Linear Intensity Modulation Research Articles

Detailed analysis of the R1f/ΔI1 WMS technique and demonstration of significantly higher detection sensitivity compared to 2f WMS for calibration-free trace gas sensing.

Recognizing that wavelength modulation spectroscopy (WMS) is particularly important in the development of high-sensitivity gas sensing systems, this paper presents a detailed analysis of the R 1f /Δ I 1 WMS technique that has recently been successfully demonstrated for calibration-free measurements of the parameters that support detecting multiple gases under challenging conditions. In this approach, the magnitude of the 1f WMS signal (R 1f ) was normalized by using the laser's linear intensity modulation (Δ I 1) to obtain the quantity R 1f /Δ I 1 that is shown to be unaffected by large variations in R 1f itself due to the variations in the intensity of the received light. In this paper, different simulations have been used to explain the approach taken and the advantages that it shows. A 40mW, 1531.52nm near-infrared distributed feedback (DFB) semiconductor laser was used to extract the mole fraction of acetylene in a single-pass configuration. The work has shown a detection sensitivity of 0.32ppm for 28cm (0.089ppm-m) with an optimum integration time of 58s. The detection limit achieved has been shown to be better than the value of 1.53ppm (0.428ppm-m) for R 2f WMS by a factor of 4.7, which is a significant improvement.

Photothermal Circular Dichroism of Single Nanoparticles Rejecting Linear Dichroism by Dual Modulation.

Circular dichroism (CD) is the property of chiral nanoobjects to absorb circularly polarized light of either handedness to different extents. Photothermal microscopy enables the detection of CD signals with high sensitivity and provides a direct absorptive response of the samples under study. To achieve CD measurements at the single-particle level, one must reduce such artifacts as leakage of linear dichroism (LD) and residual intensity modulation. We have simulated our setup with a simple model, which allows us to tune modulation parameters to obtain a CD signal virtually free from artifacts. We demonstrate the sensitivity of our setup by measuring the very weak inherent CD signals of single gold nanospheres. We furthermore demonstrate that our method can be extended to obtain spectra of the full absorptive properties of single nanoparticles, including isotropic absorption, linear dichroism, and circular dichroism. We then investigate nominally achiral gold nanoparticles immersed in a chiral liquid. Carefully taking into account the intrinsic chirality of the particles and its change due to heat-induced reshaping, we find that the chiral liquid carvone surrounding the particle has no measurable effect on the particles’ chirality, down to g-factors of 3 × 10–4.

Enhanced second-order nonlinearity and tunable plasmon induced transparency in noncoplanar Dirac semimetal system

Plasmon Induced Transparency (PIT) effect possesses the characteristics of sharp and pronounced spectral response and considerable group delay. In this work, a PIT system consists of two noncoplanar Dirac semimetal particles (DSPs) has been studied by finite-difference time-domain (FDTD) method. Tailoring of this PIT system can be achieved by structural parameters, Fermi energy (EF), refractive index (RI) of dielectric medium and polarization angle of incident light. Benefitted from its smaller particle size, the bandwidth of PIT window in this DSPs system is up to 3.94 THz, which is much broader than in bulk Dirac semimetal PIT system (about 0.36 THz). An efficient second order nonlinearity of this DSPs system is obtained for its intensive localized field, non-centrosymmetric structureunique and unique electromagnetic mode. The second-order nonlinear conversion efficiency in this system is up to 10−6 to 10−5 for its noncoplanar structure. This noncoplanar DSPs system can also achieve approximate linear light intensity modulation effect of its second harmonic waves (SHWs). This developed noncoplanar DSPs structure could provide guidance for the design of high integration Dirac semimetal THz devices.

QCL-Based Open-Path, Single-Pass Measurement of Ambient Carbon Monoxide Using R 1f /ΔI₁ WMS

This letter outlines a new robust wavelength modulation spectroscopy technique to achieve calibration-free measurements. The technique normalizes the magnitude of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1^{st}$ </tex-math></inline-formula> harmonic signal by the amplitude of the laser’s linear intensity modulation, and uses the fact that the ratio of the intensity to the amplitude of the linear intensity modulation remains invariant at every point within a system and over time despite significant and unpredictable variations in the individual quantities. The technique is demonstrated using a mid-infrared quantum cascade laser for open-path measurements of ambient carbon monoxide with a single-pass detection limit of 12 ppb-m. The method is simple to implement and performs robustly for long durations of outdoor measurements.

