IEEE J. Quant. Electron. 24, 431 - Department of Physics

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43 1
Generation of Synchronized Ultraviolet and Red
Femtosecond Pulses by Intracavity
Frequency Doubling
matching brandwidth limitation. Insertion of the doubling
crystal leaves the operating characteristics of the dye laser
virtually unperturbed, apart from minor effects detailed
below. The power of the extracted ultraviolet beam (
mW) is sufficient to be easily measured with a calibrated
power meter, and is brightly visible on a surface such as
an ordinary business card with fluoresces in response to
OR a number of years, the colliding pulse mode- ultraviolet illumination. This ease of observation and
locked dye laser operating at 620 nm[ l], with subse- measurement greatly facilitates experimental alignment of
quent modifications [2], was the only available source of the extracted ultraviolet beam. Intracavity frequency douoptical pulses less than 100 fs in duration. Recently sev- bling has been applied previously to synchronously modeeral investigators [3]-[8] have developed new femtosec- locked picosecond dye lasers [ lo]-[ 121. Our results, howond dye lasers which operate at wavelengths ranging from ever, represent the first application in the femtosecond
green to near infrared. However, femtosecond source la- doman.
sers in the blue and ultraviolet have not yet been develOur basic dye laser cavity follows the design of Valdoped; furthermore, schemes for synchronized generation manis et al. [2], which includes an intracavity configuof unamplified femtosecond pulses at differenct wave- ration of four prisms [13] for control of group velocity
lengths have not been realized. Consequently, generation dispersion. We introduce the doubling crystal near the foof femtosecond pulses at wavelengths beyond the cur- cus of an additional intracavity subresonator formed by
rently available range of sources, and/or synchronized the two slightly off-axis 5 cm focal length spherical mirgeneration of pulses at different wavelengths, must be rors, as depicted in Fig. 1. These mirrors focus the intraachieved through a cumbersome amplification process at cavity red beam to a measured diameter of approximately
a much lower repetition rate than the source laser, fol- 60 pm in the plane of incidence at the KDP crystal. We
lowed by wavelength shifting through nonlinear wave observe a nearly circular generated ultraviolet beam promixing or white light continuum generation [9].
file (minor axidmajor axis = 2/3), indicating a nearly
We have developed a unamplified source of synchro- circular fundamental beam profile at the focus. The modnized red and ultraviolet pulse trains of milliwatt average est asymmetry arises from astigmatism introduced by the
powers and 100 MHz repetition rate by intracavity fre- use of off-axis spherical mirrors throughout the laser cavquency doubling in a passively mode-locked femtosecond ity. The beam waist dimensions also vary slightly with
dye laser. While our results have been obtained with a particular cavity alignment. The doubling crystal was cut
colliding pulse mode-locked ring laser, the same tech- for Type I phase-matched second harmonic generation at
nique can extend the operation to other types of femto- Brewster angle incidence in order to minimize reflective
second dye lasers to ultraviolet wavelengths. We obtain insertion losses. By using doubling crystals which are
efficient ultraviolet generation by utilizing the high intra- compatible with antireflection coatings, normal incidence
cavity power of the fundamental red beam appropriately insertion of the crystal would also be possible. This would
focused into a KDP doubling crystal, which is thin enough allow easy insertion and removal of the crystal without
(1 mm) to minimize broadening of the second harmonic deflecting the intracavity beam path. We extract the secpulses caused by group velocity walk-off and phase- ond harmonic pulses, after a single pass of the intracavity
red pulses, through one of the subresonator mirrors, which
Manuscript received May 27, 1987; revised August 18, 1987. This work
has been coated for 99.9 percent reflectivity at 620 nm
was supported in part by the Joint Services Electronics Program under Conand
65 percent transmission at 310 nm. The beam is then
tract F49620-86-C-0045 and in part by the Texas Advanced Technology
recollimated by an extracavity fused silica lens. By using
Research Program, M . C. Downer also acknowledges support from the
Robert A. Welch Foundation and an IBM Faculty Development Award.
a similar dichroic mirror for the opposing subresonator
The authors are with the Department of Physics, University of Texas,
mirror, an equivalent ultraviolet beam could be extracted
Austin, TX 78712.
in the opposite direction.
IEEE Log Number 8717801.
Abstract-We efficiently extract an ultraviolet femtosecond pulse
train of milliwatt average power and 100 MHz repetition rate from a
colliding pulse mode-locked dye laser by intracavity frequency doubling in KDP. The ultraviolet and visible outputs, which are comparable in power and pulse duration, are perfect synchronized with each
0018-9197/88/0200-0431$01.OO @ 1988 IEEE
Fig. 1. Schematic of the subresonator for intracavity frequency doubling,
showing the bidirectional red pulse train, Brewster angle cut KDP crystal, and the dichroic output coupling mirror for the generated ultraviolet
beam. The remainder of the complete cavity configuration follows the
design depicted by Valdmanis et al. [Z].
