Frequency Doubling Of Femtosecond Pulses Research Articles

Frequency-Doubling of Femtosecond Pulses in “Thick” Nonlinear Crystals With Different Temporal and Spatial Walk-Off Parameters

We present a comparative study on frequency-doubling characteristics of femtosecond laser pulses in thick nonlinear crystals with different temporal and spatial walk-off parameters. Using single-pass second harmonic generation (SHG) of 260 fs pulses at 1064 nm from a high-average-power femtosecond Yb-fiber laser in 5-mm-long crystals of β-BaB 2O4 (BBO) and BiB3O6 (BIBO), we find that for comparable values of temporal and spatial walk-off parameters in each crystal, the optimum focusing condition for SHG is more strongly influenced by spatial walk-off than temporal walk-off. It is also observed that under such conditions, the Boyd and Kleinman theory commonly used to define the optimum focusing condition for frequency-doubling of cw and long-pulse lasers is also valid for SHG of ultrafast lasers. We also investigate the effect of focusing on the spectral, temporal, and spatial characteristics of the second harmonic (SH) radiation, as well as angular acceptance bandwidth for the SHG process, under different temporal and spatial walk-off conditions in the two crystals.

Efficient frequency doubling of femtosecond pulses with BIBO in an external synchronized cavity

We experimentally demonstrate the second harmonic generation (SHG) of infrared femtosecond pulses using a BIBO crystal placed in an external ring cavity, synchronized with an input mode-locked laser at 78MHz. A frequency doubling efficiency of 53% is achieved which is, to the best of our knowledge, the highest value ever reported for a low energy input beam of 1.4nJ/pulse. Theoretical analysis of cavity related issues such as design, fundamental mode characteristics and fidelity against misalignments are also presented. The modeling of SHG cavity enables us to estimate the cavity losses and the mode matching visibility. Such synchronized SHG cavities in pulse domain, having higher SHG conversion efficiencies compared to their continuous wave counterparts, may find potential applications in scientific areas such as in photonics, and in quantum optics.

Second harmonic generation by femtosecond Yb-doped fiber laser source based on PPKTP waveguide fabricated by femtosecond laser direct writing

The frequency doubling of femtosecond pulses from an Yb-doped fiber laser source was demonstrated in a PPKTP waveguide fabricated by femtosecond laser direct writing. The PPKTP waveguide contains a fixed period of 8.9 μm and the feomtosecond fundamental pulses have a central wavelength of 1044 nm. A maximum SHG power of 406 mW was produced, yielding a conversion efficiency of 5.6%. Numerical simulations were carried out to investigate the property of frequency doubling for femtosecond pulses. The results show that the SHG process proceeds even the quasi-phase-matching (QPM) condition is not well satisfied, which is significantly different from that of “long” pulses or CW light and is accorded with the experimental results.

Numerical method for finding 3D solitons of the nonlinear Schrödinger equation in the axially symmetric case

A system of two nonlinear Schrodinger equations is considered that governs the frequency doubling of femtosecond pulses propagating in an axially symmetric medium with quadratic and cubic nonlinearity. A numerical method is proposed to find soliton solutions of the problem, which is previously reformulated as an eigenvalue problem. The practically important special case of a single Schrodinger equation is discussed. Since three-dimensional solitons in the case of cubic nonlinearity are unstable with respect to small perturbations in their shape, a stabilization method is proposed based on weak modulations of the cubic nonlinearity coefficient and variations in the length of the focalizing layers. It should be emphasized that, according to the literature, stabilization was previously achieved by alternating layers with oppositely signed nonlinearities or by using nonlinear layers with strongly varying nonlinearities (of the same sign). In the case under study, it is shown that weak modulation leads to an increase in the length of the medium by more than 4 times without light wave collapse. To find the eigenfunctions and eigenvalues of the nonlinear problem, an efficient iterative process is constructed that produces three-dimensional solitons on large grids.

Phase matching and frequency-noncritical interactions upon frequency conversion of femtosecond pulses

The phase-matching properties of biaxial nonlinear crystals providing the highly efficient frequency doubling of femtosecond pulses are analysed. The investigation method is based on the separation of the phase-matching condition and frequency-noncritical interaction.

Frequency doubling of femtosecond pulses in walk-off-compensated N-(4-nitrophenyl)-L-prolinol

We show how to exploit the high quadratic nonlinear coefficient of the organic crystal N-(4-nitrophenyl)-L-prolinol for generation and parametric mixing of ultrashort pulses by use of tilted-pulse techniques. The effective crystal length for subpicosecond operation is shown to be enhanced from tens of micrometers to tens of millimeters. Efficient frequency doubling of 100-fs pulses is predicted in walk-off-compensated geometries with peak intensities of a few megawatts per square centimeter.

Blue light generation in single-pass frequency doubling of femtosecond pulses in KNbO 3

We study the generation of blue light in single-pass frequency doubling of femtosecond pulses in a thick KNbO 3 crystal. Depending on the input IR power, the maximum amount of generated blue light is a function of the position of the nonlinear crystal relative to the focusing lens. At certain focusing distance, the efficiency of blue light generation is substantially degraded. The issue of beam quality of the generated blue light is discussed.

Phase-matching temperature shifts in blue generation by frequency doubling of femtosecond pulses in KNbO_3

We have demonstrated temperature rises that result from nonlinear absorption in single-pass frequency-doubling experiments using femtosecond pulses and a 3-mm-thick KNbO3 crystal. These temperature changes shift the phase-matching curve and must be accounted for to optimize the conversion efficiency. We obtained a maximum second-harmonic generation (SHG) efficiency of 66% at an input power of 107 mW and a slope efficiency of ∼1.5%/mW at low input powers. We have investigated, for the first time to our knowledge, the focusing dependence of the phase-matching temperature. We have also found that the temperature-dependent SHG efficiency in femtosecond SHG experiments is significantly different from that obtained for frequency doubling of continuous-wave light.

Ultraviolet generation by first-order frequency doubling in periodically poled KTiOPO_4

First-order quasi-phase-matched frequency doubling at 780 nm in a 1-mm-thick periodically poled KTiOPO(4) crystal with an inverted-domain period of 2.95 microm has been demonstrated for the first time to the authors' knowledge. A normalized conversion efficiency of 1.1%W(-1) cm(-1) has been obtained in cw mode. Experiments show that group-velocity walk-off prevents efficient frequency doubling of femtosecond pulses in KTiOPO(4) in the UV spectral range. However, because of the large optical nonlinearity, periodically poled structures can be efficiently used for frequency doubling of cw and pulsed lasers with pulse lengths as short as ~1 ps .

High-efficiency blue generation by frequency doubling of femtosecond pulses in a thick nonlinear crystal

We have demonstrated highly efficient frequency doubling of femtosecond pulses in a thick, noncritically phase-matched KNbO(3) crystal under conditions of large group-velocity mismatch. At low power we observed a slope efficiency of ~300% nJ (-1) for harmonic conversion, and at higher powers we generated 170 mW of second-harmonic blue output for 300 mW of input light. Furthermore, we have shown that the focusing dependence for our conditions of large group-velocity mismatch is considerably different from that obtained for frequency doubling of continuous-wave light. We have also demonstrated that one can tune the spectral width of the generated blue light by varying the focusing conditions.