<PAGE>
 
                                                  ------------------------------
                                                          OMB APPROVAL
                                                  ------------------------------
                                                   OMB Number:   3235-0145
                                                   Expires:  October 31, 1994
                                                   Estimated average burden
                                                   hours per response.... 14.90
                                                  ------------------------------

                                 UNITED STATES
                      SECURITIES AND EXCHANGE COMMISSION
                            Washington, D.C. 20549

                                 SCHEDULE 13G


                   Under the Securities Exchange Act of 1934
                             (Amendment No.  1  )*
                                           -----


                            HARMONIC LIGHTWAVES INC
- --------------------------------------------------------------------------------
                               (Name of Issuer)


                                    COMMON
- --------------------------------------------------------------------------------
                        (Title of Class of Securities)


                                   413160102
- --------------------------------------------------------------------------------
                                (CUSIP Number)


Check the following box if a fee is being paid with this statement [_]. (A 
fee is not required only if the filing person: (1) has a previous statement 
on file reporting beneficial ownership of more than five percent of the 
class of securities described in Item 1; and (2) has filed no amendment 
subsequent thereto reporting beneficial ownership of five percent or less 
of such class.) (See Rule 13d-7).

*The remainder of this cover page shall be filled out for a reporting person's 
initial filing on this form with respect to the subject class of securities, and
for any subsequent amendment containing information which would alter the
disclosures provided in a cover page.

The information required in the remainder of this cover page shall not be deemed
to be "filed" for the purpose of Section 18 of the Securities Exchange Act of 
1934 ("Act") or otherwise subject to the liabilities of that section of the Act 
but shall be subject to all other provisions of the Act (however, see the 
Notes).


                               Page 1 of 6 pages

<PAGE>


- -----------------------                                  ---------------------
  CUSIP No. 413160102                   13G                Page 2 of 6 Pages
- -----------------------                                  ---------------------
 
- ------------------------------------------------------------------------------
      NAME OF REPORTING PERSON
 1    S.S. or I.R.S. IDENTIFICATION NO. OF ABOVE PERSON
      ESSEX INVESTMENT MANAGEMENT COMPANY
      04-2601915

- ------------------------------------------------------------------------------
      CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*
 2                                                              (a) [_]
                                                                (b) [_]
                                                 
- ------------------------------------------------------------------------------
      SEC USE ONLY
 3
 

- ------------------------------------------------------------------------------
      CITIZENSHIP OR PLACE OF ORGANIZATION
 4    
      MASSACHUSETTS

- ------------------------------------------------------------------------------
                          SOLE VOTING POWER
                     5   
     NUMBER OF            1,026,380
 
      SHARES       -----------------------------------------------------------
                          SHARED VOTING POWER
   BENEFICIALLY      6
                          0
     OWNED BY
                   -----------------------------------------------------------
       EACH               SOLE DISPOSITIVE POWER
                     7    
    REPORTING             1,347,450
 
      PERSON       -----------------------------------------------------------
                          SHARED DISPOSITIVE POWER
       WITH          8
                          0
- ------------------------------------------------------------------------------
      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
 9  
      1,347,450

- ------------------------------------------------------------------------------
      CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES*
10                  
 
 
- ------------------------------------------------------------------------------
      PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9
11    
      13.31%

- ------------------------------------------------------------------------------
      TYPE OF REPORTING PERSON*
12
      IA

- ------------------------------------------------------------------------------

                     *SEE INSTRUCTIONS BEFORE FILLING OUT!


                               Page 2 of 6 pages


<PAGE>
 
                         INSTRUCTIONS FOR SCHEDULE 13G

Instructions for Cover Page

(1) Names and Social Security Numbers of Reporting Persons-Furnish the full
    legal name of each person for whom the report is filed-i.e., each person
    required to sign the schedule itself-including each member of a group. Do
    not include the name of a person required to be identified in the report but
    who is not a reporting person. Reporting persons are also requested to
    furnish their Social Security or I.R.S. identification numbers, although
    disclosure of such numbers is voluntary, not mandatory (see "SPECIAL
    INSTRUCTIONS FOR COMPLYING WITH SCHEDULE 13G," below).

(2) If any of the shares beneficially owned by a reporting person are held as a
    member of a group and such membership is expressly affirmed, please check
    row 2(a). If the membership in a group is disclaimed or the reporting person
    describes a relationship with other persons but does not affirm the
    existence of a group, please check row 2(b) [unless a joint filing pursuant
    to Rule 13d-1(e)(1) in which case it may not be necessary to check row
    2(b)].

(3) The third row is for SEC internal use; please leave blank.

(4) Citizenship or Place of Organization-Furnish citizenship if the named
    reporting person is a natural person. Otherwise, furnish place of
    organization.

(5)-(9), (11) Aggregate Amount Beneficially Owned by Each Reporting Person, 
    Etc.-Rows (5) through (8) inclusive, and (11) are to be completed in
    accordance with the provisions of Item 4 of Schedule 13G. All percentages
    are to be rounded off to the nearest tenth (one place after decimal point).

