JPH0354517A - Optical isolator - Google Patents

Abstract

PURPOSE:To suppress the influence of temperature variation by combining the loss characteristic of an angle of Faraday rotation canceling an absorption loss characteristic in a multi-stage optical isolator constituted by connecting N stages of optical isolators including Faraday rotators arranged between 1st and 2nd polarizers. CONSTITUTION:In an example wherein the Faraday rotator 8 of the multi-stage optical isolator constituted by connecting N stages of optical isolators, the loss characteristic C due to temperature variation in the angle of rotation of the Faraday rotator 8 is obtained. The loss characteristic C due to the temperature variation in the angle of the Faraday rotation decreases monotoneously with temperature almost at center temperature to cancel the absorption loss characteristic B having a positive temperature coefficient. For the purpose, the absorption loss characteristic B and loss characteristic C are combined together to obtain the total loss characteristic A which is nearly flat to the temperature variation. Therefore, the loss characteristic of the optical isolators has a little variation with temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION [Objective of the Invention J (Field of Industrial Application) The present invention relates to an optical isolator applied to an optical communication system.

(Prior Art) A configuration as shown in FIG. 11 is known as an example of an optical communication system. 1 is a laser light source consisting of a semiconductor laser, etc., 2.4 is a lens, 3 is an optical isolator, and 5 is an optical fiber. 4
After being focused, the light is input into an optical fiber 5 and transmitted to a desired communication destination.

In such an optical communication system, if the transmitted laser light is reflected from the outside for some reason and is returned to the laser light source 1, the oscillation of the laser light source 1 will become unstable, causing communication problems. It starts to come. For this reason, an optical isolator 3 is used to prevent reflected light from entering the laser light source 1 and to ensure stable laser oscillation.

Figure 1 shows an example of such an optical isolator, where 6 and 7 are the first and second polarizers that are exposed from the prism, and 8
9 and 10 are polarizer holders that support the first and second polarizers 6 and 7; l is a Faraday rotator disposed between the two;
1 is a permanent magnet. According to this structure, when light is incident in the forward direction from the laser light source into the first polarized light, this incident light has a constant polarization plane with respect to the optical axis L indicated by the broken line. Only the light component with a plane of polarization perpendicular to it is passed through, and the light component with a plane of polarization perpendicular to it is not passed through. Similarly, when the reflected light is incident on the second polarizer 7 in the opposite direction, only light components having a certain plane of polarization are passed through. The direction of the polarization plane of the light that has passed through the first and second polarizers 6.7 in the forward direction and in the reverse direction is usually rotated by 45 degrees by the Faraday rotator 8, but this direction of rotation is in the direction in which the light travels. Since it is irreversible, the plane of polarization of the light exiting the Faraday rotator 8 is 9 in the forward and reverse directions.
0' will be different. Therefore, in the first and second polarizers 6.7, light from opposite directions does not return to the incident direction, but is scattered in a direction oblique to the optical axis L by the polarizers. As a result, only a portion of the polarized light in the forward direction is focused on the optical fiber, and in the opposite direction, both polarized light portions are prevented from returning to the incident direction, thereby preventing the influence of reflected light.

Furthermore, the operating principle of the optical isolator will be explained. Assuming that the angle of the plane of polarization of linearly polarized light that can pass through the first polarizer 6 is 0, that of the second polarizer 7 is ψ, and the rotation angle of the Faraday rotator 8 is θ, each element If the loss is O, the forward transmittance T, is T, = cos2(ψ
- θ, ), and the reverse transmittance T is T. = cos2(ψ
+θ, ).

Here, the conventional optical isolator has a center temperature of ψ=45°. Since the wavelength is set to θ, = 45°, the above T,
, T, are each T, = cos2 (45°-45°) = 1Tb =
cos2(45°+45°) = cos290°−0
Therefore, it can operate as an optical isolator. ．． In such an optical isolator, the first isolator acts to pass only light with a specific polarization plane among the incident light.
Alternatively, as the second polarizers 6 and 7, for example, a polarizing prism or a polarizing beam splitter made of birefringent crystal is used. Further, as the Faraday rotator 8, rare earth iron garnet or Bi (bismuth)-substituted rare earth iron garnet is often used, for example, for near-infrared wavelengths (1.3 μm or 1.5 μm).

