Place pineapple juice and carambola slices in a blender; process until smooth. Pour mixture through a sieve into a pitcher to measure 2 cups. Discard solids. Stir in ginger syrup and lemon juice. Cover and refrigerate 45 minutes or until thoroughly chilled. Fill each of 2 glasses with 1 cup crushed ice. Pour star fruit mixture over ice. Add 1/4 cup sparkling water to each glass. Garnish with carambola slices, if desired.