JP6243829B2 - 1μm band optical isolator - Google Patents

Description

  The present invention relates to a 1 μm band optical isolator, and more particularly to a 1 μm band optical isolator used for processing or marking in the field of industrial lasers.

Conventionally, CO ₂ lasers (10.6 μm) and lamp-pumped YAG lasers (1 μm) are representative devices for industrial laser processing machines for cutting, welding, marking and the like.

  In recent years, demands for processing performance have become more severe, and higher accuracy, higher output, and longer life have been demanded of laser processing machines. Among such market demands, fiber lasers are attracting attention. A fiber laser propagates 1 μm band light oscillated from a laser diode (LD) light source to a rare earth element doped fiber such as ytterbium (Yb) and amplifies it by a pumping LD, thereby producing high-precision and high-power laser light. It has the feature that it can output fiber. Compared to lamp-pumped YAG lasers of the same wavelength band, the pump light conversion efficiency is high, cooling requirements are low, and lamp pumping is unnecessary, so it is attracting attention for its advantages of low power consumption and long life. Yes.

  However, the fiber laser is characterized by its narrow emission spectrum and excellent conversion efficiency, but on the other hand, it is very sensitive to the return light from the reflected light, and it has a coupling end face to the optical fiber and high reflectivity. When the reflected light from the metal surface returns, the characteristics become unstable, and there is a risk that the LD light source unit is damaged due to high-power light emission. Therefore, for the stable operation of the fiber laser, in order to prevent the reflected light from returning to the light emitting element that is the light emitting light source, a one-way transmission function of light (forward light is applied between the light emitting light source and the workpiece). It is necessary to arrange an optical isolator that transmits light and blocks light in the opposite direction) to block the return light of reflected light from the optical fiber toward the light emitting light source (Patent Document 1).

Here, the optical isolator mainly includes a Faraday rotator, a polarizer disposed on the light incident / exit side of the Faraday rotator, and a magnet that applies a magnetic field in the light transmission direction (optical axis direction) of the Faraday rotator. Consists of three parts. When light enters the Faraday rotator in this form, a phenomenon occurs in which the plane of polarization rotates in the Faraday rotator. This is a phenomenon called the Faraday effect. The angle at which the polarization plane rotates is called the Faraday rotation angle, and its magnitude θ is expressed by the following equation.
θ = V × H × L
V is a Verde constant, a constant determined by the material of the Faraday rotator and the measurement wavelength, H is the magnitude of the magnetic field, and L is the length of the Faraday rotator in the optical axis direction. As can be seen from this equation, in a rotor having a certain Verde constant, when trying to obtain a desired Faraday rotation angle, the larger the magnetic field, the shorter the rotor length becomes. The longer the field, the smaller the magnetic field.

In general, in order to have the function of an optical isolator, the Faraday rotation angle needs to be about 45 °.
Specifically, the light incident on the optical isolator is transmitted through an incident / exit polarizer whose angle of polarization is rotated by 45 ° by a Faraday rotator. On the other hand, the return light makes use of the non-reciprocity of the Faraday rotator, the polarization plane is rotated 45 ° in the opposite direction to become an orthogonal polarization plane of 90 ° with the incident polarizer, and cannot be transmitted. The optical isolator utilizes this phenomenon to prevent the generation of return light.

JP 2012-83381 A

  In the conventional configuration, the Faraday rotator to be used, such as a terbium gallium garnet (TGG) crystal, has a Verde constant of about 0.135 min / (Oe.cm) at a wavelength of 1.06 μm. The length satisfying 45 ° needs to be about 20 mm. For this reason, in order to increase the magnetic flux density applied to the Faraday rotator, it is necessary to increase the surrounding magnet shape. As a result, there is a dimensional limitation within the apparatus, and the leakage magnetic field from the outside of the optical isolator is large and difficult to handle.

Therefore, in recent laser processing machines, especially optical isolators mounted on fiber lasers, there is a strong demand for miniaturization, and the materials used as the Faraday rotator of the optical isolators have a large Faraday effect and are used. A configuration in which the shape of the magnet to be formed is small is necessary.
That is, it is necessary to shorten the length of the Faraday rotator, but it is most realistic to solve the problem by using a crystal having a large Faraday effect, a magnet material having a high magnetic flux density, and a magnetic circuit. Further, the optical damage of the Faraday rotator due to high power light, which is a problem in a laser processing machine, is determined by the transmittance and length of the rotor, so it is more convenient that the transmittance is high and the length is short.

On the other hand, according to Patent Document 1, in order to reduce the size of the magnet, (Tb _X Y _1-X ) ₂ O ₃ (x = 0.6 to 1.0) is used as a Faraday rotator, and the magnet-shaped size is reduced. And shortening of the Faraday rotator (7.0 ≦ L ≦ 11.0 mm).
However, increasing the output power of fiber lasers for today's wider range of processing applications is steadily increasing and the output is constantly increasing. Along with this, optical isolators mounted on these high-power lasers are required to have lower loss characteristics at the same time as miniaturization, and the need for shortening the Faraday rotator is increasing.

