JP3526155B2 - Waveguide type light modulator - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type optical modulator for forming an optical waveguide on a substrate having an electro-optical effect, and applying a voltage to the optical waveguide to modulate guided light. The present invention relates to a waveguide type optical modulator having a wavelength conversion function.

[0002]

2. Description of the Related Art Conventionally, various optical modulators have been provided, for example, for modulating recording light based on an image signal in an optical scanning recording apparatus.

As one of the light modulation elements, as disclosed in Japanese Patent Laid-Open No. 4-333835, LiNb _x Ta _1-x O ₃ (0 ≦ x has an electro-optical effect).
≦ 1) A pair of optical waveguides forming a directional optical coupler are formed on a Z-cut (cut in a plane orthogonal to the Z axis) substrate of a crystal or a substrate doped with MgO, and the vicinity of these optical waveguides is formed. There is also known a waveguide type optical modulation element in which an electrode for applying a voltage is formed on the optical waveguide.

Further, in Japanese Patent Laid-Open No. 4-333835,
It is also described that a substrate having a non-linear optical effect is used as a substrate, and the modulated light guided through the optical waveguide is wavelength-converted in a wavelength conversion unit formed in the optical waveguide to shorten the wavelength.

If the waveguide type optical modulator having the wavelength conversion function is constructed so as to modulate the intensity of the guided light by varying the applied voltage, it can be used for continuous tone image recording. In this case, if the light emitted from the device has a short wavelength as described above, for example, blue modulated light can be obtained and used for color image recording, or the recording density can be increased. Is obtained.

[0006]

However, the Z-cut L
When the light modulation element is formed using the iNb _x Ta _1-x O ₃ crystal substrate, there is a problem that it is difficult to secure an extinction ratio of 20 dB or more that is generally required for continuous tone image recording. . Hereinafter, this problem will be described in detail with reference to FIGS. 7 and 8.

When a Z-cut crystal substrate is used, the relationship between the voltage applied to the electro-optical light modulator and the light output is approximately as shown in FIG. As shown in the figure, there is an offset voltage Vo of about 1/3 of the difference Vπ between the voltage V ₂ for obtaining the maximum light output and the voltage V ₁ for obtaining the minimum light output, and the light output is 0 when the applied voltage is 0. Since it does not become 0, if the modulation is performed without applying a bias voltage, the extinction ratio remains at about 5 dB. The value of this extinction ratio was confirmed by the experiment of the present inventor.

Therefore, it is conceivable to apply a DC bias voltage to the optical modulator so that the relationship between the applied voltage and the optical output is as shown by the broken line in FIG. In that case, the extinction ratio is improved to 15 dB or more. However, if such a DC bias voltage is continuously applied, an operating point fluctuation phenomenon called DC drift, that is, a phenomenon that the value of the applied voltage at which the optical output is 0 fluctuates, and the extinction ratio changes with time. It is impossible to put the device into practical use.

As a method of eliminating the DC drift,
As shown in Japanese Patent Application Laid-Open No. 7-29480, there is a method of superimposing a high frequency on the drive signal of the optical modulation element, but in this case as well, the extinction ratio remains at about 5 dB unless a bias voltage is applied. The extinction ratio improves when a bias voltage is applied,
Since the average voltage is not 0, DC drift will occur.

On the other hand, X-cut or Y-cut LiN
It is also considered to form an optical modulation element using a b _x Ta _1-x O ₃ crystal substrate. In that case, since the offset voltage is small, the extinction ratio can be increased by using the above high frequency superposition method.
It is possible to secure 15 dB or more (confirmed value by the experiment of the inventor). However, when the X-cut or Y-cut substrate is used, the wavelength conversion efficiency of the wavelength conversion unit is significantly lower than that when the Z-cut substrate is used, so that the output of the modulated light becomes extremely low.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a waveguide type optical modulator having a wavelength conversion function, which can realize both a high extinction ratio and a high wavelength conversion efficiency. To do.

[0012]

A waveguide type optical modulator according to the present invention comprises a ferroelectric crystal substrate having an electro-optical effect and a non-linear optical effect, a plurality of optical waveguides formed on this substrate, and a plurality of these optical waveguides. The optical waveguide includes an electrode for applying a voltage to at least one of the optical waveguides, and a wavelength conversion unit formed in the optical waveguide on the downstream side of the electrode in the waveguide direction. In a waveguide-type optical modulation element that modulates light passing through the wavelength conversion unit by controlling transfer of guided light between a plurality of optical waveguides, the surface thereof and the Z axis of the ferroelectric crystal serve as a substrate. What is cut so that the angle θ formed by and becomes 0 ° <θ <90 ° is used, and the wavelength conversion unit inverts the direction of spontaneous polarization of the ferroelectric crystal substrate by 180 °. Direction and flatness of spontaneous polarization It is characterized in that extending building domain reversals is composed of periodic domain inversion structure formed by periodically disposed.

