The current information capacity of communication systems based on single-mode fibers (SMFs) is approaching its physical limits. To solve this problem, spatial-division multiplexing based on mode-division multiplexing (MDM) has been intensively investigated. Due to its orthogonal characteristics, MDM can help realize more multiplexed channels, and thus the capacity of existing optical fiber communications can be enhanced. Mode converters are critical devices in optical-fiber communication systems and are essential for improving the performance of future MDM systems applicable in long-distance and high-capacity optical-fiber communication. Mode-conversion efficiency is a major index of mode converters. Mode converters based on asymmetric Y junctions on polymer platforms offer the advantages of low cost, high fabrication tolerance, and wide bandwidth. Thus, the design and fabrication of mode converters with compact structures and high mode-conversion efficiencies based on asymmetric Y junctions on polymer platforms are essential to meet the increasing demands in data traffic.

The proposed mode converter consists of two identical inversely connected asymmetric Y junctions. The stem of the Y junction is a straight two-mode waveguide designed to support only the E11i and E21i modes (i=x or y, indicating the polarization direction), which correspond to the LP₀₁ and LP_11a modes (polarized in the i direction) of the optical fiber, respectively. This two-mode core gradually branches into two single-mode cosine S bends with different widths, forming an asymmetric Y junction. Based on the mode evolution principle of the asymmetric Y junction, the parameters of the proposed mode converter based on a cascaded asymmetric Y junction (Fig. 1) are optimized in this study. These parameters include the widths of the core (w₁), arms A and D (w₂), and arms B and C (w₃), as well as the length of the arm (L), width of the Y-junction end (w₄), and distance between the two Y-junction ends (w₅). A three-dimensional finite-difference beam-propagation method (3DFD-BPM) is used to simulate the mode-conversion characteristics of the proposed mode converter. Under these device parameters, the proposed mode converter is fabricated with in-house microfabrication facilities. In addition, an experiment is conducted to characterize the mode-conversion performance of the proposed mode converter.

In the proposed mode converter, the waveguide core height is fixed at 4 μm, and w₁, w₂, and w₃ are set to 9.0, 6.3, and 2.7 μm, respectively. The refractive-index difference between the core and cladding is sufficient to achieve mode conversion. The mode-conversion efficiency between the LP₀₁ and LP_11a modes is optimal when the length of the arm , width of the Y-junction end, and distance between the two Y-junction ends are 1.5 μm, 3.1 μm, and 6.8 μm, respectively. The simulation results show that the mode-conversion efficiencies for the x polarized LP₀₁-LP_11a and LP_11a-LP₀₁ are 99.3% and 99.2%, respectively (Fig. 4). A experiment is conducted to characterize the mode-conversion performance, and the near-field spots detected by the infrared camera indicate that the device can implement mode conversion (Fig. 6). Over a wavelength of 1530?1600 nm, the insertion losses are between ~4.8 dB and ~5.8 dB and between ~3.5 dB and ~5.1 dB for the x and y polarizations, respectively (Fig. 7). To investigate further the mode-conversion and crosstalk characteristics at the asymmetric Y junction of the device, the device is cleaved at the middle position to obtain an asymmetric Y junction. The results show that, under the premise of neglecting the radiation losses of the asymmetric Y junction and propagation losses of the waveguide, the mode-conversion efficiencies are greater than ~98% and ~98.1% for the x and y polarizations over the C+L band, respectively, and the mode crosstalk is less than -17.5 dB (Fig. 9).

We propose and demonstrate a mode converter constructed using two identical asymmetric Y junctions connected inversely. Our proof-of-concept mode converter, designed for the conversion of the LP₀₁ and LP_11a modes and fabricated using an optical polymer material, has a miniature footprint of approximately 1.5 mm×14.0 μm. The results show that over the C+L band and for both polarizations, the mode-conversion efficiencies are greater than ~98%, the crosstalk is less than ~-17.5 dB, and the insertion loss is less than ~5.8 dB. Our proposed mode converter with polymeric materials is easy to fabricate and inexpensive. In particular, the same structure can be implemented with other high refractive index contrast material platforms such as lithium niobite on insulators, silicon nitride, and silicon on insulators to realize more advanced integrated photonic circuits.