Appl. Phys. B 74 [Suppl.], S225–S229 (2002)

Applied Physics B

DOI: 10.1007/s00340-002-0891-y

Lasers and Optics

m. hirasawa1,3,✉
n. nakagawa1,3
k. yamamoto1,3
r. morita1,3h. shigekawa2,3
m. yamashita1,3

Sensitivity improvement of spectral phaseinterferometry for direct electric-fieldreconstruction for the characterizationof low-intensity femtosecond pulses
1 Department of Applied Physics, Hokkaido University, Kita-13, Nishi-8, Kita-ku, Sapporo, 060-8628 Japan
2 Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, 305-8573 Japan
3 CREST, Japan Science and Technology Corporation (JST), Japan

Received: 18 September 2001/
Revised version: 14 November 2001
Published online: 16 July 2002 • © Springer-Verlag 2002
ABSTRACT A modified-SPIDER (spectral phase interferometryfor direct electric-field reconstruction) technique where external powerful optical pulses are employed as a light source of
chirped reference pulses for sum-frequency generation is proposed and is demonstrated experimentally. It provides great
improvements in sensitivity and signal-to-noise ratio, and will
enable a single-shot measurement for a weak pulse, such as
1 nJ/THz-bandwidth. The technique demonstrates high accuracy for determining the delay-time, t, for weak pulse characterization, which greatly affects the group-delay dispersion of thereconstructed spectral phase.
PACS 39.30.+w

1Introduction

In recent years, significant advances have been
made in the measurement [1–5] as well as the generation ofultrashort optical pulses [6–8]. That is, it has been demonstrated that frequency-resolved optical gating (FROG) [1, 2,
7, 8] and spectral phase interferometry for direct electric-fieldreconstruction (SPIDER) [3–5] techniques are greatly usefulfor the characterization of the temporal intensity and phaseprofiles for few-optical-cycle pulses. Particularly, the latterhas several advantages over the former as follows: a) the
measurement capability of mono-cycle pulses with the ultrabroad bandwidth exceeding one octave, and b) the fast measurement time due to no moving components and non-iterative
reconstruction algorithms. This fast measurement is usefulfor feedback control that is combined with chirp compensation using a spatial light modulator [8, 10, 11]. Furthermore,
SPIDER technique, which is based on the spectral interferometry, has the capability of the single-shot measurement.
This is unlike the standard FROG measurement as a function of the delay-time which is an average measurement ofrepetitive pulses, for example. This single-shot measurement
is essential for the characterization of few-cycle [9] or monocycle pulses because for such pulses the carrier-envelope✉ E-mail: hirasawa@eng.hokudai.ac.jp

the phase becomes significant. That is, few-cycle or monocycle pulses with the same duration but the different carrierenvelope phase interact differently with matter.
The SPIDER technique so far, however, has the disadvantage of low sensitivity, especially for the characterization ofultrashort, ultra-broadband pulses. This is due to the drastic decrease in peak intensity of the strongly-chirped reference pulse, which wastes most of energy of the pulse to bemeasured on itself. For a single-shot SPIDER measurement,
this insensitivity is a serious problem. The characterization ofa weak pulse is also important for external pulse compressionof the self-phase modulated and induced-phase modulated
output from conventional glass fibers [12] or microstructure
fibers [13, 14], the characterization of the optical nonlinear
response from absorbing materials, the characterization ofhigher-order nonlinear optical processes, and the study of ultrafast optical communication.
In order to overcome such a disadvantage, we propose a modified-SPIDER technique in this paper. Instead ofa chirped reference pulse split directly from the pulse to be
characterized, as a chirped, high intensity pulse, we employ
a powerful fundamental seed pulse directly from a regenerative amplifier. According to this idea, we demonstrate theexperimental characterization of femtosecond weak pulses
propagated through a single-mode, fused-silica fiber andevaluate the sensitivity and the signal-to-noise ratio of themodified-SPIDER apparatus compared with the conventionalone.
2

Experimental

Figure 1 illustrates the modified-SPIDER apparatus. It has two optical input parts; the one is for the pulses (testpulse i ; i = 1 − 3) to be measured, and the other the intenselight source to generate the chirped pulse.
All the employed mirrors, the periscopes PSi , the cornerreflectors ORi , and the delay stage (DSi ) were aluminumcoated, except for a flip mirror FM1. The beam splitters BSi
are broadband dielectric multilayer mirrors. The test pulse i
was reflected by the silver-coated flip mirror FM1 and was input into a Michelson interferometer to make two-pulse replicas E 1 and E 2 . The mirror FM1 is used as a switch from theconventional-SPIDER mode to the modified-SPIDER one.

