JPH11296900A - Optical device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make switchable an electrically effective numerical aperture with high light usage efficiency by converging the luminous flux diffracting/ transmitting a laser beam from a laser light source with a diffraction optical element controlled by an electric signal with a convergent optical system and making a controlled area act on a partial area of the luminous flux. SOLUTION: The laser beam 103 emitted from the laser light source 101 and made into a parallel planar wave by a collimate lens 102 is diffracted by the diffraction optical element 104 consisting of parts 105, 106 diffracting by zero degree and θ degree. The laser beam 107 transmitting through the part 106 diffracting by θdegree and diffracted by θ degree is made incident on an area excepting the circular area 110 of the convergent optical system 108, and is image-formed on points πP1 not to contribute to the image forming point P of the circular area 110 for CD. Then, the diffractive function of the part 106 diffracting by θ degree of the diffraction optical element 104 is stopped with the electric signal to make it into the part diffracting by zero degree, and effective luminous flux 111 is bypassed to be image-formed on the image forming point P and to be used for a DVD. Thus, the numerical aperture can be electrically switched with a simple constitution.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for switching an effective numerical aperture of an optical system, and more particularly, to a technique for switching an effective numerical aperture in a recent optical pickup of an optical disk apparatus.
It belongs to a technology that allows an optical pickup having different numerical apertures, such as for D and CD-ROM, to be shared by one optical pickup.

[0002]

2. Description of the Related Art To facilitate understanding of the prior art, the numerical aperture of an optical system will be briefly described. In an optical system designed with almost no aberration in geometrical optics, a point image is formed as an infinitely small spot, but in reality, the spot has a finite spread due to diffraction due to the wave nature of light. At this time, if the numerical aperture of the optical system that contributes to image formation or light collection is NA,
The physical definition of the spread of a spot is represented by k × λ ÷ NA. Here, λ is the wavelength of light, and k is a constant determined by the optical system and usually takes a value of about 1 to 2. NA is proportional to the ratio D / f between the effective entrance pupil diameter D of the optical system (generally, the diameter of the effective light beam) and the focal length f. The spread of the spot represented by this equation becomes the theoretical resolution limit and is called the diffraction limit.

[0003] As is apparent from the above equation, the theoretical resolution of an optical system largely depends on the numerical aperture. Generally, the numerical aperture of the converging optical system of an optical pickup for an optical disc is about 0.45 for a CD or CD-ROM, and about 0.6 for a DVD (digital versatile disc). Also, the thickness of the optical disc substrate may be 1.2 mm for CD and 0.6 mm for DVD, and condensing optical systems having the same numerical aperture cannot be shared between CD and DVD.

In order to solve this problem, a method of installing two optical pickups in one device, a method in which a hologram is cut into a condenser lens of the optical pickup, and two kinds of apertures are simultaneously obtained by making two focuses. A method, a method of switching an aperture by switching an effective entrance pupil diameter using a liquid crystal shutter, or the like is used.

Next, a conventional example close to the present invention is shown in FIG.
This is based on application to an optical disk. This will be described below with reference to FIG.

Linearly polarized light 703 emitted from a linearly polarized laser light source 701 and converted into a plane wave by a collimating lens 702
Is the Y-axis direction whose polarization axis 704 is parallel to the paper surface. The direction of the polarization axis 704 of the linearly polarized light 703 is 90 degrees by a 90 degree TN (twisted nematic) liquid crystal element 705.
And rotate in the X-axis direction. The condensing optical system 706 condenses the linearly polarized light 703. At this time, it is assumed that a polarizing plate 707 whose center is cut out in a round shape is installed in front of the condensing optical system 706, and its linearly polarized light transmission axis is in the Y-axis direction. At this time, due to the optical shutter function of the 90-degree TN type liquid crystal element 705 combined with the polarizing plate 707, only the linearly polarized light that has passed through the hollow central portion of the polarizing plate contributes to light collection. C
D is used in this state for reproduction.

