Ultra-narrow Bandpass Filter

Free-space quantum communication (FSQC) has many advantages over conventional long-distance communication based on the microwave or radiofrequency (RF) bands, including enhanced security and narrow beam divergence, and. Optical filters are one of the many system components essential in implementing FSQC for ground-to-satellite or satellite-to-satellite links. Optical filters will allow getting “more signal, with less background,” thereby enabling high-fidelity FSQC in the presence of the background noise (e.g., daytime sun light). For example, in FSQC between Mars and the Earth, the Sun might be in or near the line of sight, and FSQC without an ultra-narrowband filter (transmission bandwidth < 100 MHz) will suffer from background noise issues. The ability to tune the filter passband in the NIR spectral range would also enable operation flexibility. To summarize, the optical fiber should have a narrow passband (~ 100 MHz), offer frequency tuning, and provide minimal insertion loss (< 0.5 dB) and high rejection (>50 dB) of out-of-band light with a wide operational range (780 ~ 2500 nm). To address this need, Opto-Atomics Corp. (OAC) proposes to develop an Ultra-narrow Bandpass Filter (UBF), which will provide a transmission bandwidth of 100 MHz around a tunable center wavelength in the NIR range. UBF will also have optical insertion loss < 0.5 dB, and an out-of-band rejection over 50 dB.

The Mars-to-Earth FSQC is a critical enabling technology to execute NASA’s initiative to send humans to Mars. A narrowband optical technology with a transmission bandwidth <100 MHz will allow high SNR communication even in the presence of the Sun or near the line of sight of Mars. At the optical receivers, these filters will enable beam steering and provide “more signal, with less background.” Additionally, ground-to-satellite links and quantum computer communications using optical wireless communication are important in various NASA missions. Military communications require broadband capabilities at the highest level of security, which can be challenging to achieve in a dense RF operating environment. The bandwidth and security qualities of FSQC make it an attractive technology for military communications. There are also applications for UBF in the world of quantum cryptography, where it can help to enable quantum key distribution.

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