JP2735168B2 - Multi-stage depolarization circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for depolarizing a light source used in an optical communication system and an optical measurement system.

[0002]

2. Description of the Related Art In an optical amplification multi-repeating system currently under development, each repeater merely performs optical amplification mainly for compensating for attenuation of an optical signal generated during transmission. Optical signal distortion and noise light generated during transmission and for each relay accumulate, greatly affecting the transmission characteristics of the entire system.

For this reason, the measurement of the amplification characteristics of each optical amplifier used for multi-relay amplification requires ultra-high accuracy and ultra-high resolution of 0.1 dB or less. However, the insertion loss of various optical components used in the optical amplifier changes depending on the polarization state of the signal light, and a measuring instrument having a polarization dependence of about 0.5 dB such as an optical spectrum analyzer is used. It is extremely difficult to satisfy accuracy and ultra-high resolution.

Techniques for measuring the amplification characteristics of each optical amplifier used for multi-relay amplification with ultra-high accuracy and ultra-high resolution include:
Since it is extremely important in constructing an optical communication system, research has been actively conducted. The following describes the currently proposed technology.

As described above, as one means for improving the resolution and accuracy of measuring the characteristics of an optical amplifier, signal light having a degree of polarization of about 1 is depolarized or its polarization is reduced, and the measurement system and the measurement target are measured. There has been proposed a method of effectively eliminating the polarization dependence of the optical amplifier and the measurement system by reducing the optical power affected by the polarization dependence of the object.

The degree of polarization (= degree of polarization) is represented by the ratio of the polarization component power to the total power of light. That is, when the degree of polarization is 1, the light is completely polarized, and when the degree of polarization is 0, the light is not polarized. FIG. 4 is a graph showing the relationship between the original polarization dependence of the measurement system and the object to be measured and the polarization-dependent optical power fluctuation observed when light with a low degree of polarization is used. In the figure, the horizontal axis represents the magnitude of the original polarization dependence of the measurement system and the object to be measured, the vertical axis represents the polarization dependence actually measured, and the straight line in the figure represents α = 0.1 and β = β. Polarization degree 0.2
5, γ are measured values when signal light having a polarization degree of 0.5 is used.

As is apparent from FIG. 4, even if the original polarization dependence of the measurement system and the measured object is 0.5 dB, the degree of polarization is 0.1 (the polarization component is 10% of the entire signal light). In this case, the observed polarization dependence is about 0.05 dB, and when the degree of polarization is 0.5 (the polarization component is 50% of the entire signal light), the observed polarization dependence is about 0.26 dB. It is.

Thus, it is clear that the smaller the degree of polarization of the light used for measurement is, the smaller the observed polarization dependency is, and the use of light having a degree of polarization of 0.5 or less can significantly improve the measurement accuracy. ing.

The principle of non-polarization for reducing the degree of polarization of light having a large degree of polarization will be described below. The output light from the laser has a constant phase during the time generally called "coherence time" (approximately the reciprocal of the line width of the light source), and after that, oscillations with a phase different from the previous phase occur randomly. . For this reason, as shown in FIGS. 5A and 5B, when the laser beam is branched and one of them is given a time difference greatly exceeding the coherence time, a constant phase relationship exists between the two divided lights. No longer holds, no correlation.

Since the polarization is defined by the sum of orthogonal polarization components having a fixed phase relationship, the phase relationship cannot be uniquely defined. That is, if orthogonally polarized light having no correlation is synthesized with equal power, the polarization state becomes unique. , Ie, non-polarized light, and the degree of polarization is zero. At this time, if the orthogonal polarization components are not equal in power, the difference has polarization and the degree of polarization is greater than 0 by the difference power.

Further, even if the orthogonal polarization components have the same power, if there is a slight correlation between the orthogonal polarization components, the optical power having the polarization property increases accordingly, so that the degree of polarization is greater than 0 by the degree of the correlation. . Therefore, in order to generate light with a small degree of polarization, it is necessary to eliminate the correlation between orthogonally polarized lights and combine them with the same power.