In the above structure, when the optical waveguide is formed by proton exchange and annealing and the wavelength conversion section is composed of the periodic domain inversion structure formed in the optical waveguide, the angle θ is θ ≦
It is desirable to be in the range of 20 °.

When the optical waveguide is formed by proton exchange and annealing as described above, and the wavelength conversion portion is formed by the periodic domain inversion structure formed in the optical waveguide, the angle θ is 0.2. It may be in the range of ° ≦ θ, more preferably in the range of 0.5 ° ≦ θ.

More specifically, as the ferroelectric crystal substrate, more specifically, one that is cut in a plane perpendicular to an axis obtained by rotating the Y axis of the crystal in the YZ plane by 3 ° toward the Z axis, , Z
It is preferable to use one that is cut by a plane perpendicular to the axis rotated by 87 ° on the X axis side in the ZX plane.

More specifically, as the ferroelectric substance constituting the substrate, more specifically, LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1),
Alternatively, those doped with MgO or ZnO are preferably used.

Further, in the waveguide type optical modulator of the present invention, preferably, the optical waveguide is formed so as to extend in the X-axis or Y-axis direction of the substrate in the wavelength conversion section, and the periodic domain inversion formed in the optical waveguide is performed. The structure constitutes the wavelength conversion unit.

Further, in the waveguide type optical modulator of the present invention, preferably, a plurality of optical waveguides forming a directional optical coupler are provided, and the electrodes are provided with optical coupling between these optical waveguides by voltage application. Arranged to be controllable. Furthermore, the waveguide type optical modulator of the present invention is provided with a wavelength conversion unit.
The extinction ratio to the light before it is incident on is 10 dB or more.
It is desirable to be configured as follows.

[0019]

As described above, when the X-cut or Y-cut ferroelectric crystal substrate is used, the wavelength conversion efficiency of the wavelength converter is significantly lower than that when the Z-cut substrate is used. According to the research conducted by a researcher, when a substrate cut so that the angle θ formed by the substrate surface and the Z axis of the ferroelectric crystal is 0 ° <θ <90 °, a case of using a Z-cut substrate Similar wavelength conversion efficiency can be obtained.

When a substrate cut so that the angle θ between the substrate surface and the Z axis of the ferroelectric crystal is 0 ° <θ <90 °, the extinction ratio in electro-optical light modulation is 15 dB or more can be secured as in the case of using an X-cut or Y-cut substrate.

On the other hand, the intensity of the wavelength converted wave is 2 times the fundamental wave intensity.
The extinction ratio in electro-optical light modulation is proportional to the power of
When it is 10 dB, the extinction ratio of the wavelength converted wave is 20 dB. Therefore, if the extinction ratio in electro-optical light modulation is 15 dB or more as described above, the extinction ratio of 20 dB or more required for continuous tone image recording is secured for the wavelength converted wave.
Therefore, according to the waveguide type optical modulator of the present invention, the substrate is cut so that the angle θ between the substrate surface and the Z axis of the ferroelectric crystal is 0 ° <θ <90 °. ,
Both a high extinction ratio and a high wavelength conversion efficiency can be obtained.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. 1 and 2 are a perspective view and a plan view, respectively, of a waveguide type optical modulator according to a first embodiment of the present invention.

As shown in the figure, this waveguide type optical modulator is formed by, for example, proton exchange on a crystal substrate 10 of LiNbO ₃ (hereinafter, referred to as MgO-LN) doped with MgO by 5 mol%. Two channel optical waveguide
11 and 12, plate-shaped electrodes 13, 14A and 14B formed on the sides of each of the optical waveguides 11 and 12, and a wavelength conversion portion 15 formed on one optical waveguide 12.

A laser beam 21 having a wavelength of 950 nm emitted from a semiconductor laser 20 is input to one channel optical waveguide 12. In this example, this semiconductor laser 20
Are directly coupled to the substrate 10 at the end face portion of the optical waveguide 11. However, the present invention is not limited to this, and the semiconductor laser 20 may be disposed apart from the substrate 10 and the laser beam 21 converged by the optical system may be applied to the end face portion of the optical waveguide 11 and input to the optical waveguide 11. Good. The longitudinal mode of the semiconductor laser 20 is locked by a known method.

