Switchable multiwavelength thulium-doped fiber laser with cylindrical vector beam emission

Cylindrical vector beams (CVBs), characterized by their spatially non-uniform but rotationally symmetric polarization distributions, have garnered considerable attention due to their unique properties. In this Letter, we demonstrate the generation of cylindrical vector beams directly from a Yb-doped Mamyshev oscillator (MO). Both the radially and azimuthally polarized vector beams are achieved with a pulse duration of 3.6 ps and a pulse energy of 8.2 nJ at the repetition rate of 15.25 MHz. We also find that the filtered linearly polarized beam from the output CVBs can be homogeneously dechirped to 114 fs using the external grating pairs, indicating the potential intracavity pulse compression capability for the output CVBs. Simultaneously, ultrafast scalar Gaussian beams can also be obtained from another arm. These results can give insights into the experimental design and realization of high-energy and high-power structured (CVB or vortex) ultrashort lasers.

A tunable multi-wavelength narrow-linewidth thulium-doped fiber laser employing two-stage cascaded Sagnac loop mirrors is proposed and experimentally demonstrated. The designed fiber laser is composed of a pump source, wavelength division multiplex, circulator, thulium-doped fiber, polarization controllers (PCs), couplers and polarization-maintaining fibers (PMFs). Two cascaded Sagnac loops are used as the cavity reflector and filter, and the proposed filter is fabricated using two sections of PMFs with 2-m and 1-m lengths, respectively. In the experiment, the laser threshold is 110 mW, and laser can emit single, double, triple, quadruple and quintuple wavelengths in the spectral range of 1873–1901 nm through the simultaneous adjustment of the two PCs. The power fluctuations and 3-dB linewidth are less than 2.1 dB and 0.2 nm, respectively, over 10 min at room temperature, and the side-mode suppression ratio is greater than 20 dB. The proposed laser will be useful in various fields, such as spectral analysis, fiber sensing and optical communication.

We propose a chip-integratable cylindrical vector (CV) beam generator by integrating six plasmonic split ring resonators (SRRs) on a planar photonic crystal (PPC) cavity. The employed PPC cavity is formed by cutting six adjacent air holes in the PPC center, which could generate a CV beam with azimuthally symmetric polarizations. By further integrating six SRRs on the structure defects of the PPC cavity, the polarizations of the CV beam could be tailored by controlling the opening angles of the SRRs, e.g., from azimuthal to radial symmetry. The mechanism is governed by the coupling between the resonance modes in SRRs and PPC cavity, which modifies the far-field radiation of the resonance mode of the PPC cavity with the SRR as the nano-antenna. The integration of SRRs also increases the coupling of the generated CV beam with the free-space optics, such as an objective lens, promising its further applications in optical communication, optical tweezer, imaging, etc.

A switchable multi-wavelength thulium-doped fiber laser (TDFL) using a four-mode fiber based Sagnac loop filter was proposed and demonstrated. The Sagnac loop, incorporating a 4.8 m long four-mode step-index fiber, acting as a comb filter was analyzed theoretically and experimentally. By adjusting two polarization controllers, eight stable single-wavelength operations and five stable dual-wavelength operations were obtained with easy switching between the different operation modes. The wavelength range covered during the single-wavelength switchable operation with a signal-to-noise ratio of up to 55 dB was ∼28.51 nm. In dual-wavelength switchable operation, the maximum and minimum wavelength spacings were ∼21.78 nm and ∼1.50 nm, respectively. Due to the abundant wavelength resources, the proposed TDFL has great potential in wavelength division multiplexing free-space optical communications, optical fiber sensing, and photonic generation of microwave signals.

A novel self-seeded multi-wavelength Brillouin-erbium fiber laser with a transmissive Sagnac loop filter is proposed. In the laser scheme, the Brillouin pump is self-excited within the cavity, which doesn't require injection from external cavity. 52 stokes lines are obtained by adjusting polarization controller (PC) in the Sagnac loop filter. The experiment also demonstrates that the pump power has an influence on the wavelength number and power distribution of the generated multi-wavelength comb. The multi-wavelength comb is tunable in a range of 6.5nm.

We propose and experimentally demonstrate a tunable and switchable multi-wavelength erbium-doped fiber ring pulsed laser based on a nonlinear optical loop mirror (NOLM) and an improved Sagnac filter. To achieve multi-wavelength pulsed laser output, we adopt a NOLM as a quasi-saturable absorber and an improved Sagnac loop as a wavelength selected filter. The constructed laser has a maximum output wavelength number of five with a pulse repetition frequency of 40.45kHz and pulse duration of 108ns. The laser can output single-wavelength and dual-wavelength pulsed lasers within a certain wavelength tuning range and a five-wavelength pulsed laser with a constant wavelength interval of 3nm by adjusting the polarization controller. Dual-wavelength, three-wavelength, and four-wavelength pulsed lasers with various wavelength intervals are also obtained. The output performance of the constructed laser is tested with a maximum average output power of 127.45µW and minimum pulse duration of 52ns, and the stability of the laser output is also tested with a maximum power fluctuation of 0.62dB and minimum wavelength drift of 0.51nm with pump power of 350mW.

