Coherent Nyquist optical pulse transmission at nearly 1-Tb/s/λ over 1,600 km with a capacity of 21.5 Tb/s using PS-32 QAM signals.

We demonstrate the 1,600-km transmission at nearly 1-Tb/s/λ signals with a capacity of 21.5 Tb/s. Probabilistic shaping was newly applied to high-speed coherent optical Nyquist pulse transmission systems to maximize the transmission capacity. Employing a 160-GBd PS-32 QAM format, WDM signals at nearly 1-Tb/s/λ were successfully transmitted over 1,600 km with a capacity of 21.5 Tb/s.

Eigen-function division multiplexed coherent optical transmission in time domain by using higher-order Hermite-Gaussian pulses.

We describe a new multiplexing technique and its application to demultiplexing in the time domain by using higher-order Hermite-Gaussian (HG) pulses, which are solutions of the Schrödinger equation. We call this technique eigen-function division multiplexing (EDM). This method enables us to further increase the total transmission capacity by superimposing many different HG pulses in the same time slot. This technique is different from a conventional optical time domain multiplexing (OTDM) technique using interleaving, where one pulse exists only in one time slot. The transmitted EDM HG pulses can be demultiplexed by adopting the time-domain orthogonality of the HG pulses (eigen-function orthogonality). The information carried by the mth-order HG pulse (HGm pulse) can be coherently detected by a photo detector, where photo-mixing with a phase-locked HGm pulse generated by a local oscillator can realize demultiplexing. The overlap integral with a different HG pulse becomes zero due to the time domain orthogonality. First, we show numerically that such a new EDM transmission scheme in the time domain is possible. We then show experimentally that we could successfully carry out an EDM HG coherent pulse transmission with four different HG pulses (HG0, HG1, HG2, and HG3), where we report a 400∼480 Gbit/s (10 Gbaud x 4 eigen-functions x 2 pol-mux.) 32∼64 QAM EDM transmission over 300∼450 km.

Broadband injection-locked homodyne receiver for digital coherent transmission using a low Q Fabry-Perot LD.

We describe the broadband injection-locking performance of a Fabry-Perot laser diode (FP-LD) for digital coherent transmission. The dynamic locking bandwidth of the FP-LD was as wide as 28.8 GHz, which makes it possible to achieve precise carrier-phase synchronization with extremely low phase noise over a wide frequency range. By applying the FP-LD as an LO in an injection-locked homodyne receiver for digital coherent quadrature amplitude modulation (QAM) transmission, we demonstrate, for the first time, the precise demodulation of 3, 10 and 20 Gbaud 256 QAM signals even when using a widely and randomly phase-modulated transmitter laser. This is attributed to the excellent wideband dynamic injection-locking characteristics of the FP-LD.

Chromatic dispersion dependence of GAWBS phase noise compensation with pilot tone.

We describe experimental and numerical results regarding the influence of chromatic dispersion in optical fibers on guided acoustic-wave Brillouin scattering (GAWBS) phase noise compensation with a pilot tone (PT). We compared the compensation performance for GAWBS phase noise generated in an ultra-large-area fiber (ULAF) where DULAF = 21 ps/nm/km with that in a dispersion-shifted fiber (DSF) where DDSF = -1.3 ps/nm/km and found that the performance depends strongly on chromatic dispersion. The numerical analysis shows that the group delay between the signal and PT caused by chromatic dispersion reduces the GAWBS noise correlation between them, which degrades the compensation performance for GAWBS phase noise. It is clarified that a condition for effective GAWBS compensation is that the group delay between the signal and PT should be less than half the period of the GAWBS phase noise component.

Spectrally efficient pilot tone-based compensation of inter-channel cross-phase modulation noise in a WDM coherent transmission using injection locking.

We propose the precise and wideband compensation of the nonlinear phase noise caused by cross-phase modulation (XPM) among WDM channels using a pilot tone (PT) and injection locking for short-reach, higher-order QAM transmission. A high spectral efficiency is maintained by sharing a single PT among multiple channels. We describe a 60 ch, 3 Gbaud PDM-256 QAM transmission over 160 km, where the bit error rate was improved from 6 × 10-3 to 2 × 10-3 by employing the proposed XPM compensation technique, with a spectral efficiency of 10.3 bit/s/Hz. We also analyze the influence of the group delay caused by fiber chromatic dispersion that determines the compensation range achievable with a single PT. We obtained good agreement with the experimental results.