(10) Check if the aggregate amount reported as beneficially owned in row (9) 
     does not include shares as to which beneficial ownership is disclaimed
     pursuant to Rule 13d-4 [17 CFR 240.13d-4] under the Securities Exchange Act
     of 1934.

(12) Types of Reporting Person-Please classify each "reporting person" according
     to the following breakdown (see Item 3 of Schedule 13G) and place the
     appropriate symbol on the form:

                Category                                  Symbol
           Broker-Dealer                                    BD
           Bank                                             BK
           Insurance Company                                IC
           Investment Company                               IV
           Investment Adviser                               IA
           Employee Benefit Plan, Pension Fund,
            or Endowment Fund                               EP
           Parent Holding Company                           HC
           Corporation                                      CO
           Partnership                                      PN
           Individual                                       IN
           Other                                            OO

Notes:
    Attach as many copies of the second part of the cover page as are needed, 
one reporting person per page.
    Filing persons may, in order to avoid unnecessary duplication, answer items
on the schedules (Schedule 13D, 13G or 14D-1) by appropriate cross references to
an Item or items on the cover page(s). This approach may only be used where the
cover page item or items provide all the disclosure required by the schedule
item. Moreover, such a use of a cover page item will result in the item becoming
a part of the schedule and accordingly being considered as "filed" for purposes
of Section 18 of the Securities Exchange Act or otherwise subject to the
liabilities of that section of the Act.
    Reporting persons may comply with their cover page filing requirements by 
filing either completed copies of the blank forms available from the Commission,
printed or typed facsimiles, or computer printed facsimiles, provided the 
documents filed have identical formats to the forms prescribed in the 
Commission's regulations and meet existing Securities Exchange Act rules as to 
such matters as clarity and size (Securities Exchange Act Rule 12b-12).

             SPECIAL INSTRUCTIONS FOR COMPLYING WITH SCHEDULE 13G

    Under Sections 13(d), 13(g), and 23 of the Securities Exchange Act of 1934 
and the rules and regulations thereunder, the Commission is authorized to 
solicit the information required to be supplied by this schedule by certain 
security holders of certain issuers.
    Disclosure of the information specified in this schedule is mandatory,
except for Social Security or I.R.S. identification numbers, disclosure of which
is voluntary. The information will be used for the primary purpose of
determining and disclosing the holdings of certain beneficial owners of certain
equity securities. This statement will be made a matter of public record.
Therefore, any information given will be available for inspection by any member
of the public.

                               Page 3 of 6 pages


<PAGE>
 
        
     Because of the public nature of the information, the Commission can utilize
it for a variety of purposes, including referral to other governmental 
authorities of securities self-regulatory organizations for investigatory 
purposes or in connection with litigation involving the Federal securities laws 
or other civil, criminal or regulatory statutes or provisions, Social Security 
or I.R.S. identification numbers, if furnished, will assist the Commission in 
identifying security holders and, therefore, in promptly processing statements 
of beneficial ownership of securities.

     Failure to disclose the information requested by this schedule, except for 
Social Security or I.R.S. Identification numbers, may result in civil or 
criminal action against the persons involved for violation of the Federal 
securities laws and rules promulgated thereunder.

                             GENERAL INSTRUCTIONS

A.   Statements containing the information required by this schedule shall be 
filed not later than February 14 following the calendar year covered by the 
statement or within the time specified in Rule 13d-1(b)(2), if applicable.

B.   Information contained in a form which is required to be filed by rules
under section 13(f)(15 U.S.C. 78m(f)) for the same calendar year as that covered
by a statement on this schedule may be incorporated by reference in response to
any of the items of this schedule. If such information is incorporated by
reference in this schedule, copies of the relevant pages of such form shall be
filed as an exhibit to this schedule.

C. The item numbers and captions of the items shall be included by the text of
the items is to be omitted.  The answers to the items shall be so prepared as to
indicate clearly the coverage of the items without referring to the text of the 
items.  Answer every item.  If an item is inapplicable or the answer is in the 
negative, so state.

Item 1.

     (a) Name of Issuer HARMONIC LIGHTWAVES INC
     (b) Address of Issuer's Principal Executive Offices 549 BALTIC WAY 
         SUNNYVALE, CA 94089-1140
 
Item 2.

