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A Convenient Mode Locking Method of an Ion Laser December 1984

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A Convenient Mode Locking Method of an Ion Laser December 1984 (702) A Convenient Mode Locking Method of an Ion Laser December 1984 Laser Technical Note A Convenient Mode Locking Method of an Ion Laser Shunsuke NAKANISHI*, Hiroshi ITOH*, Koji KONDO* and Yukio FUKUDA** (Received November 19, 1984) This note shows convenient and low cost method for obtaining the mode locked pulses in any commercially available ion laser without the rearrangement of the cavity. In contrast to the usual cases, this mode-locker system consists of two Brewster-cut fused quartz crystals, one of which acts as the acousto-optic modulator and the other as the compensator of the displacement of the laser beam at the acousto-optic modulator. Recentry the picosecond laser has become not be used as the A. O. modulator or such a indispensable to the investigation of the ultrafast modification is very troublesome 3), a home- dynamics in solids, liquids and gases. The designed A.O. modulator should be inserted in generation methods of the picosecond laser the cavity, for example between the laser tube pulses have been developed by using various and the output mirror 4). In this method, means 1,2). A synchronously pumped mode however, it becomes necessary to remove the locked cw dye laser is the most versatile and output mirror from the cavity holder in order to reliable for the generation of the tunable pico- construct the laser cavity because the path of second pulses and easily applicable to obtaining the laser beam suffers the displacement (walk subpicosecond or high power picosecond pulses. off) at the A.O. modulator and the diameter of In the case of the synchronous pumping by an the output mirror is usually too small compared ion laser, it is necessary to achieve the active with the displacement. The rearrangement of mode locking of the ion laser. This report the cavity lacks the usablences of the Ar ion shows a convenient and low-cost method for the laser and has the disadvantage that the thermally- mode locking of the Ar ion laser which is compensated property of the cavity holder can frequently used for pumping the dye solution. not be utilized. In order to avoid these dis- This mode locker can be adapted to any com- advantages we designed a mode locker system mercial ion laser without any rearangement of as.shown in Fig.1, which does not require the the cavity if there is a small space between the rearrangement of the cavity and is applicable to mirrors of cavity and the laser tube. any types of the ion laser at the expense of a The active mode locking of the Ar ion laser small space. This mode locker system consists can be performed by inserting a Brewster-cut of two same-shaped fused quartz crystals cut at acousto-optic (A.O.) modulator in the cavity. In Brewster angle(55.5•‹) for 514.5 nm in contrast to the case where the tuning prism in the cavity can the usual one, one of which was attached with * Department of Physics, Faculty of Education, Kagawa University(1-1,Saiwai-cho ,Takamatsu 760) **Department of Physics,Faculty of Science,Kyoto University(Kitashirakawa,Sakyo-ku ,Kyoto 606) ―28― Vol.12, No.12 1984 The Review of Laser Engineering (703) Fig.1. Schematic of the mode locker system used for the mode locking of 514.5 nm of the Ar ion laser. A,B: thd fused quartz crystals cut at Brewster angle 55.5•‹, C:LiNbO3 crystal of 167ƒÊm thickness, D:a heater,E:a thermistor, F:a wedge-shaped aluminum plate. LiNbO3 crystal of 167ƒÊm thickness and acted as locker was inserted, the output power in cw A.O. modulator of the standing acoustic wave operation at 514.5 nm was not so decreased, for mode. The other crystal was introduced in example from 1.5 w to 1.3 w. order to compensate the displacement of the The output property of the mode locked laser beam at the A.O. modulator. The thick- pulses at 514.5 nm was examined by a biplaner ness of LiNbO3 crystal was determined to phototube (Hamamatsu R1328U, which has the equalize the resonant acoustic frequency (64 rise and fall time of 80 ps at the supply voltage MHz) of the LiNbO3 crystal to half the frequency of 1.5 kV) and a sampling oscilloscope separation of the longitudinal modes of our Ar (Tektronix 7904 with the sampling units 7T11 ion laser (NEC GLG3300). The two quartz and 7S11, and the sampling head S-6 of the rise crystals, which have the 3 cm height, mounted time of 30 ps). Figure 2 shows the sampling on a wedge-shaped aluminum plate whose wedge oscilloscope traces of the pulses. The pulse angle (138•‹) was precisely adjusted to obtain the repetition period was about 7. 