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problems need
configurations.
to be carefully
P. E. FLEISCHER
A. GANESAN
K. R. LAKER
Bell Laboratories
Holmdel, NJ 07733, USA
considered
for these
20th October 1981
References
1
GRAY, p. R., HODGES, D. A., and BRODERSEN, R. w. (Eds.): 'Analog
MOS integrated circuits' (IEEE Press, New York, 1981)
2
WHITE, B. j . , JACOBS, G. M., and LANDSBURG, G. F.: 'A monolithic
dual-tone multifrequency receiver'. 1979 ISSCC digest of technical
papers, pp. 36-37
3 MARTIN, K.: 'Improved circuits for the realization of switchedcapacitor filters', IEEE Trans., 1980, CAS-27, pp. 237-244
4
FLEISCHER, p. E., and LAKER, K. R.: 'A family of active switched
capacitor biquad building blocks', Bell Syst. Tech. J., 1979, 58, pp.
2235-2269
5
FLEISCHER, P. E., LAKER, K. R., MARSH, D. G., BALLANTYNE, J. P., YIAN-
NOULOS, A. A., and FRASER, D. L.: 'An NMOS analog building block
for telecommunication application', IEEE Trans., 1980, CAS-27,
pp. 552-559
6
MARSH, D. G., AHUJA, B. K., MISAWA, T., DWARAKANATH, M. R., FLEISCHER, p. E., a n d SAARI, v. R . : 'A single-chip C M O S P C M c o d e c
with filters', IEEE J. Solid-State Circuits, 1981, SC-16, pp. 308-315
7
LAKER, K. R., FLEISCHER, p. E., and GANESAN, A.: 'Parasitic insensitive
switched capacitor biquads: a review'. Proc. 1981 IEEE ISCAS, p.
423
0013-5194/81/240929-03$!.50/0
operated under pulsed conditions and results were quoted for a
point well into the pulse, after transient effects had disappeared. The influence of the external cavity on the laser spectrum under transient conditions was not reported.
In this letter we describe a practical implementation of the
short external cavity technique in a prototype laser transmitter
module. We also show that the external cavity produces stable
tunable single longitudinal mode operation of the laser even
under conditions of high speed modulation.
We have obtained single mode operation of lasers at wavelengths of 1-3 fim and 1-55 fim. The results reported here were
obtained on a GalnAsP/lnP inverted rib-waveguide laser
operating at approximately 1-3 fim.8 The threshold current of
this device was about 78 mA at a temperature of 19°C.
The laser was mounted in the transmitter module on a
copper submount which allowed access to both laser facets.
The output from one facet was butt coupled into a monomode
fibre tail which was subsequently fixed in place using a low
melting point solder.9 The coupling efficiency to thefibrewas
estimated to be about 10%, resulting in a power of 400 /iW in
the fibre tail for a current of 20 mA above threshold. A 200 /mi
radius concave spherical reflector was positioned about 200
fim from the other laser facet to form the external cavity. The
laser was not antireflection coated. The reflector was produced
by pressing a 400 fim diameter sapphire sphere into indium
alloy supported in a hypodermic needle. This resulted in a
component which could conveniently be handled, aligned and
fixed in position inside the module.
A micromanipulator was used to align the reflector with the
laser. During this process the laser spectrum was monitored by
I h = 199mA
I = 184 mA
n
EXTERNAL CAVITY CONTROLLED SINGLE
LONGITUDINAL MODE LASER
TRANSMITTER MODULE
Indexing terms: Lasers, Semiconductor lasers
Ih= 167mA
A GalnAsP/lnP laser transmitter module incorporating an
external cavity is described. It is shown that the cavity produces stable tunable single longitudinal mode operation of
the laser even under high speed modulation conditions.
High bit rate monomode optical fibre transmission systems
operating at 1-55 fim have the advantage that losses in silicabased fibre can be made very small at this wavelength.
However, the nonzero chromatic dispersion of silica at 1-55 fim
can have important consequences. Semiconductor lasers
normally operate in several longitudinal modes, especially
under modulated conditions. Studies have shown that the performance of 1-55 fim monomode optical fibre systems using
such lasers is limited at high bit rates by effects associated with
the chromatic dispersion of the fibre.1-2 It is possible to produce monomode fibre having zero total dispersion at 1-55 fim
by offsetting waveguide dispersion against material
dispersion,3 but this may increase the fibre loss4 and also
makes jointing tolerances more severe. An alternative
approaches to reduce the spectral width of the source.
