MEMS Optical Fabry–Pérot Switches

Xiang Zheng Tu

 

Ten years ago, the present author designed a MEMS optical switch array for DNA synthesis and detection, as shown in the figure 1.  A MEMS optical Fabry-Perot switch consists of a silicon substrate, a cavity and a driving circuit. The cavity is formed by a deflectable plate and a fixed plate which are separated by an air gap. The plates are constructed by dielectric thin films coated with a metal film on their opposite surfaces. The dielectric plates are transparent in the wavelength ranges 350 nm – 14000 nm. The metal films are used as both optical reflecting mirrors and electrodes connecting to the driving circuit. The air gap can be changed by applying the voltage between the two plates resulting in an electrostatic force which pulls the plates closer.

 

The principle of operation of the optical switch is illustrated in the figure 2. The input signal is incident on the left surface of the cavity. After one pass through the cavity a part of the light leaves the cavity through the right facet and a part is reflected. A part of the reflected wave is again reflected by the left facet to the right facet. If the air gap is equal to half an even multiple of the wavelength in the cavity a round trip through the cavity will be an integral multiple of the wavelength. In this case all the light waves will transmit through the right facet add in phase. Such wavelengths are called the resonant wavelengths of the cavity and the optical switch is in “on” state. Similarly, if the air gap is equal to half an odd multiple of the wavelength all the light waves will reflect by the cavity and the optical switch is in “off” state.

Optical Fabry-Perot cavities based on micro electro-mechanical systems (MEMS) are an enabling technology for hyper spectral images and micro spectrometer. MEMS optical switches are high pass filters that block the visible light and pass ultraviolet light. They are characterized by their bandwidth at which maximum transmission is 50%. A MEMS optical switch array consists of a Cartesian grid of switches. This can be used chiefly to map or "encode" the coordinate of each switch to its function. Switches in these arrays typically use a universal signal ling technique (e.g. fluorescence), thus making coordinates their only identifying feature.

Additional features of the MEMS optical switch array for DNA synthesis and detection are combination of DNA synthesis and detection, high probe density and low fabrication cost. Such DNA probes with a MEMS optical switch array can help to dramatically accelerate the identification of the estimated 80,000 genes in human DNA, an ongoing world-wide research collaboration known as the Human Genome Project. The DNA probes can quickly sequence DNA. In addition to genetic applications, the DNA probes can be used in toxicological, protein, and biochemical research. The DNA probes can also be used to rapidly detect chemical agents used in biological warfare so that defensive measures can be taken.