8 ns (shown in accurate compensation of the displacement. Fig. 2 (a)) corresponding to the rf frequency 64. The A.O. modulator was temperature controlled 459 MHz, which drived the LiNbO3 crystal with by the combination of a thermister, a heater and the power of 500 mw, and any sub-pulses the circuits which control the current to the between the main pulses was not observed. The heater. The mode locker system which could observed pulse width was about 400 ps (FWHM) be, as a whole, angle tuned to the axis of the as is shown in Fig. 2 (b). The rather scattered laser beam, was inserted between the laser tube shape in Fig. 2 (b) was mainly due to the trigger and the end mirror (of 100% reflection) of the Ar jitter caused by the amplitude change of pulses ion laser and sealed by a metal box of the of less than 5%. We expect from Fig. 2 (b) that dimension of 5•~5•~6 cm3. In the metal box the the real pulse width is about 300 ps (FWHM)by N2 gas flow was used in order to avoid the taking account of the rise and fall time of the accumulated contamination on the optical biplaner phototube and the sampling head. It is surfaces. Nevertheless four new optical surfaces slightly broader than the transform-limited pulse were introduced in the cavity when the mode width of the Doppler broadened gain band width ―29― (704) A Convenient Mode Locking Method of an Ion Laser December 1984 pumped mode locked cw dye (Rh-6G) laser with the resonator of the folded four mirrors. The tuning element in the dye cavity was only one- plate birefringent filter (a quartz plate of 0.7 mm (a) thickness) and the output coupling was performed by the mirror of 96% reflection. The threshold power of the laser oscillation of the dye laser cavity was 150 (200) mw in the mode locked (cw) operation. We could obtain the stable mode locked pulses in the region of 575 10 nm and the average output power was~6 about 10 mw at the average pumping power of 200 mw. The pulse width of the dye laser was (b) measured by the conventional background free autocorrelation technique using the phase matched second harmonic generation (SHG) in KDP crystal. The autocorrelation trace in Fig . 3 gives the pulse width of 9.4 ps at 582.2 nm by assuming a sech2 pulse shape. This trace shows Fig. 2. Sampling oscilloscope traces of the mode locked Ar ion laser pulses at 514.5 nm. (a) shows the that any subpulse does not exist in the mode portion of pulse train of the repetition time of locked pulse train. Much shorter pulse width about 7.8 ns (1 ns/div). (b) shows the pulse can be obtained by reducing the reflection coef- width (200 ps/div). ficient of the output mirror as reported of Ar ion gas. The small ringing after the main previously3). pulse was examined to change its position in In conclusion, this note described a modified time by changing the length of the coaxial cable mode locker system which easily enables us to from the phototube to the sampling head and achieve the mode locking of the Ar ion laser found that it was not the signal due to the sub- pulse, but the reflection of the signal. The average output power in the mode locked operation was examined to be about 40% of that in the cw operation. The stability of the mode locked pulses was sufficiently good after the warming up of one hour. Therefore this specially modified mode locker gives rise to the same efficiency as the usual ones, and it is in- expensive compared to purchasing the com- mercial one. Furthermore, in order to examine the utility Fig. 3. Autocorrelation trace of the mode locked cw dye laser pulse at 582.2 nm taken by the background of the mode locked pulses at 514.5 nm thus free autocorrelation technique using SHG in obtained, we constructed the synchronously KDP crystal. ―30― Vol. 12, No. 12 1984 The Review of Laser Engineering (705) and showed that it gives the useful pumping 2) R. Hochstrasser, W. Kaiser and C. V. Shank, ed. : Picosecond Phenomena 2•h, Springer Series in•g source of the synchronously pumped cw mode Chemical Physics, Vol. 14 (Springer-Verlag, Berlin, locked dye laser. We thank Sony-Tektronix Co. 1980). in Japan for loaning us the sampling units used 3) H. Nakatsuka, S. Asaka, M. Fujiwara and M . in the pulse width measurement at 514.5 nm. Matsuoka: The Review of Laser Engineering 11 (1983) 842. 4) S. Kinoshita, H. Ohta and T. Kushida: Rev . Sci. References Instrum. 52 (1981) 572. 1) S. L.
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