Compact Optical Fiber Amplifierswith Fast AGC or for Analog Signal Transmission
by Mikiya Suzuki * , Toshiharu Izumikawa * , Katsuhiko Iwashita * 2 , Kazutaka Shimoosako * 2 ,Shinichi Takashima * 3 , Toru Yoshikawa * 4 , Yasushi Aoyagi * 4 and Masaru Fukushima * 4With the growing use of optical fiber networks Furukawa Electric is working
on the development of a full range of optical fiber amplifiers. In recent years
wavelength-division multiplexing (WDM) systems have been faced with a need for add/drop
capability to allow dynamic change in the number of channels. And the frequent increases
and decreases in channels using the add/drop function brings with it rapid changes in theinput power level of optical fiber amplifiers. This makes it necessary to have amplifiers thatare capable of fast automatic gain control (AGC) to track these rapid changes in input powerlevels. Further, FTTC or FTTH systems (fiber to the curb or home respectively) have toprovide television signal transmission capability in addition to telecommunications. Since TV
transmission is generally by analog signals, a necessary specification for optical fiber amplifiers
for analog transmission is low signal distortion. This paper reports on two types of compact
optical fiber amplifiers that have been developed: the first with fast AGC for transient control, andthe second for analog signal transmission.

ABSTRACT1.

INTRODUCTION

In recent years there has been a need for flexibility in themetro access networks that connect users with backbone
networks to allow rapid modification of transmission
capacity in response to user needs. It is therefore hopedthat it will be possible to adapt wavelength-divisionmultiplexing (WDM) systems to new technologies that use
dynamic change in the number of channels in configuring
optical fiber networks. This requires the introduction of
optical add/drop multiplexers (OADMs), devices that canadd or drop any desired channel from the multiplexed
signal, and erbium-doped fiber amplifiers (EDFA) tocompensate for the loss of optical components. Frequentadding or dropping of channels using an OADM resultsin rapid changes in the input power level to the opticalfiber amplifier. If the response time of automatic gain
control (AGC) for the optical fiber amplifier is slow, it
will be unable to keep up with the rapid changes in
input power level, giving rise to transient changes inoutput power level. Since changes in output power
result in degradation of transmission quality (systemperformance), it is necessary the AGC be fast enough tosuppress these output excursions.
* Optical Components Dept., FITEL Products Div.
* 2 Transport Products Dept., Network Div.
* 3 Engineering Dept., Network Div.
* 4 FITEL Network Lab., R&D Div.

Fiber-to-the-curb or -to-the-home systems (FTTC orFTTH respectively) are expected to provide TV signaltransmission capability in addition to telecommunications,and these TV signal transmission systems also need
optical fiber amplifiers to compensate for losses in the
optical components of the distribution system. Since TV
transmission is generally by analog signals, the needfor optical fiber amplifiers with low signal distortion isparticularly acute. This is because if signal distortion isgreat, picture quality on the TV receiver will be degraded.
Equipment must also be suitable for outdoor installation(-20 to 75°C) This paper reports on two types of compactoptical fiber amplifiers that have been developed: the firstwith fast AGC for the control of transients, and the second
for analog signal transmission.

2.

COMPACT OPTICAL FIBER AMPLIFIERWITH FAST AGC

2.1 Configuration
Figure 1 and Table 1 show the full-optics configurationof a compact optical fiber amplifier with fast AGC. The
outline dimensions adopted are 150×125×20 mm(without the heatsink), the de facto standard. Figure 2
shows the appearance. This compact package houses
laser diodes, photodiodes, variable optical attenuator(VOA), erbium-doped fiber and an EDF heater to reduce
the temperature dependence of the EDF, and a printed

Furukawa Review, No. 25

2004

19

Compact Optical Fiber Amplifiers with Fast AGC or for Analog Signal Transmission��� ���������� ������������ ����Table 1��� ���������� ������������� ������� ����������� �������

Figure 1��� ����������

������������������

���

���

Full-optics configuration of compact optical fiber amplifier with fast AGC.

Full-optics configuration of compact optical fiber
amplifier with fast AGC.
����������������������� �� ����� ������������� �� ������������������ �� �������� ������������������ ������ �������������� ������� �� ��� ����������������������� �� �������� ���������circuit board for the control circuitry. There is a mid-stage
access (MSA) I/O optical port for the insertion of opticalcomponents that give rise to MSA loss, together with a25-pin D-sub, the de facto standard electrical connector.
Pulse width modulation (PWM) is adopted as thecontrol method for the laser diodes, thermoelectric cooler(TEC) and EDF heater, effecting a reduction in power
consumption. A single power supply with a voltage of+5 V is used, and serial communication to the outside of
the optical fiber amplifier is provided by RS232:TTL level.
Alarm functions include case temperature alarm, loss ofoutput power alarm, pump bias alarm, and loss of input
power alarm. Monitoring of the optical input and output
power levels is also supported. There is also a control
function from outside the optical fiber amplifier, plus
optical output shutdown and automatic power reduction(APR) to lower the output to a given level. External control
is also possible using the RS232 serial communication
port.
A digital system is used for the control of the laser
diodes etc., and can be adapted flexibly to changes in thedesired specifications by modifications in firmware. The
EDF transfer function in particular undergoes significant
change according to operating conditions, and the AGC
control parameters can be changed dynamically. The
presence of a hot swap function means that firmware
updates can be carried out even while the optical fiber
amplifier is in network service. The optical configurationof the optical fiber amplifier may differ in various ways
depending on the final user, but can be addressed byfirmware settings alone, with no need to modify thephysical circuitry. Here we have developed high-speed
digital circuitry and a proprietary control algorithm,
achieving fast AGC and realizing outstanding transientperformance.

Figure 2Appearance of compact optical fiber amplifier.

2.2 Transient Performance
The use of an OADM to effect add/drop of any desiredchannel of a WDM signal results in rapid fluctuationsin the optical fiber amplifier input power level. If theresponse time of automatic gain control (AGC) for the
optical fiber amplifier is slow, transient changes in output
power level will occur. If these changes are large there
will be significant effect on transmission quality (systemperformance), causing a degradation of the bit error
rate. Indicators of transient performance include gain
excursion, settling time and gain error. A gain excursion is
the maximum value of the change in optical fiber amplifier
gain during a rapid change in its input power level, also
referred to as overshoot or undershoot. Settling time istransient suppression time--the time until gain stabilizes.
Gain error is gain offset--the difference between the gain
before the rapid change in input level and the gain aftersettling.
We therefore, in this work, made rapid changes in
the input level of a compact optical fiber amplifier withAGC, and measured the gain transient performance. The
increases or decreases in input level that were adopted
were 6 dB (equivalent to the add/drop of from 1 to 4channels) and 15 dB (equivalent to the add/drop of from
1 to 32 channels).
Figure 3 shows transient performance at the 6-dB add
condition, with the upper waveform representing thetotal input power level and the lower representing theoutput power at one channel. Figures 4 through 6 show
analogous waveforms. The measuring conditions are:mid-stage access (MSA) loss of 10 dB, output power levelof 19 dBm, gain of 26 dB, and input power level add/drop
through-rate of 0.1 µ s or less. It was possible to obtain again excursion of 0.5 dB, a settling time of 20 µ s, and again error after settling of 0.2 dB. Figure 4 shows transient

Furukawa Review, No. 252004

20

Compact Optical Fiber Amplifiers with Fast AGC or for Analog Signal Transmission����������������������������Figure 3Transients at 6-dB add condition.

Figure 5

Transients at 15-dB add condition.

����������
����
����
������������

Figure 4

Transients at 6-dB drop condition.

performance at the 6-dB drop condition. The measuringconditions were the same as for the add condition, witha gain excursion of 0.7 dB, a settling time of 70 µ s, and again error after settling of 0.2 dB. Figure 5 shows transient
performance at the 15-dB add condition. The measuringconditions were the same as for the 6-dB add/drop add
condition, with a gain excursion of 0.7 dB, a settling timeof 70 µ s, and a gain error after settling of approximately
0 dB. Figure 6 shows transient performance at the 15-dB
drop condition. The measuring conditions were the same
as for the add condition, with a gain excursion of 0.8 dB,
a settling time of 50 µ s, and a gain error after settling ofapproximately 0 dB.
These characteristics are summarized in Table 2,demonstrating outstanding transient performance.

3.

Figure 6

Table 2

Transient performance of compact optical fiber
amplifier with fast AGC.

���� �� ����������������������� ������������ ���������������������� ������ ���� � ���� ������ ������� ���� � ���� ������� ������ ���� � �� ������� ������� ���� � �� �������� ������� ���������� ������� �������������������� ��������������� ������������ ���������� ����������

Figure 7Full-optics configuration of compact optical fiber
amplifier for analog signal transmission.

Table 3Full-optics configuration of compact optical fiber
amplifier for analog signal transmission.

COMPACT OPTICAL FIBER AMPLIFIERFOR ANALOG SIGNAL TRANSMISSION

3.1 Configuration
Figure 7 and Table 3 show the full-optics configurationof a compact optical fiber amplifier for analog signaltransmission. The outline dimensions adopted are 150×125×20 mm (without the heatsink), the same as for thecompact optical fiber amplifier with AGC. This compact
package houses laser diodes, photodiodes, couplers,and a printed circuit board for the control circuitry. Also asTransients at 15-dB drop condition.