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Applied Physics B – Lasers and Optics
Ti:Sapphire

DS1
Chirped-Pulse Source

BS4

BS5

TS
PS1

A1

FM2

DS2
PS3

A3

RO

A2

DM

RO

OPA

BS1

PS2

idler

EC

Test Pulses

E1

OR1

BS3

E2
ICCD
L S

Test pulse 1(800 nm, 90 fs, 500 nJ)

Test pulse 3(1100 nm, 200 fs, 14 nJ)

Experimental setup of the optical sources both for the chirped
pulse and for the test pulses to be evaluated by the modified-SPIDER. The
intensity of test pulse 1 was controlled by neutral density filters. The test
pulse 2 centered at 800 nm, ∆ f = 20 THz (of the test pulse 3 at 1100 nm,∆ f = 50 THz ) was output from a 3-mm-long single-mode optical fiber,where the pulse was spectrally broadened by the self-phase modulation
FIGURE 2

A4 BS2P

ND

Optical source forchirped-pulse generation(800 nm, 3.7 ps, 1.2 µ J )Test pulse 2(800 nm, 200 fs, 3.6 nJ)
silica fiber 3 mm

FM1
GDD

Spectrometer

fundamental

REGEN

DS2

τ

OR2
NC
PFM

FIGURE 1 Setup of the highly-sensitive modified-SPIDER apparatus. An
external intense optical pulse is employed as a light source of a chirped
pulse which is used for the sum-frequency generation with the pulse to bemeasured. TS, telescope; DSi, delay stages; Ai, aperture; PSi, periscopes;
BSi, beam splitters; GDD, TF5 glass, 10-cm long, group-delay dispersion
medium; DM, dichroic mirror; FM1, flipper mirror; ORi, corner reflectors;
PFM, off-axial parabolic mirror; NC, nonlinear crystal; S, slit; L, lens; P,
polarizer

When the mirror FM1 is flipped out, this apparatus becomesthe conventional-SPIDER mode. The powerful seed pulse(a dashed line) reflected by a fused-silica beam splitter BS1(96% transmission with an anti-reflection coating) was sent to
the Michelson-interferometer to produce two replicas. In addition, this powerful, reflected pulse is conveniently used for an
optical guide of the weak test pulse in the modified-SPIDER
mode and for the calibration of the delay-time τ between two
replicas. After the powerful, transmitted seed pulse passed
through a highly dispersive glass TF5 (10-cm in length),
its linear polarization was changed to the p-polarization bya periscope PS2 and the pulse passed again through the dispersive glass (round-trip group-delay dispersion of 3.46 × 104 fs2
at 800 nm). Thus, a strongly chirped pulse with a 3.7-ps duration was generated. The chirped pulse and two replicas
were focussed onto a β -barium-borate (BBO) crystal NC(type II phase-matching; 50-µm in thickness) by an off-axial
parabolic mirror PFM (5-cm focal length) to produce the
SPIDER signal of the interferometric sum-frequency wave.
After passing through a slit S, a focusing lens L and a polarizer
P, the signal spectra were measured by a 1200-groove/mm
spectrometer (SpectraPro-500i) with a 1024-pixel, intensified
CCD camera (PI-MAX, Roper Scientific), where the spectral
resolution is 0.2 nm. This system enables us to easily compare the sensitivity between the modified-SPIDER techniqueand the conventional one under the same optical components
without realignment.

The experimental setup concerning the optical sources
is shown in Fig. 2. A few percents of the 80-fs, 800-nm,
600-µJ optical pulses from a Ti:sapphire regenerative amplifier (REGEN) operating at a 1-kHz repetition rate passed
through BS4 to be used for the test pulses 1, 2 and for thereference pulse (1.2 µJ before NC in the apparatus). For generating the test pulses, one third of the pulse energy was split
at BS5. The pulse which is guided by beam splitters BS4 andBS5 was reduced to appropriate pulse energy by neutral density filters ND and was used as a test pulse (test pulse 1). To
demonstrate the characterization of the weak pulse propagating through a glass fiber, we used a single-mode, fused-silica,
3-mm long, 2.7-µm core diameter fiber (Newport, F-SPV). To
focus the input pulse into the fiber and to collimate its output,
we used a pair of non-dispersive reflective objectives (× 36)
coated with gold (in) and silver (out), respectively. Test pulse
2 was the self-phase modulated output from the fiber (3.6 nJ)
when the fundamental pulse was injected. Most of the energyof the amplified pulse from REGEN was also used for excitation of the optical parametric amplifier (OPA) system to shift
the center wavelength of the pulses (idler pulse). We abbreviate the self-phase modulated output from the fiber (centeredat 1100 nm, 14 nJ) as the test pulse 3 when the idler pulse wasinjected.
All the pulses were originated from a common light source(REGEN). Therefore, the carrier-phase differences among
them were locked [15]3
3.1

