JP5152366B2 - Isolator and variable voltage attenuator - Google Patents

Description

  The present invention relates to an isolator and a voltage variable attenuator.

In an optical head device for reading information from an optical disc such as a CD or DVD, for example, a polarizing diffraction grating 400 shown in FIG. 10 is used as a polarizing beam splitter. The diffraction grating 1 is formed of a birefringent material layer having a normal light refractive index n _o and an extraordinary light refractive index n _e (n _o ≠ n _e ) on one side of a glass substrate which is a translucent substrate 4, and the cross-sectional shape is A periodic structure with unevenness and a step difference d is formed.

The isotropic transparent material 3 having a refractive index n _s almost equal to the ordinary light refractive index n _o is filled so as to fill the irregular portion of the periodic structure, and a glass substrate which is a translucent substrate 5 is overlaid on the isotropic transparent material 3. Thus, a polarizing diffraction grating 400 is formed. Here, by setting | n _e −n _s | × d to half the wavelength λ of the incident light, the incident light of ordinary light polarized light (polarization direction giving the ordinary light refractive index) is transmitted straight without being diffracted, and abnormal light The incident light of polarized light (polarization direction giving an extraordinary light refractive index) is diffracted and does not pass straight, but becomes a polarizing diffraction grating.

When such a polarizing diffraction grating is used as an isolator for optical communication having a wavelength band of 1400 to 1700 nm, there has been a problem that a sufficient extinction ratio cannot be obtained. That is, when paying attention to a specific single wavelength λ ₀ , the amount of _first linearly polarized light (for example, ordinary polarized light) that is transmitted in a straight line is I ₁ , and the second linearly polarized light having a polarization direction orthogonal to the first linearly polarized light ( when the amount of light straightly transmitted without being diffracted for extraordinarily polarized light) and _{I 2,} the ratio _I 2 / _{I 1} (hereinafter, referred to as the extinction ratio) of -20dB or less. However, since the transmissivity of the linearly transmitted light of the extraordinary light polarization is described by cos ² (0.5 × π × λ ₀ / λ), the incident light is not diffracted as the wavelength λ is different from λ _0. Components that pass straight through are generated and the extinction ratio deteriorates.

  In addition, in order to realize a high extinction ratio for a specific single wavelength, it is necessary to accurately process the step d of the irregular periodic structure, and a polarizing diffraction grating having a high extinction ratio with high reproducibility is required. It was difficult to get.

  In view of the above circumstances, an object of the present invention is to provide an isolator and a voltage variable attenuator that can stably realize a high extinction ratio.

The present invention is an isolator including at least a polarizer and a condensing lens, and as the polarizer, incident light having a first polarization direction does not act as a diffraction grating and transmits straight, Multi-layer diffraction in which at least two polarizing diffraction gratings including a birefringent material are diffracted by acting as a diffraction grating for incident light having a second polarization direction orthogonal to one polarization direction. A multi-layer diffractive polarizer formed of a grating, and each polarizing diffraction grating includes at least one translucent substrate, the ordinary refractive index n _o formed on the translucent substrate, and A birefringent material layer made of a polymer liquid crystal having an extraordinary refractive index n _e (n _o ≠ n _e ) has been processed into a periodic concavo-convex shape whose cross-sectional shape is longer than the wavelength λ of incident light, the refractive index at least in recesses n _o or n _e is the isotropic transparent material is filled equal, unlike each further grating longitudinal direction of polarization of the diffraction grating in the polarizing diffraction grating, the incident light having the first polarization direction, the polarizer The incident light having a second polarization direction is diffracted by the polarizer and is focused by the condenser lens. An isolator is characterized in that a focal point is formed on a focal plane outside the optical axis of the condenser lens.

Further, the present invention provides a light incident surface having a first polarization direction on a light incident surface and a light emitting surface of a liquid crystal cell in which a liquid crystal layer is sandwiched between electrodes. Polarization with a birefringent material that does not act as a diffraction grating and transmits straight, and diffracts incident light having a second polarization direction orthogonal to the first polarization direction. A composite liquid crystal element in which a multilayer diffractive polarizer formed by a multilayer diffraction grating in which at least two of the above diffraction gratings are stacked is laminated, and each polarizing diffraction grating has at least one light transmitting property A birefringent material layer made of a polymer liquid crystal having an ordinary light refractive index n _o and an extraordinary light refractive index n _e (n _o ≠ n _e ) formed on the translucent substrate , Processed into periodic irregularities with a period longer than the wavelength λ of the incident light Further, wherein the refractive index at least recesses for adjusting the straight transmission light amount of a specific polarization direction by n _o or equal isotropic transparent material to n _e and is filled, the voltage applied to the liquid crystal layer A variable voltage attenuator is provided.

