Achieving high pulse energies and average powers in all-fiber amplifiers involves balancing the reduction of nonlinearities through the utilization of large mode area fibers along with fiber designs enabling enhanced higher order mode loss to avoid transverse mode instabilities. Here, we report nanosecond pulse amplification in such a ytterbium-doped polarization maintaining fiber, achieving millijoule pulse energies with kilowatt average powers at 1 MHz, polarization extinction ratios around 23.5 dB, and a measured 99.9% of in-pulse power with an M-2 of 1.1. At 500 kHz, 1.5 mJ of pulse energy was achieved with a polarization extinction ratio of 20 dB, over 99.9% of in-pulse power, and an M2 of 1.13.

We report BiEr-DFA for seamless amplification of the S+C-bands with >20 dB small signal gain and <6 dB corresponding NF over 9 THz bandwidth (1490-1560 nm). Amplifier has 23 dBm output power and 20% PCE.

We demonstrate 0.86 mJ, 2.7 ns pulses at average powers of 430 W from a polarization maintaining Yb-30/400 fiber amplifier. High higher-order mode loss supports diffraction limited output with a mode field diameter of 26 um. © 2024 The Author(s)

Polarization maintaining (PM), all-fiber amplifiers offer the benefits of alignment free and environmentally stable operation. To achieve high output powers, particularly in pulsed operation, it is necessary to balance the need to reduce deleterious nonlinear effects, often through the use of large mode area (LMA) fibers, with the onset of transverse mode instability whereby higher order modes (HOMs) mix with the desired fundamental mode output. Over the last few years, advances in high HOM loss, ytterbium-doped LMA fibers have enabled continuous wave (CW) output powers up to 5 kW and pulse energies up to 2 mJ in non-PM fibers. In CW operation, LMA PM fibers have shown up to 2 kW of average power. In this contribution, we present all-fiber nanosecond pulsed amplification in a high HOM loss, Yb-doped LMA fiber with a 26 mu m mode field diameter and 2.4 dB/m of pump absorption at 976 nm, achieving 1 mJ of pulse energy at 1 kW of average power, and 1.5 mJ of pulse energy at 750 W of average power. The polarization extinction ratios were 20 dB or higher and the M2 was near the diffraction limit. We measured the in-pulse to out-of-pulse energy and found 99.9% or more of the measured power remained in-pulse.

Large mode area (LMA) fibers with enhanced higher-order mode (HOM) loss enable average output power up to 5 kW in continuous wave (CW) operation while avoiding transverse mode instabilities in a $22 \mu \mathrm{m}$ core [1]. A $30 \mu \mathrm{m}$ core fiber utilizing this strategy achieved nanosecond pulse energy of 1 mJ at kW average powers with diffraction limited output [2]. It is desirable to extend this performance to polarization maintaining (PM) fiber for enhanced environmental stability to support applications including micro-machining, light detection and ranging, quantum computing, and sources for scientific research.

Bismuth doped fiber amplifiers (BDFA) have been pumped with single mode 1190-1260 nm lasers to amplify signals over the O-band. We report here a BDFA using 915 nm multimode laser via an ytterbium fiber conversation stage. The conversion stage transforms 915 nm pump to single mode 1150 nm light used to pump BDF. This pump provides up to 3W of 1150 nm power over 20-70 ˚C with 0.25-0.2 electrical to optical conversion (wall-plug) efficiency. Compared to directly pumped BDFAs with 1195 nm single mode semiconductor laser(s) the 915/1150 nm pumping has superior optical performance and lower power consumption. The resultant BDFA provides >20 dB gain over 1255 to 1355 nm (17.6 THz) with a maximum gain of 29.3 dB and corresponding noise figure (NF) of 4.6 dB (λ = 1300 nm, Pin = -20 dBm). The BDFA has electrical power consumption of 8.1-9.6 W over 20-70 ˚C respectively. We also show the amplifier is suitable for high-speed data transmission by amplification of 50 Gbaud/s PAM-4 WDM signals over 30 km of G.652 fiber.

We demonstrate record high energy of 2 mJ, with 4 nanosecond pulses a peak power of > 420 kW and average power of 660 W, in a fiber amplifier using a novel 26 mu m mode-field diameter Yb-doped gain fiber. The TMI threshold for this fiber was measured to be 1kW. This is achieved at a diffraction limited beam quality of M-2=1.14.

We demonstrate new, large-mode area (LMA) gain fibers with ∼25 µm mode-field diameter, and increased higher-order mode loss that enable diffraction limited, pulsed fiber lasers operating at high average power with high pulse energy. We achieved 1.6 mJ, ns pulses, with 1.2 kW average power and 370 kW peak power in one of the new Yb-doped gain fibers. In a second, higher absorption fiber, we demonstrate 2 mJ pulse energy with peak power of >420 kW at an average power of 660 W. To the best of our knowledge these are the highest demonstrated energies, powers and peak powers for any nanosecond diffraction-limited, all-fiber laser. The TMI thresholds of two of these fibers were measured to be 1.8 kW and 1 kW respectively.

We demonstrate record high energy (1.2 mJ) nanosecond pulses with peak power of >200 kW and average power of 620 W, in a fiber amplifier using a novel 25 µm mode-field diameter Yb-doped gain fiber. The TMI threshold for this fiber was measured to be 1.8 kW. This is achieved at a diffraction limited beam quality of M 2 =1.08.

Bismuth-doped phosphosilicate fibers have become the most promising gain medium for O-band amplifiers. Yet scientific challenges on understanding the nature of bismuth active centers (BACs), mechanisms of bismuth cluster formation in the phosphosilicate glass network still exist. It is likely that multiple BACs with different oxidation states in different structural sites all contribute to the broad, nonsymmetric luminescence and gain spectra. Due to the progress in the fundamental understanding of bismuth-doped phosphosilicate glass, various designs of optical amplifiers with decent performances have been demonstrated.

The combination of high efficiency, high power, excellent beam quality, and low weight make ytterbium-doped fibre lasers a critical component in high-power laser systems. For high power operation, nonlinear impairments such as stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) become limiting factors in further increases in output power beyond a certain limit.

We report the performance of new LMA Yb fibers with increased cladding absorption for pumping in the 915 nm absorption band. A 0.5 dB/m cladding-absorption Yb20/400 fiber showed negligible photodarkening loss in 400-hour laser operation at 3 kW, with 77% optical-to-optical efficiency. Low-SRS and TMI-free operation at 3.5 kW signal power was achieved with a 0.65 dB/m cladding-absorption and 20.2 μm mode-field diameter Yb fiber, tested in a co-pumped amplifier. The Raman peak was 31 dB below the signal peak at the maximum power.

We compared BDFAs directly pumped with 1195 nm single-mode semiconductor laser(s) to 915 nm multimode laser via YDF mode/wavelength conversion stage. We demonstrated that 915/1150 nm pumping have superior performance and lower power consumption compared to 1195 nm.

We report BDFA with >20 dB gain over 1255-1355 nm bandwidth (17.6 THz) with maximum gain of 29.3 dB and corresponding NF of 4.6 dB (λ=1300 nm, Pin=-20 dBm). The BDFA has electrical power consumption of 8.1-9.6 W over 20-70 °C.

We report results from a new Yb-doped gain fiber with increased higher-order-mode (HOM) loss, compared to conventional step-index fibers. The fiber had 20 µm mode-field diameter (MFD), high absorption, and high transverse-mode instability (TMI) threshold. TMI-free operation with 5 kW output power was demonstrated from a 9 m length of gain fiber, limited by pump power. The large MFD and high absorption allowed for a 7.5 m long amplifier with greater than 80% o-o efficiency and the Raman peak more than 50 dB below the signal. These results were also enabled by a new, small-size, 7+1:1 pump-signal combiner.

We demonstrate record high energy (1.6 mJ) nanosecond pulses, as well as average powers reaching 1.2 kW and peak powers of >370 kW, with a diffraction limited beam quality of M 2 =1.12. This with an all-fiber amplifier using a novel 25 µm mode-field diameter Yb-doped gain fiber. The TMI threshold for this fiber was measured to be 1.8kW.

We report new Yb-doped gain fibers with approximately 20-μm core diameter and 400-μm cladding. These fibers have mode-field diameter greater than 19 μm, and increased higher-order-mode loss compared to conventional 19-μm modefield diameter fibers. The increased higher-order mode loss allows for high transverse-mode instability thresholds in fibers that also have large MFD and high absorption. A 21-μm fiber with 6.5 m operating length, was free from transverse-modal instabilities at 3.6 kW signal power and had 81% optical-optical efficiency. In narrow-linewidth amplifier experiments, the next-generation fiber with 21 μm fiber achieved 2.74 kW output power at 6 GHz linewidth, limited by pump power. This represents almost 2x increase in the ratio of power to signal linewidth compared to existing commercially available 19.5 μm MFD fibers, achieving 510 kW/GHz.

Optical amplification beyond C- and L-bands may be a promising solution to increase the capacity of transmission systems. In this paper we reviewed different amplification technologies, including semiconductor optical amplifiers (SOA), Raman amplifiers and doped fiber amplifiers briefly discussing their advantages and disadvantages. We found that while a variety of amplification solutions are available, they have not been implemented in practical systems because, until recently, C- and L- bands EDFA amplified systems provided sufficient capacity, and the lack of a complete optical components ecosystem. The O-band (1260–1360 nm) may be a notable exception since it has been used for decades for unamplified point-to-point transmission near fiber zero dispersion wavelength. Overall capacity demand and power budget reduction caused by the bit rate increase of pluggable modules promises to make O-band amplification an attractive solution. We present simple broadband O-band bismuth doped fiber amplifier (BDFA) with characteristics similar to, or above commercially available EDFAs. We demonstrated that BDFAs can amplify the entire O-band and a single stage amplifier can deliver 20 dB gain, 18 dBm output power and 5.5 dB noise figure over the 3-dB bandwidth greater than 60 nm. We illustrated the application of BDFAs by transmitting signals from a commercial 400 Gb/s 8-channel LAN-WDM transponder, operating within the IEEE standardized band, over 50 km of legacy G.652 fiber. We also demonstrated high power BDFA capable of delivering more than 700 mW of optical power without inducing measurable nonlinear distortion to intensity modulated signal propagating over a dispersive fiber waveguide.

We review the progress of BDFA development for O-band amplification. Currently, BDFAs can provide up to 35 dB small signal gain, less than 5.5 dB noise figure, and up to 29 dBm output power over 1270-1360 nm. We discuss amplifier design issues including pump wavelength and bandwidth allocation, as well as components performance and availability. Recent transmission experiments including LAN-WDM and CWDM modules reach extension over the spooled fiber and installed cables will be presented. Future milestones in BDF/BDFA development would also be suggested. Since with the implementation of BDFAs the transmission over O-band would no longer be power budget limited, we will briefly discuss the options of chromatic dispersion compensation over O-band.

Growing capacity demand of fibre optic transmission systems increased interest to multispan transmission outside traditional C- and L- bands. In this paper we review recent progress of doped fibre amplifiers designed for 1250-1450 nm range. We will also discuss various telecom and DCI transmission scenarios.