As one of the conventional depolarizing circuits using the above depolarizing principle, a difference in the propagation speed between orthogonal polarized waves in a high birefringence optical fiber (that is, polarization dispersion) is used to generate a signal between orthogonal polarized waves. There has been a non-polarizing optical element that generates a transit time difference equal to or longer than the coherence time and realizes the above-described non-polarizing condition (for example, Japanese Patent Application Laid-Open No. Sho 59-155806, IEEE).
Journal of Lightwave Tech
noology vol. LT-1, No. 3, pp47
5-479, etc.).

Using such a non-polarizing optical element, the light source line width used in a long-distance ultra-high-speed optical communication system can be reduced to 10 lines.
In order to depolarize a narrow linewidth laser beam having a coherence time of about 0 MHz or less and having a long coherence time, the polarization dispersion of the current polarization-maintaining fiber is as small as about 2 ps / m. Become.

[0014]

In a conventional non-polarizing optical element which gives a time difference to two polarization components by using two orthogonal polarization component optical paths formed by one conventional polarization preserving fiber, for example, 100 MHz Degree of polarization with respect to the line width of 0.1
The fiber length required to achieve a transit time difference of about 7.3 nanoseconds between the orthogonal polarizations in the polarization-maintaining fiber required to achieve:
It is about 4 km, which is very long. This causes the following problems.

1) Since mode coupling occurs between orthogonal polarizations in the polarization-maintaining fiber, the separation between orthogonal polarizations in the polarization-maintaining fiber does not increase in proportion to the length of the optical fiber. The degree does not decrease. 2) It is very difficult to fabricate a polarization maintaining fiber of several km without a connection point using current manufacturing technology, and mode coupling occurs between orthogonal polarizations due to the connection. The separation between orthogonal polarizations is significantly reduced.

3) When the polarization-maintaining fiber becomes longer, the transmission loss difference between the orthogonal polarizations cannot be ignored, and the optical power between the orthogonal polarizations is not the same, so that the degree of polarization obtained cannot be reduced further. 4) Since the transmission loss of the polarization-maintaining single-mode fiber of several km is about several dB, the obtained non-polarized signal light power is small. 5) The polarization-maintaining fiber of several km requires a considerable amount, which increases the size of the apparatus and is very expensive.

If the light source line width is reduced to 1/10, the required fiber length is further increased by a factor of ten. The present invention solves the above-mentioned problems, and further provides a small-sized, low-loss, inexpensive, multi-stage depolarizing circuit that can easily and powerfully depolarize narrow line light used in current optical communication. The purpose is to provide.

[0018]

The object of the present invention is attained by adopting the following novel characteristic constitution means of the present invention. That is, the first feature of the present invention is that the input light is divided into a first polarization component and a second polarization component orthogonal to each other.
A first polarization splitting optical element for splitting the polarization component into a first polarization component, a first optical path for propagating the input light from the polarization splitting optical element while maintaining the polarization plane of the first polarization component, A second optical path that is spatially separated from the first optical path that propagates from the separation optical element while maintaining the polarization plane of the second polarization component of the input light; A first polarization combining optical element that combines the first polarization component and the second polarization component that have respectively propagated through the second optical path, and the first polarization combining optical element that is emitted by the polarization combining optical element. The polarization component and the second polarization component are arranged so as to be incident at 45 degrees with respect to the polarization axis.
A second polarization separation optical element for separating incident light into a third polarization component and a fourth polarization component orthogonal to each other, and a polarization plane of the third polarization component emitted from the polarization separation optical element. The third optical path for propagating while holding the light and the third optical path for propagating while holding the polarization plane of the fourth polarization component emitted from the polarization splitting optical element are spatially separated. A second polarization combining optical element that combines the third polarization component and the fourth polarization component that have respectively propagated through the third optical path and the fourth optical path; A first optical path for attenuating the input light disposed on at least one of the first optical path and the second optical path;
And a second means for attenuating the input light disposed on at least one of the third optical path and the fourth optical path.

