Optical parametric amplification (OPA), as well as DFG, is a three-wave mixing process in which a pump beam, a signal beam, and an idler beam at frequencies ...
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J. Opt. Soc. Am. B/Vol. 10, No. 9/September 1993
Optical parametric generation and amplification in barium borate and lithium triborate crystals J. Y. Zhangl* J. Y. Huang,' Y. R. Shen, and C. Chen' Department of Physics,University of California at Berkeley,and Centerfor Advanced Materials, Lawrence Berkeley Notional Laboratory,Berkeley,California 94720 Received November 10, 1992; revised manuscript received March 5, 1993
A picosecond, widely tunable, optical parametric generator-amplifier system can be constructed with barium borate and lithium triborate crystals pumped by third- or second-harmonic output of an active-passive modelocked Nd:YAGlaser. A tunable output of several hundred mirojoules per pulse with a conversion efficiency as high as 30% has been obtained.
The tuning range covers from 0.4 to 2.0 or from 0.6 to 2.5 Am and can be ex-
tended by sum- and difference-frequency generation to near 0.2 Am in the UV and to near SAm in the IR. The output linewidth can be narrowed to near the transform limit with the help of frequency selection by a grating.
1.
INTRODUCTION
Optical parametric oscillators and amplifiers have long been considered ideal coherent tunable sources.' Their main advantage over other tunable sources such as dye lasers is that they offer a much wider tuning range. However, advances in optical parametric devices were rather slow in the past; improvements were impeded by the lack of suitable nonlinear-optical crystals with sufficiently large nonlinearities and high laser damage thresholds. Recently it was found that barium borate (-BaB 2 O4, BBO) and lithium borate (LiB3 0 5, LBO) crystals are highly nonlinear and resistive to laser damage. They also have a wide transparent range from the IR to the deep UV2'3 (2.5-0.19 pm for BBO and 3.0-0.16 Am for LBO). They are therefore ideal for use in optical parametric generation (OPG) of widely tunable radiation.' Indeed, the discovery of BBO and LBO crystals has stimulated a tremendous amount of renewed interest in optical parametric oscillators and amplifiers in the past three years.` 9 Optical parametric generation and amplification (OPGOPA) with high-power picosecond pump pulses is an efficient means for generating high-power, high-efficiency, and widely tunable picosecond radiation. Using BBO and LBO with various OPG-OPA schemes, we have obtained
narrow-band, picosecond pulses that are tunable from --0.4 to -2.5 Am with an output of '-1 md/pulse. The total conversion efficiency can be as high as -30%,4 and the output bandwidth can be reduced to near the Fouriertransform limit.' 0 Such an optical parametric device should find many useful applications in modern spectroscopy and related fields. We outline in Section 2 the theoretical considerations involved in designing an OPG-OPA system with BBO and LBO. We then describe in Section 3 the experimental arrangement of the OPG-OPA system that we have developed with various pumping sources. Section 4 gives an account of the experimental results and their comparison
with theoretical calculations. Also in Section 4 we briefly discuss the scheme of extending the tuning range of an OPG-OPA system to the mid-IR (3-8 pAm)by difference-frequency generation (DFG). 0740-3224/93/091758-07$06.00
2. THEORETICAL CONSIDERATIONS IN OPA SYSTEM DESIGN The theory of optical parametric processes in a nonlinear medium follows the earlier work of Armstrong et al." and has been reviewed by many authors."," Here we simply sketch the results. Optical parametric amplification (OPA),as well as DFG, is a three-wave mixing process in which a pump beam, a signal beam, and an idler beam at frequencies W3, &)1, and (W)2, respectively, with £03 = (0 + (02, propagate and interact in a nonlinear medium. Under the phase-matching condition the quasi-steady-state solution of the coupledwave equations leads to""14 "15 I1(Z)
13(0)[1-
n2(Z/6,Y)]co1/wa),
I2(Z)= I2(0) + 13(0)[1 - Sn2(z/,Y)]w 2/W3, 13(z
=
2 I3(0)sn (Z/,y),
(1)
where Ii(z) is the intensity of the &wi wave at distance z; sn denotes the Jacobian elliptic function; ~ is the effective interaction length of the wave-mixing process, depending on the effective optical nonlinearity deff of the medium; and y is related to the normalized input signal intensity divided by the input pump intensity. To take into account the spatial-temporal modulation of the interacting waves, we assume the temporal and the spatial profiles of the field amplitude of a pulse to be Gaussian. The beam and the pulse envelopes are then approximated by histograms.
For each step in the histograms the signal and the idler outputs are calculated with Eq. (1). Summing up contributions from all the steps yields the total output. This model preserves the simplicity of the plane-wave approximation. It works well for input laser pulses with negligible diffraction and negligible group-velocity mismatching between interacting waves during propagation in the medium.'" In OPG signal and idler inputs are derived from spontaneous quantum noise. The theoretical analysis of OPG was described by Kleinman."7 The signal output from a crystal of length L and a pump beam of radius r has a © 1993 Optical Society of America
Vol. 10, No. 9/September 1993/J. Opt. Soc. Am. B
Zhang et al . BB01
PBPrism
BB02
amplifier according to the scheme adopted from
HWP
Delayline
PBS
K
telescope 1:4
1:3
I----- V I, DM
I NC3 I
I
I
L____I OPA
1
i! DM
I. I
M
NCZ
NC1
r---
r,
1
1
ni
%?DM
I
M
k'
j Z
LI__
PBPrism
_
I
_
OPG-OPA
Fig. 1. Schematic of the experimental arrangement of a BBO OPG-OPA system pumped at 355 nm without a grating as the dispersive device. One mirror (M in the dashed box at right) can be replaced by a grating. NC's, nonlinear crystals; DM's dichroic mirrors; PB, Pellin-Broca.
r L . Noncollinear phase matching within AfI by different frequency components broadens the output spectral width. A combined OPG-OPA system can effectively reduce the spectral width and beam divergence of the output.4 "0 The optical parametric amplifier crystal situated at a distance away from the optical parametric generator also acts as a spatial and spectral filter, because only the central part of the signal wave overlaps the pump beam and is beam divergence with a solid angle AfIl-
amplified in the optical parametric amplifier crystal. The effects become more significant if the distance increases, but the signal injection into the optical parametric amplifier is weaker and the output energy decreases. The signal injection is an important parameter for optimizing the performance of an optical parametric amplifier. The output of the optical parametric amplifier has a broad region in which the output signal appears to be insensitive to the injected signal. In this region, the optical parametric amplifier is expected to be more stable. In many applications one would like to have a narrowband tunable source. Several factors are responsible for the bandwidth of the signal and the idler waves from an OPG-OPA system.'9 Even for collinear phase matching, there is a finite signal bandwidth given by'9 Av = c[(2 ln 2)(2FoL)]"12(2TrLln2
-
nil),
(2)
where n, and n2 are the refractive indices seen by the signal and the idler waves, respectively, L is the crystal length, and ro is the maximum gain coefficient. For a 1-cm BBO crystal pumped at 0.355 Am with 2F0L = 30, Av at 0.55 m is estimated to be 4 nm (-120 cm-'). Av increases as the pump intensity I, increases, since r02 is proportional to I,. Noncollinear phase matching in a finite beam spread further enhances the bandwidth. Obviously, to improve the output bandwidth, one must resort to the scheme of OPG followed by bandwidth narrowing by dispersion and then OPA.
3.
