JPS60184225A - Optical fiber type isolator - Google Patents

Abstract

PURPOSE:To make a large-sized heavy polarizer/analyzer, Farady rotating element, or lens unnecessary to make a device small-sized and light-weight by producing an isolator directly in an optical fiber. CONSTITUTION:A thin Farady rotating element 2 consisting of a bismuth substitution gadolinium iron garnet is interposed between end faces of two optical fibers and is adhered or is inserted to grooves formed on fibers. Dielectric metallic multilayered film polarizers 3 and 3 inclined at 45 deg. to each other are formed on individual optical fibers, and such magnetic field is given that the Farady rotation angle is 45 deg.. After the Farady rotating element 2 and dielectric metallic multilayered film polarizers 3 are formed on the way of optical fibers 1, they are polished in accordance with peripheral faces of fibers. A groove 10 is cut up to a core 8 from the side of the optical fiber 1 consisting of the core 8 and a clad 9. Though the groove 10 is cut with an arch shape, dielectric films 4 and metallic films 5 are formed alternately on a bottom 11 in parallel with the bottom. Many parallel dielectric films and metallic films are formed in this manner to cover the core 9.

Description

[Detailed description of the invention] (7) Technique branch The present invention relates to an optical fiber isolator.

Light emitting diodes, laser diodes, and the like are used as light sources in optical communication systems. This is because it has the advantages of being small and capable of direct modulation.

A device using a laser diode is particularly suitable for transmission over a distance of 1%. However, the laser diode 1 has a drawback in that the presence of return light changes the oscillation conditions and makes the operation unstable.

Therefore, in an optical communication system using a semiconductor laser (laser diode) as a light source, an optical isolator is used to cut the returning light.

The optical Eitzleh light beam consists of a Faraday rotator sandwiched between a polarizer and an analyzer. A Faraday rotator rotates the plane of polarization of incident light by 45 degrees. The polarization direction of the polarizer and the polarization direction of the analyzer are shifted by 45 degrees. The light that passes through the optical isolator in the forward direction passes through the polarizer and becomes linearly polarized light with a fixed plane of polarization, and the plane of polarization is rotated by 45° by the Faraday rotator, which matches the polarization direction of the analyzer. pass. Light entering in the opposite direction cannot pass through the polarizer because its plane of polarization rotates in the opposite direction and becomes perpendicular to the polarization direction of the polarizer.

(g) Faraday rotator The Faraday rotation angle is proportional to the length table of the rotor and the magnitude of the magnetic field H applied along this length.

The proportionality constant is called the Verdet constant.

A material with a large Verdet constant and transparent to light must be selected for the Faraday rotation angle.

Materials with the Faraday effect are sometimes called magneto-optical materials. The most well-known material for this material is YI
G (yttrium, iron, garnet).

The Verdet constant is a value specific to a material, but it changes depending on the wavelength of the light that passes through it.

It is known that HOYA glass FR-5 has a large Verdet constant when magnetized by short wavelength light (0.8 μm or less). Optical isolators using this paramagnetic glass have also been put into practical use.

However, the Verdet constant of FR-5 glass changes in inverse proportion to the square of the wavelength of light (to be exact, to the 2.4th power).

For this reason, the Verdet constant becomes too small for light with a long wavelength (1.3 to 1.5 .mu.17+), and it cannot be used as an optical isolator.

Transmission loss of optical fibers and stable semiconductor lasers (In
Optical communication systems using light in the long wavelength band are particularly promising due to the existence of P-type light. Therefore, an optical isolator that is effective in this wavelength range is required.

(1) YIG optical isolator Currently, the only optical isolator actually manufactured for long wavelength bands is one using YIG. A single crystal obtained by growing a YIG single crystal and cutting it may also be used. A thick film single crystal may also be used.

FIG. 1 is a perspective view of a known YIG Faraday rotation element. On the thin substrate 50, a Gd-YIG filler plate 51 is placed.
was grown by liquid phase epitaxial growth. Optical axis 52
holds the Gd-YIG film 51 parallel to the plane, and applies a magnetic field H parallel to the optical axis 52. The polarization plane 53 of the incident light becomes a polarization plane 54 rotated by 45° on the output side.

FIG. 2 is an enlarged sectional view of an optical isolator using this YIG Faraday rotation element.

