Generation of 50-fsec pulses from a pulse-compressed, cw, passively mode-locked Ti:sapphire laser. Nobuhiko Sarukura, Yuzo Ishida, and Hidetoshi Nakano.
February 1, 1991 / Vol. 16, No. 3 / OPTICS LETTERS
153
Generation of 50-fsec pulses from a pulse-compressed, cw,
passively mode-locked Ti:sapphire laser Nobuhiko Sarukura, Yuzo Ishida, and Hidetoshi Nakano NTT Basic Research Laboratories,3-9-11,Midori-Cho Musashino-Shi, Tokyo 180,Japan Received June 27, 1990; accepted November 16, 1990 Stable pulses of less than 150 fsec are generated directly from a tunable cw passively mode-locked Ti:sapphire laser,
through a balance of self-phase modulation in the Ti:sapphire rod and negative group-velocitydispersion produced by a prism pair. After external fiber compression, 50-fsec pulses are obtained at approximately 750 nm.
The broad gain bandwidth of Ti:sapphire,l which exceeds even that of dyes, makes this material attractive for both femtosecond laser oscillators and amplifiers. Recently various attempts have been made to achieve ultrafast Ti:sapphire lasers, including conventional active mode locking with an acoustic optical modulator,2 additive-pulse mode locking,3 4 resonant passive mode locking, 5 and even self-mode locking. 6 We have
already demonstrated cw passive mode locking using organic dyes and semiconductor-doped glass as satu-
rable absorbers.7'8 Furthermore positive linear-
chirped pulses were directly generated by self-phase modulation (SPM) in the Ti:sapphire rod and groupvelocity dispersion (GVD) in the cavity, and output pulses were compressed to 1.6 psec with a grating pair.9 On the basis of these results, a prism pair was introduced into the cavity to obtain shorter pulses.10 In this Letter we describe direct generation of nearly transform-limited and tunable pulses of less than 150 fsec from a cw passively mode-locked Ti:sapphire laser. Subsequent pulse compression yields 50-fsec pulses. The experimental setup of the Ti:sapphire oscillator and fiber compressor is shown in Fig. 1. The configu-
time-bandwidth product of 0.35. Both the autocorrelation trace and spectral shape show good agreement with the sech2 pulse [open circles, Fig. 2(a)] and the spectral shape [dashed curve, Fig. 2(b)]. The average and peak output powers were, respectively, 300 mW and 21 kW (at a 101-MHz repetition rate), with 5.3-W pumping power. No modulation in the envelope of the pulse train or absence of double pulsing behavior was confirmed through the observation of a storage oscilloscope with a fast photodetector and an optical sampling scope. The tunability of 70 nm (near 730800 nm) was checked at several points; the pulse widths and center wavelengths were 320 fsec at 731 nm, 155 fsec at 761.5 nm, 165 fsec at 780 nm, and 140
fsec at 804 nm. The dependence on pumping power of the pulse properties, including the spectral width, the pulse width, the average output power, and the center wavelength, was also measured in a fixed alignment (Fig. 3). At pumping powers below 5 W, the autocorrelation trace width was near 10 psec, and the spectral width was below the resolution limit (-0.5 nm) of the detection apparatus. Both the spectra and the autocorrelation traces changed drastically at 5.5-Wpumping pow-
ration of the cavity is essentially the same as that previously reported,7 -9 except for an intracavity prism chirp compensator.1 0' 1' The Ti:sapphire laser consists of a six-mirror cavity with an additional focus for a saturable absorber, a 5% output coupler, a single-
plate birefringent filter as a tuning element, and a pair of high-dispersion Brewster prisms made of SF57 glass. With this prism pair, sufficiently large negative GVD can be obtained only with a 20-cm separation of
the prism pair. A cw Ar laser in all-lines operation was used as a pumping source. The saturable absorber dye was 1,1',3,3,3',3'-hexamethylindotricarbocyanine iodide (HITCI) at a concentration of 3.6 X 10-5 M
in ethylene glycol. The autocorrelation trace and the spectrum in optimum conditions were simultaneously monitored with a rapid-scanning autocorrelator and a spectrometer coupled with an optical multichannel analyzer. As shown in Fig. 2, 140-fsec pulses were directly obtained,
if we assume a sech2 shape. The 5.2-nm spectral width at 792.8 nm yielded a nearly transform-limited 0146-9592/91/030153-03$5.00/0