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EXPERIMENTAL ARRANGEMENT
Webriefly describe here the experimental arrangement of a high-energy, picosecond optical parametric generator-
Laubereau et al.' 9 and developed in our laboratory. The schematic layout is shown in Fig. 1, in which the OPGOPA system is pumped by the third-harmonic generation output of a 35-ps active-passive mode-locked Nd:YAG laser at 1.064 jim, operating at 10 Hz. Third-harmonic generation is obtained by frequency doubling in a 3-mm type-I BBO crystal followed by sum-frequency generation in a 5-mm type-II BBO crystal. At a pump intensity of 2 and with no antireflection coating on the two -5 GW/cm BBO crystals, the total conversion efficiency from the fundamental to the third harmonic is -30%. The pump beam at 0.355 m is then divided into two parts: one part is used to pump the front OPG-OPA stage in a double-pass geometry, and the other to pump the ensuing OPA stage after a suitable time delay. The OPG-OPA stage is meant to generate from superfluorescence a tunable seed beam that can be injected into the optical parametric amplifier to be power amplified. The pump energy for OPG-OPA is approximately 25-30% of the total pump energy and can be adjusted by the combination of a half-wave plate (HWP) and a polarizing beam splitter (PBS). The signal from the first BBO crystal (BBO1) in the OPG-OPA system is broadband with a significant beam divergence. The second BBO crystal (BBO2) is situated
-20
cm away from the first one, so
that only the central portion of the divergent output from BBO1 overlaps with the pump beam in BBO2 and is amplified. Thus BBO2 serves both as a preamplifier and as a spatial and spectral filter that improves the beam divergence and bandwidth. Angle tuning of the BBO crystals is used to change the phase-matching conditions and hence to vary the output frequency. By tilting of the two crystals in opposite directions from the axis, the beam walk-off of the pump and signal waves in the two crystals is minimized. After BBO2 the pump and the signal beams are separated by a dichroic mirror (M). The residual pump beam was reflected back into BBO2 and BBO1 after a suitable delay to provide pumping for further amplification of the signal beam, which is reflected by a mirror (or by a grating after dispersion). The output of the OPG-OPA system after this second-pass amplification can reach tens of microjoules per pulse. The output from the OPG-OPA system is then injected into a third BBO crystal (BBO3) to be further amplified. An amplification of 10 or more in BBO3 can be achieved. In our setup the BBO crystals and the grating in the system were mounted on rotation stages driven by computercontrolled stepping motors. By rotation of the crystals and the grating in synchronous steps, continuous tunable radiation from 0.42 to 2.1 m can be generated with a maximum output of -1.0 md.4 The BBO crystals used in our experiment were cut for type-I phase matching with an internal angle of 31° between the surface normal and the optical axis. The polished end faces were uncoated. The pump beam was p polarized. The effective secondorder nonlinearity of BBO in this geometry is d e~ff= 1.51 x 10-12 m/V, compared
with 0.38
X
10-l2 m/V for
KDP crystal. However, the figure of merit, defined as F = Id(21/n312, with n being the refractive index, is 4.9 X 10-25 m2 /V2 for BBO, which is much larger than F= 0.25
X
10-25 m 2 /V 2 for KDP. Moreover, the laser damage
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Zhang et al.
J. Opt. Soc. Am. B/Vol. 10, No. 9/September 1993 2.00 0 I a
0) 0
E
uc3 0"
1.60 -
0, ._
0) 0)
a
3
sc
0
1.20 -
I' 60
0.80 -
0.40 25.00
-
EWEIEI*S}}O-OU 27.50
30.00
32.50
35.00
Crystal Orientation Angle (Deg.)
Fig. 2. Signal (idler) wavelength of a type-I BBO parametric amplifier as a function of the crystal orientation (internal angle between the surface normal and the optical axis of the crystal). The pump beam is at 355 nm. The squares are the experimental data, and the dashed curve is from the phase-matching calculation. The discrepancy between theory and experiment is due to the inaccuracy of the refractive indices used in the calculation.
