JP2013195915A - Shielding mechanism of ktn optical scanner - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem that the deflection width of a deflection angle is gradually reduced in a KTN light deflector in spite of application of a burst voltage by continuous long-time application of a driving voltage and results in variation in performance of the light deflector to considerably limit the range of use of the light deflector using a KTN crystal.SOLUTION: An operation mechanism of a KTN crystal is provided which has a light shielding structure for shielding, from external light, the KTN crystal and tools and the like required for operating it. A light deflector is provided with a shielding structure which shields an electro-optic crystal like a KTN crystal from light in the band of a wavelength causing light absorption due to a trap level of the electro-optic crystal and wavelengths shorter than the wavelength causing light absorption and has at least one window through which incident light to the electro-optic crystal and emitted light from the electro-optic crystal pass.

Description

  The present invention relates to an optical deflector. More specifically, the present invention relates to a light shielding mechanism of an optical deflector using KTN.

  Currently, there is an increasing demand for light control elements for deflecting laser light in projectors and other video equipment, laser printers, high-resolution confocal microscopes, barcode readers, and the like. Conventionally, a technique for rotating a polygon mirror, a technique for controlling the deflection direction of light by a galvanometer mirror, an optical diffraction technique using an acoustooptic effect, and a micromachine technique called MEMS have been proposed.

In recent years, an optical deflector that deflects light as a result of realizing a space charge control state by injecting charges into an electro-optic crystal to generate an electric field gradient and an electro-optic effect to cause a refractive index gradient has been proposed. (Patent Document 1). Unlike a galvano mirror, a polygon mirror, a MEMS mirror, or the like, this optical deflector using an electro-optic crystal does not have a movable part, so that high-speed light deflection is possible. Examples of the electro-optic crystal include lithium tantalate niobate (KTa _1-x Nb _x O ₃ (0 <x <1): hereinafter referred to as KTN) crystal, or a material having the same effect as KTN. K _1-y Li _y Ta _1-x Nb _x O ₃ (0 <x <1, 0 <y <1) and the like are known. Hereinafter, an optical deflector using KTN will be described as an example of the electro-optic crystal as described above.

  FIG. 6 is a diagram showing a configuration of an optical deflector using KTN. Electrodes 102 and 103 are formed on the upper and lower surfaces of the KTN crystal 101. A control voltage is applied from the control voltage source 104 between the two electrodes. Incident light 105 travels in the z direction from the left end face of the KTN crystal 101 in the drawing, and is deflected in the KTN crystal 101 to obtain outgoing light 106 whose traveling direction is changed in the x-axis direction. A corresponding deflection angle θ is obtained according to the magnitude of the applied voltage from the control voltage source 104.

  Regarding the details of the electro-optic effect of KTN, the relationship between the crystal structure and the refractive index of KTN and the temperature has been analyzed (Non-Patent Document 1). Furthermore, the operation as an optical deflector is also quantitatively analyzed based on the space charge control electro-optic effect peculiar to KTN. KTN has a feature that it can be driven with a smaller driving voltage and a smaller crystal than existing materials. Furthermore, since KTN is a basic device that changes the direction of light, it is considered to be applied not only to optical communication but also to various fields that handle light. Practical application to spectroscopes and medical devices has already begun, and application to the field of processing machines and microscopes is also being studied (Non-Patent Document 1).

  As shown in FIG. 6, it is necessary to apply a control voltage to the KTN crystal 101 in order to deflect the KTN. A mechanism for maintaining the temperature of the KTN crystal at a constant operating temperature is also required. Therefore, in order to drive an optical deflector using a KTN crystal and measure its characteristics, or to operate a device using an optical deflection function, the KTN crystal is physically held under a predetermined condition. The mechanism or jig is used.

  As shown in FIG. 6, when light is deflected by KTN, a drive voltage in a signal format suitable for the application field is applied from the control voltage source 104 to the KTN crystal. Further, in order to perform a high-speed deflection operation in the KTN crystal, it is necessary to apply a burst voltage before applying the drive voltage to inject electrons into the KTN crystal and capture electrons in the trap. preferable. Here, the burst voltage means a voltage signal having a frequency sufficiently low with respect to the frequency of the driving voltage.

