This application is a PCT National Phase Application the claims priority from PCT/GB00/03796 having an International filing date of 4 Oct. 2000 which in turn claims priority from UK patent application 9923428.8 filed 4 Oct. 1999.
The invention relates to the general field of optical switching and more particularly to optical switching using multiphase or continuous phase hologram devices.
Optical fibre switching components are fundamental to modern global information systems. Single-stage matrix switches operating independently of the optical bit-rate and modulation formats, capable of reconfigurably interconnecting N optical inputs to M optical outputs (where N and M are generally, but not necessarily the same number), are particularly attractive. Many switches for achieving the required switching are limited in functional size to less than 64×64, and/or suffer from relatively poor noise performance. One method which provides good noise performance and is potentially more scalable than other optical switch technologies is to use reconfigurable holograms as elements for deflecting optical beams between arrays of optical inputs and optical outputs.
A known holographic optical switch, otherwise known as an optical shuffle, is shown in
In
To achieve switching, the input array 1 is arranged behind a first lens array 2. Each optical signal emitted by the input array enters free-space, where it is collimated by one of the lenses in first lens array 2. Each collimated beam then passes through a first hologram display device 3. The first hologram display device 3 displays a holographic pattern of phase and/or intensity and/or birefringence which has been designed to produce a specific deflection of the optical propagation directions of the beams incident upon the device. The hologram pattern may also be designed such that each optical beam experiences a different angle of deflection. The first hologram display device 3 may also have the effect of splitting an individual beam into several different angles or diffraction orders. One application for utilising this power splitting effect is to route an input port to more than one output port.
The deflected optical signals propagate in free-space across an interconnect region 4 until they reach a second hologram device 5. The hologram pattern at second hologram device 5 is designed in such a way to reverse the deflections introduced at the first hologram display device 3 so that the emerging signal beams are parallel with the system optic axis again.
The optical signals then pass through a second lens array 6 where each lens focuses its associated optical signal into the output ports of a receiver array 7. Thus the hologram pattern displayed on first hologram display device 3 and the associated “inverse” hologram pattern displayed on second hologram display device 5 determine which output fibre or fibres 7 receive optical data from which input fibre or fibres 1. The interconnect region 4 allows the signal beams to spatially reorder in a manner determined by the specific hologram patterns displayed on the first 3 and second 5 hologram display devices. The switch also operates reversibly such that outputs 7 may transmit optical signals back to the inputs 1.
The system shown in
It has been proposed to extend the optical shuffle of
Accordingly the present invention aims to address at least some of these issues.
According to a first aspect of the invention there is provided a switch comprising an integrated spatial light modulator for receiving light of a predetermined wavelength, the modulator comprising a liquid crystal layer spaced from a second layer by a layer having an optical retardance of an odd integer number of quarter-waves of said wavelength, wherein the second layer is reflective of said light of said wavelength.
In one embodiment said liquid crystal layer is a nematic crystal layer.
In another said liquid crystal layer is a π-cell.
Preferably the second layer is a metallic layer.
Advantageously the metallic layer is of Aluminium.
Conveniently said wavelength is 1.57 μm.
According to a second aspect of the invention there is provided a switch comprising an integrated spatial light modulator for receiving light of a predetermined wavelength, the modulator comprising a liquid crystal cell having a pair of opposed and mutually substantially parallel end plates disposed substantially parallel to an axial plane, and spaced apart by a liquid crystal layer providing a director angle tilt in a tilt plane substantially orthogonal to said axial plane, said liquid crystal being spaced from a second layer by an optical layer having a retardance of an odd integer number of quarter-waves of said wavelength, wherein the second layer is reflective of said light of said wavelength, and the optical layer being disposed with respect to said tilt plane such that light polarised in said tilt plane returns through said liquid crystal layer polarised substantially orthogonal to said tilt plane.
Preferably said liquid crystal layer is a nematic crystal layer.
Alternatively said liquid crystal layer is a π-cell.
Preferably the second layer is a metallic layer.
Conveniently the metallic layer is of Aluminium.
Advantageously the modulator has a glass cover disposed over said liquid crystal layer, and the metallic layer has a connection to driving circuitry for switching the modulator.
