Combine the sugar and water in a medium heavy pot and bring to a boil over high heat. Reduce the heat to medium-low and simmer until the sugar dissolves, about 2 minutes. Add the raspberries and cook, stirring occasionally, until the syrup thickens and reduces by 3/4 in volume, about 5 minutes.
Transfer to a blender or food processor, and puree on high speed. Strain through a fine mesh strainer, pressing down with a rubber spatula to extract as much juice as possible. If too thick, add water a teaspoon at a time to reach the desired consistency.
Cover and refrigerate until ready to use. (The coulis can be refrigerated in an airtight container for up to 3 days.)
Yield: 2/3 cup