GB2069713A - Beam spot scanning device - Google Patents

Abstract

A device for scanning a beam spot has a light deflecting portion 7 and a light condensing thin film lens 9 formed on a thin film waveguide 2. In this device, a chirped signal is applied to a surface elastic wave transducer constituting the light deflecting portion, whereby scanning of the beam spot is effected on or near the end surface of the thin film waveguide path. <IMAGE>

Description

SPECIFICATION Beam spot scanning device Background of the invention Field of the invention This invention relates to a device for scanning a beam spot.

Description of the prior art Apparatus for scanning a laser beam spot have heretofore comprised a rotational polygon mirror for deflecting the laser beam and an f-6 lens or the like for condensing the deflected beam into a spot moving at a linear speed. In these prior art apparatus, however, various operating portions have been separate and independent and a predetermined light path interval has been required therebetween and therefore, precise adjustment during the assembly of the apparatus has been very much complicated and the assembled apparatus has been bulky.

There are also known -apparatus which use an acoustic optical deflector instead of a rotational polygon mirror to make the apparatus compact. An example of such apparatus is disclosed in U. S. Patent No.

3,514,534, but even in that example, various operating portions are still independent and therefore, cumbersome adjustment has been required during the assembly.

On the other hand, to reproduce images from a magnetic recording medium on which, for example, TV signals are recorded, it is necessary to move the recording medium and a signal reading head relative to each other at a high speed to thereby effect scanning in order to obtain a high frequency of 4.2 MHz which is the band of NTSC signal. In the conventional VTR for domestic use, mechanical scanning devices which effect reading by a head rotated art a high speed inside of a recording medium wound on a cylinder have been typical. Generally, however, mechanical scanning devices are inferior to optical scanning devices in accuracy and durability in the case of high-speed scanning and accordingly, there have been desired compact recording or reproducing apparatus utilizing an optical scanning device which can readily realize stable high-speed scanning.Recently, attention has particularly been paid to a system which utilizes the magnetic Kerr effect or the Faraday effect to effect beam spot scanning on a magnetic recording medium and thereby accomplishes magnetic recording or reproduction. However, the aforementioned disadvantages of the conventional optical scanning devices have prevented their application to compact and high frequency band recording or reproducing apparatus.

Summary of the invention Accordingly one aspect of the present invention provides a beam spot scanning device which is compact and which does not require precise adjustment during the assembly thereof.

Another aspect of the present invention provides provides a beam spot scanning device which is compact and does not require precise adjustment during the assembly thereof and in which no focus deviation occurs during scanning, that is, the diameter of the spot does not vary during scanning.

These aspects of the present invention utilize the technique of the light integrated circuit. Recently, there is known a technique of forming a thin film lens, an AlO deflector or an E/O modulator in a thin film waveguide path formed on a base to thereby make a light integrated circuit, as explained in T. Tamir, Integrated Optics, published by Spinger Verlag, Inc. (1975). Further, a deflector utilizing the acousto-optic action in a thin film waveguide path to defract a parallel light beam is disclosed in Proc. IEEE 64.779(1976) E.G. Lean et al, Thin Film Acoustooptic Devices. Also, an example of the transducer for generating an acoustic wave in a waveguide path over a wide band is disclosed in SPIE 139, 139 (1978) C. S. Tsai, Guided Wave Optical Sytems and Devices.

A typical device according to this invention for scanning a beam spot comprises a light deflecting portion (transducer) and a light condensing lens portion integrally formed on one and the same base, and is compact and light in weight and accordingly low in cost.

The term "light" used herein includes electro-magnetic radiation in both the visible and invisible portions of the spectrum. The term "recording medium" includes magnetically and optically recorded mediums, mediums on which information is permanently recorded, and mediums capable of beng re-recorded.

The invention will become more fully apparent from the following detailed description thereof taken in conjunction with the accompanying drawings.

Brief description of the drawings Figure 1 is a perspective view of a beam spot scanning device of the type which uses a prism coupler to direct a light beam into a waveguide path.

Figure 2 is a plan view of a light deflecting portion.

Figure 3 is a perspective view of a deflecting portion for deflecting light over a wide band.

Figure 4 is a perspective view of a beam spot scanning device in which the exit end surface of the waveguide path is curved so that the diameter of a beam spot does not vary during scanning.

Figure 5 is a fragmentary view of a device which effects beam spot scanning outside of the end surface of the waveguide path.

Figure 6 is a plan view of a beam spot scanning device provided with a field-flat thin film lens.

Figure 7 is a perspective view of a device for recording TV signals on a film by beam spot scanning.

Figure 8 is a perspective view of a device for effecting scanning and recording on a metal thin film recorded medium.

Description of the preferred embodiments Figure 1 shows the beam spot scanning device according to a first embodiment of the present invention. In this beam spot scanning device, a prism coupler 3, a comb-tooth-like (interdigital) electrode 6 and a thin film lens 9 are provided on a waveguide path 2 formed on a base 1. A parallel laser beam 4 is directed as a light beam 5 into the waveguide path 2 through the prism coupler 3. The light beam 5 propagated through the waveguide path is diffracted and deflected by an ultrasonic wave surface elastic wave 7 which is excited by the comb-tooth-like electrode 6 provided on a portion of the waveguide path 2. This deflected light beam 8 is condensed by the thin film lens 9so as to form a beam spot 11 on the end surface 10 of the thin film waveguide path.That is, the end surface 10 is formed at a location substantially coincident with the focal plane of the thin film lens 9 having a power in x-z plane (shown), and the condensed light beam is condensed on or near the end surface 10 with respect to x-direction and exits from the end surface. In y-direction perpendicular to the x-z plane, the width of the spot is limited by the thickness d (usually seveal ,am) of the waveguide path. In the beam spot scanning device of the present embodiment having such a construction, the frequency of a high frequency voltage applied to the comb-tooth-like electrode 6 is varied to vary the wavelength of the ultrasonic wave surface elastic wave on the waveguide path to thereby control the angle of deflection and effect beam spot scanning on the exit end surface.

As described above, the beam spot scanning device of the present embodiment comprises a light deflector and a condenser lens provided on the same base so that a beam spot is formed and scanned on or near the exit end surface of the waveguide path thereof, and this leads to the advantages that the device is compact and that precise adjustment is unnecessary.

The portions which constitute the beam spot scanning device of the above-described embodiment will now be described in greater detail.

A material having a piezoelectric function and through which ultrasonic wave of high frequency may be efficiently propagated is suitable for the base 1, the LiNbOB, LiTaO3, ZnO or the like is desirable. In a case where the base is formed of LiNbO3, the waveguide path 2 may be formed to a thickness of several pm on the base by in-diffusing Ti under a high temperature (about 1000 C). In a case where the base is formed of LiTaO3, the waveguide path may be provided by in-diffusing Nb or Ti.Other examples are described in the aforementioned book, but the waveguide path of the present embodiment should desirably be formed of a material having a high refractive index and having such a great difference in refractive index from the base that light is propagated even if the waveguide path is made thin. The refractive index of the waveguide path formed of such a material is high and therefore, the beam spot formed on the end surface through the condenser lens can be made small in spot diameter, namely, sharp.

The deflector should desirably be one which utilizes an ultrasonic wave surface elastic wave and in the present embodiment, as shown in Figure 2, an ultrasonic wave is excited by the comb-tooth-like (interdigital) electrode 6 formed on the surface of the waveguide path having a piezoelectric property. The pitch a of the comb-tooth-like electrode is set to 1/2 of the center wavelength of the excited ultrasonic wave.For example, if the electrode pitch is set to a = 8.8 Fm in the case of LiNbO3 base, when a high frequency voltage of 200 MHz is applied thereto, an ultrasonic wave of wavelength 17.5 itm can be excited. (The velocity of the ultrasonic wave is about x x 106 mmlsec.) The band of the deflector provided by this single electrode is limited by the angle selection width of a Bragg type diffraction grating created by the excited ultrasonic wave and the band of a transducer comprising this piezoelectric material and an electrode. The band limited by the former Bragg type diffraction is given by the following equation from the aforementioned Proc IEEE 64,779 (1976), E.G. Lean et al, Thin Film Acoustooptic Devices; v = vA (1) 2n,0L s-0 L where n : refractive index ofthewaveguide path A0 wavelength of the incident light beam v : velocity of the ultrasonic wave surface elastic wave A : wavelength of the elastic wave L : width oftheelasticwave.

Also, the deflection angle AQ when the applied frequency has been biased by Av is given by the following formula: AQ - Av (2) Accordingly, in the device of Figure 1, the frequency of the signal applied to the electrode 6 is continuously repetitively varied within a predetermined range, whereby the beam spot 11 is continuously scanned in a predetermined range. The number N of scanning points which are separable from one another within this deflection range is given by the following equation: W N = Av.W (3) V where W is the width of the incident light beam. For example, when Av = 500 MHz and W = 10 mm and V = 3.5 x 106 mm/sec, N = 143.

When it is desired to further increase the number of scanning points, it is possible to utilize the aforementioned wide band deflector shown by C. S. Tsai et al (SPIE vol. 139, p. 139, 1978). This, as shown in Figure 3, comprises arranging a plurality of electrodes different in pitch at angles which satisfy the Bragg diffraction condition for the incident light in accordance with each wavelength band, causing each transducer 12 to bear a portion of the wide band, and applying to the electrodes so-called chirped signals whose frequency is continuously varied to thereby vary the wide band of 500 MHz. By this, it is possible to obtain 1250 scanning points.

Next, suitable as the thin film lens 9 is a mode index lens, Luneburg lens, Geodesic lens or the like shown in IEEE Quntum Elect vol. QE-13, p.129, 1977 (by D. W. Bakey & Van E. Wood).

A performance approximate to the theoretical resolution limit is obtained by the latter two types of lenses.

The size (diameter) bx of the beam spot in x-direction condensed by the thin film lens is given by the folowing equation in a case where the incident light beam is a parallel light beam of rectangular intensity distribution: x = 2.44 A0-f/nW 2.44 ( )F (5) n where F is the F-number given by When, in the present embodiment, F-number is set to 2.0 and the wavelength of the incident light is set to k0 = 820 nm and the refractive index of the waveguide path is set to n = 2.2, the diameter 6x of the beam spot in x-direction is 1.8 um and, by forming the film thickness of the waveguide path to 1.5 Ftm, a substantially circular beam spot can be obtained on the exit end surface 10.In Figure 1, the exit surface need not be the end surface 10 parallel to x-y plane, but by making the end surface 10 oblique, the light may be bent in y-direction and the band area of the end of the waveguide path surface parallel to x-z plane may be the exit surface.

As described above, in the beam spot scanning device of the present invention, the deflector and the condenser lens are formed on the same base and therefore, the device is compact and stable without any arrangement deviation.

Now, the image plane of the thin lens in the ordinary optical system is substantially flat and accordingly, the beam spot scans substantially in a flat plane. However, when use is made of a special lens such as a thin film lens, the image plane (the locus of the point whereat light is condensed) is greatly curved and therefore, .

the influence thereof can be neglected in a case where a narrow band is scanned, whereas it becomes a problem in a case where a very wide band is scanned.

In the case of the aforementioned three types of lenses, the position of the light-condensed point when a light beam has been deflected by A4 is deviated by A = -1 cos(A2) from the focal plane of the on-axis light beam and the beam spot scanning surface becomes curved. If A4 is small, say AQ = 60 or so, A = 1 m, and this can be neglected. When beam spot scanning is effected on the flat end surface as shown in Figure 1 in a case where the deflection angle is great, that is, the scanned band is wide, the size of the beam spot on the flat end surface will fluctuate due to focus deviation in the course of the scanning, and this is very inconvenient. Figure 4 shows an embodiment which overcomes such inconvenience.

The image plane ofthe circular thin film lens 9 as shown in Figure 4 lies in x-z plane and on a concentric circle of the thin film lens 9. Accordingly, to solve this focus deviation error, the light beam exit end surface may be made into a cylindrical end surface 15 centered at the center of the lens 9 as shown in Figure 4. By thus forming the exit surface into a shape substantially conforming to the image plane of the condensing thin film lens, any focus blur can be eliminated. While, in the first embodiment, the light beam is directed to the waveguide path through the coupling prism 3, a semiconductor laser 13 may be installed in proximity to the end surface of the waveguide path as the embodiment of Figure 4 so that the light beam may be directly directed to the waveguide path.In this case, however, the light beam becomes a divergent light in the waveguide path 2 and thus, a thin film lens 14 for collimating the light beam will become necessary. The present embodiment is effective not only for a case where the point whereat the light is condensed by the condenser lens 9 lies on the exit surface 15, but also for a case where the point whereat the light is condensed lies outside of the exit end surface 15 as shown in Figure 5. In that case, the light is condensed in x-direction, but the light is defocused (out of focus) and diverges in y-direction which is perpendicular to the thin film.Accordingly, to cause the light to be condensed both in x- and y-direction, a cylindrical lens having a condensing function only in y-direction may be provided so that the light condensing in y-direction may be coincident with the light-condensed point in x-direction.

Figure 6 shows another embodiment of the beam spot scanning device which solves the problem of focus deviation. The beam spot scanning device shown in Figure 6, as compared with the device shown in Figure 1, has a field-flat type thin film lens 35 (field flattener) provided adjacent to the exit end surface of the thin film waveguide path 2, which thin film lens 35 acts on the light beam deflected by a deflecting portion and condensed on the curved image plane near the exit end surface by the thin film lens 9 so as to form a beam spot on a straight line 36. Accordingly, this beam spot scanning device can effect field-flat scanning with respect to the surface of a recorded medium and is very preferable as a scanning device. It is also possible to endow this field-flat type thin film lens with an f-0 lens function.

