JPH06110018A - Polarization-independent composite filter device - Google Patents

Abstract

(57) [Summary] [Purpose] Transmitted light with a constant and stable power can be obtained without depending on the polarization state of incident light. [Structure] A filter 10 which is a long-wavelength pass filter is arranged obliquely with respect to incident light, and a birefringent plate 12 is arranged in front of the incident side of the filter 10 perpendicular to the optical axis. Here, the filter 10 has a structure in which a dielectric multilayer film is vapor-deposited on the incident surface 10a of a parallel plate of glass and an antireflection film is vapor-deposited on the emission surface 10b. The optical axis Ax of the birefringent plate 12 is set to the filter 1
When projected to 0, the projected image is P of the filter 10.
It is arranged so as to form an angle rotated by 45 degrees from the polarization direction to the S polarization direction.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization-independent composite filter device in which a filter obliquely arranged with respect to incident light and a birefringent plate arranged in front of the incident side are combined. . This composite filter device is useful for optical multiplexing / demultiplexing of various optical communication devices and optical measuring devices, or for selecting an optical wavelength.

[0002]

2. Description of the Related Art Filters are used in devices for optical multiplexing / demultiplexing or optical wavelength selection. Some filters of this type have a non-absorptive dielectric metal multilayer film laminated on a substrate such as glass, quartz, or quartz. To obtain the wavelength characteristic of. Therefore, various filters such as a bandpass filter, a long wavelength pass filter, and a short wavelength pass filter can be realized in an arbitrary wavelength range by changing the film configuration and the film thickness.

[0003]

Usually, the filter is arranged obliquely with respect to the incident light. The bandpass filter is arranged obliquely with respect to the incident light and the incident angle is increased, whereby the passband can be selected on the short wavelength side. As described above, when the incident angle is large, the Brewster conditions are different between the S-polarized component and the P-polarized component with respect to the filter, so that the transmission and reflection branching ratios are different for a certain wavelength.

As an example, FIG. 7 schematically shows the characteristics of a long wavelength pass filter. In the reflection wavelength region (region represented by a in FIG. 7), the reflectances of the P-polarized component and the S-polarized component are substantially constant at each wavelength, but in the transmission wavelength region (region represented by b in FIG. 7), The transmittances of the S-polarized light component and the P-polarized light component greatly change at each wavelength, and the transmittances of the S-polarized light component and the P-polarized light component are different even at the same wavelength. Also, regarding the characteristics of the short wavelength pass filter, the reflectances of the P-polarized component and the S-polarized component are almost constant at each wavelength in the reflection wavelength region, but the reflectances of the S-polarized component and the P-polarization component are approximately equal in the transmission wavelength region. The transmittance greatly changes at each wavelength, and the transmittances of the S-polarized component and the P-polarized component are different even at the same wavelength. Further, as an example, FIG. 8 schematically shows the characteristics of the bandpass filter. Initially, the filter was arranged perpendicularly to the incident light (shown by a thick line in FIG. 8), but if the filter is largely tilted in order to intentionally shift its pass band to the short wavelength side, the P polarization component Shifts to the shorter wavelength side, the maximum transmission wavelengths of the S-polarized component (passband represented by the thin line in FIG. 8) and the P-polarized component (passband represented by the broken line in FIG. 8) are shifted and the same wavelength However, the transmittances of the S-polarized component and the P-polarized component are different.

The polarization state of light transmitted through a single-mode optical fiber varies randomly depending on the optical fiber. Therefore, the polarization state of the light incident on the filter is not uniquely determined, and the ratio of the P polarization component and the S polarization component changes. Here, when the incident light has a certain wavelength within the transmission wavelength range, since the transmittances of the P-polarized component and the S-polarized component are different, the output light causes power fluctuation according to the polarization state.

For example, as shown in FIG. 7, in the long wavelength pass filter, the transmittance of the S-polarized component is 98%, the transmittance of the P-polarized component is 90%, and the power of the incident light is 100 at the wavelength λ _0. . The ratio of the S polarization component to the P polarization component is 50:
50, the output light power is 50 × 0.98 + 50 × 0.90 = 94. Then, if the ratio of the S polarization component to the P polarization component is changed to 80:20, the output light power is 80 × 0.98 + 20 × 0.90 = It becomes 96.4.

