Applications of the Vytran LFS4100 Filament Fusion Splicer

Features

• Splice Standard and Specialty Optical Fibers with Cladding Diameters up to 1.25 mm
  • Filament Fusion for Precise and Consistent Splicing
  • Wide Thermal Dynamic Range for Various Fiber Sizes and Fusion Processes
  • Fire Polishing for Enhanced Splice Strength
• True Core Imaging^® for High Resolution Fiber Alignment
• Thermally Diffuse Fiber Cores to Produce Mode Field Adapters
• Real-Time Control System and Machine-Level Programming

Build Your System

• Filament Fusion Splicer for Standard, Large-Diameter, and Specialty Optical Fiber (LFS4100)
• Choose from Four Graphite and Three Iridium Filament Assemblies (One FTAV4 Graphite Filament Pre-Installed in System)
• Choose Top and Bottom Inserts (Two Top Inserts and Two Bottom Inserts Required; See Fiber Holder Insert Tab for More Information)
• Optional Ultrasonic Cleaner for Preparing Fibers Prior to Splicing

Thorlabs' LFS4100 Vytran^® Filament Fusion Splicer for Standard, Large-Diameter, and Specialty Optical Fiber combines filament fusion technology, a high degree of user process control, and simple operation. These properties make this system ideal for volume production in manufacturing environments that demand precise, consistent performance. The splicer is designed to perform high-quality fusion splicing for fibers with claddings from Ø125 µm to Ø1.25 mm. An ultrasonic cleaner for preparing fibers for splicing can be purchased separately below.

The LFS4100 system consists of a filament-based heater, a microscopic high-resolution CCD imaging system, precision stages with multi-axis control, and a desktop computer. The filament heater has a wide temperature tuning range that extends up to 3000 °C, which makes it possible to fuse and process various fiber types and sizes. For alignment purposes, the imaging system displays a magnified fiber image with sub-micron resolution; the camera can display views of both the fiber sides and fiber ends. Using these images, the XY and rotation stages automatically manipulate the fibers to achieve optimum positioning and ensure high-quality, low-loss splicing. The LFS4100 also has the ability to rotate fibers, making it possible to align PM fibers, eccentric core fibers, and non-circular fibers.

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LFS4100 Mirror Tower and Two Fibers Ready for Splicing

Filament Fusion Splicing and Fire Polishing
Thorlabs' Vytran filament fusion technology provides a consistent, reliable method of making high-strength, low-loss splices. Precision control of the fusion process is achieved by purging the splice region with an inert gas and using a resistive graphite or iridium filament to supply the thermal input for fiber fusion. Because the fusion heat source is isolated from the environment, filament fusion splicing is not dependent on ambient conditions. The controlled conditions inside the system in combination with constant power control ensure repeatable performance for every splice.

Post-fusion, a fire polishing process significantly increases splice strength through a rapid heat treatment of the splice region. In addition, the fire polishing process provides core diffusion capabilities that can be used to adiabatically expand the mode field diameter of a fiber, producing low loss splices between markedly dissimilar fibers. For more details, please see the Fusion Splicing tab.

End-View Imaging for Rotational Alignment of Polarization-Maintaining and Specialty Fiber

Fiber Imaging and Alignment
The LFS4100 fusion splicer includes both an imaging system and an array of stepper motors for automatic positioning of fibers and the camera assembly. The imaging system consists of a camera tower with a movable objective and a ring illuminator that provides backlighting for the fibers. A mirror tower containing two sets of mirrors and an LED sits around the fibers and provides a view of both sides of the fibers as well as the fiber end faces. This mirror tower can be seen between the two fiber ends in the image to the right.

Side view images of fibers can be acquired for all fiber diameters and used for automatic or manual XY alignment with the motion control system. In addition, end-view imaging allows for rotational alignment of polarization-maintaining and other specialty fibers. However, this end-view alignment feature is only available for fibers that can be held by the cladding or fibers that have a buffer with diameter ≤1269 microns. Larger diameter buffers will not fit in the VHB00 or the VHB05, the Fiber Holding Block Top Inserts with indents for LED end-view illumination. For more difficult alignment tasks, accurate positioning can also be achieved by maximizing the power coupled from one fiber into the other as measured by an optical power meter.