According to a second feature of the present invention, in the first feature, n1 is the refractive index of the first optical path, n2 is the refractive index of the second optical path, n3 is the refractive index of the third optical path, and n4 is the refractive index of the third optical path. Is the refractive index of the fourth optical path, △ ν is the line width of the input light, and C is the speed of light, the length A1 of the first optical path, the length A2 of the second optical path, and the length of the third optical path A multi-stage depolarizing circuit in which A3 and the length A4 of the fourth optical path satisfy the condition of the following equation. | A2 × n2-A1 × n1 | ≧ 0.22 ÷ △ ν × C | A4 × n4-A3 × n3 | ≧ 0.22 ÷ △ ν × C || A2 × n2-A1 × n1 | − | A4 × n4-A3 × n3 || ≧ 0.22 ÷ △ ν × C

A third feature of the present invention is that the first or the second
The first optical path, the second optical path, the third optical path, and the fourth optical path in the above-mentioned feature are provided by a polarization-maintaining fiber or a multi-stage depolarizing circuit formed by spatial propagation.

[0021]

According to the present invention, a transmission path of two orthogonal polarization components conventionally formed by one polarization-maintaining fiber is separated by a connection between a first polarization combining optical element and a second polarization separation optical element. In each stage, each orthogonal polarization component is spatially separated (by a combination of a polarization beam splitter, a polarization plane preserving fiber, etc.) and formed. It is obtained by the physical optical path difference of the orthogonal polarization component.

For example, in the present invention, all optical paths are constituted by polarization maintaining fibers, and the difference between the first optical path and the second optical path is 1 m.
Since the transit time per 1 m of the polarization-maintaining fiber is about 5 ns, the transit time difference per 1 m between orthogonal polarization components in the conventional polarization-maintaining fiber is about 2p.
A time difference of about 1000 times can be obtained as compared with the case where a 1 m long polarization maintaining fiber utilizing s is used. Therefore, in the present invention, the required optical path length is reduced to one thousandth of the conventional value.
Below, each stage can be shortened.

Also, since there is no interaction between the orthogonal polarization components, the transit time difference between the orthogonal polarization components can be easily increased, and the degree of correlation between the orthogonal polarization components can be reduced at each stage. Is easily and stably obtained by the multiple stages.

Prior to the description of the embodiment, the basic principle of the unit depolarizing circuit of each stage used by the present invention will be described with reference to the drawings. FIG. 1 shows the configuration of a unit depolarizing circuit that achieves a degree of polarization of 0.1. In the figure, 1 denotes a light source line width Δν of 100 MHz.
A light source such as a DFB semiconductor laser or the like that oscillates (full width at half maximum) narrow line width laser light L1 and has a Lorentz spectrum spectrum distribution, and the light source 2 separates the incident light L1 into orthogonal polarization components to generate outgoing lights P1, S1. It is a polarization beam splitter (polarization separation optical element).

Reference numeral 3 denotes light P1 emitted from the polarizing beam splitter 2.
A polarization-maintaining fiber having a refractive index of 1.5 and a length of 5 cm, which constitutes an optical path C1 for preserving and propagating the polarization plane, and an optical path for preserving and propagating the polarization plane of the output light S1 of the polarization beam splitter 2 1.55m long with refractive index 1.5 constituting C2
5 and 6 are optical attenuators for adjusting the combined orthogonal polarization components P1 and S1 to have equal power, and 7 is a combination of orthogonal incident lights P1 and S1. A polarizing beam splitter (polarization combining optical element) 8 is a condenser lens for converging light. As for the optical attenuators 5 and 6, either of the two orthogonal polarization components P1 and S1 after the multiplexing may be omitted because they are used to make the power equal.