Further, for example, as shown in FIG. 5, in the band pass filter, the transmittance of the S-polarized component is 90%, the transmittance of the P-polarized component is 70%, and the power of the incident light is 100 at the wavelength λo. . The ratio of S-polarized component and P-polarized line segment is 5
If the ratio is 0:50, the output light power is 50 × 0.90 + 50 × 0.70 = 80. Then, if the ratio of the S polarization component to the P polarization component is changed to 80:20, the output light power is 80 × 0.90 + 20 ×. 0.70 = 86 Further, if the ratio of the S-polarized component and the P-polarized component is changed to 20:80, the output light power is 20 × 0.90 + 80 × 0.70 = 74.

As described above, it is clear that power fluctuations occur depending on the polarization state of incident light. Therefore, in digital optical communication or the like, an error may occur especially when the signal waveform is distorted.

To solve this drawback, it is conceivable to use a polarization maintaining fiber. Certainly, using a polarization-maintaining fiber allows the polarization direction of the light beam to be fixed and incident on the filter, but in order to obtain the desired characteristics, it must be adjusted so that it enters with a constant polarization direction, which is difficult to adjust. Is.

An object of the present invention is to provide a polarization-independent composite filter device which can obtain transmitted light of constant and stable power without depending on the polarization state of incident light.

[0011]

SUMMARY OF THE INVENTION The present invention comprises a combination of a filter using a dielectric layered film obliquely arranged with respect to incident light and a birefringent plate arranged in front of the incident side of the filter. It is a polarization-independent composite filter device.
The optical axis of the birefringent plate has a projected image of 4 from the P polarization direction of the filter to the S polarization direction when projected onto the filter.
It is adjusted to form an angle rotated by 5 degrees.

The filter of this polarization independent composite filter device is a long wavelength pass filter or a short wavelength pass filter. A first entrance port is provided on the optical path of the incident light in front of the birefringent plate of this filter device, an exit port is provided on the optical path of the exit light that passes through the filter, and is reflected by the filter and exits to the exit port. The optical multiplexer may be provided with the second incident port on the optical path of such incident light. Further, a second birefringent plate may be additionally provided on the optical path of incident light that is reflected by the filter and is emitted to the emission port.

In addition, an entrance port is provided on the optical path of the incident light in front of the birefringent plate of this filter device, and a first exit port is provided on the optical path of the exit light passing through the filter so that the incident light enters from the entrance port. The optical demultiplexer may be provided with a second emission port on the optical path of the emitted light reflected by the filter.

Alternatively, this filter may be used as a bandpass filter, an entrance port may be provided on the optical path of incident light in front of the birefringent plate, and an exit port may be used on the optical path of outgoing light passing through the filter. Good.

[0015]

[Function] The incident light is a birefringent plate, and ordinary light whose polarization directions are orthogonal to each other.
It is separated into extraordinary light and reaches the filter. Since the projection image of the optical axis of the birefringent plate on the filter forms an angle rotated by 45 degrees from the P polarization direction to the S polarization direction, the polarization directions of ordinary and extraordinary light are both P polarization direction and S polarization direction. Make an angle of 45 degrees. Therefore, the branching ratios of transmission and reflection of ordinary light and extraordinary light in the filter are the same. Therefore, even if the polarization state of the incident light is not constant and the birefringence plates have different branch ratios of ordinary and extraordinary rays, the branching ratio of transmission and reflection in the filter is constant, and the ordinary and extraordinary rays are not parallel. Therefore, since the transmitted light and the reflected light can be efficiently received, the power of the emitted light becomes constant.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an explanatory view showing an embodiment of a polarization independent composite filter device according to the present invention. A filter 10, which is a long-wavelength pass filter, is arranged obliquely with respect to incident light, and a birefringent plate 12 is arranged in front of the incident side of the filter 10 perpendicular to the optical axis. Here, the birefringent plate 12 is a parallel plate using, for example, a rutile single crystal. The filter 10 is a parallel plate of glass, and has a structure in which a dielectric multilayer film is vapor-deposited on the incident surface 10a and an antireflection film is vapor-deposited on the emission surface 10b. When the optical axis Ax of the birefringent plate 12 is projected on the filter 10, the projected image is arranged so as to form an angle rotated by 45 degrees from the P polarization direction of the filter 10 to the S polarization direction. Therefore, when the S polarization direction and the P polarization direction of the filter 10 are projected on a plane perpendicular to the optical axis, the S polarization direction is X.
When the axis, the P polarization direction are the Y axis, and the optical axis direction is the Z axis, the optical axis Ax of the birefringent plate 12 is inclined 45 degrees (clockwise when viewed from the incident side) with respect to the X axis.

