EX-10.10



 EXHIBIT 10.10 

ACACIA COMMUNICATIONS, INC. 
 2016
EMPLOYEE STOCK PURCHASE PLAN 
 The purpose of this 2016 Employee Stock Purchase Plan (this “Plan”) is to provide eligible
employees of Acacia Communications, Inc. (the “Company”) and certain of its subsidiaries with opportunities to purchase shares of the Company’s common stock, $0.0001 par value per share (the “Common Stock”), commencing at
the time set forth in the Plan. Subject to adjustment under Section 15 hereof, the number of shares of Common Stock that have been approved for this purpose is the sum of: 

(a) 700,000 shares of Common Stock; plus 

(b) an annual increase to be added on the first day of each fiscal year, commencing on January 1, 2016 and ending on
December 31, 2026, equal to the least of (i) 900,000 shares of Common Stock, (ii) 1.0% of the outstanding shares on such date and (iii) an amount determined by the Company’s Board of Directors (the “Board”). 

This Plan is intended to qualify as an “employee stock purchase plan” as defined in Section 423 of the Internal Revenue Code of 1986, as
amended (the “Code”), and the regulations issued thereunder, and shall be interpreted consistent therewith. 
 1.
Administration. The Plan will be administered by the Board or by a Committee appointed by the Board (the “Committee”). The Board or the Committee has authority to make rules and regulations for the administration of the Plan and its
interpretation and decisions with regard thereto shall be final and conclusive. 
 2. Eligibility. All employees of the Company and
all employees of any subsidiary of the Company (as defined in Section 424(f) of the Code) designated by the Board or the Committee from time to time (a “Designated Subsidiary”), are eligible to participate in any one or more of the
offerings of Options (as defined in Section 9) to purchase Common Stock under the Plan provided that: 
 (a) they are
customarily employed by the Company or a Designated Subsidiary for more than twenty (20) hours a week and for more than five (5) months in a calendar year; 

(b) they have been employed by the Company or a Designated Subsidiary for at least three (3) months prior to enrolling in
the Plan; and 
 (c) they are employees of the Company or a Designated Subsidiary on the first day of the applicable Plan
Period (as defined below). 
 No employee may be granted an Option hereunder if such employee, immediately after the Option is granted,
owns 5% or more of the total combined voting power or value of the stock of the Company or any subsidiary. For purposes of the preceding sentence, the attribution rules of Section 424(d) of the Code shall apply in determining the
stock ownership of an employee, and all stock that the employee has a contractual right to purchase shall be treated as stock owned by the employee. 







