Flexible pulse-controlled fiber laser

Abstract

Controlled flexible pulses have widespread applications in the fields of fiber telecommunication, optical sensing, metrology, and microscopy. Here, we report a compact pulse-controlled all-fiber laser by exploiting an intracavity fiber Bragg grating (FBG) system as a flexible filter. The width and wavelength of pulses can be tuned independently by vertically and horizontally translating a cantilever beam, respectively. The pulse width of the laser can be tuned flexibly and accurately from ~7 to ~150 ps by controlling the bandwidth of FBG. The wavelength of pulse can be tuned precisely with the range of >20 nm. The flexible laser is precisely controlled and insensitive to environmental perturbations. This fiber-based laser is a simple, stable, and low-cost source for various applications where the width-tunable and/or wavelength-tunable pulses are necessary.

Figure 1

(a) Laser setup. EDF, erbium-doped fiber; WDM, wavelength-division multiplexer; PC, polarization controller; LD, laser diode; CIR, circulator; FBG, fiber Bragg grating; CNT, carbon nanotube. (b) FBG system. A uniform FBG is glued in a slanted direction onto the lateral side of a right-angled triangle cantilever beam. The flexible cantilever beam is made of polyurethane. (c) Nonlinear absorption characterization of the CNT-SA. The solid curve is fitted from the experimental data (circle symbols). (d) Absorption spectra of the pure polyvinyl alcohol (PVA) and the CNT-PVA composite. The red stripe illustrates the spectral gain region of the Er³⁺-doped fiber.

Figure 2. Schematic diagram of bandwidth-tunable and wavelength-tunable operations by flexibly controlling FBG.

(a) Without the translation in the free state, (b) vertically translating the screw G_z along the direction of z-axis, and (c) horizontally translating the screw G_x along the direction of x-axis.

Figure 3. Typically reflection spectra of FBG.

(a) Vertically translating the screw G_z along z-axis. The FBG bandwidth Δλ_B is changed whereas the central wavelength approximately is fixed. Δλ_B is about 0.8, 1, 1.4, 1.8, 2.2, 3, 3.4, and 4 nm from inner to outer, respectively. (b) Horizontally translating the screw G_x along x-axis. The central wavelength of FBG is tuned with the range of >20 nm, whereas the spectral profile changes slightly.

Figure 4. Typical laser characteristics.

(a) Optical spectra at the FBG bandwidths (Δλ_B) of 0.17, 0.35, 0.71, and 1.48 nm (from inner to outer) by vertically translating the screw G_z in Fig. 1(b). (b) Autocorrelation traces of the experimental data (circle symbols) and sech²–shaped fit (solid curves). The FWHM spectral bandwidths (Δλ) and the corresponding pulse widths (Δτ) are about 0.035 nm and 73.5 ps, 0.07 nm and 35 ps, 0.216 nm and 14.6 ps, and 0.386 nm and 9.2 ps, respectively. (c) Fundamental RF spectrum with the resolution of 2 Hz and the span of 200 Hz. Inset in the top left corner: RF spectrum with the resolution of 10 Hz and the span of 80 kHz. Inset in the top right corner: oscilloscope traces with the separation of ~104.13 ns, corresponding to 9.60376 MHz of the fundamental harmonic frequency that is independent of the pump power. (d) Wideband RF spectrum up to 3 GHz.

Figure 5. (a) Pulse width Δτ and (b) spectral bandwidth Δλ with respect to the FBG bandwidth Δλ_B.

The beam is translated vertically, i.e., translating the screw G_z along the direction of z-axis.

Figure 6. Output spectra of laser by horizontally translating the screw G_x along the direction of x-axis.