JPH08136216A - Optical position displacement sensor - Google Patents

Abstract

PURPOSE: To detect the change in optical output corresponding to the displacement of a position without including higher harmonic components by making the mode interval of an inner resonator wider than the mode interval of an outer resonator and setting the realtionship of a specified expression under the specified conditions. CONSTITUTION: This sensor is constituted of a unimodal coherent surface-light- emission light source, whose beam expansion is five degrees or less, an outer resonance mirror 3 and a photodetector. The photodetector detects the interference caused by the phase difference of the returned light corresponding to the minute displacement quantity of the mirror 3 as the change in optical output. When the distance of the mirror 3 is Lo, the effective reflectivity is Re, the wavelenght of the surface light emission light source is λ, the length of an inner resonator Li, the effective refractive index is (n) and the reflectivity of the emitting surface is R2 (>0.9 and Re<R2 ), the mode interval of the inner resonator is at least 10-times broader than the mode interval of the outer resonator, and the relationship of the expression (in the expression, (q) is an integer of 20 or less) holds true.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical position displacement sensor for detecting the amount of movement of an external reflection mirror by the output fluctuation of a compound resonator laser.

[0002]

2. Description of the Related Art FIG. 8A is a diagram showing the structure of a conventional compound resonator laser, which comprises an external reflector 31, an edge emitting semiconductor laser 32 and a monitor photodetector (PD) 33. The monitor photodetector (PD) 33 detects the position change of the external reflector 31 based on the output fluctuation of the composite resonator laser. In the optical output at that time, as shown in FIG. 8B, a peak or a valley appears in the optical output signal for each wavelength λ with respect to the movement amount ΔL, but the change gradually attenuates and converges.

That is, the coefficient when the light returned from the external reflector 31 is coupled to the optical electric field in the active layer of the semiconductor laser 32 is C _n (n is the number of reflections), and the effective electric field reflectance is r _e.
given that,

[Equation 2]

Here, Li is the resonator length of the semiconductor laser 32, λ is the oscillation wavelength of the laser, θ _R is the amount of phase shift during one revolution, r _m is the reflectance of the external mirror, and r ₂ is the semiconductor laser 3
The output-side reflectance of 2 and L0 are the external cavity lengths. The coupling coefficient C _n is derived from Fresnel diffraction from F _a (Li) = a ₂ / (λL0) (2) F _b (Li) = b ₂ / (λL0) (3).

Here, a and b are near-field spot sizes of the semiconductor laser 32 and correspond to the thickness and width of the active layer of the stripe laser. For example, in the mode-controlled semiconductor laser 32, if a, b is 1 μm, and λ is 1 μm,

(Equation 3)

The beam remarkably spreads as the external cavity length L0 increases, and the coupling coefficient C increases every time reflection is repeated.
_n approaches zero. Here, considering the effective reflectance r _e ,
The second term on the right side of the equation (1) approaches zero, and the effective reflectance r _e
Is the semiconductor laser 32 as the external cavity length L0 increases.
To the output side reflectance r ₂ .

In an ordinary edge emitting semiconductor laser, the cavity length Li is 300 μm or more, the beam divergence is 30 ° in the vertical direction and 10 ° in the horizontal direction, and the spot diameters on the end face are 2 μm in the vertical direction and 6 μm in the horizontal direction, respectively. Is. The beam spreads in the direction perpendicular to the external mirror, and the phase and wavefront of the returning light differ greatly from the Gaussian distribution. In the position displacement sensor using the semiconductor laser as a light source, the amount of displacement can be detected as an optical output signal when L0 ≤ 3λ and it is several μm.

That is, as the external cavity length L0 increases, the beam spreads widely to the surroundings, the reflectance decreases, and the phase and the wavefront have a distribution at the time of returning to the laser, and the position displacement is caused by the signal. Accuracy in detecting quantity is λ / 4
It becomes the following.