  Therefore, the present invention has been made in view of the above problems, and an object thereof is to provide a small optical isolator having a short Faraday rotator.

In order to achieve the above object, the present invention is a 1 μm band optical isolator including a Faraday rotator and a magnet, and the Faraday rotator has a Verde constant at any wavelength of 0.9 to 1.1 μm. 0.45 min / (Oe · cm) or more, and the magnet includes a first hollow magnet disposed around the Faraday rotator and a second hollow magnet disposed with the first hollow magnet interposed therebetween. The second hollow magnet and the third hollow magnet are arranged so that the same magnetic poles face each other with respect to the first hollow magnet, and are applied to the Faraday rotator. The magnetic flux density B (T) is within the range of the following formula (1) and the length L (mm) of the Faraday rotator in the optical axis direction. ) Is within the range of the following formula (2), a 1 μm band optical isolator.
0.75 ≦ B ≦ 1.2 (1)
5.0 ≦ L <7.0 (2)

  As can be seen from the equation of the Faraday rotation angle of θ = V × H × L, when trying to obtain a desired Faraday rotation angle, first, the higher the value of the Verde constant, the Faraday rotator in the required optical axis direction. (Hereinafter, simply referred to as the length of the Faraday rotator), the required magnetic field, and hence the magnetic flux density can be reduced. As a result, the Faraday rotator can be shortened and the size of the magnet can be reduced.

  In the present invention, the Verde constant is 0.45 min / (Oe · cm) or more, which is a high value. Therefore, the size of the Faraday rotator and the magnet can be made smaller than that of the conventional product.

  Furthermore, in the present invention, the length L of the Faraday rotator is less than 7.0 mm, and the size of the Faraday rotator can be made smaller than the conventional product (7.0 mm or more) as described above (long). Can be shortened). If the length L of the Faraday rotator is increased from the Faraday rotation angle equation, the required magnetic flux density B decreases, so that the size of the magnet can be reduced or the number thereof can be reduced. Here, if the magnetic flux density B is made too small, the length L of the Faraday rotator needs to be made larger than before, and the loss of the optical isolator becomes large. 75T or more.

  On the other hand, if the length L of the Faraday rotator is reduced, the required magnetic flux density B increases and the size of the magnet also needs to be increased. In the present invention, since the lower limit of the length L of the Faraday rotator is 5.0 mm, the required magnetic flux density B does not become too large (1.2 T or less). Therefore, the size of the magnet can be prevented from becoming larger than necessary, and the overall size of the optical isolator can be reduced.

  Further, by arranging the first to third hollow magnets as in the present invention, the magnetic flux density where the Faraday rotator is located can be efficiently increased. As a result, it is possible to reduce the size of the magnet and hence the optical isolator.

The Faraday rotator is made of a single crystal or a ceramic material composed of (Tb _X Re _1-X ) ₂ O ₃ (x = 0.9 to 1.0), and Re is other than terbium. And at least one element selected from the group consisting of scandium and yttrium.

  The above is suitable as a material used as a Faraday rotator, and a higher Verde constant can be obtained when x is in the above range. As a result, the optical isolator can be further reduced in size.

Further, the Faraday rotator _may be comprised of a ceramic material composed of _{(Tb X Y 1-X)} 2 O 3 (x = 0.9~1.0).

  If it is such, it is low-cost and relatively easy to obtain.

  The Faraday rotator may have optical characteristics with an insertion loss of 1.0 dB or less and an extinction ratio of 25 dB or more in the length L (mm) in the optical axis direction.

  If it is such, it can be set as the optical isolator which has the optical characteristic of a low loss and a high isolation.

  The first hollow magnet, the second hollow magnet, and the third hollow magnet may be mounted inside a carbon steel casing.

  By housing in the carbon steel casing, a yoke material is formed around the magnet, so that the attractive force or attractive force of the magnet can be increased.

  As described above, according to the present invention, the Faraday rotator can be made shorter than the conventional product, the magnet can be made smaller, and the optical isolator can be further reduced in size. Further, by reducing the size of the optical isolator, the degree of freedom of the spatial dimension of the main body device having the optical isolator can be increased. Further, since the length of the Faraday rotator can be made shorter than before, it can be expected to have resistance against optical damage of the Faraday rotator which is a concern due to high power light input.

It is a cross-sectional schematic diagram which shows an example of the fundamental structure of the optical isolator of this invention. It is a graph which shows the relationship between the length of a Faraday rotator required in order to make a Faraday rotation angle into 45 degrees, and magnetic flux density. It is a graph which shows the relationship between the length of a Faraday rotator and magnetic flux density required in order to set the Faraday rotation angle to 45 degrees in Examples 1 and 2 and Comparative Example 1. 6 is a graph showing magnetic flux density distribution analysis results in the magnet configuration of Example 1; 10 is a graph showing magnetic flux density distribution analysis results in the magnet configuration of Comparative Example 1; It is explanatory drawing which shows the behavior of the polarization state of input light and return light in a polarization independent type optical isolator. It is a cross-sectional schematic diagram which shows an example of the fundamental structure of the conventional optical isolator.

