Integrated planar composite coupling structures for bi-directional light beam transformation between a small mode size waveguide and a large mode size waveguide

An optical mode transformer comprising:

• a substrate having a substrate surface;
• a lower cladding disposed on the substrate surface, the lower cladding having a vertical refractive index having a vertical value that varies according to first substantially stepwise function of a y-coordinate, the y-coordinate representing a distance from the substrate surface, the first function having a maximum value and a minimum value, and the lower cladding having a horizontal refractive index having a horizontal value that varies according to first substantially stepwise function of an x-coordinate, the x-coordinate representing a position in a dimension parallel to the substrate surface and transverse to the long axis, the first function having a maximum value and a minimum value, the lower cladding further having an upper surface;
• a transformer core disposed on the upper surface of the lower cladding, the transformer core having a core refractive index, the ratio of the core refractive index to the maximum value of the first function being at least about 1.3, the transformer core further having a first end located substantially at the small beam port, a second end defining an intermediate beam port, and an upper surface;
• and an upper cladding disposed on the upper surface of the transformer core and on the upper surface of the lower cladding, the upper cladding having a refractive index having a value that varies as a second function of the y-coordinate and of the x-coordinate, the second function having a maximum value and a minimum value, the ratio of the core refractive index to the maximum value of the second function being at least about 1.3, wherein the optical mode transformer is configured such that the transformer core has a first region with a vertical taper along the long axis, the vertical taper being a changing thickness of the transformer core in a dimension normal to the substrate surface, wherein the thickness decreases along the long axis from a first thickness value at a transition point away from the small beam port to a second thickness value at a second point near the intermediate beam port, the second thickness value being less than a critical thickness value, the critical thickness value being defined as a thickness value below which a significant portion of the energy of a light beam having a small mode size received at the small beam port and propagating in the transformer core penetrates into at least one of the upper cladding layer and the lower cladding layer, and wherein the vertical taper modifies a wavefront curvature of the light beam, thereby enlarging the small mode size.