JPH0544643B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical coupling method that is easy to manufacture and efficiently couples light from a light output element to a light input element.

A coupling method that efficiently couples the light emitted from a semiconductor laser to a single-mode optical fiber is to use two lenses. (a) Conventional co-focusing compound lens system; b) There is a second lens split type confocal compound lens system. The coupling system is shown in FIG. 1 and FIG. 4, which will be described later.

FIG. 1A shows the method (a), in which two lenses 4 and 5 having different focal lengths are arranged between the semiconductor laser 1 and the single mode optical fiber 2, and the optical fiber 2 from the semiconductor laser 1 is The light is made incident on the incident end face of the core 3. Further, by attaching a glass plate 7 having a refractive index similar to that of the core 3 of the optical fiber to the input end face 6 of the optical fiber 2, reflected light from the input end face 6 is reduced.

FIG. 1B is a diagram explaining the principle of the lens system in FIG. 1A. In the figure, 8 is the light emitting surface of the semiconductor laser, 6
denote the input end face of a single mode optical fiber. The first lens 4 is separated from the light emitting surface 8 of the semiconductor laser by approximately the focal length f ₁ of the first lens 4,
The distance between the second lens 5 (focal length f ₂ ) and the first lens 4 is the sum of their focal lengths (f ₁ +f ₂ ), and the position 6 of the entrance plane of the optical fiber is the distance between the second lens 5 and f ₂ is far away.

If we take such a lens system, the emission diameter 2w ₀ of the semiconductor laser is the ratio of the focal length of the lens system (f ₂ / f ₁ )
is magnified by
A light beam having a diameter of 2w ₂ =2w ₀ ×(f ₂ /f ₁ ) is imaged. Therefore, when the diameter of the guided light beam of the optical fiber is 2w _f , f ₂ /f ₁ should be selected so that w _f and w ₂ are approximately equal.

Next, the axis deviation characteristics of the conventional confocal compound lens system will be explained. To manufacture a semiconductor laser module, first the first lens 4 is aligned, then the second lens 5 is aligned, and the beam conversion by these lens systems is used to match the fiber. No matter what kind of lens system is used for the lens 5, the fixing accuracy of the optical fibers will be equivalent to the connection characteristics between single mode optical fibers.

Figure 2 shows a semiconductor laser with a spot size w ₀ of 1 μm and a guided light beam spot size w _f
A diagram is shown in which the horizontal axis represents the amount of deviation x in the direction perpendicular to the optical axis of the optical fiber 2, and the vertical axis represents the coupling efficiency η, for the case where a single mode optical fiber with a diameter of 5.5 μm is used. As can be seen from Figure 2, the optical fiber 2
If it moves about ±2.5 μm perpendicular to the optical axis, a 1 dB increase in coupling loss will occur, and it can be seen that the permissible amount of axis deviation in the direction perpendicular to the optical axis is small, and strict fixing accuracy is required.

Further, FIG. 3 shows a diagram in which the axis deviation amount z in the optical axis direction of the optical fiber 2 is plotted on the horizontal axis, and the coupling loss η is plotted on the vertical axis. 1dB due to a deviation in the optical axis direction of approximately 60μm
Deterioration occurs.

By the way, the coupling characteristic of two Gaussian beams with the same spot size is expressed as η(x, θ)=exp(−x ² /w ² −π ² w ² /λ ² θ ² ) (1) (Reference M. Saruwatari and K.
Nawata: Semiconductor fiber coupler:
Appl.Optics.vol.18.No.11.PP.1847-1856, 1979).
Here, x is the axis deviation in the direction perpendicular to the optical axis, θ is the angular deviation between the two beams, w is the spot size of the Gaussian beam, and λ is the oscillation wavelength of the semiconductor laser.
In addition, for example, when assembling the conventional module shown in FIG. 1, the angular deviation θ is
If the optical axis of the first lens 4 is relatively shifted in the direction perpendicular to the optical axis of This occurs when the optical axes of the integrated second lens 5 and optical fiber 2 are misaligned. From this equation, it can be seen that as w increases, the allowable axis deviation amount of x increases, but the allowable angular deviation amount decreases. In the case of currently used single mode optical fibers, the values of x and θ for 1 dB degradation are approximately ±2.5 μm and approximately ±2.2 degrees, respectively. During module fabrication, the angular misalignment can usually be set to no more than 0.5 degrees, so even if the spot size of the single mode optical fiber is increased several times, the coupling efficiency due to the angular misalignment will be reduced. It can be seen that the deterioration can be suppressed to a small level and the allowable amount of axis deviation in the direction perpendicular to the optical axis of the single mode optical fiber becomes considerably large.

