WO2006035393A2

WO2006035393A2 - Binary phase structure for the generation of a periodical light signal

Info

Publication number: WO2006035393A2
Application number: PCT/IB2005/053155
Authority: WO
Inventors: Levinus Bakker; Sjoerd Stallinga; Christopher Busch
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-09-28
Filing date: 2005-09-23
Publication date: 2006-04-06
Also published as: WO2006035393A3; TW200628844A; EP1844362A2

Abstract

The invention relates a binary phase structure for generating a periodical light signal having high pitch/width ratios. The binary phase structure comprises an array of adjacent unit cells, each unit cell being defined by a height profile having either height h1 or height h2. The height profile is derived from a two-dimensional formula giving an angle either equal to 0 (modulo 2π) or to π, depending on the spatial position in the unit cell. Use: Generation of a periodical light signal.

Description

"Binary phase structure for the generation of a periodical light signal".

FIELD OF THE INVENTION

The invention relates to a binary phase structure for the generation of a periodical light signal.

The invention may be used in the field of optical storage and microscopy.

BACKGROUND OF THE INVENTION

The use of optical storage solutions is nowadays widespread for content distribution, for example in storage systems based on the DVD (Digital Versatile Disc) standards. Optical storage has a big advantage over hard-disc and solid-state storage in that the information carrier are easy and cheap to replicate.

However, due to the large amount of moving elements in the drives, known applications using optical storage solutions are not robust to shocks when performing read/write operations, considering the required stability of said moving elements during such operations. As a consequence, optical storage solutions cannot easily and efficiently be used in applications which are subject to shocks, such as in portable devices.

Recently, optical storage solutions have thus been developed. These solutions combine the advantages of optical storage in that a cheap and removable information carrier is used, and the advantages of solid-state storage in that the information carrier is still and that its reading requires a limited number of moving elements.

Fig.l depicts a three-dimensional view of system illustrating such an optical storage solution. This system comprises an information carrier 101. The information carrier comprises a set of square adjacent elementary data areas having size referred to as s and arranged as in a matrix. Data are coded on each elementary data area via the use of a material intended to take different transparency levels, for example two levels in using a material being transparent or non-transparent for coding a 2-states data, or more generally N transparency levels (for example N being an integer power of 2 for coding a ²log(N)-states data). This system also comprises an optical element 102 for generating an array of light spots 103 which are intended to be applied to said elementary data areas.

Each light spot is intended to be successively applied to an elementary data area, for example using an actuator in charge of two-dimensionally translating the optical element 102. According to the transparency state of said elementary data areas, the light spot is transmitted

(not at all, partially or fully) to a CMOS or CCD detector 104 comprising pixels intended to convert the received light signal, so as to recover the data stores on said elementary data area.

The optical element 102 may correspond to a two-dimensional array of apertures at the input of which the coherent input light beam 105 is applied. Such an array of apertures is illustrated in Fig.2. The apertures correspond for example to circular holes having a diameter of 1 μm or much smaller.

The array of light spots 103 is generated by the array of apertures in exploiting the

Talbot effect which is a diffraction phenomenon working as follows. When a coherent light beam, such as the input light beam 105, is applied to an object having a periodic diffractive structure (thus forming light emitters), such as the array of apertures, the diffracted lights recombines into identical images of the emitters at a plane located at a predictable distance Z_T from the diffracting structure. This distance Z_T is known as the Talbot distance. The Talbot distance z_τ is given by the relation Z_T = 2.n.d² / λ, where d is the periodic spacing of the light emitters, λ is the wavelength of the input light beam, and n is the refractive index of the propagation space. More generally, re-imaging takes place at other distances z(m) spaced further from the emitters and which are a multiple of the Talbot distance z such that z(m) =

2.n.m.d² / λ, where m is an integer. Such a re-imaging also takes place for m = '/2 + an integer, but here the image is shifted over half a period. The re- imaging also takes place for m = ¹A + an integer, and for m = ³A + an integer, but the image has a doubled frequency which means that the period of the light spots is halved with respect to that of the array of apertures.