Further, in the ordinary semiconductor laser, the internal cavity length Li is 300 μm, whereas the external cavity length L0 is several wavelengths.
For the internal resonator mode interval Δf _d (= C / (2nLi)), the external resonator mode interval Δf _ex = C / (2L0)
And the former is extremely narrow. For return light of 10 ^-4 or more, which is a problem with the return light noise in the optical disk light source, mode skipping occurs between the internal resonator modes or multimode occurs, and the mode distribution noise increases, and the optical signal It is unsuitable as a displacement sensor that performs analog signal processing, because harmonic distortion due to wavelength jumps and multimode is superimposed on. It can be used for optical disc signals that detect only peak values,
As a sensor for dividing an analog signal into a large number to improve accuracy, it is considered to be a fraction of the wavelength.

[0010]

In the equation (1), the effective reflectance r _e due to multiple reflection is described. However, when it is used as a displacement sensor, the accuracy is improved when the phase delay due to multiple reflection is superimposed. Therefore, considering only the case of coupling to the semiconductor laser body by a single reflection, the coupling efficiency C _n needs to be significantly reduced in the high-order reflection. That is,
L0 needs to be sufficiently large for at least the wavelength λ.

The optical position displacement sensor of the present invention has been made by paying attention to such a problem, and an object thereof is to set the external resonator length L0 to be sufficiently large with respect to the wavelength λ so that the amount of returned light is large. An object of the present invention is to provide an optical position displacement sensor that can detect a change in the optical output corresponding to the positional displacement ΔL even if it is several percent, without including harmonic components, with an accuracy of the order of nm.

[0012]

In order to achieve the above object, the present invention comprises a single-peaked coherent surface-emitting light source with a beam spread of 5 degrees or less, an external resonator mirror and a light-receiving element. In an optical position displacement sensor in which interference due to a phase difference of return light corresponding to a slight displacement amount of a resonator mirror is detected by a light receiving element as a change in optical output, a distance L0 of an external resonator mirror, its effective light reflectance R _e , The wavelength λ of the surface emitting light source, the internal cavity length Li, the effective refractive index n thereof,
When the emission surface reflectance R ₂ (> 0.9 and _Re <R ₂ ), the internal resonator mode interval is at least 10 times wider than the external resonator mode interval, and

[Equation 4]

The relationship is established.

That is, according to the present invention, in the optical position displacement sensor, the distance L 0 of the external resonator mirror, its effective light reflectance R _e , the wavelength λ of the surface emitting light source, the internal resonator length Li,
The effective refractive index n, the emission surface reflectance R ₂ (> 0.9, and
When _Re <R ₂ ), the internal resonator mode interval is at least 10 times wider than the external resonator mode interval, and

(Equation 5)

The relationship is established.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION First, a first configuration of this embodiment will be described. The optical position displacement sensor according to the present embodiment includes a unimodal coherent surface emitting light source having a beam divergence of 5 degrees or less, an external resonator mirror, and a light receiving element, and the light receiving element is a small displacement amount of the external resonator mirror. The interference due to the phase difference of the corresponding return light is detected as a change in the optical output. The distance L0 of the external cavity mirror, its effective light reflectance R _e , the wavelength λ of the surface emitting light source, the internal cavity length Li, its effective refractive index n, and the emission surface reflectance R ₂ (> 0.9 and , R _e <
R ₂ ), the internal cavity mode spacing is at least 10 times wider than the external cavity mode spacing, and

(Equation 6)

The relation of is established.

Here, the surface emitting light source includes not only the vertical short cavity surface emitting semiconductor laser but also a vertical cavity edge emitting semiconductor laser of 10 μm or less vertically assembled.

Regarding the influence of the oscillation spectrum of the semiconductor laser on the amount of returned light, it was published in 1980 by Hiroyoshi Sangu and Isao Kobayashi in the Journal of Quantum Electronics, Vol. 16, pp. 347. "External return light effect on current injection type semiconductor laser characteristics" is explained in the complex cavity model (Roy, Lang, and Kohroh Kobayashi, “External Optic
al Feedback Effectson Semiconductor Laser Proporti
es '' J. Quantum Electron, vol 16 No.3, pp347-355, (198
0).

