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A high-power short-pulse laser diode for waveguide second harmonic generation
Solid-State ElectronicsVol. 34, No. 12, pp. 1329-1333,1991 Printed in Great Britain. All rights reserved
0038-1101/91 $3.00+ 0.00 Copyright© 1991PergamonPresspic
A HIGH-POWER SHORT-PULSE LASER DIODE FOR WAVEGUIDE SECOND H A R M O N I C GENERATION MASAHIRO KUME,~ HIROKI NAITO,l JUN OHYA, 2 ISSEY OHTA, I HmogAzu Smmzu, ~ MASARU KAZUMURAt and IwAO TERAMOTO1 tElectronics Research Laboratory, Matsushita Electronics Corporation, Takatsuki, Osaka 569, Japan 2Semiconductor Research Center, Matsushita Electric Industrial Co., Ltd., Moriguchi, Osaka 570, Japan (Received 15 April 1991)
Almtraet--This paper describes successful development of a high-power short-pulse i.r. laser diode, together with numerical analyses giving a clear understanding of the operation. The novel aspect of the present laser diode is that a saturable absorber is incorporated for gain switching into the buried-twin-ridge substrate (BTRS) structure which allows a high-stability fundamental spatial mode operation. Peak powers as high as 1.23 W and pulse widths as narrow as 34.1 ps have been obtained in an efficient 8ain-switehing operation with the high mode stability. The experimental observations are well-explicable by the numerical analyses in substantial respects. We also report picosecond-pulse blue-light generation which has been attained by applying the frequency doubling technique to the above laser in a LiNbO3 wavegulde.
I. INTRODUCTION Picosecond-pulse blue light has been required for some sophisticated optical measurements such as fluorescent lifetime measurement and temporally resolved spectroscopy. To meet this requirement, a compact coherent blue light source, which is allsolid-state, has been developed by applying the frequency doubling technique to a GaAIAs laser diode, where the Cherenkov radiation scheme is used in a proton-exchanged LiNbO 3 waveguide[1-3]. However, there is still a strong need for higherpower pulsed laser light. A high-power pulsed i.r. laser light is strongly required for second harmonic generation (SHG) because SHG capability increases in proportion to the square of the input power. Fundamental spatial mode operation of the laser diode is also required for efficiency coupling between the laser beam and the waveguide SHG element. Various investigations of short-pulse laser diodes have been published, and the most attractive ones are those using the gain switching method whereby picosecond (10-50 ps) pulses have been generated[3-5]. However, in those previous reports the gain switching has only been performed by the direct current switching which uses high peak pulse current, and the operation has been exclusively in multi-spatial mode, not meeting the above-mentioned requirement. This paper describes a high-power fundamental mode i.r. pulse-light laser diode which we have developed for blue light SHG. Fundamental mode operation has been attained in this type of laser by using the buried-twin-ridge substrate (BTRS) structure which is suitable for attaining stable fundamental spatial mode operation. Into this BTRS structure a saturable absorber region is incorporated for gain u~ ~4/t~--m
switching operation by devising a construction in which a current blocking layer is provided adjacent to the part of the active layer near the cavity end region. This construction is very simple, and is advantageous from both fabrication and analysis points of view. The more important point is that this novel gainswitching method gives a positive feedback switching. A high-peak pulse operation is obtained due to this efficient switching and due also to the use of the BTRS structure which is suitable for high-power operation. We have reported in a previous letter[16] efficient blue-light pulse generation which we attained using for the first time the present type of pumping-source laser in an LiNbO 3 waveguide. This paper gives a full description of high power characteristics of the present i.r.-light pulse laser, which is compared with numerical calculations. Thus this work gives a clear understanding of the gain switching operation under the background that the saturable absorber is provided in a very simple structure suitable for analysis. We also report picosecond-pulse blue light generation which has been attained by using the frequency doubling technique for the present i.r. laser in an LiNbO3 waveguide. 2. OPERATION PRINCIPLE AND ANALYSES
2.1. Basic operation principle
The basic operation principle of the gain switching laser is explained with the aid of Fig. 1. The current injection into the active layer is blocked by the n - G a A s current blocking layer, so that the active layer in the cavity end region serves as a saturable absorber. The optical loss in the saturable absorber
1329
MASAHIRO KUME et al.
1330 AbsorptionRegion 12
k
~
*
/
Gain Region II
l~31th0
hffi200,um
20[- i~lOl~O
~50,u m
Laser Light
t~,~L E>
- - L ~ T T T Current TTT
Current Blocking I L~F~
Facet
~Yer
p-GaAsSubstrate
F!cet z 6 - ~
0/
I
01
i
i
~
i
i
x
Fig. 1. The basic operation principle of the gain-switching
laser. region decreases with increasing light intensity. This effect, which works under the gain increase in the gain region, results in a further increase in the net gain. Consequently the peak power is higher for this gain switching laser than for conventional lasers without a saturable absorber.
2.2. Analyses using rate equaitons
/2nx\ 0- ,cos w ) ,
(1)
where No and N~ are the Fourier coefficients of the density of carriers in the gain region, and W is the width of an active layer. The carrier and photon densities are averaged over the y- and z-directions. In addition, where the laser is biased for stable operation, the light is emitted in a given longitudinal mode. This optical behavior can be modeled by one rate equation for the density of photons in the lasing mode. The rate equations are[18]: dINd °t =
eV FAS(No-No-~)-BNZo, dNt
dt
= -raS(No-
i
I
i
Time (200ps/div)
The lasing characteristics of the devices are analyzed using the familiar rate equations given in a simple model for the density of minority carriers in the gain region, for the density of those in the absorber region and for the density of photons in the lasing mode. The carrier diffusion in the active layer caused by the inhomogeneous stimulated emission at high injection level is considered[17] by the Fourier expansion of carrier density N as follows[18].
.(x)=
i
(2)
N 6 - N~)
Bt I + [2nLa ~---~ \2) ) ~Nt,2
(3)
tiN2 = FAS(N2 - No) - BN~,
(4)
Fig. 2; Numerical result of photon density as a function of time for a given injection current.
V is the volume of the active region with gain, and I the injected current. N2 and S are the density of carriers in the absorber region and the density of photons, respectively, a and b are defined as a = l~/(l~+ 12) and b = 12/(ll+/2) where lI and/2 are the length of the gain region and the length of the absorber region, respectively, e Is the electronic charge, zp the photon lifetime and C the spontaneous emission factor. A bimolecular recombination rate is also considered with B as the recombination constant. A linear gain dependence on injected carrier density G(N)=A(N-Na) is used in this calculation. The same coefficients are given for the gain and the absorber region, r is the optical confinement factor of the active layer and Ld is the diffusion length of carriers. The values of the various parameters used in the calculation are listed in Table 1. The static light-current characteristic, which is obtained by taking the time derivatives in (2-5) to be zero, displays a sizable hysteresis. The injection current l(t) is assumed to consist of a d.c. bias current Ib and of a Gaussian-shaped pulse with amplitude Ip and width Tv. The nonlinear differential eqns (2-5) are solved numerically by the Runge-Kutta--Gill A-"
dt
d._SSdt=[FA{a( N°- N G - ~ ) + b ( N 2 - N o , }
t~ 12=50 p m
× / / / s ~ ~ 2 5 ~2----0/~
Table 1. The values of the various parameters used in the calculation Parameter
Value
Parameter
Value
• V F A
1.6 x 10-19C 200× 10 × 0.1/~m 3 0.1 2.75 x 10-~em3 s -I 0.83 X lO is cm -3
B Ld W ~p C
3 x 10-1°em3s -1 3gin 6gm 1.2 x 10-12s lO -4
NG
O
0
1
m
#m /1=200 u tn Ip=10Ith °
2 3 4 5 Bias current (Ib/Ith°) Fig. 3. Calculated peak photon density as functions of the bias current.
High-power laser diode
1331 1.5 11=200 p m
ActiveLayer 1.o o
12=0 ,am m
=
0.5 • n-GaAs
~
BlockingLayer
Absorption
~ / . . . ,,~ Region I>-uaAs~utntrate
L~.~
O
. . . . .
0
I
'
'
'
12----50 /1 m '
. . . .
50 100 Bias Current (mA)
150
Fig. 4. S c h e m a t i c d r a w i n g o f the g a i n - s w i t c h i n g B T R S laser.