����������������������� �� ����� ������������� �� ������������

������ ������� �� ��� �������������

���������� �� �������� ���������

Furukawa Review, No. 25

2004

21

Compact Optical Fiber Amplifiers with Fast AGC or for Analog Signal Transmission

3.2 Optical Characteristics
Among the factors contributing to the degradation ofpicture quality in analog TV transmission are interference
due to noise and signal distortion generated by theoptical fiber amplifier. The noise figure (NF) is one
index representing amplifier noise. Amplifier noise issuperimposed on the picture signal, so that when NF islarge noise appears in the form of “snow” resulting in
an indistinct picture. Indices of signal distortion include
composite second-order (CSO) and composite triple-beat(CTB), both of which have a significant effect on picture
quality.
Accordingly we measured NF, CSO and CTB for
the compact optical fiber amplifier for analog signaltransmission. Figure 8 shows the noise figure ofthe compact optical fiber amplifier for analog signaltransmission. It plots NF against wavelength for input
power levels of 0 and -10 dB when the output power
level is +24 dBm. At operating temperatures from -20 to+65°C and signal wavelengths from 1540 to 1560 nm, anexcellent characteristics was obtained, with an NF of 4.93
dB or less at an input power of 0 dBm and 4.51 dB at -10dBm.
Figure 9 gives the CSO and CTB distortion
characteristics. It shows the characteristics for a 5-stage
cascade of compact optical fiber amplifiers for analog
signal transmission. After the 5-stage cascade it wasconfirmed that CSO was -72.5 dB and CTB was -71.2
dB.
In addition we carried out a subjective assessmentof picture quality using actual picture signals over atransmission line with a 5-stage cascade of optical fiber
amplifiers for analog signal transmission. In confirming
picture quality on the TV receiver after the 5-stage
cascade, good picture quality was obtained, with nopicture quality degradation found due to the 5-stagecascade.

���� ��� ���� ���������
���������������������������������

���� � ��������� ������ ���������������� � ��������� ������ ����������������� � �����

����� �������������� �������� ������������

���� ������ ������������

���� ������

������� ������ ��������

������������������
���������� ������������� �����
����� ���� ��������� ������� ��� ������ �������� ��������� ���� �����

���������
������

����

����

����

����

���������� ����Noise figure of compact optical fiber amplifier
for analog signal transmission.

���������� ��� ��������

���

����

����� � ��� �����

Performance of compact optical fiber amplifier
for analog signal transmission.

����������� ���� ��������� ��� �������� ��� ���� ��������Figure 8Table 4� ���� ��������� ��� ���� ������� ���� ����� � ��� ���in the optical fiber amplifier with fast AGC, the electricalconnector is a 25-pin D-sub.
The system used for the control of the laser diodes
etc. is digital, as in the case of the compact optical fiber
amplifier with fast AGC, and can be adapted flexibly tochanges in the desired specifications by modifications
in firmware. The following functions are also similar to
those of the compact optical fiber amplifier with fast AGC:
a single power supply with a voltage of +5 V is used,and serial communication to the outside is provided
by RS232:TTL level. Alarm functions include casetemperature alarm, loss of output power alarm, pumpbias alarm, and loss of input power alarm. Monitoringof the optical input and output power levels is also
supported. There is also a control function from outside
the optical fiber amplifier, plus optical output shutdown
and automatic power reduction (APR) to lower the output
to a given level. External control is also possible using
the RS232 serial communication port. Table 4 shows theoptical and electrical performance.
The amplifier can mount 980-nm multimode laser
diodes of the 2-W class, making itself possible to be used
as a clad-pump optical fiber amplifier with an optical
output of up to +31 dBm maximum.

���������
������������

����������� �� �������� �����Figure 9 CSO and CTB distortion in compact optical fiber
amplifier for analog signal transmission.

Furukawa Review, No. 25

2004

22

Compact Optical Fiber Amplifiers with Fast AGC or for Analog Signal Transmission

4.

CONCLUSION

As compact optical fiber amplifier platforms for WDM
transmission systems and for analog signal transmission,
we have developed:
1) a compact optical fiber amplifier with AGC capable offast transient control; and2) a compact optical fiber amplifier for analog signaltransmission.
It will be possible, using these platforms, to addressa variety of needs for compact optical fiber amplifiers
through optimization of the optics configuration andfirmware.

REFERENCES
1) Suzuki et al., Society Conference of IEICE, B-10-105, p.424 (2003)(in Japanese)2) Iketani et al., General Conference of IEICE, B-10-142, p.572 (2003)(in Japanese)
3) CATV Technical Committee of JEITA: Introduction to Cable TV
Technology, Corona Publisher (1994) (in Japanese)

Furukawa Review, No. 25

200423