We obtain gigawatt white-light continuum pulses that permit spectroscopic measurements with a time ... pulse, which is delayed by a stepper-motor-driven stage.
January 1983 / Vol. 8, No. I / OPTICS LETTERS
i
Femtosecond white-light continuum pulses R. L. Fork, C. V. Shank, C. Ifirlimann,
and R. Yen
Bell Laboratories,Holmdel, New Jersey 07733
W. J. Tomlinson Bell Laboratories,Allentown, Pennsylvania 18103 Received September 30,1982 We obtain gigawatt white-light continuum pulses that permit spectroscopic measurements with a time resolution of 80 fsec. These pulses extend continuously from 0.19 to 1.6 gm and have time sweeps as small as 10 fsec/1000
A.
We find temporal, spatial, and spectral properties that are consistent with self-phase modulation having a prominent role in generation of the continuum.
We previously described the use of white-light continuum pulses that permitted spectroscopicmeasurements with time resolution in the subpicosecond regime.'
We
report here white-light continuum pulses that permit spectroscopic measurements with a time resolution as short as 80 fsec. The physics of the continuum-generation process simplifies for our 80-fsec pump pulses as compared with earlier continuum generation, In particular, we find temporal, spatial, and spectral properties that are consistent with self-phase modulation within the leading and trailing edges of the pump pulse having a prominent role in generation of the continuum. The apparatus that we use for generating and measuring our continuum pulses employs only reflective
of the continuum with a series of cross-correlation functions taken over the spectral range reached by our KDP crystal (0.43-1.09 pm). We expect the physics of our continuum generation to differ from previous continuum generation because our pump pulse is approximately 2 orders of magnitude shorter in duration than in earlier work by others.t 5 The consequence is that self-phase modulation, which
increases in importance with decreasing pulse duration, 8 takes a more prominent role while less easily controlled processes, such as parametric amplification of quantum
noise5 and self focusing,6 - which do not depend explicitly on pulse duration, decrease in relative importance. Avalanche ionization is also less likely because the threshold for that effect increases with decreasing
optics and thin generating and analyzing media (Fig. 1).
pulse duration.6 38
The temporal distribution of the continuum is thus determined primarily by the generation mechanism rather than by group velocity dispersion in the gener-
Our observations are consistent with these expected changes in relative importance of the various physical mechanisms. We observe, e.g. that the temporal distribution of the continuum is typical of self-phase modulation8 ' 9 (Figs. 2 and 3). The red-shifted portion of the continuum coincides with the leading edge of the pump pulse, whereas the blue-shifted portion of the continuum coincides with the trailing edge of the pump pulse (Fig. 2). (We show the pump-pulse autocorrelation at an intensity below threshold for continuum
ating or analyzing medium or optics. A pulse at 627 nm
of 80-fsec duration from our colliding-pulse modelocked laser 2 is amplified to gigawatt power3 and divided
into a pump and a reference pulse. The pump pulse is focused at f/12 into a thin (500-gm) jet of ethylene glycol by a 25-cm-radius focusing mirror yielding intensities of 1013-10l4W/cm2 at the jet at a repetition rate of 10 Hz. Alurminum coatings are used on the mirrors, which reflect the continuum and dielectric or aluminum coatings on the other mirrors. We determine the temporal distribution of the continuum by cross-
correlating the continuum pulse with the reference pulse, which is delayed by a stepper-motor-driven
stage.
A thin (100-pm) KDP crystal is used to upconvert the continuum light. The finite bandwidth of the KDP crystal for phase-matched upconversionand the angular orientation of the c axis select a tunable region of the continuum. An OMA2 optical multichannel analyzer detects the upconverted signal and measures the upconverted wavelength. By varying the orientation of the KDP crystal, we map out the temporal distribution 0146-9592/83/010001-03$1.00J0
CRYSTAL
OPTICAL MULTICHANNELANALYZER
Fig. 1. Experimental arrangement for continuum generation
and measurement. © 1983, Optical Society of America
/ Vol. 8, No. 1 / January 1983
OPTICS LETTERS
2
I
I
the continuum in the immediate vicinity of the pump
t
does not exhibit sharply defined peaks in time, we have
\
I
I
II
Cc
+
I-
II
z
I
z /
/ -100
-300 -200
0 100 TIME(fsec)
200
300
Fig. 2. Cross-correlation traces for representative blue (+) and infrared (X) portions of the continuum. An autocorrelation of the pump pulse at low intensity is also shown (e). The blue and infrared traces have been shifted by + 12.5 and -12.5 fsec, respectively, to correct for delays in the KDP crystal.
x
UNCORRECTED DATA
*
KDPCORRECTED
v
KDPANDDISPERSiON COR¶ECQED
14 SHIFT(10 HZ) FREQUENCY -2.0
not attempted to give data in the region. The principal points to be made are as follows. (1) The red porion of the continuum occurs in the vicinity of the leading edge of the pump pulse. (2) The blue portion of the continuum occurs in the vicinity of the trailing edge of the pump pulse. (3) The chirp of the continuum is small, amounting to a temporal shift of
