JP6364706B2 - Optical module - Google Patents

Description

  The present invention relates to an optical waveguide type diffraction grating and an optical module provided with a ferroelectric crystal substrate having a domain-inverted periodic structure.

  An optical waveguide device using a nonlinear optical effect or an electro-optical effect of a ferroelectric crystal is known. With regard to this optical waveguide device, in order to obtain the target light, a device has been proposed in which a structure that acts on the incident fundamental wave is provided in the optical waveguide to perform wavelength conversion and light modulation.

  In addition, an optical waveguide composed of a ferroelectric crystal substrate with a domain-inverted periodic structure can constitute a diffraction grating having a periodic refractive index distribution induced inside the crystal substrate by the electro-optic effect, Light propagating in the optical waveguide can be controlled. For example, DBR (Distributed bragg refrector) is capable of Bragg-reflecting light of a specific wavelength in the waveguide and converting it into light propagating in the reverse direction. As an example of this, Patent Document 1 discloses a wavelength conversion element in which a polarization inversion type diffraction grating using a residual electric field is formed in an optical waveguide element.

JP 7-13008 A

  However, in the case of the polarization inversion type diffraction grating using the residual electric field as in Patent Document 1, it is difficult to control the residual electric field due to the influence of processing strain, stress, etc. during the production of the optical waveguide, and as a result, the diffraction efficiency varies. is there. Another problem is that the diffraction efficiency cannot be arbitrarily modulated.

  The present invention has been made in view of the above problems, and can reliably operate a diffraction grating formed in a ferroelectric crystal substrate, improve diffraction efficiency, and can arbitrarily modulate the optical waveguide. It is an object to provide a type diffraction grating and an optical module.

According to one aspect of the present invention , a ferroelectric crystal substrate having an optical waveguide having a core shape of a ridge type and provided with a domain-inverted periodic structure portion and a wavelength conversion portion, and the ferroelectric crystal substrate And a pair of electrode portions selectively disposed opposite each other only in a region sandwiching the domain-inverted periodic structure portion outside the cladding unit, and the polarization inversion There is provided an optical module including a diffraction grating electrode section that generates an electric field by applying a voltage to a periodic structure section.

  According to the present invention, it is possible to provide an optical waveguide type diffraction grating and an optical module capable of reliably operating a diffraction grating formed in a ferroelectric crystal substrate, improving diffraction efficiency and arbitrarily modulating the diffraction grating. can do.

It is side surface sectional drawing which shows the principal part of the optical waveguide type diffraction grating of 1st Embodiment. It is side surface sectional drawing orthogonal to an optical waveguide explaining the structure of the polarization inversion periodic structure of the optical waveguide type diffraction grating of 1st Embodiment. It is side surface sectional drawing which shows the principal part of the optical module of 2nd Embodiment. It is a top view which shows the structure of the optical module of 3rd Embodiment. It is side surface sectional drawing parallel to an optical waveguide explaining the polarization inversion periodic structure of the optical waveguide type diffraction grating which comprises the optical module of 3rd Embodiment.

  Embodiments of the present invention will be described below with reference to the accompanying drawings. However, it should be noted that the drawings are schematic and dimensional ratios and the like are different from actual ones. Accordingly, specific thicknesses and dimensions should be determined in consideration of the following description. Moreover, it is a matter of course that portions having different dimensional relationships and ratios are included between the drawings.

  Further, the following embodiments exemplify apparatuses and methods for embodying the technical idea of the present invention, and the embodiments of the present invention include the material, shape, structure, The layout is not specified as follows. The embodiments of the present invention can be implemented with various modifications without departing from the scope of the invention.

[First Embodiment]
First, the first embodiment will be described. FIG. 1 is a side cross-sectional view showing the main part of the optical waveguide type diffraction grating of the present embodiment. FIG. 2 is a side cross-sectional view orthogonal to the optical waveguide, illustrating the polarization inversion periodic structure of the optical waveguide type diffraction grating of the present embodiment.

  The optical waveguide type diffraction grating 10 of the present embodiment has a ferroelectric crystal substrate 11 provided with an optical waveguide 11g having a core shape of a ridge type and having an optical path length adjusting portion 11p and a domain-inverted periodic structure portion 11r. The insulating layer 12 (12a, 12b) disposed in a clad shape so as to sandwich the ferroelectric crystal substrate 11 is opposed to the outside of the insulating layer 12, and an electric field is applied by applying a voltage to the optical path length adjusting unit 11p. A diffraction grating electrode section 20 (20a) that generates an electric field by applying a voltage to the polarization inversion periodic structure section 11r that is disposed opposite to the outside of the insulating layer 12 and the optical path length adjustment electrode section 18 (18a, 18b) to be generated. 20b).

