JP2602155B2 - Polarization independent composite filter device - Google Patents

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 arranged obliquely 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 or optical wavelength selection of various optical communication devices and optical measuring devices.

[0002]

2. Description of the Related Art A filter is used in an optical multiplexing / demultiplexing device or an optical wavelength selecting device. This type of filter includes a non-absorbing dielectric metal multilayer film laminated on a substrate made of glass, quartz, quartz, or the like. Is obtained. Therefore, various filters such as a band pass 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 filters are arranged obliquely with respect to the incident light. As for the band-pass filter, the pass band can be selected on the short wavelength side by arranging the filter obliquely with respect to the incident light and increasing the incident angle. As described above, when the incident angle is large, the Brewster condition is different between the S-polarized light component and the P-polarized light component with respect to the filter, so that the branching ratio between transmission and reflection is 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 (the region represented by a in FIG. 7), the reflectance of the P-polarized component and the S-polarized component is substantially constant at each wavelength, but in the transmission wavelength region (the region represented by b in FIG. 7), The transmittance of the S-polarized component and the P-polarized component greatly fluctuates at each wavelength, and even at the same wavelength, the transmittances of the S-polarized component and the P-polarized component are different. Also, in the characteristics of the short-wavelength pass filter, the reflectance of the P-polarized component and the S-polarized component is almost constant at each wavelength in the reflection wavelength range, but the transmittance of the S-polarized component and the P-polarized component is substantially constant in the transmission wavelength range. The transmittance varies greatly at each wavelength, and the transmittance of the S-polarized component and the transmittance of the P-polarized component are different even at the same wavelength. Further, as an example, FIG. 8 schematically shows the characteristics of the band-pass filter. Initially, the filter was arranged perpendicular to the incident light (shown by the thick line in FIG. 8). In order to intentionally shift the pass band to the shorter wavelength side, if the filter was greatly tilted, the P polarization component Is shifted to the shorter wavelength side, the maximum transmission wavelengths of the S-polarized component (pass band indicated by a thin line in FIG. 8) and the P-polarized component (pass band indicated by a broken line in FIG. 8) are shifted to be the same wavelength. However, the transmittances of the S-polarized light component and the P-polarized light component are different.

[0005] The polarization state when light is transmitted through a single-mode optical fiber varies randomly with the optical fiber. Therefore, the polarization state of light incident on the filter is not uniquely determined, and the ratio between the P-polarized component and the S-polarized component changes. Here, when the incident light has a certain wavelength within the transmission wavelength range, the transmittance of the P-polarized component and the S-polarized component is different, so that the emitted light undergoes power fluctuations according to the polarization state.

For example, as shown in FIG. 7, in a long wavelength pass filter, when the wavelength is λ ₀ , 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. . The ratio of the S-polarized component to the P-polarized component is 50:
Assuming that the output power is 50, the power of the emitted light is 50 × 0.98 + 50 × 0.90 = 94. Next, assuming that the ratio of the S-polarized light component to the P-polarized light component is changed to 80:20, the power of the emitted light is 80 × 0.98 + 20 × 0.90 = 96.4.

For example, as shown in FIG. 5, in a band-pass filter, when the wavelength is λo, 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. . The ratio of the S-polarized light component to the P-polarized light line is 5
Assuming that 0:50, the output light power is 50 × 0.90 + 50 × 0.70 = 80. Next, assuming that the ratio of the S-polarized component to the P-polarized component is changed to 80:20, the output light power is 80 × 0.90 + 20 × 0.70 = 86 Further, assuming that the ratio of the S-polarized light component to the P-polarized light component is changed to 20:80, the output light power becomes 20 × 0.90 + 80 × 0.70 = 74.

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

In order to solve this disadvantage, it is conceivable to use a polarization maintaining fiber. Certainly, if a polarization preserving fiber is used, the light beam can be incident on the filter with a fixed polarization direction, but it must be adjusted to enter the filter in a fixed polarization direction to obtain the desired characteristics, and this adjustment is difficult. It is.

It is an object of the present invention to provide a polarization-independent composite filter device capable of obtaining transmitted light having a constant and stable power without depending on the polarization state of incident light.

[0011]

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 filter on the incident side. It is a polarization independent composite filter device.
The optical axis of the birefringent plate is such that when projected onto the filter, the projected image is shifted from the P polarization direction of the filter to the S polarization direction.
It is adjusted to make an angle rotated by 5 degrees.

The filter of the polarization independent composite filter device is a long wavelength pass filter or a short wavelength pass filter. A first incident port is provided on the optical path of the incident light in front of the birefringent plate of the filter device, an output port is provided on the optical path of the outgoing light transmitted through the filter, and the light is reflected by the filter and emitted to the outgoing port. An optical multiplexer provided with a second incident port on the optical path of such incident light may be used. Further, a second birefringent plate may be additionally provided on the optical path of the incident light which is reflected by the filter and emitted to the emission port.

