JP2003091861A - Optical pickup and optical recording device provided with the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical pickup which can suppress the variation of balance of tracking error signals and can carry out a stable tracking operation, and an optical recording device provided with the optical pickup which can stably carry out recording on a recording medium. SOLUTION: The optical recording device has a light source LD which emits a light beam, a branching element 2 which branches the light beam into a main beam and a side beam, a separation element 3 which separates the light beam, a convergence means 6 which converges the separated light beam on a signal recording surface of a recording medium 20, and a light receiving element 7 receiving a return light beam which is the light beam reflected on the signal recording surface. The device is equipped with the optical pickup 1 which has a sufficiently small light quantity of the return side beam LBS to the light quantity of the return main beam LBM, when the return light beam passes through the branching element 2 again and is branched into a return main beam LBM and a return side beam LBS. The optical recording device with the optical pickup 1 carries out recording on a recording medium 20.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup suitable for use in an optical recording apparatus for recording information on a recording medium such as an optical disk by optical recording, and an optical recording apparatus equipped with this optical pickup.

[0002]

2. Description of the Related Art A push-pull method (PP method) or a three-beam method has been conventionally used as a tracking servo method in an apparatus for reproducing an optical disk or recording on an optical disk.

A differential push-pull method (D) is a typical tracking servo system used in an optical recording apparatus for recording on a recordable optical disk.
PP method).

Here, the differential push-pull method (DPP method)
The principle of is shown in FIG. As shown in FIG. 6A, three beam spots S1, S2, S formed by the diffraction grating are formed.
3, side spots S2 and S3 are main spots S1
On the other hand, the discs are arranged so as to be displaced by half the track pitch P in the radial direction. And each spot S1, S
In order to obtain a push-pull signal with respect to S2 and S3, it is configured so that the light reception can be detected separately in each of areas A to H in the figure.

FIG. 6B shows the three spots S1 of FIG. 6A.
The structure of the detection part which light-receives and detects each area | region A-H of S2 and S3 is shown. The light receiving element corresponding to the main spot S1 is 4
It is divided into four areas corresponding to the two areas A, B, C and D.
The light receiving elements corresponding to the side spots S2 and S3 are divided into two corresponding to the two regions E, F and G and H, respectively.

The push-pull signal of the main spot S1 and the push-pull signals of the side spots S2, S3 are differentiated to obtain the tracking error signal TE (DPP signal). However, in consideration of the difference in intensity between the main spot S1 and the side spots S2, S3, a predetermined gain G1 is applied to the sum of the push-pull signals of the side spots S2, S3. That is, the tracking error signal TE is expressed by the following equation. TE = (A + D)-(B + C) + G1 {(EF) + (GH)} (1)

The differential push-pull method (DPP method) has the following two effects. As a first effect, the offset of the tracking error signal (TE signal) due to the radial tilt of the disk and the radial shift of the objective lens, which are the drawbacks of the push-pull method (PP method), can be eliminated. it can. As a second effect, it is possible to eliminate the offset of the tracking error signal at the time of recording and the offset of the tracking error signal due to the inclination (tangential skew) in the track direction of the disk, which are the drawbacks of the three-spot method.

As described above, the DPP method has an advantage that the problematic offset of the tracking error signal due to the skew of the disk and the shift of the objective lens at the time of recording can be prevented.

Here, an optical system of a conventional optical pickup is shown in FIG. This optical pickup is separated from a laser diode LD that emits a light beam, a diffraction grating (grating) 51 that splits the light beam into a main beam and a side beam, and a beam splitter 52 that splits the split light beam. A lens group (collimator lens 54, quarter-wave plate 55, objective lens 56) for focusing the beam on the signal recording surface of the disk 60, and a light receiving element 57 for receiving the return light beam separated by the beam splitter 52. Is equipped with. Further, on the other side of the beam splitter 52 with respect to the laser diode LD, a monitor light receiving element 53 for detecting a laser output.
Is provided.

FIG. 8 shows an optical system of another conventional optical pickup. This optical pickup has a configuration in which an anamorphic prism 59 is added between the collimator lens 54 and the quarter-wave plate 55 as an element for shaping the beam shape in the optical system shown in FIG. . By adding the anamorphic prism 59, the optical path is bent before and after it.

