Xuanfeng Zhou

High-Power Ultraflat Near-Infrared Supercontinuum Generation Pumped by a Continuous Amplified Spontaneous Emission Source

A high-power ultraflat near-infrared supercontinuum (SC) is generated in a section of photonic crystal fiber (PCF) pumped by an amplified spontaneous emission (ASE) source instead of continuous-wave (CW) and pulsed lasers. A low-power ASE seed at 1 μm is amplified to be 90.9 W by two fiber amplifiers and then emitted from a 10-μm-core fiber. Using this ASE source to pump a section of 100-m-long PCF, a 49.5-W near-infrared SC is obtained, and the 5-dB spectral bandwidth is 760 nm, covering from 1062 to 1822 nm. This is the reported highest power of ASE-pumped SC source. A comparative experiment is taken with a 122-W CW laser at 1090 nm to pump the same PCF. A 56.2-W SC source is generated with 5-dB spectral width of 605 nm from 1082 to 1687 nm. The conversion efficiency to SC is higher, and the spectrum is broader and flatter using the ASE source as the pump. Conclusively, pump incoherence can aid the SC generation and spectral flatness.

All-Fiber-Integrated High-Power Supercontinuum Sources Based on Multi-Core Photonic Crystal Fibers

The obstacles of power scaling the supercontinuum (SC) source based on single-core photonic crystal fiber (PCF) are analyzed. The combination of high-power fiber lasers and multi-core PCFs would be a feasible method to obtain an all-fiber-integrated high-power broadband SC source (covering visible range). In this paper, we present a comprehensive study of high-power SC generation in multi-core PCFs. Comparative experiments are performed by using a high-power pulse-repetition-rate-tunable picosecond fiber laser to pump two kinds of home-made seven-core PCFs. The influences of PCF structure (fiber dispersion property) and pulse repetition rate (pulse peak power) on the SC generation in multi-core PCFs are investigated in detail. When the picosecond fiber laser at a pulse repetition rate of 1.9 GHz is adopted as the pump, 116 W SC spanning from 800 to 1700 nm is generated in 1# seven-core PCF. Also 64 W visible SC spanning at least 500-1700 nm is demonstrated in 2# seven-core PCF at a pump pulse repetition rate of 480 MHz. The potential of extending the spectral range and scaling the output power for the SC source based on multi-core PCFs are analyzed and discussed.

Mode-field adaptor between large-mode-area fiber and single-mode fiber based on fiber tapering and thermally expanded core technique

We report a method to fabricate a mode-field adaptor (MFA) for a large-mode-area (LMA) fiber and single-mode fibers (SMFs) based on fiber tapering and the thermally expanded core (TEC) technique. Through adiabatically tapering a LMA fiber by an oxygen-hydrogen flame, the mode-field diameter is decreased effectively with nearly no extra loss, which makes it possible to realize mode-field adaptation between the LMA fiber and SMFs. Both numerical calculation and experimental results show it is workable to realize fundamental mode propagation in LMA fiber and get a low insertion loss by this method. For a LMA fiber (d=25  μm, NA=0.06) and a SMF (d=4  μm, NA=0.2), a low loss with less than 0.4 dB has been obtained both in the forward and backward directions experimentally.

Ultra-compact Watt-level flat supercontinuum source pumped by noise-like pulse from an all-fiber oscillator.

We demonstrate Watt-level flat visible supercontinuum (SC) generation in photonic crystal fibers, which is directly pumped by broadband noise-like pulses from an Yb-doped all-fiber oscillator. The novel SC generator is featured with elegant all-fiber-integrated architecture, high spectral flatness and high efficiency. Wide optical spectrum spanning from 500 nm to 2300 nm with 1.02 W optical power is obtained under the pump power of 1.40 W. The flatness of the spectrum in the range of 700 nm∼1600 nm is less than 5 dB (including the pump residue). The exceptional simplicity, economical efficiency and the comparable performances make the noise-like pulse oscillator a competitive candidate to the widely used cascade amplified coherent pulse as the pump source of broadband SC. To the best of our knowledge, this is the first demonstration of SC generation which is directly pumped by an all-fiber noise-like pulse oscillator.

Combined supercontinuum source with >200 W power using a 3 × 1 broadband fiber power combiner.

We report an incoherently combined near-infrared supercontinuum (SC) source with >200  W output power using a 3×1 broadband fiber power combiner. A broadband fiber power combiner is designed and theoretically investigated. The power transmission efficiencies of light at different wavelengths of the combiner are calculated, and the combiner is verified to be capable of combining broadband sources efficiently. Then a combiner is fabricated. Three ∼70  W near-infrared SC sources are constructed and then, using the combiner, a >200  W near-infrared SC source is obtained. Conclusively, using incoherently combining method we can obtain a high-power SC source, and the thermo-management can be realized easily. We believe that this is a suitable method to obtain a higher-power SC source.

All-fiber 7 × 1 signal combiner for high power fiber lasers

We present an all-fiber 7×1 signal combiner for high power fiber lasers. Through theoretical analysis, the fabrication method is confirmed and the taper length of the fiber bundle is chosen to be 1 cm to ensure a high transmission efficiency of the combiner. Based on the theoretical results, an all-fiber 7×1 signal combiner with high transmission efficiency is fabricated. A capillary with low refractive index is fused around the bundle of signal fibers to make an additional cladding layer. Then the fiber bundle is tapered to match the core of the output fiber and then spliced with the output fiber. The combiner is tested with a 500 W fiber laser and a temperature increase of 13°C/kW without any active cooling is observed in the combiner. The power transmission efficiency is measured to be close to 99% for each input port and the beam quality M2 is around 10.

High power incoherent beam combining of fiber lasers based on a 7 × 1 all-fiber signal combiner

Abstract. We report an experiment of incoherent beam combining based on a 7×1 all-fiber signal combiner with output power up to 6.08 kW. Properties of transmission efficiency and beam quality are analyzed by beam propagation method. Based on the calculative results, a 7×1 all-fiber signal combiner is fabricated. The handle power capacity is tested with average transmission efficiency of 98.9% and beam quality of M2≈10.

Beam Quality Analysis of Incoherent Beam Combining by a

Beam combining is one of the most potential methods to break through the output power limitations of a single fiber laser. In this letter, we report a high-power experimental result achieved by incoherent beam combining based on a 7 × 1 all-fiber signal combiner. The final output power is 6.26 kW with high beam quality (M2 ≈ 4.3). Beam quality of the incoherent combination laser is analyzed both by theory and experiment in details. Experimental measured values fit well with the simulation results. These results can demonstrate the feasibility of high-power high-brightness incoherent beam combining based on all-fiber signal combiners.

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Core mode cutoff is a useful concept not only for a tapered single-core fiber (SCF) but also for a tapered multicore fiber (MCF) to realize cladding mode transmission. In this paper, cut-off conditions of either core mode for tapered SCFs or supermodes for MCFs are theoretically investigated. Rigorous analytical formulas are derived for the modes of SCF by a three-layer waveguide model, and an approximation formula of the cut-off condition is given for the LP01 mode. The supermodes of MCFs are analyzed by the coupling mode theory, and the cut-off condition is calculated by a numerical method. Simulation results show that the in-phase supermode of MCFs has a similar cut-off condition with that of SCF. Based on this property, a convenient approximate formula is given to estimate the cut-off condition of the in-phase supermode for tapered MCFs.

All-Fiber Signal Combiner

We report a novel method to obtain high-power near-infrared broadband supercontinuum based on amplified spontaneous emission (ASE) source. Firstly two kinds of ASE seed sources, without and with parasitic laser, are amplified to be 19.1 and 19.9 W, respectively, using a fiber amplifier. Then, comparative experiments are performed by using these two sources to pump different fibers. When using the ASE source without parasitic laser to pump a piece of 1000-m single-mode fiber, six orders Stokes waves are generated to extend the spectrum to beyond 1500 nm. But the spectrum is not flat with clear discrete Stokes peaks. When a section of 150-m double-clad passive fiber is pumped by the ASE source with parasitic laser, 18.3 W supercontinuum with 20 dB spectral range covering from 1030 to 1650 nm is generated. And the continuous ASE source becomes a stochastic pulsed light owing to parasitic lasing and self-pulse effect without any artificial modulation. This method combines the merits of ASE source and pulsed light, which are simplicity and low cost of the former and high peak power of the latter.