Tunable-Focus Lens

In addition to displays, liquid crystals have also been used extensively in tunable photonic devices, such as optical phased array for laser beam steering, variable optical attenuator (VOA) for telecommunications, tunable-focus lens for camera zoom lens, LC-infiltrated photonic crystal fibers [1,2], diode laser-pumped dye-doped LC laser, just to mention a few. 

In this chapter, we only select four topics to illustrate the potential appUcations of liquid crystals in photonics and their technical challenges. The four representative subjects selected are (1) laser beam steering, (2) variable optical attenuator, (3) tunable-focus lens, and (4) polarization-independent LC devices. 

H. Ren, Y. H. Fan, S. Gauza, and S. T. Wu, Tunable-focus flat hquid crystal spherical lens, Appl. Phys. Lett. 84, 4789 (2004). 

The femtosecond transient absorption studies were performed with 387 nm laser pulses (1 khz, 150 fs pulse width) from an amplified Ti Sapphire laser system (Model CPA 2101, Clark-MXR Inc). A NOPA optical parametric converter was used to generate ultrashort tunable visible pulses from the pump pulses. The apparatus is referred to as a two-beam setup, where the pump pulse is used as excitation source for transient species and the delay of the probe pulse is exactly controlled by an optical delay rail. As probe (white light continuum), a small fraction of pulses stemming from the CPA laser system was focused by a 50 mm lens into a 2-mm thick sapphire disc. A schematic representation of the setup is given below in Fig. 7.2. 2.0 mm quartz cuvettes were used for all measurements. 

This Synchroscan [68] streak camera system has been used to study the time resolved fluorescence of trans-stilbene in the picosecond time regime. The experimental arrangement [69] is shown in Fig. 20. An acousto-optically mode-locked argon ion laser (Spectra Physics 164), modulated at 69.55 MHz was used to pump a dye laser. The fundamental of this dye laser, formed by mirrors M, M2, M3 and M4, was tunable from 565 to 630 nm using Rhodamine 6G and second harmonic output was available by doubling in an ADP crystal placed intracavity at the focal point of mirrors M5 and M6. The peak output power of this laser in the ultraviolet was 0.35W for a 2ps pulse which, when focused into the quartz sample cell of lens L, produced a typical power density of 10 KW cm-2. Fluorescence was collected at 90° to the incident beam and focused onto the streak camera photocathode with lens L3. The fluorescence was also passed through a polarizer and a bandpass filter whose maximum transmission corresponded to the peak of the trans-stilbene fluorescence. 

An ultraviolet fluorescence excitation beam was generated with an intracavity frequency doubled argon ion laser (Coherent, FRED) providing a tunable wavelength range of 229 - 264 nm. The excitation beam was focused onto the sample with a 10 cm lens to give a spot size of 60-70 pm. For fluorescence experiments, front-face illumination geometry was used. The... 

The molecular jet of molecules is crossed with a tunable dye laser and the laser-induced fluorescence is collected with a lens and focused on a PMT detector (Fig. 5). In the original experiments, a standard pulsed dye laser was used to match the 10-Hz duty cycle of the pulsed valve and the pulsed Nd YAG vaporization laser. Although this approach provides a high S/N ratio and wide spectral coverage, the resolution is limited by the laser line width of typically 0.5cm-1 (no etalon) to 0.05cm-1 with an etalon. 

The crossed-beam arrangement (Fig. 2) used in the MPD experiments consisted of nozzle beam system, multimode pulsed TEA-COj laser, pulsed tunable probe laser, fluorescence detection system. The COj laser beam of about rectangular cross-section, 2x3 cm, was focused onto the molecular beam with a lens of 25 cm focal length. Most of the laser output ( 70%) of 1 J/pulse was contained in a 0.2-jas pulse, the rest in a 2-/is long tail. The repetition rate of 10 Hz was compatible with the repetition of the pulsed probe laser. The experiments with CjHjCN have been carried out at the 10.591 fim P(20), (00°1-10°0) and with CH3NH2 at the 9.586 jam P(24), (00°l-02°0) laser line. 

Figure 2.32 shows a possible experimental arrangement for the observation of Doppler-free two-photon absorption. The two oppositely traveling waves are formed by reflection of the output beam from a single-mode tunable dye laser. The Faraday rotator prevents feedback into the laser. The two-photon absorption is monitored by the fluorescence emitted from the final state Ef into other states Em - From (2.66) it follows that the probability of two-photon absorption is proportional to the square of the laser intensity. Therefore, the two beams are focused into the sample cell by the lens L and the spherical mirror M. 

Optofluidic Lens for Tunable Optical Diverging, Collimating, and Focusing... 

If a tunable lens with variable focus is employed in the objective, many of these limitations in OCT performance may be circumvented. As the focal length is tuned with the axial scan such that the focused position always overlaps with the interference position, a number of advantages accrue (1) the DOF may be reduced so that the beam waist is small and the reflection corresponds only to a small volume in the sample, improving interference contrast (2) reflection from other parts of the sample is suppressed (3) the position of maximum intensity moves with the axial scan into the sample high lateral resolution is maintained [32]. 

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