EXHIBIT 5.1

           [Letterhead of Skadden, Arps, Slate, Meagher & Flom LLP]


                                                        December 22, 2000

Sycamore Networks, Inc.
150 Apollo Drive
Chelmsford, Massachusetts 01824

Re: Registration Statement on Form S-8
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Ladies and Gentlemen:

       We have acted as special counsel to Sycamore Networks, Inc., a Delaware
corporation (the "Registrant"), in connection with the preparation of a
Registration Statement on Form S-8 (the "Registration Statement"), for the
purpose of registering with the Securities and Exchange Commission (the
"Commission"), under the Securities Act of 1933, as amended (the "Securities
Act"), an aggregate of 25,000,000 shares of the Registrant's common stock, par
value $.001 per share ("Common Stock"), issuable pursuant to the Registrant's
1999 Stock Incentive Plan (the "Plan"). The Plan provides for the issuance of
Common Stock to the officers, directors, employees and consultants of the
Registrant and any present or future, parent or subsidiary of the Registrant.

       This opinion is being furnished in accordance with the requirements of
Item 601(b)(5) of Regulation S-K under the Securities Act.

       In connection with this opinion, we have examined originals or copies,
certified or otherwise identified to our satisfaction, of (i) the Registration
Statement, in the form to be filed with the Commission on the date hereof under
the Securities Act; (ii) a specimen certificate representing the Common Stock;
(iii) the Amended and Restated Certificate of Incorporation of the Registrant,
as presently in effect; (iv) the Amended and Restated By-Laws of the Registrant,
as presently in effect; (v) certain resolutions of the Board of Directors of the
Registrant relating to the Plan; and (vi) the Plan. We have also examined
originals or copies, certified or otherwise identified to our satisfaction, of
such records of the Registrant and such other documents, certificates and
records as we have deemed necessary or appropriate as a basis for the opinions
set forth herein.

       In our examination, we have assumed the legal capacity of all natural
persons, the genuineness of all signatures, the authenticity of all documents
submitted to us as originals, the conformity to original documents of all
documents submitted to us as certified, conformed or photostatic copies and the
authenticity of the originals of such copies. In making our examination of
documents executed or to be executed by parties other than the Registrant, we
have assumed that such parties had or will have the power, corporate or other,
to enter into and perform all obligations thereunder and have also assumed the
due authorization by all requisite action, corporate or other, and execution and
delivery by such parties of such documents and the validity and binding effect
thereof on such parties. As to any facts material to the opinions expressed
herein which we have not independently established or verified, we have relied
upon statements and representations of officers and other representatives of the
Registrant and others. We have assumed that each option or award agreement
setting forth the terms of each grant of options or other awards under the Plan
is consistent with the Plan and will be duly authorized and validly executed and




delivered by the parties thereto, and that the consideration received by the
Registrant for the Common Stock delivered pursuant to the Plan will be in an
amount at least equal to the par value of such Common Stock. We have also
assumed that, upon issuance, the stock certificates evidencing such Common Stock
will be manually signed by an authorized officer of the transfer agent and
registrar for the Common Stock and registered by such transfer agent and
registrar and will conform to the specimen stock certificate examined by us.

       Members of our firm are admitted to the Bar of the state of Delaware, and
we do not express any opinion as to the laws of any jurisdiction other than the
corporate laws of the State of Delaware and we do not express any opinion as to
the effect of any other laws on the opinion stated herein.

       Based upon and subject to the foregoing, we are of the opinion that the
shares of Common Stock issuable under the Plan have been duly authorized for
issuance by the Registrant and, when such shares of Common Stock are issued in
accordance with the terms of the Plan, such shares of Common Stock will be
validly issued, fully paid and nonassessable.

       We hereby consent to the filing of this opinion with the Commission as an
exhibit to the Registration Statement. In giving such consent, we do not thereby
admit that we are included in the category of persons whose consent is required
under Section 7 of the Securities Act or the rules and regulations of the
Commission promulgated thereunder.

       This opinion is furnished by us, as the Registrant's special counsel, in
connection with the filing of the Registration Statement with the Commission
and, except as provided in the immediately preceding paragraph, is not to be
used, circulated, quoted or otherwise referred to for any other purpose or
relied upon by any other person without our prior written permission.

                         Very truly yours,

                         /s/ Skadden, Arps, Slate, Meagher & Flom LLP