Fig. 6. M e a s u r e d pulse p e a k p o w e r s as f u n c t i o n s o f bias current.
procedure. The typical result of the numerical calculation is depicted in Fig. 2 where the injection current I, the gain FAa(No-No-N~/2), the loss 1/~p- FAb(N2- NG) and the photon density S are plotted vs time. In this calculation the current pulse amplitude lp is taken as 10 times the threshold current of the laser diode having no absorption region ~ , and the bias current lb is taken as three times ~ . The generation of a high-intensity light pulse causes a reduction of the gain by the stimulated emission, which in turn limits the duration of the optical pulse. The loss in the absorption region decreases with the increase in light, which results in higher peak photon density than that of the laser diode having no absorption region. In our calculation, these effects are varied by changing the absorption region length/2 and the bias current lb. In Fig. 3, the peak photon densities are plotted vs the bias current for a given electrical pulse height and active region length. The bias current is normalized by the threshold current ~ . There exists an optimal bias current giving a maximum peak power in agreement with previous reports[13,15]. As clearly shown in Fig. 3, an increase of the absorber region length leads to those of the pulse peak power and optimal bias current.
inherently thin enough to enlarge the lasing spot size, consequently reducing optical flux density. The front and rear facets of the laser diode are given reflectivities of 3 and 96%, respectively, in order to increase the forward output power.
3. DEVICE STRUCTURE
Figure 4 shows a schematic drawing of the gainswitching BTRS laser diode. In the absorber region, an n-GaAs layer is grown on the truncated ridge of the p-CraAs substrate so as to block the injection current[20]. This BTRS structure is suited for highpower operation[21] because an active layer formed by the liquid epitaxial growth over two ridges is
4. EXPERIMENTAL RESULTS
The measured static output power vs current characteristics are shown in Fig. 5 for the four absorber region lengths. Bistability with large hysteresis predicted by the calculation is observed [19,22]. For pulsed excitation, the laser diode is gain-switched by short electrical pulses of amplitude 20 V with 400 ps duration superposed on a d.c. bias current. The current pulse amplitude is estimated to be 0.4 A. The pulses are obtained from an impulse generator (AVTECH AVMH-2-C) with a repetition rate of 10 MHz. The 830 nm optical pulses from the laser diode are observed on a sampling optical oscilloscope (Hamamatsu OOS-01) with 10 ps resolution. The measured pulse peak powers as functions of bias current are shown in Fig. 6. The peak power increases with increasing absorber region length as predicted by the calculation. The experimental results are qualitatively in good agreement with the calculation. The optical pulse waveform at maximum peak l~ower is presented in Fig. 7. The optical pulse consists of a main peak with 1.23 W peak power and 34.1 ps pulse width and secondary peaks due to relaxation oscillation. The problem of catastrophic Optical damage (COD) which results from localized temperature rise due to the absorption of intense light is avoided because the optical pulse width is as narrow as few tens of picoseconds and the absorption region 11=200p m, 12ffiS0p m, Ib=100mA
l l = 2 0 0 p m 12ffi0,15,2. ~ ,50pro
1.23W 1
,.- 34.1ps
0 0
Fig.
' 50
100 150 Curator (mA) 5. L i g h t - c u r r e n t c h a r a c t e r i s t i c s o f B T R S lasers.
200
0 I
gain-switching
I
I
I
I
i
200ps/div Fig. 7. Optical pulse waveform of the laser diode.
1332
MAS~,HntOKut~ et al.