In particular, the Faraday rotator 3 has a so-called positive temperature coefficient, in which its absorption loss gradually increases in response to temperature changes. FIGS. 7 to 10 show the absorption loss characteristics when constructed from Bi-substituted rare earth iron garnet, and are indicated by B in each case. In addition, in FIG. 7, the Faraday rotation angle is set to 45° at 25° C., the angle formed by the two polarizers 6 and 7 is set to 45°, and the absorption loss is 0. This is the case of id8. Figure 8 shows a case where the Faraday rotation angle is set to 45° at 25°C, the angle between the two polarizers 6 and 7 is set to 45°, and the absorption loss is 0.
．． This is the case of 2 dB. Figure 9 shows the Faraday rotation angle set at 45° at 25°C, and two polarizers 6,
The angle formed by 7 is set to 45°, and the absorption loss is 0.3d.
This is the case of B. Figure 10 shows a two-stage optical isolator (the structure shown in Figure 2, in which the optical isolators in Figure 1 are connected in two stages).
At 25°C, the Faraday rotation angle is set to 45°, the angle formed by the two polarizers 6 and 7 is set to 45° (both stages), and the absorption loss is set to θ. The case of IdB is shown.

Furthermore, the Faraday rotation angle of the Faraday rotator 3 changes due to temperature changes. The Faraday rotation angle at 25°C is θ. 2, the general Faraday rotation angle θ is expressed as follows.

θ,=θ. 25 +kt (7 25℃) However, T
: Temperature (°C), kT : Temperature coefficient (deg/°C).

Since k1 is normally 0, the higher the temperature, the smaller θ becomes. Due to such a change in θ, the T. becomes more likely to change. In FIGS. 7 to 10, C shows the loss characteristics of the optical isolator due to temperature changes in the Faraday rotation angle, and in each figure, kT=-0. 07
(deg/°C) is shown as an example. Also θ,
=45°, ψ=45°. Furthermore, Figures 7 to 1
In FIG. 0, A indicates the result of addition of the respective loss characteristics B and C, which indicates the overall loss characteristic, that is, the loss characteristic of the optical isolator.

(Problems to be Solved by the Invention) Conventional optical isolators have a problem in that the reliability of the optical isolator decreases because the loss characteristics of the Faraday rotator vary with temperature changes. That is, in the past, the absorption loss characteristics of the Faraday rotator have not been taken into consideration, so the loss of the optical isolator will also vary depending on the temperature change. The absorption loss usually has a positive temperature coefficient that increases monotonically as the temperature rises, and the larger the loss, the larger the temperature change tends to be. If the loss as an optical isolator is 0.5 dB or less for a single-stage type, it can be used sufficiently, but the temperature change is 0.05 dB in the range of -10°C to +60°C.
It is necessary to keep it below.

The present invention has been made in response to the above-mentioned problems.
It is an object of the present invention to provide an optical isolator configured such that its loss characteristics hardly change due to temperature changes.

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, the present invention provides an optical system that includes a first polarizer, a second polarizer, and a Faraday rotator disposed between them. In a multi-stage optical isolator in which N stages of isolators are connected (N is an integer greater than or equal to 1), the average value of Faraday rotation angles <θFO> at the operating center temperature and operating center wavelength of the N Faraday rotators is determined by the following formula <θFO >≦44° or θFO>≧46° and here k is the temperature coefficient of the Faraday rotation angle of the Faraday rotator used (unit: deg/°C), k. is the temperature coefficient of loss (unit: dB/°C) of the Faraday rotator used.

It is characterized by satisfying the following.

