Suppression Of Amplitude Modulation Research Articles

Field-programmable gate array-based residual amplitude modulation suppression and control for compact atomic clocks.

We designed a field-programmable gate array (FPGA) fabric to provide phase modulation techniques to lock lasers to optical frequency references. The method incorporates an active residual amplitude modulation (RAM) suppression scheme that relies on complex modulation. All the required servos to construct an optical atomic clock are incorporated into the same low-cost, commercial FPGA chip. We demonstrate a reliable, long-term RAM suppression of 60 dB with the remaining RAM level at -100 dBc and an improved stability of three decades when applied on a two-photon rubidium clock.

Application of additional high-frequency modulation to reduce influence of residual amplitude modulation LiNbO3 phase modulator on fiber optical gyroscope signal

Residual amplitude modulation in LiNbO3 phase modulator is one of the key factors that limit the accuracy of high-sensitive fiber-optical sensors. A fiber-optical gyroscope is an angular velocity sensor whose sensitivity is better than 0.001 °/h. Optical light intensity changes after phase modulator is a reason for wrong phase difference that introduces an error in the angular velocity signal. Most existing residual amplitude modulation suppression methods are based on reduction of back reflections between an optical fiber and integrated optical waveguide, absorbing groove to suppress or reduce reflection on the bottom face, and algorithmic compensation. In this paper, new approach to reduce residual amplitude modulation in LiNbO3 for fiber optical gyroscope application is presented. Method feature is an application of additional differential signal modulation with uniform amplitude distribution in the input signal voltage range of the phase modulator. The proposed method allows to suppress residual amplitude modulation of the multifunctional integrated optical circuit phase modulator more than 3 times using additional triangle signal modulation with the frequency f = 200.09 MHz and power P = 36 dBm. This method is suitable for improving fiber optical gyroscope accuracy. Moreover, it could be applied for any fiber-optic sensors based on LiNbO3 phase modulator. The paper will be of interest to specialists in the field of highly sensitive fiber optical sensors, fiber, and integrated optics.

Evolution of intense narrowband noise beams

The main part of this work is focused on the experimental study of the evolution of acoustic beams with a narrowband spectrum at the input. The nonlinear interaction leads to initiation of new harmonics whose width increases with the harmonic number and a continuous power spectrum is formed at high frequencies. The experimental equipment enables us to study the beam propagation at large acoustic Reynolds numbers and analyze the spectral content of the field in a wide frequency band. For narrowband noise, the input field can be considered as a quasi-monochromatic signal with random amplitude and phase modulation. The amplitude-modulation suppression is observed for large Reynolds numbers as the wave propagates. Far from the sound transducer, the field on the sound transducer axis is a sequence of discontinuities with a universal behaviour between them. The locations of the discontinuities and their amplitudes and, hence, the statistical characteristics of the waves are determined by the phase fluctuations of the initial wave. It has been theoretically shown that the field spectrum at the discontinuous stage has a universal structure which is determined by the probability distribution of the initial wave frequency. It has been demonstrated experimentally that the theory describes sufficiently well the noise-signal spectrum form at the discontinuous stage.

Suppression of amplitude modulation induced by polarization mode dispersion using a multi-degree-of-freedom fiber filter

Polarization mode dispersion (PMD) in fibers for high-power lasers can induce significant frequency modulation to amplitude modulation (FM-to-AM) conversion. However, existing techniques are not sufficiently flexible to achieve efficient compensation for such FM-to-AM conversion. By analyzing the nonuniform transmission spectrum caused by PMD, we found that the large-scale envelope of the transmission spectrum has more serious impacts on the amount of AM. In order to suppress the PMD-induced FM-to-AM conversion, we propose a novel tunable spectral filter with multiple degrees of freedom based on a half-wave plate, a nematic liquid crystal, and an axis-rotated polarization-maintaining fiber. Peak wavelength, free spectral range (FSR), and modulation depth of the filter are decoupled and can be controlled independently, which is verified through both simulations and experiments. The filter is utilized to compensate for the PMD-induced FM-to-AM conversion in the front end of a high-power laser facility. The results indicate that, for a pulse with phase-modulation frequency of 22.82 GHz, the FM-to-AM conversion could be reduced from 18% to 3.2% within a short time and maintained below 6.5% for 3 h. The proposed filter is also promising for other applications that require flexible spectral control such as high-speed channel selection in optical communication networks.