In the structure of the optical pickup shown in FIG. 7 or 8, the light receiving element 57 has the same structure as that shown in FIG. 6B, so that the main beam and the side beam branched by the grating 51 are formed on the signal recording surface of the disk 60. After irradiating, the return light due to reflection is detected by the light receiving element 57,
A tracking error signal can be obtained by the DPP method.

By providing the optical pickup shown in FIG. 7 or 8, it is possible to configure an optical recording device for recording on the disc 60.

[0013]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
Also in the method, an offset of the tracking signal occurs in the following cases. In an optical system of an optical pickup adopting the DPP method, in order to split a light beam emitted from a laser diode LD into a main beam and a side beam, as shown in FIG. 7 and FIG. A diffraction grating (grating) is provided. Further, the light beam is condensed on the signal recording surface of the disk in the forward path from the laser diode LD to the disk, and is reflected by this surface to be return light toward the laser diode LD. In the return path from this disc to the laser diode LD, the returned light is split into two by a beam splitter, one of which passes through the light receiving element and the other of which passes through the diffraction grating again. And in FIG.
As shown in (enlarged view near the laser diode LD in FIG. 7), the return light L _{B that} has passed through the diffraction grating 51 again is returned by the branching action (diffraction action) of the diffraction grating 51 and returns to the main beam L _BM and the return side. It is split into beams L _BS .

Here, the return main beam L_BMAnd return rhino
DoBeam L_BSIs the facet of the laser diode LD
The end face 71) or the header (stem) 72 directly
And these parts 71, 72 have a finite reflectance
Therefore, return main beam L _BMAnd return side beam L
_BSFurther reflections of the
Multiple main beams and side beams are produced. This multiple
Beams interfere with each other, and multiple
Beam and side beam, and their interference intensity
Changes depending on the skew condition of the disk,
The light intensity of the upper side spot becomes unbalanced and
The king error signal is offset.

As described above, since the tracking error signal is offset when the disc is skewed, the DPP
There was a problem that the signal was out of balance and the track was in a detrack state.

Further, particularly in the optical pickup shown in FIG. 8, the directions of the three beams are set so as to form 45 degrees with respect to the radial direction (radial direction) and the tangential direction (circumferential direction) of the disc. And the magnification of the anamorphic prism 59, there is a problem that the side beam L _BS of the return light directly strikes the chip end surface (facet) 71 of the laser diode LD.
Therefore, it is expected that the DPP balance fluctuation in the skew of the disk 60 in the tangential direction becomes large.

Here, in the optical pickup of FIG. 7 and FIG.
Function of the used diffraction grating (grating) 51
Will be described with reference to FIGS. 10 and 11. Grating
51 is opposite to the beam light source LD, as shown in FIG.
Irregularities 51B forming a lattice surface are formed on the side. Outbound route
Then, as shown in FIGS. 10A and 11A, P-polarized light (S
The polarized light beam L may pass through the grating 51.
Main beam of P polarization (S polarization) by
(0th order light) L_MAnd side beam (+ 1st order light and -1st order light)
Light) L_SBranch to. Then, by the quarter wave plate 55
The P-polarized light (S-polarized light) is converted into circularly-polarized light and the disk 6
It is irradiated to 0. For the return path, see Fig. 10B and Fig. 11B.
As shown, the circularly polarized light reflected by the disk 60 is ¼
By passing through the wave plate 55, the P-polarized light (S-polarized light)
Return light L _BBecomes And again, the grating 51
By passing through, return light L_BAre P-polarized light
(S-polarized) return main beam (0th order light) L_BMAnd return service
It is branched into an id beam (+ 1st order light and −1st order light).

Further, as an evaluation of the configuration of the conventional optical pickup as shown in FIG. 8, FIG. 12 shows the measurement result of the DPP balance fluctuation. From FIG. 12, it can be seen that the DPP balance changes greatly with respect to the change in skew in the tangential direction. In addition, ± 0.
Including the region outside 6 degrees, the DPP balance changes periodically with almost the same amplitude.

In order to solve the above-mentioned problems, in the present invention, an optical pickup capable of performing stable tracking operation while suppressing fluctuations in the balance of the tracking error signal, and a recording medium provided with this optical pickup. An optical recording device capable of stable and stable recording.

