Dual-LP11 mode 4x4 MIMO-OFDM transmission ... - OSA Publishing

Dual-LP11 mode 4x4 MIMO-OFDM transmission over a two-mode fiber Abdullah Al Amin,* An Li, Simin Chen, Xi Chen, Guanjun Gao and William Shieh National ICT Australia and Centre for Energy- Efficient Telecommunications (CEET) Department of Electrical and Electronic Engineering, The University of Melbourne, Parkville, VIC 3010, Australia * [email protected]

Abstract: We report successful transmission of dual-LP11 mode (LP11a and LP11b), dual-polarization coherent optical orthogonal frequency division multiplexing (CO-OFDM) signals over two-mode fibers (TMF) using allfiber mode converters. Mode converters based on mechanically induced long-period grating with better than 20 dB extinction ratios are realized and used for interfacing single-mode fiber transmitters and receivers to the TMF. We demonstrate that by using 4x4 MIMO-OFDM processing, the random coupling of the two LP11 spatial modes can be successfully tracked and equalized with a one-tap frequency-domain equalizer. We achieve successful transmission of a 35.3-Gb/s CO-OFDM signal over 26-km twomode fiber with less than 3 dB penalty. ©2011 Optical Society of America OCIS codes: (060 2330) Fiber optics communications; (060-4080) Modulation.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

D. Qian, M. F. Huang, E. Ip, Y. K. Huang, Y. Shao, J. Hu, and T. Wang, “101.7-Tb/s (370 x 294-Gb/s) PDM128QAM-OFDM transmission over 3 x 55-km SSMF using pilot-based phase noise mitigation,” Optical Fiber Communication Conference (OFC), 2011, p. PDPB5. R. Essiambre, G. Kramer, P. J. Winzer, G. J. Foschini, and B. Goebel, “Capacity limits of optical fiber networks,” J. Lightwave Technol. 28(4), 662–701 (2010). W. Shieh and X. Chen, “Information spectral efficiency and launch power density limits due to fiber nonlinearity for coherent optical OFDM systems,” IEEE Photon. J. 3(2), 158–173 (2011). S. Chandrasekhar, X. Liu, B. Zhu, and D. W. Peckham, “Transmission of a 1.2-Tb/s 24-carrier no-guard-interval coherent OFDM superchannel over 7200-km of ultra-large-area fiber,” European Conference On Optical Communication, (ECOC), 2009, PD2.6. F. Yaman, N. Bai, Y. K. Huang, M. F. Huang, B. Zhu, T. Wang, and G. Li, “10 x 112Gb/s PDM-QPSK transmission over 5032 km in few-mode fibers,” Opt. Express 18(20), 21342–21349 (2010), http://www.opticsinfobase.org/abstract.cfm?URI=oe-18-20-21342. J. Sakaguchi, Y. Awaji, N. Wada, A. Kanno, T. Kawanishi, T. Hayashi, T. Taru, T. Kobayashi, and M. Watanabe, “109-Tb/s (7x97x172-Gb/s SDM/WDM/PDM) QPSK transmission through 16.8-km homogeneous multi-core fiber,” Optical Fiber Communication Conference (OFC), 2011, p. PDPB6. B. Zhu, T. Taunay, M. Fishteyn, X. Liu, S. Chandrasekhar, M. Yan, J. Fini, E. Monberg, and F. Dimarcello, “Space-, wavelength-, polarization-division multiplexed transmission of 56-Tb/s over a 76.8-km seven-core fiber,” Optical Fiber Communication Conference (OFC),2011, PDPB7. H. R. Stuart, “Dispersive multiplexing in multimode optical fiber,” Science 289(5477), 281–283 (2000). A. Tarighat, R. C. Hsu, A. Shah, A. H. Sayed, and B. Jalali, “Fundamentals and challenges of optical multipleinput multiple-output multimode fiber links,” IEEE Commun. Mag. 45(5), 57–63 (2007). N. Hanzawa, K. Saitoh, T. Sakamoto, T. Matsui, S. Tomita, and M. Koshiba, “Demonstration of mode-division multiplexing transmission over 10 km two-mode fiber with mode coupler,” Optical Fiber Communication Conference (OFC), 2011, p. OWA4. A. Li, A. Al Amin, X. Chen, and W. Shieh, “Transmission of 107-Gb/s mode and polarization multiplexed COOFDM signal over a two-mode fiber,” Opt. Express 19(9), 8808–8814 (2011). M. Salsi, C. Koebele, D. Sperti, P. Tran, P. Brindel, H. Mardoyan, S. Bigo, A. Boutin, F. Verluise, P. Sillard, and others, “Transmission at 2x100Gb/s, over Two Modes of 40km-long Prototype Few-Mode Fiber, using LCOS based Mode Multiplexer and Demultiplexer,” Optical Fiber Comm. Conference (OFC), 2011, p.PDPB9. R. Ryf, S. Randel, A. H. Gnauck, C. Bolle, R. J. Essiambre, P. Winzer, D. W. Peckham, A. McCurdy, and R. Lingle, “Space-division multiplexing over 10 km of three-mode fiber using coherent 6 x 6 MIMO processing,” Optical Fiber Communication Conference (OFC), 2011, p. PDPB10. I. B. Djordjevic, M. Arabaci, L. Xu, and T. Wang, “Spatial-domain-based multidimensional modulation for multi-Tb/s serial optical transmission,” Opt. Express 19(7), 6845–6857 (2011).

