Anomalous slowdown of pump light in Raman fiber lasers.

Usually, the pump light in lasers should perform fast light owing to operating in the absorption band. In this study, we observe and demonstrate anomalous slowdown of the pump light in a Raman fiber laser. Experiments show that the pump light can be slowed down to sub-nanoseconds at a repetition rate of 50-500 MHz. Theoretical analysis shows that the hole-burning effect is formed at the Raman gain spectrum in the saturation regime, which imposes on the pump light by normal dispersion. Consequently, the pump light experiences an unusual slow light effect rather than the fast light effect after absorption. We believe it has promising potentials in the improvement of ultrashort pulse generation, and may have significant influence on improving the conversion efficiency in pulse-pumped laser systems.

Passively Q-switched fiber lasers based on pure water as the saturable absorber.

We propose and demonstrate a passively Q-switched Er-doped fiber laser based on pure water as the saturable absorber (SA). The SA is made of two optical ferrules matched with a cannula, and the gap between the end-facets is filled with pure water. The nonlinear response of this SA has been characterized, and stable Q-switching operation at 1558.03 nm has been achieved. The maximum output power is 21.1 mW with 65.0 kHz repetition rate. The duration is 1.44 µs, and the pulse energy reaches 324.8 nJ. To the best of our knowledge, this is the first demonstration of the passively Q-switched laser with pure water as the SA. It provides further evidence of the possibility of liquid as an effective SA for pulsed lasers.

Bidirectional dark-soliton fiber lasers for high-sensitivity gyroscopic application.

Bidirectional mode-locked lasers are very useful in laser sensing and optical communications. Here we report a bidirectional domain wall soliton (DWS) fiber laser with an anomalous dispersion cavity. Two mode-locked dark pulse trains propagating in the opposite directions have been generated. Moreover, the specific application as a gyroscopic effect has been demonstrated by mounting this DWS laser on a rotating platform. The beat frequency of the two dark pulse DWS beams is measured as a linear function of the rotation velocity. The rotation sensitivity reaches 3.31 kHz/(deg/s), which are comparable with the one in a bright pulse laser gyroscope. Without the limit of using tunable delay lines, the DWS laser gyroscope has the advantages of simple structure, high sensitivity, and low cost, while possessing the entire superiority of a mode-locked laser gyroscope. It is more promising for the applications in modern inertia navigation systems.

Enhanced negative group velocity propagation in optical fibers with a hybrid Brillouin lasing resonator.

We report an enhanced scheme to achieve superluminal propagation at negative group velocity in optical fibers with a hybrid Brillouin lasing cavity. The hybrid cavity was constructed by introducing a pumped erbium-doped fiber to enhance the Brillouin lasing. Experimental results show that with the assistance of a hybrid cavity, the threshold power of the Brillouin lasing was reduced from 758.7 to 235.7 mW, and the time advancement was promoted to 444.4 ns after passing through 2 m highly nonlinear fiber at a modulation frequency of 1 MHz, corresponding to a group index of -44.6 and group velocity of -0.0224c. Moreover, a maximum negative group index of -21,029 was observed at the modulation frequency of 1 kHz, which, to the best of our knowledge, is the highest negative group index ever reported in optical fibers via stimulated Brillouin scattering.

Generation of individually modulated femtosecond pulse string by multilayer volume holographic gratings.

A scheme to generate individually modulated femtosecond pulse string by multilayer volume holographic grating (MVHG) is proposed. Based on Kogelnik's coupled-wave theory and matrix optics, temporal and spectral expressions of diffracted field are given when a femtosecond pulse is diffracted by a MVHG. It is shown that the number of diffracted sub-pulses in the pulse string equals to the number of grating layers of the MVHG, peak intensity and duration of each diffracted sub-pulse depend on thickness of the corresponding grating layer, whereas pulse interval between adjacent sub-pulses is related to thickness of the corresponding buffer layer. Thus by modulating parameters of the MVHG, individually modulated femtosecond pulse string can be acquired. Based on Bragg selectivity of the volume grating and phase shift provided by the buffer layers, we give an explanation on these phenomena. The result is useful to design MVHG-based devices employed in optical communications, pulse shaping and processing.

Periodical energy oscillation and pulse splitting in sinusoidal volume holographic grating.

This paper presents dynamical diffraction properties of a femtosecond pulse in a sinusoidal volume holographic grating (VHG). By the modified coupled-wave equations of Kogelnik, we show that the diffraction of a femtosecond pulse on the VHG gives rise to periodical energy oscillation and pulse splitting. In the initial stage of diffraction, one diffracted pulse and one transmitted pulse emerge, and energy of the transmitted pulse periodically transfers to the diffracted pulse and vice versa. In the latter stage, both the diffracted and transmitted pulses split into two spatially separated pulses. One pair of transmitted and diffracted pulses propagates in the same direction and forms the output diffracted dual pulses of the VHG, and the other pair of pulses forms the output transmitted dual pulses. The pulse interval between each pair of dual pulses is in linearly proportional to the refractive index modulation and grating thickness. By the interference effect and group velocity difference we give explanations on the periodical energy oscillation and pulse splitting respectively.

Pulse splitting by modulating the thickness of buffer layer of two-layer volume holographic grating.

Based on Kogelnik's coupled-wave theory and matrix optics, generation of femtosecond double pulses by modulating thickness of the buffer layer of two-layer volume holographic grating (TL-VHG) is discussed. Expressions of diffraction field when a femtosecond pulse incidents on the TL-VHG are deduced. Simulation results show when thickness of the buffer layer increases from 6mm to 11 mm or even larger, one incident pulse splits into double femtosecond pulses with the same duration and peak intensity, and pulse interval is linearly proportional to the thickness. The reason of these phenomena is due to the interference of diffraction waves reconstructed from two gratings and phase shift resulting from the buffer layer thickness. Time-delay of diffracted double pulses is explained by group time delay of periodic media. It is shown that the slope of the pulse interval with respect to the thickness of buffer layer is 2 times of that of pulse time-delay. Furthermore, we demonstrate it is possible to control the output double pulses' duration and pulse interval by varying the grating thickness.