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We determined the thermal resistance of the laser diodes as
depicted in Fig. 4 by measuring the emission wavelength as a function of electrical input power and stage temperature. We observe a
linear relationship in both cases. We find that the thermal resistance of the lasers is approximately inversely proportional to the
device diameter in agreement with a simple theoretical model. The
resistance varies from 613WW for the largest device to 4221WW
for the 4 . 5 device.
~
The heat spreading layer gives a reduction in
the thermal resistance by about a factor of 2 for all laser sizes. The
estimated average temperature increase in the active region at
maximum output power is -105K for all devices at this wavelength. The heat spreading layer pushes the thermal roll-over point
towards higher drive currents resulting in 60% higher output
power levels. The thick plated Au layer also provides additional
mechanical support for the etched pillar.
In conclusion, we demonstrated improved performance of vertical-cavity laser diodes with submilliamp threshold currents and
high output power by the integration of Au-plated heat spreading
layers. The devices should prove to be useful in low cost optical
data links and free-space communication systems.
Acknowledgment: The authors thank G. Robinson for helpful
technical discussion. This work was financially supported by
ARPA via O N R and the Optoelectronics Technology Center. T.
Wipiejewski gratefully acknowledges a fellowship from the Deutsche Forschungsgemeinschaft.
0 IEE 1995
30 November 1994
Electronics Letters Online No: I9950190
T. Wipiejewski, D.B. Young, M.G. Peters, B.J. Thiheault and L.A.
Coldren (Universiry of California, Santa Barbara. Department of
Electrical & Computer Engineering, Santa Barbara, CA 93/06, USA)
strained MQW DFB laser operating at 2.5Gbitds has been shown,
both theoretically and experimentally [l], to be capable of transmitting over 150km with a 2dB dispersion penalty. If transmission
over greater distances is required then some form of dispersion
compensation must be used. Dispersion compensation with fibre
Bragg gratings has recently been demonstrated in a low chirp
external modulator system [2, 31 but such an experiment has not
yet been performed with a directly modulated laser source. Such a
system is very attractive in view of its physical simplicity leading
to reduced system cost. In this Letter we investigate the potential
improvements in transmission distance for such a source which
may be achieved using a chirped fibre grating compensator using a
validated system model.
It is a simple matter to show that the optimum fibre length
which a chirped grating can compensate for is given by
-1
X2D
where n is the fibre refractive index (l.5), D is the fibre dispersion
(here 15ps/nm.km), h is the operating wavelength ( 1 . 5 5 ~ ) ,and B
and L, are the grating length and bandwidth respectively. If the
grating length is fixed by fabrication technology then the compensation distance is proportional to the inverse of the bandwidth.
Thus it is clear that to obtain the maximum compensation distance a narrow bandwidth grating is required. However, a penalty
will be incurred if the reflection bandwidth of the grating becomes
narrower than the signal bandwidth.
1
r-----------
References
I
I
and COLDREN, L.A.: ‘High
efficiency submilliamp vertical cavity lasers with intracavity
contacts’, IEEE Photonics Technol. Lett., 1994, 6, pp. 678-680
2 HUFFAKER, D L., DEPPE, D G , KUMAR, K., and ROGERS, T J : ‘Nativeoxide ring contact for low threshold vertical-cavity lasers’, Appl.
Phy.7. Lett., 1994, 65, pp. 97-99
3 WIPIEJEWSKL T., PANLLAFF. K., ZEEB. E., and EBELING. K.J ; ‘Tunable
extremely low threshold vertical-cavity laser diodes’, IEEE
Photonics Technol. Lett., 1993, 5, pp. 889-892
1
4
dispersion
Compensator
S C O I I . J W , THIBEAULT. R J , YOUNG. D.B.,
PETERS, M . G , YOUNG, D B , PETERS, F H , SCOTT. J . W . THIBEAULT. B . J ,
and COLDREN. L.A.: ‘17.3% peak wall plug efficiency vertical-cavity
surface-emitting lasers using lower barrier mirrors’, lEEE
Photonics Techno/. Lett., 1994, 6, pp. 31-33
5 YOUNG. DB., SCOTT, J.W., PETERS, F.H., PETERS. M.G., MAIEWSKI, M.L.,
THIBEAULT. B.J., CORZINE, s.w., and COLDREN, L.A.: ‘Enhanced
performance of offset-gain high-harrier vertical-cavity surfaceemitting lasers’, IEEE J. Quantum E/ectron., 1993, QE-29, pp.
