Single Longitudinal Mode Research Articles

Single-longitudinal-mode self-seeded Brillouin fiber laser with an ultra-narrow linewidth achieved utilizing a triple-ring resonator

What we believe to be a novel single longitudinal mode (SLM) triple-ring (T-R) self-seeded Brillouin fiber laser (BFL) featuring outstanding stability, an ultra-narrow linewidth, and a high optical signal-to-noise ratio (OSNR) is proposed and experimentally investigated in this study. The key innovation in this design is eliminating the need for an additional costly ultra-narrow linewidth pump source typically required in conventional BFL. Instead, the laser preferentially excites stimulated Brillouin scattering (SBS) in the fiber through the self-seeded cavity mode dominant in the cavity. This approach generates Brillouin stokes and leverages the Vernier effect of the T-R resonator structure to suppress multimode oscillations, ensuring the generation of a Brillouin laser with an ultra-narrow linewidth and enabling SLM operation. The experimental results show that with the output power of 980-nm LD fixed at 400 mw, the OSNR of the self-seeded BFL spectrum reaches 54 dB, and the side mode suppression ratio (SMSR) is 22 dB. The -3 dB linewidth of the self-seeded BFL can be measured by the heterodyne beat frequency method at 30 Hz, and the output power and wavelength fluctuations are lower than 1.318 dB and ±0.007 nm, respectively, during the sixty minutes observation period. Additionally, the wavelength of the self-seeded BFL can be flexibly tuned within the range of 1560 – 1575 nm. This innovative approach demonstrates significant theoretical and practical implications for the development of low-cost, high-performance BFL systems compared to traditional BFL methods.

Wavelength-Switchable 2 μm Single-Longitudinal-Mode Thulium-Doped Fiber Laser Based on Dual-Active Cavity and DLTCTR

A thulium-doped fiber laser (TDFL) with a dual-active cavity and a directly linked three-coupler triple-ring filter is designed and demonstrated. Its operational principle is analyzed, and a corresponding experimental setup is built. Eleven single-wavelength laser outputs with a single-longitudinal-mode (SLM) output near 2 μm are obtained. The laser output covers a wavelength range from 1933.95 nm to 1971.76 nm, with a continuous switchable output range of 37.81 nm and a minimum center wavelength interval of 0.22 nm. The optical signal-to-noise ratio (OSNR) of the output laser within the tuning range is >48.53 dB, and its maximum OSNR is 70.24 dB. The minimum wavelength fluctuation is 0.03 nm, and the power fluctuation is between 0.15 and 2.61 dB. A single wavelength with a center wavelength of 1933.95 nm is monitored for 75 min, and the radio-frequency spectrum is scanned 27 times within the frequency range of 0 to 400 MHz. The results demonstrate that the TDFL can operate continuously and stably in an SLM state. The linewidth and linewidth fluctuation of the TDFL are measured, and the minimum linewidth, corresponding to a measurement time of 0.001 s, is 65.14 kHz. The experimental results show that the proposed TDFL has a high OSNR and excellent wavelength-switching ability, and its SLM operation is very stable.

High average power nanosecond pulsed single longitudinal mode diamond Raman laser in the 1.6 µm waveband

High average power nanosecond pulsed single longitudinal mode diamond Raman laser in the 1.6 µm waveband

Spatially controllable stimulate Brillouin scattering in the diamond Raman laser

Diamond Raman lasers offer a significant advantage over conventional inversion lasers in achieving high-power single-longitudinal mode (SLM) output, making them highly suitable for various potential applications. However, the presence of stimulated Brillouin scattering (SBS) introduces instability and degrades the SLM mode into a multimode longitudinal mode (MLM) as power scaling increases. In this research article, we present an analytical model that elucidates the transverse-mode behavior of the SBS field within the resonator to explore the mechanism of SBS inhibition. We further investigate the relationship between the cavity ABCD matrix and the transverse mode distribution of the SBS. To validate the model, we design and build laser cavities to observe the SBS beam profile. By utilizing this model, we achieve the controllable TEMmn (m + n ≤ 9) transverse modes of SBS. This model not only acts as a guide for suppressing SBS in diamond Raman lasers, thereby increasing the power limit of SLM output, but also provides a novel approach to attain flexible transverse modes with high power, narrow linewidth, and adjustable wavelength without the need for additional modulation elements.

