WO2016001173A1

WO2016001173A1 - A broadband linear polarization scrambler

Info

Publication number: WO2016001173A1
Application number: PCT/EP2015/064756
Authority: WO
Inventors: Frans SNIK; Gerard van HARTEN; Christoph Keller
Original assignee: Universiteit Leiden
Priority date: 2014-07-01
Filing date: 2015-06-29
Publication date: 2016-01-07
Also published as: GB2527783A; GB201411719D0

Abstract

A broadband polarization scrambler comprising a retarder material layer having a plurality of through-thickness zones extending through the thickness of the layer, each zone configured to alter the polarization state of light passing through the layer differently than each adjacent zone whilst retaining the circular polarization of at least a portion of incoming circularly polarized light.

Description

A BROADBAND LINEAR POLARIZATION SCRAMBLER

Technical Field The present disclosure relates to the field of optical components and, in particular, to optical components which affect the polarization of light. Certain disclosed aspects/embodiments relate to spectrally resolved measurements (and associated apparatus such as spectrometers and circular dichroism instrumentation). Background

Many optical instruments require that the linear polarization of a beam of light is reduced to some tolerable value. For example, polarimetric and polarization-sensitive instruments may need to be calibrated with "unpolarized" light. Therefore, for light sources which are polarized to some degree, the light source may need to be depolarized before being used. Other instruments that measure circularly polarized light may need to have linear polarization strongly reduced to achieve the required sensitivity in circular polarization. Depolarization can be achieved by scrambling the (linear) polarization in several ways. For example, depolarization may be performed in the temporal, spatial or spectral domains.

In the case of temporal scrambling, an optical element that changes in time may be used, such as a rotating half-wave retarder. The measurement is then performed such that it integrates over a period in time that is long compared to the temporal changes induced by the changing optical element. It will be appreciated that a rotating half-wave plate only affects the linear polarization and preserves the circular polarization.

Spectral scrambling, for example using a combination of thick birefringent elements, converts the incoming polarization to different polarization states that rapidly vary with wavelength, such that a broadband measurement, which integrates over a wavelength range that is large compared to the spectral changes induced by the birefringent elements, can be considered as unpolarized. This spectral scrambling mechanism also strongly reduced the degree of circular polarization. The length-scale of the zones may be greater than two times the shortest wavelength of the light for which the circular polarization of at least a portion of incoming circularly polarized light is retained. The length-scale of the zones may be greater than 1 μιη. The 6 length-scale of the zones may be less than several millimetres (e.g. less than 10mm, less than 8mm, less than 5mm, less than 3mm, less than 2mm, or less than 1mm).

The listing or discussion of any background in this specification should not necessarily be taken as an acknowledgement that the background is part of the state of the art or is common general knowledge.

Summary In a first aspect, there is provided a broadband polarization scrambler comprising a retarder material layer having a plurality of through-thickness zones extending through the thickness of the layer, each zone configured to alter the polarization state of light passing through the layer differently than each adjacent zone whilst retaining the circular polarization of at least a portion of incoming circularly polarized light.

Each zone may be considered to be a pixel. As discussed below the zones may or may not be regularly shaped; and may or may not be regularly sized. Each zone may have substantially parallel sides extending through the thickness of the layer perpendicular to the surface of the layer. The substantially parallel sides may comprise smooth transitions between adjacent zones (i.e. the zones may not be separated by distinct boundaries).

Each zone may be considered to be a through-thickness portion of retarder material having a contiguous front surface area which is configured to affect the polarization of light in substantially the same way regardless of where on the contiguous front surface area the light is incident.

Each zone may be configured to retain the circular polarization of most or substantially all of the incoming circularly polarized light. The light may be configured to pass through the retarder material layer as a beam of predefined diameter, and the retarder material layer may have at least 100 through- thickness zones across the diameter of the beam.

The retarder material may be a birefringent material. Each zone may be configured to alter the linear polarization state of light passing through the layer differently than each adjacent zone whilst retaining the circular polarization of at least a portion of incoming circularly polarized light. The broadband polarization scrambler may be a transmissive optical component configured to transmit light. The broadband polarization scrambler may be configured to reside in a pupil-plane location of an optical system, such that it ensures a scrambled Point Spread Function (PSF) in the consecutive image plane. That is, each point in the PSF will still be linearly polarized to a degree that is substantially the same as the degree of linear polarization of the incoming light, but the orientation of the linear polarization will rapidly change as a function of position in the PSF. When incoherently averaging over the PSF, the linear polarization of the incoming beam will be strongly attenuated.