Hereinafter, although an embodiment of the present invention is described in detail, the present invention is not limited to this.
The optical isolator of the present invention is preferably used for laser light having a wavelength band of 0.9 to 1.1 μm. Such lasers include lamp-pumped YAG lasers.

Here, first, the basic structure and principle of a polarization-independent optical isolator will be briefly described.
An optical isolator mounted on a high-power fiber laser is required to be a polarization-independent type in which each component is resistant to high-power light and is not affected by the polarization state of propagating light. In order to meet this demand, a birefringent crystal that separates the light beam by utilizing the difference in refractive index is optimal as the polarizer used. Typical birefringent crystals include yttrium vanadate (YVO ₄ ), rutile single crystal (TiO ₂ ), calcite single crystal (CaCO ₃ ), and α-BBO crystal that are transparent at a wavelength of 1.0 to 1.1 μm ( BaB ₂ O ₄ ), which can be used.

  Further, in order to make polarization independent, it is preferable to perform flat plate processing so that the optical axis of the birefringent crystal is approximately 45 ° with respect to the optical axis, and the thickness is proportional to the separation distance of extraordinary light. Therefore, it is only necessary to accurately process each thickness to satisfy a desired beam shift amount. Two plane birefringent polarizers are arranged as input and output polarizers, and a Faraday rotator having a Faraday rotation angle of 45 ° at any wavelength of 0.9 to 1.1 μm between them and a polarization plane at the same wavelength. A polarization-independent optical isolator is configured by arranging a 45-degree rotator rotating 45 degrees and a magnet for applying a magnetic field in the optical axis direction of the Faraday rotator around the 45-degree rotator.

FIG. 6 shows the behavior of the polarization state of the input light and the return light in the polarization-independent optical isolator having the above configuration.
In accordance with Snell's law, the input light is separated into extraordinary light that shifts in the optical axis polarization direction of the incident polarizer and normal light that travels straight in the orthogonal polarization direction with respect to the optical axis. The two separated lights are respectively rotated on the plane of polarization by the Faraday rotator, and further rotated on the plane of polarization by 45 ° in the incident 45 degree optical rotator. As a result, the polarization rotation angle of the extraordinary light and ordinary light becomes 90 degrees, and when incident on the output polarizer, the extraordinary light is replaced with ordinary light and the ordinary light is replaced with extraordinary light, and only the extraordinary light is shifted in the optical axis direction. As a result, the two beams separated in the incident polarizer are coincident in the output polarizer, and exhibit a polarization-independent function.

  On the other hand, when the return light enters the output polarizer, it is separated into extraordinary light and ordinary light as described above. When the two separated lights enter the 45-degree rotator, the plane of polarization is rotated by 45 ° in the same direction as the input light, and enters the Faraday rotator. At this time, due to the nonreciprocity of the Faraday rotator, the input light is rotated by 45 ° in the opposite direction to the polarization rotation direction. As a result, since the rotation angle of the polarization plane is 0 degree, in the incident polarizer, the extraordinary light is beam-shifted by 2 degrees as extraordinary light and goes straight, and the ordinary light goes straight as normal light. As a result, the return light is divided into two separated lights, and at the incident position of the input light, each is output in parallel while maintaining the separation distance of one polarizer, so that it functions as a polarization-independent optical isolator.

The polarization-independent optical isolator of the present invention will be described in detail.
FIG. 1 is a schematic cross-sectional view showing an example of the basic configuration of the optical isolator of the present invention. As an overall configuration, first, as shown in FIG. 1, in the optical isolator 1 of the present invention, the incident polarizer 2 and the Faraday rotator 3 (here, the desired Faraday rotation angle is 45 °), And the output polarizer 4 are sequentially arranged on the optical axis 5 from the left incident side to the right output side. In FIG. 1, an incident polarizer 2 and an output polarizer 4 are fixed on an optical axis 5 by a wedge glass 6. The incident polarizer 2 is fixed to the polarizer holder 7 on the incident side. On the other hand, the 45-degree optical rotator 8 and the output polarizer 4 are fixed to the polarizer holder 7 on the output side. An optical axis 9 is shown for the incident polarizer 2 and the outgoing polarizer 4.
A magnet 10 (first hollow magnet 11, second hollow magnet 12, third hollow magnet 13) is disposed on the outer periphery of the Faraday rotator 3.
The magnet 10 is mounted inside the carbon steel casing 14.

Hereinafter, the Faraday rotator 3 and the magnet 10 according to the present invention will be described in more detail.
(About Faraday rotator)
First, the Faraday rotator 3 will be described.
The Faraday rotator according to the present invention has a Verde constant at a wavelength of 0.9 to 1.1 μm of 0.45 min / (Oe.cm) or more, preferably a Verde constant at a wavelength of 1.06 μm of 0.45 min. /(Oe.cm) or more. The Verde constant is not particularly limited as long as it is 0.45 min / (Oe.cm) or more, but it is more preferable to have a larger Verde constant. By adopting such a large Verde constant, the length of the Faraday rotator 3 and the size of the magnet 10 can be further reduced.

