JPH0854579A - Optical isolator element, optical isolator, optical isolator with optical fiber and semiconductor laser module - Google Patents

Abstract

PURPOSE:To obtain isolation optimum for diagonal incident light on the surface of an optical isolator. CONSTITUTION:This optical isolator consists of first and second double refractive crystal plates 1, 2 arranged in such a manner that their optical axes vary by a specified angle and a Faraday rotator 3 of the faraday rotating angle thetaf between these double refractive crystal plates. The angle of the optical axes of these double refractive crystal plates 1, 2 are so set that the incident polarized light in a forward direction transmits the first and second double refractive crystal plates 1, 2 as ordinary light and ordinary light or extraordinary light and extraordinary light. The angle phieff formed by the plane of polarization at which the light transmits the double refractive crystal plate 1 as the ordinary light and the plane of polarization at which the light transmits the double refractive crystal plate 2 as the ordinary light satisfies thetaf+phieff when the light is made incident from the specific diagonal direction to be actually used or the angles of the optical axes of the double refractive crystal plates 1, 2 are so set that the incident polarized light in the forward direction transmits the double refractive crystal plates 1, 2 as the ordinary light and the extraordinary light or the extraordinary light and the ordinary light. These angles are so adjusted as to satisfy thetaf=phieff. The points which do not higher the transmission of the light are provided with marks for the incident direction of the light.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical isolator element used for optical information processing, optical communication, etc., an optical isolator, and an optical isolator with an optical fiber.

[0002]

2. Description of the Related Art In an optical isolator element, a relative polarization angle of two polarizers is set to about 45 degrees, and a magneto-optical element having a Faraday rotation angle of about 45 degrees, that is, a Faraday rotator plate is inserted between them. Are fixed to each other, and have a function of transmitting light in the forward direction and blocking light in the reverse direction.
When a birefringent plate is used as the polarizer, when the two birefringent plates and the Faraday rotator are bonded together, the two birefringent plates are made incident while the laser light is perpendicularly incident on the Faraday rotator surface. It has been proposed to adjust the relative polarization angle by relatively rotating around the ray direction and fix the positional relationship of each element at a position where the amount of transmitted laser light in the opposite direction is minimized (for example, JP-A-1- 219816, 1-219817,
1-219818). This is a method for avoiding the characteristic deterioration of the product due to the dispersion of the Faraday rotation angle of the Faraday rotator and the relative polarization angle of the two polarizers.

[0003]

In order to prevent the reflected light from the optical isolator element itself from returning to its original state, the optical isolator element is usually used by allowing light to enter obliquely instead of perpendicularly (for example, 1988 Electron National Institute of Information and Communication Spring National Convention C-447, pages 1-152). When the polarizer is a birefringent plate, even if the amount of transmitted light in the reverse direction of the optical isolator element with respect to vertically incident light is set to the minimum as described above, transmission in the reverse direction depends on the direction of oblique incidence. The amount of light greatly increases.

