Since it was first proposed in 1975, the fiber optic gyroscope (FOG) has steadily improved in performance and manufacturability. Now a mainstream, high-volume manufactured product with performance approaching Ring Laser Gyros (RLG), FOGs offer substantial advantages over competitive technologies in terms of reliability, cost, and complexity. Optical fiber splicing and related processes are at the core of this achievement.
Production techniques to meet FOG performance requirements for strength, optical loss, and polarization extinction ratio (PER) include fiber preparation, splicing, and post processing. Looking ahead, miniaturization will become increasingly important in FOG design, and manufacturing automation will be critical for improved consistency, throughput, traceability and quality, and cost reduction.
FOGs are interferometric devices utilizing the well-known Sagnac effect. Figure 1 presents a simple example of such a device. Figure 1a shows an optical fiber loop of radius R with N loops. The device has an input/output (I/O) coupler that launches two counter-propagating light waves. When the waves are recombined at the detector photodiode, they will add coherently. If the loop is rotating, then a phase difference develops between them, which manifests as an intensity difference at the detector.
To understand this, imagine a clockwise rotation of the loop at a given angular speed ω. Between the time the light enters the loop and exits the loop, the I/O coupler will have moved a distance ΔL (Figure 1b). The clockwise wave will travel a distance L+ ΔL, and the counterclockwise wave will travel a distance L-ΔL. The path length difference between the two is 2ΔL, which corresponds to a phase difference between them of:
where R is the radius of the loops, N is the number of loops, λ is the wavelength of the light in the fiber, and c is the speed of light.
Assuming an exact 50/50 split of the two waves and no power loss in the fiber, the light intensity at the detector is given by:
where I0 is the input intensity.
Given the large number of splices required to produce a FOG—typically 10 to 20 per device—splicing plays a key role in optimizing performance. Splicing an optical fiber requires stripping the fiber buffer, then cleaning, cleaving, aligning, splicing, recoating, and proof (strength) testing the fiber. Using the best proven technology is critical for obtaining the required splice performance.
FOG Requirements and Splicing Techniques
Due to the nature of optical fiber, various transmission modes can be excited and there can be crosstalk between them. Since these modes will have different propagation speeds in the fiber, any crosstalk will degrade the interference pattern, decreasing sensitivity. To avoid this effect, single mode polarization-maintaining (PM) fiber is generally used. The primary parameter of interest for PM fiber is crosstalk or Polarization Extinction Ratio (PER). PER is one of the most critical splice performance metrics.
PM fiber has a “fast axis” and a “slow axis” (birefringence). Typical fiber specifications for polarization crosstalk between the two axes are on the order of 25dB/100m and multiple kilometers of fiber are typically used in a FOG. It is important that splices do not add significantly to the intrinsic fiber PER.
To maintain the PER through a splice, the fast and slow axes of the two end faces to be spliced together need to be aligned (Figure 2). End view imaging combined with Advanced Image Processing (AIP) is used to line up the stress rods in the PM fiber. If the stress rods are not symmetric about the core, the AIP will calculate the optimal rotation alignment for maximum PER and minimum loss. Using this technique, PER values >35dB are routinely achievable. These values are generally not significant compared to the intrinsic fiber PER.
For maximum performance, active feedback alignment is an option. Fiber pairs can be aligned and rotated relative to each other while observing PER on a properly calibrated power meter. When the PER is maximized, the fiber pair is fused. Since access to the ends of the fiber is required for the measurement it is not always possible to utilize this technique, so reliable passive alignment is critical.
Due to the relatively short fiber lengths used in FOGs (compared to telecommunications networks), power loss is usually not a critical parameter. However, backscattered light can cause crosstalk between the two counter-propagating waves, and reduce sensitivity. Back-reflections can either be intrinsic to the fiber (Rayleigh scattering) or extrinsic (reflections from interfaces). The effects from Rayleigh scattering can be managed by appropriate choice of the light source. Minimizing return loss from all surfaces (splices in particular) needs to be managed in production. A high-quality splice technique such as filament fusion splicing is critical to achieving excellent loss performance, and less than 0.02dBm can be routinely achieved in production.
