Ultrashort Pulse Laser Technology

Ultrashort pulse lasers are an enabling technology for a whole range of fundamental and applied research areas as well as industrial applications. The pulse width typically ranges from picoseconds down to few femtoseconds in duration. To generate such short pulses typically multiple longitudinal modes in a laser resonator are excited, such that a pulse much shorter than the cavity roundtrip time is created inside the laser resonator. The shortest pulse that can build up is then limited by the gain bandwidth of the amplifying medium. We show the progress in generating short optical pulses from various laser materials. Over the last decades, our group has made major contributions to both the understanding of the pulse generation mechanisms involved in the various types of lasers and the emerging technologies leading to shorter and shorter pulse durations.

New techniques that improve the robustness of both Ti:Sapphire and Er-fiber laser systems have been developed. The studies aim at overcoming the difficulties in achieving mode-locking and maintaining operation in an extreme regime (e.g., ultrabroadband, high repetition rate, and high intensity) where the lasers might suffer from huge nonlinearity, high mode-locking threshold, or thermal damage. In Ti:sapphire lasers, we have demonstrated a novel gain-matched output coupler for minimizing the required nonlinearity as well as the threshold for broadband modelocking, which allows us to operate the laser in a more robust parameter range with less spatial and temporal beam distortions. In high repetition rate Er-lasers, the thermal damage on the saturable Bragg reflector responsible for mode locking has been resolved by avoiding direct contact of the hot fiber core with the saturable Bragg reflector, which greatly increases the durability of the laser. With a demand for even higher repetition rates, an integrated interleaver with thermally tunable power-splitting and delay stages has been demonstrated. Based on both types of laser systems, more powerful tools for exploring science at different power levels and wavelength ranges are also being developed and studied. For example, we recently performed studies on the noise characteristic of optical parametric chirped pulse amplification (OPCPA) in order to resolve the problem of depletion of pump energy by superfluorescence noise in the case of low seed energy, ultrabroad signal bandwidth, and high desired amplified signal pulse energy. In addition, a cavity-enhanced optical parametric amplification (C-OPCPA) technique has been studied and proposed for extension the gain bandwidth of parametric amplifiers while maintaining high conversion efficiency. A highly efficient and broadband nonlinear frequency conversion technique using Cherenkov radiation from a photonic crystal fiber has also been proposed as a means to push the limits of optical pulse generation using low-energy pulses at wavelengths at which no laser media for direct emission are known. We believe these techniques can enable new possibilities in many research areas.