WO2016012818A1

WO2016012818A1 - Optical spectrometer

Info

Publication number: WO2016012818A1
Application number: PCT/HR2014/000029
Authority: WO
Inventors: Mario RAKIC; Marijan BISCAN; Zlatko KREGAR
Original assignee: Institut Za Fiziku
Priority date: 2014-07-25
Filing date: 2014-07-25
Publication date: 2016-01-28

Abstract

The present invention discloses a compact optical spectrometer setup which removes the necessity of embedding into the spectrometer a wavelength dispersive element (prism or grating) and provides higher spectral resolution together with smaller dimensions than conventional portable spectrometers. Incident light, which is defined by a single spectrum, passes through optical band-pass filter (5), optically diffusive element (4), optical filter with spatially non-uniform transmission characteristics (3) and is detected at optical sensor (2). Optical sensor (2) consists of large number of pixels (>le6) with each pixel being subjected to differently transformed spectrum of light incident on band-pass filter (5). Spectrum of incident light is then calculated at controlling device (PC) from signal measured at each pixel and spectral responses matrix whose elements correspond to signals measured at each pixel for each wavelength (in the wavelength range of interest) and are obtained using calibration by a wavelength tuneable monochromatic light source.

Description

OPTICAL SPECTROMETER

DESCRIPTION

Technical Field

The present invention relates to an optical spectrom- eter.

Previous State of the Art

Optical spectrometers are used as standalone devices or as an integral part of other, more complex, devices and have an irreplaceable role in many scientific fields as well as in industry. Throughout history the construction process of optical spectrometers has been modified, but the basic components have remained the same. Namely, most of commercially available optical spectrometers contain a diffractive element such as an optical grating or a prism. The main role of this element is to spectrally resolve incoming electromagnetic radiation, which in many applications consists of visible, but also higher (such as microwaves) and lower (such as X-rays) wavelengths. Re- solved light is subsequently detected on various kinds of optical sensors (CCD, CMOS, etc.) .

Due to improved production processes of optical elements and detector electronics, properties of optical spectrometers (such as spectral resolution, quantum efficiency and dimensions) have been significantly improved over time. However, some of these properties can be improved even further; for example, dimensions can be reduced by reducing optical path length of light inside spectrometer; resolution can be enhanced by more precise mapping of dispersed wavelengths onto optical sensor, etc. The latest generation of spectrometers is inclined towards replacing wavelength dispersive elements (grating or prism) with linearly variable filters (LVF) . These filters have a property that, depending on their thickness, they transmit different narrow band of wavelengths. Usually the thickness of the filter changes linearly over its surface. If such filter is positioned at the surface of a pixel array of an optical sensor, each pixel becomes sensitive to only a narrow band of wavelengths. This approach is somewhat similar to the one described in the present invention but with an important difference, as explained below.

Some of the previous art, closest to the disclosed invention, is described in the following paragraphs. Document WO 2014/078426 Al, inventors HRUSKA, C.R. et al . discloses a portable spectrometer which is to be used mainly for IR measurements in reflection and transmission modes. The proposed device consists of optical sensor, linearly variable filter, tapered light pipe, illumination sources and control system, and introduces several methods for improving spectral resolution and eliminating stray light. The difference to the present invention is that it uses linearly variable filter for separating incident light into constituent wavelengths and each of optical sensor's pixels is disposed in such a way to receive at least a portion of one of the constituent wavelengths. Moreover, the spectrometer can be used only if the source of spectrum to be measured is in close proximity to the detector and the sample needs to be illuminated by incorporated light sources.

Document "Design and implementation of a sub-nm resolution microspectrometer based on a Linear- Variable Optical Filter, Emadi, A. et al . , Optics Express, 20 (1) 489 (2011)" discloses a microspectrometer based on a linear variable optical filter for operation in the visible spectrum together with an iterative signal processing algorithm for enhancement of spectral resolution. The signal processing algorithm itself is similar to the one used in the present invention. Therefore, the concept of this device is closest to the present invention. The difference is that this device relies on using linearly variable filter which leads to 1:1 mapping of wavelengths to pixels (wavelength discrimination) and that only a small fraction of optical sensor (CMOS) pixels can be used. Document WO 2013/126548, inventor Bao, J et al . teaches about a spectrometer which includes a plurality of semiconductor nanocrystals where spectral discrimination is achieved by differing light absorption and emission characteristics of these semiconductor nanocrystals. The need for optical grating or prism is therefore avoided. The difference of this previous art and the present invention is that this previous art also relies on selective wavelength detection; each part of the plurality of semiconductor nanocrystals absorbs the predetermined wavelength of light and emits only a distinct wavelength of light, and the photosensitive element can be sensitive to the distinct wavelength of light. Document WO 2014/043799 Al, inventor Preston, K. et al . discloses a spectrometer comprising a dispersive element configured to generate a plurality of spa^¬ tially separated spectral components from a received optical signal, fabricated on a chip (optical sensor) itself, spectrometer thus being robust and with smaller dimensions. Optical transmission character^¬ istics of the dispersive element can be changed by changing its refractive index. The difference of this art and the present invention is that this art also relies on wavelength separation prior to detection.