  The ferroelectric crystal substrate 11 includes an optical path length adjusting unit 11p and a polarization inversion periodic structure unit 11r continuous on the outgoing light side of the optical path length adjusting unit 11p. The optical path length adjusting unit 11p and the optical path length adjusting unit 11p The polarization inversion periodic structure portion 11r is provided. The core shape of the optical waveguide 11g is a ridge type.

(Ferroelectric crystal substrate)
A single crystal of lithium niobate (LiNbO ₃ ) or lithium tantalate (LiTaO ₃ ) is used as the ferroelectric crystal substrate 11. In order to suppress optical damage, it is preferable to use a ferroelectric crystal substrate doped with at least one of Mg, Zn, Sc, and In.

Further, when designing in the short wavelength region (for example, blue light or UV light) as the incident light or the outgoing light, Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) from the viewpoint of optical damage and transparent wavelength region. The ferroelectric crystal substrate 11 is a single crystal of a single-polarized stoichiometric composition or a near-stoichiometric composition single crystal (MgOSLT) doped with MgO and having a molar fraction of 0.495 or more and less than 0.505. It is preferable to use as.

  As shown in FIG. 2, the optical waveguide 11g is a ridge type in which the propagation efficiency is increased, and the ridge height H (thickness) can be formed by thinning the ferroelectric crystal substrate 11. it can. In addition, it can also be processed into a ridge (groove) shape by mechanical processing such as dicing or dry etching processing using photolithography. In addition to the ridge type optical waveguide 11g, a planar type optical waveguide may be used.

(Electrode part)
The electrode part constituting the optical waveguide type diffraction grating 10 is divided and arranged at two or more locations along the light propagation direction, and in this embodiment, the optical path length arranged so as to sandwich the optical path length adjusting part 11p. The adjustment electrode portion 18 and the diffraction grating electrode portion 20 arranged so as to sandwich the domain-inverted periodic structure portion 11r. The optical path length adjusting electrode portion 18 and the diffraction grating electrode portion 20 are arranged to face each other in the C-axis direction of the ferroelectric crystal substrate 11. In order to achieve such an arrangement configuration facing each other, for example, a substrate-like support 16a having an optical path length adjusting electrode portion 18a and a diffraction grating electrode portion 20a formed on one side, and an optical path length adjusting electrode portion on one side. A substrate-like support 16b on which 18b and a diffraction grating electrode portion 20b are formed is used. At this time, the electrode material (the optical path length adjusting electrode portion 18 and the diffraction grating electrode portion 20) is preferably a metal thin film (Al, Cr, Ta, etc.) that can be formed by a sputtering method having excellent adhesion. . In this embodiment, by providing the optical path length adjusting electrode portion 18 and the diffraction grating electrode portion 20, the electrode is divided into two or more locations along the light propagation direction.

  In addition, when the support bodies 16a and 16b, the optical path length adjusting electrode section 18, and the diffraction grating electrode section 20 are made larger than the ferroelectric crystal substrate 11 in plan view, the support body and the electrode section The contact structure can be easily and reliably formed.

Polarization inversion periodic structure member 11r has a structure in which the polarization inversion period structure is formed by any period lambda _1, constitutes a diffraction grating driven by an electric field is applied by the diffraction grating electrode section 20 Yes.

  As a driving principle, a diffractive index distribution n ± Δn that coincides with the polarization inversion period is induced by the applied electric field E, and a diffraction grating is formed. Here, Δn is a value determined by the following equation using r33 (electro-optic constant).

Δn = n · r33 · E / 2
Note that the polarization inversion period in accordance with the incident fundamental wave has a pitch in the range of micron to submicron, and a method of applying a voltage to an electrode processed by photolithography or a method of inversion by electron beam irradiation is known. It has been.

The diffraction grating period to be formed (that is, the polarization inversion period) Λ ₁ is formed with a quarter optical path length or an odd multiple of 1/4 times the incident fundamental wave wavelength. For example, a ferroelectric with a refractive index of 2 The period formed on the crystal substrate 11 is 0.13 μm with a quarter period with respect to the fundamental wave of 1064 nm. Here, the diffraction efficiency can be adjusted by the voltage applied to the electrode.

(Insulator)
The insulating layer 12 constituting the optical waveguide type diffraction grating 10 is made of a transparent dielectric film such as SiO ₂ , SiO _x N _y (nitrogen oxide film), Al ₂ O ₃ (aluminum oxide), or an adhesive having good transparency. The optical confinement is performed by the refractive index difference from the ferroelectric crystal substrate 11 (core). Moreover, it is preferable that the thickness of the insulating layer 12 is 0.5 μm or more for the stable and high light confinement effect of the guided wave.

(Support)
As the supports 16a and 16b described above, it is preferable to use a substrate similar to the ferroelectric crystal substrate 11. For example, as a substrate similar to MgOSLT, by using a CLT substrate or a BLT substrate (a CLT substrate doped with iron), the effect of suppressing the influence of environmental temperature changes as an element can be obtained, and the device can be manufactured at low cost. It becomes possible.

