MMI thermo-optic coupler

Details MMI thermo-optic coupler MMI thermo-optic coupler

2000-04-21

2002-03-05

Schuberg, Darren (Department: 2872)

Optical waveguides

With optical coupler

Particular coupling structure

C385S129000, C385S130000, C385S131000, C385S040000

Reexamination Certificate

active

06353694

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a multi-mode (MMI) coupler, and more particularly to an MMI coupler having a polymer cladding.
BACKGROUND OF THE INVENTION
In the processing of light beams for example, in telecommunications applications, important and desired functions are the splitting and combining of light beams. In conventional optics, prisms or pellicle splitters are used for this purpose. Attempts are continually being made to reduce the dimensions of the optical components to a considerable extent. On the one hand, it is being attempted in three dimensions to realise the processing of light beams by means of interference phenomena such as holography and free space optics. On the other hand, the technique of integrated optics is developing very rapidly. In this technique, waveguides are patterned on thin-film layers. It is an object of integrated optics to realise the functionality of the components used in conventional optics by new, integrable optical elements. This research field has found important applications in the field of communication.
In fiber-optical communication, data are transmitted by means of optical signals through otpical fibers. The optical signals are processed on integrated optical chips which are placed between the fibers. To manufacture these chips, generally thin-film layers are provided on support substrates such as, for example glass, Si, InP, GaAs and subsequently structured.
In optical waveguides the light is guided through a medium referred to as the waveguide core. The guidance is realised in that the waveguide core is bounded by a reflecting transition. In cavity waveguides, a metal is used for this purpose. In dielectric waveguides, the total reflection on a surrounding medium having a smaller refractive index the waveguide cladding is used. In optical waveguides, only those modes can propagate which fulfil the Maxwell equations. The waveguides are referred to as cut-off, monomode or multimode waveguides, dependent on whether they can guide no mode, only one mode for each polarization or a plurality of modes.
In waveguides, the light propagates in the longitudinal z direction. The x direction is parallel to the waveguide layer and is defined as the horizontal, or lateral direction. Analogously, the y direction is vertical to the waveguide layer and is defined as the vertical, or transversal direction. The propagation of light on the chips is computed by means of numerical methods such as beam propagation (BPM) methods, or modal propagation analysis (MPA) methods. In some cases, semi-analytical computations such as the effective index method (EIM) are used. The Maxwell equations are often solved in a scalar approximation. These equations describe the planar optics exactly. Two polarizations can be distinguished: the TE polarization has the E vector in the x direction. and the TM polarization has the H vector in the x direction. For the most frequently used dielectric waveguides in integrated optics, the scalar approximation leads to “quasi-TE” and “quasi-TM” modes. In such waveguides, the mode forms and the “effective” indexes may be dependent on the polarization. In many cases it is very much desirable, but very difficult, to produce components which are independent of polarization. It should be noted that “monomode” waveguides often have a mode for each polarization, i.e. overall, there are often two modes in “monomode” waveguides. These are degenerate modes in normal optical glass fibers.
Important components in integrated optics are the beam splitters and combiners. Generally, one refers to N×M splitters. N×M denote the number of inputs and outputs. Ideally, these splitters should have the following properties: they should be compact (having small dimensions), independent of polarization, not very sensitive to manufacturing inaccuracies and easy to produce. Moreover, it should be possible to readily adapt the splitting or combining ratios to the various applications by geometrical changes in the design. Various beam splitters and combiners have already been realised: Symmetrical Y branches are simple solutions for 1×2 splitters with a 50%/50% intensity ratio. Asymmetrical Y branches yield other intensity ratios but, due to coupling effects, they are often polarization-dependent between the two outputs. For manufacturing Y branches, a high resolution, particularly in the sharp bifurcation, is required. They are very sensitive to manufacturing inaccuracies.
Directional couplers with two parallel waveguides separated by means of a narrow slit operate as 2×2 splitters. However the coupling length is, however, very sensitive to manufacturing parameters, particularly as regards slit width and depth. The coupling length is also very much dependent on polarization. “Two-mode” interference (TMI) couplers without a slit also operate as 2×2 splitters. The intensity ratio is, however, very much dependent on the coupling into the input and output Y branches. Consequently, they are very sensitive to manufacturing conditions. U.S. Pat. No. 5,857,039 describes a thermo-optically activated directional coupler having a polymer guide buffer layer and a heater which allows the refractive index of the polymer to be varied. Of course it is well known that polymer has a higher refractive index variation with temperature than silica and better heat confinement. The '039 patent extols the virtues of polymer over silica especially in the interguide space. When the interguide space is small in relation to the dimensions of the cross section, the guides can only be properly covered by the mineral layer, particularly in the case of silica coverings, by a so-called “FHD” technique which is extremely difficult to apply. Therefore, the '039 patent provides a solution which is tailored to providing a more practicably directional coupler.
It is somewhat obvious, in hindsight, that in a directional coupler wherein coupling of light across a cladding boundary between two closely spaced waveguides is to be accomplished, that the boundary region must be controlled to increase or decrease the coupling across this region. Ergo, in order to allow the two single mode signals to couple, or to remain isolated, in a controlled manner, this intermediary cladding region must be highly manufacturable and controllable. As the '039 patent purports, a polymer disposed between these cores, provides a practicable solution.
The use of a polymer cladding on an silica filament strand of optical fibre is well known, and has been disclosed in U.S. Pat. No. 4,116,654 issue Sep. 26, 1978. In this patent it is stated that “Where low attenuation of transmitted light in an optical fiber material is critical, the preferred material for the filamentary core is silica, since it has one of the lowest attenuations presently known. Suitable cladding materials known in the art include thermoplastic polymers which have an index of refraction lower than that of the core; and which preferably are substantially amorphous.”
A further mention of polymer cladding is found in U.S. Pat. No. 5,873,923 in the name of DiGiovanni, with reference to optical amplifiers.
In this patent, a polymer cladding is suggested in a multi-clad fibre amplifier, where the 923 patent states that “Any polymer cladding serves little purpose beyond guiding”.
Considering the teaching of DiGiovanni, and that of U.S. Pat. No. 5,857,039, it is evident that the cladding guides the light within a waveguide or optical fibre and when the relative refractive index difference between the cladding and the core is varied the confinement of light within a guide varies as well.
What is surprising however, is that significance of providing a cladding on a multi-mode coupler which operates under a very different regime. What is further surprising is how coupling within a wide MMI core is affected by varying the cladding on top. Notwithstanding, this will be explained.
In the last few years, multimode interference (MMI) couplers have become more and more popular. These components are wavegui

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