Note 9 - Employee Equity Incentive Plans (Tables)

6 Months Ended

Sep. 30, 2017

Notes Tables

Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table Text Block]

	Three Months Ended September 30,				Six Months Ended September 30,
Statement of Operations Classifications	2017		2016		2017		2016
	(unaudited)				(unaudited)
Cost of goods sold	$	107	$	96	$	200	$	196
Research, development and engineering		416		283		741		586
Selling, general and administrative		575		495		1,064		947
Stock-based compensation effect in income before income tax provision		1,098		874		2,005		1,729
Provision for income tax (1)		439		350		802		692
Net stock-based compensation effects in net income	$	659	$	524	$	1,203	$	1,037

Schedule of Employee Service Share Based Compensation Fair Value Assumptions and Methodology [Table Text Block]

	Stock Options (1)											Purchase Plan (2)
	Three Months Ended September 30,					Six Months Ended September 30,						Six Months Ended September 30,
	2017		2016			2017			2016			2017			2016
	(unaudited)					(unaudited)						(unaudited)
Weighted average estimated fair value of grant per share	$	na	$	4.99		$	6.49		$	4.40		$	2.92		$	3.94
Risk-free interest rate		na		1.3	%		1.9	%		1.3	%		0.7	%		0.3	%
Expected term in years		na		6.41			6.63			6.41			0.75			1.00
Volatility		na		42.0	%		36.8	%		41.4	%		29.9	%		42.9	%
Dividend yield (3)		na		0.0	%		0.0	%		0.8	%		0.0	%		1.4	%

Share-based Compensation, Stock Options, Activity [Table Text Block]

				Weighted Average
	Number of			Exercise Price		Intrinsic
	Shares			Per Share		Value (1)
				(unaudited)		(000)
Balance at March 31, 2017		5,097,545		$	10.71
Options granted		549,000		$	16.05
Options exercised		(1,059,795	)	$	9.72	$	8,074
Options cancelled		(89,750	)	$	11.42
Options expired		(20,250	)	$	9.46
Balance at September 30, 2017		4,476,750		$	11.59
Exercisable at September 30, 2017		2,717,250		$	10.80
Exercisable at March 31, 2017		3,300,295		$	10.34