Hua Wang

Generation of triangular-shaped optical pulses in normally dispersive fibre

We determine through numerical modelling the conditions for the generation of triangular-shaped optical pulses in a nonlinear, normally dispersive (ND) fibre and experimentally demonstrate triangular pulse formation in conventional ND fibre.

All-fiber passively mode-locked femtosecond laser using a 45º-tilted fiber grating polarization element

We report on the demonstration of an all-fiber femtosecond erbium doped fiber laser passively mode-locked using a 45º tilted fiber grating as an in-fiber polarizer in the laser cavity. The laser generates 600 fs pulses with output pulse energies ~1 nJ. Since the 45° tilted grating has a broad polarization response, the laser output has shown a tunabilty in wavelength from 1548 nm to 1562 nm by simply adjusting the polarization controllers in the cavity.

Watt-level L band superfluorescent fiber source.

An L band superfluorescent fiber source (SFS) with output power of 0.94W is presented, under 4.4W 976nm pump power. The optical conversion efficiency is about 21%. The spectrum covers the broad wavelength range from 1560nm to 1615nm. The high power L band SFS is constructed by a low power L band amplified spontaneous emission (ASE) seed source and a high power erbium-ytterbium co-doped fiber (EYDF) amplifier in double pass forward pumping configuration.

Dual-stage superfluorescent fiber source with 1.16-W output power centered at 1561 nm

A 1.16-W superfluorescent fiber source (SFS) centered at 1561 nm with a 3-dB bandwidth of 8 nm has been achieved, under the pumping of a 3.56-W 976-nm laser diode array. The optical conversion efficiency reaches 32%. The source is constructed in a dual-stage configuration. The first stage is an ASE seed source with output power about 30 mW in the C band. The second stage is a backward-pumped high-power erbium-ytterbium-codoped double-clad fiber amplifier. An interesting phenomenon has been observed: low-power ASE seed source causes laser oscillation in the SFS, yet a relatively high-power ASE seed source prevents the SFS from lasing. The Rayleigh backscattering and the saturation effect of the amplifier are considered to explain the phenomenon.

An all fibre ring cavity watt-level erbium–ytterbium co-doped fibre laser

An erbium–ytterbium co-doped fibre ring laser pumped by a 976 nm laser diode array is presented. The laser power reaches 1.07 W at 1565.8 nm under 3.5 W pump power, with an optical conversion efficiency of 30.6%. The optimal output splitting ratio is demonstrated to be 99%. Stable output with a fine-shaped laser spectrum is achieved, although no wavelength selecting component is utilized. The laser linewidth increases with pump power and approaches 0.5 nm at high pump power levels, indicating the inhomogeneous broadening property of the gain medium.

Bandwidth broadening and efficiency enhancement of a double-pass forward L-band erbium-doped superfluorescent fibre source

A novel double-pass forward L-band erbium-doped superfluorescent fibre source (SFS) with a segment of unpumped fibre between the reflector and the wavelength division multiplexing (WDM) coupler is presented. The effects of the fibre length arrangement on the output characteristics of the L-band SFS are simulated. The spectral bandwidth is broadened and the conversion efficiency is enhanced by optimizing the fibre length ratio of the unpumped section to the total erbium-doped fibre. A broadband L-band SFS is obtained experimentally, with a 3 dB bandwidth of 46 nm, a power ripple less than 1 dB, and an output power of 23.6 mW.

Generation of triangular pulses in normally dispersive fibre

Optical pulses with simple temporal intensity profiles, e.g. flat-top and triangular shapes, are attractive for various photonic applications [1]. Recently, we have introduced a method for passive nonlinear pulse shaping which relies on the combination of pulse pre-chirping and propagation in a section of normally dispersive fibre (NDF) [2]. In this scheme, nonlinearity and dispersion in the NDF lead to various reshaping processes of an initial, conventional pulse according to the chirping value and power level at the input of the fibre. In particular, we have observed that triangular-shaped pulses can be generated for sufficiently high energies and a positive initial chirp parameter (using the definition iCt² for the phase profile). Figure 1 shows the pulse reshaping processes to a nearly ideal triangular intensity profile that can take place in a NDF for different C values, as obtained from numerical simulations. There, we used the misfit parameter M between the pulse intensity profile and a triangular fit of the same energy and full-width at half-maximum R (FWHM) duration (M² = ∫ dt (|u|² − |u<inf>T</inf>|²)²/ ∫dt |u|⁴ with u<inf>T</inf> corresponding to the intensity profile |u<inf>T</inf>(t)|²=P<inf>0,T</inf>(1−|t/τ−|t|)) to define regions of triangular pulse formation in the plane energy parameter N = (T²γP<inf>0</inf>/β<inf>2</inf>)^1/2 - normalized distance ξ= β<inf>2</inf>z/T² (T and P<inf>0</inf> are the respective characteristic width and peak power of the initial pulse, and β<inf>2</inf> and γ are the respective dispersion and nonlinearity parameters of the fibre). A typical example of shaped pulse intensity profile and of the corresponding chirp (first time derivative of the phase) profile is also shown.

Double-pass bidirectional pump broadband L-band erbium-doped superfluorescent fiber source

In this paper, a novel double-pass bidirectional pump broadband L-band erbium-doped superfluorescent fiber source (SFS) is demonstrated for the first time. In this fiber source, the EDF is divided into two segments, one of which (EDF2) is bidirectional pumped by a 980nm laser diode through two wavelength division multiplexers (WDM), and the other one (EDF1) is arranged between the reflector and the first WDM. EDF1 is unpumped. The fiber length ratio of the EDF1 length to the total length is defined as RL=LEDF1/(LEDF1+ LEDF2). The pump power ratio of forward to total pump power of EDF2 is defined as K=Pforward/Ptotal. The effects of the fiber length and pump power arrangement on the output characteristics of the L-band fiber source are simulated. With an appropriate pump power ratio K and an optimal fiber length ratio RL, broadband L-band erbium-doped SFS with flat output spectrum can be obtained. Additionally, the optimal fiber length ratio RL is also depended on the pump power ratio K. When K>0.4, the optimal RL tends to be changeless. When K=0.1, the optimal RL is 0 and widest flat spectrum is achieved with a 3-dB bandwidth of 63 nm (1540nm-1603nm).

Cavity optimization of erbium-ytterbium co-doped fiber ring lasers

The cavity configurations of erbium-ytterbium co-doped fiber ring lasers (EYDFL) have been experimentally investigated. Additional attention has been paid to the mode competition effect of the laser. It is demonstrated that even in a traveling wave cavity, mode competition occurs when the cavity configuration or the output splitting ratio are incorrectly chosen. By employing the proper cavity configuration and an optimal output splitting ratio, an extremely stable ring cavity EYDFL with fine-shaped laser spectrum is obtained at 1565.8nm. Output power of 1.07 W is achieved under 3.5W 976nm pump power, with an optical conversion efficiency of 30.6%.