It has previously been shown that the multimode spectrum
of a semiconductor laser may be reduced in width and even
restricted to a single longitudinal mode by optically coupling
an external cavity to the laser.5-6 Renner and Carroll7 used
short external cavities to produce tunable single longitudinal
mode emission from GaAlAs lasers with wavelengths of approximately 830 nm. They obtained a single mode tuning
range of 43-5 A from a laser of mode spacing 1-6 A when a
concave spherical reflector of 60 fim radius was aligned with
the laser using a micropositioning device. Their lasers were
ELECTRONICS LETTERS
26th November 1981
I h =K3mA
l h = 113mA
1285
1295
1290
wavelength, nm
j822/i|
Fig. 1 Tuning range obtained by use of heater
Ih = heater current
Laser operated CW, 20 mA above threshold
Vol. 17 No. 24
931
taking the light output from thefibretail to a monochromator
equipped with a scanning mirror arrangement. This allowed
the laser spectrum to be displayed continuously on an oscilloscope as the reflector was adjusted. Once the reflector had been
positioned to produce single mode operation of the laser it was
fixed in place with a room temperature curing epoxy. Fine
control of reflector position in the completed module was
achieved thermally, by passing a current through a small
heater coil wound on the hypodermic needle. The resistance of
the coil was approximately 1 Q. The reflector produced no
noticable effect on the threshold current of the laser, showing
that the amount of power reflected back into the laser cavity
was small.7
Spectral measurements on the transmitter module were
made using a lm monochromator. During these measurements the module was mounted on a heatsink, the temperature
of which was maintained at 19-5 + 0-5°C by a thermoelectric
cooler. Fig. 1 shows the single mode tuning range obtainable
with the heater coil. The laser was operated CW at a current of
20 mA above threshold. Heater currents between those shown
generally resulted in operation in two longitudinal modes.
With the heater current correctly adjusted, single longitudinal
mode operation was maintained for laser currents ranging
from threshold to at least 1-3 times threshold. Higher currents
were not tried in case these resulted in damage to the laser. The
tuning range is seen to be about 4 nm and is cyclic with heater
current. A similar tuning range was observed under modulated
conditions.
The results of time resolved spectrum measurements are
shown in Fig. 2. The spectra in this Figure have all been normalised to the same peak height in order to show details of the
start of the pulse more clearly. The laser was modulated from
threshold by a current pulse of 6 ns duration and 11 MHz
repetition rate. Rise and fall times of the current pulse were less
than 1 ns. The detector used was a germanium APD. The
spectra of Fig. 2 are for a pulse current amplitude of 20 mA and
similar spectra were obtained for pulse amplitudes of 10 mA
and 30 mA. Fig. 2b shows results with the reflector in position
and heater current adjusted for single mode operation. It is
seen that single mode operation is obtained even during the
first nanosecond of the pulse. For comparison, Fig. 2a shows
time resolved spectra from the same module under identical
conditions except that the reflector has been removed.
The external cavity controlled laser transmitter module was
also modulated by 2 10 - 1 bit pseudorandom patterns at bit
rates of 140 Mbit/s and 280 Mbit/s. It was found that single
mode operation was maintained throughout the patterns.
The reflector with which these results were obtained is not
necessarily optimum. A smaller radius of curvature would
allow a shorter external cavity to be constructed, which would
be expected to increase the available tuning range.7 The surface quality of the present reflector could also be improved.
Temperature affects the wavelengths of the gain profile and
also of individual longitudinal modes in a semiconductor laser.
In order to maintain single longitudinal mode operation over a
range of temperature using an external cavity it will be necessary to take this into account. Active control of the reflector
position may be needed, and one way of doing this would be to
use a modification of the technique described by Malyon et
al}°
Although we see the main use for the external cavity controlled laser transmitter module to be in overcoming the limitation of chromatic dispersion in 1-55 nm monomode optical
fibre systems, there are other possible applications. The
tunable, spectrally narrow output may be advantageous in
wavelength multiplexed systems. The module may also provide
a suitable source for injection locked11 and optical heterodyne
systems.
