Xinzhu Sang

Gain and noise characteristics of high-bit-rate silicon parametric amplifiers

We report a numerical investigation on parametric amplification of high-bit-rate signals and related noise figure inside silicon waveguides in the presence of two-photon absorption (TPA), TPA-induced free-carrier absorption, free-carrier-induced dispersion and linear loss. Different pump parameters are considered to achieve net gain and low noise figure. We show that the net gain can only be achieved in the anomalous dispersion regime at the high-repetition-rate, if short pulses are used. An evaluation of noise properties of parametric amplification in silicon waveguides is presented. By choosing pulsed pump in suitably designed silicon waveguides, parametric amplification can be a chip-scale solution in the high-speed optical communication and optical signal processing systems.

Influence of Pump-to-Signal RIN Transfer on Noise Figure in Silicon Raman Amplifiers

The effect of the relative intensity noise (RIN), transferred from the pump to the signal, in 1-cm-long chip scale silicon Raman amplifiers is investigated in the presence of nonlinear losses. We show that due to the short waveguide length, the reduction in fluctuations that normally occurs due to ldquowalk-offrdquo between pump and signal waves in fiber amplifiers is inefficient in chip scale Raman amplifiers. In the counterpropagating pump configuration, which leads to minimum frequency RIN transfer, fluctuations up to 1.5 GHz are transferred from the pump to the signal. As a case study, the noise figure degrades by as much as 11 dB in the silicon waveguide with the free carrier life time of 0.1 ns, when it is pumped with a laser with a RIN value of -125 dB/Hz.

Dual-Wavelength Mode-Locked Fiber Laser With an Intracavity Silicon Waveguide

We demonstrate dual-wavelength short pulses lasing at 1540 and 1675 nm based on a silicon waveguide. The inline silicon waveguide inside a laser cavity facilitates pulse compression and laser mode-locking due to two-photon absorption (TPA) and TPA-induced free-carrier absorption at 1540 nm. Compressed pulses provide pump for stimulated Raman scattering. Raman amplification and low threshold Raman lasing are observed based on the same silicon waveguide.

Imaging by silicon on insulator waveguides

We present multiphoton imaging based on semiconductor planar waveguide technology which can be used as a transmitter and receiver simultaneously. In particular, silicon on insulator waveguides with p-i-n diode structures are used to demonstrate <5 μm resolution three-photon imaging of Er3+:Y2O3 microparticles by using 1550 nm excitation. Additional theoretical study has been performed to demonstrate the proposed scheme for three-dimensional tomography of micron-sized objects, which could be realized by using multiple transmitter-detector pairs.

Silicon based optical pulse shaping and characterization

The high-index contrast between the silicon core and silica cladding enable low cost chip-scale demonstration of all-optical nonlinear functional devices at relatively low pump powers due to strong optical confinement the in silicon waveguides. So far, broad ranges of applications from Raman lasers to wavelength converters have been presented. This presentation will highlight the recent developments on ultrafast pulse shaping and pulse characterization techniques utilizing the strong nonlinear effects in silicon. In particular, pulse compression due to two photon absorption and dual wavelength lasing and ultrafast pulse characterization based on XPM FROG measurement will be highlighted.

Silicon on sapphire and SOI photonic devices for mid-infrared and near-IR wavelengths

Conventional SOI waveguide technology, serving as the foundation of near-IR photonics, meets its limitation in mid-IR due to high loss associated with the buried oxide. Silicon-on-sapphire (SOS) waveguides are considered as a good mid-IR alternative, because the transparency window of sapphire is up to 6 μm and SOS waveguides are compatible with SOI technology. We show that properly-designed SOS waveguides can facilitate frequency band conversion between near-IR and mid-IR. An indirect mid-IR detection scheme is proposed and the mid-IR signal is down-converted to telecommunication wavelength (1.55 μm) through SOS waveguides and indirectly detected by near-IR detectors. The performance of the indirect mid-IR detection scheme is discussed. Particularly we model and compare the noise performance of the indirect detection with direct detection using state-of-the-art mid-IR detectors. In addition to advantages of room temperature and high-speed operation, the results show that the proposed indirect detection can improve the electrical signal-to-noise ratio up to 50dB, 23dB and 4dB, compared to direct detection by PbSe, HgCdTe and InSb detectors respectively. The improvement is even more pronounced in detection of weak MWIR signals. In order to further boost the performance, we also investigate mechanisms to increasing the conversion efficiency in SOS waveguide wavelength converters. The conversion efficiency can be improved by periodically cascading SOS waveguide sections with opposite dispersion characteristics to achieve quasi-phase-matching. Conversion efficiency enhancement over 30dB and the conversion bandwidth increased by 2 times are demonstrated, which may facilitate the fabrication of parametric oscillators that can improve the conversion efficiency by 50dB.

