JPH0827442B2 - Optical isolator - Google Patents

Description

The present invention relates to an optical isolator used for optical communication and optical information processing.

[Conventional technology]

The optical isolator is a device that uses the Faraday effect as one of the magneto-optical elements, and is used to block the reflected light returning from the end face of the optical fiber to the oscillator side and stabilize the oscillation of the laser light. It is what is done.

FIG. 3 shows the structure of a conventional optical isolator 1. A cylindrical permanent magnet 2 having a cylindrical hollow portion 3 is provided with a cylindrical magneto-optical element 4 which is shaped to fit inside the hollow portion 3. A polarizer 5 is attached to one surface of the magneto-optical element 4, and an analyzer 6 is attached to the other surface thereof so as to face each other.

In this optical isolator 1, a laser beam output from a laser diode 7 provided outside is made incident on a polarizer 5, and only a polarization component matching a polarization plane of the polarizer 5 is selectively passed. The magneto-optical element 4 receives this polarized component and rotates its plane of polarization by 45 degrees. This utilizes the Faraday effect due to the magnetic field generated by the permanent magnet 2.

Next, since the polarization plane of the analyzer 6 is tilted by 45 degrees with respect to the polarization plane of the polarizer 5, the laser light emitted from the magneto-optical element 4 passes through the analyzer 6 without being polarized. Then, it is incident on the optical fiber 8.

On the other hand, the incident end of the optical fiber 8 or the optical fiber 8
In the reflected light that is reflected from the output end and the like and returns to the optical isolator 1, only the polarization component that matches the polarization plane of the analyzer 6 is incident on the magneto-optical element 4, and the polarization plane is rotated by 45 degrees. Since the polarization plane after being rotated by 45 degrees is at an angle of 90 degrees with respect to the polarization plane of the polarizer 5, the reflected light passing through the magneto-optical element 4 in the opposite direction can pass through the polarizer 5. Disappear. That is, the light returned by reflection from the optical fiber 8 side cannot enter the laser diode.

As described above, the optical isolator 1 plays a role of blocking the reflected light returning from the optical fiber 8 side.
In actual use, the Faraday rotation angle changes due to the change in ambient temperature, so the reflected light returning from the optical fiber 8 does not become 90 degrees to the polarization plane of the polarizer 5 but several degrees. There will be an angular deviation of. For this reason, a part of the reflected light returned from the optical fiber 8 passes through the polarizer 5 and is re-injected into the laser diode 7, which causes a problem of changing the oscillation characteristics of the laser diode.

Therefore, as a method to prevent such deterioration of isolation characteristics, a conventional example in which the same optical isolator is connected in two stages (series) to achieve high isolation has been reported (1984, The Institute of Electronics and Communication Engineers, National Convention on Optical and Radio Waves, Proceedings 355, "Isolation characteristics improvement of isolator for single mode fiber" Fukushima et al.).

In the following description, continuous connection is used in the same meaning as serial connection.

[Problems to be solved by the invention]

However, in the conventional optical isolator in which two isolators having the same characteristics are cascaded, the isolation characteristics are improved to some extent as compared with the case of using a single isolator, but the Faraday rotation angle of the magneto-optical element is improved. The temperature dependence of is effectively doubled, so even if a cascade connection is used to achieve high isolation,
The drawback is that the required isolation cannot be ensured over a wide temperature range.

Therefore, an object of the present invention is to provide an optical isolator having a good temperature characteristic in view of the above drawbacks.

[Means for solving problems]

According to the present invention, the first polarizer and the first analyzer, and the first polarizer disposed between the first polarizer and the first analyzer and having a Faraday rotation angle of 45 degrees with respect to the used wavelength λ0. First optical isolator having a magneto-optical element, a second polarizer and a second analyzer, and a working wavelength λ0 disposed between the second polarizer and the second analyzer.
And a second optical isolator having a second magneto-optical element having a Faraday rotation angle of 45 degrees is arranged in series, and the polarization plane of the first analyzer and the polarization plane of the second polarizer are It is possible to obtain an optical isolator characterized in that are shifted from each other by a necessary angle according to the temperature. Further, according to the present invention, the first optical isolator and the second optical isolator each include a cylindrical permanent magnet having a cylindrical hollow portion, and the cylindrical permanent magnet is arranged in the axial direction of the cylindrical hollow portion. The first and second polarizers, the magneto-optical element, and the analyzer are arranged in sequence in the hollow portion, respectively, and each of the first and second polarizers, the magneto-optical element, and the analyzer is arranged along the hollow portion. It is preferable that the end surface is arranged so as to be perpendicular to the axial direction of the hollow portion.

