JP3874160B2 - Position detection device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a position detection apparatus that optically detects the position of a detection target using an interferometer.
[0002]
[Prior art]
A position detection apparatus using an interferometer has an advantage that position detection can be performed with high accuracy and without contact. FIG. 5 is a configuration diagram of a conventional example of a position detection apparatus using an interferometer.
[0003]
In FIG. 5, the position of the target mirror 1 is fixed, and the optical circuit 2 moves in the CC ′ direction in conjunction with a detection target (not shown).
The laser light source 21 emits laser light having a polarization component perpendicular to the paper surface. The lens 22 makes the light emitted from the laser light source 21 balanced light. The half mirror 23 branches the light that has passed through the lens 22 into transmitted light and reflected light.
[0004]
The mirrors 24 and 25 whose positions are fixed receive the light transmitted through the half mirror 23, reflect the received light, and return it to the half mirror 23. The mirror 24 is arranged at an angle of 45 ° with the optical axis of the laser light source 21. On the other hand, the mirror 25 is arranged at an angle of 45 ° + θa with the optical axis of the laser light source 21.
[0005]
A polarization beam splitter (hereinafter referred to as PBS) 26 reflects the light reflected by the half mirror 23. The reflected light passes through the λ / 4 plate 261 and proceeds to the target mirror 1.
When light passes through the λ / 4 plate 261 twice, the light is converted from vertical polarization to horizontal polarization and from horizontal polarization to vertical polarization. By providing the λ / 4 plate 261, the PBS 26 can select transmitted light and reflected light.
[0006]
After being reflected by the target mirror 1, the corner cube 27 reflects the light transmitted through the λ / 4 plate 261 and the PBS 26 and returns it to the PBS 26.
The phase detector 28 detects interference fringes generated by the light returned to the half mirror 23.
Let L be the distance between the target mirror 1 and the corner cube 27.
[0007]
In the position detection apparatus of FIG. 4, the light emitted from the laser light source 21 includes light that takes the following path.
Lens 22 → half mirror 23 → mirror 24 → mirror 25 → half mirror 23 → phase detector 28
The light that takes this path is referred to as light of (1).
Lens 22 → half mirror 23 → PBS 26 → λ / 4 plate 261 → target mirror 1 → λ / 4 plate 261 → PBS 26 → corner cube 27 → PBS 26 → λ / 4 plate 261 → target mirror 1 → λ / 4 plate 261 → PBS 26 → half mirror 23 → phase detector 28
The light that takes this path will be referred to as light (2).
[0008]
Since the arrangement angle of the mirror 25 is shifted by θa with respect to the arrangement angle of the mirror 24, the wavefront of the light a returning along the path (1) and the light returning along the path (2) The wavefront of b is shifted by θa. The light a and b interfere with each other due to the deviation of the wavefront to form an interference fringe S. The interference fringe pattern P shown in FIG. 6 is given by the following equation.
P = λ / θa (λ is the wavelength of the laser beam)
[0009]
The phase detector 28 detects an interference fringe by two light receiving elements arranged with a phase shifted by P / 4 pitch along the arrangement direction of the interference fringes. The light receiving element is, for example, a photodiode. FIG. 6 shows the arrangement of the two light receiving elements 281 and 282.
The two light receiving elements 281 and 282 output detection signals corresponding to the amount of received light. The detection signals of the light receiving elements 281 and 282 are sin θ and cos θ (θ is a phase). The phase θ is modulated according to the movement amount ΔL of the optical circuit unit 2.
[0010]
θ is given by the following equation.
sin θ = sin (2π · 4ΔL / λ)
cos θ = cos (2π · 4ΔL / λ)
In these equations, “4” exists because light travels back and forth between the target mirror 1 and the corner cube 27 and the optical path length changes by 4ΔL when the optical circuit unit 2 moves by ΔL.
[0011]
The comparator 29 converts the detection signals K1sinθ and K1cosθ of the light receiving elements 281 and 282 into pulse signals. The converted pulse signals are referred to as pulse A and pulse B.
The phase counter 30 integrates the phase change Δθ / 2π within the fixed time T of the pulse A and the pulse B, and obtains the phase integrated value n = Σ (Δθ / 2π).
