Dynamic Gratings Research Articles

Spectral analysis of Brillouin dynamic gratings in polarization-maintaining fibers.

Experiment observations show that the spectra of Brillouin dynamic gratings in polarization-maintaining fibers based on a polarization decoupled scheme are quite broad and usually have multiple peaks. In this paper, we measure the birefringence distribution along polarization-maintaining fibers with high spatial resolution using optical frequency-domain reflectometry. Based on birefringence measurements, both the simulation and experiment are carried out to determine the spectra of Brillouin dynamic gratings generated within the same polarization-maintaining fibers, indicating quantitatively that spectral broadening and irregularity are mainly due to phase mismatch caused by the non-uniformity of birefringence along polarization-maintaining fibers.

Low-noise Brillouin random fiber laser with Auto-tracking dynamic fiber grating based on a saturable absorption ring

A low-noise Brillouin random fiber laser (BRFL) with stabilized lasing output and narrow linewidth is proposed and demonstrated experimentally. Stimulated Brillouin scattering and Rayleigh scattering are exploited to work as the gain and random distributed feedback mechanisms, respectively. A simple unpumped Erbium-doped fiber (EDF) loop structure which consists of a section of EDF and an optical coupler is introduced into the BRFL cavity, forming a saturable absorption ring (SAR). When light with proper intensity is incident into the SAR, a dynamic fiber grating (DFG) can be built up to work as a narrow-bandwidth filter with the ability of auto-tracking the center frequency. The SAR can effectively filter out dense random longitudinal modes and suppress multi-mode resonances and frequency jitters by restricting the mode-competition-induced mode hopping within a limited frequency range of ∼ 1 MHz and auto-tracking the center frequency of light in the cavity, simultaneously. Thus, the output of BRFL with SAR is much more stabilized than existing free-running BRFLs. In the experiment, a stable lasing output with an ultra-narrow linewidth (less than2kHz) is obtained from the proposed BRFL. The measured relative intensity noise and frequency noise demonstrate that the noise level of the proposed laser is significantly lower than that of the free-running BRFLs. This proposed technique not only provides a solution to the dilemma of previously reported BRFLs with severe lasing instabilities, but also makes a step towards the development of practical and reliable BRFLs. This high-quality laser source with advantages of high stability, low cost, low threshold power, high coherence, narrow linewidth, and low noise can find useful applications in coherent optical detection, high-resolution spectrometers, microwave photonics, and optical sensing fields.

Stabilized Long-Distance Superluminal Propagation Based on Polarization-Matched Low-Noise Brillouin Lasing Resonance

This paper comprehensively investigated noise characteristics of superluminal propagation based on low-noise single-frequency Brillouin lasing oscillation with the aid of a population inversion dynamic grating. Thanks to high-degree polarization alignment between the Brillouin pump and the lased Stokes lightwaves in polarization maintaining fibers, efficient Brillouin lasing resonance with over 10-dB relative intensity noise suppression has been demonstrated to activate Brillouin loss-induced anomalous dispersion in the vicinity of pump signals, benefiting a noise-insensitive superluminal propagation along kilometer-long optical fibers with robust resistance to ambient disturbance. Consequently, sinusoidally modulated pump signals experienced the time advancement of 4634.0 ns at the group velocity of 10.63 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> . Results show that the variance of the fractional advancement with polarization maintaining fibers is 2.54 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−4</sup> which is two orders of magnitude lower than that of conventional single mode fibers. Furthermore, the dependence of the group velocity on the modulation frequency was experimentally investigated, showing good agreement with the theoretical analysis.

Recording the dynamic polarisation gratings in pure nematic liquid crystal

We report on experimental realisation of polarisation dynamic gratings recording in pure nematic liquid crystal. This work is devoted to the experimental implementation of writing polarisation dynamic gratings in a pure nematic liquid crystal. The registration mechanism is based on the photorefractive effect in a cell filled with a pure nematic crystal under the action of an applied constant electric field. This phenomenon does not require the presence of photoexcited charge carriers in the liquid crystal and is explained by the extremely high anisotropy of the liquid crystal. The article provides an elementary theoretical model of this process. The dependences of the grating writing process on the spatial frequency of the interference field and the intensity of the recording beams are investigated. It was found that the spatial grating does not coincide with the interference field – the response of the medium is non-local. However, we do not associate this property with that in photorefractive crystals. In our case, a mechanism of purely geometric mismatch is more likely due to the peculiarity of the recording mechanism. One of the fundamental properties of the described processes is their independence from the wavelength of the recording radiation, which confirms the model of the recording mechanism we have chosen.

