JPS6327811A - Optical isolator - Google Patents

Abstract

PURPOSE:To obtain high coupling efficiency by setting the angle of incidence on an optical isolator so that a projection light beam does not become elliptic. CONSTITUTION:In the basic constitution of a laser diode (LD) module constituted by using the optical isolator, a light beam 38 which is emitted by an LD 32 and passed through a lens 34 to have an axially symmetric beam spread 36 is made incident on the 1st polarizing prism 20 of the optical isolator 18 at an angle theta0 of incidence to become a light beam 40 which has an optical axis parallel to the optical axis of the light beam 38 when projected. In this case, the angle of incidence is set to theta0 so that the ellipticity epsilon of the projection beam pattern is 0, so the beam pattern 39 of the light beam 40 projected from the 2nd polarizing prism 22 becomes axially symmetrical and can be guided efficiently to a single-mode optical fiber SMF 44 by a lens 42.

Description

[Detailed description of the invention] II! ! In optical isolators used in LD modules that serve as light sources in optical fiber transmission systems, we clarified that the ellipticalization of the emitted light beam spread depends on the angle of incidence on the polarizing prism that constitutes the optical isolator. By setting the incident angle so that this does not occur, that is, so that the beam spread of the emitted light is axially symmetrical, it is possible to improve the coupling efficiency of the Lr) module.

INDUSTRIAL APPLICATION FIELD The present invention relates to an optical system for an LD module used as a light source in a long-distance, large-capacity optical fiber transmission system, and more particularly to a feedback beam separation type optical isolator constructed using a birefringent polarizing prism.

2. Description of the Related Art In recent years, as communication distances have become larger and longer, optical communication systems using essentially broadband single-mode optical fibers (hereinafter sometimes referred to as SMF) as transmission paths have come into practical use.

In this system, an LD (laser diode) provided in an LD module is driven with a time-series electrical signal to form a time-series optical pulse, which is guided into the SMF. The optical coupling structure of the LD module is, for example, as shown in FIG. is usually used.

On the other hand, in order to eliminate the limitations on transmission distance and transmission capacity caused by the wavelength dispersion of optical fibers, there are great expectations for single-wavelength operation of LDs, and wavelength selection is required in the reflective mirror section, which is a component of LDs. DFB-LDs (distributed feedback LDs) with additional functions are currently under development. However, when the LD module shown in Figure 4 is configured using such an FB-LD, the light emitted from the DFB-IO is reflected by the end face of the connected SMF or other optical system devices. The reflected feedback light of the same wavelength returns to DFB.
-There is a disadvantage that the operation of the LD becomes unstable and noise increases, and an optical isolator that can pass the optical signal in only one direction is required.

BACKGROUND OF THE INVENTION It is desirable that the optical isolator used for the above-mentioned purpose be incorporated into the LD module for ease of system configuration. , as shown in Figure 5,
It is common to insert an optical isolator 18 between lenses 12 and 14.

FIG. 6 is for explaining the schematic structure and operation of the optical isolator 18, and in the figure, 20 is a birefringent device having a crystal axis (coinciding with the main axis, the same applies hereinafter) 20a in a direction perpendicular to the plane of the paper. a first polarizing prism made of a uniaxial crystal;
Reference numeral 2 denotes a second polarizing prism made of a birefringent axle crystal having a crystal axis 22a tilted at 45° clockwise when viewed from the forward direction with respect to the crystal axis 20a, and 24 a second polarizing prism formed between these polarizing prisms 20. A Faraday rotator is inserted and applied with a uniform magnetic field H to rotate transmitted light by 45 degrees.

