Thanks to a fashionable new method of comparing richly valued stocks, Ciena ( CIEN) shareholders today find themselves several billion dollars richer than they were just last month. After shriveling last year under the weight of a failed merger with Tellabs ( TLAB) and the onset of competition from large suppliers, Ciena stock has quintupled this year, jumping 64% since Dec. 1 alone. On Friday, Ciena beat profit expectations, pushing its stock up 10 5/16, or 17%, to 71 7/8. Bulls -- and they are many -- note that Ciena has shored up its business by broadening its customer list, upgrading products and acquiring builders of more advanced fiber-optic technology. But more than management, technology or market position, Ciena owes its stubborn rise to a risky, self-perpetuating mindset among analysts and investors in today's bull market: relative valuations.