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                                              Exhibit 5.1

                                     September 24, 1999


Sycamore Networks, Inc.
10 Elizabeth Drive
Chelmsford, MA 01824

     Re:  Registration Statement on Form S-1
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Ladies and Gentlemen:

     This opinion is furnished to you in connection with a Registration
Statement on Form S-1 (File No. 333-84635) (the "Registration Statement") filed
with the Securities and Exchange Commission (the "Commission") under the
Securities Act of 1933, as amended (the "Securities Act"), for the registration
of an aggregate of 7,475,000 shares of Common Stock, $0.001 par value per share
(the "Shares"), of Sycamore Networks, Inc., a Delaware corporation (the
"Company").

     The Shares are to be sold by the Company pursuant to an underwriting
agreement (the "Underwriting Agreement") to be entered into by and among the
Company and Morgan Stanley & Co. Incorporated, Lehman Brothers Inc., J.P. Morgan
Securities Inc. and Dain Rauscher Wessels, as representatives of the several
underwriters named in the Underwriting Agreement, the form of which has been
filed as Exhibit 1.1 to the Registration Statement.

     We are acting as counsel for the Company in connection with the sale by the
Company of the Shares.  We have examined signed copies of the Registration
Statement as filed with the Commission.  We have also examined and relied upon
the Underwriting Agreement, minutes of meetings of the stockholders and the
Board of Directors of the Company as provided to us by the Company, stock record
books of the Company as provided to us by the Company, the Certificate of
Incorporation and By-Laws of the Company, each as restated and/or amended to
date, and such other documents as we have deemed necessary for purposes of
rendering the opinions hereinafter set forth.

     In our examination of the foregoing documents, we have assumed 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 copies, the authenticity of the originals of such latter documents and the
legal competence of all signatories to such documents.

     We assume that the appropriate action will be taken, prior to the offer and
sale of the Shares in accordance with the Underwriting Agreement, to register
and qualify the Shares for sale under all applicable state securities or "blue
sky" laws.

     We express no opinion herein as to the laws of any state or jurisdiction
other than the state laws of the Commonwealth of Massachusetts, the Delaware
General Corporation Law statute and the federal laws of the United States of
America.  To the extent that any other laws govern the matters as to which we
are opining herein, we have assumed that such laws are identical to the state
laws of the Commonwealth of Massachusetts, and we are expressing no opinion
herein as to whether such assumption is reasonable or correct.
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Sycamore Networks, Inc.
September 24, 1999
Page 2




     Based upon and subject to the foregoing, we are of the opinion that the
Shares have been duly authorized for issuance and, when such Shares are issued
and paid for in accordance with the terms and conditions of the Underwriting
Agreement, such Shares will be validly issued, fully paid and nonassessable.

     It is understood that this opinion is to be used only in connection with
the offer and sale of the Shares while the Registration Statement is in effect.

     Please note that we are opining only as to the matters expressly set forth
herein, and no opinion should be inferred as to any other matters.  This opinion
is based upon currently existing statutes, rules, regulations and judicial
decisions, and we disclaim any obligation to advise you of any change in any of
these sources of law or subsequent legal or factual developments which might
affect any matters or opinions set forth herein.

     We hereby consent to the filing of this opinion with the Commission as an
exhibit to the Registration Statement in accordance with the requirements of
Item 601(b)(5) of Regulation S-K under the Securities Act and to the use of our
name therein and in the related Prospectus under the caption "Legal Matters."
In giving such consent, we do not hereby admit that we are in the category of
persons whose consent is required under Section 7 of the Securities Act or the
rules and regulations of the Commission.

                                    Very truly yours,

                                    /s/ Hale and Dorr LLP

                                    HALE AND DORR LLP