JP2018084743A - Optical isolator and optical transmission device using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To maintain a stable operation characteristic, in such a mode that each element of an optical isolator is arranged obliquely to a reference surface orthogonal to a light incident direction.SOLUTION: An optical isolator 5 includes a Faraday rotor 3 and a halfwave plate 4 disposed between a first birefringent crystal 1 and a second birefringent crystal 2. On condition that an incident surface of the halfwave plate 4 is arranged orthogonally to a light incident direction, A thickness of the halfwave plate 4 at a maximum isolation value to light with a use wavelength λ is defined as T, while on condition that the incident surface of the halfwave plate 4 is arranged obliquely to a reference surface orthogonal to the light incident direction, a thickness T of the halfwave plate 4 is adjusted so as to match a substantial optical length L passing through the halfwave plate 4 with T. Further, this invention also relates to an optical transmission device including the optical isolator 5.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical isolator used as an element of an optical transmission device, and more particularly to an optical isolator effective at low cost and in a polarization-independent type and an improvement of an optical transmission device using the same.

  The optical isolator is used for, for example, a laser light source for high-speed long-distance transmission, and the forward loss in which light emitted from the laser light source is guided to the optical fiber is as small as 1 dB or less, and is guided from the optical fiber to the laser light source. The loss in the reverse direction is as large as 20 dB or more. In order to realize such characteristics, at least one polarizer and a Faraday rotator are indispensable for an optical isolator, but at least two polarizers are required particularly when high reverse loss is required. And two Faraday rotators are required. Both the polarizer and the Faraday rotator for communication components are expensive optical components, and further price reduction is required to meet the severe price reduction demand from the communication component market in recent years.

  The polarizer currently used as a standard at present is glass, but a polarization-independent type is required for an optical isolator used in the middle of an optical signal or an amplifier. A typical polarization-independent optical isolator includes, for example, two polarizers obtained by processing a birefringent crystal into a wedge shape and one Faraday rotator. However, such an optical isolator needs to be assembled by fixing individual optical components to individual metal holders and adjusting their mutual rotation directions, and is more expensive than a communication isolator.

  In addition, the polarization-independent optical isolator is sometimes used for a semiconductor optical amplifier in addition to being used for an amplifier such as an erbium-doped fiber amplifier (EDFA). In this case, a lower cost product is required, and at the same time, a product with a high mounting density is required.

  As an optical isolator useful for such an application, for example, as shown in Patent Document 1, there is one in which one Faraday rotator and one half-wave plate are arranged in parallel between at least two birefringent crystal plates. Already known.

Japanese Unexamined Patent Application Publication No. 2004-029334 (Example, FIG. 1)

By the way, this type of optical isolator is installed as an optical component between a condensing lens and an optical fiber, for example, and a semiconductor amplifier is constructed. At this time, reflected return light does not return to the semiconductor amplifier. In order to do so, it is necessary to effectively suppress reflection of each element of the optical isolator. For example, if it is assumed that the focused beam focused on the optical fiber has an aperture of 0.1 and a spread angle of 6 degrees, each element (each element) of the optical isolator is usually tilted to a spread angle of 6 degrees or more. .
However, in order to assemble a semiconductor amplifier with high gain, it has been found that there is a concern that slight reflection may impair the operating characteristics, and it is necessary to tilt each element of the optical isolator with a larger tilt angle.
Therefore, when each element of the optical isolator is inclined at an inclination angle larger than 6 degrees, for example, 8 degrees, the peak isolation value (corresponding to the maximum value of the isolation value) changes with respect to the light having the wavelength λ used. The phenomenon that operation became unstable was seen.

  A technical problem to be solved by the present invention is to maintain stable operation characteristics in an aspect in which each element of an optical isolator is inclined with respect to a reference plane orthogonal to the incident direction of light.

