Yb-doped Double-clad Fiber Research Articles

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.

Demonstration of hundred-watt-level near-diffraction-limited monolithic fiber laser near 980 nm with step-index double-cladding Yb-doped fiber

In this paper, a hundred-watt-level near-diffraction-limited step-index Yb-doped fiber (YDF) laser near 980 nm is demonstrated firstly, to the best of our knowledge. By using the 11.7-W 979-nm single-mode seed light, the in-band amplified spontaneous emission (ASE) is well suppressed and the maximum output power of 101.5 W with the beam quality (M2 factor) of 1.285 was obtained. This work does not only propose an effective method for the suppression of in-band ASE, but also provides a cost-effective solution of hundred-Watt-level near-diffraction-limited fiber lasers near 980 nm.

Requirements on double-cladding Yb-doped fiber for power scaling of diffraction-limited fiber amplifiers.

Requirements on the double-cladding Yb-doped fiber (DCYF) for power scaling of diffraction-limited fiber amplifiers are studied in this paper. By considering the limitations of thermal lens and transverse mode instability separately, it is found that the power scaling limit can be enlarged to more-than 100 kW and 80 kW, respectively, with the proper selection of pump and signal wavelengths. It is also found that the requirements on DCYF strongly depend on the wavelength and brightness of pump light. It is predicted that smaller-than 20-µm core diameter is required to achieve the 20-kW output power, as long as the 976-nm pump brightness can be high enough. The requirements on the inner-cladding diameter and cladding-to-core ratio of DCYF are also investigated.

FDTD modeling of excitation-balanced, mJ-level pulse amplifiers in Yb-doped double-clad optical fibers.

A finite-difference time-domain (FDTD) simulation of Yb-doped cladding-pumped, mJ-level, excitation-balanced fiber pulse amplifiers (EBFAs) is presented. In EBFAs, two pumps, one above (anti-Stokes pump, or ASP) and one below (Stokes pump, or SP) the signal wavelength, are utilized to reduce the net thermal energy generated due to the quantum defect. From the results of the FDTD simulation, detailed analyses on the fiber length optimization, excited Yb3+ population evolution, pump and signal power evolution, optical-to-optical (o-o) conversion efficiency, wall plug efficiency, as well as thermal energy generation are performed. For example, with an ASP at 990 nm and a SP at 975 nm, only 2.3 µJ of thermal energy is produced when generating a 2 mJ output pulse at 985 nm, whereas a pulse amplifier with only SP pumping rendering the same 2 mJ output gives more than 10 times the thermal energy. In the meantime, the system maintains an o-o efficiency of 8.43% and wall plug efficiency of 6.6%. The results here indicate the feasibility of the power-scaling of excitation-balanced laser systems, and the FDTD model will be beneficial for the design and optimization of such systems. The first half of this paper presents the FDTD model and provides an example calculation outlining the modeling procedure. The remaining half details the impact of varying laser parameters on system performance. These include pumping and input signal energies, repetition rates, and selection of the ASP, SP, and signal wavelengths. The results presented herein can also be extended to excitation balancing in other solid-state laser systems, such as Yb:YAG and Tm:YAG lasers.

Millihertz band low-intensity-noise single-frequency laser for space gravitational wave detection

A low-noise single-frequency laser is a key component of the space-based gravitational wave detector, and the intensity noise of the laser directly affects the sensitivity of the space-based GW detector. In this work, we report a low-noise single-frequency laser designed for space-based gravitational wave detector. The laser is based on a master oscillator power amplifier (MOPA), which is designed to possess a low-power, narrow-linewidth seed laser acting as master oscillator (MO) and an all polarization-maintaining fiber amplifier acting as power amplifier (PA). The amplifier that uses a robust mechanical design consists of an Yb-doped double-clad fiber forward pumped by wavelength-locked 976 nm pump laser diode (LD) to achieve 2.13 W of output power and 70 dB of signal-noise ratio (SNR). To suppress the relative intensity noise (RIN) in a millhertz regime (1 mHz–1 Hz), we characterize the power stabilization of a pump diode laser based on a proportional-integral-derivative (PID) feedback control loop where an in-loop photodetector is used. The power fluctuation can be converted into the fluctuation of the current signal by the photodiode, the current signal is converted into the voltage signal and amplified by a transimpedance circuit. Then, the voltage signal is compared with the voltage reference signal, and the error signal is achieved to adjust real-timely the drive current of the pump laser diode. This is a good way to significantly suppress the RIN of a laser at low frequencies, but the measured RIN below 4 mHz is still higher than –60 dBc/Hz. In order to further suppress the RIN to lower than 4 mHz, an active precise temperature control technology is used to suppress the thermal noise from pump LD and fiber coupler. To assess the RIN milliertz regime, we design an RIN measurement system consisting of a high-precision signal acquisition card (24 bit) and a computer program based on LabVIEW. The measurement range of the system is 2 μHz–102.4 kHz and the frequency resolution up to 2 µHz, much better than the counterparts of commercial instruments. By stabilizing the fiber amplifier pump LD current and the temperature of pump LD and the temperature of fiber coupler, the out-of-loop RINs are measured to be –63.4 dBc/Hz@1 mHz and –105.8 dBc/Hz@1 Hz , and in a milliertz regime of 1 mHz–1 Hz, the RIN is below –60 dBc/Hz. The results show that the feedback control of the fiber amplifier pump LD current and the temperature control of the key devices can effectively suppress the RIN in the millihertz frequency band, which lays a foundation for further improving the intensity noise performance in the low frequency band.

Comparative study on the continuous-wave double-cladding Yb-doped fiber amplifiers operating around 978 nm and 982 nm

The comparative study on the continuous-wave double-cladding Yb-doped fiber (DCYF) amplifiers operating around 978 nm and 982 nm is carried out in this paper. By means of numerical and experimental studies, it is revealed that compared with 978-nm amplifiers, the amplified spontaneous emission (ASE) around 1030 nm can be better suppressed in 982-nm amplifiers because of the lower requirement of population inversion for the amplification of 982-nm signal light, and thus, the larger optimum active fiber length and slope efficiency can be obtained in 982-nm amplifiers. This should be the first experimental demonstration that the slope efficiency of 982-nm amplifiers can be larger than that of 978-nm amplifiers, to the best of our knowledge. The effect of signal wavelength on the slope efficiency of amplifiers is also numerically studied. It is found that the optimum signal wavelength varies from the 982 nm to 980 nm with the increment of pump light. It is also found that the variation of optimum fiber length with the pump power in the case of signal wavelength longer than 986 nm is different from that with the signal wavelength shorter than 984 nm. The pertinent explanation is also given.

Experimental investigation on a hundred-Watt monolithic fiber laser operating near 980 nm with 20/125-μm double-cladding Yb-doped fiber

• A hundred-Watt 980-nm MOPA fiber laser is demonstrated with the 20/125-μm DCYF. • In-band ASE limits the power up-scaling of the MOPA fiber laser. • Increasing seed power is beneficial to suppress the in-band ASE. • Split-step seed loading method is presented, with which 109-W output power is achieved. • The fiber laser provides a cost-effective solution for high-power core-pumping source. In this paper, a hundred-watt monolithic fiber laser operating near 980 nm is demonstrated by using a double-cladding Yb-doped fiber (DCYF) with 20-μm core and 125-μm inner-cladding diameter. The fiber laser is fabricated with the master oscillator power amplifier (MOPA) configuration to scale up the output power. In our experiment, it is found that the power scalability of the fiber laser is limited by the in-band amplified spontaneous emission (ASE) which can be effectively suppressed by increasing the seed power. Then, with the split-step seed loading method, 109-W output power around 980 nm has been achieved with in-band and 1030-nm ASE suppressed to 17.1 and 40.6 dB, respectively. The beam quality has also been measured, and the M 2 factor is about 1.9 at the maximum output power. Our work provides a cost-effective solution to achieve hundred-Watt fiber lasers operating near 980 nm.

2 × 2 kW near-single-mode bidirectional high-power output from a single-cavity monolithic fiber laser.

Traditional monolithic fiber lasers can only achieve unidirectional high-power laser output. In this Letter, a novel high-power linear cavity fiber laser that can achieve bidirectional high-power output is proposed and demonstrated. In an ordinary laser resonant cavity, we replace the high-reflectivity fiber Bragg grating with a low-reflectivity fiber Bragg grating to realize bidirectional laser output. In our experiment, the laser cavity was composed of two fiber Bragg gratings with a reflectivity of about 10%. The pump power provided by the 976 nm laser diodes was injected into a double-clad Yb-doped fiber with core/cladding diameters of 20/400 µm. At the maximum pump power, the bidirectional output powers were 2025 W and 1948 W, respectively, and the output laser beam quality (M2 factor) at both ends was about 1.5. For the first time, to the best of our knowledge, the feasibility of a bidirectional output fiber laser that can achieve double high (2-kW-level) power was verified. Compared with a traditional unidirectional output laser, this type of bidirectional output laser can achieve a double high-power laser by employing a laser resonant cavity. Thus, the average cost and structure size can be further reduced in mass production.

Modeling of a dual-wavelength fiber amplification system for further mid-infrared generation

A dual-wavelength, two-stage Yb-doped fiber amplification system is theoretically studied for further mid-infrared generation. In this amplification system, two sections of double-clad single-mode Yb-doped fiber are used as the gain medium, and the spectrum in the wavelength range 1000–1120 nm is simulated. For the simulation of the preamplification stage, a 975 nm diode laser with a power of 6.5 W is used to pump the preamplification stage by the counter-pumping scheme, and the measured dual-wavelength seed spectra are used. For the main amplification stage simulation, the gain fiber length is 1.7 m, and a 975 nm, 7.5 W diode laser is used as the pump source. Results show that for the two-stage amplification process, the maximum value of the dual-wavelength power product can be obtained in the main amplification stage with a 1.7 m gain fiber when the 2.2 m Yb-doped fiber is selected for the preamplification stage with the counter-pumping scheme. Under the above condition, the total average power of the seed signal after amplification is calculated to be 3.2 W, with negligible ASE noise. In addition, we have compared the theoretically calculated spectral results with the experimental results of the actual main amplification stage.

Homemade 20 kW Yb-Doped Double-Cladding Fiber for Tandem Pumping

Homemade 20 kW Yb-Doped Double-Cladding Fiber for Tandem Pumping

High peak/average power picosecond pulsed MOPA system with tapered large mode area double-clad Yb-doped fiber

In this paper, a high peak/average power picosecond pulsed master‑oscillator power‑amplifier (MOPA) system utilizing tapered large mode area double-clad fiber (T-DCF) is presented. In a high average power regime, we report a pulsed MOPA laser system with average power up to 613 W at 20 MHz repetition rate with 50 ps pulse duration (613 kW of peak power and 30.6 μJ pulse energy). In high peak power regime, over 3MW of peak power pulses (at seed source bandwidth) at 1 MHz repetition rate (310 μJ pulse energy and 310 W average) was demonstrated. The outstanding properties of T-DCF for amplification of laser signals to high power/energy levels with maintaining the excellent beam quality open a new horizon for advanced laser material processing.

Demonstration of kilowatt monolithic Yb-doped fiber laser operation near 980 nm.

In this Letter, to the best of our knowledge, the first kilowatt monolithic Yb-doped fiber laser operating near 980 nm is demonstrated. The fiber laser is achieved with the help of an all-fiber amplifier fabricated with a 105/250-µm core/cladding-diameter double-cladding Yb-doped fiber. With 11-W seed light, about 1.11-kW output power was obtained with 65.3% slope efficiency. The center wavelength was around 978 nm with a 10-dB bandwidth of about 0.6 nm. About 34-dB suppression of 1030-nm amplified spontaneous emission was realized, and the output beam quality (M2 factor) was about 16.2 at maximum output power. Better beam quality can be expected by optimizing the seed light and active fiber. This work can provide significant guidance for the study and designation of high-power fiber lasers operating near 980 nm and other three-level fiber sources.

Experimental study on the 500-W-level all-fiber amplifier operating near 980 nm

In this paper, a 500-W-level all-fiber amplifier operating near 980 nm is demonstrated firstly, to the best of our knowledge. The amplifier is fabricated with a 105/250-μm core/cladding-diameter double-cladding Yb-doped fiber (DCYF). By optimizing the length of DCYF, the record 556-W output power and 50.5% slope efficiency are obtained with a 1.85-W seed light, and about 30.9 dB peak-to-peak suppression of 1030-nm amplified spontaneous emission (ASE) is realized. Furthermore, the strong in-band ASE is also observed in experiment, which limits the power up-scaling of the fiber amplifier. By analyzing the origin of the in-band ASE, it is revealed that optimizing the seed light should be beneficial to the suppression of in-band ASE. Besides, the beam quality is measured at the maximum output power and the M2 factor is about 23.6. The experimental study should be able to provide significant guidance for understanding and designing the high-power three-level fiber lasers and amplifiers.

Demonstration of 50-W-level all-fiber oscillator operating near 980 nm with the 20-μm core-diameter double-cladding Yb-doped fiber

In this paper, a cost-effective 50-W-level all-fiber oscillator operating near 980 nm is demonstrated with the 20-μm core-diameter double-cladding Yb-doped fiber (DCYF) firstly, to the best of our knowledge. By using the 15-degree angle-cleaved output fiber end-facet, the record 54.8-W output power is obtained with the slope efficiency about 15.6%. It is revealed that increasing the cleaved angle (or suppressing the optical feedback) of output fiber end-facet should be an effective way to improve the performance of the fiber oscillator. Then, by optimizing the length of DCYF, the slope efficiency is slightly improved to 15.8% because of better suppression of the amplified spontaneous emission (ASE) around 1030 nm. It is also found that the effect of pumping scheme is negligible on the performance of the fiber oscillator. The beam quality is measured and the M2 factor is about 2.5. Better beam quality can be expected by lowering the numerical aperture (NA) of the active fiber core. The all-fiber oscillator can provide a well solution for the high-power core-pump source.

Stimulated Brillouin scattering mitigation using optimized phase modulation waveforms in high power narrow linewidth Yb-doped fiber amplifiers.

We demonstrate the mitigation of stimulated Brillouin scattering (SBS) in a double-clad single mode Yb-doped optical fiber amplifier through external phase modulation of narrow linewidth laser radiation using optimized periodic waveforms from an arbitrary waveform generator. Such optimized phase modulation waveforms are obtained through a multi-objective Pareto optimization based on a comprehensive model for SBS in high power narrow linewidth fiber amplifiers using Brillouin parameters determined from controlled measurements. The ability of our approach to mitigate SBS is tested experimentally as a function of RMS linewidth of the modulated optical radiation, and we measure an enhancement in SBS threshold with respect to optical linewidth of ∼ 10 GHz-1. Furthermore, we discuss the dependence of the SBS threshold enhancement on key parameters such as the amplifier length and the period of the optimized waveforms. Through simulations we find that waveforms of sufficiently long periods and optimized for a relatively long fiber (10 m) are effective for SBS suppression for shorter fibers as well. We also investigate the effect of increase in the bandwidth and amplitude of the modulation waveform on the SBS threshold enhancement observed at higher optical linewidth.

All-fiber pulsed laser generation of 25.5 mJ and solution of parasitic oscillation.

A Q-switched, high-energy pulsed master oscillator power amplifier fiber laser utilizing the lab-built 100/400 µm double-cladding Yb-doped fiber is demonstrated. After two-stage amplification, the pulse energy was boosted to 25.5 mJ, for an average power of 510 W at a repetition of 20 kHz, yielding a slope efficiency of approximately 72.8%; the pulse duration was approximately 140 ns, and corresponding peak power was 182.1 kW. What is more, the limitation of further promotion of pulse energy was proposed: the threshold-like parasitic oscillation, which was determined by the injecting power, repetition, and fiber length, was the main restriction on power scaling in ultra-high-energy systems. Efficient solutions were proposed to suppress the parasitic oscillation by experimentally studies.

Corrections to: Experimental and Numerical Demonstration on the Gain and Power Saturation of Yb-Doped Double-Clad Fiber Amplifiers in Multiwavelength CW and Ultrafast Pulsed Operation

Presents corrections to the above named paper.

Study on tens-of-Watt all-fiber oscillator operating near 980 nm with commercially-available double-cladding ytterbium-doped fiber

In this paper, a more-than ten-Watt all-fiber oscillator operating near 980nm is demonstrated with a commercially-available 20/125-μm double-cladding Yb-doped fiber (DCYF) firstly, to the best of our knowledge. The 15-W output power is eventually obtained with the slope efficiency of 13 % and central wavelength around 978 nm. Moreover, the power scalability of the 980-nm fiber oscillator is also investigated numerically. It is revealed that more-than 20 % slope efficiency can be obtained with this kind of DCYF. It is also found that besides pump power, the cladding light stripper (CLS) is another limitation in the 980-nm fiber oscillator, and more-than 50-W output power can be obtained if more-than 80-W residual pump light can be filter out with each CLS.

Investigation of photodarkening in tandem-pumped Yb-doped fibers

Investigation of photodarkening (PD) in Yb-doped fibers tandem-pumped at 1018 nm is reported. For a homemade Yb-doped aluminosilicate double-clad fiber (YADF), the transmitted power of a 633 nm probe beam is reduced by 2.4% over 2 hours for the tandem pumping configuration at 1018 nm, which is significantly smaller than 33.3% for a laser diode (LD) pumping at 976 nm. A tandem-pumped Yb fiber amplifier also shows a much smaller decrease in the amplified output power over time than a LD-pumped Yb fiber amplifier. Based on fluorescence spectra of the YADF, we can not only associate PD of the YADF to intrinsic oxygen deficiency centers or Tm3+ impurities but also confirm the impact of the excited Yb3+ ion density on PD. The benefits of the tandem pumping in a high-power Yb fiber laser system will be discussed.

Analytical formulation of a high-power Yb-doped double-cladding fiber laser

A detailed formalism to achieve an analytical solution of a lossy high power Yb-doped silica fiber laser is introduced. The solutions for the lossless fiber laser is initially attained in detail. Next, the solution for the lossy fiber laser is obtained based on the lossless fiber laser solution. To examine the solutions for both the lossless and lossy fiber laser, two sets of values are compared with the exact numerical solutions, and the results are in a good agreement. Furthermore, steps and procedures for achieving the final solutions are explained clearly and precisely.