Del Mar Photonics
High Energy Femtosecond 1086/543 nm Fiber System Swamis 1086/543 for Nano and Micromachining in Transparent Materials and on Solid Surfaces (request a
Swamis fiber laser system generates femtosecond pulses with an energy of 1 μJ
at a wavelength of 1086 and an energy of 300 nJ at a wavelength of 543 nm. The
pulse parameters at a wavelength of 1086 nm are optimized with respect to the effective frequency doubling.
The high energy fiber system Swamis is ideal for variety of femtosecond material processing applications.
The focused radiation of femtosecond pulses with a relatively high energy (no
less than 100 nJ) makes it possible to create various micro and nanostructures
in glasses, optical crystals, and fibers [1–3] and on metal surfaces [4, 5].
Different physical principles are involved in the formation of such structures
in the optical media (e.g., modification of refractive index  or ablation
) but the corresponding laser sources exhibit similar parameters: the pulse
duration ranges from 50 to 500 fs, the focal energy density is no less than 10
mJ/cm2 (up to several joules per square centimeter for the recording of
structures in optical media), and the repetition rate ranges from 1 kHz to 10
MHz. The development of a femtosecond system with relatively high pulse energy
and repetition rate is a topical problem for the femtosecond material
processing. Several femtosecond laser systems (especially, solid state systems
[8–11]) provide a relatively high pulse energy at a low repetition rate (less
than 100 kHz).
Note that a few applications necessitate a repetition rate of greater than 1 MHz [12, 13]. The fiber systems with a pulse energy of greater than 1 μJ and a repetition rate of greater than 1 MHz were demonstrated but the amplification stages of such systems are normally based on exotic fibers with a large effective mode area [14, 15]. An increase in the repetition rate of short pulses to the gigahertz level can be due to the modulation of cw pumping  but such an approach is quite rare.
Most works devoted to the above problem are performed at a wavelength of about 800 nm (titanium sapphire laser) although shorter and longer wave lengths are more appropriate for several media.
Swamis 1086/543 fiber laser system with wavelengths of 1086 and 543 nm and pulse energies of 1.0 and 0.3 μJ, respectively, at a repetition rate of 4 MHz.
Figure 1 demonstrates the configuration of the femtosecond laser system that contains a fiber master oscillator, a fiber amplifier, a grating compressor, and a nonlinear crystal for frequency doubling.
A ring Yb doped fiber laser in which the mode locking is based on the nonlinear polarization rotation serves as the master oscillator with normal total dispersion and an increased cavity length, which provides a decrease in the pulse repetition rate . For pumping, we employ a diode laser with an output power of up to 10 W at a wavelength of 976 nm with a fiber output that allows the pumping of double clad active fibers. The laser cavity contains the following fiber elements: active Yb GTWave fiber with a core diameter of 7 μm and an inner cladding diameter of 105 μm , polarization beam splitter, polarization maintaining 50% beam splitter, Faraday isolator, two polarization controllers, and an additional fragment of fiber for a decrease in the pulse repetition rate. Thus, the laser has two outputs in which the radiation is linearly polarized.
In the mode locked laser at a pump power of about 0.5 W, both outputs provide almost identical radiation powers of about 20 mW at a wavelength of about 1086 nm. The pulse repetition rate of the master oscillator is 4 MHz. Figure 2 shows the spectrum and the noncollinear autocorrelation function of the radiation intensity from the output of the 50% beam splitter. On the assumption of the sech2 pulse shape, the corresponding pulse duration is 4.5 ps.
In the presence of the additional fragment of the 1060XP fiber with a core diameter of 6 μm (single mode fiber at a wavelength of about 1 μm), we observe significant (up to 50%) energy conversion of the laser pulses to the Stokes components with wavelengths of about 1140 and 1200 nm. For the generation at a lower repetition rate needed for the further amplification, we substitute the SMF28 fiber with a core diameter of 8.5 μm and a length of 40 m for the 1060XP fiber.
The amplification unit contains a fiber isolator, a preamplifier, and an amplifier. In the preamplifier, a polarization maintaining double clad Yb doped active fiber with a length of 4 m and a core diameter of 10 μm is pumped by a laser diode with a power of up to 9 W using a “2 + 1 × 1” fiber combiner. In the amplifier, a polarization maintaining double clad Yb doped active fiber with a length of 2.5 m and a core diameter increased to 15 μm is pumped by two semiconductor diode lasers with a total power of up to 18 W using a similar fiber combiner. The preamplifier provides an increase in the mean power of the master oscillator to a level of about 300 mW. Such an input power of the amplifier is needed for the elimination of the generation of the amplified spontaneous emission, which emerges in the vicinity of the wavelength corresponding to the maximum gain of the active fiber (about 1030 nm) at a relatively high pumping power.
Fig. 1. Block diagram of the high energy femtosecond fiber system working at wavelengths of 1086 and 543 nm: LD1–LD4, pumping diode lasers, PC1 and PC2, polarization controllers, ISO, Faraday isolator, PBS, polarization beam splitter, PD, control photodetector, M1–M5, totally reflecting mirrors, G1 and G2, diffraction gratings, and SHG, nonlinear crystal for the second harmonic generation.
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