This PDF file contains the front matter associated with SPIE Proceedings Volume 10561, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

Optical networks, for 400G and beyond, require flexibility and agility from network elements, especially from coherent transceivers. Higher baud rates are made possible using new electrooptic technologies. Multi-dimensional modulation formats provide improved tolerance to noise and fiber nonlinearities. Constellation shaping further improves these tolerances while allowing a finer granularity in selection of capacity. Frequency division multiplexing also provides improved tolerance to the nonlinear characteristics of fiber. Algorithms with reduced computation complexity allow the implementation at-speed of direct precompensation of nonlinear propagation effects. FlexEthernet and FlexOTN allow network operators to optimize capacity on their optical transport networks.

We demonstrate a high responsivity of 64 GBaud micro intradyne coherent receivers (Micro-ICR) operating at the wavelength range from 1565 nm to 1612 nm (L-band). The package body size is 12.0 mm × 22.7 mm × 4.5 mm, which is fully compliant with OIF implementation agreement. A newly developed InP-based photonic integrated circuit (InPPIC) for the L-band High-Bandwidth Micro-ICR is composed of the high-speed p-i-n-photodiode array (f^3dB > 40GHz) and a 90° hybrid waveguide consisting of MMIs which is designed to obtain high transmittance over the L-band. They are monolithically integrated through butt-joint regrowth in which a GaInAs absorption layer of the photodiode is directly connected with a GaInAsP core layer of the 90° hybrid in order to achieve the high responsivity with high optical coupling efficiency. Furthermore, the optical couplings between optical fibers and InP-PICs are realized by low optical loss micro optics. Receiver responsivities of more than 65 mA/W for signal input and local input are achieved within the L-band at 25°C. The wide 3-dB bandwidth of 40 GHz is also confirmed using high-bandwidth TIA. These values are comparable to the coherent receiver for 64 GBaud C-band applications. This receiver module is very promising for the increase of transmission capacity through expansion of operation wavelength (C-band and L-band).

Silicon photonics has undergone significant development in the last decade with some commercial successes for optical transceivers in telecommunication and data center applications. Here, we review and discuss the most successful silicon photonics devices which have been already implemented in the products, the remaining device challenges in the coming 400G transceivers, and the future of silicon photonics.

We demonstrate the hybrid integration of a multi-format tunable transmitter and a coherent optical receiver based on optical polymers and InP electronics and photonics for next generation metro and core optical networks. The transmitter comprises an array of two InP Mach-Zehnder modulators (MZMs) with 42 GHz bandwidth and two passive PolyBoards at the back- and front-end of the device. The back-end PolyBoard integrates an InP gain chip, a Bragg grating and a phase section on the polymer substrate capable of 22 nm wavelength tunability inside the C-band and optical waveguides that guide the light to the inputs of the two InP MZMs. The front-end PolyBoard provides the optical waveguides for combing the In-phase and Quadrature-phase modulated signals via an integrated thermo-optic phase shifter for applying the pi/2 phase-shift at the lower arm and a 3-dB optical coupler at the output. Two InP-double heterojunction bipolar transistor (InP-DHBT) 3-bit power digital-to-analog converters (DACs) are hybridly integrated at either side of the MZM array chip in order to drive the IQ transmitter with QPSK, 16-QAM and 64-QAM encoded signals. The coherent receiver is based on the other side on a PolyBoard, which integrates an InP gain chip and a monolithic Bragg grating for the formation of the local oscillator laser, and a monolithic 90° optical hybrid. This PolyBoard is further integrated with a 4-fold InP photodiode array chip with more than 80 GHz bandwidth and two high-speed InP-DHBT transimpedance amplifiers (TIAs) with automatic gain control. The transmitter and the receiver have been experimentally evaluated at 25Gbaud over 100 km for mQAM modulation showing bit-error-rate (BER) performance performance below FEC limit.