_L_KOZ ~-.." Wavegulde
Impulse
C r, tor I
Half-Wave Plate ,/"
~ 0 : 6 ~ [ (
.o, Filter
V"~J'-~'~[ ~aJ~plino~'T-... ~ '~
'
Wavegmde
I
m
MgO:LiNbO3 ~ / / " ~ ) 18ram
Fig. 8. Experimental setup for frequency doubling using a gain.switehing laser diode in a proton-exchanged LiNbO3 waveguide. becomes transparent in the lasing condition. This is the main cause for the high peak operation. Finally, we present the observations of the picosecond-pulse blue-light generation which have been obtained by applying the frequency doubling technique to a gain switching laser. The experimental setup is shown in Fig. 8. The laser diode optical pulse is frequency doubled in the LiNbO3 waveguide under a high efficiency, and blue light pulse is generated in the form of Cberenkov radiation. The waveguide is 0.36/~m deep, 1.6/tm wide and 18ram long, and is fabricated in an MgO-doped LiNbO 3 substrate by proton exchange with pyrophosphoric acid[23]. The 830 nm guided wave is frequency doubled during the propagation along the waveguide in the nonlinear polarization medium, giving the 415 nm harmonic wave. In this Cherenkov radiation scheme, the phasematching condition is satisfied automatically for all longitudinal mode oscillations in a laser diode. The frequency-doubled peak power as a function of laser diode peak power is plotted in Fig. 9. The frequencydoubled peak power is proportional to the square of the laser diode peak power as expected. A maximum peak power of 7.88 mW of blue light pulse under 28.7 ps width is obtained for 1.23 W laser diode peak power, which is almost a four-fold improvement compared to the laser without a saturable absorber. 10
It is found in this figure that the efficiency of coupling between the laser diode and the waveguide is almost constant. This fact means that the fundamental spatial mode is maintained in all ranges of peak power covered in the present experiment. 5. CONCLUSION We have described the generation of high-power picosecond pulses using the BTRS laser structure into which a saturable absorber region is incorporated in a very simple construction. Fundamental mode optical pulses as high as 1.23 W and pulse widths as narrow as 34.1 ps have been optained. The operation characteristics have been well explained by numerical solutions of the relevant rate equations. We have also demonstrated picosecond-pulse blue fight generation which has been attained by applying the frequency doubling technique to the above pulse laser in the proton-exchanged LiNbO 3 waveguide. A blue-light pulse of 7.88 mW peak power has been generated in the form of Cherenkov radiation. It is concluded therefore that the present gain-switching BTRS laser is useful for blue-light SHG. Acknowledgements--The authors would like to thank Dr M.
Takeshima and Dr G. Kano for their helpful discussions and encouragement. They are also grateful to G. Tohmon, K. Yamamoto and Dr T. Taniuchi for fabrication and measurement of the SHG element and for their fruitful discussions.
REFERENCES 0.1 • Without Abmt~aer
i
0.01 10
i
100
i
I
1000
LDPeakPower(mVO Fig. 9. The frequency doubled peak power as a function of laser diode power.
I. T. Taniuchi and K. Yamamoto, ECOC'86, Tu-C5, Barcelona (1986). 2. T. Taniuchi and K. Yamamoto, CLEO'87, WP6, Washington, DC (1987). 3. C. Lin, P. L. Liu, T. C. Damon and D. J. Eilenberger, Electron. Lett. 16, 600 (1980). 4. J. Au Yenng, Appl. Phys. Lett. 38, 308 (1981). 5. H. Ito, H. Yokoyama, S. Murata and H. Inaba, IEEE J. Quant. Electron. QE-17, 663 (1981). 6. J. P. van der Ziel and R. A. Logan, IEEE J. ~ m t . Electron. QE-18, 1340 (1982).
High-power laser diode 7. R. A. Elliott, H. De Xin, R. K. De Frelz, J. M. Hunt and P. G. Rickman, AppL Phys. Lett. 42, 1012 (1983). 8. E. School, D. Bimberg, H. Schumacher and P. T. Landsberg, IEEE J. Quant. Electron. QE-20, 394 (1984). 9. M. S. Demokan and A. Nacaroglu, IEEE J. Quant. Electron. QE-20, 1016 (1984).
10. J. P. van der Ziel, H. Temkin, R. A. Logan and R. D. Dupuis, IEEE J. Quant. Electron. QE-20, 1236 (1984). 11. M. Osinski and M. J. Adams, IEEE J. Quant. Electron. QE-21, 1929 (1985). 12. R. S. Tucker, J. M. Wiesenfield, A. H. Gnauck and J. E. Bowers, Electron. Lett. 22, 1329 (1986). 13. P. Paulus, R. Langenhorst and D. Jager, IEEE J. Quant. Electron. 24, 1519 (1988). 14. P. P. Vasilev, IEEE J. Quant. Electron. 24, 2386
(1988). 15. H. Liu, M. Fukazawa, Y. Kawai and T. Kamiya, IEEE J. Quant. Electron. 25, 1417 (1989).