For optimum splice strength, the choice of splice and fiber preparation technologies—stripping, cleaving, and cleaning—is critical. The dominant method in FOG production is a filament-based furnace for fusion splicing. This is an economical method based on resistive heating. It generates uniform heating in a highly consistent and repeatable manner, and provides high splice strength, typically from 100 kpsi to 200 kpsi depending on fiber diameter.
Prior to splicing, stripping the buffer without contacting the fiber cladding is essential, as any nicks or scratches on the cladding will be a failure point in the future. The preferred method is thermo mechanical stripping (TMS). The fiber is heated, a pair of blades is clamped around the fiber just outside the clad diameter, and the fiber is drawn through. This effectively removes all acrylate buffer, and is followed by ultrasonic cleaning.
The fiber ends are then cleaved using a tension-and-scribe technique, critical for producing the low cleave angle and high facet quality required to achieve a high strength splice. Poor cleaving can lead to both high loss and low strength.
After the fiber is spliced it must be recoated for protection. Using a quartz mold is preferred for repeatable high-quality recoating. The method is well known for the resulting quality and performance of the recoat, and for its manufacturing longevity. Once recoating is complete, fibers can be “proofed,” or stressed with a minimum load to assure that each splice exceeds a minimum strength—typically 100 kpsi for 80m fiber—using a high-precision pull test. In practice, due to the short lead length, it is difficult to proof test each fiber splice, so sampling methods must be used.
A single axis FOG has one coil, one I/O coupler, one source, and one detector. Other components are required as well, so the number of splices is quite large — again, typically 10 to 20 splices per device, depending on how many axes and the design details. Such large splice counts on small devices do not lend themselves to a distributed manufacturing line. A compact workstation containing all required hardware in easy reach is the preferred solution. The workstation configuration minimizes device handling and is the most practical and economical configuration for manual splicing.
Typically, the FOG is placed above or in front of the workstation (Figure 3). Each fiber pair to be spliced is cut to its proper length, then spliced. The operator can perform each step in the splice sequence with only minimum movement of the FOG itself.
Typical FOGs are currently produced using 80-mm-diameter PM fiber. There is demand for smaller diameter fibers to miniaturize FOG designs and some fibers as small as 40mm in diameter are now commercially available.
From a design point of view, 40mm fiber (Figure 4) offers the advantage of higher packing density (more fiber loops per unit volume), and smaller minimum bend radius (due to smaller fiber diameter). This can enable either higher sensitivity in the same package as a current design, or similar sensitivity in a smaller package. Both of these are desirable characteristics and provide increased design flexibility. The main tradeoffs are the fiber itself may have a higher loss, and the increased production difficulty of working with such ultrafine fibers. Individual 40mm-diameter fibers are difficult for a technician to work with, so a minimalist approach to fiber handling is even more essential. To meet this challenge, automation is required.
A fully automated splicing platform will both improve current production and support the transition to smaller diameter fiber. Any automation platform must maintain the advantages of the current workstation approach, while offering high performance, scalability, and excellent return on investment (ROI). In particular, any automation solution needs to have a high throughput, high performance, and minimum fiber movement.
Technically, the preferred approach is to maintain a fiber pair in a fixed position, minimizing fiber movement, and perform the full splice process in-situ. Vytran’s FAS-3000 is a fiber splicing workstation that can fully automate the process and the entire splice sequence (strip, clean, cleave, align, splice, recoat, and proof test) can take less than two minutes. Active feedback on the alignment is not necessary as the end face alignment on this system provides ample performance on PER and loss. Yields can also improve as operator variability is eliminated, and final quality can be improved by the 100%-in-situ proof testing. Test data is provided to a Computer Integrated Manufacturing System for statistical process control and traceability. Finally, fiber management is done before and after the full FOG splice process, which leads to a high utilization efficiency and high ROI.
Optical fiber splicing techniques, including fiber alignment, splicing, cleaving and recoating, are critical for FOG production and performance. Splicing technology and equipment architecture are key elements to efficient FOG manufacturing, and both will become increasingly important as the industry moves toward miniaturization and automation.