Mode division multiplexing reconstructive spectrometer with an all-fiber photonics lantern.

This study presents a high-accuracy, all-fiber mode division multiplexing (MDM) reconstructive spectrometer (RS). The MDM was achieved by utilizing a custom-designed 3 × 1 mode-selective photonics lantern to launch distinct spatial modes into the multimode fiber (MMF). This facilitated the information transmission by increasing light scattering processes, thereby encoding the optical spectra more comprehensively into speckle patterns. Spectral resolution of 2 pm and the recovery of 2000 spectral channels were accomplished. Compared to methods employing single-mode excitation and two-mode excitation, the three-mode excitation method reduced the recovered error by 88% and 50% respectively. A resolution enhancement approach based on alternating mode modulation was proposed, reaching the MMF limit for the 3 dB bandwidth of the spectral correlation function. The proof-of-concept study can be further extended to encompass diverse programmable mode excitations. It is not only succinct and highly efficient but also well-suited for a variety of high-accuracy, high-resolution spectral measurement scenarios.

Influence of wavelength, linewidth, and temperature on second harmonic generation of a superfluorescent fiber source.

Low-coherence tunable visible light sources have a wide range of applications in imaging, spectroscopy, medicine, and so on. Second harmonic generation (SHG) based on a superfluorescent fiber source (SFS) can produce high-brightness visible light while retaining most of the characteristics of superfluorescent sources, such as low coherence, low intensity noise and flexible tunability. However, due to the limitations in phase matching conditions, SHG based on SFS is difficult to reach an equilibrium between high efficiency and robustness of phase matching to temperature variation. In this paper, based on a spectral tunable SFS, we provide a comprehensive analysis, both experimental and theoretical, of the impact of wavelength, linewidth, and temperature on the output performance of SHG. Our findings indicate that broader linewidths adversely affect conversion efficiency, yet they enhance the capacity to withstand temperature variations and central wavelength detuning, which is an advantage that traditional SHG methods do not possess. This work may pave the way for utilizing low-coherence visible light in domains and extreme environments where robust output stability becomes imperative.

Phosphorus-doped fiber for flat octave spanning supercontinuum generation.

In a fiber supercontinuum (SC) source, the Raman scattering effect plays a significant role in extending the spectrum into a longer wavelength. Here, by using a phosphorus-doped fiber with a broad Raman gain spectrum as the nonlinear medium, we demonstrate flat SC generation spanning from 850 to 2150 nm. Within the wavelength range of 1.1-2.0 µm, the spectral power density fluctuation is less than 7 dB. Compared to a similar SC source based on a germanium-doped fiber with narrower Raman gain spectrum, the wavelength span is 300 nm broader, and the spectral power density fluctuation is 5 dB lower. This work demonstrates the phosphorus-doped fiber's great advantage in spectrally flat SC generation, which is of great significance in many applications such as optical coherence tomography, absorption spectroscopy, and telecommunication.

High power tunable Raman fiber laser at 1.2 µm waveband.

Development of a high power fiber laser at special waveband, which is difficult to achieve by conventional rare-earth-doped fibers, is a significant challenge. One of the most common methods for achieving lasing at special wavelength is Raman conversion. Phosphorus-doped fiber (PDF), due to the phosphorus-related large frequency shift Raman peak at 40 THz, is a great choice for large frequency shift Raman conversion. Here, by adopting 150 m large mode area triple-clad PDF as Raman gain medium, and a novel wavelength-selective feedback mechanism to suppress the silica-related Raman emission, we build a high power cladding-pumped Raman fiber laser at 1.2 µm waveband. A Raman signal with power up to 735.8 W at 1252.7 nm is obtained. To the best of our knowledge, this is the highest output power ever reported for fiber lasers at 1.2 µm waveband. Moreover, by tuning the wavelength of the pump source, a tunable Raman output of more than 450 W over a wavelength range of 1240.6-1252.7 nm is demonstrated. This work proves PDF's advantage in high power large frequency shift Raman conversion with a cladding pump scheme, thus providing a good solution for a high power laser source at special waveband.

