Pump Relative Intensity Noise Research Articles

Relative timing jitter in a counterpropagating all-normal dispersion dual-comb fiber laser

The counterpropagating all-normal dispersion (CANDi) fiber laser is an emerging high-energy single-cavity dual-comb laser source. Its relative timing jitter (RTJ), a critical parameter for dual-comb timing precision and spectral resolution, has not been comprehensively investigated. In this paper, we enhance the state-of-the-art CANDi fiber laser pulse energy from 1 nJ to 8 nJ. We then introduce a reference-free RTJ characterization technique that provides shot-to-shot measurement capability at femtosecond precision. The measurement noise floor reaches 1.6 × 10 − 7 f s 2 / H z , and the corresponding integrated measurement precision is only 1.8 fs (1 kHz, 20 MHz). With this characterization tool, we are able to study the physical origin of the CANDi laser’s RTJ in detail. We first verify that the cavity length fluctuation does not contribute to the RTJ. Then we measure the integrated RTJ to be 39 fs (1 kHz, 20 MHz) and identify the pump relative intensity noise (RIN) to be the dominant factor responsible for it. In particular, pump RIN is coupled to the RTJ through the Gordon–Haus effect. Finally, solutions to reduce the free-running CANDi laser’s RTJ are discussed. This work provides a general guideline to improve the performance of compact single-cavity dual-comb systems such as the CANDi laser, benefitting various dual-comb applications.

Spectral Model of High-Power Ytterbium-Raman Fiber Amplifiers

In this work, we establish a comprehensive spectral model to realize effective design of high-power ytterbium-Raman fiber amplifiers (YRFAs) employing different types of Raman-pump lasers, and propose an intuitive approach for the optimization of high-power YRFAs. Quantitative comparisons with the experimental results are given in two high-power YRFAs employing Raman-pump lasers with different spectral linewidth. The consistency of measured and simulated power ratios and spectral properties of different components in the two YRFAs confirms the fidelity of the spectral model. Further theoretical analysis reveals that the impact of Raman-pump lasers on YRFAs is mainly determined by the relative intensity noise (RIN) of Raman-pump lasers up to the cutoff frequency of the pump RIN transfer function of the YRFA. Notably, the RIN of Raman-pump lasers could provide a well reference for the superior design of Raman-pump lasers in high-power YRFAs.

Pump RIN reduction of the TMI threshold powers in Yb-doped kW class fiber amplifiers.

A 3D spatially resolved coupled mode and perturbation analysis for the transverse mode instability threshold powers in Yb-doped fiber amplifiers is extended to include threshold power reduction induced by the pump laser relative intensity noise (RIN). A static intensity grating combines with the fluctuating pump intensity to generate an inversion standing wave. The forward component of the standing wave contributes to the nonlinear coupling between the fundamental mode and the higher-order mode (HOM), thereby reducing the threshold power. It is estimated that a pump RIN level <-150dB/Hz is required to achieve quantum limited performance. The model also quantifies the impact of unwanted laser power in the HOM on the threshold power reduction. The threshold power calculations as a function of RIN and laser power in the HOM are in qualitative agreement with recently published experimental measurements.

Pump RIN coupling to frequency noise of a polarization-maintaining 2 µm single frequency fiber laser.

We investigated the frequency noise coupling mechanism of a 2 μm polarization-maintaining single frequency fiber laser (SFFL) theoretically and experimentally. The coupling of pump's relative intensity noise (RIN) to frequency noise of a single-frequency high-gain silica fiber laser is shown experimentally to be consistent with a theoretical model where thermal expansion and thermo-optic effect mediate the coupling. The measured and theoretical frequency noise of the 2 μm SFFL with three pump sources is compared. We find using a 1550 nm single frequency laser pump source produces the lowest frequency noise, less than 100 Hz/Hz at frequencies higher than 100 Hz.

