JP7742015B2 - Optical Frequency Modulator - Google Patents

Description

The present invention relates to an optical frequency modulator that performs frequency modulation of light.

Measuring devices are known that use optical heterodyne interferometry to detect the distance to and displacement of a measurement object. Optical heterodyne interferometry uses two types of light with different frequencies, one of which is used as the measurement light and the other as the reference light. The interference signal between the measurement light reflected by the measurement object and the reference light reflected by a reference surface is detected, and the distance to and displacement of the measurement object are detected based on this interference signal (see, for example, Patent Documents 1 to 3).

In optical heterodyne interferometry, known methods for generating measurement and reference light of different frequencies include using a light source capable of emitting light of two different frequencies, and splitting the light emitted from the light source into measurement and reference light and frequency-modulating one of the light beams using an optical frequency modulator (also called a frequency shifter). The former method requires the use of an expensive light source, so the latter method is often adopted from a cost perspective.

A well-known optical frequency modulator is one that includes an oscillator and multiple diffraction gratings formed on the oscillator's surface (see, for example, Patent Document 4). In this optical frequency modulator, incident light that strikes the oscillator's surface while it is vibrating (moving back and forth) is reflected by the diffraction grating, causing frequency modulation (Doppler shift), and the frequency-modulated diffracted light is then emitted as measurement light or reference light.

Japanese Patent Application Laid-Open No. 2020-106284 Japanese Patent Application Laid-Open No. 2020-056658 Japanese Unexamined Patent Publication No. 2-160221 Japanese Patent Application Laid-Open No. 2020-165700

When modulating the frequency of light using a diffraction grating, the diffraction grating must be moved in the same direction at a predetermined constant speed. In the optical frequency modulator described in Patent Document 4, the diffraction grating is moved by vibrating the oscillator. Therefore, while the oscillator is moving at a constant speed in one direction of oscillation, the diffracted light can be used as measurement light, etc.; however, while the oscillator is moving in the other direction of oscillation (the opposite direction from the one direction), the frequency of the diffracted light shifts in the opposite direction, making it impossible to use the diffracted light as measurement light, etc. Furthermore, with the optical frequency modulator described in Patent Document 4, even if the oscillator's oscillation direction switches from the other direction to the one direction, the diffracted light cannot be used as measurement light, etc., until the oscillator's moving speed reaches the above-mentioned constant speed. Therefore, the optical frequency modulator described in Patent Document 4 cannot continuously emit diffracted light of the desired frequency.

One possible solution is to use an acousto-optic element as the optical frequency modulator, but optical frequency modulators using acousto-optic elements are more expensive than optical frequency modulators using diffraction gratings, which poses the problem of increased costs.

The present invention was made in consideration of these circumstances, and aims to provide an optical frequency modulator that can continuously emit diffracted light of a desired frequency at low cost.

An optical frequency modulator for achieving the object of the present invention comprises a cylindrical rotor having a rotation axis and extending in a direction parallel to the rotation axis, the rotor rotating at a uniform speed around the rotation axis, and a plurality of transmission diffraction gratings formed on the circumferential surface of the rotor and parallel to the rotation axis, the transmission diffraction gratings frequency-modulating incident light incident on the circumferential surface of the rotor during rotation and emitting frequency-modulated diffracted light.

This optical frequency modulator allows each transmission diffraction grating to move (rotate) at a uniform speed along a rotational trajectory centered on the rotation axis, thereby maintaining a state in which each diffraction grating passes the incident position of the incident light on the rotating body at a uniform speed and continuously.

In an optical frequency modulator according to another aspect of the present invention, a transmission diffraction grating frequency-modulates incident light entering the interior of the rotor from the outer surface of the rotor.

An optical frequency modulator according to another aspect of the present invention includes a reflector disposed inside a rotor that reflects incident light that enters the rotor from outside along the rotation axis toward the rotor's inner circumferential surface, and a transmission diffraction grating frequency-modulates the incident light that enters the inner circumferential surface from the reflector and is emitted to the outside of the rotor. This allows for the maximum amount of diffracted light to be obtained without the need for complex optical adjustments.

