Micro optical devices
Microengineered Bar Code Reader
Tony Roberts, Richard Syms, Andrew Holmes, Eric Yeatman, Regina Luttge
Information acquisition by optical scanning is a powerful technique with application in a range of markets. Many systems perform scanning using rotating polygon mirrors and holographic lenses, or galvanometer-driven torsion mirrors. Miniaturisation will reduce cost and increase applications, for example by allowing an operator to wear a finger-mounted scanner. Microengineered scanners can be based on surface micromachined torsion mirrors. However, an alternative system with potential for miniaturisation uses a moving fibre or waveguide. Light emerging from the fibre is imaged to a spot by a lens, and a small displacement of the fibre tip is magnified to produce a scan line in the image plane. This displacement is generated by exciting resonance of a fibre cantilever using (for example) piezoelectric actuation.
|Moving fibre bar code reader with dual NA confocal detection.|
An unfortunate consequence of miniaturising a scanner without simultaneously reducing the focal distance is that the return power reduces as the square of the system dimension. It is therefore important to devise architectures that maximise the optical signal. We have developed a piezoelectrically actuated fibre optic resonant scanner (FORS), which uses dual numerical aperture confocal operation and optical filtering to obtain good performance from a layout suitable for eventual integration on V-grooved Si. In this system, the return signal is gathered from the cladding modes of the fibre by a mode stripping detector.
|Return signals from a) mode stripping detector, and b) adjacent conventional detector.|
Self-tuning of a Resonant Fibre Scanner using Intermittent Optical Feedback
We have demonstrated the use of an apertured mirror in front of the imaging lens as a method of providing intermittent position feedback in a single-axis resonant fibre optical scanning system. Reflection from the mirror at the extremities of the scan generates timing signals interlaced with back-scattered data from the target, and a phase locked loop and a proportional controller can be used to adjust the drive frequency and amplitude. We have examined the capture range, convergence and stability of the system theoretically. We have obtained experimental verification using a piezoelectrically actuated dual numerical aperture confocal resonant fibre scanner based on mechanically asymmetric fibre operating at 635 nm wavelength.
|Dual numerical aperture confocal resonant fibre scanner: a) system, b) idealised time variations of drive, mechanical response and detected signal; c) controller block diagram.|
|Breadboard scanner: a) schematic and b) experimental rig.|
|Time variation of drive voltage and detected signal, for a) transverse fibre error and b) mirror tilt (for misaligned system), and c) start and d) end of control loop (aligned system).|
Tailored fibre waveguides for precise two-axis Lissajous scanning
We have demonstrated a two-axis optical imaging system using a Lissajous scan pattern with non-integer frequency ratio. A waveguide with precisely tuned mechanical resonant frequencies has been constructed, by dip coating two fibres with a transparent polymer to form an asymmetric cross-section. Motion has been achieved by mounting a waveguide cantilever at 45˚ on a single piezoelectric actuator with a dual-frequency drive. Confocal signal collection has been achieved using a mode-stripping detector, and feedback signals needed for frequency and phase locking derived from intermittent reflection from an apertured mirror. The first scan axis is locked to the resonance of one of the modes, while the second scan axis is locked to the correct phase at the desired frequency ratio. Accurate acquisition of two-dimensional images has been demonstrated.
|Ideal cross-section of dual-core waveguide with a) no, b) partial and c) full polymer fillet; d) experimental guide cross-sections after different numbers of polymer coats.|
|a) Mounting of fibre and detector; b) time variation of the drive, signal and differentiated signal for the resonant mode, with points indicating timing edges and extracted zero-crossings; c) histogram of phase error.|
|a) and b) experimental line scans in the x- and y-directions; c) experimental Lissajous pattern with n = 15.|
|Images reconstructed from experimental data acquired by Lissajous scanning with n = 15, a) with no phase error, b) with a phase error of 4o, and c) from three scans with deliberate phases offsets of -4o, 0o and +4o.|
Optical Fibre Alignment Devices
Richard Syms, Helin Zou, John Stagg (Imperial College)
Deepak Uttamchandani, Jun Yao (Strathclyde U.)
