Optical measuring apparatus

Abstract

Provided is a compact, low-cost optical measuring apparatus capable of acquiring an image of a target to be measured without moving a mirror or using a wavelength-scanning light source or beam splitter. A laser beam emitted from a light source is split into first and second beams, and the first beam is focused as a signal beam onto the target by a lens for irradiation purposes, while the second beam is reflected as a reference beam by a mirror without irradiating the target. Then, a signal beam reflected by or scattered by the target is multiplexed with the reference beam and then enters interference optics, whereby three or more interference beams with different phases are generated and detected by photodetectors. Then, the detection signals are operated by a signal processing unit. During the measurement, the focus position of the first beam is moved at least in the optical axis direction.

Claims

1. An optical measuring apparatus comprising: a light source configured to emit a laser beam; an optical splitter configured to split the laser beam into a first beam and a second beam; a lens configured to focus the first beam as a signal beam onto a target to be measured to irradiate the target to be measured with the signal beam; a light reflector configured to reflect the second beam as a reference beam without irradiating the target to be measured; a lens actuator configured to move the lens; a control unit configured to control the lens actuator to move a focus position of the first beam by moving the lens at least in an optical axis direction during measurement; interference optics configured to multiplex a signal beam reflected by or scattered by the target to be measured with the reference beam, thereby generating three or more interference beams with different phases; and a plurality of photodetectors configured to detect the interference beams, wherein the control unit is further configured to control the lens actuator to move the focus position of the first beam in the optical axis direction within a scanning range such that a coherence length of the laser beam is greater than or equal to a change in an optical path length of the signal beam.

2. The optical measuring apparatus according to claim 1, wherein the lens has a numerical aperture that is greater than or equal to 0.4.

3. The optical measuring apparatus according to claim 1, wherein a position of the light reflector is fixed during measurement.

4. The optical measuring apparatus according to claim 1, wherein the photodetectors include four photodetectors, an interference phase of the signal beam and the reference beam on each of the four photodetectors differs from one another by an integral multiple of substantially 90, and pairs of interference beams are detected by a current differential detector, each pair having a difference of substantially 180 in the interference phase of the signal beam and the reference beam.

5. The optical measuring apparatus according to claim 1, wherein the control unit is configured to control the lens actuator so that movement of the focus position of the first beam is repeated in the optical axis direction.

6. The optical measuring apparatus according to claim 5, wherein the control unit is configured to control lens actuator so that the focus position returns in the optical axis direction at a position beyond an observation target area of the target to be measured.

7. The optical measuring apparatus according to claim 5, further comprising: a cover glass between the lens and the target to be measured, wherein the control unit is configured to control the lens actuator so that a surface of the cover glass is included in a movement range of the focus position in the optical axis direction, and an observed image is corrected using a signal from the surface of the cover glass.

8. The optical measuring apparatus according to claim 5, wherein an intensity of the laser beam in an area around a return position of the repetitive movement of the focus position in the optical axis direction is set lower than when the focus position is outside the area around the return position.

9. The optical measuring apparatus according to claim 5, wherein power of the laser beam is set to zero around the return position of the repetitive movement of the focus position in the optical axis direction.

10. The optical measuring apparatus according to claim 5, wherein the laser beam is pulse-modulated so that light emission occurs in synchronism with a signal acquisition timing.

11. An observation unit comprising: a light source configured to emit a laser beam; an optical splitter configured to split the laser beam into a first beam and a second beam; a lens configured to focus the first beam as a signal beam onto a target to be measured to irradiate the target to be measured with the signal beam; a light reflector configured to reflect the second beam as a reference beam without irradiating the target to be measured; a lens actuator configured to move the lens; a control unit configured to control the lens actuator to move a focus position of the first beam by moving the lens at least in an optical axis direction during measurement; an optical fiber connection unit having an optical fiber connected thereto; and optics configured to cause a combined beam to enter the optical fiber connected to the optical fiber connection unit, the combined beam having been generated by multiplexing the signal beam with the reference beam by multiplexing a signal beam reflected by or scattered by a target to be measured with the reference beam, wherein the control unit is further configured to control the lens actuator to move the focus position of the first beam in the optical axis direction within a scanning range such that a coherence length of the laser beam is greater than or equal to a change in an optical path length of the signal beam.

