A styrene copolymer of 4-vinylbenzyl thiocyanate (PST-co-VBT) was employed as recording material for optical interference patterns with periods Λ < 1 μm. Using lower intensity laser irradiation (4 mJ cm-2, λ = 266 nm), refractive index gratings were produced in PST-co-VBT by an UV-induced photoisomerization SCN → NCS. Subsequent modification of the patterns with gaseous amines yielded surface relief gratings via the formation of derivatives of thiourea. Laser irradiation with higher pulse energies (7 mJ cm-2, λ = 266 nm) directly produced surface relief gratings via laser ablation (modulation depth 30 nm). Also these gratings were reactive toward amine reagents and allowed a selective functionalization of the grooves of the relief (“reactive gratings”). Under selected conditions, a doubling of the grating frequency was achieved by postexposure modification with gaseous amines. Optically inscribed gratings in PST-co-VBT were employed as optical resonators for distributed feedback (DFB) lasing. With a laser dye (DCM) dissolved in PST-co-VBT, optically pumped DFB laser action was observed after inscribing index and relief gratings. The pumping threshold for lasing I th was 250 nJ cm-2 at λ = 532 nm.

A compact high peak power, passively Q-switched, longitudinally diode-pumped laser was specially constructed for laser ignition. Nd:YAG was chosen as laser active medium and Cr 4+ :YAG as the saturable absorber medium. For pumping, a laser diode emitting at 808 nm with an output power of up to 300 W and pulse duration of up to 500 µs was employed. Experimental studies were carried out to find the optimum laser design parameters. These are the output coupler reflectivity, initial transmission of the saturable absorber, doping concentration of the Nd:YAG, oscillator length and the pump light distribution within the Nd:YAG crystal. Single pulses at 1064 nm with energies of 6 mJ and durations under 1.5 ns were achieved in a TEM00 mode.

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Due to market demands aimed at increasing the efficiency and the power density of gas engines, existing ignition systems are rapidly approaching their limits. To avoid this, gas engine manufacturers are seeking new technologies. From the viewpoint of gas engine R&D engineers, ignition of the fuel/air mixture by means of a laser has great potential. Especially the thermodynamic requirements of a high compression ratio and a high power density are fulfilled well by laser ignition. Results of measurements on the test bench confirm the high expectations — with a BMEP of 1.8 MPa it was possible to verify NOx values of a non-optimized system of 30 ppm (70 mg/Nm3 @ 5 % O2) with very high combustion stability. Despite this, considerable developmental steps are still necessary to adapt the laser ignition concept fully to desired objectives (especially costs).

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