Lawrence R. Chen

Subwavelength grating filtering devices

We propose and simulate the characteristics of optical filters based on subwavelength gratings. In particular, we demonstrate through numerical simulations the feasibility of implementing SWG Bragg gratings in silicon-on-insulator (SOI). We also propose SWG ring resonators in SOI and verify their operation using numerical simulations and experiments. The fabricated devices exhibit an extinction ratio as large as 30 dB and a Q-factor as high as ~20,000. These fundamental SWG filters can serve as building blocks for more complex devices.

Wavelength conversion of 28 GBaud 16-QAM signals based on four-wave mixing in a silicon nanowire.

We demonstrate error-free wavelength conversion of 28 GBaud 16-QAM single polarization (112 Gb/s) signals based on four-wave mixing in a dispersion engineered silicon nanowire (SNW). Wavelength conversion covering the entire C-band is achieved using a single pump. We characterize the performance of the wavelength converter subsystem through the electrical signal to noise ratio penalty as well as the bit error rate of the converted signal as a function of input signal power. Moreover, we evaluate the degradation of the optical signal to noise ratio due to wavelength conversion in the SNW.

Subwavelength grating enabled on-chip ultra-compact optical true time delay line

An optical true time delay line (OTTDL) is a basic photonic building block that enables many microwave photonic and optical processing operations. The conventional design for an integrated OTTDL that is based on spatial diversity uses a length-variable waveguide array to create the optical time delays, which can introduce complexities in the integrated circuit design. Here we report the first ever demonstration of an integrated index-variable OTTDL that exploits spatial diversity in an equal length waveguide array. The approach uses subwavelength grating waveguides in silicon-on-insulator (SOI), which enables the realization of OTTDLs having a simple geometry and that occupy a compact chip area. Moreover, compared to conventional wavelength-variable delay lines with a few THz operation bandwidth, our index-variable OTTDL has an extremely broad operation bandwidth practically exceeding several tens of THz, which supports operation for various input optical signals with broad ranges of central wavelength and bandwidth.

Quantification of different water species in acetone using a NIR-triple-wavelength fiber laser

A fiber laser using a thulium-doped ZBLAN gain medium was used to generate laser radiation simultaneously at 1461, 1505 and 1874 nm, with > 5 mW output power at each of the wavelengths. The laser was used to quantify the near-infrared absorption of liquid water in acetone. Additionally, near-infrared spectra were recorded using a broad band source and were interpreted using parallel factor (PARAFAC) analysis to rationalize the concentration-dependent peak shifts.

Chirped Microwave Pulse Generation Using an Integrated SiP Bragg Grating in a Sagnac Loop

We demonstrate an integrated spectral shaper based on a Sagnac loop incorporating a chirped Bragg grating in silicon photonics for the photonic generation of chirped microwave pulses. The technique is based on optical spectral shaping combined with linear frequency-to-time mapping. By tuning the central wavelength of the input optical pulse, we obtain chirped microwave pulses with central frequencies ranging from ~10 GHz to ~30 GHz and an RF chirp rate of ~20 GHz/ns with both positive and negative signs.

RF-Arbitrary Waveform Generation Based on Microwave Photonic Filtering

We demonstrate RF arbitrary waveform generation based on microwave photonic filtering. We use four-wave mixing in a silicon nanowire to increase the number of taps in an N-tap microwave photonic filter (MPF). Using a programmable optical filter, we can control the tap weights and, hence, the MPF response and corresponding generated waveform. We show uniform and apodized waveforms with four taps and seven taps with tunable central frequencies.

Low crosstalk Bragg grating/Mach-Zehnder interferometer optical add-drop multiplexer in silicon photonics.

We characterize the interferometric crosstalk and system performance of two optical add-drop multiplexer (OADM) designs based on Bragg grating/Mach-Zehnder interferometers implemented in silicon-on-insulator. Both OADM designs exhibit low crosstalk and negligible crosstalk-induced power penalties over their 3 dB bandwidths. The devices are tolerant to wavelength drift and misalignment between the transmitter and OADM; moreover, their designs can be optimized further to enable high performance operation in WDM systems.

Time-delay to intensity mapping based on a second-order optical integrator: application to optical arbitrary waveform generation

We propose and validate experimentally a time-delay to intensity mapping process based on second-order optical integrators. This mapping provides dynamic control of the intensity modulation profile of a waveform based on a purely passive and linear process. In particular, we can realize linear intensity control by tuning the time-delay between two optical pulses launched into a second-order optical integrator. We suggest and experimentally prove the use of this mapping process for reconfigurable optical arbitrary waveform generation.

All-optical modulation format conversion from binary to quadrature phase-shift keying using delay line interferometer

We propose an all-optical modulation format conversion method from binary phase-shift keying to quadrature phase-shift keying using a conventional delay line interferometer. We experimentally demonstrate error-free operation of the proposed conversion method at 12 Gb/s.

Silicon Photonics for Microwave Photonics Applications

We provide an overview of recent work on developing integrated microwave photonic subsystems in silicon photonics for generating chirped microwave waveforms as well as for providing optical true time delay. First, we describe on-chip spectral shapers based on Bragg gratings (BGs), including a distributed Fabry–Perot cavity, a Michelson interferometer incorporating identical chirped BGs, and a Sagnac loop incorporating a chirped BG, for use in wavelength-to-time mapping systems. The performance of these on-chip spectral shapers is compared to those based on microring resonators as well as fiber or free-space configurations. Second, we consider the development of an index variable optical true time delay line (OTTDL) using subwavelength grating (SWG) waveguides. For SWG waveguides of the same length, incremental delays can be obtained by tailoring the group index via control over the duty cycle. We compare the performance of SWG waveguide OTTDLs to other length variable implementations in silicon.