Foundry-based waveguide-enhanced Raman spectroscopy in the visible.

Waveguide-enhanced Raman spectroscopy (WERS) is an analytical technique frequently employed for chemical and biological sensing. Operation at visible wavelengths to harness the inverse fourth power with excitation wavelength signal scaling of Raman scattering intensity is desirable, to combat the inherent inefficiency of Raman spectroscopy. Until now, WERS demonstrations in the visible have required custom materials and fabrication, resulting in high losses and low yields. In this work, we demonstrate a silicon nitride (SIN) visible WERS platform fabricated in a 300 mm complementary metal-oxide semiconductor (CMOS) foundry. We measure the propagation loss, coupling loss, WERS signal, and background for WERS spirals designed for 532 nm and 633 nm pump wavelengths. We compare these results to the state-of-the-art near-infrared WERS platform at 785 nm. Further, we theoretically validate the relative performance of each of these WERS configurations, and we discuss the optimal WERS configuration at visible wavelengths. We conclude that a configuration optimized for 785 nm pumping provides the greatest signal-to-background ratio in the fingerprint region of the spectrum, and pumping at 633 nm maximizes Stokes signal out to 3000 cm-1.

Micro-electro-mechanically tunable optical phase matching and mode conversion.

Mode-division multiplexing (MDM) enables a large increase in the information-carrying capacity of an optical network. Recently, chip-scale MDM devices that can switch different mode orders to different output waveguides have been demonstrated. However, an important milestone showing dynamically tunable mode-order conversion in a single compact device has so far not been reported. In this work, we demonstrate via simulation and measurement a new, to the best of our knowledge, approach for reconfigurable mode conversion using optical micro-electro-mechanical systems (MEMS) to locally modify the effective index in an asymmetric coupler. Modeling shows that dynamic tuning to increase or decrease the mode order is possible. Measurements on fabricated devices are consistent with simulations of reconfigurable mode conversion based on tunable phase matching. Our experimental results demonstrate reconfigurable TE0-TE2 to TE0-TE1 conversion and validate this new tunable phase-matching approach for mode-division multiplexing.

Optical and geometric parameter extraction for photonic integrated circuits.

We describe an in-situ technique to characterize the material refractive indices and waveguide geometry for photonic integrated circuits over hundreds of nanometers of optical bandwidth. By combining white light spectroscopy with unbalanced Mach-Zehnder interferometers, we can simultaneously and accurately extract the core thickness, core width, core refractive index, and cladding refractive index. This information is important for the technological maturation of photonic integrated circuit foundry fabrication. Capturing the inter-wafer and intra-wafer variation of these parameters is necessary to predict the yield of photonic components and for overall process quality control. Refractive indices are found with a 1-σ error of between 0.1% and 0.5%, and geometric parameters are found with an error of between 3 nm and 7 nm. Our analysis and validation are implemented and verified using the same waveguide layers as are used in the standard photonic wafer build, without any external techniques such as ellipsometry or microscopy.

Germanium-on-silicon waveguides for long-wave integrated photonics: ring resonance and thermo-optics.

Germanium-on-silicon (GOS) represents the leading platform for foundry-based long-wave infrared photonic integrated circuits (LWIR PICs), due to its CMOS compatibility and absence of oxides. We describe ring resonance (Q-factors between 2×103 and 1×104) and thermo-optic tunability in germanium-on-silicon waveguides throughout the long-wave-infrared. The ring resonances are characterized by Q-factors and couplings that agree with measurements of propagation loss (as low as 6 dB/cm) and simulations and are enabled by broadband edge coupling (12dB/facet over a 3 dB bandwidth of over 4 microns). We demonstrate the furthest into the infrared that ring resonators have been measured and show the potential of this platform for photonic integration and waveguide spectroscopy at wavelengths from 7 microns to beyond 11 microns.

Passive photonic integration of lattice filters for waveguide-enhanced Raman spectroscopy.

