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1550 nm 1x2 Polarization-Maintaining Fiber Optic Couplers / Taps

  • Narrowband PM Couplers with 1550 nm Center Wavelength
  • 50:50, 75:25, 90:10, or 99:1 Split Ratio
  • 2.0 mm Narrow Key FC/PC or FC/APC Connectors
  • High ≥25 dB Polarization Extinction Ratio (PER) Options

Use for Splitting Signals


90:10 PM Coupler with FC/PC Connectors and High PER


50:50 PM Coupler with FC/APC Connectors

Related Items

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1x2 PM Coupler Selection Guide
Center Wavelength Bandwidth
480 nm ±15 nm
530 nm ±15 nm
560 nm ±15 nm
635 nm ±15 nm
670 nm ±15 nm
780 nm ±15 nm
850 nm ±15 nm
980 nm ±15 nm
1064 nm ±15 nm
1310 nm ±15 nm
1480 nm ±15 nm
1550 nm ±15 nm

Click for Details

Each coupler is engraved with the Item #, serial number, and key specifications for easy identification. When the white port on the left is used as the input, the coupling ratios listed below correspond to the ratio of the measured output power from the white (signal output) port to the red (tap output) port.

PANDA PM Fiber Cross Section

Panda Style Fiber
The connector key is aligned to the slow axis of the fiber.


  • 1550 nm Polarization-Maintaining Fiber Optic Couplers
  • 50:50, 75:25, 90:10, or 99:1 Split Ratio
  • 2.0 mm Narrow Key FC/PC or FC/APC Connectors
  • ≥20.0 dB or ≥25.0 dB Polarization Extinction Ratio (PER) Options
  • Individual Test Report Included with Each Coupler
    (See the PER Measurement Tab; Click Here for a Sample Data Sheet)
  • Contact Us for Custom Wavelength, Coupling Ratio, and Connector Options

These 1x2 Polarization-Maintaining (PM) Fiber Couplers are designed for operation at 1550 nm and are available with a 50:50, 75:25, 90:10, or 99:1 coupling ratio. 1x2 couplers have only one input port for simplified use and cable management. These couplers are ideal for applications where light is split from the input port into two output ports at the specified coupling ratio; unlike WDMs, they are generally not recommended for beam combining applications. The unused port is internally terminated within the coupler housing in a manner that minimizes back reflections (please see the 1x2 Coupler Tutorial tab for details).

PM couplers are manufactured using PANDA fiber, which allows them to maintain a high polarization-extinction-ratio (PER) when light is launched along the slow axis of the fiber. As seen in the diagram to the right, stress rods run parallel to the fiber's core and apply stress that creates birefringence in the fiber's core, allowing polarization-maintaining operation. Typical applications for PM couplers include optical sensors, optical amplifiers, and fiber gyroscopes.

Thorlabs' High Polarization Extinction Ratio (PER) Couplers (highlighted in green in the tables below) provide a PER of ≥25 dB when measured with connectors while the standard PM Couplers provide a PER of ≥20 dB when measured with connectors. For specific differences between the standard and high PER cables, please refer to the Comparison tab. As with all the couplers on this page, the high PER versions have a specified operating temperature range of -40 °C to 85 °C but the actual PER will vary slightly over the entire range, as shown in the Temperature Cycling Tests section of the PER Measurement tab. Note that within an optical system the achievable PER is always limited by the element with the lowest PER within the system.

All of these 1550 nm couplers have a maximum power handling of 1 W with connectors or bare fiber and a maximum power handling of 5 W when spliced (see the Damage Threshold tab for more details). These couplers undergo extensive individual testing and verification of the PER; details of our testing procedures are provided on the PER Measurement tab. Testing results are included with a data sheet that is shipped with these couplers. A sample data sheet for the 1550 nm PM couplers can be viewed here.

These couplers are available with 2.0 mm narrow key FC/PC or FC/APC connectors, as outlined in the tables below. Fiber leads are jacketed in Ø900 µm Hytrel® tubing and the leads are 0.8 m long. Custom coupler configurations with other wavelengths, fiber types, coupling ratios, alignment axes, or port configurations are also available. Please contact Tech Support with inquiries.

