Laser light which modulated and transmitted through one-meter water bath and one-kilometer transmission distances in atmospheric media is detected by method of direct photo-detection. Also, it is analyzed as a square law detection and the experimental apparatus are designed and constructed. Consequently, in spite that the quantitative analysis remains, the availability of coherent optical communication is understood. The average attenuation with the conditions of weather is about -25dB, and the average attenuation coefficient of water is about 0.4(m-1).