Recently in CLEO 2008 Category

I wasn't at CLEO for the last day, but optics.org publisher Claire Bedrock was. There was one particular talk that caught her eye:

"For those who stayed for the final day of the conference there was a fascinating day-long symposium on hollow-core photonic crystal fibre. The final talk of the day created a particular buzz, when Brian Mangan of Crystal Fibre of Denmark presented some new results obtained in a collaboration with the Technical University of Denmark.

First up was a fibre with an antiresonant core, which achieved an attenuation of 9.3 dB/km ‐ a new record for this type of seven-cell fibre). Then Mangan presented results for two high-birefringence fibres, one with an air-fill factor of 91% and the other with a lower air-fill factor of 85%. The latter produced an impressive attenuation of 19 dB/km.

When questioned at the end about the future of photonic crystal fibre. Mangan replied that 'there are loads of talented guys in the field and I'm sure we'll find a solution to the limitations'. This seemed an appropriate note on which to end not only a fascinating symposium but an exciting conference."

In Thursday's post-deadline session, Liang Dong of fibre laser specialist IMRA showcased a new class of fibre that supports singlemode operation for fiber laser applications requiring high peak powers. According to Dong, the new all-glass fibre will extend the reach of practical ultrafast amplifiers to millijoule pulse energies, and could also lead to continuous-wave fibre lasers and amplifiers in the 10 kW range.

Dong explained that high peak powers requires singlemode operation to be achieved in a fibre with a large effective area. He pointed out that today's large-mode area fibres are limited to a core diameter of 30 µm, while photonic crystal fibres have been demonstrated with diameters of up to 100 µm.

Those figures make it even more impressive that IMRA has achieved singlemode operation in fibres with core diameters of up to 170 µm. The fibres exploit the company's "leakage channel fibre" (LCF) design, in which the core of the fibre is formed by six low-index features arranged in a hexagonal grid. Unlike photonic crystal fibre, in LCF the air holes are filled with flourine-doped silica glass, and so it can be cleaved, spliced and handled in the same way as normal optical fibre.

Versions of the fibre with core diameters of 100 µm and 170 µm were designed as passive LCFs. The 100 µm version achieves a pump absorption of more than 25 dB/m &ndash which Dong says would mean that a typical amplifier could be just tens of centimetres long – while LCFs with larger diameters could yield even shorter amplifiers.

A ytterbium-doped LCF was also fabricated with a core diameter of 50 µm. A single-stage amplifier constructed from this fibre achieved pulse energies of 600 µJ for a pulse length of 600 ps and a repetition rate of 25 kHz, which Dong says shows that "the design is a practical choice for extending the peak power in fibre lasers".

Keen-eyed delegates may have been intrigued by the signage for PhotonXpo scattered around the CLEO exhibit hall.

Well, all was revealed this morning: it's part of a rebranding exercise for the CLEO technical exhibit. "The goal is to provide the exhibit with its own identity," Colleen Morrison, the OSA's director for for public relations, told me today. "It has continued to expand, and we wanted a name that would better reflect what's out there on the show floor."

This year's CLEO attracted 320 exhibitors, but the organizers hope that renaming the event will increase numbers at the 2009 event in Baltimore, Maryland.

And that's it from me for now. I'm off to the airport for the red-eye flight to London, but look out in the next couple of days for a couple of post-deadline posts.

The growing interest in commercial photovoltaic sources was apparent at Wednesday afternoon's PhAST session. A full conference room heard Craig Cruikshank of cintelliq, a UK consultancy specializing in organic semiconductors, explain how increased research funding into organic photovoltaics (OPVs) would lead to increased efficiencies and lifetimes, but that the real challenge will be to manufacture devices in high volume and at low cost.

For the moment, he said, the efficiency of OPVs is limited to around 5%, but manufacturers such as Konarka have claimed that improved materials and device architectures will increase that figure to 20% by 2015 – which is comparable to that of silicon solar cells. But he said that widespread adoption of OPVs would not happen until the cost per watt approaches that of conventional power generation systems.

That theme was picked up by James Dietz of Plextronics, a US-based manufacturer of OPV materials. He said that falling prices for OPV systems, combined with rapidly increasing rates for electricity producing using conventional means, would enable OPVs to reach grid parity in around 2015. "Analysts predict that demand for OPVs will outstrip supply once grid parity is reached," he said.

In the meantime, said Dietz, OPVs will be limited to niche applications. One key market lies in micro-powered consumer devices, for example to reduce the battery power required in toys and handheld devices. OPVs are particularly suited to this type of power scavenging application, since they maintain good efficiency at low light levels – such as you might find indoors.

Finally, both Dietz and Cruikshank issued a warning about the accuracy of efficiency and lifetime measurements claimed by OPV manufacturers. The methodology for taking these measurements has not yet been standardized, which can lead to some misleading figures. According to Dietz, Plextronics and other companies are now working with the National Renewable Energy Laboratory (NREL) – which currently runs a certification programme for OPV measurements – to define future standards.

At Wednesday's plenary session, the OSA recognized the achievements of key optical scientists and engineers in its annual awards ceremony.

