JPH0199018A - Semiconductor laser module with optical isolator - Google Patents

Abstract

PURPOSE:To prevent the fluctuation of an optical output of a semiconductor laser by constituting the title module so that among emitted light beams which return in reverse to the semiconductor laser from an optical fiber, the light of a polarized light component being parallel to a place of polarization is condensed to other part than the active region of the semiconductor laser. CONSTITUTION:When a return light 9 from an optical fiber 7 is made incident on a double refraction crystal plate 4, it is divided into an ordinary ray 9a of a polarized light component being perpendicular to the surface containing an optical axis and the normal of the incident surface, and the abnormal ray 9b of a polarized light component being parallel to said surface. Subsequently, the ray 9a is propagated linearly as it is, but the ray 9b is propagated diagonally through a crystal plate 4, shifted to the (x) axis negative direction and the (y) axis positive direction from an origin n the (x)-(y) plane and emitted. Next, the ray 9a and 9b are rotated by 45 deg. by non- reciprocity of a Faraday rotor 6. Thereafter, by an action of a lens 3, the ray 9b is made incident on the inside of the end face of a semiconductor laser 1, and also, other part than the active region 2 of the laser 1. In such a way, the fluctuation of an optical output of the semiconductor laser is reduced, an optical output characteristic and an oscillation characteristic are stabilized, and also, the increase of a noise caused by the return light can be suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention is used for optical signal transmission such as optical communication. Semiconductor laser module with optical isolator.

This is related to optical output stabilization.

[Conventional technology]

2. Description of the Related Art In optical signal transmission systems such as optical communications, a portion of light emitted from a light source may be reflected by a transmission path or each connection of transmission optical components and returned to the light source. When a semiconductor laser is used as a light source, this returned light is

This caused the oscillation characteristics of the semiconductor laser to become unstable and noise to increase. In order to prevent deterioration of the characteristics of a semiconductor laser due to this reflected light, an optical isolator is generally used. Of the light returned from the optical fiber, the polarization component perpendicular to the polarization direction of the light emitted from the semiconductor laser is determined by Sugie et al.
As reported in 330J, noise characteristics are slightly degraded.

FIG. 6 was shown in "Utility Model Application Publication No. 7, 1987-495". It is a block diagram of a conventional semiconductor laser module with an optical isolator, (1) is a semiconductor laser, C21 is an active region of the semiconductor laser (1), (3a), (3b)
is a lens, (4a) * (4b) is a birefringent crystal plate,
(5) is an equivalent permanent magnet, C6) is a Faraday rotator to which a magnetic field is applied by permanent magnet (5), (7) is an optical fiber, and (81 detects the optical output of semiconductor laser (1)) The light receiving element (9) for this purpose is the return light from the optical fiber.

Further, FIGS. 7, 8, and 9 are explanatory diagrams of the operation of the conventional example. Note that the coordinate axis X+ in each drawing of this invention
Y and z are determined as quadratic. The X axis is.

Parallel to the polarization direction of the TE mode of the semiconductor laser, the y-axis is
The y-axis is perpendicular to the junction surface of the semiconductor laser, the direction from the substrate of the semiconductor laser to the active region is the fixed direction of the y-axis, and the Z-axis is parallel to the optical axis of the lens. In FIG.

(41 is a birefringent crystal plate 2, cut out from a uniaxial crystal such as rutile or calcite so that its optical axis is inclined to the surface,
It is a parallel plate polished, and αa is a birefringent crystal plate (
41, the optical axis al) is the ordinary ray, and aZ is the extraordinary ray.

As shown in Figure 7, when a single ray is made perpendicularly incident on a birefringent crystal plate (41), an ordinary ray +I1 whose polarization plane is perpendicular to the plane containing the optical axis and the normal to the plane of incidence. and.

2 of the above plane and the extraordinary ray aZ with a parallel polarization plane
Book splits into rays. Ordinary rays 1) 1) propagate within the birefringent crystal plate (4) as an isotropic medium, so they propagate without being refracted at its surface, but extraordinary rays 12 do not follow Snell's law. , propagates inside the birefringent crystal plate (41) obliquely to the incident light beam.However, the birefringent crystal plate (
4), the ordinary ray +Ill and the extraordinary ray a'a propagate as two parallel rays. In addition.