  According to the present invention, since the multilayer diffractive polarizer has a high extinction ratio with respect to incident light in a wide wavelength band, a high performance isolator can be obtained by using the multilayer diffractive polarizer. In addition, the composite liquid crystal element in which the multilayer diffractive polarizer and the liquid crystal cell are integrated with each other can set the voltage applied to the liquid crystal cell to a predetermined magnitude so that the linearly transmitted light amount in a specific polarization direction has a desired magnitude. Therefore, a high performance voltage variable attenuator can be obtained by using this composite type liquid crystal element.

The side view which shows the structural example of the multilayer diffraction type polarizer of the 1st embodiment of this invention. The side view which shows an effect | action when extraordinary-light polarized light injects into the multilayer diffraction type polarizer shown in FIG. The side view which shows an effect | action when normal light polarized light injects into the multilayer diffraction type polarizer shown in FIG. The top view which shows an example of the 2 types of diffraction grating pattern which comprises the multilayer diffraction type polarizer shown in FIG. The side view which shows an example which comprises the optical system in which the multilayered diffraction type polarizer shown in FIG. 1 isolate | separates a straight transmission light and diffracted light. The top view which shows an example of the condensing position of the linearly transmitted light and diffracted light in the focal plane of the condensing lens of the light which permeate | transmitted the multilayer diffraction type polarizer shown in FIG. The graph which shows an example of the wavelength dependence (calculated value) of the extraordinary light polarization | polarized-light transmittance in the multilayer diffraction type polarizer of this invention. The side view which shows the structural example of the composite type liquid crystal element of the 3rd embodiment of this invention. The side view which shows the other structural example of the composite type liquid crystal element of this invention. The side view which shows the structural example of the conventional multilayer diffraction type polarizer.

  In the present invention, first, incident light having a first polarization direction does not act as a diffraction grating and transmits straight, and for incident light having a second polarization direction orthogonal to the first polarization direction. A polarizing diffraction grating having a birefringent material that acts as a diffraction grating and diffracts is obtained, and a multilayer diffraction polarizer in which two or more polarizing diffraction gratings are stacked is obtained. By configuring the multi-layer diffractive polarizer in this way, the effect of increasing the extinction ratio is produced.

[First Embodiment]
FIG. 1 is a side view showing a first embodiment of the constitution of the multilayer diffraction type polarizer of the present invention. In each of one surface of the translucent substrate 4 and the transparent substrate 5, a birefringent material layer of the ordinary refractive index n _o and extraordinary refractive index _{n e (n o ≠ n e} ), the fast axis (ordinary The direction in which the refractive index is indicated) is aligned with the X-axis direction in FIG. Next, the birefringent material layer is divided into a diffraction grating 1 having an uneven periodic structure with a cross-sectional shape of step d ₁ and a grating pitch p _{1, and} an uneven periodic structure with a cross-sectional shape of step d ₂ and a grating pitch p _2. To a diffraction grating 2 having

Thereafter, the optical substrate 4 permeable at least to the respective concave refractive index n _{s (ordinary} refractive index n _o or equal to the extraordinary refractive index n _e) a diffraction grating polarizing isotropic transparent material 3 is filled in and After forming on the translucent substrate 5, the translucent substrate 4, the translucent substrate 5, and the translucent substrate 6 are laminated | stacked, and it is set as the multilayer diffraction type polarizer 100. FIG. Here, the meaning of at least each of the recesses may be that only the recesses are filled, or that the recesses and projections are filled. The isotropic transparent material is a transparent material having an isotropic refractive index. The longitudinal direction of the lattice, which is the groove direction of the lattice concave portion, may be parallel or orthogonal to the translucent substrate 4 and the translucent substrate 5, or at a predetermined angle. May be. Since the diffracted light generated by the diffraction grating is perpendicular to the grating longitudinal direction, the diffracted light can be generated in a desired direction by setting the grating longitudinal directions of the diffraction grating 1 and the diffraction grating 2 to a predetermined angle. .

