Half-open Cavity Research Articles

Random Raman Lasing in Diode-Pumped Multi-Mode Graded-Index Fiber with Femtosecond Laser-Inscribed Random Refractive Index Structures of Various Shapes

Diode-pumped multi-mode graded-index (GRIN) fiber Raman lasers provide prominent brightness enhancement both in linear and half-open cavities with random distributed feedback via natural Rayleigh backscattering. Femtosecond laser-inscribed random refractive index structures allow for the sufficient reduction in the Raman threshold by means of Rayleigh backscattering signal enhancement by +50 + 66 dB relative to the intrinsic fiber level. At the same time, they offer an opportunity to generate Stokes beams with a shape close to fundamental transverse mode (LP01), as well as to select higher-order modes such as LP11 with a near-1D longitudinal random structure shifted off the fiber axis. Further development of the inscription technology includes the fabrication of 3D ring-shaped random structures using a spatial light modulator (SLM) in a 100/140 μm GRIN multi-mode fiber. This allows for the generation of a multi-mode diode-pumped GRIN fiber random Raman laser at 976 nm with a ring-shaped output beam at a relatively low pumping threshold (~160 W), demonstrated for the first time to our knowledge.

Direct generation of a 635 nm red random laser based on praseodymium (Pr)-doped ZBLAN fiber.

Visible random fiber lasers have garnered significant attention due to their unique emission properties and potential applications in various fields. We first, to the best of our knowledge, demonstrateda compact all-fiber structure, red wavelength, and random fiber laser (RFL) based on a double-clad Pr-doped ZBLAN fiber. The simple half-open cavity consists of a high-reflectivity fiber pigtail mirror and the Pr-doped ZBLAN fiber. The Pr-doped ZBLAN fiber not only served as a gain medium but also offered random backward scattering. We investigated the effects of different lengths on output power and slope efficiency of the RFL. For 21 m Pr-doped fiber, the RFL emitted a maximum output power of 208.50 mW with a slope efficiency of 11.09%. For 15 m Pr-doped fiber, the maximum power decreased to 120.18 mW with the slope efficiency of 7.27%. We are also numerically simulating the output power versus the pump power at different fiber lengths based on power steady-state light propagation equations. This novel RFL has the potential for broad applications in fields such as display technology, spectroscopy, biomedical imaging, and optical sensing due to its unique properties and simple all-fiber structure.

A half-open cavity random fiber laser based on a single-mode fiber and fiber Bragg grating

A random erbium-doped fiber laser based on fiber Bragg grating (FBG) and single-mode fiber (SMF) was designed and realized experimentally in this study. For the proposed random fiber laser (RFL), the 10 km long SMF was used to generate Rayleigh scattering (RS) emissions, along with FBG to provide randomly distributed feedback in the cavity. In the experiment, the RFL laser threshold is 55 mW, as the pump power is increasingly improved to 175 mW. The RS effect is gradually apparent, and 1531.59nm laser beam was formed. The signal-to-noise ratio was 38.4 dB. The FBG was stretched for sensing applications, and the lasing wavelength was adjusted from 1531.59 to 1533.83 nm when the grating was stretched from 0 to 2000 μϵ.

High-order-mode Brillouin random fiber laser based on distributed Rayleigh scattering in all-few mode fiber half-open cavity

A high-order-mode (HOM) Brillouin random fiber laser (BRFL) based on distributed Rayleigh scattering in an all-few-mode fiber random cavity is proposed and experimentally demonstrated. Compared to traditional BRFLs oscillating in the single-mode-fiber cavity, the few-mode-fiber random cavity allows the high-order spatial mode Stokes beams to be activated and amplified via intra-mode stimulated Brillouin scattering for sufficient gain. Besides random feedback of distributed Rayleigh scattering along few-mode fibers, a few-mode fiber Bragg grating (FM-FBG) is employed to provide the one-end transverse mode-dependent reflection, which facilitates a high-purity oscillating random laser mode switchable between LP01 and LP11 mode by readily tunning the working wavelength of the Brillouin pump. Consequently, the all few-mode fiber-based BRFL emits a 600-Hz narrow-linewidth LP11 mode with a high mode purity of ∼91.3 %, which paves the way for spatial multiplexing coherent communication, high-resolution imaging, highly sensitive sensing, etc.

All-optical group velocity manipulation in Mach-Zehnder fiber interferometers incorporating with Brillouin random lasing resonance

We proposed and experimentally demonstrated a novel scheme of light group velocity control via a Mach-Zehnder fiber interferometer incorporated with Brillouin random lasing resonance in a half-open linear cavity. A theoretical model of the proposed Brillouin random lasing resonance-based Mach-Zehnder interferometer (BRLR-MZI) for the light group velocity manipulation was derived, predicting optical power-dependent transmittance spectrum shift as well as the enhancement of the temporal advancement. Thanks to Brillouin random lasing-induced fast light effect, the proposed scheme not only achieves a flexible all-optical manipulation of its transmittance spectrum but also overcomes the saturation effect for the light speed control based on a traditional MZI. In experimental validation, sinusoidally modulated signals with the modulation frequency of 160 kHz ultimately experienced a maximum advancement of 4031 ns (corresponding to the group index of 0.875), which exhibits an advancement improvement of 1364 ns compared with the traditional MZI-based scheme. It suggests a promising platform for exploring applications in fields of all-optical signal processing, microwave photonic, and ultra-sensitive fiber sensing.

Spectral characteristic of multi-wavelength random fiber laser using a microfiber knot resonator

We demonstrate a U-band multi-wavelength random Raman fiber laser (RRFL) based on a microfiber knot resonator (MKR). The RRFL has a forward-pump half-open cavity, wherein a 10-km single mode fiber provides both Rayleigh backscattering feedback and Raman gain. A MKR with a 0.18 nm free spectral range is used as the broadband comb filter. Up to 40 and 38 wavelength channels within 3 dB bandwidth were achieved from the intracavity and the end of the RRFL, respectively. The laser showed a good stability with maximum 0.38 and 0.1 dB peak power fluctuation within an hour at the two outputs, respectively. The spectral evolution with two envelopes was observed, and the impact of the MKR was discussed. The MKR is a small-size all-fiber and wavelength-insensitive broadband filter, which suits well with the broadband operation of the RRFL. The proposed RRFL has a simple structure and good potential tunability and provides guidance for flexible multi-wavelength lasers in the U-band and other wavebands, which have great potential in applications.

Tunable and switchable multi-wavelength actively Q-switched random fiber laser based on an electro-optic modulator and Sagnac loop filter.

In this article, an actively Q-switched multi-wavelength random fiber laser based on a Sagnac loop filter and electro-optic modulator is proposed and demonstrated experimentally. The random distributed feedback media is a section of a 25km long single-mode fiber. When the pump power is 350mW, the polarization angle of the Sagnac loop filter can be adjusted by polarization controller to achieve the switching of a single, double, triple, and quadruple channels laser output. In the case of a single laser channel, dual laser channels, and three laser channels output, multiple laser channels can be tuned simultaneously with a fixed wavelength interval. In addition, by changing the waveform of the external signal source, the light and dark pulses can be switched. Owing to the half-open cavity structure and the high gain of the erbium-doped fiber, the laser threshold was reduced to 25mW, and the light conversion efficiency was 0.67%. The laser is an ideal light source for medical imaging and long-distance sensing.

Bitterness quantification and simulated taste mechanism of theasinensin A from tea.

Theasinensin A is an important quality chemical component in tea, but its taste characteristics and the related mechanism are still unclear. The bitterness quantification and simulated taste mechanism of theasinensin A were researched. The results showed that theasinensin A was significantly correlated with the bitterness of tea. The bitterness threshold of theasinensin A was identified as 65 μmol/L for the first time. The dose-over-threshold (DOT) value of theasinensin A was significantly higher than that of caffeine in black tea soup. The concentration-bitterness curve and time-intensity curve of theasinensin A were constructed. The bitterness contribution of theasinensin A in black tea was higher than in oolong and green tea. Theasinensin A had the highest affinity with bitterness receptor protein TAS2R16, which was compared to TAS2R13 and TAS2R14. Theasinensin A was mainly bound to a half-open cavity at the N-terminal of TAS2R13, TAS2R14, and TAS2R16. The different binding capacity, hydrogen bond, and hydrophobic accumulation effect of theasinensin A and bitterness receptor proteins might be the reason why theasinensin A presented different bitterness senses in human oral cavity.

