A DFB laser with an external modulator integrated on the same chip — narrower spectrum, longer reach.
What an EML Is
An electro-absorption modulated laser runs its DFB section continuously at constant current and places an electro-absorption modulator immediately after it on the same indium phosphide chip. The modulator is a short section of semiconductor whose absorption edge shifts with applied voltage: transparent with no bias, strongly absorbing with a few volts on it.
Because the laser itself never changes drive current, its carrier density never changes, its refractive index never changes and its wavelength never moves. The light is gated after it has been generated rather than being generated in bursts.
Why That Buys Reach
A directly modulated laser is switched on and off, and every switching event momentarily shifts the emitted wavelength. The pulse leaves the transmitter already smeared across a range of wavelengths, and chromatic dispersion converts that wavelength spread into time spread as it travels. The eye closes with distance no matter how much power you launch.
An EML removes the mechanism rather than compensating for it. The practical result is roughly an order of magnitude more dispersion tolerance, which in product terms is the difference between a 10 km part and an 80 km one. The dispersion calculator shows the effect directly if you change the source type at a fixed distance.
What It Costs
The modulator absorbs light even in its transmissive state, so an EML delivers less optical power for the same laser drive than a DFB of the same class. It needs a driver capable of swinging several volts at the symbol rate, it is temperature sensitive enough to want a thermoelectric cooler, and the monolithic integration makes it harder to yield.
Those are the reasons EMLs cost more and draw more power than DFBs, and why they are not used where a DFB will do. Nobody puts an EML in a 10 km part that a directly modulated laser closes comfortably; the question is only ever whether the reach or the rate demands one.
Where You Meet It
Wherever reach or lane rate is demanding: 10G ER and ZR, 100G LR4 and ER4, 400G FR4 and LR4, and the majority of 800G single-mode modules. If a datasheet claims 40 km or more at 1550 nm, an EML is almost certainly behind it — the DR4 versus FR4 versus LR4 comparison shows where the boundary falls in practice.
Beyond about 80 km the answer stops being a better direct-detect transmitter and becomes coherent detection, as used in ZR and ZR+ optics. Silicon photonics offers the other alternative, integrating Mach-Zehnder modulators alongside an external laser instead.
EML reference
| Structure | a CW DFB and an electro-absorption modulator, monolithic on one InP die |
|---|---|
| Modulator physics | the Franz-Keldysh or quantum-confined Stark effect shifts the absorption edge |
| Drive | a few volts on the modulator; laser current stays constant |
| Chirp | near zero, and often deliberately slightly negative |
| Dispersion tolerance | roughly an order of magnitude better than a directly modulated laser |
| Typical extinction ratio | 8 to 13 dB |
| Cooling | normally a thermoelectric cooler |
| Cost and power | both higher than a DFB — the modulator absorbs part of the light |
| Where used | 10G ER and ZR, 100G LR4 and ER4, 400G FR4 and LR4, most 800G single-mode |
Related Terms
DFB is the laser inside it, VCSEL is the multimode counterpart, and extinction ratio is where the difference shows on a datasheet.