JPH0797016B2 - Phase shifter for laser Doppler vibrometer as a device for measuring flying height variation between magnetic disk and slider - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flying height variation measuring device between a magnetic disk and a slider in a magnetic disk device.

[0002]

2. Description of the Related Art Dynamic pressure is generated between a magnetic disk rotating at high speed and a slider carrying a magnetic head by the hydrodynamic effect of air. The dynamic pressure is used to levitate the slider above the magnetic disk, thereby controlling the gap between the magnetic disk and the slider (hereinafter referred to as the flying height). In order to improve the linear recording density of the medium, it is essential to reduce the flying height between the magnetic disk and the slider. On the other hand, if the flying height is reduced, the risk of contact between the disk and the slider increases, and the reliability of the device is significantly impaired. At present, the flying height of the slider reaches the ultimate region of 0.1 μm or less, and the requirement for the flying stability of the slider is becoming more severe. Along with this, there is an ever-increasing demand for a technique for measuring the flying height variation between the magnetic disk and the slider with higher speed and higher accuracy. There are mainly three conventional techniques for measuring the fluctuation of the flying height between the magnetic disk and the slider: 1) capacitance method, 2) optical interference method, and 3) laser Doppler method.

The electrostatic capacitance method 1) is a method of measuring the flying height by detecting the electrostatic capacitance between the electrode and the disk using a special slider provided with an electrode. In the optical interference method of 2), a transparent optical material (generally glass) is used for either one of the slider and the disk, and the change in the interference light intensity caused by the optical path difference between the light reflected on the slider surface and the light reflected on the disk surface is detected. And measure the flying height. In the laser Doppler method of 3), the flying height is measured by detecting the Doppler shift amount of the laser frequency that occurs when the surface of the vibrating object is irradiated with laser light (TA Rien).
er, et al, IEEE TRANSACTI
ONS ONMAGNETICS, VOL. 24,
NO. 6, NOVEMBER 1988, PP27
45).

[0004]

The response frequency possessed by the capacitance method and the optical interference method is at most several tens kHz. These approaches are insufficient in terms of response frequency to measure the dynamic behavior of sliders supported by air bearings with natural frequencies of tens of kHz. Also,
The capacitance method and the optical interference method have a problem that they are easily affected by spatial variations in electrical characteristics and optical characteristics on the disk surface. Further, in the capacitance method, the spatial resolution of the flying height fluctuation measurement is determined by the area of the electrode, but it is very difficult to reduce the electrode size of the slider provided with the electrode. Optical interferometry requires the use of very high light transmissive optical materials such as glass for either the slider or the disk. Therefore, it is not possible to know the fluctuation of the flying height in the system using the actual slider and disk.

On the other hand, the laser Doppler method is 1) about 1 MH
It has a high response frequency of z, 2) has a high spatial resolution of measurement, 3) has a wide dynamic range of 160 dB or more, and 4) has little influence on the measurement data due to the electrical and optical properties of the vibrating surface. , Has a number of advantages over other measurement methods such as.

However, the greatest drawback of the laser Doppler method is that the true flying height fluctuation defined by the relative movement between the disk surface and the slider cannot be measured. Originally, in the laser Doppler method, a laser obtained from one light source is divided into two by a beam splitter, and one of them is used as a measurement light and the other is used as a reference light to heterodyne-detect the reflected light of the two. It is possible to measure vibrations. However, in the case of measuring the relative vibration between the slider and the disk, it is impossible to measure them in the exact same phase because the disk surface immediately below the slider is behind the slider. Even if the two beams are made as close to each other as possible, if there is a slight phase difference between the two beams, the disc surface runout component (disc runout component) that accounts for 99% of the absolute vibration of the slider remains in the difference information between them. However, it becomes impossible to extract the relative vibration component of the slider with respect to the disk. As described above, in the conventional laser Doppler vibrometer, only the absolute vibration of the slider can be measured, and the fluctuation of the flying height of the slider with respect to the disk cannot be measured.