The present inventors investigated the change in the light beam state when the respective elements of the optical isolator (the birefringent crystal plate, the Faraday rotator, and the half-wave plate) of the pair configuration are sequentially inclined, and the following results were obtained. found.
First, since the birefringent crystal plate functions as a beam shifter, the shift amount of the beam is expected to change slightly depending on the tilted arrangement, but it is at an undetectable level experimentally, and No reduction in extinction ratio due to switching was found.
Next, when the Faraday rotator was arranged in an inclined manner and the dependency of the Faraday rotator on the tilt angle was examined, no significant change with respect to the tilt angle or a decrease in the extinction ratio due to light switching was found.
Finally, when the extinction ratio accompanying the switching of light was measured with the half-wave plate inclined, a phenomenon in which the extinction ratio decreased as the inclination angle increased was observed. Further, when examining the wavelength dependence of the extinction ratio, it was confirmed that the wavelength at which the maximum extinction ratio is obtained becomes longer as the tilt angle becomes larger.
Here, it has been found that the cause of the characteristic deterioration is that the half-wave plate does not function as intended at the wavelength used. In other words, the light beam obliquely transmitted through the half-wave plate transmits a distance whose substantial optical path length is longer than the actual thickness dimension of the half-wave plate, and accordingly, the extinction ratio of the emitted light beam is reduced accordingly. That is.
In order to solve the above-mentioned problems, the inventors of the present invention focused on selecting the thickness of the half-wave plate by correctly estimating the substantial optical path length of the light beam obliquely transmitted through the half-wave plate. It came to devise.

The first technical feature of the present invention is that a first birefringent crystal in which non-polarized light is incident in the forward direction and is divided into a plurality of polarized lights of ordinary light and extraordinary light, and the first birefringence A plurality of polarized light beams arranged between the second birefringent crystal and the first and second birefringent crystals arranged away from the crystal in the forward direction and passing through the first birefringent crystal are in the forward direction. On the other hand, between the first and second birefringent crystals, the plurality of polarized lights incident from the opposite direction are rotated by 45 degrees in the predetermined direction and rotated by 45 degrees in the direction opposite to the forward direction. A half-wave plate in which a plurality of polarized lights are rotated by 45 degrees in the same direction with respect to the traveling direction of light, and an incident surface of the half-wave plate is disposed orthogonal to the incident direction of the light. The half-wave plate thickness when taking the maximum isolation value for the light of the wavelength λ used If it is _zero, the incident surface of the half-wave plate is arranged obliquely conditions with respect to the reference plane perpendicular to the incident direction of the light, so to match the substantial optical path length that passes through the half-wave plate to T ₀ And adjusting the thickness T of the half-wave plate.

According to a second technical feature of the present invention, in the optical isolator having the first technical feature, an incident surface of the half-wave plate is inclined at an inclination angle θ with respect to a reference surface orthogonal to the incident direction of the light. Under the arranged condition, when the relative refractive index of the half-wave plate with respect to air is n and the refraction angle of the half-wave plate is α, the thickness T of the half-wave plate is
T = T ₀ × cos α (Formula 1)
However, sin α = sin θ / n
The optical isolator is selected according to the above.
According to a third technical feature of the present invention, in the optical isolator having the first technical feature, the incident surface of the half-wave plate is inclined with respect to a reference surface orthogonal to the incident direction of the light. Then, if the relative refractive index of the half-wave plate with respect to air is n, and the incident angle of light with respect to the incident normal surface of the optical isolator is Θ, the thickness T of the half-wave plate is
T = T ₀ × √ (1-sin ² Θ / n ² ) (Formula 2)
The optical isolator is selected according to the above.
According to a fourth technical feature of the present invention, in the optical isolator having any one of the first to third technical features, an inclination angle θ of the half-wave plate with respect to the reference plane is an angle exceeding 6 degrees. This is an optical isolator.

  A fifth technical feature of the present invention is that an optical isolator having any one of the first to third technical features and non-polarized light is incident on the first birefringent crystal of the optical isolator. An optical transmission device comprising: an incident-side optical component; and an emission-side optical component that receives light emitted from the second birefringent crystal of the optical isolator in the same optical path.

According to the first technical feature of the present invention, stable operation characteristics can be maintained in an aspect in which each element of the optical isolator is inclined with respect to a reference plane perpendicular to the incident direction of light.
According to the second technical feature of the present invention, it is possible to easily select the thickness of the half-wave plate from the refractive index and the inclined posture of the half-wave plate in order to maintain stable operation characteristics.
According to the third technical feature of the present invention, the thickness of the half-wave plate can be easily selected from the refractive index of the half-wave plate and the incident angle of light to the optical isolator in order to maintain stable operating characteristics. can do.
According to the fourth technical feature of the present invention, the tilt angle of each element of the optical isolator can be increased, and more stable operation characteristics can be maintained.
According to the fifth technical feature of the present invention, stable operation characteristics can be maintained in an aspect in which each element of the optical isolator is inclined with respect to a reference plane perpendicular to the incident direction of light. An optical transmission device including an optical isolator can be provided.

