Dynamic computational optical fringe mitigation in tunable laser absorption spectroscopy.

In optical spectroscopic systems where unwanted optical scattering cannot be eliminated, Fabry-Pérot etalons cause unpredictable changes in the spectral background. Frequent system calibration is then required to maintain the desired measurement accuracy, which presents a major limitation to the spectrometer. We introduce a computational approach to mitigate the adverse effects of optical fringing without hardware modifications. Motivated by experimental observations of complicated fringe behaviors, we simplify the problem by decomposing the fringe background into component etalons that can be addressed according to their individual characteristics. The effectiveness of the proposed method is demonstrated on a silicon photonic methane sensor, where accurate measurements of methane concentration are obtained from spectral data strongly affected by optical fringes.

Adaptive thermal stabilization of an integrated photonic spectrometer using parasitic interference fringes.

Parasitic fringe drift from unwanted scatterings limits the long-term stability of waveguide-based optical spectrometers. Yet their spectral features provide relevant information that can be used to improve performance of the spectrometer. We show that fringe drift can be extracted and utilized to perform accurate thermal stabilization, especially in the case of integrated waveguide sensors. In this Letter, effective stabilization of a methane silicon photonic sensor is demonstrated, and significant reduction in fringe noise is clearly observed.

Field Deployment of a Portable Optical Spectrometer for Methane Fugitive Emissions Monitoring on Oil and Gas Well Pads.

We present field deployment results of a portable optical absorption spectrometer for localization and quantification of fugitive methane (CH4) emissions. Our near-infrared sensor targets the 2ν3 R(4) CH4 transition at 6057.1 cm-1 (1651 nm) via line-scanned tunable diode-laser absorption spectroscopy (TDLAS), with Allan deviation analysis yielding a normalized 2.0 ppmv∙Hz-1/2 sensitivity (4.5 × 10-6 Hz-1/2 noise-equivalent absorption) over 5 cm open-path length. Controlled CH4 leak experiments are performed at the METEC CSU engineering facility, where concurrent deployment of our TDLAS and a customized volatile organic compound (VOC) sensor demonstrates good linear correlation (R2 = 0.74) over high-flow (>60 SCFH) CH4 releases spanning 4.4 h. In conjunction with simultaneous wind velocity measurements, the leak angle-of-arrival (AOA) is ascertained via correlation of CH4 concentration and wind angle, demonstrating the efficacy of single-sensor line-of-sight (LOS) determination of leak sources. Source magnitude estimation based on a Gaussian plume model is demonstrated, with good correspondence (R2 = 0.74) between calculated and measured release rates.

Experimental realization of deep-subwavelength confinement in dielectric optical resonators.

The ability to highly localize light with strong electric field enhancement is critical for enabling higher-efficiency solar cells, light sources, and modulators. While deep-subwavelength modes can be realized with plasmonic resonators, large losses in these metal structures preclude most practical applications. We developed an alternative approach to achieving subwavelength localization of the electric and displacement fields that is not accompanied by inhibitive losses. We experimentally demonstrate a dielectric bowtie photonic crystal structure that supports mode volumes commensurate with plasmonic elements and quality factors that reveal ultralow losses. Our approach opens the door to the extremely strong light-matter interaction regime with, simultaneously incorporating both an ultralow mode volume and an ultrahigh quality factor, that had remained elusive in optical resonators.

Distributed backscattering in production O-band Si nanophotonic waveguides.

Backscattering in integrated photonic waveguides can significantly impact the performance of optical systems. However, it has not been extensively studied in the literature and measurements on waveguides fabricated in production foundry processes are particularly lacking in view of their importance to technology. Here we experimentally measure and analyze distributed backscattering in various production O-band silicon photonic waveguides. We find the measured backscattering to scale from -18 to -36 dB/mm. Measured trends across waveguide geometry and polarization are consistent with stochastic defects on waveguide sidewalls being the dominant source of distributed backscattering in production Si waveguides. For production SiNx waveguides, both sidewall and cladding defects need to be considered to fit measured trends.

Ultralow crosstalk nanosecond-scale nested 2 × 2 Mach-Zehnder silicon photonic switch.

We present the design and characterization of a novel electro-optic silicon photonic 2×2 nested Mach-Zehnder switch monolithically integrated with a CMOS driver and interface logic. The photonic device uses a variable optical attenuator in order to balance the power inside the Mach-Zehnder interferometer leading to ultralow crosstalk performance. We measured a crosstalk as low as -34.5 dB, while achieving ∼2 dB insertion loss and 4 ns transient response.

Si⁺-implanted Si-wire waveguide photodetectors for the mid-infrared.

