Development of a Fast Gain Tilt Compensator
by Tatsuya Hatano *, Daeyoul Yoon * and Kazuyou Mizuno *In designing a network architecture to offer greater flexibility and higher reliability
in the wavelength-division multiplexing (WDM) systems of the future, an architecture that uses the optical add-drop multiplexer (OADM) for signal I/O with respect
to the transmission path has been proposed. It is known that changes in the gain of the erbiumdoped fiber amplifier (EDFA) that accompany fluctuation in the signal light input to the OADM
and changes in Raman gain generated in the transmission path occur in the sub-millisecond
order. It is also known that this gain change is gain tilt occurring in the wavelength direction, constituting a factor in the degradation of transmission characteristics due to the power level deviations among signals. We have developed a fast gain tilt compensator that is capable of operationin the microsecond order and uses a lithium niobate (LN) polarization transformer to compensate for the gain tilt due to these gain changes.

ABSTRACTINTRODUCTION

The larger volume and faster speed of data transmission
due to the rapid spread of Internet services has resulted
in active research efforts being devoted to wavelength
division multiplexing (WDM) systems. In designing a network architecture to offer greater flexibility and higher reliability for the wavelength-division multiplexing (WDM)
systems of the future, an architecture that uses the optical
add-drop multiplexer (OADM) for signal I/O with respect
to the transmission path has been proposed.
It is known that changes in the gain of the erbiumdoped fiber amplifier (EDFA) that accompany fluctuationin the signal light input to the OADM and changes inRaman gain generated in the transmission path occur inthe sub-millisecond order 1).
Figure 1 is a block diagram showing a transmission
system with an OADM, from which it may be seen thatwhen the number of signals input to the OADM is reduced
due to fiber breakage etc., there will be a change inRaman gain in the transmission fiber caused by stimulated Raman scattering (SRS). It is known that the gainchange constitutes a gain tilt generated in the wavelength
direction, and it is a factor in the degradation of transmission characteristics due to power level deviations among
signals. A variety of methods have been proposed forachieving gain tilt compensation.
Among these compensation methods are gain tilt
adjustment by controlling the internal pump laser lightsource of the EDFA, flattening of the output power levelby means of a variable optical attenuator, and a technique* Optical Transmission Components Development Dept.,
FITEL-Photonics Lab., R&D Div.

that compensates for the gain tilt using a gain tilt compensator (GTC) that can vary the slope of the wavelengthtransmittance characteristic by means of control parameters, as shown in Figure 2. Of these the GTC seems
promising, due to the fact that is capable of batch compensation of the band used and because the compensaPower level deviation
Before SRS
SRS generation
After gain tilt
generationcompensation
P
P
P

Tx

λ

OADM

Mux

λ

Tx

λ
Demux
Rx
Rx

Fiber cut

Rx

Tx

Rx

Tx

Gain tilt compensator
Transmission fiber

Optical amplifer

Figure 1

Block diagram of a transmission system with an
OADM.

Transmittance1.

Parameter aParameter b
Parameter cWavelength

Figure 2

Change in slope of transmittance characteristics of avariable GTC.

Furukawa Review, No. 28

2005

15

Development of a Fast Gain Tilt Compensatortion width is large, thereby reducing system cost.
As of the present time, techniques proposed to construct a GTC include a method that uses a Faraday rotator 2), and a technique that uses a heating means (heater,etc.) to vary the slope of the wavelength-transmittancecharacteristic of a Mach-Zehnder circuit on a planar waveguide using a material like silica glass etc 3).
Neither, however, has been able to follow gain changes
occurring at the sub-millisecond order. Accordingly, inorder to compensate for the gain tilt due to changes ingain occurring at the sub-millisecond order, we havedeveloped a GTC using a lithium niobate (LN) polarizationtransformer that is capable of operating at the microsecond order.

2.

