JPH04288509A - Optical fiber terminal with microlens and its manufacture - Google Patents

Abstract

PURPOSE:To omit optical axis adjustment and realize high coupling efficiency with small reflection loss by fusing a fiber lens directly to an optical fiber at its terminal and improving the sphericity of the lens. CONSTITUTION:An optical fiber 1 which has a spherical single-refractive-index lens 10 having length and a radius required for beam diffusion by Gaussian diffusion and also has the same external diameter is united by fusion with the tip of the optical fiber 8. The spherical part is formed by a thermal fusing method and the tip part of the optical fiber is put in the thermally fused part to form the spherical lens which has a target diameter at the tip of the optical fiber.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structure of an optical fiber terminal with a microlens for use in various optical components such as an optical switch, an optical multiplexer/demultiplexer, and an optical isolator, and a method for manufacturing the same.

0002

2. Description of the Related Art With the development of optical communications, it is desired that optical devices and optical components used be miniaturized. Especially in optical isolators, optical circulators, optical multiplexers/demultiplexers, etc.
There is a demand for miniaturization and simplification of structure when coupled with optical fiber. In recent years, distributed feedback lasers with a narrow spectral width have been used in high-speed optical communication systems.
Sensitivity to back reflections has also created a requirement for optical fiber terminations to have high return loss.

Generally, in a pigtailed optical isolator with an optical fiber, as shown in FIG. 2, light emitted from an 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. incident,
The emitted light is similarly condensed onto the optical fiber 1 and coupled.

0004

[Problems to be Solved by the Invention] Conventional collimating systems have had the problem of adjusting the optical axis position of the optical fiber and lens, and the cost of assembly equipment and the like has been high, making the optical fiber collimating product expensive. In addition, in the case of the conventional method, Figure 3
As shown in the figure, a refractive index matching agent 5 made of an organic substance is used to prevent reflection. Therefore, there are issues with weather resistance and heat resistance. Furthermore, in order to form an anti-reflection film on the light entrance/exit surface 6 in FIG. 3, an optical fiber cord is attached to the hard coating, which is heated to about 300 degrees Celsius like normal vapor deposition. However, due to the heat resistance of the optical fiber coating and gas generation, coatings using ion assist or the like were used, which hindered uniformity and low cost.

Furthermore, from the standpoint of miniaturization of optical devices, fiber collimator light with a sufficiently narrow luminous flux (for example, 200 μm or less) is required, but conventional optical fiber collimators have been able to obtain a light beam of at least 300 μm. Furthermore, with the conventional fiber collimator structure, a return loss of only about -27 dB was obtained, but the fiber tip had to be coupled to a lens system at an angle, or the coupling structure itself had to be complicated.

In recent years, attempts have been made to form microscopic collimated light. Journal of Lightwave
eTechnology Vol. LT-5 No.
9 (1987), William L. Emk
ey et al., single mode fiber (hereinafter referred to as SMF) and multimode graded index fiber (hereinafter referred to as MMGI)
We are proposing the coupling of micro-collimated light of up to about 40 μm by fusion bonding (referred to as F). Here, coupling is performed over a distance of about 3 mm with a coupling loss of 0.1 to 1.6 dB. However, in the structure using MMGIF, the expansion of the luminous flux is about 40 μm, and the coupling loss deteriorates significantly at a distance of 3 mm or more, so there is no flexibility in the collimation distance, and the distribution state of the gradient index fiber part is changed during the manufacturing process. Adjustment of the wavelength pitch must be performed while individually measuring, which has the drawback of being unsuitable for mass production and being expensive.

[0007]

[Means for Solving the Problems] As a means for solving the above-mentioned drawbacks, the present invention provides a structure in which the tip of an optical fiber having a waveguide structure in the center is made of SiO2 or a single refractive index mainly composed of SiO2, and has Gaussian diffusion. It has a structure in which fiber lenses with the same outer diameter are integrated by fusion to form a spherical lens with the length and radius necessary for beam expansion.

Specifically, a first optical fiber and a second optical fiber having the same outer diameter and a refractive index equivalent to that of the core of this optical fiber are joined, and the tip of the second optical fiber is spherical. It is an optical fiber terminal with a microlens that is formed into a microlens and has the function of controlling the radiation angle of the light beam.In the second optical fiber, the length is set within a range where the light beam does not touch the outer circumference, and The spherical lens is coated with an anti-reflection coating that matches the wavelength band in use, and a light absorber and a refractive index matching agent with a refractive index equal to or higher than these are coated on the outer periphery of the first and second optical fibers. Cover. The manufacturing method involves forming a spherical portion at the tip of the second optical fiber by a thermal melting method, and by feeding the tip into the thermal melting section, a spherical lens having a desired radius is formed at the tip.