In order to increase the spot size of the portion to be aligned, let us consider the case where the second lens 5 is integrated with the optical fiber 2, and the integrated structure is adjusted to align the axes.

The spot size w ₀ of the semiconductor laser of about 1 μm by the first lens 4 is determined by the laser oscillation wavelength λ=
Assuming that the spot size is 1.3 μm and the focal length f ₁ of the first lens 4 is 456 μm, the spot size is converted to w ₁ =189 μm using the relational expression w ₁ =λf ₁ /(πw ₀ ). Substituting this value into equation (1), the allowable axis misalignment, allowable angular misalignment x, and θ for a 1 dB increase in coupling loss are approximately ±
It becomes 87 μm, about ±3.6 minutes, and it can be seen that although the amount of axial deviation is relaxed compared to the above example, the angular deviation becomes extremely severe.

Furthermore, if the fixing accuracy of the first lens 4 and the semiconductor laser 1 in the optical axis direction is about ±10 μm, the waist position of the beam passing through the second lens 5 in the optical axis direction is (f ₂ /f ₁ ) ² × ( ±10) = positional fluctuation of approximately ±250μm. When adjusting by moving only the optical fiber 2, it is sufficient to correct this positional variation, but when the second lens 5 and the optical fiber 2 are integrated, the amount of movement required for correction is There are cases where it becomes extremely large and cannot be corrected.

Furthermore, when the parallel beam with no angular deviation is incident on the second lens 5, an image is focused on the optical axis of the second lens 5.
The optical axis of the lens 5 and the optical axis of the optical fiber 2 must be completely concentric and integrated. However, with the current machining accuracy, an error of several tens of micrometers occurs when the second lens 5 and the optical fiber 2 are integrated, so in an attempt to correct this error when aligning the axis with the parallel beam, This means that even if the lens 5 and the optical fiber 2 are moved in parallel, the correction cannot be made. In principle, an integrated second
Although it is possible to tilt the lens 5 and the optical fiber 2 and adjust the angle using a fine movement table, this adjustment is extremely complicated and is practically impossible.

Therefore, if the second lens 5 is directly integrated into the optical fiber 2, high coupling efficiency cannot be expected.

FIG. 4A is a block diagram of the above-mentioned second lens split type confocal compound lens system that has been proposed to eliminate the above-mentioned drawbacks. A spherical lens is used as the first lens 4, and two converging rod lenses (hereinafter abbreviated as Rondo lenses) 51 and 52 are used as the second lens 5, and the rod lenses 52 are brought into close contact with the optical fiber 2. In the illustrated example, the pitch lengths of the rod lenses 51 and 52 are Φ and (1/4
-Φ), and the sum is 1/4 pitch (here, 0.5 pitch is the length of the lens that images a point light source into a point light source). The rod lens 52 is integrated with the optical fiber 2 using an adhesive or the like, and the alignment of the optical fiber 2 is performed with the rod lens 52 and the optical fiber 2 fixed.

FIG. 4B is a diagram explaining the principle of the lens system shown in FIG. 4A. The coupling characteristics of this lens system are as follows: The spot size of a real image produced when the light beam emitted from the semiconductor laser 1 and whose spot size is converted to _w1 by the first lens 4 passes only through the first rod lens 51.
It is determined from w ₂₁ and its position, and the spot size w w2 of a virtual image created when a light beam of spot size w _f enters the rod lens 52 from the right from the core 3 of the optical fiber ₂ and its position. For example, if the spot size w 22 of a light ray of spot size w ₁ that is generated when it passes through the two rod lenses 51 and ₅₂ matches the spot size w _f , then the real image w ₂₁
and the virtual image w _f2 match in size and position, and in this case, the maximum coupling efficiency can be obtained.

