With the exponentially increasing demand for high-speed information transfer, the role of probabilistic shaping has become increasingly important; however, it necessitates modifications to existing digital signal processing architectures to ensure error-free transmission. In this work, we propose a two-stage carrier synchronization method combining a kernel recursive least-squares (KRLS) filter with zero-averaging blind phase search (BPS). The method jointly synchronizes phase and frequency at the receiver while also compensating for any nonlinear phase noise present in the data. Simulation results on a probabilistically shaped 64-QAM constellation at a symbol rate of 33.33 GBaud per polarization demonstrate a significant improvement in generalized mutual information (GMI) compared to other widely used algorithms under various channel conditions. At high shaping factors, where fast Fourier transform (FFT)-based frequency estimators typically fail, the proposed method maintains accurate carrier synchronization and delivers consistently high GMI. When the optical signal-to-noise ratio (OSNR) is varied by adjusting the incident optical power, our method achieves an optimal OSNR gain of 4 dB compared to the commonly used FFT + BPS combination, thereby enhancing nonlinear tolerance and also shown to extend the transmission reach to 8000 km under a normalized GMI threshold. The computational load is substantially reduced compared to traditional BPS and FFT, making the approach well-suited for real-time digital signal processing in long-haul coherent systems.

We propose and experimentally demonstrate a novel, to our knowledge, technique to achieve automatic mode-locking in nonlinear polarization rotation-based fiber lasers. The polarization dynamics of the fiber laser are studied in the framework of a support vector regression model. The feature vectors for the support vector regression are obtained by downsampling the photocurrent spectrum, acquired by an electrical spectrum analyzer, after optical to electrical down conversion of the laser output. Combining support vector regression for anomaly detection along with sliding window correlation of optimal window size, we obtain an empirical model that predicts the precise voltage to be applied to the electronic polarization controller to produce the required polarization state for automatic mode-locking at the fundamental frequency. The proposed algorithm offers fast operation with low computational complexity, making it practical for hardware realization in real-time laser control. Using this method, we demonstrate stable mode-locking in an Er-doped fiber ring laser operating at 1.55 & micro;m having a fundamental repetition rate of 185 kHz. (c) 2026 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

In this work, we investigate the impact of polarization space exploration strategies on the detection of mode-locking regimes in a rectangular pulsed passive mode-locked (PML) fiber laser. A comparative study between pseudo-random and Sobol quasi-random sampling is performed for EPC voltage selection. The Sobol sequence demonstrates superior space-filling characteristics, resulting in faster detection of mode-locking and a higher diversity of accessible regimes, including fundamental, harmonic, dual-pulse, and pulse-bunching states. Following the initial detection of mode-locking, a hill-climbing optimization method with a customized objective function is implemented to drive the system toward stable fundamental mode-locking (FML). The objective function is designed to suppress harmonic or multi-pulse regimes while simultaneously promoting pulse compression and peak amplitude enhancement. Experimental results show improvement of pulse characteristics across iterations, including reduction of pulse width and increase in peak amplitude with the increase in the objective function. The proposed method is further shown to maintain stable FML operation over a period of 65 min by adaptively adjusting the polarization for small environmental perturbations. The combined quasi-random exploration and hill climbing provides a simple, computationally efficient, and robust approach for automatic and stable FML operation in PML fiber lasers.

We propose and experimentally demonstrate a real-time automatic polarization control scheme for fiber-optic systems based on golden section search (GSS) and proportional endless reset-free (PE-RF) control action. In the proposed method, we separate the orthogonal components of the incoming state of polarization of light and use one of the polarization components to generate a feedback signal such that the power in the other component is maximized. The feedback signal is applied to waveplates of an electronic polarization controller for which the required voltage signals are derived by the application of GSS and PE-RF control algorithms, implemented on an Arduino Uno R3 microcontroller. Our proposed method is simple, cost-effective, and has a fast response to mitigate polarization fluctuations in optical fiber links. We demonstrate this by maintaining stable polarization operation of binary and 4-PAM optical links consisting of 50 km optical fiber operating at 1550 nm. The proposed method maintains a Q-factor to within ±0.5dBm with excellent eye opening over 100 min of continuous operation.