DWDM with/without super-channel based photonic networks require the use of optical carriers with equalized amplitudes and frequency stabilization of adjacent carriers to realize reliable high bandwidth optical communication systems with high spectral efficiency and long reach. Cascading of electro-optic (EO) modulators is a versatile method for generating tuneable, high repetition rate frequency combs which can be used as sources for the carriers. However, the number of lines produced with this technique is limited by the number of phase modulators. Nonlinear spectral broadening is an attractive option for bandwidth scaling; however, bandwidth scaling of single carrier combs through four wave mixing suffers from unequal comb lines or power limitations due to Brillouin scattering. A simpler technique to increase the number of comb lines would involve using multicarrier excitations for comb generation which would result in a proportional increase in the comb lines. Further, dual-carrier excitation enables an excellent temporal profile for nonlinear spectral broadening. However, since the two carriers have uncorrelated drifts, the resultant frequency combs would be unsuitable for most applications. This issue can be overcome by frequency offset locking the two lasers. Here, we demonstrate frequency offset locking (MHz accuracy) of two diode lasers spaced by 100GHz by using an optical phase locked loop which locks one laser to a RF harmonic of the other. This allows for the generation of frequency comb lines locked to each other even post nonlinear broadening. Using this technique, we demonstrate a 25GHz frequency comb with >90 lines (2THz) in the C-band.

Optical orthogonal frequency division multiplexing (OFDM) utilizes orthogonal sub-carrier channels whose symbol rates are equal to their frequency spacing. Optical OFDM is effective in increasing the spectral efficiency of optical communication, and is applicable for highly spectral-efficient and adaptive optical networks including an elastic network as well as point-to-point transmission. As transmission capacity is varied depending on the traffic and transmission distance in these adaptive optical networks, we need to develop an OFDM signal demultiplexer with high-speed processing and low-power consumption, which can flexibly deal with various symbol rate and number sub-carrier channels in the optical domain. We previously reported an integrated-optic demultiplexer for variable optical OFDM signals, which is composed of an array of variable optical attenuators (VOAs) before a slab star coupler-type optical discrete Fourier transform (DFT) circuit. However, this demultiplexer showed large loss variation (several dB) when changing its characteristics in response to various symbol rate OFDM sub-carriers. In this presentation, I propose and report a tunable optical OFDM demultiplexer that consists of an array of VOAs, optical DFT circuit, and an array of Mach-Zehnder interferometer-type tunable couplers. The newly added array of tunable couplers after the optical DFT circuit could efficiently utilize output lightwave and decrease the loss variation (intrinsically zero). I report the operating principle of the proposed tunable demultiplexer, its characteristics evaluation, and preliminary experimental results of the tunable demultiplexer fabricated with silicon waveguide technology. The size, processable channel number and symbol rate of the demultiplexer were 1 mm x 8 mm, 2 to 8 and 10 to 40 Gsymbol/s, respectively.

We review the principles underpinning the Kramers-Kronig (KK) receiver operation and its various implementations. These include direct-detection based schemes, where the information-carrying signal is transmitted along with a CW field that is necessary for the implementation of the KK algorithm, as well as other schemes, where the CW is added at the receiver, owing to the availability of a local oscillator. Polarization-multiplexing with the KK receiver will also be discussed. Finally, an up-to-date review of the experimental implementations of KK transceiver solutions will be presented.

We describe a number of experiments demonstrating and exploring the potential of combing wideband-optical comb sources with space-division multiplexed transmission systems and in particular with homogeneous-single-mode multicore fibers including Pb/s transmission using conventional receiver technology without MIMO processing and longdistance recirculating transmission. We describe experiments using synchronized parallel transmission loops to investigate joint processing of spatial super channels and multi-dimensional modulation before discussing how optical comb technology may combine with SDM fibers to allow comb re-generation across networks to enable high-order modulation with simplified DSP. Overall, these experiments demonstrate a range of scenarios where the combination of optical combs and homogeneous MCFs can be advantageous in future optical communications networks.

