Proof of concept of a Brillouin-LIDAR for Remote Sensing of Oceanic Temperature Profiles

A remote sensing technique measuring temperature profiles of the upper layer would provide valuable input to climate studies, wheather forecasts and oceanography in general [1]. Originally proposed by Hirschberg et al. [2], later demonstrated by Guagliardo et al. using pulsed lasers [3] and Hirschberg et al. using cw-lasers [4], the working principle is an extension to common airborne lidar bathymetry [5]. Light sent into the Ocean undergoes a Brillouin shift leading to additional frequency components in the back-scattered light. Specifically, the injected laser pulses are scattered off of moving density fluctuations in the water; due to the Doppler shift, the spectrum of the backscattered light is shifted to the red and blue of the original light frequency. Typical frequency shifts to be expected are in the order of 7-8 GHz located symmetrically around the wavelength of the injected laser pulses. The shift is proportional to the local speed of sound and the index of refraction, and thus a function of temperature [4,6]. Therefore, temperature information can be extracted. The temperature profile can be deduced by the timing information.

Recent developments in laser and sensor technology [7,8] brought practical systems within reach and has spurned renewed interests in such measurements [9]. However, the requirements on the complete sensor system are rather stringent: (1) Since operation from a mobile platform is intended, the sensor has to be compact, light-weight, insensitive to vibrations and exhibit relatively low power consumption. (2) In order to resolve the Brillouin-shift the laser source has to produce relatively high energy ns-pulses, preferentially near-Fourier bandwidth limited. (3) The laser radiation should be close to the absorption minimum of water, e.g. between 380 and 550 nm [10]. (4) The receiver unit must exhibit a high light gathering power, and be able to resolve the Brillouin shift.

In our approach, the intended light source is comprised of a three-stage pulsed Yb-doped fiber amplifier frequency doubled into the green spectral range [11]. The detector is based on an excited state Faraday anomalous dispersion optical filter (ESFADOF) approach [12,13].

Yb-doped fiber amplifiers can be operated in a wide spectral range between 1015 nm - 1100 nm [14], which makes it easier to match the laser with a receiver based on a molecular or atomic edge filter. Using a seeding technique near Fourier transform limited pulses can be generated [15]. Our current setup delivers Fourier transform limited pulses of 10 ns duration with a repetition rate of up to 5 kHz. After 62 dB of amplification 516 $\mu$J of pulse energy at 1064 nm have been generated. Frequency doubling is achieved with an efficiency of 26.7 %. To our knowledge these are the highest values reported for a fiber amplifier operated at theses temporal and spectral parameters. The onset of stimulated Brillouin-Scattering inside the fiber prevents so far higher output energies. However, recenty fibers became available which will allow output pulses with energies in the mJ-range in the near future.

Using this laser system we succeeded in performing the first temperature measurements employing a frequency-doubled fiber amplifier [16]. In an extension of our scheme we performed first range resolved measurements in our laboratory setup giving a proof of concept for a Brillouin lidar system. In these measurements we used a Fabry-Perot etalon as the detector for the Brillouin shifts, which cannot be used in a practical implementation due to restrictions in light gathering power and susceptibility to vibrations.

ESFADOFs allow the implementation of flexible edge filters [17]. Edge filter with steep transmission edges in the regions of interest convert small frequency shifts in the Brillouin lines to a relatively large change of transmission [18]. By exploiting the anomalous dispersion in the vicinity of an atomic transition, their filter characteristics can be tuned over a wide range and can be tailored to meet the required frequency shift of 7-8 GHz. In addition the filter shows excellent suppression of daylight.

In this contribution, we will present our recent progress concerning the laser system, our range resolved measurements as well as progress of the implementation of our ESFADOF setup.

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