JPH021289B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a Fresnel optical scanner that performs optical scanning using a thin film optical waveguide.

An optical scanner or optical deflector is one of the basic elements required when lasers are applied to optical information processing, optical recording, displays, etc. The performance of such devices is determined by response speed, resolution, power consumption, scanning format (continuous scanning, random access, etc.), and the like.

Methods of mechanically moving a mirror to deflect light using the Viezo effect, etc., and acousto-optic methods are currently in common use, but they inherently have the drawback of slow response speed. A method using an electro-optic effect is suitable for high-speed optical deflection, and various methods have been proposed.
Among these, digital optical deflectors consist of many electro-optic optical modulators using large-diameter bulk crystals, and these are arranged in series and parallel to digitally deflect light.However, the structure is complex and the applied voltage is limited. However, it has very large drawbacks and has not been put into practical use. There is also a prism array optical deflector using bulk crystals, but this method also has the same drawbacks as digital optical deflectors and is not practical. In other words, the electro-optic effect changes the refractive index by an electric field, and the amount of change is proportional to the electro-optic coefficient, electric field and electrode length, and inversely proportional to the thickness in the direction of electric field application, but the electro-optic coefficient is very Because it is small,
The method using bulk crystals has the essential drawback of requiring the aforementioned large applied voltage.

Similarly, various optical deflectors using thin film optical waveguides have been proposed in order to improve the above-mentioned drawbacks by using the electro-optic effect. An example is shown in FIG.
In this example, two electrodes 2 and 3 having a length l are arranged parallel to each other on a thin film slab type optical waveguide 1,
Another electrode 4 is arranged obliquely to these electrodes. Electrode 4 has a distance h from electrodes 2 and 3 at each end, and a distance w+2h between electrodes 2 and 3.
to have. A voltage V is applied to electrodes 2 and 3, and electrode 4 is connected to ground potential. The monochromatic plane wave 5 that has entered the single prism optical deflector having the above configuration is deflected by a predetermined angle and outputted like an output light 7 having an equal phase plane 6. Although the configuration of such a single prism optical deflector is very simple, it has the drawback that light passing near the parallel electrodes 2 and 3 generates side lobes, resulting in a reduction in S/N and resolution.

FIG. 2 shows a prism array type optical deflector in which the single prism optical deflectors shown in FIG. 1 are arranged in parallel. Here, 21 is a slab type thin film optical waveguide, 2
2A to 22E are parallel electrodes, and 23A to 23D are oblique electrodes. A voltage V is applied to the parallel electrodes 22 to 22E, and the oblique electrodes 23A to 23D are connected to the ground potential. For an incident monochromatic plane wave 24, an equiphase front 25
An output light 26 having . In this prism array type optical deflector, the deflection angle that can be used as an optical polarizer takes discrete values, so even if the deflection angle can be made large relative to the beam thickness, no improvement in resolution can be expected. There is a drawback.

FIG. 3 shows striped optical waveguides 31A to 31G.
Parallel electrodes 32 and 33 are arranged with respect to electrode 32 and
A voltage V is applied to the electrode 33 and grounded. Output light 36 having an equal phase front 35 is obtained with respect to input light 34 of a monochromatic plane wave. In this optical deflector, unlike the example shown in Figure 2, the deflection angle changes continuously with respect to the applied voltage V, but the optical output decreases as the deflection angle increases, so it is difficult to improve the resolution. be.

The above three examples are typical optical deflectors using thin-film optical waveguides, but they have a common drawback that they cannot condense light unless a convex lens is provided at the exit of the optical deflector. Therefore, when configuring a so-called optical integrated circuit that integrates an optical deflector, a photodetector, etc.,
The structure becomes complicated and is subject to restrictions due to the performance of the condensing lens.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a Fresnel optical scanner using a thin film optical waveguide, which can perform high-speed optical scanning with low applied voltage and can focus light without a convex lens.

