Portable I/sub 2/-stabilized Nd:YAG laser for international comparisons

We have established a compact and transportable I/sub 2/-stabilized Nd:YAG laser for international comparisons of laser frequency. The root Allan variance of the portable laser has reached 3.9/spl times/10/sup -14/ when the integration time is longer than 200 s. The results of an international comparison between the National Research Laboratory of Metrology (NRLM), Tsukuba, Japan and the JILA (formerly the Joint Institute for Laboratory Astrophysics), Boulder, CO, USA show that the frequency difference of the portable laser NRLM-Y1 and the JILA laser JILA-W (f/sub NRLM-Y1/-f/sub JILA-W/) was -2.5 kHz, when the cold-finger temperatures of NRLM-Y1 and JILA-W were kept at -10/spl deg/C and -15/spl deg/C, respectively. The averaged frequency offset between two NRLM lasers (f/sub NRLM-Y1/-f/sub NRLM-Y2/) was -1.1 kHz. A frequency variation of about 1.2 kHz was found for the frequency offset between two NRLM lasers, after NRLM-Y1 was taken for a round trip to Sydney for a comparison organized by the National Measurement Laboratory, (NML), Australia.