  On the contrary, when the Verde constant is less than 0.45 min / (Oe.cm), the length of the Faraday rotator necessary for setting the Faraday rotation angle to 45 ° is increased, and it is difficult to reduce the loss of the optical isolator. It is. In addition, the required size of the magnet increases, making it difficult to reduce the size.

  The Verde constant may be measured according to a standard method. Specifically, an oxide having a predetermined thickness is cut out, mirror-polished, a Faraday rotator is set on a permanent magnet with a known magnetic flux density, and a Verde constant at a wavelength of 0.9 to 1.1 μm. Measure. The measurement conditions are 25 ± 10 ° C. and measurement is performed in the atmosphere.

Further, the shape of the Faraday rotator is not particularly limited except for its length, and may be triangular or quadrangular, but is preferably cylindrical. Here, a cylindrical Faraday rotator will be described as an example.
And the length L (mm) of a Faraday rotator exists in the range of following formula (2).
5.0 ≦ L <7.0 (2)
When the length L is 7.0 mm or more, the length of the Faraday rotator is not shorter than that of the conventional product, and it is difficult to reduce the size and loss of the optical isolator.
On the other hand, if it is less than 5.0 mm, the magnetic flux density for obtaining a desired Faraday rotation angle becomes large, so that it is necessary to increase the size of the magnet, and it is difficult to reduce the size of the isolator. Therefore, the length of the Faraday rotator needs to be 5.0 mm or more and less than 7.0 mm as described above.

The Faraday rotator used in the present invention is made of a single crystal or a ceramic material composed of (Tb _X Re _1-X ) ₂ O ₃ (x = 0.9 to 1.0), and Re is terbium. It may contain at least one element selected from the group consisting of a lanthanoid element group other than the above, scandium, and yttrium. Such a thing is suitable.

Here, Re may be one kind alone, or a plurality of Re may be included in any ratio. Among these, from the viewpoint of easy availability of raw materials, Re is preferably yttrium, gadolinium, and lutetium, and more preferably yttrium. That is, (Tb _X Y _1-X ) ₂ O ₃ (x = 0.9 to 1.0) is advantageous in terms of cost and availability.

The Faraday rotator used in the present invention may contain components other than the oxide represented by the formula (Tb _X Re _1-X ) ₂ O ₃ .
Other components that can be contained in the Faraday rotator that can be used in the present invention include alkaline earth metal oxides, Group 13 element oxides, Group 14 element oxides, other Group 4 elements, Metal oxide selected from the group consisting of Group 5 elements (V, Nb, Ta, etc.), Group 6 elements (Mo, W, etc.), and Group 17 elements (F, Cl, Br, etc.) However, it is not limited to these. In addition, two or more kinds of other components may be included, and the content thereof is preferably 0.000001 to 1.0% by mass, and 0.00001 to 0.1% by mass of the entire Faraday rotator. It is more preferable.

  The above metal oxide is contained, for example, as a dopant added at the time of producing a single crystal or a sintering aid added at the time of producing a ceramic. Moreover, when manufacturing the material of the Faraday rotator, the constituent components of the crucible may be mixed as subcomponents.

As a method for producing the oxide crystal, a known method may be used, and examples thereof include a floating zone melt method, a micro pulling method, a pulling method, a skull melt method, and a Bridgman method. For each of these methods, “Bulk Single Crystal Latest Technology and Application Development” (supervised by Yoshio Fukuda, CMC Publishing, March 2006), “Crystal Growth Handbook” (“Crystal Growth Handbook” For details, see Editorial Board, Kyoritsu Publishing Co., Ltd., September 1995.
In the production of an oxide single crystal, as described above, an alkaline earth metal oxide (for example, magnesium, calcium, strontium, barium) may be doped for the purpose of stable crystallization.

The oxide represented by the formula (Tb _X Re _1-X ) ₂ O ₃ is preferably a single crystal or a ceramic, but more preferably a ceramic because it can be synthesized at a low temperature.
When producing a single crystal of oxide, it is necessary to raise the temperature to bring the raw material into a molten state. For example, terbium oxide has a melting point of about 2600 ° C. and yttrium oxide has a melting point of about 2300 ° C. When these two solid solutions are produced, it is necessary to raise the temperature to an intermediate temperature between the two melting points.
On the other hand, in the case of ceramics, it is not necessary to raise the temperature to its melting point, and if it is pressure sintered, it can be produced below the melting point. At the time of sintering, it is possible to add a sintering aid to increase the sintering density for densification. In the case of ceramics, the selection of a crucible for melting a raw material to produce a single crystal is not limited to rhenium, tungsten, or alloys thereof because of the high temperature as in the case of producing a single crystal. The manufacturing cost can be further reduced.

  As a method for producing the transparent ceramic used as the Faraday rotator, a conventionally known production method can be appropriately selected and used, and is not particularly limited. Main production methods include hot isostatic pressing, solid phase and press molding, vacuum sintering using mold molding, etc. "From optical single crystals to optical polycrystals" Applied Physics, Vol. 75, No. 5, 579-583 (2006), Takanori Yanagiya, Hideki Yagi "Current Status and Future of Ceramic Laser Materials" Laser Research, Vol. 36 9, No. 544-548 (2008) and the like.

