JP5833598B2 - Polarization combiner - Google Patents

Description

  The present invention relates to a polarization beam combiner, and more particularly to a bulk polarization beam combiner composed of a plurality of prisms.

  There is a demand for an increase in communication traffic volume via the Internet and the like. Therefore, in a wavelength division multiplexing (WDM) system, an increase in transmission speed per channel and an increase in the number of wavelengths are required. Specifically, high transmission rates such as 40 Gbit / s and 100 Gbit / s are required for transmission in the WDM system.

  However, when the modulation symbol rate is increased for higher speed, there is a problem that the dispersion tolerance is rapidly deteriorated and the transmission distance is shortened. A multilevel technology for increasing the bit rate without increasing the symbol rate. And the need for multiplexing technology is increasing. Various formats have been developed such as DQPSK (Differential Quadrature Shift Keying) modulators and DP (Dual Polarization) -QPSK optical modulators in which a plurality of Mach-Zehnder optical modulators are arranged in parallel. Wave multiplexing techniques are becoming standard.

  In the polarization multiplexing technique, a function required for the optical transmitter is to place different modulation signals on the orthogonal polarization components. There are two possible ways to shape it. One is a method (see Non-Patent Document 1) in which one polarization light is modulated, and one of the polarizations is rotated by 90 ° and then combined by a polarization beam combiner (Polarization Beam Combiner). The other is a method in which two polarization components are provided at the time when light enters the optical modulator, and after the incidence, they are separated into orthogonal components by a polarization beam splitter, and the orthogonal components are respectively modulated. (See Non-Patent Document 2).

  The former requires a polarization rotation element. In the latter configuration, the polarization rotation element is not necessary, but it is necessary to tilt the polarization direction of the input light into the optical modulator so that both polarization components have the same intensity.

At present, these technologies are realized by using an LN modulator composed of LiNbO ₃ (lithium niobate; LN). When 100-Gbit / s DP-QPSK becomes popular in the future, The size will increase. In addition, it is necessary to use a driver having a relatively high half-wave voltage and a high voltage output, and thus the power consumption of the driver is increased. In the current communication, it is required to reduce the power consumption while reducing the size, and in the future, there is a limit to solving the above problem using only the LN modulator.

  Therefore, as one means for meeting these requirements, a Mach-Zehnder type semiconductor modulator that changes the refractive index by applying an electric field to a semiconductor element and converts an input electric signal into a phase change of light has attracted attention. The semiconductor modulator has a larger relative refractive index difference between the optical waveguides to be configured and a smaller bending radius than the LN modulator, so that a small circuit layout is possible. In addition, since the drive voltage can be made smaller than that of the LN modulator, it has been attracting attention from the viewpoint of low power consumption. In these semiconductor modulators, as with the LN modulator, a high-speed modulator corresponding to a multilevel transmission format such as DQPSK has already been reported.

Hiroshi Yamazaki et al., “Integrated 100-Gb / s PDM-QPSK modulator using a hybrid assembly technique with silica-based PLCs and LiNbO3 phase modulators,” ECOC 2008, Mo.3.C.1, 2008. C. R. Doerr and L. Zhang, “Monolithic 80-Gb / s Dual Polarization On-Off-Keying Modulator in InP,” OFC, PDP19, 2008. Hatem El-Refaei and David Yevick, “An Optimized InGaAsP / InP Polarization Converter Employing Asymmetric Rib Waveguides,” Journal of Lightwave Technology, Vol. 21, No. 6, June 2003. Sugitani Akihiko, “Current Status and Trends of Recent Projectors”, Optical Technology Information Magazine “Light Edge”, USHIO Inc., No. 15, pp. 84-89, November 1998 G. L. Li and P. K. L. Yu, “Optical Intensity Modulators for Digital and Analog Applications,” Journal of Lightwave Technology, vol. 21, No. 9, pp. 2010-2030, 2003.

