共查询到20条相似文献,搜索用时 31 毫秒
1.
?. Arpali S. A. Arpali Y. Baykal H. T. Eyyubo?lu 《Applied physics. B, Lasers and optics》2011,103(1):237-244
The intensity fluctuation of a partially coherent laser beam array is examined. For this purpose, the on-axis scintillation
index at the receiver plane is analytically formulated via the extended Huygens–Fresnel diffraction integral in conditions
of weak atmospheric turbulence. The effects of the propagation length, number of beamlets, radial distance, source size, wavelength
of operation and coherence level on the scintillation index are investigated for a horizontal propagation path. It is found
that, regardless of the number of beamlets, the scintillation index always rises with an increasing propagation length. If
laser beam arrays become less coherent, the scintillation index begins to fall with growing source sizes. Given the same level
of partial coherence, slightly less scintillations will occur when the radial distance of the beamlets from the origin is
increased. At partial coherence levels, lower scintillations are observed for larger numbers of beamlets. Both for fully and
partially coherent laser beam arrays, scintillations will drop on increasing wavelengths. 相似文献
2.
H. T. Eyyubo?lu 《Applied physics. B, Lasers and optics》2011,103(3):755-762
We study the scintillation aspects of partially coherent Lorentz Gaussian (LG) beams via numerically integrating the average
and average squared intensity expressions. Within the examined range of input and propagation medium parameters, the LG beams
are generally found to offer less and less scintillations against the pure Gaussian beam, particularly when the Lorentzian
feature of the beam is emphasized more. This lower scintillation property is exhibited for collimated coherent beams with
different Lorentz widths and at on-axis and off-axis positions of the receiver plane. When focusing is introduced, at shorter
propagation distances the ordering of the beams remains as described above, but at longer propagations distances a complete
reversing of the beam order is observed. Raising the turbulence levels by increasing the structure constant inevitably causes
rises in scintillations, while preserving the advantage of LG beams over the pure Gaussian beam. Partial coherence reduces
scintillations as expected, at the same time nearly eliminating the scintillation differences between the beam types. 相似文献
3.
By using the Rytov method, we formulate and numerically evaluate the scintillations of Laguerre Gaussian beams in weak atmospheric
turbulence. Our results indicate that at on-axis positions, Laguerre Gaussian beams with zero angular mode number will have
less scintillations than fundamental Gaussian beams, where the amount of scintillations will further decrease with rising
radial mode number. When off-axis positions are considered, this situation reverses however, and the scintillations of Laguerre
Gaussian beams become generally higher than the fundamental Gaussian beam. Plotted against the source size, the on-axis scintillations
of Laguerre Gaussian beams fall below the fundamental Gaussian beam, following the same trend as the fundamental Gaussian
beam all throughout the source size range examined. 相似文献
4.
Scintillation index of partially coherent annular and partially coherent flat-topped Gaussian beams propagating in horizontal links is found at the receiver origin when these beams propagate in extremely strong atmospheric turbulence. Scintillation index of coherent versions of such beams attain unity saturation value whereas the decrease in the degree of source coherence results in the decrease of the scintillations. At a fixed degree of partial coherence, thin ring sized annular beams possess smaller scintillations than thick ones. For partially coherent flat-topped Gaussian beams, higher flatness yields smaller intensity fluctuations. In extremely strong turbulence, partially coherent annular and partially coherent flat-topped Gaussian beams have smaller scintillations when compared to partially coherent single Gaussian beam scintillations. 相似文献
5.
The scintillation index of a laser array beam is analytically derived and numerically evaluated for weak turbulence conditions.
On-axis as well as off-axis positions of the receiver plane are considered. Our graphical illustrations prove that at longer
propagation ranges and at some midrange radial displacement parameters, laser array beams exhibit less scintillations, when
compared to a fundamental Gaussian beam. However, when compared among themselves, laser array beams tend to have reduced scintillations
with rising numbers of beamlets, longer propagation wavelengths, at midrange radial displacement parameters, at intermediate
Gaussian source sizes, at bigger inner scales and smaller outer scales of turbulence. However, in this improvement, the number
of beamlets does not seem to have a major role.