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Received 25 May 2011; revised 12 Jul 2011; accepted 18 Jul 2011; published 15 Aug 2011

15 August 2011 / Vol. 19, No. 17 / OPTICS EXPRESS 16672

15. S. H. Murshid, A. Chakravarty, and R. Biswas, “Simultaneous transmission of two channels operating at the same wavelength in standard multimode fibers,” Lasers and Electro-Optics, 2008 and 2008 Conference on Quantum Electronics and Laser Science. CLEO/QELS Conference, 2008, pp. 1–2. 16. B. C. Thomsen, “MIMO enabled 40 Gb/s transmission using mode division multiplexing in multimode fiber,” Optical Fiber Communication (OFC), 2010, p.OThM6. 17. B. Franz, D. Suikat, R. Dischler, F. Buchali, and H. Buelow, “High speed OFDM data transmission over 5 km GI-multimode fiber using spatial multiplexing with 2x4 MIMO processing,” European Conference on Optical Communication (ECOC), 2010, p.Tu3.C.4. 18. J. Carpenter and T. D. Wilkinson, “Holographic mode-group division multiplexing,” Optical Fiber Communication Conference (OFC), 2011, p.OThN3. 19. A. Al Amin, A. Li, X. Chen, and W. Shieh, “LP01/LP11 dual-mode and dual-polarisation CO-OFDM transmission on two-mode fibre,” Electron. Lett. 47(10), 606–607 (2011). 20. R. C. Youngquist, J. L. Brooks, and H. J. Shaw, “Two-mode fiber modal coupler,” Opt. Lett. 9(5), 177–179 (1984). 21. S. L. Jansen, I. Morita, N. Takeda, and H. Tanaka, “20-Gb/s OFDM transmission over 4,160-km SSMF enabled by RF-pilot tone phase noise compensation,” Optical Fiber Communication Conference (OFC), 2007,p.PDP 15. 22. K. Y. Song, I. K. Hwang, S. H. Yun, and B. Y. Kim, “High performance fused-type mode-selective coupler using elliptical core two-mode fiber at 1550 nm,” IEEE Photon. Technol. Lett. 14(4), 501–503 (2002).