One cylindrical permanent magnet or a plurality of permanent magnets 5 whose magnetization directions are parallel to each other so as to surround the Gd-YIG film 51
5 is placed. Ferromagnetic yokes 56 and 56 are provided so as to face both ends of the permanent magnet 550. yoke 56
．． At the center of the end face of 56 is a gradient index lens 57.5.
7 is inserted.

The lenses 57, 57 and the YIG film 51 are arranged on the same straight line.

These members are secured within housing 58. A polarizer 59 and an analyzer 60 are attached to the end face of the gradient index lens 57.57.

YIG is transparent at wavelengths from around 1 μm to the long wavelength side, and has a large Naberde constant. The length required for 45° rotation is 2 to 3 MM (λ = 1.2 to 1.3 μm) at saturation magnetization.
That's about it.

Those shown in FIGS. 1 and 2 are T, Aoyama,
T.

Hibara, and Y, 0hta, Light
t Waveguide Tech.

This was proposed in L T-1, 280 (1973).

Although the characteristics of YIG as a Faraday rotation element are satisfactory, this optical isolator has two drawbacks.

Since the YIG film 51 operates in bulk, a lens is absolutely necessary to pass light through it. Also,
Polarizers and analyzers are large-sized elements that are difficult to align with single-mode optical fibers. Even if the optical axes match, the light passes through many media with different refractive indexes, so the loss due to reflection is extremely large.

Since a single mode fiber has an extremely narrow core diameter of about 5 μm, it is difficult to make light enter the end face of the single mode fiber.

B) Location of the problem In optical communications, when a semiconductor laser is used as a light source, an optical isolator is required to block returning light. Light propagates through optical fibers. If the light source is a semiconductor laser, a single mode optical fiber is often used.

When the optical fiber and the optical isolator are separated and there is a space between them, light is strongly reflected between media with different refractive indexes, resulting in large reflection losses.

It is desirable that the insertion loss of the optical isolator be small.

The refractive index of YIG is 2.2, and the refractive index of optical fiber is also approximately 1.
．． It is 5. When light is emitted into the air (refractive index is 1), II
Not only is it difficult to match fl+, but the loss is large.

Ultimately, an optical fiber isolator is more desirable than a simple optical isolator as an isolator for use with optical fibers.

An optical fiber isolator is an optical isolator made as part of an optical fiber. This refers to an optical fiber and an optical isolator that are connected to each other without air intervening between them.

For this purpose, not only must the Faraday rotation element be made to the dimensions of the fiber, but also the polarizer and analyzer must be made to the dimensions of the fiber.

(4) Polarizer, Analyzer Polarizers and analyzers are elements that convert light having various polarization planes into linearly polarized light having a polarization plane in a fixed direction. Polarizing prisms, polarizing plates, etc. are used. Both elements are large in size and cannot be fabricated as part of an optical fiber.

Polarizers based on new principles are needed.

The present inventor has already invented a dielectric metal multilayer film polarizer based on a completely new principle (Patent application No. 58-206801)
, filed on November 1, 1981).

A perspective view is shown in FIG.

The dielectric metal multilayer film polarizer 3 is made by laminating a large number of thin dielectric films 4 and thinner metal films 5 alternately.

In FIG. 3, the direction in which light travels is the Z axis, and the direction perpendicular to this is the XSY axis. Dielectric #4 and metal film 5 are xZ
Assume that it is parallel to the plane.

The metal film 5 has a thickness of several tens to hundreds of .mu.m, and is
The l'E body membrane 4 is several thousand angstroms, which is on the order of the wavelength of light.

The light traveling in the Z-axis direction and incident on the dielectric metal multilayer film polarizer 3 can be considered to be a linear combination 111 of the electric field Ex in the X-axis direction and the electric field Ey in the Y-axis direction.

Since the light having an electric field in the X-axis direction becomes parallel to the electric field and the metal film 5, a current is generated in the metal film. This generates Joule heat and causes energy loss, so the electric field Ex in the X-axis direction
decreases rapidly. In other words, light that has an electric field in the X-axis direction is significantly attenuated by this dielectric metal multilayer film and cannot pass through it. 5 will be perpendicular. Since the electric flux density is continuous at the interface, polarized charges are generated at the interface, and the electric field in the Y direction inside the metal becomes zero. Therefore, no current flows in the metal film,
No fever occurs either.