To perform waveguide-enhanced Raman spectroscopy (WERS) or fluorescence spectroscopy in a compact device, the optical fibers to couple the passive photonic circuit to the laser source and detector require attachment directly to the die. This necessitates the integration of edge couplers and waveguide-based filters to isolate the fiber background emission from the on-chip signal, while efficiently coupling the pump laser and detector to the input and output fibers, respectively. In this work, we experimentally demonstrate the successful integration of four-port lattice filters with sensing spirals and inverse-taper edge couplers in a passive photonic circuit. We further show that the four-port lattice filter enables the collection of backscattered on-chip Stokes signal, improving and simplifying overall system performance.

Comparison of maritime measurements of Cn2 with NAVSLaM model predictions.

In this paper, a study is made of the refractive index structure parameter Cn2, as derived from angle-of-arrival (AOA) measurements made on the beam after propagation along a 16 km slant path across the Chesapeake Bay. These measurements are compared with Cn2 estimates derived from the Navy Atmospheric Vertical Surface Layer Model (NAVSLaM), which are based upon prevailing meteorological conditions. Correlation coefficients for the reported data vary between 0.64 and 0.9. Despite the Chesapeake Bay theoretically being a difficult location for employing a Monin-Obukhov similarity theory-based model such as NAVSLaM, the agreement between the AOA Cn2 measurements and the NAVSLaM Cn2 estimates was, in many cases, good. A possible explanation of this agreement between the modeled and measured Cn2 values is that the large air-water temperature differences encountered provided such strong forcing for the NAVSLaM model that any potential violations of the Monin-Obukhov similarity theory assumptions had only a secondary influence on the Cn2 estimates.

Loss reduction in electromechanically tunable microring cavities.

Nanophotonic structures coupled with mechanics enable large effective index perturbation. To date, however, the relation between index tuning and induced optical loss has not been considered in detail. In this work we present an in-depth study of optical loss mechanisms in an electromechanically-tunable waveguide filter. Gradient electric forces modify the coupling between a microring optical cavity and a suspended micromechanical (MEMS) perturber resulting in a measured tuning greater than one free-spectral range (FSR) and an effective index tuning of 3×10-2. We examine various loss contributions and find, for certain conditions, a surprising reduction in loss with greater MEMS-induced mode perturbation. Modeling confirms the device behavior and loss mitigation is discussed.

Free space quantum key distribution using modulating retro-reflectors.

Quantum key distribution (QKD) can be used to produce a cryptographic key whose security is guaranteed by quantum mechanics. The range of fiber-based QKD links is limited, by loss, to a few hundred kilometers, and cannot be used between mobile platforms. Free space QKD can, in principle, overcome these limitations. In practice, very narrow beam divergences must be used, requiring highly accurate pointing of the transmitting terminal to the receiver. This makes deployment very difficult. Here we describe the experimental implementation of a new type of free space QKD link, using modulating retro-reflectors (MRR). The MRR-QKD link eases the pointing requirements by more than three orders of magnitude, from microradians to degrees, while maintaining the narrow beam divergence necessary for long-range communication links. The system uses new, high extinction surface-normal multiple quantum well modulators with a modulation rate of 100 MHz. A laboratory-based BB84 QKD link using multiple quantum well MRRs is demonstrated, link budgets for possible applications are discussed, and security issues are considered.

Waveguide-enhanced Raman spectroscopy of trace chemical warfare agent simulants.

We report the measurement of waveguide-enhanced Raman spectra from trace concentrations of four vapor-phase chemical warfare agent simulants: dimethyl methylphosphonate, diethyl methylphosphonate, trimethyl phosphate, and triethyl phosphate. The spectra are obtained using highly evanescent nanophotonic silicon nitride waveguides coated with a naturally reversible hyperbranched carbosilane sorbent polymer and exhibit extrapolated one-σ detection limits as low as 5 ppb. We use a finite-element model to explain the polarization and wavelength properties of the differential spectra. In addition, we assign spectral features to both the analyte and the sorbent, and show evidence of changes to both due to hydrogen bonding.

