D-Ai39 635 FIVE TIMES COMPRESSION OF MODE-LOCKED ARGON ION LAE -/

PULSES(U) CALIFORNIA UNIV SAN DIEGO LA JOLLA DEPT OFCHEMISTRY C G DUPUY ET AL_ MAR 84 TR-i7

UNCLASSIFIED N868i4-78-C-8325 F/G 29/5 NL

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OFFICE OF NAVAL RESEARCH

Contract N00014-78 C-0325

TECHNICAL REPORT NO. 17

cFIVE TIMES COMPRESSION OF

MODE-LOCKED ARGON ION LASER PULSES

BY

CHARLES G. DUPUY AND PHILIPPE BADODepartment of Chemistry

University of California, San DiegoLa Jolla, CA 92093

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41 TITLE (andSubtitle) S. TYPE Or

REPORT a PERIOD COVEREO

FIVE TIMES COMPRESSION OF MODE-LOCKED ARGON TechnicalION LASER PULSES

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Charles G. Dupuy and Philippe Bado ONR-NO0014-78 C-0325

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AREA 6 WORK UNIT NUMBERSDepartment of ChemistryUniversity of California, San DiegoLa Jolla, CA 92093

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Submitted to Optics Communications, February 1984

*.=;I t. KEY WORDS (Continue an reerse old* it noce.)oa" ind tdnilt by WMock number)

5, fiber opticpulse compression

20. ABISTRqACT (Contiue an, * revers sid i nec*oer An ent bp block nabo)

..-4e-report a five times compression of the 110 ps, 11 nJ pulses from a mode-

locked argon ion laser. The pulses are frequency broadened and chirpedduring passage through a polarization conserving single-mode optical fiber90 m in length, then compressed by a diffraction grating stage to 22 ps.

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4.

Five Times Compression of Mode-Locked Argon Ion LaserPulses

Charls G. DuW and Phike BadoDepartment of Chemistry B-014

University of California at San DiegoLa Jolla, California 92093

ABSTRACT

We report a five times compression of the 110 ps , 11 nJ pulses from amode-locked argon ion laser. The pulses are frequency broadened and chirpedduring passage through a polarization conserving single-mode optical fiber 90 min length, then compressed by a diffraction grating stage to 22 ps.

submitted to: Optics Communications, February 1984

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Present address: Laboratory for Laser Energetics, University of Rochester, 250 East River Road, Rochester.New York 14623

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Five Times Compression of Mode-Locked Argon Ion LaserPulses

Chares G. Dupw and PAlpe Bodo

Department of Chemistry B-014University of California at San Diego

La Jolla, California 92093

INTRODUCTIONFor short input pulses, such as the ones generated by mode-locked dye lasers, it is possi-

ble with a short length of optical fiber to take full advantage of the combined features of self-phase modulation and group velocity dispersion to generate linearly chirped pulses, as shownrecently by Grischkowsky, et al.14 and Shank et al.5 Pulse compressions of more than an orderof magnitude have been achieved by sending this frequency broadened and chirped output ofthe fiber onto a dispersive diffraction grating delay line.3 We demonstrate that the same tech-nique can be used to compress the longer temporal pulses from a mode-locked argon ion laser(with easy extension to frequency doubled, cw mode-locked Nd/YAG pulses) despite the lessfavorable conditions of narrower initial bandwidth and lower peak power.

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FIG. 1 Schematic diagram ofthe experimental apparatus: P- polarizer, FR. - Fara-

Iiopj\f day rotator, - agneticYMcTA field, M. 0. -microscope

M CLLET" otuectav, G grating, andC.L. - cylindrical lens

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The experimental apparatus is shown schematically in Fig. 1. The laser source is a SpectraPhysics model 171 mode-locked argon ion laser operated at 5145 A. Typical working conditionswere 850 mW output power at 80 MHz repetition rate. The pulse width (FWHM) was 110 ps,as monitored by a fast photodiode (Spectra Physics, model 403B) and a smpling oscilloscope(Tektronix model 7603, 7S12 sampling head model S-6).

The optical fiber used in these experiments was provided by Dr. Stolen of Bell Labora-tories. It has a 2.8 jam pure silica core with an elliptical borosilicate cladding and an indexdifference of 0.0089. The characteristic properties of this fiber have been described in detail byRamaswamy, Stolen et al.6 The light was coupled in and out of the 2.8 1 m core of the fiber by

0 Present addres.: Laboratory for Laser Eneraetics, University of Rochester, 250 East River Road, Rochester.New York 14623

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1.0INPUT INPUT INPUT

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FIG. 2 Frequency sectra ofthe pulses at the output f the

C15 - fiber for three djerent input>_ powrs. The nionochromator

_ resolution-limited peak due toZ non-broadened light transmit-

_N'UJj /I-- ted through the cladding wasz included for coparison.

0-5140 5145 5150"5140 5145 5150 N40 5145 5150

WAVELENGTH (4)

0 a 10x or 20x microscope objective. By careful alignment of the polarization along one of theprincipal axis of the cladding, it was possible to obtain a 90 to I conservation of polarizationand a power throughput of 25%. Due to the small core diameter the alignment is very sensitiveto vibrations, so values of 10% power throughput and 12:1 ratio of polarization were more typi-cal operating conditions. The fiber length in these experiments was 90 m and the loss at 5145Ais 40 dB/km.

A Faraday rotator consisting of two polarizers, a fixed field magnet of 2.3 kG and a lxl0cm Terbium doped glass rod (Hoya Optical FR-5) was used to decouple the laser cavity fromback scattered Rayleigh light and the reflections from the microscope objectives and fiber ends.Even with the Faraday rotator, feedback from the fiber considerably degraded the mode-lockingwhen the argon laser was operated above 850 mW.

The pulse widths were measured with a fast diode or with a background free autocorrela-tor, in which a 2 nun Uthium Formate crystal (Quantum Technology) was used to frequencydouble the 5145 A light. The signal from the photomultiplier was recorded by a computerinterfaced picoammeter (Keithely, model 416). The bandwidth of the input argon laser pulseswas measured with a confocal air-spaced Fabry-Perot etalon with a free spectral range of I cm- t

( Quanta-Ray, model FPA-I). At the output of the fiber the bandwidth of the frequencybroadened pulses was measured with the Fabry-Perot ptalon or a monochromator (Spex, model1870) operated in first order with a resolution of 0.5 A.

The initial argon pulses were close to transform limited with a bandwidth of 0.05 A and apulse width of 110 ps. Figure 2 shows the bandwidth recorded at the output of the fiber,before the grating, for three different argon ion laser input powers. At 240 mW input powerthe spectrum was still resolution-limited on the monochromator, even though the bandwidthwas already considerable broader than the 1 cm- (0.1 A) free spectral range of the Fabry-Perotetalon. At 700 mW input power the bandwidth was -2.5 A. Bandwidth of up to 4 A wasachieved with 1.1 W input power. The resolution-limited peak due to the non-broadened argonlight transmitted through the cladding was included for calibration in the spectra of Fig. 2. Inthe pulse compression experiment it was removed by mode strippers and apertures.

Since the generated bandwidth is strongly dependent on the foupling efficiency, undertypical operating conditions, the 850 mW input power yielded 2.5 A, which represents a '50fold increase over the initial bandwidth. The autocorrelation of the pulses at the output of thefiber, before the grating compressor, was essentially the same as that of the input pulses, show-ing that group velocity dispersion effect is small.

The frequency broadened pulses from the optical fiber were compressed with a dispersivedelay line using a single grating (PTR, holographic, 2400 line/mm) in a double pass

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05 PULSE FIG. 3 Autocorrelation tracesof the input and compressedU" l plses.

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DELAY (PICOSECONDS)

configuration as shown in Fig. 1. The grating was used at near grazing angle to obtain highdispersion 7 and had an efficiency of 45% per pass. As pointed out to us later by R. Stolen, thedesign of our grating compressor was not optimal. By using a mirror to reflect the light back onthe single grating, instead of using a right angle prism as did Grischkowsky et al, 3 we did notobtain a collimated output beam, and could acheive linear dispersion only in a localized spatialregion following the second pas on the diffraction grating.

