Optical coherence tomography (OCT) is a noninvasive imaging modality which can provide cross-sectional imaging of the tissues in high-resolution. Especially in retina imaging, the OCT becomes one of the most valuable imaging tools for the diagnostics of the eye diseases. Considering the scattering and absorption properties of the eye, the 1000 nm OCT system is preferred for the retina image. In this study, we describe an akinetic swept source OCT system based on pulse-modulated active mode locking (AML) fiber laser at the 1080 nm wavelength region for in-vivo human retina imaging. The akinetic AML wavelength swept fiber laser is constructed with polarization maintaining fiber which has average linewidth of 0.625 nm, a spectral bandwidth of 81.15 nm and a duty ratio of 90 % without buffering method. We successfully obtained in-vivo human retina images using proposed OCT system without the additional k-clock and the frequency shifter providing wide field of view of 43.1º. The main retina layers such as RPE can be distinguished through the OCT image with axial resolution of 6.3 m.
We propose a high speed strain measurement method using an active mode locking (AML) fiber Bragg grating (FBG) laser sensor with a chirped FBG cavity. The mode-locked frequency of the AML laser depends on both the position and Bragg wavelength of the FBG. Thus, the mode-locked frequency of cascaded FBGs can be detected independently along the cavity length of cascaded FBGs. The strain across FBGs can be interrogated dynamically by monitoring the change in mode-locked frequency. In this respect, the chirped FBG critically improves the frequency sensitivity to Bragg wavelength shift as a function of increasing dispersion in the AML cavity. The strain measurement of the FBG sensor shows a highly linear response, with an R-squared value of 0.9997.
We have demonstrated a quasi-distributed sensor using an active mode-locking (AML) laser with multiple fiber Bragg grating (FBG) reflections of the same center wavelength. We found that variations in the multiple cavity segment lengths between FBGs can be measured by simply sweeping the modulation frequency, because the modulation frequency of the AML laser is proportionally affected by cavity length.
We demonstrated a MHz speed wavelength-swept fiber laser based on the active mode locking (AML) technique and applied to interrogation system of an array of fiber Bragg grating (FBG) sensors. MHz speed wavelength sweeping of wavelength-swept fiber laser can be obtained by programmable frequency modulation of the semiconductor optical amplifier (SOA) without any wavelength tunable filter. Both static and dynamic strain measurement of FBG sensors were successfully characterized with high linearity of an R-square value of 0.9999 at sweeping speed of 50 kHz.
We demonstrate a highly linear wavenumber- swept active mode locking (AML) fiber laser for optical sensing and imaging without any wavenumber-space resampling process. In this all-electric AML wavenumber-swept mechanism, a conventional wavelength selection filter is eliminated and, instead, the suitable programmed electric modulation signal is directly applied to the gain medium. Various types of wavenumber (or wavelength) tunings can be implemented because of the filter-less cavity configuration. Therefore, we successfully demonstrate a linearly wavenumber-swept AML fiber laser with 26.5 mW of output power to obtain an in-vivo OCT image at the 100 kHz swept rate.
This paper shows experimentally dual wavelength swept at multi band (O,C-Band) property with active mode
locking (AML) method. Unlike conventional wavelength swept laser, AML wavelength swept laser does not require any
wavelength selecting filter in the cavity. The cavity has two free spectral ranges (FSRs) depend on dual path
configuration. This wavelength swept laser can be useful for wide-band fiber-optic application such as Optical
Coherence Tomography and Fiber Bragg Grating sensor system.
We performed an experiment of wavelength-swept laser based on active mode-locking on reflective
semiconductor optical amplifier (RSOA). Since this laser does not have a wavelength-selecting filter, it can
achieve a high sweeping speed and reduce the component cost and size. Compared to the conventional SOA
gain medium, RSOA shows a merit of higher efficiency of cross gain modulation due to the twice propagation
of active gain region. We applied this laser to Fiber Bragg grating (FBG) strain sensor system which can have
faster data acquisition speed than conventional wavelength-swept laser method. The linear response of FBG
peak for the applied strain is monitored using the proposed laser source.
An interferometric phase sensor is proposed and demonstrated for strain sensing based on a Sagnac
interferometer including polarization maintaining fiber (PMF). Since the conventional PMF Sagnac
interferometer includes a PMF in the loop by splicing both ends with the two ends of 50:50 coupler, it has
been difficult to apply the two-ended PMF in the practical applications. In this paper, we propose a simple
configuration by adding one more 50:50 coupler in the Sagnac loop to connect the PMF with one splicing
process for a remote sensing position. Instead of tracking the shift of periodic spectrum by optical spectrum
analyzer, higher sensitive method is proposed by interrogating a phase variation of the interferometric optical
signal from the modified Sagnac interferometer including single-ended PMF.
A novel linearized interrogation method is presented for Fourier-domain mode locked (FDML) fiber Bragg grating
(FBG) sensor system. In the ultra high-speed regime over 10 kHz modulation, only sine wave is available to scan a
center wavelength of FDML wavelength-swept laser instead of conventional triangular wave. However, sine wave
modulation has been suffered an exaggerated nonlinear filter response in demodulating the time-encoded parameter into
the absolute wavelength. The linearized demodulation is demonstrated by the third order polynomial conversion of filter
between the time-encoded parameter and wavelength-encoded parameter based on the spectral information of
polarization maintaining fiber (PMF) Sagnac interferometer.