An InP-based monolithically integrated few-mode transmitter aiming at the combination of wavelength division multiplexing (WDM) and mode division multiplexing (MDM) technologies is proposed. The core elements of the proposed transmitter are mode converters and a wavelength-mode division multiplexer that are all based on multimode interference (MMI) couplers. Simulations show that the wavelength-mode division multiplexer has a large fabrication tolerance of 30 and 0.5 μm for the length and the width of the device, respectively. A low loss below 0.26 dB for the passive parts of the transmitter is obtained in the whole C-band wavelength range.OCIS codes: 250.5300, 060.4230, 060.4510. doi: 10.3788/COL201614.080601.Mode-division multiplexing (MDM) technology has emerged as an effective solution to increase the fiber capacity in addition to the commonly used technologies, such as wavelength division multiplexing (WDM) [1][2][3] , optical time division multiplexing (OTDM), polarization division multiplexing (PDM) [4,5] , and advanced modulation formats. In a typical MDM system, mode conversion and multiplexing are indispensable for the transmitters, where the fundamental laser/fiber mode should be converted to high-order modes and multiplexed into a common output. Many structures have been proposed to realize mode conversion and multiplexing, which typically includes fiber-based technology [6] , free-space-opticsbased technology [7] , and integrated-waveguide-based technology. In terms of functionality and mass-production capability, integrated waveguide technologies are promising due to their capability to be integrated with other devices on chips. Typical structures used in integrated waveguide technologies include directional couplers [8,9] , adiabatic couplers [10] , and multimode interference (MMI) couplers [11,12] . Due to its large fabrication tolerance and low loss, the MMI-based mode converter-multiplexer will be a potential candidate for integrated few-mode systems. In addition, as a building block for photonic integrated circuit, MMI has been widely adopted in the generic foundry model [13] , enabling a high possibility of large scale integrated with active components.The principle of MMI is based on the self-imaging effect, which can reproduce single or multiple images of the input optical field profile along the propagation of the light [14] .By choosing a proper length, width, and positions of the input and output arms of the MMI, it is possible to realize mode conversion between the fundamental TE 0 mode and the first-order TE 1 mode [15] . Several groups have reported the design and realization of the MMI-based mode converter/(de)multiplexer on silicon on insulator (SOI) substrate [9,11,16] . Thanks to the large wavelength tolerance of the MMI structure, the mode converter/ (de)multiplexer can work in a wide wavelength range, which also indicates a possibility of WDM-compatible functionality. Considering the integration possibility of laser sources, modulators, mode converters, and wavelengthmode divisi...
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