Hwi Don Lee

Characterization of wavelength-swept active mode locking fiber laser based on reflective semiconductor optical amplifier

The static and dynamic characteristics of a wavelength-swept active mode locking (AML) fiber laser are presented in both the time-region and wavelength-region. This paper shows experimentally that the linewidth of a laser spectrum and the bandwidth of the sweeping wavelength are dependent directly on the length and dispersion of the fiber cavity as well as the modulation frequency and sweeping rate under the mode-locking condition. To achieve a narrower linewidth, a longer length and higher dispersion of the fiber cavity as well as a higher order mode locking condition are required simultaneously. For a broader bandwidth, a lower order of the mode locking condition is required using a lower modulation frequency. The dynamic sweeping performance is also analyzed experimentally to determine its applicability to optical coherence tomography imaging. It is shown that the maximum sweeping rate can be improved by the increased free spectral range from the shorter length of the fiber cavity. A reflective semiconductor optical amplifier (RSOA) was used to enhance the modulation and dispersion efficiency. Overall a triangular electrical signal can be used instead of the sinusoidal signal to sweep the lasing wavelength at a high sweeping rate due to the lack of mechanical restrictions in the wavelength sweeping mechanism.

Linearly Wavenumber-Swept Active Mode Locking Short-Cavity Fiber Laser for In-Vivo OCT Imaging

We demonstrate a highly linearly wavenumber-swept active mode locking (AML) fiber laser in the 1.3 μm region for in-vivo imaging in optical coherence tomography (OCT) without wavenumber space resampling. In this all-electric AML wavenumber-swept mechanism, the conventional wavelength selection filter is eliminated, and instead a suitable programmed electric modulation signal is applied directly to the gain medium. For a high sweep rate (up to 1 MHz) along the wavenumber, the fiber cavity structure is made as short as possible (0.88 m in air). A 15 ps/nm chirped fiber Bragg grating and a circulator are used for a shorter ring cavity configuration. A linewidth of 0.1 nm and tuning range of 42 nm are obtained under the mode-locking condition. Various types of wavenumber (or wavelength) tunings can be implemented because of the filterless cavity configuration. Therefore, we successfully demonstrate a linearly wavenumber-swept AML fiber laser with 26.5 mW of output power for obtaining in-vivo OCT images at a sweep rate of 100 kHz.

Linearized Wavelength Interrogation System of Fiber Bragg Grating Strain Sensor Based on Wavelength-Swept Active Mode Locking Fiber Laser

We demonstrated a linearized wavelength interrogation system of fiber Bragg grating (FBG) strain sensors, based on a wavelength-swept active mode locking (AML) fiber laser. Since the wavelength sweeping mechanism of the AML fiber laser does not depend on the wavelength selecting filter, we can easily demonstrate the wavelength-linear sweeping by applying a programmable electrical signal to the laser cavity. Both static and dynamic strain measurements of FBG sensors were characterized with high linearity, which has an R-square value of 0.9999 at a sweep rate of 50 kHz.

A Fiber Bragg Grating Sensor Interrogation System Based on a Linearly Wavelength-Swept Thermo-Optic Laser Chip

A linearized wavelength-swept thermo-optic laser chip was applied to demonstrate a fiber Bragg grating (FBG) sensor interrogation system. A broad tuning range of 11.8 nm was periodically obtained from the laser chip for a sweep rate of 16 Hz. To measure the linear time response of the reflection signal from the FBG sensor, a programmed driving signal was directly applied to the wavelength-swept laser chip. The linear wavelength response of the applied strain was clearly extracted with an R-squared value of 0.99994. To test the feasibility of the system for dynamic measurements, the dynamic strain was successfully interrogated with a repetition rate of 0.2 Hz by using this FBG sensor interrogation system.

Uniform spacing interrogation of a Fourier domain mode-locked fiber Bragg grating sensor system using a polarization-maintaining fiber Sagnac interferometer

A novel linearized interrogation method is presented for a Fourier domain mode-locked (FDML) fiber Bragg grating (FBG) sensor system. In a high speed regime over several tens of kHz modulations, a sinusoidal wave is available to scan the center wavelength of an FDML wavelength-swept laser, instead of a conventional triangular wave. However, sinusoidal wave modulation suffers from an exaggerated non-uniform wavelength-spacing response in demodulating the time-encoded parameter to the absolute wavelength. In this work, the calibration signal from a polarization-maintaining fiber Sagnac interferometer shares the FDML wavelength-swept laser for FBG sensors to convert the time-encoded FBG signal to the wavelength-encoded uniform-spacing signal.

Simultaneous Dual-Band Wavelength-Swept Fiber Laser Based on Active Mode Locking

We report a simultaneous dual-band wavelength-swept laser based on the active mode locking method. By applying a single modulation signal, synchronized sweeping of two lasing-wavelengths is demonstrated without the use of a mechanical wavelength-selecting filter. Two free spectral ranges are independently controlled with a dual path-length configuration of a laser cavity. The static and dynamic performances of a dual-band wavelength-swept active mode locking fiber laser are characterized in both the time and wavelength regions. Two lasing wavelengths were swept simultaneously from 1263.0 to 1333.3 nm for the 1310 nm band and from 1493 to 1563.3 nm for the 1550 nm band. The application of a dual-band wavelength-swept fiber laser was also demonstrated with a dual-band optical coherence tomography imaging system.

High-speed broadband frequency sweep of continuous-wave terahertz radiation.

We present a new technical implementation of a high-speed broadband frequency sweep of continuous-wave terahertz (THz) radiation. THz frequency sweeping with a kHz sweep rate and a THz sweep range is implemented using THz photomixing in which an optical beat source consists of a wavelength-swept laser and a distributed feedback laser diode. During the frequency sweep, frequency-domain THz interferograms are measured using the coherent homodyne detection employing signal averaging for noise reduction, which can give time-of-flight information via fast Fourier transform. Multiple reflections in a Si wafer and the thickness of the wafer are measured to demonstrate the potential of this method for fast THz tomography and thickness measurement.

Linearized interrogation of FDML FBG sensor system using PMF Sagnac interferometer

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.

Wavelength-spacing tuning of a dual-wavelength active-mode locking fiber laser

We demonstrate a novel concept of wavelength-spacing tuning of a dual-wavelength active-mode locking fiber laser in the 1.3-μm wavelength region. A dual-cavity configuration is implemented using a chirped fiber Bragg grating pair for wavelength-spacing tuning. Both normal and anomalous dispersion cavities were schematically designed for wavelength-spacing tuning ranging from 3.52-33.54 nm. The side-mode suppression ratio was ∼30  dB for both wavelengths. Because of the filter-less tuning mechanism, a fast repletion rate of wavelength-spacing sweeping and switching was demonstrated for frequencies up to 50 kHz.

Characterization of variable reflectivity of a polarization-maintaining fiber Sagnac mirror for long-distance remote fiber Bragg gratings cavity sensors

Variation of broadband reflectivity is experimentally characterized for a polarization-maintaining fiber (PMF) Sagnac mirror. Theoretical analysis is also demonstrated using the Jones matrices of the PMF Sagnac mirror. Variable reflectivity is useful to enhance the signal performance of a long-distance remote fiber Bragg gratings (FBGs) cavity sensor at a range of tens of km. Controlling both the reflecting bandwidth and partial reflectivity assists the effective multi-wavelength lasing from the linear-type resonance cavity with FBG mirrors at one side and a PMF Sagnac mirror at the other. The FBG sensing signal achieves a high SNR of greater than ~45 dB due to the cavity mirror effect.