Total Internal Reflection


Long distance transmission of optical signals over fiber optic cables is possible due to the phenomenon of Total Internal Reflection (TIR). When a wave, in this case light, encounters the boundary between materials with different refractive indices, the wave may be partially refracted at the boundary and partially reflected. However, if the angle of incidence is greater than the “critical angle”, the wave will not be refracted but will be totally reflected internally. This will only occur when the wave in a medium with a higher refractive index meets a boundary with a medium of lower refractive index. This is the case in fiber optic cables which are constructed from two types of glass, a core of higher refractive index and a cladding layer of lower.

Critical Angle and Snell’s Law

So, the “critical angle” is the angle greater than which total internal reflection occurs. The critical angle, Ɵc, a function of the refractive indices of the two boundary materials, is given by Snell’s law,

n1 sin Ɵi = n2 sin Ɵtsnells-law

Rearranging to solve for the angle of incidence,

Ɵi = arcsin (n1/n2 sin Ɵt)

The critical angle occurs when Ɵt = 90⁰ at which

sin Ɵt = 1. Solving for Ɵi, also the critical angle Ɵc,

Ɵc = Ɵi  = arcsin (n1/n2)


Willebrord Snellius (1580-1626), depicted here, is known in the English-speaking world as Snell.  Snell has long been credited with the law of refraction of light described above. However, this law was already known to a Persian by the name of Ibn Sahl in 984, over 600 years earlier, in the  court at Baghdad!

In the document On Burning Mirrors and Lenses, Sahl used the law of refraction to derive the correct shape of lenses that would focus light with no geometric aberrations.

Index of Refraction

The refractive index of a material is a function of the speed of light in that material. Light travels fastest in a vacuum, approximately 300,000 kilometers per second. The refractive index of a vacuum is, by definition, 1. The refractive index of a given medium is the ratio of the speed of light in a vacuum to the speed of light in that medium. It follows, the larger the index of refraction, the slower light travels in that medium.

In a standard singlemode fiber of the type used widely in communications networks, the cladding material is composed of pure silica glass with a refractive index of 1.444 and core of silica specifically doped to raise its refractive index to approximately 1.4475. Using Snell’s law to solve for the critical angle in such singlemode fibers,

Ɵc = Ɵi  = arcsin (1.444/1.4475) = 86⁰

Total Internal Reflection in Optical Fibers


The diagram above depicts the application of the law of refraction as it applies to fiber optic cables. The blue area represents the core of the fiber into which the signal is launched. Any light encountering the boundary with the cladding at less than the critical angle is refracted through the cladding (shown at the leftmost dashed vertical line). Exactly at the critical angle (in the middle of the diagram) the light travels straight down the boundary. At any incident angle greater than the critical angle, total internal reflection is the result (as shown above).

Interesting Total Internal Reflection Demonstration

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For more information about Fluxlight, please visit
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Ready for 40G but Not New Jumper Cables?

Networks and data centers have relied on 10Gbps connectivity speeds for the past 5+ years. With ever increasing network access rates (e.g., GoogleFiber and 5G Mobile) and the increasing trend to remote “Cloud” storage and computing, fiber connectivity rates are on the rise. While speculation abounds whether 40Gbps or 100Gbps (or something higher) will be the next-big-thing, 40Gbps transceivers in the QSFP (Quad Small Form-Factor Pluggable) package are a mature technology with thousands QSFP slots being shipped each month.

One thing slowing down adoption of QSFP-based 40G solutions for Short-Reach (SR) applications is that customers have had to both acquire new switching gear with the QSFP ports and switch out their interconnect cabling. Existing 10GBASE-SR SFP+ transceivers use OM3 multimode fiber terminated with LC connectors for interconnection. 40GBASE-SR4 QSFP modules are defined with an MPO-12 connector. Not only are MPO-12 jumper cables bulkier and stiffer, they also eliminate the possibility of a drop-in 40G replacement of a previous 10G installation. The lowest cost and most time-efficient upgrade for network installers/operators is to simply disconnect fiber jumpers from previous generation 10G SFP+ transceivers, slot in the 40G equipment, and plug those same jumpers right back in to 40G QSFPs.

Finally, a new 40Gbps bidirectional optical (BiDi) module provides a QSFP-based package compatible with previous 10Gbps fiber infrastructure. This 40-Gbps BiDi QSFP supports an aggregate of 40Gbps capacity over a single pair of OM3 MMF with LC connectors. Now network and datacenter operators have a migration path to 40G with no additional fiber cost. In fact, since each pair upgraded to 40G carries 4X the data, this technology achieves a 4:1 reduction in fiber cost!

