R. Claps

University of California, Los Angeles, Los Angeles, CA, United States

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Publications (29)45.49 Total impact

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    ABSTRACT: The noise figure of silicon Raman amplifiers in the presence of nonlinear losses is calculated. The impact of two-photon absorption (TPA) and free-carrier scattering on the noise figure is quantified using the quantum formulation of the Langevin approach. It is found that TPA-induced free-carrier loss degrades the noise figure by an amount that depends on the carrier lifetime. For example, in a 1-cm-long waveguide pumped at 200 MW/cm<sup>2</sup>, the noise figure is 5.2 dB for a lifetime of tau = 1.6 ns and is reduced to 3.7 dB for tau = 0.1 ns. The reduction in the noise figure along with a concomitant increase in Raman gain from 2 to 8 dB suggests that lifetimes on the order of 0.1 ns or less are needed to create a useful silicon Raman amplifier that operates in the continuous-wave mode. It is also shown that in devices that use a p-n junction for carrier sweep-out, the screening of the junction field by generated free carriers results in a sharp increase in the noise figure at high-pump intensities. These results apply to operation in the near-infrared communication wavelengths. For mid-infrared wavelengths above the two photon absorption band-edge (2.3 nm), the absence of TPA and pump-induced free-carrier absorption ensures that the amplifier has a low-noise figure.
    Journal of Lightwave Technology 05/2008; 26(7):847-852. · 2.56 Impact Factor
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    ABSTRACT: 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.
    Proc SPIE 03/2007;
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    ABSTRACT: Coherent anti-Stokes Raman scattering (CARS) is a well-known Raman scattering process that occurs when Stokes, anti-Stokes and pump waves are properly phase-matched. Using a quantum-field approach with Langevin noise sources, we calculate the noise figure for wavelength conversion between the Stokes and anti-Stokes waves in CARS and show its dependence on phase mismatch. Under phase matched conditions, the minimum noise figure is approximately 3 dB, with a correction that depends on the pump frequency, Stokes shift, refractive indices, and nonlinear susceptibilities. We calculate the photon statistics of CARS and show that the photon number distribution is non-Gaussian. Our findings may be significant for currently pursued applications of CARS including wavelength conversion in data transmission and spectroscopic detection of dilute biochemical species.
    Optics Express 12/2006; 14(23):11418-32. · 3.55 Impact Factor
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    ABSTRACT: This paper discusses the design of acoustic vibrational modes in Si/Ge planar optical waveguides and its application in creating silicon-based Raman devices with a flexible spectrum. It addresses the deficiencies of the recently demonstrated Raman-based silicon lasers and amplifiers as they relate to spectral and low efficiency limitations of bulk silicon. The treatment is for in-plane scattering in a forward scattering configuration. In addition to calculating the spectrum and the efficiency for Raman active modes, it is shown that the negligible wave-vector of the phonons involved in this type of scattering allows for the use of the bandgap “pinching” effect to arrive at specific layer thicknesses for Si and Ge that optimize the scattering efficiency.
    Superlattices and Microstructures. 01/2006;
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    ABSTRACT: Stimulated Raman scattering in SOI waveguides has received significant attention recently with the demonstration of pulsed, continuous wave Raman lasers and high gain Raman amplification. However, the limited bandwidth of the Raman signal in silicon (∼105GHz) renders this scheme unsuitable for broadband WDM amplification unless multi-pumping scheme is employed. Large pulsed gain and lasing have been reported in GeSi waveguides. The SiGe on SOI platform represents a Raman medium with a flexible gain spectrum.
    SOI Conference, 2005. Proceedings. 2005 IEEE International; 11/2005
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    ABSTRACT: This paper presents recent breakthroughs and applications of Raman based silicon photonics such as silicon Raman amplifiers and lasers. These lasers would extend the wavelength range of III-V laser to mid-IR where important applications such as laser medicine, biochemical sensing, and free space optical communication await the emergence of a practical and low cost laser.
    Group IV Photonics, 2005. 2nd IEEE International Conference on; 10/2005
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    ABSTRACT: 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.
    Journal of Lightwave Technology 07/2005; · 2.56 Impact Factor
