Source: http://aoot.osa.org/oe/abstract.cfm?uri=oe-27-6-9088
Timestamp: 2019-04-23 06:51:52+00:00

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We perform fast comb spectroscopy by dividing the probe comb into several sub-comb segments so as to produce multi-heterodyne beats focused around targeted molecular absorption lines. This concentrated scheme of comb spectroscopy is able to achieve a 30 dB signal-to-noise ratio with just a single shot measurement of 10 μs acquisition time. Such high signal sensitivity is verified by measuring separate absorption lines of H13C14N and 12CO2 gases simultaneously. In addition, atmospheric 12CO2 concentration over a 1.3 km open-air path is traced with a signal repeatability of 15 ppm at a 5 kHz update rate.
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Fig. 1 System configuration of comb segmentation spectroscopy. DFB LD: distributed feedback laser diode, EDFA: erbium-doped fiber amplifier, FBGA: fiber Bragg grating array, f: frequency, fr: repetition rate, fo: carrier-envelop offset frequency, OFS: optical frequency synthesizer, PD: photo-detector, Rb: rubidium, ∆s: spectral width of comb segments, λ-meter: wavelength meter.
Fig. 2 Data sampling. (a) Multi-heterodyne interference between sub-comb segments and their corresponding CW laser lines in the optical frequency domain. (b) Electric beats sampled in the radio-frequency domain. ∆f1: locking offset of DFB LD1, ∆f2: locking offset of DFB LD2.
Fig. 3 Experimental data. (a) Optical spectra of the source comb (black line) and two DFB LDs (red & blue lines) measured using an optical spectrum analyzer of 6.24 GHz resolution (0.05 nm). (b) and (c) Individual optical spectra of two sub-comb segments at 1550 nm and 1572 nm. Shaded areas (red & blue) produce multi-heterodyne beats over a 16 GHz bandwidth. (d) Frequency stability measurements of the Rb clock (gray) and DFB LDs (red). (e) Heterodyne RF beats measured at a sampling time of 10 μs.
Fig. 4 Simultaneous detection of two absorption lines. (a) HITRAN database of H13C14N and 12CO2 from 1525 nm to 1585 nm. (b) & (c) Measured absorption line profiles of the P10 line of hydrogen cyanide (H13C14N) at 1549.73054 nm and the R18 line of carbon dioxide (12CO2) at 1572.01847 nm with a sampling time of 200 μs. The comb-resolved spectral resolution is 0.8 pm. For comparison, the same absorption lines were measured using a tunable CW laser by scanning over a measurement time of 100 s (black). (d) Standard deviation of the measured transmittance with increasing the sampling time from 10 μs to 1 ms (orange dots). Gray dots indicate the tendency of 1/ τ .
Fig. 5 Outdoor measurement of atmospheric CO2 concentration. (a) Measurement path of a 1.3 km round-trip distance. (b) Overall measurement system setup. (c) Temporal fluctuation of the received beam intensity due to air turbulence. (d) Power spectral density of the received beam intensity. (e) Absorption spectrum obtained at 200 μs sampling. (f) Measurement plots over 12 hours (green) in comparison with those of NDIR sensors (red). Error bars indicate uncertainty. (g) Measurement standard deviation with increasing the averaging time from 10 μs to 200 μs. Gray dots and dotted line indicate the tendency of 1/ τ .

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