Single Longitudinal Mode Laser Research Articles

Switchable Dual-Wavelength Fiber Laser with Narrow-Linewidth Output Based on Parity-Time Symmetry System and the Cascaded FBG

In this paper, a dual-wavelength narrow-linewidth fiber laser based on parity-time (PT) symmetry theory is proposed and experimentally demonstrated. The PT-symmetric filter system consists of two optical couplers (OCs), four polarization controllers (PCs), a polarization beam splitter (PBS), and cascaded fiber Bragg gratings (FBGs), enabling stable switchable dual-wavelength output and single longitudinal-mode (SLM) operation. The realization of single-frequency oscillation requires precise tuning of the PCs to match gain, loss, and coupling coefficients to ensure that the PT-broken phase occurs. During single-wavelength operation at 1548.71 nm (λ1) over a 60-min period, power and wavelength fluctuations were observed to be 0.94 dB and 0.01 nm, respectively, while for the other wavelength at 1550.91 nm (λ2), fluctuations were measured at 0.76 dB and 0.01 nm. The linewidths of each wavelength were 1.01 kHz and 0.89 kHz, with a relative intensity noise (RIN) lower than −117 dB/Hz. Under dual-wavelength operation, the maximum wavelength fluctuations for λ1 and λ2 were 0.03 nm and 0.01 nm, respectively, with maximum power fluctuations of 3.23 dB and 2.38 dB. The SLM laser source is suitable for applications in long-distance fiber-optic sensing and coherent LiDAR detection.

Continuous-wave operation of InGaN tunable single-mode laser with periodically slotted structure

By introducing wavelength tunability, InGaN single longitudinal mode lasers can be used for pumping wavelength conversion devices with a small pump wavelength tolerance. A 405 nm InGaN tunable slotted laser was designed and fabricated by a simple process without high-resolution lithography and epitaxial regrowth. Continuous-wave single-mode oscillation was obtained. By current injection to active and slotted channels separately, a wavelength tuning range of 0.55 nm was successfully demonstrated for the first time in InGaN in-plane single-mode lasers.

High OSNR single-longitudinal-mode thulium-doped fiber laser based on a cascaded dual-ring cavity filter

A high optical-signal-to-noise ratio (OSNR) single-longitudinal-mode (SLM) thulium-doped fiber laser (TDFL) is proposed and has been investigated. SLM lasing was realized by utilizing a uniform fiber Bragg grating (UFBG) and a cascaded dual-ring cavity (CDRC) filter. The design, simulation, and characteristic analysis of three compound-cavity filters based on the Vernier effect are presented in detail, and provided the basis for experiments. The single-wavelength SLM lasing with a high OSNR of ∼75.10 dB, operating at 2048.44 nm was pumped by a 793 nm laser diode, and the SLM lasing was continuously monitored for 60 minutes. The peak power fluctuation and wavelength drift were less than 0.685 dB and 0.01 nm, respectively, over this time span. During the test, the relative intensity noise was below −128.49 dB/Hz above 1 MHz and the linewidth was 11.82 kHz with an integration time of 0.001 s.

All-solid-state 3-µm single-frequency Er:CaF2 laser with a stable and switchable wavelength.

We have demonstrated a 3-µm all-solid-state single-frequency laser with a stable center frequency and a switchable wavelength using the intra-cavity Fabry-Perot etalon method. Experimentally, the central wavelengths of the laser for the single-longitudinal mode are 2728 and 2794 nm, with maximum output powers of 268 and 440 mW, respectively. The corresponding single-longitudinal mode linewidths are 25 and 11 MHz. In particular, the central wavelengths of the single-longitudinal mode laser remain almost constant as the incident pump power increases. To the best of our knowledge, this study represents the first instance of using a laser diode to directly pump Er:CaF2 block single crystals for single-frequency lasers in the 3 µm region. Additionally, it achieves the highest output power of a 3-µm all-solid-state single-longitudinal mode.

