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Frequency-tunable sum- and difference-frequency generation by using two diode lasers in a KTP crystal
Optics Communications 102 (1993) 304-308 North-Holland
OPTICS COMMUNICATIONS
Frequency-tunable sum- and difference-frequency generation by using two diode lasers in a KTP crystal Weizhi Wang and Motoichi Ohtsu Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259, Nagatsuta-cho, Midori-ku, Yokohama 227, Japan
Received I 1 May 1993
Reported are the frequency-tunable sum-frequency generation at 0.52 p.m and the difference-frequency generation at 1.6 lxm by using two diode lasers at 0.78 p.m and 1.54 p.m. The influence of the walk-off effect associated with the elliptical gaussian beam of the diode laser was calculated and compared with the experimental results.
A w i d e b a n d highly coherent tunable light souce is required urgently in rapidly d e v e l o p e d fields, such as q u a n t u m optics and high resolution spectroscopy, in which m a n i p u l a t i o n o f a t o m s or ions, optical frequency and f u n d a m e n t a l physical constant measurements have been p a i d much attention. To realize a diode-laser-based optical frequency sweep generator covering the wavelengths from ultraviolet to near infrared, nonlinear frequency conversions are performed by e m p l o y i n g d i o d e laser a n d nonlinear crystals [ 1,2]. By using a K T P crystal, which has large nonlinear coefficients, larger angular and temperature acceptance widths, frequency-tunable coherent light in the green region can be o b t a i n e d in the sumfrequency generation o f diode lasers, while in the near infrared region, frequency down-conversion such as parametric amplification which uses a diode laser and a solid-state laser has been also d e m o n s t r a t e d [3 ]. In this paper, we show our recent e x p e r i m e n t results on the sum-frequency generation, in which a 0.52 ~tm coherent tunable light was o b t a i n e d by using highpower single m o d e d i o d e lasers at 0.78 lain and 1.5 txm, and c o m p a r e the theoretical calculation with the experimental result o f the influence o f the walk-off in the used K T P crystal on the nonlinear conversion efficiency associated with the elliptical gaussian beam o f the d i o d e laser. F u r t h e r m o r e , we also d e m o n s t r a t e the difference-frequency generation, for the first time 304
tO the author's knowledge, by using these two diode lasers. To extend the sum-frequency generation at a wavelength shorter than the previously reported 0.54 ~tm green light for which a m u l t i m o d e high-power 1.5 p m diode laser was used [2], we used here a highpower 1.54 lam multi-electrode corrugation-pitchm o d u l a t e d M Q W - D F B laser [4] with a m a x i m u m power o f 50 mW, and a 50 mW, 0.78 ~tm diode laser. The experimental setup was the same as our previous one [ 2 ] in which a l 0 m m length K T P was put in the 0 plane, i.e., ¢ = 0 °, the polarization o f the 1.5 ~tm laser was perpendicular to the 0 plane and the polarization o f the 0.78 ~tm laser was in the 0 plane for satisfying the type-II phase matching. A maxim u m 0.34 ~tW output at 0.52 ~tm was obtained. To investigate the angle tuning characteristics o f the used K T P crystal in the case o f sum-frequency generation, the relation between the output power at the sumfrequency and the detuning o f the matching angle o f the K T P crystal was measured and is shown in fig. 1. The acceptance angle defined by the fwhm o f the curve o f the output power versus phase mismatch was m e a s u r e d to be about 0.5 ° . We also carried out the sum-frequency generation by replacing the 0.78 p.m laser with the high-power single m o d e diode lasers in the 0.8 p.m region. The c o m b i n a t i o n o f these results has p r o v i d e d us the highly coherent tunable output from 0.51 ~tm to 0.56 ~tm corresponding to a span o f
0030-4018/93/$ 06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
Volume 102, number 3,4
OPTICSCOMMUNICATIONS
waves is insignificant compared with their inputs and the phase matching condition is satisfied, the fundamental waves are assumed to be plane waves and take forms as
I
0.34 t 1 ,i
v
El =Elo exp[
o,
r
o Q.
