It is demonstrated that an ultra-high speed window buried heterostructure GaAlAs laser fabricated on semi-insulating substrate can be used as narrow band signal transmitters in the Ku-band frequency range (12-20GHz). The modulation efficiency can be increased over a limited bandwidth by a weak optical feedback. A stronger optical feedback enables one to actively mode-lock the laser diode at a very high repetition rate up to 17.5GHz, producing pulses of = 12ps long

Lau, K. Y.

Yariv, A.

English

Caltech Authors - Main

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Microwave Optical Link In The
Frequency Range Of 10-18
Gigahertz By Direct Modulation Of
Injection Laser Diode
Lau, K., Yariv, A.
K. Y. Lau, A. Yariv, "Microwave Optical Link In The Frequency Range Of 10-18
Gigahertz By Direct Modulation Of Injection Laser Diode," Proc. SPIE 0545,
Optical Technology for Microwave Applications II,  (28 October 1985); doi:
10.1117/12.948338
Event: 1985 Technical Symposium East, 1985, Arlington, United States
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Microwave optical link in the frequency range of 10 -18 gigahertz by
direct modulation of injection laser diode
K.Y. Lau
Ortel Corporation, 2015 W. Chestnut St., Alhambra, California 91803
A. Yariv
California Institute of Technology, Pasadena, California 91125
Abstract
It is demonstrated that an ultra -high speed window buried heterostructure GaAlAs laser
fabricated on semi - insulating substrate can be used as narrow band signal transmitters in
the Ku -band frequency range (12- 20GHz). The modulation efficiency can be increased over a
limited bandwidth by a weak optical feedback. A stronger optical feedback enables one to
actively mode -lock the laser diode at a very high repetition rate up to 17.5GHz, producing
pulses of - 12ps long.
Introduction
Significant progress has been made recently in operating semiconductor laser diodes at
very high frequencies. A quantity of major significance in the small signal modulation re-
gime is the -3dB modulation bandwidth, which is a direct measure of the rate at which infor-
mation can be transmitted by the laser. However, one can obtain a high modulation depth at
frequencies beyond the -3dB point by driving the laser with sufficient RF drive power to
compensate for the drop -off in the modulation response of the laser. This technique is very
useful in generating repetitive optical pulses from a laser diode at a very high repetition
rate, although the repetition rate itself has no significance in terms of true information
transmitting capacity of the laser. A means to reduce the RF drive power required for mod-
ulating the laser to a high optical modulation depth at high repetition rate is the tech-
nique of mode -locking. The laser diode is coupled to an external cavity whose round -trip
time corresponds to the modulation frequency. The modulation frequency in this case is lim-
ited to a very narrow range around the round trip frequency of the external cavity. Recent
experimental work have extended the small signal -3dB direct modulation bandwidth of a soli-
tary laser diode to _ 12GHz using a specially developed window buried heterstructure laser
on semi - insulating substrate (BH on SI) [1], while large signal direct modulation has been
reported at repetition rates up to 10.6GHz [2]. Active mode - locking of laser diodes in an
external cavity has been reported at repetition rates up to 7.2GHz in a LiNb03 directional
coupler external cavity [3] and to lOGHz in a fiber cavity [4].
An ultra -fast laser
The laser used in this experiment is a window BH on SI laser reported previously [1].
The length of the laser is 300pm, with an active region dimension of 2pm x 0.2pm. The pres-
ence of the window near the end facet alleviates the problem of catastrophic damage and en-
able the laser to operate at very high optical power densities. The tight optical and el-
ectrical confinement along the length of the laser cavity (except at the window region) en-
ables maximum interaction between the photon and electrons to take place and results in a
very high direct modulation bandwidth. The small signal modulation bandwidth of this device
biased at an optical output power of 10mW is approximately 10GHz. Here, the "small signal"
regime is loosely defined as that when the modulation depth of the optical output is < 80 %.
The -3dB bandwidth is 10.3GHz. The response drops to -10dB at -13.5GHz and to -20dB at]BGHz.
The fall -off in the modulation response is due to a combination of the intrinsic laser re-
sponse and effects due to parasitic elements. However, at a limited frequency range at a-
bove the -3dB point, the response is relatively flat over a 1GHz band (to within ± 2dB), and
is within ± 1dB over a 100MHz bandwidth. It is thus possible to use this laser as an opti-
cal transmitter operating in a narrow bandwidth in the upper X -band frequencies.
Loss compensation and mode - locking
The intrinsic modulation response is approximately 13dB below the midband value. This
loss in modulation efficincy can be partially compensated by coupling the laser to an exter-
nal cavity of the appropriate length. In this experiment the external cavity is composed of
a short length (6.3mm) of standard graded index multimode fiber of 50pm core diameter [4,5],
wiht a high refractive index hemispherical lens attached to one end of the fiber to facili-
tate coupling. The far end of the fiber is cleaved but not metallized. The amount of opti-
SPIE Vol 545 Optical Technology for Microwave Applications Il (1985) / 23
link in the frequency range of 10 by 
la
.  
tion, . ., mbra, 
 
 , 
an ultra-high d GaAlAs laser 
-insulating substrate can be used as narrow in 
-band f  (12-  b increased over a 
   
-lock the laser diode at a very high repetition rate up to 17.5GHz, producing 
= 12 lon
 s  s  
fr   
  i a of the rate at which infor- 
la  er, de a  
 l sufficient RF drive power to 
-of  in the modulation   
l  ating itive ical lses   er ode t  y h tion 
te, ough he ition te lf as o icance n rms f ue ation 
la   
g he ser o  gh ical odulation th t gh tion te  e h- 
of mode  i coup to an external cavity whose round  
s  i th cas is lim  
   freq o th external cavity   
al lation bandwidth of a soli- 
 r iode o = Hz ng  ly loped indow ied ucture er 
mi-insulating substrate ( SI  I]/ while ge l  
  [2  ode-locking of laser  
al ity as een rted t ion tes p o GHz   O3 tional 
 [  i a fi ca [4]
tra-fast 
i a BH on SI laser reported previously [1]. 
3 y an activ region dimension of 2ym x 0.2ym.  
    