Now, assuming that the polarizing prisms 20 and 22 are made of the same material, the refractive index for the ordinary ray is n, and the refractive index for the extraordinary ray is n. Then, among the light from the light source, the ordinary ray component for the first polarizing prism 20, that is, the polarized component perpendicular to the crystal axis 20a, is the first polarized light/rhythm 20.
refractive index n. After being refracted by the Faraday rotator 24, it is rotated by 45 degrees clockwise with respect to the traveling direction (from left to right in the figure), and further rotated by 45 degrees with respect to the crystal axis 20a.
A second polarizing prism 24 having a crystal axis 22a tilted by 5°
refractive index n as an ordinary ray for the second polarizing prism 24. It is refracted and emitted. Further, among the light from the light emitting source, the extraordinary ray component directed toward the first polarizing prism 20, that is, the polarized light component parallel to the crystal axis 20a is refracted at the first polarizing prism 20 with a refractive index n, and then refracted at the Faraday rotator 24. Similarly, the second polarizing prism 22 is rotated by 45 degrees and has a refractive index n as an extraordinary ray for the second polarizing prism 22. It is refracted and emitted. The light emitted from the second polarizing prism 22 is focused by a suitable lens and guided to the core of the optical fiber.

On the other hand, the ordinary ray component toward the second polarizing prism 22 of the reflected feedback light that has been reflected and returned from the end surface of the optical fiber, etc.
That is, the polarized light component perpendicular to the crystal axis 22a has a refractive index n in the second polarizing prism 22.

After being refracted by the Faraday rotator 24, it is rotated by 45° counterclockwise with respect to the traveling direction (leftward from the cloth in the figure), and then refracted by the first polarizing prism 20 as an extraordinary ray toward the first polarizing prism 20. Rate n. It is refracted and emitted. Further, among the reflected feedback light, an extraordinary ray component to the second polarizing prism 22, that is, a polarized light component parallel to the crystal axis 22a, has a refractive index n in the second polarizing prism 22.
.

After being refracted at , the Faraday rotator 24 rotates the same 45 degrees counterclockwise, and the first polarizing prism 20 has a refractive index n as an ordinary ray for the first polarizing prism 20 . It is refracted and emitted.

In this way, the forward light from the light emitting source to the optical fiber is refracted by each polarizing prism 20.22 (a combination of n and n or a combination of n and n) and the end face of the optical fiber. Since the refraction (combination of n and n) that the reflected return light receives at the polarizing prism 22.20 is different, the optical coupling structure is designed to guide the light from the light source to the optical fiber. In this case, the reflected feedback light is separated into two polarized light components and emitted from the first polarizing prism 20, and does not return to the light emitting source.

Problems to be Solved by the Invention By the way, in FIG. 7, a light beam (optical axis 2
6) is not actually a perfectly parallel ray, but gradually spreads in the direction of travel, and its beam pattern is approximately axially symmetrical circular, that is, approximately conical. It has a beam shape of This light beam is the first
When entering the polarizing prism 20, the beam spread in the direction perpendicular to the plane of the paper is incident at the same incident angle for the beam forming the outermost layer, but the beam spread in the direction parallel to the plane of the paper is the same. The light rays forming the outer layer are incident at different angles of incidence. Therefore, regarding the spread of the light beam emitted from the second polarizing prism 22, the original beam spread is almost preserved for the beam spread perpendicular to the plane of the paper, but the original beam spread for the beam spread parallel to the plane of the paper is preserved. The beam spread of the beam will be expanded or reduced. As a result, the axial symmetry of the emitted beam pattern is disrupted and it becomes approximately elliptical (hereinafter referred to as beam ellipticalization), making it difficult to condense the beam well into an optical fiber. Therefore,
When an LD module is configured using an optical isolator as described above, the coupling efficiency (ratio of the power of light entering the optical fiber to the total power of light emitted from the LD)
There is a problem that the amount decreases.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an optical isolator that prevents a decrease in coupling efficiency due to ellipticalization of an emitted beam.

Means for Solving the Problems FIG. 1 is for explaining the present invention in detail.
As shown in Figure (a), like the conventional product, the first polarizing prism 20, the second polarizing prism 22, and the polarizing prism 22 are similar to the conventional product. Incident light 26 (only the optical axis is shown) having an axially symmetrical (circular) spread 28 enters the optical isolator 18 consisting of the Radday rotator 24.
The beam is incident at the center of the beam at an incident angle θ. For the beam spread perpendicular to the plane of the paper, each polarizing prism 20
It is not affected by the wedge (wedge part) of 22° and is emitted while spreading out in the same way on the front and back sides of the page.