1 October 1993
-
(xE/w2x-l-y2/W2y)]exp( - i ( o I t)
,
and f
o.
g
0.17
E2 =E2o exp[ -
i J r
J t i ¢
I
i
E CO
o
O.o"t ]
i
o Detuning angle (deg.)
Fig. 1. Measured relation between the output power of the sumfrequency generation and the detuning angle. The phase-matching angle is 61 °.
(x'Z/w2x+y'E/w2y)]exp(-kozt) ,
where E~o and E2o represent the field amplitudes at the beam center of the fundamental waves, Wix and w,y represent beam waists along the short- and longaxis of the elliptical gaussian beam cross-section, respectively, coordinates x' and y' represent so called extra-ordinary waves which deviate from the ordinary waves (coordinates x and y) in propagation [ 8 ]. Then we can express the generated wave in the process of frequency mixing (sum or difference) as follows: E3 = - i to3dcrf El0E2o
50 THz in the green region by using only diode lasers. A series of our experiments show that the KTP crystal in the angle matching manner is insensitive to both slight deviation from the matching angle and the temperature fluctuation so as to offer an excellent stability. However, the walk-off effect in KTP, which is due to the double refraction when non-critical phase matching is unavailable, degrades the conversion efficiency. As the walk-off effect becomes more serious when the beams participating in the frequency mixing are focused to make the diameters as small as several tens of lain, that is the case when the relatively low power single mode diode lasers are employed for the highly coherent light generation, we should pay more attention to optimizing the nonlinear conversion efficiency. Although the theoretical analysis of the nonlinear conversion involving the walk-off effect and the circular gaussian beams has been carried out by several authors [5-7], the influence of the walk-off effect associated with the elliptical gaussian beams as in the case of using diode lasers has not been documented. We present here an extension of the heuristic theory mentioned-above to the case of the type-II frequency mixing in a KTP crystal with the near field approximation and compare the calculation with the experimental results. In the case that the depletion of the fundamental
2n3c
L
X
,~-. + w2.--+w2--+~2
exp -
dz,
0
where the subscript i (i= l, 2, 3) denotes the lightwave at the frequency co~, and to, satisfies m3-- co~_+ to2, i.e., the energy conservation in the frequency conversion, and the temporal terms were neglected for simplicity, n is the refractive index, c is the speed of light in vacuum, L is the length of the crystal, and deft is the effective nonlinear coefficient. We discuss here only the case of 0-phase-matching in which ~ = 0 °, so we have x' =x-pz, and y' = y by referring to fig. 2a, where p represents the double refraction or the walk-off angle. The generated power at w3 can be calculated by
P3=½n3cEof S E3E~dxdy, c.s.
where c.s. implies that the integral is carried out over the crystal cross-section, E~ denotes the complex conjugation of E3. As the plane waves have been assumed, better approximation is expected to the case of loose beam focusing, while in the case of tight beam focusing, variation of beam diameters along the propagation inside the crystal should be taken into 305
Volume 102, number 3,4
OPTICS COMMUNICATIONS
X
X'
= ke
F..... oo,,o axis •I
L ~
W a l k - o f f in K T P crystal
(a) I
I
I
I
I
\
10
\\
"\
v
o en O.