In order to achieve such an object, the present invention has a one-dimensional Fresnel strip formed of a thin film optical waveguide, and at least near the boundary of each Fresnel zone of the one-dimensional Fresnel strip, a pair of the same length are provided. comb-shaped electrodes are arranged alternately along said boundary;
It is characterized in that a predetermined voltage is applied between the pair of comb-shaped electrodes.

According to the present invention, since Fresnel strips are used,
In addition to being able to condense light without a convex lens, there is a linear relationship between the applied voltage and the distance the focal point moves, and although the device capacity is larger than a single prism deflector, the resolution can be improved by using the three types of thin films mentioned above. It also has the advantage that it can be made higher than an optical deflector using an optical waveguide. Furthermore, it is easy to integrate and is suitable for applications such as optical recording and display, as well as for distributing signals from optical fibers.

The present invention will be described in detail below with reference to the drawings.

Typical one-dimensional Fresnel strips include:
There is a system in which the Fresnel zones are arranged so that the distance x from the center of the strip is proportional to √ to a positive integer number representing the position of the Fresnel zone, that is, the number of zones m, and this Fresnel strip is similar to a convex lens. When a monochromatic plane wave is used as incident light, it can be focused. In the present invention, such a one-dimensional Fresnel strip plate is used, and in the Fresnel optical scanner according to the present invention, the phase of light from every other Fresnel zone is electro-optically controlled, and arbitrary It is possible to configure both an amplitude type, in which the light is focused on a point, and a phase type, in which the light control method is similar to the amplitude type, but the static phases of adjacent zones differ from each other by 180°. There are two types of amplitude types: one with an opening in an odd-numbered zone and one with an opening in an even-numbered zone.

First, FIG. 4 shows an amplitude type Fresnel optical scanner with an odd-numbered zone aperture as an example of the present invention. Here, among the Fresnel zones constituting the one-dimensional Fresnel strip, odd-numbered zones m=1, 3, 5, 7... are connected to striped optical waveguides 41A, 42 having different widths.
A, 43A, 44A,..., 41B, 42B, 43
B, 44B, . . . are formed on a substrate 40 made of electro-optic crystal such as LiNbO ₃ , and interdigitated electrodes 45 are arranged at the boundaries of each zone in a staggered manner so that the polarities of adjacent electrodes are different from each other. and 46 are placed. The lengths of the intersecting electrodes 45 and 46 are all the same. If the electrode length at the intersection is l, the phase ±θ _n induced by the electro-optic effect of the m-th aperture is θ _n = πn ³ γlVξ/λ(x _n −x _n-1 ) (1) Given. In this case, m is an odd number and X ₀ =0. Here, n is the effective refractive index, γ is the electro-optic coefficient, V is the applied voltage, ξ is a coefficient representing the degree of overlap between the electric field distribution and the refractive index distribution, λ is the incident wavelength, and x _n is the m-th and m+1-th This is the distance between the zone boundary and the center O of the Fresnel optical scanner.

As will be described later, in order to increase the moving distance b of the focal point P in the x direction on the focal plane PL,
The value of l·V in formula (1) may be increased. Electrode 4
Since the electrodes 5 and 46 are formed on the substrate 40 made of crystal such as LiNbO ₃ , the size of the crystal is maximized, and if the electrode length l is made large, the same focal point movement distance can be obtained with a lower voltage.

Next, the principle of the Fresnel optical scanner of the present invention will be briefly described. When the voltage V applied to the electrode 45 is 0, the Fresnel optical scanner becomes a mere Fresnel solution, so that the input light 47 of a monochromatic plane wave is The diffracted light is concentrated at a focal point P. At other points on the focal plane PL, the phases of the lights from each optical waveguide cancel each other out, and the light intensity there is much weaker than that at the focal point P. However, when voltage V is applied to the scanner, the phase ±θ _n shown in equation (1) is given to the diffracted light from each optical waveguide, so the focal point P becomes interference that cancels each other out, and interference that strengthens each other. teeth,
It will occur at another position on the focal plane PL and the light will be scanned. Relative light intensity I at a point Q separated by b from P on the focal plane of the amplitude Fresnel optical scanner
is given by the following equation.

