Optical beam forming network is an important part in optically controlled phased array radar, which could improve the beam scanning ability with large bandwidth and direction angle. The direction of beam is usually controlled by optical switches to change relative delay of transmitting and receiving channels. Among commonly-used techniques, dispersion delay is a concision way to realize optical beam forming network. Linear dispersion is only applicable to beam forming with limited dispersion delay and channels. With the increase of delay, nonlinear dispersion delay accumulates, which distorts the beamform. Therefore, relative dispersion slope (RDS) was used as a nonlinear factor. Moreover, adjusting wavelengths of commercial lasers was raised to compensate the nonlinearity. If RDS was 0.003 nm^-1, the maximum wavelength interval stretched from 0.796 nm to 0.862 nm and wavelengths shifted -0.31 nm. In this case, maximum difference between modified and commercial laser wavelengths was 0.2 nm, which was suitable for the passband of commercial wavelength division multiplexing devices. In the meantime, ratio of main to side lobe improved from 5 dB to 12.9 dB with large scanning direction. Based on the analysis, the smaller RDS value was, the less wavelengths modifications of lasers were. Therefore, RDS is a key parameter in choosing dispersion material and adjusting wavelengths of lasers. In this way, distorted beamform could be recovered. The abilities of imaging and identifying thus could be improved in phase arrayed systems.