Micromechanical optical modulator having a reduced-mass composite membrane

• US 5,710,656 A
• Filed: 07/30/1996
• Issued: 01/20/1998
• Est. Priority Date: 07/30/1996
• Status: Expired due to Term

• Current Assignee: Avago Technologies General IP PTE Limited (Broadcom, Inc.)
• Original Assignee: Lucent Technologies, Inc. (Nokia Corporation)
• Inventors: Goossen, Keith Wayne
• Primary Examiner(s): Ben, Loha
• Application Number:
  US08/688,325
• Time in Patent Office: 539 Days
• Field of Search: 359/290, 359/291, 359/298, 359/292, 359/846
• US Class Current: 359/291
• CPC Class Codes: G02B 26/0825 the reflecting element being a flexible sheet or membrane, e.g. for varying the focus flexible mirrors for cosmetic use A45D42/24
• CPC Class Codes: G02B 26/0833 the reflecting element being a micromechanical device, e.g. a MEMS mirror, DMD G02B26/0825 takes precedence; micromechanical devices in general B81B

Abstract

A method and apparatus for a micromechanical modulator having a reduced-mass multilayer membrane is disclosed. The modulator includes a movable composite membrane suspended over a subtrate. The composite membrane consists of an overlayer and an underlayer. The overlayer is characterized by a thickness that is less than one-quarter of a wavelength of the optical signal being modulated. Under the action of a bias voltage, the movable membrane moves from a quiescent position to a second position closer to the substrate. The modulator is fabricated so that in either the quiescent position or the second position the gap between the composite membrane and the substrate is equal to d=mλ/4-n_s s when n_u is about equal to (n_s) ⁰.5, where m is an even integer, λ is the wavelength of the optical signal being modulated, n_s is the refractive index of the subtrate, n_u is the refractive index of the underlayer, n_o is the refractive index of the overlayer, and s is the thickness of the overlayer. More generally, the gap may be given by d=mλ/4-n_s s+(n_u -n_s⁰.5) (λ/10) sin (4πn_o s/λ).

Claims

1. A micromechanical optical modulator for modulating an optical signal, the signal characterized by a wavelength, comprising:

a substrate characterized by a first refractive index;

a movable layer comprising an overlayer and an underlayer, wherein

the underlayer is characterized by a thickness about equal to one-quarter of the wavelength of the optical signal as measured in the underlayer, and further characterized by a second refractive index; and

the overlayer is characterized by a thickness that is less than one-quarter of the wavelength of the optical signal as measured in the overlayer, and further characterized by a third refractive index that is about equal to the first refractive index; and

a support for positioning the movable layer in a spaced and superposed relation to the substrate defining a gap, wherein, of the overlayer and the underlayer, the underlayer is proximal to the substrate, and wherein,

the movable layer is movable between a first position wherein the gap is characterized by a first size and a second position wherein the gap is characterized by a second size, wherein, one of either the first size or the second size of the gap is given by d=mλ

/4-n_o s+(n_u -n_s⁰.5) (λ

/10) sin (4π

n_o s/λ

), where m is an even integer, λ

is the wavelength of the optical signal, n_s is the first refractive index, n_u is the second refractive index, n_o is the third refractive index, and s is the thickness of the overlayer.

2. The modulator of claim 1 further comprising a controllable voltage source for applying a voltage to the movable layer and the substrate.

3. The modulator of claim 1 wherein the movable layer is a membrane.

4. The modulator of claim 3 further comprising support arms attached at a first end to the membrane and supported at a second end by the support.

5. The modulator of claim 1 wherein the second size of the gap is given by d=mλ

/4-n_o s+(n_u -n_s⁰.5) (λ

/10) sin (4π

n_o s/λ

), where m is an even integer and the first size of the gap is in the range of about 1.2d to 1.3d.

6. The modulator of claim 5 wherein m=2.

7. The modulator of claim 1 wherein the underlayer is silicon nitride.

8. The modulator of claim 1 wherein the overlayer is polysilicon.

9. The modulator of claim 1 wherein the substrate is silicon.

10. A method for forming a micromechanical optical modulator for modulating an optical signal characterized by a wavelength, the modulator including a movable two-layer membrane having a first position and a second position relative to a substrate, which substrate is characterized by a first refractive index, comprising the steps of:

(a) forming the movable two-layer membrane having an underlayer and an overlayer so that the thickness of the overlayer is less than one-quarter of the wavelength of the optical signal as measured in the overlayer, and wherein the overlayer is characterized by a third refractive index which is about equal to the first refractive index and the underlayer is characterized by a second refractive index; and

(b) forming a support on the substrate, the support for positioning the membrane in spaced and superposed relation to the substrate, wherein the support is formed so that in one of either the first or the second position, the space between the membrane and the substrate is given by d=mλ

/4-n_o s+(n_u -n_s⁰.5) (λ

/10) sin (4π

n_o s/λ

), where m is an even integer, λ

is the wavelength of the optical signal, n_s, is the first refractive index, n_u is the second refractive index, n_o is the third refractive index, and s is the thickness of the overlayer.

11. A micromechanical optical modulator for modulating an optical signal, the signal characterized by a wavelength, comprising:

a substrate characterized by a first refractive index;

a movable layer comprising an overlayer and an underlayer, wherein

the underlayer is characterized by a thickness about equal to one-quarter of the wavelength of the optical signal as measured in the underlayer, and further characterized by a second refractive index that is about equal to the square root of the first refractive index, and

the overlayer is characterized by a thickness that is less than one-quarter of the wavelength of the optical signal as measured in the overlayer, and further characterized by a third refractive index that is about equal to the first refractive index; and

a support for positioning the movable layer in a spaced and superposed relation to the substrate defining a gap, wherein, of the overlayer and the underlayer, the underlayer is proximal to the substrate, and wherein,

the movable layer is movable between a first position wherein the gap is characterized by a first size and a second position wherein the gap is characterized by a second size, wherein, one of either the first size or the second size of the gap is given by d=mλ

/4-n_o s, where m is an even integer, λ

is the wavelength of the optical signal, n_o is the third refractive index, and s is the thickness of the overlayer.