In a non-imaging system, the scrambling takes place over the beam area (or footprint) so as to have effect in the far field.

The broadband polarization scrambler may be configured to depolarize the incoming light by more than a factor of 10 by reducing the degree of linear polarization of the incoming light, regardless of the angle of linear polarization.

The polarization scrambler may be considered to be a broadband scrambler if the scrambler is configured to depolarize linearly polarized light whilst retaining the circular polarization of at least a portion of incoming circularly polarized light, the linearly polarized light having a wavelength within a wavelength range ΔΑ of a central wavelength ^-central given by ΔΛ > 0.25A_central . That is, a polarization scrambler may be considered to be a broadband scrambler if it is configured to depolarize linearly polarized light whilst retaining the circular polarization of at least a portion of incoming circularly polarized light between the wavelengths _central + 0.2SA_central and _central - Q.2S _central, wherein ^central '^{s a} central wavelength.

The polarization scrambler may be configured to depolarize linearly polarized light by more than a factor of 10 whilst retaining the circular polarization of at least 90% of incoming circularly polarized light within a wavelength range of Αλ > 0.251_centrai. The polarization scrambler may be configured to depolarize linearly polarized light by more than a factor of 100 whilst retaining the circular polarization of at least 99% of incoming circularly polarized light within a wavelength range of ΔΛ > 0.15A_centrai. The polarization scrambler may be configured to depolarize linearly polarized light by more than a factor of 1000 whilst retaining the circular polarization of at least 99.9% of incoming circularly polarized light within a wavelength range of ΔΛ > 0.05A_centrai.

It will be appreciated that the light may be visible light (e.g. with wavelengths between 400 nm and 700 nm) and/or between 150 nm and 1 mm. In certain embodiments, the light may comprise electromagnetic radiation outside the visible spectrum, for example, ultraviolet radiation (e.g. with wavelengths between 400 nm and 10 nm) and/or infrared radiation (e.g. with wavelengths between 700 nm and 1 mm). The plurality of through-thickness zones may have different fast axis or extraordinary axis orientations. The extraordinary axis may be a fast axis or a slow axis. For a given, fixed orientation of linear polarization of the incoming light, each zone will rotate the orientation of the linearly polarized light by a different angle. For a birefringent material with a fast axis and a linear retardation of half-a-wave, the orientation will be changed by twice the angle between the orientation of the incoming light and the fast axis orientation of that particular zone. It is appreciated that, in general, the polarization eigenvectors of the through- thickness zones may correspond to elliptical polarization states and that the fast and slow axes may be defined with respect to those eigenvectors. Each through-thickness zone may be configured to act as a half-wave plate. A wave plate is configured to shift the phase between two perpendicular polarization components of the light wave. A typical wave plate has two axes in the plane of the plate: the ordinary axis, with index of refraction, n₀, and the extraordinary axis, with index of refraction n_e. For a light wave normally incident upon the plate, polarization component along the ordinary axis travels through the crystal with a speed, v₀ - c/n₀, while the polarization component along the extraordinary axis travels with a speed v_e = c/n_e. This leads to a phase difference between the two components as they exit the crystal. When n_e < n₀, the extraordinary axis may be called the fast axis and the ordinary axis may be called the slow axis. For n_e > n₀ the situation extraordinary axis may be called the slow axis and the ordinary axis may be called the fast axis. It will be appreciated that non-crystalline embodiments may be used to introduce a phase difference between different polarisation components of a light wave. It will be appreciated that not only uniaxial birefringent materials can be used, but also biaxial birefringent materials. A half-wave plate may be considered to be a layer which is configured to introduce a relative phase shift of π radians or 180° between the linear polarization components of the incoming light that are aligned with the extraordinary axis and the linear polarization components of the incoming light that are aligned with the ordinary axis as the light traverses the plate. The effect of this is that when the waves emerge from the plate there will be a relative phase shift of λ₀/2 between orthogonal linear polarization components where λ₀ is the wavelength of the light (or, more generally, a relative phase shift of (n + 1/2)A₀, where n is an integer). It will be appreciated that a perfect half-wave plate merely flips the handedness of circular polarization, regardless of its axis orientation. For broadband liquid crystal devices, the fast/slow axes do not necessarily correspond to linear polarization eigenvectors. They are slightly elliptical because of the multilayer achromatization approach. Therefore, the corresponding linear axes and retardance as a function of wavelength are usually approximated.

