JPH07294778A - Optical coupling lens - Google Patents

Abstract

PURPOSE:To realize an optical coupling lens having a high optical coupling rate. CONSTITUTION:This lens is used for optically coupling the luminous flux from a semiconductor laser or light emitting diode which is a light source 10 to the incident end face of an optical fiber 20 and is composed of a single lens. The lens face on the optical fiber side is an aspherical face symmetrical with the optical axis. The max. height of the ray passing this aspherical face, defined as hmax, the aspherical face value at the max. height hmax, defined as Z(hmax), and the radius of curvature on the optical axis of the aspherical face, defined as r, satisfy conditions: (1-1)0.8{r-sq. rt. (r<2>-hmax<2>)}>Z(hmax)>{r-sq. rt. (r<2>hmax<2>)}.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical coupling lens, and more particularly to a lens for optically coupling a light beam emitted from a semiconductor laser or a light emitting diode to an incident end face of an optical fiber.

[0002]

2. Description of the Related Art In the field of communication such as computer networks, optical communication is becoming the mainstream due to the large amount of information that can be transmitted. As a signal source for optical communication, a semiconductor laser or a light emitting diode (hereinafter, LD and L, respectively) is generally used because of their ease of control and inexpensiveness.
(Abbreviated as ED) is used. Further, an optical fiber is generally used as a transmission body that transmits the light carrying the signal.

The optical coupling lens is a lens for optically coupling the light flux emitted from the LD or LED to the incident end face of the optical fiber. For the total amount of light emitted from the light source,
The ratio of the amount of light that is reliably sent to the optical fiber and transmitted by the optical fiber is called the "optical coupling rate". The optical coupling rate by the optical coupling lens is effective for transmitting a strong signal with less noise. It is necessary to be big.

[0004]

The present invention has been made in view of the above circumstances, and an object thereof is to provide a novel optical coupling lens having a large optical coupling rate (claims 1 to 1).
4). Another object of the present invention is to provide a novel optical coupling lens having a large optical coupling rate and capable of easily performing various measurements and switching during optical coupling (claims 3 and 4).

[0005]

The "optical coupling lens" of the present invention is a lens for optically coupling the light flux emitted from the LD or LED, which is the light source, to the incident end face of the optical fiber. ~ 4).

The optical coupling lens described in claim 1 is composed of a single lens, and the lens surface of this lens on the optical fiber side is an "optical axis symmetric aspherical surface".

The maximum height of a ray passing through this aspherical surface is h
_max, maximum height: aspheric value in h _max Z (h _max),
Assuming that the radius of curvature of the aspherical surface on the optical axis is r, these conditions (1-1) 0.8 {r-√ (r ² -h _max ² )}> Z (h
_max )> {r−√ (r ² −h _max ² )} is satisfied.

In this specification, the "optical axis symmetric aspherical surface" means that the Z axis is aligned with the optical axis, the Z axis is positive toward the optical fiber side, and the coordinate is h in the direction orthogonal to the optical axis.
Where r is the radius of curvature on the optical axis, K is the conical constant, and A, B, and C are high-order aspherical coefficients, Z (h) = (1 / r) h ² / [1 + √ {1- (K + 1) (1 / r) ²
h ² }] + A · h ⁴ + B · h ⁶ + C · h ⁸ (1) A curved surface obtained by rotating the curve represented by the formula around the optical axis.

The lens surface on the light source side of the lens may be a spherical surface. Alternatively, the lens surface on the light source side,
It may be an aspherical surface having optical axis symmetry (claim 2).

An optical coupling lens according to a third aspect has a first lens and a second lens arranged from the light source side toward the optical fiber side. The "first lens" converts the light flux from the light source into a parallel light flux parallel to the optical axis. The “second lens” focuses the parallel light flux from the first lens toward the incident end face of the optical fiber.

The lens surface on the optical fiber side of the first lens is
It is an optical axis symmetric aspherical surface, and the maximum height of a ray passing through this aspherical surface is h _max , the aspherical surface value at the maximum height: h _max is Z (h _max ), and the curvature of the aspherical surface on the optical axis is When the radius is r, these are given by the condition (2-1) 0.8 {r-√ (r ² -h _max ² )}> Z (h
_max )> {r−√ (r ² −h _max ² )} is satisfied.

