Nobuhiko Kikuchi

• Hitachi, Ltd.

We present our data-collection and deep neural network (DNN)-training scheme for extracting the optical status from signals received by digital coherent optical receivers in fiber-optic networks. The DNN is trained with unlabeled datasets across multiple administrative network domains by combining federated learning and unsupervised learning. The s...

The needs for ultra-high speed short- to medium-reach optical fiber links beyond 100-Gbit/s is becoming larger and larger especially for intra and inter-data center applications. In recent intensity-modulated/direct-detection (IM/DD) high-speed optical transceivers with the channel bit rate of 50 and/or 100 Gbit/s, multilevel pulse amplitude modula...

We introduce a precise Tx-side non-linear distortion compensation scheme for higher-order PAM signals, and experimentally show its effectiveness in 28 GBaud PAM-8 and 40 GBaud THP(Tomlinson Harashima Precoding)-PAM6 signals with more than 2-dB power budget improvement.

This paper shows the concept of the elastic lambda aggregation network (E $\lamda$ AN) that will achieve an elastic common access infrastructure for the metro-access network. It proposes a service restoration technique that uses optical path switching and inter-central office handover for E $\lamda$ AN to recover from disasters. The feasibility of...

We experimentally prove the effectiveness of Tomlinson-Harashima Precoding for IM/DD Nyquist PAM signals with severe transmitter-side bandwidth limitation despite the increase of optical signal levels, and its application to 50-GBaud Faster-Than-Nyquist PAM4 is also demonstrated.

We firstly demonstrate incoherent multiplexing of two NRZ signals utilizing directly modulated lasers transmits 106-Gb/s/lane PAM-4 signals over 10 km without non-linear compensation. Skewless eye diagram of this method attained lower BERs than conventional method.

As the proof-of-concept of a next-generation 100-Gb/s/channel high-speed optical transceiver for short- to mid-range applications, we develop a first full-rate and fully-functional FPGA-based single-polarization 16APSK/16QAM transceiver prototype using optical delay- and intensity-detection. Almost all the major transceiver functions, such as multi...

We develop full-rate / fully-functional FPGA-based high-speed optical 16-APSK/QAM transceiver prototype using optical delay-detection for single-polarization 100-Gbit/s/channel short- to mid-range fiber transmission. Four channel WDM (448-Gbit/s) field trial over 18.2-km SMF is also performed.

We investigate practical equalizing filter requirement and temperature range of EML-base 102.4-Gbit/s Nyquist-PAM4 transceivers and demonstrate its wide-temperature operation (20 degree) and a 40-km SMF transmission at 1.3 mm using commercial EMLs experimentally.

40-km NZ-DSF and SSMF transmission of 100 Gbit/s/lambda Nyquist-PAM4 with the help of digital pre-equalization is demonstrated, achieving BER less than Super FEC limit (BER 2E-3).

A bidirectional multi-symbol phase estimation (BD-MSPE) scheme for differentially encoded optical signals, which can realize high-speed processing with superior BER characteristics, is proposed. Conventional MSPE needs decision feedback to correct phases of past symbols, so it is not suitable for parallelization. BD-MSPE, in contrast, uses tentativ...

The use of InP-based IQ modulator for ultra-high speed precise 28-GBaud 64QAM signal generation (336 Gbit/s with polarization multiplexing) is experimentally demonstrated using digital modulator non-linearity compensation techniques, for the first time to our knowledge.

We propose a monitoring technique for constellation distortion due to imbalance of balanced detection in incoherent optical QAM signaling, and we experimentally confirm that its features can contribute realization of adaptive balance compensation.

We experimentally confirm that distortion in constellation of received signals due to imbalance of delay detection causes BER degradation in incoherent optical QAM signaling and propose and demonstrate distortion compensation technique using digital signal processing.

We propose a new DSP configuration for optical higher-order multilevel signaling with chromatic dispersion pre-distortion both for optical delay-detection and for coherent detection with digital delay detection, which is effective to significantly reduce hardware size.

The use of digital-delay detection for coherently received higher-order QAM signals is an attractive technique to improve phase noise and IF offset tolerance. The paper reviews its concept, implementation and performance improvement with experimental results.