CMOS-compatible Si⁺-implanted Si-waveguide p-i-n photodetectors operating at room temperature and at mid-infrared wavelengths from 2.2 to 2.3 µm are demonstrated. Responsivities of 9.9 ± 2.0 mA/W are measured at a 5 V reverse bias with an estimated internal quantum efficiency of 2.7 - 4.5%. The dark current is found to vary from a few microamps down to less than a nanoamp after a post-implantation annealing of 350°C. The measured photocurrent dependence on input power shows a linear correspondence over more than three decades, and the frequency response of a 250 µm-length p-i-n device is measured to be ~1.7 GHz for a wavelength of λ = 2.2 µm, thus potentially opening up new communication bands for photonic integrated circuits.

Binary phase-shift keying by coupling modulation of microrings.

We propose a coupling-modulated microring in an add-drop configuration for binary phase-shift keying (BPSK), where data is encoded as 0 and π radian phase-shifts on the optical carrier. The device uses the π radian phase-flip across the zero coupling point in a 2 × 2 Mach-Zehnder interferometer coupler to produce the modulation. The coupling-modulated microring combines the drive power reduction of resonant modulators with the digital phase response of Mach-Zehnder BPSK modulators. A proof-of-concept device was demonstrated in silicon-on-insulator, showing differential binary phase-shift keying operation at 5 and 10 Gb/s.

Cascaded Mach-Zehnder wavelength filters in silicon photonics for low loss and flat pass-band WDM (de-)multiplexing.

We present 1-to-8 wavelength (de-)multiplexer devices based on a binary tree of cascaded Mach-Zehnder-like lattice filters, and manufactured using a 90 nm CMOS-integrated silicon photonics technology. We demonstrate that these devices combine a flat pass-band over more than 50% of the channel spacing with low insertion loss of less than 1.6 dB, and have a small device size of approximately 500 × 400 µm. This makes this type of filters well suited for application as WDM (de-)multiplexer in silicon photonics transceivers for optical data communication in large scale computer systems.

A silicon-based widely tunable short-wave infrared optical parametric oscillator.

We demonstrate a synchronously pumped optical parametric oscillator (OPO) based on parametric gain in a silicon-on-insulator photonic wire. We exploit the highly nonlinear broadband response of the photonic wire to achieve broadband single-pass amplification up to 54 dB. This allows us to construct an OPO that is tunable across a 75 nm-wide band near 2075 nm, when pumped by a picosecond pulse train at 2175 nm. Additionally we demonstrate broadband tuning across 150 nm by varying the pump wavelength and exploiting the higher order dispersion characteristics of the silicon photonic wire.

Demonstration of electrooptic modulation at 2165nm using a silicon Mach-Zehnder interferometer.

We demonstrate electrooptic modulation at a wavelength of 2165nm, using a free-carrier injection-based silicon Mach-Zehnder modulator. The modulator has a V(π)∙L figure of merit of 0.12V∙mm, and an extinction ratio of -23dB. Optical modulation experiments are performed at bitrates up to 3Gbps. Our results illustrate that optical modulator design methodologies previously developed for telecom-band devices can be successfully applied to produce high-performance devices for a silicon nanophotonic mid-infrared integrated circuit platform.

High-speed receiver based on waveguide germanium photodetector wire-bonded to 90nm SOI CMOS amplifier.

The performance of a receiver based on a CMOS amplifier circuit designed with 90nm ground rules wire-bonded to a waveguide germanium photodetector is characterized at data rates up to 40Gbps. Both chips were fabricated through the IBM Silicon CMOS Integrated Nanophotonics process on specialty photonics-enabled SOI wafers. At the data rate of 28Gbps which is relevant to the new generation of optical interconnects, a sensitivity of -7.3dBm average optical power is demonstrated with 3.4pJ/bit power-efficiency and 0.6UI horizontal eye opening at a bit-error-rate of 10(-12). The receiver operates error-free (bit-error-rate < 10(-12)) up to 40Gbps with optimized power supply settings demonstrating an energy efficiency of 1.4pJ/bit and 4pJ/bit at data rates of 32Gbps and 40Gbps, respectively, with an average optical power of -0.8dBm.

50 dB parametric on-chip gain in silicon photonic wires.

A pulsed mid-infrared pump at λ=2173 nm is used to demonstrate wideband optical parametric gain in a low-loss 2 cm long silicon photonic wire. Using dispersion engineering to obtain negative second-order (ß2) and positive fourth-order (ß4) dispersion, we generate broadband modulation instability and parametric fluorescence extending from 1911 nm-2486 nm. Using a cw probe signal to interrogate the modulation instability spectrum, we demonstrate parametric amplification >40 dB with an on-chip gain bandwidth wider than 580 nm, as well as narrowband Raman-assisted peak gain >50 dB.