12. An optical measuring apparatus comprising: a light source configured to emit a laser beam; an optical splitter configured to split the laser beam into a first beam and a second beam; a lens configured to focus the first beam as a signal beam onto a target to be measured to irradiate the target to be measured with the signal beam; a light reflector configured to reflect the second beam as a reference beam without irradiating the target to be measured; a lens actuator configured to move the lens; a control unit configured to control the lens actuator to move a focus position of the first beam by moving the lens in an optical axis direction during measurement; an optical path length modulation unit configured to modulate an optical path length of the reference beam at a faster speed than a change in an optical path length of the signal beam that occurs due to the movement of the focus position of the first beam; and a photodetector configured to detect a beam that is obtained by multiplexing a signal beam reflected by or scattered by the target to be measured with the reference beam, wherein the control unit is further configured to control the lens actuator to move the focus position of the first beam in the optical axis direction within a scanning range such that a coherence length of the laser beam is greater than or equal to a change in an optical path length of the signal beam.

13. The optical measuring apparatus according to claim 12, wherein the optical path length modulation unit includes a piezo element.

14. An optical measuring apparatus comprising: a light source configured to emit a laser beam; an optical splitter configured to split the laser beam into a first beam and a second beam; a lens configured to focus the first beam as a signal beam onto a target to be measured to irradiate the target to be measured with the signal beam; a spherical aberration correction unit arranged on an optical path of the first beam; a light reflector configured to reflect the second beam as a reference beam without irradiating the target to be measured; a lens actuator configured to move the lens; a control unit configured to control the lens actuator to move a focus position of the first beam by moving the lens at least in an optical axis direction during measurement; interference optics configured to multiplex a signal beam reflected by or scattered by the target to be measured with the reference beam, thereby generating three or more interference beams with different phases; and a plurality of photodetectors configured to detect the interference beams, wherein the control unit is further configured to control the lens actuator to move the focus position of the first beam in the optical axis direction within a scanning range such that a coherence length of the laser beam is greater than or equal to a change in an optical path length of the signal beam.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram representing the configuration of a conventional OCT apparatus;

(2) FIG. 2 is an illustration diagram of an interference signal detected with the conventional OCT apparatus;

(3) FIG. 3 is an illustration diagram of a demodulated signal of the conventional OCT apparatus;

(4) FIG. 4 is a schematic diagram showing an exemplary configuration of an optical measuring apparatus of the present invention;

(5) FIG. 5 is a schematic diagram showing an ideal movement path of the focus position;

(6) FIG. 6 is a schematic diagram showing an example of the actual movement path of the focus position;

(7) FIG. 7 is a schematic diagram showing an example of the movement path of the focus position of the optical measuring apparatus of the present invention;

(8) FIG. 8 is a schematic diagram showing another exemplary configuration of the optical measuring apparatus of the present invention;

(9) FIG. 9 is a diagram showing an example of interference optics of the optical measuring apparatus of the present invention;

(10) FIG. 10 is a schematic diagram showing another exemplary configuration of the optical measuring apparatus of the present invention;

(11) FIG. 11 is a schematic diagram showing another exemplary configuration of the optical measuring apparatus of the present invention; and

(12) FIG. 12 is a schematic diagram showing another exemplary configuration of the optical measuring apparatus of the present invention.

DETAILED DESCRIPTION

(13) Hereinafter, embodiments of the present invention will be described with reference to the drawings.

Embodiment 1

(14) FIG. 4 is a schematic diagram showing the basic embodiment of an optical measuring apparatus in accordance with the present invention.

(15) A laser beam with single-wavelength components emitted from a light source 401 is converted into a collimated beam by a collimator lens 402, and is subjected to rotary polarization by a /2 plate 403 capable of adjusting the optical axis direction, and is further split into two: a signal beam and a reference beam by a polarization beam splitter 404. The signal beam passes through a /4 plate 405 at which the optical axis direction is set to about 22.5 with respect to the horizontal direction so that the signal beam is converted from the s-polarized beam into a circularly polarized beam, and is then focused by a lens 406 with a numerical aperture of greater than or equal to 0.4. Then, the signal beam passes through a cover glass 408, and irradiates a target to be measured 409. Herein, the lens 406 is moved at least in the z direction by a lens actuator 407 under control of a control unit 430, whereby the focus position (i.e., the measurement position) of the signal beam by the lens 406 is moved. A signal beam reflected by or scattered by the target to be measured is converted into a collimated beam by the lens 406, and is converted from the circularly polarized beam into a p-polarized beam by a /4 plate 405. Then, the signal beam enters the polarization beam splitter 404.