Optical modulators and receivers in various material systems for higher-order modulation formats were discussed. LN-based modulators are useful for high-end LH systems. InP/Si-based modulators show applicability to advanced modulation formats. Powerful DSP will push InP/Si-based modulators into fields from Metro/SR applications. Many approaches in...

Application of digital-delay detection to considerably higher-order coherent multilevel signaling is verified by 100-Gbit/s PM-32QAM and 80-Gbit/s PM-16PSK transmission experiments with low implementation penalty, and their laser linewidth and IF-frequency offset tolerance is also shown.

We experimentally show 1.8-dB improvement of the tolerance to intra-channel non-linearity of coherent PM-16QAM signal using digital-delay detection over highly-dispersed links (Ex. SSMFs and long NZDSFs). The use of CD pre-distortion is also proposed to compensate its performance drop in low dispersion links.

We propose the use of receiver-side digital differential detection for coherent higher-order QAM signaling and demonstrate the improvement of laser linewidth and XPM tolerance in 320-km transmission experiment of 7-channel 80-Gbit/s PM-16QAM signals.

We present a practical receiver-side chromatic dispersion (CD) compensation scheme for higher-order multilevel signaling using optical delay-detection, and up to 40-Gbit/s 16QAM signaling experiments have been demonstrated with large tolerance to CD (±40-km SSMF) and laser phase noise (1-MHz linewidth).

Dispersion pre-distorted 40-Gbit/s 16QAM transmissions over 800-km SSMF (13300 ps/nm) using delay detection is demonstrated with large tolerance to laser phase noise (1-MHz linewidth DFB-LD) and pre-distortion error (±40-km SSMF).

Higher-order optical multilevel signaling is a key technique for future ultra-high speed signal transmission at 100 Gbit/s or beyond. In this paper, we review various multilevel transmitter and receiver designs realizing higher-order multilevel signaling with precise optical field modulation and demodulation capabilities.

We present a robust and effective chromatic dispersion (CD) compensation scheme in incoherent multilevel receivers with field-reconstruction technique. Eight-level (30-Gbit/s 8DPSK and 8QAM) signalling experiments are performed for the first time, resulting in >1200-ps CD tolerances.

We review basic concept and recent advance of the direct-detection (incoherent) higher-order multilevel signalling scheme for high-speed fibre link at 40, 100 Gbit/s or beyond, which is expected to serve as a simple alternative to the one with dual-polarization coherent detection.

In this section, we introduce the concept of direct-detection multilevel signaling technology, in which “optical delay-differential detection” is used as a main tool of signal phase detection. Significant progress toward more than four-level signaling has been achieved by the use of an “orthogonally coupled differential receiver” with digital signa...

Chromatic dispersion and differential group delay tolerance of 30-Gbit/s 8DPSK and 8QAM signaling is investigated experimentally, and we show 1.7 and 1.25 times enhancement of them in 8QAM signaling by simple receiver-side digital signal processing.

A proposal for the realization of highly sensitive optical direct-detection (incoherent) multilevel transmission with arbitrary signal constellation without optical chromatic dispersion (CD) compensation is reviewed in this paper. We introduce transmitter-side signal processing, ??CD pre-equalization,?? and ??phase preintegration?? techniques to re...

For the first time, over 100-Gbit/s direct-detection optical 16APSK (2-ASK and 8-DPSK) signaling without polarization multiplexing has been experimentally demonstrated.

Fibre nonlinear effect on incoherent multilevel signalling with co-propagating 10-G OOK channels is investigated by experiments and numerical simulations, and QAM signals with non-Euclidean decision boundaries are shown to alleviate SPM and XPM effects.

We review recently proposed advanced digital signal processings for ultra-high speed incoherent (direct-detection) multilevel signaling, realizing the use of arbitrary signal constellation, digital compensation of chromatic dispersion and high OSNR sensitivity.

Highly sensitive incoherent multilevel signaling is demonstrated with the sensitivity nearly comparable to coherent detection, and a 160-km SMF transmission of pre-distorted 40-Gbit/s 16-QAM signal without optical dispersion compensation is also performed.

Incoherent 30-Gbit/s 8-level 400-km and 35.8-Gbit/s 12-level 240-km standard-SMF transmissions are demonstrated without optical dispersion compensation for the first time.

