Mutual Phase Coherence Research Articles

Bose-Einstein Condensation of Photons in a Four-Site Quantum Ring.

Thermalization of radiation by contact to matter is a well-known concept, but the application of thermodynamic methods to complex quantum states of light remains a challenge. Here, we observe Bose-Einstein condensation of photons into the hybridized ground state of a coupled four-site ring potential. In our experiment, the periodically closed ring lattice superimposed by a weak harmonic trap for photons is realized inside a spatially structured dye-filled microcavity. Photons thermalize to room temperature, and above a critical photon number macroscopically occupy the symmetric linear combination of the site eigenstates with zero phase winding, which constitutes the ground state of the system. The mutual phase coherence of photons at different lattice sites is verified by optical interferometry.

Phase noise of Kerr soliton dual microcombs.

Dissipative Kerr soliton microcombs are believed to be a promising technique to build a dual-comb source for applications including precision laser metrology, fast laser spectroscopy, and high-speed optical signal processing. In this Letter, we conduct a detailed experimental investigation on the phase coherence between two on-chip Kerr soliton microcombs, where the underlying physical and technical origins that lead to the mutual phase noise between microcombs are analyzed. Moreover, the techniques of 2-point locking and optical frequency division are explored to enhance the dual-microcomb phase coherence, and we demonstrate the best phase noise down to -50 dBc/Hz at 1-Hz offset, -90 dBc/Hz at 1-kHz offset, and -120 dBc/Hz at 1-MHz offset. Our study provides a basic reference for both fundamental studies and practical applications of Kerr soliton dual microcombs that entail high mutual phase coherence.

Phase-coherent asynchronous optical sampling system.

Mutual phase coherence is the utmost crucial factor in an asynchronous optical sampling system. The enhancement is commonly achieved by phase-locking dual femtosecond lasers to a shared narrow-linewidth cavity-stabilized laser. Here we report such a system with a low residual optical phase jitter at a level of 0.04 rad in a Fourier frequency band from 1 Hz to 5 MHz around 1.55 µm, corresponding to a timing jitter of 30 as. The residual phase jitter reaches 1 rad at a Fourier frequency between 0.06 Hz and 0.1 Hz, from which the phase-coherence time is inferred to be at least 10 s. The multi-heterodyne beats experimentally reveal a hardware-limited phase coherence time of ∼8.155 s throughout the direct lasing spectral band.

Generation of multiple laser lines by sum-frequency mixing of continuous-wave Raman emissions from a dispersion-compensated optical cavity

Three color continuous-wave (CW) laser emissions with constant frequency separation are generated in the near-infrared (NIR) region using a dispersion-compensated optical cavity filled with hydrogen gas. By focusing these laser emissions into second-harmonic generation (SHG) crystals, multiple second harmonic signals and sum-frequency signals are generated in the near-ultraviolet (NUV) with a constant frequency spacing. Up to five colors of these NUV CW laser emissions can be generated simultaneously by using SHG crystals with different orientations. The interference between the second-harmonic signal of one NIR laser emission and the sum-frequency signal of the other two NIR emissions was observed experimentally, indicating mutual phase coherence among the NIR laser emissions. The phase coherence allows the synthesis of a train of ultrashort pulses with a THz repetition rate in both the NUV and the NIR by using the CW emission lines.

Laser speckle contrast imaging reveals large-scale synchronization of cortical autoregulation dynamics influenced by nitric oxide.

Synchronization of tubuloglomerular feedback (TGF) dynamics in nephrons that share a cortical radial artery is well known. It is less clear whether synchronization extends beyond a single cortical radial artery or whether it extends to the myogenic response (MR). We used LSCI to examine cortical perfusion dynamics in isoflurane-anesthetized, male Long-Evans rats. Inhibition of nitric oxide synthases by N(ω)-nitro-l-arginine methyl ester (l-NAME) was used to alter perfusion dynamics. Phase coherence (PC) was determined between all possible pixel pairs in either the MR or TGF band (0.09-0.3 and 0.015-0.06 Hz, respectively). The field of view (≈4 × 5 mm) was segmented into synchronized clusters based on mutual PC. During the control period, the field of view was often contained within one cluster for both MR and TGF. PC was moderate for TGF and modest for MR, although significant in both. In both MR and TGF, PC exhibited little spatial variation. After l-NAME, the number of clusters increased in both MR and TGF. MR clusters became more strongly synchronized while TGF clusters showed small highly coupled, high-PC regions that were coupled with low PC to the remainder of the cluster. Graph theory analysis probed modularity of synchronization. It confirmed weak synchronization of MR during control that probably was not physiologically relevant. It confirmed extensive and long-distance synchronization of TGF during control and showed increased modularity, albeit with larger modules seen in MR than in TGF after l-NAME. The results show widespread synchronization of MR and TGF that is differentially affected by nitric oxide.

Independent telecom-fiber sources of quantum indistinguishable single photons

Quantum-mechanically indistinguishable photons produced by independent (or equivalently, mutually phase incoherent) light sources are essential for distributed quantum information processing applications. We demonstrate heralded generation of such photons in two spatially separate telecom-fiber spools, each driven by pulsed pump waves that are measured to have no mutual phase coherence. Through Hong–Ou–Mandel experiments, we measure the quantum interference visibility of those photons to be . Our experimental results are well predicted by a quantum multimode theory we developed for such systems without the need for any fitting parameter.

Employing the mutual phase coherence of the overtones in musical sounds for audio source separation and reconstruction

Source separation from a monaural mixture of musical sounds is challenging. When only a single mixture of multiple sources is available, the method of independent component analysis may not be straightforwardly applied, and proposed source separation methods have relied on previous knowledge of the independent sources. Our method employs mutual correlations of the overtone phases of musical sounds to ascribe spectral features to individual sources. The phase versus time is computed from the analytical signal representation of the output of a set of adaptive band‐pass filters that perform initial spectral separation. A small number of cleanly separable partials, usually the fundamentals, are then employed as references to which the remaining filter‐bank outputs are correlated. To separate overlapping spectral features and ascribe the proper amount of power to each source, the phase of the reference signal is scaled to the harmonic frequency, and the amplitude of each overtone is estimated by computing the cross‐correlation of the phase trajectories. To group sonic events in time, we employ the history of the signal itself, thus eliminating the need for prior information. We discuss this separation method and the audio quality of the reconstructed source signals, with examples and a quantitative perceptual metric.

STIMULUS-INDUCED CHAOTIC SYNCHRONIZATION OF CHUA'S OSCILLATORS

The problem of phase synchronization of Chua's chaotic oscillators is investigated. We consider Chua's circuit when it exhibits a chaotic attractor and apply a single pulse stimulus. It is shown that under certain conditions the system displays self-referential phase reset (SPR) phenomenon. This is a case when the reset phase of the chaotic oscillation is independent on the initial phase, hence on the time moment when the stimulus has been applied. In an ensemble of chaotic oscillators simultaneously stimulated, the SPR yields mutual phase coherence or synchronization between the units. We describe basic dynamical mechanisms of the effect and show how it can be used for controllable cluster formation and for the control of chaotic dynamics.

Quasiperiodic Raman technique for ultrashort pulse generation.

We report the experimental demonstration of a new Raman technique that produces 200 sidebands, ranging in wavelength from 3 microm to 195 nm. By studying multiphoton ionization of nitric oxide (NO) molecules, we show mutual phase coherence among 15 visible sidebands covering 0.63 octaves of bandwidth.

Mutual coherence of supercontinuum pulses collinearly generated in bulk media

We report the collinear generation of white light pulses in bulk media by phase-locked and time-delayed intense laser pulses. The mutual phase coherence of the pump pulses is fully transferred to the two supercontinua, which thus show a highly-modulated two-pulse spectrum extending from all of the visible down to the near-infrared. We have investigated the effects of changing the relative delay between the pulses as well as the role of imbalances in the pump energy, and we have found that, at least in a limited energy interval, the mutual phase coherence of the two white-light pulses is surprisingly robust.

Phase-locked, time-delayed, harmonic pulses for high spectral resolution in the extreme ultraviolet: errata

We present experiments in the time and frequency domains aimed at confirming the measured mutual phase coherence of time-delayed, collinear harmonic pulses. We show that pairs of phase-locked harmonic pulses of medium order can be generated for peak intensities up to ∼1.5 1014 W/cm2 in xenon, demonstrating the possibility of performing high-resolution spectroscopy in the extreme ultraviolet with Ramsey-like techniques.

Phase-locked, time-delayed harmonic pulses for high spectral resolution in the extreme ultraviolet

We present experiments in the time and frequency domains aimed at confirming the measured mutual phase coherence of time-delayed, collinear harmonic pulses. We show that pairs of phase-locked harmonic pulses of medium order can be generated for peak intensities up to ~1.5 10(14)W/cm(2) in xenon, demonstrating the possibility of performing high-resolution spectroscopy in the extreme ultraviolet with Ramsey-like techniques.

Generation and applications of phase-locked white-light continuum pulses

We report on an experiment demonstrating the high degree of mutual phase coherence between two white-light continuum pulses independently generated by phase-locked ultrashort laser pulses. We show that these secondary pulses are locked in phase and exhibit surprisingly clear and stable Young's interference fringes when overlapped on a screen in the far field. We discuss the use of such phase-locked continuum pulses in the realization of a new kind of ultrabroad frequency comb for absolute frequency measurements throughout the visible spectrum.

Generation of phase-coherent laser beams for Raman spectroscopy and cooling by direct current modulation of a diode laser

A setup for generation of GHz side-bands by direct current modulation of a diode laser is described. The first order side-bands are used to inject two slave power diode lasers, in order to produce phase coherent, controllable frequency shifted, light beams, the mutual phase coherence of which has been verified (beat-note FWHM of 1 Hz, instrumentally limited). Some possible applications in the context of laser Raman cooling of cesium atoms are briefly discussed.

Phase-coherent light pulses for atom optics and interferometry

We have developed a novel source for the generation of powerful phase-coherent light pulses. Our setup uses an acousto-optic modulator (AOM) inside an external high-finesse resonator. By applying a short rf pulse to the AOM, we dump the cavity and extract a large part of the stored and enhanced power within a short optical pulse and with a controllable optical phase. In preliminary experiments we reached 100 W of peak power in a 15-ns optical pulse. The mutual phase coherence of successive light pulses is demonstrated with a molecular iodine interferometer experiment in a cell.