ACACIA COMMUNICATIONS, INC. 2009 STOCK PLAN ADOPTED ON NOVEMBER 23, 2009 AMENDED ON JUNE 29, 2010, DECEMBER 20,2011, MARCH 5, 2012, APRIL 17, 2013 AND APRIL 29,2015 TABLE OF CONTENTS

Exhibit 10.2

ACACIA COMMUNICATIONS, INC.

2009 STOCK PLAN

ADOPTED ON NOVEMBER 23, 2009

AMENDED ON JUNE 29, 2010, DECEMBER 20, 2011, MARCH 5, 2012,

APRIL 17, 2013 AND APRIL 29, 2015

TABLE OF CONTENTS

i

ii

ACACIA COMMUNICATIONS, INC. 2009 STOCK PLAN

SECTION 1. ESTABLISHMENT AND PURPOSE.

The purpose of the Plan is to offer selected persons an opportunity to acquire a proprietary interest in the success of the Company, or to increase such interest, by acquiring Shares of the Company’s Stock. The Plan provides for the direct award or sale of Shares, the grant of Options to purchase Shares and the grant of Restricted Stock Units. Options granted under the Plan may include Nonstatutory Options as well as ISOs intended to qualify under Section 422 of the Code.

Capitalized terms are defined in Section 12.

SECTION 2. ADMINISTRATION.

(a) Committees of the Board of Directors. The Plan may be administered by one or more Committees. Each Committee shall consist of one or more members of the Board of Directors who have been appointed by the Board of Directors. Each Committee shall have such authority and be responsible for such functions as the Board of Directors has assigned to it. If no Committee has been appointed, the entire Board of Directors shall administer the Plan. Any reference to the Board of Directors in the Plan shall be construed as a reference to the Committee (if any) to whom the Board of Directors has assigned a particular function.

(b) Authority of the Board of Directors. Subject to the provisions of the Plan, the Board of Directors shall have full authority and discretion to take any actions it deems necessary or advisable for the administration of the Plan. All decisions, interpretations and other actions of the Board of Directors shall be final and binding on all Participants and all persons deriving their rights from a Participant.

SECTION 3. ELIGIBILITY.

(a) General Rule. Only Employees, Outside Directors and Consultants shall be eligible for the grant of Nonstatutory Options, Restricted Stock Units or the direct award or sale of Shares. Only Employees shall be eligible for the grant of ISOs.

(b) Ten-Percent Stockholders. A person who owns more than 10% of the total combined voting power of all classes of outstanding stock of the Company, its Parent or any of its Subsidiaries shall not be eligible for the grant of an ISO unless (i) the Exercise Price is at least 110% of the Fair Market Value of a Share on the Date of Grant and (ii) such ISO by its terms is not exercisable after the expiration of five years from the Date of Grant. For purposes of this Subsection (b), in determining stock ownership, the attribution rules of Section 424(d) of the Code shall be applied.

SECTION 4. STOCK SUBJECT TO PLAN.

(a) Basic Limitation. Not more than 6,835,895 Shares may be issued under the Plan, subject to Subsection (b) below and Section 9(a).¹ All of these Shares may be issued upon the exercise of ISOs. Except as otherwise provided in Subsection (b) below, the number of Shares that were previously issued or are subject to Awards outstanding at any time under the Plan shall not exceed the sum of (i) the number of Shares previously issued under the Plan and (ii) the number of Shares that then remain available for issuance under the Plan. The Company, during the term of the Plan, shall at all times reserve and keep available sufficient Shares to satisfy the requirements of the Plan. Shares offered under the Plan may be authorized but unissued Shares or treasury Shares.

(b) Additional Shares. In the event that Shares previously issued under the Plan are reacquired by the Company without the payment of any consideration therefor in excess of the amount (if any) previously paid by the Participant to the Company in respect of such Shares, such Shares shall be added to the number of Shares then available for issuance under the Plan. In the event that an outstanding Award for any reason expires or is canceled without the payment of any consideration therefor in excess of the amount (if any) previously paid by the Participant to the Company in respect of such Shares, the Shares allocable to the unexercised or unvested portion of such Award, as applicable, shall be added to the number of Shares then available for issuance under the Plan.

SECTION 5. TERMS AND CONDITIONS OF AWARDS OR SALES OF SHARES.

