JPS60173516A - Optical system for coupling light source - Google Patents

Abstract

PURPOSE:To execute easily a polish-working and an assembly, and to execute a mass-production at a low cost by arranging two self-focusing type lenses consisting of a cylindrical transparent body having a specified refractive index n(r) in a distance of (r) from the center, in the optical axis direction, and making a numerical aperture of a light source side of an optical system larger than a numerical aperture of an emitting side. CONSTITUTION:An optical system 1 is constituted by placing two self-focusing type lenses 2, 3 so that the axes are made to coincide and each end face is made to adhere tightly. The self-focusing type lenses 2, 3 consist of a transparent cylindrical body of glass, synthetic resin, etc., and all end faces of both the lenses 2, 3 form a plane vertical to the optical axis. Also, as for both the lenses 2, 3, a refractive index n(r) in a distance of (r) from the optical axis has a refractive index distribution as shown in the equation. Particulars of the lens are selected so that numerical apertures NA1, NA2 of both the lenses 2, 3 become a relation of NA1>NA2. Also, with regard to a biquadratic term coefficient h4 in the equation of the lenses 2, 3, h4 of the first lens 2 of the light source side is made smaller than h4 of the second lens 3.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an optical system that makes a diffused light beam from a light source enter an optical transmission fiber.

[Technical background of the invention]

When making diffused light of a specific wavelength emitted from a light source such as a semiconductor laser enter a small-diameter optical transmission fiber, especially a single-mode fiber, it must be made to enter efficiently with low aberrations in order to keep wave light loss as low as possible. is necessary.

In particular, it is necessary to sufficiently correct coma aberration so that it can be used even with off-axis incidence caused by assembly errors. Further, the optical system is required to be small and lightweight.

Conventionally, a typical optical system for coupling light sources of this type is one in which a single self-focusing lens is disposed between a light source and an optical transmission fiber.

As is well known, a self-focusing lens consists of a transparent cylindrical body having a refractive index distribution where the refractive index is maximum on the central axis, gradually decreases parapolitically in the radial direction, and is minimum at the outer periphery. In the case of a single self-focusing lens as described above, controlling the refractive index distribution is extremely difficult in order to reduce the axial aberration, and even if the axial aberration can be reduced to a satisfactory level, Off-axis aberrations, especially comatic aberrations, remain large, so when the light source is shifted from the optical axis of the lens due to an error during assembly, the aberrations become thicker (and the loss of light that is not incident on the fiber increases rapidly). - For example, in the most commonly used single mode fiber with a core diameter of 10 μm, there is a problem of conventional single self-focusing of the light beam from a light source with a deviation of 0.1 mm perpendicular to the optical axis. FIG. 3 shows a spot diaphragm when the light is incident through a molded lens.

In the figure, 7 indicates the core of the single mode fiber, and the black dots indicate the positions where the light ray crosses the plane including the fiber end face.

It can be seen from FIG. 3 that the light rays incident on the lens do not converge at one point but are greatly diffused, and that there is a considerable amount of light rays that do not enter the fiber core 7.

An optical system that corrects the aberrations described above can be constructed from three or one ordinary spherical lens with a uniform refractive index, but the optical system would be quite large and would require lens polishing and polishing. There is a problem that assembly is time consuming and costs increase.

[Purpose of the invention]

The purpose of the present invention is to eliminate the above-mentioned conventional drawbacks, to reduce not only the axial aberration but also the off-axis aberration, and because all lens end faces can be flat, polishing and assembly are extremely easy and mass production is possible at low cost. An object of the present invention is to provide a lightweight light source coupling optical system.

[Structure of the invention]

In the optical system of the present invention that achieves the above object, the refractive index n(r) at a distance r from the center is no greater than the refractive index on the central axis.
As n2(r)=no2C/-(gr)2+h4(gr)4
+h6(gr)6+11s(gr)s+...] Two self-focusing lenses made of cylindrical transparent bodies are arranged in the optical axis direction, and the numerical aperture of the lens on the light source side (first lens) is set to the other side. The lens is characterized in that the numerical aperture is larger than that of the lens (second lens), and the coefficient h4 of the first lens is smaller than that of the second lens.

〔Example〕

Embodiments of the present invention illustrated in the drawings will be described in detail below.

In FIG. 1, reference numeral 1 denotes an optical system according to the present invention, and diffused emitted light S from a light source such as a semiconductor laser is focused through the optical system and enters the core of a single mode fiber B.

The optical system/ is constructed by arranging two self-focusing lens stones 3 with their axes aligned and their end surfaces in close contact with each other.