A popular winner was Ursula Keller of ETH Zurich, who was presented the Joseph Fraunhofer award for her work on ultrafast lasers, notably semiconductor saturable absorber modelocking. We have already reported on Keller's latest research into integrated modelocked VECSELs, and she clearly felt honoured to receive such an award. She pointed out that her two sons, aged 9 and 11, were in the audience "to see why Mommy is away all the time".

In her acceptance speech, Keller urged women scientists to continue to do good science, but also to remember to have a family. "You can do it," she said. "It may be tough at times, but you just need to try harder."

Other winners were:

* Eric Mazur of Harvard University was awarded the Esther Hoffman Beller Medal, which is presented for contributions to optics education. Mazur won the award for developing a teaching methodology known as "peer instruction", which promotes deeper understanding of the fundamentals of science.

* Kam Y Lau, professor emeritus at the University of California, Berkeley, was awarded the Nick Holonak Jr. Award for his work on high-speed direct modulation of semiconductor lasers through enhanced differential gain.

* Robert R Alfano of the City College of New York won the Charles H Townes award for his contributions to the discovery and investigation of supercontinuum sources, and the development of tetravalent chromium-based tunable solid-state lasers. By a happy coincidence, Alfano also celebrated his birthday today.

Finally, the 2008 Student Award, which is sponsored by New Focus, was decided on Tuesday night in a competition between seven finalists. Each finalist presented a research paper to a panel of judges, as well as to a "live" audience, and the winner was Richard Sandberg of JILA and the University of Colorado at Boulder. Sandberg receives $5000 as part of his award, while the runners up each receive $1500.

One of the most eye catching displays at this year's CLEO exhibit can be found on Paradigm Lasers' booth. Its o-Tool product offers a simple visual display that allows users to quickly and easily assess the light output from a laser source. It can be used by scientists to instantly visualize the output intensity from their light source, and by educators to demonstrate the concept of polarization.

Company president Tim Irwin told me that the o-Tool provides the same ease-of-use for optical systems as a multimeter provides for electronic systems. "Full characterization of an electrical system requires an oscilloscope, but all labs have a multimeter to quickly measure voltage and current levels," he said.

Before use, the o-Tool just needs to be plugged in and energized by shining a light at a side hole. Directing a laser at the centre of the instrument then generates a response that depends on its intensity, and an optional infrared attachment makes it possible to "see" the output from an IR source.

An additional polarization plate makes it possible to visualize the direction and intensity of a polarized light source. Irwin believes this could be a real winner in a teaching environment, since it would allow students to see the effect of waveplates and polarizing sheets on the laser output. With the basic instrument retailing for around $225, it could be a worthwhile investment.

At the CLEO exhibit today, Paul Morris of Gooch & Housego gave me a sneak preview of a prototype fibre Q-switch that is significantly smaller than an earlier device that delivers similar performance (see picture). According to Morris, the innovative device is a direct result of the integrated strategy that Gooch & Housego unveiled back in January.

The fibre Q-switch is designed to boost the output of pulsed fibre lasers operating in the 20–40 W range. According to Morris, the small size of the device was achieved in part by reducing the size of the crystal, drawing on the expertise of the company's main site in Ilminster, UK. But it also exploits fibre packaging know-how acquired from the former SIFAM business in Torquay, UK, to efficiently couple light into the crystal and so eliminates the need for bulk optics to achieve good alignment.

Morris said that Gooch & Housego is currently previewing the device to key customers in the fibre laser space. A formal launch is scheduled for Q3 – at which time a full technical spec will be released – while manufacturing of the device is slated to start by the end of the year.

The size of the market for such a fibre Q-switch is not yet clear. Manufacturers of fibre lasers either use fibre amplifiers or Q-switches to achieve pulsed operation at high powers, depending on the performance parameters needed for each application. Morris believes that the compact size of the fibre Q-switch will prove popular, while power levels below 50 W represent a sweet spot for fibre lasers to displace conventional Q-switched lasers in marking applications.

Also in the pipeline is a scalable fibre isolator technology. Isolators are needed in all fibre lasers to prevent reflected laser light from damaging the pump laser diode, but currently they are fabricated one at a time in a lab-based environment. Gooch & Housego is now working on a fibre isolator that would allow for higher throughputs, and Morris said that formal announcements are likely to be made towards the end of this year, or early in 2009 – no doubt in time for next year's Photonics West.

All-optical data processing at ultrafast speeds is the goal of researchers at Sandia National Laboratories in Alberquerque, New Mexico. In today's conference session on high-speed components, Sandia's Gordon Keeler said that the initial aim is to develop a chip-based optical platform that can perform logic operations at speeds of up to 40 GHz, but that the longer term goal is to achieve processing speeds of 100 GHz and beyond.

The critical technology in the Sandia team's research is a self-electrooptic effect device, or SEED for short. SEEDs, which were invented 25 years ago by David Miller at Bell Labs, operate as both a modulator and a high-speed photodetector. Crucially, when two of these devices are put together to form a symmetric SEED (S-SEED), they operate as a bistable device that can be switched by an optical pump.