Intensity Modulation-Normalized Calibration-Free 1f and 2f Wavelength Modulation Spectroscopy

This paper reports the development and validation of a field-deployable quantum cascade laser-based tunable diode laser spectroscopy (TDLS) sensor for in situ measurement of atmospheric CO and CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> using a new calibration-free 1f and 2f wavelength modulation spectroscopy (WMS) method. The 1f WMS and 2f WMS signals are normalized by the linear and non-linear intensity modulation components respectivelyto make the signals immune to laser intensity variations. The advantage of this method is that the normalized signals appear on a background that is independent of the laser parameters. The error in mole fraction extraction is 0.2% and 0.98% for CO and CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> respectively. The system has a single-pass detection limit of 3 ppb for CO (using 1f WMS and 4.05 m path length) and 45 ppb for CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (using 2f WMS and 20 cm path length) for an optimum integration time of 17 s and 69 s respectively. These values correspond to normalized detection limits of 12 ppb-m for CO and 9 ppb-m for CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . The mean mole fraction of CO and CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> measured in Gandhinagar, India from 19-25 July 2019 were found to be 477 ± 32 ppb and 486 ± 29 ppm respectively. These measurements demonstrate that this WMS algorithm and the sensor system are suitable for accurate long-term monitoring of greenhouse gases and air pollutants.

Injection-locked semiconductor laser-based frequency comb for modulation applications in RF analog photonics.

A linearized intensity modulator for periodic and pulsed light is proposed and demonstrated. The free carrier plasma effect has been used to modulate the refractive index of the phase section of a three-section mode-locked laser. If injection locked, the modulation induces an arcsine phase response on the three-section mode-locked laser. By introducing this mode-locked laser into a Mach-Zehnder interferometer biased at quadrature, one can realize a true linear intensity modulation. This novel laser suppresses any unwanted amplitude modulation and increases the performance of the linearized intensity modulator. Experimental results have provided a record low static Iπ of 0.39 mA and a spur-free dynamic range of 75 dB.Hz2/3.

Calibration-free 2f WMS with in situ real-time laser characterization and 2f RAM nulling.

This Letter demonstrates a new calibration-free 2f wavelength modulation spectroscopy (WMS) technique to measure gas concentration and pressure without the need for laser precharacterization. A 1650-nm laser diode is used for methane concentration and pressure measurements for pressures up to 4 bar and for a modulation index (m) of 2.2. All laser parameters such as the intensity, linear and nonlinear intensity modulation (IM), frequency modulation (FM) characteristics, the phase difference ψ1 between the FM and the linear IM, and the phase difference ψ2 between the FM and the nonlinear IM are accurately estimated in situ and in real time. This technique accounts for variations in these parameters that arise due to scanning of the laser's center wavelength, laser temperature variations, and aging of the laser. The laser is modulated at its phase quadrature frequency at which the linear IM and the FM are orthogonal to each other (ψ1=90°). This ensures that the two linear IM-dependent distorting Fourier components are orthogonal to the detection axis, and the undistorted 2f signal is recovered. This simplifies the simulation and gas parameter-extraction process. Finally, 2f RAM nulling is implemented to remove the significant absorption-independent 2f residual amplitude-modulation (RAM) signal that is seen to cause significant distortion of the 2f signal and detector saturation.

Rocking motion of an optical wing: theory.

The intensity-dependent rocking frequency of an illuminated semicylindrical refractive rod (or "optical wing") on a flat, nonslip surface is investigated. Both longitudinal and transverse radiation pressure forces (scatter and lift forces), as well as radiation pressure torque, transform the mechanical system into one having a bistable potential energy above a critical intensity. The equation of motion may be written as a parametrically driven nonlinear bistable harmonic oscillator, resulting in complex rocking dynamics. The effects of linear and sinusoidal intensity modulation schemes are explored, and experimental conditions to verify these results are discussed.

Plasmonic Spectral Engineering via Interferometric Illumination of Colloid Sphere Monolayers

A novel method is presented for complex structure fabrication, which is capable of breaking the hexagonal symmetry of the conventional colloid sphere lithography via the interferometric illumination of colloid sphere monolayers (IICSM). It is demonstrated that the perfect lateral synchronization of a linear intensity modulation originating from two-beam interference with respect to a hexagonal colloid sphere monolayer makes it possible to tune four complex structure parameters independently. Based on comparative study of hexagonal and rectangular hole doublet-arrays, which can be generated by linearly polarized light via homogeneous illumination and via IICSM, it is shown that the novel IICSM method enables plasmonic spectral engineering with higher degrees of freedom. The unique spectral properties of the complex patterns attainable via IICSM are more precisely tunable by properly selected azimuthal orientation during illumination and by the surrounding medium. It is shown that coupling phenomena between propagating and localized plasmonic modes on appropriately designed complex structures result in unique charge and near-field distribution accompanied by narrow Fano lines. Optimal configurations of complex plasmonic structures consisting of a rectangular array of hole doublets with different geometrical size parameters are presented, which ensure enhanced sensitivity in bio-detection.

Study on the Laser Characteristics for Calibration-Free Wavelength Modulation Spectroscopy

In this paper, a calibration-free wavelength modulation model via tunable diode laser absorption spectroscopy was setup. The laser modulation characteristics, modulation depth, linear modulation amplitude and frequency shift, were measured for the model setup. The experimental results show that modulation depth and the linear intensity modulation amplitude have well linear fitting to the scan voltage. Based on the laser parameters, the calibration-free model can be used to infer the gas temperature.

Theoretical and Experimental Study of a Semiconductor Resonant Cavity Linear Interferometric Intensity Modulator

The phase response of an injection locked semiconductor laser that is used as the phase modulator in a resonant cavity linear interferometric intensity modulator is studied in detail. It is shown that, signal-to-intermodulation ratio of such a modulator is affected by the injection ratio, linewidth enhancement factor of the semiconductor laser, residual amplitude modulation, depth of phase modulation, and linearity of the resonant cavity response. Experimental measurements of the signal-to-intermodulation ratio of this modulator using a semiconductor Fabry-Pérot laser as the resonant cavity are in good agreement with the theoretically predicted values.

Polarization insensitive linear intensity modulation in a Sagnac fiber-optic loop with an electro-optic cell and optoelectronic feedback

We present a modulation scheme consisting of a fiber-optic Sagnac interferometer, which has an electro-optic modulator (Pockels cell) in the loop of the interferometer. In order to diminish the nonlinearity of the Sagnac interferometer transmittance function, we monitor a very small part of the output light intensity by means of a photodiode and then amplify this signal by a constant factor ξ. This amplified signal is added to the electrical modulating signal that is being applied to the electro-optical modulator. The consequence of introducing this feedback loop is that by properly choosing the amplifying factor ξ, we can increase the linearity of the modulation. The device is insensitive to the polarization state of the input light, so it should be used for intensity modulation of nonpolarized light with up to 100% modulation depth. The theoretical and experimental examination of the device is presented.

A novel design for fiber optic interferometric fringe projection phase-shifting 3-D profilometry

A novel design for phase-shifting fiber optic interferometric fringe projection three-dimensional (3-D) profilometry is developed by using the frequency modulation capability of a single mode laser diode (LD) and is applied to automatic 3-D shape measurement. Injection current modulation is used for generating the frequency shift of the LD. The spatial synchronous phase measurement technique is developed for on-line calibration of the phase shift introduced by the frequency shift of the LD in the unbalanced two-beam fiber optic interferometric fringe projection system. Experimental analysis shows that the spatial synchronous phase measurement technique is simple and accuracy with uncomplicated system. A 10-sample phase-shifting algorithm is presented for 3-D shape measurement, which is insensitive to phase-shift errors and the linear intensity modulation of the interferogram. Experimental results are presented for the phase-shift calibration, system stability and 3-D object shape measurements.

Theoretical description based on Fourier analysis of wavelength-modulation spectrometry in terms of analytical and background signals

A theoretical description of the wavelength-modulation (WM) spectrometry technique is given. The formalism is based on Fourier analysis and can therefore correctly handle arbitrary large frequency-modulation amplitudes. It can also deal with associated intensity modulations as well as wavelength-dependent transmission effects. It elucidates clearly how various Fourier components of these entities combine with those of the line-shape function to yield separately the final analytical and background nf WM signals. Explicit expressions are given for the 2f and the 4f signals. It is shown, among other things, that the 4f technique in general gives rise to smaller background signals (and therefore larger signal-to-background ratios) than does the 2f technique when the background is dominated by etalon effects from short cavities and that a finite intensity modulation necessarily leads to an out-of-phase nf WM signal. The formalism is also able to elucidate clearly that a linear intensity modulation is not sufficient to cause any 2f background residual-amplitude-modulation signals (as was the general consensus until recently in the literature) but that 2f background signals instead can exist only in systems with either wavelength-dependent transmission or a laser with nonlinear intensity modulation.

New fiber-optic attenuator calibration technique using linear intensity modulation characteristics of a laser diode

A new fiber-optic attenuator calibration technique has been developed using linear intensity modulation characteristics of a laser diode. It does not require detector linearity to be known. The uncertainty of attenuation calibration for a typical 10 dB attenuator is 0.020 dB at two standard deviation levels. The resolution of the system is 0.001 dB and the dynamic range is 25 dB in a single step.

558 The influence of linear frequency and intensity modulation of sound impulse sequence on evaluation of simulated movement

558 The influence of linear frequency and intensity modulation of sound impulse sequence on evaluation of simulated movement

Vibration-Induced Intensity Modulation in Multimode Fiber

The mechanism of the linear intensity modulation by external mechanical vibrations on a multimode fiber has been investigated theoretically. The effect of power redistribution among the guided modes which arises from mode-coupling is evaluated for a few-mode fiber. It is shown that linear intensity modulation occurs if the vibration-induced stress of the core lacks axial symmetry. The sensitivity is given by a first order effect in mode-coupling and is significantly enhanced at the resonance condition. This linear effect is a different one from the quadratic effect due to bending loss.

Squared signal correlation and a possible acoustooptic implementation

A new method of signal detection called squared signal correlation (SSC) is introduced. The performance statistics of the SSC are derived for Gaussian signals and compared with those of the conventional correlators. It is shown that the SSC’s characteristics are compared with those of conventional correlators that use linear amplitude and linear intensity modulation. The SSC also allows for system simplicity at the expense of a slight reduction in the output SNR.

New Fiber Optic Analog Baseband Transmission Plan for Color TV Signals

A new fiber optic baseband analog transmission plan is proposed. It arranges the original signal into a pulse train for driving the laser diode to generate baseband components at the transmitter. The receiver can be just the same as those used in conventional direct linear intensity modulation systems using light emitting diodes (LED). Advantages of this new plan are (i) potentially cheap fibers can be used, since the plan requires neither an extra wide bandwidth nor special light acceptance conditions, (ii) long repeater spacings can be attained, since abundant light can be coupled into fibers using laser diodes and also the simple structures required for fibers are suitable for realizing low transmission losses, and (iii) the transmitter and receiver can be quite simple. Some candidate pulse arranging plans for generating baseband components are investigated. A pulse frequency arranging plan and pulse width arranging plan seem to be promising for efficient picture transmissions. An experiment using pulse frequency arrangement proves the theory developed here to attain a color TV transmission using a 10 km long step index low-loss fiber for a signal to noise ratio of 44 dB. Future improvements are expected by redesigning the transmitter and receiver and also by improving the mode stability of laser diodes.

Bandwidth limitation of a fibre optic system with linear intensity modulation

Bandwidth limitation of a fibre optic system with linear intensity modulation