Tunable multiwavelength synchronized femtosecond pulse trains for ultrafast spectroscopy

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15 March 1996

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OPTICS COMMUNICATIONS

ELSEWIER

Optics Communications

124 (1996) 505-511

Tunable multiwavelength synchronized femtosecond pulse trains for ultrafast spectroscopy JYhPYng

Wang*, Hsiao-Hua Liu, Meng-Ru Li

Insrinrte of Atomic and Molecular Sciences, Academia Sinica, and Department of Electrical Engineering, National Taiwan University,

Taipei, Taiwan Received 4 September

1995; revised version received 23 October 1995; accepted 24 October 1995

Abstract By using femtosecond optical techniques including self-phase modulation, spectral filtering and dispersion compensation, we constructed a tunable multiwavelength synchronized femtosecond light source from a cw modelocked femtosecond Ti:sapphire laser, and experimentally characterized its performance. Synchronized femtosecond pulses of different central frequencies can be selected independently within a 60-nm FWHM band. Pulse duration can also be programmed from 200 fs to 100 fs by controlling the bandwidth. Synchronization between pulses of different wavelengths are experimentally shown to be within a few femtoseconds. We demonstrate applications of the light source in ultrafast pump-probe spectroscopy with a sensitivity as great as lo-” in transient transmission measurements. PACS:

42.65.Re; 78.47; 42.6O.F~ Self-phase modulation; Ultrafast spectroscopy; Pump-probe; Femtosecond; Mode-locking; Synchronization

Keywords:

1. Introduction Recent development in femtosecond lasers has enabled scientists to investigate ultrafast dynamics in nature with unprecedented temporal resolution. Femtosecond lasers not only greatly enhance the capability of existing spectroscopic techniques, but also bring new ways of manipulating molecular systems by ultrafast optical pumping. Indeed, in the frameworks of pumpand-probe and nonlinear spectroscopy, femtosecond lasers have emerged as one of the most important tools for studying ultrafast dynamics. The potential of femtosecond lasers is not limited to merely producing ultrashort pulses for pump-and* Corresponding Taiwan.

author.

Address:

PO Box 23-166,

Taipei

107,

0030-4018/96/$12.00 0 1996 Elsevier Science B.V. All rights reserved SSDIOO30-4018(95)00682-6

probe or nonlinear mixing. The large bandwidth and the phase coherence among the spectral components of femtosecond laser pulses enable us to restructure the pulses optically in ways which cannot be done with conventional lasers. As examples, femtosecond timedomain waveform programming by use of dispersive optics and spatial light modulators have been demonstrated [ 11, and rapid progress toward highspeed dynamic programming is being made [ 21. Such techniques extend our Capability in ultrafast spectroscopy from pulse excitation to waveform excitation, and open up new ways toward studying coherent interactions between light and matter. Similar techniques can be applied to programming femtosecond pulses for desired waveforms in the frequency domain [ 31. Because the frequency clusters in the programmed waveform are derived from the same

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source, they represent accurately synchronized waveforms with different center (carrier) frequencies. This feature is highly desirable in multiwavelength ultrafast spectroscopy, because in addition to pulse durations, the accuracy of waveform synchronization determines the time resolution of the experimental system. In this paper we present a femtosecond laser source which produces tunable multiwavelength synchronized pulses for ultrafast spectroscopy. The input pulses are from a 130-fs cw modelocked Ti:sapphire laser, and the programming is carried out with standard femtosecond optical techniques including self-phase modulation, spectral filtering, and dispersion compensation. We characterized the system performance in its pulse durations, time-bandwidth products, multiwavelength synchronization and stability, and demonstrate its applications in ultrafast pump-probe spectroscopy with high sensitivity.

2. Working principles and experimental

setup

In multiwavelength ultrafast spectroscopy both synchronization and wavelength tunability are important. In order to produce synchronized ultrashort pulses with large wavelength separations by spectral filtering, it is desirable to have a coherent light source with a bandwidth as large as possible. Although the bandwidth of our 130-fs laser is only 12 nm, its 46-kW peak power enables us to take advantage of the ubiquitous optical Kerr effect for bandwidth expansion. Self-phase modulation in Kerr media increases the spectral width of ultrashort pulses [ 41. Unlike chaotic light such as that comes from fluorescence or plasma, the spectral components of self-phase modulated ultrashort pulse maintain definite phase relations with each other. Phase coherence over a large expanded bandwidth enables us to produce multiwavelength synchronized femtosecond pulses by spectral filtering and dispersion compensation. Large self-phase modulation and spectral expansion can be obtained by propagating high power femtosecond pulses through a short piece of single-mode optical fiber. In optical fiber, the intensity of the laser pulses is large due to the small core size. Unlike in free space propagation, in optical fiber the region of high intensity is not limited by the confocal parameter. Instead, it extends to the whole length of the fiber. Hence large

