JPS6356624A - Faraday rotator and optical isolator - Google Patents

Abstract

PURPOSE:To obtain low forward loss and high isolation by rotating the plane of polarization of incident light by Faraday effect by a specific angle of incident through a magnetooptic element and using an Nd-Fe-B magnet for the magnetooptic element. CONSTITUTION:Incident light shown by an arrow A strikes on an incidence surface where the reflecting film 10a of the magnetooptic element 10 is adhered and is projected from a projection surface through reflecting films 10c, 10b, and 10d. The plane of polarization of the projection light is rotated counterclockwise by 45 deg. from the plane of polarization of the incident light which becomes linear polarized light by being transmitted through a polarizing element 9 at the time of the projection 16. Return reflected light shown by an arrow B, on the other hand, is inputted to the element 10, reflected plural times in the element 10, and projected from the incidence surface. At this time, the return reflected light which becomes linear polarized light by being transmitted through a polarizing element 12 is rotated counterclockwise by 45 deg. as compared with the incident light. Therefore, the plane of polarization of the return reflected light which is projected is rotated counterclockwise by 90 deg. from the plane of polarization of the incident line when viewed from the side of an incidence hole 15. Therefore, it can not be transmitted through the 1st polarizer 9 and is not projected from the incidence hole 15.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] Kichiro relates to Faraday port-takes and optical isolators that utilize the Faraday effect.

[Conventional technology]

A Faraday rosator has a function of rotating the polarization plane of incident light by a predetermined angle within a magnetic field by the Faraday effect, and is used in light receiving devices for optical pickups and the like. On the other hand, an optical isolator is a device that utilizes the Faraday port and take, and has the function of transmitting light in only one direction.For example, in optical fiber transmission, it is used to stabilize a light source such as a semiconductor laser. It is used to ensure high optical transmission quality by eliminating reflected return light reflected at optical fiber connection points and outgoing ends.

Here, FIG. 3(a) is a sectional view showing a conventional optical isolator, and FIG. 3(b) is a sectional view taken along the line Xl-Xt in FIG. 3(a).

As a conventional optical isolator, for example, the one shown in Fig. 3(a)
As shown in , there is a first polarizer 1 that linearly polarizes incident light and outputs it, a substantially cylindrical magneto-optical element (Faraday medium) 2 having a Faraday effect, and a samarium element that applies a magnetic field to the magneto-optical element 2. - A device is known that basically consists of a magnet 3 made of cobalt (S m-Co) and a second polarizer 4 that transmits light emitted from the magneto-optical element 2.

Note that 5 shown in FIGS. 3(a) and 3(b) is
This is a case for storing the above-mentioned optical isolator, and 6a shown in FIG.
Reference numeral b designates a rear side plate in which an exit hole 8 is formed to output the light that has passed through the second polarizer 4.

At 83 on this optical isolator, arrow △ (Fig. 3 (a)
) The incident light such as a laser beam (for example, a laser beam with a wavelength of 830 nm) propagating from the direction (forward direction) shown in FIG. and enters the entrance surface of the magneto-optical element 2,
Next, the light is reflected by the reflection film 2a and proceeds toward the other reflection 11i2b. Next, it is reflected by the reflective film 2b and the reflective film 2
Proceed towards a and reflect again v! The light beam is reflected by the magneto-optical element 7-2, travels toward the reflection film 2b, is reflected again by the reflection film 2b, and is emitted from the output surface of the magneto-optical element 7-2. At this time,
The emitted light from the first (!2 photon 1) undergoes multiple reflections (in this example; 4 reflections) within the magneto-optical element 2 as described above, and during propagation, its polarization plane changes to the magnetic field of the Sm-Goliti stone 3. Depending on the intensity, the light is normally rotated by 45 degrees and is emitted from the output surface of the magneto-optical element 2.Next, the emitted light from the magneto-optical element 2 enters the second polarizer 4, but at this time, the 262nd photon Since the inclination (45°) of the plane of polarization that can be transmitted by the second polarizer 4 is set equal to the rotation angle (45°) of the plane of polarization of the light emitted from the magneto-optical element 2, the second polarizer 4 is a magneto-optical element. 2
Allows the emitted light to pass through. Next, the transmitted light that has passed through the second (!a photon 4) exits from the exit hole 8. On the other hand, arrow B
The reflected return light propagating from the direction shown in FIG. ), the polarization plane that can be transmitted by the first polarizer 1 becomes 9.
The light becomes linearly polarized light with a plane of polarization tilted by 0°, and is incident on the first Q photon 1. Therefore, the reflected return light is transmitted to the first polarizer 1.
cannot be passed through. As described above, the optical isolator transmits only incident light from the forward direction.

