JP3589338B2 - Optical transmission module - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical transmission device used for optical communication, and more particularly, to an optical transmission module including an optical fiber, a light receiving element, and a lens for optically coupling the optical fiber and the light receiving element.
[0002]
[Prior art]
FIG. 3 shows a conventional example of this type of optical transmission module. This optical transmission module includes an optical fiber 31, a lens 32 for condensing light emitted from the optical fiber 31, and a light receiving element (semiconductor light receiving element) 33.
[0003]
Light emitted from the optical fiber 31 is condensed by the lens 32 and forms an image at the focal point 341 on the optical axis 35. Reference numeral 34 in the figure indicates an optical path until the light emitted from the optical fiber 31 reaches the light receiving element 33.
[0004]
Here, the light receiving element 33 is positioned such that the optical axis 35 passes vertically through the center of the light receiving area and the center of the light receiving area of the light receiving element 33 is located at the focal point 341.
[0005]
Next, the operation of the optical transmission module shown in FIG. 3 will be described.
[0006]
The light emitted from the optical fiber 31 is collected by the lens 32 and enters the light receiving area of the light receiving element 33. The incident light generates carriers in the depleted light absorbing layer. The carriers are accelerated by the electric field generated by the reverse bias voltage applied to the light receiving element 33. Therefore, the photocurrent can be extracted to the outside and can be detected as an optical signal.
[0007]
By the way, in an optical transmission module used in an analog transmission system such as an optical CATV, an extremely low intermodulation distortion characteristic is required. Specifically, a target value of second-order intermodulation distortion <-80 dBc must be satisfied.
[0008]
However, in the conventional optical transmission module shown in FIG. 3, since the light receiving element 33 is located at the focal point 341, the density of carriers generated in the light receiving area is high, and the above-mentioned secondary intermodulation distortion is caused by the space charge effect. It was difficult to satisfy the target value of
[0009]
Here, the space charge effect refers to a phenomenon that the original electric field distribution is distorted because the electric field in the depletion layer is canceled by a large amount of generated carriers, and moves into the depletion layer due to the distortion of the electric field distribution in the depletion layer. Fast carriers and slow carriers are generated. As a result, secondary intermodulation distortion occurs and the transmission characteristics deteriorate.
[0010]
To solve such a problem, there is a semiconductor light receiving device disclosed in Japanese Patent Application Laid-Open No. 5-224101. FIGS. 4A and 4B show this semiconductor light receiving device.
[0011]
This semiconductor light receiving device includes an optical fiber 41, a lens 42 for condensing light emitted from the optical fiber 41, and a light receiving element 43. The light emitted from the optical fiber 41 is provided on the surface of the light receiving element 43. The light is focused rearward or forward.
[0012]
That is, in the case of FIG. 7A, the light receiving element 43 is positioned such that the light receiving area is in front of the focal point 441. In the case of FIG. 3B, the light receiving element 43 is positioned so that the light receiving area is behind the focal point 441.
[0013]
Therefore, in any case, since the defocused light beam enters the light receiving region of the light receiving element 44, the amount of light per unit area in the light receiving region decreases. For this reason, the density of carriers generated in the light receiving region decreases, and the effect of the space charge effect decreases. As a result, the secondary intermodulation distortion is reduced, and the transmission characteristics are improved as compared with the optical transmission module shown in FIG.
[0014]
[Problems to be solved by the invention]
However, the semiconductor light receiving device disclosed in JP-A-5-224101 has the following problems.
[0015]
Now, in this semiconductor light receiving device, the mounting position of the light receiving element 43 is shifted in the direction away from the focal point 441 on the optical axis (the optical axis direction in FIG. 4A, the direction opposite to the optical axis in FIG. 4B). The following describes the case where
[0016]
If the area of the light flux condensed by the lens 42 on the light receiving element 43 becomes larger than the area of the light receiving area due to a positional shift of the mounting position of the light receiving element 43 in the optical axis direction, the amount of light entering the light receiving area decreases, The optical coupling efficiency between the fiber 41 and the light receiving element 43 rapidly decreases.
