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Fiber Optic Transmission Dip Probe Bundle
Transmission Dip Probe Fiber Y-Bundle
TP22 Fiber Bundle and TPT220 Probe Tip Mounted on an RPS Probe Stand (Post Replaced with TR8 Ø1/2" Post)
Transmission Dip Probe Tips
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Schematic showing ray paths (in red) through the bundle and probe tip.
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The durable aluminum Y-joint of our bifurcated cables includes an adjustable fiber clamp, which is secured in place with an 8-32 locking screw.
Thorlabs' Transmission Dip Probe Fiber Bundle is optimized for measuring transmittance and absorbance in liquid samples. Unlike a cuvette-based setup, the probe tip is immersed within the sample; liquids can flow freely into the opening of the probe tip. This method allows measurement of the sample directly and is ideal for applications requiring real-time measurements such as monitoring chemical reactions or water quality testing (see the Application tab for more information). A complete transmission dip probe requires the purchase of a fiber bundle and one probe tip.
The Ø1/4" probe and probe tips are manufactured with 316 stainless steel and fitted with high-quality lenses and mirrors to minimize transmission loss when the probe tip is immersed in liquids. As shown in the images to the right, light from the fiber bundle is collimated at the sample end of the fiber bundle. Transmission Dip Probe Tips with varying lengths can be attached to the probe end. Within the probe tip the light passes through the sample twice; once when exiting the fiber bundle and again when reflected from the mirror fitted into the end of the probe tip. Longer path lengths can increase measurement sensitivity but will increase overall transmission loss. The transmitted and backscattered light from the sample is collected by the six fibers on the outside of the bundle that is directed to the spectrometer end of the bundle. Connecting the spectrometer and light source in this manner minimizes clipping of the beam exiting and re-entering the fiber. The spectrometer end can be rotated to optimize alignment between the fiber bundle and the spectrometer before final tightening of the SMA connector.
The TP22 Transmission Dip Probe Fiber Bundle features a wavelength range of 400 - 900 nm and is equipped with two SMA905-terminated legs. These are compatible with most spectrometers, including Thorlabs' CCD Spectrometers, and most light sources, including Thorlabs' Broadband Fiber-Coupled Light Sources. Each SMA905-terminated leg is engraved with the fiber orientation; the light source end has a single fiber, while the spectrometer end has a six-fiber bundle with a single dead fiber in a round configuration. While the location of the dead fiber will not always be in the same location, Thorlabs guarantees the dead fiber will never be in the center position. A sliding clamp at the Y-junction can be locked in place by tightening the 8-32 setscrew.
Measuring Absorbance Spectra Using a Transmission Dip Probe
Absorbance spectroscopy uses the amount of light absorbed by a sample to determine its concentration via Beer's Law. Such measurements are typically performed using a benchtop spectrometer where the sample is placed
Transmission dip probes are particularly well suited for applications that require real-time measurements such as chemical processing or environmental monitoring. Both transmitted and backscattered light are measured by the spectrometer, which means that this method has a lower dynamic range compared to benchtop methods. Careful selection of the probe tip and absorbance wavelength can help with optimizing measurement performance.
To show how the transmission dip probe can be used for these applications, the graph to the right shows the absorbance spectra measured for four different samples of food coloring mixed into
Laser-Induced Damage in Silica Optical Fibers
The following tutorial details damage mechanisms relevant to unterminated (bare) fiber, terminated optical fiber, and other fiber components from laser light sources. These mechanisms include damage that occurs at the air / glass interface (when free-space coupling or when using connectors) and in the optical fiber itself. A fiber component, such as a bare fiber, patch cable, or fused coupler, may have multiple potential avenues for damage (e.g., connectors, fiber end faces, and the device itself). The maximum power that a fiber can handle will always be limited by the lowest limit of any of these damage mechanisms.
While the damage threshold can be estimated using scaling relations and general rules, absolute damage thresholds in optical fibers are very application dependent and user specific. Users can use this guide to estimate a safe power level that minimizes the risk of damage. Following all appropriate preparation and handling guidelines, users should be able to operate a fiber component up to the specified maximum power level; if no maximum is specified for a component, users should abide by the "practical safe level" described below for safe operation of the component. Factors that can reduce power handling and cause damage to a fiber component include, but are not limited to, misalignment during fiber coupling, contamination of the fiber end face, or imperfections in the fiber itself. For further discussion about an optical fiber’s power handling abilities for a specific application, please contact Thorlabs’ Tech Support.
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Undamaged Fiber End
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Damaged Fiber End
There are several potential damage mechanisms that can occur at the air / glass interface. Light is incident on this interface when free-space coupling or when two fibers are mated using optical connectors. High-intensity light can damage the end face leading to reduced power handling and permanent damage to the fiber. For fibers terminated with optical connectors where the connectors are fixed to the fiber ends using epoxy, the heat generated by high-intensity light can burn the epoxy and leave residues on the fiber facet directly in the beam path.
