LAKE FOREST, IL--The Sixth Inter national Symposium on Display Holography convened at Lake Forest College (LFC) last July and attracted approximately 120 scientists, artists, teachers, and entrepreneurs from 18 countries. A triennial event started in 1982, this year`s conference was chaired by founder Tung Jeong and cochaired by Hans Bjelkhagen, both of Lake Forest College; honorary chairs were Emmett Leith (University of Michigan; Ann Arbor, MI) and Yuri Denisyuk (JOFFE; St. Petersburg, Russia).
During the two weeks preceding the conference, organizers arranged a five-day hands-on worksho¥for making fundamental holograms. They also offered seven one-day tutorials on photochemistry, mastering, embossing, nonsilver materials and the DuPont OmniDex photopolymer, color holog raphy, portraiture, and processing of non-Agfa silver halide.
A crisis and a renaissance
Agfa Gevaert NV (Mortsel, Belgium) announced in February that it will sto¥manufacturing silver halide material for holography. This set off a panic among holographers and helped define the emphasis on materials as a major theme of the symposium.
After the traditional Monday morning session, Reports of the Nations, the first technical session, chaired by Sylvia Stevenson of DuPont (Wilmington, DE), covered new--to most holographers--materials for holography. Materials reported on included the DuPont OmniDex photopolymer and silver-halide material from Russia, Germany, and Japan. The atmosphere turned upbeat because not only are the new materials better than those from Agfa, but prices are comparable and may be lower. In naming the symposium "Holography--a Renaissance," Jeong compared Agfa`s withdrawal to a forest fire--those who survive it will reach new heights.
A special scientific exhibition of color holograms made on Russian plates was open throughout the week (see Fig. 1 on p. 38). It attracted the attention of CNN, which internationally broadcast two-minute reports on five separate programs.
Lippmann photography--a revival
J. M. Fournier of the Rowland Institute (Cambridge, MA), W. R. Alschuler of the California Institute of the Arts (Valencia, CA), and Bjelkhagen held a session on Lippmann photography by projecting images from both old Lippmann photographs and new ones recorded on silver halide holographic plates. Gabriel Lippmann won the 1908 Nobel Prize in physics for his method of reproducing colors photographically based on the phenomenon of interference.
Two important facts emerged: the color of all Lippmann photographs that are recorded on black-and-white emulsion withstood the ravages of time, and mercury is not necessary for reflecting the light back through the emulsion to cause interference. Bjelkhagen made many Lippmann photographs with LFC student Ray Ro without mercury, which were exhibited with the color holograms.
To make Lippmann photographs, Bjelkhagen used Russian PFG-03C plates and an old camera without mercury. The 4% Fresnel reflection from the emulsion-air interface sets u¥a Fabry-Perot type of resonance in the emulsion, sufficient to produce shar¥interference patterns that result in bright color images when viewed under diffused light. This technique could become a popular method of recording archival color images in two dimensions.
In other work, Christy A. Heid of the US Army Research Laboratory (Adelphi, MD) reported on a method for real-time three-dimensional holographic imaging using photorefractive crystals. By recording a hologram on a mosaic of two cerium-doped strontium barium niobium (Ce:SBN) crystals and projecting the real image back using a phase-conjugate read beam generated from a second photorefractive crystal acting as a double-pumped phase-conjugate mirror (DPPCM), a high-quality image can be seen with a 30° field of view (see Fig. 2).
Tung H. Jeong
TUNG H. JEONG is director of the Center for Photonic Studies at Lake Forest College, Lake Forest, IL.
In mid-August, NASA awarded a contract worth more than $25 million to the University of Colorado and Ball Aerospace (both Boulder, CO) to build another ultraviolet (UV) spectrograph for the Hubble Space Telescope (HST). In 2002, the instrument will replace the Space Telescope Imaging Spectrograph (STIS) as one of the four instruments on the HST. The new Cosmic Origins Spectrograph (COS) will be a slitless Rowland spectrograph designed to provide UV spectra from 120 to 170 nm of faint objects. The instrument will be used to observe faint UV sources to determine the distribution and composition of matter in the universe.
In the beginning
The Cosmic Origins Spectrograph will obtain moderate-resolution (R of more than 20,000) spectra of faint UV sources, such as quasars and active galactic nuclei. "One of the most powerful methods for mapping out the large-scale distribution, kinematics, and chemical composition of matter in the universe is to observe spectra of distance sources such as quasars," says the project proposal. Then researchers can analyze the spectral features imprinted on the quasar`s spectra for clues about matter between the source and the detector.
The UV spectral region was chosen because it is most useful for this sort of diagnostic. Principal investigator James Green at the University of Colorado explains, "Materials emit more lines in the UV than in any other band." The instrument could allow researchers to look at composition and distribution of matter to try to understand the origins of the universe, including the origin of large-scale structures and the intergalactic medium, the formation, evolution, and ages of galaxies, and the origins of stellar and planetary systems.