The plurality of through-thickness zones may have a pseudo-random distribution of extraordinary axis orientations. A pseudo-random distribution may be considered to be a distribution wherein there is a range of extraordinary axis orientations. For example, embodiments may have 5 or more extraordinary axis orientations. In addition, a pseudorandom distribution may be such that there is a substantially uniform distribution of the orientation differences between the extraordinary axes of each zone and its adjacent zones. The spatial orientation of the fast axis in two dimensions A(x,y) may be considered pseudorandom if measurements of the extraordinary axis orientation gives a spatial auto- covariance, c{Ax, Ay), given by:

c(Ax, Ay) = mean((A(x, ) - 90) * (A(x + Δχ, y + Ay) - 90)), is substantially zero except for when |Δχ| and |Ay| are comparable to or smaller than the typical size of a zone. This equation includes the subtraction of the average orientation angle, which, for a uniform distribution of orientations between 0 and 180 degrees, is 90 degrees (hence the subtraction of 90). For such a distribution, the spatial auto-covariance c(0,0) is expected to be 2700. When Δχ and Ay are not equal to zero, however, the spatial auto-covariance is expected to have a mean of 0 with a standard deviation of 2700/Vn, where n is the number of zones.

In order to generate a substantially uniform, spatially random axis orientation pattern, a suitable, two-dimensional distribution of fast axis orientation angles may be generated with a software-based pseudo-random number generator. For example, the software-based pseudo-random number generator published in Algorithm 712, Collected Algorithms from ACM, Vol. 18, No. 4, December, 1992, pp. 434-435 may be used. For n contiguous zones, the number of zones in the 18 bins of 10 degrees width can be determined (e.g. the number of zones with extraordinary angles between 0 and 10, 10 and 20, 20 and 30 degrees ... 170 to 180 degrees). If the number of zones in each bin is substantially n/18, the distribution of extraordinary axis orientations may be considered uniform (i.e. pseudo-random). For example, if the standard deviation of the number of zones in each bin is less than 0.24-Vn the distribution of extraordinary axis orientations may be considered uniform. The scrambler may be located in an optical beam such that the linear polarization direction of different sections of the beam footprint are rotated in different directions. The pattern may be such that if the device is located in a pupil-plane, the average degree of linear polarization of the ensuing Point Spread Function (PSF) is decreased by more than a factor of 5 (or more than a factor of 10). More generally, the degree of linear polarization of the average far-field intensity pattern may be decreased by more than a factor of 5 (or more than a factor of 10). The average position of the brightness structure of the far-field pattern/PSF shall not vary with input/analysed polarization by more than 2A/D where D is the beam diameter. The scrambler may be configured such that the polarization scrambling performance does not vary significantly with a shift of the footprint (e.g. the area over which the beam impinges on the polarisation scrambler) over the pattern in either direction, nor does it depend on the precise size or shape of the beam. For example, The reduction of the degree of linear polarization by the scrambling is roughly proportional to 1/sqrt(N) where N is the number of zones covered by the beam assuming a random pattern of zone orientation and an average over the complete far-field pattern/PSF.

One implementation of such a pattern is a 2D pixelated pattern for which the fast axis orientation of each pixel is drawn from a uniform distribution between 0 and 180 degrees. Such a pattern is easily generated by using a random (or pseudo-random) number generator of a computer to provide uniformly distributed, random numbers between 0 (inclusive) and 180 (exclusive) degrees. The difference in angle of the fast axis between adjacent zones may have no preferred value. More generally, the difference between angles of the fast axis of zones with a distance that is equal to or larger than the size of the zones may have no preferred value. The zones may be irregularly shaped. The zones may have a range of different sizes (e.g. area or diameter). The retarder material layer may comprise one or more liquid crystal layers (e.g. twisted liquid crystal layers). Each twisted liquid crystal layer may have a different twist (e.g. by having a different helix angle). Each layer may have different thicknesses, and/or different refractive indices (n_e and n₀). The liquid crystal chemical component may be different for each layer.

The retarder material may comprise a plurality of liquid crystal layers configured such that the birefringence dispersion of a first layer is at least partially cancelled out by one or more second layers. This may mean that the overall effect of the retarder material layer is less dependent on wavelength than the individual liquid crystal layers, resulting in a more achromatic (or broadband) polarization scrambler. Another way of achromatizing is through stacking an odd number of identical layers at varying orientations (Pancharatnam approach). In this scenario, the plurality of liquid crystal layers may be configured such that each liquid crystal layer has one or more of different extraordinary axis orientations, twists and birefringence than the other liquid crystal layers to provide the greater bandwidth.

