Because the working media of laser is gas, and the pressure difference between the inner apparatus and environment of laser is large, so the window of output laser beam must be sealed.

由于其激光工作介质是气体，而且激光腔内的压力与外界环境压强相差很大，激光束的输出窗口必须密封。

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It says a one-rule-for-all on the number of sick days allowed each year unfairly penalises officers working in high-pressure inner city environments.

BBC: Q&A: Police reform white paper