Also, if the field-flat type thin film lens 35 is provided between the exit end surface and the thin film lens 9 and the exit end of the waveguide path is made straight as shown in Figure 1 and the light-condensed point is designed to be coincident with the exit end, then stable plane scanning will be possible on the exit end surface.

In the present embodiment, the thin film lens 9 and the field-flat type thin film lens are installed separately from each other, but if the design conditions permit, they may of course be replaced by a single thin film lens which will function as both lenses.

Description will now be made of some embodiments to which the beam spot scanning device of the present invention is applied.

Figure 7 shows an embodiment in which the beam spot scanning device of the present invention is applied for the film recording of TV images. A beam spot 17 emitted from the beam spot scanning device 16 of the present invention is directed to the surface of a film 19 by a magnifying projection lens 18. The beam spot scanning device 16 comprises the light IC portions shown in Figure 1. In this case, the base 1 is an LiNbO3 base and the waveguide path is provided by in-diffusing Ti.Now, if N = 500 and ho = 0.82 Ft and f = 15 mm and n = 2.2 and F = fl(W) = 2 and VA = 3.5 x 106 mm/sec, the then band width Av, swing angle Acp, beam spot diameter bx, scanning width f and response r will be as follows: bx = 244A0 F = 1.8 n f- = 1.8x10-3x500 = 0.9mm W = 7.5 mm = = 2tan-1 0.9/2 = 38' 15 AV = nVA A+625MHz iio W ~ r - ~~~ ~ VA - 2.1 Ltsec.

The film recording ofTV images can be accomplished by causing this scanning beam spot to be projected upon a film moved in a direction perpendicular to the beam spot scanning direction by the magnifying projection lens 18. in this case, however, the repetition frequency of the scanning line must be 15.7 MHz and high-speed scanning must be effected for each scanning line of the TV screen.

The features of the thin film waveguide type scanning device of the present invention lie in that it is suited for such high-speed scanning and that the output of the driver which drives it may be a low power.

According to the present invention, as has hitherto been described, there can be provided a beam spot scanning device which is compact, highly reliable and capable of high-speed scanning, and such beam spot scanning device is applicable in various forms and very high in applicability.

Claims (18)

1. A device for scanning a beam spot continuously comprising: a waveguide for propagating a light beam; first means for continuously deflecting said light beam in said waveguide; and second means for concentrating said light beam.

2. A device according to Claim 1, wherein said first means comprises a comb-tooth-like transducer for .exciting an acoustic wave in said waveguide, and means for applying to said transducer is a signal whose frequency varies continuously.

3. A device for scanning a beam spot repeatedly in a predetermined spatial range comprising: a waveguide for propagating a light beam; first means for deflecting said light beam in said wave-guide repeatedly within a predetermined angle range; and second means for concentrating said light beam.

4. A device according to Claim 3, wherein said first means comprises a comb-tooth-like transducer for exciting an acoustic wave in said waveguide, and means for applying to said transducer a signal whose frequency varies repeatedly within a predetermined range.

5. A device according to Claim 1 or 3, wherein said second means comprises a thin film lens formed on said waveguide.

6. A device according to Claim 5, wherein the light exit surface of said waveguide is substantially coincident with or parallel to the image plane of said thin film lens.

7. A device according to Claim 1 or 3, wherein said second means includes a field flattener.

8. A device for scanning a beam spot and recording signals on a recorded medium comprising: a waveguide for propagating a light beam; first means for deflecting said light beam in said waveguide; and second means for concentrating said light beam on the recorded medium.

9. A device according to Claim 8, wherein said first means comprises a comb-tooth-like transducer for exciting an acoustic wave in said waveguide, and means for applying a chirped signal to said transducer.

10. A device according to Claim 8, wherein said second means comprises a thin film lens formed on said waveguide.

11. A device according to Claim 10, wherein the light exit surface of said waveguide is substantially coincident with the image plane of said thin film lens and said recorded medium is positioned in proximity to said light exit surface.

12. A device for scanning a beam spot substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.

13. A device for scanning a beam spot substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.

14. A device for scanning a beam spot substantially as herein before described with reference to Figure 4 of the accompanying drawings.

15. A device for scanning a beam spot substantially as hereinbefore described with reference to Figure 5 of the accompanying drawings.

16. A device for scanning a beam spot substantially as herein before described with reference to Figure 6 of the accompanying drawings.

17. A device for scanning a beam spot substantially as hereinbefore described with reference to Figure 7 of the accompanying drawings.

18. A device for scanning a beam spot substantially as hereinbefore described with reference to Figure 8 of the accompanying drawings.