A record high current modulation efficiency of 5 GHz/[sqrt](mA) has been demonstrated in an ultralow threshold strained layer single quantum well InGaAs laser

Chen, T. R.

Eng, L.

O'Brien, J.

Yariv, A.

Zhao, B.

Zhuang, Y. H.

English

Caltech Authors - Main

2 EISELT, M., GROSSKOPF, G., LUDWIG, R., P I E P ~ ,  w., and WEBER, H. G.: 
‘Photonic ATM switching with semiconductor laser amplifier 
gates’, Electron. Lett., 1992, 28, (15), pp. 1438-1439 
SAXTOFT, c.: ‘Photonic packet switch experiment employing a 2 t 2 
switch matrix and a semiconductor laser amplifier’. Proc. Photonic 
Switching Topical Meeting, Kobe, Japan, 1990, Postdeadline 
Paper 14B-7 
4 FORTENBERRV, R. M., LOWERY, A. I., HA, w. L., and TUCKER, . s.: 
‘Photonic packet switch using semiconductor optical amplifier 
gates’, Electron. Lett., 1991, 27, pp. 1305-1306 
3 
VERY HIGH MODULATION EFFICIENCY OF 
ULTRALOW THRESHOLD CURRENT SINGLE 
QUANTUM WELL InGaAs LASERS 
T. R. Chen,  B. Zhao,  L. Eng, Y. H. Zhuang,  J. O B r i e n  
a n d  A. Yariv 
Indexing terms: Semiconductor lasers, High-speed optical tech- 
niques 
A record high current modulation efficiency of SGHz/J(mA) 
has been demonstrated in an ultralow threshold strained 
layer single quantum well InGaAs laser. 
The key for high density applications, such as optical inter- 
collnects between future supercomputers, is the ability to 
handle multigigabit per second data streams at reasonably low 
power consumption. This translates to the following require- 
ments for semiconductor lasers which are considered to be the 
most promising candidate for this kind of application: the 
individual laser must be capable of operating under low drive 
current (milliamp to microamp region), yet with a very large 
bandwidth (multi-gigahertz); the lasers can be integrated at a 
high density to allow parallel processing. Wavelength division 
multiplexing (WDM) or other multiplex technologies may also 
be applicable to  such applications. 
To characterise such an ultralow threshold and ultrahigh 
speed laser, we suggest a figure of merit, i.e. the modulation- 
current efficiency factor (MCEF), defined as 
MCEF = f 3  d J ( 1  - I th )  (1) 
where f, dB is the 3 dB bandwidth of the laser under direct 
current modulation, I is the bias current at which the modula- 
tion bandwidth is measured and Ith is the threshold current of 
the laser. It is clear that the desirable laser should have as low 
as possible Ith and as high as possible MCEF. 
The 3 dB bandwidth of a damping-free, parasitic-free semi- 
conductor laser under small signal direct current modulation 
can be expressed as [l,  21 
where f, is the resonance frequency, G is the differential gain, 
T~ is the photon lifetime, and P is the photon density at the 
active region which is proportional to the optical power. Eqn. 
3 can be derived directly from eqn. 2 under the assumption 
that optical power is linearly related to the injection current 
above threshold. The MCEF is therefore expressed as 
(4) 
where ug is the photon velocity inside the capacity, rjt is the 
internal quantum efficiency, e is the electron charge and V,,, is 
the volume of the optical mode which is equal to wLt, w is the 
lateral width of the active stripe, L is the cavity length and t is 
the effective transverse width of the optical mode. It is noted 
that eqn. 3, and therefore eqn. 4, is valid only within the linear 
regime of the L - I relation. Also the damping and/or para- 
sitic capacitance render the f 3 d B  always smaller than 1,55f,, 
especially at high bias current. 
Eqn. 4 tells us that a strained layer quantum well laser 
would he superior to  a bulk material semiconductor laser due 
t o  its higher differential gain. An improvement in the internal 
quantum efficiency and a reduction in optical mode volume 
(tight confinement) would also be desirable in obtaining high 
MCEF. 
The device used in this work is the ultralow threshold 
strained layer (SL) single quantum well (SQW) InGaAs/ 
AlGaAs laser developed recently in our laboratory. We 
adopted a graded-index separate confinement heterostructure 
(GRINSCH) and buried heterostructure (BH) to  obtain tight 
optical confinement in the transverse and lateral directions, 
respectively. The width of the InGaAs SQW is 80A and the 
width of the active stripe w is -1.5pm. The threshold cur- 
