Objectives: High brightness is a perpetual pursuit in research and
application of laser, and fiber laser coherent combination is an effective
method to improve the brightness of laser. Active phase locking technology
based on MOPA is too complicated .In comparison, passive phase locking
technology is much more simple, with high power combining efficiency and
high contrast ratio. The technology is a new development trend and one
of the most promising approaches of coherent combination.
A novel mutual injection phase locking fiber laser combination
technology based on corner cube is proposed. The phase locking principle
is analyzed and coherent combination schemes respectively utilizing
spherical corner cube and planar corner cube are introduced. The
mutual-injection phase locking coherent combination experiment is
demonstrated.
Methods：The three inner reflective faces of corner cube are perpendicular
to each other. While the laser beam is located in the effective incident
angle, it would be reflected back in the opposite direction paralleled
with the input direction, and, the input and reflected beams are
distributed symmetrically with the acme of corner cube. Two kind schemes
of multiple fiber laser coherent combination based on corner cube are
proposed.
One scheme is based on spherical corner cube: N fiber lasers are
located in parallel with each other, each fiber laser's resonator is
constructed with a plane output mirror and a Bragg grating or medium film
on the other end. Some laser outputs from the end mirror, and changes to
approximately parallel beams through the coupling lens arrays. Each beam
becomes emanative after being reflected by the spherical corner cube, then
it will be coupled into the other N-1 fiber lasers. And then, mutual
injection locking phase between the N fiber lasers is realized.
The other scheme is based on planar corner cube: by using a 6×1 fiber
coupler and a planar corner cube, a mutual injection system is built with
six paralleled fibers lasers and a Bragg grating by using a 6×1 fiber
coupler and a planar corner cube . Laser beam of each fiber laser is
reflected by the corner cube and coupled into the symmetrical fiber laser.
Meanwhile, the six lasers are connected with each other by the 6×1 fiber
coupler, and with the reflection by fiber grating, the laser energy is
re-injected into the other fiber lasers. So the six lasers have the same
phase, polarization and wavelength, the coherent beams combination are
realized.
Results: Phase locking experiment of two fiber lasers based on plane
corner cube shows us the phenomena of wavelength locking, bandwidth
compressing, far field coherence and so on.
The central wavelengthλ1 of FL1 is 1085.11nm，the bandwidth is 0.11nm，
the central wavelengthλ2 of FL2 is 1085.31nm，and its bandwidth is 0.12nm,
the differentia of the two central wavelengths is 0.2nm. While the lasers
are phase locked by mutual injection, the central wavelength is 1085.22nm
and the bandwidth is 0.21nm at 1.5A.The experiment shows that the two
individual lasers are related to each other when the system is mutual
injection phase locked by the corner cube, then, the wavelength is locked,
the two lasers have the same working state and output wavelength.
Distribution in far field is the sum of amplitudes before mutual
injection, after it clear coherence patterns appear (visibility is 0.5).
After mutual injection the FWHM is 0.34nm, which can be compressed to
0.19nm when the fiber been screwed or twisted, the compressing ratio is
44%, and the center wavelength almost stays the same. Combination
efficiency of 120% is observed after optimizing experiment structure
parameters, raising coupling efficiency between fiber and corner cube,
and controlling polarization in the resonator.
Furthermore, experiments show that the wavelength excursion of the
two lasers are inversely proportional to the inject power, and the
empirical formulas is P1  1  P2  2 .
At 7A, the max combination output power of two fiber lasers is more
than 2.4W.
Conclusions: The paper proposes two kinds of passive phase locking schemes
based on corner cube. Phenomena such as wavelength locking, bandwidth
compressing, obvious coherent distribution in far field are observed.
Mutual injection phase locking fiber laser based on corner cube is an
absolutely self-organized scheme, it has several advantages such as
simple structure, high combining efficiency, completely self-adaptive,
single polarization output and easy to magnify.