JPH0369083B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a gradient index collimating lens for collimating light emitted from a semiconductor laser into a parallel beam in an optical beam scanning device such as an optical information recording device, an optical reading device, or a laser beam printer. It is.

The collimating lens of the above device collimates the light emitted from the semiconductor laser with low aberration, so it is essential that the residual wavefront aberration is within λ/4. Here, λ is the oscillation wavelength of the semiconductor laser, and is generally 760≦λ≦850 nm. It is also required to be small, lightweight, and inexpensive.

Since the wavelength used in the above-mentioned optical beam scanning device is generally a single wavelength, the higher the transmittance of the lens as possible, the more the laser emission power can be suppressed and the reliability can be improved. Become. Therefore, as a countermeasure, it is advantageous to apply an antireflection film to the lens surface and to reduce the number of lenses as much as possible.

A collimator lens that satisfies the various requirements described above is known to be composed of three or four ordinary spherical lenses having a uniform refractive index. However, with this type of conventional spherical lens, there are 6 to 8 surfaces to be polished, and the lens diameter is very small, making polishing difficult and requiring advanced technology to combine the lenses. This will lead to an increase in costs.

The object of the present invention is to exhibit a residual wavefront aberration equal to or less than that of a collimating lens made of a conventional spherical lens system, and to have two flat light entrance/exit surfaces, which makes polishing extremely easy and inexpensive. The object of the present invention is to provide a gradient index calimeter lens for semiconductor lasers that can be mass-produced.

The collimating lens for semiconductor laser of the present invention is
The refractive index n(r) at the distance from the center r is n ² (r)=
No ² [1-(gr) ² +h ₄ (gr) ⁴ +h ₆ (gr) ⁶ +...] A graded index rod lens made of a cylindrical transparent medium, which satisfies the following conditions. do.

(1) r ₁ =∞ (2) r ₂ =∞ (3) 6.70≦Z＋f _B ≦7.30mm (4) 2ro≧2.0mm (5) 1.60≦no≦1.65 (6) 0.18≦g≦0.22mm ^-1 (7) 0.38≦h ₄ ≦0.87 (8) 1h ₆ 1＜5 (9) 0.21≦NA≦0.24 (10) 760≦λ≦850nm Under the conditions (1) to (10) above, r ₁ and r ₂ are The radius of curvature of the lens entrance surface and exit surface, Z is the lens length, f _B is the back focus on the light source (object) side, ro is the effective radius,
NA is the numerical aperture on the light source side, no is the refractive index on the central axis,
g, h ₄ and h ₆ are distribution constants, and λ is the oscillation wavelength of the semiconductor laser.

Among the above conditions, conditions (1) and (2) indicate that the entrance end face and the exit end face of the lens are flat, and since both sides are flat, it is possible to consolidate many lenses at the same time and perform flat polishing. It has the advantage of significantly improving efficiency and significantly reducing processing costs.