On the other hand, regarding the beam spread parallel to the plane of the paper, the incident angles of the rays above and below the optical axis 26 in the figure are different (the angle of incidence of the rays on the top is smaller than θ, and the angle of incidence of the rays on the bottom is smaller than θ). angle is greater than θ), each ray is refracted by the wedge of each prism 20122,
In the end, the beam spread parallel to the plane of the paper is emitted as the above-mentioned beam spread perpendicular to the plane of the paper in the form of y4. In other words, the beam spread perpendicular to the plane of the paper is not affected by the wedge of each polarizing prism 20, 22 and always moves within the optical isolator 18 with a constant spread, and the beam spread parallel to the plane of the paper is not affected by the wedge of the Under the influence, the light typically migrates within the optical isolator 18 with an extent different from the constant extent.

The inventors of the present application have focused on these facts, and have further found that the change in the beam spread parallel to the plane of paper at the time of emission depends on the incident angle θ. By setting the incident angle so as to be equivalent to the beam spread perpendicular to , the output beam is prevented from becoming elliptical.

This will be explained in more detail. Now, in FIG. 1(a), the radius of the output beam pattern 30 at a predetermined position in the direction horizontal to the plane of the paper is a, and the radius in the direction perpendicular to the plane of the paper is b.
When, ellipticity ε is defined as ε=(ab)/a. In this case, as mentioned above, b takes a constant value,
a will depend on θ. In this way, when the output beam pattern 30 has a substantially elliptical shape with its major axis in the direction horizontal to the plane of the paper, ε〉O, and when it has a substantially elliptical shape with its major axis in the direction perpendicular to the plane of the paper, it becomes ε〈0, making it axially symmetrical. Since ε-〇 occurs when the shape is circular, the above-mentioned objective can be achieved by setting 0 so that ε=O.

FIG. 1(b) shows how the ellipticity ε changes when the incident angle θ is changed. When the incident angle is smaller than 00, that is, the incident light hits the first polarizing prism 20 at a deep angle. When the beam is incident at the angle of incidence, the output beam forms a pattern extending parallel to the plane of the paper, and the angle of incidence is θ. It is clear that when the angle is larger, that is, when the incident light is incident on the first polarizing prism 20 at a shallow angle, the output beam has a pattern that is shrunk in the direction parallel to the plane of the paper.

Therefore, the incident angle is a predetermined angle θ determined by the refractive index of each member, the wedge angle, etc. By setting
The output beam pattern can be axially symmetrical.

Function: In the optical isolator of the present invention, the incident angle is set to 00' so that the ellipticity ε of the output beam pattern becomes O, so the output beam becomes an axially symmetrical approximately circular shape and is efficiently It becomes possible to focus the light onto an optical fiber.

That is, as shown in FIG. 2, in the basic configuration of an LD module configured using the optical isolator of the present invention,
The light beam 38 that is emitted from the LD 32, passes through the lens 34, and has an axially symmetrical beam spread 36 has an incident angle θ. The light beam 40 enters the first polarizing prism 20 of the optical isolator 18 and is output as a light beam 40 having an optical axis parallel to the optical axis of the original light beam 38. In this case, the ellipticity of the output beam pattern Set the angle of incidence to θ so that ε becomes O. Since the second polarizing prism 22
The beam pattern 39 of the light beam 40 emitted from the lens 42 is axially symmetrical, and the lens 42 facilitates efficient
It becomes possible to lead to 4.

Embodiments Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.

Referring to FIG. 3, the LD module shown in FIG. 2 is shown in a concrete form.

Reference numeral 46 denotes a DFB-LD that oscillates in a single mode, and in this embodiment, a DFB-LD with a wavelength of 1.31 μm is used.

48 is a spherical lens into which the light from the DFB-LD 46 is incident, and the refractive index n of the medium forming this lens is 1.8, and the radius R1 is 0.4 mm.