An input port is provided on the optical path of the incident light in front of the birefringent plate of the filter device, and a first output port is provided on the optical path of the output light passing through the filter. The optical demultiplexer may be provided with a second output port on the optical path of the output light reflected by the filter.

Alternatively, the filter may be a band-pass filter, an input port may be provided on the optical path of incident light in front of the birefringent plate, and an optical wavelength selector using an output port on the optical path of output light passing through the filter. Good.

[0015]

[Function] The incident light is ordinary light whose polarization direction is orthogonal to the birefringent plate.
It is separated into extraordinary light and reaches the filter. Since the projected image of the optical axis of the birefringent plate onto the filter forms an angle rotated by 45 degrees from the P-polarization direction to the S-polarization direction, the polarization directions of ordinary light and extraordinary light are both the P-polarization direction and the S-polarization direction. Make a 45 degree angle. Therefore, the filter has the same branching ratio of transmission and reflection of ordinary light and extraordinary light. For this reason, even if the polarization state of the incident light is not constant and the branching ratio of ordinary light and extraordinary light due to the birefringent plate is different, the branching ratio of transmission and reflection at the filter is constant, and the ordinary light and extraordinary light are parallel. Therefore, the transmitted light and the reflected light can be efficiently received, and the power of the emitted light is constant.

[0016]

FIG. 1 is an explanatory view showing one 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 made of glass, and has a structure in which a dielectric multilayer film is deposited on an incident surface 10a and an antireflection film is deposited on an emission surface 10b. When the optical axis Ax of the birefringent plate 12 is projected onto the filter 10, the projection 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 changed to X
Assuming that the axis and 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 as viewed from the incident side) with respect to the X axis.

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

Here, a specific example will be described. For example, when a linearly polarized light beam having an angle of 45 degrees with the X-axis direction is incident, the transmittance of the light beam at the filter is 94%.
%. If the incident light is circularly polarized, and the power of the incident light is 100, the branching ratio between ordinary light and extraordinary light is 5
0:50. Therefore, the power of the emitted light is 50 × 0.94 + 50 × 0.94 = (50 + 50) × 0.94 = 100 × 0.94. Assuming that the incident light is elliptically polarized light and the branching ratio between 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 is stabilized.

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 band-pass filter, is arranged obliquely with respect to the incident light, and a birefringent plate 12 is arranged in front of the filter on the incident side in the same manner as in FIG. The filter 11 is a parallel flat plate made of glass, and an anti-reflection film is deposited on its entrance surface 11a.
1b is a structure in which a dielectric multilayer film is deposited.

For example, when a linearly polarized light beam whose polarization direction forms an angle of 45 degrees with the X-axis direction is incident, the transmittance of the light beam at the filter is assumed to be 80%. The incident light is elliptically polarized light,
Assuming that the power of the incident light is 100 and the branching ratio between the ordinary light and the extraordinary light is 80:20, the power of the emitted light is 80 × 0.80 + 20 × 0.80 = (80 + 20) × 0.80 = 100 × 0.80 Assuming that the branching ratio between the extraordinary light and 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 is stabilized.

FIG. 3 is an explanatory diagram of an optical multiplexer using the polarization-independent composite filter device. This optical multiplexer includes a first incident 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 output port on the optical path of emitted light transmitted through the filter 10. Po is provided, and a second incident port P _i2 is provided on the optical path of incident light that is reflected by the filter 10 and exits to the exit port Po. The filter 10 has an 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. Each port has an optical fiber 30, 3
Lenses 35, 36, 37 for making light parallel to 1, 32
Are disposed, the optical fibers 30, 31, and 32 are single mode optical fibers, and the lenses 35, 36, and 37 are spherical lenses.

For example, a light beam having a wavelength of 1.55 μm from the first input port P _{i1 and} a light beam having a wavelength of 1.48 μm from the second input port P _i2 are polarized-independent composite filter devices 14.
Incident on. In the filter 10, the wavelength is 1.55 μm
Is in the transmission wavelength range, and the wavelength 1.48 μm is in the reflection wavelength range. This light beam exits from the optical fibers 30 and 31 and passes through the lenses 35 and 36 to become parallel light. Wavelength 1.
Since the 55 μm light beam is emitted from the single mode optical fiber 30, the polarization state changes. It is constant. On the other hand, the wavelength 1.48 μ emitted from the second input port P _i2.
The m rays are reflected by the filter 10 and are reflected at a constant rate independent of the state of polarization. Therefore, the combined light having the wavelengths of 1.55 μm and 1.48 μm does not depend on the polarization state of the incident light and does not cause power fluctuation.
This ray couples to the exit port Po.

FIG. 4 shows another embodiment of the optical multiplexer. Due to the characteristics of the filter 10, when a light beam incident from the second incident port P _i2 is reflected and a power fluctuation occurs, the birefringent plate 15 is disposed in front of the reflection surface of the filter 10 on the incident side.
Place. Then, when the optical axis of the birefringent plate 15 is projected on 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 beam entering from the second entrance port P _i2 passes through the birefringent plate 15 and is reflected by the filter 10, and is coupled to the exit port Po without power fluctuation as in the above-described embodiment.

