Showing papers by "Ansheng Liu published in 2005"

A continuous-wave Raman silicon laser

Abstract: Achieving optical gain and/or lasing in silicon has been one of the most challenging goals in silicon-based photonics because bulk silicon is an indirect bandgap semiconductor and therefore has a very low light emission efficiency. Recently, stimulated Raman scattering has been used to demonstrate light amplification and lasing in silicon. However, because of the nonlinear optical loss associated with two-photon absorption (TPA)-induced free carrier absorption (FCA), until now lasing has been limited to pulsed operation. Here we demonstrate a continuous-wave silicon Raman laser. Specifically, we show that TPA-induced FCA in silicon can be significantly reduced by introducing a reverse-biased p-i-n diode embedded in a silicon waveguide. The laser cavity is formed by coating the facets of the silicon waveguide with multilayer dielectric films. We have demonstrated stable single mode laser output with side-mode suppression of over 55 dB and linewidth of less than 80 MHz. The lasing threshold depends on the p-i-n reverse bias voltage and the laser wavelength can be tuned by adjusting the wavelength of the pump laser. The demonstration of a continuous-wave silicon laser represents a significant milestone for silicon-based optoelectronic devices.

An all-silicon Raman laser

Abstract: With the growing use of optoelectronics in information technology, manipulating light is almost as important as manipulating electrons. Unfortunately silicon, workhorse of modern microelectronics, is next to useless in optical applications. There has been a massive effort to overcome silicon's inadequacies, and ways of coaxing silicon to handle light are under development but a key component — the laser — has been problematic. Last year a silicon laser was produced, but it involved metres of optical fibre. Now workers in Intel's research labs have come up with an all-silicon laser on a single chip. The device is compact and readily integrated with other silicon components. The possibility of light generation and/or amplification in silicon has attracted a great deal of attention1 for silicon-based optoelectronic applications owing to the potential for forming inexpensive, monolithic integrated optical components. Because of its indirect bandgap, bulk silicon shows very inefficient band-to-band radiative electron–hole recombination. Light emission in silicon has thus focused on the use of silicon engineered materials such as nanocrystals2,3,4,5, Si/SiO2 superlattices6, erbium-doped silicon-rich oxides7,8,9,10, surface-textured bulk silicon11 and Si/SiGe quantum cascade structures12. Stimulated Raman scattering (SRS) has recently been demonstrated as a mechanism to generate optical gain in planar silicon waveguide structures13,14,15,16,17,18,19,20,21. In fact, net optical gain in the range 2–11 dB due to SRS has been reported in centimetre-sized silicon waveguides using pulsed pumping18,19,20,21. Recently, a lasing experiment involving silicon as the gain medium by way of SRS was reported, where the ring laser cavity was formed by an 8-m-long optical fibre22. Here we report the experimental demonstration of Raman lasing in a compact, all-silicon, waveguide cavity on a single silicon chip. This demonstration represents an important step towards producing practical continuous-wave optical amplifiers and lasers that could be integrated with other optoelectronic components onto CMOS-compatible silicon chips.

High speed silicon Mach-Zehnder modulator

Abstract: We demonstrate a silicon modulator with an intrinsic bandwidth of 10 GHz and data transmission from 6 Gbps to 10 Gbps Such unprecedented bandwidth performance in silicon is achieved through improvements in material quality, device design, and driver circuitry

Net continuous wave optical gain in a low loss silicon-on-insulator waveguide by stimulated Raman scattering.

Abstract: We observe for the first time net continuous wave optical gain in a low loss silicon-on-insulator waveguide based on stimulated Raman scattering. We show that nonlinear optical loss due to two-photon absorption induced free carrier absorption can be significantly reduced by introducing a reverse biased p-i-n diode in the waveguide. For a 4.8 cm long waveguide with an effective core area of ~1.6 µm2, we obtain a net CW Raman gain of >3dB with a pump power of ~700mW inside the waveguide.

Lossless optical modulation in a silicon waveguide using stimulated Raman scattering

Abstract: In this paper we describe a new modulation scheme using stimulated Raman scattering in conjunction with a reverse biased p-i-n diode embedded in a silicon waveguide. We show optical modulation of a weak probe beam by modulating the reverse bias voltage of the silicon waveguide excited by a strong pump beam. The probe beam modulation is due to the two-photon absorption-induced carrier density modulation in the waveguide. By tuning the probe wavelength to the Stokes wavelength, we demonstrate for the first time a lossless optical modulator in silicon with modulation speeds up to 80-MHz.

