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range. These values are in reasonable agreement with model
calculations using the effective index approach if nac, = 3.39
and nclad= 3.29 a t 1 = 1.06pm at the active layer thickness
and stripe width mentioned above as measured by SEM. The
almost symmetrical far field results from a proper dimensioning of active layer thickness and stripe width in this example.
It should be noted that the almost symmetrical far-field
pattern with low numerical aperture is advantageous in
obtaining a high coupling efficiency in simple optical systems.
are similar to results obtained from long-wavelength devices,
and indicate good lifetime behaviour.
In conclusion, DCPBH lasers lasing at a wavelength of
1.06pm with relatively low threshold currents, high quantum
efficiencies and a symmetrical far-field radiation pattern have
been fabricated and described. The strong current and light
confinement resulting from the DCPBH structure enables
high-power, room-temperature C W operation and pulsed
output powers of up to 150mW. Overall electrical power to
CW light power conversion efficiencies of up to 20% result,
which constitute a significant improvement over diodepumped N d : Y A G laser^.^ We suggest that these semiconductor DCPBH lasers can replace low-power Nd :YAG laser
devices.
T. VAN DONGEN
17th J u l y 1989
G. L. A. VAN DER HOFSTAD
P. VAN DER LAAR
M. BOERMANS
C. J. VAN DER POEL
Phillips Research Laboratories
PO Box XOOW,5600 J A Eindhoven, The Netherlands
References
1 RISK, w. P., and LENTH, w.: 'Diode laser pumped blue-light source
based on intracavity sum frequency generation', Appl. Phys. Lett.,
1989,54, pp. 789-791
0
60
180
120
300
2L0
I ,mA
IL8912)
Fig. 2 Measured C W L / l curves from uncoated DCPBH at temperatures of5-40°C
Preliminary life-test data have been obtained for 10 devices
by monitoring the threshold current during 500h C W operation at a injection current of 200mA at 30°C (output power
IOmW). N o increase in threshold current is found. The data
2
YORK, P. K., BEERNINK, K . J., FERNANOEZ, G.
E., and COLEMAN,
J. J.:
'GaInAs-GaAs strained-layer quantum well buried heterostructure
lasers (I> 1pm) by metalorganic chemical vapor deposition',
Appl. Phys. Lett., 1989.54, pp. 499-501
3
KAKIMOM, S., TAKEMOTO, A., SAKAKIBARA, Y., NAKAIIMA, Y., FUJIWARA
M.,, NAMILAKI, H., HIGUCHI, H., and YAMAMOTO, Y . : 'Wave-
length dependence of characteristics of 1.2-1.55 pm InGaAsP/InP
psuhstrate buried crescent laser diodes', IEEE J . Quantum Electron., 1988, QE24, pp. 29-34
4 MITO, I., KITAMURA, M., KOBAYASHI, K., and KOBAYASHI, K . : 'Doublechannel planar buried-heterostructure laser diode with etlective
current confinement', Electron. Lett., 1982, 18. pp. 953-954
5 BERGER, 1.. WELCH, D., SCIFRES, 0..STREIFER. W., and CROSS, P.:
'High-power, high-efficient neodymium :yttrium aluminum garnet
laser end pumped by a laser diode array', Appl. Phys. Lett., 1987,
51, pp. 1212-1214
FIRST FIELD DEMONSTRATION OF
OPTICAL SUBMARINE CABLE SYSTEM
USING LD-PUMPED Er-DOPED OPTICAL
FIBRE AMPLIFIER
Indexing terms: Optical communications, Optical transmission,
Opticalfibres, Cables
t
0
l
l
100
'
l
200
T,K
,
This is a report on the first field demonstration of an optical
submarine cable system using an in-line Er-doped optical
fibre amplifier pumped by a 1.47 pm laser diode. The experiment was performed at an ocean depth of 3000m. With a
maximum optical repeater gain of 20dB, 1.2 Gbit/s intensitymodulated optical signals were transmitted stably during and
after installation and also during recovery of the system. Furthermore, the effectiveness of an automatic-gain-control
(AGC) scheme proposed for optical amplifiers was confirmed
in a practical environment.
"
300
m
Fig. 3 Temperature dependence of threshold current of DCPBH-lasers
ai wavelengths of I = 1.06, 1.30 and I.55pm
Stripe widths of lasers are 2.2, 1.8 and 1.6 pm, respectively
0 I = 1.06pm
0 I = 1.30pm
x I = 1.55pm
1278
Introduction: Er-doped optical fibre amplifiers are promising
optical amplifiers, especially for use in 1.5 pm optical telecommunication systems. On the basis of the previous work,'-5
intense effort is now being directed towards implementation of
optical transmission systems using Er-doped optical fibre
ELECTRONICS LETTERS
14th September 1989 Vol. 25
No. 19
of the spontaneous noise in the optical amplifier (see Fig. 1).
