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Fiber Photometry Overview
Kinematic Fluorescence Filter Cube
GFP Emission Filter
BFP Dichroic Filter
470 nm Fiber-Coupled LED
Fiber Optic Cannula, Stainless Steel Ferrule
Fiber Optic Cannula, Ceramic Ferrule
Ø400 µm Core, 0.50 NA FC/PC Low‑Autofluorescence Patch Cable
Thorlabs Fiber Photometry System Components
Thorlabs offers a full line of equipment for in vivo stimulation, including implantable fiber optic cannulae, fiber optic patch cables, rotary joints, and LED and laser light sources. We are also well equipped to provide custom fiber photometry packages, including fiber-coupled light sources and custom-made cannulae. Please contact Tech Support for individual assistance regarding fiber photometry equipment selection.
The equipment needed for photometry is very similar to that used in an optogenetics experiment; however, due to the nature of fluorescence imaging, care must be taken in selecting the correct fiber optic components throughout the system to optimize signal intensity. In addition to the abovementioned common optogenetic tools, filters and dichroics with mounting hardware and multimode collimators used for fiber-to-free-space coupling are needed to filter and split the outgoing excitation signals from the incoming fluorescence signals. The collimators and rotary joints use an achromatic design to ensure that insertion loss is consistent throughout the visible wavelength spectrum that pertains to fiber photometry.
Interactive Fiber Photometry System Schematic
Click on the components or labels for more details about our optogenetics line of products. Contact Tech Support for more information about our expanding line of fiber photometry products.
Introduction to Fiber Photometry
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An example of fluorescence data showing neural activity in a GCaMP transgenic sample. GCaMP fluoresces only when bound to a calcium ion, providing insight into calcium dynamics and neuronal response patterns.
Fiber photometry, a technique related to optogenetics, provides detailed insight into the activity and behavior of neuronal populations. This technique stimulates neurons with light and measures fluorescence signals that correspond to calcium dynamics. By detecting changes in the fluorescence intensity from a genetically encoded calcium indicator (GECI), calcium dynamics related to activities and patterns in neural circuits can be measured in real time.
The technique has helped in developing treatments for neurological diseases and brain-related traumatic injuries. Its use of light offers many advantages over other methods of neuromodulation, including a temporal resolution on the order of milliseconds, closely matching natural neuronal activity; and the stimulation of multiple neuron sets independently through the use of different wavelengths.
In fiber photometry, data is collected by analyzing the change in fluorescence (ΔF) relative to an initial baseline fluorescence (F) and observing a change in signal that corresponds to a calcium transient (ΔF/F). These indicators are typically based on fluorophores like GFP, RFP, tdTomato, mCherry, etc., of which GCaMP is the most common example. GCaMP contains Green Fluorescent Protein (GFP). As such, GCaMP not only exhibits the same fluorescence patterns as standalone GFP, but also provides key insight into calcium dynamics because GCaMP only fluoresces when bound to a calcium ion. In neurons, calcium ions regulate several important processes, including neurotransmitter release and membrane excitability. Therefore, GCaMP fluorescence is tied directly to neuronal response patterns, as depicted by the graph to the right.
To excite GCaMP in a neuronal population of interest, 470 nm and 405 nm LED sources are used to simultaneously photoexcite the maximum absorption and isosbestic points of the fluorophore, respectively. Each peak on a graph of ΔF/F indicates a neuronal response to an externally applied light stimulus. When using GCaMP as the fluorophore, as is most typical for fiber photometry, the peak specifically indicates when 525 nm fluorescence is emitted, following the simultaneous application of 405 nm and 470 nm inputs.
The main advantage of this excitation scheme is that excitation at the 405 nm isosbestic point will yield calcium-independent fluorescence in contrast to the calcium-dependent fluorescence at 470 nm. By modulating the excitation sources, the two emission signals can be read independently and simultaneously, providing important information about which part of the detected signal can be attributed to real information about behavior as opposed to system noise. This system noise can be generated from the fiber optic cables, interconnects, and cannulae used in the setup. Motion artifacts pose a particular risk because they create sharp features in recordings of ΔF/F that could easily be misunderstood as spikes in calcium activity if the 405 nm isosbestic point excitation is not measured in parallel.
Thorlabs offers stock and custom fiber optic cannulae, which can be surgically mounted to the skull of the specimen using stereotactic guidance. Custom cannulae are available with stainless steel or ceramic ferrules as well as an array of different fiber types, lengths, and end terminations. See the Custom Cannula tab above for details.
For fiber photometry applications, we recommend using Ø400 µm, 0.50 NA optical fiber to maximize signal intensity and minimize autofluorescence intensity. As detailed below, we offer Low Autofluorescence Patch Cables that meet these specifications.
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Ferrule Patch Cables are ideal for connection to our Implantable Fiber Optic Cannulae.
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Fiber Optic Cannulae are available in various
fiber lengths and ferrule sizes.
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Implant Guide Assembly
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Weep Holes for Epoxy to Escape
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Mounting Surface of OGF Cannula Impant Guide
Cannula Implant Guides
These Cannula Implant Guides are designed to provide guidance and stability for a fiber optic cannula during an implantation procedure. The bottom surface of each implant guide features a roughened surface and circular groove (see image to the right) that increase the surface area available to dental cement and improves adhesion to the specimen. A 1.6 mm long protrusion on the implant guide helps stabilize the guide when implanted. Each implant is constructed using lightweight surgical titanium (≤0.11 g) which can be sterilized prior to use.
For best results when implanting a cannula, the OGL and OGF should be used with a cannula holder and stereotaxic guidance equipment. To affix the cannula within the implant guide, first insert the cannula into the receptacle of the implant guide. Then, add a small amount of cement or epoxy to the cannula via the two Ø0.8 mm weep holes (see image above). Finally, attach the cannula ferrule to an XCL (Ø1.25 mm ferrule) or XCF (Ø2.5 mm ferrule) cannula holder (see image above).
The OGL implant guide is compatible with our standard Ø1.25 mm cannulae (ceramic and stainless steel) and the OGF implant guide is compatible with our standard Ø2.5 mm cannulae (ceramic and stainless steel). When assembled, the length of the protruding fiber is reduced by 1 mm (Item # OGL) or 2 mm (Item # OGF); therefore, these implant guides cannot be used with our 2 mm long cannulae. Additionally, due to the fiber separation distance, the implant guides cannot be used with our dual-core cannulae.
Multimode Step Index Patch Cables
Thorlabs offers multimode step index fiber optic patch cables with SMA905 (straight ferrule) and FC/PC connectors. These cables are ideal for a broad range of wavelengths from 250 nm to 2400 nm.
Each patch cable includes two protective caps that shield the connector ends from dust and other hazards. Additional CAPM Rubber Fiber Caps and CAPSM Metal Threaded Fiber Caps for SMA905-terminated ends are also sold separately. All patch cables on this page are sold from stock with same-day shipping available.
The majority of our 0.50 NA patch cables have orange (Ø3 mm) PVC furcation tubing, while the Ø1500 µm core fibers are packaged in stainless steel jackets. We recommend choosing stainless steel jackets when using fibers with large core diameters (≥Ø1000 µm) or high NAs (≥0.50) in light-sensitive applications, as it is easier for stray ambient light to penetrate the Ø3 mm (Item # FT030) fiber jackets. Alternatively, custom patch cables may be purchased that use our black or stainless steel furcation tubing (e.g., FT030-BK, FT038-BK, FT061PS, and others), in order to minimize stray light entering the fiber.
These cables are not designed for applications that require the fibers to carry high optical powers, as excessive powers can cause the epoxy used in the connectors to experience catastrophic heating. Please see the Damage Threshold tab for detailed information. Thorlabs offers alternate cabling options, in addition to unconnectorized fibers, that are compatible with high optical powers. Links to some options are included in the table below.
