Performance enhancement of optical coherence tomography using photonic crystal fiber

Abstract

In this study, to improve performance of the optical coherence tomography (OCT), such as axial resolution, tomographic image quality, and acquisition rate, the OCT was proposed and demonstrated using broad bandwidth of a photonic crystal fiber (PCF)-based coupler, the k-domain linearization based on Sangac interferometer, and the dispersion tuned wavelength-swept laser. Since the axial resolution of the OCT is inversely proportional to the bandwidth of the optical source, the components in the OCT have to be improved in several properties, such as wavelength range, wavelength bandwidth, and optical loss, for high axial resolution of the OCT. The PCF-based optical interferometer fabricated with a fused biconical tapered method has optical coupling range of ultra broad bandwidth while maintaining the coupling ratio. The spectral domain (SD)-OCT with the optical interferometer based on PCFs was realized with the axial resolution of 2 m using the supercontinuum source with bandwidth of 500 nm at the center wavelength of 1350 nm. The composite interference signal with tomographic information from the sample target in the Fourier domain (FD)-OCT can be analyzed by the spectrometer. However, the spectrometer in FD-OCT has nonlinearity of the diffraction angle according to the wavenumber or inequality of wavelength swept rate in optical filter. To correct the nonlinearity of the spectrometer, the k-domain linearization has to compensate mismatched spectral information between the wavenumber and the pixel position at the CCD for high axial resolution of the SD-OCT. The k-domain linearization method by using a Sangnac interferometer based on PMF was experimentally executed for high axial resolution of the OCT using the optical source with broad bandwidth. The proposed method has ease in compensation of the nonlinearity compared to the early k-domain linearization method by using shifted slip mask and fiber Bragg grating. The resolution of the SD-OCT was achieved to ~ 2 m with a bandwidth of ~550 nm after the proposed k-domain linearization. To enhance the acquisition rate of the OCT, the dispersion tuned wavelength swept fiber laser was demonstrated and applied to the OCT. The dispersion tuned wavelength swept laser can widely tune wavelengths by controlling modulation frequency on the intensity modulator and chromatic dispersion in the laser cavity. The proposed wavelength-swept laser was applied to the OCT and high acquisition rate of 20 kHz was achieved. The birefringence fiber used in this research, a PMF and a PM-PCF, was applied to other applications, such as optical devices and sensors. In the optical devices, the long period fiber gratings (LPFG) based on PM-PCFs were proposed and fabricated with a periodic wet etching to the cladding of the PMFs. The PM-PCF-based MRLPGs have two resonant peaks because of the birefringence of the PM-PCF. The extinction ratios of two resonant peaks of the PM-PCF-based MRLPG were enhanced by increasing the applied strain without variation in their resonant wavelengths because of the photoelastic effect. We believe that the experimental results are very useful for applications to fiber optic sensors, optical switch filters, etc. In the other application in optical sensors, the simultaneous measurement method for the concentration of ethylene glycol and temperature by using a hybrid Sagnac interferometer was proposed and experimentally demonstrated. The hybrid Sagnac interferometer consisted of the D-shaped PMF-based Sagnac interferometer and the LPFG. The maximum errors of the hybrid Sagnac interferometer were measured to be ±0.012oC in temperature and ±0.576 % in concentration, respectively.