A　strengthened　acousto-optic　(AO)　coefficient　of　piezoelectric　crystal　is　defined.　AO　coupled-wave　equations　of　surface　acoustic　wave　(SAW)　to　plane　guide　optic　wave　(PGOW)　and　to　fiber　guide　optic　wave　(FGOW)　are　given　respectively.　A　diffractive　efficiency　formula　of　SAW　PGOW　AO　interaction　and　a　back-wave　efficiency　formula　of　SAW　FGOW　AO　interaction　are　given.　The　formulas　indicate　that　under　condition　of　weak　AO　interaction　intensities　of　the　diffractive　wave　and　the　back-wave　are　proportional　to　ultrasonic　powers.　Lithium　Niobate　(LN)　is　optimum　crystal　of　SAW　PGOW　AO　device,　quartz　is　optimum　crystal　of　SAW　FGOW　AO　device.　A　SAW　PGOW　AO　modulator　using　LN　and　a　SAW　FGOW　AO　modulator　using　quartz　are　designed　and　manufactured.　Some　curves　of　the　diffractive　wave　power　and　the　back-wave　power　vs.　electric　powers　of　signals　driving　the　modulators　are　measured.　SAW　AO　modulators　have　many　advantages,　for　example　small　volume,　good　stability,　low　energy　consumption　and　it　is　easy　to　integrate.　The　modulators　can　be　used　as　intensity　modulators.　The　modulators　have　applications　in　optic　communication　and　real　time　signal　processing.