MMF Links Research Articles

Mode separability in offset-based MGDM MMF links: a phase-retrieval-based approach

Mode group diversity multiplexing (MGDM)-based multimode fiber (MMF) systems have been known to be promising solutions for high-data-rate links. However, the challenge of cross-talk within these systems remains a critical concern, since coupling across mode groups diminishes data rates. Past work has shown that optimizing launch conditions and effective spatial filtering can minimize cross-talk across mode groups, although the optimization of the launching has been largely using trial-and-error-based approaches. Characterizing cross-talk limits and quantifying mode coupling requires the use of coherent imaging of the fiber, which is prohibitively complex. In this paper, we present, to our knowledge, a novel approach to characterize the received spatial signal at the MMF output, by obtaining both the amplitude and phase, using just imaging intensity measurements. This is enabled by leveraging recent advances in signal processing to recover the phase from intensity measurements using an alternating minimization-based algorithm for low-complexity phase retrieval, and presents an accurate characterization of mode coupling and estimation of cross-talk in MMF MGDM systems. Experimental validation demonstrates the effectiveness of this approach, particularly in offset-launch-based large-core MMF MGDM systems, where optimal launch parameters are identified to minimize cross-talk. Our findings highlight the significance of optimal launching and spatial filtering in maximizing mode separation, thus reducing cross-talk and enhancing overall system performance, as validated through extensive data rate experiments. The novel phase-retrieval-based fiber characterization can be extended and used to efficiently design spatial filtering solutions for MGDM, such as photonic lanterns and mode multiplexers.

VCSEL-based optical transceiver with a 90°-bent graded-index core polymer waveguide coupler for 25.78-Gb/s transmission

In this paper, we propose a low-loss assembly of optical elements [vertical cavity surface emitting laser (VCSEL)/photodiode (PD)] and a 90°-bent graded-index (GI) core polymer optical waveguide for compact high-speed optical transceivers. The air gap between the 90°-bent GI-core waveguide and VCSEL/PD chips are filled with a high-index resin, and we demonstrate a reduction not only of the Fresnel reflection loss but also of the coupling loss due to the lower divergence angle of VCSEL and fiber output beams. In this paper, the core size and NA of the 90°-bent GI-core in the polymer waveguide are redesigned to match the specific launch conditions realized with the gap filled with resins, resulting in theoretical estimates of insertion losses as low as 0.82 dB and 1.38 dB for the waveguides on the transmitter and receiver sides, respectively. Such low insertion losses are obtained not by step-index core but GI core waveguides. After redesigning the structure, 90°-bent GI-core polymer waveguides with a bending radius of 1 mm are experimentally fabricated using the Mosquito method, and we experimentally demonstrate that the insertion loss of the waveguide can be reduced by about 4 dB when the 100-µm air gap is filled with an optical adhesive (na = 1.51). In addition, an error-free 25.78-Gb/s data transmission is successfully demonstrated over a 100-m MMF link using the fabricated VCSEL based transceivers with 90°-bent GI core polymer optical waveguides.

The Modal Effect of VCSELs on Transmitting Data Rate Over Distance in OM4 Fiber

The effective modal bandwidth (EMB) limit for the multi-mode (MM) VCSEL on transmitting data rate over distance in OM4 fiber is commonly referenced to be within 4700 MHz*km. However, this limit may not be valid for the VCSELs with reduced-mode (RM) or single-mode (SM) emission spectrum for data transmission. In this work, we investigate the EMB limit of OM4 fiber through characterizing three types of 850 nm oxide-VCSELs, namely, MM, RM and SM. Each type of VCSEL is tested to the highest error-free NRZ data rate over transmitting distance up to 1 km. The MM VCSEL demonstrates 50 Gb/s (100 m) and 26 Gb/s (300 m) that are within EMB limit. In contrast, the RM and SM VCSELs demonstrate speed distance product beyond the OM4 EMB limit. The SM VCSEL delivers record speed-distance of 44 Gb/s (500 m), and 36 Gb/s (1 km) at 25 °C without the use of equalization. These results suggest the SM VCSEL-based MMF link can transmit much higher data rate over extended distance up to 2 km in data centers.

