JPH04213413A - Laser diode module - Google Patents

Abstract

PURPOSE:To widen tolerance and to facilitate the assembly of a laser diode module by providing the expanded part of a core diameter near the incident end face for laser diode light of an optical fiber for output light. CONSTITUTION:This module is so constituted that the laser beam from an LD element 1 is coupled to the expanded part 52a of the core diameter of the optical fiber 5a by receiving a condensing effect and is stopped down in spot size during the propagation of the expanded part 52a of the core so as to be guided to the non-expanded part 51a. The tolerance is widened from 1.2mum to about 3mum under the specified coupling loss if, for example, the beam spot radius of the optical fiber 5a for output is 3.1mum and the expanded part 52a expanded in the beam spot radius to 8mum is provided at one end thereof. Then, the permissible quantity to the axial misalignment arising when a ferrule holder 7 is fixed to a lens holder 4 is widened and the yield of production of the laser diode module is improved.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser diode module used as a light source for optical communications, optical sensors, etc.

0002

[Prior Art] Fig. 2 is a structural diagram of a conventional laser diode (hereinafter abbreviated as LD) module. In the figure, 1 is an LD element, 2 is an LD stem, and 3 is for condensing light. 4 is a lens holder, 5 is an optical fiber, 51 is a core of the optical fiber 5, 6 is a ferrule, 7 is a ferrule holder, and 8 is a solder or welded portion.

To assemble such an LD module, first, the condensing lens 3 is fixed to the lens holder 4 with adhesive, solder, or the like. Next, this lens holder 4 is aligned and fixed by soldering or welding to the LD stem 2 on which the LD element 1 is previously mounted so that the optical axes of the lens 3 and the LD element 1 are aligned. Next, the ferrule 6 to which the optical fiber 5 is fixed and the ferrule holder 7 that holds the ferrule 6 are aligned with respect to the LD element 1 in the optical axis direction and in the direction perpendicular to the optical axis, respectively, and soldered or Fix by welding.

0004

[Problems to be Solved by the Invention] The biggest problem in the above assembly process is that the light emitted from the LD element 1 is The problem is that the power incident on the optical fiber 5 decreases.

[0005] The allowable amount of axis misalignment (hereinafter referred to as tolerance) of this optical fiber is
In this case, it can be easily evaluated using Gaussian beam approximation.

For example, when there is an axis deviation of x in the direction perpendicular to the optical axis, which is usually the most severe, the coupling efficiency η of a Gaussian beam with a beam spot radius ω is η∝exp(-x2 /ω2). (Saruwatari, Nawata, “Semiconductor
r Laser tosingle-mode fib
er coupler”, Applied Opti
cs, vol. 18, No. 11, pp. 1
847-1856). From this, a relationship of tolerance x 1 dB ≈0.48ω when the loss increases by 1 dB is derived. That is, the tolerance is proportional to the beam spot radius ω.

[0007] In a general single mode optical fiber for communication, the wavelength λ is 1.3 μm and the beam spot radius ω is 5 μm, so the 1 dB tolerance is about 2.4 μm. It can be seen that this is quite severe considering that the normal part processing accuracy is about several tens of micrometers. Therefore, in current module assembly, highly accurate YA
It is fixed using G laser welding to realize modularization. The fixing accuracy of the YAG laser is about 1 to 2 .mu.m, which has been achieved to some extent, but it is desired to further increase the tolerance in order to improve the yield.

[0008] Furthermore, as a light source for excitation of optical sensors and fiber-type optical amplifiers, it is necessary to use an LD element with a shorter wavelength than that for general communications and an optical fiber with a large refractive index difference Δ between the core and the cladding. For such short wavelengths or refractive index difference Δ
For optical fibers with a large diameter, the beam spot size is smaller than that for communications, so the tolerance becomes even stricter according to the above equation. Therefore, it has been extremely difficult to create a module with high coupling efficiency.