1333
16. J. Ohya, G. Tohmon, K. Yamamoto, T. Taniuchi and M. Kume, Appl. Phys. Lett. f ~ 2270 (1990). 17. J. Butts and M. Danielsen, IEEE J. Quant. Electron. QE-13, 669 (1977). 18. Y. Suematsu, Semiconductor Laser and Optical Integrated Circuit, p. 197. Ohm, Tokyo (1984). 19. C. Harder, K. Y. Lau and A. Yariv, IEEE J. Quant. Electron. QE-18, 1351 (1982). 20. T. Shibutani, M. Kume, K. Hamada, H. Shimizu, K. Itoh, G. Kano and I. Teramoto, IEEE J. Quant. Electron. QE-23, 760 (1987). 21. K. Hamada, M. Wada, H. Shimizu, M. Kume, F. Susa, T. Shibutani, N. Yoshikawa, K. Itoh, G. Kano and I. Teramoto, IEEE J. Quant. Electron. QE-21, 623 (1985). 22. H. Kawaguchi and G. Iwane, Electron. Lett. 17, 167 (1981). 23. K. Yamamoto and T. Taniuchi, 0FC/I00C'87, TuH2, Washington, DC (1987).
0038-1101/91 $3.00+ 0.00 Copyright© 1991PergamonPresspic
A HIGH-POWER SHORT-PULSE LASER DIODE FOR WAVEGUIDE SECOND H A R M O N I C GENERATION MASAHIRO KUME,~ HIROKI NAITO,l JUN OHYA, 2 ISSEY OHTA, I HmogAzu Smmzu, ~ MASARU KAZUMURAt and IwAO TERAMOTO1 tElectronics Research Laboratory, Matsushita Electronics Corporation, Takatsuki, Osaka 569, Japan 2Semiconductor Research Center, Matsushita Electric Industrial Co., Ltd., Moriguchi, Osaka 570, Japan (Received 15 April 1991)
Almtraet--This paper describes successful development of a high-power short-pulse i.r. laser diode, together with numerical analyses giving a clear understanding of the operation. The novel aspect of the present laser diode is that a saturable absorber is incorporated for gain switching into the buried-twin-ridge substrate (BTRS) structure which allows a high-stability fundamental spatial mode operation. Peak powers as high as 1.23 W and pulse widths as narrow as 34.1 ps have been obtained in an efficient 8ain-switehing operation with the high mode stability. The experimental observations are well-explicable by the numerical analyses in substantial respects. We also report picosecond-pulse blue-light generation which has been attained by applying the frequency doubling technique to the above laser in a LiNbO3 wavegulde.
I. INTRODUCTION Picosecond-pulse blue light has been required for some sophisticated optical measurements such as fluorescent lifetime measurement and temporally resolved spectroscopy. To meet this requirement, a compact coherent blue light source, which is allsolid-state, has been developed by applying the frequency doubling technique to a GaAIAs laser diode, where the Cherenkov radiation scheme is used in a proton-exchanged LiNbO 3 waveguide[1-3]. However, there is still a strong need for higherpower pulsed laser light. A high-power pulsed i.r. laser light is strongly required for second harmonic generation (SHG) because SHG capability increases in proportion to the square of the input power. Fundamental spatial mode operation of the laser diode is also required for efficiency coupling between the laser beam and the waveguide SHG element. Various investigations of short-pulse laser diodes have been published, and the most attractive ones are those using the gain switching method whereby picosecond (10-50 ps) pulses have been generated[3-5]. However, in those previous reports the gain switching has only been performed by the direct current switching which uses high peak pulse current, and the operation has been exclusively in multi-spatial mode, not meeting the above-mentioned requirement. This paper describes a high-power fundamental mode i.r. pulse-light laser diode which we have developed for blue light SHG. Fundamental mode operation has been attained in this type of laser by using the buried-twin-ridge substrate (BTRS) structure which is suitable for attaining stable fundamental spatial mode operation. Into this BTRS structure a saturable absorber region is incorporated for gain u~ ~4/t~--m
switching operation by devising a construction in which a current blocking layer is provided adjacent to the part of the active layer near the cavity end region. This construction is very simple, and is advantageous from both fabrication and analysis points of view. The more important point is that this novel gainswitching method gives a positive feedback switching. A high-peak pulse operation is obtained due to this efficient switching and due also to the use of the BTRS structure which is suitable for high-power operation. We have reported in a previous letter[16] efficient blue-light pulse generation which we attained using for the first time the present type of pumping-source laser in an LiNbO 3 waveguide. This paper gives a full description of high power characteristics of the present i.r.-light pulse laser, which is compared with numerical calculations. Thus this work gives a clear understanding of the gain switching operation under the background that the saturable absorber is provided in a very simple structure suitable for analysis. We also report picosecond-pulse blue light generation which has been attained by using the frequency doubling technique for the present i.r. laser in an LiNbO3 waveguide. 2. OPERATION PRINCIPLE AND ANALYSES
2.1. Basic operation principle
The basic operation principle of the gain switching laser is explained with the aid of Fig. 1. The current injection into the active layer is blocked by the n - G a A s current blocking layer, so that the active layer in the cavity end region serves as a saturable absorber. The optical loss in the saturable absorber
1329
MASAHIRO KUME et al.