The retarder material may comprise nanostructured transparent materials (e.g. glass or diamond). The nanostructured transparent materials may comprise surface structures with length-scales smaller than the wavelength of the light. The nanostructured transparent materials may be considered to act as sub-wavelength gratings. The retarder material may comprise birefringent polymer.

The apparatus may comprise a substrate layer; and an alignment layer between the substrate and the retarder layer, the alignments layer comprising a number of alignment zones corresponding to the zones of the retarder layer.

According to a further aspect, there is provided a spectrometer configured to measure circularly polarized light, the spectrometer comprising the polarization scrambler of any preceding claim.

The spectrometer may be a spectrally multiplexed instrument (e.g. configured such that a single measurement contains information on a wide range of wavelengths). The spectrometer may be configured to perform circular dichroism measurements.

According to a further aspect, there is provided a method of manufacturing a broadband polarization scrambler, the method comprising: EP2015/064756 providing a retarder material layer having a plurality of through-thickness zones extending through the thickness of the layer, each zone configured to alter the polarization state of light passing through the layer differently than each adjacent zone whilst retaining the circular polarization of at least a portion (e.g. greater than 50% or greater than 75%) of incoming circularly polarized light.

The method of manufacture may comprise:

providing a substrate layer;

providing an alignment layer on the substrate layer, the alignments layer comprising a plurality of alignment zones;

providing one or more layers of retarder material on the alignment layer, the retarder material being configured to align with the alignment zones to form the through- thickness zones. The alignment zones of the alignment layer may be provided by illuminating zones of the alignment layer material with a polarized laser, the polarization of the laser having a particular orientation for each zone.

The present disclosure includes one or more corresponding aspects, embodiments or features in isolation or in various combinations whether or not specifically stated (including claimed) in that combination or in isolation. Corresponding means for performing one or more of the discussed functions are also within the present disclosure. Corresponding computer programs for implementing one or more of the methods disclosed are also within the present disclosure.

The above summary is intended to be merely exemplary and non-limiting.

Brief Description of the Figures A description is now given, by way of example only, with reference to the accompanying drawings, in which:-

Figures 1a-1d are a series of perspective views showing successive stages of a method of manufacture of an embodiment of a broadband polarization scrambler;

Figure 1e is a perspective view showing the effect of a portion of the broadband polarization scrambler on linearly polarized light; Figure 1f is a perspective view showing the effect of a portion of the broadband polarization scrambler on circularly polarized light;

Figure 2 is a photograph of a portion of an embodiment between crossed polarizers taken in true-colour using a microscope;

Figure 3 is a graph showing the suppression of the degree of linear polarization for a number of embodiments with different design parameters; and

Figure 4 is a graph showing depolarization results for two embodiments with three twist- retarder liquid crystal layers; and two embodiments with a single twist-retarder liquid crystal layer.

Description of Example Aspects/Embodiments

Other embodiments depicted in the figures have been provided with reference numerals that correspond to similar features of earlier described embodiments. For example, feature number 1 can also correspond to numbers 101 , 201 , 301 etc. These numbered features may appear in the figures but may not have been directly referred to within the description of these particular embodiments. These have still been provided in the figures to aid understanding of the further embodiments, particularly in relation to the features of similar earlier described embodiments.

The present invention relates to the depolarization of linearly polarized light in the spatial domain. In particular, the present invention relates to a polarization scrambler comprising a layer of retarder material having a plurality of through-thickness zones extending through the thickness of the layer, each zone configured to alter the polarization state of light passing through the layer differently than each adjacent zone. Each zone may be configured to alter the polarization state of light passing through the layer differently than each adjacent zone by having a different extraordinary axis orientation.

To scramble linear polarized light in the spatial domain, a spatially varying retarder (e.g. half-wave retarder) is introduced in the light beam, preferentially in a pupil plane, such that the measurement at a consecutive focal plane (or in the far field, or averaged over a beam) is averaged over the polarization pattern in the pupil. The spatial variation may comprise a pattern of different extraordinary axis directions with a spatial resolution scale much smaller than the beam footprint (e.g. at least 5 or at least 10 times smaller than the beam footprint diameter), and, in some embodiments, with a random sequence. To reduce the amount of linear polarization for a broadband beam, the retardance of the device should be achromatic (or broadband). Such a device may have significant advantages over temporal and spectral scramblers, as it may not impose any restrictions on the measurement time and/or the spectral range and resolution.

