OPTICAL FIBER SPLICING
Introduction Reason for jointing: - Increase fiber span (long cable runs) Connecting devices such as amplifiers, pigtails, etc. to fibers
TYPES OF SPLICING Two types of splicing Mechanical splice Temporary splice Fusion splice Permanent splice
Consideration in jointing Factors that cause loss in fiber connection: - Lateral displacement Angular displacement Longitudinal displacement (air gap) Numerical Aperture (NA) mismatch Core size mismatch End surface finishing
Lateral displacement Loss due to core center not at the same level Very critical for single mode fiber Fiber Lateral Offset
Angular displacement Light travel within 7-8 degree inside the fiber Causes 0.2 db loss for every angle displaced Angular Misalignment Fiber
Longitudinal displacement (air gap) Loss due to Fresnel reflection in air gap Causes about 0.17 db loss for each air/glass interface Fiber Air Gap
NA mismatch When light travel from high NA to lower NA fiber, it experience high loss Fiber Fiber with higher NA Fiber with lower NA
Core size mismatch Loss occurs when light travel from larger to smaller core Fiber Bottle -Neck Fiber with larger core Fiber with smaller core
End surface finishing A poor end surface finishing cause light to reflect into other that intended direction Fiber Rough surface
Fusion splicing The controlled aligning, melting and pushing together of hair-thin strands of glass resulting in a transparent, non-reflective joint. This is accomplished with a machine that performs two basic functions: aligning of the fibers and melting them together typically using an electric arc.
Example of Fusion Splicing
How to make a Fusion splice? Fusion splicing consists of four basic steps, regardless of how sophisticated a machine is. Step 1 - Prepare the fiber Step 2 - Cleave the fiber Step 3 - Fuse the fiber Step 4 - Protect the fiber Fusion splicer do each of these steps with varying degrees of accuracy. Overall, the more accurate the machine does these steps, the lower your actual splice loss will be.
How to make a Fusion splice? Preparing the fiber is accomplished by stripping away all the protective coatings, jackets, tubes, etc., until you are left with the bare fiber. The main concern here is cleanliness. A clean fiber is essential for the all important cleaving step. Cleaving the fiber properly is the key to successful splicing. It is virtually impossible to make a good fusion splice with a poor cleave.
A comparison of two cleaves shows why:
Principles of Fusion Splicer How does the fusion splicer view the fibers? Most splicers use video cameras to focus on the fibers and then display the image on an LCD screen. Typically, one camera views the fibers on the X axis (front to rear axis) and one views the Y axis (up and down axis). This allows splicers with 3-axis alignment (X, Y, and Z) to align the fibers along both the X and Y axes before bringing them together along the Z axis.
There are three ways a splicer can align the fiber, passively or actively: - Passive alignment This is a very simple process found on less expensive machines. The splicer uses some type of V-shaped groove to hold the fibers in place along the X and Y axes. The only movement of the fibers is along the Z axis as the splicer brings the fibers together. This process relies heavily on precisely-shaped V-grooves and very clean fiber. Chipped V-grooves or dirty fibers can affect the X or Y alignment to the point that the splicer cannot perform a good splice on them. Passive alignment only allows cladding alignment because the fibers cannot be moved to align the cores.
Active alignment This process involves alignment on all 3 axes. The splicer still uses a V- groove to hold the fibers in place, but in active alignment it can actually move the V-groove area along the X, Y, and Z axes to bring the fibers into alignment with each other. This 3-axis alignment allows for core alignment instead of just aligning the fibers based on their outside geometry. Due to the expensive electronics and motors required to handle this type of precision movement, this process is only found on high-end machines.