  FIG. 7 is a diagram for explaining an example of a method for driving the KTN crystal. In FIG. 7, a positive constant voltage and a negative constant voltage are applied for a certain period of time for the burst voltage, and the amplitude 201 of the voltage is the same for both positive and negative V ′ = 400 V, and the total t of the positive and negative voltage application time 202 is t. 'Is set to 5 msec, for example. When the drive voltage amplitude 203 is a sine wave of V = 400 V and the duration is 30 msec, a sufficient deflection angle swing can be maintained by repeatedly applying the burst voltage and the drive voltage.

  As shown in FIG. 7, by applying a burst voltage before the drive voltage, electrons are injected into the crystal and electrons are trapped in the trap while the burst voltage is applied. Even during the drive voltage application period 204, even if no electrons are injected, the electric field is tilted by the electrons trapped in the trap, and as a result, the refractive index is tilted due to the electro-optic effect. It is possible to realize a simple light deflection.

International Publication No. 2006/137408 Pamphlet

NTT Technical Journal November 2009, p.12-15

  However, even if a burst voltage is applied, if the drive voltage is continuously applied for a long time, there is a problem that the deflection angle swing width gradually decreases. When a driving voltage is applied continuously to the KTN crystal, for example, when a burst voltage is applied at ± 300 V and positive and negative polarities are applied for 10 seconds each, then a driving voltage (200 kHz) of ± 300 V is continuously applied for 10 minutes. When applied, the deflection angle deflection is reduced by about 20% from the start of driving. Further, when applied continuously for 30 minutes, the deflection angle deflection decreased by more than 40% from the start of driving. Reduction of the deflection angle deflection width leads to fluctuations in the performance of the optical deflector, which significantly limits the range of use of the optical deflector using the KTN crystal.

  As described above, in the case of the application of KTN in which the drive voltage is continuously applied at a high speed for a long time, there is a problem that the deflection width of the deflection angle cannot be maintained. In view of the above-described problems, the present invention provides a mechanism and an optical deflector for operating a KTN crystal capable of continuous driving for a long time while maintaining the deflection characteristics of the optical deflector using the KTN crystal during continuous driving. I will provide a.

  In order to achieve the above object, the present invention is an electro-optic crystal in which at least two electrodes are formed on opposing surfaces, and a control voltage is applied to the electrodes. In the optical deflector using an electro-optic crystal that deflects incident light that is incident substantially perpendicular to the electric field formed by the control voltage by generating a gradient of the internal refractive index distribution by the electro-optic effect. A shielding structure that shields light in a wavelength shorter than a wavelength causing light absorption due to a trap level of the electro-optic crystal and a wavelength causing the light absorption with respect to the optical crystal, An optical deflector comprising a shielding structure having at least one window through which light incident on an electro-optic crystal and light emitted from the electro-optic crystal pass.

The invention according to claim 2 is the optical deflector according to claim 1, wherein the electro-optic crystal is a lithium tantalate niobate (KTa _1-x Nb _x O ₃ (0 <x <1)) crystal, or lithium K _1-y Li _y Ta _1-x Nb _x O ₃ (0 <x <1, 0 <y <1).

  A third aspect of the invention is the optical deflector of the first aspect, wherein the band blocked by the shielding structure includes visible light having a wavelength shorter than 750 nm.

  The invention according to claim 4 is the optical deflector according to claim 1, wherein the electro-optic crystal is opposed to a non-reflecting surface formed on a surface on which incident light is incident and the electro-optic crystal is opposed to the non-reflecting surface. A high reflection surface formed on the surface, and the shielding structure has one window through which the incident light and the outgoing light reflected from the high reflection surface and emitted from the electro-optic crystal pass. Features.

  The invention according to claim 5 is the optical deflector according to claim 1, wherein the window of the shielding structure transmits only the wavelength of the target light to be deflected or transmits the wavelength of the target light. A band-pass filter that removes wavelengths shorter than the wavelength of the target light is formed.

  As described above, according to the present invention, there is provided a mechanism for operating a KTN crystal capable of continuous driving for a long time while stabilizing the deflection characteristics of the optical deflector using the KTN crystal during continuous driving. provide.

FIG. 1 is a conceptual diagram showing a configuration of a KTN optical deflector having a light shielding structure of the present invention. FIG. 2 is a diagram showing a light absorption spectrum of a KTN crystal. FIG. 3 is a diagram showing the time dependence of the deflection angle of the KTN optical deflector having the light shielding structure. FIG. 4 is a diagram illustrating a configuration of a KTN crystal in the KTN optical deflector according to the first embodiment. FIG. 5 is a diagram illustrating a configuration of a KTN optical deflector having the shielding structure of the first embodiment. FIG. 6 is a diagram showing a configuration of an optical deflector using KTN. FIG. 7 is a diagram for explaining an example of a method for driving the KTN crystal.

