IEC 62150-4:2009 specifies test and measurement procedures for relative intensity noise (RIN). It applies to lasers, laser transmitters, and the transmitter portion of transceivers. This procedure examines whether the device or module satisfies the appropriate performance specification. The procedure is applicable to single longitudinal mode (SLM). An optional section of the procedure presents a controlled return loss to the device-under-test, but is only applicable to devices coupled to SMF. Keywords: relative intensity noise (RIN), single longitudinal mode (SLM)