New “optical molecule” on-chip spectrometer

Spectrometers are used to decompose and measure spectral information of electromagnetic waves and are widely used in materials analysis, astronomical observation, and biomedical imaging.

Traditional benchtop spectrometers are based on spatially dispersive elements such as prisms or gratings, resulting in large structures and sensitivity to mechanical vibrations, which are typically only available in laboratory environments.

The new on-chip spectrometer promises to overcome these drawbacks. These spectrometers are based on integrated photonic circuits, in which the various types of optics consist of solid-state planar waveguides, so they can be intensively integrated at the chip scale and can eliminate the effects of environmental disturbances. On-chip spectrometer has application value in smart healthcare, geological exploration and Lab-on-a-chip and other fields, especially for the realization of miniaturized, portable, and even wearable intelligent sensing devices.

However, most of the reported on-chip spectrometers have a common drawback of resolution-bandwidth limitations. Specifically, for on-chip spectrometers, achieving higher resolution requires a long waveguide path, which tends to reduce the free spectral range of the output response, which in turn affects the operating bandwidth. Although the free spectral range can be extended to a certain extent by using special structures such as photonic crystal microcavities, such structures are difficult to process and the tuning efficiency is low. There is currently no universal solution to overcome this limitation.

In view of this, recently, the research group of Tsang Hanqi of the Department of Electronic Engineering of the Chinese University of Hong Kong has realized a new type of on-chip spectrometer with both high resolution and large bandwidth by using a novel “optical molecule” structure combined with computational reconstruction methods.

The results were published in Light: Science & Applications under the title “Breaking the resolution-bandwidth limit of chip-scale spectrometry by harnessing a dispersion-engineered photonic molecule.” Hongnan Xu is the first author and corresponding author of this article, and Hon Ki Tsang is the co-corresponding author of this article.

The basic composition of this structure is a pair of identical tunable microring resonators (Figure 1a).

During thermo-optical tuning, the input spectrum is filtered and sampled, which in turn generates a signal containing spectral information at the output port, and finally the input spectrum is restored by computational reconstruction (Figure 1b).

Figure 1.” How the optical molecule spectrometer works

In this process, the core problem that needs to be solved is how to resolve wavelength channels that are an integer multiple of the free spectral range. For single resonators, only one resonant mode is contained within each free spectral range, so broadband spectral reconstruction is not possible. When a pair of resonators is strongly coupled, the individual resonant modes will split into a symmetrical mode and an antisymmetric mode (Figure 1c). This phenomenon is similar to the energy level cleavage present in diatomic molecules.

It is worth noting that the cleavage strength of the resonant mode is proportional to the coupling strength between the resonant cavities. Therefore, by enhancing the dispersion of the coupling intensity, the cleavage intensity of the “optical molecule” spectral line varies with the wavelength, and based on this feature, the wavelength channels located in different free spectral ranges can be identified. Specifically, when thermo-optical tuning passes through a free spectral range, the output signal corresponding to each wavelength channel contains a pair of spikes; In this case, even for wavelength channels separated by integer multiples of the free spectral range, the spacing between spikes is still different, so the different wavelength channels can be decorrelated (Figure 1d).

Summary and outlook

In this work, the authors experimentally confirmed a spectral line resolution of 40 pm with a working bandwidth of 100 nm. At the same time, the test spectrum is generated by using a monolithic integrated filter, and the high-precision reconstruction of various characteristic spectra is verified by experiments.

The innovations and highlights of this work can be summarized as:

1. The authors propose an on-chip spectrometer that is completely different from traditional schemes. Unlike tunable filter schemes, this design is not limited by the free spectral range, allowing high resolution while greatly extending the operating bandwidth. Different from the computational “spot” spectrometer, this design does not depend on complex topologies, and has the advantages of simple structure and compact size.

2. The design idea is extensible. Under certain conditions, the number of free spectral ranges to be resolved can be further increased, further expanding the operating bandwidth and channel capacity while ensuring low power consumption.

3. The concept involved in this work stems from an extremely common phenomenon in high-quality microcavities – pattern cleavage. At the same time, the structure is based entirely on the microring resonator, a unit device that is very common in integrated photonic circuits. This makes this solution easy to process and highly versatile.

This work provides a new idea for the development of new on-chip spectrometers, and at the same time has enlightening research directions such as computational spectroscopy, and may be used in monolithically integrated spectral sensing systems.

Related paper information:

(Source: LightScience Applications WeChat public account)
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