JP2004252148A - Dielectric multilayer film filter element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a dielectric multilayer film filter element in which an optical component having the dielectric multilayer film filter element nippedly held between a couple of collimator lenses can be small-sized by reducing a reflection attenuation quantity and a ripple of the optical component. <P>SOLUTION: The dielectric multilayer film filter element 4 has a 1st end surface slanted to an optical axis and a 2nd end surface parallel thereto. Consequently, reflected light is at a different angle from the optical axis on the 1st end surface and the reflection attenuation quantity is reducible. On the 2nd end surface, primary transmitted light and high-order transmitted light which is secondary light or later are different in projection position and further the high-order transmitted light which is the secondary light or later is attenuated through an AR coat. Consequently, interference can be deterred and the amplitude of a ripple is reducible. Furthermore, collimator lenses 3 and 5 and the filter element 4 can be arranged nearly on a straight line, so the optical component can be made small-sized. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a dielectric multilayer filter element used as a component of an optical amplifier or an optical demultiplexer / multiplexer used in optical communication and a dielectric multilayer filter type optical component using the same.
[0002]
[Prior art]
A dielectric multilayer filter is used as a wavelength selection element in optical communication equipment such as an optical amplifier and an optical demultiplexer / demultiplexer, and is made of a glass or the like having a predetermined thickness. A dielectric multilayer film is formed on one or both surfaces of a substrate by a method such as vapor deposition. The transmittance or reflectance at a predetermined wavelength of the dielectric multilayer filter can be designed to a predetermined value by adjusting the refractive index, thickness, and number of layers of the film material.
[0003]
Incidentally, in an optical amplifier, the gain of an erbium-doped optical fiber (hereinafter abbreviated as "EDF") as an amplification medium has wavelength dependence, and a wavelength band of 1530 nm to 1570 nm used in optical wavelength division multiplexing communication. In (2), a gain equalizer (hereinafter, abbreviated as “GFF”) is required to equalize the optical output at each signal wavelength after amplification.
[0004]
The dielectric multilayer filter is used as a gain equalizing element in the GFF because the transmittance of each wavelength can be designed to a predetermined value over a wide wavelength range. That is, by making the transmittance at each wavelength of the dielectric multilayer film filter inverse to the gain curve of the EDF, the fluctuation of the optical signal intensity for each signal wavelength is offset, and smoothing is performed over the entire wavelength region. The obtained uniform optical signal intensity can be obtained.
[0005]
FIG. 11 shows an example of the structure of a conventionally used GFF. FIG. 11A is a plan view of the GFF, and FIG. 11B is a side view. 11, reference numeral 1 denotes a first optical fiber, reference numeral 2 denotes a first optical fiber holding member for fixing the first optical fiber, reference numeral 3 denotes a first collimator lens, and reference numeral 4 denotes a dielectric multilayer film filter. Reference numeral 5 denotes a second collimator lens, reference numeral 6 denotes a second optical fiber holding member for holding the second optical fiber, and reference numeral 7 denotes a second optical fiber.
[0006]
Reference numeral 8 denotes a housing for housing these. The optical fiber holding members 2 and 6, the collimator lenses 3 and 5, and the dielectric multilayer filter element 4 are fixed with an adhesive after the optical axis is adjusted so as to obtain good characteristics. Is contained. FIG. 12 shows an example of the structure of the filter element 4 used in FIG. 12, reference numeral 4a denotes a filter substrate, reference numeral 4b denotes a dielectric multilayer film, reference numeral 4c denotes a first end surface of the filter substrate 4a, reference numeral 4d denotes a second end surface of the filter substrate 4a, and reference numeral 4e denotes an antireflection (Anti Reflection). Reference numeral 4f denotes an optical axis of the filter element (hereinafter, abbreviated as "AR coat").
[0007]
The filter element 4 in this example is a wedge shape in which one end surface of a rectangular parallelepiped is cut off obliquely, and a dielectric multilayer film is formed on an optical axis 4f on a first end surface 4c through which incident light is transmitted first. A second end face 4d facing the first end face and from which the incident light exits the filter element is inclined with respect to the first end face, and an AR coat 4e is provided thereon.
