The Agilent 81623A optical heads are specially designed for low PDL , low spectral ripple and high return loss and are used for optical component tests. The optical heads have to be operated with the single (Agilent 81618A) or dual (Agilent 81619A) interface modules. The heads come with a magnetic D-shape adapter and provide a safe stand. They are used in lightwave mainframe models 8163A/B, 8164A/B and 8166A/B.The 81623A is the predecessor to the 81623B. Here is Agilent's description of the difference between these two models:What is the difference between the 8162xA and the 8162xB optical heads?Most of the functionality of the 8152x optical heads was carried over when Agilent Technologies first introduced the 8162xA family of optical heads. However, the 8162xA optical heads did not have an analog output available. The 8162xB models, with the exception of the 81625B (now replaced by the 81626B), all offer an analog output on the optical head.In addition, some specifications have been added. For example, the 81623B now has a specification for power variation with polarization. The 81623A did not.The main difference is that the 8162xB models (exception is the 81625B) offer an analog output, the 8162xA do not. All functionality the 8162xA offered is available for the corresponding B model.