1. Introduction Single-channel data transmission rate over 100 Gb/s has become a commercial reality, thanks to reemergence of coherent detection technologies in combination with high-speed electronic digital-to-analog and analog-to-digital converter (DAC/ADC) and digital signal processing (DSP). Together with wavelength-division multiplexing (WDM), polarization-division multiplexing (PDM) and high-order modulation schemes, the highest reported single optical fiber data transmission speed has reached over 100 Tb/s [1]. However, there is a need to continue enhancing the total data transmission capacity while keeping the signals within the available optical spectrum of the conventional Erbium doped fiber amplifier (EDFA), which translates into the requirement for increased spectral efficiency (SE, expressed in b/s/Hz). Although Shannon’s theory predicts SE to increase with higher received SNR as a result of increased transmission power, fiber nonlinearity poses a hard limit on improving channel capacity much beyond the state-of-the-art and the achievable SE rather decreases with increasing optical power after a certain transmission power level [2,3]. In order to explore avenues of further increasing data transmission capacity, the research community has therefore focused on either fibers with large core to improve nonlinear tolerance, or fibers with multiple cores, or fibers that support multiple spatial modes. Even though the first approach is convenient because it is compatible with existing single-mode fiber (SMF) components [4,5], it is either limited to a certain enlargement of effective area within the single-mode condition, or need careful design to avoid modal mixing induced crosstalk. Multi-core fiber has recently been actively explored [6,7], and also can be made compatible with SMF components with tapered multi-core couplers, but needs be precisely designed to sufficiently suppress the crosstalk among the cores. The third approach is to use multiple spatial modes of a single-core fiber, which has been predicted to increase the total capacity [8,9]. Recently a number of groups have demonstrated this approach of spatial mode division multiplexing (MDM) of LP01 and LP11 modes [10,11] and LP01 and two degenerate LP11 modes [12,13]. In addition to increased capacity or SE and higher possible tolerance to fiber nonlinearity, MDM allows the possibility of using spatial modes as an additional degree of freedom for information coding, whereby the forward error correction (FEC) codes can be made further efficient [14]. The biggest challenge in MDM is the efficient combining and splitting of higher-order spatial modes from and to the fundamental LP01 mode of SMF components. Early proposals of MDM [8,9] focused on using multimode fiber (MMF) with core diameters of 50-62.5 µm for short distances. However, these fibers may support 100 or more modes which couple with each other during transmission of even moderate distance in a random manner due to environmental perturbation, therefore practical demonstrations of MDM in MMF is limited to using a limited number of modes and using some form of spatial filtering, such as using donut-shaped photodiodes [15], MMF fused couplers [16], butt-coupling [17] or spatial light

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modulators (SLM) [18]. However, it is more practical to limit the number of supported modes to very small numbers, hence the name of few-mode fiber (FMF). It has been demonstrated that by using two-mode fibers (TMF) that support only the LP01 and two degenerate LP11 modes, well controlled coupling to SMF is achievable [10–13,19]. FMF has the potential to provide high mode selectivity, well understood modal dispersions and stable, broadband transmission performance. For example, by designing the differential modal dispersion (DMD) to be large, inter-modal mixing between the LP01 and LP11 modes in the TMF can be minimized [5], and no complex algorithm to separate the channels are required [10,11]. In addition, by combining the powerful multiple-input, multiple-output (MIMO) digital signal processing (DSP) techniques, modal and polarization mixing can be tracked and the dispersion can be equalized, as demonstrated in [12,13]. The reports in [12,13] relied on free-space mode conversion with the use of phase masks which may be bulky to realize in a practical system. In this paper, we demonstrate the application of all-fiber-based mode converters to multiplexing and demultiplexing of the twodegenerate LP11 modes (LP11a and LP11b). We also demonstrate the transmission over 26-km TMF where we apply 4x4 MIMO processing along with coherent optical orthogonal frequency-division multiplexing (CO-OFDM). CO-OFDM has the advantage of fast channel estimation by using only a few training symbols at the beginning of data frame. Together with heterodyne detection scheme that reduces the number of required ADCs, we have achieved transmission of a 35.3-Gb/s CO-OFDM signal in a bandwidth of only 5.5 GHz. Table 1. Parameters for the Step Index TMF Used in this Work (* are design values)

