Operating Principle and Applications
Castor Optics' double-clad fiber couplers (DCFCs) are designed for robust separation and combination of a single-mode and multimode signal. This overview outlines their operating principles, types, practical considerations, and key applications such as OCT, fluorescence imaging, and confocal microscopy.
Guiding both Single-Mode and Multimode Light
Double-Clad Fibers
Double-clad fibers (DCFs) are ideally suited to combine modalities requiring both single-mode (SM) and multimode (MM) light. Those fibers are characterized by two concentric light-guiding regions: the core and the inner cladding. Typically, the indices and dimensions of these regions are chosen to provide wavelength-specific single-mode propagation in the core and multimode propagation in the inner cladding.
The figure on the left displays cross-sections of a single-mode fiber (SMF), a multimode fiber (MMF), and a double-clad fiber (DCF).
Multiplexing Single-Mode and Multimode Light
Double-Clad Fiber Couplers
A key aspect of working with DCFs is efficiently addressing light from each fiber region separately. This is where the double-clad fiber coupler (DCFC) comes in. A DCFC essentially consists of a modal separator, demultiplexing single-mode and multimode signals:
- In the single-mode regime, a DCFC acts as a null coupler for which light is transmitted from one DCF port to the other ideally, quasi-losslessly.
- In the multimode regime, the coupler efficiently transfers light from one waveguide to the other, either from the DCF’s inner cladding to a second fiber or vice versa.
The figure on the right shows a DCFC with a single-mode signal (red) propagating through the core from Port A to S, and, reciprocally, from S to A. The multimode signal (blue) is coupled into the inner cladding at Port S and is transferred to the second waveguide through the coupler towards Port B.
Maximizing Multimode Transfer
Optical Étendue
The theoretical multimode transfer limit for a given structure can be approximated as the ratio of optical étendues.
The optical étendue G of a waveguide is defined as the product of the surface of its cross-section S with its numerical aperture squared (or equivalently, the solid angle): G=πS NA2 , where NA=(n12-n22 )1/2, with n1 and n2, the refractive indices of the fiber core and inner cladding.
Knowing the étendues of each waveguide within the coupler structure, the theoretical multimode transfer limit T is given by: T=GB/(GB+GA), where GA and GB are the étendues at Port A and B, respectively.
This equation highlights a theoretical limit of the DCFC: performances cannot exceed a certain point considering the NA of the fibers used. Other factors, such as intrinsic loss and fabrication constraints, also limit the maximum transfer ratio.
Understanding the Different DCFC Configurations
Since their first demonstrations, several DCFC designs have been developed to support a broader range of applications. DCFCs are primarily differentiated by their geometry, which is the key parameter for the intended application. Each geometry can then be implemented over different wavelength ranges depending on the operating requirements.
Geometry
- Large collection: a large portion of a multimode signal (i.e., incoherent signal) is collected using a large inner cladding DCF, optimizing multimode collection at Port S and its transfer to Port B. This is ideal for applications such as fluorescence collection, low-light collection, and highly multimode signal collection.
- Confocal collection: a small portion of a multimode signal (i.e., confocal or partially coherent signal) is collected using a small inner cladding DCF, specifically designed to allow high detection resolution while maximizing the intensity of the collected signal. This is ideal for applications such as confocal detection and small-spot-size illumination.
- Bidirectional: Allowing both multimode injection and collection with high efficiencies. It is ideal for applications such as LiDAR, hyperspectral imaging, or spectroscopy combined with a single-mode modality.
Each type is optimized for different imaging and detection modalities. Table I shows typical applications for each type of DCFCs. Further insights about applications can be found at castoroptics.com.
Table I
| Type | Large Collection | Confocal Collection | Bidirectional |
|---|---|---|---|
| Multimode Operation | Port S → B | Port S → B | Port S ↔︎ B |
| Typical Applications | OCT & fluorescence imaging OCT & reflectance imaging LiDAR Quantum sensing | Confocal imaging OCT & SLO | LiDAR Free space optical communication |
Wavelength range
- Castor Optics’ couplers are available for single-mode signal operation centered at 530, 780, 1060, and 1300/1550 nm.
- Multimode transmission is uniform over the entire operating range of the fiber, typically from 400 to 1600 nm.
Buyer's Guide
Selecting a DCFC requires considering the practical constraints of the intended application. The following questions can help guide that selection.
→ What is the operational wavelength of the single-mode modality?
For a specific single-mode modality, the wavelength range will define which DCF core size is best suited for the application.
→ Do you need confocal detection?
Then choose from our Confocal collection series. The inner cladding-to-core diameter ratio is specifically designed to optimize the system resolution while maximizing collection intensity.
→ Do you need to collect a low-intensity multimode signal?
Then choose from our Large collection series. Applications such as LiDAR, spectroscopy, remote sensing, hyperspectral imaging, and fluorescence light collection benefit from this DCFC configuration.
→ Do you need multimode injection AND extraction?
Then choose from our Bidirectional series. A bidirectional DCFC allows for multimode injection and extraction to be performed through the same component. Depending on the target application, it can be tailored to either favor injection performance over extraction or to optimize both directions. Our Bidirectional series also benefits from using a smaller-core multimode collection fiber, compatible with a smaller surface detector.
→ Is your application sensitive to back-reflection or cross-talk between the two modalities?
Ask for optimized connectors, such as high-angle polishing or an anti-reflective (AR) coating, to reduce back-reflections at Port S and contamination from the core to the inner cladding.
The DCFC series is available on Thorlabs.com. To learn more about how Castor Optics' innovative fiber optics technology can enable faster, farther, and brighter sensing, contact our team at sales@castoroptics.com.
By the Castor Optics Application Scientist Team