 The Company retains the discretion to determine which eligible employees may participate in an
offering pursuant to and consistent with Treasury Regulation Sections 1.423-2(e) and (f). 
 3. Offerings. The Company will make one
or more offerings (“Offerings”) to employees to purchase stock under this Plan. Offerings will begin each May 1 and November 1, or the first business day thereafter (such dates, the “Offering Commencement Dates”). Each Offering
Commencement Date will begin a six (6) month period (a “Plan Period”) during which payroll deductions will be made and held for the purchase of Common Stock at the end of the Plan Period. The Board or the Committee may, at its
discretion, choose a different Plan Period of not more than twelve (12) months for Offerings. Notwithstanding anything to the contrary, the first Plan Period shall begin on the first date that the Common Stock is publicly traded following the
Company’s initial public offering (“IPO”) and shall end on April 30, 2016. 
 4. Participation. An employee eligible
on the Offering Commencement Date of any Offering may participate in such Offering by completing and forwarding either a written or electronic payroll deduction authorization form to the employee’s appropriate payroll office at least fifteen
(15) days prior to the applicable Offering Commencement Date. The form will authorize a regular payroll deduction from the Compensation received by the employee during the Plan Period. Unless an employee files a new form or withdraws from
the Plan, his or her deductions and purchases will continue at the same rate for future Offerings under the Plan as long as the Plan remains in effect. The term “Compensation” means the amount of money reportable on the employee’s
Federal Income Tax Withholding Statement, excluding overtime, shift premium, incentive or bonus awards, allowances and reimbursements for expenses such as relocation allowances for travel expenses, income or gains associated with the grant or
vesting of restricted stock, income or gains on the exercise of Company stock options or stock appreciation rights, and similar items, whether or not shown or separately identified on the employee’s Federal Income Tax Withholding Statement, but
including, in the case of salespersons, sales commissions to the extent determined by the Board or the Committee. 
 5. Deductions.
The Company will maintain payroll deduction accounts for all participating employees. With respect to any Offering made under this Plan, an employee may authorize a payroll deduction in any percentage amount (in whole percentages) up to a maximum of
fifteen (15)% of the Compensation he or she receives during the Plan Period or such shorter period during which deductions from payroll are made. The Board or the Committee may, at its discretion, designate a lower maximum contribution rate. The
minimum payroll deduction is such percentage of Compensation as may be established from time to time by the Board or the Committee. 
 6.
Deduction Changes. An employee may decrease or discontinue his or her payroll deduction once during any Plan Period, by filing either a written or electronic new payroll deduction authorization form. However, an employee may not
increase his or her payroll deduction during a Plan Period. If an employee elects to discontinue his or her payroll deductions during a Plan Period, but does not elect to withdraw his or her funds pursuant to Section 8 hereof, funds deducted
prior to his or her election to discontinue will be applied to the purchase of Common Stock on the Exercise Date (as defined below). 

  
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 7. Interest. Interest will not be paid on any employee accounts, except to the extent that
the Board or the Committee, in its sole discretion, elects to credit employee accounts with interest at such rate as it may from time to time determine. 

8. Withdrawal of Funds. An employee may at any time prior to the close of business on the fifteenth business day prior to the end of a
Plan Period and for any reason permanently draw out the balance accumulated in the employee’s account and thereby withdraw from participation in an Offering. Partial withdrawals are not permitted. The employee may not begin participation again
during the remainder of the Plan Period during which the employee withdrew his or her balance. The employee may participate in any subsequent Offering in accordance with terms and conditions established by the Board or the Committee. 

9. Purchase of Shares. 

(a) Number of Shares. On the Offering Commencement Date, the Company will grant to each eligible employee who is then a participant in
the Plan an option (an “Option”) to purchase on the last business day of such Plan Period (the “Exercise Date”) at the applicable purchase price (the “Option Price”) up to that number of shares of Common Stock
determined by multiplying $2,083 by the number of full months in the Plan Period and dividing the result by the closing price (as determined below) on the Offering Commencement Date; provided, however, that no employee may be granted an Option which
permits his or her rights to purchase Common Stock under this Plan and any other employee stock purchase plan (as defined in Section 423(b) of the Code) of the Company and its subsidiaries, to accrue at a rate which exceeds $25,000 of the fair
market value of such Common Stock (determined at the date such Option is granted) for each calendar year in which the Option is outstanding at any time; and, provided, further, however, that the Committee may, in its discretion, set a fixed maximum
number of shares of Common Stock that each eligible employee may purchase per Plan Period which number may not be greater than the number of shares of Common Stock determined by using the formula in the first clause of this Section 9(a) and
which number shall be subject to the second clause of this Section 9(a). 
 (b) Option Price. The Board or the Committee shall
determine the Option Price for each Plan Period, including whether such Option Price shall be determined based on the lesser of the closing price of the Common Stock on (i) the first business day of the Plan Period or (ii) the Exercise
Date, or shall be based solely on the closing price of the Common Stock on the Exercise Date; provided, however, that such Option Price shall be at least 85% of the applicable closing price. In the absence of a determination by the Board or the
Committee, the Option Price will be 85% of the lesser of the closing price of the Common Stock on (i) the first business day of the Plan Period or (ii) the Exercise Date. The closing price shall be (a) the closing price (for the
primary trading session) on any national securities exchange on which the Common Stock is listed or (b) the average of the closing bid and asked prices in the over-the-counter-market, whichever is applicable, as published in The Wall Street
Journal or another source selected by the Board or the Committee; provided that, with respect to the first Plan Period, the closing price on the Offering Commencement Date shall be the initial public offering price, without regard to