[0008]
In the GFF using the filter element 4, the optical signal amplified by the EDF is emitted from the first optical fiber 1, converted into a parallel light by the first collimator lens 3, and transmitted to the dielectric multilayer filter element 4. Incident. The light beam emitted from the dielectric multilayer filter element 4 enters the second collimator lens 5 and is focused on the second optical fiber 7. The dielectric multilayer filter element 4 transmits light of a predetermined wavelength at a predetermined transmittance.
[0009]
In a GFF used for an optical amplifier, a structure in which a dielectric multilayer filter element 4 (hereinafter abbreviated as a filter element) is sandwiched between two single-core collimators is generally used. It is disclosed in Japanese Patent Application No. 2002-205697.
[0010]
By the way, as described above, in the case of using a filter element having a rectangular parallelepiped outer shape and a structure in which a dielectric multilayer film is formed on the first end face 4c, light rays incident from the first end face 4c are directly transmitted from the second end face 4d. When the light is emitted, a part of the incident light is reflected by the second end face 4d, and after being reflected a plurality of times inside the filter substrate 4a, the light is emitted from the second end face 4d. It is called the next transmitted light. The order of high-order transmitted light increases with the number of internal reflections.
[0011]
The primary transmitted light and the higher-order transmitted light are condensed by a collimator lens and are superimposed to generate interference fringes, which are called ripples.
[0012]
The ripple is generated after the incident light passes through the dielectric multilayer filter and is gain flattened. The ripple degrades the gain flatness, so that the signal wavelength interval is, for example, 10 nm or less. In the optical wavelength division multiplexing communication as described above, the light intensity fluctuates between the signal wavelengths, which has an undesirable effect.
[0013]
In order to reduce the influence of the ripple caused by this interference, the reflectivity of the second end face 4d of the filter element may be reduced to lower the intensity of second-order and higher-order transmitted light. As a method for reducing the ripple, a method of applying an AR coating to the second end face 4d is generally used.
[0014]
However, even if a high-performance AR coating is applied, its substantial reflection suppression ability is about −30 dB, so that the ripple amplitude (maximum value−minimum value) is set to 0.01 dB or less, which is the target value. In order to achieve the required -65 dB, there is a shortage of 35 dB.
[0015]
Therefore, conventionally, as shown in FIG. 12, the second end face 4d of the filter element 4 is inclined to form a wedge, and the optical axes and angles of the primary transmitted light and the secondary and higher transmitted lights are shifted to cause interference. Has been reduced. That is, the interference is suppressed by reducing the coupling efficiency of the second and subsequent transmitted light to the second collimator lens 5, thereby reducing the ripple.
[0016]
By the way, a filter element having such a wedge is manufactured by the following steps.
Step 1: An AR coat is formed on one surface of the filter substrate by a technique such as vapor deposition. The AR coat is necessary to monitor the situation when forming the dielectric multilayer film.
Step 2: A dielectric multilayer filter having predetermined characteristics is formed on the other surface of the filter substrate by a technique such as vapor deposition.
Step 3: The filter substrate subjected to the above-described processing is fixed to a horizontal pedestal for cutting with an adhesive or the like.
[0017]
Step 4: The fixed filter substrate is cut into a rectangular shape having a predetermined size in order to facilitate post-processing and to keep the element size to some extent.
Step 5: The cut square filter substrate is removed from the cutting base.
[0018]
Step 6: The rear surface side of the rectangular filter substrate removed from the pedestal is wedge-processed to a predetermined angle (for example, 0.4 °). The AR coating applied to the back surface is removed in a polishing step in wedge processing.
Step 7: The wedge-processed surface is mirror-polished, and an AR coat is applied thereon again.
Step 8: The rectangular filter substrate is fixed to a cutting pedestal using an adhesive or the like in order to form an element.
Step 9: The square filter substrate is cut into dimensions of a predetermined filter element.
