Patent Application
20240318951
The present application is based on, and claims priority from JP Application Serial Number 2023-045062, filed Mar. 22, 2023, the disclosure of which is hereby incorporated by reference herein in its entirety.
The disclosure relates to a laser interferometer.
JP-A-2007-285898 discloses a laser vibrometer as a device for measuring a vibration speed of an object. In this laser vibrometer, an object to be measured is irradiated with a laser beam, and the vibration speed is measured based on scattered laser beam subjected to a Doppler shift.
The laser vibrometer described in JP-A-2007-285898 includes a laser source unit that outputs a laser beam and a vibrator that generates a predetermined frequency. The vibrator shifts a frequency of an incident laser beam based on the vibration frequency thereof, and generates a reflected laser beam having a frequency different from that of the incident laser beam. In the laser vibrometer, the reflected laser beam is used as reference light. The scattered laser beam derived from the object to be measured and the reference light are combined and received by a photodetector, thereby electrically extracting a beat signal. Then, the vibration speed of the object to be measured is measured from the beat signal.
JP-A-2007-285898 is an example of the related art.
In the laser source, a wavelength of the output laser beam may change from an initial value due to various causes such as changes over time. In the laser vibrometer described in JP-A-2007-285898, when the wavelength of the laser beam changes from the initial value, a measured value of the vibration speed also changes. As a result, measurement accuracy of the laser vibrometer decreases as compared with an initial state.
A laser interferometer according to an application example of the disclosure includes:
Hereinafter, a laser interferometer according to an embodiment of the disclosure will be described in detail with reference to the accompanying drawings.
The laser interferometer 1 shown in
The laser interferometer 1 shown in
The sensor head unit 51 shown in
The main body unit 59 includes a calculation unit 52. The main body unit 59 can be disposed away from the sensor head unit 51, and may be accommodated in a rack or the like.
The sensor head unit 51 shown in
The interference optical system 50 shown in
The laser source 2 emits emitted light L1 (laser beam). The light receiving element 10 converts the received light into an electric signal. The optical modulator 12 includes an acousto-optic modulator 122, changes a frequency of the emitted light L1, and generates reference light L2 including a modulation signal (a laser beam including a modulation signal). The emitted light L1 incident on the object 14 is reflected as object light L3 including a sample signal which is a Doppler signal derived from the object 14 (a laser beam including a sample signal).
An optical path coupling the light splitter 41 and the laser source 2 is referred to as an optical path 18. An optical path coupling the light splitter 41 and the optical modulator 12 is referred to as an optical path 20. An optical path coupling the light splitter 41 and the object 14 is referred to as an optical path 22. An optical path coupling the light splitter 41 and the light receiving element 10 is referred to as an optical path 24. An optical path coupling the light splitter 42 and the detector 72 is referred to as an optical path 182. The “optical path” in the present specification indicates a path which is set between optical components and along which light travels.
On the optical path 18, the half-wavelength plate 92, the light splitter 42, and the collimation lens 91 are disposed in this order from the light splitter 41. The quarter-wavelength plate 94 is disposed on the optical path 20. The quarter-wavelength plate 93 is disposed on the optical path 22. The analyzer 95 is disposed on the optical path 24. The gas cell 71 is disposed on the optical path 182.
The emitted light L1 emitted from the laser source 2 passes through the optical path 18, and is partially reflected and branched by the light splitter 42. Branched light L1c passes through the optical path 182 and transmits through the gas cell 71, and then is incident on the detector 72. The emitted light L1 transmitted through the light splitter 42 is split into two by the light splitter 41. First split light L1a, which is a part of the emitted light L1, passes through the optical path 20 and is incident on the optical modulator 12. Second split light L1b, which is another part of the emitted light L1, passes through the optical path 22 and is incident on the object 14. The reference light L2 generated by shifting the frequency by the optical modulator 12 passes through the optical path 20 and the optical path 24 and is incident on the light receiving element 10. The object light L3 generated by being reflected by the object 14 passes through the optical path 22 and the optical path 24 and is incident on the light receiving element 10.
In the interference optical system 50 as described above, phase information of the object light L3 is obtained by optical heterodyne interferometry. Specifically, two types of light having different frequencies (the reference light L2 and the object light L3) are caused to interfere with each other, and the phase information of the object light L3 is extracted from obtained interference light. Then, a displacement of the object 14 is obtained from the phase information in the calculation unit 52 to be described later. According to the optical heterodyne interferometry, the extraction of the phase information is less susceptible to an influence of disturbances, in particular, an influence of stray light having a frequency that becomes noise, and is given high robustness.