Condition (3) determines the numerical aperture NA of condition (9),
Outside this range, the NA will be too large or too small. Condition (4) determines the effective diameter, which is generally 2 mm in the optical beam scanning device described above.
An effective diameter of φ is required, so the lens outer diameter is 2
A value larger than mmφ is chosen. Conditions (5) and (6) are the refractive index distribution constants necessary to satisfy the NA of condition (9) and the effective diameter of condition (4) and to obtain a constant back focus f _B , and NO is on the central optical axis. In the refractive index, g represents the gradient of the refractive index, and corresponds to the radius of curvature of the spherical surface of a general lens. If g is too large, f _B will be too short, and if g is too small, NA will be too small. conditions
(7) is a condition for the refractive index distribution constant h ₄ to reduce the spherical difference of the lens. This range varies slightly depending on the lens length Z, center refractive index NO, and g constant, but when NO=1.63 and g=0.2 as shown in Figure 2, the lens length Z must be within the range of 3.6≦Z≦4.24. Condition (9) is not satisfied. In this case, the optimum h ₄ is the area narrowed by the vertical straight lines in FIG. 2, where the wavefront aberration is within λ/8 within the double diagonal lines, and within λ/4 within the diagonal lines. For example, if Z = 3.6mm, it is 0.52
When ≦h ₄ ≦0.72, the wavefront aberration Ws becomes Ws≦λ/8.
When 0.38≦h ₄ ≦0.82, Ws≦λ/4. Condition (8) is a condition regarding the refractive index distribution constant h ₆ that affects the spherical difference near the outer periphery of the lens. If this condition is exceeded, large high-order aberrations, especially fifth-order aberrations, will occur in the peripheral areas, and spherical aberrations will significantly worsen. Condition (9) is a condition for efficiently collimating the radiation angle from a semiconductor laser into parallel light with uniform intensity. Generally, the radiation angle of a laser varies depending on the direction, and is approximately
Since many of them are around 15°, converting to HA, NA~
0.25 or less. If the NA of the lens is too small, the efficiency will decrease, and if it is too large, the radiation angle of the semiconductor laser will be different in the direction of its active layer and in the direction perpendicular to it, making it impossible to obtain a parallel beam with a circular cross section, but with an elliptical cross section. It becomes parallel light with . For this reason, the NA of the lens needs to be within the range of 0.21 to 0.24.

FIG. 1 shows the function of the gradient index lens for semiconductor lasers according to the present invention. The lens 1 is a rod lens made of a transparent cylindrical medium such as glass or synthetic resin, and the entrance end surface 4 and exit end surface 5 of this lens 1 are parallel planes perpendicular to the optical axis 0-0. Also,
2 is a semiconductor laser light source, and 3 is a lens aperture.
Light rays diffusely emitted from a light source 2 placed at a distance f _B from the incident end surface 4 of a lens 1 are collimated into parallel light by a lens 1, and after passing through an aperture 3 having an aperture with a radius of ro, information is recorded. Emitted to the medium, etc.

Numerical examples of the collimator lens of the present invention described above will be shown below.

Example 1 r ₁ =∞ r ₂ =∞ Z=3.60mm 2ro=2.0mm no=1.63 g=0.2 0.52<h ₄ <0.72 h ₆ =0.30 NA=0.21 f _B =3.50mm f _B +Z=7.10mm Implementation Example 2 (When λ=780nm) r ₁ =∞ r ₂ =∞ Z=4.24mm 2ro=2.0mm no=1.63 g=0.2 0.62<h ₄ <0.78 h ₆ =0.30 NA=0.24 f _B =2.7mm f _B +Z=6.94mm

[Brief explanation of drawings]

FIG. 1 is a sectional view showing the specifications of the lens of the present invention;
FIG. 2 is a diagram showing the range of the refractive index distribution constant h ₄ for keeping the wavefront aberration within λ/8 to λ/4. DESCRIPTION OF SYMBOLS 1... Gradient index rod lens, 2... Light source (semiconductor laser), 3... Aperture, 4... Incidence end surface, 5... Output end surface.

Claims (1)

[Claims] 1. The refractive index n(r) at a distance r from the center is
n ² (r) = no ² [1-(gr) ² +h ₄ (gr) ⁴ +h ₆ (gr) ⁶ +
…]
A collimator lens for a semiconductor laser, which is a graded refractive index rod lens made of a cylindrical transparent medium represented by: (1) r ₁ =∞ (2) r ₂ =∞ (3) 6.70≦Z＋f _B ≦7.30mm (4) 2ro≧2.0mm (5) 1.60≦no≦1.65 (6) 0.18≦g≦0.22mm ^-1 (7) 0.38≦h ₄ ≦0.87 (8) 1h ₆ 1＜5 (9) 0.21≦NA≦0.24 (10) 760≦λ≦850nm However, r ₁ and r ₂ are the radius of curvature of the entrance surface and exit surface,
Z is the lens length, f _B is the back focus on the light source (object) side, ro is the effective radius, NA is the numerical aperture on the light source side, no
is the refractive index on the central axis, g, h ₄ and h ₆ are the refractive index distribution constants, and λ is the oscillation wavelength of the semiconductor laser.