That is, in the optical isolator element 11 in which the birefringent plate 1, the Faraday rotator 3 and the birefringent plate 2 are superposed as shown in FIG. 2, the X and Y axes orthogonal to each other are taken in the plane of the optical isolator element. The Z axis is taken perpendicularly to the surface of the optical isolator element, the angle from the Z axis is θ, and the direction in the XY plane measured from the X axis is φ. The direction (θ ₁ , φ ₁ ) of the C axis (optical axis) of the birefringent plate 1 made of rutile crystal is (45 °, 0 °), and the direction of the C axis (optical axis) of the birefringent plate 2 made of rutile crystal ( θ ₂ , φ ₂ ) (45 °, 45
Angle), the incident direction (θ _in , φ _in ) of the laser light is changed, and as shown in FIG. 1, the angle formed by the polarization planes that transmit two birefringent plates as ordinary light, that is, the effective value φ of the relative angle.
_eff varies greatly. More specifically, assuming that the relative angle between the two polarizers is φ, the Faraday rotation angle of the Faraday rotator is θ _f , and the polarizer and the Faraday rotator are ideal elements, the forward transmittance is T _f = cos
² (θ _f −φ) The transmittance in the opposite direction is T _g = cos ² (θ _f
+ Φ). Isolation Iso is I in dB unit
is defined as _{so = -10log 10 (T g /} T f). θ
When _{f = φ = 45 ° T f} = 1, T g = 0, Iso = ∞
It becomes dB, and the characteristics of an ideal optical isolator can be obtained.
However, in actual manufacturing, there are variations in angle. In an optical isolator element, maximizing isolation (reducing transmissivity in the opposite direction) is of the utmost importance, and normally φ or φ is set so that θ _f + φ = 90 ° at the center wavelength of use and the center temperature of use. Adjust the angle of θ _f and assemble. (Θ _f generally changes depending on wavelength and temperature.) When a birefringent plate is used as a polarizer of an optical isolator element, θ _f + φ = 90 for vertically incident light.
Even if it is adjusted so that the angle becomes °, the effective value φ _eff for the obliquely incident light actually used is significantly different from that for vertically incident light as shown in Fig. 1 and θ _f + φ _eff ≠ 90 °, resulting in isolation. Is decreased (the transmittance in the opposite direction is increased). Here, φ _eff is defined as the angle formed by the plane of polarization that transmits the first birefringent plate as ordinary light and the plane of polarization that transmits the second birefringent plate as ordinary light.

Conventionally, the problem that the incident direction affects the isolation has not been taken up, but this is accompanied by a device for measuring the return light when the optical isolator element is incorporated in a semiconductor laser, a lens system, an optical fiber or the like. Because it is difficult. However, Japanese Patent Laid-Open No. 2-93409 points out the above-mentioned problem although it relates to a case where two polarizers are composed of polarization beam splitters. However, in this document, when assembling an optical isolator element with an optical fiber or a lens system, even if the incident direction with respect to the surface of the optical isolator element changes, the normal line of the two polarization beam splitters is set so that the variation of the isolation becomes as small as possible. It defines the relative relationship of directions,
It is not intended to obtain the optimum optical isolator characteristics at the time of assembly.

Since the characteristics of the optical isolator element change depending on the incident direction of light as described above, it is desired to optimize the optical isolator characteristics in a specific oblique incident direction that is actually used. . Therefore, it is an object of the present invention to provide an optical isolator element and an optical isolator with a fiber which have the maximum isolation in a specific oblique incident direction. An object of the present invention is to solve a problem that occurs when an optical isolator element whose isolation varies greatly depending on the direction of oblique incidence, particularly when a birefringent plate is used as a polarizer. Although the birefringent plate can be obtained at a low price, it has the above-mentioned problems, and it is particularly advantageous to solve this problem.

[0007]

According to the present invention, there are provided first and second birefringent crystal plates arranged such that their optical axes are different by a constant angle, the first birefringent crystal plate and the second birefringent crystal plate. In an optical isolator element comprising a Faraday rotator arranged between the refraction crystal plate and rotating the plane of polarization of the incident light by a certain angle, the forward polarization of the first and second birefringent crystal plates is ordinary light, The angles of the optical axes of the first and second birefringent crystal plates are set so as to transmit ordinary light, extraordinary light, and extraordinary light, and when light is incident from a specific oblique direction actually used. The angle formed by the plane of polarization that transmits the first birefringent crystal plate as ordinary light and the plane of polarization that transmits the second birefringent crystal plate as ordinary light is φ _eff, and the use center temperature of the Faraday rotator The Faraday rotation angle at the center wavelength is θ _f Then θ _f + φ _eff = 90
Provided is an optical isolator element characterized in that the thickness of a Faraday rotator or the relative angle of a birefringent plate is adjusted so as to satisfy the condition.

According to the present invention, in the same construction as described above, the first, second, and second birefringent crystal plates transmit the incident polarized light as ordinary light, extraordinary light, extraordinary light, or ordinary light. When the angle of the optical axis of the second birefringent crystal plate is set, the thickness of the Faraday rotator or the relative angle of the birefringent plate should be adjusted so as to satisfy θ _f = φ _{ef f.} A characteristic optical isolator element is provided.