We have extensive patch cable capabilities including a large, diverse stock of optical fiber and many different types of fiber connectors. If you do not see a stock cable suitable for your application, please see our Custom Patch Cables webpage to request a cable that meets your specific needs.
Neuron stimulation requires precise temporal control of the output light, typically on the order of millisecond pulses. For in vivo applications, electronic modulation of a fiber-coupled LED may be preferred over using a mechanical shutter or other modulation technique. Fiber-coupled LEDs at common wavelengths for fiber photometry can be seen in the table to the right. In a fiber photometry setup utilizing GCaMP as the fluorophore, a 470 nm LED, such as the M470F3, and a 405 nm LED, such as the M405FP1, can be used.
Our fiber-coupled LEDs can be modulated in several ways when used with our LED drivers, such as the LEDD1B. The LEDD1B is designed to drive high-power LEDs with currents up to 1200 mA. It features an adjustable LED current limit to protect the connected LED. The output current can be limited continuously from 200 mA to 1200 mA using the adjuster on the front of the unit, thereby ensuring the output current does not exceed the limit regardless of the other settings or the modulation input voltage. A typical fiber photometry system will require two synchronized drivers that can simultaneously deliver light at both the isosbestic and maximum excitation wavelengths.
This LED Driver has the same compact form factor as other T-Cube modules, and, thus, it integrates well into the platform. It is shipped attached to a removable base plate, which allows the T-Cube to be easily secured to an optical table. The power supply options compatible with the LEDD1B LED Driver can be found here.
The DC4104 is another driver that is compatible with our fiber-coupled LEDs. This driver is capable of driving up to four of our fiber-coupled LEDs via the DC4100-HUB connector hub with a current range between 0 and 1000 mA. The current of each LED can be set individually by the driver or modulated individually by four external signals through the external modulation cable that comes included with the driver. Please contact Tech Support for additional cables. This driver can be operated through a wheel selector and three buttons on the front panel or remotely via USB 2.0 and the included software package.
Please note that these patch cables cannot be sterilized using an autoclave. Users can alternatively apply a light mist of an aqueous mixture containing Virkon™ disinfectant.
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Click Here for Raw Data
Plot comparing the recovery of autofluorescence for a low-autofluorescence (AF) and standard patch cable after photobleaching at 470 nm for 12 hours. An M470F3 LED was used for excitation and autofluorescence intensity at 525 nm was measured relative to the output power from the patch cable.
Fluorescence signals are characteristically low, so it is important to maximize signal collection by selecting fibers and cannulae with large numerical apertures and core sizes. In general, Ø400 µm, 0.50 NA core fiber is recommended for adequate signal level, though, to minimize implant size in in vivo experiments, Ø200 µm core fiber can be used as well. Although smaller NAs such 0.39 NA can be used for some optogenetics applications, fibers with a 0.50 NA maximize signal intensity and are therefore recommended for fiber photometry. However, in any high NA fiber, the polymer cladding itself will exhibit some non-zero autofluorescence (AF). On the input side, filters should be used to ensure the input signal is clean, but on the output side, autofluorescence can be easily confused with fluorescence from the genetically encoded calcium indicator (GECI) being studied in the experiment since they are modulated at the same frequency.
To reduce these artifacts, Low-Autofluorescence Patch Cables should be used. These cables are manufactured with components and epoxies that reduce the emitted autofluorescence in the visible spectrum and should be photobleached before use to further minimize noise (see the graph below). This reduction makes these patch cables suitable for fiber photometry applications where high sensitivity is required to measure the changes in fluorescence that indicate neural activity within a specimen. As shown in the schematic above, low-autofluorescence cables are most important in the signal collection path of the fiber photometry system. The design of these patch cables is based on testing of the autofluorescent properties of our patch cable components, such as the bare fiber, ferrule types, and epoxies.
