Differential absorption LIDARs (DIAL) are used to monitor trace gases and require two synchronized narrow linewidth laser sources of nearly the same wavelength. Knowing the absorption profile as a function of wavelength a priori , one laser source emits near the wavelength of peak absorption of the target trace gas; the other emits in the region where target gas does not absorb. The differential absorption at the two wavelengths results in the different levels of detected optically backscattered power as a function of range, the ratio of which is used to calculate the concentration of the gas.
Fiber laser based LIDAR systems offer numerous advantages over conventional LIDAR systems that use bulky free space optics. It can be made low cost, rugged, light and compact, alignment hassle free, polarization independent, sustainable without an active cooling mechanism for low power systems, highly efficient, broadly tunable etc.. All these advantages of fiber based LIDAR systems offer a superior technology for space based remote sensing applications in airplanes and satellites, where significant cost can be reduced by using light and compact devices.We have demonstrated, a novel Q-switched dual-wavelength fiber laser seed source, for application in a DIAL system. The main advantage of the dual-wavelength fiber laser is that only one laser is needed to generate perfectly synchronized two-wavelength pulses.
Figure 2. Experimental results for dual-wavelength fiber ring laser in cw mode with tunable wavelength separation. The different wavelength spacing obtained by rotating the polarization controller in the Sagnac filter-2 are (a) 1.1-nm, (b) 2.2-nm, (c) 3.3-nm, and (d) 17.5-nm
Figure 3. Experimental results demonstrating the stability of the dual-wavelength fiber laser with time. The wavelength separation in this case is 2.2-nm and each measurement was taken at a regular interval of 15 minutes.
Figure 4. (a) Stable dual wavelength optical spectrum of Q-switched pulses at 20 kHz repetition rate and wavelength separation of 2.2 nm. (b) Q-switched pulses at repetition rate of 20 kHz.
In summary, our dual-wavelength Q-switched fiber laser demonstrates tunability in absolute wavelength over several nanometers and discrete tuning in wavelength spacing from 1.1 to 3.3 nm. Apart from the inherent advantages of fiber lasers, the experimentally measured narrow spectral linewidth (< 40 kHz) and no noticeable frequency jitter between the two wavelengths make it an ideal seed source for DIAL applications.
References:
Utkarsh Sharma, C. S. Kim and Jin U. Kang, “Highly stable, tunable dual-wavelength Q-switched fiber laser for DIAL applications,” IEEE Photonics Tech. Lett., vol. 16, 1277-1279, 2004.
Utkarsh Sharma, Chang-Seok Kim, and Jin U. Kang , "Tunable dual wavelength Q-switched fiber laser for DIAL applications," CLEO/QELS 2003, Paper: CThY7, Baltimore, Maryland (June 1-7, 2003).
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