Analytical Technologies Singapore

Imaging

Volume Bragg Tunable Filter

What is Volume Bragg Grating?

A volume Bragg grating (VBG) is a diffraction grating in which there is a periodic modulation of the refractive index through the entire volume of a photosensitive material.

As shown in figure 1, this modulation can be oriented either to transmit (a) or reflect (b) the incident beam. VBGs can be fully described by the following parameters (see Fig. 1 a): the thickness of the grating, the refractive index of the photo-termo-refractive glass it’s made of (n0), the period (⋀) of the grating (or spatial frequency f = 1/⋀), the angle (θ) between the incident beam and the normal of the entrance surface (N), and the inclination of the Bragg planes (φ) defined as the angle between the normal (N) and the grating vector (Kg).

Like shown in Fig. 1 (a), the incoming collimated light is diffracted by the volume holographic filter, and only a small fraction of the spectrum is affected. In order to select which particular wavelength will be diffracted, the angle of the filter is adjusted to meet Bragg’s condition: λB=2n0Λcos(θ+φ), where λB is the diffracted wavelength.

Like shown in Fig 1 (a), for transmission gratings, φ = π/2 (Bragg planes are perpendicular to the entrance surface). In this case, the Bragg condition becomes: λB=2n0Λsin(θ).

 

As mentioned, this condition is valid for transmission gratings and has to be altered for reflection gratings where Bragg planes are parallel to the entrance surface. For reflection gratings, φ = 0 and the Bragg condition becomes :  λB=2n0Λcos(θ). If the beam does not meet the Bragg condition, it passes through the filter undiffracted.

Bragg Tunable Filter

A Bragg tunable filter is a filter that exploits Bragg gratings in order to extract a small bandwidth of wavelengths out of a polychromatic input.

As stated by Bragg’s law, θ determines which wavelength is diffracted. Hence, by tuning the angle of the grating, we can scan the output wavelength over hundreds of nanometers (see figure 1 (a)). Since Bragg gratings are dispersive, their output is divergent. Therefore, a second pass in the grating is essential in order to recombine the diffracted beam and cancel out this divergence. The second pass helps reduces the bandwidth and provides an output parallel to the input beam. This technology allows for the detection of a whole image at one wavelength .

Fabrication Method: How is it created?

In order to create a volume Bragg grating, a photo-thermo-reflective (PTR) glass is exposed to ultra-violet laser radiation at 325 nm. The PTR glass is placed in a sea of interference which will induce migration of ions.

​This will generate a variation in the electronic density over the whole material. The variation of the charge distribution gives rise to a variation in the refractive index. When the radiation stops this variation persists and the glass is then exposed to high temperature in order to accentuate this modulation. Other materials can be used to produce volume hologram but PTR glasses offer unpolarised output (in transmission) and are highly resistant, this is why they are the most popular. **

** Read MoreVolume Bragg Gratings as Ultra-Narrow and Multiband Optical Filters

Invited Paper, Proc. of SPIE Vol. 8428 84280C-1, doi: 10.1117/12.923575.

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What is Hyperspectral Imaging?

Hyperspectral Imaging

Hyperspectral imaging is a technique combining spectroscopy and imaging, where each image is acquired at a narrow band of the electromagnetic spectrum. As an example, the human eye sees the light in three bands (red, green and blue) of the visible spectrum. At the same time, hyperspectral imaging divides the spectrum in more bands, typically covering the visible and near-infrared range.

The term hyperspectral imaging refers to the continuous acquisition of narrow bands (< 10 nm) across the electromagnetic spectrum. With our unique technology we are able to obtain bands of 2nm-4nm wide and even 0.3nm. On the contrary, multi-spectral imaging covers only a discrete number of bands, and is often performed with a filter wheel.

Through the analysis of the spectral and spatial information contained in each pixel of the image, it is possible to identify unique spectral signatures and assign them to the components of the sample under investigation. For example, the material or tissue analysand can be mapped according to its molecular components.

Hyperspectral Data Cube

The monochromatic images acquired form what we call a hyperspectral data cube, which contains both the spatial and spectral information of a sample, forming a 3-dimensional (3D) cube.

In the hyperspectral cube, the first two dimensions are spatial (x, y-axis), while the third dimension (z-axis) is the wavelength. Depending on the size of the sensor used, one single cube can represent many gigabytes of data representing an extremely rich source of information for material scientists or biomedical researchers.

Applications

Lifescience

IN VIVO NIR-II IMAGING: A DISRUPTIVE FORCE FOR PRECLINICAL RESEARCH

Hyperspectral imaging has opened up many doors across research fields and industries; in vivo imaging in the second biological window using NIR-II is one of the most recent discoveries that holds endless potentials in opening up new windows in the life science research.

Preclinical optical imaging suffers from the inability to localise signals due to complications associated with light absorption, scattering and auto-fluorescence in living tissues. In vivo optical imaging can localise a signal well when it is at the surface but not when it is deep in the organism.

Preclinical biologists still strongly desire the ability to rapidly localise optical signals in vivo, but their discussions with imaging physicists often end up in a standstill. Biologists ask: can I use optical imaging to see my mCherry cancer cells in vivo? What about my luciferase cells? The answer is: it depends on many different factors such as the temperature of the animal, the optical properties of organs, how deep they are and how many photons come out.