(a) Stock Grant or Purchase Agreement. Each direct award of Shares under the Plan shall be evidenced by a Stock Grant Agreement between the Grantee and the Company. Each sale of Shares under the Plan (other than upon exercise of an Option or settlement of Restricted Stock Units) shall be evidenced by a Stock Purchase Agreement between the Purchaser and the Company. Such award or sale shall be subject to all applicable terms and conditions of the Plan and may be subject to any other terms and conditions which are not inconsistent with the Plan and which the Board of Directors deems appropriate for inclusion in a Stock Grant Agreement or Stock Purchase Agreement. The provisions of the various Stock Grant Agreements and Stock Purchase Agreements entered into under the Plan need not be identical.

(b) Duration of Offers and Nontransferability of Rights. Any right to purchase Shares under the Plan (other than an Option) shall automatically expire if not exercised by the Purchaser within 30 days after the grant of such right was communicated to the Purchaser by the Company. Such right shall not be transferable and shall be exercisable only by the Purchaser to whom such right was granted.

(c) Purchase Price. The Board of Directors shall determine the Purchase Price of Shares to be offered under the Plan at its sole discretion. The Purchase Price shall be payable in a form described in Section 7.

2

(d) Withholding Taxes. As a condition to the award, purchase, vesting or transfer of Shares, the Grantee or Purchaser shall make such arrangements as the Board of Directors may require for the satisfaction of any federal, state, local or foreign withholding tax obligations that may arise in connection with such event.

(e) Transfer Restrictions and Forfeiture Conditions. Any Shares awarded or sold under the Plan shall be subject to such special forfeiture conditions, rights of repurchase, rights of first refusal and other transfer restrictions as the Board of Directors may determine. Such restrictions shall be set forth in the applicable Stock Grant Agreement or Stock Purchase Agreement and shall apply in addition to any restrictions that may apply to holders of Shares generally.

SECTION 6. TERMS AND CONDITIONS OF OPTIONS.

(a) Stock Option Agreement. Each grant of an Option under the Plan shall be evidenced by a Stock Option Agreement between the Optionee and the Company. The Option shall be subject to all applicable terms and conditions of the Plan and may be subject to any other terms and conditions which are not inconsistent with the Plan and which the Board of Directors deems appropriate for inclusion in a Stock Option Agreement. The provisions of the various Stock Option Agreements entered into under the Plan need not be identical.

(b) Number of Shares. Each Stock Option Agreement shall specify the number of Shares that are subject to the Option and shall provide for the adjustment of such number in accordance with Section 9. The Stock Option Agreement shall also specify whether the Option is an ISO or a Nonstatutory Option.

(c) Exercise Price. Each Stock Option Agreement shall specify the Exercise Price. The Exercise Price of an Option shall not be less than 100% of the Fair Market Value of a Share on the Date of Grant, and in the case of an ISO a higher percentage may be required by Section 3(b). Subject to the preceding sentence, the Exercise Price shall be determined by the Board of Directors at its sole discretion. The Exercise Price shall be payable in a form described in Section 7. This Subsection (c) shall not apply to an Option granted pursuant to an assumption of, or substitution for, another option in a manner that complies with Section 424(a) of the Code (whether or not the Option is an ISO).

(d) Exercisability. Each Stock Option Agreement shall specify the date when all or any installment of the Option is to become exercisable. No Option shall be exercisable unless the Optionee (i) has delivered an executed copy of the Stock Option Agreement to the Company or (ii) otherwise agrees to be bound by the terms of the Stock Option Agreement. The Board of Directors shall determine the exercisability provisions of the Stock Option Agreement at its sole discretion. All of an Optionee’s Options shall become exercisable in full if Section 9(b)(iv) applies.

(e) Basic Term. The Stock Option Agreement shall specify the term of the Option. The term shall not exceed 10 years from the Date of Grant, and in the case of an ISO a shorter term may be required by Section 3(b). Subject to the preceding sentence, the Board of Directors at its sole discretion shall determine when an Option is to expire.

3

(f) Termination of Service (Except by Death). If an Optionee’s Service terminates for any reason other than the Optionee’s death, then the Optionee’s Options shall expire on the earliest of the following dates:

(i) The expiration date determined pursuant to Subsection (e) above;

(ii) The date three months after the termination of the Optionee’s Service for any reason other than Disability, or such earlier or later date as the Board of Directors may determine (but in no event earlier than 30 days after the termination of the Optionee’s Service); or

(iii) The date six months after the termination of the Optionee’s Service by reason of Disability, or such later date as the Board of Directors may determine.

