Abstract
An extra-shortened He-Ne laser (shorter than 100 mm) does not work well because there is likely no longitudinal mode in the lasing bandwidth at some times and the output power rises and falls deeply from time to time. A method to solve this problem is presented and investigated. A piece of appropriately designed quartz crystal plate is used as a cavity mirror of a He-Ne laser, which has a larger longitudinal mode spacing than the lasing bandwidth to achieve an equal-spacing mode split. In other words, the number of longitudinal modes is doubled and the actual mode spacing is reduced to half of that without mode split. Therefore, there is always at least one mode operating in the lasing bandwidth and the power is stable at any room temperature. Such a laser with 85-mm cavity length and 0.24-mW output power is constructed.
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