In the mode division multiplexing (MDM) system, differential mode delay (DMD) restricts the quality of transmission. Thus, it is necessary to precisely measure DMD for compensation and system design. The DMD measurement method using low-coherence digital holography (LCDH) has been proposed. This method can obtain not only accurate DMD but also spatial mode fields. However, in this method, an SMF as the reference arm is needed and its length should be particularly adjusted to a fiber under test (FUT) for low-coherence interferometric measurement. We propose a DMD measurement method by reference-free low-coherence digital holography (RF-LCDH). In the proposed method, we generate a new optical path from the light emitted from the FUT, which is regard as internal-reference light. The proposed method enables us to obtain DMD and spatial mode fields without the SMF as the reference arm by using internal-reference light. In the experiment, we measured DMD of a 10-mode fiber to confirm the basic operation of the proposed method. As the result, without using additional SMFs for reference arm, the proposed method achieved the measurement accuracy which was in good agreement with that of the conventional method.

The differential modal group delay (DMGD) of a few-mode fiber (FMF) is an important parameter that decides the complexity of the digital signal processing at the receiver in space division multiplexed (SDM) systems. We use the random mode and polarisation coupling simulation model to explain the origin of statistical spread in DMGD when measured through the conventional linear techniques. We propose and experimentally demonstrate a method based on degenerate inter-modal four wave mixing (IMFWM) to accurately measure the DMGD of a few-mode fiber. Unlike most conventional linear methods, we also extract the sign of DMGD using this nonlinear method. We demonstrate IMFWM using an all-fiber architecture with offset splicing, which is low-cost, robust and non-mode selective.

Superconducting single photon detectors are promising candidates for cutting-edge applications like space-to-ground communication and quantum-key-distribution. The challenge is to transfer the assembly technology from university research to industrial processes. Especially the positioning of the optical fiber with respect to the active area of the superconducting detector are open questions. We demonstrate the operation of a superconducting nanowire single photon detector (SNSPD) in a closed cycle cryostat. The thermal coupling between superconducting detector chip and closed-cycle cryostat is investigated. The possibility to mount the optical fiber in an RIE-ICP-etched hole in a silicon carrier wafer is demonstrated. Different illumination wavelength and intensities are used to validate the SNSPD assembly in the closed cycle cryostat. Further, an advanced package of the silicon carrier wafer and the SNSPD-chip is introduced. In this package, the SNSPD-chip is mounted via flip-chip technology on the silicon carrier wafer. The striven flip-chip position accuracy of ± 1 μm ensures the accurate coupling between optical fiber and active area of the SNSPD.

As the nonlinear capacity limit of single mode fiber (SMF) transmission systems is being approached, space-division multiplexing (SDM) in multicore fibers (MCFs) or few-mode fibers (FMFs) is currently under intense investigations to achieve ultrahigh spectral efficiency per fiber. Meanwhile, a key advantage of SDM over simply increasing the number of SMFs, is its inherent device integration and resource sharing capability. This can potentially provide significant benefits in terms of the cost per bit in future optical networks. In order to efficiently address capacity scaling in a single optical fiber, few-mode and multicore erbium-doped fiber amplifiers are being developed. Critical for the implementation of SDM amplifiers is to achieve almost the same amount of gain for all spatial channels. In this respect, we have recently demonstrated multimode fiber amplifiers, supporting >15 modes, with a maximum differential modal gain of 2 dB and negligible mode mixing.

In a mode division multiplexing (MDM) transmission using few-mode fibers (FMFs), differential modal gain (DMG) in amplifiers and mode dependent loss (MDL) cause deterioration in signal quality. Therefore, the techniques which selectively attenuate multiplexed modes and equalize optical powers among mode channels are required for a long-haul MDM transmission. In this paper, we propose a selective mode attenuator using phase-intensity-phase (PIP) modulation. The PIP modulation, consisting of a cascade of two phase and one intensity modulation masks connected by optical Fourier transforms (OFTs), makes it possible to selectively attenuate multiplexed mode channels with high accuracy. In the proposed method, the intensity distributions of spatial modes are converted by the phase modulation and OFT, before doing attenuation by intensity mask located between the two phase masks. Due to the action of a pair of phase masks having phase conjugate relations, accurately selective mode attenuation can be performed even if the number of modes is increased over three. To confirm the basic operation of the proposed method, we perform a numerical simulation for power equalization among six spatial modes (LP₀₁, LP_11a, LP_11b, LP_21a, LP_21b, and LP₀₂) having different powers. The phase and intensity masks are designed by using simulated annealing. Moreover, we also evaluate modal crosstalk (MXT) characteristics and the wavelength dependence of the equalization in C-band. The results show that the optical powers of all modes are successfully equalized for any wavelengths and the MXT smaller than −25 dB were achieved between all modes.