Parameter

Core dia. [µm]

Clad. dia. [µm]

Refrac. index diff.(∆n)

Loss [dB/km]

NA*

LP01/LP11 CD* [ps/nm/km]

DMD * [ps/m]

LP01/ LP11 Aeff * [µm2]

LP11 cutoff * [nm]

Values

8.2

103

0.54%

0.25

0.17

22 /17

3

92.3/ 148.4

2323

2. All-fiber LP01-LP11 mode converter and LP11 mode multiplexer The 26-km custom designed TMF used in this work has the same parameters as the 4.5-km TMF used in [11,19], which are summarized in Table 1. The TMF is designed to provide a high DMD value of 3 ps/m between LP01-LP11 to minimize coupling probability during transmission. The mode beat length LB = 2π/(β0-β1) is estimated to be 520 µm. By employing a metallic V-groove with pitch equal to LB to create a mechanical pressure grating along the TMF length, we can build a LP01-LP11 mode converter (MC) with low-loss, high conversion efficiency, without the requirement for coupling out to free-space bulk optics [20]. Because the index grating changes only in the direction of the applied pressure, the input LP01 light couples predominately to the LP11 mode oriented in that direction. The schematic diagram of the mode conversion and multiplexing of the two orthogonal LP11 modes is shown in Fig. 1(a) and the picture of the corresponding setup is shown in Fig. 1(b). We have utilized precisiontooled steel-made v-grooves with pitch closely matched to the beating length LB and having only 20 periods, which provides the advantage of compact size and low polarization dependence. In order to prevent damage to the TMF fiber itself, 0.9-mm loose tube jacket is used as a buffer. With this simple design, the mode extinction ratio of better than 20 dB is achieved for all the 4 MCs used in the MDM transmission experiment, two for the transmitter to convert incoming signal from SMF to LP11a or LP11b, and the remaining two at the receiver to convert LP11a or LP11b back to LP01 for SMF coherent detector setup, respectively. The MCs are assisted by mode strippers (MS) to prevent unwanted LP11 mode before MC at the transmitter and after MC at the receiver. The MSs are realized by tight bending the 0.9 mm jacketed TMF fiber over 8 mm posts of about 10 rounds. The insets of Fig. 1(a) show the two orthogonal LP11 modes, (hereafter we denote LP11a and LP11b) viewed by a beam-profiler.

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Rotating FC connector

0.9 mm jacket

(a) SMF Core-center aligned splice

TMF Mode stripper Mode converter coiling V-groove

CL CL

CL

SMF

BS

TMF

TMF IR beam profiler

(b)

TMF

V-groove

LP11a BS CL LP 11b

TMF

Fig. 1. (a) Schematic of fiber grating-based LP01-LP11 mode converter and dual-LP11 mode multiplexer. The mode demultiplexer is similar by reversing the direction of the signals. Inset shows the orthogonally adjusted far-field patterns of the LP11a and LP11b modes viewed with an infra-red (IR) beam-profiler. BS: beam-splitter, CL: collimating lens. (b) Pictures of one of the mode multiplexers (left) and mode converters (right).

Because it is not easy to know beforehand the axial orientation of the TMF prior to applying the V-groove, we utilize rotation of an adjustable keyed FC connector to enable rotational correction of the two LP11 modes to be orthogonal (orientation 90° rotated) to each other during mode multiplexing and demultiplexing, in the same manner as axial alignment of polarization maintaining fiber (PMF). In order to enable coupling and splitting the LP11a and LP11b mode components in the absence of fiber-based options for this demonstration, we chose to use free-space beam splitters along with collimating lenses with numerical aperture (NA) of 0.25. In future, a fused TMF or micro-optic coupler could be utilized in order to fabricate a compact and low loss orthogonal LP11 mode multiplexer.