In FIG. 5, the axis deviation amount x in the direction perpendicular to the optical axis of the optical fiber 2 integrated with the rod lens 52 (hereinafter abbreviated as lens 52 + optical fiber 2) is plotted on the horizontal axis, and the coupling efficiency η is plotted on the vertical axis. Show the diagram. When the pitch lengths of the two rod lenses 51 and 52 are 0.06 pitch and 0.18 pitch, respectively,
w ₂₁ = 12.9 μm, and the axis deviation x and angular deviation θ that give a 1 dB loss increase are approximately ±5.9 μm and approximately ±
It will be 56 minutes.

Further, FIG. 6 shows the relationship between the amount of axis deviation z of the lens 52 and the optical fiber 2 in the optical axis direction and the coupling efficiency η. By comparing FIG. 3 and FIG. 6, it can be seen that the second lens division system is extremely loose regarding the allowable amount of axis deviation in the optical axis direction. In the second lens division system, z, which increases coupling loss by 1 dB, is extremely loose at about 400 μm.

In addition, in the second lens division system, the rod lens 52
serves as an anti-reflection plate, and the end face 6 of the optical fiber 2
Since the reflection from the lens is suppressed, the optically polished glass plate 7 in the conventional confocal system is no longer necessary.

As mentioned above, in the conventional confocal system, not only the allowable amount of axis deviation of the optical fiber 2 is small, but also the optically polished glass plate 7 for anti-reflection is required. However, the disadvantage is that the number of lenses is increased by one compared to the conventional confocal system.

Furthermore, when manufacturing a semiconductor laser module, it is necessary to adjust the mutual positions of the lens 5 and the optical fiber 2 in the conventional confocal system, and the rod lens 51 and the lens 52 + the optical fiber 2 in the second lens division system. This positioning work is difficult and significantly reduces the productivity of the semiconductor laser module.

The present invention is characterized by converging or diverging light rays emitted from the first lens to a light entrance element integrated with the second lens.The purpose of the present invention is to achieve extremely high manufacturability, the number of lenses is as small as two, and the tolerance is An object of the present invention is to provide an optical coupling method in which the amount of axis deviation is moderate. The present invention will be explained in detail below with reference to the drawings.

FIG. 7A shows an embodiment of the present invention, and FIG. 7B shows its principle. Parts corresponding to those in FIGS. 1 and 4 are given the same reference numerals. 1st lens 4
A ball lens is used as the second lens 9, and a rod lens is used as the second lens 9. Note that when a spherical lens is used as the second lens 9, it is necessary to leave a space between the lens 9 and the optical fiber 2, and an optically polished glass plate, optical adhesive, matching oil, etc. is inserted in between. Alternatively, it is necessary to apply an antireflection coating to the end face of the optical fiber 2. Hereinafter, the combination of the second lens 9 and the optical fiber 2 will be abbreviated as lens 9+optical fiber 2.

In the embodiments shown in FIGS. 7A and 7B, the distance d ₀ between the first lens 4 and the light emitting surface 8 of the LD is the focal length of the first lens 4 f ₁
For example, the relationship between the focal length f ₂ of the rod lens 9 and the distance d ₁ from the end surface of the rod lens 9 to its principal surface is f ₂ > d ₁ ). The coupling characteristics of this lens system are such that the semiconductor laser beam of spot size w ₀ is connected to the first lens 4 (focal length f ₁ , the semiconductor laser and the first lens
The spot size w ₃₁ of the real image created by converting the spot size w 31 according to the distance d ₀ ) from the optical fiber 2 and its position, and the rays of spot size w _f from the core 3 of the optical fiber 2 are transferred from the right to the lens 9 (focal length f ₂ , the optical fiber It is determined by the spot size w _f2 of the virtual image created when the light is incident on the distance d ₁ between the end surface 6 of the lens 2 and the lens 9 and its position.

These optical parameters w ₀ , f ₁ , d ₀ and
By selecting w _f , f ₂ , d ₁ , the spot sizes w ₃₁ and w _f2 of the real and virtual images and their positions can be matched, respectively, resulting in the best combination.

The distance d ₀ between the light emitting surface 8 of the semiconductor laser 1 and the first lens 4 is determined by the ray matrix (References M. Saruwatari and K. Nawata) from the condition that the virtual image spot size w _f2 and the real image spot size w ₃₁ match. , :
Semiconductor fiber coupler: Appl.Optics,
vol. 18, No. 11, PP. 1847-1856, 1979), it is calculated as follows.

