Macro-scale optical components with surface flatness better than 25 nm over large areas (more than 1 X 1 mm) are widely available. However, the flatness of optical MEMS devices (for example micro-mirrors and -diffraction gratings) is often considerably worse. In addition to static deformation caused by film stresses and stress gradients, dynamic mechanical effects, such as air drag and excitation of higher-order resonant modes, cause surface deformations that are difficult to predict using theoretical or finite- element models. These deformations can cause significant degradation to optical performance. Dynamic measurements of nanometer-scale displacements across the entire surface of a micro-mirror are difficult or impossible to perform with conventional MEMS metrology techniques such as SEM, AFM, and optical microscopy. Stroboscopic interferometry, however, can be used to measure time-slice images that show 3D motion of fast-moving MEMS devices, with vertical resolution better than 1 nm. In this paper, we report the application of this technique to dynamic characterization of fold-up surface- micromachined structures and show how the method can be used to provide new insights into the optical and mechanical behavior of scanning micro-mirror devices.
We describe a raster-scanning display system comprised of two tilt-up micromachined polysilicon mirrors that rotate about orthogonal axes. We have demonstrated a resolution of 102 X 119 pixels. The optical efficiency of our two- mirror micro-optical raster-scanning system is comparable to that of micromachined display systems developed by Texas Instruments and Silicon Light Machines. Ease of integration with on-chip light sources and lenses has the potential to reduce packaging size, complexity and cost of the display system and makes it well suited for head-mounted display applications.
The high reflectivity of VCSEL mirrors renders the device very sensitive to small changes int he reflectivity or absorption within the laser cavity. Therefore, the integration of a quantum-well absorber within the VCSEL enables compact, monolithic devices with multiple functions. As a result, the VCSEL exhibits a variety of useful characteristics under different configurations. In this work, we demonstrate the use of a quantum-well absorber within one of the VCSEL mirror stacks. With strategic design and simple biasing circuitry, we have shown experimentally (1) a VCSEL with an integrated quantum-well photodetector, (2) a self-pulsating VCSEL with a controllable quantum-well saturable absorber, and (3) a novel technique for VCSEL modulation where the quantum-well absorption is modulated, thereby modulating the laser light output. The theory, design criteria, experimental results, and potential applications for these devices are discussed.
We have constructed a novel VCSEL structure in which a voltage biased quantum well absorber is embedded in the mirror stack. Several applications for these devices are demonstrated. The devices exhibit regimes of negative differential resistance in the absorber under varying laser bias conditions. By choosing the absorber bias conditions such that the operating load line intersects the absorber IV trace three times, the laser can be made to exhibit optical bistability. The magnitude of the bistability and consequent hysteresis loop can be controlled by adjusting the absorber bias. If the bias conditions are adjusted so that the load line is tangential to the portion of the absorber IV exhibiting negative differential resistance, then the device can be made to self-pulsate. Self-pulsations at frequencies as high as 2 GHz with 700 kHz rf linewidth FWHM (full-width at half-maximum) have been obtained using these devices. Furthermore, the self-pulsation frequency can be tuned over 700 MHz by adjusting the bias conditions, representing a substantial advance over existing self- pulsating VCSELs. We have also demonstrated a novel modulation scheme using these devices, in which the drive signal is applied to the bias voltage across the absorber. Theoretical analysis of the chirping mechanism leads us to expect that this technique will minimize the chirp at high modulation frequencies, while still providing substantial modulation depth and high speeds. We achieved a minus 3 dB bandwidth of 9 GHz; the bandwidth increases with the laser bias current at a rate of 7 GHz/(root)mA.
We present the design and fabrication of surface- micromachined electrostatic-comb driven microscanners that have high angular precision over a large scan angle. When used as resonant scanners, these mirrors have fast scan rates with very low operating power. We use polysilicon microhinges, which allow the micromirrors to be lifted out of the plane of the substrate after processing is completed, to create high-aspect-ratio optical surfaces with dimensions in the hundreds of micrometers s while taking advantage of the planar surface-micromachining processing technology. Microscanners that are capable of high-speed scanning over large scan angles with high precision have been fabricated. Application of these actuated micromirrors in laser barcode scanning and optical-fiber switches have been demonstrated. These single-mirror scanners can be combined to form more complicated microscanners such as a two-mirror, two-axis raster scanner that have a wide range of applications in areas such as medicine, displays, printing, data storage, and communications.
In-building radio propagation measurements at 900MHz are used to analyze the dynamic range requirements and optimal architecture for a distributed antenna network. The ideal performance/cost ratio for the network is found to be achieved with a low-cost hybrid fiber-coax architecture. A system design procedure and field-trial results are presented.
Using an uncoated monolithic single-contact distributed feedback (DFB) laser, transmission of 2 Mb/s data at a subcarrier frequency of 35 GHz over 2.2 km of optical fiber by resonant modulation is demonstrated. Modulation response of 60 MHz with more than 1 GHz of enhancement at round trip frequency, carrier-to-noise ratio and bit-error-rate results are reported. The tolerance of the resonant round-trip frequency to the DFB facet cleaving process and the device length uncertainty due to cleaving is also addressed in detail by computer simulation.