  
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any underwriters’ discount, provided for in the underwriting agreement entered into by the Company in connection with the IPO. If no sales of Common Stock were made on such a day, the price
of the Common Stock shall be the reported price for the next preceding day on which sales were made. 
 (c) Exercise of Option. Each
employee who continues to be a participant in the Plan on the Exercise Date shall be deemed to have exercised his or her Option at the Option Price on such date and shall be deemed to have purchased from the Company the number of whole shares of
Common Stock reserved for the purpose of the Plan that his or her accumulated payroll deductions on such date will pay for, but not in excess of the maximum numbers determined in the manner set forth above. 

(d) Return of Unused Payroll Deductions. Any balance remaining in an employee’s payroll deduction account at the end of a Plan
Period will be automatically refunded to the employee, except that any balance that is less than the purchase price of one share of Common Stock will be carried forward into the employee’s payroll deduction account for the following Offering,
unless the employee elects not to participate in the following Offering under the Plan, in which case the balance in the employee’s account shall be refunded. 

10. Issuance of Certificates. Certificates representing shares of Common Stock purchased under the Plan may be issued only in the name
of the employee, in the name of the employee and another person of legal age as joint tenants with rights of survivorship, or (in the Company’s sole discretion) in the name of a brokerage firm, bank, or other nominee holder designated by the
employee. The Company may, in its sole discretion and in compliance with applicable laws, authorize the use of book entry registration of shares in lieu of issuing stock certificates. 

11. Rights on Retirement, Death or Termination of Employment. If a participating employee’s employment ends before the last
business day of a Plan Period, no payroll deduction shall be taken from any pay then due and owing to the employee and the balance in the employee’s account shall be paid to the employee. In the event of the employee’s death before the
last business day of a Plan Period, the Company shall, upon notification of such death, pay the balance of the employee’s account (a) to the executor or administrator of the employee’s estate or (b) if no such executor or
administrator has been appointed to the knowledge of the Company, to such other person(s) as the Company may, in its discretion, designate. If, before the last business day of the Plan Period, the Designated Subsidiary by which an employee is
employed ceases to be a subsidiary of the Company, or if the employee is transferred to a subsidiary of the Company that is not a Designated Subsidiary, the employee shall be deemed to have terminated employment for the purposes of this
Plan. 
 12. Optionees Not Stockholders. Neither the granting of an Option to an employee nor the deductions from his or her pay
shall make such employee a stockholder of the shares of Common Stock covered by an Option under this Plan until he or she has purchased and received such shares. 

  
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 13. Options Not Transferable. Options under this Plan are not transferable by a
participating employee other than by will or the laws of descent and distribution, and are exercisable during the employee’s lifetime only by the employee. 

14. Application of Funds. All funds received or held by the Company under this Plan may be combined with other corporate funds and may
be used for any corporate purpose. 
 15. Adjustment for Changes in Common Stock and Certain Other Events. 

(a) Changes in Capitalization. In the event of any stock split, reverse stock split, stock dividend, recapitalization, combination of
shares, reclassification of shares, spin-off or other similar change in capitalization or event, or any dividend or distribution to holders of Common Stock other than an ordinary cash dividend, (i) the number and class of securities available
under this Plan, (ii) the share limitations set forth in Section 9, and (iii) the Option Price shall be equitably adjusted to the extent determined by the Board or the Committee. 

(b) Reorganization Events. 

(1) Definition. A “Reorganization Event” shall mean: (a) any merger or consolidation of the Company with or into
another entity as a result of which all of the Common Stock of the Company is converted into or exchanged for the right to receive cash, securities or other property or is cancelled, (b) any transfer or disposition of all of the Common Stock of
the Company for cash, securities or other property pursuant to a share exchange or other transaction or (c) any liquidation or dissolution of the Company. 

