Hello, welcome to Thorlabs. My name is Kevin, and today I'm going to demonstrate single mode, fiber-to-fiber coupling using Thorlabs' FiberPorts. Now the biggest piece of advice I can offer for
single mode fiber coupling is to have patience. You're aiming to overlap an incredibly small, focused spot on an equally small mode field diameter of your single mode patch cable. For reference, the size of the typical mode field diameter
is less than the width of a human hair. I'm going to show you a methodical approach that I use to
achieve efficient coupling and hopefully less headaches overall. For my demonstration today, I have in front
of me a red, fiber-coupled laser source, two single mode patch cables in the yellow tubing,
one FC/PC multimode patch cable in the orange tubing, a power meter, a power sensor,
two identical FiberPorts on a FiberBench, two irises to help with alignment, a fiber
cleaner, and a fiber inspection scope. First, let's get familiarized with
the adjusters on the FiberPort. First, we have our X and Y Adjusters. These adjusters align the lens up-down and left-right,
laterally with respect to the fiber tip. These two adjusters have a travel range of ±0.7 mm. One important note about these two adjusters is that, in order to achieve a
compact design, there is a little bit of cross talk in between the two of them. Adjusting X or Y will move the lens in X and Y. Next up, we have our ZΘ adjusters. To understand how the ZΘ adjusters work, it's important to know
that the lens cell inside the device is mounted against a tilt plate. The tilt plate is connected to the main body of the FiberPort via contact with the
three ZΘ adjustment screws and tension springs. The naming convention for the ZΘ adjusters comes from the fact that adjusting any of the ZΘ adjusters independently will
angle the lens with respect to the fiber tip, or the Θ direction. If you adjust all three ZΘ adjusters the same amount, in the same direction,
you can achieve Z-translation of the lens with respect to the fiber tip. Adjusting all three ZΘ adjusters counterclockwise
will move the lens closer to the fiber, while moving all three clockwise will
move the lens farther from the fiber. Our FiberBenches with pre-installed
FiberPorts ship roughly aligned. But if you're working with FiberPorts that have been adjusted already,
it'll be helpful to pre-align and collimate the FiberPorts prior to coupling. The easiest way to perform pre-alignment is to remove
the FiberPort opposite the one you are collimating. I like to use a lens tissue for this part, so the mounting
screws do not fall into the optical table. In addition, pre-alignment is more easily
done if you have a visible alignment source. So, I'm going to inspect and clean our
fiber tip before placing in the FiberPort. This is helpful to ensure there
is no debris on the fiber end face. That could cause poor transmission
or poor beam quality. Now that we have attached our fiber-coupled
laser source, let's turn it on. So, I'll move our ruler closer and start with the ZΘ1 adjuster. Now, the first step of the pre-alignment procedure is to back out all three ZΘ adjusters so that they are no longer touching the tilt plate. This will ensure that the tilt plate is orthogonal to the optical axis. Now, we're going to move each ZΘ adjuster clockwise
until we see a slight movement in the far field, then back off slightly counterclockwise,
and clockwise again before we see any movement. This ensures that the adjusters
are flush with the tilt plate. So, now we are going to
collimate the first FiberPort. We're going to turn each ZΘ adjuster clockwise
the same amount until we see a collimated beam. First, we'll start with one full turn, using
the logo of our ball driver as a reference. Okay. Our beam is still a little bit divergent,
so we'll do another full turn clockwise. Let's take a look at the collimation. It's still a little bit divergent, so we can do fractions of a turn.
Ah, let's do about a quarter turn. Look again. It looks pretty collimated.
Now we'll move our ruler back. We'll block our beam as we attach our iris. And now, we're going to use only the X and Y
adjusters to center the beam on the first iris. Remember you may have to iterate between
X and Y, since they are coupled. Okay.