Mid-infrared to telecom-band supercontinuum generation in highly nonlinear silicon-on-insulator wire waveguides.

We demonstrate the generation of a supercontinuum in a 2 cm long silicon wire by pumping the wire with mid-infrared picosecond pulses in the anomalous dispersion regime. The supercontinuum extends from 1535 nm up to 2525 nm for a coupled peak power of 12.7 W. It is shown that the supercontinuum originates primarily from the amplification of background noise. A detailed analysis of the spectral components which are generated through phase-matched processes is applied to extract the group velocity dispersion and fourth-order dispersion coefficient of the silicon wire waveguide.

Low-power continuous-wave four-wave mixing in silicon coupled-resonator optical waveguides.

We demonstrate four-wave mixing in silicon-on-insulator coupled-resonator optical waveguides consisting of 35 and 65 microring resonators, using a cw pump with coupled power below 20 mW and observed parametric conversion across more than 10 THz. The conversion efficiency is enhanced by +16 dB relative to a silicon straight waveguide of equivalent length, due to the slowing factor of the coupled-resonator structure.

Drive-noise-tolerant broadband silicon electro-optic switch.

We report a broadband digital electro-optical switch, based upon a multi-stage Mach-Zehnder lattice design in silicon-on-insulator. A digital switching response is demonstrated, engineered through apodization of the coupling coefficients between stages. The digital switching behavior results in crosstalk lower than -15 dB for drive-voltage noise levels in excess of 300 mV(pp), which exceeds the noise tolerance of a conventional single-stage Mach-Zehnder switch by more than six-fold. In addition, the digital design enables a larger maximum 'on'-state extinction (below -26 dB) and lower 'on'-state free-carrier-induced insertion loss (less than 0.45 dB) than that of the single-stage switch. The noise-tolerant, low-crosstalk switch can thus play a key role within CMOS-integrated reconfigurable optical networks operating under noisy on-chip conditions.

Self-phase modulation and nonlinear loss in silicon nanophotonic wires near the mid-infrared two-photon absorption edge.

We report an experimental study of picosecond pulse propagation through a 4-mm-long Si nanophotonic wire with normal dispersion, at excitation wavelengths from 1775 to 2250 nm. This wavelength range crosses the mid-infrared two-photon absorption edge of Si at ~2200 nm. Significant reduction in nonlinear loss due to two-photon absorption is measured as excitation wavelengths approach 2200 nm. At high input power, self-phase modulation is clearly demonstrated by the development of power-dependant spectral fringes. Asymmetry and blue-shift in the appearance of the spectral fringes at 1775 nm versus 2200 nm is further shown to originate from a strong reduction in the intra-pulse density of two-photon absorption-generated free carriers and the associated free-carrier dispersion. Analysis of experimental data and comparison with numerical simulations illustrates that the two-photon absorption coefficient ß(TPA) obtained here from nanophotonic wire measurements is in reasonable agreement with prior measurements of bulk silicon crystals, and that bulk Si values of the nonlinear refractive index n(2) can be confidently incorporated in the modeling of pulse propagation in deeply-scaled waveguide structures.

Non-blocking 4x4 electro-optic silicon switch for on-chip photonic networks.

We present a 4x4 spatially non-blocking Mach-Zehnder based silicon optical switch fabricated using processes fully compatible with standard CMOS. We successfully demonstrate operation in all 9 unique switch states and 12 possible I/O routing configurations, with worst-case cross-talk levels lower than -9 dB, and common spectral bandwidth of 7 nm. High-speed 40 Gbps data transmission experiments verify optical data integrity for all input-output channels.

Statistics of light transport in 235-ring silicon coupled-resonator optical waveguides.

In contrast to recent reports of localization-impaired transport in long slow-light waveguides, we demonstrate light transport in silicon coupled-resonator optical waveguides (CROWs) consisting of up to 235 coupled microrings without localization over frequency bands that are several hundred gigahertz wide. Furthermore, from the unique statistical signatures provided by time-domain propagation delay measurements, we demonstrate the spectrally correlated nature of light propagation in CROWs.

Waveguide dispersion effects in silicon-on-insulator coupled-resonator optical waveguides.

The dispersion of the waveguides that constitute microring resonators can considerably affect the dispersion characteristics of coupled-resonator optical waveguides (CROWs). We derive expressions for CROW dispersion and group delay for silicon-on-insulator microring CROWs, showing both theoretically and experimentally the band-to-band dependence of the bandwidth and group delay on the dispersion properties of the constituent single-mode silicon waveguide.