Development of a Crystal-Type Depolarizer
by Hajime Kazami *, Shun-ichi Matsushita *2, Yoshihiro Emori *2, Takashi Murase *3,
Masamichi Tsuyuki *4, Kunihiro Yamamoto *4, Hiroshi Matsuura *3, Shu Namiki *2and Takaaki Shiba *3

Fiber Raman amplifier (hereafter called Raman amplifier) is becoming popular for
its superiority in characteristics. Because the Raman amplifier has a large polarization dependence of Raman gain, a number of designs are being proposed to reduce the influence, and a device known as depolarizer plays an important role among those designs. A depolarizer based on a 45° spliced polarization-maintaining fiber (PMF) is already in practical application. However, this depolarizer has problems such as, besides the necessity for large accommodation space, unsteady Raman gain due to the deterioration of degree of polarization (DOP),
which is caused by temperature fluctuations in the amplifier. Accordingly, the authors have developed a crystal-type depolarizer recently, and succeeded in realizing miniaturization of the device
as well as reduction of the temperature dependence of DOP. It is expected that this development
help to promote popularization of Raman amplifiers as well as demand increase hereafter.

ABSTRACT

1.

FUNCTION OF DEPOLARIZER

Depolarizer is an optical device that can convert, as illustrated in Figure 1, any incident light that is completely or
partially polarized into unplorized light.
When the end point of its electric field vector representswith respect to time a line, a circle, or an ellipse, then thelight is defined as completely polarized.
Unpolarized light is comprised of an even distribution of
all sorts of polarized light, and partially polarized lightrefers to a mixture of the completely polarized light andthe unpolarized light.
DOP is used to represent the light amount ratio of completely polarized light of certain light with respect to its
total light amount. Therefore, the DOP for completely
polarized light and unpolarized light are 100 % and 0 %,respectively, and the DOP for partially polarized light
ranges between 0 % and 100 %.

2.

PROBLEMS WITH CONVENTIONALTECHNOLOGIES

Depolarizers for optical communications based on PMF
are already in practical application. In this technology, an
all-fiber depolarizer of Lyot-type is extensively used 1), 2), in

which two lengths of PMF with a ratio of 1:2 in length arespliced together to have a 45° between the polarization
axes of the fibers.
But the all-fiber depolarizer of Lyot-type has two problems: one is deterioration in DOP due to temperaturechanges and the other is related with accommodation
space.
The deterioration of DOP due to temperature changes isprobably caused by the changes in the state of polarization of the light that is incident on the depolarizer under
the influence of temperature changes in the amplifier.
Figure 2 shows the DOP changes in a conventional depolarizer when the temperature is changed between -10°C
and 70°C. It is seen that the DOP changes between 4 %and 15 % maximum.
With regard to accommodation space, irrespective of
the increasing requirement for saving of the space to promote size reduction of amplifiers, all-fiber depolarizers
need a large accommodation space since they use a fiber
as long as more than 20 m, which is subject to allowablebending radius. When a heat-insulating package is used
to decrease the temperature changes in DOP, space saving becomes more difficult.

Completely
polarized light
Crystal*Okano Electric Wire Co., Ltd.
*2 Optical Transmission Sub-Systems Development Dept.,
FITEL-Photonics Lab.
*3 Optical Components Development Dept., FITEL-Photonics
Lab.
*4 Optical Components Dept., FITEL Products Div.

Figure 1

Unpolarized
light

Schematic of depolarizer.

Furukawa Review, No. 23

2003

44

Development of a Crystal-Type Depolarizer

100

60

15

40

10

20

5

0

20

40

25
20

60

15

40

10

20

0

5

0

-20

0

-20
0

60

20

40

Time (min)Temperature dependence of DOP of conventional
depolarizer.

Table 1Figure 3

Target characteristics of depolarizer.
Max.

Unit

Operating temperature

-10

70

°C

Operating humidity

0

85

%RH

Insertion loss

0.80

dB

@Top

DOP

7

%

@Top

dB

@Top

Package dimension

50
φ5.5✕ 60

Temperature dependence of DOP of a depolarizer
equipped with polarizer.

0

Min.

Return loss

60

Time (min)Remarksmm

λ6-5
Optical Power (dBm)

Figure 2

80

Temp.

20

0

DOP

Temp. (°C)

DOP (%)80

DOP (%)

DOP
Temp.

25

100

30

Temp. (°C)

30

δλ

λ5
-10

λ3

-15

-20

FWHM

λ4

λ2
λ1

-25
Wavelength

3.

