JPH08278472A - Optical variable delay line - Google Patents

Abstract

PURPOSE: To provide a small-sized optical variable delay line with high precision, high sureness and a wide variable range. CONSTITUTION: Input light 1 is made incident on a polarization rotating means 20 after passing through an input/output separation means 10. The polarization rotating means 20 rotates a polarization plane of incident light by 0 or 90 degrees according to a control signal to emit it to a birefringence plate 30. And so on, the polarization plane of the incident light is controlled by respective polarization rotating means when the incident light passes through successively like the polarization rotating plane 21 →the birefringence plate 31 →the polarization rotating plate 22 →the birefringence plate 32 → the polarization rotating plate 23 → the birefringence plate 33. Then, this light signal is reflected by a mirror 40, and is made incident on the input/output separation means 10 again by following the same optical path as the forward path to be emitted from the position different from the incident position of the input light 1 as output light 2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical delay line for delaying an optical signal, and more particularly to an optical variable delay line whose delay time can be changed.

[0002]

2. Description of the Related Art In an electric circuit handling a high speed signal, a variable delay line for changing the propagation delay time of the signal is used in order to adjust the phase between different signals. In the case of handling a high-speed optical signal also in an optical circuit, a similar optical variable delay line is necessary to adjust the propagation delay time between signals. In order to adjust the propagation delay time of the optical signal, it is necessary to directly or indirectly change the optical path length (length of the optical path through which the optical signal propagates). For example, when an optical fiber is used as a transmission medium, a direct method is to switch optical fibers having different lengths with an optical switch to change the delay amount. As an indirect method, there is a method of applying an external force (mechanical, electromagnetic, thermal) to the optical fiber to change the propagation constant of the optical fiber to adjust the delay amount.

[0003]

However, in the case of the direct method, it is not easy to adjust the length of the optical fiber with an accuracy of a centimeter or less, and therefore, it is highly accurate and highly accurate (several tens of picoseconds or less). ), There was a problem that the delay time could not be adjusted. Further, there is a limit to miniaturization by combining a mechanical switch (currently available, stable and low loss optical switches are limited to mechanical optical switches) and an optical fiber. On the other hand, in the indirect method,
Since the rate of change of the propagation constant with respect to the external force of the optical fiber is small, it is possible to adjust the delay time with high accuracy (picoseconds or less), but there is a problem that it is not easy to adjust the delay time in the time domain of the order of several tens of picoseconds. It was

An object of the present invention is to overcome the above problems and provide an optical variable delay line having a small size, high accuracy, high accuracy, and a wide variable range.

[0005]

In order to achieve the above object, according to the invention of claim 1, in an optical variable delay line for adjusting a propagation delay time of an optical signal, an input optical signal and an output optical signal are separated. It serves as an output separation means, a polarization rotation means for rotating the polarization plane of the linearly polarized optical signal incident through the input / output separation means by a predetermined angle, and a transmission medium for the optical signal passed through the polarization rotation means. A birefringent plate and a mirror that reflects an optical signal that has passed through the birefringent plate, and the propagation delay time of the optical signal in the birefringent plate by controlling the rotation angle of the polarization plane in the polarization rotating means. Controlled.

According to the invention described in claim 2,
The variable optical delay line described in (1) includes a plurality of the polarization rotating means and the plurality of birefringent plates. In the invention according to claim 3, in the variable optical delay line according to claim 2, the thickness ratio between the plurality of birefringent plates is a power of 2.

In the invention according to claim 4, the invention according to claim 1
In the variable optical delay line according to any one of 1 to 3, a polarization that splits one optical signal having an arbitrary polarization into two orthogonal polarization components or combines two orthogonal polarization components to generate one optical signal. The separating / combining means and the wave plate for rotating the polarization plane of one polarization component of the polarization separating / combining means by 90 degrees are provided.

According to the invention of claim 5, claim 1
In the variable optical delay line according to any one of 4 to 4, the input / output separating means and the mirror are removed, and a lens for combining output light is provided instead. According to a sixth aspect of the invention, in the variable optical delay line according to the first to fourth aspects, all the constituent element groups excluding the input / output separating means and the mirror are defined as a first constituent element group, Furthermore, a second component group having the same configuration as the first component group was provided, and the second component group was arranged in the opposite direction to the optical axis with respect to the first component group.

[0009]

In the variable optical delay line according to the present invention, the linearly polarized input light passes through the input / output separation means, and then the polarization plane of the light is rotated by 0 or 90 degrees. Go straight or diagonally and after being reflected by the mirror,
It again passes through the polarization rotating means in the opposite direction and is emitted from the input / output separation means from a position different from the incident position. Therefore, it is possible to control the propagation delay time of the optical signal in the birefringent plate by controlling the rotation angle of the plane of polarization of the incident light by the polarization rotating means, and the delay time is the birefringent plate. Since it can be determined by the thickness of the birefringent plate, it is possible to easily obtain a highly accurate and extremely short delay time depending on the thickness and processing accuracy of the birefringent plate.

An optical variable delay line according to a second aspect is the first aspect.
Since the optical variable delay line described above includes a plurality of the birefringent plates, the delay time can be varied by controlling the rotation angle of the polarization plane with each birefringent plate. In the variable optical delay line according to claim 3, since the ratio of the thicknesses of the plurality of birefringent plates is a power of 2 in the variable optical delay line according to claim 2, 2 ⁿ kinds of delay times can be controlled, Also,
The delay time can be represented by a binary number.