I24 (1996) 505-511

26.cm f\

(0

fiber I\

6 nJ/pulse

v 20x objective

20x objective

Fig. 1. Schematicdiagramof the multiwavelengthsynchronizedfemtosecond light source.

amount of self-phase modulation can be produced even though the glass materials in the fiber do not have particularly large Kerr nonlinearity. In our laser system the expanded spectrum is dispersed spatially with prism pairs. Using slits to select the desired bands and the same prism pairs to compensate for dispersion, we produce two synchronized pulses whose central wavelengths and pulse durations are independently tunable. A schematic diagram of the multiwavelength femtosecond laser is shown in Fig. 1. The energy of the 130-fs pulses from the modelocked Ti:sapphire laser is 6 nJ. We use 20 X objective lenses to couple the laser beam into a short piece of single-mode silica optical fiber (newport model F-SV). The core size is 4 p,rn and the cutoff wavelength is 580 + 30 run. The coupling efficiency is about 50%, and the peak intensity in the fiber is 1.8 X 10” W/cm*. After the fiber we split the beam into two, and use prism pairs made of SF- 14 glass to disperse the expanded spectrum. Although grating pairs would make the system much more compact, low loss gratings are much more expensive than glass prisms. At the end of the prism pairs we use simple slits

to filter the spectrum. The positions of the slits determine the carried frequencies (central wavelengths), while the widths of the slits determine the bandwidths (hence the pulse durations) of the two beams. Because the tuning is done outside of the laser cavity, it can be easily automated. The reflecting mirrors at the back of the slits are slightly tilted down, so that the retrore-

J. Wang et al. /Optics

Communications

ftected beams can be separated out. The prism pairs also function as group-velocity dispersion compensators [ 51. By adjusting the distance between the prisms, we eliminate the group-velocity dispersion caused by the optical fiber and other optics to restore the minimum pulse durations. Because fiber coupling is sensitive to environmental fluctuations, we take special care in our setup to ensure stable coupling. (i) We use stable stainless-steel mounts and stages (Klinger SB 18 and MR5.16) to hold the fiber and the objective lenses. The mounts sit on 9cm diameter solid aluminum bases which are clamped on the optical table. Fine adjustment of the fiber position is made with differential micrometers. It is our daily experience that mechanical stability is most important for stable fiber coupling. (ii) Optical feedback generally causes fluctuations in the laser output power. We wedge cleave the fiber by about 7” to prevent feedback to the input laser. (iii) To eliminate the effect of air current, we seal the beam path completely inside a Plexiglas box. The box encloses the fiber coupling assembly as well as the pump-probe experimental setup. (iv) We fix the whole piece of fiber, not just the ends, on solid block mounts to avoid polarization fluctuations due to twisting or bending. These measures have proved to be effective that the power fluctuation after the fiber coupling remains the same as the < 2% r.m.r. input fluctuation. The FWHM spectral width of the input pulses is 12 nm. The time-bandwidth product is 0.73, which corresponds to 2.33 times of the Fourier transform limit for hyperbolic secant pulses [ 61. With external prism pairs to compress the pulses, the time-bandwidth product can be reduced to 0.53. Since the initial chirp of the input pulses is not significant compared with the much larger chirp acquired in the optical fiber, it is not essential to compress the chirp before launching the pulses into the fiber. Fig. 2 shows the spectra of the 130-fs pulses before and after passing the optical fiber. The input laser wavelength is tuned at 800 nm. The shapes of the spectra do not depend on me input wavelength. Two fiber lengths, 7 cm and 26 cm, are used. For both fiber lengths the FWHM spectral width increases from 12 nm to about 60 nm. With the 7-cm fiber the spectrum has the characteristic flat-top shape of seIf-phase modulated pulses [7], whereas with the 26-cm fiber the shape of the spectrum is near Gaussian.

124 (1996) 505-511

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Fig. 2. (a) Spectrum of the cw modelocked Ti:sapphire laser. (b) Spectrum after the 7-cm fiber. (c) Spectrum after the 26-cm fiber.

