JPH06138410A - Optical isolator - Google Patents

Abstract

PURPOSE:To obtain the polarization dependence type optical isolator which provides about 40dB isolation by one stage. CONSTITUTION:The polarization dependence type optical isolator consists principally of a polarization beam splitter 13, a total reflecting prisms 14, a Faraday rotator 11, a 45 deg. rotary polarizing plate 12, a polarization beam splitter 15, and a total reflecting prism 16 and four polarizing members 17-19 are added. Return light in a reverse direction (from right) is substantially cut off by the glass members 17 and 18. More effects can be obtained by providing 90 deg. rotary polarizing plates between the polarization beam splitter 13 and total reflecting prism 14, and polarization beam splitter 15 and total reflecting prism 16.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization-independent optical isolator used in the fields of optical communication and optical measurement.

[0002]

2. Description of the Related Art In a large-capacity optical communication system using an optical fiber, in order to amplify an optical signal that is attenuated during transmission, conventionally, the optical signal is first converted into an electric signal and then amplified. Then, an electric amplifier that once again converts it into an optical signal has been used. However, in recent years, the development of an optical fiber amplifier that amplifies an optical signal as it is without converting the optical signal into an electric signal has been advanced.

An optical fiber amplifier is a device that guides an optical signal to be amplified into an optical fiber doped with a rare earth element such as Er and injects a laser beam for excitation to amplify the original optical signal. An optical fiber amplifier used in a large-capacity optical communication system can usually obtain an amplification factor of about 30 dB. The optical fiber amplifier has characteristics such as a high C / N ratio as compared with the conventional electric optical signal amplifier, and is expected to be replaced by the electric optical signal amplifier in the future.

An optical isolator is an essential element in an optical fiber amplifier because of its structure. This is because if return light is generated in the laser light source for excitation, the oscillation of the laser light source is disturbed and becomes noise, which is amplified inside the optical fiber and interferes with the signal light. On the other hand, when the amplified signal light enters the transmission optical fiber as return light, it interferes with the signal light attenuated by long-distance transmission and disturbs the waveform of the optical signal. Will be needed. That is, in the optical fiber amplifier, optical isolators are required at a total of two places, that is, the connection end of the optical fiber on the incident side and the emission end of the laser light source for excitation.
Further, at least the optical isolator at the optical fiber connection end on the incident side of the signal light is required to be a polarization independent type. This is because the signal light inside the optical fiber is generally considered to be elliptically polarized light, and therefore, when a polarization-dependent optical isolator is used, a large attenuation of the signal light cannot be avoided. In a large-capacity optical communication system using optical fibers, the amount of attenuation when transmitting through an optical isolator must be minimized in order to prevent transmission loss of signal light as much as possible.
Therefore, it is considered that the polarization-dependent isolator cannot be used for this purpose.

From the above description, it is expected that the demand for the polarization-independent optical isolator will be greatly increased by the practical use of the optical fiber amplifier. However, there are the following problems regarding the optical characteristics. I'm holding.

[0006]

Since the polarization-independent optical isolator has a more complicated internal structure than the polarization-dependent optical isolator, it is difficult to design an optical isolator having a large isolation. For this reason, while the polarization-dependent optical isolator generally achieves isolation of about 40 dB, the conventional polarization-independent optical isolator normally has a level of 30 dB per stage.
Only the following isolation was obtained. Since this value is not sufficient for an optical isolator used in an optical fiber amplifier, conventionally, measures such as using polarization independent optical isolators in series in two stages have been taken. In this case, the price of the optical isolator as a whole is expensive because it is used in two stages, and the insertion loss of the transmitted light is significantly increased as compared with the case where only one stage is used. Therefore, it has been desired to develop a one-stage type optical isolator whose isolation is about the same as that of the polarization dependent type. The present invention proposes a structure of a single-stage polarization-independent optical isolator with high isolation that can solve this problem.

FIGS. 6 and 7 are perspective views showing the appearance and configuration of a conventional polarization-independent optical isolator using the above-mentioned glass polarization beam splitter and having insufficient isolation alone. Rotor, 45 ° optical rotation plate 12, polarization beam splitter 13, total reflection prism 1
4, a polarization beam splitter 15, and a total reflection prism 16. Reference numeral 23 indicates the optical axis, 24 indicates the direction of the external magnetic field, and 29 indicates the total reflection surface. Illustration of a permanent magnet for applying an external magnetic field, an external housing, etc. is omitted.