(2) Consequences of a Reorganization Event on Options. In connection with a Reorganization Event, the Board or the Committee may take
any one or more of the following actions as to outstanding Options on such terms as the Board or the Committee determines: (i) provide that Options shall be assumed, or substantially equivalent Options shall be substituted, by the acquiring or
succeeding corporation (or an affiliate thereof), (ii) upon written notice to employees, provide that all outstanding Options will be terminated immediately prior to the consummation of such Reorganization Event and that all such outstanding
Options will become exercisable to the extent of accumulated payroll deductions as of a date specified by the Board or the Committee in such notice, which date shall not be less than ten (10) days preceding the effective date of the
Reorganization Event, (iii) upon written notice to employees, provide that all outstanding Options will be cancelled as of a date prior to the effective date of the Reorganization Event and that all accumulated payroll deductions will be
returned to participating employees on such date, (iv) in the event of a Reorganization Event under the terms of which holders of Common Stock will receive upon consummation thereof a cash payment for each share surrendered in the
Reorganization Event (the “Acquisition Price”), change the last day of the Plan Period to be the date of the consummation of the Reorganization Event and make or provide for a cash payment to each employee equal to (A) (1) the
Acquisition Price times (2) the number of shares of Common Stock that the employee’s accumulated payroll deductions as of immediately prior to the Reorganization Event could purchase at the Option Price, where the Acquisition Price is
treated as the fair market value of the Common Stock on the last day of the applicable Plan Period for purposes of determining the Option Price under Section 9(b) hereof, and where the number of shares that could be purchased is subject to the
limitations set forth in 

  
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Section 9(a), minus (B) the result of multiplying such number of shares by such Option Price, (v) provide that, in connection with a liquidation or dissolution of the Company,
Options shall convert into the right to receive liquidation proceeds (net of the Option Price thereof) and (vi) any combination of the foregoing. 

For purposes of clause (i) above, an Option shall be considered assumed if, following consummation of the Reorganization Event, the
Option confers the right to purchase, for each share of Common Stock subject to the Option immediately prior to the consummation of the Reorganization Event, the consideration (whether cash, securities or other property) received as a result of the
Reorganization Event by holders of Common Stock for each share of Common Stock held immediately prior to the consummation of the Reorganization Event (and if holders were offered a choice of consideration, the type of consideration chosen by the
holders of a majority of the outstanding shares of Common Stock); provided, however, that if the consideration received as a result of the Reorganization Event is not solely common stock of the acquiring or succeeding corporation (or an affiliate
thereof), the Company may, with the consent of the acquiring or succeeding corporation, provide for the consideration to be received upon the exercise of Options to consist solely of such number of shares of common stock of the acquiring or
succeeding corporation (or an affiliate thereof) that the Board determines to be equivalent in value (as of the date of such determination or another date specified by the Board) to the per share consideration received by holders of outstanding
shares of Common Stock as a result of the Reorganization Event. 
 16. Amendment of the Plan. The Board may at any time, and from
time to time, amend or suspend this Plan or any portion thereof, except that (a) if the approval of any such amendment by the shareholders of the Company is required by Section 423 of the Code, such amendment shall not be effected without
such approval, and (b) in no event may any amendment be made that would cause the Plan to fail to comply with Section 423 of the Code. 

17. Insufficient Shares. If the total number of shares of Common Stock specified in elections to be purchased under any Offering plus
the number of shares purchased under previous Offerings under this Plan exceeds the maximum number of shares issuable under this Plan, the Board or the Committee will allot the shares then available on a pro-rata basis. 

18. Termination of the Plan. This Plan may be terminated at any time by the Board. Upon termination of this Plan all amounts in the
accounts of participating employees shall be promptly refunded. 
 19. Governmental Regulations. The Company’s obligation
to sell and deliver Common Stock under this Plan is subject to listing on a national stock exchange (to the extent the Common Stock is then so listed or quoted) and the approval of all governmental authorities required in connection with the
authorization, issuance or sale of such stock. 
 20. Governing Law. The Plan shall be governed by Delaware law except to the extent
that such law is preempted by federal law. 

