[Basic skills in optics] Method of coupling light from space to optical fiber [continued]

[Basic Optics Skills] Method of Coupling Space Light to Optical Fiber [Continued]

1. Supplement to the previous article

(Previous article: [Basic Optics Skills] Method of Coupling Space Light to Optical Fiber [Video]) The previous article summarized the basic method of coupling free space light into single-mode fiber very well. Here are some points that should be paid attention to when coupling to help everyone understand and master it. First of all, before coupling, try to ensure that the optical path is parallel to the optical platform before it reaches the focusing aspheric lens or laser objective lens, and at the same time, it should be as parallel as possible to the row of screw holes on the platform.

Then use a white screen with cross concentric circles (concentric circles drawn on cardboard) behind the focusing lens to observe whether the direction of the light path has changed. This step mainly allows the incident light to pass through the center of the lens to avoid causing serious aberrations and reducing the quality of the focal spot. The reason why concentric circles are used is because the light beam diverges quickly after the focusing lens.

It is easier to find the center of the light spot using such gradually increasing concentric circles. Of course, if it is infrared light, you need to use an infrared observer or infrared card to assist. In this way, most novices can probably understand that fiber coupling systems generally fix the lens and adjust the position of the fiber end face to achieve optimal coupling. If you adjust the lens, it is easy to make the direction of the light beam change greatly after passing through the lens.

Figure 1. Concentric white screen

Another key point in the fiber coupling process is to estimate the position of the lens focus as accurately as possible, but do not place the fiber input end too precisely near the focal plane (assuming it is a plane, at least in the paraxial case). If the optical fiber input end is too far away from the focal plane, the spot power density will be too small. If it is too close, the spot power will be too small. It is difficult to detect whether the light beam is in contact with the fiber core, and it is easy to feel that it is completely out of focus. So this one requires a little bit of estimating ability. These are described in detail in previous articles. When roughly searching for the fiber core position, you can directly observe the brightness of the fiber incident end face under laser irradiation. When part of the light enters the fiber core, the entire fiber head will darken. Similarly, infrared light requires the use of an infrared observer.

2. A new tool that facilitates spatial coupling of optical fibers

The focus of this article is that the author came across an artifact for adjusting optical fiber coupling two years ago, which is a cheap and high-quality pen-type USB digital microscope. It specifically looks like the picture below. I won’t tell you what brand it is. It’s not my product and I wasn’t given any promotion fees. You can find it by searching USB microscope on a certain website.

Figure 2. USB Microscope

The reason why I like it so much is because it is really convenient and cheap, so I never forget to take two of them to my friends in the UK when I go abroad (paying out of my own pocket). The nominal magnification is

500. Whether it can reach this much, how it is calibrated to 500 times, etc. is not important. We only look at whether there is light coming out, not the details of the fiber core. This microscope has an outer diameter of 12mm and can be easily mounted on half-inch clamps commonly used in optical laboratories using its own adjustment mount. For specific use, control the average power of the spatial light below 15mW, and then place the microscope directly and horizontally facing the output end of the optical fiber.

At this time, the output end face of the optical fiber can be seen on the computer screen. In most cases, you can see bright light at the output end immediately. If not, you can basically see part of the light output by scanning the X and Y axes of the translation stage, although at first the light may only be transmitted by the cladding of the optical fiber. Then adjust the X and Y axes of the fiber optic translation stage, and you can quickly find the best lateral position.

At this time, the microscope field of view is the roundest and brightest, and may even be saturated. In order to protect the microscope, you can take away the microscope at this time and use a white screen, infrared card, photodiode or even a power meter to continue optimizing the coupling at the fiber input end. When the power is not high, there is generally no need to worry about the laser damaging the microscope, especially when the Z-axis has not been optimized.

The author has coupled it dozens of times and has never worn it out. Besides, even if it breaks, it only costs about 200 yuan, so the risk is controllable. The advantage of using a microscope to observe fiber output is that CMOS is much more sensitive than a white screen or an infrared card. Imaging directly against the fiber output head can collect the output light energy very well, and the imaging method can provide spatial information on the light field distribution of the output end face compared to the single-point photodetector method.

The cheap price means that users don’t have to worry too much about the microscope being damaged by lasers. The shortcoming is that this microscope is powerless for wavelengths above 1100 nanometers. I believe users are not willing to use expensive infrared CCDs for long-wavelength spatial light coupling. But at least within the wavelength range of 400nm-1100nm, the method introduced in this article can still greatly improve the speed of fiber coupling adjustment.