Development of a High-Precision DOP Measuring Instrument
by Tatsuya Hatano *, Takeshi Takagi *, Kazuhiro Ikeda * and Hiroshi Matsuura *In response to the need for higher speed and greater capacity in optical communication, studies are being carried out on high-speed digital transmission technologies, but at rates of 40 Gbps specifically, the deterioration in transmission characteristics dueto the effect of polarization mode dispersion (PMD) has led to a need for PMD compensators. It isknown that the amount of PMD and the degree of polarization (DOP) are interrelated, so that it ispossible to compensate for PMD by monitoring the DOP of the signal light. Accordingly, in aneffort to devise a PMD monitor for PMD compensators, we have developed a high-precision DOP
measuring instrument that is low in cost and accommodated in a compact package. This paper
reports on the results obtained by producing a prototype of this instrument.

ABSTRACT

1.

Incident light

INTRODUCTION

Polarization controller

In response to the need for higher speed and greater
capacity in optical communication, studies are being carried out on high-speed digital transmission technologies.
Deterioration in transmission characteristics (bit error rate,or BER) due to the effect of polarization mode dispersion(PMD) is not a significant problem at the bit rates usedheretofore, but at 40 Gbps specifically, this has led to aneed for PMD compensators. PMD occurs as a result of adifference in group delay between two orthogonal polarization modes due to increased ellipticity of the singlemode fiber core, lateral pressure or localized temperaturechange. A PMD compensator is a device that compensates for the effects of PMD occurring in the transmissionpath, and to do this the amount of PMD must be monitored. Since in general there is a correlation between
PMD and DOP, a DOP monitor may be considered a PMDmonitor. Figure 1 shows an example of PMD compensation.
In a 10-Gbps transmission testing system, PMD wasproduced by actually using a variable DGD, and the DOPof the signal light was measured. At 10 Gbps the timeinterval between adjacent bits is 100 psec, but even whena PMD of 50 psec--half that value--is applied, DOP is in
the area of 80 %, not much of a deterioration. The BER,
however, does deteriorate significantly, and so in considering its use to improve BER as the PMD monitor of thePMD compensator, it is necessary that DOP be measured
with high precision at conditions when it is greater than 80%. Accordingly the authors have developed a compact,
high-performance, low-cost DOP measuring instrument asa PMD monitor for a PMD compensator, and report on itherewith.
*

Optical Components Development Dept., FITEL-PhotonicsLab.

DGD emulator

DOP monitor

RxFeedback

Figure 12.

Example of PMD compensation.

OPERATING PRINCIPLE OF HIGH-PRECISION DOP MEASURING INSTRUMENT

The high-precision DOP measuring instrument represents
the state of polarization of incident light by Stokes parameters (S0, S1, S2, S3) that are defined by Equation (1),and outputs S0, S1 and S2. The Stokes parameters are calculated by using, together with Equation (2), four values ofcurrent (It, I0, I45, Iq45) that are output from the structure
described in the following.
In this, the expressions indicating the electrical currents
obtained by photoelectric conversion of the light incident
on the DOP measuring instrument are: It for the totality of
all incident light, I0 for the light of one of the four split
beams of incident light that has been passed through a 0deg polarizer, I 45 for the light that has been passed
through a 45-deg polarizer, and I q45 for light passed
through the 1/4 wave plate and 45-deg polarizer. Here therelative angle between the crystal axes of the 0-deg and45-deg polarizers is 45 deg, and the setting for the reference angle is arbitrary.
Following is an explanation of Poincare sphere representation. This is used because, as all states of polarization (SOP) are represented in a single coordinate system,

Furukawa Review, No. 23

200339Development of a High-Precision DOP Measuring Instrument

lcp

Optical section
Optical splitter

Electrical circuitry

Photodiode

GPIB interface

-45°c

V2α2ω

H

Incident
light+45°

pA meterP

1/4 wave plate0-deg polarizer
rcp

Figure 2Figure 3

Poincare sphere representation.

CPULCD

45-deg polarizer

Schematic diagram of high-precision DOP measuring
instrument.

it makes it easier to comprehend changes in the SOP.
Figure 2 shows a Poincare sphere representation in which
the axes of the rectangular coordinates are defined as S1for the horizontal linearly polarized component, S2 for thelinearly polarized component at 45 deg from the horizontal, and S3 for the right-handed circularly polarized component.
S0 = < Ex 2 > + < Ey 2 >S1 = < Ex 2 > - < Ey 2 >S2 = < 2 . Ex . Ey . cosδ >S3 = < 2 . Ex . Ey . sinδ >(1)

where: δ = Φ y - Φ x
S0 is the total intensity
S1 is the horizontal linearly polarized component
S2 is the linearly polarized component at 45 degfrom the horizontal
S3 is the right-handed circularly polarized component
S0 = ItS1 = 2 . I0 - It
S2 = 2 . I45 - It
S3 = 2 . Iq45 - It

(2)

