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Broadly tunable fiber laser system has been demonstrated at room temperature.
The IR fiber laser system consists of two high peak power individual pulse fiber lasers
and their difference frequency generation (DFG) in Periodically Poled Lithium Niobate
(PPLN) nonlinear optical crystal. Both lasers were operating at 20 kHz Pulse Repetition
Rate and 200 ns pulse duration with diffraction limited beam quality. The first laser is a
CW, < 16 GHz spectral bandwidth, tunable in 1050-1081 nm spectral range and Acousto-
Optically extra cavity amplitude modulated Yb fiber laser ring oscillator - high power
two stage amplifier which produced 280 mW of average and 70W of Peak power,
respectively. The second laser is a DFB CW, single 1556 nm wavelength, < 1MHz
spectral bandwidth and extra cavity amplitude modulated Er fiber laser oscillator - high
power single stage amplifier with 160 mW of output average power and 40W of peak
power. Synchronized pulses from two fiber lasers have been combined and fiber coupled
into a single polarization maintained fiber using a fiber WDM combiner and then fibercoupled
into the NovaWave Technologies, commercial DFG laser module which
employed 50 mm PPLN crystal. The DFG stage of the system produced tunable radiation
in 3236.4-3545.4 nm spectral range (309 nm). The difference frequency generation has a
9 mW average power, 20 kHz pulse repetition rate and 200 ns pulse duration which
corresponds to 2.25 W of peak power. The demonstrated pulse DFG conversion
efficiency is 0.2 W/W2 (20%/W) which is ~ 100 times higher than that of CW operation.
Further scaling of IR laser power was limited by optical damage of PPLN crystal and
fiber lasers combining optics. Using a PPLN-MgO crystal and additional fiber laser
amplifier stages based on Large Mode Area gain fibers is expected to allow us to achieve
damage free difference frequency generation with up-to 100 mW of average power and
peak power of up to 25 W.
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