Fiber Optic Connectors:
Expanded Beam vs. Physical Contact
By Soren Grinderslev, Ph.D.,Tyco Electronics

Ruggedness has always been a challenge to fiber optic systems. The thin glass strand transmitting the optical signal is well protected within the cable, but the exposed fiber ends transmitting the signal through a mating interface are susceptible to contamination and mechanical damage. For protection and alignment reasons, the fiber is usually terminated via an epoxy bond to a precision ceramic ferrule, followed by an end face polish. Generally, there are two main methods of transmitting an optical signal between two fibers:

1. Physical contact connectors

2. Expanded beam connectors

The following review focuses on a comparison of these two concepts

Physical Contact
The first method makes use of physical contact (PC) between two fibers. In this concept, the ferrules are mated within a precision sleeve to assure radial alignment between the two ferrules in order to minimize optical misalignment losses. The optical beam will exit the fiber core in a cone-shaped diverging manner (figure 1).

Thus, to captivate the emitted beam and to eliminate optical losses, the receiving fiber must be in close proximity, preferably in physical contact, with the emitting fiber (figure 2). The PC concept uses spring force to assure that the fibers are always in physical contact. The design allows the ferrules/sleeve arrangement to float relative to the external housing.

There are many different types of PC connectors. The simplest ones are one-channel connectors such as LC, ST, FC, and SC connectors. The ferrules are either a 1.25 or a 2.5mm ceramic ferrule. Their housings consist mostly of low-cost molded polymeric components. Even though these connector types require mating in an adaptor, they are the cheapest solution and can provide excellent optical performance. Their applications, however, are limited to indoor use in a controlled and friendly environment. Often the application is a simple separable connection within a larger system, or an application in which the connector is mounted on a PC board coupled to an active component.

Multi-fiber PC connectors are found in the same category. These can be the MT-style connectors with rectangular ferrules made from a composite material, which have the fibers linearly spaced on a 250mm pitch and in several rows for the higher fiber counts. They come with 8-, 12-, 24-, and 72-channel counts, but have the same application limitations as the single-channel connectors.

The ferrule assembly and mating sleeves of both types of PC connectors can also be designed into a sealed, rugged, circular-style connector, very similar to those known from electrical connectors. Most notably is the MIL-C-38999 series III and M28876 circular shell connector, which can contain up to 72 fiber channels. Due to the rugged design, they are suitable for a wide range of applications in indoor as well as outdoor applications and are therefore subjected to a more challenging environment. Examples of discrete termini are shown in figures 3 and 4.

Expanded Beam
The second method introduces a beam expansion/contraction at the respective fiber end faces and allows for an air gap in the optical pathway (figure 5).

The expanded beam concept makes use of optical lenses (typically a 3mm ball lens) to expand and collimate the beam emitted from the launch fiber. The expanded beam remains collimated across the mechanical interface planes until the receiving lens focuses the beam onto the receiving fiber. For each connector half, the ferrule is fixed relative to the lens inside a housing, which aligns with a similar lens housing located in the mating half via pins. The mating force of the lens housings is maintained with wave springs, and the unit is flexibly suspended within the connector shells via O-rings.

The most popular EB connector types are PRO BEAM connectors from Tyco Electronics, which are available in three different sizes: PRO BEAM Mini connectors, PRO BEAM Jr. connectors, and PRO BEAM Sr. connectors (see figure 6). Channel counts for the EB design are 1, 2, 4, and 8. The PRO BEAM Mini connector version is also used with a MIL-C-38999 Series III, size 11 circular shell (see figure 7), and in the ARINC 600 connector, which can hold up to 128 EB channels. Since these connectors are used in rugged environments, they are usually terminated to a MIL-Tactical-type cable, as well as an avionic/flight grade cable.

The simplicity of the PC design provides a lower SM insertion loss (typically 0.2dB) than what is achieved by the SM EB concept (typically 0.7dB). A major advantage of the EB concept, however, is that there is no mechanical contact between the optical elements of the two mated connectors, and there is no need for use of fragile alignment sleeves. This makes the concept especially suitable for use in rugged environments, which are subjected to vibrations such as avionics, industrial, and military applications.

Figure 8 shows the Tyco Electronics connector with LuxCis optical termini in a general purpose rectangular shell, as well as LC-based ODVA-conforming connectors and the PRO BEAM connector series designs. Connectors including combinations of both concepts in the same shell are also available. 

In summary, the advantages of the beam expansion are:

• No contact between optical elements of the mated connectors

• Less sensitivity to soil and contamination particles

• Connector misalignment and vibration have little effect on the signal loss

• Consistent performance during repeated matings

Fresnel and Return Loss
When an optical signal crosses from one transparent medium to another, a fraction of the light is reflected at the interface. These types of reflections are called Fresnel reflections. As an example, a transition from a glass fiber to air will result in a Fresnel reflection loss of 0.17dB. If the reflected energy is large enough, it can result in instabilities and quality reduction of the signal from the laser source.