[0020]

The optical pickup of the present invention includes a light source for emitting a light beam, a branching element for branching the light beam into a main beam and a side beam, and a separating element for separating the light beam. It has a converging means for condensing the light beam on the signal recording surface of the recording medium and a light receiving element for receiving the return light reflected by the signal recording surface, and the return light passes through the branching element again. Then, when the light is branched into the return main beam and the return side beam, the light amount of the return side beam is sufficiently smaller than the light amount of the return main beam.

According to the configuration of the optical pickup of the present invention described above, when the return light passes through the branching element again and is branched into the return main beam and the return side beam, the return side beam with respect to the light amount of the return main beam is returned. With a sufficiently small amount of light, it is possible to suppress fluctuations in the balance of the tracking error signal (DPP signal) due to multiple interference due to reflection of the return side beam around the light source. As a result, it is possible to prevent the tracking error signal from being offset due to, for example, the skewed state of the disk that is the recording medium.

The optical recording apparatus of the present invention comprises a light source for emitting a light beam, a branching element for branching the light beam into a main beam and a side beam, and a separating element for separating the light beam.
It comprises a converging means for condensing the separated light beam on the signal recording surface of the recording medium and a light receiving element for receiving the return light reflected by the signal recording surface, and the return light is branched again. When the light beam is split into a main beam and a return side beam after passing through, the optical pickup has a light amount of the return side beam that is sufficiently smaller than the light amount of the return main beam. It is what you do.

According to the configuration of the above-described optical recording apparatus of the present invention, the optical pickup of the present invention is provided to perform optical recording on the recording medium, so that fluctuations in the balance and offset of the tracking error signal are suppressed. Therefore, it is possible to perform stable tracking servo and perform recording on the recording track of the recording medium.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION According to the present invention, a light source for emitting a light beam, a branching element for branching the light beam into a main beam and a side beam, a separating element for separating the light beam, and a recording of the separated light beam. It comprises a converging means for condensing on the signal recording surface of the medium, and a light receiving element for receiving the return light reflected by the signal recording surface, and the return light passes through the branching element again and returns to the main beam. When the light is branched into the return side beam, the light amount of the return side beam is sufficiently smaller than that of the return main beam.

According to the present invention, in the above optical pickup, the branching element is composed of at least a polarization hologram whose diffraction efficiency changes depending on the polarization direction and a quarter wavelength plate, and the polarization hologram is closer to the light source side than the quarter wavelength plate. It is configured to be placed in.

Further, according to the present invention, in the above optical pickup, the polarization hologram and the quarter-wave plate are integrated to form a branching element.

According to the present invention, a light source for emitting a light beam, a branching element for branching the light beam into a main beam and a side beam, a separating element for separating the light beam, and a signal recording of the separated light beam on a recording medium. It has a converging means for condensing on the surface and a light receiving element for receiving the return light reflected by the signal recording surface, and the return light passes through the branching element again and returns to the main beam and the return side beam. Equipped with an optical pickup in which the light quantity of the return side beam is sufficiently smaller than the light quantity of the return main beam when branched to
This is an optical recording device for performing optical recording on a recording medium by this optical pickup.

The present invention also provides the above optical recording device,
The branching element includes at least a polarization hologram whose diffraction efficiency changes depending on the polarization direction and a quarter-wave plate, and the polarization hologram is arranged on the light source side with respect to the quarter-wave plate.

The present invention also provides the above optical recording device,
The polarization hologram and the quarter-wave plate are integrated to form a branching element.

FIG. 1 shows a schematic configuration diagram of an optical pickup (a diagram showing an optical system) as an embodiment of the present invention. This optical pickup 1 includes a laser diode LD as a light source for emitting a light beam, a polarization grating 2 as a branching element for splitting the light beam into a main beam and a side beam, and a beam splitter 3 as a splitting element for splitting the split beam. And a lens group (collimator lens 5, anamorphic prism 9, objective lens 6) for focusing the separated beam on a signal recording surface of a disk (optical disk or the like) 20 which is a recording medium, and a beam splitter 3. The light receiving element 7 receives the return light beam reflected from the separated disk 20. The objective lens 6 transmits the light beam to the disc 2
Converging means for converging on the signal recording surface of 0 is configured. The anamorphic prism 9 is an element that shapes the beam shape of the light beam.