Fiber Holding Block Inserts and Filaments
The LFS4100 is designed to accept Fiber Holding Block Inserts that can clamp onto a range of fiber cladding or buffer diameters. Nine top and ten bottom inserts are available separately below. Two top and two bottom inserts are required to operate the fusion splicer. The Fiber Holder Inserts tab has a selection guide to aid in choosing which inserts are required based on the cladding or buffer diameter of the fiber to be spliced. Please note that the only top inserts that allow end-view imaging for the alignment of polarization-maintaining and specialty fiber are the VHB00 and VHB05 (sold below).

The LFS4100 also requires a filament assembly to perform splicing operations. The filaments are omega-shaped resistive heaters capable of achieving the high temperatures required for splicing large-diameter fibers. Proper filament selection is essential for optimum splice performance. Graphite filaments are capable of heating up to the high temperatures required to splice standard and large-diameter fibers, making them ideal for most applications. By contrast, iridium filaments are preferred for splicing mid-IR fluoride fiber and other softer glasses because of their lower operating temperature. We supply filaments mounted in complete filament assemblies, which are held to the splice head of the LFS4100 by two screws. One FTAV4 Graphite Filament Assembly comes pre-installed with the LFS4100; additional graphite or iridium filaments can be purchased below.

Complementary Fiber Processing Systems
The Vytran LFS4100 Splicer, FPS300 Fiber Preparation Station, GPX3400, GPX3600, GPX3800, GPX3850, GPX4000LZ Glass Processing Systems, LDC Series of Fiber Cleavers, and PTR Series of Fiber Recoaters, Proof Testers, and Recoaters with Proof Testers make up a suite of complementary products for both early process development and volume manufacturing. The FPS300 Fiber Preparation Station, LDC Series of Fiber Cleavers, and LFS4100 Splicer have interchangeable Fiber Holder Transfer Bottom Inserts (sold below) to ensure precise positioning within each tool. This makes splicing with the LFS4100 easier and faster because it facilitates repeatable fiber positioning when the splicer is used in conjunction with the other Vytran products. The operator can transfer fibers from one tool to another in the same transfer insert and the ends of the fibers will fall in the same place every time. More information about transfer inserts can be found below.

Fusion Splicing is the process of joining two optical fibers end-to-end using heat. The goal is to fuse the two fibers together in such a way that light passing through the fibers is not scattered or reflected by the splice while ensuring that the splice and the region surrounding it are almost as strong as the original fiber. The LFS4100 uses a resistive graphite or iridium filament shaped like an inverted omega to provide the heat necessary to fuse fibers together.

Fiber Alignment
Before fusion, the two fibers to be spliced must be aligned by the edge, core, or end-view alignment methods using the images captured by the camera tower or by the active alignment method. The edge and core alignment methods use side view images to determine the positions of the fibers and align them in the X-Y plane. In edge alignment mode, the system identifies the fiber edges and automatically repositions the fibers using the XY stepper motors. Successive images are analyzed, and fiber repositioning continues until adequate alignment is achieved. The core alignment method is similar to the edge alignment method, except the system attempts to align the fiber cores instead of the fiber edges. This process can only be used when there is a clearly visible image of the cores that is not distorted by the fiber claddings.

The end view alignment method is used for polarization-maintaining fibers such as 3M elliptical-core fiber (PM or PZ), panda fiber, bow-tie style fiber, and for hybrid splices between any of these. For this method, images of the fiber ends are analyzed and used to automatically control both XY and rotational position. Alternatively, the active alignment method can be used for fiber that has a high core eccentricity. In this case, alignment is achieved by maximizing the power transmission between the two fibers. For more information about these advanced alignment methods, please see the Software tab.

