Compatibility of Bend Optimized Multimode Fibers White Paper. Issued September 2009

Transcription

1 Compatibility of Bend Optimized Multimode Fibers White Paper Issued September 2009

2 Optimized Low Bend for the Use of Fibers In The Home The current migration of optical fibers closer to and even into the home has increased service providers interests in new fibers with improved bending performance. After the successful introduction of low bend SM fibers, several leading fiber manufacturers have recently launched bend optimized versions of their premium high bandwidth (OM3, OM4) multimode fiber products, so as to reduce or even eliminate challenges encountered during fiber installations. To confirm their promises, we examined bend optimized OM3 fiber performances of different companies in real world conditions when compared to standard multimode fibers. The investigations proved that these bend optimized fibers are good candidates for loss and bend critical applications (such as indoor wiring) because of their higher immunity to bending losses, without loosing performances or compatibility to other standard high bandwidth multimode fibers. New fibers need improved bending performance for the use in the home Roger Krähenbühl, Ph.D. R+D Technology Fiber Optics Division HUBER+SUHNER AG 9100 Herisau, Switzerland Hanspeter Schiess Product Unit Manager Fiber Optics Division HUBER+SUHNER AG 9100 Herisau, Switzerland Claudio Cecchin Product Unit Manager Fiber Optics Division HUBER+SUHNER AG 9100 Herisau, Switzerland 2

3 New Challenges Created from new Applications Introduction Optical fibers have already been used for a long time in environments where small bends were not a major requirement. Recently, indoor and data center applications with tighter restrictions have led to investigations in the bending performance of optical fibers. First, the optical network market has seen a flurry of introductions of various grades of low bend radii singlemode fibers [1], with the consequence of recent updates to the low bend fiber standard (ITU G.657). Despite the absence of a bend insensitive multimode fiber standard, recently, extended band width (ISO/IEC OM3, OM4) multimode fiber (MMF) with enhanced macro bending performance were introduced [2][3], designed to reduce and, in some cases, eliminate the cable bending challenges encountered in installations in local area networks (LAN), data centers and other enterprise applications. Tighter restructions for the bending performance due to the use of optical fibers in indoor applications Compared with singlemode optical fibers, multimode fibers have a larger core that guide multiple modes (or rays) of light simultaneously. Modes traveling at the outside edge of the core have a longer distance to go than modes that travel near the center of the core (modal dispersion). To minimize this modal dispersion and thereby maximize bandwidth, special care is taken in the profile design of the refractive index [3]. Proper design (parabolic shape) will slow down modes with a shorter distance so that all modes arrive at the end of the fiber as close in time as possible. Such improvements resulted in products that fulfill bandwidth requirements as given in the OM3 and the just recently published OM4 standards. Important profile design When you bend a traditional optical fiber, the tendency is for the light to keep going straight and escape from the fiber. As the bend radius decreases, the amount of light that leaks out of the core increases. This escaped light causes signal degradation and increases the potential for transmission errors. To enhance bend performance in multimode fibers a unique approach is required (as compared to singlemode fiber) to influence the many modes of light traveling down the core without adversely affect fiber bandwidth (modes arriving simultaneously). After attempts to address bending in multimode fibers reaching only low bandwidth performance, new bend optimized multimode fibers now are able to deliver superior bandwidth performance in an ultra-bendable package without requiring any adjustments to standard field installation, termination monitoring, or maintenance procedures. The bend improvement in these new multimode fibers is achieved through the use of a specially engineered optical barrier (trench) that traps the many modes within the fiber core. This optical trench ensures that the outer modes, which traditionally have a higher tendency to "leak" out of a multimode fiber when it is subjected to bends, stay put [4]. By keeping these modes within the core of the fiber, much less of the information carrying signal is lost and more information gets to the end user without dropped packets or corrupted data. Preventing the escape of the light 3