Broadband opto-electro-mechanical effective refractive index tuning on a chip.

Photonic integrated circuits have enabled progressively active functionality in compact devices with the potential for large-scale integration. To date the lowest loss photonic circuits are achieved with silica or silicon nitride-based platforms. However, these materials generally lack reconfigurability. In this work we present a platform for achieving active functionality in any dielectric waveguide via large-scale opto-electro-mechanical tuning of the effective refractive index (Δneff≈0.01-0.1) and phase (Δϕ>2π). A suspended microbridge weakly interacts with the evanescent field of a low-mode confinement waveguide to tune the effective refractive index and phase with minimal loss. Metal-coated bridges enable electrostatic actuation to displace the microbridge to dynamically tune nEFF. In a second implementation we place a non-metallized dielectric microbridge in a gradient electric field to achieve actuation and tuning. Both approaches are broadband, universally applicable to any waveguide, and pave the way for adding active functionality to many passive optical materials.

Irradiance correlations in retro-reflected beams.

Communications links that utilize modulating retro-reflectors can make use of turbulence-induced fade information available at the remote data-signal terminal in order to optimize the data transfer rate. Experiments were conducted to measure the irradiance in both the direct and the retro-reflected beams. Both on-axis and off-axis components were recorded in order to further study the enhancement in the scintillation index observed in the retro-reflected beam. Measurements were made over a 1.8 km terrestrial range at AP Hill, Virginia. The degree of correlation of the received irradiance between the direct and double-passage beams is found to approach 90% on-axis and 70% outside of the Fresnel zone radius. The scintillation index in the retro-reflected beam is enhanced on-axis due to reciprocal optical paths. The measured scintillation indices, and the correlation of the retro-reflected beam with the direct beam, are compared with a point source, point scatterer, and point receiver model in the strong scintillation approximation.

Suspended photonic waveguide devices.

This article describes recent research at the U.S. Naval Research Laboratory that focuses on the use of micro- and nanomachining techniques for photonic waveguide devices. By selectively etching a sacrificial layer that the waveguide core is supported by, in whole or in part, the waveguide obtains enhanced properties and functionality, such as mechanical flexibility, index contrast, birefringence, and evanescent field depth. We describe how these properties enable unique waveguide applications in areas such as cavity optomechanics, displacement sensing, electro-optics, and nonlinear optics.

InAlAs/InGaAs avalanche photodiode arrays for free space optical communication.

In free space optical communication, photodetectors serve not only as communications receivers but also as position sensitive detectors (PSDs) for pointing, tracking, and stabilization. Typically, two separate detectors are utilized to perform these tasks, but recent advances in the fabrication and development of large-area, low-noise avalanche photodiode (APD) arrays have enabled these devices to be used both as PSDs and as communications receivers. This combined functionality allows for more flexibility and simplicity in optical system design without sacrificing the sensitivity and bandwidth performance of smaller, single-element data receivers. This work presents the development of APD arrays rated for bandwidths beyond 1 GHz with measured carrier ionization ratios of approximately 0.2 at moderate APD gains. We discuss the fabrication and characterization of three types of APD arrays along with their performance as high-speed photodetectors.

Mid-infrared difference-frequency generation in suspended GaAs waveguides.

We experimentally demonstrate mid-infrared difference-frequency generation in suspended 181 nm thick GaAs waveguides. Generation of the idler at wavelengths between 2800 and 3150 nm is enabled by form-birefringent phase-matching in ultrahigh index-contrast waveguides. Nonlinear mixing has a measured efficiency of 0.4 W⁻¹ in a 1.2 mm long waveguide using a CW signal tunable between 1490 and 1620 nm and a CW pump tunable between 1018 and 1032 nm at powers of a few mW.

Trace gas absorption spectroscopy using functionalized microring resonators.