An optical variable delay line according to a fourth aspect is the first aspect.
In the variable optical delay line according to any one of 1 to 3, an optical signal having arbitrary polarization is separated into two orthogonal polarization components, and one of the separated polarization components is polarized by a wave plate. Since the light is incident on the variable optical delay line after being rotated by a certain degree, it is possible to delay light of arbitrary polarization and control the delay time, so that a more practical variable optical delay line can be configured.

An optical variable delay line according to a fifth aspect of the present invention is the optical variable delay line of the first aspect.
In the variable optical delay line according to any one of 1 to 4, a condensing lens is provided instead of the mirror, and the condensing lens synthesizes the output light of the optical variable delay line. By arranging the short surface of the optical fiber to be the output terminal, the output light of the variable optical delay line can be directly incident on the optical fiber.

According to a sixth aspect of the present invention, there is provided an optical variable delay line according to the first aspect.
In the variable optical delay line according to any one of 1 to 4, the round-trip optical path formed by using a mirror is replaced by a polarization separating means, a polarization rotating means, and a birefringent plate, instead of the mirror. Since the same component as the outward component is arranged in the opposite direction to the optical axis, the optical path in the component arranged instead of the mirror is completely symmetrical with the forward component, Thus, the variable optical delay line that does not depend on the polarization state of the incident light in the optical path in one direction can be configured.

[0014]

First, the function of a birefringent plate, which is a basic constituent element of the variable optical delay line of the present invention, will be described with reference to FIG. In FIG. 10, a light beam incident perpendicularly to the longitudinal direction of the birefringent plate 100 goes straight or obliquely through the birefringent plate according to the polarization direction (or polarization plane) thereof.
For example, as shown in (a), an input light beam 101 having an electric field oscillation direction perpendicular to the paper surface goes straight in the birefringent plate 100 as an ordinary ray (a ray that follows the law of refraction).

Further, as shown in (b), the input light beam 101 having an electric field oscillation direction parallel to the plane of the paper and the plane of incidence of the birefringent plate is an extraordinary ray (a ray which does not follow the law of refraction) and is a birefringent plate. The inside of 100 is skewed at a specific angle with respect to the optical axis. Therefore, by controlling the polarization direction of the light beam incident on the birefringent plate, the optical path in the birefringent plate can be selected to be either straight or oblique. When the light beam incident on the birefringent plate is elliptically polarized light having both polarization components, or the like, light in each polarization direction travels by separating each optical path.

Next, with reference to FIG. 11, the principle of the variable optical delay line composed of the polarization rotating means, the birefringent plate and the mirror will be described. In FIG. 11, the polarization rotating means for rotating the polarization plane of the incident light beam by 0 degree or 90 degrees is provided on one surface of the birefringent plate, and the mirror for totally reflecting the light beam is provided on the other surface opposite to the polarization rotating means. An optical variable delay line that changes the propagation delay time can be realized.

That is, an input light beam of linearly polarized light (in which the oscillation direction of the electric field is assumed to be perpendicular to the paper surface in the figure) incident on the polarization rotating means has its polarization plane rotated by 0 ° or 90 ° by the polarization rotating means. . As a result, when the light beam passes through the polarization rotation means and is incident on the birefringent plate, an ordinary ray (when the polarization rotation angle by the polarization rotation means is 0 degree) or an extraordinary ray (when the polarization rotation angle is 90 degrees) ). As described above, the ordinary ray goes straight and the extraordinary ray goes obliquely in the birefringent plate.

Further, the light beam is totally reflected by the mirror and goes backward in the birefringent plate in the left direction in FIG. At that time, in the case of an ordinary ray and the case of an extraordinary ray, the same optical paths as the respective optical paths before reflection are reversed. Then, it again passes through the polarization rotation means in the opposite direction and is output in the left direction. At that time, the polarization rotation means returns the polarization plane to the polarization plane at the time of incidence. That is, the ordinary ray in the birefringent plate is 0
The extraordinary ray undergoes a 90 degree polarization rotation. As a result, in any case, the oscillation direction of the electric field of the output light beam is perpendicular to the paper surface as at the time of input.

When considering a system as described above, in which a light beam (optical signal) is incident on the polarization rotating means, is reflected by a mirror, and passes through the polarization rotating means again and is emitted to the outside, the system is considered. The time required for an optical signal to pass through depends on the rotation angle of the polarization plane by the polarization rotation means. That is, as described above, in the birefringent plate, the propagation speed of light depends on the plane of polarization. For example, in the case of a birefringent plate made of calcite, the ordinary ray has a higher refractive index than the extraordinary ray, so that the propagation speed of the ordinary ray is slower than that of the extraordinary ray.

In FIG. 11, the extraordinary ray travels longer than the ordinary ray because the extraordinary ray obliquely travels in the birefringent plate, but in reality, the angle formed by both rays is about 6 degrees (the birefringent plate from the calcite crystal. The angle is between 0 and about 6 degrees depending on how to cut out), which is small, and the optical path lengths of both rays can be considered to be almost equal. Therefore, the time required for an optical signal to pass through this system (propagation delay time) is longer when it passes as an ordinary ray than when it passes as an extraordinary ray. That is, by controlling the rotation angle of the plane of polarization by the polarization rotation means (selection between ordinary rays and extraordinary rays),
It is possible to control the propagation delay time of the optical signal. In this way, FIG. 11 functions as an optical variable delay line capable of switching the propagation delay time in two steps.