According to this, the oscillation threshold condition of the compound resonator laser is expressed by the following equation.

[0021]

(Equation 7)

Where g is the linear gain per unit length, α
₀ is the absorption coefficient per unit length, Li is the internal cavity length of the semiconductor laser, r ₂ is the reflectance of the semiconductor laser end face, and r ₃ is the reflectance of the external reflector.

[0023]

(Equation 8)

From this equation, it is clear that the phase and the threshold gain change depending on the parameter Z.

Furthermore, the effective reflectivity r _eff of the Fabry-Perot resonator defined by the laser mirror and the external resonator.
Is defined as follows.

[0026]

[Equation 9]

That is, the additional gain G ₀ required by the returning light is G _min = log _e x ₁ and G _max = log _e x _2.
Between the two changes depending on the phase of the reflected electric field. Further, the phase φ of the parameter Z when r ₃ <r ₂ is ±
Vary between φ _max .

[0028]

[Equation 10]

When the spectral spread Δf is described for the reflectance R _e of the power of light, not the electric field,

[Equation 11]

Since the angular frequency ω shifts with respect to the change of the phase φ of the complex return light quantity parameter Z, this maximum spectral spread Δf is the external resonator mode interval Δf _ex.
If it is smaller than the above, it is possible to suppress multimode generation due to power coupling between the external resonator mode and the internal resonator mode.

That is,

(Equation 12)

On the other hand, if the internal cavity mode spacing is not wider than the external cavity mode spacing by at least 10 times or more, the peak gain of the active layer shifts or the distributed reflector's gain increases when the amount of returned light is set to several percent. Since the wavelength selectivity is not steep, there is a risk of mode jump between the internal cavity modes.

[0033]

(Equation 13)

Here, the cavity length nLi is nLi <1 in consideration of the quality of epicrystals on crystal growth and uniform injection of current.
It is considered to be 0 μm, and the value of q is an integer of 20 or less. Therefore, L0> 10nLi = 5qλ (q ≦ 20, integer) The second configuration of this embodiment will be described below. In the second configuration, in the optical position displacement sensor of the first configuration, the temperature rise amount of the light source when measuring the position displacement amount is ΔT,

[Equation 14]

The relationship of is established.

Here,

(Equation 15)

The oscillation wavelength shift amount of the surface emitting laser may be due to the change in the laser gain in the internal cavity due to the return light, or to the shift to the longer wavelength side due to the temperature rise in the active layer. To be In the surface emitting laser, the distributed reflector 2 is attached to the outside of the clad layer sandwiching the active layer, and the internal resonator (the upper and lower clad layers become the resonator).
The wavelength selectivity of the reflection mirror 2 works against the change of the bandgap in the active layer with respect to the above wavelength shift, and the wavelength shift amount due to the temperature rise is about 1/5 of that of a normal semiconductor laser. I keep it to. However, the shift amount Δλ _T toward the long wavelength side accompanying the temperature increase ΔT promotes coupling with the external resonator mode.

Therefore, in addition to the conditions described in the first structure, it is necessary to add a shift amount due to temperature rise for designing.
If this is described by an equation, it becomes the following equation.

[0039]

[Equation 16]

The third configuration of this embodiment will be described below. In the third configuration, in the optical displacement sensor of the first configuration, nLi = λ, emission surface reflectance R ₂ > 0.95, and

[Equation 17]

The relationship of is established.

In order to operate the surface emitting semiconductor laser with low current and high efficiency, it is necessary to reduce the threshold value. 1
In 990, the Institute of Electronics, Information and Communication Engineers, Quantum Electronics Research Group, Technical Report OQE90-62, the surface emitting laser low The thresholding method will be described below.

Threshold current density J of surface emitting semiconductor laser
_th is the threshold condition,

(Equation 18)

Is represented by Where g _th is the threshold gain,

[Formula 19]

Is the average surface emitting laser mirror reflectance, Li
And d are the internal cavity length and the active layer thickness, respectively. α _ac and α _ex represent the absorption loss of the active layer and the cladding layer, respectively. ξ is the optical confinement coefficient of the active region and is expressed by the following equation.

