Abstract
Optical spectroscopy is a fundamental tool in numerous areas of science and technology. Much effort has focused on miniaturizing spectrometers, but thus far at the cost of spectral resolution and broad operating range. Here we describe a compact spectrometer that achieves both high spectral resolution and broad bandwidth. The device relies on imaging multimode interference from leaky modes along a multimode tapered optical fibre, resulting in spectrally distinguishable spatial patterns over a wide range of wavelengths from 500 to 1,600 nm. This tapered fibre multimode interference spectrometer achieves a spectral resolution down to 40 pm in the visible spectrum and 10 pm in the near-infrared spectrum (corresponding to resolving powers of 10(4)-10(5)). Multimode interference spectroscopy is suitable in a variety of device geometries, including planar waveguides in a broad range of transparent materials.
Highlights
Optical spectroscopy is a fundamental tool in numerous areas of science and technology
Imaging of the far-field interference pattern I(x,y,z,o) 1⁄4 |E(x,y,z,o,t)|2 and subsequent basis mapping enables resolving power in excess of R 1⁄4 105 and broad bandwidth BB1. This tapered fibre multimode interference (MMI) (TFMMI) imaging approach enables unique performance compared with other spectroscopy tools (Fig. 1)
To verify that the images are distinguishable, we evaluated their cross-correlation by computing every permutation of the inner product between two different wavelengths vector ui and uj: for an ideal TFMMI of long length, ui Á ui 1⁄4 1 and ui Á uj 1⁄4 0 for all iaj after normalization
Summary
A non-adiabatic taper angle causes light at frequency o to couple to a large number of spatial modes with different propagation constants, resulting in a unique interference profile that can be imaged as a ‘fingerprint’, or basis state, of a given frequency o. Imaging of the far-field interference pattern I(x,y,z,o) 1⁄4 |E(x,y,z,o,t)|2 and subsequent basis mapping enables resolving power in excess of R 1⁄4 105 and broad bandwidth BB1. This tapered fibre MMI (TFMMI) imaging approach enables unique performance compared with other spectroscopy tools (Fig. 1). We coupled light into the TFMMI using a butt-coupled single-mode fibre with infibre polarizer (OZ Optics) This coupling was long-term stable and robust against coupling misalignment, mechanical vibrations and polarization drifts, as will be described in the section below. The images associated with each wavelength formed the column vectors in our calibration matrix that converts between spatial patterns and input frequencies
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