6×10 Gb/s polarization- and mode-multiplexed transmission without MIMO equalization for data center applications is experimentally demonstrated, over a 200-m elliptical core with two PANDA stress rods few-mode fiber (EP-FMF). This fiber simultaneously breaks the degeneracies of space and polarization in a mode group. A large effective index difference (Δn_eff=6.6×10^-4 ) between adjacent modes among 6-modes is achieved over the entire C band, which translates to lower crosstalk than previous fiber designs. A polarization crosstalk of <-16 dB over 200 m and loss less than 0.63 dB/km were achieved.

Recent developments in autonomous vehicle instrumentation has led to advancements in laser based LiDAR sensors. For these type of systems, sensor performance is dependent on fast response time and good contrast (SNR) at long distances. Careful design for the optical transmitter assembly is paramount to achieving good device performance, and has become a primary differentiator between emerging LiDAR technologies. These systems rely heavily on both the source emitter quality as well as the conditioning and delivery of collimated beams using passive optical components. Most available sources, from semiconductor to fiber lasers, have the necessary spectral characteristics, but lack the required beam quality for long distance propagation and detection. In order to overcome these limitations, LightPath leverages decades of experience in precision molded aspheres and fiber delivery systems to achieve quality collimation and astigmatic beam correction. These capabilities are applicable to a broad range of optical system configurations, and therefore transcend the differences in LiDAR system architecture. We will explore the basic optical requirements for emerging LiDAR transmitter systems and discuss their common reliance on precision optical components.

Optical Physics Company (OPC) has developed under SBIR Phase II funding an advanced laser pointing sensor system (LPSS) for future integration into the NASA Integrated Radio and Optical Communications (iROC) program. This would enable deep space pointing of high rate communications terminals to earth terminals via beaconless lasercom. The heart of the LPSS is OPC’s patented interferometric star tracker which can provide better than 300 nrad pointing accuracy. This paper will report on the LPSS technology and the upcoming ground field testing to verify the pointing accuracy required for deep space high rate laser communications.

We discuss modulation format design techniques used to approach Shannon capacity limit. The techniques include the use of multi-dimensional coded modulation with iterative decoding, probabilistic, and geometric constellation shaping. We also show how the use of coded modulation enables adaptive non-linear compensation techniques to further improve performance in nonlinear transmission systems.

Coherent optical transponder is ubiquitous and dominant in long-haul optical network. Bit error rate (BER) versus optical signal to noise ratio (OSNR) determines the transmission distance for coherent optical transponder. However, a complicated setup is needed for this measurement, which limits this measurement to laboratory environment. We have developed an accurate model to predict BER versus OSNR at various receiver optical power (ROP) under assumption of additive white Gaussian noise (AWGN). The model has three parameters, which are related to noise floor, filter mismatching, and OSNR value without noise loading. We determined the first two parameters through curve fitting of BER vs. ROP curve. We determined the third parameters through design verification test (DVT). We validated the model over 100 channels within extended C band. Furthermore, we expanded the model to high modulation format 16- ary quadrature amplitude modulation (QAM). We investigated the influence of high baud rate, like 45G, 56G, 64G and 86G. The model works well for both high modulation format and high baud rate. The influence of baud rate on the fitting parameters are discussed. Since one can measure BER versus ROP using built-in components of coherent optical transponder, BER versus OSNR can be monitored during in-field deployment based on this accurate model. In addition, one can monitor OSNR based on BER reversely. No extra hardware or DSP processing algorithm is needed for this OSNR monitoring scheme. The monitoring accuracy is further improved with consideration of chromatic dispersion (CD), polarization mode dispersion (PMD) and nonlinearity impairment.

