## 3 Experimental Results
The dependence of the time-averaged optical spectra of the MOPA on the current \(I_{\rm TS}\) is shown in Figure 5b as a color-scale mapping. Over the whole range of TS currents, the peak wavelength of the MOPA does not change and the spectral width remains \(<10\) pm because of the fixed seeding wavelength. A comparison between optical spectra emitted by the DFB laser at \(P\approx 20\) mW and the MOPA at \(P=16.3\) W is given in Figure 5c. The peak wavelength of 964.02 nm and the measured spectral full width at half maximum (FWHM) below 10 pm given by the resolution limit of the optical analyzer coincide for both the MO and the MOPA. The side mode suppression ratio (SMSR) of 55 dB of the DFB laser is larger  
Figure 4: **(a)** CW optical output power (left axis) and voltage (right) versus the injection currents to the two branches of the Y-DFB laser (\(I_{\rm G1}\): dotted lines, \(I_{\rm G2}\): solid lines) at \(T=25\)\({}^{\circ}\)C (black) and \(35\)\({}^{\circ}\)C (red). **(b)** CW optical spectra of the two branches at currents of \(I_{\rm G1}=I_{\rm G2}=130\) mA at \(T=25\)\({}^{\circ}\)C. **(c)** Superimposed pseudo-color mappings of CW optical spectra of the two branches in dependence on the injection currents at \(T=25\)\({}^{\circ}\)C (left) and \(T=35\)\({}^{\circ}\)C (right). than that of the MOPA, where the ASE leads to a deterioration of the SMSR to 40 dB, which is still an excellent value for such a MOPA device. The results for a seeding wavelength of 967.50 nm, corresponding to an operation of the second branch of the Y-DFB laser (\(I_{\rm G1}=130\) mA, \(I_{\rm G2}=0\) mA) shown in Figure 6 are very similar to those shown in Figure 5. When changing the temperature to \(T_{\rm MO}=35\)\({}^{\circ}\)C and fixing all other conditions, the same optical peak power of 16.3 W for the wavelength pair 964.33 nm and 967.81 nm is obtained (not shown here). The widths of the optical spectra given in this paper are limited by the resolution of the optical spectrum analyzer (Advantest Q8384) used. It can be expect  
Figure 5: (**a**) Temporal behavior of the optical power emitted by the MOPA system at a seed wavelength of 964.02 nm, (\(I_{\rm G1}=130\) mA, \(I_{\rm G2}=0\) mA). (**b**) Color-scale mapping of the time-averaged optical spectrum of the MOPA in dependence of the amplitude of the current pulses injected into the TS for a fixed DFB laser current of \(I_{\rm G1}=130\) mA, \(I_{\rm G2}=0\) mA. (**c**) Comparison between the optical spectra of the DFB laser and the MOPA system for \(I_{\rm G1}=130\) mA, \(I_{\rm G2}=0\) mA and \(I_{\rm TS}=18.4\) A.  
Figure 6: (**a**) Temporal behavior of the optical power emitted by the MOPA system at a seed wavelength of 967.50 nm (\(I_{\rm G1}=130\) mA, \(I_{\rm G2}=0\) mA). (**b**) Color-scale mapping of the time-averaged optical spectrum of the MOPA in dependence of the amplitude of the current pulses injected into the TS for a fixed DFB laser current of \(I_{\rm G1}=0\) mA, \(I_{\rm G2}=130\) mA. (**c**) Comparison between the optical spectra of the DFB laser and the MOPA system for \(I_{\rm G1}=0\) mA, \(I_{\rm G2}=130\) mA and \(I_{\rm TS}=18.4\) A.  
linewidths are significantly smaller. Indeed, CW-operated DFB and DBR lasers emitting at 1064 nm having similar epitaxial layer structures, ridge waveguides and cavity lengths measured by the self-delayed heterodyne measurement technique in our lab revealed spectral linewidths below 1 MHz (0.003 pm), using commercially available low-noise current sources. For the DFB laser, a minimum intrinsic (Lorentzian) linewidth of 22 kHz and a total (FWHM) linewidth of 234 kHz were found [27]. A DBR laser exhibited even smaller values of 2 kHz (intrinsic linewidth) and 180 kHz (total linewidth) [28]. Although the spectral linewidth is not deteriorated by the PA as shown in [29] for a CW operated PA, in our case, the optical spectrum is enlarged since the Fourier transform limit of an 8 ns long pulse is of the order 100 MHz (0.3 pm). The SMSR is deteriorated too because of the additional ASE emitted by the PA.