In order to solve the above problems, a method and apparatus for correcting the phase difference caused by the positional relationship between the measurement light and the reference light by providing a phase shifter on the optical axis of the return of the measurement light of the laser Doppler vibrometer. Has been already proposed by the inventor of the present application (Japanese Patent Application No. 4-124022). An object of the present invention is to provide a phase shifter having a configuration suitable for a flying height fluctuation measuring device, and thereby to optimally set the phase difference of the measurement light with respect to the reference light, so that the flying height fluctuation between the magnetic disk and the slider can be suppressed. On the other hand, it is to enable highly accurate and highly sensitive measurement over a wide frequency band.

[0008]

A phase shifter composed of an optical fiber according to the present invention and a movable corner cube is provided on a measurement optical axis of a laser Doppler vibrometer to optimally set a phase difference of the measurement light with respect to a reference light. As a result, it becomes possible to perform high-precision and high-sensitivity measurement over a wide band against variations in the flying height between the magnetic disk and the slider.

There are the following two means for changing the relative phase difference between the measurement light and the reference light. One is a method of relatively changing the optical path lengths of both lights. the other one is,
A method of passing one light through an electro-optical element, generally fsec
This method is intended for phase differences on the order of. The phase difference of light, which is a problem in measuring the flying height of the slider, is
It is determined by the positional relationship of each spot of the reference light and the peripheral speed of the disc, and its order is μsec.
Longer than. Therefore, in slider flying height measurement,
The former method of relatively changing the optical path lengths of the measurement light and the reference light is judged to be appropriate. The optical phase shifter of the present invention comprises an optical fiber and a movable corner cube. The return light of the measurement light is once received by the optical fiber, and then the light emitted from the optical fiber is received by the movable corner cube whose relative distance to the optical fiber can be freely changed. The light returning from the movable corner cube is returned to the return optical axis of the measuring light via the optical fiber. By adjusting the length of the optical fiber as the coarse adjustment portion and the relative distance between the optical fiber and the movable corner cube as the fine adjustment portion, the relative phase difference between the measurement light and the reference light can be arbitrarily changed.

The procedure of the flying height measuring method using a laser Doppler vibrometer having an optical phase shifter on the return optical axis of the measuring light is as already proposed (Japanese Patent Application No. 4-12402).
2).

[0011]

In the slider flying height variation measurement of the magnetic disk device, an optical phase shifter composed of an optical fiber and a movable corner cube is provided on the return optical axis of the measurement light of the laser Doppler vibrometer, and the position of the measurement light with respect to the reference light is set. By optimally setting the phase difference, the following effects can be brought about. 1) With the conventional laser Doppler vibrometer, only the absolute vibration of the slider can be measured, but by eliminating the phase difference between the measurement light and the reference light, the disc runout component is removed from the absolute vibration of the slider, and the slider flying height fluctuation It becomes possible to extract only the components. 2) As a result, the problems of the capacitance method and the optical interference method are overcome. Specifically, it is possible to perform measurement using an actual slider and an actual disk, and it is possible to measure variations in slider flying height in a frequency band well over 100 kHz with high sensitivity and high spatial resolution. These effects of the present invention are
The present invention significantly advances the development of a magnetic disk device having high reliability and high recording density.

[0012]

EXAMPLES The present invention will be described in more detail with reference to the following examples. Example FIG. 1 shows a schematic diagram of an optical system of a laser Doppler vibrometer in which a phase shifter according to an example of the present invention including an optical fiber and a movable corner cube is provided on a measurement optical axis. In the figure, 1 is a laser oscillation tube, and in this device, 1 mW of He-Ne
It uses a laser. 2a and 2b are beam splitters, and the oscillated laser is split into two beams, that is, measurement light and reference light by the beam splitter 2a. Reference numeral 3 is a frequency shifter called a Bragg cell, and the frequency of the reference light is shifted in advance to the high frequency side with respect to the measurement light in order to distinguish the vibration direction (+-). In addition, polarization beam splitters 4a and 4b, a quarter-wave plate 5, a lens system 6, and a detector 7 are provided. The optical phase shifter of the present invention is composed of the optical fiber 8 and the movable corner cube 9.

