Up-conversion of optical signals with multi-longitudinal-mode pump lasers

Multi-longitudinal-mode lasers have been believed to be good candidates as pump sources for optical frequency conversion. However, we present a semiclassical model for the frequency conversion of optical signals with a multimode pump laser, which shows that fluctuations of the instantaneous pump power limit the conversion efficiency. In an experiment, we up-converted a 1.55-$\mu$m optical signal in a periodically poled lithium niobate waveguide using a multi-longitudinal-mode laser and observed a maximum conversion efficiency of 68%, in good agreement with our theoretical model. Compared to single-mode pumping, multimode pumping is not a suitable technique for attaining stable near-unity-efficiency frequency conversion. However, our results could find application in the characterization of the spectral or temporal structure of multi-longitudinal-mode lasers.

Figure 1

(a) Spectral and (b) temporal representation of a two-mode laser (solid) with fixed phases between the oscillating modes; (c) conversion efficiency computed with Eq. (6) vs normalized average pump power ${\overline{P}}_p/P_{\max }$ for two-mode (solid) and single-mode (dashed) pump lasers. (d)–(f) Same as (a)–(c) for a pump laser with the spectral characteristics used in the experiment.

Figure 2

Schematic of experimental setup. PC, polarization controller; VATT, variable attenuator; DM, dichroic mirror.

Figure 3

Optical spectrum of Tm-doped fiber laser used in this experiment, after 20-dB optical attenuation.

Figure 4

Signal depletion vs normalized pump power $P_p/P_{\max }$ for either single-mode optical pump at 1.8 $\mu$m (data from [11]) or Tm-doped fiber laser pump at 1.94 $\mu$m.

Figure 5

RF spectrum of signal light at 1555.3 nm transmitted through the waveguide for 0 (pump off) or 3.47 dB (pump on) depletion. When the pump is on, its rf spectral content is transferred onto the depleted signal, and also onto the generated target radiation. Inset 1: Zoom of rf spectrum between dc and 200 MHz, showing seven rf spectral peaks with a mode spacing of 26 MHz.