Tunable bandpass optical filters for spectroscopy come in many possible configurations, including diffraction gratings, acousto-optic, liquid-crystal, volume Bragg gratings, tilted multilayer filter-coated plates, and flat bandpass filters with bandpass center wavelength linearly varying across the filter. Many of these require complex and costly extra optics; the last two, which rely on multilayer thin-film coatings, are simpler, but have their own drawbacks. For example, flat filters tuned by tilting have a limited tuning range unless several individual filters are used, and linearly varying filters must be physically large to accommodate broadband tuning, and must be mounted to a translation stage as well.
Researchers at the University of Alberta (Edmonton, AB, Canada) have been developing a new sort of wavelength-tunable filter, suitable for imaging systems, based on frustrated total internal reflection (FTIR), in which two flat optical surfaces are brought close enough together that light within a passband resonantly tunnels from one element through the small air gap to the other element (each surface is coated with a few-layer high-index-contrast admittance-matching thin-film coating). With proper design, tilting this micrometer-scale gap region with respect to the incoming light tunes a passband across a large spectral region.
The first-generation device was based on prisms,1 while the second-generation device was built using cylindrical lenses, with tunability across the 1000 to 1800 nm spectral region.2 Now, the group has developed an extremely compact FTIR-based tunable filer consisting of two half-ball lenses placed together with a small gap to form a sphere that is rotated for tuning.3 The filter itself can form the heart of a spectrometer when coupled into an aperture or fiber (and can simultaneously serve as the lens that couples light into the fiber). And, because the ball lens can become just another lens in an optical system, it can be used as the tunable filter in a compact hyperspectral imager (see figure).
Flat-top passband
The device design consists of two half-spheres forming a 10 mm ball lens, with one version intended for a 400–700 nm (visible) tuning range and a second intended for a 1000–1800 nm (near-IR) tuning range, with full width at half maximum (FWHM) passband (which has a nicely shaped flat top) on the order of 2 to 4 nm. The air-gap filter plane can be tilted up to an angle of 60° away from normal incidence to match or exceed the 0.2 numerical aperture (NA) of a typical compact commercial spectrometer (the acceptance angle of the ball lens filter declines at angles away from normal incidence), or even more if desired if the input is from a single-mode fiber with an NA of 0.13. The air gap is simultaneously adjusted using piezo-positioners to optimize tunability. It should be noted that the output is strongly polarized due to high rejection of transverse magnetic-polarized light.
An experimental device was built using two stock N-BK7 half-ball lenses from Edmund Optics (Barrington, NJ) with thin-film stacks coated on the two flat surfaces; piezo-positioners from Thorlabs (Newton, NJ) allowed the air gap to be adjusted. Setups using either a fiber input or an imaging-system configuration were tested. The visible-range filter could access the 450–700 nm range; the 400–450 nm range was experimentally inaccessible due to the resting air gap being too large. The near-IR setup could access the full spectral region, but had difficulties with maintaining collimation below 1100 nm due to suboptimal performance of the aluminum jig used to hold the components. However, neither of these problems were fundamental. A typical figure for variation in insertion loss for the device across the tuning region was about 3 dB.
An imaging system was put together using two achromatic doublet lenses with a 19 mm focal length; the system sharply imaged a 1951 USAF resolution target. By augmenting this system with an infinity-corrected objective lens with a 33 mm focal length and an adjustable iris, a hyperspectral imager was created that could selectively image LEDs of different colors (red, green, and blue, along with a white LED for comparison).
The researchers note that the ability to provide a flat-top passband shape is one of the winning attributes of their tunable-filter approach; simplicity is another.
REFERENCES
1. A. Melnyk et al., Opt. Lett., 41, 1845–1848 (2016).
2. T. R. Harrison et al., Opt. Express, 27, 23633–23644 (2019).
3. T. R. Harrison et al., Appl. Opt. (2020); https://doi.org/10.1364/ao.398936.