Examination of Potential Terrestrial System Effects From Raman Pumps Below Cable Cutoff in G.654.E Fibers

ITU G.654.E was recently approved as a new optical fiber standard for terrestrial systems. While G.654-compliant fibers have been used extensively in undersea submarine networks for some time, the G.654.E standard is optimized for terrestrial systems and, in particular, it enables the use of large effective area fibers to extend the reach of new high data rate signals with multilevel modulation formats. The new G.654.E fiber standard increases the maximum cable cutoff wavelength specification to the lower edge of the C- band, 1530 nm. We analyze potential signal impairments that may arise from Raman amplification with pump wavelengths below cable cutoff. We study the case where Raman pump relative intensity noise (RIN) is caused only by Raman pump multipath interference and show that its effect is negligible compared to the contribution from inherent Raman pump RIN that is constant along a fiber propagation length. We find negligible effects are expected even under worst-case condition.

Passive optimization of pump noise transfer function by narrow band-pass filtering in femtosecond fiber lasers

Fluctuation of pump power is one of the major sources of temporal and intensity noise in femtosecond fiber lasers. In this work, the transfer functions between the relative intensity noise (RIN) of the pump laser diode (LD) and the output RIN, between the RIN of the pump LD and timing jitter of femtosecond fiber lasers are systematically studied. It is demonstrated, for the first time to our knowledge, that the amplitude of the pump RIN transfer function can be effectively decreased by an intra-cavity narrow band-pass filter. In particular, for normal-dispersion lasers, the 3-dB bandwidth of the transfer function can also be narrowed by two-thirds, with a steeper falling edge. Furthermore, with the narrow band-pass filtering, the transfer function is almost independent of the net intra-cavity dispersion due to amplifier similariton formation. The proposed scheme can effectively isolate the pump-induced noise without the need of complex active pump LD control and intra-cavity dispersion management, thus providing an easy way for practical high-power, high-stability femtosecond fiber laser design and related high-precision applications outside the laboratory.

Characterization and Optimization of Unrepeatered Coherent Transmission Systems Using DRA and ROPA

We present a detailed investigation of the singe-channel 100G polarization-multiplexed quadrature-phase-shift-keying-based unrepeatered coherent transmissions using various order distributed Raman amplifiers (DRAs). The impact of main factors on the system performance is discussed including intrachannel nonlinearity, amplified spontaneous emission, double Rayleigh backscattering, and pump relative intensity noise (RIN)-induced impairments. After optimizing the pump power configuration, it is found that compared to a first- and a third-order DRA, a second-order DRA can provide the best performance owing to relative high optical signal-to-noise ratio (OSNR) and low pump RIN-induced penalty. By using the Giles model, the optimization of a remote optically pumped amplifier is performed in terms of the Erbium-doped fiber length and its farthest position, with given input signal's power and OSNR.

Efficient evaluation of impairment induced by distributed fiber Raman amplifier using error vector magnitude techniques in unrepeated coherent communication system

We investigate the impairment induced by relative intensity noise (RIN) of Raman pump in an ultra-long unrepeated multi-level modulated coherent optical communication system. By adopting error vector magnitude (EVM) techniques, we proposed a simple and high efficient numerical method to calculate and analyze the impact of Raman pump RIN on the coherent receiver system. Both intensity and phase noise are taken into consideration in our numerical simulations when choosing Raman pump lasers with different RIN and using different signals. Our simulation result shows that higher-order phase-modulated signal is more sensitive to RIN of the Raman pump. Comparing to the phase noise, intensity noise induced by RIN of the Raman pump can generally be ignored. Apart from the well-known walk-off parameter, nonlinear parameters and Raman-gain coefficient also play important roles in the complex noise transfer process. Our calculation makes it possible to quickly and accurately evaluate the hybrid distributed fiber Raman amplification (DFRA) along with remotely-pumped erbium-doped fiber amplification (EDFA) in ultra-long unrepeated transmission systems.

