JP2001133646A - Optical waveguide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical waveguide which reduces changes in the coupling efficiency of the optical waveguide and reduces variance of coupling efficiency between optical waveguides of a multi-beam light source, using a simple constitution. SOLUTION: The width of the core of an optical waveguide, which approximately minimizes the guide mode diameter is so selected that the guide mode diameter cannot change greatly, in spite of a slight variance in size of the core of the optical waveguide due to production errors or the like.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical waveguide, and more particularly, to an optical waveguide configured as a light source of a laser beam printer.

[0002]

2. Description of the Related Art With the development and digitization of information networks in recent years, there has been a strong demand for faster laser beam printers. One of the means for increasing the speed of the laser beam printer is to increase the speed of rotation of the scanning polygon mirror. However, at present, when the number of rotations of the polygon mirror approaches 50,000, distortion of the polygon surface occurs due to centrifugal force, and it is said that there is a limit to further speeding up the rotation of the polygon mirror. Therefore, in order to further increase the drawing speed of the laser beam printer, the surface of the photoconductor is scanned with a plurality of laser beams.

[0003] Specifically, for example, Japanese Patent Application Laid-Open No. 10-2824.
No. 41, US Pat. No. 4,637,679, US Pat.
As described in US Pat. No. 47038, US Pat. No. 4,958,893, etc., a configuration in which a plurality of laser beams are optically deflected to appropriate intervals and adjusted using a polarizing beam splitter, a half mirror, reflection of a prism surface, and the like. Has been proposed or adopted. However, in these methods, when the number of laser beams is large, alignment becomes difficult, and there is a disadvantage that components become large and cost becomes too high. .

Therefore, there is a demand for a method of forming a so-called multi light source in which a plurality of laser light sources are arranged at a fine pitch. The method is described in, for example, JP-A-54-73.
No. 28, a method using a so-called array laser in which a plurality of laser diodes are formed on a substrate as a plurality of laser light sources, a method using light emitted from an optical fiber as a secondary light source, emitting light from an incident side There is a method using an optical waveguide in which the pitch on the side is narrowed.

However, in the method using an array laser, the pitch at which the laser diodes are arranged is set to a small value of 100 μm or less in order to bring a plurality of laser beam spots sufficiently close in consideration of the state of image formation on the photoreceptor surface. It is desirable that the interval is, but to form a laser diode on the substrate at such a small pitch,
Difficult with the problem of fever. Therefore, it is considered that the other method using an optical fiber or an optical waveguide is effective.

[0006]

However, when a plurality of laser beams are used, in order to obtain good printing results in the laser beam printer, it is essential that the light amounts of the laser beams on the photoreceptor surface are uniform. Becomes However, in the case of using the above-described method of using the optical waveguide in which the pitch on the emission side is narrower than that on the incidence side, if the light intensity of the laser beam emitted from each optical waveguide varies, the light amount on the photoconductor surface becomes uneven. Can't get a clear image. Therefore, it is necessary to minimize variations in light intensity between the beams.

There are a number of factors that cause the light intensity of each beam to vary, including variations in the intensity of light emitted from a semiconductor laser as a light source. One of the factors is the incidence of an optical waveguide. There are variations in the coupling efficiency at the time. This will be described below. First, when light enters the optical waveguide from the end face of the optical waveguide, if the optical axis of the incident light and the optical axis of the optical waveguide match, the coupling efficiency is determined by the beam diameter of the incident light and the waveguide of the optical waveguide. It greatly depends on the relationship between the wave mode diameters.

[0008] The waveguide mode diameter referred to here indicates the entire width of 1 / e ² from the maximum value of the intensity in the intensity distribution of the fundamental mode of the guided light. Specifically, FIG.
As shown in the figure, in a configuration in which a clad 102 is formed around a square core 101 in a plane perpendicular to the optical axis of the optical waveguide, the width direction of the core is defined as the x-axis direction, and the height direction is defined as the y-axis. Assuming that the direction is the direction, the intensity distribution indicated by the contour line 103 generally indicates a Gaussian distribution in any direction, and the waveguide mode diameters are represented by w _2x and w _2y , respectively.

At this time, considering that both the incident light and the guided mode are almost Gaussian beams, the coupling efficiency is approximated by the following equation. efficient = [2 / {( _w1x / _w2x ) + ( _w2x / _w1x )}] x [2 / {( _w1y / _w2y ) + ( _w2y / _w1y )}] (1)

Here, the subscripts x and y will be described again. FIG. 2 is a diagram schematically illustrating a cross section of the optical waveguide. Here, the lower clad 102 is formed on the upper surface of the substrate 104.
b is formed, and a core 101 and an upper clad 102a are formed on the upper surface so as to surround the core 101. Here, in a plane perpendicular to the optical axis of the optical waveguide, when the direction parallel to the cladding film is the x direction and the deposition direction of the film is the y direction, the suffixes x and y are quantities in the respective directions. Represents that.

