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                                                                     EXHIBIT 5.1



                                February 5, 2001



Board of Directors
CIENA Corporation
1201 Winterson Road
Linthicum, MD 21090


Ladies and Gentlemen:

                We are acting as counsel to CIENA Corporation, a Delaware
corporation (the "COMPANY"), in connection with its registration statement on
Form S-3, as amended (the "REGISTRATION STATEMENT"), filed with the Securities
and Exchange Commission, relating to the proposed public offering of shares of
the Company's common stock, par value $.01 per share. This opinion letter is
furnished to you at your request to enable you to fulfill the requirements of
Item 601(b)(5) of Regulation S-K, 17 C.F.R. Section 229.601(b)(5), in connection
with the Registration Statement.

                For purposes of this opinion letter, we have examined copies of
the following documents:

                1.      An executed copy of the Registration Statement.

                2.      The Third Restated Certificate of Incorporation of the
                        Company, as certified by the Secretary of the State of
                        the State of Delaware on January 29, 2001 and by the
                        Secretary of the Company on the date hereof as then
                        being complete, accurate, and in effect.


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                3.      The Amended and Restated Bylaws of the Company, as
                        certified by the Secretary of the Company on the date
                        hereof as then being complete, accurate, and in effect.

                4.      Resolutions of the Board of Directors of the Company
                        adopted by the written consent on January 14, 2001 and
                        at a board meeting on February 4, 2001, as certified
                        by the Secretary of the Company on the date hereof as
                        being complete, accurate and in effect, relating to the
                        filing by the Company of the Registration Statement and
                        related matters, and resolutions of the 2001 Pricing
                        Committee of the Board of Directors (the "PRICING
                        COMMITTEE") of the Company adopted on February 1, 2001
                        and February 5, 2001, as certified by the Secretary of
                        the Company on the date hereof as being complete,
                        accurate and in effect, authorizing the issuance and
                        sale of up to 11,500,000 shares of common stock (the
                        "SHARES") and arrangements in connection therewith.

                5.      The proposed form of Underwriting Agreement among the
                        Company and Goldman, Sachs & Co., Morgan Stanley Dean
                        Witter, Banc of America Securities LLC and Robertson
                        Stephens, filed as Exhibit 1.01 to the Registration
                        Statement (the "UNDERWRITING AGREEMENT").

                In our examination of the aforesaid documents, we have assumed
the genuineness of all signatures, the legal capacity of all natural persons,
the accuracy and completeness of all documents submitted to us, the authenticity
of all original documents, and the conformity to authentic original documents of
all documents submitted to us as copies (including telecopies). This opinion
letter is given, and all statements herein are made, in the context of the
foregoing.

                This opinion letter is based as to matters of law solely on the
Delaware General Corporation Law, as amended. We express no opinion herein as to
any other laws, statutes, ordinances, rules, or regulations. As used herein, the
term "Delaware General Corporation Law, as amended" includes the statutory
provisions contained therein, all applicable provisions of the Delaware
Constitution and reported judicial decisions interpreting these laws.

                Based upon, subject to and limited by the foregoing, we are of
the opinion that following: (i) execution and delivery by the Company of the
Underwriting Agreement, (ii) effectiveness of the Registration Statement, (iii)
issuance of the Shares pursuant to the terms of the Underwriting Agreement, and
(iv) receipt by the Company of the consideration for the Shares as specified in
the resolutions adopted by the Pricing Committee, the Shares to be issued will
be validly issued, fully paid and nonassessable.

                This opinion letter has been prepared for your use in connection
with the Prospectus Supplement and speaks as of the date hereof. We assume no
obligation to advise you of any changes in the foregoing subsequent to the
delivery of this opinion letter.

                We hereby consent to the filing of this opinion letter as
Exhibit 5.1 to the Registration Statement and to the reference to this firm
under the caption "Legal Matters" in the prospectus relating to the Shares
constituting a part of the Registration Statement. In giving this consent, we do
not  thereby admit that we are an "expert" within the meaning of the Securities
Act of 1933, as amended.


                                                Very truly yours,


                                                HOGAN & HARTSON L.L.P.