1330 AbsorptionRegion 12
k
~
*
/
Gain Region II
l~31th0
hffi200,um
20[- i~lOl~O
~50,u m
Laser Light
t~,~L E>
- - L ~ T T T Current TTT
Current Blocking I L~F~
Facet
~Yer
p-GaAsSubstrate
F!cet z 6 - ~
0/
I
01
i
i
~
i
i
x
Fig. 1. The basic operation principle of the gain-switching
laser. region decreases with increasing light intensity. This effect, which works under the gain increase in the gain region, results in a further increase in the net gain. Consequently the peak power is higher for this gain switching laser than for conventional lasers without a saturable absorber.
2.2. Analyses using rate equaitons
/2nx\ 0- ,cos w ) ,
(1)
where No and N~ are the Fourier coefficients of the density of carriers in the gain region, and W is the width of an active layer. The carrier and photon densities are averaged over the y- and z-directions. In addition, where the laser is biased for stable operation, the light is emitted in a given longitudinal mode. This optical behavior can be modeled by one rate equation for the density of photons in the lasing mode. The rate equations are[18]: dINd °t =
eV FAS(No-No-~)-BNZo, dNt
dt
= -raS(No-
i
I
i
Time (200ps/div)
The lasing characteristics of the devices are analyzed using the familiar rate equations given in a simple model for the density of minority carriers in the gain region, for the density of those in the absorber region and for the density of photons in the lasing mode. The carrier diffusion in the active layer caused by the inhomogeneous stimulated emission at high injection level is considered[17] by the Fourier expansion of carrier density N as follows[18].
.(x)=
i
(2)
N 6 - N~)
Bt I + [2nLa ~---~ \2) ) ~Nt,2
(3)
tiN2 = FAS(N2 - No) - BN~,
(4)
Fig. 2; Numerical result of photon density as a function of time for a given injection current.
V is the volume of the active region with gain, and I the injected current. N2 and S are the density of carriers in the absorber region and the density of photons, respectively, a and b are defined as a = l~/(l~+ 12) and b = 12/(ll+/2) where lI and/2 are the length of the gain region and the length of the absorber region, respectively, e Is the electronic charge, zp the photon lifetime and C the spontaneous emission factor. A bimolecular recombination rate is also considered with B as the recombination constant. A linear gain dependence on injected carrier density G(N)=A(N-Na) is used in this calculation. The same coefficients are given for the gain and the absorber region, r is the optical confinement factor of the active layer and Ld is the diffusion length of carriers. The values of the various parameters used in the calculation are listed in Table 1. The static light-current characteristic, which is obtained by taking the time derivatives in (2-5) to be zero, displays a sizable hysteresis. The injection current l(t) is assumed to consist of a d.c. bias current Ib and of a Gaussian-shaped pulse with amplitude Ip and width Tv. The nonlinear differential eqns (2-5) are solved numerically by the Runge-Kutta--Gill A-"
dt
d._SSdt=[FA{a( N°- N G - ~ ) + b ( N 2 - N o , }
t~ 12=50 p m
× / / / s ~ ~ 2 5 ~2----0/~
Table 1. The values of the various parameters used in the calculation Parameter
Value
Parameter
Value
• V F A
1.6 x 10-19C 200× 10 × 0.1/~m 3 0.1 2.75 x 10-~em3 s -I 0.83 X lO is cm -3
B Ld W ~p C
3 x 10-1°em3s -1 3gin 6gm 1.2 x 10-12s lO -4
NG
O
0
1
m
#m /1=200 u tn Ip=10Ith °
2 3 4 5 Bias current (Ib/Ith°) Fig. 3. Calculated peak photon density as functions of the bias current.