The polarization plane preserving fiber 3 for optically connecting the polarization beam splitter 2 and the polarization beam splitter 7,
Numeral 4 indicates that the respective polarization axes coincide with the polarization axes of the polarization beam splitter 2 and the polarization beam splitter 7 in order to make the polarization state in the optical paths C1 and C2 difficult to change with environmental changes such as temperature and vibration. To be placed. In the following description, the polarization axes of the polarization beam splitters 2 and 7 are defined as an axis perpendicular to the paper (hereinafter, referred to as a “vertical axis”) and an axis perpendicular to the vertical axis in a plane parallel to the paper (hereinafter, referred to as “vertical axis”). , “Horizontal axis”).

The difference between the times (traveling times T1 and T2) during which the two outgoing lights P1 and S1 from the polarizing beam splitter 2 pass through the optical paths C1 and C2 (traveling time difference Ts = T2-
T1) is determined by a desired degree of polarization. FIG.
The time T normalized by the reciprocal of the line width Δν of the light source 1 having a Lorentzian spectrum distribution like an FB semiconductor laser and light emitted from such a light source are separated into orthogonal polarization components, and two orthogonal polarization components are separated. The relationship between the degrees of polarization obtained when one of the wave components is delayed by a certain time T and then the two orthogonal polarization components are multiplexed with equal power is shown. From this, for example, to achieve a polarization degree of 0.5 (-3 dB), 0.22 / Δν
The difference between the transit times Ts is a degree of polarization of 0.1 (−10 dB).
It is understood that a running time difference Ts of 0.73 / Δν or more is required to achieve the above.

Therefore, the equation for calculating the lengths of the optical paths C1 and C2 is as follows. | A2 × n2-A1 × n1 | ≧ Ts × C where A1 is the length of the optical path C1, A2 is the length of the optical path C2,
n1 is the refractive index of the optical path C1, n2 is the refractive index of the optical path C2, △
ν represents the line width of the input light, and C represents the speed of light. Since it is considered that the degree of polarization is 0.5 or less, which effectively works as the depolarizing circuit, Ts may be set to 0.22 ÷ △ ν to achieve this. Therefore, the following equation can be said to be a practical configuration. | A2 × n2-A1 × n1 | ≧ 0.22 ÷ △ ν × C

Accordingly, for example, when the light source line width Δν is 100 MHz as in the present basic principle, the transit time difference Ts required to achieve the degree of polarization of 0.1 is about 7.3 nanoseconds, and the polarization-maintaining fiber 3 Is very short and the transit time T1 can be regarded as almost 0 second, the polarization maintaining fibers 3 and 4
Is about 1.5, the required length of the polarization-maintaining fiber 4 is a value obtained by adding at least about 1.5 m to the length of the polarization-maintaining fiber 3.

Therefore, according to the basic principle, since the polarization maintaining fibers 3 and 4 have a refractive index of about 1.5, if the length of the polarization maintaining fiber 3 is 5 cm, the polarization maintaining fiber 4 is not required.
Is required to be about 1.55 m. If the length of the polarization-maintaining fiber 3 is 10 cm, the length of the polarization-maintaining fiber 4
Will need to be about 1.6 m long.

The operation principle of the basic principle will be described below.
In the optical paths C1 and C2, optical attenuators 5 and 6 are used, respectively, to use a polarization component parallel to the vertical axis (hereinafter referred to as “vertical component”) in which the loss from the input end of the polarization beam splitter 2 to the output end of the polarization beam splitter 7 is And the polarization component parallel to the horizontal axis (hereinafter referred to as “horizontal polarized light”).

When a linearly polarized wave (input light L 1) whose polarization axis is rotated by 45 degrees from the vertical axis in a plane perpendicular to the plane of the paper from the light source 1 is input to the polarization beam splitter 2, the light is horizontally aligned with the vertical axis of the polarization beam splitter 2. It is equally incident on the axis. Accordingly, the first depolarizing circuit is used when the input light is fixed so that it can be input to the polarization beam splitter 2 in a polarization state close to a linearly polarized wave rotated by 45 degrees from the vertical axis in a plane where the polarization axis is vertical. Can more easily obtain the effects of the invention.

