JPS62198184A - Method for detecting semiconductor laser mode hopping - Google Patents

Abstract

PURPOSE:To readily detect a mode hopping by observing the light emitting point position of a semiconductor laser by a suitable method, and detecting at least one of longitudinal and lateral movements of the light emitting point position. CONSTITUTION:A mode hopping detector 32 has a half mirror 34 for branching a laser beam 16, a focusing lens 36 for focusing a laser beam branched by the mirror 34 at a predetermined position, and a 2-dimensional position detector 38 disposed at the focusing position. According to this detector 32, the moving state (moving distance and direction) of lateral directions (X, Y direction) of the light emitting point position in a semiconductor laser 10 can be detected, and information regarding the moving state is input to a semiconductor laser 2-dimensional position controller 42 for forming a light emitting point position correcting unit 40 in combination with the detector 32. The controller 42 finely moves the laser 10 in a direction for cancelling the movement of the light emitting point position due to mode hopping based on information from the detector 32.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method for detecting mode hopping in a semiconductor laser.

(Technical Preparation of the Invention 11j and Prior Art) Recording various information such as image information on a recording medium by scanning a light beam on the recording medium, or reading various information recorded on the recording medium. In optical scanning, a laser beam emitted from a semiconductor laser may be used as the scanning light beam.

However, it is known that semiconductor lasers cause mode hopping depending on their driving conditions. In other words, in a semiconductor laser, different modes (laser wavelengths) compete under certain driving conditions (predetermined combination of temperature and light intensity), and under that driving condition, one of the different modes will be forced to move from one to the other. A phenomenon occurs in which mode hopping from one mode to another or from the other to one is repeated in a reciprocating manner. FIG. 5 is a conceptual diagram showing an example of driving conditions under which this mode hopping occurs, and mode hopping occurs under the driving conditions corresponding to the dotted portions in the figure.

It is also known that when the mode hopping occurs, the amount of light of the laser beam fluctuates, causing mode hopping noise. The magnitude of the laser beam light amount fluctuation due to this mode hopping is usually about 1%, and when it is large, it can reach about 2 to 3%. Therefore, if the optical scanning to be performed using a laser beam is, for example, one that handles continuous Rf image information, which generally requires the maximum variation in the laser beam light amount to be approximately 0.2% or less, In this case, if mode hopping as described above occurs during the optical scanning, unevenness in the read or recorded continuous l@tonal image information will occur, which is undesirable.

However, as a result of intensive research into the mode popping of the semiconductor laser, the inventors of the present invention found that when mode popping occurs, not only the amount of the laser beam but also the light emitting point position (beam waist position) of the semiconductor laser changes. It was discovered that the light emitting point position moves in the vertical direction, which is the laser beam emission direction, and in the horizontal direction perpendicular to the longitudinal direction.

This movement of the light emitting point position is undesirable because it causes jitter and pitch unevenness in optical scanning.

As described above, a phenomenon called mode hopping exists in semiconductor lasers, and this mode hopping causes changes in the laser beam emission state that are unfavorable for optical scanning, such as changes in the amount of laser beam light and changes in the position of the light emitting point.

Accordingly, when performing optical scanning with a semiconductor laser, if -mode hopping occurs, the mode hopping is avoided by zero. It is desirable to take measures such as taking appropriate measures to correct changes in the laser beam emission state such as changes in the laser beam light intensity or changes in the light emitting point position due to mode hopping by some method. The condition is to detect mode hopping that occurs in the semiconductor laser (detection of mode hopping here means detection of the occurrence of mode hopping or detection of a change in the laser beam emission state due to mode hopping). It is necessary to.

(Object of the Invention) In view of the above circumstances, an object of the present invention is to provide a semiconductor laser mode hopping detection method that can easily detect mode hopping in a semiconductor laser.

(Structure of the Invention) In order to achieve the above object, the semiconductor laser mode hopping detection method according to the present invention detects the light emitting point position of the semiconductor laser by an appropriate method, and moves the light emitting point position in the vertical direction and horizontally. The present invention is characterized in that mode hopping of the semiconductor laser is detected by detecting at least one of the directional movements.

The above-mentioned movement detection of the light emitting point position may be simply detecting that the light emitting point has moved, or may also detect the movement state (moving direction, amount of movement, etc.) of the light emitting point. .

Detection of mode hopping in the case of detection of mere movement means detection of occurrence or occurrence of mode hopping. Furthermore, in the case of detection that includes the moving state, the above-mentioned detection of mode hopping is not limited to detecting the occurrence of mode hopping, but also includes detection of changes in the emission state of the laser beam due to mode hopping. It turns out.