In the case of an optical modulator using a silica-based planar lightwave circuit in which an optical waveguide mainly composed of SiO ₂ glass is formed on a Si substrate, a groove is formed on the Si substrate and a wavelength is formed there. Polarization rotation can be obtained simply by inserting a plate, but in the case of a semiconductor optical modulator, the waveguide loss is large when the groove is cut. Although a semiconductor polarization rotation element has also been reported (see Non-Patent Document 3), it is difficult to manufacture with a high yield, and a special additional process is required. Therefore, in a semiconductor optical modulator, it is difficult to rotate the polarization on the substrate, and there is a need for a polarization synthesizer that combines after rotating one of the polarizations by 90 ° after the modulation in the semiconductor optical modulator. It has been.

  Therefore, it is conceivable to use a conventional polarization multiplexer / demultiplexer as shown in FIG. 1 (see Non-Patent Document 4). The optical element 100 in FIG. 1 is configured by combining a polarization beam splitter (PBS) cube 101 and a first prism 102, and the first prism 102 is a prism having a total reflection surface 102A. When the optical element 100 is used as a polarization beam combiner, the TE mode light incident on the PBS cube 101 is transmitted as it is, and the TM mode light incident on the first prism 102 is totally reflected by the total reflection surface 102A. Then, the light is reflected by the reflective film 101A of the PBS cube 101 and emitted from the PBS cube 101 together with the TE mode light. In order to perform polarization synthesis by entering TE mode light into the PBS cube 101 and the first prism 102, a half-wave plate may be provided on the incident surface of the first prism 102.

However, merely providing a half-wave plate in the optical element of FIG. 1 requires phase compensation in the TE mode and the TM mode, and the optical element becomes large. FIG. 2 shows a top view of an optical element further provided with an optical path adjusting prism 103 necessary for phase compensation. When H-K9L is used as the glass material of the PBS cube 101, the first prism 102, and the optical path adjusting prism 103, and quartz is used as the material of the half-wave plate 104, the length L of the optical path adjusting prism 103 determined by the following equation is It becomes about 6 mm and becomes 8 mm or more as a whole.
L × n _H−K9L = L ₁ × n _air + L ₂ × n _qurtz + L ₃ × n _H-K9L
(Formula 1)
For example, the thickness L ₂ of the quartz half-wave plate 104 is 0.35 mm, the refractive index n _qrtz is 1.531788, and the distance L ₃ from the total reflection surface 102A to the reflection film 101A is equal to the length of one side of the PBS cube 101. If the refractive index n _H-K9L of the glass glass material H-K9L is _set to 1.5009922, the length L can be determined from the relationship L = L ₁ + L ₂ . When L ₃ = 2 mm, the length L of the phase adjusting prism 103 is about 6.28353 mm, and the total length of the polarization multiplexer is added to the length L ₃ = 2 mm of the PBS cube 101 to be 8.2853 mm. Also become. Moreover, since the compensation accuracy depends on the processing accuracy of the glass material, it is not so high.

Further, if the size of one cube is reduced, the phase difference caused by the reflection path is reduced, so that the phase compensation length is shortened, and the optical element can be miniaturized. However, the size of the collimated beam of light is typically about 0.6 mm to 0.8 mm for the full 1 / e ² width. When two beams travel in parallel, the width of the cube used as a reflection mirror is the full width of the beam to completely suppress the amount of light entering the adjacent optical path due to misalignment due to mutual interference or misalignment. Twice as much as is necessary. For this reason, with the above structure, it is difficult to reduce the size, particularly to reduce the size.

  The present invention has been made in view of such problems, and an object of the present invention is to compensate for the phase difference between two input lights in a bulk polarization synthesizer for two input lights having the same polarization direction. However, it is to suppress the enlargement of the element.

