A novel semiconductor laser formed by monolithically integrating an array of active stripes with a passive planar waveguide bearing an etched-in diffraction grating is reported. Laser emission occurs from different stripes at different, precisely predetermined, wavelengths. It is expected that this laser will find widespread application in wavelength division multiplexed networks

Bhat, R.

Caneau, C.

Chang-Hasnain, C.

Hayes, J. R.

Koza, M. A.

LeBlanc, H. P.

Poguntke, K.

Scherer, A.

Soole, J. B. D.

Caltech Authors - Main

(iii) Back  propagat ion [2]. This network has 64 input neurons, 
20 hidden neurons and 26 output neurons. Only the hidden 
and output neurons are mapped onto individual PES. The 
algorithm provides learning with momentum. The training 
and test sets comprises 26 characters that use an 8 x 8 bit 
map. 
(iv) A R T - 1  [2]: This network consists of 90 neurons in total, 
of which 64 are in the classification layer, and 26 are in the 
recognition layer. Each neuron is mapped onto individual 
PES. The training and test sets comprise 26 characters that 
use an 8 x 8 bit map. 
M e t h o d :  We developed low level (assembly) program code to 
implement each of these algorithms on a systolic linear array. 
We allowed the programmer to use an unlimited number of 
registers, but required that the solutions were coded without 
being extravagant in the use of registers. 
From this we analysed the dynamic usage of each register 
and calculated the penalty incurred by having to access the 
registers via external (PE-local) memory. 
Resu l t s .  Fig. 1 shows the proportion of total access for the ’n’ 
most frequently used registers. When there are no registers 
available to hold data, then the data need to be stored in 
memory. Accessing these data incurs a time penalty. This 
penalty is determined as an extra three clock cycles for every 
access that goes to memory instead of a register. The three 
cycles involve saving the old address register, loading the 
address of the value into the address register and reading the 
value at the given address into a data register. Fig. 2 shows 
2 4 6 8 10 12 11-16 
s ndmber of general purpose registers 131 1 
Fig. 1 Register access compared to overall access 
-U- BAM + Hopfield -*- hack propagation + ART 1 
2 1.0 
0 
I- 
e 
a , l  
i 0 .4  I-.. - ’ ’ ’ ’ ’ .. Lpi~- ~ ~ 
2 4 6 8 10 1 2  14 16 
number of general pbrpose registers LETE 
Fig. 2 Increused speed with increased number of registers 
-m- BAM + Hopfield -*- hack propagation 
e-- ART 1 
the speed degradation that occurs when the number of regis- 
ters is less than the total required by the algorithm. The 
curves start at 2 as this is the minimum number of registers 
required for the PE to function. 
Analys i s :  From the graphs it can he seen that a small number 
of registers account for the majority of accesses. Keeping 
speed within 5% of the maximum obtainable, the register 
requirements are shown in Table 1. 
It can be seen from this Table that both the back propaga- 
tion and ART-1 algorithms require an equally large number 
of registers. For a general purpose architecture, such as the 
TNP,  the use of six general purpose registers gives a good 
compromise between speed and the processor complexity. 
Table 1 REGISTER REQUIREMENTS 
Number of Relative 
Algorithm registers speed 
Hopfield 2 0.9873 
BAM 2 0.9967 
Back propagation 6 0.9528 
ART-I 6 0.9651 
Acknowledgments .  We would like to thank SERC for their 
support for this work. 
16th July 1992 
J. Hunter and S. Jones (Department of Electronic and Electrical Engin- 
eering, Loughborough Uniuersitp of Technology, Loughborough, Leices- 
tershire LEI1 3 T U ,  United Kingdom) 
References 
I JONES. s., SAMMUI,  K., and HUNTER, I.: ‘Toroidal neural network 
processor. architecture, operation, performance’, in RAMACHER, U,, 
R U r K E R T ,  Cl., and NOSSEK, J .  A (Eds.): Proc. 2nd Int. Cod. on 
Microelecrronics for Neural Networks, Munich, Oct. 1991, pp. 
163-169 (ISBN 3-927527-46.7) 
LIPPMANN, R. P ‘An introduction to computing with neural nets’, 
IEEE A S S P  Maguzine, April 1987, pp. 4-22 
KOSKO, B.: ‘Bidirectional associative memories’, IEEE Truns., 1988, 
SMC-18, ( l ) ,  pp 49-60 
2 
3 
MULTISTRIPE ARRAY GRATING 
INTEGRATED CAVITY (MAGIC) LASER: A 
NEW SEMICONDUCTOR LASER FOR W D M  
APPLICATIONS 
J. B. D. Soole, K. Poguntke,  A. Scherer, H. P. LeBlanc, 
C. Chang-Hasnain,  J. R. Hayes, C .  Caneau,  R. Bhat  
a n d  M. A. Koza 
Indexing terms. Semiconductor lasers, Lusers 
A novel semiconductor laser formed by monolithically inte- 
