Optical MEMS switching technology has attracted attention in managing data flow due to its compactness and
robustness. It allows hundreds of optical channels to be switched by micro-mirrors with very low power consumption.
Furthermore, the ability to switch signals independent of data rates, formats, wavelengths and protocols is advantageous
in many real world environments such as internet peering exchanges, undersea cable landing locations and data centers.
All of these applications require a highly reliable and stable switching system. Dielectric isolation has a huge impact on
major failure modes of capacitive MEMS devices such as breakdown and charging. This issue becomes more
challenging in electrostatic MEMS optical switches since they usually operate at relatively high voltages. The charges
trapped in this dielectric layer could cause interference in the electric field, resulting in erratic responses of the steering
mirrors and instability of pointing accuracy over temperature and time, which greatly degrades the system performance.
Aiming at reducing charging and preventing high voltage breakdown, a dielectric charging guard has been developed by
using an oxide "fence" with extended breakdown path length that is shielded by conductive sidewalls of the silicon
interposer. In this paper, the reliability tests as well as the performance impact to the optical switch will be presented,
including characterizations of breakdown voltage, leakage current, and charging verses temperature. The test results
demonstrate highly repeatable switching accuracy of micro-mirrors with very low drift at varied temperature. Failures
induced by fabrication will also be discussed.
growth of data and video transport networks. All-optical switching eliminates the need for optical-electrical conversion
offering the ability to switch optical signals transparently: independent of data rates, formats and wavelength. It also
provides network operators much needed automation capabilities to create, monitor and protect optical light paths. To
further accelerate the market penetration, it is necessary to identify a path to reduce the manufacturing cost significantly
as well as enhance the overall system performance, uniformity and reliability. Currently, most MEMS optical switches
are assembled through die level flip-chip bonding with either epoxies or solder bumps. This is due to the alignment
accuracy requirements of the switch assembly, defect matching of individual die, and cost of the individual components.
In this paper, a wafer level assembly approach is reported based on silicon fusion bonding which aims to reduce the
packaging time, defect count and cost through volume production. This approach is successfully demonstrated by the
integration of two 6-inch wafers: a mirror array wafer and a "snap-guard" wafer, which provides a mechanical structure
on top of the micromirror to prevent electrostatic snap-down. The direct silicon-to-silicon bond eliminates the CTEmismatch
and stress issues caused by non-silicon bonding agents. Results from a completed integrated switch assembly
will be presented, which demonstrates the reliability and uniformity of some key parameters of this MEMS optical
An effort to develop a miniaturized multichannel optical fiber Bragg grating sensor interrogator was initiated in 2006 under the Small Business Innovative Research (SBIR) program. The goal was to develop an interrogator that would be sufficiently small and light to be incorporated into a health monitoring system for use on tactical missiles. Two companies, Intelligent Fiber Optic Systems Corporation (IFOS) and Redondo Optics, were funded in Phase I, and this paper describes the prototype interrogators that were developed. The two companies took very different approaches: IFOS focused on developing a unit that would have a high channel count and high resolution, using off-the-shelf components, while Redondo Optics chose to develop a unit that would be very small and lightweight, using custom designed integrated optical chips. It is believed that both approaches will result in interrogators that will be significantly small, lighter, and possibly even more precise than what is currently commercially available. This paper will also briefly describe some of the sensing concepts that may be used to interrogate the health of the solid rocket motors used in many missile systems. The sponsor of this program was NAVAIR PMA 280.
Fiber gratings are proving to provide versatile discrete sensor elements for structural health monitoring systems. For example, they outperform traditional resistive foil strain gages in terms of temperature resistance as well as multiplexing capability, relative ease of installation, electromagnetic interference immunity and electrical passivity. However, the fabrication method and post-fabrication processing influences both performance and survivability in extreme temperature environments. In this paper, we compare the performance and survivability when making strain measurements at elevated temperatures for a range of fabrication and processing conditions such as UV-laser and electric-arc writing and post-fabrication annealing. The optimum method or process will depend on the application temperatures (e.g., up to 300°C, 600°C or 1000°C), and times at these temperatures. As well, other sensing requirements, including the number of sensors, measurand and sensitivity may influence the grating choice (short or long period).
