JP2001272639A - Optical device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical device which has two functions of an optical modulator and an optical isolator, and can realize the miniaturization and cost reduction. SOLUTION: An antenna 56 on the magnetic ferrite single crystal 54 of the optical device 50 is connected to a high frequency input terminal 58. An external magnetic field H is applied to the magnetic ferrite single crystal 54. A laser light source, a polarizer 60 and a first lens 62 are provided on one side of the magnetic ferrite single crystal 54 and a second lens 64, an analyzer 66, a third lens 68, an optical fiber 70 and a photodiode 72 are provided on the opposite side. The optical path length of the magnetic ferrite single crystal 54 is selected so that a vibration direction (b) of light in an emission position to an incidence position of the magnetic ferrite single crystal 54 may rotate at an angle applied integral multiple of 90 degrees to 45 degrees and the analyzer 66 is adjusted so that the transmissivity to such the polarized light may become the maximum.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical element,
In particular, the present invention relates to an optical element that uses a magnetic material such as a ferrite single crystal and obtains light intensity modulated by a high frequency signal such as a microwave or a millimeter wave of several hundred MHz or more.

[0002]

2. Description of the Related Art In optical communication, information is transmitted by using light as a carrier and modulating it with a signal in a microwave band. At a communication rate up to about 10 Gb / s, internal modulation can be performed by a current injected into a semiconductor laser as a light source. However, when the band is higher than this, external light modulation needs to be performed using a semiconductor, lithium niobate, or the like. That is, in high-speed optical communication, the optical modulator is one of the important optical elements that are key. As one of such optical modulators, conventionally, yttrium iron garnet (YI
G) transmits light through a magnetic ferrite single crystal, etc., and modulates the light intensity with microwaves or millimeter waves by applying microwaves or millimeter waves to it using a thin wire transducer or microstrip line. Optical modulators have been considered (for example, J. Kurumida and N.
S. Chang; IEEE Trans. Magn. Vol. 34 (1998) p. 1399
And Makoto Tsutsumi, Tetsuya Ueda; IEICE Technical Report Vol.98 No.123 MW98-41, O
PE98-33 (1988) p.45). FIG. 4 shows the configuration of such an optical modulator.

FIG. 4 is an illustrative view showing one example of a conventional optical modulator as a background of the present invention. Optical modulator 10 shown in FIG.
Includes a plate-like magnetic garnet single crystal 12 as a magnetic material. At the center of one main surface of the magnetic garnet single crystal 12, a microstrip line (transducer) 1
4 are provided. An output terminal of a microwave signal generator 16 is connected to one end of the microstrip line 14 via a microwave amplifier 18. The other end of the microstrip line 14 is connected to the terminating resistor 20. Further, in the vicinity of the magnetic garnet single crystal 12,
A permanent magnet (not shown) is provided. This permanent magnet
This is for applying a DC magnetic field in a direction parallel to the magnetic garnet single crystal 12 and perpendicular to the microstrip line 14. A laser light source 22, a polarizer 24, and a first lens 26 are provided outside the one side surface of the magnetic garnet single crystal 12 so as to approach the magnetic garnet single crystal 12 in this order. Laser light source 22
Is for generating a laser beam. Polarizer 2
Numeral 4 is for converting the laser light generated by the laser light source 22 into linearly polarized light in a specific direction. First lens 2
Numeral 6 is for condensing the laser light generated by the laser light source 22 into the magnetic garnet single crystal 12. Further, outside the other side surface of the magnetic garnet single crystal 12, the second lens 28, the analyzer 30, and the third lens 32
And a photodiode 34 are provided so as to be separated from the magnetic garnet single crystal 12 in this order. The second lens 28 is for converting a laser beam transmitted through the magnetic garnet single crystal 12 into a parallel beam. Analyzer 30
Is for transmitting linearly polarized light in a specific direction of the laser light, and is installed in a crossed Nicol relationship with the polarizer 24. The third lens 32 is for condensing the laser light transmitted through the analyzer 30. The photodiode 34 is for detecting a signal of a laser beam. The output terminal of the photodiode 34 is connected to the input terminal of the photocurrent amplifier 36.

