Dispersion, coherence and noise of Fourier domain mode locked lasers

We report on the effect of chromatic dispersion on coherence length and noise of Fourier Domain Mode Locked (FDML) lasers. An FDML laser with a sweep range of 100nm around 1550nm has been investigated. Cavity configurations with and without dispersion compensation have been analyzed using different widths of the intra-cavity optical band-pass filter. The measurements are compared to non-FDML wavelength swept laser sources. Based on these observations, a simple model is developed providing a connection between timing, photon cavity lifetime and characteristic time constant of the filter. In an optimized configuration, an instantaneous laser linewidth of 20pm is observed, corresponding to a 10texttimes narrowing compared to the intra-cavity optical band-pass filter. A relative intensity noise of -133dBc/Hz or 0.200MHz detection bandwidth during sweep operation is observed. For optimum operation, the filter drive frequency has to be set within 2ppm or 120mHz at 51kHz.