Dielectric-Enhanced Metal Coatings for MEMS Tunable Filters

Abstract

The present invention concerns the use of hybrid metal-dielectric optical coatings as the end reflectors of laser cavities and/or in the mirror structures used in other optical resonators, such as Fabry-Perot tunable filters, along with the use of such Fabry-Perot tunable filters in wavelength swept sources such as lasers. Hybrid metal-dielectric optical coatings have reflectivity spectra that can be broader than pure dielectric coatings, offer optical reflectivities higher than metal, as high as pure dielectric coatings, eliminate mirror transmission that can cause parasitic light reflections, and use fewer layers and thus have lower mass and higher mechanical resonant frequency for movable mirror applications An important characteristic of these coatings concerns the non-reflected light. Pure dielectric coatings offer high reflectivity, while the non-reflected portion of the light is transmitted by the coating to the substrate, for example. When metal is added to the optical coating, the non-reflected portion of the light is absorbed by the metal and is not transmitted to the substrate or outside the cavity. Hybrid metal-dielectric coatings have broader and more uniform spectral reflection. Tunable lasers with performance enhanced by the hybrid metal-dielectric coatings can be used in optical coherence tomography and spectroscopic analysis applications.

What is claimed is: 
     
         1 . A method for fabricating a MEMS tunable filter, comprising:
 depositing a first dielectric mirror on a deflectable membrane device;   depositing a second dielectric mirror on a fixed spacer substrate;   bonding the deflectable membrane device to the fixed spacer mirror device to create an optical cavity defined by the first mirror and the second mirror;   wherein one or both of the first mirror and the second mirror comprises a metal layer under the dielectric mirror.   
     
     
         2 . A method as claimed in  claim 1 , wherein depositing the first dielectric mirror and depositing the second dielectric mirror each comprise depositing alternating layers of high refractive index material and low refractive index material. 
     
     
         3 . A method as claimed in  claim 2 , wherein the dielectric layers comprise 6 or more layers. 
     
     
         4 . A method as claimed in  claim 2 , wherein the dielectric layers comprise 10 or more layers. 
     
     
         5 . A method as claimed in  claim 2 , wherein the dielectric layers comprise tantalum pentoxide and silicon dioxide. 
     
     
         6 . A method as claimed in  claim 2 , wherein the metal layer comprises silver, gold, or aluminum. 
     
     
         7 . A laser, comprising:
 a laser cavity;   a gain medium in the laser cavity; and   two mirrors defining the laser cavity, wherein at least one of the mirrors comprises dielectric enhanced metal mirror comprising dielectric layers and a metal layer.   
     
     
         8 . A laser as claimed in  claim 7 , wherein the gain medium is a semiconductor optical amplifier. 
     
     
         9 . A laser as claimed in  claim 7 , further comprising a tunable filter, wherein the dielectric enhanced metal mirror is one of the two mirrors of the tunable filter. 
     
     
         10 . A laser as claimed in  claim 7 , further comprising a tunable filter defining one end of the laser cavity, wherein the dielectric enhanced metal mirror is a mirror of the Fabry-Perot tunable filter that is outermost one of the two filter mirrors, with respect to the laser cavity. 
     
     
         11 . A laser as claimed in  claim 7 , further comprising a tunable filter defining one end of the laser cavity, wherein the dielectric enhanced metal mirror is a mirror of the Fabry-Perot tunable filter that is supported on a deflectable membrane device that is bonded to a fixed spacer substrate. 
     
     
         12 . An optical coherence analysis system, comprising:
 an interferometer that combines swept optical signal returning from a reference path and a sample to generate an interference signal;   a detection system that detects the interference signal; and   a tunable laser that generates the swept optical signal, wherein the laser comprises a laser cavity, a gain medium in the laser cavity, and a tunable filter defining one end of the laser cavity, wherein the tunable filter comprises an optical cavity defined by two mirrors, wherein both of the mirrors comprise dielectric layers and an outermost one of the two mirrors, with respect to the laser cavity, further comprises a metal layer under the dielectric layers.   
     
     
         13 . An optical coherence analysis system, comprising:
 an interferometer that combines swept optical signal returning from a reference path and a sample to generate an interference signal;   a detection system that detects the interference signal; and   a tunable laser that generates the swept optical signal, wherein the laser comprises a laser cavity, a gain medium in the laser cavity, and a tunable filter, wherein the tunable filter comprises two mirrors, wherein one of the two mirrors comprises dielectric layers and a metal layer under the dielectric layers.   
     
     
         14 . A tunable laser spectroscopic analysis system, comprising:
 an optical probe that delivers a wavelength tunable optical signal to a sample;   a detection system that detects the tunable optical signal from the sample; and   a tunable laser that generates the tunable optical signal, wherein the laser comprises a laser cavity, a gain medium in the laser cavity, and a tunable filter, wherein the tunable filter comprises two mirrors, wherein one of the two mirrors comprises dielectric layers and a metal layer under the dielectric layers.