curve of BBO with the pump beam at 0.355 Am is presented in Fig. 2, where the experimental data are seen to agree well with the theoretical prediction. The tuning curve of LBO with the pump beam at 0.355 pumis shown in Fig. 3, also in fair agreement with theory. With a high pump intensity, a pump-to-signal efficiency of 30% can be reached in both cases. However, because of the much higher energy threshold required for the onset of superfluorescence in a LBO crystal, it requires a pump intensity of 5 GW/cm2 or higher for the OPG-OPA system to operate in the saturated regime with a stable output. Such a high pumping intensity could cause damage to many optical components. It is advisable to use LBO only in OPArather than in OPG. The frequency output of a BBO optical parametric generator pumped at 532 nm is highly sensitive to the orientation of the crystal. It takes a crystal rotation of only about 1.5° to cover a tuning range from 0.6 to 2.5 jAm. Therefore its output bandwidth is very broad. For many applications bandwidth narrowing is needed and will be
discussed in Subsection 4.C. With LBO pumped at 2
threshold of BBO is -8 GW/cm or higher for the 15-ps pulses at 355 nm. This permits pumping of BBO at high power. The BBO crystals in Fig. 1 can be replaced by LBO. The effective second-order nonlinearity of LBO [d()= 0.87 x 10-12 m/V] is lower than that of BBO. The figure of merit of LBO is -60% of that for BBO. For this reason the pump intensity for the onset of superfluorescence for LBO is 2.6 GW/cm2 , as compared with 1.0 GW/cm2 for BBO. Therefore the pumping intensity needed for a LBO OPG-OPA system is much higher than that for BBO. However, the laser damage threshold of LBO is also much
higher (more than 10 GW/cm2 for 15-ps pulses at
0.355 .m). This permits pumping of LBO at a much higher intensity and compensates for the disadvantage of its lower nonlinearity. In our experiment no damage was observed with a 5-GW/cm2 pumping intensity even over a long term. The other advantage of using LBO as the nonlinear medium is its large acceptance angle and low walkoff effect. This permits the use of a smaller pump-beam diameter and a longer crystal length. Our experimental results4 showed that the tuning range of the LBO OPGOPA system pumped by third-harmonic generation of YAG is approximately the same as that of BBO. A recent study 20 suggested that the tuning ranges from 0.405 to 2.4 Aum, which is somewhat broader than that of BBO. The OPG-OPA system in Fig. 1 can also be pumped by the second-harmonic output of a Nd:YAGlaser. Using 20ps pump pulses of more than 5 mJ/pulse at 0.532 pumwith a beam diameter of 2-2.5 mm in the BBO crystals (10 mm long each and cut at 210),we find that the output from the OPG-OPA stage is as high as 200-300 ttJ/pulse. This output is already enough for many applications, so that the additional OPA stage is often not needed.
0.532 pum,phase matching can be achieved by either angle
tuning or temperature tuning the crystal. The latter with noncritical phase matching is most attractive, because it drastically increases the angle of beam acceptance and reduces the beam walk-off. It also permits the use of a LBO crystal with a small cross section. The angle-tuning curve and the temperature-tuning curve of LBO pumped at 0.532 gum are shown in Figs. 4(a) and 4(b), respectively. The tuning curves of LBO in Fig. 4 exhibit a retracing behavior; i.e., the signal frequency does not increase or decrease with the crystal angle or temperature monotonically. We then find that in some region the output of parametric generation can simultaneously have two signal frequencies together with two idler frequencies. The appearance of this retracing behavior in LBO can be under-
stood from the phase-matching condition. It is the consequence of a peculiar behavior of LBO that, for beams propagating along the X axis of the crystal, the phase mismatching versus the signal frequency between a pump wave polarized along the Y axis and the signal and the idler waves polarized along the Z axis exhibit a clear mini-
mum.7' 8 The double-valued phase-matching charac2.00
1.60 -c
1.20
C a) 0
0.80
0.40 I=--
25.00
4. EXPERIMENTAL RESULTS AND COMPARISON WITH THEORY A. Frequency Tuning The OPG-OPA system using BBO or LBO crystals has an extremely broad frequency tuning range. The tuning
30.00
35.00
40.00
45.00
Internal Angle 5p (Deg.)
Fig. 3. Angular tuning of the output wavelength of a type-I LBO parametric amplifier with the pump beam at 355 nm. The squares and the circles are the experimental data from the signal and the idler waves, and the solid curve is from the phasematching calculation.
Zhang et al. 3.00
Vol. 10, No. 9/September 1993/J. Opt. Soc. Am. B -
2 .c 1.75 C
0
0.50 _ 6.0
14.0
10.0
1 8.0
Internal Angle (Deg.)