FEATURES OF DEVELOPED PRODUCTAND TARGET CHARACTERISTICS

The depolarizer developed this time has the following features to solve the problems mentioned above.
With respect to DOP deterioration, the depolarizer was
designed to provide a polarizer in front of a birefringent
crystal to stabilize the state of polarization of the incident
light, aiming at a significant reduction in the deteriorationof DOP.
With regard to accommodation space, a birefringent
crystal was used in the design since a significant spacesaving by all-fiber depolarizers was found to be difficult.
Moreover, through optimization of the length of the birefringent crystal, the accommodation space was successfully reduced to about one tenth of that for conventional
depolarizers.
Table 1 shows the target characteristics of this development program.
The upper limit of DOP was set to a value such that thepolarization dependence of Raman gain (PDRG) became
equivalent to or less than the polarization dependent loss
(PDL) of general optical components 3).

4.

DESIGN

4.1 Design for Reduction of DOP Deterioration
The DOP deterioration of a depolarizer is caused by thechanges in the state of polarization of the light incident on
the birefringent crystal, so that the temperature change inan amplifier is the main cause of deterioration after the

Figure 4

Output spectrum of pumping laser diode.

depolarizer is assembled into the amplifier. Consequently,
it is possible to reduce the DOP deterioration by insertinga polarizer in front of the birefringent crystal thereby stabilizing the state of polarization of the incident light.
Figure 3 shows the temperature dependence of DOP
when a polarizer is inserted in front of the birefringent
crystal. By comparing Figure 3 with Figure 2, it can beseen that whereas the DOP without polarizer changes up
to 15 %, the DOP with polarizer changes by about 2 %,and the maximum DOP is 4 % or less.
4.2 Design for Saving of Accommodation Space
Basically it is very difficult to design a low-attenuation, as
well as compact depolarizer maintaining its ability of converting any polarized light into unpolarized light. However,in the case of a depolarizer for pumping lasers, it is possible to make the depolarizer low in attenuation and compact in size taking advantage of the features of pumping
lasers.
The output light of a Fabry-Perot laser diode has, as
shown in Figure 4, the following features:a) Completely linear polarized lightb) Multiple intensity peaks with a period of FP-mode
interval, i.e.,δλc) Full width half maximum (FWHM) of intensity distribution is around 1 nm
Thus from the feature a), it is understood that every
wavelength component in the wavelength spectrum of the

Furukawa Review, No. 23

2003

45

Development of a Crystal-Type Depolarizer

Γ (λ)–Γ (λ+δλ) = (2m+1)π

S3

π

S1
λ 1 λ 3 λ 5 ...

λ 2 λ 4 λ 6 ...

DOP reduction represented in the Stokes space.

100

80

DOP (%)

FWHM 1.6nm
60

FWHM 0.8nm
FWHM 0.4nm

40

20

0
0

L1 (π)

L2 (2π)L3 (3π)L4 (4π)L5 (5π)Length of birefringent crystal

Figure 6

DOP (%)

20

10
4%
0
0

Relationship between birefringent crystal length and
DOP with FWHM as a parameter.

0.2

0.4

0.6

0.8

1

1.2

FWHM (nm)Figure 7

Relationship between FWHM and DOP with phase difference as a parameter.

100
90
80

δλ 0.30nm

δλ 0.25nm

δλ 0.20nm

70
60
50
40
30
20
10
0
0

(2)

Figure 6 shows the relationship between the length ofbirefringent crystal and the DOP. It can be confirmed thatthe DOP takes a local minimal value at every phase difference of (2m+1)π and that the DOP becomes smaller the

Figure 5

Phase difference=5π30

L(0.20)Thus the length of crystal that gives a phase differenceof (2m+1) π between the adjacent peaks after passing
through the crystal can be obtained from equation (2).

Phase difference=3π

L(0.25)(1)Phase difference=π40

L(0.30)Γ (λ) = 2π L ∆ n/λ50

DOP (%)

pumping light traces, since the laser output undergoes
phase differences when passing through the birefringent
crystal, a great circle on the Poincare sphere as the phasedifference changes.
Taking the feature b) into consideration further, polarization vectors which are weighted by the light intensity of theadjacent peaks in the pumping light are summed, if a birefringent crystal is provided to give a phase difference of(2m+1)π between the adjacent FP-mode peaks, so thatthe totality of the vectors become minimum as illustrated
in Figure 5 using the Stokes space. When this procedure
is repeated over the entire wavelengths, the sum of thevectors approaches the zero, resulting in DOP decrease.
Below will be presented the design taking advantage ofthe features of the pumping laser diode. First, the designusing features a) and b) will be described.
The phase change Γ(λ) of certain light (wavelength is λ)after passing through a birefringent crystal can be
expressed, if the refractive index difference of crystal be∆n and the length be L, by the equation (1).