Experimental verification of relative phase noise in Raman amplified coherent optical communication system

The intensity fluctuation of a Raman pump laser source will not only transfer to the signal's intensity through stimulated Raman scattering effect, which has been investigated comprehensively from both theoretical and experimental perspective, but also affect signal's phase through cross-phase modulation (XPM) effect. The phase noise is characterized by relative phase noise (RPN) parameter. Although the analytical model of RPN was proposed and the simulations were performed, no experimental verification has been carried on so far. In order to verify the statistical property of RPN, we conducted an experiment to measure the RPN transfer function using a continuous-wave signal and a modulated Raman pump laser diode. The measurement result for the co-pumping configuration has achieved good agreement with the theoretical calculation, validating the proposed theory. In addition to the measurement, we found clear Raman pump induced phase modulation in the experiment using a CO-OFDM signal, demonstrating that RPN will occur in a practical transmission system. With the measured RPN transfer function and the prior knowledge of Raman pump relative intensity noise, we can predict the RPN induced system performance degradation in a theoretical manner.

Noise Properties in SESAM-Based Mode-Locked Laser With Intracavity Pump Reflection Coating

We investigate the intensity noise and microwave phase noise properties in an SESAM-based mode-locked fiber laser operating at $1.55~\mu \text{m}$ with intracavity pump reflection coating. Two lasers with similar design are made for comparison with one having a pump reflection coating whereas the other one has not. The noise conversion from pump relative intensity noise (RIN) to the RIN and phase noise of two lasers is evaluated experimentally. It is found that although the laser with pump reflection coating has much lower self-starting pump power and higher slope pumping efficiency, the pump-RIN-to-laser-RIN conversion scales as the square of the slope efficiency at low-frequency region and thus is increased by $\sim 7$ dB in the laser with coating. The pump-RIN-to-laser-phase-noise conversion is also increased due to the existence of the slow saturable absorber effect.

Relative Phase Noise-Induced Phase Error and System Impairment in Pump Depletion/Nondepletion Regime

Although the pump relative intensity noise transfer phenomenon in traditional Raman amplified amplitude-shift keying fiber communication systems has been investigated intensively, the stochastic intensity fluctuation of Raman pump laser will interplay with fiber nonlinearity and leads to additional phase noise of signal in the multilevel modulated coherent optical communication system. Such phase noise is defined as relative phase noise (RPN). In this paper, we present comprehensive analysis regarding the characteristics and impairments of RPN through both theoretical derivation and Monte Carlo simulation. We derive the analytical expressions of phase error variance for M-ary PSK and 16QAM modulation formats in consideration of RPN using decision-aided maximum-likelihood phase estimation algorithm. The analysis can be used to predict RPN induced impairment theoretically. Bit error rate (BER) performance is evaluated and compared for QPSK, 8PSK, 16PSK, and 16QAM modulation formats, and the result shows that in the context of RPN 16QAM signal outperforms 16PSK which has the same spectral efficiency. Finally, we extend the analytical result obtained in pump nondepletion regime to depletion regime via numerical analysis. It is observed by comparison that Q-factor penalty will increase in depletion regime.

Relative phase noise induced impairment in CO-OFDM optical communication system with distributed fiber Raman amplifier.

In this Letter, we demonstrate that the interplay between Raman pump relative intensity noise and cross-phase modulation leads to a relative phase noise (RPN) that brings non-negligible performance degradation to coherent optical orthogonal frequency-division multiplexing (CO-OFDM) transmission systems with co-pumped Raman amplification. By theoretical analysis and numerical simulation, we proved that RPN brings more system impairment in terms of Q-factor penalty than the single carrier system, and relatively larger walk-off between pump and signal helps to suppress the RPN induced impairment. A higher-order modulated signal is less tolerant to RPN than a lower-order signal. With the same spectral efficiency, the quadrature-amplitude modulation format shows better tolerance to RPN than phase-shift keying. The reported findings will be useful for the design and optimization of Raman amplified CO-OFDM multi-carrier transmission systems.