A Study of Residual Amplitude Modulation Suppression in Injection Locked Quantum Cascade Lasers Based on a Simplified Rate Equation Model

Using results that come out of a simplified rate equation model, the suppression of residual amplitude modulation in injection locked quantum cascade lasers with the master laser modulated by its drive current is investigated. Quasi-static and dynamic expressions for intensity modulation are used. The suppression peaks at a specific value of the injection ratio for a given detuning and linewidth enhancement factor. The intensity modulation suppression remains constant over a range of frequencies. The effects of injection ratio, detuning, coupling efficiency and linewidth enhancement factor are considered.

Optical carrier sinusoidal modulation suppressed by ultra-long semiconductor optical amplifier gain saturation

The amplitude modulated sinusoidal pattern over an optical carrier is suppressed by a deeply saturated ultra-long semiconductor optical amplifier. The extinction-ratio of the modulated signal is reduced from 16.8 to 3.9 dB at 15 GHz. The characterization of the amplitude modulation suppression is presented, with good results for broadband operation (15 nm at 1557 nm), high-speed (up to 35 GHz) and high input extinction-ratio (up to 16.8 dB). © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:1286–1290, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26010

频率转换过程中强度调制的形成机制及抑制方法

频率转换过程中强度调制的形成机制及抑制方法

频率转换过程中强度调制的形成机制及抑制方法

频率转换过程中强度调制的形成机制及抑制方法

频率转换过程中强度调制的形成机制及抑制方法

频率转换过程中强度调制的形成机制及抑制方法

All optical linearization technique of DFB‐LD based on optical injection locking for RoF system

AbstractA novel all optical linearization technique using optical injection locking is proposed for radio‐over‐fiber system. When slave laser diode (SLD) is operated under injection locking condition, the modulated optical signals from master laser diode (MLD) are suppressed by residual amplitude modulation suppression. However, these signals are not completely suppressed by imperfection of residual amplitude modulation suppression and remained in the cavity of SLD. By appropriately controlling the phase and amplitude of these signals to have out‐of phase and the same magnitude condition compared with those of direct modulated optical signals from SLD, the efficiency of linearization of DFB‐LD can be more enhanced by optical cancellation in the SLD. By using the proposed technique, 17 dB enhancement of carrier‐to‐interference ratio was experimentally achieved up to frequency response of DFB‐LD compared with that of free‐running case. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 2403–2406, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22800

Stabilization, injection and control of quantum cascade lasers, and their application to chemical sensing in the infrared

Quantum cascade lasers (QCLs) are a relatively new type of semiconductor laser operating in the mid- to long-wave infrared. These monopolar multilayered quantum well structures can be fabricated to operate anywhere between 3.5 and 20 μm, which includes the molecular fingerprint region of the infrared. This makes them an ideal choice for infrared chemical sensing, a topic of great interest at present. Frequency stabilization and injection locking increase the utility of QCLs. We present results of locking QCLs to optical cavities, achieving relative linewidths down to 5.6 Hz. We report injection locking of one distributed feedback grating QCL with light from a similar QCL, demonstrating capture ranges of up to ±500 MHz, and suppression of amplitude modulation by up to 49 dB. We also present various cavity-enhanced chemical sensors employing the frequency stabilization techniques developed, including the resonant sideband technique known as NICE-OHMS. Sensitivities of 9.7×10 −11 cm −1 Hz −1/2 have been achieved in pure nitrous oxide.

Suppression of amplitude modulation of pulses generated from a subharmonically mode-locked semiconductor laser

A feedforward modulation technique is used to suppress amplitude modulation associated with 40-GHz pulses generated from a subharmonically mode-locked semiconductor laser. It is shown that the amplitude modulation can be suppressed by as much as 20 dB. The improvement of amplitude modulation is shown for different subharmonic numbers.

AM suppression with low AM-PM conversion with the aid of a variable-gain amplifier

This paper proposes the use of a variable-gain amplifier instead of a hard limiter for amplitude modulation (AM) suppression with low AM-PM (phase modulation) conversion. A hard limiter shows phase shift variations through input-amplitude dependent changes in output waveform, combined with bandwidth limitations. It is shown that these can be kept small only for limiter bandwidths much larger than the input frequency. A linear amplifier with variable gain used for AM suppression does not suffer from this problem. A CMOS variable-gain amplifier with gain-insensitive phase shift has been designed for this purpose. The benefits and limitations of the technique are explored with reference to an experimental 2.5 /spl mu/m BiCMOS chip for a television IF demodulator. Experimental and simulation results indicate that the AM-PM conversion can be kept below 0.5/spl deg/ at 40 MHz over an input amplitude range of 20 dB, where typical hard limiters show 3-5/spl deg/. This is achieved with an amplifier bandwidth of 80 MHz, while a hard limiter would need a bandwidth of more than 600 MHz to obtain similar results.

Residual amplitude modulation suppression using deeply saturated erbium-doped fiber amplifiers

Residual amplitude modulation due to wavelength dithering has been reduced significantly. A power penalty of 3.5 dB is reduced to only 0.2 dB by employing a deeply saturated erbium-doped fiber amplifier with a 25-kHz response frequency as a booster amplifier.

Amplitude Modulation Suppression in FM Systems

Inadequacies of some limiter concepts are discussed and a limiter analysis in terms of low-index modulation theory is presented. The analysis of a particular circuit proves the possibility of perfect amplitude modulation suppression with small loss to the frequency modulated signal. Experimental data are presented which verify the theory and demonstrate the practicability of the device analyzed.

Discrimination of a synchronized oscillator against interfering tones and noise

An account is given of the discrimination provided by a synchronized oscillator against unwanted signals accompanying the synchronizing tone; the synchronized oscillator is more specifically a non-linear regenerative tuned circuit. The discrimination is partly due to the frequency response of the system, but has also an important contribution from the non-linear behaviour of the circuit, provided that the wanted (sychronizing) signal has a greater amplitude, after allowing for frequency response, than the unwanted (interfering) signals.Section 2 gives a general description of the phenomena of synchronization and non-linear discrimination, illustrated by experimental observations of frequency response and non-linear discrimination in resistance-capacitance-tuned oscillators. Both single-frequency and noise interferences are considered. It is concluded that very large amounts of discrimination can be obtained when the natural frequency of the oscillator is very close to the synchronizing frequency, so that only very small amplitudes of the latter are needed to maintain synchronism. In more practical cases, where reasonable amounts of variation of both natural and synchronizing frequencies must be allowed for, only smaller amounts of discrimination due to nonlinearity can be obtained, but 10 dB or more is quite feasible. This amount will be important when the interfering frequencies are so close to the synchronized frequency that the amount of frequency discrimination is negligible. For instance, use is made of these effects in the homodyne and synchrodyne demodulators. It is shown that when the synchronizing tone is not dominant or is absent no suppression of the interference or narrowing of its spectrum is obtained.Section 3 gives a mathematical analysis of the effects, on the assumption that the non-linear law of the system has no terms above the cubic, and that the feedback loop has uniform amplitude and phase response over the frequency band covered by the applied signals. Equations are developed for the improvement of signal/noise ratio and suppression of single-tone interference. Interference due to amplitude modulation and phase modulation of the synchronizing tone is also dealt with, and it is shown that the suppression of amplitude modulation is greater than that of signals merely accompanying the synchronizing tone, but that phase modulation is not suppressed at all.