Further, as shown in FIG. 12, a plurality of polarization rotating means and birefringent plates are alternately arranged, and the polarization rotation angle of each polarization rotating means is independently controlled, whereby the propagation delay time is switched in multiple stages. Possible optical variable delay lines can also be constructed. When the variable optical delay line is constructed by the above method, the delay step is mainly determined by the thickness of the birefringent plate. Therefore, if the thickness of the birefringent plate is reduced, it is possible to shorten the delay step from several tens femtoseconds (1 femtosecond is 10 ⁻¹⁵ seconds) to several hundreds femtoorders.
If the number of delay steps is increased by increasing the number of stages of the birefringent plate and the polarization rotation means in the configuration of 2, it is possible to extend the variable range to several tens of picoseconds to several hundreds of picoseconds or more.

For example, the separation angle between the ordinary ray and the extraordinary ray is about 6
When a calcite birefringent plate with a thickness of 1 mm in degrees is used, the delay step for an optical signal with a wavelength of 1.55 μm is about 0.
56 picoseconds (picoseconds are 10 ^-12 seconds) with a thickness of 10
In the case of mm, it is about 5.6 picoseconds. Therefore,
If the number of stages is 10, then a variable range of about 6 to 60 picoseconds can be obtained. Also, the processing accuracy of optical components such as birefringence plate,
Since it is easy to set the assembly accuracy to several tens of μm or less, the setting accuracy of the delay time of 0.03 picoseconds or less can be easily obtained.

Embodiments of the present invention will be described below with reference to the drawings. [First Embodiment] FIG. 1 is a block diagram of an embodiment of the variable optical delay line according to the first and second aspects. First, the correspondence relationship between the respective parts will be described. Reference numeral 1 is input light, 2 is output light, 3 is a line showing an optical path of an optical signal, 10 is input / output separating means, and 20 to 23.
Is a polarization rotating means, 30 to 33 are birefringent plates, 40 is a mirror, 301 to 335 are light input / output positions at the right end of each birefringent plate, broken lines are settable optical paths for ordinary rays or extraordinary rays, and thick solid lines. Is an example of the optical path. In addition, the solid line arrow indicates the forward light beam propagation direction, the broken line arrow indicates the backward light beam propagation direction, the black circle indicates the electric field vibration direction of the light (normal ray) perpendicular to the paper surface, and the arrows with arrows on both sides. Indicates the electric field direction (extraordinary ray) of light parallel to the paper surface.

Next, the function of each component will be described with reference to FIG. Input light 1 of linearly polarized light is
After passing through the input / output separating means, it enters the polarization rotating means 20 as linearly polarized light having a polarization plane perpendicular to the paper surface. The polarization rotation means 20 has the function of rotating the polarization plane of the incident light by 0 degree or 90 degrees in accordance with the control signal. That is, when the rotation angle of the polarization plane is 0 degree, the polarization plane of the output light of the polarization rotation means 20 is the same as that at the input, and the vibration direction of the electric field is perpendicular to the paper surface (ordinary ray in the birefringent plate). In the case of degrees, the polarization plane of the output light is perpendicular to the input light, and the vibration direction of the electric field is parallel to the paper surface (extraordinary ray).

The light that has passed through the polarization rotation means 20 enters the birefringent plate 30 and propagates rightward inside the birefringent plate 30. At that time, the light (ordinary ray) whose electric field vibration direction is perpendicular to the paper surface is transmitted through the optical path 3
The light (extreme ray) whose electric field vibration direction is parallel to the plane of the drawing travels straight along 01 and travels obliquely along the optical path 302. These lights then enter the polarization rotation means 21, which causes the plane of polarization to rotate by 0 or 90 degrees. At that time, when the rotation angle of the plane of polarization is 0 degree, the light which is an ordinary ray is output as an ordinary ray and the light which is an extraordinary ray is output as an extraordinary ray.

However, in the case of 90 degrees, the ordinary ray is converted into an extraordinary ray, and the extraordinary ray is converted into an ordinary ray. Next, the light that has entered the birefringent plate 31 from the position 301 exits from the position 311 or 312 depending on whether the light is an ordinary ray or an extraordinary ray. On the other hand, the light incident on the birefringent plate 31 from the position 302 is emitted from the position 311 or 312 depending on whether the light is an ordinary ray or an extraordinary ray. Similarly, the light is converted into the polarization rotation means 2
2. The light passes through the birefringent plate 32, the polarization rotation means 23, and the birefringence plate 33, and the light is transmitted from the positions 331 to 331 at the right end of the birefringence plate 33 in accordance with the angle of polarization plane rotation by the polarization rotation means 22 and 23.
Reach any of 335.

Further, the light is totally reflected by the mirror 40, and thereafter proceeds to the left in the figure. At that time, the light is birefringent plate 3
From 3 to the polarization rotation means 20, the optical path exactly the same as the outward path before total reflection by the mirror 40 is followed to the left, and reaches the optical path 3. Then, the input / output separation unit 10 emits the output light 2 from a position different from the incident position of the input light 1.

As described above, by controlling the rotation angle of the plane of polarization by the polarization rotating means 20-23, the plane of polarization of the light inside the birefringent plates 30-33 is changed to be an ordinary ray or an extraordinary ray. As a result, the required propagation time (delay time) can be changed from the input of light to the input / output separation means to the output thereof. This is because the propagation velocity of light inside the birefringent plate differs depending on whether it is an ordinary ray or an extraordinary ray. For example, in the case of a calcite birefringent plate, since the refractive index for an ordinary ray is larger than the refractive index for an extraordinary ray, the ordinary ray has a lower propagation speed than the extraordinary ray.