The Optionee may exercise all or part of the Optionee’s Options at any time before the expiration of such Options under the preceding sentence, but only to the extent that such Options had become exercisable before the Optionee’s Service terminated (or became exercisable as a result of the termination) and the underlying Shares had vested before the Optionee’s Service terminated (or vested as a result of the termination). The balance of such Options shall lapse when the Optionee’s Service terminates. In the event that the Optionee dies after the termination of the Optionee’s Service but before the expiration of the Optionee’s Options, all or part of such Options may be exercised (prior to expiration) by the executors or administrators of the Optionee’s estate or by any person who has acquired such Options directly from the Optionee by beneficiary designation, bequest or inheritance, but only to the extent that such Options had become exercisable before the Optionee’s Service terminated (or became exercisable as a result of the termination) and the underlying Shares had vested before the Optionee’s Service terminated (or vested as a result of the termination).

(g) Leaves of Absence. For purposes of Subsection (f) above, Service shall be deemed to continue while the Optionee is on a bona fide leave of absence, if such leave was approved by the Company in writing and if continued crediting of Service for this purpose is expressly required by the terms of such leave or by applicable law (as determined by the Company).

(h) Death of Optionee. If an Optionee dies while the Optionee is in Service, then the Optionee’s Options shall expire on the earlier of the following dates:

(i) The expiration date determined pursuant to Subsection (e) above; or

(ii) The date 12 months after the Optionee’s death, or such earlier or later date as the Board of Directors may determine (but in no event earlier than six months after the Optionee’s death).

All or part of the Optionee’s Options may be exercised at any time before the expiration of such Options under the preceding sentence by the executors or administrators of the Optionee’s estate

4

or by any person who has acquired such Options directly from the Optionee by beneficiary designation, bequest or inheritance, but only to the extent that such Options had become exercisable before the Optionee’s death (or became exercisable as a result of the death) and the underlying Shares had vested before the Optionee’s death (or vested as a result of the Optionee’s death). The balance of such Options shall lapse when the Optionee dies.

(i) Post-Exercise Restrictions on Transfer of Shares. Any Shares issued upon exercise of an Option shall be subject to such special forfeiture conditions, rights of repurchase, rights of first refusal and other transfer restrictions as the Board of Directors may determine. Such restrictions shall be set forth in the applicable Stock Option Agreement and shall apply in addition to any restrictions that may apply to holders of Shares generally.

(j) Pre-Exercise Restrictions on Transfer of Options or Shares. An Option shall be transferable by the Optionee only by (i) a beneficiary designation, (ii) a will or (iii) the laws of descent and distribution, except as provided in the next sentence. If the applicable Stock Option Agreement so provides, a Nonstatutory Option shall also be transferable by gift or domestic relations order to a Family Member of the Optionee. An ISO may be exercised during the lifetime of the Optionee only by the Optionee or by the Optionee’s guardian or legal representative.

(k) Withholding Taxes. As a condition to the grant or exercise of an Option, the Optionee shall make such arrangements as the Board of Directors may require for the satisfaction of any federal, state, local or foreign withholding tax obligations that may arise in connection with such grant or exercise. The Optionee shall also make such arrangements as the Board of Directors may require for the satisfaction of any federal, state, local or foreign withholding tax obligations that may arise in connection with the vesting or transfer of Shares acquired by exercising an Option or any similar event.

(l) No Rights as a Stockholder. An Optionee, or a transferee of an Optionee, shall have no rights as a stockholder with respect to any Shares covered by the Optionee’s Option until such person becomes entitled to receive such Shares by filing a notice of exercise and paying the Exercise Price pursuant to the terms of such Option.

(m) Modification, Extension and Assumption of Options. Within the limitations of the Plan, the Board of Directors may modify, extend or assume outstanding Options or may accept the cancellation of outstanding Options (whether granted by the Company or another issuer) in return for the grant of new Options for the same or a different number of Shares and at the same or a different Exercise Price. The foregoing notwithstanding, no modification of an Option shall, without the consent of the Optionee, impair the Optionee’s rights or increase the Optionee’s obligations under such Option.

