Document and Entity Information - shares	3 Months Ended
	Dec. 30, 2017	Feb. 05, 2018
DEI [Abstract]
Entity Registrant Name	COHERENT INC
Trading Symbol	COHR
Entity Central Index Key	0000021510
Document Type	10-Q
Document Period End Date	Dec. 30, 2017
Amendment Flag	false
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q1
Current Fiscal Year End Date	--09-29
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		24,821,704