An optical frequency modulator according to another aspect of the present invention includes a drive unit that controls the rotation speed and direction of the rotor.

The present invention can continuously emit diffracted light of the desired frequency at low cost.

1 is a schematic diagram of a measurement device including an optical frequency modulator according to the present invention. FIG. 1 is a schematic diagram of an optical frequency modulator according to a first embodiment. FIG. 3 is an enlarged view of the area within the dotted circle A in FIG. 2 . FIG. 10 is an explanatory diagram for explaining a modified example of the diffraction grating of the first embodiment. FIG. 10 is a schematic diagram of an optical frequency modulator according to a second embodiment. FIG. 10 is a schematic diagram of an optical frequency modulator according to a third embodiment. FIG. 7 is an enlarged view of the area within the dotted circle A1 in FIG. 6. FIG. 10 is a schematic diagram of an optical frequency modulator according to a fourth embodiment. FIG. 11 is a front view of a rotating body of an optical frequency modulator according to a fifth embodiment, as viewed from the axial side of the rotating shaft. FIG. 13 is a cross-sectional view of a rotor of an optical frequency modulator according to a fifth embodiment.

[Measuring equipment]
1 is a schematic diagram of a measurement device 10 equipped with an optical frequency modulator 16 of the present invention. As shown in Fig. 1, the measurement device 10 measures the distance to a measurement object 9 and the displacement (including minute displacement) of the measurement object 9 using an optical heterodyne interferometry method (optical heterodyne interferometer). Note that the measurement object 9 is provided with a retroreflective member 9a such as a corner cube reflector or a three-surface mirror for measurement by the measurement device 10.

The measurement device 10 includes a laser light source 12, a half mirror 14, an optical frequency modulator 16, a half mirror 18, a light receiving unit 20, and a calculation device 22.

The laser light source 12 emits laser light L of a predetermined frequency (wavelength) toward the half mirror 14. The half mirror 14 splits the laser light L incident from the laser light source 12 into measurement light L1 and reference light L2 of the same frequency. The half mirror 14 then emits the measurement light L1 toward the optical frequency modulator 16 and the reference light L2 toward the half mirror 18.

The optical frequency modulator 16, which will be described in more detail below, frequency-modulates (shifts the frequency of) the measurement light L1 incident from the half mirror 14 using Doppler shift, and emits modulated measurement light L1A with a different frequency from the reference light L2. The amount of frequency shift of the modulated measurement light L1A is not particularly limited and is set to a general value that allows measurement using optical heterodyne interferometry. The modulated measurement light L1A, frequency-modulated by the optical frequency modulator 16, is emitted toward the retroreflective member 9a and is retroreflected by the retroreflective member 9a before entering the half mirror 18.

The half mirror 18 outputs to the light receiving unit 20 an interference signal SG (beat signal) between the modulated measurement light L1A incident from the retroreflective member 9a and the reference light L2 incident from the half mirror 14.

The light receiving unit 20 uses a known photodiode or the like, detects (receives) the interference signal SG incident from the half mirror 14, and outputs a detection signal of the interference signal SG to the calculation device 22.

The calculation device 22 performs a predetermined calculation process on the detection signal of the interference signal SG input from the light receiving unit 20, and calculates the distance from the measurement device 10 to the measurement object 9 (retroreflective member 9a). This makes it possible to measure the distance to the measurement object 9 and the displacement of the measurement object 9. Note that the specific calculation method performed by the calculation device 22 is publicly known technology, so a detailed explanation will be omitted here.

The following describes the first to fourth embodiments of the optical frequency modulator 16.

[Optical frequency modulator of the first embodiment]
FIG. 2 is a schematic diagram of the optical frequency modulator 16 of the first embodiment. FIG. 3 is an enlarged view of the area within the dotted circle A in FIG. 2. As shown in FIGS. 2 and 3, the optical frequency modulator 16 frequency-modulates the measurement light L1 by Doppler shifting to emit modulated measurement light L1A. The optical frequency modulator 16 includes a first mirror 30, a first lens 32, a rotating body 34, a driver 36, a second lens 38, and a second mirror 40. Note that the configuration of the optical frequency modulator 16, other than the rotating body 34 and the driver 36, can be modified as appropriate.