Therefore, the propagation delay time becomes longer when the ordinary ray passes through all the optical paths than when the ordinary ray passes. When the optical paths of the ordinary ray and the extraordinary ray are mixed, the intermediate delay time occurs when only one of them is used. When the thicknesses of the birefringent plates 30 to 33 are all the same, the delay step Δt is constant. As shown in FIG. 1, in the case of a four-stage structure having four polarization rotation means and birefringent plates, t ₀ , t ₀ + Δt, t ₀ + 2Δt, t ₀ + 3Δt, t
Five types of delay times of ₀ + 4Δt can be taken. Similarly, in a structure of n stages (n is a positive integer), n + 1 types of delay times can be taken.

At this time, the delay time t is represented by t = t ₀ + kΔt {k is an integer, k = 0 to n} (1). The unit delay time Δt changes by changing the thickness d of the birefringent plate. Now, as a birefringent plate, d = 1.
When a calciner of 0 mm is used and a liquid crystal which will be described later is used as the polarization rotating means, the delay time step Δt is about 5.6 picoseconds when the wavelength is 1.55 μm The propagation time difference of the extraordinary ray).

Incidentally, the birefringent plates 30 to 3 in this embodiment.
No. 3 is not limited to calcite, and any material having similar birefringence and satisfying the necessary conditions can be used.

Further, in the present embodiment, on the premise that the ordinary ray and the extraordinary ray are spatially separated at a constant separation angle and travel in different optical paths in the birefringent plate, for ease of explanation. I've explained. However, the essence of the invention is
It does not necessarily mean that the ordinary ray and the extraordinary ray travel in different optical paths, but the propagation speeds of the ordinary ray and the extraordinary ray are different. Therefore, it is not necessary to use a birefringent plate cut at a specific angle with respect to the optical axis of the crystal so as to maximize the separation angle. May be (may be 0 degrees). This also applies to the following examples.

[Second Embodiment] FIG. 2 is a block diagram of an embodiment of the variable optical delay line according to the present invention. First, the correspondence relationship between the respective parts will be described. 1 is input light, 2 is output light, 3
Is a line showing the optical path of an optical signal, 10 is an input / output separating means, 20
22 to 22 are polarization rotation means, 30 to 32 are birefringent plates, 40 is a mirror, 301 to 327 are light input / output positions at the right end of each birefringent plate, broken lines are settable optical paths for ordinary rays and extraordinary rays, The thick solid line is an example of the optical path.

The solid arrow indicates the forward light beam propagation direction, the broken arrow indicates the backward light beam propagation direction, the black circles indicate the electric field vibration direction (normal ray) of the light perpendicular to the paper surface, and the arrows on both sides. The arrow with indicates the electric field direction (extraordinary ray) of light parallel to the paper surface. The configuration of the present embodiment has a birefringent plate 30.
It is exactly the same as the configuration of FIG. 1 except that the thicknesses of .about.32 differ from step to step, and more specifically, the thickness ratio between the birefringent plates is a power of 2. Therefore, the description of the propagation of light in the variable optical delay line is omitted.

The characteristics newly generated by setting the thickness ratio between the birefringent plates to a power of 2 will be described below. In the configuration of FIG. 1, the number of types of delay time obtained by the n-stage variable optical delay line is n + 1 types, whereas in the configuration of FIG. 2, the number is 2 ⁿ types. That is, the delay time in each stage can be expressed in two states, that is, whether the light passes through the stage as an ordinary ray or an extraordinary ray, and therefore, when the thickness ratio of the birefringent plate is a power of 2, the delay time of the whole is The delay time can be represented by a binary number. For example, in the case of a three-stage structure, the thickness ratio of the birefringent plate is 4: 2: 1 and the delay time has eight types. The minimum delay step in this case is determined by the thickness of the thinnest birefringent plate.

[Third Embodiment] FIG. 3 is a block diagram of an optical variable delay line according to a fourth embodiment of the present invention. First, the correspondence relationship between the respective parts will be described. 1 is input light, 2 is output light, 1
Reference numeral 1 is an input / output separating means, 20 to 22 are polarization rotating means, and 30
˜32 is a birefringent plate, 40 is a mirror, 50 is a polarized light separating / combining means, 60 is a ½ wavelength plate, and 301 to 327 and 30.
Reference numerals 1'-327 'denote light input / output positions at the right end of each birefringent plate, and 501 and 501' denote polarization splitting / combining means 5, respectively.
Input and output positions of the ordinary ray and the extraordinary ray at the right end of 0, the broken line is an optical path that can be set for the ordinary ray or the extraordinary ray, and the thick solid line is an example of the optical path.

The solid arrow indicates the forward light beam propagation direction, the broken arrow indicates the backward light beam propagation direction, the black circles indicate the electric field vibration direction (normal ray) of the light perpendicular to the paper surface, and the arrows on both sides. The arrow with indicates the electric field direction (extraordinary ray) of light parallel to the paper surface. The configuration of the present embodiment is obtained by adding the polarized light separating / combining means 50 and the ½ wavelength plate 60 to the configuration of FIG. Although only linearly polarized light is allowed as the input light 1 in the configuration of FIG. 2, the configuration of FIG.
The light of arbitrary polarization can be used as the input light 1. This is a feature of this embodiment in that a more practical variable optical delay line can be constructed.