We demonstrate pulse width and wavelength tunable Er-doped mode lock laser (PWT-MLL) using pulsed modulation as opposed to sinusoidal modulation. We develop a theoretical model based on discrete coupled modes and solve it numerically to study the effect of pulsed modulation on the pulse width tunability of our proposed PWT-MLL. We also study the impact of intra-cavity filter on the performance of our laser. Using two types of optical bandpass filters (OBPFs)—a fixed filter with a 3 dB bandwidth of 0.7 nm and a tunable filter with a 3 dB bandwidth of 0.25 nm—we achieve both temporal and wavelength tunability at a fixed repetition rate. The pulse width of the laser is tunable from 2.3 ns to 370 ps in the case of a tunable filter and from 700 ps to 91 ps in the case of a fixed filter. The wavelength can be tuned across the entire C-band. Our results demonstrate that supermode noise suppression ratio (SMSR) and signal-to-noise ratio (SNR) are significantly influenced by pulse modulation duty cycles. Using Tunable Fiber Bragg grating (TFBG), the average SMSR was approximately 27 dB across all duty cycles. In contrast, the Dense Wavelength Division Multiplexing (DWDM) filter maintained a high SMSR only down to a 20 % duty cycle before declining sharply. In both configurations, the average SNR was close to 50 dB.

We demonstrate an all-fiber broadband photon pair source based on four wave mixing (FWM) process in dispersion flattened highly non-linear fiber (DF-HNLF). The fiber exhibits a zero dispersion slope near 1550 nm, allowing phase-matched FWM over entire S, C, and L bands and thus efficient generation of signal and idler photons. A comparative theoretical study between conventional dispersion-shifted fiber (DSF) and DF-HNLF highlights the spectral range differences in the generation of photon pairs. We measure coincidence counts at three different sets of wavelengths. To study the effects of Raman scattering, which acts as noise source in these types of fibers, we calculate the correlation g((2))(tau) at different pump powers. We use stimulated emission tomography to characterize the generation of photon pairs across the S, C, and L bands. We show that DF-HNLF is an ideal medium for generating correlated photons over a broad spectral range (>100nm), making it suitable for frequency-multiplexed quantum communication systems. We estimate the photon pair generation efficiency to be 0.05 W-2/pulse.

In this article, we propose and experimentally demonstrate a novel synchronization method for quantum key distribution (QKD) systems. The method consists of maximizing the visibility of frequency-domain interference of optical sidebands about an optical carrier at the receiver node. The sidebands are generated by phase modulation of the optical carrier by an radio-frequency (RF) signal whose phase can be dynamically varied. The phase-variable RF signal is generated by the field-programmable gate array (FPGA) at the transmitter and the receiver using GTX transceivers. In order to facilitate this, we use square waveforms for RF signal instead of the conventional sinusoidal signals. We derive mathematical expressions for sideband power as a function of the phase difference between RF signals at transmitter and receiver. The phase is adjusted using dynamic phase shifter module, implemented by the FPGA. We propose a complete workflow that allows transmitter and receiver synchronization to within 12.6 ps directly over the quantum channel of QKD systems. Once synchronized, the same system can be switched over to quantum transmission by user-defined time delay. The workflow was implemented on a Xilinx Kintex-7 KC705 FPGA board. We studied the robustness of our technique by evaluating the stability of the interferometer over an operation of 10 min with standard deviation of interference to be less than 9% of the mean detection amplitude.