The first mirror 30 reflects the measurement light L1 incident from the half mirror 14 toward the rotating rotor 34, which will be described later. As a result, the measurement light L1 (corresponding to the incident light in this invention) is incident at a predetermined angle of incidence on a predetermined position (incident position P) on the rotating rotor 34.

The first lens 32 is positioned on the optical path of the measurement light L1 between the first mirror 30 and the rotating body 34, and emits the measurement light L1 incident from the half mirror 14 toward the rotating body 34.

The rotating body 34 is a cylindrical body having a rotation axis C and extending in a direction parallel to this rotation axis C. This rotating body 34 receives a driving force from the drive unit 36 and rotates around the rotation axis C.

The drive unit 36 is a known actuator composed of, for example, a motor and gears, and controls the rotation direction and rotation speed of the rotating body 34. This drive unit 36 rotates the rotating body 34 in one direction at a uniform speed around the rotation axis C.

The rotating body 34 has an outer peripheral surface 35 (corresponding to the curved surface of the present invention) formed along the axial direction of the rotation axis C (hereinafter simply referred to as the axial direction). This outer peripheral surface 35 has multiple diffraction gratings 42 formed (equally spaced) along the axial direction parallel to the rotation axis C. As a result, while the rotating body 34 continues to rotate at a uniform speed, each diffraction grating 42 passes continuously at a uniform speed through the incident position P of the measurement light L1, which is incident on the rotating body 34 at a predetermined angle of incidence.

Each diffraction grating 42 is a blazed diffraction grating, a well-known reflection-type diffraction grating. When measurement light L1 is incident on the diffraction grating 42 at a predetermined angle of incidence while passing through the incident position P at a constant speed, the diffraction grating 42 generates specularly reflected light LR and diffracted light (at least first-order diffracted light) that is reflected by the diffraction grating 42 and Doppler-shifted (frequency-modulated). In this embodiment, the first-order diffracted light (or second-order or later diffracted light, not shown) is used as modulated measurement light L1A. Note that the emission angle of modulated measurement light L1A, i.e., the amount of frequency shift of modulated measurement light L1A, is arbitrary. Therefore, the rotational speed of the rotor 34, the rotational direction of the rotor 34, the angle of incidence of measurement light L1, the blazed angle of the diffraction grating 42, and the pitch of the diffraction grating 42 are determined according to the desired frequency shift amount.

The second lens 38 is positioned on the optical path of the modulated measurement light L1A emitted from the rotating body 34, and emits this modulated measurement light L1A toward the second mirror 40. The second mirror 40 reflects the modulated measurement light L1A incident from the second lens 38 toward the retroreflective member 9a. As a result, as shown in Figure 1 above, the modulated measurement light L1A retroreflectively reflected by the retroreflective member 9a enters the half mirror 18.

As described above, in the optical frequency modulator 16 of this embodiment, by forming multiple diffraction gratings 42 around the axis on the outer circumferential surface 35 of the cylindrical rotor 34, each diffraction grating 42 can be moved at a uniform speed (rotated at a uniform speed) along a rotational trajectory centered on the rotation axis C simply by rotating the rotor 34 at a uniform speed. Therefore, during the uniform rotation except when the rotor 34 starts and stops rotating, each diffraction grating 42 continues to pass the incident position P at a uniform speed. This allows the rotor 34 to continuously emit modulated measurement light L1A of the desired frequency without interruption, without using an expensive acousto-optic element. As a result, modulated measurement light L1A (diffracted light) of the desired frequency can be continuously emitted at low cost.

(Modification of Diffraction Grating)
Fig. 4 is an explanatory diagram illustrating a modified example of the diffraction grating 42 of the first embodiment. In the above-described first embodiment, a blazed diffraction grating has been described as an example of the reflective diffraction grating 42, but as shown in Fig. 4, for example, a reflective diffraction grating 42A may be formed by forming the outer peripheral surface 35 of the rotor 34 from glass or the like and forming a plurality of grooves 35a parallel to the rotation axis C in the direction around the axis on this outer peripheral surface 35. In other words, the type of reflective diffraction grating formed on the outer peripheral surface 35 is not particularly limited.