Further, according to the present invention, a mark indicating the incident direction of light is provided to an optical isolator element having a structure in which the optical isolator characteristic is optimum when the light is incident from a specific oblique direction under the conditions of actual use. Is provided.
Alternatively, when the optical isolator element of the present invention is fixed to an outer shape holder having an outer peripheral surface parallel to the light incident direction by inclining with respect to the light incident direction, an optical isolator fixed under optimum conditions can be obtained. . Normally, the optical isolator element is marked with the incident polarization direction.
There is no difference of 180 degrees. As shown in FIG. 1, the optical isolator characteristics actually change greatly depending on the incident direction. The optical isolator element of the present invention or the optical isolator using the same is obtained by constructing the optical isolator element so as to be optimum for a particular incident direction of light and then marking the incident direction.

The present invention will be described in detail below. As described above, there is an angle variation in actual manufacturing. In optical isolators, maximizing isolation (reducing transmissivity in the opposite direction) is of utmost importance. Normally, at the operating center wavelength and operating center temperature, θ
Assemble by adjusting the angle of φ or θ _f so that _f + φ = 90 °. (Θ _f generally changes depending on wavelength and temperature.) When a birefringent plate is used as a polarizer of an optical isolator element, even if it is adjusted so that θ _f + φ = 90 ° with respect to vertically incident light, It decreased when the effective value phi _eff is normally incident light greatly differs θ _f + φ _eff ≠ 90 ° next to the isolation of phi as shown in FIG. 1 is an obliquely incident light to be used for (the reverse direction of the transmission rate It has already been explained that it will increase). Here, φ _eff is defined as the angle formed by the plane of polarization that transmits the first birefringent plate as ordinary light and the plane of polarization that transmits the second birefringent plate as ordinary light. Therefore, it is necessary to satisfy θ _f + φ = 90 ° for obliquely incident light that is actually used.

Principle of the Present Invention Now, in the case of a birefringent plate, there are two cases in which forward incident polarized light passes through the birefringent plate as ordinary light and as extraordinary light.
There is a street. When incident light in the forward direction passes through the first and second birefringent plates as ordinary light, ordinary light or extraordinary light, and extraordinary light, φ = φ _eff , and θ _f + φ = 90 °, θ _f
It may be + φ _eff = 90 °. For this purpose, the thickness of the Faraday rotator or the relative angle of the birefringent plate may be adjusted. Meanwhile, first the forward direction of the incident polarized light, the second birefringent plate ordinary, extraordinary light or abnormal light, in case the transmission is _{φ = 90 ° -φ eff θ f} + φ = 90 ° as the ordinary light _May be θ _f = φ _eff . For this purpose, the thickness of the Faraday rotator or the relative angle of the birefringent plate may be adjusted. Here, 0 ° <θ _f <90 °, 0 ° <φ _eff <
It shall be set at 90 °. Further, in this manner, the Faraday rotator plate and the first and second birefringent plates are manufactured, and these are adhered to each other to form an optical isolator element. Finally, the incident surface of the optical isolator element or the element is formed. The incident direction of light can be marked on the outer shape holder of the incorporated optical isolator by a method such as vapor deposition and printing. Here, the incident direction is the angle θ from the direction normal to the incident surface of the birefringent plate.
It is what is specified by the angular orientation phi _in the _in the incident surface inner surface, not related to the polarization direction that is conventionally indicated. A specific example of this will be described in Example 1.

The present invention also integrates an optical fiber having an oblique end face and an optical isolator element, and fixes the optical isolator element after rotating the optical isolator element around the optical fiber axis at an angle that optimizes the optical isolator characteristics. A fiber optic isolator is provided. An optical fiber having an oblique end face can receive light in a substantially constant narrow range as shown in FIG. 4. Therefore, if the optical isolator element and the optical fiber are integrally fixed, the incident direction to the optical isolator element is almost fixed. . If the characteristics of the optical isolator element are optimized in this direction, then when the laser diode and this optical isolator with optical fiber are coupled to the lens, the incident direction to the optical isolator element becomes almost the same, so the lens Since the coupling direction of the system and the laser diode is inevitably determined, it is convenient that the system can be used in an optimum state. Therefore, the optical isolator characteristic uses this incident direction, and the angle of the optical isolator element is 180 ° apart.
When the effective value φ _eff of the relative angle between the two birefringent plates is obtained in one direction, different values are obtained depending on the incident direction. Among them, the optical isolator element is rotated around the axis of the optical fiber by an angle at which the isolation is increased, and the relationship with the optical fiber is fixed in that state. A specific example of this will be described in Example 2.

