Ultrafast Laser: Octavius Laser Series
The Octavius series of ultrafast lasers provide, depending on the model, octave-spanning spectra with pulse durations less than 6 fs and up to 750 mW of output power at a repetition rate of 1 GHz or 85 MHz. The octave-spanning spectrum allows for carrier offset frequency stabilization. With an install base of more than 20 systems worldwide in applications reaching from calibration of Echelle spectrometers for exo-planet search (Ch. Li, et al.Nature,Vol 452, 2008) to seeding of OPCPA systems (Hemmer et al. Proc. SPIE, Vol. 7578, 757818 (2010)) or single molecule spectroscopy the Octavius has proven itself as a state-of the art and reliable research tool.
Technology
The Octavius is a soft aperture Kerr-lens mode locked (KLM) Ti:Sa laser emitting the shortest possible pulses. The laser cavities incorporate octave-spanning Dispersion Compensation Mirror (DCM) pairs, which are required for smooth, high-precision group delay control over an octave-wide bandwidth. The design of these unique mirror pairs requires more than 200 coating layers.
The spectral bandwidth required to provide sub-2-cycle pulses is greater than the gain spectrum of the Ti:Sa laser medium. The high intensity of the intra-cavity circulating laser pulses and a carefully set temporal focus are used to create the necessary additional bandwidth through Self-Phase Modulation (SPM) in the Ti:Sa crystal. These uniquely-engineered cavities exploit the interplay between strong KLM in the crystal and the dispersion generated by the DCM pairs to provide stable ultra-short pulses that are not possible with standard cavity designs. The spectral content, at one octave of bandwidth, enables long-term stability and a robust f/2f carrier envelope offset frequency beat detection with more than 30 dB signal-to-noise ratio in a 100 kHz bandwidth.
Mechanical Design
Ease of use and mechanical robustness were at the forefront of the design for the Octavius lasers. Unlike typical laser designs, which use traditional translation stages for tuning and alignment, the alignment of the Octavius-1G is controlled using a unique flexure stage design that eliminates the various materials typically used for springs, bearings, and frames while still maintaining unprecedented accuracy and repeatability. Custom tooling and fixtures guarantee a stress-free machining during production and therefore minimize drifts and misalignment of the laser cavity caused by stress relaxation. Two independent, all-solid-state temperature controllers for the laser cavity and the crystal, respectively, further increase the long-term stability of the cavity alignment. The oscillator requires no water cooling, which reduces short-term noise due to vibration caused by turbulent water flow.
The crystal and pump lens positions, as well as the curved mirror separation, are all tracked using optical linear position encoders that offer ~1 μm resolution. Therefore, it is easy to reestablish modelocked operation, and switching between multiple working configurations becomes seamless.
Acknowledgments
The unique DCM pairs incorporated in the Octavius were developed in close collaboration with Franz Kärtner, a professor at the Massachusetts Institute of Technology and IQE member of the board.