[0012]
The relative distance L ′ from the start of integration is
L ′ = n × λ / 4
It becomes. Further, the phase counter 30 determines the moving direction of the detection target from the preceding and following relationship between the phases of the pulse A and the pulse B.
[0013]
FIG. 7 is a waveform diagram of each signal in the apparatus of FIG. 5, and the horizontal axis indicates the relative distance L ′.
[0014]
[Problems to be solved by the invention]
Depending on operating conditions such as temperature and current, the semiconductor laser diode 24 may change its transmission mode (mode hop) and change its transmission wavelength (λ). When the mode hop point is in the vicinity of the normal operating point, there is a possibility of mode hopping with a slight change in conditions.
[0015]
When position detection is performed using a semiconductor laser as an interferometer, if a mode hop occurs during length measurement, the wavelength before the mode hop is λ0, the wavelength after the mode hop is λ1, and the phase integrated value n after the mode hop As
ΔL = n × (λ0−λ1) / 4
Measurement error will occur.
Therefore, the problem to be solved by the present invention is to realize a position detection device that can predict the possibility of mode hopping and avoid this in advance.
[0016]
[Means for Solving the Problems]
In order to achieve such a problem, the invention according to claim 1 of the present invention is:
An optical circuit unit that generates interference fringes with a constant pitch that moves in conjunction with the movement of the detection target by using interference of light;
A phase detector that detects interference fringes by a plurality of light receiving elements arranged to be shifted along the arrangement direction of the interference fringes and generates two types of modulation signals that are modulated according to the amount of movement of the detection target;
In a position detection device comprising a phase counter that calculates the amount of movement by integrating the phase difference of the phase detector output,
A semiconductor laser diode provided as a light source of the optical circuit section;
A current source that drives the semiconductor laser diode with an operating current defined by the control input voltage ;
Mode hop control means for varying the operating current according to the control input voltage ,
The mode hop control means detects a mode hop of the light source based on a change in the phase counter output when the operating current is changed by the control input voltage.
[0017]
The invention according to claim 2
The mode hop control means determines the mode hop of the light source based on a change in the phase counter output when the control input voltage is changed with a constant amplitude and the operating current is changed with the optical circuit portion fixed. It is characterized by detecting.
[0018]
The invention described in claim 3
The mode hop control means records the phase counter output in a state where the optical circuit unit is fixed, and changes the control input voltage a plurality of times with a constant amplitude to change the operating current and then returns to the original state. The phase counter output and the recording output are compared, and if the two are the same, the current control input voltage is maintained. .
[0019]
The invention according to claim 4
A switching means is provided in which an operating point that is approximately ½ the mode hop interval away from the current operating point is used as the changed operating point.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing a configuration of an embodiment of a position detection apparatus according to the present invention. The same reference numerals are given to the same elements as those of the conventional apparatus described in FIG.
[0022]
The characteristic part of the present invention is that the mode hop control means 31 is provided, the control input voltage V defining the operating current of the semiconductor laser diode 24 is changed, and the change of the integrated output n of the phase counter 30 is monitored.
[0023]
Assuming that the wavelength of the laser is λ0, sin θ and cos θ are
sin θ = sin (2π × 4L / λ0)
cos θ = cos (2 × 4L / λ0)
It becomes. When the optical circuit unit 2 is fixed and the current I is changed by the control input voltage V, λ0 slightly changes according to the current.
[0024]
FIG. 2 shows waveform changes of sin θ and cos θ corresponding to the change of the current I. In the range (1) where the operating point is normal and mode hopping does not occur, when the current is restored, the wavelength returns to λ0, so that the phase also returns to the same phase state as before the current change, and the integrated value n of the phase counter 30 is also It does not change.
[0025]
On the other hand, at the current value at which the mode hop occurs, the wavelength changes instantaneously from λ0 to λ1, so that the waveform changes discontinuously as shown in (2) and (3). The phase change δθ at this time is
δθ = 2π × 4L (1 / λ0-1 / λ1)
It becomes. The phase change at this time exceeds 2π if L is sufficiently large, and the phase counter 30 cannot perform normal phase integration.
4L> δθ / {2π (1 / λ0−1 / λ1)}
Therefore, even if the current is returned to the original state, n does not return to the original phase integrated value. This makes it possible to detect the presence or absence of a mode hop near the operating point.