[0013]

【Example】

Example 1 An optical isolator was produced using a rutile plate as a birefringent crystal plate, a magnetic garnet as a Faraday rotator, and a magnet that applies a saturation magnetic field to the magnetic garnet. In the configuration of the optical isolator element in FIG. 2, θ ₁ = θ ₂ = 45 °, φ ₁ =
The angle was 0 ° and φ ₂ = 45 °. Incident angle of light θ
Consider a case where _in = 4 ° and φ _in = 0 °. From FIG. 1, in this case, φ _eff = 46.2 °. 1) The incident polarized light in the forward direction passes through the first and second rutile plates as ordinary light, ordinary light (or extraordinary light,
The Faraday rotation angle is positively rotated with respect to the light in the forward direction so that it is transmitted as extraordinary light), and the size of the rotation angle is θ _f = 90 ° −46. When it was manufactured so that 2 ° = 43.8 °, an isolation of 40 dB was obtained. On the other hand, in the fabricated optical isolator, θ _in = 4 °, φ _in =
When 180 ° light is incident, the isolation is 2
It was significantly deteriorated to 7 dB. This optical isolator specifically has a structure shown in FIG. 5 as an example. The optical isolator element 11 is supported by a cylindrical element holder 17, and the element holder is supported by the outer shape holder 10. The outer shape holder 10 has a cylindrical permanent magnet 18 for applying a fixed magnetic field to the optical isolator element 11. The optical isolator element 11 is provided with a mark 19 indicating the direction of oblique incidence, a mark 13 on the front end surface of the outer shape holder, or a mark 14 (or both) on the peripheral surface of the front end portion, so that light is mistakenly emitted from the opposite direction. It is convenient because it can be prevented from entering. 2) The Faraday rotation angle rotates negative with respect to the forward light so that the forward incident polarized light passes through the first and second rutile plates as ordinary light and extraordinary light (or extraordinary light, ordinary light), and The size of the rotation angle is the use center wavelength 1310 nm, the use center temperature 2
When it was manufactured so that θ _f = 46.2 ° at 5 ° C., isolation of 40 dB was obtained. On the other hand, when the light of θ _in = 4 ° and φ _in = 180 ° was incident on the produced optical isolator, the isolation was greatly deteriorated to 27 dB. Figure 5 shows the outline holder of this optical isolator.
By providing a mark indicating the direction of oblique incidence, as in the case shown in (1), it is convenient to prevent light from accidentally entering from the opposite direction.