[0019]
Prior art documents relating to such a filter element and an optical component using the same include, for example, Japanese Patent Application No. 2002-205697 filed by the present applicant. (Filed on July 15, 2002)
[0020]
[Problems to be solved by the invention]
However, in the filter element which has been subjected to the wedge processing, in order to prevent the light reflected on the first end face from being incident on the first collimator lens again and deteriorating the return loss, FIG. As shown in b), it was necessary to tilt the second collimator lens with respect to the filter element.
[0021]
However, since the filter element, the collimator lens, and the fiber holding member cannot be arranged on a straight line, the size of the housing 8 for housing these optical components increases. The GFF is housed in the optical amplifier. However, if the GFF is too large, the size of the optical amplifier itself becomes large, and the degree of freedom in designing the optical amplifier is reduced. For the above-mentioned reason, the GFF is required to be miniaturized, and it is not desirable to increase the size of the optical component.
[0022]
Further, in the first collimator lens and the filter element, the optical axis adjustment work is performed to reduce the return loss, and in the second collimator lens and the filter element, the optical axis adjustment operation is performed to minimize the insertion loss. This is necessary, and there is a problem that the manufacturing cost increases.
[0023]
Further, as described above, in the manufacture of a wedge-shaped filter element, a wedge working step and an AR coat reworking step are added as compared with the case where no wedge processing is performed, so that the manufacturing cost increases. There was a problem.
[0024]
[Means for Solving the Problems]
In order to solve this problem, an invention according to claim 1 is a dielectric multilayer filter element having a hexahedral outer shape, wherein a first end face provided with a dielectric multilayer film and a second end face parallel to the first end face are provided. Is a dielectric multilayer filter element which is inclined at a predetermined angle with respect to the direction of the optical axis.
[0025]
According to a second aspect of the present invention, in the dielectric multilayer filter element according to the first aspect, an anti-reflection coating is applied to a second end face opposite to the first end face on which the dielectric multilayer film is formed. Is a dielectric multilayer filter element.
[0026]
According to a third aspect of the present invention, in the method of manufacturing a dielectric multilayer filter element, the filter substrate on which the dielectric multilayer film is formed is cut at a predetermined angle with respect to the cutting direction. This is a method for manufacturing a membrane filter element.
[0027]
According to a fourth aspect of the present invention, there is provided an optical component comprising the dielectric multilayer filter element according to the first or second aspect, a pair of collimator lenses and an optical fiber holding member, wherein the pair of collimators is provided. An optical component characterized in that the dielectric multilayer filter is sandwiched between lens lenses.
[0028]
According to a fifth aspect of the present invention, in the optical component according to the fourth aspect, an optical element is sandwiched between the dielectric multilayer filter and one of the collimator lenses. .
[0029]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail. 1 and 2 show examples of the filter element. FIG. 2 shows a cross section of FIG. The description of reference numerals 4a to 4f in FIGS. 1 and 2 is the same as that shown in FIG. Reference numeral 4h in FIG. 2 denotes a lower bottom surface parallel to the optical axis, and 4g denotes an upper bottom surface parallel to the optical axis. Also, t is the thickness of the filter element 4, and l is the width of the filter element 4. theta ₁ is the incident angle of the incident light, theta ₂ is a tilt angle of the filter element 4.
[0030]
The filter element 4 of this example has a hexahedral outer shape as shown in FIG. 1, and a dielectric multilayer film 4b is formed on one end face (first end face) 4c. The first end face 4c and the second end face 4d facing the first end face 4c are parallel to each other, and the other four faces constitute a prism having a rectangular cross section.
[0031]
Further, as shown in FIG. 1, the first end face 4c on which the dielectric multilayer film is formed is inclined with respect to the optical axis 4f of the filter element. The inclination angle theta ₂ is, for example, a 1 ° to 20 °. However, in FIG. 1 and FIG. 2, this angle is emphasized for ease of understanding.
[0032]
Further, in FIG. 2, the straight line A-A is perpendicular to the first end surface 4c, the angle theta ₁ of the straight line A-A and the incident light beam (i.e. the direction of the optical axis) is the incident angle of the incident light. A straight line B-B is perpendicular vertical bottom 4g filter element, to 4f, the linear B-B and the angle of the end face 4c is a tilt angle theta ₂ of the filter element. The incident direction is a direction along the optical axis 4f, the upper and lower bottom surface 4g of the filter element, since it is parallel to 4f, theta ₁ and theta ₂ are equal.
