CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	3 Months Ended				6 Months Ended		12 Months Ended
	Dec. 28, 2019	Sep. 28, 2019	Dec. 29, 2018	Sep. 29, 2018	Dec. 28, 2019	Dec. 29, 2018	Jun. 29, 2019
OPERATING ACTIVITIES:
Net income	$ 49.1	$ 47.6	$ 16.3	$ 47.4	$ 96.7	$ 63.7
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation expense	28.7		26.0		60.3	45.7
Stock-based compensation					36.9	30.5
Loss on early extinguishment of debt					8.0	0.0
Unrealized loss on derivative liability	0.0		(10.9)		0.0	(8.8)
Loss on disposal of property, plant and equipment					4.7	0.0
Amortization of acquired intangibles	18.6		5.2		37.4	6.0
Amortization of debt discount and debt issuance costs					11.3	8.8
Amortization of inventory fair value adjustment in connection with Oclaro acquisition					4.3	1.3
Other non-cash income					(0.5)	(0.1)
Changes in operating assets and liabilities:
Accounts receivable					(54.0)	(0.8)
Inventories					44.3	0.5
Operating lease right-of-use assets, net					6.6
Prepayments and other current and non-currents assets					4.8	(10.7)
Income taxes, net					10.4	5.8
Accounts payable					(18.7)	16.5
Accrued payroll and related expenses					11.4	(1.6)
Operating lease liabilities					(6.9)
Accrued expenses and other current and non-current liabilities					(7.2)	2.8
Net cash provided by operating activities					249.8	159.6
INVESTING ACTIVITIES:
Payments for acquisition of property, plant and equipment					(47.4)	(53.9)
Payment for asset acquisition					(3.0)	(0.7)
Payment for Oclaro acquisition, net of cash acquired					0.0	(619.8)
Proceeds from deposit related to sale of product lines					2.5	0.0
Purchases of short-term investments					(51.3)	(100.1)
Proceeds from maturities and sales of short-term investments					157.0	102.1
Net cash provided by (used in) investing activities					57.8	(672.4)
FINANCING ACTIVITIES:
Repurchase of common stock					(200.0)	0.0
Proceeds from the issuance of 0.50% Convertible Notes due 2026, net of issuance costs					1,042.9	0.0
Tax payments related to restricted stock					0.0	(2.4)
Payment of dividends - Series A Preferred Stock					0.0	(0.7)
Payment of acquisition related holdback					0.0	(1.0)
Proceeds from employee stock plans					4.9	4.7
Proceeds from term loan, net of issuance costs					0.0	491.0
Principal payments on finance leases					(5.8)	(4.6)
Proceeds from the exercise of stock options					0.3	0.0
Repayment of term loan					(497.5)	0.0
Net cash provided by financing activities					344.8	487.0
Effect of exchange rates on cash and cash equivalents					0.0	(0.2)
Increase (decrease) in cash and cash equivalents					652.4	(26.0)
Cash and cash equivalents at beginning of period		$ 432.6		$ 397.3	432.6	397.3	$ 397.3
Cash and cash equivalents at end of period	$ 1,085.0		$ 371.3		1,085.0	371.3	$ 432.6
Supplemental disclosure of cash flow information:
Cash paid for taxes					3.9	0.4
Cash paid for interest					10.2	0.7
Supplemental disclosure of non-cash transactions:
Unpaid property, plant and equipment in accounts payable and accrued expenses					9.4	21.7
Issuance costs in current liabilities					0.7	0.3
Right-of-use assets obtained in exchange for new operating lease liabilities					2.2
Issuance of common stock upon conversion of Series A Preferred Stock					0.0	79.4
Net transfer of assets from property, plant and equipment, net to assets held-for-sale					2.1
Issuance of common stock and replacement awards in connection with Oclaro acquisition					$ 0.0	$ 460.1