Here, for example, the refractive index _{n s} is using approximately equal isotropic transparent material 3 and the ordinary refractive index _{n o,} the retardation value _| is _{× d 2 | n e -n s} | × d 1 and _| n e -n _s Steps d ₁ and d ₂ that are (m + ½) times the wavelength of incident light (m is 0 or a positive integer) are preferable for the following reason. The reason for this is that the ratio of the amount of light transmitted in a straight line to the incident light having the second polarization direction is minimized, and a high extinction ratio is obtained. Here, (m + 1/2) times may include a change in magnification within ± 10% of (m + 1/2), and the effect in the present invention does not change.

When extraordinary polarized light (S-polarized light) is incident on such a multilayer diffractive polarizer 100, the polarizing diffraction gratings comprising the diffraction grating 1 and the diffraction grating 2 in the present invention are as shown in FIG. The concavo-convex periodic structure acts as a phase modulation type diffraction grating having a refractive index _ne and a refractive index n _s to generate diffracted light. Hereinafter, the diffraction grating 1 means a polarizing diffraction grating 1 composed of the diffraction grating 1, and the same applies to the diffraction grating 2.

In the multilayer diffractive polarizer, the retardation values | n _e −n _s | × d ₁ and | n _e −n _s can be used to suppress the wavelength dependence of diffraction efficiency and reduce the processing step. | × d ₂ is preferably ½ times the wavelength of the emitted light (corresponding to m = 0). Here, as described above, a magnification change within ± 10% of ½ times may be included. That is, the magnification may be in the range of 0.55 to 0.45.

  Here, a part of the extraordinary light polarized light (S-polarized light) that has been transmitted straight without being diffracted by the diffraction grating 1 is diffracted by the diffraction grating 2, so that the extraordinary light polarized light that passes straight through the multilayer diffraction polarizer 100 is extremely high. Slightly.

On the other hand, when ordinary light polarized light (P-polarized light) is incident on the multilayer diffractive polarizer 100, the diffraction grating 1 and the diffraction grating 2 in the present invention may have an irregular periodic structure as shown in FIG. This is equivalent to a medium having a refractive index of n _s , and incident light is transmitted straight without being diffracted.

  Therefore, by laminating the diffraction grating 1 and the diffraction grating 2 having a transmittance of ordinary light polarization of 90% or more and a transmittance of extraordinary light polarization having a polarization direction orthogonal to the polarization direction of ordinary light polarization of 5% or less. In the incident light of extraordinary light polarization, a multilayer diffractive polarizer in which the linearly transmitted light is 0.5% or less of the incident light is obtained.

Here, as shown in the two types of diffraction grating patterns in FIG. 4 and FIG. 1, the diffraction grating 1 is a linear grating having a grating pitch p ₁ and an angle in the longitudinal direction of the grating θ ₁ with respect to the X axis. Is a linear lattice having a lattice pitch p ₂ and an angle in the lattice longitudinal direction of θ ₂ with respect to the X axis.

In general, when the light diffracted by the diffraction grating 1 is also diffracted by the diffraction grating 2 and superimposed on the straight transmitted light, the result is that the straight transmitted light component increases and the extinction ratio deteriorates. However, if the grating pitch p ₁ is different from the grating pitch p ₂ or the angle θ ₁ and the angle θ _{2 in the} grating longitudinal direction are different, such a change in the extinction ratio can be prevented. That is, setting so that the grating pitch or the grating longitudinal direction of each diffraction grating as a constituent element does not coincide with each other does not cause the multiple diffracted lights of the diffraction grating 1 and the diffraction grating 2 to be superimposed on the straight transmitted light, and the extinction ratio is reduced. It is preferable without causing deterioration. Even if the steps d ₁ and d _{2 of the} respective diffraction gratings are the same, the extinction ratio does not deteriorate as long as p ₁ and p ₂ or θ ₁ and θ ₂ are different.

  FIG. 5 is a side view showing an example of an optical system configuration in the case where an isolator having a high extinction ratio is formed using the multilayer diffraction polarizer 100 of the present invention. When parallel light in which ordinary light polarization and extraordinary light polarization are mixed is incident on the multilayer diffractive polarizer 100 and the condenser lens 7 is arranged on the exit side, the ordinary light polarized light transmitted straight through the multilayer diffractive polarizer 100 is The light is condensed on the focal plane on the optical axis of the condenser lens 7. On the other hand, the extraordinary light polarized light diffracted by the multilayer diffraction polarizer 100 is condensed on the focal plane outside the optical axis of the condenser lens 7.