(n) Company’s Right to Cancel Certain Options. Any other provision of the Plan or a Stock Option Agreement notwithstanding, the Company shall have the right at any time to cancel an Option that was not granted in compliance with Rule 701 under the Securities Act. Prior to canceling such Option, the Company shall give the Optionee not less than 30 days’ notice in writing. If the Company elects to cancel such Option, it shall deliver to the Optionee consideration with an aggregate Fair Market Value equal to the excess of (i) the Fair Market

5

Value of the Shares subject to such Option as of the time of the cancellation over (ii) the Exercise Price of such Option. The consideration may be delivered in the form of cash or cash equivalents, in the form of Shares, or a combination of both. If the consideration would be a negative amount, such Option may be cancelled without the delivery of any consideration.

SECTION 7. PAYMENT FOR SHARES.

(a) General Rule. The entire Purchase Price or Exercise Price of Shares issued under the Plan shall be payable in cash or cash equivalents at the time when such Shares are purchased, except as otherwise provided in this Section 7.

(b) Services Rendered. At the discretion of the Board of Directors, Shares may be awarded under the Plan in consideration of services rendered to the Company, a Parent or a Subsidiary prior to the Award.

(c) Promissory Note. At the discretion of the Board of Directors, all or a portion of the Purchase Price or Exercise Price (as the case may be) of Shares issued under the Plan may be paid with a full-recourse promissory note. The Shares shall be pledged as security for payment of the principal amount of the promissory note and interest thereon. The interest rate payable under the terms of the promissory note shall not be less than the minimum rate (if any) required to avoid the imputation of additional interest under the Code. Subject to the foregoing, the Board of Directors (at its sole discretion) shall specify the term, interest rate, amortization requirements (if any) and other provisions of such note.

(d) Surrender of Stock. At the discretion of the Board of Directors, all or any part of the Exercise Price may be paid by surrendering, or attesting to the ownership of, Shares that are already owned by the Optionee. Such Shares shall be surrendered to the Company in good form for transfer and shall be valued at their Fair Market Value as of the date when the Option is exercised.

(e) Exercise/Sale. To the extent that a Stock Option Agreement so provides, and if Stock is publicly traded, all or part of the Exercise Price and any withholding taxes may be paid by the delivery (on a form prescribed by the Company) of an irrevocable direction to a securities broker approved by the Company to sell Shares and to deliver all or part of the sales proceeds to the Company.

(f) Other Forms of Payment. To the extent that a Stock Purchase Agreement or Stock Option Agreement so provides, the Purchase Price or Exercise Price of Shares issued under the Plan may be paid in any other form permitted by the Delaware General Corporation Law, as amended.

SECTION 8. TERMS AND CONDITIONS OF RESTRICTED STOCK UNITS.

(a) Restricted Stock Unit Agreement. Each grant of Restricted Stock Units under the Plan shall be evidenced by a Restricted Stock Unit Agreement between the recipient and the Company. Restricted Stock Units granted under the Plan shall be subject to all applicable terms and conditions of the Plan and may be subject to any other terms and conditions

6

that are not inconsistent with the Plan and which the Board of Directors deems appropriate for inclusion in a Restricted Stock Unit Agreement. The provisions of the various Restricted Stock Unit Agreements entered into under the Plan need not be identical.

(b) Payment for Restricted Stock Units. No cash consideration shall be required of the recipient in connection with the grant of Restricted Stock Units.

(c) Vesting Conditions. Restricted Stock Units may or may not be subject to vesting, as determined in the discretion of the Board of Directors. Vesting may occur, in full or in installments, upon the satisfaction of the vesting conditions specified in the Restricted Stock Unit Agreement, which may include continued Service with the Company or a Parent or Subsidiary, achievement of performance goals and/or such other criteria as the Board of Directors may determine. A Restricted Stock Unit Agreement may provide for accelerated vesting upon specified events.

(d) Voting and Dividend Rights. The recipient of Restricted Stock Units shall have no voting rights. Prior to settlement or forfeiture, any Restricted Stock Unit granted under the Plan may, at the discretion of the Board of Directors, carry with it a right to dividend equivalents. Such right entitles the recipient to be credited with an amount equal to all cash dividends paid on one Share for each Restricted Stock Unit held by the recipient at the time the cash dividend is paid. Dividend equivalents may be converted into additional Restricted Stock Units. Settlement of dividend equivalents may be made in the form of cash, in the form of Shares, or in a combination of both. Prior to distribution, any dividend equivalents that are not paid shall be subject to the same conditions and restrictions as the Restricted Stock Units to which they attach.

