A comparative study of an Yb-doped fiber gain-managed nonlinear amplifier seeded by femtosecond fiber lasers

In this work, we show that the nonlinear evolution of femtosecond seed pulses with different parameters (temporal and spectral shapes, repetition rate, pulse energy) in an Yb-fiber amplifier leads to gain-managed nonlinear amplification, enabling robust generation of high-peak-power and nearly transform-limited pulses after external compression. We demonstrate a compressed pulse duration of 33 fs with an energy of 80.5 nJ and a peak power of 2.29 MW for a source with a repetition rate of 30 MHz. For a second seed source with a repetition rate of 125 MHz, we obtained a pulse duration of 51 fs with an energy of 22.8 nJ and a peak power of 420 kW. Numerical simulations incorporating rate equations and nonlinear propagation in the amplifier provide evolutions that agree well with the experimental results. The discrepancies in the amplifier’s absorption edge appearing at low repetition rates and higher pump powers are attributed to the temperature dependence of the amplifier’s gain cross-sections. Here, we experimentally verify this attribution and thus underline the importance of accounting for the fiber core temperature for precise modelling of the short-wavelength spectral edge of the output pulses in nonlinear Yb-fiber amplifiers. We also measure, for the first time, the relative intensity noise of an amplifier operating in the gain-managed nonlinear regime. The measurements reveal a significant contribution of the amplification process to the overall output noise of the system.

Er-Doped Tapered Fiber Amplifier for High Peak Power Sub-ns Pulse Amplification

A tapered Er-doped fiber amplifier for high peak power pulses amplification has been developed and tested. The core diameter changed from 15.8 µm (mode field diameter (MFD) 14.5 µm) to 93 µm (MFD 40 µm) along 3.7 m maintaining single-mode performance at 1555 nm (according to the S2-method, the part of the power of high-order modes does not exceed 1.5%). The amplification of 0.9 ns pulses with spectral width below 0.04 nm up to a peak power above 200 kW (limited by self-phase modulation) with a slope pump-to-signal conversion efficiency of 15.6% was demonstrated.

New coaxial low-pass filters with ultra-wide and spurious free stopband

Modern space communication systems often need high-power low-frequency (UHF, L-, S-, and C-band) low-pass filters (LPFs) with wide stopbands extending to Ka-band and beyond. Current design approaches frequently fail to meet these requirements completely. This paper proposes a new coaxial LPF concept and design methodology. The LPF consists of an array of cavity elements, which operate with transverse electromagnetic mode (TEM) and transverse magnetic (TM)-coupled resonances, and thus achieve a frequency response with a reflection zero at DC and transmission zeroes at targeted stopband locations. The design method is based on positioning the cavities in a quasi-periodic order, which efficiently spreads the transmission zeroes over the stopband, while keeping the characteristic impedance matched to the input/output interfaces over the passband. This design concept yields an ultra-wide, continuous and modal spurious-free stopband, while maintaining a low insertion loss, high peak power capacity, and low sensitivity to production tolerances.

The generation of femtosecond optical vortex beams with megawatt powers directly from a fiber based Mamyshev oscillator

Numerous approaches have been developed to generate optical vortex beams carrying orbital angular momentum (OAM) over the past decades, but the direct intracavity generation of such beams with practical output powers in the femtosecond regime still remains a challenge. Here we propose and experimentally demonstrate the efficient generation of high-peak-power femtosecond optical vortex pulses from a Mamyshev oscillator (MO) based on few-mode polarization-maintaining (PM) ytterbium-doped fibers (YDFs). By employing an appropriate intracavity transverse spatial mode selection technique, ultrafast pulses carrying OAM with selectable topological charge of l = ±1 are successfully generated with an average output power of ∼5.72 W at ∼24.35 MHz repetition rate, corresponding to a single pulse energy of ∼235 nJ. The chirped pulses can be compressed to ∼76 fs outside the cavity, leading to a pulse peak power of ∼2.2 MW. To the best of our knowledge, this is by far the highest pulse energy and peak power for optical vortex pulses ever generated directly from a fiber oscillator. This unprecedented level of performance should be of great interest for a variety of applications including materials processing and imaging.