H.K. Tsang

Optical dispersion, two-photon absorption and self-phase modulation in silicon waveguides at 1.5 μm wavelength

We measure linear optical dispersion, nonlinear refraction and two-photon absorption in a silicon waveguide at 1.54 μm wavelength. The total dispersion in the silicon waveguide was found to be −9.1 fs/(nm cm). At 1.54 μm wavelength, the two-photon absorption coefficient was found to be 0.45 cm/GW and a π phase shift from self-phase modulation was observed in optical pulses of 60 W peak-coupled power, generated from an amplified gain-switched laser diode.

Silicon waveguide two-photon absorption detector at 1.5 μm wavelength for autocorrelation measurements

We describe the use of a silicon waveguide for two-photon absorption measurements in an autocorrelator which achieved a peak-power average-power sensitivity of 1 (mW)2 at 1.56 μm wavelength and which was used to measure optical pulses generated by a mode-locked fiber ring laser. The experimental results agreed with a theoretical model of the two-photon-induced photocurrent generation inside the waveguide.

Role of free carriers from two-photon absorption in Raman amplification in silicon-on-insulator waveguides

We show experimentally that free carriers generated by two-photon-absorption in silicon-on-insulator (SOI) waveguides can introduce large losses which limit the usable pump power for Raman amplification at telecommunication wavelengths. The measured pump loss agreed with a theoretical model of the free-carrier absorption arising from two-photon-induced free carrier generation inside the waveguide.

Efficient Raman amplification in silicon-on-insulator waveguides

We describe a silicon-on-insulator waveguide Raman amplifier which achieves a large fiber-to-fiber optical gain of 6.8dB using stimulated Raman scattering in a 1.7-cm-long silicon waveguide. By using picosecond pulse pumping at 1557.4nm wavelength, high net optical gain at the first-order Stokes wavelength of 1694.6nm was measured. The optical loss from two-photon absorption generated free carriers was reduced by using a low pulse duty cycle and picosecond pulse pumping.

Integrated polarization beam splitter in high index contrast silicon-on-insulator waveguides

We design, fabricate, and test a novel transverse-electric/transverse-magnetic (TE/TM) polarization beam splitter which is based on the dependence of birefringence on rib width in silicon-on-insulator waveguides. The polarization beam splitter has high TE to TM extinction ratio, low excess loss and wide operation bandwidth. The simple fabrication method involves only single etch step.

Nonlinear absorption and Raman scattering in silicon-on-insulator optical waveguides

We study the nonlinearities in silicon-on-insulator (SOI) optical waveguides, which include two-photon absorption (TPA), free-carrier absorption, and spontaneous and stimulated Raman scattering (SRS). We show experimentally that free carriers generated by TPA in the SOI waveguides produce large optical loss at room temperature. The experimental results confirmed the presence of relative optical signal amplification from SRS, but it was found that net gain was hardly achieved because the stimulated Raman gain was less than the induced loss from TPA-generated free carriers at room temperature with continuous-wave pumping source in a SOI rib waveguide. We also experimentally investigated the temperature dependence of Raman scattering in the SOI waveguide and observed the Raman gain to be greater than TPA-generated free-carrier absorption loss at 77 K.

Nonlinear absorption and Raman gain in helium-ion-implanted silicon waveguides.

We study whether helium ion implantation can reduce the carrier lifetime and thus reduce the density of two-photon-absorption-generated free carriers. Our experiments and theoretical model show that helium ion implantation into silicon waveguides can successfully reduce the free-carrier losses and allow net gain to be attained by cw-pumped stimulated Raman scattering without requiring reverse bias to remove the photogenerated free carriers.

Investigation of 4-ASK modulation with digital filtering to increase 20 times of direct modulation speed of white-light LED visible light communication system

In this demonstration, we propose and experimentally investigate the quaternary-amplitude-shift-keying (4-ASK) modulation with digital filtering to enhance the direct modulation speed of white-light light-emitting-diode (LED) in visible light communication (VLC) system. Here, an ordinary LED commercially available for lighting application with a direct modulation speed of 1 MHz is used. Data rate of 20 Mbit/s can be achieved in a 1 m free space transmission without using optical blue filter. In the previous studies, the transmission rate of LED VLC could only be increased by 2 to 10 times of the direct modulation speed of the white-light LED if using electrical equalization only. Moreover, the adaptive-controlled FIR filter makes the system closer to the matched filtering condition for reducing the inter-symbol-interference (ISI) for the LED VLC. A recorded 20 times enhancement of the direct modulation speed of white-light LED VLC system is demonstrated by using digital filter only and without using optical blue filter.

Polarization-independent DPSK demodulation using a birefringent fiber loop

A robust polarization-independent technique for demodulating optical differential phase-shift-keying (DPSK) signals using a birefringent-fiber-loop (BFL) is proposed and demonstrated. The BFL has a differential group delay between its fast and slow optical axes, providing interference which allows the phase modulation to be converted to intensity modulation and then directly detected by a photodiode. Error-free bit-error-rate measurements and clear demodulated eyes were observed. The temperature stability of the scheme improves with the bit rate of the DPSK signal.

In-line channel power monitor based on helium ion implantation in silicon-on-insulator waveguides

We propose and demonstrate an in-line channel power monitor (ICPM) based on helium ion implanted silicon waveguides. The implanted waveguide can detect light at below-bandgap wavelengths (1440-1590 nm) which are normally not detectable by silicon. We study the enhanced photoresponse of helium ion implanted samples which were annealed at 200 degC, 300 degC, or 350 degC for different durations. Optical absorption and photodetector current measurements were performed for each sample. The ICPM can provide the same function as a waveguide tap coupler and a hybrid-integrated conventional photodiode