JPH04313711A - Optical fiber terminal with microlens - Google Patents

Abstract

PURPOSE:To enable the adjustment of an optical axis to be omitted and to obtain high coupling efficiency with less reflection losses by directly fusing a fiber lens to an optical fiber and improving the sphericity of the lens. CONSTITUTION:The tip of an optical fiber terminal is constituted of a non-doped silica fiber lens introducing part 7 at the terminal, a main single mode fiber 8, a ferrule 9 for protecting the tip part, and a tip spherical lens 10. Namely, the optical fiber 7 of the same outside diameter having the spherical lens 10 consisting of a single refractive index, having the length to become at least >=80mum the beam expanstion by Gauss diffustion at the exit end and having >=200mum radius of curvature is integrated by fusing at the tip of the optical fiber 8. The optical fiber terminal consists of the integral structure which is fused with the light introducing part 7 of the same outside diameter as the outside diameter of the single mode fiber 8 and focuses the exit light with the spherical lens 10 at the end on the opposite side in such a manner and, therefore, this terminal is excellent in terms of reliability and in addition, there is substantially no attenuation of reflection as there are no parallel boundaries in the ray passage.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the structure of an optical fiber terminal with a microlens for use in various optical components such as optical switches, optical multiplexers/demultiplexers, optical isolators, etc., and for optical connectors.

0002

[Background Art] With the development of optical communications, there is a desire for miniaturization of optical devices, optical components, etc. used, and optical isolators, optical circulators, optical multiplexers/demultiplexers, etc. are miniaturized by being coupled with optical fibers. There is a need for simplification of the structure. In addition, in recent years, high-speed, high-density optical communication systems are sensitive to back reflections, but because they use distributed feedback lasers with extremely narrow spectral linewidths, the ends of optical fibers have high reflection attenuation. It has also come to be required to have quantity.

Generally, in the case of a pigtail optical isolator with optical fibers at both ends, the light emitted from the optical fiber 1 is converted into parallel light by a ball lens 2 or a gradient index lens 3 and sent to an optical device 4, as shown in FIG. Optical coupling is performed by making the light incident on the optical fiber 1, and condensing the light onto the optical fiber 1 in the same manner after exiting.

0004

[Problems to be Solved by the Invention] In the conventional optical coupling system as shown in FIG. 2, there is a problem in that the optical axis position of the optical fiber and the lens must be adjusted within a submicron range, which increases the cost of assembly equipment, etc. This makes optical system products including optical fiber collimators and optical fiber coupling systems expensive. Furthermore, in the conventional method, as shown in FIG. 3, antireflection is performed using a refractive index matching agent 5 made of an organic substance, which has disadvantages in weather resistance and heat resistance.

Furthermore, in order to form an anti-reflection film on the light entrance/exit surface 6 in FIG. 3, it is necessary to attach an optical fiber wire to the surface. Therefore, the hard coating that is generally performed while heating to about 300°C, which is used to form a strong reflective film, cannot be used, and only low-temperature deposition can be performed while reinforcing it with ion assist, etc., and the durability and
This was a factor that hindered uniformity and price reduction.

In addition, from the standpoint of miniaturizing optical devices, a collimator light with a sufficiently narrow luminous flux, for example, 200 μm or less, is required, but with conventional technology, the coupling loss increases, so the collimator light is only about 300 μm at the most. It wasn't practical. Furthermore, in conventional optical coupling systems, there is always a parallel end face, and return loss can only be obtained up to about -27 dB.In reality, the fiber tip must be coupled to a lens system at an angle, or the coupling itself must have a complicated structure. I had to.

In order to solve the above-mentioned drawbacks of conventional optical coupling systems, attempts have been made in recent years to form microfiber collimated light. Journal of Light
twave Technology Vol. LT-
5 No. 9 (1987) by William L. E
proposed coupling of microfiber collimator light up to a parallel beam of approximately 40 μm by fusing a multimode gradient index fiber (hereinafter referred to as GIF) to a single mode fiber (hereinafter referred to as SMF) by Mkey et al. space to 0
．． It has been reported that optical coupling can be obtained with a coupling loss of 1 to 1.6 dB.

However, in the structure using SMF and GIF,
It is theoretically impossible to expand the luminous flux beyond the core diameter of the GIF, and the maximum limit is 50 to 62.5 μm, and it cannot be increased further than this, and a distance of 3 mm or more will cause significant coupling loss deterioration, so optical There is no flexibility in the coupling distance, and the GIF must be manufactured while individually measuring the refractive index distribution state and wavelength pitch adjustment during the manufacturing process, making it expensive and unsuitable for mass production. On the contrary,
In JP-A-61-264304, Kevin J.
Warbrick proposes SMF and undoped silica fiber lens optics.