[0033]
Next, a method for manufacturing the filter element according to the present invention will be described. FIG. 3 shows an example of a procedure for producing a filter element. 3, reference numeral 11 denotes a filter substrate having an upper bottom surface and a lower bottom surface parallel to each other, reference numeral 12 denotes an AR coat, reference numeral 13 denotes a dielectric multilayer film, reference numeral 14 denotes an inclined base for fixing the filter substrate, and reference numeral 15 denotes It is a filter element. The manufacturing process of the filter element according to the present invention is as follows.
[0034]
Step 1: An AR coat 12 is applied to one surface of the filter substrate 11 because it is necessary to monitor the state of film formation when forming a dielectric multilayer film. The substrate size is, for example, an outer diameter of 100 mm and a thickness of 2 mm.
Step 2: The dielectric multilayer filter 13 is formed on the other surface of the filter substrate 11 by a technique such as vapor deposition.
[0035]
Step 3: The filter substrate 11 is fixed to the inclined surface of the cutting pedestal 14 having a horizontal surface and a surface inclined at a predetermined angle opposite thereto with an adhesive or the like.
Step 4: The horizontal surface of the pedestal 14 to which the filter substrate 11 is fixed is mounted on a cutting device (not shown). The cutting device generally employs a method in which a disc-shaped grindstone is rotated, and this is applied to a sample to grind and cut. However, the pedestal 14 is mounted so that the grinding direction is perpendicular to the horizontal plane of the pedestal 14. Is attached to the cutting device. By doing so, the cut surfaces of the filter substrate to be cut are respectively parallel and inclined with respect to the upper and lower bottom surfaces of the filter substrate. The filter element 15 is obtained by cutting the filter substrate 11 into a dice pattern.
[0036]
As shown in FIG. 1, the filter element 15 manufactured in this manner has a hexahedral shape that is inclined with respect to the optical axis and includes two surfaces parallel to each other and four surfaces parallel to the optical axis and parallel to each other. Having. The size of the cut filter element is, for example, 1.3 mm square.
[0037]
As shown in the manufacturing process of FIG. 3, the filter element according to the present invention does not require a wedge forming step, and has five steps (a step of cutting into a size that can be easily processed, a step of forming a wedge) as compared with the above-described conventional method. Therefore, the steps of fixing the filter substrate to the pedestal, forming the wedge, removing the filter substrate, and recoating the AR substrate after forming the wedge can be reduced. That is, the cost can be significantly reduced as compared with the conventional method.
[0038]
Next, a GFF using such a filter element will be described. FIG. 4 shows an example of a GFF using such a filter element. FIG. 4A is a plan view, and FIG. 4B is a side view. FIG. 5 shows ray tracing diagrams on the incident side and the outgoing side.
[0039]
4A and 4B and the description of the reference numerals 1 to 8 in FIG. 5 is the same as that shown in FIG. In the structure of the present invention, the light beam emitted from the first collimator lens 3 is incident on the dielectric multilayer filter 4b along the optical axis 4f in parallel with the upper and lower bottom surfaces 4g and 4h of the filter element 4. Since the first end face 4c is inclined with respect to the incident light beam, the reflected light on the filter surface is reflected at an angle different from the incident direction of the first collimator lens 3, and is reflected by the first fiber 1. No light is collected. Therefore, the return loss does not deteriorate.
[0040]
Further, the intensity of the secondary transmitted light is sufficiently attenuated by the AR coat 4e provided on the second end face 4d, even though the primary transmitted light and the secondary transmitted light passing through the filter element 4 become parallel. . Further, since the second-order and higher-order transmitted light is emitted to a position different from the first-order transmitted light, the high-order transmitted light coupled to the second collimator lens 5 is sufficiently reduced, and the first-order transmitted light is reduced. Interference between the light and the second and higher transmitted light is negligibly small. Therefore, almost no ripple occurs due to interference between the primary transmitted light and the secondary and higher transmitted light.