Inspection of the documents shows that any of the above cited documents alone or in combination with each other left the essential features of the present invention undisclosed, i.e. the use of spectral filter characterised with spatially non-uniform transmission characteristic of a wide spectral range, with each pixel of optical sensor thus being sensitive to multiple wavelengths.

Technical Problem

Generally, conventional portable spectrometers have rather limited spectral resolution which is mainly defined by the embedded wavelength-dispersive element (such as prism or optical grating) and the number of pixels which comprise the optical sensor. Moreover, the necessity of embedding wavelength-dispersive el- ement limits the minimum size of spectrometer and its robustness.

The first technical problem solved by the present invention is related to spectral resolution; the present invention provides higher spectral resolution by using an optical sensor consisting of orders of mag^¬ nitude larger number of pixels and a filter with spa^¬ tially non-uniform transmission characteristic of a wide spectral range which is attached directly to the optical sensor. The second technical problem solved by the present invention is related to industrial applicability; it enables building much more robust portable optical spectrometer of smaller dimensions and lower production cost.

Summary of the invention Accordingly, the essence of the present invention is the ability to analyse optical spectrum using a simple and compact device. Present invention therefore discloses an optical spectrometer which essential components are a spatially non-uniform optical filter (SNOF) and an optical sensor, including a plurality of pixels, where: SNOF is positioned between a light source and the optical sensor, with each part of the SNOF having a unique transmission characteristic and each characteristic being such that it allows trans- mission of a range of wavelengths of incoming light. Therefore, each pixel of optical sensor is exposed to a range of wavelengths comprising the incoming light. By using signals measured at each pixel of the optical sensor and the spectral response matrix the spectrum of incident light is subsequently mathematically re^¬ constructed. Spectral response matrix elements cor^¬ respond to signals measured at each pixel for each wavelength (in the wavelength range of interest) and are obtained using calibration by a wavelength tune- able monochromatic light source. During measurements, SNOF illumination conditions should be similar to those during calibration; extremely large differences in illumination conditions should be avoided, i.e. geometry of the incoming light beam used for calibration should be replicated during measurements. Also, the wavelength range in which calibration is performed defines the wavelength range in which the spectrum can be reconstructed.

The advantage of the present invention is that it provides a possibility of producing compact, cost effective and high resolution spectrometers which can be used as a standalone device or as a part of a complex analytical laboratory device.

Brief Description of the Figures

Fig. 1A is a cross-sectional perspective view of the preferred embodiment of the present invention. It consists of a housing 1, an optical sensor 2 consisting of a plurality of pixels, a spectral filter (SNOF) 3, an optically diffusive element (ODE) 4, an optical band-pass filter 5 and accompanying electronics 6. Fig. IB is a cross-sectional view of preferred embodiment of the present invention. The numberings are the same as in Fig. 1.

Fig. 2 schematically shows a process by which collin- ear incident light with uniform cross section passes through the optical band-pass filter 5, optically diffusive element 4, SNOF 3 and reaches the optical sensor 2.

Fig. 3 shows a comparison of a simulated input and reconstructed spectra in the case when optical sensor 2 consists of 600 pixel groups with each group having 1000 pixels and each pixel signal being subjected to random white noise amounting from -15% to +15% of its real value. The spectrum is reconstructed in 510 points with calibrating tuneable laser having FWHM of 0.5 nm and wavelength step of 1 nm.