AWG

2 OFDM symbol dela y MS1

MC1

MS2

MC2

Tx 1

SMF

PMF

CL

1550.9nm 3dB

OFDM Tx

PBS

3dB

PDM emulation

Tx 2

MDM emulation

Mode conversion and multiplexing

26 km TMF MC3

MS3

BPF1

Rx 1

PBS

BS

CL

CL

CL MC4

MS4

BPF2

Rx 2

PBS

Pol. Demux. Mode demultiplexing and conversion

MS4

6dB

BS

CL

IQ Mod.

CL

LD

3dB

PD1

ADC1

3dB

PD2

ADC2

3dB

PD3

ADC3

3dB

PD4

ADC4

PCs

4x4 MIMO OFDM DSP

1 OFDM symbol dela y

Heterodyning

Fig. 2. Experimental setup for the 4x4 MIMO-OFDM transmission over TMF. The colors delineate fiber-based and free-space optics. PDM: polarization division multiplexing, MDM: mode division multiplexing, PBS/C: polarization beam splitter/combiner, MS: LP11 mode stripper, MC: LP01-LP11 mode converter, BS: beam splitter (non-polarizing cube), CL: collimating/ focusing lens, ADC: analog-to-digital converter. BPF: band-pass filter (optical). PC: polarization controller.

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3. Dual-LP11-mode transmission experimental setup In this work we focus on the demonstration of the MDM transmission feasibility instead of high data rates using the proposed transmission method, which will be targeted in future works. The end-to-end transmission experiment setup is depicted in Fig. 2. In order to maintain simplicity in the receiver setup, we have chosen to adopt heterodyne polarization and mode-diversity coherent detection condition so that only 4 sets of photodetectors (PD) and a real-time oscilloscope with 4-channels of ADC are sufficient to realize the 4 MIMO receivers. Similarly at the transmitter, we use a single optical OFDM generator consisting two DACs of an arbitrary waveform generator (AWG), and then optically subdivide and recombine it twice to emulate the 4 MIMO transmitters. This is schematically depicted in Fig. 3(a). First the laser output from an external cavity laser is modulated with an optical IQ modulator to generate OFDM signal employing 4 training symbol (TS) slots, of which only the first slot is populated with 1 TS and the remaining 3 are empty. When the signal is split and recombined on orthogonal polarizations with one OFDM symbol delay between them, dual-polarization transmitter is emulated [11]. Then the signal is again split with a 3-dB SMF coupler and the two branches are delayed by two OFDM symbols length to de-correlate them and adjust the location of the TS so that all the 4 tributaries have an orthogonal set of TSs, which can be utilized to compute the 4x4 MIMO channel matrix. The offline DSP processing steps at the OFDM transmitter are shown in Fig. 4(a). The two branches of PDM OFDM signal are then transferred from SMF to the LP11a and LP11b modes of the transmission TMF by utilizing the mode multiplexer shown in Fig. 1. After the TMF transmission, the mode demultiplexer splits the randomly oriented incoming mode patterns to two orthogonal LP11 modes by using beamsplitter, and converts them back to SMF components by using MCs. Any residual LP01 mode components excited during mode multiplexing or fusion splicing on the TMF are eliminated, as the MCs convert them to LP11 and then subsequent MSs strip them off. After this the two PDM tributaries are further split to orthogonal polarizations by PBS. The 4 tributaries are mixed with 4 LO branches using 3-dB couplers into 4 PDs with trans-impedance amplifiers. RF spectrum of one of the 4 receivers is depicted in Fig. 3(b).The signal-signal intermixing products falling on the lower frequency regions are avoided by inserting a frequency guard band between the signal and the LO in the heterodyne scheme. The 4 channel RF signals on an intermediate frequency (IF) of around 7.7 GHz are then directly sampled by 50 GSa/s ADCs and digitally down-converted and filtered into baseband 4 OFDM tributaries. The 4x4 MIMO-OFDM receiver processing steps are shown in Fig. 4(b), which include frequency estimation and phase-noise compensation by RF pilot-tone method [21], timing synchronization and MIMO-channel estimation based on zero-forcing algorithm using TS. After estimating the 4x4 channel matrices for each subcarrier, channel equalization, demodulation and bit error rate (BER) evaluation processes follow. TS

