Xinlun Cai

Orbital angular momentum vertical-cavity surface-emitting lasers

Harnessing the orbital angular momentum (OAM) of light is an appealing approach to developing photonic technologies for future applications in optical communications and high-dimensional quantum key distribution (QKD) systems. An outstanding challenge to the widespread uptake of the OAM resource is its efficient generation. In this work we design a new device that can directly emit an OAM-carrying light beam from a low-cost semiconductor laser. By fabricating micro-scale spiral phase plates within the aperture of a vertical-cavity surface-emitting laser (VCSEL), the linearly polarized Gaussian beam emitted by the VCSEL is converted into a beam carrying specific OAM modes and their superposition states, with high efficiency and high beam quality. This new approach to OAM generation may be particularly useful in the field of OAM-based optical and quantum communications, especially for short-reach data interconnects and QKD.

Orbital Angular Momentum Divider of Light

Manipulation of orbital angular momentum (OAM) of light is essential in OAM-based optical systems. In particular, the OAM divider, which can convert the incoming OAM mode into one or several new smaller modes in proportion at different spatial paths, is very useful in OAM-based optical networks. However, this useful tool has rarely been reported. Here, we put forward a passive OAM divider based on coordinate transformation. The device consists of a Cartesian to log-polar coordinate converter and an inverse converter. The first converter converts the OAM light into a rectangular-shaped plane light with a transverse phase gradient, and the second converter converts the plane light into multiple diffracted light. The OAM of zeroth-order diffracted light is the product of the input OAM and the scaling parameter. The residual light is the output from other diffracted orders. Furthermore, we extend the scheme to realize equal N-division of OAM and arbitrary division of OAM. The ability of dividing OAM shows huge potential for OAM-based classical and quantum information processing.

Spin and orbital angular momentum and their conversion in cylindrical vector vortices

The generation of light beams carrying orbital angular momentum (OAM) has been greatly advanced with the emergence of the recently reported integrated optical vortex emitters. Generally, optical vortices emitted by these devices possess cylindrically symmetric states of polarization and spiral phase fronts, and they can be defined as cylindrical vector vortices (CVVs). Using the radiation of angularly arranged dipoles to model the CVVs, these beams as hybrid modes of two circularly polarized scalar vortices are theoretically demonstrated to own well-defined total angular momentum. Moreover, the effect of spin-orbit interactions of angular momentum is identified in the CVVs when the size of the emitting structure varies. This effect results in the diminishing spin component of angular momentum and purer OAM states at large structure radii.

Generation of photonic orbital angular momentum superposition states using vortex beam emitters with superimposed gratings

An integrated approach to produce orbital angular momentum (OAM) superposition states has been demonstrated. Superposition states between two vector OAM modes have been achieved by integrating a superimposed angular grating in one silicon micro-ring resonator.

High-dimensional quantum key distribution based on multicore fiber using silicon photonic integrated circuits

Quantum key distribution provides an efficient means to exchange information in an unconditionally secure way. Historically, quantum key distribution protocols have been based on binary signal formats, such as two polarization states, and the transmitted information efficiency of the quantum key is intrinsically limited to 1u2009bit/photon. Here we propose and experimentally demonstrate, for the first time, a high-dimensional quantum key distribution protocol based on space division multiplexing in multicore fiber using silicon photonic integrated lightwave circuits. We successfully realized three mutually unbiased bases in a four-dimensional Hilbert space, and achieved low and stable quantum bit error rate well below both the coherent attack and individual attack limits. Compared to previous demonstrations, the use of a multicore fiber in our protocol provides a much more efficient way to create high-dimensional quantum states, and enables breaking the information efficiency limit of traditional quantum key distribution protocols. In addition, the silicon photonic circuits used in our work integrate variable optical attenuators, highly efficient multicore fiber couplers, and Mach-Zehnder interferometers, enabling manipulating high-dimensional quantum states in a compact and stable manner. Our demonstration paves the way to utilize state-of-the-art multicore fibers for noise tolerance high-dimensional quantum key distribution, and boost silicon photonics for high information efficiency quantum communications.Silicon chip-to-chip high-dimensional quantum key distributionQuantum key distribution (QKD) enables ultimate secure communication guaranteed by quantum mechanics. Most of QKD systems are based on binary encoding utilizing bulky, discrete, and expensive devices. Consequently, a large scale deployment of this technology has not been achieved. A solution may be by photonic integration, which provides excellent performances and are particularly suitable for manipulation of quantum states. The Center for Silicon Photonics for Optical Communication (SPOC) led by Prof. Leif Katsuo Oxenløwe at the Technical University of Denmark demonstrated an integrated solution for manipulation of new high-dimensional quantum states using spatial degrees of freedom (the cores of a multicore fiber). We achieved the first silicon chip-to-chip decoy-state high-dimensional QKD, which is suitable for longer transmission distance with higher secret key rate, better resilience to noise, and higher information efficiency.

