Single- and double-stage acousto-optical Ti:LiNbO 3 TE-TM converters used in wavelength-selective polarimeter and polarization-independent depolarizer

Abstract

A Ti:LiNbO3 acoustooptical TE-TM converter followed by a 45 linear polarizer allows to measure the Stokes parameters. A wavelength-selective demonstration unit with 9 MHz electrical measurement bandwidth, roughly 0.04 rad accuracy and 250 GHz optical bandwidth has been developed.An acoustooptical TE-TM converter whose transducer is placed at 1/3 ofthe interaction length forms a polarization-independent optical depolarizer, suitable to suppress polarization hole burning in long haul EDFA links. We present a double-stage depolarizer with 0.03 residual degree-of-polarization, 1 10 GHz optical bandwidth and 4 ps depolarization time.1. POLARIMETER IntroductionSome optical polarimeters contain O, 450 and circular polarizers for measurement ofthe respective Stokes parameters. Another approach is a rotating-waveplate polarimeter' . The introduction of optical communication techniques such as wavelength-division multiplexing, minimization of polarizationdependent optical nonlinearities, and polarization shift keying requires a wavelength-selective polarimeter. It can be made from an optical filter plus a conventional, say, rotating-waveplate polarimeter. A similar but simpler approach is to realize both in one component: A Ti:LiNbO3 acoustooptical TE-TM converter4'5 followed by a 450 linear polarizer allows to measure the Stokes parameters wavelength-selectively6. We have developed a computer-controlled demonstration unit with 9 MHz electrical measurement bandwidth, roughly 0.04 rad accuracy and 250 GHz optical bandwidth. We further discuss how weighted acoustical coupling and lock-in detection can improve selectivity and sensitivity. TheoryA piezoelectrical interdigital transducer that is driven by an RF signal generates a moving acoustical density grating in an integrated-optical TE-TM converter (Fig. 1 a). The grating transforms an incident TB wave in a frequency-shifted TM wave and vice versa. The Jones matrix of the device in usual TB-TM coordinates (electrical and acoustical angular frequency o, retardation p = 2 id, coupling constant IC, interaction length 1) M( ) = [ cos/2 _jet sin /2 (1) [_jeJ)t sin p/2 cosp/2 O-819418005/95/$6.OO SPIE Vol. 2449 /365 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/22/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx