Abstract. Lidar is an effective tool for high-altitude atmospheric measurement in which
a weak absorption line for the target gas is selected to ensure a large
optical depth. The laser frequency stabilization to the line center is
required, and a sub-Doppler (sD) spectroscopy of the target line is preferred
as a frequency reference. In this paper, a novel universal sD noise-immune cavity-enhanced optical heterodyne molecular
spectroscopy
(NICE-OHMS)
instrumentation based on a fiber-coupled optical single-sideband
electro-optic modulator (f-SSM) for the potential application in atmospheric
lidar for different target gases with different types of lasers is reported.
The f-SSM can replace all frequency actuators in the system, so as to
eliminate the individual design of feedback servos that often are tailored
for each laser. The universality of the instrumentation was demonstrated by
the alternative use of either an Er-doped fiber laser or a whispering-gallery-mode laser. Then the instruments based on both lasers were used to produce
the sD signals of acetylene, which worked as a frequency reference to
stabilize the laser. By performing the lockings, relative frequency
stabilizations of 8.3×10-13 and 7.5×10-13 at
an integration time of 240 s were demonstrated.
A novel methodology to directly predetermine the relative wavelength response (RWR) of a DFB laser, in terms of a combined current linearly scanned wavelength response and current modulated wavelength response (CMWR), in wavelength modulation spectroscopy (WMS) is presented. It is shown that the assessed RWR can be used to mimic the measured response with standard deviation of discriminations that are below 3.4 × 10 −3 cm −1 under a variety of conditions. It is also shown that its performance supersedes two commonly used assessment models of the CMWR but is slightly worse than that of the third model, however with the benefit of solely using a single fitting parameter (the concentration) instead of more. When the novel method is applied to the assessment of CO 2 concentration in a Herriot-type multipass cell by using the technique of calibration-free WMS, the results show that there is virtually no difference compared to that by use of the best of the other methods. It is concluded that the novel method is more robust and simplifies the retrieval process of gas concentration.
A novel technique for performing fiber pigtailed DFB laser and linear Fabry-Pérot cavity based optical feedback cavity enhanced absorption spectroscopy (OF-CEAS) is proposed. A fiber-coupled electro-optic modulator (f-EOM) with x-cut y-propagation LiNbO3 waveguide is employed, instead of PZT used in traditional OF-CEAS, to correct the feedback phase, which improves the compactness and applicability of OF-CEAS. Through the efficient and real-time control of the feedback phase by actively changing the input voltage of the f-EOM, a good long-term stability of the signal has been achieved. Consequently, a detection sensitivity down to 7.8×10−10 cm−1, better than the previous by PZT based OF-CEAS, has been achieved over the integration time of 200 s, even by use of a cavity with moderate finesse of 2850.
The laser frequency could be linked to an radio frequency through an external cavity by the combination of Pound-Drever-Hall and Devoe-Brewer locking techniques. A stable and tunable optical frequency at wavelength of 1.5 μm obtained by a cavity with high finesse of 96,000 and a fiber laser, calibrated by a commercial optical frequency comb, has been demonstrated. The locking performances have been analyzed by in-loop and out-loop noises, indicating that the absolute frequency instability could be down to 50 kHz over 1 s and keep to less than 110 kHz over 2.5 h. Then, the application of this stabilized laser to the direct absorption spectroscopy has been performed. With the help of balanced detection, the detection sensitivity, in terms of optical density, can reach to 9.4×10-6.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.