Sub-kHz high-order mode Brillouin random fiber laser based on long-period fiber grating and distributed Rayleigh scattering in a half-open linear cavity.

We demonstrate a narrow-linewidth high-order-mode (HOM) Brillouin random fiber laser (BRFL) based on a long-period fiber grating (LPFG) and distributed Rayleigh random feedback in a half-open linear cavity. The single-mode operation of the laser radiation with sub-kilohertz linewidth is achieved thanks to distributed Brillouin amplification and Rayleigh scattering along kilometer-long single mode fibers whilst a few mode fiber-based LPFGs enable the transverse mode conversion among a broadband wavelength range. Meanwhile, a dynamic fiber grating (DFG) is embedded and incorporated to manipulate and purify the random modes, which hence suppresses the frequency drift resulting from random mode hopping. Consequently, the random laser emission with either high-order scalar or vector modes can be generated with a high laser efficiency of 25.5% and an ultra-narrow 3-dB linewidth of 230 Hz. Furthermore, the dependence of the laser efficiency and frequency stability on the gain fiber length are also experimentally investigated. It is believed that our approach could provide a promising platform for a wide range of applications such as coherent optical communication, high-resolution imaging, highly sensitive sensing, etc.

Highly-stable optical injection wavelength locking of Er-doped random fiber lasers

A highly-stable and effective Er-doped random fiber laser (RFL) is proposed and experimentally demonstrated based on external optical injection locking technology. By adjusting the optical injected wavelength, the lasing wavelength can be selected precisely and emitted separately within the spectral bandwidth of 1555.5 nm to 1563.5 nm. The wavelength and output power fluctuation are less than 0.004% and 2.9%, respectively, indicating that optical injection locking of the RFL is quite stable over a long time. The optical injection locking technology is used in the half-open cavity incoherent RFL systems based on the random distributed Rayleigh scattering scheme, which provides an effective method to control or select the wavelength emission with high stability in a RFL.

Frequency Comb Generation Based on Brillouin Random Lasing Oscillation and Four-Wave Mixing Assisted with Nonlinear Optical Loop Mirror

A frequency comb generator (FCG) based on dual-cavity Brillouin random fiber lasing oscillation in the 1.5 μm telecon spectral window is established and experimentally demonstrated. In the half-open main cavity of the dual cavity, the stimulated Brillouin scattering in highly nonlinear fiber (HNLF) and Rayleigh scattering in single-mode fiber are employed to provide sufficient Brillouin gain and the randomly distributed feedback, respectively, for random mode resonance. The sub-cavity includes an Er-doped fiber amplifier to couple back and boost lower-order Stokes and anti-Stokes light for the cascade of stimulated Brillouin scattering to generate multiple higher-order Stokes and anti-Stokes light. Meanwhile, efficient four-wave mixing is stimulated in the HNLF-based main cavity, further enhancing the number and intensity of the resonant Stokes and anti-Stokes light. By taking advantages of the unique transmission characteristics of nonlinear optical loop mirrors, the power deviation between Stokes and anti-Stokes lines is further optimized with 17 orders of stable Stokes lines and 15 orders of stable anti-Stokes lines achieved within the 10 dB power deviation, with maximum optical signal-to-noise ratio (OSNR) of ~22 dB and ~17 dB and minimum OSNR of ~10 dB and ~7.5 dB for Stokes and anti-Stokes lines, respectively. In addition, the dynamic characteristics of the proposed FCG have been experimentally investigated. Such an FCG with fixed frequency spacing will find promising applications in fields of optical communication, microwave, optical sensing, etc.

High-spectral-purity random Raman fiber laser oscillator with full-open cavity counter-pumped by random lasing

Raman fiber lasers with high spectral purity and simple structure are essential for the high-power fiber lasers beyond the conventional emission band of active dopants. In this paper, we demonstrate a compact random Raman fiber laser (RRFL) oscillator with spectral purity of over 99%. The RRFL is generated in a full-open cavity which is counter-pumped by a random ytterbium-doped fiber laser with half-open cavity. They share the same piece of passive fiber to form the cavity, a fiber Bragg grating separates the signal light and pump light for the output. This configuration promises the compactness and high spectral purity of the RRFL. Leveraging the spatiotemporal gain-modulation, the RRFL can provide both continuous-wave output and stable pulsed output through adjusting the pump power. The output laser of continuous-wave operation and pulsing operation exhibits spectral purity of 99.4% and 99.5% around 1134 nm, respectively. This work offers a simple and robust solution for high-spectral-purity Raman fiber lasers of both continuous-wave operation and pulsed operation of hundred-watt level.

Kilowatt-level supercontinuum generation in a single-stage random fiber laser with a half-open cavity

Abstract The random distributed-feedback fiber laser (RFL) is a new approach to obtain a high-power stable supercontinuum (SC) source. To consider both structure simplification and high-power SC output, an innovative structure achieving a kilowatt-level SC output in a single-stage RFL with a half-open cavity is demonstrated in this paper. It consists of a fiber oscillator, a piece of long passive fiber and a broadband coupler, among which the broadband coupler acting as a feedback device is crucial in SC generation. When the system has no feedback, the backward output power is up to 298 W under the pump power of 1185 W. When the feedback is introduced before the pump laser, the backward power loss can be reduced and the pump can be fully utilized, which could promote forward output power and conversion efficiency significantly. Under the maximum pump power of 1847 W, a 1300 W SC with spectrum ranging from 887 to 1920 nm and SC conversion efficiency of 66% is obtained. To the best of our knowledge, it is the simplest structure used for high-power SC generation, and both the generated SC output power and the conversion efficiency are highest in the scheme of the half-opened RFL output SC.

Effect of Linewidth on the Relative Intensity Noise in Random Distributed Feedback Raman Fiber Lasers.

We experimentally explore the relation between spectral linewidth and RIN transfer in half-open cavity random distributed feedback Raman lasers, demonstrating for the first time the possibility of adjusting the pump-to-signal RIN transfer intensity and cut-off frequency by using spectral filtering in the reflector section. We apply this approach to a 50-km laser system, operating in the C-Band, reliant on a standard single-mode fiber. We obtained a minimum bandwidth of 13 pm, which translates into a visible RIN cut-off at 800 MHz.

Towards optimal conversion efficiency of Brillouin random fiber lasers in a half-open linear cavity.

We proposed and demonstrated an unprecedented high-efficiency Brillouin random fiber laser (BRFL) by fiber length optimization in a half-open linear cavity. In terms of the trade-off between Brillouin gain saturation and weak distributed Rayleigh feedback strength, optimal laser efficiency associated to proper fiber length in a BRFL was theoretically predicted. As a proof-of-concept, a unidirectional-pumped BRFL with a half-open linear cavity was experimentally conducted, in which a fiber Bragg grating at one end of gain fiber served as a high-reflection mirror while Rayleigh scattering enabled distributed feedback for random lasing resonance. Results show that the optimal fiber length of ∼3.4 km in the BRFL offers sufficient Rayleigh scattered random feedback whilst alleviating the Brillouin gain saturation to a large extent. Consequently, an optimal laser efficiency of 77.0% in the BRFL was experimentally demonstrated, which reaches the state-of-the-art high record. Laser characteristics, including the linewidth, statistics and frequency jitter were also systematically investigated. It is believed that such efficient BRFL could provide a promising platform for inspiring new explorations of laser physics as well as potentials in long-haul coherent communication and fiber-optic sensing.

1.7 <bold>μ</bold>m self-synchronized picosecond pulsed random Raman fiber laser

Aiming at the structural complexity of the current 1.7 μm band short pulse lasers, we propose and implement a self-synchronized picosecond pulsed random Raman fiber laser. The half-open Raman cavity based on random distributed feedback is pumped by a 1 578 nm pulsed fiber laser to achieve a picosecond pulse output with a central wavelength of 1 695 nm and an average power of 224 mW. The composite cavity formed by distributed Rayleigh scattering automatically satisfies the synchronous pumping condition without the need for precise matching of cavity length or complex feedback control in the system. By inserting a wavelength division multiplexer in the cavity, the random sub-cavity noise is suppressed and the stability of the output pulse is improved. To the best of our knowledge, this is the first realization of a random pulse fiber laser operating at 1.7 μm, which can be widely used in bioimaging and material processing. <fig fig-type="abstract-image" id="F1" orientation="portrait" position="float"><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1000-2618-2022-39-4-363/7A9ED982-4F80-49df-B290-3AA8759A767B-F001.jpg"/></fig>

Low threshold and high spectral purity 1.7 μm random fiber laser based on hybrid gain

1.7 μm band lasers have shown great potential in various applications including gas monitoring and pumping mid-infrared lasers. However, due to the lack of an effective gain mechanism, the lasing performance of current sources in this band, in terms of the threshold, the spectral purity, as well as the reliability, is still inferior to their counterparts at ∼1 μm or ∼1.5 μm and hinders their actual applications. Here, we propose the first demonstration of an efficient 1.7 μm random fiber laser with hybrid gain in a common cavity structure to the best of our knowledge. In the hybrid gain approach, the lasing threshold is greatly reduced benefiting from the active gain of thulium fiber, while the optical efficiency is dramatically boosted accompanied by the nonlinear gain of stimulated Raman scattering. Besides, the optimized cavity design based on two backward pumped half-open cavities can produce a completely pure 1.7 μm lasing for the whole power scaling range. This work enriches the gain mechanism of the 1.7 μm band laser and provides high spectral purity and low lasing threshold as well as excellent robustness for realistic applications.

A Broadband Dual-Antenna Pair Based on Half-Open Cavity With Horizontally Polarized Radiation for Wi-Fi 6/6E Application

A broadband and omnidirectional coverage dual-antenna pair with horizontally polarized (HP) radiation for Wi-Fi 6/6E application is presented in this article. The dual-antenna pair is composed of two open cavity antennas (OCA) with the apertures partially sealed. By sealing the aperture partially, the dimension of the proposed half-OCA (HOCA) is reduced dramatically without changing the matching bandwidth and the radiation performance. Compared with other OCAs with the omnidirectional radiation, the proposed dual-antenna pair realizes omnidirectional coverage in azimuth plane by the back-to-back combination of two unidirectional OCAs, which also makes the antenna have multiple-input multiple-output (MIMO) performance. Besides, a broadband covering both 5 and 6 GHz bands of Wi-Fi 6/6E is also achieved. A prototype was fabricated and tested to validate the concept. In well agreement with the simulated results, the measured bandwidth with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S_{11} &lt; -10$ </tex-math></inline-formula> dB for two ports are 5.03–7.35 GHz and 5.09–7.53 GHz, respectively, with the isolation higher than 22.67 dB. Besides, the measured gain > 5 dBi and ECC < 0.05 are achieved over the operating band. The proposed antenna shows great potential in the Wi-Fi 6/6E application.

Tunable and switchable multi-wavelength random distributed feedback fiber laser based on SMF and a cascaded Sagnac ring filter.

A tunable and switchable multi-wavelength random distributed feedback fiber laser based on cascaded Sagnac loops is proposed and experimentally demonstrated. The random distribution feedback of the laser is provided by the Rayleigh scattering generated by the single-mode fiber (SMF). The cascaded Sagnac loops act as a filter and a reflector in the half-open cavity laser. The single-, dual-, three-, and four-wavelength channels can be realized by adjusting the angle of the polarization controller at the pump power of 300 mW. In the single-, dual-, and three-wavelength channels, the wavelength spacing can be maintained, and the laser wavelength position can be changed at the same time. The maximum wavelength tuning ranges of single-, dual-, and three-wavelength outputs are about 4.5 nm, 2.6 nm, and 1 nm, respectively. The proposed multi-wavelength random fiber laser has the advantages of simple structure and low threshold, and it has good application prospects in remote sensing and imaging systems.

Comparison of multimode GRIN-fiber Raman lasers with FBG and random DFB cavity

Abstract Raman lasing in multimode GRIN fibers is accompanied by sufficient improvement of the output beam quality in comparison with that for pump radiation, which offers opportunities to wavelength-agile fiber lasers of new type. Here we compare power scaling and brightness enhancement capabilities of Raman laser based on multimode GRIN-fiber of 62.5/125 um core/cladding diameters pumped by ∼700 W multimode source with beam quality M2∼10, performed in two different cavity configurations: 1) linear cavity based on two fiber Bragg gratings and 2) half-open cavity with one FBG and random distributed feedback via Rayleigh backscattering along the GRIN fiber.