[0046]

(Equation 20)

D / Li · Γ (d) and ξ (D) are the vertical and horizontal confinement coefficients of the active layer, respectively. Further, the threshold gain g _th and the threshold carrier density N _th are experimentally approximated by g _th = A ₀ N _th −α _in (28). A ₀ is a parameter,
α _in is the waveguide loss. Further, in the case of a quantum well active layer, J _th = edB _eff N _th ² (29) Here, e is an electron charge and B _eff is a parameter.

From these equations, the threshold current density J
_th can be approximated by the following equation.

[0049]

[Equation 21]

Here, B _eff = 1 × 10 ^-10 cm ³ /
S, A ₀ = 2 × 10 ^-16 cm ² , α _in = 400 cm ^-1 ,
The reflectance R ₀ when ξ (D) to 1 is used as a parameter,
FIG. 6 shows the relationship between the active layer thickness d and the threshold current density J _th .

To reduce the threshold current density J _th ,
The multiple quantum well active layer is more advantageous than the bulk crystal in the active layer in terms of steep gain and wavelength stability.
Also, due to the uniformity of current injection into the active layer, the multiplicity is d _a ≦ 0.1 μm for the entire active layer thickness. From FIG. 6, the condition is that the reflectance R ₂ of the exit facet is> 0.95.

[0052]

[Equation 22]

It becomes Now, in the vertical cavity surface emitting semiconductor laser, the condition required for low threshold current operation is that the position of the active layer is the position of the antinode of the standing wave. That is, since light is generated by recombination of carriers in the active layer, the presence of the active layer that increases the gain at the antinode position of the standing wave is a condition for smoothing laser oscillation. Since the active layer also serves as a light absorbing medium for laser oscillation light, the number of antinodes of standing waves in the active layer is suppressed as much as possible. Therefore, the optimum condition is that the active layer is at the antinode position of the standing wave and the amplitude is large on both end faces of the resonator, and nLi = λ.

By setting the minimum standing wave number of the internal resonator length Li = λ / n,

(Equation 23)

Since the spectral spread of the active layer and the distributed reflector is 1/10 or less of the wavelength λ, there is no mode hopping between the internal cavity modes.

The fourth configuration of this embodiment will be described below. In the fourth configuration, in the optical position displacement sensor of the third configuration, the emission surface reflectance R ₂ is 0.98 ≦ R ₂ <1, and the external mirror reflectance R _ex ≦ 0.4.

[0057] From FIG. 6, the monitor side and R ₁ to 0.99, if a value of R ₀ and 0.98 ≦ R ₀ <1, the threshold current density is minimum d _a is several hundred nm Value, and the internal cavity length is nLi = .lambda., And from n.about.3.4, Li.about..lambda. / N
= Λ / 3.4 = 0.25 to 0.30 μm, for example, if the cladding layer is symmetrically formed to have a thickness of 0.1 μm and the active layer is sandwiched, the condition of the third configuration is satisfied. Since the threshold current density J _th is close to the minimum value, the temperature rise ΔT at the time of detecting the position displacement can be suppressed to the maximum, for example, several degrees Celsius or less.

When Li = λ / n is a minimum value from the equation (11), G ₀ may be reduced to reduce the absolute value of the return light amount parameter Z in the equation (7). Therefore,

[Equation 24]

When R ₂ is fixed, becomes an increasing function of R _e . Further, φ _max also becomes a monotonically increasing function of R _{e when} R ₂ is fixed. Therefore, when R _ex which is about 10% of the maximum value of the peak value of G ₀ is obtained, R _ex ≦ 0.4 as shown in FIG. 7.

This embodiment will be further described below with reference to the drawings.