Exploiting the Talbot effect allows generating an array of light spots at a relatively large distance from the array of apertures (a few hundreds of μm, expressed by z(m)), without the need of optical lenses.

Instead of using an array of apertures, a phase/amplitude structure may be used. Using

Fourier optics, it is rather straightforward to calculate the phase and the amplitude structure, which has to be used in order to create a certain array of light probes. However, these calculations in general result in very complicated amplitude and phase structures. These complicated structures are very difficult, if not impossible to manufacture. Alternatively, instead of using an array of apertures, a binary phase structure may be used. With such known binary phase structure, the width of the spots compared to their pitch, sometimes called the compression ratio, is however too high to be applied to an information carrier as described above. Indeed, such known binary phase structure cannot give high pitch/width ratios.

OBJECT AND SUMMARY OF THE INVENTION It is an object of the invention to propose a binary phase structure for generating a periodical light signal having high pitch/width ratios.

To this end, the binary phase structure comprises an array of adjacent unit cells, each unit cell being defined by a height profile taking either a first height value or a second height value. The height profile is derived from a two-dimensional formula giving an angle either equal to 0 (modulo 2π) or to π, depending on the spatial position on the unit cell.

The periodical light signal advantageously correspond to an array of light spots having different possible shapes, the spots having a full-width-half-maximum much smaller than reported in literature. In the context of an information carrier as depicted above, a certain requirement for the intensity profile of the light spots is needed, i.e. the light profile must be that of an array of spots, each spot matched to the size of bits on the information carrier. There is no restriction on the phase profile at the position of the information carrier. This freedom in the choice of the phase profile at the position of the information layer offers the opportunity to optimize the binary phase structure for efficiency, manufacturability, and resemblance to the targeted periodical light signal. Moreover, the height profile of each unit cell is easily characterized by a single equation.

The distance zθ between the binary phase structure and the optical spots can also be freely defined, contrary to the use of an array of apertures where such a distant is determined by the pitch of the apertures.

From the required laser power point of view, it is beneficial to have no absorption in the phase-amplitude structure, i.e. that the phase-amplitude structure is a pure phase-structure.

From the manufacturing point of view, a two-level binary phase structure is beneficial since a master for such a structure can be made by writing the required height pattern in resist via a lithography process, or in using an Electron Beam Pattern Generator (EBPG), to produce the phase profile in for instance SiO₂. The master can then easily be replicated by embossing.

Detailed explanations and other aspects of the invention will be given below.

BRIEF DESCRIPTION OF THE DRAWINGS

The particular aspects of the invention will now be explained with reference to the embodiments described hereinafter and considered in connection with the accompanying drawings, in which identical parts or sub-steps are designated in the same manner: Fig.1 depicts a system for reading an information carrier,

Fig.2 depicts a known optical element dedicated to generate an array of light spots, Fig.3 illustrates a three-dimensional view of a binary phase structure according to the invention, Fig.4 illustrates a cross-section of a system comprising a binary phase structure according to the invention,

Fig.5 illustrates a first example showing the amplitude variations of a periodical light signal generated by a binary phase structure according to the invention,

Fig.6 illustrates a second example showing the amplitude variations of a periodical light signal generated by a binary phase structure according to the invention,

Fig.7 illustrates a top-view of a periodical light signal as illustrated by Fig.5, Fig.8 illustrates a top-view of a periodical light signal as illustrated by Fig.6, Fig.9 depicts a cross-section of a unit cell according to the invention, Fig.10 illustrates a top-view of a periodical light signal generated by an alternative binary phase structure according to the invention,

Fig.l 1 illustrates a top-view of a unit cell according to the invention, Fig.12 illustrates a top-view of a binary phase structure according to the invention, Fig.13 depicts a system comprising a binary phase structure according to the invention. Fig.14 depicts various apparatus comprising a system as depicted by Fig.13.

