Pump Phase Modulation Research Articles

Enhancing the sensitivity of spin-exchange relaxation-free magnetometers using phase-modulated pump light with external Gaussian noise

An optical pumping scheme is proposed for reducing the gradient of electron spin polarization and suppressing light source noise in a spin-exchange relaxation-free magnetometer. This is achieved by modulating only the phase of a narrow-linewidth pump light field with external Gaussian noise. Compared to the absence of phase modulation, the uniformity of electron spin polarization was improved by over 40%, and the light-frequency noise suppression ratio of the magnetometer was enhanced by 4.3 times. Additionally, the response of the magnetometer was increased by 54%, resulting in a sensitivity of 0.34 fT/Hz1/2 at 30 Hz. The applicability of this scheme can extend to other optical pumping experiments involving large atom ensembles requiring uniform electron spin polarization distribution, which is beneficial for developing ultra-high sensitivity and high stability magnetometers essential for magneto-cardiography and magneto-encephalography research applications.

Online digital compensation of pump dithering induced phase and amplitude distortions in transmission links with cascaded fibre-optical parametric amplifiers

We present an advanced online digital signal processing (DSP) method for correcting the phase and amplitude distortions caused by the phase modulation of the pump source and its interaction with the dispersive fibre channel in transmission systems using cascaded fibre-optical parametric amplifiers. The proposed algorithm is numerically demonstrated to achieve significant (up to 3.7 dB for a four-tone pump-phase modulation scheme) Q2-factor performance improvement over conventional DSP in 16 quadrature-amplitude modulation signal transmission.

Soliton stabilization in microresonators with high order dispersion via pump phase modulation

We theoretically and numerically investigate the effects of third-order dispersion on microresonator solitons in anomalous dispersion with an extended normalized Lugiato–Lefever equation. The temporal position drift of solitons in the microresonator induced by the third-order dispersion is observed. The soliton temporal location stabilization is realized via pump phase modulation. Moreover, the final stable cavity soliton temporal position is deviated to the maxima of the pump phase modulation field profile which is the stable position of the solitons without high-order dispersion. We also study the effectiveness of the pump phase modulation in different modulation parameters. The research method sheds light on the cavity soliton stabilization, on-chip ultrashort pulse formation and optical information processing.

Turnkey generation of Kerr soliton microcombs on thin-film lithium niobate on insulator microresonators powered by the photorefractive effect

Generation of Kerr soliton microcombs on microresonators enables power-efficient, phase-coherent, and broadband frequency teeth generation, which has revolutionized a wide range of scientific areas such as astronomy, metrology, spectroscopy, communications, etc. However, compared with a conventional frequency scanning method that requires a complex start-up and feedback control, turnkey generation of soliton microcombs remains challenging and a more promising solution is desired. Here, we propose for the first time and numerically demonstrate that turnkey generation of soliton microcombs can be achieved on thin-film lithium niobate on insulator (LNOI) microresonators for polarization along the ordinary axis of lithium niobate (LN) for which the photorefractive (PR) effect dominates. The PR effect shows power-dependent refractive index change, which is strong and opposite to that of the Kerr effect and thermal effect, thus enables the self-routing and converge of the total pump-resonator detuning into the existence region of soliton. Our results show that initiated with a certain amount of initial pump-resonator detuning on either blue- or red-detuned side, generation of soliton microcombs can self-start, self-route, and finally get stable without any artificial frequency scanning. Moreover, we show that deterministic and turnkey generation of single soliton microcombs can be achieved by leveraging a phase-modulated pump laser. Thanks to the inherent electro-optic effect of LNOI, a lab-on-a-chip device with monolithically integrated high-speed phase modulators and high-Q microresonators is feasible.

Group Velocity Control of Gaussian Pulse Signal With Pump Phase Modulation

Pump phase modulation is an important method for signal group velocity control. By controlling the phase difference between pump and signal, the fast and slow light can be changed easily, and the degree of signal delay or advancement can be greatly improved. In this letter, the fast and slow light of a Gaussian signal with pump phase modulation is studied for the first time. The Gaussian pulse signal is similar to the actual network transmission signal, so it is significant to investigate it. Unlike a sinusoidal signal, the duty ratio of a Gaussian signal can be adjusted, leading to the difference in pump phase modulation of the Gaussian signal. The experimental and simulation results show that the smaller the duty ratio of the Gaussian signal is, the smaller the impact of the pump phase adjustment on the fractional delay and advancement will be. Moreover, it is also found that at a certain duty ratio, two signal peaks appear within a certain adjustment range of phase difference.

