The present invention relates to photopolymer materials sensitive to infrared, near infrared, red and green light radiation for initiating polymerization and to applications of such photopolymer, like holographic polymer dispersed liquid crystal (H-PDLC) or reversible dye doped photopolymer (RDDP) materials, for making optical devices. The invention relates to holographic polymer dispersed liquid crystal and reversible dye materials having improved electrical and optical switching properties.
Infrared (IR) diode sources are largely used in integrated photonic circuits and the list of their applications grows very rapidly. The management of the radiation of these sources requires the fabrication of optical elements such as lenses, interconnects, modulators, etc. Holographic diffractive elements are emerging as very promising for these applications. These include wavelength selective holographic interconnects (1 to N, or M to N), couplers, lenses, mirrors. Today, a new class of Holographic Photopolymer Dispersed Liquid Crystal Materials (H-PDLCs) is considered to be one of the most viable technologies for the development of reflective color displays, switchable holographic optical elements (such as Bragg gratings for Wavelength Division Multiplexing (WDM) devices), switchable-focus lenses, etc. See for example Crawford et al., xe2x80x9cReflective color LCDs based on H-PDLC and PSCT technologiesxe2x80x9d, J Soc. Information Display, 1996, 5(1); Domash et al., xe2x80x9cElectronically switchable waveguide Bragg gratings for WDM routingxe2x80x9d, 1997 Digest of the IEEE. /LEOS Summer Topical Meetings: Vertical-Cavity Lasers/Technologies for a Global Information Infrastructure/WDM Components Technology; and Domash et al., xe2x80x9cSwitchable-focus lenses in holographic polymer dispersed liquid crystalxe2x80x9d, Proceedings of the SPIExe2x80x94The International Society for Optical Engineering, vol. 2689, (Diffractive and Holographic Optics Technology III, San Jose, Calif., USA, 1-2 Feb. 1996.) SPIE-Int. Soc. Opt. Eng, 1996. p. 188-94.
Commercially available holographic materials are generally sensitive in the UV/visible part of the spectrum only. Thus, the actual fabrication of above-mentioned elements requires an initial recording step, where a UV-Visible laser source is used, and then, a further adaptation or adjustment of the obtained element for utilization with near IR wavelengths (e.g. 800-850 nm or 1300-1500 nm), which are used in local or long distance communication systems. Due to strong astigmatism and divergence of the used diode lasers, this work is difficult and has poor efficiency. It would therefore be highly desirable to provide in situ recording of holographic diffractive elements with lasers, which are already integrated in the given photonic circuit, thus providing self-alignment of the photonic circuits. Thus, there is a need to extend the sensitivity of these materials up to communication wavelengths. Namely, for in situ holographic recording of optical components with diode lasers operating in the 800-855 nm, among which there are Vertical Cavity Surface Emitting Lasers (VCSEL), it is important to have H-PDLC material with suitable holographic characteristics, which include high sensitivity and diffraction efficiency of the recording, low scattering and noise level, and low switching voltage.
Up to now, the sensitivity of the existing H-PDLC materials has been extended up to 790 nm (Natarajan et al., Report on Photonics West 98, San Jose, January 1998). Photopolymerizable materials have been recently proposed with sensitivity in the 800-850 nm area, for example in co-pending application Ser. No. 09/503,207, filed on Feb. 14, 2000. See also EP 0 223 587; EP 0 387 087; EP 0 389 067; U.S. Pat. No. 4,343,891; Chatterjee et al., J. Am. Chem. Soc., 1990, 112, 6329; Schuster et l, Photochem. Photobiol. A: Chem., 1992, 65, 191; Chatterjee et al., J. Am. Chem. Soc., 1988, 110, 2326; Cooper et al., J. Am. Chem. Soc., 1963, 85, 1590; and Noiret et al., Pure and Applied Optics, 1994, 3(1), 55-71. Imaging applications with low resolution (such as printing plates) were also successfully explored. Some of the materials were the subject of the study for holographic gratings recording and only very low level of performance was achieved (7% of diffraction efficiency at sensitivity of about 300-500 mJ/cm2), as reported in the mentioned Noiret et al. article supra. This low level of performance makes such polymers impractical for commercial application. All the above materials still suffer from limitations such as spatial resolution, diffraction efficiency etc.
In accordance with the present invention, there is now provided a photopolymerizable formulation sensitive to light in a green to infrared region of the optical spectrum comprising:
a) a photopolymerizable monomer, oligomer or mixtures thereof;
b) a photoinitiator sensitive to light in the green to infrared region;
c) an additive for increasing the refractive index and decreasing the viscosity of the formulation; and
d) an optional filler having optical properties selected to contrast with optical properties of a polymer resulting from photopolymerization of the monomer, oligomer or mixtures thereof.
Preferably, the formulation comprises filler, which is a liquid crystal or a reversible dye, or combinations thereof, or any other filler materials such as mesogens having polar or functional groups. The liquid crystal preferably has a polar group attached to ends of the molecule chain elements, whereby droplets of LC matter are more efficiently formed in the polymer dispersed in the liquid crystal material. Larger droplets allow for more efficient optical state switching.
In a second aspect of the invention, there is provided a process for making an optical device comprising the steps of:
preparing a photopolymerizable formulation sensitive to light from green to infrared region of the optical spectrum, the formulation comprising
a) a photopolymerizable monomer, oligomer or mixtures thereof;
b) a photoinitiator sensitive to light in the region
c) an additive for increasing the refractive index and decreasing the viscosity of the formulation; and
d) an optional filler having optical properties selected to contrast with optical properties of a polymer resulting from photopolymerization of the monomer, oligomer or mixtures thereof;
applying a layer of the formulation on an optical element;
first exposing the optical element comprising the formulation to a light source emitting light in the region, to polymerize the formulation and produce a recording pattern on the optical element; and
optionally second exposing the optical element to the light to polymerize any remaining portion of the formulation not polymerized during first exposing, thereby producing said optical device.
Finally, in a third aspect of the present invention, there is provided a process of making an optical connection between at least two waveguides or light guides of an optical element by photopolymerization, the process comprising the steps of:
preparing a photopolymerizable formulation sensitive to light from green to infrared region of the optical spectrum, the formulation comprising:
a) a photopolymerizable monomer, oligomer or mixtures thereof;
b) a photoinitiator sensitive to light in the green to infrared region;
c) an additive to increase the refractive index and decrease the viscosity of the formulation; and
d) an optional filler;
applying a sufficient amount of the formulation between the waveguides or light guides of the optical element to be connected; and
exposing the optical element to a light source emitting light in the region to polymerize the formulation, thereby making an optical connection between the at least two waveguides or light guides.
Preferably, the light is transmitted during the exposing through at least one of the at least two waveguides or light guides. While a filler is also optional in the case of optically coupling waveguides or light guides, a filler having the desired optical properties is preferred to reduce the amount of polymer used and to impart desirable physical properties to the formulation.