Orbital angular momentum modes emission from a silicon photonic integrated device for km-scale data-carrying fiber transmission

: We experimentally demonstrate orbital angular momentum (OAM) modes emission from a high emission efficiency OAM emitter for 20-Gbit/s quadrature phase-shift keying (QPSK) carrying data transmission in few-mode fiber (FMF). The device is capable of emitting vector optical vortices carrying well-defined OAM efficiently with the efficiency of the device >37%. Seven modes propagate through a 2-km two-mode and a 3.6-km three-mode FMF with measured optical signal-to-noise ratio (OSNR) penalties less than 4 dB at a bit-error rate (BER) of 2 × 10 − 3 . The demonstrations with favorable performance pave the way to incorporate silicon photonic integrated devices as transceivers in an OAM-enabled optical fiber communication link


Introduction
The unabated exponential growth of global internet traffic is driving an ever-increasing demand for higher data transmission capacity and more efficient spectral usage in transmission links [1].To break the coming capacity crunch, a great many of research efforts have been made to investigate different physical properties of a light wave for data transmission, including frequency/wavelength, amplitude, phase, polarization, time.Thus, various advanced multilevel modulation formats and multiplexing techniques, i.e. m-ary quadrature amplitude modulation (m-QAM), orthogonal frequency-division multiplexing (OFDM), wavelength-division multiplexing (WDM), time-division multiplexing (TDM), and polarization-division multiplexing (PDM), have been widely used to increase the transmission capacity [2][3][4][5][6][7].Meanwhile, the multiplexing of multiple independent spatial channels, i.e. space-division multiplexing (SDM), has been proposed as a promising technology to boost attractive increase in transmission capacity by exploring the spatial domain of a light wave [1,8].There are several different types of orthogonal modal basis sets that are potential candidates for such SDM systems.In fiber optical communications, few-mode fiber (FMF) and multi-core fiber (MCF) are well-known promising candidates enabling efficient SDM [9][10][11][12].In addition to FMF and MCF, another SDM focusing on the spatial phase structure of light beams, known as orbital angular moment (OAM) multiplexing, has also shown its potential use in both free-space, underwater and fiber optical communications to improve the transmission capacity .An OAM beam is characterized by a helical phase front of exp( ) ilθ in which l is the topological number and θ refers to the azimuthal angle [38].Owing to the helical phase structure, an OAM beam features a doughnut intensity profile with a phase singularity at the beam center.The unlimited topological charge values of OAM and the inherent orthogonality between different OAM states facilitate an alternative multiplexing technique, i.e.OAM-division multiplexing.
So far, most of OAM transmission experiments rely on complex and bulky optical components, which are slow to respond, and cumbersome to use [17][18][19][20][21][22][23][24][25][26][27][28][29][31][32][33][34][35][36][37].This severely limits the prospect of its widespread use in future practical systems.In this paper, we experimentally demonstrate a FMF link based on a micro-meter-sized highly efficient silicon integrated optical vortex beam emitter.The device is capable of emitting vector optical vortices carrying well-defined OAM efficiently with the efficiency of the device >37% [39,40].Using this device, seven modes, each modulated by 20-Gbit/s quadrature phase-shift keying (QPSK) signal have been successfully transmitted through 2-km two-mode FMF (LP 01 and LP 11 ) and 3.6-km three-mode FMF (LP 01 , LP 11 and LP 21 ), respectively.In the silicon integrated optical vortex beam emitter, a micro-ring resonator with angular grating patterned along the inner wall is coupled to an access waveguide for optical input.Figure 1(a) shows the scanning electron microscopy (SEM) image of the fabricated device (R = 7.5 um).The operation principle of this integrated device is to couple the rotating whispering gallery mode (WGM) in the micro-ring resonator to a vertically propagating cylindrical symmetric vector vortex mode.By matching the wavelength of the light with the micro-ring resonance, and by detuning from the grating Bragg wavelength, this device is capable of emitting a propagating field with desired vortex topological charge.The fabricated high emission efficiency OAM emitter is based on substrate transfer technology, and the sketch of the device is depicted in Fig. 1(b).An aluminum mirror is introduced below the micro-ring resonator by wafer bonding technology to the structure to effectively improve the emission efficiency [39,40].

