# Polymer/Silica Hybrid Waveguide Thermo-Optic VOA Covering O-Band
## 3 Fabrication and Characterizing  
To fabricate the proposed VOA, a 15-\(\upmu\)m-thick SiO\({}_{2}\) buffer layer was thermally oxidized on a Si substrate. Then, the negative photoresist SU-8 2002 was spin-coated as the core layer on the silica buffer layer, at a rotational speed of 600 r/min, and pre-baked with two steps, including 10 min baking at 60 \({}^{\circ}\)C and 20 min at 90 \({}^{\circ}\)C. Then, the coated chip was exposed to ultraviolet (UV) light from a mercury discharge lamp (peak emission wavelength, 365 nm; irradiance, 23 mW/cm\({}^{2}\), ABM-USA, Inc., San Jose, CA, USA) for 4 s and post-baked with two steps, including 10 min baking at 65 \({}^{\circ}\)C and 20 min at 95 \({}^{\circ}\)C. Time of exposure should be adjusted for a good pattern-transfer, from the photomask to the SU-8 thin film, according to the energy density. After that, we removed the unexposed SU-8 polymer and hard-baked it at 120 \({}^{\circ}\)C. Finally, a 7-\(\upmu\)m-thick PMMA upper cladding was spin-coated on the waveguide cores and baked at 125 \({}^{\circ}\)C for 2.5 h. A 500-nm-thick aluminum film was thermally evaporated over the upper-cladding. A 20-\(\upmu\)m-width and 2000-\(\upmu\)m-length microheater was formed, above the modulation arm, via another UV photolithography and wet etching process. The fabricated VOA is shown in Figure 3a. To characterize the fabricated VOA, the light from the tunable laser source (TSL-550, Santec, Japan) is launched into the device by the edge coupling system with single mode fiber (SMF). The polarization of light output from the tunable laser is adjusted by the polarization controller. The output power is monitored by an optical power meter (MPM200, Santec, Japan). The switch is tuned by applying electric power using source meter instruments (Keithley 2450, Tektronix, Beaverton, OR, USA). When applying the driving voltage on the microheater, the current and the transmission at 1310 nm are measured, as shown in Figure 3b,c. After a linear fitting of the current-voltage (I-V) curve, the microheater shows 1.745 \(\Omega\). An extinction ratio (ER) of 18.64 dB is measured while applying 8.72 mW power to the microheater. Then, we measure the normalized transmission spectra of the VOA,  
Figure 2: **(a)** The schematic of the VOA; **(b)** Calculated transmission versus modulation arm temperature change; **(c)** Calculated spectra of the VOA, while the temperature change of the modulation arm is 0 K and 1.8 K.  
under 0.00 mW and 8.72 mW, as shown in Figure 3d. The fiber-to-fiber loss of 12.43 dB includes the propagation loss and coupling loss. The propagation loss mainly results from the material absorption in the near-infrared wavelengths region and surface scattering. The coupling loss primarily comes from the surface roughness of the end facet and the fiber-waveguide mode field mismatch. The attenuation is larger than 6.99 dB over the O-band, which shows a significant attenuation over the O-band. The maximum attenuation is 23.65 dB at 1279 nm. However, the attenuation is not as significant as the simulation. The reason for this is the deviation of the fabrication process and the loss from the waveguide deteriorating the performance of the VOA. To measure the dynamic characteristics, a 1 kHz square wave signal was applied to the microheater, with two probes by a signal generator (SDG6032X-E, Siglent, China), and the output power was coupled into a high-speed photodetector. The driving voltage of the switch and the detected optical response were simultaneously observed on an oscilloscope (DS4024, RIGOL, China). Figure 4 shows the image of the oscilloscope with both the input driving signal (red) and the output optical signal (blue). The 10-90% rise and 90-10% fall time of the modulator are 184 us and 180 us, respectively. Figure 4: Time response of the broadband VOA.