关于光激射器的线宽

本文从描述光激射器工作的基本方程出发,讨论了光激射器光束线宽的“短期”(short-time)部分。通常都以下式描写这部分线宽:△v_s=(8πhv(△v′)²)/P,(1)其中△v′是谐振腔电磁模的半宽度;v为光束频率;P为输出功率。我们的主要结论可以归结为以下三点:(1)线宽主要是由与电磁模耦合的耗散体系引起的,与分子耦合的耗散体系的贡献是可以忽略的;(2)在单模近似下,只要分子的线宽比电磁模的线宽大得多,则(1)式是正确的;(3)如果多模谐振腔中激发模与其他模之间的相互作用是强的,则式(1)将被推广为 △v_s=(8πhv(△v′)²)/P+(4hv(△v′))/P somefromn=(λ′≠λ)(r_λλ′²/(rλ′),(2) 其中r_λλ′是相关弛豫系数;r_λ′是模λ′的线宽。在某些情况下,式(2)中第二项可以比其第一项还大。所以,这可能是目前关于线宽的实验结果与由式(1)计算结果不相符合的原因之一。

ON THE LINEWIDTH OF LASERS

In this paper, the "short-time" part of the linewidth of a laser beam has been considered with the fundamental equations which describe the behaviours of lasers. This linewidth is described usually by △v_s=(8πhv(△v′)²)/P, (1) here △v′ is the half-width of the electromagnetic mode of cavity, v the beam frequency, and P the output power. The main results can be summarized in the following way: (1) The linewidth is raised mainly from the dissipative system coupled to the electromagnetic mode, and the contribution of the dissipative system coupled to the molecules is negligible. (2) In the single-mode approximation formula (1) is correct, only if 1/T₂?1/(2π)△v′,1/T₂ being the homogeneous linewidth of molecules. (3) If the inter-actions between the exciting mode and other modes in the multimode cavity are strong, then formula (1) will be generalized to the following: △v_s=(8πhv(△v′)²)/P+(4hv(△v′))/P somefromn=(λ′≠λ)(r_λλ′²/(rλ′),(2) here γ_λλ′ is the correlative relaxation coefficient and γ_λ′ is the half-width of mode λ′. In certain case the second term in formula (2) may be much larger than its first term. Therefore, it is one of reasons for the disagreement between the experimental results about the linewidth and the prediction from formula (1).