From Then to Now (and Beyond): The Progression of Multimode Optics

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Multimode optics, including multimode VCSEL-based transceivers and multimode fiber, have been dominating data center short-reach links. Why? Because they can offer:

• Lower link costs
• Less power consumption
• Higher resistance to fiber misalignment and dirt at connections

There are innovations in multimode optics on the horizon that will address many challenges, helping maintain and improve the appeal of multimode optics in the years to come. Let's discuss them here.

VCSEL: The Light Source of Multimode Optics

VCSEL stands for 'vertical-cavity surface-emitting laser.' Because of its low cost, low power consumption and ability to be produced at high volume in production facilities, it is the light source used for multimode optical transmission.

VCSEL Structure

A VCSEL is typically comprised of 40 to 60 layers of alternating semiconductor materials, each λ/ 4 deep; the bottom and top mirrors of the cavity are made with distributed Bragg reflectors (DBRs).

Economic Advantages of Multimode Optics

When compared to singlemode optics, multimode optics continue to be the cost-effective choice for shorter-reach data center applications. The cost of multimode fiber cable is higher than that of singlemode fiber cable, but multimode transceivers are what bring the price down:

1. VCSEL costs are very low compared to singlemode laser sources. According to LightCounting research, the price for 10G multimode SFP+ (300 m/400 m reach) was 2.6 times lower than 10G singlemode SFP+ (10 km reach) in 2014. That price difference may come closer in the next few years (2017 to 2020), but it will still be at least two times less as per the forecast.
2. More than 65% of optical transceiver volume was made up of multimode transceivers for 0 m to 300 m reach in 2014. We expect this number to be above 60% in the next five years.
3. As reported by various fiber cable vendors, OM3 and OM4 multimode fiber are currently used as the main trunk cable installed in data centers; more than 90% of multimode fiber cable link lengths are <100 m.
4. Since 2015, some hyperscale data centers have deployed singlemode fiber infrastructure in their gigantic footprints. Despite reach limitations, multimode transceivers and multimode fiber cable can fulfill the majority of data center reach requirements (up to 400 m for 10G and 40G, and up to 100 m for 100G) in enterprise data centers, multi-tenant data centers and Tier 2/Tier 3 cloud data centers

Technical Advantages of Multimode Optics

As technology improves and demand increases, price comes down. There are many technical advantages and technology improvements offered by multimode optics that increase efficiency and reduce costs even further:

1. VCSEL has very high electrical-optical slope efficiency; hence, it is very power efficient.
2. VCSEL devices, with emission aperture diameters of ~5-15µm and in the 850 nm wavelength range, can be easily coupled with multimode fiber (with a core diameter of 50 µm) with low loss and high misalignment tolerance.
3. VCSEL devices can be tested directly on wafer and don't require hermetic packaging or optical isolators like singlemode laser sources do; therefore, the costs are much lower.
4. During the past 15 years, multimode fiber cabling systems, together with VCSEL transceivers, have been the most popular and cost-effective short-reach interconnect solution. These types of systems are extensively deployed in data center environments.

New Challenges for High-Speed VCSEL

Despite the success of multimode optics, ever since Gigabit VCSEL-based transceivers were produced, speculations have always indicated that VCSELs could not reach higher speeds.

Thanks to industry-wide efforts, the 10G VCSEL has become a commodity (the cost of a 10G VCSEL is less than $1). The 25G VCSEL started high-volume production in 2016. Researchers have already shown directly modulated VCSEL operation of up to 71G with digital signal processing. Currently, the commercial 50G VCSEL is being developed.

Although operating speeds could be improved by a new VCSEL structure and material science, multimode optics also encounter another technical hurdle: bandwidth limitations due to modal and chromatic dispersion in multimode fiber.

• The reach in OM4 multimode fiber has been reduced from 400 m at 10G to 100 m at 25G using the non-return-to-zero (NRZ) modulation format.
• If operating 50G VCSEL with NRZ, then the reach could be further reduced to 50 m, which would make VCSEL a much less favorable choice for next-generation high-speed links.

New Horizons for Multimode Optics

Admitting the challenges associated with higher-speed multimode optics, the industry and standards organizations, such as IEEE, TIA and ISO/IEC, have been working together to maintain the appeal of multiple optics, particularly for next-generation data center applications that will no doubt have growing bandwidth requirements.

• Digital signal processing (DSP) is an advanced technique that compensates for signal distortion and removes noise with the help of a CDR. Forward error correction (FEC) encoding is a mathematical way to recover the signal from detection error, with a defined max bit error rate (BER) threshold. Both DSP and FEC have been incorporated into next-generation VCSEL-based transceivers to overcome bandwidth limitations.
• The IEEE 802.3cd taskforce has adopted PAM4 modulation for 50G data transmission in multimode optics, with a signaling rate of 26.5625 GBaud, which paved the way for next-generation 50G, 100G and 200G multimode optics and keeps supporting a minimum reach of 100 m in OM4 multimode fiber.
• An exciting new approach to improve data rates carried over single multimode fiber is to use shortwave wavelength division multiplexing (SWDM). The newly developed OM5 (wideband) multi mode fiber is an ideal fiber choice for these applications to lower fiber count and improve fiber utilization and permanent link longevity.
• Some proprietary vendor solutions support longer reach, e.g. 100GBASE-eSR4 can support up to 300 m in OM4, and 100G-SWDM4 supports up to 150 m in OM5. This extended reach can cover the majority of enterprise and multi-tenant data center applications.
• If your organization is planning next-generation Ethernet deployment, and has unanswered questions, please contact Belden, your trusted data center partner, for assistance.