Hereinafter, each unit of the interference optical system 50 will be further described.
The laser source 2 is a laser source that emits the coherent emitted light L1. As the laser source 2, a light source having a line width of MHz band or less is preferably used. Specifically, examples thereof include gas lasers such as a He—Ne laser, and semiconductor laser elements such as a distributed feedback-laser diode (DFB-LD), a fiber Bragg grating-laser diode (FBG-LD), a vertical cavity surface emitting laser (VCSEL) and a Fabry-Perot laser diode (FP-LD).
The laser source 2 is particularly preferably a semiconductor laser element. Accordingly, a size of the laser source 2 can be particularly reduced. Therefore, a size of the laser interferometer 1 can be reduced. In particular, in the laser interferometer 1, the size and the weight of the sensor head unit 51 accommodating the interference optical system 50 is reduced, which is useful in that operability of the laser interferometer 1, such as a degree of freedom in disposing the sensor head unit 51, can be improved. In the following description, a case where the laser source 2 is a semiconductor laser element will be described as an example.
The laser source 2 shown in
The collimation lens 91 is an optical element disposed between the laser source 2 and the light splitter 41, and an example thereof is an aspherical lens. The collimation lens 91 collimates the emitted light L1 emitted from the laser source 2. The collimation lens 91 may be omitted when the emitted light L1 emitted from the laser source 2 is sufficiently collimated, for example, when a gas laser such as the He—Ne laser is used as the laser source 2.
On the other hand, it is preferable to provide the collimation lens 91 when the laser source 2 is a semiconductor laser element. Accordingly, since the emitted light L1 becomes collimated light, it is possible to prevent an increase in sizes of various optical components that receive the emitted light L1, and it is possible to reduce a size of the laser interferometer 1.
The emitted light L1, which becomes the collimated light, passes through the half-wavelength plate 92 to be converted into linearly polarized light having an intensity ratio of P-polarized light and S-polarized light of, for example, 50:50, and is incident on the light splitter 41.
The light splitter 41 is a polarizing beam splitter disposed between the laser source 2 and the optical modulator 12 and between the laser source 2 and the object 14. The light splitter 41 transmits P-polarized light and reflects S-polarized light. With this function, the light splitter 41 splits the emitted light L1 into the first split light L1a which is light reflected by the light splitter 41 and the second split light L1b which is light transmitted through the light splitter 41.
The first split light L1a, which is the S-polarized light reflected by the light splitter 41, is converted into circularly polarized light by the quarter-wavelength plate 94, and is incident on the optical modulator 12. The first split light L1a incident on the optical modulator 12 is subjected to a frequency shift of fm [Hz] and is emitted as the reference light L2. Accordingly, the reference light L2 includes a modulation signal having the frequency fm [Hz]. The reference light L2 is converted into the P-polarized light when passing through the quarter-wavelength plate 94 again. The P-polarized light as the reference light L2 is transmitted through the light splitter 41 and the analyzer 95 and is incident on the light receiving element 10.
The second split light L1b, which is the P-polarized light transmitted through the light splitter 41, is converted into the circularly polarized light by the quarter-wavelength plate 93, and is incident on the object 14 in a moving state. The second split light L1b incident on the object 14 is subjected to a Doppler shift of fd [Hz] and is reflected as the object light L3. Accordingly, the object light L3 includes a sample signal having the frequency fd [Hz]. The object light L3 is converted into the S-polarized light when passing through the quarter-wavelength plate 93 again. The S-polarized light as the object light L3 is reflected by the light splitter 41 and is transmitted through the analyzer 95, and is incident on the light receiving element 10.
Since the emitted light L1 is coherent, the reference light L2 and the object light L3 are incident on the light receiving element 10 as the interference light. Therefore, in other words, the light splitter 41 has a function of splitting the emitted light L1 into one part (first split light L1a) and the other part (second split light L1b), a function of irradiating the optical modulator 12 with the first split light L1a and irradiating the object 14 with the second split light L1b, and a function of mixing the reference light L2 returning from the optical modulator 12 and the object light L3 returning from the object 14. Accordingly, since the laser beam can be split and mixed by the light splitter 41, a space of the interference optical system 50 can be saved, which can contribute to the reduction of the size of the laser interferometer 1.