Length of birefringent crystal

Figure 8

Relationship between birefringent crystal length and
DOP for various pumping lasers suggesting crystal
length optimization.

larger the phase difference, i.e., the order m.
Next will be described the design that exploits the feature c). Figure 7 shows the relationship between the
FWHM of light intensity and the DOP, from which it can be
confirmed that, in the case of a pumping laser with a
FWHM value of about 1 nm, a sufficiently small DOP canbe obtained even at a small phase difference of π . The
length of birefringent crystal to give such a phase difference can be calculated by equation (2) by letting m be 0,
and referring to Figure 6, the length turns out to be thesmallest to result in a local minimal of DOP.
4.3 Design Customization
By means of the design procedures described above, it is
now possible to optimize the depolarizer design to make itsuitable for various pumping lasers with different δλ and
FWHM values.
As an example, the relationships between the DOP andbirefringent crystal length for three pumping lasers with a
FWHM value of 0.8 nm and δλ of 0.20 nm, 0.25 nm, and0.30 nm are calculated and the results are shown in
Figure 8. It can be seen from the figure that the length ofbirefringent crystal can be optimized to deal with pumping
lasers of different characteristics, and that the length has
to be optimized in order to implement high-performance
pumping light sources.

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2003

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Development of a Crystal-Type Depolarizer

Optical Power (dBm)0Port 2
Port 1-5

δλ =0.25nm
FWHM=0.8nm

-10

-15

-20

-25

Wavelength

Figure 9

Appearance of depolarizer module.

Figure 12 Output spectrum of pumping laser diode used in prototype module.

10
Sample01
Sample0270

DOP (%)

Temp.
6

4
25
2

Temperature (°C)

8

Table 2

-10

0
0

20

40

Characteristics of developed depolarizer.
Min.

Max.

Operating temperature

-10

70

Operating humidity

0

Typ.

Unit

Remarks

°C

85

%RH

Insertion loss

0.80

0.50

dB

@Top

DOP

7

4

%

@Top

dB

@Top

60

Time (min)Figure 10 Temperature dependence of DOP of prototype depolarizer.

Return loss

50

Package dimension

φ5.5 ✕ 60

55

mm

1.0
Sample01
Sample0270

IL (dB)

Temp.
0.6

0.4
25

Temperature (°C)

0.8achieved, in the -10~+70°C temperature range, such as
DOP≤4 %, DOP fluctuations≤2 %, and insertion loss≤0.8
dB. Table 2 summarizes typical performance of the developed product.

0.2
-10

7.

IN CONCLUSION

0.0
0

20

40

60

Time (min)Figure 11 Temperature dependence of insertion loss of prototype depolarizer.

5.

DEVICE STRUCTURE

Figure 9 shows an appearance of the developed depolarizer module. The outer diameter is 5.5 mm and the length
60 mm, achieving the targeted miniaturization. The device
structure is such that the pumping light of complete polarization incident on the port 1 with a PMF passes through apolarizer and a birefringent crystal, and subsequently exits
as unpolarized light from the port 2 with a single mode
fiber (SMF).

We have developed a depolarizer --an essential device for
Raman amplifiers, which are expected to enjoy anincreasing demand in future. By designing a new structurethat take advantage of the features of pumping lasers forthe amplifiers, we have achieved a goal of realizing acrystal-type depolarizer with a significant size reductionand excellent DOP characteristics than ever before.
In particular, the temperature dependence of DOP has
been improved to 4 % or less over the entire temperature
range. It is hoped that the developed product is extensively used in future thereby satisfying the stringent requirements for gain performance of Raman amplifiers as thecapacity in optical communications is enhanced.

REFERENCES

6.

DEVICE CHARACTERISTICS

Below will be described the optical characteristics of the
prototype module.
Figures 10 and 11 show the temperature dependence of
DOP and insertion loss of the module, respectively. Usinga pumping laser whose characteristics are shown in
Figure 12, good performance characteristics were

1) K.Bohm et al.: Performance of Lyot Depolarizers with
Birefringent Single-Mode Fibers, J of Lightwave Tech., LT-1, 7174(1983)
2) S.Matsushita et al.: Design of Temperature Insensitive
Depolarizer for Raman Pump Laser Diode, OFC2002, WB3(2002)
3) Y.Emori et al.: Cost-effective depolarized diode pump unit
designed for C-band flat-gain Raman amplifiers to control EDFA
gain profile, OFC2000, FF4(2000)

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