Polarization maintaining buffered Fourier domain mode-locked swept source

A polarization maintaining buffered Fourier domain mode-locked (FDML) swept source at center wavelength of 1310 nm for multiplying the scanning rate of FDML swept source was demonstrated. The scanning rate of the buffered FDML swept source was doubled without sacrificing the output power of the swept source by combining two orthogonally polarized outputs with a polarization beam combiner (PBC). The stability of the swept source was improved significantly because the polarization state of the laser beam inside the cavity is maintained without any polarization controllers. With the linear polarization states of the output laser beam, the buffered FDML swept source is also ready to be used in a PSOCT system. The swept source is capable of a tuning range of more than 150 nm at a 102 kHz sweeping rate. An FDOCT system was developed with the built swept source.


INTRODUCTION
Optical coherence tomography (OCT) is a noninvasive, noncontact imaging modality that uses coherent gating to obtain high resolution, cross-sectional images of tissue microstructure [1]. The development of Fourier domain OCT (FDOCT) including swept source OCT (SSOCT) [2][3][4] and spectral domain OCT (SDOCT) [5][6][7] has significantly increased the imaging speed and sensitivity of OCT systems. Compared with a SDOCT system which uses a large array line scan camera, SSOCT proves to be a better choice at 1.3 m due to its simpler system design since no spectrometer is required. Furthermore, narrower spectral line widths can be achieved without crosstalk, which results in a larger imaging range.
To build a high speed SSOCT system, a swept source with a fast scanning speed is required. Recently, the new swept sources utilizing the Fourier domain mode-locked (FDML) technique have attracted much attention because broad sweep ranges, narrow instantaneous line widths, unprecedented sweep rates and high phase stability can be obtained by extending the laser cavity and periodically driving the optical bandpass filter synchronously with the optical round-trip time of the propagating light wave in the laser cavity [8][9][10][11][12]. For tailoring the output and for multiplying the sweeping rate of FDML lasers, buffered FDML was developed by combining two time-delayed copies of a sweep with a fiber coupler. Highly stable, unidirectional wavelength sweeps without sacrificing the sweeping rate can be achieved with this technique [10,11]. However, only half of the output power can be used because the output light is coupled from one output arm of a 50/50 fiber coupler. In addition, polarization of a conventional FDML laser is wavelength dependent due to the long single mode fiber used in the cavity. Therefore one needs to carefully adjust polarization controllers to control the polarization states of light within the cavity to achieve the best performance for a given gain medium, usually a polarization dependent semiconductor optical amplifier (SOA) to maintain optimum lasing and hence a stable output power and bandwidth. Polarization maintaining (PM) fibers combined with fiber polarizers were used to build a non buffered FDML swept source without the use of polarization controllers [13]. However, the detailed characterization of the source was not given.
In this manuscript, a PM buffered FDML swept source is demonstrated. The scanning rate is doubled without sacrificing the output power of the swept source by combining two orthogonally polarized outputs with a polarization beam combiner (PBC). In addition, the stability and robustness of the swept source are significantly improved because the polarization state of the laser beam inside the cavity is maintained without the usage of any fiber polarization controllers. With the linear polarization states of the output laser beam, the buffered FDML swept source is also ready to be used in a PM fiber based PSOCT system [14]. Figure 1 shows the setup of the PM buffered FDML swept source. The swept source is composed of a PM ring along with two single mode (SM) delay lines. In the ring portion, a PM fiber pig-tailed SOA (Covega, Inc.) is used as the gain medium while a PM fiber Fabry-Perot tunable filter (FFP-TF, LambdaQuest, Inc.) provides the wavelength selection. Two associated PM isolators are used to force unidirectional propagation in the cavity. The delay lines are formed with two lengths of SM optical fibers, each 1 km long. Two Faraday mirrors are placed at the end of each SM fiber respectively to rotate the polarization by 90° and reflect the light back to the ring cavity. The light from the SOA is linearly polarized along the slow axis of the output PM fiber. 30% of the Light is coupled out by a PM coupler and the remaining 70% of the light is transmitted through port 1 of a polarization beam splitter (PBS) to the first 1 km long SM delay line fiber. The polarization state of the reflected light is orthogonal to that of the input light at port 3 of the PBS. Thus the reflected light is coupled back to port 2 of the PBS and linearly polarized along the slow axis of the PM output fiber of the PBS. Subsequently 50% of the light is tapped out by a 50/50 PM coupler. The remaining 50% of the light is transmitted through port 1 of the second PBS to the other 1 km long SM fiber. The reflected light is coupled back to port 2 of the second PBS and transmitted through the PM fiber pig-tailed FFP-TF to the SOA.

EXPERIMENTAL SETUP
The FFP-TF is driven at a frequency of 51 kHz which is synchronous to the round-trip time of light in the cavity. A forward (short to long) and a backward (long to short) wavelength sweep are generated during each drive cycle. The SOA is modulated to suppress and replace the forward wavelength sweep with a delayed backward wavelength sweep because the forward wavelength sweep generates increased noise at high sweep rates [10]. The two orthogonally polarized outputs are combined with a polarization beam combiner (PBC). The sweep rate is doubled to be 102 kHz with two sweeps during one drive cycle while the output power of the swept source is not sacrificed.  In a conventi single mode fiber polariza decreases in approach in stability of th the swept so fluctuation is ws the tempor output light is   The output light from the FDML laser is coupled into an SSOCT system. The fringe signals collected by photodetectors are digitized by a 250 M samples/second 12 bit analog-to-digital data acquisition converter (ATS 9350, AlazarTech Inc.) and transferred to a computer for processing. Parallel computing algorithms with a dual-quad-core high speed processor based workstation are used to achieve real time processing and display [15]. Figure 5 shows the point spread function (PSF) measured with a partial reflector placed at a depth of 1 mm. The axial resolution of the SSOCT system is measured to be 9.4 µm in air corresponding to an effective axial resolution of 6.7 µm in tissue (n = 1.4). The sensitivity of the SSOCT system is measured with a -40.5 dB partial reflector as the sample. As shown in figure 6, the sensitivity is 107.5 dB at a depth of 0.25 mm and decreases by 6 dB at a depth of 2.25 mm. The corresponding instantaneous linewidth of the swept source is calculated to be 0.33 nm. The increasing width of the PSFs at larger depths results from residual errors in spectral calibration which is for conversion from time to wavenumber space. To illustrate the performance of the system in biological tissues, a human finger is imaged as shown in figure 7. 2D imaging of 5 mm × 2 mm area is processed and displayed at a rate of 200 frames/ second (512 A-lines per frame). The fixed pattern near a depth of 500 µm is due to spectral modulations generated by a crosstalk between fast and slow axis of the PM components which are connected with PM fiber connectors.

SUMMARY
A PM buffered FDML swept source was developed to double the scanning rate of the source without sacrificing the output power by combining two orthogonally polarized outputs with a PBC. The swept source is capable of an average power of 12 mW and an edge to edge tuning range of 152 nm at a 102 kHz sweeping rate. The axial resolution of the SSOCT system was measured to be 9.4 µm in air. The SSOCT system is capable of maximum sensitivity of 107.5 dB and an imaging range of 2.25 mm.