A new hybrid optical fiber sensing system is proposed, which uses optical filter and coherent detection method to separate the mixed Rayleigh, Brillouin and Raman signal. In the sensing system the optical Raman signal is filtered out by corresponding optical fiber grating, and then by an optical local oscillator the optical Rayleigh and Brillouin signals are converted to radio frequency (RF) signals through a balanced photodetector (BPD) with two phase-inverse output ports. Then the Rayleigh RF from one output port of the BPD is filtered out by a RF filter with center frequency of 200MHz and bandwidth of 5MHz, and then the power of the RF Rayleigh signals are extracted by a RF power detector. And for the RF Brillouin signal extraction, a band pass filter with center frequency of 11.45GHz and bandwidth of 1.7GHz is used to filter the RF signals from the other output port of the BPD, so that the RF Brillouin signals are purified and then by a mixer and a sweeping RF source the details of the RF Brillouin spectra are characterized. The method can make the hybrid optical fiber sensing system more compact, and enhance the sensing performance, which benefits for the cost reduction.
Contaminations existing inevitably in high-power laser facilities modulate laser beams and decrease beam quality. This study set up a detection system to study the mechanism of initial filamentary damage in optical components induced by surface contaminations. The effect of ordinary solid particles, liquid particles, and solid-liquid mixed particles on the near-field intensity distribution of laser beam was studied and analyzed statistically. The experiment results show that pure solid particles make the beam generate diffraction rings with dark center usually in the shadow of the particles which is a weak intensity modulation; pure liquid particles focus the localized beam into a bright spot rapidly, but it is diffracted away soon; solid-liquid mixed particles cause diffraction rings with strongly bright center, but the high local intensity can be diffracted away only after a longer distance, which is one of the reason that induces the initial filamentary damage to optical components. The research results can predict the likelihood of component damage, and the corresponding preventive measures help to keep the safe operation of high-power laser facilities.
Surface shape of optical components is an essential factor of the laser beam quality. Different types of surface correspond to different characteristics of the laser focal spot. Striated surface shape is one of common and typical cases of optical component surfaces in laser facilities, which have attracted great attention. For learning the impact of the component on focal spot in the far-field, a model component with the similar features was introduced in the study. Intensity distributions of focal spot in the far-field was simulated after laser beam went through the model component. Effects of the modulation depth and the modulation period on spot morphology were presented. Furthermore, the relations between these optical specifications and focal spots with some requirements had been analyzed. The results can enhance our understanding about striae degrees of optical elements and have reference values to guide the processing and the use of large-aperture components correctly.