grating an array of active stripes with a passive planar wave- 
guide hearing an etched-in diffraction grating is reported. 
Laser emission occurs from different stripes at different, pre- 
cisely predetermined, wavelengths. It is expected that this 
laser will find widespread application in wavelength division 
multiplexed networks. 
Introduct ion:  Mult~wavelength networks are being actively 
investigated for a range of applications in telecommunications 
and for use in computer optical interconnects [l, 21. ‘High 
density’ wavelength division multiplexed (HDWDM) systems 
employing direct detection techniques are currently attracting 
a great deal of attention. HDWDM systems experiments 
reported to-date have generally operated in the 1.5pm low 
loss fibre band and employed signal channels that are evenly 
spaced in wavelength [ I ,  21. 
One of the greatest challenges in setting up any HDWDM 
network is obtaining laser sources with the precise emission 
wavelengths required by the system. The wavelengths of all 
sources throughout the network need to  be accurately placed 
to within a small fraction of the interchannel spacing. Ease of 
manufacture is also highly desirable to make low cost com- 
ponents available for wide-scale HDWDM implementation. 
We report a new form of semiconductor laser in which the 
lasing wavelength can be accurately set at the design stage. By 
activating different parts of the same device, lasing can be 
made to  occur on any one of a comb of different wavelengths, 
the spacing between which can be very accurately predetermi- 
ned. Laser fabrication involves only standard photolitho- 
ELECTRONICS LETTERS 10th September 1992 Vol. 28 No. 19 1805 
graphic masking techniques, dry and wet-chemical etching. 
Just one regrowth is employed, that of a semi-insulating 
current blocking layer. By virtue of its wavelength precision 
and simplicity of fabrication, we believe that this laser has 
significant potential for wide-scale implementation in 
HDWDM networks. 
M A G I C  laser: The laser may be regarded as a monolithic 
two-dimensional realisation of an external grating laser where, 
instead of a single active element and a rotating grating, we 
have a fixed integrated grating and an array of active stripes. 
When any one of the stripes is injection pumped it lases at a 
wavelength determined by its position relative to that of the 
grating. As this geometry is accurately defined by a photo- 
lithographic mask, precise design-stage definition of the lasing 
wavelengths is possible. We call the integrated laser a ‘multi- 
stripe array grating integrated cavity (MAGIC)’ laser [3]. 
A similar multistripe integrated laser has been suggested as 
a potential source of multiwavelength radiation [4], and an 
equivalent bulk-optic demonstration has recently been report- 
ed [ 5 ] .  We will consider multiwavelength operation of the 
MAGIC laser in another publication. 
A schematic diagram of the MAGIC laser reported in this 
Letter is given in Fig. 1. The optical resonator is similar to 
p. 1 Fig. 1 Schematic diagram of MAGIC laser 
As Indicated. shaded bar is being injection pumped 
that of our previously discussed grating-based InP 
multiplexer/demultiplexer [SI. In the MAGIC laser a planar 
InP/InGaAsP/InP double heterostructure waveguide forms 
the body of the laser between the diffraction grating (on the 
right) and the active waveguiding stripes (on the left). The 
external facet of the active stripes together with the grating 
form the reflective boundaries of the resonant cavity. The 
grating is formed by a reflective wall vertically etched down 
through the waveguide core. It is curved in the plane of the 
device and focuses retrodiffracted light onto the ‘internal’ ends 
of the active stripes. These stripes comprise a conventional 
InCaAs/lnGaAsP multiquantum well (MQW) active region 
that lies directly on top of the waveguiding core, the latter 
being continuous with the rest of the optical resonator body. 
This arrangement assures good coupling of the optical mode 
between the active stripe and the planar guide region [7]. 
Semi-insulating InP provides current-blocking for the electri- 
cally pumped stripes and also forms an upper cladding to the 
planar waveguide. 
Fabrication: The laser structure was grown by low-pressure 
metal organic vapour deposition (MOCVD) on an n+-InP 
substrate. First, the waveguide core and laser structure was 
grown: a 0.3pm n-InGaAsP (Ag  = 1.3pm) guide core, fol- 
lowed by a thin InP etch-stop layer, a 6 InGaAs we11/6 
1nCaAsP (ig = 1.3pm) barrier undoped MQW active region, 