We experimentally investigate the generation of cylindrical vector beams with noise-like pulses (NLPs) and their bound state in a ytterbium-doped fiber laser. A homemade mode-selective coupler serves as the mode converter, which consists of a pre-tapered single-mode fiber and a two-mode fiber fabricated using a weak-fusion technique, exhibiting an insertion loss of approximately 0.55 dB. Switching between radially and azimuthally polarized beams was achieved by adjusting the polarization controller, with measured mode purity exceeding 94.5% across different operational states. Numerical simulations were employed to elucidate the formation mechanisms of both NLPs and their bound states. The demonstrated laser system shows promising potential for applications in laser material processing and mode-division multiplexing systems.

Generation of multiwavelength bismuth-doped fiber laser based on all-fiber Lyot filter

A simple scheme of a switchable multi-wavelength and dual-scale soliton fiber laser is proposed by cascading three chirped fiber Bragg gratings (CFBGs). By managing the cavity dispersion and controlling the losses of attenuators, quadruple-wavelength switchable pulses are generated, including one dissipative soliton (DS) and three conventional solitons (CSs). The central wavelength of the DS is 1565 nm with a pulse duration of 12.5 ps which can be compressed to ~571 fs after a 70 m single mode fiber external to cavity. Three CSs are achieved at 1539.5, 1544.5, and 1549.5 nm with pulse durations of 9.6, 8.7, and 8.8 ps, corresponding to the CFBG-based comb spectral filtering. These results demonstrate that the proposed method offers potential opportunities to exploit the flexibly switchable multi-wavelength CS/DS-pulse sources at picosecond and femtosecond scales.

Tunable and switchable multiwavelength fiber laser incorporating tapered few-mode fiber Bragg gratings

We present the generation of the nanosecond cylindrical vector beams (CVBs) in a two-mode fiber (TMF) and its applications of stimulated Raman scattering. The nanosecond (1064 nm, 10 ns, 10 Hz) CVBs have been directly produced with mode conversion efficiency of ∼18 dB (98.4%) via an acoustically induced fiber grating, and then the stimulated Raman scattering signal is generated based on the transmission of the nanosecond CVBs in a 100-m-long TMF. The transverse mode intensity and polarization distributions of the first-order Stokes shift component (1116.8 nm) are consistent with the nanosecond CVBs pump pulse.

A study on the effect of balance and imbalance fraction ratio of nonlinear optical loop mirror (NOLM) to produce multiwavelength erbium-doped fiber laser (MWEDFL) is presented. Based on the study, generation of multiwavelength fiber laser with 60dB OSNR value and 0.42nm mode spacing are realized from the effect of the laser interference using sagnac loop mirror and light interference It is shown that the transmittance value for imbalance fraction ratio obtained from the experiment is 0.36 thus fraction ratio of NOLM is an important parameter in producing multiwavelength fiber laser.

We propose and experimentally demonstrate a multi-wavelength thulium-doped fiber laser using two cascaded single-mode-four-mode-single-mode (SFS) fiber interferometers and a 150-m-long highly nonlinear fiber. The transmission properties of SFS fiber interferometers with two different four-mode fiber (FMF) lengths are analyzed theoretically. The interferometer with a longer FMF is used to select specific wavelengths, and the one with a shorter FMF is used to stimulate more laser lines. By adjusting the polarization controller and increasing the pump power, the number of output laser channels and the wavelength spacings can be changed and switched. By selecting the proper length of the FMFs, 31 lasing lines are achieved with covering a wavelength range of $\sim 22$ nm. The power fluctuation and the wavelength drift of the 31-wavelength operation for the proposed fiber laser are within 0.706 dB and 0.02 nm, respectively.

We present a method for achieving the cylindrical vector beams (CVBs) and optical vortex generation in a solid-core ring fiber (SCRF) by using an acoustically induced fiber grating (AIFG). Theoretical analysis shows that the SCRF can introduce a refractive index difference of ~10-5 between the first group of adjacent high-order degenerate vector modes. In the experiment, the transmission spectra of AIFG constructed with the SCRF depicts three separate resonance peaks with 3-dB bandwidth and resonance wavelength interval of 0.16 and 2.5 nm, respectively, revealing that the high-order degenerate vector modes have been effectively separated. On the basis of the separation of the degenerate vector mode, the CVBs and the ±1-order optical vortex are further experimentally generated at three separate wavelengths using the AIFG.

A widely tunable microwave photonic notch filter with adjustable bandwidth based on multi-wavelength fiber laser is proposed and demonstrated. The multi-wavelength fiber laser generates the multi-taps of the microwave photonic filter (MPF). In order to obtain notch frequency response, a Fourier-domain optical processor (FD-OP) is introduced to control the amplitude and phase of the optical carrier and phase modulation sidebands. By adjusting the polarization controller (PC), different numbers of taps are got, such as 6, 8, 10 and 12. And the wavelength spacing of the multi-wavelength laser is 0.4 nm. The bandwidth of the notch filter is changed by adjusting the number of taps and the corresponding bandwidths are 4.41 GHz, 3.30 GHz, 2.64 GHz and 2.19 GHz, respectively. With the additional phase shift introduced by FD-OP, the notch position is continuously tuned in the whole free spectral range (FSR) of 27.94 GHz. The center frequency of the notch filter can be continuously tuned from 13.97 GHz to 41.91 GHz.