STOCK PURCHASE AGREEMENT

Exhibit 10.12

STOCK PURCHASE AGREEMENT

This Stock Purchase Agreement (this “Agreement”) is entered into as of [                    ], by Acacia Communications, Inc., a Delaware corporation (the “Company”), and [                    ] (the “Purchaser”).

SECTION 1. ACQUISITION OF SHARES.

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

(b) Consideration. The Purchaser agrees to assign to the Company certain intellectual property rights, pursuant to that certain Technology Assignment Agreement dated as of the date hereof between the Company and the Purchaser (the “Assignment Agreement”), in exchange for the Purchased Shares. The Company and the Purchaser agree that the fair market value of such consideration is $[        ], or $[        ]per Purchased Share. The Purchase Price is agreed to be at least 100% of the Fair Market Value of the Purchased Shares. Payment shall be made on the transfer date by execution and delivery of the Assignment Agreement.

(c) Defined Terms. Capitalized terms not defined above are defined in Section 12 of this Agreement.

SECTION 2. RIGHT OF REPURCHASE.

(a) Scope of Repurchase Right. Until they vest in accordance with Subsection (b) below, [                    ] of the Purchased Shares shall be Restricted Shares and shall be subject to the Company’s Right of Repurchase. The Company, however, may decline to exercise its Right of Repurchase or may exercise its Right of Repurchase only with respect to a portion of the Restricted Shares. The Company may exercise its Right of Repurchase only during the Repurchase Period following the termination of the Purchaser’s Service. The Right of Repurchase may be exercised automatically under Subsection (d) below. If the Right of Repurchase is exercised, the Company shall pay the Purchaser an amount equal to the Purchase Price for each of the Restricted Shares being repurchased.

(b) Lapse of Repurchase Right. The Right of Repurchase shall lapse with respect to [                    ] of the Purchased Shares when the Purchaser completes each month of continuous Service following [                    ]. In addition, the following rules shall apply if the Company is subject to a Change in Control before the Purchaser’s Service terminates:

(i) At all times after the Change in Control, the vested portion of the Restricted Shares shall be determined by adding six months to the Purchaser’s actual Service.

(ii) If the Purchaser is subject to an Involuntary Termination after the Change in Control, then the Right of Repurchase shall lapse in full and all Restricted Shares shall become vested.

(c) Escrow. If requested by the Company, the certificate(s) for Restricted Shares shall, upon issuance, be deposited in escrow with the Company to be held in accordance with the provisions of this Agreement. If requested by the Company, any additional or exchanged securities or other property described in Subsection (f) below shall immediately be delivered to the Company to be held in escrow. All ordinary cash dividends on Restricted Shares (or on other securities held in escrow) shall be paid directly to the Purchaser and shall not be held in escrow. Restricted Shares, together with any other assets held in escrow under this Agreement, shall be (i) surrendered to the Company for repurchase upon exercise of the Right of Repurchase or the Right of First Refusal or (ii) released to the Purchaser upon his or her request to the extent that the Shares have ceased to be Restricted Shares (but not more frequently than once every six months). In any event, all Purchased Shares that have ceased to be Restricted Shares, together with any other vested assets held in escrow under this Agreement, shall be released within 90 days after the earlier of (i) the termination of the Purchaser’s Service or (ii) the lapse of the Right of First Refusal.

(d) Exercise of Repurchase Right. The Company shall be deemed to have exercised its Right of Repurchase automatically for all Restricted Shares as of the commencement of the Repurchase Period, unless the Company during the Repurchase Period notifies the holder of the Restricted Shares pursuant to Section 9 that it will not exercise its Right of Repurchase for some or all of the Restricted Shares. During the Repurchase Period, the Company shall pay to the holder of the Restricted Shares the purchase price determined under Subsection (a) above for the Restricted Shares being repurchased. Payment shall be made in cash or cash equivalents and/or by canceling indebtedness to the Company incurred by the Purchaser in the purchase of the Restricted Shares. The certificate(s) representing the Restricted Shares being repurchased shall be delivered to the Company properly endorsed for transfer.

(e) Termination of Rights as Stockholder. If the Right of Repurchase is exercised in accordance with this Section 2 and the Company makes available the consideration for the Restricted Shares being repurchased, then the person from whom the Restricted Shares are repurchased shall no longer have any rights as a holder of the Restricted Shares (other than the right to receive payment of such consideration). Such Restricted Shares shall be deemed to have been repurchased pursuant to this Section 2, whether or not the certificate(s) for such Restricted Shares have been delivered to the Company or the consideration for such Restricted Shares has been accepted.

(f) 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 of such transaction exchanged for, or distributed with respect to, any Restricted Shares shall immediately be subject to the Right of Repurchase. Appropriate adjustments to reflect the exchange or distribution of such securities or property shall be made to the number and/or class of the Restricted Shares. Appropriate adjustments shall also be made to the price per share to be paid upon the exercise of the Right of Repurchase, provided that the aggregate purchase price payable for the Restricted Shares shall remain the same. In the event of a merger or consolidation of the Company with or into another entity or any other corporate reorganization, the Right of Repurchase may be exercised by the Company’s successor.

(g) Transfer of Restricted Shares. The Purchaser 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 Purchaser may transfer Restricted Shares to one or more members of the Purchaser’s Immediate Family, to a trust established by the Purchaser for the benefit of the Purchaser and/or one or more members of the Purchaser’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 Purchaser transfers any Restricted Shares, then this Agreement shall apply to the Transferee to the same extent as to the Purchaser.

(h) Assignment of Repurchase Right. The Board of Directors may freely assign the Company’s Right of Repurchase, in whole or in part. Any person who accepts an assignment of the Right of Repurchase from the Company shall assume all of the Company’s rights and obligations under this Section 2.

SECTION 3. RIGHT OF FIRST REFUSAL.

(a) Right of First Refusal. In the event that the Purchaser proposes to sell, pledge or otherwise transfer to a third party any Purchased Shares, or any interest in Purchased Shares, the Company shall have the Right of First Refusal with respect to all (and not less than all) of such Purchased Shares. If the Purchaser desires to transfer Purchased Shares, the Purchaser shall give a written Transfer Notice to the Company describing fully the proposed transfer, including the number of Purchased Shares proposed to be transferred, the proposed transfer price, the name and address of the proposed Transferee and proof satisfactory to the Company that the proposed sale or transfer will not violate any applicable federal or state securities laws. The Transfer Notice shall be signed both by the Purchaser and by the proposed Transferee and must constitute a binding commitment of both parties to the transfer of the Purchased Shares. The Company shall have the right to purchase all, and not less than all, of the Purchased 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 10 days after receiving the Transfer Notice, the Purchaser may, not later than 90 days after the Company received the Transfer Notice, conclude a transfer of the Purchased 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 and state securities laws and not in violation of any other contractual restrictions to which the Purchaser 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 Purchaser, 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 Purchased 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 Purchased 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 Purchased 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 of such transaction exchanged for, or distributed with respect to, any Purchased 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 Purchased 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 Purchaser desires to transfer Purchased Shares, the Company shall have no Right of First Refusal, and the Purchaser 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 Purchaser’s Immediate Family or to a trust established by the Purchaser for the benefit of the Purchaser and/or one or more members of the Purchaser’s Immediate Family. If the Purchaser transfers any Purchased 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 Transferee to the same extent as to the Purchaser.

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

(i) The Purchaser is acquiring and will hold the Purchased 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.

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

(iii) The Purchaser 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 Purchaser acknowledges and understands that the conditions for resale set form in Rule 144 have not been satisfied and that the Company has no plans to satisfy these conditions in the foreseeable future.

(iv) The Purchaser will not sell, transfer or otherwise dispose of the Purchased 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 Purchaser agrees that he or she will not dispose of the Purchased Shares unless and until he or she has complied with all requirements of this Agreement applicable to the disposition of Purchased 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 Purchased 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 Purchased Shares under state securities law.

(v) The Purchaser 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 Purchased Shares, and the Purchaser has had an opportunity to ask questions and receive answers from the Company regarding the terms and conditions of the issuance of the Purchased Shares.

(vi) The Purchaser 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 Purchaser is able, without impairing his or her financial condition, to hold the Purchased Shares for an indefinite period and to suffer a complete loss of his or her investment in the Purchased Shares.

(b) Securities Law Restrictions. Regardless of whether the offering and sale of Shares under this Agreement 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 Purchased 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 Purchaser 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 Purchased Shares without the prior written consent of the Company or its underwriters. 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 underwriters. In no event, however, shall such period exceed 180 days. 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 Purchased 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, and the Purchaser shall be subject to this Subsection (c) only if the directors and officers of the Company are subject to similar arrangements.

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

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 Purchaser and the Purchaser’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 shall confer upon the Purchaser 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 Purchaser) or of the Purchaser, 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 Purchased 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 purchase. The form for making the Code Section 83(b) election is attached to this Agreement as Exhibit I. The Purchaser should consult with his tax advisor to determine the tax consequences of acquiring the Purchased Shares and the advantages and disadvantages of filing the Code Section 83(b) election. The Purchaser 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 Purchaser requests the Company or its representatives to make this filing on his or her behalf.

SECTION 8. LEGENDS.

All certificates evidencing Purchased 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 CERTAIN REPURCHASE RIGHTS 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.”

“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 Purchased 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 or (ii) deposit with the United States Postal Service, by registered or certified mail, with postage and fees prepaid. Notice shall be addressed to the Company at its principal executive office and to the Purchaser at the address that he or she most recently provided to the Company in accordance with this Section 9.

SECTION 10. ENTIRE AGREEMENT.

This Agreement constitutes the entire contract between the parties hereto with regard to the subject matter hereof. It supersedes any other agreements, representations or understandings (whether oral or written and whether express or implied) which 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 Purchase Agreement.

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

(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 Purchaser of the Company’s confidential information or trade secrets, which use or disclosure causes material harm to the Company;

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

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

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

(vi) A continuing failure by the Purchaser to perform reasonably assigned duties after receiving written notification of such failure from the Board of Directors; or (viii) A failure by the Purchaser 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 Purchaser’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) “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 members of the Board of Directors.

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

(h) “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.

(i) “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.

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

(k) “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.

(l) “Purchased Shares” shall mean the Shares purchased by the Purchaser pursuant to this Agreement.

(m) “Purchase Price” shall mean the dollar value for which one Share may be purchased pursuant to this Agreement, as specified in Section 1(b).

(n) “Repurchase Period” shall mean a period of 180 consecutive days commencing on the date when the Purchaser’s Service terminates for any reason, including (without limitation) death or disability.

(o) “Restricted Share” shall mean a Purchased Share that is subject to the Right of Repurchase.

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

(q) “Right of Repurchase” shall mean the Company’s right of repurchase described in Section 2.

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

(s) “Service” shall mean service as an Employee, Consultant or member of the Board of Directors.

(t) “Share” shall mean one share of Stock.

(u) “Stock” shall mean the Common Stock of the Company, with a par value of $0.0001 per Share.

(v) “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.

(w) “Transferee” shall mean any person to whom the Purchaser has directly or indirectly transferred any Purchased Share.

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

IN WITNESS WHEREOF each of the parties has executed this Agreement, in the case of the Company by its duly authorized officer, as of the day and year first above written.