The output of a wavelength-swept laser (WSL) based on a fiber Fabry–Pérot tunable filter (FFP-TF) tends to shift the peak wavelength due to external temperature or heat generated by the FFP-TF itself. Therefore, when measuring the output of WSL for a long time, it is very difficult to accurately measure a signal in the temporal domain corresponding to a specific wavelength of the output of the WSL. If the wavelength variation of the WSL output can be predicted through the peak time information of the forward scan or the backward scan from the WSL, the variation of the peak wavelength can be compensated for by adjusting the offset voltage applied to the FFP-TF. This study presents a successful stabilization method for peak wavelength variation in WSLs by adjusting the offset voltage of the FFP-TF with closed-loop control. The closed-loop control is implemented by measuring the deviation in the WSL peak position in the temporal domain using the trigger signal of the function generator. The feedback repetition rate for WSL stabilization was approximately 0.2 s, confirming that the WSL output and the peak position for the fiber Bragg grating (FBG) reflection spectrum were kept constant within ±7 μs at the maximum when the stabilization loop was applied. The standard deviations of WSL output and reflection peak positions were 1.52 μs and 1.59 μs, respectively. The temporal and spectral domains have a linear relationship; the ±7 μs maximum variation of the peak position corresponded to ±0.035 nm of the maximum wavelength variation in the spectral domain. The proposed WSL system can be used as a light source for temperature or strain-dependent sensors as it compensates for the WSL wavelength variation in applications that do not require a fast scanning rate.

Cholesteric liquid crystals (CLCs) can be applied to various physical and chemical sensors because their alignment structures are changed by external stimuli. Here, we propose a CLC device fabricated by vertically forming the helical axis of the CLC between the cross-sections of two optical fiber ferrules. An optical fiber temperature sensor was successfully implemented using the proposed optical fiber ferrule-based CLC device. A wideband wavelength-swept laser with a center wavelength of 1073 nm and scanning range of 220 nm was used as a light source to measure the variations in the reflection spectrum band according to the temperature change in the CLC cell. The wavelength variation of the reflection spectrum band according to the temperature applied to the CLC cell was reversible and changed linearly with a change in the temperature, and the long-wavelength edge variation rate according to the temperature change was −5.0 nm/°C. Additionally, as the temperature applied to the CLC cell increased, the reflection spectrum bandwidth gradually decreased; the reflection spectrum bandwidth varied at a rate of −1.89 nm/°C. The variations in the refractive indices with temperature were calculated from the band wavelengths of the reflection spectrum. The pitch at each temperature was calculated based on the refractive indices and it gradually decreased as the temperature increased.

Broadband wavelength-swept lasers (WSLs) are widely used as light sources in biophotonics and optical fiber sensors. Herein, we present a polygonal mirror scanning wavelength filter (PMSWF)-based broadband WSL using two semiconductor optical amplifiers (SOAs) with different center wavelengths as the gain medium. The 10-dB bandwidth of the wavelength scanning range with 3.6 kHz scanning frequency was approximately 223 nm, from 1129 nm to 1352 nm. When the scanning frequency of the WSL was increased, the intensity and bandwidth decreased. The main reason for this is that the laser oscillation time becomes insufficient as the scanning frequency increases. We analyzed the intensity and bandwidth decrease according to the increase in the scanning frequency in the WSL through the concept of saturation limit frequency. In addition, optical alignment is important for realizing broadband WSLs. The optimal condition can be determined by analyzing the beam alignment according to the position of the diffraction grating and the lenses in the PMSWF. This broadband WSL is specially expected to be used as a light source in broadband distributed dynamic FBG fiber-optic sensors.

Wavelength-swept laser (WSL) is a light source for sensing dynamic physical quantities in optical fiber sensors and has the advantage of a fast scan speed over a wide wavelength range. However, WSLs based on the fiber Fabry–Pérot tunable filter (FFP-TF) has difficulty maintaining a stable scanning wavelength band due to its thermal instability. Additionally, fluctuations in the intensity of the WSL due to temperature variations in the gain medium within the WSL resonator affect the accuracy of the sensor. In this study, we report two approaches for FFP-TF-based WSL stabilization. First, we implement a feedback mechanism to stabilize the center wavelength of the WSL by adjusting the voltage of the FFP-TF based on measurements in the temporal domain. Second, we propose a method to rapidly measure the signal in the temporal domain and compensate for intensity variations due to the temperature changes in the gain medium. The effectiveness of the proposed WSL stabilization and amplitude correction techniques was evaluated using a set of five fiber Bragg gratings (FBGs) maintained under consistent conditions. Over a measurement period of 15 h, the wavelength stability of the WSL was confirmed to be within a maximum standard deviation of 66.7 pm. Additionally, the peak intensities of the FBGs were successfully corrected for fluctuations due to the temperature changes in the gain medium. These results demonstrate the effectiveness of the method in maintaining the precision and stability of a WSL output over a long period of time and should enable rapid and accurate measurement of the wavelength and peak intensity of FBG-based sensors.

The scanning speed stability of a wavelength-swept laser (WSL) with a polygon-scanner-based wavelength filter (PSWF) is affected by the speed of the brushless direct current (BLDC) motor driving a polygonal scanner mirror (PSM). In particular, the PSM running at low speeds experiences fluctuations in rotational speed, causing the output of the WSL to fluctuate in the temporal domain. The speed variation of the WSL causes critical measurement errors in the fiber Bragg grating (FBG) interrogator sensors. In this study, we successfully corrected the output error of the FBG array sensor caused by the PSM rotational speed fluctuations in a WSL running at a low scanning rate. The FBG array used five FBGs, and the time intervals of the FBGs were corrected according to the PSM rotation speed variation based on the periodic interval of the array signal. A WSL with a scanning rate of 1.0 kHz suggested a 97% improvement after compensating for rotational speed variations of the PSM.