JPH04113302A - Waveguide type optical circuit and its manufacture - Google Patents

Abstract

PURPOSE:To adjust the stress birefringence value of an optical waveguide by denaturing the surface of a substrate nearby a core part by laser light irradiation as to a circuit formed by arranging a clad layer and a single-mode optical waveguide, which is embedded therein and provides optical propagating operation, on the substrate. CONSTITUTION:The silicon substrate l is irradiated with the laser beam 24 from a YAG laser light source 23 along the ring-shaped optical waveguide 2 and the denaturing of the irradiated part of the silicon substrate l progresses. For the purpose, while an XY moving table 22 is moved, the irradiation with the laser beam 24 is carried on to form a denatured area 11. At the same time, tunable DFB semiconductor laser light is guided in a sample 21 as diagnostic light from an input fiber 3b and the resonance wavelength characteristic of signal light which is guided out of an output fiber 4b is monitored; and the irradiation with the beam 24 is stopped when the resonance peaks of TE polarized light and TM polarized light are completely put over lapped, and then the polarization dependency of a ring resonator can completely be removed to enable easy polarization characteristic control.

Description

Detailed Description of the Invention [Industrial Application Field 1] The present invention relates to a waveguide type optical circuit in which an optical waveguide is disposed on a substrate and a method for manufacturing the same. The present invention relates to a waveguide type optical circuit that has desired polarization dependence or polarization independence by adjusting refractive properties, and a method for manufacturing the same.

[Prior Art 1] Various optical circuit components are required in the fields of optical communication and optical signal processing. Optical circuit components can be broadly classified into 1) bulk type, 2) fiber type, and 3) waveguide type depending on their form.

The bulk type is constructed by combining microlenses, prisms, interference film filters, etc., but it requires a long time to assemble and adjust, and has problems in terms of price and size.

The fiber type is constructed using the optical fiber itself through polishing, fusing, and stretching processes, and although it has the advantage of being relatively compact, it has problems such as a lack of productivity and scalability. be.

On the other hand, the waveguide type has the advantage of being able to be mass-produced on a flat substrate using a photolithography process, and is attracting attention as a future optical circuit component due to its reproducibility and possibility of compact integration.

A waveguide type optical circuit is basically constructed from an optical waveguide formed on a flat substrate. Generally, an optical waveguide on a flat substrate exhibits birefringence, in which the refractive index nTM of polarized light perpendicular to the substrate surface (TM mode) and the refractive index nTE of polarized light horizontal to the substrate surface (mad mode) are slightly different. In many waveguide optical circuits, it is required to adjust this birefringence with high precision in order to achieve desired polarization characteristics.

For example, a silica-based optical waveguide that can be fabricated on a silicon substrate can have a core cross-sectional dimension of about 5 to IO μm, matching that of commonly used single-mode optical fibers. It is expected to be a means of realizing practical integrated optical circuits with excellent consistency, but due to the difference in thermal expansion coefficient between silicon substrate and quartz glass,
Birefringence value B・(+1tM−T++
) is shown. In order to realize a waveguide optical circuit without polarization dependence or, in some cases, a waveguide optical circuit with the desired polarization dependence, the birefringence of the silica-based guide waveguide described above has traditionally been adjusted. The waveguide type optical circuit configuration obtained is patent application 1986-116.
No. 938.

FIGS. 4(a) and 4(b) show a plan view and an enlarged view of this plan view along line AA', respectively, of a silica-based waveguide optical ring resonator as an example of such a conventional waveguide optical circuit. FIG. Here, 1 is a silicon substrate, 2 is a ring-shaped leading waveguide portion (core portion) formed on the silicon substrate so as to be embedded in the silica-based glass cladding layer 1b, 3 and 4.
are the human power optical waveguide (core part) and the output optical waveguide (core part), respectively. The ring-shaped optical waveguide 2 and the input/output optical waveguides 3 and 4 are optically coupled by directional couplers 5a and 5b, respectively, with a coupling rate of about 5 to 20%.

On the crat layer 11 along the ring-shaped optical waveguide 2,
A thin film heater 6 as a thermo-optic effect phase shifter is also installed. A stress imparting film 7 is further loaded along the ring-shaped optical waveguide 2, and a portion of it is trimmed so that the optical resonance characteristics of the present ring resonator match the 7M polarization and the TE polarization.

In FIG. 4(a), a part of the signal light incident on the input optical waveguide 3 from the input boat 3a is guided to the ring-shaped optical waveguide 2 via the directional coupler 5a.

When the optical path length around the ring matches an integral multiple of the signal light wavelength, a resonance phenomenon occurs, and the resonant wavelength signal light is extracted from the output boat 4a of the output optical waveguide 4 via the directional coupler 5b. Here, the resonance condition is the thin film heater phase shifter 6
This can be controlled by finely adjusting the effective optical circumference of the ring by applying current to the ring and making use of the temperature dependence of the refractive index of silica-based glass.