On the other hand, in the case of reproducing a DVD, a 90 ° TN
An electric field in the Z direction is applied to the type liquid crystal element 705 to bring it to a homeotropic state described later. In this state, since the liquid crystal element has no optical rotation, the linearly polarized light 703 can pass through the polarizing plate, and the numerical aperture is larger than that in the previous state. Even in this state, the amount of light is lost due to the light absorbing action of the polarizing plate. In addition, since the central part of the polarizer is hollowed out, there is a phase difference between the linearly polarized light transmitted through the polarizer and the other linearly polarized light due to the difference in the refractive index between the polarizer and air, and the light reaches the diffraction limit. It becomes difficult to do. Therefore, if the same type of polarizing plate whose linearly polarized light transmission axis is in the X-axis direction is provided in the hollowed-out central portion, the problem of the phase difference is solved, but the light amount is further lost.

The condensed light spot 708 condensed by the condensing optical system 706 is reflected by the optical disk 709 and returns almost in the same optical path as the incident light.
1 is condensed by another condensing optical system 712 and a condensed spot 7
13 is detected by the light detection element 714.

[0009]

However, installing two pickups in one device complicates the device configuration and is disadvantageous in terms of space. In addition, when a hologram is engraved on the condenser lens to make it a bifocal point, one of them will always generate an unnecessary condensed spot that is not used.
Light utilization efficiency decreases. This causes a problem of reduction in writing speed in a device requiring a large amount of light, such as a DVD-R, ie, a rewritable DVD.
Similarly, the same problem occurs in the method using the liquid crystal shutter.

Accordingly, the present invention provides an optical device comprising a laser light source for emitting laser light and a condensing optical system for condensing laser light, wherein a diffractive optical element capable of electrically controlling a diffraction function is provided in an optical path. In addition, an object of the present invention is to provide an optical device which has high light utilization efficiency and can switch an electrically effective numerical aperture by applying a diffraction function to a partial area of a laser beam condensed by a condensing optical system. And

[0011]

An optical device according to the present invention comprises a laser light source for emitting at least a laser beam, a diffractive optical element for diffracting the laser beam, and a condensing optic for condensing a light beam transmitted through the diffractive optical element. In the optical device composed of the optical system, the diffraction function of the diffractive optical element is controlled by an electric signal, and the controlled area acts on a partial area of the light beam condensed by the condensing optical system. I do.

The diffractive optical element is composed of a part that diffracts the incident laser light by 0 degrees and a part that diffracts by θ degrees, and the part that diffracts by 0 degree is included in the effective light beam used by the focusing optical system. Acting on a substantially circular area centered on the optical axis, the portion diffracted by θ degrees acts on an area other than the circular area in the effective light beam, and the diffraction function of the diffractive optical element is controlled by an electric signal. It is characterized by.

A linearly polarized laser beam is used as the laser beam, and a liquid crystal diffractive optical element is used as the diffractive optical element which can be controlled by an electric signal. The liquid crystal diffractive optical element is composed of a parallel alignment type liquid crystal element. The direction of the polarization axis of the linearly polarized laser light substantially coincides with the direction of the liquid crystal molecule alignment axis of the liquid crystal diffraction optical element, and the phase modulation capability of the parallel alignment type liquid crystal element is a half wavelength plus the wavelength of the linearly polarized laser light. It is characterized in that it is about an integral multiple of the wavelength or about an integral multiple of the wavelength.

[0014]

(First Embodiment) FIG. 1 shows a first embodiment of the present invention. For simplicity, FIG. 1 is drawn by projecting onto a two-dimensional YZ plane. Actually optical axis 1
The rotation reference is the rotation axis 09. In addition, parts of the detection optical system that are not directly related to the present invention are omitted. The laser light 103 emitted from the laser light source 101 and converted into a parallel plane wave by the collimator lens 102 passes through the diffractive optical element 104 and is diffracted by the diffractive optical element 104. The diffractive optical element 104 is a part 1 that diffracts incident light by 0 degree.
05 and a portion 106 that diffracts by θ degrees.