PACS 42.25.Dd; 42.25.Ja; 42.25.Kb 相似文献
6.
Scintillation evaluations for Laguerre-Gaussian (LG) beams for slant paths are made using Rytov approximation. On- and off-axis scintillation is formulated and calculated up to several tens of kilometers of slant distances for different zenith angles. Scintillation index variations against radial receiver point and different source sizes are also investigated. In all cases evaluated, it is found that LG beams with higher radial mode numbers result in less scintillation than Gaussian beam. Kolmogorov spectrum function is utilized in the scintillation calculations. 相似文献
7.
Propagation and coherence properties of higher order partially coherent dark hollow beams in turbulence 总被引:3,自引:1,他引:2
Halil Tanyer Eyyubolu 《Optics & Laser Technology》2008,40(1):156-166
We formulate and evaluate in terms of graphical outputs, source and receiver plane expressions, the complex degree of coherence, beam size variation and power in bucket performance for higher order partially coherent dark hollow beams propagating in turbulent atmosphere. Our formulation is able to cover square, rectangular, circular, elliptical geometries for dark hollow and flat-topped beams in one single expression. From the graphical outputs of the receiver plane, it is observed that higher order partially coherent dark hollow beams will initially develop an outer ring around a central lobe, but will eventually evolve towards a Gaussian shape as the propagation distance is extended. It is further observed that stronger turbulence levels and greater partial coherence have similar effects on beam profile. During propagation, modulus of complex degree of coherence of partially coherent dark hollow beams appears to rise above that of the source plane values, reaching as high as near unity. Beam size analysis shows that, among the types examined, (nearly) flat-topped beam experiences the least beam expansion. Power in bucket analysis indicates that lowest order square fully coherent dark beam offers the best power capturing. 相似文献
8.
The scintillation aspects of dark hollow (DH) and flat topped (FT) beams propagating in the turbulent atmosphere containing the non-Kolmogorov power spectrum are investigated. It is found that low scintillations will occur when the exponent of the power spectrum is just above the numeric value of 3. Initially, the rises in scintillations will take place as the exponent becomes larger, but later the scintillation reductions will be experienced as the exponent grows further, eventually minimum scintillations will be seen when the exponent has reached the value of 4. This will be the case, for scintillation variations against propagation distance, source size, wavelength, inner and outer scales of turbulence. Furthermore, it is found that at the small source sizes, DH beams will offer less scintillation than FT beams, while at the large source sizes, the reverse will be applicable. 相似文献
9.
In strong atmospheric turbulence, the asymptotic on-axis scintillation behaviors of Laguerre Gaussian (LG) beams are examined.
To arrive at the strong-turbulence solution, we utilize the existing filtering approach for weak-turbulence solutions and
our recently reported weak-turbulence scintillation index formula for LG beams. In the limiting case, our solution correctly
predicts the asymptotic strong-turbulence behavior of Gaussian beam wave scintillation. Investigation of the scintillations
versus the propagation distance, source size, wavelength and refractive index structure parameter lead to the conclusion that
the LG beams with higher order radial modes can provide less scintillation. The results are applicable to long-haul atmospheric
optical communication links. 相似文献
10.
11.
H.T. Eyyubolu Y. Baykal E. Sermutlu Y. Cai 《Applied physics. B, Lasers and optics》2008,92(2):229-235
For a weak turbulence propagation environment, the scintillation index of the lowest order Bessel–Gaussian beams is formulated.
Its triple and single integral versions are presented. Numerical evaluations show that at large source sizes and large width
parameters, when compared at the same source size, Bessel–Gaussian beams tend to exhibit lower scintillations than the Gaussian
beam scintillations. This advantage is lost however for excessively large width parameters and beyond certain propagation
lengths. Large width parameters also cause rises and falls in the scintillation index of off-axis positions toward the edges
of the received beam. Comparisons against the fundamental Gaussian beam are made on equal source size and equal power basis.
