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Liquid Crystal Waveguide Technology

Numerous applications require active control over light including: robotic-vision, optical computing, telecommunications, holographic data-storage, remote sensing, cold-atom optics, industrial process analysis, and many more.  In response to this need, a diverse array of technologies has been investigated and developed over the past several decades:  micro-electro mechanical systems (MEMs),[1] photonic crystals,[2] thermo-optics,[3] and electro-optic materials such as inorganic crystals and organic poled-polymers.[4]  While tremendous progress has been made, there are still numerous applications, such as beamsteering and large optical phase delay, where bulky and power-consumptive macroscopic opto-mechanical techniques are still the best.  This is, at least in part, because typical electro-optic approaches do not realize sufficient control over light to replace traditional opto-mechanics, and MEMs techniques are still inherently mechanical, which imposes vibration and inertia design challenges, and they also provide only limited control over optical phase.

Over the past several decades one of the most technically and commercially successful approaches for light control has been liquid-crystal (LC) optics.  LCs have the world largest electro-optic response (Dn > 0.2 over 5 volts for a typical LC, which corresponds to 105-106 pm/V, i.e., several orders of magnitude larger than any other approach), are environmentally stable, and inexpensive.[5]  This has enabled the now > $100 billion a year display market. A typical “display-like” LC-optic is shown in Figure 1.  The light traverses a thin (< 20 micron) LC layer that is sandwiched between glass sheets.  Transparent electrodes are used to apply an electric field, which, in combination with polarizers, may be used to either block or transmit the light.

Figure 1:  A Typical LC-Optic, such as is used in the ubiquitous LC-Display

While undeniably potent for information displays, this traditional LC-optic has two significant limitations.  First, the light must transmit through transparent electrodes, which in turn limits the total optical power that may be controlled. Second, and arguably more significant, the LC layer must be extremely thin.  The LC- material is rendered a single-domain crystal via thin alignment layers.  The LC-molecules adjacent these alignment layers (shown in red in Figure 1) are highly ordered, which means low scattering loss, and fast.  If one were to make the LC cell thicker, the bulk LC material (shown as blue in Figure 1) would become prohibitively slow and opaque.  Therefore, even though the LC material has a tremendous electro-optic effect, the necessarily short interaction length mitigates this effect.  In order to circumvent these limitations we have invented and are developing the LC-clad waveguide architecture, as shown in Figure 2.

Figure 2 A) The basic geometry of an LC-waveguide.  The light is confined to a core and the LC is an electro-optic upper cladding. As the index of refraction of the upper cladding is tuned the “effective index” of the guided mode is also tuned. B) A side view of a liquid crystal waveguide.  In a slab waveguide the light is guided in the x dimension, but is free to propagate as Gaussian beams, sheets, or even 1D images in the plane.

 

Rather than transmit through an LC cell, which by design must be thin (typically < 20 mm), we utilize the LC as an active cladding layer in a waveguide architecture, i.e., the light skims along the surface of an LC layer, as shown in Figure 2.  This electro-evanescent architecture circumvents limitations of traditional LC-optics: i) the light never crosses a transparent electrode, ii) the light only interacts with the well-behaved LC-surface layer via the evanescent field, and iii) the interaction length is now decoupled from the LC-layer thickness.  Example operation of a device is given in Figure 3. This device exhibited more than 1 millimeter of voltage tunability over optical phase.  We know of no other technology that can provide similar performance.  Furthermore, the LC waveguide switching time is faster than normal liquid crystals by about one order of magnitude. Typical relaxation times for LC waveguides are on the order of 500 microseconds.

 

Figure 3  A) The transmission of an LC waveguide between polarizers.  The figure was recorded over a longer sweep time so that individual waves could be observed. This is a plot of the relative phase shift induced by the LC-waveguide, each minima or maxima on the plot corresponds to one wavelength (1.3 mm) of optical phase change.  B) The tunable optical phase delay versus applied voltage. For this device greater than one millimeter of  optical phase delay (OPD) was achieved. The inset shows a picture of the device (next to a nickel for scale).

Electro-optic beamsteering for micro-ladar applications is one important field that is benefiting from LC waveguides.  We have demonstrated analog angle tuning of 80° in a device with only two control voltages.  Figure 4 shows a  device where each electrode is comprised of a series of prism shaped electrodes shorted together.  The refractive index under each electrode can be tuned by as much as 0.05 giving a deflection at each prism by Snell’s law.

Figure 4 — Two dimensional electro-optic beamsteerer based on liquid crystal clad waveguide technology.

Videos of LC-waveguide beamsteerer devices in action are shown in our downloads section.

 

The LC-waveguide architecture is a new electro-optic approach that provides unprecedented voltage control over optical phase (> 1 mm).  This previously unrealizable level of control makes possible new devices with remarkable performance attributes.  To date we have demonstrated: ultra-wide field of view  non-mechanical laser beamsteerers,  FTIR spectrometers on a chip with < 5 nm resolution, chip-scale widely tunable lasers (nearly 40 nm tunability demonstrated), ultra-low power (< 5 mWatts) tunable micro-ring filters and Mach-Zehnder switches, and many more, as shown in Figure 5. All of these devices may be in small LCD-like packages that can ultimately be as low cost as a calculator display.

Figure 5 — LC-Waveguides provide a platform for a new class of photonic devices.

1.         Wu, M.C., O. Solgaard, and J.E. Ford, Optical MEMS for Lightwave Communiation. Journal of Lightwave Technology, 2006. 24(12): p. 4433-4454.

2.         Summers, C.J., C.W. Neff, and W. Park, Active Photonic Crystal Nano-Architectures. Journal of Nonlinear Optical Physics and Materials, 2003. 12(4): p. 587-597.

3.         Brainard, R., B. Fondeur, and D.J. Dougherty. Advances in Planar Lightwave Circuits for Wavelength Routing Applications. in OSA Integrated Optics Conference. 2006.

4.         Jin, D., et al., Material development and processing for electro-optic device systems. Proceedings of SPIE, Organic Photonic Materials and Devices V, 2003. 4991: p. 610-620.

5.         Khoo, I.-C. and S.-T. Wu, Optics and nonlinear optics of liquid crystals. 1993: World Scientific Publishing.