FIG. 5 is an explanatory diagram of an optical demultiplexer using the polarization-independent composite filter device. In this optical demultiplexer, an input port Pi is provided on the optical path of the incident light in front of the birefringent plate 12 of the polarization-independent composite filter device 14 using the long-wavelength pass filter 10, and the light of the outgoing light transmitted through the filter 10 is provided. The configuration is such that a first output port P _o1 is provided on a path, and a second output port P _o2 is provided on an optical path of output light reflected by the filter 10. The arrangement of each member is the same as that of the optical multiplexer.

For example, a light beam having a wavelength of 1.55 μm and a wavelength of 1.48 μm emitted from the input port Pi is coupled to each output port without depending on the polarization state of the incident light and without power fluctuation. That is, the light having a wavelength of 1.55 μm is
After passing through the 0, the first output port P _o1, wavelength 1.48μ
m is reflected by the filter 10 and then exits the second exit port P
_{Demultiplexed} coupling to _o2 .

FIG. 6 is an explanatory diagram of an optical wavelength selector using the polarization-independent composite filter device. Here, an optical wavelength selector having a wavelength tuning mechanism used to select a light beam having an appropriate wavelength from a wavelength-multiplexed multiplexed light beam and to extract a light beam having an appropriate narrow bandwidth from a light beam having a wide bandwidth will be described. . Therefore, although it has a rotation or tilt function for wavelength tuning, the rotation or tilt mechanism is not described in detail here (for example, a pulse motor or the like). This optical wavelength selector is a bandpass filter 12.
Birefringent plate 1 in polarization-independent composite filter device 16 using
In this configuration, an input port Pi is provided on the optical path of the incident light in front of 2, and an output port Po is provided on the optical path of the output light transmitted through the filter. Note that the filter 11 is mounted on a substrate 17 having a hole for transmitting light, and a rotating shaft pin 18 is vertically set on the substrate 17 so as to be incident on the XY plane as described above. Arrange so that the corner is variable. Each port has the same arrangement as in the above example.

For example, a light beam having a center wavelength of 1.55 μm and a half 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 to have 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 about 10 degrees by a rotation mechanism to select a wavelength of 1.552 μm. In this case, since the inclination angle of the filter with respect to the incident light beam is large, the transmittances of the S-polarized light and the P-polarized light are different, which may cause power fluctuation. As described above, a light beam having a wavelength of 1.552 μm and a half-value width of 0.004 μm that has passed through the polarization-independent composite filter device 14 is coupled to the output port Po without power fluctuation regardless of the polarization state of the incident light. I do.

According to the present invention, in the case where power fluctuation occurs in reflected outgoing light, the second birefringent plate may be arranged as described above. Further, the present invention is not limited to a long wavelength pass band filter, but may use a filter such as a band pass filter or a short wavelength pass filter.
Particularly, when used as an optical multiplexer or an optical 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, a birefringent plate is arranged in front of the filter, and the projected image of the ordinary and extraordinary light on the filter and the P and S polarization directions form 45 degrees. , The branching ratio of the transmission and reflection of the ordinary light and the extraordinary light becomes the same.
By receiving these two rays together, even if the polarization state of the incident light changes and the ratio of ordinary to extraordinary light fluctuates, the power of the light is stabilized without depending on the polarization state of the incident light. And can be emitted.

[Brief description of the 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 the polarization-independent composite filter device according to the present invention.

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

FIG. 6 is a diagram showing one 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 light component and a P-polarized light component of a long wavelength pass filter.

FIG. 8 is a schematic diagram of a transmittance characteristic of an S-polarized light component and a P-polarized light component of a band-pass filter.

[Explanation of symbols]

 10 Filter 12 Birefringent plate Ax Optical axis s S polarization direction p P polarization direction

Claims (5)

(57) [Claims] The filter comprises a combination of a filter using a metal or dielectric layered film obliquely arranged with respect to incident light and a parallel plate birefringent plate arranged in front of the filter on the incident side. The optical axis of the refraction plate is such that when projected onto 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 having an input port, an output port provided on an optical path of output light transmitted through the filter, and a second input port provided on an optical path of incident light reflected by the filter and output to the output port. . 3. 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 input port is provided, an output port is provided on an optical path of output light transmitted through the filter, and a second birefringent plate is provided on an optical path of the input light reflected by the filter and output to the output port. An optical multiplexer having a second incident port on the 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 input port is provided on an optical path of incident light in front of the birefringent plate. An optical demultiplexer having a first output port provided on an optical path of transmitted outgoing light, and a second output port provided on an optical path of outgoing light incident from the input port and reflected by a filter. 5. The polarization-independent composite filter device according to claim 1, wherein the filter is a band-pass filter, wherein an incident port is provided on an optical path of the incident light in front of the birefringent plate, and light of the outgoing light transmitted through the filter. An optical wavelength selector having an output port on the road.