February 14, 2009

Lasers, Holograms and Laser Communication Roundup

1. Photonic chip breaking the terabit-per-second barrier.

Researchers from Australia, Denmark, and China have combined efforts to show the feasibility of terabit-per-second Ethernet over fiber-optic cables. The solution involves a photonic chip that uses laser light for switching signals, and a form of the exotic material type, chalcogenide. One of the key breakthroughs researchers made wasn't so much in speed but in practicality. By using relatively traditional methods to etch a circuit out of a glassy form of a chalcogenide, arsenic trisulfide (As2S3), researchers were able to reduce the waveguide that demultiplexed an incoming signal from tens of meters down to 5 cm. Eggleton said that silicon-based chips could also be used in principle to achieve similar, but slower, results, but their ultimate goal was to create fully photonic chips in the same foundries that now make CMOS (complementary metal oxide semiconductor) integrated circuits.

"It's years to complete," Eggleton said, taking these research efforts into a production technology. But these demonstrations "are starting to establish this is a serious proposition."

2. Audience interaction with 'hologram' enabled by spinning-mirror system

To an assembled audience, the system displays a dynamic, three-dimensional volumetric image of the speaker's head in real time and enables two-way communication between the display and observers. Behind the scenes, a high-speed projector projects patterned light onto the face of the remote speaker at 120 frames/s. Meanwhile, two video cameras record the changing pattern of light on the face from slightly different points of view. From this, a computer reconstructs
the 3-D shape of the face 30 times per second, and then by texture mapping that geometry, produces a 3-D model that updates at the frame rate of the video. This image is projected onto a flat brushed aluminum surface molded into the shape of an upside-down "V," which spins at 15 times per second–thus providing 30 passes of the surface every second. Each observer gets a different view of the speaker's face–as does every viewer's left and right eye. What the speaker sees is a flat screen showing video of the viewing audience. Thus, the speaker can interact with specific audience members. At a recent trade show, Paul Debevec, research associate professor and associate director of graphics at ICT, explained how the system operated at a
recent trade show (see video at The video shows a glass cage surrounding the "hologram" that, according to the researchers,
prevented curious observers from getting too close and clipping their fingers on the spinning mirror. Once the system is further refined, they expect to be able to do away with the glass

3. Ultrafast laser creates photonic crystal in diamond

A metamaterial consisting of periodic regions of high electrical conductivity in a 3 × 3 × 1.5 mm synthetic-diamond crystal has been fabricated by researchers at Kyoto University (Kyoto, Japan); the regions are created by focusing 230 fs pulses at 1 kHz from a modelocked Ti:sapphire laser to a beam-waist diameter of 2 µm and an energy fluence of 28.5 J/cm2 within the crystal. Potential uses of the metallo-dielectric photonic crystal include wire-grid polarizers and terahertz metamaterials

4. Sufrace plasmnos switched at Terahertz rates

Efforts to effect such switching have been ongoing in recent years, making use of a wide range of media including thermoelectric and electro-optic materials, quantum dots, and photochromic molecules. However, switching the packets of electron oscillations has so far been limited to submillisecond timescales, reaching down into the nanosecond regime in 2007. Now, Southampton's Kevin MacDonald and his colleagues have demonstrated a jump some five orders of magnitude over the prior best efforts, modulating plasmon pulses in a metal-dielectric waveguide at the 100-femtosecond level–a modulation bandwidth in the terahertz range.

"Our result represents a significant advance in the achievable modulation bandwidth for active plasmonics," says MacDonald, though he admits that the demonstration is more at the "proof-of-principle" stage than toward immediate practical applications. In fact, the advance may be too dramatic for its own good. "Some would argue that femtosecond switching is actually too fast for the envisioned next-generation applications of plasmonics in data transport and processing," he says.

5. Plasmonic LED appraches 10 GHz modulation

OPTICAL COMMUNICATIONS: Plasmonic LED approaches 10 GHz modulation speed Even though the device with a 40 nm gap between the quantum well and silver layer achieves a modulation speed of only 3.6 GHz, the researchers expect that a device fabricated with higher free-carrier densities on the order of 10**19 cm-3 could easily achieve a modulation speed of 10 GHz. The next step for the research team is to study electrically driven devices with good external efficiencies. "LEDs capable of 10 Gbit/s modulation speed can be the next-generation low-cost optical source for very short distance links within computer racks," says Michael Tan, a senior scientist at Hewlett Packard Laboratories. "The cost of the optical components is one of the largest mitigating factors for introducing photonics inside the box. These LEDs are much easier to manufacture and would cost 50 to 100 times less than today's data communication VCSELs."