Here, the reason why the incident light that is emitted from the first polarizer 1 and enters the magneto-optical element 2 is subjected to multiple reflections within the magneto-optical element 2 as described above will be described below.

The plane of polarization of the light incident on the magneto-optical element 2 is
While propagating inside, it rotates by a predetermined angle (usually 45°) due to the magnetic field strength of the Sm-Go magnet 3. This rotation angle θ (Faraday rotation angle θ) is determined by the formula θ=Vl)-1 (where ■ is Weerde's constant, i is the propagation optical path length of the incident light in the magneto-optical element, and H is the magnetic field strength). It will be done. Here, as the magneto-optical element 2, paramagnetic glass containing rare earth ions or the like has been used, but one with a sufficiently large value of Weerde's constant V has not yet been obtained. Therefore, in order to obtain a sufficient Faraday rotation angle θ, the incident light is multiple-reflected within the magneto-optical element 2 to lengthen the propagation optical path length, thereby obtaining a predetermined Faraday rotation angle θ. There is.

[Problem that the invention seeks to solve]

However, there is a limit to increasing the length of the propagation optical path by multiply reflecting the incident light within the magneto-optical element 2 and obtaining a predetermined Faraday rotation angle θ. This is because as the number of multiple reflections increases, the interference and scattering of incident light within the magneto-optic cable 2 becomes more severe, leading to an increase in forward loss in the optical isolator and isolation ((reverse loss)
This is because it causes a decrease in (forward loss)).

Furthermore, in order to multiple-reflect incident light within the magneto-optical element 2 and obtain an optical isolator with small forward loss and high isolation, the magneto-optical element 2 must be manufactured with high precision processing. However, this precise processing is extremely difficult and takes a long time.

By the way, next, as shown in FIG. 4(a), a through hole 3a for fitting and inserting the magneto-optical element 2 is formed in a 3m-Col1 stone 3 (dimensions: Width w1
=25m, height hl'''25m, side length (jl-=
20 Cocoon 9 Diameter of through hole 3a: 8Mφ) Through hole 3a
The magnetic field strength distribution along the central axis within is shown by curve a in FIG. 4(b). As shown by curve a, 3
The maximum magnetic field strength along the central axis in the through hole 3a of the m-co magnet 3 is 25000 e (2500 eels), and the magnetic field strength approximately equal to this maximum magnetic field strength is as follows.
This is a narrow region of ±4 feet from the center along the central axis of the through hole 3a, and it is not possible to apply a stable and strong magnetic field to the magneto-optic cord 2. Therefore, in the conventional optical isolator, the forward loss increases and the isolation margin decreases.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide an optical isolator with low forward loss and high isolation, and to provide a Faraday rotator with low forward loss. With the goal.

[Means for solving problems]

The present invention has been made to achieve the above-mentioned objects, and a first invention provides a magneto-optical element that rotates the polarization plane of incident light by a predetermined angle within a magnetic field by the Faraday effect, and a magnetic field applied to the magneto-optical element. Apply N d-F e-B
A second aspect of the present invention is a Faraday rotator comprising: a first polarizer that converts incident light into linearly polarized light and outputs the polarized light; a magneto-optical element that rotates the plane of polarization by a predetermined angle by the Faraday effect within a magnetic field; a Nd-Fe-8 magnet that applies a magnetic field to the magneto-optical element; This is an optical isolator characterized by comprising a second polarizer that transmits linearly polarized light rotated by an angle.

〔Example〕

Hereinafter, first, an optical isolator according to an embodiment of the second invention will be described in detail.

FIG. 1(a) is a sectional view showing the optical isolator according to this embodiment, FIG. 1(b) is a sectional view taken along the line X2-X2 in FIG. A perspective view showing the N d-F e-B llfl stone used in the optical isolator according to the example, and the same figure (b) is the N d-F e-B
FIG. 3 is a diagram showing the magnetic field strength distribution along the central axis inside a through hole of la stone.

As shown in FIG. 1(a), the optical isolator according to this embodiment includes a first polarizer 9, a magneto-optical element 10, and an Nd-Fe-8 magnet 11 that applies a magnetic field to the magneto-optical element 10. It basically consists of a second polarizer 12. Note that 13 shown in FIGS. 1(a) and 1(b) is a case that houses the optical isolator, and 14a shown in FIG. 1(a) is a front side plate that receives incident light. An entrance hole 15 is formed therein, and the illustrated rear side plate 14b is formed with an emission hole 16 through which the emitted light is emitted.

The first polarizer 9 and the second polarizer 12 are polarizers consisting of a polarizing beam splitter that converts incident light into linearly polarized light and outputs it.As will be described later, the plane of polarization that can be transmitted by the second polarizer 12 is , are arranged so as to be inclined at 45 degrees counterclockwise with respect to the plane of polarization that can be transmitted by the first polarizer 9.

