Stable and Fast Wavelength Switching in Digitally Tunable Laser Using Chirped Ladder Filter

We propose a digitally tunable ring laser incorporating a chirped ladder filter and a ring resonator. The widely tunable ladder filter selects one channel from the periodic outputs of the ring resonator. Therefore, it is easy to change the lasing wavelength of this device. To obtain stable lasing operation, we employ chirping in the ladder filter. The chirped ladder filter has one dominant passband, and other peaks corresponding to the free-spectral range are suppressed. The device is monolithically integrated by using the InP-InGaAsP material system. The device is capable of 34-channel (100-GHz spacing) digitally tunable laser operation, and the sidemode suppression ratio is more than 40 dB. The switching time for any wavelength is less than 8 ns, and the frequency drift caused by the thermal transient is less than 500 MHz.