(e) Form and Time of Settlement of Restricted Stock Units. Settlement of vested Restricted Stock Units may be made in the form of (i) cash, (ii) Shares or (iii) any combination of cash and Shares, as determined by the Board of Directors. The actual number of Restricted Stock Units eligible for settlement may be larger or smaller than the number included in the original award, based on predetermined performance factors. Vested Restricted Stock Units shall be settled in such manner and at such time(s) as specified in the Restricted Stock Unit Agreement. Until Restricted Stock Units are settled, the number of such Restricted Stock Units shall be subject to adjustment pursuant to Section 9.

(f) Modification, Extension and Assumption of Restricted Stock Units. Within the limitations of the Plan, the Board of Directors may modify, extend or assume outstanding Restricted Stock Units. The foregoing notwithstanding, no modification of a Restricted Stock Unit shall, without the consent of the Participant, impair the Participant’s rights or increase the Participant’s obligations under such Restricted Stock Unit.

(g) Forfeiture. Unless a Restricted Stock Unit Agreement provides otherwise, upon termination of the Participant’s Service or upon such other time specified in the Restricted Stock Unit Agreement, any unvested Restricted Stock Units shall be forfeited to the Company. For this purpose, Service shall be deemed to continue while the Participant is on a bona fide leave of absence, if such leave was approved by the Company in writing and if continued crediting of Service for this purpose is expressly required by the terms of such leave or by applicable law (as determined by the Company).

7

(h) Death of Recipient. Any Restricted Stock Units that become distributable after the recipient’s death shall be distributed to the recipient’s beneficiary or beneficiaries. Each recipient may designate one or more beneficiaries for this purpose by filing the prescribed form with the Company, and may thereafter change such designation by filing the prescribed form with the Company at any time. If no beneficiary is designated or if no designated beneficiary survives the Participant, then any Restricted Stock Units that become payable after the Participant’s death shall be distributed to his or her estate.

(i) Creditors’ Rights. A holder of Restricted Stock Units shall have no rights other than those of a general creditor of the Company. Restricted Stock Units represent an unfunded and unsecured obligation of the Company, subject to the terms and conditions of the applicable Restricted Stock Unit Agreement.

(j) Transferability of Restricted Stock Units. Restricted Stock Units shall be transferable by a Participant only by (a) beneficiary designation, (b) a will or (c) the laws of descent and distribution.

SECTION 9. ADJUSTMENT OF SHARES.

(a) General. In the event of a subdivision of the outstanding Stock, a declaration of a dividend payable in Shares, a combination or consolidation of the outstanding Stock into a lesser number of Shares, a reclassification, or any other increase or decrease in the number of issued shares of Stock effected without receipt of consideration by the Company, proportionate adjustments shall automatically be made in each of (i) the number of Shares available for future grants under Section 4, (ii) the number of Shares covered by each outstanding Option or Restricted Stock Unit and (iii) the Exercise Price under each outstanding Option. In the event of a declaration of an extraordinary dividend payable in a form other than Shares in an amount that has a material effect on the Fair Market Value of the Stock, a recapitalization, a spin-off, or a similar occurrence, the Board of Directors at its sole discretion may make appropriate adjustments in one or more of (i) the number of Shares available for future grants under Section 4, (ii) the number of Shares covered by each outstanding Option or Restricted Stock Unit or (iii) the Exercise Price under each outstanding Option; provided, however, that the Board of Directors shall in any event make such adjustments as may be required by Section 25102(o) of the California Corporations Code.

(b) Mergers and Consolidations. In the event that the Company is a party to a merger or consolidation, all Shares acquired under the Plan and all Awards outstanding on the effective date of the transaction shall be treated in the manner described in the agreement of merger or consolidation, which need not treat all Awards in an identical manner. The agreement of merger or consolidation shall provide for one or more of the following with respect to each Award:

(i) The continuation of the outstanding Award by the Company (if the Company is the surviving corporation).

8

(ii) The assumption of the outstanding Award by the surviving corporation or its parent, provided that the assumption of an Option shall be in a manner that complies with Section 424(a) of the Code (whether or not the Option is an ISO).

(iii) The substitution by the surviving corporation or its parent of an equivalent award for the outstanding Award (including, but not limited to, an award to acquire the same consideration paid to the holders of Shares in the transaction), provided that the substitution of an Option shall be in a manner that complies with Section 424(a) of the Code (whether or not the Option is an ISO).

