mode field converters (original) (raw)

Definition: optical devices which allow for efficient coupling between modes of different sizes

Alternative term: mode field adapters

Categories: article belongs to category fiber optics and waveguides fiber optics and waveguides, article belongs to category photonic devices photonic devices

Related: modeseffective mode areamode radiusfibersfiber jointstapered fiberswaveguides

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Contents

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:

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 area 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.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is a mode field converter?

A mode field converter, or mode field adapter, is an optical device used to expand or contract a light mode in its transverse dimensions. It is typically used to efficiently transfer light between two different waveguides, such as optical fibers, which have substantially different mode characteristics.

Why is a mode field converter often needed for coupling different optical fibers?

Efficient coupling requires that the mode radii, shapes, and wavefronts of the two fibers match closely. A significant mismatch, for example between a standard fiber and a large mode area fiber, causes high insertion loss, which a mode field converter can minimize.

What are common methods for making a mode field converter for fibers?

One method is to create a tapered fiber by heating and pulling it to produce a gradual diameter change. Another technique is to use a thermally expanded core (TEC), where the fiber end is heated to make the core material diffuse, thus enlarging the mode field.

How can light from a laser diode be efficiently coupled into a fiber?

This requires converting the diode's tiny mode to the larger mode of the fiber. While a simple lens can be used for free-space coupling, a more compact solution is an integrated, adiabatically tapered waveguide on the photonic integrated circuit itself.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains 11 suppliers for fiber mode field adapters. Among them:

DK Photonics, supplier of fiber mode field adapters

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fiber mode field adapters

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.

CSRayzer Optical Technology, supplier of fiber mode field adapters

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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, supplier of fiber mode field adapters

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fiber mode field adapters

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, supplier of fiber mode field adapters

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fiber mode field adapters

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.

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