Exhibit 10.5

ACACIA COMMUNICATIONS, INC. 2009 STOCK PLAN:

SUMMARY OF STOCK GRANT (FOR SERVICES)

The Transferee is acquiring shares of the Common Stock of Acacia Communications, Inc. on the following terms:

By signing below, the Transferee and the Company agree that the acquisition of the Transferred Shares is governed by the terms and conditions of the 2009 Stock Plan and the Stock Grant Agreement. Both of these documents are attached to, and made a part of, this Summary of Stock Grant. The Transferee agrees to accept by email all documents relating to the Company, the Plan or this grant and all other documents that the Company is required to deliver to its security holders (including, without limitation, disclosures that may be required by the Securities and Exchange Commission). The Transferee also agrees that the Company may deliver these documents by posting them on a website maintained by the Company or by a third party under contract with the Company. If the Company posts these documents on a website, it shall notify the Transferee by email of their availability. The Transferee acknowledges that he or she may incur costs in connection with electronic delivery, including the cost of accessing the internet and printing fees, and that an interruption of internet access may interfere with his or her ability to access the documents. This consent shall remain in effect until the Transferee gives the Company written notice that it should deliver paper documents.

ACACIA COMMUNICATIONS, INC. 2009 STOCK PLAN:

STOCK GRANT AGREEMENT (FOR SERVICES)

SECTION 1. ACQUISITION OF SHARES.

(a) Transfer. On the terms and conditions set forth in the Summary of Stock Grant and this Agreement, the Company agrees to transfer to the Transferee the number of Shares set forth in the Summary of Stock Grant. The transfer shall occur at the offices of the Company on the date of transfer set forth in the Summary of Stock Grant or at such other place and time as the parties may agree.

(b) Consideration. The Transferee and the Company agree that the Transferred Shares are being issued to the Transferee as consideration for a portion of the services performed by the Transferee for the Company. The value of such portion is agreed to be not less than 100% of the Fair Market Value of the Transferred Shares.

(c) Stock Plan and Defined Terms. The transfer of the Transferred Shares is subject to the Plan, a copy of which the Transferee acknowledges having received. The provisions of the Plan are incorporated into this Agreement by this reference. Capitalized terms are defined in Section 12 of this Agreement.

SECTION 2. FORFEITURE CONDITION.

(a) Scope of Forfeiture Condition. All Transferred Shares initially shall be Restricted Shares and shall be subject to forfeiture to the Company. The Transferee shall not transfer, assign, encumber or otherwise dispose of any Restricted Shares without the Company’s written consent, except as provided in the following sentence. The Transferee may transfer Restricted Shares to one or more members of the Transferee’s Immediate Family or to a trust established by the Transferee for the benefit of the Transferee and/or one or more members of the Transferee’s Immediate Family, provided in either case that the Transferee agrees in writing on a form prescribed by the Company to be bound by all provisions of this Agreement. If the Transferee transfers any Restricted Shares, then this Agreement shall apply to the Subsequent Transferee to the same extent as to the Transferee.

(b) Vesting. The Forfeiture Condition shall lapse and the Restricted Shares shall become vested in accordance with the vesting schedule set forth in the Summary of Stock Grant. In addition, (i) if the Company is subject to a Change in Control before the Transferee’s Service terminates, then at all times after the Change in Control the portion of the Shares that are Restricted Shares and subject to the Forfeiture Condition shall be determined by adding six (6) months to the Transferee’s actual Service; and (ii) if the Transferee is subject to an Involuntary Termination at any time following a Change in Control then the Forfeiture Condition shall immediately lapse with respect to 100% of the then Restricted Shares.

(c) Execution of Forfeiture. The Forfeiture Condition shall be applicable only if the Transferee’s Service terminates for any reason, with or without cause, including (without limitation) death or disability, before all Restricted Shares have become vested. In the event that the Transferee’s Service terminates for any reason, the certificate(s) representing any remaining Restricted Shares shall be delivered to the Company. The Company shall make no payment for Restricted Shares that are forfeited.

(d) Additional Shares or Substituted Securities. In the event of the declaration of a stock dividend, the declaration of an extraordinary dividend payable in a form other than stock, a spin-off, a stock split, an adjustment in conversion ratio, a recapitalization or a similar transaction affecting the Company’s outstanding securities without receipt of consideration, any new, substituted or additional securities or other property (including money paid other than as an ordinary cash dividend) which are by reason of such transaction distributed with respect to any Restricted Shares or into which such Restricted Shares thereby become convertible shall immediately be subject to the Forfeiture Condition. Appropriate adjustments to reflect the distribution of such securities or property shall be made to the number and/or class of the Restricted Shares.

