Marcel W. Pruessner

Thermo-optic tuning and switching in SOI waveguide Fabry-Perot microcavities

Compact silicon-on-insulator (SOI) waveguide thermo-optically tunable Fabry-Perot microcavities with silicon/air Bragg mirrors are demonstrated. Quality factors of Q=4,584 are measured with finesse F=82. Tuning is achieved by flowing current directly through the silicon cavity resulting in efficient thermo-optic tuning over 2 nm for less than 50 mW applied electrical power. The high-Q cavities enable fast switching (1.9 mus rise time) at low drive power (<10 mW). By overdriving the device, rise times of 640 ns are obtained. Various device improvements are discussed.

Integrated waveguide Fabry-Perot microcavities with silicon/air Bragg mirrors.

We demonstrate in-plane microfabricated Fabry-Perot cavities with cryogenically etched silicon/air distributed Bragg reflector (DBR) mirrors and integrated silicon-on-insulator rib waveguides. Several DBR configurations and cavity lengths were measured. Various devices exhibit Q=26963, FWHM=0.060 nm, finesse F=489, free spectral range FSR=81.7 nm, and DBR mirror reflectance R=99.4%. Thermo-optic tuning over 6.7 nm is also demonstrated.

Mechanical property measurement of InP-based MEMS for optical communications

We investigate mechanical properties of indium phosphide (InP) for optical micro-electro-mechanical systems (MEMS) applications. A material system and fabrication process for InP-based beam-type electrostatic actuators is presented. Strain gradient, intrinsic stress, Young’s modulus, and hardness are evaluated by beam profile measurements, nanoindentation, beam bending, and electrostatic testing methods. We measured an average strain gradient of δe0/δt = 4.37 × 10 −5 m −1 , with an average intrinsic stress of σ0 =− 5. 4M Pa for [0 1 1] beams. The intrinsic stress results from arsenic contamination during molecular beam epitaxy and (MBE) can be minimized by careful MBE growth and through the use of stress compensating layers. Nanoindentation of (1 0 0) InP resulted in E = 106. 5G Pa and H = 6.2 GPa, while beam bending of [0 1 1] doubly clamped beams resulted in E = 80.4 GPa and σ0 =− 5.6 MPa. We discuss the discrepancy in Young’s modulus between the two measurements. In addition, we present a method for simultaneously measuring Young’s modulus and residual stress using beam bending. Electrostatic actuation in excess of 20 V is demonstrated without breakdown.

InP-based optical waveguide MEMS switches with evanescent coupling mechanism

An optical waveguide MEMS switch fabricated on an indium phosphide (InP) substrate for operation at 1550 nm wavelength is presented. Compared to other MEMS optical switches, which typically use relatively large mirrors or long end-coupled waveguides, our device uses a parallel switching mechanism. The device utilizes evanescent coupling between two closely-spaced waveguides fabricated side by side. Coupling is controlled by changing the gap and the coupling length between the two waveguides via electrostatic pull-in. This enables both optical switching and variable optical coupling at voltages below 10 V. Channel isolation as high as -47 dB and coupling efficiencies of up to 66% were obtained with switching losses of less than 0.5 dB. We also demonstrate voltage-controlled variable optical coupling over a 17.4 dB dynamic range. The devices are compact with 2 /spl mu/m/spl times/2 /spl mu/m core cross section and active area as small as 500 /spl mu/m/spl times/5 /spl mu/m. Due to the small travel range of the waveguides, fast operation is obtained with switching times as short as 4 /spl mu/s. Future devices can be scaled down to less than 1 /spl mu/m/spl times/1 /spl mu/m waveguide cross-sectional area and device length less than 100 /spl mu/m without significant change in device design.

Monolithic suspended optical waveguides for InP MEMS

We present a novel waveguide design for InP microelecromechanical systems. The substrate is removed from underneath the waveguide by sacrificial etching, and the suspended waveguide is supported by lateral tethers. This allows segments of the waveguide to be moved and prevents substrate leakage loss in the fixed segments of the waveguides. A single-mask fabrication process is developed that can be extended to more complex devices employing electrostatic actuation. Fabricated suspended waveguides exhibit a loss of 2.2 dB/cm and tether pairs exhibit 0.25-dB additional loss.