(iv) Full exercisability of the Option and full vesting of the Shares subject to the Option, followed by the cancellation of the Option. The full exercisability of the Option and full vesting of the Shares subject to the Option may be contingent on the closing of such merger or consolidation. The Optionee shall be able to exercise the Option during a period of not less than five full business days preceding the closing date of such merger or consolidation, unless (A) a shorter period is required to permit a timely closing of such merger or consolidation and (B) such shorter period still offers the Optionee a reasonable opportunity to exercise the Option. Any exercise of the Option during such period may be contingent on the closing of such merger or consolidation.

(v) The cancellation of the outstanding Award and a payment to the Participant with respect to each Share subject to the Award (including both vested and unvested Shares) as of the merger or consolidation equal to the excess of (A) the Fair Market Value of a Share as of the closing date of such merger or consolidation over (if applicable) (B) the per-Share Exercise Price of the Award (such excess, if any, the “Spread”). Such payment shall be made in the form of cash, cash equivalents, or securities of the surviving corporation or its parent with a Fair Market Value equal to the Spread. Subject to Section 409A of the Code, such payment may be made in installments and may be deferred until the date or dates when the Award would have become vested. Such payment may be subject to vesting based on the Participant’s continuing Service, provided that the vesting schedule shall not be less favorable to the Participant than the schedule under which the Award would have vested. If the Spread applicable to an Award is zero or a negative number, then the Award may be cancelled without making a payment to the Participant. For purposes of this Paragraph (v), the Fair Market Value of any security shall be determined without regard to any vesting conditions that may apply to such security. In the event that a Restricted Stock Unit is subject to Section 409A of the Code, the payment described in this Section 9(b)(v) shall be made on the settlement date specified in the applicable Restricted Stock Unit Agreement, provided that settlement may be accelerated in accordance with Treasury Regulation 1.409A-3(j)(4).

Any action taken under this Section 9(b) must either preserve an Award’s status as exempt from Section 409A of the Code or comply with Section 409A of the Code.

9

(c) Reservation of Rights. Except as provided in this Section 9, a Participant shall have no rights by reason of (i) any subdivision or consolidation of shares of stock of any class, (ii) the payment of any dividend or (iii) any other increase or decrease in the number of shares of stock of any class. Any issuance by the Company of shares of stock of any class, or securities convertible into shares of stock of any class, shall not affect, and no adjustment by reason thereof shall be made with respect to, the number of Shares subject to an Award or the Exercise Price of Shares subject to an Option. The grant of an Award pursuant to the Plan shall not affect in any way the right or power of the Company to make adjustments, reclassifications, reorganizations or changes to its capital or business structure, to merge or consolidate or to dissolve, liquidate, sell or transfer all or any part of its business or assets.

SECTION 10. MISCELLANEOUS PROVISIONS.

(a) Securities Law Requirements. Shares shall not be issued under the Plan unless the issuance and delivery of such Shares comply with (or are exempt from) all applicable requirements of law, including (without limitation) the Securities Act, the rules and regulations promulgated thereunder, state securities laws and regulations, and the regulations of any stock exchange or other securities market on which the Company’s securities may then be traded. The Company shall not be liable for a failure to issue Shares that is attributable to such requirements.

(b) No Retention Rights. Nothing in the Plan or in any right or Award granted under the Plan shall confer upon the Participant any right to continue in Service for any period of specific duration or interfere with or otherwise restrict in any way the rights of the Company (or any Parent or Subsidiary employing or retaining the Participant) or of the Participant, which rights are hereby expressly reserved by each, to terminate his or her Service at any time and for any reason, with or without cause.

(c) Treatment as Compensation. Any compensation that an individual earns or is deemed to earn under this Plan shall not be considered a part of his or her compensation for purposes of calculating contributions, accruals or benefits under any other plan or program that is maintained or funded by the Company, a Parent or a Subsidiary.

(d) Governing Law. The Plan and all awards, sales and grants under the Plan shall be governed by, and construed in accordance with, the laws of the State of Delaware, as such laws are applied to contracts entered into and performed in such State.

(e) Tax Matters.

(i) As a condition to the award, grant, issuance, vesting, purchase, exercise or transfer of any Award, or Shares issued pursuant to any Award, granted under this Plan, the Participant shall make such arrangements as the Board of Directors may require or permit for the satisfaction of any federal, state, local or foreign withholding tax obligations that may arise in connection with such event.

