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FP, VCSEL, DFB, DML, and EML are all “SFP transmitters” (lasers/light sources) of ethernet optical transceiver modules, responsible for converting electrical signals into optical signals. The “receiver” of the module fiber optic is the PIN and APD.
These five types of lasers represent the evolution of optical communication light sources from short to long distances, and from low to ultra-high speeds. Their core differences are as follows:
Difference between Structure and Luminescence Mode
- VCSEL (vertical cavity surface emitting laser): VCSEL emits light vertically upward from the chip surface. The spectrum is wide and can only be used with multimode fiber, specializing in short distances of several hundred meters. With low cost and low power consumption, it dominates in data centers and massive consumer electronics such as mobile facial recognition.
- FP (Fabry Perot laser): FP emits light from the side of the chip edge. It is an early edge emitting laser that emits light containing multiple wavelengths (multiple longitudinal modes), like a “shotgun”, with severe dispersion during long-distance transmission, suitable for cost sensitive scenarios with distances not exceeding 20 kilometers.
- DFB (Distributed Feedback Laser): Built in with a “grating” on top of FP, it forcefully filters out other clutter and only outputs a pure wavelength (single longitudinal mode). The spectrum is extremely narrow, making it very suitable for single-mode long-distance transmission.
- DML (Direct Modulation Laser): It is a working mode of DFB laser itself. Control the laser’s “on” and “off” (1 and 0) by directly adding or subtracting current.
- EML (Electro Absorption Modulation Laser): EML is composed of a DFB laser and an EA (Electro Absorption Modulator). The DFB laser is responsible for constant illumination, while the EA modulator behind it is responsible for high-speed “opening and closing” like a shutter to cut off or release light.
Modulation method: DML vs EML (High Speed Battle)
At high speeds of 10G, 25G, 100G, and even 400G/800G, DML and EML are mainly used, which are currently the two most core technology routes for high-speed optical modules:
- DML (Direct Modulation): DML generates signals by directly changing the current of the switched laser. Its advantages are simple design, low power consumption, and low cost. But the disadvantage is that directly switching the current will change the characteristics of the laser, causing slight jitter in the output light wavelength (known as “chirp” Chirp). At high speeds and long distances, the combination of jitter and fiber dispersion can cause severe signal distortion, thus limiting transmission distance and speed. It is typically suitable for medium to short distances within 10 km.
- EML (External Modulation): It separates the laser and modulator. The laser (usually DFB) remains constantly on, providing a stable and pure light source, and then an independent electroabsorption modulator (EAM) is used externally to complete the signal “writing”. This is like keeping a projector (laser) on all the time, using a high-speed shutter (modulator) to control the output of light. Although this method consumes more power and is expensive, the signal quality it emits is extremely high and almost distortion free, making it a key technology for achieving long-distance (over 40 km) and high-speed (over 10G) transmission.
Comparison of Five Laser Devices
For a more intuitive display of the 5 lasers’ difference, please refer to the table below:
| Laser type | Full name | Extreme speed | Transmission distance | core principle | advantage | shortcoming | Typical application scenarios |
| VCSEL | Vertical cavity emission | 50G/100G | ≤100m | Light is emitted perpendicular to the surface of the chip, forming a circular spot. | Extremely low cost and minimal power consumption | Low power, short distance (≤ 500m) | Internal data center (multi-mode SR) |
| FP | Fabry Perot | 1.25G | ≤20km | The simplest edge emitting laser, with multiple longitudinal modes and a wide spectrum. | Cheap and capable of single-mode operation | Large dispersion, short transmission distance (≤ 20km) | Traditional Gigabit Base Station/Gigabit BIDI |
| DFB | Distributed feedback | 25G/50G | ≤80km | Directly modulating the current to emit light has a simple circuit. | Pure wavelength, long-distance main force | Limited distance (≤ 10-20km), poor performance at high speeds | 10G/25G long-distance, CWDM wavelength division |
| DML | Direct modulation | 25G/50G | ≤10km | Built in grating, achieving single longitudinal mode and extremely narrow spectrum. | DFB directly adjusts the current, with high cost-effectiveness | Complex craftsmanship and high price | 100G/400G data center mid-range (LR4) |
| EML | Electroabsorption modulation | 100G/200G | 10km – 80km | DFB laser+EAM external modulator, the laser is constantly on. | DFB plus modulator, the signal is extremely perfect | Expensive price and high power consumption | 10G single-mode long-distance (ZR), telecommunications backbone network |
It is worth noting that DFB is the physical structure of the chip and is commonly used as a light source for DML; And EML also integrates a DFB laser internally, but it uses a different modulation method. Therefore, DFB and EML are often considered as two separate solutions in the industry.
How to choose the suitable laser for your fiber transceiver?
In practical applications, you can choose the appropriate optical module based on transmission requirements and costs.
① Cost analysis: VCSEL<FP<DFB (DML)<EML.
② Looking at speed and distance:
- Gigabit (1.25G) era: VCSEL for short distance, FP for medium distance, DFB for long distance (40K-120KM).
- In the era of 10G/25G: FP has been eliminated. Short distance VCSEL is used, DML (DFB) is used for 10 kilometers, and EML is mandatory for distances over 40 kilometers.
- In the era of 100G/400G/800G: DML is used for 2-10 kilometers, EML is used for 10-40
| Application scenarios | Mainstream solution (laser type) | typical model |
| Short distance (≤500m), such as data center server interconnection | VCSEL (multimode) | 1G-SX, 10G SR, 100G SR4, 400G SR4 |
| Medium distance (≤ 10-20km), such as urban area network access | DML (usually FP/DFB chip) | 1.25G-LX, 10G-LR |
| Long distance (≤ 40km), such as metropolitan area network convergence | DFB (belonging to DML) | 1.25G-EX, 10G-ER |
| Ultra long distance (≥ 40km), such as backbone network, 5G fronthaul | EML (including DFB chip) | 10G-ER, 100G-ER4 |
Conclusion
These five types (FP, VCSEL, DFB, DML, EML) are actually divided from two different dimensions:
- Divided by chip structure: FP, VCSEL, DFB, EML are the types of laser chips that determine the “quality” of light.
- Divided by modulation method: DML and EML are two modulation methods that determine how electrical signals are “written” into light.
Simply put, DML is a “method”, while FP, VCSEL, and DFB are the “specific tools” that adopt this method; EML is another more advanced “method” that comes with specialized “tools” (DFB laser+modulator).
