Abstract:
An apparatus for detecting the relative displacement of a surface to be detected, and an apparatus and method for recording information on a hard disc of a hard disk drive using such a detecting apparatus, the detecting apparatus comprising an interference optical system for condensing a light beam on the surface to be detected, and making the reflected light from the surface to be detected interfere with the condensed light beam to thereby form an interference light beam, light receiving means for receiving the interference light beam and outputting bright and dark signals attributable to the relative displacement of the surface to be detected, and condensed light information supplying means for separating part of the reflected light from the surface to be detected from the optical path until the reflected light arrives at said light receiving means, and utilizing the separated light beam to detect the condensed state of the incident light beam onto the surface to be detected or make the condensed state observable.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a displacement detecting apparatus capable of detecting a positional fluctuation of an object in a non-contact manner, where the positional displacement is minute displacement in the order of nanometers, and an information recording apparatus using such a displacement detecting apparatus. 
     2. Related Background Art 
     FIG. 1A of the accompanying drawings shows a perspective view of an information recording reproducing apparatus according to the prior art. The apparatus is comprised of a hard disc drive  1  for writing a servo track signal from a signal generator (SG)  48  into a hard disc, and a rotary positioner system  2  for effecting highly accurate rotary positioning. The hard disc drive  1  comprises a disc-shaped hard disc  3 , a slider  4  having a magnetic head at its tip end, a magnetic head arm  5 , a voice coil motor  6 , a voice coil motor driver  7 , a spindle  8 , etc. Also, the rotary positioner system  2  comprises a push rod  9 , a push rod arm  10 , a positioning control motor  11 , a rotary encoder  12  for detecting the amount of rotation of the rotary shaft of the control motor  11 , a signal processor  13  for analyzing the detection output from the rotary encoder  12  and sending a positioning command signal to the servo track signal writing position of the magnetic head, a control motor driver  14  for driving the control motor  11  by the command signal of the signal processor  13 , etc. 
     With such a construction, the writing and reading of magnetic information are effected on any track on the surface of the hard disc  3  being rotated at a high speed, by the arcuately operating magnetic head arm  5  through the magnetic head. At this time, in order to effect highly accurate positioning, the cylindrical surface of the push rod  9  is pushed against the side of the magnetic head arm  5 , and the push rod arm  10  is rotated by the control motor  11  while feedback control is effected by the system of the rotary encoder  12 , the signal processor  13  and the control motor driver  14 , and positioning is effected while the magnetic head arm  5  is sequentially finely fed through the intermediary of the push rod  9 . At this time, in order to effect contact reliably, usually some electric current is supplied to the voice coil motor  6  and pushing is also effected against the push rod  9  from the magnetic head arm  5  side. 
     FIG. 1B of the accompanying drawings shows a perspective view of another highly accurate positioning apparatus. This detecting apparatus is comprised of a laser source  15 , mirrors  16 ,  17 , a beam splitter  18 , a retro-reflector  19 , like a corner cube provided on a magnetic head arm  5 , and a light receiving element  20 . Movement of the magnetic head is measured with high accuracy not by the magnetic head arm  5  being mechanically pushed, but by optical means. 
     In this apparatus, by the utilization of a Michelson-type interferometer comprising the laser source  15 , the mirrors  16 ,  17 , the beam splitter  18  and the retro-reflector  19 , the interference light of two light beams passed from the retro-reflector  19  via the mirror  16  and the mirror  17  is detected by the light receiving element  20 , thereby to obtain positional information of the magnetic head arm  5 . On the basis of the obtained detection signal, a signal processor  13  issues a command, and an electric current to be supplied to a voice coil motor  6  is controlled by a voice coil motor driver  7 , thereby to directly move the magnetic head arm  5  and effect appropriate control. 
     FIG. 1C of the accompanying drawings shows a perspective view of the optical system of an optical-type sensor unit  20  according to the prior art, and in the optical-type sensor unit  20 , there are successively arranged a multimode laser diode light source  21 , a collimator lens  22 , a non-polarizing beam splitter  23 , and a probe-shaped polarizing prism  24  having a polarizing beam splitter surface  24   a  and a reference reflecting mirror surface  24   b  on which reflecting evaporated film is formed. In the reflecting direction of the non-polarizing beam splitter  23 , there are arranged a quarter wavelength plate  25 , a beam diameter limiting opening plate  26 , a beam amplitude dividing diffraction grating  27  having staggered grating structure, polarizing plate analyzers  28   a  to  28   d  disposed with their polarization azimuths deviated by 45° from one another, and light receiving elements  29   a  to  29   d.    