In order to achieve such an object, according to a first aspect of the present invention, there is provided a first prism having a PBS cube and a first reflecting surface adjacent to the PBS cube and parallel to a reflecting film of the PBS cube. A second prism having a second reflecting surface adjacent to the PBS cube and perpendicular to the reflecting film of the PBS cube and a third reflecting surface facing the second reflecting surface; and an optical path adjusting cubes adjacent to the prism, the PBS cube, the first prism, the second prism, and the optical path adjustment cube made of the same material, provided in the optical path adjustment cube and a half-wave plate, is provided on the second prism, further comprising a compensation plate consisting of a half wavelength the same material as the plate, the first and second input light having a first polarization direction, it Are having entered into the compensation plate and the half-wave plate, the first input light is transmitted through the compensation plate after being reflected by the third reflecting surface, it is reflected by the second reflecting surface The optical path length of the PBS cube reaching the reflective film and the second input light are transmitted through the half-wave plate and converted into the second polarization direction, and then transmitted through the optical path adjustment cube. The polarization beam combiner is characterized in that the optical path length reflected by the first reflecting surface and reaching the reflecting film of the PBS cube is equal .

  According to a second aspect of the present invention, in the first aspect, the first prism includes a first cube having the first reflecting surface, and the second prism is formed in the PBS cube. A second cube having the second reflective surface adjacent and perpendicular to the reflective film of the PBS cube; and adjacent to the second cube and parallel to the second reflective surface of the second cube. And a third cube having the third reflecting surface, and the compensator is provided on the third cube.

  According to a third aspect of the present invention, in the second aspect, the first polarization direction is a TE mode, the second polarization direction is a TM mode, and the first reflection surface is a TM mode. The reflection film is 94% or more and 96% or less with respect to light, and the third reflection surface is 94% or more and 96% or less with respect to TE mode light.

  According to a fourth aspect of the present invention, in the third aspect, to detect the TM mode light transmitted through the first reflecting surface and the TE mode light transmitted through the third reflecting surface, respectively. The light receiving element is further provided.

  According to a fifth aspect of the present invention, in the second aspect, the second reflecting surface is a total reflection mirror formed by an interface between the glass material of the second cube and air.

  According to the present invention, in a bulk polarization synthesizer for two input lights having the same polarization direction, two optical paths are appropriately designed by using a PBS cube and a plurality of prisms having reflecting surfaces. An increase in the size of the element can be suppressed while compensating for the phase difference of the input light.

It is a figure which shows the general-purpose polarization multiplexer / demultiplexer known conventionally. FIG. 2 is a top view of an optical element in which the polarization multiplexer / demultiplexer of FIG. 1 is further provided with an optical path adjusting prism necessary for phase compensation. It is a figure which shows the polarization beam combiner which concerns on the 1st Embodiment of this invention. It is a figure for demonstrating the function of the polarization beam combiner of FIG. It is a figure which shows the modification of 1st Embodiment. It is a figure which shows another modification of 1st Embodiment. It is a figure which shows the polarization beam combiner which has a light receiving element which concerns on 2nd Embodiment. It is a figure for demonstrating one form of the manufacturing method of the polarization beam combiner which concerns on this invention. It is a figure for demonstrating one form of the manufacturing method of the polarization beam combiner which concerns on this invention. It is a figure for demonstrating one form of the manufacturing method of the polarization beam combiner which concerns on this invention. It is a figure for demonstrating another form of the manufacturing method of the polarization beam combiner which concerns on this invention.

  Hereinafter, the present invention will be described in detail with reference to the drawings.

(First embodiment)
FIG. 3 shows a polarization beam combiner according to the first embodiment of the present invention. The polarization beam combiner 300 is adjacent to the PBS cube 301, the first prism 302 adjacent to the PBS cube 301 and having the first reflecting surface 302A parallel to the reflective film 301A of the PBS cube 301, and the PBS cube 301. The second prism 303 having the second reflecting surface 303A perpendicular to the reflecting film 301A of the PBS cube 301 and the third reflecting surface 303B facing the second reflecting surface 303A, and adjacent to the first prism 302 An optical path adjustment cube 304, a compensation plate 305 provided in the second prism 303, and a half-wave plate 306 provided in the optical path adjustment cube 304 are provided.

