EML vs. VCSEL vs. CW laser: which optical transceiver laser is right for you?
Laser technology is the most expensive part of an optical transceiver, roughly 50% of the module's total cost. Picking the wrong one means you're either overpaying or underperforming, so it's worth understanding what each type actually does well.
This article compares three laser technologies used in modern optical transceivers:
- •Electro-absorption Modulated Lasers (EML)
- •Vertical-Cavity Surface-Emitting Lasers (VCSEL)
- •Continuous Wave (CW) lasers.
Technical comparison: EML vs. VCSEL vs. CW laser
| Parameter | VCSEL | EML | CW Laser |
|---|---|---|---|
| Full Name | Vertical-Cavity Surface-Emitting Laser | Electro-absorption Modulated Laser | Continuous Wave Laser |
| Wavelength | 850 nm (multimode fiber) | 1470-1610 nm (C-band, ~1550 nm) | 1310-1550 nm (single-mode) |
| Transmission Distance | Short-reach: Up to 300m at 10 Gbps | Long-reach: 10-80 km (up to 40 km typical for 100G) | Medium to long-reach: 10-40+ km with external modulation |
| Speed/Data Rate | Up to 100G per lane (200G/lane emerging) | >25 Gbps, high-speed (>40 GHz capable, 200G/lane available) | Up to 200G per lane with silicon photonics modulation |
| Modulation Type | Direct modulation | Integrated electro-absorption modulator (EAM) with CW DFB laser | External modulation (silicon photonics or Mach-Zehnder) |
| Chromatic Dispersion | Minimal (multimode fiber) | Very low (stable wavelength, low chirp) | Minimal (chirp-free external modulation) |
| Relative Cost | Low | High | Medium |
| Power Consumption | Very low | Higher (can exceed 4.5W per module) | Moderate (depends on modulator type) |
Best use cases for each laser type
VCSEL: best laser for short-reach data center transceivers
- •Data center server interconnects (100G SR4, 800G SR8)
- •Short-reach parallel fiber links within racks
- •High-density, high-volume deployments where cost matters
- •Energy-efficient data centers (low power consumption)
VCSEL is the go-to for anything inside a data center where distances stay under a few hundred meters. The cost per port is low, power draw is minimal, and the manufacturing process is mature enough that supply is rarely an issue. If you're buying QSFP28 SR4 or QSFP-DD SR8 modules, there's a VCSEL inside.
EML: best laser for long-haul and 800G optical transceivers
- •Long-haul and metro networks (10G, 40G, 100G Ethernet)
- •Telecom systems requiring >10-40 km reach
- •High-speed applications >25 Gbps where signal quality matters most
- •800G/1.6T AI cluster interconnects (DR8, 2xFR4 modules in QSFP-DD and OSFP form factors)
- •Applications that need minimal dispersion compensation
EML packs a laser and modulator onto a single chip, which gives it cleaner modulation at high speeds compared to directly modulated alternatives. That's why you'll find EML in most 800G DR8 and 2xFR4 modules shipping today. The downside: it's expensive and, as of 2026, very hard to get.
CW laser: the silicon photonics alternative
- •Silicon photonics-based transceivers (common in 400G DR4 and newer 800G modules)
- •Future 1.6T optical transceiver modules using external modulators
- •Coherent optical communication systems
- •Wavelength Division Multiplexing (WDM) systems
A CW laser produces a steady, unmodulated beam. The actual data encoding happens externally on a silicon photonics chip (typically a Mach-Zehnder modulator). Because the laser and modulator are separate, the laser is simpler to build than an EML, and the modulation chip can be made in standard semiconductor fabs. That's a big deal for scaling production, and it's the main reason cloud providers have been moving to CW-based designs while EML supply stays locked up.
So which laser is better: EML, VCSEL, or CW?
Looking at the comparison, the takeaway is pretty straightforward: each laser fits a different job. Distance alone narrows your options fast. VCSEL lasers are built for short-reach, while EML and CW handle medium to long-range. You can compare cost, power draw, and specs all day, but if your link is 500 meters, VCSEL wins. If it's 40 km, it doesn't matter how cheap VCSEL is.
There is no "best" laser technology, just the right one for your requirements. Think of them as different tools in a toolbox: VCSEL is your everyday screwdriver, EML is your precision instrument, and CW is your versatile multi-tool.
Is a Ferrari better than a pickup truck? For lap times, yes. For hauling lumber, absolutely not. The "best" laser is the one that meets your distance, budget, timeline, and performance requirements, not the one with the highest specs on paper.
Where does DML fit in? (DML vs. EML)
Worth a quick mention: DML (Directly Modulated Laser) sits between VCSEL and EML in both reach and cost. A DML modulates by varying the drive current directly, so there's no separate modulator on the chip. That keeps it cheaper and lower-power than EML. The tradeoff is more chromatic dispersion at speed, so DML typically tops out around 10 km at 25 Gbps. You'll find DML inside many 100G QSFP28 modules used for 2-10 km links. If VCSEL can't reach but EML is overkill, DML is usually the right call.
