Pulse pattern manipulation of dichromatic soliton complexes by a twistable tapered-fiber filter.

Soliton complexes highlight the particle-like dynamics of dissipative pulses. However, simple and reliable manipulation of bound solitons remains challenging, particularly for all-polarization-maintaining (PM) configurations that are free from random polarization perturbations. Here, we report controllable pulse patterns of robustly coexisting dichromatic soliton complexes in an all-PM fiber laser based on a twistable tapered-fiber filter. According to the twist angle, dichromatic pulses are switched between different patterns, and components at each wavelength can be independently manipulated, extending encodings from the time to the frequency domain. To the best of our knowledge, it is the first experimental demonstration of dual-wavelength soliton complexes that different pulse patterns coexist at separated wavebands.

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.

Broadband similariton generation in a mode-locked Yb-doped fiber laser.

We experimentally demonstrated a broadband similariton generated from a mode-locked Yb-doped fiber laser with dispersion compensation. The broadest spectrum was obtained by pushing the net dispersion to its limitation and fully exploiting the gain bandwidth. The spectrum was 115 nm broad in 10 dB bandwidth and 36 nm broad in 3 dB bandwidth. The output was 105 mW optical power at 545 mW pump power. Simulation combined with experiment was performed to investigate and confirm the mode-locking regime of the laser. Experimental observations agreed well with the numerical simulation. We believe our study provides a practical route for designing broadband mode-locked fiber lasers.

Polarization-maintaining all-fiber tunable mode-locked laser based on a thermally controlled Lyot filter.

We propose and demonstrate for the first time, to the best of our knowledge, a thermally controlled all polarization-maintaining (PM) fiber Lyot filter. This filter is implemented in an all-PM mode-locked fiber laser to achieve wavelength tunability. When operating in the single-wavelength tunable mode, the center wavelength can be tuned across a range from 1546 nm to 1571 nm. Furthermore, the laser can also operate in a dual-wavelength mode with center wavelengths at 1545 nm and 1571 nm. The temperature sensitivity achieved in our all-PM fiber Lyot filter is 0.602 nm/°C, which is over 46 times higher than other fiber-based filters such as a fiber Bragg grating filter (0.013 nm/°C). This highly stable and versatile wavelength-tunable all-PM fiber mode-locked laser is a promising source for various applications requiring wavelength tunability and/or dual-wavelength output, such as coherent Raman microscopy and dual-comb spectroscopy.

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.

2.5 GHz harmonic mode locking from a femtosecond Yb-doped fiber laser with high fundamental repetition rate.

We experimentally demonstrated a high repetition rate harmonic mode-locked fiber laser with a high signal-to-noise ratio (SNR) and super mode suppression ratio. A novel approach using a laser with a high fundamental repetition rate and low harmonic order was presented. 2.5 GHz harmonic mode-locking from an Yb-doped fiber laser was realized with a short cavity length corresponding to a 167 MHz fundamental repetition rate. The laser operated at anomalous net dispersion regime and generated soliton-like pulses at 1050 nm. At the harmonic order of 15th, the laser had a stable output of 773 fs at 2.5 GHz with the average power of 48 mW under a subwatt pump power of 406 mW, and the time jitter was 2 ps. A high RF SNR over 70 dB was measured. A super mode suppression ratio was confirmed larger than 60 dB. This is, to the best of our knowledge, the highest SNR and super mode suppression ratio achieved in harmonic mode-locked fiber lasers with an over 2 GHz repetition rate.

Amplitude-modulated continuous-wave laser rangefinder employing Bessel-Gauss beamforming.

We propose an amplitude-modulated continuous-wave laser rangefinder employing Bessel-Gauss beamforming. Our Bessel-Gauss beam was generated by a newly proposed doublet configuration, which is simple and low cost. Such a beam was propagated >2m with a mainlobe having the diameter <1mm. We have conducted proof-of-concept ranging experiments employing the amplitude-modulated continuous-wave scheme with the Bessel-Gauss beam and obtained ranging results of a measurement distance up to 2 m. To the best of our knowledge, this is the first attempt to apply zeroth-order Bessel-Gauss beamforming to laser rangefinders.

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.

Mid-infrared high-Q germanium microring resonator.

Germanium is a promising material for mid-infrared (MIR) integrated photonics due to its CMOS compatibility and wide transparency window covering the fingerprint spectral region (2-15 µm). However, due to the limited quality and structural configurations of conventional germanium-based integration platforms, the realization of high-Q on-chip germanium resonators in the MIR spectral range remains challenging to date. Here we experimentally demonstrate an air-cladding MIR germanium microring resonator with, to the best of our knowledge, the highest loaded Q-factor of ∼57,000 across all germanium-based integration platforms to date. A propagation loss of 5.4 dB/cm and a high extinction ratio of 22 dB approaching the critical coupling condition are experimentally realized. These are enabled by our smart-cut methods for developing high-quality germanium-on-insulator wafers and by implementing our suspended-membrane structure. Our high-Q germanium microring resonator is a promising step towards a number of on-chip applications in the MIR spectral range.