For example, an example of designing using equation (2) will be described below.
First, the lens 9 is a 0.18 pitch rod lens, and the optical fiber 2 is a spot size w _f =.
A 5 μm single mode fiber is provided as a condition.
Therefore, the spot size w _f2 of the virtual image formed by the lens 9 + optical fiber 2 can be determined as follows using equations (3), (4), and (5).

Here, λ is the wavelength (1.3 μm), f is the focal length of the lens 9, and h is the distance from the end surface to the principal surface of the lens 9.

f=1/n ₀ √Asin(2πP) (4) h=1/n ₀ √Atan(π・P) (5) Here, n ₀ is the axial refractive index of lens 9, √ is the focusing parameter, and P is The pitch lengths of lens 9 are 1.59, 0.32 (mm ^-1 ), and 0.18, respectively.

In other words, from equations (4) and (5), f=2.17mm, h=1.25mm
The value of is obtained. Here, the distance h from the end surface to the main surface represents the distance d ₁ from the optical fiber end surface 6 to the equivalent center of the lens 9, and since d ₁ < f, it is difficult to form a virtual image. Recognize. The spot size w _f2 of the virtual image is calculated using these values and formula (3)
Using this, w _f2 =12.9 μm is obtained.

Next, as the first lens 4, the focal length f ₁ is f ₁ = 456μ
m ball lens is used as a semiconductor laser, its spot size w ₀ = 1 μm, and the oscillation wavelength λ = 1.3 μm.
shall be used.

Using equation (2) from the numerical values given above, the distance d ₀ between the light emitting surface 8 and the first lens 4 is obtained as d ₀ =491 μm.

In the present invention, the glass plate 7 in the conventional confocal system or the lens 51 in the second lens division system is not necessary, and when a semiconductor laser package containing the lens 4 is used, only the lens 9 + the optical fiber 2 are required for adjustment. This greatly improves module manufacturing efficiency. Further, since the setting deviation in the x direction perpendicular to the optical axis of the lens 4 is relatively small, the deterioration in coupling efficiency due to the angular deviation remaining after the lens 9 + optical fiber 2 is translated in parallel to correct the positional deviation is small. In a conventional confocal system, the deterioration of the coupling efficiency due to angular deviation is smaller than when the lens 5 and the optical fiber 2 are integrated, because the spot size of the light after exiting from the lens 4 is 189μ in the conventional confocal system.
This is because in the present invention, the size is 12.9 μm, which is less than 1/10 of the size. Note that if it is desired to also correct the above-mentioned angular deviation, the lens 9 + optical fiber 2 may be tilted. In this case as well, since the spot size is approximately 12.9 μm, adjustment is relatively easy.

FIG. 8 shows a diagram in which the axis deviation amount x of the lens 9+optical fiber 2 in the direction perpendicular to the optical axis is plotted on the horizontal axis, and the coupling efficiency η is plotted on the vertical axis. Here, it is assumed that the lens 9 is a 0.18 pitch rod lens. At this time, the axial deviation x and angular deviation θ that cause a 1 dB increase in loss are approximately ±6.3 μm and approximately ±52 minutes, respectively.

Also, lens 9 + optical fiber 2 in the optical axis direction
FIG. 9 shows the relationship between the axis deviation amount z and the coupling efficiency η. z which gives a loss increase of 1 dB is approximately 420 μm. In this way, the allowable axis misalignment in both the optical axis direction and the direction perpendicular to the optical axis is about the same as that of the second lens division system, that is, the one shown in Fig. 4, despite the fact that there is one less lens. The focusing system is much looser than that of FIG.

Furthermore, in the present invention, in order to prevent the misalignment of the central axes of the second lens 9 and the optical fiber 2 that occur when the second lens 9 and the optical fiber 2 are integrated, the integrated second lens 9 and the optical fiber 2 are It has the advantage that it can be easily handled by adjusting perpendicular to the axis. This can be achieved because the light emitted from the first lens 4 is a convergent beam. As a result, optical adjustment is not necessary when integrating the second lens 9 and the optical fiber 2, and the accuracy of machining is sufficient. That is, in the present invention, even if there is a misalignment of the central axes of about several tens of micrometers between the second lens 9 and the optical fiber 2, the coupling efficiency is hardly affected.