4 New Challenges Created from new Applications Such bend optimized fibers claim to obtain extremely low bending loss at both 850nm and 1300nm, while keeping the advantages of existing OM3/OM4 fibers. These fibers should be bendable down to a radius of 7.5mm with less than 0.2dB added loss at 850nm. At a 15mm radius the added loss is less than 0.1dB. This is up to a 10x improvement in bend loss compared with traditional multimode bending standards (Table 1). To verify these claims, prove their usability and interoperability to standard MMF, we examined the most widely used products. We took the bend optimized fiber type MaxCap -BB-OM3/OM4 of Draka, ClearCurve OM3 fiber of Corning and LaserWave FLEX of OFS, to execute different tests and simulate daily use while comparing them to standard OM2 and OM3 fibers. Extremely low bending loss IEC ITU G Bend optimized Bending radius 37.5 mm 15 mm 15 mm 7.5 mm Number of turns Max 850 nm 0.5 db 1.0 db 0.1 db 0.2 db Max 1300 nm 0.5 db 1.0 db 0.3 db 0.5 db Table 1 Multimode bend standards and specifications given by the fiber manufacturers for the new bend optimized fibers. For the following tests these types of fibers and standard OM2/OM3 of the same manufacturers were used to fabricate properly but neutrally marked 0.9mm semi-tight tubes (Figure 1, left) and 2.0mm indoor type simplex cables (Figure 1, right). Figure 1 Bend optimized fibers (BO1-BO3) and standard OM2/OM3 fibers of different manufacturers in 0.9 mm semi-tight tubes (left) and in 2.0 mm indoor type simplex cables (right). 4

5 Bending Performance Bending performance To determine the bending performance of all 0.9 mm type tube samples, standardized bending loss tests (IEC E11A) were carried out. Thereby the induced fiber attenuations for different number of turns, on mandrels with different radii at selected wavelength were monitored. The non-linear behavior of the results obtained for consecutive number of turns around a mandrel with r = 5 mm at λ = 850 nm (Figure 2, left) is an indication of the mode filtering (outer modes are less guided) effect of MMF bending. Together with the bending radius dependency (Figure 2, middle), it is apparent that bend optimized fibers (reddish lines) present a well improved bending loss behavior compared to standard OM2 and OM3 fibers (bluish lines) and clearly fulfill the performance specification (0.1 db for two turns around r = 7.5 mm at λ = 850 nm) as given in Table 1. The wavelength dependences (Figure 2, right), show a variation between the bend optimized fibers at longer wavelengths (e.g. BO3 shows worst transmission at 850 nm and best at 1625 nm), this could be an indication of slightly different bend optimized refractive index profiles. All of these bending measurements clearly indicate that the bend optimized fibers of all chosen manufacturers could make fiber installations less critical, and more flexible as they allow bending around tighter corners. Bending loss test Figure 2 Change of optical transmission for the different variants of fibers in 0.9 mm tubes depending on number of turns (right), mandrel radius (middle) and on wavelength (right). 5

6 Mechanical Characteristics Mechanical characteristics In real world installations fibers are often squeezed and crushed during and after installation through staples and other supporting features and they might have to endure temperature changes. To check and compare the mechanical and environmental reliability of the different fiber samples, crush tests (according IEC E3) and temperature cycling tests (according to IEC F1) using the 0.9mm tubes and the 2.0 mm cables were executed. Selected results are given in Figure 3. Crush tests Temperature cycling test In contrast to the case for SM fibers [1], bend optimized MM-fibers demonstrated no loss improvement during crush tests. Crushing the optical cable will not introduce macro bending into the fiber but micro bending. It is believed that micro bending has a random mode mixing effect (in contrast to mode filtering), resulting in the chaotic loss curves. The temperature change test showed a more similar behavior as in the macro bending tests. For this case, it is assumed that micro bending and macro bending are produced within the cable due to mantel material shrinkages during the low temperature cycles. As a result, the use of bend optimized fibers for manufacturing of optical cables improves only to a certain degree the optical performance under variable environmental conditions. Change of Attenuation [db] Crush Test: N/cm; 3x3x1 min OM2 OM3 A OM3 B λ=850nm BO1 BO2 BO Time [min] 50 Compressive Stress [N/cm] Change of Attenuation [db] Temperature Cycling Cables: Average OM2 BO1 OM3 A OM3 B BO2 BO3 λ=850nm Time [h] Temperature [C] Figure 3 Change of optical transmission for the three types of fibers in 0.9 mm tubes during short-term crush tests (left) and in 2.7 mm cables during temperature cycling (right). 6

7 Splicing Investigations Splicing investigations All indoor optical communication systems have to be connected to the legacy world, which typically operates using standard MMF. Sometimes this connection is performed by fusion splicing. However, bend optimized MMF may not splice seamlessly to other standard MMF due to dissimilar index profiles and number of modes within the fiber. To confirm the spliceability of the investigated fibers a systematic splicing field test (Figure 4, upper left, power meter loss measurements using encircled flux launch conditions according to IEC/PAS 62614) was carried out between all bend optimized to the standard OM3 fiber at 850nm wavelength. The measured loss distribution of the acquired splice loss data (80 connections per set) are summarized in Figure 4 (right). Splicing field test Although to some degree a splice boundary is clearly visible (Figure 4, pictures in the lower left), the spliceability of the three bend optimized fibers to standard OM3 fibers was very well confirmed, as only one or less splice among the 80 splices per combination failed. Furthermore, the measured splice loss was well within the allowed values and the small mean loss difference (0.02 db) between the different configurations can be explained by the limits (+/-0.01 db) given by the measurement setup. Figure 4 Test setup (upper left), photograph of the splices(lower left) and distribution of the measured loss distribution at λ=850 nm for the different splice configurations (right). 7

