Align Fiber Collimators to Create Free Space Between Single Mode Fibers | Thorlabs Insights
ฝัง
- เผยแพร่เมื่อ 2 มิ.ย. 2024
- Two collimators, inserted into a fiber optic setup, provide free-space access to the beam. The first collimator accepts the highly diverging light from the first fiber and outputs a free-space beam, which propagates with an approximately constant diameter to the second collimator. The second collimator accepts the free-space beam and couples that light into the second fiber. Some collimation packages, including the pair used in this demonstration, are designed for use with optical fibers and mate directly to fiber connectors.
Ideally, 100% of the light emitted by the first fiber would be coupled into the second fiber, but some light will always be lost due to reflections, scattering, absorption, and misalignment. Misalignment, typically the largest source of loss, can be minimized using the alignment and stabilization techniques described in this video.
In this demonstration, the first fiber is single mode. The optical power incident on the second collimator, as well as the power output by the second fiber, are measured. When the second fiber is multimode with a 50 µm diameter core, alignment resulted in 91% of the power incident on the second collimator being measured at the fiber output. This value was 86% when the second fiber was single mode. Some differences in collimator designs, and their effects on the characteristics of the collimated beams, are also discussed.
For more information about the modified thin lens equation we used to model Gaussian beams, see
www.thorlabs.com/newgrouppage... .
00:00 - Introduction
00:41 - Characteristics of Collimated Beams
02:32 - Reduce Degrees of Freedom
03:46 - Baseline Power Measurement
04:37 - Coarse Alignment Using a Multimode Fiber
06:38 - Attach Single Mode Fiber to Second Collimator
08:28 - First Alignment Approach: Misalign & Maximize
10:02 - Second Alignment Approach: Misalign & Misalign
Components used in this demonstration include:
- S1FC635 Fiber-Coupled 635 nm Laser: www.thorlabs.com/newgrouppage...
- TC12FC-633 Triplet Fiber Optic Collimator: www.thorlabs.com/newgrouppage...
- KM100 Kinematic Mount: www.thorlabs.com/newgrouppage...
- AD12NT Adapter (Between Collimator and Mount): www.thorlabs.com/newgrouppage...
- S130C Power Sensor: www.thorlabs.com/newgrouppage...
- SM1A29 Thread Adapter (Gives Power Sensor SM1 Threads): www.thorlabs.com/newgrouppage...
- S120-FC Fiber Adapter Cap with SM1 Threads: www.thorlabs.com/newgrouppage...
- PM400 Power Meter: www.thorlabs.com/newgrouppage...
- P1-630A-FC-1 Single Mode FC/PC Patch Cable: www.thorlabs.com/newgrouppage...
- P5-630A-PCAPC-1 Single Mode Hybrid Patch Cable: www.thorlabs.com/newgrouppage...
- M42L01 Step Index Multimode Fiber Patch Cable: www.thorlabs.com/newgrouppage...
- FCC-7020 Fiber Connector Cleaner: www.thorlabs.com/newgrouppage...
- FSR3 Storage Reel for Patch Cables: www.thorlabs.com/newgrouppage...
- TR2 2" Post: www.thorlabs.com/newgrouppage...
- PH2 0.5" Post Holder: www.thorlabs.com/newgrouppage...
For more photonics how-to videos, visit www.thorlabs.com/newgrouppage... - วิทยาศาสตร์และเทคโนโลยี
Very useful demostration
hoping for more videos like this
We’re glad you’re enjoying our videos! There are more on the way.
As an optical engineer, I can hold my breath and open my eye widely when watching the host adjusting his kinematic, like what I do when adjusting my system :)) Thanks anyway for a very informative video :)
We are glad you liked the video! Are there any techniques not discussed in the video that you found to be useful as you were learning to align optical systems?
You guys are doing so good work on these tutorials. Top-notch quality - thank you!
Thanks! We try to turn conversations we have in the lab into videos, and it is great to hear the approach is helpful! Please let us know if you have suggestions for other video topics 😊
@@thorlabs I would suggest making tutorial how to align beam expander in to the beam path
@zedius24 Thanks for your response! We’d like to better understand your suggestion. We have a Video Insight ( th-cam.com/video/xuzuFPl8rh0/w-d-xo.html ) that demonstrates the design and construction of a beam expander using a laser speckle technique. Does this video provide the information you had in mind, or was your suggestion something different?
