Effective Modal Bandwidth Research Articles

Differential mode delay and modal bandwidth measurements of multimode fibers using frequency-domain method

• A frequency-domain method is developed to measure the DMD of multimode fibers. • Vector network analyzer is used to measure the complex transfer functions. • The time-domain output pulses can be calculated from CTF with proper procedures. • Frequency-domain method yields DMD results equivalent to the time domain method. • The frequency domain method can conduct DMD measurements at very low optical power. We report a frequency-domain method for measuring the differential mode delay (DMD) and bandwidth of multimode fibers (MMFs). Using a frequency domain instrument, vector network analyzer (VNA), the method measures the complex transfer functions (CTFs) of multimode fibers for a given set of launch conditions. As a result of the fiber length, which is long relative to sampling points used in VNA, the frequency sampling is under-sampled and the measured CTF is aliased. Aliased CTF behaves very differently from fully sampled CTF with dependence on the sampling frequency step. With proper procedures, the CTF can be transformed to local time frame centered around DMD into a modified CTF. The time-domain output pulses can be further recovered through an inverse Fourier transform. By recovering output pulses using complex transfer functions of a multimode fiber at controlled launch offsets, one can subsequently follow the same procedures of existing time domain methods to determine the effective modal bandwidth of the multimode fiber under test (FUT). As a result of the method, DMD and modal bandwidth measurements can be undertaken using frequency domain instruments rather than time domain instruments in a mathematically equivalent manner for multimode fibers. The measurement method utilizes CW light sources that are readily available in a wide range of wavelengths, thus providing greater flexibility for measuring the modal bandwidth of MMFs at different wavelengths, including WDM applications other than 850 nm. The very high sensitivity of the VNA allows the measurement at much lower optical power compared to the time domain instrument. Several measurements at 850 nm have been conducted using the frequency domain method and validated by comparison with the results from the time domain method. The method is suitable for measuring MMFs from a short length of a few hundred meters to a very long length close to 20 km. We have also measured an MMF at 910 nm using the method to demonstrate its flexibility in adopting different wavelengths.

Fabrication of Y-Branched GI Core Polymer Waveguide and its Application to CWDM MUX Device for Multimode Fiber

This paper describes the characteristics of graded-index (GI) core Y-branched polymer optical waveguides as optical couplers for multimode fibers (MMFs) in coarse wavelength division multiplexing links with a restricted mode launch function. In this paper, we compare two GI-core Y-branched waveguides fabricated using different methods: the Mosquito method and the imprint method. Error free transmission is confirmed when transmitting a 25.6125-Gbps NRZ optical signal (PRBS31) at 850-nm wavelength through a 100-m long OM3 fiber using the waveguide as an optical coupler. We also discuss the function of a restricted mode launch exciter for a 1-km long MMF. From the effective modal bandwidth analysis, realizing higher bandwidth-distance product in long MMFs should be possible by reducing the core size of the waveguide coupler.

Modal Bandwidth and Single-Mode VCSEL Transmission Capability Over Multimode Fibers

Single-mode VCSELs have been demonstrated with great transmission capability over multimode fibers in the literature through restricted launch, which is beyond what is allowed by the effective modal bandwidth (EMB) defined for multimode transmission with the encircled flux launch condition. However, the impact of fiber index profile errors and their interactions with launch conditions have not been thoroughly studied. We conduct a detailed experimental study to gain insight on the impacts of the fiber index profiles and launch conditions on the system performance of single-mode VCSELs transmitting over multimode fibers. Experimentally, our results show that a launch spot roughly matching the fiber LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> mode field leads to very high bandwidth and robust performance, largely independent of the imperfection of the index profiles and therefore can perform well in very high data rates. For a restricted launch without mode matching, the performance highly depends on the quality of the fiber, in line with EMB. We found that the center offset tolerance is around <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2~\mu \text{m}$ </tex-math></inline-formula> for a mode matched launch. We also conducted single-mode VCSEL transmission experiments over 600-m and 1000-m multimode fibers, which agree well with the modal bandwidth results.

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.

High-Speed Performance Evaluation of Graded-Index Multicore Fiber Compatible With Multimode and Quasi-single Mode Operation

A comprehensive study on the optical characteristics and high-speed transmission performance of a hexagonally arrayed seven-core fiber supporting both multimode transmission at 850 nm and quasi-single mode transmission at 1310 and 1550 nm are presented. We investigated its optical and transmission characteristics such as inter-crosstalk, modal bandwidth, chromatic dispersion, and misalignment tolerance. The results show that the multicore fiber (MCF) has a high bandwidth at 850 nm while remaining a comparable performance with standard single-mode fiber (SSMF) at 1310 and 1550 nm. The worst inter-core crosstalk is less than -43 dB over a 10 km-long MCF connected with a pairwise fanin/fan-out devices. The minimum effective modal bandwidth of 8.44 GHz·km is achieved with the MCF for multimode operation, while the chromatic dispersion of the MCF at 1550 nm only increases by 7% compared with that of SSMF. In experiment, we successfully performed the 7 × 25 Gb/s multimode transmission over a 300 meter-long MCF at 850 nm and quasi-single mode transmission over a 12.4 and 10 km-long fiber at 1310 and 1550 nm, respectively, with an OOK modulation format. There was no obvious degradation observed when all cores operated simultaneously. Moreover, we carried out 7 × 50 Gb/s PAM4 transmission over a 300 meter-long and 10 km-long MCF at 850 and 1310 nm, respectively, as well as 7 × 40 Gb/s PAM4 transmission over a 10 km-long MCF at 1550 nm. A BER which is well below the forward error correction (FEC) threshold (5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> ) was reached for different MCF links.

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.

Comparison on OM5-MMF and OM4-MMF Data Links With 32-GBaud PAM-4 Modulated Few-Mode VCSEL at 850 nm

Based on the pre-emphasis 4-level pulse amplitude modulation (PAM-4) format at a symbol rate of 32 GBaud, the comparisons among optical PAM-4 data transmissions through back-to-back (BtB), 100-m OM4 multi-mode fiber (MMF), and 100-m OM5-MMF links are discussed in detail by employing the directly modulated few-transverse-mode (FM) vertical cavity surface emitting laser (VCSEL) chip at 850 nm as the carrier. The bare FM-VCSEL chip exhibits an oxide-confined aperture of 5 μm to provide 3–4 transverse modes with low threshold current (Ith) of 0.25 mA and high optical power of 1.2 mW at a bias current of 38 Ith. The impendence of 65 Ω and the 3-dB bandwidth of 24 GHz are obtained by setting the DC point of the FM-VCSEL at 6.25 mA. With the bias current optimization and the pre-emphasis, the receiving bit error ratio (BER) of the 32-GBaud PAM-4 data meets the KP4-forward error correction criterion of 2.4 × 10−4, which enables the FM-VCSEL successfully achieving the 64-Gbit/s data stream transmission through the BtB and 100-m OM4-/OM5-MMF links. As compared to the BtB transmission with a BER of 2.5 × 10−8, replacing OM4 with OM5 for the 64-Gbit/s PAM-4 transmission reduces the receiving power penalty from 3.41 to 0.86 dB and improves the BER from 3.8 × 10−5 to 3.8 × 10−6. That is mainly attributed to the higher effective modal bandwidth and compensated modal and chromatic dispersions of the OM5-MMF as compared to the OM4-MMF when delivering the FM-VCSEL carried PAM-4 data stream.

The Impact of Effective Modal Bandwidth on 100G SWDM Transmission Over 250 m OM5 and Left-Tilt OM4 Multimode Fibers

We demonstrate 100G SWDM4 transmission over 250 m OM5 and left-tilt OM4 multimode fibers. The OM5 fibers are compliant to TIA-492AAAE and the left-tilt OM4 fibers are compliant only to TIA-492AAAD and have a bandwidth peak at a wavelength longer than 850 nm. The “left-tilt” descriptor refers to the leftward leaning slope of the differential mode delay peaks. This is an indication that the peak bandwidth is at a wavelength longer than the measurement wavelength. To measure the margin to the forward error correction limit, 24 250 m channels were constructed from two links of 100 m and 150 m using either left-tilt OM4 or OM5 fiber. Error-free performance was achieved with each of the 24 OM5 channels. None of the 24 left-tilt OM4 channels achieved error-free performance. To further characterize performance, the effective modal bandwidth was measured for 6 left-tilt OM4 fibers and 12 OM5 fibers from the same spools as the fibers used in the margin measurements. The OM4 fibers all have a left tilt at 850 nm and the OM5 fibers have either a left or right tilt. Bit error ratio (BER) versus received power was then measured over 250 m of these fibers. With the OM4 fibers, the long wavelength channels limited the system performance due to low modal bandwidth at longer wavelengths despite the left tilt at 850 nm. We demonstrate the relationship between effective modal bandwidth at 953 nm and BER performance at 910 and 940 nm.

