Laser-inscribed 3-D waveguide (3DW) technology has been applied to realize photonic-lantern spatial multiplexer (SMUX), which potentially enables lossless mode (de)multiplexing. However, the index ...contrast Δn between the transparent substrate and the spatially-inscribed waveguides by femto-second laser pulses is generally smaller than 6 × 10 -3 . It is simulated that a 3DW SMUX with Δn <; 6 × 10 -3 is able to provide a low mode-dependent loss (MDL) and coupler insertion loss (CIL) when coupling to a three-mode few-mode fiber (FMF). However, in sixand 15-mode cases, to guide all supermodes, few-mode region created by coupled or merged waveguides at the FMF side of the 3DW SMUX cannot be very small. This results in mode-profile mismatch between the 3DW SMUX and a low differential-group-delay FMF, which is generally with Δn around 1 × 10 -2 . Instead of employing bulky imaging optics, uptapering FMF is proposed to enlarge the modes of the FMF for minimizing the mode-profile mismatch. Simulation shows MDL and CIL enhancement can be achieved by the uptapering solution. It also points out that as mode number increases to 15, although uptapering FMF still improves coupling performance, low MDL <; 1dB cannot be acquired with Δn <;= 6 × 10 -3 . In order to achieve a lossless mode (de)multiplexing for 15 spatial modes, Δn = 1 × 10 -2 is required fora 15-core 3DW SMUX. Moreover, a fully-packaged dual channel 3DW coupling circuit including two six-mode SMUXes is introduced and experimentally demonstrated. MDL = 7 dB and CIL <; 8 dB is achieved in a loop through measurement with the two SMUXes. Mode-profile mismatch due to the limited Δn of the 3DW device is solved by uptapering FMF with a factor of 1.4.
Indoor wireless traffic is evolving at a staggering pace, and is quickly depleting radio spectrum resources. Optical wireless communication (OWC) offers powerful solutions for resolving this imminent ...capacity crunch of radio-based wireless networks. OWC is not intended to fully replace radio wireless techniques such as WiFi, but to complement these and offload their high traffic loads. After discussing OWC's application domains, this paper gives a tutorial overview of two major directions in OWC: wide-coverage visible light communication which builds on LED illumination techniques and shares capacity among multiple devices, and communication with narrow 2-D steered infrared beams which offers unshared high capacity to devices individually. In addition, supporting techniques for wide field-of-view receivers, device localization, bidirectional hybrid optical/radio networks, and bidirectional all-optical wireless networks are discussed.
The free-space narrow infrared beams can offer unprecedented data capacity to devices individually, as they can provide non-shared connections that have a large link power budget. By means of a fully ...passive module based on a high port count arrayed waveguide grating router (AWGR), many infrared beams can be 2D steered individually using wavelength tuning. By applying the defocusing techniques, a compact beam steering module has been realized. A simultaneous communication at up to 112 Gbit/s PAM-4 per beam has been shown with an 80-ports AWGR, thus offering a total wireless throughput beyond 8.9 Tbit/s. The wireless provisioning of multiple ultrahigh-definition video streams has been demonstrated in a proof-of-concept laboratory setup.
Free-space indoor optical communication deploying pencil beams can offer ultra-high wireless capacity individually per user device. By means of two-dimensional (2D) diffractive modules, such as a ...pair of crossed gratings, 2D steering of multiple beams by just tuning the wavelength of each beam can be achieved. The design aspects of an indoor system fed via an intelligent optical fiber backbone network are discussed. 2D angular beam steering over a 6° × 12° area was achieved by wavelength tuning from 1505 to 1630 nm. System experiments using PAM-4 modulation have shown a capacity of 32 Gbit/s per infrared beam. With radio-overfiber techniques and optical carrier recovery from the downstream signal, 10 Gbit/s upstream transmission of a 60 GHz radio signal has been shown using adaptive DMT modulation.
Non-uniform pulse amplitude modulation (PAM) utilizes unequal distances between its modulation levels. In a multilevel PAM symbol, multiple bits are encoded. Due to the unequal level spacing, some ...bits can be decoded successfully at a lower received optical power than others. This is well suited for practical passive optical network (PON) deployments wherein the optical powers received by the different optical network units (ONUs) typically vary over a broad range. Thus, more ONUs in the PON can successfully decode non-uniform PAM-4 and PAM-8 than standard PAM-4/8, thereby increasing the aggregated capacity of the network. In systems where signal-dependent noise makes up a significant part of the total received noise level, the non-uniform PAM constellation can be adapted to take this signal-dependent variance into account. In doing so, a lower unequal level spacing can be used, decreasing the received optical power required to successfully decode all the bits in the PAM symbol. The impact of non-uniform PAM on the network throughput is presented by comparison of the experimental results with the actual loss distribution of a commercially deployed PON.