Alternative Fiber Coupler & Splitter Options
Double-Clad Couplers Single Mode Couplers Single Mode PLC Splitters Multimode Couplers Polarization-Maintaining Couplers Wavelength Division
Multiplexers (WDM)
2x2 1x2 2x2 1x4 1x8 1x16 1x2 2x2 1x2 2x2 1x4
Item #a Description Qty.
Light Source (Not Shown)
S5FC1005P PM Benchtop SLD Source, 1550 nm 1
P1-1550PM-FC-1 Patch Cable, FC/PC, 1550 nm, PM Panda Style, 1 m 1
Linear Polarizer Module
PAF-X-11-PC-Cb FiberPort, FC/PC,
1050 nm - 1620 nm
CP08FP(/M) Cage Plates for Mounting FiberPorts 2
LPNIR050-MP2 Linear Polarizer 1
CRM1P(/M) Cage Rotation Mount 1
SM1A6T Adapter with External SM1 Threads and Internal SM05 Threads 1
ER2-P4 2" (50.8 mm) Long Cage Rods, 4 Pack 1
Analyzer Module
PAF-X-11-PC-Cb FiberPort, FC/PC,
1050 nm - 1620 nm
CP08FP(/M) Cage Plates for Mounting FiberPorts 1
LPNIR050-MP2 Linear Polarizer 1
CRM1P(/M) Cage Rotation Mount 1
SM1A6T Adapter with External SM1 Threads and Internal SM05 Threads 1
CP33(/M) SM1-Threaded (1.035"-40)
Cage Plate
PM122D Digital Power Meter, 700 - 1800 nm 1
ER2-P4 2" (50.8 mm) Long Cage Rods, 4 Pack 1
  • Item list does not include the posts, post holders, clamping forks, breadboard, or fiber component tray shown in the photo to the left.
  • This previous-generation FiberPort is no longer available. A suitable replacement is the PAF2P-11C FiberPort.

Click to Enlarge
Setup to Measure the Extinction Ratio of a
1550 nm PM Coupler

Measurement of Polarization Extinction Ratio (PER)

The polarization extinction ratio (PER) is a measure of how well a polarization-maintaining (PM) fiber or device can prevent cross coupling between the different polarization axes of the fiber. External stress on a fiber from sources such as heating, bending, or pulling can cause the PER to change.

There are two accepted techniques for measuring PER in a fiber coupler. The most common method uses a low-coherence (unpolarized or circularly polarized) broadband light source and measures the extinction ratio with a linear polarizer and power meter. An alternative method uses a narrowband, high-coherence light source and measures the PER with a polarimeter.

Thorlabs uses the power meter method to characterize the extinction ratio performance of the PM fiber couplers sold on this page. An example setup is shown in the image to the right with itemized component list in the table. A broadband light source is input into a linear polarizer module, which allows the user to set the polarization of light entering the input leg of the fiber coupler. The output from one of the legs is sent to the analyzer module, which contains another polarizer and the power meter for measuring the output. Alternatively, the analyzer module can be replaced with an extinction ratio meter
(Item # ERM100).

The PER is measured using the following test procedure:

  • Prepare the fiber end faces of the PM coupler to connect to the measurement setup.
    • For bare fiber ends, strip and cleave the fibers. Use a bare fiber terminator, such as the BFT1, to create a temporary fiber termination.
    • For terminated fiber ends, clean and inspect the connector end faces.
    • Attach a fiber optic light trap to any fiber leads not being measured.
  • Adjust the polarizers in the linear polarizer and analyzer modules sequentially until a minimum power value is measured by the power meter. Record the measured value as Pmin.
  • Rotate the analyzer rotation mount by 90°. Then record the measured value as Pmax.

After Pmin and Pmax are measured, the extinction ratio can be calculated:

Click to Enlarge
7-hour temperature cycling test performed on a standard PN1550R5A1 PM fiber coupler shows that the PER measured for the white-white and white-red paths remains stable over a wide temperature range.

The White-White Path Follows the Input to Signal Output and the White-Red Path Follows the Input to Tap Output

Temperature Cycling Tests

Traditional PM couplers typically exhibit diminished polarization extinction ratio (PER) performance when used at sub-zero (°C) temperatures due to the contraction of the adhesives that are used to assemble the device. This effect disrupts the polarization state of light within the coupler and leads to a decrease in PER. Soft adhesives can be used to mitigate the impact of cold-temperature operation but can create reliability issues at higher temperatures. At high temperatures, adhesives can soften permanently, which changes the optical properties of the coupler.

Unlike traditional coupler manufacturing, Thorlabs uses a proprietary packaging process and design for our standard PM couplers as well as careful selection of adhesives to enable operation over a very wide temperature range (from -40 °C to 85 °C) without significant changes to PER and other optical specifications.

1550 nm PM Coupler Comparison

The graphs below depict sample data comparing Thorlabs' 1550 nm high polarization extinction ratio (PER) PM couplers to our standard 1550 nm PM Couplers. Our high PER couplers are specified to have an extinction ratio of 25 dB or higher at 1550 nm, whereas the standard cables are only guaranteed to have an extinction ratio of at least 20 dB. 

