Abstract
We describe a fiber Raman amplifier for nanosecond and sub-nanosecond pulses centered around 1260 nm. The amplification takes place inside a 4.5-m-long polarization-maintaining phosphorus-doped fiber, pumped at 1080 nm by 3-ns-long pulses with a repetition rate of 200 kHz and up to 1.75 kW peak power. The input seed pulses are of sub-mW peak-power and minimal duration of 0.25 ns, carved out of a continuous-wave laser with sub-MHz linewidth. We obtain linearly polarized output pulses with peak powers of up to 1.4 kW, corresponding to peak-power conversion efficiency of over 80%. An ultrahigh small signal gain of 90 dB is achieved, and the signal-to-noise ratio 3 dB below the saturation power is above 20 dB. No significant temporal and spectral broadening is observed for output pulses up to 400 W peak power, and broadening at higher powers can be reduced by phase modulation of the seed pulse. Thus, nearly-transform-limited pulses with peak power up to 1 kW are obtained. Finally, we demonstrate the generation of pulses with controllable frequency chirp, pulses with variable width, and double pulses. This amplifier is thus suitable for coherent control of narrow atomic resonances, especially for the fast and coherent excitation of rubidium atoms to Rydberg states. These abilities open the way towards several important applications in quantum non-linear optics.
Highlights
Coherent excitation of alkali atoms to Rydberg levels [1] enables novel quantumoptics applications, such as the generation of non-classical light or the realization of deterministic photonic quantum gates [2]
Strong, sub-ns pulses are required for obtaining coherent excitations and for reaching the quantum nonlinear optics regime [6]
The same figure shows traces of pump pulses with two different peak powers, Ppump = 1310 W and Ppump = 1760 W. These are measured without the BPF, without any seed pulses, and in the polarization exhibiting minimal Raman scattering (45◦ off the fast axis of the PDF)
Summary
Coherent excitation of alkali atoms to Rydberg levels [1] enables novel quantumoptics applications, such as the generation of non-classical light or the realization of deterministic photonic quantum gates [2]. Shorter pulses require even higher peak powers and may excite more than a single Rydberg level, as the typical separation between neighbouring high-n levels is a few GHz [1] For these reasons, ps and fs pulses, which around this wavelength can be generated using an optical parametric oscillator [13] or a chromium:foresterite mode-locked laser [14], are not suitable for this application. In order to reach peak powers of a few-hundred Watts with low noise in a compact design, we have developed a pulsed-pump Raman fiber amplifier based on a commercially-available phosphorus-doped fiber [18, 19, 20, 21, 22, 23, 24, 25, 26] This fiber has an additional Raman gain band, due to P=O double bonds, at 40±0.3 THz (1330±10 cm−1). We demonstrate the generation of amplified pulses with controllable duration up to 1 ns, pulses with controllable chirp rate, and double pulses of up to 1.6 ns temporal separation
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