February 1, 2000 / Vol. 25, No. 3 / OPTICS LETTERS183

Generation of intense ultrabroadband optical pulses
by induced phase modulation in
an argon-filled single-mode hollow waveguide
Naoki Karasawa and Ryuji Morita
Department of Applied Physics, Hokkaido University, and Core Research for Evolutional Science and Technology,
Japan Science and Technology Corporation, Kita-13, Nishi-8, Kita-Ku, Sapporo 060-8628, Japan

Hidemi Shigekawa
Institute of Applied Physics, University of Tsukuba, and Core Research for Evolutional Science and Technology,
Japan Science and Technology Corporation, 1-1-1 Tennodai, Tsukuba 305-8573, Japan

Mikio Yamashita
Department of Applied Physics, Hokkaido University, and Core Research for Evolutional Science and Technology,
Japan Science and Technology Corporation, Kita-13, Nishi-8, Kita-Ku, Sapporo 060-8628, Japan
Received August 26, 1999
We experimentally demonstrate the generation of intense ultrabroadband optical pulses whose spectrum ranges
from 300 to 1000 nm (700-THz bandwidth) with a well-behaved spectral phase and 23-mJ pulse energy by anovel, simple setup utilizing induced phase modulation (IPM) in an argon-f illed single-mode hollow waveguide.
Fundamental as well as second-harmonic pulses produced by one common femtosecond pulse from a Ti:sapphirelaser-amplif ier system are copropagated in the hollow waveguide. The effect of the delay time between thetwo input pulses on the IPM spectral broadening is clarif ied and confirmed to agree with the theoretical result.
It is found that the compressed pulse duration from this pulse is 1.51 fs if its phase is completely compensatedfor.  2000 Optical Society of America
OCIS codes: 060.4370, 060.7140, 320.5520, 320.5540, 320.7140.

Generation of an ultrabroadband optical pulse with af luent frequency dependence of the phase is importantfor creating a monocyclelike optical pulse and for shaping multiwavelength optical pulses. Recently, generation of an ultrabroadband optical pulse by use of
induced phase modulation1 (IPM) as well as self-phasemodulation (SPM) in a glass fiber was proposed,2,3 andbroadband pulse generation from 480 to 900 nm wasexperimentally demonstrated by use of a fundamental pulse and a signal pulse from an optical parametric amplifier based on a femtosecond Ti:sapphire laser
system.4
Single-mode capillary fibers filled with noble gases
have been used to broaden the spectrum of the opticalpulses through dispersive SPM,5,6 and sub-5-fs, highpowered pulses have been generated by use of thistechnique.6 Also, 8-fs pulses at 400 nm have been
generated with a similar approach.7 Because of thechemical stability of noble gases, the damage threshold at the capillary fiber is relatively high8 共⬃100 mJ兲
compared with that of a glass fiber. Also, the large
walk-off length is advantageous for spectral broadening by use of IPM. We have analyzed nonlinear chirpfrom SPM and IPM in the case of two-pulse propagation in a single-mode capillary fiber and found that bychoice of a proper delay time between pulses it is possible to generate ultrabroadband pulses whose spectrum
covers the near infrared to the near ultraviolet, witha f luent frequency dependence of the phase by meansof a practical setup.9 Since the second-harmonic andthe fundamental waves are generated from one com0146-9592/00/030183-03$15.00/0

mon femtosecond pulse, the carrier-phase difference
between their waves is constant.10,11 This enables us
to synthesize constructively two spectrally broadened
waves at the fiber output. In this Letter an experimental verification of this analysis is presented.
The experimental setup is shown in Fig. 1. The
output beam of a Ti:sapphire laser-amplif ier system(Femtopower 1000; center wavelength, ⬃790 nm; pulsewidth, 24 fs; repetition rate, 1 kHz; pulse energy,
1 mJ) is passed through a 0.5-mm-thick b-barium
borate (BBO) crystal 共C1 兲. Second-harmonic pulsesgenerated by type I phase matching are separated
from fundamental pulses by two harmonic separators(HS’s). We rotate the polarization direction of the fundamental pulses by 90± by use of two periscopes (PS) tomatch it with that of the second-harmonic pulses. TheFig. 1. Experimental setup of ultrabroadband opticalpulse generation. See text for definitions.
 2000 Optical Society of America