     With such a construction, divergent light from the multimode laser diode light source  21  is made into a loosely condensed light beam L by the collimator lens  22 , and is transmitted through the non-polarizing beam splitter  23  and then passes through the probe-shaped polarizing prism  24 , and is divided into polarized components in the polarizing beam splitter surface  24   a . An S-polarized light beam reflected by the polarizing surface  24   a  emerges from the end surface of the probe-shaped polarizing prism  24  and is condensed near the beam waist of the measuring surface  5   a  of the magnetic head arm  5 , and the reflected light thereof becomes a divergent spherical wave and passes along the original optical path and returns to the probe-shaped polarizing prism  24 . On the other hand, a P-polarized light beam transmitted through the polarizing surface  24   a  is condensed at a position deviating from the beam waist on the reference reflecting mirror surface  24   b  in the end portion, and the reflected light thereof passes along the original path and likewise returns to the probe-shaped polarizing prism  24 . 
     These two light beams are re-combined on the polarizing surface  24   a  of the probe-shaped polarizing prism  24  and become linearly polarized light beams orthogonal to each other, and do not directly interfere with each other and become bright and dark signals, but yet when these two light beams are reflected in the non-polarizing beam splitter  23  and transmitted through the quarter wavelength plate  25 , the linearly polarized light beams orthogonal to each other are converted into oppositely circularly polarized light beams, and these two light beams have their vibration surfaces vector-combined and are re-converted into a linearly polarized light beam rotated by the fluctuation of the phase difference therebetween. 
     This rotated linearly polarized light beam is amplitude-divided into four light beams by the phase diffraction grating  27 , and these four divisional light beams are transmitted through the polarizing plate analyzers  28   a  to  28   d , whereby they are converted into interference light beams in which the timing of light and darkness shifts by 90° each in terms of phase, and are received by the respective light receiving elements  29   a  to  29   d . On the basis of the light reception signals of these light receiving elements  29   a  to  29   d , a minute fluctuation of the position of the measuring surface  5   a  of the magnetic head arm  5  is detected with high accuracy of 1 nm or less. 
     In the above-described rotary positioner system  2  of FIG. 1A, however, vibration due to rotation or the like of the hard disc  3  is transmitted to the magnetic head arm  5 , and is further transmitted to the control motor  11  through the cylindrical surface of the push rod  9 . Therefore, highly accurate positioning is hindered and the capability of writing information such as a high-density servo track signal is reduced. For this reason, as a method of detecting minute displacement, there is known an electrostatic capacity sensor or the like utilizing impedance, e.g. electrostatic capacity, between the measuring surface  5   a  of the magnetic head arm  5  and the push rod  9  of the measuring probe. However, in this case there is a problem in that, if the area of the measuring surface  5   a  is small, the measuring resolving power will be reduced and the output will drift. 
     Also, in the above-described optical positioning apparatus of FIG. 1B, it is necessary to place the retro-reflector  19  like a corner cube on the magnetic head arm  5 , and it requires much care to secure the space therefor and mount and dismount the retro-reflector. There also is a problem in that a control characteristic due to bulkiness and increased weight is aggravated, and is affected by an environmental fluctuation such as the fluctuation of air. 
     The optical type sensor unit of FIG. 1C is a useful one which has solved the above-noted problems peculiar to the prior art. In this unit, the sensor probe comprising the polarizing prism  24  is small and, therefore, the detecting position range thereof is as small as 100, μm or less. Further, the set position of the sensor probe is proximate to the measuring surface  5   a  of the magnetic head arm  5  and, therefore, it is necessary to adjust the direction and position of the sensor probe during the setting thereof, to look for an appropriate signal location, and to set the sensor probe so that the level of the signal may become greatest. 
