications including coherent receivers, high sensitivity detection, laser linewidth narrowing, millimeter and THz wave generation and optical frequency synthesis [3-5]. In previous works, offset locking ranges up to 25 GHz [6], large loop bandwidth exceeding 1 GHz [2] and residual OPLL phase noise variance as low as 0.03 rad 2 [1] were demonstrated for the chip-scale OPLLs. However, these OPLLs consume almost 3 W of electrical power [2], being unsuitable for the real life applications. In this work, a chip-scale heterodyne OPLL with a total power consumption of 1.3 W is designed and demonstrated utilizing a novel indium phosphide (InP)-based photonic integrated circuit (PIC) and commercial-off-the-shelf (COTS) electronic ICs. The PIC receiver contains a widely-tunable (50 nm) compact Y-branch laser, a 180° hybrid (MMI) and two photodiodes. This is offset locked to narrow-linewidth (100 kHz) external-cavity laser (ECL) up to a range of 17.4 GHz with an RF synthesizer. The low power consumption PIC is integrated with COTS electronic ICs in order to realize the highly-integrated OPLL. An optical microscope image and the schematic of the receiver PIC is shown in Fig. 1(a) and (b), respectively. The PIC incorporates a compact Y-branch laser formed between a high-reflectivity coated back mirror and a pair of Vernier tuned front mirrors. The output from one mirror leads to the coherent receiver used for offset locking, while the other output forms the optical output signal from the backend integrated system. The Y-branch laser has a compact cavity with short gain and mirror sections, requiring low current and therefore low drive power. It is tuned via Vernier effect and has been designed for high efficiency at 30º C ambient. The measured tuning range exceeds 50 nm with >50 dB side-mode suppression ratio. The low power receiver PIC is connected with SiGe-based COTS ICs including a limiting amplifier and digital XOR as a mixer/phase detector. The limiting amplifier has a 3-dB bandwidth of 17 GHz with 30 dB of differential gain. The digital XOR operates up to at least 12.5 GHz input RF frequencies. The limiting amplifier limits the signal coming from photodiode pair to logic levels, which enables the system to be insensitive to any optical intensity fluctuations. A second order dual-path loop filter was used to get high loop bandwidth. This was achieved by employing a fast feedforward path which increases the system frequency acquisition range. Fig. 1(b) and (c) displays the architecture and a microscope image of the whole OPLL system, respectively. The PIC, electronic ICs and the loop filter are all integrated on an aluminum nitride (AlN) carrier, and wire-bonded. The system size is approximately 1.8 cm by 1.6 cm. Total delay is less than 300 ps, and the loop bandwidth is approximately 500 MHz.

Heterodyne locking of a fully integrated optical phase-locked loop with on-chip modulators

We design and experimentally demonstrate a highly ­integrated heterodyne optical phase ­locked loop (OPLL) consisting of an InP-­based coherent photonic receiver, high­-speed feedback electronics and an RF synthesizer. Such coherent photonic integrated circuits contain two widely ­tunable lasers, semiconductor optical amplifiers, phase modulators, and a pair of balanced photodetectors. Offset phase­ locking of the two lasers is achieved by applying an RF signal to an on­chip optical phase modulator following one of the lasers and locking the other one to a resulting optical sideband. Offset locking frequency range >16 GHz is achieved for such a highly­sensitive OPLL system which can employ up to the third ­order ­harmonic optical sidebands for locking. Furthermore, the rms phase error between the two lasers is measured to be 8°.

An Optical Phase-Locked Loop Photonic Integrated Circuit

IEEE/OSA Journal of Lightwave Technology, 2010

We present the design, fabrication, and results from the first monolithically integrated optical phase-locked loop (OPLL) photonic integrated circuit (PIC) suitable for a variety of homodyne and offset phase locking applications. This InP-based PIC contains two sampled-grating distributed reflector (SG-DBR) lasers, semiconductor optical amplifiers (SOAs), phase modulators, balanced photodetectors, and multimode interference (MMI)-couplers and splitters. The SG-DBR lasers have more than 5 THz of frequency tuning range and can generate a coherent beat for a wide spectrum of frequencies. In addition, the SG-DBR lasers have large tuning sensitivities and do not exhibit any phase inversion over the frequency modulation bandwidths making them ideal for use as current controlled oscillators in feedback loops. These SG-DBR lasers have wide linewidths and require high feedback loop bandwidths in order to be used in OPLLs. This is made possible using photonic integration which provides low cost, easy to package compact loops with low feedback latencies. In this paper, we present two experiments to demonstrate proof-of-concept operation of the OPLL-PIC: homodyne locking and offset locking of the SG-DBR lasers.