(e) Termination of Rights as Stockholder. If Restricted Shares are forfeited in accordance with this Section 2, then the person who is to forfeit such Restricted Shares shall no longer have any rights as a holder of such Restricted Shares. Such Restricted Shares shall be deemed to have been forfeited in accordance with the applicable provisions hereof, whether or not the certificate(s) therefor have been delivered as required by this Agreement.

(f) Escrow. Upon issuance, the certificates for Restricted Shares shall be deposited in escrow with the Company to be held in accordance with the provisions of this Agreement. Any new, substituted or additional securities or other property described in Subsection (d) above shall immediately be delivered to the Company to be held in escrow, but only to the extent the Transferred Shares are at the time Restricted Shares. All regular cash dividends on Restricted Shares (or other securities at the time held in escrow) shall be paid directly to the Transferee and shall not be held in escrow. Restricted Shares, together with any other assets or securities held in escrow hereunder, shall be (i) surrendered to the Company for forfeiture and cancellation in the event that the Forfeiture Condition or Right of First Refusal applies or (ii) released to the Transferee upon the Transferee’s request to the extent the Transferred Shares are no longer Restricted Shares (but not more frequently than once every six months). In any event, all Transferred Shares that have vested (and any other vested assets and securities attributable thereto) shall be released within 60 days after the earlier of (i) the termination of the Transferee’s Service or (ii) the lapse of the Right of First Refusal.

(g) Part-Time Employment and Leaves of Absence. If the Transferee commences working on a part-time basis, then the Company may adjust the vesting schedule set forth in the Summary of Stock Grant. If the Transferee goes on a leave of absence, then the Company may adjust the vesting schedule set forth in the Summary of Stock Grant in accordance with the Company’s leave of absence policy or the terms of such leave. Except as provided in the preceding sentence, Service shall be deemed to continue while the Transferee is on a bona fide leave of absence, if (i) such leave was approved by the Company in writing and (ii) continued

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crediting of Service is expressly required by the terms of such leave or by applicable law (as determined by the Company). Service shall be deemed to terminate when such leave ends, unless the Transferee immediately returns to active work.

SECTION 3. RIGHT OF FIRST REFUSAL.

(a) Right of First Refusal. In the event that the Transferee proposes to sell, pledge or otherwise transfer to a third party any Transferred Shares, or any interest in Transferred Shares, the Company shall have the Right of First Refusal with respect to all (and not less than all) of such Transferred Shares. If the Transferee desires to transfer Transferred Shares, the Transferee shall give a written Transfer Notice to the Company describing fully the proposed transfer, including the number of Transferred Shares proposed to be transferred, the proposed transfer price, the name and address of the proposed Subsequent Transferee and proof satisfactory to the Company that the proposed sale or transfer will not violate any applicable federal, State or foreign securities laws. The Transfer Notice shall be signed both by the Transferee and by the proposed Subsequent Transferee and must constitute a binding commitment of both parties to the transfer of the Transferred Shares. The Company shall have the right to purchase all, and not less than all, of the Transferred Shares on the terms of the proposal described in the Transfer Notice (subject, however, to any change in such terms permitted under Subsection (b) below) by delivery of a notice of exercise of the Right of First Refusal within 30 days after the date when the Transfer Notice was received by the Company.

(b) Transfer of Shares. If the Company fails to exercise its Right of First Refusal within 30 days after receiving the Transfer Notice, the Transferee may, not later than 90 days after the Company received the Transfer Notice, conclude a transfer of the Transferred Shares subject to the Transfer Notice on the terms and conditions described in the Transfer Notice, provided that any such sale is made in compliance with applicable federal, State and foreign securities laws and not in violation of any other contractual restrictions to which the Transferee is bound. Any proposed transfer on terms and conditions different from those described in the Transfer Notice, as well as any subsequent proposed transfer by the Transferee, shall again be subject to the Right of First Refusal and shall require compliance with the procedure described in Subsection (a) above. If the Company exercises its Right of First Refusal, the parties shall consummate the sale of the Transferred Shares on the terms set forth in the Transfer Notice within 60 days after the Company received the Transfer Notice (or within such longer period as may have been specified in the Transfer Notice); provided, however, that in the event the Transfer Notice provided that payment for the Transferred Shares was to be made in a form other than cash or cash equivalents paid at the time of transfer, the Company shall have the option of paying for the Transferred Shares with cash or cash equivalents equal to the present value of the consideration described in the Transfer Notice.