A non-polarizing beam splitter may be used instead of the polarizing beam splitter. In this case, since the half-wavelength plate 92, the quarter-wavelength plate 93, the quarter-wavelength plate 94, and the like are unnecessary, the size of the laser interferometer 1 can be reduced by reducing the number of components. A light splitter other than the beam splitter may be used.
The light splitter 42 is a non-polarizing beam splitter disposed on the optical path 18. The light splitter 42 reflects a part of the emitted light L1 and branches the reflected light onto the optical path 182 to generate the branched light L1c. In addition, the light splitter 42 transmits the remaining part of the emitted light L1 and causes the remaining part of the emitted light L1 to be incident on the light splitter 41. A branching ratio is not particularly limited, but is preferably set such that an amount of the transmitted light is larger than that of the reflected light, specifically, a ratio of the reflected light is about 10% or less.
The S-polarized light and the P-polarized light that are orthogonal to each other are independent of each other, and an interference-induced beat does not appear by simply superimposing the S-polarized light and the P-polarized light. Here, a light wave obtained by superimposing the S-polarized light and the P-polarized light is passed through the analyzer 95 inclined by 45° with respect to both the S-polarized light and the P-polarized light. By using the analyzer 95, light having mutually common components can be transmitted and the interference can be caused. As a result, in the analyzer 95, the reference light L2 and the object light L3 interfere with each other, and the interference light having a frequency of |fm-fd| [Hz] is generated.
When the interference light is incident on the light receiving element 10, the light receiving element 10 outputs a photocurrent (light receiving signal) corresponding to an intensity of the interference light. By demodulating the sample signal from the light receiving signal using a method to be described later, the movement of the object 14, that is, a displacement and a speed can be finally obtained. Examples of the light receiving element 10 include a photodiode and a phototransistor. The light received by the light receiving element 10 is not limited to the interference light described above as long as the light includes the sample signal and the modulation signal. In addition, “demodulating the sample signal from the light receiving signal” in this specification includes demodulating the sample signal from various signals converted from the photocurrent (light receiving signal).
The optical modulator 12 shown in
A drive signal Sd output from the signal generation unit 54 is input to the acousto-optic modulator 122. By appropriately setting the drive signal Sd, it is possible to control the modulation signal in the acousto-optic modulator 122.
The signal generation unit 54 shown in
The signal generation unit 54 may be, for example, a signal generator such as a function generator or a signal generator, or a signal generator based on numerical control.
The gas cell 71 seals a gas absorbing light having a predetermined wavelength. Examples of the sealed gas include gaseous alkali metals such as cesium and rubidium, halogens such as gaseous iodine, rare gases such as krypton, and also hydrogen cyanide and acetylene. These atoms or molecules absorb or emit light having a predetermined wavelength. The gas cell 71 may be provided with a temperature adjustment mechanism (not shown). Accordingly, a vapor pressure of the gas can be sufficiently increased even when a size of the gas cell 71 is further reduced. As a result, the size of the gas cell 71 can be reduced.
Table 1 below shows examples of combinations of gases (atoms or molecules) sealed in the gas cell 71 and wavelengths of light with which the gases are irradiated.
85Rb (D2 line), 87Rb (D2 line)
85Rb (D1 line), 87Rb (D1 line)
The branched light L1c branched by the light splitter 42 is incident on the gas cell 71. Then, the gas sealed in the gas cell 71 is irradiated with the branched light L1c. Accordingly, the atoms and molecules constituting the gas transition from a ground state to a state having higher energy (excited state) according to energy of the branched light L1c.
As shown in
The cesium atoms at the ground level transition to the excitation level by absorbing a CsD1 line shown in
For example, the cesium atoms at the ground level of F=4 transition to the excitation level of F′=3 by absorbing energy between levels indicated by an arrow (1) in
Further, the cesium atoms at the ground level of F=3 transition to the excitation level of F′=3 by absorbing energy between levels indicated by an arrow (3) in
Resonance wavelengths corresponding to the transitions of the arrows (1) to (4) in
As described later, the light source controller 8 adjusts a current input to the laser source 2, specifically, a bias current. When the bias current is changed, an amount of the emitted light L1 emitted from the laser source 2 is changed and a center wavelength (center frequency) is changed accordingly. Therefore, when the branched light L1c is incident on the gas cell 71 and a change in an amount of light emitted from the gas cell 71 is observed, an absorption spectrum as shown in
As shown in
In
The detector 72 detects the amount of the light emitted from the gas cell 71. Then, an emitted light amount detection signal SL1 corresponding to the amount of the light is output. Examples of the detector 72 include a photodiode and a phototransistor.
