JP6386964B2 - Optical deflector - Google Patents

Description

  The present invention relates to an optical deflector. More specifically, the present invention relates to an optical deflector that uses a change in refractive index distribution inside an electro-optic crystal due to an electro-optic effect, and the amount of light deflection is uniform in the cross section of the light beam. The present invention relates to an optical deflector.

  2. Description of the Related Art Conventionally, an optical deflector, which is an optical device that changes the direction of light, is used in various fields such as a display element of a projector, a laser printer, a high-resolution confocal microscope, and a barcode reader.

  As an optical deflector, a technology that rotates a polygon mirror, a technology that controls the deflection direction of light using a galvanometer mirror, and a device that uses MEMS (Micro Electro Mechanical System) are widely used. In these systems, since the mirror is mechanically driven, there is a limit to light deflection at high speed.

  Therefore, an optical switch or an optical deflector using a material having an electro-optic effect that can be deflected by applying an electric field has been devised.

  When a voltage is applied to the electro-optic crystal, the refractive index of the crystal changes due to the electro-optic effect, and the beam can be deflected.

  In recent years, in crystals that exhibit a specific electro-optic effect, a space charge distribution is formed in the crystal when a voltage is applied, and the resulting non-uniform electric field distribution induces a gradient of refractive index, and the path of light rays that are orthogonal to this gradient. The phenomenon of bending is found.

  When an electrode that is in ohmic contact with the electro-optic crystal is formed and a voltage is applied to the electrode, charge is injected into the electro-optic crystal, a space charge control state is established, and an electric field is tilted inside the crystal. The gradient of the electric field can cause a gradient of the refractive index and deflect the beam of light that passes through the crystal (see, for example, Patent Document 1 below).

  In such an electro-optic crystal, all the places in the crystal have a deflection action, so the actions at various places on the light propagation path are accumulated, and the deflected light is emitted from the crystal. As a result, high speed operation is possible and the change in deflection angle can be greatly increased.

(Configuration of conventional planar light deflector)
As one form of a conventional optical deflector utilizing the refractive index gradient in the electro-optic crystal, a KTN (KTa _1-x Nb _x O ₃ , 0 ≦ x ≦ 1) crystal or a KLTN in which lithium is added to a KTN crystal An electrode as shown in FIG. 1 is formed using (K _1-y Li _y Ta _1-x Nb _x O ₃ , 0 ≦ x ≦ 1, 0 ≦ y ≦ 1) crystal (hereinafter also referred to as KTN crystal). A planar light deflector arranged on the same crystal plane has been devised to realize a light deflection operation (see Non-Patent Document 1 below).

  In the conventional optical deflector using the electro-optic crystal of FIG. 1, the light beam of the incident light 106 is incident in the y-axis direction from the center of the front side surface of the flat electro-optic crystal substrate 100, and the opposite side surface of the substrate To emit a light beam deflected in the x-axis direction within the substrate surface.

  On the upper surface of the electro-optic crystal substrate 100, on the light beam path of the incident light 106, an electrode pair composed of two electrodes 101, 102 is provided across the path, and a voltage is applied between the two electrodes 101, 102. As a result, an electric field is generated in the electro-optic crystal 100, and the exit direction of the incident light 106 can be deflected as shown in the drawing using the refractive index gradient generated in the electro-optic crystal by the electric field.

  In this conventional planar light deflector, the crystal thickness in the direction orthogonal to the deflection is reduced under the electrodes, and the distance between the electrodes necessary for maintaining the beam diameter is secured, so the electrodes are arranged on the opposing surface. Compared to a conventional optical deflector, it has a feature that the capacitance of the optical deflector can be greatly reduced while maintaining the same deflection angle.

  For this reason, the dielectric loss during high-frequency operation can be greatly reduced, and the heat generated by the dielectric loss can be greatly reduced.

  Note that the dielectric constant of the KTN crystal greatly depends on the temperature, and since the deflection characteristics are greatly influenced by the dielectric constant, it is important to keep the temperature of the KTN crystal constant during the deflection operation in order to obtain a stable operation. is there. From this point, it can be said that the planar light deflector is an optical deflector suitable for high-frequency operation.