Fig. 1. Schematic of the femtosecond cw passively modelocked Ti:sapphire laser system with a fiber compressor. SA, saturable-absorber dye jet; BF, birefringent filter. © 1991 Optical Society of America
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OPTICS LETTERS / Vol. 16, No. 3 / February 1, 1991 0"C&27
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SPM was confirmed, and the pulse width showed thresholdlike change with pumping power. Thus one possible explanation for the above properties is a solitonlike balance of SPM and negative GVD or dispersive pulse shaping, which has already been reported for colliding-pulse mode-locked ring dye lasers." Previously there have been discussions on the pulseshaping process and solitonlike behavior.'4' 7 However, the existence of the time-dependent absorption and gain in the cavity might produce some deviation from the soliton in the fiber described by the nonlinear Schr6dinger equation. Therefore, in the interpretation of the observed properties of femtosecond Ti:sapphire lasers, there should be more discussion and systematic experiments on various aspects, including the mode-locking mechanism of Ti:sapphire lasers or the pulse-shaping process' 8 and the output stability. Because the peak output power is high enough to produce shorter pulses, a pulse compression experiment using fiber and prism pairs was performed (Fig. 1).19 The length of the polarization-preserving fiber with a 4-!tm core diameter was -39 cm, and the two high-dispersion SF59 glass prisms were separated by 90 cm. The autocorrelation trace and the spectrum after optimum compression are shown in Fig. 4. The -170-fsec input pulses were compressed to 50-fsec pulses (sech2 pulse shape) with no wings. The average
Fig. 2. (a) Autocorrelation trace and (b) spectrum of pulses obtained directly from the cavity. The autocorrelation function and spectrum for the sech2 shape are plotted as the open circles and dashed curve, respectively.
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er, although the output power (160 mW) increased by 2 a
only -30 mW over that of the 5-W pumping case. In
the region of the 5.5-9-W pumping power level, a slight decrease in the pulse width near 200 fsec and a slight increase in the spectral width were observed. In addition, the center wavelength shifted toward the blue side. Above this pumping power mode locking was not stable. Here the intracavity peak intensity and the GVD produced by the prism pair should be noted for discussion. The peak intensity in the Ti:sapphire rod is over 20 GW/cm2 , assuming 1-mrad beam divergence in full angle. That is high enough to permit SPM in the Ti:sapphire rod, in the case without intracavity chirp compensation.9 The GVD produced by a prism pair is calculated to be -1.8 X 103fsec2 (negative) at 775 nm for the 200-mm separation and the 4.5-mm glass path length (for 190-fsecpulses at 775 nm) and the parameters of the SF57 glass (n = 1.761, dn/dX = -7.25 X 10-2 Am'-, and d2n/dX 2 = 2.83 X 10-1' pm 2 at 775 nm).' 2
On the other hand, the Ti:sapphire rod might be the major positive GVD element, because of its 20-mm length. The GVD of the rod is estimated to be 1.2 X 103 fsec2 from the dispersion parameter of sapphire' 3 (d 2n/dX 2 = 7.43 X 10-2 Am- 2 at 775 nm). In total, excess negative GVD (-6 X 102 fsec 2) should exist.
To summarize the experimental results, the obtained femtosecond pulse and spectrum were close to the sech2 -shape assumption and almost transform limited, the existence of excess negative GVD and
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Fig. 3. (a) Dependence of the spectral width and pulse width on pumping power. (b) Dependence of output power and center wavelength on pumping power. All data were taken in a fixed alignment. At 10-W pumping power, both stable and unstable mode-locked operation were observed; the data points in parentheses correspond to unstable mode locking.
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a
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o.{
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730
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Fig. 4. (a) Autocorrelation trace and (b) spectrum of pulses after fiber compression.
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