B t
20
I
I
30
I
I
I
40
J-
50
I
I
I
t
60
B e a m w a i s t (IJm) (b)
Fig. 2. (a) Coordinates in the crystal, ko and k= represent ordinary and extra-ordinary waves in the case of ~ = 0, p represents the walk-off angle. (b) Relative output powers versus beam diameters. Curve A and curve B correspond to the cases that the walk-offoccurs along the long- and the short-axis of the elliptical gaussian beam, respectively. One of the participating beams is circular and its diameter is equal to the short-axis diameter of the ellipticalbeam. account for evaluating the total output power. However, in c o m p a r i s o n o f the difference o f the walk-off effect between the case that the walk-off takes place along the short-axis o f the elliptical b e a m cross-section a n d the case o f the long-axis, such a calculation offered a good a p p r o x i m a t i o n . Figure 2b show the calculation results. Curves A and B correspond to the a b o v e - m e n t i o n e d two cases. F o r the purpose o f comparing the calculation with the experiment, a circular gaussian b e a m which corresponds to the 1.5 ~tm diode laser and an elliptical gaussian b e a m which corresponds to the 0.78 Ixm d i o d e laser were used in calculation, while the ratio o f long-axis short-axis o f the elliptical gaussian b e a m was 3 : 1 and the walk-off an306
1 October 1993
gle was p = 3 X 10 -2 rad for the used K T P crystal. It is shown that when the walk-off occurs along the longaxis o f the elliptical beam, the output power can be m o r e than d o u b l e d with respect to the case that the walk-off occurs along the short-axis o f the elliptical beam. To confirm our theoretical estimation o f the walk-off influence associated with the elliptical gaussian b e a m shape o f the diode laser, we measured the power differences through changing the relative orientation between the crystal and the elliptical b e a m cross-section. The m e a s u r e m e n t was carried out by using half-wave plates to change the polarizations of the laser beams and rotating the 0 plane of the K T P crystal. Two sets o f the measured values in fig. 2b are n o r m a l i z e d to the smaller one for comparison, corresponding to the experimental condition that lenses with focal lengths o f 80 m m and 200 m m were used to focus the two collimated fundamental beams. Enh a n c e m e n t factors by arranging the walk-off along the long-axis o f the elliptical b e a m were 2.13 and 2.26, respectively, and were in fair accordance with the calculated values o f 2.9 a n d 2.3, respectively. F o r optimizing the b e a m parameters in frequency conversion o f diode lasers, we also calculated the relation between the output power and the long- to short-axis ratio r = Wlx/Wly (W2x/W2y -~ 1 ), using the b e a m d i a m e t e r as a parameter. Figure 3 shows the calculation result. Curves A, B and C correspond to the cases when the beam diameter o f the circular one, which is assumed to be equal to the short-axis dia m e t e r o f the elliptical one, is 30, 60 and 100 ~m, respectively. It is noted that the m a x i m u m output does not happen at r = l, which means that the elliptical b e a m is preferable to the circular one, such an effect was also confirmed in other work [9 ]. W i t h the increase o f the b e a m diameter, the m a x i m u m trends to r = 1, implying that the circular beams can offer good conversion efficiency in the case of large b e a m diameters. Because the influence o f the walkoff effect also d e p e n d s on the b e a m diameters, the o p t i m u m value o f r for the conversion efficiency varies as the focusing condition is different. In order to d e m o n s t r a t e the feasibility o f the difference-frequency generation using two diode lasers for our intended continuous-wave highly coherent frequency sweep generator [3], we used the same laser in the above experiment to generate the coherent light at 1.6 ktm. The experiment setup is shown
Volume 102, number 3,4 i
OPTICS COMMUNICATIONS
I
I
i
i
J
i
I
i
1 October 1993
Fu6-T}0
M
1.51Jm CL M
=
10
0.78 IJm CL
HW
ML Matching angle
e~
\
"~
"
(3.
~ 0
KTP
.
