Why Chalcogenide Glass Fiber?

Chalcogenide glasses are made from mixture of the chalcogen elements: sulfur (S), selenium (Se), and tellurium (Te). The addition of other elements, such as arsenic, germanium, and antimony establish crosslinking between the chains of chalcogens to facilitate stable glass formation. It offers promising properties, such as transmission in the mid and far infrared, low phonon energies, high refractive index, and very large nonlinearities as compared to silica glasses.

Chalcogenide glasses are thermally stable, chemically durable, mechanically flexible, and easily fiberized. The spectral range of infrared transmission of chalcogenide glass fiber (1-12 µm) is much broader than that of silica glass fiber (UV-2 µm). Chalcogenide glass fibers have very high refractive index and very large nonlinearities (1,000 times that of silica fiber), allowing for the development of super continuum in the mid-infrared. Chalcogenide glass fibers are the ideal candidates for mid-infrared applications that require high-power laser delivery, chemical sensing, thermal imaging, and temperature monitoring.

Arsenic-containing chalcogenide glasses, such as arsenic sulfide (As2S3) and arsenic selenide (As2Se3), have excellent chemical and physical properties for active optical devices. The minimum optical loss in As2S3 and As2Se3 are 0.1dB/m between 2 and 10µm.  Among any infrared optical material in use today (silica, fluoride, germanium, zinc selenide, and zinc sulfide), As2S3 glass has the lowest thermal change in refractive index.  For this reason, lenses or windows made from As2S3 glass do not show optical distortion when subjected to the intense IR radiation from lasers such as YAG, ER/YAG or CO.  The low thermal change in refractive index is thought to be the basis for the fact that 700 µm fibers made from As2S3 glass have been reported to transmit more than 100 W of laser energy from a CO laser emitting at 5.4 µm. In short, arsenic-containing chalcogenide, glasses are the most-used material for infrared optical fibers.

Comparison for As2S3 and As2Se3 chalcogenide glass fibers, silica fiber and fluoride fiber

Composition

As2S3

As2Se3

SiO2

Fluoride

Density (g/cm3)

3.2

4.62

2.2

4.33

Coefficient of Thermal Expansion (10-6/ºC)

24

21

0.55

17.2

Refractive Index (wavelength)

2.395 (3 µm)

2.8275 (3 µm)

1.455 (0.7 µm)

1.499 (0.589 µm)

ΔN/ΔT@10 µm (10-6 /ºC)

<1

~30

12

-15

Transmission Range (µm)

1-6

1-10

0.24-2

0.25-4

Optical Loss (dB/m) (Wavelength)

≈ 0.1-0.2 (3 µm)

0.5 (4.55 µm)

800 (3 µm)

0.05 (3 µm)

Glass Transition Temperature (ºC)

185

170

1120

265


Are Chalcogenide Glass Fibers Safe to Use?

Chalcogenide glasses are very stable.  According to the best of our knowledge, using chalcogenide fibers below 100°C is safe for handling, storage, and transportation. Above approximately 100 ºC, it is recommended to work under fume hood environment.  Chalcogenide glass fibers have high stability to atmospheric moisture and do not crystallize.  The Safety Data Sheet describes chalcogenide glass as nonhazardous and not absorbable through the skin.  For medical applications, there is no data for chalcogenide glass fibers used in vivo.  Related Material Safety Data Sheets for the chalcogenide glasses can be found at IRflex Corporation’s website: www.irflex.com.

What About the Chemical Resistance of Chalcogenide Glass?

Chalcogenide glasses are insoluble in water, concentrated hydrochloric acid, non-oxidizing acids, alcohol, acetone, gasoline, and toluene. They are soluble in strong alkaline solutions, such as KOH.

Is Chalcogenide Glass Like Fluoride Fiber?

Chalcogenide glass is insoluble in water, concentrated hydrochloric acid, non-oxidizing acids, alcohol, acetone, gasoline, and toluene, so it has higher chemical durability than that of ZBLAN glass (the most common fluoride glass). Fluoride fiber is more fragile and is sensitive to moisture, so its lifetime is uncertain.

Do chalcogenide glass fibers have the same mechanical strength as silica glass fiber?