1761
sponding to a conversion efficiency of -30%. The output also depended on the energy of the injected signal beam. Figure 5 shows the output versus the input at three different wavelengths (460, 570, and 660 nm). Because amplification is in the pump-depletion limit, the output increases only slowly when the input increases by more than 5 orders of magnitude. This agrees well with our theoretical calculation outlined in Section 2 and shown in Fig. 5. Operating an optical parametric amplifier in the nearly flat response region certainly provides a more stable output. The tunable input signal can also be amplified in an angle- or temperature-tuned LBO OPA system. Results from an angle-tuned 16-mm LBO crystal are comparable with those from the BBO system mentioned above.4 C.
Bandwidth Narrowing
As is discussed above, the bandwidth of an optical
3.00
2
Begs ~
~ ~
parametric generator output is rather broad. It is advisable to narrow the bandwidth before the output is injected into an optical parametric amplifier. We have studied this possibility and the limitation of the bandwidth reduction by using a BBO OPG-OPA system pumped by a 0.532pm beam. This system has a very large inherent output bandwidth because its signal frequency is sensitive to the crystal orientation (rotating the crystal 1.50would result in
e-
cJ 1 .75 .C
al C
a frequency change of tens of thousands of wave numbers). i
0.50 50
100
3 1 50
-~~~~~~~S 200
250
Temperature ( C ) Fig. 4. Signal (idler) wavelength of a type-I LBO optical parametric amplifier (e - o + o) as a function of (a) the crystal orientation (internal angle between the surface normal and the optical X axis of the crystal) and (b) the crystal temperature. The pump beam is at 532 nm. The squares are the experimental data, and the dashed curves are from the phase-matching calculation (see Sellmeier equation given in Refs. 7 and 8). The dotted curve in (a) is calculated
from the Sellmeier equation given in
Ref. 22. The solid curve is calculated from the Sellmeier equation given in Ref. 8.
teristics can be observed directly from optical parametric fluorescence from LBO along the pump-beam direction if the orientation or the temperature of LBO is properly adjusted. With the crystal oriented at 0 = 900 and 4)= 90 at T = 20'C and at 0 = 90° and (A = 0° at T = 113'C, one can see that the phase-matching peaks appear at 0.72 and 0.77 pm and at 0.71 and 0.79 pumfor the two cases, respectively. 7 '8 B.
Energy Conversion
To study energy conversion in an optical parametric amplifier, we can inject a signal beam into the nonlinear crystal and measure the amplified output. In our experiment in OPA pumped at 0.355 prm,the injected signal beam came from a tunable optical parametric generator using BBO with type-I phase matching, as described in Section 3. The OPG-OPA stage generated a stable and tunable output of 50 p.J/pulse. With a 15-mm BBO crystal used for OPA, the output was continuously tunable 4 from 0.43 to 2.1 pum, limited by the cross section of the
crystal. At the pumping intensity of 2.8 GW/cm2 the output of the signal branch was 0.7 mJ at 460 nm, corre-
The configuration of our modified OPG-OPA system is shown in Fig. 6. The first stage consisted of two 5 mm x 5 mm x 10 mm BBO crystals that could be tilted in opposite directions. With a total pump energy of 5 mJ or higher at 0.532 ptm and the beam diameter telescoped
down to -2.5 mm to yield a pumping intensity of
3 GW/cm2 , the single-pass output from this stage was -0.5 mJ or more and had a bandwidth of tens of nanometers. The single-pass output was then separated from the pump beam by a dichroic mirror, and after being expanded by a 10:1 telescope, it was directed
onto a 1800-
groove/mm grating with a diffraction efficiency of -90%.
The first-order diffracted beam was reflected back onto the grating to be diffracted again. The frequencyselected beam was then fed back together with the timedelayed reflected pump beam into the BBO crystals for a
1.00
0.75
-
0.50
-
0) C Id
,X
,
-t]-- la- _43- -+'
.
0.25
0
0.0010
4
l
-2
1
10
2
Injected Signal Energy (microjoule)
Fig. 5. Output energy versus injected energy for a BBO optical parametric amplifier. The pump energy is 2.4 mm at 355 nm with a 15-ps pulse width, focused to a 2.7-mm beam spot. Triangles, squares, and crosses refer to output wavelengths at 460, 570, and 650 nm, respectively.
1762
Mhang et al.
