Adaptive mode control based on a fiber Bragg grating

Experimental investigation of point-by-point off-axis Bragg gratings inscribed by a femtosecond laser in few-mode fibers

Off-axis Bragg gratings with varied horizontal and vertical distances off the center in a step-index two-mode fiber were fabricated by 800 nm infrared-femtosecond laser pulses through a point-by-point technique. In this article, we experimentally investigate these gratings via measuring the transmitted power and the reflected intensity profiles under different input polarization, with multiple characteristics reported for the first time to the best of our knowledge. To highlight, we find that the birefringence induced to the LP₀₁ reaches its maximum magnitude at an intermediate offset, followed by the fast and slow axes switching at a further slightly increased offset. We also show that the peak reflectivity of the LP₁₁ exhibits strong polarization dependence, with the much stronger peak reflectivity constantly corresponding to the polarization perpendicular to the damage-point-to-center line, whereas the peak reflectivity of the LP₀₁ has almost no polarization dependence. Moreover, we report that the reflected mode patterns of the cross-coupling of the LP₀₁ and LP₁₁ are linked to the direction of linear polarization, through which one can selectively excite an arbitrarily oriented LP₁₁ by merely altering the polarization.

Simultaneous Temperature and Strain Measurements Using Polarization-Maintaining Few-Mode Bragg Gratings

Simultaneous measurement of temperature and strain was demonstrated using a polarization-maintaining few-mode Bragg grating (PM-FMF-FBG) based on the wavelength and phase modulation of the even LP11 mode. The wavelength shift sensitivity and the interrogated phase sensitivity of the temperature and strain were measured to be 10 pm·°C⁻¹ and 0.73 pm·με⁻¹ and −3.2 × 10⁻² rad·°C⁻¹ and 4 × 10⁻⁴ rad·με⁻¹, respectively, with a discrimination efficiency of 98%. The polarization interference led to selective polarization excitation of the reflection spectra, and the calculated phase sensitivity agreed with the experimental results.

High-power, cladding-pumped all-fiber laser with selective transverse mode generation property

We demonstrate, to the best of our knowledge, the first cladding-pumped all-fiber oscillator configuration with selective transverse mode generation based on a mode-selective fiber Bragg grating pair. Operating in the second-order (LP₁₁) mode, maximum output power of 4.2 W is obtained with slope efficiency of about 38%. This is the highest reported output power of single higher-order transverse mode generation in an all-fiber configuration. The intensity distribution profile and spectral evolution have also been investigated in this paper. Our work suggests the potential of realizing higher power with selective transverse mode operation based on a mode-selective fiber Bragg grating pair.

Mode-division-multiplexing of absorption-based fiber optical sensors

We theoretically consider the possibility of using a few-mode fiber (FMF) and mode-division-multiplexing (MDM) to construct a quasi-distributed network of absorption-based fiber optical sensors. In this design, we utilize the low-attenuation fundamental linearly polarized (LP) mode for signal transmission, and a high-attenuation LP mode for absorption-based sensing. We develop a matrix formalism and use it to analyze the performance of such a MDM sensor network. We demonstrate that such a sensor network can indeed combine high sensitivity with large scale multiplexing, which is very difficult to achieve in traditional single-mode-fiber-based sensor networks.

Adaptive mode control of a few-mode fiber by real-time mode decomposition

A novel approach to adaptively control the beam profile in a few-mode fiber is experimentally demonstrated. We stress the fiber through an electric-controlled polarization controller, whose driven voltage depends on the current and target modal content difference obtained with the real-time mode decomposition. We have achieved selective excitations of LP01 and LP11 modes, as well as significant improvement of the beam quality factor, which may play crucial roles for high-power fiber lasers, fiber based telecommunication systems and other fundamental researches and applications.