Page 509 - High Power Laser Handbook

476 Fi b er L a s er s Pulsed Fiber Lasers 477

The net effect of such occurrence is that the in-core confinement of
high-order modes may increase, resulting in BQ degradation. 28

16.3.2 Amplified Spontaneous Emission Management
The possibility of suppressing or managing ASE by means of special
characteristics engineered in gain fibers has attracted some interest
over the years, though research efforts in this area have not yet yielded
a practically viable and effective recipe. A relatively simple design
solution is to lower the core NA, which (in the case of total internal
reflection wave guidance) quadratically reduces the fraction of spon-
taneous emission captured in the fiber core.
Another interesting recent development has been the introduc-
tion of microstructured gain fibers, which can incorporate photonic
stop bands for light that propagates in specific wavelength intervals,
thus acting as a distributed spectral filter. This concept has been dem-
29
onstrated by Goto et al. in the suppression of ~1030-nm ASE within
a Yb-doped solid-core photonic band-gap fiber.
A more common approach to managing ASE for high-pulse-
energy generation entails proper fiber laser and amplifier architec-
ture design. As shown in other laser sources, a widely applicable ASE
mitigation strategy is gain staging. This solution is naturally sup-
ported by master oscillator power amplifier (MOPA) architectures
and consists of distributing the desired optical gain over several
pieces of doped fiber separated by band-pass spectral filters centered
at the signal wavelength. The filters allow for cumulative amplifica-
tion of the signal pulses across the entire amplifier chain, while
rejecting, after each stage, all ASE generated outside their pass band.
This concept, which requires the signal spectral bandwidth to be
much narrower than the ASE spectrum, is frequently realized in gain
media embedded within amorphous hosts, such as glass fibers, where
energy transitions incur severe Stark broadening and often exhibit
line widths of tens of nanometers.
Of course, spectral filtering is not effective for ASE that lies within
the filter pass band (in-band ASE), which is best managed by time-
domain techniques. A common solution is to insert an electro-optic or
acousto-optic amplitude modulator at some point along the amplifier
chain. The modulator is then used as a gating optical switch that pro-
vides high transmission in a short-time window centered at each
pulse and low transmission at any other time. If the modulator exhib-
its sufficiently high on/off extinction (e.g., ~20 dB or higher) and the
time window is a sufficiently small fraction of the pulse repetition
period, then substantial suppression of in-band ASE is possible.
A drawback to this approach is that it increases the system complexity
and parts count. However, this drawback is somewhat mitigated by
the fact that in a well-engineered MOPA source, most in-band ASE is
generated in the high-gain, low-power first stages of the amplifier