Data symbols LP11a X-pol

One symbol delay

LP11a Y-pol

Two symbol delay

LP11b X-pol LP11b Y-pol

(b)

10

Spectral Spectralpower Power[dB] [dB]

(a) (a)

(b)

0

RF pilot

Inter -mixing products

-10 -20 -30 -40 -50 -60 0

5

10

15

Frequency [GHz] Frequency [GHz]

20

Fig. 3. (a) Time-domain transmitted signal traces for the four tributaries, depicting method of emulating 4x4 MIMO transmitter from single source by 4-way splitting and multiplexing with delays of integer number of OFDM symbol length. TS: training symbol. (b) Received spectrum of one of the 4 channels after 26-km TMF transmission, depicting RF pilot used for phase noise compensation and frequency guard band to avoid signal-signal intermixing.

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(a)

TX 1 TX 2 TX3 TX 4 delay IQ modulator I

DAC DAC

Q

4 TS insertion

(b)

RX 1 RX2 RX3 RX 4 ADC ADC ADC ADC Freq. sync. and RF pilotbased phase noise cancelling Timing sync. and serial-toparallel conv. CP removal

1/8 CP insertion 1024 point IFFT Subcarrier mapping QPSK or 8QAM modulation PRBS (21 5 -1) data loading

1024 point FFT Subcarrier demapping Channel estimation Channel equalization and phase estimation Data recovery and BER calculation

Fig. 4. Schematic diagram of 4x4 MIMO DSP program: (a) offline digital signal processing at transmitter, and (b) offline digital signal processing at receiver.

Because the two LP11 modes of the TMF have relatively low DMD between them, even very small environmental perturbation causes the signal to rotate and couple between them. Then as the MCs at the receiver are mode-selective, this would cause power fading if we only used one of the LP11 modes. For dual-LP11 mode transmission, we solve this problem by orthogonal LP11 mode demultiplexing and MIMO channel tracking, as also reported in [12,13]. However, in contrast to previous approaches of employing long memory finite impulse response (FIR) filters, the use of MIMO-OFDM gives the advantage of simplicity in channel equalization employing one-tap equalizer. In order to periodically update channel estimation, we use TS after every 2.5 µs. We measure a channel delay spread of 10ns in our 26 km TMF (corresponding to 0.38 ps/m). Because the DMD between LP11 and LP01 much larger than this value (ref. Table 1), we conclude this to be caused by residual DMD between the two LP11 modes. We will investigate the cause of relative large DMD between the two degenerate LP11 modes in our prototype TMF in future works. In this work we minimize the effect of the channel delay spread by inserting cyclic prefix of 12.8 ns as guard interval to prevent inter-symbol interference from DMD. 4. Dual-LP11 mode, 4x4 MIMO-OFDM transmission experiment results After building the mode multiplexing and demultiplexing subsystem, we record the achieved end-to-end transmission losses among the PDM transmitters (Tx1 and Tx2) and PDM receivers (Rx1 and Rx2), because of lack of a suitable TMF amplifier, the transmission distance will be limited if the mode conversion loss is too high. The MCs are each estimated to cause only a combined excess loss between 2.8 to 4 dB. The loss of the MC and free-space couplers can be avoided partially by realizing efficient all-fiber mode couplers [22]. However, this will require specialized fusion splicing or evanescent coupling methods. Even though the excess losses are relatively high at this early stage of TMF development, because of the short distance and increased effective area the transmission power can be increased and the received OSNR is still above 30 dB in our case. Therefore, power loss due to mode conversion is not the fundamental limitation transmission over such moderate distances of TMF. Next we proceed to measure the back-to-back 4x4 MIMO-OFDM transmission performance without the long TMF, but including the two transmitters and two receivers with mode conversion (we call this TMF “B2B” and use as a benchmark for evaluating transmission impairments). Because of heterodyning and direct ADC sampling, in order to avoid the influence of signal-signal intermixing products in the low-frequency regions, we used increased LO-signal ratio of 20 dB and optimize the frequency guard band. We find a large delay spread for the 26-km TMF and increased spatial mode coupling, therefore we use a longer symbol length of 115.2 ns with 1/8 CP length to verify the feasibility of overcoming DMD and penalty in dual-LP11 mode transmission over 26-km TMF. We chose QPSK modulation for all subcarriers. The total data rate is 35.3 Gb/s, taking into consideration CP and TS overheads. Figure 5 (a) shows the

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performance of the 26-km TMF transmission as compared to TMF B2B. For B2B, the required optical signal-to-noise ratio (OSNR) to achieve a BER of 1x10−3 is found to be 11.5 dB, with 2 dB of variations among tributaries. Increased BER variation may have been caused by channel estimation error due to using long symbols; however this variation further increases to almost 4 dB after 26 km transmission. This is also evident in the constellations of Fig. 5 (b), where LP11b components showed poorer performance and also from the BER-vsOSNR plots in Fig. 5 (a) after 26 km transmission. Together with the influence of DMD, the polarization dependent loss and the power imbalance in the two transmitter side MC may be playing a role in BER variation, and we will investigate ways to mitigate this by power preemphasis and optimization of MC in future works. On average the required OSNR penalty after 26-km TMF transmission is 3 dB. Therefore we conclude that by using OFDM signals with longer CP, the spatial mode mixing and random rotations are still correctable in a 4x4 MIMO transmission on LP11a/b modes in relatively long TMF spans. This indicates that spatial mode diversity among the degenerate LP11 modes in TMF can be harnessed to double the capacity compared to SMF. It is also noteworthy that the LP01-LP11 mode converters required to interface TMF with SMF components can be realized based on fiber-based compact with low complexity. As the data bandwidth is only 5.5 GHz, even after considering 7% overhead for FEC, the achieved net SE is 5.9 b/s/Hz, which is only possible for QPSK through the use of the two spatial modes. (a)

B2B LP11b-X B2B LP11b-Y B2B LP11a -X B2B LP11a -Y 26km LP11b-X 26km LP11b-Y 26km LP11a -X 26km LP11a -Y

1.E-01

BER

1.E-02 1.E-03

(b)

LP11b,X-pol

LP11b,Y-pol

LP11a,X-pol

LP11a,Y-pol

LP 11a X-pol

LP 11b X-pol

LP11a Y-pol

LP11b Y-pol

1.E-04 1.E-05 1.E-06 10

After 26 km TMF, 20 dB OSNR 15

20

25

OSNR [dB]

Fig. 5. (a) Measured OSNR sensitivity for QPSK, 35.3-Gb/s 4x4 MIMO-OFDM transmission over TMF back-to-back (B2B) and 26-km TMF. (b) Constellation of the received signal tributaries after 26-km TMF transmission.

6. Summary We report successful mode-multiplexed dual-polarization transmission on two degenerate LP11 modes of a two-mode fiber using all-fiber based LP01-LP11 mode converter. Mechanically induced fiber gratings show stable mode conversion between LP01-LP11 with extinction ratios over 20 dB. Using free-space coupling of the two LP11 modes, we have realized 4x4 MIMO CO-OFDM transmission over up to 26-km of a two-mode fiber. After mode- and polarization-diversity heterodyne detection, we have successfully demonstrated transmission of a 35.3-Gb/s CO-OFDM signal over 26-km TMF fiber with QPSK modulation, with less than 3-dB penalty compared to TMF back-to-back detection.

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