Fiber Optic Transmission Dip Probe Bundle
- Fiber Y-Bundle and Mirrored Tips for Making Transmission Dip Probes
- Bundle Legs for Light Source, Sample, and Spectrometer
- 2 mm to 20 mm Long Probe Tips
- 400 - 900 nm Wavelength Range
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.
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Typical transmission dip probe setup using Thorlabs' TP22 transmission dip fiber bundle and probe tip, CCD spectrometer, and broadband fiber-coupled light source. Note: The positioning of the dead fiber in the spectrometer end of the bundle is random, but will not be in the center of the fiber bundle.
- Fiber Optic Y-Cable Bundle and Tips for Transmission Dip Probes
- Measure Absorbance or Transmission In Situ
- 400 - 900 nm Wavelength Range
- Fiber Bundles with Adjustable Fiber Clamp at Y-Joint
- Light Source End Illuminates Sample
- Sample Leg Equipped with a Ø1/4" Probe
- Spectrometer End Carries Reflected Light from Sample
- Reinforced Stainless Steel Tubing and Strain Relief Sleeve
- SMA905 Connectors Engraved with Fiber Orientation
- Probe Tips with Varying Lengths from 2 mm to 20 mm (Sold Separately)
- One Fiber Bundle and One Probe Tip Required
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.
Thorlabs offers a fiber probe stand and holder (sold below), which holds a sample end probe during immersion within the sample medium. They are easily adjustable and do not allow the fiber to move during measurements. The holders allow the probe to be held vertically or at a 45° angle.
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
|Damage at the Air / Glass Interface|
|Intrinsic Damage Threshold|
|Preparation and Handling of Optical Fibers|
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.
|Estimated Optical Power Densities on Air / Glass Interfacea|
|Type||Theoretical Damage Thresholdb||Practical Safe Levelc|
|~1 MW/cm2||~250 kW/cm2|
|10 ns Pulsed
|~5 GW/cm2||~1 GW/cm2|
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 Fibers
The effective area for single mode (SM) fiber is defined by the mode field diameter (MFD), which is the cross-sectional area through which light propagates in the fiber; this area includes the fiber core and also a portion of the cladding. To achieve good efficiency when coupling into a single mode fiber, the diameter of the input beam must match the MFD of the fiber.
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
SMF-28 Ultra Fiber: Area = Pi x (MFD/2)2 = Pi x (5.25 µm)2 = 86.6 µm2 = 8.66 x 10-7 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)
7.07 x 10-8 cm2 x 250 kW/cm2 = 1.8 x 10-5 kW = 18 mW (Practical Safe Level)
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)
8.66 x 10-7 cm2 x 250 kW/cm2 = 2.1 x 10-4 kW = 210 mW (Practical Safe Level)
Effective Area of Multimode Fibers
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.
Bend losses occur when a fiber is bent to a point where light traveling in the core is incident on the core/cladding interface at an angle higher than the critical angle, making total internal reflection impossible. Under these circumstances, light escapes the fiber, often in a localized area. The light escaping the fiber typically has a high power density, which burns the fiber coating as well as any surrounding furcation tubing.
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.
A second damage mechanism, called photodarkening or solarization, can occur in fibers used with ultraviolet or short-wavelength visible light, particularly those with germanium-doped cores. Fibers used at these wavelengths will experience increased attenuation over time. The mechanism that causes photodarkening is largely unknown, but several fiber designs have been developed to mitigate it. For example, fibers with a very low hydroxyl ion (OH) content have been found to resist photodarkening and using other dopants, such as fluorine, can also reduce photodarkening.
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
These general cleaning and operation guidelines are recommended for all fiber optic products. Users should still follow specific guidelines for an individual product as outlined in the support documentation or manual. Damage threshold calculations only apply when all appropriate cleaning and handling procedures are followed.
All light sources should be turned off prior to installing or integrating optical fibers (terminated or bare). This ensures that focused beams of light are not incident on fragile parts of the connector or fiber, which can possibly cause damage.
