Spatial modes have received substantial attention over the last decades and are used in optical communication applications. In fiber-optic communications, the employed linearly polarized modes and phase vortex modes carrying orbital angular momentum can be synthesized by fiber vector eigenmodes. To improve the transmission capacity and miniaturize the communication system, straightforward fiber vector eigenmode multiplexing and generation of fiber-eigenmode-like polarization vortices (vector vortex modes) using photonic integrated devices are of substantial interest. Here, we propose and demonstrate direct fiber vector eigenmode multiplexing transmission seeded by integrated optical vortex emitters. By exploiting vector vortex modes (radially and azimuthally polarized beams) generated from silicon microring resonators etched with angular gratings, we report data-carrying fiber vector eigenmode multiplexing transmission through a 2-km large-core fiber, showing low-level mode crosstalk and favorable link performance. These demonstrations may open up added capacity scaling opportunities by directly accessing multiple vector eigenmodes in the fiber and provide compact solutions to replace bulky diffractive optical elements for generating various optical vector beams.
Optical vortices have attracted increasing interest since the early description as the basic properties of ‘dislocations in wave trains’1. The following research into optical vortices became the core of what is now known as ‘singular optics’2. Optical vortices feature doughnut intensity profiles possessing phase or polarization singularities, commonly referred to as phase vortices or polarization vortices. An optical phase vortex has a helical phase front that carries an orbital angular momentum (OAM)3. An optical polarization vortex, which is a type of vector beam, has spatially variant polarizations and resultant undetermined polarization and null intensity at the beam center4. These characteristics are different from those of traditional plane waves with spatially uniform phases and polarizations. In the past decades, optical vortices have been extensively studied in a variety of fields such as optical manipulation, trapping, tweezers, microscopy, imaging, material processing, astronomy, and quantum processing3, 4, 5, 6, 7, 8, 9, 10, 11, 12. Beyond these diverse developments, optical vortices, as one type of spatial mode accessing the spatial domain of light waves, have recently been exploited in free-space and fiber optical communications13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, either by encoding M-ary information as high-dimensional vortex states for modulation13, 16, 17, 23, 25, 26 or by employing optical vortices as information carriers for multiplexing14, 15, 18, 19, 20, 21, 22, 24, 25, 26. The latter approach using OAM mode multiplexing for improved transmission capacity is analogous to the well-known space-division multiplexing in fiber optical communications based on linearly polarized (LP) modes with homogeneous polarizations27. Remarkably, OAM modes (optical phase vortices) and LP modes can be synthesized using different linear combinations of fiber vector eigenmodes. Actually, somehow, the fiber vector eigenmodes manifest optical polarization vortices. For example, the transverse magnetic (TM01) mode in a fiber with no magnetic field in the direction of propagation is a radially polarized mode, while the transverse electric (TE01) mode in a fiber with no electric field in the direction of propagation is an azimuthally polarized mode. Both the radially polarized mode and azimuthally polarized mode are optical polarization vortices with polarization singularity at the beam center. Accordingly, it will be interesting and worthwhile to exploit optical polarization vortices and directly use fiber vector eigenmode multiplexing for data transmission.
Various techniques for the generation of optical vortices have been demonstrated, including laser cavities, mode converters, spatial light modulators, spiral phase plates, q-plates, fibers, and metamaterials4, 5, 19, 24, 25, 26. Perhaps the most convenient way to generate optical vortices is to use the commercially available spatial light modulators28, which, however, are relatively bulky and expensive despite the impressive performance. Recently, integrated OAM-carrying optical vortex emitters have been reported29, 30. Photonic integration is clearly the trend and the key enabler toward compact, reliable and low-cost optical devices that are required in fiber-optic communications. In this scenario, a laudable goal is to employ photonic integrated devices to generate optical polarization vortices (vector vortex modes) for the excitation and multiplexing transmission of fiber vector eigenmodes.