  Therefore, by disposing the aperture stop 8 having an opening on the focal plane on the optical axis of the condenser lens 7, an isolator that transmits only ordinary light polarization and blocks abnormal light polarization is obtained. Here, only the component of ordinary light polarization can be detected by arranging a photodetector having a light receiving portion corresponding to the opening instead of the aperture stop 8. Further, if the core portion of the optical fiber for optical transmission is arranged instead of the opening portion, only ordinary polarized light can be transmitted.

4, in the diffraction grating 2 of the grating pitch p ₂ in the grating longitudinal direction of the angle theta ₁ with the diffraction grating 1 and the grating longitudinal direction of the angle theta ₂ of the grating pitch _{p 1, θ 1 = θ 2} = 0 to °, FIG. 6 shows an example of the condensing position of the linearly transmitted light and the diffracted light generated on the focal plane of the condensing lens 7 in FIG. 5 when p ₂ is twice as large as p ₁ .

  Ordinary polarized light (P-polarized light) is not diffracted by the diffraction grating 1 and the diffraction grating 2 (0th-order diffracted light is further converted to 0th-order diffracted light) and is collected at a position indicated by ◎ on the optical axis. This is expressed as 0th order × 0th order. Also, extraordinary light polarization (S-polarized light) diffracted by the diffraction grating 1 and the diffraction grating 2 as light having the same sign and the same order (± 1st-order diffracted light is further diffracted into ± 1st-order diffracted light, respectively) Is condensed at a position indicated by Δ or ▽. This is expressed as first order × first order and −1st order × −1 order. The same applies hereinafter. The extraordinary light polarized light that is diffracted as ± first-order light by the diffraction grating 1 but is not diffracted by the diffraction grating 2 and becomes zero-order diffracted light is condensed at a position indicated by ◯.

  Further, it is diffracted by the diffraction grating 2 as ± first-order light but not diffracted by the diffraction grating 1 (0th-order diffracted light), and is diffracted by different orders and different orders by the diffraction grating 1 and the diffraction grating 2. The extraordinary light polarization (which is diffracted into + 1st order diffracted light or diffracted into + 1st order diffracted light into + 1st order diffracted light) is collected at a position indicated by □.

The diffraction direction of the extraordinary light polarization is determined by the angles θ ₁ and θ ₂ of the grating longitudinal direction of the diffraction grating 1 and the diffraction grating 2, and the distance from the optical axis of the condensing position of the diffracted light is the wavelength of the incident light and the grating pitch p _1. , P ₂ and the focal length of the condenser lens 7.

[Second Embodiment]
Next, it is preferable to make the steps d ₁ and d _{2 of} the birefringent material layers constituting the diffraction grating 1 and the diffraction grating 2 different. Furthermore, the ratio of the wavelength lambda ₁ when the wavelength of the incident light is in the range of lambda ₁ to [lambda] _2, and lambda _2, the difference △ n of the ordinary refractive index and an extraordinary refractive index of the birefringent material layer, lambda ₁ It is preferable that d ₁ and d ₂ exist between / (2 × Δn) and λ ₂ / (2 × Δn). A relatively high extinction ratio can be obtained even for incident light having a wide wavelength band.

A second embodiment of the multilayer diffractive polarizer of the present invention will be described. When extraordinary light polarized light having a wavelength λ is incident on the multilayer diffractive polarizer of this embodiment, the transmittance η ₀ of straight transmitted light (0th-order diffracted light) that is not diffracted by the diffraction grating 1 and the diffraction grating 2 is η ₀ = ( cos (φ / 2)) ^{2 is} approximately described. Here, φ = 2 × π × △ n × d / λ, △ n = | n e -n s |> 0 in, _{n o} and _{n s} is substantially equal, The diffraction grating 1, d _{= d} 1, In the diffraction grating 2 and d = _{d 2.}

When the wavelength of the incident light is in the range of lambda ₁ to [lambda] _2, in order to achieve a high diffraction efficiency at this wavelength band, the _{d 1} and _{d 2 λ 1 / (2 ×} △ n) and lambda ₂ / ( It is effective to have a different value between 2 × Δn). When the wavelength of the incident light is in the range of 1400 to 1700 nm, a concavo-convex periodic structure is formed using a birefringent material with Δn = 0.15, and the steps are d ₁ = 4.8 μm and d ₂ = 5. FIG. 7 shows the result of calculating the wavelength dependence of the straight-line transmittance η ₀ of extraordinary light polarization when .5 μm is set. Note that λ ₁ /(2×Δn)=4.67 μm and λ ₂ /(2×Δn)=5.67 μm, and d ₁ and d ₂ are values therebetween.