Towards chip-scale optical frequency synthesis based on optical heterodyne phase- locked loop

An integrated heterodyne optical phase-locked loop was designed and demonstrated with an indium phosphide based photonic integrated circuit and commercial off-the-shelf electronic components. As an input reference, a stable microresonator-based optical frequency comb with a 50-dB span of 25 nm (~3 THz) around 1550 nm, having a spacing of ~26 GHz, was used. A widely-tunable on-chip sampled-grating distributed-Bragg-reflector laser is offset locked across multiple comb lines. An arbitrary frequency synthesis between the comb lines is demonstrated by tuning the RF offset source, and better than 100Hz tuning resolution with ± 5 Hz accuracy is obtained. Frequency switching of the on-chip laser to a point more than two dozen comb lines away (~5.6 nm) and simultaneous locking to the corresponding nearest comb line is also achieved in a time ~200 ns. A low residual phase noise of the optical phase-locking system is successfully achieved, as experimentally verified by the value of 80 dBc/Hz at an offset of as low as 200 Hz. Hall, and S. T. Cundiff, " Carrier-envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis, " Science 288(5466), 635–639 (2000). 2. J. Castillega, D. Livingston, A. Sanders, and D. Shiner, " Precise measurement of the J = 1 to J = 2 fine structure interval in the 2(3)P state of helium, " Phys.

Offset Locking of a Fully Integrated Optical Phase-Locked Loop Using On-chip Modulators

We demonstrate an integrated heterodyne optical phase-locked loop for potential RF remoting. Offset-locking of the two on-chip lasers is achieved by applying a RF signal to an on-chip optical phase modulator and locking to an optical sideband. Over the years, microwave/millimeter wave photonic link technology, supporting the fiber optic remoting of RF signals based on remote heterodyne detection (RHD) [1], has attracted a great deal of attention for a wide range of applications including delay lines over phased-array antenna feeders and backbone networks for cellular phone systems [2]. Such a fiber-optic link at the transmitter or base station requires two widely-tunable lasers with slightly different wavelengths, the phases of which must be strictly correlated. The strict phase correlation between these two lasers can be achieved by using a heterodyne optical phase-locked loop (OPLL) [3] transmitter configuration. There are two techniques that could be adopted for such heterodyne locking. As a first technique, the RF signal can be applied to an electronic mixer following optical detection in such a coherent photonic receiver and the RF difference frequency used for offset locking [4,5]. Another technique is to apply the RF to an on-chip optical modulator monolithically integrated on the photonic receiver following the tunable laser and to achieve the locking using an optical sideband [6]. Use of higher-order sidebands are also possible enabling higher offset frequencies than available from the RF source or the electronics. In this work, we report the second technique using an InP-based photonic integrated circuit with an on-chip optical phase modulator following the LO laser for applying the offset. Outputs from the LO and signal lasers are combined into a pair of photodetectors that provide inputs to agile and highly-sensitive feedback electronics that control the phase section of the LO for locking. A net loop bandwidth of 500 MHz was obtained, and an offset locking frequency range ~16 GHz is achieved in the system, which can employ up to the third-order-harmonic optical sidebands for locking, yielding a locking range as high as 48 GHz. Figure 1(a) shows a schematic of the coherent optical receiver PIC used for OPLL. It includes two widely-tunable sampled-grating distributed Bragg reflector (SG-DBR) lasers that are to be offset locked, MMI couplers, semiconductor optical amplifiers (SOAs), photodetectors, and several RF-modulators. The chip size is 7 mm × 0.5 mm. As can be seen, light from each laser is first equally divided into two paths using 1 × 2 MMIs. One half from each laser is guided into a 2 × 2 MMI, which serves as a 180 degree hybrid to feed the two photodetectors for the feedback loop. Each input arm of the 2 × 2 MMI contains a phase modulator (M) that can be used for applying RF to generate optical sidebands. After adding the fields from these two lasers in the MMI coupler, light is detected in a pair of photodetectors (D) to provide a differential output. The other half from each laser travels through semiconductor-optical-amplifiers (SOAs) to increase their amplitude and modulators for applying possible information. They are then combined in a 2 × 2 MMI at the right side of the OPLL-PIC. In the current experiment this is used for monitoring of the interference between the two SG-DBR lasers by coupling into an optical fiber. An optical microscope photo of the fully-processed PIC on an InGaAsP/InP material platform is shown in Fig. 1(b). The process used to fabricate the devices is quantum-well intermixing (QWI) that creates self-aligned passive regions by intermixing the quantum-wells with their barriers and surrounding waveguide material by a patterned diffusion of implanted phosphorus ions after a first growth. Details of the processing steps for the well-established QWI-based material structure can be found elsewhere [7]. Two widely tunable on-chip SG-DBR lasers along with all of the other optical components were used to form the heterodyne OPLL. One of the lasers was used as a master (or signal) laser, while the other as a slave (or LO) laser to be offset phase-locked to the former. Prior to combining the outputs of these two lasers using a 2 × 2 optical coupler, the output of slave SG-DBR laser is phase-modulated for offset-locking using an integrated on-chip modulator and envelope detected using a pair of balanced on-chip photodetectors. The current output from the

Integrated Semiconductor Laser Optical Phase Lock Loops

IEEE Journal of Selected Topics in Quantum Electronics, 2018

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.