The above-mentioned behavior of the optical isolator is known and will be clarified in the description of the embodiments of the invention of the present application, and therefore it is omitted here, and the isolation of the conventional optical independent optical isolator will be described below. I will explain mainly the inferiority compared to the polarization dependent type, but before that,
It should be noted that the isolation of this type of optical isolator has its upper limit determined by the optical element having the smallest extinction ratio among the constituent optical elements. This problem of reduced isolation results from the fact that the extinction ratios of the two types of linearly polarized light that are separated are not necessarily high. The forward incident light that has entered the polarization beam splitter 13 made of glass along the optical axis 23 is transmitted light (Tp) whose optical path is parallel to the incident light.
Light) and reflected light (Ts light) whose optical path is vertical are separated into two types of linearly polarized light. The extinction ratio of Tp light is generally 30 dB to 4
Although it is 0 dB, the extinction ratio of Ts light is much lower, generally about 20 dB to 25 dB. Considering that the optical characteristics are further deteriorated when assembling the optical isolator, it is impossible to obtain an isolation of 30 dB or more with the polarization independent optical isolator having the shape shown in the conventional example.

In some cases, a rutile polarization element is used as the polarization separation means, but the polarization separation characteristic of the element itself is 40 dB.
Although it is sufficiently high at ˜50 dB, there is a problem in the polarization separation system of the rutile polarization element, and it cannot be said that the optical characteristics as an optical isolator are sufficient. This is because, in the polarization splitting method using the rutile polarization element, the incident light is split into two types of linearly polarized light that are parallel to each other. Normally, the separation distance between linearly polarized light cannot be set too large due to the structure. This is because it is impossible to completely separate the two types of linearly polarized light inside the optical isolator.

As described above, in the conventional polarization-independent optical isolator, it is structurally impossible to obtain the same isolation as that of the polarization-dependent optical isolator. Therefore, it cannot be used in an optical fiber amplifier or the like. Therefore, the optical isolators were used in two layers.

On the other hand, the present invention is intended to provide a polarization-dependent optical isolator capable of obtaining isolation of about 40 dB in one stage.

[0012]

In contrast to the polarization-independent type, the polarization-dependent type optical isolator generally uses polarizing glass as a polarizing element to obtain an isolation of about 40 dB. The extinction ratio of polarizing glass is 45 dB to 50
The optical element having a high dB and therefore directly controlling the isolation value is a Faraday rotator having an extinction ratio of about 45 dB. Further, it can be seen from this optical characteristic value that it is possible to sufficiently manufacture an optical isolator having an isolation of about 40 dB. From this fact, a polarizing glass is placed inside a polarization-independent optical isolator that uses a polarization beam splitter, and the extinction ratio of the separated linearly polarized light that passes through the inside of the optical isolator is improved on the way. It was conceived that the isolation of the dependent optical isolator could be improved to at least 40 dB or more, and the initial purpose could be achieved.

That is, according to the present invention, the first polarization beam splitter and the first total internal reflection prism which is paired with the first polarization beam splitter and is installed on the lateral side surface of the polarization beam splitter are used to form two forward incident light beams. A beam splitting means for splitting into parallel linearly polarized light, a Faraday rotator provided on the optical path of the two linearly polarized lights, a 45 ° optical rotation plate, and
And a beam combining means for pairing the polarized beam splitter and a second total reflection prism installed on the lateral side surface of the polarizing beam splitter, for combining the two linearly polarized lights to emit forward outgoing light. In a polarization-independent optical isolator provided, the light is incident on the optical path of each of the two linearly polarized lights between the beam splitting means and the Faraday rotator and between the 45 ° optical rotation plate and the beam combining means. An optical isolator comprising a polarizing glass having a polarization direction that minimizes the attenuation of two separated linearly polarized lights is obtained.

According to the present invention, in the above optical isolator of the present invention, between the first polarization beam splitter and the first total reflection prism and between the second polarization beam splitter and the second total reflection prism. A first 90 ° optical rotation plate and a second 90 ° optical rotation plate are respectively mounted between the reflecting prisms, and on the optical path of the first linearly polarized light and the second linear polarization.
The two polarization glasses disposed between the beam separation means and the Faraday rotator on the linearly polarized light path of (1) as a single element, and between the 45 ° optical rotation plate and the beam combining means. An optical isolator is obtained in which the two polarizing glasses arranged in the above are configured as a single element.

According to the above construction, the return light from the opposite direction is substantially blocked by the two polarizing glasses arranged between the 45 ° optical rotation plate and the beam combining means.