7, a single diffraction grating 1 and the extraordinarily polarized light with respect to the diffraction grating 2 rectilinear transmittance eta ₀ de respectively △ and □ and, the rectilinear transmittance eta ₀ of multilayer diffraction type polarizer 100 as a whole extraordinarily polarized beam Indicated by ○. The incident light of ordinary light polarization is hardly diffracted, and 90% or more of the incident light is transmitted in a straight line, so that the isolator has an extinction ratio of −35 dB or less in the wavelength band of 1400 to 1700 nm.

  In addition, a higher extinction ratio can be obtained by further laminating the multilayer diffractive polarizer 100 of the present invention in series.

[Third Embodiment]
Next, FIG. 8 shows a side view of a configuration example of a composite type liquid crystal element which is the third embodiment of the present invention in which the multilayer diffraction polarizers 110 and 120 are combined with the liquid crystal cell 130. The liquid crystal cell 130 includes a translucent substrate 61 and a translucent substrate 62 in which transparent electrode films 71 and 72 made of, for example, ITO and alignment films (not shown) made of, for example, polyimide that have been subjected to alignment treatment are formed on one side of the substrate. For example, a liquid crystal layer 9 of nematic liquid crystal in which alignment directions of liquid crystal molecules are aligned is formed into cells by a sealing material 10.

  As shown in FIG. 8, the multilayer diffractive polarizer described in the above embodiment is laminated on the surface of at least one light transmitting substrate of a liquid crystal cell in which a liquid crystal layer is sandwiched between electrodes. It is preferable to use the composite liquid crystal element configuration as described above because the element can be downsized and a stable extinction ratio can be obtained.

Here, for example, a rectangular wave AC voltage is applied to the transparent electrode films 71 and 72 from the external AC power supply 11. In addition, by setting the alignment treatment direction of the alignment film to the X-axis direction in FIG. 8, the liquid crystal molecules are aligned in the X-axis direction parallel to the substrate surface in the liquid crystal cell 130 when no voltage is applied. When the difference between the ordinary refractive index n _o (LC) and the extraordinary refractive index n _e (LC) of the liquid crystal molecules is Δn (LC), the thickness d (LC) of the liquid crystal layer is set to the wavelength λ of the incident light. Δn (LC) × d (LC) = λ / 2.

  The multilayer diffraction polarizers 110 and 120 joined to the liquid crystal cell 130 using a transparent adhesive (not shown) or the like are the multilayer diffraction polarizations described in the first and second embodiments. The fast axis direction (direction in which the ordinary refractive index is given) of the birefringent material layer constituting each multi-layer diffractive polarizer is multi-layer diffracted with respect to the X-axis direction in the XY plane of FIG. The multi-layer diffractive polarizer 120 is formed at an angle of 45 °, while the multi-layer diffractive polarizer 120 is formed at an angle of 45 °. That is, the two diffraction gratings of the multilayer diffractive polarizer 110 are linear gratings, and the longitudinal direction of the grating is at an angle of 45 ° with respect to the X-axis direction. The two diffraction gratings are also linear gratings, and the longitudinal direction of the gratings is at an angle of 135 ° with respect to the X-axis direction.

  When light having a wavelength λ enters the composite liquid crystal element 200 having such a configuration from the multi-layer diffractive polarizer 110 side, the first linearly polarized light whose polarization direction forms an angle of 45 ° with the X axis is the multi-layer diffraction. The second linearly polarized light, which is transmitted without being diffracted by the polarizing polarizer 110 and whose polarization direction forms an angle of 135 ° with the X axis, is diffracted and transmitted by the multilayer diffracting polarizer 110, and passes through the liquid crystal cell 130. Incident.

  When no voltage is applied to the liquid crystal cell 130, the liquid crystal cell 130 acts as a phase plate having a phase difference π with respect to the first and second incident linearly polarized light. That is, since it acts as a half-wave plate, the straight transmitted light that is not diffracted by the multilayer diffractive polarizer 110 is converted into linearly polarized light having an angle of 135 ° with the X axis, and is diffracted by the multilayer diffractive polarizer 110. The transmitted light is converted into linearly polarized light having an angle of 225 ° with the X axis.