This is one reason why it is advantageous for PC connectors to have physical contact between the fibers. Because the mated fibers are made from the same material, they have the same refractive properties; hence, there are practically no Fresnel reflections observed from the mating interface.

For the EB design, the optical signal must cross several optical surfaces which have different refractive indexes, resulting in a substantial Fresnel loss. To remedy this, anti-reflective (AR) coatings are commonly applied to these surfaces. Good AR coatings can practically eliminate the Fresnel loss.

The ratio of reflected to incident optical power is termed Return Loss (RL). The RL is measured in dB and describes the amount of power returned from a refractive index discontinuity relative to the amount incident on that interface. The RL is a positive number, with higher numbers indicating better performance (less reflected signal). Analytically, the RL can be expressed as:                    

RL(dB) = 10log[((n₁-n₂)/(n₁+n₂))²]

Here, n₁ and n₂ represents the index of refraction in medium 1 and medium 2, respectively.

Tyco Electronics manufactures an expanded beam connector series in which the SM fiber is in physical contact with the AR-coated lens. This combination typically yields a mated RL >31dB. By comparison, the PC connection easily yields a mated RL of 45 - 50dB, if a proper polish has been applied.

While the anti-reflective coating of the lenses in the EB design will eliminate most of the reflections, the performance depends on wavelength, unlike the PC interface. It is necessary to apply the AR coating to specific wavelengths or wavelength intervals; for example, SM at 1310nm, 1550nm, or dual 1310nm-1550nm.

A special case to consider is if the connectors are disconnected with the power still on. For the EB launch connector, the RL will remain above 34 dB, while the PC launch connector will drop to 15dB and send a strong and potentially damaging Fresnel reflection back towards the source.

A common way to avoid the unmated low RL for the PC connector is to apply an angle polish (APC), often 8 degrees, to the ferrule end faces. The angle on the end face will cause most of the reflected light to be directed into the fiber cladding, where it will die out instead of being guided down the fiber core. If this solution is chosen, it requires a keying arrangement of the ferrules in order to properly align the angle polished surfaces of the ferrules in the two mating halves. The advantage of this design is that the connectors have an inherent RL of 65dB or better both in mated and unmated condition. It should be added that the APC concept also can be incorporated into the EB connector with the same RL result.

Ferrule End Face Finish
A factor which has an influence on both the IL and RL is the end face geometry of the ferrule. For the PC connection, it is essential for optimized performance that the geometry follows the specifications outlined in the Telcordia GR-326 document. The three most important metrics are:  

• Radius of Curvature (ROC)

• Apex Offset (AO)

• Fiber Height (FH)

If these requirements are followed along with a good polishing quality, optimum IL and RL will be achieved.

The EB design is more forgiving. For the SM design, where the fiber makes contact with the ball lens, the fiber geometry can exceed the GR-326 specified limitations and still have excellent performance. This is due to the fact that mating the large ferrule ROC (7mm ≤ ROC ≤ 25mm) with a relative small ball lens ROC (R = 1.5mm) will most often assure proper fiber/lens contact. For the MM version of the EB design, there is no contact between the fiber and the lens, thus, there are no strict requirements for the ferrule geometry of this version.

Contamination and Cleaning
All fiber optic paths must be kept clean to assure proper transmission of the optical signal.

When disconnected in a hostile environment, the fiber and the alignment features of the PC connection are totally exposed and subject to contamination. For this concept, the cleaning is applied directly to the fiber and to the internal surface of the alignment sleeve. For this concept, there will always be the risk that any contaminants being trapped between the mated ultra-fine polished fibers can easily become embedded into the glass surface or create scratches, which will increase the optical losses and deteriorate the return loss. This is more pronounced the smaller the fiber core is. Therefore, single-mode fibers with a core size of approximately 10um are more at risk for detrimental damage than, for example, a multimode fiber with a core size of 62.5um. To ensure proper cleanliness, the surfaces should be inspected under magnification prior to mating. This connector type performs best in a controlled environment. It should be mentioned that some PC connectors are designed for easy cleaning access. For example, the LuxCis connectors have a removable sandwich layer which contains the alignment sleeve. This provides easy cleaning access to the ferrules.

Disconnection of the EB connectors in a hostile environment does not present a problem because the alignment mechanism is enclosed and protected by the lens housing at all times. Thus, no cleaning of the actual fiber is necessary once assembled. Instead, the field cleaning procedure needs only to be applied to the external lens surface. An untrained operator can perform a simple wipe of the exposed lens surface, which will be sufficient in most cases (see figure 9).

