Acoustic-induced switching of the reflection wavelength in a fiber Bragg grating

Spectro-temporal evolution of mode-locked lasing in fiber frequency-shifted feedback laser

As a type of mode-locked fiber laser, fiber frequency-shifted feedback lasers (FSFLs) have been rarely studied regarding the buildup process of mode locking. We carried out an experimental investigation to show the unique characteristics of the process, such as the direct evolution of the Q switched mode-locked state to the mode-locked state, the unique phenomenon of spectral center frequency oscillation, evolution to a double-pulse state in harmonic mode locking, millisecond stabilization time, and exponentially decreasing change of relative phase per round trip. For the first time, to the best of our knowledge, the double-beam heterodyne method is applied to measure the evolution of laser spectra with pulse width in the nanosecond range. Understanding the unique buildup mechanism may aid in the engineering and application of FSFLs.

All-fiber frequency shifter consisting of a fiber Bragg grating modulated via an acoustic flexural wave for optical heterodyne measurement

We present an all-fiber frequency shifter (AFFS) consisting of a fiber Bragg grating (FBG) modulated via an acoustic flexural wave for optical heterodyne measurement. The AFFS can efficiently generate the frequency-shifted signal due to the resonance peak with a high-reflection efficiency and being completely separated from the reflection spectrum of the original FBG, simultaneously. The experimental result shows that the minimal measurable vibration amplitude and the resolution of the all-fiber optical heterodyne measurement setup constructed with the AFFS are 0.06 nm and 30 pm in the region of tens to hundreds of kilohertz, respectively.

All-fiber 10 MHz acousto-optic modulator of a fiber Bragg grating at 1060 nm wavelength

Acousto-optic modulation of a 1 cm fiber Bragg grating at 10.9 MHz frequency and 1065 nm wavelength is demonstrated for the first time. A special modulator design is employed to acoustically induce a dynamic radial long period grating which couples power of the fundamental mode to the higher-order modes supported by the Bragg grating. A modulated reflection band with a depth of 16 dB and 320 pm bandwidth has been achieved. The results indicate a higher modulation frequency compared to previous studies using flexural acoustic waves. In addition, the reduction of the grating length and the modulator size points to compact and faster acousto-optic modulators.

Spectra analysis of nonuniform FBG-based acousto-optic modulator by using Fourier mode coupling theory

Fourier mode coupling theory was first employed in the spectral analysis of several nonuniform fiber Bragg grating (FBG)-based acousto-optic modulators (NU-FBG-AOMs) with the effects of Gaussian-apodization (GA), phase shift (PS), and linear chirp (LC). Because of the accuracy and simplicity of the algorithm applied in this model, the modulation performances of these modulators can be acquired effectively and efficiently. Based on the model, the reflected spectra of these modulators were simulated under various acoustic frequencies and acoustically induced strains. The simulation results of the GA-FBG-AOM and PS-FBG-AOM showed that the wavelength spacing between the primary reflection peak and the secondary reflection peak is proportional to the acoustic frequency, and the reflectivity of reflection peaks depends on the acoustically induced strains. But for the LC-FBG-AOM, the wavelength spacing between the neighboring reflection peaks increased linearly and inversely with the acoustic frequency, and the extinction ratio of each peak relates to the acoustically induced strain. These numerical analysis results, which were effectively used in the designs and fabrications of these NU-FBG-AOMs, can broaden the AOM-based application scope and shed light on the performance optimization of optical wavelength-division multiplex system.

Time-frequency analysis of long fiber Bragg gratings with low reflectivity

A new technique to investigate the spatial distribution of the reflection spectrum along fabricated long weak fiber Bragg gratings (FBG) is experimentally demonstrated, together with its potential applications for distributed fiber sensing and broadband signal processing. A short pulsed coherent light signal is launched into a FBG and the signal frequency is scanned through the FBG reflection spectrum. When the pulse duration is set much shorter than the transit time through the grating a time-resolved reflected signal can be obtained for each signal frequency. It informs about the distribution of the refractive index periodic perturbation along the entire FBG length, hence the uniformity or frequency chirp information of the fabricated FBG. This technique has been implemented to demonstrate a distributed temperature sensing system with high spatial resolution and to also realize a robust all-fiber tunable delay line for broadband signals.

Research on FBG-based longitudinal-acousto-optic modulator with Fourier mode coupling method

Fourier mode coupling model was first applied to achieve the spectra property of a fiber Bragg grating (FBG)-based longitudinal-acousto-optic modulator. Compared with traditional analysis algorithms, such as the transfer matrix method, the Fourier mode coupling model could improve the computing efficiency up to 100 times with a guarantee of accuracy. In this paper, based on the theoretical analysis of this model, the spectra characteristics of the modulator in different frequencies and acoustically induced strains were numerically simulated. In the experiment, a uniform FBG was modulated by acoustic wave (AW) at 12 different frequencies. In particular, the modulator responses at 563 and 885.5 KHz with three different lead zirconate titanate (PZT) loads applied were plotted for illustration, and the linear fitting of experimental data demonstrated a good match with the simulation result. The acoustic excitation of the longitudinal wave is obtained using a conic silica horn attached to the surface of a shear-mode PZT plate paralleled to the fiber axis. This way of generating longitudinal AW with a transversal PZT may shed light on the optimal structural design for the FBG-based longitudinal-acousto-optic modulator.

Dynamic index modulation mechanism in polarization-maintained fiber Bragg gratings induced by transverse acoustic waves

A novel index modulation mechanism of polarization-maintained fiber Bragg gratings based on the microbend of stress members induced by a transverse acoustic wave is proposed and investigated experimentally. The index modulation leads to a series of ghost gratings with specific polarization, whose wavelengths can be tuned by the acoustic wave frequency and whose intensities depend on the vibration direction of the transverse acoustic wave. Our method provides a novel way to achieve polarization-dependent narrowband acousto-optic tunable filters.

Analysis of solving multimode-coupled equations and its improvement for modulated fiber Bragg gratings

An efficient numerical method to solve multimode-coupled equations with two point boundary conditions is improved. Our method [Abrishamian et al., Opt. Fiber Technol.13, 32-38 (2007)] based on theoretical matrix integration of coupled differential equations and then solving the system of equations by use of initial or final conditions would be straightforward and thus beneficial in comparison with previously used fundamental matrix methods that depend strongly on the initial guess. However, we found that the new analysis depends on how accurately the integrals of the matrix element are calculated. For accuracy in the matrix integration it is required to divide the system of equations into a large number of subsections. Then, the reflectivity calculated is found to be comparable to experimental data reported so far. The present method is highly applicable for simulation of any type of fiber Bragg gratings modulated by long period gratings.

Numerical analysis and optimization of optical spectral characteristics of fiber Bragg gratings modulated by a transverse acoustic wave

Acoustic waves that impinge transversely on a fiber Bragg grating (FBG) induce periodic microbends of the fiber, which modulate the phase index and lead to the changes of optical spectral characteristics of the FBG. We investigated the spectral characteristics of a FBG modulated by a transverse acoustic wave. The corresponding theoretical model is presented by modifying the multimode coupled equations. A fast algorithm based on the Newton-Raphson method is proposed to simulate numerically the spectral characteristics of such a FBG. Our numerical results are in excellent agreement with the known experimental results. For the first time, to our knowledge, the known experimental results have been reproduced by numerical simulations. Moreover, the optimization of the reflective spectra of such a FBG is also discussed. From the perspective of inherent physical mechanisms, the exceptional spectral characteristics of such a FBG are discussed as well.