[0017]
Here, when a certain minimum target value is set for the optical coupling efficiency, the allowable amount of the mounting position in the optical axis direction is determined.
[0018]
However, the structure of the semiconductor light receiving device shown in FIG. 4 has a new problem that the adjustment of the position of the light receiving element 43 in the optical axis direction cannot be simplified because the allowable mounting position of the light receiving element 43 in the optical axis direction cannot be increased.
[0019]
That is, although there is an advantage that the transmission characteristics can be improved as compared with that shown in FIG. 3, it is necessary to prevent a sharp decrease in the optical coupling efficiency, so that the position adjustment of the light receiving element 44 in the optical axis direction is very troublesome. There is a problem that it becomes something.
[0020]
The present invention has been made in view of such a situation, and together with the improvement of the transmission characteristics, the permissible mounting position of the light receiving element in the optical axis direction can be increased, and as a result, the light receiving element in the optical axis direction can be increased. It is an object to provide an optical transmission module that can simplify position adjustment.
[0021]
[Means for Solving the Problems]
An optical transmission module according to the present invention includes an optical fiber , a lens whose optical axis is aligned with the optical axis of the optical fiber, for condensing light emitted from the optical fiber, and a lens. A light receiving element arranged to receive the light to be emitted in a defocused state , and wherein the lens has two or more secondary intermodulation distortions less than -80 dBc on the optical axis. This is a lens having a focal point, and the light receiving element is arranged between these focal points, thereby achieving the above object.
[0022]
Hereinafter, the operation of the present invention will be described.
[0023]
First, in the present invention, since the light receiving element is disposed between a plurality of focal points, a light beam enters the light receiving region in a defocused state. Therefore, since the influence of the space charge effect can be reduced, the transmission characteristics can be improved as compared with the case of FIG.
[0024]
In addition, according to the present invention, since a lens having a plurality of focal points is used as the lens, the allowable mounting position of the light receiving element in the optical axis direction can be increased as compared with the semiconductor light receiving device of FIG. The reason will be described below with reference to FIG. 2 by taking a case where a lens having two focal points is used as an example.
[0025]
Here, a curve E1 in FIG. 2 shows a change in optical coupling efficiency between the optical fiber and the light receiving element when the lens 42 having one focus as shown in FIG. 4, and a curve E2 has two focuses. 5 shows a change in optical coupling efficiency when a lens is used.
[0026]
The permissible mounting positions in the optical axis direction determined by the minimum target value E0 of the optical coupling efficiency are Z1 and Z2 in curves E1 and E2, respectively. The permissible amounts of the system using the lens 2 having two focal points (see FIG. 1) are shown. Is larger (Z2> Z1).
[0027]
This is because the curves E21 and E22 of the light coupling efficiency between the light emitted from the regions 2-1 and 2-2 of the lens 2, ie, the lenses 2-1 and 2-2 (see FIG. 1) and the light receiving element, respectively. When the two curves E21 and E22 are superimposed because of the displacement in the axial direction, the maximum value of the optical coupling efficiency decreases, but the flat portion where the optical coupling efficiency does not change much increases.
[0028]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be specifically described with reference to the drawings.
[0029]
(Embodiment)
1 and 2 show an embodiment of the optical transmission module of the present invention. As shown in FIG. 1, the optical transmission module includes an optical fiber 1, a lens 2 for condensing light emitted from the optical fiber 1, and a light receiving element (semiconductor light receiving element) 3. However, this lens 2 is a lens having two focal points, and this point is clearly different from the configuration of the conventional example shown in FIGS. The details of the lens 2 will be described below.
[0030]
As shown in FIG. 1, the lens 2 is a compound lens in which two lenses 2-1 and 2-2 are coaxially arranged, and each of the lenses 2-1 and 2-2 has a different position on the optical axis 5. It has focal points 4-1 and 4-2. The light receiving element 3 is disposed between the focal points 4-1 and 4-2 on the optical axis 5, and the optical axis 5 passes through the center of the light receiving area of the light receiving element 3 vertically.