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OPTICS LETTERS / Vol. 25, No. 3 / February 1, 2000

fundamental and the second-harmonic pulses are combined by use of dichroic mirror (DM) and collimatedby a spherical mirror with a 20-cm focal length. Acapillary fiber with a 34-cm length and a 0.1-mm inner diameter is positioned in a chamber filled withargon gas with a pressure of 3.3 atm. The chamber
has two 1-mm-thick sapphire windows (S1 and S2 ). A
retroref lector (RR) is used to adjust the delay time ofthe second-harmonic pulses with respect to the fundamental pulses. The input energies of the pulses
are adjusted by use of variable neutral-density filters(V ND’s). Both input pulses are redirected by a f lip
mirror (F1 ) and evaluated by use of a transient-grating
frequency-resolved optical gating (TG-FROG) apparatus.12 To include the inf luence of the sapphire window, S1 , we adjust the beam sizes and pass the beams
through the equivalent sapphire plate, S3 , before they
enter the TG-FROG apparatus. The measured timedependent intensities and phases are used for numerical calculations. To calibrate the delay time between
the fundamental and the second-harmonic pulses, weuse a 10-mm-thick BBO crystal 共C2 兲 to generate sumfrequency pulses from these pulses. To avoid the necessity of positioning the crystal inside the chamber
we redirect the pulse beams by use of a f lip mirror,
F2 , and focus them at the crystal. The crystal is positioned such that the product of the path length andd 苷 1兾vg2 2 1兾vg1 , and vgi , the group velocity of thepulse i (the subscripts 1 and 2 represent the fundamental and the second-harmonic, respectively), is equivalent to the original path to the fiber entrance, i.e.,
共a 1 b兲 3 dair 苷 c 3 dAr, 3.3atm in Fig. 1. Also, S3 ispositioned so that the path length from F1 to S1 isequal to that from F1 to S3 . The delay position thatgives the largest sum-frequency signal is determinedas the delay time 0 共Td 苷 0兲, which corresponds tothe situation in which both the fundamental and thesecond-harmonic pulses coincide at the fiber input end.
A negative Td corresponds to the case in which thesecond-harmonic pulse enters the fiber before the fundamental pulse.
Fundamental pulses of 40.8-mJ energy and secondharmonic pulses of 37.8-mJ energy are focused at thefiber entrance. Figures 2(a)–2(c) show the spectra at
the output of a fiber. In Fig. 2(a), two spectra obtained when the two pulses are propagated separately
are shown. In this case the spectral broadening isdue to only SPM. When the delay time between two
copropagated pulses is adjusted so that these pulses
overlap inside the fiber, we observe larger spectral
broadening owing to IPM and SPM [Figs. 2(b) and2(c)]. Inside the fiber, d 苷 185 fs兾m, and thus the
propagation-time difference between the fundamental
and the second-harmonic pulses for the entire fiber
length of 34 cm is 63 fs (185 fs兾m 3 0.34 m). Figure 2(b) shows the spectrum obtained when the twopulses meet near the fiber entrance 共Td 苷 13 fs兲. It
can be observed that the spectral intensity near bothedges [the spectral intensity at shorter wavelengths
than the second harmonic (less than ⬃400 nm) andlonger wavelengths than the fundamental (more than⬃800 nm)] increases, but it does not increase much at
the middle. Figure 2(c) shows the spectrum when thetwo pulses meet near the fiber exit 共Td 苷 280 fs兲. In
this case, unlike in Fig. 2(b), an intensity increase of
the spectrum can be observed in the middle section
共450 550 nm兲; in particular, a new peak can be observed near 480 nm. The best overlap of the spectrum
between two pulses, with more-homogeneous broadening from 300 to 1000 nm (700-THz bandwidth), is observed at this delay time. This delay-time dependenceof the spectral broadening agrees well with our previous theoretical prediction.9 Also, the peak powersthat are required for spectrum overlap of the two pulses
are consistent with those of our theoretical analysis.9
The total output pulse energy is 23.0 mJ (29.3%
eff iciency), corresponding to 11.9 and 11.1 mJ for
the fundamental and the second-harmonic pulses,respectively. These corresponding output energies
give energy throughput of 29.2% 共11.9/40.8兲 for thefundamental and 29.4% 共11.1/37.8兲 for the secondharmonic pulse. However, the coupling loss at thefiber input is unknown. To determine this quantity
we perform numerical calculations to obtain thespectral broadening at various input-pulse energies
for SPM (Ref. 9) and compare them with spectra
obtained from SPM experiments. In the calculations the temporal intensity and the phase prof ile asmeasured by TG-FROG for the fundamental and thesecond-harmonic pulses are used. The calculations
include accurate dispersion effects resulting from theargon gas13 and the waveguide mode14 and a SPM nonlinear term with self-steepening. In the calculations
the following parameters are used: lc 苷 767.95 nm,Tp 苷 24 fs, P0 苷 1.03 GW (values of P0 苷 0.12, 0.29, and0.57 GW are also used, but the calculations are notshown directly here), and n2 苷 2.99 3 10223 m2 兾W forFig. 2. (a) – (c) Experimental and (d) – (f ) calculated spectra when the fundamental and the second-harmonic pulses
are propagated separately and are copropagated in afiber: (a), (d) SPM only; ( b), (e) IPM 1 SPM, Td 苷 13 fs;(c), (f ) IPM 1 SPM, Td 苷 280 fs. In the inset in (c), thetemporal intensity of the inverse Fourier transform of thespectrum, assuming a constant phase, is shown.