The light source controller 8 controls the wavelength of the emitted light L1 (laser beam) emitted from the laser source 2 based on the emitted light amount detection signal SL1.
The light source controller 8 shown in
The low-frequency oscillator 81 oscillates at a low frequency of, for example, about several Hz to several hundred Hz and outputs a low-frequency signal.
The emitted light amount detection signal SL1 is input to the detection circuit 82. Since modulation is applied to the bias current by the modulation circuit 83 to be described later, the emitted light amount detection signal SL1 includes a result of sweeping the bias current in a range determined by an amplitude of the low-frequency signal. That is, the wavelength (frequency) of the emitted light L1 oscillates in a predetermined range, and an oscillation of the change in the emitted light amount according to the oscillation is included. Therefore, in the detection circuit 82, a relationship between the bias current and the emitted light amount is obtained, and an absorption spectrum AS3 shown in
A horizontal axis of the absorption spectrum AS3 in
As shown in
As described above, the detection circuit 82 synchronously detects the emitted light amount detection signal SL1 at the frequency of the low-frequency signal. Accordingly, the error signal ES shown in
The modulation circuit 83 applies modulation to the error signal ES using the low-frequency signal. Then, the error signal ES to which the modulation is applied is output.
The light source drive circuit 84 adjusts the bias current input to the laser source 2 using the error signal ES applied with the modulation. For example, when the voltage of the error signal ES is positive, the bias current is decreased, and when the voltage of the error signal ES is negative, the bias current is increased. The light source drive circuit 84 sweeps the bias current with a predetermined width. According to the above operation, even if the amount of the light emitted from the gas cell 71 deviates from the minimum value, the deviation is fed back to the bias current and the amount of the light emitted from the gas cell 71 is adjusted to approach the minimum value. As a result, the center wavelength (center frequency) of the emitted light L1 is locked to any peak top of the absorption peaks P1 to P4 and is stabilized.
The absorption peaks P1 to P4 have a Doppler spread associated with movement of atoms and molecules sealed in the gas cell 71. For example, when the gas includes cesium atoms, the Doppler spread is about 380 MHz at 60° C. In order to lock the center wavelength of the emitted light L1 to the peak top, it is preferable to make a frequency width of the modulation smaller than the Doppler spread. That is, a modulation width of the bias current is preferably set such that the frequency width of the modulation is less than the Doppler spread. In addition, in consideration of more stable operation, the frequency width of the modulation is preferably set to ½ of the Doppler spread or less, and more preferably set to ⅓ of the Doppler spread or less. On the other hand, a lower limit value is appropriately set so as to stabilize feedback control, and is set to, for example, 1/20 of the Doppler spread or more.
In step S102 shown in
In step S104, a desired absorption peak is detected, and a value of the bias current is set.
In step S106, the error signal ES is observed by synchronous detection of current modulation of the laser source 2.
In step S108, it is determined whether the voltage of the error signal ES is positive. When the voltage is positive, the process proceeds to step S110. In step S110, the bias current is adjusted to decrease. On the other hand, when the voltage is negative, the process proceeds to step S112. In step S112, the bias current is adjusted to increase.
In step S114, it is determined whether the voltage of the error signal ES is converged near zero. When not converged, the process returns to step S106. When converged, the flow ends.
Then, the above flow is repeated as necessary. Accordingly, the center wavelength (center frequency) of the emitted light L1 can be stabilized. Such feedback control of the bias current is useful in that a response in stabilizing the center wavelength is relatively high.
The feedback control described above may be executed all the time, or may be executed as necessary. For example, when measurement is performed on the object 14, the feedback control may be stopped. Accordingly, the feedback control can be prevented from influencing measurement accuracy.
The temperature adjuster drive circuit 85 outputs a temperature adjustment signal to the temperature adjuster 212. Accordingly, heat generation or heat absorption of the temperature adjuster 212 is adjusted, and the temperature of the laser source 2 is controlled to a target temperature. The temperature adjuster drive circuit 85 shown in
The main body unit 59 shown in
Examples of the processor include a central processing unit (CPU) and a digital signal processor (DSP). Instead of a method in which the processor executes software, a method in which a field-programmable gate array (FPGA), an application specific integrated circuit (ASIC), or the like implements the above-described functions may be adopted.
