JPS60179709A - Optical coupling device - Google Patents

Abstract

PURPOSE:To prevent generation of interferential distortion and noise in transmitting signal light, by setting the interval between the laser light emitting surface of a semiconductor laser and the reflecting surface of the laser light to a specific length. CONSTITUTION:The interval L between the laser light emitting surface of a semiconductor laser and reflecting surface of the laser light is set so as to satisfy the formula I . An optical coupling section is composed of a semiconductor laser 15, laser light emitting surface 15a, active layer 15b, optical fiber 16, light receiving surface 16a, convergent lens 17, and front spherical section 18 of the optical fiber 16. The optical coupling device of this invention is formed by constituting the optical coupling section in the same way as applied to conventional ones and by positioning and fixing each section by means of well known fitting structures. The interval L is set to a value which satisfies the Formula I and, at the same time, the convergent lens 17 or front spherical part 18 having a focal length corresponding to the interval L is selected.

Description

Detailed Description of the Invention (a) Technical Field of the Invention The present invention relates to an optical coupling device between a semiconductor laser and an optical fiber, and particularly relates to a laser beam emitting surface of a semiconductor laser in an optical coupling section and a reflection in an optical coupling circuit. surface (receiving surface of optical fiber,
This invention relates to an optical coupling device in which the distance between the light beam and the light source (others) is set at an appropriate distance to prevent the adverse effects of reflected light.

(b) Background of the Technology Optical coupling devices between semiconductor lasers and optical fibers are often used in optical communication transmission systems. This optical coupling between a semiconductor laser and an optical fiber is very important in creating an optical communication transmission system. That is, even if a low-loss optical fiber is made, if the coupling between the semiconductor laser, which is a light emitting element, and the optical fiber is poor at the optical coupling part, the transmission efficiency will be extremely reduced. For this reason, in order to efficiently input the light emitted from the semiconductor laser into the optical fiber, it is necessary to use a focusing lens in the optical coupling circuit of the optical coupling section, or to devise the shape of the tip of the optical fiber (the shape of the light receiving section). is being attempted. However, this type of optical coupling device has good transmission efficiency and the transmitted signal light is of good quality, that is, it is capable of transmitting stable signal light without coherent distortion or noise. It is hoped that it will be something.

Prior Art and Problems Conventional optical coupling devices have a conventional optical coupling device in which a lens is connected to a lens according to its focal length in an optical coupling section between a semiconductor laser and an optical fiber, in order to input the laser light emitted from the semiconductor laser into the optical fiber as efficiently as possible. An optical coupling circuit is constructed by simply placing it at a predetermined position, and it connects the laser light emitting surface of the semiconductor laser to the reflecting surface of this laser light (the light receiving surface of an optical fiber, the light transmitting window of the semiconductor laser package, etc.) No consideration was given to the coherence of the reflected light, which is related to the distance (interval) between the two. Therefore, conventional optical coupling devices have the problem that the characteristics of the semiconductor laser are often changed by the reflected light, and the reflected light in the optical coupling circuit causes coherent distortion and noise in the transmitted signal light. There was a problem in that it was easy for this to occur.

(→Purpose of the Invention In view of the problems of the prior art described above, the object of the present invention is to etc)
change in characteristics due to reflected light of the semiconductor laser,
An object of the present invention is to provide an optical coupling device that can suppress interference distortion and noise in transmitted signal light by setting intervals such that the coherence due to reflected light in an optical coupling circuit is minimized.

((E) Structure of the invention And in order to achieve the above object, according to the present invention,
In an optical coupling section between a semiconductor laser and an optical fiber in an optical coupling device, a laser light emitting surface of the semiconductor laser;
When the distance between the laser beam and the reflective surface is L, this L
However, C: speed of light (3X 108 m/s, ), Δsit: frequency difference corresponding to longitudinal mode interval, m: integer (0, 1, 2
, 3, 4, -), x knee 0.25<X<0.25.

(F) Embodiments of the Invention Hereinafter, embodiments of the present invention will be described in detail based on the drawings.

1 to 4 are detailed illustrations of the present invention. 1 to 3 are diagrams for explaining the present invention in detail, and FIG. 4 is a diagram showing a main part of an embodiment of the present invention, that is, an optical coupling section.