The Faraday rotator that can be used in the present invention has a length L in the optical axis direction (5.0 ≦ L <7.0) in order to realize an optical isolator function used at a wavelength of 0.9 to 1.1 μm. ) Preferably has optical characteristics such that the insertion loss is 1.0 dB or less and the extinction ratio is 25 dB or more.
The insertion loss is preferably lower if it is 1.0 dB or less, and the extinction ratio is preferably higher if it is 25 dB or more, but at least within the above range, it has low loss and high isolation optical characteristics. An optical isolator can be manufactured.
The optical characteristics such as insertion loss and extinction ratio can be measured at a wavelength of 1.06 μm according to a conventional method. The measurement conditions are 25 ± 10 ° C., and measurement is performed in the atmosphere.

The Faraday rotator that can be used in the present invention has a transmittance (light transmittance) of 70% at a wavelength of 1.06 μm and a length Lmm (5.0 ≦ L <7.0) in the optical axis direction. The above is preferable. If the transmittance of the Faraday rotator is 100% or less, the higher one is preferable, and the upper limit is not particularly limited.
The transmittance is measured by the intensity of light when light having a wavelength of 1.06 μm is transmitted through a Faraday rotator having a thickness of Lmm. That is, the transmittance is expressed by the following formula.
Transmittance = I / Io × 100
(In the above formula, I represents transmitted light intensity (intensity of light transmitted through a sample having a thickness of Lmm), and Io represents incident light intensity.)
In addition, when the transmittance | permeability of the oxide obtained is not uniform and the transmittance | permeability fluctuate | varies with a measurement location, let the average transmittance | permeability of arbitrary 10 points | pieces be the transmittance | permeability of this oxide.

(About magnets)
Next, the magnet 10 (first hollow magnet 11, second hollow magnet 12, and third hollow magnet 13) will be described.
As shown in FIG. 1, a first hollow magnet 11 is disposed around the Faraday rotator 3, and a second hollow magnet 12 and a third hollow magnet 13 are disposed with the first hollow magnet 11 interposed therebetween. ing. The second and third hollow magnets 12 and 13 are arranged so that the same magnetic poles face each other with respect to the first hollow magnet 11.

When the Faraday rotator is cylindrical, the first to third hollow magnets are preferably hollow cylindrical, and the central axis of the Faraday rotator and the central axis of the hollow part of the first to third hollow magnets. Is preferably coaxial. Further, the outer diameter of the Faraday rotator and the inner diameter of the hollow portion of the first to third hollow magnets are substantially the same, and it is preferable to perform alignment after assembling the optical isolator. With this arrangement, the Faraday rotator is arranged at the center of the first hollow magnet.
Further, the first to third hollow magnets are arranged so that these hollow portions are coaxial with the optical axis.

By the way, in general, in an optical isolator, it is preferable to arrange a magnet so that the magnetic flux density where the Faraday rotator is located is increased.
Here, as a conventional example, there is an arrangement example as in Patent Document 1. FIG. 7 shows an example of a basic configuration of a conventional optical isolator including a magnet arrangement example.
In the conventional optical isolator 101 described in Patent Document 1, the first hollow magnet 111 is disposed around the Faraday rotator 103 (Verde constant is 0.27 min / (Oe · cm) or more), and the first hollow magnet 111 is disposed at both ends thereof. Two hollow magnets 112 and a third hollow magnet 113 are arranged in the orientation shown in FIG. The second hollow magnet 112 and the third hollow magnet 113 are each divided into four.

However, if the configuration as shown in FIG. 7 is used, the magnetic flux density can be made relatively large, but there is a problem that the number of parts increases and the manufacture of the optical isolator becomes complicated.
On the other hand, the magnet configuration of the present invention is extremely excellent from the viewpoint that the optical isolator can be easily manufactured and the cost can be reduced.

  Moreover, it is preferable that the magnet used for the optical isolator is as small as possible. Therefore, the type of magnet used in the present invention is not particularly limited, but a neodymium-iron-boron (NdFeB) magnet is preferable because it exhibits a high magnetic flux density.

Further, the numerical range of the magnetic flux density B (T) applied to the Faraday rotator 3 in the present invention is represented by the following formula (1) as described above. The process of deriving this numerical range will be described.
0.75 ≦ B ≦ 1.2 (1)

The magnetic flux density B required to obtain an optical isolator with a Faraday rotation angle of 45 ° was determined. Here, a Faraday rotator made of (Tb _X Y _1-X ) ₂ O ₃ (x = 0.9 to 1.0) was prepared. Then, the length L of the Faraday rotator was changed, and the magnetic flux density B required at that time was obtained. The result is shown in FIG.
The length L of the Faraday rotator is in the range of 5.0 to 7.0 mm. When x = 1.0 in the above composition formula (Verde constant is 0.50 min / (Oe · cm)), x = 0. .9 shows the magnetic flux density distribution at a time of 0.9 (Verde constant is 0.46 min / (Oe · cm)).
And from this FIG. 2, it turns out that said required magnetic flux density B is the range of 0.75-1.2 (T). In this way, the numerical range of the magnetic flux density B in the present invention was derived.