The use of arbitrary signal constellation for incoherent optical multilevel signaling has been demonstrated with newly proposed transmitter-side digital phase pre-integration technique for the first time by up to 16 QAM signaling experiments.

Multi-level transmission having advantage of wider dispersion tolerance and narrower bandwidth, is one of the strong candidate for ultra high-speed serial interface. We review recent progress in multi-level systems, especially, incoherent multi-level receiver.

Generation and incoherent detection of a 30-Gbit/s 8-level staggered APSK signal using two-state MLSE detection is successfully demonstrated with a 140-km fiber transmission. Its better constellation design results in 1.5-dB sensitivity improvement from 8-DPSK signaling.

In order to explore the potential of optical multilevel signaling for high-speed optical fiber networks, an incoherent 50 Gbit/s 32-level optical multilevel signaling (combination of QASK and 8-DPSK) is demonstrated, for the first time. The application of an ISI-suppressed modulation technique, an orthogonally-coupled differential detection techniq...

An intersymbol interference (ISI)-suppressed optical multilevel modulation technique that is applicable to a wide range of binary and multilevel signaling is proposed. It employs binary phase-shift keying modulations that are generated by Mach-Zehnder intensity modulators as basic building blocks, and complex multilevel modulations are synthesized...

We propose a novel high-sensitive and cost-effective bias control scheme for MZ modulator with minimizing monitored power of backward-traveling light, and demonstrate its effectiveness with the experiments of 10-Gbit/s CSRZ and 43-Gbit/s RZ-DQPSK signals.

The optical multilevel signaling is a promising candidate to realize next-generation high-speed transceivers beyond 40 Gbit/s, or ultimate-capacity DWDM system. This paper discusses advanced modulation/demodulation schemes toward the practical implementation of more than 16-level signaling.

The feasibility of 50 Gbit/s 32-level optical multilevel signaling for high-speed optical fiber transmission is demonstrated in a 100-km DSF transmission for the first time, with digital joint coupled differential detection and digital signal processing.

We used highly refractive silicon etalons to develop a tunable dispersion compensator with plusmn250-ps/nm tunable range and low

Highly refractive silicon etalons have been used to reduce the insertion loss from 4.4 to 2.3 dB and a 40 Gbit/s tunable dispersion compensator with a plusmn250 ps/nm tunable range and <plusmn1.5 ps group-delay ripple has been developed. Owing to the low-group-delay ripple, a 1000 ps/nm tunable range in a double-pass configuration is demonstrated

We propose a novel concept of incoherent optical field detection receiver, and successfully demonstrate its feasibility in 240-and 400-km SMF transmissions with numerical chromatic dispersion compensation of 10-Gbit/s DQPSK and 5-Gbit/s DPSK signals. Introduction In the next-generation high-capacity optical fiber networks with the channel capacity...

A simple cancellation technique of SPM-induced phase shift in amplitude- and phase-modulated signalling is proposed and experimentally demonstrated in 100-km transmissions of 40-Gbit/s 16-level and 50- Gbit/s 32-level signals with >6.8 dB increase of input power.

Rapidly evolving multilevel signalling is a serious candidate of the next generation modulation format for high-speed and high-capacity optical communication links. In this paper, we review the recent progress of multilevel signalling technologies.

A virtually ISI-free optical multilevel modulation technique applicable to wide-range of multilevel formats is proposed. Superior QASK modulation performance is experimentally demonstrated with 3-dB sensitivity improvement in 40-Gbit/s 16-level amplitude- and phase-shift keying (APSK) modulation.

We investigate the impact of non-ideal modulating condition on multilevel APSK modulations, and find 4-% angular misalignment of orthogonal PSK components or residual chirp of 0.2 in ASK modulator causes 1-dB penalty of PSK components.

The optical multilevel modulation is one of the attractive candidates to significantly increase the channel bit rate and total capacity of future optical fiber communications. We review various multilevel modulation schemes proposed to date, including our experimental results of the 8-level and 16-level APSK modulation/demodulation schemes at 10 Gs...

We propose an optical eye pattern measurement technique of phase-modulated optical signals with high timing resolution applicable to multilevel signals and binary intensity-modulated signals generated by external modulators, for the first time, to our knowledge.

We experimentally demonstrated the dispersion compensation in a 40 Gb/s non-return-to-zero optical transmission, using photonic crystal coupled-cavity waveguides (PhC CCWs). In this experiment, the PhC CCWs consisted of one-dimensional SiO2/Ta2O5 thin film structures. The deteriorated optical signal after traveling a single-mode fiber (SMF) was com...

The characteristic degradation mechanism of the cross-phase modulation (XPM) effect on multilevel amplitude and differentially phase-shift keying (APSK) signals in dense wavelength-division-multiplexing (DWDM) transmissions is analyzed and experimentally investigated, for the first time. Based on the results, we successfully demonstrate a 0.48-Tb/s...

The performance of the chromatic dispersion monitoring method using statistical moments of asynchronously sampled waveform histograms is investigated in detail. We show its robustness against signal-to-noise ratio degradation and its wide monitoring range using experiments and a newly developed numerical simulation method. We also show the effectiv...

A practical 16-ary amplitude-phase-shift-keying (16APSK) optical modulation/demodulation scheme (4 bit/symbol) with a combination of multilevel amplitude and phase modulations is proposed, and its feasibility is successfully demonstrated for the first time. The spectral width of return-to-zero 16APSK signal is only 0.27 nm (33.75 GHz) at 20 dB down...

The XPM effect on optical 8-ary APSK (amplitude- and phase-shift keying) multi-level signals is experimentally investigated, for the first time. Its feasibility is demonstrated in 0.48 Tbit/s (30 Gbit/s/ch) 50-GHz-spaced 16-channel dense-WDM 160-km unrepeatered transmission.

A Raman fiber oscillator used for optical amplification is demonstrated to have lower double Rayleigh scattering, transient spikes, cross-phase modulation, and higher saturation input threshold compared with a conventional discrete Raman amplifier at similar operating conditions. This could be a promising technology for deployment in practical syst...

We experimentally evaluate the tolerance of 8-state/symbol optical modulation (binary ASK and QPSK) to chromatic dispersion and SPM, and investigate the cause of sensitivity degradation induced by simultaneous modulation of amplitude and phase.

We fabricate a compact integrated transmitter, in which an EA modulator is directly mounted on a driver-IC with a chip size of 1.9-mm×2.4-mm and demonstrate 40-Gbit/s operation of this modulator at a low driving voltage (V_p-p=0.7 V). The modulation efficiency, defined by f_{3 dB}/V_p-p' is a high 50 GHz/V. We further re...

Raman fiber oscillator used for intracavity amplification is demonstrated to have lower double Rayleigh scattering, transient spikes, cross phase modulation, and higher saturation input threshold compared with conventional discrete Raman amplifier at similar operating conditions and pump power.

A 4-level DD-PolSK system with phase modulators and alternative allocation of constellations, which led to a greatly simplified transceiver architecture was demonstrated. The anisotropy of the LiNbO 3 crystals was used to perform polarization modulation by launching the light at a 45° angle to the principal axis of the crystal. The avoidance of int...

To reduce the initial introduction cost of fiber Raman amplifiers, a novel in-service wavelength-band upgrade scheme is proposed. In this scheme, new pump lasers are added to an existing Raman amplifier already carrying wavelength-division-multiplexing (WDM) signals, and all the pump laser driving currents are changed synchronously through the tran...

The novel adaptive technique for compensating the fiber chromatic dispersion in high-speed optical-fiber transmission by using the higher order polarization-mode dispersion generated when two pieces of polarization-maintaining fibers are connected. Changes in signal wavelength and polarization state are tracked by an automatic polarization controll...

The basic property of degree of polarization (DOP) degradation of optical signal induced by polarization mode dispersion (PMD) in high-speed optical fiber transmission is investigated in detail. The DOP of the optical signal reflects the degree of waveform degradation caused by PMD, therefore, it is proposed to be used as the control signal judging...

We propose a new compact interferometric sensor for displacement measurement which utilizes the output intensity undulation caused by the optical feedback effect of a frequency modulated laser diode (LD). In order to stabilize the undulation signal, the relationship between the feedback rate and the oscillation behaviour of the LD was investigated...

The authors analyse the effect of cross-phase modulation (XPM) on wavelength division multiplexing (WDM) transmissions by experiments and numerical simulations, and provide the XPM limitation in a normalised and closed form formula. They also investigate the effect of XPM on dispersion compensated transmissions. It is shown that the location of dis...

