Joseph E. Ford

Wavelength add-drop switching using tilting micromirrors

This paper describes a single-mode optical fiber switch which routes individual signals into and out of a wavelength multiplexed data stream without interrupting the remaining channels. The switch uses free-space optical wavelength multiplexing and a column of micromechanical tilt-mirrors to switch 16 channels at 200 GHz spacing from 1531 to 1556 nm. The electrostatically actuated tilt mirrors use an 80 V peak-to-peak 300 KHz sinusoidal drive signal to switch between /spl plusmn/10/spl deg/ with a 20 /spl mu/s response. The total fiber-to-fiber insertion loss for the packaged switch is 5 dB for the passed signals and 8 dB for added and dropped signals, with 0.2 dB polarization dependence. Switching contrast was 30 dB or more for all 16 channels and all input and output states. We demonstrate operation by switching 622 Mb/s data on eight wavelength channels between the two input and output ports with negligible eye closure.

Optical MEMS for Lightwave Communication

The intensive investment in optical microelectromechanical systems (MEMS) in the last decade has led to many successful components that satisfy the requirements of lightwave communication networks. In this paper, we review the current state of the art of MEMS devices and subsystems for lightwave communication applications. Depending on the design, these components can either be broadband (wavelength independent) or wavelength selective. Broadband devices include optical switches, crossconnects, optical attenuators, and data modulators, while wavelength-selective components encompass wavelength add/drop multiplexers, wavelength-selective switches and crossconnects, spectral equalizers, dispersion compensators, spectrometers, and tunable lasers. Integration of MEMS and planar lightwave circuits, microresonators, and photonic crystals could lead to further reduction in size and cost

Dynamic spectral power equalization using micro-opto-mechanics

We present a voltage-controlled spectral attenuator for gain shaping and power equalization in wavelength division multiplexed single-mode fiber systems. A micro-opto-mechanical modulator array, where electrostatic deflection of a silicon nitride quarter-wave dielectric layer suspended over a silicon substrate creates a column of variable reflectivity mirrors, is packaged using bulk optics and a diffraction grating to disperse the input spectrum across the device and collect the reflected light into a separate output fiber. The packaged component has 9-dB excess loss, 20-dB dynamic range and 10-/spl mu/s response. We demonstrate equalization of the amplified spontaneous emission spectrum from an erbium-doped fiber amplifier and of individual laser signals with 10-dB initial variation to less than 0.5-dB variation over a 24-nm passband-free spectrum.

Planar micro-optic solar concentrator

We present a new approach to solar concentration where sunlight collected by each lens in a two-dimensional lens array is coupled into a shared, planar waveguide using localized features placed at each lens focus. This geometry yields a thin, flat profile for moderate concentration systems which may be fabricated by low-cost roll manufacture. We provide analyses of tradeoffs and show optimized designs can achieve 90% and 82% optical efficiency at 73x and 300x concentration, respectively. Finally, we present preliminary experimental results of a concentrator using self-aligned reflective coupling features fabricated by exposing molded SU-8 features through the lens array.

Incremental recording for photorefractive hologram multiplexing.

We investigate an incremental recording technique for multiplexed hologram storage in photorefractive crystals, in which each hologram is recorded with multiple short exposures. The performance is theoretically compared with that of scheduled (single exposure per hologram) recording. Our analysis shows that this technique systematically controls the signal uniformity and can also decrease the total recording time. We present an experimental demonstration with LiNbO(3) using a binary orthogonal phase-code addressing technique.

A tunable dispersion compensating MEMS all-pass filter

A tunable dispersion compensating filter based on a multistage optical all-pass filter with a microelectromechanical (MEM) actuated variable reflector and a thermally tuned cavity is described. A two-stage device was demonstrated with a tuning range of /spl plusmn/100 ps/nm, 50-GHz passband and a group delay ripple less than /spl plusmn/3 ps. The device has negligible polarization dependence and is suitable for single or multiple channel compensation. An off-axis, two-fiber package with an excess loss <2 dB/stage avoids the need for a circulator. By cascading four stages, a passband to channel spacing ratio of 0.8 is obtained that allows both 40 Gb/s nonreturn-to-zero (NRZ) and return-to-zero (RZ) signals to be compensated.

Micromechanical fiber-optic attenuator with 3 /spl mu/s response

Optomechanical fiber-optic attenuators are bulky and slow. The mechanical antireflection switch (MARS) modulator offers a high-speed alternative for applications including dynamic gain control in fiber amplifiers. This paper describes a compact electrically controlled variable attenuator using a micromechanical device where electrostatic deflection of a silicon nitride quarter-wave dielectric layer suspended over a silicon substrate creates a variable reflectivity mirror. This device is packaged with two fibers in one ceramic ferrule placed in contact with a gradient index (GRIN) collimation lens, so that the input light reflects from the modulator in the collimated beam plane and couples into the output fiber. Using a 300 /spl mu/m diameter MARS attenuator and a 500 /spl mu/m diameter collimation lens, the total insertion loss at 1550 nm was 3.0 dB with no applied voltage, increasing to 31 dB at 35.2 V. The polarization dependent loss was less than 0.06 dB. Full attenuation with more than 100 mW input power produced no damage. The response time was 2.8 /spl mu/s to move from maximum to minimum transmission and 1.1 /spl mu/s to return to maximum transmission.

POLARIZATION-SELECTIVE COMPUTER-GENERATED HOLOGRAMS : DESIGN, FABRICATION,AND APPLICATIONS

We constructed polarization-selective computer-generated holograms that apply an independent phase profile during readout by horizontal and vertical light polarizations. These elements are composed of two surface-relief-etched birefringent substrates joined face to face. We describe the design methodology for arbitrary birefringent substrate and gap materials. We show how these holograms are fabricated with standard microelectronics technology and discuss the effects of etching and alignment errors on performance. We demonstrated a diffraction efficiency of 60% with a polarization contrast ratio of >100:1 using a multilevel phase hologram made from two birefringent lithium niobate substrates. We also showed that a single-layer SiO(2) thin-film antireflection coating on all surfaces can reduce reflections from the high-index substrates without significant effect on hologram performance. We also consider some possible applications of this technology and demonstrate experimentally a dual focal-length lens and a self-interconnecting binary 2 × 2 polarization switch.

Polarization-selective computer-generated holograms

We demonstrate polarization-selective computer-generated holograms with independent phase profiles for the two orthogonal linear polarizations. The holograms are made of two surface-relief-etched birefringent substrates joined face to face. We describe their design and fabrication and present experimental results for dual binaryphase computer-generated holograms fabricated in lithium niobate. The first-order diffraction efficiency varied from 6% to 25%, with as much as 40:1 contrast between polarizations. Such elements can be used in compact optoelectronic systems or combined with electro-optic polarization rotators to make electrically controlled optical elements.

Time-integrating interferometry using photorefractive fanout

We have experimentally demonstrated a single-beam interferometer that effectively subtracts an exponentially weighted history of the input from the current value, thus functioning as a novelty filter. The single-beam interferometer uses signal depletion due to noise amplification (fanout) in a specially cut crystal of photorefractive BaTiO(3). To demonstrate its real-time operation we used a Hughes liquid-crystal light valve to convert a video image into a phase- and/or amplitude-modulated input signal. Potential applications of this interferometer include image-clutter removal, motion detection and tracking, edge enhancement, and image time differentiation.