Fixed V-Groove (passive) - Precise groove in materials used to align fibers - Alignment of fiber based on shape of outside bare glass
The different between splicing multimode than single-mode fiber? Larger cores. Multimode fiber generally has cores of either 50 µm or 62.5 µm in diameter. Single-mode fiber has a core of only around 9 µm. The fiber core and cladding are both made out of silica but the core has dopants added to it to alter its reflective qualities. This difference in material and size of the core means that the core and cladding are not affected by the fusion current in the same way. Due to this difference, in multimode splicing the fusion current and fusion time must be adjusted (typically to a longer time and cooler arc than on a single-mode fusion splice) to keep from completely burning back the fibers.
Multimode fibers also have a much higher tendency to allow air bubbles to form during the splicing process. This can result in a deformed or completely open splice. For this reason you can expect a remake rate of about 5% when multimode splicing. This number used to be as high as 10-15% but recent improvements in the way splicers prepare the end-face during the prefusion stage have lowered that rate.
What happens if we splice 50 µm to 62.5 µm fiber? When transmitting from the 62.5 µm fiber to the 50 µm fiber you will experience a one-time loss of approximately 3.0 db strictly due to the differences in core diameter, numerical aperture, and scattering coefficient. The splice quality could also slightly affect the total loss. The analogy of two pipes with different inner diameters may help visualize that light "leaks" when passing from the large pipe to small pipe, but negligible "leaks" occur when water flows in the opposite direction. When transmitting from the 50.0 µm fiber to the 62.5 µm fiber you can expect only the loss associated with the splice itself.
What happens if we splice single- mode to multimode fiber? This is typically not done. Since the core sizes and glass structure are significantly different, it is very difficult to obtain a good splice. Even if a good splice were made, a one-time loss of > 10 db would occur due to the mismatched cores.
What adjustments of the splicer affect a splice the most? The two adjustments that most affect a fusion splice are: - Changing the fusion current or Changing the fusion time. To a certain degree different combinations of these two parameters can give you the same results. For example, increasing the fusion time slightly and decreasing the fusion current slightly can give you the same results as decreasing the time and increasing the current.
Environmental factors that affect a splice. Cold and hot temperatures can be compensated for by increasing or decreasing the fusion current. Humidity does not generally affect single-mode splicing. Humidity does affect multimode splicing. Reduce the current if necessary. Humidity can affect the cleanliness of the electrodes so check them more often than usual in humid conditions.
Environmental factors that affect a splice. Splicing at high altitudes (3,000 ft) requires a higher fusion current. Remember, most batteries experience decreased life in cold areas.
Operation of Fusion Splicers Splicer Operation It is awkward at first to hold, strip, cleave, and place the fiber in the clamps. Practice makes perfect. Here are five general steps to complete a fusion splice:
1. Strip, Clean, & Cleave a. Strip Strip fiber to appropriate length per your splicer's instruction manual b. Cleaning Clean the fiber with Fiber- Clean towelettes or a lint-free wipe and isopropyl alcohol so that the fiber squeaks
c. Cleaving Place fiber (after stripping and cleaning it) in cleaver using the fiber guide to position it Align the fiber in the cleave area to cleave at the proper length Depress the cleaver arm gently Remove and safely discard the fiber scrap
2. Load Splicer Position tip of fiber near electrodes Do not bump tips into anything Ease placement by bowing fibers in groove
3. Splice Fibers READ The Manual! PRACTICE! Don't expect to be a pro after one splice Place first cleaved fiber in v-groove with fiber tip near the electrodes Close the fiber clamps Repeat on opposite side for second fiber Select program on fusion splicer Initiate fuse cycle (can be manual or automatic)
4. Diagnose and Correct If Errors Occur Cleaver - wipe blade and clamps periodically - operate slowly; it's not a stapler! Alignment - clean V-grooves, guides, clamps when offsets occur Electrodes - clean at the start of each day; Video System - clean LEDs, prisms/mirrors, cameras, and protective disk High Loss - fibers not aligned, poor geometry, dirty electrodes, or wrong parameters Multimode fiber - bubbles and neck downs are frequent occurrences: expect about 80% yield on most splicers Titan fiber - difficult to cleave: deeper score, splices hotter: reduce current settings for ribbon A "Good Splice" is determined by: User Skill: cleanliness, operation of equipment, ability to recognize and correct poor preconditions Splicer: V-groove, cladding alignment vs. core alignment, proper settings Fiber: good geometry quality