Figure 1 a-1 d are a series of perspective views showing successive stages of the method of manufacture of an embodiment of a broadband polarization scrambler.

Figure 1 a is a perspective view of a transparent substrate 101 , which in this case is glass. Unto this substrate 101 is provided a layer 102 of alignment layer material (e.g. a polymer layer) The alignment layer may be a thin, photo-sensitive polymer film that has a strong orientational photo-chemical reaction in response to the local direction of linearly polarized UV light such as ROP-103/2CP (Rolic Technologies Ltd.).

The layer 102 of alignment layer material is then irradiated with a linearly-polarized laser (e.g. a UV laser) to produce an alignment layer 103 comprising a number of zones 103a, 103b, 103c as shown in figure 1 c. In this case, to generate the different zones 103a, 103b, 103c, the orientation of the linearly polarized laser light is kept constant with respect to the orientation of the substrate covered with the alignment layer material when irradiating within a particular zone and changed when irradiating within a neighbouring zone. That is, the alignment zones 103a, 103b, 103c of the alignment layer are provided by illuminating zones of the alignment layer material with a polarized laser, the polarization of the laser having a different orientation for each zone. As can be seen in figure 1 c, the alignment layer zones 103a, 103b, 103c in this case are irregularly shaped and have different sizes. One or more strata of retarder (e.g. birefringent) material are then successively deposited on the alignment layer to form a retarder material layer 104 having a plurality of through- thickness zones 104a, 104b, 104c extending through the thickness of the layer, each zone configured to alter the polarization state of light passing through the layer differently than each adjacent zone whilst retaining the circular polarization of at least a portion of incoming circularly polarized light.

In this case, the retarder material comprises liquid crystal. Such liquid crystals are described in R.K. Komanduri er a/. (Komanduri, Ravi K., Kristopher F. Lawler, and Michael J. Escuti, "Multi-twist retarders: broadband retardation control using self-aligning reactive liquid crystal layers", Optics express, 21 , 1 , (2013), 404). As the first stratum of retarder material is added the liquid crystal is configured to align with respect to the particular zone of the underlying alignment layer. As each subsequent stratum is added, the liquid crystal of that stratum is configured to align with respect to the underlying liquid crystal stratum. In this way, zones 104a, 104b, 104c are formed in the retarder material layer which correspond to the zones 103a, 103b, 103c of the alignment layer. Using such retarder materials may provide the desired zonal (or pixelated) pattern, whilst achieving broadband performance over fairly large angles of incidence (within 5 degrees). It will be appreciated that the pattern may have the same or different pixel dimensions along different axes.

Each of the through-thickness retarder zones 104a, 104b, 104c, in this case, have different extraordinary axis orientations. Each of the through-thickness retarder zones 104a, 104b, 104c, in this case, is a broadband half-wave plate. That is, each of the through-thickness zones is configured to introduce a relative phase of π radians or 180° between linearly polarized light components aligned with the extraordinary axis and linearly polarized light components aligned with the ordinary axis (the ordinary axis being the axis perpendicular to the extraordinary axis and to the normal of the retarder surface'). Using a half-wave plate means that when the light waves emerge from the plate there will be a relative phase shift of A/2 between the component of the wave parallel to the extraordinary axis and the component of the wave perpendicular to the extraordinary axis, where λ is the wavelength of the light (or, more generally, a relative phase shift of (n + 1/2)A, where n is an integer). The scrambler is configured such that this equation holds approximately for any wavelength λ within the broadband range.

For linearly polarized light, this phase shift has the effect of rotating the direction of the linear polarization of the light through 2Θ where Θ is the angle between the polarization of the light and the extraordinary axis of the retarder material. In this case, different zones have different extraordinary axis orientations, so the angle Θ is different for the different zones.