PACS 42.25.Dd; 42.25.Bs; 42.68.Bz; 42.68.-w 相似文献
12.
Based on the propagation law of cross-spectral density function,
studied in this paper are the coherence vortices of partially
coherent, quasi-monochromatic singular beams with Gaussian
envelope and Schell-model correlator in the far field, where our
main attention is paid to the evolution of far-field coherence
vortices into intensity vortices of fully coherent beams. The
results show that, although there are usually no zeros of
intensity in partially coherent beams with Gaussian envelope and
Schell-model correlator, zeros of spectral degree of coherence
exist. The coherence vortices of spectral degree of coherence
depend on the relative coherence length, mode index and
positions of pairs of points. If a point and mode index are kept
fixed, the position of coherence vortices changes with the
increase of the relative coherence length. For the low coherent
case there is a circular phase dislocation. In the coherent
limit coherence vortices become intensity vortices of fully
coherent Laguerre--Gaussian beams. 相似文献
13.
Halil T. Eyyubo?lu 《Applied physics. B, Lasers and optics》2012,106(1):161-169
We provide the scintillation formulation for a multiwavelength source. Within this context, the scintillation aspects of Gaussian,
cos, cosh and annular Gaussian beams are investigated. For all situations examined, it is found that for a source comprising
many wavelengths, there will be less scintillations as compared to a single wavelength source of the lowest wavelength and
but the reverse will be true if the comparison is with respect to the single wavelength source of the highest wavelength.
The same is observed at all propagation distances, source sizes, on-axis and off-axis positions considered. Additionally,
it is seen that the scintillation characteristics of multiwavelength sources will follow similar trends of single wavelength
sources. The analysis is based on the Rytov approximation, therefore our results are valid for conditions of weak atmospheric
turbulence. 相似文献
14.
H. T. Eyyubo?lu 《Applied physics. B, Lasers and optics》2010,98(1):203-210
As an extension of our previous study, the area scintillation aspects of Bessel Gaussian and modified Bessel Gaussian beams
of zeroth order are investigated. The analysis is carried out on the basis of equal source sizes and equal source powers.
It is found that, when compared on equal source size basis, modified Bessel Gaussian beams always have less area scintillations
than a Gaussian beam, while Bessel Gaussian beams exhibit more area scintillations. Comparison on equal source power basis,
however, removes the advantage of modified Bessel Gaussian beams, that is, their area scintillations become nearly the same
as those of the Gaussian beam. On the other hand, for the case of equal source powers, Bessel Gaussian beams with larger width
parameters continue to have higher area scintillations than the Gaussian beam. We provide graphical illustrations for profiles
of equal source size beams, equal source power beams and the curves to aid the selection of equal source power beams. 相似文献
15.
Halil T. Eyyubo?lu 《Applied physics. B, Lasers and optics》2011,103(3):763-769
For the strong atmospheric turbulence regime, the asymptotic on-axis scintillation behavior of annular, cosh and cos Gaussian
beams is theoretically derived and illustrated with numerical examples. It is observed from the plots that annular Gaussian
beams exhibit more scintillations than a Gaussian beam, regardless of the amplitude coefficient and source size settings.
For small source sizes, cosh Gaussian beams seem to have an advantage over Gaussian beams in terms of reduced scintillation,
but for large source sizes a switchover occurs where cos Gaussian beams assume the advantage. Analysis of the effect of inner
scale value shows that scintillations increases for all beams as the inner scale increases. 相似文献
16.
