External Frequency Modulation Research Articles

A Modified Oxazolone Dye Dedicated to Spectroscopy and Optoelectronics

Here we present a newly synthesized bifunctional organic chromophore with appealing spectroscopic and nonlinear optical features. The positions of absorption and emission maxima of the dye vary with increasing solvent polarity and exhibit positive solvatochromism. The determined change in the dipole moment upon excitation based on the Bilot and Kawski theory is 5.94 D, which corresponds to the intermolecular displacement of a charge equal to 1.24 Å. An investigated organic-based system represents a significant, repeatable, and stable over time optical signal modulation in the manner of the refractive index value. Its magnitude is varied both by optical pumping intensity as well as by external frequency modulation, which indicates that such system is an alluring and alternative core unit for optoelectronic devices and complex networks. Then, the same active system, due to the nonresonant mechanism of higher harmonics of light inducement, can provide second and third harmonic signals. According to the introduced laser line spatial modifications (parallel or perpendicular polarization directions), it is resulted in output SHG signal with magnitude varied about 100%. Its magnitude is noticeably small; however, to construct sensitive optical sensors or infrared indicators, such feature may guarantee satisfying circumstances.

Incoherent Frequency 12-tupling Microwave Signal Generation Scheme Based on Cascade Modulators

Frequency-multiplication technology based on microwave photonic principles can be used to generate microwave and millimeter wave signals with a wide frequency tuning range. However, the existing cascaded external modulation frequency-tupling scheme needs to ensure the phase coherence of the modulated Radio Frequency (RF) signal, while the phase modulation directly limits the frequency tuning range of the external modulation frequency multiplication. In this paper, a novel approach for generating an incoherent frequency 12-tupling signal with cascade modulation is proposed. The structure of cascaded dual-parallel Mach-Zehnder modulators can generate a frequency 12-tupling signal. The proposed structure uses no filter or phase control of the RF driving signal. Microwave photonic frequency-tupling was realized under incoherent conditions. Software simulations and experiments validated the proposed structure and proved that it can generate frequency 12-tupling microwave signals under incoherent conditions. Both the frequency range and reliability of the frequency-tupling system has been improved by the proposed structure.

Phase Locking of a Gyrotron with Low-Frequency Voltage and Current Fluctuations by an External Monochromatic Signal

We show that during phase locking of the oscillations of a megawatt-level gyrotron by a relatively weak external monochromatic signal, the phase and frequency modulation of the gyrotron radiation due to the low-frequency fluctuations of the accelerating voltage and current decreases severalfold compared to the autonomous regime. The calculations were performed using the example of two megawatt-level gyrotrons, namely, a gyrotron developed at the Institute of Applied Physics of the Russian Academy of Sciences and operated at the TE28,12 mode with a frequency of 170 GHz [1] and a gyrotron with twice as high frequency and operation-mode indices (345 GHz and TE56,24), whose ability to generate a megawatt power in the case of frequency and phase locking by an external signal is shown theoretically [2].

Synchronization of Marangoni waves by temporal modulation of interfacial heat consumption

When a thin film consisting of two layers of different liquids is heated from above, some oscillations of its interfaces can appear spontaneously due to the temperature dependence of interfacial tensions (Marangoni convection). These oscillations have a certain natural frequency. We investigate the possibility of changing that frequency by a periodic modulation of heating or cooling of the interface between the liquid layers. We found that in some intervals of the modulation frequency a synchronization occurs: the natural frequency of oscillations is shifted in such a way that it becomes commensurate to the external modulation frequency.

Tunable laser-induced fluorescence with signal correlation algorithm for dental caries detection with controlled ablation

The variation of chemical and optical properties of sound and effected caries lesion areas produce changes of light reflected and emitted energy as function in wavelength after human teeth interact with laser beam. In this study, custom non-invasive laser-induced fluorescence technique with optical sensor, associated with correlation algorithm was employed for caries detection and ablation control. Tunable argon laser source of 488‐nm and 5 mw was employed for tooth excitation. While sound tooth demonstrated high fluorescence emission at 528 nm; decayed tooth demonstrated low fluoresce emission intensity. Novel digital signal processing algorithm using correlation technique was employed to recognize and localize the caries position. Sound tooth response was employed as a reference. Cross-correlation of emitted signal amplitude from scanned sound position to reference normal sound of tooth will produce high correlation intensity values (maximum values). In the mean time, caries area will develop low intensity values; consequently low correlation to reference will be obtained (low intensity, minimum vales). Threshold for correlation value was developed to convert the analog correlation signal to logic signal (1, 0). This novel approach can offer automated precise drilling using the same laser source after external frequency modulation at 10 Hz and power up to 500 mw to achieve precise caries removal.

External serrodyne modulation for the suppression of low-frequency noise in quadrature interferometry.