Implementation and simulation of direct Doppler wind measurement technology under regime of multi-longitudinal mode laser

<sec>Single-longitudinal-mode (SLM) direct Doppler wind lidar (DDWL) requires complex techniques of the seed injection, high precision frequency stability, and frequency locking to provide an output of the stable frequency SLM laser, resulting in a complex structure of DDWL. To reduce the technical difficulty and structural complexity of the excitation light source of DDWL, a multi-longitudinal mode (MLM) DDWL is proposed. In the MLM DDWL, a free-running MLM laser is directly used as an excitation light source and quadri-channel Mach-Zender interferometer (QMZI) with four periodic outputs is adopted as a spectral discriminator.</sec><sec>Firstly, for the typical Nd:YAG pulsed laser, the scattering spectra of atmospheric elastic echo excited by the MLM laser are analyzed which are coincident with the longitudinal modes of the MLM laser. The peaks of atmospheric elastic echo scattering spectra excited by the MLM laser overlap with each other. The overlaping degree is affected by the laser radiation linewidth, laser optical resonator length, laser center wavelength, and type of scattering particles. In addition, the scattering spectra of atmospheric elastic echo excited by each longitudinal mode of the MLM laser have the Doppler frequency shift introduced by atmospheric wind. Therefore, it is necessary to select an optical interferometer with the periodic transmittance curve as the spectral discriminator of MLM DDWL.</sec><sec>Subsequently, a QMZI is designed as the spectral discriminator to achieve high-precision measurement for the Doppler frequency shift of scattering spectra of atmospheric elastic echo excited by the MLM laser. The designed QMZI has four periodic output channels and the phase difference between adjacent channels is π/2. The mathematical model of the transmittance function of the QMZI is established. The effective transmittance of the QMZI for atmospheric elastic echo scattering spectrum excited by the MLM laser is analyzed based on the partial coherence theory of quasi-monochromatic light interference and the polarization effect of light. On this basis, the data inversion algorithm of MLM DDWL is constructed.</sec><sec>Finally, the simulation experiments of wind measurement are carried out. The QMZI simulation model is built by the non-sequential mode of Zemax optical simulation software. The atmospheric elastic echo scattering spectra excited by the MLM laser are configured by the SPCD files of Zemax optical simulation software under different theoretical wind speeds ranging from -50 to 50 m/s, laser optical resonator lengths (<i>L</i> = 30 mm and 300 mm), and laser center wavelengths (<i>λ</i> = 1064 nm, 532 nm, and355 nm). The SPCD files are fed to the QMZI simulation model as input signals. At the same time, the ray tracing on input signal is performed based on the principle of Monte Carlo simulation s, and the output signals of the four channels of the QMZI simulation model are recorded to retrieve the atmospheric wind information. The simulation results show that the proposed MLM DDWL can achieve high-precision measurement of atmospheric wind information. With the laser optical resonator length of 300 mm and laser center wavelengths <i>λ</i> = 1064 nm, <i>λ</i> = 532 nm, <i>λ</i> = 355 nm, the maximum detectable wind speeds of MLM DDWL are about 50 m/s, 30 m/s, and 20 m/s, and the wind measurement errors can be controlled within 2.5 m/s, 3.0 m/s, and 4.0 m/s, respectively. When the center wavelength of each laser is 532 nm, and the lengths of laser optical resonator are 30 mm and 300 mm, then the maximum detectable wind speeds of MLM DDWL are about 50 m/s and 30 m/s, and the wind measurement errors can be controlled within 2.0 m/s and 3.0 m/s, respectively. Therefore, the longer the laser center wavelength and the shorter the laser optical resonator length, the larger the wind measurement range will be and the smaller the wind measurement error.</sec>

Trapezoidal plane-mirror cavity for a single-longitudinal-mode Nd:YAG laser

Diode-laser-pumped all-solid-state lasers are widely used for their high efficiency and excellent monochromaticity. However, existing figure-8 ring cavity single-longitudinal-mode (SLM) lasers, which use two concave cavity mirrors, produce elliptical output spots. Here, we present an SLM Nd:YAG laser with a trapezoidal cavity and planar mirrors, resulting in a circular output spot. Two of the cavity mirrors are Brewster thin-film polarizers (BTFPs), which along with a Faraday rotator and half-wave plate ensure unidirectional operation of an s-polarized intracavity beam. Our design achieves two outputs at 1064 nm: 945-mW s-polarized light from the output mirror and 652-mW p-polarized light from the BTFP cavity mirror. This design has advantages for the manufacturing and downsizing of high-power SLM lasers with isotropic gain media.