Since the loss from the output end of the polarization beam splitter 2 to the output end of the polarization beam splitter 7 is adjusted so that the vertically polarized light and the horizontally polarized light match, the output light L2 synthesized by the polarization beam splitter 7 is adjusted. , L3 (orthogonal polarization components) have the same power, and a traveling time difference Ts of about 0.73 times the reciprocal of the light source line width is given in the optical path C2. Therefore, the degree of polarization of the output light is 0.1 or less.

Next, the case where the input light L1 having an arbitrary polarization state is incident will be described. Since an arbitrary polarization state is represented by the sum of orthogonal polarization states, when the input light L1 having an arbitrary polarization state is input to the polarization beam splitter 2, it is incident on the vertical axis and the horizontal axis of the polarization beam splitter 2. However, if the optical attenuators 5 and 6 are used, the output lights L2 and L2 synthesized by the polarization beam splitter 7 are used.
L3 can be equal power.

Therefore, if the power reduction of the output lights L2 and L3 can be tolerated to some extent, the degree of polarization of the output lights L2 and L3 can be set to 0.1 or less. According to this basic principle, in order to avoid a decrease in output power as much as possible, the input polarization state can be input to the polarization beam splitter 2 in a polarization state close to a linearly polarized wave rotated by 45 degrees from the vertical axis in a plane perpendicular to the plane of the paper. Need to be fixed.

The polarization preserving fibers 3 and 4 used in the optical path C1 or C2 are such that the optical path C1 or C2 is sufficiently short so that the polarization state in the optical path hardly changes with environmental changes such as temperature and vibration. If necessary, a mirror or the like may be used. Further, the optical attenuators 5 and 6 can be omitted when the output lights L2 and L3 combined by the polarization beam splitter 7 can have the same power for a fixed input polarization state. Further, the condenser lens 8 is not always necessary unless the light spreads. Based on the basic principle of the unit depolarizing circuit used in each of the above stages, the operation of the present invention will now be described.

[0037]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. FIG.
3 is a block diagram showing a configuration of a multi-stage depolarizing circuit of the present embodiment. The difference from the unit depolarizing circuit is that, in this embodiment, the unit depolarizing circuit has a two-stage configuration via the half-wave plate 10 so that the output power does not decrease even if the input light is in an arbitrary polarization state. It is to have done.

In this embodiment, the light source line width Δν is 100 MHz.
Since it is a depolarizing circuit that obtains a degree of polarization of 0.1,
The required transit time difference Ts is about 7.3 ns, and the polarization maintaining fibers 3, 4, 12, 13 have a refractive index of about 1.5,
The lengths of the polarization maintaining fibers 3 and 12 are each 5 cm,
The length of the polarization maintaining fiber 4 is about 1.55 m, and the length of the polarization maintaining fiber 13 is 3.05 m. These lengths can be arbitrarily selected according to the formula described later. For example, the lengths of the polarization maintaining fibers 3 and 12 are each set to 10
cm, the length of the polarization preserving fiber 4 is about 1.6.
m, the length of the polarization-maintaining single-mode fiber 13 is about 3.1 m.

The present embodiment is configured as described above. Hereinafter, the description will be made focusing on the use of the unit depolarizing circuit of each stage. The configuration from the light source 1 to the polarization beam splitter 7 is the same as that of the unit depolarizing circuit, and the description is omitted. Further, the configuration from the polarization beam splitter 11 to the polarization beam splitter 16 (hereinafter, the configuration between them is referred to as a “second stage”) is also applied to the configuration from the polarization beam splitter 2 to the polarization beam splitter 7 of the unit depolarizing circuit ( Hereinafter, the configuration during this period is referred to as “first stage”).