Examples of the memory include a hard disk drive (HDD), a solid state drive (SSD), an electrically erasable programmable read-only memory (EEPROM), a read-only memory (ROM), and a random access memory (RAM).
Examples of the external interface include a digital input and output port such as a universal serial bus (USB) and an Ethernet (registered trademark) port.
Examples of the input unit include various input devices such as a keyboard, a mouse, a touch panel, and a touch pad. Examples of the display unit include a liquid crystal display panel and an organic electro luminescence (EL) display panel.
The input unit and the display unit may be provided as necessary, and may be omitted.
A function of the light source controller 8 described above is also implemented by the same hardware as described above.
For example, a preprocessing unit and a demodulation unit disclosed in JP-A-2022-38156 may be applied to the preprocessing unit 53 and the demodulation processing unit 55.
The preprocessing unit 53 performs preprocessing on the light receiving signal based on the reference signal Ss. In the preprocessing, the light receiving signal is divided into two signals PASS1 and PASS2, one of which is then multiplied by the reference signal, and then the two signals PASS1 and PASS2 are added to output a preprocessed signal.
The demodulation processing unit 55 demodulates the sample signal corresponding to a speed and a position of the object 14 based on the reference signal Ss from the preprocessed signal output from the preprocessing unit 53.
The demodulated signal output unit 57 performs phase connection by performing, for example, phase unwrapping processing on the demodulated signal output from the demodulation processing unit 55. Accordingly, the position of the object 14 is calculated. Therefore, the laser interferometer 1 is a displacement meter. Further, the speed can be obtained from the position of the object 14. Therefore, the laser interferometer 1 is a speedometer.
When the displacement of the object 14 is measured by the laser interferometer 1, a measured displacement d is expressed by the following Equation (A).
In Equation (A), λ is the wavelength of the emitted light L1, n is a refractive index of an atmosphere, and φ is a phase of the sample signal demodulated by the demodulation processing unit 55.
As can be seen from the above Equation (A), the measured displacement d is influenced by the wavelength λ of the emitted light L1. For example, consider a situation where the initial wavelength λ becomes λ′ due to changes over time or the like. When the measured displacement in this case is d′, the measured displacement d′ is expressed by the following Equation (A′).
Here, a change rate when the initial measured displacement d changes to the measured displacement d′ is denoted by Δd/d. The change rate Δd/d of the measured displacement is expressed by the following Equation (B).
Accordingly, it is understood that the change rate LA/A of the wavelength influences the change rate Δd/d of the measured displacement. Accordingly, if the wavelength of the emitted light L1 can be stabilized, accuracy of the measured displacement can be improved.
Next, a first modification of the laser interferometer 1 will be described.
Hereinafter, the first modification will be described. In the following description, differences from the above embodiment will be mainly described, and the description of similar matters will be omitted. In
The first modification is the same as the above embodiment except that the light source controller 8 adjusts the temperature of the laser source 2 instead of adjusting the bias current.
In the light source controller 8 shown in
The temperature adjuster drive circuit 85 shown in
As shown in
According to the above operation, even if the amount of the light emitted from the gas cell 71 deviates from the minimum value, the deviation is fed back to the temperature of the laser source 2 and the amount of the light emitted from the gas cell 71 is adjusted to approach the minimum value. As a result, the center wavelength (center frequency) of the emitted light L1 is locked to any peak top of the absorption peaks P1 to P4 and is stabilized.
In step S202 shown in
In step S204, a desired absorption peak is detected, and a value of the temperature of the laser source 2 is set.
In step S206, the error signal is observed by synchronous detection of temperature modulation of the laser source 2.
In step S208, it is determined whether the voltage of the error signal is positive. When the voltage is positive, the process proceeds to step S210. In step S210, the temperature of the laser source 2 is adjusted to decrease. On the other hand, when the voltage is negative, the process proceeds to step S212. In step S212, the temperature of the laser source 2 is adjusted to increase.
In step S214, it is determined whether the voltage of the error signal is converged near zero. When not converged, the process returns to step S206. When converged, the flow ends.
Then, the above flow is repeated as necessary. Accordingly, the center wavelength (center frequency) of the emitted light L1 can be stabilized.