PRINCIPLE AND CONFIGURATIONIn this section we discuss the principle by which a transmittance slope waveform is generated, and describe an
LN polarization transformer.
2.1 Principle of Transmittance Slope Generation
Figure 3 shows the principle of generation of transmittance slope in this GTC. When linear polarized light in the
X-axis direction impinges on a doubly refracting crystal, aretardation ∆ dependent on the wavelength is generated
between the orthogonally polarized components of theincident light. Figure 4 shows optical output power P for acase in which the light having this retardation ∆ impinges
on the polarizer and only that polarized component of agiven direction is extracted. Figure 4 shows the transmitx

tance characteristics when the input-side polarizer angle
θ shown in Figure 3 is 0, 45 and 90° with a dashed line,
solid line and dot-dash line respectively.
Optical output power P varies sinusoidally with respect
to retardation ∆. Further the amplitude of the sinusoidal
wave can be controlled by changing the state of polarization of light incident on the doubly refracting crystal, making it possible to change the slope centered at the wavelength that is the node. The state of polarization of theincident light is changed by an LN polarization transformer, which is described in Section 2.2 below. The retardation ∆ (wavelength) that constitutes the node in Figure 4
sets the range of retardation ∆ (wavelength) used anddetermines the length of the doubly refracting crystal,
thereby making control possible.
2.2 LN Polarization Transformer
Figure 5 is a schematic diagram of the LN polarizationtransformer used in this GTC. It is made of an X-cut LN
crystal substrate measuring 4.5 × 20.5 × 0.5 mm. As thefigure shows, when a voltage is applied in the X-axis
direction of the LN crystal, the refractive index of the LN
crystal axis changes in accordance with the electro-optical effect. Taking advantage of this it is possible to change
the state of polarization of the incident light that propagates in the Z direction. The LN transformer is fabricatedby depositing an SiO2 buffer layer on the voltage application plane of the LN crystal, and an Au film is depositedon the both faces to make electrodes.
Figure 6 shows the change in the refractive index ellipsoid of an LN polarization transformer under applied voltX

Zz
Optical output
power PyW

Vs

Au electrodes

Y

Polarizer

θ

Buffer layer (SiO2)∆

L

Doubly refracting crystal
PolarizerdCrystal axis
Incident light component

Output optical power strength P (a. u.)

Figure 3

Principle of transmittance slope generation using adoubly refracting crystal.

Incident light

Figure 5

LN crystal

Schematic diagram of LN polarization transformer.
X

1.0

X’

no
θ = 0°π4θ = 45°0.5

θ = 90°1V
γ 22 n o 3
2
d
Y

no
0

0

π

2π

Retardation ∆ (rad)Figure 4

Typical transmittance characteristics obtained bypolarization manipulation using doubly refracting
crystal.

1V
γ 22 n o 32dFigure 6

Y’

Change in the refractive index ellipsoid of an LN
polarization transformer under applied voltage.

Furukawa Review, No. 28

2005

16

Development of a Fast Gain Tilt Compensator

age. Here, assuming that the polarization state of the incident light is the X-axis direction of the LN crystal, when
there is no voltage applied to the LN crystal, the polarization state does not change and the light is output as is.
When there is a voltage applied to the LN crystal, a portion of the incident light is transformed to the Y-axis direction and when the retardation generated by the LN crystal
becomes π, it becomes equivalent to a λ/2 waveplate disposed at an axis angle of 45°, and complete polarizationtransformation from the X-axis angle to the Y-axis angle iscarried out.
The retardation ∆ generated in the two major axis directions is given by∆=

2πV
γ 22 n03 L
λ
d

X

Z

2.3 Configuration
Figure 8 is a schematic showing the structure of the GTC.
The optical components of which it is made are the input
and output collimators, rutile, a λ/2 waveplate (axis angle
22.5°), λ/4 waveplate (axis angle 0°), λ/4 waveplate (axisangle 90°), LN polarization transformer, and a quartz crystal and LN crystal forming a high-order waveplate.

Light path 2

Figure 8

1st crystal

High-order waveplate
Polarization transformer unit

λ /2 WP (-22.5° )Schematic diagram of fast gain tilt compensator(GTC).