  As a result, the linearly transmitted light that is not diffracted by the multilayer diffractive polarizer 110 enters the multilayer diffractive polarizer 120 as ordinary light polarization, and thus travels straight without being diffracted. On the other hand, the light diffracted by the multilayer diffractive polarizer 110 is diffracted because it enters the multilayer diffractive polarizer 120 as extraordinary light polarization. Accordingly, of the incident light to the composite liquid crystal element 200, the first linearly polarized light is transmitted straight without being diffracted, and the second linearly polarized light having a polarization direction orthogonal to the polarization direction of the first linearly polarized light is diffracted. Is emitted. Here, since the diffraction grating constituting the multilayer diffractive polarizer 110 and the diffraction grating constituting the multilayer diffractive polarizer 120 are different from each other in the longitudinal direction of the grating, the generated multiple diffracted light is on the optical axis through which the light travels straight. Does not overlap with light.

In addition, when a voltage is applied to the liquid crystal cell 130, a voltage is applied to the transparent electrodes 71 and 72. Therefore, a director of liquid crystal molecules in the liquid crystal cell 130 (abnormal light refractive index _ne (LC) is relative to the substrate surface). aligned in the vertical direction. Accordingly, since the liquid crystal layer acting as a substantially uniform ordinary refractive index n _{o (LC)} layer, regardless of the polarization direction of the incident light, while the incident light was kept the polarization state without causing phase difference change Exit.

  As a result, the linearly transmitted light that is not diffracted by the multilayer diffractive polarizer 110 is diffracted because it enters the multilayer diffractive polarizer 120 as extraordinary light polarization. On the other hand, the transmitted light diffracted by the multilayer diffractive polarizer 110 is not diffracted because it enters the multilayer diffractive polarizer 120 as ordinary light polarized light. Therefore, both the first linearly polarized light and the second linearly polarized light of the incident light to the composite liquid crystal element 200 are diffracted and emitted. That is, incident light is diffracted regardless of its polarization state, and does not exist on the optical axis through which light passes straight.

  Accordingly, FIG. 5 shows that the straight transmitted light and the diffracted light are separated by turning on and off the voltage applied to the liquid crystal cell 130. In FIG. 5, by arranging the composite liquid crystal element 200 in place of the multilayer diffractive polarizer 100, a polarizing switching element with a high extinction ratio can be realized. In addition, since a linearly transmitted light amount in a specific polarization direction can be adjusted to a desired magnitude by setting a voltage having a predetermined magnitude without turning on and off the applied voltage, it functions as a voltage variable attenuator.

  FIG. 9 shows another configuration example of the composite liquid crystal element 300 in which the multilayer diffraction polarizer 120 and the liquid crystal cell 130 are combined. On the light incident side of the composite liquid crystal element 300, a polarization conversion element 15 in which a half-wave plate 14 is bonded to a prism on which the polarization separation film 12 and the total reflection mirror 13 are formed is disposed.

  Of the two linearly polarized light incident on the polarization conversion element 15, one linearly polarized light is transmitted through the polarization separation film 12, and the other linearly polarized light whose polarization direction is orthogonal is reflected by the polarization separation film 12 and the total reflection mirror film 13. After that, the polarization plane (polarization direction) is rotated by 90 ° by the half-wave plate 14, and becomes linearly polarized light having the same polarization direction as one side and enters the composite liquid crystal element 300. As a result, a switching element or attenuator with little insertion loss can be realized regardless of the polarization state of incident light.

  In this embodiment, the alignment direction of the liquid crystal molecules of the liquid crystal cell is parallel, but by making the alignment treatment directions of the alignment films of the translucent substrate 61 and the translucent substrate 62 form a specific angle, A structure in which the alignment of liquid crystal molecules is twisted around an axis in the thickness direction of the liquid crystal layer may be employed. Also, by selecting the alignment treatment method of the alignment film and the liquid crystal material, a so-called hybrid in which the alignment direction of the liquid crystal molecules is perpendicular to one light-transmitting substrate surface and parallel to the other light-transmitting substrate surface. An oriented structure may be adopted.