Next, the operation will be described with reference to FIG.
When the input light 1 of arbitrary polarization passes through the input / output separating means 11, it enters the polarization separating / combining means 50. After passing through the polarization splitting / combining means 50, the input light is split into two polarization components orthogonal to each other (light having an electric field oscillation direction perpendicular to the paper surface, light having an oscillation direction parallel to the paper surface on the lower side). . Of these, the lower component is the polarized light separating / combining means 5
Since the light passes through the half-wave plate 60 located immediately after 0, its plane of polarization is rotated by 90 degrees, and the electric field vibration direction changes perpendicularly to the plane of the paper (the same polarization as the upper side).

These two lights are incident on the variable optical delay line main body portion from the polarization rotating means 20 to the mirror 40 from the polarization rotating means 20 from the left side in a completely equal manner, and are independently propagated to the right direction and reflected by the mirror 40. This time, the light travels to the left, and again exits from the polarization rotating means 20 to the left. (The operation of this optical variable delay line main body is completely equivalent to the configuration of FIG. 2, and therefore its explanation is omitted.)

Further, the upper light is incident on the polarization separating / combining means 50 as it is, and the lower light is the light having the electric field oscillation direction parallel to the paper surface by rotating the polarization plane 90 degrees by the ½ wavelength plate 60. After that, the light enters the polarized light separating / combining means 50. After passing through the polarized light separating / combining means 50, these are combined into one light, which is separated from the input light 1 by the input / output separating means 11 and emitted to the left as the output light 2.

Next, the required time (propagation delay time) from the input light 1 entering the input / output separating means 11 from the left to the output light 2 being output to the left will be described.
In this embodiment, the light of arbitrary polarization is polarized / separated by the polarization separation / combination means 5.
The polarized light components are orthogonally separated by 0, pass through the optical variable delay line body portion (from the polarization rotation means 20 to the mirror 40) having exactly the same optical path length, and are again synthesized by the polarization splitting / synthesizing means 50. . Therefore, the propagation delay times of the two polarization components are exactly the same, and the polarization state of the output light 2 is the same as the polarization state of the input light 1 (because the phase difference between both polarization components is the same at the time of input and at the time of output). That is, an optical variable delay line that does not depend on the polarization state can be configured.

[Fourth Embodiment] FIG. 4 is a block diagram of an optical variable delay line according to a fifth embodiment of the present invention. First, the correspondence relationship between the respective parts will be described. 1 is input light, 2 is output light, 2
0 to 23 are polarization rotation means, 30 to 32 are birefringent plates, 51
Is a polarization separating means, 52 is a polarization combining means, and 60 and 61 are 1
/ 2 wave plate, 70 is a lens, 301-314 and 301 '
˜314 ′ is the light input / output position at the right end of each birefringent plate, 511 and 511 ′ are the output positions of the ordinary ray and the extraordinary ray at the right end of the polarization separation means 51, and the broken line can be set for the ordinary ray or the extraordinary ray. The optical path and the thick solid line are examples of the optical path. The solid arrows indicate the light beam propagation direction, the black circles indicate the electric field oscillation direction of the light perpendicular to the paper surface (ordinary ray), and the arrows with arrows on both sides indicate the electric field direction of the light parallel to the paper surface (extraordinary ray). .

The configuration of the present embodiment is such that the input / output separating means and the mirror are removed from the configuration of FIG. 3, and instead the polarization rotating means 23 for converting the polarized light into an ordinary ray on the output side, one polarization is converted into an extraordinary ray. A half-wave plate 61, a polarization combining means 52 for combining two polarized lights, and a lens 70 for collecting output light are added. In the first to third embodiments, the input light is input from the left side, the traveling direction of the light is changed by 180 degrees by the mirror, and the output light is also extracted to the left side, whereas in the present embodiment, the input light is input from the left side. The input light is
Take out to the right through 0.

The input light 1 is split into two mutually orthogonal polarization components by the polarization splitting means 51 (the construction is the same as the polarization splitting / combining means in FIG. 3). Of these polarization components, light having an electric field oscillation direction perpendicular to the paper surface is directly incident on the main body of the variable optical delay line (from the light control element 20 to the birefringent plate 32). The light having the electric field vibration direction parallel to the other paper surface is incident on the main body portion of the variable optical delay line with the polarization plane rotated 90 degrees by the half-wave plate 60 and the electric field vibration direction being perpendicular to the paper surface.

The operation of the main body of the variable optical delay line is as follows. In FIG.
The light is emitted to the right from the birefringent plate 32 with the same delay as the operation up to. The two lights emitted from the birefringent plate 32 have the same polarization and are ordinary rays or extraordinary rays. The polarization rotation means 23 rotates the polarization plane so that these two lights are always output as ordinary rays. That is, when both inputs are ordinary rays, the plane of polarization is rotated by 0 degrees, and when the input is extraordinary rays, the plane of polarization is rotated by 90 degrees.