FIG. 1A is a configuration diagram corresponding to the above-mentioned first configuration. In the figure, a distributed Bragg reflector 2 is provided outside an internal resonator (length Li) formed of an active layer 7 sandwiched between clad layers. An external resonator mirror 3 attached to the movable portion is provided in front of the emitting side of the surface emitting semiconductor laser 1. Further, on the monitor side of the surface emitting semiconductor laser 1, an antireflection (AR) coating film 4 is formed.
The photodetector 6 is provided via the. The laser light emitted at the laser oscillation angular frequency ω is reflected by the external mirror 3 and returns to the surface-emitting semiconductor laser 1 again, the time τ becomes the phase delay ωτ.

The phase difference between the return light and the emitted light causes interference. The change in the optical output due to this interference can be understood by detecting the light that has passed through the AR coat film 4 on the monitor side of the surface emitting semiconductor laser 1 by the photodetector 6. That is, as shown in FIG. 1B, the output signal is a sine wave signal in which a moving distance ΔL has a period of λ and peaks or troughs appear. An antireflection (AR) coating film 5 is also attached to the surface of the photodetector 6 so that a composite resonator due to reflection from the photodetector surface is not formed on the monitor side.

Next, the laser oscillation wavelength λ is 0.98 μm,
The internal cavity length is nLi = 3λ, and the beam of the surface-emitting semiconductor laser is close to a parallel beam, but considering its spread, the coupling parameter ρ is set to 0.85. That is, an example of a case where it is assumed to be described by externally effective reflectivity R _e = ρ ₂ R _ex in FIG.

The reflectance R ₁ on the monitor side of the surface emitting semiconductor laser 1 is as shown in FIG.
Should approach 1 as in. However, in order to detect the output change of the surface emitting semiconductor laser 1 on the monitor side, considering that the detection sensitivity and the output are about 1 mW, the reflectance R ₁ is considered to be a practical value of about 0.98. To be

Regarding the emitting facet reflectivity R ₂ , the surface emitting semiconductor laser 1 has a cavity length Li which is two orders of magnitude shorter than that of normal facet emitting, so that it is necessary to increase the number of feedbacks to obtain a gain. It is practically preferable to design the reflectance of the reflector to be a value of about 0.98.

Therefore, the moving distance ΔL can be set in a wide range (up to 1 m).
m), when L0 is shorter than that in Fig. 2,
There is no problem because the external resonator spacing becomes large at C / 2L0, and the range of single axis mode oscillation at L0 to 1 mm should be considered. If R ₂ = 0.98, then R _ex ≦ 0.2. In the short-cavity laser, log _e (1 / r ₂ ² ) and log _{e for} the second term and the third term on the right side of the equation (11).
x (= G ₀ ), and since Li is several μm, | G
_{If 0} | ≧ 10 ⁻³ , the gain g is given by the formula (1
0), φ ₁ and g in the equation (11) are changed, and the optical output is changed. Calculating G ₀ when R _ex = 0.2 gives G
_max is 12 × 10 ⁻³ and G _min is −5.6 × 10.
^It becomes ^-3 , and it can be seen that the return light greatly changes the gain g.

On the other hand, when R _ex > 0.2, the maximum value Δω _max of the angular frequency shift and the value of φ _max do not become so large. However, in the surface emitting semiconductor laser 1, the angular frequency ω _D of the mode interval of the internal cavity is larger than the ordinary edge emitting laser by nearly two orders of magnitude in terms of the cavity length Li.
As a result, Δω _max becomes two orders of magnitude larger, and the power is coupled to the external resonator mode, and the mode becomes multimode.

Therefore, at ΔL˜1 mm, R ₂ = 0.
Under the condition of 98, by designing R _ex ≦ 0.2,
A sinusoidal signal as shown in FIG. 1B is obtained, and the sensor becomes a high-precision sensor that can be subdivided.

Next, the second structure will be further described.
In the first configuration, the output surface reflectance R ₂ = 0.98,
Wavelength shift coefficient with temperature rise of surface emitting laser

(Equation 25)

Is 0.6 angstrom / ° C., Δ
FIG. 3 shows an example of the external resonator length L0 capable of single-axis mode oscillation at T = 0 ° C., 5 ° C., 10 ° C., and 20 ° C.
When the temperature rise of the element reaches 20 ° C., the amount of shift to the long wavelength side due to the temperature rise rather than changing the reflectance R _ex of the external mirror dominates the power coupling with the external resonator mode. Will be done. Displacement measurement range ΔL in this case
Is around 100 μm.

Therefore, it is important to reduce the operating current value of the surface emitting semiconductor laser 1 and suppress the heat generation of the sensor light source as much as possible. For example, when the peak value of the wavelength characteristic of the reflectance of the distributed reflector is shifted in advance to the long wavelength side with respect to the peak value of the gain spectrum distribution of the active layer, it is possible to prevent heat generation in the active layer. Since the temperature coefficient of the distributed reflector is remarkably reduced in the shift to the long wavelength side, a method of setting the spectral shift amount to Δλ _T ≦ 1 to 2 nm (in the range of 20 to 30 ° C.) is also conceivable.

The only way to extend the displacement measuring range ΔL to 1 mm is to make ΔT at ± 1 ° C. and to have a low reflection of R _ex ≦ 0.15. External resonator length L0 against temperature rise
The use range of is significantly narrowed. For example, there is a method of detecting a positional displacement by a method in which a relatively long pulse (for example, 2 μs, a duty ratio of 100) and a rise in element temperature is suppressed to ± 1 ° C., but continuous positional displacement and subdivision Accuracy is reduced.

Next, the third structure will be further described.
When the position of the multi-quantum well active layer is the position of the antinode of the standing wave, the internal cavity length Li is one wavelength effective refractive index n = 3.34, and the oscillation wavelength λ of the laser when there is no returning light is 0.98 μm. , Li = λ / n to 0.293 μm. Therefore, the active layer thickness d _a,

(Equation 26)

The above coefficient 1/3 is a confinement coefficient when the difference in refractive index between the active layer and the cladding layer is small and the electric field distribution spreads to both sides. However, in reality, the active layer thickness d _a is thinner than this, and the quantum well effect is strengthened. It should be noted that λ / 2n is the period of the standing wave in the active region.

Under the condition of the active layer thickness d _a <0.05 μm and the range of single quantum well width d _a > 0.008 μm, FIG.
As shown in, the threshold current density J _th of the surface emitting semiconductor laser 1 has a minimum value.

The reflectance R ₂ of the emission surface is 0.95, 0.9
FIG. 4 shows an example of the relationship of the external resonator length L0 in which the single-axis mode is maintained in the optical position displacement sensor of this embodiment in the case of 8, 0.99, and 0.995.

When the internal resonator length Li is shortened, the range of the external resonator length L0 is narrowed. This means that the term G ₀ / Li in the third term on the right side of the equation (11) increases, the gain variation Δg with respect to the return light increases, and Δω _max also increases the internal resonator mode angular frequency ω _D. However, the shift amount of the spectrum is also larger than that of the equation (20).

The output power value when the output surface reflectance R ₂ is 0.995 is a very small value, but in this case, the reflectance R _ex of the external mirror is changed to 300 μm.
It can handle a wide range up to 1 mm.

However, if the reflectance on the output side and the monitor side is significantly increased, the sensitivity of the light receiving element for detecting the change in the optical output caused by the phase interference due to the returning light is required,
In some cases, a detector using a multiplication effect such as an avalanche photodiode is required.

In practical use, the reflectance of the emitting surface R ₂ = 0.98
Considering the above, as shown in FIG. 5, the allowable measurement range does not become extremely narrow with respect to the temperature rise of the element. For example, even if the temperature rise of the element is ΔT = 20 ° C., R _ex ≧
At 0.5, it is only about 20% narrower. On the other hand, in the case of the high reflectance mirror R _ex > 0.9, even if the temperature rise is ΔT = 20 ° C., it does not change even if the L 0 becomes short by about 10 μm.