According to another aspect of the invention there is provided a method of switching a light beam having a first component polarised in a first direction and a second component polarised in a second direction orthogonal to the first, the method comprising providing a device having a liquid crystal layer and an optical retardance, the liquid crystal being responsive to a variable drive voltage to provide a corresponding variation in director angle tilt; and further comprising: applying a variable drive voltage to said liquid crystal device; applying said beam to said liquid crystal device to provide an intermediate beam having a variable phase delay applied to said first component and an at least substantially fixed phase delay to said second component; by said retardance, rotating the polarisation of said intermediate beam; applying the resultant light to said liquid crystal device whereby a component of said resultant light polarised in said first direction receives said variable phase delay and a component of said resultant light polarised in said second direction receives said at least substantially fixed phase delay.
Preferably the rotating step comprises rotating said polarisation through 90 degrees whereby at least substantially equal amounts of variable phase delay are applied to each of said first and second components.
Advantageously the rotating step comprises a step of reflecting said intermediate beam back along its incoming path.
According to yet another aspect of the invention there is provided an optical switch comprising a plurality of input optical fibres for providing plural input light beams, a plurality of optical receivers for receiving output light beams, a first and a second reflective spatial light modulator, and drive circuitry for forming a respective plurality of switching holograms on each spatial light modulator, said holograms being selected to couple each said input optical source to a respective desired optical receiver, wherein each spatial light modulator incorporates a liquid crystal device for modulating the phase of light travelling through said liquid crystal device, a reflector device for returning light back through said liquid crystal device and a device, disposed between said liquid crystal device and said reflector device, for rotating the polarisation of light by 90 degrees, wherein the optical switch has an axis of symmetry and the spatial light modulators are disposed on opposite sides of said axis, each said switching hologram on said first spatial light modulator being operative to deflect said input light beams to said switching holograms on said second spatial light modulator and each said switching hologram on said second spatial light modulator being operative to deflect said light beams to a respective optical receiver.
Preferably each said input optical fibre is directed towards a respective switching hologram on said first spatial light modulator, and each said optical receiver comprises an output optical fibre, wherein each output optical fibre is directed towards a respective switching hologram on said second spatial light modulator.
In one embodiment the first and second spatial light modulators are disposed such that a respective zero-order beam reflected from each switching hologram on said first spatial light modulator is incident on a respective switching hologram on said second spatial light modulator.
Preferably a half wave plate is disposed between said first and second spatial light modulators.
Alternatively the switching holograms are spaced apart on said first and second spatial light modulators and the first and second spatial light modulators are disposed such that a respective zero-order beam reflected from each switching hologram on said first spatial light modulator is incident on a spacing between two adjacent switching holograms on said second spatial light modulator.
Advantageously a half wave plate is disposed between said first and second spatial light modulators.
Conveniently the switch further comprises respective optical systems disposed between said input fibres and said first spatial light modulator and between said output fibres and said second spatial light modulator, wherein each said optical system comprises two confocal lenses, the input and output fibres being disposed in respective planes and a focal plane of a first lens of each optical system coinciding with the plane of the associated fibres.
Preferably the input and output fibres are disposed in respective planes and the optical switch further comprises respective arrays of microlenses, said microlenses being disposed in front of each fibre plane such that each microlens corresponds to a respective fibre, and respective optical systems disposed between said input fibres and said first spatial light modulator and between said output fibres and said second spatial light modulator, wherein each said optical system comprises two confocal lenses, and a focal plane of a first lens of each optical system coinciding with the output focal plane of the associated microlens array.
Advantageously said optical fibres are thermally expanded core (TEC) fibres.
In another embodiment the first and second spatial light modulators are mutually offset so that no zero order beams from the first spatial light modulator is incident on the second spatial light modulator.
Conveniently at least one optical receiving element is disposed in a region receiving said zero-order beams from said first spatial light modulator, whereby input signal may be monitored.
Advantageously, the or each element is a fibre. Alternatively other elements such as receiver diodes could be used.
Preferably each switching hologram provides a repeating pattern on its spatial light modulator, whereby the repeating patterns on the two SLMs satisfy the relation:
θ_{2}(u)=θ_{1}(−u)
where θ_{2 }(u) is the repeating pattern on the second SLM and θ_{1}(−u) is the repeating pattern on the first SLM, and the angle of incidence is such that the Poynting vector of the input light beam incident on the first SLM, and of the light beams leaving the second SLM, is in the plane of tilt of the director.
