OM4 Fiber Research Articles

26.5625 Gbaud PAM-4 short-reach transmission at 75°C using 850 nm VCSELs with strategic oxide guiding layer positioning.

This article presents an all-epitaxy approach to reduce the root mean square spectral width (ΔλR M S) of 850 nm oxide-confined vertical cavity surface-emitting lasers (VCSELs) with a large aperture of 7 µm through strategic optimization of the oxide guiding layer within the epitaxy structure. At 75°C, the VCSEL demonstrates a ΔλR M S of ∼0.3 nm at a bias current of 7.5 mA. Furthermore, the VCSEL achieves successful transmission of 26.5625 Gbaud PAM-4 modulation over a short-reach (SR) OM4 fiber link while maintaining a TDECQ budget below the 4.5 dB specified by 50G IEEE Ethernet standards.

Noise-like pulses generation and its beam self-cleaning effect in an all-fiber multimode cavity with a figure-8 scheme

The beam profiles of multimode mode-locked fiber lasers are highly multimode, the speckled beam profiles limit their potential application. To improve the beam profiles of multimode mode-locked fiber lasers, we proposed an all-fiber figure-8 multimode cavity and experimentally demonstrated the generation of noise-like pulses (NLPs) and beam self-cleaning of NLPs in the cavity at 1.5 µm. The laser was mode locked by a nonlinear amplifying loop mirror, an Er/Yb co-doped fiber provided gain, and 50 m OM4 fiber introduced enough nonlinear phase shift difference. The scheme generated NLPs with 159 mW average power, 2.78 MHz repetition rate and 57.19 nJ pulse energy. The packet duration of the NLPs increased from 0.61 ns to 2.99 ns when the pump power was increased from 2 W to 9 W. Nonlinear beam self-cleaning with a near Gaussian beam profile output was achieved in the multimode fiber laser. The M2 was measured to be 2.41 at the pulse energy of 6.83 nJ. We also obtained spatiotemporal mode-locked pulses with 1.03 ps duration and speckled beam profiles. This is the first experimental demonstration of NLPs in multimode fiber lasers, and beam self-cleaning of NLPs was observed for the first time. The results enriched nonlinear dynamics in multimode fiber lasers.

2.4-Tb/s/λ*km fundamental mode transmission over OM2 fiber employing Kramers-Kronig receiver.

We propose a digital-carrier Kramers-Kronig (DC-KK) scheme based high-speed multimode fiber short-reach optical interconnect system with fundamental mode transmission. After optimization of the parameters, including the roll-off factor of the root-raised-cosine (RRC) filter, and the guard interval (GI) between signal and carrier tone, as well as the carrier signal power ratio (CSPR), 200-Gb/s 32-quadrature amplitude modulation (32QAM) signal transmission over 12-km OM2 fiber has been experimentally demonstrated with a bit error ratio (BER) below the soft-decision forward error correction (SD-FEC) threshold of 4 × 10-2. To the best of our knowledge, this is the highest experimental record of single lambda bitrate-distance-product (SLBDP) achieved by direct-detection (DD)-based transmission over a standard multimode fiber (MMF). The proposed scheme has potential to improve the system performance without replacing massive deployed legacy MMFs for future large-capacity data center interconnects (DCIs).

C-band coherent transmission at 1.6 TB/s over 3.5 km of OM4 fiber using a fundamental mode operation

We demonstrate the highest, to the best of our knowledge, capacity obtained for an OM4 fiber reaching 1.6 Tb/s for 3.5 km of OM4 fiber. The result is obtained using a highly selective modal multiplexer exciting the LP01 mode and a state-of-the-art coherent transmission setup operating in the C-band. The results indicate that there is no need to replace legacy optical fibers to increase their capacity, which may be an extremely important factor when considering the costs of upgrading existing networks.

Coherent Communication Over Multi Mode Fibers for Intra-Datacenter Ultra-High Speed Links

Multimode fibers (MMFs) links using vertical cavity surface-emitting lasers (VCSELs) are still the solution of choice for today’s data center shorter distances thanks to their low cost and robustness. Currently, the market is moving towards 400G-capable systems using multiple lanes at 50 Gbps per lane, based on intensity modulation (IM) and direct detection (DD). A technology switch will probably be required to overcome the 100 Gbps per lane limit that will likely be the bottleneck of future IM-DD MMF-based links. In this manuscript we propose an intra-data center system based on coherent detection and MMF, analyzing experimentally and analytically the performance of polarization multiplexed coherent communication. We show that the advantages enabled by the coherent technology can be fully exploited in an MMF-based link when central launch is ensured. In particular, we study in detail the effect of lateral offsets introduced by the connectors at the interface between two MMF facets, in terms of net optical loss and signal-to-noise ratio (SNR) degradation. A statistical analysis is performed analytically exploiting a large database of OM3 and OM4 fibers modal delays and results are presented for various combinations of fiber length, type and number of MMF-to-MMF connections along the link. We show quantitative results on the maximum acceptable lateral offsets in the MMF connectors, highlighting that in practical conditions average offsets of up to 3 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> m can be tolerable.

