从调制的基本概念出发，指出声光驻波调制器件为宽带频率调制型。讨论了调制度概念的成立条件和适用范围。理论上求出其成立条件为v＜0.9(v=2πL△u/λ为衍射光强I(θ，t)中的自变量)。鉴于一般驻波型声光器件都是含有一定的行波成份，但行波声场和驻波声场在拉曼-奈斯衍射范围内衍射角均遵从sin θmi=±mλ/Λ，故两者在空间上是无法分开的。文中给出了从频域上测量调制度M的一种新方法。并给出了零级和一般衍射光的测量值。

Based on the fundamental concept of modulation, we have analysed the related facts and concluded that the modulation of acousto-optic sounding wave modulator is a broad frequency modulation. It is shown that the criteria of broad band modulation of this type are v<0.9 (v=2πL△μ/λ is the independent variable of diffracted light beam I(θ,t) ).A certain part of travelling wave is generally contained in the diffracted light beam of ordinary standing wave modulators. But in the range of Raman-Nath diffraction the diffraction angles abide by the formula sin θmi=±mλ/Λ, so that diffracted beams by both standing wave modulation and travelling wave appear in the same space range. In this paper a method of sprectal analysis for measuring the modulation index in the frequency-domain is proposed. The formulas of measurable values for zero-order and first-order diffracted light beams are given and verified experimentally.