Mode locking of excimer laser pumped dye lasersMasayoshi Watanabe, Shuntaro Watanabe, and Akira Endoh Citation: Applied Physics Letters 45, 929 (1984); doi: 10.1063/1.95464 View online: http://dx.doi.org/10.1063/1.95464 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/45/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Simultaneous third and fourth harmonic modelocking of a Qswitched flashlamppumped Nd:YAG laser Rev. Sci. Instrum. 67, 3783 (1996); 10.1063/1.1147277 New feedback mechanism for reducing timing jitter between pulses from two synchronously pumped modelocked lasers Rev. Sci. Instrum. 66, 5165 (1995); 10.1063/1.1146144 An amplified tunable picosecond dye laser based on an activepassive modelocked Nd:YAG laser Rev. Sci. Instrum. 60, 2592 (1989); 10.1063/1.1140676 Mode locking and Q switching of a diode laser pumped neodymiumdoped yttrium lithium fluoride laser Appl. Phys. Lett. 54, 403 (1989); 10.1063/1.100976 Continuous wave modelocked neodymium:phosphate glass laser Appl. Phys. Lett. 45, 1171 (1984); 10.1063/1.95092

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>­~

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z w ~ z

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FIG. 5. Far-field patterns lateral to the junction plane levels at varying opti­cal output powers per facet.

The capability of a regrowth following the implantation process demonstrates a large flexibility of growth by MOCVD.

Finally it should be noted that Be implantation is ad-

vantageous in producing integrated devices, ·as previously demonstrated in Ref. 10.

The author is indebted to Miss Hadas Shtrikman and to the staff of the Microelectronics Research Center for making this research possible. This work was supported by the Is­raeli fund for applied and industrial research at the universi­ties.

JR. D. Dupuis, J. Cryst. Growth 55, 213 (1981); D. R. Scifres, R. D. Burn­ham, and W. Streifer, Appl. Phys. Lett. 38, 915 (1981).

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Mode locking of excimer laser pumped dye lasers Masayoshi Watanabe, Shuntaro Watanabe, and Akira Endoh The InstituteforSolid State Physics, The University of Tokyo, Roppongi 7-22-1 Minato-ku, Tokyo 106, Japan

(Received 5 June 1984; accepted for publication 14 August 1984)

Picosecond pulses have been generated over a broad spectral range with passive mode locking of dye lasers pumped by a long-pulse XeCllaser. The rapid reduction of pulse duration to less than lOps was confirmed to occur within a few tens of round trips. The characteristics of mode locking were intensively investigated at the multiple wavelengths of rare-gas halide lasers such as KrF, ArF, and XeCllasers, resulting in pulse durations of 5.5 ps at 497 nm and 7 ps at 580 nm, respectively.

High-power and picosecond(ps) excimer laser systems have proved to be usefully applicable to nonlinear optics 1,2 at the vacuum-ultraviolet and extreme-ultraviolet wavelength regions, solid state physics, biological studies, and photo­chemistry. For the generation ofps pulses at the wavelengths of rare-gas halide lasers, the wavelength conversion from a mode-locked glass laser3 or dye lasers4-7 has been employed, Recently, mode-locked dye lasers synchronously pumped by an Ar,6 or yttrium aluminum garnet 7 laser have been suc­cessfully adapted to ps excimer laser systems. If a rare-gas halide laser itself can be used for the generation of ps pulses, the cost and complexity, and the difficulty of exact synchro­nism between a ps generator and excimer amplifiers would be reduced. Then, these systems would be open to a wide­spread use for various applications. Along this line, we have investigated direct mode locking of XeCllasers. 8,9 Recently, the generation of a single ps pulse at 308 nm has been demon-

strated by using the distributed feedback dye laser system pumped by a commercially available XeCI laser. 1O Mode locking of dye lasers pumped by a rare-gas halide laser would also offer the possibility to generate ps pulses at multiple wavelengths of rare-gas halide lasers using a single arrange­ment. But pulse durations less than a few tens of nanose­conds of commercially available excimer lasers were not large enough for this purpose.