@@thorlabs I had another idea: what if you buy an beam expander and want to align it's position to optical path? What are the tips for ajusting procedure and suggestes mounts.
@zedius24 Thanks for providing more information about your suggestion for a video demonstrating the alignment of a beam expander in a setup. It is now on our list of topics!
This is really great stuff!!! Very useful indeed!!! I wish they were around when I was a student! Thanks a lot!
We're very excited you've found the videos useful! Are there any other topics or demonstrations that would have saved you time at any point? We love to receive recommendations for future videos.
@@thorlabs anything about aligning laser resonators
@Michal Pietrzak Great suggestion! Thanks for sharing it with us 😊
Great explanation!
We're glad you found it helpful!
Very nice and useful demonstration. Can you please add some videos on PAF coupler's alignment with single photons (SPDC photons). We generally use those couplers.
Thank you, and great suggestion - we are very glad you asked!
I second this!
It’s very interesting! Regarding the fixed focus collimator package, is it the first kind or the second kind? I guess it’s the first kind, right? The waist is pretty close to the housing. While I was learning this technique in the lab, I was told to tune the distance between the collimating lens and fiber end, till the waist towards infinity to make a homogeneous beam. In reality we just move the waist to several meters away. It’s similar to the second kind, just to increase the Rayleigh length, right? Can you tell me the exact position of the fiber end with respect to the focal length of lens in these two collimating methods?
@Thinker Doer Yes, the collimator package used in this video positions the fiber's end face at the input focal plane, similar to the left-side illustration at 2:20. The collimated beam waist is at the output focal plane, and the collimated beam has the longest possible Rayleigh range and lowest possible divergence. There are actually three lenses inside each collimator used in this demonstration. The effective focal length of the lens assembly is 12 mm at 633 nm, and the collimated beam waist is a little over 9.5 mm from the output side of the collimator's housing. Separating the collimators by ~19 mm would maximize the coupling between the fibers, but many applications require more free space.
As you mentioned, tuning the separation between the input fiber end face and a collimating lens can increase the distance to the collimated beam waist but never by an infinite amount. However, doing this actually shortens the Rayleigh range of the collimated beam by as much as half. Since an adjustable collimator was not used in this video, a case similar to the right-side illustration at 2:20 can be modeled using a single thin collimating lens whose focal length (f = 12 mm) is the same as the collimator's. The light from the input fiber has a fairly short Rayleigh range (zR) of ~0.025 mm, since the fiber's mode field diameter is ~0.0045 mm at 633 nm. The maximum distance (~2.9 m) to the collimated beam waist is provided when the separation between the fiber end face and lens is ~12.025 mm (f + zR = 12 mm + 0.025 mm).
There's a modified thin-lens equation for Gaussian beams (described here: www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=14511) that comes in handy for estimating beam parameters like these.
What if you were to translate one of the collimators closer, or further away, does the coupling efficiency stay the same?
The coupling efficiency actually will change, and this is due to the characteristics of the laser light. While it may appear that the first collimator outputs a plane wave with a constant beam diameter, this is not the case. Instead, the output beam has a waist, at which its diameter is smallest, and the diameter of the beam increases (diverges) on either side of the waist (for more information, see: www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=14489 ). The second coupler collects the light and focuses it to a location that changes with the spacing between the two collimators. As a result, the coupling efficiency changes based on the distance between the collimators. Oftentimes it is less important to maximize the coupling efficiency than to provide the free space required for the application.
To achieve the highest coupling efficiency, it's necessary to know the position of the beam waist with respect to the collimator's housing. This specification is typically provided with the collimator. The spacing between the two collimators' housings should then be the sum of their beam waist distances. When they are spaced by this distance, the light focuses on the endface of the second single-mode fiber, which optimizes the coupling conditions (for more information, see: www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=14205 ). The coupling efficiency will decrease if the spacing is either increased or reduced from the sum of the beam waist distances, since any changes to the spacing will shift the position of the beam waist image with respect to the fiber's endface.
Hi what is the wavelength you are using for this set up?
@VishalDhurgude A fiber-coupled laser with a 635 nm wavelength was used during this demonstration. Links that provide more information about this laser and the other components used in this demonstration are included in the video’s description. (Click the “Show More” button just above the comments to expand the description text.)
Can we align 1550nm laser through TC12FC collimator, with this concept?
Absolutely! This approach can be used with both a TC12FC collimator and 1550 nm light.
Since 1550 nm is not a visible wavelength, you may find it useful to use an IR viewing card ( www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=296 ) to track the beam spot while performing rough alignment. The procedure demonstrated in the video applies once some coupled light is measured by the power meter.
For those unfamiliar with the TC12FC, it includes three lenses in its design, instead of the single-lens design of the collimator used in the demonstration. The TC12FC's three lenses can be modeled as a single lens with low aberrations, and this collimator will provide a beam that also follows the figures in this video.
Rayleigh range = 3.90 mm. How is it calculated?
@PasinKuncharin We calculated the Rayleigh range of the beam output by the collimator using the values noted on the diagram at 1:54 and the Gaussian beam equations that are summarized here: www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=14511.
We first calculated the Rayleigh range of the input beam ( zR_in = (pi/wavelength)*Wo^2 , where Wo is one half of the input fiber's mode field diameter ). To calculate the waist radius of the beam output by the lens ( Wo’ = m*Wo ), we then used the magnification ( m = Wo’/Wo = focal length/ zR_in) and the lens’ 11 mm focal length. The Rayleigh range of the output beam is then ( zR_out = (pi/wavelength)*Wo’^2 ).
How to calculate beam waist?
@Sahil Nazir Pottoo The size and position of the beam waist output by the collimator depends on the diameter of the input beam waist, the focal length of the collimator, and the wavelength of light. The collimated beam waist diameters and positions for all of Thorlabs’ collimators are provided, but these values can also be calculated. If the fiber is single mode, the diameter of the input beam waist is the mode field diameter (see the link below for more information about this parameter). If you’re using a commercial fiber collimator, the focal length should be provided. When the single mode fiber’s core is placed at one focal point of the collimator, the location and diameter of the beam waist of the collimated output light can be estimated using a combination of Gaussian beam equations and the modified thin lens equation (see the link below for more information).
For those who are interested, the modified thin lens equation takes into account the Gaussian nature of the beam, while the standard thin lens equation does not. The modified thin lens equation can be used to estimate the output beam waist position by treating the input beam waist from the fiber as the object and the output collimated beam waist as the image.
Mode Field Diameter: www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=14203
Modified Thin-Lens Equation: www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=14511
How does the intentional misalignment and then compensation improve the coupling over that which was initially achieved ?
After the initial alignment, the fiber collimators are not perfectly parallel to each other. The alignment is good enough that some light gets into the second fiber, but since the two collimators are angled somewhat away from one another, there's also a significant amount of light that is not coupled into the fiber. Aligning the collimators to directly face one another maximizes the amount of coupled power. The intentional misalignment and compensation is the approach Bill uses when he wants to incrementally walk the two fiber collimators into alignment.
The misalignment step is an attempt to move one collimator into better alignment, and the compensation step to improve the second collimator's alignment with respect to the first. After each step in the walk, a peak power is found. These peak powers can be thought of as local maxima pointing in the direction of the greatest achievable coupled power. As long as each local maximum power is larger than the previous, the adjustments are improving the alignment. When no adjustment further increases the coupled power, the power peak is assumed to have reached a global maximum with optimal alignment of the collimator. However, if the light coupled between the two fibers is still not within the expected range than we would suggest trying a larger move in all axes to see if any direction provides a power jump.
when Beam Waist Away from Housing, g=f+ZR, b=g*f/g-f = f+f^2/ZR ,so can u tell me please how did you achieve this result f+f^2/2*ZR
@free.codm The equation we include in the video was derived using a modified version of the standard lens equation. The standard lens equation (using your notation: 1/g + 1/b = 1/f ) does not model the behavior of Gaussian beams. But a modified version of that lens equation can provide a good Gaussian beam model. This modified lens equation (again using your notation: 1/[g+(ZR^2)/(g-f)] + 1/b = 1/f, where f is the lens’ focal length, g is the distance to the object beam waist, b is the distance to the image beam waist, and ZR is the Rayleigh Range of the object beam waist) was described by Sidney Self in an Applied Optics paper (“Focusing of spherical Gaussian beams”, Applied Optics, 22 (1983) 658) and is summarized here:
www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=14511 .
thank u very much
Nice video. Can you also make a video for the beam coupling into fiber in both directions using FiberBench and FiberPort?
As a matter of fact, alignment of a couple of FiberPorts on a FiberBench is the subject of the video we're currently working on! We plan to release it this month (Feb 2022) and hope you like it also :)