Mode Field Converters
Author: the photonics expert Dr. Rüdiger Paschotta (RP)
Definition: optical devices which allow for efficient coupling between modes of different sizes
Alternative term: mode field adapters
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DOI: 10.61835/3dp Cite the article: BibTex plain textHTML Link to this page! LinkedIn
What is a Mode Field Converter?
A mode field converter (or mode field adapter) is an optical device which can be used for expanding or contracting a mode in the transverse spatial dimensions. Typically, it is used to efficiently transfer light from one waveguide device to another, having substantially different mode characteristics. In many cases, the involved waveguides have single-mode characteristics.
Efficient coupling (meaning coupling to the desired mode with small insertion loss) is possible only when the mode radii and transverse mode shapes of the two waveguides are approximately equal. (The article on fiber joints contains a formula for estimating the minimum coupling loss for Gaussian mode profiles.) Note that one also requires matching wavefronts; it is not sufficient, for example, to let light diverge (which implies curved wavefronts) until it has the same mode radius as the target waveguide. As the article on mode matching explains, one can use an overlap integral involving complex mode functions for calculating the coupling efficiency.
Varying Requirements
The requirements on mode field converters can be quite diverse. Some examples:
- Some adapters require only a moderate amount of mode field adjustment, while others require a drastic change in mode size.
- The involved fiber cladding diameters can vary.
- In some cases, polarization-maintaining characteristics are needed.
- A high power handling capability is essential in some cases.
Examples of Mode Field Converters
Coupling Between Fibers
For various applications, such as high-power fiber lasers and amplifiers, there are large mode area fibers which have a substantially larger effective mode area than standard single-mode fibers. Simply using fusion splicing to couple them could lead to a large insertion loss on the order of 10 dB. Therefore, one often uses a mode field converter, which typically provides an adiabatic transfer, where the mode size is gradually varying between the values of input and output fiber. Note that the transition must be sufficiently slow to allow for adiabatic mode adjustment without coupling to other modes.
There are different methods for fabricating such mode field adapters. One method involves pulling of a fiber (in a heated state) such that one obtains a gradually changing diameter (→ tapered fibers), which also affects the core diameter and thus the mode radius of the fundamental mode. Heating may be done with a flame or with a CO2 laser. Note that the two ends will then have substantially different total fiber diameters. This is not ideal for coupling of two fibers which both have the standard 125-μm cladding diameter, since splicing is then more difficult, but it can be just appropriate for coupling fibers with different diameters. Also note that the numerical aperture stays constant, while ideally it should get higher on the side with smaller mode size. However, a non-ideal numerical aperture over a limited length (where bending is avoided) may be well tolerable.
An alternative technique is using a thermally expanded core (TEC). Here, the end of a single-mode fiber is heated to a high temperature for some amount of time, where the glass softens and the core material is able to diffuse, while the overall fiber structure is largely preserved. Effectively, this leads to an enlargement of the core and simultaneously a reduction in core refractive index, i.e., to a reduced numerical aperture. It is possible to obtain a gradual change in core characteristics such that the mode field expands adiabatically towards the fiber end, such that it becomes compatible with the mode field of the large mode area fiber. Splicing is then not problematic, since the fiber diameter is preserved.
It is also possible to combine the techniques of thermal core expansion and tapering. This is appropriate particularly in cases where the large mode are fiber has an increased cladding diameter. One then applies thermal core expansion to the single-mode fiber and tapering to the large mode area fiber.
Coupling Between Small Waveguide and Fiber
A mode converter could expand the very tiny mode of the waveguide in a laser diode or in a photonic integrated circuit to a size which fits to the mode of an optical fiber. Here, the effective mode area may be increased quite drastically – from the order of 1 μm2 to roughly 100 μm2, as is appropriate for a usual single-mode fiber. This allows one to efficiently couple light from the laser diode (or photonic chip) into the fiber.
In principle, a mode field converter may simply be made of a single optical lens. Here, the light exiting the first waveguide first expands in free space, and then it is refocused to a spot with appropriate size by the lens. The input of the second waveguide is placed at that beam focus.
In many situations, however, it is desirable to avoid any free-space propagation and also to obtain a monolithic and more compact solution. Therefore, other technical approaches for mode field converters are often used. An often used type of device is an adiabatically tapered waveguide just as explained above. Interestingly, the mode size does not necessarily increase together with the core size; the relation can be the opposite in a certain weak guiding regime. Different solutions have been developed, partly not at all based on tapered waveguides. For example, some nanotechnology devices exploit surface plasmons.
Related Articles
Suppliers
The RP Photonics Buyer's Guide contains 11 suppliers for fiber mode field adapters. Among them:
CSRayzer Optical Technology
CSRAYZER’s Mode Field Adaptors (MFA) can be used to match the mode field diameters of fibers with low signal loss and minimal degradation of beam quality (M2).
O-E Land
O/E Land's OEMFA-100 mode field adapter is a key component for efficient power coupling, e.g. between large mode area (LMA) and standard single-mode fibers. Coupling losses can be below 0.5 dB.
PhiX
Spot size converters (SSCs) provide an efficient coupling from arrays of optical fibers to photonic integrated circuits (PICs). They reduce the mode field using lithographically defined waveguides. PHIX offers V-groove fiber arrays with a pre-attached spot size converter.
DK Photonics
Our mode fields adapters allow one to combine fibers with different core diameters and NA with reduced splicing loss, usually <0.5 dB or even <0.3 dB.
Questions and Comments from Users
2023-09-04
Is the insertion loss always the same for forward and backward-propagating light, for free-space or tapered-fiber mode converters?
The author's answer:
Surely the insertion loss is the same in both directions for waveguide devices. For free-space mode converters, it is in principle the same situation, but generally you get in the output a superposition of modes, and would need to count only the power in the desired output mode to specify the insertion loss.
2022-06-08
Isn't there any method to transfer energy in higher modes of an MM fiber into the fundamental mode of an SM fiber? What will happen to light in the higher modes of a 200-μm core fiber if the fiber core is adiabatically thinned to 8 μm?
The author's answer:
One can make mode converters which can couple energy from higher-order modes into lower-order modes, but not without losing energy of the latter.
A tapered fiber as you described will eventually get into the single-mode regime. By then, the optical power in the higher-order modes will have been lost into the cladding.