Previous 40-Gbps Short Reach Transceivers
As defined in the QSFP MSA, previous short-reach (SR) transceivers for 40-Gbps connectivity in a QSFP form factor, such as QSFP SR4, use 4 parallel fiber strands in each direction, for a total of 8 active fibers per MPO-12 link. One result is 4 fiber strands in each cable are wasted. Figure 1 shows the cabling for existing short-reach 40-Gbps QSFP cabling.

Figure 1. Earlier 40-Gbps Transceivers (MMF, 40GBASE-SR4)

Beyond the four wasted fibers, the 12-fiber ribbon in larger and substantially more ridged than the typical 2mm duplex fiber jumpers used with previous generation 10Gbps SFP+ transceivers. This compounds an already often tightly congested cabling environment.

The 40-Gbps QSFP BiDi Solution
The QSFP BiDi Transceiver, Fluxlight PN: QSFP-40G-SR-BD (for Cisco compatible version) is a short-reach optical transceiver that delivers 40 Gbps over either a duplex OM3 or OM4 MMF connection. Connections of up to 100 meters on OM3 MMF or up to 150 meters on OM4 MMF are supported. The QSFP BiDi Transceiver is identified by a gray bail latch or pull tab. Figure 2 shows a Fluxlight QSFP BiDi Transceiver.

Figure 2. Fluxlight QSFP BiDi Transceiver (QSFP-40G-SR-BD)


The QSFP BiDi Transceiver combines the four 10Gbps data lanes from the host device into two 20-Gbps channels. Each of these 20Gbps channels are transmitted and received simultaneously on two wavelengths over a single MMF strand. The result is a full duplex link totaling 40-Gbps link over a single pair of MMF strands. Figure 3 shows is a depiction of these two bi-direction 20Gbps links forming a total 40Gbps on two fibers.

Figure 3. Bidirectional Transmit and Receive Concept of the QSFP BiDi Transceiver


For More Information
For more information about Fluxlight’s Cisco-compatible 40-Gbps BiDi Transceiver, please visit
For information about QSFP 40Gbps BiDi solutions compatible with other vendors equipment, or about transceiver in general, please call 888-875-7574 or email: or

Migration to 40Gbps & 100Gbps Ethernet


As new applications, devices and architectures continue to demand ever higher speed networks, 40Gbps and 100Gbps Ethernet links are rapidly coming on line. In fact, volume is starting to build and switch port as well as transceiver prices are dropping rapidly. If you have not stepped up to these new technologies there are a number of new concepts, components and terminology with which you will want to become familiar. Of course there are a number new form factors for optical transceivers supporting a variety of transmission distances and fiber types. These form factors are not the primary focus of this post (they will be presented in separate blog post). One major trend is the use of multiple parallel optical paths (including parallel fibers) to achieve the total 40Gbps or 100Gbps interface transfer rate. A second trend is using multiple wavelengths and integrated wavelength division multiplexing to achieve multiple paths but on single fibers.

Because of modal dispersion issues, at speeds of 25Gbps or more, it is very challenging to achieve reaches of more than 5 or 10 meters on multimode fibers. Therefore, most multimode transceiver implementations at 40G/100G use multiple parallel links each running at rates of 10Gbps or 25Gbps. While some may be familiar with the use of multi-fiber ribbon cables terminated with MPO connectors for interconnection between with fiber patch panels, these cables are now being used directly on 40G and 100G transceivers.  Another result of moving more to parallel paths is expanded use of Direct Attached Cables (DACs, both fiber and copper). While DACs were somewhat popular as a low-cost alternative to fiber transceivers at 10Gbps, there increased bulk was a significant concern in some applications, like high density data centers. With the advent of ribbon fiber with MPO connectors on 40G/100G optical transceivers, the ‘bulk’ difference between Copper and Fiber connections is substantially lessened.


Since 40/100G Ethernet multimode QSFP modules use parallel optics technology which requires data transmission across multiple fibers simultaneously, multi-fiber connectors are needed. MPO (often referred to as MTP which is actually a brand name from US Conec) is the specified connector type for multi-mode 40/100G Ethernet. The fiber cable used with these connectors is typically OM3 although singlemode (OS1) is also in what are referred to as PSM (Parallel Single Mode) interfaces. Of course these connectors and cables are backward is compatible with legacy 1Gbps and 10Gbps interfaces.

MPO2The MPO connector is quite robust and features a keying mechanism so the connector only mates in on position. A common problem with previous duplex connectors (like dual-LC or dual-SC) is reversing of transmit and receive. Since the MPO connector contains both transmit and receive fibers and can only be assembled in a single orientation, this problem should be eliminated. A nice feature of the MPO connector is the audible locking “click” which occurs when the connector is seated. Also, the connector retention mechanism is very reliable. Once seated the connector cannot be separated without firmly grasping and pulling back on the plastic sleeve (shown in aqua in figure above).

