Optical Readout for In-Plane Micro Displacements
Publication Type:Conference Paper
Source:Pacific Northwest Microsystems and Nanotechnology Meeting (PANOMINO), University of Washington Friday Harbor Labs, Friday Harbor, WA, USA (2008)
The common way to sense in-plane micro-displacements is by measuring induced capacitive variations, translated into an output voltage through a transimpedance circuit . A large number of elementary “fingers” attached to the movable structure, interdigitated with stationary ones, are needed to achieve higher sensitivity levels. This has several negative effects: area increase (limiting the scalability of the sensing structures), an increase in damping and mechano-thermal noise , pull-in limitations, parasitic mechano-electrical interactions, etc.
Interferometric principles have been applied to (micro-)displacement measurements for more than two decades -, but present instruments are only capable of sensing out-of-plane movements. In commercial equipments, the complementary in-plane measurements are addressed by other techniques, such as video stroboscopy, with lower resolution levels than those achievable by interferometry .
For all these reasons, in-plane micro-displacement sensing techniques of high sensitivity, low noise, and scalable to very small movable structures are desirable. The present work proposes an interferometric, Integrated-Optics (IO) Micro-Opto-Electro-Mechanical System (MOEMS), with the block diagram illustrated in Fig. 1.
Large cross-section Silicon ridge waveguides (RWG)  have been designed for single-mode behaviour and single-mode fibre compatibility at a wavelength =1550 nm. Such RWGs have been laid out in a Y shape to create a modified Mach-Zehnder interferometer (MZI). Light from an external source is injected in the front-end of the Y-shaped RWG, split equally between its two back-end branches, and shed onto a fixed and an electrically-actuated microstructure respectively, which act as partial reflectors (Fig. 2). A fraction of the light reflected by each of them is collected by the respective RWG branches, and their superposition in the stem of the Y-shaped RWG gives rise to an interferometric signal at its front end. The optical path difference (associated with the relative position of the fixed and the movable reflectors) determines the intensity of the interferometric signal. The back-propagating light is directed to a 1550nm PIN photodiode by using an optical circulator; subsequently, the generated photocurrent is amplified and filtered by the low-noise readout electronic circuitry.
A system-level analysis of the mechanical resonator was carried out in Coventorware® software, yielding a resonant frequency of 18.2 kHz. Based on this result and the working wavelength, a readout circuitry based on an FGA04 PIN photodiode has been designed and simulated in Multisim® software. The attenuation in the RWG has been calculated by numerical simulations in Beamprop® for the TE-like fundamental mode, yielding a 2.4 dB attenuation value (Fig. 3). Matlab® simulations have been carried out to predict the interferometric signal, using Gaussian Optics to account for the variable distance-dependent losses in the RWG-air-reflector interfaces. The MZI shows an average insertion loss of 15.5 dB. The displacement-dependent interferometric signal attenuation has been approximated as an exponential decay , with an attenuation coefficient =0.0035 [ ].
A noise analysis for the photodiode and the readout circuit preamplifying stage shown in Fig. 5 was carried out to calculate an equivalent noise current at the photodiode (Fig. 6). Such analysis showed that the noise current is in the order of nA for the bandwidth of interest. With an appropriate optical input power, it is possible to overcome the noise of the electrical front end. A resolution of 7 pm for a 32 kHz bandwidth has been predicted. The first prototypes of this device are being fabricated.