laser4-研深圳大学 高等激光物理学期末复习课件

laser4-研深圳大学 高等激光物理学期末复 习课件
From amplifiers to oscillators
the laser: an optical amplifier
a source of light
amplifier : bandwidth and gain saturation
amplifiers with positive feedback
1 tsp
2
n 2 8
g( )
Amplifier bandwidth
The frequency response is proportional to the
lineshape function of the given transition.
In the case of homogeneous broadening
The frequency response is proportional to the
lineshape function of the given transition.
#
G(z) n 21( , 0 )
21
(
,
0
)
B21g
(
)
h
c
21( , 0 )
A21c 2
8 2
g( )
21( , 0 )
the exponential growth of the intensity At high power levels ,this is not correct,
it will produce gain saturation.
#
Gain saturation
The intensity increases the rate of stimulated emission increases population from the upper level is reduces the degree of population inversion is reduced the gain is also reduced. #
power sources.
#
Amplifier bandwidth
Bandwidth : amplifier’s frequency response
In the optical amplifier
the frequency response is determined by the
frequency dependence of the gai来自百度文库 coefficient
Electronic amplifiers can only be powered by electrical energy
Optical amplifiers can be ‘pumped’ by electrical,
optical, chemical, mechanical and even nuclear
Phase is not relevant in electronic amplifiers.
➢ Frequency response
In optical amplifiers the basic frequency selection is determined by the energy levels in atoms
➢ both are characterised by a gain coefficient, which is frequency dependent;
➢ both are influenced by noise, gain saturation, and
other non-linearities.
G() is a Lorentzian function
I→0
( )2
G(
, 0 )
G( 0 )
(
2 0 )2
( 2
)2
2 G( 0 )
gN ( , 0 )
2
(
1
0
)2
(
2
)2
gN
(
0
)
2
Gain saturation
At low power, optical amplify function I(z) = I(0)exp(Gz)
#
Differences between optical and electronic amplifiers
➢ Phase
The optical amplifier increases the magnitude of the optical field while maintaining its phase.
Once the number of photons overtakes the number of excited atoms, the exponential growth comes to an end.
#
Gain saturation
The population difference is a function of the energy density : (ν=ν0)
optical oscillators.
laser
#
Light amplifiers
Comparing optical amplifiers with electronic amplifiers
Similarities:
➢ both rely on an external power source to supply energy;
Electronic amplifiers rely on electronic circuits made
of capacitors, inductors, etc
#
Differences between optical and electronic amplifiers
➢ The source of external power
Gain saturation
The intensity increases in the amplifier, it stimulates more and more excited atoms to emit photons.
This can only go on as long as the number of stimulating photons is less than the number of excited atoms.