On the contrary, when the emission surface reflectance R ₂ is 0.95, the operating current of the surface emitting semiconductor laser 1 also rises by two digits from FIG. 6, which is not only practically impossible but also the single axis mode oscillation. Also from the point of, it is possible to measure only displacement of a distance of several tens of wavelengths. Considering the wavelength shift due to temperature rise, only a distance of several wavelengths can be detected like a normal semiconductor laser emitting edge light.

The fourth structure will be further described below. When the light output on the monitor side is 1% of the inside of the resonator and a change in the light output on the inside can be detected. When a light receiving element is used, the reflectance R ₂ of the reflection mirror on the output side is 0.
98 ≦ R ₂ <1. The average reflectance in this range is

[Equation 27]

As shown in FIG. 6, if the active layer has a multi-quantum well structure of 100 Å ≦ d _a ≦ 500 Å, the minimum threshold current density J _{th is 5} kA / cm ² in this region. . To make the divergence of the beam close to a parallel beam and making it a monomodal coherent light,
The diameter is about 12 μmφ and the threshold current is 5.
At 6 mA, the output is about 1 mW because it is a displacement sensor. Therefore, the operating current does not exceed 10 mA, and in this respect as well, the temperature rise of the element can be suppressed low.

When a displacement sensor in the range of 100 times (˜100 μm) of the oscillation wavelength λ is realized with high accuracy, the reflectance R _ex of the external resonator mirror is set to 0.4 or more as shown in FIG. , R ₂ ≧ 0.98, practical use is possible in the range of 200 μm to 400 μm. Further, when R ₂ = 0.98, even if the element temperature rises by 20 ° C., a highly accurate sinusoidal displacement signal can be detected within a range of 100 μm.

The technical idea of the following configuration is derived from the above-described specific embodiment.

(1) It consists of a single-peaked coherent surface-emitting light source with a beam divergence of 5 degrees or less, an external resonator mirror and a light-receiving element, and it depends on the phase difference of the return light corresponding to the minute displacement of the external resonator mirror. In an optical position displacement sensor that detects interference as a change in optical output by a light receiving element, the distance L0 of the external resonator mirror, its effective light reflectance R _e , the wavelength λ of the surface emitting light source, the internal resonator length Li, and its effective refraction. Rate n,
When the emission surface reflectance R ₂ (> 0.9 and _Re <R ₂ ), the internal resonator mode interval is at least 10 times wider than the external resonator mode interval, and

[Equation 28]

An optical position displacement sensor characterized in that the following relationship is established.

(2) In the optical position displacement sensor having the configuration (1), the temperature rise amount of the light source when measuring the displacement amount is ΔT, and the wavelength shift coefficient is

[Equation 29]

An optical position displacement sensor characterized in that the following relationship is established.

(3) In the optical position displacement sensor having the configuration (1), nLi = λ, the emission reflectance R ₂ > 0.95, and

[Equation 30]

An optical position displacement sensor characterized by satisfying the relationship of:

(4) In the optical position displacement sensor having the configuration (3), the light position is characterized in that the emission surface reflectance is 0.98 ≦ R ₂ <1 and the external mirror reflectance R _ex ≦ 0.4. Displacement sensor.

The effects of the above configurations (1) to (4) are as follows.

(1) The external resonator is designed by designing the reflectance R ₂ of the surface emitting light source and the effective reflectance R _e (<R ₂ ) of the external reflector so as to satisfy the condition of the above configuration (1). Since L0 can be made sufficiently large with respect to the oscillation wavelength λ and the shift amount accompanying the phase change of the spectrum is smaller than the external resonator mode interval, a multimode between the external resonator mode and the internal resonator mode can be realized. However, even if the amount of returned light becomes several percent, it is suppressed. The optical position displacement sensor of this embodiment includes
The displacement sensor signal does not include harmonic distortion, and the signal can be subdivided, and the accuracy can be up to several tenths or less of the λ order.
It becomes a high-precision sensor on the order of nm.

(2) Taking into consideration the shift amount to the long wavelength side due to the temperature rise of the light source during measurement, the spectral variation (spreading) of the wavelength of the internal cavity is taken into consideration in the variation of the external cavity length L0. By making it narrower than the external resonator mode interval,
It is possible to suppress multi-mode. That is, according to the present embodiment, it is possible to suppress the harmonic distortion included in the displacement sensor signal,
The signal can be subdivided, and high accuracy on the order of nm can be provided.

