JP2005141075A - Phase mask for manufacturing grating - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress the 0th-order light component transmitting without diffracted to the minimum and to prevent noises in the reflection spectrum of a transferred optical waveguide grating. <P>SOLUTION: The phase mask 21 has a grating type pattern with repetition of recessed grooves 26 and projections 27 having a square cross section on one surface of a transparent substrate, and is used to form a grating in an optical waveguide 22 by interference fringes of diffracted light of UV exposure light 23 by the repeated pattern. The phase mask 21 is constituted in such a manner that the cross sectional dimensions of the repeated pattern of the recessed grooves 26 and the projections 27 satisfy the relation of d/d<SB>0</SB>≥1.03 and f≤0.48, wherein d is the groove depth of the recessed groove, w is the width of the projection, Λ is the period of repetition, λ is the exposure wavelength, n<SB>2</SB>is the refractive index of the transparent substrate, n<SB>1</SB>is the refractive index of the atmosphere, d<SB>0</SB>is defined by d<SB>0</SB>=λ/ä2(n<SB>2</SB>-n<SB>1</SB>)}, and f is defined by f=w/Λ. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a phase mask for producing a diffraction grating, and more particularly to a phase mask for producing a diffraction grating using an ultraviolet laser beam in an optical fiber used for optical communication or the like.

  Optical fiber has revolutionized global communications, enabling high-quality, high-capacity transoceanic telephone communications, but conventionally, a refractive index profile has been created periodically in the core along this optical fiber, A black diffraction grating, and the wavelength of the diffraction grating for optical communication is determined by determining the reflectivity level and width of the wavelength characteristics according to the period and length of the diffraction grating and the size of the refractive index modulation. It is known that it can be used as a splitter, a narrow-band high-reflection mirror used for a laser or a sensor, a wavelength selection filter for removing an extra laser wavelength in an optical fiber amplifier, and the like.

  However, since the attenuation of quartz optical fiber is minimized and the wavelength suitable for the long-distance communication system is 1.55 μm, in order to use the optical fiber diffraction grating at this wavelength, the grating interval needs to be about 500 nm. It is said that it is difficult to make such a fine structure in the core at first, and side polishing, photoresist process, holographic exposure, reactive ions are used to create a black diffraction grating in the core of the optical fiber. Many stages of complicated processes, such as beam etching, were taken. For this reason, the production time was long and the yield was low.

  However, recently, a method of irradiating an optical fiber with an optical fiber and directly changing the refractive index in the core to make a diffraction grating has been known, and this method of irradiating the ultraviolet light does not require a complicated process. It has come to be implemented gradually as technology advances.

  In the case of this method using ultraviolet light, since the grating interval is as small as about 500 nm as described above, an interference method in which two light beams interfere with each other (a single diffraction grating surface is formed by condensing a single pulse from an excimer laser). There are a method of writing each point), a method of irradiating using a phase mask having a grating, and the like.

  The interference method that causes the two light beams to interfere with each other has a problem in the quality of the beam in the lateral direction, that is, spatial coherence, and the method of writing by each point requires precise step control of submicron size. In addition, it is required to write a lot of surfaces by taking in a small amount of light, and there is a problem in workability.

  For this reason, an irradiation method using a phase mask has been attracting attention as a method that can cope with the above problem. However, as shown in FIG. 1A, this method has a concave groove on one surface of a quartz substrate. Using a phase mask 21 provided at a predetermined depth with a predetermined repetition period, KrF excimer laser light (wavelength: 248 nm) 23 is irradiated with the mask 21 to cause a change in refractive index directly on the core 22A of the optical fiber 22, A grating is formed. In FIG. 1A, the interference fringe pattern 24 in the core 22A is shown in an easily enlarged manner. FIG. 1B and FIG. 1C show a sectional view of the phase mask 21 and a part of a top view corresponding thereto. The phase mask 21 has a binary phase type diffraction grating-like structure in which a concave groove 26 having a repetition period Λ and a depth d is provided on one surface, and a ridge 27 having substantially the same width is provided between the concave grooves 26. It is.

The depth d of the groove 26 of the phase mask 21 (the difference in height between the ridge 27 and the groove 26) d is selected so as to modulate the phase of the excimer laser beam (beam) 23 as the exposure light by π radians. The 0th-order light (beam) 25A is suppressed to 5% or less by the phase mask 21, and the main light (beam) emitted from the mask 21 includes the plus-first-order diffracted light 25B including 35% or more of the diffracted light. And the first-order diffracted light 25C. Therefore, irradiation of interference fringes with a predetermined pitch is performed by the plus first-order diffracted light 25B and minus first-order diffracted light 25C, and a refractive index change at this pitch is brought into the optical fiber 22.
“SPIE” Vol. 883 (1988), p. 8-11

  The cross-sectional shape of the phase mask 21 is such that the depth d of the groove 26 shifts the phase of the exposure light 23 having the wavelength λ by π radians as described above, and the width w of the ridge 27 and the repetition period Λ. It is said that the diffraction efficiency of the 0th-order light component 25A is minimized when the duty ratio f of the ridge 27 defined by the ratio w / Λ is 0.5 (scalar diffraction theory).

  However, if the period becomes the wavelength order due to the miniaturization of the repetition period, the 0th-order light component 25A is transmitted by about 3%. Therefore, the 0th-order light component 25A becomes noise during transfer, and the transferred optical waveguide diffraction There was a problem that noise was generated in the reflection spectrum of the grating.

  The present invention has been made in view of such problems of the prior art, and its purpose is to reduce the zero-order light component that is transmitted without being diffracted as much as possible in the reflection spectrum of the transferred optical waveguide diffraction grating. It is an object of the present invention to provide a phase mask for producing a diffraction grating that is free from noise.

The phase mask for producing a diffraction grating of the present invention that achieves the above object is provided with a repetitive pattern of concave grooves and ridges having a substantially rectangular cross section in the shape of a grating on one surface of a transparent substrate, and ultraviolet rays generated by the repetitive pattern. In the phase mask in which the diffraction grating is formed in the optical waveguide by the interference fringes of the diffracted light of the exposure light, the cross-sectional shape dimension of the repeated pattern of the concave groove and the convex stripe is d: groove depth of the concave groove, w: Width of ridge, Λ: repetition period, λ: exposure wavelength, n ₂ : refractive index of transparent substrate, n ₁ : refractive index of atmosphere, d ₀ = λ / {2 (n ₂ −n ₁ )}, f = When w / Λ,
d / d ₀ ≧ 1.03, f ≦ 0.48
It is characterized by satisfying the relationship.

In this case, the exposure light is S-polarized light,
d / d ₀ ≧ 1.03, f ≦ 0.47
It is desirable to satisfy this relationship.

The exposure light is P-polarized light,
d / d ₀ ≧ 1.05, f ≦ 0.48
It is desirable to satisfy this relationship.

Also, the exposure light is randomly polarized,
d / d ₀ ≧ 1.04, f ≦ 0.48
It is desirable to satisfy this relationship.

  Note that it is desirable to use synthetic quartz as the transparent substrate.

  The present invention also includes an optical fiber grating produced by exposure using the above-described phase mask for producing a diffraction grating.

  According to the phase mask for producing a diffraction grating of the present invention, the cross-sectional shape of the repetitive pattern of the concave grooves and ridges is substantially rectangular, and the cross-sectional shape dimension of the repetitive pattern of the concave grooves and ridges is the transmission zero-order light diffraction efficiency. Since it is optimized so as to be minimized, an interference fringe having a predetermined pitch is formed in an optical waveguide such as an optical fiber by irradiating this phase mask for producing a diffraction grating with ultraviolet exposure light, and interference between ± 1st order diffracted lights. By producing the diffraction grating by generating the refractive index change in the pitch of the interference fringes in the optical waveguide, a high-quality diffraction grating that does not generate noise in the reflection spectrum can be produced.