In this embodiment, the polarization axis of the used light is rotated by 45 degrees in the vertical plane through the polarization preserving fiber 9 between the first stage and the second stage. Wave plate 1
0 is arranged. The polarization plane preserving fiber 9 between the polarization beam splitter 7 and the half-wave plate 10, which is the output of the first stage, is arranged so that its polarization axis matches the polarization axis of the polarization beam splitter 7. Accordingly, the half-wave plate 10 output from the first-stage polarization beam splitter 7
The optical axes of the polarization components parallel to the vertical axis and the horizontal axis, which are input to the second stage, can be easily matched even at the input end of the second-stage polarization beam splitter 11. The optical attenuators 5 and 6 in the optical paths C1 and C2 are used to control the loss from the input end of the polarizing beam splitter 2 to the output end of the half-wave plate 10 by the polarization component parallel to the vertical axis and the polarization component parallel to the horizontal axis. It is adjusted so that it matches with the component.

Propagation time (travel time) of each optical path C1 to C4
Are defined as T1 to T4, respectively, and as shown in the basic principle, assuming that a transit time difference required for a desired degree of polarization obtained based on FIG. 2 is Ts, | T2-T1 |> Ts | T4-T3 |> Ts || T4-T3 |-| T2-T1 ||> Ts, the lengths of the optical paths C1 to C4 are configured.

Accordingly, the length A1 of the optical path C1, the length A2 of the optical path C2, the length A3 of the optical path C3, and the length A4 of the optical path C4
Can be obtained from the following condition. | A2 × n2-A1 × n1 | ≧ Ts × C | A4 × n4-A3 × n3 | ≧ Ts × C || A2 × n2-A1 × n1 | − | A4 × n4-A3 × n3 || ≧ Ts × C, n1 is the refractive index of the optical path C1, n2 is the refractive index of the optical path C2, n3 is the refractive index of the optical path C3, n4 is the refractive index of the optical path C4,
Δν represents the line width of the input light, and C represents the speed of light.

Since it is considered that the degree of polarization of 0.5 or less works effectively as a depolarizing circuit, in order to achieve this,
Ts = 0.22 ÷ △ ν. Therefore, the following equation can be said to be a practical configuration. | A2 × n2-A1 × n1 | ≧ 0.22 ÷ △ ν × C | A4 × n4-A3 × n3 | ≧ 0.22 ÷ △ ν × C || A2 × n2-A1 × n1 | − | A4 × n4-A3 × n3 || ≧ 0.22 ÷ △ ν × C

The optical attenuators 14 and 15 are connected to the polarization beam splitter 16 from the incident end of the polarization beam splitter 11.
Can be omitted if the loss to the emission end can be matched between the polarization component parallel to the vertical axis and the polarization component parallel to the horizontal axis.

The polarization-maintaining fiber 1 used in the optical path C3 or the optical path C4 as in the unit depolarizing circuit.
The mirrors 2 and 13 may be configured as mirrors if the optical path C3 or the optical path C4 is sufficiently short so that the polarization state in the optical path is unlikely to change with environmental changes such as temperature and vibration. Further, the optical attenuators 14 and 15 can be omitted when the output lights L2 and L3 combined by the polarization beam splitter 16 for the fixed input polarization state can have the same power.

The operation of this embodiment will be described below. Light source 1
Linearly polarized light having a plane of polarization parallel to the vertical axis (the input light L
When 1) is incident on the polarizing beam splitter 2, it is emitted from the polarizing beam splitter 7 as linearly polarized light having a plane of polarization parallel to the vertical axis (output light L 1 ′), and the polarization axis is rotated by 45 degrees by the half-wave plate 10. And the polarizing beam splitter 1
1 are equally incident on the vertical axis and the horizontal axis, and are emitted to the polarization maintaining fibers 12 and 13 (emitted light P2 and S2).