2013-2022
Increasing the transmission distance of a
directlv modulated laser with a chirped fibre
Bragg grating dispersion compensator
P.L. Mason, R.V. Penty and I.H. White
Indexing terms: Gratings in fibres. Oprical di.vper.sion,Distributed
feedhack lasers
The dependence of dispersion penalty on transmission length has
been investigated in an optical communications system employing
a directly modulated ZSGbitis DFB laser source and a fibre
grating dispersion compensator. The 2dB penalty transmission
distance is -520km along standard monomode fibre, representing
a great improvement over the 150km range expected for a similar
uncompensated system.
Introduction: Recent developments in DFB laser design and fabri-
cation have led to an increase in the distance over which such a
device may transmit in standard optical fibre. For example, a
ELECTRONICS LEl7ERS
16th February 1995
rn
Fig. 1 Functional block diagram of system model
System model: Fig. 1 shows the structure of the optical communi-
cations system model used in this work 141. All elements of the
model have been validated so that their experimental performance
matches the theoretical one. The source under consideration is a
strained MQW 2M8 D F B laser with a linewidth enhancement factor of +2 [5]. The laser is directly modulated with a 2.5Gbitis
pseudorandom N R Z signal. The optical electric field is passed on
to a linear dispersion fibre model where it undergoes distortion.
The fibre grating model is then applied to the distorted signal and
the result is displayed as an eye diagram. The grating is assumed
to be apodised, that is, its reflectivity has a nonuniform profile
with respect to length. This has been found to improve the performance of gratings in dispersion compensation 161. Both Gaussian and tanh profiles have been suggested, so simulations have
been performed with both apodisation schemes. The grating coupling coefficient varies from K = 1m (Om-’ for the tanh profile) at
the ends to x = loom-l in the centre.
’
The grating length L, was kept fixed at 5cm which represents
the current fabrication limit 121, so that the only variable in the
grating design was the reflection bandwidth B. The system model
was run for a range of grating bandwidths and in each case the
optimum length of optical fibre L, was used according to eqn. 1
Then the dispersion penalty of the system as a function of fibre
link length was obtained from the eye diagrams.
Because we are modelling a real system it is assumed that the
laser temperature may be controlled to 0.1 “C. Temperature variations in the laser give rise to a wavelength change of -0.lnmiOC
[SI. Thus the centre frequency of the laser is aligned to within
+/-0.6125GHz of the centre reflection frequency of the grating.
For each choice of grating bandwidth the wavelength detuning
was selected to determine the lowest and the highest penalty,
because the group delay characteristics of narrow bandwidth gratings tend to be nonlinear. Fig. 2 shows the nonlinear nature of the
group delay of 5cm, 8GHz bandwidth gratings with tanh and
Gaussian apodisation.
Vol. 31
No. 4
28 1
References
500r--
,
I
I
24001
Z
l
O
;no
-5
O
L
,
---
A1
I
2
,
3
-2
-1
0
1
2
3
4
5
relative optical frequency,GHz
Fig. 2 Group delay characteristic of Scm, 8GHz bandujidth grating with
tunh and Gaussian apudisation
~
~
----
m
-4
-3
tanh
Gaussian
4
-
5
6
7
J.E.A.: Private communication, 19th October 19%
ws., EPWORTH, R.E., ERICHENO. T , and CHEW.c . ~ :
‘Practical dispersion equalizer based on fibre gratings with a bitrate
length product of 1.6TBis.km’. Proc. ECOC 1994, Vol. 4, Postdeadline papers, pp. 11-14
HILL, K.O., 1 H E R l A U L T . S.. MALO, B., BILODEAU. C , KITAGAWA. .r,
JOHNSON, D . C ,
ALBERT. J , TAKIGUCHI. K., KATAOKA. T., and
HAGIMOTO. K.: ’Chirped in-fibre Bragg grating compensators linearisation of dispersion characteristics and demonstration of
dispersion compensation in 100km. lOGbit/s optical fibre link’.
Electron. Lett., 1994, 30, pp. 1755-1756
MASON. P.I., PENTY.
R . V , and WHITE. I H.: ’Multiple stage dispersion
compensation in long haul optical fibre systems using chirped fibre
Bragg gratings’, Electron. Lett., 1994. 30, pp. 12&124S
AGRAWAL. G P , and DUTTA. N K.: ‘Semiconductor lasers (Van
Nostrand Reinhold, New York, 1993, 2nd Edn.), p. 468, p. 348
OUELLETTE.