Data traffic demands continue to increase worldwide, driving requirements for higher spectral efficiency systems and higher individual channel capacities. To enable terrestrial transmission systems to keep up with demand, the ITU recently adopted the G.654.E standard for optical fiber with larger effective area for terrestrial use. To keep macrobend loss performance the same as for conventional G.652 fiber, the cable cutoff wavelength specification for the new fiber class was increased to the lower edge of the C-band. We examine here several aspects of G.654.E fiber in terrestrial systems including modeled and experimentally measured transmission reach, the use of Raman amplification with pump wavelengths below cable cutoff, and the transmission of optical supervisory channels (OSC) at wavelengths below cable cutoff. We demonstrate significant transmission reach increases for 200 Gb/s PM-16QAM channels of at least 55% compared to standard single-mode fiber in a re-circulating loop experimental configuration. Addressing the practical questions of OSC and Raman pumps propagating below cable cutoff, we demonstrate experimentally and through extensive modeling that negligible impact is expected and observed in both cases.

Optical hybrids enable the unambiguous measurement of both the amplitude of an optical signal and its phase relative to a reference. A fusion-tapering technique is used to produce monolithic all-fiber 3x3 optical hybrids. By a symmetry argument, theory predicts that an equilateral triangular 3x3 coupler must form a 120° hybrid whenever a power equipartition is obtained during tapering. Precision-machined holding clamps constrain three SMF-28 fibers to an equilateral triangle geometry. An oxygen-propane micro-torch is used for the fusion and tapering steps. Fabricated devices are characterized with respect to insertion loss and relative phases at different wavelengths. Fabricated devices exhibit excess loss less than 1 dB from 1300 to 1600 nm, the coupling ratio is 33,2 ± 2,6% at 1550 nm, the design center wavelength. The relative phases are measured within 120 ± 10° and 240° ± 10° across the whole C-band from 1530 to 1565 nm. Compared to previous work, all-fibre hybrids are fabricated without an outer glass tube, exhibit lower excess loss and good phase tolerance over the whole C-band.

To expand the submarine transmission capacity, higher OSNR is important. Higher OSNR can be realized by advanced optical fibers having low transmission loss and low nonlinearity. We successfully realized the lowest transmission loss of 0.1419 dB/km at 1560 nm wavelength. It was the first time to realize a 0.14 dB/km in 50 years history of silica-based optical fiber. This epoch-making result was enabled by reducing Rayleigh scattering. In addition, microbending loss was kept low using a new soft inner coating in spite of a 147μm² large effective area. This achievement will contribute to further expansion of submarine transmission capacity.

A sharp growth in the transmission capacities of photonic transport networks has necessitated an increase in the spectral efficiency and number of wavelength channels. Advanced photonic transport technology in the T-band (thousand band: 1000–1260 nm, available bandwidth of 61.9 THz) is a promising solution to expanding the usable wavelength channels for optical communication to increase the available capacity. The available bandwidth in the T-band is approximately five times that of the conventional C- and L-bands, and, therefore, we propose an ultra-broadband photonic transport system that employs waveband multiplexing technology which is compatible with both novel and conventional wavebands. Wavelength-tunable quantum dot (QD) laser technologies aid the availability of the T-band with its wide wavelength tunability. An endlessly single-mode holey fiber (HF) is used as a wave-band multiplexing transmission line in the O- and T-bands. Optimizing the spectral efficiency is key to realizing high-capacity transmission using advanced digital signal processing (DSP) technology with a coherent detection scheme. In the study, we successfully demonstrated error-free transmission with 20-Gb/s quadrature phase-shift keying (QPSK) using homodyne detection receivers in the O- and T-bands simultaneously over a 4-km long HF. The measured bit error rates (BERs) were within the forward error correction limit of 2×10^-3 in both the O- and T-bands. The large number of wavelength channels with conventional use of the O- and T-bands should help increase the total capacity of an optical fiber to meet the growth in demand of optical communication data traffic.