However, in this case as well, the curvature of the lens portion is limited to 62.5 μm for reasons of diffraction loss, so the obtained luminous flux is approximately 60 μm, which is structurally at most 80 μm of the diameter of the silica fiber. %, which is too narrow to insert an optical device. i.e. 60
On the other hand, a luminous flux on the order of μm is too narrow and is not suitable for combining Gaussian beams. Therefore, the practical problem is how to realize a light beam of 60 to 200 μm.

0010

[Means for Solving the Problems] The present invention essentially consists of S
An optical fiber terminal for optical coupling is proposed, which is composed of an MF, an undoped silica fiber beam expanding section, and an undoped silica ball lens. A specific structure is constructed by joining a first optical fiber and a second optical fiber having the same outer diameter and equivalent to the refractive index of the core portion. The second optical fiber has a spherical lens formed at its tip having a diameter larger than its outer diameter, and when it passes through the spherical lens, the light beam is transmitted to at least 62.5 μm, which is at least half the diameter of the optical fiber, preferably 62.5 μm. expands to more than 80 μm,
Further, in order to exhibit the effect of converting the curved surface portion of the spherical lens into a parallel beam or light having an emission angle depending on the application, the radius of curvature is at least 200 μm.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1(a) is a sectional view of the tip end of an optical fiber according to the present invention. It consists of an undoped silica fiber lens introduction section 7 at the tip, a main SMF fiber 8, a ferrule 9 for protecting the tip, and a spherical lens 10 at the tip. FIG. 1(b) shows the transmission state of light. If the distance to the beam waist point of the light emitted from the SMF is z, then the Si
Assuming that the refractive index of O2 is n, the degree of spread of the Gaussian beam by propagating through the fiber lens introduction section and the ball lens section is expressed by equation 1.

[Math 1]

In other words, by controlling the length L of the second optical fiber and taking into consideration the diameter of the optical fiber or the spread of the spherical lens part, it is possible to expand the emitted light flux to a greater extent, and as a result, the beam waist distance can be reduced. Even if the size is increased, optical coupling can be achieved with a small coupling loss. Figure 4
are the calculated radii of curvature R and L of the spherical lens and the distance z to the beam waist, which provide the maximum coupling efficiency. As can be seen from this figure, z = 2.5 mm or more (i.e. distance between lenses: 2z = 5 mm or more) can be achieved when L≧1000 μm and the curvature R of the spherical lens is also 250 μm.
This is when conditions equal to or more than m are satisfied. At this time, the luminous flux of the emitted light is 90 μm or more, and the luminous flux can be made thicker than that of a conventional fiber collimator, so that the distance between the lenses can be increased accordingly.

From the above, if the diameter of the Gaussian distribution light beam at the exit end of the lens is 80 μm or more, it is possible to obtain the high coupling efficiency that is the objective of the present invention, and the light can be converted into parallel rays or finite beams. In order to focus the beam to have a beam waist of , it is necessary that the radius of curvature of the light focusing ball lens be at least 200 μm. This is shown in Figure 1 (
In b), it can be estimated from the ray matrix of the light from the SMF to the beam waist, and is derived from the relational expression of Equation 2.

[Math 2]

Furthermore, from the ray equation of the Gaussian beam, equation 3
Then, the distance z to the beam waist can be calculated.

[Formula 3] However, a=λ/πnw02. Furthermore, from equations 2 and 3 and the Gaussian beam ray formula,

[Math 4] can be introduced, and the result shown in FIG. 4 can be obtained.

FIG. 5 shows the beam radius in the Z-axis direction after exiting from the ball lens. When R=165μm, the beam at the exit end is 60μm, but the luminous flux at the beam waist position (=2Wz) is about 3
Although it is narrowed down to 0 μm and deviates from the gist of the present invention, R = 200
Above μm, the luminous flux becomes moderate, for example, R = 265 μm.
Then, 2Wz=92μm, 2z=5.2mm, and further, when R=340μm, 2Wz=118μm, 2z=9.
0 mm, and the necessary conditions are met.

