Peng Huei Lim

Photonic Time-Stretched Analog-to-Digital Converter Amenable to Continuous-Time Operation Based on Polarization Modulation With Balanced Detection Scheme

In this paper, we show analytically and experimentally that a polarization modulator which supports TE and TM modes of opposite phase modulation indexes can be utilized to reject dispersion-induced even-order distortions in a photonic Time-Stretched Analog-to-Digital Converter (TS-ADC). The output of the polarization modulator propagates through a single dispersive channel. This makes the present scheme amenable to continuous operation. Based on the virtual time gating principle, the continuous-time RF signal is time-stretched by a factor of 4 and segmented into four channels prior to digitization. For a single channel, differential operation is achieved by using a polarization beam-splitter that generates complementary pulses which are fed to a balanced detector. The differential operation helps to reject dispersion-induced even-order distortions and the balanced detection assists in the suppression of second-order distortion as well as improving the signal-to-noise ratio (SNR) by 6 dB. Using a 10 bit electronic ADC with a sampling rate of 2 GSamples/s, we demonstrate digitization of RF signals up to a frequency of 950 MHz and obtain ~ 3.56 effective number of bits (ENOB) with a single channel at ~ 31.6% of the electronic ADCs peak-to-peak full scale voltage. With adequate backend digitizing hardware, a four-channel continuous-time TS-ADC with a sampling rate of 8 GSamples/s can be realized to handle RF frequencies as high as 4 GHz.

An optically tunable wideband optoelectronic oscillator based on a bandpass microwave photonic filter

An optoelectronic oscillator (OEO) with wideband frequency tunability and stable output based on a bandpass microwave photonic filter (MPF) has been proposed and experimentally demonstrated. Realized by cascading a finite impulse response (FIR) filter and an infinite impulse response (IIR) filter together, the tunable bandpass MPF successfully replaces the narrowband electrical bandpass filter in a conventional single-loop OEO and serves as the oscillating frequency selector. The FIR filter is based on a tunable multi-wavelength laser and dispersion compensation fiber (DCF) while the IIR filter is simply based on an optical loop. Utilizing a long length of DCF as the dispersion medium for the FIR filter also provides a long delay line for the OEO feedback cavity and as a result, optical tuning over a wide frequency range can be achieved without sacrificing the quality of the generated signal. By tuning the wavelength spacing of the multi-wavelength laser, the oscillation frequency can be tuned from 6.88 GHz to 12.79 GHz with an average step-size of 0.128 GHz. The maximum frequency drift of the generated 10 GHz signal is observed to be 1.923 kHz over 1 hour and its phase noise reaches the -112 dBc/Hz limit of our measuring equipment at 10 kHz offset frequency.

Ultra-compact low loss polarization insensitive silicon waveguide splitter

We observe that the cascaded typical Y junctions will introduce unwanted periodic fringes over the spectrum in practical systems when they link with multimode waveguides. To solve the problem, we design and experimentally demonstrate a wavelength insensitive multimode interferometer (MMI) based 3-dB splitter which has all the merits of Y-splitters such as polarization insensitivity and ultra-compactness. The splitter has a footprint of 1.5 × 1.8 µm(2), nearly one order smaller than the previously reported MMI splitters. The measured excess losses for TE and TM modes at telecom wavelength are as low as -0.11 dB and -0.18 dB respectively.

Passively mode-locked III-V/silicon laser with continuous-wave optical injection.

We demonstrate electrically pumped two-section mode locked quantum well lasers emitting at the L-band of telecommunication wavelength on silicon utilizing die to wafer bonding techniques. The mode locked lasers generate pulses at a repetition frequency of 30 GHz with signal to noise ratio above 30 dB and 1 mW average output power per facet. Optical injection-locking scheme was used to improve the noise properties of the pulse trains of passively mode-locked laser. The phases of the mode-locked frequency comb are shown to be coherent with that of the master continuous-wave (CW) laser. The radio-frequency (RF)-line-width is reduced from 7.6 MHz to 150 kHz under CW optical injection. The corresponding pulse-to-pulse jitter and integrated RMS jitter are 29.7 fs/cycle and 1.0 ps, respectively. The experimental results demonstrate that optical injection can reduce the noise properties of the passively mode locked III-V/Si laser in terms of frequency linewidth and timing jitter, which makes the devices attractive for photonic analog-to-digital converters and clock generation and recovery.

