A n apparatus that includes two optical parametric chirped pulse amplification stages has been built as a means of generating few-cycle, common-envelope (CE)-phase-stable, high-energy optical pulses. [CE phase is the phase of an optical carrier-signal waveform relative to the pulse envelope waveform. CE phase is an important property of fewcycle pulses, and CE-phase stability is essential in typical applications involving few-cycle pulses.] The apparatus can generate two-cycle (14-fs-duration) pulses at a nominal middle wavelength of 2 μm, and two-cycle (5-fs-duration) pulses at a nominal middle wavelength of 800 nm at a repetition rate of 1 kHz, without need for pulse compression by an external apparatus. The apparatus is intended for use in high-harmonic generation (HHG) of extreme ultraviolet (EUV) and soft x-rays in the near term, extending to production of attosecond EUV and soft-x-ray pulses in the longer term. Moreover, this apparatus is expected to enable exploration of generation of fewand single-cycle laser pulses over the wavelength range from 700 to 2.6 μm.

Two OPA Stages are combined with pulse-stretching, pulse-compressing, pulse-synchronizing, and ancillary optical amplification stages to generate two-cycle optical pulses.
The apparatus (see figure) includes an octave-spanning Ti-doped-sapphire laser that can be directly CE-phase-stabilized. The broadband (wavelengths from <570 nm to >1,140 nm) pulsed output of this laser is fed to an MgO-doped, periodically poled lithium niobate (MgO:PPLN) crystal having a poling spatial period of 13.1 μm, wherein CE-phase-stable broadband (wavelengths from 1.6 to 2.4 μm) seed pulses are generated by mixing [difference- frequency generation (DFG)]. The seed pulses are preamplified in a neodymium-doped yttrium lithium fluoride (Nd:YLF) regenerative amplifier, the output of which constitutes the pump pulses for two optical parametric amplifier (OPA) stages.

To ensure efficient transfer of energy from pump pulses to seeded signal pulses, it is necessary to match the duration of the signal pulses with that of the pump pulses. For 30 ps pump pulses and 14 fs signal pulses, this corresponds to a stretching factor of ≈2×103. The needed match is accomplished by means of an ultracompact stretcher compressor unit that includes a first stretching stage, preceding the first OPA stage, comprising a Brewster-cut, 150-mm block made of a commercially available high-optical-quality, optically isotropic fused-silica (quartzglass) product. After sufficient amplification of signal pulses in the first OPA stage, there is a second stretching stage (which also serves as a pulse-shaping and higher-order-dispersion-compensation stage) comprising a programmable acousto-optic dispersion filter based on a 45-mm TeO2 crystal. Following amplification in the second OPA stage, pulses are compressed in a 30-mm silicon block.

In addition to matching the durations of the pump and seeded signal pulses, it is necessary to synchronize the pulses because any temporal mismatch adversely reduces the efficiency of optical parametric amplification and distorts the signal pulses. The Ti-doped sapphire laser is synchronized with the Nd:YLF regenerative amplifier by injection seeding of this amplifier: A portion of the broadband DFG output from the MgO:PPLN crystal in the wavelength range of 1,050 ±10 nm is coupled into an yttrium-doped fiber amplifier (YDFA), the output of which seeds the Nd:YLF regenerative amplifier for generating 30-ps pulses having a nominal middle wavelength of 1,047 nm.

This work was done by Franz X. Kärtner of the Massachusetts Institute of Technology for the Air Force Research Laboratory.

This Brief includes a Technical Support Package (TSP).
Apparatus Generates CE-Phase-Stable Two-Cycle Optical Pulses

(reference AFRL-0022) is currently available for download from the TSP library.

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This article first appeared in the October, 2007 issue of Defense Tech Briefs Magazine.

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