The side-coupled cladding-pumped fiber has many advantages in terms of system design, ease of pump injection, signal extraction and power scalability over traditional double cladding fiber. We demonstrate a 290-W all home-made side-coupled cladding-pumped fiber laser with the slope efficiency of 67.8%. To the best of our knowledge, the slope efficiency is the highest for the published high power fiber laser based on the DSCCP fiber. The slope efficiency of the home-made side-coupled cladding-pumped fiber amplifier is mainly limited by the background loss, pump light coupling and pump light absorption. The results show that the power ratio of signal light to residual pump light is near 20 dB. The pump light is not absorbed enough that the slope efficiency can be further improved by optimizing the amplifier structure.
Great progress has been made in fiber laser technology especially the high power fiber laser. One of the key
techniques to acquire higher output power is coupling more pump laser into the double-cladding fiber using the fiber
combiner. Fiber splices exist in both manufacture of the combiner and integration of the fiber components. The optical
waveguide structure of the splice point has great effect on the insertion loss and modal content for the fiber laser system.
Thus it is important to use proper method to compute the insertion loss of the splice points. This is also vital in the
manufacture of fiber combiner because the structure must be precisely controlled in order to acquire low insertion loss
for the signal arm of the combiner to ensure the capability of sustaining high power laser.
Generally speaking, there are two common methods to compute the insertion loss of splice points: the mode field
diameter (MFD) and the modal overlap integral (MOI). The MFD method is simple but its accuracy is relatively lower,
while the MOI is more accurate than the MFD but also more complicated. We use both two methods to compute the
insertion loss of the signal arm of a (6+1) ×1 fiber combiner. The result shows that the MFD method is appropriate when
there is only fundamental mode at the splice point. At the mode field matched point, the insertion loss is 0dB when using
the MFD method while 0.29dB when using the MOI method. This indicates that the MOI method is more accurate than
the MFD method to predict the minimum insertion loss and the optimal structure. Meanwhile, the MOI method can
explain the different insertion loss for the co-propagating situation and the counter-propagating situation for the fiber
combiner which cannot be explained by the MFD method. If there are higher order modes passing through the splice
point, the MFD method is also inappropriate.
Passive coherent beam combining, which uses optical feedback from the fibers to force all to lock in phase, seems to be a
promising way for high power output. Fused-taper fiber laser array is one of the passive methods, in which phase locking
is realized by mutual energy coupling, without any active phase control. In this paper, we use a dynamical model to
express phase-locking evolution of the fused-taper fiber laser array theoretically. In the numerical analysis of 3- and 7-
fused-taper fiber arrays, we start the simulations beginning with each laser in an off-state, that is, the initial electric fields
are chosen at random of order 0.1 and the phases are uniformly distributed. The results show that the array can achieve a
relative in-phase state. The coupling degree varies with the distance between the two neighbor fibers. The simulations
also show that the fiber grating reflectivity, the pumping parameters and the coupling length have significant effect on
the stability of the coherent combing.
Coherent combination of fiber lasers through mutual injection locking is demonstrated experimentally in this paper. By
moving the mutual injection couplers from the output port to the high reflection feedback port of the lasers, a modified
combining configuration is constructed with obviously enhanced slope efficiency as compared with the conventional
one. The laser efficiency increases from 29.7% to 37.8% by this modification. The corresponding maximum output
power enhancement of the combined laser is 26.6%. This modification increases not only the individual child laser
powers but also the combining efficiency. The physical connotation of the modification on the improvement of the laser
performance has been discussed.