Semiconductor lasers are potentially devices of great importance for optical transmission as well as short pulse generation for various sampling, characteration and dispersion measurements. Since semiconductor lasers are currently driven devices, it is relatively easy to modulate the optical output and to generate short pulses, on the order of 10 ps long, by directly modulating the injection current into the laser. This paper will present some recent developments in injection lasers which are capable of being analog or digitally modulated at rates up to 10 GHz, as well as generating short optical pulses at repetition rates from several hundred megahertz to tens of gigahertz

Lau, K. Y.

Yariv, A.

English

Caltech Authors - Main

PROCEEDINGS OF SPIE
SPIEDigitalLibrary.org/conference-proceedings-of-spie
Analog, Digital And Short Pulse
Modulation Of Ultrafast Gallium
Aluminum Arsenide Semiconductor
Lasers
Lau, K., Yariv, A.
K. Y. Lau, A. Yariv, "Analog, Digital And Short Pulse Modulation Of Ultrafast
Gallium Aluminum Arsenide Semiconductor Lasers," Proc. SPIE 0533,
Ultrashort Pulse Spectroscopy and Applications,  (2 April 1985); doi:
10.1117/12.946547
Event: 1985 Los Angeles Technical Symposium, 1985, Los Angeles, United
States
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Analog, digital and short pulse modulation of ultrafast gallium aluminum arsenide
semiconductor lasers
K.Y. Lau
Ortel Corporation
2015 W. Chestnut St., Alhambra, California 91803
A. Yariv
California Institute of Technology, Pasadena, California 91125
Introduction
Semiconductor lasers are potentially devices of great importance for optical transmiss-
ion as well as short pulse generation for various sampling, characteration and dispersion
measurements. Since semiconductor lasers are currently driven devices, it is relatively ea-
sy to modulate the optical output and to generate short pulses, on the order of lOps long,
by directly modulating the injection current into the laser. This paper will present some
recent developments in injection lasers which are capable of being analog or digitally mod-
ulated at rates up to 10GHz, as well as generating short optical pulses at repetition rates
from several hundred megahertz to tens of gigahertz.
Approach to high speed lasers
A basic analysis shows that the small signal modulation bandwidth is
1
r 2>r 1 Tp
where Tp is the photon lifetime, given by Tp= 1 /v(d+l /L In 1 /R) where v is the group velocity
of the light, a is the distributed loss, L is the length of the cavity and R is the mirror
reflectivity, Po is the steady state photon density and A is the differential optical gain
constant. The modulation bandwidth of the laser is widely accepted to be equal to fr. A
means to increase the modulation speed of semiconductor lasers is to decrease the cavity
length and hence the photon lifetime. The actural device used was a buried heterostructure
laser fabricated on a semi -insulating substrate (BH on SI) [3]. In addition to a low lasing
threshold (typically S 15mA) which is necessary to avoid excessive heating due to the short
cavity length, these lasers possess very low parasitic capacitance [4] which is detrimental
at high speeds. The modulation bandwidth can be pushed to beyond 8GHz by operating near the
catastrophic damage point. A second method to increase the modulation bandwidth is to in-
crease the optical gain constant by lowering the operating temperation of the laser. The
relative slopes of the plots of bandwidth versus the square root of the emitted optical pow-
er ( ) yield values for the relative change in A as the temperature is varied. The ratio
of the slope at 22 °C to that at -50 °C is 1.34. This factor is fairly consistent (between
1.3 and 1.4) among all the lasers tested, even including those from different wafers. This
infers that the intrinsic differential optical gain increases by a factor of =1.8 by cooling
from 22 °c to -50 °C, which is consistent with theoretical gain calculations [5]. A third
parameter to be tackled is the optical power density Po. However, an increase of the opti-
cal power can bring about undue degradation or even catastrophic damage to the laser unless
the structure of the laser is suitably designed. One common means of raising the ceiling
of the reliable operating power of semiconductor lasers is by means of a large optical cav-
ity [6]. The mechanism responsible for a higher catastrophic damage power in these devices
is by lowering the optical power density at the active layer, since such damage commonly or-
iginates from the active layer near the crystal facet. This maneuver, however, serves litt1,
to increase thè modulation bandwidth because the quantity of concern here, the photon dens-
ity within the active region (P0 in Eq. (7)), remains unchanged. A laser suitable for high
speed operation should therefore be one with a tight optical confinement in the active re-
gion along the entire length of the laser, with a transparent window at the end regions able
to withstand much larger photon densities without catastrophic damage. The use of a trans-
parent window structure to increase the catastrophic damage level has previously been demon-
strated [7,8]. Using a window buried heterostructure laser fabricated on a semi - insulating
substrate, we have demonstrated for the first time a direct modulation bandwidth exceeding
10GHz for a semiconductor laser operating at room temperature. The device used is structur-
ally similar to the BH on SI laser except that here the end regions near the facets is cov-
ered by a layer of unpumped GaA1As which forms a transparent window. The optical wave prop-
agates freely in the end window region. As a result of diffraction, only a small amount of
light reflected from the crystal facet couples back into the active region. The theoretical
value of the effective reflectivity, assuming a fundamental gaussian beam profile, is _ 5%
for L = 5}im. The actual values of L for the devices fabricated lie around this value. The
106 / SPIE Vol. 533 Ultrashort Pulse Spectroscopy and Applications (19851
 
