Micromechanical optical modulator having a reduced-mass composite membrane

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=m.lambda./4-n.sub.o s, where m is an even integer, .lambda. is the wavelength of the optical signal, n.sub.o is the third refractive index, and s is the thickness of the overlayer.

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=m.lambda./4-n.sub.s s+(n.sub.u -n.sub.s.sup.0.5) (.lambda./10) sin (4.pi.n.sub.o s/.lambda.), wherein m is an even integer, .lambda. is the wavelength of the optical signal portion received by the modulator, n.sub.s is a refractive index of the subtrate, n.sub.u is a refractive index of the underlayer, n.sub.o is a refractive index of the overlayer, and s is the thickness of the overlayer.

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=m.lambda./4-n.sub.s s+(n.sub.u -n.sub.s.sup.0.5) (.lambda./10) sin (4.pi.n.sub.o s/.lambda.), wherein m is an even integer, .lambda. is the wavelength of the optical signal portion received by the modulator, n.sub.s is a refractive index of the subtrate, n.sub.u is a refractive index of the underlayer, n.sub.o is a refractive index of the overlayer, and s is the thickness of the overlayer.