(A) is explanatory drawing which shows the basic component in the outline | summary of embodiment of the optical isolator to which this invention is applied, (b) is explanatory drawing which shows the aspect which inclinedly arrange | positioned the optical isolator shown to (a), (c) ) Shows a method for selecting the thickness of the half-wave plate of the optical isolator in the inclined arrangement shown in (b). 1 is an explanatory diagram illustrating an overall configuration of an optical isolator according to a first embodiment. (A) is explanatory drawing which shows the selection method 1 of the thickness of the half wavelength plate used in Embodiment 1, (b) is the description which shows the selection method 2 of the thickness of the half wavelength plate used in Embodiment 1. FIG. (A) is explanatory drawing which shows the structural example of the optical isolator used when selecting the substantial optical path length in Fig.3 (a) (b), (b) is an optical isolator shown in (a) by inclination-angle (theta). It is explanatory drawing which shows the aspect arrange | positioned inclined. It is explanatory drawing which shows each wavelength characteristic of the optical isolator shown to Fig.4 (a) (b). (A) is explanatory drawing which shows the structure of the optical isolator which concerns on Example 1, (b) is explanatory drawing which shows the wavelength characteristic of the optical isolator shown to (a). (A) is explanatory drawing which shows the structure of the optical isolator which concerns on the comparative example 1, (b) is explanatory drawing which shows the wavelength characteristic of the optical isolator shown to (a). (A) is explanatory drawing which shows the structure of the optical isolator which concerns on the comparative example 2, (b) is explanatory drawing which shows the wavelength characteristic of the optical isolator shown to (a).

Outline of Embodiment FIG. 1A shows an outline of an embodiment of an optical transmission apparatus incorporating an optical isolator to which the present invention is applied.
In the figure, the optical isolator 5 has a first birefringence in which unpolarized light is incident on the forward direction and is divided into a plurality of polarized light Bm (Bm ₁ and Bm _{2 in} this example) of ordinary light and extraordinary light. A first birefringent crystal 2 disposed away from the first birefringent crystal 1 in the forward direction, a first birefringent crystal 1 and a second birefringent crystal 1; A plurality of polarized lights Bm that have passed through the birefringent crystal 1 are rotated 45 degrees in the predetermined direction with respect to the forward direction, and a plurality of polarized lights Bm (Bm ₃ and Bm _{4 in} this example) incident from the reverse direction are forward. Arranged between the Faraday rotator 3 rotated 45 degrees in the direction opposite to the direction and the first and second birefringent crystals 1 and 2, and a plurality of polarized lights Bm in the same direction with respect to the traveling direction of light And a half-wave plate 4 rotated by 45 degrees.
In this example, as the optical transmission device, the optical isolator 5 described above, the incident-side optical component 7 that enters the non-polarized light with respect to the first birefringent crystal 1 of the optical isolator 5, and the optical isolator 5 includes an optical component 8 on the emission side that receives light emitted from the second birefringent crystal 2 in the same optical path.
Here, as the optical components 7 and 8, a semiconductor laser, a condensing lens, an optical fiber, or the like may be appropriately selected.

In such technical means, the first and second birefringent crystals 1 and 2 are linearly polarized light components orthogonal to each other and have different refractive indexes, and the light rays are separated into two, which can be appropriately selected including calcite. is there.
Further, the arrangement order of the Faraday rotator 3 and the half-wave plate 4 is arbitrary with respect to the incident direction of light.
Here, the operation principle of the optical isolator 5 in this example will be briefly described.
In FIG. 1A, an optical isolator 5 includes two birefringent crystals 1 and 2 that function as beam shifters, and each has, for example, an optical axis that rises to the right on the page.
In FIG. 1A, assuming that a non-polarized forward light beam is incident on the optical isolator 5 from the right side, it is a component that vibrates in the vertical direction (here, corresponding to the vertical direction perpendicular to the horizontal direction). When the extraordinary light component (corresponding to Bm ₂ indicated by the dotted line in FIG. 1A) passes through the first birefringent crystal 1, a beam shift occurs. It is assumed that this extraordinary light component is rotated 45 degrees clockwise by the Faraday rotator 3 and has a declination of, for example, 22.5 degrees with respect to the crystal axis of the half-wave plate 4. At this time, if the half-wave plate 4 is adjusted to an appropriate thickness with respect to the wavelength of the incident light, the light incident on the half-wave plate 4 is further rotated 45 degrees clockwise and transmitted through the half-wave plate 4. The polarization direction of the emitted light oscillates in the horizontal direction. When horizontal light is incident on the second birefringent crystal 2, it becomes an ordinary light component and passes through the second birefringent crystal 2 without causing a beam shift.