DETAILED DESCRIPTION OF THE INVENTION

Fig.3 illustrates a three-dimensional view of a binary phase structure 301 according to the invention. The binary phase structure 301 comprises an array of adjacent unit cells, such as unit cells 302a-302b-302c-302d. Each unit cell defines a specific binary phase profile having characteristics which will be described in the following.

For example, the shape of the unit cells may be rectangular (as illustrated with unit cells having dimension p_x and p_y, which also correspond to the repetition period), or hexagonal.

Fig.4 illustrates a cross-section of a system comprising a binary phase structure 401 according to the invention placed at a first height zl for generating a periodical light signal 403 at a second height z2, when a uniform collimated laser beam 404 is applied to the binary phase structure 401. The quantity (z2-zl) is referred to as zθ.

The binary phase structure 401 comprises an array of adjacent unit cells 402a-402b- 402c-402d, each unit cell defining a specific binary phase profile whose characteristics will be described in the following.

The periodical light signal 403 may be seen as an elementary light signal which is periodically repeated with a period p. The periodical light signal 403 generated by the binary phase structure 401 is constructed from a collective effect of the adjacent unit cells. In other words, an elementary light signal derives from light coming from a plurality of unit cells of the binary phase structure, and the other way around, a single unit cell of the binary phase structure generates light which contributes to a plurality of elementary light signals. It is noted that the period of the unit cells is the same as the period of the periodical light signal 403, provided the binary phase structure 401 is illuminated with a plane wave.

To define the binary phase profile of each unit cell, the periodical light signal 403 to be generated is first modelled by a targeted periodical light signal having ideal variations at said second height z2. Different patterns for the targeted periodical light signal may be considered.

Fig.5 is a first example showing the amplitude variations of targeted periodical light signal of a first type. The targeted periodical light signal is formed of a bi-dimensional square elementary targeted signal which is periodically repeated along x and y directions with a period p = Px = Py. The width of the light spots is 2d. Fig.6 is a second example showing the amplitude variations of targeted periodical light signal of a second type. The targeted periodical light signal is formed of a bi-dimensional circular targeted elementary signal which is periodically repeated along x and y directions with a period p = p_x = p_y. The diameter of the light spots is 2d. It can be shown that for generating the periodical light signal formed of a targeted elementary signal periodically repeated, the phase profile for each unit cell may be defined by the following two-dimensional expression:

where: m_max is the number of layers in between the binary phase structure and the height at which the periodical light signal is generated (e.g. a substrate layer stacked on the binary phase structure, the propagation layer such as air, a substrate layer staked on an information carrier to which is applied the periodical light signal ...), zθ_m is the thickness of the layer having index m. It is noted that the distance zθ between

, the binary phase structure and the periodical light signal is defined by zθ =

. n_m is the refractive index of the layer having index m, int is the operator for rounding down to an integer, arg is an operator for calculating the complex argument (between -π and π), i is the complex operator / = V- I , λ is the wavelength of the light beam to be applied to the binary phase structure, p_x and py correspond to the period in the x and y directions of the unit cells, with - p_x / 2 < x < p_x / 2 and - p_y / 2 < y < p_y / 2

index j_L is defined by j_L = int — *- ,

index l_L is defined by l_L = int — ,

where Δx and Δy are the smallest details in the x and >> directions of the unit cell (the smallest details corresponding to the dimensional sampling of the binary phase structure), φ(x,y) is a phase term characterizing the incident wave. In the case of plane wave applied perpendicularly to the binary phase structure <p(x,y) = 0, in the case of plane wave applied with an angle a compared to the x direction of the binary phase structure φ(x,y) =

(where λ' is the wavelength of the light in the area at the illumination side of the binary phase structure). O_ji are the Fourier coefficients of the targeted elementary signal.