Each patch cable is 1 m long and incorporates a Ø400 µm, 0.50 NA multimode fiber (Item # FP400URT) and is available with three connector configurations. One end is equipped with an FC/PC connector, while the other is equipped with an FC/PC connector, Ø1.25 mm stainless steel ferrule, or a Ø2.5 mm stainless steel ferrule. Black jacketing along the cable minimizes light leakage. Each patch cable includes two protective caps that shield the ferrule ends from dust and other hazards when not in use. Additional plastic, metal, or threaded caps for the connector and ferrule ends are sold separately. If the fiber ends become dirty from use, we offer a selection of inspection tools, as well as fiber optic cleaning products. Similar to our standard optogenetics patch cables, the patch cables with a ferrule end can be mated to a fiber optic cannula using an interconnect or mating sleeve; see the selection guide for compatible products.
We can custom fabricate cables, including armored cable for protection from specimen damage and fan-out cables for incorporating multiple light sources into one fiber optic implant. If you do not see a stock cable suitable for your application, please see our Custom Patch Cables webpage to request a cable that meets your specific needs.
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The APD440A2 Avalanche Detector is recommended for fiber photometry applications.
Thorlabs offers Avalanche Photodetectors (APD) for use in fiber optics systems. These devices feature a variable gain that can be controlled by a knob on the right side of the housing. SM1 threads are ideally matched with our internally SM1 threaded fiber adapters, such as S120-FC2 and S120-SMA, since they create a light-tight path that allows the fiber tip to be brought as close as possible to the detector element.
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The RJ1 rotary joint has terminated ends and accepts a wide variety of FC/PC patch cables.
Rotary Joint Commutator
The collimators and RJ1 rotary joints are designed specifically for use with high NA fibers and use an achromatic design to ensure that insertion loss is consistent throughout the visible wavelength spectrum that pertains to fiber photometry. The selection of the filters and dichroics will depend upon the specific GECI being studied, with 470 nm (gCaMP) and 565 nm (rCaMP) being common examples.
The RJ1 rotary joint commutator has been shown not to add any fluorescence and has minimal rotational variation, which is critical when working with low signal levels. The achromatic design of the RJ1 ensures that insertion loss and rotational variation is very similar throughout the visible wavelength spectrum (400-700 nm), so again the isosbestic point can be used to differentiate motion artifacts from rotation from real calcium transients. The pigtailed rotary joints have higher rotational variation and are therefore not recommended for fiber photometry.
Thorlabs offers interconnects and mating sleeves for making connections between our line of optogenetics patch cables and fiber optic cannulae. These ferrule mating components provide low-loss coupling and are compatible with both stainless steel and ceramic (zirconia) ferrules. Interconnects are designed to facilitate easy connections and disconnections from an implanted cannula, requiring >80% less force to disconnect compared to mating sleeves. On the other hand, mating sleeves are preferred for very lightweight (~0.18 g), low-profile connections between a patch cable and cannula.
Excitation and emission filters are marked with an arrow that shows the recommended direction of light propagation.
Dichroic filters are marked on the side with the dichroic coating. Light should be incident on this side for best performance.
Fluorescence Imaging Filters and Dichroics
These excitation, emission, and dichroic filters are designed specifically for use in fluorescence imaging applications. They are fabricated at industry-standard dimensions that make them compatible with filter cubes from all major manufacturers. We offer individual filters and filter sets targeted at common fluorophores: BFP, CFP, WGFP, GFP, FITC, Alexa Fluor® 488, YFP, tdTomato, TRITC, Texas Red, mCherry, and Cyanine (CY3.5). In addition, the Fluorophores tab provides information on the alternative fluorophores suitable for these filters. These filters are also available pre-installed into our microscope filter cubes.
Each filter is housed in a black anodized aluminum ring, which makes handling easier and enhances the blocking OD by limiting scattering. These filters can be mounted in our extensive line of filter mounts and wheels. As the mounts are not threaded, Ø1" retaining rings will be required to mount the filters in one of our internally-threaded SM1 lens tubes. For customers who wish to use these filters in Thorlabs, Olympus, or Nikon fluorescence microscopes, Thorlabs manufactures a family of Drop-In Microscope Filter Cubes.
