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Monolithic Ytterbium doped fiber amplifier

For the development of high power optical parametric chirped pulse amplifiers (OPCPA), the synchronization of the seed and pump source is very important. The most advanced systems use all-optical schemes for the pump-seed synchronization. Because of their potential to deliver very high spatial quality output beams, figer lasers are very attractive to use as a pump/seed front-end for high power OPCPA systems. However, the most attractive feature of fiber lasers, their potential of eliminating the need for optical alignment, is lost in almost all fiber laser demonstrated delivering the required level of output power for such front-ends. The aim of this project is to fill this void, to develop a monolithic fiber laser delivering tens of microjoule pulses with good spatial and temporal characteristics.

 

1. Monolithic fiber chirped pulse amplifier

Below a schematic sketch of our fiber laser is shown.

 

Fig.1: Schematic of our monolithic Ytterbium-doped fiber chirped pulse amplifier.

The output pulses from an Ytterbium-doped fiber oscillator are stretched in 480 m of polarization maintaining single mode fiber. After stretching, the pulses are amplified at the full repetition rate of the oscillator in the first single mode pre-amplifier. After the first pre-amplifier, the repetition rate is reduced from 80 MHz to 100 kHz with an accousto-optic modulator (AOM). Then the pulses are amplified further in a second single mode pre-amplifier. After the second pre-amplifier, a second AOM is used to reduce the background due to amplified spontaneous emission. Using a mode-field adaptor, the pulses are lounched into the final large mode area amplifier stage. The output pulses are then compressed in negative dispersion compressor, using grisms, consisting of prisms and reflection gratings.

 

2. Carrier envelope phase stable OPA seeded by a monolithic fiber chirped pulse amplifier

In a proof-of-principle experiment we have build a self phase stabilized optical parametric amplifier (OPA) seeded by our monolithic fiber laser.

 

Fig.2: Schematic representation of the passively stabilized OPA. On the right hand side the beam profile of the signal beam after the second OPA stage is shown.

The compressed pulses from the fiber laser are frequency doubled in a thin BBO crystal. The 520 nm pulses are split in two, one part is used to generate a white-light continuum as a seed for the first OPA stage, and the second part is used as a pump for the first OPA stage. Since the pump and seed are derived from the same pulse, and in an OPA the idler absorbs the difference of the pump and seed phase, the generated idler wave is self phase-stabilized. The idler can be further amplified in a second OPA stage, pumped by the remaining fundamental light after the second harmonic generation. Below the tunability and phase stability of the OPA output is demonstrated.

 

Fig.3: (a) Tunability of the signal wavelength of the first OPA stage. (b) Idler spectra measured for two different signal wavelengths, and spectral broadening of the phase stable idler.

 

Fig.4: Phase stability of the idler.

 

 

Last update Nov. 21, 2009