White-light Signal Research Articles

Phase-error-compensation-based surface recovery algorithm using spectrum selection for white light interferometry.

White light interferometry is a well-established surface recovery technique. In this paper, a white light signal processing algorithm based on phase error compensation using spectrum selection is proposed. The derived nonlinear phase distribution from the correlogram is modeled as the combination of random errors and systemic deviations. By developing a new, to the best of our knowledge, recovery algorithm, the phase noise can be separated from the linear map and significantly attenuated. Based on the proposed algorithm, the spectrum features of white light LEDs and halogen lamps are investigated in detail. The inner products defined by three selected points are employed to generate a coefficient to evaluate the linearity of an unwrapped phase map within a certain spectrum region. The optimal spectrum range corresponding to the best measurement performance can then be located where the coefficient approximates 1 and the spectrum energy stays relatively high. The simulations are carried out under different levels of SNR and scan step noises, which show that the new method can effectively reduce additional disturbance from the recovered topography. In experiments, the system with the proposed method is first calibrated by a step height standard (VLSI, 182.7±2.0nm) with the repeatability of 0.44 nm. A silicon wafer and three roughness standards are also tested to further verify the robustness of the new method.

In-situ measurement of aligning intensity and rotational temperature in field-free molecular alignment via white light generation

We determine the aligning intensity and rotational temperature in field-free molecular alignment measured via white light signal. Relying on temporal dependencies of the time-resolved alignment signature obtained via white-light generation by femtosecond laser pulses in N2 molecules, we derived the aligning pump intensity and rotational temperature using the occurrence times of the local alignment and anti-alignment around the half and full revivals. Results proved that the aligning intensity and rotational temperature can be obtained simultaneously by any two reverse local alignment extrema in the evolution of field-free molecular alignment.

Gravitational wave detectors with broadband high frequency sensitivity

Gravitational waves from the neutron star coalescence GW170817 were observed from the inspiral, but not the high frequency postmerger nuclear matter motion. Optomechanical white light signal recycling has been proposed for achieving broadband sensitivity in gravitational wave detectors, but has been reliant on development of suitable ultra-low loss mechanical components. Here we show demonstrated optomechanical resonators that meet loss requirements for a white light signal recycling interferometer with strain sensitivity below 10−24 Hz−1/2 at a few kHz. Experimental data for two resonators are combined with analytic models of interferometers similar to LIGO to demonstrate enhancement across a broader band of frequencies versus dual-recycled Fabry-Perot Michelson detectors. Candidate resonators are a silicon nitride membrane acoustically isolated by a phononic crystal, and a single-crystal quartz acoustic cavity. Optical power requirements favour the membrane resonator, while thermal noise performance favours the quartz resonator. Both could be implemented as add-on components to existing detectors.

Double end-mirror sloshing cavity for optical dilution of thermal noise in mechanical resonators

The sensitivity of gravitational wave detectors at high frequencies is currently limited by quantum shot noise. It has been shown theoretically that white light signal recycling using optomechanical negative dispersion filter cavities can increase the gain-bandwidth product of gravitational wave detectors that is usually limited by conventional signal recycling. This sensitivity enhancement is most pronounced at high frequencies above 500 Hz. This technology could be implemented in current facilities or future detectors, but requires thermal noise dilution of the mechanical resonator. Here we provide a theoretical analysis of a double end-mirror sloshing (DEMS) cavity to achieve strong thermal noise dilution with low radiation pressure noise and suppression of optical anti-damping. We demonstrate experimentally that the DEMS cavity can be tuned to a regime that is expected to support these favorable conditions.

Enhanced detection of high frequency gravitational waves using optically diluted optomechanical filters

Detections of gravitational waves (GW) in the frequency band 35 Hz to 500 Hz have led to the birth of GW astronomy. Expected signals above 500 Hz, such as the quasinormal modes of lower mass black holes and neutron star mergers signatures are currently not detectable due to increasing quantum shot noise at high frequencies. Squeezed vacuum injection has been shown to allow broadband sensitivity improvement, but this technique does not change the slope of the noise at high frequency. It has been shown that white light signal recycling using negative dispersion optomechanical filter cavities with strong optical dilution for thermal noise suppression can in principle allow broadband high frequency sensitivity improvement. Here we present detailed modelling of AlGaAs/GaAs optomechanical filters to identify the available parameter space in which such filters can achieve the low thermal noise required to allow useful sensitivity improvement at high frequency. Material losses, the resolved sideband condition and internal acoustic modes dictate the need for resonators substantially smaller than previously suggested. We identify suitable resonator dimensions and show that a 30 $\mu$m scale cat-flap resonator combined with optical squeezing allows 8 fold improvement of strain sensitivity at 2 kHz compared with Advanced LIGO. This corresponds to a detection volume increase of a factor of 500 for sources in this frequency range.

Ultra-low dissipation resonators for improving the sensitivity of gravitational wave detectors

Broadband enhancement of the sensitivity of gravitational wave detectors can be achieved by the use of negative dispersion filters to create white light signal recycling cavities. This filter should have mechanical frequency of 300 kHz or higher and T/Qm∼6×10−10 K, in order to achieve appreciable sensitivity enhancement in the range of 1–2 kHz. This paper investigates the possibility of using optical dilution of GaAs/AlGaAs-coated Si and GaAs “cat-flap” micro-resonators to achieve such performance. We analyse the loss contributions to such resonators, particularly thermoelastic loss, suspension loss and acceleration loss. Sufficient reduction of thermoelastic loss is possible when operating near the zero thermal expansion point with temperature control of ∼1 K for both materials. Acceleration loss and suspension losses can be minimised in the frequency range 104–105 Hz, allowing Q-factors in the range 1011–1012, but these are reduced at the target 400 kHz frequency. Results are subject to assumptions regarding material losses. Fabrication techniques for creating GaAs and SiNx suspended silicon cat-flap resonators are presented.

White-light interferometers with polarizing optics for length measurements with an applicable zero-point detection

For absolute length and form measurements at a large working distance (>150 mm) two special interferometers, a tandem interferometer and a Michelson interferometer with achromatic polarizing optics are constructed. In our experiments, both consist of a combination of one low-coherence interferometer and one laser interferometer. For the low-coherence interferometer part, a simple white-light source with less than 100 µW optical power output is chosen. It bases upon a low-cost fiber-coupled near-infrared LED with a large spectral width (FWHM > 68 nm at 825 nm). The use of achromatic polarizing optics such as broadband polarizing beamsplitters and achromatic quarter-wave plates in the low-coherence interferometer parts increases the contrast level of the white-light signal fringe pattern to nearly 100%. Furthermore, the fringe pattern in a polarized interferometer has no subsignatures and is unique. Hence, different algorithms are tested for signal processing and automated zero-point detection of the white-light signature. The software for an automated measurement is tested in a standard room without thermal control and without damped oscillation. Therefore, in experiments with the tandem interferometer, it was possible to measure the zero-point position of a white-light signature with a peak-to-peak difference of 154 nm under uncontrolled environmental conditions without thermal stabilization. The white-light Michelson interferometer with polarizing achromatic optics allows zero-point detections with a standard deviation (mean value) of less than 15 nm. The drift is proved through measurement results.

Linear negative dispersion with a gain doublet via optomechanical interactions.

Optical cavities containing a negative dispersion medium have been proposed as a means of improving the sensitivity of laser interferometric gravitational wave detectors through the creation of white-light signal recycling cavities. Here we demonstrate that negative dispersion can be realized using an optomechanical cavity pumped by a blue detuned doublet. We used an 85-mm cavity with an intracavity silicon nitride membrane. Tunable negative dispersion is demonstrated, with a phase derivative dφ/df from -0.14 Deg·Hz(-1) to -4.2×10(-3) Deg·Hz(-1).

PHOTOMORPHOGENIC DEVELOPMENT AND CARBOHYDRATE COMPOSITION OF YOUNG WATERMELON PLANTS AS AFFECTED BY INDIVIDUAL OR MULTIPLE FR SIGNALS DURING THE DARK PHASE OF THE PHOTOPERIOD

Treatment of young watermelon (Citrullus lanatus cv. Sugar Baby) plants with individual and multiple FR light (15 min) treatments during the dark phase of the photoperiod influenced plant growth and development (i.e., petiole elongation, internode elongation, and reduced petiole angles) as compared to plants not treated with FR signals. The timing for the most effective light signal for inducing a growth response was when the signal was delivered immediately after the plant entered into the dark phase of the photoperiod. Decreasing growth responses to FR signals were observed as the signals were delayed after the plant entered into the dark phase. Multiple FR signals during the dark phase slightly increased growth responses as compared to plants that received the signal immediately after the light period. Young watermelon plant growth responses to FR light signals do not appear to be photoperiodic as plants similarly treated with a white light signal did not generate growth responses. Tissue analysis of petioles, leaves, stems, and cotyledons from plants treated with individual and multiple FR signals suggested that carbohydrate composition, distribution, and diurnal fluctuation were affected by the light quality treatments.

Coherence measurement of a grating-based white-light optical signal processor

A two-beam interference technique for coherence measurement in the Fourier plane of a grating-based white-light optical signal processor is presented. The visibility measurement is obtained with a scanning photometer at the output plane of the processor. The degree of coherence as a function of slit separation, due to source size, input object size, and the spatial frequency of the sampling grating, is plotted. The results show that the degree of coherence increases as the spatial frequency of the sampling grating increases. However, this improvement of coherence is somewhat more effective in the direction perpendicular to the light dispersion. Thus, this white-light signal processing technique is more effective in one dimension. Since this white-light processor is capable of processing the signal with the entire spectral band of the light source, it is very suitable for color image processing.