(16) Meanwhile, the reference beam passes through a /4 plate 410, and is converted from the p-polarized beam into a circularly polarized beam. Then, the reference beam becomes incident on and is reflected by a stationary mirror 411, and is converted from the circularly polarized beam into a s-polarized beam, and then enters the beam splitter 404.

(17) The signal beam and the reference beam are multiplexed by the polarization beam splitter 404, whereby a combined beam is generated. The combined beam is guided to interference optics 412 that include a half beam splitter 413, a /2 plate 414, a /4 plate 419, condenser lenses 415 and 420, and Wollaston prisms 416 and 421.

(18) The combined beam that has entered the interference optics 412 is split into two: a transmitted beam and a reflected beam by the half beam splitter 413. The transmitted beam passes through a /2 plate 414 at which the optical axis is set to about 22.5 with respect to the horizontal direction, and is focused by a condenser lens 415, and is further split into two by a Wollaston prism 416, whereby a first interference beam and a second interference beam having a phase difference of 180 are generated. The first interference beam and the second interference beam are detected by a current differential photodetector 417, whereby a signal 418 that is proportional to the intensity difference between the first interference beam and the second interference beam is output.

(19) Meanwhile, the reflected beam passes through the /4 plate 419 at which the optical axis is set to about 45 with respect to the horizontal direction, and is focused by the condenser lens 420, and is further split into two by a Wollaston prism 421, whereby a third interference beam and a fourth interference beam having a phase difference of 180 are generated. The third interference beam and the fourth interference beam are detected by a current differential photodetector 422, whereby a signal 423 that is proportional to the intensity difference between the third interference beam and the fourth interference beam is output. The thus generated signals 418 and 423 are input to a signal processing unit 424 and are operated, whereby a signal that is proportional to the amplitude of the signal beam is obtained.

(20) The operation principle mentioned above will be specifically described using formulae. Provided that the Jones vector of the combined beam at a time point when it enters the interference optics 412 is represented as follows:

(21) $\begin{array}{cc}\left(\begin{array}{c}{E}_{\mathrm{sig}}\\ E_{\mathrm{ref}}\end{array}\right),& \left(1\right)\end{array}$
the Jones vector of the combined beam that has passed through the half beam splitter 413 and has further passed through the /2 plate 414 is represented as follows.

(22) $\begin{array}{cc}\left(\begin{array}{cc}1\text{/}\sqrt{2}& -1\text{/}\sqrt{2}\\ 1\text{/}\sqrt{2}& 1\text{/}\sqrt{2}\end{array}\right)\left(\begin{array}{c}{E}_{\mathrm{sig}}\text{/}\sqrt{2}\\ E_{\mathrm{ref}}\text{/}\sqrt{2}\end{array}\right)=\frac{1}{2}\left(\begin{array}{c}{E}_{\mathrm{sig}}-E_{\mathrm{ref}}\\ E_{\mathrm{sig}}+E_{\mathrm{ref}}\end{array}\right)& \left(2\right)\end{array}$

(23) The combined beam represented by Formula (2) is split into two: p-polarized components and s-polarized components by the Wollaston prism 416, which are then differentially detected by the current differential photodetector 417. Thus, the detection signal 418 is represented as follows.

(24) $\begin{array}{cc}\begin{array}{c}I=\frac{1}{4}{.Math.E_{\mathrm{sig}}+E_{\mathrm{ref}}.Math.}^2-\frac{1}{4}{.Math.E_{\mathrm{sig}}-E_{\mathrm{ref}}.Math.}^2\\ =.Math.E_{\mathrm{sig}}.Math..Math.E_{\mathrm{ref}}.Math.\cos \left({}_{\mathrm{sig}}-{}_{\mathrm{ref}}\right)\end{array}& \left(3\right)\end{array}$

(25) Herein, .sub.sig and .sub.ref are the phases for when the complex numbers E.sub.sig and E.sub.ref are represented on the polar coordinates. For simplicity purposes, the conversion efficiency of the detector is assumed to be 1.

(26) Meanwhile, the Jones vector of the combined beam that has been reflected by the half beam splitter 413 and has further passed through the /4 plate 419 is represented as follows.

