JPS62257120A - Faraday rotator - Google Patents

Abstract

PURPOSE:To decrease absorption loss by setting the thickness of a Faraday rotator within a range where the total loss of light absorption loss and loss due to deviation in the angle of rotation of linear polarized light on a return way. CONSTITUTION:When the thickness of the Faraday rotator 4 is expressed by mum while the absorption coefficient of a magnetooptic material constituting the Faraday rotator 4 is denoted as A db/mum and an angle of Faraday rotation per mum is denoted as B deg./mum, the absorption loss to the quantity of light arriving at a detector D and the loss due to deviation in the angle of rotation of the light on the return way are so represented that the absorption loss of the Fara day rotator is 2AT (dB) and the loss due to the deviation from 90 deg. of the plane of polarization of light on the return way is -10log cos<2>(90-2Bt). In this case, the magnetooptic material whose thickness t(mum) is set to a minimum value -10log cos<2>(90-2Bt)+2AT is used. The angle of rotation is set to <=90 deg. and the thickness is set so that the total loss of the absorption loss and loss due to the deviation in angle of rotation is minimum. Consequently, the absorption loss is suppressed and a low-loss Faraday rotator for an optical head is obtained.

Description

Detailed Description of the Invention [Summary] In a Faraday rotator, which rotates the plane of polarization by reciprocating linearly polarized light in the Faraday rotator and separates the light on the return path from the light on the outgoing path, light absorption loss and By setting the thickness of the Faraday rotator in the region where the total loss due to rotational angle deviation of linearly polarized light on the return path is minimized,
Reduce absorption losses.

[Industrial application field]

An optical head that records/reproduces information on a magneto-optical disk or the like requires functions such as writing (recording), erasing, and reproducing. For this purpose, the surface of the magneto-optical disk is irradiated with laser light and the reflected light is ejected. At this time, attempts have been made to use a Faraday rotator to change the plane of polarization of the reciprocating light in order to separate the incident light and the reflected light so that only the reflected light can be detected. The present invention relates to a Faraday rotator that is used to change the plane of polarization of linearly polarized light in a round trip.

[Conventional technology]

FIG. 3 is a side view of an optical head using a conventional Faraday rotator. Semiconductor laser LD and magneto-optical disk 1
Polarizer PBS, 45゛ Faraday rotator 2
and an objective lens 3 are provided. Also polarizer PB
A light detector D is disposed at the reflected light detection position of S.

The linearly polarized light emitted from the semiconductor laser LD is passed through the polarizer PB.
By transmitting S as P-polarized light and passing through the Faraday rotator 2 in the magnetic field H, the plane of polarization is rotated by 45°, and the objective lens 3 illuminates the magneto-optical disk surface 1. After the reflected light is collimated by the objective lens 3, it is further rotated by 45 degrees by the Faraday rotator 2, and becomes linearly polarized light that is orthogonal to the incident light. Therefore, it enters the polarizer PBS as S-polarized light, is reflected, and is detected by the detector D.

[Problem that the invention seeks to solve]

A 45° Faraday rotator is used to convert P-polarized light into S-polarized light in this way, but current Faraday rotators have large losses due to light absorption at the wavelengths used in magneto-optical disks. . 4 to change the plane of polarization by 90°
A 5° Farade single rotator requires a thickness of about 100 μm, but even with this thickness, the amount of light is reduced by about half due to light absorption loss. Therefore, the light intensity of the detection light is low, making it impractical.

A technical object of the present invention is to solve such problems in conventional Faraday rotators and to realize a Faraday rotator with less light absorption loss.

Means for Solving Problem C] FIG. 1 is a diagram illustrating the basic principle of a Faraday rotator according to the present invention. The horizontal axis represents the thickness of the Faraday rotator, and the vertical axis represents the optical loss. The straight line L1 represents the light absorption loss due to the Faraday rotator, and the curve L2 represents the optical loss due to the rotational angle deviation of the polarization plane from 90°. In the present invention, both edges L
The thickness of the Faraday rotator is set within the region A where the total value of 1 and L2 is the smallest.