(ii) Unless otherwise expressly set forth in an Award Agreement, it is intended that awards granted under the Plan shall be exempt from

10

Section 409A of the Code, and any ambiguity in the terms of an Award Agreement and the Plan shall be interpreted consistently with this intent. To the extent an award is not exempt from Section 409A of the Code (any such award, a “409A Award”), any ambiguity in the terms of such award and the Plan shall be interpreted in a manner that to the maximum extent permissible supports the award’s compliance with the requirements of that statute. Notwithstanding anything to the contrary permitted under the Plan, in no event shall a modification of an Award not already subject to Section 409A of the Code be given effect if such modification would cause the Award to become subject to Section 409A of the Code unless the parties explicitly acknowledge and consent to the modification as one having that effect. A 409A Award shall be subject to such additional rules and requirements as specified by the Board of Directors from time to time in order for it to comply with the requirements of Section 409A of the Code. In this regard, if any amount under a 409A Award is payable upon a “separation from service” to an individual who is considered a “specified employee” (as each term is defined under Section 409A of the Code), then no such payment shall be made prior to the date that is the earlier of (i) six months and one day after the Participant’s separation from service or (ii) the Participant’s death, but only to the extent such delay is necessary to prevent such payment from being subject to Section 409A(a)(1). In addition, if a transaction subject to Section 9(b) constitutes a payment event with respect to any 409A Award, then the transaction with respect to such award must also constitute a “change in control event” as defined in Treasury Regulation Section 1.409A-3(i)(5) to the extent required by Section 409A of the Code.

(iii) Neither the Company nor any member of the Board of Directors shall have any liability to a Participant in the event an Award held by the Participant fails to achieve its intended characterization under applicable tax law.

SECTION 11. DURATION AND AMENDMENTS.

(a) Term of the Plan. The Plan, as set forth herein, shall become effective on the date of its adoption by the Board of Directors, subject to the approval of the Company’s stockholders. If the stockholders fail to approve the Plan within 12 months after its adoption by the Board of Directors, then any grants, exercises or sales that have already occurred under the Plan shall be rescinded and no additional grants, exercises or sales shall thereafter be made under the Plan. The Plan shall terminate automatically 10 years after the later of (i) the date when the Board of Directors adopted the Plan or (ii) the date when the Board of Directors approved the most recent increase in the number of Shares reserved under Section 4 that was also approved by the Company’s stockholders. The Plan may be terminated on any earlier date pursuant to Subsection (b) below.

(b) Right to Amend or Terminate the Plan. The Board of Directors may amend, suspend or terminate the Plan at any time and for any reason; provided, however, that any amendment of the Plan shall be subject to the approval of the Company’s stockholders if it (i) increases the number of Shares available for issuance under the Plan (except as provided in Section 9) or (ii) materially changes the class of persons who are eligible for the grant of ISOs.

11

Stockholder approval shall not be required for any other amendment of the Plan. If the stockholders fail to approve an increase in the number of Shares reserved under Section 4 within 12 months after its adoption by the Board of Directors, then any grants, exercises or sales that have already occurred in reliance on such increase shall be rescinded and no additional grants, exercises or sales shall thereafter be made in reliance on such increase.

(c) Effect of Amendment or Termination. No Shares shall be issued or sold under the Plan after the termination thereof, except upon exercise of an Option or settlement of a Restricted Stock Unit (or any other right to purchase Shares) granted under the Plan prior to such termination. The termination of the Plan, or any amendment thereof, shall not affect any Share previously issued or any Award previously granted under the Plan.

SECTION 12. DEFINITIONS.

(a) “Award” shall mean any award granted under the Plan, including an Option, Restricted Stock Unit or other right to acquire Shares under the Plan.

(b) “Award Agreement” shall mean a Stock Option Agreement, Stock Grant Agreement, Stock Purchase Agreement or Restricted Stock Unit Agreement.

(c) “Board of Directors” shall mean the Board of Directors of the Company, as constituted from time to time.

(d) “Code” shall mean the Internal Revenue Code of 1986, as amended.

(e) “Committee” shall mean a committee of the Board of Directors, as described in Section 2(a).

(f) “Company” shall mean Acacia Communications, Inc., a Delaware corporation.

(g) “Consultant” shall mean a person who performs bona fide services for the Company, a Parent or a Subsidiary as a consultant or advisor, excluding Employees and Outside Directors.

(h) “Date of Grant” shall mean the date of grant specified in the applicable Stock Option Agreement, which date shall be the later of (i) the date on which the Board of Directors resolved to grant the Option or (ii) the first day of the Optionee’s Service.