In this article, we propose and demonstrate the multiplexing and kilometer-scale data transmission of two fiber vector eigenmodes (TM01 and TE01) generated via integrated optical vortex emitters. Two fiber vector eigenmodes, each carrying a 10-Gbit s−1 quadrature phase-shift keying (QPSK) or a 20-Gbit s−1 16-ary quadrature amplitude modulation (16-QAM) signal, are generated using silicon microring resonators etched with angular gratings and then multiplexed for transmission though a 2-km large-core fiber (LCF) link. The measured crosstalk between two fiber vector eigenmodes is approximately −16 dB. The obtained results show favorable performance of data-carrying fiber vector eigenmode excitation and multiplexing transmission.
Materials and methods
Concept and principle of direct fiber vector eigenmode multiplexing transmission
The concept and principle of direct fiber vector eigenmode multiplexing transmission seeded by integrated optical vortex emitters are illustrated in Figure 1. High-order fiber vector eigenmodes TM01 and TE01, which are supported in a LCF, are considered (Supplementary Information). Instead of using commercially available bulky spatial light modulators, two compact integrated optical vortex emitters are employed to generate TM01 and TE01 modes feeding the LCF for fiber vector eigenmode multiplexing transmission. When each fiber vector eigenmode carries an independent data information channel, the multiplexing of two orthogonal fiber vector eigenmodes can double the transmission capacity through the fiber link, at the output of which the two fiber vector eigenmodes are separable from each other with low-level crosstalk.
Integrated optical vortex emitters
The designed and fabricated integrated optical vortex emitters are silicon microring resonators with the inner sidewall etched as angular gratings that can extract the confined whispering gallery modes (WGMs) in the microring resonator into radiated optical vortices29. The so-called Euler bends31 as access waveguides for increased effective interaction length are adopted to couple the incident Gaussian light from the single-mode fiber (SMF) to the WGMs in the microring resonator. Hence, such integrated emitters, which are significant to miniaturization, perform the same function as traditional diffractive optical elements (for example, spatial light modulators) for converting Gaussian light to optical vortices. Instead of using well-studied optical vortices that carry OAM29, here, we focus on two specific cylindrical vector beams (polarization vortices), which are generated by the integrated optical vortex emitters, that is, TM01 and TE01 modes, for the fiber vector eigenmode multiplexing transmission. More details about the device principle, design and fabrication are given in the Supplementary Information.
Results and discussion
Characterization of integrated optical vortex emitters
First, we characterize the performance of the fabricated integrated optical vortex emitters (Supplementary Information). We consider the case when the wavelength-dependent azimuthal order of the WGM in the microring resonator is equal to the number of etched grating elements in the inner sidewall of the microring resonator. Note that there is a mode splitting, which originates from the Bragg reflection-induced strong cross-coupling between the degenerate clockwise and counter-clockwise travelling WGMs inside the microring resonator (Supplementary Information). The mode splitting causes the splitting of the radiation spectrum, which is measured by scanning the incident laser wavelength. We fabricate two integrated optical vortex emitters (Chip1 and Chip2) and measure their radiation spectra, as shown in Figure 2. The radius of the two chips is 7.5 μm. One can clearly see the splitting of the radiation spectrum. The shorter and longer wavelength resonances of the split spectrum are associated with the radially polarized mode (TM01) and the azimuthally polarized mode (TE01), respectively. As shown in Figure 2a, the original radiation wavelengths of the two chips are different. To facilitate identical wavelengths of the TM01 mode from Chip1 and the TE01 mode from Chip2 for the fiber vector eigenmode multiplexing transmission, thermal tuning of Chip2 is performed to shift its radiation wavelength (Supplementary Information), as shown in Figure 2b.