Damage Mechanisms on the Bare Fiber End Face
Damage mechanisms on a fiber end face can be modeled similarly to bulk optics, and industry-standard damage thresholds for UV Fused Silica substrates can be applied to silica-based fiber. However, unlike bulk optics, the relevant surface areas and beam diameters involved at the air / glass interface of an optical fiber are very small, particularly for coupling into single mode (SM) fiber. therefore, for a given power density, the power incident on the fiber needs to be lower for a smaller beam diameter.
The table to the right lists two thresholds for optical power densities: a theoretical damage threshold and a "practical safe level". In general, the theoretical damage threshold represents the estimated maximum power density that can be incident on the fiber end face without risking damage with very good fiber end face and coupling conditions. The "practical safe level" power density represents minimal risk of fiber damage. Operating a fiber or component beyond the practical safe level is possible, but users must follow the appropriate handling instructions and verify performance at low powers prior to use.
Calculating the Effective Area for Single Mode and Multimode Fibers
As an example, SM400 single mode fiber has a mode field diameter (MFD) of ~Ø3 µm operating at 400 nm, while the MFD for SMF-28 Ultra single mode fiber operating at 1550 nm is Ø10.5 µm. The effective area for these fibers can be calculated as follows:
SM400 Fiber: Area = Pi x (MFD/2)2 = Pi x (1.5 µm)2 = 7.07 µm2 = 7.07 x 10-8 cm2
To estimate the power level that a fiber facet can handle, the power density is multiplied by the effective area. Please note that this calculation assumes a uniform intensity profile, but most laser beams exhibit a Gaussian-like shape within single mode fiber, resulting in a higher power density at the center of the beam compared to the edges. Therefore, these calculations will slightly overestimate the power corresponding to the damage threshold or the practical safe level. Using the estimated power densities assuming a CW light source, we can determine the corresponding power levels as:
SM400 Fiber: 7.07 x 10-8 cm2 x 1 MW/cm2 = 7.1 x 10-8 MW = 71 mW (Theoretical Damage Threshold)
SMF-28 Ultra Fiber: 8.66 x 10-7 cm2 x 1 MW/cm2 = 8.7 x 10-7 MW = 870 mW (Theoretical Damage Threshold)
The effective area of a multimode (MM) fiber is defined by the core diameter, which is typically far larger than the MFD of an SM fiber. For optimal coupling, Thorlabs recommends focusing a beam to a spot roughly 70 - 80% of the core diameter. The larger effective area of MM fibers lowers the power density on the fiber end face, allowing higher optical powers (typically on the order of kilowatts) to be coupled into multimode fiber without damage.
Damage Mechanisms Related to Ferrule / Connector Termination
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Plot showing approximate input power that can be incident on a single mode silica optical fiber with a termination. Each line shows the estimated power level due to a specific damage mechanism. The maximum power handling is limited by the lowest power level from all relevant damage mechanisms (indicated by a solid line).
Fibers terminated with optical connectors have additional power handling considerations. Fiber is typically terminated using epoxy to bond the fiber to a ceramic or steel ferrule. When light is coupled into the fiber through a connector, light that does not enter the core and propagate down the fiber is scattered into the outer layers of the fiber, into the ferrule, and the epoxy used to hold the fiber in the ferrule. If the light is intense enough, it can burn the epoxy, causing it to vaporize and deposit a residue on the face of the connector. This results in localized absorption sites on the fiber end face that reduce coupling efficiency and increase scattering, causing further damage.
For several reasons, epoxy-related damage is dependent on the wavelength. In general, light scatters more strongly at short wavelengths than at longer wavelengths. Misalignment when coupling is also more likely due to the small MFD of short-wavelength SM fiber that also produces more scattered light.
To minimize the risk of burning the epoxy, fiber connectors can be constructed to have an epoxy-free air gap between the optical fiber and ferrule near the fiber end face. Our high-power multimode fiber patch cables use connectors with this design feature.
Determining Power Handling with Multiple Damage Mechanisms
When fiber cables or components have multiple avenues for damage (e.g., fiber patch cables), the maximum power handling is always limited by the lowest damage threshold that is relevant to the fiber component. In general, this represents the highest input power that can be incident on the patch cable end face and not the coupled output power.
As an illustrative example, the graph to the right shows an estimate of the power handling limitations of a single mode fiber patch cable due to damage to the fiber end face and damage via an optical connector. The total input power handling of a terminated fiber at a given wavelength is limited by the lower of the two limitations at any given wavelength (indicated by the solid lines). A single mode fiber operating at around 488 nm is primarily limited by damage to the fiber end face (blue solid line), but fibers operating at 1550 nm are limited by damage to the optical connector (red solid line).
In the case of a multimode fiber, the effective mode area is defined by the core diameter, which is larger than the effective mode area for SM fiber. This results in a lower power density on the fiber end face and allows higher optical powers (on the order of kilowatts) to be coupled into the fiber without damage (not shown in graph). However, the damage limit of the ferrule / connector termination remains unchanged and as a result, the maximum power handling for a multimode fiber is limited by the ferrule and connector termination.