5. Remove and Protect Splice Remove completed splice from splice area Use Heat-Shrink oven (or mechanical protection) to protect the splice Place splice tray in adjustable tray holder and insert protected splice into splice tray
Which splice protection option should we choose? This question is all about time, money, and accessibility. Here's a list of the pro and contra of each choice. Comparison of Splice Protection Options: - Type Consumable Cost Per Each Time Needed To Complete Future Accessibility Of One Fiber RTV Gel Low Low Difficult Heat-shrink Medium High Easy Quick Fold-over Options Low to High Low Easy
Silicone gel - After completion, each splice is placed in a groove in the splice tray and then covered with gel that hardens. Heat-shrinks - A heat-shrink is a small plastic tube with a small rigid bar running through it. Prior to splicing, slide the heat-shrink down one of the fibers. After the splice, you slide the heat-shrink up to cover the joint and place it into a heat-shrink oven (usually supplied with the fusion splicer). After it shrinks down around the splice and the oven cycles off, remove the splice and place it into a heat-shrink splice tray. Quick-Fold Protection Options - A piece of formed plastic or metal is folded around the splice. This can be done after the splice is completed and with no electrical power.
How often should We do routine maintenance? In most situations, the beginning of the day is the best time to do routine maintenance. This is especially true if we are doing a large number of splices per day, such as 100+ per day. For smaller amounts per day, routine cleaning and general maintenance can be performed every 2-3 days. The best rule of thumb is to pay attention to splicer and when we see our splice loss performance begin to change, clean it.
How often do fusion splicers need to be repaired & when it should be replaced? If we follow the routine cleaning and care necessary for our splicer and cleaver (5 minutes/day) then we will rarely need to send the machine to be repaired. Most of the consumable items on a splicer (esp. electrodes and V-grooves) can be replaced in the field. Like having a 10-year old computer, if it can handle the work you are doing, there is no need to switch. But imagine how much easier our job would be if we did buy a new machine.
How to test our splice is acceptable? A one-way OTDR reading is an inaccurate method for measuring single-mode splices. Single-mode fiber splices MUST be measured bidirectionally to make an accurate reading due to differences in mode field diameters. The most accurate way to measure splice loss is to use a power through test set-up by first referencing a continuous piece of fiber, breaking it, then resplicing it and reading the loss from the meter. This is not practical in the field, and only applies in a lab environment.
How is the splice loss estimation done? There are two ways to analyze the outcome of a fusion splice: - using video cameras or Optical power analysis.
Video Cameras Analysis This is generally done through the use of two video cameras. The cameras take a detailed picture of the splice area after the alignment has been done but prior to the fusion process. This is the "before" picture. Upon completion of the splice the cameras take another picture of the splice, the "after" picture. Then the computer compares the two pictures to see if the "after" picture was what it expected.
Video Cameras Analysis If there is a deformation of some type at the splice point then it will estimate a splice loss based on the severity of that deformation. The estimation is based on historical data of similar splices that were completed and tested in a lab with a complete end-to-end test setup. Cleave angle and fiber offset are also used to estimate the splice loss.
Optical Power Analysis The only way to truly measure a splice loss is to actually inject light into the splice point and measure the loss at that point. For example, the Corning Cable Systems LID-SYSTEM unit on the M90 fusion splicer does that. It partially bends the fiber around a mandrel and injects light into the fiber on one side of the splice point. It uses the same process on the second fiber to retrieve the light and measure how much was lost along the way. Since the distance is too short to worry about the intrinsic fiber loss, the only loss remaining is due to the splice itself.