. 
 
.  9
. 
91125
importa for optical transmiss 
  ll  and dispersion 
 it is relativel ea 
  ate t or of lOps long, 
in t la  p so  
a c o being analog or digitally mod- 
lO a sho opt pulses at repetition rates 
to tens of gigahertz
 
f = 
e Tp is the photon lif time, given by Tp=l/v(*<+l/L In 1/R)  
 l i th lengt of the cavity and R is the mirror 
ivity, O i t optical gain 
 la i accepted to be equal to fr.  
 s     
life  heter  
a semi-in (BH o SI    
<. ) heating due to the short 
th, [4 which is detrimental 
 8G by operating near the 
 bandwidth is to in- 
 of the lase   
 o th emitted optical pow- 
 ( ) f t re ch in as the temperature is varied.  
at 2 at -50°  (betwee  
 4) from different waf   
 b a factor of =1.8 by cooling 
°c to -50°C, which is consistent with theoretical gain [5].  
0 . ,  
 damag to the laser unless 
 th cei  
 i of a large optica cav- 
[6  for a highe catastrophic damage power in these devices 
l sinc such damage commonly or- 
 f   however, serves li tle 
e conce her the photon dens- 
(P 0 in Eq. ),  f  
confinement in the active re- 
 at the end regions able 
da   
lev has previously been demon- 
[7,  buried heterostructure laser fabricated on a semi-  
rate, e  exc  
lO      
r  th face s  
   l   
re  a s  
r   
a funda gaussian beam profi is = 5% 
= Su  li aroun this va  
 SPIE Vol. 533 Uftrashort Pulse Spectroscopy and pplications (1985)
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threshold current of window BJ on SI lasers ranges from 14mA to 25mA. The catastrophic dam-
age threshold in these devices is beyond 120mW under pulse operation. Under cw operation,
the maximum operating power is limited by heating to 50mW. The -3dB bandwidth can be pushed
to beyond 10GHz when operating above 15mW. The bandwidth characteristics of a large opti-
cal cavity BH on SI lasers is also investigated, and found to be substantially lower at cony
parable output power levels. This bears witness to the fact that the optical power density
at the active region of these devices is lower than that in lasers of conventional construct-
ion.
Short pulse generation
There are various means of generating short pulses from a semiconductor laser. The
easiest way is to directly modulate the laser diode with short, intense current pulses or by
a large -amplitude sinusoidal signal. Optical pulses on the order of 20ps can be generated
in this manner. If shorter pulses are desired, the method of mode - locking can be used. The
laser is coupled to an external optical cavity, and by means of active (directly modulating
the laser at the round trip frequency of the external cavity) or passive (by some saturable
absorning defects in the laser) optical pulses below lops can be generated at the round trip
frequency of the external cavity. The need for very sensitive alignment in coupling the
laser to the external cavity is a drawback in practical application of mode - locked laser
systems. One way of generating short pulses of below lOps without the need of external
pulse of RF pumping, or sensitive optical alignment is to apply optoelectronic feedback to
a self -pulsing semiconductor laser. Highly stable optical pulses (with a frequency jitter
of <30kHz) of pulsewidths between 10 to 20 ps can be generated, with a very high repetition
rate from 1 -5 GHz which can be electrically tuned.
The lasers used in this experiment were similar to the high speed GaA1As lasers de-
scribed above, which possesses the highest direct modulation speed reported thus far. Self -
pulsation was induced in these otherwisw well- behaved lasers by momentarily operating the
lasers beyond the catastrophic damage level. It must be emphasized here that it is not nec-
essary for one to induce damage to a laser for it to self -pulse; a highly reliable way of
accomplishing the task is by a double contact laser structure reported previously. This
frequency (f) of pulsation varies with the pump current (i) following a dependence
as in conventional relaxation oscillation in lasers indicated in the last section.
Next, the output of the laser is coupled to a high speed GaAs pin photodiode, the out-
put of which is amplified by a 2 -4GHz, 20dB gain amplifier, which in turn drives the inject-
ion laser. The intensity output from the laser now pulsates, with a much reduced linewidth,
at an integral multiple of fo which is closest to the "natural" self -pulsation frequency of
the solitary laser (without optoelectronic feedback), where fo is the inverse of the total
delay in the feedback loop, - 20 ns in this case. This result can be explained by inter-
preting the self -pulsing laser as a device with a very high gain in a narrow frequency band
around the self -pulsing frequency, and hence oscillation commences at the integral multiple
of fo which has the highest open -loop gain. As the natural self -pulsing frequency is tuned
by varying the pump current, the pulsation frequency jumps from one integral multiple of fo
to the next. The pulsation repetition rate can be steptuned from 1.2GHz to 4.5GHz by vary-
ing the pump current.
The exact optical pulsewidth was measured by a standard optical doubling autocorrela-
tion technique using LiI03 as the non -linear medium. During these measurements, the loop
gain is fixed at a value of 7. The pulsewidth varies between 20 to 12 ps within the repe-
tition -rate tuning range. In contrast, when the optoelectronic loop is opened, the pulse
width of the self -pulsing laser is 57ps.
Next, the wideband microwave amplifier (1 -6GHz, 20dB gain) is replaced by a high gain
(39.5dB) narrow band amplifier (1.8 -2GHz) which produces a loop gain of 65. Under this in-
creased loop gain, the measured pulsewidth falls below lOps, the shortest being 9.5ps at a
pulse repetition rate of 2GHz.
Digital modulation
The results described in Section 2 of this paper are concerned with small signal modula-
tion. By "small signal" one means that the modulation depth in the optical output of the
laser is substantially less than one. In wideband digital telecommunication, the on -off
ratio in the laser output should be as large as possible to avoid excessive power penalty.
The modulation response of a laser under large modulation is different from (and generally
worse than) the small signal response, although the latter is often a good indicator of the
former as long as the laser is maintained above threshold at all times. Ikegami and Suemat-
su [9] studied large signal effects by direct numerical integration of the rate equations.
The result is quite similar to the small signal response except that the relaxation oscilla-
tion occurs at a lower frequency. The reason for this is that the laser cannot be swiftly
brought from a state of very low optical level to a high level due to the low rate of stim-
SPIE Vol. 533 Ultrashop Pulse Spectroscopy and Applications (1985) / 107
2  da  
  