  As described above, the diffraction efficiency of each diffraction order of the binary phase type diffraction grating (phase mask) 21 as shown in FIGS. 1B and 1C is determined by the vector diffraction theory (Non-patent Document 1). It can be determined strictly.

Accordingly, the cross-sectional shape of the phase mask 21 that minimizes the 0th-order light diffracted light 25A when the exposure light 23 is incident on the phase mask 21 perpendicularly is obtained by this vector diffraction theory. The results are shown in FIGS. As a result, the horizontal axis is a value Λ / λ obtained by normalizing the repetition period Λ of the ridge 27 with the exposure wavelength λ, and FIG. 2 shows that the depth d of the concave groove 26 is the phase of the wavelength λ of the exposure light 23 by π. radians shifting depth _{d 0 (= λ / {2} (n 2 -n 1)}; n 2 is the refractive index of the transparent substrate, n ₁ is the refractive index of air) vertically value d / d ₀ normalized with In FIG. 3, the duty ratio f of the ridges 27 is the vertical axis. This result was obtained for the case of λ = 248 nm, n ₂ = 1.508 (synthetic quartz), and n ₁ = 1. In the case of S-polarized light, the polarization plane is orthogonal to the concave groove 26, in the case of P-polarized light, the polarization plane is parallel to the concave groove 26, and AVE is in the case of random polarization. The results are shown in Table 1 below.

Table 1. Optimum conditions for each polarization that minimizes the diffraction efficiency of transmitted zero-order light S-polarized light Λ / λ 1.61 2.02 2.42 2.82 3.23 4.03 8.06 12.10
─────────────────────────────────────
d / d ₀ 1.19 1.08 1.12 1.12 1.07 1.05 1.03 1.02
f 0.251 0.363 0.400 0.396 0.408 0.435 0.469 0.479
─────────────────────────────────────
Zero-order light diffraction efficiency (%) 1.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
─────────────────────────────────────
○ P-polarized light Λ / λ 1.61 2.02 2.42 2.82 3.23 4.03 8.06 12.10
─────────────────────────────────────
d / d ₀ 1.23 1.21 1.19 1.16 1.13 1.10 1.05 1.03
f 0.373 0.381 0.437 0.434 0.449 0.461 0.483 0.489
─────────────────────────────────────
Zero-order diffraction efficiency (%) 3.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0
─────────────────────────────────────
○ Random polarization Λ / λ 1.61 2.02 2.42 2.82 3.23 4.03 8.06 12.10
─────────────────────────────────────
d / d ₀ 1.12 1.13 1.15 1.14 1.09 1.08 1.04 1.03
f 0.368 0.389 0.413 0.414 0.434 0.449 0.476 0.484
─────────────────────────────────────
Zero-order diffraction efficiency (%) 4.4 0.8 0.3 0.4 0.3 0.2 0.0 0.0
───────────────────────────────────── ─.

  By the way, light having a wavelength of 1700 nm or less is used for communication using an optical fiber. Since this wavelength is about 1200 nm in an optical fiber having a refractive index of 1.45, interference fringes (black diffraction grating) with a pitch of about 600 nm or less are required for reflection. In order to produce such interference fringes with the arrangement of FIG. 1, the repetition period Λ of the ridges 27 of the phase mask 21 is about 1200 nm. If the exposure is performed with ultraviolet rays from an argon fluoride excimer laser having a wavelength λ = 193 nm, the phase mask 21 in the range of Λ / λ ≦ 7 is required.