Concept and principle
For the state of polarization (SOP) of the source, since WGMs and the angular grating structure are both cylindrically symmetric, the radiated beams should maintain this symmetry and should be cylindrical vector (CV) beams.In the devices, for quasi-transverse electric (TE) WGMs, the radiated near field is predominantly azimuthally polarized with its Jones vector E CV written as sin exp( ) cos . The radiated CV vortex beam can be described as the superposition of two orthogonal scalar vortices, as E CV can be further decomposed into ( ) , which consists of a righthand circularly polarized (RHCP) beam with topological charge of 1 l + and a left-hand circularly polarized (LHCP) beam with 1 l − [41][42][43][44].The vector beam is emitted into free space and decomposed by a quarter-wave plate (QWP) and a polarizer.After passing through the QWP, the LHCP beam and RHCP beam are converted to two orthogonal linearly polarization beams.Then the linearly polarized OAM beam we need is picked out by a polarizer.The linearly polarized beam then couples into and propagates through the FMF, followed by the detection and imaging to form a complete FMF transmission link.The FMF designed to support two modes is 2 km while the other one designed to support three modes is 3.6 km.The experimental setup of OAM modes emission from a highly efficient silicon photonic integrated device for transmission in FMF is shown in Fig. 2. At the transmitter side, a 20-Gbit/s QPSK signal is generated by a tunable laser followed by an optical I/Q modulator, which is modulated by an arbitrary waveform generator (AWG).Then the signal is coupled into the input waveguide of the micro-ring resonator.A polarization controller (PC) is used to launch light in the quasi-TE mode in the emitter chip waveguide, and the power is monitored by a power meter placed at the output port of the waveguide.The vortex vector modes with topological charge 1, 1, 2 l = − − are excited when the center wavelength of the tunable laser is 1529.02nm, 1552.32 nm and 1564.02nm, respectively.While the vortex mode with topological charge 0 l = is splitting to two modes (TM 01 and TE 01 ) as the strongest crosscoupling occurs at the wavelength of 1538.9 nm and 1541.96nm respectively due to the second order Bragg reflection.The emitted beam is collimated by a 40X objective lens after emission.Then the collimated circularly polarized beams are converted by a QWP and filtered by a polarizer.The linearly polarized beam is coupled into the FMF by a 10X objective lens.After propagating through the 2 km or 3.6 km FMF, the beam is collimated Remarkably, in the proof-of-concept experiments, the designed and fabricated silicon integrated optical vortex beam emitter generates different OAM modes at different wavelengths.With future improvement, in order to further realize OAM mode multiplexing transmission, different OAM modes at the same wavelength are highly desired.Fortunately, such OAM mode multiplexer can be implemented based on a multimode microring resonator with angular grating [45].Moreover, we can also use different emitters to generate different OAM modes at the same wavelength by thermally tuning their radiation wavelengths [46].

Conclusion
In summary, we experimentally demonstrate a FMF link based on a micro-meter-sized highly efficient silicon integrated optical vortex beam emitter.The device is capable of efficiently emitting vector optical vortices carrying well-defined OAM (efficiency>37%).Using this device, seven modes, each modulated by 20-Gbit/s QPSK signal have been successfully transmitted through 2-km two-mode FMF and 3.6-km three-mode FMF, respectively.The obtained results indicate impressive emission and transmission performance, which opens a door to further employ silicon photonic integrated devices as transceivers in OAM-based optical fiber communication systems.

Fig. 1 .
Fig. 1.(a) Measured SEM image of the fabricated device (R = 7.5 μm ) with an angular grating patterned along the inner wall of a micro-ring resonator.(b) Schematic illustration of the device with an angular grating patterned along the inner wall of a micro ring resonator and an Al mirror layer.(c) Concept of OAM modes emission from the device for transmission in FMF.

Fig. 3 .
Fig. 3. Relative refractive index profiles of the (a) two-mode and (b) three-mode FMF.

Fig. 9 .
Fig. 9. Measured BER versus OSNR of (a) 2 km two-mode FMF link (b) three-mode FMF link based on a micro-meter-sized highly efficient silicon integrated optical vortex beam emitter.