JPH0420835A - Apparatus for measuring noise of returned light - Google Patents

Abstract

PURPOSE:To maintain optical performance and to simplify an apparatus by providing only one condenser lens between an optical monitor and a beam splitter, and arranging the lens so that the center of the lens is located at the outside of total incident luminous flux in the optical monitor. CONSTITUTION:The diameter of a condenser lens 21 is made larger than the maximum diameter of the total luminous flux from a beam splitter 4. An optical axis 24 of the lens is located at the outside of the luminous flux formed of rays 22 and 23. An optical monitor 16 is arranged at the focal point of the lens 21. The light receiving surface of the monitor 16 is made to agree with the optical axis 24 of the lens 21. In this constitution, the luminous flux formed of the rays 22 and 23 at the outermost sides is condensed at the monitor 16, and rays 25 and 26 at the outermost sides of the reflected luminous flux advance in the symmetrical direction with respect to incident light and the optical axis 24 of the lens 21. Therefore, as far as the optical axis 24 is located at the outside of the luminous flux formed of the rays 22 and 23, the luminous flux does not return to the beam splitter 4. When only one lens 21 having the large diameter is used in this way, the optical system wherein the luminous flux formed on the rays 22 and 23 and the luminous flux formed of the reflected rays are not overlapped can be constituted.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical system for a return light noise measuring device of a laser diode (hereinafter abbreviated as LD).

[Conventional technology]

When an LD is used for recording and reading compact disks, magneto-optical disks, etc., the light emitted from the LD is reflected from the surface of these disks and returns to the LD, and this returned light modulates the LD oscillation, making the amount of emitted light constant. It may disappear.

This is called return optical noise and interferes with stable recording and reading. Therefore, it is important to measure and evaluate the return light noise of the LD. Figure 4 shows a schematic diagram of the configuration of a conventional return light noise measuring device and its optical system. An overview of the device and its optical system will be explained.

In FIG. 4, for convenience of explanation, the light emitted from the LDI is treated as one light, and its path is represented by a dotted line, and its traveling direction is shown by an arrow.
It traces the progress of light along with each optical component. First, light emitted from the LDI passes through a collimator lens 2 to become light 3, and is split by a beam splitter 4 into light 5 in the same direction as light 3 and light 6 perpendicular to light 3. If the intensity of light 3 is 1, then light 5. The intensity of light 6 is about 172 each
It is.

The light 5 passes through the variable neutral density filter 7, has its light intensity weakened, and is focused by the condensing lens 8 onto the reflective surface of the plane [9].

And a plane! It is reflected by I9, becomes light 5a in the opposite direction, and returns to beam splitter 4. 1/ of the intensity of light 5a
2 becomes light 3a and light 10 in a direction perpendicular to the light 3a by the beam splitter 4, and the light 3a returns to the light output point LD1 using the collimator lens 2. The light 10 having the same intensity as the light 3a returns to the light monitor 12 by the condenser lens 11.
The light is focused on.

On the other hand, the light 6 leaves the polarizing beam splitter 13 and further passes through the polarizing beam splitter 13.
The light is focused onto a light monitor 16 by a condenser lens 15 using a 74-wavelength plate 14 .

In this way, the light 6 that does not depend on the light intensity change due to the neutral density filter 7 enters the light monitor 16, and the light 6 returns to the light monitor 12.
The light 10 that has been attenuated by passing through the neutral density filter 7 twice is incident. Polarizing beam splitter 13 and 174 wave plate 14
With this combination, the optical monitor 16
This prevents the light 6a reflected from the beam splitter 4 from returning to the beam splitter 4, and makes it possible to control the intensity of the returning light when the attenuation rate by the attenuation filter is large. The dimmed lights 3a and 5a are transmitted through the LD 1. The polarizing beam splitter 13 and the 174-wave plate 1 cut the light 6a reflected from the optical monitor 16 in order to return it to the optical monitor 12.
This is because if the light 6a is not cut here, only the sufficiently attenuated returning light will not go to the LDI and the returning light monitor 12. The DC component of the signal generated by the optical monitor 16 is proportional to the light emission intensity of the LDI, and the AC component corresponds to return light noise, which is usually evaluated using a spectrum analyzer.

