JPH0621485A - Semiconductor optical detection module - Google Patents

Abstract

PURPOSE:To achieve a higher multiplication rate and reduce a dark current upon multiplication by providing an APD chip and optical fibers, and irradiating an optical detection part on the ADP chip with light emitted from the optical fibers in a spot size of a predetermined ratio or more of an optical detection diameter. CONSTITUTION:A positional relation among single mode optical fibers 1, a rod lens 3, and an APD chip 5 is adjusted such that a spot size of an optical beam 21 is 50% or more of an optical detection diameter on an optical detection part 5a of the ADP chip 5. Such irradiation with the optical beam 21 on the optical detection part 5a of the ADP chip as becoming 50% or more of an optical detection part diameter is because of a reason that optical intensity per unit area at the optical detection part 5a is reduced and a space charge effect is restricted. When the optical beam 21 is out of the optical detection part 5a, a quantum efficiency is reduced and response is deteriorated owing to absorption outside an depletion layer of photons. Accordingly, taking into consideration the reliability of an optical coupling, the upper limit of the spot size of the optical beam 21 is taken to be 90% of the optical detection part.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor light receiving module, and more particularly to a semiconductor light receiving module using an avalanche photodiode (APD) chip.

[0002]

2. Description of the Related Art A semiconductor light receiving element is usually used by being optically coupled with an optical fiber when used for optical fiber communication or optical fiber measurement. To make such optical coupling easy, a semiconductor light receiving element is used. It is customarily used as a semiconductor light receiving module in which an element chip and an optical fiber are fixed at a position where the optical coupling efficiency is maximized.

As such a semiconductor light receiving module,
There are two types of semiconductor light receiving elements used for optical fiber communication and optical fiber measurement: PIN PD and APD. Among them, the APD has a function of amplifying photocurrent inside the element itself, so that a high receiving sensitivity is required especially in optical fiber communication, or an optical fiber pulse tester (OTDR) for detecting a breaking point of an optical fiber. Used in optical measuring instruments.

In such application fields, in order to achieve excellent performance, the APD element is required to have a low dark current at the time of multiplication and be capable of realizing a high photocurrent multiplication factor.
In particular, when it is used for OTDR, it operates at a high multiplication factor of about 30, and it is necessary to suppress the dark current at this time to a low value of 100 nanoamperes or less.

FIG. 4A shows an example of a conventional semiconductor light receiving module, which is a single mode optical fiber 1 attached to a holder 4, a rod lens 3 and a ferrule 2.
The slide ring 8, the stem 6 having the APD chip 5 mounted thereon, and the cap 7. The rod lens 3 is for narrowing the light beam 21 emitted from the single mode optical fiber 1 to the light receiving portion 5a on the APD chip 5 having a diameter of several tens of microns (usually 30 to 100 microns).

In order to obtain a high degree of reliability in the stability of optical coupling, the light beam is usually narrowed down to the smallest possible beam diameter on the light receiving portion, and the positions of the optical fiber 1, rod lens 3 and APD chip 5 are positioned. When using a single mode optical fiber with a core diameter of 10 microns,
As shown in FIG. 4B, the light beam 21 is adjusted and fixed so as to have a positional relationship such that the light beam 21 is narrowed down to a diameter of about 7 microns on the light receiving portion 5a of the APD chip 5. The slide ring 8 is for fixing the ferrule 2 and the holder 4 in an arbitrary positional relationship.

[0007]

However, in the conventional semiconductor light receiving module, the photocurrent multiplication factor of the APD element is such that when the incident light intensity is as high as several hundreds of microwatts, the center of the multiplication area inside the APD element. It is known that the electric field strength is lowered in the vicinity due to the space charge effect, so that the increase of the photocurrent is suppressed and thus the electric field strength is lowered.

In optical fiber communication and optical fiber measurement, the incident light intensity is usually as low as several microwatts,
When the light beam is squeezed and irradiated onto the light receiving portion, that is, when the light intensity per unit area becomes strong, the multiplication factor is suppressed due to the influence of the space charge effect.

That is, in the above-described conventional semiconductor light receiving module, the light beam 2 is received on the light receiving portion 5a of the APD chip.
1 has been narrowed down to a diameter of 7 μm, and there is a drawback that it is difficult to obtain a high multiplication factor because an increase in photocurrent is suppressed by receiving the above-mentioned space charge effect. When the light beam is narrowed to a diameter of 7 microns on the light receiving portion of the APD chip, for example, in InGaAs APD, the dark current at the multiplication factor 30 is about several hundred nanoamperes if the incident light intensity is about 1 microwatt. This value poses a problem in applications such as OTDR, where a low dark current of 100 nanoamperes or less is required as a dark current at a multiplication factor of 30.

In view of the above-mentioned drawbacks, a technical object of the present invention is to provide a semiconductor light receiving module which achieves a high multiplication factor and reduces the dark current at the time of multiplication.

[0011]

A semiconductor light receiving module of the present invention has an APD chip and an optical fiber, and the light emitted from the optical fiber is 50% or more of the light receiving diameter in a light receiving portion on the APD chip. It is characterized by being irradiated with a spot diameter of.

[0012]

Embodiments of the present invention will now be described with reference to the drawings.

Embodiment 1 FIG. 1A shows a semiconductor light receiving module which is an embodiment of the present invention. According to this embodiment, the semiconductor light receiving module 1
0 is a holder 4, a rod lens 3, a single mode optical fiber 1 attached to a ferrule 2, a slide ring 8, a stem 6 having an APD chip 5 mounted thereon,
It is composed of a cap 7. The function of each part is the same as the conventional one, but the positional relationship between the single mode optical fiber 1, the rod lens 3 and the APD chip 5 is such that the spot diameter of the light beam 21 is the light receiving part of the APD chip 5 as shown in FIG. It is adjusted to be 50% or more of the light receiving diameter on 5a. For example, when the light receiving portion 5a has a diameter of 50 μm, the beam diameter is adjusted to be 25 μm or more. The light beam 21 is irradiated onto the light receiving portion 5a of the APD chip 5 so as to have a diameter of 50% or more of the light receiving portion in order to reduce the light intensity per unit area in the light receiving portion 5a and suppress the space charge effect. Is. When the light beam 21 is expanded as much as possible and applied to the light receiving portion of the APD chip 5, the effect of suppressing the space charge effect becomes more remarkable.
If it is out of the range of a, the quantum efficiency is lowered and the response is deteriorated due to absorption of photons out of the depletion layer. There is.

Regarding the stability of the optical coupling, the rod lens 3 and the holder 4 are fixed by soldering, the ferrule 2 and the slide ring 8, the slide ring 8 and the holder 4, and the cap 7 (or stem 6) and the holder 4. YAG fixed between
High reliability can be obtained by performing laser welding.

As described above, in the semiconductor light receiving module of this embodiment, the light emitted from the optical fiber is irradiated with the spot diameter of 50 to 90% of the light receiving portion on the APD chip, as shown in FIG. Since the required multiplication factor can be obtained with a lower bias voltage, the dark current value at a certain multiplication factor is reduced. For example, the dark current value at a multiplication factor of 30 is about 100 nanoamps when the spot diameter of the light beam on the light receiving portion is 50% of the light receiving diameter, as compared to the conventional several hundred nanoamps.
When the light beam is further expanded to 80% of the light receiving diameter, it becomes several tens nanoamperes, which is reduced by about one digit compared with the conventional case (see FIG. 2).

In this embodiment, as the optical fiber, a single-mode optical fiber is used, which has a small core diameter so that the emitted light is usually narrowed to a small beam diameter. However, a multi-mode optical fiber can also be used.

Second Embodiment Next, a second embodiment of the present invention will be described with reference to FIG. In the present embodiment, as shown in FIG. 3, unlike the first embodiment, there is no rod lens, the curved lens 9 having a relatively large aberration is fixed to the cap 7, and the curved lens 9-APD. As for the distance between the chips 5, the light beam 21 is AP
It is determined to give a multiplication factor that cannot be reduced to less than 50% of the light receiving portion of D. This type of curved lens integrated with the cap is originally a lens with relatively large aberration, but since it can be used without problems in an optical system that expands a light beam like the present invention, it can be used like a rod lens. It is possible to omit expensive optical components.

Further, in the first embodiment, in the optical coupling work in the module manufacturing process, even if the position is adjusted so that the optical coupling efficiency is maximized, the beam diameter of the light beam on the light receiving portion becomes to what extent. It is difficult to confirm whether or not the manufacturing process is complicated. On the other hand, in the second embodiment, it is only necessary to adjust the position so as to maximize the optical coupling efficiency between the optical fiber and the APD.

As described above, in the second embodiment, the number of parts and man-hours can be reduced as compared with the first embodiment, so that the cost can be reduced.

[0020]

As described above, in the semiconductor light receiving module of the present invention, the light emitted from the optical fiber is irradiated to the light receiving portion on the APD chip with a spot diameter of 50% or more of the light receiving diameter, so that it is high. The photocurrent multiplication factor can be obtained, and the dark current at the time of multiplication can be reduced.

[Brief description of drawings]

FIG. 1A is an embodiment of a semiconductor light receiving module of the present invention. (B) is a figure which shows the optical coupling system of (a).

FIG. 2 is a graph illustrating a difference in characteristics between the present invention and a conventional semiconductor light receiving module.

FIG. 3 shows a second embodiment of the semiconductor light receiving module of the present invention.

FIG. 4A is a diagram showing an example of a conventional semiconductor light receiving module. (B) is a figure which shows the optical coupling system of (a).

[Explanation of symbols]

 1 single mode optical fiber 2 ferrule 3 rod lens 4 holder 5 APD chip 5a light receiving part 6 stem 7 cap 8 slide ring 9 curved lens 10 semiconductor light receiving module 21 light beam

Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location 8422-4M H01L 31/10 B

Claims (1)

[Claims] 1. A semiconductor light receiving module having an avalanche photodiode chip having a predetermined light receiving diameter and an optical fiber for emitting light and optically coupling the light to the avalanche photodiode chip with a predetermined spot diameter. The semiconductor light receiving module, wherein the fiber has a spot diameter of 50% or more of the light receiving diameter of the avalanche photodiode chip.