When the focal lengths of the first and second lenses are f ₁ and f ₂ , respectively, they satisfy the condition (2-2) 0.15 <| f ₁ / f ₂ | <0.25. Be satisfied.

Also in the optical coupling lens according to claim 3,
An optical axis symmetric aspherical surface can be adopted for one or more lens surfaces other than the lens surface on the optical fiber side of the first lens. For example, as in the optical coupling lens according to the fourth aspect, the lens surface on the optical fiber side of the first lens and the lens surface on the light source side of the second lens can be aspherical surfaces that are symmetrical about the optical axis.

[0014]

As is well known, the optical fiber in which the light beam from the light source is coupled by the optical coupling lens of the present invention is generally a core portion having a high refractive index: n _{1 at} the axial core portion, and the periphery thereof, It has a structure in which a cladding layer having a low refractive index: n ₂ is surrounded.

With the input end face of the optical fiber (perpendicular to the axial direction of the fiber) being orthogonal to the optical axis of the optical coupling lens, the light beam entering the core of the incident end face from the optical coupling lens with respect to the above-mentioned axis center. If the angle is θ, the condition under which this light beam is transmitted by the optical fiber, that is, the condition that the light beam incident on the core part is totally reflected to the core part side at the boundary part between the core part and the clad layer is well known. As shown in “sin θ <
√ (n ₁ ² −n ₂ ² ) ”, and the critical angle of θ: θ ₀ is generally around 6 degrees in a single mode fiber.

That is, in order to improve the optical coupling efficiency, the light flux from the optical coupling lens must be incident on the core portion of the optical fiber at a convergence angle of about 6 degrees or less.
At this time, if the optical imaging lens has spherical aberration, the optical coupling rate is greatly affected.

The conditions (1-1) and (2-1) are conditions for controlling the spherical aberration of the optical coupling lens from the viewpoint of the optical coupling rate. When the lower limits of the conditions (1-1) and (1-2) are exceeded, the aspherical surface is not used and spherical aberration is insufficiently corrected.
If the spherical aberration is insufficiently corrected, a large number of rays are incident on the core portion of the optical fiber at an angle exceeding the above-mentioned critical angle, and the optical coupling rate is reduced.

If the upper limits of the conditions (1-1) and (2-1) are exceeded, spherical aberration will be overcorrected, and at the entrance end face,
The number of light rays protruding from the core portion increases, and the optical coupling rate also decreases.

In the optical coupling lens of the third and fourth aspects, the light flux from the light source is made into a parallel light flux parallel to the optical axis by the first lens, and then is condensed by the second lens toward the incident end face of the optical fiber. It Therefore, various measurements and switching can be performed by utilizing the portion of the parallel light flux between the first lens and the second lens.

With respect to the optical coupling lens of the present invention, LDs and LEDs used as light sources emit divergent light flux. This divergent light beam has a normal distribution type light intensity distribution in which a bell-shaped curve is drawn and the light intensity decreases with an increase in the divergence angle. 1/2 of divergence angle range with 1/2 intensity
As is well known, the angle region of 10) is 10 to 20 degrees.

Considering a conical area formed by connecting light rays inclined by 30 degrees with respect to the optical axis around the origin of divergence of a divergent light beam emitted from such a light source, the conical area is radiated. The emitted light amount is 95% or more of the total emitted light amount.

Therefore, in order to sufficiently take in the light from the light source into the optical coupling lens, the numerical aperture: NA is 0.5 (=
About 33 degrees) is sufficient. The light flux thus captured must be converged toward the incident end face of the optical fiber at the convergence angle of about 6 degrees.

When the optical coupling lens is composed of the first and second lenses as in the optical coupling lens of the third and fourth aspects, the ratio of the focal length of the first lens to the focal length of the second lens. Is a reciprocal of the ratio of NA of each lens, that is, | f ₁ / f ₂ | ≈ (NA of the second lens) in order to sufficiently take in the light from the light source and to efficiently enter the optical fiber. ) / (NA of the first lens).

