Suppress Mode Competition Research Articles

Switchable Single- to Multiwavelength Conventional Soliton and Bound-State Soliton Generated from a NbTe2 Saturable Absorber-Based Passive Mode-Locked Erbium-Doped Fiber Laser.

As a member of transition metal dichalcogenides (TMDs), NbTe2 has a work function of 5.32 eV and a band gap of 0 eV at the Fermi level, which enables it to possess broadband absorption characteristics and has huge potential in optoelectronic devices. In this work, a combination of liquid phase exfoliation (LPE) and optical deposition methods (ODMs) were used to fabricate a NbTe2 saturable absorber (SA). Based on the NbTe2 SA, a ring passive mode-locked erbium-doped fiber laser (PML-EDFL) was constructed by adding NbTe2 SA into the laser cavity. A switchable single- to multiwavelength (dual/triple/quadruple) conventional soliton (CS) and a bound-state soliton (BS) were observed for the first time. The results reveal that NbTe2 SA has excellent saturable absorption characteristics (modulation depth of 2.6%, saturation intensity of 177.4 MW/cm2, and unsaturated loss of 63.8%) and can suppress mode competition and stabilize multiwavelength oscillation. This study expands the applications of NbTe2 nanosheets in ultrafast optoelectronics. The proposed switchable PML-EDFL has extensive applications in high-capacity all-optical communication, high-sensitivity optical fiber sensing, high-precision spectral measurements, and high-energy-efficiency photon neural networks.

Influences of distributed losses on start current, quality factor Q and frequency in multi-mode multi-frequency coaxial gyrotrons

The primary problem that gyrotrons encounter is mode competition during their development, especially multi-mode multi-frequency gyrotrons. In this paper, the influence of distributed losses on start current, quality factor Q and resonant frequency were studied. Four types of distributed losses are discussed, which each type can also increase start oscillation current of operating and competing modes. But the amplitudes of increase are different, which makes the start oscillation current of operating and competing modes further separated. By separating start oscillation currents, they suppress competing modes and enhance devices stability. The adverse effect of distributed losses is that they reduced Q value and raised resonant frequency slightly. But they have little effect on other performance of gyrotrons. The start oscillation current of multi-mode multi-frequency coaxial gyrotrons with distributed losses has the ability to suppress mode competition and improve stability of operation. So it is a new method of suppressing modes competition that deserves consideration.

Switchable multi-wavelength actively Q-switched erbium-doped fiber laser based on nonlinear polarization rotation and Sagnac filter

This letter describes a switchable multi-wavelength actively Q-switched erbium-doped fiber (EDF) ring laser based on nonlinear polarization rotation (NPR) structure with Sagnac filter and electro-optic modulator (EOM). The intensity-dependent loss (IDL) generated by the constructed NPR structure can suppress the mode competition caused by EDF, combined with a comb filter Sagnac loop which composed of only 2-m-long polarization maintaining fiber (PMF), the constructed laser generates nine-wavelengths laser output with a constant wavelength interval of 3 nm. When the NPR structure is removed, the constructed laser generates quadruple-wavelengths laser output with 3 nm constant wavelength interval. It is proved that the effect of the NPR can suppress mode competition to achieve multi-wavelength laser output. In the laser with the constructed NPR structure, the nine-wavelengths laser output has a maximum wavelength shift of 0.21 nm and a maximum power fluctuation of 0.69 dB, while when the NPR structure is removed from the constructed laser, for the quadruple-wavelengths laser output, the maximum wavelength shift is 0.16 nm and the maximum power fluctuation is 0.29 dB. When the constructed laser with or without NPR structure, the minimum Q-switched pulse width and maximum average output power are 1.42 μs, 1.24 μs and 3.2μW, 10.2μW, respectively.

Generation of 12th order harmonic mode-locking in a Nd-doped single-mode all-fiber laser operating at 0.9 µm

Based on the Nd-doped single-mode fiber as the gain medium, an all-fiber 12th harmonic mode-locked (HML) laser operating at the 0.9 µm waveband was obtained for the first time, to the best of our knowledge. A mandrel with a diameter of 10 mm was employed to introduce bending losses to suppress mode competition at 1.06 µm, which resulted in a suppression ratio of up to 54 dB. The 1st–12th order HML pulses with the tunable repetition rate of 494.62 kHz–5.94 MHz were obtained in the mode-locked laser with a center wavelength of ∼904 nm. In addition, the laser has an extremely low threshold pump power of 88 mW. To the best of our knowledge, this is the first time that an HML pulse has been achieved in a 0.9 µm Nd-doped single-mode all-fiber mode-locked laser with the advantages of low cost, simple structure, and compactness, which could be an ideal light source for two-photon microscopy.