(Effects of the Invention) As described above, the semiconductor laser mode small popping detection method according to the present invention detects the movement of the light emitting point position of the semiconductor laser.

As described above, the present inventors discovered that when mode hopping occurs in a semiconductor laser, the position of the laser beam emission point moves.

Therefore, by detecting the movement of the light emitting point position of the semiconductor radar as in the present invention, it is possible to easily detect the occurrence of mode hopping.

Furthermore, if the movement state is also detected when detecting the movement of the light emitting point position, information regarding changes in the Radhe beam emission state caused by mode hopping can also be obtained. That is, in addition to information regarding the movement of the light emitting point position among changes in the laser beam emission state, the correlation between other changes in the laser beam emission state, such as changes in the amount of laser beam light, and the movement of the light emission imaginary W1 is determined in advance. If investigated, it is possible to obtain information regarding various other changes in the laser beam emission state based on the light emitting point position movement information.

By obtaining information regarding the occurrence of mode hopping and changes in various laser beam emission states caused by mode hopping, we will take various measures to eliminate inconveniences caused by mode hopping based on that information. It is possible to take the following steps.

(Actual Embodiment) Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a perspective view showing a semiconductor laser. As shown, the semiconductor laser 10 emits a laser beam 16 from its active @ 12 light emitting region, ie, a light emitting point 14 .

As described above, this light emitting point 14 is caused by mode hopping in the vertical direction (J), which is the emission direction (Z direction) of the laser beam 16, and in the horizontal direction (X), which is perpendicular to the vertical direction.

(Y direction). A phenomenon in which the light emitting point 14 moves simultaneously in the vertical and horizontal directions, but only in either direction, has not been found.

FIG. 2 is a conceptual diagram (not shown) of a recording optical scanning device equipped with a mode hopping detection device for carrying out an embodiment of the method according to the present invention. This device modulates a laser beam 1G emitted from a semiconductor laser 10 with an optical modulator 18 such as an AOM, and directs the modulated laser beam onto a recording medium 22 using a rotating polygon 1iJ120, which is an optical deflector.
24 is a collimator lens,
2G is an fθ lens, 28.degree. 30 is a cylindrical lens, and the pair of cylindrical lenses 28 and 30 function to correct the tilt of the rotating polygon #A20.

Additionally, this device is a mode hopping detection device 32 for implementing an embodiment of the present invention, that is, a method for detecting the lateral direction (X, Y direction) movement state of the semiconductor laser light emitting point position.
It is equipped with the following.

This mode hopping detector @32 is a laser beam 1
Half mirror as a beam splitter that branches 6
34, an imaging lens 3G that images the laser beam split by the half mirror 34 at a predetermined position, and a two-dimensional position detector 38 disposed at the imaging position. As the two-dimensional position detector 38, any device that can detect the two-dimensional position of the light beam may be used, such as a continuous position detection photodiode, a four-part photodiode, a photodiode array, and a vidicon. .

According to this mode hopping detector @32, the movement state of the light emitting point position in the semiconductor laser 10 in the lateral direction (X, Y direction) ffl! (Amount of movement, direction of movement, etc.) can be detected, and information regarding this movement state is collected by the light emitting point position correction device 4 in combination with this mode hopping detection device 32.
0 is input to the semiconductor laser two-dimensional position control device 42 .

The two-dimensional position RA control device r142 is a device that can move the semiconductor laser 10 by a minute amount in the lateral direction (X, Y direction), and detects the light emitting point by mode hopping based on the information from the mode hopping detection device 32. To move the semiconductor laser 10 in a direction that cancels the positional movement, for example, when the light emitting point moves by 6 in the upward direction in the figure (upward in the Y direction), move the semiconductor laser 10 downward in the figure (downward in the Y direction) by δ. It operates to move only.

According to l1itllll of the present inventors, this shift δ of the light emitting point position (this movement takes various directions such as Y direction only, Y direction only, x, composite direction) depends on which mode mode hopping is Although it depends on which mode the mode is to be transferred from, the maximum value is approximately 0.1 μm. Calculating the change Δ in the Radhe beam position on the scanning surface of the scanning medium 22 due to this shift all δ, we get: 8−δx(f2/f,) However, “1: Focal length f2 of the collimator lens 24
: Calculated by the focal length of the rθ lens 2G, and in the typical case of a laser printer, usually f = 3.5 ~ 10 am, f 2-2
Since it is 50 to 500M, the Δmaro is about 2.5 to 15 μm. If the pixel size is 80 μm, this movement amount Δ corresponds to a pixel shift of 0.19 pixels, that is, jitter or pitch unevenness.