Further, of the two outgoing lights from the polarization rotating means 23, the lower light is directly incident on the polarization combining means 52 (the construction is the same as the polarization separating / combining means in FIG. 3), but in the figure. The polarization plane of the upper light is rotated 90 degrees by the ½ wavelength plate 61, the electric field oscillation direction becomes parallel to the paper surface, and enters the polarization combining means 52. At the right end of the polarized light combining means 52, the two lights are combined and emitted as one light. When the light is emitted from the polarization beam combiner 52, the light is emitted at the same angle regardless of the position from which the light is emitted. Therefore, after passing through the lens 70 which is a convex lens, the light is condensed at the focal point. Therefore, by arranging the end face of the optical fiber serving as the output terminal at the focal position of the lens 70, the output light 2 can be incident on the optical fiber. In this way,
This embodiment also makes it possible to construct an optical variable delay line that does not depend on the polarization state.

[Fifth Embodiment] FIG. 5 is a block diagram of an optical variable delay line according to a sixth embodiment of the present invention. First, the correspondence relationship between the respective parts will be described. 1 is input light, 2 is output light, 2
0 to 25 are polarization rotation means, 30 to 35 are birefringent plates, 51
Is a polarization separating means, 52 is a polarization combining means, and 60 and 61 are 1
The half wave plates 301 to 351 and 301 'to 351' are light input / output positions 511 and 51 at the right end of each birefringent plate.
1'is the output positions of the ordinary ray and the extraordinary ray at the right end of the polarization separating means 51, 521 and 521 'are the input positions of the ordinary ray and the extraordinary ray at the left end of the polarization combining means 52, respectively, and the broken line is the ordinary ray or the extraordinary ray. The light path that can be set, and the thick solid line is an example of the light path. The solid arrows indicate the light beam propagation direction, the black circles indicate the electric field oscillation direction of the light perpendicular to the paper surface (ordinary ray), and the arrows with arrows on both sides indicate the electric field direction of the light parallel to the paper surface (extraordinary ray). .

In the structure of this embodiment, the input / output separating means and the mirror are removed from the structure shown in FIG. 3, and the main part of the variable optical delay line (a group of constituent elements composed of the polarization rotating means and the birefringent plate) is added. Set (however, rotated 180 degrees with respect to the optical axis)
A half wave plate 61 and a polarized light combining means 52 are added. Note that in FIG. 5, the number of stages of the polarization rotation means or the birefringent plate in the two sets of constituent elements is reduced to two in order to simplify the drawing.

In this embodiment, the operation until the input light 1 enters the polarization separation means 51 and reaches the right end of the first component group (polarization rotation means 20 to the birefringence plate 31) is as shown in FIG. It is completely equivalent to the operation up to the mirror. Further, the second component group composed of the birefringent plate 34 to the polarization rotation means 25 is arranged line-symmetrically with respect to the boundary line between the birefringent plates 31 and 34 with respect to the first component group. Therefore, if the polarization rotation angles of the polarization rotation means at the symmetrical positions between the first and second constituent element groups are equal (the rotation angles of the polarization planes by the polarization rotation means 20 and 25 are equal, and the polarization rotation means is the same). The rotation angles of the planes of polarization of 21 and 24 are equal), and the optical path in the second component group is completely symmetrical to the optical path in the first component group.

Further, a polarization combining system (comprising the polarization combining means 52 and the half-wave plate 61) having the same structure as the polarization separation system (comprising the polarization separating means 51 and the half-wave plate 60) but oriented in the opposite direction. Thus, the two emitted lights from the second component group are combined and output to the right as output light 2. That is, in FIG. 5, the round-trip optical path formed by using a mirror is replaced with a mirror in FIG. An optical variable delay line that does not depend on the polarization state is configured. (Functionally, it is completely equivalent to the third embodiment.)

As a special case of the present embodiment, when the separation angle between the ordinary ray and the extraordinary ray in the birefringent plate is 0 degree (when a crystal cut out in a plane including the optical axis is used),
First component group (polarization rotating means 20 to birefringent plate 31)
Since all the emitted light of 1) follows the same optical path, the second component group (the birefringent plate 34 to the polarization rotation means 25) can be omitted. However, the delay step is halved.

Next, a concrete configuration example of the polarization separating / combining means, the polarization rotating means, and the input / output separating means which are the constituent elements of the first to fifth embodiments will be described. Polarization separation / combination means
Specifically, it can be realized by the configuration as shown in FIG. In FIG. 6, 1, 5, 6 are input lights, 2, 7, 8 are output lights, 36,
37 is a birefringent plate, 62 is a half-wave plate, 361, 36
Reference numeral 1'denotes an input / output position of light at the right end of the birefringent plate 36.

FIG. 6A shows the polarization splitting operation when the input light 1 of arbitrary polarization is incident from the left side. The input light 1 is separated into polarization components (ordinary ray and extraordinary ray) orthogonal to each other in the birefringent plate 36, the extraordinary ray obliquely travels, and the ordinary ray advances straight. Then, the extraordinary ray enters the ½ wavelength plate 62 from the position 361, and the ordinary ray enters the ½ wavelength plate 62 from the position 361 ′. The polarization plane of the light passing through the half-wave plate 62 is 90
The ordinary ray is converted into an extraordinary ray and the extraordinary ray is converted into an ordinary ray because of the rotation. As a result, in the birefringent plate 37,
The light coming from the position 361 goes straight as an ordinary ray, and the light coming from the position 361 ′ skews as an extraordinary ray. Then, they are emitted rightward from the right end of the birefringent plate 37.

Of the two orthogonal polarization components separated in the birefringence plate 36, the upper component in the figure advances as an extraordinary ray in the birefringence plate 36 and an ordinary ray in the birefringence plate 37. , The lower component proceeds as an ordinary ray in the birefringent plate 36 and as an extraordinary ray in the birefringent plate 37. At this time, since the birefringent plates 36 and 37 have the same thickness, the optical path lengths of the upper component and the lower component are equal. Therefore, the propagation delay time from when the upper component and the lower component are incident on the birefringent plate 36 to when they are emitted from the birefringent plate 37 is equal.