40GBASE-SR4 Ethernet uses a 12 position MTP/MPO connector interface. In this connector all twelve fibers are in a single row. The four leftmost fibers are used for transmit data, the middle four fibers are left unused, and the four rightmost fibers are used for receive data.

MPO Pins

Three options are defined for the 100GBASE-SR10 Ethernet interface. This first two, shown below, use two separate 12 position MTP/MPO connectors, one for all of the Tx fibers and the other for the Rx fibers. Neither of these options appear to be actually used in commercial implementations. The 3rd option utilizing a 24 position MPO has become the de facto standard in the marketplace. The MPO is arranged in two rows of 12. The middle 10 fibers of each row are used while the outermost fibers at each end of the rows are vacant. 10 fibers in the upper row for transmitting data and the remaining 10 fibers in the lower row for receiving data.

MPO Pins2


40G/100G DACs

Direct Attached Cables (DACs) are assemblies composed of a fixed length of copper or fiber cable with standard pluggable transceiver module(s) permanently fixed to each of its ends. There are many variations of DACs for a variety of 40G and 100G applications with the following being the most common:DACsThere are a number of other variations available including 100G QSFP28 to 10X 10G SFP+ breakout cables and a 100G QSFP28 to 4X 25G QSFP.

The primary advantage of using DACs versus paired transceivers with a length of cable is cost savings. The biggest disadvantage is the fixed length of the assemblies. In applications where location of equipment is carefully planned in advance and link lengths are known quite accurately and will remain fixed over the life of the installation (for example, a datacenter build) DACs can be a very economical alternative.


As IT infrastructures migrate to 40G and 100G speeds, network designers must carefully weigh alternative implementations of such links. Direct Attached Cables (DACs) are a low cost alternative to more flexible pluggable transceivers interconnected by fiber. New MPO connectors with their multi-fiber ribbon cables may be new an unfamiliar versus the ubiquitous LC and SC connectors of the past. However, by understanding the keying and pinout arrangements of MPO connections, reasonable multimode and low cost singlemode connections are possible even at 40G and 100G speeds.


Solution to your 10G Copper

10GBase-T words sm

Almost five years after SFP+ form factor 10Gbps transceivers emerged in volume, usable 10GBASE-T SFP+ transceivers are finally becoming available. While 10GBASE-T ports are quite commonplace on Network Interface Cards (NICs) and on a variety of network appliances (firewalls, storage devices, etc.), they are quite uncommon on the switches and routers to which many of these servers and appliance would like to connect. The reason is that these interfaces have consumed far too much power to operate in pluggable SFP+ slots. Of course this mismatch has led to numerous connectivity problems. All of the fixed 10BASE-T to SFP+ 10G ports cases (shown in blue) in the figure below have been impossible without some kind of external device, perhaps an expensive media converter, involved.


With the availability of copper 10G SFP+ modules, all of the above scenarios are straightforward. Simply plug the 10GBASE-T SFP+module in an available slot and connect with a CAT-6A/7 cable.

Fluxlight 10Gbps Copper SFP+ Key Features

  • Supports Links up to 30m using Cat 6a/7 Cable
  • SFF-8431 and SFF-8432 MSA Compliant
  • IEEE 802.3az Compliant
  • Low Power Consumption (2.5W MAX @ 30m)
  • Fast Retrain EMI Cancellation Algorithm
  • Low EMI Emissions
  • I2C 2-Wire Interface for Serial ID and PHY Register Access
  • Auto-negotiates with other 10GBase-T PHYs
  • Supports 100/1000Base-T using Cat 5e cable or better
  • MDI/MDIX Crossover
  • Multiple Loopback Modes for Testing and Troubleshooting
  • Built-in Cable Monitoring and Link Diagnostic Featureso
    • Cable Length Measurements
    • Opens/Shorts
  • Robust Die Cast Housing
  • Bail Latch Style ejector mechanism
  • Unshielded and Shielded cable support



Introduction to BiDi Optical Transceivers


In the past few years a new class of pluggable optical transceivers have been developed that send and receive optical signals end-to-end over a single fiber strand. This reduces by half the amount of fiber required for that same total data transmission. This factor-of-two improvement can lead to substantial cost savings especially in campus environments with large numbers of connectivity endpoints.

Bi-Directional transceivers, called BiDi’s for short, use two different wavelengths to achieve transmission in both directions on just one fiber. The modules are deployed in pairs, one for the upstream (“U”) direction and another for the downstream (“D”). The standard defining these parts is the IEEE 802.3ah Gigabit Ethernet 1000BASE-BXnn (nn= transmission reach in kilometers) specification for point-to-point Ethernet in the First Mile (EFM) applications.



The value of the BiDi solution derives from the reduction in the use of fibers by a factor of two. There are many situations in real-world networks where this reduction is extremely important if not absolutely required. As mentioned above, the IEEE802.3ah specification defining BiDi’s mentions point-to-point Ethernet in the First Mile (EFM) applications. In addition BiDi transceivers can be of great use in any situation where only limited fibers or limited conduit space is available. Other common applications include: digital video and Closed Circuit TeleVision (CCTV) applications and high-density switch-to-switch port interconnection.

BiDi Technology…how they work…

As mentioned above BiDi transceivers are deployed in matched pairs, one for the upstream (“U”) direction and another for the downstream (“D”), each part transmitting at a different wavelength. The figure below depicts the details of such a matched set of BiDi transceivers. In this example, the two wavelengths utilized by the BiDi pair are 1310nm and 1490nm. Typically the “Upstream” or “U” transceiver transmits at the shorter of the two wavelengths and the “Downstream” or “D” module the longer wavelength.


The key additional technology present in BiDi’s that is not present in standard 2-fiber transceivers is the “Diplexer”. The Diplexer acts simultaneously couples the locally transmitted wavelength onto the single fiber while “splitting” off the received wavelength so it is directed at the receiver.

Economic Case for BiDi’s

The value of the BiDi solution derives from the reduction in the use of fibers by a factor of two. There are many situations in real-world networks where this reduction is extremely important if not absolutely required. As mentioned above, the IEEE802.3ah specification defining BiDi’s mentions point-to-point Ethernet in the First Mile (EFM) applications. In addition BiDi transceivers can be of great use in any situation where only limited fibers or limited conduit space is available. Other common applications include: digital video and Closed Circuit TeleVision (CCTV) applications and high-density switch-to-switch port interconnection.

The simplest economic case for BiDi’s is probably a campus environment requiring fiber connectivity to a large number of endpoints. For example, most universities campuses are spread over a fairly wide area and required high-speed (read: fiber) connectivity between campus core resources (e.g., databases, computing resources, common internet access, etc.) and a large number of classrooms, dorm rooms, and faculty and administrative offices. The following is a simple economic model to demonstrate the savings possible in such an environment using BiDi versus standard 2-fiber transceivers.

So, for a campus environment where average link length is greater than 800 feet, the BiDi solution is the right decision. In an real world example, a large university campus lighting 400 GbE fiber links with an average length of 1600 feet used BiDi’s to save $32,000 versus using 2-fiber transceivers.

Fluxlight’s BiDi Offering

FluxLight, Inc. offers BiDi transceivers in the SFP form-factor supporting 1GbE for all major switch brands (e.g., Cisco, HP, Juniper, Extreme, Brocade-Foundry, etc.) and for most 2nd and 3rd tier brands. 10GbE SFP+ BiDi’s are also available for most of the larger brands. If the switch vendor you use is not in this list, please contact us as we can generally cross-reference one of our solutions to your switch.

We offer a 1Gbps SFP BiDi’s to cover a wide range of distances including: 10km, 20km, 40km, 80km and 120km, all of which are ROHS compliant. To aid in turn-up and maintenance of BiDi links, all FluxLight BiDi transceivers support Digital Diagnostics Monitoring (DDM as defined in standard SFF-8472) allowing real-time monitoring of parameters of the SFP, such as optical output power, optical input power, temperature, laser bias current, and transceiver supply voltage. Our 10Gbps BiDi transceivers are available supporting reaches of 20km, 40km and 60km.

Global Fiber Optic Market to reach $4.4 billion by 2021

With a compound annual growth rate of 7.40% from 2015 through 2021 and an increasing demand for high bandwidth, fiber optical components will be rising in demand for the years to come. As the increase in demand for cloud based services increases as well, so will the amount of fiber components in data centers. The advances in medicinal technology have also led to a major increase in demand for fiber optics in the health sector.

The main fiber optic component markets types are LC, SC, ST, MPO/MTP and a few others. The main industries in demand for fiber optical components are the Telecom Industry, Datacom (Data Centers), DWDM systems, Lasers, and now Medicine Technologies.

Geographically, the fiber optic market has been established in North America, Latin America, Europe, Asia Pacific, the Middle East, and Africa. In 2014 it was North America that reigned supreme with revenue in the fiber market.

To take a look at results for 2015 and projections by the Transparency Mkt. Research group, request a sample study here.

Broadband in America

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FCC Broadband in America results