F : ‘Dispersion cancellation using linearly chirped
Bragg grating filters in optical waveguides’. Opt. Lett.. 1987, 12.
pp. 847-849
CELLS. J A.J , GIBBON. M A., THOMPSON. Ci H.B., WHITE. I PI.. PENTY. K V..
WRIGHT. A.P., SAUNDERS. R.A. ARMISTEAD. c.J., and K I M B E R . EM.:
‘Improving the system performance of integrated multiquantum
well laser modulators with negative chirp’. To be presented at
OFC’95
WHITEAWAY,
GARTHE. D., LEE.
I
200
300
LOO
500
600
fibre length. krn
a
Fig. 3 (bmnparizm of performance of tanh and Gaussian grating compensutors in opfimised long haul systems
The two separate lines for each grating show the hest and worst possible operation of the compensator given the uncertainty in the laser
wavelength of +1-0.61 GHz
tanh
~
Gaussian
-
Low threshold current and high output
power operation for 1.5pm GRINSCH
strained MOW laser diode
~~~
Results and discussion: Fig. 3 shows a graph of dispersion penalty
against fibre length for a system which is dispersion compensated
with a Gaussian or tanh apodised 5cm fibre k a g g grating. Both
best case and worse case scenarios are plotted, implying that the
actual penalty might lie anywhere between these lines. Because the
extent of the detuning cannot be determined at any given time due
to the uncertainty of the laser wavelength, estimates of dispersion
penalty must always represent the worst case scenario. It is evident
therefore that for the grating under consideration the tanh apodisation profile provides compensation with a significantly lower
penalty than the Gaussian profile. Penalty uncertainties as a result
of detuning are relatively constant with the tanh profiling, whereas
the Gaussian profiling gives rise to a wide variation. A 2dB penalty is incurred after -520km if compensation is effected with a
tanh profiled grating. This represents a SUbStantkal improvement
over the 150km possible in an uncompensated system. It also
compares favourably with results recently obtained with an
uncompensated 2.5GbiUs integrated laser modulator system,
where near penalty-free transmission over 360 km of standard fibre
has been achieved [7].
Conclusions: Modelling leads us to expect that a 2SGbitis strained
MQW DFB laser could transmit over 520km of fibre with the aid
of a chirped fibre Bragg grating compensator. This is a significant
improvement in performance when compared with an uncompensated system using the same source.
Ackowledgments: The authors would like to thank J.A.J. Fells for
his work with the system model and they also acknowledge the
financial support of the RACE WELCOME programme, under
which the modulator model was developed.
T. Kunii, Y . Matsui, Y . Katoh and T. Kamijoh
Indexing terms: Seniicuniluctur junction lasrrs. Semimndurtor
quanfum ne//s, Grndient indrr optics
~
Introduction: Laser diodes (LDs) for optical access networks have
been extensively investigated. The lasing characteristics of these
LDs must be improved if low-cost optical access networks are to
be realised. Low threshold current operation and short carrier lifetime are important for achieving a small turn-on delay time. High
output power operation is promising for improving the performance of networks and reducing the cost of packaging. Excellent
lasing characteristics have been reported for 1.3pm LDs by several
groups [I, 21. Although threshold currents of less than 2 mA have
~
the output power characteristics o f
been achieved for 1 . 5 LDs,
these lasers have been insufficient for practical application [3 - 51.
We have developed 1 Spm GRINSCH Strained multiquantum-well
(MQW) LDs for low-threshold, high output power operation.
This Letter reports the lasing characteristics of these LDs, which
also exhibited a short carrier lifetime.
o-InGaAs
3 November 1994
Electronics Letters Online No: 19YSO224
,-
~
Band diagram
Well: 0.7% compressive strained InCaAs, 408, barrier
Barrier: 1 . 3 1 p InGaAsP. 190A
b Device structure
U
P.L. Mason, R.V. Penty and I.H. White (Optoelectronics Group,
School of Physics. Vniversity of Bath. Bath BA2 7A Z United Kingdom)
282
i
n
~
p-inp
b
n-rnPsubstrate
Fig. 1 Bund diagram u / a c f i v e layer and device .structure
U
0 IEE 1995
~
A low threshold current of 1.XmA and large slope efficiency of
0.30mW/mA were obtained using 1.5m GRINSCH strained
MQW laser diodes. High-temperature operation was confirmed.
and the threshold current at 80°C was 5.0mA with a slope
efficiency of 0.lXmWimA. The device had a short carrier lifetime
of less than 1.5511s.
ELECTRONICS LETTERS
16th February 1995
Vol. 31
No. 4
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