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Each filter cube can hold a fluorescence filter set: one dichroic mirror, one excitation filter, and one emission filter.
Thorlabs' Fluorescence Filter Cubes are designed to hold a fluorescence filter set (dichroic mirror, excitation filter, and emission filter) in a cage-system-compatible cube for home-built fiber photometry setups. The insert is designed to hold a rectangular optic, such as a dichroic mirror or filter. In addition, the cube insert has four round ports for emission or excitation filters. These filters can be mounted in two possible configurations and are held using the included retaining rings, which can be tightened or loosened with our spanner wrenches (sold separately). At least one emission port is oriented at a 3° angle with respect to the face in order to reduce undesired reflections. To help keep track of your filters, spaces are provided on the top to write labels for the mounted filters and mirrors. The cube insert is designed to be inserted in a single orientation with respect to the cube base, so it is important to orient the full cube assembly in the proper direction. The 30 mm cage cubes and cage cube inserts on this page are available with either a left- or right-turning orientation.
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High-NA Achromatic Collimators like the F950FC-A are compatible with high-NA multimode fiber, making them ideal for fiber photometry.
Thorlabs' High-NA Achromatic Collimators are designed for use with high-NA multimode fiber and are therefore ideal for applications such as fiber photometry. These triplet collimators feature a meniscus lens and an achromatic doublet for high performance across the visible spectrum with low spherical aberration. The optics are broadband antireflection (AR) coated for 350 to 700 nm at the air-to-glass interfaces to minimize losses caused by surface reflections. Each collimator is factory aligned at 473 nm. The collimator housing includes either an FC/PC 2.2 mm wide key connector or an SMA905 port; typically, FC/PC collimators will offer better coupling efficiency.
The multimode collimators are compatible with the AD15F and AD15NT adapters for setups requiring SM1-threaded or Ø1" smooth bore compatibility; additional collimator mounting adapters are also available. When using these collimators as a free-space coupler, precise alignment is needed for good coupling efficiency. For autofluorescence and fiber photometry applications, our fluorescence filter cube is ideal for directing a free-space collimated beam towards fluorescence imaging filters. It is also possible to use a kinematic tip-and-tilt mount paired with an XYZ adjustable platform (such as our KM100V Kinematic V-Mount and MT3(/M) XYZ Translation Stage). Please note that using these collimators without a kinematic mount will introduce 1 dB of loss or more.
Thorlabs offers a wide variety of multimode patch cables that can be used with these collimators, including lightweight cables for optogenetics and low-autofluorescence cables for fiber photometry applications.
We also offer a line of aspheric fiber collimators, including our fixed collimators and our FiberPort adjustable collimation packages, that are well suited for use with a wide range of wavelengths. For our complete line of collimation and coupling options, please see the Collimator Guide tab.
Thorlabs offers a variety of fiber-coupled light sources that can be used for in vivo stimulation. Our Fiber-Coupled LEDs offer a durable, economic solution with a variety of wavelength choices. Our Fiber-Coupled Lasers, Pigtailed Laser Diodes, and Benchtop Laser Diodes provide higher power at the cannula tip, and our Multichannel Light Sources offer a variety of wavelengths in a compact unit.
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Multimode Optical Fiber
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SMA905 Connector Components
Cable and Cannula Building Supplies
Thorlabs stocks individual components for building custom fiber optic cannulae and patch cables, including optical fiber, ferrules, connectors, tubing, and connectorization tools. Our FN96A connectorization manual, which is free to download, gives clear instructions on how to add connectors to optical fiber.
Optogenetics Selection Guide
Thorlabs offers a wide range of optogenetics components; the compatibility of these products in select standard configurations is discussed in detail here. Please contact Technical Support for assistance with items outside the scope of this guide, including custom fiber components for optogenetics.