In the first modification described above, effects same as those of the above embodiment can also be obtained.
In the first modification, an amount of change in the amount of the light when the wavelength of the emitted light L1 is adjusted by a predetermined amount can be reduced as compared with the above embodiment. Accordingly, it is possible to prevent a change in the amount of the interference light incident on the light receiving element 10 of the interference optical system 50 as compared with the above embodiment. As a result, it is possible to prevent variation over time of the S/N ratio (signal-to-noise ratio) of the light receiving signal.
For example, a case where a general surface emitting laser element is used as the laser source 2 and the center wavelength of the emitted light L1 is adjusted to be longer by 0.1 nm is considered as an example. Table 3 below shows a change width of the current or the temperature and a change width of the light amount associated with adjustment when the adjustment of increasing the center wavelength of the emitted light L1 by 0.1 nm is performed under the current wavelength control (the above embodiment) and the temperature wavelength control (the first modification) in this example.
In the above embodiment, it is necessary to increase the bias current by 0.38 mA, and accordingly, the amount of the emitted light L1 increases by 11.5%. Meanwhile, in the first modification, it is necessary to increase the temperature of the laser source 2 by 1.7° C., and accordingly, the amount of the emitted light L1 decreases by 0.7%. As described above, the amount of change in the amount of the light in the first modification is reduced to 1/10 of that in the above embodiment or less.
The above calculation examples were calculated using various parameters of the surface emitting laser element shown in Table 4 below.
Next, a second modification of the laser interferometer 1 will be described.
Hereinafter, the second modification will be described. In the following description, differences from the above embodiment and the modification will be mainly described, and the description of similar matters will be omitted. In
The second modification is the same as the first modification except that the light source controller 8 adjusts the bias current based on the light output of the laser source 2 in addition to adjusting the temperature of the laser source 2.
The interference optical system 50 shown in
The light splitter 43 is a non-polarizing beam splitter disposed on the optical path 182 branched by the light splitter 42. The light splitter 43 reflects a part of the branched light L1c and branches the reflected light onto an optical path 184 to generate branched light Lid. In addition, the light splitter 43 transmits the remaining part of the branched light L1c and causes the remaining part of the branched light L1c to be incident the gas cell 71. A branching ratio is not particularly limited. The arrangement of the light splitter 43 is not limited to the above.
The light output detector 73 detects an amount of the branched light Lid branched onto the optical path 184 by the light splitter 43. In this specification, the amount of the light is referred to as a “light output”. The light output is substantially proportional to the amount of the emitted light L1 emitted from the laser source 2, and is used to monitor the amount of the emitted light L1. The light output detector 73 outputs a light output detection signal SL2 corresponding to the amount of the branched light Lid. Examples of the light output detector 73 include a photodiode and a phototransistor.
The APC unit 86 outputs a light output control signal for controlling the bias current based on the light output detection signal SL2, so that the light output of the laser source 2 becomes constant. The light source drive circuit 84 shown in
As described above, in the second modification, a configuration of feedback control of the bias current using the APC unit 86 is added to the configuration (feedback control of the temperature) of the first modification. Accordingly, the center wavelength (center frequency) of the emitted light L1 emitted from the laser source 2 can be stabilized, and the light output can be controlled to be constant.
In step S302 shown in
In step S304, a desired absorption peak is detected, and a value of the temperature of the laser source 2 is set.
In step S306, the error signal is observed by synchronous detection of temperature modulation of the laser source 2.
In step S308, it is determined whether the voltage of the error signal is positive. When the voltage is positive, the process proceeds to step S310. In step S310, the temperature of the laser source 2 is adjusted to decrease. On the other hand, when the voltage is negative, the process proceeds to step S312. In step S312, the temperature of the laser source 2 is adjusted to increase.
In step S314, it is determined whether the voltage of the error signal is converged near zero. When not converged, the process returns to step S306. When converged, the process proceeds to step S322.
In step S322, a voltage value of the light output detection signal SL2, which is a voltage signal output from the light output detector 73, is measured. An initial value of the voltage value is stored in advance in a memory (not shown).
In step S324, it is determined whether the measured voltage value of the light output detection signal SL2 is larger than the initial value. When the measured voltage value is larger than the initial value, the process proceeds to step S326. In step S326, the bias current is adjusted to decrease. On the other hand, when the measured voltage value is smaller than the initial value, the process proceeds to step S328. In step S328, the bias current is adjusted to increase.