In this paper, a crosstalk-mitigated transmission scheme in arrayed waveguide grating router (AWGR) based two dimensional infrared beam-steered optical wireless communication (OWC) system is proposed ...for indoor applications. By creating polarization orthogonality between the odd and even AWGR channels, high crosstalk tolerance between spectrally overlapping AWGR channels is realized experimentally. Because two signals with orthogonal polarization states will not beat with each other in a photodiode. The optical crosstalk on the orthogonal polarization state will not generate a beat note upon detection and thus crosstalk in the electrical domain can be largely reduced. Reduced crosstalk leads to a reduction in the required spectral guard band and/or an improved tolerance to spectral overlap, which allows higher spectral efficiency. Moreover, the port number of an AWGR can be increased by simply shortening the spatial gap between adjacent output waveguides on a chip. The higher port number can support the high spatial resolution of the steered OWC system. This technique can also tolerate the wavelength misalignment between AWGRs and lasers, which relaxes the design of low crosstalk AWGRs and high wavelength stable lasers. A 20 Gbit/s data rate, four-level pulse amplitude modulation OWC transmission has been experimentally demonstrated over 1.2-m free-space link. The experimental results show that the proposed scheme can maintain stable, low crosstalk impact with an apparent improvement of the responsivity.
High-speed photonic networks using digital signal processing (DSP) techniques are flourishing nowadays to meet the high-bandwidth requirements of modern bandwidth-thirsty applications in a ...cost-effective manner. However, the additional latency introduced by DSP is hindering the latency-critical applications. In this paper, a FPGA-based real-time low-latency four-level pulse amplitude modulation (PAM-4) receiver including digital adaptive equalization (DAE) is designed and implemented by using a latency-reducing parallel architecture. The DSP-introduced latency in the receiver end is analyzed in detail. As for DAE parallel implementation, a novel re-allocation scheme is proposed to cope with the issue of the dependency of the output on the successive input samples, and a look-ahead computation technique is introduced to improve the adaptive update efficiency. A real-time PAM-4 receiver is demonstrated in an experimental fiber link with 2.5 Gbit/s data rate for the performance evaluation. Compared with offline processing with MATLAB, the BER performance has little deterioration at 7% FEC limit of 1 × 10−3. With the help of the proposed deep-parallel technique, the DSP-introduced latency is reduced to 0.4μs on average, which better meets the requirements of latency-sensitive user cases in 5G networks. Furthermore, the real-time PAM-4 receiver could be flexibly reconfigured for various scenarios with low-latency requirements, and the latency-efficient parallel technique as well as the latency analysis method can also be extended to high-speed hardware implementation for data rates up to 100 Gbit/s or more.
•A deep-parallel technique is proposed to reduce the total DSP-introduced latency.•A data re-allocation scheme is proposed to enable the deep-parallel implementation.•A look-ahead computation technique is proposed for the deep-parallel implementation.•Two parallel architectures with different parallel depths are compared.•The DSP-introduced latency is analyzed in detail.•A FPGA-based deep-parallel real-time PAM-4 receiver is experimentally demonstrated.
To unlock the cost benefits of space division multiplexing transmission systems, higher spatial multiplicity is required. Here, we investigate a potential route to increasing the number of spatial ...mode channels within a single core few-mode fiber. Key for longer transmission distances and low computational complexity is the fabrication of fibers with low differential mode group delays. As such in this work, we combine wavelength and mode-division multiplexed transmission over a 4.45 km low-DMGD 6-LP-mode fiber by employing low-loss all-fiber 10-port photonic lanterns to couple light in and out of the fiber. Hence, a minimum DMGD of 0.2 ns (maximum 0.357 ns) is measured after 4.45 km. Instrumental to the multi-mode transmission system is the employed time-domain-SDM receiver, allowing 10 spatial mode channels (over both polarizations) to be captured using only 3 coherent receivers and real-time oscilloscopes in comparison with 10 for conventional methods. The spatial channels were unraveled using 20 × 20 multiple-input multiple-output digital signal processing. By employing a novel round-robin encoding technique, stable performance over a long measurement period demonstrates the feasibility of 10x increase in single-core multi-mode transmission.
Due to eye safety regulations, the allowable transmitted power in an optical wireless communication system is limited. Maximization of the optical power collected at the receiver is required in order ...to achieve the link power budget needed for maximum-speed data transfer. A large optical aperture at the receiver yields efficient power collection. Large-area top-illuminated photodiodes can on the one hand collect much light, but on the other hand inherently have a large capacitance and, thus, a reduced electrical bandwidth. To completely break this optical-electrical tradeoff, we propose a new class of optical receivers, i.e., cascaded aperture optical receivers. Such an optical receiver decouples the light collection function from the light detection one by using two separate apertures: the first function is done by surface grating coupler(s) feeding the received light into a waveguide, and the second one by an ultrahigh speed waveguide-coupled photodiode. These two apertures can be engineered independently to optimize the overall optical and electrical properties of the receiver. Empowered by an integrated cascaded aperture optical receiver fabricated on our InP membrane platform, we successfully demonstrated an indoor optical wireless communication system with a 200 Gb/s (5λ × 40 Gb/s) capacity.
Scalability of packet switched cross-connects that utilize all-optical signal processing is a crucial issue that eventually determines the future role of photonic signal processing in optical ...networks. After reviewing several labeling techniques, we discuss label stacking and label swapping techniques and their benefits for scalable optical packet switched nodes. All-optical devices for implementing the packet switch based on the labeling techniques will be described. Finally, we present a 1×4 all-optical packet switch based on label swapping technique that utilizes a scalable and asynchronous label processor and label rewriter. Error-free operation indicates a potential utilization of the swapping technique in a multihop packet-switched network.
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