2 µm band four-wavelength-switchable narrow linewidth fiber laser enabled by fs-laser direct-written polarization-dependent parallel fiber Bragg gratings

We propose and experimentally demonstrate a four-wavelength-switchable single-longitudinal-mode (SLM) narrow linewidth thulium–holmium co-doped fiber laser (THDFL) using two polarization-dependent parallel fiber Bragg gratings (PD-PFBGs). The PD-PFBGs, fabricated using femtosecond (fs) laser direct-writing technology in a standard single-mode fiber (SMF) via a point-by-point method, are used as a four-channel reflection filter. Two FBGs are inscribed in parallel in the fiber core along the axial direction and are uniquely positioned symmetrically on either side of the centerline. This configuration enables polarization-dependent multi-channel filtering capability, which further allows for polarization-control-based four-wavelength-switchable operations of the THDFL. SLM lasing is accomplished by utilizing a simple dual-ring sub-cavity filter. An exceptional output performance of the THDFL is achieved, including an optical signal-to-noise ratio (SNR) of >72 dB, maximum power and wavelength fluctuations of 0.350 dB and 0.024 nm, respectively, and a linewidth of <2 kHz, for all four single-wavelength operations lasing at ∼2000 nm. These performance indicators suggest that the THDFL can be applied in free-space optical communication, atmospheric monitoring, and Lidar.

Single-longitudinal-mode lasing based on enhanced scattering in eccentric-hole microstructured optical fiber resonators

In this paper, we propose a dual-core eccentric-hole fiber (DCEHF) microresonator to achieve single-longitudinal-mode (SLM) laser emission based on enhanced pump light field induced by scattering effect of the two fiber cores. The directional dependences of the scattered pump field distribution on incident angle as well as overlap integrals between scattered pump light fields and WGMs have been theoretical investigated, and the calculation results show that when the incident angle of the pump light is close to 180°, significant discrepancies would occur in the overlap integrals between WGMs and the scattered field. And consequently, specific mode having the largest overlap integral would outperform through the mode competition process to excite SLM lasing. We have experimentally obtained SLM lasing emission when the DCEHF-based microresonator is rotated by 206°, which is in good accordance with our calculation results. Additionally, experimental observation of the output laser spectrum shows that the proposed fiber microcavity supports highly stable SLM laser emission. The proposed microresonator based on DCEHFs possesses several desirable merits such as compact size, good laser stability, single-mode operation, and good compatibility with existent fiber-optic systems, which make it a good candidate for applications as functional photonic device in future micro-optics systems.

Switchable and tunable orthogonal single-polarization dual wavelength SLM fiber laser based on parity-time symmetric system and PM-FBG

A dual-wavelength narrow linewidth orthogonal single polarization fiber laser based on a parity-time (PT) symmetric system is proposed and demonstrated, which achieves the function of tunable and switchable wavelength, and single longitudinal mode (SLM) operation. The PT symmetric system composes of a single physical loop, which includes two polarization controllers (PCs) and a polarization beam splitter (PBS). By modulating PCs to match gain, loss and coupling coefficient, the switchable wavelength and stable SLM laser output have been achieved. Through stretching the polarization maintaining fiber Bragg grating (PM-FBG), the wavelength adjustment range from 1549 nm to 1551 nm is obtained. In the process of tuning, the full width at half maxima (FWHM) linewidths and polarization states at each wavelength have been measured and keep basically stable. The measured FWHM linewidths are less than 13.66 kHz and 11.89 kHz. Further, the stable orthogonal single polarization state has been realized. The degree of polarization (DOP) at each wavelength exceeds 97% and basically remains unchanged during 20 min.

