Self-frequency-shifted solitons in a polarization-maintaining, very-large-mode area, Er-doped fiber amplifier.

We demonstrate soliton self-frequency-shifted, femtosecond-pulse amplification in a newly-developed, polarization-maintaining, Er-doped, very-large-mode-area fiber amplifier. The PM-VLMA Er fiber had a core diameter of 50 µm, an effective area of ~1050 µm2, and Er absorption of 50 dB/m. The measured birefringence beat length of the PM-VLMA Er fiber was 14.1 mm. The soliton wavelength could be shifted by more than 90 nm. The soliton generation process resulted in remarkably clean, 86 fs pulses with 21 nJ energy at 1650 nm and 244 kW peak power from an all-fiber, fusion spliced system without bulk-optics for pulse compression. The polarization extinction ratio of the soliton was greater than 40 dB, and the M2 was 1.1. The fully polarization-maintaining fiber laser system provides robust and stable soliton generation. Peak-to-peak variation in the soliton wavelength, measured over the course of an hour was only 0.03% and pulse energy variation was only 0.5%.

High energy, 1572.3 nm pulses for CO2 LIDAR from a polarization-maintaining, very-large-mode-area, Er-doped fiber amplifier.

We demonstrate the first polarization-maintaining, very-large-mode-area, Er-doped fiber amplifier with ~1100 µm2 effective area. The amplifier is core pumped by a Raman fiber laser and is used to generate single-frequency, one-microsecond, pulses with pulse energy of 541 µJ, peak power of 700 W, M2 of 1.1, and polarization extinction > 20 dB. The amplifier operates at 1572.3 nm, a wavelength useful for trace atmospheric CO2 detection.

Axicons for mode conversion in high peak power, higher-order mode, fiber amplifiers.

Higher-order mode fiber amplifiers have demonstrated effective areas as large as 6000 µm2, allowing for high pulse energy and peak power amplification. Long-period gratings are used to convert the fundamental mode to the higher-order mode at the entrance to the amplifier, and reconvert back to the fundamental at the exit, to achieve a diffraction limited beam. However, long period gratings are susceptible to nonlinearity at high peak power. In this work, we propose and demonstrate axicons for linear bulk-optic mode conversion at the output of higher order mode amplifiers. We achieve an M2 of less than 1.25 for 80% mode conversion efficiency. Experiments with pulsed amplifiers confirm that the mode conversion is free from nonlinearity. Furthermore, chirp pulse amplifier experiments confirm that HOM amplifiers plus axicon mode convertors provide energy scalability in femtosecond pulses, compared to smaller effective area, fundamental mode fiber amplifiers. We also propose and demonstrate a route towards fiber integration of the axicon mode convertor by fabricating axicons directly on the tip of the fiber amplifier end-cap.

Scaling the effective area of higher-order-mode erbium-doped fiber amplifiers.

We demonstrate scaling of the effective area of higher-order mode, Er-doped fiber amplifiers. Two Er-doped higher-order mode fibers, one with 3800 µm(2) A(eff) in the LP(0,11) mode, and one with 6000 µm(2) effective area in the LP(0,14) mode, are demonstrated. Output beam profiles show clean higher order modes, and S(2) imaging measurements show low extraneous higher order mode content. CW and pulsed amplifier experiments are reported. Nanosecond pulses are amplified to 0.5 mJ pulse energy with 0.5 MW peak power.

Measuring higher-order modes in a low-loss, hollow-core, photonic-bandgap fiber.

We perform detailed measurements of the higher-order-mode content of a low-loss, hollow-core, photonic-bandgap fiber. Mode content is characterized using Spatially and Spectrally resolved (S2) imaging, revealing a variety of phenomena. Discrete mode scattering to core-guided modes are measured at small relative group-delays. At large group delays a continuum of surface modes and core-guided modes can be observed. The LP11 mode is observed to split into four different group delays with different orientations, with the relative orientations preserved as the mode propagates through the fiber. Cutback measurements allow for quantification of the loss of different individual modes. The behavior of the modes in the low loss region of the fiber is compared to that in a high loss region of the fiber. Finally, a new measurement technique is introduced, the sliding-window Fourier transform of high-resolution transmission spectra of hollow-core fibers, which displays the dependence of HOM content on both wavelength and group delay. This measurement is used to illustrate the HOM content as function of coil diameter.

A higher-order-mode erbium-doped-fiber amplifier.

We demonstrate the first erbium-doped fiber amplifier operating in a single, large-mode area, higher-order mode. A high-power, fundamental-mode, Raman fiber laser operating at 1480 nm was used as a pump source. Using a UV-written, long-period grating, both pump and 1564 nm signal were converted to the LP(0,10) mode, which had an effective area of 2700 microm(2) at 1550 nm. A maximum output power of 5.8 W at 1564 nm with more than 20 dB of gain in a 2.68 m long amplifier was obtained. The mode profile was undistorted at the highest output power.

Diffraction limited amplification of picosecond pulses in 1170 microm2 effective area erbium fiber.

Robust fundamental mode propagation and amplification of picosecond pulses at 1.56 microm wavelength is demonstrated in a core-pumped Er fiber with 1170 microm2 effective area. Record peak power exceeding 120 kW, and 67 nJ pulse energy are achieved before the onset of pulse breakup. A small increase in input pulse energy results in a temporal collapse of the pulse center to 58 fs duration, with peak powers approaching 200 kW.