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The stub heights are designed to obtain a hyperbolic secant distribution of the refractive index to achieve the required phase velocity variation in space , hence the gradient-index lens behavior .
 engineering feature: designed; concept or principle: distribution; concept or principle: phase; concept or principle: variation; manufacturing process: lens
Given options, please tell me the categories of all the listed entity words.Output format is "type1: word1; type2: word2". 
Option: manufacturing standard, machanical property, manufacturing process, material, engineering feature, process parameter, machine or equipment, application, process characterization, enabling technology, biomedical, concept or principle 
Text: {0} 
 Entities: designed, distribution, phase, variation, lens 
Answer:
 engineering feature: designed; concept or principle: distribution; concept or principle: phase; concept or principle: variation; manufacturing process: lens