Comparison of High-Speed PAM4 and QAM-OFDM Data Transmission Using Single-Mode VCSEL in OM5 and OM4 MMF Links

To realize the superiority of data transmission with reduced modal dispersion in OM4- and OM5-mulitmode fiber (MMF), a single transverse mode (SM) vertical cavity surface emitting laser (VCSEL), directly encoded with large-capacity data formats for transmissions in OM5-MMF and OM4-MMF, are compared. The SM-VCSEL contains a 12-μm mesa formed by double-oxidized AlGaAs layers, which confines a current-flow area within an aperture of 3 μm. The SM-VCSEL is lasing with a carrier-to-noise ratio of 34.4 dB and a linewidth of 0.05 nm. The SM-VCSEL is biased at 10Ith to provide a modulation bandwidth of 21.4 GHz and a relative intensity noise of -138 dBc/Hz. By encoding the SM-VCSEL with four-level pulse amplitude modulation (PAM-4) at 64 Gbit/s, the bit error ratio (BER) of 32-GBaud PAM-4 data is improved from 7.9 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> to 4.9 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> under a KP4-FEC criterion by replacing OM4-MMF with OM5-MMF. After OM5-MMF transmission, a bathtub BER plot shows bottom-eye, middle-eye, and top-eye jitter tolerances of 9.3, 10.6, and 7.1 ps, which are much wider than 6.9, 10.1, and 6.5 ps for OM4-MMF, respectively. When encoding the 16-QAM OFDM at 100 Gbit/s, OM5-MMF allows data transmission at 96-Gbit/s with a corresponding error vector magnitude, signal-to-noise ratio, and BER of 16.7%, 15.4 dB, and 3.6 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> under preleveling at a slope of 0.3 dB/GHz. Because of the high effective modal bandwidth and low modal dispersion of the OM5-MMF, a relatively low receiving power penalty of 0.1 dB between 100-m and back-to-back (BtB) transmissions is obtained with either the pre-emphasized PAM-4 or the preleveled QAM-OFDM data format. By contrast, the receiving power penalty is 1.04 dB between 100 m and BtB cases during OM4-MMF transmission.

A Novel Equalizer for 112 Gb/s CAP-Based Data Transmission Over 150 m MMF Links

In this paper a novel feedforward and decision feedback equalizer is proposed for the first time for use in short-reach high-speed communication links based on carrierless amplitude and phase (CAP) modulation. The proposed new equalizer mitigates crosstalk between the in-phase (I-) and quadrature (Q-) channels resulting from the non-linear phase response of the link enabling significant improvement in the link performance when operating at high data rates. The structure of this novel equalizer is introduced, its operation is described, and simulation and experimental studies on short-reach high-speed MMF links using 850 nm multimode VCSELs are presented. Data transmission tests are carried out over 150 m of OM4 MMF at 112 Gb/s using CAP modulation and the proposed CAP equalizer, and a bit-error-rate (BER) within the hard-decision forward error-correction (HD-FEC) threshold of 3.8 × 10−3 is achieved. The link is also tested when a conventional equalizer is used instead of the CAP equalizer and when adaptive discrete multitone (DMT) modulation is applied. The performance of the link is compared for these different transmission schemes and it is shown that the use of the novel equalizer and CAP modulation outperforms the other two schemes. The proposed equalizer can be implemented with low additional complexity, providing a potential cost-effective solution enabling >100 Gb/s single-lane data transmission in short-reach MMF-based links.

DSP-Enabled 100 Gb/s PAM-4 VCSEL MMF Links

Short reach optical interconnects must support higher data rates to manage the increasing needs of end users and the commensurate increase in storage and computation within and between data centers. VCSEL-based MMF links provide a power efficient solution which is achieved in part by maximizing the data rate per laser source. We design an archetype system to explore the challenges and benefits of signal processing with currently available technology. By careful use of signal processing at both the transmitter and receiver, we experimentally demonstrate 107 Gb/s PAM-4 transmission over 105 m of wideband MMF using 850 and 940 nm VCSELs and thus show a path to 400 Gb/s and eventually 1 Tb/s multimode fiber links.

VCSELを用いたMMF長距離伝送の現状と今後の展望

VCSELを用いたMMF長距離伝送の現状と今後の展望

Noise in VCSEL-Based Links: Direct Measurement of VCSEL Transverse Mode Correlations and Implications for MPN and RIN

The correlation properties of vertical-cavity surface-emitting laser (VCSEL) transverse modes determine the noise impairments of VCSEL-based high-speed MMF links. Relative intensity noise and mode partition noise originate from the distinct and aggregate fluctuations of the lasing transverse modes of VCSELs. These noise mechanisms are often the limitations in VCSEL-based optical links particularly 25 Gbaud and higher OOK and PAM-4 links and are an integral part of any link budget including those of the IEEE 802.3 and fiber channel T11.2 standards efforts. Here, we report the direct measurement of the correlation properties of high-speed VCSELs. We show that the corresponding computed mode partition noise (MPN) and relative intensity noise (RIN) powers are inconsistent with the conventional models used by standards groups yet are consistent with link measurements.