0.2pm of quaternary grading, 0.9pm of p-lnP, and a final 
0.2pm p+-InCaAsP (E.# = 1.3pm) top contact layer. The 
multistripe array was then formed by etching down to the top 
of the waveguide core, and 1.0pm of semi-insulating Fe : InP 
re-grown on the etched surface. chemically assisted ion beam 
etching [SI was then used to etch the -3pm deep vertical- 
walled diffraction grating. After metallising the grating, top 
contacts were formed to the stripes and the wafer was thinned 
and cleaved across the stripe array to provide the laser output 
facet. 
The fabricated laser measured - 14 x 3mm2, with a stripe/ 
grating separation of IOmm. The stripes were typically 
-2” long and spaced at -40pm, their exact placing being 
chosen to linearise the laser output wavelengths. Active region 
widths were 6-7 pm. The diffraction geometry employed was a 
standard 9 mm radius Rowland circle; the grating operated in 
16th order and was blazed for retroreflection. 
Laaer characteristics The laser was examined under pulsed 
operation Each active stripe was pumped separately No tem- 
perature stabillsation was employed 
Lasing was observed from the active stripes, over a spectral 
range from -1500 to  1535nm Current thresholds were 80- 
100 mA, and the lasing intensity was typically > 25 dB above 
the background spontaneous emission The emissions were 
TE polarised The typical emission spectrum of a stripe, taken 
with an 0 1 nm resolution optical spectrum analyser, is shown 
in Fig 2 
I 
-LO I 
1510 1515 1520 
“‘21 wavelength, nm 
Fig. 2 Typical luser emissionfrom injection pumped stripe 
Taken with 0.1 nm resolution optical spectrum analyser 
The measured wavelength of the laser emission from each 
stripe was in accordance with its geometric placing with 
respect to the diffraction grating. The absolute wavelength 
values were within 3nm of the original design values. The 
laser wavelengths are plotted in Fig. 3. The wavelength 
spacing i s  -4nm. The wavelength placings are linear to  
within measurement accuracy. Individual variations are 
k 0.09 nm, or - 2% of the channel spacing, as measured from 
a scan of an optical spectrum analyser with 0.1 nm resolution, 
This very high degree of wavelength linearity and spacing 
accuracy is a direct consequence of photolithographically 
determined geometric definition of the lasing cavity. 
Scanning Fabry-Perot measurements revealed that the 
lasing of each stripe occurred predominantly on a single longi- 
tudinal mode. The emission was > 8 d B  above the next most 
intense of the 8-9 other longitudinal modes contained within 
a -0.27nm emission band. The longitudinal mode spacing 
and the output spectral band are in accord with the laser 
cavity length and active stripe width, respectively. Reduction 
of the active width to a value typical of single mode buried 
lasers is expected to produce single frequency operation of the 
MAGIC laser. 
Summary: We have presented a new monolithic semicon- 
ductor laser: the multistripe array grating integrating cavity 
(MAGIC) laser. Laser emission occurs from each stripe at a 
wavelength determined by its position relative to an inte- 
grated diffraction grating. This permits accurate determi- 
nation of the lasing wavelengths at the design stage. We have 
realised a MAGIC laser that lases over 35 nm in the 1.5 pm 
15LO I 
11904 I 
c 2 4 6 8 10 
stripe pumped 
Fig. 3 Lasing wavelength against stripe 
I a06 ELECTRONICS LErrERS lOrh September 1992 Vol. 28 No. 19 
fibre band at accurately predefined wavelengths. We expect 
that this MAGIC laser will find widespread application in 
HDWDM networks. 
Acknowledgments :  We gratefully acknowledge helpful dis- 
cussions with C. E. Zah. W. Chan, F. Favire, and B. Pathak, 
the assistance of J. Ringo and P. J. Rickmann, and the loan of 
equipment from W. Way, E. Goldstein, and B. Yoo. 
24th July 1992 1992 
J. B. D. Soole, K. Poguntke,' A. Scherer, H .  P. LeBlanc, C. Chang- 
Hasnain,t J. R. Hayes, C .  Caneau, R. Bhat and M. A. Koza (Bellcore, 
331 Newman Springs Road, Red Bank N J  07701. U S A )  
* O n  leave from Dept of Physics, University of Surrey, United 
Kingdom 
t Present address, Ginzton Lab. Stanford University, Stanford CA. 
USA 
References 
BRACKETT, C.: 'Dense wavelength division multiplexing networks: 
principles and applications', IEEE J .  Sel. Areas Commun., 1990, 
SAC-8, pp. 948-964 
DONO, N. R., GREEN, P. E., JUN.,  LIU,  K., RAMASWAMI, R ,  and TONG, F. 
F . X .  'A wavelength division multiple access network for computer 
communication', IEEE J .  Sel. Areas Cummiin.. 1990, SAC-8, pp 
983-994 
HASNAIN, C .  HAYES, J .  R., BHAI, R., CANEAU, c. and KOZA, M. A: 'A 
