For a detailed explanation of the Company's stock plans, refer to Note 14, Employee Benefits and Share-Based Compensation, of the audited Consolidated Financial Statements and accompanying Notes included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2020, filed with the SEC on February 24, 2021.

The following table sets forth the total share-based compensation expense recognized in the Company’s Condensed Consolidated Statements of Operations for the three and nine months ended September 30, 2021 and 2020:


	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020

	(In thousands)
Cost of product and service revenues	$	1,909	$	1,758	$	5,890	$	5,658
Research and development	1,998		1,577		5,429		5,199
Selling, general, and administrative	9,759		7,689		27,158		22,177
Total share-based compensation expense	$	13,666	$	11,024	$	38,477	$	33,034


	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
Expected life, years	0.5 - 2.0		0.5 - 2.0		0.5 - 2.0		0.5 - 2.0
Expected volatility, %	27.4% - 46.6%		30.4% - 53.5%		27.4% - 53.5%		30.4% - 53.5%
Risk-free interest rate, %	0.1% - 1.5%		0.1% - 2.7%		0.1% - 2.6%		0.1% - 2.7%
Dividend yield, %	—	%	—	%	—	%	—	%

For the nine months ended September 30, 2021 and 2020, employees purchased approximately 287,000 and 333,000 shares of common stock, respectively, under the ESPP at a weighted-average price of $62.14 and $48.77, respectively. As of September 30, 2021, the unrecognized compensation cost related to the shares to be purchased under the ESPP was approximately $3.1 million and is expected to be recognized over a weighted-average period of 1.5 years.

The following assumptions were used to value stock options granted pursuant to the Company’s 2009 Equity Incentive Plan (as amended, the “2009 Plan”) for the three months ended September 30, 2020, and the nine months ended September 30, 2021 and 2020. There were no stock options granted during the three months ended September 30, 2021.


	Three Months Ended September 30,		Nine Months Ended September 30,
	2020		2021		2020
Expected life, years	4.7		4.9		4.7
Expected volatility, %	41.2	%	30.1	%	39.0	%
Risk-free interest rate, %	0.3	%	0.6	%	0.7	%
Estimated forfeiture rate, %	5.7	%	7.9	%	5.7	%
Dividend yield, %	—	%	—	%	—	%


	Number of Shares	Weighted-Average Exercise Price		Weighted-Average Remaining Years	Aggregate Intrinsic Value

	(In thousands, except per share data)
Outstanding at December 31, 2020	3,932	$	62.50	7.8	$	226,160
Granted	150	125.80
Exercised	(648)	55.70
Expired	(11)	60.64
Forfeited	(180)	74.79
Outstanding at September 30, 2021	3,243	$	66.11	7.1	$	266,946
Exercisable at September 30, 2021	1,640	$	51.96	5.8	$	158,170
Vested and expected to vest at September 30, 2021 and thereafter	3,104	$	65.35	7.0	$	257,873

The weighted-average fair value per share of options granted during the three months ended September 30, 2020 was $23.75, and the weighted-average fair value per share of options granted during the nine months ended September 30, 2021 and 2020 was $33.89 and $25.30, respectively. The intrinsic value of options exercised during the three months ended September 30, 2021 and 2020 was $26.0 million and $4.1 million, respectively, and during the nine months ended September 30, 2021 and 2020 was $56.8 million and $16.5 million, respectively.

As of September 30, 2021, total unrecognized compensation cost related to unvested stock options was $38.4 million, which is expected to be recognized over a weighted-average vesting period of 2.4 years.


	Number of Shares	Weighted-Average Grant Date Fair Value		Weighted-Average Remaining Years	Aggregate Intrinsic Value

	(In thousands, except per share data)

Outstanding at December 31, 2020	580	$	72.87	1.6	$	69,670
Granted (Awarded)	409	145.44
Vested (Released)	(157)	72.39
Forfeited	(54)	74.75
Outstanding and unvested at September 30, 2021	778	$	111.00	1.7	$	115,456

As of September 30, 2021, total unrecognized compensation cost related to RSUs was $73.4 million, which is expected to be recognized over the remaining weighted-average vesting period of 3.1 years.


	Number of Shares	Weighted-Average Grant Date Fair Value

	(In thousands, except per share data)

Outstanding at December 31, 2020	21	$	68.11
Granted (Awarded)	11	137.36
Vested (Released)	(21)	68.11

Outstanding and unvested at September 30, 2021	11	$	137.36

As of September 30, 2021, total unrecognized compensation cost related to RSAs was $1.0 million, which is expected to be recognized over the remaining weighted-average vesting period of 0.7 years.


	Number of Shares	Weighted-Average Grant Date Fair Value Per Unit

	(In thousands, except per share data)
Outstanding at December 31, 2020	155	$	74.26
Granted	51	156.79
Vested	(41)	70.68
Forfeited	(7)	64.06
Outstanding and unvested at September 30, 2021	158	$	102.29

As of September 30, 2021, total unrecognized compensation cost related to PSUs was approximately $7.0 million, which is expected to be recognized over the remaining weighted-average vesting period of 1.2 years.


			Number of Shares

			(In thousands)
Share options outstanding			3,243
Non-vested restricted stock awards			947
Shares authorized for future issuance			1,514
ESPP shares available for future issuance			919
Total shares reserved for future issuance			6,623

On August 2, 2016, the Company’s Board of Directors (the “Board”) authorized a stock repurchase program providing for the repurchase of up to $50.0 million of the Company’s common stock (the “2016 Repurchase Program”). The 2016 Repurchase Program is in addition to the stock repurchase program approved by the Board on November 4, 2014 providing for the repurchase of up to $50.0 million of the Company’s common stock (the “2014 Repurchase Program”). As of September 30, 2021, the maximum dollar value of shares that may yet be purchased under the two repurchase programs was $54.9 million. The stock repurchase programs do not obligate the Company to repurchase any specific number of shares, and the Company may terminate or suspend the repurchase programs at any time.

On September 17, 2020, the Board authorized a one-time stock repurchase transaction providing for the repurchase of up to $75.0 million of the Company’s common stock in privately negotiated transactions concurrently with the issuance of the Notes, described in Note 10, Convertible Senior Notes. In September 2020, the Company repurchased 749,300 shares of its common stock from purchasers of the Notes in the offering in privately negotiated transactions effected through one of the initial purchasers or its affiliate at an average price of $70.78 per share for an aggregate purchase price of approximately $53.0 million. There will be no further repurchases under this one-time authorization.

During the three and nine months ended September 30, 2021 and 2020, the Company did not repurchase any of its outstanding common stock, including under the 2014 Repurchase Program and the 2016 Repurchase Program, other than the separately-authorized one-time stock repurchase concurrent with the offering of the Notes in September 2020.