4 CHINESE OPTICS LETTERS / Vol. 5, No. 1 / Jan uary 10, 2007
Study on the approximative formula for the far-field of
a Gaussian beam under circular aperture diffraction
and its divergence
Hongzhan Liu (
), Ruisheng Liang (
), and Xuguang Huang (
ÊÊÊ
)
Laboratory of Photonic Information Technology, South China Normal University, Guangzhou 510006
Received August 10, 2006
The approximative formula for the far-field diffraction of a Gaussian beam through a circular aperture is
obtained by using the superposition of Gaussian beams instead of the aperture function, and the explicit
expression for calculating the beam divergence is also gained. Using the formula, the influences of the
aberrations on the far-field wavefront and the beam’s divergence are researched, and the results show that
the large aberrations badly affect the far-field wavefront and the divergence. It is suggested that the
aberrations and the diffractions should be avo ided when designing the transmitter.
OCIS codes: 050.0050, 050.1970.
The semiconductor laser diode communication system
has been studied for many years
[1−3]
. It is very interest-
ing and will be extensively applied in the future. We are
doing the relative research in this field
[4−10]
.Wehavede-
signed a transmitter for breadboard model. The optical
beam out of the transmitter propagates over 45000 km,
so the beam wavefronts are required to have high qual-
ity and the divergence must be reached the diffraction-
limited. But the actual wavefronts are badly affected by
the aberrations and the diffraction of the emitter aper-
ture.
When the Gaussian beam emits through a circular
aperture, the beam is diffracted. Now we assume that
the normal amplitude of the Gaussian beam is
a(r
0
)=exp(−
r
2
0
ω
2
0
), (1)
where ω
0
is the waist size of the Gaussian beam. Let the
added focused phase aberration is represented by err(r
0
).
According the Ref. [11]
err(r
0
)=exp(−j2π
Δω
λ
)
=exp(−j2π
Δω
m
λ
)exp(j2π
Δω
m
r
2
0
λR
2
0
), (2)
where R
0
notes the radius of the circular aperture and
Δω
m
represents the maximum of the focused aberration.
Let the aperture function is represented by circ(r
0
), then
the amplitude of the aberration Gaussian beam after the
aperture is expressed as
b(r
0
)=a(r
0
)err(r
0
)circ(r
0
)
=exp(−
r
2
0
ω
2
0
)exp(−j2π
Δω
m
λ
)exp(j2π
Δω
m
r
2
0
λR
2
0
)circ(r
0
).
(3)
Using the diffraction theory, the wavefront of optical
far-field can be expressed as
A(r, z)=
2π
jλz
exp(j
2π
λ
z)
×
∞
0
exp(j
2π
λ
r
2
+ r
2
0
2z
)J
0
(
2π
λ
rr
0
z
)b(r
0
)r
0
dr
0
=
k
jz
C
R
0
exp(jk
r
2
+ r
2
0
2z
)exp(−
r
2
0
ω
2
0
)
× exp(
jkΔω
m
r
2
0
R
2
0
)J
0
(k
rr
0
z
)circ(r
0
)r
0
dr
0
, (4)
where C =exp(jkz)exp(−jkΔω
m
), k =
2π
λ
,andJ
0
is
the zero order Bessel function.
Equation (4) is changed into non-dimension expression
as
A(τ,η)=
2C
jη
1
0
exp(j
τ
2
+ ζ
2
η
)exp(−
R
2
0
ω
2
0
ζ
2
)
× exp(jkΔω
m
ζ
2
)J
0
(2
τζ
η
)circ(ζ)ζdζ
=
2C
jη
1
0
exp(j
τ
2
+ ζ
2
η
)exp(−B
0
ζ
2
)
×J
0
(2
τζ
η
)circ(ζ)ζdζ, (5)
here, B
0
=(R
2
0
/ω
2
0
) − jkΔω
m
, τ = r/R
0
, ζ = r
0
/R
0
,
η = z/L. L = kR
2
0
/2 notes the Rayleigh distance of the
circular aperture with radius R
0
. Equation (5) represents
the far-field wavefront of the Gaussian beam out of the
circular aperture. It is very difficult to get the analytic
expression of the far-field wavefront. In order to research
the far-field, it is requested to carry the numerical inte-
gration of Eq. (5). It is also difficult to obtain the ana-
lytic expression of the far-field. To solve this difficulty,
we will expand the circular function circ(ζ) according to
1671-7694/2007/010004-04
c
2007 Chinese Optics Letters