  The present invention provides a light shielding mechanism for a KTN crystal, which is provided with a light shielding structure for shielding a KTN crystal and jigs necessary for operating the crystal from light from the outside. More specifically, the optical deflector has a wavelength that causes light absorption caused by the trap level of the electro-optic crystal and a wavelength shorter than the wavelength that causes light absorption for an electro-optic crystal such as a KTN crystal. And a shielding structure having at least one window that allows light incident on the electro-optic crystal and light emitted from the electro-optic crystal to pass therethrough. The shielding structure includes not only a box having a light shielding effect covering KTN and the entire jig, but also a structure made of a material formed on the KTN crystal. The inventors have empirically noted that visible light striking the KTN crystal from the outside influences the deflection width of the deflection angle.

  In the present invention, the term jig means a mechanism or structure that physically holds the KTN crystal. Furthermore, the jig includes a circuit / mechanism (electrical wiring for driving voltage, temperature control circuit, etc.) for performing a deflection operation by KTN. Thus, the jig can also be part of the mechanism in the device that contains the KTN crystal that utilizes the light deflection function. It can also be a mechanism for use in experimental applications for evaluating the deflection characteristics of KTN crystals.

  FIG. 1 is a conceptual diagram showing a configuration of a KTN optical deflector having a light shielding structure of the present invention. A jig 1 including a KTN crystal shown as a rectangular parallelepiped is fixed on an adjustment table 2. The jig 1 includes a KTN crystal and all mechanisms for holding and operating the KTN crystal, and is conceptually represented as a rectangular parallelepiped. The adjusting table 2 is for adjusting the vertical position of the incident light to the KTN crystal in the jig 1 or the outgoing light from the KTN crystal, and is not essential. The jig 1 may include a mechanism capable of adjusting the height.

  The KTN optical deflector of the present invention is characterized in that the entire jig 1 is covered with a shielding box 3 that is a shielding structure. The shielding box 3 may be made of any material as long as it has an effect of shielding at least visible light and light having a shorter wavelength than visible light. The shielding box 3 has an incident window 6 for entering light into the KTN crystal and an exit window 7 for emitting deflected light. Furthermore, it has a wiring window 8 for inputting and outputting wiring for applying a control voltage to the KTN crystal and wiring 7 for temperature control of the entire KTN crystal and the jig.

  As will be described later, depending on the configuration of the optical path for deflection in the KTN crystal, the entrance window 6 and the exit window 7 can be used as a single window. Further, the wiring window 8 can be disposed on the lower surface of the adjustment table 2 through the inside of the adjustment table 2 from directly below the jig 1 in order to suppress the incidence of visible light or the like as much as possible.

  With the adjusting table 2, the position of the incident light 1 on the KTN crystal included in the jig 1 can be adapted to the incident window 6, and the positions of the deflected outgoing lights 5 a and 5 b can be adapted to the outgoing window 6. The shapes and sizes of the entrance window 6 and the exit window 7 do not interfere with the incident light 4 and the exit lights 5a and 5b and should be as small as possible. (I want an idea for the next limited section)

  FIG. 2 is a diagram showing a light absorption spectrum of a KTN crystal. The horizontal axis indicates the wavelength, and the vertical axis indicates the absorption coefficient α. As is clear from FIG. 2, it is essential to block light having a wavelength shorter than the band gap wavelength (400 nm) in the KTN crystal. This is because light having a wavelength shorter than the band gap wavelength is changed from the state of normal deflection operation by electrons excited by the light. Naturally, it is sufficient to shield light having a band gap wavelength or less, as long as visible light in the range of 400 to 750 nm in which light absorption that is considered to be caused by the trap level remains can be shielded. Therefore, the material of the shielding box 3 may be metal, glass, plastic, acrylic or the like provided with a band filter capable of shielding light having a wavelength of 750 nm or less.