The condition (2-2) defines this condition. When the upper limit is exceeded, the amount of light taken in from the light source by the first lens becomes small, or the critical angle of the light beam incident on the optical fiber is: Many exceed θ ₀ and the optical coupling rate decreases. On the other hand, when the value goes below the lower limit, the NA of the first lens becomes unnecessarily large, which makes it difficult to correct aberrations by the lens, or the light beam diameter at the incident end face of the optical fiber becomes large, so that a portion protruding from the core portion may be left out. It grows up.

[0025]

EXAMPLES Specific examples will be described below.

The radius of curvature of the i-th lens surface counted from the light source side (the radius of curvature on the optical axis in the case of an aspherical surface symmetric with respect to the optical axis) is represented by "r _i (for the lenses according to claims 1 and 2). I =
1, 2 and i = 1 to 1 with respect to the lenses according to claims 3 and 4.
4) ”, counting from the light source side, the i-th lens surface and the i + th lens surface
The surface distance on the optical axis from the first lens surface is represented by “d _i (i = 1 for the lenses described in claims 1 and 2, i = 1 to 3 for the lenses described in claims 3 and 4). The refractive index of the j-th lens material counted from the light source side for light having a wavelength of 1300 nm is represented by “n _j (j = 1 for the lenses described in claims 1 and 2, claims 3, 4). For the lens described, j =
1, 2) ".

The distance on the optical axis from the light emitting end surface of the LD or LED, which is the light source, to the first lens surface is "d".
₀ (i = 0) ", and the distance on the optical axis from the lens surface closest to the optical fiber to the incident end surface of the optical fiber is" d ₂ (i = 2) ”and the inventions described in claims 3 and 4 are represented by“ d ₄ (i = 4) ”.

Regarding the aspherical surface, the shape is specified by giving the radius of curvature on the optical axis, the conical constant K in the equation (1), and the high-order aspherical surface coefficients A, B, and C.

The first embodiment is an embodiment of the invention according to claim 1,
The second embodiment is an embodiment of the invention described in claim 2.

Example 1 i r _i d _i j n _j 0 0.385 1 1.000 1.983 1 1.76817 2 -1.000 5.927.

Aspherical second surface: K = -1.07116, A = 0.481868
^{× 10¯ 2, B = -0.308759 ×} 10 ~ 1, C = 0.3821
57.

Value of conditional expression 0.8 {r−√ (r ² −h _max ² )} = − 0.132, Z (h _max ) = − 0.148 {r−√ (r ² −h _max ² )} = − 0.165
.

Example 2 i r _i d _i j j n _j 0 0.390 1 1.000 1.975 1 1.76817 2 -1.000 5.857.

Aspheric surface First surface: K = -0.15273 × 10 ² , A = -0.1
57173 Second surface: K = -0.958719, A = -0.1355
63 × 10 to ¹ , B = 0.707348 × 10 to ² , C = −0.4127
26 × 10 ~ ² .

Value of conditional expression 0.8 {r−√ (r ² −h _max ² )} = − 0.160 Z (h _max ) = − 0.182 {r−√ (r ² −h _max ² ). } = -0.200
.

Embodiment 3 is an embodiment of the invention according to claim 3,
Embodiment 4 is an embodiment of the invention described in claim 4.

Example 3 i r _i d _i j n _j 0 0.140 1 0.900 1.793 1 1.76817 2 -0.900 5.0 3 3.970 0.969 2 1.76817 4 ∞ 4.592.

Aspheric surface Second surface: K = -1.00752, A = -0.98532.
2 × 10 to ¹ , B = −0.180772 × 10 to ¹ , C = 0.5383
19 × 10 ~ ¹ .

Value of conditional expression 0.8 {r−√ (r ² −h _max ² )} = − 0.122, Z (h _max ) = − 0.143 {r−√ (r ² −h _max ² _{)} = - 0.152 | f 1} / f 2 | = 0.199
.

Example 4 i r _i d _i j n _j 0 0.140 1 0.900 1.793 1 1.76817 2 -0.900 5.0 3 3.970 0.969 2 1.76817 4 ∞ 4.592.

Aspheric surface Second surface: K = -1.05375, A = -0.90721
× 10 to ¹ , B = -0.195993 × 10 to ¹ , C = 0.703
04 × 10 ~ ¹ third surface: K = 0.132517 × 10, A = 0.1
19979 × 10 ~ ¹ , B = -0.546435 × 10 ~ ²
.