Scaling the modulation bandwidth and phase efficiency of a silicon optical modulator

Abstract: We present an optimized design and detailed simulation of an all-silicon optical modulator based on a silicon waveguide phase shifter containing a metal-oxide-semiconductor (MOS) capacitor. Based on a fully vectorial Maxwell mode solver, we analyze the modal characteristics of the silicon waveguide. We show that shrinking the waveguide size and reducing gate oxide thickness significantly enhances the phase modulation efficiency because of the optical field enhancement in the voltage induced charge layers of the MOS capacitor, which, in turn, induce refractive index modulation in silicon due to free carrier dispersion effects. We also analyze the device speed by transient semiconductor device modeling. As both optical absorption and modulation bandwidth increase with increasing doping concentration, we show that, with a nonuniform doping profile in the waveguide, balance between the device operation speed and optical loss can be realized. Our simulation suggests that a TE-polarized optical phase modulator with a bandwidth of 10 GHz and an on-chip optical loss less than 2 dB is achievable in silicon.

Phase modulation efficiency and transmission loss of silicon optical phase shifters

Abstract: This paper focuses on understanding the phase efficiency and optical loss of MOS-capacitor-based silicon waveguide phase shifters. A total of nine designs have been fabricated using poly-silicon and characterized at wavelengths around 1.55 /spl mu/m. Detailed comparison of design parameters shows that scaling down the waveguide dimensions, placing the capacitor gate oxide near the center of the optical mode, and reducing the oxide thickness significantly enhance phase modulation efficiency. Our best design to date demonstrates a /spl pi/-radian phase shift with 0.8-cm device length and 3-V drive. This phase shifter has a transmission loss of 15 dB, the primary source of which is the poly-silicon regions inside the device. An improved material can reduce loss to as little as 4 dB.

Customized drive electronics to extend silicon optical modulators to 4 gb/s

Abstract: The data transmission bandwidth of a metal oxide semiconductor (MOS) capacitor Si optical modulator is extended from 1 to 4 Gb/s through the introduction of custom-designed low-impedance drive circuitry Two distinct drive circuits were produced and tested-the first targeting 25 Gb/s data rate and 3 dB extinction ratio (ER), and the second having reduced voltage swing (13 V single-ended swing) while achieving an open eye at 4 Gb/s The speed, power, and ER data collected are used to build a quantitative discussion of the challenges in achieving a power-efficient free-carrier modulator at bit rates above 1 Gb/s

Integration challenge of silicon photonics with microelectronics

Abstract: In this presentation we discuss some practical issues and challenges associated with processing silicon photonic devices in a high volume CMOS manufacturing environment and the challenges of integrating the photonic devices with microelectronics

Two-photon absorption generated carrier lifetime reduction in semiconductor waveguide for semiconductor based raman laser and amplifier

Abstract: A semiconductor based Raman laser and/or amplifier with reduced two-photon absorption generated carrier lifetimes. An apparatus according to embodiments of the present invention includes optical waveguide disposed in semiconductor material and a diode structure disposed in the optical waveguide. The optical waveguide is to be coupled to a pump laser to receive a first optical beam having a first wavelength and a first power level to result in emission of a second optical beam of a second wavelength in the semiconductor waveguide. The diode structure is to be biased to sweep out free carriers from the optical waveguide generated in response to two photon absorption in the optical waveguide.

Method and apparatus for polarization insensitive phase shifting of an optical beam in an optical device

Abstract: An apparatus and method for modulating a phase of optical beam independent of polarization. In one embodiment, an apparatus according to embodiments of the present invention includes a first region of an optical waveguide disposed in semiconductor material, the first region having a first conductivity type, and a second region of the optical waveguide disposed in the semiconductor material, the second region having a second conductivity type opposite to the first conductivity type. The apparatus also includes a substantially V shaped insulating region disposed between the first and second regions of the optical waveguide, wherein a vertex of the substantially V shaped insulating region forms an intersecting line that is substantially parallel to an optical path of an optical beam to be directed through the optical waveguide.

Characterization of a silicon Raman laser

Abstract: With a reverse biased p-i-n structure embedded in a silicon waveguide, we efficiently reduced the nonlinear loss due to two photon absorption induced free carrier absorption and achieved continuous-wave net gain and lasing in a silicon waveguide cavity on a single chip. We report here the laser characterization for different cavity lengths from 1.6 to 8 cm. With a pump wavelength at 1550 nm, the laser output at 1686 nm is single mode with over 55 dB side mode suppression and has less than 80 MHz linewidth. The lasing threshold depends on the p-i-n reverse bias voltage. With 25V bias, the threshold pump power is ~180 mW. The slope efficiency is ~4.3% for a single side output and a total output power of >10 mW can be reached at a pump power of 500 mW. The laser wavelength can be tuned by adjusting the wavelength of the pump laser. In addition to the laser line at Stokes wavelength, a narrow linewidth anti-Stokes line at 1434.3 nm is also generated in the laser cavity through parametric conversion process.

