Patent Description:
This section illustrates useful background information without admission of any technique described herein being representative of the state of the art.

Sensitivity of optical detection, for example in laser spectroscopy, can be improved by increasing the optical pathlength within the sample being analyzed. Such an increase of optical pathlength is conveniently obtained using a multipass cell. Conventional multipass cells, e.g. Herriott cells, are well known. Such conventional multipass cells require a large volume and are accordingly unsuitable for applications with limited space and/or low available sample volume.

Previously a multipass cell with circular beam pattern has been disclosed in patent publication <CIT>. Such a multipass cell can be fitted into a relatively low volume. However, such a cell is difficult to produce even with modest optical surface quality and accordingly such a cell is expensive.

A circular multipass cell constructed from separate sections has also been previously presented in a journal article "<NPL>. However, such a cell, while more cost-effective, requires very tasking alignment of the separate mirrors and problems arise with diffraction on the mirror edges.

Furthermore, concave mirrors opposite to one another have been disclosed in patent publication <CIT> for use in focusing and pumping laser light from several sources into a sample region of a Raman spectrometer.

<CIT> describes a monolithic multipass cell for gas spectroscopy, in which two parabolic mirrors are arranged parallel to each other in such a way that multiple reflection of the light beam results in the light beam being guided parallel to the plane of the base plate.

<CIT> describes an annular multipass cell for absorption-based gas sensing. <CIT> describes a multipass optical cell with a toroidal ring mirror configured to reflect light in one radial plane.

<CIT> describes a multipass cell for particle detection comprising two cylindrical mirrors configured to provide a sheet light illumination.

<CIT> describes a multipass photo-acoustic cell comprising two spherical mirrors arranged at the walls of the cavity for concentrating the reflected light towards the common optical axis.

It is the aim of the current invention to provide a multipass cell that mitigates for example the above problems of the state of the art and/or provides a high optical path to sample volume ratio.

According to a first aspect of the present invention, there is provided a multipass cell, comprising.

The first mirror and the second mirror may be configured in such a way that prior to the first reflection from the first or second mirror and after the last reflection from the first or second mirror the beam propagates substantially in the same single plane.

The multipass cell may further comprise at least a first channel configured to allow a beam to enter the cavity.

The multipass cell may further comprise at least a second channel configured to allow a beam to exit the cavity.

The multipass cell may further comprise means for introducing a sample in to the cavity.

The means for introducing a sample into the cavity may comprise a fluid inlet channel and a fluid outlet.

The multipass cell may further comprise a cover configured to cover the cavity.

The means for introducing a sample into the cavity may comprise perforations and/or porous material in the cover.

According to a second aspect of the present invention, there is provided an optical detection system, comprising:.

The optical source may comprise a tunable laser source configured to direct a laser beam of adjustable frequency or modulated intensity into the cavity.

The optical detection system may comprise a laser absorption spectrometer or a photoacoustic detector.

The present invention and its potential advantages are understood by referring to <FIG> of the drawings. In this document, like reference signs denote like parts or steps.

<FIG> show a schematic top and side view, respectively, of a multipass cell <NUM> according to an embodiment of the invention. The multipass cell <NUM> comprises a body <NUM>. The body <NUM> is made of a suitable material, in an embodiment material such as plastic, glass, metal or composite materials or a composition comprising several materials. A cavity <NUM> is formed in the body <NUM> of the multipass cell <NUM>. The cavity <NUM> has a rectangular form. In a further embodiment the cavity is thinner in the middle than at the ends thereof in order to minimize the volume thereof.