The shortest pulses were obtained with a grating-grating distance of 1.5 :± .1 m, which isclose to the value of 2 m calculated for the 15 degree incident angle.8 Figure 3 shows the auto-correlation traces of the input and of the compressed pulses. Due to poor alignment of theautocorrelator, the autocorrelation trace of the compressed pulse was asymmetric. For otheroperating conditions secondary peaks symmetrically centered with respect to the main peakhave been recorded. Such secondary peaks are expected for pulses which are not linearlychirped over the full bandwidth, as is likely to be the case here due to the narrow initialbandwidth, low peak power and moderate fiber length.9

In order to compress the 110 ps pulses, the grating stage must introduce a -3 cm pathlength difference between the leading blue edge and the trailing red edge. After compressionby the grating stage the beam was a band '2 mm high and "30 mm wide. A cylindrical tele-scope was used to reduce the asymmetry in the beam to 2 x 5 mm. An alternative way toreduce the astigmatism is to greatly increase the beam diameter before the grating stage, thenreduce it with spherical lenses. clearly a scheme for linear compression without beam shapedeformation is desirable. A possible candidate might be the near resonant absorption in metalvapor used by Nakatsuka and Grischkowsky to compress dye laser pulses. 10

In summary we have demonstrated the use of optical fiber and a diffraction grating stageto compress by a factor of five the 110 ps, 5145 A pulses from a mode-locked argon ion laser.Given the large bandwidth generated in the optical fiber an additional factor of 10 in compres-sion is theoretically possible. A lower loss single mode fiber would be desirable to take fulladvantage of this technique.

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! We thank the National Science Foundation, Chemistry and the Office of Naval Research,Chemistry for the financial support which has made this work possible, and the Swim NationalFoundation for fellowship support for P. Bado.

* The authors would like to thank R. Stolen (BeD Labortories) for his advice, encourage-mert and critique of our work, s well as for the loan of the opticad fiber. We also thank R.Byer And T. Kane (Stanford University) for the Ioan of the Terbium glm rod and R. BoW(Spectra Physics) for the loan of the autcorreato crystal. We ae ao indebted toK. t. Wil-son for the use of his laboratores.

1. Hlroki Nakatsuka, D. Grischkowsky, and A. C. Man, "Nonlinear picosecond-pulse pro-pation through optical fibers positive group velocity dispersion," Py. Rev. L4r.a, vol.47, pp. 910-913 (1981).

2. D. Grischkowsky and A. C. Balant, "Optical pulse compression basd on enhanced fre-quency chirpins," AA. .ym Lem.. vol. 41, pp. 1-3 (1982).

,S. 3. B. Nikolaus and D. Grischkowsky, "12x pulse conrsion using optical fibers," Appi.mPs. LM, ., vol. 42, pp. 1-2 (1983).

4. 3. Nikolaus and D. Grischkowsky, "90-fs tunable opicl pulses obtained by two-staepulse compression," A . Phs La., vol. 43, pp. 228-230 (193).

5. C. V. Shank, t. L. Fork, R. Yen, R. H. Stolen, and W. J. Tomlinson, "Compression offemtosecond optical pulses," AAp. Pys. L., vol. 40, pp. 761-763 (192).

6. V. Ramaswamy, R. H. Stolen, M. D. Divino, and W. 1leibel, "Wirefrinsence in ellipticallyclad borosilicate single-mode fibers," ApL Op., vol. 13, pp. 4080.406 (1979).

7. J. D. McMullen, "Analysis of compression of frequency chirped optical pulses by astrongly dispersive grating pair," App. Op., vol. 13, pp. 737-741 (1979).

S. E. B. Treacy, "Optical pulse compression with diffraction pratinga," IEEE . OumumEkc e., vol. QE-S, pp. 454-458 (1969).

9. R. H. Stolen and Chinlon Lin, "Self-phase-modulatin in silica optical fibers," PAys. Rev.A, vol. 17, pp. 1448-1453 (1978).

10. Hifoi Nakatsuka and D. Grischkowsky, "Recompresmion of optical pulses broadened bypassae throush optical fibers," Opt. Lou., vol. 6, pp. 13-15 (1981).

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