(c) Additional or Exchanged Securities and Property. In the event of a merger or consolidation of the Company with or into another entity, any other corporate reorganization, a stock split, the declaration of a stock dividend, the declaration of an extraordinary dividend payable in a form other than stock, a spin-off, an adjustment in conversion ratio, a recapitalization or a similar transaction affecting the Company’s outstanding securities, any securities or other property (including cash or cash equivalents) that are by reason

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of such transaction exchanged for, or distributed with respect to, any Transferred Shares subject to this Section 3 shall immediately be subject to the Right of First Refusal. Appropriate adjustments to reflect the exchange or distribution of such securities or property shall be made to the number and/or class of the Transferred Shares subject to this Section 3.

(d) Termination of Right of First Refusal. Any other provision of this Section 3 notwithstanding, in the event that the Stock is readily tradable on an established securities market when the Transferee desires to transfer Transferred Shares, the Company shall have no Right of First Refusal, and the Transferee shall have no obligation to comply with the procedures prescribed by Subsections (a) and (b) above.

(e) Permitted Transfers. This Section 3 shall not apply to (i) a transfer by beneficiary designation, will or intestate succession or (ii) a transfer to one or more members of the Transferee’s Immediate Family or to a trust established by the Transferee for the benefit of the Transferee and/or one or more members of the Transferee’s Immediate Family, provided in either case that the Transferee agrees in writing on a form prescribed by the Company to be bound by all provisions of this Agreement. If the Transferee transfers any Transferred Shares, either under this Subsection (e) or after the Company has failed to exercise the Right of First Refusal, then this Agreement shall apply to the Subsequent Transferee to the same extent as to the Transferee.

(f) Termination of Rights as Stockholder. If the Company makes available, at the time and place and in the amount and form provided in this Agreement, the consideration for the Shares to be purchased in accordance with this Section 3, then after such time the person from whom such Shares are to be purchased shall no longer have any rights as a holder of such Shares (other than the right to receive payment of such consideration in accordance with this Agreement). Such Shares shall be deemed to have been purchased in accordance with the applicable provisions hereof, whether or not the certificate(s) therefor have been delivered as required by this Agreement.

(g) Assignment of Right of First Refusal. The Board of Directors may freely assign the Company’s Right of First Refusal, in whole or in part. Any person who accepts an assignment of the Right of First Refusal from the Company shall assume all of the Company’s rights and obligations under this Section 3.

SECTION 4. OTHER RESTRICTIONS ON TRANSFER.

(a) Transferee Representations. In connection with the issuance and acquisition of Shares under this Agreement, the Transferee hereby represents and warrants to the Company as follows:

(i) The Transferee is acquiring and will hold the Transferred Shares for investment for his or her account only and not with a view to, or for resale in connection with, any “distribution” thereof within the meaning of the Securities Act.

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(ii) The Transferee understands that the Transferred Shares have not been registered under the Securities Act by reason of a specific exemption therefrom and that the Transferred Shares must be held indefinitely, unless they are subsequently registered under the Securities Act or the Transferee obtains an opinion of counsel, in form and substance satisfactory to the Company and its counsel, that such registration is not required. The Transferee further acknowledges and understands that the Company is under no obligation to register the Transferred Shares.

(iii) The Transferee is aware of the adoption of Rule 144 by the Securities and Exchange Commission under the Securities Act, which permits limited public resales of securities acquired in a non-public offering, subject to the satisfaction of certain conditions, including (without limitation) the availability of certain current public information about the issuer, the resale occurring only after the holding period required by Rule 144 has been satisfied, the sale occurring through an unsolicited “broker’s transaction,” and the amount of securities being sold during any three-month period not exceeding specified limitations. The Transferee acknowledges and understands that the conditions for resale set forth in Rule 144 have not been satisfied and that the Company has no plans to satisfy these conditions in the foreseeable future.