After that's centered, we can open the iris. We'll close the second iris. If we've done our pre-alignment procedure correctly, um,
it should be pretty close to the center of this second iris, which we've set up along the same row of holes and at
the same optical height as our fiber bench previously. Um, so, it looks like we
are slightly off-center. So, we can use the ZΘ adjusters,
um, to center on that second iris. And the reason we're doing this is because we weren’t able to move each ZΘ adjuster exactly the same amount during the collimation process, which is okay. Now we'll close the first iris, make sure it's still centered. Looks good. And now we've successfully
aligned our first FiberPort. Now, let's align the second one. Let's turn the laser off for now. Now, ideally, you'd want to align this FiberPort
in the same fashion you did with the first one. But, since we're going to be adjusting this FiberPort almost immediately after pre-alignment and collimation, we don't have to be as accurate with collimation. Let's do the same with our second FiberPort.
First, let's swap over the single mode cable. Next, we'll move our ruler over to ensure the beam
is blocked, and we'll turn the laser source back on. We'll use a business card for this one. And we're going to do the
same steps we did previously. We're going to move each ZΘ adjuster counterclockwise
until we see no movement of the beam in the far field. Then we're going to move each ZΘ clockwise
until we see a tiny bit of movement. When we do, we'll back off and then again
clockwise, so we see no movement of the beam, but we know our ZΘ adjuster
is flush with the tilt plate. Now that we've completed the first step of pre-alignment,
let's turn each ZΘ adjuster clockwise. If you remember from the first FiberPort, we turned it
two clockwise, and then a little bit more. So, let's start off with
two full turns clockwise. Now the output does look pretty well-collimated. We don't have to be too precise with this part, because when we're coupling in, we're going to adjust those ZΘ adjusters again anyway. Another way you can align the, ah, the lateral position of the lens is by moving the X and Y adjusters and looking at the output through the bulkhead of the first FiberPort. When you see that the output is pretty concentric, um,
you know you're roughly in the correct position laterally. Now that we're done with pre-alignment and collimation
of both FiberPorts, let's move on to fiber coupling. So, we'll turn our laser off, move our ruler,
and swap the single mode cable back to the first one. Now, we'll turn our laser back
on and measure a baseline. I've adjusted our fiber-coupled laser such that it has approximately
1 mW output for ease of calculating coupling efficiencies for later. Now, let's attach our multimode cable. Today, I'm using a 50 µm core multimode cable. It's a little bit easier to couple into than single mode fiber, and it will leave us with a smaller area of uncertainty for when we switch to our single mode fiber. Let's clean our tips, turn our laser off, and attach the second tip. Now, we'll turn our laser back on. So, we've done our pre-alignment, ah, procedure pretty well;
we have a very high coupling efficiency right off the bat. Normally, you'd use the X and Y adjusters on your
coupling FiberPort to try to maximize this value first. It's not going to help us
too much, but let's see. Yup, so adjusting the X and Y adjusters didn't help too much,
uh, but now we'll move on to the ZΘ adjusters. When we're adjusting the ZΘ adjusters, first choose one,
I like to choose the ZΘ1 adjuster, then choose a direction. Let's choose clockwise. We're going to rotate the adjuster in that
direction until we see a maximum value. If adjusting a single adjuster does not
increase the power, but decrease the power, I’ll put the adjuster back where it was. So, that adjuster did not seem to help. We'll move on to the ZΘ2 adjuster. Well, that adjuster did not seem to help. Then, ZΘ3. That one does seem to help, so
we'll move until we see a max. We're going to continue moving in that direction until
we get to about 90-95% of that maximum value. And move slightly past it. And continue. Back to ZΘ1. Did not seem to increase our optical
power, so we'll put it back where it was. ZΘ2 seems to help slightly. Went a little bit too far, so I'll back back up. Now ZΘ3. ZΘ1 did not help, so I'll
put it back where it was. ZΘ2. So now all three ZΘ adjusters seem to decrease
the optical power if we move clockwise, so we'll move each ZΘ adjuster slightly counterclockwise
to the absolute max and move on. For our alignment, it doesn't seem
to help much, which is a good sign. Once you've hit about 90-95% coupling efficiency with your
multimode fiber, you can swap out to your single mode fiber. So again, turn the laser off. And now we'll inspect and
clean our single mode cables. You may notice that our single mode
cable has a green boot on one end. That denotes an FC/APC connector. The reason for this is because the power meter sensor head
has a very high Fresnel reflection with respect to air. The FC/APC connector will prevent any back reflections from going into
our system and destabilizing our laser. Now that we have our single mode cable attached, let's turn our laser on and hopefully achieve 100% coupling efficiency. We have not.
