JPH07117610B2 - Optical attenuator - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical attenuator used for optical fiber communication, and particularly to provide an optical attenuator having a great effect when used for transmitting an analog optical signal. It is a thing.

2. Description of the Related Art An optical attenuator is an optical attenuator that is inserted in the middle of an optical fiber transmission line to attenuate optical power.
It is used for evaluation of optical transmission systems. Fig. 3 shows the general configuration of an optical variable attenuator that is widely used as an attenuator (reference document Mukai, Kurita et al .; "Optical circuit device" FUJITSU, vol.29, No.3 (1978)). First optical fiber 1
The light from 7 is collimated by the lens 18, attenuated by the rotary type continuously variable attenuation filter 19 and the step variable attenuation filter 20, and focused by the lens 22 to form the second optical fiber.
It is configured to be combined with 23. The prism 20 is used as a front-side operation for ease of use and is not essential.

Problems to be Solved by the Invention When a laser diode (LD) light is attenuated by using the optical attenuator as shown in the conventional example, reflected light generated from the optical components of the attenuator may be a problem. . Drive the LD with a digital signal. In the case of so-called digital optical transmission, even if there is reflected light from each part, it does not particularly hinder the transmission. However, in the case of so-called analog optical transmission in which the LD is directly luminance-modulated with an analog signal, the influence of this reflected light is very serious. This is because in the case of analog transmission, the requirements for noise and distortion become stricter than in the case of digital transmission. For example, when the reflected light from the component enters the fiber and is re-injected into the LD, so-called reflection noise is generated in the LD or distortion is varied. In addition, noise and distortion may occur due to multiple reflections between components.

In the configuration shown in the conventional example, the light emitted from the first fiber is collimated by the two lenses and then focused on the second fiber. Basically, the fiber axis and the optical axis of the lens coincide with each other, and a light attenuator (usually one of parallel plate glass with a metal film evaporated) or a prism is inserted in the part where the light is parallel. To be done. In the reference, by arranging these attenuators and prisms diagonally,
Care is taken so that the reflected light from this part does not return. However, actually, in such a configuration, the reflected light from the end face of the input / output fiber and the reflected light from the end face of the mating fiber become a big problem. In addition, since it is necessary to arrange the optical element at an angle to the lens axis or the fiber axis,
The holder becomes complicated.

Means for Solving the Problems The fiber axis direction and the optical axis of the lens are parallel to each other, and the center position of the fiber emission or incidence end is on the focal plane of the lens and on the lens main plane. Is an optical attenuator having two sets of end face oblique fibers and lenses, which are arranged so that the lens optical axis intersects with the center of emitted light or incident light.

Action The present invention has the above-described configuration, and since the end faces of all the optical components forming the optical attenuator and the traveling direction of light have a certain angle, all the reflected light from the end faces is re-injected into the fiber. It will not be done. Also, the multiple reflection light between the end faces of the component does not enter either the input / output fiber or does not enter the transmission mode in the fiber even when it enters.

Example FIG. 1 is a schematic diagram showing the basic configuration of an optical attenuator in an example of the present invention. In FIG. 1, reference numeral 1 denotes a method such as polishing the output end face at an angle θ _{1 with} respect to a plane perpendicular to the fiber axis.
I slanted it. It is the first fiber for the optical attenuator input. The core part is indicated by diagonal lines. The center of the optical output at the fiber output end (point A in the figure) is on the focal plane (F ₁ ) of the first lens 2, and the optical axis (0 ₁ ) of the first lens,
A certain distance h away.

The light emitted from the fiber is collimated by the lens 1. H ₁ is the principal plane of the first lens. It is arranged so that the optical axis of the first lens and the center of the light emitted from the fiber intersect on the main plane H ₁ (point B). The light passing through the first lens is focused by the second lens 3 and the second fiber 4
Is guided to the core part of. The positional relationship between the second lens 3 and the second fiber 4 is exactly the same as that between the first fiber 1 and the first lens 2. That is, conversely, when light is emitted from the second fiber, this light is parallel to the second lens, and the light center is on the optical axis O ₂ of the second lens and on the main plane H ₂ . It will intersect at point C. F ₂ is second
Is the focal plane of the lens, and point D indicates the center of the light emitting end of the second fiber.

Here, the relationship between the inclination angle θ ₁ of the fiber end face and the distance h between the optical axis of the lens and the center of the fiber end face will be described.
The tilt angle θ ₁ of the fiber end face is selected so that the Fresnel reflected light component generated at this end face is not transmitted in the opposite direction.
If the refractive index of the fiber is n, the angle θ ₂ formed by the light emitted from the oblique end face with the fiber axis is θ ₂ = sin ⁻¹ (n · sin θ ₁ ) −θ ₁ (1). Here, if the focal length of the lens is f, and h = f tan θ ₂ (2), the fiber output light intersects the lens optical axis on the main plane of the lens.

As shown in FIG. 1, two sets of a pair of diagonal end face fibers and lenses having such a positional relationship are arranged so as to be point symmetrical. The light attenuating element is placed between the two lenses. In the conventional optical attenuator, the large amount of reflected light that is re-injected into the fiber in the opposite direction out of the reflected light generated at each end face is caused by the part where the light is parallel and the reflection from the other end face of the fiber. Light. This is because the reflected light from this portion is mostly focused on the core portion of the fiber. In the configuration of the present invention, the reflected light from the fiber end surface (point D) of the partner is transmitted through the two lenses in opposite directions and is focused at point A, but the incident angle to the fiber becomes very large. , There is no reverse transmission in the fiber. Next, the part where the light is parallel (H ₁ and H _{2 in} Fig. ₁₎
The reflected light from (between) passes through the lens 2 and is focused at a position on the focal plane F ₁ that is completely different from the point A, so that it does not re-enter the core portion of the fiber. Also, the first
In the figure, for the sake of clarity, the lens is represented by a normal thin lens, but for optical fiber transmission, a GRIN (gradient distribution type) rod lens of gradient index type is used as a lens with a half pitch. The closest one is used. Since the end surface of the rod lens is a plane perpendicular to the optical axis, when such a lens is used, the structure of the conventional example is easily affected by the reflected light from the end surface. However, in the present invention, even if the GRIN rod lens is used, as described above,
The reflected light from all end faces is never transmitted in the opposite direction. Therefore, when used for analog optical transmission, there is no deterioration in transmission characteristics due to the insertion of the optical attenuator, and extremely low noise and low distortion transmission is possible.

Next, a method of adjusting the amount of light attenuation will be described. In the configuration of the present invention, since the reflected light is not transmitted in the opposite direction at all, as described above, it is possible to insert the optical attenuating element composed of parallel planes between the two lenses. In this case, as the light attenuating element, it is possible to use an ordinary rotatable continuously variable attenuating filter in which a metal is vapor-deposited on one surface of parallel flat glass. Moreover, since the insertion angle of this filter may be perpendicular to the optical axis of the lens, it is very easy to mount.

As a second adjusting method, there is a method of changing the coupling efficiency from the first fiber to the second fiber by shifting the distance between the optical axes of the two sets of fibers and the lens. In this case, the direction of displacement is perpendicular to the optical axis of the lens, which has the advantage of simplifying the adjustment mechanism.

Further, as a merit when the configuration according to the present invention is taken, it is only necessary to fabricate the one in which the oblique end face fiber and the lens are integrated. There is also a good thing.

The next embodiment is shown in FIG. This is a metal film 7 on the end surface of two GRIN rod lenses 5, 6 with a length close to 1/2 pitch.
It is used by vapor-depositing and bonding as shown in the figure. This case corresponds to the case where the distance 1 between the lens main surfaces in FIG. 1 becomes zero. Reference numeral 8 is a holder for holding the lens and at the same time placing the two optical adapters 9 and 14 in a predetermined positional relationship. The hole direction of the lens and the both end faces of the holder are vertical. An optical connector 10 in which a tip of a connector plug 11 is polished obliquely is fitted into the optical adapter 9. connector
Reference numeral 10 is shown by a halftone dot in the figure, and is integrated with the connector plug 11. In the figure, the connector portion is drawn only on the right side portion, but the connector is also arranged on the left side portion so that the diagonal end faces are parallel. In the figure, only the connector plug 15 is shown. Plug center line 12,16
Is a fiber. Reference numeral 13 is a screw for fixing the adapter 9 to the holder 8.

As the optical connector 10 and the adapters 9 and 14, those which are usually on the market can be used, and it suffices to polish the tip of the connector plug by a predetermined angle. As for the attachment of the adapter, first, one of them is attached so that the center of the holder hole and the center of the adapter are displaced by the amount represented by the formula (2), then the optical connector is inserted, and the connector is vapor-deposited on the other one. The adapter may be fixed to the holder 8 at the point where the passing power is maximized while closely contacting the holder 8.

In this case, the lens itself may be a general rod lens, and general adapters and connectors can be used.
Further, the holder part is also easy to process because the lens hole and the adapter hold surface are vertical and there is no need for complicated angle control.

EFFECTS OF THE INVENTION As described above, according to the present invention, since the reflected light or the multiple reflected light is not transmitted through the fiber, it is possible to provide an optical attenuator that does not affect the analog transmission characteristics at all. You can In addition, the manufacturing procedure, process, and assembly are simple.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a basic configuration of an optical attenuator according to an embodiment of the present invention, FIG. 2 is a specific configuration diagram of the attenuator, and FIG. 3 is a configuration diagram of an attenuator of a conventional example. 1,4 …… Fiber, 2,3 …… Lens, 5,6 …… Rod lens, 7 …… Vaporized film, 8 …… Holder, 9,14 …… Adapter, 10 …… Connector, 11,15 …… Connector plug.

Claims (4)

[Claims] 1. A fiber having two oblique end faces and two lenses, wherein the fiber axis direction and the lens optical axis are parallel to each other, and the center position of the fiber exit (or entrance) end is the focal plane of the lens. The optical attenuator characterized in that the lens and the fiber are arranged so that the lens optical axis and the outgoing (or incident) light center intersect with each other on the principal plane of the lens. 2. An optical attenuator according to claim 1, wherein a rotatable continuously variable attenuation filter is inserted perpendicularly to the lens optical axis between the two lenses. 3. An optical attenuator according to claim 1, wherein the attenuation amount is changed by adjusting the position of a set of a lens and a fiber having an oblique end face. 4. A GRI having a length of about 1/2 pitch as a lens
2. The optical attenuator according to claim 1, wherein an N rod lens is used, an attenuation film is provided at one end of the first lens, and the attenuation film is brought into close contact with one end of the second lens.