(3) A surface emitting semiconductor laser of low current operation can be realized, the operating current is several mA, and the temperature rise of the light source during displacement detection can be suppressed low. In this embodiment, the internal cavity mode spacing is λ / 2, and mode hopping does not exist because the wavelength characteristic of the distributed reflector is several tenths of the wavelength (laser oscillation does not occur under this condition). By designing the coupling between the external resonator modes under the conditions described in the configuration (1), it is possible to provide a highly accurate position displacement sensor on the order of nm.

(4) Threshold current density J _th ≦ 10 kA
/ In cm ^{2, and} unimodal surface emitting laser few degrees beam spread is about emission diameter of 10 [mu] m, the threshold current 8m
Since it is less than A, and the optical output of 1 mW or less is sufficient,
The operating current of the surface emitting laser is 10 mA or less. Since it is driven with a low current, the temperature rise of the light source during displacement detection is
In addition to being suppressed to several degrees Celsius or less, it is possible to detect a sinusoidal subdivided position displacement signal in the range of the movement amount ΔL ≧ 100λ, and it can be expected that the accuracy is in the nm order.

[0098]

According to the present invention, even if the external cavity length L0 is set sufficiently large with respect to the wavelength λ and the amount of returned light is several percent, the change in the optical output corresponding to the positional displacement ΔL is converted into a harmonic component. It becomes possible to detect with accuracy of nm order without including it.

[Brief description of drawings]

1A is a schematic configuration diagram of an optical position displacement sensor of the present invention, and FIG. 1B is an output signal diagram thereof.

FIG. 2 is a diagram showing a relationship between a reflectance L _{ex of} an external reflection mirror and a distance L0 of an external mirror capable of single-axis mode oscillation.

FIG. 3 is a diagram showing a relationship of a distance L0 of an external mirror capable of single-axis mode oscillation with respect to an external reflection mirror reflectance R _ex in consideration of a rise in temperature of a sensor light emitting portion.

FIG. 4 is a diagram showing the relationship between the reflectance L _ex of the external reflection mirror and the distance L0 of the external mirror capable of single-axis mode oscillation.

FIG. 5 is a diagram showing the relationship between the external reflection mirror reflectance R _{ex and} the distance L 0 of the external mirror capable of single-axis mode oscillation when the rise in the temperature of the sensor light emitting part is taken into consideration.

FIG. 6 is a diagram showing a relationship between a threshold current density and an active layer thickness dependency of a surface emitting semiconductor laser.

FIG. 7 is a diagram showing the relationship between the amplitude coefficient G ₀ of the return light parameter Z and the phase φ ₀ / π of the displacement amount.

8A is a schematic configuration diagram of a conventional optical position displacement sensor, and FIG. 8B is an output signal diagram thereof.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Surface emitting semiconductor laser, 2 ... Distributed reflection mirror, 3 ... External reflection mirror, 4, 5 ... Non-reflective coating film, 6 ... Photodetector, 7 ... Active layer, 31 ... External reflection mirror, 32 ... Normal end surface Light emitting mirror, 33 ... Photodiode.

Claims (1)

[Claims] 1. Interference due to a phase difference of return light corresponding to a minute displacement amount of an external resonator mirror, which is composed of a unimodal coherent surface emitting light source with a beam spread of 5 degrees or less, an external resonator mirror and a light receiving element. In the optical position displacement sensor for detecting the light output change by the light receiving element, the distance L0 of the external resonator mirror, its effective light reflectance Re,
The wavelength λ of the surface emitting light source, the internal cavity length Li, the effective refractive index n thereof, the emission surface reflectance R ₂ (> 0.9 and _Re <R
₂ ), the internal cavity mode spacing is at least 10 times wider than the external cavity mode spacing, and An optical position displacement sensor, characterized in that the relationship is established.