Then, we verify the generation of TM01 and TE01 modes using the two chips. The measured near-field intensity distributions of the TM01 mode from Chip1 and the TE01 mode from Chip2 are shown in Figure 3a and 3b, respectively. One can clearly see the doughnut intensity profiles due to polarization singularity with an undefined polarization and the resultant null intensity at the beam center. After passing through a rotating polarizer, the measured near-field intensity distributions of the TM01 and TE01 modes are shown in Figure 3a1–3a4 and Figure 3b1–3b4 respectively. According to the directions of the polarizer axis marked with white arrows and the lobe-like intensity profiles (Supplementary Movies S1 and S2), one can confirm that the radiated beams from Chip1 and Chip2 are TM01 and TE01 modes, respectively. The intensity distribution of the multiplexed TM01 and TE01 modes is shown in Figure 3c. The radiated beams from the two chips are sent to a 2-km LCF link. After the fiber transmission, the measured intensity distributions of the TM01 mode, TM01 mode through a rotating polarizer (Supplementary Movie S3), TE01 mode, TE01 mode through a rotating polarizer (Supplementary Movie S4), and multiplexing of the TM01 and TE01 modes are shown in Figure 3d, 3d1–3d4, 3e, 3e1–3e4 and 3f, respectively. The observed intensity distributions indicate good quality of the TM01 and TE01 modes, which are radiated from the two chips and transmitted through a km-scale LCF. The slight tilt of the measured intensity distributions after the rotating polarizer may be due to the unwanted spurious components being radiated from the chips or excited during the fiber transmission. In addition to the rotating polarizer, a mode decomposition tool based on q-plates can also be used to verify the TM01 and TE01 modes (vector modes)32.
Performance of data-carrying fiber vector eigenmode multiplexing transmission
We further set up an experimental configuration (Supplementary Information) to study the system performance of data-carrying fiber vector eigenmode (TM01, TE01) multiplexing transmission through a 2-km LCF. The TM01 and TE01 modes emitted from Chip1 and Chip2 carry 10-Gbit s−1 QPSK or 20-Gbit s−1 16-QAM signals. The measured spectra of the demultiplexed TM01 and TE01 modes after the QPSK or 16-QAM-carrying fiber vector eigenmode multiplexing transmission are shown in Figure 4a and 4b, respectively. These spectra overlap with one another, occupying the same bandwidth. The measured bit-error rate (BER) curves as a function of the received optical signal-to-noise ratio (OSNR) for the QPSK or 16-QAM-carrying fiber vector eigenmode multiplexing transmission are shown in Figure 4c and 4d, respectively. Insets in Figure 4c and 4d depict the constellations of QPSK and 16-QAM, respectively. In the experiment, polarization controllers on a large-core fiber (PC-LCF) are employed to assist the mitigation of the crosstalk between the TM01 and TE01 modes (Supplementary Information). The measured mode crosstalk at the output of LCF is approximately −16 dB. The measured TM01- or TE01-only results without crosstalk (w/o crosstalk) are also depicted in Figure 4 for comparison with those with crosstalk (w/ crosstalk). For the 10-Gbit s−1 QPSK signals, the measured OSNR penalties of the demultiplexed TM01 and TE01 modes are approximately 1.5 dB w/o crosstalk and 3.2 dB w/ crosstalk at a BER of 2 × 10−3, which is the enhanced forward error correction (EFEC) threshold. For the 20-Gbit s−1 16-QAM signals, the measured OSNR penalties of demultiplexed TM01 and TE01 modes are approximately 4.5 dB w/o crosstalk and 7.7 dB w/ crosstalk at a BER of 1.5 × 10−2, which is the limit for a hard-decision forward-error correction (HD-FEC) with a 20% overhead. The obtained results indicate a successful implementation of a km-scale data-carrying fiber vector eigenmode multiplexing transmission seeded by integrated optical vortex emitters with a favorable performance.
