296 Characterization of Optical Devices 4. Modulation speed requirement, which requires short Dt. This refers to the requirement that within the differential delay time Dt, there should be negligi- ble intensity variation of the optical signal induced by modulation. Therefore, the differential delay must be much smaller than the inverse of the maximum modulation frequency f m : Dt ( 1=f m . As an example, for a 40 GHz modula- tion frequency, the differential delay should be much shorter than 25 ps which corresponds to the arm length difference of approximately 0.8 mm in the air. In the design of the chirp measurement setup, we must consider all four of these requirements and make tradeoffs. Figure 3.2.17 shows an example of the measured waveforms of intensity modulation and the associated frequency modulation of an unbalanced LiNbO 3 external modulator [14]. This wave- form-dependent frequency chirp usually cannot be precisely characterized by small-signal frequency-domain measurements. Intensity (linear)