Semiconductor lasers usually have high-speed modulation (for example, several tens to several hundred MHz, or several tens to several hundred Mb/s).
) is used. When such high-speed modulation is performed, the semiconductor laser oscillates in longitudinal multimodes, as shown in FIG. FIG. 1 is a diagram showing the oscillation spectrum of a longitudinal multimode when the horizontal axis represents the wavelength (λ) region and the vertical axis represents the optical power.

In the figure, Δλt indicates the interval between longitudinal multimodes, that is, the wavelength difference, and numerals 11, 12, and 13 indicate each longitudinal mode. The frequency difference ΔνL corresponding to this longitudinal mode spacing (Δλt) is calculated by the following formula, dνZ = + 22 dλt...
・・・・・・(1), However, C: Speed of light (3X108 le 4
) Fλ: Wavelength, entangled. In addition, dν in the above equation (1)
t is ΔνL, and dλt is Δλt in FIG. For example, in a GaAtAs/GaAs semiconductor laser having a wavelength of 0.8 μm, ΔI/l is about 200 GHz.

FIG. 2 is an explanatory diagram of the reflection state of light within the optical coupling circuit in the optical coupling section. In the same figure, reference numeral 15 is a semiconductor laser, 15a is a laser beam emitting surface, 16 is an optical fiber, 16
a indicates a light receiving surface, and L indicates an interval (distance) between the laser beam emitting surface 15a and the light receiving surface 16a. In the same figure,
A part of the light P emitted from the semiconductor laser 15 is transmitted to the light receiving surface 1.
6a (which also serves as a reflecting surface in this case) returns the reflected light R' reflected by the output surface 15a by a round trip distance, that is, a distance of 2L from the reflected light R reflected by the exit surface 15a.
It returns to the semiconductor laser 15 with a delay of a time corresponding to . Further, most of the output light P is directly transmitted to the optical fiber 16 as transmitted light (incident light) T, but a part of it is reflected by the light receiving surface 16a and the output surface 15a and then transmitted to the optical fiber 16. It is transmitted as reflected and transmitted light T'.

The delay time L between this reflected and transmitted light T' and the above-mentioned feedback reflected light R' is τ-- (where L is the above-mentioned interval and C is the speed of light). The degree of coherence between the returned reflected light R' and the reflected light R that are delayed by this delay time τ, and between the reflected transmitted light T' and the directly transmitted light T is expressed by a coherence function C(τ). .

In Figure 3, the horizontal axis shows the delay time τ9, and the vertical axis shows interference! (degree of coherence) The coherence function C (τ
). As shown in the figure, in a semiconductor laser that oscillates in longitudinal multi-mode, the coherence degree periodically has a plurality of peaks (pt + p2 + p3 . . . ). And these peaks (pl.

p21P3...) is shown by the attenuation function e-rT (where γ is the width of one longitudinal mode, see Figure 1),
Attenuates gradually. Now, the value of τ for each peak is τ-0 for peak I)1 and τ-1/Δsit1 for peak p2.
At peak p3, τ=2/Δνt, . and,
When m=0.1, 2.3, . . . ], the degree of coherence becomes minimum (nearly close to 0). Therefore, by selecting the delay time, interference between R and R', and between T and 77 in FIG. 2 can be minimized.

The spacing at which such minimum coherence can be obtained is L = −'−” (2m +1) ・・・・・・・・・
・・・・・・・・・(2) 4Δν, where C: speed of light (3×108Iv/S), Δsit: frequency difference corresponding to longitudinal mode interval, m: integer (0, 1, 2
,3,...).

However, each peak in Figure 3 (p1+P2+p3
+...) the midpoint between them, 0 the aforementioned τ-both (2m+1
Since the degree of coherence is very small before and after ), there is no problem in practice even if the interval corresponding to the predetermined range of τ is selected before and after this. Therefore, the practical spacing is the following equation, which has a wider range than equation (2) above, L=,,'7(
2m+1+x) ・・・・・・・・・・・・・・・ (
3) However, x: 0.25 (measured by x (a numerical value in the range of 0, 25).

