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The double-clad (DC) fiber converts the poor beamquality of high-power large-area pump diodes from the 1st cladding to laser light at another wavelength guided in an active single-mode core. High-power coherent MOPA (Master Oscillator Power Amplifier) sources (several 10W CW or several 100W in pulsed regime) will soon be achieved. Unfortunately it also brings nonlinear effects which quickly impairs output signal distortions. Stimulated Brillouin scattering (SBS) and optical parametric amplification (OPA) have been shown to be strong limitations.
Based on amplifier modeling and experiments we discuss the performances of these sources.
This design makes the system compacter and achieves a 200g system weight. In addition to its low volume, the fully fibered architecture allows designing a building block rangefinder with the collimator sub-system on one side and the laser and electronics cards module on the other side. Both are linked up by only an optical fiber. This kit format enables the rangefinder to better fit in any available space in higher level systems such as gimbals and multi-function imagers. Besides, no alignment is needed, and no parallax error is possible: the alignment between channels is guaranteed by design over the whole range.
The emission/reception channels of the first prototype has a 28mm diameter 80mm focal length lens, and a 1.55μm 100μJ pulsed laser firing in a burst mode. The rangefinder is set in a class 1 configuration, and measures at 1Hz. The achieved Extinction Ratio is 30dB, which is equivalent to a range on NATO targets of 7km. The achieved ER being class 1M at 5Hz is even 32dB, which is equivalent to a range of 8.5km on NATO targets.
More configurations are reported in this article with their associated performance.
To reach long range within a short acquisition time, coherent wind Lidars require high power (~kW), narrow linewidth (few MHz) pulsed laser sources with nearly TF limited pulse duration (~1μs). Eyesafe, all-fiber laser sources based on MOPFA (master oscillator, power fiber amplifier) architecture offer many advantages over bulk sources such as low sensitivity to vibrations, efficiency and versatility. However, narrow linewidth pulsed fiber lasers and amplifiers are usually limited by nonlinear effects such as stimulated Brillouin scattering (SBS) to 300W with commercial fibers. We investigated various solutions to push this limit further. For example, a source based on a new fiber composition yielded a peak power of 1120W for 650ns pulse duration with excellent beam quality. Based on these innovative solutions we built a Lidar with a record range of 16km in 0.1s averaging time.
In this proceeding, we present some recent results obtained with our wind Lidars based on these high power sources with record ranges. EDR measurements using the developed algorithm based on structure function calculation are presented, as well as its validation with simulations and measurements campaign results.
For the sake of demonstration, this new technique is experimentally applied twice for continuous wave second-harmonic-generator (SHG) combination: i) combining 2 SHG of 1.55-μm erbium-doped fiber amplifiers in PPLN crystals generating 775-nm beams; ii) combining 2 SHG of 1.064-μm ytterbium-doped fiber amplifiers in LBO crystals generating 532-nm beams. Excellent CBC efficiency is achieved on the harmonic waves in both these experiments, with λ/20 and λ/30 residual phase error respectively.
In the second experiment, I/Q phase detection is added on fundamental and harmonic waves to measure their phase variations simultaneously. These measurements confirm the theoretical expectations and formulae of correlation between the phases of the fundamental and harmonic waves. Unexpectedly, in both experiments, when harmonic waves are phase-locked, a residual phase difference remains between the fundamen tal waves. Measurements of the spectrum of these residual phase differences locate them above 50 Hz, revealing that they most probably originate in fast-varying optical path differences induced by turbulence and acoustic-waves on the experimental breadboard.
Recent advances in very highly nonlinear chalcogenide photonic crystal fibers and their applications
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