In summary, we successfully transmitted l0-Gbit/s seven-channel WDM signals over 140-km of dispersion shifted fibre (DSF) with equal channel spacing of 0.8 nm by the bit-phase arranged RZ technique

We analyze the effect of cross-phase modulation (XPM) on IM/DD WDM transmission using numerical simulation, and provide the XPM limitation on WDM transmission with 1-dB penalty in a normalized form. We also investigate the effect of XPM on the dispersion compensated transmission in terms of the location and amount of compensation.

The authors investigate the fibre non-linear effect, self-phase modulation (SPM) and modulation instability (MI), in long distance (>1000 km) dispersion-compensated (DC) conventional optical fibre transmission using numerical simulation and experiments. They compare the results with those in the dispersion-shifted fibre (DSF) transmissions and clar...

Summary form only given. In summary, we performed a l0-Gbit/s four-channel transmission and verified that the cross phase modulation (XPM) combined with typical variation of fiber dispersion can significantly degrade the WDM transmission performance over dispersion shifted fiber (DSF), for the first time.

We introduce a novel method for generating nearly transform-limited picosecond optical pulses at multi-gigahertz repetition rates. Frequency-chirped optical pulses are first produced using a gain-switched semiconductor diode laser, and they are subsequently amplified by a semiconductor optical amplifier. The red frequency chirp in the gain-switched...

WDM transmission experiment at 80 Gbit/s (8×10 Gbit/s, 0.8 nm channel spacing) over 1171 km of standard singlemode fibre has been performed with a 90.1 km recirculating loop incorporating a dispersion compensation fibre and gain-equalising optical bandpass filters

We propose an approximate analytical evaluation technique of self-phase-modulation (SPM) in cascaded optical amplifier systems. The effect of SPM on optical pulse distortion is represented by the increase in the α-parameter of the transmitter. The waveform distortion is approximated by assuming that the pulse is transmitted over a dispersive fiber...

The authors demonstrate 10 Gbit/s dispersion-compensated long distance transmission over conventional (non-dispersion-shifted) fibres in a recirculating loop incorporating dispersion compensation fibres (DCF). We obtain a bit error rate below 10^-9 at 2245 km and show the feasibility of conventional fibre transmission over more than 2000...

The authors propose a novel WDM transmission technique combined with polarisation-division multiplexing (PDM) which doubles the frequency utilisation efficiency. This technique is used to transmit a 10 Gbit/s four-channel WDM signal with 0.5 nm channel spacing over 340 km and the signal is demultiplexed using an optical filter having 0.5 nm FWHM an...

The authors have successfully demonstrated 10 Gbit/s four-channel densely spaced WDM transmission over 500 km of dispersion-shifted optical fibre with a technique for suppressing FWM crosstalk. This result shows the feasibility of future WDM transmission systems

A highly efficient (2 V peak to peak for 22 dB extinction ratio) and extremely low-chirp (<64 pm) integrated electroabsorption modulator/DFB laser is demonstrated by reducing both electrical and optical crosstalk between the modulator and the laser. The device allowed 2.5 Gbit/s penalty-free transmission over repeaterless 200 km-long 1.3 mu m-optim...

For future wavelength-division-multiplexed (WDM) networks, optical frequency conversion will enable the flexible and efficient use of optical frequency bandwidth. However, the signal degradation at frequency converters limits the maximum size of the network. Noise due to optical frequency conversion using nearly degenerate four wave mixing in a sem...

A novel phase noise cancellation method for optical frequency conversion using nearly degenerate four wave mixing in a semiconductor optical amplifier is proposed. A -44 dBm sensitivity at BER=10^-9 is achieved for 155 MBit/s FSK transmission with 1750 GHz (14 nm) frequency conversion.

A compact optical displacement sensor is reported which is based on the light undulation caused by the optical feedback on a frequency modulated laser diode. For applying this effect to the metrology, the relationship between the feedback rate and the oscillation behavior of the laser diode was investigated experimentally. As the result, it was fou...

The possibility of a non-contact optical distance/displacement sensor, making use of optical intensity variations caused by optical feedback effects in laser diodes, was investigated. A compact sensor system and a simple signal processing technique were proposed to realise the sensor system. The experimental results indicated that the resolutions w...