Optical fiber channels present flat channel response per wavelength in general, owing to the ultra-wide available bandwidth of optical fiber and optical amplifiers. However, the recent transport capacity upgrade per wavelength from 10 to >100 Gbaud has given rise to severe power fading at the high frequency range. Especially, the modern optical networks may rely on a massive number of reconfigurable optical add and drop multiplexers (ROADM) to enhance the network flexibility with low latency. These cascaded ROADMs bring about a well-known filter-narrowing effect that has become a severe issue in the deployed networks. This strongly limits the channel bandwidth, and leads to an optical channel with colored signal-to-noise ratio (SNR). To address this issue, we utilize the water-filling, an optimum power allocation that determines the capacity of colored-SNR Gaussian channels, and proposes multicarrier entropy loading to offer a theoretically optimum strategy to approach the Shannon capacity. Within each subcarrier, probabilistic constellation shaping is exploited to design Gaussian sources. Compared with the conventional uniform-entropy modulation, entropy loading possesses fundamental advantages on channel coding: it maximizes the channel mutual information under fixed channel coding rate when the system operates below the channel capacity, and approaches the capacity with less coding overhead. Entropy loading can be generalized to any applications under colored-SNR Gaussian channels beyond the optical communication.

With the advent of the coherent age the implementation of massive digital signal processors (DSP) co-integrated with high speed AD and DA converters became feasible allowing for the realization of huge flexibility of transponders. Today the implementation of variable transponders is mainly based on variable programming of DSP to support different modulation formats and symbol rates. Modulation formats with high flexibility are required such as pragmatic QAM formats and hybrid modulation formats. Furthermore, we report on an implementable probabilistically shaping technique allowing for adjusting the bitrate. We introduce fundamental characteristics of all modes and describe basic operation principles. The modifications of the operational modes are enabled simply by switching between different formats and symbol rates in the DSP to adjust the transponders spectral efficiency, the bitrate and the maximum transmission distance. A fine granularity in bitrate and in maximum transmission distance can be implemented especially by hybrid formats and by probabilistically shaped formats. Furthermore, latter allow for ~+25% increase of the maximum transmission distance due their operation close to the Shannon limit as a consequence of their 2D Gaussian like signal nature. If the flexibility and programmability of transponders is implemented, it can be utilized to support different strategies for the application. The variability in symbol rate is mainly translated into variability in bitrate and in bandwidth consumption. Contrary the variable spectral efficiency translates into a variation of the maximum transmission reach and of the bitrate. A co-adjustment of both options will lead to a superior flexibility of optical transponders to address all requirements from application, transponder and fiber infrastructure perspective.

The paper proposes the use of Visible Light Communication (VLC) in Vehicular Communication Systems for vehicle safety applications. A smart vehicle lighting system that combines the functions of illumination, signaling, communications, and positioning is presented. The system aims to ensure the communication between a LED based VLC emitter and an on-vehicle VLC receiver. A traffic scenario is stablished. Vehicle-to-vehicle (V2V) and Infrastructure-to-Vehicle (I2V) communications are analyzed. For the V2V communication study, the emitter was developed based on the vehicle head lights, whereas for the study of I2V communication system, the emitter was built based on streetlights. The VLC receiver is used to extract the data from the modulated light beam coming from the white RGB-LEDs emitters. The VLC receiver is based on amorphous SiC technology and enhances the conditioning of the signal enabling to decode the transmitted information. The [p(SiC:H)/i(SiC:H)/n(SiC:H)/p(SiC:H)/i(Si:H)/n(Si:H)] tandem photodetectors are located at the roof-top of the vehicle, for I2V communications, and at the tails for V2V reception. Clusters of emitters, in a square topology, are used in the I2V transmission. The information and the ID code of each emitter in the network are sent, simultaneously, by modulating the individual chips of the trichromatic white LED. Free space is the transmission medium. An on-off code is used to transmit data. An algorithm to decode the information at the receivers is set. The proposed system was tested. The experimental results, confirmed that the proposed cooperative VLC architecture is suitable for the intended applications.