In addition, the component that oscillates in the horizontal direction among the non-polarized forward light rays incident on the optical isolator 5 from the right side in the figure is the ordinary light component (solid line in FIG. 1A) with respect to the first birefringent crystal 1. It shows an equivalent) to Bm _1, the first relative to the birefringent crystal 1 passes through without causing beam shift, when rotated 45 degrees in the clockwise direction by the Faraday rotator 3, the half-wave plate 4 crystal axis The angle of deviation is 67.5 degrees. In this state, if the half-wave plate 4 is adjusted to an appropriate thickness with respect to the wavelength of the incident light, the polarization direction of the light transmitted through the half-wave plate 4 oscillates in the vertical direction. Then, when light that vibrates in the vertical direction enters the second birefringent crystal 2, this becomes an extraordinary light component, causes a beam shift, and passes through the second birefringent crystal 2.
As described above, if the Faraday rotator 3 and the half-wave plate 4 function ideally, the vertical polarization and the horizontal polarization incident from the right side in FIG. Any two birefringent crystals 2 cause a beam shift in the same way. As a result, when incident light passes through all elements of the optical isolator 5, all components travel on the same optical path.

Considering light rays propagating in the opposite direction on the same optical path, in FIG. 1A, a path of a component that oscillates vertically and a component that oscillates horizontally from the left side of the second birefringent crystal 2 in the opposite direction. Will be analyzed.
Here, the vibration in the vertical direction is an extraordinary light component (corresponding to Bm ₄ indicated by a two-dot chain line in FIG. 1A), and causes a beam shift in the second birefringent crystal 2. When this is incident on the half-wave plate 4, it has a deviation angle of 67.5 degrees from the crystal axis of the half-wave plate 4. Therefore, the polarization direction of the light after passing through the half-wave plate 4 is an orientation of −45 degrees. Since the Faraday rotator 3 further rotates 45 degrees counterclockwise with respect to this polarization, the polarization direction of the light after passing through the Faraday rotator 3 is the vertical direction. For this reason, when the light beam incident on the first birefringent crystal 1 from the left side is a vibration in the vertical direction, this is an extraordinary light component, so that the light beam transmitted through the first birefringent crystal 1 is further subjected to a beam shift. Wake up. That is, the extraordinary light component (Bm ₄ ) of the light incident on the optical isolator 5 from the opposite direction is emitted through two beam shifts.

On the other hand, an ordinary light component (corresponding to Bm ₃ shown by a one-dot chain line in FIG. 1A) that is a horizontal vibration does not cause a beam shift in the second birefringent crystal 2 or the first birefringent crystal 1. The half-wave plate 4 rotates 45 degrees clockwise, and the Faraday rotator 3 rotates 45 degrees counterclockwise and transmits.
In this way, the light beam incident on the optical isolator 5 from the opposite direction causes a beam shift in either the first or second birefringent crystal 1 or 2, or does not cause a beam shift in either of the half wavelengths. Since it passes through the plate 4 and the Faraday rotator 3 with the polarization direction of the light returned to its original state, the light is emitted from a position deviated from the position of the light beam incident from the forward direction. That is, it can be said that the optical isolator 5 functions correctly if all the components of the light traveling in the reverse direction are correctly rotated by 180 degrees, or even if rotated, the original state is eventually restored.