At this time, when the incident light and the guided mode are symmetric with respect to the optical axis, it is not necessary to consider the direction, and the coupling efficiency is expressed by the following equation. efficient = 4 / ｛(w ₁ / w ₂ ) + (w ₂ / w ₁ )｝ ² (2)

FIG. 3 is a graph showing the dependence of the coupling efficiency on the waveguide mode diameter. Here, the dependence of the coupling efficiency on the waveguide mode diameter w ₂ when the incident beam diameter w ₁ = 8 μm in the equation (2) is shown. In the figure, the horizontal axis represents the waveguide mode diameter, and the vertical axis represents the coupling efficiency. In the figure, the coupling efficiency is maximum when the waveguide mode diameter w ₂ = 8 μm. That is, the coupling efficiency is the best when the incident beam diameter w ₁ and the waveguide mode diameter w ₂ match, and the coupling efficiency deteriorates as the difference between the diameters of the two increases. Here, since the waveguide mode diameter of the optical waveguide depends on the size of the core, if the size of the core differs from the design value due to a manufacturing error or the like, the coupling efficiency also differs from the design value.

On the other hand, the incident beam diameter w _{1 is changed} to the waveguide mode diameter w ₂
It is known that when the distance is larger than the above, a decrease in the coupling efficiency with respect to the optical axis shift is suppressed. Here, FIG. 4 is a graph showing a change in coupling efficiency with respect to an optical axis shift. In the figure, the horizontal axis represents the optical axis deviation, and the vertical axis represents the normalized coupling efficiency. The coupling efficiency here is w ₁ ,
In each case where the relative magnitude of w ₂ is different, the peak time is normalized as 1.

As shown in FIG. ₁ , when w ₁ <w ₂ (specifically, w ₁ = 2.5 μm, w ₂ = 5.4 μm), the optical axis shift is obtained as shown by the solid line curve a. The coupling efficiency is rapidly degraded due to the That is, the so-called tolerance is severe.
Also, w ₁ ≒ w ₂ (specifically, w ₁ = 5.7 μm, w ₂ =
In the case of (5.4 μm), as shown by the curve b by the broken line, the degradation of the coupling efficiency due to the deviation of the optical axis becomes relatively moderate, but still severe. Further, w ₁ > w ₂ (specifically, w ₁
= 11.0 μm, w ₂ = 5.4 μm), the coupling efficiency is most moderately degraded due to the optical axis shift, as shown by the dotted line curve c.

However, if the incident beam diameter w ₁ = 8 μm and, for example, the waveguide mode diameter w ₂ = 5 μm in order to utilize such a property, as shown in FIG. Since the inclination is large, the change in the coupling efficiency with respect to the change in the waveguide mode diameter is large. On the other hand, if the waveguide mode diameter is w ₂ = 8 μm, which has the best coupling efficiency, the change in the coupling efficiency with respect to the change in the waveguide mode diameter is small, but the deterioration of the coupling efficiency due to the optical axis shift becomes severe. Would.

As described above, in the waveguide-type multi-beam light source, even if each waveguide is designed with the same parameters, if the cores of the fabricated waveguides are different, the light having the same beam diameter can be obtained. Is incident, the coupling efficiency of the waveguides varies for the above-described reason.

In addition, the waveguide mode diameter of the manufactured optical waveguide is measured every time, and the coupling efficiency is not varied.
There is also a method of adjusting the diameter of the incident beam to each optical waveguide, but this requires a lot of trouble and leads to an increase in cost.

In view of the above problems, the present invention can reduce the change in the coupling efficiency of an optical waveguide with a simple configuration. It is an object of the present invention to provide an optical waveguide capable of reducing variation in coupling efficiency.

[0019]

In order to achieve the above object, the present invention is characterized by having a core satisfying at least one of the following conditional expressions. In _{t a <t <t b h} a <h <h b where, t: core width h: a core height. However, the waveguide mode diameters in the width direction at the predetermined t _a and t _b are the same, and t _a <t ₀ <t _b with respect to the core width t ₀ at which the width in the width direction is minimum. . Further, a predetermined h _a, a waveguide mode size in the height direction is the same, respectively, in h _b, h _a relative core height h ₀ that the mode diameter in the height direction is minimized <h ₀ <h _b .