A new geometry of high-force electrothermal actuator is demonstrated using micro-electro-mechanical systems (MEMS) technology. The actuator consists of two sets of inclined, parallel, suspended beams, which are tethered at one end and linked together at the other. The first set is divided into two halves, which are connected in series and heated electrically. The second set acts as a tether for the first, so that differential thermal expansion gives rise to a lateral deflection. The design can be scaled easily to increase the actuation force, which is sufficient to deflect a cantilevered optical fibre. A bi-directional fibre alignment device is formed from two opposed actuators, which are designed to grip the fibre, and a set of fixed mounting features. Prototype devices are demonstrated by deep reactive ion etching of bonded silicon on insulator, and in-plane alignment of single-mode fibre is demonstrated.
|Fibre alignment actuator after deep reactive ion etching.|
|Power-displacement characteristics for alignment actuator, with and without an inserted optical fibre.|
|View of the tip region of two back-to-back actuators, showing 15 micron deflection of the left-hand fibre.|
Latching Optical Alignment Stages
Richard Syms, Helin Zou, John Stagg
Latching MEMS positioning stages have been constructed using deep reactive ion etching and undercut of bonded silicon-on-insulator material. Because the fabrication process allows large (ca 100 micron) feature heights, the stages can support external loads such as micro-optical components, that lie in the milligram range. Electrothermally driven linear translation stages with a travel of > 100 micron have been demonstrated. Latching is performed by a rack and tooth mechanism driven by shape bimorph actuators. A rack period of 10 micron may be accurately achieved with a structural depth of 100 micron. A Vernier mechanism based on multiple offset rack and tooth latches is used to increase the precision beyond this value, which is limited by pattern transfer. The mechanism has been modified to allow in-plane rotation, using a primary linear drive to act on a tangential drive pin attached to a secondary passive rotary table. A further modification to allow latched out-of-plane rotation based on an inserted tilt-mirror component has also been demonstrated.
|Rack-and tooth mechanism of a latching translation stage|
|Latching rotation stage|
|Latching tilt stage|
3-D Self Assembly of Micro-Optomechanical Devices
Richard Syms, BCO Technologies
Self-assembly is being used in the construction of 3-dimensional micro-optomechanical devices. We have developed a simple surface micromachining process for 3D MOEMS based on mechanical parts formed in industry-standard bonded silicon-on-insulator (BSOI) material, which are rotated out of plane by the surface tension force of thick pads of commercial photoresist. The BSOI material consists of a 5 microns thick layer of single crystal Si, which is bonded to a 2 microns layer of thermal oxide on a conventional 4” Si wafer. The mechanical parts are defined by lithography and wet etching, and are released by freeze drying. The peak process temperature is 150oC, making the method compatible with microelectronics. Simple mechanical latches ensure that motion ceases when the parts have rotated through 45o; and assembly accuracy is currently around 0.5o. The method is being used to build micro-optical components such as reflecting mirrors. The optical quality of these components has been measured with a Zygo interferometer, and has been found to be approximately half a wavelength (at near infrared wavelengths) over a 1 mm2 area.
|1 mm x 1 mm self-assembled 45 degree micro-mirror.|
|Flatness profile of self-assembled 45 degree mirror.|
|Detail of latching mechanism.|
Self-Assembled Electrostatic Torsion Mirror Microscanners
Surface tension self-assembly of three-dimensional frames made from bonded silicon on insulator material has been used as a method of constructing torsion mirror micro-scanners. The scan element consists of a small (ca 450 mm x 450 mm) mirror mounted on a torsion bar, which is excited into resonant oscillation by a skewed electrostatic comb drive at the base. Under static conditions, the device will deflect an optical beam travelling parallel to the wafer surface through 90o. When the electrodes are driven with a sinusoidal drive voltage, the mirror system oscillates at the mechanical resonant frequency and deflects the beam to produce a scan line suitable for bar-code reading applications. High quality factors (up to 60) and large optical scan angles (up to 16o) have been achieved, without causing the frame to collapse or even vibrate noticeably. The drive voltages are currently high (175 V) but are expected to be reduced with improved electrode design.
|Self-assembled 3-D electrostatic torsion mirror scanner.|
|Scan angle versus drive frequency for an electrostatic torsion mirror scanner.|
|Detail of skewed comb-drive electrodes. The interelectrode spacing is 2 microns.|
Self-Assembled Micro-Scanners with Self-Assembled Electrodes
The skewed electrode layout used in simple self-assembled 3D microscanners suffers from a number of drawbacks. Firstly, the drive voltage is high, because of the weak electrostatic field. Secondly, there is a trade-off between the voltage and the scan angle, because a reduction in separation between the halves of the comb can only be performed by lengthening the moving electrode fingers, reducing the angle of turn before they struck the substrate. Finally, only mirror axes parallel to the substrate could be used, to avoid the electrodes clashing.