Value of conditional expression 0.8 {r−√ (r ² −h _max ² )} = − 0.122, Z (h _max ) = − 0.144 {r−√ (r ² −h _max ² )} = − 0.152
.

│f ₁ / f ₂ │ = 0.199
.

1 to 4 show the optical coupling states of the optical coupling lenses of Examples 1 to 4 in order. In these figures, reference numeral 10 indicates a light source and reference numeral 20 indicates an optical fiber.

The "optical coupling rate" in the above Examples 1 and 2 was 58% (-2.4 dB) in Example 1 and Example 2
Is 63% (-2 dB). The optical coupling by one ball lens (the amount of aspherical surface in Examples 1 and 2 is 0) is 9% (-10.5 dB).

The optical coupling rates in Examples 3 and 4 were 67% (-1.7 dB) in Example 3 and 69% in Example 4.
(-1.6 dB). Ball lens (Examples 3 and 4)
When the optical coupling is performed by the combination of the first lens in (1) with an aspheric amount of 0) and the convex lens (the second lens in Example 3), it is 24% (-6.2 dB).

In each of the first to fourth embodiments, the light source is a semiconductor laser that emits light having a wavelength of 1300 nm,
The optical fiber is assumed to have a diameter of 0.01 mm, which is a general single-mode fiber.

[0048]

As described above, according to the present invention, a novel optical coupling lens can be provided. Since the optical coupling lens of the present invention is configured as described above, it is possible to effectively optically couple the light flux from the LD or LED, which is the light source, to the optical fiber with a high optical coupling rate. 4).

Further, in the optical coupling lens according to the third and fourth aspects, since the light flux is a parallel light flux between the first lens and the second lens, various measurements and switching can be performed in this portion. it can.

[Brief description of drawings]

FIG. 1 is a diagram showing an optical coupling state of an optical coupling lens of Example 1.

FIG. 2 is a diagram showing an optical coupling state of an optical coupling lens of Example 2.

FIG. 3 is a diagram showing an optical coupling state of an optical coupling lens of Example 3.

FIG. 4 is a diagram showing an optical coupling state of an optical coupling lens of Example 4.

[Explanation of symbols]

 10 Light source (LD, LED) 20 Optical fiber

Claims (4)

[Claims] 1. A lens for optically coupling a light beam emitted from a semiconductor laser or a light emitting diode, which is a light source, to an incident end face of an optical fiber, the lens surface being formed by a single lens Is an axisymmetric aspherical surface, the maximum height of a ray passing through the aspherical surface is h _max , the aspherical surface value at the maximum height: h _max is Z (h _max ), on the optical axis of the aspherical surface. When the radius of curvature of r is r, these are given by the condition (1-1) 0.8 {r-√ (r ² -h _max ² )}> Z (h
_max )> {r−√ (r ² −h _max ² )}. 2. The optical coupling lens according to claim 1, wherein the lens surface on the light source side is also an aspherical surface symmetric with respect to the optical axis. 3. A lens for optically coupling a light beam emitted from a semiconductor laser or a light emitting diode as a light source to an incident end face of an optical fiber, wherein the light beam from the light source is a parallel light beam parallel to an optical axis. And a second lens for condensing the parallel light flux from the first lens toward the incident end surface of the optical fiber, and the lens surface of the first lens on the optical fiber side has an optical axis symmetry. It is an aspherical surface, and the maximum height of a ray passing through the aspherical surface is h _max , the aspherical surface value at the maximum height: h _max is Z (h _max ), and the radius of curvature of the aspherical surface on the optical axis is r. to time, these are conditions (2-1) 0.8 {r-√ (r 2 -h max 2)}> Z (h
_max )> {r−√ (r ² −h _max ² )}, and when the focal lengths of the first and second lenses are f ₁ and f ₂ , respectively, these conditions (2-2) 0 An optical coupling lens characterized by satisfying 0.15 <| f ₁ / f ₂ | <0.25. 4. The optical coupling lens according to claim 1, wherein the lens surface on the light source side of the second lens is also an aspheric surface symmetric with respect to the optical axis.