(About other parts)
As described above, the magnet 10 is mounted in the carbon steel casing 14. By housing in the carbon steel casing 14, a yoke material is formed around the magnet 10, so that the attractive force or attractive force of the magnet can be increased.

EXAMPLES Hereinafter, although an Example and a comparative example are shown and this invention is demonstrated more concretely, this invention is not limited to these.
Example 1
An optical isolator 1 according to the present invention as shown in FIG. 1 was produced. Here, a Faraday rotation angle of 45 ° was produced.
A rutile single crystal is used as the entrance polarizer 2 and the exit polarizer 4, and its light transmission surface is processed into a 10 mm-thick parallel plate, and its optical axis 9 is inclined by 47.8 degrees with respect to the optical axis 5. ing. In FIG. 1, the inclination direction is depicted as in the figure. Further, this flat plate polarizer is provided with an antireflection film having a center wavelength of 1.06 μm on the light transmission surface, and a wedge having an inclination angle of 5 degrees to prevent the reflected light from the light transmission surface from returning to the incident light path. The bottom surface of the polarizer was bonded and fixed on the glass 6 and mounted on the polarizer holder 7. Further, the Faraday rotator 3 provided with an antireflection film having a center wavelength of 1.06 μm is positioned at the center of the hollow portion of the first hollow magnet 11, and all of the second hollow magnet 12 and the third hollow magnet 13 are combined. The magnetic field distribution formed by the magnet was fixed at the maximum position. The 45-degree optical rotator 8 arranged after the Faraday rotator 3 in the order of the incident optical path uses a half-wave plate made of artificial quartz, and like the polarizer and the Faraday rotator, An antireflection film having a center wavelength of 1.06 μm is applied.

  Here, as the behavior of the polarization plane of the input light, the ordinary light and the extraordinary light whose polarization planes are separated by 0 degrees and 90 degrees in the incident polarizer 2 are rotated 45 degrees clockwise by the Faraday rotator 3, respectively. The optical axis of the half-wave plate was set to 22.5 degrees in the plane so that the angle of this polarization plane was further rotated 45 ° clockwise. When ordinary light and extraordinary light are transmitted through the half-wave plate in this configuration, both polarization planes are rotated 45 ° clockwise, so that the ordinary light and extraordinary light are respectively rotated by 90 degrees. As a result, since the outgoing polarizer 4 has an optical axis in the same direction as the incident polarizer 2, ordinary light is beam-shifted as extraordinary light, and extraordinary light goes straight as ordinary light and both beams coincide with each other and are polarized. Independence is achieved.

The beam shift amount described above depends on the thickness of the parallel plate polarizer and is about 1 mm in the present embodiment in which the thickness is 10 mm. Since the return light is also separated and emitted 1 mm above and below the incident position, considering the optical isolator function, the maximum beam diameter (1 / e ² ) can correspond to φ1.0 mm. Also, when handling a larger beam diameter for the purpose of reducing the power density of high power light, etc., the effective area of the Faraday rotator is secured and the thickness of the parallel plate polarizer is an arbitrary size of 10 mm or more. Just do it.

Regarding the magnet magnetic circuit, the second hollow magnet 12 and the third hollow magnet 13 each have a counter magnetic force with respect to the first hollow magnet 11, and the outer shape may be a cylinder, a square, or a polyhedron.
In the first embodiment, each magnet has a cylindrical shape with the same outer diameter, and is inserted into the external housing 14 simultaneously with the polarizer holder 7 with the same outer diameter, and the side surface portion of the polarizer holder 7 is screwed. Alternatively, each magnet can be fixed without a gap by fixing with a roll pin or the like. As a result, it is possible to mount with high reliability without requiring an adhesive or the like for fixing the entire magnet.

Next, details of the Faraday rotator 3 in the first embodiment will be described.
The material was terbium oxide, and Tb ₂ O ₃ ceramics with a composition of x = 1.0 was used. As a method for producing a transparent ceramic, first, a mixed powder of raw material powders (Tb ₂ O ₃ and other components) is prepared. Note that the method of preparing the mixed powder, high purity powder materials _(Tb 2 _{O 3} and other components) was used to a purity of 99.99 wt% or more.
Further, the terbium oxide is not limited to Tb ₂ O ₃ , and Tb ₄ O ₇ can also be used, but Tb ₂ O ₃ is used here because of excellent crystallinity of the obtained oxide. did.

Next, a solvent, a binder, a plasticizer, a lubricant, and the like are added to the obtained mixed powder and wet mixed to form a slurry. At this time, a predetermined amount of a sintering aid is added, and the resulting slurry is treated with a spray dryer, dried, and then formed into a cylindrical shape.
In addition, after shaping | molding, it is preferable to perform a degreasing process by heating (preferably 400-600 degreeC), and was performed at 400 degreeC here in air | atmosphere.

