The use of a silicon-germanium platform for the development of optically active devices will be discussed in this paper, from the perspective of Raman and Brillouin scattering phenomena. Silicon-Germanium is becoming a prevalent technology for the development of high speed CMOS transistors, with advances in several key
parameters as high carrier mobility, low cost, and reduced manufacturing logistics. Traditionally, Si-Ge structures have been used in the optoelectronics arena as photodetectors, due to the enhanced absorption of Ge in the telecommunications band. Recent developments in Raman-based nonlinearities for devices based on a silicon-on-insulator platform have shed light on the possibility of using these effects in Si-Ge architectures. Lasing and amplification have been demonstrated using a SiGe alloy structure, and Brillouin/Raman activity from acoustic phonon modes in SiGe superlattices has been predicted. Moreover, new Raman-active branches and inhomogeneously broadened spectra result from optical phonon modes, offering new
perspectives for optical device applications. The possibilities for an electrically-pumped Raman laser will be outlined, and the potential for design and development of silicon-based, Tera-Hertz wave emitters and/or receivers.
Silicon Photonics is emerging as an attractive technology in order to realize low cost, high density integrated optical circuits. Realizing active functionalities in Silicon waveguiding structures is being pursued rigorously. In particular, the Stimulated Raman scattering process has attracted considerably attention for achieving on-chip light generation, amplification and wavelength conversion. This paper reviews some of the recent efforts in using the Raman nonlinear process to realize amplifiers, and lasers. First the prospects of Raman process in realizing high gain amplifiers are discussed theoretically. Following this experimental results on amplification with gains as high as 20dB are presented. Some of the recent results in realizing pulsed and CW lasers with reverse-biased carrier sweep out are presented. The paper is concluded by highlighting some of the applications of the Raman process in Silicon in realizing mid-IR sources and also the use of SiGe as a flexible Raman medium are discussed.
Although the Raman effect is nearly two orders of magnitude stronger than the electronic Kerr nonlinearity in silicon, under pulsed operation regime where the pulse width is shorter than the phonon response time, Raman effect is suppressed and Kerr nonlinearity dominates. Continuum generation, made possible by the non-resonant Kerr nonlinearity, offers a technologically and economically appealing path to WDM communication at the inter-chip or intra-chip levels. We have studied this phenomenon experimentally and theoretically. Experimentally, a 2 fold spectral broadening is obtained by launching ~4ps optical pulses with 2.2GW/cm2 peak power into a conventional silicon waveguide. Theoretical calculations, that include the effect of two-photon-absorption, free carrier absorption and refractive index change indicate that up to >30 times spectral broadening is achievable in an optimized device. The broadening is due to self phase modulation and saturates due to two photon absorption. Additionally, we find that free carrier dynamics also contributes to the spectral broadening and cause the overall spectrum to be asymmetric with respect to the pump wavelength.
In silicon, direct electronic transitions leading to light emission have a low probability of occurrence due to the momentum mismatch between upper and lower electronic levels. Until recently, this had prevented the realization of the long waited silicon optical amplifier and laser. Raman scattering, which describes the interactions of light with vibrational levels, can be used as a way to bypass the indirect band structure of silicon and to obtain amplification and lasing. The Raman approach is very appealing because device can be made in pure silicon with a spectrum that is widely tuneable though the pump laser wavelength. While a new research topic, amplifiers with pulsed gain of 20dB and CW gain of 3 dB have already been demonstrated. Using parametric Raman coupling, wavelength conversion from 1550nm to 1300nm has been achieved. A distinguishing feature of silicon Raman devices, compared to fiber devices, is the electronic modulation capability. By integrating a p-n junction with the silicon gain medium, electrically switched lasers and amplifiers have already been demonstrated. These have many exciting applications. For example, the laser can be directly modulated to transmit data, and can be part of a silicon optoelectronic integrated circuit. At the same time, electrically switched amplifiers represent loss-less optical modulators.
Scaling properties of two photon absorption, free carrier scattering, Raman scattering and Kerr effect in silicon waveguides is reported. It is shown that the dependence of minority carrier lifetime on waveguide dimensions has a profound impact on the performance of nonlinear optical devices built using silicon waveguides.
We demonstrated conversion of optical signals from 1550nm band to the 1300nm band in silicon waveguides. The conversion is based on parametric Stokes to anti-Stokes coupling using the Raman susceptibility of silicon. Achieving high conversion efficiency requires phase matching in the waveguides as well as means to reduce
waveguide losses including the free carrier loss due to two photon absorption.
Silicon-On-Insulator integrated optics boasts low loss waveguides and tight optical confinement necessary for the design of nanophotonic devices. In addition, the processing is fully compatible with capabilities of standard silicon foundries. Because of crystal symmetry, silicon does not possess 2nd order nonlinear optical effects. However, the combination of nanoscale geometries with the high refractive index contrast creates high optical intensities where 3rd order effects may become important, and in fact, may be exploited. In this context, we study the two main nonlinear processes that can occur in silicon waveguides, namely Stimulated Raman Scattering (SRS) from zone-center optical phonons and Two-Photon Absorption (TPA). Because of the single crystal structure, the Raman gain coefficient in silicon is several orders of magnitude larger than that in the (amorphous) glass fiber while its bandwidth is limited to approximately 100GHz. To achieve Raman gain in the 1550nm region requires the pump to be centered at around 1427nm. We discuss the Raman selection rules in a silicon waveguide and present the design of an SOI Raman amplifier. We show that by causing pump depletion, TPA can limit the amount of achievable Raman gain. TPA also limits the maximum optical SNR of the silicon amplifier.