Infrared Frequency Mixing

Starting from a commercial fiber laser, we are investigating a number of frequency conversion experiments in the near to mid-infrared. The fiber laser outputs 500-fs chirped pulses (~30 nm FWHM) with ~3.5 nJ pulse energy. The pulses are coupled into a polarization-maintaining (PM) optical fiber with a core diameter of ~9.5 um in which an optical soliton is launched.

Due to the anomalous dispersion of the fiber at 1550 nm, the soliton experiences a continuous red-shift in its central wavelength. This effect, known as the soliton self-frequency shift, is a function of both power and propagation length. This shift produces a multi-peaked output spectrum in which the peak separation can be tuned by adjusting input power. Figure 1 shows this shift as a function of input power for a 20-m fiber.

Figure 1: Sample tuning of pulse splitting observed in an optical fiber, as a function of the input pump power.

This tunable separation can be used to drive a number of nonlinear processes. The simplest of these is sum frequency generation, as shown in Figure 2 by way of a SHG FROG trace. This source could also be used for tuneable difference frequency generation in the mid-IR.

Figure 2: SHG FROG trace of the output of a 20-m polarization-maintaining fiber. The soliton peak is delayed in time almost 55 ps relative to the fundamental.

We are further investigating the use of this source for coherent anti-Stokes Raman (CARS) microscopy, with the 1550-nm light serving as the pump beam and the soliton-shifted light serving as the Stokes beam. The tunability of this particular source could allow it to be used to investigate Raman-active transitions up to 1000 cm^-1.