Combined Data Detection Scheme for Zero-Padded OFDM Signals in MMF Links

In this letter, we propose a receiver scheme for zero-padded orthogonal frequency division multiplexing (OFDM) that combines low complexity from overlap-and-add equalizer and low error rate provided by successive interference cancellation data detection from optimal ordering vertical Bell Laboratories layered space-time (V-BLAST) architecture. Results of numerical simulations on multimode optical fiber links show that the proposed scheme improves the error rate performance of zero forcing (ZF) equalization receiver, reaching results similar to V-BLAST. For example, the proposed scheme can reach 33.9 Gb/s in a 600-m link, whereas the ZF receiver would reach 29.06 Gb/s and cyclic prefix OFDM only 19.37 Gb/s. These results are obtained with a reduction in computational complexity (measured in number of real products) of 86% in detection and 66% in preprocessing with respect to the ZF receiver, and 75% and 97% with respect to the V-BLAST receiver.

Robust and tunable 16.375Gb/s dual-band optical OFDM transmissions over directly modulated VCSEL-based 200m OM2 MMFs.

Utilizing low-cost, 2.2GHz modulation bandwidth, uncooled and standalone directly modulated VCSEL (DM-VCSEL)-based real-time dual-band optical OFDM (OOFDM) transmitters, aggregated 16.375Gb/s transmissions of OOFDM signals having bandwidths approximately 3.8 times higher than the VCSEL manufacturer-specified modulation bandwidths, are experimentally demonstrated, for the first time, over 200m OM2 MMF links based on intensity modulation and direct detection. The aggregated signal transmission capacities of the aforementioned links vary by just 8% for various OM2 MMFs ranging from 100m to 500m, and by just 10% over a 1GHz passband carrier frequency detuning range. Such dual-band OOFDM adaptability-induced excellent performance robustness and large passband frequency tunability can significantly relax the requirements on VCSEL modulation bandwidth for achieving specific transmission performances for cost-sensitive application scenarios such as data centers.

Modeling of Mode Coupling in Multimode Fibers With Respect to Bandwidth and Loss

A fast, accurate and simple field coupling model is presented which is capable of describing mode coupling effects due to bends and splices in multimode fibers with parabolic index profile as well as the coupling losses induced by this process. This model is validated numerically by comparing the results to the well-known coupled amplitude theory model yielding the same relative bandwidth increase behavior as long as the coupling losses are the same. It is shown, that the number of discrete segments used in this model can be reduced considerably as long as the coupling losses are kept constant. The effect of mode coupling on the differential group delay, mode dependent loss, bandwidth gain and impulse response width reduction are analyzed. It is shown that the relative bandwidth gain induced in MMF links induced by the coupling process is independent of fiber parameters or number of guided modes; it can be fully characterized by coupling induced losses. The model is compared to well-known results given by power coupling models and a good agreement is observed for high steady state loss values.

Performance comparison of 850-nm and 1550-nm VCSELs exploiting OOK, OFDM, and 4-PAM over SMF/MMF links for low-cost optical interconnects

We experimentally compare the performance of two commercially available vertical-cavity surface-emitting laser diodes (VCSELs), a multi-mode 850-nm and a single-mode 1550-nm, exploiting on–off keying/direct detection (OOK/DD), and orthogonal frequency division multiplexed (OFDM) quadrature phase-shift keying (QPSK)/16-ary quadrature amplitude modulation (16QAM) with direct detection, over SMF (100m and 5km) and MMF (100m and 1km) short-range links, for their potential application in low-cost rack-to-rack optical interconnects. Moreover, we assess the performance of quaternary-pulse amplitude modulation (4-PAM), for the 1550-nm transmitter over SMF and MMF links and we compare it to the data-rate equivalent NRZ-OOK. The extensive performance comparison under various transmission scenarios shows the superiority of 1550-nm single-mode VCSEL compared to its multi-mode 850-nm counterpart. Moreover, OFDM/DD and 4-PAM in conjunction with low-cost, inexpensive VCSELs as transmitters prove to be an enabling technology for next-generation WDM, point-to-point, short-reach, SMF/MMF optical interconnects and potential candidates to substitute NRZ-OOK. Nevertheless, the sensitivity requirements are higher in that case, whereas these advanced, spectrally-efficient modulation formats become severely degraded when transmitted over MMF links, especially, when employing the inexpensive 850-nm VCSELs as transmitter. Finally, we compare the performance of the point-to-point links under investigation to the performance of a semiconductor optical amplifier (SOA)- based, scalable permutation switch fabric, the Optical Shared MemOry Supercomputer Interconnect System (OSMOSIS).