In a preferred embodiment, the output fibres are secured together in an array by a glue containing black pigment to attenuate misaligned light.
In another preferred embodiment, the output fibres are secured together to form an array and the spacing between the fibres of the array is occupied by interstitial fibres which serve to accept and guide away cross talk from the switching zone.
Non-limiting embodiments of the invention will now be described with reference to the accompanying drawings, in which:
In the various figures, like reference signs indicate like parts.
The length of the minor axis 112 is n_{0}, for all values of θ. Hence the component of the field that is parallel to the y-axis experiences refractive index n_{O }whatever the tilt angle θ is, and therefore the phase delay caused to it by the cell is independent of the voltage across it (ordinary wave). On the contrary, the length of the major axis does depend on the tilt angle θ, and so the x-component of the field (extraordinary wave) experiences different refractive index n for different values of the tilt angle.
The length of the major axis 111 is n(θ), and is given by equation (1):
It follows that n_{o}≦n(θ)≦n_{e}. The relative phase delay between the two components is then given by the equation (2):
Δφ=k_{0}dΔn (2)
In equation (2), d is the thickness of the liquid crystal cell, k_{0 }the wavenumber of the field in free space and Δn is given by Δn=n(θ)−n_{o}. Since Δn is a function of the voltage across the cell, equation (2) shows that the applied voltage can continuously control the phase difference between the two components across the cell.
It will be understood by those skilled in the art that it is desirable to provide phase modulation that is not sensitive to polarisation, and devices and methods for achieving this will now be described for the situation of normally incident light:
Expression (3) shows a mathematical representation of an arbitrary polarisation state as the superposition of two orthogonal, linearly polarised waves:
where the amplitudes, E_{0Y}(t) and E_{0X}(t), and phases ε_{X}(t) and ε_{Y}(t) vary slowly, remaining essentially constant over a large number of oscillations. For unpolarised light the relative amplitude, E_{0Y}(t)/E_{0X}(t) and relative phase, ε_{Y}(t)−ε_{X}(t), vary rapidly compared to the coherence time of each linearly polarised component, i.e. the two waves are mutually incoherent. For randomly polarised light the relative amplitude and phases vary slowly with respect to the coherence time, i.e. the two waves are mutually coherent. Hence the above representation is valid for any light wave.
Such light could be modulated by applying the same phase delay to both of these components. However, the configuration in
For the first pass, on the way towards the quarter wave plate and mirror, the polarisation component polarised in the x direction (E_{0X}(t) exp j ε_{X}(t)) experiences a refractive index n(θ), where θ depends on the applied voltage, while the component in the y direction (E_{0Y}(t) exp j ε_{Y}(t)) does not, and instead experiences a refractive index, n_{0}, that is independent of the applied voltage. The orientation of the quarter wave plate is such that these two polarisation components are exchanged. For the 2^{nd }pass, on returning back through the liquid crystal, the component E_{0X}(t) exp j ε_{X}(t)) is now polarised in the y direction, and therefore experiences a refractive index n_{0}, while the component E_{0Y}(t) exp j ε_{Y}(t) is now polarised in the x direction, and experiences a refractive index n(θ). In this way both components gain overall the same amount of phase delay through the system since they both experience one pass under a refractive index n(θ) and one pass under a refractive index n_{o}.
In particular (equations 4 and 5):
E_{0X }component: Δφ_{OX}=Δφ_{1ST-PASS}+Δφ_{2ND-PASS}=kn(θ)d+kn_{0}d (4)
E_{OY }component: Δφ_{OY}=Δφ_{1ST-PASS}+Δφ_{2ND-PASS}=kn_{0}d+kn(θ)d (5)
The system may be described mathematically (equation 6) in terms of Jones matrices, with the result that (as expected):
It should however be noted that the light exits the system in the opposite orthogonal state. This Jones matrix result uses the convention that the y-axis is inverted on reflection from the mirror. The mathematical result confirms that both components of the output light have the same phase change (in agreement with equation 4 and 5) and therefore polarisation insensitive phase modulation is feasible.