Two types of polysilicon surface-micromachined actuators designed for moving hinged micromirrors are described. An electrostatic comb-drive actuator comprised of interdigitated capacitors has been used to move a mirror at frequencies of at least a kHz. Impact-actuated linear vibromotors allow mirrors to travel over large (> 100 micrometers ) ranges with submicron positioning.
Analog fiber-optic links can be used as a connecting infrastructure for modern wireless communication systems. For existing wireless networks, which operate at frequencies below 3GHz, the possibility of using low-cost, commercially available optoelectronic components is very attractive. Also, the development of practical millimeter-wave optical transmitters may be critical to the deployment of future high frequency (> 20 GHz) radio distribution systems. The linearity performacne of low-cost microwave, and millimeter-wave optical trnamitters will be assessed for the wireless application.
In this paper, we introduce a novel self-routed wavelength-addressable switching network (SWANET) that provides wavelength-transparent optical data paths between end-point, configured by wavelength-coded optical signals. The network is based on a new scheme for encoding destination addresses using a sequence of wavelengths. This allows large networks to be constructed using a moderate number of available wavelengths. The multistage switch architecture may be used either as a circuit switch or as a packet switch. We describe the architecture of SWANET and the design of its switch nodes. We also analyze the effect of fiber dispersion on the transmission time of the header and evaluate the tradeoffs involved in minimizing the header transmission time.
We have designed and built integrated, movable micromirrors for on-chip alignment in silicon- optical-bench technology. The mirrors are fabricated using surface micromachining with three polysilicon layers. A polysilicon-hinge technology was used to achieve the required vertical dimensions and functionality for alignment in hybrid photonic integrated circuits. The positioning accuracy of the mirrors is measured to be on the order of 0.2 micrometers . This precision is shown theoretically and experimentally to be sufficient for laser-to-fiber coupling. In the experimental verification, we used external actuators to position the micromirror and obtained 45% coupling efficiency from a semiconductor laser (operating at 1.3 micrometers ) to a standard single-mode optical fiber. The stability and robustness of the micromirrors were demonstrated in shock and vibration tests that showed that the micromirrors will withstand normal handling and operation without the need for welding or gluing. This micromirror technology combines the low-cost advantage of passive alignment and the accuracy of active alignment. In addition to optoelectronic packaging, the micromirrors can be expected to find applications in grating-tuned external-cavity lasers, scanning lasers, and interferometers.
This paper provides a brief description of several techniques used to efficiently transport millimeter-wave signals over optical fiber. Transmission of millimeter-wave signals is demonstrated using resonant modulation of monolithic semiconductor lasers, feedforward optical modulation, and by stabilization of the mode-locked frequency of a passively mode- locked laser using an optoelectronic phase-locked loop.
The frequency detuning-linewidth characteristics of the mode-locked frequency of a monolithic, passively mode-locked laser at millimeter wave frequency have been investigated. By changing the applied biases to the gain and absorber sections, a tuning range of about 20 MHz was observed at a mode-locked frequency of 47.61 GHz, while keeping the mode-locked linewidth to below 20MHz. The gain current variation needed for this tuning range is about 20 mA, while the absorber bias voltage affects the tuning only weakly.
TeraNet is an experimental optical network which offers user access rates as high as one Gigabit per second (Gbps). The network provides both circuit switching and ATM packet switching in a hierarchical fashion. Circuit switching is accomplished through the use of tunable optical WDMA/Subcarrier-FDMA channels. An overlaid multihop technique that uses a sub-set of the underlying optical circuits, constructs a packet switching environment. Circuit switched services interface to the passive optical 'ether' through Media Interface Units (MIU). A 3 X 3 switching node called Network Interface Unit (NIU), with 1 Gbps capacity per port, is used as an ATM access port to the packet switching layer. The ATM overlaid network incorporates a traffic control architecture that supports multiple traffic classes. Output buffers and bandwidth are shared according to a resource allocation concept called Asynchronous Time Sharing (ATS).
Detailed analysis has been performed of modal noise reduction by superposition of high frequency modulation. Data obtained revealed that, for both direct and external modulation, the modal noise reduction factor depends on the modulation frequency and fiber length. The fiber mode coupling, chromatic dispersion, and the mode partition noise have been neglected in the analysis. Experimental results confirmed that, for multimode fiber, the modal noise reduction depends almost only on the modulation depth.
Recent advances in high speed semiconductor lasers[l] have made
possible the use of optical transport and control in microwave
systems such as phased arrayed radar[21, various microwave
subcarrier multiplexed networks scheines[3J as well as cable
television distribution[4). For a typical laser with a differential
efficiency of 0.4mW/mA and emitting xmw of CW power, a microwave
power of (-8+2Olog10x)dBm is required in a 50k) system for full
optical modulation. This is much higher than typical received signal
powers from microwave antennae, and considerable cost is incurred
in providing the necessary amplification at the undersirable
location of an antenna horn.
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