  
lO abov 15mW   
  lower at com- 
lev  d  
is lower than that in lasers of conventional construct- 
fr a semicondu lase   
la sh intense current pulses or by 
-amplitude sinuso  2 c b genera  
 pulses are desired, the method of mode-locking   
  (directl modu  
t e c o passiv (by some saturable 
lO ca b generated at the round trip 
  
cavity is a drawback in practical application of mode- la  
    l 
g, i t ap optoelectronic feedback to 
-pulsing semic  (wi j  
Hz)  ths      e nerated, with  ry gh etition 
-5 GHz which can  
 l  - 
d ove, th fa  f  
i thes otherwisw well- operating the 
s leve   
dam to a laser for it to self  rel way of 
la str reported previ   
cy ) f (i  a  
i la indi in the last sectio
, l i co to a high speed GaAs pin photodiode the out- 
b a 2-4GHz, dB er, d the inject- 
 t la n wi a much reduced linewidt  
fo which is closest to the "natural" self frequency of 
( f f i th inverse of the total 
l  =  c  in  
-pulsing laser as a device with  frequency band 
-pulsing frequency, and hence int multiple 
 highest open  self-pulsing   
 f j fro o integral multiple of fo 
  z ry- 
 l rela- 
using LiI0 3 as the non  th loo  
a of 7. 2 t 12 p within the repe- 
-rate  st, i th p  
self-pulsing  = 
t, (l-6GHz, 20dB gain) is replaced by a high gain 
.5dB) (1.8-2GHz) wh l gain of 65.  
 in, l shorte being 9.5ps at a 
 = 
2 t ar concerned with small signal modula- 
 in the optical output of the 
tha on  l telecommunication, the on  
  e power pena  
la is different from (and generally 
) a th latt is often a good indicator of the 
  t   
] o the rate equat  
 th relaxation oscilla  
     y 
le t a h leve due to the low rate of stim-
I  Vol. 533 /trashort l  pectroscopy and Ap lications (1985) / 10
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ulated emission at the low -power state. This can be illustrated by modulating a laser,
which is biased at a high optical level, with two closely spaced negative -going current
pulses. When the optical modulation depth is below 70 -80%, optical output is a faithful
reproduction of the current pulse input up to a pulse spacing of 200ps, which is limited by
the current pulse width. When the optical modulation depth exceeds _ 85 %, the laser re-
sponds sluggishly when going from the "low" to the "high" state, and severe relaxation osc-
illation occurs. This is obviously not satisfactory for a pulse -code modulation transmiss-
ion. The slow rate of the optical output in the large signal regime has been calculated
from the rate equations by Kaminow and Tucker [10], and the relaxation oscillation associat-
ed with large modualtion depth has recently been observed by Eisenstein et al. [11]. The
basic conclusion of this section is that when high speed modulation is desired, the optical
modulation depth should be maintained below = 70 %, but when power penalty must be minimized
in a digital telecommunication system, modulation speed will be sacrificed.
References
1. H.W. Yen and M. Barnoski, Appl. Phys. Lett., 32, 182, 1978.