(Function, effect)
Hereinafter, the operation and effect of the present embodiment will be described. In the optical waveguide type diffraction grating 10 of this embodiment, an electric field is generated in the optical path length adjusting unit 11p by the optical path length adjusting electrode unit 18, and an electric field is generated in the polarization inversion periodic structure unit 11r by the diffraction grating electrode unit 20. Can do. Therefore, the optical waveguide type diffraction grating 10 can be used as a distributed bragg refrector (DBR).

  In order to use the optical waveguide type diffraction grating 10 as a DBR, a voltage is applied to the optical path length adjusting electrode section 18 and the diffraction grating electrode section 20 to polarize part of the incident fundamental wave propagating through the optical path length adjusting section 11p. Reflected by the inversion periodic structure portion 11r (Bragg reflection) to form an external resonator. At this time, in order to adjust to a desired oscillation wavelength (resonator wavelength), the optical path length is adjusted by applying an electric field. That is, the voltage applied to the optical path length adjusting electrode portion 18 and the diffraction grating electrode portion 20 is adjusted.

  Therefore, an arbitrary electric field can be generated by applying an arbitrary voltage to an electrode (optical path length adjusting electrode portion 18) different from the diffraction grating electrode 20, and the optical path length can be individually increased without affecting the diffraction efficiency. Adjustments can be made. Therefore, the diffraction grating 10 formed in the ferroelectric crystal substrate 11 can be reliably operated, and the optical waveguide type diffraction grating 10 that can improve the diffraction efficiency and can be arbitrarily modulated is realized.

  In addition, since the voltage required to change the refractive index of the optical path length adjusting unit 11p can be suppressed to 5 V or less by thinning the ferroelectric crystal substrate 11, the optical waveguide type diffraction grating 10 has low power and It can be used as a device operating at high speed.

[Second Embodiment]
Next, a second embodiment will be described. FIG. 3 is a side cross-sectional view showing the main part of the optical module of the present embodiment.

  The optical module 30 of the present embodiment includes a ferroelectric crystal substrate 31 having a core shape of a ridge type, an optical waveguide 31g having a polarization inversion periodic structure portion 31c and a wavelength conversion portion 31c, and a ferroelectric material. An insulating layer 32 (32a, 32b) made of an insulator arranged so as to sandwich the crystal substrate 31 is disposed opposite to the outside of the insulating layer 32, and a voltage is applied to the domain-inverted periodic structure portion 31r to generate an electric field. And a diffraction grating electrode 38 (38a, 38b). The domain-inverted periodic structure part 31c is equivalent to the domain-inverted periodic structure part 11r, and the insulating layer 32 is equivalent to the insulating layer 12.

With this configuration, the incident fundamental wave wavelength is narrowed and stabilized by the diffraction grating, and the fundamental wave is wavelength-shifted by a polarization inversion structure with a quasi phase matching (QPM) period Λ ₂ formed on the same optical waveguide. It is converted and emitted.

For example, the QPM period Λ _{2 in the} case of wavelength conversion from the fundamental wave 1064 nm to 532 nm is 8 μm, and in the wavelength conversion to the third harmonic (wavelength is 355 nm), Λ ₂ is 2 μm.

  Further, by increasing the QPM order or reducing the plate thickness of the ferroelectric crystal substrate 31, the polarization inversion aspect ratio can be kept small, and the polarization inversion periodic structure portion 31c can be made uniform. Furthermore, by forming two or more steps of wavelength conversion periods from infrared light to UV light in the same element, the optical module 30 can be configured to perform wavelength conversion in a cascaded (stepwise) manner.

  As compared with the optical waveguide type diffraction grating 10 of the first embodiment, the polarization inversion periodic structure portion 11r, the insulating layer portion sandwiching the polarization inversion periodic structure portion 11r, and the diffraction grating electrode portion 20 are provided. The length adjusting portion 11p, an insulating layer portion sandwiching the length adjusting portion 11p, and an optical path length adjusting electrode portion 18 are used, and the polarization inversion periodic structure portion 11r is used as the polarization inversion periodic structure portion 31c of the present embodiment. Is also possible.

[Third Embodiment]
Next, a third embodiment will be described. FIG. 4 is a plan view showing the configuration of the optical module of the present embodiment. FIG. 5 is a side sectional view parallel to the optical waveguide for explaining the polarization inversion periodic structure of the optical waveguide type diffraction grating constituting the optical module of the present embodiment.

  The optical module 50 according to the present embodiment includes a plurality of optical waveguide type diffraction gratings 51 and multiplexing means 52 that superimposes the light emitted from each optical waveguide type diffraction grating 51.

  Compared with the optical waveguide type diffraction grating 10 of the first embodiment, the optical waveguide type diffraction grating 51 includes a polarization inversion periodic structure portion 11r, an insulating layer portion sandwiching the polarization inversion periodic structure portion 11r, and the diffraction grating electrode portion 20, and includes an optical waveguide type. In the diffraction grating 10, the optical path length adjusting portion 11 p, the insulating layer portion sandwiching the optical path length adjusting portion 11 p, and the optical path length adjusting electrode portion 18 are not provided. That is, the optical waveguide type diffraction grating 51 includes a domain-inverted periodic structure 11r that forms the optical waveguide 11gh, and an insulating layer 12 (12ah, 12bh) made of an insulator disposed so as to sandwich the domain-inverted periodic structure 11r. A diffraction grating electrode section 20 (20a, 20b) is provided opposite to the outer side of the layer 12 and generates an electric field by applying a voltage to the domain-inverted periodic structure section 11r. The optical waveguide type diffraction grating 51 includes supports 16ah and 16bh having diffraction grating electrode portions 20a and 20b on one side, respectively.

  The multiplexing means 52 includes a continuous path 54 that is continuous with each optical waveguide 11gh, and a single combined path 56 that the continuous paths 54 join and connect.

  In the optical module 50 of the present embodiment, the diffraction efficiency of propagating light can be arbitrarily varied by applying an arbitrary electric field to the electrode part 20 for the diffraction grating. Therefore, the optical waveguide of each optical waveguide type diffraction grating 51 The propagation light of 11 gh, that is, the propagation light of the branched optical waveguide can be modulated at a high speed to a desired light intensity.

In this optical module 50, the diffraction grating periods Λ _R , Λ _G , Λ _{B for the} incident light L _R (red light), L _G (green light), L _B (blue light) according to each optical waveguide type diffraction grating 51. Are formed on the ferroelectric crystal substrate, and the voltage values to the respective diffraction grating electrode portions 20 formed so as to face each other through the insulating layer 12 are adjusted by forming the polarization inversion periodic structure portions 11rR, 11rG, and 11rB on the ferroelectric crystal substrate. The white output light at the final end is monitored and fed back to be reflected in the applied voltage value, and white output light having a desired ratio intensity can be obtained.

  This optical module 50 can be used for a display and a projector light source. The three primary color lights of red light, green light, and blue light are respectively incident on a three-branch type optical waveguide 11gh, and are combined in a combined channel 56 to be white. Getting output light. At this time, a well-balanced white output light can be obtained by adjusting the intensity ratios of the incident red light, green light, and blue light.

  As described above, in these embodiments, the input / output end faces of the optical waveguide type diffraction grating and the optical module are optically polished, and the AR coating for improving the entrance / exit efficiency and the end face cut-off processing for preventing the return light are performed. It is preferable.

  In addition, the optical waveguide type diffraction grating and the optical module described in these embodiments can be used for a laser light source device, a display, a projector light source device, and the like that emit visible light and UV light.

DESCRIPTION OF SYMBOLS 10 Optical waveguide type diffraction grating 11 Ferroelectric crystal substrate 11g Optical waveguide 11gh Optical waveguide 11p Optical path length adjustment part 11r Polarization inversion periodic structure part 12, 12a, 12b, 12ah, 12bh Insulating layer (cladding part)
18, 18a, 18b Optical path length adjusting electrode part 20, 20a, 20b Diffraction grating electrode part 30 Optical module 31 Ferroelectric crystal substrate 31c Wavelength converting part 31r Polarization inversion periodic structure parts 32, 32a, 32b Insulating layer (cladding part) )
38, 38a, 38b Diffraction grating electrode section 50 Optical module 51 Optical waveguide type diffraction grating 52 Multiplexing means

Claims (3)

A ferroelectric crystal substrate having an optical waveguide in which a core shape is a ridge type, and a polarization inversion periodic structure portion and a wavelength conversion portion are provided;
A clad portion made of an insulator disposed so as to sandwich the ferroelectric crystal substrate;
Diffraction consisting of a pair of electrode portions that are selectively arranged opposite each other only in a region sandwiching the domain-inverted periodic structure portion outside the cladding unit, and applying an electric voltage to the domain-inverted periodic structure unit to generate an electric field A grid electrode;
An optical module comprising: 2. The optical module according to claim 1, wherein the ferroelectric crystal substrate is made of MgO-doped LiTaO ₃ or LiNbO ₃ . In the ferroelectric crystal substrate, the molar fraction of Li ₂ O / (Ta ₂ O ₅ + Li ₂ O) is in the range of 0.495 or more and less than 0.505, and a single polarization constant ratio doped with MgO. 3. The optical module according to claim 1, wherein the optical module is composed of a single crystal of lithium tantalate having a composition or a specific ratio composition.

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