In general θ may vary with z, in which case the index n(θ) in (6) should be replaced by (expression 7):
The foregoing principle can be applied to an array of modulating elements. A plane wave front of arbitrarily polarised light, which normally impinges on to such an array of pixels, each of which is characterised by a specific value of tilt angle (by the application of different voltages across it), or a specific distribution of tilt angles, can be spatially phase modulated.
Referring now to
AS seen in
The quarter-wave plate can be deposited on the pixel array by spin-coating a proper reactive monomer, which can be polymerised by exposure to ultraviolet light. In the cell of
A second embodiment is shown in
Referring to
An embodiment of a spatial light modulator in accordance with
Referring to
Although the above discussions are in the context of an integral retarder, it is also possible to use a non-integral retarder, such as a non-integral quarter wave plate. The following description is not therefore limited to an integral quarter wave plate.
Referring now to
Routing from input fibre fC to output fibre fB is achieved by configuring input hologram hC to deflect the input beam to output hologram hB, so that the angle of reflection typically differs from the incident angle θ_{in}. Output hologram hB deflects the beam incident on it to output fibre fB. In between each hologram and its corresponding fibre there is an optical system, embodiments of which are described later herein, that has the function of presenting beams of appropriate diameter to the hologram.
In order to minimise the system losses, it is desirable to have as few lenses as possible in the optical system. A first optical system, for use with the switch of
In a co pending patent application an embodiment using reflective SLMs has the beam passing twice through a lens (off-axis) positioned immediately in front of the SLM.
The system of
An advantageous option is to use both a microlens array and larger spot-size fibres in the fibre array.
As will be clear to those skilled in the art, the required number of pixels in each row of the hologram, M, may be calculated using the beam spot size of the hologram and the maximum beam steering angle, and the cross talk requirement.
The requirements of optimum performance suggest the use of either standard fibres with a microlens array or fibres with larger than standard spot size.
As known to those skilled in the art, a quarter-wave plate will only work perfectly for one particular wavelength, giving rise to errors at other wavelengths. Deviations from the theoretical also result from fabrication tolerances in the quarter-wave plate thickness and birefringence, and from misalignments between the plate orientation and the plane of tilt of the liquid crystal.
It can be shown that these effects produce zero-order (i.e. undiffracted) polarisation-dependent crosstalk in a switch configuration due to the component of incident light in the y polarisation direction.
For incident light polarised in the x direction, it can be shown that the result of the errors is to produce a diffraction order at twice the angle of the intended main diffraction order. The amplitude of this doubled-order crosstalk varies with the polarisation state of the input light, and hence the effect is to generate polarisation-dependent crosstalk.
Reference to
As the hologram array is regular, such that the set of tilt angles is quantised into units of Mp/L, where M is the number of pixels in each row of the hologram, p is the pixel pitch, and L is the distance between the holograms, therefore to route to a fibre n_{x }long in the x direction, and n_{Y }along in the y direction, the beam deflection at the input hologram is given by equation 8:
Also the beam deflection at the output hologram is (equation 9):
Referring now to
The effect of the quarter-wave plate tolerances is to route a beam 145 of amplitude a_{YY }from hologram hA on input SLM 140 to hologram hB on output SLM 141, where a_{YY }is the fraction of incident light polarised in the y direction which remains in that state after transition through the first SLM 140. Analogous effects at the second SLM 141 cause a beam 146 of net amplitude of up to (a_{YY})^{2 }to pass into the zero-order output from hologram hB. As a result of the system geometry, the zero-order beam 146 reaches output fibre fC. Hence the effect of the y polarised light that remains in this polarisation state is to cause crosstalk in fibre fC of maximum amplitude (a_{YY})^{2 }from the signal entering the switch at fibre fA. The remainder of the light from hologram hA directed to hologram hB has amplitude a_{YY}(1−a_{YY}). This light will be subject to the intended deflection angle introduced by hologram hB, and will form a light beam 147. Let the distance in hologram units between holograms hA and hC on first, input SLM 140 be (d_{X},d_{Y}). What happens next depends on the design of the system. For the basic system (microlens-free system), the beam will enter output fibre fC at a tilt angle. The system may be designed such that this light (of maximum amplitude a_{YY}(1−a_{YY})) is partially attenuated by the limited angular acceptance of the output fibre (or offset acceptance, depending on the optical architecture). It may be shown that the attenuation, α_{TILT}, due to this tilt is given by equation 10:
α_{TILT}=(d_{X}^{2}+d_{Y}^{2})α_{T} (10)
where
where C is the clipping parameter at the hologram, such that Mp=C.ω_{HOL}, where ω_{HOL }is the beam spot-size at the hologram. With a switch configured for maximum wavelength range, the worst-case value of d_{X}^{2}+d_{Y}^{2 }is unity. To improve crosstalk suppression, α_{TILT }should be as high as possible: thus performance is improved by increasing the value of the ratio of the spot-size to the fibre separation.