A Dynamic Grating with Tunable Duty Cycle and Height

A Dynamic Grating with Tunable Duty Cycle and Height

Gain switched linearly polarized single-frequency pulsed fiber laser

Single-frequency pulsed fiber lasers have aroused intense interest due to their excellent performances in terms of good coherence, compact structure and good beam quality, which have been widely used in different areas, such as coherent LIDAR, nonlinear frequency conversion, and remote sensing. In this paper, a linearly polarized single-frequency pulsed fiber laser is reported. The narrow linewidth all-fiber oscillator is built by using an all-polarization-maintaining ring cavity structure. A section of unpumped polarization-maintaining ytterbium doped fiber is fused in the cavity as a saturated absorber to produce ultra-narrow bandwidth dynamic grating, which can be used for longitudinal mode selection. Thus, the laser can realize single-frequency operation. A 976-nm semiconductor laser is used as a pump source to generate a hybrid pump laser, which contains both pulsed component and continuous component. As a result, a long-pulse laser is achieved with a repetition rate of 10–90 kHz and a pulse duration of 1–8 μs. At the same time, the effects of pump power and repetition rate on the time domain, the frequency domain and the power characteristics of the output laser are investigated. It is found that there is an optical bistability in the frequency domain characteristic of the output laser within a certain power range. And the influence of the laser power characteristic on the longitudinal mode selection mechanism of dynamic grating is analyzed. Finally, through parameter optimization, single-frequency pulsed fiber laser is achieved with a center wavelength of 1064 nm, a linewidth of about 23.5 MHz, a repetition rate of 10–90 kHz, a pulse duration of 4–8 μs, and a polarization extinction ratio of about 29 dB.

Three modes of the nonstationary holographic current excitation in a gallium oxide crystal

We report the nonstationary holographic current excitation in a β-Ga2O3 crystal at light wavelength . The material demonstrates insulating properties and high transparency for visible light, but this, however, does not prevent the dynamic space-charge grating formation and the holographic current observation for various external electric fields —zero, dc and ac ones. The signal amplitude is measured and analyzed vs. the frequency of phase modulation, spatial frequency and electric field value. The main photoelectric parameters such as specific photoconductivity, sensor responsivity and diffusion length of carriers are determined for the blue region of spectrum.

Experimental observation of chaotic Brillouin dynamic grating.

We experimentally observe the local Brillouin dynamic grating (BDG) based on a chaotic laser in a polarization-maintaining fiber for the first time, to the best of our knowledge. The grating length of the chaotic BDG can be adjusted by changing the optical spectral width of the chaotic laser. The characteristics of the reflection spectrum versus the grating length are further analyzed, which agrees with the theory of fiber Bragg grating. Temperature distributed measurements based on the chaotic BDG have been demonstrated with a spatial resolution of an order of centimeter.

Nonstationary holographic currents in a β-Ga2O3 crystal at wavelength λ = 457 nm

We report the excitation of nonstationary holographic currents in a monoclinic gallium oxide crystal. Although the crystal is almost transparent and insulating for a visible light, the dynamic space-charge gratings are recorded and holographic currents are observed for both the diffusion and drift modes. The anisotropy along the [100] and [010] directions is revealed, namely, there is a small difference in the transport parameters and a pronounced polarization dependence of the signal. The crystal’s photoconductivity, responsivity and diffusion length of electrons are estimated for the light wavelength λ = 457 nm.

Ultrahigh spectral resolution single passband microwave photonic filter.