Light having an electric field Ey in the Y-axis direction is difficult to attenuate.

As a result, the dielectric metal multilayer film only transmits light having an electric field perpendicular to the film surface. This is therefore a polarizer. Since the electric field in the X-axis direction attenuates extremely quickly, the thickness T of the dielectric metal multilayer film in the Z direction may be extremely small.

Since light with an electric field in the Y-axis direction corresponds to the 7M mode, and light with an electric field in the X-axis direction corresponds to the TE mode, it can be said that this polarizer passes the 7M mode but not the TE mode.

When the dielectric film is thin, the attenuation is large for both TMXTE, but
Its damping ratio is also large. When the dielectric film becomes thick, 2 to 3 μm, the attenuation ratio decreases, but the attenuation constant itself becomes low.

The thickness of the metal film may be 50 to 150 angstroms. As the thickness of the metal film increases, the attenuation of the TM wave increases.

The metal film is formed by evaporating aluminum or silver to form a dielectric fd.
The film can be formed by sputtering quartz.

(Force) Hismuth-substituted YIG It was discovered that a material in which Y in YIG (yttrium iron garnet) was replaced with B1 had a large bellbelt proboscis. H,T
akeuchi, in Shinagawa, an
jS, Taniguchi JJAP 12, 465
(1978') gave measurements of the Faraday rotation coefficient of YIG substituted with bismuth as shown in FIG.

The molecular formula is represented by Y 3-xB i x Fe 5012. The replacement kiX of B1 is a parameter. The horizontal axis is the wavelength of light (ttm). The vertical axis is the Faraday rotation coefficient (104 dag/cm). This is the rotation coefficient when a magnetic field is applied to obtain a saturation magnetic flux, and is not the help constant itself.

When Y- is replaced by B1, the sign of Faraday rotation changes, and when the amount of substitution X is increased, Faraday rotation increases. This increase in the rotation angle is particularly remarkable in the short wavelength region of 611 m or less. Even in the range of 0.6 to 0.8 μm, when the bismuth substitution amount X is increased, the Faraday rotation coefficient increases. However, one measurement in the long wavelength range, for example 1.8 μm, is not shown. From the trend of this graph, it is inferred that Faraday rotation is small in the long wavelength range.

The maximum amount of rare earth iron garnet that can be replaced by Bi differs depending on the rare earth element. Gadolium iron garnet has the largest amount of rare earth elements that can be replaced by B1.

H, Takeuchi, S, Ito, 1. Mi
Kami, and S., Taniguchi, J.
Appl Phys, 44, 4789 (1973
) is Gd 2 in which one of the three gadolinium atoms of the gadolinium iron net is replaced with bismuth.
Bx.

Fe, 0.2 was prepared and its Faraday rotation coefficient and absorption coefficient were measured. FIG. 5 is a graph showing the results.

The horizontal axis is the wavelength of light (μm), and the vertical axis is the Faraday rotation coefficient (
103deg/a), and absorption coefficient (α-1).

The solid line is ccf2Bi]Fe5. o12 The broken line is YIG data for comparison.

The absorption coefficient is also Y for bismuth-substituted gadolinium iron garnet.
IG hasn't changed much either. Absorption is 0 for light with a wavelength of 1.1 μm or more.

On the other hand, compared to YIG, the 7Alade-M conversion coefficient is
The bismuth-substituted gadolinium iron garnet is about an order of magnitude larger. The Faraday rotation coefficient decreases as the wavelength of light increases. However, as the wavelength of light becomes shorter, the absorption coefficient increases.

Since it is used as a Faraday rotation element in an optical isolator, it is necessary not only to have a large Faraday rotation coefficient, but also to have a small absorption coefficient.

Therefore, we will consider the figure of merit divided by the Faraday rotation coefficient. The unit is deg/dB. Figure 6 is G
It is a graph showing the wavelength dependence of the figure of merit of d 2 B i lFe 501□ and YIG. Both are 0.8μ
It has one maximum at about m. This is because the minimum absorption coefficient is around 0.8/1 m.

The figure of merit increases rapidly at 1.0 μη1 or more. This is due to the fact that the absorption is close to 0.

Importantly, bismuth-substituted gadolinium iron garnet has been used as an isolator material for 1.1-1.3 μm light in YI.
This means that it is more than 10 times better than G. For light with a wavelength of 1.3 μm, Faraday ~ rotation coefficient rotation, O
X 1103de/n, absorption coefficient is 0.05G
, the figure of merit is 4000deg/dB.

Then, the length required to obtain a Faraday rotation of 45° is 225 μm. Furthermore, as the substitution position of B1 is increased, the Faraday rotation coefficient increases.
Monthly, as reported by. - e.g. GdI, 1
The Faraday rotation coefficient of 15B41.15FeSO1'f is about 2800 deg/a at a wavelength of 1.3 μm. In this case, the length required to obtain a Faraday rotation of 45° is 196 μm.

When making optical fiber isolators, no lenses are used,
It is necessary to attach a Faraday rotation element directly between the optical fibers. Since no lens is used, the loss is particularly large when the core diameter is small, such as a single mode fiber. However, this decreases if the spacing between the optical fibers is small. Since the difference in refractive index between the core and cladding is small, the aperture angle of a single mode fiber is small.

As shown in Fig. 7, two optical fibers 1.1 are placed facing each other,
Assume that the connection is made using a connection medium 2 having a refractive index of n.

How the loss of optical power due to diffraction changes depending on the distance between the end faces of the optical fiber was calculated and shown in a graph in FIG. The horizontal axis is the optical fiber end face distance D (l
tm), and the vertical axis is the loss (dB) due to diffraction. Both input and output optical fibers have a core diameter of 9.177 m%, and the value Δ, which is the difference in refractive index between the core and cladding divided by the core refractive index, is 0.
．． At 24%, the wavelength of light is 1.3 μm. If the refractive index of the fiber core is 1.46, the normalized frequency V is 2.
It is 2. Since it is smaller than 2.4, it is a single mode fiber.

The parameter is the refractive index n of the connecting medium 2, n-1,1
, 46, 2, 2 is shown. Gd5-xBix
The refractive index for λ; 1.3 of Fe5o,2 is YrGO
Assuming that the value is equal to 2.2, from the graph of n=2.2, the diffraction loss when the interval is 225 μm is 1.95 dB. This is a surprisingly small value. This means that the refractive index n is 2.
This is due to the large value of 2.

Fresnel reflection loss is caused by reflection on the front and back surfaces.
It can be evaluated as 0.36 dB.

Then, the total loss caused by inserting the bismuth-substituted gadolinium iron garnet Faraday rotator (225 μm) into the optical fiber is about 2 dB.

If it is desired to reduce the total loss, it is possible to reduce the loss due to Fresnel reflection by attaching an anti-reflection film to the Faraday rotation element. Furthermore, as shown in FIG. 13, if the anti-reflection treated Faraday rotator element is divided into a plurality of pieces so that the total thickness is equal to a Faraday rotation angle of 45 degrees, the loss due to diffraction can be reduced. This is because diffraction loss increases nonlinearly with the spacing.

(-+) Production of Gd5-xBixFe50,2 Bismuth-substituted gadolinium iron garnet is produced here by a flux method. Gd2O3, Bi2O3 and F
'e2o3 are mixed and melted in a platinum crucible at 1800°C for 5 hours. After this, 2.7° to 10.00°C
It was prepared by cooling at the same rate.

The lattice constant is 12.572, and the saturation magnetization at room temperature is 9.2.
It is 5 emu/g.

(ri) Proposed bismuth-substituted gadolinium iron garnet as an optical isolator element is a highly prepared material,
It was predicted that reducing the amount of hismuth substitution with -F would make it ferromagnetic, but it was not possible to create a material with a large amount of bismuth.

Proposals have already been made to use this material as a Faraday rotation element in an optical isolator.

By the Physical Properties Research Group of the NHK Broadcasting Fees Research Institute (
(Nikkan Kogyo Shimbun, October 1981, 31B) That proposal is being made. According to Kone, the Gatorini mentioned above
It is said that a bismuth-substituted gadolinium-iron garnet was produced in which 40% of gadolinium was replaced with bismuth, going one step further than replacing one-third of the shim element with bismuth.