The outgoing light beams P2 and S2 from the polarizing beam splitter 11 synthesized by the polarizing beam splitter 16 are given equal traveling power difference T2 about 1.5 times the reciprocal of the light source line width Δν on the optical path C4. Therefore, the output light has a polarization degree of 0.1 or less. On the other hand, when linearly polarized light having a plane of polarization parallel to the horizontal axis is incident on the polarizing beam splitter 2 from the light source 1, the degree of polarization is set to 0.
1 or less output light can be obtained from the polarizing beam splitter 16.

Since an arbitrary polarization state is represented by the sum of orthogonal polarization states, when light having an arbitrary polarization state is incident on the polarization beam splitter 2, it becomes a linear polarization component parallel to the vertical axis and the horizontal axis. Each orthogonal polarization component can be considered by decomposing the polarization beam splitter 1 according to the above operation principle.
It becomes unpolarized light with 6 outputs.

At this time, the traveling time difference T2 is equal to the traveling time difference Ts.
Therefore, a transit time difference Ts of at least 0.73 times the reciprocal of the light source line width is generated between the orthogonal polarization components of the output light obtained at the output of the polarization beam splitter 16. It is understood that the degree of polarization of L2, L3, L4, and L5 is at least about 0.1. Therefore, according to the present invention, output light having a degree of polarization of about 0.1 or less can be obtained regardless of the polarization state of the input light L1.

In the half-wave plate 10 used in this embodiment, for example, the output end of the polarization plane preserving fiber 9 is rotated by 45 degrees so that the polarization axes of the polarization beam splitter 7 and the polarization beam splitter 11 are 45 degrees. If it can be made to have an angle, it can be omitted. Further, the polarizing beam splitter 7 and the half-wave plate 1
The polarization plane preserving fiber 9 between 0 and 0 makes the optical axes of the polarization components parallel to the vertical axis and the horizontal axis emitted from the polarization beam splitter 7 and incident on the half-wave plate 10 coincide with each other after the output end. It may be omitted if possible.

The polarization preserving fibers 3, 4, 12, and 13 used in the optical paths C1, C2, C3, and C4 are connected to the optical path C1,
If C1, C2, C3, and C4 are sufficiently short and the polarization state in the optical path is unlikely to change with environmental changes such as temperature and vibration, all of them can be omitted.

The optical attenuators 5, 6, 14, and 15 convert the loss from the input ends of the polarization beam splitters 2 and 11 to the input ends of the polarization beam splitters 7 and 16 into a polarization component parallel to the vertical axis and a horizontal axis. This can be omitted if it is unnecessary for matching with parallel polarization components, or if output light may change due to a change in the polarization state of incident light.

In the present embodiment, the configuration of the second stage is shown, but n stages may be used. In that case, the polarization axis of the light between the (n−1) -th stage and the n-th stage is 45 ° in a plane perpendicular to the paper surface.
It must be configured to be rotated degrees.

[0054]

According to the present invention, a polarization beam splitter and a polarization-maintaining fiber (or mirror) for separating input light into orthogonal polarization components instead of a high birefringence optical fiber.
Is used in each stage to independently delay the orthogonal polarization components in each stage without any interaction, and then in each stage, the orthogonal polarization components are combined with equal power by the polarization beam splitter. Optical path length (length of polarization-maintaining fiber)
Can be reduced to 1 / 1,000 or less of the conventional one, and as a result, a much smaller, low-loss and inexpensive multi-stage depolarizing optical circuit can be obtained.

Also, since there is no interaction between the orthogonal polarization components at each stage, the transit time difference between the orthogonal polarization components can be easily increased, and the degree of correlation between the orthogonal polarization components can be reduced. Easily and stably obtained by means of steps. Therefore, the implementation of the present invention greatly improves the measurement accuracy and resolution of the optical amplifier characteristics, and the transmission characteristics are greatly improved if an optical amplification multi-repeating system is constructed using this as a signal light source. Is big.