[0041]
Also, as can be seen by comparing the structure of the conventional device shown in FIG. 11B with the structure of the present invention shown in FIG. 4B, in the present invention, the first and second filter elements are provided with respect to the filter element. It is not necessary to tilt the collimator lens, and the collimator lenses can be arranged substantially linearly, so that the optical axes of the respective optical elements can be easily aligned, and the size of the housing can be reduced.
[0042]
Here, the incident angle theta ₁ to the filter element 4 according to the present invention with a second collimating the secondary transmission light - an example of calculation of the relationship between the coupling efficiency to Tarenzu shown FIG. A commercially available “Selfoc” lens is used as the lens, the beam diameter of the incident light on the filter element 4 (the diameter of the incident parallel light beam) is 400 μm, and the return loss of the AR coat on the second end face is −30 dB. The curve a shows the calculation result when the filter element thickness t is 1 mm, and the curve b shows the calculation result when t is 2 mm.
[0043]
From FIG. 6, in order to reduce the coupling efficiency to −35 dB or less, an incident angle of 8 ° or more is required when the thickness t of the filter element is 2 mm and 16 ° or more when the thickness t of the filter element is 1 mm. You can see that there is.
[0044]
However, in general, when the incident angle θ ₁ is increased, the value of polarization dependent loss (hereinafter, abbreviated as “PDL”) increases, so that the incident angle θ ₁ (that is, the inclination angle θ _2). It is better to increase the thickness t of the filter element than to increase the value of).
[0045]
In order to solve the problem of the PDL, it is preferable to use a quartz rod lens having a beam diameter of about 140 μm incident on the filter element 4 as the collimator lens. Here, when the beam diameter is 140 μm, the thickness t of the filter element is 2 mm, and the return loss of the AR coat 4 e is −30 dB, the relationship between the incident angle θ ₁ and the coupling efficiency of the secondary transmitted light to the light receiving element. Is shown in FIG.
[0046]
In FIG. 7, a curve c shows a calculation result when the thickness t of the filter element is 1 mm, and a curve d shows a calculation result when the thickness t is 2 mm. From FIG. 7, in order to reduce the coupling efficiency to −35 dB or less, when the element thickness t is 2 mm, the incident angle is 4.2 ° or more, and when the chip thickness t is 1 mm, the incident angle is 8.2 ° or more. It can be seen that θ ₁ is necessary.
[0047]
As described above, by using the quartz rod lens as the collimator lens, the overall size of the GFF can be further reduced, so that the degree of freedom when housed in the optical amplifier can be increased. When a collimator having a beam diameter of 400 μm is used, the size of a general optical component is 5.5 mm in outer diameter × 40 mm in length, whereas when a quartz rod lens having a beam diameter of 140 μm is used. The outer diameter is 2.5 mm and the length is 25 mm. The far, bi - beam diameter 140 .mu.m, has been described 400μm ones, Other bi - be of beam diameter, same effect by the inclination angle theta ₂ of the filter element to the appropriate angle to obtain Can be
[0048]
Next, specific examples of the GFF according to the present invention will be described. FIGS. 8 and 9 show an example of the characteristics of the prototype GFF. The structure of the prototype GFF is shown in FIG. 4, and the structure of the filter element is that shown in FIG. The thickness t of the filter element is 1 mm to 1.4 mm, and the width 1 is 0.5 mm to 2 mm. Further, the inclination angle of the filter element is 2.5 ° to 4 °. The filter element used has a dielectric multilayer film on the first end face and an AR coat on the second end face. The collimator lens is a quartz rod lens having an outer diameter of 400 μm and a beam diameter of 140 μm.
[0049]
In FIG. 8, a curve e is a target loss characteristic for flattening a gain characteristic of the EDF, and a curve f is a loss characteristic of a prototype GFF. It can be seen that the loss characteristics of the GFF match well with the target loss characteristics. The curve g in FIG. 9 shows the gain residual of the GFF (the difference between the curve e and the curve f). The gain wavelength dependence of the erbium-doped optical fiber is sufficiently smoothed, and the gain residual is obtained. Shows an excellent value of 0.21 dB. Furthermore, the amplitude of the ripple is 0.01 dB or less, which indicates that the filter element according to the present invention sufficiently reduces the ripple.