The self-focusing lens λ, 3 is made of a transparent cylindrical body made of glass, synthetic resin, etc., and all end faces of both lenses 2, 3 are planes perpendicular to the optical axis. In addition, both lenses 2 and 3 have refractive indexes n (r) at a distance r from the optical axis of n, 2(
r)-no2 (/-(gr)2+h4(gr)'+h
6 (gr) 60 + hs (gr) 8 +...] ... (
It has a refractive index distribution expressed by the formula 1).

In the above equation (1), no is the refractive index on the optical axis, L11
4+h6. ha is a refractive index distribution constant.

And the aperture of both lenses 2 and 3 @NA, 11 NA2 is N
Lens specifications are selected so that AI>NA2.

Due to the above numerical aperture relationship, the light beam emitted from the light source l at a relatively large diffusion angle is efficiently captured by the seventh lens 2, which has a large numerical aperture, and the second lens 3, which has a relatively small numerical aperture. Since the light is emitted and focused at a gentler angle than when it enters from the light source, the light source light can be efficiently entered into a single mode fiber with a relatively small numerical aperture.

Regarding the linear term coefficient h4 in the equation ( ]) for the lenses 2 and 3, h4 of the first lens on the light source side is made smaller than h4 of the second lens.

This coefficient h4 will be described in more detail.

A typical numerical example of the central refractive index no and the distribution constant g of the self-focusing lenses 2 and 3 is no=8g.

Select g=0.22×4Zm m-1, determine the distribution coefficient h4 of one lens, and calculate the distribution coefficient h4 of the other lens such that the spherical aberration is O from the aberration calculation. Then, calculate the amount of unsatisfactory sine condition of the combined optical system l of both lenses 2 and 3, and if this unsatisfactory amount is not approximately Ovc, change the value of the initially set distribution coefficient h4 and repeat the above calculation until the amount of unsatisfactory sine condition is approximately OK. I repeated this process until I found the optimal combination of h4 for both lenses. The results are shown in FIG. Regarding the numerical aperture NA2 of the exit side lens 3, it is assumed that Nl2 = 0.10 considering the optimum spot size of the single mode fiber, and the numerical aperture NA1 of the input side lens 2 is such that the loss at the time of incidence from the light source is /db or less. So that Nl1=0
．． It is set at 39. In the graph of Figure 2, the horizontal axis is the total lens length Z of the combined optical system and the lens length z1 of the first lens.
The vertical axis is the value of the linear term coefficient h4 in equation (1). Also, the parameter Sl is the light source and the first
lens! is the distance between the plane of incidence and the two people.

Three curves A11 at the bottom of the graph in Figure 2
A2 and A3 are respectively 5l=o, zamm, /. 3
It shows the value of h4 of the light source side lens 2 at 63mmT2゜omm, and the three curves Bl and B! above are shown. i, B3 are the curves AI, A2. The value of h4 of the optimal fiber-side lens 3 (the amount of unsatisfactory sine condition is approximately O) corresponding to the value of h4 on A3 is shown.

As is clear from Fig. 2, in the combined optical system of both lenses 2 and 3, aberrations can be sufficiently suppressed under a wide range of conditions by making the h4 value of the light source side lens 2 smaller than the h4 value of the fiber side lens 3. Can be set to a small value.

As shown in the graph, the desirable range of h4 of the light source side lens 2 is -2≦h4≦0.2,
Further, the desirable range of h4 of the fiber-side lens 3 is O5≦h4≦2.0.

To give seven numerical examples, the distance to the incident surface 2A of lens 2 is 1IIS1-/, 3mm, the distance from the exit surface 3A of lens 3 to the fiber end face is 52=10.399mm, and the central refraction of both lenses 2 and 3. Rate no = HatSza, constant g-0 of equation (1), 22zallmm'-'9 Lens length zl = Z
2=2.9110mm, the distribution coefficient of the light source side length is h4=-〇, 7. Let h6=O1h8-O, and each coefficient of the fiber side lens 3 is h4=/, /t,
h6=0. /2, h6=/, 4'7K By selecting K, the aberration of the combined optical system can be minimized.

In the optical system l in the numerical example above, the light source is 0.1 in the direction perpendicular to the optical axis.
Figure 1 shows the spot diaphragm on the end face of the single mode fiber when it is shifted by mm.

As is clear from a comparison between FIG. 3 and FIG. 1, the optical system according to the present invention has extremely small off-axis aberrations, so even if the light source position is slightly deviated from the optical axis of the optical system, the light source light can be reliably transmitted. It enters the fiber core and has very high coupling efficiency.

