E. J. R. Kelleher

Tm-doped fiber laser mode-locked by graphene-polymer composite

We demonstrate mode-locking of a thulium-doped fiber laser operating at 1.94 μm, using a graphene-polymer based saturable absorber. The laser outputs 3.6 ps pulses, with ~0.4 nJ energy and an amplitude fluctuation ~0.5%, at 6.46 MHz. This is a simple, low-cost, stable and convenient laser oscillator for applications where eye-safe and low-photon-energy light sources are required, such as sensing and biomedical diagnostics.

Tunable Q-switched fiber laser based on saturable edge-state absorption in few-layer molybdenum disulfide (MoS₂).

We fabricate a few-layer molybdenum disulfide (MoS₂) polymer composite saturable absorber by liquid-phase exfoliation, and use this to passively Q-switch an ytterbium-doped fiber laser, tunable from 1030 to 1070 nm. Self-starting Q-switching generates 2.88 μs pulses at 74 kHz repetition rate, with over 100 nJ pulse energy. We propose a mechanism, based on edge states within the bandgap, responsible for the wideband nonlinear optical absorption exhibited by our few-layer MoS₂ sample, despite operating at photon energies lower than the material bandgap.

Wideband saturable absorption in few-layer molybdenum diselenide (MoSe₂) for Q-switching Yb-, Er- and Tm-doped fiber lasers.

We fabricate a free-standing molybdenum diselenide (MoSe2) saturable absorber by embedding liquid-phase exfoliated few-layer MoSe2 flakes into a polymer film. The MoSe2-polymer composite is used to Q-switch fiber lasers based on ytterbium (Yb), erbium (Er) and thulium (Tm) gain fiber, producing trains of microsecond-duration pulses with kilohertz repetition rates at 1060 nm, 1566 nm and 1924 nm, respectively. Such operating wavelengths correspond to sub-bandgap saturable absorption in MoSe2, which is explained in the context of edge-states, building upon studies of other semiconducting transition metal dichalcogenide (TMD)-based saturable absorbers. Our work adds few-layer MoSe2 to the growing catalog of TMDs with remarkable optical properties, which offer new opportunities for photonic devices.

Nanosecond-pulse fiber lasers mode-locked with nanotubes

We demonstrate that mode-locking of ytterbium fiber lasers with a carbon nanotube saturable absorber can produce pulses ranging from 20 ps to 2 ns at repetition rates between 21 MHz and 177 kHz, respectively, depending on cavity length. Nonlinear polarization evolution is not responsible for mode-locking. Even in the nanosecond regime, clean single pulses are observed and the pulse train exhibits low jitter. Combined with extremely large chirp, these properties are suited for chirped-pulse amplification systems.

Few-layer MoS 2 saturable absorbers for short-pulse laser technology: current status and future perspectives [Invited]

Few-layer molybdenum disulfide (MoS2) is emerging as a promising quasi-two-dimensional material for photonics and optoelectronics, further extending the library of suitable layered nanomaterials with exceptional optical properties for use in saturable absorber devices that enable short-pulse generation in laser systems. In this work, we catalog and review the nonlinear optical properties of few-layer MoS2, summarize recent progress in processing and integration into saturable absorber devices, and comment on the current status and future perspectives of MoS2-based pulsed lasers.

Generation and direct measurement of giant chirp in a passively mode-locked laser.

We evaluate the shape and chirp of nanosecond pulses from a fiber laser passively mode locked with a nanotube-based saturable absorber by using a synchronously scanning streak camera and a monochromator to directly measure the pulse spectrogram. We show that the stable sech(2) output pulse possesses a predominantly linear chirp, with a residual quartic phase and low noise. Comparison with analytical mode-locking theory shows a good quantitative agreement with the master equation mode-locking model.

Passive synchronization of all-fiber lasers through a common saturable absorber

We present the synchronization of two all-fiber mode-locked lasers, operating at 1.0 μm and 1.54 μm, coupled through the use of a shared single-wall carbon nanotube absorber. Both lasers operate in the soliton-regime, achieving a synchronized repetition rate of 13.08 MHz. The broadband absorption range of the single-wall carbon nanotubes allows the stable mode-locking behavior at 1 μm and 1.5 μm. The nonlinear coupling effects between two energy states of the carbon nanotube absorber result in stable synchronized pulses for hours of operation, with a large cavity mismatch of 1400 μm.

Observation of timing jitter reduction induced by spectral filtering in a fiber laser mode locked with a carbon nanotube-based saturable absorber

A self-starting passively mode-locked fiber laser with a carbon nanotube-based saturable absorber and a fiber-based bandpass filter (BPF) is proposed. Incorporation of a BPF into the cavity leads to a great reduction of its timing jitter from 84.8 to 29.1 fs (10 Hz-3 MHz). This happens because the filtering effect can weaken the fluctuation of the central wavelength induced by the quantum noise, being one of the important contributions to timing jitter in the optical amplifying process.

CW-pumped short pulsed 1.12 μm Raman laser using carbon nanotubes

We demonstrate passive mode-locking of a Raman fiber laser at 1.12 μm, using a nanotube-based saturable absorber. A regular train of pulses, with a duration of 236 ps at the fundamental repetition frequency of the cavity, are generated by the all-normal dispersion oscillator. Importantly, this simple system is pumped with a continuous wave Yb fiber laser, removing the need for complex synchronous pumping schemes, where pulse-shaping depends on the action of the saturable absorber and a balance of dissipative processes. These results illustrate the flexibility of combining Raman amplification with a nanotube-based absorber for wavelength versatile pulsed sources.

Nonlinear coupling of relative intensity noise from pump to a fiber ring laser mode-locked with carbon nanotubes

Pump relative intensity noise (RIN) has been recognized as a major source of noise in mode-locked lasers. The coupling of RIN from the pump to the output of a passively mode-locked fiber laser (PMFL) is systematically investigated using a pump modulation technique. It is found that the linear RIN coupling ratio from pump to PMFL is decreased with an increase in modulation frequency and is independent of modulation power. Moreover, the nonlinear RIN coupling from pump to PMFL is clearly demonstrated with a square wave modulated pump. The nonlinear RIN coupling ratio is noise power dependent. An exponential decay model based on the view of gain modulation is proposed and explains well the behavior of the nonlinear coupling phenomena.