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1x4 Single Mode Fiber Optic Couplers
1550 nm 1x4 Wideband Coupler, FC/APC Connectors
1064 nm 1x4 Narrowband Coupler, FC/PC Connectors
Use for Splitting Signals
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1x4 Wideband Single Mode Coupler Mounted on FCQB Base (Available Below)
Thorlabs' Single Mode 1x4 Fiber Optic Couplers allow a user to split a single input signal evenly into four output signals. Several center wavelength options are available (see the table to the right for details). Narrowband couplers have a ±15 nm bandwidth, dual-window couplers have a ±40 nm bandwidth around each center wavelength, and wideband couplers have a ±50 nm or ±100 nm bandwidth. 1x4 couplers are manufactured using three 50:50 fiber couplers to split the signal from the input port (see the 1x4 Coupler Tutorial for details); they cannot be used in reverse to combine light from four sources. The unused ports on these internal 50:50 fiber couplers are terminated in a manner that minimizes back reflections.
For our wideband 1x4 couplers, Thorlabs provides an individual test data sheet with each coupler that includes coupling data and performance graphs. These graphs, which show data within the design bandwidth and also show measured data outside of the specified bandwidth including the entire wavelength range where the coupling ratio meets the specified tolerance. Sample data sheets for our 1x4 wideband couplers can be viewed below. Please note that the data sheets for the 630 nm and 1064 nm 1x4 narrowband couplers do not include performance graphs, but a typical performance plot is included in the spec sheets below.
Each single mode coupler is contained in a compact 100 mm x 80 mm x 10 mm housing that includes four through holes for mounting the device to our FCQB mounting base (available separately below). Narrowband and wideband couplers use a labeled red housing and 0.8 m fiber leads jacketed using Ø900 µm Hytrel® tubing. Dual-window couplers feature either a labeled red housing and 0.8 m fiber leads jacketed using Ø900 µm Hytrel® tubing, or a labeled black housing and 0.8 m fiber leads jacketed using Ø3 mm yellow furcation tubing. Couplers are offered from stock with 2.0 mm narrow key FC/PC or FC/APC connectors. Custom coupler configurations with other wavelengths, fiber types, coupling ratios, connectors, or port configurations are also available. Please contact Tech Support with inquiries.
Definition of 1x4 Fused Fiber Optic Coupler Specifications
This tab provides a brief explanation of how we determine several key specifications for our 1x4 couplers. 1x4 couplers are manufactured using three 50:50 couplers internally to split the input signal evenly among four outputs (as shown in the schematic below). Any unused ports are terminated using a propietary method that reduces back reflections. 1x4 couplers are not recommended for light combining applications and should only be used to split light. For combining light of different wavelengths, Thorlabs offers a line of wavelength division multiplexers (WDMs). The ports on our 1x4 couplers are configured as shown in the schematic below.
Excess loss in dB is determined by the ratio of the total input power to the total output power:
Pinput is the input power and Pport1+Pport2+Pport3+Pport4 is the total output power. All powers are expressed in mW.
Optical Return Loss (ORL) / Directivity
The directivity refers to the fraction of input light that is lost in the internally terminated fiber ends within the coupler housing. It can be calculated in units of dB using the following equation:
where Pt1, Pt2, and Pt3 are the optical powers (in mW) in the internally terminated fiber ends shown in the image above. This is the result of back reflections at each coupler junction and represents a loss in the total light output at the output ports. For a 1x4 coupler with an even split, the directivity is equal to the optical return loss (ORL).
The insertion loss is defined as the ratio of the input power to the output power at one of the output legs of the coupler. Insertion loss is always specified in decibels (dB). It is generally defined using the equation below:
where Pin and Pout are the input and output powers (in mW). For our 1x4 couplers, the insertion loss specification is provided for each output port. To define the insertion loss for a specific output (e.g., port 1 or port 2), the equation is rewritten as:
Insertion loss inherently includes both coupling (e.g., light transferred to the other output legs) and excess loss (e.g., light lost from the coupler) effects. The maximum allowed insertion loss for each output is specified. Because the insertion loss in each output is correlated to light coupled to the other outputs, no coupler will ever have the maximum insertion loss in all outputs simultaneously.