In step S330, it is determined whether the voltage value of the light output detection signal SL2 is converged near the initial value. When not converged, the process returns to step S322. When converged, the flow ends.
Then, the above flow is repeated as necessary. Accordingly, both the center wavelength (center frequency) of the emitted light L1 and the light output can be stabilized.
Next, a third modification of the laser interferometer 1 will be described.
Hereinafter, the third modification will be described. In the following description, differences from the above embodiment and the modification will be mainly described, and the description of similar matters will be omitted. In
The third modification is the same as the second modification except that the light output detector 73 is provided in a package PKG of the laser source 2.
The laser source 2 shown in
In
In the third modification described above, effects same as those of the second modification can also be obtained.
Next, a fourth modification of the laser interferometer 1 will be described.
Hereinafter, the fourth modification will be described. In the following description, differences from the above embodiment and the modification will be mainly described, and the description of similar matters will be omitted. In
The fourth modification is the same as the above embodiment except that the configuration of the optical modulator 12 is different and the low-frequency oscillator 81 is omitted.
The optical modulator 12 shown in
The signal generation unit 54 shown in
Examples of the vibrator 30 and the oscillation circuit 542 include a vibrator and an oscillation circuit disclosed in JP-A-2022-38156.
In the light source controller 8 shown in
In the fourth modification described above, effects same as those of the above embodiment can also be obtained. In addition, the vibrator 30 has features such as small size, light weight, and low power consumption as compared with the AOM and the EOM. Therefore, according to the fourth modification, it is possible to reduce the size, weight, and power consumption of the laser interferometer 1.
Although the modification is an example in which the low-frequency oscillator 81 is omitted, a signal output from the low-frequency oscillator 81 may be used as in the other modifications, instead of using the reference signal Ss. That is, a configuration example in which the low-frequency oscillator 81 is added to the modification is also one of the modifications.
Next, a fifth modification of the laser interferometer 1 will be described.
The fifth modification has a configuration obtained by combining the first modification and the fourth modification described above. That is, the fifth modification includes a configuration that enables the feedback control of the temperature of the laser source 2, and the optical modulator 12 including the vibrator 30. Accordingly, it is possible to add effects such as small size, light weight, low cost, and low power consumption to the effects of the first modification.
Next, a sixth modification of the laser interferometer 1 will be described.
The sixth modification has a configuration obtained by combining the second modification and the fourth modification described above. That is, the sixth modification includes a configuration that enables the two types of feedback control described above, and the optical modulator 12 including the vibrator 30. Accordingly, it is possible to add effects such as small size, light weight, low cost, and low power consumption to the effects of the second modification.
Next, a seventh modification of the laser interferometer 1 will be described.
The seventh modification has a configuration obtained by combining the third modification and the fourth modification described above. That is, the seventh modification includes a configuration that enables the two types of feedback control described above in a more space-saving manner, and the optical modulator 12 including the vibrator 30. Accordingly, it is possible to add effects such as small size, light weight, low cost, and low power consumption to the effects of the third modification.
As described above, the laser interferometer 1 according to the above embodiment and the modifications includes the interference optical system 50, the gas cell 71, the detector 72, and the light source controller 8. The interference optical system 50 includes the laser source 2 that emits the emitted light L1, which is the laser beam, and causes the laser beams, for example, the reference light L2 and the object light L3 to interfere with each other. The gas cell 71 seals the gas absorbing light having a predetermined wavelength, and allows the branched light L1c (laser beam) to be incident. The detector 72 detects the amount of the light emitted from the gas cell 71, and outputs the emitted light amount detection signal SL1. The light source controller 8 controls the wavelength of the emitted light L1 based on the emitted light amount detection signal SL1.
According to such a configuration, it is possible to adjust driving of the laser source 2 by using gas sealed in the gas cell 71 to absorb the light, so that the center wavelength (center frequency) of the emitted light L1 is constant. Accordingly, it is possible to implement the laser interferometer 1 in which the wavelength of the emitted light L1 is stabilized. In such a laser interferometer 1, for example, the accuracy of measured displacement of the object 14 can be improved.
The light source controller 8 preferably controls the wavelength of the emitted light L1 (laser beam) by adjusting a current (bias current) input to the laser source 2 based on the emitted light amount detection signal SL1.
According to such a configuration, it is possible to implement the laser interferometer 1 having a relatively high response for stabilizing the center wavelength.