1.0
Transmittance (a. u.)

-5
-10
-15
-20
-25

0

Applied voltage (V)
Vπ
Vπ/2
0

0.5

0.0

-30
-35
-150 -100 -50

2nd crystal

The incident light is separated by a polarization beam
splitter into light path 1 and light path 2, both of which arein the same polarization state due to their respective λ/2
waveplates, and by passing through the polarizationtransformer unit are subjected to change in polarization
associated with the applied voltage. Then, after impinging
on the high-order waveplate that generates the sinusoidal
waveform of the transmittance, only a portion of the polarized component is output. The polarization transformer
unit comprises the LN polarization transformer and two
λ/4 waveplates, of which the one whose axial direction istilted at 45° to the X-axis direction is given an applied voltage offset such that when the applied voltage is 0 V, theamount of transmittance slope is 0. The high-order waveplate comprises a combination of crystals of two kinds—
quartz and LN. The ratio of the respective crystal lengths
is optimized, to cancel the temperature characteristics ofthe crystals. And the lengths of light path 1 and light path
2 are the same, in a structure that is so designed thatpolarization mode dispersion (PMD) does not occur.
Figure 9 shows the ideal transmittance characteristicsof the GTC, in which λ1, λ2, λ3, λ4, and λ5 are the wavelengths at which the retardation at the high-order waveplate are -π, -π/2, 0, π/2, and π.
It was so arranged that at wavelength λ2, transmittanceat an applied voltage of 0 V was 1.
Using one part of the C-band wavelengths, we wereable to change the slope of the transmittance characteristic by changing the applied voltage.

0
Output optical power (dBm)

λ /4 WP (45° )Light path 1(1)

where λ represents the wavelength of the incident light,
and by using the characteristic values for the LN crystal(namely electro-optical coefficient γ22 (5.4 pm/V), refractive
index n0 (2.14), thickness d and length L), and taking theapplied voltage as V, we can find the retardation ∆. At awavelength of 1.55 µm, voltage Vπ at which retardation ∆is π will be 366 V.
Figure 7 shows the results of measuring Vπ. In this evaluation a light linearly polarized in the X-axis direction is
input to the LN polarization transformer, and only thecomponent of the output light polarized in the X-axis
direction was transmitted using the polarizer to be measured for its power under changing applied voltage V,
and the applied voltage when the extinction ratio reacheda maximum was taken as Vπ. As can be seen from Figure
7, the extinction ratio reached its maximum at an applied
voltage of 350 V, substantially agreeing with the calculated value. Note that the voltage applied to the prototype
GTC module fabricated in this work ranged from -70 V to
+70 V.

λ1

λ2

λ3

λ4

λ5

Wavelength (µm)

50 100 150 200 250 300 350 400
Applied voltage (V)Figure 7

λ /2 WP (22.5° ) λ /4 WP (90° ) LN polarization transformer

Y

Figure 9

Ideal transmittance characteristics of GTC.

Measured values of the extinction ratio for LN polarization transformer.

Furukawa Review, No. 28

2005

17

Development of a Fast Gain Tilt Compensator

PROTOTYPING RESULTS

Fitting

3.1 Transmittance Characteristics
Figure 10 shows the wavelength vs. transmittance characteristics of the GTC module that we fabricated in thiswork. When the applied voltage was varied to -70 V, 0 V
and +70 V, the slope of the transmittance characteristicchanged, and at a center wavelength of 1553 nm, transmittance was -3.5 dB.

Relationship between Applied Voltage and Slopeof Transmittance Characteristic
Figure 13 shows the relationship between applied voltage
and the slope of the transmittance characteristic for the
prototyped GTC module. At slopes of 0.094 dB/nm and-0.093 dB/nm, the applied voltages were -70.1 V and+68.4 V respectively.
3.4 Response Characteristic
Figure 14 shows values measured for the response of the
wavelength vs. transmittance characteristic to changes inapplied voltage. In the evaluation, the total optical outputof the module was detected by photodiodes and convert0
Applied voltae
0V

Transmittance (dB)