〔Example〕

Next, an embodiment of the present invention will be described with reference to the drawings.

As shown in Fig. 1, the optical isolator consists of a polarizer 10,
Faraday rotation angle 4 at analyzer 11, permanent magnet 9, and wavelength λ ₀
A first optical isolator 19 composed of a magneto-optical element 12 having 5 degrees, a magneto-optical element 16 having a Faraday rotation angle of 45 degrees at a wavelength λ ₀ , a polarizer 14, an analyzer 15, and a permanent magnet 13. And a second optical isolator 20 composed of In this case, the first optical isolator 19
The plane of polarization of the analyzer 11 and the plane of polarization of the polarizer 14 of the second optical isolator 20 are arranged so as to be offset by several degrees. In this embodiment, the used wavelength λ ₀ is 1310 nm. Also, the polarizer 1
0,14, a lotion prism is used as the analyzer 11,15,
As the magneto-optical elements 12 and 16, liquid phase epitaxially grown bismuth-substituted potassium iron garnet [(GdBi) ₃ (Fe
AlGa) ₅ O ₁₂ ]. In this embodiment, the angle deviation between the polarization plane of the analyzer 11 of the first optical isolator 19 and the polarization plane of the polarizer 14 of the second optical isolator 20 is set to 7 degrees to secure the required isolation of 40 dB. , And the temperature characteristics were set to be the best.

Now, when the reflected light from the optical fiber 18 returns to the optical isolators arranged in cascade with the angles of the polarization planes of the first optical isolator 19 and the second optical isolator 20 thus shifted, Of these, only the component that matches the polarization plane of the analyzer 15 of the second optical isolator 20 passes through the analyzer 15,
Then, after the plane of polarization is rotated by 45 degrees by the magneto-optical element 16, the light passes through the polarizer 14 and the analyzer 11 of the first optical isolator 19
Is incident on. At this time, since the polarization plane of the analyzer 11 of the first optical isolator 19 is displaced by 7 degrees from the polarization plane of the polarizer 14 of the second optical isolator, the analyzer of the first optical isolator 19 is arranged. Only the component matching the polarization plane of 11 passes through the analyzer 11. Next, the polarization plane is rotated by 45 degrees by the magneto-optical element 12, and then reaches the polarizer 10. This time,
Since the polarization plane of the reflected light is at a position rotated by 90 degrees with respect to the polarization plane of the polarizer 10, the reflected light is blocked by the polarizer 10 and does not reach the laser diode 17.

Now, the effect of improving the temperature characteristics of the optical isolators arranged in cascade is as follows.

Generally, the isolation of an optical isolator is
It is given by the equation (1).

Isolation = -10iog (sin ² Δθ) [dB] (1) where Δθ is the deviation of the Faraday rotation angle from 45 degrees.

The temperature dependence of the Faraday rotation angle of the magneto-optical element is expressed by the equation (2).

Δθ = P · Δλ + q · ΔT (2) P: fluctuation amount of Faraday rotation angle per unit wavelength q: fluctuation amount of Faraday rotation angle per unit temperature Δλ: deviation from operating wavelength λ ₀ ΔT: room temperature ( In the optical isolator of this embodiment, the first optical isolator 19 and the second optical isolator 20, which rotate the polarization direction by 45 degrees at the used wavelength λ ₀ , are shifted in advance by 7 degrees and are cascaded. The isolation at this time is expressed as follows.

Isolation = −10log (sin ² Δθ ₁ ) −10log (sin ₂ Δθ ₂ ) (3) where Δθ ₁ = PΔλ + qΔT (4) Δθ ₂ = PΔλ + qΔT + Δθ ₃ (5) Δθ ₃ : first The angle deviation between the polarization direction of the analyzer of the optical isolator and the polarization direction of the polarizer of the second optical isolator.

Magneto-optical elements 12 and 16 ((DgBi) ₃ (FeAlGa) ₅ O in this example
Fig. 2 shows the effect of improving the temperature characteristics by cascading the optical isolators according to [ ₁₂ ].

In FIG. 2, a broken line a indicates a temperature characteristic of isolation when the optical isolator is used alone, and a solid line b indicates the first optical isolator 19 and the second optical isolator 20.
This is the temperature characteristic of the isolation when continuously arranged so that the polarization directions thereof coincide with each other. On the other hand, the alternate long and short dash line c represents the temperature characteristic of the isolation when the first optical isolator 19 and the second optical isolator 20 are arranged in cascade with the polarization directions of the first optical isolator 19 and the second optical isolator 20 shifted by 7 degrees from each other. Is.