Recent development in silicon photonics: 2.5 Gb/s silicon optical modulator and silicon Raman laser

Abstract: Due to the mature silicon fabrication technology and vast existing infrastructures, silicon photonics has a chance to offer low cost solutions to telecommunications and data communications. It could also enable a chip-scale platform for monolithic integration of optics and microelectronics circuits for applications of optical interconnects for which high data streams are required in a very small footprint. Two key building blocks needed for any silicon based optoelectronics are silicon based light source and high-speed optical modulator. This paper gives an overview of recent results for a fast (>1GHz) silicon modulator and a silicon Raman laser. We present optical characterization of a high speed metal-oxide-semiconductor (MOS) capacitor-based silicon optical modulator. We show that a Mach-Zehnder interferometer (MZI) structure with a custom-designed driver circuit results in the realization of a silicon modulator transmitting data at 2.5 Gb/s with an extinction ratio of up to 2.8 dB. In addition we show that by reducing the waveguide dimensions one can improve the phase efficiency. In addition, as single crystal silicon possesses higher (four orders of magnitude) Raman gain coefficient as compared to silica, it is possible to achieve sizeable gain in chip-scale silicon waveguide for optical amplification and lasing. With a 4.8 cm long waveguide containing a reverse biased p-i-n diode, we demonstrate lasing operation using a pulsed pump laser. We achieve ~10% slope efficiency. We in addition model a continuous-wave silicon Raman laser and show that higher conversion efficiency and lower threshold power can be realized with optimised cavity device design.

Reduced loss ultra-fast semiconductor modulator and switch

Abstract: A fast optical modulator or switch with reduced optical loss is disclosed An apparatus according to aspects of the present invention includes an optical splitter disposed in a semiconductor material An optical beam having a first wavelength is split by the optical splitter into first and second portions First and second optical waveguides disposed in the semiconductor material are optically coupled to the optical splitter The first and second portions of the optical beam are to be directed through the first and second optical waveguides, respectively The first optical waveguide is also optically coupled to receive a pump optical beam The pump optical beam has a pump wavelength and a pump power level to amplify and phase shift the first portion of the optical beam of the first wavelength in the first optical waveguide A diode structure is disposed in the first optical waveguide and is selectively biased to sweep out free carriers from the first optical waveguide generated in response to two photon absorption in the optical waveguide An optical coupler is disposed in the semiconductor material and is optically coupled to the first and second optical waveguides to combine the first and second portions of the optical beam

Avalanche photodetector with reflector-based responsivity enhancement

Abstract: An avalanche photodetector is disclosed. An apparatus according to aspects of the present invention includes an absorption region including a first type of semiconductor. The first type of semiconductor material has a graded doping concentration of a dopant material within the absorption region. A multiplication region is proximate to and separate from the absorption region. The multiplication region includes a second type of semiconductor material in which there is an electric field. The electric field is to multiply the free charge carriers created in the absorption region. A reflector is disposed proximate to the multiplication region such that the multiplication region is between the absorption region and the reflector. The reflector is to reflect unabsorbed light that reaches the reflector from the absorption region back to the absorption region.

Enhanced polarization-independent optical ring resonators on silicon-on-insulator

Abstract: Recently, we have realised a polarisation independent optical racetrack resonator whose resonant dips for TE and TM align to better than 1pm. The devices had a Free Spectral Range (FSR) of only several hundred picometres. This in large part was to the relatively large bend radius (~ 400μm) designed and fabricated with initial focus on producing low bend loss devices. Modelling of the bend loss of the same dimension devices shows that the bend radius can be reduced significantly (down to ~25μm) to produce race track ring resonator with an FSR that is approximately 400% larger than that of those previously fabricated, whilst retaining polarisation independence. This paper will focus on the proposed enhancement of these devices as well as the impetus for their investigation.

Recent Results in Silicon Photonics

Abstract: Silicon photonics has recently attracted attention. This presentation gives an overview of recent advances in this area including demonstration of 10 Gbps data transmission using a silicon modulator and a continuous-wave Raman silicon laser.

Gain modulation in a silicon waveguide using stimulated Raman scattering

Abstract: We demonstrate a novel 75-MHz optical modulator in an SOI waveguide. Modeling of the p-i-n shows device speed is limited by decreasing internal resistance of the p-i-n due to two-photon generated free carriers.