JP2832842B2 - Optical isolator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical isolator for preventing noise caused by return light in a single mode optical oscillator such as a semiconductor laser often used for a compact disk or a laser disk. is there.

[Conventional technology]

In optical communication, return light returning to an oscillating semiconductor laser causes mode hopping of the semiconductor laser and causes noise that makes laser oscillation light unstable. If the oscillation light of the semiconductor laser becomes unstable as described above, it becomes difficult to transmit an optical signal accurately. Therefore, in order to prevent noise generated by such return light, it is important to block the return light. One of the methods for blocking the return light is an optical isolator. The feature of the optical isolator is that the polarization of the emitted light is rotated by 45 degrees using a Faraday rotation element. The polarization of the returned light is further rotated by 45 degrees, and the polarization of the returned light is made orthogonal to the polarization of the first emission. This is a simple principle.

The shape of such an optical isolator is roughly divided into two.
Classified into types. One is to interpose a Faraday rotator with a polarizing crystal, make the polarization of the return light orthogonal to the polarization axis of the polarizing crystal on the exit light side, and block the light to the semiconductor laser with the polarizing crystal. Using only a Faraday rotator without using a polarizing crystal and simply making the polarization of the return light orthogonal to the polarization of the emitted light, the return light is applied to the semiconductor laser chip (oscillation in principle). Return light of a polarization orthogonal to the polarization cannot cause noise.)
The former is often used for optical communication, and the latter is often used for pickup of a laser disk or the like.

At present, a garnet thick film such as (GdBi) ₃ (FeAlGa) ₅ O ₁₂ is widely used as a Faraday rotator for an optical isolator, but the garnet thick film is sensitive to a change in temperature of a use environment. In addition, there is a disadvantage that the Faraday rotation angle which is the main characteristic is changed. Another disadvantage is that the Faraday rotation angle fluctuates even when the wavelength used changes. If the Faraday rotation angle deviates from the set value, isolation (reverse loss), which is a main characteristic of the optical isolator, is deteriorated, and the ability to block return light is greatly reduced.
Although the temperature characteristic of the Faraday rotation angle varies depending on the material, ΔG _F / ΔT (deg / ° C.) (provided that the Faraday rotation angle θ _{F is} 45 deg and the setting wavelength λ 0.78 μm) is (GdBi) ₃
(FeAlGa) ₅ O ₁₂ at −0.10 deg / ° C. The wavelength characteristic of the Faraday rotation angle is (GdBi) ₃ when the working wavelength is 0.78 μm.
(FeAlGa) ₅ O ₁₂ and Δθ _F / θ _F = 0.5 (% / 10 ⁻³ μm). From this, the Faraday rotation angle of 45 deg (GdBi) measured at room temperature in the operating temperature range of 0 to 60 ° C.
The garnet film thickness of ₃ (FeAlGa) ₅ O ₁₂ has a Faraday rotation angle fluctuation of about ± 3.0 deg. This is (GdBi) ₃ (FeAl
Considering the Faraday rotation performance of Ga) ₅ O ₁₂ −6500 deg / cm, it can be seen that this corresponds to a thickness variation of ± 4.6 μm. The fluctuation range of the set wavelength of the semiconductor laser is ± 0.78 μm at the set wavelength.
There is about 10nm. The fluctuation of the Faraday rotation angle due to this wavelength fluctuation width is ± 2.25 deg, and (GdBi) ₃ (FeAlGa) ₅ O
Given the Faraday rotation performance -6500deg / cm of _12, ± 3.
This corresponds to a thickness variation of 46 μm.

Considering these facts, the thickness of the garnet thick film must be considered to vary by ± 4.0 μm depending on the usage. However, in the conventional method, the setting use wavelength 0.
Since the thickness of the garnet thick film is determined at 78 μm and the use environment temperature is 25 ° C., the Faraday rotation angle is slightly changed depending on the use environment of the semiconductor laser incorporating the Faraday rotation element. Further, when the film thickness is individually controlled in accordance with the wavelength of the semiconductor laser to be incorporated and the use environment, there is a disadvantage that productivity is extremely deteriorated and cost is increased.

[Problems to be solved by the invention]

In consideration of productivity and cost, it is desirable to adopt a method that enables a garnet thick film to be produced with uniform specifications, processed into a Faraday rotating element, and corrected when incorporated as a semiconductor laser. In this case, it is more important to stabilize the performance when incorporated than the maximum performance. The present invention
In view of the above, it is an object of the present invention to provide an optical isolator that can obtain a constant Faraday rotation angle by correcting the incident position of a laser beam with respect to a change in a use environment or a semiconductor laser.

B. Means for Solving the Problems According to the present invention, a Faraday rotator and a magnet for magnetically saturating the Faraday rotator are incorporated in an encapsulation lid for enclosing a semiconductor laser, and the polarization of oscillation light of the semiconductor laser is increased. An optical isolator for a semiconductor laser that prevents the generation of noise of the semiconductor laser by making the polarization of the reflected return light orthogonal to
The thickness of the Faraday rotator element is set such that the thickness of the central portion becomes a Faraday rotation angle of 45 °, and the thickness decreases toward one end and increases toward the other end. An optical isolator for a semiconductor laser is provided, which has one end surface inclined at a constant angle and is attached to the sealing lid with the inclined one end surface facing the inside of the sealing lid.