Switchable and stable erbium-doped fiber laser using the reconfigurable Lyot ring filter with WS2 film

A novel switchable and stable erbium-doped fiber laser (EDFL) using the reconfigurable Lyot ring filter (RLRF) with tungsten disulfide (WS2) film is proposed and experimentally demonstrated. The principle of RLRF is that the interference of the polarized lights with phase differences produces a comb spectrum with obvious envelope and uniform fluctuation. The transmission spectrum of RLRF can be changed by adjusting the polarization controllers (PCs) in RLRF. In the experiment, the actual length of the polarization-maintaining fiber (PMF) is 4 m. By adjusting PC1 adjacent to PMF, 2 to 5 wavelengths can be switched. Adjusting PC2 in the ring can change the equivalent length of the PMF in RLRF. When the equivalent length is 8 m, 5 to 8 wavelengths can be switched with a wavelength interval of 0.68 nm. The nonlinear material WS2 film is used as a means to suppress mode competition in the laser cavity, and the fiber laser has a stable output. The switching of the proposed EDFL is simple and flexible, and can be used in wavelength division multiplexing (WDM) systems.

725 nm watt-level injection-locked continuous-wave Ti:sapphire laser for a mercury optical lattice clock.

We develop a watt-level 725 nm continuous-wave tunable narrow linewidth injection-locked Ti:sapphire laser. The seed laser is frequency-locked on a Fabry-Perot cavity to narrow the linewidth and stabilize the frequency. Because the wavelength of the seed laser is located at the edge of the gain profile of the Ti:sapphire crystal, it is difficult to injection-lock the Ti:sapphire laser at 725 nm. A cavity mirror, which has a long-pass-filter coating with a sharp edge, is used in the Ti:sapphire cavity to suppress mode competition from the long wavelength side. This method effectively reduces the power requirement of the seed laser at 725 nm, and the Ti:sapphire laser can be injection-locked at higher output power. As a result, a 1 W output power with a 27% slope efficiency is achieved in the injection-locked laser, and a 210 mW UV laser is obtained in a subsequent second-harmonic generation stage. To the best of our knowledge, this is the shortest wavelength for the watt-level injection-locked continuous-wave Ti:sapphire laser. This laser will be used in a mercury optical lattice clock in the future.

High-Efficiency Phase-Locking of Millimeter-Wave Magnetron for High-Power Array Applications

We introduced a phase-locking mechanism of millimeter-wave magnetrons coupled through a carefully designed coupling bridge. For such a structure, two Ka-band 22-vane rising-sun magnetron circuits operated in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula> -mode were phase-locked tightly as a whole through a double-ridged coupler. Moreover, the double-ridged coupler was optimized to suppress mode competition between zero-phase and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula> -phase difference mode, which achieved high-efficiency phase-locking. Total output power of the two identical magnetrons nearly doubled the output power of a free-running magnetron and efficiencies of them were nearly the same. Extension of this phase-locking mechanism to high-power applications appears feasible with more elements in an array of the millimeter-wave magnetrons.

Terahertz gyrotrons with inhomogeneous magnetic fields to suppress mode competition and enhance efficiency

Gyrotrons are promising radiation sources for bridging the terahertz gap. They are based on the instability of electron cyclotron maser, where the harmonic operation is generally necessary to alleviate the need for a strong magnetic field. Unfortunately, the performance of a harmonic gyrotron is extremely sensitive to mode competition and magnetic tuning. In this study, to achieve highly efficient and mode-selective gyrotrons, inhomogeneous magnetic fields are applied to introduce a specified longitudinal distribution of the detuning frequency between the terahertz wave and the gyrating electron beam. This detuning frequency has different influences on the oppositely traveling forward wave (FW) and backward wave (BW) inside the cavity, from which optimized magnetic-field profiles for FW-favored and BW-favored interaction circuits are generalized accordingly. It is proposed that a negatively tapering magnetic field converts the energy-transfer rate of the FW interaction into a positive value, leading to highly efficient FW interaction. By contrast, a positively tapering magnetic field reduces the detuning frequency of BW interaction and extends its effective length. By controlling the detuning frequency, a scenario of suppressed mode competition is proposed in a 330-GHz second-harmonic gyrotron. A universal understanding of the influence of an inhomogeneous magnetic field—i.e., the detuning frequency—on the interaction dynamics would help to develop efficient and broadband tunable terahertz gyrotrons.