High-power laser diode
1331 1.5 11=200 p m
ActiveLayer 1.o o
12=0 ,am m
=
0.5 • n-GaAs
~
BlockingLayer
Absorption
~ / . . . ,,~ Region I>-uaAs~utntrate
L~.~
O
. . . . .
0
I
'
'
'
12----50 /1 m '
. . . .
50 100 Bias Current (mA)
150
Fig. 4. S c h e m a t i c d r a w i n g o f the g a i n - s w i t c h i n g B T R S laser.
Fig. 6. M e a s u r e d pulse p e a k p o w e r s as f u n c t i o n s o f bias current.
procedure. The typical result of the numerical calculation is depicted in Fig. 2 where the injection current I, the gain FAa(No-No-N~/2), the loss 1/~p- FAb(N2- NG) and the photon density S are plotted vs time. In this calculation the current pulse amplitude lp is taken as 10 times the threshold current of the laser diode having no absorption region ~ , and the bias current lb is taken as three times ~ . The generation of a high-intensity light pulse causes a reduction of the gain by the stimulated emission, which in turn limits the duration of the optical pulse. The loss in the absorption region decreases with the increase in light, which results in higher peak photon density than that of the laser diode having no absorption region. In our calculation, these effects are varied by changing the absorption region length/2 and the bias current lb. In Fig. 3, the peak photon densities are plotted vs the bias current for a given electrical pulse height and active region length. The bias current is normalized by the threshold current ~ . There exists an optimal bias current giving a maximum peak power in agreement with previous reports[13,15]. As clearly shown in Fig. 3, an increase of the absorber region length leads to those of the pulse peak power and optimal bias current.
inherently thin enough to enlarge the lasing spot size, consequently reducing optical flux density. The front and rear facets of the laser diode are given reflectivities of 3 and 96%, respectively, in order to increase the forward output power.
3. DEVICE STRUCTURE
Figure 4 shows a schematic drawing of the gainswitching BTRS laser diode. In the absorber region, an n-GaAs layer is grown on the truncated ridge of the p-CraAs substrate so as to block the injection current[20]. This BTRS structure is suited for highpower operation[21] because an active layer formed by the liquid epitaxial growth over two ridges is
4. EXPERIMENTAL RESULTS
The measured static output power vs current characteristics are shown in Fig. 5 for the four absorber region lengths. Bistability with large hysteresis predicted by the calculation is observed [19,22]. For pulsed excitation, the laser diode is gain-switched by short electrical pulses of amplitude 20 V with 400 ps duration superposed on a d.c. bias current. The current pulse amplitude is estimated to be 0.4 A. The pulses are obtained from an impulse generator (AVTECH AVMH-2-C) with a repetition rate of 10 MHz. The 830 nm optical pulses from the laser diode are observed on a sampling optical oscilloscope (Hamamatsu OOS-01) with 10 ps resolution. The measured pulse peak powers as functions of bias current are shown in Fig. 6. The peak power increases with increasing absorber region length as predicted by the calculation. The experimental results are qualitatively in good agreement with the calculation. The optical pulse waveform at maximum peak l~ower is presented in Fig. 7. The optical pulse consists of a main peak with 1.23 W peak power and 34.1 ps pulse width and secondary peaks due to relaxation oscillation. The problem of catastrophic Optical damage (COD) which results from localized temperature rise due to the absorption of intense light is avoided because the optical pulse width is as narrow as few tens of picoseconds and the absorption region 11=200p m, 12ffiS0p m, Ib=100mA
l l = 2 0 0 p m 12ffi0,15,2. ~ ,50pro
1.23W 1
,.- 34.1ps
0 0
Fig.
' 50
100 150 Curator (mA) 5. L i g h t - c u r r e n t c h a r a c t e r i s t i c s o f B T R S lasers.
200
0 I
gain-switching
I
I
I
I
i
200ps/div Fig. 7. Optical pulse waveform of the laser diode.
1332
MAS~,HntOKut~ et al.