The multipass cell <NUM> further comprises a first mirror 30a and a second mirror 30b at opposite ends of the cavity <NUM>, i.e. the first mirror 30a at a first end and the second mirror 30b at the other, opposite, end. The first and second mirrors 30a, 30b comprise concave spherical, toroidal, or cylindrical mirrors. In an embodiment, the mirrors 30a, 30b are positioned parallel to each other and perpendicular to the centerline of the cavity <NUM>. In a further embodiment, the first and/or the second mirror is tilted with respect to the other mirror and/or with respect to a cross-sectional axis of the cavity <NUM>. The mirrors 30a, 30b in an embodiment comprise materials such as dielectric material, aluminium and copper and are in an embodiment coated, e.g. with gold, silver or with a coating applied by thin film technology, to improve reflectivity. The mirrors 30a, 30b are easy to manufacture with high optical quality and cost-effectively.

The multipass cell <NUM> further comprises a first channel <NUM> and a second channel <NUM> configured to allow electromagnetic radiation, in an embodiment UV, visible or infrared light such as a laser beam, to enter and exit the cavity <NUM>. In an embodiment, the first <NUM> and second <NUM> channel are positioned on the same side of the cavity <NUM> as shown in <FIG>. In a further embodiment, the first <NUM> and second <NUM> channel are positioned on opposite corners of the cavity <NUM>. In a still further embodiment, the first <NUM> and second <NUM> channel are positioned at the same end of the cavity <NUM> on opposite sides thereof. In a yet further embodiment, the multipass cell <NUM> comprises only a single channel configured to allow light to enter and exit the cavity <NUM>. In a yet further embodiment, the multipass cell <NUM> comprises a channel on each corner of the cavity. The first channel <NUM> is positioned in such a way that the light enters the cavity next to the outer edge of the first 30a or second 30b mirror, i.e. a beam entering the cavity passes close to the edge of the first 30a or second 30b mirror. The exact distance of the beam entering the cavity <NUM> from the edge of the mirror 30a, 30b depends on the angle in which the beam is to enter the cavity <NUM> and on the desired position at which the beam hits the opposite mirror 30a, 30b.

The multipass cell <NUM> further comprises means for introducing a sample into the cavity <NUM>. In an example embodiment, the means comprise a fluid inlet <NUM> and a fluid outlet <NUM> configured to convey a fluid sample into and out of the cavity <NUM>. In a further embodiment, the multipass cell <NUM> comprises a cover, or a lid, <NUM> configured to cover the cavity <NUM> and in an embodiment to seal the cavity <NUM> in fluid tight manner. In a further example embodiment, the cover <NUM> is porous and/or perforated in order to allow fluid, for example gas, to enter and exit the cavity <NUM>.

The first 30a and second 30b mirror and the first channel <NUM> are aligned in such a way that a beam, such as a beam of infrared laser, entering the cavity through the first channel <NUM> next to the outer edge of the first mirror 30a hits the second mirror 30b next to the opposite edge thereof. In an embodiment, the first 30a and the second 30b mirror are positioned at a distance corresponding to twice the radius of curvature of the first 30a and the second 30b mirror in such way that the beam is focused between the first 30a and the second 30b mirror. The first 30a and second 30b mirror are configured to reflect the beam entering the cavity next to an outer edge of the first 30a or the second 30b mirror a predetermined number of times so that the beam propagates substantially in a single plane between the first and the second mirror 30a, 30b and forms a substantially linear reflection spot pattern on the first 30a and the second 30b mirror. In an embodiment, the beam propagates in the cavity <NUM> prior to the first reflection from the first or second mirror 30a, 30b and after the last reflection from the first or second mirror 30a, 30b substantially in the same single plane. After several reflections, the beam exits the cavity next to the outer edge of the first 30a or the second 30b mirror depending on the position of the second channel <NUM>. The number of reflections and the exit position depends on the alignment of the mirrors and the optical parameters thereof. In the following the propagation of a beam in the multipass cell <NUM> is explained further with reference to <FIG>.

<FIG> shows a schematic principle view of a multipass cell according to an embodiment of the invention. A beam <NUM> enters the cavity <NUM> just outside the edge of the first mirror 30a and hits the opposite edge of the second mirror 30b. The first 30a and second 30b mirror are configured to reflect the beam entering the cavity next to an outer edge of the first 30a or the second 30b mirror a predetermined number of times so that the beam <NUM> propagates substantially in a single plane between the first and the second mirror 30a, 30b and forms a substantially linear reflection spot pattern on the first 30a and the second 30b mirror. After several reflections, the beam <NUM> exits the cavity just outside the edge of the second mirror 30b on the same side of the cavity from which it entered.