(Function) The optical isolator is configured by combining the absorption loss characteristic with temperature change with the loss characteristic due to temperature change in the Faraday rotation angle that offsets this loss characteristic. Loss characteristics can be made almost flat against temperature changes. As a result, the loss characteristics of the optical isolator hardly change due to temperature changes, so the reliability of the optical isolator can be improved.

(Example) Examples of the optical isolator of the present invention will be described below.

First, the principle of the present invention will be explained in detail.

Assume that the angle of the plane of polarization of linearly polarized light that can pass through the first polarizer 6 is O, that of the second polarizer 7 is ψ, and the rotation angle of the Faraday rotator 8 is θ.

Normally, the loss of the polarizer is small and the temperature change is also small, so if we ignore the loss of the polarizer (express the loss in dB)
Forward loss L. is L H =-10 10gto(Cot2(ψ1θp
)) + LR...(1) Reverse direction loss L, is Lb = - 10 Io(to(cos2(ψ+or
)) + LR...Q) Here, Li is the absorption loss of the Faraday rotator (unit: dB) Considering this temperature change, the center temperature of use T can be approximated.

Nearby L R = L to + k * (T-To)
”{3), where k
, is a coefficient representing the temperature change in absorption loss.

L. : Operating center temperature T. Absorption loss at, T: temperature.

Also, θ is T in the same way considering temperature changes. θ, = θr near
o+kP(T-TO)-(4
) can be written. Here, k is a coefficient representing a temperature change in the Faraday rotation angle. (θ, .: Rotation angle at operating center temperature To) First, the reverse direction loss is at center temperature T. Since it should be determined so that it is maximum at To, ψ + θ, = 90° T in equation (4). Then θ,=θ,.

So in the end ψ10. = 90°, that is, ψ=90° −θ,. ...(5)
It is.

Substituting (3), (4), and (5) into (1), the temperature change of L at To is obtained by differentiating (6) with T and substituting T=To. 0 1516 k (7) As can be seen, , xlan (90° −2θro) + kR... (
7) When θ-o=45° as in the conventional case, the temperature change in the absorption loss of the Faraday rotator directly becomes the temperature change in the forward loss of the optical isolator, which poses a problem.

In the present invention, almost kg tan(90°-20,.) =6.596 ...(8) k1 i.e. ...(9) Since it is set so that at T=To A new optical isolator can be obtained.

For example, an example will be shown in which Bi-substituted rare earth iron garnet is used as a Faraday rotor.

This shows an embodiment in which the Faraday rotator 8 in the optical isolator of FIG. Loss characteristics due to changes can be obtained.

Figure 3 shows the Faraday rotation angle θ at 25°C. =47
°, the angle between the two polarizers 6 and 7 is 43°, and the absorption loss is 0.1 dBSkr = - 0.07°/°C, k R
= 0. This is the case of 0008 dB/T:. Figure 4 shows the Faraday rotation angle θ at 25°. =49°, 2
The angle formed by the two polarizers 6 and 7 is 41°, and the absorption loss is 0.
2 dB, k, =-0.07°AI', kR
=0.0016dB/°C. Figure 5 is 2
Faraday rotation angle θ, at 5°C. =51° The angle formed by the two polarized lights is 39°, and the absorption loss is 0.3d.
B, kF = - 0.07°, Q:, km = 0.
This is the case of 0024 dB/t.

In addition, in FIGS. 3 to 5, B is a Faraday rotator 8.
In this absorption loss characteristic B, the absorption loss gradually increases as the temperature rises, and has a so-called positive temperature coefficient.

Furthermore, in FIGS. 3 to 5, A represents each loss characteristic B.
This shows the result of addition of and C, and shows the overall loss characteristics, that is, the loss characteristics of the optical isolator.