Photonic Generation of Frequency-Tunable Microwave Signals Using an Array of Uniformly Spaced Optical Combs

We show analytically as well as experimentally a photonic technique for generation of frequency-tunable microwave signals. The technique involves propagating an array of uniformly spaced optical combs with identical comb profile into a length of dispersive single-mode fiber (SMF) prior to photodetection. A new scheme that involves a supercontinuum source and a Fourier-domain programmable optical processor as a spectral filter is proposed to generate the required array of optical combs. For an initial optical comb with a fixed repetition rate and a given length of standard SMF, our scheme shows that by choosing appropriate values of the number of optical combs, the number of comb lines in each optical comb, as well as the frequency spacing between the adjacent optical combs, we can generate a microwave signal whose frequency can be tuned to any integer multiple of the optical combs repetition frequency that lies within the bandwidth of the photodetector. Based on the proposed scheme, using a 2 GHz supercontinuum source as an initial optical comb, we demonstrate the generation of stable microwave signals tunable to 8, 10, and 12 GHz.

Generation of flat supercontinuum for time-stretched analog-to-digital converters

Flat supercontinuum (SC) generation with less than 1 dB fluctuation in the telecommunication band is demonstrated using a regenerative active mode-locked fiber laser together with a dispersion-flattened highly nonlinear fiber. Such SC is highly desirable in the context of a photonic time-stretched ADC system due to its flatness, bandwidth and brightness. Experimental results presented here agree well with the simulations.

Time- and wavelength-interleaved optical pulse train generation based on dispersion spreading and sectional compression.

A method to generate time- and wavelength-interleaved optical pulse trains based on dispersion spreading and sectional compression is proposed and demonstrated. A 4×2  GHz time- and wavelength-interleaved pulse train is generated from an input 2 GHz mode-locked pulse train. The advantages of the proposed scheme are its simplicity and robustness, since no microwave component or multiwavelength laser source is required. In addition, we demonstrate supercontinuum generation of an ultraflat 18 nm bandwidth spectrum with less than 0.5 dB fluctuation over the 3.2 nm central bandwidth.

Design and characterization of low loss 50 picoseconds delay line on SOI platform

We design and experimentally demonstrate 50 picoseconds (ps) low loss delay line on 300 nm SOI platform. The delay line unit consists of straight rib waveguide and strip bend section linked by a transition taper waveguide. Low propagation loss of ~0.1 dB/cm is achieved on the straight rib waveguide. With taking into account both low loss and desirable delay, a complete design and characterization process for passive delay line is presented. Our measurement results show that about 0.7 dB excess loss is achievable for 50 ps delay. The loss can be further reduced by adjusting the layout parameters.

Photonic Generation of Tunable Continuous-Wave Microwave Signals Using a Temporally-Stretched and Chirped Pulse-Train

We show analytically and experimentally that direct photodetection of a temporally-stretched and chirped optical pulse-train together with its time-delayed replica can be exploited to realize the generation of tunable continuous-wave microwave signals. The proposed scheme utilizes the chromatic dispersion of the standard single-mode-fiber (SMF) to temporally-stretch and chirp an optical pulse-train in such a way that the pulses overlap with each other. The temporally-stretched pulse-train is then sent to a Mach-Zehnder Interferometer (MZI) where it first splits and then recombines with a time-delayed replica of itself at the output. Proper management of both the dispersion in the system as well as the relative time-delay of the two arms of the MZI enables one to tune the frequency of the generated microwave signal to any integer multiple of the pulse sources repetition frequency which is within the bandwidth of the photodetector. Based on the proposed scheme, using an initial 2 GHz pulse-train, we demonstrate generation of tunable continuous-wave microwave signals from 4 GHz to 14 GHz.

Characterization of silicon electro-optical modulators for microwave photonic links

In this submission, we present the characterization of a silicon electro-optical modulator using Mach-Zehnder interferometer structure. We characterize the transmission spectra upon various DC and RF voltage supplies with different input optical power. We show the slope efficiency dependence on the optical input power. We also measured the spur-free dynamic range of the modulator using two-tone method, showing an optical power dependent SFDR with maximum value of ∼ 7.5 dBc.