Double-clad Research Articles

10-Watt diode-pumped red Pr:ZBLAN double-clad fiber laser

10-Watt diode-pumped red Pr:ZBLAN double-clad fiber laser

Continous-wave and passively Q-switched laser at 1.36 µm of a Nd3+-doped phosphate fiber with a Yb3+-doped inner cladding.

All-fiber continuous-wave (CW) and passively Q-switched lasers at 1.36 µm (4F3/2 → 4I13/2) by a Nd3+-doped double cladding phosphate fiber are demonstrated for the first time, to the best of our knowledge. To suppress the competitive 0.9 and 1.05 µm emission, the Yb3+ ions are intentionally introduced into the inner cladding of this Nd3+-doped fiber. CW laser at 1.36 µm with a signal-to-noise ratio over 50 dB is realized by using two fiber-type dielectric films as cavity mirrors. A 4.0% CW laser efficiency is obtained when the Nd3+-doped fiber length is 33 mm. The emission at 0.9 and 1.05 µm is well-suppressed, and no parasitic laser is observed. Taking a commercial semiconductor saturable absorber (SA) mirror as the SA, the compact 1.36-µm Q-switched laser is demonstrated, and the repetition rate of output pulses can be tuned from 230 to 522 kHz with a narrowest pulse duration of 152 ns. Our results may provide a promising way to realize 1.3 µm laser oscillation in Nd3+-doped fibers.

Ultrafast visible fiber laser mode-locked by frequency-shifted feedback of an α-BaTeMo2O9 crystal acousto-optic modulator.

We report the first demonstration, to the best of our knowledge, of visible mode-locked fiber laser using frequency-shifted feedback (FSF) with a visible α-BaTeMo2O9 (α-BTM) crystal acousto-optic modulator (AOM). First, an α-BTM crystal is used as the visible high-quality AOM with a high diffraction efficiency of 85%, a fast rise/fall time of 79/98 ns, and a low insertion loss of 0.2 dB at 635 nm. Then, the 635 nm FSF mode-locked fiber laser is achieved using a Pr3+:doped ZBLAN double-clad fiber as a visible gain medium and the α-BTM AOM as a frequency-shifting element. Self-starting mode-locking at 635 nm directly generates red laser pulses with a pulse duration of 45 ps and a repetition frequency of 31.8 MHz. Furthermore, we investigate the evolution of mode-locking dynamics as the α-BTW-AOM feedback-frequency, which helps further understand the FSF dynamics by directly generating visible ultrashort pulses.

1.8 mJ pulses at 1560 nm based on a 55 µm core pure-erbium-doped VLMA fiber.

Despite the increasing demand for high-energy erbium lasers for LIDAR imaging applications, the scaling of the current Er-Yb co-doped technology is still hindered by a 1 µm parasitic emission. In this study, we present the first, to the best of our knowledge, utilization of the REPUSIL powder synthesis method to fabricate a 55 µm double-clad fiber with a remarkably large modal area. Doped solely with erbium and free of ytterbium, its estimated cladding absorption is 2 dB/m at 976 nm, enabling short amplification lengths. We demonstrate its potential through direct amplification of a GHz femtosecond laser (M2 = 1.12) and notably, the generation of 1.8 mJ, 37 ns Q-switched pulses in oscillator configuration (M2 = 1.46).

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.

Modeling of rare-earth-doped double-clad fiber laser based on rate equations with spatially averaged parameters

A mathematical model of a rare-earth doped double-clad fiber laser based on rate equations with spatially averaged parameters laser has been developed. Analytical expressions for the threshold absorbed pump power, slope efficiency and laser output power have been obtained for the CW regime. It has been shown that the results of output power calculations using the analytical model for CW double-clad ytterbium and neodymium doped lasers are in a good agreement with the results obtained using numerical models described in literature. A quantitative criterion for the applicability of the presented analytical model for calculating the parameters of double-clad fiber lasers has been proposed. The applicability of the proposed model to describe the transient stage of the operation of a fiber laser under continuous pumping has been demonstrated.