The ring-shaped optical waveguide 2 made of a silica-based optical waveguide on a silicon substrate 1 has a slightly different optical circumference between 7M polarized light and TE polarized light due to the above-mentioned birefringence. Since the resonance conditions do not match, ring resonators generally exhibit polarization dependence, but in the conventional example shown in FIG. 4, such polarization dependence is eliminated by loading the stress applying film 7 and trimming it. It is. That is, as shown in FIG. 4(b), the stress applying film 6 loaded on the upper cladding layer 1b of the ring-shaped optical waveguide 2 exerts stress on the ring-shaped optical waveguide core part 2 located below, and the substrate 1 to compensate for a part of the stress exerted on this core part 2,
It has the effect of adjusting the stress-induced birefringence value that the optical waveguide 2 is exposed to. If necessary, the birefringence value can be finely adjusted by trimming a part of the stress applying film 7 by laser beam irradiation. Regarding the birefringence adjustment method using this stress applying film 7, please refer to the literature (Masao Kawachi, "Application to silica-based optical waveguides and integrated optical components", Optics Vol. 18,
12 (1989) [) [), 681-686).

[Problems to be Solved by the Invention] In the structure of the waveguide ring resonator equipped with the above-mentioned stress imparting film, it is possible to provide a resonator that eliminates polarization dependence, but in practice, the following problems occur. There was a problem. In other words, with conventional waveguide optical circuits capable of controlling birefringence and their manufacturing methods, the formation of stress-applying films requires complicated additional processes such as etching and film deposition, which complicates the manufacturing process of integrated optical circuits. I had a problem with it.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a waveguide optical circuit and a method for manufacturing the same, which eliminate the above-mentioned drawbacks of the prior art and enable easier birefringence control of an optical waveguide. The object of the present invention is to provide a waveguide type optical circuit that has polarization dependence or, conversely, has no polarization dependence.

1 Means for Solving the Problem 1 In order to achieve such an object, the waveguide type optical circuit of the present invention is a waveguide type optical circuit formed by disposing a single mode optical waveguide on a substrate. The one-mode optical waveguide has a cladding layer on the substrate, and a core part embedded in the cladding layer and having a light propagation function, and a laser beam is applied to the surface of the substrate near the core part. A region metamorphosed by light irradiation is provided, and the stress acting on the core from the substrate is irreversibly changed depending on the shape and distribution of the metamorphic region, thereby adjusting the stress birefringence value of the single mode optical waveguide. It is characterized by being configured as follows.

Here, the substrate may be a silicon substrate, and the single mode optical waveguide may be a quartz optical waveguide containing 5i02 as a main component.

The manufacturing method of the present invention is a method for manufacturing a waveguide type optical circuit in which a single mode optical waveguide is arranged on a substrate. a step of forming a mode optical waveguide, and modifying a desired portion of the substrate by laser irradiation, irreversibly changing the stress exerted by the substrate on the core portion, and changing the stress birefringence value of the single mode optical waveguide. and a step of adjusting.

[Function] In the present invention, in order to control birefringence, the substrate near the optical waveguide core is modified by laser beam irradiation, thereby irreversibly changing the stress exerted from the substrate on the optical waveguide core. , unlike conventional technology, the birefringence value of the optical waveguide can be adjusted simply by modifying the substrate by laser trimming without providing special stress-releasing grooves or stress-applying films. Therefore, simple polarization characteristic control is possible.

[Embodiment 1] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIGS. 1(a) and 1(b) are a plan view showing the configuration of a waveguide ring resonator as an embodiment of the waveguide optical circuit of the present invention, and FIGS. FIG.

In FIGS. 1(a) and (b), the cladding layer 1b on the silicon substrate 1 is made of 8102 glass with a thickness of about 50 μm, and the core parts of the ring-shaped optical waveguide 2 and the input/output optical waveguide 34 are formed on the cladding layer 1b. Buried cross-sectional size 6μ
It is SiO□-GeO3-based glass of m×6 μm. At the top of the ring-shaped optical waveguide 2, which has a diameter of about 13mm, there is a
Similar to the conventional examples shown in a) and (b), a thin film heater phase shifter 6 is installed, but unlike the conventional examples, the present invention is characterized in that no stress applying film is loaded. be. In place of the stress imparting film, in this embodiment, a substrate metamorphic region 11 is provided near the surface of the silicon substrate 1 at the bottom of the ring-shaped optical waveguide 2, and by appropriately determining the shape and distribution of this metamorphic region 11, the ring-shaped The birefringence value of the optical waveguide 2 is adjusted to achieve desired polarization-independent ring resonance characteristics.