Such an advantage of the present invention is that, as already mentioned in the conventional example shown in FIG. Compared to this, it is extremely significant.

In addition, in the present invention, variations in the position of the beam waist w ₂₂ after passing through the second lens due to a fixing error in the optical axis direction of the first lens 4,
By adjusting the distance between and the second lens 9,
We have confirmed that it can be easily corrected.

Furthermore, in this embodiment, when the second lens 9 is integrated with the optical fiber, the end faces of both are bonded together while matching the refractive index with an optical adhesive, so that the entrance end face 6 of the optical fiber is Reflections are reduced. In this way, the optically polished glass plate 7 for antireflection, which was necessary in the conventional confocal system, is no longer necessary.

In the above embodiment, the light emitting surface 8 of the semiconductor laser 1
The case has been described in which the distance d ₀ between the lens and the first lens 4 is set larger than the focal length f ₁ of the first lens 4, and the pitch number of the second lens 9 is shorter than 1/4 pitch. However, by setting d ₀ smaller than f ₁ , the second lens 9
It is also possible to make the length longer than 1/4 of the number of pitches.
In this case, the spot size w ₃₁ after exiting the first lens 4 becomes a virtual image, and the optical fiber 2
The spot size w _f2 generated by entering the second lens 9 from the core 3 becomes the real image. The coupling efficiency from the semiconductor laser 1 to the optical fiber 2 is determined by the spot size w ₃₁ of the virtual image after exiting from the first lens 4 and the spot size w 31 of the real image generated by entering the second lens 9 from the core 3 of the optical fiber 2. It can be calculated as a combination with _f2 .

Furthermore, if we assume that w ₃₁ and w _f2 are equal (spot size matching), then the semiconductor laser 1 and the first lens 4 when expanding the spot size w ₀ of the semiconductor laser 1 to w ₃₁ (=w _f2 ) The distance d ₀ from

Note that the sign before the radical in equation (6) is negative, unlike equation (2), because the output beam of the first lens 4 is
This is because formula (2) is a convergent beam, whereas formula (6) is a divergent beam. Therefore, equation (6)
In the case of a diverging beam, the semiconductor laser 1 and the first
The distance d ₀ from the lens 4 is the focal length f ₁ of the first lens 4
This means that it has become smaller.

Note that there is no difference in performance such as coupling efficiency between the converging beam system and the diverging beam system.

FIG. 10 shows an embodiment in which a light output element and a light input element are arranged opposite to the embodiment shown in FIG. An amplifier can be realized. Here, the first lens is rod lens 1.
3. The second lens constitutes a ball lens 13.

In addition, in FIG. 10, the ball lens 10, the semiconductor laser 11 for light incidence, and a fixing jig are used to integrate them so that their relative positions are fixed.

FIG. 11 shows an embodiment in which a rod lens 9 is used in place of the ball lens 10 in the embodiment shown in FIG. 10, and an optical fiber 2 is used in place of the semiconductor laser 11, making it possible to realize a passive optical circuit.

Note that in FIGS. 10 and 11, the optical fiber 12 and the rod lens 13 may be brought into close contact with each other. Furthermore, the ball lens 10 and the rod lens 13 may be replaced in the embodiment shown in FIG.

LD module with built-in isolator using YIG bulb (Saruwatari/Sugie, 1986 Institute of Electronics and Communication Engineers
316), YIG has a high refractive index and the focal point of the ball lens falls inside the ball, so the light rays emitted from the YIG ball 14 become parallel rays, as shown in Figure 12A. The result will be the limit. For this reason, a conventional confocal lens configuration cannot be used.
In FIG. 12A, 15 is a magnet and 16 is a polarizer.

In the present invention, even if a lens with a high refractive index is used as the first lens, a configuration is possible. The configuration in this case is shown in FIG. 12B. As a result, even in the case of an isolator using a YIG sphere, it is possible to improve the manufacturing efficiency and increase the allowable amount of axial deviation of the optical fiber 2 with the second lens integrated therein.