DOP = {(S12 + S22 + S32 )1/2}/S0(3)Figure 3 is a schematic diagram of the high-precision
DOP measuring instrument, which is divided into the optical section and the electrical circuitry.
The optical section comprises an optical fiber collimator(not shown), an optical splitter, polarizers, and four photodiodes (PD), enclosed in a modular package measuring
70 mm in length, 26 mm in width, and 8 mm in height(Figure 4).
From their definitions, the Stokes parameters can becalculated based on measurements of the intensity of thefour transmitted light beams. The following description of
the optical section and electrical circuitry is based onFigure 3.
The incident light is fed by the optical fiber collimator tothe splitter, where it is split into four. The first of these isreceived directly by the photodiode resulting in the output
It. The second passes through the polarizer having a crystal angle of 0 deg and impinges on the photodiode resulting in output I0. The third passes through the polarizer

Figure 4Appearance of Stokes analyzer module.

having a crystal angle of 45 deg and impinges on the photodiode resulting in output I45. The fourth passes throughthe 1/4 wave plate and the polarizer having a crystal angleof 45 deg and impinges on the photodiode, resulting inoutput Iq45. At this point the crystal axis of the polarizer
that is at a 45-deg angle to that of the 0-deg polarizer isrotated by an angle of 45 deg in a plane perpendicular tothe direction of travel of the incident light. The crystal axisof the 1/4 wave plate agrees with that of the 0-deg polarizer.
The four current outputs described above (It, I0, I45 andIq45) are then measured by the pA meter on the electricalside. The CPU performs the calculations shown inEquations (2) and (3) to find the Stokes parameters andDOP, and the results are output from the GPIB interface
and displayed on the LCD panel.
The electrical circuitry primarily comprises a 32-bit CPU,EPROM, GPIB controller, and equipment for measuring
minute currents (a pA meter with 4-channel measuring
capability, minimum resolution of 100 pA, maximum current of 1 mA and maximum sampling rate of 1 msec).
These elements are mounted on a circuit board measuring 257 mm in length and 182 mm in width. To further
economize on space, the optical section module is also
mounted on the same circuit board. The board on which
the optical module is mounted is enclosed in a casing,shown in Figure 5, together with a power supply, LCD
panel, and various setting switches.

Furukawa Review, No. 23

200340Development of a High-Precision DOP Measuring Instrument

15SOP (deg)129630020

40

6080100120140160

Displacement in polarizer crystal axis angle (min)Figure 5

Figure 8

Appearance of high-precision DOP measuring instrument.

Simulation result 2 (on SOP).

1.2
1.15
1.1

ASSEMBLYIn assembling the optical section of this instrument theprecision with which the crystal axes of its 1/4 wave plate
and the 0-deg and 45-deg polarizers has a major impact
on DOP precision.
We therefore conducted a simulation of the effect on
SOP and DOP precision of a displacement in the rotational angle of the 1/4 wave plate, in a case in which there
was no displacement in the crystal axis angles of two 45deg polarizers and a 10-minute displacement between thecrystal axes of a 0-deg polarizer and 45-deg polarizer.
The results are shown in Figures 6 and 7.
We also conducted a simulation of the effect on SOPand DOP precision of a displacement in the crystal axesof a 0-deg polarizer and 45-deg polarizer, in a case inwhich there was no displacement in the crystal axis
angles of two 45-deg polarizers and a 20-minute displace-

15SOP (deg)129630020406080100120140160

180

Displacement in rotational angle of 1/4 wave plate (min)Figure 6DOP

3.

1.05
10.95
0.9
0.85
0.8
020406080

100120140160

Displacement in polarizer crystal axis angle (min)Figure 9

Light source

Simulation result 2 (on DOP).

Polarizer

1/4 waveplate

1/2 waveplateθ0

θ1

θ2

DOP measuring
instrument

λ=1550nm

Figure 10 Measurement system for evaluation.

ment in the rotational angle of the 1/4 wave plate. The
results are shown in Figures 8 and 9.
As a result of these simulations it was found that toachieve a DOP precision of ±1 %, the displacement must
be within 20 min for the rotational angle of the 1/4 wave
plate and within 10 min for polarizer angle. The method ofalignment currently in use involves determining the angleof the crystal axis with respect to the 1/4 wave plate andpolarizer holders, with respect to the configurations of therespective components.

Simulation result 1 (on SOP).

4.

1.2

EVALUATION

1.15
1.1

DOP1.05
10.95
0.9
0.85
0.8020406080100120

140160

Displacement in rotational angle of 1/4 wave plate (min)Figure 7

Simulation result 1 (on DOP).

180

Direction and size are both related to Stokes parameter
precision, and the evaluation was performed as shown in
Figure 10. With respect to SOP, which indicates direction,
the 1/4 wave plate and 1/2 wave plate were rotatedsequentially by angles θ1 and θ2 from a certain Stokes
vector reference state (1, 0, 0), and a comparison wasmade between the rotational angles on a Poincare spherecalculated from angles θ1 and θ2 and the SOP rotation
angles measured on a Poincare sphere using the DOP
measuring instrument developed here. With respect toFurukawa Review, No. 23

200341

Development of a High-Precision DOP Measuring Instrument

Variable-wavelength
light source

DOP measuring
instrument

DUT

Controller computer

Figure 11 Changes in measured Stokes parameter.