Optical generation of tunable and narrow linewidth radio frequency signal based on mutual locking between integrated semiconductor lasers

An integrated on-chip optical device consisting of two distributed feedback (DFB) lasers and one multimode semiconductor ring laser (SRL) has been numerically investigated. In this optical circuit, the two DFB lasers are injected into the SRL, and with the presence of the four-wave mixing effect and optical feedback, the three semiconductor lasers achieve mutual-locking state. The beating between the output optical spectral lines can generate readily tunable radio frequency signals with high spectral purity.

High index contrast integrated optics in the cylindrical coordinate

Recently the developments of high contrast optics, such as high contrast grating (HCG), have attracted much attention. Much of the existing work has been focused on structures that can be characterized as ‘Cartesian’, i.e., which are easily described by functions that are separable in the Cartesian coordinates. Yet optical fields with cylindrical rather than Cartesian symmetries, such as Laguerre-Gaussian (LG) modes and their relatives including both the scalar LG modes and cylindrical vectorial (CV) modes, can be more efficiently manipulated by high contrast structures that have the same kind of cylindrical symmetries, hence best described in a polar or cylindrical coordinate. An example of such a structure is the angular grating based silicon photonics micro-ring optical vortex emitter device we reported. An efficient treatment of cylindrical high contrast structures requires the decomposition of Fourier components of both the field and the structure in the cylindrical coordinates, so that the coupling process between the field Fourier components via the structure can be calculated. We have implemented a semi-analytical model that fully describes the 3D vectorial coupling process using a transverse spatial Fourier analysis in the cylindrical space. This model can deal with HCG structures in cylindrical coordinates with high precision and fast speed, enabling rapid yet accurate simulation of the coupling of planar waveguide modes with optical vortex modes carrying photonic orbital angular momenta and allowing optimization of the emission coefficient and emitted beam quality. The details of the method and optimized silicon photonics integrated OAM emitter devices will be presented.

Compact tunable electromagnetically induced transparency and Fano resonance on silicon platform

We propose and demonstrate an on-chip coupling resonant system to generate electromagnetically induced transparency (EIT)-like effect and Fano resonance on silicon platform. It is composed of a microring resonator (MRR) and two cascaded Sagnac-loop mirrors (SLMs) assisted Fabry-Perot (FP) cavity on silicon-on-insulator. According to the coupling conditions of the MRR, two cases are studied theoretically. When the MRR is over coupling, EIT-like transmission can be observed. In contrast, Fano resonances can be generated by the condition of under coupling. In the experiment, the add-drop MRR is under coupling, leading to a sharp asymmetric line shape for Fano resonance. The resonance wavelength of the MRR can be dynamically tuned based on thermal-optic effects by tuning the micro-heater. The experiment results show Fano resonances with maximum extinction ratio (ER) of 23.22 dB and maximum slope rate (SR) of 252 dB/nm. Moreover, the wavelength of Fano resonance can be shifted widely with a tuning efficiency of 0.2335 nm/mW.

Integrated optical vortex beam receivers

A simple and ultra-compact integrated optical vortex beam receiver device is presented. The device is based on the coupling between the optical vortex modes and whispering gallery modes in a micro-ring resonator via embedded angular gratings, which provides the selective reception of optical vortex modes with definitive total angular momentum (summation of spin and orbital angular momentum) through the phase matching condition in the coupling process. Experimental characterization confirms the correct detection of the total angular momentum carried by the vortex beams incident on the device. In addition, photonic spin-controlled unidirectional excitation of whispering-gallery modes in the ring receiver is also observed, and utilized to differentiate between left- and right-circular polarizations and therefore unambiguously identify the orbital angular momentum of incident light. Such characteristics provide an effective mode-selective receiver for the eigen-modes in orbital angular momentum fiber transmission where the circularly polarized OAM modes can be used as data communications channels in multiplexed communications or as photonic states in quantum information applications.

Scalable mode division multiplexed transmission over a 10-km ring-core fiber using high-order orbital angular momentum modes

We propose and demonstrate a scalable mode division multiplexing scheme based on orbital angular momentum modes in ring core fibers. In this scheme, the high-order mode groups of a ring core fiber are sufficiently de-coupled by the large differential effective refractive index so that multiple-input multiple-output (MIMO) equalization is only used for crosstalk equalization within each mode group. We design and fabricate a graded-index ring core fiber that supports 5 mode groups with low inter-mode-group coupling, small intra-mode-group differential group delay, and small group velocity dispersion slope over the C-band for the high-order mode groups. We implement a two-dimensional wavelength- and mode-division multiplexed transmission experiment involving 10 wavelengths and 2 mode groups each with 4 OAM modes, transmitting 32 GBaud Nyquist QPSK signals over all 80 channels. An aggregate capacity of 5.12 Tb/s and an overall spectral efficiency of 9 bit/s/Hz over 10 km are realized, only using modular 4x4 MIMO processing with 15 taps to recover signals from the intra-mode-group mode coupling. Given the fixed number of modes in each mode group and the low inter-mode-group coupling in ring core fibres, our scheme strikes a balance in the trade-off between system capacity and digital signal processing complexity, and therefore has good potential for capacity upscaling at an expense of only modularly increasing the number of mode-groups with fixed-size (4x4) MIMO blocks.