Curved surface not being smooth when exported in STL format

Hello, I used Splines and extrude to make a D shape. Although it looks smooth in Solidworks, the resulting 3D print does not. I can see a series of rectangular segments making up the curved surface. This shows up when I viewed the STL file using a viewer. How to create smooth surfaces so that they not only look smooth in Solidworks but also look smooth when printed using a 3D printer? Is there a setting to create smooth curve surfaces in STL format? 

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