Figure 1e shows linearly polarized light passing through a portion of the retarder material layer of figure 1 d. In figure 1 e, the polarization of the light is shown using arrows with solidly coloured heads. The portion of the retarder material layer comprises at least part of three retarder zones 104x, 104y, 104z each with different extraordinary axis orientations. In figure 1e, the extraordinary axis orientation is shown using arrows with lined heads. The polarization of the incoming beam 111 a is aligned with the extraordinary axis of the top left zone 104z so the polarization of the portion of beam passing through the top left zone is unaffected by the retarder material. That is the polarization of the outgoing beam 1 11 b that has passed through the top left zone 104z remains unchanged. In contrast, the polarization of the incoming beam 111a makes an angle Θ of approximately 45° with the extraordinary axis of the bottom zone 104z so the polarization of the portion of outgoing beam 11 1 b passing through the bottom zone 104z has been rotated by approximately 90°. Likewise, the polarization of the incoming beam 111a makes an angle Θ of approximately -10° with the extraordinary axis of the top right zone 104y so the polarization of the portion of outgoing beam 111 b passing through the top right zone 104y has been rotated by approximately -20°. In this way the linear polarization of different spatial zones of the beam are changed.

In contrast, for circularly polarized light, the phase shifts of the half-wave plate zones have the effect of inverting the handedness of the light independent of the orientation of the fast/slow axis. That is, circularly polarized light will remain circularly polarized: left-handed circularly polarized light will emerge as right-handed circularly polarized light; and right- handed circularly polarized light will emerge as left-handed circularly polarized light. Because circularly polarized light will remain circularly polarized regardless of the orientation of the extraordinary axis, the magnitude of the circularly polarized light will not be diminished in the same way as the magnitude of the linearly polarized light.

Figure 1f shows an incoming beam 112a of left-handed circularly polarized light passing through the same portion of the retarder material layer comprising at least part of three retarder zones 104x, 104y, 104z each with different extraordinary axis orientations, as is shown in figure 1e. In figure 1f, the extraordinary axis orientation is shown using arrows with lined heads and the polarization of the light is shown using arrows with solidly coloured heads. Because the effect of the half-wave plate zones on circularly polarized light is independent of the extraordinary axis orientation, the beam emerging from the retarder layer remains circularly polarized, albeit with the handedness reversed. That is, the outgoing beam 112b comprises left-handed circularly polarized light.

In this case, the plurality of through-thickness zones have a random distribution of extraordinary axis orientations. This may help reduce the effect of interference effects from neighbouring zones.

Figure 2 is a photograph of a front surface of a portion of a spatially variant half-wave broadband polarization scrambler comprising a retarder material layer having a plurality of through-thickness zones extending through the thickness of the layer, each zone configured to alter the polarization state of light passing through the layer differently than each adjacent zone whilst retaining the circular polarization of at least a portion of incoming circularly polarized light. The photograph was taken of the broadband polarization scrambler in between crossed polarizers in true-colour using a microscope. The length scale of the zones, some of whose boundaries in this case result in distinct dark curved lines, is approximately 10 μιη. In this case, not all boundaries show up as dark lines because the boundaries are visible only when the difference in orientation between adjacent pixels is large. As shown in figure 2, the scale bar corresponds to 100 μηι. The length-scale of the zones may be determined using a line segment passing through a number of zones by dividing the length of the line segment by the number of zones intersected by the line segment. It will be appreciated that the length-scale may be determined by averaging over several such line segments. It will be appreciated that there may be a range of zone sizes within certain embodiments of a broadband polarization scrambler.

It will be appreciated that the boundary between contiguous zones may be transition regions which have a finite width (for example, a transition region may be a region in which the extraordinary axis orientation smoothly varies from one zone to an adjacent zone). That is, there may not be clear or sharp boundaries between the zones in certain embodiments. It will be appreciated that the scrambler may be configured such that the boundary width is smaller than the length scale of the zones. In other embodiments the boundary width may be comparable to the length scale of the zones.

Figure 3 is a graph showing the suppression of the degree of linear polarization as a function of relative retardance between the ordinary and extraordinary axis. Relative retardance is the number of wavelengths by which the ordinary axis is retarded with respect to the extraordinary axis. Therefore, a retardance of λ₀/2 corresponds to a half- wave plate. The results 331 -335 for a number of embodiments are shown. In particular, embodiments having: 100 zones (331 ); 316 zones (332); 1000 zones (333); 3162 zones (334); and 10000 zones (335) across the diameter of the beam are shown. That is, figure 3 shows the depolarization effects achieved by a circular array of zones that each have a random orientation of the retarder material's fast (or extraordinary) axis. As figure 3 shows, depolarization for linear polarization by more than three orders of magnitude can be achieved by passing a circular beam of light through a circular broadband polarization scrambler embodiment which has approximately 1000 zones across its diameter (each zone being a few microns in size). The length scale of the zones may be configured to be large with respect to the wavelength of the light. For example, the length scale of the zones may be configured to be at least 10 times the wavelength of the longest wavelength in the broadband range. This may reduce the effect of scattering, diffraction and interference effects.