Example 2 In the configuration of the optical isolator element of FIG. 2, a rutile plate is used as a polarizer, and θ ₁ = θ ₂ = 45 °, φ ₁ = 0 °,
φ ₂ = 45 °, Faraday rotator rotation angle θ _f = 45 °
And This optical isolator element is integrated with an optical fiber to form an optical isolator 12 with a fiber having the structure shown in FIG.
Was configured. In FIG. 3, 4 is an optical fiber core wire, 5 is an element wire exposed from the core wire 4, 6 is a ferrule that supports the optical fiber, 7 is a ceramic capillary that can be regarded as a part of a ferrule that supports the element wire 4, 8 Is a permanent magnet for applying a saturated or unsaturated magnetic field to the Faraday rotator 3, 9 is an obliquely formed fiber end face, 10 is an element holder made of a non-magnetic or soft magnetic material, and 11 is a light having the configuration of FIG. It is an isolator element. In FIG. 3, the end face of the fiber was set to a slope of 8 °. In this case, the incident angles θ _in and φ _in of the focused beam on the optical isolator element that maximize the coupling efficiency with the optical fiber are θ _in = about 3.7 ° and φ _in = 90 °. The second optical fiber 16 and the lens 15 are arranged as shown in FIG. 6, and the position of each element is adjusted so that the amount of light transmitted from the second optical fiber 16 to the first optical fiber 4 is maximized. At this time, as described above, the light incident on the first optical fiber 4 (and the optical isolator element 11) is a focused beam with θ _in = 3.7 ° and φ _in = 90 °. In this state, the characteristics of the optical isolator with optical fiber were evaluated. Consider a case where the incident polarization direction in the forward direction is the Y direction and the light in the forward direction passes through the rutile plates 1 and 2 as ordinary light. In this case, the directions of the optical axes of the rutile plates 1 and 2 are φ ₁ = 0 °, φ ₂ = 45 ° or φ ₁ = 180 °, φ ₂ = where the optical isolator element is rotated 180 ° about the optical fiber as the central axis. It is necessary to arrange the optical isolator element in the direction of 225 °. As can be seen from FIG. 1, the effective value φ _eff of the relative angle with respect to the two directions is different from about 0.9 °, and therefore the isolation characteristic is different between the two directions. Therefore, the optical isolator element is fixed in one of the two directions having the larger isolation (the amount of transmitted light in the opposite direction is smaller). By assembling the optical isolator in this way, good characteristics can be stably obtained even if there is a variation in angle. The results of actual trial production are shown below. The standard deviation of the manufacturing variation of the Faraday rotation angle θ _f was 0.2 °, and the standard deviation of the manufacturing variation of the relative angle of the rutile plate was 0.4 °. At this time, assembling in the direction in which the isolation is larger among the two directions as described above, the standard deviation of the manufacturing variation of the peak wavelength (wavelength at which the isolation is maximum) is 4.1 nm, which is small and stable. Could be made. On the other hand, when the prototype was manufactured without considering the orientation as in the conventional case, the standard deviation of the manufacturing variation of the peak wavelength was 12
It was as large as nm. As shown in FIG. 7, an optical isolator with an optical fiber thus manufactured, a semiconductor laser module including a semiconductor laser and a lens, was prototyped. Here, the position of each element is adjusted so that the coupling efficiency from the semiconductor laser to the optical fiber is maximized. At this time, the light incident on the optical fiber (and the optical isolator element) is a focused beam with approximately θ _in = 3.7 ° and φ _in = 90 °. When adjusted in this way, the direction of the beam of light that can be received by the optical fiber with maximum efficiency is uniquely determined. Therefore, when combined as shown in FIG. Will be realized. Therefore, in this case, the characteristics of the optical isolator optimized as described above are almost realized also in this module, and stable dynamic characteristics can be realized as a semiconductor laser module.

FIG. 8 shows another embodiment. In the figure, the outer diameter holder 20 has a rectangular shape, a square shape, or a cylindrical shape, and its outer periphery is formed parallel to the light incident direction. An inclined surface 22 is formed on the inner surface of the outer diameter holder, and the optical isolator element 11 is inclined and attached thereto. This inclination angle is matched with the inclination angle of the light examined in the embodiment of FIG. In this example, unlike the embodiment shown in FIG. 5, it is sufficient that the light incident direction is along the axis of the outer shape holder, and it is not necessary to indicate the light incident direction.

[0016]

According to the present invention, it is possible to set the optical isolator element so as to maximize the isolation at an oblique incident angle. Further, according to the present invention, an optical fiber and an optical isolator element are combined so as to maximize the isolation at an oblique incident angle, and this optical isolator with a fiber is optimal when combined with other necessary elements such as a laser light source. It is possible to easily set the arrangement.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between incident angles θ _in and φ _{in of} light and effective values of relative angles of two polarizers.

FIG. 2 is a perspective view showing a configuration of an optical isolator element and a direction of incident light and a direction of a polarizer.

FIG. 3 is a sectional view showing an embodiment of an isolator with a fiber of the present invention.