February 1, 2000 / Vol. 25, No. 3 / OPTICS LETTERS

fundamental pulses, and lc 苷 404.84 nm, Tp 苷 58 fs,
P0 苷 0.46 GW (0.073, 0.20 GW are also used butnot shown), and n2 苷 4.78 3 10223 m2 兾W for secondharmonic pulses, where lc is the center wavelength, Tp
is the pulse width, P0 is the peak power, and n2 is thenonlinear refractive index.15 The effective core area16of the EH11 mode is calculated to be 0.477pa2 , where ais the radius of the capillary fiber. Here we assume
that the spatial prof ile of the electric field for its modeis J0 共2.405r兾a兲,14 where J0 is the zeroth-order Bessel
function. The nonlinear refractive index is taken from
Ref. 17, and we use different values of n2 for the fundamental and the second-harmonic waves, taking into
account the dispersion of this n2 parameter. The theoretical propagation loss that is due to the waveguideitself14 is 52% for the fundamental wave and 17% for
the second-harmonic wave. If we consider only thisloss, the net input energy evaluated from the output
energy 共11.9 mJ兲 for the fundamental wave is 24.8 mJ
共11.9 mJ兾0.48兲. The calculated spectral broadening
obtained by use of SPM with this value 共24.8 mJ兲 agrees
with the experimental broadening. We have also verified this agreement, using different input-pulse energies. Thus the coupling loss at the fiber entrance (inmicrojoules) is evaluated to be 39% 关共40.8 2 24.8兲兾40.8兴.
For the second-harmonic wave, the net input energy
evaluated from output energy by consideration of thetheoretical waveguide loss is 13.4 mJ 共11.1 mJ兾0.83兲.
However, it was necessary that we set 26.7 mJ as thepractical input-pulse energy to obtain similar broadening in SPM experiments. Therefore, the coupling loss
(in microjoules) becomes 29% 关共37.8 2 26.7兲兾37.8兴 forthis case. This coupling loss has also been conf irmed
for different input-pulse energies. The reason for thisdifference is probably defects such as microbending,whose effect becomes remarkable for shorter wavelengths, inside the fiber.14 In Figs. 2(d)–2(f ), thenumerically calculated spectra obtained with theparameters thus determined are shown. In Fig. 2(d),the calculated spectra for SPM for the fundamental andthe second-harmonic waves are shown. In Figs. 2(e)and 2(f ), the calculated spectra for IPM 1 SPM at
two different delay times are shown. It can be seenthat the relative spectral intensity around the central
part from 400 to 800 nm is larger in Fig. 2(f ) thanin Fig. 2(e). In addition, the more-homogeneous
spectrum broadens from 300 to 1000 nm in Fig. 2(f )compared with that in Fig. 2(e). These behaviors
agree with those shown by the experimental spectra.
In Figs. 2(e) and 2(f ), the calculated wavelengthdependent phases are shown. It can be seen that thephase variations are relatively smooth, which is important for phase compensation for pulse compression andsynchronous multicolor shaping.
In the inset of Fig. 2(c), the inverse Fourier transform of the experimental spectrum of Fig. 2(c) when its
spectral phase is set to be constant is shown. The evaluated pulse duration is 1.51 fs. Since this spectrum isgenerated by the interaction between two pulses, thechirp is not expected to be linear, and thus it is necessary to use a method that can compensate for the185

nonlinear chirp. One such method is the use of an
adaptive spatial light modulator, which has the potential for generating a monocyclelike pulse.
In conclusion, ultrabroadband (300–1000 nm) optical pulse generation by use of IPM as well as SPM ina gas-f illed single-mode hollow waveguide has been experimentally demonstrated. By utilizing fundamental
as well as second-harmonic pulses from a femtosecond
Ti:sapphire laser-amplifier system in an argon-f illedcapillary fiber, we observed pulses whose spectrum
covers the near infrared (1000 nm) to the near ultraviolet (300 nm). To our knowledge this is the broadest coherent spectrum (700-THz bandwidth) with awell-behaved spectral phase that has been obtained.
The coupling loss was evaluated by comparison of
the SPM experimental and calculated spectra. The
IMP 1 SPM spectral broadening behavior as a function of the delay time has been experimentally clarified, and agreement with the theoretical analysis has
been conf irmed.
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