Other income (expense), net (Details) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Other income (expense), net
Foreign exchange gain (loss)	$ 169,000	$ 1,573,000	$ (434,000)
Income from local China government subsidy	2,557,000	1,710,000	1,125,000
Other income (expense)	(547,000)	204,000	(182,000)
Total other income, net	$ 2,179,000	$ 3,487,000	$ 509,000