New optical isolator with spectral light conversion and laser radiation generation
Dr. habil. Rudi Danz
Feldfichten 46, D-14532 Kleinmachnow, Germany, e-mail: firstname.lastname@example.org
The concept relates to optical isolators for optical communication and computer engineering, for optical information processing and information transfer, for free-space optic communication as well as for optical laser and measurement engineering.
An optical isolator, also called an optical diode, represents an optical component that permits light transmission in one direction only (forward direction) and that is largely optically impermeable in the opposite direction (backward direction) (optically unidirectional diode behavior). Optical isolators are important for the optical technologies and serve to prevent optical feedback that might affect, for example, the function of lasers, and that prevent further disturbances of optical systems due to reflected and scattered light.
The function of the most frequently used optical isolators is based on magneto-optical Faraday rotators and polarizers that, if arranged appropriately, exhibit optical diode behavior. Faraday isolators have a complex design, are expensive and work only with polarized light. Moreover, their disadvantages are that they require an operating voltage for their operation and that their spectral scope of application is limited.
Optical isolators with spectral light conversion
In the following, new optical isolators with far-reaching technical advantages are presented, with a spectral filter 1 and a spectral light converter 2 as optical components being optically and mechanically firmly linked, thus forming a physical unit as a new optical component with optical diode or isolator functions. In the process, the spectral properties of the spectral filter 1 and of the spectral light converter 2 are chosen in such a way, that light rays, impinging in forward direction 3 on the optical isolator, can pass through the optical isolator spectrally converted with a modified wavelength, whereas they are optically absorbed or blocked out in backward direction 4 (Fig. 1).
The function of the new optical isolator is based on the spectral properties of the spectral filter and light converter components used for its design. If, according to Fig. 1, light rays with a present spectral bandwidth impinge in forward direction 3 on the spectral light converter 2, they are absorbed in the spectral light converter 2 as excitation radiation and emitted as emission radiation by fluorescence and/or laser effects with a spectrally longer wavelength (down-conversion) or a spectrally shorter wavelength (up-conversion), with the wavelengths of the excitation radiation and the emission radiation distinguishing from each other by Stokes shift, for example. The spectral properties of the spectral filter 1 and the spectral light converter 2 are coordinated so that excitation radiation impinging on the spectral light converter 2 is heavily absorbed or blocked out by the spectral filter 1, whereas the radiation emitted from the spectral light converter 2 and distinguishing in its wavelength from the excitation radiation can pass through the spectral filter 1 and thus the optical isolator. Thus, an optical isolator is formed, which lets the excitation radiation, spectrally converted and with modified wavelength, pass through in forward direction 3, while being absorbed or blocked out in backward direction. Due to its different optical transmission behavior in dependence on the light irradiation direction, the presented component exhibits optical diode behavior.
Polymer layers or molds, doped with laser dyes, such as rhodamines, pyromethenes or perylenes and put on the spectral filter by a thin film process or with a bonding technique, are suitable to be used as spectral light converters for an application of the optical isolator within the visible spectral range. If, for example, a polymer layer doped with RED 300 laser dye is excited by green LED irradiation or laser irradiation at 532 nm, red fluorescence or laser radiation will be obtained within the spectral range between 600 nm and 630 nm. A red long pass filter that allows red radiation to pass through but absorbs green radiation heavily is used as a spectral filter.
In order to generate emission radiation within the near infrared spectral range (NIR), erbium ion doped sol gel matrices, for example, can be used as spectral light converters. After excitation with an argon ion laser at 488 nm, the doped layers fluoresce at 1531 nm and 1550 nm. Black or red glasses are used as spectral filters here. They allow NIR radiation to pass through at high transmission and they heavily block out excitation radiation at 488 nm.
Up-converter materials as nanocomposites convert NIR radiation into visible radiation. In this case, the optical isolator is composed of the NIR converter and suitable short pass filters that are transparent to visible radiation and absorb NIR radiation. The demonstrated spectral filters and converters to be used in the optical isolator cover its NIR function as well.
Technology/ Application example
For the fabrication of a large-scale optical isolator for blue LEDs, a yellow fluorescence layer as spectral converter, consisting of yellow 083 fluorescence dye, is applied in a polymer solution to a glass substrate. After that, a yellow paint as spectral long pass filter is applied to the fluorescence layer on the glass substrate, which absorbs heavily in the blue spectral range at 470 nm (optical density equals approx. 4) and is permeable in the yellow spectral range.
If in this arrangement the blue LED irradiation, at first impinges in forward direction, after penetration through the glass substrate, on the yellow fluorescence layer, the blue LED irradiation will be very efficiently converted into yellow light and then pass through the paint acting as a spectral filter and with that through the optical isolator consisting of the fluorescing layer and the yellow paint. Blue LED irradiation, impinging at first in backward direction on the paint layer, is completely absorbed by the paint and thus blocked out by the isolator unit. The yellow paint acts on the blue irradiation like a black wall, while the blue irradiation, impinging at first on the fluorescence layer, is capable of passing at high transmission through the yellow paint and thus the isolator unit, spectrally converted with modified wavelength.
The demonstrated optical isolator unit absorbs the blue LED light or lets it pass through, depending on the irradiation direction, thus exhibiting optical diode properties.
The new optical isolators work purely optically and do not require any operating voltage. They cover a wide spectral range, from UV up to NIR, the latter being important for the optical communication technology. The demonstrated optical isolators can be manufactured easily and cost-effectively, both the large-scale and the micro-optical design, and they also work with non-polarized radiation. They are geared for diffuse and laser radiation applications.
R. Danz, “New optical isolator with spectral light conversion and laser radiation generation”,
German Utility Model, application of 27 October 2011, no. 20 2011 107 213.0
Fraunhofer Institute for Applied Polymer Research in Potsdam, Technical University Graz,
Fraunhofer Heinrich Hertz Institute Berlin
Dr. habil. Rudi Danz, physicist, Feldfichten 46, D-14532 Kleinmachnow, Germany
e-mail: email@example.com Phone: 493320324956