A compact circular waveguide system can connect circular waveguides through a bend while avoiding excessive interaction between the orthogonal modes of the circular waveguides. A compact bend system with circular waveguide input and output can be achieved by providing short quarter wave transformers. The quarter wave transformers can be positioned at the transitions between the circular waveguides and a single-mode quasi-rectangular waveguide segment. Within the single-mode quasi-rectangular waveguide segment, a bend can be formed without concern for mixing of the orthogonal modes of the circular guided wave. The undesired mode of propagation can be substantially reduced or eliminated within the quarter wave transformers with a resistive mode suppressor. The compact system can be machined out of a single block of material from the outside flange faces.

An optical waveguide comprises a core, an inner cladding laterally surrounding the core, and an outer cladding laterally surrounding the inner cladding, wherein the core, inner cladding, and outer cladding have a depressed well configuration. The waveguide operates in three or more wavelength bands, wherein a first wavelength band is centered at about 1300 nm, and wherein a second wavelength band is centered at about 1625 nm. The waveguide can have bend losses that are less than or equal to 0.7 dB/turn when measured over a 10 mm radius bend at 1625 nm. The waveguide can also have bend losses that are less than or equal to 1.0 dB/turn when measured on a 5 mm radius bend at 1625 nm and bend losses that are less than or equal to 1.5 dB/turn when measured on a 5 mm radius bend at 1650 nm.

　コア及びクラッドからなる光導波路において、そのコア形状が中途に曲率の反転を有さず、かつ両端における曲率がゼロに漸近することを特徴とする曲線光導波路。コア及びクラッドからなる光導波路において、そのコア形状が中途に曲率の反転を有さず、かつ一端における曲率がゼロに漸近し、他端における曲率半径が有限な曲線光導波路。このような曲線光導波路に別のコア形状の光導波路を光学的に接続して配置した光導波路、ならびにこのような曲線光導波路を用いた光学装置である。                                                                     

Waveguide bends are specially designed according to a matching condition in order to suppress mode distortion and other undesirable effects. Generally, an optical field in a waveguide bend (58) is describable by first and second bend modes which are in phase at the start of the bend, but which propagate at different velocities through the bend, thereby coming out of phase and into phase with each other in beats. According to the teaching of the invention, the bend (58) is structured having regard to its length and curvature (1, R, θ) to ensure that at its end the first and second bend modes are substantially in phase with each other having completed approximately an integer number of beats. By being in phase at the end of the bend, the two modes are able to properly reconstruct the first mode of the straight waveguide and propagate on with a minimum of distortion, whether it be into a straight section, a further curved section of arbitrary curvature, into a free space propagation region or whatever. This approach can be used to suppress mode distortion, transition losses and other negative effects of waveguide bends, without inserting small lateral jumps as proposed in the prior art. A number of device applications are discussed, including couplers, Y-branches and Mach-Zehnder interferometers, all of which include waveguide bends.

　本発明は、中途に軸ずれ構造を有さないＳ字型のコア形状を有する曲線光導波路にあって、一端における曲率が０であり、他端における曲率が有限（＞０）であることを特徴とする曲線光導波路に関し、係る曲線光導波路により、光スプリッタや方向性結合器等においても両端の接続部で光損失が最小限となり、かつオフセットを設ける必要がない形状の光導波路を提供することができる。

A bend (B) in a wave guide (G) is formed having two angled cuts (201, 202) on either side of a middle pivot point (207). The angled cuts (201, 202) extends through a first side of a cladding layer (101) of the wave guide formed with at least one inner layer (102) and one outer layer (101). The wave guide (G) is then bent at the angled cuts about the middle pivot point (207) to make a desired angle bend. The middle pivot point (207) has a reflective angled surface (401) such that light (404) propagating through the wave guide (G) will be reflected and turned at the desired angle.