Abstract
We describe the design and performance characterization of spectrally tunable microengineered notch filters operating in the longwave infrared from 8 to 12 micron using quantum cascade lasers (QCLs) tunable over the full spectral range. The filter design is based on the guided mode resonance phenomenon. The device structure consists of a subwavelength dielectric grating on top of a planar waveguide using high-index transparent dielectric materials, i.e., germanium (Ge) and zinc selenide (ZnSe) with refractive indices of 4.0 and 2.4, respectively. The filters are designed to reflect the incident broadband light at one (or more) narrow spectral band while fully transmitting the rest of the light. Spectral tuning of the reflected wavelength is achieved by changing the angle of incidence by mechanically tilting the filter. Filters based on one-dimensional (1-D) gratings are polarization dependent and those based on two-dimensional (2-D) gratings are close to polarization independent. Simple two-layer antireflection coatings were applied to minimize reflections from the nonpatterned side of the filter substrate. Our experimental setup consisted of a commercial QCL system operating at room temperature, a microengineered filter, and an uncooled broadband sensor. We present the filter design and detailed characterization experiment, and compare the theoretical and experimental results for 1-D filters.
Highlights
There is a need to develop compact tunable notch or laserline rejection transmission filters operating in the longwave infrared (LWIR) region
We used a 1-mm-thick zinc selenide (ZnSe) substrate with refractive index of 2.41, a Ge waveguide and grating with refractive index equal to 4.0, and a simple two-layer AR coating consisting of Ge and ZnSe layers applied to the bottom of the substrate
Λx is the grating period, W is the width of Ge grating groove equal to the fill factor times the grating period, hwg is the thickness of the planar Ge waveguide, hg is the thickness of the Ge grating groove, hs is the thickness of the ZnSe substrate, hAR1 is the thickness of Downloaded From: https://www.spiedigitallibrary.org/journals/Optical-Engineering on 02 Nov 2021 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use
Summary
There is a need to develop compact tunable notch or laserline rejection transmission filters operating in the longwave infrared (LWIR) region. Such a filter will efficiently block a single or multiple discrete narrow wavelength band(s), while transmitting light in the rest of the spectral region. Tunable filters are used for a wide variety of applications in imaging and sensing Such tunable filters include acoustooptic tunable filters, liquid-crystal tunable filters, FabryPerot filters, and so on that operate over many spectral regions including LWIR. Existing spectral filter technologies are inadequate for notch filtering applications as they transmit only a narrow band of light and reject the rest of the light in the spectral region. Filters based on one-dimensional (1-D) gratings are polarization-dependent and those based on
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
