Method for forming and apparatus comprising optical waveguides leading to a free space coupler region

1. A method of forming a plurality of optical waveguides leading to at least one free space coupler region comprising the steps of:

utilizing a single mask defining monotonically converging mask openings terminating at a interface region with a free space coupler region;

forming a plurality of longitudinal optical waveguides as ridge waveguides by etching a groove between adjacent ridge waveguides to form them; and

employing said mask to form the grooves with an anisothropic etch to a first depth where the cross-sectional width of a groove between adjacent ridge waveguides along their longitudinal length monotonically decreases up to the interface region and terminating at the interface region with the free space coupler region, followed by etching with the same mask with an isothropic etch to a second depth where the depth of a groove between adjacent ridge waveguides along their longitudinal length monotonically decreases in depth up to the interface region and terminating at the interface region with the free space coupler region.

2. The method of claim 1 wherein said optical waveguides and free space coupler region are silicon-based or InP-based.

3. The method of claim 1 wherein said optical waveguides and free space coupler region are formed as part of a power coupler, a star coupler, a multi-mode interference (MMI) coupler, an arrayed waveguide grating (AWG) or an Echelle grating.

4. The method claim 1 wherein the second etched depth is substantially non-existent at the interface region.

5. A method of reducing insertion loss between a plurality of optical waveguides coupled to a free space coupler region at an interface region, comprising the steps of:

forming said waveguides as ridge waveguides having a longitudinal length and uniform width to the free space coupler region by etching a groove between adjacent ridge waveguides up to the interface region; and

controlling the etched depth of the groove along the longitudinal length of adjacent ridge waveguides so that the depth of the groove monotonically decreases in depth to the free space coupler region.

6. The method of claim 5 comprising the further step of forming the etched grooves using a combination of anisothropic and isothropic etched steps.

7. The method of claim 6 wherein the combination of anisothropic and isothropic etched steps are carried out employing a single mask.

8. The method of claim 5 further comprising the step of forming the etched depth between the ridge waveguides to be substantially non-existent at the interface region.

9. The method of claim 5 wherein the ridge waveguides and free space coupler region are formed as part of a power coupler, a star coupler, a multi-mode interference (MMI) coupler, an arrayed waveguide grating (AWG) or an Echelle grating.

10. The method of forming a plurality of optical waveguides leading to free space coupler region comprising the steps of:

employing a mask to form longitudinal grooves to define adjacent ridge waveguides extending longitudinally to and optically coupled a free space coupler region;

applying an anisothropic etch using the mask to form a first groove depth where the formed first grooves monotonically diminish in width along their longitudinal length to the free space coupler region; and

applying an isothropic etch using the same mask to form a second groove depth beyond the first groove depth where the formed second grooves monotonically diminish in depth along their longitudinal length to the free space coupler region.

11. The method of claim 10 where the width along longitudinal extent of the ridge waveguides remains the same.