     (a) Name of Person Filing ESSEX INVESTMENT MANAGEMENT COMPANY
     (b) Address of Principal Business Office or, if none, Residence 125 HIGH 
         STREET, BOSTON, MA 02110
     (c) Citizenship U.S.A.
     (d) Title of Class of Securities COMMON
     (e) CUSIP Number 413160102

Item 3.  If this statement is filed pursuant to Rule 13d-1(b), or 13d-2(b),
         check whether the person filing is a:

     (a) [_]  Broker or Dealer registered under Section 15 or the Act
     (b) [_]  Bank as defined in section 3(a)(6) of the Act
     (c) [_]  Insurance Company as defined in section 3(a)(19) of the act
     (d) [_]  Investment Company registered under section 8 of the Investment
              Company Act
     (e) [X]  Investment Adviser registered under section 203 of the Investment
              Advisers Act of 1940
     (f) [_]  Employee Benefit Plan, Pension Fund which is subject to the
              provisions of the Employee Retirement Income Security Act of 1974
              or Endowment Fund; see (S)240.13d-1(b)(1)(ii)(F)
     (g) [_]  Parent Holding Company, in accordance with (S)240.13d-1(b)(ii)(G)
              (Note: See Item 7)
     (h) [_]  Group, in accordance with (S)240.13d-1(b)(1)(ii)(H)

Item 4. Ownership

     If the percent of the class owned, as of December 31 of the year covered by
the statement, or as of the last day of any month described in Rule 13d-1(b)(2),
if applicable, exceeds five percent, provide the following information as of 
that date and identity those shares which there is a right to acquire.

     (a) Amount Beneficially Owned 1,347,450
     (b) Percent of Class 13.31%



                               Page 4 of 6 pages

<PAGE>
 
(c)  Number of shares as to which such person has:
     (i) sole power to vote or to direct the vote                  1,026,380
    (ii) shared power to vote or to direct the vote                0
   (iii) sole power to dispose or to direct the disposition of     1,347,450
    (iv) shared power to dispose or to direct the disposition of   0

Instruction: For computations regarding securities which represent a right to 
acquire an underlying security see Rule 13d-3(d)(1).

Item 5. Ownership of Five Percent or Less of a Class
   If this statement is being filed to report the fact that as of the date 
hereof the reporting person has ceased to be the beneficial owner of more than 
five percent of the class of securities, check the following [_].

Instruction: Dissolution of a group requires a response to this item.

Item 6. Ownership of More than Five Percent on Behalf of Another Person
   If any other person is known to have the right to receive or the power to 
direct the receipt of dividends from, or the proceeds from the sale of, such 
securities, a statement to that effect should be included in response to 
this item and, if such interest relates to more than five percent of the class,
such person should be identified. A listing of the shareholders of an investment
company registered under the Investment Company Act of 1940 or the beneficiaries
of employee benefit plan, pension fund or endowment fund is not required.

Item 7. Identification and Classification of the Subsidiary Which Acquired the 
        Security Being Reported on By the Parent Holding Company
   If a parent holding company has filed this schedule, pursuant to Rule 
13d-1(b)(ii)(G), so indicate under Item 3(g) and attach an exhibit stating the 
identity and the Item 3 classification of the relevant subsidiary. If a parent 
holding company has filed this schedule pursuant to Rule 13d-1(c), attach an 
exhibit stating the identification of the relevant subsidiary.

Item 8. Identification and Classification of Members of the Group
   If a group has filed this schedule pursuant to Rule 13d-1(b)(ii)(H), so 
indicate under Item 3(h) and attach an exhibit stating the identity and Item 3 
classification of each member of the group. If a group has filed this schedule 
pursuant to Rule 13d-1(c), attach an exhibit stating the identity of each member
of the group.

Item 9. Notice of Dissolution of Group
   Notice of dissolution of a group may be furnished as an exhibit stating the 
date of the dissolution and that all further filings with respect to 
transactions in the security reported on will be filed, if required, by members 
of the group, in their individual capacity. See Item 5.

Item 10. Certification
   Following certification shall be included if the statement is filed pursuant 
to Rule 13d-1(b):
   By signing below I certify that, to the best of my knowledge and belief, the
securities referred to above were acquired in the ordinary course of business 
and were not acquired for the purpose of and do not have the effect of changing 
or influencing the control of the issuer of such securities and were not 
acquired in connection with or as a participant in any transaction having such 
purposes or effect.


                                   SIGNATURE

  After reasonable inquiry and to the best of my knowledge and belief, I certify
that the information set forth in this statement is true, complete and correct.

                                                DECEMBER 2, 1996
                                        ----------------------------------------
                                                         Date


                                        ----------------------------------------
                                                       Signature

                                         CHARLES H. RITT III    VICE PRESIDENT
                                        ----------------------------------------
                                                       Name/Title


                               Page 5 of 6 pages


<PAGE>
 
     The original statement shall be signed by each person on whose behalf the 
statement is filed or his authorized representative. If the statement is signed 
on behalf of a person by his authorized representative other than an executive 
officer or general partner of the filing person, evidence of the 
representative's authority to sign on behalf of such person shall be filed with 
the statement, provided, however, that a power of attorney for this purpose 
which is already on file with the Commission may be incorporated by reference. 
The name and any title of each person who signs the statement shall be typed or 
printed beneath his signature.

Note: Six copies of this statement, including all exhibits, should be filed with
the Commission.

     Attention: Intentional misstatements or omissions of fact constitute 
Federal criminal violations (See 18 U.S.C. 1001)






                                  Page 6 of 6