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A Brief Overview Of The Differences In Spectral Properties Of VCSEL, FP And DFB

Time: 2023-12-13
• Background
        Semiconductor lasers are among the core components of optical modules.
        The three most common types are VCSEL, FP, and DFB lasers.
        This article briefly explains the differences in their spectral characteristics.
• VCSEL Laser
        VCSEL stands for Vertical-Cavity Surface-Emitting Laser.
        The most mature and widely deployed wavelength is 850 nm.
        A typical VCSEL spectrum is shown below:
VCSEL spectrogram
Figure 1 VCSEL Spectrogram
        VCSEL lasers exhibit the following spectral features:
        • VCSELs operate in multiple transverse modes, producing a spectrum with multiple peaks.
        • The intensity and wavelength spacing of these peaks are relatively asymmetric.
        The spectral width is narrow, typically less than 0.6 nm.
• FP Laser
        FP stands for Fabry–Pérot Laser.
        It is mainly used at the 1310 nm wavelength band.
        A typical FP spectrum is shown below:
VCSEL spectrogram
Figure 2 FP Laser Spectrogram
        FP lasers have these spectral characteristics:
        • FP lasers are multi-longitudinal-mode devices, with significantly more spectral peaks than VCSELs.
          The side peaks on both sides of the main peak show symmetrical amplitude variation, and the wavelength spacing between adjacent side peaks is uniform.
• DFB Laser
        DFB stands for Distributed Feedback Laser.
        It is commonly used across the 1270 nm ~ 1610 nm wavelength range.
        A typical DFB spectrum is shown below:
VCSEL spectrogram
Figure 3 DFB Laser Spectrogram
        DFB lasers feature:
        • DFB lasers are single-longitudinal-mode devices, with only one strong main peak and very weak side modes.
        SMSR (Side Mode Suppression Ratio) is used to evaluate the power difference between the main mode and side modes.
        A higher SMSR indicates better spectral purity.
        DFB lasers typically require an SMSR greater than 30 dB, while SMSR is not a specified parameter for VCSEL and FP lasers.
        The spectral width is extremely narrow, usually less than 1 nm.
• Summary
        VCSEL, FP, and DFB lasers differ clearly in operating wavelength, spectral pattern, spectral width, and mode structure.
        • These distinct characteristics allow for easy identification among the three types.
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