We detect trace gases at parts-per-billion levels using evanescent-field absorption spectroscopy in silicon nitride microring resonators coated with a functionalized sorbent polymer. An analysis of the microring resonance line shapes enables a measurement of the differential absorption spectra for a number of vapor-phase analytes. The spectra are obtained at the near-infrared overtone of OH-stretch resonance, which provides information about the toxicity of the analyte vapor.

Broadband near-infrared emission in silicon waveguides.

Silicon photonic integrated circuit foundries enable wafer-level fabrication of entire electro-optic systems-on-a-chip for applications ranging from datacommunication to lidar to chemical sensing. However, silicon's indirect bandgap has so far prevented its use as an on-chip optical source for these systems. Here, we describe a fullyintegrated broadband silicon waveguide light source fabricated in a state-of-the-art 300-mm foundry. A reverse-biased p-i-n diode in a silicon waveguide emits broadband near-infrared optical radiation directly into the waveguide mode, resulting in nanowatts of guided optical power from a few milliamps of electrical current. We develop a one-dimensional Planck radiation model for intraband emission from hot carriers to theoretically describe the emission. The brightness of this radiation is demonstrated by using it for broadband characterization of photonic components including Mach-Zehnder interferometers and lattice filters, and for waveguide infrared absorption spectroscopy of liquid-phase analytes. This broadband silicon-based source can be directly integrated with waveguides and photodetectors with no change to existing foundry processes and is expected to find immediate application in optical systems-on-a-chip for metrology, spectroscopy, and sensing.

Atmospheric turbulence effects measured along horizontal-path optical retro-reflector links.

The scintillation measured over close-to-ground retro-reflector links can be substantially enhanced due to the correlations experienced by both the direct and reflected echo beams. Experiments were carried out at China Lake, California, over a variety of ranges. The emphasis in this paper is on presenting the data from the 1.1 km retro-reflecting link that was operated for four consecutive days. The dependence of the measured irradiance flux variance on the solar fluence and on the temperature gradient above the ground is presented. The data are consistent with scintillation minima near sunrise and sunset, rising rapidly during the day and saturating at irradiance flux variances of ~10. Measured irradiance probability distributions of the retro-reflected beam are compared with standard probability density functions. The ratio of the irradiance flux variances on the retro-reflected to the direct, single-pass case is investigated with two data sets, one from a monostatic system and the other using an off-axis receiver system.

Integrated waveguide-DBR microcavity opto-mechanical system.

Cavity opto-mechanics exploits optical forces acting on mechanical structures. Many opto-mechanics demonstrations either require extensive alignment of optical components for probing and measurement, which limits the number of opto-mechanical devices on-chip; or the approaches limit the ability to control the opto-mechanical parameters independently. In this work, we propose an opto-mechanical architecture incorporating a waveguide-DBR microcavity coupled to an in-plane micro-bridge resonator, enabling large-scale integration on-chip with the ability to individually tune the optical and mechanical designs. We experimentally characterize our device and demonstrate mechanical resonance damping and amplification, including the onset of coherent oscillations. The resulting collapse of the resonance linewidth implies a strong increase in effective mechanical quality-factor, which is of interest for high-resolution sensing.

Demonstration of a mode-conversion cavity add-drop filter.

We experimentally demonstrate a new type of add-drop filter incorporating an asymmetric Y-branch waveguide coupler and a shifted-grating mode-conversion cavity. The device relies on mode separation in the asymmetric Y-branch and wavelength-selective mode conversion upon reflection from the shifted-grating cavity. Add-drop functionality is demonstrated in a three-port integrated silicon-on-insulator device.

Probability density of irradiance fluctuations observed over terrestrial ranges.

The irradiance fluctuations imposed on a laser beam that has propagated over horizontal terrestrial paths in the range of 2 to 24 km are compared to lognormal (LN) and gamma-gamma (GG) distributions. For the direct links reported here the irradiance fluctuations follow a LN distribution except in cases of weak turbulence, characterized by a scintillation index of less than 1 and a Fried parameter larger than the receiver aperture, in which case the GG distribution gives an improved fit. In very weak turbulence the difference between the two distributions is insignificant.