Further, in this type of optical isolator 5, since there is a concern that the presence of reflected return light impairs the operation characteristics, each element of the optical isolator 5 is made incident on each element as shown in FIG. In many cases, the surface is inclined at an inclination angle θ with respect to a reference plane orthogonal to the light incident direction.
Here, the inclination angle θ of each element of the optical isolator 5 may be appropriately selected. However, if the inclination angle θ described above is set to be large in order to reduce the influence of the reflected return light, the isolation value is increased. A phenomenon was observed in which the operation of the optical isolator 5 became unstable on the contrary.
As described above, as a result of studying this factor, the present inventors focused on the substantial optical path length of the light incident on the half-wave plate 4 and, as shown in FIG. When the thickness of the half-wave plate 4 when taking the maximum isolation value with respect to the light of the wavelength λ used is T ₀ under the condition that the incident surface is arranged orthogonal to the incident direction of light, FIG. As shown in c), under the condition that the incident surface of the half-wave plate 4 is inclined with respect to the reference plane orthogonal to the light incident direction, the substantial optical path length L passing through the half-wave plate 4 is set to T ₀ . The thickness T of the half-wave plate 4 is adjusted so as to match.

Next, a typical aspect or a preferable aspect of the optical isolator according to the present embodiment will be described.
Here, as a first method for specifically selecting the thickness of the half-wave plate 4, as shown in FIG. 1C, a reference plane in which the incident surface of the half-wave plate 4 is orthogonal to the incident direction of light. If the relative refractive index of the half-wave plate 4 with respect to air is n and the refraction angle of the half-wave plate 4 is α, the thickness T of the half-wave plate 4 is
T = T ₀ × cos α (Formula 1)
However, sin α = sin θ / n
The method of selecting by is mentioned.
In this method, the substantial optical path length L is calculated from the inclined posture of the half-wave plate 4, and the substantial optical path length L is considered to depend on the refraction angle α of the half-wave plate 4. This is based on the fact that it is uniquely determined by the inclination angle θ and the relative refractive index n with respect to air.

In addition, as a second method for specifically selecting the thickness of the half-wave plate 4, as shown in FIG. 1C, the incident surface of the half-wave plate 4 is set to a reference plane orthogonal to the light incident direction. If the relative refractive index with respect to the air of the half-wave plate 4 is n and the incident angle of light with respect to the normal to the incident surface of the optical isolator 5 is θ, the thickness T of the half-wave plate 4 is ,
T = T ₀ × √ (1-sin ² Θ / n ² ) (Formula 2)
The method of selecting by is mentioned.
In this example, the substantial optical path length L of the half-wave plate 4 is calculated from the relative refractive index n of the half-wave plate 4 and the incident angle Θ of light with respect to the incident surface normal of the optical isolator 5. Even if each element of the isolator 5 is not inclined with respect to the vertical plane orthogonal to the horizontal plane, the light incident angle Θ with respect to the normal surface of the optical isolator 5 is not 0. By calculating the substantial optical path length L of the half-wave plate 4 by using the incident angle Θ, the thickness T of the half-wave plate 4 can be determined.
Thus, from the viewpoint of further reducing the influence of the reflected return light, the half-wave plate 4 has an incident surface at an inclination angle θ (or incident angle Θ) with respect to a reference plane orthogonal to the light incident direction. Any configuration may be used as long as it is tilted, and the tilt angle θ (or incident angle Θ) is preferably more than 6 degrees.

Hereinafter, the present invention will be described in more detail based on embodiments shown in the accompanying drawings.
Embodiment 1
FIG. 2 is an explanatory diagram showing the overall configuration of the optical isolator according to the first embodiment.
In the figure, an optical isolator 20 is incorporated in an optical transmission device (not shown), and has a cylindrical holder 21 in which a cylindrical magnetic pole (not shown) is incorporated, and a space portion in the cylindrical holder 21. Includes a first birefringent crystal 22 that functions as a beam shifter in which non-polarized light is incident in the forward direction and is divided into a plurality of polarization components of ordinary light and extraordinary light, and a first birefringent crystal in the forward direction of the light. A half-wave plate 23 which is disposed downstream of the refractive crystal 22 and rotates the polarization component transmitted in the forward direction or the reverse direction in a predetermined direction, for example, 45 degrees clockwise, and the order transmitted through the half-wave plate 23. A Faraday rotator 24 that rotates a plurality of polarization components in a predetermined direction, for example, 45 degrees clockwise, while rotating a polarization component incident from the opposite direction 45 degrees counterclockwise; Farah against direction Disposed downstream of the over rotor 24, the second birefringent crystal 25 which acts as a beam shifter in the same manner as the first birefringent crystal 22 is obtained by sequentially disposed.