The launched pump power was controlled t o stabilise the
output optical signal level. The maximum launched pump
power under control was about 40mW.
amplifiers. In this letter we report the first field demonstration
of an optical submarine cable system using a n LD-pumped
Er-doped optical fibre amplifier.
Experimental s y s t e m : A schematic diagram of the experimen-
tal system is shown in Fig. I. The system consisted of 23.5 km
optical submarine cable and a n optical amplifier repeater, and
was installed at a n ocean depth of 3000m near Hachijo
Island, Japan. The cable consisted of 1.3 pm zero-dispersion
1.5 pm loss-minimised single-mode optical fibres. The fibres
were spliced into each other in the repeater t o make a 47 kmlong repeater system. The maximum tension applied t o the
cable was about 1.5tonne during installation and about
2 tonne while holding the cable. The loss of the loop, including
splicing and connecting, was 11.1 dB. The signal light source
was a DFB-LD emitting at 1.536pm and which was directly
intensity-modulated at 1.2 Gbit/s by a 2'-l pseudorandom
binary sequence with NRZ format. At the receiver end, the
signal light power, pump light power, optical spectrum of the
signal light and bit error rate performance were measured
simultaneously.
Results and discussion: The total connection loss of the amplifier, including splicing and insertion losses of each optical
component, was around 4dB. The input signal level in the
Er-doped fibre was estimated t o be -23dBm. The modulation index of the pilot tone was 2.9%, which produced a net
optical gain (defined as IO log ( P J P J , see Fig. 1) of 14dB. The
maximum net optical gain obtained with AGC was 20dB.
Fig. 2 shows the long-term optical power change during
installation (measured at port A, see Fig. 1). Even though
environmental conditions changed considerably (i.e. temperature range of about 20"C), the output signal power
change was less than I d B with AGC as opposed to 4 d B
25r
[
I
& l o r -
Repeater configuration: The fibre amplifier used in the repeater consisted of a 60m-long Er-doped pure silica optical fibre
(dopant concentration:
100ppm). A forward pumping configuration was adopted. The amplifier was optically pumped
with light from a high-power FP-type L D emitting at 1.47pm
through a polarising beam-splitter (PBS). The incoming signal
light was mixed with the linearly polarised pump light using
an interference lilter (WDM in Fig. 1).
-
polarisation
fluctuation
AGC scheme: For long-term stable operation of a system,
repeaters must have a n AGC function. In this experiment, the
data signal amplitude was slightly sinusoidally modulated at
IOkHz (pilot tone) so that the output optical signal level of
the optical amplifier could be monitored a t the repeater by
extracting the 10 kHz pilot tone from a considerable amount
KDD
time, h
Fig. 2 Stability ofoutput signal power
1
Maru
-3000 m
repeater
4
--
sea
4
+
WMP
E r - doped fibre
6
10 k H z
optical
spect r u m
omlyser
submarine
cable
I
1 high -power
i
+
( 1 47pm)
LD
J
port A
power
meter
optical repeater
"
'
4
monitor
Fig. 1 System diagram
WMP: wavelength multiplexer, WDMP: wavelength demultiplexer, PBS: polarising beam-splitter, PPG: pulse pattern generator, P, = input
signal level at repeater, P,, = output signal level at repeater, FC: fibre coupler, JB: joint box, EB: end box
ELECTRONICS LETTERS
14th September 1989
Vol. 25 No. 19
1219
without AGC. The results show that the AGC worked effectively in a practical environment. As is shown in the inset of
Fig. 2, the short-term output signal power change was quite
small with and without AGC (upper Figure, measured a t port
A), while the state of polarisation of the signal light in the
fibre changed significantly (lower Figure, measured at port B,
3 d B bandwidth of polaristion fluctuation frequency was
about 10 Hz). This stable operation indicates that a n Er-doped
optical fibre amplifier with a polarised pump light source is
less sensitive t o signal polarisation change. It is therefore clear
that pump light source redundancy can be provided using a
PBS in a practical system. Fig. 3 shows the bit error rate
(BER) performance measured before, during and after installation. The input optical signal power Pi was stable during the
experiment. There was a power penalty of around 1.2dB
observed at a BER of lo-’ due to dispersion (0.7dB) and
amplified spontaneous noise (0.5dB). The inset in Fig. 3 shows
Acknowledgment: We thank Mr. K. Furusawa and Mr. H.
Taga of K D D R&D Laboratories for their assistance in the
experiment. We also thank Dr. K. Nosaka, Mr. K. Komuro
and Dr. T. Yamamoto of K D D for their continued encouragement. We are grateful to Kokusai Cable Ship Co. Ltd. for
their help in the experiment, and thank Sinestu Chemical Co.
Ltd. for their co-operation in making Er-doped optical fibres.