Furthermore, a change in the light emitting point position also causes problems other than the jitter and pitch unevenness described above. That is, the light emitting point position is δ-1,5
When moving by μm, the collimator lens 2
If the laser beam 16 passing through the AOM 18 is deflected and the focal length of the lens is r t -4 m, the angle of the laser beam 16 incident on the AOM 18 will be shifted by about 5.2", and as a result, the angle of incidence on the AOM 18 will be slightly different from the Bragg condition. Off, AOM
This results in a disadvantage that the diffraction efficiency of 18 is reduced.

Note that in the mode hopping detection device 32,
It is desirable to expand the amount of movement of the laser beam imaging position on the two-dimensional position detector 38 to be about 10 times or more the amount of movement of the light emitting point position on the semiconductor laser 10. For that purpose, for example, [3/"5≧10 (however, "
3 may be the focal length of the imaging lens 36>. The resolution of the position detector 38 is about 0.1 to 0.2 μm, and unless the amount of movement is expanded by about 10 times, the light emitting point position can be accurately moved vJ! This is because there is a possibility that 1 cannot be detected.

FIG. 3 is a conceptual diagram showing a recording optical scanning device comprising a mode hopping detection device 144 for carrying out another embodiment of the method according to the present invention.

This optical scanning device differs from the one shown in FIG. 2 only in a light emitting point position correction device 4G including a mode popping detector fi44 for implementing the above-mentioned other actual tSS, and the components common to both are the same. have the same number.

This mode hopping detection device 44 is a device for implementing another embodiment, that is, a method for detecting the vertical (two-direction) movement state of the semiconductor laser light emitting point position, and includes a collimator lens 24 and a half mirror 34. , the half mirror
A vertical movement detector @4B receives the laser beam branched by 34 and detects the vertical movement state of the light emitting point.

A specific example of the vertical movement detection device 48 is shown in FIG.
b), shown in (0). All of these specific examples utilize the fact that when the light emitting point position changes in the vertical direction, the parallelism of the laser beam 16 after passing through the collimator lens 24 changes.

The vertical movement detection device 48 shown in FIG. Opening member 5 having an opening 52a for passing through
2, and a photodetector 54 that detects the amount of laser beam that has passed through the aperture 52a.

As mentioned above, if the light emitting point position changes in the vertical direction, the parallelism of the laser beam after passing through the collimator lens 24 will change. For example, if the light emitting point position moves to the left in the figure, the parallelism of the laser beam after passing through the collimator lens 24 will change. The laser beam 1G tends to converge, and therefore the branched laser beam that passes through the condenser lens 50 is condensed onto Al above the original condensing point A, thereby increasing the amount of light passing through the opening 52a. In addition, when the light emitting point position moves to the right side in the figure, the condensing point becomes lower, At, and as a result, the aperture 5
The amount of light passing through 2a decreases. Therefore, the photodetector 54
By changing the amount of laser beam received by
Vertical movement of the light emitting point position can be detected.

A device 48 shown in FIG. 4(b) is obtained by removing the condenser lens 50 from the specific example shown in FIG. 4(a). In this case too,
According to the principle explained in the case of FIG. 4(a), the opening 52a
The amount of light of the branched laser beam passing through is detected by a photodetector 54, and the vertical movement of the light emitting point position is detected based on the change in the amount of received light.

Note that in this specific example, there is no condenser lens 50, so 1. Compared to the case of FIG. 4(a), the ratio of the change in the amount of received light to the amount of change in the position of the light emitting point is smaller.

The device 48 shown in FIG. 4(C) has a central portion 5 as shown.
6a and an outer peripheral portion 56b provided with a predetermined gap between the two portions of the circular photodetector 56. In this specific example, when the light emitting point position changes in the vertical direction and the parallelism of the laser beam 1G changes, this causes the central portion 56a and the outer peripheral portion 56 to change.
Since the ratio of the amount of light received by the laser beams b and B is different, the vertical movement of the light emitting point position is detected based on the change in the ratio of the amount of received light.

The direction of change in the amount of received light in FIGS. 4(a) and (b) above,
That is, the increase/decrease in the amount of received light and the direction of change in the ratio of received light amounts in FIG. This corresponds to the direction movement, and the rate of change in the amount of received light and the ratio of received light amounts corresponds to the amount of change in the position of the light emitting point. ! 1lfft can be detected.