Watt-level long-term stable ultra-narrow linewidth 1064 nm single-longitudinal-mode ring cavity fiber oscillator

This study presents a high-power, single-longitudinal-mode (SLM) fiber oscillator with a ring cavity design, operating at 1064 nm. Utilizing a double-cladding ytterbium-doped fiber as the gain medium, the system incorporates a fiber Bragg grating Fabry-Perot cavity and a dual coupler ring for step-by-step filtering to achieve SLM operation. With a pump power of 4.19 W, the oscillator delivered an output power of 1.01 W and narrowed the linewidth to 147 Hz, with the potential for further power increase. The oscillator demonstrated excellent longitudinal-mode stability, maintaining mode-hop-free operation for 8.7 hours after temperature stabilization. To the best of our knowledge, this work represents the longest duration of mode-hop-free operation and the narrowest laser linewidth achieved by a free-running ring-cavity SLM fiber oscillator at watt-level output powers.

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.

Stable and Tunable Erbium Ring Laser by Rayleigh Backscattering Feedback and Saturable Absorber for Single-Mode Operation

This work demonstrates a high-quality erbium-doped fiber (EDF) ring laser in the L-band gain range by combining the Rayleigh backscattering (RB) feedback signal and unpumped EDF induced saturable absorber (SA) filter. The optical filter effect induced by the RB feedback injection and EDF SA could generate single-longitudinal-mode (SLM) behavior and shrink the linewidth to sub-kHz. The output linewidth, power, and optical-signal-to-noise ratio (OSNR) of the fiber ring laser were also shown within the 42 nm wavelength bandwidth of 1565.0 to 1607.0 nm. Also, the instabilities of output power and central wavelength of each lasing lightwave were analyzed with a measurement time of 45 min.

Programmable multi-wavelength distributed feedback laser array integrated in a liquid crystal polymer waveguide.

Liquid crystal (LC) distributed feedback (DFB) lasers hold significant potential for integrated photonics applications. However, limitations in wavelength spacing for wavelength switching, device size, and compatibility with other technologies have impeded advancements of the LC DFB laser in integration and responsiveness. Herein, we propose a thin-film multi-wavelength DFB laser array utilizing high-resolution patterned programmable nematic LC polymers, enabling rapid switching with high-resolution wavelength spacing between wavelength division multiplexing channels while maintaining a stable single longitudinal mode (SLM) for each laser. The underlying physical mechanism involves modulating the effective refractive index of the DFB laser by varying the LC molecules' orientation angles between adjacent regions of the LC grating to achieve wavelength modulation. Additionally, a specialized LC waveguide design connects the DFB lasers, facilitating wavelength modulation as well as straight-line and bending propagation of the laser. Furthermore, the laser array demonstrates a relatively low energy threshold, facilitating its applications in high-integration scenarios.

High-power semiconductor laser with a narrow linewidth based on transverse photonic crystal

Broad-area lasers (BA) are practical for producing high output power. However, under a high current operation, high-order modes are easily excited, resulting in the broadened linewidth. Here, based on mode engineering of double-side transverse photonic crystals (TPCs) combined with a longitudinal high-order surface grating, a narrow-linewidth electrically-pumped broad-area laser with high power emission only using I-line lithography is demonstrated. By matching the high-order modes of the wide main waveguide with TPC bands, the effective volume of the high-order modes is expanded, while the fundamental mode remains unchanged. Then, single-lateral mode operation is achieved by selective pumping only for the main waveguide due to the significant distinction in modal gain between the fundamental mode and the high-order modes. In addition, a 27-order grating is constructed above the main waveguide to keep the laser operating in single-longitudinal mode. In the experiment, the device shows an output power of 115 mW, a lasing wavelength of 1552.94 nm with a side-mode suppression ratio (SMSR) of 59.26 dB, a narrow linewidth of 443 kHz, and a relative intensity noise (RIN) < -135 dB/Hz at 600 mA, thus has the potential to meet the needs in fields such as coherent optical communication and LiDAR.

