COMMITMENTS AND CONTINGENCIES Operating Leases We have non-cancelable operating lease agreements for office, research and development and manufacturing space in the United States and foreign locations. We also have operating leases for certain equipment, automobiles and services in the United States and foreign jurisdictions. These lease agreements expire at various dates through 2026, and certain agreements contain provisions for extension at substantially the same terms as currently in effect. Lease escalation clauses, rent abatements and/or concessions, such as rent holidays and landlord or tenant incentives or allowances, are typically included in the determination of straight-line rent expense over the lease term. Future minimum lease payments for the next five fiscal years as of September 29, 2017 , are as follows (in thousands): 2018 $ 11,116 2019 9,001 2020 5,985 2021 3,828 2022 3,113 Thereafter 7,999 Total minimum lease payments $ 41,042 Rent expense incurred under non-cancelable operating leases was $10.9 million , $7.0 million and $6.5 million in fiscal years 2017 , 2016 and 2015 , respectively. Asset Retirement Obligations We are obligated under certain facility leases to restore those facilities to the condition in which we or our predecessors first occupied the facilities. We are required to remove leasehold improvements and equipment installed in these facilities prior to termination of the leases. As of the end of fiscal years 2017 , 2016 and 2015 , the estimated costs for the removal of these assets are recorded as asset retirement obligations was $2.3 million , $4.3 million and $1.3 million , respectively. Unused Letter of Credit As of September 29, 2017 , we had outstanding unused letters of credit from a bank aggregating $0.4 million . Purchase Commitments As of September 29, 2017 , we had outstanding non-cancelable purchase commitments aggregating to $ 1.8 million pursuant to inventory supply arrangements. Litigation From time to time we may be subject to commercial disputes, employment