The light rays transmitted through the birefringent plate 12 are separated into ordinary light and extraordinary light whose polarization directions are orthogonal to each other to become parallel light. Since the optical axis Ax of the birefringent plate 12 is tilted by 45 degrees (clockwise when viewed from the incident side) with respect to the X axis, the polarization direction of ordinary light is tilted by -45 degrees with respect to the X axis (viewed from the incident side). The polarization direction of the extraordinary light is inclined at 45 degrees (clockwise when viewed from the incident side). Most of the ordinary and extraordinary light that reaches the filter 12 is transmitted, and the rest is reflected, if the wavelength is in the transmission wavelength range. In FIG. 1, the ordinary transmitted light is To,
The transmitted light of extraordinary light is represented by Te, the reflected light of ordinary light is represented by Ro, and the reflected light of extraordinary light is represented by Re. When the polarization direction of the linearly polarized light in the transmission wavelength range is changed from 0 degree to 90 degrees with respect to the X-axis direction,
The transmission / reflection branching ratio of the filter changes accordingly. Since the ordinary light and the extraordinary light have their polarization directions orthogonal to each other, they have the same angle and the same transmittance only when they form an angle of 45 degrees with respect to the X-axis direction. By receiving these two light rays together, the power of the emitted light becomes constant even if the polarization state of the incident light changes and the branching ratio between the ordinary light and the extraordinary light changes.

A specific example will be described below. For example, when a linearly polarized light ray having an angle of 45 degrees with the X-axis direction is incident, the transmittance of the light ray through the filter is 94.
%. If the incident light is circularly polarized light and the power of the incident light is 100, the branching ratio between the ordinary light and the extraordinary light is 5
It is 0:50. Therefore, the output light power is 50 × 0.94 + 50 × 0.94 = (50 + 50) × 0.94 = 100 × 0.94. If the incident light is elliptically polarized and the branching ratio of the ordinary light and the extraordinary light is 80:20, the power of the emitted light is 80 × 0.94 + 20 × 0.94 = (80 + 20) × 0.94 = 100 × 0.94. . As described above, even if the branching ratio between the ordinary light and the extraordinary light varies depending on the polarization state of the incident light, the power of the emitted light becomes stable.

FIG. 2 is an explanatory view showing another embodiment of the polarization independent composite filter device according to the present invention. A filter 11, which is a bandpass filter, is arranged obliquely with respect to the incident light, and a birefringent plate 12 is arranged in the preceding stage on the incident side of the filter in the same manner as in FIG. The filter 11 is a parallel plate of glass, and an antireflection film is vapor-deposited on the incident surface 11a of the filter 11 to form an exit surface 1a.
It has a structure in which a dielectric multilayer film is deposited on 1b.

For example, when a linearly polarized light ray having an angle of 45 degrees with the X-axis direction is incident, the transmittance of the light ray at the filter is 80%. The incident light is elliptically polarized,
Assuming that the power of incident light is 100 and the branching ratio between ordinary and extraordinary light is 80:20, the power of emitted light is 80 × 0.80 + 20 × 0.80 = (80 + 20) × 0.80 = 100 × 0.80 And the branch ratio of the extraordinary light is 80:20, the power of the emitted light is 20 × 0.80 + 80 × 0.80 = (80 + 20) × 0.80 = 100 × 0.80. As described above, even if the branching ratio between the ordinary light and the extraordinary light varies depending on the polarization state of the incident light, the power of the emitted light becomes stable.

FIG. 3 is an explanatory view of an optical multiplexer using the polarization independent composite filter device. This optical multiplexer is provided with a first entrance port P _i1 on the optical path of incident light in front of the birefringent plate 12 of the polarization-independent composite filter device 14, and an exit port on the optical path of outgoing light passing through the filter 10. In this structure, Po is provided, and the second incident port P _i2 is provided on the optical path of incident light that is reflected by the filter 10 and is emitted to the emission port Po. The filter 10 has the X-axis, as described above.
When the Y axis and the Z axis are taken, they are arranged so as to form 22.5 degrees with respect to the XY plane. Optical fiber 30, 3 in each port
Lenses 35, 36, 37 for making parallel light with 1, 32
, And the optical fibers 30, 31, 32 are single mode optical fibers, and the lenses 35, 36, 37 are spherical lenses.