LD-pumped all-solid-state single-longitudinal-mode Pr3+:LiYF4 laser at 915 nm

We report an InGaN laser diode (LD) pumped 915 nm single-longitudinal-mode (SLM) Pr3+:LiYF4 (Pr:YLF) laser for the first time. The all-solid-state 915 nm SLM laser is achieved by inserting two pieces of Fabry–Perot (F-P) etalons with different thicknesses in the resonant cavity. The maximum power of the SLM laser is ∼ 115 mW with the narrowest spectral linewidth of ∼ 110 MHz and the beam quality Mx2 and My2 factors are measured to be 1.16 (x) and 1.10 (y), respectively. In addition, we perform a theoretical simulation of combined F-P etalons transmittance. The experimental result is well-matched with the theoretical simulation. The first achievement of the 915 nm SLM Pr:YLF laser further expands the near-infrared laser applications.

A 0.45 pm of linewidth, single-longitudinal-mode Tm:YLF laser based on reflecting Bragg gratings and Fabry-Perot etalons

Based on the reflecting Bragg gratings (RBG) combined with the Fabry-Perot (F–P) etalon mode selection technique, a diode-pumped single-longitudinal-mode (SLM) Tm:YLF laser output has been obtained. The optimal insertion angle of double F–P etalons is simulated and analyzed theoretically. Fine-tuning the insertion angle of the F–P etalon to match the central wavelength of the RBG. The out wavelength of the Tm:YLF laser is locked in 1907.8 nm. Finally, under the condition of a resonator length of 55 mm and a pump power of 10.15W, an SLM laser output with an output power of 250 mW and an output linewidth of 37.5 MHz (about 0.45pm) is obtained. The power instability is less than 2% within 300s.

Diode-Pumped Single-Longitudinal-Mode Pr3+:YLF Laser Based on Combined Fabry–Perot Etalons at 522.67 nm

We create a rate equation theoretical model of a continuous-wave end-pumped Pr3+:YLF SLM laser that characterizes the output properties of a single-longitudinal-mode (SLM) green laser. After inserting two Fabry–Perot (F–P) etalons with thicknesses of 0.3 mm and 0.5 mm and angles of 1.42° and 0.69° into the cavity, a single-longitudinal-mode green laser was generated. The maximum output power in single-longitudinal mode was 183 mW. The maximum absorbed pump power was 6.2 W. The corresponding linewidth is about 18 MHz. This work presents a simple method for generating a single-longitudinal-mode laser in the green spectral region, providing a practical approach for various green-laser-related applications.

Narrow linewidth fiber laser with multiple unpumped EDF fiber loops and self-injection feedback

In this paper, we proposed a narrow linewidth single-longitudinal-mode (SLM) fiber laser, which was realized by employing a triple-cascaded unpumped erbium doped fiber (EDF) Sagnac loops structure with Rayleigh backscattering (RBS) feedback. Narrow linewidth SLM laser output can be achieved by using the unpumped EDF loops. However, it is generally difficult to achieve laser emission with a linewidth below 1 kHz and a power of tens of milliwatts. In this paper, we propose a fiber laser with multiple unpumped Er-doped fiber loops and self-injection feedback. The cascade of multi-unpumped Er-doped fiber rings is employed to reduce the linewidth of the laser, while the self-injection feedback based on RBS is used to improve the stability of the laser and further compress the laser linewidth. By only utilizing three unpumped Er-doped fiber rings cascaded, we obtain an output beam with a linewidth of 723.05 Hz and laser power of 21.52 mW with 0.30 dB fluctuation during a period time of 30 min at the pump power of 170.6 mW. After introducing the feedback mechanism based on RBS into the fiber laser, an output laser with a linewidth of 530.11 Hz and a laser power of 8.57 mW with 0.23 dB fluctuation in 30 min can be obtained at the pump power of 268.68 mW. Compared with the fiber laser with one unpumped Er-doped fiber ring, the linewidth is reduced more than two times.

UV generation via periodically poled MgO:LiTaO3 circular waveguide crystal and diode-based master oscillator power amplifier.

Lasers with emission wavelengths in the near-ultraviolet (UV) spectral range have been used in many applications across various fields, and the demand for these lasers has been on the rise. For example, in medicine, near-UV light has been used for fluorophore excitation. Although laser diodes emitting in this region exist, single longitudinal mode lasers emitting at 380nm with high optical power are limited. One of the solutions to this problem is the use of second harmonic generation by a non-linear crystal. In this work, single-longitudinal-mode laser emission at 380.5nm with an optical power of up to 13mW has been achieved. The emission was realized by frequency doubling using a periodically poled circular waveguide crystal of stoichiometric L i T a O 3 doped with MgO (PPMgSLT) pumped by a master oscillator power amplifier with optical power up to 5W. A distributed Bragg reflector ridge waveguide laser diode at 761nm was used as the master oscillator and a tapered amplifier as the power amplifier.

Wavelength-Tunable Single-Longitudinal-Mode Narrow-Linewidth Thulium/Holmium Co-Doped Fiber Laser with Phase-Shifted Fiber Bragg Grating and Dual-Coupler-Ring Filter

A wavelength-tunable single-longitudinal-mode (SLM) narrow-linewidth thulium/holmium co-doped fiber laser (THDFL) was developed in this study. The lasing wavelength was determined by combining a phase-shifted fiber Bragg grating (PS-FBG) and a uniform FBG (UFBG). SLM oscillation was achieved by incorporating a dual-coupler ring filter with the PS-FBG. At a pump power of 2.0 W, the THDFL exhibited excellent SLM lasing performance with a stable optical spectrum. It operated at an output wavelength of ~2050 nm with an optical signal-to-noise ratio of >81 dB, an output power fluctuation of 0.15 dB, a linewidth of 8.468 kHz, a relative intensity noise of ≤−140.32 dB/Hz@≥5 MHz, a slope efficiency of 2.15%, and a threshold power of 436 mW. The lasing wavelength tunability was validated experimentally by stretching the PS-FBG and UFBG simultaneously. The proposed THDFL had significant potential for application in many fields, including free-space optical communication, LiDAR, and high-precision spectral measurement.

Polarimetric parity-time symmetry fiber laser with switchable and tunable orthogonal single-polarization narrow-linewidth output

Parity-time (PT) symmetry has garnered significant attention in the field of photonics owing to its remarkable properties. High-performance lasers, which require simultaneous gain and loss, serve as an ideal platform for harnessing the exceptional properties of PT symmetry. In this study, we present and experimentally verify a polarimetric PT-symmetry fiber laser that uses a polarization-maintaining fiber Bragg grating (PM-FBG). This design allows for tunable, switchable dual-wavelength operation with narrow linewidth and orthogonal single-polarization characteristics. By converting two equal-length yet distinct polarization states into two spatial subspaces sharing a common wavelength space, we can achieve PT symmetry. Regulating the polarization controllers (PCs) enables interconversion between PT-symmetry phases. During the PT-symmetric phase (or PT exact phase), a dual-wavelength laser is realized with wavelength fluctuations below 0.098 nm and power fluctuations below 2.175 dB. At this stage, light propagates through both ordinary and extraordinary wavelength-spaces. During the PT-broken phase, one waveguide has a net gain and the other incurs a net loss, leading to a single longitudinal mode (SLM) laser output. The wavelength and power fluctuations of left-wavelength λ1 and right-wavelength λ2 remain below 0.002 nm and 0.770 dB over a 20-minute period, during single-wavelength operation. By adjusting the axial strain on the PM-FBG, the laser output can be tuned within the wavelength range of 1549.754 nm to 1551.690 nm, while maintaining a linewidth variation of less than 0.130 kHz. As the wavelength is adjusted, the azimuth sum of λ1 and λ2 consistently remains close to 90 degrees, accompanied by degrees of polarizations exceeding 99%.