A series of measurements including the following comparative examples was carried out using a disk with a 5-inch sputtered carbon protective film and a radius of 5
IB at 0mm position with disk rotation speed 3600rpm
It was carried out by flying an M3380 type slider. Also,
The measuring light is applied to the trailing edge on the back of the slider,
The reference light was applied to the disk surface adjacent to the same track as the measurement light.

In the present embodiment, 5.26 μsec was determined as the optimum set phase difference of the optical phase shifter from the relationship of (set phase difference of optical phase shifter) vs (fundamental frequency component of measurement signal).

Next, the phase shifter 1 in the embodiment of FIG.
An ordinary laser Doppler vibrometer having a configuration in which 0 is not interposed was taken as a comparative example and compared with the example of FIG.
The measurement results of the slider floating amount in these Examples and Comparative Examples were as follows. The slider flying height fluctuation waveform obtained from the example has a maximum amplitude of about 10 nm and an amplitude of 1
A natural vibration component of 20 kHz to 30 kHz due to an air bearing of nm or less is actualized. On the other hand, the waveform observed in the comparative example has a maximum amplitude of about 200 nm, the vibration component of 60 Hz, which is the same as the disk rotation frequency, is dominant, and it is almost impossible to recognize other frequency components. Under the measurement conditions of the comparative example, the slider having the average flying height of about 200 nm vibrates in the fluctuation range of 200 nm in the steady flying state, and the measurement by the vibrometer of the comparative example is incorrect. That is clear. Considering that the main frequency component of vibration is 60 Hz, the residual component of the disc runout is definitely observed in the comparative example, not the fluctuation waveform of the flying height of the slider.

[0016]

As described above, the optical phase shifter including the optical fiber of the present invention and the movable corner cube is provided on the measurement optical axis of the laser Doppler vibrometer to optimally set the phase difference of the measurement light with respect to the reference light. As a result, the disc runout component can be removed from the absolute vibration component of the slider. Along with this, it is possible to realize highly accurate measurement of slider flying height variation while utilizing the high performance of the laser Doppler vibrometer.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an optical system of a laser Doppler vibrometer in which an optical phase shifter of the present invention is provided on a measurement optical axis.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Laser oscillation tube 2a, 2b Beam splitter 3 Bragg cell 4a, 4b Polarization beam splitter 5 1/4 wavelength plate 6 Lens system 7 Detector 8a, 8b Optical fiber 9 Movable corner cube 10 Optical phase shifter

Claims (2)

[Claims] 1. A phase shifter for adjusting an optical path length of return measurement light in a laser Doppler vibrometer to control a phase of the return measurement light with respect to a return reference light, wherein incident light is guided to a corner cube by a first optical fiber. ,
The light reflected by the corner cube is returned to the optical path of the return measurement light by a second optical fiber, and the phase is controlled by adjusting the distance of the corner cube with respect to the first and second optical fibers. A phase shifter characterized by that. 2. A laser beam output from one laser light source is split into two, one laser beam is made incident on a magnetic disk surface as a reference beam, and the other laser beam is made incident on a slider as a measuring beam. A laser Doppler vibrometer for measuring the fluctuation of the flying height of the slider with respect to the magnetic disk surface by comparing the frequency of the return reference light reflected by the magnetic disk surface with the frequency of the return measurement light reflected by the slider. In the optical path of the return measurement light in, in the phase shifter for controlling the phase of the return measurement light with respect to the return reference light, receiving the return measurement light from the optical path of the return measurement light at one end,
A first optical fiber that emits the light from the other end, a corner cube that reflects the light received from the first optical fiber, the light reflected by the corner cube at one end, and the other light A second optical fiber which is emitted from an end and is returned to the optical path of the return measurement light; and a distance between the light emitting surface of the corner cube with respect to the emitting end of the first optical fiber and the incident end of the second optical fiber. And a means for performing the phase shifter.