Precise measurements and their analysis of GAWBS-induced depolarization noise in various optical fibers for digital coherent transmission.

We undertake precise measurements of guided acoustic wave Brillouin scattering (GAWBS) depolarization noise caused by the TR2,m mode (torsional and radial mode) in various fibers and analyze the results. And we describe the influence of the noise on digital coherent transmission. We first show that the TR2,m mode is distributed over a wider bandwidth when the effective core area Aeff of the fiber is smaller. We then describe the strong mode-number dependence of the depolarization power generated from the profile of the refractive index change induced by the TR2,m mode. We also use two methods to measure the polarization crosstalk (XT) induced by the depolarization, namely, a direct detection method with a photodiode and a conventional power detection method with an optical spectrum analyzer. The results of the two methods agree well, and the XT increase is inversely proportional to the fiber Aeff and proportional to fiber length. Finally, we evaluate the influence of the GAWBS-induced XT on the BER characteristics in a coherent QAM transmission, where we find that the influence of the TR2,m mode is much weaker than that of the R0,m mode (radial mode). That is, the error-free transmission distance in standard single-mode fiber is extended to 8600 km for 256 QAM signal assuming hard-decision FEC with a 7% overhead. This distance is seven times longer than that obtained with the R0,m mode.

Injection-locked 256 QAM WDM coherent transmissions in the C- and L-bands.

We demonstrate WDM 256 QAM coherent transmissions with injection locking in the C- and L-bands and compare the transmission performance in the two bands. Although four-wave mixing (FWM) is more significant in an L-band EDFA than in a C-band EDFA, the FWM did not accumulate through the transmission and the FWM components were hidden by the ASE noise level. Since the FWM was weakened by the decorrelation of the WDM signals during the transmission, the transmission performance in the L-band was the same as that in the C-band. The injection locking circuit enabled precise carrier-phase synchronization between a data signal and a local oscillator regardless of the transmission band. By using this circuit, we successfully transmitted 58.2 and 57.6 Tbit/s 256 QAM WDM signals over 160 km with a spectral efficiency of 12 bit/s/Hz in the C- and L-bands, respectively.

GAWBS phase noise characteristics in multi-core fibers for digital coherent transmission.

We describe the guided acoustic-wave Brillouin scattering (GAWBS) phase noise characteristics in multi-core fibers (MCFs) used for a digital coherent optical fiber transmission both experimentally and analytically. We first describe the GAWBS phase noise in an uncoupled four-core fiber with a 125 µm cladding and compare the phase noise spectrum with that of a standard single-mode fiber (SSMF). We found that, unlike SSMF where the R0,m mode is dominant, off-center cores in MCF are affected by higher-order TRn,m modes. We then report measurement results for GAWBS phase noise in a 19-core fiber with a 240 µm cladding. The results indicate that the cores exhibit different spectral profiles depending on their distance from the center of the fiber, but the amount of phase noise generated in each core is almost identical. These results provide a useful insight into the space division multiplexing transmission impairments in digital coherent transmissions using MCF.

Theoretical and experimental analyses of GAWBS phase noise in various optical fibers for digital coherent transmission.

We describe the fiber structural dependence of guided acoustic-wave Brillouin scattering (GAWBS) phase noise in a digital coherent optical fiber transmission. We present theoretical and experimental analyses of GAWBS phase noise spectra in three types of optical fibers and show that the GAWBS resonant modes are distributed over a wider bandwidth as the effective core area of the fiber becomes smaller. We also use a vector signal analysis to show phase fluctuations caused by GAWBS. On the basis of these analyses, we show that the GAWBS phase noise fluctuation has a Gaussian distribution, which is used to evaluate its influence on the BER characteristics in a coherent QAM transmission. As a result, we found that the error-free transmission distance in SSMF is limited to 4600, 1200, and 340 km with 64, 256, and 1024 QAM, respectively, assuming a hard-decision FEC with a 7% overhead. These results provide useful insights into the influence of GAWBS on digital coherent transmission.

Suppression of large error floor in 1024 QAM digital coherent transmission by compensating for GAWBS phase noise.

There is a large error floor in an ultra multi-level digital coherent transmission signal of 1024 QAM or higher, and we have yet to determine its origin. In this paper, we show that this large error floor results from guided acoustic-wave Brillouin scattering (GAWBS) phase noise. We prove experimentally that such an error floor can be greatly reduced by compensating for the GAWBS noise with a phase modulation technique. We show that the BER of a 1024 QAM signal was reduced from 8.7 × 10-4 to 2.7 × 10-4 after a 160 km transmission with GAWBS noise compensation. Furthermore, we successfully extend the transmission distance from 160 to 240 km with a 7% overhead forward error correction.

Single-channel 15.3 Tbit/s, 64 QAM coherent Nyquist pulse transmission over 150†km with a spectral efficiency of 8.3 bit/s/Hz.

We report the first single-channel 15.3 Tbit/s, 1.28 Tbaud, 64 QAM transmission using 670 fs coherent Nyquist pulses. We newly constructed an optical gate to improve the signal-to-noise ratio (SNR) of the homodyne detection signal, a coherent spectral expansion technique, and an optical phase-locked loop (OPLL) circuit with a 0.6 deg. phase noise. We also constructed an active 70 fs timing stabilization circuit between the OTDM signal and Nyquist LO pulse to realize precise homodyne detection. With these new techniques, we successfully achieved a record speed of 15.3 Tbit/s in a single channel transmission over 150 km with a spectral efficiency of 8.3 bit/s/Hz.

Single-channel 10.2 Tbit/s (2.56 Tbaud) optical Nyquist pulse transmission over 300 km.

We describe a single-channel 10.2 Tbit/s online transmission using non-coherent ultrashort optical Nyquist pulses. A 10.2 Tbit/s signal was generated at a symbol rate of as fast as 2.56 Tbaud with a polarization-multiplexed DQPSK format. We developed a new ultrafast optical sampler for Nyquist OTDM demultiplexing with a nonlinear optical loop mirror, an RZ-CW conversion technique to improve the SNR, and an active stabilization technique providing stable long-term demultiplexing operation. With precise higher-order dispersion compensation up to fourth order, a 10.2 Tbit/s signal was transmitted over 300 km for the first time as a real-time demonstration with a spectral efficiency of 2.5 bit/s/Hz. We also report a 10.2 Tbit/s transmission over 225 km with a spectral efficiency of 3.7 bit/s/Hz, which we realized by reducing the roll-off factor to zero.

Single-channel 7.68 Tbit/s, 64 QAM coherent Nyquist pulse transmission over 150 km with a spectral efficiency of 9.7 bit/s/Hz.

We achieved a record capacity of 7.68 Tbit/s in a single-channel OTDM transmission with a 9.7 bit/s/Hz spectral efficiency, where a polarization-multiplexed 640 Gbaud, 64 QAM coherent Nyquist pulse has been transmitted over 150 km. In this scheme, a 1.39 ps optical Nyquist pulse with an OSNR of 53 dB at a 0.1 nm resolution was generated by combining a mode-locked laser and a highly nonlinear fiber and used at both the transmitter and receiver. Phase synchronization was achieved between these pulse sources with an advanced optical phase-locked loop based on the higher harmonics of the mode-locked laser mode. In addition, we suppressed a nonlinear phase rotation at an EDFA in the transmitter by broadening the pulse width with second-order dispersion and recompressed it to the original pulse width before a 150 km transmission link. We succeeded in a bit error rate below 2 x 10-2 for all tributaries.

Observation of guided acoustic-wave Brillouin scattering noise and its compensation in digital coherent optical fiber transmission.

We describe the first observation of guided acoustic-wave Brillouin scattering (GAWBS) phase noise in a digital coherent optical fiber transmission. GAWBS noise, which is a forward lightwave generated by thermally excited vibration modes in a cylindrical fiber structure, occurs coherently not only in a signal at a single carrier frequency, but also in modulated wide-band optical signals. Since the signal-to-GAWBS-noise ratio is independent of signal power, it has caused problems in various fields including quantum optics. We point out that GAWBS noise exists even in a digital coherent transmission system such as quadrature amplitude modulation (QAM) and degrades the transmission performance since the phase noise is inevitably included within the bandwidth of the transmitted data. We propose two analogue and one digital method to compensate for the GAWBS noise and demonstrate improved performance in a QAM digital coherent transmission.

Experimental and numerical comparison of probabilistically shaped 4096 QAM and a uniformly shaped 1024 QAM in all-Raman amplified 160 km transmission.

We describe an experimental and numerical comparison of a probabilistically shaped (PS) 4096 QAM signal and a uniformly shaped 1024 QAM signal. Both modulation formats have the same transmission rate and a spectral efficiency of 15.3 bit/s/Hz. In the computational simulation, we compared the generalized mutual information (GMI) of both modulation formats with bit-wise soft decision decoding and bit-wise hard decision decoding. For bit-wise hard decision decoding with an overhead of 7%, a shaping gain of 1.8 dB was attained. Then we successfully transmitted a single-channel PS-4096 QAM signal for the first time in an all-Raman amplified 160-km link, in which the transmission performance was improved compared with a uniformly shaped 1024 QAM with the same transmission rate. Transmissions with a high QAM multiplicity were achieved by using an optical phase-locked loop (OPLL) and a frequency stabilized fiber laser locked to an acetylene absorption line. Thanks to a shaping gain based on a bit-wise hard decision decoder, the 1.9-dB power margin, which agreed with the simulation result to within 0.1 dB, was increased after transmission.

Low-loss and reflection-free fused type fan-out device for 7-core fiber based on a bundled structure.

We describe a fused type fan-out device for 7-core fiber based on a bundled structure, which has no taper structure and a highly accurate core arrangement. We evaluated the repeatability of the splice loss characteristics of the fan-out device by splice testing 10 samples, resulting in an average splice loss of as low as 0.3 dB with a deviation of 0.048 dB. The crosstalk between the center and outer cores was less than -52 dB. Furthermore, the power damage threshold was higher than 1 W and the amount of Fresnel reflection at the splice point between the fiber bundle and the 7-core fiber was lower than the Rayleigh scattering level because of the arc fusion splicing.

Single-channel 3.84 Tbit/s, 64 QAM coherent Nyquist pulse transmission over 150 km with a spectral efficiency of 10.6 bit/s/Hz.

We report a polarization-multiplexed 320 Gbaud, 64 QAM coherent Nyquist pulse transmission with a frequency-stabilized mode-locked laser and a modified digital back-propagation method for pulse transmission. Using a combination consisting of a mode-locked laser and a pulse shaper, we obtained a Nyquist pulse with a high OSNR of 51 dB. We achieved error free operation under a back-to-back condition with the OSNR improvement. By developing a new digital back-propagation method for pulse propagation, we achieved a bit error rate below the 7% forward error correction limit of 2x10-3 for all the tributaries of the OTDM signal data after a 150 km transmission. As a result, single-channel 3.84 Tbit/s data were successfully transmitted over 150 km with a spectral efficiency of 10.6 bit/s/Hz.

440 fs, 9.2 GHz regeneratively mode-locked erbium fiber laser with a combination of higher-order solitons and a SESAM saturable absorber.

We report a regeneratively FM mode-locked erbium fiber laser operating at 9.2 GHz, in which a SESAM saturable absorber and a higher-order soliton effect are combined to generate a 440 fs pulse train. Higher-order solitons have a narrow center peak while accompanied by pedestals on the wing, but such components can be well suppressed by the saturable absorber, which enables very efficient pulse narrowing. As a result, 440 and 480 fs transform-limited sech pulses were successfully obtained at output powers of 15 and 45 mW, respectively.

Roll-off factor dependence of Nyquist pulse transmission.

We evaluate the dependence of system performance on the roll-off factor, α, of a Nyquist pulse in a single-channel 1.28 Tbit/s-525 km transmission both experimentally and analytically. Low α values are preferable in terms of spectral efficiency and tolerance to chromatic dispersion and polarization-mode dispersion, while a strong overlap with neighboring symbols results in larger nonlinear impairments. On the other hand, a Nyquist pulse with high α values also suffers from nonlinearity due to higher peak power. As a result, we found experimentally that the optimum α value is 0.4~0.6, which agrees well with the analysis.

Single-channel 40 Gbit/s digital coherent QAM quantum noise stream cipher transmission over 480 km.

We demonstrate the first 40 Gbit/s single-channel polarization-multiplexed, 5 Gsymbol/s, 16 QAM quantum noise stream cipher (QNSC) transmission over 480 km by incorporating ASE quantum noise from EDFAs as well as the quantum shot noise of the coherent state with multiple photons for the random masking of data. By using a multi-bit encoded scheme and digital coherent transmission techniques, secure optical communication with a record data capacity and transmission distance has been successfully realized. In this system, the signal level received by Eve is hidden by both the amplitude and the phase noise. The highest number of masked signals, 7.5 x 10(4), was achieved by using a QAM scheme with FEC, which makes it possible to reduce the output power from the transmitter while maintaining an error free condition for Bob. We have newly measured the noise distribution around I and Q encrypted data and shown experimentally with a data size of as large as 2(25) that the noise has a Gaussian distribution with no correlations. This distribution is suitable for the random masking of data.