Optical coherence tomography (OCT) is an emerging technology for biomedical imaging [1
]. Recent development of Fourier domain OCT has significantly increased the imaging speed and sensitivity [2
]. Two methods have been developed to employ the Fourier domain techniques: a spectrometer based system that uses a high speed line scan camera [3
], or a swept laser source based system that uses a fast wavelength scanning laser [5
]. Although a high speed imaging line scan camera is readily available at 800 nm, the large array line scan camera at 1.3 μm has to be custom made, which makes the swept light source based system a better choice at 1.3 μm. In addition, swept source OCT (SSOCT) has the advantage of a simple system design since no spectrometer is required. Furthermore, narrow spectral line width can be achieved without crosstalk, which results in a larger imaging range.
A key component in high-resolution OCT imaging is an optical source with a broad bandwidth because the axial resolution is inversely proportional to the bandwidth of the light source. Although a number of broadband optical light sources have been developed to improve resolution in time domain and spectrometer based Fourier domain OCT, few broadband high-speed swept light sources have been reported [2
The speed of the conventional swept light source is limited by both the tuning speed of the filter and laser cavity lifetime. Recently, R. Huber et al. [9
] demonstrated a high-speed wavelength swept laser called a Fourier domain mode-locked (FDML) laser in order to increase the sweeping speed of the swept laser. This FDML laser is not limited by the laser lifetime because a long dispersion delay line fiber is inserted into the laser cavity, and one or more wavelength sweeps are stored in the laser cavity [9
]. The FDML laser provides a sweeping rate of up to 370 kHz [10
]. The FDML based swept source has the advantage of high-speed, narrow linewidth, and high phase stability [11
]. However, the current reported FDML swept source has limited bandwidth (< 100 nm at FWHM bandwidths), which corresponds to an axial resolution of 10 μm in air [9
]. More recently, three-dimensional endomicroscopy using FDML laser has been reported. The reported FDML use a single SOA with a full tuning range of 160 nm and FWHM bandwidth of less than 100 nm, corresponding to an of axial resolution of 7.1 μm. [13
] In this paper, we design a FDML laser by combining two SOAs as gain media [14
]. We successfully demonstrate a FDML wavelength swept laser which has an over 136.5 nm FWHM bandwidth and an over 160 nm edge-to-edge spectral bandwidth with a scanning rate of 45.6 kHz. The measured axial resolution of the system is 6.6 μm in air. The maximum sensitivity of the swept laser system is measured with 100.7 dB and is decreased by 8.6 dB at a 2 mm imaging depth. We demonstrate OCT imaging using an SSOCT system, which is built based on the developed FDML swept laser.