It is noted that the targeted elementary signal T_1?J(x,y) is thus expressed by:

In the case of square elementary signals as depicted in Fig.5, the Fourier transform used to get Fourier coefficients αy is applied on a signal as illustrated by Fig.7.

Fig.7 is a top view of a signal having level 1 in black area, and level 0 otherwise. The edges of the unit cell are shown in dotted line for information purpose only. It can be shown that Fourier coefficients a,i are defined by:

(2)

In the case of circle elementary signals as depicted in Fig.6, the Fourier transform used to get Fourier coefficients a_β is applied on a signal as illustrated by Fig.8. Fig.8 is a top view of a signal having level 1 in black area, and level 0 otherwise. The edges of the unit cell are shown in dotted line for information purpose only. It can be shown that Fourier coefficients a,ι are defined by:

where Ji is the Bessel function of the first kind of order one.

A angle map is thus obtained from the phase profile characterized by (1) giving an angle either equal to 0 (modulo 2π) or to π, depending on the position (x,y). This angle map is used to derive a height map for each unit cell.

A unit cell is such that it has a first height hi for a first angle value, and a second height h2 for a second angle value, as illustrated in Fig.9 by a cross-section of a unit cell. The height difference Δh = |hl - h2| is used to create a phase difference of π, and is expressed by:

Δh = |hl - h2| = 0.5λ / (n-n₀) (4)

where: n is the refractive index of the unit cell material, no is the refractive index of the medium adjacent to the unit cell (i.e. air having index no = 1), λ is the wavelength of the light beam applied to the binary phase structure.

For example, if the index of refraction is about n = 1.5, and n₀ = 1, then the height difference Δh is approximately equal to the wavelength λ, e.g. 400 nm.

In other words, the structure of a unit cell is given by the two-dimensional height function H(x,y) defined by the following relation:

(5)

Each unit cell may be made as a height profile in for instance quartz, or alternatively, as an index of refraction modulation in a polymer material.

According to an alternative embodiment of the invention, instead of defining a binary phase structure with adjacent rectangular unit cells, the binary phase structure is defined with adjacent hexagonal unit cells. Expression (1) still applies. In the case of square elementary signals as depicted in Fig.5, the Fourier transform used to get Fourier coefficients a,ι is applied to a signal as illustrated by Fig.10 which is a top view of a signal having level 1 in black area and level 0 otherwise (consisting of 1 full square and 6 half squares). This signal is included in a rectangular shown in bold dotted lines. The edges of the adjacent hexagonal unit cells are shown in dotted line for describing their relative position with the square elementary signals.

More generally, other shapes for the unit cells may be chosen as long as the signal to which is applied the Fourier transform to get series of Fourier coefficients α_7/ has a rectangular symmetry (as shown on Fig.10 with the rectangular in bold dotted lines). Fig.1 1 illustrates by an example the cross-section of a unit cell 1201 according to the invention.

The elementary binary phase structure 1201 is made of a material having a first height hi in black areas and having a second height h2 in white areas, so that a phase-shift of π is ensured between these two areas.

Fig.12 illustrates by an example the top view of a binary phase structure 1201 according to the invention.

The binary phase structure 1201 is made of a plurality of unit cells as the one illustrated by Fig.11 (only an array of 4*4 unit cells are shown for sake of understanding), which are placed adjacent in the plane (x,y). The binary phase structure 1201 is intended to generate a periodical light signal, as illustrated by the system depicted in Fig.5.

To avoid a deterioration of the periodical light signal at the edges of the binary phase structure, due to the fact that the periodicity of the unit cells is suddenly broken, only the central part of the periodical light signal facing the binary phase structure may be considered. Alternatively, for a given targeted periodical light signal, the number of unit cells may be increased so that this targeted signal faces only a subset of unit cells. In other words, the number of unit cells defining the binary phase structure has to be somewhat larger than the number of elementary signals which are intended to be used in an application, such applying these elementary signals to an information carrier.

Alternatively, instead of using only two different heights hi and h2, a third height h3 could be defined, this third height having no influence on the phase profile of the unit cell.

