JPS63139318A - Ld module optical system - Google Patents

Abstract

PURPOSE:To suppress the influence of a reflected feedback light to the minimum by using a polarization plane holding fiber as an optical fiber, placing 1/4 wavelength plate between said fiber and an optical isolator, and setting a reflected light returned to a semiconductor laser LD to a deflecting surface being different from an output light of the LD. CONSTITUTION:An optical isolator 1 is placed in a lens system 3 of an LD module optical system, and between deflecting prisms 11, 12 having a birefringence property, in which crystal axes are inclined by 45 deg. to each other, a Faraday rotor 13 for turning an incident light by 45 deg. in a uniform magnetic field is interposed and installed. By such a lens system 3, an output light from a semiconductor laser LD2 is coupled onto an optical axis of a polarization plane holding fiber 4. Between this polarization plane holding fiber 4 and the isolator 1, a 1/4 wavelength plate 5 is placed. Also, the holding polarization plane of the polarization plane holding fiber 4 and the crystal axis of the wavelength plate 5 are positioned so as to attain parallel or vertical to the polarization plane of the output light of the LD2, and the influence of a reflected feedback light is minimized.

Description

[Detailed Description of the Invention] Overview In an LD module configured using a reflection-feedback light separation type optical isolator, a polarization maintaining fiber is used as an optical fiber serving as a transmission path, and a polarization maintaining fiber is used as a transmission path. In addition to arranging a quarter wavelength plate,
The maintained polarization plane of the polarization-maintaining fiber and the crystal axis of the quarter-wave plate are parallel or perpendicular to the polarization plane of the output light of the LD, and the reflected feedback light that returns to the LD without being separated by the optical isolator is The plane of polarization is made perpendicular to the plane of polarization of the output light of the LD. This reduces the influence of reflected feedback light on the LD.

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 in particular to an optical system for an LD module that is constructed using a reflective feedback light separation type optical isolator.
This relates to the D module optical system.

In recent years, with the increase in long-distance and large-capacity communication, single-mode optical fiber (hereinafter abbreviated as SMF), which has an essentially wide band, has been developed.
An optical communication system that uses the transmission line as a transmission path is in the practical stage. In this system, the LD provided in the LD module
(semiconductor laser) is driven with a time-series electric signal to form a time-series optical pulse, which is guided to the SMF. The optical coupling structure of the LD module is, for example, as shown in FIG.
6 is commonly 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.
DFB-LD with a wavelength selection function added to the JJ mirror section (
Distributed feedback type LD) etc. are currently under development.

However, when an LD module like the one shown in Figure 3 is constructed using an LD with excellent coherence such as a DFB-LD, the light emitted from the LD is emitted from the end face of the connected SMF or other optical system devices. There is a disadvantage that the operation of the LD becomes unstable and noise increases due to the reflected feedback light of the same wavelength that is reflected and returned by the LD. Therefore, an optical system using an optical isolator that allows optical signals to pass in only one direction is required.

BACKGROUND OF THE INVENTION Optical isolators used for the above-mentioned purposes are usually incorporated into LD modules due to the ease of system configuration. As shown in FIG. 2, an optical isolator 28 is inserted between lenses 22 and 24. In this case, the distance between the lenses 22 and 24 is 1 from the viewpoint of coupling efficiency.
The distance is preferably 0 m+ or less, and it is difficult to apply a large optical isolator constructed using a Glan-Thompson type or U-tion type polarizer. For this reason, a reflective feedback light separation type optical isolator, which is suitable for miniaturization and uses a birefringent polarizing prism, has been proposed.

FIG. 5 is for explaining the schematic structure and operation of an optical isolator 28 suitable for miniaturization.

In the figure, 30 is a first polarizing prism made of a birefringent center-axis crystal having a crystal axis (corresponding to the main axis, the same applies hereinafter) 30a in a direction perpendicular to the plane of the paper, and 32 is the crystal O & 30.
A second polarizing prism 34 is also made of a birefringent uniaxial crystal having a crystal axis 32a tilted 45° clockwise when viewed from the forward direction with respect to the polarizing prism 30.a.
It is inserted between 32 and a uniform magnetic field H is applied to transmit the transmitted light to 45.
Each shows a Faraday rotator rotating the optical axis.