The range of pulse duration that can be tuned with the slits are determined by two factors. In the lower end the pulse duration is limited by uncompensated high order dispersion to about 100 fs. Effects of high order dispersion become important as the bandwidth increases. More precise compensation schemes must be used to ease this limit [ 8,9]. In the upper end the pulse duration is limited by the beam size of a single frequency component to about 200 fs. As the slits become smaller than the single-frequency beam size, further decrease of slit size does not make the spectrum narrower. The limit can be eased by using more dispersive optics such as grating-pairs or by increasing the prism separation, or by using narrow bandwidth interference filters. Band selection also affects the power of the output beams. For a lo-nm bandwidth the average power of the selected beams are about 30 mW. The minimum length of fiber required for bandwidth expansion can be estimated theoretically from the peak power and pulse duration of the input laser [7,12]. Longer fiber does not produce proportionally larger

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Communications 124 (1996) 505-511

self-phase modulation, because as the bandwidth increases, group-velocity dispersion starts to broaden the pulse and reduce the peak power rapidly. Because we use large prism separations to raise the upper limit of the pulse duration set by the beam size, a fiber length longer than sufficient for bandwidth expansion is employed to balance off the large negative groupvelocity dispersion from the prisms. Alternatively, one can use interference filters to select the desired bands, then the fiber can be just long enough to provide desired bandwidth expansion. The trade-off is that one cannot use interference filters to continuously tune either the central wavelength or the pulse duration. A particular interference filter must be used for each desired central wavelength and pulse duration. Since stimulated Raman scattering is also an important nonlinear process in optical fiber that competes with self-phase modulation, one should avoid using excessively long fiber. According to Ref. [ lo] and Ref. [ 111, the critical power of stimulated Raman scattering in our 26cm fiber is 69 kW. The initial peak power of the pulses in the fiber is 23 kW, much lower than the critical power. As the pulses propagate along the fiber, the pulse duration is increased by dispersion and selfphase modulation, hence the peak power decreases. Therefore it is safe to say that stimulated Raman scattering is not important under our experimental conditions.

3. Experimental

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Fig. 3. Autocorrelation measurement of the 760-nm pulse train. Inner curve: Gaussian fit. Outer curve: hyperbolic-secant fit. Dots: experimental data.

show the intensity autocorrelation functions of a 760nm beam and a 820~nm beam selected from the expanded spectrum. Both autocorrelation functions are fit with Gaussian and sech’ curves. Under the Gaussian fit the FWHM pulse durations are 155 fs and 146 fs respectively, and under the sech’ fit the pulse durations are 139 fs and 130 fs. The fittings show that the pulses have negligible residual energy in the wings, and the quality of the pulses is comparable to the output of ordinary cw modelocked lasers. This two figures show that high quality pulses can be obtained even when the selections are made at the wings of the expanded spec-

results

Because of the tunability of both wavelength and pulse duration, it is impossible to show pulses of all the possible configurations. Experimentally we observed that the pulse characteristics are similar for various configurations, therefore we present the analysis of a typical configuration in detail rather than presenting many similar data. The configuration we choose is two synchronized pulse trains at 760 nm and 820 nm when the input wavelength is 790 nm. These two wavelengths have nothing special except that 760 nm is the pump wavelength and 820 nm is the probe wavelength of our experiment described in Section 5 which demonstrates the potential of this system. The FWHM bandwidths are 10 nm for both beams. The separation of the two wavelengths are set as large as possible to show the system at its limit of performance. Fig. 3 and Fig. 4

Fig. 4. Autocorrelation measurement of the 820-nm pulse train. Inner curve: Gaussian fit. Outer curve: hyperbolic-secant fit. Dots: experimental data.

J. Wang et al. /Optics

1.o

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Communications

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Fig. 5. Crosscorrelation measurement compared with that of perfectly synchronized hyperbolic-secant and Gaussian pulses. Inner curve: perfectly synchronized Gaussian pulses. Outer curve: perfectly synchronized hyperbolic-secant pulses. Dots: experimental data.

trum where the clipped spectra are highly asymmetric and the nonlinear chirp are largest [ 121. Synchronization between the two output pulse trains with different central frequencies can be verified by crosscorrelation measurements. Assume the pulse shapes are either Gaussian or hyperbolic-secant, we calculate the expected crosscorrelation from the autocorrelations of the individual pulse trains. The result is compared with experimental data. In Fig. 5 the measured crosscorrelation between the 760-nm beam and the 820-nm beam is compared with the expected crosscorrelation for both Gaussian and hyperbolic-secant pulses. The agreement between the data and the theoretical curve shows that the jitters between the two pulse trains are smaller than a few femtosecond, which is the sensitivity limit of our crosscorrelation measurements. The small deviation between the experimental data and theoretical curves at the right wing is likely due to slight asymmetry in the pulse wings which cannot show up in the autocorrelation curves. Data in Fig. 3-5 are taken with a Hamamatsu R928 photomultiplier, a wideband electronic amplifier, and an A/D converter directly. The bandwidth of the electronics is about 100 kHz. Lock-in detection or signal averaging schemes are not used. The data show that the “raw” signal-to-noise ratio of the experimental system is smaller than 2%, which can be further improved by lock-in detection and signal averaging. Because the