This means that S-SEEDs can be exploited as all-optical logic gates, in particular to perform both NAND and NOR operations. And, according to Keeler, more complex logic circuits can be realized by cascading several S-SEEDs together using micro-optics.

Earlier work at Sandia demonstrated fast switching times for S-SEEDs fabricated in AlGaAs that operate at 865 nm. Now, however, the team has developed devices in a different material system that deliver similar performance at the more useful telecom wavelength of 1550 nm.

The telecom-compatible S-SEEDs are made from InAlGaAs quantum well structures grown on InP substrates. According to Keeler, the quantum wells are made shallow enough that a few picojoules of energy from a modelocked fibre laser is sufficient to switch the state of the logic gate.

Using this scheme, Keeler and his Sandia colleagues have demonstrated switching times at 1550 nm of down to 5.5 ps, which should yield an operating frequency of 40 GHz. Simulations suggest that all-optical switching in this system shouldbe possible at speeds of up to 100 GHz, but will ultimately be limited by the carrier dynamics in the system.

An intriguing notion was presented this afternoon by Max Shtein of the University of Michigan. Rather than fabricating organic light-emitting diodes as discrete planar devices, Shtein has shown how organic emitters can be formed on 3D structures such as cylindrical fibres and micron-scale cantilevers for use in atomic force microscopy (AFM).

Once you think about it, the idea is obvious: organic materials have been fêted for their flexible nature, and the weak van der Waals forces that bind organic molecules together are weak enough for the organic layers that make up an OLED to conform to non-planar substrates.

The problem, says Shtein, is the need in conventional OLED structures for transparent electrodes. Indium tin oxide (ITO) is the usual choice, but remains an expensive option for large-area deposition. And ceramic materials that also combine optical transparency with good electrical conductivity must be deposited at high temperature – which increases the fabrication cost – and are too brittle to be used on curved substrates.

The solution for Shtein and his team is to replace the transparent electrodes either side of the organic layers with thin metal films. Tests on a planar metal–organic–metal (MOM) structure emit light efficiently, but the spectral content varies with viewing angle. In contrast, forming the MOM structure on a cylindrical fibre substrate yields uniform colour output at all viewing angles.

More intriguing still is the idea of depositing OLED structures onto an AFM tip. This enables localized current injection at the vertex of the tip – largely because the sharpness of the tip concentrates the electric field and so increases the current density – which leads to controlled light emission at the vertex of the tip.

What's more, says Shtein, the fabrication process is scalable, and so could allow OLED structures to be deposited onto an array of AFM tips in a single process run. "The ability to fabricate nanoscale light sources on commercial AFM cantilevers could enable new applications in high-resolution optical scanning microscopy, as well as nanoscale optical and chemical sensing," he said.

The first fibre-based laser to deliver picosecond pulses at high average power levels took pride of place when I visited Coherent's booth this morning. According to Matthias Schulze, Coherent's director for product marketing, the Talisker laser is targeted at industrial micromachining applications where high precision and high throughput are critical for success.

Schulze told me that nanosecond Q-switched lasers operating at ultraviolet wavelengths have become the industry standard for machining materials such as silicon at the micron scale. But he says that in some cases the Talisker laser could offer the same performance at infrared wavelengths, which would eliminate the need for costly UV beam delivery systems.

Indeed, the Talisker laser can switch operation between three wavelengths. UV output at 355 nm enables ultraprecise machining with an average power output of 4 W, while the laser can also produce visible output at 532 nm and infrared at 1064 nm – in which case the average power output rises to 18 W. In each case, the pulse repetition rate is 200 kHz, which equates to a pulse width of less than 15 ps.

According to Schulze, this flexibility will allow customers to evaluate which wavelength works best for their particular application. "If infrared picosecond pulses provide the same precision as a UV nanosecond laser, Talisker would make it possible to increase throughput and lower the system cost without compromising on precision," said Schulze.

According the Schulze, the combination of ultrafast operation and high output power Talisker is only possible with a hybrid design. A fibre-based laser oscillator provides a stable and robust source of picosecond pulses, while a free-space amplifier boosts the peak power to levels some ten times greater than is possible with a fibre laser on its own.

Schulze says that the laser could be used for micromachining applications in the microelectronics, biomedical, and solar cell industries, and that customers are currently evaluating its use with a range of materials. He also mentioned that Talisker is just the first in a series of fibre-based products that Coherent will be launching to address materials processing applications at the micron scale.

* Also new for CLEO, and aimed more at the research community, are two new ultrafast amplifiers that now offer pulse widths of less that 25 fs. The Legend Elite USX-HE offers pulse energies of more than 2.5 mJ, while the Duo-USX model delivers pulse energies in excess of 5 mJ.

According to Marco Arrigoni, director of marketing for Coherent's scientific business, this is the first amplifier to achieve such short pulse widths – which are needed in research applications such as ultrafast spectroscopy, high-harmonic generation, and attosecond physics. This performance is achieved by a regenerative amplifier module combined with Coherent's BandMax technology, which ensures stable short-pulse generation.