  FIG. 3 is a diagram showing the time dependence of the deflection angle of the KTN optical deflector having the light shielding structure. It is shown in comparison with a case without a light shielding structure. A burst voltage (± 300 V, 10 seconds) is applied to a KTN crystal (capacitance 1.71 nF, relative dielectric constant εr = 15000), and then an alternating voltage of ± 300 V and 200 kHz is continuously applied as a drive voltage. In this case, the time variation of the maximum deflection width of the deflection angle is shown. In the case of not having the light shielding structure, the deflection angle deflection width decreased by 40% or more in 30 minutes, whereas in the KTN optical deflector of the present invention having the light shielding structure, the deflection angle deflection width was several percent. It is suppressed to decrease. Therefore, the present invention can be practically and sufficiently applied to applications in which the KTN optical deflector is continuously driven.

  Next, a more specific configuration of the KTN optical deflector having a light shielding structure will be described.

  In the present embodiment, a configuration in which the light incident window and the light exit window are combined into one will be described. That is, an electro-optic crystal such as a KTN crystal has a non-reflective surface formed on a surface on which incident light is incident, and a highly reflective surface formed on a surface facing the non-reflective surface of the electro-optic crystal, The shielding structure has one window through which incident light and outgoing light reflected from the highly reflective surface and emitted from the electro-optic crystal pass. First, the configuration of this KTN crystal used in this example will be described.

  FIG. 4 is a diagram illustrating a configuration of a KTN crystal in the KTN optical deflector according to the first embodiment. The incident light 4 enters the KTN crystal 10 from the back of the drawing to the front (z direction) in FIG. After the incident light 4 is deflected in the KTN crystal, it is emitted from the same plane as the incident light. In FIG. 4, electrodes 11a and 11b for applying a driving voltage are formed on the upper and lower surfaces of the KTN crystal 10 to cause deflection in the x-axis direction.

  The back surface where light enters and exits is a non-reflective coated surface 12 (AR surface: Anti-Refection). On the other hand, the front surface is a highly reflective coated surface 13 (HR surface: High Reflection). With the configuration having the AR surface 12 and the HR surface 13, incident light incident on the AR surface 13 is deflected in the KTN crystal and reflected by the HR surface 13. The reflected light is further deflected again in the KTN crystal and is emitted from the AR plane 12.

  FIG. 5 is a diagram illustrating a configuration of a KTN optical deflector having the shielding structure of the first embodiment. The KTN crystal shown in FIG. 4 is used. FIG. 5A is a view of the surface including the deflection surface from the side surface of the KTN crystal. (B) is the figure which looked at the AR surface 12 of the KTN crystal | crystallization from the light source (not shown) side of incident light. (C) is the figure which looked at the surface parallel to the application electrodes 11a and 11b of the drive voltage of a KTN crystal | crystallization. In FIG. 5, the KTN crystal is drawn in detail, but the jig 1 and the adjustment table 2 including the KTN crystal are drawn in a simple rectangular parallelepiped. The jig 1 and the adjustment stand 2 are entirely covered by a light shielding box 3 that is a shielding structure. In this embodiment, as clearly shown in FIG. 5B, the light shielding box 3 is provided with one window 20 through which light enters and exits. Further, the KTN optical deflector of the present embodiment includes a mirror 21 that changes the direction of the outgoing lights 5a and 5b.

  The overall size of the jig 1 and the adjusting table 2 is approximately 3 mm square. The size of the shielding box 3 is approximately 6 mm square.

  The incident light 4 passes through the window 20 and enters the KTN crystal from the AR plane 12. The light traveling in the KTN crystal is deflected in the x-axis direction by the drive voltage applied to the electrodes 11a and 11b. The light deflected in the KTN crystal is reflected by the HR plane 13 and travels in the reverse direction on the z axis, and is again deflected in the KTN crystal. The deflected light is then emitted from the AR surface 12, passes through the window 20, is further changed in the direction of travel by the mirror 21, and is used as emitted light 5a, 5b. As can be seen from FIG. 5 (c), when the incident light 4 is incident with a slight inclination from the z-axis, the outgoing lights 5a and 5b are also inclined and emitted from the KTN crystal, and thus interfere with the incident light 4. The outgoing lights 5a and 5b can be easily extracted without any problems.

  By using the KTN crystal having the configuration shown in FIG. 4 and using one light input / output window of the light shielding box 3, the light shielding box 3 can be used more effectively. In FIG. 5, a wiring window 8 for taking in and out the wiring 9 for application of control voltage and temperature control is provided on the side surface of the light shielding box 3. The wiring window 8 can be formed on the bottom surface. In the configuration of the optical path of the optical deflector shown in FIG. 5, the total reflection mirror 21 is used, but a different optical path may be formed using a half mirror or the like. In this case, the incident light 4 is not necessarily incident obliquely.