[0004] In the optical modulator 10 shown in FIG. 4, the optical system (magnetic garnet single crystal 12 and the configuration on both sides thereof) is provided.
Is almost the same as the optical system for measuring the transmission type magneto-optical effect such as the Faraday effect. That is, by applying a microwave to the magnetic garnet single crystal 12 (microstrip line (transducer) 14), the light and the microwave are coupled in the magnetic garnet single crystal 12, and the polarization state of the light is modulated by the microwave. I do. This is converted into a change in intensity by the Faraday optical system and detected. In the optical modulator 10 shown in FIG. 4, when no microwave is applied, the transmitted light of the magnetic garnet single crystal 12 remains DC, and the detected optical output is constant. On the other hand, the optical modulator 1 shown in FIG.
At 0, when a microwave is being applied, the modulated AC component is superimposed on the DC component. In the optical modulator 10 shown in FIG. 4, when an external magnetic field is not applied to the magnetic garnet single crystal 12 or is considerably weak even if it is applied, the microwave propagates as it is and light modulation by the microwave is performed. The magnetic garnet single crystal 12
When a sufficiently strong external magnetic field is applied, the magnetostatic wave is excited, and light modulation by the magnetostatic wave is performed.

[0005]

An optical modulator 1 shown in FIG.
0, the third lens 32 and the photodiode 34
Return light from the end face of an optical component such as
Since laser oscillation becomes unstable when the light enters the optical path, an optical isolator is used to prevent this. In such an optical isolator, in either the bulk type or the waveguide type, the non-reciprocity of rotation in the direction of light oscillation due to the Faraday effect is utilized in many cases, and the Faraday medium is represented by YIG. In most cases, magnetic ferrite is used. However, conventionally, since the optical modulator and the optical isolator are separately formed, it is not only a hindrance to miniaturization and integration of the optical element, but also an increase in the number of parts causes an increase in cost.

Therefore, a main object of the present invention is to provide an optical element which has two functions, that is, a function of an optical modulator and a function of an optical isolator, and can be reduced in size and cost. is there.

[0007]

Means for Solving the Problems As a result of intensive studies, the present inventors have found that in a light modulator that modulates transmitted light of a magnetic material by applying a microwave to the magnetic material, the transmitted light of the magnetic material is modulated. It has been found that a certain relationship is maintained between the DC component of the A. and the AC component generated by the modulation. Based on this knowledge, the two functions of the optical modulator and the optical isolator
Succeeded in developing an optical element that combines two functions. The optical element according to the present invention is an optical element using a magnetic material, wherein a portion functioning as an optical modulator is configured using the magnetic material, and as an optical isolator utilizing the Faraday effect using the magnetic material. An optical element having a functioning portion. In the optical element according to the present invention, the light is modulated by the high-frequency signal when passing through the magnetic body, and after the light passes through the magnetic body, 45 ° + 90 ° × n in the vibration direction of the light.
(N is an integer) non-reciprocal rotation occurs. However, the vibration direction of light is defined by the vibration direction of the electric field of the electromagnetic wave of light. That is, assuming that the light oscillates clockwise by 45 degrees when the light travels in the forward direction through the optical element, the light oscillates counterclockwise when the light travels in the reverse direction. When the light rotates 45 degrees and the light reciprocates through the optical element, a total of 90 degrees of rotation occurs in the vibration direction of the light. In the optical element according to the present invention, the high-frequency signal includes a microwave or millimeter wave signal. Further, in the optical element according to the present invention, the magnetic material is, for example, a ferrite single crystal, and the ferrite single crystal is, for example, yttrium iron garnet or a component in which a part of the composition is replaced with another element. In the present invention, the one in which a part of the composition is replaced by another element means, in addition to the one in which a part of the Y and / or Fe element is replaced by another element, the Y element is, for example, another rare earth element or Bi element. And the like, all of which are replaced. Further, the optical element according to the present invention includes, for example, a transducer provided on a magnetic body, a high-frequency input terminal connected to the transducer,
A magnet for applying an external magnetic field to the magnetic material, a laser light source for generating laser light, a polarizer for converting the laser light generated by the laser light source into linearly polarized light in a specific direction, and a polarizer for specifying a specific direction. A first lens for condensing the linearly polarized laser light into the magnetic material, a second lens for converting the laser light transmitted through the magnetic material into parallel rays, and a second lens for parallelizing the laser light. An analyzer for transmitting linearly polarized light in a specific direction of the laser light converted into a light beam, and a third lens for condensing the laser light transmitted through the analyzer,
An optical fiber for transmitting the laser light condensed by the third lens and a photodiode for detecting a signal of the laser light transmitted by the optical fiber are included.