FIG. 6B shows a polarization combining operation when two input lights 5 and 6 having polarization planes orthogonal to each other are incident from the left side. The input light 5 having an electric field oscillation direction perpendicular to the paper surface is an ordinary ray in the birefringent plate 37 and therefore goes straight, and the input light 6 having an electric field oscillation direction parallel to the paper surface is an extraordinary ray in the birefringent plate 37. Because it is, it skews. Since their polarization planes rotate 90 degrees when passing through the half-wave plate 62, ordinary rays are converted into extraordinary rays and extraordinary rays are converted into ordinary rays. As a result, in the birefringent plate 36, the light incident from the position 361 skews as an extraordinary ray, and the light incident from the position 361 ′ goes straight as an ordinary ray. Then, they are combined at the left end of the birefringent plate 36 and emitted to the left as the output light 2 having an arbitrary polarization. In this way, the configuration of FIG. 6 functions as a means for separating / combining the polarization components.

The polarized light separating / combining means can also be realized by the configuration shown in FIG. 7 (however, in the figure, the combining operation is completely opposite to the separating operation, so that it is omitted). In FIG. 7, 1 is input light, 7 and 8 are output light, 36 and 37 are birefringent plates, and 361 and 361 ′ are light input / output positions at the right end of the birefringent plate 36. Reference numerals 371 and 371 ′ are light input / output positions at the right end of the birefringent plate 37. Figure 7
(A), (b) is a side view and a plan view of a polarization splitting / combining means, respectively.

FIGS. 7A and 7B show the polarization splitting operation when the input light 1 of arbitrary polarization is incident from the left side. In this configuration, the planes including the optical paths of the separated ordinary ray and extraordinary ray are arranged so that the birefringent plates 36 and 37 are at a right angle. As shown in FIG. 7B, the input light 1 is separated into polarization components (ordinary ray and extraordinary ray) orthogonal to each other in the birefringent plate 36, the extraordinary ray obliquely passes, and the ordinary ray advances straight. Then, the ordinary ray and the extraordinary ray enter the birefringent plate 37 from the position 361 and the position 361 ′, respectively. In the birefringent plate 37, the light coming from the position 361 skews as an extraordinary ray, and the light coming from the position 361 ′ goes straight as an ordinary ray, as shown in FIG. Then, they are emitted rightward from the right end of the birefringent plate 37.

Of the two orthogonal polarization components separated in the birefringent plate 36, the ordinary ray advances as an extraordinary ray in the birefringent plate 37 and the extraordinary ray advances as an ordinary ray in the birefringent plate 37. . At this time, since the birefringent plates 36 and 37 have the same thickness, the optical paths of the two separated components are equal to each other. Therefore, the propagation delay times from the two polarization components entering the birefringent plate 36 to the exit from the birefringent plate 37 are equal. Here, the synthesizing operation is an operation that is completely opposite to the separating operation, and it suffices to replace the input light with the output light and consider the light path in the opposite direction. Thus, the configuration of FIG. 7 also functions as a means for separating / combining the polarization components.

The polarization rotation means changes the polarization rotation angle (0 degree or 90 degrees) according to a control signal (eg, electric field or magnetic field) applied from the outside. As a specific example, consider a polarization rotating means using a liquid crystal as shown in FIG.
In the figure, 80 is a liquid crystal, 81 and 82 are transparent electrodes, 8
3 and 84 are isotropic transparent media, and 85 and 86 are spacers. In a liquid crystal cell (for example, a twisted nematic liquid crystal {TN liquid crystal} cell) having a structure shown in the figure and subjected to an appropriate alignment treatment, the polarization rotation angle changes depending on the voltage applied between the transparent electrodes 81 and 82.

That is, when light passes through the liquid crystal cell once from the left or right, when the applied voltage is V1 (zero voltage), the polarization is rotated by 90 degrees (equivalent to a ½ wavelength plate), and V2
At the time of (AC voltage of about several volts), polarization rotation does not occur (polarization rotation angle: 0 degree). Further, the polarization rotating means can be configured by using a liquid crystal other than the twisted nematic liquid crystal (for example, a super twisted nematic liquid crystal or a ferroelectric liquid crystal). Further, the polarization rotating means can be configured by using a material other than liquid crystal (for example, a dielectric crystal or a transparent ceramic such as PLZT). In addition,
The isotropic transparent media 83 and 84 can be made of an isotropic transparent material such as glass.

As the input / output separating means, one having no polarization dependency is required, and for example, a half mirror can be used. However, in that case, a theoretical loss of 3 dB occurs. To remove such a loss, a known circulator circuit as shown in FIG. 9 may be used. In this circulator, an input light 1 having an arbitrary polarization plane incident from a point X is split into two polarization components (p-polarized light and s-polarized light) orthogonal to each other by a polarization beam splitter 91, and the Faraday rotators 95 are independently separated from each other. And the half wave plate 96.

When passing through the Faraday rotator 95 (rotating the polarization plane by 45 degrees) and the ½ wavelength plate 96 (rotating the polarization plane by 45 degrees), the respective polarization components are 90 degrees (45 degrees + 45 degrees).
The polarization plane rotates, and the p-polarized component is converted into s-polarized light and the s-polarized component is converted into p-polarized light. Further, these are combined again by the polarization beam splitter 92 and output from the point Z toward the main body of the variable optical delay line.