5
"'-.. B
\
\\
0=52.1 ° ~ = 0
L P F
\
Detection
1 I
1
I
I
I
I
5
I
I
I
(a)
I
10
Ratio of long- to s h o r t - a x i s diameter
Fig. 3. Calculated relation between relative output power and the long- to short-axis ratio r= w~,,/w~y.The walk-off occurs along the long-axis of the elliptical gaussian beam. Curves A, B and C correspond to w2x=w2y=w,y=30 lam, 60 lam and 100 ~tm, respectively. in fig. 4a. The polarization o f the 0.78 lain laser was p e r p e n d i c u l a r to the 0 plane and the polarization o f the 1.5 lam laser was in the 0 plane. The phase matching angle was 52.1 ° which was in agreement with the calculated value. The b e a m s were arranged to obtain the higher efficiency based on the above theoretical calculation. Figure 4b shows the measured spectral profiles o f the three participating light waves by using an optical spectrum analyzer (Anritsu MS9702B). A 0.3 IxW o u t p u t was obtained. It is the first report o f the different-frequency generation by using only d i o d e lasers. The tunable range can be larger than 5 T H z (1.58-1.62 lam) by controlling the o p e r a t i o n t e m p e r a t u r e s and currents o f the lasers. Since the coherence o f d i o d e lasers is conveniently i m p r o v e d by optical a n d / o r electrical feedback techniques [ 10 ], the difference-frequency generation by d i o d e lasers can p r o v i d e us the highly coherent cw tunable light in the near infrared region. In s u m m a r y , we reported our e x p e r i m e n t a l progress in realization o f the diode-laser-based w i d e b a n d highly coherent light source. The sum-frequency generation in K T P was e x t e n d e d to 0.52 lam with a 0.34 /aW o u t p u t power by using a 0.78 ~m a n d a 1.54 lain high power single m o d e laser. F o r o p t i m i z i n g the nonlinear frequency conversion efficiency in the case
50roW
+
40roW
1.o
+
0.31JW
IA °.°
785.0nm
1538.0nm
1603.0rim
O.5nm/div
0.5nm/div
O.2nm/div
(b)
Fig. 4. (a) Experimental setup for the difference-frequency generation. HW: half-wave plate; F: filter; P: polarizer; DM: dichroic mirror; L: lens; CL: collimating lens. (b) Spectral profiles of the fundamental waves at 0.78 .am and 1.54 .am and the generated wave at 1.6 I~m measured by an optical spectrum analyzer.
o f using diode lasers, the influence o f the walk-off effect was calculated and c o m p a r e d with the experimental results. Difference-frequency generation at 1.6 ~tm with a frequency tunable range larger than 5 T H z in the same K T P was also achieved by using the above two lasers. Although the output power in our engaged tunable light source system is still low, it can be ready for some p r i m a r y experiments on spectroscopy and even on the advanced quantum optics. F o r example, observation o f iodine molecule absorption lines by using the sum-frequency generation is under investigation for stabilizing the diode laser frequency and providing a frequency link to the absolute frequency reference in our w i d e b a n d frequency sweep generator. The authors would like to thank Dr. M. Okai o f 307
Volume 102, number 3,4
OPTICS COMMUNICATIONS
H i t a c h i C o r p . f o r d i s c u s s i o n o n t h e h i g h - p o w e r 1.5 ~tm m u l t i - e l e c t r o d e D F B laser.
References [1] M. Ohtsu, K. Nakagawa, C-H Shin, H. Kusuzawa, M. Kourogi and H. Suzuki, CLEO'90 (Optical Society of America, Washington, DC, 1990) paper CME5. [ 2 ] W. Wang, K. Nakagawa, Y. Toda and M. Ohtsu, Appl. Phys. Lett. 61 ( 1992 ) 1886; in this paper there is a mistake in the phase matching angle of the sum-frequency generation; it should be 59 °.
308
1 October 1993
[3] W. Wang and M. Ohtsu, Optics Lett., in press. [4 ] M. Okai, T. Tsuchiya, K. Oumi, N. Chinone and T. Harada, IEEE Photon. Tehnol. Lett. 2 (1990) 529. [5] G.D. Boyd and D.A. Kleinman, J. Appl. Phys. 39 (1968) 3597. [6 ] J.-J. Zondy, Optics Comm. 81 ( 1991 ) 427. [7] K. Asaumi, Appl. Phys. B 54 (1992) 265. [8] V.G. Dmitriev and D.N. Nikogosyan, Optics Comm. 95 (1993) 173. [ 9 ] T. Taira, Jpn. J. Appl. Phys. 31 ( 1992 ) L 682. [ 10 ] M. Ohtsu, K. Nakagawa, M. Kourogi and W. Wang, J. Appl. Phys., in press.
