Double Modulation Scheme Research Articles

Double-modulation stimulated Raman scattering: how to image up to 16-fold faster.

A stimulated Raman microscope is conventionally performed by modulating either the pump or Stokes beam and demodulating the other. Here, we propose a double modulation scheme that modulates both beams at fm and 2fm. Exploiting aliasing and reduction of the repetition rate, we show that the proposed double modulation scheme amplifies the signal amplitude by a factor of 1.5, 2, and 4 for different modulation frequencies and experimental realizations for the same average power at the sample. By deriving the noise power for different sources, we show that the double modulation scheme can perform stimulated Raman scattering (SRS) imaging with an up to 16-fold speed improvement as compared with single beam modulation.

Coherent population trapping with polarization modulation

Coherent population trapping (CPT) is extensively studied for future vapor cell clocks of high frequency stability. In the constructive polarization modulation CPT scheme, a bichromatic laser field with polarization and phase synchronously modulated is applied on an atomic medium. A high contrast CPT signal is observed in this so-called double-modulation configuration, due to the fact that the atomic population does not leak to the extreme Zeeman states, and that the two CPT dark states, which are produced successively by the alternate polarizations, add constructively. Here, we experimentally investigate CPT signal dynamics first in the usual configuration, a single circular polarization. The double-modulation scheme is then addressed in both cases: one pulse Rabi interaction and two pulses Ramsey interaction. The impact and the optimization of the experimental parameters involved in the time sequence are reviewed. We show that a simple seven-level model explains the experimental observations. The double-modulation scheme yields a high contrast similar to the one of other high contrast configurations like push-pull optical pumping or crossed linear polarization scheme, with a setup allowing a higher compactness. The constructive polarization modulation is attractive for atomic clock, atomic magnetometer, and high precision spectroscopy applications.

Background-Free Stimulated Raman Spectroscopy and Microscopy

We propose a three-color, double-modulation scheme for the background-free detection of stimulated Raman scattering (SRS). We call the scheme stimulated Raman gain and opposite loss detection (SRGOLD). It exploits the symmetric nature of potential parasitic signals (cross phase modulation, two-photon absorption, and thermal effects) to the end of suppressing them. Conversely, the antisymmetric nature of SRS provides for a twofold increase in the magnitude of the SRS signal. We experimentally demonstrate SRGOLD spectroscopy and microscopy on test samples as well as on mice skin samples.

Complete polarization control of light from a liquid crystal spatial light modulator

We present a method to generate complete arbitrary spatially variant polarization modulation of a light beam by means of a parallel aligned nematic liquid crystal spatial light modulator (SLM). We first analyze the polarization modulation properties in a transmission mode. We encode diffraction gratings onto the SLM and show how to achieve partial polarization control of the zero order transmitted light. We then extend the technique to a double modulation scheme, which is implemented using a single SLM divided in two areas in a reflective configuration. The polarization states of the transmitted beam from the first pass through the first area are rotated using two passes through a quarter wave plate. The beam then passes through the second area of the SLM where additional polarization information can be encoded. By combining previously reported techniques, we can achieve complete amplitude, phase and polarization control for the diffracted light that allows the creation of arbitrary diffractive optical elements including polarization control. Theoretical analysis based on the Jones matrix formalism, as well as excellent experimental results are presented.

Measurement range enlargement in Brillouin optical correlation-domain reflectometry based on double-modulation scheme

We have demonstrated a double-modulation scheme to enlarge the measurement range of Brillouin optical correlation-domain reflectometry for fiber-optic distributed strain sensing. In this scheme, the frequency of the laser output is simultaneously modulated with two different frequencies. In the experiment, 53-cm resolution and 1.5-km measurement range were simultaneously obtained. Furthermore, 27-cm resolution and 1.5-km measurement range were also simultaneously achieved when a noise-floor compensation technique was employed.

Femtosecond spin dynamics of epitaxial Cu(111)/Ni/Cu wedges

The femtosecond spin dynamics in ferromagnetic metals is studied using time-resolved magneto-optics on an epitaxial Cu(111)/Ni/Cu wedge. A novel and sensitive double modulation scheme is introduced. The induced Kerr rotation and ellipticity are separately measured, and display a contrasting behavior during the first 500 fs after excitation. This proves the absence of a direct relation between the magneto-optical signal and the magnetization. A systematic study of the spin dynamics along the nickel wedge shows a surprisingly small influence of interface scattering, magnetic anisotropy, and morphology on the dynamics.

Stark-modulation-enhanced FM-spectroscopy

Diode lasers are a versatile tool for fundamental atomic and molecular spectroscopy and have increasing applications in gas analysis and diagnostics. In order to obtain quantum limited performance, the problem of the suppression of time-dependent unwanted background structures superimposed to the desired signal from the spectral feature under investigation has to be addressed. Sample modulation enhanced FM-spectroscopy as described in this paper combines laser frequency modulation (FM) with a sample modulation, based on the Stark effect in molecular spectra. The transitions 1 10–1 11 at 1746.2 cm −1 and 5 33–4 32 at 1759.9 cm −1 in the v 2-band of H 2CO have been used in experiments with static and AC electric fields up to 2 kV cm −1. An intercomparison of the FM technique and the FM-Stark double modulation scheme demonstrates the remarkable potential of this technique for background suppression.

Excited state absorption in Yb 3+–Er 3+-codoped phosphate glasses (ZnO–Al 2O 3–La 2O 3–P 2O 5) around the 4I 13/2→ 4I 15/2 emission spectral range

We are involved in Yb 3+–Er 3+-codoped phosphate glass research for eye-safe laser source near 1540 nm. Undoped and doped glasses of the composition: ZnO–Al 2O 3–La 2O 3–P 2O 5 have been synthesised. Absorption from the 4I 15/2 ground state, emission spectra and decay time measurements have been performed at room temperature on these materials. In addition to the usual spectroscopic characterisations, the excited state absorption 4I 13/2→ 4I 9/2 Er 3+ transition located at the long wavelength edge of the 4I 13/2→ 4I 15/2 emission transition have also been investigated at room temperature. The excited state absorption spectra were obtained with a pump and probe technique employing a double modulation scheme in the infrared wavelength region from 1300 to 1900 nm. Furthermore, the line strength of the excited state absorption transition have been calculated in order to compare it with the experimental.

Infrared excited-state absorption and stimulated-emission cross sections of Er3+-doped crystals

Detailed spectra of the excited-state absorption and the stimulated-emission cross-sections of Er3+ -doped Y3Al5O12, YAlO3, LiYF4, and KYF4 crystals are analyzed in the infrared wavelength regions from 700 to 2100 nm. The spectra were measured with a pump and probe technique employing a double modulation scheme. For Er (2%): Y3Al5O12 also the stimulated emission at 3 µm and the reabsorption due to excited-state absorption from the lower laser level are investigated.

TEST OF QUANTUM ELECTRODYNAMICS USING ULTRA-HIGH SENSITIVE INTERFEROMETERS

For strong fields, quantum electrodynamics tells us that the vacuum is refractive. This can be described by Euler-Heisenberg effective Lagrangian and for B ~ 12 T , the effective index deviates from one of the order of 10-21. Schemes to measure this effect are proposed using ultra-high sensitive interferometers of the type developed for gravity-wave detection. A double modulation scheme is promising in measuring the effect to 1%. For E ~ 5 × 107 V/m and B ~ 12 T , the mixed electric-magnetic effect is also within the reach of detectability.

16 QAM Carrier Recovery PLL for Service-Channel Transmission Using FSK Additional Modulation

This paper describes the design and performance of the 16 QAM carrier recovery PLL for service-channel transmission using FSK additional modulation. When FSK additional modulation is applied, tracking phase error occurs and corrupts the 16 QAM signal. To reduce the phase error without widening the loop noise bandwidth is an important issue in the 16 QAM-FSK double modulation scheme. An analytical expression to estimate the peak phase error due to class 4 partial response FSK is established, By using the approximation formula, an optimization of the carrier recovery PLL is presented. Moreover, experiments and computer simulations are carried out for the 200 Mbit/s 16 QAM and 64 kbit/s SC double modulation system. As a result, it is demonstrated that a 0.1 dB 16 QAM equivalent power loss and an 8 dB frequency deviation margin of SC transmission performance can be obtained.

Sub-picosecond pulse generation by a new synchronous double-modulation scheme

Sub-picosecond pulse generation by a new synchronous double-modulation scheme

Optical coherence modulation by ultrasonic waves 1: Dependence of partial coherence on ultrasonic parameters

Quantitative control of partial coherence by ultrasonic waves is considered theoretically and experimentally. The double coherence modulation factor obtained is space-invariant or not according to whether a light beam passes twice the same path or a different path in a progressive ultrasonic field by reflection at a plane mirror or at a right-angle prism. A double-modulation scheme using a plane mirror reflection system is found to be more effective than a single-modulation scheme for coherence control of low levels of ultrasonic power. The coherence of light is quantitatively varied with the input voltage applied to the ultrasonic transducer.

Elastic and inelastic contributions to secondary electron yield structure

The secondary electron yield of solid surfaces contains weak but complicated structure, which is greatly enhanced by differentiation with respect to primary electron energy. Auger electron appearance potential features, corresponding to yield changes due to core level excitations, and diffraction features are the major contributions to this structure. The magnitude of the diffraction features decreases with primary energy and depends strongly on temperature and surface order. Surprisingly, even for polycrystalline materials the diffraction features extend out to about 400 eV and completely obscure appearance potential features in this region. We have measured the energy distribution of electrons contributing to each of these kinds of features. The diffraction structure is due almost entirely to elastic electrons while the appearance potential peaks result primarily from electrons with energy less than 30 eV. This difference can be exploited using a double modulation scheme with four-grid LEED optics to separate the two kinds of features. This extends the useful range of Auger electron appearance potential spectroscopy down to energies below 200 eV.

Self-sustained Oscillations in Pulse Modulated Feedback Systems

In this paper, the problem of computing self-sustained oscillations in feedback systems with double modulation scheme, combining the pulse width modulation law and pulse frequency modulation law, is considered. A method based on state variable techniques is developed by which symmetric periodic solutions can be determined. Suitable examples are given to demonstrate the analytical procedure.