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 200 Gbit/s, 10 Gsymbol/s-1024 QAM injection-locked coherent transmission over 160 km with a pilot-assisted adaptive equalizer.

We demonstrate a 10 Gsymbol/s, 1024 quadrature amplitude modulation (QAM) 160 km coherent transmission with an injection locking technique. Our newly developed, pilot-assisted adaptive equalizer has greatly improved the precision of waveform distortion compensation, and this has enabled us to increase the symbol rate to 10 Gsymbol/s in a 1024 QAM transmission. Thus, we could realize a 200 Gbit/s, 1024 QAM transmission over 160 km with a potential spectral efficiency of 12.6 bit/s/Hz.

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.

8 kHz linewidth, 50 mW output, full C-band wavelength tunable DFB LD array with self-optical feedback.

We describe a 1.5 µm, narrow linewidth, high output power wavelength tunable LD that employs a self-optical feedback circuit. By incorporating an optical circulator-based feedback circuit in a DFB LD array, we have successfully reduced the oscillation linewidth from several MHz to less than 8 kHz over the full C-band range. A high output power of　approximately 50 mW and a low relative intensity noise of less than -130 dB/Hz were simultaneously achieved. Furthermore, by employing a partial reflection mirror as a self-optical feedback circuit, we have also realized a full C-band wavelength tunable LD with a linewidth of less than 11 kHz and a simple laser configuration.

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.

1.5µm, mode-hop-free full C-band wavelength tunable laser diode with a linewidth of 8 kHz and a RIN of -130 dB/Hz and its extension to the L-band.

We demonstrate a narrow-linewidth, low-intensity noise, widely wavelength-tunable LD for which we adopted a long external cavity configuration (ECLD) with a tunable optical filter. By employing a simultaneous tuning circuit between the optical filter and the Fabry-Perot mode caused by the external cavity, the oscillation wavelength was successfully tuned over the full C-band without mode hopping. The tunable optical filter bandwidth was 0.55 nm (1530~1570 nm). We achieved a linewidth of less than 8 kHz obtained with a delayed self-heterodyne method. On the other hand, the linewidth estimated from the white noise level of the optical frequency modulation noise power spectrum of the laser output was as narrow as 314 Hz. A relative intensity noise (RIN) below -130 dB/Hz was simultaneously achieved over the C-band range. With the same configuration except for an optical filter with a filter bandwidth of 0.6 nm for L-band operation (1570~1610 nm), we also realized a full L-band wavelength tunable ECLD with a linewidth of less than 7.7 kHz and a RIN of below -126 dB/Hz.

42.3 Tbit/s, 18 Gbaud 64 QAM WDM coherent transmission over 160 km in the C-band using an injection-locked homodyne receiver with a spectral efficiency of 9 bit/s/Hz.

We demonstrate a 235-channel wavelength division multiplexing (WDM), polarization-multiplexed (pol-mux) 18-Gbaud 64 QAM coherent transmission of 160 km over the full C-band. By applying an injection-locked homodyne detection circuit to WDM coherent transmission, we have achieved low noise optical carrier-phase locking between transmitted data and a local oscillator over the full C-band range. As a result, a potential capacity of 42.3 Tbit/s data with a spectral efficiency of 9 bit/s/Hz was transmitted.

Color-image reconstruction for two-wavelength digital holography using a generalized phase-shifting approach.

We propose a color-image reconstruction method for two-wavelength digital holography using generalized phase-shifting digital holography (GPSDH). In this method, color interference fringes are captured by a digital camera with a Bayer array color filter, and phase shifting is simultaneously performed for all wavelengths. Color interference fringes are separated into three monochromatic interference fringes using a color-separation method that suppresses the color-filter crosstalk. The object wave is extracted from each monochromatic interference fringe using GPSDH, which prevents problems due to the phase shift wavelength dependence. Image reconstruction is performed using a shifted Fresnel transform-based method, in which the color reconstructed image is obtained by directly superposing the reconstructed images for all wavelengths. We verify the proposed method through optical experiments with a two-wavelength digital holography system. The results show that the dual-color image can be successfully reconstructed without chromatic aberration.

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.

1.55 µm hydrogen cyanide optical frequency-stabilized and 10 GHz repetition-rate-stabilized mode-locked fiber laser.

We describe a 1.55 µm hydrogen cyanide (HCN) optical frequency and repetition rate stabilized mode-locked fiber laser, where the optical frequency was locked to the P(10) HCN absorption line and the repetition rate was locked to 9.95328 GHz by using a microwave phase-locked loop. The optical frequency stability of the laser reached 5 x 10-11 with an integration time τ of 1 s. With a bidirectional pumping scheme, the laser output power reached 64.6 mW. To obtain a short pulse train, the average dispersion in the cavity was managed so that it was zero around 1.55 µm, resulting in a 0.95 ps pulse train. In addition, the stabilization of the optical frequency and the repetition rate, meant that the entire spectral profile remained the same for 24 hours.

320 Gbit/s, 20 Gsymbol/s 256 QAM coherent transmission over 160 km by using injection-locked local oscillator.

We demonstrate 20 Gsymbol/s, 256 QAM polarization multiplexed (pol-mux) 320 Gbit/s coherent transmission. By employing an LD-based injection locking circuit, we achieved low noise optical carrier-phase locking between the LO and the data signal. Furthermore, frequency domain equalization and digital back-propagation enabled us to realize precise compensation for transmitted waveform distortions. As a result, a 320 Gbit/s data was successfully transmitted over 160 km with a potential spectral efficiency of 10.9 bit/s/Hz. This is the highest symbol rate yet achieved in a pol-mux 256 QAM coherent transmission. In addition, we also describe a pol-mux 256 QAM transmission at a symbol rate of 10 Gsymbol/s.