Patent ID: 6917631

Claim:
A source of pulses of coherent radiation at a wavelength of approximately 1 μm, comprising: a laser oscillator system operable to generate laser pulses at a defined repetition rate; and a cascade of sequentially arranged optical amplifiers arranged to receive and amplify the laser pulses, each optical amplifier comprising an amplifier pump source for producing amplifier pump light and an amplification medium comprising Yb 3+ -doped optical fiber having a cladding and a core and arranged to receive the amplifier pump light and the laser pulses, the cascade comprising a first pre-amplifier followed by a second pre-amplifier followed by a power amplifier, the amplification media of the pre-amplifiers comprising single mode optical fibers arranged to receive amplifier pump light into the core of the optical fibers, each pre-amplifier further comprising a Yb 3+ -doped indirectly pumped optical fiber coupled to the optical fiber of its amplification medium to absorb amplified spontaneous emission arising from the amplification medium, and arranged so that it does not receive amplifier pump light, and the amplification medium of the power amplifier comprising a large mode area optical fiber having an inner cladding and arranged to receive amplifier pump light into the inner cladding.