2.9 K VCSEL demonstrates 100 Gbps PAM-4 optical data transmission

A cryogenic packaged vertical-cavity-surface-emitting laser (VCSEL) with microwave input and coupled with OM4 fiber for optical output was developed and demonstrated laser operation from 295 to 2.9 K. Record 2.9 K optical data links of 100 Gbps pulsed amplitude modulation 4-level have been demonstrated with an energy per bit ∼153.4 fJ/bit when TDECQ = 3.79 dB. The energy/bit is limited by the link path bandwidth resulted from the input cable loss to 2.9 K VCSEL and can be further improved. Thus, the VCSEL optical data link is proven to be a viable solution for cryogenic communication and computing.

A Converter for Transmission With Singlemode Performance on OM2/3/4/5 Multimode Fibers

A converter from a singlemode fiber to the fundamental mode of an OM2, OM3, OM4, or OM5 type multimode fiber is presented and is used to transmit 100 GbE with no detected errors over 1600 m multimode fiber using commercial transceivers. The converter is based on splicing a short length of few-mode fiber between the singlemode fiber and the multimode fiber. The few-mode fiber is designed so that the fundamental mode of the few-mode fiber has a good mode overlap with the fundamental mode of the multimode fiber. To suppress any higher order modes in the few-mode fiber, an optimized bend based mode stripper is used. The converter has a low loss of 0.35 dB, lower than previous reported solution. The converter do, opposite to previous suggested solutions, not include any free space part or mechanical connections, which assures low loss and cross talk as well as good long term stability.

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.

Ultra-low jitter clock distribution for the trigger electronics of the New Small Wheel for the ATLAS experiment

The Large Hadron Collider (LHC) at CERN plans to have a series of upgrades to increase its instantaneous luminosity to 7.5 × 1034 cm−2 s−1. The increased luminosity drastically impacts the ATLAS trigger and readout data rates. The inner-most station of the ATLAS muon spectrometer, the so-called Small Wheels is being replaced with a New Small Wheel (NSW) system, consisting of Micromegas and small-strip Thin Gap Chambers (sTGC) detectors. The on-detector radiation levels required radiation tolerant electronics. The lower radiation levels on the rim of the NSW allowed utilizing commercial electronic chips, such as Field Programmable Gate Arrays (FPGAs), in the trigger chain of the sTGC detectors. Those FPGAs require an ultra-low jitter clock for the proper operation of their Gigabit transceivers (4.8 Gb/s serial links). The initial design was based on a clock provided by a radiation tolerant ASIC designed at CERN, but due to its intrinsic jitter and consequent marginal error rate on the transmission lines, a different approach had to be chosen. An additional clock source based on commercial jitter cleaners, fanout chips and optical transmitters driving dedicated fibers was built. The new scheme provides 64 low-jitter clocks (32 main and 32 redundant) from the radiation-protected area (USA15) to the trigger electronics over 60 m of OM3 fiber.

Superconducting Processor Modulated VCSELs for 4K High-Speed Optical Data Link

Superconducting processor based on single flux quantum (SFQ) technology is a promising solution for cryogenic computing with high-speed operation at extremely low power. However, energy-efficient data link from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 4\text{K}$ </tex-math></inline-formula> operation to room temperature (RT) users has been remained as a key challenger to be resolved. In this work, we report the 5K performance of 51.56 Gb/s PAM-4 optical data link from a VCSEL, coupled by free-space optics inside cryostat to RT receiver through cryostat optical window. Finally, a superconducting bit pattern generator (BPG) is used to modulate a VCSEL at 4 K: 12.5 Gb/s NRZ error-free optical data transmission and 20 Gb/s NRZ open eye-diagrams are demonstrated via an OM4 fiber coupled directly to the VCSEL at 4 K inside cryostat and linked to the outside receiver at RT.