Hepta-cross-ring based erbium laser with stable single-mode oscillation in L-band range

In this research, a hepta-cross-ring based erbium-doped fiber (EDF) laser is designed and studied to achieve continuous-wave (CW) and single-longitudinal-mode (SLM) tunability in L-band region. The hepta-cross-ring design can introduce the mode-filter function caused by the Vernier effect to effectively suppress dense multi-longitudinal-mode (MLM). Due to the L-band erbium gain medium, the wavelength-tuning range of the fiber laser is achieved from 1567.6 to 1612.0 nm. Furthermore, the output implementations of optical signal to noise ratio (OSNR), corresponding power, stability and wavelength linewidth in the proposed fiber laser are also measured and discussed.

Sub-picometer level all-solid-state narrow linewidth single-frequency Pr3+:LiYF4 laser in the near-infrared spectral region.

We report an all-solid-state near-infrared single-frequency (single longitudinal mode, SLM) Pr3+:LiYF4 (Pr:YLF) laser with the spectral linewidth at the sub-picometer level. The SLM lasers with center wavelengths of 868 and 907 nm are realized in Pr:YLF crystal for the first time to the best of our knowledge. The maximum output powers of SLM lasers at 868 and 907 nm are 102 and 213mW, corresponding to the narrowest spectral linewidths of 82 MHz (0.21 pm) and 94 MHz (0.26 pm), respectively. At the maximum output power, the beam quality factors in the x and y directions are measured as 1.25 and 1.16 at 868 nm and 1.21 and 1.13 at 907 nm, respectively. The output power stabilities of the 868 and 907 nm SLM lasers are calculated as 1.39% and 0.87%, respectively. The successful realization of 868 and 907 nm all-solid-state SLM lasers makes up for the gap that the Pr:YLF SLM lasers developed in the past are focused on the visible region, enriches the types of near-infrared (NIR) SLM lasers, and can provide practical applications in biomedicine, cold atom physics, and optical atom manipulation.

Vibration-Induced Sweeping Operation in Fiber Lasers

A new vibration-based mechanism of optical frequency/wavelength sweeping in fiber lasers induced by optical path length modulation of a laser cavity section is proposed. The mechanism is implemented for an erbium-doped fiber ring laser with a saturable absorber. Without the vibrations, the laser generates a single longitudinal mode (SLM) radiation. We show experimentally for the first time that mechanical vibrations of the laser cavity section can lead to mode dynamics in both frequency and time domains. The possibility of obtaining various mode dynamics, such as vibration-induced sweeping in a wavelength range of up to 2.2 nm or SLM generation with periodic mode hopping between two fixed longitudinal modes depending on the pump wavelength, is experimentally shown. In this vibration-based approach, the interval between changes in the laser cavity modes has good stability, because it directly relates to the vibration period.

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-power single-longitudinal-mode visible Pr:YLF ring lasers

We present the development and characterization of high-power single-longitudinal-mode (SLM) Pr:YLF lasers across the visible spectrum using a ring cavity design integrated with an optical diode (OD) and Fabry-Perot etalons. Our study achieved notable single-frequency outputs at 639 nm (red), 604 nm, and 607 nm (orange), and 522 nm (green). Specifically, we obtained a maximum output of 1.4 W at 639 nm, while the orange lasers produced outputs of 435 mW at 604 nm and 1.03 W at 607 nm. The green laser achieved a maximum output of 313 mW at 522 nm. The narrowest linewidths recorded were 7.8 MHz for red, 9.4 MHz and 7.6 MHz for orange, and 9.7 MHz for green. These results demonstrate the feasibility of using a ring cavity combined with an F-P etalon to achieve high-power, narrow-linewidth SLM operation across a significant portion of the visible spectra, offering potential applications in precision spectroscopy, optical communications, and metrology.

Long wavelength distributed feedback tapered quantum cascade lasers

We present a study on distributed feedback tapered quantum cascade lasers emitting at 14 µm. The fabricated lasers with taper angles between 0° and 3° exhibited output powers scaling in accordance with the active zone volume increase. Reduced divergence angles as small as 4.2° have been obtained with diffraction limited-beam quality (M2 ≈ 1). With a first order Bragg-grating for single longitudinal mode selection, continuous wave operation was demonstrated up to almost room temperature with side-mode suppression ratios greater than 20 dB.