[Action]

The present invention relates to an optical isolator in which a garnet thick film, which is a Faraday rotator, is attached as a window material of a sealing lid (hereinafter, referred to as an LD cap) of a semiconductor laser and magnetically saturates the garnet thick film. The Faraday rotation element to be used is attached to the LD cap with a constant inclination such that the Faraday rotation angle is 45 deg at the center. This inclination also has a role of reducing near-end reflection of the emitted light. When incorporating the LD cap thus formed into a semiconductor laser, the Faraday rotation angle that fluctuates according to subtle changes in the set value of the wavelength, the environment, etc., is changed, and the position of the laser light incident on the Faraday rotation element is shifted by shifting the position of the LD cap. Fine adjustment by correction. Since the laser beam that oscillates and passes through the window material of the LD cap has a constant beam diameter, the laser area irradiated on the window material may take into account the slight inclination of the Faraday rotator attached as the window material. If the inclination is constant, it is almost constant with respect to the correction of the laser beam incident position. Therefore, even if the set wavelength or use environment fluctuates, the LD cap is shifted and adjusted, and the Faraday rotation angle within a certain area of the Faraday rotation element (for example, the center point of the laser area) under that environment is corrected to be constant. Then, since the inclination angle is constant, the extinction ratio of each of the optical isolators thus corrected as a whole is constant.

Therefore, if the present invention is used, it is not necessary to individually preset the Faraday rotation angle of the Faraday rotation element accurately in response to a minute setting wavelength or a change in the use environment. Thereby, the productivity and stability of characteristics of the optical isolator are greatly improved.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
As an example of the present invention, the garnet film thickness used in the production test of the optical isolator was for 0.78 μm wavelength, and the insertion loss was
0.3dB (value at center), Faraday rotation angle 43.8 to 46.
(TbBi) ₃ Fe ₅ O ₁₂ with 2 deg (however, 45 deg at the center position) and an extinction ratio of 40 dB (value at the center) (GdBi) ₃ (FeAlG
a) A garnet thick film with improved temperature characteristics over ₅ O ₁₂ . FIG. 1 is a front view of an optical isolator showing one embodiment of the present invention. Using the above-mentioned garnet thick film, on one surface, the same garnet substrate 1 used for growing the garnet thick film was processed to a predetermined size while being attached without being removed, and a certain amount was fixed on the other surface. The Faraday rotator 2 with a slope and an anti-air reflection coating on both sides is LD.
LD cap body 3 with inclined surface as window material of cap body 3
And an optical isolator was fabricated.

FIG. 2 is a front view of a semiconductor laser using the optical isolator shown in FIG. A Faraday rotator 2 of an optical isolator manufactured as described above is mounted on a pedestal 6 provided with a semiconductor laser chip 4 and an electrode 5 connected to the semiconductor laser chip 4 in a predetermined manner.
Of the LD cap body 3 and the edge of the pedestal 6 were joined to each other, and an optical isolator was incorporated to produce a semiconductor laser. At the time of this assembling, when the polarized light having the same angle as the emitted laser light passing through the Faraday rotator 2 returns as the return light and passes through the Faraday rotator 2 again, the polarization of the laser light is reflected by the semiconductor laser chip 4. The optical isolator was fine-tuned by rotating and moving it so that it was orthogonal to the polarization.

The semiconductor laser device thus produced is forcibly injecting the reflected light having the oscillation light polarization and the polarization of 45 degrees (the same angle as the polarization when passing through the Faraday rotator 2) into the semiconductor laser. However, no mode hopping was observed at 1% or less with respect to the emitted light, and a relative noise figure (RIN) of -140 dB / Hz or less was stably obtained. Thereby, it was confirmed that the optical isolator of the present invention was functioning efficiently in the device. Therefore, by using this optical isolator for a semiconductor laser, it is not necessary to accurately set in advance the Faraday rotation angle of each Faraday rotator according to the set wavelength of the semiconductor laser and the minute fluctuation due to the use environment. Fine adjustment of the position may be performed when the optical isolator is incorporated, and the productivity and the stability of characteristics of the optical isolator itself can be greatly improved, and the yield of the semiconductor laser can be greatly improved.

C. Effects of the Invention [Effects of the Invention] As described above, according to the present invention, the productivity and stability of characteristics of the optical isolator itself are significantly improved,
It is possible to provide an optical isolator that greatly improves the yield of semiconductor lasers incorporating the optical isolator.

[Brief description of the drawings]

FIG. 1 is a front view of an optical isolator showing one embodiment of the present invention. FIG. 2 is a front view of a semiconductor laser using the optical isolator shown in FIG. DESCRIPTION OF SYMBOLS 1 ... board | substrate, 2 ... Faraday rotation element, 3 ... LD cap main body, 4 ... semiconductor laser chip, 5 ... electrode, 6
……pedestal.

Claims (1)

(57) [Claims] An Faraday rotator and a magnet for magnetically saturating the Faraday rotator are incorporated in an encapsulation lid for enclosing the semiconductor laser, and the polarization of the reflected return light is made orthogonal to the polarization of the oscillation light of the semiconductor laser. In a semiconductor laser optical isolator that prevents the generation of noise, the Faraday rotation element adjusts the thickness of the central part by a Faraday rotation angle of 45 degrees.
厚, and has one end surface that is constantly inclined so that the thickness is thinner toward one end and the thickness is increased toward the other end, and the inclined one end is An optical isolator for a semiconductor laser, wherein the optical isolator is attached to the sealing lid toward the inside of the sealing lid.