[Second embodiment]
Fig. 5 is a schematic diagram of an optical frequency modulator 16 of the second embodiment. As shown in Fig. 5, the optical frequency modulator 16 of the second embodiment has basically the same configuration as the optical frequency modulator 16 of the first embodiment, except that it includes a rotor 34A instead of the rotor 34. Therefore, components that are the same in function or configuration as those of the first embodiment are designated by the same reference numerals, and descriptions thereof will be omitted.

The rotating body 34A of the second embodiment is a cylindrical body extending in a direction parallel to the rotation axis C, and rotates at a uniform speed around the rotation axis C upon receiving a driving force from the drive unit 36. This rotating body 34A has the same outer peripheral surface 35 and multiple diffraction gratings 42 (or diffraction gratings 42A) as in the first embodiment. Therefore, as in the first embodiment, each diffraction grating 42 can be moved (rotated) at a uniform speed along a rotational trajectory around the rotation axis C. As a result, the optical frequency modulator 16 of the second embodiment achieves the same effects as the first embodiment.

[Third embodiment]
FIG. 6 is a schematic diagram of an optical frequency modulator 16 according to a third embodiment. FIG. 7 is an enlarged view of the dotted circle A1 in FIG. 6 . In the above-described embodiments, reflective diffraction gratings 42, 42A are formed on the outer peripheral surface 35 of the rotor 34, 34A. However, in the third embodiment, as shown in FIGS. 6 and 7 , a plurality of transmissive diffraction gratings 42B are formed on the outer peripheral surface 35 or the inner peripheral surface 37 (or both) of the rotor 34B. The optical frequency modulator 16 according to the third embodiment has basically the same configuration as the optical frequency modulator 16 according to the above-described embodiments, except for the rotor 34B and the diffraction grating 42B. Therefore, components identical in function or configuration to those according to the above-described embodiments are designated by the same reference numerals, and their description will be omitted.

The rotating body 34B of the third embodiment is a cylindrical body similar to the rotating body 34A of the second embodiment, and receives a driving force from the drive unit 36 to rotate at a constant speed around the rotation axis C. A plurality of transmission-type diffraction gratings 42B are formed on the outer peripheral surface 35 of this rotating body 34B along the direction around the axis.

Each diffraction grating 42B is composed of multiple slits 35b (which may be light-transmitting portions such as glass) parallel to the rotation axis C, and multiple slits are formed at a predetermined pitch around the axis on the outer peripheral surface 35 (inner peripheral surface 37). Note that the transmission diffraction grating 42B is not limited to the one shown in FIGS. 6 and 7; various known shapes (e.g., transmission blazed diffraction gratings, ruled lines, etc.) may be used instead. As in the above embodiments, when measurement light L1 is incident at a predetermined angle on the diffraction grating 42B passing through the incident position P at a uniform velocity, modulated measurement light L1A, which is Doppler-shifted diffracted light (at least first-order diffracted light) that passes through the diffraction grating 42B, is emitted into the interior of the rotor 34B. This modulated measurement light L1A is then guided from the interior of the rotor 34B to the retroreflective member 9a via multiple optical components such as mirrors, prisms, and lenses (not shown).

In this way, in the third embodiment, the modulated measurement light L1A of the desired frequency can be emitted continuously and without interruption from the rotating body 34B, thereby achieving the same effects as the above embodiments.

[Fourth embodiment]
Figure 8 is a schematic diagram of an optical frequency modulator 16 of a fourth embodiment. In the above-described embodiments, the optical frequency modulator 16 includes a cylindrical or columnar rotor 34, 34A, 34B, but as shown in Figure 8, the optical frequency modulator 16 of the fourth embodiment includes a non-cylindrical rotor 34C. Note that the optical frequency modulator 16 of the fourth embodiment has basically the same configuration as the optical frequency modulator 16 of the above-described embodiments except for the rotor 34C. Therefore, components that are identical in function or configuration to those of the above-described embodiments are designated by the same reference numerals, and their description will be omitted.

The rotating body 34C is a semi-cylindrical body (or semi-cylinder) extending in a direction parallel to the rotation axis C, and has a curved surface 50 (outer peripheral surface) that forms part of a circumference centered on the rotation axis C when viewed from the axial side of the rotation axis C (the side perpendicular to the plane of the paper in Figure 8). The rotating body 34C receives a driving force from the drive unit 36 and rotates at a uniform speed around the rotation axis C.

Multiple diffraction gratings 42 (or diffraction gratings 42A) are formed on the curved surface 50 along the axial direction. If the rotor 34C is formed in a semi-cylindrical shape, a transmission-type diffraction grating 42B similar to that of the third embodiment may be formed instead of the reflection-type diffraction gratings 42, 42A. This allows the rotor 34C to continuously emit modulated measurement light L1A of the desired frequency without interruption while the diffraction grating 42 passes through the incident position P at a constant speed in accordance with the rotation of the rotor 34C. As a result, in the fourth embodiment, modulated measurement light L1A (pulsed light) of the desired frequency can be emitted from the optical frequency modulator 16.

In the fourth embodiment, the rotor 34C is formed as a semi-cylindrical body (semi-cylinder), but there is no particular limitation to this shape as long as it has a curved surface (outer peripheral surface) that forms part of a circumference centered on the rotation axis C, such as a quarter-cylindrical body (quarter-cylinder).

Fifth Embodiment
Fig. 9 is a front view (top view, side view) of the rotor 34B of the optical frequency modulator 16 according to the fifth embodiment, as viewed from the axial direction of the rotation axis C. Fig. 10 is a cross-sectional view of the rotor 34B of the optical frequency modulator 16 according to the fifth embodiment.

In the third embodiment, multiple transmission diffraction gratings 42B are formed on the rotor 34B, and the measurement light L1 entering the interior of the rotor 34 from the outer peripheral surface 35 side of the rotor 34B is frequency-modulated by each diffraction grating 42B. In contrast, in the fifth embodiment, the measurement light L1 emitted to the exterior of the rotor 34 from the inner peripheral surface 37 side of the rotor 34B is frequency-modulated by each diffraction grating 42B.

As shown in Figures 9 and 10, the optical frequency modulator 16 of the fifth embodiment has basically the same configuration as the optical frequency modulator 16 of the third embodiment, except that measurement light L1 enters the interior of the rotor 34B from outside the rotor 34B along the rotation axis C, and a reflector 54 is fixedly disposed inside the rotor 34B. Therefore, components that are functionally or structurally identical to those of the third embodiment are designated by the same reference numerals, and their description will be omitted.

The reflector 54 is, for example, a mirror or a pentaprism, and is arranged (fixed) on the rotation axis C inside the rotor 34B so that it cannot rotate around the axis. The reflector 54 reflects at a right angle the measurement light L1 that enters the rotor 34B from outside along the rotation axis C, causing this measurement light L1 to be incident on the inner circumferential surface 37 of the rotor 34B. Note that the reflection direction of the measurement light L1 by the reflector 54 may be changed as appropriate, as long as this measurement light L1 can be incident on the inner circumferential surface 37.

Each diffraction grating 42B of the fifth embodiment, like each diffraction grating 42B of the third embodiment, is a transmission diffraction grating formed on the outer peripheral surface 35 or inner peripheral surface 37 (or both) of the rotor 34B, and uses slits 35b (see Figure 7), a transmission blazed diffraction grating, rulings, etc. Each diffraction grating 42B frequency-modulates the measurement light L1 incident on the inner peripheral surface 37 from the reflector 54 while the rotor 34B is rotating. As a result, the frequency-modulated modulated measurement light L1A is emitted to the outside of the rotor 34B.

In this way, in the optical frequency modulator 16 of the fifth embodiment, a reflector 54 is placed inside the rotor 34B, and the measurement light L1 is transmitted from the inside to the outside of the rotor 34B. This measurement light L1 is then frequency-modulated by each diffraction grating 42B, thereby obtaining the maximum amount of modulated measurement light L1A without the need for complex optical adjustments.

For example, in the optical frequency modulator 16 of the first and second embodiments described above, in which a reflective diffraction grating 42 is formed on the outer surface 35 of a cylindrical or cylindrical rotor 34, two types of angle adjustment are required to output the maximum amount of modulated measurement light L1A: adjusting the angle of incidence of the measurement light L1 onto the diffraction grating 42 and adjusting the blazed angle of the diffraction grating 42. Furthermore, if the diffraction grating 42 is a blazed diffraction grating, the measurement light L1 is likely to be diffused when the measurement light L1 is incident obliquely onto the diffraction grating 42. Therefore, as shown in FIG. 1, it is necessary to position a first lens 32 in the optical path of the measurement light L1 and a second lens 38 in the optical path of the modulated measurement light L1A. Furthermore, in this case, it is necessary to adjust the optical axis of the first lens 32 parallel to the optical path of the measurement light L1 and the optical axis of the second lens 38 parallel to the optical path of the modulated measurement light L1A.

For example, in a simple transmission-type optical frequency modulator (see Figure 2 of JP 2004-287029 A) that has a disk and multiple transmission-type diffraction gratings formed along the same circumference centered on the central axis of the disk, when measurement light L1 is incident perpendicularly on the disk (transmission-type diffraction grating), modulated measurement light L1A (zeroth-order diffracted light) with the maximum light intensity is obtained in the same direction. However, even in this case, two types of angle adjustment are required: adjusting the angle of incidence of measurement light L1 with respect to the disk, and adjusting the angle of the disk itself.

In contrast, the optical frequency modulator 16 of the fifth embodiment can output the maximum intensity of modulated measurement light L1A by simply adjusting the angle of the reflector 54, allowing the measurement light L1 to be perpendicularly incident on the inner surface 37 (diffraction grating 42B). Furthermore, when the measurement light L1 is incident precisely along the rotation axis C into the interior of the rotor 34B, the angle of the reflecting surface of the reflector 54 is also set to 45 degrees with respect to the rotation axis C. As a result, the maximum intensity of modulated measurement light L1A can be obtained without complex optical adjustments. Furthermore, when an optical element capable of refracting light 90 degrees, such as a pentaprism, is used as the reflector 54, adjusting the angle of the reflector 54 becomes easier. Furthermore, the position of the first lens 32 and the second lens 38 is also easier to calculate the emission direction of the modulated measurement light L1A (diffracted light) than when a blazed diffraction grating is used, facilitating adjustment of their placement (position).

[others]
In each of the above embodiments, the measurement light L1 is frequency-modulated by the optical frequency modulator 16, but the reference light L2 may also be frequency-modulated.

In the above embodiments, an optical frequency modulator 16 used in a measurement device 10 that measures the distance to and displacement of a measurement object 9 using optical heterodyne interferometry has been described as an example, but the present invention can be applied to optical frequency modulators used for a variety of purposes.

9 Measurement object 9a Retroreflective member 10 Measuring device 12 Laser light source 14 Half mirror 16 Optical frequency modulator 18 Half mirror 20 Light receiving unit 22 Calculating device 30 First mirror 32 First lens 34, 34A to 34C Rotating body 35 Outer surface 35a Groove 35b Slit 36 Driving unit 37 Inner surface 38 Second lens 40 Second mirror 42, 42A, 42B Diffraction grating 50 Curved surface 54 Reflector C Rotation axis L Laser light L1 Measurement light L1A Modulated measurement light L2 Reference light LR Reflected light P Incident position SG Interference signal

Claims (2)

a cylindrical rotor having a rotation axis and extending in a direction parallel to the rotation axis, the rotor rotating at a constant speed around the rotation axis;
a plurality of transmission type diffraction gratings formed on the circumferential surface of the rotating body and parallel to the rotation axis, the transmission type diffraction gratings frequency-modulating incident light that is incident on the circumferential surface of the rotating body during rotation and emitting frequency-modulated diffracted light;
Equipped with
The optical frequency modulator includes a transmission type diffraction grating that modulates the frequency of the incident light that enters the interior of the rotating body from the outer peripheral surface of the rotating body . 2. The optical frequency modulator according to claim 1 , further comprising a drive unit for controlling the rotation speed and direction of the rotor.