Multi-Wavelength SOA Fiber Laser With Ultra-Narrow Wavelength Spacing Based on NPR Effect

A multi-wavelength semiconductor optical amplifier (SOA) fiber laser with ultra-narrow wavelength spacing is experimentally demonstrated in this paper. The SOA is used as the primary gain medium due to its inhomogeneous broadening and nonlinear polarization rotation (NPR) effect that allows to achieve stable multi-wavelength output with ultra-narrow wavelength spacing. The combination of PC (polarization controller)-SOA-PC-polarizer and nonlinear optical loop mirror (NOLM) serve as the intensity equalizer for inducing IDL to suppress mode competition effect furtherly, while the NOLM mainly achieves power equalization and improve SMSR in this laser. The Lyot-Sagnac loop mirror is designed to realize different channel spacing, when the length of PMF is 71 m, stable multi-wavelength output has been achieved at room temperature. The number of lasing lines is up to 184 with wavelength spacing of 0.08 nm within 10 dB bandwidth from 1549.75 nm to 1664.25 nm, and all peaks experience some minor power fluctuation, and the maximum value is about 5 dB. The side mode suppression ratio (SMSR) is about up to 22 dB. Moreover, the maximum value of optical signal-to-noise ratios (OSNR) is 10.7 dB.

Multi- and dual-wavelength Thulium-doped fluoride fiber laser assisted by four-wave mixing in S-band region

In this work, multi-wavelength lasing in the S-band region using a Sagnac loop filter in a thulium-doped fluoride fiber laser (TDFFL) was demonstrated for the first time. The Sagnac loop served as a comb-like wavelength filter, while a 10-km long dispersion compensating fiber (DCF) was utilized to induce the four-wave mixing (FWM) effect in the cavity. The FWM effect was used to suppress mode competition in the TDFFL as well as to enhance the number of lasing lines. At its optimum polarization state, 17 stable lasing lines at peak powers of less than 10 dB from the highest peak power were observed, with these lasing lines having a uniform wavelength spacing of 0.23 nm. The multi-wavelength operation exhibited minimal wavelength drifts and power fluctuations of less than 0.01 nm and 1.2 dB respectively. A tunable dual-wavelength laser output was also demonstrated, with a constant wavelength spacing throughout the tuning range.

Study of the beam–wave interaction of a 220 GHz coaxial-cavity gyrotron with two electron beams

In this paper, the mode selection, ohmic loss quality factor, mode competition, and beam–wave interaction of a 220 GHz coaxial-cavity gyrotron with two electron beams are studied by using the linear and nonlinear theories of a gyrotron. Research shows that a coaxial-cavity gyrotron with two electron beams has more advantages than a coaxial-cavity gyrotron with one electron beam in mode selection, suppressing mode competition and increasing output power, and the eigenvalue and the ohmic loss quality factor of modes have similar monotonicity in the direction of the gyrotron axis. Results find that (1) the current ratio K of the gyrotron has a greater influence on the beam–wave interaction efficiency of the two electron beams, (2) under B = 8.90 T, I = 136 A, α = 1.40, and K = 0.45, the coaxial-cavity gyrotron with two electron beams can operate steadily in the TE51,32− mode, which has an efficiency of 37.67% and an output power of 4.53 MW at f = 220.04 GHz, and (3) the total ohmic loss power of the gyrotron accounts for 1.3% of the output power.

Multi-wavelength fiber laser based on highly erbium-doped fiber and nonlinear effects in SMF

In this paper, a kind of multi-wavelength fiber laser based on highly erbium-doped fiber (HEDF) is introduced. A new type of HEDF is used as the gain medium and four-wave Mixing (FWM) in single mode fiber(SMF) is used to suppress mode competition of the laser. And then, a nonlinear optical fiber loop mirror(NOLM) is added to ring cavity to optimize the experimental structure. The flatness of the output spectrum and side mode suppression ratio(SMSR) are studied. At last, the stability of the experimental system is analyzed by taking two peak light waves near the center wavelength as an example.