Thereafter, firing was performed in a vacuum furnace. Firing conditions are preferably 1500 to 2000 ° C., firing time is 1 to 50 hours, and the degree of vacuum during firing is preferably ¹ Pa or less, and more preferably 1 × 10 ⁻¹ Pa or less.
Here, it was performed at 1600 ° C. and 1 × 10 ⁻² Pa for 3 hours.

Moreover, in order to raise transparency further after said baking, it processed by the hot isostatic pressing (HIP) method. The treatment temperature is preferably higher than the firing temperature, and is 1600 to 2000 ° C. Moreover, the process pressure at this time can be 10-1000 MPa. Although processing time is not specifically limited, It can carry out in 50 hours or less.
Here, it was performed at 1700 ° C. and 180 MPa for 3 hours.

The Tb ₂ O ₃ ceramic thus obtained was subjected to peripheral grinding and optical polishing of both end faces of the cylinder, and antireflection coating was applied to both end faces at a center wavelength of 1.06 μm. Thereafter, when optical measurement was performed at the same wavelength of 1.06 μm, it was found that the insertion loss was 0.3 dB, the extinction ratio was 35 dB, and the Verde constant was 0.50 min / (Oe.cm). .
The sample measured at this time had a shape with an outer diameter of 3.5 mm and a length of 5.5 mm.

  Here, the shape of the magnet was considered. Specifically, when the length of the Faraday rotator is 5.5 mm, the magnetic flux density required for the Faraday rotation angle to be 45 ° is obtained, and in the magnet arrangement in the present invention as shown in FIG. Then, the shape of the magnet capable of obtaining the necessary magnetic flux density was obtained.

First, with respect to the Faraday rotator of Example 1, when the sample length (the length of the Faraday rotator in the optical axis direction) is changed, the magnitude of the magnetic flux density at which the Faraday rotation angle is 45 ° is shown in FIG. .
Here, for Example 1, when the magnetic flux density at which the Faraday rotation angle is 45 ° is calculated from the sample length (5.5 mm) and the Verde constant (0.50 min / (Oe.cm)), the required magnetic flux density Is about 9800 (Oe) (= 0.98 (T)).

Next, the magnetic flux density distribution of the magnet when the magnet was arranged as shown in FIG. 1 was obtained by magnetic field analysis using the outer diameter of the magnet as a parameter.
The finite element method (JMAG-Designer) was selected as the analysis method, the magnet material was a neodymium-iron-boron (NdFeB) magnet manufactured by Shin-Etsu Chemical Co., Ltd., and the material of the external housing 14 was carbon steel. The simulation results are shown in FIG.

Here, the horizontal axis Position Z in FIG. 4 indicates the position along the optical axis 5 (the central axis of the hollow portion of the hollow magnet), and the zero point is the entire hollow magnet unit (first hollow magnet, second hollow magnet, This corresponds to the center of the length of the third hollow magnet). When the length (mm) of the first hollow magnet is indicated by MT and the lengths of the second and third hollow magnets are indicated by MO, MO is obtained from the following equation.
MO = (65−MT) / 2 (the length of all hollow magnet units is 65 mm)

As a result, the overall shape of all the hollow magnets satisfying the expressions (1) and (2) was an inner diameter φ4.0 mm, an outer diameter φ25 mm, and a length 65 mm.
In order to satisfy the magnetic flux density 9800 (Oe) (= 0.98 (T)) required for the Faraday rotator 3 (sample length 5.5 mm, outer diameter 3.5 mm) of the first embodiment, as shown in FIG. The optimum was when MT = 20 (mm).

In summary, in Example 1, each numerical value is as follows.
Faraday rotator shape (Verde constant: 0.50 min / (Oe · cm), sample length: 5.5 mm, outer diameter: 3.5 mm)
Magnetic flux density applied to the Faraday rotator: 0.98T
Overall shape of magnet (inner diameter: 4.0 mm, outer diameter: 25 mm, length: 65 mm (of which the length of the first hollow magnet is 20 mm))

(Example 2)
The optical isolator of the present invention is the same as in Example 1 except that the Faraday rotator 3 has a composition of x = 0.9 and (Tb _0.9 Y _0.1 ) ₂ O ₃ ceramics is used. Produced.
In Example 2, the numerical values are as follows.
Faraday rotator shape (Verde constant: 0.46 min / (Oe · cm), sample length: 5.5 mm, outer diameter: 3.5 mm)
Magnetic flux density applied to the Faraday rotator: 10600 [Oe] = 1.06T
Overall shape of magnet (inner diameter: 4.0 mm, outer diameter: 25 mm, length: 65 mm (of which the length of the first hollow magnet is 15 mm))

(Comparative Example 1)
The conventional optical isolator of FIG. 7 having a magnet arrangement as in Patent Document 1 was produced.
A ceramic of (Tb _0.6 Y _0.4 ) ₂ O ₃ was prepared as the Faraday rotator 103, and the Verde constant was measured. As a result, the Verde constant was 0.30 min / (Oe.cm). In addition, the length of the Faraday rotator 103 in Patent Document 1 is actually 7 mm or more, but here it is easy to compare the effectiveness of the magnet configuration of Examples 1 and 2 (possibility of miniaturization of the magnet size). As described above, the length of the Faraday rotator was set to less than 7 mm (specifically, 6.5 mm) as in Examples 1 and 2.

