Regenerative amplifier with frequency synthesizer

#The present invention relates generally to amplification of ultrashort seed pulses in a regenerative amplifier. The invention relates in particular to a regenerative amplifier including a gain-element pumped by optical pump pulses, and a Pockels cell controlled such that output pulses can be delivered at a rate or in a sequence different from but related to the rate at which optical pump pulses are delivered to the regenerative amplifier.

#Ultrashort pulses are generally considered to be pulses having a duration of about 10 picoseconds (ps) or less. One method of providing ultrashort pulses is to generate seed pulses using an ultrafast laser and amplify these seed pulses in a regenerative amplifier by a process generally referred to as chirped-pulse amplification (CPA). An ultrafast laser is generally regarded as a laser capable of delivering pulses at a rate from about 1 Hertz (Hz) up to 300 Kilohertz (KHz) or greater. In chirped pulse amplification, an ultrashort “seed” pulse is positively “chirped” (frequency modulated). The positive chirp stretches the duration of the pulse, thereby reducing the peak power in the pulse. The pulse is then amplified in a regenerative amplifier. Reducing the peak power in the seed-pulse enables the seed pulse to be amplified by several orders of magnitude, while keeping the peak power in the amplified pulse below levels that can cause damage or undesirable nonlinear optical effects in optical components of the amplifier. Following amplification, the amplified pulse can be negatively chirped to reduce the duration and increase the peak power of the pulse.

#Gain-media used in the regenerative amplifiers include Ti:sapphire (Ti:Al2O3), alexandrite (Cr:Be2O3), coloquirites (for example Cr:LiSAF and Cr:LiCAF), neodymium-doped YAG (Nd:YAG), and ytterbium-doped YAG (Yb:YAG) in bulk (crystal) form. All of these gain-media exhibit thermal lensing effects when pumped at high average power levels, for example, at an average pump power of 20 Watts (W) or greater. Typically, in commercially available regenerative amplifiers, pumping of the gain-medium is effected by pulses delivered by a Q-switched, pulsed laser. Delivery of the pump pulses is synchronized with delivery of the seed pulses, and the rate of delivery of output pulses is determined by the rate of delivery of pump pulses. Accordingly, if the output pulse delivery rate is changed, the pump pulse delivery rate is also changed. Changing the delivery rate of the pump pulses changes the average power delivered to the gain medium. This changes the thermal load on the gain-medium and correspondingly the thermal lensing effect in the gain-medium. Changing thermal lensing effects can lead to inconsistency of beam quality in the output pulses. This can make it difficult to focus the pulses to a spot of consistent dimensions. There is a need for a regenerative amplifier in which output pulses can be provided at different frequencies while minimizing thermal lensing changes in the gain-medium.

#The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the present invention.

#FIG. 1 schematically illustrates a preferred embodiment of a laser system in accordance with the present invention including a regenerative amplifier optically pumped by a pump laser and including a Pockels cell, a seed pulse laser providing seed pulses and electronic pulses corresponding to the seed pulses, a frequency divider for dividing the frequency of the electronic pulses and delivering the frequency-divided electronic pulses as synchronizing pulses for triggering the pump laser, and a pulse selector arranged to also receive the synchronizing pulses and to control the Pockels cell to deliver amplified output pulses from the regenerative amplifier in a sequence thereof related to the frequency of the synchronizing pulses.

#FIGS. 2A-D are graphs schematically illustrating one temporal relationship of the electronic pulses, synchronizing pulses, Pockels cell activation status and output pulses in the regenerative amplifier of FIG. 1, with output pulses delivered in a continuous train.

#Referring now to the drawings, wherein like features are designated by like reference numerals, FIG. 1 schematically illustrates a laser system 20 in accordance with the present invention. It should be noted here that apparatus for temporally stretching seed pulses prior to amplification and temporally compressing the amplified seed pulses is not shown in FIG. 1 to simplify illustration. Such stretching and compressing is well known in the art and a description thereof is not necessary for understanding principles of the present invention. Laser system 20 includes a regenerative amplifier 22 having a resonator 24 terminated by mirrors 26 and 28. Resonator 24 includes a gain-medium or gain-element 30, a polarizing beamsplitter 32 and a Pockels cell 34. Gain-element 30 is preferably formed from a material selected from the group consisting of Ti:Al2O3, Cr:Be2O3, Cr:LiSAF, Cr:LiCAF, Nd:YAG, and Yb:YAG. This preference, however, should not be construed as limiting the present invention. Pockels cell 34 is operated by a driver or voltage source 36 via electrodes 38 and 40. Gain-element 30 is optically pumped by pump light 42 delivered by a pump laser 44. Operation of pump laser 44 is controlled by a controller 45. Pump light 42 from the pump laser is directed by a turning mirror 46 through a lens 48 that directs the pump light through mirror 26 and focuses the pump light into gain-element 30.

#Seed pulses to be amplified are generated by a seed pulse laser. Seed pulse laser 50 is passively mode-locked and delivers seed pulses at a pulse repetition frequency (PRF) F of about 100 Megahertz (MHz) or less. Radiation in the seed pulses is plane polarized. A beamsplitter 52 directs a portion of each seed pulse to a detector 54, for example, a photodiode. Detector 54, accordingly, generates a signal S1 including a train of electronic pulses at the frequency F of the seed pulses. The signal is transmitted to a frequency divider circuit 56. Frequency divider circuit 56 selects every Nth pulse (where N is an integer greater than 1) from the seed-pulse-frequency signal S1 to provide a second signal S2 including a train of electronic pulses at a PRF of F/N. Signal S2 can be defined as a synchronizing signal, and pulses thereof are used to synchronize two operations in laser system 20.

#A first of these operations is triggering pump laser 44 to deliver pump pulses to gain-element 30. Signal S2 is delivered to pump laser controller 45 which operates pump laser 44 to deliver a pump-pulse in response to every pulse in the signal, i.e., the PRF of pump pulses is F/N. Controller 45 is arranged to trigger the pump-pulses such that a seed pulse to be amplified arrives in gain-element 30 coincident with the leading edge of the pump pulse. The pump-pulse frequency F/N is the maximum frequency at which seed pulses can be amplified.