Phase-modulated radio over fiber multimode links

We present the first experimental demonstration of a phase-modulated MMF link implementing high-frequency digital transmission in a cost-effective solution based on direct detection. Successful subcarrier transmission of QPSK, 16-QAM and 64-QAM data channels for bit rates up to 120 Mb/s through a 5 km MMF link is achieved over the microwave region comprised between 6 and 20 GHz. The overall capacity of the proposed approach can be further increased by properly accommodating more passband channels in the operative frequency range determined by the phase-to-intensity conversion process provided by the dispersive nature of the optical fiber. In this sense, our results show the possibility of achieving an aggregate bit rate per length product of 144 Gb/s · km and confirm, in consequence, the possibility of broadband phase-modulated radio over fiber transmission through MMF links suitable for multichannel SCM signal distribution.

Holographic mode-selective launch for bandwidth enhancement in multimode fiber

With rapidly growing bandwidth demands in Local Area Networks, it is imperative to support next generation speeds beyond 40 Gbit/s. Various holographic optimization techniques using spatial light modulators have recently been explored for adaptive channel impulse response improvement of MMF links. Most of these experiments are algorithmic-oriented. In this paper, a set of lenses and a spatial light modulator, acting as a binary amplitude filter, played the pivotal role in generating the input modal electric field into a graded-index MMF, rather than algorithms. By using a priori theoretical information to generate the incident modal electric field at the MMF, the bandwidth was increased by up to 3.4 times.

Optimization of adaptively modulated optical OFDM modems for multimode fiber-based local area networks [Invited

Focus Issue on Orthogonal-Frequency-Division Multiplexed Communications Systems and Networks The impact of a wide range of different parameters of various components involved in optical modems using adaptively modulated optical orthogonal-frequency-division multiplexing (AMOOFDM) on the transmission performance of AMOOFDM signals is explored thoroughly, in single-channel, unamplified, multimode-fiber (MMF)-based, intensity modulation and direct detection (IMDD) transmission links. Practically available optimum component parameters are identified, based on which the AMOOFDM modems are optimized. It is shown that the optimized AMOOFDM modems enable a >70% increase in the capacity versus reach performance without compromising link loss margins, in comparison with that achieved without modem optimization. In addition, the validity of the identified optimum parameters and the feasibility of the optimized AMOOFDM modems are also statistically verified for implementation in the vast majority of installed MMF links. Statistical investigations show that the optimized AMOOFDM modems can support >50Gbits/s signal transmission over 300m in 99.5% of already installed MMF links with loss margins of >7dB. Furthermore, it is also confirmed statistically that the optimized AMOOFDM modems have excellent performance flexibility and great robustness to various fiber and/or system implementation-related impairments.

Maximizing the Transmission Performance of Adaptively Modulated Optical OFDM Signals in Multimode-Fiber Links by Optimizing Analog-to-Digital Converters

Based on a comprehensive theoretical model of a recently proposed novel technique known as adaptively modulated optical orthogonal frequency-division multiplexing (AMOOFDM), investigations are undertaken into the impact of an analog-to-digital converter involved in the AMOOFDM modem on the transmission performance of AMOOFDM signals in unamplified intensity-modulation and direct-detection (IMDD) multimode-fiber (MMF)-based links. It is found that signal quantization and clipping effects are significant in determining the maximum achievable transmission performance of the AMOOFDM modem. A minimum quantization bit value of ten and optimum clipping ratio of 13 dB are identified, based on which, the transmission performance is maximized. It is shown that 40-Gb/s-over-220-m and 32-Gb/s-over-300-m IMDD-AMOOFDM signal transmission at 1550 nm with loss margins of about 15 dB is feasible in the installed worst case 62.5-mum MMF links having 3-dB effective bandwidths as small as 150 MHz middot km. Meanwhile, excellent performance, robustness to fiber types, and variation in launch conditions and signal bit rates is observed. In addition, discussions are presented of the potential of 100-Gb/s AMOOFDM signal transmission over installed MMF links

Penalty free subcarrier modulated multimode fiber links for datacomm applications beyond the bandwidth limit

The use of coherent electrical detection in subcarrier modulated MMF links is shown for the first time to provide sufficient enhancement in receiver sensitivity to compensate fully for excess passband loss. This allows subcarrier modulated MMF links to be operated without effect on the link power budget.

Three-wavelength-division-multiplexed multichannel subcarrier-multiplexing transmission over multimode fiber with potential capacity of 12 Gb/s

We investigate the feasibility of 10-Gb/s delivery over multimode-fiber (MMF) local area networks by using the externally modulated multiple-wavelength-division-multiplexed subcarrier-multiplexed technique. In a system experiment, three externally modulated distributed-feedback lasers with different wavelengths (1551.1, 1553.6, and 1556.1 nm) are used to carry a total of 300 random-phased continuous-wave carriers for simulating 300 channels of 256-QAM signals. The transport of a potential capacity of 12 Gb/s (=40 Mb/s/spl times/300 channels) over a 4-km MMF link is demonstrated and analyzed.