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5fs high power OPCPA laser for attosecond pulse production
Recently there has been growing interest in the generation of intense femtosecond pulses with durations in the few cycle regime. Optical Parametric Chirped Pulse Amplification (OPCPA) provides a method for scaling ultrashort pulses to much higher energies than are available directly from conventional CPA laser systems. In OPCPA, a short pulse is chirped before being amplified parametrically in a non-linear crystal. OPAs have extremely large gain bandwidths that can allow amplification of the full bandwidth of a 5 fs pulse, removing the need for post-amplification spectral broadening and enabling orders of magnitude higher pulse energies. Minimal thermal deposition in the OPA medium allows for high average power operation and high optical quality due to the absence of thermal lensing.
High average power OPCPA requires a high average and peak power pump laser and efficient conversion of pump to signal. We are building a diode-pumped Nd:YLF amplifier which will produce pulse energies up to ~200mJ with pulse durations of ~40ps, at repetition rates up to 1 kHz.
We have designed a stretcher and compressor system capable of stretching a pulse with 400 nm bandwidth out to 20 ps and then recompressing down to 5 fs. The 20 ps chirped pulse will be amplified in two broad bandwidth OPA stages.
When complete, this system will be integrated into the attosecond XUV beamline at Imperal college, enhancing the capabilities of that experimental facility.
High average power pump laser
While the pulse energy and peak intensity of the pump laser are not insignificant, the predicted average powers of close to 200W have been the more stringent specifications and have dictated certain elements of the laser design. We have chosen ND:YLF as the laser medium due to its low non-linear index, low thermal lensing and natural birefringence. The laser takes the form of a front end 1 kHz Nd:YLF oscillator providing 1 mJ 40 ps pulses at 1047 nm, followed by a home-built diode pumped multipass amplifier.
Detailed analysis of the operation of the amplifier has been addressed to enable achievement of a maximum pulse energy without exceeding the limits set by self-focusing and thermal fracture. The final design of our amplifier uses five, 1 kW laser diode arrays from Dilas to pump a 100 x 5 mm deep etched Nd:YLF rod at 15 W cm-1.
Recent tests on the complete amplifier system, with 3-pass amplification and a non-optimum beam diameter, have produced output pulse energies of up to 130mJ Further work is under way to achieve the design energy of >200 mJ.
Optical parametric chirped pulse amplification
Extensive modelling has be undertaken to optimise the design of our OPCPA stages. The overall system must achieve a saturated gain of ~107 with a reasonably flat profile over a 400nm bandwidth. Most modelling to date has centred around a two stage amplifier setup with single-pass small signal gain of ~106 in the first amplifier and 104 in the second amplifier.
Our simulations show that LBO is a good candidate for the OPA medium, with a gain bandwidth of 700-1150 nm, corresponding to a pulse duration of 5.8fs. Predicted extraction efficiency from the pump is close to 30%, which will give few cycle pulses with energies up to 10mJ after compression.
We are now looking in detail at the exact modelling of our current setup, and considering adding a third stage to the amplification setup. We will have initial data from gain testing by the end of February 2006, and this will further guide our OPA stage design.
We have developed a design for a stretcher compressor system capable of stretching an 850 nm pulse with 200 nm FWHM (~400 nm edge-to-edge) bandwidth to 20 ps and recompressing to 5 fs. To achieve the fine balance of positive and negative dispersive elements we have designed a system which includes a prism stretcher, grating stretcher, grating compressor and a programmable acousto-optic phase filter (Dazzler). Testing of this system has shown we can stretch the unamplified pulses from our current oscillator to 10ps (bandwidth 250nm edge to edge) and recompress them to within 0.5fs of their 11 fs transform limit.
Novel features of this design include the use of transmission gratings in both the stretcher and compressor which have extremely high efficiency over the full bandwidth of the pulses. Also, we have chosen to use an 8 element custom designed lens in the stretcher, as it is extremely difficult to design a reflective stretcher with low chromatic aberrations over our large bandwidth.
Work will continue on developing and building the full OPCPA laser system, with amplified low rep rate pulses available by the end of April 2006. As these will be from our 11fs oscillator we will then work on producing a spectrum broad enough to seed the full gain bandwidth of the OPA stages, while in tandem with this the pump laser is brought up to full rep rate operation. As soon as the system is operational it will be moved to Imperial College and integrated into the beamline there as a drive laser for high harmonic generation to produce attosecond pulses.
Publications and conference presentations arising from this work
 A 5 fs OPCPA laser system for attosecond pulse production E Springate, Y Tang, P Bates, IN Ross, RA Smith, JWG Tisch and JP Marangos, CLEO Europe, Munich, Germany (June 2005).
 A 5 fs high average power OPCPA laser system for attosecond pulse production P Bates, E, Springate, Y Tang, IN Ross, RA Smith, JWG Tisch and JP Marangos, UK High Power Laser Users Meeting, Abingdon, UK, (Dec 2005).
 High-power Nd:YLF Amplifier Development , Y Tang, M Divall, IN Ross, E Springate and GJ Hirst, CLF Annual Report , (2004 - 2005)
 Design of a Stretcher-Compressor System for High Energy 5 fs Pulses , P Bates, E Springate, Y Tang, IN Ross, CLF Annual Report (2004 - 2005.)