And that's okay. When swapping from your multimode cable to your single mode cable,
it's not uncommon if you're reading nW. But the first step in this process would be, again, to maximize the value that you see, using only the X and Y adjusters on the coupling FiberPort. After you've maximized your value here, we're going to do a similar process as the multimode fiber, where you pick an adjuster, let's say ZΘ1, pick a direction, let's say clockwise, and move that adjuster. If you do see a maximum, move slightly past it, to about 90-95%. If you do not see an increase in power, put the adjuster
back where it was, and skip the adjuster for now. And repeat the process until all three adjusters
do not increase your optical power in a row. So, we're not really seeing
much increase in optical power, so we'll move each adjuster counterclockwise. So, regardless of the direction that we adjust any of the ZΘ adjusters,
we're seeing about the same optical power. Um, one step I like to try is to unscrew the nut of
the connector and slightly pull back on it. If, while you're removing the fiber, you see a jump in optical power, you know you need to move your lens a little bit farther from the fiber tip. Oh! So, we did see a big jump there. So, we'll put our connector back on, screw it down tight. Now we're going to move a large movement of each
of the ZΘ adjusters clockwise in order to move the lens further from the fiber tip. Let's try a half a turn. It's important in this step to try and adjust each
ZΘ adjuster in as equal of an amount as possible. The power should drop as you're adjusting the first two, but,
when you adjust the third one, should hopefully come back. Now we may not have adjusted each of the three adjusters exactly the same amount, so let's just maximize all three to see where we are. So, it seems our optical power dropped slightly.
Um, it is possible we've moved too far. Let's try our trick again. Okay, so we do see the power does increase a lot. So, we're just stuck in a different local maximum that happens to be a little bit smaller than the local maximum we were at before. So, we still want to move, clockwise,
let's try another half turn. Again, let's maximize. This still isn't close to what we saw
before, so let's continue in that direction. Do another half turn. Again, let's maximize. Now let's try our trick one more time. Okay, that time it didn't seem to help. So, let's try to move our coupling
FiberPort counterclockwise, now. If we reach a maximum, we'll move
slightly past that maximum value. It's not uncommon to make four or five
rounds until you see a local max. Don't be afraid to do a larger movement. Just remember which direction you go, just in case you don't increase. Here, I'm going about 1/8 of a turn. Again, let's maximize. Let's move another 1/8 of a turn. Maximize again. We'll keep going counterclockwise
another 1/8 of a turn. Maximize again. Let's keep going counterclockwise
another 1/8 of a turn. Maximize. Now that we're in the mW range, ah, to prevent further
jitter in our power meter, let's fix the gain. Let's continue moving counterclockwise. Let's try another 1/8 of a turn. Try maximizing. And, again, counterclockwise 1/8 of a turn.
Maximize. You can try adjusting the ZΘ adjusters on your collimating FiberPort, let's say, counterclockwise, and we'll see if we can reach a max and move past it. So, it seems our optical power dropped slightly. Let's try clockwise, starting with the ZΘ1. So, we're at a local max. We can also adjust each of the ZΘ adjusters the exact same
amount to translate the lens on the collimating FiberPort. Let's do 1/8 of a turn counterclockwise. Let's try maximizing. Our power went down, that's perfectly fine. That just means we'll flip directions
and start going clockwise. Make up what we just did, and a little more,
so we'll try a quarter of a turn clockwise. And we're maximizing. Okay.
Let's do another 1/8 of a turn clockwise. Again, let's maximize. So, it looks like we went a little bit too far.
So, let's try 1/16 of a turn counterclockwise. Let's again maximize. Again, let's try another 1/16
of a turn counterclockwise. And we'll maximize. Let's maximize the three adjusters on the coupling side, followed by the collimating side, just to see what local maximum we're at. That seemed to help slightly, so
let's just maximize all six again. And let's try one more time. With our chosen laser and patch cables, coupling
efficiencies of 70-80% can be expected. I hope this video was insightful and will
be helpful in one of your applications. One point to keep in mind is that
adjusting FiberPorts is a skill. As you've seen today, patience is required. If you have any further questions,
please feel free to contact Tech Support.
