JPH04303814A - Optical isolator - Google Patents

Abstract

PURPOSE:To obtain mean characteristic by low insertion loss and high isolation by adjusting the film thickness of a Faraday rotation element so that the sum of the angles of Faraday rotation with different wavelengths is almost 90 deg.. CONSTITUTION:A polarizer 1 which passes polarized light in a constant oscillating direction, the Faraday rotation element 2 which rotates the oscillating direction transmitting the polarizer 1 by almost 45 deg. by the Faraday effect, an analyzer 3 arranged in which the transmitting direction of the polarized light is arranged inclining by 45 deg. in a Faraday rotating direction against the polarizer 1 and which transmits only the polarized light rotated by almost 45 deg. from the Faraday rotation element 2, and a magnet which generates the Faraday effect by supplying a saturation magnetic field to the Faraday rotation element 2 are provided, and they are used for incident light with two different wavelength lambda1, lambda2. The angles of Faraday rotation in the wavelength lambda1, lambda2 are set at theta1=45 deg.+alpha, and theta 2=45 deg.-alpha, and the film thickness L of the Faraday rotation element 2 can be adjusted and formed so that the angle between (theta1>theta2) and theta1+theta2 can go to almost 90 deg..

Description

[Detailed description of the invention]

0001

FIELD OF THE INVENTION This invention relates to optical isolators that operate at both two different wavelengths.

0002

[Background Art] In the case of measuring instruments used in the field of optical communications, such as optical pulse testers, optical signals such as backscattered light returned from an optical fiber under test or reflected light from a connection point are transmitted to a laser diode. Repeated multiple reflections between the light emitting end face of the laser diode inside the module,
As a result, it becomes impossible to precisely measure various characteristics of the optical fiber to be measured, such as optical fiber loss, optical fiber connection loss, and abnormal points in the optical fiber. Further, in the case of a stabilized light source, the oscillation of the laser diode becomes unstable due to the influence of the return light on the emitted light from the laser diode, and as a result, it becomes impossible to obtain a stable wavelength and optical output. Therefore, it has been proposed to incorporate an isolator inside the laser diode module. To explain further, the optical isolator includes a polarizer and an analyzer whose transmission polarization directions are arranged at 45 degrees to each other;
The device includes a Faraday element having a Faraday rotation angle of 45° between the polarizer and the analyzer, and a magnet that generates a magnetic field around the Faraday element.

The linearly polarized light transmitted through the polarizer in the forward direction (the direction in which light is emitted from the light source) is subjected to the Faraday effect when passing through the Faraday element, and the polarization direction is rotated by 45 degrees before transmitting through the analyzer. On the other hand, among the reflected light that is incident on the optical isolator in the opposite direction (the direction opposite to the direction in which the light is emitted from the light source), the linearly polarized light that has passed through the analyzer is faraday when the polarization direction is in the forward direction. It is further rotated by 45° in the same direction as the rotation, and is perpendicular to the direction of polarization transmitted by the polarizer, thereby blocking its passage from the polarizer.

0004

However, in the optical isolator described above, as shown in FIG. 3, the Faraday rotation angle due to the Faraday effect has a wavelength dependence that decreases exponentially as the wavelength becomes longer. Therefore, the Faraday rotation angle is 45 for a certain wavelength λ1.
It is necessary to fabricate a Faraday element so that
Also, the Faraday rotation angle is 45° for wavelength λ2.
It was necessary to create a Faraday element so that In other words, it is necessary to prepare an optimal optical isolator according to the wavelength of the light source used, and one optical isolator cannot be used at a plurality of desired wavelengths.

Furthermore, when the Faraday rotation angle is set to 45° at wavelength λ1 (λ2), different wavelengths λ2 (λ
In 1), the Faraday rotation angle is different from 45°, and as shown in [Table 1] below, at wavelength λ2 (λ1), forward loss increases and isolation decreases, resulting in a significant deterioration of the optical isolator characteristics. There was a problem.

[0006]

[Table 1]

By the way, isolation is maximum at a certain wavelength when the Faraday rotation angle of the Faraday element is 45° and the relative angle at which the polarizer and analyzer are arranged is 45° in the Faraday rotation direction. In addition, as mentioned above, the wavelength dependence of the rotation angle of the Faraday element decreases exponentially, so each wavelength λ1, λ
Even if an isolator with an intermediate wavelength of 2 (λ3=(λ1+λ2)/2) is manufactured, the same characteristics cannot be obtained for λ1 and λ2. For example, 1310nm (λ1) and 1550nm (λ2)
When an isolator is manufactured with an intermediate wavelength of 1430 nm (λ3), the wavelength is 1310 nm (λ1) as shown in Figure 4.
It can be seen that the isolation at 1,550 nm (λ2) is different from that at 1,550 nm (λ2), and that the isolation at 1,310 nm is particularly deteriorated.

[0008] The present invention has been made in view of the above problems, and its purpose is to
The object of the present invention is to provide an optical isolator that has substantially the same characteristics due to low insertion loss and high isolation, and is suitable for use at a plurality of desired wavelengths.