J. Opt. Soc. Am. B/Vol. 10, No. 9/September 1993
r -
I
pumping intensities above 3 GW/cm2, and the stability was found to be poor. It is important to center the narrow-band input fre-
- - - G- Grating
M5
quency of the seed beam at the maximum of the parametric gain of the optical parametric amplifier. Otherwise, the signal output from the optical parametric amplifier could have a spectrum with two peaks, one at the input frequency and the other near the frequency with the maximum parametric gain. Reducing the pump intensity in OPA and increasing the pump intensity in OPG will emphasize the peak at the input frequency and reduce the
other peak.
LBO
The output also becomes more stable,
and both the bandwidth and the output energy are nearly optimized. Other factors that can affect the bandwidth are the divergence of the pump and the input signal beams in the optical parametric amplifier. Careful adjustment of the telescopes for the pump and the seed beams is important. Finally, the bandwidth also depends on the signal frequency. Near the degeneracy point of the tuning curve, the bandwidth is appreciably broader as shown in Fig. 9.
HWP 3.0
l
. .
r a. .
, . . . . . . . .
en
2.0
YAG
Laser u)
Fig. 6. Schematic of the experimental arrangement of a BBO OPG system pumped at 532 nm with a grating for bandwidth reduction, together with a AgGaS2 DFG system that extends the output of the optical parametric generator to the mid-IR.
second-pass amplification. The output bandwidth after amplification was approximately 0.12-0.15 nm at a pump intensity of 2.2 GW/cm2 . When a single diffraction from the grating was used, the bandwidth increased to approximately 0.25 to -0.3 nm at the same pump intensity. We could not further improve the bandwidth by increasing the beam size to cover more lines on the grating because of the short pulse durations (-25 ps for the pump beam and 15 ps for the signal beam). A large beam area on the plane grating would have resulted in a reflected signal pulse width longer than the pump pulse. Only the part overlapping temporally with the pulse pulses could be amplified. As is mentioned in Section 2, the pumping intensity can affect the bandwidth significantly. This was confirmed
experimentally. Figure 7 shows the observed output bandwidth of our OPG-OPA system as a function of
pumping intensity.
B
U-
1.0 F.T
limit
lV n-f
1.0
2.0 Pump-
Beam
3.0 Intensity
4.0
5.0
(GW/cm 2 )
Fig. 7. FWHM bandwidth of the signal wave from a BBO OPGOPA system as a function of the pump intensity. The pump beam is at 532 nm. The solid line shows the transform-limited bandwidth.
D...................
250
q
w 0)
200
0)
l
150 U)
.
At a pumping intensity below
1.5 GW/cm2 the bandwidth is -0.02 nm, which is close to the Fourier-transform-limited bandwidth. With increasing pumping intensity it increases rapidly to a saturated value of -0.23 nm. Thus, for the output bandwidth to be reduced a low pump power must be used, but this will yield a low output. The output energy of the OPG-OPA system as a function of pump intensity is shown in Fig. 8. It is seen that at pumping intensity below 2 GW/cm2 the output is less than 100 p.J, compared with 250 p.J at
100
0
l ............ 50n -
1
2
Pump-
3
beam
Intensity
4
5
(GW/cm 2 )
Fig. 8. Signal output energy versus pump intensity for a type-I BBO OPG-OPA system pumped at 532 nm. The signal wavelength is 808.8 nm.
Vol. 10, No. 9/September 1993/J. Opt. Soc. Am. B
Mhanget al.
for this application is AgGaS2, which is transparent from 0.532 to 12 gm. It has a high second-order nonlinearity and has been shown to be an effective nonlinear medium
1.2
1.0-
°_:
for generating mid-IR radiation by OPO,2 1 OPA,22or DFG.23
.: 0.8 -
.) 0.6 -
a U..
0.4 -
n
|
|
> l
5000
9000
7000 Wavelength
11 00 0
(A)
Fig. 9. FWHM bandwidth of the signal wave from the narrowband BBO OPG-OPA system as a function
length.
of the signal wave-
The pump beam is at 532 nm with an intensity of 2
1.5 GW/cm .
120
>1
I-
60
ci
r. _Q
0
8.0
6.0
Idler Wavelength (m) Fig. 10. Idler pulse energy of a type-I AgGaS2 differencefrequency generator versus the idler wavelength. The DFG results from mixing of the fundamental output of the Nd:YAGlaser at 1.064 ,umwith the idler output from the 532-nm pumped BBO OPG-OPA. The 1.064-,um pump beam has an intensity of 2 300 MW/cm (3 mJ in a 6-mm spot).