(iv) The Transferee will not sell, transfer or otherwise dispose of the Transferred Shares in violation of the Securities Act, the Securities Exchange Act of 1934, or the rules promulgated thereunder, including Rule 144 under the Securities Act. The Transferee agrees that he or she will not dispose of the Transferred Shares unless and until he or she has complied with all requirements of this Agreement applicable to the disposition of Transferred Shares and he or she has provided the Company with written assurances, in substance and form satisfactory to the Company, that (A) the proposed disposition does not require registration of the Transferred Shares under the Securities Act or all appropriate action necessary for compliance with the registration requirements of the Securities Act or with any exemption from registration available under the Securities Act (including Rule 144) has been taken and (B) the proposed disposition will not result in the contravention of any transfer restrictions applicable to the Transferred Shares under applicable state law.

(v) The Transferee has been furnished with, and has had access to, such information as he or she considers necessary or appropriate for deciding whether to invest in the Transferred Shares, and the Transferee has had an opportunity to ask questions and receive answers from the Company regarding the terms and conditions of the issuance of the Transferred Shares.

(vi) The Transferee is aware that his or her investment in the Company is a speculative investment that has limited liquidity and is subject to the risk of complete loss. The Transferee is able, without impairing his or her financial condition, to hold the Transferred Shares for an indefinite period and to suffer a complete loss of his or her investment in the Transferred Shares.

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(b) Securities Law Restrictions. Regardless of whether the offering and sale of Shares under the Plan have been registered under the Securities Act or have been registered or qualified under the securities laws of any State, the Company at its discretion may impose restrictions upon the sale, pledge or other transfer of the Transferred Shares (including the placement of appropriate legends on stock certificates or the imposition of stop-transfer instructions) if, in the judgment of the Company, such restrictions are necessary or desirable in order to achieve compliance with the Securities Act, the securities laws of any State or any other law.

(c) Market Stand-Off. In connection with any underwritten public offering by the Company of its equity securities pursuant to an effective registration statement filed under the Securities Act, including the Company’s initial public offering, the Transferee or a Subsequent Transferee shall not directly or indirectly sell, make any short sale of, loan, hypothecate, pledge, offer, grant or sell any option or other contract for the purchase of, purchase any option or other contract for the sale of, or otherwise dispose of or transfer, or agree to engage in any of the foregoing transactions with respect to, any Transferred Shares without the prior written consent of the Company or its managing underwriter. Such restriction (the “Market Stand-Off”) shall be in effect for such period of time following the date of the final prospectus for the offering as may be requested by the Company or such underwriter. In no event, however, shall such period exceed 180 days plus such additional period as may reasonably be requested by the Company or such underwriter to accommodate regulatory restrictions on (i) the publication or other distribution of research reports or (ii) analyst recommendations and opinions, including (without limitation) the restrictions set forth in Rule 2711(f)(4) of the National Association of Securities Dealers and Rule 472(f)(4) of the New York Stock Exchange, as amended, or any similar successor rules. The Market Stand-Off shall in any event terminate two years after the date of the Company’s initial public offering. In the event of the declaration of a stock dividend, a spin-off, a stock split, an adjustment in conversion ratio, a recapitalization or a similar transaction affecting the Company’s outstanding securities without receipt of consideration, any new, substituted or additional securities which are by reason of such transaction distributed with respect to any Shares subject to the Market Stand-Off, or into which such Shares thereby become convertible, shall immediately be subject to the Market Stand-Off. In order to enforce the Market Stand-Off, the Company may impose stop-transfer instructions with respect to the Transferred Shares until the end of the applicable stand-off period. The Company’s underwriters shall be beneficiaries of the agreement set forth in this Subsection (c). This Subsection (c) shall not apply to Shares registered in the public offering under the Securities Act.

(d) Rights of the Company. The Company shall not be required to (i) transfer on its books any Transferred Shares that have been sold or transferred in contravention of this Agreement or (ii) treat as the owner of Transferred Shares, or otherwise to accord voting, dividend or liquidation rights to, any Subsequent Transferee to whom Transferred Shares have been transferred in contravention of this Agreement.

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SECTION 5. SUCCESSORS AND ASSIGNS.

Except as otherwise expressly provided to the contrary, the provisions of this Agreement shall inure to the benefit of, and be binding upon, the Company and its successors and assigns and be binding upon the Transferee and the Transferee’s legal representatives, heirs, legatees, distributees, assigns and transferees by operation of law, whether or not any such person has become a party to this Agreement or has agreed in writing to join herein and to be bound by the terms, conditions and restrictions hereof.