[0009]

[Means for Solving the Problems] In order to achieve the above object, an optical isolator according to the present invention includes a polarizer 1 that transmits polarized light in a specific vibration direction, and a vibration direction of the polarized light transmitted through the polarizer using the Faraday effect. A Faraday rotation element 2 rotated by approximately 45 degrees, and a direction in which polarized light is transmitted is inclined at 45 degrees in the Faraday rotation direction with respect to the polarizer, and only polarized light rotated by approximately 45 degrees is transmitted from the Faraday rotation element. An optical isolator used for incident light of two different wavelengths λ1 and λ2, which is equipped with an analyzer 3 that causes a saturation magnetic field to be applied to the Faraday rotation element to produce a Faraday effect, and is used for incident light of two different wavelengths λ1 and λ2. The Faraday rotation angle is θ1 = 45° + α, and the Faraday rotation angle at wavelength λ2 is θ2 = 45° − α (θ1
>θ2), and the film thickness L of the Faraday rotation element is adjusted so that θ1+θ2 is approximately 90°.

0010

[Operation] The Faraday rotation angle at wavelength λ1 is θ1=
45° + α, Faraday rotation angle at wavelength λ2 is θ
2=45°-α (θ1>θ2), and the film thickness L of the Faraday rotation element 2 is adjusted and formed so that θ1+θ2 becomes approximately 90°. This makes it possible to obtain low insertion loss and approximately the same isolation near the two wavelengths λ1 and λ2.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a perspective view showing the schematic structure of an optical isolator according to the present invention. The optical isolator according to this embodiment includes a polarizer 1, a Faraday rotation element 2, an analyzer 3, and a magnet 4.

The polarizer 1 receives light of a predetermined wavelength from a light source (not shown), and allows only linearly polarized light having a specific vibration direction to pass therethrough and enters the Faraday rotation element 2 . The Faraday rotation element 2 rotates the vibration direction of the linearly polarized light that has passed through the polarizer 1 by approximately 45 degrees due to the Faraday effect. This Faraday rotation element 2 has two different wavelengths λ1.
, λ2, the Faraday rotation angle θ1 at the wavelength λ1 is θ1 = 45° + α, and the Faraday rotation angle θ2 at the wavelength λ2 is θ2 = 45° − α (θ1>θ2)
The film thickness is adjusted so that θ1+θ2 becomes approximately 90°.

[0013] Here, the Faraday rotation angle θ is expressed by the following formula. θ=V・L・H V: Verdet constant (physical property value) L: Length of the Faraday rotation element in the optical axis direction H: Strength of the saturation magnetic field applied to the Faraday rotation element In the above formula, V
Since and H are constant, the Faraday rotation angle is adjusted by changing the film thickness of the Faraday rotation element 2 corresponding to L. The analyzer 3 is arranged such that the direction in which the linearly polarized light is transmitted is inclined by 45° to the Faraday rotation direction with respect to the polarizer 1, and only the linearly polarized light rotated by approximately 45° from the Faraday rotation element 2 is transmitted. The magnet 4 is composed of, for example, a permanent magnet or an electromagnet, and applies a saturation magnetic field to the Faraday rotation element 2 to produce the Faraday effect.

[0014] Here, wavelength λ1=1310 nm, λ2=
1550 nm, α=9° (λ1+α=53.6°,
The isolation characteristics when set to λ2-α=36.4° are shown in Figure 2, and the insertion loss characteristics are shown in Table 2 below.
Shown below.

[0015]

[Table 2]

From this, wavelength λ1=1310 nm, λ2
= 1550 nm isolation is both 16.
The same characteristics can be obtained at 5 dB. In addition, the insertion loss can be lowered than in the past, and the characteristics can be improved.

By the way, in the optical isolator according to this embodiment, the selected wavelengths are λ1=1310 nm, λ2=
1550 nm and α=9°, but the two wavelengths are selected appropriately depending on the light source used, and low insertion loss and almost the same isolation can be obtained near each wavelength λ1 and λ2. Needless to say, high isolation can be similarly achieved with low insertion loss at wavelengths between λ1 and λ2.

[0018]

As explained above, according to the optical isolator of the present invention, almost the same characteristics due to low insertion loss and high isolation can be obtained at different wavelengths, and it can be used at a plurality of desired wavelengths. We can provide optical isolators suitable for

[Brief explanation of drawings]

FIG. 1 is a perspective view showing a schematic configuration of an optical isolator according to the present invention.

[Figure 2] In the same isolator, wavelength λ1 = 1310
Diagram showing the isolation characteristics when setting nm, λ2 = 1550 nm, α = 9°

[Figure 3] Diagram showing the relationship between Faraday rotation angle and wavelength

[Figure 4] A diagram showing the isolation characteristics when the isolation peak is aligned with the center wavelength in a conventional optical isolator.

[Explanation of symbols]

1 Polarizer 2 Faraday rotation element 3 Analyzer 4 Magnet

Claims (1)

[Claims] [Claim 1] A polarizer (
1), a Faraday rotation element (2) that rotates the vibration direction of the polarized light transmitted through the polarizer by approximately 45 degrees due to the Faraday effect, and a Faraday rotation element (2) in which the direction of transmission of the polarized light is tilted by 45 degrees in the Faraday rotation direction with respect to the polarizer. an analyzer (3) that is disposed so as to transmit only polarized light rotated by approximately 45 degrees from the Faraday rotation element; and a magnet (4) that applies a saturation magnetic field to the Faraday rotation element to produce a Faraday effect. In the optical isolator configured and used for incident light of two different wavelengths λ1 and λ2, the wavelength λ1
The Faraday rotation angle at θ1 = 45° + α, the Faraday rotation angle at wavelength λ2 is θ2 = 45° - α (θ1 > θ2), and the film thickness (L) of the Faraday rotation element is set so that θ1 + θ2 is approximately 90°. An optical isolator characterized in that the optical isolator is formed by adjusting.