    
( on)  
s mum ction ween e on d ctrons o ke ce  lts   
 of this devic  
lOm is approximately lOGHz  , "s si  
as that when the modulation depth of the optical output is < 80%. 
i 10.3  d to -lOdB at -13.5GHz and to -20dB at 18GHz  
-of  in the modulation response is due to a combination of he  
  s  , freq ran at a- 
 int,    I  (  ,  
 + I ban    l a a o  
 i a narrow bandwidth in the u per X fre
compensation and mode-
.  
  b coupling the laser to an exter- 
leng  is compo o  
(6  g index multim fiber of 50ym core diameter [4,5], 
 
 f i b no m  
I  Vol.  ti l echnology for Microwave Ap lications II (1985) /
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cal feedback into the laser in this arrangement is expected to be below 1%, and produces no
substantial reduction in lasing threshold or differential quantum efficiency. The feedback,
however, induces a broad resonance in the frequency response at - 16GHZ - the round trip
frequency of the fiber cavity. The full width of the resonance is about 1.5GHz, measured at
the upper anc lower -3dB points. At the peak of the resonance the modulation efficiency is
enhanced by =10dB. The -3dB bandwidth of the resonance is approximately 1.5GHz.
In a separate experiment the far end of a fiber is cleaved and is butted to a gold
mirror. This induces a very sharp resonance in the modulation response of the laser. When
the laser is driven on resonance by a microwave source whose power output is < 6dBm, the op-
tical output is not fully modulated and the laser is operating in the small signal regime.
As the microwave drive power is increased to > 10dBm the optical modulation depth approaches
unity and the optical waveform becomes pulse -like. The characteristics of the optical puls-
es cannot be resolved by the photodiode, whose output appears to be sinusoidal since only
the fundamental frequency (17.5GHz) of the modulated laser light can be detected with reson-
able efficiency. An autocorrelator using optical second harmonic generation is used to ob-
serve the optical output. At a microwave drive power levels of 4dBm, the optical output is
sinusoidal while at a 14dBm drive the output is pulse -like with a FWHM width of 12.4ps.
This, in effect, is active mode - locking of a laser diode at a repetition rate of 17.5GHz.
The spectrum of the laser consists of a large number (7) of longitudinal modes of the laser
diode since there is no frequency selective element (such as an etalon) in the external cav-
ity. The width of the individual mode is mainly determined by frequency chirping due to
heavy carrier modulation and does not seem to correspond to the transformed value of the op-
tical pulsewidth.
Conclusion
In conclusion, we have demonstrated that suitable constructed high speed laser diodes
can be used as narrow band signal transmitters in the Ku -band frequency range (12- 20GHz).
The modulation efficiency can be increased over limited bandwidth by a weak optical feedback.
A stronger optical feedback enables one to actively mode -lock the laser diode at a very high
repetition rate up to 17.5GHz, producing pulses of 12ps long.
This research was supported by the Defence Advance Research Project Agency, the Nation-
al Science Foundation under the Optical Communication Program and by the Army Research Off-
ice.
References
1. K.Y. Lau, N. Bar -Chiam, I. Ury and A. Yariv, to be published in Appl. phys. Lett.,
September 1984.
2. C. Lin, C.A. Burrus, G. Eisenstein, R.S. Tucker, P. Besomi and R.J. Nelson, paper
WJ4, Optical Fiber Communication conference, New Orleans, 1984.
3. R.C. Alferness, G. Eisenstein, S.K. Korotky, R.S. Tucker, L.L. Buhl, I.P. Kaminow
and J.J. Veselka, paper WJ3, Optical Fiber Communication conference, New Orleans, 1984.
4. R.S. Tucker, G. Eisenstein and I.P. Kaminow, Electron. Lett., 19, 552, 1983.
5. L. Figurroa, K.Y. Lau, H.W. Yen and A. Yariv, J. Appl. Phys., 51, 3062, 1980.
6. J.E. Bower and C.A.Burrus, paper M -1, Ninth Inter. Semicond. Laser Conf., Rio de
Janeiro, 1984.
7. R.A. Linke, paper M -4, Ninth Inter. Semicond. Laser Conf., Rio de Janeiro, 1984.
24 / SPIE Vol 545 Optical Technology for Microwave Applications ll (1985)
     
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band signal transmi ters in the Ku frequency range (12- . 
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z,  = l
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 Y. ,  -Chiam, I. Ury and A. Yariv, to be published l. . , 
  , . ,  ,     
, e, , 
 . s,  ,  , .  I  
 .J. eselka, r J3, l   ence, s, .
 . ,   ,  1
  a, .    ,   5 3
 . p M-l, Ninth Inter. emicond. .,  
198
 . M-4, Ninth ter. d. Rio de Janeiro, 1984.
  S l.  Optical Te II (
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Microwave Optical Link In The Frequency Range Of 10-18 Gigahertz By Direct Modulation Of Injection Laser Diode

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Microwave Optical Link In The Frequency Range Of 10-18 Gigahertz By Direct Modulation Of Injection Laser Diode

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