Arrays of electronically addressable lasing pixels may form the high-brightness, large-screen projection displays of the future. Such lasing pixels can be created by sandwiching a layer of polymer-dispersed liquid crystal (PDLC) inside a submillimeter laser cavity. The PDLC layer can act as a spatially patterned Q-switch to produce laser emission with a selected transverse pattern. In the "off" (or zero-voltage) state, the PDLC acts as a high-loss cavity element due to scatter from randomly oriented liquid-crystal droplets, thereby eliminating the optical feedback required for lasing. When voltage is applied, the droplets align longitudinally and the PDLC becomes transmissive, bringing the laser above threshold.
Nabil Lawandy and collaborators at Brown University (Providence, RI) demonstrated the technique with a 2 × 2-mm, 10-line display of addressable lasing pixels. To fabricate the PDLC cell, the group produced a 4:1 mixture of TL205 liquid crystal and PN393 polymer, applied a 5-µm-thick layer between a pair of glass plates, and cured it with ultraviolet light from a mercury lamp. Addressable pixels were formed by patterning the glass with 1-mm-wide indium tin oxide (ITO) strips, spaced 0.1 mm apart, then rotating the plates so that the ITO lines were orthogonal with respect to each other. By applying voltage to the proper combination of ITO lines on the two plates, each pixel in the array can be independently activated.
Dye cell in a sandwich
The gain medium consists of a 500-µm-thick, 1 × 2-cm dye flow cell containing Rhodamine 6G in 0.25-mM ethylene glycol. When pumped by 50-mJ pulses at 532 nm from a Nd:YAG laser operating at a pulse-repetition rate of 20 Hz, the dye produces output at 560 nm. To form the optical cavity, the dye cell is sandwiched between the PDLC cell and a dichroic mirror; the glass/air interface of the liquid crystal cell acts as the other cavity mirror and as a 4% output coupler. The researchers pumped the entire region of the PDLC cell with an apodized beam and ap plied voltage to selectively activate array pixels.
The display produces about 400 mW/cm2 of intensity; 400 mW corresponds to about 250 lumens of optical power. The conversion efficiency for the unoptimized de vice is about 45%; with proper output coupling, Lawandy estimates that efficiencies could run as high as 70%.
Unlike galvanometer-scanned projectors that serially refresh pixels at a rate of 30 Hz, the lasing pixel device simultaneously refreshes the entire image electrically at the switching rate of the PDLC. Pixels can be as small as 20 µm if diffraction effects are controlled by a sufficiently short cavity length, potentially increasing the fill factor over conventional displays.
Lawandy plans to eliminate the patterning step by using a commercially available 300 × 300 liquid crystal produced for flat-panel displays; the lasing pixel display will project CD-ROM images at video rates. To increase resolution, the subsequent design will incorporate a liquid-crystal element produced for active matrix displays. Lawandy expects the group to produce a display with 100,000 pixels/cm2 by year's end.
Lawandy, now at Spectra Science Corp. (Providence, RI), and the Brown University group recently won an $800,000 Phase II SBIR grant from the Fast Track program of the US Ballistic Missile Defense Organization as part of a multimillion-dollar project for the development of a 2-in. (diagonal) display with more than 100,000 addressable pixels; a private investor has contributed an additional $200,000.
Kristin Lewotsky | Associate Editor (1994-1997)
Kristin Lewotsky was an associate editor for Laser Focus World from December 1994 through November 1997.