Note that, as mentioned above, the amount of laser beam light changes by several percent due to mode hopping, so in the vertically moving detection device N4B, the change in the amount of laser beam light due to the vertical movement of the light emitting point position has a variation width of about several percent. You need to set it so that it exceeds it. For this purpose, for example, Figure 4 (a)
), the focal length ratio between the collimator lens 24 and the condensing lens 50 may be appropriately selected. In addition, in the case of FIG. 4 <a>, there is no need to take such consideration.

The mode hopping detecting device 44 in FIG. 3 constitutes the above-mentioned light emitting point position correcting device 46 together with the semiconductor laser vertical position v4 control device 5B, and the output of the mode hopping detecting device [44 is for controlling the vertical position! The information is input to the device 58. The vertical position control @flsB is a device that can move the semiconductor laser 10 by a minute amount in the vertical direction (two directions), and the output from the mode hopping detection device @44 (
The semiconductor laser is moved in a direction that cancels the vertical movement of the light emitting point due to mode hopping based on the information (information regarding the vertical movement of the Fe light point). For example, when the light emitting point moves by δ to the right in the figure, the semiconductor laser It operates to move the laser 10 by δ in the left direction in the figure.

The embodiments described in FIGS. 2 and 3 above detect only the horizontal movement and the vertical movement of the light emitting point position, respectively, but the method according to the present invention detects movement in both directions simultaneously. It may be something that you do. Such a method can be implemented, for example, by simultaneously providing both mode hopping detection devices fi 32.44 shown in FIGS. 2 and 3. Further, as an example of the use of such a method of simultaneously detecting movement in both directions, for example, a semiconductor laser is used to cancel the movement of the light emitting point in both directions based on the movement information of the light emitting point detected by this method It is conceivable to move 10 in both directions.

The method according to the present invention is constructed based on the discovery that when mode hopping occurs as described in -1, the light emitting point position moves in the vertical and horizontal directions, and the movement of the light emitting point position is detected. The mode hopping of the semiconductor laser is thereby detected.

Detection of the movement of the light emitting point position is not limited to detecting the moving state of the light emitting point position in the above-described embodiment;
Detection of mode hopping in this case means detection of occurrence of mode hopping, and the method according to the present invention in such a case is a typical one. Specifically, it can be used to change the driving conditions of a semiconductor laser to avoid mode popping, but it can of course also be used for various other purposes.

In addition, the movement detection of the light emitting point position is performed as described above! In the case where the movement state of the light emitting point position is detected as in the M mode, the detection of mode hopping is the detection of a change in the laser beam emission state due to mode hopping, and the method according to the present invention in this case also It can be used for various purposes.

A typical example of the purpose is to move the semiconductor laser in order to cancel the movement of the light emitting point position by directly using the information regarding the state of movement of the light emitting point position described above, but there are also other methods, such as moving the recording medium. It may be used for the purpose of canceling the movement of the light emitting point position, or the correlation between the movement of the light emitting point position and a change in the laser beam output layer other than the movement of the light emitting point position, such as a change in the amount of laser beam light, may be determined in advance. It may be used for the purpose of canceling the change in the laser beam emission state after checking. Examples of such usage include, for example, changing the driving conditions of the semiconductor laser so as to change the light intensity of the laser beam in order to cancel changes in the laser beam light intensity depending on the state of movement of the light emitting point position, or It is conceivable to add appropriate correction to the read signal when reading information.

Furthermore, the method according to the present invention can be used not only for the above-mentioned optical scanning for recording, but also for optical scanning for reading.

The method according to the present invention can be modified in various ways without departing from the gist thereof, and is not limited to the embodiments described above.

[Brief explanation of drawings]

FIG. 1 is a perspective view without showing a semiconductor laser; FIGS. 2 and 3 are conceptual diagrams showing an optical scanning device equipped with a device for implementing an embodiment of the present invention; FIG. 4(a) , (b) and (C) are diagrams each showing a specific example of the vertical movement detection device in FIG. 3, and FIG. 5 is a conceptual diagram showing an example of mode hopping generation drive conditions. 10... Semiconductor laser 14... Light emitting point position 16... Laser beam

Claims (1)

[Claims] Detecting mode hopping in the semiconductor laser by detecting at least one of the vertical movement of the light emitting point position of the semiconductor laser in the laser beam emission direction and the horizontal movement perpendicular to the vertical direction. Features a semiconductor laser mode hopping detection method.