Referring to
Referring to
The third embodiment is most appropriate in the presence of good surface flatness on the SLM. For the case of offset loss, it reduces as the ratio of the spot-size to the fibre separation is increased. In any final design there will be an optimum value of this ratio to obtain the overall required system performance.
Referring now to
Now consider the polarisation-dependent doubled orders, in a 2-D system. Let these be approaching the output hologram at deflection angles (equation 11):
In the zero-order aligned system (
In a preferred embodiment, the attenuation of beams arriving between the output fibres is increased by adding black paint to the glue holding the fibres together inside the fibre array. It will be understood that other absorbers could also be used. In another embodiment, the spacing between the fibres of the array is occupied by interstitial fibres which serve to accept and guide away cross talk from the switching zone.
The amplitude of the doubled-order beam is at most a_{XX}. In the absence of a central half-wave plate, there will be a beam of maximum amplitude a_{XX}^{2 }coming out at deflection angles (with reference to beams focused directly into an output fibre) given by equation 12:
The worst-case scenario is that c_{X}=2n_{X}, and c_{Y}=2n_{Y}. In this case for the zero-order aligned system (
In the presence of a central half-wave plate, a weak beam, of maximum amplitude a_{XX}a_{YY}, will be reflected as a zero-order reflection, and will therefore come out at deflection angles given by equation 13:
Firstly consider what happens in the zero-order aligned system (
Now consider what happens in the zero-order interleaved system (
Now consider the remaining light in the incident doubled order. Without a central half-wave plate, this beam will have a maximum amplitude of a_{XX}(1−a_{XX}), while in the presence of a central half-wave plate, this beam will have a maximum amplitude of a_{XX}(1−a_{YY}). With or without the central half-wave plate, this beam is deflected by the intended deflection angle, and so leaves the output hologram at a deflection angle given by equation 14:
For the zero-order aligned system, the worst-case is for either c_{X}=n_{X }or c_{Y}=n_{Y}, but not both. Assume that one of these is true. The minimum attenuation is when |c_{X}−n_{X}|=1 or when |c_{Y}−n_{Y}|=1 and so the beam will be attenuated by a tilt loss of α_{T}. For the zero-order interleaved system, the minimum attenuation is when |c_{X}−n_{X}|=½ and |c_{Y}−n_{Y}|=0. The minimum attenuation is then 0.25α_{T}, added to the offset loss. If additionally, the output SLM 141 is offset by an odd integer number of hologram heights, then the offset loss is doubled from that previously defined, and the minimum value of |c_{Y}−n_{Y}|becomes ½, so the tilt attenuation is increased to 0.5 α_{T}.
To maintain desired back reflection conditions off-normal incidence is preferable: it is likely to occur in any event due to the geometrical constraints of the system. However the closer to normal incidence, the better is the performance.
Where the beam has off-normal incidence, the phase of the reflection coefficient from the mirror of the SLM becomes polarisation-dependent, due to plasmon resonances in the metal mirror. The effect is to increase the fraction of light in each polarisation state that remains in that state after passing back through the quarter-wave plate. Another effect of off-normal incidence through the quarter-wave plate is to change, for the worse, both the effective thickness and also the birefringence. Hence a consequence of off-normal incidence is to increase the strength of the polarisation-dependent crosstalk into the zero and doubled orders.
Given off-normal incidence, it now becomes necessary to choose the plane of incidence. In this section the effects of off-normal incidence, but still in the x–z plane, are investigated.