In the KDP crystal, the intracavity red pulses ( - 5 nJ
in energy) reach a peak intensity of nearly lo9 W/cm2. For
a crystal thickness of 1 mm at the phase-matching angle,
a second harmonic conversion efficiency of approximately
0.3 percent, or an ultraviolet pulse energy of about 15 pJ,
is readily computed using standard formulas [14]. This
computation assumes Gaussian spatial and temporal profiles for the incident and second harmonic pulses, and
takes into account the partial phase mismatch in the wings
of the pulse spectrum. The result shows that the intracavity pulse parameters are almost ideally suited for achieving the maximum second harmonic conversion efficiency
in a 1 mm crystal consistent with the continuing operation
of the dye laser. Our somewhat lower measured ultraviolet pulse energy of 7 k 2 pJ, or about lo7photons/pulse,
results primarily from ultraviolet reflective losses at the
dichroic extraction mirror, the fused silica recollimation
lens, and the KDP exit face. With more efficient doubling
crystals, conversion efficiencies as high as 1-2 percent
should be readily achievable.
With appropriate adjustments of the intracavity prisms
after insertion of the doubling crystal, we preserve a typical red pulsewidth of 60-70 fs, as shown by the background-free autocorrelation traces with and without the
doubling crystal in Fig. 2(a). Similar compensation for
the broadening effect of dispersive intracavity elements
has been observed previously with acoustooptic cavity
dumpers [ 151. Furthermore, insertion of the doubling
crystal does not limit the frequency bandwidth of the fundamental red pulses, as shown by the comparison of spectra with and without the doubling crystal in Fig. 2(b). The
small spectral shift evident in Fig. 2(b) results from slight
intracavity mirror adjustments required upon insertion of
the doubling crystal, rather than from an inherent effect
of the doubling crystal itself. We have observed slight
shifts in both directions or no shift at all, depending on
the particular cavity alignment. Both the pulse duration
and bandwidth measurements were performed with the intracavity doubling crystal tuned for maximum ultraviolet
output power. Detuning from the optimum phase-matching angle had no observable effect on either quantity.
Fig. 2. (a) Autocorrelation traces and (b) spectral intensity profiles of the
fundamental red pulses without the intracavity doubling crystal (dashed
curves) and with the intracavity doubling crystal tuned for maximum
output power (solid curves). A sech' temporal pulse profile has been
assumed in arriving at the pulse duration shown in (a). (c) Spectral intensity profile of the generated ultraviolet pulse.
These observations contrast sharply with previous observations of significant pulse broadening and bandwidth
limitation caused by intracavity doubling crystals in synchronously mode-locked picosecond dye lasers [lo]-[ 121.
We attribute the more favorable performance of our frequency-doubled laser to the presence of a saturable absorber, which compensates the pulse broadening effect of
power-dependent loss in the doubling crystal. Quantitative analysis of this compensating effect is in progress.
The measured frequency spectrum of the ultraviolet
beam is shown in Fig. 2(c). The full width at half maximum of 5 nm is consistent with a transform-limited pulse
duration of 40 fs. Group velocity walk-off and phasematching bandwidth limitation in the doubling crystal,
however, broaden the ultraviolet pulse from this ideal
transform-limited duration. We have quantitatively accounted for these effects by calculating the freqency-dependent amplitude and phase of the second harmonic pulse
for each spectral component of a 70 fs, Gaussian fundamental input pulse using standard formulas [14]. The
Fourier transform of the resulting second harmonic pulse
spectrum yields the temporal ultraviolet pulse envelope.
We find that the ultraviolet pulse is broadened to approximately 170 fs, for a 1 mm KDP crystal, in agreement
with similar calculations by others [16]. The same calculation shows, however, that a 0.1 mm LiI03 crystal
would yield 90 fs ultraviolet pulses, and that a 0.22 mm
P-BaB204crystal would yield 95 fs pulses with the same
conversion efficiency as a 1 mm KDP crystal.
Insertion of the doubling crystal has no discernible effect on the power of the red beam nor on the mode-locking stability of the dye laser. We observe a 50 percent
increase in the practical argon laser pump power to between 3.0 and 4.0 W, an increase caused by residual intracavity reflection losses from the crystal, minor diffraction, absorption, and scattering losses caused by optical
imperfections in the crystal, and losses due to second harmonic generation. In addition, we observe a broadening
of the hysteresis which normally characterizes the dye
laser threshold as the pump power is ramped upwards and
downwards. Normally, the “turn-on” threshold, which is
observed as the pump power increases from zero, is approximately 0.3 W higher than the “turn-off’ threshold,
observed as the pump power decreases with the dye laser
opening. With the doubling crystal in place, this hysteresis widens to approximately 1-1.5 W.
The dye laser can “blink” off unpredictably when operated at a pump power within this hysteresis window.
We attribute this behavior to a thermooptic lensing effect,
normally caused by the heating of the gain jet by the pump
laser, and augmented here by the heating of the KDP crystal by the intracavity beam. A more pronounced hysteretic
effect of similar origin has been observed and analyzed
previously with intracavity frequency doubling crystals in
solid-state lasers [ 171, [18] with higher intracavity power.