References

1 - a housing

2 - an optical sensor 3 - a spatially non-uniform optical filter (SNOF)

4 - an optically diffusive element (ODE)

5 - an optical band-pass filter

6 - accompanying electronics

Detailed Description of Preferred Embodiment With reference to Fig. 1A and IB, in one possible embodiment the present invention comprises a protective housing 1 in which a CCD optical sensor 2 consisting of plurality of pixels, a spatially non-uni- form optical filter (SNOF) 3, an optically diffusive element (ODE) 4 and an optical band-pass filter 5 are embedded. The housing 1 is attached to accompanying electronics 6.

SNOF 3 is either a thin film, with spatially non- uniform transmission characteristic of a wide spectral range, deposited by processes of pulsed laser deposition on a transparent substrate or on an optical sensor 2, or a slice of transparent material with its bulk having spatially non-uniform transmission char- acteristics.

When measuring the spectrum, the light coming from light source passes through elements 5, 4 and 3 and then is detected at 2. Referring to Fig. 2, in the process_, of detection each of elements 5, 4, 3 and 2 has a following role:

- optical band-pass filter 5 selects wavelength region of interest of incident light and removes un^¬ wanted wavelengths.

- ODE 4, by the process of diffuse reflections inside its bulk, ensures better homogeneity of light intensity over the light beam cross-section which passes through it and is positioned between light source and SNOF 3 in such way as to ensure spatially uniform illumination of SNOF 3.

- SNOF 3, which receives light passing through optical band-pass filter 5 and ODE 4, is characterised by the fact that it has different transmission characteristics for each part of its surface (or of bulk in the case of a thick filter) . The light beam which passes through SNOF 3 is therefore differently absorbed depending on the position at which it passes through the filter. As shown in Fig. 2, the incident light beam described with a single spectrum is, by passing through SNOF 3, transformed into a light beam which is characterised by the fact that each point of its cross-section is described with differently transformed incident spectrum. SNOF 3 therefore ensures that each pixel of optical sensor 2 receives differently transformed incident spectrum.

- optical sensor 2, consisting of plurality of pixels, detects light which passes through band-pass filter 5, ODE 4 and SNOF 3 and converts it into electrical signal. Due to spatially non-uniform transmis^¬ sion characteristics of SNOF 3, each pixel receives differently transformed incident spectrum consisting of multiple wavelengths. The signal detected at the optical sensor 2 is then digitized by the accompanying electronics 6 and sent to a controlling device (PC) over a USB connection. Analog to digital conversion should be as efficient as possible, preferably 16-bit. The power to the device is also delivered through the same USB connection . As the present invention does not include a wavelength dispersive (discriminating) optical element, there is no linear correspondence between signals measured at the optical sensor 2 and the intensity of wavelengths comprising the incident spectrum. Spectrum of incident light is therefore calculated (reconstructed) at the controlling device (PC) from signals measured at N pixels of optical sensor 2 and N x M spectral response matrix T. Single column of matrix T corresponds to signals measured at N pixels of the optical sensor 2 when this sensor is exposed to single (narrow band) wavelength light beam. The other columns of the matrix T are then obtained by changing the wavelength of the light beam by discrete steps and measuring the corresponding signals at N pixels of optical sensor 2. The number M is equal to the number of different wavelengths and their cumulative wavelength range is equal to the transmission range of the band-pass filter 5. The source of monochromatic light beam could be, for example, a tuneable laser. The number of columns M of the matrix T is then equal to the number of different central wavelengths to which the laser is sequentially tuned. Therefore, in the present embodiment the tuneable range of the laser should be equal to or larger than wavelength range defined by band-pass filter 5.

In practice it is impossible to obtain purely mono- chromatic light; even a laser light has a certain spectral width. In the present embodiment it is preferred for there to be a certain spectral overlap of neighbouring laser wavelengths. For example, for laser light with full width at half maximum (FWHM) of 0.5 nm, the wavelength shift (step) to the next wavelength can be 1 nm.

The overall number M of different wavelengths which are sequentially incident onto optical sensor 2 should be equal, or preferably less, than the number of pix- els N. The calculated spectrum is therefore defined in the same number of points M. It should be noted that spectral response matrix elements are unique for a given assemblage of optical sensor 2, SNOF 3, op^¬ tically diffusive element 4 and band-pass 5 and they are determined in the production process.