The one electrode 13 is connected to a modulation drive circuit 25 which receives the image signal S, and the electrodes 14A and 14B are grounded. The wavelength conversion unit 15 has a periodic domain inversion structure in which the domain inversion unit 16 in which the spontaneous polarization (domain) of the MgO-LN substrate 10 is inverted is repeated at a predetermined period.

A method of manufacturing the waveguide type optical modulator having the above structure will be described below. As shown in FIG. 3, the substrate 10 is a ferroelectric substance M having an electro-optical effect and a non-linear optical effect.
Obtained by cutting and polishing a gO-LN ingot 10 'with a plane perpendicular to an axis Y'rotated by Y = 3 ° in the YZ plane on the Z axis side by θ = 3 ° (3 ° Y It is a cut substrate) and is monopolarized to have a thickness of 0.3 mm, for example. Note that the angle θ is exaggerated in FIG.

On the surfaces 10a and 10b of the MgO-LN substrate 10 thus formed, as shown in FIG. 4, comb electrodes are formed, respectively.
30, comb-shaped electrode on the + Z side with the plate electrode 31 attached
When a pulse voltage is applied between both electrodes 30 and 31 such that 30 is a positive potential and the plate electrode 31 located on the −Z side is a negative potential, as shown in FIG. The direction of the spontaneous polarization of the substrate 10 which was oriented in the opposite direction is inverted in the voltage application portion, and the domain inversion portion 16 is formed. The direction of the spontaneous polarization is inclined by θ = 3 ° with respect to the substrate surface 10a, so that the polarization direction of the domain inversion portion 16 is also inclined with respect to the substrate surface 10a.

The surfaces 10a, 10 of the MgO-LN substrate 10 are
a direction parallel to b and orthogonal to the X axis, and the substrate surface 10a,
The directions perpendicular to 10b are the Z-axis direction and the Y-axis direction, respectively.
Since the angle is 3 ° with respect to the axial direction, these directions will be referred to as Z ′ direction and Y ′ direction, respectively, for convenience.

Next, on the MgO-LN substrate 10, the channel optical waveguides 11 and 12 are formed by well-known photolithography.
A mask of metal (Ta in this example) having an opening corresponding to the shape is formed. After that, the MgO-LN substrate 10 is
After dipping in pyrophosphoric acid heated to 160 ° C. for 64 minutes for proton exchange to remove the Ta mask with an etching solution, it is annealed in air at 350 ° C. for 1 hour.

By the above processing, the channel optical waveguides 11 and 12 as shown in FIGS. 1 and 2 are formed. At this time, the channel optical waveguide 12 is formed such that the portion where the domain inversion portions 16 are formed extends along the direction in which the domain inversion portions 16 are arranged.

Next, the -X plane and the + X plane including the end faces of the channel optical waveguides 11 and 12 of the MgO-LN substrate 10 are optically polished to complete the waveguide type optical modulator.

As shown in FIGS. 1 and 2, in the waveguide type optical modulator having the above structure, the laser beam 21 is input to one of the channel optical waveguides 12 and guided therethrough, and in accordance with a potential difference V described later, The portion where the two optical waveguides 11 and 12 are close to each other (coupling portion) is transferred to the other channel optical waveguide 11.

The laser beam 21 guided through the channel optical waveguide 12 has a wavelength of ½ in the wavelength conversion section 15, that is,
Converted to the blue second harmonic wave 22 of 475 nm. That is, in the wavelength conversion unit 15, the laser beam 21 as the fundamental wave and the second harmonic 22 thereof are formed by the periodic domain inversion structure in which the domain inversion unit 16 is periodically repeated in the waveguide direction of the laser beam 21. Phase matching (so-called pseudo phase matching) is performed. In this example, the length of the optical waveguide 12 in the portion where the periodic domain inversion structure is formed, that is, the interaction length L for wavelength conversion is 10 mm.

Here, the electrodes 14A and 14B are held at the ground potential in the coupling portion of the two channel optical waveguides 11 and 12. On the other hand, a variable voltage is applied to the electrode 13 from the modulation drive circuit 25, and the transfer of the laser beam 21 is controlled according to the potential difference V between the electrode 13 and the electrodes 14A and 14B.

That is, when the potential difference V is 0 (zero), the laser beam 21 guided through the channel optical waveguide 12
Are basically transferred to the optical waveguide 11, and as the potential difference V increases, the transfer amount decreases, and conversely, the amount of light guided through the optical waveguide 12 increases. When V is a predetermined value Vπ, there is no transition and the laser beam 21 basically guides all through the optical waveguide 12. Therefore, by controlling the voltage applied to the electrode 13 based on the image signal S, the intensity of the laser beam 21 guided through the channel optical waveguide 12 can be modulated, and thus the blue second harmonic wave 22 is converted into the image signal. The intensity can be modulated based on S.