     Also, when the direction of the measuring surface  5   a  is predetermined, there is adopted a method of supporting the sensor probe on an X stage or the like with its direction made substantially perpendicular to the measuring surface  5   a , setting it in a direction to take in reflected light, approximating the sensor probe to the measuring surface  5   a  and looking for an appropriate signal location, and setting the sensor probe at the center of the range thereof. In this case, the detecting position range of the sensor probe is small and therefore, when the signal is to be caught to thereby determine the center of the signal, the sensor probe is gradually reciprocated several times. Outside the detecting position range, there is no signal from this sensor probe, and even within the detecting position range, the direction of the light condensing position cannot be known. Consequently, for example, to automatize measurement, a further improvement becomes necessary. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of what has been described above, and an object thereof is to provide a displacement detecting apparatus capable of discriminating the position of the surface of reflecting means in which the surface of the reflecting means and polarizing separating means are in a proper positional relation. 
     Another object of the present invention is to provide a displacement detecting apparatus which can detect the position of an object with high reliability without providing a discrete member on the object side, and makes positioning of high accuracy and high resolving power possible. 
     Still another object of the present invention is to provide an information recording apparatus capable of writing a servo track signal highly accurately into a hard disc. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a perspective view of a servo track signal writing apparatus according to the prior art. 
     FIG. 1B is a perspective view of a sensor unit. 
     FIG. 1C is a perspective view of a non-contact distance sensor unit. 
     FIG. 2A is a perspective view of a servo track signal writing apparatus according to a first embodiment of the present invention. 
     FIG. 2B is a perspective view of a non-contact distance sensor unit. 
     FIGS. 3A,  3 B and  3 C are illustrations of the shape of a converged light on a four-division sensor. 
     FIG. 4 is a perspective view of a non-contact distance sensor unit according to a second embodiment of the present invention. 
     FIG. 5 is a perspective view of a non-contact distance sensor unit according to a third embodiment of the present invention. 
     FIG. 6 is a perspective view of a non-contact distance sensor unit according to a fourth embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will hereinafter be described in detail with reference to several embodiments thereof shown in FIGS. 2A,  2 B,  3 A to  3 C and  4  to  6 . 
     FIG. 2A shows a perspective view of an information recording-reproducing apparatus according to a first embodiment of the present invention which is comprised of a hard disc drive  40  for writing in a servo track signal, and a rotary positioner unit  41  for effecting highly accurate rotation positioning. The hard disc drive  40  comprises a hard disc  42  having a magnetic recording medium vapor-deposited on the surface thereof, a spindle  43  providing the center of rotation of the hard disc  42 , a magnetic head slider  44  having a magnetic head fixed thereto, a magnetic head arm  45  holding the magnetic head slider  44 , a voice coil motor  46 , etc. 
     The magnetic head arm  45  having a rotary shaft  0  is mounted outside the hard disc  42  normally rotated at a high speed about the spindle  43 , the substantially rectangular parallelepiped-shaped magnetic head slider  44  is mounted on the tip end of the magnetic head arm  45 , and the magnetic head on the tip end of the magnetic head slider  44  is disposed with a gap of 0.5, μm or less relative to the surface of the hard disc. The magnetic head is adapted to be arcuately moved in a substantially radial direction on and relative to the hard disc  42  by the rotation of the magnetic head arm  45 . Also, the rotary shaft of the voice coil motor  46  is connected to the rotary shaft  0  of the magnetic head arm  45 , and the output of a voice coil motor driver  47  is connected to the voice coil motor  46 . The output of a signal generator  48  generating a signal to be written into the hard disc  42  is connected to the magnetic head of the magnetic head slider  44 . 
     The rotary positioner unit  41  is disposed under the hard disc drive  40 , and a probe supporting (positioning) arm  49 , a motor  50  and a rotary encoder  51  of high resolving power are connected together coaxially with the rotary shaft  0  of the magnetic head arm  45 . An optical type position detecting sensor unit  52  is mounted on the probe supporting arm  49 , the optical sensor probe  53  of the position detecting sensor unit  52  is inserted in a slot-like opening, not shown, in the base plate of the hard disc drive  40 , and the end surface of the tip end portion of the optical sensor probe  53  is disposed with a spacing of the order to 300 μm with respect to the side of the magnetic head arm  45 . The output of the position detecting sensor unit  52  is connected to the voice coil motor drive  45  through a signal processor  54 , and the probe supporting arm  49  is adapted to be rotatively moved about a rotary shaft coaxial with the rotary shaft  0  of the magnetic head arm  45 . The output of the rotary encoder  51  is connected to the motor  50  through a signal processor  55  and a motor driver  56 . 