NIR-II in vivo imaging is not impacted in the same way by drawbacks of light propagation in living tissues, thus enabling real-time imaging of optical probes much deeper in the organism and with much higher resolutions.

One of the breakthroughs in the field of in vivo SWIR imaging has been the demonstration that both NIR-I and NIR-II probes can work well for this application. There is an abundance of probes for the new imaging modality and many of them remain to be validated. he ball is back in the court for biologists to take. No longer will biologists need to accept the “oh well, I guess it depends” answer when asking an optical imaging physicist if it is possible to localise their probes in vivo.

MATERIAL SCIENCE

IN VIVO NIR-II IMAGING: A DISRUPTIVE FORCE FOR PRECLINICAL RESEARCH

Semiconductor materials are present in a vast collection of devices, ranging from transistors to solar cells, multiprocessors to light emitting diodes. In order to improve further the next generation of such devices, researchers need to study the fundamental properties of semiconductor materials and perform quality control measurements. To do so, accurate characterisation systems and methods are paramount, and hyperspectral imagers possess the essential modalities to perform these tasks through the rapid collection of highly valuable spatial and spectral data.

The intrinsic specificity of Raman scattering confers to this imaging platform the ability to measure the uniformity and morphology of a wide variety of materials. It also allows a fast identification of the composition and stoichiometry, while providing spatial distribution of stresses and constraints. This platform was successfully used to characterise various properties of chalcogenide glasses, GaAs and GeSn/Ge/Si thin films, the identification of MoS2 layers, and the analysis of constraints in a Si wafer covered by SiO2.

Photoluminescence and electroluminescence are also widely used characterisation technique, providing a fast spatial distribution of key properties of semiconductors samples. It was successfully employed to characterise different defects present in SiC pin diodes and to study the uniformity of optoelectronic properties in CIGS, CIS and GaAs solar cells.

To understand more about the types of hyperspectral imaging systems we provide, click here.

If you wish to know how hyperspectral imaging can help improve your research, contact us.

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Raster scanning VS Global Imaging

Raster scanning is done by taking sequential measurements of the spectra of adjacent regions of a sample by moving the sample point by point or line by line until the region of interest has been covered.

On the other hand, imaging consists of focusing the image of a sample on an array detector and measuring for each pixel the intensity of light at one particular wavelength, much like taking a photograph, but at a single wavelength. In some applications, the power of the laser used in imaging can be orders of magnitude stronger than in mapping, since the power is spread on the whole region instead of a single point or line, thus avoiding the damaging of a sample. Imaging also permits a higher resolution, reduces the acquisition time by orders of magnitude and requires no prior knowledge of the sample contrary to mapping.

With global imaging, the gain in acquiring 3D data, 2D spatial and 1D spectral, is important since only a  few monochromatic images are required to cover the complete spectral range where one needs to take the full spectrum for each point or line in the image with other technologies

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Laser Doppler Velocimetry (LDV)

Laser Doppler Velocimetry (LDV) is a technique which allows the measurement of velocity at a point in a flow field with a high temporal resolution. Whenever a micron-sized liquid or solid particle entrained in a fluid passes through the intersection of two laser beams, the scattered light received from the particles fluctuates in intensity.
 
Laser Doppler Velocimetry (LDV) makes use of the fact that the frequency of this fluctuation is equivalent to the Doppler shift between the incident and scattered light, and is thus proportional to the component of particle velocity which lies in the plane of two laser beams.

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Rayleigh scattering

Rayleigh scattering is the elastic scattering of light by particles much smaller than the wavelength of the light. That is the case for gas phase molecules and, therefore, this method is suited for laser imaging in gases. Rayleigh scattering of sunlight by atmospheric molecules is the reason for the observed blue colour of the sky, because the scattering efficiency varies inversely with the fourth power of the wavelength. 

For a single component gas with known scattering cross section the Rayleigh signal is directly proportional to the gas density. The scattered light is almost at the same wavelength as the incident light, i. e. Rayleigh scattering is not species selective. Rayleigh scattering requires either constant gas composition or known mole fractions of all major species for the density measurement of a gas mixture. In some cases Rayleigh scattering is stronger for one species than another, and it can be used to image mixing processes such as fuel – air mixing.

When gas composition and pressure are known Rayleigh imaging allows to measure planar temperature fields (Rayleigh Thermometry). Rayleigh scattering is much weaker than Mie scattering but more than two orders of magnitude stronger than Spontaneous Raman Scattering. Incandescence from soot and Mie scattering are processes that can totally obscure the Rayleigh signal.

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Interferometric Mie Imaging

IMI imaging of single droplets

Interferometric Mie Imaging (IMI) is a sizing technique for the evaluation of the diameter of spherical particles, droplets and bubbles similar to PDI. The working principle is based on the out-of-focus imaging of particles illuminated by a laser light sheet. The optical setup of a standard IMI system consists of a laser-light sheet and a digital camera with a high quality lens. Moving the camera chip to an out-of-focus position an interference fringe pattern becomes visible.

The visible fringe pattern corresponds exactly to the far field scattering which can be calculated by the Mie theory. The number of fringes within the aperture image depends on the diameter of the droplet and the aperture angle. With increasing particle size, the number of fringes increases. The exact size of the particles is determined by analysis of the fringe patterns. The size of the aperture image is a measure for the z-position of the particle along the line-of-sight.

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