124 (1996) 505-511

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mechanism we use to generate new wavelengths is completely passive, the stability of the output beams is about the same as the input beam. Supercontinuum generated by amplified femtosecond pulses [ 131 has also been used for multiwavelength ultrafast spectroscopy [ 14,151. However, many femtosecond amplifiers are limited in repetition rates, which make lock-in detection ineffective. Energy fluctuation in amplifier systems is also much larger than in cw modelocked lasers. For experiments which do not require large pulse energy or large wavelength separation, the system described in this paper is a viable alternative to femtosecond amplifiers because it offers comparable average power and better stability, yet the cost and complexity are much less. Time-bandwidth product is also an important quantity for characterizing ultrashort pulses. For pulses of a fixed temporal width, it tells how much excessive bandwidth the pulses have over the Fourier transform limit and vice versa. Fig. 6 shows some typical data of the time-bandwidth products at various central wavelengths and pulse durations. Variations of the data are due to the fact that the characteristics of the input pulses, such as the pulse duration and the chirp parameter, depend on the fine tuning of the Ti:sapphire laser and change slightly from run to run. The data show that across the wavelength tuning range the time-bandwidth products of the output pulses stay close to that of the input pulses. Better time-bandwidth should be achievable with a combination of prism pairs and grating pairs

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products of the output pulses at various operating configurations. Round dots: data for 26-cm fiber and prism pairs. Cross: data for 7-cm fiber and lo-nm bandwidth interference filters.

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to eliminate third order dispersion, with a trade-off in power loss and system complexity [ 81.

4. Comparison trains

with other synchronized

pulse

Synchronized, independently tunable femtosecond pulses can also be obtained from synchronously pumped optical parametric oscillators or dye lasers. parallel operation of two synchronously pumped parametric oscillators has been reported recently. The timing jitters between the two pulse trains were L-50 fs [ 161. A femtosecond dye laser synchronously pumped by a femtosecond Ti:sapphire laser has also been demonstrated. The jitters between the two lasers were reported to be less than 100 fs [ 171. In parallel synchronously pumped dye lasers the jitters were reported to be about 1 ps [ 181. In comparison, the timing jitters of our system are an order of magnitude smaller. The result is hardly a surprise because with self-phase modulation the process of generating new frequency components is completely passive. No optical gain media or resonant cavities are involved. On the other hand our system has the disadvantages that the wavelength tuning range and output power is smaller than synchronously pumped parametric oscillators and dye lasers.

5. Demonstration

of pump-probe

experiments

As a demonstration of the system in real ultrafast pump-probe experiments, we measured the transient transmission change of the hexamethyl-indotricarbocyanine iodide (HITCI) dye in ethylene glycol solution. Sun et al. have employed a similar experimental setup to study carrier heating effects in quantum well diode lasers [ 191. Instead of using flow cells, we use a jet nozzle to suspend the dye solution in air to avoid cumulative thermal and photo-degradation effects on cell surfaces. The solution is maintained at a low concentration to reduce effects from triplet states. The dye solution is pumped at 760 nm, and transient transmission is probed at 780 nm, 800 nm, and 820 nm. The result for the 820 nm probe is shown in Fig. 7, together with the crosscorrelation curve which marks the zero delay point. A detailed analysis and interpretation of the response curve is beyond the scope of this paper.

-7

0

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(~9

Fig. 7. Transienttransmissionchange of dilute HITCI solution at 820 nm after pumped at 760 nm. The crosscorrelation curve marks the zero delay point between the pump and the probe.

Nevertheless the data clearly show the rapid population transfer in the sub-picosecond time scale. With narrow band lock-in detection and a balanced subtractor to remove background noises [ 201, we can detect changes in transmission in the 10e5 range.

6. Conclusion In conclusion, by use of self-phase modulation, spectral filtering, and dispersion compensation, we constructed a multiwavelength synchronized femtosecond light source from a modelocked Ti:sapphire laser, and characterized it experimentally. Width-tunable femtosecond pulses can be selected within the 60-nm spectrum expanded by self-phase modulation. The central position of the expanded spectrum can also be tuned from 750 nm to 950 nm by tuning the input laser. Synchronization between pulses of different wavelengths are verified to be within a few femtoseconds. General characteristics of the output pulses are similar to that of the input, except for smaller pulse energy. Although the principles we utilized to construct such a light source have been discovered for more than a decade, nevertheless we find that the simplicity and stability of such systems offer a practical solution for ultrafast spectroscopy experiments that do not require large pulse energy or widely separated wavelengths. We are using this light source to investigate the population transfer and relaxation of large molecules in soluand we believe such multiwavelength tions, femtosecond light sources can be very useful for many other ultrafast spectroscopy experiments.

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Communications

Acknowledgements We thank the National Science Council of the Republic of China in Taiwan for financial support of this research project (contract NSC 83-0208-M-001107).

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