The relationship between the sample length of Comparative Example 1 and the magnitude of the magnetic flux density at which the Faraday rotation angle is 45 ° is shown in FIG. 3 together with the results of Example 1 and Example 2.
As can be seen from FIG. 3, in Comparative Example 1, when the sample length is 6.5 mm, the magnetic flux density at which the Faraday rotation angle is 45 ° is about 13800 (Oe) (= 1.38 (T)). all right.
FIG. 5 shows the result of analyzing the magnetic flux density distribution of the magnet configuration (see Patent Document 1) shown in FIG. The overall shape of the magnet corresponding to the magnetic flux density required by the (Tb _0.6 Y _0.4 ) ₂ O ₃ ceramic (sample length 6.5 mm, outer diameter 3.5 mm) of Comparative Example 1 is The outer diameter was 4.0 mm, the outer diameter was 35 mm, and the length was 65 mm. MT = 15 (mm) was optimal.

That is, when the comparative example 1 was put together, each numerical value was as follows.
Faraday rotator shape (Verde constant: 0.30 min / (Oe · cm), sample length: 6.5 mm, outer diameter: 3.5 mm)
Magnetic flux density applied to the Faraday rotator: 1.38T
Overall shape of magnet (inner diameter: 4.0 mm, outer diameter: 35 mm, length: 65 mm (of which the length of the first hollow magnet is 15 mm))

Here, when the overall shapes of the magnets of Example 1 and Comparative Example 1 were compared, it was found that a size reduction of 50% in volume ratio could be realized. Therefore, this invention can make each component and structure more compact.
When the magnet shapes of Example 2 and Comparative Example 1 were compared, it was found that a size reduction of 50% in volume ratio could be realized.

The Verde constant is 0.45 min / (Oe · cm) or more, and in the configuration of the present invention that satisfies the formulas (1) and (2) (shaded area in FIG. 3), the (Tb _0.6 Unlike conventional products using Y _0.4 ) ₂ O ₃ ceramics, it is possible to reduce the magnetic flux density necessary to obtain a desired Faraday rotation angle. Therefore, it is not necessary to use the conventional magnet configuration as shown in FIG. 7 (see Patent Document 1), and the number of parts can be reduced, which contributes to downsizing and cost reduction of the optical isolator.
In addition, since the length of the Faraday rotator can be shortened to less than 7 mm as described above, it contributes to the reduction of the loss of the optical isolator.

  The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, and the present invention has substantially the same configuration as the technical idea described in the claims of the present invention, and any device that exhibits the same function and effect is the present invention. It is included in the technical scope of the invention.

DESCRIPTION OF SYMBOLS 1 ... Optical isolator, 2 ... Incident polarizer, 3 ... Faraday rotator,
4 ... Outgoing polarizer, 5 ... Optical axis, 6 ... Wedge glass, 7 ... Polarizer holder,
8 ... Optical rotator, 9 ... Optical axis, 10 ... Magnet,
11 ... 1st hollow magnet, 12 ... 2nd hollow magnet,
13 ... Third hollow magnet, 14 ... Carbon steel case.

Claims (5)

A 1 μm band optical isolator comprising a Faraday rotator and a magnet,
The Faraday rotator has a Verde constant at a wavelength of 0.9 to 1.1 μm of 0.45 min / (Oe · cm) or more,
The magnet comprises a first hollow magnet disposed around the Faraday rotator, a second hollow magnet and a third hollow magnet disposed across the first hollow magnet,
The second hollow magnet and the third hollow magnet are arranged so that the same magnetic poles face each other with respect to the first hollow magnet,
The magnetic flux density B (T) applied to the Faraday rotator is within the range of the following formula (1), and the length L (mm) of the Faraday rotator in the optical axis direction is represented by the following formula (2). A 1 μm band optical isolator characterized by being in the range.
0.75 ≦ B ≦ 1.2 (1)
5.0 ≦ L <7.0 (2) The Faraday rotator is made of a single crystal or ceramic material composed of (Tb _X Re _1-X ) ₂ O ₃ (x = 0.9 to 1.0), and Re is a lanthanoid other than terbium. 2. The 1 μm band optical isolator according to claim 1, comprising at least one element selected from the group consisting of an element group, scandium, and yttrium. The Faraday _{rotator, (Tb X Y 1-X} ) 2 O 3 (x = 0.9~1.0) according to claim 1 or claim, characterized in that is made of a formed ceramic material by The 1 μm band optical isolator according to 2.   The Faraday rotator has optical characteristics with an insertion loss of 1.0 dB or less and an extinction ratio of 25 dB or more in a length L (mm) in the optical axis direction. Item 4. The 1 μm band optical isolator according to any one of Items 3 to 3.   5. The first hollow magnet, the second hollow magnet, and the third hollow magnet are mounted inside a carbon steel casing, according to any one of claims 1 to 4. 1 μm band optical isolator.

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