  The function of the polarization beam combiner 300 will be described with reference to FIG. Two input lights in the first polarization direction, for example, the TE mode, are incident on the compensation plate 305 and the half-wave plate 306, respectively. The first input light transmitted through the compensation plate 305 is totally reflected by the third reflecting surface 303B of the second prism 303, and then totally reflected by the second reflecting surface 303A facing the third reflecting surface 303B. Is done. The first input light then enters the PBS cube 301 and passes through the reflective film 301A. The second input light incident on the half-wave plate 306 is converted from the TE mode, which is the first polarization direction, to the TM mode, which is the second polarization direction, and then passes through the optical path adjustment cube 304. 1 is incident on one prism 302. Then, after being totally reflected by the reflecting surface 302A of the first prism 302, it is reflected by the reflecting film 301A of the PBS cube 301 and is emitted from the PBS cube 301 together with the first input light in the TE mode.

  By adopting such a configuration, it is possible to suppress an increase in the size of the element while compensating for the phase difference between the two input lights in the bulk polarization synthesizer for the two input lights having the same polarization direction. For example, as in the example of FIG. 2, H-K9L as the glass material of the PBS cube 301, the first prism 302, the second prism 303, and the optical path adjustment cube 304, as the material of the compensation plate 305 and the half-wave plate 306 When quartz is used, a triangular mirror with a side of 2 mm or two rows of cube mirrors and a quartz half-wave plate with a length of about 0.35 mm are used, so the total length of the polarization multiplexer is 4 It is about .35 mm, which is about half of the length of 8.28 mm in the conventional structure, and it can be seen that the increase in size can be suppressed.

  In FIG. 3, a cubic prism is illustrated as the PBS cube 301, but the PBS cube included in the polarization beam combiner according to the present embodiment includes a PBS cube structure in a larger prism. It is also included. For example, as shown in FIG. 5, a polarization beam combiner can be configured by combining three components. The general-purpose PBS block 501 includes a PBS cube structure centered on the reflective film 501A. The PBS block 501 may be bonded to the first block 502 on the first surface 501B, and may be bonded to the optical path adjusting cube 503 on the second surface 501C.

  An example as shown in FIG. 6 is also conceivable. The polarization beam combiner 600 shown in FIG. 6 is assembled with cubic prisms as parts, and the structure of the first prism 302 and the second prism 303 is different from that of the polarization beam combiner 300 shown in FIG. In the polarization beam combiner 600, the first prism 302 of FIG. 3 includes a first cube 602 having a first reflecting surface 602A, and the second prism 303 of FIG. 3 is adjacent to the PBS cube 301. , A second cube 603 ′ having a second reflective surface 603A perpendicular to the reflective film 301A of the PBS cube 301, and a second reflective surface 603A of the second cube 603 ′ adjacent to the second cube 603 ′. And a third cube 603 ″ having a third reflecting surface 603B parallel to the first cube 602, the second cube 603 ′ and the third cube 603 ″ as shown in FIGS. In addition, these cube structures can also be manufactured.

  The major difference from the example of FIG. 3 is that in FIG. 3, the first reflecting surface 302A, the second reflecting surface 303A, and the third reflecting surface 303B are all formed at the interface between the glass material and air. In contrast to the reflecting mirror, in FIG. 6, the first reflecting surface 602A, the second reflecting surface 603A, and the third reflecting surface 603B are formed by coating a reflecting film.

  The compensation plate 305 and the half-wave plate 306 are quartz plates made by the same processing. The compensation plate 305 has a crystal axis (C axis) of 0 degree, and the half-wave plate 306 has a crystal axis of 45 degrees. It can be cut out.

  In addition, a compound semiconductor laser using a normal [100] substrate often oscillates basically in the TE mode. In addition, an electro-optic effect and a light absorption effect, which are the principle of operation of a semiconductor optical modulator, are also obtained. The Franzkeldish effect and the quantum effect such as QCSE have a higher modulation efficiency for TE mode light than TM mode light (see Non-Patent Document 5). Therefore, when the polarization directions of the two input lights are in the TE mode, the significance of the polarization synthesizer according to the present embodiment that can suppress the increase in size of the element while compensating for the phase difference between the two input lights is very significant. Big.

  Further, although the term “total reflection” is used, it is difficult to actually realize 100% reflectance as described in the third embodiment. Note that terminology is used.

(Second Embodiment)
When the polarization beam combiner is configured as shown in FIG. 6, it is possible to perform monitoring by partially extracting TE mode light and TM mode light. When the first reflective surface 602A is coated with, for example, 95% reflectance for TM mode light, and the third reflective surface 603B is coated with, for example, 95% reflectance for TE mode light, FIG. As shown in FIG. 7, by arranging the light receiving elements 701 and 702, both polarizations can be monitored. There are cases in which the light intensity is controlled by monitoring each independently.

(Third embodiment)
With reference to FIGS. 8 to 10, an embodiment of a method for manufacturing a polarization beam combiner according to the present invention will be described.

  First, a large glass material having a controlled thickness is cut, and a required coating is applied to one side of the cut thin plate (FIG. 8). Next, these thin plates are bonded and cut out at 45 degrees (FIG. 9). The same process is performed for different coating combinations and bonded together (FIG. 10). For example, the surface treatments (1) to (5) in the figure are respectively applied to an optically polished surface without coating, a 94% to 100% reflective film for TE mode, a 94% to 100% reflective film for TE mode, and for PBS. If the reflective film is a 95% to 100% reflective film with respect to the TM mode, as shown in FIG. 10, polarization synthesis of TE mode and TM mode input light can be performed. By manufacturing in this way, the polarization beam combiner according to the present invention can be mass-produced at a low price. In this manufacturing method, since a thick plate manufactured by the same polishing is cut out and used only with a different coating, a manufacturing error is similarly applied to each of the TE side and the TM side. High-precision phase compensation becomes possible.

As a general principle, it is possible to create a highly reflective film with a reflectance of 100% for each of the TE mode and TM mode, but in reality, the relationship between the absorption and refractive index of the glass material, the wavelength of the light used, In addition, there is no coating material having an ideal refractive index, and depending on the wavelength band, the TM mode can reach nearly 100%, but the TE mode can be in a highly reflective state only up to about 95%. In the wavelength band of light of 1550 nm, this is exactly the case. With silicon dioxide SiO ₂ , titanium dioxide TiO ₂ , magnesium fluoride MgF, amorphous Si, etc., which are usually used for coating, the TE mode is 93% in the cube structure. Whether it can be increased to about 95% by adding a metal film to it. For this reason, in this structure which reflects twice in the polarization multiplexer, the insertion loss is about 10% with respect to the TE mode. Of these, even if 5% is the light extraction for monitoring described in the second embodiment, the remaining 5% is a loss.

  For this reason, when it is desired to keep the insertion loss within 0.5 dB with respect to light in each polarization state, 95% reflection is caused particularly on the third reflection surface ((2) in FIG. 10) on the TE mode side. When the rate is set and 5% is used as the monitor light, the second reflecting surface ((3) in FIG. 10) must be as close to 100% as possible. In general, unless the optical output of the optical device has more margin than necessary, an insertion loss of the optical component of 0.5 dB or more is not permitted in order to maintain the characteristics of the transmitter. For this reason, the second reflecting surface for light in the TE mode is required to be as close to 100% as possible. For the same reason, the ratio of light that can be used as monitor light is not more than 6%. For this reason, the first reflecting surface ((1) in FIG. 10) and the third reflecting surface ((2) in FIG. 10), which are reflecting surfaces for the monitor light, are approximately 95% in consideration of manufacturing errors ( 94% to 96% is preferable). All other reflective surfaces must be designed to be as close to 100% as possible.