As noted above, the retarder material layer may comprise a plurality of liquid crystal layers (e.g. twisted liquid crystal layers) configured to increase the bandwidth of the half-wave retardation. Figure 4 are depolarization results for two embodiments with three twist- retarder liquid crystal layers 441 , 442 and two embodiments with a single twist-retarder liquid crystal layer 443, 444. It will be appreciated that the layers cooperate to increase the bandwidth as is described by the Pancharatnam principle (a stack of retarders with different orientations of their fast axes minimize the variation in retardance and fast axis orientation as a function of wavelength of the stack. For liquid crystal retarders, this principle is generalized and achromatic solutions can be obtained by stacking layers of liquid crystals with different orientations, twists and birefringence, the "multi-twist retarder" principle described by Komanduri et al. 2013), together with dispersion compensation.

As the results indicate, the single twist-retarder embodiments exhibit larger depolarization for two wavelengths and significantly less depolarization for other wavelengths. In contrast, the multiple twist-retarder embodiments show more even depolarization performance over the range of wavelengths measured (between 400 and 800 nm).

Although the above described embodiments have used liquid crystals as the retarder materials, other materials are available. For example, the retarder structures may comprise nanostructured transparent materials (e.g. diamond or glass), or birefringent polymer. See, for example:

Hsu et al. ("Full-Stokes imaging polarimeter using an array of elliptical polarizer"

Optics Express, Vol. 22, Issue 3, pp. 3063-3074 (2014));

Samoylov et al. ("Achromatic and super-achromatic zero-order waveplates" Journal of Quantitative Spectroscopy and Radiative Transfer, Vol. 88, Issues 1-3, pp. 319-325 (2004))

Kikuta ef al. ("Achromatic quarter-wave plates using the dispersion of form birefringence" Applied Optics, Vol. 36, Issue 7, pp. 1566-1572 (1997)); and

Zhao ef al. ("Liquid-crystal micropolarimeter array for full Stokes polarization imaging in visible spectrum" Optics Express, Vol. 18, Issue 17, pp. 17776-17787 (2010)) for more details on alterative retarder structures.

As described above, embodiments may retain at least a portion of the circularly polarized input light, regardless of the orientation or patterning of the retarder's extraordinary axis. This means that circular polarization is not depolarized, while linear polarization is. This property can be used in instruments that measure small circular polarization effects. An example of this is circular dichroism (CD) spectroscopy, which is widely used to characterize chiral molecules.

An issue in such measurements is that the probe beam may be linearly polarized to a degree that can be significantly larger than the degree of circular polarization to be measured. This unwanted linear polarization can be due to the light source itself, or created by optical elements within the instrument.

Furthermore, linear polarization can be confused with circular polarization if the polarization detection scheme is not 100% effective in measuring circular polarization. To deal with the latter issue, many CD instruments employ a piezo-elastic modulator (PEM) for polarization modulation, which allows for a frequency-selective detection of circular polarization only. In addition, optical elements may convert linear polarization into circular ("cross-talk").

A broadband polarization scrambler for linear polarization in CD instruments (and indeed any instrument that needs to measure circular polarization effects that are much smaller than the linear polarization present in the instrument) may offer considerable improvement in the measurement efficiency. First, it may mitigate the need for an active component for polarization scrambling, which reduces the instrument complexity and cost and enables much higher measurement speeds. Second, it may enable the use of imperfect polarization modulators that are partially sensitive to linear polarization but simultaneously operate over a wide wavelength range. Therefore, an optimized instrument can be designed that yields an instantaneous spectral measurement with a multi-pixel detector, in contrast to the scanning single-pixel monochromator or Fourier-transform spectrometers that are best suited for PEM modulation, which may be inherently relatively narrowband. It will be appreciated that such a spectrometer may be useful for pharmaceutical/chemical companies that produce chiral molecules (e.g. amino acids, sugars) and that need to verify that their chemical process produces substances with the correct handedness. For example, using the incorrect enantiomer in a drug formulation could have serious adverse health effects. Broadband polarization scrambler embodiments may offer a cheaper solution that requires fewer or no moving parts. By making CD instrumentation more efficient by allowing simultaneous measurements over a large wavelength range, such measurements may be sped up by several orders of magnitude and may be implemented in production lines.