For propagation in turbulent atmosphere, the radius of curvature variations for annular, dark hollow and flat topped beams
are examined under a single formulation. Our results show that for collimated beams, when examined against propagation length,
the dark hollow, flat topped and annular Gaussian beams behave nearly the same as the Gaussian beam, but have larger radius
of curvature values. Increased partial coherence and turbulence levels tend to lower the radius of curvature. Bigger source
sizes on the other hand give rise to larger radius of curvature. Dark hollow and flat topped beams have reduced radius of
curvature at longer wavelengths, whereas the annular Gaussian beam seems to be unaffected by wavelength changes; the radius
of curvature of the Gaussian beam meanwhile rises with increasing wavelength. 相似文献
17.
18.
C. Z. ?il H. T. Eyyubo?lu Y. Baykal Y. Cai 《Applied physics. B, Lasers and optics》2009,95(4):763-771
Within the context of a general beam formulation, beam wander characteristics of cos and cosh-Gaussian beams are derived and
numerically evaluated. In our graphs, the fundamental Gaussian beam is used as a benchmark for comparisons. The associated
plots reveal that at small source sizes, a cos-Gaussian beam has the lowest beam wander, while this property is enhanced with
increasing values of the displacement parameter. At large source sizes however, this advantage is taken over by cosh-Gaussian
beam. Joint examination against the changing source sizes and propagation lengths shows that the range of source sizes, where
the beam wander of cos-Gaussian beam remains lower, is enlarged as we go toward higher propagation lengths. Asymmetric beams
tend to exhibit higher beam wanders both at small and large source sizes, but for the intermediate source size ranges, the
beam wanders of asymmetric beams will fall below those of the symmetric beams. Explanations concerning these behaviors are
offered. A historical account of beam wander formulation is also included. 相似文献
19.
Average relative power transmittance is evaluated, by incorporating atmospheric turbulence, for partially coherent cosh-Gaussian, cos-Gaussian, Gaussian and annular beams. For all the collimated versions of these beams, against the increasing propagation length, there is a typical trend of the decrease in the relative average power transmittance with incremental drop being much less for partially coherent cos-Gaussian beams. The change in the transmittance versus the propagation length will be similar to the corresponding collimated cases, when these beams are focused at a certain focal length. Also partially coherent beams are less sensitive to propagation length changes, except for cos-Gaussian case. Partially coherent cosh-Gaussian beams exhibit a drop in the transmittance as the displacement parameter of the beam is made larger, whereas this trend is just the opposite for partially coherent cos-Gaussian beams. When examined versus the source size, for all the four types of beams, the transmittance has a similar behavior, i.e., it becomes high at small source sizes, falling with increasing source size, and following a dip, it starts to rise, eventually approaching the plane wave limit of unity. The occurrence of the dip coincides with the smallest source size for cosh-Gaussian, with the largest for cos-Gaussian, and about the same source size for Gaussian and annular beams. In general, the average relative power transmittance of coherent beam is affected much more than the partially coherent beams against the variations in source properties. 相似文献
20.
Yongtao Zhang Lin Liu Fei Wang Yangjian Cai Chaoliang Ding Liuzhan Pan 《Optical and Quantum Electronics》2014,46(2):365-379
With the help of the Rayleigh-Sommerfeld diffraction integral, an analytical formula for the spectral density of a nonparaxial polychromatic partially coherent (NPPC) standard or elegant Laguerre-Gaussian (LG) beam propagating through free space is derived. The evolution properties of the intensity distribution of a NPPC standard or elegant LG beam are studied numerically. It is found that the behavior of the intensity distribution of the NPPC standard or elegant LG beam is determined by the initial beam parameters (i.e., beam waist size, spatial coherence width and beam orders). Furthermore, we also study the influence of the initial beam parameters on the spectral change of a NPPC standard or elegant LG beam upon propagation in free space. It is shown that the on-axis spectral shift of a NPPC standard LG beam is almost the same with that of a NPPC elegant LG beam when the initial beam parameters of two type beams are identical. However, the difference between the off-axis spectral shift between a NPPC standard LG beam and a NPPC elegant LG beam is enhanced with the decrease of beam waist size or with the increase of spatial coherence width and beam mode orders. 相似文献