Ultra-broadband mode converters based on length-apodized long-period waveguide gratings

Abstract

We propose an ultra-broadband mode converter based on the structure of a length-apodized long-period grating, where π-phase shifts are introduced at strategic locations of the grating profile. Using a 3-section length-apodized grating structure, we design and fabricate an LP-LP and an LP-LP mode converter with a sidewall grating and a surface grating formed along a polymer channel waveguide, respectively. The fabricated LP-LP and LP-LP mode converters provide a conversion efficiency higher than 99% over a bandwidth of ~120 nm and ~150 nm, respectively, or a conversion efficiency higher than 90% over a bandwidth of ~180 nm and ~300 nm, respectively. The transmission characteristics of these devices are weakly sensitive to polarization and temperature variations. These mode converters can find applications in ultra-broadband mode-division-multiplexing transmission systems based on few-mode fibers and the design principle can be applied to general grating-based mode-coupling devices for a wide range of applications.

Three-dimensional long-period waveguide gratings for mode-division-multiplexing applications

“…The proposed 3D LPWG structure together with the fabrication process greatly expands the range of devices that can be formed with waveguide gratings, especially for MDM applications. Previously reported techniques for increasing the bandwidths of LPWGs, such as length-apodization [16] and cladding-profile control [28], are applicable to the proposed 3D LPWG structure.…”

“…Such a cladding-mode LPWG has been explored as a basic structure for the realization of a wide range of wavelengthselective devices, which include band-rejection filters [3-6], bandpass filters [7], wavelength add-drop multiplexers [8], thermo-optic [9, 10] and electro-optic [11,12] tunable filters, etc. In recent years, the LPWG structure has been applied to the construction of mode converters [13][14][15][16], where the grating is designed to enable coupling between the fundamental mode and a higher-order mode of a few-mode waveguide. Mode converters are important devices for the development of the mode-division-multiplexing (MDM) technology, where different spatial modes of a few-mode fiber carry different signal channels.…”

“…MDM is a promising technology to increase the transmission capacity of fiber communication systems [17,18] and can also be applied to optical sensing systems to improve measurement accuracy and facilitate multi-parameter measurements [19]. While LPWGs are usually conceived as strongly wavelength-sensitive devices, ultra-broadband grating-based mode converters with bandwidths well over 100 nm have been demonstrated recently by using special grating profiles [16]. Such mode converters are suitable for application in MDM systems that transmit wavelength-division-multiplexed signals.…”

“…A conventional LPWG is a two-dimensional (2D) structure, as the corrugations extend over the entire width (for a surface grating) or height (for a sidewall grating) of the waveguide core. Because of that, mode converters based on 2D gratings only work for modes with certain symmetry properties [14][15][16]. An approach to realizing more advanced mode-conversion functions is to cascade carefully matched surface and sidewall gratings [15], but the use of multiple gratings adds significant complexity to the design and the fabrication of the device, not to speak of increasing the total length and the propagation loss of the device.…”

Novel Insights into Orbital Angular Momentum Beams: From Fundamentals, Devices to Applications

“…However, LPFGs also have many disadvantages compared with the mode selective coupler and the photonic lantern, for example, the narrow bandwidth and one LPFG can only convert the fundamental mode (the zeroth order mode) to a phase-matching higher order mode. Thus, there have been many attempts to increase the bandwidth of an LPFG, such as chirped LPFG [69], length-apodized LPFG [70], the cascading of several LPFGs [43], operating an LPFG at its turning point along its phase-matching curve [71], shortening the length of the LPFG, namely decreasing the number of grooves [72], and so on. However, chirping can significantly decrease the mode conversion efficiency and the bandwidth increase is still limited.…”