Filament Fusion
Once fiber alignment has been achieved, the splice head is repositioned so that the filament is centered under the fiber ends. Power is then applied to the filament to raise its temperature to a level hot enough to soften the glass and fuse the fibers together, typically about 2000 °C. The filament would oxidize if it were brought to such a high temperature in air, so high-purity argon gas is used to purge the splicing chamber of oxygen during the filament fusion process. In order to keep the fibers clean and improve splice strength, the purging gas is set to flow over the fibers at a high rate during the fusion process. The splicing system includes a high-purity PTFE gas line and a gas regulator with an attached connector that fits a gas tank (not included) with a CGA-580 output port. An additional DIN 477 Number 6 connector is also included for the regulator. Zero-grade Argon gas with purity of >99.999% is recommended, and research-grade argon is preferred.

The fiber ends are allowed to heat up for a defined time before they are fused in order to remove any surface defects in the fiber ends and increase the plasticity of the glass. The user can control the power to the filament and the length of time the fibers heat up before fusion. The system's computer-controlled stepper motors push the fibers together over the pre-push distance before heating and then further move the fibers through the hot push distance to produce a splice with low loss and high strength. The total splice time, which can be adjusted by the user, is typically set between 2 and 15 seconds; common splice settings for several fiber cladding diameters are given in the table above. For special applications, the system can automatically run several splice steps in a sequence.

Fire Polishing
The fire polishing process significantly increases splice strength through a rapid post-fusion heat treatment of the splice region. When a fusion splice is made, silica evaporates off of the hot center region of the splice and condenses on either side of the joint where the fiber is cooler. The condensed silica deposits act as surface flaws, lowering the splice strength. Our fire polishing process removes or minimizes these deposits, thereby improving splice strength. In addition, the fire polishing process provides core diffusion capabilities that can be used to adiabatically expand the mode field diameter of a fiber. Through this thermally expanded core (TEC) process, low-loss fusion splices can be achieved between markedly dissimilar fibers, such as those typically used in fiber laser applications.

Fiber Holder Inserts Selection Guide

Fiber Holder Inserts, which are designed to hold various sizes of fibers within the LFS4100 splicer, must be purchased separately. The bottom inserts have V-grooves to hold the fiber, while the top inserts each feature a recessed, flat surface that clamps the fiber against the V-groove in the bottom insert. Each top and bottom insert is sold individually, as the fiber diameter clamped by the left and right holding blocks may not be the same. Two top inserts and two bottom inserts are required to operate the splicer.

The table below indicates the minimum and maximum diameters that can be accommodated by different combinations of top and bottom inserts. It also indicates how far offset the fiber will be for recommended combinations of top and bottom inserts. Note that this outer diameter may be the fiber cladding, jacket, or buffer. If one side of the fiber is being discarded, it is preferable to clamp onto the cladding of this section except in special cases (such as non-circular fiber) where the coating or buffer may be preferable. Sections of fiber that are not being discarded should always be clamped on the coating or buffer in order to avoid damaging the glass. This may require different sets of fiber holder inserts to be used in the left and right holding blocks. In this case, it is important to minimize the difference in the offsets introduced by the left and right sets of inserts when attempting to produce high quality splices.

Each fiber holder insert can accommodate a range of fiber sizes.