In summary, by exploiting the optical polarization vortices generated from integrated optical vortex emitters, we report data-carrying fiber vector eigenmode multiplexing transmission through a 2-km LCF. Such fiber vector eigenmode multiplexing transmission can open up added capacity scaling opportunities by directly using the multiple vector eigenmodes of a specially designed multi-mode fiber, paving an alternative yet straightforward way for fiber-based spatial mode multiplexing. The integrated optical vortex emitters can provide truly compact solutions to replace bulky diffractive optical elements for the generation of fiber-eigenmode-like optical polarization vortices. Beyond the proof-of-concept demonstration on two fiber vector eigenmode (TM01 and TE01) multiplexing transmission seeded by silicon microring resonators with inner sidewall angular gratings, there are significant challenges and potential solutions towards a large number of fiber vector eigenmode multiplexing transmission.
Though the proper structural design of the LCF and the use of PC-LCF led to reduced mode crosstalk and resultant two fiber vector eigenmode multiplexing transmission, a specialty fiber that supports many fiber vector eigenmode multiplexing transmission with low-level crosstalk is still a challenge. Conventional weakly guiding multi-mode fibers support many vector eigenmodes, which, however, suffer strong mode coupling due to their similar effective refractive indices, especially in the same mode group. Fortunately, the high-index-contrast ring-core/air-core fiber can obtain negligible mode crosstalk by lifting the degeneracy between the neighboring fiber vector eigenmodes33, 34 and achieving an effective refractive index difference larger than 10−4, which is also the typical value of birefringence in the polarization-maintaining fiber (PMF). Moreover, the elliptical-core fiber can also help the mitigation of mode crosstalk35, 36, 37. Hence, the optimized design of a specialty fiber borrowing the ideas from the ring-core/air-core fiber, elliptical-core fiber, and PMF may enable full-vectorial multiple fiber eigenmode multiplexing transmission that is free of multiple-input multiple-output (MIMO) digital signal processing.
Though the silicon microring resonators with inner sidewall angular gratings generate fiber-eigenmode-like TM01 and TE01 vector modes, the (de)multiplexer of vector modes using a single photonic integrated chip is another challenge. Fortunately, integrated building blocks, including chip-scale Mach-Zehnder interferometers, phase shifters, and 2-dimensional (2D) gratings, can facilitate flexible spatial amplitude/phase/polarization manipulation38, 39. By employing a 2D grating array arranged in a circle, with each 2D grating manipulating its local polarization38, it is possible to build up a vector mode generator and (de)multiplexer (TM01, TE01) on a single photonic integrated chip. Moreover, high-order vector modes may also be generated and (de)multiplexed using a single photonic integrated chip composed of the building blocks with an appropriate layout for the desired spatial amplitude/phase/polarization manipulation. Hence, the optimized design of monolithic integration that properly incorporates building blocks may enable flexible arbitrary vector mode manipulation for full-vectorial multiple fiber eigenmode multiplexing transmission.
This can be the future envisaged roadmap toward practical applications in vector mode communications. It holds the potential to develop many data-carrying fiber vector eigenmode multiplexing transmission seeded by compact photonic integrated circuits.
This work was supported by the National Basic Research Program of China (973 Program) under grants 2014CB340004, 2014CB340001 and 2014CB340003, the National Natural Science Foundation of China (NSFC) under grants 11690031, 61761130082, 11574001, 11774116, 11274131, 61222502, 61575224 and 61622510, the Royal Society-Newton Advanced Fellowship, the National Program for Support of Top-notch Young Professionals, the Program for New Century Excellent Talents in University (NCET-11-0182), the Program for HUST Academic Frontier Youth Team, the Project ROAM (H2020-ICT-2014-1—Contract Number: 645361), and the Project Cornerstone (EPSRC-EP/L021129/1). The authors also acknowledge the support of the technical staff of the JWNC at Glasgow University and Shuhui Li and Yifan Zhao at Huazhong University of Science and Technology.
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Note: Supplementary Information for this article can be found on the Light: Science & Applications’ website(http://www.nature.com/lsa).