SFP: The multimode module autopsy

A dissimilar laser technology versus the single-mode sibling trades off operating range for cost, at least theoretically. But it’s still faster than the LED-based predecessor.

Last month’s premiere small form-factor pluggable (SFP) module teardown of a single-mode optical transceiver showcased either a Fabry-Perot– or distributed-feedback-(DFB) based semiconductor diode laser operating at 1310 nm.

I’m not sufficiently versed in the technology to visually differentiate between the two—knowledgeable reader insights are welcomed—but I know enough to know that today’s 850 nm multimode optical module uses neither of them.

Requoting my previously published tutorial on SFPs and their siblings:

SFP modules most commonly run at the following wavelengths (all are center frequencies):

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

The distinction between multimode and single-mode fiber optics is important to comprehend and keep in mind, as the two technologies are not interchangeable (although some modules will work with both associated cable material types).

Multimode was historically much less expensive to implement, at the trade-off of lower usable transmission distance. It features a comparatively larger cable core (50 to 62.5 microns) that lets multiple light signals travel down different paths at the same time, and it usually uses lower-cost LEDs or vertical-cavity surface-emitting lasers (VCSELs).

Single-mode features a comparatively tiny core (about 8 to 10 microns), which allows only a single ray of light to pass straight through without bouncing off the edges, and it uses focused lasers as a light source. Its historical cost disadvantage versus “multimode” has more recently decreased, due in part to the availability of non-proprietary, widely compatible modules. And as noted earlier, it generally specifies much longer usable transmission distances.

Different emitter, same dim red glow

The “uses lower-cost LEDs or vertical-cavity surface-emitting lasers (VCSELs)” excerpt from the above is strictly accurate, although in the modern era it’s half-obsolete. Near-infrared LEDs were indeed leveraged with early sub-Gbit 850 nm SFP modules.

However, as a helpful reference I came across notes, “their maximum rate of about 622 Mb/s meant that they could not support 1 Gb/s or higher transmission.” Therefore, I expect to find an 850 nm VCSEL transmitter inside today’s teardown victim, along with a spectrally tuned variant of the earlier 1310-nm receiver.

I first visually confirmed the 850 nm spectrum association by plugging the module into a SFP cage-inclusive multi-port switch in a dark room and (quickly!) discerning the light coming out of the module’s transmit connector.

You might ask why it’s perceptible (and smartphone camera-capturable) at all, given that the human visual spectrum range extends only to around a 750 nm wavelength (corresponding to a ~400 THz frequency). To answer that question, I’ll refer you to another excerpt from my earlier requote: “all are center frequencies”. Even though 850 nm (~3.5 THz) itself may be beyond the range of 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, another SFP module (versus higher-speed SFP+), was bundled with a media converter I’ll keep as a spare for potential future lightning-strike galvanic isolation purposes.

Initial perspectives

I’ll as-usual start with some overview shots, as-usual accompanied by a 0.75″ (19.1 mm) diameter U.S. penny for size comparison purposes.

Top.

This time accompanied by 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).

Right side (as before, with “right” referencing its orientation when inserted in a network switch or other gear’s SFP port).

Bottom (as was the case last time, note the retractable retaining pin).

And left side.

The SFP connector end, from both right-side-up and upside-down module perspectives.

Same goes for the optical fiber connector end, first with the dust cap still in place.

And now post-dust cap removal.

Dissection time

As before, bending back on the side tab you might have noticed earlier (if not, please go back and see), holding the outer and inner enclosure pieces together is our pathway to the insides. There are two of them, 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 we saw last month, fell out as the enclosure halves separated.

And there we are.

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

Free at last.

Here’s the bottom of the PCB.

The optical receiver module is 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, with the 850 nm VCSEL-based transmitter below it and conversely looking very different than 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? Let’s take a closer look.

The larger of the two has the following four-line topside mark; see also my notes in brackets.

1138142
780KF VJ [The second character might instead be the letter O]
PHL [There’s another three-digit code to the right, but it’s obscured by the yellow paint. In a photo I found online, it’s “141” but seemingly not in this case; I think that the first character is “B”]
e4 [This seems to be a company logo, given that it’s surrounded by a circle. Next to it is, judging from other photos I’ve seen, the two-character code “ES”, albeit again yellow paint-obscured]

A Google search on the phrase “1138142 IC” suggests that STMicroelectronics is the supplier but doesn’t reveal anything about the chip’s function. Reader insights are welcomed!

The smaller IC is marked as follows; again, note my comments in brackets.

532R
B809 [As before, the third character here may instead be the letter “O”]

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

In closing, the SFP interface and LC (Lucent Connector) ends of the PCB are visually identical to the ones found in last month’s module, so I’ve decided to spare you the photo redundancy. See last month’s teardown images for representations of how these ones also look. And as always, please share your thoughts on what I’ve found today in the comments!

—Brian Dipert is the associate editor, as well as a contributing editor, at EDN.

Related Content

The post SFP: The multimode module autopsy appeared first on EDN.

Different emitter, same dim red glow

Initial perspectives

Dissection time

Become a member

Become a member

Become a subscriber

Become a sponsor