GHz optoacoustic mode-locking of fibre lasers

The intense optoacoustic interactions in solid-core PCFs with high air-filling fractions – the core acts rather like an optically driven quartz oscillator – can be used to force fibre lasers to mode-lock at a high harmonic of their round-trip frequency [Kang (2013)]. This passive GHz mode-locking mechanism turns out to be extremely stable, resulting in lasers that can oscillate without interruption for weeks, and probably indefinitely. We have been perfecting this technique over the recent several years, and now can produce sub-100 fs pulses at repetition rates of 2 GHz at 1550 nm. The system works at any wavelength where the core is transparent, and can be used to store data long-term.

Wavelength-tunable GHz mode-locking

These optomechanical effects can be used to mode-lock fibre lasers at the GHz frequencies. By inserting a short length of PCF into a soliton fibre laser, highly stable soliton trains can be obtained with pulse durations of ~1 ps at a repetition rate of 2.122 GHz, corresponding to the 126th harmonic of the cavity round-trip frequency (16.8 MHz) [Pang (2015)]. The robustness of the passive locking between the GHz-rate pulse train and optically-driven acoustic resonance enables smooth and repeatable tuning of both the laser central wavelength and the pulse duration [He (2015)]. By adjusting the transmission wavelength of a tunable optical filter inside the laser cavity, the central wavelength of the laser could be continuously tuned over 30 nm without destroying the optoacoustic mode-locking. The duration and 3 dB bandwidth of the laser pulses could also be slightly tuned by adjusting the laser pump power, without losing their fundamental soliton character. 

(a) Experimental set-up of the soliton fibre laser and SEM photograph of the solid-core PCF. (b) Typical pulse train recorded over 30 min using oscilloscope operating in infinite persistence mode. (c) RF spectrum of the pulse train measured by a real-time spectrum analyser (RSA). (d) Optical spectra of the soliton laser at wavelengths between 1535 nm and 1565 nm in 5 nm steps. (e) Measured pulse duration (black squares) and 3-dB bandwidth (blue circles) versus estimated pulse energies in the laser cavity. The fitting curves are based on the assumption that the pulses are fundamental solitons.

Sub-100-fs pulses

The pulse duration of this optoacoustic mode-locking fibre laser was considerably shortened using careful intra-cavity dispersion management, using a dispersion-compensating fibre (DCF) with large normal dispersion to shift from the soliton regime to a unique "stretched-soliton" regime [He (2016)]. While strong temporal breathing was observed in the reconfigured system over one cavity round-trip, the pulse duration at the laser output could be as short as ~85 fs, resulting in an all-fibre pulsed-light source with the sought-after characteristics of simultaneous GHz repetition rate and ultra-short pulse duration.

(a) Experimental set-up of the stretched-soliton fibre laser and SEM photograph of the solid-core PCF. (b) Measured optical spectra at output port-1 (blue line) and port-2 (red line). (c) Measured autocorrelation trace at output port-2 with fitting curve, assuming a sech2 pulse shape.

Mode-locked Tm-doped fibre-laser

The optoacoustic mode-locking scheme has also been used to mode-lock a Tm-doped fibre laser, producing stable GHz-rate pulse trains at 1.85 µm wavelength [Pang (2016)]. A short length of specially designed nano-bore PCF was inserted into the Tm-doped fibre laser cavity to provide an acoustic core resonance at 1.453 GHz. Stable pulse trains could be obtained at a repetition rate of 1.446 GHz repetition rate, with durations of tens of ps, high supermode noise suppression ratio and low pulse timing jitter.

(a) Experimental set-up of the Tm-doped fibre laser and SEM photograph of the nano-bore PCF. (b) Typical pulse train recorded by an oscilloscope operating in the infinite persistence mode. (c) RF spectrum of the pulse train measured by an RSA. (d) Measured (black solid line) and simulated (dashed red line) optical spectra of the laser. The conventional split-step Fourier method was used in the numerical simulations.

Long-term bit storage

High-harmonic mode-locking, in which each cycle of vibration is occupied by one optical pulse, is merely one example of a stable state of an optomechanically-coupled laser. With an appropriate erasure process, an encoded soliton sequence can be stably preserved in a freely-running laser cavity over many hours, corresponding to transmission of GHz-rate soliton sequences over tens of billions of km in this fibre laser loop [Pang (2016a)].

Storage of a soliton sequence in the laser cavity for 100 h. Readout was performed over one cavity round-trip every 30 min.