Page 503 - High Power Laser Handbook

470 Fi b er L a s er s Pulsed Fiber Lasers 471

16.2.2 Amplified Spontaneous Emission
A fraction of the spontaneous emission (isotropically distributed over
the entire solid angle) from excited-state rare earth ions in the fiber is
captured in the core and becomes bidirectionally guided through it
and amplified via stimulated emission when in the presence of exist-
ing population inversion. In the interval between pulses, when only
the pump beam is present in the fiber and the population inversion is
being built up (i.e., when energy is being stored in the core), such ASE
will continue to grow in power to the extent that, beyond a certain
point, any further absorbed amount of pump power is simply ren-
dered into ASE and does not lead to any additional increase in the
excited-state population. As a general rule, this begins to happen
when the stimulated and spontaneous emission rates become compa-
rable. By limiting the population inversion, the ASE limits the achiev-
able small-signal gain G and ultimately the energy extractable by the
incoming pulse, which is given by
E = GE sat (16.6)
ext
Here E is the fiber saturation energy, which is, in turn, given by
sat
ε A
E sat = (16.7)
σ a + σ e Γ
where σ and ε are the absorption (or emission) cross section of the core
a(e)
dopant and the photon energy, respectively, which are both calculated at
the pulse wavelength; A is the mode field area; and Γ is the transverse
overlap integral between mode field and doping distribution.
Especially unfavorable are low PRF regimes, in which a longer
time is available for ASE buildup, resulting in substantial degrada-
tion of the pulse energy versus pump power conversion efficiency.
For example, in typical Yb-doped large mode area (LMA) fibers (e.g.,
~25 μm/0.06 [core diameter/numerical aperture (NA)]) that are CW
pumped at 975-nm wavelength, PRF values less than 10 kHz repre-
sent challenging pulse formats from the standpoint of efficiency. 12
In addition to its energy-clamping consequences, the portion of
ASE that copropagates with the signal also degrades the pulse con-
trast (i.e., the pulse-to-CW background power ratio), whereas back-
ward-propagating ASE may be harmful for components located in
the amplifier front end (such as in the master oscillator). All such det-
rimental effects are greatly exacerbated if back reflections are present,
which results in ASE multipassing of the fiber gain medium. Ulti-
mately, parasitic lasing at the ASE gain peak will occur. The threshold
pump power for such an effect can be quite low if ASE and sources of
unwanted optical feedback are left unmanaged. However, parasitic
lasing eventually sets in within any fiber amplifier of high enough
gain, because even with the implementation of optical isolators and