Towards phase-stabilized Fourier domain mode-locked frequency combs

Abstract

Fourier domain mode-locked (FDML) lasers are some of the fastest wavelength-swept light sources, and used in many applications like optical coherence tomography (OCT), OCT endoscopy, Raman microscopy, light detection and ranging, and two-photon microscopy. For a deeper understanding of the underlying laser physics, it is crucial to investigate the light field evolution of the FDML laser and to clarify whether the FDML laser provides a frequency comb structure. In this case, the FDML would output a coherent sweep in frequency with a stable phase relation between output colours. To get access to the phase of the light field, a beat signal measurement with a stable, monochromatic laser is performed. Here we show experimental evidence of a well-defined phase evolution and a comb-like structure of the FDML laser. This is in agreement with numerical simulations. This insight will enable new applications in jitter-free spectral-scanning, coherent, synthetic THz-generation and as metrological time-frequency ruler.

Tracking of Gaussian pulse release length inside mode-locked semiconductor laser amplifier

“…The spectrum of the electric field and the temporal intensity at the stretched pulse following the CFBG are both explained by these Figures. The findings made it abundantly evident that the electric field distribution along the gradient line was centered in the central of the wave guide [29]- [31]. To effectively enter mode-locked, you must first: because the side reflection restricted the bandwidth on the spectrum, selecting the facet reflectivity of the external laser cavity was very critical.…”

900 nm swept source FDML laser with kW peak power

“…The 1064 nm laser is combined in free space with a 1300 nm FDML laser using a dichroic mirror (1200 nm Techspec shortpass filter, Edmund Optics) (Figure 1, red). FDML lasers operate in cw mode, while periodically changing their wavelength over an adjustable span with high sweeping frequencies (here 419 kHz) and a narrow instantaneous linewidth [7]- [9]. Polarization control of the MOPA is achieved using a half-waveplate, whereas the polarization of the FDML laser is adjusted using a fiber loop polarization controller.…”

828 kHz retinal imaging with an 840 nm Fourier domain mode locked laser

“…Fourier domain mode locked (FDML) lasers (design detailed in section 2 ) are, on the other hand, favored for their long coherence length - besides their high sweep rates [ 37 , 38 ]. Thanks to their long fiber cavity, a complete sweep is optically “stored” in the laser for several hundreds of roundtrips, improving the coherence of the laser.…”