12. The modulator of claim 11 further comprising a controllable voltage source for applying a voltage to the movable layer and the substrate.

13. The modulator of claim 11 wherein the movable layer is a membrane.

14. The modulator of claim 13 further comprising support arms attached at a first end to the membrane and supported at a second end by the support.

15. The modulator of claim 11 wherein the second size of the gap is given by d=mλ

/4-n_o s, where m is an even integer and the first size of the gap is in the range of about 1.2d to 1.3d.

16. The modulator of claim 15 wherein m=2.

17. The modulator of claim 11 wherein the underlayer is silicon nitride.

18. The modulator of claim 11 wherein the overlayer is polysilicon.

19. The modulator of claim 11 wherein the substrate is silicon.

20. A micromechanical modulator for modulating an optical signal characterized by a wavelength, comprising:

a two-layer membrane consisting of an underlayer and an overlayer, the two-layer membrane supported above a substrate and forming a gap therebetween, wherein,

the membrane is capable of moving between a first position wherein the gap has a first size and a second position wherein the gap has a second size, wherein the underlayer has a thickness about equal to one-quarter of the wavelength of the optical signal, as measured in the underlayer, and the overlayer has a thickness less than one-quarter of a wavelength of the optical signal, as measured in the overlayer, and one of either the first or the second size of the gap is given by d=mλ

/4-n_s s+(n_u -n_s⁰.5) (λ

/10) sin (4π

n_o s/λ

), wherein m is an even integer, λ

is the wavelength of the optical signal, n_s is a refractive index of the subtrate, n_u is a refractive index of the underlayer, n_o is a refractive index of the overlayer, and s is the thickness of the overlayer.

21. A method for forming a micromechanical optical modulator for modulating an optical signal characterized by a wavelength, the modulator including a movable two-layer membrane having a first position and a second position relative to a substrate, which substrate is characterized by a first refractive index, comprising the steps of:

(a) forming the movable two-layer membrane having an underlayer and an overlayer so that the thickness of the overlayer is less than one-quarter of the wavelength of the optical signal as measured in the overlayer, and wherein the overlayer is characterized by a third refractive index that is about equal to the first refractive index and the underlayer is characterized by a second refractive index that is about equal to the square root of the third refractive index; and

(b) forming a support on the substrate, the support for positioning the membrane in spaced and superposed relation to the substrate, wherein the support is formed so that in one of either the first or the second position, the space between the membrane and the substrate is given by d=mλ

/4-n_o s, where m is an even integer, λ

is the wavelength of the optical signal, n_o is the third refractive index, and s is the thickness of the overlayer.

22. An optical communications network comprising:

a head end terminal;

a transmitter for launching optical signals, intended for at least one of a plurality of individual subscribers, into an optical medium;

at least one receiver for receiving optical signals from at least one of the plurality of individual subscribers; and

a plurality of optical network units, each of said optical network units including

a receiver for receiving optical signal portions intended for one of the plurality of individual subscribers, and

a modulator for modulating a portion of the received optical signal portions with information for transmission to the head end terminal, the modulator having a two-layer membrane consisting of an underlayer and an overlayer, the two-layer membrane supported above a substrate and forming a gap therebetween, the membrane capable of moving between a first position wherein the gap has a first size and a second position wherein the gap has a second size, wherein the underlayer has a thickness about equal to one quarter wavelength of the optical signal portion received by the modulator, as measured in the underlayer, and the overlayer has a thickness less than one quarter wavelength of the optical signal portion, as measured in the overlayer, and one of either the first or the second size of the gap is given by d=mλ

/4-n_s s+(n_u -n_s⁰.5) (λ

/10) sin (4π

n_o s/λ

), wherein m is an even integer, λ

is the wavelength of the optical signal portion received by the modulator, n_s is a refractive index of the subtrate, n_u is a refractive index of the underlayer, n_o is a refractive index of the overlayer, and s is the thickness of the overlayer.

23. A method of operating an optical communications network, comprising the steps of:

(A) launching, at a head end terminal, an optical signal intended for at least one of a plurality of individual subscribers into an optical medium;

(B) receiving portions of optical signals launched into the optical medium at an optical network unit; and

(C) modulating a portion of the received optical signal portions, for transmission to the head end terminal, with a modulator, the modulator including

a two-layer membrane consisting of an underlayer and an overlayer, the two-layer membrane supported above a substrate and forming a gap therebetween, the membrane capable of moving between a first position wherein the gap has a first size and a second position wherein the gap has a second size, wherein the underlayer has a thickness about equal to one quarter wavelength of the optical signal portion received by the modulator, as measured in the underlayer, and the overlayer has a thickness less than one quarter wavelength of the optical signal portion, as measured in the overlayer, and one of either the first or the second size of the gap is given by d=mλ

/4-n_s s+(n_u -n_s⁰.5) (λ

/10) sin (4π

n_o s/λ

), wherein m is an even integer, λ

is the wavelength of the optical signal portion received by the modulator, n_s is a refractive index of the subtrate, n_u is a refractive index of the underlayer, n_o is a refractive index of the overlayer, and s is the thickness of the overlayer.

24. The method of claim 23, further including the step of:

receiving, at the head end terminal, the modulated optical signal.