Generation of synchronized trains of picosecond laser pulses at two wavelengths in a single-cavity synchronously mode-locked dye laser

Synchronized trains of picosecond laser pulses of two different wavelengths constitute a powerful tool to study fast processes in materials. In the last several years this scheme was used to study vibrational dephasing of molecular monolayers on surfaces,’ optical nonlinearities in the polydiacetylene PTS conjugated polymer,“‘3 and the solvent dependance of the rotational diffusion of Cresyl Violet,” among other phenomena. To generate the synchronized pulse trains utilized in these studies, two separate dye lasers were pumped in parallel by the same pump laser. ’ Alternatively, an electro-optic modulator has been introduced in a single synchronously mode-locked dye laser to obtain dual wavelength operation by interleaving the pulse train amplification in the dye medium.5 The generation of synchronized picosecond pulses at two wavelengths has also been demonstrated using two cavities that share a common gain medium in a synchronously mode-locked continuous wave dye laser.“” Pulses of 5 ps of duration were obtained in an arrangement in which each cavity had an overall length twice that of the argon ion pump laser to give sufficient separation between the beams to allow two mirrors to be mounted in separate translation stages. The average jitter between pulses of different wavelength was 20 PS.~ In a different dual-cavity arrangement, passive mode locking of one cavity acted as a gain modulator for the second one to generate two independently tunable wavelengths.’ In another approach a dye laser having a Rhodamine 6G and a Cresyl Violet jet introduced in the same cavity produced 4-5 ps pulses at two tunable wavelengths.8 In this scheme the laser beam produced by pumping the Rhodamine jet with a mode locked argon ion laser pumped a second jet containing Cresyl Violet. Herein we report the simultaneous generation of picosecond pulses at two different wavelengths from a single dye laser cavity of the type commonly used to generate femtosecond pulses at a single wavelength by hybrid mode locking. Tunable dual wavelength operation was achieved mixing two dyes in a single jet to increase the bandwidth of the gain medium and utilizing twointracavity prisms to compensate for the difference in transit time of the pulses corresponding to the two different wavelengths. Selection of the laser frequencies was obtained by means of an intracavity spatial filter. By making use of a laser configuration available in many laboratories, this dual wavelength laser scheme has advantages of simplicity and improved pulse synchronization associated with a single cavity. The dye laser consists of a dual-jet linear cavity arrangement,’ which includes a pair of interactivity prisms for group velocity dispersion compensation. For dualwavelength operation, the absorbent jet was turned off and the gain medium was prepared mixing Rhodamine 590, 1.4X lop3 M, and DCM, 3.1 X lo-’ M, in a solvent composed of 68% ethylene glycol and 32% benzene alcohol. A small amount of Kiton Red, 0.2~ 10d3 M, was also added to assist in dissolving the DCM. The dye laser was pumped by a frequency-doubled mode-locked cw Nd-YLF laser with an average power of 850 mW at 527 nm and a pulse duration of 40 ps at a repetition rate of 76 MHz. The absorption of DCM is negligible at wavelengths above 595 nm, the lowest wavelength produced by the Rhodamine dye, consequently the pumping of both dyes is almost exclusively due to the 527 nm radiation from the frequencydoubled Nd:YLF laser. Wavelength selection was achieved by introducing a spatial mask at the high retlectivity end of the cavity, where the wavelengths are dispersed by the prisms. At this location a gold wire 12 pm diameter and two lateral metallic blades were positioned for wavelength selection. Synchronized pulses of less than 5 ps of duration in the orange and in the red, separated in wavelength by approximately 30 run, were obtained. The alignment and cavity length were adjusted in order to obtain similar pulsewidths at both wavelengths. A spectrum of the output beam of the dual wavelength laser is shown in Fig. 1. The pulses centered at 607 nm had a bandwidth of 4.6 nm and an average power of 24 mW, while the pulses centered at 640 nm had a bandwidth of 9.2 nm and an average power of 13 mW. It was possible to adjust the relative power of the two wavelength components by varying the dye concentrations. Figures 2 (a) and 2(b) show the autocorrelation traces of the pulses corresponding to each wavelength. The autocorrelation traces were generated one at the time by adjusting the angle of a LIO, crystal to generate the second harmonic of each wavelength component. Tunability with 10 nm was readily achieved for both pulse trains while maintaining the pulse widths below 6 ps. No effort was made to extend the tunability beyond this range, as it was not required by our particular application. Figure 2(c) shows a cross-correlation signal of 5 ps pulse trains at 603 and 630 nm. The cross-correlation protile was generated by monitoring the variation of the sum frequency signal in a KDP crystal, as the relative delay of