In the present embodiment, in particular, instead of the grating 51 of FIG. 8, a polarization grating 2 is provided as a branching element for branching a light beam into a main beam and a side beam, and an optical pickup 1 is constructed.

A schematic configuration diagram (cross-sectional view) of this polarization grating 2 is shown in FIG. This polarization grating 2
Is the first transparent material 11, the polarization hologram 12, and 1 /
The four-wave plate 13 and the second transparent material 14 are laminated.

The polarization hologram 12 is constructed so that the diffraction efficiency changes depending on the polarization direction of the light beam, that is, the diffraction efficiency changes between P-polarized light and S-polarized light. Specifically, the hologram pattern of the polarization hologram 12 is formed so that the diffraction efficiency changes depending on the polarization direction of the light beam. Then, the polarization hologram 12 is arranged so that the diffraction efficiency is relatively high for one polarization (for example, P polarization) corresponding to the light beam from the laser diode LD, and is very small for the other polarization (for example, S polarization). Arrange and configure.

Comparing the configuration of the optical pickup of the present embodiment shown in FIGS. 1 and 2 with the configuration of the optical pickup shown in FIG. 8, since the quarter wave plate 13 is provided in the polarization grating 2, In FIG. 8, the quarter-wave plate 55 between the objective lens 56 and the anamorphic prism 59 is eliminated.

The function of the polarization grating 2 as a branching element will be described with reference to FIGS. 3 and 4. First, in the outward path, as shown in FIGS. 3A and 4A, the P-polarized (or S-polarized) light beam L is converted into the polarization hologram 12.
Of the main beam (0th order light) L _{M of} P-polarized light (or S-polarized light) and the side beam (+
1st-order light and -1st-order light) L _S , but further divided into 1/4
By passing through the wave plate 13, P-polarized light (or S-polarized light) is converted into circularly polarized light, and the disk 20 is irradiated with the polarized light.

On the other hand, in the return path system, as shown in FIGS. 3B and 4B, the circularly polarized light reflected by the disk 20 first passes through the quarter-wave plate 13 of the polarization grating element 2 to obtain a quarter-wavelength. The polarization direction of the plate 13 is changed from that of the outward path and becomes S-polarized light (or P-polarized light) and enters the polarization hologram 12. Here, the polarization hologram 12 has a relatively high diffraction efficiency for P-polarized light (or S-polarized light) corresponding to the light beam L as described above, whereas S-polarized light (or P-polarized light) whose polarization directions differ by 90 degrees. The diffraction efficiency with respect to is very small. Therefore, the return light L _B whose polarization direction is S-polarized (or P-polarized) different from the outward system is a return side beam (+ 1st-order light and −1st-order light) when entering the polarization hologram 12. Even if is branched, it cannot pass through the polarization hologram 12. As a result, the light amount ratio of the side beam to the main beam (0th order light) becomes extremely small.

Since the light quantity ratio of the side beam (return side beam) is extremely small as described above, reflection of the return side beam on the facet 71 and the header 72 as shown in FIG. 9 is also greatly reduced. This makes it possible to reduce fluctuations in the DPP balance due to multiple interference of reflected light.

The polarization grating 2 serving as a branching element
Preferably has a configuration satisfying the following conditions. That is, the light quantity ratio of the m-th order diffracted light in the forward path system is I (± m), and the light quantity ratio of the m-th order diffracted light in the return path system is J (± m) (where ΣI (m) = 1, ΣJ (m) = 1) Then, the configuration satisfies the following (2) and (3). 0.078 ≦ I (± 1) / I (0) ≦ 0.11 (2) J (± 1) ≦ 0.015 (3)

Equation (2) shows that in order to perform stable tracking servo, it is desirable that the light amount ratio of the side beam in the forward path system be set to an appropriate amount (0.078 to 0.11). In order to avoid the problem of the DPP balance variation due to the interference of the return side beams described above, the formula (3) sets the light amount ratio of the ± first-order light to 0.015 or less and sets the light amount of the return side beams to the return main beam. It has been shown that it is desirable to make it sufficiently small.

