High-resolution Fourier-transform spectrometer chip with microphotonic silicon spiral waveguides

Abstract

We report a stationary Fourier-transform spectrometer chip implemented in silicon microphotonic waveguides. The device comprises an array of 32 Mach-Zehnder interferometers (MZIs) with linearly increasing optical path delays between the MZI arms across the array. The optical delays are achieved by using Si-wire waveguides arranged in tightly coiled spirals with a compact device footprint of 12 mm 2 . Spectral retrieval is demonstrated in a single measurement of the stationary spatial interferogram formed at the output waveguides of the array, with a wavelength resolution of 40 pm within a free spectral range of 0.75 nm. The phase and amplitude errors arising from fabrication imperfections are compensated using a transformation matrix spectral retrieval algorithm. In a typical configuration, a waveguide array of MachZehnder interferometers (MZIs) with increasing path differences are used to implement the SHS concept [9,10]. For such a geometry, the source power spectrum and the output interferogram are related by the cosine FT. A similar MZI array geometry, including phase-correction circuits using independent heaters for each MZI, has also been demonstrated [13]. However, when long optical path delays are required for high spectral resolution, similar configurations yield prohibitively large devices.In this Letter, we present a compact FT spectrometer chip, in which a high spectral resolution of 40 pm with a compact device size is achieved by using tightly coiled spiral waveguide structures in an MZI array. Furthermore, a spectral retrieval algorithm with phase and amplitude error compensations is demonstrated for the first time to the best of our knowledge, obviating the need for dedicated phase correction circuits. The FT spectrometer is implemented as an array of N MZIs in silicon-on-insulator (SOI) waveguides (Fig. 1). Each MZI comprises a reference arm of constant length and a delay arm with a spiral waveguide. The length of the delay arm, i.e., spiral length, linearly increases by ΔL across the array. The high refractive index contrast of the SOI platform and the waveguide bend radius of ∼5 μm readily allows the making of spirals with geometrical lengths of over a centimeter within an area only a few hundred micrometers in diameter.For a given input spectral distribution, the dispersive property of the MZI array results in a wavelengthdependent spatial interferogram at the outputs of the array. The relation between the input spectral distribution and the interferogram Ix i is unambiguous within

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“…The first on-chip SHFTS was proposed in Michelson configuration [10] and was subsequently implemented as an array of MZIs with linearly increasing optical path delays (OPDs) [11][12][13][14][15]. These MZIs constitute a discrete Fourier-transform in which the output of each MZI corresponds to a point in the spatial interferogram.…”

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“…In recent years, there has been a strong effort to develop novel spectrometers that are compact and have high resolving powers and operational bandwidths 5,[12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] . In addition to miniature grating spectrometers 12 , resonant structures such as nanocavities [13][14][15][16][17][18] have also been employed to separate spectral components into unique detectors.…”

“…Recently, a silicon-based spectrometer 23 that measures the multimode transmission profile from a disordered photonic crystal structure achieved a resolving power comparable to typical grating spectrometers, but it requires coupling to a subwavelength input waveguide and has a limited bandwidth from 1,500 to 1,525 nm. A high-resolution Fourier transform spectrometer 24 has also been demonstrated on-chip, but its narrow free spectral range limits its operation bandwidth to 0.75 at 1,550 nm. Although some of these recently developed compact spectrometers can already match the spectral resolution of grating spectrometers, a compact device with both high resolving power and broad operating bandwidth remains elusive.…”

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“…This configuration does not require any moving element, unlike a conventional FT spectrometer, and its resolution is limited by the maximum measured optical path delay, without requiring an interaction between the measured signal and a local oscillator as in coherent spectroscopy [14]. SHFT spectrometers can be implemented as an interferometer array integrated on a photonic chip [15][16][17][18], allowing multiple input apertures for an increased radiant throughput compared to planar waveguide devices with a single input waveguide, such as arrayed waveguide gratings [19]. Furthermore, this allows us to individually characterize the transfer function of each interferometer element of the array, and in principle it enables the computational compensation of deviations from the ideal design that may arise from fabrication limitations or imperfections.…”

“…Furthermore, this allows us to individually characterize the transfer function of each interferometer element of the array, and in principle it enables the computational compensation of deviations from the ideal design that may arise from fabrication limitations or imperfections. However, maximum optical path delays in such integrated spectrometer chips are limited to a few centimeters [15][16][17][18].…”