Please note that these are rough estimates of power levels where damage is very unlikely with proper handling and alignment procedures. It is worth noting that optical fibers are frequently used at power levels above those described here. However, these applications typically require expert users and testing at lower powers first to minimize risk of damage. Even still, optical fiber components should be considered a consumable lab supply if used at high power levels.
In addition to damage mechanisms at the air / glass interface, optical fibers also display power handling limitations due to damage mechanisms within the optical fiber itself. These limitations will affect all fiber components as they are intrinsic to the fiber itself. Two categories of damage within the fiber are damage from bend losses and damage from photodarkening.
A special category of optical fiber, called double-clad fiber, can reduce the risk of bend-loss damage by allowing the fiber’s cladding (2nd layer) to also function as a waveguide in addition to the core. By making the critical angle of the cladding/coating interface higher than the critical angle of the core/clad interface, light that escapes the core is loosely confined within the cladding. It will then leak out over a distance of centimeters or meters instead of at one localized spot within the fiber, minimizing the risk of damage. Thorlabs manufactures and sells 0.22 NA double-clad multimode fiber, which boasts very high, megawatt range power handling.
Even with the above strategies in place, all fibers eventually experience photodarkening when used with UV or short-wavelength light, and thus, fibers used at these wavelengths should be considered consumables.
General Cleaning and Operation Guidelines
Tips for Using Fiber at Higher Optical Power
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Custom 1-to-4 Fan-Out Cable
Custom Fiber Bundles
Thorlabs is pleased to offer custom straight and fan-out fiber bundles with random or mapped fiber configurations. The table below outlines some of our current bundle production capabilities. We are in the process of expanding these production capabilities, so do not hesitate to inquire if you do not see the bundle that you require described here.
Some custom bundles will require techniques outside of our usual production processes. As a result, we cannot guarantee that we will be able to make a bundle configuration to fit the requirements of your specific application. However, our engineers will be happy to work with you to determine if Thorlabs can produce a fiber bundle that fulfills your needs. To receive a quote, please provide a drawing or draft of your bundle configuration.
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Custom Silica Fiber Bundle with SMA905 Connectors
Our cable engineers are available to help manufacture a bundle for your application.
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|Item #||Fiber Configuration|
(Click for Plot)
|NAc||Minimum Bend Radius|
|Short Termd||Long Terme|
|TP22||High OH||400 - 900 nm||FG200UEA||SMA905
|200 ± 4 µm||220 ± 2 µm||0.22 ± 0.02||19 mm||53 mm|
|Wavelength Range (Click for Plot)||400 - 900 nm|
|Length (Distance from Probe Lens to Mirror)||2 mm||5 mm||10 mm||20 mm|
|Optical Path Length||4 mm||10 mm||20 mm||40 mm|
|Mirror Reflectance (Click for Plot)a||Ravg >99% (400 - 900 nm)b|
These Transmission Dip Probe Tips attach to the end of the transmission dip probe fiber bundle sold above. The probe tips feature a sample opening to allow a liquid sample to flow freely within the measurement region during a measurement. The end of each tip is equipped with a front-surface dielectric mirror (-E02 coating) to reflect the light from the probe bundle. A protective coating on the mirror surface allows it to be immersed in liquids. For repeated uses, probe tips can be cleaned using acetone and an ultrasonic cleaner. Please note that the end cap that mounts the dielectric mirror is epoxied to the housing of the probe and should not be removed.
Tips with 2 mm, 5 mm, 10 mm, or 20 mm lengths are available. A longer length increases the interaction length of light with the sample medium within the probe tip; therefore, using a probe tip with a longer path length has the benefit of increasing the signal-to-noise ratio for low-absorbance samples. Conversely, a shorter probe tip may be useful for resolving high-absorbance samples. In general, using a longer path length increases transmission loss of the probe (see the wavelength range plot in the table above). Choosing the right light source and probe length for your experiment is important for optimizing measurement performance.
Thorlabs' RPS Adjustable Fiber Optic Probe Stand is designed to hold Ø1/4" fiber bundle probes above a sample at 45° for diffuse measurements or 90° for specular measurements. Each stand is comprised of an adjustable fiber holder arm (also available separately), a Ø1/2" optical post with engraved metric height scale, and a Ø6" (Ø152.4 mm) base with engraved concentric circles and grid pattern.
The Ø1/4" probe on the sample leg is held in place on the RPA arm using a TS25H thumbscrew. The arm's height can be adjusted using a TS25H thumbscrew, which has a spring-loaded, retractable Delrin®† tip. The spring-loaded tip provides sufficient force to hold the arm in place as final positional adjustments are being made, allowing for precise height adjustments. When using the RPS stand, samples up to 55 mm (2.16") tall can be accommodated using the included optical post. For taller samples, the included post can be easily replaced with a longer imperial or metric Ø1/2" post (see the photo to the right). The post is secured to the base by an M6 cap screw on the underside of the base, which can be removed using a 3/16" or 5 mm
Replacement RPA post holder arms are also available separately. These arms can also be used to mount Ø1/4" probes within custom optomechanical setups by securing them to Ø1/2" posts.
†Delrin® is a registered trademark of Dupont Polymers, Inc.