For more information about the methods used to measure the PER, please refer to the PER Measurement tab.

Definition of 1x2 Fused Fiber Optic Coupler Specifications

This tab provides a brief explanation of how we determine several key specifications for our 1x2 couplers. 1x2 couplers are manufactured using the same process as our 2x2 fiber optic couplers, except the second input port is internally terminated using a proprietary method that minimizes back reflections. For combining light of different wavelengths, Thorlabs offers a line of single mode wavelength division multiplexers (WDMs). The ports on our 1x2 couplers are configured as shown in the schematic below.

1x2 CouplerPicture


Excess Loss

Excess loss in dB is determined by the ratio of the total input power to the total output power:

Excess Loss

Pport1 is the input power at port 1 and Pport2+Pport3 is the total output power from Ports 2 and 3. All powers are expressed in mW.


Polarization Dependent Loss (PDL)

The polarization dependent loss is defined as the ratio of the maximum and minimum transmissions due to polarization states in couplers. This specification pertains only to couplers not designed for maintaining polarization. PDL is always specified in decibels (dB), and can be calculated with the following equation:

Polarization Dependent Loss Equation

where Pmax is the maximum power able to be transmitted through the coupler when scanning across all possible polarization states. Pmin is the minimum transmission across those same states.


Optical Return Loss (ORL) / Directivity

The directivity refers to the fraction of input light that is lost in the internally terminated fiber end within the coupler housing when port 1 is used as the input. It can be calculated in units of dB using the following equation:

Directivity or Return Loss

where Pport1 and Pport1b are the optical powers (in mW) in port 1 and the internally terminated fiber, respectively. This output is the result of back reflection at the junction of the legs of the coupler and represents a loss in the total light output at ports 2 and 3. For a 50:50 coupler, the directivity is equal to the optical return loss (ORL).


Insertion Loss

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 (signal or tap). Insertion loss is always specified in decibels (dB). It is generally defined using the equation below:

Insertion Loss

where Pin and Pout are the input and output powers (in mW). For our 1x2 couplers, the insertion loss specification is provided for both signal and tap outputs; our specifications always list insertion loss for the signal output first. To define the insertion loss for a specific output (port 2 or port 3), the equation is rewritten as:

Insertion Loss

Insertion Loss

Insertion loss inherently includes both coupling (e.g., light transferred to the other output leg) and excess loss (e.g., light lost from the coupler) effects. The maximum allowed insertion loss for each output, signal and tap, are both specified. Because the insertion loss in each output is correlated to light coupled to the other output, no coupler will ever have the maximum insertion loss in both outputs simultaneously.

Calculating Insertion Loss using Power Expressed in dBm
Insertion loss can also be easily calculated with the power expressed in units of dBm. The equation below shows the relationship between power expressed in mW and dBm:


Then, the insertion loss in dB can be calculated as follows:

Insertion Loss


Click to Enlarge

A graphical representation of the coupling ratio calculation.

Coupling Ratio

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 (ports 2 and 3) to the sum of the total power of both output ports as a function of wavelength. Path A represents light traveling from port 1 to port 2 while Path B represents light traveling from port 1 to port 3. It is not impacted by spectral features such as the water absorption region because both output legs are affected equally.


Click to Enlarge

A graphical representation of the Uniformity calculation.


The uniformity is also calculated from the measured insertion loss. Uniformity is the variation (in dB) of the insertion loss over the bandwidth. It is a measure of how evenly the insertion loss is distributed over the spectral range. The uniformity of Path A is the difference between the value of highest insertion loss and the solid red insertion loss curve (in the Insertion Plot above). The uniformity of Path B is the difference between the solid blue insertion loss curve and the value of lowest insertion loss.

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.

Power Handling Limitations Imposed by Optical Fiber
Click to Enlarge

Undamaged Fiber End
Power Handling Limitations Imposed by Optical Fiber
Click to Enlarge

Damaged Fiber End

Damage at the Air / Glass Interface

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
(Average Power)
~1 MW/cm2 ~250 kW/cm2
10 ns Pulsed
(Peak Power)
~5 GW/cm2 ~1 GW/cm2
  • All values are specified for unterminated (bare) silica fiber and apply for free space coupling into a clean fiber end face.
  • This is an estimated maximum power density that can be incident on a fiber end face without risking damage. Verification of the performance and reliability of fiber components in the system before operating at high power must be done by the user, as it is highly system dependent.
  • This is the estimated safe optical power density that can be incident on a fiber end face without damaging the fiber under most operating conditions.

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
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 µm= 7.07 x 10-8 cm2

 SMF-28 Ultra Fiber: Area = Pi x (MFD/2)2 = Pi x (5.25 µm)2 = 86.6 µm= 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. The