Fiber Insert Selection Chart

1. First, select the bottom insert that matches your fiber size most closely.
   Example: For an Ø800 µm fiber, the VHF750 insert is the closest match, since it is only 50 µm smaller.
2. On the chart below, look to the right of your chosen bottom insert. Select a compatible top insert based on the fiber diameter size range shown in each cell.
   Example: For the Ø800 µm example fiber from step 1, the green cell is in the 750 µm groove column for the VHA05 and VHB05 top inserts, which have two grooves. The numbers listed in the green cell indicate that this combination of inserts is good for fibers from 728 to 963 µm in diameter. Our Ø800 µm fiber is within this range, so this is a good choice. There are several other options that will accommodate an Ø800 µm fiber as well, but the green shading in the chart indicates that the 750 µm groove in either the VHA05 of VHB05 provides the best fit. Of these two top insert options, the VHB05 is compatible with end-view imaging but the VHA05 is not.
3. The second line of numbers in each cell shows the range of offsets that can be expected for any given combination of top and bottom inserts. When selecting inserts for the right and left fiber holding blocks, try to minimize the offsets between the pairs of inserts on each side.
   Example: If we choose a VHF750 bottom insert and the Ø750 µm groove in the VHA05 top insert, we can use fiber as small as 728 µm, in which case the center of the fiber would sit 23 µm below the surface of the bottom insert. We could also clamp a fiber as large as 963 µm, in which case the center of the fiber would sit 213 µm above the surface of the bottom insert. We could interpolate to find the offset expected for our hypothetical 800 µm fiber, but it turns out that in a 60° V-groove, the offset is equal to the diameter difference. So, that means that the center of our fiber is going to sit 50 µm above the bottom insert surface because it is 50 µm larger than the fiber that the bottom insert was designed for (800 - 750 = 50).
4. Holding blocks designed for fibers less than 1000 µm in diameter have vacuum holes, designed to aid in aligning small fiber within the groove, while bottom inserts for fibers of Ø1000 µm or larger do not have these holes. The LFS4100 has a vacuum pump that provides a small holding force via these holes, keeping small fibers in place as the clamps are lowered. Inserts with vacuum holes are indicated by a superscript "c" in the table below.

• These inserts are dual sided to accomodate two different ranges of fiber diameters.
• The VHB00 and VHB05 top inserts are equipped with an indent for LED illumination of the fiber end faces.
• These bottom inserts have vacuum holes to aid in aligning small fibers when used with the LFS4100 Fiber Splicer.

Each LFS4100 Filament Fusion Splicer is shipped with a desktop computer that comes with the GUI software for operating the system. This software is used to control stepper motor motion, camera image acquisition, argon flow rate, and fiber alignment mode. In addition, it is used to control splicing parameters such as splice power, splice time, pre-push distance, hot push distance, and hot push delay (see the Fusion Splicing tab for more information). An abbreviated library of splice process files for common splicing procedures is included with the software. The GUI and splice library software also enable users to create their own splice files for new processes and customize existing files as necessary. Please contact Tech Support for inquiries regarding the availability of additional splice files for your specific application.

The video to the right highlights some of the main features of the software and the sections below describe some of the fiber splicing and tapering parameters that can be programmed through the software GUI.

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Screenshot of PM Fiber Alignment Configuration Window

End-View Alignment
End-view alignment is used for splicing polarization-maintaining fibers such as elliptical-core fiber (PM or PZ), PANDA or bow-tie polarization-maintaining fiber, or a hybrid splice between any of these. These types of fiber require a rotational alignment in addition to the XY alignment to align the stress regions within the cladding region.

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Multi-Stage Splicing Configuration

The end-view alignment process is initiated by pulling the fibers back so that an end-view mirror can be inserted between two fiber end faces. An LED illuminates the fiber cladding, allowing the software to image the fiber end. Then, the image of the fiber end face is displayed and used to automatically align the cores of the two fibers. PM alignment parameters can be set for each fiber type as shown in Figure 1. This window consists of four parameters: diameter (fiber cladding), fiber type, and two PM geometry parameters for both the left and right fiber. If these parameters are not known, it is possible to directly measure them using the displayed image of the fiber end face.

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Active X-Y Alignment Scan Properties

Multi-Stage Splicing
For special applications, the software can run several splice steps in a sequence. Users can independently set the splice parameters for each step, as shown in the screenshot to the left. Alternatively, multiple independent splice files from the user's library can be executed in order. The system will then perform a complex splicing function according to the sequence of the selected splice files.

Active X-Y Alignment
The active alignment method is typically used for fiber that has a high core eccentricity. In this case, standard imaging methods cannot always ensure proper alignment of the fiber cores. Instead, the cores are aligned using the output from an optical power meter as feedback to maximize the power transmission between the two fibers. This is done by scanning one fiber across the other with a given scan step size and taking a power meter reading at each position. At the end of the scan, the fiber is moved back to the position at which the optical power was either maximized or minimized. Parameters for this scan, such as step size and fiber offset position, can be set within the software to ensure accurate alignment, as shown in the image to the right.