Broadband tunable Raman fiber laser with monochromatic pump.

Raman fiber laser (RFL) has been widely adopted in astronomy, optical sensing, imaging, and communication due to its unique advantages of flexible wavelength and broadband gain spectrum. Conventional RFLs are generally based on silica fiber. Here, we demonstrate that the phosphosilicate fiber has a broader Raman gain spectrum as compared to the common silica fiber, making it a better choice for broadband Raman conversion. By using the phosphosilicate fiber as gain medium, we propose and build a tunable RFL, and compare its operation bandwidth with a silica fiber-based RFL. The silica fiber-based RFL can operate within the Raman shift range of 4.9 THz (9.8-14.7 THz), whereas in the phosphosilicate fiber-based RFL, efficient lasing is achieved over the Raman shift range of 13.7 THz (3.5-17.2 THz). The operation bandwidths of the two RFLs are also calculated theoretically. The simulation results agree well with experimental data, where the operation bandwidth of the phosphosilicate fiber-based RFL is more than twice of that of the silica fiber-based RFL. This work reveals the phosphosilicate fiber's unique advantage in broadband Raman conversion, which has great potential in increasing the reach and capacity of optical communication systems.

Phosphosilicate Fiber-Based Low Quantum Defect Raman Fiber Laser with Ultrahigh Spectral Purity.

The phosphosilicate fiber-based Raman fiber laser (RFL) has great potential in achieving low-quantum defect (QD) high-power laser output. However, the laser's performance could be seriously degraded by the Raman-assisted four-wave mixing (FWM) effect and spontaneous Raman generation at 14.7 THz. To find possible ways to suppress the Raman-assisted FWM effect and spontaneous Raman generation, here, we propose a revised power-balanced model to simulate the nonlinear process in the low-QD RFL. The power evolution characteristics in this low-QD RFL with different pump directions are calculated. The simulation results show that, compared to the forward-pumped low-QD RFL, the threshold powers of spontaneous Raman generation in the backward-pumped RFL are increased by 40% and the Raman-assisted FWM effect is well suppressed. Based on the simulation work, we change the pump direction of a forward-pumped low-QD RFL into backward pumping. As a result, the maximum signal power is increased by 20% and the corresponding spectral purity is increased to 99.8%. This work offers a way for nonlinear effects controlling in low-QD RFL, which is essential in its further performance scaling.

Low quantum defect random Raman fiber laser.

The random Raman fiber laser (RRFL) has attracted great attention due to its wide applications in optical telecommunication, sensing, and imaging. The quantum defect (QD), as the main source of thermal load in fiber lasers, could threaten the stability and reliability of the RRFL. Conventional RRFLs generally adopt silica fiber to provide Raman gain, and the QD exceeds 4%. In this letter, we propose and demonstrate a phosphosilicate-fiber-based low-QD RRFL. There is a strong boson peak located at the frequency shift of 3.65 THz in the phosphosilicate fiber we employed. By utilizing this boson peak to provide Raman gain, we demonstrated an 11.71 W temporally stable random Raman laser at 1080 nm under a pump wavelength of 1066 nm. The corresponding QD is 1.3%, less than one third of the QD of the common silica-fiber-based RRFL. Compared with the full-cavity low-QD Raman fiber laser, this cavity-less low-QD RRFL has lower and flatter noise in the high frequency area (>100 kHz). This work provides a reference for suppressing thermal-induced effects, such as thermal-induced mode instability, thermal noise, and even fiber fusing in RRFLs.

Seeing the beam cleanup effect in a high-power graded-index-fiber Raman amplifier based on mode decomposition.