(27) $\begin{array}{cc}\left(\begin{array}{cc}i\text{/}\sqrt{2}& 1\text{/}\sqrt{2}\\ 1\text{/}\sqrt{2}& i\text{/}\sqrt{2}\end{array}\right)\left(\begin{array}{c}{E}_{\mathrm{sig}}\text{/}\sqrt{2}\\ E_{\mathrm{ref}}\text{/}\sqrt{2}\end{array}\right)=\frac{1}{2}\left(\begin{array}{c}i\left(E_{\mathrm{sig}}-i{E}_{\mathrm{ref}}\right)\\ E_{\mathrm{sig}}+i{E}_{\mathrm{ref}}\end{array}\right)& \left(4\right)\end{array}$

(28) The combined beam represented by Formula (4) is split into two: p-polarized components and s-polarized components by the Wollaston prism 421, which are then differentially detected by the current differential photodetector 422. Thus, the detection signal 423 is represented as follows.

(29) $\begin{array}{cc}\begin{array}{c}Q=\frac{1}{4}{.Math.E_{\mathrm{sig}}+i{E}_{\mathrm{ref}}.Math.}^2-\frac{1}{4}{.Math.E_{\mathrm{sig}}-i{E}_{\mathrm{ref}}.Math.}^2\\ =.Math.E_{\mathrm{sig}}.Math..Math.E_{\mathrm{ref}}.Math.\sin \left({}_{\mathrm{sig}}-{}_{\mathrm{ref}}\right)\end{array}& \left(5\right)\end{array}$

(30) The following operation is performed on the above outputs with the signal processing unit 424, whereby a signal that is independent of the phase and is proportional to the absolute value of the amplitude of the signal beam is obtained.
|E.sub.sigE.sub.ref={square root over (I.sup.2+Q.sup.2)}(6)

(31) Next, a lens movement method and a laser intensity control method will be described. Hereinafter, the optical axis direction will be indicated by z-direction, a direction that is parallel with the paper surface will be indicated by x-direction, and a direction that is perpendicular to the paper surface will be indicated by y-direction.

(32) A two-dimensional image (zx image) of a target to be measured can be obtained by, for example, repeatedly moving the lens 406 in the z-direction by controlling the lens actuator 407 of the scanning unit with the control unit 439, and moving the lens 6 by a predetermined amount in the x-direction (i.e., about a spot diameter of the focused signal beam) each time the lens 406 reaches the return position. FIG. 5 shows an ideal movement path of the focus position of the signal beam at this time. However, as the lens actuator does not always move linearly, the lens may be tilted at an end of a stroke in the xy direction, and thus the actual movement path may become distorted as shown in FIG. 6. When the movement path is distorted as described above, the acquired image will be curved and a correct image cannot be obtained. Thus, in this embodiment, the surface of the cover glass 408 is included in the measurement area (i.e., the scanning range in the optical direction) as shown in FIG. 7. Accordingly, it becomes possible to acquire a correct image by acquiring a signal from the surface of the cover glass 408 each time scanning in the z-direction is performed and correcting a curved image so that the surface of the cover glass 408 becomes flat.

(33) A three-dimensional image of the target to be measured can be acquired by, after acquiring the zx image with the aforementioned scanning method, repeating the procedures of moving the lens in the y-direction by a predetermined amount (i.e., about a spot diameter of the focused signal beam). Alternatively, it is also possible to, after acquiring the zx image by moving the lens, repeat the procedures of moving the target to be measured or the entire OCT apparatus in the y-direction using a motor-driven stage or the like. Although the z-scan is performed by moving the lens 406 in this embodiment, it is also possible to move the focus position by inserting at least one lens in front of the lens 406 and moving the inserted lens.

(34) The movement speed of the focus position is relatively slow in an area around the return position of the scanning. Thus, there is a possibility that the amount of exposure to light may increase in that area and thus the damage to the target to be measured may increase. To suppress such damage, in this embodiment, the laser intensity at the return position of the scanning is set relatively low. It is also possible to set the laser power at the return position of the scanning to zero. Herein, as another method of suppressing the damage to the target to be measured due to the exposure to light, it is also possible to apply pulse modulation to the laser beam so that light emission occurs in synchronism with the signal acquisition timing. Accordingly, as the exposure does not occur in a period other than when a signal is acquired, it is possible to reduce the average amount of exposure.

(35) In this embodiment, the measurement position is moved in the optical axis direction by moving the lens. Thus, it is not necessary to move a reference beam mirror unlike in the time domain OCT. Accordingly, it is possible to avoid a tilt of the reference beam mirror that would otherwise occur due to a mechanical movement of the reference beam mirror. Further, as a wavelength-scanning light source or a beam splitter is not necessary unlike in the Fourier domain OCT, the apparatus can be provided with a compact size and low cost.

(36) Further, a laser source is used that emits a longer coherence laser beam than a change in the optical path length of the signal beam that would occur due to the movement of the lens in the optical axis direction. Accordingly, even when an optical path length difference is generated between the signal beam and the reference beam as a result of the lens having been moved in the optical axis direction, it is possible to suppress a decrease in the interference amplitude.

(37) Further, four interference beams that differ from one another in the interference phase of the signal beam and the reference beam by an integral multiple of about 90 are generated and detected, and an operation is performed on such detection signals to obtain a signal represented by Formula (6) that is independent of the interference phase. Thus, it is possible to demodulate even a signal that would not be able to be demodulated through the envelope detection used for the conventional time domain OCT.

Embodiment 2

(38) FIG. 8 is a schematic diagram showing another embodiment of the optical measuring apparatus in accordance with the present invention. It should be noted that members that are the same as those shown in FIG. 4 are denoted by the same reference numerals, and description thereof will be omitted.

(39) This embodiment is the same as Embodiment 1 in the structure in which a laser beam emitted from a light source 401 is split into two: a signal beam and a reference beam, and the split beams are multiplexed again to generate a combined beam. The generated combined beam is first split intoprimary diffraction rays by a diffraction grating 802, whereby a first split combined beam and a second split combined beam are generated. Such combined beams pass through a phase plate 803 that is arranged so that the phase difference between the s-polarized components and the p-polarized components of the first split combined beam differs from the phase difference between the s-polarized components and the p-polarized components of the second split combined beam by 90. After that, the polarization directions of the beams are rotated by a /2 plate 804 that is set at about 22.5 with respect to the horizontal direction, and then, the beams are subjected to polarization split by a Wollaston prism 805, whereby four interference beams whose interference phases differ from one another by substantially 90 are generated. Such interference beams are focused by a condenser lens 806, and pairs of interference beams are differentially detected by a detector 807, each pair having a phase difference of 180. The detection signals are operated by a signal processing unit 424, whereby a signal that is independent of the phase and is proportional to the absolute value of the amplitude of the signal beam is obtained. The functions of the interference optics 801 are the same as those of the interference optics 412 in Embodiment 1. Thus, description thereof will be omitted. In this embodiment, the interference optics are more compact than those in Embodiment 1. Thus, a more compact OCT apparatus can be provided.

(40) In Embodiment 1 and the embodiment shown in FIG. 8, information on the amplitude and the phase of each polarized component of the signal beam is acquired from the intensities of four interference beams. As the parameters that determine the intensities of interference beams are the following three: (1) signal beam intensity, (2) reference beam intensity, and (3) the phase difference between the signal beam and the reference beam, it is in principle possible to acquire information on the amplitude and the phase by detecting the intensities of three or more interference beams with different phases.

(41) Accordingly, interference optics 901 shown in FIG. 9, for example, can be used as the interference optics. Hereinafter, the function of the interference optics 901 will be described. A combined beam that has entered the interference optics 901 is split into three: a first split combined beam, a second split combined beam, and a third split combined beam by non-polarization beam splitters 902 and 903. Among them, the first split combined beam passes through a phase plate 904 at which the s-polarized beam has a phase difference of 120 generated with respect to the p-polarized beam; the second split combined beam passes through a phase plate 905 at which the s-polarized beam has a phase difference of 240 generated with respect to the p-polarized beam; and the three split combined beams pass through polarizers 906, 907, and 908 that allow only 45 linearly polarized beams to pass therethrough, respectively, and are focused by condenser lenses 909, 910, and 911, and are further detected by detectors 912, 913, and 914. Outputs 915, 916, and 917 of such detectors are represented as follows.

