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References
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BRILLOUET, F., CLEI, A,, BOUADMA, N., LEFEVRE, R., AZOULAY, R.,
ALEXANDRE, F., and DUHAMEL, N.: ‘GaAs integrated laser and elec-
tronic technology’, Proc. SPIE, 1985,587, p. 164
BRILLOUET, F., RAO, E. v. K., and ALEXANDRE, F.: French Patent No.
8600089, 1986
KUMABE, H., TANAKA, T., NAMIZAKI, H., TAKAMIYA, s., ISHII, M., and
SUSAKI, w.: ‘High temperature single mode cw operation with a
junction up TJS laser’, J p n . J . Appl. Phys., 1979, 18, p. 371
FURUYA, A,, MAKIUCHI, M., WADA, o., FUJII, T., and NOBUHARA, H.:
‘AlGaAs/GaAs lateral injection current MQW laser using impurity disordering’, Jpn. J . Appl. Phys., 1987,26, p. L134
SUZUKI, Y., MUKAI, s., YAJIMA, H., and SATO, T.: ‘Transversejunction
buried heterostructure AlGaAs diode laser’, Electron. Lett., 1987,
23, pp. 3 8 4 3 8 5
RAO, E. v. K., THIBIERGE, H., BRILLOUET, F., ALEXANDRE, F., and
AZOULAY, R . : ‘Disordering of AlGaAs/GaAs quantum well structure by donor sulfur diffusion’, Appl. Phys. Lett., 1985,46, p. 867
TUCKER, R., and KAMINOW, I . : ‘High frequency characteristics of
directly modulated InGaAsP ridge waveguide and buried heterostructure laser’, J . Lightwave Technol., 1984, LT-2, p. 385
semiconductor laser with a 500 pm-long cavity. The end-facets
were antireflection-coated with a single layer of SiO, with one
quarter wavelength thickness. Before AR-coating the lasing
threshold current was 16 mA. Microscope objectives were
used to couple the light into and out of the amplifier, and
optical isolators with approximately 30 dB of isolation were
inserted between the amplifier and the fibre at both ends. The
coupling efficiency between the fibre and the amplifier was
measured to be - 5 7 d B at the input and - 5 3 d B at the
output. The signal light source was an external cavity laser
(ECL) continuously tunable over the whole wavelength range
studied here, and a fibre polarisation controller was used to
keep the input signal TE-polarised. A second ECL was used
as a local oscillator in the noise measurements.
polarisation
controller
nmnlifwr
- r
-
isolator
polarisation
controller
WAVELENGTH DEPENDENCE OF NOISE
FIGURE OF A TRAVELLING-WAVE
GalnAsP/lnP LASER A M P LIFIER
Indexing terms: Semiconductor lasers, Noise
Fig. 1 Experimental set-up
Heterodyne noise measurements on a travelling-wave semiconductor laser amplifier show that the excess noise factor
decreases with increasing wavelength. When the signal wavelength is changed from 1.48pm to 1 . 5 5 ~ the
~ 1 noise figure
decreases from 10.5 dB to 6.0 dB.
Introduction: Semiconductor laser amplifiers are attractive
devices for fibre-optic communication systems. Ideally, every
amplifier will degrade the optical signal to noise ratio (SIN) by
3dB in a system, but for a practical device the degradation is
larger. In direct detection systems using laser amplifiers, the
dominant noise sources are normally spontaneousspontaneous (SP-SP) beat noise and signal-spontaneous beat
noise.’ However, the SP-SP beat noise component can be
suppressed by optical filtering. In a heterodyne system having
a reasonably large local oscillator power, the major noise
source is the local oscillator-spontaneous emission (LO-SP)
beat noise.’ In both these cases the signal to noise ratio will
be given by * 3
’
SIN
=
Pin Gc
2N,,Khv(GC - 1)Bo
The unsaturated amplifier gain was measured at resonant
and antiresonant conditions at several different wavelengths.