Design and Simulation of a Multi-Sheet Beam Terahertz Radiation Source Based on Carbon-Nanotube Cold Cathode.

Carbon nanotube (CNT) cold cathodes are proving to be compelling candidates for miniaturized terahertz (THz) vacuum electronic devices (VEDs) owning to their superior field-emission (FE) characteristics. Here, we report on the development of a multi-sheet beam CNT cold cathode electron optical system with concurrently high beam current and high current density. The microscopic FE characteristics of the CNT film emitter is captured through the development of an empirically derived macroscopic simulation model which is used to provide representative emission performance. Through parametrically optimized macroscale simulations, a five-sheet-beam triode electron gun has been designed, and has been shown to emit up to 95 mA at 3.2 kV. Through careful engineering of the electron gun geometric parameters, a low-voltage compact THz radiation source operating in high-order mode is investigated to improve output power and suppress mode competition. Particle in cell (PIC) simulations show the average output power is 33 W at 0.1 THz, and the beam–wave interaction efficiency is approximately 10%.

Theoretical Study of a ${W}$ -Band-Covering Frequency-Tunable Gyrotron

The gyrotron has already demonstrated the capability of generating high-power coherent electromagnetic (EM) radiation at high frequencies and finds application in fusion plasma heating and magnetic resonance spectroscopy. In this article, we propose a ${W}$ -band gyrotron, which uses a so-called multimode switching scheme to realize ultrabroadband frequency tuning capability, nearly covering about 70% of the ${W}$ -band (75–110 GHz) range. The tuning strategy used to suppress mode competition and stabilize gun parameters in such an open-cavity multimode switching gyrotron is presented. Theoretical study shows that the multimode switching gyrotron can generate a frequency tuning range much wider than a conventional step-tuning gyrotron or a single-mode tuning gyrotron. In addition, another technology that uses a slot-assisted circuit to select only axis-symmetrical TE operating modes is presented. Using such a circuit, a tuning range of about 25 GHz in the ${W}$ -band is obtainable. The proposed multimode switching gyrotron is a promising ultrabroadband source for millimeter-wave and terahertz-wave applications.

A high-efficiency dual-band relativistic Cerenkov oscillator based on dual electron beams

High efficiency and high frequency are key research topics for the development of high power microwave devices. There are, however, serious issues with mode competition in relativistic Cerenkov devices based on a single electron beam, which can lead to low efficiency in the beam-wave interaction process. In this paper, a high-efficiency relativistic Cerenkov oscillator based on a dual electron beam is proposed. The hollow slow wave structure is embedded in the inner conductor of the coaxial waveguide to obtain a double coaxial electromagnetic structure, which can suppress mode competition and improve the beam-wave interaction efficiency of the dual-band radiation. Moreover, each band of radiation is independent of the other, such that high frequency output radiation is generated. Using the particle-in-cell method, the physical processes involving the beam-wave interaction in the oscillator are studied with the aim of designing a high-efficiency double-electromagnetic structure. Simulation results indicated that the output power levels for the X-band and Ka-band were 1.50 GW and 710 MW, respectively, and the efficiencies were 34.7% and 30.7%, respectively, for a diode voltage of 550 kV and a guiding magnetic field of 0.85 T.

结构紧凑的双波长连续波掺铒光纤激光器

Multiwavelength erbium-doped fiber lasers can be applied to many fields, such as wavelength division multiplexing optical communication and others attracting considerable attention. To meet the requirements of different applications, we propose a compact dual-wavelength continuous-wave erbium-doped fiber laser based on nonlinear amplifying loop mirror. An all-polarization-maintaining fiber cavity is adopted in which there are only three intracavity devices besides fiber itself:a wavelength division multiplexer, a 2×2 fiber coupler and a fiber reflector. The intensity-dependent loss effect induced by the nonlinear amplifying loop mirror is used to equalize the intensity in the cavity. The input laser with higher power suffers a higher loss than the one with lower power. This feature can be used to suppress mode competition and achieve stable multiwavelength oscillation. With a pump power of 260 mW, dual-wavelength erbium-doped fiber laser can be achieved, with wavelengths of 1 560.5 nm and 1 563.2 nm, respectively. The side-mode suppression ratio is 46.8 dB. As pump power increases, the laser can operate in single-, dual-and triple-wavelength regions in proper order. As the multi-wavelength lasing oscillation is a balance between intensity-dependent loss and mode competition, the intensity change breaks the original balance state, resulting in the change in number and wavelength of lasing lines. This laser is simply structured and easy to operate. It can have applications in many fields.

