Revisiting of RAC devices

The development of RAC devices usually is focused on 2 goals, to minimize the insertion loss and phase deviation of S21(f) from ideal quadratic dependence. For large bandwidth RAC filter, to have the balance between the 2 goals, amplitude weighting technique is needed. One hand it can reduce too high reflectivity at certain frequency so as to control the multiple reflections which will cause phase diviation; and on another hand it can realise a flat passband so to have good compressed pulse shape. To avoid the profile depth etching, we concentrated on methods of weighting on reflecting structures. Comparing different type of reflectors and different methods of weighting their reflectivity, we found that the best results can be obtained using grooves of uniform depth along the structure and weighting by variation of the duty cycle of grooves. For typical down chirp device with B=400 MHz, T=20 us this approach results in the variation of the groove width duty cycle from about 10% to fully 50%.Then the narrowest width is about 220 nm, i.e. very close to the critical dimension of photo lithography(100 nm). Therefore we have used the standard e-beam lithography equipment for precise exposure of the weighted groove array. With this method, BT=8000 RAC filters with total insertion loss 55 dB and RMS phase deviation 15 degrees was developed.