About 60% of the instrument lifetime would be spent obtaining spectra from quasars, while the rest of the time would be devoted to investigating the ages of globular clusters and looking for heavy elements between stars and the formation of stars and objects in our solar system.
The designers expect that the COS will be able to obtain spectra from hundreds of distant quasars. Ground-based systems cannot be used for UV astronomy because the Earth`s atmosphere absorbs UV radiation. The STIS currently installed in the HST can obtain spectra from only a few dozen quasars.
The STIS, the designers say, achieves "exquisite spatial resolution, a wide wavelength range, and considerable versatility." The COS, in comparison, provides higher sensitivity and resolution of faint UV objects by focusing on high throughput and narrowing the spectral band covered.
To obtain the spectra of faint sources, the spectrograph design maximizes throughput and consequently uses as few optical elements as possible. The instrument is a Rowland spectrograph with a concave aspheric grating (see figure on p. 44). In a Rowland spectrograph, the entrance aperture and detector lie on the circle described by the curvature of the concave grating.
The input aperture is circular, covering a 2-arc sec angle, positioned at a point that passes 87% of the aberrated image collected by the telescope. The lack of a slit means that the grating re-images the source (rather than the slit) onto the detector, allowing users to resolve two point sources separated by only 0.72 arc sec.
The detector is a windowless microchannel-plate array with an opaque cesium iodide (CsI) photocathode and a double delay-line readout. The sensitive surface of the detector is bowed to lie on the Rowland circle.
At the most efficient wavelength of about 130 nm, the total end-to-end throughput of the system, including the HST and the spectrograph, is 8.6%. The COS offers 20 times the spectral resolution and efficiency (expressed in counts/photon) of the other UV spectrographs that have been installed on the HST.
The high-sensitivity grating covers the entire wavelength range from 123 to 200 nm at a resolution of 2500 to 3500. Two high-resolution gratings with an order-of-magnitude higher resolution split the wavelength range from 115.0 to 144.9 nm and 140.5 to 177.4 nm. All three gratings are aspheres designed to compensate for the spherical aberration of the HST primary mirror.
Cost and lifetime
The original estimate for the cost to build the instrument is $25 million, but Green notes that this estimate will be revised upward because NASA has asked for some design changes. The intended lifetime of the HST has been extended by five years until 2010, so the instrument, which was originally designed to last for three years, now must be designed to last for a total of eight years.
YVONNE CARTS-POWELL is a science writer based in Belmont, MA
Visible femtosecond pulses achieved in single crystal
Turkish researchers have reported visible femtosecond pulses obtained with an ultrafast optical parametric oscillator (OPO). Pumped by a femtosecond Ti:sapphire laser, the self-doubling OPO based on a potassium titanyl phosphate (KTP) substrate has so far yielded tunable output in the range from 530 to 600 nm and a conversion efficiency of 30%, reports Orhan Aytür, an assistant professor at Bilkent University (Ankara, Turkey).
While intracavity second-harmonic generation has been achieved previously using a second nonlinear crystal, Aytür said his grou¥has come u¥with a method to achieve second-harmonic or sum-frequency generation within the OPO crystal itself (see Fig. 1). The researchers published their initial results in March and presented a device that was pumped to emit blue light at this year`s Conference on Lasers and Electro-Optics (CLEO `97; Baltimore, MD; paper CThY6).1
The original device was pumped at 745 nm with 150-fs pulses by a modelocked Ti:sapphire laser. At this pum¥wavelength, the 5-mm-long KT¥crystal was phase-matched for parametric generation at 1080 nm, and frequency doubling produced a green (540-nm) output beam with 29% output efficiency. Angle and pum¥wavelength tuning provided output tunability in the 530 to 600-nm range. The device reported at CLEO was pumped at 828 nm and phase-matched for 1175 nm, producing a 487-nm output.
The system was constructed in the form of a ring cavity consisting of four mirrors (see Fig. 2). The KT¥crystal was placed at the intracavity focus, between two 100-mm-radius-of-curvature concave mirrors. Doubling efficiency was made possible by placing a half-wave retarder in the cavity between two flat mirrors. Intracavity frequency doubling was achieved by rotating the retarder and thereby rotating the intracavity signal polarization. According to Aytür, rotating the intracavity signal polarization couples a portion of the p-polarized signal beam to the s-polarization, which allows second-harmonic generation to take place in the KT¥crystal.
Possible applications for the visible femtosecond pulses include time-resolved spectroscopy and pump-probe experiments.
1. T. Kartaloglu, K. G. Köprülü, and O. Aytür Opt. Lett. 22, 280 (1997).