Pixels with similar spectral response functions, as determined by calibration, can be grouped together in groups containing one or more pixel with their then signals averaged. To make calculation of spectrum easier to understand, the following paragraphs describe it in its mathematical form. Generally, spectral response matrix is equivalent to N x M matrix denoted by ^λχτ":

where Ti(Aj) represents signal measured at pixel "i" of the optical sensor 2 when the device is illuminated by wavelength "j" (which belongs to transmission wavelength range of SNOF 3) . Signal "P" measured at the optical sensor 2 while the optical sensor 2 is being illuminated with unknown spectrum " S " is therefore represented by matrix-vector multiplication:

where the elements of vector "P" correspond to signals measured at pixels 1,...,N of the optical sensor 2 and the elements of vector "S" correspond to intensities of incident spectrum at wavelengths Ι,.,.,Μ.

The unknown vector "S" is thus calculated by solving the above linear system. As number M is generally not equal to N, the system is solved using, for example, Least Squares Method. In preferred embodiment system is over-determined, i.e. N > M and the best fit so^¬ lution vector is an approximation of the incident spectrum. Fig. 3 presents comparison of a simulated input and reconstructed spectra in the case when optical sensor 2 consists of 600 pixel groups with each group having 1000 pixels and each pixel signal being subjected to random white noise amounting from -15% to +15% of its real value. The spectrum is reconstructed in 510 points with calibrating tuneable laser having FWHM of 0.5 nm and wavelength step of 1 nm.

The wavelength range which can be measured by the preferred embodiment of the present invention is defined by the overlap of transmission wavelength ranges of elements 5, 4 and 3 and the optical sensitivity of optical sensor 2. Moreover, the distance between two neighbouring wavelengths in the reconstructed spec- trum is given by dividing this measurable wavelength range by the number M. By increasing M, the wavelength distance between points in reconstructed spectrum decreases therefore influencing spectral resolution of the device.

Industrial Applicability

Industrial applicability of the said invention is widespread as the present invention discloses a device for analysing optical spectrum. It can be used as an optical spectrometer device connected to a computer, as a part of a more complex device or as a standalone spectrometer system (such as Raman spectrometer, spectroradiometer or spectrophotometer) . These applications are common in industry and in scientific laboratories. Moreover, due to lower production costs these optical spectrometers can be employed in places in which conventional optical spectrometers haven't been much used. For example, large number of these devices can be employed for monitoring processes at assembly lines.

Claims

CLAI S

1. An optical spectrometer device comprising:

- a spatially non-uniform optical filter (SNOF) (3) ;

- an optical sensor (2); characterised in that:

- SNOF (3) is positioned between light source and optical sensor (2) ;

- each spatial part of SNOF (3) has a unique transmission characteristic and each characteristic is such that it allows transmission of a range of wavelengths;

- optical sensor (2) consists of a plurality of pixels ;

- spectrum of incident light coming from the light source is calculated from signals meas^¬ ured at each pixel of optical sensor (2) and a ~N x M spectral response matrix, where spectral response matrix elements correspond to signals measured at N pixels of optical sensor (2) when the optical sensor (2) is being sequentially subjected to a number M of different narrow wavelength bands;

- M different narrow wavelength bands are obtained using a tuneable monochromatic light source and their central wavelengths fall into transmission wavelength range of SNOF (3); - spectral response matrix elements are unique for a given assemblage of optical sensor (2) and SNOF (3) .

2. An optical spectrometer device according to claim 1 further comprising an optically diffusive element (4) characterised in that it is positioned between -light source and SNOF (3) in such way as to ensure spatially uniform illumination of SNOF (3) .

3. An optical spectrometer device according to claims 1 or 2 further comprising an optical bandpass filter (5), for selecting wavelength region of incident light to be measured and removing unwanted wavelengths.

4. An optical spectrometer device according to claims 1, 2 or 3, further comprising a protective housing for embedding elements (2), (3), (4) and

(5) .

5. An optical spectrometer device according to claim 4 characterised in that the SNOF (3) can be replaced while not disturbing the other elements.

6. An optical spectrometer device according to claim 4 characterised in that the optical band-pass filter (5) can be replaced while not disturbing the other elements.

7. An optical spectrometer device according claim 1, further comprising a battery or a cable or any- kind of wireless power receiver that provides power to the device.

8. An optical spectrometer device according to, claim 7, further comprising accompanying electronics (6) for converting measured signals to digital data and data communication with a controlling device.

9. An optical spectrometer device according to claim 1, further comprising a passive cooling unit for limiting the thermal influence on the signal reading from optical sensor (2) .

An optical spectrometer device according to claim 1 characterised in that the signals from pixels with similar spectral response functions can be grouped together in groups containing one or more signal and averaged.