-1

-70V

70V

-2

∆ max
b
Slope = b/a

Figure 12 Definition of slope and ripple of transmittance characteristics.
0.15
0.10
0.05
0.00
-0.05
-0.10
-0.15
-70 -60 -50 -40 -30 -20 -10 0

10 20 30 40 50 60 70

Applied voltage (V)Figure 13 Relationship between applied voltage and slope oftransmittance characteristic for prototyped GTC module.
140
120
100
80
60
40
20
0
-20

-3

a

Ripple = ∆ min+∆ max

Slope (dB/nm)

3.3

∆ min

Applied voltage (V)

3.2 Linearity of Transmittance Characteristic
Figure 11 plots ripple against the slope of the transmittance characteristic for the prototyped GTC module,
where the slope and ripple of the transmittance characteristic are defined as shown in Figure 12.
At a slope of 0.093 dB/nm, ripple was a maximum of
0.36 dB for the prototyped GTC module.

Mesured

0

5

10

15

0
-1
-2
-3
-4
-5
-6
-7
-8
-9
20

Monitor voltage (V)3.

time (µs)-4Figure 14 Response of transmittance characteristic to change
in applied voltage.

-5
-6
1533

1538

1543

1548

1553

1558

1563

1568

1573

Wavelength (nm)Figure 10 Transmittance characteristics of prototyped GTC
module.
0.40
0.35
Ripple (dB)

0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.1 -0.08 -0.06 -0.04 -0.02

00.02 0.04 0.06 0.08

0.1

Slope (dB/nm)

Figure 11 Ripple in transmittance characteristics for prototyped
GTC module.

ed to electrical current, and using a current-voltage converter circuit was monitored as a voltage. The responsetime of the monitored voltage was measured while applied
voltage was varied from 0 to 100 V. We were able to confirm that the monitored voltage followed changes inapplied voltage in a highly responsive manner.
We also determined the amount of change in theapplied voltage and monitored voltage for a given periodof time (0.2 µs), and calculated both the rise time of thevoltage at which the rate of change reached a maximum
and the time at which the voltages became constant. The
time required for the amount of voltage change to reach amaximum was 5.45 and 8.7 µs in the case of applied voltage, and 5.9 and 8.8 µs in the case of monitored voltage.
In terms of response time we may assign a value ofapproximately 0.45 µs.
Table 1 shows calculated values and the results
obtained for the prototyped GTC module. Except for maxi-

Furukawa Review, No. 28

2005

18

Development of a Fast Gain Tilt Compensator

Table 1

Characteristics of prototype GTC module.
Item

Calculated value Measured result

Min. transmittance at center
wavelength (dB)

3.3

Max. ripple (dB)

0.27

0.36

Max. slope voltage (V)

-67

-70.1

Max. slope value (dB/nm)

0.094

0.094

Min. slope voltage (V)

64

68.4

Min. slope value (dB/nm)

3.5-0.093-0.093

Voltage at 0-dB/nm slope (V)

-1

-0.5

PDL (dB)—≤0.1

PMD (ps)0

0.02

Return loss (dB)—>40

Dispersion (ps/nm)—+0.6/-0.42

Transition time (µs)

—

0.45

Overall dimensions W × L × H—

16×72×8.5

mum ripple, the results for the various characteristics
were substantially in agreement with the calculated values.
3.5 Temperature Dependence of Characteristics
Figure 15 shows the temperature dependence of transmittance for the prototyped GTC module. The fluctuations
in transmittance between 1535 and 1573 nm at voltagesof from -70 V to +70 V applied to the module were compared at temperatures of 25, 40, 55, 70, and 80°C and the
maximum fluctuation was plotted. Maximum fluctuation

was 0.2 dB at -70 V. Figure 16 shows the temperaturedependence of the ripple characteristics for the prototyped GTC module. Measurements were made at temperatures of 25, 40, 55, 70, and 80°C. At a slope of -0.1
dB/nm, the change in ripple was 0.02 dB.
Table 2 shows calculated values and the results
obtained for temperature dependence of transmittanceand ripple for the prototyped GTC module. Measurements
were made at temperatures of 25, 40, 55, 70, and 80°C;
calculations were made for temperatures of -5, 10, 25,
40, 55, and 70°C. As Table 2 shows, the measured values
were substantially in agreement with the calculated values.

