Pump-power-controlled L-band wavelength-tunable mode-locked fiber laser utilizing nonlinear polarization evolution in all-polarization-maintaining fibers.

We present for the first time, to the best of our knowledge, the pump-power-controlled, all-polarization-maintaining (all-PM), all-fiber configured, wavelength-tunable mode-locked fiber laser in the L-band (1565 to 1625 nm). A tuning range over 20 nm (1568.2  to 1588.9 nm) is attained simply by varying the pump power between 45 and 115 mW. Our work represents the first demonstration of wavelength tuning in all-PM configured nonlinear polarization evolution (NPE) lasers. The non-mechanical and electrically controllable tuning method offers ease of use and cost efficiency within an advanced all-PM, all-fiber design, indicating promising adaptability to diverse wavelength bands.

Ranging disambiguation of LiDAR using chirped amplitude-modulated phase-shift method.

Ranging ambiguity is the major challenge in most LiDAR techniques with amplitude modulation, which limits the performance of range detection due to the tradeoff between the ranging precision and the unambiguous range. Here we propose a novel disambiguation method using a laser with chirped amplitude modulation (sweeping modulation frequency), which can in theory infinitely expand the unambiguous range and completely solve the ranging ambiguation problem. The usage of the earlier proposed Chirped Amplitude-Modulated Phase-Shift (CAMPS) technique enables us to detect the phase-shift of chirped signals with high precision. Incorporating this technique with the proposed disambiguation method, the absolute distance well beyond the conventional unambiguous range can easily be found with merely <1% frequency sweep range. When certain conditions are met, the Non-Mechanical Spectrally Scanned LiDAR (NMSL) system employing the CAMPS method and the Dispersion-Tuned Swept Laser (DTSL) can also realize disambiguation in non-mechanical line-scanning measurement.

All-polarization-maintaining, efficiently wavelength-tunable, Er-doped mode-locked fiber laser with a compact reflective Lyot filter.

Wavelength tunable mode-locked fiber lasers have highly potential applications in precision spectroscopy, nonlinear microscopy and photonic sensing. Here, we present a compact and thermal-sensitive reflective Lyot filter and utilize it for all-polarization-maintaining efficiently wavelength-tunable Er-doped carbon-nanotube-mode-locked laser for the first time, to the best of our knowledge. The output wavelength of the laser can be tuned from 1544.46 nm to 1572.71 nm, with a wide tuning range of 28.25 nm, and a remarkable tuning efficiency of 0.589 nm/°C, when the angle-spliced fiber is only 8.2 cm and the free spectral range of the filter is 31.32 nm. Dual-wavelength mode-locking is also achieved at boundary temperatures when increasing the pump power. Due to its compact size and reflection configuration, the proposed reflective Lyot filter is promising for realizing highly efficient wavelength tuning and multiwavelength generation in all-polarization-maintaining fiber lasers where reflective filters are needed.

L-band mode-locked fiber laser using all polarization-maintaining nonlinear polarization rotation.

For the first time, to the best of our knowledge, in the soliton regime, we demonstrate an L-band fiber laser mode-locked by all polarization-maintaining (all-PM) nonlinear polarization rotation (NPR). A numerical study suggests that lengthening the NPR section boosts modulation depth and lowers saturation power of the artificial saturable absorber (SA). With the longest NPR section to date (21 m), the laser emits 1.25-ps soliton pulses at 1584.2 nm and a 3.9-MHz repetition rate. Our laser provides a promising L-band seed source, exhibiting improved repeatability and stability compared with non-PM L-band pulse fiber lasers.

Optical single-sideband modulation for a coherent Doppler lidar.

The first, to the best of our knowledge, implementation of optical single-sideband modulation (OSSBM) in coherent Doppler lidar (CDL) is demonstrated for use with short pulse widths. It is shown that by providing more bandwidth through a higher intermediate frequency (IF), the OSSBM CDL addresses cross talk between the IF and baseband spectra, reducing spectral distortion. Ultimately, a 4-ns pulse width with a 1-GHz IF is achieved and the effectiveness of the OSSBM CDL is confirmed through velocimetry experiments. This represents a several fold improvement over current fiber-based CDL implementations.

Fiber-based dynamically tunable Lyot filter for dual-wavelength and tunable single-wavelength mode-locking of fiber lasers.

We propose and demonstrate a novel dynamically tunable fiber-based Lyot filter for the realization of a dual-wavelength mode-locked fiber laser, operating at center wavelengths of 1535 nm and 1564 nm. The same laser cavity can also be operated in a single-wavelength mode-locked regime with a wavelength tuning range of 30 nm, from 1532 nm to 1562 nm. The proposed dynamically tunable Lyot-filter provides a simple setup for laser mode-locking using a single laser cavity design to generate dual-wavelength pulses, with the flexibility to also allow the generation of single-wavelength pulses with a continuously-tunable center wavelength.

Laser mode locking using a single-mode-fiber coil with enhanced polarization-dependent loss.

A laser mode-locking phenomenon based on polarization-dependent loss (PDL) using a passive fiber coil is demonstrated. We propose using a fiber coil operating in low-V-number regime to achieve an enhanced bend-induced PDL and to maintain a reasonable bend loss. A mode-locked thulium doped all-fiber laser is shown using the low-V-number fiber coil. The results indicate that a moderate amount of PDL at 1 dB is sufficient to initiate and sustain a stable CW mode-locking operation. To the best of our knowledge, this is the first demonstration of a CW mode-locked fiber laser based on PDL enabled by a fiber coil.

Optimization of a dispersion-tuned wavelength-swept fiber laser for optical coherence tomography.

We optimized parameters of a dispersion-tuned wavelength-swept fiber laser by numerically analyzing dynamic characteristics. The optimized laser is experimentally demonstrated and applied to the swept-source optical coherence tomography (SS-OCT) system. The dispersion-tuned wavelength-swept laser (DT-WSL) is a unique tunable fiber laser, whose lasing wavelength can be tuned rapidly without any mechanical tunable filters. Although the wavelength of a DT-WSL can be swept rapidly and widely, the broadening of the instantaneous spectral width at a high sweep rate has been a critical drawback for SS-OCT applications. Numerical simulations have shown that higher modulation frequencies for active mode-locking lead to narrower instantaneous spectral widths. However, a lower modulation frequency is needed to achieve a wider wavelength tuning range. Pulse modulation is employed to solve the trade-off between instantaneous spectral width and wavelength tuning range. In this paper, the characteristics of a sinusoidally modulated and a pulse-modulated DT-WSL are compared numerically and experimentally. The numerical simulation results show that a pulse-modulated laser can achieve spectral widths as narrow as that of the sinusoidally modulated laser with >5 GHz modulation frequency, even when the pulse modulation frequency is as low as 500 MHz. We also study the difference in the laser characteristics with different sweep directions and discover that a positive wavelength sweep leads to a narrower instantaneous spectral width. We also experimentally confirmed that pulse modulation can indeed achieve a narrower spectral width, as expected from our numerical simulation results. The pulse-modulated DT-WSL is then used in an SS-OCT system and successfully achieves a coherence length of 1.3 mm, whereas that of a sinusoidally modulated DT-WSL is limited to only 0.7 mm. Furthermore, we experimentally compare the performance difference in OCT imaging with different wavelength sweep directions, and the results proved that it is advantageous to apply a positive wavelength sweep, as predicted by our numerical simulation.