In other words, according to this embodiment, as is clear from FIGS. 3 to 5, the loss characteristic C due to temperature change in the Faraday rotation angle decreases monotonically with temperature near the center temperature, and has a positive temperature coefficient. The absorption loss characteristic B is changed so as to offset this loss characteristic. Therefore, by combining the absorption loss characteristic B and the loss characteristic C, the overall loss characteristic A becomes a characteristic that is almost flat against temperature changes. Therefore, the loss characteristics of the optical isolator hardly change due to temperature changes, so the reliability of the optical isolator can be improved.

Note that θ, determined by equation (9). A sufficient effect can be obtained even if there is a deviation of about ±1 degree.

Further, when θFO<46° of 44°, there is almost no effect compared to the conventional method.

The above is a description of the case where the optical isolator has one stage. When two or more stages of optical isolators are connected in series, there are many degrees of freedom, which cannot be expressed in a simple formula such as equation (9).
The principle is the same as the one-stage case.

In conclusion, the average value of the rotation angle of the Faraday rotator is approximately (
9) as long as it satisfies the formula.

FIG. 2 shows an embodiment in which the optical isolators shown in FIG. 1 are connected in two stages, and the characteristics shown in FIG. 6 can be obtained. In Figure 6, at 25°C, the Faraday rotation angle is set to 44° in the first stage and 50' in the second stage, and the angle formed by the two polarizers 6 and 7 is set to 46° in the first stage and 50' in the second stage. 40°
and the absorption loss is 0.1 dB in the ■ stage and the second stage.
The case is shown below. Note that ky=-0.07°/1. k
R=0. This is the case of 0008 da/°C. Also in this embodiment, the overall loss characteristic A of the optical isolator becomes a substantially flat characteristic with respect to temperature changes, and the same effects as in the previous embodiment can be obtained. Of course, even if the one-stage optical isolator shown in Fig. 3 is simply made into two stages, the first stage,
A similar effect can be obtained by setting both the Faraday rotation angles of the second stage to 47 degrees, but the average value of the rotation angles may also be set to 47 degrees as shown in FIG. In the case of multiple stages like this, the average value of the rotation angles is important.

Above kp=-0, 07 deg/'C and kR>
Although an example has been shown only for o, it is clear that by using equation (9), appropriate values of θ and 0 can be obtained in other cases as well.

[Effect of the Invention J As described above, according to the present invention, an optical isolator is constructed by combining the absorption loss characteristic with the loss characteristic due to temperature change of the Faraday rotation angle that cancels out this loss characteristic. Therefore, fluctuations in the loss characteristics of the optical isolator due to temperature changes can be almost suppressed.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing an embodiment of an optical isolator to which the present invention is applied, FIG. 2 is a cross-sectional view showing another embodiment of the present invention, and FIGS. FIG. 6 is a loss characteristic diagram of the optical isolator obtained by the embodiment shown in FIG. 2, FIGS. 7 to 10 are loss characteristic diagrams of the conventional optical isolator, and FIG. FIG. 1 is a configuration diagram showing an example of an optical communication system. 3... Optical isolator, 6... First polarizer, 7...
...Second polarizer, 8...Faraday rotator. 1l 11 6 Kuzu 1 Town Shima Otonoko Snow W Be&轡→K Kaikin Cb

Claims (1)

[Claims] N stages of optical isolators including a first polarizer, a second polarizer, and a Faraday rotator disposed between them are connected (
(N is an integer greater than or equal to 1) In the multi-stage optical isolator, the average value of Faraday rotation angles <θ_F_O> at the operating center temperature and operating center wavelength of the N Faraday rotators is determined by the following formula: <θ_F_O>≦44° or <θ_F_O 〉≧4
6゜and 44゜-1/2tan＾-＾1(6.596×
(k_R/k_F))≦〈θ_F_O〉≦46°-1/
2tan^-^1 (6.596×(k_R/k_F))
Here, k_F is the temperature coefficient of the Faraday rotation angle of the Faraday rotator used (unit: deg/℃), and k_R is the temperature coefficient of the loss of the Faraday rotator used (unit: dB/℃).
℃). An optical isolator that satisfies the following.