Results and Discussion
Sensitivity comparison

Figure 3 shows the SPIDER signal intensity as
a function of energy of the input pulse to be characterized formodified (open circles) and conventional (open squares) techniques. The plotted relative intensity of the signal was evaluated from the intensity of the positive AC(+τ ) componentof the inverse Fourier transform of the observed interferometric SPIDER signal, which corresponds to the amplitude of theoscillating interferometric part. In the case of the modifiedSPIDER technique, the energy of input test pulse 1 (90 fs,
800 nm) was varied while the energy of the 90-fs, 800-nm

10

0

10

) sti n u . br a(ytisnetn i la ngiS

Signal intensity (arb. units)

HIRASAWA et al.

10

10

-1

10

-2

10

-3

10

-4

10

Sensitivity improvement of SPIDER

0

~1

-1

S227

modified SPIDERSPIDER
(a)

-2

10
100
1000
Chirped-pulse energy(nJ/pulse)

(b)
(c)

0.1

1

10

Input-pulse energy (nJ/pulse)

100

FIGURE 3 SPIDER signal intensities of the conventional SPIDER and themodified SPIDER as a function of the pulse energy to be measured. Inset
shows the signal intensity as a function of chirped-pulse energy

chirped reference pulse (1.2 µJ) was maintained. In the caseof the conventional technique, the energy of the 90-fs, 800-nm
input pulse had been varied before the energy of the input
pulse was split. The energy for the chirped reference pulse is
96 percents of the incident energy at BS4. From Fig. 3 we findthat the intensity for the modified-SPIDER mode indicatesa linear function of input energy, while the intensity for theconventional mode indicates a quadratic dependence. This result suggests that the modified-SPIDER technique has a great
advantage for the characterization of the weak intensity pulse.
In Fig. 3 we also plot three broken lines (a), (b), and (c) that
represent the experimental noise levels for (a) a single-shot
case, (b) a 5000-pulse-accumulation case (corresponding to
the five-second measurement time), and (c) a 100 000-pulseaccumulation measurement. The sensitivity of the modifiedSPIDER technique is hundred times higher than that of theconventional one. For a single-shot measurement, the modified one is ten times higher and has 1 nJ/THz-bandwidth.
The signal intensity D(ω + ωc ) for both conventional- andmodified-SPIDER techniques is written as [3]of Fig. 3 shows the signal intensity of the modified SPIDER
measured as a function of energy of the chirped reference
pulse for a fixed test pulse energy of 100 nJ. This linear dependence indicates the good agreement with the prediction
mentioned above.
All the analyses of the electric-field reconstruction fortest pulse 1 were carried out using the measured Ω/2π =3.87 THz and τ = 746 fs in the same manner as ref. [5]. It
was confirmed that the reconstructed temporal-intensity profile with the 90-fs duration agrees with the results of the independent measurement of the autocorrelation trace under theassumption of a sech2 shape.
3.2

Weak pulse characterization

For the demonstration of the high sensitivity ofthe modified-SPIDER technique, we characterized the weakpulse propagated through a glass fiber for two different-color
pulses (test pulse 2 and test pulse 3), as shown in Figs. 4 and 5.
Figure 4A is the experimental results of the modified-SPIDER
signal (solid curve), and the sum-frequency-wave spectra of
the replica 1 and 2 (dotted curves) for the 800-nm, 3.6-nJ, selfphase-modulated pulse, where the signal was accumulated for
100 000 pulses. They were measured with an excellent signalto-noise ratio. Figure 4B is the corresponding spectral inten-D(ω + ωc) ∝ |χ (2) E c (ωc )E unk (ω) × χ (2) E c (ωc + Ω)E unk (ω)|× cos[φ(ω + ωc + Ω) − φ(ω + ωc) + τ × (ω + ωc )],where E unk (ω) and φ(ω) are the spectral amplitude and phaseof the input-pulse electric field to be characterized at an angular frequency of ω, respectively, E c (ωc ) is the spectral amplitude of the electric field of the chirped reference pulse atan angular frequency of ωc , Ω is the spectral shear and χ (2)is the second-order susceptibility of the BBO crystal. The
quadratic dependence of the sensitivity for the conventionalSPIDER technique results from the fourth-order dependenceof the D(ω + ωc ) on all electric fields E i (i = c and unk). For
the modified-SPIDER technique we employed the external
E c as the chirped pulse independent of the test pulse. Thus,
the signal was proportional to the square of the electric field
E unk of the test pulse, that is, the linear dependence of thetest pulse intensity. The equation also suggests that the linear dependence of the signal to the intensity of the chirped
pulse, which is chosen to be much more powerful, comparedwith that of the conventional-SPIDER technique. The inset

