A photonic instrument is proposed to boost the resolution for ultraviolet/optical/infrared spectral analysis and spectral imaging allowing the detection of narrow (0.00007-to-0.07-picometer wavelength resolution range) optical spectral signatures of chemical elements in space and planetary atmospheres. The idea underlying the proposal is to exploit the advantageous spectral characteristics of whispering-gallery-mode (WGM) resonators to obtain spectral resolutions at least three orders of magnitude greater than those of optical spectrum analyzers now in use. Such high resolutions would enable measurement of spectral features that could not be resolved by prior instruments.
Tunable single-mode WGM resonators would be incorporated into optical spectrum analyzers as shown in the block diagram in Figure 1. The center of the spectral window of the spectrum analyzer will be tuned to the carrier frequency of interest. The rough snapshot of the signal under study will be taken. After that, the WGM filter will be inserted in front of the spectrum analyzer. The internal scanning of the spectrum analyzer will be switched off, while the WGM filter will be scanned through the frequency window. The narrow-band spectral features of the signal will be resolved as the result. In particular, for the purpose of measuring abundances of selected isotopes (e.g., isotopes of carbon) in compounds in outer space and in atmospheres of Earth and other planets, an instrument equipped according to the proposal could measure narrow (width < 10 MHz) optical spectral signatures of compounds (e.g., CO2) containing such isotopes.
The advantageous spectral characteristics of WGM resonators include high resonance quality factors (see Figure 2) and clean spectra. In addition, relative to other tunable optical resonators that have similar free spectral ranges and Q values, tunable single-mode WGM resonators can be tuned over wider frequency bands and exhibit much greater rejection ratios. A tunable single-mode WGM resonator incorporated into a spectrum analyzer according to the proposal would have a power consumption of no more than a few milliwatts, would have a mass of about 100 g, would have no moving parts, and could be operated autonomously. In addition to being key components of contemplated new high-resolution optical spectrum analyzers, tunable single-mode WGM resonators could be retrofit to current optical spectrum analyzers to improve their performances.
This work was done by Anatoliy Savchenkov, Andrey Matsko, Dmitry Strekalov, and Lute Maleki of Caltech for NASA’s Jet Propulsion Laboratory.
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