A two-part optical coupling system is provided for use in an optical transmitter (TX) module. The two-part optical coupling system has first and second optical parts and an air gap at the interface between the two optical parts. A portion of the light produced by a laser diode of the optical TX module is refracted at the air gap and is coupled by the optical coupling system into an end of an optical fiber. A second portion of the light produced by the laser diode is reflected by the air gap and is coupled by the optical coupling system onto a monitor photodiode of the optical TX. The electrical signal produced by the monitor photodiode may be used to monitor and adjust the optical output power level of the laser diode. Because the air gap is formed at the interface between the first and second optical parts, the process of forming the air gap can be incorporated into the optical coupling system manufacturing process without increasing the cost of the manufacturing process.