In addition, the laser interferometer 1 may include the light emitter 200 and the temperature adjuster 212 that controls the temperature of the light emitter 200. In this case, the light source controller 8 has a function of controlling the wavelength of the emitted light L1 (laser beam) by adjusting the output of the temperature adjuster 212 based on the emitted light amount detection signal SL1.
According to such a configuration, it is possible to reduce the amount of change in the amount of the light when adjusting the wavelength of the emitted light L1. Accordingly, it is possible to prevent a change in the amount of the interference light incident on the light receiving element 10 of the interference optical system 50. As a result, it is possible to prevent variation over time of the S/N ratio of the light receiving signal.
The laser source 2 may include the package PKG that accommodates the light emitter 200. In this case, the temperature adjuster 212 is provided in the package PKG.
According to such a configuration, the size of the interference optical system 50 can be further reduced.
The laser interferometer 1 may further include the light output detector 73. The light output detector 73 detects the light output of the laser source 2 and outputs the light output detection signal SL2. In this case, the light source controller 8 controls the amount of the emitted light L1 (laser beam) by adjusting the current (bias current) input to the laser source 2 based on the light output detection signal SL2.
According to such a configuration, two types of feedback control (feedback control of temperature and bias current) can be performed. Accordingly, the center wavelength (center frequency) of the emitted light L1 emitted from the laser source 2 can be stabilized, and the light output can be controlled to be constant.
The interference optical system 50 may include the vibrator 30 irradiated with the first split light L1a (laser beam) and the oscillation circuit 542 that outputs the reference signal Ss using the vibrator 30 as a signal source. In this case, the laser interferometer 1 includes the optical modulator 12 that superimposes the modulation signal on the first split light L1a.
According to such a configuration, when the light receiving signal including the modulation signal and the reference signal Ss are subjected to calculation in the calculation unit 52, the influence of disturbances contained in both signals can be canceled out or reduced in the process of the calculation. As a result, the calculation unit 52 can accurately obtain the position and the speed of the object 14 even when subjected to the disturbance. In addition, it is possible to reduce the size, weight, and power consumption of the laser interferometer 1.
The light source controller 8 may operate using the reference signal Ss.
According to such a configuration, the configuration of the light source controller 8 can be simplified, and the size of the laser interferometer 1 can be further reduced and the cost can be reduced.
The laser source 2 is preferably a semiconductor laser element.
Accordingly, a size of the laser source 2 can be particularly reduced. Therefore, a size of the laser interferometer 1 can be reduced.
Although the laser interferometer according to the disclosure has been described above based on the embodiment and the modifications thereof shown in the drawings, the laser interferometer according to the disclosure is not limited to the above embodiment and the modifications thereof, and the configuration with the units can be replaced with any similar configuration. In addition, any other components may be added to the laser interferometers according to the above embodiment and the modifications thereof. Further, two or more of the above embodiment and the modifications thereof may be combined.
The laser interferometer according to the disclosure can be applied to, for example, a vibrometer, a tilt meter, a distance meter (length measuring device), and the like in addition to the displacement meter and the speedometer described above. In addition, examples of applications of the laser interferometer according to the disclosure include: an optical comb interference measurement technique that enables distance measurement, 3D imaging, spectroscopy, and the like; an optical fiber gyro that implements an angular velocity sensor, an angular acceleration sensor, and the like; and a Fourier spectrometer including a moving mirror device.
Two or more of the laser source, the optical modulator, and the light receiving element may be placed on a same substrate. Accordingly, it is possible to easily reduce a size and weight of an optical system, and to improve ease of assembly.
Further, although the above embodiment and the modifications thereof include the so-called Michelson interference optical system, the laser interferometer according to the disclosure can be applied to a laser interferometer having another type of interference optical system, for example, a Mach-Zehnder interference optical system.
Although the interference light of the reference light L2 and the object light L3 is incident on the light receiving element in the above embodiment, any laser beam including the sample signal and the modulation signal may be incident on the light receiving element, and thus, the optical path followed by the laser beam is not limited to the above embodiment. For example, the interference optical system may be formed such that the laser beam emitted from the laser source is incident on the light receiving element sequentially through the optical modulator and the object. In contrast, the interference optical system may be formed such that the laser beam emitted from the laser source is incident on the light receiving element sequentially through the object and the optical modulator.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2023-045062 | Mar 2023 | JP | national |