8 Optical Connectivity Optical connectivity The outer world can, besides fusion splicing, also be reached by optical connectors. Due to the different manufacturing process of the bend optimized fiber types, the question has arisen, if during the assembly process the endface will behave in a similar manner as for the standard MMF and if it will match the geometrical and quality criteria s. The manufactured 0.9mm tubes containing the three types of bend optimized OM3 fibers were assembled with LC-PC and SC-APC connectors using standard assembly processes (blind test, not knowing what type of fiber they were using). Fortunately, during the assembly process no special care was taken nor had to be. Closer examination under a high performance microscope (using 400x magnification) using white light for backside core illumination showed the slight difference of the modal distribution (see Figure 5). The variations of the visible faint light rings ("halo") around the 50m core indicate the slight different index profiles (trench location) for the bend optimized fiber among the three different manufacturers. Blind test Figure 5 Microscope picture of the fiber endfaces in the assembled ferrules (400x magnification). The measured endface geometries (radius, apex offset, fiber protrusion) of the fiber core were not distinguishable between the different samples and they stayed well within the tolerance range given by IEC /2. To prove the compatibility of the connectorized fiber assemblies to standard MMF their loss distributions were investigated at λ = 850 nm in a similar way as the splicing (encircled flux launch conditions). Mating loss results (LC-connector) for all combinations are given as box plots in Figure 6. Here the individual distributions resemble each other nicely and it can be stated that the connector performances are well compatible to standard OM3 fibers. Figure 6 Box plot (left) of the loss distribution for the different configurations indicating mean (horizontal line), 75 % (box) and 98 % (vertical line) range of measurement data. 8

9 Bandwidth Investigations Bandwidth investigations As mentioned it is very challenging to control bend loss and other waveguide properties simultaneously. Especially challenging is to keep the bandwidth performance, as even minor change from the optimum shape of the ideal graded index profile leads to degradations. To confirm the bandwidth behavior, the different fibers are characterized using differential mode delay (DMD). The DMD technique [5] incorporates stepping the output of a singlemode, fiber pigtailed 850 nm pulsed laser across the core of the fiber under test (FUT) in small (1 2 µm) increments. Exit pulses from the fiber at each radial launch position are then collected by a detector and sampling oscilloscope (blind test results for 300 m of OM3 and the three bend optimized fibers are shown in Figure 7 on the left). From these curves, using well defined weighting sets and transformation algorithm, the effective modal bandwidth (EMB) and overfilled modal bandwidth (OMBc) can be calculated. The calculated results for all measured fibers are given in the table in Figure 7. Calculation of the effective modal bandwith The measurements confirmed the specifications given by the fiber manufacturer. The difference (ripple) for high offset launch might be due to deviations in the index profile at the outer core range. To confirm the usability of these fibers in real system applications, 10Gb/s transmission bit error rate (BER) measurements for different configurations were made. A variable optical attenuator is inserted in the fiber loop of the optical data link to induce transmission degradation. Bit error rate are then measured for 300 m fibers on standard transport spools (FUT) at different attenuation levels. Figure 7 DMD measurement curves (left) and resulting effective modal bandwidth (EMB) and overfilled modal bandwidth (OMBc). 9

10 Bandwidth Investigations As BER test depend heavily upon applied components (transceivers) in the setup (Figure 8, upper left), two different transmitter/receiver (Tx/Rx) sets were used. Results (Figure 8, left) for these "worst" (lines) and "best" (dots) case show an 3 db power penalty for the different pairs. Whereas all bend optimized and standard OM3 fibers can transmit almost failure free 10Gb/s even at low optical power levels, the OM2 fiber fails (as expected since not rated for 10Gb/s transmission). A further parameter that normally influence the optical bandwidth is the launching condition. To simulate general use a simple but effective means, i.e. a cascade of 5 patchcords (PCs) using standard (low cost) MMF connectors was introduced in front of the FUT. The major effect of this cascade is increasing the mode power in higher order mode groups propagating more in the outer core region. The influence on the result is shown in Figure 8, (right) indicating that apart from fiber BO2, all the BER tested on the fibers are sensitive to change in launch condition (dot to dash-dots). BO1 shows the worst penalty increase of about 0.3 db, which is in line with the observed larger outer DMD of this fiber. The launching condition To simulate the fiber layout in a datacenter environment with many in- and outlets in fiber cabinets, an extended lay-out was made consisting of a cascade of 10 r = 5 mm full turns in the launching end of the FUT. Between each of the loops a fiber distance of about 1 m was present. The effect is usually positive as the added loss due to the heavy bending attenuates preferentially the higher order modes. This clearly shows (Figure 8, right, dash-dots to line) for fiber BO1 where the power penalty improves by about 0.3 db due to the inserted bends. Note that this is about equal to the added power penalty originating from the insertion of the launching configuration. This behavior of Fiber BO1 in contrast to BO2 (little influence due to inserted bends) can again be understood from its larger outer DMD performance. Simulation of the fiber layout in a datacenter environment Figure 8 BER measurement setups (upper row) and corresponding measurement curve. 10