We have overcome these difficulties by further self-assembly operations, using a new mechanism that allows parts as small as the fixed half of the electrode to be reconfigured. The mechanism is based on two cranks attached to a movable part DRIVE near its hinge. Motion is prevented when the cranks reach the substrate. The accuracy of this mechanism is low, but it is sufficient for sub-component assembly. We have used it to construct mirror scanners with a rotation axis orthogonal to that of earlier devices, driven by an electrostatic drive that is reconfigured into a staggered comb.
|Close-up of self-assembled, staggered comb-drive electrodes.|
|Layout of scanner with self-assembled electrodes.|
|Completed scanner with self-assembled electrodes|
Self-assembled Microlens Arrays
We have developed a new surface micromachining process for the fabrication of refractive microlens arrays. The lenses are designed for collimating optical beams that travel parallel to the substrate in micro-opto-electro-mechanical systems (MOEMS). The lens mount is formed in the device layer of a bonded silicon-on-insulator wafer, and rotated out of plane by a surface tension torque obtained by melting rectangular pads of thick photoresist. The lenses themselves are formed by melting circular pads of the same resist, with diameters currently up to 80 microns. The melted pads are constrained only by an annular ring at their perimeter, so that both the free liquid surfaces can be reshaped into spherical surfaces by the internal Laplace pressure, thus forming a biconvex lens. Correct alignment of the mount normal to the substrate is achieved by sequential self-assembly of a supporting frame. The lenses have conventional optical properties, with focal lengths of ca 50 microns, and have been used in imaging and fibre coupling experiments.
|Fully assembled refractive microlens array.|
|Close-up of microlenses.|
|Microlenses used in an optical fibre coupling experiment.|
Self-Assembled Corner Cube Retroreflection Modulators
Youngki Hong, Richard Syms (Imperial College)
Kris Pister, Lixia Zhou (Berkeley Sensors and Actuators Center)
Active and passive micromachined corner cube reflectors (CCRs) with high surface flatness and precise assembly have been formed by out-of-plane rotation of bonded silicon parts powered by the surface tension of thick photoresist pads. A two-mask fabrication process is used, and an angular alignment accuracy of < 0.18° has been achieved by minimising the number of movable parts and controlling the assembly with mechanical limiters. Active CCRs based on torsion mirror scanners with a Q-factor of 20 have been demonstrated. When used as a retro-reflection data transmitter, a data rate of up to 200 bit/s and a signal-to-noise ratio of 30 dB have been obtained with a drive voltage of 30 V.
|Assembly sequence for fabrication of a corner cube reflector|
|Completed CCR retroreflection modulator with skewed electrostatic comb drive.|
|Step response of CCR retroreflection modulator.|
Latching Variable Optical Attenuators
Richard Syms, Helin Zou, John Stagg (Imperial College)
David Moore, Billy Boyle (Cambridge U.)
A multi-state latching variable optical attenuator (VOA) has been demonstrated using MEMS technology. The mechanism is used to fix the position of a shutter inserted into the optical path between two single-mode fibres. The mechanism and fibre mounts are fabricated in 85 micron thick silicon using bonded silicon-on-insulator material, by deep reactive ion etching. The device can be continuously adjusted or latched into a discrete set of attenuation states using a rack-and-tooth mechanism driven by electrothermal shape bimorph actuators. Electromechanical and optical characterisation has been performed to demonstrate a latching VOA function, with a maximum attenuation of 30 dB. A variant has been demonstrated with a compound latch, to allow coarse and fine control of the attenuation.
|Latchable MEMS variable optical attenuator formed by deep reactive ion etching of silicon|
|Mechanism of compound latching VOA|
|Transmission obtained through compound MEMS VOA in different latched states.|
Richard Syms, Helin Zou, John Stagg, Hadi Veladi
An iris-type variable aperture fabricated using microelectromechanical systems (MEMS) technology is described. The device contains a number of shutter blades, which are each driven by a separate microactuator, and translated synchronously to create a variable polygonal aperture. The optical performance of devices with different numbers of blades is compared using simple analytic models and diffraction theory. The mechanism is simulated by finite element analysis. Four-blade devices driven by buckling mode electrothermal actuators are formed by double-sided patterning and deep reactive ion etching of bonded silicon-on-insulator and characterised experimentally. Symmetric deflections are obtained, and used to create a square pupil. Variable attenuation is demonstrated using optical fibres with thermally expanded cores.