     With such a construction, a servo track signal from the signal generator  48  is written as magnetic information from the magnetic head of the arcuately moved magnetic head arm  45  into any position (track) on the surface of the hard disc  42  being rotated at a high speed. At this time, the rotated position of the position detecting sensor unit  52  is detected by the rotary encoder  51 , and on the basis of this detection data, the motor  50  is rotatively driven through the signal processor  55  and the motor driver  56 . By the feedback control of this type, the position detecting sensor unit  52  is rotatively positioned. 
     The surface of the hard disc  42  is divided into a plurality of circular ring-shaped tracks of different radii concentric with the center of rotation thereof, and further, each of the plurality of circular ring-shaped tracks is divided into a plurality of arcs, and finally magnetic recording and reproduction are effected time-serially on the plurality of arcuate areas along the circumferential direction thereof. 
     To increase the recording capacity of the hard disc  42 , it is necessary to make recorded information onto the hard disc  42  high in density, and to make the recorded information high in density, it is effective to narrow the width of the concentrically divided tracks to thereby improve the recording density in the radial direction. The recording density in the radial direction is represented by track density t/i (track/inch) per length of 1 inch, and the present day recording density is of the order of 10000 t/i, which means that the track interval is about 3 μm. In such a minute track pitch, it is necessary to position the magnetic head radially of the hard disc  42  with resolving power 0.05 μm which is about {fraction (1/50)} of the track width, and write in servo track signals in advance. For this purpose, the technique of successively writing in servo track signals while effecting the positioning of high resolving power within a short time becomes necessary. 
     FIG. 2B shows a perspective view of the optical system of the position detecting sensor unit  52 , and a multimode laser diode light source  61 , a collimator lens  62  and the optical sensor probe  53  are successively arranged, and the sensor probe  53  is comprised of a polarizing prism having a polarizing beam splitter surface  53   a  and a reference reflecting mirror surface  53   b  formed with reflecting deposited film. In the reflecting direction of a non-polarizing beam splitter  63 , there are successively arranged an aperture mirror  64  comprising a reflecting portion  64   b  having a beam diameter limiting opening  64   a  at the center thereof, a quarter wavelength plate  65 , a beam amplitude dividing phase diffraction grating  66  having staggered grating structure, polarizing plate analyzers  67   a  to  67   d  disposed with their deflecting directions deviated by 45° from one another, and light receiving elements  68   a  to  68   d.    
     Also, in the reflecting direction of the aperture mirror  64 , there are arranged a polarizing plate  69  transmitting therethrough reflected light from the surface  45   a  to be measured on the side of the magnetic head arm  45  and intercepting a light beam reflected by the reference reflecting mirror surface  53   b , a condensing lens  70 , a cylindrical lens  71  and a four-division sensor  72 , whereby a focus detecting optical system is formed. 
     With such a construction, the divergent light from the multimode laser diode light source  61  is made into a loosely condensed light beam L by the collimator lens  62 , and this light beam L is transmitted through the non-polarizing beam splitter  63 , and thereafter is divided into polarized components by the polarizing surface  53   a  of the sensor probe  53 . An S-polarized light beam reflected by the polarizing surface  53   a  emerges from the end surface of the sensor probe  53  and is condensed near the beam waist of the surface  45   a  to be measured, and the reflected light thereof becomes a divergent spherical wave and passes along the original optical path and returns to the sensor probe  53 . On the other hand, a P-polarized light beam transmitted through the polarizing surface  53   b  is incident on the reference reflecting mirror surface  53   a  on the end portion of the sensor probe  53  lying at a location deviating from the beam waist, and the reflected light thereof likewise returns to the sensor probe  53 . 
     Here, the optical path lengths of the light beams in wave optics are set so as to be substantially equal lengths of optical paths within the coherent distance of the multimode laser diode light source  61 . For example, when the width of the sensor probe  53  formed of glass is of the order of 2 mm, the light beam reflected by the polarizing surface  53   a  travels by 1 mm through the glass, and thereafter travels by 0.3 mm through the air and is illuminated onto the surface  45   a  to be measured on the side of the magnetic head arm  45 . Accordingly, the reciprocal wave optical length of the optical path from the sensor probe  53  to the reflecting surface  45   a  to be measured is L 1 =(1×1.5+0.3)×2=3.6 mm, it being understood that the refractive index of the glass is 1.5. On the other hand, the light beam transmitted through the polarizing surface  53   a  travels by 1.2 mm through the glass, and is illuminated onto the reference reflecting mirror surface  53   b  on the end portion of the glass. Accordingly, the reciprocal wave optical length of the optical path is L 2 =(1.2×1.5)×2=3.6 mm. 
