Speed of structured light pulses in free space

AbstractA plane monochromatic wave propagates in vacuum at the velocity c. However, wave packets limited in space and time are used to transmit energy and information. Here it has been shown based on the wave approach that the on-axis part of the pulsed beams propagates in free space at a variable speed, exhibiting both subluminal and superluminal behaviours in the region close to the source, and their velocity approaches the value of c with distance. Although the pulse can travel over small distances faster than the speed of light in vacuum, the average on-axis velocity, which is estimated by the arrival time of the pulse at distances z ≫ ld (ld is the Rayleigh diffraction range) and z > cτ (τ is the pulse width) is less than c. The total pulsed beam propagates at a constant subluminal velocity over the whole distance. The mutual influence of the spatial distribution of radiation and the temporal shape of the pulse during nonparaxial propagation in vacuum is studied. It is found that the decrease in the width of the incident beam and the increase in the central wavelength of the pulse lead to a decrease in the propagation velocity of the wave packet.

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Effect of adverse weather conditions and pointing error on the performance of 2-D WH/TS OCDMA over FSO link

Journal of Optical Communications ◽

10.1515/joc-2020-0127 ◽

2020 ◽

Vol 0 (0) ◽

Author(s):

Bithi Mitra ◽

Md. Jahedul Islam

Keyword(s):

Free Space ◽

Atmospheric Condition ◽

Optical Power ◽

Weather Conditions ◽

Free Space Optical ◽

Central Wavelength ◽

Pointing Error ◽

Adverse Weather ◽

Code Division Multiple ◽

Ber Performance

AbstractIn this paper, the performance of two-dimensional (2-D) wavelength-hopping/time-spreading (WH/TS) optical code division multiple access (OCDMA) system over free space optical (FSO) channel is analyzed in the presence of pointing error and different weather conditions. Prime code scheme is employed for both wavelength-hopping and time-spreading to address user code-matrix. The operating central wavelength of 1550 nm is considered to demonstrate the bit error rate (BER) performance of the proposed system as a function of various system parameters. The required optical power of the proposed system is determined to maintain a BER value of 10−9. The numerical evaluation interprets that the BER performance is highly dependent on transmission length, transmitted power, pointing error angle as well as the number of simultaneous user. It is also observed that the 2-D OCDMA system over free space needs minimum required optical power in case of rainy atmospheric condition, but it is maximum for foggy atmospheric condition.

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Non-Standard Hierarchies of the Runnings of the Spectral Index in Inflation

10.20944/preprints201701.0133.v1 ◽

2017 ◽

Author(s):

Chris Longden

Keyword(s):

Spectral Index ◽

Field Model ◽

Variable Speed ◽

Speed Of Light ◽

Microwave Background ◽

Gravity Effects ◽

Alternative Scenarios ◽

Single Field ◽

Canonical Quantum Gravity ◽

Canonical Quantum

Recent analyses of cosmic microwave background surveys have revealed hints that there may be a non-trivial running of the running of the spectral index. If future experiments were to confirm these hints, it would prove a powerful discriminator of inflationary models, ruling out simple single field models. We discuss how isocurvature perturbations in multi-field models can be invoked to generate large runnings in a non-standard hierarchy, and find that a minimal model capable of practically realising this would be a two-field model with a non-canonical kinetic structure. We also consider alternative scenarios such as variable speed of light models and canonical quantum gravity effects and their implications for runnings of the spectral index.

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