###### Abstract  
A microring resonator with the multimode interference coupler is fabricated on the polymer platform by using UV-based soft nanoimprint technique. A unique class of fluorinated polymer, perfluoropolyether (PFPE), is employed for the fabrication of the flexible soft mold. By optimizing the proportion between Ormocore and the thinner maT, the microring resonator is fabricated almost without residual layer. The fabricated device with a Q-factor up to \(2.3\times 10^{4}\) is demonstrated for very-near-infrared wavelengths, which shows high potential for sensing applications.  
microring resonator; polymer waveguide; multimode interference coupler (MMI); UV-based soft nanoimprint 2019/09/2019/11  
chemical synthesis [19; 20]. As for the fabrication, several simple and productive processes have been proposed, such as UV lithography [21], direct laser writing [22], and nanoimprint lithography (NIL) [23]. Among them, NIL has drawn much attention as it can achieve high resolution and high yield at the same time with simple processes. By using a mechanical imprinting process, one can avoid the diffraction limit which is inevitable in the UV lithography and reach a high resolution. Since it is based on the replication technology, the quality of the master mold is very important. The traditional NIL utilizes a hard mold which is fabricated on the silicon or metal platform. However, after several imprints, the hard mold usually gets damaged and contaminated, which influences its reutilization. In order to solve this problem, UV-based soft nanoimprint lithography (UV Soft NIL) is proposed [24]. The soft mold which is replicated from the hard mold is introduced to do the imprinting. As the soft mold normally has low surface energy, the replication process will not bring damage to the hard mold and the demolding process can also be much easier. On the other hand, as UV curing is chosen, high pressure and temperature are no longer needed. Polydimethylsiloxane (PDMS) has demonstrated its advantages as soft mold in the UV Soft NIL due to its advantages such as flexible backbone structure, high degree of toughness, and large elongation. However, in recent years, a new material, Polyterafluoroethylene (PFPE) is proposed as soft mold [25]. It has lower surface energy and ideal elastic modulus which makes it a better option for the soft mold. The resolution of the UV Soft NIL can reach 20 nm, and it also can achieve a high yield by using the roll-to-roll process [26; 27].  
In this work, a microring resonator with MMI coupler was demonstrated on the low-cost polymer platform. OrmoCore (Microresist Technology), which has a low loss at short wavelengths was utilized. The very-near-infrared region (around 890 nm) was chosen as the working wavelengths, since the absorption loss of water in this wavelength region is much lower than at the optical fiber communication wavelengths. In addition, the light sources and detectors are also cheaper. The microring resonator was fabricated by Soft UV NIL which is a cost-effective and high-throughput technique. PFPE with very low surface energy was used to fabricate the soft mold. A 3 dB MMI coupler was designed to couple the light between the straight waveguide and the microring. In order to reduce the thickness of the residual layer to a desired level, the volume ratio of OrmoCore and the thinner maT was optimized. The fabricated device with a Q-factor up to \(2.3\times 10^{4}\) showed high potential for sensing applications.