It is reported that the Faraday rotation coefficient for light with a wavelength of 1', 15/7 m was 8200 deg/ctn. The length required to obtain a rotation angle of 45° is 140 μm.
It is.

The optical isolator made by this is 5 mm x 5 mm.
It is reported that the insertion loss was less than 1 dB.

This report does not mention polarizers or analyzers.

Also, considering its size and low insertion loss, it is inferred that this optical isolator uses a lens for coupling. Also, the polarizer may be one that uses a conventional polarizing plate.

It is thought that a conventional polarizing plate is treated with anti-reflection processing, and that the optical isolator simply performs bulk operation using a laser beam.

(k) Structure of the Invention As shown in a schematic perspective view in FIG. 9, the optical fiber type optical system of the present invention includes (1) one or more thin Faraday rotation elements 2 made of bismuth-substituted gadolinium iron garnet; (2) insert the two optical fibers between the end faces of the optical fibers 1.1 and glue them or insert them into the grooves created in the fibers;
(3) A magnetic field application mechanism that forms dielectric metal multilayer film polarizers 3.3 tilted at 45 degrees to each other, and is provided to surround the Faraday rotation element 2 and provides a magnetic field such that the Faraday rotation angle is 45 degrees. has been established.

Although the magnetic field application mechanism is not shown in FIG. 9, it can be constructed from a permanent magnet and a yoke.

In FIG. 9, two optical fibers 1.1 are butted together in a Faraday rotation element 2. In FIG. As already explained, the thickness of the Faraday rotation element 2 is 100 to 250 μm.
It is.

The dielectric metal multilayer film 3 can be formed in the middle of the optical fiber 1. In this case, the A part and B part of the optical fiber are
Originally, it was a single continuous fiber.

Alternatively, the dielectric metal multilayer 3 can be made of a separate material and applied as a thin strip between the end faces of the optical fiber. In this case, the optical fiber A,! :The part B is originally separate.

After forming the Faraday rotation element 2 and the dielectric metal multilayer polarizer 3 in the middle of the optical fiber 1, they are polished to match the circumferential surface of the fiber, resulting in a shape as shown in FIG.

(Production of optical fiber type inlator First, the 10th
As shown in the figure, a groove 10 is cut from the side of an optical fiber 1 consisting of a core 8 and a cladding 9 up to the core 8. The groove 1o is cut in an arcuate shape, but the bottom surface 11 must be flat. Preferably, the sides 12.12 of the groove are also flat.

In order to flatten the bottom surface 11 and side surfaces 12 of the groove 10, a directional etching method such as ion milling, sputter etching, or reactive ion etching is suitable.

Alternatively, the groove 10 can be mechanically cut out using a sharp knife.

Once the arcuate groove 10 is formed in this way, the dielectric film 4 and the metal film 5 are alternately formed on the bottom surface 11 so as to be parallel to the bottom surface 11.

For example, a layer of fused silica is formed into the groove 10 by sputtering to a thickness of 4,000 to 5,000 layers. On top of this, AI is deposited to a thickness of 50 to 200.
Add it so that it is about the size of a person. This process is repeated to form a large number of parallel dielectric films and metal films. The formation of the multilayer film must be repeated at least until the core 9 is formed. After the formation of the multilayer film including the core is completed, the rest may all be made of a dielectric film.

It is also possible to insert a polarized analyzer made of a thin flake of an alternating multilayer film of metal and electric material into a groove formed in the optical fiber.

The width of r:/710 may be several μm to several tens of lrn.

FIG. 11 is an enlarged perspective view of a dielectric metal multilayer polarizer fabricated in the middle of an optical fiber.

What is shown in FIG. 9 is a conceptual diagram of an optical fiber type isolator, which is actually covered and reinforced.

Also, since bismuth-substituted gadolinium iron garnet is a single crystal, it is not mechanically strong and is difficult to mold, so it must be inserted as a thin piece into the groove of the optical fiber created by the above procedure, or it can be inserted into the optical fiber end face as a rectangular thin piece. Alternatively, the periphery may be surrounded by adhesive 14.

Figure 12 shows an optical core f using a permanent magnet as a magnetic field applying mechanism.
An example of a bar type isolator is shown. The magnetic field of the permanent magnet 15 is guided by a ferromagnetic yoke 16, and becomes an axial magnetic field in the Faraday rotation element 2.