It is noted that for generating a given targeted periodical light signal, the height profile of each unit cell can be slightly modified, without the characteristics of the periodical light signal which is generated varies in large proportions. In particular, this may be the case with a non-perfect accuracy of manufacturing the binary phase structure. However, even slightly modified, the pattern practically obtained for the height profile always has a high correlation factor with the theoretical pattern given by (5).

Fig.13 depicts a three-dimensional view of a system for reading/writing an information carrier 1301.

This system differs from Fig.l in that a binary phase structure 1302 according to the invention is used for generating an array of light spots 1303, instead of using an array of apertures. A detector 1304 is used for recovering the level of data stored on the information carrier. A collimated laser beam 1305 is applied to the binary phase structure 1302 for generating the light spots.

As illustrated in Fig.14, the system depicted by Fig.13 may advantageously be implemented in a reading apparatus RA (e.g. home player apparatus ...), a portable device PD (e.g. portable digital assistant, portable computer, a game player unit...), or a mobile telephone MT. These apparatus and devices comprise an opening (OP) intended to receive an information carrier IC to which is intended to be applied the array of light spots, in view of for reading/writing data.

The binary phase structure in accordance with the invention may be used in a microscope. Microscopes with reasonable resolution are expensive, since an aberration- free objective lens with a reasonably large field of view and high enough numerical aperture is costly. Scanning microscopes solve this cost issue partly by having an objective lens with a very small field of view, and scanning the objective lens with respect to the sample to be measured (or vice- versa). The disadvantage of this single-spot scanning microscope is the fact that the whole sample has to be scanned, resulting in cumbersome mechanics. Multi-spot scanning microscopes solve this mechanical problem, since the sample does not have to be scanned over its full dimensions, the scanning range is limited to the pitch of the lens array that is used. The main disadvantage of the lens array is the fact that the free working distance is of the same order as the pitch of the lens array, which puts a constraint on the lens array pitch, since the free working distance should be large enough to have the possibility to insert relevant samples in the focal plane. With the binary phase structure of the invention, the Talbot-effect is used in order to increase the distance from the phase structure to the optical spots. Moroever aberrations present in the spots are significantly reduced, and the spots are thus small enough to be used in a microscopic apparatus.

In a microscope in accordance with the invention, a sample is illuminated with the spots that are created by the binary phase structure, and a camera takes a picture of the illuminated sample. The working distance can be appropriately tuned to the samples to be measured by this microscope. The large working distance is the main advantage of this system. Another advantage is that the lens and the camera only need to resolve the light coming from two adjacent spots. This means that the lens and the camera can be of relatively low quality. By scanning the spots over the sample and taking pictures at several positions, high-resolution data are gathered. A computer may combine all the measured data to a single high-resolution picture of the sample.

The focus distance can be controlled manually, by looking at a detail of the picture of the sample. This can be cumbersome, though, as the detected image is only a sub-sample of the entire image (with the detection probes having a pitch of e.g. 15 micron). It can also be performed automatically, as is done in a digital camera (finding the position in which the picture has the maximum contrast). Note that the focusing of the imaging system is not critical, only the position of the sample with respect to the probes is critical and should be optimized. The intensity of the light spots varies somewhat across the field of view due to variations in the illumination light intensity as well as variations in the probe generating array. In order to be able to detect grey-scale images with uniform dependence on the optical properties of the sample, a calibration step may required. In principle, this can be done once at the end of the manufacturing process where the calibration parameters are measured and saved, but for a higher precision it might be advantageous to do this more frequently, e.g. by using a homogeneously substrate before performing the actual measurement.