  In the present embodiment, the fast axes of the birefringent material layers of the multilayer diffractive polarizer 110 and the multilayer diffractive polarizer 120 are orthogonal to each other, but may be parallel to each other. In this case, the transmittance of the linearly transmitted light is minimum when no voltage is applied to the liquid crystal cell and is maximum when a voltage is applied. Further, by patterning the transparent electrode films 71 and 72 of the liquid crystal cell and applying a voltage independently to each pattern electrode, the spatial transmittance of the straight transmitted light according to the patterning shape can be adjusted.

The multilayer diffraction polarizer of this example will be described with reference to FIG. On each side of the light-transmitting substrate 4 and the light-transmitting substrate 5 made of a glass substrate, a birefringent material layer having a high ordinary light refractive index n _o = 1.55 and an extraordinary light refractive index n _e = 1.70. A molecular liquid crystal layer was formed, and linear diffraction gratings 1 and 2 were formed by photolithography and etching techniques. The grating pitches p ₁ and p ₂ of the diffraction gratings 1 and 2 are 20 μm and 40 μm, respectively, the respective grating longitudinal directions are parallel, and the respective concave depths of the polymer liquid crystal layers of the diffraction gratings 1 and 2, that is, the step d _1. and _{d 2} were set to 4.8μm and 5.6μm, respectively.

Further, an isotropic transparent material 3 made of a homogeneous transparent resin having a refractive index n _s = 1.55 is filled in a concave portion processed into a concavo-convex shape of a polymer liquid crystal layer, and a translucent substrate 6 made of a glass substrate is laminated. Thus, the multilayer diffraction polarizer 100 in which the polarizing diffraction grating composed of the diffraction grating 1 and the polarizing diffraction grating composed of the diffraction grating 2 were laminated was manufactured. Here, the polymer liquid crystal layer was produced by injecting a liquid crystal monomer solution between the substrates on which the alignment film (alignment treatment completed) was formed, and irradiating with ultraviolet rays to polymerize and solidify the liquid crystal monomer. Further, an antireflection film is formed at the interface between the light-transmitting substrate 4 and the light-transmitting substrate 6 and air.

  When parallel light having a wavelength of 1400 to 1700 nm is incident on the multilayer diffractive polarizer 100 thus obtained, ordinary light polarization is hardly diffracted, and 97% of the incident light is transmitted straight through, and the polarization direction is ordinary light. The extraordinary light polarized light orthogonal to the polarized light was almost diffracted and 0.05% or less passed straight. As shown in FIG. 5, the converging lens 7 is used to condense the light transmitted through the multilayer diffractive polarizer 100 on the focal plane, and only the straight transmitted light on the optical axis is the core of the optical fiber (not shown). It was made to form an image. As a result, in the wavelength band of 1400 to 1700 nm, an isolator having a high extinction ratio of -30 dB or less of the extraordinary light polarization to the ordinary light polarization was obtained.

  As described above, the multi-layer diffractive polarizer described above has a high extinction ratio with respect to incident light in a wide wavelength band. Therefore, by using such a multi-layer diffractive polarizer, a high-performance isolator is used. Is obtained.

  Furthermore, the composite type liquid crystal element in which the multilayer diffractive polarizer and the liquid crystal cell are integrated can realize a switching operation having a high extinction ratio by turning on and off the voltage applied to the liquid crystal cell. By using the composite liquid crystal element, a high-performance switching element can be obtained. In addition, by setting the applied voltage to a predetermined level, the amount of linearly transmitted light in a specific polarization direction can be adjusted to a desired level. An attenuator is obtained.

DESCRIPTION OF SYMBOLS 1, 2 Polarizing diffraction grating 3 Isotropic transparent material 4, 5, 6, 61, 62 Translucent substrate 7 Condensing lens 8 Aperture stop 9 Liquid crystal layer 10 Sealing material 11 External AC power supply 12 Polarization separation film 13 All Reflective mirror 14 Half-wave plate 15 Polarization conversion element 71, 72 Transparent electrode film 100, 110, 120, 300 Multilayer diffractive polarizer 130 Liquid crystal cell 200 Composite liquid crystal element

Claims (7)