The influence of the reflected light on the laser diode differs depending on the type of laser diode, the loss of the coupling circuit, etc., but for optical communication applications, isolation of 30 dB or more is usually required, and depending on the application, higher isolation is required. Although required, it was confirmed that in the present embodiment, a temperature improving effect of about 14% can be obtained at an isolation of about 40 dB as compared with the case of simply continuing (solid line b in FIG. 2). Especially high temperature (0 ℃ or more) which is usually a problem
It can be seen that in, the temperature characteristic improvement effect of about 70% can be obtained. The embodiment of the present invention is conceivable with some modifications other than the representative embodiment described above. For example, in this embodiment, the first optical isolator 19 having a polarizer and an analyzer 19 is used.
And the second optical isolator 20 is arranged in cascade. However, the second optical isolator 20 is arranged in the second optical isolator 20 except the analyzer 11 of the first optical isolator 19.
The first optical isolator in the polarizer 14 of the optical isolator 20
The configuration may be such that 19 analyzers are substituted. In this case, there is no functional difference from the present embodiment, but it is extremely advantageous in terms of reducing the thickness.

Although the Rochon prism is used as the polarizer and the analyzer in this embodiment, a material exhibiting birefringence such as calcite or rutile may be used in terms of cost reduction.

Further, in the case of an optical module in which the optical isolator is brought close to the radar zion and fiber-coupled with a lens, the output light from the laser diode is generally linearly polarized, and the first optical isolator 19 Polarizer 10
The configuration may be omitted. In this case, the reflected light is re-injected into the laser diode as linearly polarized light corresponding to the TM mode, but in the resonator of the laser diode, TE
Since it is occupied by mode light, it is not affected at all.

Further, the permanent magnets may be integrated as well as the case where the permanent magnets are arranged separately as in the present embodiment.

Further, in this embodiment, as a Faraday rotator (GdBi)
_Although it is limited to the case where ₃ (FeAlGa) ₅ O ₁₂ is used, another magnetic thin film or a structure using YIG may be used.

Further, in this embodiment, the wavelength λ ₀ used is 1310 nm, but the configuration of the optical isolator according to the present invention is not limited to the above wavelength, and 1310 nm ± 60 nm, 1550 nm ± 60 nm, or other wavelengths used. Needless to say, is also effective.

〔The invention's effect〕

As described above, according to the present invention, an optical isolator having good isolation temperature characteristics can be obtained.

[Brief description of drawings]

FIG. 1 is a conceptual diagram of a configuration example of an optical isolator according to the present invention, FIG. 2 is a correlation diagram between isolation and temperature characteristics of this embodiment and a conventional example, and FIG. 3 is a configuration of an optical isolator of a conventional example. is there. 1 ... Optical isolator, 2 ... Permanent magnet, 4 ... Magneto-optical element, 5 ... Polarizer, 6 ... Analyzer, 7 ... Laser diode, 8
...... Optical fiber, 9,13 ...... Permanent magnet, 10 ...... First polarizer, 11 ...... First analyzer, 12 ...... First magneto-optical element, 14
...... Second polarizer, 15 ・ ・ ・ Second analyzer, 16 ・ ・ ・ Second magneto-optical element, 17 ・ ・ ・ Laser diode, 18 ・ ・ ・ Optical fiber, 19 ・ ・ ・ First optical isolator, 20 ・ ・ ・… Second optical isolator.

Claims (2)

[Claims] 1. A first polarizer and a first analyzer, and a first polarizer arranged between the first polarizer and the first analyzer and having a Faraday rotation angle of 45 degrees with respect to a used wavelength λ0. A first optical isolator having a magneto-optical element, and a second optical isolator
And a second analyzer, and a second magneto-optical element that is disposed between the second polarizer and the second analyzer and has a Faraday rotation angle of 45 degrees with respect to the used wavelength λ0. Two optical isolators are arranged in series, and the plane of polarization of the first analyzer and the plane of polarization of the second polarizer are offset from each other by a necessary angle according to temperature. Optical isolator. 2. The optical isolator according to claim 1, wherein each of the first optical isolator and the second optical isolator has a cylindrical permanent magnet having a cylindrical hollow portion. In order along the axial direction of the cylindrical hollow portion,
The first and second polarizers, the magneto-optical element, and the analyzer are arranged in the hollow portion, and the respective end faces of the first and second polarizers, the magneto-optical element, and the analyzer are An optical isolator, which is arranged so as to be perpendicular to the axial direction of the hollow portion.