An optical chip that spatially separates wavelengths

A conventional planar grating spectrometer consists of a sophisticated arrangement of optical components and is rather large. To reduce the size, various micro-electro-mechanical system (MEMS) fabrication techniques have been proposed, yielding MEMS-based spectrometers that use moving parts for wavelength separation. Unfortunately, these moving parts are always associated with mechanical instability problems.

In our work, we use a concave micro-grating embedded inside a slab waveguide without any moving parts, allowing us to produce a real chip-based spectrometer. Our design has two advantages: first, the size of the spectrometer is dramatically reduced; and second, the mechanical instability resulting from moving parts is effectively removed.

The micro-grating has a circular cylindrical surface profile in a Rowland circle configuration that allows it to be used as a spectrometer or as a wavelength dispersion device.^1–4 In the wavelength dispersion plane of such a grating, the divergent rays of different wavelengths coming from the entrance slit are spatially separated and focused on different locations of the Rowland circle. This is illustrated in Figure 1, which depicts the working principle of a reflective circular cylindrical grating.

Figure 1. Working principle of the Rowland circle configuration for a reflective circular cylindrical grating. The light source is located at point A. A beam of specific wavelength focuses at a point B_o, the Gaussian image point. Beams of different wavelengths focus at different locations on the Rowland circle.

In our design, the grating is blazed to increase the diffraction efficiency of the specified diffraction order.^5,6 Our chip-spectrometer covers wavelengths from 1475 nm to 1625 nm and calls for a spectral resolution of 0.9nm when using a 10μm−entrance slit.

The benefit of using lithography to fabricate blazed gratings is obvious: it offers the freedom to produce a grating with any specified profile and blaze angle. In our work, we use deep x-ray lithography (DXL) to fabricate the concave micro-grating, a technique capable of producing a high aspect ratio, high spatial resolution and excellent lithographic quality for optical applications.⁷

For our design, a total of 2584 grating lines were generated. The grating height is 125μm and the grating pitch is 3μm. Figures 2 and 3 show scanning-electron-microscope images of the concave micro-grating fabricated using our DXL technology. The surface roughness of the side wall measured by atomic force microscopy is 5nm, which far exceeds the standard for optical applications. We also measured the spectral resolution of the grating, which yielded a value of 1.1nm, in very good agreement with the design value of 0.9nm.

Figure 2. Scanning-electron-microscope image of the concave micro-grating structure. The grating height was 125μm. A total of 2584 grating lines are generated by the DXL process.

Figure 3. Scanning-electron-microscope image of the top view of the concave micro-grating. The grating pitch is 3μm.

Our work demonstrates the successful implementation of a spectrometer-chip scheme based on a micro-grating embedded in a slab waveguide. It also shows that DXL technology is powerful enough to fabricate concave micro-gratings. Subsequent aberration calculations and ray tracing measurements have also confirmed that the chip-based spectrometer functions as designed. The incorporation of these advances in fabrication approaches will undoubtedly produce low cost polymer-based spectrometer chips with good spectral resolution.