Extra-short He-Ne lasers based on mode split

An extra-short He-Ne laser (shorter than 100 mm) does not work well because there is likely no longitudinal mode in the lasing bandwidth sometimes and the output power rises and falls deeply from time to time. A method to solve this problem has been presented and investigated. A piece of quartz crystal plate appropriately designed is used as a cavity mirror of a He-Ne laser, which has a longitudinal mode spacing larger than the lasing bandwidth to make the equal-spacing mode split. In other words, the number of longitudinal modes is doubled and the actual mode spacing is reduced to the 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 out-put power has been made.