The laser beam 107 transmitted through the part 106 diffracted by θ degrees and diffracted by θ degrees is transmitted through the optical axis 109 of the focusing optical system 108.
Is incident on a region other than the substantially circular region 110 (shown by oblique lines) centered at. This circular area 110 is a part of the effective light beam 111 that should originally enter the light-collecting optical system 108,
It can be seen that the opening is smaller than the opening 11. Here, the opening by the effective light beam 111 is set for DVD, and the opening by the circular region 110 is set for CD.

As shown in FIG. 1, the luminous flux in the region diffracted by θ degrees is ± f from the optical axis 109 in the Y-axis direction according to the principle of optical imaging.
An image is formed at a point ± P1 shifted by × tan (θ). Here, f is the focal length of the light collecting optical system 108. Ie CD
Does not contribute to the image forming point P of the light transmitted through the circular region 110 set as the aperture for use. It can also be seen that if the value of the diffraction angle θ is sufficiently large, the light beam in the region diffracted by θ degrees will not be incident on the condensing optical system 108 at all. The effective aperture in these states is for a CD.

Next, the diffraction function of the portion 106 diffracted by θ degrees of the diffractive optical element 104 is stopped by an electric signal to make the portion diffracted 0 degrees. In this state, the effective light beam 111 passes through the diffractive optical element 104 as it is, and is imaged on the image forming point P by the condensing optical system 108. Therefore, in this state, the effective aperture is for DVD.

As is apparent from the above description, the effective numerical aperture of the condensing optical system 108 can be switched by controlling the diffractive function of the diffractive optical element 104 with an electric signal.

(Second Embodiment) Next, FIG. 2 illustrates a second embodiment of the present invention. Basically, it is the same as the first embodiment shown in FIG. 1, but a liquid crystal diffractive optical element made of a parallel alignment type liquid crystal is used as a diffractive optical element which can be easily controlled by an electric signal. First, in order to facilitate understanding of the present embodiment, the operation of the parallel alignment type liquid crystal,
The diffraction phenomenon and the like will be briefly described.

FIGS. 3A and 3B schematically show the structure and operation of a general parallel alignment type liquid crystal element which can be electrically controlled. Glass substrate 30 coated with a transparent electrode
1 sandwich the liquid crystal molecules 302. The direction of the orientation axis 303 of the entrance-side and exit-side glass substrates is the Y-axis direction. The liquid crystal molecules 302 have the property of aligning the major axis direction with the alignment axis direction and the property of behaving as a continuum from FIG.
As shown in (a), the liquid crystal molecules 302 are arranged in parallel, and this is called parallel alignment or homogenous alignment.

A linearly polarized light 304 is applied to the parallel alignment type liquid crystal element.
Is incident, when its polarization axis is in the same direction as the orientation axis 303, the linearly polarized light 30
4 propagates along the long axis direction of the liquid crystal molecules 302 while maintaining linearly polarized light. In this case, assuming that the refractive index in the major axis direction of the liquid crystal molecules 302 is n1 and the thickness of the liquid crystal layer is d, the optical path length of the linearly polarized light 304 traveling in the liquid crystal layer is n1 × d.

Next, when an electric field in the Z-axis direction is applied to the liquid crystal molecules through the transparent electrode coated on the glass substrate 301, the major axis of the liquid crystal molecules 302 is the direction of the electric field as shown in FIG. Stand still in the Z-axis direction. This state is called homeotropic. In this case, linearly polarized light 3 traveling in the liquid crystal layer
04 also propagates while maintaining linearly polarized light. At this time, assuming that the refractive index of the liquid crystal molecules 302 in the minor axis direction is n2, the optical path length of the linearly polarized light 304 traveling in the liquid crystal layer is n2 × d. That is, the linearly polarized light 30 before and after the voltage is applied.
This means that the refractive index for No. 4 has been changed from n1 to n2, and thus the optical path length has been changed by (n1−n2) × d. It is also possible to create these intermediate states by controlling the applied voltage. It is also known that it is better to apply a minute voltage to the liquid crystal layer just before the liquid crystal starts to move by an electric field in order to make the liquid crystal layer close to an ideal homogenous state.

