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?
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 😊
@zedius24 Thanks for your response! We’d like to better understand your suggestion. We have a Video Insight ( ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-xuzuFPl8rh0.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!
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.
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.
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 :)
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.
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.
@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.)
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.
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.
@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 ).
@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
@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 .