FIGS. 4(a), (b), (e), and (d) show the main parts of the embodiment of the optical coupling device of the present invention, in which the spacing is selected and set according to the above-mentioned formula (3). FIG. 3 is a diagram schematically showing an optical coupling section. In these figures, 15 is a semiconductor laser, 15a is a laser beam emitting surface, 15b is an active layer, 16 is an optical fiber, 16a is a light receiving surface (in this case, it also acts as a reflective surface), 17 is a focusing lens, and 18 Reference numeral 19 indicates a spherical tip of the optical fiber (16) (corresponding to a spherical lens), and 19 indicates a light transmission window when the semiconductor laser (15) is installed in a package or the like. The structure of the optical coupling section shown in each of these figures is the same as the conventional one, and each part is positioned and fixed by a known mounting structure to form an optical coupling device. Therefore, an overall view of the optical coupling device is omitted. Each of these embodiments sets the spacing to (3
) is set to a value that satisfies the equation, and the converging lens 17 or the spherical tip 18 is selected and configured to have a focal length corresponding to this interval. Note that the interval L' in the diagram (C) indicates the interval to the reflective surface of the light transmission window 190, and is added to distinguish it from another interval.
This is substantially the same as the interval and can be determined from the above equation (3). In addition, the reflected light R in FIG. 2 mentioned above,
R' indicates interference with light inside the semiconductor laser 15, and changes the characteristics of the semiconductor laser 15 very sensitively even to weak reflection. The transmitted lights T 1 and T' in FIG. 2 are transmitted to the optical fiber 16 as transmitted light and 1+ signal light.

(G) Structure of the Invention As explained above in detail, the optical coupling device of the present invention includes:
By setting the distance between the laser light emitting surface of the semiconductor laser and the laser light reflecting surface in the optical coupling part according to the above equation (3), the transmission efficiency of the optical power of the transmitted signal light can be improved. In addition, it is highly effective in that changes in characteristics due to reflected light from the semiconductor laser, as well as interference distortion and noise in the transmitted signal light, can be suppressed to a minimum, making it possible to transmit high-quality signal light. This contributes to improving the performance of the device.

[Brief explanation of drawings]

1 to 3 are detailed explanatory diagrams of the present invention, in which FIG. 1 shows the spectrum of longitudinal multimode oscillation of a semiconductor laser, and FIG. 2 shows the light inside the optical coupling circuit in the optical coupling section. Fig. 3 is a diagram of the coherence function C(τ), Fig. 4 (a) and (b). (c) and (d) are diagrams showing a main part (optical coupling section) of an embodiment of the optical coupling device according to the present invention. 15... Semiconductor laser, 15=a... Laser light emitting surface, 16... Optical fiber, 16a... Light receiving surface (also acts as a reflecting surface in this case), 17... Focusing lens, 18... Spherical tip (corresponding to a spherical lens), 19...
Light transmitting window of the package of the semiconductor laser (15) (in this case, also acts as a reflecting surface), L... between the laser light emitting surface (15a) of the semiconductor laser (15) and the laser light reflecting surface. Interval (distance), c...speed of light (3 cut 08 + v
's), Δνt... Frequency difference corresponding to the longitudinal mode interval, m... Integer (0, 1, 2, 3,...), x...
-0,25 (x (numerical value in the range of 0,25) Patent applicant Fujitsu Limited Patent agent Akira Aoki Patent attorney Kazuyuki Nishidate Patent attorney 1) Yukio Patent attorney Akiyuki Yamaguchi 2 Figure 3 Figure 4 Procedural amendment September 7, 1985 Manabu Shiga, Commissioner of the Japan Patent Office 1, Indication of the case 1981 Patent application No. 035127 2, Title of invention Optical coupling device 3, Amendment Relationship with the case involving the person who filed the patent application Patent applicant name C522) Fujitsu Ltd. 4. Agent (3 others) 5. Subject of amendment + 11 "Detailed description of the invention" column 6 of the specification Contents of amendment

Claims (1)

[Claims] 1. In an optical coupling part between a semiconductor laser and an optical fiber in an optical coupling device, when L is the distance between the laser beam emitting surface of the semiconductor laser and the reflecting surface of the laser beam, Noshiga, 1=possible (2″+1+゛)・However, C: Speed of light (3×108 Iv/s), Δsit: Frequency difference corresponding to longitudinal mode interval, m: Integer (0, 1, 2, 3,4,...)x ;-0,25(x
(A photocoupler acid characterized by being set to have a numerical value in the range of 0.25.