<FIG> shows a further schematic principle view of a multipass cell according to an embodiment of the invention. A beam <NUM> enters the cavity <NUM> just outside the edge of the first mirror 30a and hits the opposite edge of the second mirror 30b. The first 30a and second 30b mirror are configured to reflect the beam entering the cavity next to an outer edge of the first 30a or the second 30b mirror a predetermined number of times so that the beam <NUM> propagates substantially in a single plane between the first and the second mirror 30a, 30b and forms a substantially linear reflection spot pattern on the first 30a and the second 30b mirror. After several reflections, the beam <NUM> exits the cavity just outside the edge of the second mirror 30b on the opposite side of the cavity from which it entered.

<FIG> shows a further schematic principle view of a multipass cell according to an embodiment of the invention. A beam <NUM> enters the cavity <NUM> just outside the edge of the first mirror 30a and hits the opposite edge of the second mirror 30b. The first 30a and second 30b mirror are configured to reflect the beam entering the cavity next to an outer edge of the first 30a or the second 30b mirror a predetermined number of times so that the beam <NUM> propagates substantially in a single plane between the first and the second mirror 30a, 30b and forms a substantially linear reflection spot pattern on the first 30a and the second 30b mirror. After several reflections, the beam <NUM> exits the cavity just outside the edge of the first mirror 30a on the opposite side of the cavity from which it entered.

<FIG> shows a further schematic principle view of a multipass cell according to an embodiment of the invention. A beam <NUM> enters the cavity <NUM> just outside the edge of the first mirror 30a and hits the opposite edge of the second mirror 30b. The first 30a and second 30b mirror are configured to reflect the beam entering the cavity next to an outer edge of the first 30a or the second 30b mirror a predetermined number of times so that the beam <NUM> propagates substantially in a single plane between the first and the second mirror 30a, 30b and forms a substantially linear reflection spot pattern on the first 30a and the second 30b mirror. After several reflections, the beam <NUM> exits the cavity just outside the edge of the first mirror 30a on the same side of the cavity from which it entered. In an embodiment, the multipass cell <NUM> further comprises an optical element <NUM>, such as a beamsplitter or a mirror, configured to reflect the light exiting the cavity <NUM> into a desired direction, for example away from the source of the beam <NUM> towards e.g. a detector element.

<FIG> shows a schematic principle view of reflection spot patterns of a multipass cell according to an embodiment of the invention. <FIG> shows only the first mirror 30a with two example forms thereof. The first 30a and second 30b mirror are configured to reflect the beam entering the cavity next to an outer edge of the first 30a or the second 30b mirror a predetermined number of times so that the beam propagates substantially in a single plane between the first and the second mirror 30a, 30b and forms a substantially linear reflection spot pattern on the first 30a and the second 30b mirror. Accordingly, the cavity <NUM> need not have a large height, i.e. volume, as the beam propagation does not require height in comparison with traditional multipass cells with an elliptic reflection spot pattern.

The number of reflections, and accordingly the optical pathlength attained, depends on the alignment of the mirrors 30a, 30b, the optical parameters of the mirrors 30a, 30b and the size of the cavity <NUM>, as well as the entry angle of the beam <NUM>. The number of reflections is accordingly chosen in accordance with the use of the multipass cell, i.e. the needed pathlength and the size of the cavity required. The minimum size of the cavity is somewhat limited by diffraction depending on the wavelength of the used radiation. An example minimum length of the cavity <NUM> is for example with mid-infrared radiation, i.e. wavelengths of about <NUM> to <NUM>, approximately from <NUM> to <NUM>. In an embodiment, the minimum height of the cavity is in the region of a few millimeters, for example <NUM> millimeters.