The optical element according to the present invention has both the function of an optical modulator and the function of an optical isolator. further,
In the optical element according to the present invention, since the magnetic substance involved in the function of the optical modulator and the magnetic substance involved in the function of the optical isolator are the same magnetic substance, the function of the optical modulator and the function of the optical isolator are different. Despite having both functions, downsizing and cost reduction can be achieved.

The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description of embodiments of the present invention with reference to the accompanying drawings.

[0010]

FIG. 1 is an illustrative view showing one example of an optical element according to the present invention. Optical element 50 shown in FIG.
Includes a ground plate 52 made of metal, for example. On the ground 52, a plate-shaped magnetic ferrite single crystal 54 is provided as a magnetic material. At the center of one main surface of the magnetic ferrite single crystal 54, a thin wire antenna 56 is provided as a transducer. One end of the antenna 56 is connected to the high frequency input terminal 58. The other end of the antenna 56 is connected to the ground plate 52.

In the vicinity of the magnetic ferrite single crystal 54, for example, a cylindrical or block-shaped permanent magnet (not shown) is provided. This permanent magnet is for applying a DC magnetic field H in a direction parallel to the magnetic ferrite single crystal 54 and perpendicular to the antenna 56. In order to apply the DC magnetic field H to the magnetic ferrite single crystal 54, an electromagnet may be used instead of a permanent magnet.

Further, a laser light source (not shown), a polarizer 60 and a first lens 62 are provided outside the one side surface of the magnetic ferrite single crystal 54 so as to approach the magnetic ferrite single crystal 54 in this order. Can be The laser light source is for generating laser light. Polarizer 6
0 is for converting the laser light generated by the laser light source into linearly polarized light in a specific direction. First lens 62
Is for condensing the laser light generated by the laser light source into the magnetic ferrite single crystal 54.

A second lens 64, an analyzer 66, and a third lens 64 are provided outside the other side surface of the magnetic ferrite single crystal 54.
And one end of the optical fiber 70 are arranged so as to be separated from the magnetic ferrite single crystal 54 in this order. The second lens 64 is for converting the laser light transmitted through the magnetic ferrite single crystal 54 into a parallel light beam. The analyzer 66 is for transmitting linearly polarized laser light in a specific direction. The third lens 68 is for condensing the laser light transmitted through the analyzer 66. The optical fiber 70 is for transmitting a laser beam. A photodiode 72 is provided near the other end of the optical fiber 70. The photodiode 72
This is for detecting a signal of a laser beam.

In the optical element 50, the light that has been converted into linearly polarized light having a vibration direction in the direction a by the polarizer 60 is the first light.
And is incident on the magnetic ferrite single crystal 54. A sufficiently strong external magnetic field H is applied to the magnetic ferrite single crystal 54 in parallel with the light propagation direction.
As a result, the Faraday effect occurs, so that the light oscillation direction rotates in the magnetic ferrite single crystal 54. The light transmitted through the magnetic ferrite single crystal 54 is transmitted to the second lens 64
After being returned to the parallel light beam by the above, the light enters the analyzer 66. The optical path of the magnetic ferrite single crystal 54 is such that the vibration direction b of the light at the emission position from the magnetic ferrite single crystal 54 is rotated by 45 degrees (or an angle obtained by adding an integral multiple of 90 degrees to 45 degrees) with respect to the incident position. Length is selected, analyzer 6
6 is adjusted so that the transmittance for such polarized light is maximized.

In the optical element 50, a high-frequency signal such as a microwave can be applied to the magnetic ferrite single crystal 54 from the high-frequency input terminal 58 through the thin wire antenna 56, and a sufficiently strong external magnetic field H is applied. Therefore, a magnetostatic wave is excited in the magnetic ferrite single crystal 54. In this case, the magnetostatic wave excited here is a magnetostatic backward volume wave (MSBVW) due to the relationship between the traveling direction of a high-frequency signal such as a microwave and the direction in which the external magnetic field H is applied. Therefore, light is modulated in the magnetic ferrite single crystal 54 by the magnetostatic backward volume wave. The light transmitted through the magnetic ferrite single crystal 54 modulated by the magnetostatic backward volume wave is condensed by the third lens 68 and
0 and is transmitted through the optical fiber 70. Light is emitted from the optical fiber 70 and enters the photodiode 72, where an optical signal is converted into an electric signal and is electrically detected. At this time, the frequency band of the photodiode 72 needs to be wider than the frequency band of a high-frequency signal such as a microwave serving as a light modulation source. If not, the modulated optical signal cannot be detected by the photodiode 72. Thus, the optical element 50 shown in FIG.
Functions as an optical modulator.