No.  Chalcogenide glass fibers are softer glass fibers than silica glass (SiO2) fibers used in telecommunications, which are essentially pure silicon dioxide, and possess 10 times the tensile strength.  Therefore, special attention and different methods should be applied in handing chalcogenide glass fiber. 

Coating used on IRflex’s commercial available fibers and its temperature range?

Coating is the protective layer applied directly to the glass during the fiber draw process.  The standard silica fiber uses dual urethane acrylate coatings made up of 2 layers.  The softer, inner layer cushions the fiber during bending and allows for ease of stripping, and the outer layer is a higher modulus for protection from abrasion.

In some applications, when exceptional adhesion to the fiber or smaller form factor is required, an outer single acrylate layer is used alone.  We use standard telecom fiber coating.  As our initial design requirement for the application, IRflex’s chalcogenide glass fibers are coated with a single layer of acrylate.  The temperature range for this acrylate is -40 to +85 ºC.  For example:  coating on our IRF-S-5, IRF-S-7, IRF-S-9, IRF-S-10, IRF-S-50/85 and IRF-S-100.

Some of our fibers are offered with other kinds of acrylate coating choices to enable premium fiber performance for client’s special applications, where the operation temperature can withstand -20 º C to +130 ºC and the supplier also claims that the coating can withstand -40 º C to 200 º C for a brief period.  For example: coating on our IRF-S-6.5, IRF-S-200, IRF-Se-100, IRF-Se-100(R&), IRF-Se-150 and IRF-Se-300.

What to use to remove coating?

The chalcogenide fibers supplied by IRflex are currently equipped with a standard single layer of acrylate coating.  The fibers can be stripped chemically using a suitable solvent.  The less aggressive and safer way is to soak the fiber end to be stripped, submerging only the length of fiber that will be stripped, in dichloromethane (CH2CI2) (commonly known as methylene chloride) liquid to soften the acrylate jacket.

Prior to using dichloromethane, please follow manufacturers’ guidelines for safe handling and storage of this solvent, read the Material Safety Data Sheet (MSDS) carefully.  Avoid contact with skin and use only with adequate ventilation.

The dichloromethane causes the coating to swell after approximately 30 to 60 seconds in the solution.  Once the coating swells and stretches out, using a razor blade gently score the swollen coating without touching the fiber.  Gently pulling on the swollen coating propagates the cut and removes the coating from the fiber.

However, the coating cannot be removed by using mechanical strippers as for telecom silica fibers.

Damage threshold of the fiber?

The amount of power our chalcogenide glass fibers can carry depends on the fiber’s diameter and the laser induced damage threshold.

The effective damage threshold can be affected by several factors: inclusions in the glass, cleave quality, surface cleanliness and wavelength of laser used.

The theoretical laser induced damage threshold (LIDT) of the chalcogenide glass fibers have been calculated as 3GW/cm2 for a 10 nanoseconds pulse.  In the case of continuous wave power, the fibers have been tested up to 40W without damage.

Connectors used with IRflex’s fiber?

The IRF-S-100 fiber is offered with cable assembling configuration.  Our standard cable uses SMA and FC/UPC connectors.  Customers can choose any combinations of connector and jacket.

For most applications, FC/UPC or FC/APC connectors will be sufficient.  In terms of high power handling, FC type connectors can typically handle up to 5W of average optical powers before thermal induce degradation takes place.  For higher optical power, we recommend using SMA type connectors.  For average powers up to 50W, a standard SMA-905 connector is offered as the connector ferrule consists of solid stainless steel.  The fiber at the tip of the connector is cantilever to increase power capabilities.

When considering connector type, please also take into account how much power our fiber can carry.  The amount of power our chalcogenide glass fibers can carry depends on the fiber’s diameter and damage threshold. The standard IRflex multimode fiber (IRF-S-100) has a core diameter of 100µm and damage threshold of 3 GW/cm2 for a 10 ns pulse.  In the case of continuous wave power, the fiber has been tested up to 40W without damage.

What is the difference between SMA Amphenol connector and SMA connector?

Amphenol is the manufacturer of the connector and they use stainless steel ferrules.  Some SMA connectors from other manufacturers are not with stainless steel ferrules.