Next, the wavelength conversion efficiency and extinction ratio of this waveguide type optical modulator, and the above-mentioned offset voltage Vo
The results of examining (see FIG. 7) will be described. The extinction ratio is a value before wavelength conversion. Substrate 10 described above for comparison
Instead of the MgO-LN ingot, as shown in FIG.
Substrate obtained by cutting and polishing 10 "in a plane perpendicular to the axis Z" which is obtained by rotating the Z axis in the ZX plane by 87 ° to the X axis side (87
(Z-cut substrate) (second embodiment of the present invention), as well as the conventionally known Z-cut substrate, X-cut substrate, and Y-cut substrate, wavelength conversion efficiency, Extinction ratio and offset voltage Vo
I checked. In addition, these waveguide type optical modulation elements are basically configured in the same manner as in the first embodiment except points other than the cut orientation of the substrate. However, when the 87 ° Z-cut substrate is used, the optical waveguide is formed so as to extend in the Y-axis direction. The results are shown below (Table 1).

[0037]

[Table 1]

In this (Table 1), the wavelength conversion efficiency is indicated by the conversion efficiency η. The conversion efficiency η is the power of the fundamental laser beam 21, P _FM (mW), the second harmonic 22
When the power of P _SHG (mW) and the interaction length are L (mm), η (% / Wcm ² ) = P _FM / P _SHG × 1000 × (10 / L)
It is given by ² .

As shown here, in the waveguide type optical modulators of the first and second embodiments of the present invention, wavelength conversion efficiency as high as that when using the Z-cut substrate is obtained. On the other hand, the extinction ratio is 15 dB or more as in the case of using the X-cut substrate or the Y-cut substrate.
Therefore, for the second harmonic 22, for the reasons described above,
20 dB or more, which is generally required for continuous tone image recording, is secured.

[0040]

[0041]

[0042]

[0043]

Further, in the present invention, the above-mentioned JP-A-7-2
As shown in Japanese Patent No. 94860, by superposing a high frequency on the voltage applied to the electrode 13, the above-mentioned problem of DC drift can be solved and an extinction ratio of 20 dB or more can be obtained.

Further, the embodiment using the 3 ° Y-cut substrate and the 87 ° Z-cut substrate, that is, the substrate in which the angle θ between the substrate surface and the Z axis is 3 ° has been described above.
The present invention is not limited to the substrate in which θ is 3 °, but 0 ° <θ
Basically, the same effect as described above can be obtained by using all the substrates in the range of <90 °. However, in the case where the optical waveguide is formed by proton exchange and annealing and the wavelength conversion part is configured by the periodic domain inversion structure, in order to obtain wavelength conversion efficiency almost equal to that when using the Z-cut substrate, 0.2 ° ≦ It is desirable to use a substrate in the range of θ ≦ 20 °, more preferably in the range of 0.5 ° ≦ θ ≦ 20 °.

That is, when the optical waveguide is formed by proton exchange and annealing, the range of θ at which the waveguide mode is not cut off is 20 ° or less. In this case, the field distribution of the guided mode is 1.2 μ at the shallowest.
It will be about m. Correspondingly, in order to make the depth of the domain inversion portion 1.2 μm or more, θ should be 0.2 ° or more. On the other hand, in order to facilitate the light input to the optical waveguide, it is desirable that the field distribution of the guided mode be 1.4 μm or more, and correspondingly, the domain inversion depth should be 1.4 μm.
In order to make m or more, θ may be 0.5 ° or more.

Further, the waveguide type optical modulator of the present invention is not limited to the above-mentioned proton exchange optical waveguide, but may be any other Ti.
It is also possible to provide by providing a diffusion optical waveguide or the like.

The substrate forming the optical waveguide is also the above-mentioned M.
Not only the LiNbO ₃ substrate doped with gO, but also other LiNbO ₃ substrates and LiNbO doped with ZnO
₃ substrate, may be LiTaO ₃ substrate, the MgO and ZnO utilizing doped LiTaO ₃ substrate.

[Brief description of drawings]

FIG. 1 is a perspective view of a waveguide type optical modulator according to a first embodiment of the present invention.

FIG. 2 is a plan view of the above-mentioned waveguide type optical modulator.

FIG. 3 is a schematic diagram illustrating a cut state of a substrate used in the above-mentioned waveguide type optical modulator.

FIG. 4 is a schematic perspective view showing a state in which the above-mentioned waveguide type optical modulator is produced.

FIG. 5 is a schematic perspective view showing a domain inversion portion formed in the above-mentioned waveguide type optical modulator.

FIG. 6 is a schematic diagram illustrating a cut state of a substrate used in a second embodiment of the present invention.

FIG. 7 is a graph showing a schematic relationship between an applied voltage and an optical output of the electro-optical light modulator.

FIG. 8 is a graph illustrating a bias voltage in the electro-optical light modulator.

[Explanation of symbols]

10 MgO-doped LiNbO ₃ substrate 11, 12, 50, 51 channel optical waveguide 13, 14A, 14B electrode 15 wavelength converter 16 domain inversion 20 semiconductor laser 21 laser beam (fundamental wave) 22 second harmonic 25 modulation drive circuit 30 Comb-shaped electrode 31 Flat plate electrode 60, 61 3dB coupler section

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-6-273817 (JP, A) JP-A-4-333835 (JP, A) JP-A-2-13929 (JP, A) 1983 ULTRASONICS SY MPOSIUM, 1983, 527-530 (58) Fields investigated (Int.Cl. ⁷ , DB name) G02F 1/37

Claims (11)

(57) [Claims] 1. A ferroelectric crystal substrate having an electro-optical effect and a non-linear optical effect, a plurality of optical waveguides formed on the substrate, an electrode for applying a voltage to at least one of these optical waveguides, A wavelength conversion portion formed on the optical waveguide on the downstream side of the electrode in the waveguide direction, and controlling the transfer of the guided light between the plurality of optical waveguides according to the applied state of the voltage. As a result, in the waveguide type optical modulation element that modulates the light passing through the wavelength conversion unit, the angle θ formed by the surface of the substrate and the Z axis of the ferroelectric crystal is 0 ° <θ <90 °. And the wavelength conversion unit inverts the direction of spontaneous polarization of the ferroelectric crystal substrate by 180 ° and extends the direction parallel to the direction of spontaneous polarization.
1. A waveguide type optical modulation element, comprising a periodic domain inversion structure in which periodic domain inversion portions are periodically arranged. 2. The optical waveguide is formed by proton exchange and annealing, the wavelength conversion portion is formed of a periodic domain inversion structure formed in the optical waveguide, and the angle θ is θ ≦ 20 °. 2. The waveguide type optical modulation element according to claim 1, wherein 3. The optical waveguide is formed by proton exchange and annealing, the wavelength conversion portion is formed by a periodic domain inversion structure formed in the optical waveguide, and the angle θ is 0.2 ° ≦ θ 3. The waveguide type optical modulator according to claim 1, wherein the waveguide optical modulator is in the following range. 4. The waveguide type optical modulator according to claim 3, wherein the angle θ is in a range of 0.5 ° ≦ θ. 5. The ferroelectric crystal substrate is cut in a plane perpendicular to an axis obtained by rotating the Y axis of the crystal in the YZ plane by 3 ° to the Z axis side. Claim 1
4. The waveguide type optical modulator according to any one of 1 to 4. 6. The ferroelectric crystal substrate is cut on a plane perpendicular to an axis obtained by rotating the Z axis of the crystal in the ZX plane by 87 ° to the X axis side. Claim 1
4. The waveguide type optical modulator according to any one of 1 to 4. 7. The ferroelectric material is LiNb _x Ta.
7. The waveguide type optical modulator according to claim 1, which is _{1-xO 3} (0 ≦ x ≦ 1) or one doped with MgO or ZnO. 8. The optical waveguide is formed in the wavelength conversion section so as to extend in the X-axis direction of the substrate, and the wavelength conversion section is constituted by a periodic domain inversion structure formed in the optical waveguide. The waveguide type optical modulator according to any one of claims 1 to 7, which is characterized in that. 9. The optical waveguide is formed in the wavelength conversion section so as to extend in the Y-axis direction of the substrate, and the wavelength conversion section is constituted by a periodic domain inversion structure formed in the optical waveguide. The waveguide type optical modulator according to any one of claims 1 to 7, which is characterized in that. 10. A plurality of optical waveguides forming a directional optical coupler are provided as the optical waveguides, and the electrodes are arranged so that optical coupling between these optical waveguides can be controlled by applying a voltage. The waveguide type optical modulator according to any one of claims 1 to 9, wherein 11. The light before entering the wavelength conversion unit
The extinction ratio to be 10 dB or more.
11. The waveguide type optical modulator according to any one of 1 to 10.