The Studies Of Phase Characteristics And Control Of Fiber Laser Beams

High power and beam quality laser has been becoming a hot field for a long time. Inthe past, lamp or diode laser pumped bulk solid-state medium laser was one of mainmethods for realizing the desired high power output. However, the thermal effectwithin the bulk gain medium has prevented this bulk laser from achieving a muchhigher output. As a contrast, fiber lasers developed in recent years have shown someexcellent characters that the bulk solid state lasers don't have, such as a higherquantum conversion efficiency, better output beam quality and higher output powerwith simple air-cooling mainly due to its large exterior surface-bulk volume rate. It hasbeen reported lately that the IPG Corporation has achieved thousand-watt level outputpower using a single fiber by a multi-branch pumped structure. Nevertheless, for thepurpose of achieving greater output power and avoiding the restriction of the lowerdamage threshold value of a single fiber, many research efforts have been put on fiberlaser beams combining techniques. Among those research activities the simplest one iscombining several beams together spatially. However, unless all the beams are of thesame polarization, wavelength and locked phase, diffraction limited beam with muchhigher output power density can not be achieved. A more effective way to achievehigher output is coherently combining the fiber laser beams. It can be realized, forinstance, by splitting a low power single frequency laser beam from a seed lasersource into N beams, one of them is used as a reference beam, while all others arerespectively injected into their corresponding dual-cladding fiber amplifiers. If allamplified beams can be kept as with the same phase and polarization direction, thepower density of the combined beam of these amplified beams in the far field is N2times of that of a single beam, assuming that all amplified beams are of the samepower.High power laser beam with beam quality approaching diffraction extreme would beachieved by coherent combining techniques, witch make the techniques would beapplied widely. At present, the studies about it are not plenitude especially in ourcountry. It is not easy to make several laser beams combining coherently with all thefactors, such as intensity, phase, polarization, keeping equality. Especially eliminatingphase fluctuations of the beams transmitted in fibers is a critical problem in multibeam coherent combining techniques. Obviously, one of critical concerns forsuccessfully doing coherent combining is to determine the phase fluctuationcharacteristics of the beam amplified by the fiber amplifiers relative to the referencebeam for the sake of determining requirement of any potential phase control system.As a part of the efforts in the research work on this issue, phase changing rates of laserbeam in a single mode polarization maintaining fiber (PMF), a dual claddingYb-doped fiber with rectangular inner cladding (DCF) and a dual cladding Yb-dopedlarge-mode-area fiber (LMF) are studied by means of interference under situationswith and without exterior disturbance. At the same time, the other important work thata phase control system which suit for MOPA coherent combining system is designedin according to above result.The result of our study shows that the frequency scope of phase fluctuation of beamstransmitted through all these three kinds of fibers PMF DCF LMF are respectively inthe range of 100~200Hz, 100~300Hz, 80~400Hz in normal laboratory environment.The frequency scope of phase fluctuation of beams transmitted through DCF understatic pressure, vibration condition and changing temperature condition arerespectively in the range of 100~600Hz, 100~300Hz and100~500Hz. The frequencyscope of phase fluctuation of beams transmitted through LMF under static pressure ,vibration condition and changing temperature condition are in the range of 150~600Hz,100~400Hz and100~1000Hz respectively. In a brief, the result of our study is that thefrequency scope of phase fluctuation of beams transmitted in all these three kinds offibers are mainly in the range of 100~1000Hz. This means in order to realize a stableof the laser beams from multiple fibers/fiber amplifiers, the band width of anypotential phase control system has to be over 1 KHz at least.Based on the requirements of the phase control system in MOPA coherent combiningand above conclusion after investigating and compare several phase control methods,we designed a LiNbO3 wave-guide phase control system. Photoelectric detection,phase control circuit, LiNbO3 crystal drive part and LiNbO3 wave-guide constitute ofphase control loop. We use a diode pumped Nd3+:YVO3 single frequency linearpolarization laser with center wavelength at 1064nm laser as seed source, the spectrumline width of which is less than 5MHz. The source beam is divided by a beam splitterinto two beams: one is used as reference beam while another (named as transmittingbeam) is injected into fiber. The reference beam propagates in the free space, while thetransmitting beam transmits through the fiber and after output interferes with thereference beam. It is assumed that the phase of the reference beam doesn't changewith time while the phase of the transmitting beams exchanging with time. Theintensity of the interfered light field changes with the phase difference between thereference beam and transmitting beam. The rate of changing depends on the phasefluctuating rate of the transmitting beam in the fiber compare to the reference beam. Aphotodiode is used to detect the intensity of interfered beam, and then the signal is sentto the phase control circuit, after rectified it is amplified by LiNbO3 crystal drive part.As a result the phase of the beam transmit through LiNbO3 wave-guide is controlled.Experiments proved that the phase fluctuations could be controlled by the phasecontrol system, with the results of the phase matching rate reaches 0.87, and theundulate rate of coherent beam intensity is 3.8%.