SECTION 6. NO RETENTION RIGHTS.

Nothing in this Agreement or in the Plan shall confer upon the Transferee any right to continue providing services to the Company for any period of specific duration or interfere with or otherwise restrict in any way the rights of the Company or of the Transferee, 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.

SECTION 7. TAX ELECTION.

The acquisition of the Transferred Shares may result in adverse tax consequences that may be avoided or mitigated by filing an election under Code Section 83(b). Such election may be filed only within 30 days after the date of transfer set forth in the Summary of Stock Grant. The form for making the Code Section 83(b) election is attached to this Agreement as an Exhibit. The Transferee should consult with his or her tax advisor to determine the tax consequences of acquiring the Transferred Shares and the advantages and disadvantages of filing the Code Section 83(b) election. The Transferee acknowledges that it is his or her sole responsibility, and not the Company’s, to file a timely election under Code Section 83(b), even if the Transferee requests the Company or its representatives to make this filing on his or her behalf.

SECTION 8. LEGENDS.

All certificates evidencing Transferred Shares shall bear the following legends:

“THE SHARES REPRESENTED HEREBY MAY NOT BE SOLD, ASSIGNED, TRANSFERRED, ENCUMBERED OR IN ANY MANNER DISPOSED OF, EXCEPT IN COMPLIANCE WITH THE TERMS OF A WRITTEN AGREEMENT BETWEEN THE COMPANY AND THE REGISTERED HOLDER OF THE SHARES (OR THE PREDECESSOR IN INTEREST TO THE SHARES). SUCH AGREEMENT GRANTS TO THE COMPANY CERTAIN RIGHTS OF FIRST REFUSAL UPON AN ATTEMPTED TRANSFER OF THE SHARES AND IMPOSES CERTAIN FORFEITURE CONDITIONS UPON TERMINATION OF SERVICE WITH THE COMPANY. THE SECRETARY OF THE COMPANY WILL UPON WRITTEN REQUEST FURNISH A COPY OF SUCH AGREEMENT TO THE HOLDER HEREOF WITHOUT CHARGE.”

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“THE SHARES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED, AND MAY NOT BE SOLD, PLEDGED, OR OTHERWISE TRANSFERRED WITHOUT AN EFFECTIVE REGISTRATION THEREOF UNDER SUCH ACT OR AN OPINION OF COUNSEL, SATISFACTORY TO THE COMPANY AND ITS COUNSEL, THAT SUCH REGISTRATION IS NOT REQUIRED.”

If required by the authorities of any State in connection with the issuance of the Transferred Shares, the legend or legends required by such State authorities shall also be endorsed on all such certificates.

SECTION 9. NOTICE.

Any notice required by the terms of this Agreement shall be given in writing. It shall be deemed effective upon (i) personal delivery, (ii) deposit with the United States Postal Service, by registered or certified mail, with postage and fees prepaid or (iii) deposit with Federal Express Corporation, with shipping charges prepaid. Notice shall be addressed to the Company at its principal executive office and to the Transferee at the address that he or she most recently provided to the Company in accordance with this Section 9.

SECTION 10. ENTIRE AGREEMENT.

The Summary of Stock Grant, this Agreement and the Plan constitute the entire contract between the parties hereto with regard to the subject matter hereof. They supersede any other agreements, representations or understandings (whether oral or written and whether express or implied) that relate to the subject matter hereof.

SECTION 11. CHOICE OF LAW.

This Agreement 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.

SECTION 12. DEFINITIONS.

(a) “Agreement” shall mean this Stock Grant Agreement.

(b) “Board of Directors” shall mean the Board of Directors of the Company, as constituted from time to time or, if a Committee has been appointed, such Committee.

(c) “Cause” shall mean a good faith determination by the Board of Directors of any of the following:

(i) An unauthorized use or disclosure by the Transferee of the Company’s confidential information or trade secrets, which use or disclosure causes material harm to the Company;

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(ii) A material breach by the Transferee of any material agreement between the Transferee and the Company;

(iii) A material failure by the Transferee to comply with the Company’s written policies or rules after receiving written notification of such failure from the Board of Directors;

(iv) The Transferee’s conviction of, or plea of “guilty” or “no contest” to, a felony under the laws of the United States or any state thereof;

(v) The Transferee’s gross negligence or willful misconduct in the course of performing Service;

(vi) A continuing failure by the Transferee to perform reasonably assigned duties after receiving written notification of such failure from the Board of Directors; or

(vii) A failure by the Transferee to cooperate in good faith with a governmental or internal investigation of the Company or its directors, officers or employees, if the Company has requested the Transferee’s cooperation.

