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                                                                     EXHIBIT 5.1

                             HOGAN & HARTSON L.L.P.
                            111 South Calvert Street
                            Baltimore, Maryland 21202


                                  June 11, 1999

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-4 (the
"REGISTRATION STATEMENT") filed with the Securities and Exchange Commission
relating to the registration of 16,000,000 shares of the Company's common stock,
par value $.01 per share (the "SHARES"), pursuant to the terms of the Agreement
and Plan of Merger between the Company and Omnia Communications, Inc. ("OMNIA"),
which provides for the merger of Omnia with and into the Company, with the
Company surviving the merger (the "MERGER"). 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 April 19, 1999 and by the Secretary of
                 the Company on the date hereof as being complete, accurate and
                 in effect.

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


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          4.     Resolutions of the Board of Directors of the Company adopted
                 at a meeting held on March 14,1999, 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
                 the Shares.

          5.     Executed copies of the Agreement and Plan of Merger, dated
                 March 15, 1999, by and among the Company and Omnia
                 (the "Merger Agreement"), relating to, among other things,
                 the issuance of the Shares.

          In our examination of the aforesaid documents, we have assumed the
genuineness of all signatures, the legal capacity of all natural persons, the
authenticity, 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 Delaware
corporate law. We express no opinion herein as to any other laws, statutes,
ordinances, rules, or regulations.

          Based upon, subject to and limited by the foregoing, we are of the
opinion that following (i) effectiveness of the Registration Statement, (ii)
approval of the Merger by the Omnia stockholders, and (iii) the effectiveness of
the Merger pursuant to the General Corporation Law of the State of Delaware, the
Shares will be validly issued, fully paid and nonassessable.

          This opinion letter has been prepared solely for your use in
connection with the Registration Statement 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 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.