Scaling VCSEL-MMF Links to 1 Tb/s Using Short Wavelength Division Multiplexing

We demonstrate a path to 1-Tb/s VCSEL-based links using a wideband multimode fiber (WBMMF/OM5), which provides an extended effective modal bandwidth ranging from 844 to 948 nm. Using 850-, 880-, 910-, and 940-nm vertical cavity surface emitting lasers (VCSELs), we quantify the benefits of electronic pulse-shaping and channel equalization for both pulse-amplitude modulations (PAMs), i.e., PAM-2 and PAM-4. We demonstrate that all these VCSELs support an error-free performance (bit error rate <10−12) at 54 Gb/s for PAM-2 and at 84 Gb/s for PAM-4 over 100 m of OM5 without forward error-correction (FEC) or receiver equalization. Furthermore, all VCSELs support 100-Gb/s PAM-4 transmission over 100 m of OM5 with FEC and receiver equalization. Importantly, all signal filtering investigated is readily implemented in SiCMOS.

VCSEL-Based PAM-4 Links up to 62 Gbit/s Over OM3, OM4, and WB-MMF: Performance Comparison at 850 nm and 1050 nm

We present an analysis of pulse-amplitude modulation 4 (PAM) based multimode fiber (MMF) links using VCSELs at 850 and 1050 nm across a wide range of fiber profiles with effective modal bandwidth spanning from 2 to 10 GHz·km. We demonstrate speeds >51.56 Gbit/s over a set of standard OM3 and OM4 fibers with bit error rate (BER) 50 Gbit/s using PAM-4. We perform VCSEL characterization and through a comparison with our analytic link model, identify various link penalties including those due to the relative intensity noise and mode partition noise of the VCSEL.

A Record 1-km MMF NRZ 25.78-Gb/s Error-Free Link Using a 1060-nm DIC VCSEL

We report successful achievement of a record long distance error-free transmission of 1000-m 50-μm-core multimode fiber (MMF) in a non-return-to-zero 25.78-Gb/s modulated vertical cavity surface emitting laser-based optical link using four-channel miniature parallel-optical modules. The optical power is centralized within the core of MMF by an active alignment assembly. The restricted launch condition gives a very high effective modal bandwidth (EMB) of >9 GHz · km based on the calculations and the required performance data specified in TIA-FOTP-220. An EMB of 11.5 GHz · km provides a very good agreement between the calculated and measured power penalty characteristics. Such a long distance low-cost MMF link, operating at 1060-nm wavelength is potentially an alternative solution to PSM4.

Statistical Study of Graded-Index Perfluorinated Plastic Optical Fiber

Plastic fiber-based optical links have been proposed for a wide variety of high-speed short reach applications. Graded-index (GI) multimode plastic optical fiber (POF) that supports 10 Gb/s over 50 m is commercially available and 40 Gb/s over 100 m has been demonstrated. However, unlike single-mode fiber and like silica multimode fiber (MMF), GI POF exhibits a range of performance that depends on launch condition and the accuracy of the index profile. Here, we report on the first statistical study of a large set of GI POF. We examine >; 40 samples of 62.5 μm core POF with four different cabling types that represent current-generation commercially available POF. We assess performance in terms of the 10 GbE requirements and quantify loss, chromatic dispersion, differential modal delay, effective modal bandwidth (EMB), and eye closure penalty. We find that nearly all of the measured 100 m fibers support the strict 10 GbE vertical-cavity surface emitting laser source EMB requirement, and that many fibers can support >; 40 Gb/s over 100 m. We thus demonstrate that GI POF can reliably support 10 Gb/s and larger bit rates to distances of 100 m.