11 Reliability Considerations and Conclusion Reliability considerations So far we ve only seen the benefits of using bend optimized MMF for optical cable installations. However, signal transmission reliability of an installed optical link must be viewed in terms of both optical loss and mechanical stability. With traditional rules for fiber management at large bend radii, fiber breakage has not been an issue. However, as fiber bend radii drop, fiber lifetime becomes an important consideration. Just take into account, that the stress on the fiber at 5mm bend radius is 2 times higher than at 10 mm bend radius and more than 6 times higher than on a fiber following the traditional 30 mm bend radius. Similar as for low bend SM fibers [1], the stress behavior can be calculated according to IEC leading to the expected failure probability during the lifetime of a bend fiber. If we consider a rack in a datacenter having 1000m of fiber bend at r = 30 mm, one out of 10 million racks will fail during a 20 year lifetime due to fiber break. If the fibers in the same racks are bent at r = 10 or even r = 5 mm we would see two fails among 100 or one among 10 racks, respectively. Viewing both optical loss and mechanical stability Conclusion Handling and installation testing has confirmed that no differences exist between standard 50-µm fiber and the bend optimized 50-µm OM3 fiber (of three different manufacturers) in terms of termination and splicing methods. Therefore, new products enabled with the bend optimized fibers are able to provide the standard performance that network designers expect and the added benefit of enhanced bend performance without sacrifices. The new bend optimized fibers have been extensively tested and compared in respect to the optical properties of standard multimode fibers. They have clearly proven better performance under bending, to some extent better mechanical and environmental behavior, as well as their compatibility to standard MMF. Normal splicing and manufacturing processes can be used to assemble connectors with these fibers while reaching similar optical performances. Taking reliability considerations into account, it can be concluded that bend optimized multimode fibers are recommended for bend and loss critical applications as they are a real advance to current standard multimode fibers, but they should be handled with appropriate care as all optical glass fibers. Bend optimized fibers guarantee the expected performance Note: For all of the above described investigations we used for standard: OM3 of Draka, for the bend optimized OM3 fibers: MaxCap -BB-OM3 of Draka, ClearCurve of Corning, and LaserWaveFlex of OFS ; for 0.9 mm tubes: 01-G50/CW-E9-F of HUBER+SUHNER, and for 2.0 mm cables: 01-G50/CWJH-E20-F of HUBER+SUHNER. Acknowledgement: The authors would like to thank the three fiber manufacturer for providing their bend optimized fibers, Draka Communications(G. Kuyt) for taking the DMD measurements, and HHI Berlin for making the BER test References: [1] R. Krähenbühl, et al., "Usability of low bend fibers for optical connectivity", HUBER+SUHNER white paper, [2] L. Molin, et al., "Low bending sensitivity of regular OM3/OM4 fibers in 10GbE applications", OFC 2010, JThA55. [3] Y. Sun, et al., "Specialty high bandwidth multimode fiber for optical interconnection", SPIE, vol. 7631, ACP 2009, TuZ3. [4] D. Donlagic, "A low bending loss multimode fiber transmission system", Optics Express, vol.17(24), p , [5] "Differential mode delay measurement of multimode fiber in the time domain",tia/eia , Dec

12 HUBER+SUHNER is certified according to ISO 9001, ISO 14001, ISO/TS and IRIS. WAIVER It is exclusively in written agreements that we provide our customers with warrants and representations as to the technical specifications and/or the fitness for any particular purpose. The facts and figures contained herein are carefully compiled to the best of our knowledge, but they are intended for general informational purposes only. HUBER+SUHNER AG Fiber Optics Division Degersheimerstrasse Herisau/Switzerland Tel Fax Bend optimized-fibers/4291/4743/ hubersuhner.com