2 and 3 and Table 1 above, as such a phase mask 21, when S-polarized light is used, d / d ₀ ≧ 1.03, f ≦ 0.47, when P-polarized light is used. , D / d ₀ ≧ 1.05, f ≦ 0.48, and when using random polarization, it is necessary to satisfy the conditions of d / d ₀ ≧ 1.04 and f ≦ 0.48. I understand. When the polarization state is not questioned, it can be said that d / d ₀ ≧ 1.03 and f ≦ 0.48 must be satisfied. By using the phase mask 21 in the above range, a high-quality diffraction grating that does not generate noise in the reflection spectrum can be produced in an optical waveguide such as an optical fiber.

  When obtaining the results of Table 1 above, the optimum diffraction efficiency of transmitted 0th-order light of S-polarized light and P-polarized light is substantially 0 (less than 0.05%) when Λ / λ is 2.02 or more. I found out that

  Next, examples of the phase mask for producing a diffraction grating according to the present invention will be described.

  FIG. 4 is a cross-sectional view showing an example of the manufacturing process of the phase mask 1. 4, 5 is a phase mask blank, 2 is a quartz substrate, 4 is a chromium thin film, 4A is a chromium thin film pattern, 4B is a chromium thin film opening, 6 is an electron beam resist, 6A is a resist pattern, and 6B is a resist opening. , 7 is an electron beam (beam), 1 is a phase mask, 3 is a groove having a rectangular wave shape in cross section, and 8 is a ridge having a rectangular wave shape in cross section.

  First, as shown in FIG. 4A, blanks 5 in which a chromium thin film 4 having a thickness of 20 nm was formed on a quartz substrate 2 by sputtering were prepared. The chromium thin film 4 serves to prevent charge-up when the electron beam resist 6 in the subsequent process is irradiated with the electron beam 7 and serves as a mask when the concave groove 3 is formed on the quartz substrate. The control of the thickness is also important in terms of resolution, and a thickness of 10 to 20 nm is appropriate.

  Next, as shown in FIG. 4B, an electron beam resist ZEP (manufactured by Nippon Zeon Co., Ltd.) was used as the electron beam resist 6 and applied to a thickness of 400 nm and dried.

Thereafter, as shown in FIG. 4C, the electron beam resist 6 is exposed to an electron beam drawing apparatus MEBES 4500 (manufactured by Applied Materials) with an exposure amount of 1.2 μC / cm ² , and the portion corresponding to the groove 3 is an electron. Exposure was by beam 7.

  After exposure, baking is performed at 90 ° C. for 5 minutes (PEB: Post Exposure Baking), and then the electron beam resist 6 is developed with 2.38% concentration of TMAH (tetramethylammonium hydroxide), as shown in FIG. A desired resist pattern 6A was formed. Note that the post-exposure bake (PEB) is for selectively increasing the sensitivity of the portion irradiated with the electron beam 7.

Next, using the resist pattern 6A as a mask, dry etching using CH ₂ Cl ₂ gas was performed to form a chromium thin film pattern 4A as shown in FIG.

Next, as shown in FIG. 4F, the quartz substrate 2 was etched by a depth of 272 nm or 244 nm using CF ₄ gas with the chromium thin film pattern 4A as a mask. The depth is controlled by controlling the etching time, and the etching can be performed by controlling the depth in the range of 200 to 400 nm.

  Thereafter, the resist pattern 6A is peeled off with sulfuric acid at 70 ° C., and then the chromium thin film pattern 4A is removed by etching with a ceric ammonium nitrate solution, followed by a cleaning process, as shown in FIG. 4 (g). A phase mask 1 in which lines (protrusions 8) and spaces (concave grooves 3) are arranged in a rectangular wave cross-sectional shape was obtained.

The phase mask 1 has a pitch of 600 nm, Λ / λ = 2.42, applicable to the present invention (i) depth 272 nm (d / d ₀ ≈1.12), and duty ratio f of the ridge 8 f = Two types, 0.400, and Comparative Example (ii), depth 244 nm (d ₀ ) and duty ratio f = 0.500 of the ridge 8 were prepared. Then, using each of (i) and (ii) as a phase mask, an ultraviolet laser beam having a wavelength of 248 nm is used to enter the phase mask 1 perpendicularly, and the polarization becomes S-polarized light with respect to the diffraction grating of the phase mask. In this way, a fiber-optic grating was produced by transferring it into an optical waveguide of a Ge-doped photosensitive optical fiber.

