We present a new approach to defining the beam quality of a broadband terahertz beam as it propagates through a first-order optical system. A new definition to beam variance is exhibited, taking into account the spectral weight function of the beam. The near-field far-field propagation product is presented by the general averaged ~2, marked as (M? ~(Broadband)) ~2. This parameter is invariant while the beam propagates through optical systems, and it obeys the rule of (M? ~(Broadband)) ~2≥1. Moreover, we show that, in the special case of monochromatic radiation, the definitions we made are reduced to the known expressions of single frequency radiation. We demonstrate a practical procedure for measuring the beam quality of a broadband radiation. The importance of this procedure is emphasized by our experimental results.
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