Widely wavelength-tunable high-repetition-rate femtosecond pulse source with highest average power up to 28 W

We have proposed and demonstrated a high-repetition-rate ultrashort pulse fiber amplification system based on a wavelength-tunable oscillator. This fiber amplification system produces an average power exceeding 20 W in bursts of 200 pulses with a 578 MHz intra-burst pulse repetition rate and a 1 MHz burst repetition rate. The center wavelength of the amplified pulses can be tuned from 1030 to 1080 nm. By utilizing pre-chirp management nonlinear amplification technique, the achieved shortest pulse duration is 27 fs with an average power of 25 W at 1032 nm. For all the amplified pulses with different wavelengths, the pulse duration after optimal compression is below 60 fs. To the best of our knowledge, this is the first time that a widely wavelength-tunable high-power laser with a repetition rate exceeding 100 MHz and a pulse duration of several tens of femtoseconds has been realized. Additionally, using only common double-cladding Yb-doped fiber as the gain fiber, without any large-mode-area Yb-doped photonic crystal fiber or rod-type Yb-doped fiber, makes the system compact and reliable due to the simple fusion operation.

Hexagonal-inner-cladding fluorotellurite fiber based on hot-extrusion

Laser output power can be significantly enhanced by a double-cladding fiber, as it possesses a large area of inner-cladding for pumping. However, the coupling efficiency of the double-cladding fiber (DCF) is impacted by the shape of the inner cladding. In this study, the simulation results showed that, the hexagonal inner cladding fiber exhibits the highest absorption efficiency of 94.07 % among the fibers with various typical shapes using the three-dimensional ray tracing method at a length of 100 mm. For the experiments, a type of fluorotellurite (TBY, TeO2-BaF2-Y2O3) glass was chosen for its high capacity of rare-earth ions adoption. Subsequently, a finely structured erbium-doped hexagonal DCF was fabricated based on the hot-extrusion method for the first time, with a minimum loss of 1.25 dB/m at 1310 nm. Additionally, the coupling efficiency of the hexagonal DCF was recorded to be 39.47 % at a fiber length of 53 cm, based on the energy distribution experiment. The damage threshold of the hexagonal DCF at 980 nm could be increased to above 26.5 W, nearly doubling that of the single-cladding fiber. Furthermore, a wider fluorescence spectrum with a full width at half maximum (FWHM) of 30 nm was demonstrated by the hexagonal double-cladding fiber, which indicates its potential for high-power laser applications.

Ultracompact all-fiber self-transceiving ultrasonic probe with an enhanced working distance.

All-optical ultrasonic probes exhibit notable benefits in ultrasonic detection and imaging. Typically, two separate optical fibers are used for excitation and detection, yet limited research has explored the integration of both functionalities within a single fiber. In this Letter, to our knowledge, a new method for fabricating an all-fiber self-transceiving ultrasonic probe is proposed with a lateral dimension of less than 500 µm. Double cladding fiber (DCF) is spliced with a short segment of thin-diameter single-mode fiber (TDSMF), which is then embedded into a fiber bubble to form a Fabry-Perot cavity, and the bubble surface is coated with a composite material layer. The pulsed laser propagates through the inner cladding of DCF and leaks from the splicing point of DCF-TDSMF, inducing the material excitation for efficient ultrasound generation. The core-guided detection laser is directed to the TDSMF end, entering the bubble microcavity and inducing an optical interference for weak echo detection. The emitting functionality produces an ultrasound with a -6 dB bandwidth of 17.5 MHz and a peak frequency of 6.29 MHz, which is well-matched with the fiber microcavity's response frequency of 3.29 MHz. Through self-transceiving experiments, low-noise pulse-echo signals are captured at varying working distances of up to 3.78 cm. The proposed probe exhibits great potential in biomedical and industrial fields due to its all-fiber miniaturization and enhanced-distance detection capability.

Characteristic frequencies of transverse electric modes in a double negative slab waveguide with Kerr-type nonlinearity

The evolution equation for guided transverse electrical modes in a slab waveguide having a double negative core is derived for the electrical field. Special solutions of the dispersion equation for the case with double positive symmetric cladding are searched for. Relevant eigenmodes are formulated in terms of characteristic frequencies as a new method, and these frequencies corresponding to the oscillating guided modes are found for different wave numbers and core widths assuming the lossless Drude model can be used for the core medium with a Kerr-type nonlinearity. Results show that normalized characteristic frequencies increase with increasing wave numbers and mode numbers.

Picosecond laser with Yb-doped tapered low birefringent double clad fiber

Abstract In this study, we present results of development fiber picosecond laser by using an active ytterbium tapered double clad optical fiber with mode field diameter of 35 μm and low intrinsic birefringence (1.45 * 10−8 rad m−1). We have demonstrated 1040 nm/50 ps optical source with tunable repetition rate (within range of 1–20 MHz) and an average power of 160 W (peak power 160 kW, 8 μJ per pulse) delivering Gaussian beam with excellent quality (M 2 ∼ 1.11/1.22).