The power-handling capability of an optical fiber is directly linked to the quality of the fiber/connector end face. Always inspect the fiber end prior to connecting the fiber to an optical system. The fiber end face should be clean and clear of dirt and other contaminants that can cause scattering of coupled light. Bare fiber should be cleaved prior to use and users should inspect the fiber end to ensure a good quality cleave is achieved.
If an optical fiber is to be spliced into the optical system, users should first verify that the splice is of good quality at a low optical power prior to high-power use. Poor splice quality may increase light scattering at the splice interface, which can be a source of fiber damage.
Users should use low power when aligning the system and optimizing coupling; this minimizes exposure of other parts of the fiber (other than the core) to light. Damage from scattered light can occur if a high power beam is focused on the cladding, coating, or connector.
Tips for Using Fiber at Higher Optical Power
Optical fibers and fiber components should generally be operated within safe power level limits, but under ideal conditions (very good optical alignment and very clean optical end faces), the power handling of a fiber component may be increased. Users must verify the performance and stability of a fiber component within their system prior to increasing input or output power and follow all necessary safety and operation instructions. The tips below are useful suggestions when considering increasing optical power in an optical fiber or component.
Splicing a fiber component into a system using a fiber splicer can increase power handling as it minimizes possibility of air/fiber interface damage. Users should follow all appropriate guidelines to prepare and make a high-quality fiber splice. Poor splices can lead to scattering or regions of highly localized heat at the splice interface that can damage the fiber.
After connecting the fiber or component, the system should be tested and aligned using a light source at low power. The system power can be ramped up slowly to the desired output power while periodically verifying all components are properly aligned and that coupling efficiency is not changing with respect to optical launch power.
Bend losses that result from sharply bending a fiber can cause light to leak from the fiber in the stressed area. When operating at high power, the localized heating that can occur when a large amount of light escapes a small localized area (the stressed region) can damage the fiber. Avoid disturbing or accidently bending fibers during operation to minimize bend losses.
Users should always choose the appropriate optical fiber for a given application. For example, large-mode-area fibers are a good alternative to standard single mode fibers in high-power applications as they provide good beam quality with a larger MFD, decreasing the power density on the air/fiber interface.
Step-index silica single mode fibers are normally not used for ultraviolet light or high-peak-power pulsed applications due to the high spatial power densities associated with these applications.
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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
|Custom Bundle Capabilities|
|Straighta||Fan Out (2 or More Legs)a,b|
|Single Mode||Standard (320 to 2100 nm), Ultra-High NA (960 to 1600 nm),
Photosensitive (980 to 1600 nm)
|Multimode||0.10 NA Step Index (280 to 750 nm), 0.22 NA Step Index (190 to 2500 nm),
0.39 NA Step Index (300 to 2200 nm), Multimode Graded Index (750 to 1450 nm),
Multimode ZrF4 (285 nm to 4.5 µm)
|Tubing Optionsc||Thorlabs' Stock Furcation Tubing, Stainless Steel Tubing or Black Heat Shrink Tubing|
|Connectors||SMA905 (Ø2 mm Max Cored), FC/PC (Ø800 µm Max Cored),
Ø1/4" Probe, or Flat-Cleaved Unterminated Fiber
|Length Tolerancee||±0.14 m|
|Round or Linear|
|Angle Polishing||On Special Request. Available for up to Ø105 µm Core on Single Fiber End.
Please Inquire for More Information.
Our cable engineers are available to help manufacture a bundle for your application.
Please contact firstname.lastname@example.org with your custom bundle requests.
Please provide a drawing or draft of your custom bundle to expedite quote processing.
|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|
- Probe Tips with Opening for Measuring Transmission or Absorption in Liquid Samples
- 2 mm, 5 mm, 10 mm, or 20 mm Length
- Broadband Mirror with -E02 Coating at End of Probe Tip
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.
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RPS fitted with a TR8 post to mount a transmission dip probe.
- Securely Hold Fiber Optic Probes with Ø1/4" Probe Sample Leg
- Orient Fiber Probe at 90° or 45° with Respect to the Sample
- Adjustable Height Arm Accommodates Samples up to at least 55 mm (2.16") Tall
- Ø6" (Ø152.4 mm) Base with Engraved Grid and Concentric Circles
- Replacement Arm Assembly Also Available
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.