On problem of the determination of operational mode of towed line array with distorted shape

The distortion of the array shape is one of the main factors which result the performance degeneration from the ideal situation of towed line array (TLA). Based on the ordinary array shape distortion, the directivity function of TLA is presented in this paper. An algorithm for precisely determning the coordinates of each element by means of the inverse elliptic function is derived. A fast approximation of recursive formula for solving distorted array shape is given. According to the comparison between ideal directivity and the directivity of distorted array, the criterion for making decision of operational mode of TLA sonar is presented. So that the performance prediction problem in TLA is solved. The results of system simulation in computer show a good agreement with the theoretical analysis.

Experimental study of mode distortion induced by stimulated Raman scattering in high-power fiber amplifiers

The experimental investigation of mode distortion induced by stimulated Raman scattering(SRS)in a high-power fiber amplifier,which includes the evolutions of optical spectra,spatial beam profiles,and time-frequency characteristics,has been carried out in detail.Temporal-frequency characteristics have been studied for the first time,to the best of our knowledge,by using a low-speed camera and high-speed photodiode traces,which revealed that temporal-frequency characteristics of SRS-induced mode distortion are different from traditional dynamic mode instability(MI).The experimental results show that the output beam profile remains stable before the mode distortion occurs and fluctuates obviously after the onset of SRS-induced MI but on a time scale of seconds,which is much lower than that of Yb-gain-induced MI featuring millisecond-level beam profile fluctuation.It also shows that the mode distortion became measurable in company with the onset of inter-mode four-wave mixing(IM-FWM)when the ratio of Raman light reaches 3%;further,the beam quality factor M2degrades gradually from 1.4 to 2.1 as the ratio of Raman light increases.The mode distortion is accompanied by an obvious temperature increase of the output passive fiber,which further confirms that the mode distortion originates from SRS.The cause of the mode distortion induced by SRS has been explained in the context of core-pumped SRS effect,and the investigation on the accompanying IM-FWM effect indicates that the main content of the SRSinduced high-order mode is the LP21 mode.

Effective suppression of mode distortion induced by amplifiers stimulated Raman scattering in high-power fiber

Mode distortion induced by stimulated Raman scattering(SRS)has become a new obstacle for the further development of high-power fiber lasers with high beam quality.Here,an approach for effective suppression of the SRS-induced mode distortion in high-power fiber amplifiers has been demonstrated experimentally by adjusting the seed power(output power of seed source)and forward feedback coefficient of the rear port in the seed source.It is shown that the threshold power of the SRS-induced mode distortion can be increased significantly by reducing the seed power or the forward feedback coefficient.Moreover,it has also been found that the threshold power is extremely sensitive to the forward feedback power value from the rear port.The influence of the seed power on the threshold power can be attributed to the fact that the seed power plays an important role in the effective length of the gain fiber in the amplifier.The influence of the forward feedback coefficient on the threshold power can be attributed to the enhanced SRS configuration because the end surface of the rear port together with the fiber in the amplifier constitutes a half-opening cavity.This suppression approach will be very helpful to further develop the high-power fiber amplifiers with high beam quality.