I=1/2 [ _M 〓 〓 ^m=1 δ _n {cosθ _n (∫ ^vm- 〓 ^v _vn-1 〓 _v cosπ／2v ² dv＋∫ ^vm
+ 〓 ^v _vn-1+ 〓 _v cosπ／2v ² dv) −sinθ _n (∫ ^vm- 〓 ^v _vn-1- 〓 _v sinπ／2v ² dv−∫ ^v
m+ 〓 ^v _vn-1+ 〓 _v sinπ／2v ² dv)}] ² +1/2〔 _M 〓 〓 ^m=1 δ _n {cosθ _n (∫ ^vm- 〓 ^v _vn-1- 〓 _v sinπ／2v ² dv＋ ^∫v
m+ 〓 ^v _vn-1+ 〓 _v +sinπ/2v ² dv) +sinθ _n (∫ ^vm- 〓 ^v _vn-1+ 〓 _v cosπ/2v ² dv−∫ ^-
vm+ 〓 ^v _vn-1- 〓 _v cosπ/2v ² dv)}] ² (2) Here, M is the number of zones. δ _n is a coefficient indicating the position of the aperture, and in the case of an odd number of apertures, δ _n = 1 m = odd number, δ _n = 0 m = even number, (3) and in the case of an even number of apertures, the relationship is the opposite.
Further, v, v _n and Δv are each given by the following equations.

v=x√2 (4) v _n =x _n √2 (5) Δv=b√2 (6) Here, x is a variable indicating the distance from the center O,
Δv represents the distance from the focal point P (anonymous number), and Δv=0 corresponds to the focal point P.

5A-D and 6A-D show the numerical calculation results of the light distribution on the focal plane PL of the odd-numbered aperture type Fresnel optical scanner when the number of zones m is 50 and 20, respectively _. The cases of π, −2π/3, −π/3, and 0 are shown. Although detailed calculations are omitted, as can be seen from both figures, the moving distance of the focal point P is proportional to the electrically induced phase. Since this phase is proportional to the applied voltage v, for example, from equation (1), it can be seen that there is a linear relationship between the magnitude of the voltage v and the moving distance of the focal point.

7A-D and 8A-D show the numerical calculation results of the light distribution on the focal plane of even-numbered aperture Fresnel optical scanners with 50 and 20 zones, respectively _.
The cases of 1.37π, −0.91π, −0.46π, and 0 are shown.
According to Figures 5 A to D to Figures 8 A to D, when the number of zones is large, the odd number aperture type is better because it can obtain a large light intensity, but when the number of zones is small, the even type with small side ropes is better. It can be seen that the open type is superior in terms of S/N.

FIG. 9 shows an example of the configuration of an even-numbered aperture type Fresnel optical scanner according to the present invention, in which the same parts as in FIG. 4 are given the same reference numerals. In this example, even zone m
= Optical waveguides 41A, 42A, ..., 4 at 2, 4, 6
1B, 42B, . . . are formed, and the details are omitted here. With this Fresnel optical scanner, the focal point PL when the number of zones is 20
Actual measurements of the above light distribution are shown in FIGS. 10A to 10C. Figure 10 A, B, and C each have v=-30 volts θ ₂ =
−2.5π), 0 volts (θ ₂ =0), v=40 volts (θ ₂
= +3.3π). In this example, the amount of movement of the focal point is smaller than the calculation results shown in FIGS. 8A to 8D under the same conditions, but it can be improved by increasing the accuracy of electrode placement. Also, the side ropes shown in Figures 10A-C can be reduced by increasing the number of zones.

Figure 11 shows an example of a phase _- type Fresnel optical scanner according to the present invention.
A conventional slab type optical waveguide 50 is formed by vapor-depositing Ti or the like, and the dot portions 51A, 52A, and the like shown in FIG.
As shown by 53A, . . . , 51B, 52B, 53B, . . . , more or less Ti is deposited over a predetermined length l ₀ in only a portion of the odd-numbered zone or even-numbered zone than in the other portions. Depending on the amount of Ti deposited,
Since the refractive index of the diffused optical waveguide 50 can be controlled, excessive or under-deposited portions 51A, 5
Length of 2A, 53A, 51B, 52B, 53B _{l 0}
By appropriately adjusting , the phase of the input monochromatic plane wave 54 passing through the even and odd zones can be made to differ by 180 degrees. Narrow comb-shaped intersecting electrodes 55 and 56 are arranged as close as possible to the boundary of each Fresnel zone of the phase-type Fresnel strip plate thus formed. Applying a voltage v to the electrode 55,
Electrode 56 is connected to ground potential.

In this example, as in the case of the amplitude-type Fresnel optical scanner, the lengths l ₀ of the intersections of the electrodes are all the same, so the phase ±θ _n of the light induced by the electro-optic effect is (1 ) is given by the formula. In this case, m is an integer of 1 or more.

Light intensity at focal plane PL of phase-type Fresnel optical modulator
Ip is given by the following equation, excluding a small amount of light near the Fresnel zone boundary.

Ip=1/2 [ _M 〓 〓 ^m=1 {cos (δ _n π + θ _n )∫ ^vm- 〓 ^v _vn-1- 〓 _v cosπ／2v ² d
v)＋cos(δ _n π −θ _n )∫ ^vm+ 〓 ^v _vn-1+ 〓 _v cosπ／2v ² dv−sin(
δ _n π+θ _n )∫ ^vm- 〓 ^v _vn-1- 〓 _v sinπ／2v ² dv −sin(δ _n π−θ _n )∫ ^vm+ 〓 ^v _vn-1+ 〓 _v sinπ／2
v ² dv}] ² 1/2 [ _M 〓 ^m=1 {cos (δ _n π +θ _n )∫ ^vm- 〓 ^v _vn-1- 〓 _v sinπ／2v ² dv＋cos(
δ _n π−θ _n )∫ ^vm+ 〓 ^v _vn-1+ 〓 _v sinπ／2v ² dv＋sin(δ
_n π +θ _n )∫ ^vm- 〓 ^v _vn-1- 〓 _v cosπ／2v ² dv＋sin(
δ _n π−θ _n )∫ ^vm+ 〓 ^v _vn-1+ 〓 _v cosπ／2v ² dv}] ² (7)
Here, δ _n =1 m=even number, δ _n =0 m=odd number, (8).

Number of zones M= calculated from equation (7) in Figure 12 A to D
Focal plane PL of phase-type Fresnel optical scanner when 50
The numerical calculation results of the light distribution will be shown for the cases of θ ₁₀ =−π, −2π/3, −π/3, and 0. From this, it can be seen that the phase difference type provides 3.4 to 4.8 times the light intensity compared to the amplitude type. As described above, in the case of the phase type, the light intensity is large and the S/N ratio is good, but this is within a range where the influence of light near the boundary of the Fresnel zone can be ignored. Therefore, in the phase type, it is necessary to increase the zone width in order to increase the number of zones, and it is difficult to construct an optical scanner with a large number of zones at a low voltage.

Next, the Fresnel optical scanner of the present invention and a typical conventional optical deflector will be explained. Compare the resolution with a single prism optical deflector. The resolution N _F of the Fresnel optical scanner of the present invention is given by the following equation for both amplitude type and phase type.

On the other hand, the resolution N _P of a single prism optical deflector is N _P =1.39n ³ γlv/λwln (w/h) (10). Here w and h are as shown in FIG. In a single prism type optical deflector, since w>>h is required structurally, there is a limit to the resolution. On the other hand, in the Fresnel optical scanner of the present invention, the resolution can be improved by increasing the number of zones M. However, since the capacity of the device increases as the number of zones increases, it is subject to restrictions from frequency characteristics.

As is clear from the above, according to the present invention, high-speed optical scanning and light focusing can be performed without a lens.

In the first embodiment of the present invention shown in FIG. 4, the odd-numbered Fresnel zones are striped optical waveguides, and thin electrodes are arranged at the boundaries of the Fresnel zones. The light can be focused at any point on the focal plane of the optical scanner.