[Problem to be solved by the invention]

As described above, the conventional return light noise measuring device has three optical components arranged between the beam splitter 4 and the optical monitor 16: the polarizing beam splitter 13, the quarter-wave plate 14, and the condensing lens 15. Therefore, the problem is that it is not easy to align the optical axes between these parts with high precision.
The device itself is also expensive.

The present invention was made in view of the above points, and its purpose is to simplify the device by reducing the number of optical parts, and to maintain the same performance as the conventional one without allowing the reflected light from the optical monitor to enter the measurement system. An object of the present invention is to provide a return light noise measuring device having the following features.

[Means to solve the problem]

In order to solve the above-mentioned problems, the return light noise measuring device of the present invention includes a light monitor and a beam splitter 7 that receive a parallel light flux emitted from an LD and passed through a collimator lens in a direction perpendicular to the parallel light flux by a beam splinter. A single condensing lens is used between the beam splitter and the beam splitter, and the center of this condensing lens is positioned outside of the total luminous flux that exits the beam splitter and enters the optical monitor. .

[Function] The returning light noise measuring device of the present invention has a groove structure as described above, and the condensing lens disposed between the beam splitter and the optical monitor has a radius at least larger than the maximum diameter of the total luminous flux coming out of the beam splitter. Use a large lens so that the optical axis of this condensing lens is located outside the light beam, and place the light monitor at the sharp point of the condensing lens! Therefore, instead of using three optical components: a polarizing beam splitter, a quarter-wave plate, and a condensing lens, a condensing lens with a large diameter is used.
By simply using a beam splitter, the light beam that exits the beam splitter and is focused on the optical monitor by the condensing lens does not overlap with the light beam that is reflected from the optical monitor, so there is no possibility that the reflected light will get mixed into the optical system such as a beam spotter. Therefore, this device including the optical system can be simplified, and the performance equivalent to that of the conventional device can be maintained.

〔Example〕

Figure 1 shows the return light noise measurement tube of the present invention! 4 is a schematic diagram showing the configuration of an optical system thereof, and the same reference numerals are used for parts common to those in FIG. 4. Similarly to FIG. 4, FIG. 1 also shows the path of light with dotted lines and its direction of travel with arrows, but in FIG. 1, the outline of the entire luminous flux emitted from the LDI is shown. In FIG. 1, the light emitted from the LDI passes through a collimator lens 2, and the outermost rays 17 and 18 of the luminous flux enter a beam splitter 4, and further pass through a variable neutral density filter 7, a condensing lens 8, and a plane mirror 9. The reflected light rays 19 and 20 are condensed by a condensing lens 11 onto a return light monitor 12 . On the other hand, by the beam splitter 4, the outermost ray of the total luminous flux traveling in the direction perpendicular to the lights &117 and 18, respectively, is a ray 22 that passes through the condenser lens 21 disposed between the beam splitter 4 and the light monitor 16.
．． 23 and is focused on the optical monitor 16. As described above, the device of the present invention differs from the conventional device in that the device of the present invention shown in FIG. The reason is that a condensing lens 21 is used instead.

This condensing lens 21 includes at least the beam splitter 4
It is necessary to use a lens with a radius larger than the maximum diameter of the total luminous flux coming out of the condenser lens 21, and its installation method is such that the optical axis 24 of the condensing lens 21 is located outside the luminous flux formed by the light rays 22 and 23. The light monitor 16 must be placed at the focal point of the condensing lens 21, and the light receiving surface of the light monitor 16 must be aligned with the optical axis 24 of the condensing lens 21.

In this way, the outermost light! The light flux formed by 22 and 23 is focused on the light monitor 16, and the outermost light &125 and 26 of the light flux reflected from the light monitor 16 is focused on the condenser lens 2.
Since the incident light travels in a direction symmetrical with respect to the optical axis 24 of the condenser lens 21, as long as the optical axis 24 of the condenser lens 21 is outside the light flux formed by the light rays 22 and 23, this light flux will pass through the beam splitter 4.
There is no turning back, i.e. the polarizing beam splitter 13
．． Instead of using the three optical components of the 1/4 wavelength plate 14 and the condensing lens 15, just one condensing lens 21 with a large diameter is used, and the light beam [22 and 23 and the reflected light beam 25] The key point of the present invention is that the apparatus including the optical system is configured so that the light beams formed by the light beam and the light beam 26 do not overlap.

FIG. 2 shows parts common to those in FIG. 1 with the same reference numerals, and is basically a schematic diagram of a device having a waste optical system as in FIG. Since there is a possibility that the light flux generated returns to the beam splinter 4, in order to avoid this, a light shielding plate 27 is provided on the condensing lens 21 to prevent the reflected light 25.26 from the optical monitor 16.
This is to reliably prevent this light flux from returning to the beam subrifter 4.

The schematic diagram shown in FIG. 3 is basically the same as FIG. 1 or 2, but in this case, instead of the condensing lens 21,
A light condensing partial lens 28 with a portion thereof removed is used, and a light shielding plate 27 is also used. At least the outermost rays 22, 23 from the beam splinter 4 to the optical monitor 6
Since there is no need for lenses other than the part through which the light beam formed by
In this figure, the condensing partial lens 28 is cut into 1/2 or 1/4 size using the diameter as a cutting line, for example. Even if this is done, the performance of the optical system will not change at all.

[Effects of the invention; Conventionally, the optical system of the return light noise measuring device includes a polarizing beam splitter and one optical system between the beam splitter and the optical monitor.
Equipped with three optical components: 74 wavelength plate and condensing lens! However, as described in the embodiment, the present invention replaces the three optical components with just one lens with a large diameter, and also uses a beam splitter. All you need to do is position the lens so that its optical axis is outside of the light beam emitted from the lens, so all you have to do to adjust the optical system is align the optical monitor, and the optical performance can be maintained sufficiently. In addition, the device is simplified, which makes a major economic contribution.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing the configuration of a return light noise measuring device of the present invention and its optical system, and FIG. 2 is a schematic diagram showing the configuration of a return light noise measuring device of the present invention provided with a light shielding plate and its optical system. A schematic diagram, Fig. 3 is a schematic diagram showing the configuration of the return light noise measuring device of the present invention and its optical system, which is provided with a partially removed condensing lens and a light shielding plate, and Fig. 4 is a schematic diagram showing the configuration of the optical system thereof. A schematic diagram showing the configuration of an optical noise measuring device and its optical system. 1: LD, 2: Collimator lens, 3.3a, 5.5
a. 6.6a, 10 knee light, 4: beam splitter, 7: variable neutral density filter, 8, 11, 15, 21: condensing lens 9: plane mirror, 12: light monitor, 13: polarization beam splitter, 14: 1/4 wavelength plate, 16:
Optical monitor, 17.18.19, 20, 22, 23.25
, 26: Outermost ray of light 24: Optical axis, 27: Light shielding plate, 28: Condensing partial lens.

Claims (1)

[Claims] 1) A collimator lens that receives light emitted from a laser diode and converts it into a parallel beam; a beam splitter that separates the parallel beam into a straight direction and a direction perpendicular to the parallel beam; this beam splitter a neutral density filter that attenuates the light beam in the straight direction emitted from the beam splitter; a plane mirror that reflects the attenuated light beam and returns the straight light beam as a light beam in the opposite direction to the beam splitter; A return light noise measuring device comprising: a light monitor that receives each of the light flux in the direction and the attenuated light flux in the opposite direction; an optical system disposed between the beam splitter and each of the light monitors, A return light noise measuring device characterized in that a center of a condensing lens in the optical system through which the vertical light beam from the beam splitter is incident is located outside of the light beam that enters and passes through this lens. 2) The return light noise measuring device according to claim 1,
A return light noise measuring device characterized in that a light shielding plate is attached to the remaining part of the area through which the light beam of the condensing lens passes. 3) The return light noise measuring device according to claim 1,
A return light noise measuring device characterized in that the remaining portion of the area through which the light beam of the condensing lens passes is removed.