As shown in (al and (1)l) in Figure 7, the birefringent crystal plate (4
10 If the direction of the optical axis is different, this birefringent crystal plate (4)
The direction of the extraordinary ray a2 propagating inside is also different. The optical axis of the birefringent crystal plate (4a) is indicated by the white arrow (LOI) in FIG.
As shown in FIG.
2 is oriented so as to be emitted in the y-axis angular direction of the ordinary ray 01). Further, the optical axis of the birefringent crystal plate (4b) is the eighth
As shown by the white arrow αa in the figure (bl), there is a plane that includes the optical axis and the normal to the plane of incidence of the semiconductor laser light.

From the y-z plane, the polarization plane of the semiconductor laser beam is rotated by the Faraday rotator in the same direction.
parallel to the plane rotated by 5 degrees, and

The extraordinary ray 0z of light propagating in the forward direction is the ordinary ray +Ill
It is oriented to emit light in the direction of the y-axis. Figure 9 (a
In FIG. 6, t is the distance E between the semiconductor laser (1) and the birefringent crystal plate (4a) of the semiconductor laser light propagating in the forward direction.
, birefringent crystal plate (4a) and Faraday rotator (61 F
, G between the Faraday rotator (6) and the birefringent crystal plate (4b)
.

At each point H between the birefringent crystal plate (4b) and the original fiber (71), its position and polarization direction are shown.

In addition, FIG. 9 (bl is in FIG. 6. In the opposite direction, that is,
E, F, G of the return light from the optical fiber propagating in the direction from the optical fiber to the semiconductor laser.

H shows one position at each point and its polarization direction. Depending on the polarization characteristics of the semiconductor laser, the semiconductor laser light is as shown in Fig. 9 (
As shown in alE, the light is mostly polarized in a direction parallel to the X axis. This light polarized in the X-axis direction is such that the optical axis a1 of the birefringent crystal plate (4a) is
Since it is included in the -z plane, it becomes an ordinary ray, and as shown in Fig. 9 (
propagates linearly like al F.

The light is incident on the Faraday rotator (6). This Faraday rotator (61) rotates the plane of polarization by 45° as shown in FIG. 9(a)G.

The birefringent crystal plate (4b) is also rotated by 45° in the same direction as the direction of rotation of the plane of polarization by the Faraday rotator (6), as shown in FIG. 8fbl, so as shown in FIG. 9falH. propagates through the birefringent crystal plate (4b) as an ordinary ray,
Coupled to optical fiber (7). By the way, optical fiber (
The return light from 71 does not have polarization characteristics as shown in FIG. 9 fblH. When the return beam from this optical fiber (7) enters the birefringent crystal plate (4b), an ordinary ray (9a) with a polarization component perpendicular to the plane containing the optical axis and the normal to the plane of incidence ( !: *Divided into an extraordinary ray (9b) with a polarization component parallel to the above plane.

Therefore, the ordinary ray (9c) propagates straight as it is, but the extraordinary ray (9b) propagates obliquely within the birefringent crystal plate (4b), and as shown in FIG. 9(b)G, x - Emit light to a position shifted from the origin in the X-axis stopping direction and the y-axis angular direction within the y-plane. Next, the planes of polarization of the ordinary ray (9a) and the extraordinary ray (9b) are rotated by 45° in the same direction as the direction in which the plane of polarization of the forward semiconductor laser beam is rotated due to the non-reciprocity of the Faraday rotator +61. . Note that the extraordinary ray (9b) is the ninth
As shown in figure (bl F), under the action of lens (3a) and lens (3b) *
The birefringent crystal plate (4a
). Therefore, the ordinary ray (9a) and the extraordinary ray (9b) at the birefringent crystal plate (4b) are as follows.

The plane of polarization is rotated by the Faraday rotator (6).

Conversely, in the birefringent crystal plate (4a), the ordinary ray (9a) becomes the extraordinary ray, and the extraordinary ray (9b) becomes the ordinary ray. The ordinary ray (9a) on the birefringent crystal plate (4b) becomes an extraordinary ray on the birefringent crystal plate (4a).
4a) propagates obliquely, as shown in Figure 9 (bl E).

