JPS62196622A - Collimater lens for semiconductor laser - Google Patents

Abstract

PURPOSE:To reduce return light to a semiconductor laser emitting point by forming the beam projecting end face of a collimater lens consisting of a refractive index distribution type rod lens obliquely from the optical axis of the lens. CONSTITUTION:In the collimater lens 20 consisting of the refractive index distribution type rod lens, the optical axis 21 of the lens 20 is arranged so as to coincide with the beam projecting axis 12 of the semiconductor laser 10. Laser beams 14 radiated from the light emitting point 10a of the laser 10 are turned to parallel beams 14' by the lens 20. Although the incident end face 20a of the lens 20 is vertical to the optical axis 21, the outgoing end face 20b is obliquely formed with an angle phi from the optical axis 21. Thereby, the beams 14'' reflected by the outgoing end faces 20b are advanced to the semiconductor laser 10 side through an optical path different from that of the incident beams to the collimeter lens 20. Thereby, the beams 14'' are not returned to the light emitting point 10a.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of the Invention) The present invention relates to a collimator lens that converts a diverging beam emitted from a semiconductor laser into a parallel beam, and more specifically to a semiconductor laser capable of reducing light returning to a light emitting point of a semiconductor laser. This invention relates to a laser collimator lens.

(Technical Background and Prior Art of the Invention) Conventionally, optical scanning devices that scan by deflecting a light beam with an optical deflector have been widely put into practical use, for example, in various scanning recording devices, scanning recording devices, and the like.

Semiconductor lasers have been conventionally used as one means for generating light beams in such optical scanning devices. This semiconductor laser is small compared to gas lasers, etc.
Disappear cheaply! It has many advantages, such as low power consumption and direct modulation by changing the drive current.

As is well known, the light beam emitted from this semiconductor laser is a diverging beam, so many devices that use this semiconductor laser, including the various scanning recording devices and reading devices mentioned above, use this diverging beam. It passes through a collimator lens and becomes a parallel beam. As this collimator lens, in addition to a normal convex lens, a gradient index rod lens (well known in Japan under the trade name "Selfoc lens") is often used. In other words, such a rod lens is small and inexpensive, and is extremely suitable for taking advantage of the characteristics of semiconductor lasers, which are small and inexpensive as described above.

However, when such a rod lens is used as a collimator lens, there is a problem that a large amount of light returns to the semiconductor laser light emitting point, which increases the noise of the semiconductor laser. This point will be explained in detail below with reference to FIG. The semiconductor laser 10 and the collimator lens 11 made of a rod lens as described above are arranged so that the beam emission axis 12 and the lens optical axis 13 are aligned. Entrance end surface 11a and exit end surface 1 of collimator lens 11
1b are perpendicular to the lens optical axis 13, respectively.

The laser beam emitted from the light emitting point 10a of the semiconductor laser 10 is a diverging beam 14, which enters the collimator lens 11 and is converted into a parallel beam 14' by the collimator lens 11. Here, the incident end face 11
A part of the beam is reflected at the output end face 11b. As shown in the figure, the rate at which light reflected at the input end face 11a returns to the light emitting point 10a is low, but the rate at which light reflected at the output end face 11b returns to the light emitting point 10a is extremely high. That is, if the light reflectances of both end surfaces 11a and 11b are both R, and the relative positions of the semiconductor laser 10 and the collimator lens 11 are ideally adjusted, then the light returning from the output end surface 11b to the light emitting point 10a is l is a ratio of (1-R)2R to the amount of light emitted from the semiconductor laser 10. For example, in the case of R-0,005, the proportion of returned light reaches approximately 0.005.

(Object of the Invention) The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a collimator lens for a semiconductor laser that can reduce the return light to the semiconductor laser light emitting point. It is.

(Structure of the Invention) The collimator lens for a semiconductor laser of the present invention is characterized in that, in the collimator lens made of the gradient index rod lens as described above, the beam output end face is formed obliquely with respect to the optical axis of the lens. It is something to do.

(Embodiments) Hereinafter, the present invention will be described in detail based on an actual RAM shown in the drawings.

FIG. 1 shows a collimator lens for a semiconductor laser according to one embodiment of the present invention. The collimator lens 20 made of a gradient index rod lens is arranged so that its optical axis 21 is aligned with the beam emission axis 12 of the semiconductor laser 10. A laser beam (divergent beam) 14 emitted from the light emitting point 10a of the semiconductor laser 10 is converted into a parallel beam 14' by the collimator lens 20. Here, the entrance end surface 20a of the collimator lens 20 is perpendicular to the lens optical axis 21, but the exit end surface 20b is formed obliquely to the lens optical axis 21 by an angle φ.

Therefore, the beam 14'' reflected at the output end face 20b travels to the semiconductor laser 10 side through a different optical path from that of the beam incident on the collimator lens 20, and does not return to the semiconductor laser light emitting point 10a. If the reflected light from the collimator lens 20 does not return to the light emitting point 10a, the semiconductor laser 10 will not generate noise due to the returned light as described above.

A preferred value of the inclination φ of the exit end face 20b with respect to the lens optical axis 21 will be described below. Collimator lens 2
0, SELFOC lens W manufactured by Nippon Sheet Glass Co., Ltd.
20E15B083N is used. The specifications of this SELFOC lens are as follows.