Ultrafast pulse characterization using XPM in silicon

Due to the high-index contrast between the silicon core and silica cladding, the silicon waveguide allows strong optical confinement and large effective nonlinearity, which facilitates low cost chip scale demonstration of all-optical nonlinear functional devices at relatively low pump powers. One of the challenges in ultrafast science is the full characterization of optical pulses in real time. The time-wavelength mapping is proven to be a powerful technique for real time characterization of fast analog signals. Here we demonstrated a technique based on the cross-phase modulation (XPM) between the short pulse and the chirped supercontinuum (SC) pulse in the silicon chip to map fast varying optical signals into spectral domain. In the experiment, when 30 nm linearly chirped supercontinuum pulses generated in a 5 km dispersion-shifted fiber at the normal regime and 2.4 ps pulse are launched into a 1.7 cm silicon chip with 5 μm2 modal area, a time-wavelength mapped pattern of the short pulses is observed on the optical spectrum analyzer. From the measured spectral mapping the actual 2.4ps temporal pulse profile is reconstructed in a computer. This phenomenon can be extended to full characterization of amplitude and phase information of short pulses. Due to time wavelength mapping this approach can also be used in real time amplitude and phase measurement of ultrafast optical signals with arbitrary temporal width. The high nonlinearity and negligible distortions due to walk off make silicon an ideal candidate for XPM based measurements.

Research on QPM and Its Cascaded Structure Applied in FOPA

Abstract In the research of optical communication, Optical Parametric Amplification (OPA) has been an important point. As a Four Wave Mixing (FWM) effect based nonlinear process, OPA also requires Phase Matching (PM). Rigorous PM in practical research requires extremely harsh conditions. Quasi Phase Matching (QPM) and its cascaded structure can solve that problem, which would construct an overall phase matching. In the first part of this thesis, a QPM mechanism of segmented High NonLinear Fibre (HNLF) and inserted phase shifter for pumps, was proposed in application of FOPA. The phase shifters would “correct” the phase mismatching after every amplified signal by HNLF section. In this structure the phase matching was always kept in the vicinity of the initial matching value. The signal gain was flatter, and 6.4-9.5 dB higher than that of the non-QPM structure. In second part, a cascaded FWM+OPA structure was used to realize the copier-FOPA according to the cascaded QPM scheme. In the copier part, the information of signal wave was copied into the generated idler wave, before the signal light was amplified. Not only the copy, a 160 nm (Phase Insensitive, PI)/170 nm (Phase Sensitive, PS) gain bandwidth was obtained, which improved greatly comparing to that of the conventional. In FOPA part, the signal flatter gain decreases by approximately 14 dB (PI)/15 dB (PS), with fluctuation down to <0.1 dB (PI) / <0.2 dB (PS). The gain bandwidth decreases to 135 nm (PI) / 150 nm (PS), which is tens of nanometers wider than conventional FOPA.

Improving the axial resolution in time-reversed ultrasonically encoded (TRUE) optical focusing with dual ultrasonic waves

Focusing light inside highly scattering media beyond the ballistic regime is a challenging task in biomedical optical imaging, manipulation, and therapy. This challenge can be overcome by time reversing ultrasonically encoded (TRUE) diffuse light to the ultrasonic focus inside a turbid medium. In TRUE optical focusing, a photorefractive crystal or polymer is used as the phase conjugate mirror for optical time reversal. Accordingly, a relatively long ultrasound burst, whose duration matches the response time of the photorefractive material, is used to encode the diffuse light. With this long ultrasound burst, the resolution of the TRUE focus along the acoustic axis is poor. In this work, we used two transducers, emitting two intersecting ultrasound beams at 3.4 MHz and 3.6 MHz respectively, to modulate the diffuse light within their intersection volume at the beat frequency. We show that light encoded at the beat frequency can be time-reversed and converge to the intersection volume. Experimentally, TRUE focusing with an acoustic axial resolution of ~1.1 mm was demonstrated inside turbid media, agreeing with the theoretical estimation.

High-Repetition-Rate Pulsed-Pump Optical Parametric Amplification in Silicon Waveguides

The net parametric gain evolution inside the silicon waveguides for high speed optical communications is investigated. Pulsed-pump parametric amplification can be a chip-scale solution for high bit rate DWDM systems with pulse width <;1 ps.