An isolator comprising at least a polarizer and a condenser lens,
As the polarizer, it does not act as a diffraction grating for incident light having the first polarization direction and transmits straight, and diffracts for incident light having the second polarization direction orthogonal to the first polarization direction. A multi-layer diffractive polarizer formed by a multi-layer diffraction grating having at least two laminated polarizing diffraction gratings that function as a grating and diffract the birefringent material;
Wherein each polarizing diffraction grating, the at least one light-transmitting comprises a substrate, the translucent ordinary refractive index is formed on a substrate n _o and an extraordinary refractive index _{n e (n o ≠ n e} ) birefringent material layer comprising a polymer liquid crystal, are processed into a periodic uneven larger period than the wavelength of the incident light λ its cross-sectional shape, also the refractive index in the at least recess is n _o or n _e Is filled with an isotropic transparent material, and the longitudinal direction of the grating in the polarizing diffraction grating is different in each polarizing diffraction grating,
Incident light having a first polarization direction travels straight through the polarizer and is focused on the focal plane of the optical axis of the condenser lens by the condenser lens;
Incident light having a second polarization direction is diffracted by the polarizer and focused on a focal plane outside the optical axis of the condenser lens by the condenser lens. Retardation value | n of the birefringent material layer of each of the polarizing diffraction gratings, where d is the uneven step of the birefringent material layer of each of the polarizing diffraction gratings _e -N _o | × d is (m + 1/2) times the wavelength λ of incident light (m is 0 or a positive integer).
The isolator according to claim 1. The multilayer diffraction grating has a cross-sectional shape of a step d. ₁ A first polarizing diffraction grating having a birefringent material layer processed into a periodic uneven shape and a step shape d ₂ And a second polarizing diffraction grating having a birefringent material layer processed into a periodic uneven shape of
Step d ₁ And step d ₂ Is the wavelength range of the incident light. ₁ To λ ₂ For λ ₁ / (2 × | n _e -N _o |) And λ ₂ / (2 × | n _e -N _o |) And different values
The isolator according to claim 1 or 2. Acts as a diffraction grating for incident light having the first polarization direction on the light incident surface and light emitting surface of the liquid crystal cell in which the liquid crystal layer is sandwiched between the translucent substrates with electrodes. A polarizing diffraction grating comprising a birefringent material that diffracts by acting as a diffraction grating with respect to incident light that passes straight through and has a second polarization direction orthogonal to the first polarization direction, A composite liquid crystal element in which a multilayer diffraction polarizer formed of two multilayer diffraction gratings is stacked,
Each polarizing diffraction grating of the multi-layer diffractive polarizer laminated on the light incident surface and the light emitting surface includes at least one light transmissive substrate, and ordinary light refraction formed on the light transmissive substrate. A birefringent material layer made of a polymer liquid crystal having a refractive index n _o and an extraordinary light refractive index n _e (n _o ≠ n _e ) is processed into a periodic uneven shape having a period larger than the wavelength λ of incident light. are, also, are equal isotropic transparent material the refractive index is at least recess is n _o or n _e is filled,
A voltage variable attenuator, wherein the amount of linearly transmitted light in a specific polarization direction is adjusted by a voltage applied to the liquid crystal layer. A grating longitudinal direction or a grating pitch in each polarizing diffraction grating of the first multilayer diffraction grating forming the multilayer diffraction polarizer laminated on the light incident surface is different from each other,
A grating longitudinal direction or a grating pitch in each polarizing diffraction grating of the second multilayer diffraction grating forming the multilayer diffraction polarizer laminated on the light exit surface is different from each other,
The longitudinal direction of each polarizing diffraction grating of the first multilayer diffraction grating and the longitudinal direction of each polarizing diffraction grating of the second multilayer diffraction grating are different from each other.
The voltage variable attenuator according to claim 4. Retardation value | n of the birefringent material layer of each of the polarizing diffraction gratings, where d is the uneven step of the birefringent material layer of each of the polarizing diffraction gratings _e -N _o | × d is (m + 1/2) times the wavelength λ of incident light (m is 0 or a positive integer).
The voltage variable attenuator according to claim 4 or 5. Each of the multilayer diffraction gratings has a step d in cross-sectional shape. ₁ A first polarizing diffraction grating having a birefringent material layer processed into a periodic uneven shape and a step shape d ₂ And a second polarizing diffraction grating having a birefringent material layer processed into a periodic uneven shape of
Step d ₁ And step d ₂ Is the wavelength range of the incident light. ₁ To λ ₂ For λ ₁ / (2 × | n _e -N _o |) And λ ₂ / (2 × | n _e -N _o |) And different values
The voltage variable attenuator according to any one of claims 4 to 6.

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