Tapered waveguide couplers are related to standard fibre couplers (power splitters), with the main difference usually being that an approximately adiabatic taper is introduced into one or both of the waveguides [1-3]. If the cross-sectional variation of the device is sufficiently slow, power transfers between the waveguides provided that the propagation constants of the modes cross somewhere along the taper. The advantages of these devices compared with standard directional couplers include much lower wavelength-dependence and increased dimensional tolerances, although at the expense of increased device footprint.

 

Figure 1: (a) Schematic of a tapered waveguide coupler with counter tapered cores A and B, and (b) the propagation constant crossover. The precise taper shape is immaterial, provided it varies sufficiently slowly [1].

Standard directional couplers on the other hand rely on the interference of normal modes and therefore tend to be very sensitive to the precise parameters, such as wavelength. As suggested the parameter sensitivity of standard directional couplers can however be resolved by introducing approximately adiabatic tapers into one or both of the waveguides. The case of a single-mode tapered waveguide coupler is represented in the schematic of Fig. 1. In this case the waveguide cores are counter tapered. In a tapered coupler the propagation constant difference between the two waveguides and the coupling coefficient, vary along the length of the device.

The principles of tapered waveguide couplers also extend beyond the realms of single-mode waveguides and also apply to few- or multi-mode waveguides. Moreover, the power transfer can be between different modes of non-identical cores. Therefore, provided the adiabatic criterion is satisfied, and a propagation constant crossover point does exist for the relevant modes, tapered waveguide couplers can be used for splitting the modes of a multi-moded fibre. Tapered waveguide couplers are at the heart of Modular Photonics’ LPMUX technology.

References:

  1. N. Riesen and J. D. Love, “Tapered velocity mode-selective couplers,” Journal of Lightwave Technology 31, 2163-2169 (2013).
  2. N. Riesen and J. D. Love, “Ultra-broadband tapered mode-selective couplers for few-mode optical fiber networks,” IEEE Photon. Technol. Lett. 25, 2501-2504 (2013).
  3. S. Gross, N. Riesen, J. D. Love, and M. J. Withford, “Three‐dimensional ultra‐broadband integrated tapered mode multiplexers,” Laser & Photonics Reviews 8, L81-L85 (2014).