JP3264711B2 - Optical isolator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical isolator that does not depend on the polarization direction of light in optical fiber communication or the like.

[0002]

2. Description of the Related Art With the advance of optical communication using a semiconductor laser as a signal light source, high-speed and high-density signal transmission exceeding several hundred megahertz, which has been impossible until now, has been put to practical use. The remarkable progress has made it possible to transmit vast amounts of information through optical fibers without the need for photoelectric conversion, and the technological advances in optical isolators independent of the polarization direction inserted between the fibers and methods for introducing pumping light for optical amplification. There is a growing demand for cost reduction and economical reduction, and various proposals have been made and some of them have been put to practical use.

[0003] Fig. 2 shows that the light incidence and emission surfaces are parallel planes, so that the optical coupling is relatively easy, and that the advantages of the components which require only two types of components, a birefringent crystal plate and a Faraday rotator, are shared. FIG. 2 is an actual example of an optical isolator whose optical characteristics do not depend on the polarization direction of light (hereinafter, referred to as a polarization-independent optical isolator), and is a schematic diagram tracing a forward light transmission state. The structure is a structure using a Faraday rotator F and three birefringent crystal plates R1, R2, R3 (Japanese Patent Publication No. 60-51690).
Reference).

[0004]

However, in the configuration of FIG. 2, since the propagation paths of the ordinary light and the extraordinary light are different, a phase shift occurs between the two rays, which causes the polarization dispersion of the signal characteristic. The polarization dispersion characteristics induced when actually propagating through a polarization independent optical isolator used between fibers are:
In general, it is desirable to suppress the signal resolution to 0.5 ps (picoseconds) or less from the resolution of the signal. However, in the case of the configuration shown in FIG. 2, there is a difference in the light propagation speed between the ordinary light and the extraordinary light, and polarization dispersion always occurs. .

[0005]

In general, the phase shift based on the difference in the refractive index between the ordinary light and the extraordinary light of the birefringent crystal plate has a relationship represented by the following equation (1). ω the light of the angular velocity, t e _', t _o each extraordinary light, the propagation time of the ordinary

(Equation 1)

(Equation 2) And the phase delay τ is expressed by Expression 2 using the relationship of Expression 1. Wherein, n e _', n _o is the ordinary light, the refractive index of the extraordinary light, the relationship led the thickness of the crystal d, as the optical wavelength λ of light velocity c and use.

[0006] However n _{e 'is} intended to depend on the angle θ between the optical axis of the crystal and the beam direction, n _e when the refractive index of extraordinary light and n _e' is derived from the relationship of Equation 3.

(Equation 3) Therefore, the angle formed from the Formula 3 and cut the optical axis of the birefringent crystal plate becomes n _{e '=} n _e. From these relationships, the chromatic dispersion δ is given by Equation 4.

(Equation 4)

The above relationship will be described with reference to the configuration of FIG. 2. In FIG. 2, if the thickness of the second and third birefringent crystal plates is d, the thickness of the first birefringent crystal plate is √2d. When tracking the state of polarization propagation that occurs when the light propagates through the first to third birefringent crystal plates, first, the light beam incident on the first birefringent crystal plate R1 is separated into ordinary light and extraordinary light. Next, the second birefringent crystal plate R2
When R2 propagates to R1, only the extraordinary light moves because R2 is in a state where the direction of the optical axis is mirror-symmetric with respect to R1 and is rotated by 45 ° with the ray propagation axis as the rotation axis. Next, since the third birefringent crystal plate R3 has a mirror-symmetrical arrangement with respect to R2, R3
What is an ordinary ray until 2 becomes an extraordinary ray, and as a result, as for the phase delay of the ordinary ray and the extraordinary ray, the component caused by R1 is retained until the end.

When the fourth birefringent crystal plate R4 is inserted and compensated for the purpose of compensating for the phase delay of R1, ordinary light and extraordinary light cannot be separated again. It is necessary to select a crystal orientation that has no separation and has different ray propagation speeds of ordinary light and extraordinary light. Therefore, R4 satisfies all the requirements when the optical axis direction is perpendicular to the ray propagation axis, and has a crystal cut surface in which the ray speed surface of ordinary light and extraordinary light becomes an ellipsoidal cut surface as shown in FIG. It becomes a parallel plate.

However, birefringent crystals include a positive substance in which ordinary light propagates quickly and a negative crystal in which an extraordinary light propagates quickly, and therefore both can be used in the present invention. Various combinations are also conceivable for the position where the refraction crystal plate is arranged. These combinations are peripheral elements of the present invention and are not particularly mentioned, but FIG. 1 shows an example of them. However, the main point of the present invention lies in the orthogonality between the optical axis direction of the birefringent crystal plate used for polarization dispersion compensation and the propagation direction of the signal beam, and is not an invention relating to the sign of the birefringent crystal plate or the location of the birefringent crystal plate.