Furthermore, in the case of the configuration in which even-numbered Fresnel zones are used as striped optical waveguides, as shown in FIG. 9, when the number of zones is small, the S/N ratio is superior to that of a scanner with odd-numbered apertures. be.

In the embodiment shown in Figure 11, a slab-type optical waveguide is used, in which intersecting electrodes in which positive and negative electrodes are arranged as close as possible to the boundaries of Fresnel zones, and even and odd zones are arranged. The refractive index of the part of the waveguide corresponding to the even or odd zone is changed so that the static phase difference of the light is 180°. Similar to the amplitude type, the monochromatic plane wave can be moved anywhere on the focal plane. The light can be focused on the point.
The light intensity at this time increases 3.4 to 4.8 times that of the amplitude type.

As explained above, since the Fresnel optical scanner of the present invention can scan a monochromatic plane wave, it can be applied to laser displays, optical recording, or connection and distribution of optical fiber signals. Furthermore, since the optical scanner of the present invention is small and lightweight and does not require a lens, it also has the advantage that such applied devices can be constructed in a small size. Further, the scanner of the present invention can be used as an optical modulator by fixing the photodetector in a fixed position. Furthermore, the optical scanner of the present invention can also be applied to optical information processing such as one-dimensional Fourier transform.

[Brief explanation of drawings]

FIGS. 1 to 3 are plan views showing three examples of conventional optical deflectors, FIG. 4 is a plan view showing an example of the Fresnel optical scanner of the present invention, FIGS. 5 A to D, and FIGS. 6 A to 3. D, FIGS. 7A to D, and 8A to D are light intensity distribution diagrams in the focal plane of the Fresnel optical scanner of the present invention, FIG. 9 is a perspective view showing another example of the present invention, and FIG. 10A -C are light intensity distribution diagrams in the focal plane showing an actual measurement example of the optical scanner shown in Fig. 9, Fig. 11 is a plan view showing still another example of the present invention, and Figs. FIG. 3 is a light intensity distribution diagram on the focal plane of an optical scanner. DESCRIPTION OF SYMBOLS 1... Slab type optical waveguide, 2, 3... Parallel electrode, 4... Oblique electrode, 5... Monochromatic plane wave, 6... Equal phase plane, 7... Output light, 21... Slab type optical waveguide, 22A ~22E...Parallel electrode, 23A~23
D...Diagonal electrode, 24... Monochromatic plane wave, 25...
Equal phase plane, 26... Output light, 31A to 31G...
Striped optical waveguide, 32, 33...parallel electrodes, 34...monochromatic plane wave, 35...equal phase plane, 3
6... Output light, 40... Electro-optic crystal substrate, 41
A, 42A, 43A, 44A, 41B, 42B,
43B, 44B...stripe optical waveguide, 4
5, 46...Cross electrode, 47...Monochromatic plane wave, 5
0...Slab type optical waveguide, 51A, 52A, 53
A, 51B, 52B, 53B...portions with different refractive indexes, 54...monochromatic plane wave, 55, 56...cross electrodes.

Claims (1)

[Scope of Claims] 1. A one-dimensional Fresnel strip formed of a thin-film optical waveguide, at least near the boundary of each Fresnel zone of the one-dimensional Fresnel strip, each having the same length.
A Fresnel optical scanner characterized in that pairs of comb-shaped electrodes are arranged alternately along the boundary, and a predetermined voltage is applied between the pair of comb-shaped electrodes. 2. In the Fresnel optical scanner according to claim 1, the thin-film optical waveguide is formed of a plurality of striped optical waveguides, and either the odd-numbered or even-numbered Fresnel zone is an aperture. Features Fresnel optical scanner. 3. In the Fresnel optical scanner according to claim 1, the thin film optical waveguide is a slab type optical waveguide, and the pair of comb-shaped electrodes are a pair of linear electrodes arranged adjacent to each other. A Fresnel optical scanner comprising: