# Fast Reconfigurable SOA-Based Wavelength Conversion of Advanced Modulation Format Data +
## 3 Linewidth Characterization of SGDBR Pump Lasers for Wavelength Conversion  
Among the tunable lasers, the SGDBR laser has been proven to be a suitable fast tunable laser candidate [42], and the structure of this type of laser is shown in Figure 3a. It is comprised of one gain section, one phase section, and two grating sections. Due to the period in which the two gratings are sampled differently, each grating has different period of reflection maxima. Therefore, the Vernier effect is enabled and enhances the overall tuning range considerably. The function of the phase section is to allow the cavity modes to be shifted independently of the grating's reflection peak, achieving continuous tuning. As the data is modulated on the phase of the laser in the transmission systems employing advanced modulation formats, the phase noise of the lasers becomes an important factor in determining the transmission performance [43]. For the SGDBR laser, both the gain section and the passive tuning sections can contribute to the overall phase noise of the laser [44]. In order to characterize the phase noise of the SGDBR laser, the frequency modulation (FM) noise spectrum, which has been proved to be a very suitable measurement of the phase noise of lasers to be employed in coherent systems [45], can be obtained by the technique using a coherent receiver with a narrow-linewidth LO [37]. As shown in Figure 3b, white noise from the gain section and low frequency carrier noise from the passive tuning sections are observed. Due to the advance in digital signal processing (DSP), the low frequency excess phase noise can be compensated. A 2 nd-order PLL scheme has been used in the carrier phase estimation to track the excess phase noise [46], which presents the ability to employ fast tunable laser for higher order modulation formats. Apart from that, the phase noise of the SGDBR laser is strongly dependent on the injection currents on each section, and the linewidth varies between several 100's kHz to several MHz with different injected currents. The output wavelength and phase noise of the SGDBR laser as a function of the injected currents in the front section are measured and shown in Figure 4, with 90 mA current on the gain section and 0 mA current on the phase and back section. The white noise and carrier noise are represented by the high frequency linewidth and low frequency linewidth, which are calculated from the FM-noise spectrum by the simple equation: \(\Delta\upsilon=\pi\cdot\cdot S(\prime)\)[47]. The high frequency linewidth is related to the FM noise level in the frequency range beyond 500 MHz, whereas the low frequency linewidth contributed by the passive tuning sections is extracted by the mean value of the FM-noise in the frequency range under 50 MHz. It can be observed from Figure 4, by changing the currents into a grating section, that the entire reflection comb of the grating section shifts in wavelength and the laser wavelength jumps not only to an adjacent longitudinal mode, but can also jump by several  
Figure 2: Four-wave mixing (FWM) scheme with dual co-polarized pumps. (**a**) The \(X\) and \(Y\) axes denote orthogonal states of polarization. In the dual pump FWM scheme, the polarization state of the wavelength-converted idler is that of the input signal provided both pumps have the same polarization. (**b**) Spectral locations of the pumps, signal, and idlers. nm to another super-mode at a wavelength where the reflection peaks of the Vernier-tuned gratings have re-aligned. By increasing the currents on the grating section, the low frequency linewidth and high frequency linewidth are found to present an opposite trend. The low frequency linewidth usually increases until the wavelength jump occurs, while the trend for the high frequency linewidth is decreased. As most low frequency phase noise can be compensated by the DSP, the operation points of the SGDBR laser with less high frequency noise are preferred to perform data transmission experiments.