The MEMS-FPI (short for microelectromechanical system-based Fabry-Perot interferometer) spectrum sensor is built upon the optical design first introduced in 1899 by French physicists, Charles Fabry and Alfred Perot. This optical design centers around the use of two parallel mirrors or reflective surfaces surrounding a cavity. As the volume of this cavity changes, so too does the wavelength of light that passes through it.
Figure 1. Internal structure of the MEMS-FPI
In the case of the MEMS-FPI spectrum sensor from Hamamatsu Photonics, the FPI is incorporated into the tunable filter design. Micromachined into this tunable filter are laminations of dielectric coatings (such as SiO2, SiN, or poly-Si), which make up the upper and lower mirrors. In between these mirrors is an air gap. As seen in Figure 1, light passes through a band-pass filter and then travels through the tunable filter. When voltage is applied to the MEMS-FPI spectrum sensor, an electrostatic force is produced between the two mirrors in the filter. The higher the applied voltage, the higher the electrostatic force and, therefore, the closer the mirrors are to one another; the shallower the air gap between them, the shorter the wavelength of light transmitted (and vice versa). The MEMS-FPI spectrum sensor contains an InGaAs PIN photodiode to collect the transmitted near-infrared (NIR) light. Depending on the version of the spectrum sensor used and its corresponding band-pass filter, light in the following bands can be detected (Table 1):
When applying voltage to a MEMS-FPI spectrum sensor, it is of utmost importance to keep in mind the possibility of pull-in phenomenon. Pull-in occurs when the electrostatic forces used to draw the mirrors closer together exceeds the spring force used to pull the upper mirror away from the lower mirror. If this happens, the mirrors effectively stick to one another—a process that is very difficult, if not impossible, to reverse. When this happens, the wavelengths measured become unreliable as the spectrum sensor is no longer accurately correlating a certain mirror distance to the wavelength transmitted. As seen in Figure 2, factors such as applied voltage and operating temperature contribute to the likelihood of pull-in occurring.
Figure 2. (a) Pull-in phenomenon’s relationship to applied voltage (note: pull-in occurs at the dotted line). (b) Pull-in phenomenon’s relationship to operating temperature
To ensure the correct voltage is applied to the spectrum sensor, the appropriate calibration coefficients need to be applied. These coefficients are device-specific, so each spectrum sensor head and each evaluation board (as pictured in Figure 3) must have its calibration coefficients applied to ensure proper use and accurate results.
Figure 3. MEMS-FPI spectrum sensor and evaluation board (sold separately)
Figure 4. MEMS-FPI module (containing MEMS-FPI spectrum sensor, evaluation circuitry, and lamp)
To simplify handling of the MEMS-FPI spectrum sensor, Hamamatsu Photonics has introduced the MEMS-FPI module. Built-in capabilities such as preprogrammed calibration coefficients remove the need for users to manually upload individual coefficients—instead, these values are loaded into the EEPROM of the board housed in the module and automatically read once the device is identified in the evaluation software. To streamline optical design, this device contains a built-in lamp: users can either operate the MEMS-FPI with the lamp on for reflection measurements or with the lamp off for transmission measurements using an external source. As seen in Figure 4, in its standard form, the module can be used for free-space measurement. However, a separate SMA fiber adapter is available for those looking for fiber-coupled measurements as well.
An additional feature of the module that is not included in the sensor with evaluation board option is the incorporation of temperature compensation functions. As discussed with regards to the pull-in phenomenon, temperature plays a significant role in the position of the mirrors in the tunable filter and the corresponding measured wavelength. To correct for any inaccuracies due to changes in temperature, temperature correction algorithms have been designed and integrated into the software, thus providing added reliability and temperature stability in MEMS-FPI performance.
With sensitivity in various bands of the NIR range, the MEMS-FPI can be used to observe key absorption lines for water, organic compounds, and inorganic compounds (Figure 5). This means that applications such as those listed below are possible:
Figure 5. MEMS-FPI spectral coverages and related target compounds
Using their expertise in the manufacture of MOEMS (microoptoelectromechanical system) devices, Hamamatsu Photonics has achieved a compact design of the FPI. With its small form factor, the MEMS-FPI spectrum sensor can be integrated into portable solutions for its given application. For even quicker start-up, users can turn to the MEMS-FPI module; with the MEMS-FPI spectrum sensor, evaluation circuitry, built-in lamp for reflection measurements, and USB micro-B connector, all contained within a hand-held housing, this device shortens the time between the physical setup of optics and electronics, and measurement.
Compared to other interferometer spectral sensors (such as the MEMS-based FTIR engine), the MEMS-FPI excels in its compact design. When comparing between an interferometer-based spectrum sensor, such as the MEMS-FPI, and array-based spectrometers, we suggest users consider the specifications in Table 2 below when assessing which option may work for them.
Table 2. General comparison of MEMS-FPI and other NIR spectral sensors
As an Applications Engineer at Hamamatsu Corporation, Stephanie Butron enjoys learning more about the variety of projects and applications Hamamatsu’s customers are working on, and understanding better how Hamamatsu can help. When she isn’t helping people focus in on their research or project, Stephanie enjoys focusing her camera lens on the sights around her. From still lives to portraits, Stephanie tries to find new ways to look at the world.
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