The light is emitted at a position shifted from the origin in the y-axis stop direction within the x-y plane. In addition, the extraordinary ray (9b) at the birefringent crystal plate (4b)
) becomes an ordinary ray in the birefringent crystal plate (4a), so
It propagates in a straight line. As shown in Fig. 9), all of the return light from the binary fiber returns with a shift in the positive direction of the y-axis.

In the semiconductor laser module with an optical isolator shown in FIG. 6, the optical axis is as shown in FIG.
When the birefringent crystal plates (4a) and (4b) are arranged, the return light (9) from the optical fiber does not return to the active region (21) of the semiconductor laser (1), as shown by the dotted line in FIG. As shown in FIG. 10, a part of the return light (9) from the single fiber enters the light receiving element (81).Generally, in a semiconductor laser module, as shown in FIG.

The light output of the semiconductor laser (1) is received by the light receiving element (81).

The optical output of the semiconductor laser (1) is kept constant by converting it into an electrostatic signal and feeding it back to the power supply +13 of the semiconductor laser. Therefore, the return light (9) from the optical fiber may be returned to the light receiving element +81, and the intensity of the return light (9) may vary depending on the state of the transmission path or the connection of the transmission optical components. If it fluctuates, the semiconductor laser (1)
The light output will also vary.

[Problem that the invention seeks to solve]

In the semiconductor laser module with an optical isolator as described above, the returned light from the optical fiber is incident on the light receiving element that detects the optical output of the semiconductor laser.

Therefore, due to fluctuations in the return light from the original fiber due to the state of the transmission line or the connection of the transmission optical components, the optical output detected by the light receiving element changes, and the power supply of the semiconductor laser is adjusted to cancel this change. Since feedback is required, there is a problem in that the optical output of the semiconductor laser fluctuates.

This invention was made in order to solve the above-mentioned problems.

By entering the light receiving element. Fluctuations in the optical output of the semiconductor laser can be prevented. The purpose is to obtain a semiconductor laser module with an optical isolator.

[Means for solving problems]

A semiconductor laser module with an optical isolator according to the present invention is configured such that the return light from a single fiber is incident on a location other than the active region of the semiconductor laser.

[Effect]

In the semiconductor laser module with an optical isolator according to the present invention, by arranging the birefringent crystal plate in a specific manner, the return light from the optical fiber is transmitted to the semiconductor laser, and
In addition, the light enters a location other than the active region and is reflected and scattered within the semiconductor laser, so that almost no return light from the optical fiber enters the light receiving element.

[Embodiments of the invention]

FIG. 1 is a block diagram showing an embodiment of the present invention. In the figure, (1) is a distributed feedback semiconductor laser that has a highly reflective metal or dielectric thin film on one end face.

(2) is the active region of the semiconductor laser (1), (31 is a lens, (41 is a birefringent crystal plate such as a positive crystal, and (5) is a permanent magnet.

(6) is a Faraday rotator to which a magnetic field is applied by the permanent magnet (5), (7) is the original fiber, (81 is the light receiving element that detects the optical output of the semiconductor laser (1), (9) is the optical fiber, <(7).The optical axis of the birefringent crystal plate (41) is aligned with the normal line of the optical axis and the plane of incidence of the semiconductor laser beam, as shown by the white arrow aIII in FIG. from the y-z plane to the Faraday rotator (61).

The extraordinary ray of light that is parallel to the plane rotated by 45 degrees in the same direction as the direction in which the semiconductor laser light undergoes rotation of the polarization plane and propagates in the forward direction is on the y-axis angular side of the ordinary ray. It is oriented to emit light.

The operation of the original isolator-equipped semiconductor laser module configured as described above will be described with reference to FIG.

FIG. 3 (at is A between the semiconductor laser (1) and the lens (31) of the semiconductor laser light propagating in the forward direction in FIG.
, B between lens +31 and Faraday rotator (6), C2 between Faraday rotator (6) and birefringent crystal plate 141, trap each point between birefringent crystal plate (41) and optical fiber (71).The position and polarization. The direction is also shown in FIG. 3(b).