International Publication No. 2006/137408 Pamphlet JP 2012-150409 A

Atsuko Atsumi, 5 others “New structure KTN planar light deflector for high frequency operation” The 62nd JSAP Spring Meeting, 11p-A13-5, March 2015 Shogo Yagia et.al., “Improvement of Coherence Length in a 200-kHz Swept Light Source equipped with a KTN Deflector” Proc. Of SPIE Vol. 8213, Optical Coherence Tomography and Coherence Domain Optical Methods in Biomedicine XVI, 821333 (February 9 , 2012) J. Miyazu et al., “New beam scanning model for high-speed operation using KTa1-xNbxO3 Crystals”, APEX, Vol. 4, Issue 11, pp. 115101-1-111501-3, 2011.

  On the other hand, the deflection amount in the beam cross section of the deflected light beam by the conventional planar light deflector reflects the non-uniformity in the z-axis direction of the refractive index gradient in the crystal as the crystal thickness increases. There was a problem that it was not uniform.

(Electric field distribution inside a conventional planar light deflector)
FIG. 2 shows the distribution of the electric field strength in the x-axis direction, as seen from the beam cross section (xz plane) of the incident light 106 of the conventional planar light deflector of FIG. It can be seen that the electric field strength in the x-axis direction decreases with increasing distance from the electrode surface in the z-axis direction of the substrate thickness, and the electric field strength also changes in the beam cross-sectional area of the central incident light.

  This tendency becomes conspicuous as the crystal thickness increases, and the deflection angle in the beam cross-sectional area varies, which limits the use of large incident beam light.

  For this reason, for example, a laser printer or the like has a problem that the beam spot shape is changed by the optical deflector in the surface of the photosensitive member, and the exposure state is deteriorated.

  Further, for example, in the wavelength swept light source described in Non-Patent Document 2, the wavelength selectivity becomes non-uniform within the beam cross section, that is, the wavelength selectivity becomes gradual, and stable laser oscillation is difficult to obtain. There was also a problem.

When the electron density filled in the trap of the KTN crystal is N _trap , the deflection angle obtained when the optical signal passes through the KTN crystal is expressed by the following formula (1) (Non-patent Document 3).

In the above equation, the deflection angle θ is the maximum deflection angle deflection width in the x-axis direction when a sine wave is applied as a deflection voltage. n is the refractive index of KTN, and L is the length of the electrode in the y-axis direction in FIG. g ₁₁ is an electro-optic constant, e is an elementary electric quantity, and ε is a dielectric constant.

  E is the electric field strength in the x-axis direction inside the crystal. Since the deflection amount is proportional to the electric field as expressed by the above equation, when the electric field strength E is distributed in the beam cross section, the deflected beam after passing through the deflector must not be uniformly deflected in the cross section. become.

  When deflecting light using electron injection into an electro-optic crystal, there is a lens effect that collects the deflected light beam even when the electric field strength is uniform, but a compensation lens must be placed outside the crystal. Thus, it is possible to correct the beam shape. (Patent Document 2)

  However, when the electric field strength is non-uniform in the beam cross section orthogonal to the deflection, the deflection amount itself is not constant in the beam cross section.

  Since the deflection amount continuously changes in the beam cross section, the amount of lens effect differs at each point in the beam cross section, that is, aberration is generated.

  In order to correct the aberration due to the lens effect and the nonuniformity of the deflection amount in the additional optical system, there is a problem that a complicated and large-scale lens configuration is required.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a non-uniform deflection amount in the deflection beam section in an optical deflector that causes non-uniform light deflection in the deflection beam section. An object of the present invention is to provide an optical deflector with compensated characteristics.

In order to achieve such an object, the present invention provides the invention described in claim 1,
An optical deflector that enters a light beam from one side surface of an electro-optic crystal substrate and emits a light beam deflected in the direction of the substrate surface from the opposite side surface,
An electrode pair consisting of two electrodes is provided on the substrate surface on the light beam path on the incident side, with the light beam path interposed therebetween,
On the opposite substrate surface under the light beam path on the emission side, an electrode pair consisting of two electrodes is provided with the light beam path in between,
1 or more setting an electrode structure composed of the two electrode pairs along the optical beam path,
A voltage is applied to the two electrodes constituting the electrode pair such that all electrode pairs generate an electric field in the same in-plane direction,
2. The optical deflector according to claim 1, wherein the two electrode pairs constituting the electrode structure are arranged so as not to overlap each other when viewed from the normal direction of the substrate surface along the light beam path .
It is what.

  The optical deflector of the present invention having the above configuration keeps the deflection amount of the deflected light beam uniform in the beam cross section by using two or more optical deflectors having electrode pairs arranged on the same plane. be able to.