Structural Health Monitoring (SHM) is becoming an increasingly important tool for the maintenance, safety and integrity of aerospace structural systems. Immune to electromagnetic interference, Fiber Bragg Grating (FBG) optical sensor matrices are light-weight and multiplexable, allowing many sensors on a single fiber to be integrated into smart structures. Highly sensitive to minute strains, they can facilitate maximum SHM functionality, with minimum weight and size. Consequently, these optical systems, in conjunction with advanced damage characterization algorithms, are expected to play an increasing role in extending the life and reducing costs of new generations of structures and airframes. In this paper, we discuss the development of both hardware and algorithms to detect, locate and quantify delamination in composite laminated beam structures. We present an integrated SHM system including (a) the capability of interrogating over 50 FBG sensors simultaneously with sub-picometer resolution at over 50 kHz, (b) an FBG-sensor/piezo-actuator matrix smart skin design and methodology, and (c) damage detection location and quantification algorithms based on mode shape or other relevant advanced algorithmic-based damage diagnosis and prognosis techniques. Comparison with other SHM systems (e.g., based on piezo-electric (PVDF) and Scanning Laser Vibrometer sensors) demonstrates better signal-to-noise and damage detection for our FBG system.
We report on the use of a high-speed wavelength division multiplexing (WDM) technique for multiplexing Fiber Bragg Grating (FBG) sensors applied to structural Vibration Control for the measurement of strain, permitting many sensing devices along a single optical fiber at different locations collecting samples at 5000 Hz with microstrain resolution. In this demonstration, a cantilevered flexible aluminum beam is used as the object for vibration control. A piezoceramic patch surface-bonded to the cantilevered end of the beam is used as an actuator to suppress the beam vibration. Various active vibration controllers such as positive position feedback (PPF), strain rate feedback (SRF), proportional plus derivative (PD), pole placement, and sliding mode based robust control are tested by using the fiber optical sensor for feedback purpose. Experiments successfully demonstrate that the signals from the fiber optic sensor can be used for active feedback control of the beam vibration.
An optical logic system is being developed to test the packaging and operation of free space optical logic systems. It uses symmetric-self-electro-optic effect devices as the logic elements and vertical cavity surface emitting lasers (VCSELs) to provide the optical inputs. This paper discusses the issues involved with incorporating the VCSEL array into the system. Issues that are investigated include beam combining, electrical drive, and VCSEL polarization. We find that current experimental devices are appropriate for early system experiments although issues such a multimode operation and the resulting current dependent polarization need to be addressed for practical systems.
We demonstrate the application of optical amplifiers and polymer-dispersed liquid-crystal (PDLC) shutters to electrically reconfigurable fiber optic delay line signal processors. Two 8-tap finite impulse response (FIR) fiber filter modules were fabricated. These two modules can also be interconnected in a cascaded or parallel configuration to implement a 16-tap fiber FIR filter. An erbium-doped fiber amplifier with a peak gain of about 20 dB was used to compensate for the large optical losses involved in the filters due to the large tap numbers. The relatively inexpensive PDLCs were used to realize electrically reconfigurable analog tap weights. The individual fiber filters were then evaluated for their impulse and frequency responses. The fabricated filters used single-mode fibers and fiber components and were polarization independent to within 0.5 dB. The sampling frequency was about 200 MHz, which can easily be upgraded into the gigahertz range. The tapping extinction ratio was about 13 dB with subkilohertz tunability speed. The amplified spontaneous emission noise can limit the filtering performance unless appropriate spectral filtering is included before detection. These optically amplified electrically reconfigurable fiber signal processors have the potential to lead to the realization of complex programmable and adaptive optical systems.