CLEO '99 Tech Briefs: Pulse-Front-Matched OPA Produces Record Short Pulses
Tilted pump configuration with noncollinear optical parameteric amplifier produces 4.7-fs, 5-µJ output pulses.
By: Kristin Lewotsky
BALTIMORE, MD—The Conference on Lasers and Electro-Optics (CLEO '99) convened this week with flocks of scientists and researchers presenting their latest development. It just wouldn't be CLEO if the technical sessions didn't feature at least one paper touting the world's shortest pulses, and indeed, this year's conference didn't disappoint, as researchers from the University of Tokyo, in collaboration with researchers from Hamamatsu Photonics (Shizuoka, Japan), logged in with a non-collinear optical parametric amplifier system capable of producing jaw-droppingly short pulses of 4.7 fs (3.5 fs for sech2 data fit).
This work appears to just nose out a recently-reported titanium-doped sapphire (Ti:sapphire) system demonstrated by a group from the Massachusetts Institute of Technology (MIT; Cambridge, MA) that produced pulses estimated between 4.3 and 5.4 fs in duration (see Tech Briefs: Ti:sapphire System With Advanced Dispersion Control Produces Sub-5-fs Pulses).
Noncollinear OPAs
The temporal duration of a pulse is inversely proportional to the spectral bandwidth; in other words, the broader the bandwidth, the shorter the duration of the final pulse. Noncollinear optical parametric amplification allows group velocity matching between the signal and idler beams of the system—equivalent to achromatic phase-matching with the spectral angular dispersion of the idler. This effect produces a extremely broad gain bandwidth, and hence the potential for extremely short pulses.
According to the University of Tokyo researchers, pulse-front tilting caused by the noncollinear interaction broadened the pulse width of the noncollinear optical parametric amplifier (NOPA) in previous experiments. Led by A. Shirakawa, the group solved this problem by using a tilted pump configuration, achieve pulse-front-matched (PFM) amplification and shortened pulses.
System design
In the experimental system, a Ti:sapphire regenerative amplifier pumped a Type I beta barium borate (BBO) crystal (see Figure 1, at top). Using a noncollinear internal angle of 3.7° and a crystal angle of 31.5°, the researchers obtained pulses with 160-THz bandwidths. A 45° fused-silica prism and a telescope induced PFM interaction, leading to transform-limited, post-compression pulses.
Pulse compression took place in two phases: pre-compression prior to amplification, followed by post-amplification compression. The pulse compressor incorporated ultra-broadband chirped mirrors (UBCMs) designed to induce group delay while performing higher-order dispersion compensation. Consisting of a 40-layer stack of titanium oxide (TiO2) and silicon oxide (SiO2), the components provide –40±15 fs2 of group delay dispersion (GDD) between 600 and 700 nm.
The group used an UCBM pair with a single round trip for pre-compression, and a mirror pair with three round trips for post-compression; the pulse compression unit also incorporated a 45° fused silica prism pair, and chromium-coated beamsplitters in a fringe-resolved autocorrelator.
As with the MIT results, the pulse duration varies depending on the fit used. In this case, a sech2 fit yields a pulse duration of 3.5 fs, while a Fourier transform fit yields a 4.7 fs duration (see Figure 2).
References
1. Shirakawa et. al., "Visible Sub-5-fs Pulse Generation by Pulse-Front-Matched Optical Parametric Amplification," paper #CTuF4, Proceedings of the Conference on Lasers and Electro-Optics (CLEO '99; Baltimore, MD), May 23-28, 1999.
2. Shirakawa et. al., "Sub-5-fs visible pulse generation by pulse-front-matched noncollinear optical parametric amplification," Applied Physics Letters 74[16], pp. 2268-2270, (1999).