If becomes small is also small; further is insensitive to the sign of the velocity. To circumvent this problem a frequency shifter such as Bragg cell (BC), an electro-optic device capable of increasing the frequency of light, is put in the path of one of the beams. A typical value of this increase is about 40 MHz. The modified LDV arrangement is shown in Figure 3.7.

Figure 3.7: Use of a Bragg Cell.

The Doppler shift equations for rays 1 and 2 now become

where is the shift frequency introduced by the Bragg cell. Hence

Note that as remains finite and can be conveniently determined since its value is around . Values of are interpreted as referring to velocities in the negative direction.