FIG. 4 shows a diffraction phenomenon of light by a general binary type approximately transparent phase type diffraction grating.
For simplicity, it is drawn in a sectional view projected on a plane. Thickness d having different refractive indices n1 and n2 repeatedly at pitch P
When the laser beam 402 is incident on the phase type diffraction grating 401, the output laser beam is diffracted by the diffraction effect. Here, for the sake of simplicity, the laser beam 402 is the phase type diffraction grating 4.
It is assumed that the light is incident perpendicular to 01. At this time, the zero-order light 403, which is light that normally passes through as it is, and θ
The first-order light 404 and the -1st-order light 405 diffracted in the direction and the −θ direction are generated (higher-order diffracted light having a larger diffraction angle is also generated, but is ignored because the ratio is small). At this time, the diffraction angle θ is determined by Sin (θ) = λ / P. Where λ
Is the wavelength of the laser light 402.

At this time, the areas of the regions n1 and n2 with respect to the laser beam 402 are substantially equal, and the optical path length difference (n1-n
2) When xd is λ / 2 + nλ (n: 0, 1, 2,...), This is called a Ronchi grating, and it is known that the zero-order light 403 disappears. The optical path length difference (n1-n2) ×
When d is mλ (m: 1, 2, 3,...) and the refractive index is repeatedly changed at a pitch P, the refractive index is continuously and smoothly changed from n1 to n2.
It is known that only 04 occurs. Actually, n1
It is also known that an almost ideal blazed grating can be obtained by changing stepwise from 16 to n2 in 16 steps or more, and this is called a multilevel binary grating. In general, a phase type diffraction grating is advantageous in that it has higher light utilization efficiency than an amplitude type diffraction grating having an opaque portion.

FIGS. 5A and 5B illustrate the cross-sectional structure of a liquid crystal diffraction optical element 501 that can be electrically controlled. The liquid crystal molecules 502 are aligned in parallel with the major axis direction coincident with the Y axis direction, and the refractive index in the major axis direction is n1 and the refractive index in the minor axis direction is n2. Further, on one side of the glass substrate, stripe-shaped transparent electrodes 503 are formed at a pitch P. A transparent electrode is coated on almost the entire surface of the other glass substrate. At this time, the linearly polarized laser light 504 in the Y-axis direction enters the liquid crystal diffraction optical element 501.

At this time, as shown in FIG. 5A, when no voltage is applied to the liquid crystal diffraction optical element 501, the refractive index for the linearly polarized light 504 becomes n1 uniformly. Therefore, no diffraction occurs, and the linearly polarized light 504 passes through to become the output light 508. Strictly speaking, slight diffraction occurs due to the transparent electrode 503.
If the refractive index in the major axis direction is the same, diffraction by the transparent electrode 503 does not occur.

Next, as shown in FIG.
When a sufficient voltage is applied to the liquid crystal molecule 3 from the power supply, the liquid crystal molecules 502 in that portion are brought into a homeotropic state by an electric field in the Z-axis direction. As a result, the linearly polarized light 504 has a structure in which the refractive index repeats n1 and n2 at the pitch P. Therefore, it functions as a binary-type phase-diffraction grating completely identical to that in FIG. 4, and has a zero-order light 505, a first-order light 506, and a −1-order light 507.
Occurs. At this time, if the condition of the Ronchi grating is satisfied, the zero-order light 505 is not generated. Similarly, if the above-described condition of the multilevel binary grating is satisfied, the primary light 5
06 only. However, for multi-leveling, it is necessary to cut the transparent electrode 503 at a finer pitch and change the voltage stepwise to apply the voltage.

Next, a second embodiment of the present invention will be described with reference to FIG. For simplicity, FIG. 2 is drawn by projecting on a two-dimensional plane, which is a YZ plane. Actually, it becomes a rotation reference with the optical axis 209 as a rotation axis. Basically the same as the embodiment shown in FIG. 1, but a liquid crystal diffractive optical element 205 is used as an electrically controllable diffractive optical element, and its basic structure and operation are the same as those shown in FIGS. Is the same. The direction of the polarization axis of the laser beam 203 which is linearly polarized light and the direction of the liquid crystal alignment axis of the liquid crystal diffraction optical element 205 substantially coincide with each other, and are both Y-axis directions.

Emitted from the linearly polarized laser light source 201,
The laser light 203 converted into a parallel plane wave by the collimator lens 202 is transmitted through the liquid crystal diffraction optical element 204 and causes diffraction. The liquid crystal diffractive optical element 204 includes a portion 205 that diffracts incident light by 0 degrees, and a portion 206 that functions as a diffractive element by applying a voltage from a power source and diffracts incident light by θ degrees.

The laser beam 207 transmitted through the portion 206 diffracted by θ degrees and diffracted by θ degrees is applied to the optical axis 209 of the focusing optical system 208.
Is incident on a region other than the substantially circular region 210 (shown by oblique lines) centered at. This circular area 210 is a part of the effective light beam 211 that should originally be incident on the condensing optical system 208, and the effective light beam 2
It can be seen that it is smaller than the opening created by 11. Here, the opening by the effective light beam 211 is set for DVD, and the opening by the circular region 210 is set for CD.

As shown in FIG. 2, the light flux in the region diffracted by θ degrees is ± f from the optical axis 209 in the Y-axis direction according to the principle of optical imaging.
An image is formed at a point ± P1 shifted by × tan (θ). Here, f is the focal length of the condensing optical system 208. Ie CD
Does not contribute to the image forming point P of the light transmitted through the circular region 210 set as the aperture for use. Furthermore, since the light is obliquely incident on the outer peripheral portion of the lens at an angle θ, the aberration may be large and the light may not be focused. If the value of the diffraction angle θ is sufficiently large, θ
It can also be seen that the luminous flux in the region diffracted to a degree is not incident on the light collecting optical system 208 at all. The effective aperture in these states is for a CD.

Next, the voltage application is stopped and the liquid crystal diffraction optical element 2
04, the diffraction function of the part 206 diffracted by θ degrees is stopped and 0
To the part where diffraction occurs. In this state, the effective light beam 211 passes through the liquid crystal diffraction optical element 204 as it is, and is imaged at the image forming point P by the condensing optical system 208. Therefore, in this state, the effective aperture is for DVD.

FIG. 6 shows the electrode shape of the liquid crystal diffractive optical element 204. A circular area 601 having a diameter r is provided at the center, and an annular area 602 in which a plurality of concentric annular zones having the same center as the circular area 601 is arranged at a pitch P is provided at an outer peripheral portion thereof. A lead electrode line 603 is arranged in the annular zone 602 and connected to the electrode portion 604. When an appropriate voltage is applied from the electrode section 604 to the annular zone 602, the annular zone 602 functions as a diffractive optical element. The circular area 601 is an area that is diffracted by 0 degrees, that is, a transparent area.

In FIG. 2, the portion 205 diffracted by 0 degrees has the same basic effect even if it has no electrode or liquid crystal layer, and does not diffract incident light even in a transparent region which is simply hollowed out. However, in this case, the optical path length is changed as compared with the region where the liquid crystal layer is located, so that when used for DVD, partial phase modulation of incident linearly polarized light occurs. Therefore, the condensing optical system 2
There is a possibility that correction must be made using 10 or other optical system.

Here, the effect of the specific diffraction angle θ and the shift of the focal position of the diffracted light from the optical axis 209 in the Y-axis direction (distance ± P1 from P in FIG. 2) will be calculated. In the manufacture of a general liquid crystal element, the electrode pitch is about 20 microns, the focal length of a condenser lens for an optical pickup is about 4 mm, and the wavelength of a semiconductor laser as a light source is 0.6.
Since the diffraction angle θ is about 5 microns, the diffraction angle θ
in (θ) = λ / P), and the value of θ is 1.86.
Degree, and therefore the amount of deviation is about 130 microns from the above-described equation (± f × tan (θ)). For an optical disk device,
Since the focused spot diameter is 1 micron or less, it is a sufficient shift amount.

In FIG. 2, a portion 206 of the liquid crystal diffractive optical element 204 that diffracts by θ degrees does not generate zero-order light if it satisfies the condition of the Ronchi grating described in the paragraph 21, and if it satisfies the condition of the multilevel binary grating described above. Only primary light is generated. When used as a Ronchi grating, ± first-order lights which are diffracted in ± θ directions are generated. In this case, as is clear from FIG. 2 and the above-described equation of the amount of deviation (± f × tan (θ)), the point P1 is shifted from the portion symmetrical with respect to the primary light and the optical axis 209 in the −θ direction. The diffracted -1st order light substantially overlaps. Similarly, -P
At one point, the primary light and the −1st-order light diffracted in the −θ direction from a portion symmetrical with respect to the optical axis 209 substantially overlap. The first-order light and the -1st-order light have a relative phase difference of half a wavelength, so that when they overlap with each other and interfere with each other, they can be eliminated, which is convenient.

Since the liquid crystal diffractive optical element 204 is used as a phase type diffraction grating which does not require a polarizing plate or the like, no loss of light quantity occurs in principle. In the actual measurement, the light quantity loss was about 15%, but it can be reduced to 10% or less by applying a non-reflection coating to the liquid crystal glass substrate.

[0038]

As is clear from the above description, the optical device using the liquid crystal diffractive optical element according to the present invention has a simple configuration and can easily switch the numerical aperture easily without losing a large amount of light. it can. This is DVD-R,
That is, in a digital versatile disk optical system capable of writable or rewritable, the present invention is effective for an optical pickup having a function of reproducing a CD. This is because increasing the light output of a semiconductor laser as a light source is a difficult problem. Further, since the light is removed by diffraction, the removed light does not diffuse at random as compared with the method using scattering or the like, so that it is less likely to be scattered noise, that is, stray light noise. Further, even if the wavelength of the laser light changes, it can be easily handled by controlling the amount of phase modulation of the liquid crystal element by an electric signal.

The liquid crystal element of the present invention is small in size and very simple in structure, and therefore easy to manufacture, as compared with a liquid crystal display panel for a personal computer or the like having a current complicated matrix pixel structure. In this embodiment, a liquid crystal element is used as the electrically controllable diffractive optical element. However, a solid crystal such as bismuth silicon oxide (BSO) or lithium niobate, or an electro-optical ceramic such as PLZT may be used. However, these materials have an effective operating voltage of hundreds to thousands of volts, so that they are more difficult to drive than liquid crystals having an effective operating voltage of several volts.

[Brief description of the drawings]

FIG. 1 is a configuration example of an optical device according to a first embodiment of the present invention.

FIG. 2 is a configuration example of an optical device according to a second embodiment of the present invention.

FIG. 3 is a diagram illustrating an operation of an electrically controllable parallel alignment type liquid crystal element in a second embodiment of the present invention.

FIG. 4 is a diagram showing a light diffraction phenomenon by a general binary phase diffraction grating.

FIG. 5 is a diagram showing a basic cross-sectional structure of a liquid crystal diffractive optical element that can be electrically controlled in a second embodiment of the present invention.

FIG. 6 is a diagram illustrating a transparent electrode shape of a liquid crystal diffractive optical element according to a second embodiment of the present invention.

FIG. 7 is a diagram illustrating a configuration example of an optical device according to a conventional technique.

[Explanation of symbols]

 101, laser light sources 201, 701, linearly polarized laser light sources 102, 202, 702, collimate lenses 103, 203, 402, laser light 703, linearly polarized light 104, diffractive optical elements 204, 501, liquid crystal diffractive optical element 704, polarization axis 705, 90 degree TN type liquid crystal element 105, 205, 0 degree diffracted part 106, 206, θ degree diffracted part 107, 207, θ degree diffracted laser beam 108, 208, 706, 712, condensing optical system 707, Polarizing plates 109, 209, optical axes 110, 210, 601, circular regions 111, 211, effective light beams 708, 713, condensed spot 709, optical disk 710, light separating element 711, separated light beam 714, light detecting element 301, Glass substrates 302, 502, liquid crystal molecules 303, alignment axis 304, linear polarization 401, phase type diffraction gratings 403, 505, zero-order light 404, 506, first-order light 405, 507, minus first-order light 503, transparent electrode 504, linearly polarized laser light 508, emission light 602, annular zone 603, extraction Electrode wire 604, electrode part

Claims (8)

[Claims] 1. An optical apparatus comprising at least a laser light source for emitting laser light, a diffractive optical element for diffracting the laser light, and a condensing optical system for condensing a light beam transmitted through the diffractive optical element. An optical device, wherein a diffraction function of an optical element is controlled by an electric signal, and a controlled area acts on a partial area of a light beam condensed by a condensing optical system. 2. An optical apparatus comprising at least a laser light source for emitting laser light, a diffractive optical element for diffracting the laser light, and a condensing optical system for condensing a light beam transmitted through the diffractive optical element. The optical element is composed of a part that diffracts the incident laser light by 0 degree and a part that diffracts by 0 degree, and the part that diffracts by 0 degree is substantially circular centered on the optical axis in the effective light beam used by the condensing optical system. The optical device, wherein the portion that acts on a region and diffracts by θ degrees acts on a region other than the circular region in the effective light beam, and a diffraction function of the diffractive optical element is controlled by an electric signal. 3. The optical device according to claim 1, wherein a liquid crystal diffractive optical element is used as the diffractive optical element that can be controlled by an electric signal. 4. A linearly polarized laser beam as a laser beam, and a liquid crystal diffractive optical element as a diffractive optical element controllable by an electric signal, wherein the liquid crystal diffractive optical element comprises a parallel alignment type liquid crystal element. 2. The liquid crystal diffraction optical element according to claim 1, wherein the direction of the polarization axis of the polarized laser light substantially coincides with the direction of the liquid crystal molecule alignment axis of the liquid crystal diffraction optical element.
The optical device according to the item. 5. A linearly polarized laser beam as a laser beam, and a liquid crystal diffractive optical element as a diffractive optical element controllable by an electric signal, wherein the liquid crystal diffractive optical element comprises a parallel alignment type liquid crystal element. The direction of the polarization axis of the polarized laser light substantially coincides with the direction of the liquid crystal molecule alignment axis of the liquid crystal diffraction optical element, and the phase modulation amount of the parallel alignment type liquid crystal element is a half wavelength of the wavelength of the linearly polarized laser light plus an integer of the wavelength. 2. The optical device according to claim 1, wherein the optical device is set to be approximately twice or an integral multiple of the wavelength. 6. The optical device according to claim 2, wherein a liquid crystal diffractive optical element is used as the diffractive optical element that can be controlled by an electric signal. 7. A linearly polarized laser beam as a laser beam, and a liquid crystal diffractive optical element as a diffractive optical element which can be controlled by an electric signal, wherein the liquid crystal diffractive optical element comprises a parallel alignment type liquid crystal element. 2. The liquid crystal diffraction optical element according to claim 1, wherein the direction of the polarization axis of the polarized laser light substantially coincides with the direction of the liquid crystal molecule alignment axis of the liquid crystal diffraction optical element.
The optical device according to the item. 8. A linearly polarized laser beam as a laser beam, and a liquid crystal diffractive optical element as a diffractive optical element controllable by an electric signal, wherein the liquid crystal diffractive optical element is composed of a parallel alignment type liquid crystal element. The direction of the polarization axis of the polarized laser light substantially coincides with the direction of the liquid crystal molecule alignment axis of the liquid crystal diffraction optical element, and the phase modulation amount of the parallel alignment type liquid crystal element is a half wavelength of the wavelength of the linearly polarized laser light plus an integer of the wavelength. 3. The optical device according to claim 2, wherein the optical device is set to about twice or an integral multiple of the wavelength.