Broad, Tunable and Stable Single-Frequency Erbium Fiber Compound-Ring Lasers Based on Parallel and Series Structures in L-Band Operation

In this demonstration, we present two erbium-doped fiber (EDF) lasers, with series and parallel three sub-ring configurations, respectively, to achieve tunable channel output and stable single longitudinal mode (SLM) operation in the L-band range. Here, the fiber ring cavity contains the L-band EDF as a gain medium. Based on the measured results of the two quad-ring structures of the EDF lasers, tunable output bandwidth for the two lasers can be obtained from 1558.0 to 1618.0 nm simultaneously. All the 3 dB linewidths measured for both fiber lasers are 312.5 Hz over the effective wavelength output range. Furthermore, the related optical signal-to-noise ratio (OSNR), output power, output stabilities of the central wavelength and power, and equal output power range of the two proposed EDF lasers are also examined and discussed.

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.

Tunable and switchable dual-wavelength SLM narrow-linewidth fiber laser with Lyot filter cascaded by double-ring cavity

An erbium-doped fiber (EDF) laser with single longitudinal mode (SLM), narrow linewidth, and switchable dual-wavelength based on a Lyot filter cascaded dual-ring cavity structure (DRCs) has been designed and constructed. In the experimental system, an Employed passive DRCs enhances the longitudinal mode spacing, while the Lyot filter effectively mitigates mode competition within the laser cavity. The composite filter structure facilitates stable dual-wavelength output while ensuring a consistent SLM output. Then, experimental results reveal optical signal-to-noise ratios (OSNR) with 62 dB for single-wavelength and 55 dB for dual-wavelength. Additionally, the power and wavelength fluctuations for both single-wavelength and dual-wavelength remained stable throughout monitoring periods of 20 min and 1 h, respectively. Meanwhile, a maximum tuning range of 2 nm is achieved by stretching a polarization-maintaining fiber Bragg grating (PM-FBG) using a fiber displacement stage. The linewidths of each wavelength are 2.8 kHz and 2.6 kHz, respectively. During the process of wavelength tuning, the fluctuations of linewidth do not exceed 0.3 kHz and 0.45 kHz, respectively. Simultaneously, the polarization states of each wavelength are measured, which reveals orthogonal single polarization. The relative intensity noise (RIN) is measured below −133 dB/Hz at frequencies above 1 MHz.

High-speed GaN-based laser diode with modulation bandwidth exceeding 5 GHz for 20 Gbps visible light communication

Visible light communication (VLC) based on laser diodes demonstrates great potential for high data rate maritime, terrestrial, and aerial wireless data links. Here, we design and fabricate high-speed blue laser diodes (LDs) grown on c-plane gallium nitride (GaN) substrate. This was achieved through active region design and miniaturization toward a narrow ridge waveguide, short cavity length, and single longitudinal mode Fabry–Perot laser diode. The fabricated mini-LD has a low threshold current of 31 mA and slope efficiency of 1.02 W/A. A record modulation bandwidth of 5.9 GHz (−3 dB) was measured from the mini-LD. Using the developed mini-LD as a transmitter, the VLC link exhibits a high data transmission rate of 20.06 Gbps adopting the bit and power loading discrete multitone (DMT) modulation technique. The corresponding bit error rate is 0.003, satisfying the forward error correction standard. The demonstrated GaN-based mini-LD has significantly enhanced data transmission rates, paving the path for energy-efficient VLC systems and integrated photonics in the visible regime.

High-power and narrow-linewidth nanosecond pulsed intracavity crystalline Raman laser operating at 1.7 µm

We report on a high-power and narrow-linewidth nanosecond pulsed intracavity crystalline Raman laser at 1.7 µm. Driven by an acousto-optically Q-switched 1314 nm two-crystal Nd:YLF laser, the highly efficient cascaded YVO4 Raman laser at 1715nm was obtained within the well-designed L-shaped resonator. Thanks to the absence of spatial hole burning in the stimulated Raman scattering process, significant spectral purification of second-Stokes radiation was observed by incorporating a fused silica etalon in the high-Q fundamental cavity. Under the repetition rate of 4 kHz, the highest average output power for single longitudinal mode operation was up to 2.2 W with the aid of precision vibration isolation and precision temperature controlling, corresponding to the pulse duration of ∼2.8 ns and the spectral linewidth of ∼330 MHz. Further increasing the launched pump power, the second-Stokes laser tended toward be always multimode, and the maximum average output power amounted to 4.8 W with the peak power of ∼0.8 MW and the spectral linewidth of ∼0.08 nm. The second-Stokes emission was near diffraction limited with M2 < 1.4 across the whole pump power range.