What kind of SMA connector would I need?

There are two different types of SMA connectors: 905 or 906.  The 905 is actually a straight connector while the 906 is a SMA connector that has one end slightly smaller.  If you use the right coupler you can actually join the two different SMA connectors together without any problems.

How to connect with a SMA connector?

You cannot be forceful at all when using a SMA connector.  With little effort, you can actually damage it and be forced to put another one on and start all over again.  Hooking up a SMA connector to any device is very straightforward.  It is best to start by lightly hand tightening and make sure you are not stripping the threads.  Once you have a solid connection then you can proceed to use a wrench to make a secure fit.

You also need to pay special attention to the entire connector as you are tightening your SMA connector to make sure you are not damaging your connection.  This could severely shorten the life of your connector and give you horrible signal quality.  So just be sure to pay special attention to installation and make sure that you have a very snug connection so that your signal quality does not suffer and your SMA connector will last longer.

What is the difference between a PC and a UPC polish?

A UPC (Ultra Physical Contact) is a higher quality polish then a PC (Physical Contact) polish.  The UPC polish has less scratches and the higher quality as measured by the improved return loss.  UPC connectors mate to PC connectors without any problem.

What causes insertion loss in a connector?

Insertion loss in connectors typically comes from misalignment of the fibers or bent fibers in the connectors.  The misalignment comes from eccentricity of the core in the fiber, eccentricity of the hole in the ferrule and most of all the eccentricity of the fiber in the hole.  Bending of the fiber typically comes from the fiber being constrained at the back of the connector while the ferrule moves back during mating.

What is the refractive index of IRflex’s fiber?

2.38 at 10µm, 2.43 at 4µm and 2.44 at 2µm.

What is Fiber Bundle Imager?

Current infrared cameras are commercially available for thermal imaging systems in the range of 2-14 µm. These cameras are made of a high-resolution uncooled focal plane array (FPA) that is a two-dimensional array of infrared detectors used to create a thermal image.  Ferroelectric detectors and microbolometers are two uncooled technologies used today.  A typical infrared camera has a high resolution (320 x 240 pixels).  These cameras have pixel size of 50 µm squared, a length of 5.5 in., a width of 4.5 in. and a height of 4.5 in., and weight of 3 lb.  The camera includes a lens to create an image on the FPA.  Some critical military applications require high spatial resolution IR fiber bundle for imaging in areas inaccessible to the cameras because of size and/or harsh environments.  This is the case where IR imaging in the range of 1-12 µm is needed for spatial analysis of combustion processes in turbines combustors and afterburners.  Coherent fiber bundles are commonly used in the visible and near infrared (0.4 to 1 µm) to remotely transfer images to cameras. Unfortunately, there exists no coherent Fiber bundle imager (FBI) for IR imaging in the range of 1-12 µm.

IRflex is committed to develop innovative processes to produce low-cost coherent infrared imaging fiber bundles that are of minimum bend radius and attenuation over the spectral range of 1 to 12 µm.  Recent developments in IR optical fibers and the current state-of-the-art fabrication processes of coherent fiber bundle offers the potential to develop a technology for the IR FBI. The benefits of the IR fiber optic bundle are illustrated in Figure C-1.  A scene or object is imaged through a lens and transmitted through the flexible fiber optic bundle to a camera. The novel FBI will enable IR imaging in areas where space is limited and/or in harsh environments.

Flexible Fiber Optic Bundle

Figure C-1: Coherent fiber bundle to transmit an image of a remote camera.


IRCM

Infrared countermeasures (IRCM): a device designed to prevent a heat or plume-seeking missile from reaching its target.  It generally consists of a flare, laser or other bright illumination source and optics combination that, when placed on an aircraft, confuses a missile's target acquisition system.

CO2 laser

The carbon dioxide laser (CO2 laser) was one of the earliest gas lasers to be developed (invented by Kumar Patel of Bell Labs in 1964).  The CO2 laser produces a beam of infrared light with the principal wavelength bands centering around 9.4 and 10.6 mm.

QCL

Quantum cascade lasers (QCLs) are semiconductor lasers that emit in the mid to far infrared portion of the electromagnetic spectrum.