On the contrary, the light returning from the main body of the variable optical delay line enters the circulator from the Z point, and the light entering the polarization beam splitter 92 is split into p-polarized light and s-polarized light, It passes through the wave plate 96 and the Faraday rotator 95. At this time, in the Faraday rotator 95, the polarization plane rotates 45 degrees in the same direction as the forward path with respect to the traveling direction, whereas in the ½ wavelength plate 96, the polarization plane rotates 45 degrees in the reverse direction with respect to the traveling direction. The plane of polarization rotates.

For this reason, the polarization rotation due to the Faraday rotator and the half-wave plate is canceled, and as a result, the p-polarized light remains p-polarized light and the s-polarized light remains s-polarized light (here, the forward light and the backward light are distinguished from each other. In order to achieve
incident on a polarizing beam splitter 91 (denoted as s ′),
The combined light is emitted from the point Y. In this way, the input / output separating means independent of the polarization state can be constructed. The Faraday rotator is a magneto-optical rotator, and actually requires a magnet that gives a magnetic field parallel to the optical path, but it is omitted in the figure for simplicity.

[0065]

As described above, according to the present invention,
It is possible to realize an optical variable delay line that is compact, highly accurate, highly accurate, and has a wide variable range. In addition, the variable optical delay line that does not depend on the polarization state of the input light can be easily configured.

[Brief description of drawings]

FIG. 1 is an explanatory diagram illustrating a configuration of an optical variable delay line according to a first exemplary embodiment of the present invention.

FIG. 2 is an explanatory diagram for explaining a configuration of an optical variable delay line in the second embodiment.

FIG. 3 is an explanatory diagram for explaining a configuration of an optical variable delay line in the third embodiment.

FIG. 4 is an explanatory diagram illustrating a configuration of an optical variable delay line according to the fourth embodiment.

FIG. 5 is an explanatory diagram illustrating a configuration of an optical variable delay line according to the fifth embodiment.

FIG. 6 is an explanatory diagram for explaining a specific configuration of polarized light separating / combining means in the first to fifth embodiments of the present invention.

FIG. 7 is an explanatory diagram for explaining another specific configuration of the polarization splitting / combining means in the first to fifth embodiments.

FIG. 8 is an explanatory diagram for explaining a specific configuration of the polarization rotating means in the first to fifth embodiments.

FIG. 9 is an explanatory diagram for explaining a configuration of a known circulator used as an input / output separating means in the first to fifth embodiments.

FIG. 10 is an explanatory diagram for explaining a configuration of basic components of the variable optical delay line.

FIG. 11 is an explanatory diagram for explaining the operation principle of the variable optical delay line.

FIG. 12 is an explanatory diagram illustrating a configuration of an optical variable delay line including a plurality of stages.

[Explanation of symbols] 1, 5, 6 ... Input light, 2, 7, 8 ... Output light, 3 ...
Optical path 10,11 ... Input / output separation means 20-25 ...
Polarization rotating means, 30-37 ... Birefringent plate, 40 ... Mirror, 50 ... Polarization separating / combining means, 51 ... Polarization separating means, 52 ... Polarization combining means, 60-62 ... 1/2 wavelength Plate, 70 ... lens, 80 ... liquid crystal, 81, 82 ... transparent electrode, 83, 84 ... isotropic transparent medium, 85, 86 ...
... Spacer, 90 ... Input / output light to delay line, 91, 9
2 ... Polarizing beam splitter, 93, 94 ... Right angle prism, 95 ... Faraday rotator, 96 ... 1/2 wave plate, 97 ... Light beam path, 100 ... Birefringent plate, 1
01 ... light beam, 102 ... electric field vibration direction, 103
……optical axis

Claims (6)

[Claims] 1. An optical variable delay line for adjusting a propagation delay time of an optical signal, wherein an input / output separating means for separating an input optical signal and an output optical signal, and a linearly polarized light incident through the input / output separating means. Polarization rotating means for rotating the polarization plane of the optical signal by a predetermined angle, a birefringent plate serving as a transmission medium for the optical signal passed through the polarization rotating means, and a mirror for reflecting the optical signal passed through the birefringent plate. An optical variable delay line, comprising: controlling a rotation angle of a polarization plane in the polarization rotating means to control a propagation delay time of an optical signal in the birefringent plate. 2. The variable optical delay line according to claim 1, wherein each of the polarization rotating means and the birefringent plate is provided in plural. 3. The variable optical delay line according to claim 2, wherein the thickness ratio between the plurality of birefringent plates is a power of two. 4. A polarization separating / combining means for separating one optical signal having an arbitrary polarization into two orthogonal polarization components or combining two orthogonal polarization components to generate one optical signal. 4. The variable optical delay line according to claim 1, further comprising: a wavelength plate that rotates a polarization plane of one polarization component of the polarization splitting / combining means by 90 degrees. 5. The variable optical delay line according to claim 1, wherein the input / output separating means and the mirror are removed, and a lens for synthesizing output light is provided instead. 6. A first constituent element group is all constituent element groups excluding the input / output separating means and the mirror,
Further, a second component group having the same configuration as the first component group is provided, and the second component group is arranged in the direction opposite to the optical axis with respect to the first component group. The variable optical delay line according to any one of claims 1 to 4.