Example 1 : Transmissive system

This system consists of an illumination device, a probe generator (binary phase profile), a sample stage, an imaging device (e.g. lens, fiber optic face plate, mirror), and a camera (e.g. CMOS, CCD). Light is generated in the illumination device, is focused into an array of foci, it is transmitted (partly) through the sample to be measured, the transmitted light is imaged onto the camera by the imaging system. The sample is positioned in a sample stage, which can reproducibly move the sample in the focal plane of the foci and perpendicular to the sample. A position measurement system can be implemented into the stage, or it can be implemented in the system. The advantage of this system is that the spots can be aligned to the camera, and that this alignment is not changed during scanning. It is also possible to keep the sample stationary, and scan the probe generator (and for instance the camera). Furthermore, it is possible to keep everything stationary, and scan the spots by changing the angle of illumination of the probe- generator.

Example 2: Reflective system

Fig. 15 shows such a reflective microscope. This system consists of an illumination device, a probe generator (binary phase profile), a sample stage, an imaging device (e.g. lens, fiber optic face plate, mirror), and a camera (e.g. CMOS, CCD). Light is generated in the illumination device, is focused into an array of foci, in the foci, the sample reflects the light, which is captured on the probe-generator side and it is imaged onto the camera by the imaging system. A beam splitter is used for creating separate illumination and detection branches. This beam splitter can be a 50% mirror. For a fluorescence microscope, the beam splitter can be a dichroic mirror. All the T-ROM probe scan options are possible.

Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in the claims. Use of the article "a" or "an" preceding an element or step does not exclude the presence of a plurality of such elements or steps.

Claims

1. A binary phase structure for generating, from an input light beam, a periodical light signal formed of a targeted elementary signal periodically repeated, said binary phase structure comprising an array of adjacent unit cells, each unit cell being defined by a height profile H(x,y) expressed in a plane (x,y) by:

where:

hi and h2 are two height values of said unit cell such that |hl - h2| results in a phase shift of π, m_max is the number of layers in between the binary phase structure and the height at which the periodical light signal is generated, zθ_m is the thickness of the layer having index m, n_m is the refractive index of the layer having index m, int is the operator for rounding down to an integer, arg is an operator for calculating the complex argument, i is the complex operator / = V-T , λ is the wavelength of the input light beam to be applied to the binary phase structure, p_x and py correspond to the periods in the x and y directions of the unit cells, with - p_x / 2 < x < p_x / 2 and - p_y / 2 < y < p_y / 2

index j_L is defined by

index 1_L is defined by

where Δx: and Ay are the smallest details in the x and y directions of the unit cell, φ(x,y) is a phase term characterizing the incident input light beam, O_ji are the Fourier coefficients of the targeted elementary signal.

2. A binary phase structure as claimed in claim 1 , wherein the unit cells are rectangular.

3. A binary phase structure as claimed in claim 1, wherein the unit cells are hexagonal.

4. A binary phase structure as claimed in claim 1, 2 or 3, wherein the Fourier coefficients O_f/ are expressed by :

for generating a periodical light signal composed of a single square signal two-dimensionally repeated.

5. A binary phase structure as claimed in claim 1, 2 or 3, wherein the Fourier coefficients ci_ji are expressed by :

where Ji is the Bessel function of the first kind of order one

for generating a periodical light signal composed of a single circle signal two-dimensionally repeated.

6. A system for reading/writing data on an information carrier (1301), said system comprising: a binary phase structure (1302) as claimed in claim 1, 2, 3, 4 or 5, for generating an array of light spots (1303) from an input light beam (1305), said array of light spots (1303) being intended to be applied to said information carrier (1301), - a detector (1304) for detecting data from an array of output light beams generated by said information carrier (1301) in response of said array of light spots (1303).

7. A portable device comprising a system as claimed in claim 6.

8. A mobile telephone comprising a system as claimed in claim 6.

9. A game player unit comprising a system as claimed in claim 6.

10. A microscope for imaging a sample, said microscope comprising: a binary phase structure as claimed in claim 1, 2, 3, 4 or 5, for generating an array of light spots from an input light beam, said array of light spots being intended to be applied to said sample, an imaging device for imaging said sample, and a camera for detecting said imaged sample.