・Lens length Z -3,163m ・Refractive index distribution n (r
) -1,599sec (0,298)
r-1,599 (1-0,0444r 2 )・Diameter 2
．． 0im ・Pitch number 0.15 The refractive index distribution of the gradient index rod lens is n(r)=n
o<I Ar2/2) (no is the refractive index on the lens optical axis, r is the distance from the lens optical axis, and A is a constant) It can be approximated by the formula, and in this case, the trajectory of the meridional ray will be as shown in Figure 3. . The ray matrix at this time is given by the following equation.

When using the above-mentioned Selfoc lens, no =
1,599, p = 0.0888, Z = 3.
163m5+, so from the above equation (1), as shown in FIG. Angle Q (rad) at a distance r2 from the optical axis 21
Suppose that the beam is emitted at and collimated. In this case (from equation 2, ;, r2=lIO, 294+t, 698θ.

Q--0,193+ 0.58801 Therefore, 12 votes 0.8513 (wear), θt = 0.
3282 (rad) In this case, the position IS (distance from the incident end surface 20a) where the light emitting point 10a should be placed is: S = r 1 /lan θ 1 - L468
2 (ago> Next, as shown in Figure 5, exit II side 2
Consider a case where 0b is tilted by a small angle φ with respect to the lens optical axis 21.

In this case, the beam 14'' that is reflected by the output end face 20b and travels inside the collimator lens 20 toward the semiconductor laser 10 is 2φ with respect to the lens optical axis 21 in the vicinity of the output end face 201).
form an angle. This angle is approximately 3φ as the outside angle of the collimator lens 20 (that is, assuming the average refractive index of the Selfoc lens is 1.5, 1.5φ).
5X 2φ-3φ). From the above equation (2), therefore, from this (3 equation), (from the gradient equation ri −0,5006−5,094φθ1
-0.3286 + 1.764φ...(61
(From formula 9 r1' −−0,5006−5,094φ
θ1' = -0,3286+ 1.764φ...
(On the other hand, the focus point of the returning beam 14″ and the beam exit axis 12
The distance Δ between is Δ=(r,'tanθ1 rttanθ1゛)/(ta
n (31-tan θt') −
(8) From the above equations (6), (7) and (8), the above φ
and the value of Δ is determined. Here, we give a specific value of the slope φ,
Find the distance Δ at that time.

, when φ=0,002 rad, rl -0,4904tm, θ1-0.3321
radrl' m+ 0.5108,,
θ,' −−0,3251rad, °, Δ−(−0,
1762+ 0.1653 ＞/ 0.6819
-1 0.016 rad or less Similarly, when φ-0,004 rad Δ=-0,032 aw
When φ-0,006rad Δ−−− 0.04+
When 38φ-0,0tOrad Δ--0.080
When mφ-0,015rad Δ--0,120m
When φ-0.02 rad, Δ-0.160 am or more The relationship between the slope φ and the distance Δ is as shown in FIG.

The size of the light emitting point 10a of the semiconductor laser 10 is usually PN.
Q is in the range of 2 to 5 μm in the direction along the bonding surface and 2 to 0.8 μm in the direction perpendicular to the PN bonding surface.

On the other hand, the beam 14'' that is reflected by the output end surface 20b of the collimator lens 20 and returns to the semiconductor laser 10 side is considerably affected by the aberration of the collimator lens (Selfoc lens) 20, the wavelength variation of the laser beam 14 emitted from the semiconductor laser 10, etc. The blur will be several times (3 to 5 times) the area of the light emitting point 10a.For safety reasons, if the distance Δ is set to about 10 times the light emitting point size, that is, about 50 μm, the light will return to the light emitting point 10a. From the above figure 6, when Δ-50μm, φ-0.0062
rad = 6.2 mrad. That is, the inclination φ of the output end surface 20b of the collimator lens 20 is set to 5 mrad.
With the above settings, it is possible to reliably prevent the return light from entering the light emitting point 10a.

Of course, the above value of φ-5m rad or more is a desirable value when the size of the light emitting point 10a, the degree of blur of the beam 14'', etc. are within the ranges mentioned above, and if these conditions change, it will naturally change. The desired value of the slope φ also varies.

(Effects of the Invention) As described above in detail, the collimator lens for a semiconductor laser of the present invention can reliably prevent returning light from entering the semiconductor laser light emitting point, and can prevent the generation of noise in the semiconductor laser due to this returning light. It is possible to suppress it.

[Brief explanation of drawings]

FIG. 1 is a schematic side view showing a collimator lens for a semiconductor laser according to an embodiment of the present invention, FIG. 2 is a schematic side view showing a conventional collimator lens, and FIGS. 3.4 and 5 are a schematic side view showing a collimator lens of the present invention. FIG. 6 is a graph showing the relationship between the distance from the semiconductor laser beam emission axis to the return light focusing point and the inclination of the rod lens emission end surface according to the present invention. DESCRIPTION OF SYMBOLS 10... Semiconductor laser 10a... Light emitting point of semiconductor laser 20... Collimator lens 20b... Outgoing end surface 21 of collimator lens... Lens optical axis

Claims (1)

[Claims] A collimator lens for a semiconductor laser consisting of a gradient index rod lens and converting a diverging beam emitted from a semiconductor laser into a parallel beam, characterized in that a beam emitting end face is formed obliquely with respect to the optical axis of the lens. Collimator lens for semiconductor laser.