[Effect]

The rotation angle of the light passing through the Faraday rotator increases as the thickness of the Faraday rotator increases, but as the thickness increases, the loss due to light absorption increases as shown by +X L L. Therefore, to reduce absorption loss, the smaller the thickness, the better. However, as the thickness increases, the rotation angle also increases, the rotation angle deviation decreases, and the loss caused by the rotation angle deviation of the polarization plane on the return path is reduced. Therefore, both absorption loss and loss due to rotational deviation of the plane of polarization are minimum in region A.
By setting the thickness to , the rotational angle deviation loss increases, but the absorption loss is improved more than that, so the overall loss can be suppressed to the minimum.

〔Example〕

Next, examples will be used to explain how the Faraday rotator according to the present invention is actually implemented. The second section is a side view of an example using the Faraday rotator 4 according to the present invention. The Faraday rotator 2 in FIG. 3 has a rotation angle of 45 degrees, but the Faraday rotator 4 of this embodiment has a rotation angle of 45 degrees.
It is smaller than 5° and is 31.5°.

In this way, since the Farade-rotation angle is smaller than 45°, light leaks from the polarizer PBS in the direction of the laser LD, but the associated loss is cos ``θ (θ is 45
Since the loss is proportional to the angle of deviation from .degree., even if .theta. is 5.degree., the loss is as small as 0.13 dB. However, the absorption loss decreases by 11% when θ is 5°, so the overall loss becomes smaller.

Now, if the absorption coefficient of the magneto-optical material constituting the Faraday rotator is AdB/μm, the Faraday rotation angle per μm is B°/μm, and the thickness of the Faraday rotator is expressed as μm, then the amount of light reaching the detector D is On the other hand, the absorption loss and the loss due to the rotational angle deviation of the light on the return path are expressed as follows.

Absorption loss of Faraday rotator: 2At (dB) Loss knee due to deviation from 90° of polarization plane on return path 1101o
cos” (90-2Bt) As a specific representative value,
A: 0.1 dB/μm, B: 0.7°/μm. At this time, when the respective losses are calculated for each thickness, the results are as shown in Table 1.

Table 1 As is clear from this table, as the thickness of the Faraday rotator increases, the rotation angle increases and the absorption loss also increases. On the contrary, the loss due to the deviation in the rotation angle of the plane of polarization on the return trip is reduced. In order to obtain a 45° Farade single rotor as in the past, a thickness of 64 μm is required.
Thickness 4 with minimum total loss (5,50 dB) according to the present invention
If 5pm is adopted, 0.
Total loss is reduced by 9dB.

〔Effect of the invention〕

As described above, according to the present invention, in the case of a Faraday rotator that converts P-polarized light into S-polarized light, the rotation angle is smaller than 90°, and the total loss due to absorption and rotation angle deviation is minimized. By increasing the thickness, absorption loss can be suppressed, and a Fpraday rotator for an optical head with lower loss than before can be obtained.

[Brief explanation of drawings]

Fig. 1 is a diagram explaining the basic principle of the Faraday rotator according to the present invention, Fig. 2 is a side view showing an embodiment of the Faraday rotator according to the present invention, and Fig. 3 is a diagram illustrating an optical system using a conventional Faraday rotator. FIG. In the figure, 1 is the magneto-optical disk surface, LD is the semiconductor laser, D is the detector, PBS is the polarizer, 21 or 45° Faraday rotator, 3 is the objective lens, and 4 is a Faraday rotator thinner than the 45° Faraday rotator. are shown respectively. -4ri Figure 1: The Crimson Jōkakukai of Zakucho I Figure 2

Claims (2)

[Claims] (1) In a Faraday rotator, which rotates the plane of polarization by reciprocating linearly polarized light in the Faraday rotator and separates light on the return path from light on the outward path, the absorption coefficient of the magneto-optical material is AdB/μm, Faraday rotation coefficient is B°/μ
m, -10log cos^2 (90-2Bt
) A Faraday rotator characterized by using a magneto-optical material whose thickness (μm) is set to a value that minimizes +2At. (2) Claim (1) characterized in that the Faraday rotator is a part of an optical disk head and separates reflected light from the optical disk from incident light.
Faraday rotator as described in section.