Microwave photonic filters (MPFs) with only one ultra-narrow passband are able to provide high frequency selectivity and wide spectral range, and they are of great importance in radio-frequency (RF) signal processing. However, currently all MPFs are limited by trade-offs between key parameters such as spectral resolution and range, tunability, and stability. Here, we report the first demonstration of a single passband MPF with unprecedented performance including ultrahigh spectral resolution of 650 kHz, 0-40 GHz spectral range, and high stability of center frequency drifting within ±50 kHz. This record performance is accomplished by breaking the amplitude equality of a phase-modulated signal via a Brillouin dynamic grating (BDG) which has an ultra-narrow reflection spectrum of sub-MHz. The results point to new ways of creating high performance microwave photonic systems, such as satellite and mobile communications, radars, and remote-sensing systems.

Sub-kHz-Linewidth Wavelength-Tunable Single-Frequency Ring-Cavity Fiber Laser for C- and L-Band Operation

In this paper, a sub-kHz-linewidth wavelength-tunable single-frequency ring-cavity fiber laser operating in C- and L-band is demonstrated experimentally. In our design, a single-longitudinal mode is achieved in a wide tuning range by a narrow-band filtering device composed of a fiber Fabry-Pérot interferometer and a 1.6-m-long un-pumped Er <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped fiber as a dynamic grating. Under the control of a fiber Fabry-Pérot tunable filter, a wavelength tuning range of 76.69 nm from 1535.50 to 1612.19 nm with a tuning interval of about 0.2 nm is finally achieved. In the entire tuning range of the fiber laser, the laser linewidth is less than 310 Hz. The measured optical signal-to-noise ratio is over 40 dB and the relative intensity noise is lower than -143 dB/Hz at frequencies above 1 MHz. Our results suggest that such fiber laser with narrow-linewidth and superior wavelength tunability can serve as a promising candidate applied in optical communication system.

Spacially resolved coupled mode analysis for TMI threshold powers in quantum and Rayleigh scattering limits.

A 3D spatially resolved coupled mode and perturbation analysis for the transverse mode instability (TMI) threshold powers in Yb-doped fiber amplifiers is presented in this paper. Threshold powers are computed in the quantum and thermal Rayleigh scattering limits and are compared with those calculated by other coupled mode analyses. Quantum-limited threshold powers are found to be more than three times greater than those calculated with coupled-mode analyses that use uniform and/or average gain approximations. The analysis presented here includes pump depletion, gain saturation, and transverse hole burning. Simulations are applied to co-, cnt-, and bidirectionally pump amplifier configurations. The appearance of TMI is attributed to the formation of a dynamic thermal grating, which enables the exchange of optical power between the fundamental mode (FM) and higher-order mode (HOM). The sole approximation made is that the power in the HOM is much less than that in the FM. A distributed thermal Rayleigh scattering model is introduced that includes a ray-optic representation of the fiber mode structure that relates the Rayleigh power captured by the HOM to the waveguide structure. The location and strength of the thermal gratings are identified to assist in the application of mitigation techniques.

Frequency Conversion of Lasers in a Dynamic Plasma Grating

When a dynamic medium in which the laser propagates changes its refractive index in time, the laser changes its frequency while keeping its wavevector unchanged to fullfil the dispersion relation. This is usually applied to upshift the laser frequency with ionizing plasma. We propose an alternative technique to modify light frequency. A transient plasma grating can be generated by two identical counterpropagating laser pulses via strongly coupled stimulated Brillouin scattering (SC SBS). The rapid evolution of the plasma grating affects the wave-dispersion relation and a band gap develops around the laser frequency, dependent on the grating amplitude. As a result, the lasers convert their frequency downward to the low edge of the band gap, while a free-traveling laser converts its frequency to both the upper and lower edge of the band gap. Depending on the considered setup, practical applications of this technique include either laser-frequency downshift or spectral splitting can be exploited. The former can be used for Raman amplification in plasma and the latter for dual-color x-ray generation by Thomson and/or Compton scattering.

Recent Progress in Distributed Brillouin Sensors Based on Few-Mode Optical Fibers.