Now, assuming that the polarizing prisms 30 and 32 are made of the same material, and if the refractive index for the ordinary ray is n and the refractive index for the extraordinary ray is n8, then the normal to the first polarizing prism 30 of the light from the light emitting source is n8. The light beam component, that is, the polarized light component perpendicular to the crystal axis 30a, is transmitted through the first polarizing prism 30.
After being refracted with a refractive index n at
A second polarizing prism 32 having a crystal axis 32a tilted by 5°
refractive index n as an ordinary ray for the second polarizing prism 32. It is refracted and emitted. Further, among the light from the light emitting source, the extraordinary ray component for the first polarizing prism 30, that is, the polarized light component parallel to the crystal axis 30a, is refracted at the first polarizing prism 30 with a refractive index n8, and then refracted at the Faraday rotator 34. The second polarizing prism 32 has a refractive index n as an extraordinary ray for the second polarizing prism 32. It is refracted and emitted. Light parallel to the incident light emitted from the second polarizing prism 32 is condensed by an appropriate lens and guided to an optical fiber. Note that when this optical isolator 28 is applied to an LD module that uses a DFB-LD as a light emitting source, the polarization plane (plane including the electric field vector) of the DFB-LD that emits linearly polarized light is set to the first polarizing prism 30. The light from the DFB-LD is guided to the optical fiber through a single optical path so as to increase the coupling efficiency.

On the other hand, the anti-tJ that is reflected back from the end face of the optical fiber, etc.
Among the four feedback lights, the ordinary ray component for the second polarizing prism 32, that is, the polarized component perpendicular to the crystal axis 32a, has a refractive index n in the second polarizing prism 32.

After being refracted by the Faraday rotator 34, it is rotated counterclockwise by 45° in the opposite direction (leftward from the cloth in the figure), and then refracted by the first polarizing prism 30 as an extraordinary ray toward the first polarizing prism 30. It is refracted at a rate n and emitted. Further, the extraordinary ray component of the reflected returning light to the second polarizing prism 32, that is, the polarized light component parallel to the crystal axis 32a has a refractive index n in the second polarizing prism 32.

After being refracted at , it is similarly rotated by 45 degrees counterclockwise at the Faraday rotator 34 , and is refracted at the first polarizing prism 30 as an ordinary ray to the first polarizing prism 30 with a refractive index n 0 and exits.

In this way, the forward light from the light emitting source to the optical fiber undergoes refraction at each polarizing prism 30.32 (a combination of n and n or a combination of n and n: at this time, the incident light are parallel), and the reflected feedback light reflected from the end face of the optical fiber, etc., is refracted by the polarizing prism 32, 30 (n).

and n. combination: In this case, the incident light and the outgoing light are not parallel), so if the optical coupling structure is designed to guide the light from the light source to the optical fiber,
The reflected feedback light is separated into two polarized light components and emitted from the first polarizing prism 30, and is prevented from returning to the light emitting source.

Problems to be Solved by the Invention However, in the optical isolator configured as described above, the crystal axis 30a of each polarizing prism 30,32.

It is practically difficult to set the angle formed by the angle 32a and the optical rotation angle of the Faraday rotator 34 to exactly 45°, and some deviation from 45° will occur.

Therefore, the second polarizing prism 3 in the anti-oiJ feedback light
After being rotated by the Faraday rotator 34, most of the ordinary ray components of No. 2 are separated from the incident light as extraordinary rays of the first polarizing prism 30. An ordinary ray component of the polarizing prism 3o is generated, and since this is parallel to the incident light, it returns to the LD. Further, the extraordinary ray component of the second polarizing prism 32 in the reflected feedback light is rotated by the Faraday rotator 34, and then most of it is separated from the incident light as an ordinary ray of the first polarizing prism 30. For the same reason, a slight abnormal ray component of the first polarizing prism 30 is generated, and since this is also parallel to the incident light, it returns to the LD. Such a small amount of reflected feedback light may cause deterioration of noise characteristics, especially when a highly coherent LD such as a DFB-LD is used as a light source.

The present invention was created in view of these points, and an object of the present invention is to provide an LD module optical system in which the influence of reflected feedback light is minimized.

11111 Artificial light prism 1 Among the small amount of reflected return light mentioned above, the 1st Q optical prism 3
0 ordinary ray (linearly polarized light parallel to the paper surface: hereinafter referred to as 1-E polarized light) component and the extraordinary ray (of the first polarizing prism 30).
Straight I perpendicular to the paper! When we investigated the influence of the polarized light (hereinafter referred to as TMt2 light) component on the LD, which is the light emitting source, it became clear that the influence of the TE polarized light component, which coincides with the polarization plane of the LD, is much greater. Taking note of this fact, it is only necessary to suppress the TE acid component of the light reflected back to the LD.

Therefore, the present invention provides an optical isolator in which a Faraday rotator that rotates incident light by 45 degrees in a uniform magnetic field is inserted between birefringent polarizing prisms whose crystal axes are tilted at 45 degrees, and an output of an LD. In an LD module optical system including a lens system that couples light to an optical fiber and arranged on the optical axis,
The optical fiber is a polarization-maintaining fiber, and a quarter-wave plate is arranged between the optical isolator and the polarization-maintaining fiber,
The present invention provides a patented Joule optical system characterized in that the polarization-maintaining plane of the polarization-maintaining fiber and the crystal axis of the quarter-wave plate are parallel or perpendicular to the polarization plane of the output light of the LD.

Note that although the terms "polarized light" and "polarized wave" refer to the same concept, they are used differently in this specification according to common usage.

The TE polarized light emitted from the working LD is divided into four parts by an optical isolator.
The light is rotated by 5 degrees and enters the quarter-wave plate as linearly polarized light whose TE polarized light component and TM polarized light component have the same imaging width and phase. The crystal axis (principal axis) of the quarter-wave plate is TE
Since it is set to be parallel to the polarization plane or the TM polarization plane, the linearly polarized light has its T-polarized light component and T-polarized light component.
One of the M polarized light components is emitted from the quarter-wave plate with a phase delay of π/2 relative to the other, that is, as circularly polarized light. Maintained polarization plane of polarization maintaining fiber (
The two planes (including the core central axis that are orthogonal to each other) are set to coincide with the TE polarization plane or the TM polarization plane, so the circularly polarized light is input to the polarization maintaining fiber with a practical coupling loss. .

The reflected feedback light in the opposite direction that travels in the forward direction inside the polarization-maintaining fiber and is reflected by the end face, etc., has its polarization state preserved, and when it passes through the quarter-wave plate again, Of the TE polarized light component and the TM polarized light component, the phase of the polarized light component whose phase is shifted by π/2M in the forward direction is further delayed by π/2.

Therefore, the phase of one polarized light component is delayed by π in total, and the reflected feedback light emitted from the quarter-wave plate enters the optical isolator as linearly polarized light perpendicular to the original linearly polarized light. Since this linearly polarized light is an extraordinary ray of the polarizing prism on the opposite direction entrance side of the optical isolator, it is rotated by 45 degrees counterclockwise toward the traveling direction by the Faraday rotator and becomes an ordinary ray of the polarizing prism on the reverse direction exit side. , that is, T
The light is separated from the optical axis and output from the optical isolator as E-polarized light.

At this time, the TM polarized light generated due to the deviation of the optical rotation angle, etc., returns to the LD without being separated from the optical axis.

In this way, all the TE polarized light components of the reflected feedback light are separated from the optical axis, and only the TM polarized light returns to the LD, so that the influence of the reflected feedback light on the LD is minimized.

Embodiments Hereinafter, preferred embodiments of the present invention will be explained in detail based on the drawings.

FIGS. 2(a) and 2(b) show a configuration in which the 1/4 wavelength plate has been removed from the LD module optical system of the present invention in order to facilitate understanding of the operation of the LD module optical system. It is something that

2 is an LD that outputs linearly polarized light, such as a DFB-LD;
The polarization plane is arranged so as to be parallel to the plane of paper (TE polarization plane). 3 is a lens system consisting of a first lens 3a and a second lens 3b arranged at confocal positions with respect to each other;
The output light of the LD 2 is coupled to the polarization maintaining fiber 4. Each lens 3a.

The lens 3b can be configured by combining a spherical lens, a Green rod lens (gradient index rod lens), or the like. The polarization-maintaining fiber 4 is a single mode type having a non-axisymmetric refractive index distribution or asymmetric residual stress distribution, and its main polarization-maintaining fiber 4 is a polarization-maintaining fiber 4 having a TE polarization plane clockwise in the forward direction. °It is made to match the rotated surface. In addition, the structure of the embodiment of the present invention, which will be described later,
In this case, the retained polarization plane is made to coincide with the TM polarization plane.

The optical isolator 1 is composed of a first polarizing prism 11 and a second polarizing prism 12 made of rutile, for example, and a Faraday rotator 13 inserted between these polarizing prisms 11 and 12. rays are collimated. The crystal axis 11a of the first polarizing prism 11 is set to be perpendicular to the paper plane (TM polarization plane), and the crystal axis 12a of the second polarizing prism 12 is set clockwise in the forward direction with respect to the crystal axis 11a. The direction is set so that the direction is tilted by 45 degrees. The Faraday rotator 13 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 13 in the direction of the arrow in the figure. And Faraday rotator 1
3 is set so that the polarization plane of the light passing through the Faraday rotator 13 is rotated (optically rotated) by 45° in the forward direction. The reason why the magnetic field applied to the Faraday rotator 13 is made sufficiently strong is to prevent the rotation angle from changing greatly due to changes in the magnetic field.

The progression of light in the forward direction will be explained using FIG. 2(a). The TE polarized light emitted from the LD 2 is first collimated by the lens 3a and enters the optical isolator 1. This TE polarized light is refracted as an ordinary ray by the first prism 11, rotated by 45° clockwise in the direction of travel by the Faraday rotator 13, and refracted by the second prism 12 as an ordinary ray, and then combined with the incident ray. Emitted in parallel. This light beam is linearly polarized light in which the amplitude and phase of the TE polarized light component and the TM polarized light component are equal to each other. This linearly polarized light is condensed by a lens 3b, and then connected to a polarization maintaining fiber 4.
is combined with In this case, since the plane of polarization maintained by the polarization-maintaining fiber 4 coincides with the plane of polarization of the linearly polarized light, coupling loss does not pose a practical problem. Within the polarization maintaining fiber 4, this polarization state is maintained up to the output end (not shown). Therefore, the reflected feedback light reflected from one end of the polarization-maintaining fiber 4 also becomes linearly polarized light (see Figure 2).
b)), this plane of polarization corresponds to the plane of polarization of forward linearly polarized light. This linearly polarized light is again collimated by the lens 3b, enters the optical isolator 1, and is refracted by the second polarizing prism 12 as an ordinary ray. Faraday rotator 1
3, the light is now rotated counterclockwise by 45° in the opposite direction and passes through the first polarizing prism 11 as an extraordinary ray. For this reason, the 1M beam emitted from the optical isolator 1
The polarized light is not parallel to the incident light and is not focused onto the LD 2 by the lens 3a. On the other hand, the angle of rotation is 4
The TE polarized component of the reflected feedback light caused by the deviation from 5° passes through the first polarizing prism 11 as its ordinary ray, so it becomes parallel to the incident light and returns to LD 2 via the lens 3a. Put it away.

In this way, by applying the polarization maintaining fiber 4 to the LD module optical system, all the TM62 light components of the reflected feedback light are separated, and the small amount of reflected feedback light returning to the LD 2 is all TE polarized light components. is clear. In other words, by applying the polarization maintaining fiber 4,
The small amount of reflected feedback light that returns to the LD 2 can be made into linearly polarized light, making it possible to reduce the intensity of the reflected feedback light compared to the prior art. However, the linearly polarized light returning to LD2 is TE polarized light, and its plane of polarization matches the plane of polarization of the output light of LD2. Taking these points into consideration, in the embodiment of the present invention, a 174-wave plate is used so that the plane of polarization of the reflected feedback light incident on the optical isolator 1 is perpendicular to the plane of polarization of the forward output light of the optical isolator 1. In this way, all the reflected feedback light returning to LD2 becomes 1M polarized light.

FIGS. 1(a) and 1(b) show an LD module optical system showing a preferred embodiment of the present invention, and FIG.
), In addition to the optical system of (b), a quarter-wave plate 5 is arranged between the lens 3b and the polarization-maintaining fiber 4, and the polarization plane maintained by the polarization-maintaining fiber 4 is changed to a TE polarization plane or a TM polarization plane. It will match.

The quarter-wave plate 5 is made of a birefringent uniaxial product, and the phase of the extraordinary ray of the quarter-wave plate 5 lags behind the phase of the ordinary light by a quarter wavelength, that is, π/2. As such, its length is set. The crystal axis of the quarter-wave plate 5 is set to be parallel to the TE polarization plane or the TM polarization plane (for convenience, it is referred to as the TM polarization plane).

In FIG. 1(a), the light that has passed through the optical isolator 1 is linearly polarized light obtained by rotating TE polarized light by 45° clockwise in the forward direction, as in FIG. The amplitude and phase of the component and the 1'M polarization component are equal to each other. Since the crystal axis of the quarter-wave plate 5 is set parallel to the TM polarization plane, the linearly polarized light has a TM polarization component delayed in phase by π/2 with respect to the TE polarization component, that is, , and is emitted from the quarter-wave plate 5 as circularly polarized light. This circularly polarized light can be considered to be a combination of two linearly polarized lights with equal amplitude and a phase shift of π/2, so
It can be coupled to the polarization maintaining fiber 4 without causing a large loss.

The reflected feedback light in the opposite direction that has traveled in the forward direction in the polarization maintaining fiber 4 and reflected at the end face, etc. has its polarization state preserved, and when passing through the 174-wave plate 5 again, it is reflected in the forward direction. The phase of the TM polarized light component whose phase is delayed by π/2 is further π/2i,! Neru. Therefore, the phase of the TM polarized light component is delayed by a total of π, and the reflected feedback light emitted from the 174-wave plate is treated as linearly polarized light having a vibration plane perpendicular to the original linearly polarized light. incident on .

Since this linearly polarized light is an extraordinary ray of the second polarizing prism 12, it is rotated by 45° counterclockwise toward the traveling direction by the Faraday rotator 13, and becomes an ordinary ray of the first polarizing prism 11, that is, TE polarized light. The light is separated from the optical axis and emitted from the optical isolator 1. For this reason, T.E.
The polarized light is collected by the lens 3a and does not return to the LDl. At this time, the TM polarized light caused by the deviation of the optical rotation angle is not separated from the optical axis and returns to D1, but as mentioned above, the TM@light is the output light of LDl (TE
Since the plane of vibration is different from that of polarized light, there is no risk of deterioration of noise characteristics, etc.

In this way, all the TE polarized light components of the reflected feedback light are separated from the optical axis, and only the TM polarized light returns to the LD, so that the influence of the reflected feedback light on the LD is minimized.

Effects of the Invention As detailed above, according to the present invention, the slight reflected light that returns to the LD despite the use of an optical isolator is converted into a straight line having a polarization plane different from the polarization plane of the output light of the LD. This has the effect that it is possible to provide an LD module optical system in which only polarized light can be used and the influence of reflected feedback light is minimized.

[Brief explanation of the drawing]

FIGS. 1(a) and 1(b) are block diagrams of an LD' module optical system showing a preferred embodiment of the present invention, and FIG. 2(a),
(b) is an explanatory diagram when the quarter wavelength plate is removed from FIGS. 1(a) and (b). FIG. 3 is a diagram showing an example of a conventional LD module optical system. 5 is a diagram showing an example of a conventional improved LD module optical system. FIG. 5 is a diagram for explaining the configuration and operation of an optical isolator suitable for miniaturization. 1.28... Optical isolator, 2.20... LD, 3... Lens system, 4...
Polarization maintaining fiber, 5... 1/4 wavelength plate, 11.30... First polarizing prism, 12.32... Second polarizing prism, 13.34... Faraday rotator. 20: LD 22, 24: L, > 26: SMF Conventional sword LD module round rice row ε showing 3 whistles I recently improved LD mozo construction C academic ball tase diagram Figure 4

Claims (1)

[Claims] A birefringent polarizing prism (with crystal axes tilted by 45 degrees)
An optical isolator (1) consisting of a Faraday rotator (13) that rotates incident light by 45° in a uniform magnetic field is inserted between 11) and (12), and an optical isolator (1) that connects the output light of the LD (2) to an optical fiber. Lens system to be combined (
3) arranged on the optical axis, the optical fiber is a polarization-maintaining fiber (4), and a 1/2
A four-wavelength plate (5) is arranged so that the polarization plane of the polarization-maintaining fiber (4) and the crystal axis of the quarter-wave plate (5) are parallel or perpendicular to the plane of polarization of the output light of the LD (2). An LD module optical system characterized by: