Zero-frequency refractivity of water vapor, comparison of Debye and van-Vleck Weisskopf theory

Grischkowsky, D.; Yang, Yuchen; Mandehgar, Mahboubeh

doi:10.1364/oe.21.018899

Cited by 16 publications

(17 citation statements)

References 26 publications

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“…Due to the excellent agreement between ϕ(ν) and ϕ mod (ν), the frequency dependence of L eff (ν) highlights the smaller contributions that we have neglected thus far, i.e., the effective frequency dependence of L THz . We identify three main contributions: (a) Standing waves within the Si lenses give rise to a modulation of L eff with a period of about 4.1 GHz, see inset of [23,24]. The absorption lines are very well resolved even for this comparably short path in air.…”

Section: Resultsmentioning

confidence: 91%

Group Delay in THz Spectroscopy with Ultra-Wideband Log-Spiral Antennae

Langenbach

Roggenbuck

Mayorga

et al. 2014

J Infrared Milli Terahz Waves

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We report on the group delay observed in continuous-wave terahertz spectroscopy based on photomixing with phase-sensitive homodyne detection. We discuss the different contributions of the experimental setup to the phase difference ϕ(ν) between transmitter arm and receiver arm. A simple model based on three contributions yields a quantitative description of the overall behavior of ϕ(ν). Firstly, the optical path-length difference gives rise to a term linear in frequency ν. Secondly, the ultra-wideband log-spiral antennae effectively radiate and receive in a frequencydependent active region, which in the most simple model is an annular area with a circumference equal to the wavelength. The corresponding term changes by roughly 6π between 100 GHz and 1 THz. The third contribution stems from the photomixer impedance. In contrast, the derivative ∂ ϕ/∂ν is dominated by the contribution of periodic modulations of ϕ(ν) caused by standing waves, e.g., in the photomixers' Si lenses. Furthermore, we discuss the Fourier-transformed spectra, which are equivalent to the waveform in a time-domain experiment. In the time domain, the group delay introduced by the log-spiral antennae gives rise to strongly chirped signals, in which low frequencies are delayed. Correcting for the contributions of antennae and photomixers yields sharp peaks or "pulses" and thus facilitates a time-domain-like analysis of our continuous-wave data.

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Section: Resultsmentioning

confidence: 91%

Group Delay in THz Spectroscopy with Ultra-Wideband Log-Spiral Antennae

Langenbach

Roggenbuck

Mayorga

et al. 2014

J Infrared Milli Terahz Waves

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“…The remainder comes from the 557-GHz line. Grischkowsky et al (2013) show that the full theoretical calculation behind Eq. (13.70), based on Van Vleck-Weisskopf line shapes and incorporating all water lines from 22.2 GHz through 30 THz, gives agreement with the empirical expression for refractivity without any ad hoc corrections.…”

Section: Origin Of Refractionmentioning

confidence: 96%

Propagation Effects: Neutral Medium

Thompson

Moran

Swenson

2017

Astronomy and Astrophysics Library

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The neutral gas in the atmosphere has a significant effect on signals passing through it. We are concerned with three types of effects. First, the large-scale structures in the media give rise to refractive effects. These effects, which can be analyzed in terms of geometrical optics and Fermat's principle, are the deflection of the radio waves and the change of the propagation velocity. Second, radiation can be absorbed. Finally, radiation can be scattered by the turbulent structure of the media. The phenomenon of scattering results in scintillation, or seeing.In the troposphere, water vapor plays a particularly important role in radio propagation. The refractivity of water vapor is about 20 times greater in the radio range than in the near-infrared or optical regimes. The phase fluctuations in radio interferometers at centimeter, millimeter, and submillimeter wavelengths are caused predominantly by fluctuations in the distribution of water vapor. Water vapor is poorly mixed in the troposphere, and the total column density of water vapor cannot be accurately sensed from surface meteorological measurements. Uncertainties in the water vapor content are a serious limitation to the accuracy of VLBI measurements. Small-scale (< 1 km) fluctuations in water vapor distribution limit the angular resolution of connected-element interferometers in the absence of wavefront correction techniques. Furthermore, spectral lines of water vapor cause substantial absorption at frequencies above 100 GHz and usually render the troposphere highly opaque at frequencies between 1 and 10 THz (300 and 30 m). Thus, any discussion of the neutral atmosphere must be primarily concerned with the effects of water vapor. Propagation in the neutral atmosphere from the point of view of radio communications is discussed by Crane (1981) and Bohlander et al. (1985).Our interest in the propagation media arises because the media degrade interferometric measurements of radio sources. Alternately, observations of radio sources can be used to probe the characteristics of the propagation media. Radio interferometric measurements have been used widely for this purpose.

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“…Figure 3 compares the IFFT compensated pulse with the (A 2 and A 3 ), dispersion compensated pulse, which shows excellent compensation. Figure 4a shows the complete phase calculation of β(ω)z without the constant (zero frequency) refractivity term [3][4][5][6], compared to the 3-A parameter fit of Φ A , with (ω o /2π) = 852 GHz, marked by the vertical line. The difference between the two plots (residuals) is shown in Fig.…”

Section: Mahboubeh Mandehgar and D Grischkowskymentioning

confidence: 99%

“…For our case, we remove the strong linear phase ramp as follows: Φ(ω,z) = (ω/c) (n(ω) -n(0) -2)z = β(ω)z, for which (n(0) -1) = (61.06 x 10 -6 ) [6]. Figure 1 shows the recently studied input THz bit sequence (011010) for the 852 GHz channel and the measured and calculated output THz bit sequence [4].…”

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confidence: 99%

Dispersion compensation of the THz communication channels in the atmosphere

Mandehgar

Grischkowsky

2015

2015 40th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz)

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We demonstrate that dispersive compensation can be achieved for the communication channels within the atmospheric THz windows using the long-path THz-TDS system. However, the THz pulse broadening cannot be eliminated due to the bandwidth reduction of the propagating THz pulse due to the frequency dependent absorption of the channels.There are many applications for relatively short-length, high bit-rate THz links in the atmosphere [1][2][3][4][5]. A recent study presented an experimental and theoretical characterization of short-length, high bit-rate links for the seven THz communication channels in the atmosphere below 1 THz [4]. Here, using the complete theoretical approach for the absorption and dispersion of water vapor [4-6], we show the potential to increase the bit-rate distance product, by dispersion compensation. However, we also show that the frequency dependent absorption within the channel significantly reduces the bandwidth of the transmitted signal. This reduction also broadens the THz data pulses, and consequently dispersion compensation cannot eliminate all of the observed pulse broadening with propagation. THz pulses experience distortion and broadening when they propagate through the dispersive atmosphere, which leads to overlapping of adjacent pulses in the bit sequence. Here, we analyze the increase of the performance of the Channel 7 at 852 GHz, with dispersion-compensation to demonstrate achievable high data rates of the future THz wireless network.Dispersion management compensates phase dispersion in communication channels. Such compensation has been studied since 1980, and is used in optical fibers to significantly increase the data rates in optical communications. It is also important to understand the full capacity of the THz channels without being limited by dispersion and to be able to estimate the dispersion tolerance of the THz channels.When the electric field of the input THz pulse, E(0,ω), (expressed as a function of frequency) passes through the dispersive atmosphere, the phase changes due to the resonance lines of water vapor. The output complex spectrum E (z,ω), is given by the product of the input field with the phase function and the attenuation, as

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Zero-frequency refractivity of water vapor, comparison of Debye and van-Vleck Weisskopf theory

Cited by 16 publications

References 26 publications

Group Delay in THz Spectroscopy with Ultra-Wideband Log-Spiral Antennae

Group Delay in THz Spectroscopy with Ultra-Wideband Log-Spiral Antennae

Propagation Effects: Neutral Medium

Dispersion compensation of the THz communication channels in the atmosphere

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