Multi-order concept improves microspectrometer resolution

The most challenging aspect of conceiving a compact, optical spectrometer is to simultaneously optimize the competing demands of high spectral resolution, large spectral bandwidth, and volume miniaturization. Generally, optimization of one property will negatively impact on the others. The external conditions for production require robust, very compact measuring equipment. “Look, don't touch!” is an apt description of the main principle behind in- and at-line process inspection for noncontact and nondestructive measuring methods. Spectral sensors are ideal for this purpose and offer a tremendous range of applications in production technologies, including in pharmacy, biotechnology, chemistry, and the glass or solar industries.

In recent years, different setups of micro- and miniature spectrometers have been introduced^1–4 that offer small volume but otherwise exhibit performance limitations in resolution, spectral bandwidth, and numerical aperture. They are sometimes relatively complex and difficult to manufacture in volume.

We have been working on a new approach to designing an imaging microspectrometer⁵ based on a multi-order concept that offers a spectral resolution better than 2.5nm, access to a large wavelength range from the blue visible to the near-IR regime (400–1030nm), and a compact optical volume of 11×6×5mm³. The multi-order approach does not record the total spectral bandwidth in a single step. Instead, recording is done by subsequent focusing of adjacent partial spectral intervals in different diffraction orders into the detector line array's image plane (see Figure 1). When increasing the diffraction order, the lateral position of each individual interval is shifted with respect to the detector area and spectral stretching of the intervals is induced. To allow detection of a gapless spectrum over the multiple diffraction orders, we must maintain continuity at the transition from one diffraction order to the next.

Figure 1. Schematic of the multi-order concept. The concave grating disperses and images the light coupled into the spectrometer's entrance slit. A different wavelength (λ) range is focused on the detector, depending on the specific diffraction order (shown are the 10, 7, and 5 orders). The lower inset shows the magnified wavelength distribution for different detector diffraction orders.

The most essential optical component in the setup is the imaging grating with a 5mm diameter and an image distance (lens focal length) of f = 8.6mm. For fabrication, we chose a holographic recording process. Compared to other manufacturing technologies, holography allows us to generate optical elements with high diffraction efficiencies and spatial frequency over an extended field in a single exposure step, both on planar and curved substrates.

Generally, holographic recording of an imaging diffraction grating requires two coherent point sources with nearly diffraction-limited quality. Both point sources create a fringe pattern that is captured as a latent image in a photosensitive resist material covering the substrate's concave surface. The grating's surface profile is formed in a subsequent development process. For our new multi-order microspectrometer, the fundamental challenge is the need to provide two diffraction-limited point sources in close proximity (distance <1mm). Because of the limited available space to adjust the recording optics, we used an alternative lithography concept based on introducing a supplementary hologram. The latter allows neutralization of optical aberrations and is generated in a setup reversed to the final recording geometry. In the end configuration, the supplementary hologram provides two diffraction-limited point sources at the exact predetermined positions.

To demonstrate the broad wavelength capability of the multi-order microspectrometer, we investigated the transmission of an optical color glass filter. We compared the results to data measured with a commercially available reference spectrometer⁶ that offers a limited resolution of 8nm. The upper curve in Figure 2 shows the entire spectral bandwidth from the deep blue visible to the near-IR range. A more detailed view (see Figure 2, lower diagrams) shows enlarged measurements of the multi-order microspectrometer and reference instrument in selected spectral intervals. For the microspectrometer, different diffraction orders are involved in the individual intervals. Each interval shows a distinct similarity in the overall characteristics between both curves. Particularly, in the intervals from 460 to 485nm and from 505 to 550nm, the improved resolution of the microspectrometer is evident compared to the reference instrument.

Figure 2. Broadband overview (upper curve) and detailed comparison of the performance of our multi-order microspectrometer to that of a commercially available reference instrument (Carl Zeiss MicroImaging Spectrometermodul MCS). The microspectrometer shows a broadband capability with an increased spectral resolution. More details in the lower spectral curves show different diffraction orders in the blue (minisp_blue) and green (minisp_green) ranges.

To make a microspectrometer available for in- and at-line process inspection, the multi-order concept seems very promising. Our results show a broad wavelength capability combined with high spectral resolution. Our next steps will be to prepare an appropriate manufacturing technology and incorporate the optical setup into a handheld instrument.