Fundamental mode transmission around 1310-nm over OM1 and OM2 multimode fibers enabled by a universal fiber modal adapter

Recently there has been an interest in upgrading legacy multimode fibers such as 50 μm OM2 fiber to higher data rates through single mode transmission. A fiber modal adapter hosting a modal conditioning single-mode fiber (MCSMF) has been demonstrated as one robust solution to convert OM2 links into single mode transmission links for higher data rate transmission. Legacy multimode fibers also include OM1 fiber which has higher index contrast between the core and cladding and a larger core of 62.5 μm diameter. In this paper, we conduct a study to understand the MCSMF mode field diameter and connector offset tolerance requirements for OM1 fiber, which affect the transmission performance of single mode transmission. We find that despite very different refractive index profiles between OM1 and OM2 fibers, the fiber modal adapter concept is still applicable to OM1 and only one modal conditioning single-mode fiber design is needed to accommodate the needs for both fiber types. The detailed tolerances to the mode field diameter mismatch and the connector offset are numerically studied. The transfer functions for 1000-m of OM1 and OM2 with the adapters are measured, which show very high modal bandwidths at tens of GHz in contrast to very low modal bandwidths without the adapters. Transmission at 25 Gb/s using a 25G LR transceiver, 100 Gb/s using a 100G CWDM transceiver, and 400 Gb/s using a 400G LR transceiver are demonstrated with error free performance and open eye diagrams over 1000-m of OM1 and OM2. We find that the approach of using MCSMF can support fundamental mode transmission up to 100 Gb/s over this distance without obvious limitations.

Statistical Analysis of 100 Gbps per Wavelength SWDM VCSEL-MMF Data Center Links on a Large Set of OM3 and OM4 Fibers

We present a detailed statistical study on achievable reach of 100 Gbps data center optical links based on vertical cavity surface emitting lasers (VCSEL) and multimode fibers (MMF). Based on the characterization of the spectral and spatial properties of eight lasers and of the modal and dispersion behavior of a large set of 20233 OM3 and OM4 modeled fibers (obtained by properly extending an initial set of 500 measured fibers), we compute the resulting frequency responses of all of the VCSEL-MMF combinations. Then, we feed them to a numerical tool modeling PAM-4 transmission at 100 Gbps net bit rate per wavelength. Our model analyzes performance at distances up to 400 meters, using three different adaptive equalizers at the receiver and considering two forward error correction overheads. We show that 100 Gbps operation is feasible for 99% of the simulated links reaching up to 120 m over OM4 at 850 nm and using a decision feedback equalizer (DFE). Aggregated data rates of 200 Gbps and 400 Gbps per fiber using Shortwave Wavelength Division Multiplexing (SWDM) are achievable for 99% of the links reaching 80 m over OM4 using two wavelengths and feed-forward equalizer (FFE) and four wavelengths and maximum likelihood sequence estimation (MLSE)-based equalizer, respectively.

2.6 K VCSEL data link for cryogenic computing

Cryogenic computing requires an energy-efficient optical data link from 4 K to the end-user at room temperature. Laser spectra and light–current–voltage family curves over a wide temperature range from 2.6 to 295 K are reported on oxide-vertical cavity surface emitting lasers coupled with OM4 fiber. Non-linear shifting of the lasing wavelength vs junction temperatures is measured with the wavelength shifting coefficients &amp;lt;0.005 nm/K at 2.6 K. 12.5 Gb/s non-return-to-zero error-free data transmission at 2.6 K to the error detector at 295 K is demonstrated as a viable solution for cryo-computing.

Large core multimode fiber with high bandwidth and high connector tolerance for broadband short distance communications

We propose a large core multimode fiber with 100-µm core diameter for short distance communication that is compatible with existing transceivers designed for 50-µm diameter core multimode fibers. The fiber exhibits a bandwidth that is over four times higher than the 50-µm OM4 fiber, low bending loss, and large connector offset tolerance. We have studied the bandwidth capability through both detailed modeling and experiments. A prototype fiber was fabricated, and it achieves 24 GHz km peak modal bandwidth in the 850 nm window, and the wavelength window with bandwidth above 5 GHz km is more than 100 nm. The bending losses of the fiber measured at 850 and 1300 nm with an encircled flux launch condition exceed the requirements for bending insensitive multimode fibers. The fiber has much higher connector offset tolerance compared to conventional 50-µm diameter core fibers as has been exhibited by modeling and experimental study. The additional loss caused by up to 10-µm connector offset is only 0.3 dB. Transmission experiments using 25 G SR and 100 G shortwave wavelength division multiplexing transceivers over such a fiber show error-free performance over 500 and 300 m, respectively. The proposed large core multimode fiber can address versatile needs in short distance communication including the bandwidth for high data rate transmission and a wide wavelength window for wavelength multiplexed transmission. The large connector tolerance and low bending loss can also enable low cost, compact connectivity solutions with passive alignment and ease of handling.

Development and Characterization of 100 Gb/s Data Communication VCSELs

Multi-mode vertical-cavity surface-emitting lasers (VCSELs) with improved bandwidth and noise performance have been developed for 100 Gb/s data communication links over multi-mode fiber. The VCSELs are designed for the peak multi-mode fiber bandwidth at 850 nm to maximize reach. The small-signal modulation 3dB bandwidth exceeds 25 GHz over temperature and relative intensity noise is below -145 dB/Hz. Transmission of 53.125 GBd PAM-4 signals over a 100m OM4 fiber is demonstrated by using equalization on both transmit and receive side. The transmitter and dispersion eye closure quaternary (TDECQ) penalty is below 4 dB for VCSEL operating up to 75°C.

Modal-Chromatic Dispersion Interaction Effects for 850 nm VCSEL Channels at 100 Gb/s per Wavelength

We present a theoretical model and experiments to estimate and measure the magnitude of modal and chromatic dispersion interaction (MCDI) over multimode optical channels using vertical-cavity surface-emitting laser (VCSEL) based transceivers. The effects of MCDI and bit error rate performance are also evaluated for PAM-4 signals at data rates from 100 Gbps to 112 Gbps. A section of the article is dedicated to the characterization of the spectral coupling between VCSEL and fiber, including a multimode and a VCSEL with a dominant fundamental mode that has shown the potential to increase the reach of MMF channels. For the experiments, several types of OM4 and OM3 fibers, with accurate bandwidth characterization over a broad range of wavelengths were utilized. The reported analysis applies to future Ethernet and Fibre Channel applications requiring ≥100 Gb/s per wavelength over multimode fibers.

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.

Terabit Mode Division Multiplexing Discrete Multitone Signal Transmission Over OM2 Multimode Fiber

In this paper, we experimentally demonstrated Tb/s mode division multiplexed (MDM) transmission over 20m OM2 multimode fiber (MMF) using three linearly polarized modes, wavelength division multiplexing, intensity modulation and direct detection (IM-DD). The multi-plane light conversion (MPLC) based mode multiplexer/demultiplexer were adopted and its performance has been characterized. Three mode channels with the mode crosstalk less than −20 dB were selected to implement the MDM-MMF based transmission. Each mode channel was modulated by discrete Fourier transform spread discrete multi-tone (DFT-S DMT) scheme with 2048 subcarriers via 40 GHz Mach-Zehnder modulator (MZM). We obtained the error-free transmission of 135 Gb/s per mode per wavelength and total 1.62 Tb/s WDM-MDM below the forward error correction (FEC) limit of 3.8 × 10−3 at the received optical power (ROP) of > − 4.5 dBm with 86 GSa/s digital to analog converter (DAC). To the best of our knowledge, it is the highest bit rate per channel for the MDM based on conventional OM2 fiber transmission rather than the mode group division multiplexing (MGDM) based with cost-effective IM-DD and no multiple-in-multiple-out (MIMO) technology. The experimental results show that the MDM-MMF based transmission has the potential in the high-speed large-capacity short-reach optical interconnects.

850-nm Single-Mode Vertical-Cavity Surface-Emitting Lasers for 40 Gb/s Error-Free Transmission up to 500 m in OM4 Fiber

A single-mode (SM) vertical-cavity surface-emitting laser (VCSEL) constructed using a multi-oxidation structure was demonstrated. With the help of an undoped bottom mirror for reduction of optical absorption, the SM-VCSEL possessed a threshold current of 0.6 mA. A pure single mode is observed at an operating current of 4 mA. For back-to-back transmission, the maximum bandwidth is 26.6 GHz, and the highest bit-rate is 48 Gb/s for on-off keying modulation. Under single-mode operation, 40 Gb/s error-free transmission up to 500 m in OM4 fiber was achieved in this work.

28-Gbps interconnect based on a plastic optical engine providing reduced modal dispersion

A plastic optical engine that reduces modal dispersion was proposed and implemented for a 28-Gbps interconnect; its dependence on the angular distribution of the focused beam for a transmitter was examined. The beam coupler was designed using two focused beam numerical apertures (FNAs) for transmitter. The modal dispersion of the smaller FNA was superior to the larger FNA by 6.2 ps at the backshell temperature of 75 °C for a 150-m OM3 fiber. The manufactured optical engine with smaller FNA supported effective fiber bandwidth for error-free data transmission over a 150-m-long OM3 fiber at 28 Gbps. Decreasing the FNA suppresses modal dispersion, thus enabling the use of longer OM3 fibers.