4.

The characteristics of the GTC module are impacted by
the positioning accuracy of the angle between the crystal
axes of its λ/2 and λ/4 waveplates and the thickness
accuracy of these optical components.
Figures 17 and 18 show the results of simulations of the
distribution of transmittance and ripple characteristics forthe developed GTC module in the case of random errorsof ±1° in axis angle and ±1 nm in thickness for the λ/2
waveplate (axis angle 22.5°) and λ/4 waveplate (axisangle 45°).
In this prototype, positioning was accomplished byobtaining the angle of the crystal axes between the waveTable 2

Temperature dependence of characteristics (dB).

Item evaluatedCalculated value

Measured result

Max. change in ripple

0.015

0.02

0.20

Max. change in loss

0.119

0.2

0.15

400

Frequency

0.10
0.05
0.00
-70 -60 -50 -40 -30 -20 -10 0

10 20 30 40 50 60 70

100

350

Frequency

300

Accumulation

250

60

200
40

150
100

Applied voltage (V)

2

50
0

Figure 15 Temperature dependence of transmittance.

-3.4

-3.3

-3.2

-3.1

-3.0 -2.9

-2.8

0

-2.7

Data section (Transmittance)

0.45

Figure 17 Result of simulation of transmittance.

0.35

450
400

0.30
Frequency

0.25
0.20
0.15
0.10
0.05
0.00
-0.12 -0.10 -0.08 -0.06 -0.04 -0.02

00.02 0.04 0.06 0.08 0.10 0.12

Slope (dB/nm)

Figure 16 Temperature dependence of ripple.

350
300

100

Frequency

80

Accumulation

60

250
200

40

150
100
50
0

20
0.20 0.22

0.24

0.26

0.28

0.30 0.32

0.34

Accumulation (%)

Temperature
25°C
80°C

0.40

Ripple (dB)80

Accumulation (%)

0.25
Maximum fluctuation oftransmittance (dB)DISCUSSION

0

Data section (Ripple)

Figure 18 Result of simulation of ripple.

Furukawa Review, No. 28

2005

19

Development of a Fast Gain Tilt Compensatorplates and one side of their holders. It is estimated that
positioning accuracy of the axis angles will be approximately ±1°. The thickness tolerance for the λ/2 and λ/4
waveplates was taken as ±1 µm.
For the results shown in Table 1, transmittance at thecenter wavelength for the developed GTC module was 2.9
dB, when calculated excluding module loss of 0.60 dB.
This was within the range of transmittance distribution
shown in Figure 17, and is thought to be due to errors in
the positioning accuracy or thickness accuracy of the
optical components. For ripple, on the other hand, themeasured result was 0.36 dB, outside the range for ripple
distribution shown in Figure 18. This is attributed to inadequate accuracy in the positioning of the optical components, which needs to be improved.

5.

REFERENCES
1) P. M. Krummrich, et al., “Compensation of Raman transients in
optical networks”, Technical Digest OFC2004, MF82
2) T. Naito, et al., “Active Gain Compensator in Large-Capacity, LongHaul WDM Transmission System”, Proceedings of OAA’99, Paper
WC53) H. Hatayama, et al., “Variable attenuation slope compensator(VASC) using silica-based planar lightwave circuit technology for
active gain slope control in EDFAs”, Technical Digest OFC2000,
WH74) A. Yariv and P. Yeh, “Optical Waves in Crystals”, (Wiley, New York,1984)

CONCLUSION

To compensate for the millisecond-order gain tilt that isdue to the Raman gain changes in the transmission fiber
caused by stimulated Raman scattering (SRS) and due tochanges in EDFA gain, we have developed a gain tilt
compensator using an LN polarization transformer that is
capable of microsecond-order operation, and we haveconfirmed that its transmittance characteristics andresponse is approximately 0.45 µs.

Furukawa Review, No. 28

200520