[0031]
In the above configuration, light emitted from the optical fiber 1 is condensed by the lenses 2-1 and 2-2, respectively, and forms an image at the focal points 4-1 and 4-2 on the optical axis 5.
[0032]
Since the optical transmission module of the present embodiment includes the lens 2 having two focal points as the condensing lens, the optical transmission module in the optical axis direction of the light receiving element 3 as compared with the semiconductor light receiving device of FIG. Can be increased.
[0033]
The reason will be described below with reference to FIG. However, FIG. 2 shows the relationship between the relative position of the lens 2 and the light receiving element 3 and the coupling efficiency to the light receiving element 3.
[0034]
Here, a curve E1 in FIG. 2 shows a change in optical coupling efficiency between the optical fiber 1 and the light receiving element 3 when the lens 42 having one focus as shown in FIG. 5 shows changes in optical coupling efficiency when a lens 2 having the following is used.
[0035]
The permissible mounting positions in the direction of the optical axis 5 determined by the minimum target value E0 of the optical coupling efficiency are Z1 and Z2 in the curves E1 and E2, respectively, which is the permissible amount of the system using the lens 2 having two focal points in FIG. Is larger (Z2> Z1).
[0036]
This is because the curves E21 and E22 of the light coupling efficiency between the light emitted from the lenses 2-1 and 2-2 and the light receiving element 3 in the regions 2-1 and 2-2 of the lens 2, respectively, in the direction of the optical axis 5. Since the two curves E21 and E22 overlap each other, the maximum value of the optical coupling efficiency decreases, but the flat portion where the optical coupling efficiency changes little increases.
[0037]
In addition, in the light transmission module of the present embodiment, similarly to the semiconductor light receiving device of FIG. 4, a light beam enters the light receiving region of the light receiving element 3 in a defocused state, and the effect of the space charge effect can be reduced. It is possible to improve the transmission characteristics as compared with the case of.
[0038]
As a result, according to the optical transmission module of the present embodiment, in addition to the improvement of the transmission characteristics, the allowable amount of the mounting position of the light receiving element in the optical axis direction can be increased, and as a result, the position adjustment of the light receiving element in the optical axis direction can be performed. The effect that simplification is possible can be obtained.
[0039]
In the embodiment shown in FIG. 1, the lens 2 having two focal points is used as the lens. However, the same effect can be obtained even when a lens having three or more focal points is used for the same reason as described above. It is possible.
[0040]
【The invention's effect】
According to the optical transmission module of the present invention described above, a lens having two or more focal points on the optical axis is used as the lens, and the light receiving element is arranged between these focal points. Along with the improvement of the characteristics, the permissible amount of the mounting position of the light receiving element in the optical axis direction can be increased, and as a result, the adjustment of the position of the light receiving element in the optical axis direction can be simplified.
[Brief description of the drawings]
FIG. 1 is a sectional view of an optical transmission module according to an embodiment of the present invention.
FIG. 2 is a graph showing a change in optical coupling efficiency with respect to a relative position between a lens and a light receiving element.
FIG. 3 is a sectional view showing a conventional example of an optical transmission module.
4A and 4B show another conventional example of an optical transmission module, in which FIG. 4A is a cross-sectional view when a light receiving area of a light receiving element is located in front of a focal point, and FIG. FIG. 3 is a cross-sectional view in a case where is positioned behind a focal point.
[Explanation of symbols]
REFERENCE SIGNS LIST 1 Optical fiber 2 Lens 2-1 2-2 Lens 2 constituting lens 2 Light receiving element 4-1 Focus 4-2 created by lens 2-1 Focus 5 created by lens 2-2 Optical axis

Claims (1)

An optical fiber , a lens whose optical axis is aligned with the optical axis of the optical fiber for condensing light emitted from the optical fiber, and a light focused by the lens in a defocused state. And a light receiving element arranged to receive light at a second order, wherein the second order intermodulation distortion is less than -80 dBc .
An optical transmission module in which the lens has two or more focal points on an optical axis, and the light receiving element is arranged between the focal points.

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