At 70mA the maximum amplifier gain was 31.9 dB at 1.51 pm
wavelength. The facet reflectivity of the amplifier was calculated from the measurement of the gain undulation. When the
reflectivity is small ( R < 1%)it is given by
(3)
where R = J ( R , R , ) is the product of the input and output
amplitude reflectivities, and G,,, and Gcminare the resonant
and antiresonant amplifier gains, respectively. By using eqn. 3
the reflectivity is calculated as a function of wavelength and
the result is shown in Fig. 2. The reflectivity is seen to range
from a minimum of 3.6 x
at 1.544pm wavelength to
6x
at 1.48pm wavelength.
To measure the amplifier noise characteristics, the output
from the amplifier was heterodyned with the second ECL in
Fig. 1. The heterodyned signal was detected with a pin
GaInAsP diode followed by commercial high-speed amplifiers
and the signal was observed on a spectrum analyser. Suprim-
where P i , is the input signal power, G , is the amplifier gain,
N,, is the spontaneous emission factor, hv the photon energy,
and Bo the baseband filter bandwidth. When G , $ 1 the excess
noise factor K is given by4
where R , is the input facet reflectivity, G , is the amplifier
single pass gain, r is the optical confinement factor, g is the
gain coefficient, and a is the waveguide loss. The noise figure,
which is defined as the degradation of the optical signal to
noise ratio before and after the amplifier, is given by 2N,,K,
and can be used as a quality factor of the amplifier. A noise
figure of 5 2 d B has been reported by Saitoh and Mukai.’ In
this letter we report on a measurement of the wavelength
dependence of the product N,,K made on a low reflectivity
1.5 pm laser amplifier.
Experimental: The experimental set-up is shown in Fig. 1. The
optical amplifier was made from a GaInAsPIInP CSBH
ELECTRONICS LETTERS 21st January 1988
Vol. 24
No. 2
148
149
150
151
152
153
I54
155
wavelength,pm
Fig. 2 Mirror reflectivity against wavelength
The solid circles are results obtained from the gain measurements
made at 70 mA and the open squares from those made at 60 mA
99
posed on the signal was broadband noise, caused mainly by
SP-SP and LO-SP beat noise. The SP-SP beat noise was
deconvolved by measuring the total noise levels with and
without the local oscillator. The LO shot noise was very small
and therefore neglected. The advantage of this heterodyne
measurement method is that only the true input power has to
be known to give the correct excess noise factor, and the result
is independent of the output coupling efficiency, the gain in
the electrical circuits, and any optical filtering. The carrier to
LO-SP beat noise ratio was measured at three different currents over the whole amplifier spectrum. Eqn. l was used to
calculate the noise enhancing product N,, K, and the result is
given in Fig. 3. N,, K ranges from 2.0 at the long wavelength
side of the amplifier spectrum up to approximately 5.6 at the
short wavelength side, that is the noise figure ranges from
6.0dB to 10.5dB. The noise enhancement is seen to be
roughly independent of the amplifier pumping rate when it is
below threshold, in agreement with the results in Reference 2.
The wavelength dependence of the reflectivity as seen in Fig. 2
has only a minor influence on the K factor. Instead, the variation of N,, K is mainly attributed to the decrease in the waveguide loss and the spontaneous emission factor with
increasing wavelength.
and MUKAI, T.: ‘1 5pm GaInAsP traveling-wave semiconductor laser amplifier’, I E E E J. Quantum Electron., 1987,
QE-23, pp. 101G1020
6 INOUE, K., MUKAI, T., and SAITOH, T.: ‘Gain saturation dependence
on signal wavelength in a travelling-wave semiconductor laser
amplifier’,Electron. Lett., 1987, 23, pp. 328-329
5
GENERATION OF 7 . 8 ELECTRICAL
~ ~
TRANSIENTS ON A MONOLITHIC
NONLINEAR TRANSMISSION LINE
Indexing terms: Transmission lines. Transients, Integrated circuits
Picosecond electrical transients were generated by shockwave formation on a GaAs monolithic nonlinear transmis~ ~ generated by a
sion line. An output fall time of 7 . 8 was
single line driven at 15GHz (20ps input fall time), while
3.7 : 1 compression to 10.1 ps was attained for two cascaded
lines.