For example, a light beam having a wavelength of 1.55 μm from the first incident port P _{i1 and} a light beam having a wavelength of 1.48 μm from the second incident port P _i2 are polarization independent composite filter devices 14
Incident on. Wavelength of 1.55 μm in filter 10
Is in the transmission wavelength range, and the wavelength of 1.48 μm is in the reflection wavelength range. This light beam is emitted from the optical fibers 30 and 31, passes through the lenses 35 and 36, and becomes parallel light. Wavelength 1.
The 55 μm light beam is emitted from the single-mode optical fiber 30, so the polarization state changes. However, as described above, when passing through the polarization-independent composite filter device 14, the output power is changed regardless of the polarization state of the incident light. It is constant. On the other hand, the wavelength of light emitted from the second incident port P _{i2 is} 1.48 μ.
The light ray of m is reflected by the filter 10, but it is reflected at a constant rate almost without depending on the polarization state. Therefore, the combined light beam having the wavelength of 1.55 μm and the wavelength of 1.48 μm does not depend on the polarization state of the incident light and does not cause power fluctuation.
This ray is coupled into the exit port Po.

FIG. 4 shows another embodiment of the optical multiplexer. Due to the characteristics of the filter 10, when a power fluctuation occurs when a light ray incident from the second incident port P _i2 is reflected, the birefringent plate 15 is provided on the incident side front stage of the reflecting surface of the filter 10.
To place. Then, when the optical axis of the birefringent plate 15 is projected onto the filter 10, the projection image is adjusted so as to form an angle rotated by 45 degrees from the P polarization direction of the filter 10 to the S polarization direction. The light ray incident from the second incident port P _i2 passes through the birefringent plate 15 and is reflected by the filter 10 to be coupled to the emission port Po without power fluctuation, as in the above embodiment.

FIG. 5 is an explanatory view of an optical demultiplexer using the polarization independent composite filter device. This optical demultiplexer is provided with an incident port Pi on the optical path of incident light in front of the birefringent plate 12 of the polarization-independent composite filter device 14 using the long-wavelength pass filter 10 so that the light emitted from the filter 10 is transmitted. The first emission port P _o1 is provided on the path, and the second emission port P _o2 is provided on the optical path of the emitted light reflected by the filter 10. The arrangement of each member is similar to that of the optical multiplexer.

For example, a light beam having a wavelength of 1.55 μm and a light beam having a wavelength of 1.48 μm emitted from the incident port Pi does not depend on the polarization state of the incident light and is coupled to each emission port without power fluctuation. That is, a light beam having a wavelength of 1.55 μm is filtered by the filter 1
After passing 0, a wavelength of 1.48 μm is output to the first output port P _o1.
The light beam of m is reflected by the filter 10 and is then transmitted to the second emission port P.
_Decouple to _o2 .

FIG. 6 is an explanatory view of an optical wavelength selector using the polarization independent composite filter device. Here, we will focus on an optical wavelength selector that has a wavelength tuning mechanism that is used to select a light beam of an appropriate wavelength from wavelength-multiplexed combined light beams or to extract a light beam of a wide bandwidth from a light beam of an appropriate narrow bandwidth. . Therefore, although it has a rotation or tilt function for wavelength tuning, the rotation or tilt mechanism will not be described in detail here (for example, a pulse motor or the like). This optical wavelength selector is a bandpass filter 12
Polarization-independent composite filter device 16 using
In the configuration, an entrance port Pi is provided on the optical path of incident light in front of 2, and an exit port Po is provided on the optical path of outgoing light that passes through the filter. The filter 11 is attached to a substrate 17 having a hole for transmitting a light beam, and a rotary shaft pin 18 is vertically provided on the substrate 17 and is incident on the XY plane as described above. Arrange so that the corners are variable. Each port has the same arrangement as the above example.