As explained above, according to the method of the present invention,
With the same number of lenses as in the conventional confocal system, it is possible to obtain the same amount of permissible axis deviation as in the second lens division system, which has a large number of lenses, so that a highly reliable optical coupling device can be constructed. Its manufacturability is significantly improved.

Note that a glass plate for antireflection, which is necessary in conventional confocal systems, is no longer necessary.

Furthermore, if a semiconductor laser coupling device for a multimode optical fiber is constructed using the method of the present invention, the allowable amount of axis deviation will be extremely relaxed, so that further improvement in manufacturing efficiency can be expected.

[Brief explanation of drawings]

Figures 1A and B are configuration examples of conventional confocal compound lens systems and explanatory diagrams of their principles. Figures 2 and 3 are illustrations of conventional confocal systems in the direction perpendicular to the optical axis and the axis in the optical axis direction. Diagrams explaining the deviation characteristics, Figure 4 A, B
is an explanatory diagram of a configuration example of a second lens split type confocal compound lens system and its principle, and Figures 5 and 6 show the axis deviation characteristics of the second lens split system in the direction perpendicular to the optical axis and in the optical axis direction. Figures 7A and 7B are explanatory diagrams of an embodiment of the present invention and its principle, and Figures 8 and 9 are diagrams illustrating axis deviation characteristics in the direction perpendicular to the optical axis and in the optical axis direction. , FIG. 10, and FIG. 11 are diagrams of other embodiments of the present invention, FIG. 12A is a configuration diagram of a conventional LD module, and FIG. 12B is a configuration diagram of an LD module to which the present invention is applied. DESCRIPTION OF SYMBOLS 1... Semiconductor laser for light emission, 2... Optical fiber for light input, 3... Core of optical fiber, 4... First lens, 5... Second lens in conventional confocal system, 51... Third lens A first focusing rod lens of a two-lens division system, 52... A second focusing rod lens of a second lens division system, 6... An end face of an optical fiber, 7
... Glass plate, 8 ... Light emitting surface of semiconductor laser, 9
...Second lens (focusing rod lens) in an embodiment of the present invention, 10...Second lens (spherical lens) in an embodiment of the present invention, 11...Semiconductor laser for light incidence (optical amplifier), 12... ...Light-emitting optical fiber, 13...First in the embodiment of the present invention
Lens, 14...YIG ball, 15...Magnet, 16...
...Polarizer.

Claims (1)

[Scope of Claims] 1. An optical coupling method using two lenses for coupling a light output element and a light input element, at least one of which is composed of a single mode optical waveguide, comprising: the light output element, a first lens; , by arranging the second lens and the light input element in this order, and making the distance between the light output element and the first lens larger than the focal length of the first lens, light is emitted from the light output element. The first ray of light
It is formed as a convergent beam after exiting the lens, the distance between the second lens and the light input element is smaller than the focal length of the second lens, and the light is also output from the light input element in the opposite direction. The parameters of the second lens and the light entrance element are selected so that the spot size of the virtual image of the diverging beam matches the spot size of the real image of the converging beam after exiting the first lens, and under these conditions the second lens and the spot size of the real image of the convergent beam are selected. By integrating the two so as to fix the relative position with the light entrance element, and adjusting the position of the integrated second lens and the light entrance element with respect to the convergent beam emitted from the first lens, An optical coupling method characterized in that a light beam emitted from the light output element is made to enter the light input element. 2. An optical coupling method using two lenses for coupling a light output element and a light input element, at least one of which is a single mode optical waveguide, the light output element, a first lens, a second lens and the By arranging the light input elements in this order and making the distance between the light output element and the first lens smaller than the focal length of the first lens, the light rays emitted from the light output element are directed to the first lens.
Formed as a diverging beam after exiting the lens, assuming that the distance between the second lens and the light input element is greater than the focal length of the second lens, and that the light is emitted from the light input element in the opposite direction. The parameters of the second lens and the light entrance element are selected so that the spot size of the real image of the converging beam matches the spot size of the virtual image of the diverging beam after exiting the first lens, and under these conditions, By integrating the two so as to fix the relative position with the light entrance element, and adjusting the position of the integrated second lens and the light entrance element with respect to the diverging beam after exiting from the first lens, An optical coupling method characterized in that a light beam emitted from the light output element is made to enter the light input element.