Powerful Single-Frequency Laser System based on a Cu-laser pumped Dye Laser

Powerful Single-Frequency Laser System based on a Cu-laser pumped Dye Laser V.I.Baraulya, S.M.Kobtsev, S.V.Kukarin, V.B.Sorokin Novosibirsk State University Pirogova 2, Novosibirsk, 630090, Russia ABSTRACT Reported are the results of development, prototyping and investigation of a powerful laser system based on a frequencystabilized dye jet laser pumped with a copper vapour laser. Short- and long-term frequency instability of the system is approximately ± 50 MHz at output power 4.5 W in the 610 620 nm spectrum range. In case two amplification stages are used, the overall efficiency of the system is higher than 15%. It can be further bettered by raising the pump power of the output stage. The region of smooth output frequency tuning of the stabilized master oscillator is as wide as 8 GHz, while maintaining the scanning rate of up to 1 GHz/sec. The system allows pumping of the master oscillator as well as amplifiers with either open beam of a copper laser or by way of fibre delivery. When pumping the master oscillator through a fibre, for the first time we focused the radiation at the exit of the fibre by means of a modified Fraunhofer objective lens. This type of optical system creates the focal spot twice as small as the fibre diameter while maintaining minimum spherical aberrations. Keywords: pulsed dye laser, copper vapour laser, single-frequency, powerful laser system. Laser systems based on a dye laser pumped with a copper-vapour laser feature a possibility of generation and efficient amplification of narrow-band or single-frequency radiation at a given wavelength in the visible or UV (frequencydoubled) ranges. The spectrum structure of the powerful output pulsed radiation delivered by the system (after the amplifier stage) copies the spectrum structure of the master oscillator radiation, that s why a special attention to improving the master generator parameters is required. In the simplest schemes of the narrow-band or single-frequency master oscillator (dye laser), a short (down to 10 mm) dispersion resonator is used, its diffraction grating being set at a grazing incident angle 1-4. Switching to the single-frequency mode of the pulsed dye laser poses a number of problems, the majority of which are connected with frequency stability of the single-frequency generation. Additional drift and jitter of the laser output frequency can be, for instance, introduced by mechanical and acoustic perturbations, by ambient temperature instability, changes in the intensity distribution and position of the pump beam. To minimise such perturbations, output frequency stabilizers are used, based on reference interferometer 5, based on computer-controlled cavity lengths 6,7, as well as using an electronic servo loop against secondary modes 8,9, pumping the master oscillator through a fibre 10 and measures to prevent backfeed between the oscillator and subsequent amplifier stages, etc. Implementation of reference dye lasers with short- and long-term generation line width of about 100 MHz and narrower (down to the theoretical limit determined by the pulse duration) is a fairly complicated task. Research in this direction has been actively carried out during the last few years. In this paper, we present the results of development and study of a powerful single-frequency laser system based on a dye jet frequency-stabilised (spectral width of the generation line of about 100 MHz) laser pumped with a Cu-vapour pulsed laser. The diagram of implemented master oscillator used in our system is given in Fig. 1. Short laser cavity (about 5 cm) formed by two totally reflective mirrors and holographic diffraction grating set to a grazing incident angle (2400 rulings/mm, by American Holographics). The laser active medium is pumped across the resonator axis as a free-flowing jet at around 10 m/s. The jet dimensions are 4x0.5 mm, the jet itself is formed in a high-precision quartz nozzle. The jet is

tilted with respect to the resonator axis at approximately 10. Longitudinal pumping of the laser is made through a dichroic mirror of the cavity. This mirror transmitted 87% of the green line of the copper vapour laser. The dye laser can be pumped both by an open copper laser beam (through a lens with 65-mm focal length) and through a quartz fibre, 400 µm dia. When pumping through the fibre, the radiation at the exit of the fibre is focused with a modified Fraunhofer objective lens (focal length 40mm). One of the elements in such a lens is parabolic. This type of lens introduces relatively small spherical aberrations and allows one to collect up to 90 95% of the input radiation within a spot of the required size. Fig. 1. Design of the implemented master oscillator: 1, 2 resonator mirrors, 3 - diffraction grating, 4 dye jet, 5 lens, 6 - piezoelectric positioner, 7 reference interferometer, 8 photodetectors, 9 electronic module, 10 - modified Fraunhofer objective. When using open pumping beam, the pump beam waist inside the dye laser active medium was about 90 µm, pump beam waist for second pumping method (through the fibre) was 240 µm. It is relevant to mention that minimising the size of amplifying volume of the dye laser active medium (i.e., achieving a relatively small pump beam waist inside the dye solution jet) plays a key role in producing efficient single-frequency generation, along with the cavity angular dispersion. The concept of the dispersion resonator that we used is based on change in radiation direction as the wavelength changes. Therefore, the active volume of the dye jet is analogous to a spectrometer slit, and the size of the active volume determines the resonator selectivity (or, respectively, the resolution of the equivalent spectrometer). With R6G dye solution, the dye laser output power in the single-frequency mode was up to 170 mw when pumped with open copper laser beam (green line, 8 10 ns pulse duration, 10 15 khz repetition rate), and up to 80 mw when pumped through a fibre. The overall efficiency from the radiation of copper laser green line into the single frequency dye laser output (R6G solution) amounts to 5% when pumping with an open beam and around 4% when pumping through a fibre. The pulse duration of the dye laser output did not exceed 5 ns. The laser output spectrum was controlled with two scanning Fabry-Perot interferometers. One of them was a planeparallel interferometer with variable free dispersion range (its base could be adjusted within 0.2 20 mm range) and finesse of about 20. The other was a confocal interferometer with free dispersion range 5 GHz (3 GHz) and finesse of around 100. In Fig. 2, we showed the transmission spectrum of the confocal interferometer with free dispersion range 3 GHz for the radiation of the proposed master oscillator dye laser. This spectrum was recorded with the help of an ADC over the 0.05-second period of time.