End-coupled optical waveguide MEMS devices in the indium phosphide material system

We demonstrate electrostatically actuated end-coupled optical waveguide devices in the indium phosphide (InP) material system. The design of a suitable layer structure and fabrication process for actuated InP-based waveguide micro-electro-mechanical systems (MEMS) is reviewed. Critical issues for optical design, such as coupling losses, are discussed and their effect on device performance is evaluated. Several end-coupled waveguide devices are demonstrated, including 1 × 2 optical switches and resonant sensors with integrated optical readout. The 1 × 2 optical switches exhibit low-voltage operation (<7 V), low crosstalk (−26 dB), reasonable loss (3.2 dB) and switching speed suitable for network restoration applications (140 µs, 2 ms settling time). Experimental characterization of the integrated cantilever waveguide resonant sensors shows high repeatability and accuracy, with a standard deviation as low as σ = 50 Hz (0.027%) for fresonant = 184.969 kHz. By performing focused-ion beam (FIB) milling on a sensor, a mass sensitivity of Δm/Δf = 5.3 × 10−15 g Hz−1 was measured, which is competitive with other sensors. Resonant frequencies as high as f = 1.061 MHz (Qeffective = 159.7) have been measured in air with calculated sensitivity Δm/Δf = 1.1 × 10−16 g Hz−1. Electrostatic tuning of the resonator sensors was also examined. The prospect of developing InP MEMS devices monolithically integrated with active optical components (lasers, LEDs, photodetectors) is discussed.

Cryogenic etch process development for profile control of high aspect-ratio submicron silicon trenches

A cryogenic etch process using low temperature (T⩽−100°C) and SF6 and O2 gases is presented for fabricating high aspect ratio silicon microstructures, including photonic devices and micro- and nanoelectromechanical systems. The process requires only a single electron beam resist mask and results in open area etch rates of 4μm∕min. Various etch process parameters, including O2 flow, rf forward power, substrate temperature, and chamber pressure were studied, and the resulting effect on the etch quality was evaluated in terms of sidewall verticality and surface roughness. The optimized process uses low temperature (T=−110°C) and low chamber pressure (P=7mTorr) and enables sidewall verticality greater than 89.5° with roughness of 1–10nm. A silicon etch selectivity of 26:1 was obtained for 380nm thick electron beam resist. Using the optimized process, a silicon-on-insulator Fabry-Perot optical cavity with integrated rib waveguides and deeply etched silicon/air distributed Bragg reflector mirrors was fabricated...

In-plane microelectromechanical resonator with integrated Fabry–Pérot cavity

A silicon-on-insulator in-plane microelectromechanical resonator coupled to a high-Q (Q≈4,200), high finesse (FMax=265) optical Fabry–Perot microcavity is presented. The cavity utilizes high reflectance dry-etched silicon/air distributed Bragg reflectors. By suspending one of the Bragg mirrors to a microbridge resonator, the mirror can be displaced and the cavity is tuned. Using electrostatic actuation, bidirectional cavity tuning from −12.1to+17.0nm (29.1nm total range) is demonstrated near 1601nm wavelength. The device also enables measurement of thermal-mechanical noise with sensitivity better than 10fm∕Hz1∕2 and may find application in high resolution sensors.

Detecting Low-Power RF Signals Using a Multimode Optoelectronic Oscillator and Integrated Optical Filter

We have assembled and characterized a multimode optoelectronic oscillator with integrated optical filter for detecting low-power radio-frequency (RF) signals. The system can selectively amplify RF signals from 1 to 6 GHz. The input signals can be as low as - 83 dBm with a compression dynamic range of 72 dB. Using an integrated silicon optical filter facilitates channelization of the amplified RF signal from 1 to 3 GHz by reducing the gain for signals above 3 GHz. Future system improvements are discussed.

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.