Varun Raghunathan

All optical switching and continuum generation in silicon waveguides

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.

Influence of nonlinear absorption on Raman amplification in Silicon waveguides

We model the TPA-induced free carrier absorption effect in silicon Raman amplifiers and quantify the conditions under which net gain may be obtained. The achievable Raman gain strongly depends on the free carrier lifetime, propagation loss, and on the effective Raman gain coefficient, through pump-induced broadening.

Anti-Stokes Raman conversion in silicon waveguides

The first observation of parametric down-conversion in silicon is reported. Conversion from 1542.3nm to 1328.8nm is achieved using a CW pump laser at 1427 nm. The conversion occurs via Coherent Anti-Stokes Raman Scattering (CARS) in which two pump photons and one Stokes photon couple through a zone-center optical phonon to an anti-Stokes photon. The maximum measured Stokes/anti-Stokes power conversion efficiency is 1x10-5. The value depends on the effective pump power, the Stimulated Raman Scattering (SRS) coefficient of bulk silicon, and waveguide dispersion. It is shown that the power conversion efficiency is a strong function of phase mismatch inside the waveguide.

Raman-based silicon photonics

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.

Phase-matching and nonlinear optical processes in silicon waveguides

The efficiency of four-wave-mixing arising from Raman and non-resonant nonlinear susceptibilities in silicon waveguides is studied in the 1.3 - 1.8microm regime. The wavelength conversion efficiency is dominated by the Raman contribution to the nonlinear susceptibility, and high conversion efficiencies can be achieved under the phase-matching condition. In this context, dispersion in silicon waveguides is analyzed and it is shown that phase-matching is achieved in properly engineered waveguides where birefringence compensates for material dispersion. Finally the sensitivity of the phase mismatch to fabrication-induced errors in waveguide dimensions is quantified.

Parametric Raman wavelength conversion in scaled silicon waveguides

The benefits of using submicrometer modal-dimension silicon waveguides in realizing high-efficiency parametric Raman wavelength conversion are demonstrated theoretically and experimentally. The combined effects of Raman nonlinearities and free-carrier losses induced by two-photon absorption (TPA) are analyzed using the coupled-mode theory. The analysis indicates that scaling down the lateral dimensions increases the conversion efficiency of the Raman process and reduces the effective lifetime of free carriers and hence ameliorates the free-carrier losses. The feasibility of data conversion is demonstrated by coherent transfer of the analog radio-frequency (RF) signal from Stokes to anti-Stokes channels. The conversion efficiency, and hence signal-to-noise ratio (SNR), and bandwidth of the conversion process are found to be limited by the phase mismatch between the pump, Stokes, and anti-Stokes fields. The dispersion properties of submicrometer waveguides are also studied from the point of view of achieving phase matching and enhancing the conversion efficiency.

Prospects for Silicon Mid-IR Raman Lasers

This paper presents the case for the silicon Raman laser as a potential source for the technologically important midwave infrared (MWIR) region of the optical spectrum. The mid-IR application space is summarized, and the current practice based on the optical parametric oscillators and solid state Raman lasers is discussed. Relevant properties of silicon are compared with popular Raman crystals, and linear and nonlinear transmission measurements of silicon in the mid-IR are presented. It is shown that the absence of the nonlinear losses, which severely limit the performance of the recently demonstrated silicon lasers in the near IR, combined with unsurpassed crystal quality, high thermal conductivity and excellent optical damage threshold render silicon a very attractive Raman medium, even when compared to the very best Raman crystals. In addition, silicon photonic technology, offering integrated low-loss waveguides and microcavities, offers additional advantages over todays bulk crystal Raman laser technology. Using photonic crystal structures or microring resonators, the integrated cascaded microcavities can be employed to realize higher order Stokes emission, and hence to extend the wavelength coverage of the existing pump lasers. Exploiting these facts, the proposed technology can extend the utility of silicon photonics beyond data communication and into equally important applications in biochemical sensing and laser medicine

Wavelength conversion in silicon using Raman induced four-wave mixing

Conversion of digital- and analog-modulated optical signals from the 1550nm band to the 1300nm band is demonstrated in silicon waveguides. The conversion is based on parametric Stokes to anti-Stokes coupling using the Raman susceptibility of silicon.

Raman amplification and lasing in SiGe waveguides

We describe the first observation of spontaneous Raman emission, stimulated amplification, and lasing in a SiGe waveguide. A pulsed optical gain of 16dB and a lasing threshold of 25 W peak pulse power (20 mW average) is observed for a Si1-xGex waveguide with x=7.5%. At the same time, a 40 GHz frequency downshift is observed in the Raman spectrum compared to that of a silicon waveguide. The spectral shift can be attributed to the combination of composition- and strain-induced shift in the optical phonon frequency. The prospect of Germanium-Silicon-on-Oxide as a flexible Raman medium is discussed.

20 dB on-off Raman amplification in silicon waveguides

We report on-off optical gains of 20 dB in silicon-on-insulator waveguides. Free carrier and two photon absorption of 4 dB is measured indicating an intrinsic Raman gain of 24 dB