Fig. 2.. Single-mode dye laser output monitored by a 3 GHz free spectral range scanning confocal interferometer (finesse=100). Approximately 500 laser pulses occurred during the sweep. In order to stabilize the dye laser frequency, a confocal Fabry-Perot interferometer was used. It was placed in a thermostat analogous to the one described in 11. Interferometer temperature instability didn t exceed 0.02 C. The system of laser frequency stabilization includes two photodetectors, which register peak radiation intensity before and after the reference interferometer (the technique of frequency stabilization on the slope of the reference interferometer transmittance peak). The free dispersion range of the reference interferometer is 5 GHz, its finesse being 2. The response time of the system s control ring was 0.15 sec. The error signal of the output frequency gets delivered through a piezoelectric positioner to the cavity mirror located in front of the diffraction grating. When the dye laser is installed in a massive stand with a relatively low external acoustic and mechanical perturbations, the frequency stabilization system mostly serves to reduce the output frequency drift to 100 150 MHz per hour, while having no effect on short-term (seconds) laser output line width (which is also no more than 90 120 MHz). When the master oscillator is used in a noisy environment (the stand is not stable enough, high levels of mechanical and acoustic noise), the short-term output frequency instability may be as high as 500 600 MHz. However, the frequency stabilisation system can bring it down to 100 MHz and lower in such environment. Here and in all the above discussion we define the frequency instability as the magnitude frequency instability measured simultaneously with two scanned Fabri-Perot interferometers and a high-precision wavelength meter Angström with relative accuracy of 30 MHz. Results obtained with these devices check well with each other. In Fig. 3, the dependence of the generation frequency (in cm -1 ) upon time recorded with the radiation wavelength meter Angström is presented. The registered drift of the laser frequency (about 100 MHz/hour) may be attributed both to residual drift of the reference interferometer transmission peak, and to the transmission peak drift in the most precise Fiso interferometer incorporated into the wavelength meter. The latter is more probable, in our opinion, since the wavelength meter interferometers are not thermostabilized, however the wavelength meter Angström has a built-in temperature correction of the measured wavelength by means of a temperature sensor in the finest Fiso interferometer. Fig. 3. Frequency instability of the system radiation. Short-term laser frequency jitter is ± 45-60 MHz, long-term drift of the center frequency is not more than 100 MHz/hour.

To scan the dye laser generation frequency continuously within a region that exceeds the cavity dispersion range (3 GHz), we used a method based on a modification of the pivot-technique 12,13. This method is being patented at the moment. This new technique extends the continuous scanning range of the dye laser frequency to 8 GHz and is capable of easy broadening it further to 0.5 cm -1 and more. When scanning continuously the dye laser generation frequency, the scan control voltage is applied to the piezoelectric positioner of the reference interferometer, while the laser cavity synchronously follows it by way of the frequency stabilization system locked onto the reference interferometer. This ensures the small generation line width of the dye laser while scanning its generation frequency within the range of 8 GHz and more at a rate of up to 1 GHz/sec. To amplify the radiation of the master oscillator, we used a two-stage amplifier with original self-built cells. The cells measure 20-mm long and have about 2.5-mm 3 active medium volume (first stage) and 15.7 mm 3 (second stage). The dye solution is pumped through the cells at the rate of 3 and 12 litres/min respectively. The employed amplifier cells feature an enhanced homogeneity of the solution flow rate along the cell. The cell of the second amplifier stage can pass up to 20 l/min. In Fig. 4, a diagram of the developed powerful single-frequency laser system based on a dye laser pumped with a copper vapour laser is shown. Fig. 4. Schematic of the developed powerful single-frequency laser system: 1 dye laser-master oscillator, 2 - first amplifier stage, 3 - second amplifier stage, 4 dye flow systems, 5 reference interferometer, 6 - scanning Fabry-Perot interferometer. Working spectral range of the system depends on the intended usage and amounts to around 610 620 nm. The master oscillator utilises a mixture of dyes RB and R101 (ethylene glycol solution). In the amplifiers we used alcohol solution of Phenalemine 512. Total pump power in both green and yellow lines (intensity ratio 2:1) amounts to 27 W (pulse repetition rate 11.5 khz). This power is further distributed among the system components as follows: 4 W is fed into the master oscillator dye laser, 5 W is input into the first amplifier stage, and 18 W is delivered into the second amplifier stage. The output power of the system after the first amplifier stage is about 0.5 W. After the second stage, it is raised up to 4.5 W. When the pump power of the first amplifier stage is increased to 22 W the system s output power after this stage gets boosted to 2.5 W. Spontaneous emission of the amplifiers was minimised with the help of special matching adjustments made to the arrival times of the amplified pulses and pump pulses to the amplifier cells. Shutting the master oscillator off shows that the spontaneous output of the system does not exceed 100 mw behind stopped-down aperture with amplified beam size.

In Fig. 5, an overview of the developed laser system is shown. The system features a compact design, the required table area being no more than 150x80 cm 2. Fig. 5. Photographic representation of the the developed powerful single-frequency laser system. ACKNOWLEDGEMENTS We thank the Coherent Technology JSC (Novosibirsk, Russia) for providing the necessary Cu-vapour laser system. This research was supported by the Tekhnoscan JSC (Novosibirsk, Russia, www.tekhnoscan.ru). Dr. S.Kobtsev s e- mail address is kobtsev@lab.nsu.ru. REFERENCES 1. I. Shoshan, N.N.Danon, U.P.Oppenheim. Narrowband operation of a pulsed dye laser without intracavity beam expansion, Appl. Phys., 48, N11, pp. 4495-4497, 1977. 2. M.G.Littman. Sigle-mode pulsed tunable dye laser, Appl. Optics, 23, N24, pp. 4465-4468, 1984. 3. I.T.McKinnie, A.J.Berry, T.A.King. Stable, efficient, single-mode operation of a high repetition rate grazing incidence dye laser, J. Modern Optics, 38, N9, pp. 1691-1701, 1991. 4. Y. Maruyama, M. Kato, T.Arisawa. Copper vapor laser pumped single-mode grazing incidence dye laser using dye jet, Jap. J. Appl. Phys., 30, N 4B, pp. 748-750, 1991. 5. Y.Maruyama, M.Kato, A.Sugiyama, T.Arisawa. A narrow linewidth dye laser with double-prism beam expander, Opt. Commun., 81, N1-2, pp. 67-70, 1991. 6. D.J.Binks, L.A.W.Gloster, T.A.King, I.T.McKinnie. Frequency locking of a pulsed single-longitudinal-mode laser in a coupled-cavity resonator, Appl. Optics, 36, N36, pp. 9371-9377, 1997. 7. P.L.Stricklin, D.C.Jacobs. Long-term wavelength stabilization of a commercial pulsed dye laser, Appl. Optics, 31, N33, pp. 6983-6986, 1992. 8. A.F.Bernhardt, P.Rasmussen. Design criteria and operation characteristics of a single-mode dye laser, Appl. Phys., B26, pp. 141-146, 1981. 9. I.L.Bass, R.E.Bonanno, R.P.Hackel, P.R.Hammond. High-average-power dye laser at Lawrence Livermore National Laboratory, Appl. Optics, 31, N33, pp. 6993-7006, 1992. 10. S.V.Vasil'ev, V.A.Mishin, T.V.Shavrova. Single-frequency dye laser with fibre-optic pump, Quantum Electronics, 27, N2, pp. 126-128, 1997. 11. B.V.Bondarev, A.V.Karablev, S.M.Kobtsev, V.M.Lunin. Frequency-stabilized CW Dye Laser with precise automatic tuning for high-resolution spectroscopy, Atm. Optics, 2, N12, pp. 1132-1136, 1989. 12. K.Liu, M.G.Littman. Novel geometry for single-mode scanning of tunable lasers, Opt. Letters, 6, N3, pp. 117-118, 1981. 13. G.Z.Zhang, K.Hakuta. Scanning geometry for broadly tunable single-mode pulsed dye laser, Opt. Letters, 17, N14, pp. 997-999, 1992.