Then, for example, by selecting the refractive index and thickness of each component 11, 12, 13, 14 of the polarization grating 2, I (± 1) / I (0) = 0.081, J (± 1 ) = 0.
It can have a characteristic of 006.

Here, as an evaluation of the present embodiment, FIG.
FIG. 5 shows the measurement result of the DPP balance fluctuation with respect to the change in the skew (degree) of the disk 20 in the optical pickup 1 of FIG. It can be seen that the change in the DPP balance with respect to the change in the skew in the tangential direction is smaller than that in the case of the conventional configuration (FIG. 12).

According to the above-described present embodiment, the optical pickup 1 is configured by using the polarization grating 2 formed by laminating the polarization hologram 12 and the quarter-wave plate 13 as the branch element. , In the forward path, the laser beam L is split to split the main beam L _M and the side beam L
_S and _S can be generated to perform tracking servo using the three-spot method or the DPP method. Then, in the return path system, the amount of the return side beam L _BS that is the branched light (diffracted light) of the return light L _B can be reduced by the action of the quarter-wave plate 13 of the polarization grating 2 and the polarization hologram 12.

As a result, the intensity of multiple interference caused by the return side beam L _BS reflected on the facets and the header of the semiconductor laser LD can be weakened, and the offset amount of the DPP signal corresponding to the skew angle of the disk 20 can be reduced. Can be made smaller. As a result, fluctuations in the balance of the DPP signal are suppressed to a small level, and stable tracking operation can be performed.

By constructing an optical recording device including the optical pickup 1 of the present embodiment, an optical recording device capable of stably performing recording with a light beam on a recording medium such as the disk 20 is configured. can do.

Although the polarization grating 2 is provided in place of the grating 51 of the optical pickup shown in FIG. 8 in the above-described embodiment, the present invention can be applied to other configurations.

The present invention can be applied to, for example, an optical pickup (FIG. 7) without an anamorphic prism. However, when the anamorphic prism is provided, the return side beam L _BS is multiplied by the anamorphic prism, and the problem in the configuration using the conventional grating becomes apparent. Therefore, the effect of applying the present invention is greater. Become. In the future, the need for beam shaping with an anamorphic prism or the like will increase as the recording density further increases, so the importance of the present invention will increase.

Further, although the polarization grating 2 is arranged between the laser diode LD and the beam splitter 3 in FIG. 1, the position of the polarization grating 2 serving as a branching element is not limited to the position shown in FIG. The polarization grating 2 may be arranged at the position. That is, the polarization grating can be arranged at any position between the objective lens and the laser diode. However, it is arranged so that the beam enters perpendicularly to the polarization grating so that there is no interference due to refraction or reflection inside. When the anamorphic prism is provided, it is desirable to dispose the polarization grating on the laser diode side of the anamorphic prism.

Further, in FIG. 2, polarization holograms 12 and 1
The polarization grating 2 is configured by directly laminating and integrating the quarter wavelength plate 13 with each other. The polarizing hologram 12 and the quarter wavelength plate 13 are laminated with a transparent material such as glass interposed therebetween. They may be integrated together to form a polarization grating.

Further, the polarization hologram and the quarter-wave plate are configured as separate optical components, and the polarization hologram is arranged between the objective lens and the laser diode so as to be on the light source side. It is also possible to function as a branching element by combining two parts. For example, with respect to the conventional optical pickup configuration shown in FIG.
It is conceivable that a polarization hologram is arranged in place of the grating 51, and the quarter wave plate 55 is left between the objective lens 56 and the anamorphic prism 59. Also in this case,
The polarization hologram is formed by forming a hologram pattern so that the diffraction efficiency changes depending on the polarization direction.

As compared with the case where the polarization hologram and the quarter wave plate are laminated as described above, the number of parts is reduced and the size of the optical pickup is reduced as compared with the case where the polarization hologram and the quarter wavelength plate are laminated. It has the advantage that it can be realized.

The present invention is not limited to the above-mentioned embodiments, and various other configurations can be adopted without departing from the gist of the present invention.

[0052]

According to the present invention described above, tracking servo using the three-spot method or the DPP method can be performed, and the light quantity of the return side beam which is the branched light (diffracted light) of the return light can be reduced. be able to.