_L_KOZ ~-.." Wavegulde
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MgO:LiNbO3 ~ / / " ~ ) 18ram
Fig. 8. Experimental setup for frequency doubling using a gain.switehing laser diode in a proton-exchanged LiNbO3 waveguide. becomes transparent in the lasing condition. This is the main cause for the high peak operation. Finally, we present the observations of the picosecond-pulse blue-light generation which have been obtained by applying the frequency doubling technique to a gain switching laser. The experimental setup is shown in Fig. 8. The laser diode optical pulse is frequency doubled in the LiNbO3 waveguide under a high efficiency, and blue light pulse is generated in the form of Cberenkov radiation. The waveguide is 0.36/~m deep, 1.6/tm wide and 18ram long, and is fabricated in an MgO-doped LiNbO 3 substrate by proton exchange with pyrophosphoric acid[23]. The 830 nm guided wave is frequency doubled during the propagation along the waveguide in the nonlinear polarization medium, giving the 415 nm harmonic wave. In this Cherenkov radiation scheme, the phasematching condition is satisfied automatically for all longitudinal mode oscillations in a laser diode. The frequency-doubled peak power as a function of laser diode peak power is plotted in Fig. 9. The frequencydoubled peak power is proportional to the square of the laser diode peak power as expected. A maximum peak power of 7.88 mW of blue light pulse under 28.7 ps width is obtained for 1.23 W laser diode peak power, which is almost a four-fold improvement compared to the laser without a saturable absorber. 10
It is found in this figure that the efficiency of coupling between the laser diode and the waveguide is almost constant. This fact means that the fundamental spatial mode is maintained in all ranges of peak power covered in the present experiment. 5. CONCLUSION We have described the generation of high-power picosecond pulses using the BTRS laser structure into which a saturable absorber region is incorporated in a very simple construction. Fundamental mode optical pulses as high as 1.23 W and pulse widths as narrow as 34.1 ps have been optained. The operation characteristics have been well explained by numerical solutions of the relevant rate equations. We have also demonstrated picosecond-pulse blue fight generation which has been attained by applying the frequency doubling technique to the above pulse laser in the proton-exchanged LiNbO 3 waveguide. A blue-light pulse of 7.88 mW peak power has been generated in the form of Cherenkov radiation. It is concluded therefore that the present gain-switching BTRS laser is useful for blue-light SHG. Acknowledgements--The authors would like to thank Dr M.
Takeshima and Dr G. Kano for their helpful discussions and encouragement. They are also grateful to G. Tohmon, K. Yamamoto and Dr T. Taniuchi for fabrication and measurement of the SHG element and for their fruitful discussions.
REFERENCES 0.1 • Without Abmt~aer
i
0.01 10
i
100
i
I
1000
LDPeakPower(mVO Fig. 9. The frequency doubled peak power as a function of laser diode power.
I. T. Taniuchi and K. Yamamoto, ECOC'86, Tu-C5, Barcelona (1986). 2. T. Taniuchi and K. Yamamoto, CLEO'87, WP6, Washington, DC (1987). 3. C. Lin, P. L. Liu, T. C. Damon and D. J. Eilenberger, Electron. Lett. 16, 600 (1980). 4. J. Au Yenng, Appl. Phys. Lett. 38, 308 (1981). 5. H. Ito, H. Yokoyama, S. Murata and H. Inaba, IEEE J. Quant. Electron. QE-17, 663 (1981). 6. J. P. van der Ziel and R. A. Logan, IEEE J. ~ m t . Electron. QE-18, 1340 (1982).
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16. J. Ohya, G. Tohmon, K. Yamamoto, T. Taniuchi and M. Kume, Appl. Phys. Lett. f ~ 2270 (1990). 17. J. Butts and M. Danielsen, IEEE J. Quant. Electron. QE-13, 669 (1977). 18. Y. Suematsu, Semiconductor Laser and Optical Integrated Circuit, p. 197. Ohm, Tokyo (1984). 19. C. Harder, K. Y. Lau and A. Yariv, IEEE J. Quant. Electron. QE-18, 1351 (1982). 20. T. Shibutani, M. Kume, K. Hamada, H. Shimizu, K. Itoh, G. Kano and I. Teramoto, IEEE J. Quant. Electron. QE-23, 760 (1987). 21. K. Hamada, M. Wada, H. Shimizu, M. Kume, F. Susa, T. Shibutani, N. Yoshikawa, K. Itoh, G. Kano and I. Teramoto, IEEE J. Quant. Electron. QE-21, 623 (1985). 22. H. Kawaguchi and G. Iwane, Electron. Lett. 17, 167 (1981). 23. K. Yamamoto and T. Taniuchi, 0FC/I00C'87, TuH2, Washington, DC (1987).