The coupling efficiency η of an optical waveguide having a core having a size within the above-mentioned conditional expression with respect to the beam diameter of incident light is characterized by satisfying the following expression. η _max = 0.5η _min where η _max : maximum value of coupling efficiency η η _min : minimum value of coupling efficiency η

More preferably, the coupling efficiency η of an optical waveguide having a core having a size within the above conditional expression with respect to the beam diameter of the incident light satisfies the following expression. η _max = 0.95 η _min

The present invention is characterized in that at least one of the following expressions is satisfied. t = t ₀ h = h ₀

Further, the invention is characterized by having a plurality of cores each satisfying at least one of the following conditional expressions. In _{t a <t <t b h} a <h <h b where, t: core width h: a core height. However, the waveguide mode diameters in the width direction at the predetermined t _a and t _b are the same, and t _a <t ₀ <t _b with respect to the core width t ₀ at which the width in the width direction is minimum. . Further, a predetermined h _a, a waveguide mode size in the height direction is the same, respectively, in h _b, h _a relative core height h ₀ that the mode diameter in the height direction is minimized <h ₀ <h _b .

Further, the coupling efficiency η of the optical waveguide having a plurality of cores having a size within the range of the conditional expression with respect to the beam diameter of the incident light satisfies the following expression for each core. I do. η _max = 0.95η _min where η _max : maximum value of coupling efficiency η η _min : minimum value of coupling efficiency η

Further, each core satisfies at least one of the following equations. t = t ₀ h = h ₀

[0026]

Embodiments of the present invention will be described below. In the present invention, the size of the core of the optical waveguide is selected so that the waveguide mode diameter does not greatly change even if the size of the core slightly varies due to a manufacturing error or the like. FIG.
Is a graph showing the relationship between the core size and the waveguide mode diameter. Here, the horizontal axis represents the core width, and the vertical axis represents the waveguide mode diameter. In general, as shown in the figure, the waveguide mode diameter of the optical waveguide is minimized at a certain core width, and the waveguide mode diameter increases as the core width increases or decreases. This is because if the core width is less than
This is because light seeps from the core to the cladding.

By selecting such a core width near the minimum waveguide mode diameter, the waveguide mode diameter does not greatly change, the variation in the coupling efficiency of the optical waveguide is suppressed, and the manufacturing accuracy is improved. As a result, the yield of the optical waveguide device is improved. Furthermore, in the waveguide type multi-beam light source, the manufacturing accuracy is relaxed and the variation in the mode diameter of the waveguide mode of each optical waveguide is small, so that the same coupling efficiency is obtained in all the optical waveguides, and the intensity of each beam is obtained. Can be suppressed. Note that the graph in the case of taking the core height described later on the horizontal axis has the same shape as the case of the core width and only the position is different, so the illustration is omitted, and the horizontal axis represents the core width. I also discuss using the graph when taken.

More specifically, referring to FIG. 5, it is desirable that an optical waveguide having a substantially rectangular core satisfy at least one of the following conditional expressions (3) and (4). . _{t a <t <t b (} 3) h a <h <h b (4) where, t: core width h: a core height.

[0029] However, given t _a, the guided mode diameter in the width direction of the t _b are the same (w _2x), the core width t ₀ the mode diameter in the width direction becomes a minimum _(w 2xmin) t _a <t ₀ <
t _b . The predetermined h _a, the waveguide mode size in the height direction of h _b is the same (w _2y), the core height h ₀ that the mode diameter in the height direction is minimized _(w 2ymin) h _a
<H ₀ <h _b .

Further, the coupling efficiency η of the optical waveguide having a core having a size within the range of the conditional expression (3) or (4) with respect to the beam diameter of the incident light satisfies the following expression (5). It is desirable to do. η _max = 0.5η _min (5) where, η _max : maximum value of coupling efficiency η η _min : minimum value of coupling efficiency η

Further, the coupling efficiency η of the optical waveguide having a core having a size within the range of the conditional expression (3) or (4) with respect to the beam diameter of the incident light satisfies the following expression (6). It is desirable to do. η _max = 0.95 η _min (6)

It is desirable that at least one of the following equations (7) and (8) is satisfied. t = t ₀ (7) h = h ₀ (8)

In an optical waveguide having a plurality of substantially rectangular cores, the respective cores are respectively defined by the conditional expressions (3) and (4).
It is desirable to satisfy at least one of the following.

Further, the coupling efficiency η of an optical waveguide having a plurality of cores having a size within the range of the conditional expression (3) or (4) with respect to the beam diameter of the incident light is calculated by the above expression ( It is desirable to satisfy 6).

It is desirable that each core satisfies at least one of the above equations (7) and (8).