EML laser shortage 2026: what's happening with the supply chain
The optical transceiver industry is dealing with a severe EML laser shortage, driven by the explosion in AI data center buildouts. Here's what's happening:
- •Demand surge - Global shipments of 800G+ optical transceivers are expected to hit 24 million units in 2025, then jump 2.6x to nearly 63 million units in 2026.
- •Nvidia's EML capacity lock-in - Nvidia has pre-allocated production capacity at major EML suppliers (Lumentum, Coherent/Finisar, Mitsubishi, Sumitomo, Broadcom) to secure supply for its own GPU cluster interconnects. This has pushed delivery timelines for everyone else past 2027.
- •Limited supplier base - EML production has an extremely high barrier to entry because of the complex integration of modulation functions on a single chip. You can't just spin up a new fab.
- •Most impacted modules - 800G DR8, 2xFR4, and 1.6T transceivers that rely heavily on 100G EML technology.
CW laser as an EML alternative
CW lasers have become the main alternative to EML, but their production has its own problems. The simpler chip structure (no integrated modulation) means more companies can make them, which helps. But capacity expansion is slow because equipment lead times are long. Coherent, one of the largest CW suppliers, expects to expand InP fab capacity 5x by 2026, but that won't happen overnight. Downstream steps like die-cutting and aging tests are often outsourced, which adds more bottleneck risk. Still, cloud service providers are moving to CW + silicon photonics solutions in growing numbers as a way around the EML crunch.
VCSEL supply, meanwhile, has stabilized and is readily available after the 2022-2023 shortage
Optical transceiver cost implications
The EML shortage is creating real cost pressure. According to McKinsey production of 800G optical transceivers is projected to fall 40-60% short of demand through 2027, with EML laser constraints being a primary factor. Prices will probably keep climbing. On top of the laser cost itself, manufacturers are spending more on redesigning modules and qualifying alternative suppliers, which adds to overall expenses.
CW lasers with silicon photonics are where a lot of the money is going right now. Not because CW necessarily outperforms EML, but because people need parts they can actually get. It comes down to weighing the upfront cost of switching technology against the risk of waiting months (or years) for EML components that may never arrive on time.
And this brings us back to the distance question. For short-reach applications under 500 meters, VCSEL is still the obvious pick for cost and power efficiency. In the 2-10 km range, DML offers a cost-effective middle ground. For anything over 10 km, it makes sense to evaluate both CW with silicon photonics and traditional EML, but don't just compare specs. Factor in whether you can actually get the parts.
The bottom line
The laser market is going through its biggest shift in a decade. VCSEL handles short-reach reliably. EML is still the performance king for long-haul, if you can get it. And CW + silicon photonics is where things are heading, mostly because it scales better and doesn't depend on a handful of constrained suppliers. Match the technology to what your network actually needs, not what looks best on a datasheet.
FAQ:
What laser is used in 800G optical transceivers?
Depends on the module type. Most 800G DR8 and 2xFR4 transceivers use EML lasers for their clean modulation over longer distances. But with the current EML shortage, more 800G modules are shipping with CW lasers and silicon photonics modulators instead. VCSEL-based 800G modules (SR8) exist too, but only for short-reach data center links under 100 meters.
Why is there an EML laser shortage in 2025?
Two things happened at once. AI data center buildouts pushed 800G+ transceiver demand way past what EML suppliers can produce. And Nvidia pre-allocated huge chunks of EML manufacturing capacity at Lumentum, Coherent, and others to secure its own GPU cluster supply. That pushed delivery timelines for everyone else past 2027. New EML manufacturers can't easily enter the market either, because integrating a laser and modulator on one chip is extremely hard to do at scale.
What is a CW laser in an optical transceiver?
CW stands for Continuous Wave. The laser just produces a constant beam of light, nothing fancy. The actual data encoding happens on a separate silicon photonics chip, usually a Mach-Zehnder modulator. Because the laser doesn't need a built-in modulator like EML does, it's simpler to manufacture. That's the main reason CW has become the go-to alternative during the current EML supply crunch.
What is the difference between EML and DML lasers?
EML has a laser and modulator built into one chip. That gives it cleaner signals at high speeds with low chirp, but it costs more. DML (Directly Modulated Laser) skips the separate modulator and just varies the drive current to encode data. Simpler, cheaper, but noisier over distance. In practice: DML handles 2-10 km links at up to 25 Gbps just fine. For longer distances (10-80 km) or higher speeds, you need EML.
Is VCSEL better than EML?
Wrong question, honestly. They do different things. VCSEL is cheaper, sips power, and works great for data center links under 300-500 meters. EML costs more and draws more power, but it can push clean signals 10-80 km. You wouldn't compare a city car to a long-haul truck. Short reach? VCSEL. Long reach? EML or CW with silicon photonics
What is silicon photonics and how does it relate to CW lasers?
Silicon photonics means building optical components on silicon chips using the same fabrication methods as regular semiconductors. In a transceiver, the silicon photonics chip handles data modulation and a separate CW laser provides the light. The reason this matters: you can manufacture silicon photonics chips at scale in existing semiconductor fabs, which is much easier to ramp than EML production. That's why this combination keeps showing up in 400G DR4 and 800G module designs.