The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole, in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein.

While there have been shown and described and pointed out fundamental novel features of the invention as applied to preferred embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices and methods described may be made by those skilled in the art without departing from the scope of the invention as defined by the claims.

Claims

1. A broadband polarization scrambler comprising a retarder material layer having a plurality of through-thickness zones extending through the thickness of the layer, each zone configured to alter the polarization state of light passing through the layer differently than each adjacent zone whilst retaining the circular polarization of at least a portion of incoming circularly polarized light.

2. The polarization scrambler of claim 1 , wherein the plurality of through-thickness zones have different extraordinary axis orientations.

3. The polarization scrambler of any preceding claim, wherein each through-thickness zone is a half-wave plate.

4. The polarization scrambler of any preceding claim, wherein the plurality of through- thickness zones have a pseudo-random distribution of extraordinary axis orientations.

5. The polarization scrambler of claim 4, wherein the pseudo-random distribution comprises a substantially uniform distribution of the orientation differences between the extraordinary axes of each zone and its adjacent zones.

6. The polarization scrambler of claim 4, wherein the pseudo-random distribution has a spatial auto-covariance c(Ax, Ay), given by:

c(Ax, Ay) = mean {A{x, y) - 90) * (A(x + Ax, y + Ay) - 90)), of substantially zero except for when |Δχ| and |Ay| are comparable to or smaller than the typical size of a zone, where A(x,y) is the spatial orientation of the extraordinary axis in two dimensions x, y.

7. The polarization scrambler of claim 4, wherein the pseudo-random distribution comprises substantially n/18 zones within each of 18 bins of 10 degrees width and/or the standard deviation of the number of zones within each bin is less than 0.24Vn, where n is the number of contiguous zones.

8. The polarization scrambler of any preceding claim, wherein the zones are irregularly shaped.

9. The polarization scrambler of any preceding claim, wherein the zones have a range of different sizes.

10. The polarization scrambler of any preceding claim, wherein the retarder material comprises one or more liquid crystal layers.

11. The polarization scrambler of any preceding claim, wherein the retarder material comprises a plurality of liquid crystal layers configured such that the bandwidth of the plurality of liquid crystal layers is greater than any of the individual layers.

12. The polarization scrambler of claim 11 , wherein the plurality of liquid crystal layers are configured such that the birefringence dispersion of a first layer is at least partially cancelled out by one or more second layers to provide the greater bandwidth.

13. The polarization scrambler of claim 11 , wherein the plurality of liquid crystal layers are configured such that each liquid crystal layer has one or more of different extraordinary axis orientations, twists and birefringence than the other liquid crystal layers to provide the greater bandwidth.

14. The polarization scrambler of any preceding claim, wherein the retarder material comprises nanostructured transparent materials.

15. The polarization scrambler of any preceding claim, wherein the retarder material comprises birefringent polymer.

16. A spectrometer configured to measure circularly polarized light, the spectrometer comprising the polarization scrambler of any preceding claim.

17. The spectrometer of claim 16, wherein the spectrometer is configured to measure circular dichroism.

18. The polarization scrambler of any preceding claim, wherein the zones are separated by zone boundaries.

19. The polarization scrambler of any preceding claim, wherein the width of the zone boundaries are commensurate with the length-scale of the zones.

20. The polarization scrambler of any preceding claim, wherein the width of the zone boundaries are smaller than the length-scale of the zones.

21. The polarization scrambler of any preceding claim, wherein the light is configured to pass through the retarder material layer as a beam of predefined diameter, and wherein the retarder material layer has at least 100 through-thickness zones across the diameter of the beam.

22. A method of manufacturing a broadband polarization scrambler, the method comprising:

providing a retarder material layer having a plurality of through-thickness zones extending through the thickness of the layer, each zone configured to alter the polarization state of light passing through the layer differently than each adjacent zone whilst retaining the circular polarization of at least a portion of incoming circularly polarized light.

23. The method of claim 22, wherein the method comprises:

providing a substrate layer;

providing one or more layers of retarder material on the alignment layer, the retarder material being configured to align with the alignment zones to form the through- thickness zones.

24 The method of claim 23, wherein the alignment zones of the alignment layer are provided by illuminating zones of the alignment layer material with a polarized laser, the polarization of the laser having a particular orientation for each zone.