The magneto-optical element 10 is made of substantially cylindrical paramagnetic glass (e.g.
Manufactured by HOYA@ FR-5, dimensions: 8sφ, t=20
．． ). As shown in FIG. 1(a), an antireflection film 10a is coated on the upper part of one bottom surface of the magneto-optical element 10 to serve as an incident surface, and a first film is formed on the inclined surface of the lower part of the bottom surface of the magneto-optical element 10. A reflective film 10b is applied, a second reflective film 10c is applied to the inclined surface of the upper part of the other bottom, and an anti-reflection film 10d is applied to the lower part of the other bottom. and becomes the exit surface.

Nd-Fe-B magnet 11 (e.g. NIE manufactured by Sumitomo Special Metals Co., Ltd.
OMAX magnet) has a rectangular parallelepiped shape (this example; width W2 = 25 #I + 1. height h2 = 25) as shown in Fig. 2 (a).
m.

The side length d2=20 m), and a d through hole 11a (this example; 8-way φ) for fitting and inserting the magneto-optical element 10 is formed across the center of the bottom surface.

Note that the Nd-Fe-3 magnet 11 has the same shape as the sm-com stone 3 described in the prior art section, and the magneto-optical element 10 is fitted and inserted into the through hole 11a, and the adhesive (not shown). This N d-F
The magnetic field strength distribution along the central axis inside the through hole 11a of the e-B magnet 11 is shown by the curved line in FIG. 2(b),
Its maximum magnetic field strength is 3500Qe (3,5K Oe)
T: "Yes. Also, N d-F e-B Vi 11 stones 1
1 has a uniform magnetic field strength approximately equal to its maximum magnetic field strength over a wide area of ±61 MI from the center along the central axis in the through hole 11a, and provides a magnetic field of stable strength to the magneto-optical element 10. can be applied.

Case 13. The front side plate 14a and the rear side plate 14b are made of aluminum. Further, in the inner main surface of the front side plate 14a, a first
A polarizer 9 is fixedly attached to the rear side plate 14 on the negative side.
A second polarizer 12 is fixedly attached to a portion of the inner main surface of b near the main surface where the exit hole 16 is formed. At this time, the second polarizer 12 is attached so that its transmissible polarization plane is inclined at 45° counterclockwise with respect to the transmissible polarization plane of the first polarizer 9, and the entrance hole is 15° first polarizer 9. vi1 pneumatic chordae 10. The second polarizer 12 and the exit hole 16 are arranged so as to be adjusted on the optical axis with respect to the incident light.

Next, the operation of the optical isolator according to this embodiment will be explained.

As shown in Figure 1(a), the direction of the arrow (forward direction)
Incident light (e.g., laser light with a wavelength of 830 nm) propagating from the polarizer passes through the entrance hole 15 and enters the first polarizer 9 . The part of the incident light that has passed through the first polarizer 9 and has become linearly polarized light enters the incident surface covered with the antireflection film 10a of the magneto-optical element 10, is reflected by the second reflective film 10C, and is reflected in the first polarized light. The light travels toward the reflective film 10b, is then reflected by the first reflective film 10b, and travels toward the emission surface coated with the antifouling film 10 (j).
It advances C and emits from the other exit surface. At this point, the polarization plane of the output light emitted from the output surface is N
The d-F e-B Ia magnet 11 is rotated counterclockwise to a predetermined angle (45° in this example) due to the magnetic field strength. Then, the emitted light emitted from the emitting surface of the magneto-optical element 10 is transmitted through the second polarizer 12 and emitted from the emitting hole 16 .

On the other hand, as shown in FIG. 1(a), the reflected return light incident from the direction of the arrow mark (reverse direction) passes through the exit hole 16 and enters the second polarizer 12. A portion of the reflected return light that has passed through the second polarizer 12 and has become linearly polarized light enters the magneto-optic element 10 from the output surface of the magneto-optic element 10, and is subjected to multiple reflections within the magneto-optic element 10 (this example; (reflected twice) and exits from the incident surface. At this time, the portion of the reflected return light that has passed through the second polarizer 12 and has become linearly polarized light already has a polarization plane rotated by 45° counterclockwise when viewed from the entrance hole 15 side, and When viewed from the entrance hole 15 side, the polarization plane of the portion of the reflected return light emitted from the surface is at a predetermined angle (in this example;
5°) is rotating counterclockwise. Therefore, the entrance hole 15
When viewed from the side, the plane of polarization of the portion of the reflected return light emitted from the incident surface is rotated 90° counterclockwise with respect to the plane of polarization that can be transmitted by the first polarizer 9. Therefore, the portion of the reflected return light that has passed through the second polarizer 12 and has become linearly polarized light cannot pass through the first polarizer 9 and is not emitted from the entrance hole 15 .

