Open the Shell: Inside the VCSEL, Receiver, and two ICs of an 850 nm multimode SFP

ItInsightsWorld begins this teardown with a small puzzle instead of the housing: a transceiver whose rated emission band sits beyond what the unaided eye is supposed to register still throws off an unmistakable red pinpoint. Chasing that anomaly leads from the transmit port, past the latch and hinges, to a vertical-cavity surface-emitting laser die, a wavelength-matched receiver, and the handful of parts holding them in place.

Read as a sequel, the framing is useful: relative to the single-mode sibling from the previous installment, the different laser approach theoretically exchanges operating range for lower cost, yet it remains faster than the LED-based predecessor.

The first small form-factor pluggable (SFP) module teardown, published Last month, covered a single-mode optical transceiver built around a semiconductor diode laser at 1310 nm — one that could have been Fabry-Perot– or distributed-feedback-(DFB) based.

Telling those two constructions apart by eye is not something I can claim to do with confidence (knowledgeable reader insights are welcomed), but the 850 nm multimode module on the bench today uses neither of them.

Center wavelengths and the cable they belong to

Requoting my previously published tutorial on SFPs and their siblings for context — all are center frequencies:

• 850 nm (“multimode”)
• 1300 nm (“multimode”) / 1310 nm (“single-mode”)
• 1550 nm (“single-mode”)

Placing each band correctly matters, because multimode and single-mode fiber optics belong to two technologies that are not interchangeable, even though some modules can work with both associated cable material types.

Start with the economics: multimode implementation cost was historically much lower, and the price of that economy was lower usable transmission distance. A comparatively larger cable core, 50 to 62.5 microns, allowed multiple light signals to travel different paths simultaneously, and the light sources it tolerates are cheap — it uses lower-cost LEDs or vertical-cavity surface-emitting lasers (VCSELs).

The single-mode picture inverts almost every one of those traits. Its comparatively tiny core, about 8 to 10 microns, lets only a single ray pass straight through without bouncing off edges, and it relies on focused lasers. The historical cost disadvantage versus multimode has more recently decreased, partly because non-proprietary, widely compatible modules are available, and the format generally specifies much longer usable transmission distances.

A different emitter, the same dim red glow

The excerpt “uses lower-cost LEDs or vertical-cavity surface-emitting lasers (VCSELs)” remains strictly accurate, though in the modern era it is half-obsolete — near-infrared LEDs were indeed used in early sub-Gbit 850 nm SFP modules.

A helpful reference note is blunter about the ceiling: “their maximum rate of about 622 Mb/s meant that they could not support 1 Gb/s or higher transmission.” So the expectation going in was an 850 nm VCSEL transmitter inside today's teardown victim, joined by a spectrally tuned variant of the earlier 1310-nm receiver.

The 850 nm spectrum association got a visual confirmation first: I inserted the module into a SFP cage-inclusive multi-port switch in a dark room and quickly discerned the light from the module's transmit connector.

Why that glow is perceptible — or capturable by a smartphone camera — is a fair question, given that the human visual spectrum range extends only to around a 750 nm wavelength, corresponding to a ~400 THz frequency. The answer lies in the earlier excerpt “all are center frequencies”: even though 850 nm, or ~3.5 THz, may be beyond human visual perception, the transmitter's bell curve(-ish) output profile seemingly still has a modicum of signal strength at the visual wavelength end of its overall distribution.

Today's patient is another SFP module, rather than a higher-speed SFP+, and it was bundled with a media converter that I'll keep as a spare for potential future lightning-strike galvanic isolation purposes.

Every external view before the first pry

Overview shots come first, and, as usual, a 0.75″ (19.1 mm) diameter U.S. penny provides size comparison.

Start with the top face.

That view pairs with a close-up of the embossed transmit and receive direction arrows, also mentioned in Last month's teardown and more generally found in all optical modules I've seen.

Next is the right side, as before, with right referencing its orientation when inserted in a network switch or other gear's SFP port.

Then the bottom; as was the case last time, note the retractable retaining pin.

The left side rounds out the four faces.

Both perspectives of the SFP connector end follow, seen right-side-up and upside-down.

The optical fiber connector end gets the same treatment, first with the dust cap still in place.

And now, after dust cap removal.

Dissection, step by step

Bending back the side tab you may have noticed earlier — if not, go back and look — is what holds the outer and inner enclosure pieces together and, once released, opens the pathway to the insides. There are two tabs, one on each side, although only the left side one was initially depressed.

As usual, the multiple hinge and associated pieces, conceptually similar to those seen Last month, fell out as the enclosure halves separated.

With those loose parts cleared, the interior is exposed.

One screw is all that keeps the PCB attached to the metal assembly surrounding it.

Free at last.

Underside of the PCB next.

Board level: emitter, receiver, and two ICs

The optical receiver module sits on top at the left edge, visually similar to the one found in Last month's single-mode SFP module, albeit tuned for a different incoming signal wavelength; below it is the 850 nm VCSEL-based transmitter, which conversely looks very different from its 1310 nm laser predecessor shown in the following comparison photo.

Now for the PCB topside of today's patient.

The ordering is obviously reversed in this case: transmitter at top left and receiver below it. And what of the two ICs on this side? A closer look answers that.

The larger of the two carries the following four-line topside mark; my notes appear in brackets.

• 1138142
• 780KF VJ [the second character might instead be the letter O]
• PHL [there is another three-digit code to the right, but yellow paint obscures it; in a photo found online it is 141, but this unit does not appear to show 141; I think the first character is B]
• e4 [this seems to be a company logo, given that it is surrounded by a circle; next to it is, judging from other photos, the two-character code ES, albeit again obscured by yellow paint]

Running a Google search on the phrase 1138142 IC suggests that STMicroelectronics is the supplier but does not reveal anything about the chip's function. Reader insights are welcomed!

The smaller IC is marked on two lines, 532R and B809, with the same caveat as before: the third character may instead be the letter O.

As before with the single-mode module, I assume this is an EEPROM storing data such as the module's serial number, vendor name, compatibility codes, and calibration data.

Two of this board's edges go undocumented here by design: the SFP interface end and the LC, or Lucent Connector, end are visually identical to the ones found in Last month's module, so the earlier teardown images stand in for those redundant shots. What this run actually adds is the part worth opening a module for — a VCSEL where an LED once sat, a receiver retuned away from 1310 nm toward 850 nm, the pair of ICs carrying the module's identity, and a red dot that theory says we should never have seen.