     Next, the beam waist which is the condensed position of the light beam is set to a position of 0.3 mm emerging from the sensor probe  53 . Thereby, the position of the wave source of the divergent spherical wave reflected by the surface  45   a  to be measured and the reference reflecting mirror surface  53   b  appears to deviate in the direction of the optical axis. That is, looking into the interior of the sensor probe  53  from the laser diode light source  61  side, the condensing point (wave source) of the magnetic head arm  45  is seen at a position of LP=(1+0.3×1.5)=1.45 mm from the division surface of the sensor probe  53 . On the other hand, the position of the divergent spherical wave source from the reference reflecting mirror surface  53   b  is seen at a position of L 2 ′=1.2×2−1.45=0.95 mm from the beam dividing polarizing surface. However, both are positions seen in the glass. 
     Thus, both divergent spherical wave sources deviate by 0.5 mm from each other in the glass, and when the two light beams are superposed on each other, the wave fronts thereof do not completely coincide with each other, and when for example, the polarized lights of the two are combined together, concentric circular interference fringes are obtained. In that case, when the phases of the wave fronts of the two are fluctuated by the relative movement of the magnetic head arm  45 , concentric circular interference fringes gush out or are drawn in from the center. As regards these concentric circular interference fringes, the interference fringe portion of substantially one color in the central portion is obtained wide because the amount of deviation between the two divergent spherical waves in the direction of the optical axis is as small as about 0.5 mm. Therefore, an appropriate opening  64   a  is provided in the aperture mirror  64  so as to take out only the substantially one-color portion, and by taking out part of the light beam, it becomes possible to handle the subsequent portion as a substantially plane wave. 
     The two light beams combined in the sensor probe  53  in this manner are linearly polarized lights orthogonal to each other and therefore, actually, do not directly interfere with each other and become bright and dark signals. However, when these two light beams are reflected in the non-polarizing beam splitter  63  and transmitted through the quarter wavelength plate  65 , the linearly polarized lights orthogonal to each other are converted into circularly polarized lights of opposite directions, and the vibrating surfaces of the two are vector-combined, whereby by the fluctuation of the phase difference therebetween, they are converted into a rotating linearly polarized light. 
     This rotating linearly polarized light is amplitude-divided into four light beams by the phase diffraction grating  66 . That is, by this amplitude division, all the light beams are entirely equally divided in their properties such as shape, intensity unevenness and defect and therefore, the influences they are given all become equal even if the interference fringes become not one color or are reduced in contrast for some reason or other. Particularly, in the reflected light from the magnetic head arm  45 , the wave front is disturbed by minute uneven structure and intensity unevenness occurs strongly, but the ways of disturbance of the wave fronts and the states of intensity unevenness of the four light beams are all equal. 
     The light beams divided into four are transmitted through the polarizing plate analyzers  67   a  to  67   d  disposed with their polarization azimuths deviated by 45° from one another, whereby they are converted into interference lights of which the timing of light and the timing of darkness deviate by 90° from each other in terms of phase. In a state in which the reductions in contrast by the influences of the disturbance of the wave fronts and intensity unevenness are all equally influenced, respective light and dark light beams are received by the respective light receiving elements  68   a  to  68   d.    
     The signals of the light receiving element  68   a  and the light receiving element  68   b  having a phase  25  difference of 180° therebetween are differentially detected, and this signal from which a DC component such as reduction in contrast due to disturbance or the like of the wave front has been substantially removed is defined as the A phase signal. Likewise, the signals of the light receiving element  68   c  and the light receiving element  68   d  having a phase difference of 180° therebetween are differentially detected, and this signal from which a DC component such as reduction in contrast due to disturbance or the like of the wave front has been substantially removed is defined as the B phase signal. The A phase signal and the B phase signal have a phase difference of 90° therebetween, and their Lissajous pattern observed by an oscilloscope in X-Y mode becomes circular. The size of the circle which is the amplitude of this Lissajous pattern fluctuates due to minute unevenness in the magnetic head arm  45 , but the central position thereof does not fluctuate and therefore, no essential error occurs to the phase detection for measuring the relative distance. 
