1.2 Spatial Coherence of Rhodamine 6G Laser Radiation …
15
laser pumping. Then, changing the diameter of the radiating surface of “reference”
heat source using variable slit, the coincidence has been achieved of localization
region sizes of interference patterns produced during radiation of “reference” heat
source and the laser under consideration. In this case, spatial coherence of laser
radiation corresponds to spatial coherence of radiation of heat homogeneous circular
light source.
The presence of lens led to the increase of efficiency of angular sizes of laser
source that is equivalent to spatial coherence degradation of light.
The results of the studies have shown that in both cases (in the first case, there was
no Fabry–Perot interferometer in the resonator, and in the second case, the Fabry–
Perot interferometer was included into the resonator) Spatial coherence of dye laser
radiation focused by the lens (8) with the focal distance F = 19 cm in front of input
mirror of the Mach–Zehnder interferometer (v = 30 cm
−1 , f = 11.6 cm, p = 3.6 cm)
corresponds to spatial coherence of radiation of heat homogeneous arc source with
the diameter d less than 0.1 mm. It can be easily shown that to the source of such a
size coherence region of laser end corresponds, the diameter of which is (Fig. 1.4b):
D ≈ F
λ
d
,
(1.21)
i.e., for our case F = 19 cm, λ = 5.8 × 10
−5 cm, d ≤ 10
−2 cm, D ≤ 1.2 mm. It should
also be noted that the present method can be used only for the study of light sources,
which have stable mode structure, as spatial coherence function determination (as in
any interference method) is carried out by the set of interferograms.
Comparison with heat source is reasonable when spatial coherence function under
consideration (a priori) is homogeneous, in the opposite case averaged values ˜
y will
be received.
For measuring spatial coherence function of 6G laser radiation with the selective
cavity (see Fig. 1.1) by a holographic method, the scheme similar to the mentioned
one in the works [42–44, 69] was used. The installation diagram can be seen in
Fig. 1.5.
Laser end with fivefold increasing was designed onto the hologram and diffusive screen. Near the laser end, the rectangular diaphragm (2) of 2 × 3 mm
2 was
put, which made it possible to link point coordinates of laser end on the hologram,
diffusive screen and reconstructed image. The distance from the screen to hologram
is 100 mm, and average angle between reference and object beams is about 12°.
Hologram registration was made on the “Mikrat 900” film, and the hologram size is
10 × 15 mm
2 .
In Fig. 1.6a, the scheme of wave front reconstruction and intensity registration
is shown. The hologram was exposed by narrow beam of He–Ne laser (with the
diameter of 1 mm), which could move in two mutually perpendicular directions
x and y. Photomultiplier tube, which was used for the measuring intensity along
with the diaphragm (diameter 0.5 mm) put in front of it, could also move along the
reconstructed image what made it possible to measure spatial coherence function of
laser end points.
15
laser pumping. Then, changing the diameter of the radiating surface of “reference”
heat source using variable slit, the coincidence has been achieved of localization
region sizes of interference patterns produced during radiation of “reference” heat
source and the laser under consideration. In this case, spatial coherence of laser
radiation corresponds to spatial coherence of radiation of heat homogeneous circular
light source.
The presence of lens led to the increase of efficiency of angular sizes of laser
source that is equivalent to spatial coherence degradation of light.
The results of the studies have shown that in both cases (in the first case, there was
no Fabry–Perot interferometer in the resonator, and in the second case, the Fabry–
Perot interferometer was included into the resonator) Spatial coherence of dye laser
radiation focused by the lens (8) with the focal distance F = 19 cm in front of input
mirror of the Mach–Zehnder interferometer (v = 30 cm
−1 , f = 11.6 cm, p = 3.6 cm)
corresponds to spatial coherence of radiation of heat homogeneous arc source with
the diameter d less than 0.1 mm. It can be easily shown that to the source of such a
size coherence region of laser end corresponds, the diameter of which is (Fig. 1.4b):
D ≈ F
λ
d
,
(1.21)
i.e., for our case F = 19 cm, λ = 5.8 × 10
−5 cm, d ≤ 10
−2 cm, D ≤ 1.2 mm. It should
also be noted that the present method can be used only for the study of light sources,
which have stable mode structure, as spatial coherence function determination (as in
any interference method) is carried out by the set of interferograms.
Comparison with heat source is reasonable when spatial coherence function under
consideration (a priori) is homogeneous, in the opposite case averaged values ˜
y will
be received.
For measuring spatial coherence function of 6G laser radiation with the selective
cavity (see Fig. 1.1) by a holographic method, the scheme similar to the mentioned
one in the works [42–44, 69] was used. The installation diagram can be seen in
Fig. 1.5.
Laser end with fivefold increasing was designed onto the hologram and diffusive screen. Near the laser end, the rectangular diaphragm (2) of 2 × 3 mm
2 was
put, which made it possible to link point coordinates of laser end on the hologram,
diffusive screen and reconstructed image. The distance from the screen to hologram
is 100 mm, and average angle between reference and object beams is about 12°.
Hologram registration was made on the “Mikrat 900” film, and the hologram size is
10 × 15 mm
2 .
In Fig. 1.6a, the scheme of wave front reconstruction and intensity registration
is shown. The hologram was exposed by narrow beam of He–Ne laser (with the
diameter of 1 mm), which could move in two mutually perpendicular directions
x and y. Photomultiplier tube, which was used for the measuring intensity along
with the diaphragm (diameter 0.5 mm) put in front of it, could also move along the
reconstructed image what made it possible to measure spatial coherence function of
laser end points.