Furthermore, a plurality of Faraday rotation elements 2 may be provided as shown in FIG. 13. The total Faraday rotation angle should be 45°. This will further reduce diffraction loss.

(c) Effects Since the present invention creates an isolator directly into the optical fiber, it has the following effects.

(1) No large and heavy polarized analyzer, Faraday rotation element, or lens is required. Compact and lightweight (2) There is no need to repeat difficult tasks such as axis alignment.

(3) Since the light propagating through the fiber does not exit into the air, Fresnel reflection loss can be significantly reduced.

(4) Provide an isolator that can be effectively used for light having a wavelength of 0.8 to 1.5 μm.

(5) Easy to handle.

(iii) Applications The present invention can be used in optical communication systems using semiconductor lasers as light sources, optical measurement equipment such as optical fiber gyros, and other applied equipment.

[Brief explanation of drawings]

FIG. 1 is a perspective view of a Faraday rotator using Gd-YIG film. FIG. 2 is a sectional view of a known optical isolator using a Gd-YIG film as a Faraday rotation element. FIG. 3 is a perspective view of a dielectric metal multilayer polarizer invented by the present inventor. FIG. 4 is a graph of the measured value of the Faraday rotation coefficient with respect to wavelength when Y in YIG is replaced with bismuth and the bismuth substitution amount X is used as a parameter. Figure 5 shows bismuth-substituted gadolinium iron garnet Gd2
Absorption coefficient (α-1) and Faraday rotation coefficient (1) for light wavelength (μm) of Bt 1Fe, 0.2 and YIG
03deg/clR). The solid line is data for bismuth-substituted gadolinium iron garnet, and the broken line is data for YIG. Figure 6 shows bismuth-substituted gadolinium iron garnet and YI
It is a graph showing the figure of merit of G versus the wavelength of light (μm). The unit of figure of merit is (deg/clB). FIG. 7 is a cross-sectional view schematically showing a state in which two optical fibers are placed facing each other at a distance and coupled by a connecting medium 2 having a refractive index of n. No distinction is made between the core and the cladding.0 Figure 8 calculates the diffraction loss between the optical fibers shown in Figure 7, fl+! Graph shown as a function of distance. The parameter is the refractive index of the connecting medium. The wavelength of light is 1.
It is a single mode fiber with a core diameter of 3 μm and a core diameter of 9.1 μm, a cladding refractive index difference ratio Δ−0,0024, and a normalized frequency V=′2.2. FIG. 9 is a schematic perspective view of the optical fiber type isolator of the present invention. FIG. 10 is a perspective view showing an optical fiber with an arcuate groove formed therein. FIG. 11 is a perspective view of a dielectric metal multilayer film formed in an arcuate groove. FIG. 12 is a sectional view of an optical fiber isolator equipped with a magnet to apply a magnetic field to a Faraday rotation element. FIG. 13 is a perspective view showing an embodiment of an optical fiber type isolator provided with two Faraday rotation elements. 1... Optical fiber 2... Faraday rotation element 3...
... Dielectric metal multilayer film polarizer 4 ... Dielectric film 5 ... Metal film 8 ... Core 9 ... Land 10 ... Groove 11 ... Bottom surface of groove 12 ... ... Groove side surface 14 ... Adhesive 15 ... Permanent magnet 16 ... Yaw 17 ... Through hole

Claims (1)

[Claims] Two single-mode optical fibers L1 each consisting of a core 8 and a cladding 9 surrounding it, bismuth-substituted gallium iron garnet+-rl are crystallized. Or one or ' inserted into a groove cut into the tip fiber.
A Faraday rotary element 2 of FM number and each single mode optical fiber 1.1 of [in an arcuate groove 10 having a helix angle of 45° in the groove cut from the first side to the core 8. The dielectric film 4 and the metal film 5 are alternately laminated, or the dielectric film 4 and the metal film 5 are alternately laminated. A magnetic field in the direction of the i#I+ line is applied to the dielectric metal multilayer film polarizer 3 bonded to the end face of the arbitrary Hat of the optical fiber 1.1 and the Faraday rotation element 2 to form a Faraday rotation element 2. An optical fiber isolator characterized by an IS consisting of a magnetic field application mechanism that rotates the polarization plane of passing light by 45 degrees.