Khanh Kieu

Femtosecond laser pulse generation with a fiber taper embedded in carbon nanotube/polymer composite

We propose and demonstrate a new saturable absorber based on a fiber taper embedded in a carbon nanotube/polymer composite. Greater than a 10% reduction in absorption (due to saturation) is directly measured for our saturable absorber. Using an embedded fiber-taper saturable absorber, we built an all-fiber mode-locked ring laser, which produces 594 fs/1.7 nJ pulses with a repetition rate of 13.3 MHz.

Sub-100 fs pulses at watt-level powers from a dissipative-soliton fiber laser

We report a mode-locked fiber laser that exploits dissipative-soliton pulse shaping along with cladding pumping for high average power. The laser generates 31 nJ chirped pulses at 70 MHz repetition rate, for an average power of 2.2 W. After dechirping outside the laser, 80 fs pulses, with 200 kW peak power, are obtained.

Soliton Thulium-Doped Fiber Laser With Carbon Nanotube Saturable Absorber

We report stabilization of a thulium-holmium codoped fiber soliton laser with a saturable absorber based on carbon nanotubes. The laser generates transform-limited 750-fs pulses with 0.5-nJ energy.

Biconical Fiber Taper Sensors

We present several simple sensitive fiber-optics-based sensors that utilize a biconical fiber taper. A displacement sensor with 100-nm accuracy, a temperature monitor with sensitivity DeltaT~1degC, and a refractive-index sensor capable of measuring Deltan~1.42times10-5 are demonstrated using tapers made from a standard single-mode fiber

All-fiber normal-dispersion femtosecond laser.

Spectral filtering of a chirped pulse can be a strong pulse-shaping mechanism in all-normal-dispersion femtosecond fiber lasers. We report an implementation of such a laser that employs only fiber-format components. The Yb-doped fiber laser includes a fiber filter, and a saturable absorber based on carbon nanotubes. The laser generates 1.5-ps, 3-nJ pulses that can be dechirped to 250 fs duration outside the cavity.

Scaling of dissipative soliton fiber lasers to megawatt peak powers by use of large-area photonic crystal fiber

We report an all-normal dispersion femtosecond laser based on large-mode-area Yb-doped photonic crystal fiber. Self-starting mode-locked pulses are obtained with an average power of 12 W at 84 MHz repetition rate, corresponding to 140 nJ of chirped pulse energy. These are dechirped to a near transform-limited duration of 115 fs. Experimental results are consistent with numerical simulations of dissipative soliton intra-cavity pulse evolution, and demonstrate scaling of 100 fs pulses to megawatt peak powers.

High-power picosecond fiber source for coherent Raman microscopy

We report a high-power picosecond fiber pump laser system for coherent Raman microscopy (CRM). The fiber laser system generates 3.5 ps pulses with 6 W average power at 1030 nm. Frequency doubling yields more than 2 W of green light, which can be used to pump an optical parametric oscillator to produce the pump and the Stokes beams for CRM. Detailed performance data on the laser and the various wavelength conversion steps are discussed, together with representative CRM images of fresh animal tissue obtained with the new source.

All-fiber bidirectional passively mode-locked ring laser.

We report the design and operation of a novel all-fiber bidirectional passively mode-locked ring laser. An erbium-doped fiber was chosen as the active element in a ring cavity arrangement. A short segment of a fiber taper embedded in carbon nanotubes/polymer composite, acting as a saturable absorber, was used to enable bidirectional mode locking. The laser generates two stable femtosecond pulse trains in opposite directions. A beat note of about 2 MHz, having a bandwidth of less than 2 kHz, is measured when the pulses propagating in opposite directions are (temporally) overlapped at a photodetector. We believe this device will find important applications in precision rotation sensing.

Fiber laser using a microsphere resonator as a feedback element.

We show that a glass microsphere resonator can be used as a wavelength-selective mirror in fiber lasers. Due to their high quality factor (Q approximately 10(8)), microsphere resonators possess a narrow reflection bandwidth. This feature enables construction of single-frequency fiber lasers even when the laser cavity is long. Nonlinear effects (such as stimulated Raman lasing) were also observed in our setup at relatively low pump powers.

Transition dynamics for multi-pulsing in mode-locked lasers

We consider experimentally and theoretically a refined parameter space in a laser system near the transition to multi-pulse modelocking. Near the transition, the onset of instability is initiated by a Hopf (periodic) bifurcation. As the cavity energy is increased, the band of unstable, oscillatory modes generates a chaotic behavior between single- and multi-pulse operation. Both theory and experiment are in good qualitative agreement and they suggest that the phenomenon is of a universal nature in mode-locked lasers at the onset of multi-pulsing from N to N +1 pulses per round trip. This is the first theoretical and experimental characterization of the transition behavior, made possible by a highly refined tuning of the gain pump level.