A microspectrometer has a circular geometry, and is designed with the Fresnel diffraction equation. This enables a dramatic miniaturization of the optical parts of a spectrometer over 100 times by volume. Therefore, it enables the construction of spectrometer arrays such as 100×100 microspectrometers for tunable multispectral or hyper-spectral imaging. It can be used for a massive, simultaneous spectral scan from multiple optical sources such as 10,000 optical fibers.
Two laser beams of 532 nm (green) and 633 nm (red) wavelengths were mixed and combined at a beam mixer. After the beam mixing, the beam looks like a yellow color to a human eye. This mixed pseudo-yellow beam passes a beam chopper operating at 191 Hz to modulate the beam for the lock-in amplification. A spatial filter was used to get a uniformly mixed TEM00 beam in a wide beam diameter of about 1 cm. Then, the mixed beam enters the Fresnel ring grating whose diameter is only 750 μm. The ring grating separates the lights according to the wavelengths, and only the selected photons of the desired wavelength can pass the aperture slit of 25 μm in diameter.
The number of passed photons is measured as the current of a photon detector. The signal current is delivered to the lock-in amplifier. The lock-in amplifier picks up only the 191 Hz frequency- locked signal from photon detector with respect to the reference signal from the beam chopper, and it rejects unwanted room lights and other stray lights. The computer receives the data from the lock-in amplifier and sends commands to an XYZ stage actuator to change the optical distance Z, between the ring grating and the aperture slit. A spectral scan is made with the linear actuator movement. The sharpness of the focal point and the spectral resolving power were increased with the increasing numbers of rings in the grating.
Because the size of each microspectrometer is small, a dense array of spectrometers can be fabricated in a small area. Such an array can simultaneously capture the multiple spectral signals from various sources.
This work was done by Yeonjoon Park, Glen King, Sang Choi, and James Elliott of Langley Research Center. LAR-17661-1