In Fig. 1, the return light (9) from the source fiber (7) propagating in the opposite direction is at each point A, B, C, and D. Its position and polarization direction are shown. As shown in FIG. 3 (alA, H), depending on the polarization characteristics of the semiconductor laser (1) 1,
Its polarization direction is almost parallel to the X axis. As shown in Figure 3 fat C, the Faraday rotator (
6), the polarization direction is rotated by 45 degrees, but as shown in Figure 2, the plane containing the optical axis and the normal to the plane of incidence of the birefringent crystal plate (4) is perpendicular to the polarization direction rotated by 45 degrees. Therefore, as shown in Figure 3 (al D).

It propagates in the birefringent crystal plate (4) as an ordinary ray and is coupled to the optical fiber (7). On the other hand, the return light (9) from the optical fiber (7) does not have polarization characteristics as shown in Figure 3 (bl D). When incident on the crystal plate (41), a polarized light component perpendicular to the plane containing the optical axis and the normal to the plane of incidence [ordinary ray (9a)] and a polarized light component parallel to the plane [extraordinary ray (9b)] The ordinary ray (9a) is divided into .

However, as shown in Figure 3 (blc), the extraordinary ray (9b) propagates obliquely within the birefringent crystal plate (4) and travels from the origin to the X-axis within the x-y plane. angular direction, y
The light is emitted with a deviation in the direction of the shaft stop. Next, the ordinary ray (9c) and the extraordinary ray (9b) are generated by rotating the polarization of the semiconductor laser light in the forward direction, as shown in Figure 3 (1b) B, due to the non-reciprocity of the Faraday rotator (6). Rotate 45 degrees in the same direction. Then, as shown in Figure 3 1bl A, the lens (
31, the extraordinary ray (9b) shifted from the origin of the x-y plane is moved to a position symmetrical to the origin, that is,
Semiconductor laser (1) shifted in the X-axis stop direction and the Y-axis valve direction.
incident on . Note that the ordinary ray (9a) is a semiconductor laser (
However, since the polarization direction of the ordinary ray (9a) is perpendicular to the polarization direction of the semiconductor laser beam, it does not increase noise at the semiconductor laser aperture. By arranging the birefringent crystal plate (41) as described above, out of the return light (9) from the optical fiber, the semiconductor laser (1
) can be made to enter the active region (21) of the semiconductor laser (1) within the end face of the semiconductor laser (1) and into the active region (21) of the semiconductor laser (1). The return light (9) from the optical fiber (7) that has entered the optical fiber (1) is reflected at the end face of the semiconductor laser (1) having a thin film with high reflectance, and is almost always sent to the light receiving element (81).
is not incident on . Therefore, the optical output of the semiconductor laser (1) is stabilized.

In the above embodiment, the optical axis of the birefringent crystal plate (41) is oriented so that the semiconductor laser beam propagates as an ordinary ray, as shown in FIG. As shown in FIG. 4, the plane including the optical axis and the normal to the plane of incidence of the semiconductor laser light is from the yz plane by the Faraday rotator (6).

The extraordinary ray of light that is parallel to the plane rotated by 45 degrees and propagates in the forward direction in the opposite direction to the direction in which the semiconductor laser beam undergoes rotation of the polarization plane is in the y-axis fixed direction of the ordinary ray. The semiconductor laser light may propagate as extraordinary light, and in this case, the optical fiber (7)
Of the return light (9) from the semiconductor laser (1), the origin of the polarization component parallel to the polarization plane of the emitted light from the semiconductor laser (1) is
) within the end face of and.

The light is incident on a portion other than the active region (2) of the semiconductor laser fi+.

In addition, as shown in FIG. 5, not only the Faraday rotator (61) and the optical fiber (7) but also the
) and the Faraday rotator (6), a birefringent crystal plate (4
a), the birefringent crystal plate (4a) between the semiconductor laser (1) and the Faraday rotator (6) allows the semiconductor laser [1 ) is also within the end facet of the semiconductor laser (1), and the polarization component perpendicular to the polarization direction of the emitted light of the semiconductor laser (1)
In addition, in the above embodiments, a case where a positive crystal is used as the birefringent crystal plate (41) is used, but a negative crystal is used as the birefringent crystal plate (41). may also be used, and if the direction of the optical axis when using a negative crystal is symmetrical to the direction of the optical axis when using a positive crystal and the normal to the plane of incidence, it is completely different from the above example. The same action and effect can be obtained.

〔Effect of the invention〕

As described above, according to the present invention, the birefringent crystal plate is arranged so that the return light from the optical fiber is coupled to a location other than the active region of the semiconductor laser. This has the effect of eliminating fluctuations in the optical output of the semiconductor laser due to the return light coupling to the light receiving element, stabilizing the optical output characteristics and oscillation characteristics of the semiconductor laser, and suppressing noise increase due to the return light.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing a semiconductor laser module with an optical isolator according to an embodiment of the present invention. FIGS. 2 and 3 are explanatory diagrams explaining the present invention in detail, FIGS. 4 and 5 are explanatory diagrams and configuration diagrams showing other embodiments of the present invention, and FIG. 6 is a conventional A configuration diagram showing a semiconductor laser module with an optical isolator, FIGS. 7, 8, and 9 are explanatory diagrams explaining the operation of a conventional example, and FIG. FIG. 2 is a configuration diagram of a semiconductor laser module. In the figure, (1) is a semiconductor laser, (3) is a lens, and (4)
is a birefringent crystal plate, (5) is a permanent magnet, (6) is a Faraday rotator, (7) is an optical fiber, and (8) is a light receiving element. In addition, in FIG. 9, the same reference numerals indicate the same or equivalent parts.

Claims (5)

[Claims] (1) A semiconductor laser, a light receiving element that detects the optical output of the semiconductor laser, a lens, a birefringent crystal plate made of at least one positive crystal or a negative crystal, and a device that rotates the polarization plane of incident light by 45 degrees. In a semiconductor laser module comprising a Faraday rotator that emits light, a permanent magnet that magnetizes the Faraday rotator, and an optical fiber, among the emitted light of the semiconductor laser that returns from the optical fiber to the semiconductor laser, the semiconductor laser with an optical isolator, characterized in that the light having a polarization component parallel to the polarization plane of the emitted light is condensed on a portion other than the active region of the semiconductor laser, although within the end face of the semiconductor laser. Semiconductor laser module. (2) The semiconductor laser module with an optical isolator according to claim 1, wherein the semiconductor laser is a distributed feedback semiconductor laser. (3) The optical isolator according to claims 1 and 2, wherein the end face of the semiconductor laser on the side opposite to the optical fiber side has a thin film of metal or dielectric with high reflectance. Semiconductor laser module with. (4) A lens, a Faraday rotator, and a parallel plate made of birefringent crystal are arranged in this order between the semiconductor laser and the optical fiber, and the birefringent crystal plate is A plane including the optical axis and the normal to the incident surface is perpendicular to the polarization plane of the output light of the semiconductor laser that has passed through the Faraday rotator, and the center of the active region of the end face of the semiconductor laser is The origin, the direction parallel to the optical axis of the lens is the Z axis, the direction perpendicular to the junction surface of the semiconductor laser and from the substrate to the active region of the semiconductor laser is the y axis, and the axis perpendicular to the yz plane is the x axis. When, the y-coordinate of the intersection of the extension line of the optical axis of the birefringent crystal plate and the x-y plane is positive when a positive crystal is used as the birefringent crystal plate, and positive when a negative crystal is used as the birefringent crystal plate. A semiconductor laser module with an optical isolator according to claims 1, 2, and 3, wherein: is negative. (5) A lens, a Faraday rotator, and a parallel plate made of birefringent crystal are arranged in this order between the semiconductor laser and the optical fiber; A plane including the optical axis and the normal to the incident surface is parallel to the polarization plane of the output light of the semiconductor laser that has passed through the Faraday rotator, and the origin is the center of the active region of the end face of the semiconductor laser. , the direction parallel to the optical axis of the lens is the Z-axis, the direction perpendicular to the junction surface of the semiconductor laser and from the substrate to the active region of the semiconductor laser is the positive y-axis, and the axis perpendicular to the y-z plane is the x-axis. When, the y-coordinate of the intersection of the extension line of the optical axis of the birefringent crystal plate and the x-y plane is negative when a positive crystal is used as the birefringent crystal plate, and negative when a negative crystal is used as the birefringent crystal plate. A semiconductor laser module with an optical isolator according to claims 1, 2, and 3, wherein: is positive.