1. A calibration device, for calibrating an optical time domain reflectometer of the type comprising a light source for generating pulses of light, a light detector for generating electrical signals in response to light received thereby, an optical coupler for coupling the light source and the light detector to one end of an optical fiber whereby light pulses from the light source are introduced into the optical fiber by way of said one end and electrical signals are generated by the light detector in response to light leaving the optical fiber by way of said one end, and display means for providing a display of the time relation between light pulses generated by the light source and electrical signals generated by the light detector, the calibration device comprising: a first optical transmission member having two opposite ends, one of which is connected, in use, to the coupler of the reflectometer to be tested; optical directional coupling means having at least first, second and third ports and being connected by way of its first port to the other of said two opposite ends of the first optical transmission member, the coupling means directing light received by way of the first port to the second port but not substantially to the third port, dividing light received by way of the second port and directing it to the third port and to the first port, and directing light received by way of the third port to the second port; and a second optical transmission member having a known propagation characteristic and having two opposite ends connected respectively to the second and third ports of the coupling means. whereby the nature of the display provided by the display means of the reflectometer when the reflectometer is operative and is connected to the calibration device can be utilized, in conjunction with the known propagation characteristic of the second optical transmission member, to calibrate the reflectometer.