     Also, by the light being condensed on the side of the magnetic head arm  45 , the influence of one color deviation which is the fluctuation of the interference state by the angular deviation of the side of the magnetic head arm  45  is avoided. That is, even if angular deviation due to light being condensed is present, the main emerging direction of the divergent spherical wave only somewhat deviates and the spherical wave itself being eclipsed is avoided, and the overlapping state of the wave fronts of the two divergent spherical waves does not change and therefore, the interference state is obtained stably. Accordingly, adjustment of the side of the magnetic head arm  45  and the illuminating light beam becomes unnecessary and this sensor unit can be obtained as an interference type position detecting sensor which is very easy to handle. 
     Further, the parallelism deviation of the illuminating position is not concerned in the phase deviation between the divergent spherical waves, but yet a minute change in the uneven state of the side of the magnetic head arm  45  conforming to the illuminating position results in the fluctuation of the amplitude of an interference signal. However, the central position of the Lissajous pattern does not fluctuate and therefore, no essential error occurs to the phase detection. 
     The positional relation between the magnetic head arm  45  and the optical type position detecting sensor unit  52  does not deviate as long as the distance between the two is kept constant, because both are rotatively moved about the same rotary shaft  0 . In reality, completely the same shaft is impossible and therefore, when the two are rotating, the angular deviation and parallelism deviation of the relative positional relation occur due to a shaft deviation error, but in the present embodiment, no essential problem will arise even if the alignment deviation and parallelism deviation shown above occur. 
     The finally detected signal has its principle based on the gauge interference by the reciprocal optical path and is therefore a sine wave-like signal having a half of the wavelength of the laser diode light source  61  as its period. When a laser diode light source  61  having a wavelength of 0.78 μm is used, there is obtained a sine wave signal having a period of 0.39 μm, and by counting the wave number thereof, it is possible to detect the fluctuation of the relative distance. Also, there is obtained a sine wave signal of two phases having a phase difference of 90° therebetween and therefore, by electrically dividing the signal by the use of a conventional electrical phase dividing apparatus, it is possible to detect relative positional deviation of fine resolving power. That is, if the signal is electrically divided into 4096, the relative positional deviation can be detected by minimum 0.095 nm. Accordingly, if an electric current is supplied to the voice coil motor  46  for driving the magnetic head arm by an appropriate control apparatus so that the relative positional deviation may become zero, the relative position can be stably maintained by the order of several times as great as ±0.095 nm, that is, servo can be applied. 
     If use is made of a highly accurate rotary positioner unit  41  containing therein a rotary encoder  51  generating a sine wave signal of 81000 cycles per one full rotation, and capable of dividing the signal into 2048 and positioning, the sensor probe  53  of the position detecting sensor unit  52  mounted near the magnetic head arm  45  having a radius of 30 mm can be positioned with resolving power several times as great as ±1.4 nm. That is, the stability of the relative position of the position detecting sensor unit  52  itself is of the order of several times as great as ±0.095 nm as described above and therefore, the positioning resolving power of the two put together becomes about equal to the performance of the highly accurate rotary positioner unit  41  itself. 
     By adding to the highly accurate rotary positioner unit  41  a servo mechanism for keeping the position of the end surface of the magnetic head arm  45  constant by the use of the position detecting sensor unit  52 , as described above, there can be accomplished stable and highly accurate positioning which is free of the influence of disturbance. 
     The interference between the reflected light beam from the side of the magnetic head arm  45  and the reflected light beam from the reference reflecting mirror surface  53   b  is obtained within the coherent distance of the multimode laser source  61 . In the case of a laser diode light source  61  of a single mode, the coherent distance is long, but mode hop may be caused and the phenomenon of the interference phase hopping may occur and therefore, it is preferable to make the lengths of the optical paths substantially equal to each other by the use of the multimode laser source  61 , and use it with an optical path length difference equal to or less than the coherent distance. 