As the pump power and intracavity beam power change,
the effective focal length of the thermally induced index
lenses change, thus changing the optical cavity alignment
and the lasing threshold. Although the light power absorbed in the KDP crystal is three orders of magnitude
smaller than the gain jet, the thermooptic lensing effect is
larger in magnitude because of its tenfold smaller thermal
conductivity (0.021 W . cm-’ K-I), its ten-fold larger
thermal index gradient d n / d T (3.34 x
K - I ) , and
its threefold greater thickness. Furthermore, the jet is
flowing at approximately 7 m * sC1, which removes the
heat lo4 times faster than by heat conduction. The major
practical effects of the increased thermooptic effects are a
higher required pump power and a temporary increase in
the tendency of the dye laser to “blink” off when first
brought into operation. However, once a thermal steady
state is reached within several minutes, this latter tendency typically disappears, and any residual thermooptic
effect can be compensated by adjustment of the subresonator mirror separation. These minor effects can be alleviated by using a doubling crystal with smaller residual
absorption, higher thermal conductivity, or smaller
dn / d T .
In summary, intracavity frequency doubling provides a
simple, inexpensive, and virtually non-perturbative
method of extending the operation of femotosecond lasers
to ultraviolet wavelengths. Such wavelength-extended femtosecond lasers should be applicable to time-resolved
photoionization and photoemission experiments. In addition, the tighter focusability of ultraviolet beams can enhance the spatial precision of electrooptic sampling and
optoelectronic switching in the micron and submicron
gaps between microelectronic circuit elements. Type I
phase-matched sum frequency mixing of 620 nm fundamental pulses with 310 nm second harmonic pulses to
generate 205 nm pulses has become possible for the first
time with the recent development of the P-BaB204crystal
[ 191, [20]. The intracavity frequency-doubled output of
the colliding pulse mode-locked laser may serve as a seed
pulse for injection into the recently developed XeCl subpicosecond pulse amplifer [21]. To date, such amplifiers
have required injection pulses of nanojoule to microjoule
energies, formed by first amplifying the fundamental dye
laser output before frequency doubling [22], in order to
compete successfully with amplified spontaneous emission (ASE) within the XeCl gain cell. Development of
improved methods for suppression of ASE, such as spatial
filtering, and use of lower gain amplifier cells, however,
may permit injection with the lower energy pulses from
the source described here, thus circumventing the need
for a visible wavelength dye amplifier.
R. L. Fork, B . I . Greene. and C . V . Shank, “Generation of optical
pulses shorter than 0.1 psec by colliding pulse mode locking,” Appl.
Phys. Lerf., vol. 38, no. 9 , pp. 671-672, 1981.
Valdmanis, R . L. Fork, and J . P. Gordon, “Generation of optical
pulses as short as 27 femtoseconds directly from a laser balancing
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pp.254-256, 1987.
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8 , no. 1, pp. 1-3, 1983.
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[I41 A. Yariv, Quantum Electronics. New York: Wiley, 1975, ch. 16.
[I51 M. C. Downer, R . L . Fork, and M. Islam, “ 3 MHz amplifier for
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[16] See, for example, A. M. Weiner, “Effect of group velocity mismatch
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[22] J. H . Glownia, J. Misewich, and P. P. Sorokin, “16Ofs XeCl excimer
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Glenn Focht was born in Kingsville, TX, in 1952
He received the B B A. degree from Texas Tech
University, Lubbock. in 1974, and the M.S degree in physics from Southwest Texas State University, San Marcos, in 1978.
He then worked at NASA-JSC in the Flight
Operations Directorate, Flight Training as an Astronaut Instructor, teaching the astronauts about
the on-board systems of the Space Shuttle In 1983
he enrolled in the University of Texas, Austin,
where he I S currently a Ph.D candidate in physics. His present research interests are in ultrafast opitcal interactions with
M. C. Downer received the Bachelor’s degree in
physics from the University of Rochester, Rochester, NY, in 1976 and the M A degree ds a Marshall Scholar from Oxford University, Oxford,
England In 1983 he received the Ph D degree in
applied physics from Harvard University, Cambridge, MA, for two-photon qpectroscopic studies
of rare earth crystals and for studies ot atomic collisional effects in coherent Raman spectroxopy
He first became involved in ultrafast laser
spectroscopy as a poctdoctoral fellow at AT&T
Bell Laboratories, where he developed the technique of strobe photogrdphy
on a femtosecond time scale, and made numerous contributions to femtosecond spectroscopy of semiconductors and biological molecules In 1985
he joined the faculty of the Department of Physics, University of Texas.
Austin, where he received a Trull Centennial Professorship and dn IBM
Faculty Development Award His current research focu5es on the development of new femtosecond measurement techniques dnd their application
to problems in condensed matter physics and microelectronic devices

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