|Pupil region of MEMS iris after deep reactive ion etching.|
|Iris mechanism with the pupil fully enlarged.|
|Iris variable optical attenuator under test.|
Fibre-pigtailed Electrothermal MEMS Iris VOA
Richard Syms, Hadi Veladi, Helin Zou
We have demonstrated a high performance variable optical attenuator (VOA) based on an electrothermally actuated iris with a square pupil. The device is fabricated from two separate dies formed by deep reactive ion etching of bonded silicon-on-insulator material. The iris die is inserted into an elastic clamp on a baseplate die carrying spring-mounted fibre alignment features, allowing the iris to be held in the optical path between two expanded core fibres. A novel aperture that dynamically reduces the clearances between the shutter blades is used to achieve an extinction of 25 dB and a wavelength dependent loss of ± 1 dB at 1550 nm wavelength. Synchronous blade motion is achieved using thermally-optimised, folded electrothermal actuators with undercut hot arms, which reduce the operating power to 240 mW and allow a high mechanical resonant frequency. Optical analysis is carried out using a scalar model and diffraction theory. Thermomechanical analysis is performed using a one-dimensional model and finite element simulation (ANSYS). Good agreement is obtained between the models and with experiment.
MEMS Photonic Band Gap Filters
Ariel Lipson, Eric Yeatman
In recent years, photonic band gap (PBG) structures have attracted much attention. Their wide stop band, due to the large index of refraction difference, reflects all light frequencies that lie within this gap. Constructing PBG-based devices from silicon is favourable since silicon is well studied, low cost and electronically integrable. One PBG device of interest is a fibre-based in-line filter. By constructing a Fabry-Perot filter between two fibres, where each mirror is a 1D PBG structure and the main cavity is half the transmitted wavelength, a wide band stop can be achieved with a narrow pass band. Non-silicon devices of this structure have been previously fabricated, using dielectric stacks, but these suffer from more complicated packaging and alignment, or smaller band-stops, and have limited integration potential. If silicon is used, only three silicon-air pairs are needed for each mirror, making the device extremely compact and potentially low loss. An in-plane design, where the PBG structure is etched into the substrate and the fibres are incorporated in-plane, eases the integration process and allows construction of a high quality filter with alignment features for the fibres. In order for the device to have good optical properties, the PBG structure must be made extremely vertical, accurate in dimensions and with optically smooth surfaces.
|Schematic of fixed photonic bandgap filter.|
|3D representation of the MEMS actoator.|
|Prototype electrostatically tunable photonic band gap filter|
MOEMS Tunable External Cavity Laser
Richard Syms, Anke Lohmann, Weibin Huang
Tunable lasers are essential elements of DWDM and wavelength-routed optical systems. Monolithic sources currently exist as multi-section distributed Bragg reflector (DBR) lasers. However, the powers available from monolithic tunable sources are often lower than fixed wavelength devices. Furthermore, tuning requires a complex control algorithm based on stored calibration data, which may become inaccurate as the laser ages. An alternative is offered by external cavity lasers, which may have the advantage of higher output power, a simpler relationship between control signals and the emission wavelength, reduced modal noise during tuning, and reduced sensitivity to ageing.
|Vertically etched blazed grating with integral elastic suspension and electrostatic comb drives, formed in BSOI with a 100 µm thick bonded Si layer.|
We have developed a miniature grating-tunable external cavity laser diode constructed using micro-opto-electro-mechanical systems (MOEMS) technology. The tuning element is a vertically etched blazed grating mounted on a compound flexure. The flexure is deflected using comb electrostatic drives to rotate and translate the grating. A preliminary mechanical design based on beam bending theory has verified by finite element analysis. The tuning element has been prototyped using deep reactive ion etching of bonded silicon-on-insulator (BSOI) material. Using this element, Littrow external cavity lasers with > 500 mW single mode output power have been demonstrated, with a side-mode suppression ratio of 30 dB. Mechanical tuning over 100 nm has been demonstrated. Electrical tuning over a smaller range (currently 30 nm) has also been demonstrated, and work is continuing to improve tuning performance.
|Littrow external cavity laser, with a MOEMS tuning element.|
|Emission spectra at different wavelengths from MOEMS Littrow external cavity laser.|