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We have investigated the polarisation-maintenance capability of 50µm graded-index multimode fibres (MMF) in the C-band. Using both circularly, and linearly polarisation-multiplexed signals centre-launched into the MMF, we found that orthogonality is sufficiently maintained in both cases to yield typically 10dB optical isolation between the channels. Using orthogonally circular polarisation-multiplexed signals, we show experimentally that two 3 and 5Gb/s data channels may be transmitted error free over 50µm multimode fibre spans up to 3km in length.
©2003 Optical Society of America
1. Introduction
Multimode fibres (MMF) are widely used for computer data links. Typically, installed spans are in the 300–500m range, and operate at 1.25Gb/s. Originally, optical sources were at 850nm, but the use of 1300nm is now a matter of standardisation, especially with the 10 Gigabit Ethernet standard now ratified [1]. Until recently, MMF modal bandwidth limitations were seen as an absolute limit to system performance [2, 3]. However, we have lately shown that subcarrier multiplexing (SCM) techniques [4], in conjunction with WDM can provide significant bandwidth enhancement, when used in a C-band HDWDM context [5, 6].
In this paper, we now show how polarisation-multiplexing may be used to further enhance bandwidth capacity. In particular, we have investigated the propagation of orthogonal circularly- or linearly-polarised light down two different spans of graded index MMF, using the single-mode into multimode centre-launch technique [7]. We found that the orthogonality of the two input polarisations is maintained to a sufficient extent to allow polarisation multiplexing. Accordingly, we built a source delivering two orthogonal polarisations on the same wavelength and were able to transmit simultaneously two signals at 3 and 5Gb/s respectively, over MMF lengths of up to 3km. It is important to note that bandwidths as high as 5Gb/s are not expected as a standard in all types of MMF. It was made possible in this particular fibre due to its good quality and to the single-mode into multimode launching condition, offset launch not being required in this case.
2. Preservation of polarisation orthogonality in MMF
Polarisation multiplexed signals are demultiplexed using a combination of polarisation controller and polarisation beam splitter (PBS). Referring to Fig. 1(a), the isolation between the demultiplexed channels depends on the degree of orthogonality, defined as the relative orientation angle Δδ=δ2-δ1 between the two polarisations (Δδ=90° for perfect orthogonality), where the polarisation orientation angles δ1, δ2 are defined with respect to the PBS. The cross-talk for each channel, expressed in terms of Δδ and ψ, is defined as
where ψ=δ1 +½·(δ2-δ1) is the mean orientation of the two states of polarisation (SOP’s) with respect to the PBS, P 1 and P 2 are the respective powers for each polarisation signal. From a practical point of view, an optical cross-talk ratio of typically 10dB (corresponding to Δδ=78.5°) between the two channels is desirable in order to achieve robust error free transmission, though slightly smaller values are still acceptable.
In the case of two perfectly orthogonal channels (Δδ=90°) and assuming equal power in each channel, P 1=P 2, cross-talk is due to misalignment between the polarisation multiplexed signals and the PBS, and therefore to the angle ψ. For angles Δδ larger than 78.5°, demultiplexing is simply achieved by rotating the polarisations such that ψ=45°. Simultaneous recovery of the two polarisation multiplexed signals is still possible when Δδ<78.5°, with the cross-talk ratio going down to 0dB as Δδ gradually decreases to 0°, degrading the eye and finally preventing transmission. In principle, replacing the simple PBS design by a combination of 3dB coupler and two pairs of polarisation controller and analyser would allow the demultiplexing of both channels without cross-talk, independently of their degree of orthogonality. However, this is obtained at the expense of system complexity and detected optical power, the received power decreasing to 0 as the Δδ angle approaches 0°.
Fig. 1. (a) Schematic of the orientations of the two polarisation multiplexed signals with respect to the PBS (the PBS is assumed aligned to the x-axis; (b) Polarisation ellipse at the output of 300m and (c) 3km of 50µm MMF for 2 orthogonal circular input SOP’s, as measured with a Stokes analyser.
To analyse the degree of orthogonality, we used a combination of polarisation controller and Stokes analyser and measured the SOP at the end of 300m and 3km MMF for circular left and right, as well as for linear vertical and horizontal input SOP’s. As expected, the intrinsic birefringence of the MMF, likewise any contributions due to bending and temperature effects, randomly changed the output SOP.
The orthogonality between the two SOP’s was higher than 84° (i.e., higher than the threshold value of 78.5°) for 300m of MMF, as shown in Fig. 1(b), thus allowing demultiplexing of the channels. In the 3km case, the degradation of orthogonality (Fig. 1(c)). resulted in a theoretical channel isolation marginally above 10dB (Fig. 2), thus slightly reducing the quality of the transmission with respect to the 300m case, as will be shown later (see Fig. 6 for the corresponding eye diagrams).
Although orthogonality was better preserved for linear input SOP’s than for circular input SOP’s, circular polarisations were found to be more stable over time and were therefore used for the transmission experiments. The poorer performance of the linear polarisations over time is not unexpected [8, 9], because linear polarised light centre-launched into MMF will excite polarisation modes parallel and orthogonal to the original polarisation sense. However, with circular polarisation, power is present in two orthogonal spatial directions by definition, and any depolarisation effects are less evident at the far end of the fibre.
Figure 3 indicates that, due to the high localisation of the SOP distributions, a very high degree of polarisation (DOP) was still present at the MMF output. This is attributed to the on-axis launch condition, which is believed to predominantly excite the fundamental mode [7]. Therefore, a polarisation controller can be used at the output of the fibre to realign the SOP’s to the axis of the PBS, allowing complete demultiplexing.
Fig. 3. Poincar sphere representation of antipodal input and quasi-antipodal output polarisations before and after 3km of 50µm MMF.
3. Transmission of polarisation-multiplexed data
3.1 Experimental set-up
Figure 4 shows a schematic diagram of the experiment. The transmission segment of the system (upper left section of Fig. 4) was entirely single-mode, and consisted of a tunable C-band laser, the output of which was split and passed to two Mach-Zehnder intensity modulators (MZM’s) after alignment of the input polarisation states. Two separate channels at 3Gb/s and 5Gb/s, 27 - 1, p.r.b. data were added, the frequency difference being used for signal discrimination purposes. The data bandwidth was only restricted by the available equipment, and could be greater than 10Gb/s, although the ultimate limitation is the bandwidth of the MMF itself. Subsequently, the MZM’s outputs were aligned to antipodal linear-polarisation states, and passed to a PBS acting as a polarisation-preserving combiner. Interference between the optical channels at the PBS was prevented by use of a dispersion-shifted fibre (DSF) delay line. After amplification, one polarisation was set to a circular-polarisation state. Due to the polarisation orthogonality introduced by the PBS, this automatically aligned the other channel polarisation to the corresponding antipodal state. The composite signal was then centre-launched into 50m MMF spans ranging from 300m to 3km. At the MMF output, the two emergent quasi-orthogonal channels were transformed to horizontal and vertical linear polarisations with the help of a combination of bulk optics quarter-wave and half-wave plates and demultiplexed by the PBS. The two demultiplexed signals were each coupled back into MMF pigtailed photodiodes, as shown in Fig. 4 [10]. Bit error rate (BER) measurements could only be performed over short time periods, ranging typically from 30 to 60s, since no feed-back loop was available on the last polarisation controller preceding the PBS based demultiplexing unit.
3.2 Transmission of polarisation multiplexed data up to 3km of MMF
Using the specifically designed source described above, we transmitted circularly-polarised multiplexed signals down samples of 300m of MMF. Data rates as high as 3Gb/s and 5Gb/s were achieved in 50µm graded index MMF, with error free transmission. The corresponding eye diagrams (Fig. 5) were wide open and showed good time stability.
Fig. 5. (a) Eye diagrams for the 3Gb/s circularly left polarised and (b) 5Gb/s circularly right polarised channels after polarisation demultiplexing at the end of a 300m sample of MMF.
Error free transmission was also achieved through 3km of MMF, though with slightly degraded eyes due to the loss of orthogonality and the resulting reduction of channel isolation ratio (see Figs. 1(c) and 2). Due to the longer fibre length, the polarisation state was less stable as well, requiring fine tuning after approximately every 30s. This was attributed to multimode polarisation mode dispersion (PMD) effects; these being intrinsically more complex than the bimodal PMD found in SMF. Figure 6 depicts the typical eye diagrams at the end of 3km of MMF, following polarisation demultiplexing.
BER measurements were carried out for the 5Gb/s channel, with and without the polarisation multiplexed 3Gb/s channel. The corresponding curves are shown in Fig. 7. As expected, addition of the orthogonal polarisation channel results in negligible intrinsic power penalty for the back-to-back case. However, the reduction of orthogonality results in a 2.6dB power penalty over 300m, and 3.6dB power penalty over 3km, for a BER=10-9, each with respect to the appropriate single channel cases as also shown in Fig. 7. We note that the sensitivity of the receiver was limited by the use of a standard broadband detector, which was not designed for low noise detection.
Fig. 6. (a) Eye diagrams for the 3Gb/s circularly left polarised and (b) 5Gb/s circularly right polarised channels after polarisation demultiplexing at the end of a 3km sample of MMF.
Fig. 7. BER curves measured at 5Gb/s for a circular left polarised channel, with and without associated 3Gb/s orthogonally-polarised multiplexed channel.
4. Discussion
The loss of orthogonality between the channels obviously produces cross-talk, which reduces the channel selectivity and the reliability of the transmission, and is expected to accompany degradation of the baseband bandwidth. This could be overcome by using polarisation multiplexing in combination with either sub-carrier multiplexing, for high bandwidth has been shown to be available in many different fibres [6], or an offset launch technique, which extends the baseband bandwidth [11].
As is well known, bandwidth reduction is mainly due to the number of modes and the coupling between them. It affects the polarisation maintaining properties of the fibre, resulting in a reduction of the DOP [7]. Using a single-mode into multimode launching technique results in a reduced set of excited modes [11]. In this case, the spread of the DOP remains confined (as shown in Fig. 3) and thus polarisation multiplexing can be expected to at least double the capacity of the link.
5. Conclusion
We have investigated the polarisation preservation capabilities of MMF and found that the degree of polarisation as well as the antipodality between two orthogonal input SOP’s is maintained up to 3km in samples of 50µm graded index MMF. We demonstrated the feasibility of polarisation multiplexing in the baseband bandwidth of such fibres. Using two circularly (left and right) polarisation multiplexed channels modulated at 3Gb/s and 5Gb/s respectively, we were able to transmit error free data up to 3km. This offers a good opportunity to upgrade existing MMF links in legacy networks.
References and links
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