6
5
I
4
vi
Q
2 3
2
148
149
1 5 0 151 152 1 5 3
wavelength. p m
155
(384131
154
Fig. 3 Measured excess noise product against wavelength
The triangles are obtained from measurements made with
I = 50mA, the squares with I = 60mA and the circles with
I = 70mA
Discussion: The results above show that for noise considerations it is preferable to use an optical amplifier on the longer
wavelength side of the amplifier spectrum. Another reason for
using this wavelength region is that the amplifier saturation
output power is higher due to the gain peak shift at gain
saturation.6 In our case, at 60mA injection current, the 3dB
saturation output power was measured to increase from
-0.3dBm at 1.49pm wavelength to + 5 3 d B m at 1.55pm
wavelength.
In this letter we have described measurements on a low
reflectivity laser amplifier. The lowest reflectivity was
3.6 x
and the highest gain 31.9dB. The noise figure
2N,,K ranges from 6.0dB to 10.5dB when the signal wavelength changes from 1.55pm to 1.48pm, thus showing that
the long wavelength side of the amplifier gain spectrum will
give the highest signal to noise ratio. N,,K will be further
reduced in a device with a lower waveguide loss a.
M. G. OBERG*
N. A. OLSSON
AT&T Bell Laboratories
Murray Hill, NJ 07974, U S A
SAITOH, T.,
Picosecond pulse generators are central components for widebandwidth time-domain electronic measurements, including
waveform sampling and picosecond metrology, and in highspeed analogue applications. Step-recovery diodes, generating
1 10 V transitions of 35 ps rise time, are used for gating diode
sampling bridges while tunnel diodes, with ~ 0 . V2 transitions
of 25ps rise time, frequently generate the test pulse in timedomain reflectometers. Fast electrical transients can also be
generated by nonlinear wave pr~pagation.’-~Previously, we
had proposed a GaAs monolithic nonlinear transmission line
for compression of a 25 ps input to 4 ps output fall time, and
had demonstrated compressioi: from 5OOps to loops on a
20 : 1 scale modeL4 We now report generation of electrical
waveforms with 7.8 ps fall time on a monolithic structure.
The circuit diagram is shown in Fig. la. A transmission line
with characteristic impedance Z, > SOL! is periodically
loaded, at electrical spacings of T (in units of time), by
Schottky varactor diodes, producing a synthetic transmission
line whose characteristic impedance Z,(u) = ,/[L/CAu)] and
group delay T(u)= n , / [ L C A u ) ] are voltage-dependent. L =
Z , T and C, = r / Z , are the transmission-line inductance and
capacitance per section of line, and n is the number of
diodes in the line. The total capacitance per section is CAu) =
(C,(u) + CJ, where the capacitance of a step-junction diode
with junction potential 4 is C j u ) = Cjo/J(l - u/4).
RL
a
10th November 1987
Schottky
varactor
/diode
4 ~ -;
-;
~
-
* Institute of Microwave Technology, Box 1084, 16421 Kista. Sweden
References
YAMAMOTO, Y.: ‘Noise and error rate performance of semiconductor laser amplifiers in PCM-IM optical transmission system’,
I E E E J . Quantum Electron., 1980, QE-16, pp. 1073-1081
2 OLSSON, N. A,: ‘Heterodyne gain and noise measurement of a
1.5 p n resonant semiconductor laser amplifier’, I E E E J . Quantum
Electron., 1986, QE-22, pp. 671-676
3 GAGLIARDI, R. M., and KARP, s.: ‘Optical communications’ (John
Wiley & Sons, New York, 1976), pp. 173-181
4 HENRY, c. H.: ‘Theory of spontaneous emission noise in open resonators and its application to lasers and optical amplifiers’, J .
Lightwave Technol., 1986, LT-4, pp. 288-297
ohmic
metal
Schottky /
interconnect
metal
/\
1
100
~-
,
C
I
d
Fig. 1 Nonlinear transmission line
a Circuit diagram
b Layout
c Diode top view
d Diode cross-section
ELECTRONICS LETTERS 21st January 1988
Vol. 24
No. 2
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