     Generally, the full width of the coherent distance is given by λ0×2/Δλ and therefore, if the center wavelength is λ0=780 nm and the full width of a half value of the multimode spectrum envelope is Δλ=6 nm, the full width of the coherent distance is about ±50 μm about the equal optical path length. Also, generally, the laser diode light source  61  has its wavelength fluctuated by the fluctuation of the ambient temperature. Taking a laser diode light source  61  having the center wavelength of 780 nm and a temperature coefficient of 0.06 nm/° C. as an example, when the optical path length difference ΔL=50 μm, the deviation of the measured value by the temperature fluctuation of 1° C. is of the order of −5 nm. If design is made such that the distance is kept constant in the vicinity of the peak of coherency, an optical path length difference of ±10 μm can be realized, and the measurement error in that case is ±1 nm, and this value is accuracy sufficient as a servo track writer. 
     Generally, in a laser gauge interferometer, if the optical path is separate and the sensor probe  53  is exposed to air, the signal output is unstable due to fluctuation or the like. In the present embodiment, most of the interference optical path is a common optical path and is separated into two optical paths near the tip end of the sensor probe  53 , but since the optical paths are short and in a glass medium, design is made such that the influence of fluctuation or the like becomes very small. 
     Also, the linearly polarized light beams reflected by the side of the magnetic head arm  45  and the reference reflecting mirror surface  53   b  are re-combined by the polarizing surface  53   a  of the sensor probe  53 , and are reflected by the non-polarizing beam splitter  63 , whereafter part of them is reflected by the reflecting portion  64   b  of the aperture mirror  64  and is sent to the focus detecting optical system. In the focus detecting optical system, only the reflected light from the side of the magnetic head arm  45  is first taken out by the deflecting plate  69 , and by the condensing lens  70  and the cylindrical lens  71 , the beam waist of the condensed illuminating light to the side of the magnetic head arm  45  is condensed at a condensing position αx determined by the condensing lens  70  with respect to a direction indicated by arrow x, and is condensed at a condensing position αy determined by the condensing lens  70  and the cylindrical lens  71  with respect to a direction indicated by arrow y. A position at which the sizes of the light beams in x direction and y direction become the same exists intermediately of these condensing positions αx and αy, and the four-division sensor  72  is disposed at a position whereat the sizes of the light beams in x direction and y direction when the side of the magnetic head arm  45  is at the center of the beam waist become the same. 
     FIGS. 3A to  3 C show the shape of the condensed illuminating light on the side of the magnetic head arm  45  on the four-division sensor  72 . When the side of the magnetic head arm  45  is at the center of the beam waist of the condensed illuminating light, the shape of the condensed illuminating light beams circular as shown in FIG. 3B, and when the side of the magnetic head arm  45  approaches the sensor probe  53 , a point αy approaches the four-division sensor  72  and a point αx goes away from the four-division sensor  72  and as a result, the shape of the conveyed light on the side of the magnetic head arm  45  on the four-division sensor  72  becomes an ellipse long sideways in the x direction as shown in FIG.  3 C. When, conversely, the side of the magnetic head arm  45  goes away from the sensor probe  53 , the shape of the converged light becomes a vertically long ellipse as shown in FIG.  3 A. 
     When an error signal ( 72   a + 72   c )−( 72   b + 72   d ) is calculated from the outputs of the four areas  72   a - 72   d  of the four-division sensor  72 , this value becomes O at the in-focus point, and becomes smaller than O when the side of the magnetic head arm  45  approaches the sensor probe  53 , and becomes greater than O when the side of the magnetic head arm  45  goes away from the sensor probe  53 . From this error signal ( 72   a + 72   c )−( 72   b + 72   d ), it can be determined whether the side of the magnetic head arm  45  is set at a measuring position, or if it deviates from the measuring position, i.e., whether it is near or far can be discriminated. Further, from the overall output ( 72   a + 72   b + 72   c + 72   d ) of the four areas  72   a - 72   d  of the four-division sensor  72 , it can be determined whether the direction of the side of the magnetic head arm  45  is properly set. 
     The position detecting sensor unit  52  is arranged based on light interference as its principle and can therefore detect positional displacement with high resolving power by a sine wave-like signal, but when in FIG. 2A, the optical sensor probe  53  of the position detecting sensor unit  52  mounted on the probe supporting arm  49  is disposed near the side of the magnetic head arm  45 , the detection range is as small as 100 μm or less and the absolute position is not known and therefore, the optical sensor probe  53  needs to be correctly set at a location from which a signal is output. 