FIGURE 4 A Measured SPIDER signal and replica spectra of the test pulse
2, where Ω/2π = 3.62 THz, τ = 746 fs), and B spectral phase and intensity.
(center wavelength 800 nm, pulse energy 3.6 nJ). C Reconstructed temporal
intensity profile and phase

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Applied Physics B – Lasers and Optics

and complicatedly-chirped pulses. The conventional-SPIDER
technique is poor in retrieving such pulses because the spectral shear Ω is not well-defined for those pulses. The stronglychirped pulse has a wide temporal width, which dims thecorresponding temporal frequency of the chirped reference
pulse. To circumvent this situation, the spectrally-resolved,
modified-SPIDER technique is promising. A chirped pulse
selected independently of the test pulse greatly simplifies thecharacterization of the pulse in the entire spectral region.
Finally, we would like to point out a practical merit of
the modified-SPIDER technique concerning the accurate andeasy determination of the delay-time τ between two replicas,
as we mentioned in the experimental section. In the SPIDER
algorithm, the accuracy of the group-delay dispersion of theretrieved pulse depends on that of the delay-time. For a weakpulse, however, the accuracy of the delay-time can hardly
be guaranteed in the conventional SPIDER technique. In themodified-SPIDER technique, a high intensity, well-behaved
pulse is available for the determination of the delay-time instead of the fragile, weak one.
All the advantages of the modified-SPIDER technique,highly sensitive, high-signal-to-noise ratio, etc. will enable us
the single-shot characterization of a weak pulse with an ultrabroad band.
4

FIGURE 5 A Measured SPIDER signal and replica spectra of the test pulse
3, where Ω/2π = 4.14 THz, τ = 746 fs), and B spectral phase and intensity.
(center wavelength 1100 nm, pulse energy 14 nJ). C Reconstructed temporal
intensity profile and phasesity and phase, and C the temporal intensity and phase, whichwere retrieved using the measured Ω/2π = 3.62 THz andτ = 746 fs. Similarly, Fig. 5 A shows the experimental results,
B the retrieved spectral intensity and phase, and C the temporal intensity and phase for the 1100-nm, 14-nJ, self-phasemodulated pulse (Ω/2π = 4.14 THz and τ = 746 fs), respectively. Available maximum energy (1.2 µJ) of the chirped
pulse was only limited by a technical problem at present, thatis, under the present experimental setup we wasted most of energy to excite the OPA system mentioned above. As a result,when we use a more powerful chirped pulse, a weaker pulse
can be characterized.
In addition to the high sensitivity, the modified-SPIDER
technique has another advantage in signal-to-noise ratio,that is, the great reduction of the background light. In bothconventional- and modified-SPIDER techniques, the main
origin of the background light is due to the second harmonic
generation at the nonlinear crystal and its scattering at surface.
For the modified-SPIDER technique unlike the conventional
one, we can shift the frequency region of the SPIDER signal tothe arbitrary frequency region to avoid the background by the
independent choice of an appropriate chirped-pulse frequency
ωc .
The further merits of the modified-SPIDER are the possibility of expansion of target pulses, especially for strongly-

Conclusion

A new modified-SPIDER technique, where the external powerful optical pulse was used for the sum-frequency
generation, was proposed and demonstrated experimentally.
It was shown that it provides great improvements in sensitivity, on the basis that the developed apparatus has a hundredtimes greater sensitivity for a 100 000-pulse-accumulation
measurement in comparison with the conventional-SPIDER
apparatus. It will enable us a single-shot measurement fora weak pulse, such as 1 nJ/THz-bandwidth. Our apparatus also demonstrated that the phase of the 3.6-nJ, 200-fs
self-phase-modulated pulse from a single-mode, fused-silica
fiber is easily reconstructed. In addition, the novel technique
has the other advantage in the signal-to-noise ratio because
the modified-SPIDER signal can be shifted to the arbitrary
frequency region to avoid the strong background secondharmonic wave. The technique also measured the delay-timeτ highly accurately. This delay-time greatly affects the groupdelay dispersion of the reconstructed spectral phase. The developed technique is very useful for the characterization of
weak pulses such as the externally compressed pulse using
a glass fiber, the pulse characterizing the optical nonlinear
response from absorbing materials, and the pulse based onhigh-order nonlinear optical processes.
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