  In FIG. 10, the surface treatment of the second reflecting surface ((3) in FIG. 10) has been described as an example in which the coating of the 100% reflecting film for the TE mode is used. However, it is difficult to actually set the reflectivity for the TE mode to be close to 100%, and in consideration of manufacturing tolerance, it may be about 95%. In this case, when the TE mode is reflected twice by the reflective film, about 10% of light is lost. Therefore, as a surface treatment of the second reflecting surface ((3) in FIG. 10), a concave portion may be provided as shown in FIG. 11 instead of coating the reflecting film. By providing the recess, a total reflection mirror is formed at the interface between the glass material and the air, and the reflection loss and hence the insertion loss can be reduced. In addition, when it is not necessary to extract light for monitoring, similarly as the surface treatment of the third reflecting surface ((2) in FIG. 10), the rear surface of the surface that becomes the reflecting surface, not the coating of the reflecting film. By providing a recess (indicated by a dotted line in FIG. 11) and forming a layer having an air refractive index, a total reflection surface can be obtained.

DESCRIPTION OF SYMBOLS 300 Polarization combiner 301 PBS cube 301A Reflective film 302 1st prism 302A 1st reflective surface 303 2nd prism 303A 2nd reflective surface 303B 3rd reflective surface 304 Optical path adjustment cube 305 Compensation plate 306 Half wavelength Plate 501 PBS block 501A Reflective film 501B First surface 501C Second surface 502 First block 503 Optical path adjusting cube 600 Polarization combiner 602 First cube 602A First reflective surface 603 'Second cube 603 ”Third cube 603A Second reflecting surface 603B Third reflecting surface 701, 702 Light receiving element

Claims (5)

PBS cube,
A first prism having a first reflective surface adjacent to the PBS cube and parallel to the reflective film of the PBS cube;
A second prism having a second reflecting surface adjacent to the PBS cube and perpendicular to the reflecting film of the PBS cube and a third reflecting surface facing the second reflecting surface;
And an optical path adjusting cubes adjacent to the first prism,
The PBS cube, the first prism, the second prism, and the optical path adjustment cube are made of the same material,
A half-wave plate provided in the optical path adjustment cube ;
Provided on the second prism, further comprising a compensation plate consisting of the half-wave plate of the same material,
When first and second input lights having a first polarization direction are incident on the compensation plate and the half-wave plate, respectively.
The first input light passes through the compensation plate, is reflected by the third reflecting surface, is then reflected by the second reflecting surface, and reaches the reflecting film of the PBS cube; and
The second input light is transmitted through the half-wave plate and converted into a second polarization direction, and then transmitted through the optical path adjustment cube and reflected by the first reflection surface, A polarization beam combiner characterized by having an equal optical path length to the reflection film. The first prism includes a first cube having the first reflecting surface;
The second prism is adjacent to the PBS cube, has a second reflecting surface perpendicular to the reflective film of the PBS cube, is adjacent to the second cube, and is And a third cube having the third reflecting surface parallel to the second reflecting surface of the cube, and the compensator is provided on the third cube. The polarization synthesizer described. The first polarization direction is a TE mode, and the second polarization direction is a TM mode,
The first reflection surface is a reflection film of 94% or more and 96% or less with respect to TM mode light,
3. The polarization beam combiner according to claim 2, wherein the third reflection surface is a reflection film of 94% or more and 96% or less with respect to TE mode light.   4. The polarization according to claim 3, further comprising a light receiving element for detecting TM mode light transmitted through the first reflective surface and TE mode light transmitted through the third reflective surface. Wave synthesizer.   3. The polarization beam combiner according to claim 2, wherein the second reflecting surface is a total reflection mirror formed by an interface between the glass material of the second cube and air.

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