For example, a light beam having a center wavelength of 1.55 μm and a half value width of 0.020 μm is incident on the polarization independent filter device 16 from the incident port Pi. Since it is desired to use the filter 11 in a wide band, it is assumed that the filter 11 is designed with a center wavelength of 1.56 and a half value width of 0.004 μm at normal incidence. And
The selected light beam has a wavelength of 1.552 μm and a half width of 0.004 μm.
And The filter 11 is rotated by about 10 degrees by a rotating mechanism to select the wavelength of 1.552 μm. In this case, since the inclination angle of the filter with respect to the incident light is large, the transmittances of the S-polarized light and the P-polarized light are different, which may cause a power fluctuation. As described above, the light beam having the wavelength of 1.552 μm and the half width of 0.004 μm transmitted through the polarization-independent composite filter device 14 is not dependent on the polarization state of the incident light and is coupled to the output port Po without power fluctuation. To do.

In the present invention, the second birefringent plate may be arranged as described above when the power fluctuation is generated in the reflected outgoing light. Further, the present invention is not limited to the long wavelength pass band filter, but a filter such as a band pass filter or a short wavelength pass filter can be used.
In particular, when used as an optical multiplexer or demultiplexer, a long wavelength pass band filter or a short wavelength pass filter is mainly used, and when used as an optical wavelength selector, a band pass filter is mainly used.

[0029]

According to the present invention, the birefringent plate is arranged in front of the filter, and the projected image of the polarization directions of the ordinary and extraordinary rays on the filter and the P polarization direction and the S polarization direction form 45 degrees. The branching ratios of transmission and reflection of ordinary and extraordinary light are the same.
By receiving these two rays together, the power of the light is stabilized without depending on the polarization state of the incident light, even if the polarization state of the incident light changes and the ratio of the ordinary light to the extraordinary light changes. Can be emitted.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of a polarization-independent composite filter device according to the present invention.

FIG. 2 is a diagram showing another embodiment of a polarization-independent composite filter device according to the present invention.

FIG. 3 is a diagram showing an embodiment of an optical multiplexer according to the present invention.

FIG. 4 is a diagram showing another embodiment of the optical multiplexer according to the present invention.

FIG. 5 is a diagram showing an embodiment of an optical demultiplexer according to the present invention.

FIG. 6 is a diagram showing an embodiment of an optical wavelength selector according to the present invention.

FIG. 7 is a schematic diagram of transmission / reflectance characteristics of an S-polarized component and a P-polarized component of a long wavelength pass filter.

FIG. 8 is a characteristic diagram of the transmittance of the S-polarized component and the P-polarized component of the bandpass filter.

[Explanation of symbols]

 10 filter 12 birefringent plate Ax optical axis s S polarization direction p P polarization direction

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Mayumi Hironaga 5-36-11 Shimbashi, Minato-ku, Tokyo Fuji Electric Chemical Co., Ltd.

Claims (5)

[Claims] 1. A combination of a filter using a layered film of a metal or a dielectric arranged obliquely with respect to incident light and a parallel-plate birefringent plate arranged in front of the entrance side of the filter. The optical axis of the refraction plate is such that when the image is projected on the filter, the projected image is S from the P polarization direction of the filter.
A polarization-independent composite filter device adjusted to form an angle rotated by 45 degrees in the polarization direction. 2. The polarization independent composite filter device according to claim 1, wherein the filter is a long wavelength pass filter or a short wavelength pass filter.
An optical multiplexer provided with a second incident port on the optical path of the incident light which is provided on the optical path of the outgoing light passing through the filter and is reflected by the filter to be emitted to the outgoing port. . 3. A polarization-independent composite filter device according to claim 1, wherein the filter is a long-wavelength pass filter or a short-wavelength pass filter.
An incident port is provided, an emission port is provided on the optical path of outgoing light that passes through the filter, and a second birefringent plate is provided on the optical path of incident light that is reflected by the filter and is emitted to the outgoing port. An optical multiplexer provided with a second incident port on its incident side. 4. The polarization independent composite filter device according to claim 1, wherein the filter is a long wavelength pass filter or a short wavelength pass filter, and an entrance port is provided on the optical path of the incident light in front of the birefringent plate to provide the filter. An optical demultiplexer in which a first emission port is provided on the optical path of transmitted emission light, and a second emission port is provided on the optical path of emission light which is incident from the incident port and is reflected by a filter. 5. The polarization-independent composite filter device according to claim 1, wherein the filter is a band-pass filter, and an entrance port is provided on the optical path of the entrance light in front of the birefringent plate, and the exit light is transmitted through the filter. An optical wavelength selector that provides an emission port on the road.