〔Effect of the invention〕

As explained above in the embodiments, the optical system of the present invention has very small off-axis aberrations, so even if there is some misalignment between the light source and the optical axis of the optical system due to assembly errors, etc. , the wave is efficiently transmitted within the tiny core of a single-mode fiber with almost no optical loss. At the same time, the tolerance for alignment between the light source and the optical axis of the optical system increases, so
The assembly work becomes easier as well.

Furthermore, since it is a combination of two self-focusing lenses, the polishing of the lens surface can be done by plane processing. Therefore, even lenses with minute diameters can be processed extremely easily. It is easier to manufacture than optical systems.

(9)

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing an embodiment of the present invention, FIG. 2 is a graph showing the optimum combination range of the linear term coefficient h4 of the first lens and the second lens in the optical system of the present invention, and FIG. The figure shows the spot diaphragm on the fiber end face when the source light is incident with a deviation from the lens optical axis, and Figure 3 shows the case when a conventional single self-focusing lens is used. FIG. q shows the case where the optical system of the present invention is used. / Optical system for optical coupling 2.3 Self-focusing lens
Light source ' The fiber Figure 1 Figure 2 I oω Procedural amendment March 7, 1980 Commissioner of the Patent Office Patent application filed February 20, 1980 (2) 2 Invention. Name: ro-small...1. -・Optical system for light source coupling 3 Relationship with the case of the person making the correction Patent applicant address Name of Doshomachi Gu-chomeza, Higashi-ku, Osaka-shi, Osaka Prefecture Name
(IIoo) Nippon Sheet Glass Co., Ltd. Representative Nobuo Sagara Agent address Shinbashi Sumitomo Building, No. 3 Shinbashi S-chome, Minato-ku, Tokyo Sponsored by 6 Specification subject to amendment 7, Contents of amendment (1) Specification The entire claims on page 1 are amended as shown in the attached sheet. (2) In the specification, page 1, line 1g to page S, line 1, the statement "Lens on the light source side (first lens)... made smaller than the second lens" has been changed to [the light source of the optical system]. The numerical aperture on the side is made larger than the numerical aperture on the exit side, and the coefficient h4 of the lens located on the light source side (first lens) is made smaller than the other lens (second lens). ” he corrected. (3) In the 1st and 2nd lines of page 6 of the specification [Both lenses 2
．． 3... Lens specifications are selected. ] is [The light source side numerical aperture NA1 and the output side numerical aperture N of optical system l
The optical system specifications are selected so that A2 satisfies the relationship NAl>NA2. ” he corrected. (4) In the specification, page 5, line 5 to line 2, K [by the first lens... by the second lens 3] is replaced with "Efficiently captured and corrected at the incident surface of the optical system." 5) Correct the phrase "output side lens 3" on page 7, line S of the specification to read "output side of optical system/." (6) “Incidence side lens 2” on page 7, line 7 to line g.
Correct it to read "Incidence side of 1 optical system l". (7) On page 7, line 9, K “/db” should be changed to l/dBJ.
and correct it. (8) On page 73 of the specification letter, [lens length Z1=22=
J is corrected as "Length of first lens z1=Length of second lens z2=". (9) Correct the phrase ``/, Optical system for optical coupling'' to ``/, Optical system for light source coupling'' in Line 10 of Part 10 of the specification. 2. Claims (1) The refractive index n(r) at a distance r from the center is n
"(r)=nO"(/-(gr)"10h4(gr)4
+h6(gr)6+hs) (gr)" Two self-focusing lenses made of cylindrical transparent bodies represented by cylindrical rods are arranged in the optical axis direction, and the numerical aperture on the light source side of the optical system is set on the output side. A lens having a numerical aperture larger than the numerical aperture and located on the light source side with respect to the coefficient h4 (
A light source coupling optical system characterized in that a seventh lens (seventh lens) is smaller than the other lens (second lens). (2. In claim 1, the coefficient h4 of the first lens is within the range of -2≦h4≦0.2. (3) In claim 1, , h of the second lens
4 is 0. An optical system for light source coupling that satisfies the relationship h4≦2.0. (3) (Attachment l)

Claims (1)

[Claims] (1) The refractive index n(r) at a distance r from the center is n
2(r)=no2 (/-(gr)”+h4(gr)4
+h6(gr)6+h6(gr)8+...] λ self-focusing lenses made of cylindrical transparent bodies are arranged in the optical axis direction, and the numerical aperture of the lens on the light source side (first lens) is set to the other side. An optical system for light source coupling, characterized in that the numerical aperture of the seventh lens (second lens) is larger than that of the second lens, and the coefficient h4VC of the seventh lens is smaller than the second lens. (2. In claim 1, the coefficient h4 of the first lens is within the range of -2≦h4≦0.2. (3) In claim 1, , h of the first lens
4 is a light source coupling optical system within the range of OoS≦h4≦2.0.