[0016]

1 and 2 are an external perspective view and an internal structure of an optical isolator according to an embodiment of the present invention.
In these two figures, the same components as those used in FIGS. 6 and 7 are given the same numbers. In this embodiment, like the conventional example, a magnetic garnet film is used as the Faraday rotator 11 and one optical element made of quartz is used as the 45 ° optical rotation plate 12, and two glass polarization beam splitters 13 are used. , 15 and two total reflection prisms 15 and 16 were used. 4 particularly used in the present invention
Each of the polarizing glasses 17 to 20 is a plate-shaped glass of a type that absorbs a non-polarized component of transmitted light, and is an optical element manufactured by United States Uning under the trade name of "polar core". The extinction ratio is about 45 dB to 50 dB, which is a very high value compared to other polarization separation elements. Although not shown, a magnetic field applying means by a permanent magnet is provided in the vicinity of the Faraday rotator, and the Faraday rotator 11 which is a magnetic garnet film in the figure is magnetically saturated. The Faraday rotation angle is 45 °. The extinction ratio of each optical element is about 45 dB for the Faraday rotator 11 and 45 dB to 50 dB for the 45 ° optical rotation plate 12. On the other hand, the glass polarization beam splitters 13 and 15 are
The extinction ratio of light is 30 dB to 40 dB, and that of Ts light is 20 dB to 25 dB. Furthermore, total reflection prism 1
In 4 and 16, it is 30 dB to 40 dB. As a whole, the extinction ratio of this optical isolator can be 40 dB or more.

Since the isolation of the optical isolator is determined by the optical element having the smallest extinction ratio among the constituent optical elements, if the optical isolator does not include the polarizing glass, the value is 20 dB to 25 dB or it. It is the following.

Next, the behavior of the optical isolator described above when elliptically polarized transmitted light is incident on the optical isolator described above will be described. The incident light from the forward direction is
First, the light enters along the optical axis 23 in the figure, and at the polarization beam splitter 13 which is a polarization separation prism, only the polarization component perpendicular to the paper surface is reflected in a direction perpendicular to the plane and separated into two different polarization components. It Polarizing beam splitter 13
The polarized component horizontal to the paper surface that has been transmitted through is as follows: polarizing glass 18, Faraday rotator 11, 45 ° optical rotation plate 1
2. It passes through the polarizing glass 20 in order, and finally reaches the polarization separation prism 15, where it is combined with the polarization component perpendicular to the previously separated paper surface and emitted from the optical isolator. On the other hand, the polarization component perpendicular to the plane of the paper separated by the polarization splitting prism 13 in the direction perpendicular to the sheet is next made incident on the total reflection prism 14 and is totally reflected in the direction perpendicular to the polarization maintaining the polarization direction. Polarizing glass 17, Faraday rotator 1
The light is transmitted through the 1, 45 ° optical rotation plate 12 and the polarizing glass 19 in order and reaches the total reflection prism 16. Here, the light is totally reflected again in the direction at right angles, reaches the polarization separation prism 15, and is combined with the polarization components that are horizontal to the mutually separated paper surface. In the meantime, the directions of the polarization planes of the respective polarization components that are transmitted through the optical element are subjected to Faraday rotation of 45 ° clockwise with respect to the traveling direction when transmitting through the Faraday rotator 11 in this embodiment, Further, when passing through the 45 ° optical rotation plate 12, it is configured to rotate counterclockwise by 45 °.
Therefore, the directions of the polarization planes of the respective polarization components reaching the polarization separation prism 15 and the total reflection prism 16 are the same as the directions when the polarization separation prism 13 is emitted.

The insertion loss of the transmitted light in the forward direction that passes through the isolator through the above process is a slight loss that is received when transmitting each optical element and a loss that is caused by an optical positional deviation that occurs when the optical isolator is assembled. Furthermore, the optical loss due to the installation of the polarizing glass in the optical path is integrated. Of these, only optical loss is peculiar to the optical isolator of the present invention. The outline is that the extinction ratio transmitted through the prisms is 20 dB to 40 d, respectively.
When the polarization component of the transmitted light which is B is transmitted through the polarization glass immediately after that, only the component perpendicular to the polarization plane is attenuated and 45
This is because linearly polarized light of about dB is obtained, and the attenuation amount at this time is about 0.02 dB in calculation. Similar losses are
The linearly polarized light transmitted through the 45 ° optical rotation plate 12 also occurs when it is transmitted through the polarizing glass again, but the amount of attenuation is smaller than that in the above case, and even if they are summed, they do not cause a large insertion loss. Therefore, it can be said that the insertion loss in the forward direction of the optical isolator according to the present embodiment is approximately equal to the insertion loss of the conventional optical isolator (about 0.2 to 0.3 dB).