(d) “Change in Control” shall mean (i) the consummation of a merger or consolidation of the Company with or into another entity or (ii) the sale, transfer or other disposition of all or substantially all of the Company’s assets. The foregoing notwithstanding, a merger or consolidation of the Company shall not constitute a “Change in Control” if immediately after such merger or consolidation a majority of the voting power of the capital stock of the continuing or surviving entity, or any direct or indirect parent corporation of such continuing or surviving entity, will be owned by persons who were the Company’s stockholders immediately prior to such merger or consolidation in substantially the same proportions as their ownership of the voting power of the Company’s capital stock immediately prior to such merger or consolidation.

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

(f) “Committee” shall mean (i) the consummation of a merger or consolidation of the Company with or into another entity or (ii) the sale, transfer or other disposition of all or substantially all of the Company’s assets. The foregoing notwithstanding, a merger or consolidation of the Company shall not constitute a “Change in Control” if immediately after such merger or consolidation a majority of the voting power of the capital stock of the continuing or surviving entity, or any direct or indirect parent corporation of such continuing or surviving entity, will be owned by persons who were the Company’s stockholders immediately prior to such merger or consolidation in substantially the same proportions as their ownership of the voting power of the Company’s capital stock immediately prior to such merger or consolidation.

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

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(h) “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.

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

(j) “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.

(k) “Forfeiture Condition” shall mean the forfeiture condition described in Section 2.

(l) “Immediate Family” shall mean any child, stepchild, grandchild, parent, stepparent, grandparent, spouse, sibling, mother-in-law, father-in-law, son-in-law, daughter-in-law, brother-in-law or sister-in-law and shall include adoptive relationships.

(m) “Involuntary Termination” shall mean the termination of the Transferee’s Service by reason of:

(i) The involuntary discharge of the Transferee by the Company (or the Parent or Subsidiary employing him) for reasons other than Cause; or

(ii) The voluntary resignation of the Transferee following any of the following events, if such event occurs without the consent of the Transferee: (A) a change in the Transferee’s position with the Company (or the Parent or Subsidiary employing him or her) that materially reduces his or her level of authority or responsibility (provided that the Transferee’s level of authority or responsibility shall not be deemed to be reduced merely because there are additional employees more senior to the Transferee as a result of the Change in Control), (B) a reduction in the Transferee’s base salary by more than 10%, unless such reduction is made in connection with a commensurate reduction in the compensation of employees of the Company generally, or (C) receipt of notice that the Transferee’s principal workplace will be relocated more than 50 miles.

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

(o) “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.

(p) “Plan” shall mean the Acacia Communications, Inc. 2009 Stock Plan, as amended.

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(q) “Restricted Share” shall mean a Transferred Share that is subject to the Forfeiture Condition.

(r) “Right of First Refusal” shall mean the Company’s right of first refusal described in Section 3.

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

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

(u) “Share” shall mean one share of Stock, as adjusted in accordance with Section 8 of the Plan (if applicable).

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

(w) “Subsequent Transferee” shall mean any person to whom the Transferee has directly or indirectly transferred any Transferred Shares.

(x) “Subsidiary” shall mean any corporation (other than the Company) in an unbroken chain or 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.

(y) “Summary of Stock Grant” shall mean the document so entitled to which this Agreement is attached.

(z) “Transferee” shall mean the individual named in the Summary of Stock Grant.

(aa) “Transfer Notice” shall mean the notice of a proposed transfer of Transferred Shares described in Section 3.

(bb) “Transferred Shares” shall mean the Shares acquired by the Transferee pursuant to this Agreement.

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EXHIBIT I

SECTION 83(b) ELECTION

This statement is made under Section 83(b) of the Internal Revenue Code of 1986, as amended, pursuant to Treasury Regulations Section 1.83-2.

Within 30 days after the date of transfer, this election must be filed with the Internal Revenue Service Center where the Transferee files his or her federal income tax returns. The filing should be made by registered or certified mail, return receipt requested. The Transferee must (a) file a copy of the completed form with his or her federal tax return for the current tax year and (b) deliver an additional copy to the Company.