Tunable single-longitudinal-mode narrow linewidth Brillouin fiber laser based on PT symmetry

A tunable single-longitudinal-mode (SLM) narrow linewidth Brillouin fiber laser based on parity-time (PT) symmetry is proposed and experimentally demonstrated. In the proposed scheme, the dual selecting-mode mechanism is employed to achieve the SLM operation. The first, a narrow-band gain curve induced by stimulated Brillouin scattering (SBS) can be acted as a filter for selecting mode preliminary in the fiber ring cavity. The second, a PT-symmetric structure is formed by using a Sagnac loop, in which by adjusting a single polarization controller, the PT symmetry can be broken, resulting in the SLM oscillation. The SBS effect can be further beneficial to the linewidth narrowing. Using the proposed scheme, the SLM lasing with an optical signal to noise ratio of 50.1 dB and an ultra-narrow linewidth of 300 Hz at 1547.90 nm is obtained. Compared with the original linewidth of pump laser, the linewidth-reduction ratio can be up to 680 times. The available wavelength ranges from 1528.4 to 1563.4 nm, covering 35 nm. The key advantage of the proposed laser is that by constructing a simple Sagnac loop, the SLM selection based on PT symmetry and linewidth narrowing benefiting from SBS effect are accomplished simultaneously. Moreover, it is worth noting that the wavelength tunability is virtually unrestricted, only depending on the pump wavelength.

Convolution Error Reduction for a Fabry–Pérot-Based Linewidth Measurement: A Theoretical and Experimental Study

Linewidth measurement of a short pulse single-longitudinal mode laser with a low repetition rate has been a big challenge. Although the Fabry–Pérot (FP) etalon in combination with a beam profiler is an effective approach to measure the linewidth, the convolution error introduced by the inherent transmission spectrum width of an FP restricts the measurement accuracy. Here, the source of convolutional errors of the FP etalon-based linewidth measurement is analyzed, and the convolutional fitting method is proposed to reduce the errors. The results show that the linewidth measurement using the FP cavity with low reflectance (95%) can achieve the same resolution as that with high reflectance (99.5%) based on this convolution error reduction method. The study provides a simple approach to accurately measuring the linewidth of pulsed lasers, even with low energy.

Locked-energy at first-order Stokes stimulated Raman scattering for an aqueous sulfuric acid solution

We introduced a locked-energy coherent Stokes radiation scheme based on multi-mode stimulated Raman scattering (SRS) of aqueous sulfuric acid solutions. SRS of pure sulfuric acid has only one vibrational characteristic peak. Mixing sulfuric acid with water leads to the appearance of bisulfate, and the energy transfer between adjacent vibrational energy levels in a 17.0 mol/l aqueous sulfuric acid solution leads to generation of five different vibrational modes in a solution. The competition between these five SRS modes prevents the vibrational energy from being transferred to high-order Stokes waves, thereby locking most of the energy in the first-order Stokes SRS and outputting a very strong, broadband, and coherent Raman radiation ranging from 555 to 570 nm. This study demonstrates the possibility of producing an intense single longitudinal mode laser.

Microwave photonic filter with a sub-kHz bandwidth based on a double ring Brillouin fiber laser

A scheme to achieve a sub-kHz bandwidth microwave photonic filter (MPF) based on a double-ring Brillouin fiber laser (DR-BFL) is proposed and experimentally verified in this study. The stimulated Brillouin scattering gain spectrum can be efficiently narrowed by the DR-BFL, which comprises a main-ring cavity with a 100-m single-mode fiber (SMF) and a sub-ring cavity with a 10-m SMF. A single passband narrow bandwidth MPF is accomplished by the DR-BFL with a single longitudinal mode laser output using the Vernier effect. The bandwidth of the MPF at the center frequency of 10.735 GHz is 114 Hz, and the corresponding Q-factor is 9.42 × 107, which is the highest Q value to the best of our knowledge. Since the measured linewidth of the DR-BFL is 1.2 Hz at the −20-dB power point, the theoretical bandwidth of the DR-BFL-based MPF is 0.16 Hz. This design provides a new idea for realizing high-precision signal extraction in future communication or sensing systems.