The most straightforward method for in vivo light stimulation of a specimen is to use a single fiber optic with a single LED light source. The single wavelength LED is powered by an LED driver, and then the illumination output is fiber-coupled into a patch cable, which connects to the implanted cannula. See the graphics and expandable compatibility tables below for the necessary patch cables and cannulae to create this setup. To choose the appropriate LED and driver, see below or the full web presentation.
Click on Each Component for More Information
Click to See Ø1.25 mm (LC) Ferrule Compatible Patch Cables, Cannulae, and Interconnects
Click to See Ø2.5 mm (FC) Ferrule Compatible Patch Cables, Cannulae, and Interconnects
The ability to accurately and simultaneously direct light to multiple locations within a specimen is desired for many types of optogenetics experiments. For example, bilateral stimulation techniques typically target neurons in two spatially separated regions in order to induce a desired behavior. In more complex experiments involving the simultaneous inhibition and stimulation of neurons, delivering light of two different monochromatic wavelengths within close proximity enables the user to perform these experiments without implanting multiple cannulae, which can increase stress on the specimen.
Bilateral stimulation can be achieved with several different configurations depending on the application requirements. The sections below illustrate examples of different configurations using Thorlabs' optogenetics products.
Thorlabs' RJ2 1x2 Rotary Joint Splitter is designed for optogenetics applications and is used to split light from a single input evenly between two outputs. The rotary joint interface allows connected patch cables to freely rotate, reducing the risk of fiber damage caused by a moving specimen. See the graphic and compatibility table below for the necessary cables and cannulae to create this setup. For LEDs and drivers, see below or the full web presentation.
Click to See Ø1.25 mm (LC) Ferrule Components Recommended for Use with RJ2 Rotary Joint Splitter
Click to See Ø2.5 mm (FC) Ferrule Components Recommended for Use with RJ2 Rotary Joint Splitter
If the intent is for one LED source to connect to two cannulae for simultaneous light modulation, then a bifurcated fiber bundle can be used to split the light from the LED into each respective cannula. For dual wavelength stimulation (mixing two wavelengths in a single cannula) or a more controlled split ratio between cannula, one can use a multimode coupler to connect one or two LEDs to the cannulae. If one cable end is left unused, the spare coupler cable end may be terminated by a light trap. See the graphic and compatibility table below for the necessary cables and cannulae to create this setup. For LEDs and drivers, see below or the full web presentation.
Click on Each Component Below for More Information
For bilateral stimulation applications where the two cannulas need to be placed in close proximity (within ~1 mm), Thorlabs offers dual-core patch cables and cannulae that are designed for this specific application. Each core is driven by a separate light source, enabling users to stimulate and/or supress nerve cells in the same region of the specimen. See the graphic and compatibility table below for the necessary cables and cannulae to create this setup. For LEDs and drivers, see below or the full web presentation.
Click on Each Component for More Information
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Our fiber-coupled LEDs are ideal light sources for optogenetics applications. They feature a variety of wavelength choices and a convenient interconnection to optogenetics patch cables. Thorlabs offers fiber-coupled LEDs with nominal wavelengths ranging from 280 nm to 1050 nm. See the table to the right for the LEDs with the most popular wavelengths for optogenetics. A table of compatible LED drivers can be viewed by clicking below.
Custom Fiber Optic Cannula
Thorlabs offers custom cannulae with the following options. Place a quote request through the following form to receive a quote for your custom cannula order.
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
The table below displays all of the fluorophores that are compatible with our filter sets. The filter set item numbers are listed across the top row and the fluorophores are listed down the first column. Scroll through the table to view fluorophore compatibility with our filter sets.
Click on the below to view the filter set transmission with the absorption and emission spectra of the fluorophore. The key to the right details the meaning of all check marks in the table below. Please note that absorption and emission spectra are unavailable if any is red.
Fiber Collimator Selection Guide
Click on the collimator type or photo to view more information about each type of collimator.