Y. NAMIHIRA
K. MOCHIZUKI
H. WAKABAYASHI
-6
K D D Meguro R&D Laboratories
1-23 Nakameguro 2-chome, Meguro-ku, Tokyo 153, Japan
References
7
10 -
and PAYNE, D. N.: ‘Low-noise
erbium-doped tibre amplifier operating at 1.54pm’. Electron. Lett.,
1987,23, pp. 10261028
2 GILES, c . R., DESURVIRE, E., TALMAN, J. R., SIMPSON, J. R., and BACKER,
P. e.: ‘2Gbitis signal aplification at 1 = 1.53pm in an erhiumdoped single-mode fibre amplifier’, J . Lightwaue Technol., 1989,
LT-7, pp. 651456
3 EDAGAWA, N., MOCHIZUKI, K., and WAKABAYASHI, H.: ‘267km,
1.2 Ghit/s optical transmission experiment using two in-line LDpumped Er-doped optical fibre amplifiers and an electroabsorption modulator’. To be presented at IOOC‘89 (int. conf. on
integrated optics and optical fiber commun.), Kobe, Japan, 1989,
21B4-1
W
m
8-
16g-
-do-I1
10 1
6
-35
l
-30
2
1
MEARS, R. I., REEKIE, L., JAUNCEY, I. M.,
4
HAGIMOTO, K., IWATSUKI.,, K., TAKADA, A., NAKAZAWA, M., SARUWATARI, M., AIDA, K., NAKAGAWA, K., and H ~ R I G U C H IM.
, : ‘250km non-
I
-LO
overage received optical signal
power,dBm
repeated transmission experiment at l.SGb/s using LD pumped
Er’+-doped fibre amplifiers in IM/direct detection system’, Electron. Lett., 1989, 25, pp. 662464
Em
Fig. 3 Bit error rate performance
x
6th July 1989
S. YAMAMOTO
10 -
1
Conclusion: We have for the first time demonstrated the use of
a n in situ optical submarine cable system using a n LDpumped Er-doped optical fibre amplifier. The feasibility of an
optical amplifier repeater system was confirmed through this
experiment.
N. EDAGAWA
1 05-
1
the stability of the BER performance during installation. This
stable operation was confirmed even with the considerable
polarisation changes mentioned above. The BER performances before and after installation are depicted using 0
and 0 in Fig. 3, respectively. These results show that the SNR
at the output of the repeater did not degrade significantly
during installation. The BER measured a t a received level of
throughout the experiment.
-34dBm was less than
5
back-to-back
after installation
Inset shows stability of BER performance during installation
OPTICAL HETERODYNE RECEIVER
PROVIDING R A N D O M CHANNEL
SELECTION
Indexing terms: Optical communications, Optical receiuers.
Semiconductor lasers
The letter describes a digitally tuned optical heterodyne
receiver which can randomly access the channels of an FDM
optical communication system. The receiver can identify
missing channels, and is insensitive to variation of the LO
laser frequency relative to the tuning current.
Recently developed tunable monolithic DBR lasers’-3 provide
the possibility t o realise a random access optical heterodyne
receiver4 The result relies on the frequency/tuning current
relationship of the LO laser. It is obtained by storing in the
memory of a computer (controlling the tuning current) a set of
current values corresponding t o the desired set of tuning frequencies. This result requires that the above relationship
remains time-invariant. Unfortunately, this condition is
unlikely t o be satisfied after a few hours of operation. This
1280
WELTER, R., LAMING, R. I., VODHANEL, R. S., SFSA, W. B., MAEDA, M.
w., and WAGNER,
R. E.: ‘Performance of an erbium-doped fibre
amplifier in an 16-channel coherent broadcast network experiment’. CLEO89 (conf. on lasers and electro-optics),Baltimore,
USA, 1989, Postdeadlinepaper PD 22
0 before installation
letter presents a random access digitally tuned optical heterodyne receiver which is insensitive to the drifts of the L O laser
frequency relative to the tuning current. A schematic diagram
of the receiver is shown in Fig. 1. The received F D M signal
consists of six FSK channels at 200Mbit/s spaced by a frequency interval Af of 2.2 GHz. The received signal is heterodyned by a balanced-photomixer at a n I F frequencyf,, of
700 MHz. The resulting IF signal is demodulated by a delaydiscriminator. A fraction of the demodulated signal is used
(after filtering) as the error signal for the AFC circuit. This
signal feeds a servo control circuit which is simply an active
integrator’ with a switch across the capacitance activated by
the computer (Fig. 2). The integrator output signal is added to
the voltage tuning the laser. This connection closes the AFC
feedback loop. Frequency-locking is activated by opening the
switch across the capacitance. For a tuning current i yielding
the L O laser free-running frequency f ( i ) , the laser frequency
varies during the locking process according to
ELECTRONICS LETTERS
14th September 1989
Vol. 25
No. 19
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