In this example, the optical isolator 20 has a Y axis in which the light incident surfaces of the cylindrical holder 21 and each of the elements 22 to 25 are orthogonal to the X axis when the light traveling direction is the X axis (corresponding to the horizontal axis in this example). It is inclined with respect to an inclination angle θ (in this example, θ = 6 degrees to 10 degrees) with respect to (corresponding to a vertical axis in this example).
Further, in this example, the thickness T of the half-wave plate 23 is determined in advance as a substantial optical path length L of the light Bm transmitted through the half-wave plate 23 as shown in FIGS. 2 and 3A. The value T ₀ is selected.
As shown in FIG. 3A, when the relative refractive index of the half-wave plate 23 with respect to the air is n and the refraction angle of the half-wave plate 23 is α, the thickness T of the half-wave plate 23 is expressed by the following equation. It is represented by 1.
T = T ₀ × cos α (Formula 1)
However, sin α = sin θ / n.
Here, supplementing the expression of the proviso, the light incident surface of the half-wave plate 23 is inclined with respect to the Y axis at an inclination angle θ, and in this example, the light Bm traveling along the X axis. Is based on the fact that the relative refractive index n is the sine ratio of the incident angle θ to the refraction angle α.

Moreover, in this example, the optical isolator 20 is an aspect in which the light incident surfaces of the cylindrical holder 21 and the elements 22 to 25 are inclined with respect to the Y axis at an inclination angle θ. Even if it is not inclined with respect to the Y axis, as shown in FIG. 3B, if the incident direction of the light Bm is inclined with respect to the incident surface normal of the optical isolator 20, the optical isolator 20 is provided. It is possible to obtain a state substantially the same as the aspect (see FIG. 2) in which these are physically inclined.
Therefore, in the embodiment shown in FIG. 3B, the relative refractive index of the half-wave plate 23 with respect to air is n, the refraction angle of the half-wave plate 23 is β, and the incident angle of the light Bm with respect to the incident surface normal of the optical isolator 20. If Θ is assumed, the thickness T of the half-wave plate 23 is expressed by the following formula 2.
T = T ₀ × √ (1-sin ² Θ / n ² ) (Formula 2)
Here, since n = sin Θ / sin β, Expression 2 is expressed by Expression 2 ′ below.
T = T ₀ × √ (1-sin ² β) (Formula 2 ′)
FIG. 3B shows an aspect in which the optical isolator 20 is not inclined with respect to the Y axis. For example, it is assumed that the optical isolator 20 is inclined with respect to the Y axis at an inclination angle θ. However, if the incident angle Θ of the light Bm with respect to the incident surface normal of the optical isolator 20 is different from the inclination angle θ, the thickness T of the half-wave plate 23 may be selected using Equation 2. .

Further, T ₀ used in the above-described formulas 1 and 2 is selected as follows.
As shown in FIG. 4A, in the optical isolator 20 ′, the light incident surfaces of the cylindrical holder 21 and the elements 22 to 25 are arranged orthogonal to the X axis (corresponding to the horizontal axis in this example). As shown in FIG. 5, the thickness T ₀ of the half-wave plate 23 is selected so as to obtain a wavelength characteristic that maximizes the isolation value with respect to the used wavelength λ (nm). What should I do?
The entire optical isolator 20 ′ (see FIG. 4A) including the half-wave plate 23 having the thickness T ₀ selected in this way is inclined, and as shown in FIG. When used as an optical isolator 20 ″ in which the light incident surfaces of the elements 22 to 25 are inclined with respect to the Y axis at an inclination angle θ, the light traveling direction is inclined with respect to the incident surface normal of the elements 22 to 25 Since the light is inclined at the angle θ, the light transmitted through the half-wave plate 23 is refracted, and the substantial optical path length transmitted through the half-wave plate 23 becomes longer than the thickness T ₀ of the half-wave plate 23. In this inclined arrangement mode, when the wavelength characteristics are examined, as shown by a one-dot chain line in Fig. 5, there is a tendency that the isolation value with respect to the used wavelength λ (nm) decreases.
On the other hand, in the optical isolator 20 (see FIG. 2) according to the present embodiment, when the wavelength characteristic is examined, it is confirmed that the wavelength characteristic with the maximum isolation value with respect to the used wavelength λ (nm) is obtained. Has been.

