Narrowband diamond Raman lasers for remote sensing of trace gases
Final Report Abstract
A fundamental advantage of lasers is their ability to produce a large number of photons in a single optical mode, yet this is achieved in only a minor fraction of devices due to the instability mechanism called spatial hole burning. Mitigation using intracavity dispersive elements, injectionseeding, ring lasers or short gain media have been developed in the past decades; however, these strategies complicate design and restrict performance range. Compact and powerful singlelongitudinal mode (SLM) lasers emitting at specific absorption lines in the near-infrared spectral region are required for remote sensing of atmospheric trace gases. In the course of the project, it was shown that the nonlinear optical process of stimulated Raman scattering provides a spatial hole burning free gain which enables the generation of SLM output that is intrinsically stable. The underlying mechanism was exploited for the development of two compact Raman laser configurations which were realized as external standing-wave cavities containing CVD diamond as Raman-active gain medium. Efficient frequency conversion of a tunable Yb-fiber-amplified laser emitting around 1064 nm to the first- and second-order Stokes components yielded Watt-level SLM output around 1240 nm and 1486 nm wavelength, respectively. The first Stokes diamond Raman laser provided tunable SLM output around 1240 nm with power up to 4 W and frequency stability of 80 MHz without use of any mode-selective elements or cavity length stabilization. Thermal effects were found to play an important role in longitudinal mode competition since they introduce a change in Raman shift of several hundreds of MHz/K and alter the optical cavity length through thermal expansion and thermo-optic effects. A numerical model was developed to simulate the complex coupling between Stokes power and thermally-induced path length changes. Active stabilization of the cavity length is anticipated to enable SLM output with greater frequency stability and output power. Selective amplification of the second-order Stokes component in a second Raman laser configuration produced narrowband output in the eye-safe spectral region around 1486 nm, where the implementation of a volume Bragg grating, which provided optical feedback into the Raman cavity, improved the frequency stability to 40 MHz. The Raman laser was employed for absorption measurements of ammonia as well as of water vapor in the ambient air. Simulations based on spectroscopic parameters line from HITRAN database showed good agreement with the experimental detection of water vapor, thus confirming the feasibility of the developed concept in terms of LIDAR applications. In conclusion, the results foreshadow a novel approach for greatly extending the power and wavelength range of SLM sources which is of particular interest not only for remote sensing applications, but also for laser cooling and gravitational-wave detection as well as for the fields of nonlinear optics and spectroscopy in general. Therefore, the outcomes of the project are likely to be more far-reaching than anticipated in the early stages of the project. It is also assumed that the interest will be heightened since the advance has been achieved using the unique optical material diamond which offers enormous power scalability due to its beneficial thermal properties.
Publications
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“Exploiting Spatial-Hole-Burning-Free Raman Gain to Realize High-Power Single-Longitudinal Mode Oscillators,” OSA Technical Digest, paper ATh3A.2 (2015)
O. Lux, S. Sarang, O. Kitzler, and R. P. Mildren
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“High-power single-longitudinal mode and tunable diamond Raman laser at 1240 nm,” SPIE Photonics West, LASE conference, Solid State Lasers XXV, paper 9726-51 (2016)
S. Sarang, O. Lux, O. Kitzler, R. Williams, A. McKay, and R. Mildren