(i) “Disability” shall mean that the Optionee is unable to engage in any substantial gainful activity by reason of any medically determinable physical or mental impairment.

(j) “Employee” shall mean any individual who is a common-law employee of the Company, a Parent or a Subsidiary.

(k) “Exchange Act” shall mean the Securities Exchange Act of 1934, as amended.

12

(l) “Exercise Price” shall mean the amount for which one Share may be purchased upon exercise of an Option, as specified by the Board of Directors in the applicable Stock Option Agreement.

(m) “Fair Market Value” shall mean the fair market value of a Share, as determined by the Board of Directors in good faith. Such determination shall be conclusive and binding on all persons.

(n) “Family Member” shall mean (i) any child, stepchild, grandchild, parent, stepparent, grandparent, spouse, former spouse, sibling, niece, nephew, mother-in-law, father-in-law, son-in-law, daughter-in-law, brother-in-law or sister-in-law, including adoptive relationships, (ii) any person sharing the Participant’s household (other than a tenant or employee), (iii) a trust in which persons described in Clause (i) or (ii) have more than 50% of the beneficial interest, (iv) a foundation in which persons described in Clause (i) or (ii) or the Participant control the management of assets and (v) any other entity in which persons described in Clause (i) or (ii) or the Participant own more than 50% of the voting interests.

(o) “Grantee” shall mean a person to whom the Board of Directors has awarded Shares under the Plan.

(p) “ISO” shall mean an employee incentive stock option described in Section 422(b) of the Code.

(q) “Nonstatutory Option” shall mean a stock option not described in Sections 422(b) or 423(b) of the Code.

(r) “Option” shall mean an ISO or Nonstatutory Option granted under the Plan and entitling the holder to purchase Shares.

(s) “Optionee” shall mean a person who holds an Option.

(t) “Outside Director” shall mean a member of the Board of Directors who is not an Employee.

(u) “Parent” shall mean any corporation (other than the Company) in an unbroken chain of corporations ending with the Company, if each of the corporations other than the Company owns stock possessing 50% or more of the total combined voting power of all classes of stock in one of the other corporations in such chain. A corporation that attains the status of a Parent on a date after the adoption of the Plan shall be considered a Parent commencing as of such date.

(v) “Participant” shall mean an individual who holds an Award granted under the Plan.

(w) “Plan” shall mean this Acacia Communications, Inc. 2009 Stock Plan, as amended from time to time.

13

(x) “Purchase Price” shall mean the consideration for which one Share may be acquired under the Plan (other than upon exercise of an Option), as specified by the Board of Directors.

(y) “Purchaser” shall mean a person to whom the Board of Directors has offered the right to purchase Shares under the Plan (other than upon exercise of an Option or settlement of a Restricted Stock Unit).

(z) “Restricted Stock Unit” shall mean a bookkeeping entry representing the equivalent of one Share, as awarded under the Plan.

(aa) “Restricted Stock Unit Agreement” shall mean the agreement between the Company and the recipient of a Restricted Stock Unit that includes the terms, conditions and restrictions pertaining to such Restricted Stock Unit.

(bb) “Securities Act” shall mean the Securities Act of 1933, as amended.

(cc) “Service” shall mean service as an Employee, Outside Director or Consultant.

(dd) “Share” shall mean one share of Stock, as adjusted in accordance with Section 9 (if applicable).

(ee) “Stock” shall mean the Common Stock of the Company.

(ff) “Stock Grant Agreement” shall mean the agreement between the Company and a Grantee who is awarded Shares under the Plan that contains the terms, conditions and restrictions pertaining to the award of such Shares.

(gg) “Stock Option Agreement” shall mean the agreement between the Company and an Optionee that contains the terms, conditions and restrictions pertaining to the Optionee’s Option.

(hh) “Stock Purchase Agreement” shall mean the agreement between the Company and a Purchaser who purchases Shares under the Plan that contains the terms, conditions and restrictions pertaining to the purchase of such Shares.

(ii) “Subsidiary” shall mean any corporation (other than the Company) in an unbroken chain of corporations beginning with the Company, if each of the corporations other than the last corporation in the unbroken chain owns stock possessing 50% or more of the total combined voting power of all classes of stock in one of the other corporations in such chain. A corporation that attains the status of a Subsidiary on a date after the adoption of the Plan shall be considered a Subsidiary commencing as of such date.

14

EXHIBIT A

SCHEDULE OF SHARES RESERVED FOR ISSUANCE UNDER THE PLAN

E-1