JPS61158306A - Numerical aperture convertor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention [Field of Industrial Application] This invention relates to optical fibers, and in particular to an aperture that minimizes losses due to numerical aperture mismatch when connecting optical fibers with cores of different diameters. It is related to number conversion KA@.

[Technical background of the invention] The light collection ability of an optical fiber is determined by the numerical aperture (NA) of the optical fiber.
It is called. In an optical fiber whose refractive index changes stepwise, its NA is expressed by the following formula.

where: nl - refractive index of the core of the optical fiber n2 - refractive index of the cladding of the optical fiber For graded refractive index optical fibers, the formula for NA is much more complex and is not shown here.

The greater the difference in refractive index between the core and the cladding or medium surrounding the core, the greater the NA of the optical fiber. It can be seen that since the optical fiber that emits more light emits over a wider range of angles, the optical fiber with a larger NA can collect a larger proportion of its light. Similarly, if the light source is large compared to the optical fiber, a large diameter optical fiber is preferred.

The angular distribution of the radiated output power is e.g.
The overall numerical aperture of the output optical system can be reduced by utilizing optical fibers with . A large area receiving device can be obtained by using an optical fiber with a large area core and therefore a large NA.

It has already been recognized in fiber optics that by selecting an output optic with a sufficiently low value of the numerical aperture, the required narrow output power angular distribution can be obtained. The numerical aperture through the rest of the system must be at least as large as the effective output numerical aperture to avoid losses.

If the numerical aperture through the rest of the system is less than the output device's numerical aperture, a loss occurs, which is equal to 2
It is proportional to the ratio of the power. Therefore, it is important in conventional fiber optic systems that all numerical apertures are matched to avoid unnecessary losses.

The use of large area fiber optic receivers typically precludes bidirectional operation of the system.

If the “forward” direction of the system is adopted,
Radiation exits the system at standard numerical aperture and is received by a larger area receiver. Then, “
In the opposite direction of operation, a large area is used as the output device, resulting in losses when light is transmitted from a large NA to a small N,A. Typically, the numerical aperture of an optical fiber is increases with the increase in the core dimension and area of the optical fiber.Thus, bidirectional operation of systems utilizing both forward and reverse directions is hampered by losses caused by area mismatch and numerical aperture mismatch.These Losses occur primarily in the reverse direction, going from a high numerical aperture, large area optical fiber to a low numerical aperture, small area optical fiber.Loss due to area mismatch is proportional to the ratio of the square of the core radius.

Problem to be Solved by the Invention The aim of the invention is therefore to avoid the drawbacks of the prior art.

More specifically, it is an object of the invention to provide a device for reducing the numerical aperture of an optical fiber emitting light in order to match the numerical aperture of an optical receiver.

Another object of the invention is to preserve the numerical aperture of the basic system while reducing the output power angular distribution.

Yet another object of the invention is to provide a numerical aperture conversion device that provides a large area receiving optical fiber that can be used without changing the numerical aperture of the underlying system.

An additional object of the invention is to substantially match the core diameters of two different optical fibers having different core diameters.

[IIAMJ of the Invention In order to achieve the above objects and other objects that will be made clear by the following description, the present invention provides an apparatus for matching the numerical aperture of a light emitting light waveguide to the numerical aperture of a light waveguide receiving device. It is characterized by

[Example] Referring to the drawings, a numerical aperture conversion device 1 is constituted by an optical waveguide 3 having a core 7 and at least one cladding @B. The figure also shows a further optical waveguide 2 with a central core 4 and at least one cladding layer 5, and a further optical waveguide 11.

The light waveguide 3 has a tapered portion 9 over most of its length which tapers in the direction of the end face forming an interface 6 with the end face of the light waveguide 2 .

The optical waveguide 3 has an interface 6 including a core 7 and a cladding H8.
is tapered such that its outer diameter at is substantially equal to the diameter of the core 4 of the light waveguide 2 . Optical waveguide 3
The tapered portion 9 is hereinafter referred to as the "horn".

10 in the figure shows the interface between the numerical aperture conversion device 1 and the optical waveguide 11. In FIG. This light waveguide 11 may be a light receiving device such as a photodetector or a lens. The light waveguide 11 may also be spaced a certain distance from the end face of the light waveguide 3 such that the light emitted from the end face of the light waveguide 3 traverses a certain free space to reach the light waveguide 11. good.

In one operation of the numerical aperture conversion device 1, the optical waveguide 2 is an optical fiber, and together with the numerical aperture conversion device 1 connected thereto, forms a transmitting device for transmitting light to the optical waveguide 11, and the optical waveguide 2 is an optical fiber. The wave body 11 then operates as an optical receiver. The shape of the tapered portion 9 allows light to propagate from the light waveguide 2 in the form of an optical fiber through the light waveguide 3 in the following manner. That is, the light propagating through the core 4 of the optical waveguide 2 toward the optical waveguide 3 passes through the boundary surface 6.
reach. Light passes through the interface 6 and is transmitted by both the core 7 and the cladding layer 8 of the light waveguide 3. The angle of the tapered portion 9 is such that a portion of the light propagating in the cladding layer 8 in the cladding mode enters the core 7 and continues to propagate in the optical waveguide 3 along the core 7 in the core mode. The steepness of the angle at which the light is reflected along the tapered portion 9 is reduced as the light propagates from the narrower portion to the thicker portion of the tapered region. Therefore, the angle of incidence and the angle of reflection increase. Also, the angle between the line perpendicular to the interface 6 and the incident and reflected optical paths also decreases.

Therefore, a reduction in numerical aperture occurs. Loss due to alpha profile mismatch typically occurs at the interface, but it is minor.

With this device, a given light emitting device with a given numerical aperture can be changed to a light emitting device with a smaller numerical aperture without changing the numerical aperture of either the light transmitting device or the light receiving device. In this device, the interface 10
The outer diameter of the optical waveguide 11 at is preferably the same as or larger than the outer diameter of the optical waveguide 3 at the interface 10.

The numerical aperture conversion device of the present invention has been described above with respect to the optical waveguides 2 and 3 that operate as an optical transmitter, and the optical waveguide 11 that operates as an optical receiver. It can also be used for propagation. That is, in that case, the optical waveguide 11 constitutes an optical transmitter, and the optical waveguides 2 and 3 constitute an optical receiver. This modification will be explained below.

Referring again to the drawings, the tapered end of the tapered portion 9 of the optical waveguide 3 abuts the core 4 of the receiving optical waveguide 2. In this case as well, the outer diameter of the portion including the core 7 and the cladding layer 8 provided at the interface 6 of the tapered portion 9 of the optical waveguide 3 is approximately equal to the diameter of the core 4 of the optical waveguide 2 at the interface 6. The diameter of the core l of the optical waveguide 3 is the diameter of the core 4 of the optical waveguide 2.
substantially all of the light from the core enters the core 4. If there is light propagating through the cladding layer 8 in the tapered portion, that light also enters the core 4.

Since the light now propagates through the tapered portion from the thicker side to the thinner side, the incident and reflection angles of the light propagating along the core and cladding layer boundaries decrease. Also, the angle between the incident and reflected light and a line perpendicular to the interface 6 increases. Therefore, the numerical aperture of the transmitting optical fiber increases as it approaches the interface 6.

For the propagation of light in this direction, it is necessary that the cross-sectional diameters of the light waveguides 2j5 and 3 at the interface 6 are substantially equal. In this way large area receiving devices can be used without changing the numerical aperture of either the receiving or transmitting system.

The boundary surface 10 has been described as being located between the aperture modification device W11 and the optical waveguide 11. However, the numerical aperture conversion device can also be formed in the end region of the light waveguide 11, thereby eliminating the interface 10.

In conventional systems, when attempting to transmit light between two optical fibers, it is necessary to use optical fibers with the same numerical aperture, and typically the core of the transmitting optical fiber is The diameter must be smaller than the core diameter of the receiving optical fiber to avoid loss of light at the interface. Therefore, the apparatus of the present invention makes it possible to eliminate the need for matching the numerical aperture of the optical fiber and/or matching the diameter of the optical fiber core at the interface without redesigning the system or incurring significant light or signal loss. It has the effect of being able to do it without having to do it.

Also, heretofore, bidirectional operation has not been possible when the optical fibers have different numerical apertures. In this case, if almost all of the light from the transmitter is coupled into the higher numerical aperture receiver fiber in the "forward" direction, then in the opposite direction of transmission, the light from the transmitter is coupled into the larger numerical aperture fiber (in this case the transmitter). Light loss occurs at the interface from the side) to the small numerical aperture optical fiber. According to the present invention, this problem is also solved.

One method of forming a tapered "horn" is to heat a length of optical fiber, e.g., 8 to 12 inches, to a softening temperature, e.g., with a propane flame. During heating, the optical fiber is pulled uniformly and the flame is moved uniformly along the optical fiber.

In this way, a tapered region is formed. When the narrow end of the taper reaches a cross-sectional diameter that is slightly smaller than the cross-sectional diameter of the optical fiber to be connected, the taper operation is stopped and the optical fiber is cooled. The end of the taper is scored and broken at a desired cross-sectional diameter using a scribing method. Any other suitable means for forming the tapered region may be used.

Although the principle of this invention has been explained above in connection with a specific device, this explanation is merely an example, and does not limit the technical scope of this invention as described in the claims. It should be clearly understood that there is no such thing.

[Brief explanation of the drawing]

The figure shows one embodiment of the numerical aperture conversion device of the present invention. 1...Numerical aperture conversion VRr11.2, 3, 11...
- Optical waveguide, 4.7... Core, 5.8... Cladding layer, 6.10... Boundary surface.

Claims (10)

[Claims] (1) Comprising a core and a cladding layer surrounding the core, the core and the cladding layer forming an elongated optical waveguide structure having two end faces, and this optical waveguide structure extending from one of the end faces. A numerical aperture conversion device characterized in that the optical waveguide has a tapered portion in at least a large portion in the longitudinal direction. (2) Two elongated optical waveguides each having a different numerical aperture and two end faces, and an optical waveguide structure tapering toward one of the optical waveguides. A numerical aperture conversion device comprising: the numerical aperture matching means. (3) The tapered optical waveguide structure is separated from the other of the optical waveguides and inserted between them, and the tapered optical waveguide structure guides the optical waveguide in its boundary region. 3. Numerical aperture conversion device according to claim 2, which is aligned with each of the bodies. (4) The numerical aperture conversion device according to claim 2, wherein at least one of the optical waveguides is an optical fiber, which comprises a core and a cladding layer surrounding the core. (5) The numerical aperture conversion device according to claim 4, wherein the boundary region has an outer diameter substantially equal to the diameter of the core of the one optical fiber. (6) The numerical aperture conversion device according to claim 2, wherein one of the optical waveguides is an optical transmitter, and the other optical waveguide is an optical receiver. (7) The numerical aperture conversion device according to claim 6, wherein the one optical waveguide has a core having a larger diameter than the other optical waveguide. (8) The numerical aperture conversion device according to claim 6, wherein the other optical waveguide has a core having a larger diameter than the one optical waveguide. (9) The tapered end region has a thin end and a thick end disposed on the opposite side thereof, and the tapered end region has a thin end and a thick end disposed on the opposite side thereof, and the tapered end region has a cladding of the tapered end region from the thin end to the thick end. The tapered end region has a length sufficient to ensure that light propagating through the tapered end region is transmitted to the core of the tapered end region for continuous propagation. A numerical aperture conversion device according to claim 2. (10) two optical waveguides having cores of different diameters; and an end region of one of the optical waveguides having a tapered portion, the diameter of the core of one of the optical waveguides being equal to the diameter of the core of the optical waveguide. means for matching the diameter of the other core.