MAGIC laser for WDM applications'. Post-deadline paper, 
1118-7, Device Research Conference, Boston, 22nd- 24th April 
1992 
KIRKBY, P.: 'Multichannel wavelength-switched transmitters and 
receivers-new components for broad-band networks and distrib- 
uted switching systems', J .  Lightwave Technol., 1990, LT-8, pp. 
202-2 1 1 
'Demonstration of a I x 2 multichannel grating cavity laser for 
wavelength division multiplexing (WDM) applications', Electron 
Lett., 1990, 26, pp. 832-834 
BHAT, R., and KOZA, M. A.: 'Monolithic InPhGaAsPflnP grating 
spectrometer lor the 1.48-1.56pm wavelength range', Appl. Phys. 
Lett., 1991,58, pp. 1949-1951 
KOCH, I. L., and KOREN, U,: 'Semiconductor lasers for coherent 
optical fiber communications', J. Lighrwave Technol., 1990, LT-8, 
pp. 214-293 
SCHERER, A., JEWELL, J. L., LEE, Y. H., HAREISON, J. P., and FLOREZ, L. 
I.: 'Fabrication of microlasers and microresonator optical 
switches', Appl. Phys. Lett., 1989, 55, pp. 2724-2725 
SOOLE, 1. B. D., POGUNIKE, K., SCHERER, A.. LEBLANC, H. P., CHANG- 
WHITE, 1. H., NYAIRO, K. 0.. KIRKBY, P. A., and ARMISTEEAD, C. J ' 
SOOLE, J .  B. U,, SCHERER, A., LEBLANC, H. P., ANDRtAUAKIS, Ei. C. ,  
SECOND ORDER DISTORTION IN Er-DOPED 
FIBRES 
J. H. Povlsen, A. Bjarklev, T. Rasmussen and 
K. Rottwitt 
Indexing terms: Distortion, Optica1,fihres 
It is shown that AM signals introduce travelling inversion 
waves in the Er-doped fibre amplifier. A lormula for the 
induced distortion is found. Especially for Er-doped fibres 
with high numerical aperture the distortions may reach mag- 
nitudes which are problematic in relation to AM-FDM 
optical video links. 
Introduction: Several papers indicate that the distortion in 
EDFAs is so small that they in the future can be used in 
AM-FDM optical analogue video transmission systems [l-31. 
Most of the attention has focused on second order distortion 
caused by a coupling between the gain tilt of the EDFA with 
the chirp of the signal laser, whereas the distortion caused by 
the internal gain dynamics of the EDFA has so far been 
neglected. Considering a lifetime of -1Oms for the excited 
Er ion this seems reasonable, but without a proof it must be 
considered as a postulate. Some results for the gain dynamics 
are presented in Reference 4, but these are not safely extrapo- 
ELECTRONICS LETTERS 10th September 1992 Vol. 28 
lated to distortion magnitudes relevant for optical video trans- 
mission systems. Owing to these considerations we have 
performed a first order perturbation calculation on the rate 
equation. Surprisingly, we find distortion with magnitudes 
comparable to the distortion related to the gain slope. We 
therefore hope that our results can link some of the conflicting 
results mentioned in Reference 3. 
Theory: The ratio x between Er ions in the excited state and 
total number of Er ions, known as the inversion, can be evalu- 
ated in time according to the differential equation 
where S ,  and S, are the absorption and emission strengths, 
respectively. These are related to the excited state lifetime 7, 
and the photon intensity of the signal N, ,  the photon intensity 
of the pump N, and the spectral photon intensity of the 
forward and backward propagating amplified spontaneous 
emission n,,,(u) by 
Sa = u,AcJNs + uAuP)Np + 
0 
where ue and uo are the cross-sections for emission and 
absorption and U is the optical frequency. We assume that the 
photon intensity N ,  varies in time around its time mean value 
(N,)  and consider the following identity: 
N i t )  = Nto' + 6 N y ) ( r )  
N:" = ( N , )  N W )  = N,(r)  - ( N , )  (3) 
6 = 1  
where 6 serves as a perturbation parameter. Expanding x in a 
Taylor series in 6 
we find, by inserting eqns. 2-4 into eqn. 1 and collecting terms 
of the same order in 6, that: 
Zero order : 
First order: 
where the zero order emission and absorption strengths SC,' 
SLo) are obtained from eqn. 2 with N ,  = N:O1. Because the 
intermodulation is known to be very weak at modulation fre- 
quencies relevant for analogue transmission [l-31 we consider 
only the first order correction. By Fourier transforming x(') 
we obtain from eqn. 5 
.x"'(co) = [( 1 - x'0')u.(u,) - x~"~u,(v,)]NJ"(o) 
i o  + SLo) + SLo' 
No. 19 
(6)  
1807 


Multistripe Array Grating Integrated Cavity (MAGIC) Laser: A New Semiconductor Laser for WDM Applications

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Multistripe Array Grating Integrated Cavity (MAGIC) Laser: A New Semiconductor Laser for WDM Applications

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