[0010]

FIG. 1 is a schematic view of the structure of a non-polarization optical isolator to which the present invention is applied. The basic configuration employs a rutile crystal as a birefringent crystal plate, and the cutout angle of the birefringent crystal plate is about 45 ° with respect to the optical axis of the rutile crystal. The rutile crystal is a positive uniaxial crystal, and the optical axis direction of the crystal is shown in FIG.
Place in the direction described inside. Each dimension is 3mm for R1
X 3mm, thickness 2mm, R2, R3 dimensions 3mm x 3m
m and a thickness of about 1.4 mm. The Faraday rotator F used was a Bi-substituted rare earth iron garnet grown on a GGG substrate by the LPE method having a size of 3 mm × 3 mm, a thickness of about 350 μm, and a used wavelength λ = 1550 nm.

Since the wavelength dispersion by the Faraday rotator is extremely weak, when estimating only the contribution in the birefringent crystal plate, the birefringent crystal plate R cut out at 45 ° to the ray axis is used.
1 of extraordinary refractive index n _{e 'is,} theta = 45 ° in equation 3, n _o and n _e of the refractive index at 1550nm respectively 2.453 and 2.70
Assuming that ne _′ = 2.572, the polarization dispersion δ when substituted into Equation 4 is δ = 0.79 × 10 ^−12, that is, about 0.8 ps. Actually, the polarization dispersion measurement by the AC synchronous detection method with the interference light intensity was 0.9 ps, which was almost the expected value.

[0012] Of course, from the relational expression of polarization dispersion, if the thickness of the birefringent crystal plate is reduced, the polarization dispersion can be suppressed to a lesser extent, but the beam separation width that controls the reverse insertion loss as an optical isolator is small. As a result, the extinction ratio is degraded, and is meaningless. Considering the configuration of FIG. 3, R4 cut out in the optical axis direction is an ordinary light component in R1 to compensate for the delay of the extraordinary light component in R1, and further delays the light beam propagated to R4 as an extraordinary ray. since polarization dispersion may be adjusted to the thickness d _P4 which can be extinguished, when estimated using a _{n e '= n e = 2.709} ,

(Equation 5)

That is, if R4 has a thickness of about 930 μm, polarization dispersion due to phase delay can be compensated. Actual thickness about 930
A birefringent crystal plate R4 cut out on a plane perpendicular to the optical axis of μm was manufactured, and an optical isolator having the configuration shown in FIG. 1 was assembled. The insertion loss of this optical isolator was 0.6 dB and the extinction ratio was 43.4 dB. Since the light coupling efficiency does not show any significant deterioration due to the insertion of R4, no re-separation of ordinary light and extraordinary light when transmitted through R4 is observed. When the polarization dispersion was measured in the same manner, it was found that δ = 0.05 ps, and it was found that the compensation contribution of the polarization dispersion of R4 showed an almost ideal effect.

[0014]

According to the present invention, the polarization dispersion in the polarization independent optical isolator having a structure which is relatively simple and suitable for industrial scale production can be almost eliminated, and a high speed high speed optical amplifier will be introduced in the future. It contributes to improving the reliability of a propagated signal in a high-density optical communication system, and can be easily manufactured without employing special components and a special assembly system of the polarization independent optical isolator of the present invention. A great contribution can also be expected to be able to reduce the economic burden for constructing the optical transmission system as a ripple effect.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a configuration of an optical isolator of the present invention.

FIG. 2 is a schematic diagram of a configuration of a conventional polarization-independent optical isolator.

FIG. 3 is an explanatory diagram showing a relationship between an optical axis and a cut surface of a birefringent crystal plate for polarization dispersion compensation according to the present invention.

[Explanation of symbols]

 R Birefringent crystal plate F Faraday rotator

Claims (2)

(57) [Claims] 1. A polarization independent optical isolator comprising a plurality of birefringent crystal plates having an optical axis inclined with respect to the light transmission direction, a Faraday rotator, and a permanent magnet for Faraday rotator magnetization. A birefringent crystal plate having an optical axis perpendicular to the optical axis and having a crystal thickness in the light transmission direction necessary to compensate for a phase delay between an ordinary ray and an extraordinary ray generated when the light passes through the optical component is arranged. A polarization independent optical isolator characterized in that: 2. The thickness ratio in the light transmission direction is √2: 1: 1.
Faraday rotator saturated with a permanent magnet between the first and second birefringent crystal plates of the first, second and third birefringent crystal plates having an optical axis inclined with respect to the light transmission direction. A crystal having an optical axis perpendicular to the light propagation direction and having a light transmission direction necessary for compensating for a phase delay between an ordinary ray and an extraordinary ray that occurs when a light ray passes through an optical component. A polarization independent optical isolator, wherein a birefringent crystal plate having a thickness is arranged.