Switchable Single-Longitudinal-Mode Fiber Laser Based on $\theta$ -Shaped Microfiber Filter

In this letter, we design a wavelength-switchable single-longitudinal-mode (SLM) fiber laser based on a reflective $\theta $ -shaped microfiber resonator ( $\theta $ -MR). By cascading the $\theta $ -MR with a fiber Fabry–Perot interferometer, Vernier effect can be generated for SLM filtering. Due to the reflective nature of the $\theta $ -MR, a section of unpumped erbium-doped fiber can be directly inserted in front of the $\theta $ -MR to further suppress-mode competition. A SLM lasing spectrum with central wavelength of 1564.685nm, and sidemode suppression ratio as high as 56.6 dB is achieved. Stability test shows that the proposed fiber laser has a good long-term stability. Additionally, the lasing wavelength of the proposed laser is switched to 1564.690nm, 1559.392nm, and 1558.99nm when temperature around the $\theta $ -MR is set as 30°C, 40°C, and 50°C, respectively, verifying the wavelength switch ability of the $\theta $ -MR-based SLM fiber laser.

Switchable dual-wavelength SLM narrow linewidth fiber laser based on nonlinear amplifying loop mirror

We demonstrate a stable switchable dual-wavelength single longitudinal mode (SLM) narrow linewidth ytterbium-doped fiber (YDF) laser using a nonlinear amplifying fiber loop mirror (NALM) at 1064nm. The NALM of intensity-dependent transmission acts as a saturable absorber filter and an amplitude equalizer to suppress mode competition and the fiber Bragg grating (FBG) pair is used as one wavelength selection component. By properly adjusting the polarization controllers (PCs), the switchable dual-wavelength SLM fiber laser can be operated steadily at room temperature. The optical signal-to-noise ratio (OSNR) is better than 50dB for both lasing wavelengths. Meanwhile, the linewidth of the fiber laser for each wavelength is approximate 17.07kHz and 18.64kHz with a 20dB linewidth, which means the laser linewidth is approximate 853Hz and 932Hz FWHM. Correspondingly, the measured relative intensity noise (RIN) is less than −120dB/Hz at frequencies over 5.0MHz.

Fabrication and Characterization of 2 × 2 Microfiber Coupler for Generating Two Output Stable Multiwavelength Fiber Lasers

A compact 2 × 2 microfiber coupler (MC) is proposed and experimentally demonstrated for the generation of stable multiwavelength fiber laser in the C-band region. The 2 × 2 MC is fabricated by twisting and fusing together two lateral dual single mode fibers using the flame-brushing technique. The 2 × 2 MC has a waist diameter of 8 μ m with a coupling ratio of 38:62 and 45:55 at ports 1 and 2, respectively. The 2 × 2 MC exploits super mode coupling to produce spatial mode beating and suppress mode competition of the homogeneous gain medium in a fiber ring laser. The channel spacing of the output wavelengths is immune to changes in the external environment and can be extracted from the peaks of the 2 × 2 MC's periodic oscillation. The proposed laser is capable of generating stable single, dual, and triple lasing wavelengths from its two output ports over a range of more than 29 nm, which is further enhanced by the polarization rotation mechanism. The generating lasing wavelengths have a 3 dB line-width of less than ∼30 pm and peak-to-floor ration of about 58 dB at a pump power of 38 mW.

A highly stable and switchable dual-wavelength laser using coupled microfiber Mach-Zehnder interferometer as an optical filter

The generation and switching of dual-wavelength laser based on compact coupled microfiber Mach-Zehnder interferometer (CM-MZI) is reported. The CM-MZI is constructed by overlapping two portions of a single tapered optical fiber which has a diameter of 9μm as to create multi-mode interference and also to produce spatial mode beating as to suppress mode competition in the homogeneous gain medium. The system is able to generate a dual-wavelength laser output that can be switched with the aid of the polarization rotation technique. Four dual-wavelength oscillation pairs are obtained from the interference fringe peaks of the CM-MZI comb filter with a switched channel spacing of 1.5nm, 3.0nm, and 6.0nm. The wavelength spacing is stable at different pump powers. The lasing wavelength has a 3-dB linewidth of about 30pm and peak-to-floor ration of about 55dB at a pump power of 38mW.