  As an evaluation result of the optical fiber grating, those produced by Comparative Example (ii) had low characteristics, and those of the optical fiber grating produced by the phase mask according to (i) of the present invention were high. The reason for this is that such a transfer is considered to be noise in the first-order or higher-order interference of transmitted diffracted light, and the transmitted zero-order light is considered to be noise, and the phase mask according to (ii) is compared with the transmitted zero-order light diffraction efficiency of about 10%. In contrast, the contrast of interference fringes is reduced due to transfer noise, and the phase mask according to (i) has a relatively low transmission zero-order light diffraction efficiency of about 1%, and the transfer noise is small. This is probably because the contrast of the interference fringes was high.

  As described above, the phase mask for producing a diffraction grating of the present invention has been described based on the principle and examples. However, the present invention is not limited to these examples, and various modifications are possible.

It is a figure for demonstrating the phase mask of this invention used for optical fiber processing. It is a figure which shows one result which calculated | required the cross-sectional shape of the phase mask from which 0th-order light diffracted light becomes the minimum by the vector diffraction theory. It is a figure which shows another result which calculated | required the cross-sectional shape of the phase mask from which 0th-order light diffracted light becomes the minimum by the vector diffraction theory. It is sectional drawing which shows the manufacturing process of Example 1 of the phase mask by this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Phase mask 2 ... Quartz substrate 3 ... Concave groove 4 with a rectangular cross section ... Chrome thin film 4A ... Chrome thin film pattern 4B ... Chrome thin film opening 5 ... Phase mask blank 6 ... Electron beam resist 6A ... Resist pattern 6B ... Resist opening Part 7 ... Electron beam (beam)
8 ... ridges 21 having a rectangular wave shape in section ... phase mask 22 ... optical fiber 22A ... optical fiber core 23 ... KrF excimer laser light (exposure light)
24 ... Interference fringe pattern 25A ... 0th order light diffracted light 25B ... Plus 1st order diffracted light 25C ... Minus 1st order diffracted light 26 ... Groove 27 ... Projection

Claims (6)

A concave pattern having a substantially rectangular cross section in cross section and a convex stripe are provided on one surface of the transparent substrate, and a diffraction grating is formed in the optical waveguide by interference fringes of diffracted light of ultraviolet exposure light by the repeated pattern. The cross-sectional shape dimensions of the groove and ridge repetitive pattern are d: groove depth of the groove, w: width of the ridge, Λ: repetition period, λ: exposure wavelength, n ₂ : refractive index of transparent substrate, n ₁ : refractive index of atmosphere, d ₀ = λ / {2 (n ₂ −n ₁ )}, f = w / Λ
d / d ₀ ≧ 1.03, f ≦ 0.48
A phase mask for manufacturing a diffraction grating, characterized by satisfying the relationship: The exposure light is S-polarized light;
d / d ₀ ≧ 1.03, f ≦ 0.47
The phase mask for producing a diffraction grating according to claim 1, wherein the following relationship is satisfied. The exposure light is P-polarized light,
d / d ₀ ≧ 1.05, f ≦ 0.48
The phase mask for producing a diffraction grating according to claim 1, wherein the following relationship is satisfied. The exposure light is randomly polarized,
d / d ₀ ≧ 1.04, f ≦ 0.48
The phase mask for producing a diffraction grating according to claim 1, wherein the following relationship is satisfied. The phase mask for producing a diffraction grating according to any one of claims 1 to 4, wherein the transparent substrate is made of synthetic quartz. An optical fiber grating produced by exposure using the phase mask for producing a diffraction grating according to any one of claims 1 to 5.

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