[0050]
Further, as shown in FIG. 10, an optical element such as an isolator element or a dielectric multilayer filter element having another function is inserted between the filter element 4 and the second collimator lens 5. A composite optical component with additional functions can be configured.
[0051]
【The invention's effect】
As described above, the dielectric multilayer filter element according to claim 1 reduces the return light in the incident direction by the inclined first end face, and the second end face is parallel to the first end face. This reduces ripples due to multiple reflections inside the element. Further, in the dielectric multilayer filter element of the second aspect, the ripple reduction effect is further enhanced by applying an AR coating to the second end face. According to a third aspect of the present invention, a method of manufacturing a filter element includes a step of fixing a filter substrate to a base inclined at a predetermined angle with respect to a cutting direction when cutting a filter substrate, and cutting the filter substrate. It is a reduced number. An optical component according to a fourth aspect is an optical component having a structure in which collimator lenses on the incident side and the output side are arranged before and after the filter element according to the first or second aspect. Can be arranged on a straight line, so that the size can be reduced and the ripple can be reduced. According to a fifth aspect of the present invention, in the optical component of the fourth aspect, an optical element such as an isolator is inserted between the dielectric multilayer filter and the second collimator lens to further expand the function. is there.
[Brief description of the drawings]
FIG. 1 is a perspective view showing an example of a dielectric multilayer filter element of the present invention.
FIG. 2 is a view showing a cross section of the dielectric multilayer filter element shown in FIG. 1;
FIG. 3 is a diagram illustrating an example of a manufacturing process of the dielectric multilayer filter element of the present invention.
FIG. 4 is a schematic configuration diagram showing an example of a dielectric multilayer filter element type optical component of the present invention.
FIG. 5 is a diagram showing ray tracing of the dielectric multilayer filter element type optical component of the present invention.
FIG. 6 is a table showing a relationship between an incident angle of a light beam and a coupling efficiency to a second collimator lens in the dielectric multilayer filter element according to the present invention.
FIG. 7 is a table showing a relationship between an incident angle of a light beam and a coupling efficiency to a second collimator lens in the dielectric multilayer filter element according to the present invention.
FIG. 8 is a chart showing a loss characteristic of a GFF according to the present invention and a target loss characteristic calculated from a gain characteristic of an EDF to be flattened.
FIG. 9 is a table showing gain residuals of the GFF according to the present invention.
FIG. 10 is a schematic configuration diagram showing an example of a composite optical component according to the present invention.
FIG. 11 is a perspective view showing an example of a conventional dielectric multilayer filter element.
FIG. 12 is a schematic configuration diagram showing an example of a conventional dielectric multilayer filter element type optical component.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... First optical fiber, 2 ... First optical fiber holding member, 3 ... First collimator lens, 4 ... Filter element, 4b ... Dielectric multilayer film, 4c ... First end face, 4d ... Second end face, 4e ... AR coat, 5 ... Second collimator lens, 6 ... Second optical fiber holding member, 7 ... · Second optical fiber, 8 ··· housing, 9 ··· optical element

Claims (5)

The dielectric multilayer filter element has a hexahedral outer shape, and the first end face provided with the dielectric multilayer film and the second end face parallel thereto are inclined at a predetermined angle with respect to the direction of the optical axis. A dielectric multilayer filter element characterized by the above-mentioned. 2. The dielectric multilayer filter element according to claim 1, wherein an anti-reflection coating is applied to a second end face opposite to the first end face on which the dielectric multilayer film is formed. . A method for manufacturing a dielectric multilayer filter element, comprising cutting a filter substrate on which a dielectric multilayer film is formed at a predetermined angle with respect to a cutting direction. 3. An optical component comprising the dielectric multilayer filter element according to claim 1 and a pair of collimator lenses and an optical fiber holding member, wherein the dielectric multilayer filter element is provided between the pair of collimator lenses. An optical component characterized in that a membrane filter is sandwiched. 5. The optical component according to claim 4, wherein an optical element is interposed between the dielectric multilayer filter and one of the collimator lenses.

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