[0009]

[Example] Figure 1(a) shows the tip of the optical fiber terminal of the present invention.
It was prepared as shown in the Example. It is composed of a tip SiO2 fiber lens 7, a main pigtail fiber 8, a ferrule 9 for protecting the tip, and a tip lens 10. 9 can also be omitted. Figure 1(b) shows the state of light transmission, and shows the convergence point (beam waist) of the light emitted from the SMF.
The degree of spread of the Gaussian beam as it propagates through the fiber lens 7 is expressed by Equation 1, where the distance between them is z, and the refractive index of SiO2 at the wavelength λ is n.

##EQU00001## That is, by controlling L, it is possible to enlarge the optical fiber to near or beyond the diameter of the optical fiber.

[0010] S with an outer diameter of 125 μm, which is the same as the SMF main line with a core diameter of 10 μm, an outer diameter of 125 μm, and a core refractive index of about 1.47.
Use iO2 fiber. At a wavelength of 1.31 μm, L
According to equation 1, the maximum length of Lmax should be approximately 1.1 mm or less. In FIG. 1(b), the ray matrix of light passing from the SMF to the beam waist has the relationship shown in equation 2, where R is the curvature of the tip lens of the SiO2 fiber.

[Equation 2] Furthermore, from the ray equation of the Gaussian beam, the distance z to the beam waist can be obtained from Equation 3.

[Math 3] However, a=λ/πnw02.

From equations 2 and 3 and the Gaussian beam ray formula,

[Equation 4] Therefore, the expanded beam diameter was calculated from Equation 1 to be 80 μm, and L=0.7 mm was selected. In this case, the beam diameter will be twice the beam diameter of 40 μm according to the conventional proposal. S
The curvature R of the spherical lens of the iO2 fiber has a large effect on the beam waist position z and the beam diameter 2Wz. Table 1 shows the relationship.

0012

[Table 1]

Therefore, when the curvature R=200 μm, the beam waist position is approximately 1.8 mm and the beam diameter is 88 μm. When the beam waist is the center of symmetry and the same optical system is measured at the opposite position, the lens-to-lens distance is 3.6 m.
The coupling loss was 0.5 dB at m. The amount of reflection loss is determined by the antireflection film 11 and the refractive index matching agent 5 as shown in FIG. 1(c).
We were able to achieve -45dB.

To manufacture the tip, as shown in FIG. 5, a quartz fiber 7 was fed into the arc discharge thermal melting section 13 from above the melting section to form a curvature R=200 μm. In order to make the volume (cylinder) of the quartz fiber feed length h equal to the volume of the tip sphere, use the formula 5.
h was calculated from

[Equation 5] As a result, the feed length h = 2.73 mm and the curvature R = 200
μm was obtained.

[0015]

According to the present invention, since the fiber lens can be directly fused to the optical fiber, it becomes a highly reliable optical component unlike the adhesive method, and since there is no interface, it is possible to realize high coupling efficiency with little reflection loss. Unlike conventional fiber collimators, it has the advantage of being able to be supplied at a low price as an optical fiber terminal system that also includes a pigtail portion, without having to be adjusted to a high tolerance with a lens system.

When producing a spherical fiber, it is easy to control the volume of the quartz fiber at the length of the quartz fiber to match the volume of the sphere using the thermal melting method, and since the sphere is formed using surface tension, concentric A product with good sphericity can be produced without deterioration of the sphericity, and there are no problems such as polishing scratches or damaged layers during polishing, and time wasted can be eliminated. Furthermore, since the fiber collimator has a high return loss due to its structure, there is no need to change the internal configuration or adjust the optical axis, unlike conventional fiber collimators. Due to these features, it can be applied to a wide range of applications, especially as optical isolators, optical switches, optical multiplexers/demultiplexers, etc. By further adjusting the curvature, it can be used as an optical fiber array coupling part.

[Brief explanation of the drawing]

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

FIG. 2 is a schematic diagram of an optical fiber optical system.

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

FIG. 4 is a schematic diagram showing arc discharge thermal melting 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 12 Arc discharge part

Claims (6)

[Claims] Claim 1: A first optical fiber and a second optical fiber having the same outer diameter and a single refractive index equivalent to the core of this optical fiber are joined, and the tip of the second optical fiber is shaped into a spherical shape. An optical fiber terminal with a microlens that controls the radiation angle of a light beam by forming a microlens. 2. The optical fiber terminal with a microlens according to claim 1, wherein the length of the second optical fiber is set within a range in which the light beam does not come into contact with the outer periphery of the optical fiber. 3. The optical fiber terminal with a microlens according to claim 1, wherein an antireflection film is formed on the spherical portion of the tip of the second optical fiber. 4. The optical fiber terminal with a microlens according to claim 1, wherein the outer peripheries of the first and second optical fibers are coated with a light absorbing agent or a refractive index matching agent having a refractive index equivalent to or higher than these. . 5. The method for manufacturing a microlens according to claim 1, wherein the spherical portion at the tip of the second optical fiber is formed by a thermal melting method. 6. The method of manufacturing a microlens according to claim 5, wherein a spherical lens having a desired radius is formed at the tip of the second optical fiber by feeding the tip into the thermal melting section.