Heat a small skillet. Add the ancho and chipotle chiles and toast over moderate heat until fragrant, about 3 minutes. Transfer to a work surface and let cool completely, about 10 minutes. Crumble the toasted chiles into 1/4-inch pieces.
Meanwhile, in a medium saucepan, combine the sugar with the water and bring to a simmer over moderately high heat. Reduce the heat to moderate and cook until a richly browned caramel forms, about 16 minutes. Add the crumbled chiles and the garlic and simmer until fragrant, about 30 seconds. Carefully stir in the lemon, orange and lime juices and boil over high heat until reduced to a syrupy glaze, about 12 minutes. Remove from the heat. Stir in the fish sauce and let cool to room temperature before serving.
The sauce can be refrigerated for up to 5 days. Bring to room temperature before serving.