The Performance Testing Of Phase Modulator In Coherent Combining

Coherent beam combining of low power lasers or amplifiers plays an importantpart in obtaining high power and high beam quality laser. Optical phase modulatorwhose performance directly determine the success of coherent combining is a keycomponent to achieve this technology. On base of this, the performance of phasemodulator in coherent combining is studied in detail in this paper.On the basis of multi-dithering technique, the performance requirement of phasemodulator in high power coherent combining include the following four aspects: higherdamage threshold, higher linear frequency response bandwidth, low insertion loss andlow half-wave voltage.Based on the above analysis, the property of lithium niobate phase modulator,acousto-optic frequency shifter and piezoelectric ceramic phase modulator is studied inthis paper. The main work of this paper are shown as follow:1. On account of the lower damage threshold of regular commercial lithiumniobate phase modulator and high damage threshold of magnesium-dopes lithiumniobate crystal, a design of volume phase modulator is proposes.2. In view of the low insertion loss and high damage threshold of acousto-opticfrequency shifter and the relationship between frequency and phase, a program that theacousto-optic frequency shifter is used as the phase control device is put forward. Thedetailed theoretical analysis and the whole control circuit program and part of the circuitschematic are also put forward.3. Compared to lithium niobate phase modulation, PZT phase modulator has thefollowing advantages: (1) It can greatly reduced the difficulty of production of the seedsource and amplifiers duo to the low insertion loss and high damage threshold. (2) Aslong as there is corresponding optical fiber, PZT phase modulation can extend easily toany wavelength because of the low cost. However, the available bandwidth, generallyless than 100KHz, is limited by the capacitor property of piezoelectric ceramic. Thehigher-power of amplifier is, the higher frequency of noise generated by the amplifier is,which can be corrected by higher frequency of modulation signal. Several new design ofPZT phase modulator, made to increase the frequency response bandwidth to 500KHz,can meet the frequency requirement of correcting amplifier noise.The half-wave voltage of PZT phase modulator is enhanced when the frequency ofmodulation signal is higher. Because the phase modulation signal and phase controlsignal is loaded to the same phase modulator, the control precision will be greatlyreduced when the modulation signal amplitude is close to or greater than the phasecontrol signal amplitude. A back-end control scheme can effectively solute this problembecause PZT phase modulator can be applied to the back-end of amplifier duo to its high damage threshold.In summary, the performance requirement of phase modulator in coherentcombining and the advantages and disadvantages of various kinds of phase modulatorand corresponding recommendations are put forward in this paper, which provide areference of selection of phase modulator in coherent combining.