<FIG> shows a schematic top view of an optical detection system <NUM> according to an embodiment of the invention. In embodiment, the optical detection system is a laser spectrometer. In a further embodiment the optical detection system is a laser absorption spectrometer. In a still further embodiment, the optical detection system is a photoacoustic detector. The optical detection system <NUM> comprises a multipass cell <NUM> as hereinbefore described with reference to <FIG>. The optical detection system <NUM> further comprises a source <NUM>, for example a laser source configured to direct a beam into the cavity <NUM>. In an embodiment, the source <NUM> comprises a tunable laser source, for example a mid-range infrared source, configured to direct a laser beam with adjustable frequency into the cavity <NUM> via the channel <NUM>. The laser source <NUM> is of a conventional type. In a further embodiment, the laser source comprises a light emitting diode instead of a laser source. In a still further embodiment, the optical detection system <NUM> comprises several sources configured to direct a beam into the cavity <NUM> via a single or several channels.

The optical detection system <NUM> further comprises a detector element <NUM> configured to receive the beam exiting the cavity <NUM> via the channel <NUM>. The detector element <NUM> comprises in an embodiment photodetectors and reference photodetectors and an interferometer. The detector element <NUM> is of a conventional type. In a further embodiment, the source <NUM> is configured to function as the detector element as well using e.g. so called self-mixing technique. In a still further embodiment the detector element <NUM> comprises an acoustic detection arrangement for photoacoustic detection configured to receive the acoustic signal generated by light absorption in the cavity <NUM>.

The optical detection system <NUM> with the multipass cell <NUM> according to the invention is used for example in absorption laser spectrometry for detecting isotopes in a gas. In such an application a small sample volume with large pathlength is required with small interferences. In a further example, the optical detection system <NUM> with the multipass cell <NUM> is used in detecting temporal variations of concentration in which large sample exchange rates possible due to the small volume of the cavity <NUM> are required. In a still further example, the absorption laser spectrometry <NUM> with the multipass cell <NUM> is used in studying sample containing only a few molecules of the substance to be detected, which detection is possible due to the small amount of sample required as the volume of the cavity <NUM> is small and the pathlength large. In a yet further example, the multipass cell <NUM> according to the invention is scaled in such a way that it is installed in a handheld device, such as a mobile communications device, which is possible due to the small volume of the cavity <NUM>.

Without in any way limiting the scope, interpretation, or application of the claims appearing below, a technical effect of one or more of the example embodiments disclosed herein is a simple alignment of the mirrors, i.e. the adjustment of distance and tilt thereof is straightforward. Another technical effect of one or more of the example embodiments disclosed herein is a robust multipass cell not sensitive to slight misalignment. Another technical effect of one or more of the example embodiments disclosed herein is a cost-effective multipass cell with components being easy to manufacture with high optical quality. A still further technical effect of one or more of the example embodiments disclosed herein is scalability, i.e. the provision of large pathlength in small volume.

Claim 1:
A multipass cell (<NUM>), comprising
a body (<NUM>);
a cavity (<NUM>) formed within the body (<NUM>);
at least a first channel (<NUM>) configured to allow a beam to enter the cavity (<NUM>);
a first concave spherical or toroidal mirror (30a) at a first end of the cavity (<NUM>); and
a second concave spherical or toroidal mirror (30b) at the opposite end of the cavity; wherein
the first mirror (30a) and the second mirror (30b) are configured to reflect a beam entering the cavity just outside an outer edge of the first (30a) or the second (30b) mirror through the first channel (<NUM>)
a predetermined number of times so that the beam
propagates substantially in a single plane between the first (30a) and the second (30b) mirror forming a substantially linear reflection spot pattern on the first (30a) and the second (30b) mirrors; and wherein
the first mirror and the second mirror are positioned at a distance corresponding to twice the radius of curvature of the first (30a) and the second (30b) mirrors in such a way that the beam is focused between the first and the second mirror; and wherein
the cavity (<NUM>) has a rectangular form.