Huihui Cheng

Theoretical and experimental analysis of instability of continuous wave mode locking: Towards high fundamental repetition rate in Tm 3+ -doped fiber lasers.

With increasing demand on a laser source in the gigahertz pulse repetition rate regime, clarification on the mechanism of instability in high repetition rate fiber lasers - a promising alternative to solid state lasers - is of great importance and can potentially offer guideline for continuous wave (CW) mode locking. Here we present a theoretical approach together with relevant experimental corroboration to analyze the instabilities. By means of appropriate approximations, regimes from Q-switched mode locking, CW mode locking and pulsation are theoretically identified. Meanwhile, a critical curve that characterizes pump level for triggering Q-switched mode locking and pulsation for different repetition rates is given by virtue of both analytical and numerical procedures. In experiment, a passively mode-locked fiber laser with 1.6 GHz fundamental repetition rate is realized. The three regimes and corresponding pump power intervals are revealed, which are in consistence with theoretical prediction. Pulsation, as a relatively exotic phenomenon in GHz fiber laser, is well reproduced by the present model, which further verifies the accuracy of the approach as well as enriches the nonlinear dynamics.

5 GHz fundamental repetition rate, wavelength tunable, all-fiber passively mode-locked Yb-fiber laser

A passively mode-locked Yb3+-doped fiber laser with a fundamental repetition rate of 5 GHz and wavelength tunable performance is demonstrated. A piece of heavily Yb3+-doped phosphate fiber with a high net gain coefficient of 5.7 dB/cm, in conjunction with a fiber mirror by directly coating the SiO2/Ta2O5 dielectric films on a fiber ferrule is exploited for shortening the laser cavity to 2 cm. The mode-locked oscillator has a peak wavelength of 1058.7 nm, pulse duration of 2.6 ps, and the repetition rate signal has a high signal-to-noise ratio of 90 dB. Moreover, the wavelength of the oscillator is found to be continuously tuned from 1056.7 to 1060.9 nm by increasing the temperature of the laser cavity. Simultaneously, the repetition rate correspondingly decreases from 4.945874 to 4.945496 GHz. Furthermore, the long-term stability of the mode-locked operation in the ultrashort laser cavity is realized by exploiting temperature controls. This is, to the best of our knowledge, the highest fundamental pulse repetition rate for 1-μm mode-locked fiber lasers.

High-repetition-rate ultrafast fiber lasers

Multi-gigahertz fundamental repetition rate, tunable repetition rate and wavelength, ultrafast fiber lasers at wavelengths of 1.0, 1.5, and 2.0 µm are experimentally demonstrated and summarized. At the wavelength of 1.0 µm, the laser wavelength is tuned in the range of 1040.1-1042.9 nm and the repetition rate is shifted by 226 kHz in a 3-cm-long all-fiber laser by controlling the temperature of the resonator. Compared with a previous work where the maximum average power was 0.8 mW, the power in this study is significantly improved to 57 mW under a launched pump power of 213 mW, thus achieving an optical-to-optical efficiency of 27%. For comparison, a similar temperature-tuning technique is implemented in a Tm3+-doped ultrafast oscillator but, as expected, it results in a broader tunable range of 14.1 nm (1974.1-1988.2 nm) in wavelength as compared with the value of 1.8 nm for the wavelength of 1.0 µm. The repetition rate in the process is shifted by 294 kHz. For the high-frequency range from 100 kHz to 10 MHz, the value of integrated timing jitter gradually increases with an increase in temperature. Finally, to the best of our knowledge, for the first time, a new method for tuning wavelength and repetition rate is proposed and demonstrated for a femtosecond fiber laser at the wavelength of 1.5 µm. Through fine rotation of the alignment angle between the Er/Yb:glass fiber and a semiconductor saturable absorption mirror, the peak wavelength can be tuned in the range of 1591.4-1586.1 nm and the repetition rate is shifted by 60 kHz.