In interferometry, reaching a high signal-to-noise ratio at low frequencies can be challenging when the additive noise is nonstationary. Although this problem is typically solved by inserting a frequency shifter into one of the arms, in some cases, the interferometer cannot or should not be modified in this way. This Letter presents an alternative solution, based on external serrodyne frequency modulation, which is comparable to the typical approach in terms of complexity and performance yet does not require the modification of a passive interferometer. We demonstrate a prototype that achieves frequency shifting at 500 kHz with 89% power efficiency, leading to the wideband suppression of low-frequency additive noise by more than 19 dB. This enables a fully passive measurement of the thermoconductive noise of a 100 m single-mode fiber.

Quantum dephasing induced by non-Markovian random telegraph noise

We theoretically study the dynamical dephasing of a quantum two level system interacting with an environment which exhibits non-Markovian random telegraph fluctuations. The time evolution of the conditional probability of the environmental noise is governed by a generalized master equation depending on the environmental memory effect. The expression of the dephasing factor is derived exactly which is closely associated with the memory kernel in the generalized master equation for the conditional probability of the environmental noise. In terms of three important types memory kernels, we discuss the quantum dephasing dynamics of the system and the non-Markovian character exhibiting in the dynamical dephasing induced by non-Markovian random telegraph noise. We show that the dynamical dephasing of the quantum system does not always exhibit non-Markovian character which results from that the non-Markovian character in the dephasing dynamics depends both on the environmental non-Markovian character and the interaction between the system and environment. In addition, the dynamical dephasing of the quantum system can be modulated by the external modulation frequency of the environment. This result is significant to quantum information processing and helpful for further understanding non-Markovian dynamics of open quantum systems.

Influence of mode competition and external wave frequency modulation on gyrotron frequency locking

Influence of mode competition and external wave frequency modulation on gyrotron frequency locking

Frequency-stabilized laser system at 1572 nm for space-borne CO2 detection LIDAR

A frequency-stabilized laser system at 1572 nm for space-borne carbon dioxide (CO2) detection LIDAR to realize the precise measurement of the global atmospheric CO2 concentration is presented in this Letter. A distributed-feedback laser diode serves as the master laser (ML) and is wavelength locked to the CO2 line center at 1572.0179 nm using the external frequency modulation technique. The root mean square frequency drift is suppressed to about 50 kHz at an average time of 0.1 s over 8 h. Based on optical phase-locked loops, an online seeder and an offline seeder are offset locked to the reference laser at 1572.024 and 1572.081 nm, respectively, retaining virtually the same frequency stability as the ML.

Frequency-modulated microwave generation with feedback stabilization using an optically injected semiconductor laser.

Generation of frequency-modulated continuous-wave (FMCW) microwave signals is investigated using the period-one (P1) dynamics of a semiconductor laser. A modulated optical injection drives the laser into P1 oscillation with a modulated microwave frequency, while adding feedback to the injection reduces the microwave phase noise. Using simply a single-mode laser, the tunability of P1 dynamics allows for wide tuning of the central frequency of the FMCW signal. A sweep range reaching 7.7GHz is demonstrated with a sweep rate of 0.42GHz/ns. When the external modulation frequency matches the reciprocal of the feedback delay time, feedback stabilization is manifested as an increase of the frequency comb contrast by 30dB for the FMCW microwave signal.

Stochastic and Nonlinear Control of Attractor Preference

Studying the response of multistable systems to stochastic and harmonic perturbations, we observe a surprising and counterintuitive asymptotic behavior, including nonmonotonic dependence in preference of some coexisting attractors with respect to the noise amplitude, i.e., the existence of a certain noise level for which the most probable size of the basin of attraction is larger than for other noise values, while the average size always decreases as the noise amplitude increases. Since the probabilistic size of the basin of attraction depends on the amplitude and frequency of external modulation applied to a system parameter, noise, periodic modulation, and a combination of both provide an efficient control of attractor preference in systems with multiple coexisting states. Such a control is demonstrated in the delayed feedback logistic map with two coexisting attractors and in the Hénon map with three attractors.

Stochastic control of attractor preference in a multistable system

When talking about the size of basins of attraction of coexisting states in a noisy multistable system, one can only refer to its probabilistic properties. In this context, the most probable size of basins of attraction of some coexisting states exhibits an obvious non-monotonous dependence on the noise amplitude, i.e., there exists a certain noise level for which the most probable basin’s size is larger than for other noise values, while the average size always decreases as the noise amplitude increases. Such a behavior is demonstrated through the study of the Hénon map with three coexisting attractors (period 1, period 3, and period 9). Since the position of the probabilistic extrema depends on the amplitude and frequency of external modulation applied to a system parameter, noise, periodic modulation and a combination of both provide an efficient control of attractor preference in a system with multiple coexisting states.

Actively Q-switched mode-locked Yb-doped fiber laser with high mode-locking contrast

Reliable Q-switched and mode-locked operation of Yb-doped fiber lasers is investigated by employing an acousto-optic modulator (AOM) in a linear cavity configuration. The widths and shapes of the Q-switched envelopes mainly depend on the external modulation frequency of the AOM and near symmetric Q-switched pulses can be generated. Within the Q-switched envelopes, harmonic mode-locked pulses with near 100% modulation depth at the 40 MHz repetition rate are experimentally observed.

1.5-$\mu$m Frequency-Stabilized $\lambda/4$-Shifted DFB LD Employing an External Fiber Ring Cavity With a Linewidth of 2.6 kHz and an RIN of $-$135 dB/Hz

We present a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> frequency-stabilized λ/4-shifted distributed feedback laser diode (DFB LD) with an external fiber ring cavity. By controlling the λ/4-shifted DFB passband to track one Fabry-Pérot mode associated with the fiber ring cavity, the oscillation frequency was continuously tuned over 1.2 GHz without mode-hopping. The optical frequency was stabilized to a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> absorption line with an external frequency modulation scheme that enabled us to realize an output beam without additional modulation. As a result, we obtained a frequency stability as high as 1.4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-10</sup> for τ = 1 s and 1.3 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-11</sup> for τ = 100 s. In addition, we simultaneously achieved a linewidth of 2.6 kHz and a relative intensity noise of less than -135 dB/Hz.

Quasienergy description of the driven Jaynes-Cummings model

We analyze the driven resonantly coupled Jaynes-Cummings model in terms of a quasienergy approach by switching to a frame rotating with the external modulation frequency and by using the dressed atom picture. A quasienergy surface in phase space emerges whose level spacing is governed by a rescaled effective Planck constant. Moreover, the well-known multiphoton transitions can be reinterpreted as resonant tunneling transitions from the local maximum of the quasienergy surface. Most importantly, the driving defines a quasienergy well which is nonperturbative in nature. The quantum-mechanical quasienergy state localized at its bottom is squeezed. In the Purcell-limited regime, the potential well is metastable and the effective local temperature close to its minimum is uniquely determined by the squeezing factor. The activation occurs in this case via dressed spin-flip transitions rather than via quantum activation as in other driven nonlinear quantum systems such as the quantum Duffing oscillator. The local maximum is, in general, stable. However, in presence of resonant coherent or dissipative tunneling transitions the system can escape from it and a stationary state arises as a statistical mixture of quasienergy states being localized in the two basins of attraction. This gives rise to a resonant or an antiresonant nonlinear response of the cavity at multiphoton transitions. The model finds direct application in recent experiments with a driven superconducting circuit quantum electrodynamics setup.

Coupling between Collective Excitations of a Bose–Einstein Condensate and Modulated Interactions

We investigate collective excitations of a Bose–Einstein condensate in the presence of temporal modulation of repulsive interactions, and analytically demonstrate that the modulated interaction can drive the condensate to oscillate with the external modulation frequency, and that the interaction couples with the eigen modes of the condensate collective excitations, which was previously considered to be independent of interaction. When the external modulation frequency approaches or is far away from the eigen frequency of the density monopole mode, the condensate shows resonant or beating behaviour.

40-GHz, 100-fs stimulated-Brillouin-scattering-free pulse generation by combining a mode-locked laser diode and a dispersion-decreasing fiber

A 40-GHz, 100-fs pulse train was successfully generated by soliton compression of a mode-locked laser diode (MLLD) pulse with a dispersion-decreasing fiber. The MLLD had a longitudinal mode linewidth as broad as 60 MHz, which made it possible to suppress stimulated Brillouin scattering and achieve stable, ultrahigh-speed pulse compression without applying external frequency modulation.

Controlling the morphology of polymer blends using periodic irradiation.

Morphology is the decisive factor controlling practical properties such as impact strength or transparency in multiphase polymeric materials. The co-continuous structure formed by polymers has been of great interest to material scientists because of their superiority over those with random morphology. Although a number of efforts--including forcibly freezing the spinodal structure of polymer blends--have been made to produce materials with co-continuous structures, an efficient method for controlling their regularity is still lacking. Here, we demonstrate a novel method using periodic photo-crosslinking to control the length-scale distribution of the spinodal structure in binary polymer blends. It was found that the period distribution of the resulting co-continuous structure became significantly narrow under this periodic forcing. Also, there exists a particular irradiation frequency at which the periodic structure exhibits a minimum, indicating the existence of an ordering process driven by the external modulation frequency. Our findings reveal an easy way to produce polymer materials that is not only useful for optical applications, but also promising for biological separation, such as hemodialysis.

Demonstration of negative hysteresis in an injection-locked diode laser

We report the first, to our knowledge, experimental observation of negative hysteresis in an injection-locked diode laser. The sign reversal is observed at injection current levels close to the free-running laser current threshold ( I th). The bistable loop width shows dependence on the operating temperature and external modulation frequency.

Rotationally resolved electronic absorption spectra of triacetylene cation in a supersonic jet

The rotationally resolved cold spectra (Trot≈17 K) of the A2Πg←X2Πu 000 and 401 transitions of triacetylene cation have been recorded in a supersonic jet. A discharge modulation in combination with the external frequency modulation of a cw ring laser is used to enhance detection sensitivity. The advantage of obtaining the spectra of such species under jet conditions is shown by comparison with measurements in a cell of the 401 vibronic transition. The combined analyses of the rotational structure yields rotational and spin–orbit constants for the ν4=1 level of the A2Πg state.