Variational approach to study soliton dynamics in a passive fiber loop resonator with coherently driven phase-modulated external field.

We report a detailed semianalytical treatment to investigate the dynamics of a single cavity soliton (CS) and two copropagating CSs separately in a Kerr mediated passive optical fiber resonator which is driven by a phase-modulated pump. The perturbation is dealt with by introducing Rayleigh's dissipation function in the framework of a variational principle that results in a set of coupled ordinary differential equations describing the evolution of individual soliton parameters. We further derive closed-form expressions for quick estimation of the temporal trajectory, drift velocity, and the phase shift accumulated by the CS due to the externally modulated pump. We also extend the variational approach to solve a two-soliton interaction problem in the absence as well as in the presence of the externally modulated field. In the absence of a phase-modulated field, the two copropagating solitons can attract, repulse, or propagate independently depending on their initial delay. The final state of interaction can be predicted through a second-order differential equation which is derived by the variational method. While in the presence of the phase-modulated field, the two-soliton interaction can result in annihilation, merging, breathing, or a two-soliton state depending on the detuning frequency and the pump power. Variational treatment analytically predicts these states and portrays the related dynamics that agrees with the full numerical simulation carried out by solving the normalized Lugiato-Lefever equation. The results obtained through this variational approach will enrich the understanding of complex pulse dynamics under a phase-modulated driving field in passive dissipative systems.

Kerr-microresonator solitons from a chirped background

Optical frequency combs based on solitons in nonlinear microresonators open up new regimes for optical metrology and signal processing across a range of expanding and emerging applications. In this work, we advance these combs toward applications by demonstrating protected single-soliton formation and operation in a Kerr-nonlinear microresonator using a phase-modulated pump laser. Phase modulation gives rise to spatially/temporally varying effective loss and detuning parameters, leading to an operation regime in which multi-soliton degeneracy is lifted and a single soliton is the only observable behavior. We achieve direct, on-demand excitation of single solitons as indicated by reversal of the characteristic “soliton step.” Phase modulation also enables precise, high bandwidth control of the soliton pulse train’s properties, and we measure dynamics that agree closely with simulations. We show that the technique can be extended to high-repetition-frequency Kerr solitons through subharmonic phase modulation. These results will facilitate straightforward generation and control of Kerr-soliton microcombs for integrated photonics systems.

FOPA Pump Phase Modulation and Polarization Impact on Generation of Idler Components

Fiber optical parametric amplifiers (FOPA) are well known for the variety of potential applications in all-optical signal processing. Many of these applications use the idler spectral components that are produced by the four wave mixing parametric process. The performance of parametric amplifiers and, therefore, also of generation of idler spectral components, is highly dependent on different factors, including stimulated Brillouin scattering (SBS) and polarization of the input signal. The aim of this article is to investigate the influence of pump phase modulation of parametric amplifiers, used for SBS mitigation, and the impact of mutual state of polarization mismatch of the input signal and the pump on the generation of idler spectral components. The obtained results have shown that it is crucial for systems with FOPAs to ensure that the input signal and the pump are co-polarised, as polarization dependant gain of 16.7 dB and 33.5 dB difference in idler generation was observed in the performed simulations. It was also found that applying the phase-modulation of the pump for SBS mitigation has caused 54 % idler spectral broadening. Therefore additional measures must be taken to avoid inter-channel crosstalk among the neighbour channels. DOI: http://dx.doi.org/10.5755/j01.eie.22.4.15924

Phase-shifted Brillouin dynamic gratings using single pump phase-modulation: proof of concept.

Two novel phase-shifted Brillouin dynamic gratings (PS-BDGs) are proposed using single pump phase-modulation (SPPM) in a polarization maintaining fiber (PMF) for the first time to our knowledge. Firstly, based on the stimulated Brillouin scattering (SBS), a transient PS-BDG with a 3-dB bandwidth of 354MHz is written by a 2-ns pump1 pulse and a 100-ps pump2 pulse, where the phase of pump1 pulse is shifted with π from its middle point through phase modulation. Then, with a high repetition rate of 250MHz for both pump pulses, an enhanced PS-BDG with a deep notch depth is obtained and its notch frequency can be easily tuned by changing the phase shift. We demonstrate a proof-of-concept experiment of the transient PS-BDG and show the notch frequency changing by tuning the phase shift. The proposed PS-BDGs have important potential applications in microwave photonics, all-optical signal processing and RoF (radio-over-fiber) networks.

Spectral broadening and shaping of nanosecond pulses: toward shaping of single photons from quantum emitters.

