Optics: measuring and testing – Of light reflection
Reexamination Certificate
2001-07-20
2003-11-11
Pyo, Kevin (Department: 2877)
Optics: measuring and testing
Of light reflection
C250S23700G
Reexamination Certificate
active
06646744
ABSTRACT:
The present invention relates to the formation and manufacturing of diffractive optical input and output grating couplers in surface plasmon resonance (SPR) sensor chips. The SPR sensor chips are being employed in optical bio-/chemical sensor systems, where the function of the SPR sensor chips is to measure bio-/chemical compounds in liquids or in gasses.
In surface plasmon resonance (SPR) sensors, it is important to have an efficient and reliable coupling without critical alignment between the following three components: 1) the light source (LS); 2) the sensing domain (SD), and 3) the optical detector (OD). The SD is defined as the region where interaction between the light and the surface plasmon occurs. It comprises an SPR metal film (typically a few tenths of nanometers) and a superstrate of one or more bio-/chemically active sensing areas. The optical coupling between the LS and the SD can be defined as the input coupling (IC) and the optical coupling between the SD and the optical detector can be defined as the output coupling (OC).
SPR sensors can be divided into three main concepts (A, B and C) according to the integration between the 5 components as defined above, LS, SD, OD, IC and OC. A is the discrete concept, where all 5 components are separated mechanically and the IC and the OC are achieved by means of optical components such as lenses, mirrors, optical fibres, and filters or diffractive optical elements (DOEs). B is the sensor chip concept, where the OD and the IC and OC are integrated in an SPR sensor chip and the optoelectronic components LS and OD are either discrete components and may be mounted on the same electrical circuit board, or they may be integrated on the same optoelectronic chip. C is the integrated optics sensor concept, where all 5 components are integrated on the same chip.
Concept A has traditionally dominated the commercial market of SPR sensors (e.g. products such as BIAcore and IBIS). The disadvantages of concept A are large, bulky and expensive systems, which usually require substantial service. For bio-/chemical sensors, concept C has attracted a great deal of interest within the last 20 years. Typically, an SPR sensor based on this concept consists of optical waveguides as ICs and OCs directing the light from an integrated LS to the SD disposed in a microchannel, and an integrated OD detects the output light beam. Although it is rather a hybrid solution than full integrated one, Texas Instruments has approached this concept and have commercialised an SPR system marketed under the name Spreeta, where all components are integrated in the same housing—see EP 0 797 090. The Spreeta system has a fairly low production cost, but the disadvantages are an unhandy housing and the fact that the user has to dispose all optical and electro-optical components, when replacing the bio-/chemistry on the SD.
The present invention relates to concept B and it has the advantages of the user friendliness of A and the simple optics and low price of C. In fact, since only the sensor chip is being replaced, the production costs can be very low. The present invention is an oblique angle holographic method of formation of ICs and OCs as diffraction gratings integrated on an SPR sensor chip. The formation of ICs and OCs as diffraction gratings is being made in such a manner that the coupling between the LS, the SD and the OD can be established without critical alignment. This enables the light to be accurately directed to the SD with an appropriate angle, focal length and focal point size. The present oblique angle holographic method is particularly suitable for formation of diffraction gratings that have large diffraction angles, as high as ~80° from the plane of incidence.
For SPR sensing, the angle of incident light to the SPR film is lying in the range 40° to 80°. One method to couple light into an SPR sensor chip is using a high index prism and an index matching gel (U.S. Pat. No. 5,313,264). In the present invention, where index-matching gels are eliminated, diffraction gratings employed as ICs and OCs are disposed onto an SPR sensor chip.
For an SPR sensor chip with mutually parallel flat topside surface and flat backside surface, the diffraction condition that light rays from each grating spacing of the diffraction grating interfere constructively yields the following expression for the grating spacing a
p
of the p'th grating element;
a
p
=
m
λ
n
g
x
p
y
i
[
1
+
(
x
p
y
i
)
2
]
1
/
2
,
(
1
)
where &lgr; is the wavelength of light, m is the diffract on order, x
p
and y
i
are the horizontal and vertical distances between the focal point of the diffraction grating and the position of the p'th grating element with p=0 being the first element and p=N being the last element in the grating. In case of one or more reflection points (M) of light between the p'th grating element and the focal point of the diffraction grating, y
i
has to be multiplied by M+1.
For SPR sensing, the large angle of incidence of the light beam puts high requirements to the accuracy of the formation of the diffraction gratings. This is evident from the following example, where we assume that the input light beam has a wavelength of 670 nm, the substrate of the SPR sensor chip has a refractive index of 1.65 (e.g. a polymer substrate with high refractive index), and the SD comprises an SPR gold film with a superstrate on the top having a refractive index of 1.46 (e.g. a polymer membrane for ion-detection). As a result, the SPR angle is ~73°. In order to cover this angle in the angle span of the input light beam, the SPR sensor chip may have the following dimensions: y
i
=2 mm, x
p=0
=8 mm and x
p=N
=5 mm resulting in an aperture of 3 mm. Assuming a diffraction order of m=1, eqn. (1) yields a
0
=418 nm and a
N
=437 nm. The number of grating periods is ~7050 and the difference in the grating spacing between two neighbouring grating elements is ~(a
N
-a
0
)}/7050=0.003 nm. For maximum diffraction efficiency, the depths of the gratings (d) are approximately ~100 nm for reflection gratings and ~800 nm for transmission gratings.
In practice, it would be acceptable to divide the aperture into sections of grating elements with a fixed periodicity in each section. Dividing the full aperture into 100 sections of grating elements, the minimum difference in the grating spacing from one section to the neighbouring section can then be increased to 0.3 nm. However, it is noted that for m=1, the requirement, of the method of formation of the diffraction grating are still very severe.
Alternatively, one can increase the diffraction order m, and the dimensions of the diffraction gratings scale accordingly. On the other hand, by choosing a large value of m, e.g. m>10, it becomes more difficult to optimise the performance of the diffraction grating. There is therefore a need in the art of a method, which accurately forms diffraction gratings on SPR sensor chips working in low order diffraction modes, m<10.
There are numerous methods of formation of diffraction gratings. The gratings are either directly fabricated on a substrate, e.g. glass or they are made in a mould, and the structure in the mould is subsequently transferred to another substrate, usually a transparent plastic like acrylics or polycarbonate. There are mechanical methods like single-point diamond turning [P. P. Clark, and C. Londoño, Opt. News 15, p. 39-40 (1989)], where a diamond tool with a radius as small as a few micrometers is translated incrementally to grind the desired grating profile into the substrate. A more coarse mechanical method is plunge-cut diamond turning [J. Futhey and M. Fleming,
Superzone diffractive lenses,
Vol. 9 of 1992 OSA Technical Digest Series, pp. 4-6], where a diamond with a triangular or trapezoidal profile is rotated and transfers the profile to the substrate. The groove dimensions with these methods are limited to the micron range and
Pedersen Henrik Chresten
Thirstrup Carsten
Pyo Kevin
Vir A/S
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