The squares are the experi-
mental data, and the solid curve is theoretical, from Eq. (1), for a 10-mm single crystal
We have generated narrow-band, high-power, picosecond tunable radiation in the mid-IR from 3.0 to 8.0 m by DFG between the 1.064-,umlaser output and the tunable idler output (1.65-1.22 gm) of our OPG-OPA system using AgGaS2. The AgGaS2 crystal used was cut at 42° for type-I phase matching. By tuning the idler output of the OPG-OPA system and rotating the angle of the AgGaS2 crystal (by more than 14°) simultaneously by computercontrolled rotation stages for phase-matching, a narrowband mid-IR radiation from 3 to 8 m was generated. The tuning range can be extended to 10 pm if type-II phase matching is used. With a 10-mm-long AgGaS2 crystal pumped by 3 mJ in a 6-mm spot (300 MW/cm2) at 1.064 gm and 10-20 tJ of tunable IR from the OPG-OPA system, the DFG output was -120 ,J at 4 pumand decreased to 10 ,J at 8 ,m. This is shown in Fig. 10 together with a theoretical calculation using Eq. (1). The agreement between theory and experiment is quite satisfactory. However, the output of DFG at 3 ,m was only -90 ,J (not shown in Fig. 10), much lower than the theoretical prediction. This is due to the limited beam size that could be transmitted through our tilted crystal. 5.
Q)
4.0
1763
of AgGaS 2 with a nonlinearity
of d36 =
SUMMARY
We have described here practical OPG-OPA systems using BBO and LBO crystals pumped by the second- and thirdharmonic output of an active-passive mode-locked Nd:YAG laser. These systems generate high-energy, picosecond pulses tunable from 0.4 to 2.5 Am. The pump-to-signal conversion efficiency can be as high as 30%. With the output further engaged in second-harmonic, sum-frequency, and difference-frequency generation, the tuning range of the system can be further extended to near 0.2 ,m in the UV and to near 8 ,m in the mid-IR. The output bandwidth can be narrowed to nearly the transform limit of the pulses. The observed characteristics of the system agree fairly well with theoretical predictions.
14 pm/V The Fresnel loss of the crystal surfaces has been taken into account in the calculation.
ACKNOWLEDGMENTS D. FREQUENCY EXTENSION TO THE UV BY SECOND-HARMONIC GENERATION AND TO THE MID-IR BY DIFFERENCE-FREQUENCY GENERATION The tunable output in the visible range from the OPGOPA system can be converted to the UV by secondharmonic generation.' 9 Using a 5-mm-long BBO crystal, we have frequency doubled the tunable output of our BBO OPG-OPA system pumped at 355 nm down to 0.22 ,um with a conversion efficiency of -10%. A higher conversion efficiency is expected if a 15-mm LBO crystal is used
with a tightly focused input beam. On the other hand, it is also possible to mix the near-IR tunable output from our BBO OPG-OPA system with the 1.064-,umlaser beam in a nonlinear crystal to generate by DFG tunable radiation down to the mid-IR. The nonlinear crystal most suitable
This research was partially supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Material Science Division of the U.S. Department of Energy under contract DE-AC03-765F00098, the Faculty Research Committee of Georgia Southern University, E. I. du Pont de Nemours & Co., and the Chinese Academy of Sciences. We also acknowledge contributions from Rodney Chin, Haitian Zhou, Jiwu Ling, and B. Wu to this research.
*Present address, Department of Physics, Georgia Southern University, Statesboro, Georgia 30460. tPresent address, Institute of Electro-Optical Engineering, Chiao Tung University, Hsinchu, Taiwan, China. 'Present address, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou, China.
10. J. Y.Zhang, H. T. Zhou, J. Y.Huang, Y. R. Shen, and C. Chen,
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5. 6.
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