Tightly confined, low-loss waveguides in highly nonlinear materials permit nonlinear optical interactions to occur over much shorter distances than do fibers The nonlinear interactions are further enhanced in resonators Both theory and experiment of enhanced four-wave mixing in micro-ring resonators are presented that can be used for many applications A conversion efficiency of 14% achievable with only 10-mW peak pump power is predicted under realizable conditions The experiment, the first one to the authors’ knowledge in nonlinear optics performed in microrings, shows, even in a lossy GaAs/AlGaAs ring, a 26-dB improvement in the conversion efficiency compared with that of an equivalent straight waveguide, in agreement with theory

Wavelength conversion in GaAs micro-ring resonators.

First demonstration of cross phase modulation based interferometric switch is presented in silicon on insulator waveguides. By using Mach-Zehnder interferometric configuration we experimentally demonstrate switching of CW signal ~25 nm away from the pump laser. We present the effect of free carrier accumulation on switching. Additionally, we theoretically analyze the transient effects and degradations due to free carrier absorption, free carrier refraction and two photon absorption effects. Results suggest that at low peak power levels the system is governed by Kerr nonlinearities. As the input power levels increase the free carrier effects becomes dominant. Effect of free carrier generation on continuum generation and power transfer also theoretically analyzed and spectral broadening factor for high input power levels is estimated.

All optical switching and continuum generation in silicon waveguides

This paper reviews recent progress in a new branch of silicon photonics that exploits Raman scattering as a practical and elegant approach for realizing active photonic devices in pure silicon. The large Raman gain in the material, enhanced by the tight optical confinement in Si/SiO2 heterostructures, has enabled the demonstration of the first optical amplifiers and lasers in silicon. Wavelength conversion, between the technologically important wavelength bands of 1300 and 1500 nm, has also been demonstrated through Raman four wave mixing. Since carrier generation through two photon absorption is omnipresent in semiconductors, carrier lifetime is the single most important parameter affecting the performance of silicon Raman devices. A desired reduction in lifetime is attained by reducing the lateral dimensions of the optical waveguide, and by actively removing the carriers with a reverse biased diode. An integrated diode also offers the ability to electrically modulate the optical gain, a unique property not available in fiber Raman devices. Germanium-silicon alloys and superlattices offer the possibility of engineering the otherwise rigid spectrum of Raman in silicon.

Raman-based silicon photonics

Optically transparent wavelength converters, in which the output is a wavelength-converted replica of the input, may be required to improve performance and ease management in future "mixed-mode" wavelength division multiplexed networks. Four-wave mixing (FWM) in semiconductor optical amplifiers (SOA's) is an attractive optically transparent wavelength conversion technique because it allows pump tunability. So far, three schemes for polarization-insensitive FWM in SOA's have been demonstrated, using two copolarized pumps, two orthogonal pumps, and polarization diversity. This paper presents a comparison of their output signal-to-noise ratio, and hence their ultimate tunability. A simple analytical model for FWM in SOA's is used to predict that when each scheme has the same inputs and produces the same frequency shift, the polarization-diversity scheme has the highest output signal-to-noise ratio of the three schemes, and hence promises the greatest tunability.

Tunability of polarization-insensitive wavelength converters based on four-wave mixing in semiconductor optical amplifiers

We report ~22 dB of Raman gain in single mode As2Se3 chalcogenide glass fiber using 15 ps optical pump pulses from 1470 nm to 1560 nm. We employ a novel technique of cross-phase modulation induced sideband amplification to map out the Raman gain spectrum of this glass, and investigate the role of both degenerate and non-degenerate (ND) two-photon absorption (TPA). We find that for materials such as As2Se3 where the Raman gain coefficient (gR) and TPA are comparable, it is critical to know and account for the role of both of these in order to achieve appreciable Raman gain. This is highlighted by our results, where we achieve significantly higher Raman gain at the longest pump wavelength (1560 nm), despite the fact that the Raman gain coefficient itself (gR) is smallest at this wavelength. This occurs because the TPA is significantly larger for shorter wavelengths in As2Se3. We conclude, therefore, that for Raman gain applications in As2Se3, L-band operation is strongly favored over C-band operation.

Two-photon absorption effects on Raman gain in single mode As2Se3 chalcogenide glass fiber

We analyze the effect of nonlinear loss on four‐wave mixing (FWM) conversion efficiency. Maximum conversion efficiency is geometry independent and equal to e−2|χ(3)/Im{χ(3)}|2 when nonlinear loss dominates. Optimum device length and operating conditions are obtained and theoretical results are verified with a picosecond pulse FWM experiment.

Optimization of four-wave mixing conversion efficiency in the presence of nonlinear loss

Results on wavelength shifters based on four-wave mixing that operate in the 1.5-/spl mu/m regime are reported. These devices utilize the near-bandedge resonant enhancement in the third-order nonlinearity in passive InGaAsP-InP quantum-well waveguides. Over the erbium-doped fiber-amplifier gain band approximately -20 dB conversion efficiency was obtained with /spl ap/18-dBm CW pump power. The conversion efficiency was nearly constant for wavelength shifts up to /spl ap/26 nm (3.3 THz), limited by the phase-matching bandwidth of the 8.5-mm-long device.

Nondegenerate four-wave mixing wavelength conversion in low-loss passive InGaAsP-InP quantum-well waveguides

We have demonstrated wavelength conversion in the 1.5 μm regime using the near band gap nonlinearity of semiconductor quantum wells. In a 7.5 mm long passive InGaAsP/InP single quantum well waveguide, a wavelength shift of 20 nm has been obtained with −11 dB conversion efficiency using picosecond pulses. We confirmed the ultrafast nature of the nonlinearity by measuring the conversion efficiency as a function of the frequency shift. An order of magnitude increase in the value of n2 close to the band gap is observed compared with the off‐resonance value.

Short‐pulse wavelength shifting by four wave mixing in passive InGaAsP/InP waveguides

Wavelength conversion devices (WCD), which allow the transcription of information carried on one set of optical wavelengths onto another, are useful for wavelength division multiplexing optical networks. A common approach is to utilize nondegenerate four-wave mixing (FWM) in semiconductor optical amplifiers or dispersion-shifted silica fibers. Here we describe FWM in passive InGaAsP/InP quantum-well waveguides using the near-bandedge excitonic resonance nonlinearity. Compared with FWM in active semiconductors, passive semiconductors require higher optical pump power, but are simpler, do not depend on free-carrier effects, and therefore have the potential advantage of simultaneous ultrahigh speed and broadband direct optical-to-optical conversion. Compared to silica fibers, semiconductors have several orders of magnitude lower latency, and have the potential for compact, monolithic integration.

H. Q. Le

Papers

Optimization of four-wave mixing conversion efficiency in the presence of nonlinear loss

Nondegenerate four-wave mixing wavelength conversion in low-loss passive InGaAsP-InP quantum-well waveguides

Short‐pulse wavelength shifting by four wave mixing in passive InGaAsP/InP waveguides

Nondegenerate four-wave mixing wavelength conversion in low-loss passive InGaAsP/InP waveguides