DRDO LASTEC is working on the state of art technology required for Pulsed Fiber Laser. In picture experimental setup for fiber laser .
Pulsed Fiber Lasers (PFLs) are rapidly replacing the conventional Q-switched bulk solid state lasers for generating nano second pulses at multi-kHz repetition rates. Such lasers are required for various long range applications like remote sensing, imaging, LIDAR, etc. Advantages offered by fiber based solutions include simpler thermal management, higher efficiency, flexible pulse format and waveguide defined beam quality independent of power level. Fully integrated structure in fiber laser provides compact, robust, alignment free laser that is compatible with highly efficient fiber pigtailed high power pump diodes and various fiber integrated devices such as FBG, fiber couplers, etc.
LASTEC has initiated work on this very challenging domain of developing different pulsed fiber laser sources involving Ytterbium and Erbium doped fibers to generate 1.0 μm and 1.5 μm wavelengths respectively at high repetition rates. These sources are required for long range applications like target illumination, dazzling, etc
A fiber laser is pumped by high power multi mode single emitter diode bars, typically through a cladding surrounding a single mode core. This single mode core is typically 5 to 12 μm in diameter. The double clad fiber consists of an inner single mode core doped with the appropriate rare earth ions such as Ytterbium, Erbium, Neodymium and Thulium. The cladding is made of undoped glass that has a lower index of refraction. The pump light is injected into the cladding and propagated along the structure, passing through the active core and producing a population inversion.
The emission wavelength is a function of choices in the doped fiber and by type of reflector, e.g. Bragg Grating. Traditionally, intense nanosecond laser pulses are generated with Q-switching techniques. However, a low power pulsed seed source, e.g. again switched laser diode may be employed whose output may be amplified to substantial energies with a fiber amplifier chain. With this approach one can easily change the pulse repetition rate and output pulse energy without changing the pulse duration, or can change the pulse duration and shape without affecting other pulse parameters. In a Q-switched laser, such flexible parameter control is not possible; e.g., lower pulse energy usually implies longer pulses.
Critical Areas
To get high beam quality, single mode waveguide structures are required. A small core clad numerical aperture is used to maintain the single mode transmission but it also leads to intense power density in a small core area, which brings strong nonlinear effects such as Stimulated Raman Scattering (SRS), Stimulated Brillouin Scattering (SBS) and Amplified Spontaneous Emission (ASE), which can limit the maximum output power of fiber lasers. The use of large core or Large Mode Area (LMA) fibers helps in reducing optical intensity and prevent these parasitic effects. Recently peak powers ~1 MW have been reported in combination with multi-watt average powers. The LMA in conventional fibers, however, leads to multimode propagation leading to degradation of beam quality. Therefore novel fibers called Photonic Crystal Fibers (PCF) have been the subject of intense research in recent years
Development of pulsed fiber laser sources would require several critical components with high damage thresholds to sustain high peak power densities. These components include LMA doped delivery fibers, high power pump laser diodes, isolators, pump combiners, couplers, filters, etc. Significant advances in high power diode pump lasers, refinement of power scaling and energy storage techniques, fiber component fabrication such as FBG are opening up the way for the development of active fiber systems that can deliver tens of watts of single transverse and longitudinal-mode output power, millijoule pulse energies, and ultra short pulses with peak powers in the 10-100 MW region.
Furthermore, advances in nonlinear optical materials are permitting these high-power fiber laser outputs to be efficiently converted to the visible and near infrared (IR) spectral regions. This would require Polarisation Maintaning (PM) components.
For the industrial, scientific medical and defence applications fiber lasers offer wide wavelength range. Availability of narrow line widths, polarized or unpolarised emissions, short pulse duration's, single mode operation, insensitivity to environmental conditions and compact size are some of the advantages only fiber lasers can accomplish.