Shares of Ciena fell Thursday even after the networking company reported better-than-expected earnings and raised its guidance for the fiscal year. High expectations may have been the culprit.

Ciena posted adjusted earnings of $1.35 a share for its fiscal first quarter, surpassing analysts' consensus estimate of $1.17, according to FactSet. Revenue totaled $1.43 billion, up 33% from last year and above Wall Street's call for $1.4 billion.

Shares dropped 4% in premarket trading Thursday. The stock was up 47% this year as of Wednesday's close, riding a wave of demand for cabling and communications systems at artificial-intelligence data centers.

"We delivered a very strong fiscal first quarter, driven by focused execution and unprecedented, broad-based demand as we enable customers to monetize their AI investments," CEO Gary Smith said in a statement.

Ciena lifted its fiscal-year revenue forecast to $5.9 billion to $6.3 billion from a previous range of $5.7 billion to $6.1 billion. The company expects an adjusted gross margin of 43.5% to 44.5%, up at the midpoint from 42% to 44% previously.