Figure 12 PMD measuring system.
Table 1

Performance of developed DOP measuring instrument.

Measured wavelengths1480~1640 nm

Input power-35 to 0 dBm

Sampling time

8 msec (typ)

DOP precision± 2.5 %

SOP precision± 2 deg

InterfaceGPIBPower consumption

7.5 W

Power supply

100 VAC

Overall dimensions
Output data

210 × 99 × 350 mm

Figure 13 Program for PMD measurement.

Stokes parameter, DOP
10

PMD (psec)

86
4
20
1547

1548

1549

1550

1551

1552

1553

Wavelength (nm)Figure 14 Results of PMD measurements using 9.5-m PMF.

7060

PMD (psec)

DOP, which represents size, after the state of polarizationof the light emitted by the light source (DOP = 1) rotatedon a Poincare sphere, the DOP was measured and wascompared with the original DOP that is unity, as shown inFigure 11.
Figure 11 shows the changes, represented on aPoincare sphere, in a Stokes parameter measured by theDOP measuring instrument when the 1/4 wave plate rotational angle θ1 shown in Figure 10 was set at 0 deg, ±10deg, ±20 deg, ±30 deg, ±40 deg, and ±45 deg, and the1/2 wave plate rotational angle θ2 was increased from 0deg to 90 deg in 10-deg steps.
The Stokes parameter obtained by the DOP measuring
instrument developed here and the angular displacementwith respect to the calculated values found from angles θ1and θ2 of the crystal axes of the 1/4 wave plate and 1/2
wave plate respectively were from +1.2 deg to -2.0 deg,
and the results of DOP measurement were from 0.973 to1.016. Table 1 shows the performance of the DOP measuring instrument developed here.

5040
30
20
100
1547

1547.21547.41547.6

1547.81548

Wavelength (nm)Figure 15 Results of PMD measurements using 50-m PMF.

5.

APPLICATIONS

As a typical application for the DOP measuring instrument
developed here, we designed a PMD measuring system.
As shown in Figure 12 it comprises a variable-wavelength
light source, the device under test (DUT), the DOP measuring instrument and a controller computer. Using theprogram shown in Figure 13, measurements were madeby the Poincare sphere method, in which PMD is determined by applying a 360-deg phase difference to a cycleof the path traced by SOP on the sphere. By means of thewavelengths at any two arbitrary points on the Poincare

sphere and the rotational angles of the corresponding
Stokes vectors, PMD, ∆τ may be calculated using
Equation (4). Polarization maintaining fibers (PMFs) withlengths of 9.5 m and 50 m were used as the DUT.
The results of these measurements are shown inFigures 14 and 15, and Table 2.

∆τ = (∆Φ/360).(λn.λn+1/C .(λn+1 - λn))(4)where: ∆Φ is the rotational angle on the Poincare
sphere of the Stokes vectors corresponding to
two wavelengths (deg)Furukawa Review, No. 23

2003

42

Development of a High-Precision DOP Measuring Instrument

Table 2Comparison of values of PMD using different measuring instruments.
PMD (psec)

DUT

By Furukawa

By other manufacturer'sinstrument

instrument

1st try

9.70

9.37

2nd try

9.66

9.46

1st try

62.28

63.32

2nd try

62.20

63.45

9.5-m PMF

50-m PMF

100

DOP (%)

9590
85
80

C is the speed of light (3.0×108 m/s)
λn is the light source wavelengthn is the n-th wavelength
Another promising application of the DOP measuring
instrument developed here is as the monitor in a PMD
compensating system. Figure 16 shows changes in DOP
with respect to DGD. In the measuring system a 10-Gbps
signal is input sequentially to a polarization controller anda variable DGD, and then goes to the DOP measuring
instrument for measurement. There is a correlation
between DGD and DOP, and a PMD compensator finds
applications as a PMD monitor. Figure 17 shows theappearance of a PMD compensator.
The incident light is passed through the polarization
controller, PMF and optical coupler, and then output. The
beams split by the optical coupler are received by theStokes analyzer module, DOP is monitored and applied
as feedback to the polarization controller.

757001020304050607080

DGD (psec)

Figure 16 Change in DOP with respect to DGD.

6.

CONCLUSION

A low-cost, high-precision DOP measuring instrument has
been developed, comprising a Stokes analyzer module
having an optical fiber collimator, optical splitter, 1/4 wave
plate, polarizers and photodiodes, which, together with theelectrical circuitry, comprising a 32-bit CPU, EPROM,
GPIB controller and equipment for measuring minute currents, is mounted on a single circuit board, and packaged
with the power supply, LCD panel, setting switches, etc. ina compact unit casing. It is expected to be in demand as a
PMD monitor for PMD compensators.

Figure 17 Appearance of PMD compensator.

Furukawa Review, No. 23200343