Further, in the optical element 50, the third lens 68, the optical fiber 70 and the photodiode 72
It is inevitable to some extent that light is reflected on each end face even if a non-reflection coating is applied to each end face. When the return light generated by this reflection enters the laser light source,
Laser oscillation may become unstable.
However, in this optical element 50, the Faraday rotation angle is 4
A polarizer 62 and an analyzer 6 are sandwiched by a magnetic ferrite single crystal 54 having an angle obtained by adding an integral multiple of 90 degrees to 5 degrees.
6 has the above-described relationship, and thus also functions as an optical isolator. That is, in the optical element 50,
When light enters the analyzer 66 from the third lens 68 side,
The light is also converted into linearly polarized light by the analyzer 66 and is incident on the magnetic ferrite single crystal 54, and the magnetic ferrite single crystal 5
While propagating through 4, the vibration direction rotates due to the Faraday effect. At this time, due to the non-reciprocity of the Faraday effect, the oscillation direction rotates counterclockwise as the light travels, and at the emission position of the magnetic ferrite single crystal 54, the oscillation direction rotates 45 degrees counterclockwise with respect to the incidence position. However, light having such a vibration direction cannot pass through the polarizer 60. Therefore, in the optical element 50, the incident light from the polarizer 60 to the magnetic ferrite single crystal 54 can be transmitted, while the analyzer 66 transmits the magnetic ferrite single crystal 5
Light incident on 4 is blocked. Therefore, in the optical element 50, it is possible to prevent the return light from being incident on the laser light source.

As described above, the optical element 50 shown in FIG. 1 has two functions of an optical modulator function and an optical isolator function.

[0018]

EXAMPLE An optical element 50 having the structure shown in FIG. 1 was experimentally manufactured and its characteristics were measured. However, the light transmitted through the analyzer 66 is not coupled to the optical fiber 70 but directly to the photodiode 7.
2 was incident. In addition, a pure YIG bulk single crystal grown by the floating zone method was used as the magnetic material. This was cut into a size of 1 mm × 5 mm × 10 mm, and each surface was mirror-polished. A semiconductor laser having a wavelength of 1.3 μm was used as a light source, and the light was allowed to enter the crystal from one of the 1 mm × 5 mm surfaces. The vibration direction of the incident light was fixed so as to coincide with the normal direction of the plate surface of the plate crystal. Crystal 5
On a surface of mm × 10 mm, a thin-line antenna for applying a microwave was installed. In a YIG single crystal, a high-frequency signal such as a microwave was made parallel to a light propagation direction. Further, an external magnetic field H was applied in the same direction as the light propagation direction or in the opposite direction.

At some values of the external magnetic field H, the modulation frequency by the microwave is changed from 1 GHz to 4.5 G.
FIG. 2 is a graph showing light modulation characteristics in the example when the frequency is changed to Hz. In the graph of FIG. 2, the horizontal axis indicates the frequency of the applied microwave, and the vertical axis indicates the light modulation amplitude. The three curves in the graph of FIG. 2 correspond to the intensity of the external magnetic field H applied in the same direction as the light propagation direction.
According to the graph shown in FIG.
It was confirmed that light modulation was maximized at z.

FIG. 3 is a graph showing the optical isolation characteristics of the embodiment when the direction of the external magnetic field H is changed. In the graph shown in FIG. 3, the horizontal axis represents the applied external magnetic field H, and the vertical axis represents the light intensity detected by the photodiode. The two curves in the graph shown in FIG. 3 correspond to the case where the external magnetic field H is applied in the same direction as the light propagation direction and the case where the external magnetic field H is applied in the direction opposite to the light propagation direction. I have. According to the graph shown in FIG. 3, when the applied external magnetic field H is about 27800 A / m or more, 40 dB between transmitted light intensities depending on the application direction of the magnetic field.
The above optical isolation was confirmed.

As described above, in this embodiment, it is possible to realize two functions of the function of the optical modulator and the function of the optical isolator with a single optical element. Therefore, in this embodiment, there is a remarkable effect that the size of the optical element can be reduced and the number of parts can be reduced as compared with the conventional case where these optical elements must be prepared separately.

