JPS63224385A - Semiconductor laser coupler - Google Patents

Abstract

PURPOSE:To easily obtain a high coupling efficiency by connecting a short step index fiber to the end face of a single mode fiber, and forming the end of the short fiber in a spherical curved surface. CONSTITUTION:A short step index fiber 6 having the same outer diameter as that of a single mode fiber 3 is connected to the end face of the fiber 3, and the end of the fiber 6 is formed in a spherical curved surface 8. Accordingly, even if a laser light 2 radiated from a semiconductor laser 1 is largely extended, it is passed through the fiber 6 having the spherical curved surface connected to the end of the fiber 3 to sufficiently condense laser light at a core of the fiber 3. Thus, the laser 1 is separated from the end face of the fiber 3, and even if the radius of curvature of the curved surface formed at the end face is increased, high coupling efficiency of the laser 1 to the fiber 3 can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a coupling device for a semiconductor laser and a single mode fiber used, for example, in optical fiber communication.

[Conventional technology]

FIG. 3 is a cross-sectional view showing a coupling portion between a semiconductor laser and a single mode fiber in a conventional semiconductor laser coupling device disclosed, for example, in document 928 of the 1985 Seiko Communication Society National Conference.

In the figure, 1 is a semiconductor laser, 2 is a laser beam emitted from the semiconductor laser 1, 3 is a single mode fiber, and 4 is a single mode fiber.
5 is a core portion of the single mode fiber 3; 5 is a spherical curved portion having a lens function for focusing the laser beam 2 formed at the tip of the single mode fiber 3 onto the core portion 4; and 6a is the curved portion 5. This is a tapered portion provided to facilitate formation.

Next, the operation of the above conventional semiconductor laser coupling device will be explained. Laser light 2 emitted from semiconductor laser 1 spreads into the air and is emitted. The laser beam 2 spread and emitted in this manner is focused on the core portion 4 by the curved surface portion 5 formed at the tip of the single mode fiber 3, and efficient coupling between the semiconductor laser 1 and the single mode fiber 3 is achieved. .

The above is the general operation of the conventional semiconductor laser coupling device.Next, this coupling relationship will be explained in more detail using the lN4 diagram.

FIG. 4 is an enlarged sectional view of a main part for explaining the coupling between the semiconductor laser and the single mode fiber in the semiconductor laser coupling device of FIG. 3. FIG. In FIG. 4, the laser beam 2 emitted from the semiconductor laser 1 has a spot size ω, assuming that the laser beam 2 oscillates in a single mode.
It is a Gaussian beam of LD. Further, the propagation mode of the laser beam 2 in the single mode fiber 3 is a single mode, and the electric field distribution of the single mode is Gaussian with a spot size ωF. Therefore, if the Gaussian beam with a spot size ωLD emitted from the semiconductor laser 1 is exchanged with a Gaussian beam with a spot size ωF using a lens and input into the single mode fiber 3, the coupling efficiency between the semiconductor laser 1 and the single mode fiber 3 can be calculated theoretically. It becomes 100%. General average semiconductor laser spot size ωL
D is 1 μm, and the spot size ωF of the single mode fiber is 5 μm. In this way, the coupling system of the semiconductor laser and the single mode fiber uses a lens system about 5 times larger. Here, in order to obtain the theoretical coupling efficiency of 100% using the conventional single-mode fiber 3ζ shown in Figure 4, we will find a theoretical formula for what kind of coupling system should be used. . The lens used is a spherical curved portion 5 having a radius of curvature rc and forming a single lens formed at the tip of a single mode fiber 3 as shown in FIG. Here, this single lens is approximated by a thin lens. Then, the Gaussian beam can be transformed using the commonly used A, B, C, D matrix representation of the lens [Reference (11 Kogeln
ik, H: l-Imaging of 0pti
cal mode -reronatora with
1internal Lenses J Be1l 5
yst.

Tech, J., 44.3. P, 455 (1965)
, Literature (2) Teiji Uchida, Atsushi Ueki: “Optics I for 1st Child Engineers, nJ [Child Communication Society Journal, Vol. 62,
A5.

P, 538 (1979), Reference (3) Part I Sakuraba: “Quantum Electronics” Morikita Publishing Co., Ltd.]. From this, in FIG. 4, the spot size ω of the laser beam converted by the curved surface portion 5 is shown. is expressed by the following formula.

Here, λ is the wavelength e of the laser beam 2, and NC is the refractive index of the core portion 4.

Next, ω. = ωF, the radius of curvature of the curved surface portion 5 and the distance between the semiconductor laser 1 and the tip of the single mode fiber 3 for the coupling efficiency to be 100% are determined by the following equations (4) and 3, respectively. It becomes as shown in equation (5).

Next, the actual values are substituted into the above equations (4) and (5) to obtain r2*D:. Here, we assume a generally used laser diode with a wavelength of 1.3 μm and a quartz fiber as the original communication light source using the single mode fiber 3, and assume that λ-1.3μ7Fl a Nc-1,45
Use. Also, as mentioned earlier, ωLD”1 μm, ω
F-5 μm is used. Substituting these values, the radius of curvature r'c of the curved surface portion 5 is 5.5 μm, and the distance D' between the semiconductor laser and the tip of the single mode fiber 3 is 11.8 μm.
This results in a rather small value, which causes the following inconvenience.

First, since the semiconductor laser and the end face of the single mode fiber 3 are very close to each other, the amount of laser light 2 emitted from the semiconductor laser that is reflected by the curved surface portion 5 and returns increases, resulting in the output of the semiconductor laser. This has the drawback that it becomes unstable and generates noise. In addition, since the end faces of the semiconductor laser and the single mode fiber 3 are close, the optical axis 14 of both is close! There is a drawback that when performing I, they collide with each other and are easily damaged. Furthermore, since the radius of curvature rc of the curved surface portion 5 is very small, it is very difficult to form this curved surface portion 5, and a tapered portion 6 must be formed in order to form this curved surface portion 5. There is a manufacturing defect.

[Problem that the invention seeks to solve]

Since the conventional semiconductor laser coupling device described above is configured as described above, in order to obtain high coupling efficiency, the radius of curvature rc of the curved surface portion 5 of the single mode fiber 3 is made small, and the semiconductor laser 1 is connected to the single mode fiber. 3 to 10μ
The curved surface portion 5 of the single mode fiber 3 has to be brought as close as 500 m, and the characteristics of the semiconductor laser 1 may deteriorate due to the reflected light from the single mode fiber 3, and the semiconductor laser 1 may be easily damaged. There were problems such as difficulty in forming. In addition, in order to solve such problems, in the conventional semiconductor laser coupling device, the radius of curvature rc of the curved surface portion 5 is intentionally made large, and the semiconductor laser 1
is kept away from the end face of the single mode fiber 3, but this poses a problem in that the coupling efficiency decreases.

This invention was made to solve these problems, and it is possible to increase the radius of curvature of the curved surface of the single mode fiber, to move the semiconductor laser away from the end face of the single mode fiber, and to achieve high coupling efficiency. The purpose is to obtain a semiconductor laser coupling device that can be used.

[Means for solving problems]

The semiconductor laser coupling device according to the present invention connects a short step-index fiber having the same outer diameter as the single-mode fiber to the end of the single-mode fiber.
In addition to being connected to a surface, the tip of this short step index fiber is formed into a spherical curved surface.

[Effect]

In the semiconductor laser coupling device of the present invention, even when the laser light emitted from the semiconductor laser spreads widely,
By passing the laser beam through a short step-index fiber having a spherical curved surface connected to the tip of the single-mode fiber, the laser beam can be sufficiently focused on the core portion of the single-mode fiber. Therefore,
Even if the semiconductor laser is moved away from the end face of the single mode fiber and the radius of curvature of the curved surface portion formed on this end face is increased, a high 1@ coupling efficiency of the semiconductor laser and the single mode fiber can be obtained.

〔Example〕

FIG. 1 is a sectional view showing a coupling portion between a semiconductor laser and a single mode fiber in a semiconductor device/coupling device which is an embodiment of the present invention, and each reference numeral 1, 2, 3.4 is shown in FIG. 3 above. This is the same as the conventional device. In the figure, 6 is a short step index fiber having the same outer diameter as the single mode fiber 3, 7 is the core portion of the step index fiber 6 which has a diameter larger than the diameter of the core portion 4 of the single mode fiber 3, and 8 is a step index fiber. This is a spherical curved portion formed on the end face of the fiber 6.

Next, the operation of the semiconductor laser coupling device which is an embodiment of the present invention will be described. A laser beam 2 emitted from a semiconductor laser 1 propagates while spreading in the air, and enters a spherical curved portion 8 formed on the end surface ζ of a short step index fiber 6. The laser beam 2 is refracted by the curved surface portion 8, propagates while converging in the core portion 7 of the step index fiber 6, and is transferred to the single mode fiber 3.
is inputted to the core section 4 of. As described above, since the diameter of the core portion 7 of the step index fiber 6 is larger than the diameter of the core portion 4 of the single mode fiber 3, the laser beam 2 can be focused at this portion. Therefore, by appropriately selecting the length of the step-index fiber 6, the curved surface portion 8 of the step-index fiber 6 can receive any spread laser light.

Here, being able to receive the expanded laser beam 2 means that the semiconductor laser 1 can be moved away from the curved surface portion 8 appropriately.

Next, when the coupling efficiency between the semiconductor laser 1 and the single mode fiber 3 becomes 100%, the length of the step index fiber 6 IJOp curved portion 8 surface curvature radius rc, the distance between the semiconductor laser beam 1 and the curved surface portion 8 The results of determining the relationship between D0 and D0 are shown below. This result was obtained from the Gaussian beam transformation formula in the paraxial region as described above, and is as follows:
7) The equations for determining Do are established.

Next, numerical examples based on each of the above formulas will be shown. Here, as mentioned earlier, ωF-5μm, ωLD-1μm, N
Use the value of c-1,45. Currently, the outer diameter of a standard single mode fiber is 125 μm. Further, the standard outer diameter of the step index fiber is 125 μm. Therefore, here the outer diameter is 125 μm.
Consider connecting a short step index fiber 6 with an outer diameter of 125 μm to a single mode fiber 3 of . As a method of processing the end face of the step index fiber 6 into a spherical shape, methods such as polishing and melting can be considered, but processing of the curved surface portion 8 having a radius of curvature equal to or close to the radius of the step index fiber 6 rc% is possible. can be done relatively easily.

Therefore, in the case of forming the curved surface part 8 with a radius of curvature rcm of 62.5 μm, from the above equation (6) and the force equation (force equation), , D, are calculated as follows: -120
8 μm, D6wall 1 μm. This value is the value shown in the example of the conventional device above, r2w5.5 am, D' =
This is a sufficiently large value compared to 11.8 μm, and as a result, the semiconductor laser can be moved away from the end face of the single mode fiber 3 without reducing the coupling efficiency, thereby preventing deterioration of the semiconductor laser due to reflected light. Can be done. In addition, in the above example, r, −62
, 5 μm, but LaO* Do is r
By changing c, a more appropriate value can be set. By the way, this is possible because
This is because the step index fiber 6 is connected to the end face of the single mode fiber 3, but since the outer diameters of the single mode fiber 3 and the step index fiber 6 are the same, automatic fusion splicing device using grooves (not shown), it can be easily manufactured mechanically.

By the way, in the coupling system of the semiconductor laser and the single mode fiber 3 described above, if the optical axis of the lens formed on the end face of the step index fiber 6 and the optical axis of the single mode fiber 3 do not match, axis misalignment will occur. A loss occurs and high coupling efficiency cannot be obtained. Therefore, it is necessary to align the optical axis of the lens with the optical axis of the single mode fiber 6. This alignment of the optical axes is obtained as follows. First, the optical axes of the single mode fiber 3 and the step index fiber 6 can be easily aligned by mechanically butting them together on the groove and fusing them, since fibers with the same outer diameter are used as described above. can get. The axis misalignment accuracy at this time is several μm, since the outer diameter accuracy of commercially available general fibers is about 1 to 2 μm.
High accuracy within m.

Next, regarding the formation of the curved surface part 8 on the end face of the step index fiber 6, for example, if a method is used in which the formed part is melted in the direction of the earth's gravity, it will naturally align with the optical axis of the step index fiber 6. A curved surface portion 8 forming a lens having an optical axis is obtained. As mentioned above, the optical axes of the single mode fiber 3 and the step index fiber 6 coincide with each other with high precision. Therefore, the optical axis of the lens formed by the curved surface portion 8 and the optical axis of the single mode fiber 3 coincide with each other with high precision. As described above, by using the step index fiber 6, the mechanical precision of its outer diameter can be utilized, so that the semiconductor laser 1 can be prevented from deteriorating due to the reflected light as described above, and moreover, a semiconductor laser can achieve high coupling efficiency. 1 and single mode fiber 3 can be easily obtained.

In the above embodiment, the step index fiber 6 is connected to the end face of the single mode fiber 3, but a quartz rod or a glass rod may be used instead of the step index fiber 6.

Further, in the above embodiment, the case where the spherical curved surface portion 8 with the radius of curvature rc is formed on the end face of the step index fiber 6 is shown, but the spherical curved surface portion 8 may be formed into an aspherical surface. This case will be explained using FIG. 2.

FIG. 2 is a cross-sectional view showing a coupling portion between a semiconductor laser and a single mode fiber in a semiconductor laser coupling device according to another embodiment of the present invention, and the same or equivalent parts as shown in FIG. 1 are denoted by the same reference numerals. It is displayed using. In the figure, reference numeral 9 denotes an aspherical curved surface section as shown by the broken line in FIG. 2, which is formed in place of the spherical curved surface section 8 shown in FIG. The reason why such an aspherical curved surface portion 9 is used is that in a lens system using a spherical curved surface portion 8, the laser beam 2 incident at a distance from the original axis is coupled to the single mode fiber 3. This is because the laser beam 2 passes through the center (optical axis) of the spherical curved section 8 and the laser beam 2 passes outside, emitting the semiconductor laser 1 and then reaching the core section 4 of the single mode fiber 3. This is because the time lag occurs, which causes wavefront aberration that distorts the wavefront of the Gaussian beam.

In order to prevent this wavefront aberration, the spherical curved surface 8 is replaced with an aspherical curved surface 9 so that the arrival time of the laser beam 2 is the same no matter where on the spherical curved surface 8 it passes. It is possible to do so. The shape of this aspherical curved surface portion 9 is a shape that satisfies the following equation (8).

11+ NC" h -Do 11 Nc" Lo...
・・・・・・・・・・・・・・・・・・・・・・・・(8)
Here, double is the distance between the point where the laser beam 2 enters the spherical curved surface portion 8 of the step index fiber 6 and the emission point of the laser beam 2 from the semiconductor laser 1.

l is the distance between the point where the laser beam 2 enters the spherical curved surface portion 8 and the end surface of the core portion 4 of the single mode fiber 3.

〔Effect of the invention〕

As explained above, the present invention provides a semiconductor laser coupling device in which a short step-index fiber having the same outer diameter as a single-mode fiber is connected to an end face of the single-mode fiber, and the tip of the short step-index fiber is connected to an end face of the single-mode fiber. Since the curved surface of the single mode fiber is formed on a spherical curved surface, the radius of curvature of the curved surface of the single mode fiber can be increased, the semiconductor laser can be moved away from the end face of the single mode fiber, and deterioration due to reflected light of the semiconductor laser can be prevented. without occurring,
This provides an excellent effect in that a semiconductor laser coupling device having extremely high payment efficiency can be easily realized with high precision.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing a coupling portion between a semiconductor laser and a single mode fiber in a semiconductor laser coupling device which is an embodiment of the present invention, and FIG. M2 is a cross-sectional view showing a semiconductor laser in a semiconductor laser coupling device which is another embodiment of the present invention. 3 is a cross-sectional view showing a coupling portion between a semiconductor laser and a single-mode fiber in a conventional semiconductor laser coupling device, and FIG. 4 is a cross-sectional view showing a coupling portion between a semiconductor laser and a single-mode fiber in a conventional semiconductor laser coupling device FIG. 2 is an enlarged sectional view of a main part for explaining coupling between a semiconductor laser and a single mode fiber. In the figure, l... Semiconductor laser, 2... Laser light, 3... Single mode fiber, 4.7... Core portion, 5.8... Spherical curved surface portion, 6... Step index fiber, 6a...Tapered part, 9...Aspherical curved part. In each figure, the same reference numerals indicate the same or equivalent parts.

Claims (5)

[Claims] (1) In a coupling device for a semiconductor laser and a single mode fiber, a core portion having the same outer diameter as the single mode fiber and at least larger than the diameter of the core portion of the single mode fiber is attached to the end face of the single mode fiber. A semiconductor laser coupling device characterized in that a short step index fiber having a diameter and a spherical curved portion on the semiconductor laser side is connected. (2) The refractive index of the short step index fiber connected to the end face of the single mode fiber is N_
C, the length is L_0, the radius of curvature of the spherical curved part is r_c
, the spot size of the propagating light of the single mode fiber is ω_F, and the spot size of the emitted light of the semiconductor laser at the semiconductor output end face is ω_L_D, then the following equation (1), L_0=[(r_c・N_C)/ (N_C-1)] [1
+(ω_F)/(ω_L_D)]…………………………
The semiconductor laser coupling device according to claim 1, wherein (1) is satisfied. (3) If the distance between the tip of the end face of the spherical curved part of the short step index fiber and the end face of the semiconductor laser is D_0, then the following equation (2), D_0=[
r_c/(N_C-1)][1-(ω_L_D/ω_F
)] ………………………………… so that (2) holds,
3. The semiconductor laser coupling device according to claim 2, wherein the semiconductor laser is arranged. (4) The shape of the spherical curved surface of the short step index fiber has a radius of curvature r_c near the optical axis.
Claim 1 or 2, or 3, characterized in that it is a spherical surface and is aspherical outside the optical axis.
The semiconductor laser coupling device described in Section 1. (5) The distance between the point where the laser beam emitted from the semiconductor laser enters the spherical curved surface of the short step index fiber and the emission point of the laser beam from the semiconductor laser is l_1, and the laser beam If the distance between the point of incidence on the spherical curved surface portion and the end face of the core portion of the single mode fiber is l_2, then l_1+N_C・
The semiconductor laser coupling according to claim 4, wherein the spherical curved surface portion of the short step index fiber is formed into an aspherical surface so that the relationship l_2=D_0+N_C·L_0 is established. Device.