According to the optical isolake of this embodiment, since the Nd-Fe-B magnet 11 has a strong magnetic field strength, the portion of the incident light that has passed through the first polarizer 9 and has become linearly polarized light is inside the magneto-optical element 1o. Is it possible to get the specified fu by just reflecting twice? Radhe rotation angle θ
(45° in this example), and the number of multiple reflections within the magneto-optical element 10 can be reduced. Furthermore, as described above, the N d-F e-13 magnet 11 has a uniform magnetic field strength in the through hole 11 a over a wide area from the center along the central axis, and the magneto-optical element 10 has a substantially uniform magnetic field strength. A magnetic field of stable strength can be applied. Therefore, the first Q photon 9
Interference and scattering of the portion of the incident light that has become linearly polarized light after passing through the magneto-optical element 10 is suppressed, and the optical isolator according to this embodiment has low forward loss and high isolation.

The second invention is not limited to the embodiments described above.

As the first and second polarizers 9 and 12, other than a polarizing beam splitter, a polarizer such as a Glan-Thompson prism, a Rochon prism, or a Glan-Foucault prism may be used. Moreover, if the first and second polarizers 9 and 12 are not provided, it will not function as an optical isolator, but will become a Faraday rotator according to the embodiment of the first invention, and a Faraday rotator with small forward direction loss will be obtained.

In this example, paramagnetic glass is used as the magneto-optical element 10, but a magneto-optical element made of a ferromagnetic material such as yttrium iron garnet (YIG) may also be used, and its shape and dimensions may be adjusted as necessary. You may choose. Furthermore, in this embodiment, the case where the portion of the incident light that has passed through the first polarizer 9 and becomes linearly polarized light is reflected twice within the magneto-optic cable 10 has been exemplified, but the number of reflections is limited to this number of times. Instead, the magneto-optical element 10 may be precisely processed over a long period of time to cause the light to be reflected three or more times. In addition, in this embodiment, the polarization plane of the output light emitted from the output surface of the magneto-optical element 10 is compared with the polarization plane of the portion of the incident light that passes through the first polarizer 9 and becomes Vi line light. Although the case where the polarization plane is rotated counterclockwise by 45° is described, other rotation angles and polarization planes may be used at the expense of some forward loss. Furthermore, the antireflection films 10a and 10b do not necessarily need to be deposited.

Further, it goes without saying that the shape 9 dimensions and magnetic field strength of the N d-F e-B 'di stone 11 may be appropriately selected depending on conditions such as the desired rotation angle of the plane of polarization. Furthermore, in the embodiment, the N d-F e-8 magnet 11 was arranged so that the N pole was located on the left side in FIG. Of course, this is a good thing.

In addition, in this example, the wavelength of the incident light is 830.
Although a nm laser beam is illustrated, the wavelength can be appropriately selected depending on the magnetic field strength.

In addition, Nd-Fe-B magnets can be used in waveguide-type optical isolators made by growing crystals on a substrate (for example, gadolinium-gallium-garnet (GGG)) to form magneto-optical elements, or in fiber-type optical isolators. Good too.

〔Effect of the invention〕

According to the optical isolator of the second invention, forward loss is small and high isolation is achieved. Further, according to the Faraday low data of the first invention, the forward loss is small.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an optical isolator according to an embodiment, in which (a) is a cross-sectional view, and (b) is a cross-sectional view taken along line X2-X in (a).
FIG. 2(a) is a perspective view showing an Nd-Fe-8 magnet used in the optical isolator according to the embodiment, and FIG. 2(b) is a sectional view showing a 2-line cross section. Fig. 3 is a diagram showing a conventional optical isolator, and Fig. 3 (a) is a cross-sectional view;
FIG. 4(b) is a cross-sectional view taken along the line X+-X1 in FIG. 4(a), and FIG. 4(a) is a perspective view showing Sm-Co1 stone used in a conventional optical isolator; (b) is S
FIG. 3 is a diagram showing the magnetic field strength distribution along the central axis in the through hole of the m-Co magnet.

Claims (2)

[Claims] (1) A magneto-optic cable that rotates the polarization plane of incident light by a predetermined angle within a magnetic field by the Faraday effect, and an Nd-Fe-B magnet that applies a magnetic field to the magneto-optic element. Faraday Roseta. (2) a first polarizer that converts incident light into linearly polarized light and outputs it; and a magneto-optical element that rotates the plane of polarization of the light emitted from the first polarizer by a predetermined angle within a magnetic field by the Faraday effect;
It comprises an Nd-Fe-B magnet that applies a magnetic field to the magneto-optical element, and a second polarizer that transmits linearly polarized light that is emitted from the magneto-optical element and whose plane of polarization has been rotated to a predetermined angle. Features of optical isolators.