A brief overview of the differences in spectral properties of VCSEL, FP and DFB

      ⦁ Background.
      Semiconductor laser is one of the core parts of the optical module, there are mainly three types of VCSEL, FP and DFB, what is the difference between their spectral characteristics, this article makes a brief introduction.
      ⦁ VCSEL laser
      VCSEL full name vertical cavity surface emitting laser, the most mature process is currently 850nm wavelength, the following is a typical spectral diagram:

Figure 1  VCSEL spectrogram

      VCSEL lasers have several characteristics:
      a. VCSEL lasers are multi-transverse mode, the spectrum has multiple peaks, and the intensity and wavelength spacing of multiple peaks are not very symmetrical;
      b. The spectral width is relatively small, usually less than 0.6nm.
      ⦁ FP laser
      FP full name Fabry-Perot laser, the main application wavelength of 1310nm, the following is a typical spectral diagram:

Figure 2  FP laser spectral diagram

      FP spectrum has several characteristics:
      a. FP lasers are multi-longitudinal mode, the spectrum has multiple wave peaks, the number of wave peaks is significantly more than the VCSEL, the amplitude change of the secondary peaks on the left and right sides of the main peaks is more symmetrical, and the wavelength spacing between the secondary peaks is also more uniform.
      b. the spectral width is relatively large, usually less than 4nm.
⦁ DFB laser
      DFB full name Distributed Feedback Laser, the main application wavelength of 1270 ~ 1610 nm, the following is a typical spectral diagram:

Figure 3 Spectral diagram of DFB laser

      DFB laser has several characteristics: 
      a.  DFB laser is a single longitudinal mode, only 1 main peak, the other sub-wave peaks are very small in amplitude.
      SMSR (Side Mode Suppression Ratio) is used to measure the intensity difference between the main peak and the secondary peak, the larger the Side Mode Suppression Ratio, the better, usually the DFB laser SMSR requirements are greater than 30dB, VCSEL and FP lasers do not have SMSR this measure.
      b. the spectral width is very small, usually less than 1nm.
In summary, VCSEL, FP and DFB laser wavelength, spectral shape and spectral width and other parameters have certain differences, the combination of these characteristics we can easily identify.
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