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Sensing & Measurement

High-resolution spectroscopy with microparticle arrays

A new spectral device that exploits optical resonances achieves wavelength determination with an accuracy better than 1pm.
19 April 2010, SPIE Newsroom. DOI: 10.1117/2.1201003.002865

Highly accurate characterization of the wavelength of a given light source is very important for many technical purposes. However, most systems commonly used for this are large and very expensive. We have developed a small and less expensive spectral device that exploits the properties of spherical microparticles. Such particles can be fabricated with high accuracy and exhibit very sharp optical resonances that are known as morphology-dependent resonances (MDRs) or whispering-gallery modes. MDRs strongly depend on both particle size and operational wavelength and are, therefore, very suitable for spectral applications.1,2

In our setup (see Figure 1),3 we used a glass prism to arrange an array of spherical poly(methyl methacrylate) microparticles with diameters between 80 and 100μm. The array is illuminated through the side wall of the prism to generate an evanescent coupling. Because of particle-size variations, some particles are in resonance at a given wavelength while others are not, leading to brightness variations. The intensity distribution varies with wavelength in a characteristic way.

Figure 1. Experimental setup. FC: Fiber coupler. OF: Optical fiber. CO: Collimation optics.

Wavelength calibration of the particle array is done by recording a movie with the CCD camera while scanning a tunable diode laser. The laser is scanned at 0.02nm/s and the camera captures images at approximately 80 frames/s, which results in a theoretical resolution of approximately 0.25pm. To keep the amount of data manageable, we only record the particle intensities. We take pictures at different wavelengths to relate frame number with wavelength, which we subsequently compare with the database. The accuracy that can be achieved is limited by that of the wavemeter (approximately 0.2pm). The latter is used to measure the wavelength during the calibration process. Since readout of the wavemeter's signal is too slow to constrain the wavelength directly, we perform calibration in two steps (see Figure 2). Wavelength determination of an unknown light source is done by comparing the resonator picture with the database, for which we use a correlation function that achieves a global minimum at the light source's wavelength.

Figure 2. Calibration. Step 1: A movie is recorded while scanning the calibration laser and storing the intensities of all particles in the database. λ: Wavelength. a.u.: Arbitrary units. Step 2: Relating frame number to the wavelength measured by the wavemeter (λWM).

The accuracy of our wavelength determination depends sensitively on the linewidth of the light source. We investigated its effect on the performance of the resonator array. Since we cannot change the linewidth of the light source experimentally, we numerically generated pictures of a light source by adding data sets from the diode laser at slightly different wavelengths (weighted using a Gaussian function). Figure 3 shows results for different linewidths. (The central wavelength is 681.5nm.) The wavelength can be determined correctly for relatively broad sources up to 0.2nm width. (The maximum acceptable linewidth is related to the number and size distribution of spheres in the array.)

Figure 3. Correlation function (r) for different linewidths (Δλ: Full width at half maximum).

In summary, we developed a new spectral device based on optical resonances with a accuracy of better than 1pm. We showed that the wavelength of relatively broad-line sources can be determined accurately. The scanning range of the calibration laser limits us to a small wavelength region of 10nm width. We will next extend the usable range by employing a tunable dye laser.

This work was supported by the German Research Council (DFG, grant Schw184/48-1).

Thomas Weigel, Ralf Nett, Gustav Schweiger, Andreas Ostendorf
Department of Laser Application Technology and Measurement Systems
Ruhr University Bochum (RUB)
Bochum, Germany

Thomas Weigel studied physics in Bochum. He joined the RUB in 1997 and obtained his doctoral degree in 2004. His main research focus is on optical simulations and optical resonances.

Ralf Nett studied mechanical engineering in Duisburg (Germany). He joined the RUB in 1992.

Gustav Schweiger studied applied physics in Graz and Vienna (Austria), and started his professional career in aerospace sciences in Cologne (Germany). He joined the University Duisburg-Essen in 1974, and he became professor and department chair in 1991. He has (co-)authored more than 75 peer-reviewed papers. He is also co-author of a book on airborne microparticles and has contributed chapters to several other scientific books. Since 2008 he is professor emeritus.

Andreas Ostendorf holds the Chair of Laser Applications Technology. He has (co-)authored more than 40 peer-reviewed papers and contributed to five book chapters. He has chaired several international laser conferences, ranging from nanophotonics to laser-materials processing. His main research focus is on the interaction of pulsed laser radiation with biological and technical matter. He is a Fellow of SPIE and of the Laser Institute of America (LIA) and a member of the German Academic Society of Laser Technology (WLT). In 2008, he was president of the LIA.