Relative phase noise induced impairment in M-ary phase-shift-keying coherent optical communication system using distributed fiber Raman amplifier

We show for the first time, to the best of our knowledge, that, in a coherent communication system that employs a phase-shift-keying signal and Raman amplification, besides the pump relative intensity noise (RIN) transfer to the amplitude, the signal's phase will also be affected by pump RIN through the pump-signal cross-phase modulation. Although the average pump power induced linear phase change can be compensated for by the phase-correction algorithm, a relative phase noise (RPN) parameter has been found to characterize pump RIN induced stochastic phase noise. This extra phase noise brings non-negligible system impairments in terms of the Q-factor penalty. The calculation shows that copumping leads to much more stringent requirements to pump RIN, and relatively larger fiber dispersion helps to suppress the RPN induced impairment. A higher-order phase-shift keying (PSK) signal is less tolerant to noise than a lower-order PSK.

Noise conversion from pump to the passively mode-locked fiber lasers at 15 μm

We characterize the noise conversion from the pump relative intensity noise (RIN) to the RIN and phase noise of passively mode-locked lasers at 1.5 μm. Two mode locking mechanisms, nonlinear polarization rotation (NPR) and semiconductor saturable absorber mirror (SESAM), are compared for noise conversion for the first time. It is found that the RIN and the phase noise of both types of lasers are dominated by the noise converted from the pump RIN and thus, can be predicted with the measured pump RIN and noise conversion ratios. The SESAM laser is found to show an excess noise conversion from the laser RIN to the laser phase noise due to the slow saturable absorber effect.

Nonlinear coupling of relative intensity noise from pump to a fiber ring laser mode-locked with carbon nanotubes

Pump relative intensity noise (RIN) has been recognized as a major source of noise in mode-locked lasers. The coupling of RIN from the pump to the output of a passively mode-locked fiber laser (PMFL) is systematically investigated using a pump modulation technique. It is found that the linear RIN coupling ratio from pump to PMFL is decreased with an increase in modulation frequency and is independent of modulation power. Moreover, the nonlinear RIN coupling from pump to PMFL is clearly demonstrated with a square wave modulated pump. The nonlinear RIN coupling ratio is noise power dependent. An exponential decay model based on the view of gain modulation is proposed and explains well the behavior of the nonlinear coupling phenomena.

Experimental Investigation and Analytical Modeling of Excess Intensity Noise in Semiconductor Class-A Lasers

Excess intensity noise in a low-noise single-frequency class-A VECSEL is experimentally investigated over the frequency range 10 kHz-18 GHz. An analytical model is derived, based on multimode Langevin equations, to describe the observed laser excess noise over the whole bandwidth. From 50 MHz to 18 GHz, class-A operation leads to a shot noise limited relative intensity noise (RIN), namely -155 dB/Hz for 1-mA detected photocurrent, except at harmonics of the cavity free spectral range (FSR). At these frequencies, the excess noise is shown to be due to the amplified spontaneous emission contained in the nonlasing side modes. The measured levels of excess noise correspond to side mode suppression ratios (SMSRs) ranging from 70 to 90 dB, in agreement with the model. At low frequencies, 10 kHz-50 MHz, the observed excess noise spectrum has the expected Lorentzian shape. Its bandwidth increases with the pumping rate to an upper limit given by the cavity photon lifetime. Below this cutoff frequency, we show that the pump RIN is the dominant source of noise, while it is filtered by the laser dynamics above. Finally, our model permits to design a semiconductor class-A laser with an intensity noise limited to the shot noise level over the whole 10 kHz-18 GHz bandwidth.