  The shape of the window 20 may be small as long as the incident light 4 and the outgoing lights 5a and 5b can be input and output without interference. That is, the length of the window 20 in the x-axis direction is a length corresponding to the maximum deflection angle determined by the relationship between the position of the KTN crystal on the z-axis and the position of the window 20 and the maximum deflection angle of the emitted light. Just do it.

  According to the KTN optical deflector having the configuration of the present embodiment, the effect of the light shielding box can be enhanced by using one window for inputting and outputting light.

  In the first embodiment, a single window is formed in the shielding box, and the window 20 uses a gap formed by physically removing a part of the shielding box 3. However, any window that has a shielding effect against light in a predetermined wavelength band may be used. Therefore, if a band-pass filter that selectively transmits only the light to be deflected is disposed in the window 20 in the optical deflector shown in FIG. 5, further reduction in the deflection width of the deflection angle due to visible light or the like is suppressed. be able to. In other words, it is realized by forming a filter that transmits only the wavelength of the target light to be deflected or a bandpass filter that transmits the wavelength of the target light and removes a wavelength shorter than the wavelength of the target light in the window of the shielding structure. it can.

  An equivalent effect can also be obtained by forming on each surface of the KTN crystal a material having a filter function that selectively transmits only the wavelength of light to be deflected with respect to the KTN crystal itself. It is done. A surface to which a control voltage is applied in the KTN crystal (corresponding to the yz plane in FIG. 3) is not necessary because an electrode is formed, but two opposing planes (x- For the y-plane), a material partially having a filter function can be formed except for a portion necessary for light input / output. Furthermore, a material having a filter function can be formed over the entire two opposing surfaces parallel to the deflection surface (corresponding to the xz plane in FIG. 3). Therefore, the shielding box 3 in Example 1 is for the entire jig including the KTN crystal, but is formed of a material having a filter action that is directly formed on the KTN crystal or arranged in contact with the KTN crystal. Note that it can be extended to objects (including membranes, layers).

  As described above in detail, according to the present invention, an optical deflector using a KTN crystal stabilizes and maintains the deflection characteristics during continuous driving, and a KTN crystal capable of continuous driving for a long time is obtained. A mechanism to operate can be provided.

  The present invention can be used for optical instruments. In particular, it can be used for an optical instrument using a KTN crystal.

DESCRIPTION OF SYMBOLS 1 KTN crystal and jig 2 Adjustment stand 3 Light shielding box 6, 7, 8, 20 Window 9 Wiring 10, 101 KTN crystal 11a, 11b, 102, 103 Electrode 12 AR surface 13 HR surface 21 Mirror 104 Control voltage source

Claims (5)

An electro-optic crystal in which at least two electrodes are formed on opposite surfaces, and formed by the control voltage by applying a control voltage to the electrodes and generating a gradient of the internal refractive index distribution by the electro-optic effect. In an optical deflector using an electro-optic crystal that deflects incident light that is incident substantially perpendicular to the electric field generated,
A shielding structure that shields light of a wavelength that causes light absorption due to a trap level of the electro-optic crystal and light in a shorter wavelength side than the wavelength that causes light absorption with respect to the electro-optic crystal. An optical deflector comprising: a shielding structure having at least one window through which incident light to the electro-optic crystal and outgoing light from the electro-optic crystal pass. The electro-optic crystal is a lithium tantalate niobate (KTa _1-x Nb _x O ₃ (0 <x <1)) crystal or K _1-y Li _y Ta _1-x Nb _x O ₃ (to which lithium is added. 2. The optical deflector according to claim 1, wherein either 0 <x <1 or 0 <y <1).   The optical deflector according to claim 1, wherein the band blocked by the shielding structure includes visible light having a wavelength shorter than 750 nm. The electro-optic crystal has a non-reflective surface formed on a surface on which incident light is incident, and a highly reflective surface formed on a surface facing the non-reflective surface of the electro-optic crystal,
2. The optical deflector according to claim 1, wherein the shielding structure has one window through which the incident light and the outgoing light reflected from the highly reflective surface and emitted from the electro-optic crystal pass.   A filter that transmits only the wavelength of the target light to be deflected or a bandpass filter that transmits the wavelength of the target light and removes a wavelength shorter than the wavelength of the target light is formed in the window of the shielding structure. The optical deflector according to claim 1.

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