EXHIBIT 5.1

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


                                                                   April 7, 2003

Sycamore Networks, Inc.
220 Mill Road
Chelmsford, Massachusetts 01824

Re: Registration Statement on Form S-8

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 1,228,092 shares of the Registrant's common stock, par
value $.001 per share ("Common Stock"), issuable pursuant to the Registrant's
1999 Employee Stock Purchase Plan (the "Plan"). The Plan provides for the
issuance of up to 2,250,000 shares of Common Stock to the eligible employees
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 on Form S-8, 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 amended to date and
presently in effect; (iv) the Amended and Restated By-Laws of the Registrant,
as amended to date and presently in effect; and (v) 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 latter documents. 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 subscription agreement indicating each employee's election to participate
in the Plan and authorizing payroll deductions 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 Commonwealth of
Massachusetts, 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, delivered and paid for 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