In this paper, we present the improved stimulated Brillouin-induced self-heterodyne (ISISH) method that overcomes the effect of amplified spontaneous emission (ASE) noise of erbium doped fiber amplifier (EDFA) on linewidth measurements. In addition, we eliminate the beating effects of additional sideband during the measurement of narrow laser linewidth. Moreover, we experimentally demonstrate the effect of amplified spontaneous Brillouin scattering (ASBS) noise on the variation of the measured linewidth of the laser under test (LUT) with varying pump power. We measure the Brillouin amplifier gain, which is essential for the ISISH technique, as 13.5, 12.7 and 9.5 dB at the pump powers of 16, 12 and 8 mW, respectively. These results are consistent with the analytical values of 14, 13 and 12.5 dB at the same variations in pump powers as previously reported. Experimental results show the 20 dB linewidths of 4, 6.7 and 10 kHz at pump powers of 16, 12 and 8 mW, respectively. We also perform the stimulated Brillouin-induced self-heterodyne (SISH) method. However, in SISH, the obtained linewidth of the LUT differs by approximately 4, 4.3 and 5 kHz at the pump powers of 16, 12, and 8 mW, respectively. Furthermore, we investigate the effect of detuning on LUT linewidth measurement in the ISISH method. The 20 dB linewidths at 5 and 10 MHz detuned from the Brillouin peak at 16 mW pump power are 20 and 25 kHz, respectively. These values are significantly larger than the measured linewidth at the pump power of 16 mW without detuning.

In this paper, we present a data-aided feedback technique based on the recursive least squares (RLS) algorithm to synchronize the local oscillator with the remote carrier by jointly estimating the carrier frequency offset (CFO) and phase noise (PN), even for large CFO ranges with better laser linewidth tolerance. The CFO and PN are modeled together by a linear regression model, and the filter coefficient vector is recursively learned using the RLS algorithm. Numerical simulation results on a polarization division multiplexed 16QAM, 224 Gbps data rate system with multiple spans demonstrate a significant improvement in Q-factor over other widely used techniques such as blind phase search and fourth-order periodogram maximization. Furthermore, the proposed technique has been shown to achieve synchronization of the carrier at a CFO as high as 10 GHz and exhibits a laser linewidth tolerance as high as 1.45 MHz in the low CFO range. Analysis of our proposed technique shows that computational complexity is much less compared to other widely used techniques.

We propose and experimentally demonstrate a novel dual-pump feedback-based polarization-insensitive wavelength conversion technique using four-wave mixing (FWM) in highly nonlinear fiber (HNLF). By feeding the residual pumps at the output of the HNLF back to the fiber in orthogonal polarization state with respect to the original forward path pumps, we achieve approximately 6 dB improvement in FWM conversion efficiency with negligible polarization sensitivity. A simple theoretical treatment of polarization independent operation is presented. The experimental results closely match the theory and simulation results. Comprehensive comparisons with co-polarized dual pumps, orthogonal pumps, and single pump schemes highlight the significant reduction in polarization sensitivity from over 8 dB in the single pump scheme to approximately 0.7 dB, by the proposed scheme. As an application to the proposed technique, we experimentally demonstrate successful wavelength conversion of 10 and 20 Gbps 4-PAM format signals from 1552.58 nm to 1554.28 nm.

In this paper, we experimentally demonstrate a low-power, broadband, and narrow linewidth SBS-FWM OFC with tunable line spacing of 50 to 600 GHz. The SBS-FWM OFC spans a bandwidth of $\approx$100 nm over the C$+$L-band region using $\approx$15 dBm pump power. The OFC is obtained by cascaded FWM process in highly nonlinear fiber (HNLF), seeded by SBS pumps in an all-fiber ring cavity setup. The linewidth of the SBS-FWM comb lines is measured to be $\approx$20 MHz which is largely determined by the SBS pump properties. By using the optical Vernier effect, the linewidth of comb lines is reduced by a factor of more than 2000 to below 20 kHz. We utilize the SBS-FWM OFC to study the performance of binary and 4-PAM transmission at 1 and 10 Gbps in a back-to-back (B2B) setup. The side-modes caused by the FP-cavity, formed by the HNLF modules in the ring cavity in our setup, affects the performance of the B2B transmission of PAM system.

In this paper, we experimentally demonstrate an all-fiber broadband tunable optical frequency comb (OFC) operating in the C-band. The OFC is generated by broadening a power-equalized stimulated Brillouin scattering (SBS)-based seed comb (SBS-OFC) using four-wave mixing (FWM) in a highly nonlinear fiber (HNLF). The seed SBS-OFC is obtained from a pump and Stokes power recycling cavity, which yields ≈15 comb lines with 10.8 GHz line spacing having 16 dBm average power. The seed SBS-OFC is further power-equalized by a Brillouin-assisted power equalization (BAPE) technique to minimize the high pump contribution at the recycling cavity output. The power-equalized seed SBS-OFC, which has low-power of -4.5d B m at the BAPE cavity output, is propagated down a dual-pass 200 m dispersion flattened HNLF. At the HNLF output, we obtain ≈140 comb lines within a 12 nm bandwidth having 10.8 GHz line spacing. We demonstrate wavelength tunability over a span of 35 nm by using a tunable laser source as the Brillouin pump. We also observe and measure a secondary OFC generated during the power-equalization process by placing a 10% coupler inside the BAPE cavity. Our experimental results closely match the trends obtained in the simulation.

In this paper, we report simulation and experimental results of generation of a broadband tunable optical frequency comb (OFC) source operating in the C-band, using FWM in highly-nonlinear fiber (HNLF). The OFC is seeded by a low-power and powerequalized SBS-based OFC. The seed SBS-OFC is generated by the pump and Stokes power recycling technique which yields ≈15 comb lines having 10.8 GHz line spacing and 16 dBm average power. The seed SBS-OFC is further power-equalized by the Brillouin-assisted power equalization (BAPE) cavity setup to minimize the high pump contribution at its output. The low-power and power-equalized output (< -3 dBm) is propagated down a dual-pass 200 m dispersion flattened HNLF. At the HNLF output we obtain 100 lines within a span of 8 nm having 40 dB power variation. Approximately 140 lines are observed in 12 nm bandwidth with 10.8 GHz line spacing. The wavelength tunability over a span of 35 nm is demonstrated by using a tunable laser source as the Brillouin pump. We also measure a secondary OFC generated during the power-equalization process by placing a 10% tap coupler inside the BAPE cavity. Our experimental results closely match the trends obtained in the simulation.

We describe an all-stimulated Brillouin scattering (SBS) fiber-based setup for the generation, amplification, and isolation of frequency components from an optical frequency comb (OFC). The cascaded SBS-OFC is obtained by utilizing pump and Stokes power recycling techniques. A total of >15 comb lines within a 45 dB bandwidth, having an average power of 16 dBm, is observed in the 1550 nm wavelength region of operation. By implementing a polarizer-analyzer setup exploiting the weak birefringence in silica fibers, we amplified and isolated the first Stokes component of the generated comb. The isolated component at 1550.03 nm was amplified by ≈55dB. In order to verify the isolation of a single comb line, the SBS-OFC is intensity modulated using sinusoidal signals of different frequencies, and the modulation is detected after the comb line isolation. We also observe that with the increase in Brillouin pump power during comb line isolation, the spontaneous Brillouin noise acts as a limitation to the selective amplification process.

We propose 1 state and 2 state multi-step Kalman filters (MKFs) to estimate and compensate CFO, LPN and NLPN in long-haul coherent fiber-optic communication systems. The proposed filters generate state estimates once every m symbols and therefore operate at a reduced sampling rate compared to conventional KFs that perform symbol by symbol processing. No computations are performed to obtain phase estimates of the intermediate m-1 samples; instead, the present and previous estimates are averaged and used to derotate the intermediate m-1 samples which are then demodulated to recover the transmitted symbols. This reduces the computational load on the receiver DSP. Further, in order to improve estimation accuracy, we adaptively vary the process noise covariance Q. Simulation results of 200 Gbps PDM 16 QAM system over 12 spans shows that the proposed 1 state MKF can reduce the sampling rate requirement by a factor of m=20 with Q-factor degradation of 1.32 dB compared to single-step KF at linewidth of 100 kHz. The 2 state MKF tracks PN and CFO with a maximum step size of m=10 for a CFO of 100 MHz at linewidth of 100 kHz. We also study the dynamic performance of the proposed algorithms by applying step change to CFO. The 2 state MKF with adaptive Q is able to track a step change of 400 MHz of CFO with m=1 and 3 with high estimation accuracy but slower convergence time compared to the non-adaptive 2 state MKF. Finally, we study the computational requirements of the proposed MKFs and show that they offer significant reduction in computations compared to single-step KF thus making the proposed filters suitable for hardware implementation.

In this work, the capability of a wave plate is exploited to modify the state of polarization, as it provides a controlled phase shift between the two polarization components of propagating light. A half-wave plate is used to stabilize the state of polarization of light (632.8 nm) at the distal end of a multimode fiber of core diameter 62.5 μm.

The paper describes the process of optical frequency comb generation using cascaded stimulated Brillouin scattering in optical fibers. The cascaded stimulated Brillouin scattering process is induced by the SBS-pump recycling technique in a single mode fiber. The single mode fiber is placed inside a recirculating cavity, with a loop mirror placed at the terminal end of the fiber. The pumps are obtained from four wave mixing process in a semiconductor optical amplifier. We have achieved a total of 8 comb lines − 5 lines within 6 dB power variation. The comb lines are separated by approximately 11 GHz (~0.085 nm).

The Kalman filter is often used for tracking and estimation of effects such as LPN and NLPN in long haul coherent optical communication systems. However, real-time symbol-by-symbol estimation of these parameters is computationally challenging. We use a multi-rate Kalman filtering scheme that allows for different sampling and state update rates in the system. This scheme achieves high Q-factor by making use of maximum available samples while reducing computational load. Simulations are performed for 200 Gbps PDM-16-QAM system by transmitting 20000 symbols over 800 km optical channel. The filter has Q-factor of 17.25 dB with state estimates being updated after every 20 samples. The filter shows more than 1 dB improvement in Q-factor when compared to a KF where the intermediate samples are not utilised for phase estimation.

In this paper, a semi-analytical method to calculate intercore crosstalk (ICXT) in bent and twisted multicore fibers (MCFs) is proposed. The modified refractive index of bent MCF is first computed using geometrically exact beam theory (GEBT) and conformal mapping technique. The computed refractive index profile is used to obtain mode field profiles of bent MCF by solving the wave equation using vectorial finite element method in COMSOL. To take twisting of MCF into account, outer cores of the MCF are approximated to be helically bent around the central core. Using the mode field profiles, mode coupling coefficient between the MCF cores is calculated for different bend radii which is then used to calculate ICXT by using closed form analytical expressions reported in the literature. The ICXT values obtained using our proposed approach are in very good agreement with the experimental values reported in the literature. The effect of change in core pitch and change in relative refractive index contrast of MCF on the mode coupling coefficient between the MCF cores is analysed. The influence of twisting rate of MCF on the ICXT values for different bend radii is studied and it is shown that, ICXT is constant with respect to twisting rate of MCF for bending radii greater than the critical bending radius of the MCF. The proposed approach is also extended to calculate ICXT between different modes of trench assisted few-mode MCF (FM-MCF). The ICXT values obtained for the trench assisted FM-MCF follow the expected trend.

Coherent optical communication system performance is often limited by LPN (laser phase noise) and NLPN (nonlinear phase noise). A n-step Kalman filter to track and estimate LPN and NLPN is proposed for 400 Gbps PDM-16-QAM coherent optical system. Simulations show that for 100 kHz laser linewidth, reducing the sampling factor of filter by 10, Q-factor of 14.5 dB can be achieved over 1200 km transmission and <; 1 dB degradation is observed for n = 20. Q-factor curves indicate that multi-step Kalman filter (MKF) performs better than linear step Kalman filter. MKF with m = 10 can successfully mitigate the LPN and NLPN for the proposed system with maximum laser linewidth tolerance upto 1 MHz over 40 × 80 km. Results show that MKF can optimally track carrier phase noise and Kerr nonlinearities, thereby validating our proposed filter design.