Based on a Czerny-Turner monochromator modified with a Gradient Index (GRIN) collimator input and microlens focused output, the dispersion related performance is heavily dependent on the microlens capabilities. The GRIN coupled fiber is fabricated in-house with a 50 ��M multimode fiber butt coupled with a 1.8 mm diameter 0.25 GRIN lens (Thorlabs and Newport), epoxy bonded to a glass pipette tube. Its divergence is around 1.8 degrees at 2 cm. As the diffracted rays from the actuated grating traverses the multi-layered packaging, the microlens translates the angularly dispersed wavelengths into focal positions at the exit plane, where a physical slit sits before the detector. Because of this crucial role in angle-to-position translation, the diaphragmed microlens has significant performance impact on the microspectrometer.

Figure 1.Cross-sectional view of the microspectrometer, showing the 5 mm silicon packaging with microlens, the MEMS grating, and other optical components.A number of fabrication techniques have been developed for microlens in imager arrays, waveguide coupling, endoscope microoptics, and biologically inspired structures [8-11]. General classes of passive microlens fabrication include 1) lithograph patterned material reflow, 2) diffractive microlens, and 3) mold formation. The reflow of thick AZ photoresist is a well-explored method for producing spherical profiles of patterned structures [12]. However, reflown resist microlenses are supported on opaque bulk substrates and cannot perform as a suspended lens. Diffractive microlenses produce artifacts and unnecessarily decrease transmittance [13].

Material molding presents the best method to form a suspended microlens for our microspectrometer. We use a modified molding method similar to other recent studies [14] to realize a unique suspended microlens for our system.2.?Diaphragmed Microlens Fabrication and PackagingMicrolensThe microlens was fabricated of polydimethylsiloxane, PDMS via soft-lithography using a molding technique (Figure 2a). First, a pattern of circular disks (1.2 mm diameter) was transferred to a double-layer AZ4620 Entinostat photoresist on silicon substrate in preparation for the designed lens reflow.Figure 2.(a) Process flow of the microlens starts from photoresist reflow and is realized through soft-lithography molding. (b) Photomicrograph of a finished microlens, top down.Second, the double-layer resist was reflown into a semi-spherical profile via contact hotplate heating at 145��C. Third, the lens mount wafer, fabricated earlier, was bonded to the lens substrate via thin coat of AZ5214-E. Next, premixed and degassed (via centrifugation at 3.5 kRPM) PDMS was cured over the wafer complex through gradual curing from 60-90��C for 30 minutes.