FIG. 4 is a diagram showing an oblique incident surface of an optical fiber and an incident direction capable of receiving light.

FIG. 5 is a perspective view showing an example of an optical isolator having a mark indicating an incident direction.

FIG. 6 shows a measurement system of an optical isolator with an optical fiber.

FIG. 7 shows an application example in which a semiconductor laser is combined with an optical isolator with an optical fiber according to the present invention.

FIG. 8 is a cross-sectional view showing another embodiment of the optical isolator of the present invention in which the outer shape holder 20 is made as small as possible.

[Explanation of symbols]

 1, 2: birefringent plate (polarizer) 3: Faraday rotator 4: optical fiber core wire 5: optical fiber element wire 6: ferrule 7: capillary 8: permanent magnet 9: fiber end surface 10: outer shape holder 11: optical isolator Element 17: Element holder 20: Outer shape holder

Claims (6)

[Claims] 1. A first and a second birefringent crystal plate arranged such that their optical axes differ by a constant angle, and a first birefringent crystal plate and a second birefringent crystal plate. In an optical isolator element consisting of a Faraday rotator that rotates the plane of polarization of incident light by a certain angle, the incident polarization in the forward direction causes the first and second birefringent crystal plates to be ordinary light, ordinary light or extraordinary light, and extraordinary light. The angles of the optical axes of the first and second birefringent crystal plates are set so as to be transmitted, and the first birefringent crystal plate is used when light is incident from a specific oblique direction actually used. _{Is defined as an} angle between the plane of polarization that transmits as ordinary light and the plane of polarization that transmits through the second birefringent crystal plate as ordinary light as φ _eff, and the Faraday rotation angle at the use center temperature and the use center wavelength of the Faraday rotator is θ. _{If f} , then θ _f + φ
An optical isolator element characterized by satisfying _eff = 90 °. 2. A first and a second birefringent crystal plate arranged such that their optical axes differ by a constant angle, and a first birefringent crystal plate and a second birefringent crystal plate. In an optical isolator element composed of a Faraday rotator that rotates the plane of polarization of incident light by a certain angle, the incident polarization in the forward direction is converted into ordinary light, extraordinary light or extraordinary light, and ordinary light by the first and second birefringent crystal plates. The angles of the optical axes of the first and second birefringent crystal plates are set so as to be transmitted, and the first birefringent crystal plate is used when light is incident from a specific oblique direction actually used. _{Is defined as an} angle between the plane of polarization that transmits as ordinary light and the plane of polarization that transmits through the second birefringent crystal plate as ordinary light as φ _eff, and the Faraday rotation angle at the use center temperature and the use center wavelength of the Faraday rotator is θ. _{If f} , then θ _f = φ
An optical isolator element characterized by satisfying _eff . 3. An optical isolator using the optical isolator element according to claim 1 or 2, wherein a mark for an incident direction of light is provided at a position where light transmission is not hindered. 4. An outer shape holder having an outer peripheral surface parallel to the light incident direction, and the optical isolator element according to claim 1 fixed to the outer shape holder while being inclined with respect to the light incident direction. Optical isolator. 5. A first and a second birefringent crystal plate arranged such that their optical axes differ by a constant angle, and a first birefringent crystal plate and a second birefringent crystal plate. An optical isolator element comprising a Faraday rotator for rotating the polarization plane of the incident light by a certain angle; and an optical isolator with the optical isolator element and a first optical fiber having a slanted end face, The position of the optical fiber is adjusted so that the amount of transmitted light in the forward direction from the second optical fiber to the first optical fiber is maximized by a lens inserted between the optical fiber and the first optical fiber. When the amount of transmitted light in the reverse direction from the first optical fiber to the second optical fiber is measured in this state, the optical isolator element is rotated 180 ° about the first optical fiber as the central axis. Optical fiber with optical isolator, wherein the reverse direction of the transmission light amount is increased when is. 6. The optical isolator with an optical fiber according to claim 5, a semiconductor laser, and a lens, and the position of each element is adjusted so that the coupling efficiency from the semiconductor laser to the optical fiber is maximized. Semiconductor laser module.