Mode suppression of 53 dB and pulse repetition rates of 2.87 and 36.4 GHz in a compact, mode-locked fiber laser comprising coupled Fabry-Perot cavities of low finesse (F = 2)

Multiplication of the pulse repetition frequency (PRF) of a compact, mode-locked fiber laser by a factor as large as 25 has been achieved with two coupled Fabry-Perot (FP) resonators of low finesse (F = 2). Reducing the FP finesse by at least two orders of magnitude, relative to previous pulse frequency multiplication architectures, has the effect of stabilizing the oscillator with respect to pulse-to-pulse amplitude, dropped pulses, and other effects of cavity detuning. Coupling two Fabry-Perot cavities, each encompassing a 3.3-3.6 cm length of fiber, in a hybrid geometry resembling that of the coupled-cavity laser interferometer has yielded side mode suppressions ≥ 50 dB while simultaneously doubling the laser PRF to 2.87 GHz. Pulses approximately 3.9 ps in duration (FWHM) are emitted at intervals of 27.5 ps, and in groups (bursts) of pulses separated by 350 ps. Thus, the PRF within the pulse bursts is 36 GHz, a factor of 25 greater than the free spectral range for a conventional mode-locked cavity having a length of 6.9 cm. Experimental data are in accord with simulations of the phase coherence and temporal behavior of the mode-locked pulses.

Composite filtering effect in a SESAM mode-locked fiber laser with a 3.2-GHz fundamental repetition rate: switchable states from single soliton to pulse bunch: erratum

We present the erratum regarding the x-axis label in two figures, a numerical correction and a mathematical symbol correction.

Investigation of rectangular shaped wave packet dynamics in a high-repetition-rate ultrafast fiber laser

We identify a new regime where laser pulses represent the dynamics of rectangular-shaped wave packets (RSWPs) in a passively mode-locked Tm3+-doped fiber laser. In this regime the laser consists of a train of mode-locked pulses underneath a rectangular-shaped envelope. The density of pulses within a RSWP can be as high as 2.8 GHz, which is consistent with cavity fundamental repetition rate. The effects of small-signal gain value, pulse repetition rate, and net dispersion on the RSWP performance are analyzed. These results imply that this new regime particularly favors high-repetition-rate ultrafast lasers. We further reproduce the phenomenon from using numerical simulations and understand such behavior by referring to the nonlinear dynamics.

High-power and near-shot-noise-limited intensity noise all-fiber single-frequency 1.5 μm MOPA laser

An all-fiber high-power and broad-frequency-band near-shot-noise-limited kHz-linewidth (Δν ~1.7 kHz) single-frequency master-oscillator power amplifier (MOPA) laser at 1.5 μm is demonstrated. To significantly suppress the intensity noise of seed laser and mitigate the detrimental effects of amplified spontaneous emission and stimulated Brillouin scattering in fiber amplifiers, more than 23 W of a stable low noise single-frequency laser output is achieved with a relative intensity noise of < -150 dB/Hz @0.5 mW (near to the shot-noise limit: -152.9 dB/Hz) in the frequency band from 0.1 to 50 MHz. It is believed that the achieved laser performance of ultra-low intensity noise and high-power output make the laser source become a promising candidate in further applications, such as cold atom optical lattice, quantum key distribution, and gravitational wave detection.

kHz-order linewidth controllable 1550 nm single-frequency fiber laser for coherent optical communication

A kHz-order linewidth controllable 1550 nm single-frequency fiber laser (SFFL) is demonstrated for the first time to our best knowledge. The control of the linewidth is realized by using a low-pass filtered white Gaussian noise (WGN) signal applied on a fiber stretcher in an optical feedback loop. Utilizing WGN signals with different signal amplitudes An and different cutoff frequencies fc, the linewidths are availably controlled in a wide range from 0.8 to 353 kHz. The obtained optical signal-to-noise ratio (OSNR) is more than 72.0 dB, and the relative intensity noise (RIN) at frequency greater than 40 MHz reaches -148.5 dB/Hz which approaches the shot noise limit (-152.9 dB/Hz). This kHz-order linewidth controllable SFFL is meaningful and valuable, for optimizing the receiver sensitivity and bit error rate (BER) performance of the coherent optical communication system based on high-order quadrature amplitude modulation (QAM).