     Therefore, the present embodiment adopts two steps. In a first step, the motor  50  or the voice coil motor  46  is controlled and driven by the error signal ( 72   a + 72   c )−( 72   b + 72   d ) from the focus detecting optical system, and the optical sensor probe  53 , or the side of the magnetic head arm  45 , is controlled so that the condensed illuminating light may be the center of the beam waist on the side of the magnetic head arm  45 . At this time, by checking the overall output ( 72   a + 72   b + 72   c + 72   d ) of the four areas  72   a - 72   d  of the four-division sensor  72 , it can be determined whether the direction of the side of the magnetic head arm  45  is properly set. 
     Next, as a second step, the relative position of the optical sensor probe  53  and the side of the magnetic head arm  45  is detected by the position detecting sensor unit  52  using light interference, and the positioning of the rotary positioner unit  41  is effected in such a manner that the movement of the rotary positioner unit  41  and the movement of the side of the magnetic head arm  45  are operatively associated with each other in non-contact while the voice coil motor  46  is controlled so that the relative position may become constant, whereby a servo track signal is stably written into the hard disc  42 . 
     FIG. 4 shows a second embodiment of the present invention, and in FIG. 4, the same reference numerals as those in the first embodiment designate the same members as those in the first embodiment. A polarizing plate  69 , a condensing lens  70  and a cylindrical lens  71  are disposed in the reflecting direction of the reflecting portion  64   b  of an aperture mirror  64 , and design is made such that a light beam is turned back by a mirror  73 , the image plane distance is lengthened to thereby make the image magnification great and the condensed shape onto the four-division sensor  72  becomes large. 
     While the present embodiments uses a method of deviating the focus position relative to x and y directions using the cylindrical lens  71  when focus detection is effected, and detecting the focus state using the shape of the light beam in the four-division sensor  72 , other conventional focus detection methods may also be used. 
     FIG. 5 shows a third embodiment of the present invention which is designed such that the beam waist of the converged light to the side of the magnetic head arm  45  is reflected by the reflecting portion  64   b  of the aperture mirror  64 , and is imaged on a two-dimensional sensor  81  by an imaging lens  80 . The output of this two-dimensional sensor  81  is displayed on a display monitor  82 , and the quantity of reflected light and the imaged state are observed to thereby effect the check-up of a proper position. 
     FIG. 6 shows a fourth embodiment of the present invention, in which in order to intercept a light beam reflected by the reference reflecting mirror surface  53   b , the aperture mirror  64  is disposed in front of the quarter wavelength plate  65 , and the polarizing plate  69  is disposed in a direction to transmit therethrough the reflected light from the side of the magnetic head arm  45  and intercept the light beam reflected from the reference reflecting mirror surface  53   b . The light beam is turned back by a mirror  83 , and the image plane distance is lengthened and the image magnification is made great so that the output from the two-dimensional sensor may be easier to observe. 
     As described above, in a displacement detecting apparatus, provision is made of the function of using a part of the reflected light from relatively moved reflecting means to detect the condensed state of the light beam on the surface of the relatively moved reflecting means, whereby it is possible to detect any position change with high reliability without attaching thereto any special index or the like, and by the use of other excess light beam for the detection of the position displacement of the light beam being used by the sensor unit utilizing interference, the positional relation with the relatively moved reflecting means and the propriety of the direction can be known, and the setting of the optical sensor probe can be effected easily. 
     Also, in another displacement detecting apparatus, provision is made of the function of using a part of the reflected light from relatively moved reflecting means to make a state in which the light beam is condensed on the surface of the relatively moved reflecting means observable, whereby position displacement can be detected with high reliability and also, a proper position and a proper posture can be set easily. 
     In an information recording apparatus, rotary means is given the head arm position displacement detecting function and the function of detecting the condensed state of light condensed on the side of a head arm, and design is made such that the movement of the head arm is operatively associated with the positional fluctuation of the rotary means in non-contact, whereby by the use of an excess light beam which does not affect the interference light beam detection of reflected light obtained by measuring light emitted from a light source being applied to a rotating object, the absolute positions of the rotating object and an interferometer can be automatically detected, and during each positioning, a servo track signal can be stably written into a hard disc and therefore, high accuracy can be realized.