In conclusion, we have shown that the short external cavity
provides a simple and effective method for producing dynamic
single longitudinal mode operation of a semiconductor laser,
and one that can be included in a compact laser transmitter
module. A module which employs such a technique should find
considerable application in future optical fibre systems.
5ns
Acknowledgments: The inverted rib-waveguide laser was
supplied by Standard Telecommunication Laboratories Ltd.
under a British Telecommunications development contract.
We wish to thank M. R. Matthews for many helpful discussions and the Director of Research of British Telecom for permission to publish this letter.
2ns
K. R. PRESTON
K. C. WOOLLARD
K. H. CAMERON
6th October 1981
British Telecom Research Laboratories
Martlesham Heath, Ipswich, Suffolk IP5 7RE, England
References
1 MACH1DA, S., YAMADA, J., MUKAI, T., HORIKOSH1, Y., TSUCHIYA, H., a n d
MIYA, T.: '1-5 nm optical transmission experiments using very lowloss single-mode fibres', Electron. Lett., 1979, IS, (8), pp. 219-221
2 NAKAGAWA, K., and ITO, T.: 'Detailed evaluation of an attainable
repeater spacing for fibre transmission at 1-3 (im and 1-55 ^m wavelengths', ibid., 1979, 15, (24), pp. 776-777
Ins
3 COHEN, L. G., LIN, CHINLON, and FRENCH, w. G.: 'Tailoring zero
chromatic dispersion into the 1-5—1-6 /xm lowloss spectral region of
single-mode fibres', ibid., 1979, 15, (12), pp. 334-335
4 AINSLIE, B. j . , BEALES, K. j . , DAY, c. R., and RUSH, j . D.: interplay of
design parameters and fabrication conditions on the performance
of monomode fibers made by MCVD\ IEEE J. Quantum Electron.,
1981, QE-17, pp. 854-857
05ns
5 BOGATOV, A. P., ELISEEV, P. G., IVANOV, L. P., LOGGINOV, A. S., MANKO,
^/v\fvV-W
1285
1290
wavelength, nm
M. A., and SENATOROV, K. YA.: 'Study of the single-mode injection
laser', ibid., 1973, QE-9, pp. 392-394
1285
1290
wa/elength, nm
6 VOUMARD, c , SALATHE, R., and WEBER, H.: 'Single-mode operation
Fig. 2 Time resolved spectra
a Without reflector
b With reflector
Times shown are measured from the start of the optical pulse
All spectra have been normalised to same peak height
Laser modulated from threshold, pulse current 20 mA
6 ns pulse duration, 11 MHz repetition rate
932
of diode lasers coupled to hemispherical resonators', Opt. Commun.,
1975, 13, pp. 130-133
7 RENNER, D., and CARROLL, j . E.: 'Simple system for broad-band
single-mode tuning of DH GaAlAs lasers', Electron. Lett., 1979,15,
(3), pp. 73-74
8 TURLEY, S. E. H., HENSHALL, G. D . , GREENE, P. D . , KNIGHT, V. P.,
MOULE, D. M., and WHEELER, S. A.: 'Properties of inverted rib-waveguide lasers operating at 1-3 /jm wavelength', ibid., 1981, 17, (23),
pp. 868-870
ELECTRONICS LETTERS 26th November 1981
Vol.17
No. 24
9 CAMERON, K. H., CHIDGEY, P. J., PRESTON, K. R., SMITH, D . W., a n d
MATTHEWS, M. R.: 'A laser transmitter module for monomode fibre
transmission systems'. Presented at IEE colloquium on optical
fibre systems, London, 29 May 1981
10 MALYON, D. J., SMITH, D. w., and BERRY, R. w.: 'Wavelength sensed
where n20 is a real constant, t]0 and r]3 are complex, and y3 is a
distance. Below stripe centre y = 0, so that n2 = n2o — 1o,
where rj0 is the complex index at that location. If g0 (with
g0 > 0) is the optical power gain at y = 0, then
mode control of semiconductor lasers'. Electron. Lett., 1980, 16,
(19), pp. 744-746
11 MALYON, D. J., SMITH, D. w., and BERRY, R. w.: 'Spectrum stabilised
laser transmitter'. Presented at the third international conference
on integrated optics and optical fiber communication (IOOC 81),
San Francisco, California, April 27-29, 1981
0013-5194/81/240931 -03$ 1.50/0
= 9o(b + i)/2k
(3)
where k0 is the free-space wave number and b is the antiguiding
factor, which is commonly taken to be between 1 and 4. As
|>| -• oo, where the injected charge distribution is zero, the
band-to-band absorption txbb (with <xbb > 0) remains and
therefore
= (0o + zbb){b + 0/2/co
(4)
In fact, tj3 is the net difference in complex refractive index
between y = 0 and y = co; it represents the complete effect of
the injected charges.