[0026]
FIG. 3 is a flowchart showing a procedure of the operation of the mode hop control means 31, and the processing is executed at regular intervals or on demand. First, the optical circuit unit 2 is fixed by the command R, and the phase counter output n at that time is recorded as X1.
[0027]
In the next step, the laser current I is repeatedly changed with a predetermined amplitude by the control input voltage V and then returned to the original value. The phase counter output n at this time is recorded as X2.
In the next step X1 = X2? If X1 = X2, it is determined that the operating point is normal and the current is not changed. If X1 ≠ X2, the operating point is determined to be near the mode hop point, and the operating current is changed in the next step.
[0028]
In addition to the method of changing the operating current, a change width ΔI of one time is set in advance and the current value is changed stepwise until X1 = X2 is satisfied as shown in the loop P of FIG. It is also possible to employ means for measuring an appropriate change current value in advance and switching to this current.
[0029]
FIG. 4 is an explanatory diagram when the switching means is employed. From the relation of the laser wavelength λ to the current I, it can be seen that the mode hop point has periodicity. (A) shows a case where the operating point Ia is near the center of the mode hop.
[0030]
Ib is an operating point for shifting when a mode hop danger is detected, and is set at a position of ΔImh / 2 that is approximately ½ of the current width ΔImh of the mode hop occurrence.
[0031]
(B) shows a case where the operating point when the risk of mode hop occurrence is detected is shifted to Ib. With such a setting, the current value farthest from the mode hop generation point is set as a new operating point. can do.
[0032]
【The invention's effect】
As is apparent from the above description, according to the present invention, a special laser light measurement system is prepared by monitoring the change in the phase counter output when the optical circuit unit is fixed and the laser current is changed. Therefore, it is possible to realize a position detection device that can easily check the occurrence of a mode hop and shift to a safe operating point in advance.
[Brief description of the drawings]
FIG. 1 is a configuration diagram showing an embodiment of a position detection apparatus according to the present invention.
FIG. 2 is a waveform diagram of mode hopping when the laser diode current is changed.
FIG. 3 is a flow chart for explaining the operation of mode hop control means in the apparatus of the present invention.
FIG. 4 is an explanatory diagram relating to operating point switching when a mode hop occurs in the device of the present invention.
FIG. 5 is a configuration diagram of a conventional position detection device.
6 is a diagram showing a pattern of interference fringes in FIG. 5. FIG.
7 is a waveform diagram of each signal in FIG. 5. FIG.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Target mirror 2 Optical circuit part 24 Laser light source 28 Phase detector 29 Comparator 30 Phase counter 31 Mode hop control means

Claims (4)

An optical circuit unit that generates interference fringes with a constant pitch that moves in conjunction with the movement of the detection target using the interference of light;
A phase detector that detects interference fringes by a plurality of light receiving elements arranged to be shifted along the arrangement direction of the interference fringes, and generates two types of modulation signals that are modulated according to the amount of movement of the detection target;
In a position detection device including a phase counter that calculates the amount of movement by integrating the phase difference of the phase detector output,
A semiconductor laser diode provided as a light source of the optical circuit section;
A current source that drives the semiconductor laser diode with an operating current defined by the control input voltage ;
Mode hop control means for varying the operating current according to the control input voltage ,
The position detection device, wherein the mode hop control means detects a mode hop of the light source based on a change in the phase counter output when the operating current is changed by the control input voltage. The mode hop control means determines the mode hop of the light source based on a change in the phase counter output when the control input voltage is changed with a constant amplitude and the operating current is changed with the optical circuit portion fixed. The position detection apparatus according to claim 1, wherein the position detection apparatus detects the position detection apparatus. The mode hop control means records the phase counter output in a state where the optical circuit unit is fixed, and changes the control input voltage a plurality of times with a constant amplitude to change the operating current and then returns to the original state. The phase counter output and the recording output are compared, and if the two are the same, the current control input voltage is maintained. The position detection device according to claim 1. 4. The position detecting apparatus according to claim 3, further comprising switching means for setting the operating point that is approximately ½ of the mode hop interval away from the current operating point as the changed operating point.

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