New method uses noise to make spectrometers more accurate

Date:
October 13, 2020
Source:
University of California - Davis
Summary:
Optical spectrometers are instruments with a wide variety of
uses. By measuring the intensity of light across different
wavelengths, they can be used to image tissues or measure the
chemical composition of everything from a distant galaxy to a
leaf. Now researchers have come up a with a new, rapid method for
characterizing and calibrating spectrometers, based on how they
respond to 'noise.'


FULL STORY ========================================================================== Optical spectrometers are instruments with a wide variety of uses. By
measuring the intensity of light across different wavelengths, they
can be used to image tissues or measure the chemical composition of
everything from a distant galaxy to a leaf. Now researchers at the UC
Davis Department of Biomedical Engineering have come up a with a new,
rapid method for characterizing and calibrating spectrometers, based on
how they respond to "noise."

========================================================================== Rendering of prism and spectrum Optical spectroscopy splits light
and measures the intensity of different wavelengths. It is a powerful
technique across a wide range of applications. UC Davis engineers Aaron
Kho and Vivek Srinivasan have now found a new way to characterize and cross-calibrate spectroscopy instruments using excess "noise" in a
light signal.

Spectral resolution measures how well a spectrometer can distinguish light
of different wavelengths. It's also important to be able to calibrate
the spectrometer so that different instruments will give reliably
consistent results. Current methods for characterizing and calibrating spectrometers are relatively slow and cumbersome. For example, to measure
how the spectrometer responds to different wavelengths, you would shine multiple lasers of different wavelengths on it.

Noise is usually seen as being a nuisance that confuses measurements. But graduate student Aaron Kho, working with Vivek Srinivasan, associate
professor in biomedical engineering and ophthalmology, realized that
the excess noise in broadband, multiwavelength light could also serve
a useful purpose and replace all those individual lasers.

"The spectrometer's response to noise can be used to infer the
spectrometer's response to a real signal," Srinivasan said. That's because
the excess noise gives each channel of the spectrum a unique signature.



========================================================================== Faster, more accurate calibration Instead of using many single-wavelength lasers to measure the spectrometer's response at each wavelength, the
new approach uses only the noise fluctuations that are naturally present
in a light source with many wavelengths. In this way, it's possible to
assess the spectrometer's performance in just a few seconds. The team
also showed that they could use a similar approach to cross- calibrate
two different spectrometers.

Kho and Srinivasan used the excess noise method in Optical Coherence
Tomography (OCT), a technique for imaging living eye tissue. By increasing
the resolution of OCT, they were able to discover a new layer in the
mouse retina.

The excess noise technique has similarities to laser speckle, Kho
said. Speckle -- granular patterns formed when lasers are reflected off surfaces -- was originally seen as a nuisance but turns out to be useful
in imaging, by providing additional information such as blood flow.

"Similarly, we found that excess noise can be useful too," he said.

These new approaches for characterization and cross-calibration will
improve the rigor and reproducibility of data in the many fields that
use spectrometers, Srinivasan said, and the insight that excess noise
can be useful could lead to the discovery of other applications.

The work was published Oct. 6 in Light Science & Applications. Additional authors on the paper are Tingwei Zhang, Jun Zhu and Conrad Merkle,
all at the UC Davis Department of Biomedical Engineering. The work was supported by the NIH and the Glaucoma Research Foundation.


========================================================================== Story Source: Materials provided by
University_of_California_-_Davis. Original written by Andy Fell. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Aaron M. Kho, Tingwei Zhang, Jun Zhu, Conrad W. Merkle, Vivek J.

Srinivasan. Incoherent excess noise spectrally encodes broadband
light sources. Light: Science & Applications, 2020; 9 (1) DOI:
10.1038/s41377- 020-00404-6 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201013134259.htm

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