Here, the metamorphic region 1 is a region formed by irradiating a laser beam with high energy density from above the substrate 1 through the cladding layer 1b to cause metamorphism in a desired portion of the surface of the substrate 1. . The present inventors have discovered that in this metamorphosed region 11, the crystallinity of the silicon substrate is destroyed by laser beam irradiation, resulting in a material state different from that of the untransformed substrate region, causing a change in stress distribution in the surrounding area. As a result of the stress distribution change, the stress-induced birefringence experienced by the optical waveguide core above the metamorphic region changes, and the desired birefringence adjustment can be achieved without using the conventional stress imparting film.

Next, one embodiment of the manufacturing process of the waveguide type optical circuit of the present invention will be described in detail using the above-mentioned ring resonator as an example.

First, the quartz-based optical waveguide, which is the main body of the ring resonator, was formed by a known combination of glass film deposition by flame hydrolysis reaction and microfabrication by reactive ion etching. That is, first, a lower cladding layer made of 3102-based glass and a core layer made of 5iO3-GeO3-based glass were sequentially deposited on silicon substrate 1. This accumulation includes
A flame deposition method using flame hydrolysis reaction of glass forming raw material gas such as silicon tetrachloride was used. Next, unnecessary portions of the core layer were removed by a photolithography process using reactive ion etching, leaving a desired optical waveguide core pattern. Subsequently, an upper cladding layer was again formed by the flame hydrolysis reaction deposition method so as to embed the core pattern. The total thickness of the lower cladding layer and the upper cladding layer is approximately 50 μm, and the cross-sectional dimension of the core portion is 6 μm.
μm×6gm, relative refractive index difference between core and cladding △
was set at 0.75%.

A thin film heater 6 having a width of 50 μm and a length of 10 μm was formed on the ring-shaped optical waveguide 2 having a diameter of 13 mm thus formed by a vacuum evaporation method using Ni and Cr as vapor deposition sources to form a phase shifter.

FIG. 2 shows a silicon substrate 1 at the bottom of a ring-shaped optical waveguide 2.
FIG. 3 is an explanatory diagram of a laser trimming device used in the present invention to cause metamorphosis in a desired portion of the substrate to form a metamorphic region 11. FIG. Here, 21 is a waveguide type optical ring resonator sample on the silicon substrate 1, 22 is an XY moving table, and 23 is a YA
G: a laser light source; 24 is a laser beam;

A laser beam 24 from a YAG laser light source 23 is irradiated along the ring-shaped leading waveguide 2 . Silicon substrate 1
The portion irradiated by the laser beam 24 instantaneously reaches a high temperature due to the laser heating action, and metamorphosis progresses.

Therefore, while driving the XY moving table 22, the laser beam 24
By continuing the irradiation, a metamorphosed region 11 was formed. The YAG laser used here was a microscope-mounted Q-switch YAG laser light source L-1,1A (manufactured by HOYA Corporation), and irradiated one surface of the silicon substrate via a long focal length lens with a magnification of 50 times. In preparation for the laser light source 23, the iris was adjusted so that the width of the metamorphic region 1 was approximately 30 μm.

Simultaneously with the laser beam irradiation, a tuner pull DFB with a wavelength of 1.55 μm is applied to the sample 21 via the input fiber 3b.
Semiconductor laser light is introduced as diagnostic light, and output fiber 4
The resonant wavelength characteristics of the signal light extracted through b was monitored.

At the stage before irradiation with the laser beam 24, reflecting the birefringence of the ring-shaped optical waveguide 2, the resonant wavelength characteristics are as follows.
The resonance peaks of TE polarized light and TM polarized light do not match. As the silicon substrate 1 undergoes metamorphosis by irradiation with the laser beam 24, the resonance peaks of the TE polarized light and the 7M polarized light eventually begin to approach each other. By stopping the irradiation of the laser beam 24 when the resonance peaks completely overlapped on the wavelength axis, the polarization dependence of the ring resonator could be completely eliminated. The final length of the metamorphic region 11 depends on the initial state of the sample, but
The length was approximately several mm.

Since the oscillation wavelength of the YAG laser light source 23 is 1.06 μm, it is not absorbed in the region of the optical waveguide 2 made of silica-based glass (it is efficiently absorbed only in the silicon substrate 1. The optical output of the main body of the YAG laser light source 23 used was 9 mJ/pulse, the pulse width was 8 nsec, and the peak power was 1.IMW, but if the laser light intensity was too strong, the metamorphosis of the silicon substrate 1 would proceed too rapidly. Since there is a risk of damaging the optical waveguide core section 2, it is important to keep the laser beam intensity to the minimum necessary by adjusting the iris and attenuator when irradiating the laser beam 24.