It is important for the EB performance that good perpendicularity of the mating reference plane to the optical axis is maintained. From a manufacturing point of view, this requires control of the machining process and the metrology procedures. During use, it is important to maintain cleanliness of the mechanical mating plane of the lens housings. These must be kept clean and free of any protruding irregularities, as a tilt between the lens housings will cause angular misalignment and an increase in loss. 

Wear
Because the PC connection allows physical contact between the fibers, these surfaces will eventually get worn or scratched. This shows up as a continuous degradation of the insertion and return loss. The degradation is dependent on how carefully the fibers are cleaned before each mating. The damage can be accelerated if the connection is subjected to a contaminated and vibrational environment as discussed above.

The EB design (non-contacting optics) has a low sensitivity to these effects as described previously, which is one of the top reasons this connector type is preferred in rugged environments. The durability of the EB connector is superior to that of the PC connection.

Cost Differential
Both the PC and the EB designs are offered in multiple packaging versions, which will ultimately affect the price. The system designer will evaluate these and select the connector type which best meets the optical, environmental, and cost requirements. Thus, to make a cost comparison depends on the application. If, for example, a low-cost SC-style PC connector satisfies the requirements, a comparative EB solution will be several hundreds of percent more expensive. However, if a cost comparison of a common 4-channel SM PC 38999 III shells (with M29504 ferrules) is made with a mated pair of Tyco Electronics PRO BEAM JR 4-channel SM EB connector pair, the EB design only has a 20 percent premium.

For this specific PC design, the housing can have up to 48 channels. This will reduce the cost per channel relative to the EB design (with max. 8 channels), since all the channels (ferrules) will be sharing the same shell housing.

Repair, Replacement
The same repair issues appear to exist for both designs. If individual cables are used, a defective termination can simply be replaced. If all the fibers are contained in one cable, a defective channel will usually cause the entire set of terminations to be cut off and re-terminated. This is necessary in order to keep the same break-out length for all fibers.

Finally, the PC design allows the terminations to be removed for re-polish if this is deemed necessary. This is not an issue for the EB design where the fibers are protected by the lens housing.

Water exposure and condensation
Any time a connector of any type is exposed to water or subjected to condensation, it should be cleaned to assure proper performance. The PC connection will be the least affected, as the physical contact between the fibers works to its advantage. Due to the spherical polish of the ferrule end face, any water droplets or deposits will be squeezed out from the area of contact during mating, and the performance is not affected. There is a possibility for problems if the water contains contamination particles and the fiber is scratched during the cleaning process.

For the EB design, the situation is different. The fact that water or droplets can obstruct the collimated beam and thereby reduce the performance will require that the mating interface and lenses be wiped dry. Once cleaned, the EB design returns to its original performance level.

Figure 10 shows a cleaning sequence of an expanded beam connector which accidentally has been dropped in mud.

Because the EB design is frequently used in rugged hostile environments where rapid temperature changes can occur, questions are often asked about the influence of lens condensation on the performance. Testing has shown that if the connectors remain mated during the temperature transition, there is no degradation of performance. If the connectors are opened and a large temperature differential exists, condensation will form on the exposed lens surfaces. However, feedback from users in geophysical, fighter jet, and field tactical applications all agree that a simple wipe before mating will return the connectors to their original performance level.

Likewise, exposure to snow or frost of either connector type will require cleaning.

    
    
Figure 10: Cleaning sequence of a disconnected expanded beam connector after having been submerged in mud.

Relative Performance Ratings
The following figure is a selection matrix reflecting the discussions from above:

The system designer must determine which categories best apply to his/her specific application and make a connector selection based on the individual scores. It should be noted that the overall scores shown in the table were achieved using equal weight for all categories. If, for example, the IL score has a higher importance to the user than the durability score, it is important to include such considerations into the decision process.

Soren Grinderslev, Ph.D., graduated with a doctorate from the Danish Academy of Technical Sciences and has more than 25 years experience in the field of fiber optics. He holds a position as principal with Tyco Electronics in the Fiber Optics Lab, has been awarded 17 patents, and has authored several papers. He is currently on assignment for Tyco Electronics in Shanghai, PRC.

Tyco Electronics Ltd. is a leading global provider of engineered electronic components, network solutions, undersea telecommunication systems, and wireless systems, with 2008 sales of $14.8 billion to customers in more than 150 countries. Tyco Electronics designs, manufactures, and markets products for the automotive, appliance, aerospace, defense, telecommunications, computer and consumer electronics industries. With nearly 8,000 engineers and worldwide manufacturing and customer support capabilities, Tyco Electronics' commitment is their customers' advantage. More information on Tyco Electronics can be found at www.tycoelectronics.com and www.tycoelectronics.com/adm.

PRO BEAM and Tyco Electronics are trademarks.

LuxCis is a trademark of Radiall, S.A. Product is manufactured under license from Radiall, S.A.

Bishop & Associates, Inc. © 2010