Due to the beam cleanup effect, brightness enhancement (BE) can be achieved in a Raman fiber amplifier (RFA) based on multimode (MM) graded-index (GRIN) fiber. In this Letter, a novel, to the best of our knowledge, diagnostic tool of mode decomposition (MD) based on a stochastic parallel gradient descent algorithm is demonstrated to observe the beam cleanup effect in a GRIN-fiber-based RFA for the first time, to our knowledge. During output power boosting up to 405 W at 1130 nm, the output beam quality factor M2 improves from 3.45 to 2.88, with a BE factor of 10.5. The MD results based on the near-field beam profiles from RFA indicate that the modal weight of the fundamental mode increases from 74.5% to 87%, confirming that the fundamental mode dominates with higher Raman gain. Moreover, the beam quality is found to be limited by the existence of a higher-order (Laguerre-Gaussian) LG10 mode, which is insensitive to the beam cleanup effect. The correlation coefficient reaches over 0.98 for all MD results. Thus, the accuracy of the MD method is high enough to provide further valuable insight into the physics of spatiotemporal beam dynamics in MM GRIN fiber.

Kilowatt level Raman amplifier based on 100 µm core diameter multimode GRIN fiber with M2 = 1.6.

In this Letter, we demonstrate a high-power Raman fiber amplifier with excellent beam quality based on graded-index fiber. The Yb-doped fiber laser (YDFL) and bandwidth-tunable amplified spontaneous emission (ASE) source are employed as the pump source to compare the laser performance separately. When the ASE with a bandwidth of 8 nm is employed, a maximum power of 943 W at 1130 nm is achieved, which is twice that pumped by YDFL. The beam quality factor M2 at maximum output power is 1.6, with a brightness enhancement (BE) factor of 27. To the best of our knowledge, this is the best beam quality and BE factor based on pure Raman gain with output power of over 100 W.

Over 400 W graded-index fiber Raman laser with brightness enhancement.

The power scaling on all-fiberized Raman fiber oscillator with brightness enhancement (BE) based on multimode graded-index (GRIN) fiber is demonstrated. Thanks to beam cleanup of GRIN fiber itself and single-mode selection properties of the fiber Bragg gratings inscribed in the center of GRIN fiber, the efficient BE is realized. For the laser cavity with single OC FBG, continuous-wave power of 334 W with an M2 value of 2.8 and BE value of 5.6 were obtained at a wavelength of 1120 nm with an optical-to-optical efficiency of 49.6%. Furthermore, the cavity reflectivity is increased by employing two OC FBGs to scale the output power up to 443 W, while the corresponding M2 is 3.5 with BE of 4.2. To our best knowledge, it is the highest power in Raman oscillator based on GRIN fiber.

Hundred-watt-level phosphosilicate Raman fiber laser with less than 1% quantum defect.

Quantum defect (QD)-induced high thermal load in high-power fiber lasers can largely affect the conversion efficiency, pose a threat to the system security, and even prohibit the further power scaling. In this Letter, we investigate evolutions and influences of the reflectivity of the output coupler, the length of phosphosilicate fiber, and the pump bandwidth, and demonstrate a hundred-watt-level low-QD Raman fiber laser (RFL). The RFL enabled by the boson peak of phosphosilicate fiber achieves a maximum power of 100.9 W with a reduced QD down to 0.97%; the corresponding conversion efficiency reaches 69.8%. This Letter may offer not only an alternative scheme for a high-power, high-efficiency fiber laser, but also great potential on the suppression of thermal-induced effects such as thermal mode instability and the thermal lens effect.

High power tunable multiwavelength random fiber laser at 1.3 µm waveband.

Multiwavelength fiber lasers, especially those operating at optical communication wavebands such as 1.3 µm and 1.5 µm wavebands, have huge demands in wavelength division multiplexing communications. In the past decade, multiwavelength fiber lasers operating at 1.5 µm waveband have been widely reported. Nevertheless, 1.3 µm waveband multiwavelength fiber laser is rarely studied due to the lack of proper gain mechanism. Random fiber laser (RFL), owing to its good temporal stability and flexible wavelength tunability, is a great candidate for multiwavelength generation. Here, we reported high power multiwavelength generation at 1.3 µm waveband in RFL for the first time. At first, we employed a section of 10 km G655C fiber to provide Raman gains, as a result of which, 1.07 W multiwavelength generation at 1.3 µm waveband with an optical to signal noise ratio of ∼33 dB is demonstrated. By tuning the pump wavelength from 1055 nm to 1070 nm, tunable multiwavelength output covering the range of 1300-1330 nm can be achieved. Furtherly, we realized 4.67 W multiwavelength generation at 1.3 µm waveband by shortening the fiber length to 4 km. To the best of our knowledge, this is the highest output power ever reported for multiwavelength fiber lasers.

Cascaded telecom fiber enabled high-order random fiber laser beyond zero-dispersion wavelength.

Four-wave mixing induced spectral broadening near the zero-dispersion wavelength (ZDW) of the fiber is a bottleneck factor that limits the further wavelength extending in cascaded random fiber lasers (RFLs). In this Letter, we successfully suppress the spectral broadening near the ZDW of the fiber in the cascaded RFL by simply combining two kinds of commercial telecom fibers with different ZDWs, G655C fiber with ZDW around 1.52 µm and G652D fiber with ZDW around 1.31 µm. As a result, an 8th order Stokes light component at 1721 nm with a maximum output power of 2.1 W and a spectral purity of 96.94% is realized in this telecom-fiber-based cascaded RFL. This work provides a reference of nonlinear effect management in fiber lasers as well as affords a cost-effective way with great potential of realizing high-power widely tunable fiber lasers.

Power scaling of Raman fiber amplifier based on the optimization of temporal and spectral characteristics.

We comprehensively study the effects of temporal and spectral optimization on single-mode Raman fiber amplifiers. Amplified spontaneous emission sources and ytterbium-doped fiber lasers are employed as seed or pump lasers for comparison, and passive fibers are utilized as gain media. The influences of various parameters of the laser on 2nd order Raman threshold and maximum output power are investigated experimentally, including bandwidth, seed power, wavelength separation between pump and seed laser, and temporal stability. With the 190 m passive fiber, the output power increases from 99.5 W to 142.4 W, corresponding to 43.1% improvement through the optimization of seed laser power, pump wavelength and temporal performance of pump source in this amplifier, which has guidance on the establishment of high-power single-mode Raman fiber amplifiers.

Dual-wavelength random distributed feedback fiber laser with wavelength, linewidth, and power ratio tunability.

Owing to the special power distribution property, a random distributed feedback Raman fiber laser can achieve a high power spectrally flexible output with a low power spectrally tuning device. Here, an all-fiberized linearly polarized dual-wavelength random distributed feedback Raman laser with wavelength, linewidth, and power ratio tunability is demonstrated. By adopting two watt-level bandwidth adjustable optical filters, a spectrum-manipulable dual-wavelength output with nearly a 10 W output power is achieved. The wavelength separation can be tuned from 2.5 to 13 nm, and the 3 dB linewidth of the output can be doubled by increasing the bandwidth of the optical filter. The power ratio of each laser line can be tuned from 0 to nearly 100% with the help of two variable optical attenuators. A maximum output power of 9.46 W is realized, with a polarization extinction ratio up to 20.5 dB. The proposed dual-wavelength fiber laser can be employed as a pump source in frequency tunable, bandwidth adjustable terahertz microwave generation, and mid-infrared optical parametric oscillators.

Broadband pumping enabled flat-amplitude multi-wavelength random Raman fiber laser.

A flat-amplitude multi-wavelength random Raman fiber laser with broad spectral coverage and a high optical signal-to-noise ratio (OSNR) is challenging and of great interest. In this Letter, we theoretically and experimentally proved that broadband pumping can help realize a broader, flat-amplitude multi-wavelength random Raman fiber laser. The influence of pump bandwidth, tunability of the spectral envelope, and channel spacing are investigated. As a result, with a 40 nm pump bandwidth, a spectral coverage of 1116-1125 nm with 19 laser lines and 31 dB OSNR is achieved, and the standard deviation in the peak intensities of the central nine lines is ${\sim}{1}.{1}\;{\rm dBm}$∼1.1dBm. This technique can also be applied to the multi-wavelength Raman (or random Raman) fiber lasers at other wavelengths and provide a reference for multi-wavelength applications in sensing, communication, and optical component testing.

All-fiberized cascaded random Raman fiber laser with high spectral purity based on filtering feedback.

Cascaded random Raman fiber lasers (CRRFLs) with simple configuration and high spectral purity have become a great candidate for power scaling over the 1.1 µm-2 µm spectral band. Recently, CRRFLs with high spectral purity over 90% have been proposed by applying a highly temporal-stable pump source or a free-space short-pass filter, at the cost of increased system complexity. In this work, pumped directly by a Yb-doped fiber oscillator at 1080 nm, an all-fiberized and simplified CRRFL with a short-pass optical filter based on bending fiber and a thin-film wavelength division multiplexer is demonstrated. The transmission loss of the filter for 5th Stokes order at 1440 nm is up to 70 dB. Spectral purity over 92% for all the first four Stokes orders is achieved. The highest output power is 15 W for the 4th Stokes order at 1341 nm.

Tunable random Raman fiber laser at 1.7 µm region with high spectral purity.

We demonstrate a tunable, high order cascaded random Raman fiber laser (RRFL) with high purity at 1.7 µm band by using a high power amplified spontaneous emission source (ASE) with both wavelength and linewidth tunability as pump source. The influence of the spectral bandwidth of the ASE source on the spectral purity of the output at 1.7 µm band is investigated. By adjusting the spectral bandwidth of the ASE source to the optimized 20 nm, output power >14 W with spectral purity up to 98.29% at 1715 nm is achieved. As far as we know, this is the highest spectral purity ever reported for a RRFL at 1.7 µm region. Furthermore, by adjusting the central wavelength of ASE source, the output of the RRFL can be tuned from 1695 to 1725 nm with >10 W output power. What's more, the spectral purity is above 92% over a tuning range from 1705 to 1725 nm.

Phosphosilicate fiber-based dual-wavelength random fiber laser with flexible power proportion and high spectral purity.

Phosphosilicate fiber has the inherent advantage of generating dual-wavelength output owing to the two Raman gain peaks at the frequency shifts of ∼13.2 THz (silica-related) and 39.9 THz (phosphorus-related), respectively. The frequency shift of 39.9 THz is often adopted to obtain long wavelength laser, while the control of Stokes light at 13.2 THz has attracted much attention currently. In this paper, a dual-wavelength random distributed feedback Raman fiber laser (RDFL) with over 100 nm wavelength interval and continuously tunable power proportion was presented based on phosphosilicate fiber for the first time. Through using the filtered amplified spontaneous emission (ASE) source as the pump source, the spectral purity of the Stokes light could be as high as 99.8%. By tuning two manual variable optical attenuators (VOAs), the power proportion of the silica-related Stokes light could range from ∼0% to 99.0%, and the maximum value is limited by the generation of second order Stokes light. Although the power handling capability of the VOA is merely 2 W, over 23 W total output power of the Stokes light was obtained thanks to the particular power distribution property of RDFL. This experiment demonstrates the potential to achieve a flexible high-power and high-spectral purity dual-wavelength RDFL output.

All-fiberized transverse mode-switching method based on temperature control.

In this paper, an all-fiberized transverse mode-switching method was proposed based on temperature control of few-mode (FM) fiber Bragg gratings (FBGs). Two types of fibers were selected to fabricate the FBG pair in order to match the reflection peaks of the desired mode. The temperature-dependence property of the FM FBGs has been utilized to tune the reflection spectra. Through temperature control, 20 W level output power was obtained when the output laser was switched between the LP11 mode and the LP01 mode in both an all-fiberized ytterbium-doped laser and a Raman laser, which is increased by ∼2 orders of magnitude compared with previous demonstrations (almost less than 100 mW).