FIGS. 3(a) and 3(b) are cross-sectional views showing the metamorphic region of the laser beam irradiation part in the present invention. Third
Figure (a) shows the case where the laser beam 24 is irradiated directly under the core part 2, and the laser beam 24 with a beam diameter of 10 μm is scanned three times to form a metamorphic region 11 with a width of 30 μm. When the metamorphic region was provided directly under the core portion 2 in this manner, there was a tendency for the resonance peak of TE polarized light to shift on the wavelength axis.

FIG. 3(b) shows laser beams 2 on both sides near the core part 2.
This is the case when 4 was irradiated. The metamorphic region 1 is scanned once with each laser beam 24 with a beam diameter of 10 μm.
1a and llb were formed. In the case of FIG. 3(b), on the contrary, there was a tendency for the 7M polarized light to move on the wavelength axis.

By performing metamorphism in this way, the birefringence value of the optical waveguide 2 changes by about I x 10-' in the case of FIG. 3(a), and by 0.5 in the case of FIG. 3(b). A change of about Xl0-' was observed. In any case, the desired polarization-independent resonance characteristics were achieved by laser trimming while monitoring the resonance wavelength characteristics.

In the above embodiments, a ring resonator was used as an example of an optical circuit to which the present invention is applied, but the present invention is not limited to this. It can also be applied to make polarization independent, or conversely, to configure a waveguide polarization beam splitter.

Further, the target waveguide system is not necessarily limited to a quartz-based optical waveguide on a silicon substrate (it can also be applied to a multi-component glass optical waveguide, etc.).

The point is that the optical waveguide is subjected to stress birefringence from the substrate, and any combination in which the substrate absorbs the laser beam and undergoes metamorphosis is within the scope of the present invention.

[Effect of the Invention 1 As explained above, in the present invention, the substrate near the core of the optical waveguide is modified by laser beam irradiation to change the stress state, thereby changing the stress-induced birefringence value of the optical waveguide. The polarization characteristics of a waveguide optical circuit can be accurately controlled. In the present invention, the stress state of the substrate itself is changed by laser trimming, and a special stress-applying film or the like is not required, so that polarization characteristics can be easily controlled.

The present invention is extremely effective for accurately configuring waveguide optical circuits for optical communications, optical sensors, optical signal processing, etc. in which polarization characteristics play an important role.

According to the present invention, since the birefringence value can be set accurately, it is expected to play a particularly important role in providing integrated optical devices for coherent optical communications that treat light waves like microwaves.

[Brief explanation of the drawing]

FIGS. 1(a) and 1(bl) show a waveguide ring resonator which is an embodiment of the waveguide optical circuit of the present invention, respectively.
FIG. 2 is a plan view and an enlarged sectional view taken along line segment AA′ of this plan view; FIG. Figures (a) and (b) show the sea 41 in the present invention.
f, VM Kokudai, Fig. 4, showing two examples of the beam irradiation on the Kuyūro plane.
a) and (b) are a plan view and an enlarged sectional view taken along line segment AA' of this plan view, respectively, showing a ring resonator with a stress-applying film as an example of a conventional waveguide type optical circuit. 7... Stress applying film, 11, lla, 1lb-transformed region, 21... Waveguide type optical circuit sample, 22... XY moving table, 23... YAG laser light source, 24... Laser beam . DESCRIPTION OF SYMBOLS 1... Silicon substrate, 1b... Clad layer, 2... Ring-shaped optical waveguide, 3... Input optical waveguide, 4... Output optical waveguide, 3a... Input boat, 4a... Output boat, 3b...human powered optical fiber, 4b...output optical fiber, 5a, 5b...directional coupler, 6...thin film heater phase shifter,

Claims (1)

[Claims] 1) A waveguide type optical circuit including a single mode optical waveguide disposed on a substrate, wherein the single mode optical waveguide includes a cladding layer on the substrate and a cladding layer embedded in the cladding layer. A core portion having a light propagation effect is provided on the surface of the substrate near the core portion, and a region metamorphosed by laser beam irradiation is provided, and the shape and distribution of the metamorphosed region 1. A waveguide type optical circuit, characterized in that the stress acting on the core portion is irreversibly changed to adjust the stress birefringence value of the single mode optical waveguide. 2) The waveguide type optical circuit according to claim 1, wherein the substrate is a silicon substrate, and the single mode optical waveguide is a quartz-based optical waveguide whose main component is SiO_2. 3) A method for manufacturing a waveguide optical circuit in which a single mode optical waveguide is disposed on a substrate, which includes a single mode optical waveguide embedded in a cladding layer on the substrate and including a core portion having an optical propagation function. and a step of modifying a desired portion of the substrate by laser irradiation to irreversibly change the stress exerted by the substrate on the core portion to adjust the stress birefringence value of the single mode optical waveguide. A method for manufacturing a waveguide optical circuit, comprising: