High-capacity carrier signals with high-order modulation are being considered for data center interconnection (DCI) applications as they can reduce the cost per bit by decreasing the number of ...devices and increasing fiber capacity. However, signal quality degradation due to device imperfections affecting the frequency response has limited the capacity per carrier of PDM-256QAM signals to under 400 Gbps. Here, it is possible to use a calibration method for dealing with device imperfections wherein a fixed equalizer separately calibrates the frequency response of the transmitter and receiver. Our previous study showed that PDM-256QAM signals at up to 48 GBaud could be transmitted over 100 km, while it used an experimental setup with total 3-dB analog bandwidth of 13 and 20 GHz for transmitter and receiver, respectively. A micro-integrable tunable laser assembly with a typical linewidth of 20 kHz, maximum under 100 kHz. Actual DCI applications, however, are expected to use dense wavelength division multiplexing (WDM), so in this study, we examined transmission of 10-WDM 48-GBaud PDM-256QAM signals in a 50-GHz grid and evaluated the WDM penalties. The resulting spectral efficiency of 600-Gbps/carrier WDM transmissions was as high as 12.02 bit/s/Hz.
This paper presents analytical results on longitudinal power profile estimation (PPE) methods, which visualize signal power evolution in optical fibers at a coherent receiver. The PPE can be ...formulated as an inverse problem of the nonlinear Schrödinger equation, where the nonlinear coefficient (and thus signal power) is reconstructed from boundary conditions, i.e., transmitted and received signals. Two types of PPE methods are reviewed and analyzed, including correlation-based methods (CMs) and minimum-mean-square-error-based methods (MMSEs). The analytical expressions for their output power profiles and spatial resolution are provided, and thus the theoretical performance limits of the two PPE methods and their differences are clarified. The derived equations indicate that the estimated power profiles of CMs can be understood as the convolution of a true power profile and a smoothing function. Consequently, the spatial resolution and measurement accuracy of CMs are limited, even under noiseless and distortionless conditions. Closed-form formulas for the spatial resolution of CMs are shown to be inversely proportional to the product of a chromatic dispersion coefficient and the square of signal bandwidth. With MMSEs, such a convolution effect is canceled out and the estimated power profiles approach a true power profile under a fine spatial step size.
In this paper, we discuss the beat-noise induced by a distributed Raman amplifier system with forward pumping and propose a technique to suppress it. To construct a Raman amplifier, several ...techniques are required. First, the pump light must be depolarized due to the polarization dependence of the Raman gain. Second, the intensity noise of the pump light must be suppressed to stabilize the Raman gain. Due to the fast response of Raman amplifiers, especially forward-pumped, the instability of the pump power induces relative intensity noise (RIN) in the amplified signal light. To achieve depolarization, an orthogonally polarized pump light emitted from two different laser sources was widely used in conventional Raman pump units. These sources often have a fiber Bragg grating (FBG) to lock their wavelengths. In this paper, we show that a FBG can induce fluctuations in Raman gain. We also show that an orthogonally polarized pump laser light can induce beat noise in the amplified signal. We propose a pump unit that can suppress this beat noise. We measure the RIN of the amplified light using our proposed technique and demonstrate its advantages over a conventional Raman pump unit. Finally, we present experimental results of an optical data transmission with a forward-pumped Raman amplifier. Utilizing our proposed noise suppression technique, the signal-to-noise ratio of the probabilistically-shaped 36-QAM signal after a 1,920-km transmission is improved compared with the results using a conventional Raman pump unit.
We investigate the random intensity noises that occur in forward-pumped distributed Raman amplifier systems. First, we show pump-to-signal intensity noise transfer characteristics, which strongly ...depend on the group velocities of pump light and signal light in optical fiber used as a gain medium. When signal light is in the C-band, dispersion shifted fiber (DSF) transfers much larger noise compared with standard single mode fiber. Next, we discuss the origin of the noise induced in pump light. We define the concept of "synthesized polarization" and show that fluctuation in the state of synthesized polarization (SOSP) can induce a large gain instability even if the relative intensity noise (RIN) of each pump laser source is negligible experimentally. Next, we propose a novel optical depolarizer for pump light. It can simultaneously depolarize pump light generated by multiple laser sources. Moreover, it can manage optical phases precisely to suppress the gain instability induced by the fluctuation in SOSP. Finally, we present a measurement of the Q factor of a 16-QAM signal (32 Gbaud) after 35-km transmission, with and without a forward-pumped Raman amplifier. Two pump laser sources with RINs of -134 to -126 dB/Hz were depolarized by the proposed depolarizer, and the generated on-off gain was 7 dB. Though amplified signal light was in the C-band and the gain medium was DSF, 16-QAM transmission specifications were successfully improved, and acceptable noise was observed for 5 hours operation.
Ultra-wideband (UWB) wavelength division multiplexed (WDM) transmission using high-order modulation formats is one of the key techniques to expand the transmission capacity per optical fiber. For UWB ...systems, the nonlinear interaction caused by inter-band stimulated Raman scattering (SRS) must be considered. Therefore, we have proposed and demonstrated a scheme to optimize the fiber input powers for UWB transmission systems considering the signal power transition caused by the inter-band SRS. We demonstrated a single-mode capacity of 150.3 Tb/s using the proposed power optimization scheme with 13.6-THz UWB in the S -, C -, and L -bands over 40-km transmission. Spectral efficiency of 11.05 b/s/Hz was achieved with 272-channel 50-GHz spaced WDM signals of 45-GBaud polarization division multiplexed 128 quadrature amplitude modulation.
High throughput signals with high spectral efficiency (SE) are attracting interest for their application to data center interconnection application. For such application, a high baud rate and ...high-order modulation are desirable. This article describes each of the three steps in designing a high SE signal with a net rate of 1-Tbps/λ in detail. First, the baud rate dependency of the signal-to-noise ratio (SNR) is obtained by applying methods such as precise digital calibration technique. Test signals of PDM -16 quadrature amplitude modulation (QAM), PDM-64QAM, and PDM-256QAM were compared to illustrate the modulation format dependence on signal SNR. Clipping ratio optimization and joint digital equalization and optical equalization were performed to maximize the SNR. Second, the achievable capacity is calculated from the obtained baud rate dependency of the SNR. Symbol rate is selected to maximize the achievable capacity. WDM-grid and the Nyquist pulse shaping roll-off factor is selected to achieve a high SE. Third, the rate parameter of probabilistically shaped (PS) signal are determined to generate 1-Tbps/λ net rate signal. A high fiber capacity of 41-Tbps is successfully demonstrated with C-band WDM transmission over 100 km with 41-WDM 96-Gbaud PDM-PS-256QAM signals in a 100-GHz grid.
In polarization-multiplexed digital coherent systems, polarization dependent loss (PDL) is a dominant factor limiting the capacity and distance of fiber-optic transmission systems. In this paper, we ...propose polarization-wise power profile estimation (PPE) based on a linear least squares algorithm to estimate the value and location of excessive PDLs. The method visualizes polarization-wise signal power profiles at a coherent receiver with high sensitivity. We also experimentally demonstrate that PPE can detect PDL in an optical multi-span transmission link using the method. The experiment was performed using dual-polarization transmission over a 3-span × 50-km link, with a PDL resolution of 1-3 dB and a spatial step size of 1 km. In high power conditions, all estimated PDL values agree with actual values with estimation errors of well below 0.5 dB.
This paper presents a linear least squares method for fiber-longitudinal power profile estimation (PPE), which estimates the optical signal power distribution throughout a fiber-optic link at a ...coherent receiver. The method finds the global optimum in the least squares estimation of the longitudinal power profiles; thus, its results closely match the true optical power profiles and locate loss anomalies in a link with high spatial resolution. Experimental results show that the method achieves accurate PPE with an RMS error of 0.18 dB from OTDR. Consequently, it successfully identifies a loss anomaly as small as 0.77 dB, demonstrating the potential of a coherent receiver in locating even splice and connector losses. The method is also evaluated under WDM conditions with optimal system fiber launch power, highlighting its feasibility for use in practical operations. Furthermore, the fundamental limit for stable estimation and the spatial resolution of least-squares-based PPE are quantitatively discussed in relation to the ill-posedness of the PPE by evaluating the condition number of the nonlinear perturbation matrix.
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