Experimental study on depolarized GAWBS spectrum for optomechanical sensing of liquids outside standard fibers.

We report an experimental study on the spectral dependence of depolarized guided acoustic-wave Brillouin scattering (GAWBS) in a silica single-mode fiber (SMF) on acoustic impedance of external materials. The GAWBS spectrum was measured when the acoustic impedance was changed from 1.51 to 2.00 kg/s·mm2. With increasing acoustic impedance, the linewidth increased; the dependence was almost linear with an acoustic impedance dependence coefficient of 0.16 MHz/kg/s·mm2. Meanwhile, with increasing acoustic impedance, the central frequency linearly decreased with an acoustic impedance dependence coefficient of -0.07 MHz/kg/s·mm2. These characteristics are potentially applicable to acoustic impedance sensing.

Baud-rate flexible clock recovery and channel identification in OTDM realized by pulse position modulation.

We propose a novel scheme of OTDM utilizing pulse position modulation, where optical null headers (ONH) are inserted between the signal pulses periodically to allow channel identification. The ONH also achieves in-band clock distribution through the generation of high contrast pilot tone on the signal power spectra, enabling baud-rate flexible clock recovery. Using the novel scheme, clock recovery with a timing jitter of less than 200 fs is achieved at different baud rates up to 344 Gbaud. We demonstrate stable clock recovery with channel identification in 344-Gb/s OTDM transmissions over dispersion managed 3-km SMF.

Low-Temperature PECVD Growth of Germanium for Mode-Locking of Er-Doped Fiber Laser.

A low-temperature plasma-enhanced chemical vapor deposition grown germanium (Ge) thin-film is employed as a nonlinear saturable absorber (SA). This Ge SA can passively mode-lock the erbium-doped fiber laser (EDFL) for soliton generation at a central wavelength of 1600 nm. The lift-off and transfer of the Ge film synthesized upon the SiO2/Si substrate are performed by buffered oxide etching and direct imprinting. The Ge film with a thickness of 200 nm exhibits its Raman peak at 297 cm-1, which both the nanocrystalline and polycrystalline Ge phases contribute to. In addition, the Ge thin-film is somewhat oxidized but still provides two primary crystal phases at the (111) and (311) orientations with corresponding diffraction ring radii of 0.317 and 0.173 nm, respectively. The nanocrystalline structure at (111) orientation with a corresponding d-spacing of 0.319 nm is also observed. The linear and nonlinear transmittances of the Ge thin-film are measured to show its self-amplitude modulation coefficient of 0.016. This is better than nano-scale charcoal and carbon-black SA particles for initiating the mode-locking at the first stage. After the Ge-based saturable absorber into the L-band EDFL system without using any polarized components, the narrowest pulsewidth and broadest linewidth of the soliton pulse are determined as 654.4 fs and 4.2 nm, respectively, with a corresponding time-bandwidth product of 0.32 under high pumping conditions.

In-situ monitoring of optical deposition of carbon nanotubes onto fiber end.

Carbon nanotubes (CNTs) emerged as an attractive material for nonlinear optical devices. Their quasi-one-dimensional structure provided their unique nonlinear characteristics. However, one of their drawbacks is the handling method. We have proposed and demonstrated optical manipulation of CNTs to deposit them onto cores of optical fiber ends with a simple technique. Although the method is very simple, it requires precise control of the optical power. The method does not posses controllability of the CNT-layer properties. In this paper, we employed optical reflectometry to solve these problems. A 15 microm diameter circular region was area-selectively coated by CNTs using highly uniform solution. The preferentially-deposited CNTs were directly, for the first time, observed by a field emission scanning electron microscope (FE-SEM).

Optically formed carbon nanotube sphere.

Carbon nanotubes (CNTs) offer vast possibilities for future ultra-fast photonic devices. One of the biggest challenges to realize the devices is handling of carbon nanotubes. To achieve efficient handling, we have proposed and demonstrated the optical deposition of CNTs onto optical fiber ends. We found that a sphere-shaped super-structure made of carbon nanotubes can be fabricated by changing the light intensity. Chemical functionalization was not necessary in contrast to the conventional techniques of fabricating super-structures. We demonstrated physical manipulation of the sphere, and realized a passively mode-locked fiber laser employing the sphere.