Ashish Bhardwaj

An Optical Phase-Locked Loop Photonic Integrated Circuit

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.

Highly integrated optical heterodyne phase-locked loop with phase/frequency detection.

A highly-integrated optical phase-locked loop with a phase/frequency detector and a single-sideband mixer (SSBM) has been proposed and demonstrated for the first time. A photonic integrated circuit (PIC) has been designed, fabricated and tested, together with an electronic IC (EIC). The PIC integrates a widely-tunable sampled-grating distributed-Bragg-reflector laser, an optical 90 degree hybrid and four high-speed photodetectors on the InGaAsP/InP platform. The EIC adds a single-sideband mixer, and a digital phase/frequency detector, to provide single-sideband heterodyne locking from -9 GHz to 7.5 GHz. The loop bandwith is 400 MHz.

5-Gbit/s BER performance on an all fiber-optic add/drop device based on a taper-resonator-taper structure

We present an all fiber-optic add/drop device based on a taper-resonator-taper structure with improved characteristics. Several gigahertz bandwidths are observed using microspheres having diameters ranging from 30 to 50 /spl mu/m. Extinction ratios as high as 26 dB of the dropped channel are obtained due to nearly ideal coupling and phase matching between the fiber tapers and the small resonator. This is the first time that bit-error rate (BER) measurements have been performed on such couplers. For a device with an optical bandwidth of 3.8 GHz, the BER shows less than 2-dB penalty at 5 Gbit/s and no signs of an error floor.

Monolithically Integrated Gain-Flattened Ring Mode-Locked Laser for Comb-Line Generation

We demonstrate broadband comb-line generation from an integrated multiple quantum well InGaAsP/InP passively mode-locked laser (MLL) with a gain flattening filter (GFF) based on an asymmetric Mach-Zehnder interferometer. The intracavity filter flattens the nonuniform gain profile of the semiconductor material providing a more uniform net cavity gain. The GFF MLL has a -10 dB comb span of 15 nm (1.88 THz), the widest spectral width yet demonstrated for an integrated QW MLL at 1.55 μm . The measured optical linewidth at the center of the comb is 29 MHz, the -20 dB RF linewidth 500 KHz, while the output spectrum is phase-locked to produce 900 fs pulses at a repetition rate of 30 GHz with 4.6 ps integrated jitter from 100 Hz to 30 MHz.

High Linearity InP-Based Phase Modulators Using a Shallow Quantum-Well Design

Phase modulator nonlinearity is a major problem for implementing an optical phase-locked loop (OPLL) phase demodulator. In this letter, we report an improved InP phase modulator design that uses a detuned shallow multiquantum-well structure. The phase modulator shows high linearity and low optical loss. Its phase IP3 and optical loss per unit length are ~ 4 π/mm and ~ 0.9 dB/mm, respectively. This phase modulator design is thus suitable for implementing the OPLL linear phase demodulator.

A Monolithically Integrated ACP-OPLL Receiver for RF/Photonic Links

The first monolithically integrated optical phase-locked loop (OPLL) employing attenuating-counter-propagating waves is presented. It demonstrates the highest dynamic range among monolithically integrated OPLLs. Its performance is limited by the bandwidth and linearity of the photodetectors used in the OPLL.

Indium Phosphide Photonic Integrated Circuits for Coherent Optical Links

We demonstrate photonic circuits monolithically integrated on an InP-based platform for use in coherent communication links. We describe a technology platform that allows for the integration of numerous circuit elements. We show examples of an integrated transmitter which offers an on-chip wavelength-division-multiplexing source with a flat gain profile across a 2 THz band and a new device design to provide a flatted gain over a 5 THz band. We show coherent receivers incorporating an integrated widely tunable local oscillator as well as an optical PLL. Finally, we demonstrate a tunable optical bandpass filter for use in analog coherent radio frequency links with a measured spurious-free dynamic range of 86.3 dB-Hz2/3 as well as an improved design to exceed 117 dB-Hz2/3.

Narrow linewidth sampled-grating distributed Bragg reflector laser with enhanced side-mode suppression

We demonstrate a thermally tuned SGDBR laser integrated with semiconductor optical amplifier and spectral filter achieving 70kHz spectral linewidth, 50dB SMSR, and +17dBm fiber- coupled output power over a 41nm wavelength range across the C-band.

Theory and Design of THz Intracavity Gain-Flattened Filters for Monolithically Integrated Mode-Locked Lasers

We present the theory and design of a tunable gain-flattening filter for integrated mode-locked lasers (MLLs). The filter provides the inverse of the semiconductor spectral gain profile and produces a broad flattened net gain. This improves the performance of MLLs by allowing more modes to lase simultaneously. We demonstrate a gain-flattened MLL with a record 10 dB bandwidth of 2.08 THz, the widest frequency comb span for an integrated quantum-well-based laser at 1.55 μm. Gain-flattening theory is used to extend the integrated comb span to 40 nm. We use scattering matrices to investigate feed-forward filters based on asymmetric Mach-Zehnder interferometers (MZIs). We compare MZI filters designed for a fixed coupling value to those that use an active gain arm to adjust the extinction ratio. Tunable zero placement of these filters is achieved using a passive phase tuning arm. The optimized gain-flattening filter has a 5 dB extinction ratio and a 70 nm free-spectral-range. When the filter is incorporated into a ring MLL, simulations predict a 40 nm, i.e., 5 THz, comb span with a power variation <; 3.5 dB.

InP photonic integrated circuit for 2D optical beam steering

InP photonic integrated circuit for 2D (5°×10°) optical beam steering has been demonstrated for the first time. Design, fabrication, and preliminary results are presented.