FIG. 6 shows that L=950 μm and R=26 according to the present invention.
A pair of optical fiber terminals in which a spherical lens silica fiber of 5 μm and 2z=7.0 mm is fused to an SMF with a core diameter of 10 μm are faced, and the coupling loss with respect to displacement in the X-axis direction is actually measured. It was confirmed that the minimum difference was 0.4 dB when no antireflection film was mounted on each other, and that a positional deviation of about 20 μm from front to back was acceptable. The return loss is shown in Figure 1 (c
), the anti-reflection film 11 and the refractive index matching agent 5 are applied.
45dB was achieved, which is equivalent to the conventional non-reflection connector. At this time, conventional non-reflection connectors physically abut each other and performance deteriorates when they are repeatedly connected and disconnected, but in the fiber system of the present invention, the light beam is propagated through space, so no physical damage occurs at all.

[0017] Furthermore, the conventional microfiber collimator is
The distance that can be propagated with high coupling efficiency is about 3 mm or less, which is shorter than the practical distance of 3 to 10 mm according to the present invention, and as shown in FIGS. The permissible positional deviation is narrow, which is contrary to the effect that a wide permissible region can be obtained in the numerical range of the present invention not only in the direction of the light beam (Z-axis direction) but also in the direction perpendicular to the light beam (XY-axis). That is, in FIG. 7, it can be estimated from the above calculation formula that the larger the lens curvature, the gentler the allowable position for the coupling loss in the Z-axis direction.

Similarly, FIG. 8 shows the tolerance range regarding the lens curvature and positional deviation in the X-axis direction. Obviously, as the curvature increases, the permissible positions become looser and manufacturing becomes easier. FIG. 9 shows the actually measured coupling loss of the optical system of the present invention with respect to the positional shift in the X-axis direction, and compares it with the case (2) estimated from calculation. At the same time, a case (2) of a collimator system using a commercially available gradient index lens is also illustrated. Although the coupling system of the present invention has a narrower degree of freedom, commercially available optical systems have a luminous flux of as much as 700 μm, which is completely different in spirit from the present invention. What can be seen from this figure is that the coupling system of the present invention can achieve a coupling loss equal to or lower than that of a commercially available collimator coupling system.

[0019]

[Effects of the Invention] The present invention has an integrated structure in which the SMF is fused at the light introduction part with the same outer diameter and the emitted light is focused by the ball lens at the opposite end, and is different from the conventional bonding system. In addition to being superior in terms of reliability, there is almost no return loss because there are no parallel interfaces in the optical path. In addition, since it is an integral structure, there is no need to adjust the optical axis between the fiber and lens, and it is easy to connect to other optical systems, so it is especially useful for optical isolators, optical circulators, optical switches, optical multiplexers and demultiplexers, and optical connectors. It is most suitable for etc. Furthermore, since the curvature can be adjusted, it can be applied to optical fiber array coupling parts, and can be applied to a wide range of applications.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing an optical fiber terminal of the present invention.

FIG. 2 is a schematic diagram of a fiber optic system.

FIG. 3 is a cross-sectional view of a conventional optical fiber collimator.

FIG. 4 shows calculated values for an optical fiber collimator that provides maximum coupling efficiency in the present invention.

FIG. 5 shows the beam radius in the Z-axis direction after exiting from the ball lens of the present invention.

FIG. 6 shows actual measured values of coupling loss with respect to displacement in the X-axis direction of the optical fiber collimator of the present invention.

FIG. 7 shows calculated values of coupling loss with respect to lens curvature and positional deviation in the Z-axis direction according to the present invention.

FIG. 8 shows calculated values of coupling loss with respect to lens curvature and positional deviation in the X-axis direction according to the present invention.

FIG. 9 shows a comparison between calculated values and actual values of coupling loss with respect to positional deviation in the X-axis direction of the optical system according to the present invention.

[Explanation of symbols]

1 Optical fiber 2. Ball lens 3. Gradient index lens 4. Optical device 5 Refractive index matching agent 7 SiO2 fiber lens 8 Pigtail fiber main line 9 Ferrule for tip protection 10 Tip lens 11 Anti-reflection film

Claims (1)

[Claims] Claim 1: A second light beam formed by a first optical fiber, a light introduction part that is equivalent to the core part of this optical fiber, has a single refractive index, and has the same outer diameter, and a light focusing ball lens part. In a structure in which the fiber is fused to the first optical fiber on its light introduction side, the length of the second optical fiber is such that the luminous flux of the Gaussian distribution light propagated through it is at least 80% at the output end.
1. An optical fiber terminal with a microlens, which has a length that expands to more than μm, and has a radius of curvature of a light focusing ball lens of 200 μm or more.