rents for as-cleaved lasers are below 1.5mA in the cavity 
length range 250-600 pm. The lowest threshold current mea- 
sured was l mA. Details of the laser structure and fabrication 
will be presented elsewhere. 
To  examine the high speed performance of the lasers, a 
mesa of 10pm was etched around the active stripes to  reduce 
the parasitic capacitance. We used a short cavity laser 
(1201m) to demonstrate the high modulation efficiency. The 
threshold current of the laser was 3.5mA; it dropped to 
2.4 mA when the back facet mirror was dielectrically coated to 
R 10.95 .  The L - I characteristics of the laser are shown in 
Fig. 1. The laser displayed excellent dynamic performance. 
R r  = O  95 
R ,  =O 30 
1 2  = I  E 1 0  I / 
U 
7 8 9  
current , mA (597111 
Fig. 1 L-I characteristics of low threshold InGaAs SQW laser 
Cavity length L = 1 2 0 p  
Small signal direct current modulation response curves at dif- 
ferent bias current are shown in Fig. 2. A 3dB bandwidth of 
5 GHz was evident at a bias current of only 1 mA above 
threshold. At a bias of lOmA above threshold, a bandwidth of 
13 GHz was demonstrated. 
f, dB as a function of square root of the incremental current 
above threshold is plotted in Fig. 3. At low bias, f, d B  increases 
linearly with J(1 - The relation becomes sublinear when 
the bias current increases further, indicating that damping 
and/or parasitics can no longer be neglected. A very high 
value of MCEF of 5 GHz/J(mA) was deduced from the linear 
ELECTRONlCS LETTERS 19th August 1993 Vol. 29 No. 17 1525 
part of the curve. T o  our knowledge, this is the highest value 
ever reported in edge emitting semiconductor lasers. In pre- 
vious work, the MCEF values are 2-3GHz/J(mA) in high 
speed bulk lasers [3, 41 and 2,4GHz/J(mA) in high speed 
QW lasers [5-71. f 3 d B  was also plotted as a function of the 
square root of the average output power in Fig. 4. A slope of 
10GHz/J(mA) was obtained. 
15 
I I I I I I I I I  
N 
12 
frequency, GHr 159712 
Fig. 2 Modulation response curves of SQW laser at different bias 
Current 
Bias current above threshold indicated next to curves 
For different cavity length and different mirror coatings, the 
measured MCEF is somewhat different due to the variation in 
V,,, and G .  In a laser with a cavity length of 15Opm and 
threshold current of 1.85mA, an  MCEF of 4.6GHz/J(mA) 
was measured. 
In conclusion, a very high modulation-current eficiency 
factor (MCEF) of 5GHz/J(mA) was demonstrated in a very 
low threshold short cavity SL-SQW InGaAs/AlGaAs laser. 
14 
N z 1 2  
minimum is ensured. Computer simulation results are pre- 
sented which show fast convergence and high spectral 
immovement. 
5 
2 10 
e 
2 8  
U 
4 -  
B 2 -  0
0 
Acknowledgment:  This work was supported by the Advanced 
Research Project Agency, by the Ofice of Naval Research and 
by the Air Force Ofice of Scientific Research. 
0 IEE 1993 2nd Ju ly  1993 
T. R. Chen, B. Zhao, L. Eng, Y. H. Zhuang, J. OBrien and A. Yariv 
( T .  J .  Watson, Sr. Laboratories of Applied Physics, 128-95, California 
Institute of Technology, Pasadena, California 91125, USA)  
References 
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2 
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7 
Introduction: High spectral eficiency modulation methods 
such as QAM are highly sensitive to power amplifier nonlin- 
iarities. An effective solution is to linearise the power ampli- 
fier by using predistortion of the input signal [l,  21. Because 
of changes with time, temperature and because of different 
operating channels, it is necessary that the predistorter he 
adaptive. Adaptive linearisation methods with polynomial 
functions have been previously reported [2]. These methods 
employ out-of-band intermodulation distortion (IMD) to esti- 
mate the predistorter polynomial coeficients. It has been 
;:i/ 
FAST ADAPTIVE PREDISTORTION 
LI N EAR IS ER USlN G POLY N O M  I AL 
FUNCTIONS 
M. Ghaderi, S. Kumar and D. E. Dodds 
: 10 


Very High Modulation Efficiency of Ultralow Threshold Current Single Quantum Well InGaAs Lasers

Direct amplitude modulation of short-cavity GaAs lasers up to X-band frequencies’,

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Very High Modulation Efficiency of Ultralow Threshold Current Single Quantum Well InGaAs Lasers

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