2. G. Lutes, Jet Propulsion Lab, Telecommun. and Data Acquisition Progress Report, 42,
59, 1980.
3. N. Bar -Chaim, J. Katz, I. Ury, A. Yariv, "Buried heterostructure AlGaAs laser on
semi -insulating substrate ", Electron. Lett., 17, pp. 108 -109, 1981.
4. I. Ury, K.Y. Lau, N. Bar -Chaim, and A. Yariv, "Very high frequency GaA1As laser -
field effect transistor monolithic integrated circuit ", Appl. Phys. Lett., 41, pp. 126 -128,
1982
5. H.C. Casey and M.B. Panish, Heterostructure Lasers, part A, pg. 174, Academic Press
1978.
6. H. Kressel and J.K. Butler, Semiconductor Lasers and Heterojunction LED's, Academ-
ic Press, New York, 1975, pp. 230.
7. S. Takahashi, T. Kobayashi, H. Saito and Y. Furukawa, "GaAs -AlGaAs DH lasers with
buried facets ", Jap. J. Appl. Phys., 17, pp. 865 -870, 1978.
8. H. Blauvelt, S. Margalit and A. Yariv, "Large optical cavity A1GaAs buried heter-
ostructure window laser ", Appl. Phys. Lett., 40, pp. 1029 -1031, 1982.
9. I.P. Kaminow, "High speed modulation of semiconductor lasers ", Conf. on Optical
Fiber Communication, paper NJ1, New Orleans, 1984.
10. R.S. Tucker and I.P. Kaminow, "high speed switching transients in directly modulat-
ed InGaAsP lasers ", CLEO '84, paper FF1, Anaheim, 1984.
11. G. Eisenstein, U. Koren, R.S. Tucker, B.L. Kasper, A.H. Gnauck and P.K. Tien, "High
speed analog and digital modulation of 1.5111m wavelength three channel buried crescent
InGaAsP lasers ", Appl. Phys. Lett., 45, pp. 311 -313, 1984.
108 / SPIE Vol 533 Ukrashort Pulse Spectroscopy and Applications (1985)
at the low  r, 
a a high optical level, with two closely spaced negative ing current 
 ulation depth is below 70 0%, i a faithf  
20 which is limited by 
 depth exceeds = 85%,  
 " s a sever relaxatio os  
 no satisfactory for a pulse  
 in th lar signal regime has been calculated 
[1 f oscillation assoc  
et al    
i d the opti  
sho be maintained below = 70%, st be minimized 
 sa
 .  3 1 197
 . , ion Progress Repor 42, 
 
  -Chaim, J. Katz, I. Ury, A. riv, AlGaAs laser on 
-insulating s ", Electron. p 108-109, 
  L N. Bar-Chaim,  "Very high frequency GaAlAs laser  
monolithic integrated circuit A Phys. Le t., 41, p. 126  
.
 .  A p 174 Academic Pre  
 .  a Heterojunction LED' Academ- 
 ss,  2
 . H. Saito and Y. Furukawa, "GaAs las wi  
", Jap. . 17, pp. 865
  a A Y "Larg optica cavity AlGaAs buried heter- 
laser", Appl.  4 p 1029-1031, .
 . "H spee modulation of semiconductor lasers Opt  
, l 1
  I "hig spe switching transients in directly modulat- 
", CLEO 4,  19
  ,   . , . r, H. . ,  
ulation of l.Sly crescent 
", Appl. . 45 pp. 3 1 , 
  SPI  l. 533 Ukrashort Pulse S tr   ppli ti  (1985)
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Analog, Digital And Short Pulse Modulation Of Ultrafast Gallium Aluminum Arsenide Semiconductor Lasers

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Analog, Digital And Short Pulse Modulation Of Ultrafast Gallium Aluminum Arsenide Semiconductor Lasers

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