  
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 21. Issuance of Shares. Shares may be issued upon exercise of an Option from authorized
but unissued Common Stock, from shares held in the treasury of the Company, or from any other proper source. 
 22. Notification upon
Sale of Shares. Each employee agrees, by entering the Plan, to promptly give the Company notice of any disposition of shares purchased under the Plan where such disposition occurs within two years after the date of grant of the Option pursuant
to which such shares were purchased. 
 23. Special Provisions for First Plan Period. The following provisions of this
Section 23 shall apply with respect to the first Plan Period notwithstanding any provision of the Plan to the contrary: 
 Every
eligible employee shall automatically become a participant in the Plan for the first Plan Period at the highest percentage of Compensation permitted under Section 5. No payroll deductions shall be required for the first Plan Period; however, a
participant may, at any time after the effectiveness of the Plan’s Registration Statement on Form S-8, elect to have payroll deductions up to the aggregate amount that would have been credited to his or her account if a deduction of fifteen
percent (15%) of the Compensation that he or she received on each pay day during the first Plan Period had been made (the “Maximum Amount”) or decline to participate by filing an appropriate subscription agreement. 

Upon the automatic exercise of a participant’s option on the Exercise Date for the first Plan Period, a participant shall be permitted to
purchase shares with (i) the accumulated payroll deductions in his or her account, if any, (ii) a direct payment from the participant, or (iii) a combination thereof; provided, however that the total amount applied to the purchase may
not exceed the Maximum Amount. 
 24. Grants to Employees in Foreign Jurisdictions. The Company may, to comply with the laws of a
foreign jurisdiction, grant Options to employees of the Company or a Designated Subsidiary who are citizens or residents of such foreign jurisdiction (without regard to whether they are also citizens of the United States or resident aliens (within
the meaning of Section 7701(b)(1)(A) of the Code)) with terms that are less favorable (but not more favorable) than the terms of Options granted under the Plan to employees of the Company or a Designated Subsidiary who are resident in the
United States. Notwithstanding the preceding provisions of this Plan, employees of the Company or a Designated Subsidiary who are citizens or residents of a foreign jurisdiction (without regard to whether they are also citizens of the United States
or resident aliens (within the meaning of Section 7701(b)(1)(A) of the Code)) may be excluded from eligibility under the Plan if (a) the grant of an Option under the Plan to a citizen or resident of the foreign jurisdiction is prohibited
under the laws of such jurisdiction or (b) compliance with the laws of the foreign jurisdiction would cause the Plan to violate the requirements of Section 423 of the Code. The Company may add one or more appendices to this Plan describing
the operation of the Plan in those foreign jurisdictions in which employees are excluded from participation or granted less favorable Options. 

  
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 25. Authorization of Sub-Plans. The Board may from time to time establish one or more
sub-plans under the Plan with respect to one or more Designated Subsidiaries, provided that such sub-plan complies with Section 423 of the Code. 

26. Withholding. If applicable tax laws impose a tax withholding obligation, each affected employee shall, no later than the date of
the event creating the tax liability, make provision satisfactory to the Board for payment of any taxes required by law to be withheld in connection with any transaction related to Options granted to or shares acquired by such employee pursuant to
the Plan. The Company may, to the extent permitted by law, deduct any such taxes from any payment of any kind otherwise due to an employee. 

27. Effective Date and Approval of Shareholders. The Plan shall take effect as of the effectiveness of the registration statement
relating to the Company’s initial public offering subject to approval by the shareholders of the Company as required by Section 423 of the Code, which approval must occur within twelve months of the adoption of the Plan by the Board. 

Adopted by the Board of Directors on 

October 23, 2015 

Approved by the stockholders on 

January 29, 2016 

  
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