Optical: systems and elements – Absorption filter – Neutral or graded density
Reexamination Certificate
2001-12-12
2003-11-25
Ullah, Akm Enayet (Department: 2874)
Optical: systems and elements
Absorption filter
Neutral or graded density
C385S008000, C385S031000, C385S042000, C359S579000, C359S886000
Reexamination Certificate
active
06654188
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to the field of thermooptically active materials used in optical components and technology. More particularly, the present invention relates to the fields of optical waveguides, optical filters, optical switches, laser optics, neutral density filters, optical attenuators, flat panel displays, and projection display devices.
A persistent difficulty in the field of optical polymer materials is the high attenuation of such materials when they are in their inactive, or maximally transmissive state, sometimes denoted the “ON-state”. For example, many polymer plastics have been developed for use in passive optical waveguides for telecommunication signals in the near infrared (NIR), particularly at 1300 nm and 1550 nm. Even though such polymers are intended to be optimally transmissive at all times (i.e. always in their ON-state), attenuation levels of up to 0.5 dB/cm are typical for conventional “low loss” materials. The absorption in such polymers is usually due to the presence of carbon-hydrogen bonds that exhibit strongly absorbing anharmonic vibrational resonance overtone bands in the NIR. The halogenated analogs of carbon-hydrogen polymers, wherein the hydrogen is substituted by a higher mass halogen such as fluorine, bromine, iodine, or chlorine, exhibit much weaker NIR absorption and are therefore more highly transmissive. Halogenated polymers have been tested as passive waveguide media that exhibit improved attenuation levels of under 0.1 dB/cm. In an embodiment of the present invention, preferred for applications in the NIR, the composition comprises halogenated polymers in order to realize low attenuation.
So-called “active” polymers are used in optics in order to manipulate characteristics of optical signals by means of external control inputs, which affect the optical properties of the polymers. The polymer compositions are suitable for use in thermooptically active optical components because of their large thermooptic coefficient for the index of refraction, dn/dT, or rate of change of refractive index with temperature. In an example of an electro optically active polymer composition for use in an optical component, radio frequency (rf) electrical control inputs, rather than thermal inputs, are used to allow high-speed phase modulation of optical signals propagating within the polymer. An important figure of merit for any such actively controlled polymer medium is that it have minimal total absorption when switched to the ON-state. Ideally, it will have the best achievable ON-state absorption performance for the passive polymer media mentioned above, or a figure of merit, usually denoted in the art as “insertion loss”, of less than 0.1 dB/cm.
The expected absorption for the prior art electrooptically controlled chromophoric polymers is at least 10 dB for a 1 cm active polymer length or an insertion loss of 10 dB/cm. These materials, when used in a Mach-Zehnder interferometer configuration can provide a maximum absorption in the OFF-state of 30 dB. The difference in these two attenuation levels is a second important figure of merit for any switched medium, denoted the “extinction ratio”. In this case, the difference between the maximum OFF-state absorption and the minimum ON-state absorption yields an extinction ratio of 20 dB. Since their device has an active polymer length of 1 cm, their extinction ratio per unit length is 20 dB/cm. Extinction ratio performance for the present invention was measured to be 22 dB/cm, a level equal to or improved over prior art media.
Other optical switches described in the prior art are comprised of thermooptically controlled polymer materials that exhibit insertion loss of 0.6 dB/cm and are limited to an extinction ratio of 20 dB total. In the prior art, U.S. Pat. No. 6,208,798 discloses a polymer cladding that is thermooptically controlled to permit leakage and hence attenuation of optical signals from the waveguide core; a preferred embodiment is described wherein the insertion loss is 3 dB for a design which provides an extinction ratio of 20 dB. The materials of the present invention, when configured as the core of a simple thermooptically controlled waveguide, rather than the more complex configuration of a Y-branch waveguide, a Mach-Zehnder interferometer, or a thermooptically controlled leaky waveguide cladding, would provide an insertion loss of below 0.1 dB/cm and 22 dB of extinction ratio for every centimeter of waveguide length. Thus, the present invention promises a significant reduction in complexity, and a substantial improvement in performance for thermooptically controlled devices over that of the prior art.
A further advantage of the present invention is that it exhibits a high thermooptic control coefficient. That is, for a modest change in the control parameter (temperature), a large change in signal attenuation is realized. The measured thermooptic control coefficient for compositions of the present invention is in the range of 6 dB/cm/° C. When combined with the low insertion loss of the present invention of less than 0.1 dB/cm, this figure of merit implies that for a relatively small change in temperature along a waveguide, a relatively large change in attenuation can be achieved. For example, a ten-centimeter long waveguide attenuator using a composition of the present invention would exhibit an insertion loss of 1 dB and 60 dB/° C. thermooptic coefficient. So, with a control temperature change of only 0.2° C., such a device could switch 10 dB in attenuation. Allowing such a small change in temperature is a significant advantage over the prior art because it permits, for a given thermooptic heating/cooling power per unit length, a more rapid switching speed in any device utilizing compositions of the invention.
Yet another advantage of the present invention is that it enables design of components that have attenuation that is smoothly varying and varies only gradually across its wavelengths of operation. Some thermooptically controlled devices must be optimized for performance in narrow bands of spectrum because their attenuation performance relies on resonance or nulling phenomena. For example, devices which rely on the thermooptic change of refractive index of a polymer waveguide arm of a Mach-Zehnder interferometer must be optimized for a center wavelength. Variation of operating wavelength by more than one quarter of the center wavelength value renders the device unable to achieve its ideal phase cancellation point. In the prior art disclosed in U.S. Pat. No. 6,165,389, a switching medium is described which depends on a volume phase transition phenomenon in the thermooptically active material; a preferred embodiment exhibits variation in optical absorption which is strongly resonant in wavelength. By contrast to the foregoing prior art examples, the present invention relies on bulk absorption due to a “cloud point” phase transition phenomenon; in addition, its absorption characteristics vary only slowly across the entire wavelength band from 300 nm to 2000 nm without any rapidly varying resonance attenuation features. Using compositions of the present invention, devices can be configured which exhibit only small changes in optical absorption across wavelength, which we denote “insertion loss flatness”. A preferred embodiment of the invention, for example, achieves a worst case insertion loss flatness of less than 0.01 dB
m in the vicinity of NIR telecommunication bands over the wavelength range of 1200 nm to 1600 nm.
A further characteristic of some optical polymer media such as liquid crystal polymer media is that they exhibit birefringence, meaning that their index of refraction is significantly different for the two orthogonal signal polarizations. This in turn leads to a deleterious feature known as polarization dependent loss (PDL). In some applications such as polarization independent waveguide switches, or free space neutral density filters, it would be desirable to realize behavior for a switch medium that is non-birefringent in na
Barlow Josephs & Holmes, Ltd.
Lightspan, LLC
Rahll Jerry T.
Ullah Akm Enayet
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