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    ABSTRACT: A model describing dimensional scaling of carrier lifetime and hence nonlinear optical absorption is presented. It is shown that nonlinear absorption, at a given optical intensity, can be mitigated with proper design of waveguide dimensions.
    Lasers and Electro-Optics, 2005. (CLEO). Conference on; 06/2005
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    ABSTRACT: 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.
    Optics Express 05/2005; 13(7):2459-66. · 3.55 Impact Factor
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    ABSTRACT: The lifetime of photogenerated carriers in silicon-on-insulator rib waveguides is studied in connection with the optical loss they produce via nonlinear absorption. We present an analytical model as well as two-dimensional numerical simulation of carrier transport to elucidate the dependence of the carrier density on the geometrical features of the waveguide. The results suggest that effective carrier lifetimes of ⩽ 1 ns can be obtained in submicron waveguides resulting in negligible nonlinear absorption. It is also shown that the lifetime and, hence, carrier density can be further reduced by application of a reverse bias pn junction.
    Applied Physics Letters 02/2005; 86(7):071115-071115-3. · 3.79 Impact Factor
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    ABSTRACT: In this paper, we study the dispersion properties of silicon waveguides as the waveguide dimensions are varied, from the point of view of achieving phase matching and hence enhancing the CARS process
    Information Sciences - ISCI. 01/2005;
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    ABSTRACT: Raman induced four-wave mixing is used to demonstrate wavelength conversion in silicon waveguides with sub-micron modal area. The properties of submicron waveguides that lend themselves to high efficiency conversion are also discussed.
    Lasers and Electro-Optics Society, 2004. LEOS 2004. The 17th Annual Meeting of the IEEE; 12/2004
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    ABSTRACT: 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.
    Proc SPIE 08/2004;
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    ABSTRACT: 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.
    Optics Express 07/2004; 12(12):2774-80. · 3.55 Impact Factor
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    ABSTRACT: Parametric coupling enabled by the Raman susceptibility is used to demonstrate wavelength conversion in silicon waveguides. Influence of phase matching and means to achieve phase matched waveguides are discussed.
    Lasers and Electro-Optics, 2004. (CLEO). Conference on; 06/2004
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    ABSTRACT: This chapter is organized in two parts. In part one, we present the theory of Spontaneous and Stimulated Raman Scattering (SRS), as well as that of Coherent Anti Stokes Raman Scattering (CARS) in silicon. The treatment of these phenomena in silicon is more complex than that in silica fiber, due to crystal symmetry considerations. We show that, because of the intrinsically large Raman coefficient in silicon, and the tight optical confinement in silicon-on-SiO2 waveguides, both amplification and wavelength conversion can be achieved in waveguides with millimeter scale lengths. In part two, we review our recent demonstration of spontaneous Raman emission, optical gain, and wavelength conversion in silicon waveguides. The chapter concludes by discussing practical issues and methods for realizing commercially viable devices. Keywords42.70.-a; 84.40.Ua; 85.60.-q; 42.82.-m; 85.85.+j
    02/2004: pages 1999-1999;
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    ABSTRACT: 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.
    Optics Express 02/2004; 12(1):149-60. · 3.55 Impact Factor
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    IEICE Electronic Express. 01/2004; 1:298-304.
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    ABSTRACT: This study demonstrates wavelength conversion and intrinsic Raman amplification. This work shows that phase matching is required to achieve high conversion efficiencies. In addition, reduction of waveguide cross section, while maintaining low propagation loss, is highly desirable for both wavelength conversion and optical amplification. The resulting increase in light intensity enhances the Raman interaction and the reduction in free carrier lifetime diminishes the two-photon absorption-induced free carrier losses.
    Group IV Photonics, 2004. First IEEE International Conference on; 01/2004
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    ABSTRACT: Conversion of digital- and analog-modulated optical signals from the 1550 nm band to the 1300 nm band is demonstrated in silicon waveguides. The conversion is based on parametric Stokes to anti-Stokes coupling using the Raman susceptibility of silicon.
    Applied Physics Letters 01/2004; 85. · 3.79 Impact Factor