For a laser with a filling factor Fx, which is the fraction of
modal power in the active region, we find8 that
SPONTANEOUS EMISSION FACTOR OF
NARROW-STRIPE GAIN-GUIDED DIODE
LASERS
G(y) = cosh" (y/y3)
where
Indexing terms: Lasers and applications, Semiconductor lasers
The spontaneous emission factor of narrow-stripe gainguided diode lasers, which is of importance in determining
time-response and longitudinal-mode structure, is shown to
be one order of magnitude greater than previously supposed.
The spontaneous emission ratio C,1
C=
4
defined as
H = 0-5 - (0-25 -
occurs in the rate equations and is of great significance in
determining the behaviour of diode lasers. Specifically, it
influences the pulsed response and modulation behaviour, the
abruptness with which the laser commences oscillation, the
noise properties and the longitudinal-mode spectrum.
Recently, Petermann 5 recognised that the lateral phase
dependence of the laser mode substantially affects the magnitude of C. If G(y) denotes the complex lateral-mode field
distribution, then a multiplicative factor
If \G(y)\2dy\
f
G2(y) dy
(1)
arises in the definition of C. Thus, for two lasers, the first
utilising real refractive index waveguidance and the second
gain guidance, but both with the same dimensions, charge distributions and modal intensity variations (not field), the values
of C will differ by K. Evidently, K = 1 for the first device since
G(y) is real, whereas K exceeds unity when G(y) is complex, as
is the case for gain-guided lasers.
In Reference 5, a complex lateral-modal dependence employing a Gaussian approximation for the gain-guided mode
was evaluated and a value of K = 6 obtained. For the Vgroove laser,6 a value of K = 11 was quoted. Although these
factors increase C by approximately one order of magnitude,
they appear insufficient to explain some aspects of device behaviour, at least according to theory, 7 which is not very sensitive
to C in its range of values. However, if a more accurate representation of the lateral mode is utilised, as we show, much
larger values of K can occur. Specifically, the lateral-mode
decay in gain-guided lasers is known to be exponential as
opposed to Gaussian. Thus the power in the modal tails
decreases less rapidly than is given by the Gaussian representation and the spontaneous emission coupling to the mode is
thereby enhanced substantially.
In our analysis,8 the refractive index of the active region is
written as
"2 = "20 - 1o + Is tanh 2 (y/y3)
ELECTRONICS LETTERS 26th November 1981
2n20rj3k20ylrxY'2
(6)
and the only parameter as yet unspecified is gQ. In our calculation, g0 is fixed by requiring that the modal gain just compensates for the mirror and internal losses. On substituting eqn. 5
in the formula for K, eqn. 1, changing the integration variable
to « = cosh" 2 (y/y3), and evaluating the integral in terms of
complex gamma functions,9 K becomes
rate of spontaneous emission into one oscillating mode
total rate of spontaneous emission
K =
(5)
K=
(7)
Again, utilising a convenient relationship (see formula 6.1.25,
p. 256 of Reference 9), we obtain
K
~}=[o
[n-Still)?
and it remains to evaluate this product for various combinations of the parameters.
Consider a laser with Tx = 0-5, R^ = R2 = 0-3, L = 400 /im,
internal losses of 20 cm" 1 , Xo = 0-825 /mi, n20 = 3-6 and an
equivalent modal index of 3-45. Furthermore, the antiguiding
factor b is set equal to 3 and abb = 200 cm" 1 . Fig. 1 illustrates
the results for K and the modal near-field full-width halfpower
10
Fig. 1 Spontaneous emission factor K and near-field mode width against
(2)
Vol.17
No. 24
933
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