FIG. 3 is a diagram for explaining the behavior when the elliptically polarized return light enters the above optical isolator from the opposite direction. Similarly, the return light from the opposite direction is incident from the right side along the optical axis 23 in the figure, and is reflected by the polarization beam splitter 15 in the direction perpendicular to only the polarization component perpendicular to the paper surface. It is separated into polarization components different from each other. The polarization component horizontal to the paper surface that has passed through the polarization separation prism 15 as it is passes through the polarization glass 20, the 45 ° optical rotation plate 12, and the Faraday rotator 11, and finally reaches the polarization glass 18 where it is absorbed by the polarization glass. . On the other hand, the polarization component perpendicular to the plane of the paper separated by the polarization beam splitter 15 in the direction perpendicular to the sheet is next incident on the total reflection prism 16 and is totally reflected in the direction orthogonal to the polarization direction while maintaining the polarization direction. Polarizing glass 19, 45 ° optical rotation plate 1
2. Polarizing glass 1 through the Faraday rotator 11 in order
7 is reached, where it is likewise absorbed. Since the return light from the opposite direction has a Faraday rotation in the counterclockwise direction, the direction of the polarization plane of each polarization component passing through the optical element during this time is first changed to the signal direction when passing through the 45 ° optical rotation plate 12. On the other hand, when it is rotated counterclockwise by 45 °, when it is transmitted through the Faraday rotator 11, it is also counterclockwise rotated by 45 °. Therefore, the orientations of the polarization planes of the respective polarization components reaching the polarizing glass 18 and the polarizing glass 17 are perpendicular to the orientation when exiting the polarization separation prism 15,
Therefore, the directions of the polarization planes of the above polarizing glasses are also orthogonal to each other. Therefore, each of the polarization components of the return light separated by the polarizing glass 18 and the polarizing glass 17 is absorbed and does not reach the polarizing beam splitter 13 and the total reflection prism 14 on the left side of the optical path in the figure.

Here, the reverse insertion loss of the optical isolator of the embodiment is as follows. The components of the return light absorbed by the polarizing glass 18 and the polarizing glass 17 are only the components, which are perpendicular to the polarization plane, of the polarization components of the separated returning light reaching the respective polarizing glasses. The extinction ratio of the separated return light is 45 dB to 50 dB after passing through the polarizing glass 19 and the polarizing glass 18,
Thereafter, when passing through the two optical elements of the Faraday rotator 11 and the 45 ° optical rotation plate 12, the extinction ratio of the Faraday rotator influences and decreases to about 45 dB. Therefore, the attenuation amount of the returning light by the polarizing glass 18 and the polarizing glass 17 is about 40 dB to 45 dB, including the reduction due to the assembly error of the optical isolator. Two
The respective return lights passing through the two polarizing glasses finally enter the polarization beam splitter 13, are combined there, and are emitted to the forward direction side. Since the amount of attenuation of the return light at this time is small, the intensity of the return light finally emitted from the optical isolator is about 40 dB to 45 dB. This numerical value is the isolation value of the optical isolator in the present invention, which is improved by about 10 dB as compared with about 30 dB of the conventional optical isolator shown in FIGS. 6 and 7. From these facts, the isolator in FIG.
A one-stage polarization-independent optical isolator having an isolation value of dB or more is constructed.

FIG. 4 is a diagram showing the configuration of another embodiment of the present invention, in which the optical axes of the incident light and the emitted light in the forward direction of the optical isolator are shifted in parallel. In this embodiment, the directions of rotation of the polarization planes of the forward incident light in the Faraday rotator 11 and the 45 ° optical rotation plate 12 are the same, and they are separated while passing through these two optical elements. The polarization planes of the polarization components of each incident light are rotated by 90 °.

FIG. 5 is a diagram showing the configuration of still another embodiment of the present invention. This optical isolator is characterized in that the two types of transmitted polarized light that have been separated are both passed through the same polarizing glass. As shown in FIG.
A 90 ° optical rotation plate 21 is provided between the total reflection prism 14 and the total reflection prism 14, and a 90 ° optical rotation plate 22 is also provided between the polarization beam splitter 15 and the total reflection prism 16.
The incident light from the forward direction is first split into two types of linearly polarized light whose polarization planes are orthogonal to each other in the polarization beam splitter 13, and of these, it is emitted in a direction perpendicular to the incident optical path.
The polarized light component perpendicular to the paper surface then enters the 90 ° optical rotation plate 21, where the plane of polarization is rotated by 90 °. Further, it is reflected by the total reflection prism 14 and enters the polarizing glass 17 together with the other separated polarized component. Due to the existence of the 90 ° optical rotation plate 21, the directions of polarization planes of the linearly polarized lights which are parallel to each other are the same, and therefore both can pass through the same polarizing glass 17. After that, the linearly polarized light reflected by the total reflection prism 16 has its plane of polarization again rotated by 90 ° in the 90 ° optical rotation plate 22, and the polarized beam splitter 15 outputs another linear polarization.
The two separated polarization components are combined and emitted from the optical isolator.

When return light is incident on this optical isolator from the opposite direction, the polarization plane of the separated linearly polarized light is 90 ° on the optical rotation plate 22 as in the case of the incident light in the forward direction.
Then, the two kinds of separated linearly polarized light are finally attenuated in the polarizing glass 17 together. Also in this case, the isolation of the optical isolator is 40 dB or more, as in the case of the shapes of the other embodiments. Further, the advantage of the shape of the polarization-independent optical isolator of this shape is that since the constituent elements, that is, the polarizing glasses 17 and 18 need only be provided one for each of the two optical paths, it is possible to fix the assembly process and position adjustment. The process can be simplified as compared with the shapes of other optical isolator embodiments.

The polarization-independent three types of optical isolators described in the above examples are single-stage type and have isolation of 40 dB or more, and the insertion loss thereof is the same as that of the conventional optical isolator. Is about the same. This cannot be realized by the conventional optical isolator.

[0026]

As described above, by installing a plurality of glass polarizers having a high extinction ratio in the optical paths of both linearly polarized lights, a one-stage polarization-independent light having an isolation of 40 dB or more can be obtained. It is possible to construct an isolator. With the realization of this optical isolator, originally 40 dB
Although the above isolation is sufficient, it is possible to use the one-stage isolator having this shape in the industrial field where the two-stage polarization-independent optical isolator has been used. , A significant cost reduction in that field can be realized.

[Brief description of drawings]

FIG. 1 is a perspective view showing an external appearance of an optical isolator that is an embodiment of the present invention. The external magnetic field applying means is omitted.

2 is an exploded view of the optical isolator in FIG. 1 and forward incident light enters the optical isolator from the left side of the figure. It is the figure which showed the behavior which all penetrates to the right side.

FIG. 3 is a diagram showing a behavior in which return light in the opposite direction in FIG. 2 enters an optical isolator and is all absorbed inside the optical isolator.

FIG. 4 is an exploded configuration diagram of an optical isolator that is another embodiment of the present invention.

FIG. 5 is an exploded configuration diagram of an optical isolator which is still another embodiment of the present invention.

FIG. 6 is a perspective view showing an example of the appearance of a conventional optical isolator.

FIG. 7 is an exploded configuration diagram of the optical isolator in FIG.

[Explanation of symbols]

 11 Faraday rotator 12 45 ° optical rotation plate 13 Polarizing beam splitter 14 Total reflection prism 15 Polarizing beam splitter 16 Total reflection prism 17 Polarizing glass 18 Polarizing glass 19 Polarizing glass 20 Polarizing glass 21 90 ° Optical rotating plate 22 90 ° Optical rotating plate 23 Optical axis 24 Direction of external magnetic field

Claims (2)

[Claims] 1. A first polarization beam splitter and a first total reflection prism which is paired with the first polarization beam splitter and is installed on a lateral side surface of the polarization beam splitter, and transforms forward incident light into two parallel linearly polarized lights. Beam separating means for separating, a Faraday rotator provided on the optical path of the two linearly polarized lights,
Similarly, it is composed of a 45 ° optical rotation plate, a second polarization beam splitter and a second total reflection prism that is paired with the second polarization beam splitter and is installed on the lateral side of the polarization beam splitter. In a polarization-independent optical isolator comprising a beam combining means for emitting emitted light, between the beam separating means and the Faraday rotator, and the 45 ° optical rotation plate on the optical path of each of the two linearly polarized lights. An optical isolator comprising: between the beam combining means, a polarizing glass having a polarization direction that minimizes the attenuation amount of two linearly polarized light beams into which incident light is separated. 2. A first mirror 9 is provided between the first polarizing beam splitter and the first total reflection prism and between the second polarizing beam splitter and the second total reflection prism.
Attach the 0 ° optical rotation plate and the second 90 ° optical rotation plate respectively,
Further, the two polarizing glasses disposed between the beam separating means and the Faraday rotator on the optical path of the first linearly polarized light and the optical path of the second linearly polarized light are configured as a single element. The optical isolator according to claim 1, characterized in that the two polarizing glasses arranged between the 45 ° optical rotation plate and the beam combining means are configured as a single element.