Miniature Broadband NIR Spectrometer Based on FR4 Electromagnetic Scanning Micro-Grating

Abstract

This paper presents a miniaturized, broadband near-infrared (NIR) spectrometer with a flame-retardant 4 (FR4)-based scanning micrograte. A 90° off-axis parabolic mirror and a crossed Czerny-Turner structure were used for creating an astigmatism-free optical system design. The optical system of the spectrometer consists of a 90° off-axis parabolic mirror, an FR4-based scanning micrograte, and a two-color indium gallium arsenide (InGaAs) diode with a crossed Czerny-Turner structure optical design. We used a wide exit slit and an off-axis parabolic mirror with a short focal length to improve the signal-to-noise ratio (SNR) of the full spectrum. We enabled a miniaturized design for the spectrometer by utilizing a novel FR4 micrograte for spectral dispersion and spatial scanning. The spectrometer can detect the full near-infrared spectrum while only using a two-color InGaAs diode, and thus, the grating scanning angle of this spectrometer is small when compared to a dual-detector-based spectrometer. In addition, the angle signal can be obtained through an angle sensor, which is integrated into the scanning micrograte. The real-time angle signal is used to form a closed-loop control over the scanning micrograte and calibrate the spectral signal. Finally, a series of tests was performed. The experimental results showed that the spectrometer has a working wavelength range of 800-2500 nm. The resolution is 10 nm at a wavelength range of 800-1650 nm and 15 nm at a wavelength range of 1650-2500 nm. Similarly, the stability of these two wavelength ranges is better than ±1 nm and ±2 nm, respectively. The spectrometer's volume is 80 × 75 × 65 mm³ and its weight is 0.5 kg. The maximum spectral fluctuation does not exceed 1.5% and the signal-to-noise ratio is 284 after only one instance of averaging.

Keywords: flame-retardant 4 (FR4); micro-NIR spectrometer; scanning grating micromirror.

Figure 1

(a) Schematic drawing of the assembled flame-retardant 4 (FR4)-based electromagnetic scanning micrograting; and, (b) a photograph of the prototype of the FR4 scanning micrograting integrated with a driving coil and a pair of sensors.

Figure 2

Drawing of the optical layout and assembly of the spectrometer.

Figure 3

(a) The root mean square (RMS) spot x of the previous optical structure; (b) the RMS spot x of the modified crossed Czerny–Turner optical design.

Figure 4

Two full scans within one mirror oscillation and the beginning of the spectrum of the top InGaAs (i.e., 800–1650 nm) and the end of the spectrum of the bottom InGaAs (i.e., 1650–2500 nm), which both correspond to the position where the angle signal output is 0. Their intersection points correspond to the position where the angle signal output is the largest.

Figure 5

The correlated spectrum stitched from the wavelength of the halogen lamp.

Figure 6

(a) Wavelength range of the prototype; and, (b) An array-detection-based near-infrared spectrometer and the prototype were tested using a mercury lamp experiment and the illustration indicates the highest resolution of the prototype.

Figure 7

Ten measurements of spectral peak fluctuation at 1136.9 nm and 1961.0 nm.