Commercial light-emitting diodes (LEDs) are only capable of 1 GHz maximum modulation speeds because of slow carrier recombination, limiting them to applications in short-haul optical communication links.

6. Digital holography combines optical-fiber beams coherently

An experimental proof-of-concept was demonstrated by the researchers for three single-mode polarization-maintaining passive fibers at 1.06 µm using a Nd:YAG laser with a 10 kHz bandwidth expanded to a collimated beam. The first beamsplitter creates a reference beam and feeds the fiber array. A second one separates the reference in two arms: one is used to record the hologram on the CCD, the other generates the conjugated segmented wavefront by diffracting through the SLM and is fed back to the fibers to phase lock the output beams for coherent combination.
The research team is currently working to demonstrate phase locking for a larger number of fibers. The team says that this concept has high potential for extending the performance of fiber lasers–in particular for the pulsed regime–and will be useful for applications such as light detection and ranging and free-space communications that require both high-energy and high-spatial/spectral-quality fiber

7. High-power fiber-laser beams are combined incoherently

Combining laser beams can boost power at the target far above that produced by a single laser. Incoherent beam combining achieves propagation efficiencies of greater than 90%, while avoiding the complexities of coherent or spectral beam combining. [Naval Research Laboratory.] They achieved propagation efficiencies greater than 90% at a kilometer in range, with a total power of 2.8 kW on a target with a 10 cm radius.

The high beam quality and efficiency of fiber lasers make them ideal candidates for directed-energy applications. Although a number of companies manufacture high-power fiber lasers, IPG Photonics (Oxford, MA) currently holds the record, producing more than 3 kW per fiber of single-mode (M2 approximately 1) laser radiation.2 Another company, Nufern (East Granby, CT), expected to have a 1 kW single-mode fiber laser available in 2008. 3 These multikilowatt single-mode fiber lasers are robust, compact, nearly diffraction-limited, have high wall-plug efficiency, random polarization, and large bandwidth. A 1 kW single-mode IPG fiber-laser module, emitting at 1.07 µm, has a dimension of approximately 60 × 33 × 5 cm (excluding power supply), weighs about 20 lb, has a wall-plug efficiency of about 30%, and has
an operating lifetime in excess of 10,000 hours.

To operate in a single mode, the core of the fiber must be sufficiently small. For example, the IPG single-mode 1 kW fiber lasers have a core radius of about 15 microns. Multimode IPG fibers, on the other hand, operating at 10 kW and 20 kW per fiber, have core radii of about 100 and 200 µm and a beam quality M2 of about 13 and 38. These higher-power fiber lasers with larger values of M2 have a more limited propagation range. In 2008, IPG is expected to have a single-mode fiber laser operating at 5 kW.

Incoherent beam combining of fiber lasers is readily scalable to higher total power levels. For multiple incoherently combined fiber lasers, the total transmitted power scales as the number of lasers, while the radius of the beam director scales as the square root of the number of lasers. A 500 kW laser system, for example, could consist of 100 fiber lasers (5 kW/fiber) and have a beam director radius of about
40 cm. Excluding the power supply, the fibers and pump diodes would occupy a volume of about 8 m**3.

We at NRL have recently completed a proof-of-concept field demonstration of long-range incoherent beam combining at the Naval Surface Warfare Center in Dahlgren, VA. These experiments used four IPG single-mode fiber lasers having a total output power of 6.2 kW. In the initial experiments, we transmitted a total of about 3 kW, and delivered about 2.8 kW to a 10-cm-radius target at a range of 1.2 km.
The fiber lasers were operated at half power because of thermal issues in the beam director, which can be readily corrected in the next series of experiments.

Propagation experiments using the NRL fiber lasers on a 3.2 km range at full power are presently taking place at the Starfire Optical Range (Albuquerque, NM). These experiments will verify our computer model of incoherent beam combining and will help us devise closed-loop techniques to compensate for wandering of the beam centroid due to air turbulence. We will also investigate the effects of thermal blooming, which can be an important limitation under certain conditions. This
latter investigation will use a stagnation tube to eliminate the cooling effects of transverse airflow.

8. Talbot external cavity coherently combines ten laser diodes

A method of phase-locking an array of ten index-guided tapered laser diodes has been devised by researchers at CNRS and the Alcatel-Tales III-V Lab (both in Palaiseau, France) and the University of Nottingham (Nottingham, England). The Talbot effect (a near-field diffraction effect in which a grating is repeatedly imaged at regular distances from the grating) is exploited by using a slightly tipped volume Bragg grating (VBG) as the output mirror; the tip selects an in-phase self-image and places it back on the diode array, sending light back into the diodes for phase-locking. output power of 1.7 W in its in-phase single main lobe mode.

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