Reference numeral 50 denotes an optical isolator, which is composed of a first polarizing prism 52 made of rutile, a second polarizing prism 54, and a Faraday rotator 56 interposed between these polarizing prisms 52 and 54. Crystal axis 5 of first polarizing prism 52
2a is normally oriented perpendicular to the plane of polarization where most of the light emission power of the DFB-LD 46 is concentrated (in this embodiment, the direction perpendicular to the plane of the paper), and the crystal axis 54a of the second polarizing prism 54 is , is set to be tilted clockwise by 45° when viewed from the forward direction with respect to the crystal axis 52a.

The Faraday rotator 56 can be made of YIG, for example, and a sufficiently strong permanent magnetic field H1 of, for example, 2300 Gauss or more is applied to the Faraday rotator 56 in the direction of the arrow. And the appearance of the Faraday rotator 56 is
The plane of polarization of the light passing through the Faraday rotator 56 is set to rotate (optically rotate) by 45'. The purpose of applying a sufficiently strong magnetic field to the Faraday rotator 56 is to prevent the rotation angle from changing significantly due to changes in the magnetic field.

58 and 60 are Green rod lenses (gradient index rod lenses) arranged in parallel on the optical axis of the beam in order to condense the beam emitted from the optical isolator 50 onto the SMF 62; The radius R8 of the Greenrod lens 58.60 is 0.9 m, and the center refractive index is n. is 1.592, and the refractive index distribution coefficient i is 0.324. Further, the pitch P58 of the Grinrod lens 58 is 0.04 to 0.06, and the pitch P6o of the Grinrod lens 60 is 0.16 to 0.06.
Each length is set to be 20.

SMF62 is generally widely used, and its core diameter and cladding diameter are 10 μm and 1125 μm, respectively.

In the LD module configured as described above, the incident angle θ to the optical isolator 50. When actually used with the angle set at 40 degrees, the coupling efficiency was approximately 30% to 40%, the intrusion loss of the optical isolator 50 was less than 0.5 dB in the forward direction, and more than 30 dB in the reverse direction, and the separation angle of the reflected feedback light was Good results were obtained, with the angle being 10° or more.

Effects of the Invention As detailed above, according to the present invention, since the incident angle to the optical isolator is set so that the emitted light beam does not become ovalized, an LD module with high coupling efficiency can be realized. This has the effect of making it possible.

[Brief explanation of the drawing]

FIGS. 1(a) and 1(b) are diagrams explaining the principle of the present invention;
The figure shows Lr constructed using the optical isolator of the present invention.
) Basic configuration diagram of the module; FIG. 3 is a schematic configuration diagram of an LD module showing an embodiment of the present invention; FIG. 4 is a diagram showing an example of the optical configuration of a conventional LD module; FIG. 5 is a diagram showing an example of the optical configuration of a conventional LD module; A diagram showing an example of the optical configuration of a conventional improved LD module, FIG. 6 is a diagram for explaining the configuration and operation of a conventional optical isolator, and FIG. 7 shows the problems of the conventional optical isolator. It is a figure for explaining. 10...LDl 12.14...Lens, 16.
44.62...SMF (single mode fiber), 18.50
... Optical isolator, 20.52... First polarizing prism, 22.54... Second polarizing prism, 24.56... Faraday rotator, 48... Spherical lens, 58.60...・Grinrod lens. Figure 1 Detailed explanation of the present invention 18 32: LD 44: SMF Basic configuration diagram of LD module 46: LD 50: Optical eye router 62: SMF Fig. 3 shows an example of the present invention Fig. 10: LD 12, +4° lens 16: SMF Fig. 7 shows an example of a conventional LD module Fig. 4 shows an example of a conventional improved LD module Fig. 5

Claims (1)

[Scope of Claims] A birefringent first polarizing prism (20) into which the light from the light emitting element is incident, a Faraday rotator (24) which rotates the incident light by 45 degrees, and the first polarizing prism (20). ) and birefringent second polarizing prisms (22) whose crystal axes are tilted at 45 degrees to each other are sequentially arranged on the optical axis. An optical isolator characterized in that an incident angle to a first polarizing prism (20) is set so that the polarizing prism (20) is axially symmetrical.