Calculating Insertion Loss using Power Expressed in dBm
Then, the insertion loss in dB can be calculated as follows:
Insertion loss (in dB) is the ratio of the input power to the output power from each leg of the coupler as a function of wavelength. It captures both the coupling ratio and the excess loss. The coupling ratio is calculated from the measured insertion loss. Coupling ratio (in %) is the ratio of the optical power from each output port to the sum of the total power of all output ports as a function of wavelength. It is not impacted by spectral features such as the water absorption region because all output legs are affected equally.
The uniformity is also calculated from the measured insertion loss. Uniformity is the variation (in dB) of the insertion loss over the bandwidth as a function of wavelength. It is a measure of how evenly the insertion loss is distributed over the spectral range. The uniformity is defined as the difference between the insertion loss in one output leg at a given wavelength and the highest or lowest value of insertion loss over the specified wavelength range in that same output leg.
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
These 1x4 Wideband Fiber Optic Couplers are designed for splitting a single input signal at 560 nm equally into four output signals. The couplers have an operating bandwidth of ±50 nm and are available with 2.0 mm narrow key FC/PC or FC/APC connectors. They have a max power level of 100 mW with connectors or bare fiber and 250 mW when spliced (see the Damage Threshold tab for more details).
These 1x4 Narrowband Fiber Optic Couplers are designed to split a single input signal at 630 nm equally into four output signals. The couplers have an operating bandwidth of ±15 nm and are available with 2.0 mm narrow key FC/PC or FC/APC connectors. They can handle a max power of 300 mW with connectors or unterminated (bare) fiber and 0.5 W when spliced (see the Damage Threshold tab for more details).
These 1x4 Wideband Fiber Optic Couplers are designed for splitting a single input signal at 850 nm equally into four output signals. The couplers have an operating bandwidth of ±100 nm and are available with 2.0 mm narrow key FC/PC or FC/APC connectors. They can handle a max power of 500 mW with connectors or unterminated (bare) fiber and 2 W when spliced (see the Damage Threshold tab for more details).
These 1x4 Fiber Optic Couplers are designed for splitting a single input signal at 1064 nm equally into four output signals. The couplers feature an operating bandwidth of ±15 nm or ±100 nm and are available with 2.0 mm narrow key FC/PC or FC/APC connectors. These narrowband and wideband couplers can handle a max power of 1 W with connectors or bare fiber and 5 W when spliced (see the Damage Threshold tab for more details).
These 1x4 Wideband Fiber Optic Couplers are designed for splitting a single input signal at 1300 nm equally into four output signals. The couplers have an operating bandwidth of ±100 nm and are available with 2.0 mm narrow key FC/PC or FC/APC connectors. They can handle a max power of 1 W with connectors or bare fiber and 5 W when spliced (see the Damage Threshold tab for more details).
These 1x4 Dual-Window Fiber Optic Couplers are designed for splitting a single input signal at 1310 nm or 1550 nm equally into four output signals. The couplers have operating bandwidths of ±40 nm and are available with 2.0 mm narrow key FC/PC or FC/APC connectors. They can handle a max power of 1 W with connectors or bare fiber and 5 W when spliced (see the Damage Threshold tab for more details).
These 1x4 Wideband Fiber Optic Couplers are designed for splitting a single input signal at 1550 nm equally into four output signals. The couplers have an operating bandwidth of ±100 nm and are available with 2.0 mm narrow key FC/PC or FC/APC connectors. They can handle a max power of 1 W with connectors or bare fiber and 5 W when spliced (see the Damage Threshold tab for more details).
Our FCQB mounting base provides two 2.25" long clearance slots for 1/4" (M6) cap screwsto secure the base to an optical table or other tapped surface. The two clearance slots are located 4" (101.6 mm) apart at opposite edges of the mounting base. Four M2 taps between the clearance slots are positioned to align with the through holes in Thorlabs' RGB wavelength combiners, 1x4 SM Couplers, or 1x4 PM Couplers. Four M2 screws are included.