We experimentally demonstrate spectral broadening and shaping of exponentially-decaying nanosecond pulses via nonlinear mixing with a phase-modulated pump in a periodically poled lithium niobate (PPLN) waveguide. 1550 nm pump light is imprinted with a temporal phase and used to upconvert a weak 980 nm pulse to 600 nm while simultaneously broadening the spectrum to that of a Lorentzian pulse up to 10 times shorter. While the current experimental demonstration is for spectral shaping, we also provide a numerical study showing the feasibility of subsequent spectral phase correction to achieve temporal compression and reshaping of a 1 ns mono-exponentially decaying pulse to a 250 ps Lorentzian, which would constitute a complete spectrotemporal waveform shaping protocol. This method, which uses quantum frequency conversion in PPLN with >100:1 signal-to-noise ratio, is compatible with single photon states of light.

Linearly configured BOCDA system using a differential measurement scheme

We experimentally demonstrate a linearly configured Brillouin optical correlation domain analysis (BOCDA) system enhanced by a differential measurement scheme. On-off control of the pump phase modulation with an intentional loss at the end of a fiber under test is applied for the acquisition of a Brillouin gain spectrum. This application leads to a four-fold enhancement of the spatial resolution and doubling of the measurement range in comparison with the former system under the same modulation parameters.

Narrowband flat-top Brillouin gain spectrum and low distortion amplification based on pump phase modulation

AbstractWe theoretically and experimentally investigate the conditions of obtaining a narrowband flat-top Brillouin gain spectrum (BGS) based on single-frequency and multi-frequency phase modulations. Using the unequal-amplitude spectral lines, the flat-top BGS can be realized by controlling the intensity ratio and the frequency separation between them. In experiment, we obtain the flat-top gain spectra with the bandwidths of 40 MHz and 125 MHz, and with the top fluctuation of less than 0.21 dB. Based on this, we also achieve low distortion Brillouin amplification of a probe signal pulse and spectrum.

A Comprehensive Study of Actively Mode-Locked Fiber Optical Parametric Oscillators for High-Speed Pulse Generation

We perform a comprehensive study on the implementation of active harmonic and rational harmonic mode-locking of a fiber optical parametric oscillator (FOPO). In particular, the effect of pump parameters and cavity designs on the characteristics of the generated pulses are analyzed. Two methods of frequency multiplication in FOPOs are discussed and compared and we show that we can generate optical pulses with very high-repetition-rate (up to 240 GHz) using rational harmonic mode-locking. We find that optical pulses with short duration can be obtained using pump pulses with small duty cycle, large pump power, and an intra-cavity optical filter with proper bandwidth. In addition, optical pulses with low root-mean-square timing jitter are achievable when no pump phase modulation is applied and hybrid gain of erbium-doped fiber amplifier and parametric amplifier is used. High supermode noise suppression ratio is achieved by taking advantage of the fast parametric gain and the combined effect of self-phase modulation and spectral filtering. We conclude that synchronous-pumping of an actively mode-locked FOPO offers many advantages in terms of reducing the cavity loss, increasing the modulation frequency, enabling rational harmonic mode-locking, and applications to all-optical clock recovery.

Optical Hexadecimal Coding/Decoding Using 16-QAM Signal and FWM in HNLFs

We propose a simple approach to manipulating the constellation of a multilevel modulation signal in the optical domain. By exploiting degenerate four-wave mixing (FWM) in highly nonlinear fibers and adopting 16-ary quadrature amplitude modulation (16-QAM) signal, we demonstrate 10-Gbaud/s optical variable symbol-wise hexadecimal coding/decoding assisted by a continuous-wave (CW) pump or a phase-modulated pump. The former takes the coding through the phase conjugation of degenerate FWM, and the latter offers enhanced coding via the combined contributions from the phase modulation of the pump and the phase-conjugated FWM. The penalty of optical signal-to-noise ratio for coding/decoding is measured to be ${ dB with a CW pump and ${ dB with a $(0, \pi/4)$ phase-modulated pump at a bit-error rate of 2e-3 (enhanced forward error correction threshold). The dependence of coding/decoding performance on the phase modulation depth of the pump and the signal/pump misalignment is also investigated. Moreover, considering that a hexadecimal number denotes four binary numbers, we develop the symbol-wise hexadecimal coding/decoding to general binary coding/decoding, i.e., 10-Gbaud/s symbol-wise hexadecimal coding/decoding is accompanied by 40-Gbit/s binary coding/ decoding.

Theoretical Study of High Repetition Rate Short Pulse Generation With Fiber Optical Parametric Amplification

In this paper, we study theoretically the generation of high repetition rate short pulses using fiber optical parametric amplification. We show that the pulse shape and duration depend on the signal location relatively to the pump frequency. We demonstrate that in order to get the shortest pulse width, the signal must be located at one of the extremities of the gain spectrum associated with the pump peak power. We derive the analytical expression of the pulse shape in this case and compare it to the exponential gain regime case. Using numerical simulations, we also analyze the impact of walk-off and pump phase modulation that is required to suppress Stimulated Brillouin Scattering and derive guidelines to design stable train of short and high repetition rate pulses.

Localized and stationary dynamic gratings via stimulated Brillouin scattering with phase modulated pumps

A novel technique for the localization of stimulated Brillouin scattering (SBS) interaction is proposed, analyzed and demonstrated experimentally. The method relies on the phase modulation of two counter-propagating optical waves by a common pseudo-random bit sequence (PRBS), these waves being spectrally detuned by the Brillouin frequency shift. The PRBS symbol duration is much shorter than the acoustic lifetime. The interference between the two modulated waves gives rise to an acoustic grating that is confined to narrow correlation peaks, as short as 1.7 cm. The separation between neighboring peaks, which is governed by the PRBS length, can be made arbitrarily long. The method is demonstrated in the generation and applications of dynamic gratings in polarization maintaining (PM) fibers. Localized and stationary acoustic gratings are induced by two phase modulated pumps that are polarized along one principal axis of the PM fiber, and interrogated by a third, readout wave which is polarized along the orthogonal axis. Using the proposed technique, we demonstrate the variable delay of 1 ns-long readout pulses by as much as 770 ns. Noise due to reflections from residual off-peak gratings and its implications on the potential variable delay of optical communication data are discussed. The method is equally applicable to the modulation of pump and probe waves in SBS over standard fibers.

Phase-squeezing properties of non-degenerate PSAs using PPLN waveguides

We investigate the phase squeezing characteristics of non-degenerate phase-sensitive-amplifiers (PSAs) based on periodically-poled-lithium-niobate (PPLN) waveguides. We implement two PSA configurations with phase insensitive idler generation performed in both highly-non-linear-fiber (HNLF) and PPLN waveguides. In both cases we demonstrate regeneration of a noisy BPSK signal, despite net signal attenuation in the phase sensitive PPLN, and show that the level of phase squeezing varies with the phase sensitive dynamic range (PSDR). We observe that weak idler generation in the PPLN limits the achievable PSDR and that use of HNLF for idler generation leads to the largest PSDR. However, in phase regeneration measurements we observe that the pump phase modulation, required to overcome stimulated Brillouin scattering, adds significant amplitude noise, which increases with the PSDR.

Mode-locking and Q-switching in multi-wavelength fiber ring laser using low frequency phase modulation

We describe experimental investigation of pulsed output from a multi-wavelength fiber ring laser incorporating low frequency phase modulation with large modulation amplitude. The Erbium-doped fiber (EDF) ring laser generated more than 8 wavelength channels with the help of a phase modulator operating at 26.2 kHz and a periodic intra-cavity filter. For most cases, the laser output is pulsed in the form of mode-locking at 5.62 MHz and/or Q-switching at harmonic and sub-harmonic of the phase modulation frequency. Chaotic pulse output is also observed. The behavior of the output pulses are described as functions of pump power and phase modulation amplitude. The relative intensity noise (RIN) value of a single wavelength channel is measured to be under -100 dB/Hz (-140 dB/Hz beyond 1.5 GHz).

Preconversion phase modulation of input differential phase-shift-keying signals for wavelength conversion and multicasting applications using phase-modulated pumps

We demonstrate a method to use phase-modulated pumps for multicasting of return-to-zero differential phase-shift keying (RZ-DPSK) signals. The method uses phase modulation of the input signal along with the co-phase-modulated pumps prior to wavelength conversion. Nine copies of a 50 Gbit/s RZ-DPSK data signal are generated via four-wave mixing with an average conversion efficiency of ∼0 dB. An average receiver sensitivity penalty of ∼1.9 dB is observed at a bit-error rate of 10(-9) with a worst case of 2.3 dB. The effects of synchronization errors and phase-modulation frequencies up to 5 GHz are investigated.

Slowing atoms using optical cavities pumped by phase-modulated light

We theoretically investigate the possibility of slowing and cooling a collisionless atomic gas using a phase-modulated pump field. We consider a single-pump ring cavity with asymmetric cavity losses containing a beam of N polarizable pointlike particles. We show using analytical and numerical analysis that different regimes of the atom-light interaction can be identified and for properly chosen parameters this can lead to effective slowing of the atomic ensemble.