Hereinafter, a procedure for manufacturing the optical waveguide of the present invention will be described. Quartz, a polyimide resin, an epoxy resin, or the like is used as a material of the film forming the clad layer and the core layer of the optical waveguide. In the present embodiment, a quartz-based material is used as an example. First, TEOS (Tetra Ethyl Ortho Silicate: Si (OC) is basically used as a film material.
₂ H ₅ ) ₄ ) and SiO ₂ by low-temperature plasma CVD.
Are formed. It is well known that doping SiO ₂ changes the refractive index.

For example, doping SiO ₂ with Ge increases the refractive index, and doping F decreases the refractive index. At this time, in accordance with (cladding layer / core layer / cladding layer) as the configuration of each layer of the optical waveguide, (SiO ₂ /
SiO _2: Ge / SiO ₂₎ doped with Ge to the core layer with increased refractive index as if, or (SiO _2:
A configuration is conceivable in which the cladding layer is doped with F to reduce the refractive index, as in F / SiO ₂ / SiO ₂ : F).
Here, a configuration in which F is doped will be described as an example.

FIG. 6 is a graph showing an example of how the refractive index changes with the doping amount of F. Here, the gas flow rate of C ₂ F ₆ at 0 ° C. and 1 atm (unit: scc
m: standard cubic centimeter per minute), and the refractive index of SiO ₂ formed on the vertical axis is taken. However, the film forming conditions are as follows: substrate temperature: 350 ° C. gas pressure: about 80 Pa TEOS / O ₂ : 12/100 sccm RF power: 300 w

Here, the substrate temperature refers to the temperature of a substrate on which a quartz substrate, which will be described later, is used when manufacturing an optical waveguide. TEOS / O ₂ is a flow rate of a mixture of these material gases and carrier gas. And R
F power is high frequency power applied in the atmosphere. As shown in the figure, first, when the doping amount of F is 0, the refractive index is maintained at about 1.473, but C ₂ F ₆
The refractive index gradually decreases as the flow rate of F, that is, the doping amount of F increases. When the flow rate is 30 sccm, the refractive index is 1.44.
It is as follows. Therefore, by adjusting the flow rate of C ₂ F ₆ , that is, the doping amount of F, it is possible to form the cladding layer in which the refractive index is reduced to a predetermined value.

FIG. 7 is a schematic view showing an example of a specific process for manufacturing an optical waveguide. In the figure, the specific configuration of the optical waveguide is shown from the light emission side. Here, a quartz substrate 11 having the same thermal expansion coefficient as that of the quartz substrate 11 is used so that cracks and the like due to thermal stress do not occur in the optical waveguide. First, as shown in FIG. 1A, a lower cladding layer 12a is formed on the upper surface of a quartz substrate 11. This is F-doped SiO
_{The two} films are formed to a thickness of about 15 μm using TEOS mixed with C ₂ F ₆ under the following film forming conditions.

RF power: 700 w Deposition temperature: 400 ° C. TEOS: 20 sccm O ₂ : 680 sccm C ₂ F ₆ : 37 sccm Gas pressure: about 53 Pa

Next, as shown in FIG. 3B, a core layer 13 is formed on the upper surface of the lower cladding layer 12a. This is the C ₂ F ₆ stops gas supply of TEOS containing, after evacuation of residual gas, a non-doped SiO ₂ film, C ₂ F ₆
Approximately 3 μm by TEOS without mixing
m.

RF power: 700 w Deposition temperature: 400 ° C. TEOS: 20 sccm O ₂ : 680 sccm Gas pressure: about 53 Pa

Further, as shown in FIG. 3C, an amorphous silicon film 14 is formed on the upper surface of the core layer 13 by a sputtering method to a thickness of 0.6 μm as a mask material. And
As shown in FIG.
A resist is applied to the upper surface by 0.2 μm and patterned by photolithography so as to have a core shape of the optical waveguide to form a resist 15.

In the state (d), reactive ion etching (RIE) using SF ₆ gas is performed.
By patterning the amorphous silicon film 14 and removing the resist 15 by ashing, the remaining portion becomes a mask 14a made of amorphous silicon as shown in FIG. Then, in the state of (e),
When the core layer 13 is patterned by RIE using CHF ₃ gas, the remaining portion becomes the core 13a as shown in FIG. The design value of the core width manufactured at this time is 2.2 μm.

Further, in the state of FIG.
An upper clad layer 12b is formed on the upper surface of 2a so as to cover the core 13a. This is because the F-doped SiO ₂ film is formed under the same film forming conditions as the lower clad layer 12a described above for about 15 minutes.
It is formed to a thickness of μm. As a result, as shown in FIG. 1G, the lower clad layer 12a and the upper clad layer 12b are integrated, the clad 12 surrounding the core 13a is formed, and an optical waveguide is manufactured.

The core width of the optical waveguide fabricated here was measured and found to be 2.4 μm. At this time, the wavelength 780
When the waveguide mode diameter was measured for light of nm,
It was 56 μm. Incidentally, the theoretical value of the waveguide mode diameter when the core width is 2.2 μm is 3.548 μm. Also, when a light having a beam diameter of 5 μm was made incident on this optical waveguide and the coupling efficiency was measured, it was 89.3%, which was only 0.2% smaller than the theoretical coupling efficiency of 89.1%. .

[0048]

As described above, according to the present invention,
With a simple configuration, it is possible to reduce the change in the coupling efficiency of the optical waveguide, and further, in a multi-beam light source,
It is possible to provide an optical waveguide capable of reducing variation in coupling efficiency between the optical waveguides.

More specifically, even if the core width slightly deviates from the design value due to a manufacturing error of the optical waveguide or the like, there is almost no change in the waveguide mode diameter. Little change in coupling efficiency occurs. Therefore, in the waveguide-type multi-beam light source, the coupling efficiency between the waveguides can be made the same, so that the intensity variation of the output light due to the coupling efficiency at the time of incidence can be suppressed.

[Brief description of the drawings]

FIG. 1 is a view for explaining the definition of a waveguide mode diameter.

FIG. 2 is a diagram schematically showing a cross section of an optical waveguide.

FIG. 3 is a graph showing the dependence of coupling efficiency on the waveguide mode diameter.

FIG. 4 is a graph showing a change in coupling efficiency with respect to an optical axis shift.

FIG. 5 is a graph showing a relationship between a core size and a waveguide mode diameter.

FIG. 6 is a graph showing an example of how the refractive index changes with the doping amount of F.

FIG. 7 is a schematic view showing an example of a specific manufacturing process of an optical waveguide.

[Explanation of symbols]

 101 core 102 clad 103 contour 104 substrate

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2C362 AA10 AA43 AA45 AA47 BA25 BA58 2H046 AA02 AD09 AZ04 AZ08 AZ09 2H047 KA03 PA05 PA21 PA24 QA04 QA05 RA04

Claims (7)

[Claims] 1. An optical waveguide having a core satisfying at least one of the following conditional expressions. In _{t a <t <t b h} a <h <h b where, t: core width h: a core height. However, the waveguide mode diameters in the width direction at the predetermined t _a and t _b are the same, and t _a <t ₀ <t _b with respect to the core width t ₀ at which the width in the width direction is minimum. . Further, a predetermined h _a, a waveguide mode size in the height direction is the same, respectively, in h _b, h _a relative core height h ₀ that the mode diameter in the height direction is minimized <h ₀ <h _b . 2. The coupling efficiency η of an optical waveguide having a core having a size within the range of the conditional expression with respect to the beam diameter of incident light.
The optical waveguide according to claim 1, wherein the following formula is satisfied. η _max = 0.5η _min where η _max : maximum value of coupling efficiency η η _min : minimum value of coupling efficiency η 3. The coupling efficiency η of an optical waveguide having a core having a size within the range of the conditional expression with respect to the beam diameter of the incident light.
The optical waveguide according to claim 1, wherein the following formula is satisfied. η _max = 0.95η _min where η _max : maximum value of coupling efficiency η η _min : minimum value of coupling efficiency η 4. The optical waveguide according to claim 1, wherein at least one of the following expressions is satisfied. t = t ₀ h = h ₀ 5. An optical waveguide having a plurality of cores each satisfying at least one of the following conditional expressions. In _{t a <t <t b h} a <h <h b where, t: core width h: a core height. However, the waveguide mode diameters in the width direction at the predetermined t _a and t _b are the same, and t _a <t ₀ <t _b with respect to the core width t ₀ at which the width in the width direction is minimum. . Further, a predetermined h _a, a waveguide mode size in the height direction is the same, respectively, in h _b, h _a relative core height h ₀ that the mode diameter in the height direction is minimized <h ₀ <h _b . 6. The coupling efficiency η of an optical waveguide having a plurality of cores having a size within the conditional expression range with respect to the beam diameter of incident light, wherein each core satisfies the following expression. The optical waveguide according to claim 5, wherein η _max = 0.95η _min where η _max : maximum value of coupling efficiency η η _min : minimum value of coupling efficiency η 7. The method according to claim 5, wherein each core satisfies at least one of the following expressions.
2. The optical waveguide according to 1. t = t ₀ h = h ₀