On the other hand, as a method to solve the above problem,
LD with a lens on the optical fiber side to increase tolerance
Modules are also known. FIG. 3 shows its module structure.

In this LD module, in addition to the configuration shown in FIG. 2, a convergent rod lens 9 is installed in the ferrule holder 7, and this rod lens 9 and the optical fiber 5 are integrated, so that light to the LD element 1 is Align and secure. This increases the beam spot size on the optical fiber side by the magnification of the lens, making it possible to relax the tolerance.

However, the structure shown in FIG. 3 increases the number of parts and assembly steps, making it difficult to achieve miniaturization and economy.

The present invention has been made in view of the above circumstances, and its purpose is to provide a laser diode module that increases tolerance, facilitates assembly, has a simple structure, and is suitable for mass production. There is a particular thing.

[0013]

[Means for Solving the Problems] In order to achieve the above object, claim 1 provides a laser diode module for condensing light emitted from a laser diode element onto an optical fiber and outputting the same, The laser diode of the output optical fiber
A portion with an enlarged core diameter is provided near the light incident end face. Further, in a second aspect of the present invention, the output optical fiber is constituted by a single mode optical fiber in which the difference in refractive index between the core and the cladding is greater than 0.3%.

In the third aspect of the present invention, the core diameter is enlarged so that the beam spot radius at the end face of the optical fiber is 3 to 20 μm at the laser diode emission wavelength.

[0015]

According to the first aspect of the present invention, the laser light emitted from the LD element is subjected to a condensing action and is coupled to the light incident end face of the enlarged core diameter portion of the optical fiber. The spot size of the light incident on the optical fiber is gradually narrowed down while propagating through the expanded portion of the optical fiber, and is guided to the core of the non-expanded portion and output.

Furthermore, according to claim 2, the spot size is smaller than that of the commonly used refractive index difference of 0.3%.

Further, according to claim 3, tolerance is reduced and a practical module is realized.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a structural diagram showing an embodiment of an LD module according to the present invention, and the same components as those in FIG. 2 showing a conventional example are designated by the same reference numerals.

That is, 1 is L with an oscillation wavelength of 0.98 μm.
D element; 2 is an LD stem on which the LD element 1 is mounted; 3a is an aspherical glass lens; 4 is a lens holder; Enlarged part 5 where the spot radius ω is expanded to 5 μm
2a is a single mode optical fiber, 51a is the core of the optical fiber 5a, 6 is a ferrule, 7 is a ferrule holder, and 8 is a welding fixing part.

Further, the optical fiber 5a has a refractive index difference Δ of 0.
．． 8%, the cutoff wavelength λc is 0.89 μm, and the end of the enlarged portion 52a is terminated with respect to the ferrule 6, and the end face (light incident end face) 53a thereof faces the LD element 1. Fixed. The enlarged portion 52a of the optical fiber 5a can be easily manufactured by directly heating the fiber with a gas burner and diffusing the heat, and once the conditions (temperature, heating time, etc.) are set, the spot size can be reproducibly It is possible to have good control.

[0021] Furthermore, the beam spot radius ω mentioned above is
The radius of the position where the beam intensity is 1/e2 of the center. Also, to explain the difference between the core diameter and beam spot diameter, the core diameter is the size of the part of the optical fiber where the refractive index is high, and it depends on the refractive index distribution of the optical fiber. Uniquely determined. On the other hand, the beam spot diameter indicates the spread of light rays propagating within the optical fiber.
This varies depending on the wavelength even in the same optical fiber, and generally becomes smaller as the wavelength becomes shorter. The value varies depending on the core diameter and refractive index distribution, but roughly speaking, as the core diameter increases, the spot size also increases.

To assemble the LD module having such a structure, first, the aspherical glass lens 3a for condensing light is fixed to the lens holder 4 with adhesive. Next, the lens holder 4 is attached to the LD stem 2 on which the LD element 1 is mounted in advance.
Align so that the optical axes of a and LD element 1 match, and
Weld and fix by laser. Next, the ferrule 6 to which the optical fiber 5a is fixed is placed in a Y position relative to the ferrule holder 7.
While welding with AG laser, the ferrule holder 7
are aligned with the LD element 1 in the optical axis direction and in the direction perpendicular to the optical axis, respectively, and welded and fixed to the lens holder 4 using a YAG laser.

Actually, the module of FIG. 1 is constructed with a refractive index difference Δ
(0.8%) despite being a large optical fiber.
It was possible to produce the product with a yield similar to that of the above.

Next, the operation of the above configuration will be explained.

The laser beam emitted from the LD element 1 is condensed by the aspherical glass lens 3a and coupled to the light incident end face 53a of the optical fiber 5a with a high coupling rate. The spot size of the light incident on the optical fiber 5a is gradually narrowed while propagating through the expanded portion 52a of the core 51a, and is guided to the non-expanded portion of the core 51a and output.

Next, FIGS. 4 and 5 show how the coupling loss and the tolerance in the direction perpendicular to the optical axis at 1 dB loss change by enlarging the beam spot size of the optical fiber. Let's consider using it.

FIG. 4 is a diagram showing the results of measuring the coupling loss and the change in tolerance in the direction perpendicular to the optical axis with respect to the beam spot radius in the example of FIG. As mentioned above, the optical fiber used in this measurement had a refractive index difference Δ of 0.
．． 8%, and the cutoff wavelength λc is 0.89 μm. The beam spot radius ω of the optical fiber in the portion of the core 51a that is not expanded is 3.1 μm, and the beam spot radius ω of the enlarged portion 52a is 5 μm. Further, the measured wavelength of the LD element 1 was 0.98 .mu.m, and the coupling system used an aspherical glass lens 3a designed for a communication fiber with a beam spot radius .omega.=5 .mu.m.

As can be seen from FIG. 4, even if the beam spot radius ω is expanded, the coupling loss does not increase, and the tolerance is expanded approximately in direct proportion to the beam spot radius ω. For example, the tolerance when the beam spot radius ω is not expanded is 1.2 μm, but when the beam spot radius ω is expanded to 8 μm, the tolerance increases to about 3 μm.

Note that the value of the coupling loss is the value of the communication LD (
The reason why this is worse than that (normally about 2 to 3 dB) is that the deviation of the emitted beam of this LD element from the Gaussian beam is large.

FIG. 5 shows the coupling system of FIG. 1, where the optical fiber 5a has a refractive index difference Δ of 2.3% and a cutoff wavelength λ.
When using an LD element with c of 0.73 μm and a measuring LD element with a wavelength of 0.81 μm, we measured the coupling loss with respect to the beam spot radius and the change in tolerance in the direction perpendicular to the optical axis. It is a figure showing the result.

Since the beam spot radius ω of this optical fiber before expansion is approximately 1.6 μm, which is very small, the lens used in this measurement cannot condense the light, resulting in a large coupling loss. Therefore, by enlarging the beam spot radius ω, both tolerance and coupling loss can be greatly improved.

As can be seen from FIG. 5, even if the beam spot radius ω is expanded to 12 μm, the coupling loss does not increase, and the tolerance in the direction perpendicular to the optical axis is expanded. however,
When the beam spot radius ω is increased, it is necessary to increase the distance between the lens and the fiber in order to correspondingly increase the image magnification of the coupling system. This increases the module size, but on the other hand, it is possible to insert an optical isolator etc. into this gap to increase functionality, increasing the degree of freedom in design by increasing the beam spot radius ω. Can be expanded.

However, if the beam spot radius ω is enlarged, the tolerance for angular deviation becomes strict, so it is not desirable to excessively enlarge the beam spot radius ω.

The influence of the angular shift can be calculated by Gaussian beam approximation as in the case of the axis shift, and the coupling efficiency η for the angular shift θ can be expressed as η∝exp(-π2 θ2 ω2 /λ2). From this formula, the beam spot radius ω is 5 μ
m, angular shift θ1dB ≒2.2° with 1dB loss increase
(λ=1.3 μm), but when the beam spot radius ω is 20 μm, θ1 dB≈0.55. In practical terms, 20 μm is considered to be the limit. Further, if the thickness is less than 3 μm, the tolerance becomes strict, which is not preferable.

In fact, using an optical fiber having the same structure as shown in FIG. 1 but with the beam spot radius ω expanded to 5 μm, we were able to fabricate a module capable of coupling with a coupling loss of 3 dB.

As explained above, according to this embodiment,
Since the optical fiber 5a with an enlarged beam spot radius ω is used and the light from the LD element 1 is made to enter the enlarged portion 52a, tolerance during assembly can be relaxed and a significant improvement in manufacturing yield can be expected. . In particular, even LD modules using short wavelength LD elements or optical fibers with a large refractive index difference Δ can be easily manufactured with high coupling efficiency.

Furthermore, if the beam spot radius ω is selected close to the communication fiber value, it is possible to use the same parts, manufacturing equipment, and manufacturing process as for conventional communication LD modules, allowing for common use of equipment. There is also the advantage of being able to Furthermore, by controlling the value of the beam spot radius ω, the degree of freedom in designing the optical system, module dimensions, etc. can be expanded, so that miniaturization and higher functionality can be achieved.

[0038]

As described above, according to claim 1, claim 2, or claim 3, tolerance during assembly can be relaxed and manufacturing yield can be significantly improved.

In particular, even an LD module using a short wavelength LD element or an optical fiber with a large refractive index difference can be easily manufactured with high coupling efficiency.

Furthermore, if the core diameter, specifically the beam spot radius ω, is selected close to the communication fiber value, the same parts, manufacturing equipment, and manufacturing process as for conventional communication LD modules can be used. Another advantage is that equipment can be shared. Furthermore, by controlling the value of the beam spot radius ω, the degree of freedom in designing the optical system, module dimensions, etc. can be expanded, and miniaturization and high functionality can be achieved.

[Brief explanation of the drawing]

FIG. 1 is a structural diagram showing one embodiment of an LD module according to the present invention.

FIG. 2 is a structural diagram of a conventional LD module.

FIG. 3 is a structural diagram of a conventional LD module equipped with a lens for enlarging the beam spot size on the optical fiber side.

FIG. 4 is a diagram showing the results of measuring changes in coupling loss and optical axis vertical tolerance with respect to the beam spot radius in FIG. 1;

[Figure 5] Optical fiber refractive index difference Δ, cutoff wavelength λc
FIG. 5 is a diagram showing the results of measuring changes in coupling loss and optical axis vertical tolerance with respect to beam spot radius when the wavelength of the measured LD element is set to a value different from that in FIG. 4.

[Explanation of symbols]

1...LD element 2...LD stem 3a...Aspherical glass lens for condensing light 4...Lens holder 5a...Single mode optical fiber 51a...core 52a...Enlarged section 53a...Light incidence end face 6... Ferrule 7... Ferrule holder 8...Solder or welded fixed part

Claims (3)

[Claims] 1. A laser diode module for condensing light emitted from a laser diode element onto an optical fiber and outputting the same, wherein the laser diode of the output optical fiber is
1. A laser diode module characterized in that a portion with an enlarged core diameter is provided in the vicinity of a light incident end face. 2. The laser diode module according to claim 1, wherein the output optical fiber is a single mode optical fiber having a refractive index difference between the core and the cladding of more than 0.3%. 3. The beam spot radius at the end face of the optical fiber is 3 to 20 at the laser diode emission wavelength.
3. The laser diode module according to claim 1, wherein the core diameter is expanded to .mu.m.