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Fig. 4  Simulated ASE spectra of (a) SLD, (b) FP laser diode, and (c) DFB laser diode  

Fig. 3 Schematic laser structures, (a) SLD, (b) FP laser diode, and (c) DFB laser diode   

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Simulated Amplified Spontaneous Emission (ASE) Spectra

(a) Super Luminescent Diode (SLD) (Fig. 3(a))

An example of the simulated ASE spectrum is shown in Fig. 4(a). There is no optical feedback mechanism to make lasing since both cavity facets are Anti-reflection (AR) coated. The spectrum shape is determined by optical gain spectrum and spontaneous emission spectrum.

(b) Fabry-Perot (FP) Laser Diode (Fig. 3(b)) 

An example of the simulated ASE spectrum just below threshold of lasing is shown in Fig. 4(b). Two cavity facets provide the optical feedback to make multiple peaks of longitudinal modes. The mode spacing is determined by the cavity length. The peak to valley ratio of the multiple ASE peaks is related to the optical gain at that wavelength.

(c) DFB Laser Diode (Fig. 3(c))

​An example of the simulated ASE spectrum just below threshold of lasing is shown in Fig. 4(c). The grating behaves like a filter to select the longitudinal modes,  in that the optical feedback is enhanced around the Bragg wavelength. However, the phase condition is not met at the Bragg wavelength (1550 nm), but met at the two wavelengths slightly shifted from the Bragg wavelength. Therefore, the two modes have equal threshold gain, giving two mode lasing.