Sub-volt high-speed silicon MOSCAP microring modulator driven by high-mobility conductive oxide.

Silicon microring modulator plays a critical role in energy-efficient optical interconnect and optical computing owing to its ultra-compact footprint and capability for on-chip wavelength-division multiplexing. However, existing silicon microring modulators usually require more than 2 V of driving voltage (Vpp), which is limited by both material properties and device structures. Here, we present a metal-oxide-semiconductor capacitor microring modulator through heterogeneous integration between silicon photonics and titanium-doped indium oxide, which is a high-mobility transparent conductive oxide (TCO) with a strong plasma dispersion effect. The device is co-fabricated by Intel's photonics fab and our in-house TCO patterning processes, which exhibits a high modulation efficiency of 117 pm/V and consequently can be driven by a very low Vpp of 0.8 V. At a 11 GHz modulation bandwidth where the modulator is limited by the RC bandwidth, we obtained 25 Gb/s clear eye diagrams with energy efficiency of 53 fJ/bit.

High-performance silicon-on-insulator grating coupler with completely vertical emission.

We study the physical concept of utilizing a critical coupling to obtain a high-performance grating coupler with completely vertical emission on a silicon-on-insulator substrate. Following our design strategy, we numerically show that when our grating coupler is coupled to a standard single-mode fiber operating at 1310 nm wavelength, a -1.46 dB coupling loss, a 20 nm spectral full-width-half-maximum, and a -24 dB back reflection can be achieved at the same time without full optimization. A practical design that largely relaxes the stringent lithography requirement is also proposed and presented.

Efficient broadband silicon-on-insulator grating coupler with low backreflection.

We design and fabricate an efficient broadband grating coupler on a 400 nm thick silicon-on-insulator wafer. The measured coupling loss is 3 dB when coupling to a single-mode fiber at 1310 nm wavelength with TE polarization. The spectral FWHM and backreflection are determined to be 58 nm and -27 dB, respectively.

Characterization of efficient wavelength conversion by four-wave mixing in sub-micron silicon waveguides.

We characterize silicon waveguide based wavelength converters using a commercial semiconductor optical amplifier (SOA) based wavelength converter as a benchmark. Conversion efficiency as high as -5.5 dB can be achieved using a 2.5 cm long sub-micron silicon-on-insulator rib waveguide. Comparison with the SOA reveals that silicon offers broader conversion bandwidth, higher OSNR, and negligible channel crosstalk. The impact of two-photon absorption and free carrier absorption on the conversion efficiency and the dependence of the efficiency on the rib waveguide dimensions are investigated theoretically. Using a nonlinear index coefficient of 4x10(-14) cm(2)/W for silicon, we obtain good agreement between simulations and measurements.

Multichannel dispersion compensation using a silicon waveguide-based optical phase conjugator.

We experimentally demonstrate dispersion compensation using a silicon-based optical phase conjugator. We achieve simultaneous transmission of four dense wavelength division multiplexing (DWDM) channels spaced at 100 GHz and operating at 10 Gbits/s over 320 km of standard fiber. The measured power penalty at bit error rate of 10(-9) is less than 0.3 dB.

Raman amplification of 40 Gb/s data in low-loss silicon waveguides.

We demonstrate on-chip Raman amplification of an optical data signal at 40 Gb/s in a silicon-on-insulator p-i-n rib waveguide. Using 230 mW of coupled pump power, on/off gain of up to 2.3 dB is observed, while signal integrity is maintained. In addition, the gain is measured as a function of signal wavelength detuning from the Stokes wavelength. The Lorentzian linewidth of the Raman gain profile is determined to be approximately 80 GHz. This provides applicability for the selective amplification of individual DWDM optical channels.

High efficiency wavelength conversion of 10 Gb/s data in silicon waveguides.

Efficient wavelength conversion via four-wave-mixing in silicon-on-isolator p-i-n waveguides has been realized. By reverse biasing the p-i-n diode structure formed along the silicon rib waveguide, the nonlinear absorption due to two photon absorption induced free carrier absorption is significantly reduced, and a wavelength conversion efficiency of -8.5 dB has been achieved in an 8 cm long waveguide at a pump intensity of 40 MW/cm2. A high-speed pseudo-random bit sequence data at 10 Gb/s rate is converted to a new wavelength channel in the C-band with clear open eye diagram and no waveform distortion. Conversion efficiency as functions of pump power, wavelength detuning, and bias voltages, have been investigated. For shorter waveguides of 1.6 cm long, a conversion bandwidth of > 30 nm was achieved.

Monolithic integrated Raman silicon laser.

We present a monolithic integrated Raman silicon laser based on silicon-on-insulator (SOI) rib waveguide race-track ring resonator with an integrated p-i-n diode structure. Under reverse biasing, we achieved stable, single mode, continuous-wave (CW) lasing with output power exceeding 30mW and 10% slope efficiency. The laser emission has high spectral purity with a measured side mode suppression exceeding 70dB and laser linewidth of <100 kHz. This laser architecture allows for on-chip integration with other silicon photonics components to provide a highly integrated and scaleable monolithic device.

Demonstration of wavelength conversion at 40 Gb/s data rate in silicon waveguides.

We report an efficient wavelength conversion via four-wave-mixing in reverse biased silicon-on-isolator p-i-n rib waveguides and demonstrate, for the first time, the conversion of a high-speed optical pseudo-random bit sequence data at 40 Gb/s. Results give a wavelength conversion efficiency of -8.6dB using a 8cm long waveguide with clear open eye on the wavelength converted signal . Conversion efficiency as functions of pump power and bias voltages has also been investigated. We show a slope efficiency close to 2 as predicted by theory.

A continuous-wave Raman silicon laser.

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.

The possibility of light generation and/or amplification in silicon has attracted a great deal of attention 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 nanocrystals, Si/SiO2 superlattices, erbium-doped silicon-rich oxides, surface-textured bulk silicon and Si/SiGe quantum cascade structures. Stimulated Raman scattering (SRS) has recently been demonstrated as a mechanism to generate optical gain in planar silicon waveguide structures. In fact, net optical gain in the range 2-11 dB due to SRS has been reported in centimetre-sized silicon waveguides using pulsed pumping. 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 fibre. 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.

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

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 microm2, 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.

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.

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

We observe for the first time net optical gain in a low loss silicon waveguide in silicon-on-insulator (SOI) based on stimulated Raman scattering with a pulsed pump laser at 1.545 microm. We show that pulsed pumping with a pulse width narrower than the carrier recombination lifetime in SOI significantly reduces the free carrier generation rate due to two-photon absorption (TPA) in silicon. For a 4.8 cm long waveguide with an effective core area of ~1.57 microm2, we obtained a net gain of 2 dB with a pump pulse width of ~17 ns and a peak pump power of ~470 mW inside the waveguide.

Quantum-limited optical phase detection at the 10(-10)-rad level.

Interferometric detection of gravitational waves at a level of astrophysical interest is expected to require measurement of optical phase differences of < or = 10(-10) rad. A fundamental limit to the phase sensing is the statistics of photon detection--Poisson statistics for light in a coherent state. We have built a laboratory-scale interferometer to achieve and investigate the phase detection sensitivity required for the Laser Interferometer Gravitational-Wave Observatory. With 70 W of circulating power, we have obtained a phase sensitivity of 1.28 x 10(-10) rad/square root(Hz) at frequencies above 600 Hz, limited by quantum noise. Below 600 Hz, excess noise above the quantum limit is seen, and we present our investigations into the sources of this excess. Compared with the results of previous such experiments, the phase sensitivity over the full 100-Hz-10-kHz band of interest has been improved by factors of up to 100, with a factor-of-2.5 improvement in the quantum-limited level.