In this letter, we report passive mode locking of dye lasers pumped by a long-pulse( 150 ns) XeCllaser, which was previously used in the experiments of direct mode locking. 8.9

The pulse duration of several picoseconds was achieved at the wavelengths around 497 and 580 nm (chosen because these are the double wavelength of a KrF laser and the triple wavelength of an ArF laser, respectively). The performance of mode locking was also investigated around 616 nm (the double wavelength ofaXeCllaser), To our knowledge, this is

929 Appl. Phys. Lett. 45 (9), 1 November 1984 0003-6951/84/210929-03$01.00 © 1984 American Institute of Physics 929

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STDUiE SCOPI

, , • .-.iITOi -.-t

DPIIClL DIIAY

FIG. I. Experimental setup for passive mode locking of dye lasers pumped by a 150-ns XeCllaser.

the first demonstration ofps pulses from a mode-locked dye laser pumped by an excimer laser. By streak camera mea­surements it was confirmed that the pulse shortening to sev­eral picoseconds was obtained within a few tens of round trips.

The long-pulse XeCI laser used as a pumping source was equipped with three coaxial pulse-forming lines (PFL's) of a two-way electrical transit time of 200 ns and the pulse duration of 150 ns was obtained with the output energy of 400 mJ at the optimized condition. A schematic diagram of the experimental setup is shown in Fig.l. The dye laser cav­ity was formed by a total reflector and a 70% output coupler. The cavity length was usually 32 cm except the case de­scribed later in this letter, then the 150-ns pulse duration was able to give a several tens of round trips. The active length of the laser dyes, which were circulated in the quartz cell, was 10 mm. A saturable absorber cell with the thickness of 0.5 or 1.5 mm was located in contact with the total reflector. The lasing wavelength was tuned by an intracavity, air gap Fa­bry-Perot etalon with the gap spacing of 7 f.lm. The pulse duration was measured by a streak camera (Hamamatsu C1370) with a temporal analyzer. The mode-locked pulse train was simultaneaously observed by a biplanar photo­diode (Hamamatsu RlI93U-02) and a storage oscilloscope (Tektronix 7834). The characteristics of mode locking were intensively investigated around the above-mentioned three wavelengths.

First, the experiment was carried out around the blue­green region by using a 9 X 10 - 3 M methanolic solution of Coumarin 480 with DOC I (3,3/ -diethyloxacarbocyanine io­dide) as a saturable absorber in the 2 X 10-4 M ethylenegly­col 90%-methanol 10% solution. II

•12 Figure 2 shows the

mode-locked pulse train at the wavelength 471 nm. The buildup of mode locking was very rapid and complete modu­lation was seen to occur at the early part of the pulse train. The total duration of the mode-locking pulse train was 90 ns.

'" ~-+-+--+-. co GO

~ f-+--+--ti1Ml

T jme 20ns/dj,

FIG. 2. Mode-locked pulse train at 471 nm in 20 ns per division with Cou­marin 480 as a laser dye and DOCI as a saturable absorber.

930 Appl. Phys. Lett., Vol. 45, No.9, 1 November 1984

(/)

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z ., --~ 20 -I

I

L~~I --'----, _~~ ° 20 40 60 80 100

TIME(ns)

I ~---L.j ° 10 20 30 40 50

NUMBER OF ROUND TRIPS

FIG. 3. Pulse duration dependence on time and number of round trips from the start of the pulse train at 471 nm.