Influence of polarization on pump-signal RIN transfer and cross-phase modulation in copumped Raman amplifiers

Pump-to-signal relative intensity noise (RIN) transfer in copumped Raman amplifiers is analyzed. By means of simulations and experiments, the authors call attention to the impact of the polarized nature of stimulated Raman scattering on RIN transfer. Although depolarizing the Raman pump enables polarization-independent signal gain, the authors demonstrate that proper modeling of RIN transfer requires including the polarization dependence of the Raman effect and the polarization mode dispersion of the fiber. A second key phenomenon is emphasized. Besides the RIN transfer induced by Raman gain, pump RIN is also transferred to the signal via nonlinear gain. This phenomenon can be addressed as pump-signal cross-phase modulation since it essentially generates phase noise on the signal. For the first time, in this paper, a model of pump-to-signal RIN transfer that includes polarization and nonlinear gain is presented. The authors reveal the key influence of these factors that had been uncared for so far. Their analysis is supported by experimental data. Signal noise characteristics (RIN spectrum and high-resolution optical power spectrum) measured at the output of a copumped Raman amplifier were presented. The Raman pump is a pair of semiconductor diodes in the first two experiments and then a Raman fiber laser. For these two kinds of depolarized or unpolarized Raman pumps that are commonly used in the field, the authors emphasize the necessity of considering polarization for the good assessment of RIN transfer

Nonlinear phase shift scanning method for the optimal design of Raman transmission systems

In this paper, an efficient algorithm for the search of optimum design parameters and corresponding transmission quality factor (Q) for a Raman amplified transmission system is presented. Taking the nonlinear phase shift (NPS) as the first-order key design parameter for the determination of the remaining secondary system parameters, and solving the nonlinear Schroumldinger equation (NLSE) as a function of NPS to obtain the optimum (Q) factor, the multiparameter, time-consuming fiber Raman amplifier (FRA) system design process can be reduced to a highly efficient and precise semianalytic one-dimensional optimization problem. As an application example for the suggested optimization algorithm, the authors show the design process for the determination of the system design parameters (input powers to single mode fiber (SMF), dispersion-compensating fiber (DCF), distributed Raman gain, and forward Raman pumping ratio) for a single channel 10-Gb/s 2000-km transmission link. In addition, for the first time within the author's knowledge, they assess the requirements of pump relative intensity noise (RIN), as a function of the pumping direction/span-length changes, to study the shift in the optimum design parameters. Results show a Q-factor improvement for the system more than 1.16 dB/4.89 dB at a 100-km/200-km span length with their design method, when compared to previous optimization method. Discussion on the application to dense wavelength division multiplexing (DWDM) system is also presented

Q penalties due to pump phase modulation and pump RIN in fiber optic parametric amplifiers with non-uniform dispersion

We present an experimental and numerical investigation of the quality factor degradation (ΔQ) due to pump phase modulation and pump relative intensity noise (RIN) in single- and double-pumped fiber optical parametric amplifiers (FOPAs). These penalties are investigated in FOPAs constructed with fibers having constant and randomly varying zero dispersion wavelength (λ0) along the fiber.We show that pump phase modulation in uniform fibers (constant λ0) produces large Q penalties, while in fibers with variations of λ0, these penalties can be strongly reduced in both, single and double-pumped FOPAs. We also show that Q penalties due to pump phase modulation tend to disappear in FOPAs made with very short fibers (L<0.1km) and very low dispersion slope (S0<0.002ps/nm2-km).The Q penalties due to pump RIN, which for typical FOPA parameters are much smaller than those due to pump phase modulation, are also reduced in non-uniform fibers (varying λ0), but do not depend on fiber length neither on fiber dispersion slope.

Dynamic analysis of multiple wavelength DFB fiber lasers

The behavior of multi- and single-wavelength distributed feedback fiber lasers is simulated in a comprehensive dynamic model. The evolution of the spatial distribution of gain, saturation-induced gain gratings, and spontaneous emission is taken into account. Stability and relative intensity noise (RIN) of the different laser types and laser modes are compared, and the effect of varying degree of pump RIN and quenching of spontaneous emission lifetimes is analyzed.