SiON based high-resolution spectrometer-on-a-chip

The incident light is coupled into the input waveguide by the lens fiber, and then diverges into the slab waveguide region. After reflected by the echelle grating teeth facets, the different wavelength components of the incident light converge to the different output waveguides.

The researchers, led by Prof. Jianjun He, from Zhejiang University in China, chose SiON as waveguide core layer material to obtain high index contrast, and combined the high performance spectral analysis function of echelle diffraction grating, resulting in an integrating spectrometer working around 850 nm with a resolution of 0.25 nm and a size of 9 mm by 6 mm. It is published in Chinese Optics Letters, Volume 11, No. 3, 2013 (/col/abstract.cfm?uri=col-11-3-032501)

Lab-on-a-chip is a hot topic in current scientific community. The objective of lab-on-a-chip is to accomplish the preparation, reaction, detection, and analysis of samples in a traditional laboratory on a chip, thus making the above mentioned series of operations chip-based, integrated, automated, low-cost and high efficient. As a key functional element for sample detection and analysis, spectrometer-on-a-chip will strengthen the ability of lab-on-a-chip to a considerable extent. In response to this demand, the researchers have fabricated a high-resolution spectrometer-on-a-chip on silicon-oxynitride (SiON) waveguides based on echelle diffraction grating, working around 850 nm.

They chose 3-stigmatic point method to design the basic structure of the grating to satisfy the device requirement for dispersion, resolution and phase error. The 3-stigmatic point method means that there are three zero-phase-error points along the output facet, which reduces the overall phase error compared to the traditional 1- or 2-stigmatic point methods. Besides, the researchers set up a scalar diffraction model to simulate the light propagation in the grating. Based on the simulation results, they optimized the grating structure and thus improve the spectral analysis performance. For the experimental fabrication, two challenges exist. The first one is to solve the problem about SiON core waveguide film deposition by plasma enhanced chemical vapor deposition (PECVD), concerning the issue of control of refractive index, thickness and uniformity. The second one is to optimize the deep etching process for SiON core layer and SiO2 cladding layer so as to obtain vertical and smooth sidewall, hence reducing the device insertion loss and crosstalk, and enhancing the resolution as well.

"As the signal processing is mainly achieved by electron, to make our on-chip spectrometer more robust," Prof. Jianjun He adds, "We will try to integrate photodetector onto our chip and convert the optical signal into electrical signals getting closer to fully functional lab-on-a-chip."

氮氧化硅基高分辨率芯片光谱仪

说明：通过透镜光纤耦合到输入波导中的入射光在平板波导区域中发散后，被衍射光栅齿面反射，不同波长分量的入射光汇聚到不同的输出波导中。

浙江大学何建军教授课题组利用SiON作为波导芯层从而获得高折射率差，并结合刻蚀衍射光栅优秀的光谱分析功能，在9 mm×6 mm大小的芯片上首次制成了工作于850nm附近波段的高达0.25nm波长分辨率的集成光谱仪。该研究成果将发表在Chinese Optics Letters 2013年第3期上(/col/abstract.cfm?uri=col-11-3-032501)。

芯片实验室(Lab-on-a-chip)是当前科学界的一个热门研究领域。其目标在于在一个芯片上完成传统实验室所进行的样品制备、反应、检测、分析等操作，从而实现这些操作的芯片化、集成化、自动化、低成本化和高效化。光谱仪，作为样品检测和分析的重要功能元件，若能将它芯片化将会大大增强芯片实验室的功能。针对这一前景，研究人员利用氮氧化硅波导平台，基于刻蚀衍射光栅，制作了工作于850nm附近波段的小尺寸高分辨率芯片光谱仪。

为了达到器件对于色散、分辨率、像差等性能参数的基本要求，研究人员使用三点法对光栅进行结构设计。三点法表示光栅的输出面上拥有三个零像差点，与传统的一点法和两点法相比，器件的整体像差表现得到了优化。同时，他们还建立了标量衍射模型对光场在光栅中的传播进行仿真，从而可以根据仿真结果对光栅结构进行优化，进而提高光谱分析性能。在实验制作上，首先需要解决利用等离子化学气相沉积方法进行氮氧化硅波导芯层薄膜生长的相关问题，具体涉及到折射率、厚度、均匀性等薄膜参数的控制。其次，需要优化氮氧化硅波导芯层和二氧化硅波导包层的深刻蚀工艺，获得垂直、光滑的侧壁，从而可以降低器件损耗和串扰，并提高分辨率。

当前，信号的处理工作主要还是通过电子来完成。因此，为了使芯片光谱仪更加实用，研究人员表示，下一步需要将光电探测器集成到光谱仪芯片上，将输出信号由光信号变为电信号，使得芯片实验室的实现变得更加触手可及。