Example 1
Example 1 embodies the optical isolator 20 according to the first embodiment.
In this example, as shown in FIG. 6A, the optical isolator 20 has a cylindrical holder 21 in which a light traveling direction is an X axis (corresponding to a horizontal axis in this example) and a cylindrical magnetic pole (not shown) is incorporated. The elements 22 to 25 are juxtaposed along the forward light traveling direction, and the cylindrical holder 21 and the light incident surface of each element 22 to 25 are perpendicular to the X axis (in this example, on the vertical axis). Are inclined at an inclination angle θ (in this example, θ = 10 °).
And in this example, the following are used as each element 22-25.
First birefringent crystal 22: 11 mm square plate, thickness 0.5 mm, optical axis 90 ° / 47 °
Half-wave plate 23: 11 mm square plate, thickness T (T = 0.0761 mm), optical axis 45 °
Faraday rotator 24: GTD1310LE (model number), 11 mm square plate Second birefringent crystal 25: 11 mm square plate, thickness 0.5 mm, optical axis 90 ° / 47 °
In this example, light having a used wavelength λ of 1310 nm is used, and L = T ₀ = 0.0766 mm is selected as the substantial optical path length L.
For this reason, for example, T = 0.0761 mm is selected as the thickness T of the half-wave plate 23 in accordance with Equation 1 (or Equation 2).
Here, when the relative refractive index n of the half-wave plate 23 with respect to the air is 1.46, since the inclination angle θ is 10 °, sin10 = 0.174, and the sine value of the refraction angle α is sinα = 0. 174 ÷ 1.46 = 0.119. Since cos α = √ (1-sin ² α), cos α = √0.986 = 0.993. Therefore, T = 0.0766 × 0.993 = 0.0761.
Further, the wavelength characteristics of this example are shown in FIG. According to the figure, it was confirmed that the maximum isolation value (42 dB in this example) was maintained with respect to light having a wavelength of use λ (1310 nm in this example <environmental temperature 25 ° C.>).

◎ Comparative Example 1
In Comparative Example 1, as shown in FIG. 7A, the optical isolator 20 ′ shown in FIG. 4A is embodied.
In the figure, the optical isolator 20 ′ is disposed so that the light incident surfaces of the cylindrical holder 21 and the elements 22 to 25 are orthogonal to the X axis corresponding to the light traveling direction (corresponding to the horizontal axis in this example). It is a horizontal arrangement | positioning aspect.
And in this example, the following are used as each element 22-25.
First birefringent crystal 22: 11 mm square plate, thickness 0.5 mm, optical axis 90 ° / 47 °
Half-wave plate 23: 11 mm square plate, thickness T (T = 0.0766 mm), optical axis 45 °
Faraday rotator 24: GTD1310LE (model number), 11 mm square plate Second birefringent crystal 25: 11 mm square plate, thickness 0.5 mm, optical axis 90 ° / 47 °
In this example, the thickness T of the half-wave plate 23 is T ₀ as the substantial optical path length L.
Here, the wavelength characteristic of this example is shown in FIG. According to the figure, it was confirmed that the maximum isolation value (42 dB in this example) was maintained with respect to light having a wavelength of use λ (1310 nm in this example <environmental temperature 25 ° C.>).

◎ Comparative Example 2
As shown in FIG. 8A, the comparative example 2 embodies the optical isolator 20 ″ shown in FIG. 4B.
In the figure, an optical isolator 20 ″ tilts the entire optical isolator 20 ′ of Comparative Example 1, and the light incident surfaces of the cylindrical holder 21 and the elements 22 to 25 are inclined with respect to the Y axis by an inclination angle θ (in this example). (θ = 10 °).
And in this example, the following are used as each element 22-25.
First birefringent crystal 22: 11 mm square plate, thickness 0.5 mm, optical axis 90 ° / 47 °
Half-wave plate 23: 11 mm square plate, thickness T (T = 0.0766 mm), optical axis 45 °
Faraday rotator 24: GTD1310LE (model number), 11 mm square plate Second birefringent crystal 25: 11 mm square plate, thickness 0.5 mm, optical axis 90 ° / 47 °
Here, the wavelength characteristic of this example is shown in FIG. According to the figure, it was confirmed that the isolation value is reduced to 30 dB with respect to light having a wavelength of use λ (1310 nm in this example <environmental temperature 25 ° C.>). In this example, the maximum isolation value (42 dB in this example) is obtained with respect to light having a use wavelength λ of 1330 nm <ambient temperature 25 ° C.>.