OPTICS COMMUNICATIONS
Frequency-tunable sum- and difference-frequency generation by using two diode lasers in a KTP crystal Weizhi Wang and Motoichi Ohtsu Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259, Nagatsuta-cho, Midori-ku, Yokohama 227, Japan
Received I 1 May 1993
Reported are the frequency-tunable sum-frequency generation at 0.52 p.m and the difference-frequency generation at 1.6 lxm by using two diode lasers at 0.78 p.m and 1.54 p.m. The influence of the walk-off effect associated with the elliptical gaussian beam of the diode laser was calculated and compared with the experimental results.
A w i d e b a n d highly coherent tunable light souce is required urgently in rapidly d e v e l o p e d fields, such as q u a n t u m optics and high resolution spectroscopy, in which m a n i p u l a t i o n o f a t o m s or ions, optical frequency and f u n d a m e n t a l physical constant measurements have been p a i d much attention. To realize a diode-laser-based optical frequency sweep generator covering the wavelengths from ultraviolet to near infrared, nonlinear frequency conversions are performed by e m p l o y i n g d i o d e laser a n d nonlinear crystals [ 1,2]. By using a K T P crystal, which has large nonlinear coefficients, larger angular and temperature acceptance widths, frequency-tunable coherent light in the green region can be o b t a i n e d in the sumfrequency generation o f diode lasers, while in the near infrared region, frequency down-conversion such as parametric amplification which uses a diode laser and a solid-state laser has been also d e m o n s t r a t e d [3 ]. In this paper, we show our recent e x p e r i m e n t results on the sum-frequency generation, in which a 0.52 ~tm coherent tunable light was o b t a i n e d by using highpower single m o d e d i o d e lasers at 0.78 lain and 1.5 txm, and c o m p a r e the theoretical calculation with the experimental result o f the influence o f the walk-off in the used K T P crystal on the nonlinear conversion efficiency associated with the elliptical gaussian beam o f the d i o d e laser. F u r t h e r m o r e , we also d e m o n s t r a t e the difference-frequency generation, for the first time 304
tO the author's knowledge, by using these two diode lasers. To extend the sum-frequency generation at a wavelength shorter than the previously reported 0.54 ~tm green light for which a m u l t i m o d e high-power 1.5 p m diode laser was used [2], we used here a highpower 1.54 lam multi-electrode corrugation-pitchm o d u l a t e d M Q W - D F B laser [4] with a m a x i m u m power o f 50 mW, and a 50 mW, 0.78 ~tm diode laser. The experimental setup was the same as our previous one [ 2 ] in which a l 0 m m length K T P was put in the 0 plane, i.e., ¢ = 0 °, the polarization o f the 1.5 ~tm laser was perpendicular to the 0 plane and the polarization o f the 0.78 ~tm laser was in the 0 plane for satisfying the type-II phase matching. A maxim u m 0.34 ~tW output at 0.52 ~tm was obtained. To investigate the angle tuning characteristics o f the used K T P crystal in the case o f sum-frequency generation, the relation between the output power at the sumfrequency and the detuning o f the matching angle o f the K T P crystal was measured and is shown in fig. 1. The acceptance angle defined by the fwhm o f the curve o f the output power versus phase mismatch was m e a s u r e d to be about 0.5 ° . We also carried out the sum-frequency generation by replacing the 0.78 p.m laser with the high-power single m o d e diode lasers in the 0.8 p.m region. The c o m b i n a t i o n o f these results has p r o v i d e d us the highly coherent tunable output from 0.51 ~tm to 0.56 ~tm corresponding to a span o f
0030-4018/93/$ 06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
Volume 102, number 3,4
OPTICSCOMMUNICATIONS
waves is insignificant compared with their inputs and the phase matching condition is satisfied, the fundamental waves are assumed to be plane waves and take forms as
I
0.34 t 1 ,i
v
El =Elo exp[
o,
r
o Q.
1 October 1993
-
(xE/w2x-l-y2/W2y)]exp( - i ( o I t)
,
and f
o.
g
0.17
E2 =E2o exp[ -
i J r
J t i ¢
I
i
E CO
o
O.o"t ]
i
o Detuning angle (deg.)
Fig. 1. Measured relation between the output power of the sumfrequency generation and the detuning angle. The phase-matching angle is 61 °.
(x'Z/w2x+y'E/w2y)]exp(-kozt) ,
where E~o and E2o represent the field amplitudes at the beam center of the fundamental waves, Wix and w,y represent beam waists along the short- and longaxis of the elliptical gaussian beam cross-section, respectively, coordinates x' and y' represent so called extra-ordinary waves which deviate from the ordinary waves (coordinates x and y) in propagation [ 8 ]. Then we can express the generated wave in the process of frequency mixing (sum or difference) as follows: E3 = - i to3dcrf El0E2o
50 THz in the green region by using only diode lasers. A series of our experiments show that the KTP crystal in the angle matching manner is insensitive to both slight deviation from the matching angle and the temperature fluctuation so as to offer an excellent stability. However, the walk-off effect in KTP, which is due to the double refraction when non-critical phase matching is unavailable, degrades the conversion efficiency. As the walk-off effect becomes more serious when the beams participating in the frequency mixing are focused to make the diameters as small as several tens of lain, that is the case when the relatively low power single mode diode lasers are employed for the highly coherent light generation, we should pay more attention to optimizing the nonlinear conversion efficiency. Although the theoretical analysis of the nonlinear conversion involving the walk-off effect and the circular gaussian beams has been carried out by several authors [5-7], the influence of the walk-off effect associated with the elliptical gaussian beams as in the case of using diode lasers has not been documented. We present here an extension of the heuristic theory mentioned-above to the case of the type-II frequency mixing in a KTP crystal with the near field approximation and compare the calculation with the experimental results. In the case that the depletion of the fundamental
2n3c
L
X
,~-. + w2.--+w2--+~2
exp -
dz,
0
where the subscript i (i= l, 2, 3) denotes the lightwave at the frequency co~, and to, satisfies m3-- co~_+ to2, i.e., the energy conservation in the frequency conversion, and the temporal terms were neglected for simplicity, n is the refractive index, c is the speed of light in vacuum, L is the length of the crystal, and deft is the effective nonlinear coefficient. We discuss here only the case of 0-phase-matching in which ~ = 0 °, so we have x' =x-pz, and y' = y by referring to fig. 2a, where p represents the double refraction or the walk-off angle. The generated power at w3 can be calculated by
P3=½n3cEof S E3E~dxdy, c.s.
where c.s. implies that the integral is carried out over the crystal cross-section, E~ denotes the complex conjugation of E3. As the plane waves have been assumed, better approximation is expected to the case of loose beam focusing, while in the case of tight beam focusing, variation of beam diameters along the propagation inside the crystal should be taken into 305
Volume 102, number 3,4
OPTICS COMMUNICATIONS
X
X'
= ke
F..... oo,,o axis •I
L ~
W a l k - o f f in K T P crystal
(a) I
I
I
I
I
\
10
\\
"\
v
o en O.
B t
20
I
I
30
I
I
I
40
J-
50
I
I
I
t
60
B e a m w a i s t (IJm) (b)
Fig. 2. (a) Coordinates in the crystal, ko and k= represent ordinary and extra-ordinary waves in the case of ~ = 0, p represents the walk-off angle. (b) Relative output powers versus beam diameters. Curve A and curve B correspond to the cases that the walk-offoccurs along the long- and the short-axis of the elliptical gaussian beam, respectively. One of the participating beams is circular and its diameter is equal to the short-axis diameter of the ellipticalbeam. account for evaluating the total output power. However, in c o m p a r i s o n o f the difference o f the walk-off effect between the case that the walk-off takes place along the short-axis o f the elliptical b e a m cross-section a n d the case o f the long-axis, such a calculation offered a good a p p r o x i m a t i o n . Figure 2b show the calculation results. Curves A and B correspond to the a b o v e - m e n t i o n e d two cases. F o r the purpose o f comparing the calculation with the experiment, a circular gaussian b e a m which corresponds to the 1.5 ~tm diode laser and an elliptical gaussian b e a m which corresponds to the 0.78 Ixm d i o d e laser were used in calculation, while the ratio o f long-axis short-axis o f the elliptical gaussian b e a m was 3 : 1 and the walk-off an306
1 October 1993
gle was p = 3 X 10 -2 rad for the used K T P crystal. It is shown that when the walk-off occurs along the longaxis o f the elliptical beam, the output power can be m o r e than d o u b l e d with respect to the case that the walk-off occurs along the short-axis o f the elliptical beam. To confirm our theoretical estimation o f the walk-off influence associated with the elliptical gaussian b e a m shape o f the diode laser, we measured the power differences through changing the relative orientation between the crystal and the elliptical b e a m cross-section. The m e a s u r e m e n t was carried out by using half-wave plates to change the polarizations of the laser beams and rotating the 0 plane of the K T P crystal. Two sets o f the measured values in fig. 2b are n o r m a l i z e d to the smaller one for comparison, corresponding to the experimental condition that lenses with focal lengths o f 80 m m and 200 m m were used to focus the two collimated fundamental beams. Enh a n c e m e n t factors by arranging the walk-off along the long-axis o f the elliptical b e a m were 2.13 and 2.26, respectively, and were in fair accordance with the calculated values o f 2.9 a n d 2.3, respectively. F o r optimizing the b e a m parameters in frequency conversion o f diode lasers, we also calculated the relation between the output power and the long- to short-axis ratio r = Wlx/Wly (W2x/W2y -~ 1 ), using the b e a m d i a m e t e r as a parameter. Figure 3 shows the calculation result. Curves A, B and C correspond to the cases when the beam diameter o f the circular one, which is assumed to be equal to the short-axis dia m e t e r o f the elliptical one, is 30, 60 and 100 ~m, respectively. It is noted that the m a x i m u m output does not happen at r = l, which means that the elliptical b e a m is preferable to the circular one, such an effect was also confirmed in other work [9 ]. W i t h the increase o f the b e a m diameter, the m a x i m u m trends to r = 1, implying that the circular beams can offer good conversion efficiency in the case of large b e a m diameters. Because the influence o f the walkoff effect also d e p e n d s on the b e a m diameters, the o p t i m u m value o f r for the conversion efficiency varies as the focusing condition is different. In order to d e m o n s t r a t e the feasibility o f the difference-frequency generation using two diode lasers for our intended continuous-wave highly coherent frequency sweep generator [3], we used the same laser in the above experiment to generate the coherent light at 1.6 ktm. The experiment setup is shown
Volume 102, number 3,4 i
OPTICS COMMUNICATIONS
I
I
i
i
J
i
I
i
1 October 1993
Fu6-T}0
M
1.51Jm CL M
=
10
0.78 IJm CL
HW
ML Matching angle
e~
\
"~
"
(3.
~ 0
KTP
.
5
"'-.. B
\
\\
0=52.1 ° ~ = 0
L P F
\
Detection
1 I
1
I
I
I
I
5
I
I
I
(a)
I
10
Ratio of long- to s h o r t - a x i s diameter
Fig. 3. Calculated relation between relative output power and the long- to short-axis ratio r= w~,,/w~y.The walk-off occurs along the long-axis of the elliptical gaussian beam. Curves A, B and C correspond to w2x=w2y=w,y=30 lam, 60 lam and 100 ~tm, respectively. in fig. 4a. The polarization o f the 0.78 lain laser was p e r p e n d i c u l a r to the 0 plane and the polarization o f the 1.5 lam laser was in the 0 plane. The phase matching angle was 52.1 ° which was in agreement with the calculated value. The b e a m s were arranged to obtain the higher efficiency based on the above theoretical calculation. Figure 4b shows the measured spectral profiles o f the three participating light waves by using an optical spectrum analyzer (Anritsu MS9702B). A 0.3 IxW o u t p u t was obtained. It is the first report o f the different-frequency generation by using only d i o d e lasers. The tunable range can be larger than 5 T H z (1.58-1.62 lam) by controlling the o p e r a t i o n t e m p e r a t u r e s and currents o f the lasers. Since the coherence o f d i o d e lasers is conveniently i m p r o v e d by optical a n d / o r electrical feedback techniques [ 10 ], the difference-frequency generation by d i o d e lasers can p r o v i d e us the highly coherent cw tunable light in the near infrared region. In s u m m a r y , we reported our e x p e r i m e n t a l progress in realization o f the diode-laser-based w i d e b a n d highly coherent light source. The sum-frequency generation in K T P was e x t e n d e d to 0.52 lam with a 0.34 /aW o u t p u t power by using a 0.78 ~m a n d a 1.54 lain high power single m o d e laser. F o r o p t i m i z i n g the nonlinear frequency conversion efficiency in the case
50roW
+
40roW
1.o
+
0.31JW
IA °.°
785.0nm
1538.0nm
1603.0rim
O.5nm/div
0.5nm/div
O.2nm/div
(b)
Fig. 4. (a) Experimental setup for the difference-frequency generation. HW: half-wave plate; F: filter; P: polarizer; DM: dichroic mirror; L: lens; CL: collimating lens. (b) Spectral profiles of the fundamental waves at 0.78 .am and 1.54 .am and the generated wave at 1.6 I~m measured by an optical spectrum analyzer.
o f using diode lasers, the influence o f the walk-off effect was calculated and c o m p a r e d with the experimental results. Difference-frequency generation at 1.6 ~tm with a frequency tunable range larger than 5 T H z in the same K T P was also achieved by using the above two lasers. Although the output power in our engaged tunable light source system is still low, it can be ready for some p r i m a r y experiments on spectroscopy and even on the advanced quantum optics. F o r example, observation o f iodine molecule absorption lines by using the sum-frequency generation is under investigation for stabilizing the diode laser frequency and providing a frequency link to the absolute frequency reference in our w i d e b a n d frequency sweep generator. The authors would like to thank Dr. M. Okai o f 307
Volume 102, number 3,4
OPTICS COMMUNICATIONS
H i t a c h i C o r p . f o r d i s c u s s i o n o n t h e h i g h - p o w e r 1.5 ~tm m u l t i - e l e c t r o d e D F B laser.
References [1] M. Ohtsu, K. Nakagawa, C-H Shin, H. Kusuzawa, M. Kourogi and H. Suzuki, CLEO'90 (Optical Society of America, Washington, DC, 1990) paper CME5. [ 2 ] W. Wang, K. Nakagawa, Y. Toda and M. Ohtsu, Appl. Phys. Lett. 61 ( 1992 ) 1886; in this paper there is a mistake in the phase matching angle of the sum-frequency generation; it should be 59 °.
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1 October 1993
[3] W. Wang and M. Ohtsu, Optics Lett., in press. [4 ] M. Okai, T. Tsuchiya, K. Oumi, N. Chinone and T. Harada, IEEE Photon. Tehnol. Lett. 2 (1990) 529. [5] G.D. Boyd and D.A. Kleinman, J. Appl. Phys. 39 (1968) 3597. [6 ] J.-J. Zondy, Optics Comm. 81 ( 1991 ) 427. [7] K. Asaumi, Appl. Phys. B 54 (1992) 265. [8] V.G. Dmitriev and D.N. Nikogosyan, Optics Comm. 95 (1993) 173. [ 9 ] T. Taira, Jpn. J. Appl. Phys. 31 ( 1992 ) L 682. [ 10 ] M. Ohtsu, K. Nakagawa, M. Kourogi and W. Wang, J. Appl. Phys., in press.