Brillouin scattering is a dominant inelastic scattering observed in optical fibers, where the energy and momentum transfer between photons and acoustic phonons takes place. Narrowband reflection (or gain and loss) spectra appear in the spontaneous (or stimulated) Brillouin scattering, and their linear dependence of the spectral shift on ambient temperature and strain variations is the operation principle of distributed Brillouin sensors, which have been developed for several decades. In few-mode optical fibers (FMF’s) where higher-order spatial modes are guided in addition to the fundamental mode, two different optical modes can be coupled by the process of stimulated Brillouin scattering (SBS), as observed in the phenomena called intermodal SBS (two photons + one acoustic phonon) and intermodal Brillouin dynamic grating (four photons + one acoustic phonon; BDG). These intermodal scattering processes show unique reflection (or gain and loss) spectra depending on the spatial mode structure of FMF, which are useful not only for the direct measurement of polarization and modal birefringence in the fiber, but also for the measurement of environmental variables like strain, temperature, and pressure affecting the birefringence. In this paper, we present a technical review on recent development of distributed Brillouin sensors on the platform of FMF’s.

Observation and evaluation of dynamic population gratings formed by two‐wave mixing in an erbium‐doped fiber amplifier

AbstractDynamic population gratings formed by two‐wave mixing in an erbium‐doped fiber amplifier are reported. One of the counter‐propagating fields is intensity‐modulated to distinguish reflection from the grating from amplified transmission fields. The modulation‐frequency dependence of the reflection is studied under single‐wavelength and two‐wavelength input conditions to separate reflection from transient gain. The results are analyzed by measuring the gain saturation and recovery times using self‐probe and pump–probe techniques.

Single-frequency all-polarization-maintaining ytterbium-doped bidirectional fiber laser.

We reported an all-polarization-maintaining single-frequency ytterbium-doped bidirectional fiber laser for the first time, to the best of our knowledge. Single-frequency operation was achieved by a stable dynamic grating in the active fiber of a proper length owing to the bidirectional operation of the laser. The fiber laser possesses a linewidth of 7.43 kHz, a slope efficiency of 47.9%, and a great long-term stability.

Slope-Assisted Brillouin-Based Distributed Fiber-Optic Sensing Techniques

Brillouin-based fiber-optic sensing has been regarded as a powerful distributed measurement tool for monitoring the conditions of modern large civil and geotechnical structures, since it provides continuous environmental information (e.g., temperature and strain) along the whole fiber used for sensing applications. In the past few decades, great research efforts were devoted to improve its performance in terms of measurement range, spatial resolution, measurement speed, sensitivity, and cost-effectiveness, of which the slope-assisted measurement scheme, achieved by exploiting the linear slope of the Brillouin gain spectrum (BGS), have paved the way for dynamic distributed fiber-optic sensing. In this article, slope-assisted Brillouin-based distributed fiber-optic sensing techniques demonstrated in the past few years will be reviewed, including the slope-assisted Brillouin optical time-domain analysis/reflectometry (SA-BOTDA/SA-BOTDR), the slope-assisted Brillouin dynamic grating (BDG) sensor, and the slope-assisted Brillouin optical correlation domain analysis/reflectometry (SA-BOCDA/SA-BOCDR). Avenues for future research and development of slope-assisted Brillouin-based fiber-optic sensors are also prospected.

Characteristics of Reflection Spectrum of Brillouin Dynamic Grating in Single Mode Fibers

Characteristics of Reflection Spectrum of Brillouin Dynamic Grating in Single Mode Fibers

布里渊动态光栅的研究进展

布里渊动态光栅的研究进展

Wavelength-flexible all-polarization-maintaining self-sweeping fiber laser based on intracavity loss tuning

We reported a wavelength-flexible all-polarization-maintaining self-sweeping fiber laser based on the intracavity loss tuning brought by the bent optical fiber. The bidirectional cavity structure achieved the self-sweeping effect due to the appearance of the dynamic grating in the active fiber with the spatial hole burning effect. Under this, a section of fiber was bent into a circle for adjusting the loss of the cavity. With a descending diameter of bent fiber circle, the sweeping range moves to the shorter wavelength and covers a wide range from 1055.6 to 1034.6 nm eventually. Both the initial wavelength of self-sweeping regime and the threshold of the fiber laser show exponential correlation with the diameter of the circular fiber. Our work provides a compact and low-cost way to achieve the broad wavelength-flexible self-sweeping operation.