Incidentally, in the optical element 50 shown in FIG.
It is preferable to use YIG grown by the floating zone method as the magnetic ferrite single crystal 54. Further, in order to adjust the value of the saturation magnetization without deteriorating the high frequency characteristics, a part of the Fe component of YIG may be replaced with another element such as In or Ga. Also,
In order to obtain a large Faraday effect per unit length, Y
A part of the Y component of the IG may be replaced with Bi or the like. The length of the magnetic ferrite in the optical path direction is
It is better to be as short as possible for miniaturization, but if it is too short, it becomes difficult to handle. Therefore, it is appropriate to set the Faraday rotation angle in the magnetically-saturated magnetic ferrite to an angle obtained by adding an integral multiple of 90 degrees to 45 degrees.

As the magnetic substance, a thin film single crystal grown by a liquid phase epitaxial method or the like may be used in addition to the bulk single crystal.

Further, as a means for applying a high-frequency signal such as a microwave, a microstrip line or the like may be used instead of a thin-line antenna.

The above-mentioned optical element can be constituted by providing a means for applying a high-frequency signal such as a microwave to a magnetic material of a bulk type optical isolator. In addition to the above, various types of waveguide type have been proposed (for example, see Magnetic Material Handbook (Asakura Shoten, 1999) p.799). According to the present invention, in a waveguide type optical isolator, a means for applying a high-frequency signal such as a microwave is provided to a magnetic ferrite film functioning as a non-reciprocal mode conversion element exhibiting the Faraday effect. It is possible to configure an optical element.

[0026]

According to the present invention, it is possible to obtain an optical element having both the function of an optical modulator and the function of an optical isolator and capable of reducing the size and cost.

[Brief description of the drawings]

FIG. 1 is an illustrative view showing one example of an optical element according to the present invention;

FIG. 2 is a graph showing light modulation characteristics in an example.

FIG. 3 is a graph showing optical isolation characteristics in an example.

FIG. 4 is an illustrative view showing one example of a conventional optical modulator as a background of the present invention;

[Explanation of symbols]

 Reference Signs List 50 optical element 52 ground plate 54 magnetic ferrite single crystal 56 antenna 58 high frequency input terminal 60 polarizer 62 first lens 64 second lens 66 analyzer 68 third lens 70 optical fiber 72 photodiode IB incident light a incident light Vibration direction of TB Transmitted light of magnetic ferrite single crystal b Vibration direction of transmitted light of analyzer

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Takashi Fujii 2-26-10 Tenjin, Nagaokakyo-shi, Kyoto, Japan F-term in Murata Manufacturing Co., Ltd.

Claims (6)

[Claims] 1. An optical element using a magnetic material, wherein a portion functioning as an optical modulator using the magnetic material is configured, and a portion functioning as an optical isolator utilizing the Faraday effect using the magnetic material. An optical element comprising: 2. When light is modulated by a high-frequency signal when passing through the magnetic material, the light is transmitted through the magnetic material, and then 45 degrees + 90 degrees × n (n is an integer) in the vibration direction of the light. 2. The optical element according to claim 1, wherein non-reciprocal rotation occurs. 3. The optical modulator according to claim 1, wherein the high-frequency signal includes a microwave or millimeter-wave signal. 4. The magnetic body is a ferrite single crystal.
The optical element according to claim 1. 5. The optical element according to claim 4, wherein the ferrite single crystal is obtained by substituting yttrium iron garnet or a part of its composition with another element. 6. A transducer provided on the magnetic material, a high-frequency input terminal connected to the transducer, a magnet for applying an external magnetic field to the magnetic material, a laser light source for generating laser light, and the laser light source A polarizer for converting the laser light generated in step 1 into linearly polarized light in a specific direction; a first lens for condensing the laser light converted into linearly polarized light in a specific direction by the polarizer in the magnetic body; A second lens for converting the laser light transmitted through the body into parallel rays, an analyzer for transmitting linearly polarized light in a specific direction of the laser light parallelized by the second lens, and transmitting the analyzer Third for condensing the focused laser light
A lens for transmitting the laser light condensed by the third lens; and a photodiode for detecting a signal of the laser light transmitted by the optical fiber. Item 6. The optical element according to any one of Items 5.