(42) $\begin{array}{cc}\begin{array}{c}{I}_1=.Math.\frac{1}{\sqrt{3}}{E}_{\mathrm{sig}}+\frac{1}{\sqrt{3}}{E}_{\mathrm{ref}}.Math.\\ =\frac{1}{3}{.Math.E_{\mathrm{sig}}.Math.}^2+\frac{1}{3}{.Math.E_{\mathrm{ref}}.Math.}^2+\frac{2}{3}.Math.E_s.Math..Math.E_r.Math.\cos \left({}_{\mathrm{sig}}-{}_{\mathrm{ref}}\right)\end{array}& \left(7\right)\\ \begin{array}{c}{I}_2=.Math.\frac{1}{\sqrt{3}}{E}_{\mathrm{sig}}+\frac{1}{\sqrt{3}}{}^{\frac{}{3}}{E}_{\mathrm{ref}}.Math.\\ =\frac{1}{3}{.Math.E_{\mathrm{sig}}.Math.}^2+\frac{1}{3}{.Math.E_{\mathrm{ref}}.Math.}^2+\frac{2}{3}.Math.E_s.Math..Math.E_r.Math.\cos \left({}_{\mathrm{sig}}-{}_{\mathrm{ref}}-\frac{}{3}\right)\end{array}& \left(8\right)\\ \begin{array}{c}{I}_3=.Math.\frac{1}{\sqrt{3}}{E}_{\mathrm{sig}}+\frac{1}{\sqrt{3}}{}^{\frac{2}{3}}{E}_{\mathrm{ref}}.Math.\\ =\frac{1}{3}{.Math.E_{\mathrm{sig}}.Math.}^2+\frac{1}{3}{.Math.E_{\mathrm{ref}}.Math.}^2+\frac{2}{3}.Math.E_s.Math..Math.E_r.Math.\cos \left({}_{\mathrm{sig}}-{}_{\mathrm{ref}}-\frac{2}{3}\right)\end{array}& \left(9\right)\end{array}$

(43) By performing operation of the following formula on such signals, it is possible to obtain a signal that is independent of the interference phase.

(44) $\begin{array}{cc}.Math.E_s.Math..Math.E_r.Math.=\sqrt{{\left(I_1-\frac{I_2-I_3}{2}\right)}^2+3{\left(\frac{I_2-I_3}{2}\right)}^2}& \left(10\right)\end{array}$

Embodiment 3

(45) FIG. 10 is a schematic diagram showing another embodiment of the present invention. It should be noted that members that are the same as those shown in FIG. 4 are denoted by the same reference numerals, and description thereof will be omitted.

(46) A laser beam emitted from a light source 401 is converted into a collimated beam by a collimator lens 402, and is then subjected to polarization rotation by a /2 plate 403, and is further split into two: a signal beam and a reference beam by a polarization beam splitter 404. The signal beam passes through a /4 plate 405 so that it is converted from the s-polarized beam into a circularly polarized beam, and is then focused by a lens 406 with a numerical aperture of greater than or equal to 0.4. Then, the signal beam passes through a cover glass 408 and irradiates a target to be measured 409. Herein, a signal beam reflected by or scattered by the target to be measured 409 is converted into a collimated beam by the lens 406, and is converted from the circularly polarized beam into a p-polarized beam by the /4 plate 405. Then, the signal beam enters the polarization beam splitter 404.

(47) Meanwhile, the reference beam passes through a /4 plate 410, and is converted from the p-polarized beam into a circularly polarized beam. Then, the reference beam becomes incident on and is reflected by a mirror 411 that is fixed to a piezo element 1001 through adhesion. Then, the reference beam passes through the /4 plate 410, and is converted from the circularly polarized beam into a s-polarized beam, and then enters the polarization beam splitter 404.

(48) The signal beam and the reference beam are multiplexed by the polarization beam splitter 404, whereby a combined beam is generated. The combined beam is guided to interference optics 1002 that include a 212 plate 1003, a condenser lens 1004, and a Wollaston prism 1005. The combined beam passes through the /2 plate 1003 at which the optical axis is set at about 22.5 with respect to the horizontal direction, and is then focused by the condenser lens 1004. Then, the combined beam is split into two by the Wollaston prism 1005, whereby a first interference beam and a second interference beam having a phase difference of 180 are generated. The first interference beam and the second interference beam are detected by a current differential photodetector 417, and a signal 418 that is proportional to the intensity difference between the two beams is output. The output signal herein is represented by the following formula.
I=4|E.sub.sigE.sub.ref|cos(.sub.sig.sub.ref)(11)

(49) In this embodiment, the piezo element 1001 is driven during the measurement to modulate the position of the mirror 411, whereby the optical path length of the reference beam is modulated at a faster speed than a change in the optical path length of the signal beam that would occur due to the movement of the lens 406. Accordingly, the detected interference signals can be demodulated into a desired signal through envelope detection. Thus, functions that are similar to those in Embodiment 1 can be implemented using less detectors than in Embodiment 1.

