High-order mode selection in Yb:YAG ceramic laser- Manasadevi Priyamvadha Thirugnanasambandam
Yb-doped gain media have been investigated for decades and have recently gained
importance as a vital option in the construction of both high-power and ultra-shortpulse lasers. Yb-doped gain medium used in developing diode-pumped terawatt
facilities has shown the importance of further studies on this active medium. Veryhigh optical efficiency, diffraction-limited beam quality, and output powers ofmore than 1 kW have been achieved with ytterbium-doped double-clad fiber lasers
and amplifiers. Thin-disk lasers, which most often work with Yb:YAG crystals,
can also generate well above 1 kW diffraction-limited output with high beam
quality, or even higher powers with non-diffraction-limited beam quality. Various
Yb-doped gain media have been used in mode-locked lasers for the generation offemtosecond pulses; the by far highest average output powers of 80 W have beenobtained with a passively mode-locked thin-disk Yb:YAG laser. For passive mode
locking, problems can arise in the form of Q-switching instabilities. This tendency
is a consequence of the relatively small laser cross sections of ytterbium-dopedmedia. Therefore, some of the broadband ytterbium-doped gain media are not very
suitable for passively mode-locked lasers, particularly at high power levels, but
can still be very useful in regenerative amplifiers. Relatively large cross sections
are found for tungstate crystals. Some ytterbium-doped crystals have a fairly broad
amplification bandwidth, but the emission curve is not very smooth; it exhibits
several maxima. In such cases, wide wavelength tunability may still be achieved,
but the realization of very short pulses with mode locking is difficult.
On the other hand, in recent years, considerable interest has been shown in thegeneration of laser beams containing phase singularities. These beams are best
described in terms of Laguerre-Gaussian modes which have an azimuthal phaseterm and possess a well defined orbital angular momentum per photon. The
identification of light beams with a well defined orbital angular momentum, whichis distinct from the spin component, has led to a number of exciting studies. Theseinclude the transfer of the orbital angular momentum to macroscopic objects, themode transformation upon nonlinear frequency doubling, and the interaction ofthese beams with atomic systems. The methods to generate such beams have been
widely studied in detail and these have involved either using phase elements in
Nd-doped lasers or extra-cavity diffraction plates. While Yb- has been proving
itself as a potential gain medium, there are not many works that have tried to bringthem into the scene as a valuable source for generating high-order circular modes.
With high-order modes gaining importance for their wide range of applications
especially in the optical toolbox for trapping and tweezing; it is important that westudy the role Yb-doped lasers play in generating such modes. Using an intracavity lens inside an Yb:YAG ceramic laser cavity, we have succeeded ingenerating the highest mode orders in Laguerre-Gauss beams experimentallydemonstrated so far. We also generate beams in doughnut-like and Bessel-likeprofiles that exhibit near-diffraction free characteristics of their central lobes forover tens of meters. Radial and azimuthal polarized doughnuts of high polarization
purity and high-order modes were also generated from Yb:YAG ceramic lasercavity.
This works demonstrates an economical and less complicated laser scheme togenerate high-order modes from an Yb:YAG ceramic laser cavity with uniqueproperties and hence varied applications. The intra-cavity mode selecting element
is a simple plano-convex glass lens and it has been shown to offer excellent mode
selection in scalar Laguerre-Gauss modes, modes with near-diffraction freecharacteristics, and radial and azimuthally polarized modes from within theYb:YAG cavity.
The current study bridges two different regimes that have not been brought
together so far; and the end-product is a quasi-three level laser system to realize
circular mode selection. In case of a laser resonator with intra-cavity lens, thestability conditions of the cavity as defined by ray transfer matrix are enclosedbetween the focusing and imaging conditions of the lens at the resonator mirrors.
In the presence of strong spherical aberration, the focal length of the lens varies
continuously from its axis to the periphery and is no more a defined point but aline. When such a lens is placed at focal length distance from the cavity mirror andshifted along the resonator axis to span across the line focus, the cavity conditions
provide continuous selection in modes of increasing mode diameter at the output
coupler because of the stability condition shifting to rings of increasing radiusalong the refracting surface of the lens, generating pure hollow scalar LaguerreGauss modes of increasing azimuthal orders as high as 28 (order limited only by
the diameter of the cavity optics). Generation of the whole lineage of LaguerreGauss modes from a single scheme has been reported by the proposed scheme forthe first time.
The laser scheme with aberrated intra-cavity lens can also be set up such that thecavity conditions offer stability for both focusing and imaging conditions
simultaneously. Coupling of the cavity modes, enhanced by aperture guiding at thegain element and long cavity length, generates beams which exhibit neardiffraction free properties in their central parts (maximum or minimum). The
coupling of these two modes in the cavity has also been confirmed experimentally.
Near-diffraction free properties associated with central parts of doughnut-like andmulti-ring modes are reported. The near-diffraction free properties of these beams(divergence of their central parts 10 times smaller than the diffraction limit that is
associated with the mode radius at the output and constant power confined within
the center) exist for over 40 m of propagation distance. When the intra-cavity lensin combination with a birefringent crystal (positive or negative birefringence)inside the Yb:YAG ceramic laser cavity, the stability region of the cavity between
the imaging and focusing conditions of the lens is split into two regions; onecorresponding to the extraordinary ray and the other to the ordinary ray. The
splitting of the stability region allows mode selection in azimuthal and radiallypolarized modes. The transition point from azimuthal to radial polarization as thelens is shifted along the resonator axis was clearly distinguishable when the widthof the stability region (enclosed between focusing and imaging conditions)matched the birefringence shift. Azimuthal and radial polarized doughnut modes
were generated with high polarization purity. Polarization extinction ratio as highas 100:1 in certain cases have been reported. Propagation factor M2 was estimatedto vary between 2-2.5 (theoretical value of M2 for doughnut LG*01 = 2). Apartfrom these doughnut modes, new polarization distribution patterns whereinazimuthal and radial polarizations existed together in a single output beam weregenerated for the first time. When the intra-cavity lens had pronounced sphericalaberrations, the cavity offered mode selection in high-order azimuthal and radialpolarized modes.
The experimental work has brought into limelight the realization of certain highorder modes which were only theoretically investigated till date. High-ordermodes have wide applications related to angular momentum transfer, material
processing and many more. Near-diffraction free beams generated from thescheme can be seen as a valuable replacement to Bessel beams which has only avery limited propagation distance. Radial and azimuthal polarized modes haveextensive applications in the optical toolbox for micromanipulation and some of
the new polarization distributions are added tools for realizing new phenomena.