It is a figure which shows the structure of the conventional planar light deflector. It is a figure which shows the electric field distribution inside the conventional planar light deflector. It is a figure which shows the structure of the optical deflector concerning one Embodiment of this invention. It is a side view of the optical deflector concerning one Embodiment of this invention. 1 is a diagram illustrating a configuration of an optical deflector according to Example 1. FIG. It is a figure which shows the deflection amount distribution of the crystal | crystallization thickness direction in Example 1 in contrast with a prior art example. 6 is a diagram illustrating a configuration of an optical deflector according to Embodiment 2. FIG. FIG. 6 is a diagram illustrating a configuration of an optical deflector according to a third embodiment.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(Embodiment of optical deflector of the present invention)
FIG. 3 is a diagram showing a configuration of an embodiment of the optical deflector of the present invention.

  4 is a side view corresponding to the cross section of the substrate on the yz plane including the direction of the light beam of the incident light 306 in the optical deflector of the present invention shown in FIG.

In this embodiment, an electro-optic effect crystal substrate 300 is used as an optical deflector.
As the electro-optic crystal, for example, an electro-optic crystal as disclosed in Patent Document 2 can be used.

  As described in Patent Document 2, in the KTN crystal and the KLTN crystal doped with lithium, charge is injected into the crystal in accordance with the electric field by applying voltage.

  As an electrode for applying a voltage, an electrode pair comprising two metal electrodes 302 and 303 on the light beam path on the incident side of the substrate surface of the electro-optic crystal substrate 300 (the upper surface of the substrate in FIG. 3) with the incident light path interposed therebetween. Is provided. Similarly, an electrode pair composed of two metal electrodes 304 and 305 is provided on the emission side below the light beam path on the substrate surface opposite to this surface (the lower surface of the substrate in FIG. 3).

  The structure consisting of these two electrode pairs is put together to constitute one electrode structure.

  A voltage is applied between the electrodes constituting the electrode pair in order to inject electric charges into the crystal or to deflect light with respect to each electrode pair constituting these electrode structures. When a voltage is applied, a space charge distribution formed by injected charges or a trap charge distribution generated by trapping the injected charges in the electro-optic crystal is generated in the crystal.

  A non-uniform electric field distribution due to the charge distribution causes a refractive index gradient, and a phenomenon occurs in which the path of the light beam orthogonal to the gradient is bent.

  When the light beam of the incident light 306 is incident from the center of the front side surface of the electro-optic crystal substrate 300 in the y-axis direction in the xy plane as shown in the figure, the electro-optic is generated by the voltage applied between the electrodes 302 and 303 constituting the electrode pair. The incident light 306 is deflected in the x-axis direction by the electric field in the x-axis direction generated in the crystal substrate.

  However, as described above with reference to FIG. 2 regarding the conventional example, the electric field strength in the x-axis direction depends on the position in the z-axis direction (substrate thickness direction), and the electric field strength decreases with increasing distance from the electrode even in the same yz plane. End up. As a result, in the light deflection only by the electrodes 302 and 303 constituting the electrode pair on the incident side, the deflection amount is not constant within the light beam cross section, and the cross sectional shape of the light beam changes.

  In order to correct this, in the present embodiment, after the light beam of the incident light 306 is deflected by the electric field between the electrodes 302 and 303, the light beam is formed on the emission side of the opposite surface (the back surface and the bottom surface of the substrate in FIG. 3) of the electrodes 302 and 303. Further deflection is caused by the electric field between the electrode pair composed of the electrodes 304 and 305.

  The electrodes 304 and 305 are formed on the surface opposite to the electrodes 302 and 303, but the applied voltage is applied so that an electric field is generated between the electrodes 302 and 303 and between the electrodes 304 and 305 in the same direction in the x-axis direction. As a result, the deflection direction when the light beam 306 is transmitted between the electrodes 302 and 303 is the same as the deflection direction when the light beam 306 is transmitted through the electrodes 304 and 305, and the amount of deflection can be increased additionally.

  The amount of deflection of the incident light within the beam cross section increases in the vicinity of the electrode and decreases with increasing distance from the electrode in the z-axis direction. Therefore, when the beam is transmitted between the electrodes 302 and 303, the amount of deflection is larger in the vicinity of the upper surface where the electrodes 302 and 303 are formed in the beam cross section than in the vicinity of the lower surface.

  On the other hand, when the beam is transmitted between the electrodes 304 and 305, the deflection amount in the vicinity of the lower surface on which the electrodes 304 and 305 are formed becomes larger than that in the vicinity of the upper surface. That is, by reversing the deflection amount distribution in the beam section between the first half and the second half of the optical deflector, the deflection amount distribution in the beam section can be canceled and the uniformity of the deflection amount can be improved.

  In the above embodiments, two pairs of electrodes are formed at different positions along the incident light path on the opposite surfaces on the same electro-optic crystal substrate. However, the intent of the present invention is to provide a deflection amount within the beam cross section. It is to use light deflection whose distribution is reversed. In other words, two or more individual electro-optic crystal substrates can be alternately arranged along the beam path so that the electrode surfaces are opposite to each other, and it is possible to improve the deflection uniformity within the beam section by the same light deflection operation. It is.

  Next, a more specific example will be described regarding the above-described optical deflector of the present invention.

[Example 1]
A first embodiment of the present invention will be described with reference to FIG.

In the first example, an optical deflector using a KTN crystal (KLTN crystal) was produced. Here, the size of the KTN crystal substrate 500 was processed to be 4 × 3.2 × 0.3 mm ^3, and Ti / Pt / Au electrodes were vapor-deposited on the front and back surfaces of 4 × 3.2 mm ² to prepare two sets of electrode pairs. The electrode size is ² × 1 mm ² , and the distance between the electrodes on the same surface is 1 mm.

  In the size of this electrode pair, there is no overlapping portion of the two pairs of electrode pairs along the optical system path, but it is of course possible to partially overlap each other.

  The crystal was adjusted to have a dielectric constant of 17,500 in the cubic region.

  In order to inject electrons into the crystal and capture them at the trap site, the electrodes 503 and 505 were grounded, and a DC voltage was applied to the electrodes 502 and 504 for 10 seconds each. As for the amplitude of the DC voltage, a voltage of +350 V was applied for 10 seconds, and then a voltage of -350 V was applied for 10 seconds. Subsequently, a sinusoidal AC voltage having an amplitude of 350 V and a frequency of 200 kHz was applied to each electrode pair. The applied AC voltage was in phase so that the deflection angle by the two electrode pairs was maximized.

  As incident light 506, a polarized light beam whose electric field vibration is parallel to the x-axis direction is incident along the y-axis at the center of the z-direction substrate thickness on the side surface of the crystal substrate and the center between the x-direction electrodes. Incident light was deflected in the range of ± 16.5 mrad (= ± 0.95 °) in the x-axis direction in the xy plane as it propagated through the crystal.

(Distribution of deflection amount in the crystal thickness direction)
FIG. 6 is a diagram showing the deflection amount distribution in the crystal thickness direction at the center between the electrodes in comparison with the conventional configuration and the present embodiment. In the conventional configuration, a set of electrode pairs is produced on the same surface, and the size is 4 × ¹ mm ² . A voltage similar to that in this example was applied between the electrodes. The electrode surface of the conventional configuration is the origin in the thickness direction in the crystal.

  In the conventional configuration, the electric field intensity distribution decreases in the thickness direction, the deflection amount in the beam cross section also decreases, and the non-uniformity is remarkable (in-plane distribution of deflection amount: 3 mrad). A uniform deflection amount (in-plane distribution of deflection amount: 0.6 mrad) was obtained in the crystal thickness direction.

  In the conventional structure, the electric field strength distribution decreases in the thickness direction, the amount of deflection in the beam cross section also decreases, and the non-uniformity is remarkable, but in this embodiment, a uniform amount of deflection is obtained in the crystal thickness direction. You can see that

[Example 2]
FIG. 7 illustrates a second embodiment of the present invention.

  In the second embodiment, an optical deflector using two separate KTN crystal (KLTN crystal) substrates 710 and 720 was manufactured.

Here, each of the KTN crystals 710 and 720 was processed to have a size of 2 × 3.2 × 0.3 mm ^3, and Ti / Pt / Au was vapor-deposited on a 2 × 3.2 mm ² surface to prepare two pairs of electrodes 712, 713, 724, and 725. The electrode size is ² × 1 mm ² , and the distance between the electrodes on the same surface is 1 mm.

  In Example 2, since two separate KTN crystals (KLTN crystals) 710 and 720 can be independently controlled in temperature, two KTN crystals with slightly different crystal compositions K, Li, Ta, and Nb ( KLTN crystal) can be used.

  In Example 2, the temperatures were individually adjusted so that the dielectric constants of the two KTN crystals 710 and 720 were 17,500 in the cubic region, respectively. The two KTN crystals are arranged in series at an interval of 0.5 mm in the light propagation direction so that incident light is transmitted through the center of the substrate width x-direction electrode pair and the center of the z-direction substrate thickness when no deflection operation is performed. .

Also in Example 2, in order to inject electrons into the crystal and capture them at the trap site, the electrodes 713 and 725 were grounded, and a DC voltage was applied to the electrodes 712 and 724 every 10 seconds.
As for the amplitude of the DC voltage, a voltage of +350 V was applied for 10 seconds, and then a voltage of -350 V was applied for 10 seconds. Subsequently, a sinusoidal AC voltage having an amplitude of 350 V and a frequency of 200 kHz was applied to each electrode pair. The applied AC voltage was in phase so that the deflection angle by the two electrode pairs was maximized.

  As incident light 706, polarized light whose electric field vibration is parallel to the x-axis direction is incident. In this example, as in Example 1, a uniform deflection amount was obtained in the crystal thickness direction.

[Example 3]
FIG. 8 illustrates a third embodiment of the present invention.

  In Examples 1 and 2, the amount of deflection is made uniform in the beam cross section by the electrode structure composed of two pairs of electrodes. However, two or more pairs of electrodes may be produced and a plurality of electrode structures may be provided. .

  As shown in FIG. 8, odd-numbered electrodes 802 and 803 counted from the incident side were formed on the upper surface, and even-numbered electrodes 804 and 805 were formed on the lower surface (back surface) as a pair.

In the configuration of the third embodiment, since the length of each electrode in the y-axis direction can be shortened compared to the first and second embodiments, the deviation of the deflection amount in the z-axis direction per individual electrode pair can be reduced. Since it becomes possible to cancel the deviation accurately, it is possible to further improve the uniformity of the deflection amount.
As in Examples 1 and 2, improvement in deflection amount uniformity within the beam cross section was confirmed.

  In the configuration of Example 3, a plurality of electrode structures are formed on both surfaces of a single electro-optic crystal substrate 800. However, the electro-optic crystal substrate 800 is composed of a plurality of separate substrates, A pair of electrode structures having a pair of electrodes on the electro-optic crystal substrate, or a pair of two electrode pairs on the front and back sides, that is, the optical deflectors of the first and second embodiments are provided on the light beam path. Of course, it may be configured in the form of a plurality of arrangements.

100, 300, 500, 710, 720, 800 electro-optic crystal substrate
101,102,302,303, 502,503, 712,713, 802,803 Electrode (surface, upper surface side)
304,305, 504,505, 724,725, 804,805 Electrodes (back side, bottom side)
106, 306, 506, 706, 806 Incident light, light beam

Claims (6)

An optical deflector that enters a light beam from one side surface of an electro-optic crystal substrate and emits a light beam deflected in the direction of the substrate surface from the opposite side surface,
An electrode pair consisting of two electrodes is provided on the substrate surface on the light beam path on the incident side, with the light beam path interposed therebetween,
On the opposite substrate surface under the light beam path on the emission side, an electrode pair consisting of two electrodes is provided with the light beam path in between,
One or a plurality of electrode structures comprising the two electrode pairs are provided along the light beam path ,
A voltage is applied to the two electrodes constituting the electrode pair such that all electrode pairs generate an electric field in the same in-plane direction,
The optical deflector, wherein the two electrode pairs constituting the electrode structure are arranged so as not to overlap each other when viewed from the normal direction of the substrate surface along the light beam path . The optical deflector according to claim 1 .
An optical deflector characterized in that an electro-optic crystal substrate is configured separately for each electrode pair or electrode structure. The optical deflector according to claim 1 or 2 ,
The center of the light beam which injects into the said optical deflector is located in the center of the board | substrate surface direction of the two electrodes which comprise an electrode pair, and is located in the center of a board | substrate thickness direction, The optical deflector characterized by the above-mentioned. The optical deflector according to any one of claims 1 to 3 ,
An optical deflector characterized in that all the intervals between two electrodes constituting the electrode pair are equal, and all the intervals in the light beam incident direction of the electrode pair are also equal. The optical deflector according to any one of claims 1 to 4 , wherein
An optical deflector, wherein the electrode is made of a material that makes an ohmic contact with a carrier that contributes to electrical conduction of an electro-optic crystal. The optical deflector according to any one of claims 1 to 5 ,
The electro-optic crystal constituting the electro-optic crystal substrate is a potassium tantalate niobate (KTa _1-x Nb _x O ₃ (0 ≦ x ≦ 1)) crystal, K _1-y Li _y Ta _1- added with lithium. _{x Nb x O 3 (0 ≦} x ≦ 1,0 ≦ y ≦ 1) crystal, (Pb, La) (Zr , Ti) a light deflector, characterized in that either of the O _3.