Polovodičový frekvenčně stabilizovaný laser jako hlavní oscilátor pro Prague Asterix Laser System (PALS)

Petr Jedlicka, Ondrej Cíp and Josef Lazar

The design of a new laser master oscillator is a result of co-operation with the PALS Research Centre in Prague, Czech Republic. The principal experimental resource at PALS is the Asterix IV high-power iodine laser system. This instrument was developed at the Max Planck Institute for Quantum Optics in Garching, Germany, and with the latest upgrade in 1991 provided an irradiation facility at the 1 kJ energy level until May 1997. It has been exploited by the wide international research community within the European Large-Scale Facilities scheme.

A large-scale reconstruction of the power laser is now underway, which aims to increase the peak optical power output from the present 2 TW to 5 PW. This can be achieved by implementation of the technique known as OPCPA (Optical Parametric Chirped Pulse Amplification). This became possible when ultrashort pulse generating mode-locked lasers appeared. The final duration of the laser pulses will be reduced by several orders and the peak power increased dramatically by means of non-linear optical mixing of femtosecond pulses generated by a Titanium:Sapphire laser and power pulses from the iodine laser.

Our part of this project was to assemble a new "seed laser", the first low-power, high-stability continuously working laser that will stand at the front end of the entire power laser system, a cascade of optical amplifiers. The pulses will be generated by an electro-optic shutter.

The key demand for optical stability arises from the relatively narrow spectral profile of the iodine amplifying media of the optical amplifiers, which had to be kept within a range of tens of MHz of the optical frequency. From the viewpoint of fundamental laser metrology this is not such a strict requirement, but the laser here had to be a fully autonomous system, completely maintenance-free and capable of operating in the harsh environment of the power pulsed laser full of strong electromagnetic interference arising from the huge discharging capacitors and pumping flashlamps. It can be considered extremely lucky that the desired wavelength of 1315 nm falls within the range of optical telecommunication bands, which allowed us to use a commercially available laser diode with a DFB (Distributed FeedBack) structure featuring single-frequency operation, narrow line width and easy current and temperature tunability of the emission wavelength.

The set of subsequent optical amplifiers is intended to begin with fibre-coupled amplifiers, so the design of our optical set-up was fibreoptic from the very beginning. Output light from a laser diode with an integrated Faraday optical isolator was split by a fibre coupler by a ratio of 90 % to 10 %. The smaller part of the output power was used for frequency stabilization of the laser, while the larger part comprised the useful output. The 10 % of the light from the coupler was collimated into an absorption cell filled with pure iodine. We used the same transition in the dissociated iodine that drives the power amplifiers to derive the signal for frequency stabilization. The dissociation of iodine molecules into atomic iodine was achieved in our cell by thermal dissociation. The cell was heated and thermostatized at 600 oC.

The control electronics are fully digital and based on powerful microcontrollers that calculate the derivative signal of the absorption lines, generate modulation signal, perform some digital filtering and produce a control signal for the servo-loop of the laser frequency. With a narrow "locking range" of the absorption lines we proposed a system of two servo-loops, where the laser itself is locked by thermal tuning to a certain temperature and a slow frequency drift is controlled with respect to the dispersion signal of the detected absorption line. An autonomous algorithm for self-diagnostics and selection of the proper absorption line to lock the laser is applied.

The laser system has already been successfully tested several times at the PALS laboratory under real conditions and is now prepared for final implementation

The thermostatized iodine cell (top) with electronics covering the temperature stabilization, laser diode drivers and closed -loops for frequency locking to selected absorption lines.