Embodiment 4

(50) FIG. 11 is a schematic diagram showing another embodiment of the present invention. It should be noted that members that are the same as those shown in FIG. 4 are denoted by the same reference numerals, and description thereof will be omitted.

(51) The optical measuring apparatus in this embodiment includes an optical observation unit 1101, a photodetector unit 1104, and a polarization maintaining optical fiber 1103 connecting them. The polarization maintaining optical fiber 1103 is removably fixed to a fiber connecting portion 1107 of the optical observation unit 1101 and a fiber connecting portion 1108 of the photodetector unit 1104. This embodiment is the same as Embodiment 1 in the structure and function in which a laser beam emitted from a light source 401 is split into two, and the split beams are multiplexed again to generate a combined beam. The generated combined beam is coupled to the polarization maintaining optical fiber 1103 by a condenser lens 1102. The combined beam transmitted to the photo detector unit 1104 by the polarization maintaining optical fiber 1103 is converted into a collimated beam by a collimator lens 1105, and then enters interference optics 412. The configurations and functions of the units following the interference optics 412 are the same as those in Embodiment 1. Thus, description thereof will be omitted.

(52) In this embodiment, the photodetector unit 1104 and the optical observation unit 1101 are connected with the polarization maintaining optical fiber 1103. Thus, when a large target to be measured, such as a human body, is measured, the measurement can be performed by moving only the optical observation unit 1101 closer to the target to be measured 409, and thus the measurement becomes easier. In addition, as the polarization maintaining optical fiber 1103 can be easily attached or detached, it is possible to replace only the photodetector unit when the photodetector unit fails, for example. Thus, replacement of the whole apparatus is not necessary.

Embodiment 5

(53) FIG. 12 is a schematic diagram showing another embodiment of the present invention. It should be noted that members that are the same as those shown in FIG. 4 are denoted by the same reference numerals, and description thereof will be omitted.

(54) This embodiment differs from Embodiment 1 in that a spherical aberration correction unit 1204, which includes first and second correction lenses 1201 and 1202 and a correction motor 1203, is provided on the optical path of the signal beam. The spherical aberration correction unit 1204 is adapted to correct spherical aberrations of a signal beam that are generated in the target to be measured. Specifically, the position of the first correction lens 1201 in the optical axis direction is changed by the correction motor 1203 that is controlled by the control unit 430, and the relative positional relationship between the first correction lens 1201 and the second correction lens 1202 is thus changed, whereby spherical aberrations are corrected. In this embodiment, two lenses are used as the spherical aberration correction means. However, it is also possible to use liquid crystal optical elements and the like. The configurations and functions of the components other than the spherical aberration correction unit 1204 are similar to those in Embodiment 1. Thus, description thereof will be omitted herein.

(55) In this embodiment, spherical aberrations can be corrected. Thus, it is possible to suppress degradation of S/N and a decrease in the spatial resolution due to a decrease in the signal intensity even at a deep portion of the target to be measured. Thus, it is possible to suppress degradation of an image of a deep portion of the target to be measured.

(56) It should be noted that the present invention is not limited to the aforementioned embodiments, and includes a wide variety of variations. For example, although the aforementioned embodiments have been described in detail to clearly illustrate the present invention, the present invention need not include all of the configurations described in the embodiments. It is possible to replace a part of a configuration of an embodiment with a configuration of another embodiment. In addition, it is also possible to add, to a configuration of an embodiment, a configuration of another embodiment. Further, it is also possible to, for a part of a configuration of each embodiment, add, remove, or substitute another configuration.

REFERENCE SIGNS LIST

(57) 401 Light source 403, 414 /2 plate 404 Polarization beam splitter 405, 410, 419 /4 plate 406 Lens 407 Lens actuator 408 Cover glass 409 Target to be measured 412, 801, 901, 1002 Interference optics 413 Half beam splitter 416, 421 Wollaston prism 417, 422 current differential photodetector 424 Signal processing unit 802 Diffraction grating 1001 Piezo element 1101 Optical observation unit 1103 Polarization maintaining optical fiber 1104 Photodetector unit 1204 Spherical aberration correction unit