Ahmed Serageldin had worked on a radar system used to defend against ballistic missiles and had pleaded guilty to keeping classified national defense documents about that system at his home without authorization.
Serageldin apologized for his behavior in court Wednesday.
"I was sloppy, but I have always been loyal to my job and to the country," Serageldin said before the sentence was handed down.
Federal prosecutors in Boston had asked for five years in prison.
"He took over 3,100 digital documents," Assistant U.S. Attorney Scott Garland said, adding he also took more than 100 physical documents. More than 500 of those documents were labeled classified and "exposed" to anyone who wanted to see them.
"Defendant deserves this serious sentence because he deliberately endangered national security, at the very least by keeping national defense information where it was viewable and available to others," prosecutors wrote in a sentencing memorandum.
The judge said five years was too much and questioned whether the offense was as serious as prosecutors portrayed.
"You have no evidence that he actually transmitted it to a foreign power," said Judge Patti Saris.
"That is correct we do not have that evidence," Garland replied.
"That's very important," Saris said. "He's not a traitor or a spy. He basically misused classified information in his home."
Defense attorneys called Serageldin, 67, of Sharon, Massachusetts, a "loyal American" who deserved nothing more than three years probation.
"He didn't transfer secret documents anywhere," defense attorney George Vien said. "He worked on them at home."
Capt. Jason Hall, who oversees the Dual Band Radar for the United States Navy, told the judge that Serageldin's conduct "could degrade the effectiveness" of the radar system.
"Compromise of this sensitive technical information would diminish the strategic advantage of successor radars," Hall said.
While Saris said Serageldin intended no harm, she cited the volume of documents he kept at home when she imposed her 18-month sentence.
"I don't think you intentionally harmed the United States, but you put it at risk," the judge said.