Similar performance of mode locking was observed between 471 and 488 nm by using a single etalon with the free-spec­tral range(FSR) of 17 nm. In order to investigate the evolu­tion of mode locking, the individual pulse duration was mea­sured as successive round trips. The pulse duration dependence on time and number of round trips from the start of the pulse train is shown in Fig.3. A rapid reduction of the pulse duration was observed and a pulse duration of 7 ps was obtained at 55 ns from the start of the pulse train. A further reduction of a pulse duration was not observed at the later part of the pulse train. Since the spectral tuning range was limited by FSR (17 nm) of the etalon, we added a second air gap etalon with a little different FSR into the cavity to obtain mode locking at 497 nm. To accommodate this eta­lon, the cavity length was increased to 43 cm. When the concentration of DOC I was reduced to 1.2 X 10-4 M, a simi­lar performance of mode locking was observed as shown in Fig.2. Figure 4 shows a typical streak recording of pulse form at 497 nm, indicating a pulse duration of 5.5 ps. From the measured bandwidth of 3 A, a time-bandwidth product is 1.9. This value is by a factor of 3 larger than that of trans­form limited operation when a Lorentzian-shaped pulse is assumed. Since the gain of Coumarin 480 and the absorption cross section of DOCI are large in the blue-green region,

t-o! .. 1---_9_0-'--p_s --1

FIG. 4. Streak recording of pulse form at 497 nm from the Coumarin 480 laser mode locked by DOCI, showing a pulse duration of 5.5 ps. Two pulses separated by 90 ps optical delay line are shown.

Watanabe, Watanabe, and Endoh 930

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lOpS

-If-

FIG. 5. Streak recording of pulse form at 580 nm from the rhodamine 590 laser mode locked by DQOCI, showing a pulse duration of 7 ps.

broadly tunable ps pulses will be available in this spectral range. 11-13

The mode-locked pulse train at 580 nm was obtained by replacing a laser dye and a saturable absorber with rhoda­mine 590 and DQOCI (1,3' dietyl' 4,2' -quinolyoxacarbocyan­ine iodide), respectively. The mode-locked pulse train simi­lar to Fig.2 was also observed around 580 nm when the concentrations of rhodamine 590 and DQOCI were 2.5 X 10-3 M in methanol and 2.5 X 1O- 5M in ethanol, re­spectively. Figure 5 shows a typical streak recording of pulse form at 580 nm which gives a pulse duration of 7 ps. Mode locking at 580 nm was also examined by using DODCI (3,3'­diethyloxadicarbocyanine iodide) as a saturable absorber. Although an instantaneous buildup of the pulse train was observed as with DQOCI, the pulse evolution in the train was found to be insufficient to suppress the burst of sub­pulses. 14 This result is explained partly by the difference in the relaxation times of the saturable absorbers l5

; the relaxa­tion time (120 ps) ofDQOCI is an order of magnitude shorter than that of DODC!. 16

For mode locking at 616 nm, various pairs oflaser dyes and saturable absorbers were investigated. But the absorp­tion cross sections of DODCI and DQOCI employed in this experiment were not large enough at this wavelength to give a fast evolution of the mode-locked pulse train. Therefore, the shortest pulse duration was limited to 150 ps with the combination of rhodamine 590 and DODCI, which gave much shorter pulse duration in the earlier experiments of Oashlamp4 or A~ laser pumped dye lasers. However, when

931 Appl. Phys. Lett., Vol. 45, No.9, 1 November 1984

the wavelength was tuned at 591 nm, the pulse duration was reduced to 20 ps because the absorption cross section of the saturable absorber and the gain of the laser dye are reasona­bly large at this wavelength. This result confirms that the buildup of mode locking within a few tens of round trips depends considerably on the cross section of a saturable ab­sorber at the wavelength concerned.

In conclusion, it was confirmed that the pulse duration decreased to the ps region within a few tens of round trips in passive mode locking of a dye laser pumped by a 150-ns XeCllaser with pairs of conventional laser dyes and satura­ble absorbers. This fact enables us to get the simple ps light sources for the ps rare-gas halide laser system under con­struction. The pulse durations of 5.5 and 7 ps were obtained around 497 and 580 nm, respectively. The pulse duration was still 150 ps around 616 nm. The mode-locked dye laser pumped by an excimer laser would become applicable for various fields as a conventional, high-repetition rate ps light source from the blue to red spectral region.

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Watanabe, Watanabe, and Endoh 931

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