Assume the Poynting vector of the incident light to be in the xOz plane, with a polarisation component E_{0Y}(t) exp j ε_{Y}(t) in the y direction, and E_{0XZ}(t) exp j ε_{xZ}(t) in the xOz plane (in a direction mutually orthogonal to y and the Poynting vector).
Let the light be incident at an angle θ_{INC }to the mirror, as shown in
E_{OXZ }component:
Δφ_{OXZ}=Δφ_{1ST-PASS}+Δφ_{2ND-PASS}=kn(θ_{D}−θ_{INC})d+kn_{0}d (15)
E_{OY }component:
Δφ_{OY}=Δφ_{1ST-PASS}+Δφ_{2ND-PASS}=kn_{0}d+kn(θ_{D}+θ_{INC})d (16)
Therefore the phase-modulation now has a weak polarisation dependence, which increases with the angle of incidence, and is given approximately (to second order) by equation 17:
In a cell in which the tilt angle is varying (as in 7), the polarisation dependence of the phase modulation is given by equation 18:
The rate of change of n with respect to director angle is easily shown to be (equation 19):
Note that for tilt angles in the range 0 to π/2, this derivative is always negative, while for tilt angles in the range π/2 to π, the derivative is always positive. For a pi cell, the tilt angle θ varies between 0 and π. Hence the polarisation-dependent phase modulations may partially cancel.
An important property of this plane of incidence, is that of the directions of the two polarisation modes. Bearing in mind that these are given by the directions of the minor and major axes of the ellipse formed by the intersection of the plane perpendicular to the Poynting vector, with the index ellipsoid, if the Poynting vector is in the x0z plane, then the minor axis is always in the y direction and the major axis is always in the x0z plane (and parallel of course to the x0z component of the incident light). Therefore the polarisation states of the y polarised and orthogonal components of the incident light are not changed inside the liquid crystal, and therefore proper polarisation component exchange should still take place at the quarter-wave plate and mirror.
Returning now to the polarisation-dependence, the effect on a beam-steering device, is to introduce a polarisation-dependence into the amplitude (but not the output angle) of each diffraction order, where this polarisation-dependence is a function of the angle of incidence. Now consider an N×N switch using two such devices, and let the SLM shown in
where u is the position co-ordinate of each pixel, and φ(u) is the intended phase modulation, as defined immediately before equation (13). Hence for the input SLM, the y polarised component of the incident field is diffracted into orders of amplitude b_{L1}, while the orthogonal component is diffracted into orders of amplitude a_{L1}. For a well-designed hologram, almost all of the power will go into a single diffraction order.
It is assumed that the input and output SLMs are made in the same way. Now consider pixels in the two SLMs applying the same nominal phase modulation (for a normally incident beam), and hence having the same tilt angle, θ_{D}. Due to the geometry of the arrangement of SLMs etc, the beam entering the 1st SLM is parallel to the beam leaving the second SLM, as shown in
The y polarised component of the field incident on the 1st SLM, is polarised in the x0z plane on leaving the 1st SLM, and due to the half-wave plate is again y polarised on entering the second SLM. This component perceives the ordinary index n_{0 }on propagation towards the mirror. On propagation away from the mirror, the index perceived is given by an effective tilt angle of θ=θ_{D}−θ_{INC}. Hence the total phase delay for this component is given by (equation 22):
E_{OY }component:
Δφ_{OY}=Δφ_{1ST-PASS}+Δφ_{2ND-PASS}kn(θ_{D}−θ_{INC})d+kn_{0}d (22)
Similarly, it can be shown that for the orthogonal polarised component (in the x0z) plane of the beam incident on the 1st SLM, the phase modulation at the second SLM is given by (equation 23):
E_{OXZ }component:
Δφ_{OXZ}=Δφ_{1ST-PASS}+Δφ_{2ND-PASS}=kn_{0}d+kn(θ_{D}+θ_{INC})d (23)
At the second SLM, and assuming substantially flat SLMs, the hologram is substantially complementary to that at the first SLM. Let the intended phase modulation at the second SLM be φ_{c}(u), and let the director angle be θ_{c}(u). If at the input SLM, the hologram is designed to maximise the output into the L'th diffraction order, then at the output SLM, the hologram should maximise the output into the −L'th diffraction order. For this output SLM therefore, the Fourier coefficient b_{-L2 }that defines the amplitude of the main diffraction order for the y polarised component of the field incident on the 1st SLM is given by (equation 24):
while the Fourier coefficient for the main diffraction order from the output SLM for the orthogonal component of the field incident on the 1st SLM is given by (equation 25):
The overall holographic switching efficiency for the y polarised component of the field incident on the 1st SLM is given by (equation 26):
η_{OY}=|b_{L1}|^{2}|b_{-L2}|^{2} (26)
while the overall holographic switching efficiency for the orthogonal component of the field incident on the 1st SLM is given by (equation 27):
η_{OXZ}=|a_{L1}|^{2}|a_{-L2}|^{2} (27)
Now consider the hologram patterns, and let the local director angle, θ_{D}(u) be expressed in terms of some fundamental repeating pattern, θ_{1}(u) (equation 28):
Given that the intended or mean phase modulation on the 1st SLM, φ(u), depends on the local director angle (equations 1 and 7), then it must also show periodicity with the same period Ω, as must any derivatives with respect to θ_{D}(U) (equation 19). Therefore, taking into account the effects of off-normal incidence as in equations 20, 21 etc, the net phase modulation will still be periodic with the same period. Hence we may define H^{−}(u) such that (equation 29):
where u_{0 }is some (arbitrary) origin. This origin affects the phase, but not the magnitude, of the diffraction orders. The magnitude of a_{L1 }may be obtained in terms of H(u) using Fourier series analysis (equation 30):
Similarly, let θ_{c}(u) be the director angle on the 2nd hologram, and express it in terms of another fundamental repeating pattern, θ_{2}(U) (equation 31):
Therefore, using the same arguments as above, the phase modulation on the second SLM must also be periodic with period Ω, and so we may define G^{−}(u) such that (equation 32):
where u_{1 }is another arbitrary origin. Hence we may calculate the magnitude of b_{-L2 }(equation 33):
If we let G^{−}(u)=H^{−}(−u), and make the substitution u'=−u it is clear that (equation 34):
|a_{L1}|=|b_{-L2}| (34)
Physically this may be achieved by making the repeating pattern θ_{2}(U) on the second SLM equal to θ_{1}(−u) on the first SLM. In which case (from equation(1)), φ_{c}(u)=φ(−u) as required. Now consider the other two amplitude coefficients. At the first SLM, define a periodic phase modulation H^{+}(u), and use the same origin (equation 35)
hence we obtain b_{L1 }(equation 36):
Now, at the second SLM define a periodic phase modulation G^{+}(u), to obtain a_{-L2 }(equation 37, 38):
Again, as we have already chosen that (equation 39)
θ_{2}(u)=θ_{1}(−u) (39)
then, automatically, φ_{c}(u)=φ(−u), in which case G^{+}(u)=H^{+}(−u), and therefore (equation 40)
|b_{L1}|=a_{-L2}| (40)
Combining (36) and (40) we may obtain (equation 41):
|a_{L1}∥a_{-L2}|=b_{L1}∥b_{-L2}|(41)
Hence, if the basic periodic patterns on the two SLMs are chosen to satisfy (39), and the angle of incidence is such that the Poynting vector of the light incident on the first SLM, and leaving the second SLM, is in the plane of tilt of the director (in this case the x0z plane), the overall switch efficiencies can become polarisation-independent (equation 42):
η_{0Y}=η_{0XZ} (42)
Note that this analysis neglects the change in beam direction between holograms due to diffraction-induced beam-steering. This may create some polarisation-dependent loss, but it is expected that the configuration described is still the optimum, as it cancels the polarisation-dependence of the system as a whole due to the angle of incidence.
Given that the two orthogonal components perceive different phase modulation at each plane, the holograms must be designed that the worst-case unwanted diffraction orders do not cause unacceptable crosstalk.
There have thus been described devices and systems for optical switching which are polarisation insensitive. Embodiments of the invention as described are capable of high performance in respect of cross talk.
Number | Date | Country | Kind |
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9923428 | Oct 1999 | GB | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/GB00/03796 | 10/4/2000 | WO | 00 | 4/3/2002 |
Publishing Document | Publishing Date | Country | Kind |
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WO01/25840 | 4/12/2001 | WO | A |
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0878729 | Nov 1998 | EP |