A monolithic optical displacement measurement microsystem

-

9:30am 9:45am ThA3

Monolithic Optical Interferometers for MEMS Applications H.P. Zappe, D. Hofstetter and B. MaisenhBlder Paul Scherrer Institute Badenerstrasse 569 8048 Zurich Switzerland

For high-resolution optical measurements, interferometry is often the technique of choice. The ability to perform optical interferometry inside a microsystem may prove to be lucrative for many MEMS systems, where small size, high physical robustness and reduced needs for optical alignment are of primary relevance. For these reasons, the combination of photonic integrated circuits (PICs) with MEMS may lead to microsystems of considerable physical ruggedness, high functionality and small size. PICs are monolithically integrated semiconductor optical circuits, typically including lasers, waveguide circuits and photodetectors, and often modulators and grating structures, As with most ICs, volume semiconductor batch fabrication is an attractive feature. Due to their suitability for light emission and their efficient electro-optic behavior, the 111-V materials are almost exclusively employed; considerable interesting and challenging work then revolves about the hybrid assembly of such PICs with Sibased MEMS structures. We discuss here two monolithically integrated interferometric sensor systems of interest for MEMS applications. The first is an optical displacement measurement microsystem on a chip; this system consists of a Michelson interferometer with an integrated DBR laser, waveguide photodetector and phase modulators. The second is configured as a Mach-Zehnder interferometer and is used for high-resolution refractometry with applications in biological and chemical sensing. Both of these sensor PICs employ the same optical and optoelectronic devices and share closely related fabrication technologies. The integrated interferometers were based on a GaAs/AlGaAs double heterostructure laser substrate, with one or more quantum wells in the active region; a selective vacancy-enhanced disordering process was used to define transparent and absorbing regions on the single substrate such that only one epitaxy step was required. A distributed Bragg reflector (DBR) laser was used as a light source, employing a 3'd order (A = 385 nm), holographically defined grating etched into ,a recess above the waveguide core and emitting at 820 nm. Monomode operation, essential for a clean interference characteristic, was achieved; the discrete device had a linewidth of 500 kHz, implying a coherence length of 600 m. Typical output powers were 5 mW with threshold current densities on the order of 1.3 kA/cm2. The waveguide photodetectors had responsivities of 0.6 A/W and dark currents of 500 pA for device areas of 3x500 pm. Phase modulation relied on the quantum-confined Stark effect (QCSE). The displacement measurement chip, shown in Figure 1, employs an optical measurement beam, emitted from the upper arm of the interferometer, which is reflected from a m external object and returned to the interferometer by the same waveguide; this reflection interferes with a reference beam, returned from the cleaved facet at the edge of the chip, and the interference signal is measured by the detector. Examination of a typical interference characteristic shows that one interference fringe is seen for each 408 nm of displacement, as measured by the photodetector current. Length resolution is thus better than 35

Current work is studying the expected Figure 1: Michelson interferometers for displacement improvement in resolution to below measurement. DBR lasers and photodetectors are on the 10 nm using this structure. left. The lower device also has two phase modulators on the right. Total chip length is 2 mm. An integrated optical transducer for the translation of environmental refractive index shifts into optical intensity changes, using the same optoelectronic components, is also under development. This PIC is based on a Mach-Zehnder interferometer and is used for refractometric biological/chemical sensing. One arm of the interferometer, shown in the SEM photograph of Figure 2, contains a dielectric waveguide sandwich of which the core is exposed. A change in the refractive index of either a chemically selective film deposited on this waveguide or a fluid in contact with the core, leads to a relative phase shift in the one interferometer arm and thus an interference signal. Measurements using this configuration have demonstrated a refractive index resolution of 3x 10-5for a 2 mm long sensor region, a value currently limited by system drift. Both of these monolithically integrated sensor PICs may be of interest for systems applications involving MEMS structures. Numerous hybridization techniques suitable for assembly and mounting of a PIC onto a Si substrate have been developed: the use of passive, physical alignment features, etched into both parts, is probably of particular relevance due to their simplicity and low additional cost. The displacement sensor, because of its reduced size and eliminated need for critical optical alignment, may be used for precision measurement in more complex opto-mechanical micrssystems including movable electrostatic actuators, diaphragms and resonant beams; the sub- 100 nm resolution currently attained is likely to be sufficient. Improvements in resolution, currently underway, may make the determination of AFM-tip movement appealing, suggesting means for the fabrication of more compact and robust hybrid AFM heads. The integrated optical refractometer may be employed individually as a chemical sensor or in arrays as portion of an optical sensor microsystem. The use of Si MEMS-based flow cells, micropumps or fluid-handling systems may ease the development of compact and rugged chemical diagnostic units, The ability to easily fabricate multi-sensor arrays is a particularly attractive feature of such a PICIMEMS hybrid.

Figure 2: Mach-Zehnder interferometers for refractometric measurements. The length of the sensor region, in the lower arm, is 2 mm. The photodetector is visible at the right. 36

View publication stats