  According to the optical isolator according to the present embodiment, even if each element of the optical isolator is arranged to be inclined with respect to a reference plane orthogonal to the incident direction of light, the substantial optical path that passes through the half-wave plate By selecting the thickness of the half-wave plate so as to optimize the length, it is possible to minimize the influence of the reflected return light and easily obtain the maximum isolation value for the light of the wavelength used. . As described above, an optical isolator having stable operating characteristics can be provided, and a high-accuracy optical transmission device can be provided by combining such an optical isolator with other optical components.

DESCRIPTION OF SYMBOLS 1 1st birefringent crystal 2 2nd birefringent crystal 3 Faraday rotator 4 Half wavelength plate 5 Optical isolator 7 Incident side optical component 8 Outgoing side optical component 20 Optical isolator 20 'Optical isolator 20 "Optical isolator 21 Tube Holder 22 first birefringent crystal 23 half-wave plate 24 Faraday rotator 25 second birefringent crystal Bm (Bm _{1 to} Bm ₄ ) polarization θ tilt angle T half-wave plate thickness T ₀ half-wave plate in advance Determined thickness L The actual optical path length of the half-wave plate

Claims (5)

A first birefringent crystal in which unpolarized light is incident with respect to the forward direction and is divided into a plurality of polarized lights of ordinary light and extraordinary light;
A second birefringent crystal disposed in a forward direction away from the first birefringent crystal;
A plurality of polarized light arranged between the first and second birefringent crystals and having passed through the first birefringent crystal is rotated 45 degrees in a predetermined direction with respect to the forward direction, and is incident from the reverse direction. A Faraday rotator whose polarization is rotated 45 degrees in the direction opposite to the forward direction,
A half-wave plate disposed between the first and second birefringent crystals and having a plurality of polarized lights rotated by 45 degrees in the same direction with respect to the traveling direction of light,
The thickness of the half-wave plate when taking the maximum isolation value with respect to the light having the wavelength λ used is T ₀ under the condition that the incident surface of the half-wave plate is arranged orthogonal to the incident direction of the light. In this case, under the condition that the incident surface of the half-wave plate is inclined with respect to a reference plane orthogonal to the incident direction of the light, the half-wave plate is adjusted so that a substantial optical path length passing through the half-wave plate is adjusted to T _0. An optical isolator characterized by adjusting a thickness T of a wave plate. The optical isolator according to claim 1,
Under the condition that the incident surface of the half-wave plate is inclined at an inclination angle θ with respect to a reference plane perpendicular to the incident direction of the light, the relative refractive index of the half-wave plate with respect to air is n, When the refraction angle is α, the thickness T of the half-wave plate is
T = T ₀ × cos α (Formula 1)
However, sin α = sin θ / n
An optical isolator characterized by being selected by: The optical isolator according to claim 1,
Under the condition that the incident surface of the half-wave plate is inclined with respect to a reference plane orthogonal to the incident direction of the light, the relative refractive index to the air of the half-wave plate is n, and the light with respect to the incident surface normal of the optical isolator If the incident angle of is Θ,
The thickness T of the half-wave plate is
T = T ₀ × √ (1-sin ² Θ / n ² ) (Formula 2)
An optical isolator characterized by being selected by: The optical isolator according to any one of claims 1 to 3,
The optical isolator is characterized in that an inclination angle θ of the half-wave plate with respect to the reference surface is more than 6 degrees. An optical isolator according to any one of claims 1 to 3,
An optical component on the incident side for entering non-polarized light to the first birefringent crystal of the optical isolator;
An optical component on the emission side for receiving light emitted from the second birefringent crystal of the optical isolator in the same optical path;
An optical transmission device comprising:

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