Optical Fiber Probe with Integrated Micro-Optical Filter for Raman and Surface-Enhanced Raman Scattering Sensing.

Optical fiber Raman and surface-enhanced Raman scattering (SERS) probes hold great promise for in vivo biosensing and in situ monitoring of hostile environments. However, the silica Raman scattering background generated within the optical fiber increases in proportion to the length of the fiber, and it can swamp the signal from the target analyte. While filtering can be applied at the distal end of the fiber, the use of bulk optical elements has limited probe miniaturization to a diameter of 600 µm, which in turn limits the potential applications. To overcome this limitation, femtosecond laser micromachining was used to fabricate a prototype micro-optical filter, which was directly integrated on the tip of a 125 µm diameter double-clad fiber (DCF) probe. The outer surface of the microfilter was further modified with a nanostructured, SERS-active, plasmonic film that was used to demonstrate proof-of-concept performance with thiophenol as a test analyte. With further optimization of the associated spectroscopic system, this ultra-compact microprobe shows great promise for Raman and SERS optical fiber sensing.

Dual Mach-Zehnder Interferometer Based on DCF and FCF for Temperature and Strain Measurement

In this paper, a dual Mach-Zehnder interferometer for measuring both temperature and strain is proposed and verified by experiments. The sensor configuration involves cascading a four-core fiber and a double-clad fiber between two single-mode fibers. By exploiting the different responses of the two Mach-Zehnder interferometers to temperature and strain, we construct a matrix using two selected resonance dips from the transmission spectra, so that both temperature and strain can be measured simultaneously. The experimental results show the sensor’s remarkable performance, with the maximum temperature sensitivity of −94.2 pm/°C and the maximum strain sensitivity of 2.68 pm/µε. The maximum temperature error and strain error are found to be ±0.35 °C and ±4.8 µε, respectively. Compared with other optical fiber sensors, the sensor has high sensitivity, a simple structure, and ease to manufacture and implement, making it a structure choice for applications in quality inspection of materials.

A double clad ASE Re-injected hybrid TDFA and HDFA amplifier with ±1.44 dB GF

Abstract Current developments in wireless communications accentuate the exigency of high gain, low noise figure (NF) and minimum gain fluctuations (GF) based optical amplifier in 1.9–2.15 μm eye safe wavelength band. Hybrid thulium (Tm) doped fiber amplifier (TDFA) and holmium (Ho) doped fiber amplifier (HDFA) are the perfect candidates to extend the bandwidth towards 2 μm long wavelengths. In this work, amplifier spontaneous emission (ASE) re-injected double clad TDFA-HDFA hybrid amplifier using Fiber bragg gratings (FBGs) is presented for the amplification of 64 wavelength division multiplexed (WDM) channels covering 63 nm wavelength window. Further, performance of proposed amplifier with and without ASE reinjection is evaluated in terms of Gain, NF, GF and optical signal to noise ratio (OSNR). Effects of single and double clad on the Gain are studied and optimization of Tm and Ho doped fiber length, pump powers, and input power has also been accomplished. The GF as low as ±1.44 dB, Gain >28 dB, NF < 5.08 dB, output power 4.2 W are achieved in ASE re-injected configuration using 785 nm pump at 55 dBm in TDFA and 1,900 nm pump at 20 dBm in HDFA. It is noticed that double clad and ASE reinjection in TDFA and HDFA offer better GF.

Broadband flat supercontinuum generated by chalcogenide double-clad step-index fiber with special dispersion

To achave a broader supercontinuum in the mid-infrared band, a chalcogenide double-clad step-index fiber is proposed. Comprising As2Se3, AsSe2, and As2S5 glass, this fiber effectively confines light within the core, minimizing fiber dispersion fluctuations. By iteratively adjusting the dispersion curve of the fiber, enhancement of the supercontinuum spectrum generation is possible. Spectral broadening primarily results from four-wave mixing and self-phase modulation. Raman scattering within the pulse generates unstable higher-order solitons, which, upon breakup, contribute to spectrum broadening. This study explores the effects of pulse laser parameters and fiber length on supercontinuum generation, obtains regular cognition and offers theoretical analyses of the results. Through optimization of parameters, a mid-infrared supercontinuum with an 11 μm width is achieved. These findings underscore the potential of chalcogenide double-clad step-index fiber for generating a mid-infrared supercontinuum light source. The designed fiber structure should be helpful in supercontinuum sources, bio-molecule sensing, and spectroscopy.

All-optical output power stabilization of double-clad Er:Yb amplifiers using 1064 nm signal injection

We present a novel technique for all-optical stabilization of the output power of telecom band amplifiers based on double-clad (DC) erbium-ytterbium (Er:Yb) doped fibers. The proposed method relies on the utilization of the undesirable energy back-transfer effect which occurs in DC Er:Yb active fibers. A feedback loop is built based on a low-power 1064 nm laser, which is coupled into the Er:Yb fiber along with the 1550 nm seed. The auxiliary 1 µm signal partially extracts the Yb-ion energy, altering the gain at the design wavelength. The stabilization technique was verified for two types of fibers, to demonstrate the feasibility of our approach regardless of the type of DC Er:Yb fiber used, its characteristics, and the amplifier configuration. A long-term stability improvement by a factor of 117 was achieved, reaching a deviation of 0.8 µ W, for 100 s integration time.

Dual-wavelength mid-infrared laser operation at 2.8 μm and 3.6 μm in Er3+ doped fluoride fiber

This study presents a pioneering experimental demonstration of dual-wavelength laser operation at 2.8 μm and 3.6 μm in an Er3+-doped ZrF4-BaF2-YF3-AlF3 double-cladding fluoride glass fiber, fabricated in-house. The achievement is realized through the implementation of a novel dual-wavelength pumping scheme utilizing 980 nm and 1150 nm lasers. This innovative scheme enables the excitation of the lower energy level of the 2.8 μm laser transition (4I11/2) to the upper energy level of the 3.6 μm transition (4F9/2) via an excited state absorption process, resulting in simultaneous emission at 2.8 μm and 3.6 μm. The maximum laser output powers at 2.8 μm and 3.6 μm are measured at 0.96 W and 0.32 W, respectively. This groundbreaking work marks the first experimental realization of dual-wavelength laser operation at 2.8 + 3.6 μm in Er3+-doped fluoride fiber, contributing significantly to the advancement of laser technology in this spectral range.

Exploring the potential of a bismuth-erbium-vanadium co-doped optical fiber as a stable Q-switcher in the 1550 nm region

This paper experimentally demonstrates the ability of a bismuth-erbium-vanadium co-doped optical fiber as a saturable absorber (BEV-FSA) to generate stable Q-switched pulses in the 1550 nm region. The laser cavity delivered passively Q-switched pulses exploiting an erbium-ytterbium co-doped double-clad fiber (DC-EYDF) as the laser gain medium while maintaining an all-fiber cavity configuration. The output parameters of the laser such as output power, repetition rate, pulse duration, and pulse energy have been studied in detail with the variation of pump power. The output spectrum centered at 1550.86 nm throughout the experiment. The system produced stable pulses with a minimum pulse width of 1.83 µs with a pulse energy≈0.4µJ at the highest pump power of 2.31 W. The highest achievable repetition rate was 47.5 kHz with an of SNR≈53dB.

Ytterbium-Doped Double-Clad Fiber with High Uniformity of Concentration Distribution and Suppressed Photodarkening

Photodarkening (PD) effect in ytterbium-doped fiber (YDF) has a significant impact on the high-power operational stability of fiber lasers, which seriously hinders the power scaling. In this paper, the relationship between ytterbium ions uniformity and the photodarkening effect in the YDF was investigated, and the fabrication process allowing improving the ytterbium ions uniformity in the core of preforms for suppressing the photodarkening effect was developed. The Modified Chemical Vapor Deposition (MCVD) method combined with Chelate Vapor Deposition (CVD) technology was adopted for multi-layer fiber core deposition, and an all-gas-phase technical process was proposed to improve the ytterbium ions uniformity in the Al/P co-doped glass matrix. The 25/400 μm YDFs obtained by this technology achieved stable 3.5 kW laser output power for 8 h with suppressed PD and nonlinear effects.

Double-clad optical fiber as a single-point sensor of imaging quality for scanning laser system

We have developed a single-point optical sensor of imaging quality based on a double-clad optical fiber for laser beam scanning imaging systems. The optimal imaging quality, primarily affected by the accurate focusing, is determined by maximizing the ratio of the optical power coupled to the single-mode core and the inner multi-mode cladding of the double-clad fiber collecting the light scattered from the sample. We present simulations of the sensor’s performance at different levels of defocus and validate it experimentally by optimizing focus when imaging a scattering phantom and a living human eye with a scanning laser ophthalmoscope. In contrast to the existing image-based optimization procedures, our sensor provides feedback independent of illumination intensity or sample reflectivity that can be obtained for each image point, thus exceeding the imaging framerate. Importantly, our sensor can seamlessly be integrated with most laser scanning imaging systems, providing online feedback for correction optics.