As a result, the intensity of multiple interference caused by the return side beam being reflected by the facets and headers of the semiconductor laser can be weakened, and the offset amount of the DPP signal corresponding to the skew angle of the disk can be reduced. it can. Therefore, according to the present invention, the fluctuation in the balance of the tracking error signal can be suppressed to be small, and stable tracking operation can be performed. Accordingly, it is possible to configure an optical recording device that can perform stable recording on the recording medium.

In particular, when a polarization hologram whose diffraction efficiency changes depending on the polarization direction and a quarter-wave plate are arranged so that the polarization hologram is on the light source side to form a branching element, the branching light The light amount of the return side beam, which is light), can be reduced more effectively. further,
When the polarization hologram and the quarter-wave plate are integrated to form a branch element, the number of parts can be reduced and the optical pickup can be downsized.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram (a diagram showing an optical system) of an optical pickup according to an embodiment of the present invention.

2 is a schematic configuration diagram (cross-sectional view) of a polarization grating of the optical pickup of FIG.

3A and 3B are views for explaining the function of a polarization grating in the optical pickup of FIG.

4A and 4B are diagrams illustrating a function as a branching element in the polarization grating in FIGS.

5 is a graph showing changes in the skew (degree) of the disc in the tangential direction of the optical pickup of FIG.
It is a figure which shows the fluctuation | variation of PP balance.

FIG. 6 is a diagram showing the principle of the A and B differential push-pull methods (DPP method).

FIG. 7 is a diagram showing an optical system of a conventional optical pickup.

FIG. 8 is a diagram showing an optical system of another conventional optical pickup.

9 is an enlarged view of the vicinity of the laser diode in FIG. 7.

10A and 10B are views for explaining the function of the diffraction grating (grating) in the optical pickup of FIGS.

11A and 11B are diagrams illustrating a function as a branching element in the grating of FIG.

12 is a diagram showing a change in DPP balance with respect to a change in skew (degree) in the tangential direction of the disc in the optical pickup of FIG.

[Explanation of symbols]

1 optical pickup, 2 polarization grating, 3
Beam splitter, 5 collimator lens, 6 objective lens, 7 light receiving element, 9 anamorphic prism, 11 first transparent material, 12 polarization hologram, 13 1/4 wavelength plate, 14 second transparent material, 20 disk, LD laser diode

Continued front page    F-term (reference) 5D118 AA13 BA01 CD03 CG04 CG23                       DA20 DA35                 5D119 AA20 AA29 BA01 EA02 JA23                       JA32 LB05                 5D789 AA20 AA29 BA01 EA02 JA23                       JA32 LB05

Claims (6)

[Claims] 1. A light source for emitting a light beam, a branching element for branching the light beam into a main beam and a side beam, a separating element for separating the light beam, and a signal recording of the separated light beam on a recording medium. A converging means for converging light on a surface and a light receiving element for receiving return light reflected by the signal recording surface, and the return light passes through the branching element again and returns to the main beam. When the light is branched into the return side beam, the light amount of the return side beam is sufficiently smaller than the light amount of the return main beam. 2. The branching element comprises at least a polarization hologram whose diffraction efficiency changes depending on the polarization direction and a quarter wavelength plate, and the polarization hologram is arranged on the light source side with respect to the quarter wavelength plate. The optical pickup according to claim 1, wherein: 3. The optical pickup according to claim 2, wherein the polarization hologram and the quarter-wave plate are integrated to form the branching element. 4. A light source for emitting a light beam, a branching element for branching the light beam into a main beam and a side beam, a separating element for separating the light beam, and a signal recording of the separated light beam on a recording medium. A converging means for converging light on a surface and a light receiving element for receiving return light reflected by the signal recording surface, and the return light passes through the branching element again and returns to the main beam. And an optical pickup in which the light amount of the return side beam is sufficiently smaller than the light amount of the return main beam when the light is branched into the return side beam. The optical pickup performs optical recording on the recording medium. Optical recording device. 5. The branching element comprises at least a polarization hologram whose diffraction efficiency changes depending on the polarization direction and a quarter wavelength plate, and the polarization hologram is arranged on the light source side with respect to the quarter wavelength plate. The optical recording device according to claim 4, wherein 6. The optical recording device according to claim 5, wherein the polarization hologram and the quarter wavelength plate are integrated to form the branching element.