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Efficient high power generation of pulsed red light via four-wave-mixing in a large-mode-area, endlessly single-mode photonic-crystal fiber

In the past, generation of high power, pulsed, red light was performed using dye lasers, OPOs or second harmonic generation of the 1.3µm transitions from neodymium based solid state lasers [1]. Using FWM in photonic-crystal fibers (PCF) in combination with pulsed 1µm laser sources has so far only accessed the spectral region around 730nm [2]. We present a simple approach to generate picosecond or nanosecond pulses in the visible by efficient FWM in an endlessly single-mode large mode area (LMA) PCF fiber. At a moderate pump peak power of 40kW the conversion efficiency can reach 35%. When pumping the fiber at 1030/1064nm, the resulting wavelength in the visible is 626/673nm, making it interesting for many applications like photodynamic therapy and fluorescence spectroscopy [3]. The experimental setup used to demonstrate the effect consists of the laser source emitting 50ps pulses at 1064nm at a repetition rate of 100kHz with variable peak power. This light is coupled into an endlessly single mode LMA PCF fiber with a length of ∼1m and a core diameter of 10µm. The spectrum at the output of the fiber shows two narrow peaks, the pump wavelength at 1064nm and the FWM generated idler wavelength at 673nm. No supercontinuum trace or Raman scattering can be observed as can be seen in figure 1. At an input peak power of 30kW, the conversion efficiency starts to saturate at ∼35% as shown in figure 2. This high conversion efficiency can be explained by the excellent modal overlap provided by the endlessly single mode nature of the fiber at all wavelengths. Our simulations show that the contribution of the waveguide dispersion for phase matching is almost negligible when employing fused silica PCF fibers with core diameters ≥10µm. This means that with bigger cores even higher peak powers could be handled while still fulfilling the phase matching conditions. In our institute, a master oscillator fiber amplifier system employing a LMA PCF fiber amplifier with a core diameter of 40µm delivered up to 700mW at 632nm wavelength, 100ps pulse duration and a repetition rate of 100kHz. The conversion efficiency from 1064nm to 632nm was ∼18%. The phase match diagram for the LMA PCF fiber with a 10µm core used in our experiment is presented in figure 3. It indicates a broad tuning range from ∼600 to ∼750nm when using a tuneable pump source covering the spectrum between 1020 and 1100nm, e.g. a tuneable pulsed fiber laser.