Further, in the case of the two-stage configuration, by arranging the polarization beam splitter so that the first polarization component and the second polarization component are incident at 45 degrees with respect to the polarization axis, incident light can be reduced. It exhibits excellent usefulness such that the output power can be prevented from decreasing even in an arbitrary polarization state.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a unit depolarizing circuit of each stage used by the present invention.

FIG. 2 is a graph illustrating the principle of operation, showing the relationship between the transit time difference and the degree of polarization.

FIG. 3 is a block diagram showing a configuration of an embodiment of the present invention.

FIG. 4 is a graph showing the relationship between the original polarization dependence of the measurement system and the object to be measured and the polarization-dependent optical power fluctuation observed when light with a low degree of polarization is used.

FIG. 5 is a graph showing the concept of depolarization.

[Explanation of Codes] C1, C2, C3, C4 ... optical path L1, L1 '... input light L2, L3, L4, L5 ... output light P1, P2, S1, S2 ... light separated into orthogonal polarization components 1 ... Light source 2, 7, 11, 16 ... polarization beam splitter 3, 4, 9, 12, 13 ... polarization plane preserving fiber 5, 6, 14, 15 ... optical attenuator 8 ... condenser lens, 10 ... half-wave plate

Continued on the front page (72) Inventor Shigeyuki Akiba 2-3-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo International Telegraph and Telephone Corporation (72) Inventor Hiroharu Wakabayashi 2-3-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo International Telegraph and Telephone Corporation (56) References JP-A-63-113519 (JP, A) JP-A-61-112123 (JP, A) JP-A-62-196604 (JP, U)

Claims (3)

(57) [Claims] A first polarization splitting optical element for splitting input light into a first polarization component and a second polarization component orthogonal to each other; and a first polarization splitting optical element for splitting the input light from the first polarization splitting optical element. A first optical path for propagating while maintaining the polarization plane of the polarization component; and a first optical path for propagating from the polarization splitting optical element while maintaining the polarization plane for the second polarization component of the input light. And a second optical path spatially separated from the first optical path, and the first polarization component and the second polarization component propagated through the first optical path and the second optical path, respectively. A first polarization combining optical element, and the first polarization component and the second polarization component emitted by the polarization combining optical element are arranged so as to be incident at 45 degrees with respect to a polarization axis; A second polarization splitting optical element for splitting incident light into a third polarization component and a fourth polarization component orthogonal to each other; A third optical path for propagating while maintaining the polarization plane of the emitted third polarization component, and propagating while maintaining the polarization plane of the fourth polarization component emitted by the polarization splitting optical element; A fourth optical path spatially separated from the third optical path, the third polarization component and the fourth polarization component propagated through the third optical path and the fourth optical path, respectively; A second polarization-combining optical element for multiplexing the first optical path, a first means for attenuating the input light disposed on at least one of the first optical path and the second optical path, and a third optical path. A second means for attenuating the input light disposed on at least one of the fourth optical paths. 2. The length A1 of the first optical path, the length A2 of the second optical path, the length A3 of the third optical path, and the length A4 of the fourth optical path satisfy the following condition. The multi-stage depolarizing circuit according to claim 1, wherein | A2 × n2-A1 × n1 | ≧ 0.22 ÷ △ ν × C | A4 × n4-A3 × n3 | ≧ 0.22 ÷ △ ν × C || A2 × n2-A1 × n1 | − | A4 × n4-A3 × n3 || ≧ 0.22 ÷ △ ν × C where n1 is the refractive index of the first optical path, and n2 is the refraction of the second optical path. , N3 is the refractive index of the third optical path, n4 is the refractive index of the fourth optical path, Δν is the line width of the input light, and C is the speed of light. 3. The multi-stage optical system according to claim 1, wherein the first optical path, the second optical path, the third optical path, and the fourth optical path are polarization maintaining fibers or spatial propagation. Non-polarizing circuit