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Mode filters such as an intracavity tilted Fabry-Perot etalon, or an internal metallic film used to obtain a single-frequency output in the Nd:YAG laser, only work well for relatively short lasers (∼15 cm) and at reduced values of laser gain or output power level. Two new types of mode selectors for such single-frequency operation that largely overcome these difficulties are investigated here. These consist of a double metallic-film etalon in the laser cavity, or alternatively a combination of an intracavity tilted Fabry-Perot etalon and a single metallic-film reflector etalon. Both methods are considered in some detail and some experimental results are given on the single-frequency operation of a Nd:YAG laser with such mode filters. The relative advantages of the new mode selectors are enumerated and it is shown that the laser output may be taken through the metallic-film reflector etalon without serious loss.