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02 Nov 2004
French scientists have developed a low-cost method for growing an efficient microlens at the end of optical fibre. James Tyrrell reports on the road to commercialization.
From Opto & Laser Europe November 2004
Researchers in France have come up with a low-cost way of fabricating custom-shaped polymer tips to enhance the light-gathering capability of optical fibre. Triggered by low-power laser light, the small tip grows inside a drop of photosensitive liquid deposited at the end of an optical fibre. The tip, which behaves like a microlens, can dramatically boost the fibre's coupling efficiency to optical components such as laser diodes, or act as a low-loss microscope probe.
Research director Pascal Royer and his colleague Renaud Bachelot, based at the Laboratoire de Nanotechnologie et d'Instrumentation Optique (Universit? de Technologie de Troyes, France), hit upon the idea while working with a team of Centre National de la Recherche Scientifique (CNRS) photo-chemists based in Mulhouse, France. The research has led to the formation of a company - LovaLite - which opened for business in October.
Tip growth process
Bachelot and his colleagues follow a simple process to produce their tips. Firstly, they cleave the fibre, wash it in acetone (to remove any dust and organic waste) and then check its optical properties. Next, using a pipette, they deposit a drop of photosensitive liquid formulation at the end of the fibre.
This photosensitive formulation contains, among other things, a sensitizer dye (eosin) and an acrylate monomer. When the eosin absorbs laser light it promotes the release of radicals which initiate polymerization of the monomer. The system is particularly sensitive to visible light in the 450-550 nm range, which means that the process can be driven by an argon laser (514nm) or the green line (542nm) of a He-Ne laser.
The shape of the drop and the tip can be controlled by adjusting the composition and viscosity of the formulation. The scientists are able to manipulate the drop's radius of curvature simply by raising or lowering the temperature to change the viscosity.
Exposing the formulation to a pulse of green laser light that is guided along the core to the end of the fibre (typically 2s in duration) initiates photopolymerization and creates a robust polymer tip within the drop.
Sometimes the team can actually see the tip growing. "It is very beautiful, we can visualize the growth of the tip by observing the yellow fluorescence from the eosin," Bachelot told OLE. "For example, in the case of high eosin concentrations, this speed is quite slow and we can see the tip growing in real time. Sometimes we can guess if a tip will be good or not simply by observing the fluorescence."
The final stage in the process is to wash the drop with methanol to remove any unpolymerized material from the tip. The team typically grows tips that measure between 15 and 150?m in length and have a radius of curvature from around 0.2 to 2?m. They offer transmission greater than 80% and a polarization dependent loss of less than 0.1dB.
Encouraged by their first results, Bachelot and his colleagues went on to study the process in detail. "The first point is that tip growth relies on the growth of a waveguide," explained Bachelot. "As the light propagates in the drop of formulation, the refractive index is increased [from 1.48 to 1.52] by photopolymerization."
The team noticed that instead of diverging, the tip was tending to converge. Hoping to illustrate the effect more clearly, they dipped the end of a singlemode fibre into a thick layer of formulation. They discovered that sending a long pulse of laser light down the fibre produced a thin probe-like tip 500?m in length. Light was being self-guided through the solution.
Another key player in the process turned out to be oxygen. Photopolymerization begins only when absorbed energy is greater than a threshold value - Eth - which increases in the presence of oxygen. This means that photopolymerization at the boundary between the air and the drop of photosensitive formulation is very selective. For short exposure times, only the centre of the laser's Gaussian beam is able to trigger the polymerization, which produces a sharp tip. Flatter tips with a radius corresponding to the geometry of the drop require a longer dose of light.
The French team is currently applying its technology in three areas, two of which involve using the tip as a probe for optical scanning microscopy. If operated in the far-field domain, the tip acts as a microlens for illuminating or collecting light from the sample surface. Using their tips, Bachelot and his colleagues have demonstrated an imaging resolution of around λ/2.
For near-field microscopy, which offers much higher resolution (in Bachelot's case around λ/20), the tips have to be modified as currently their radius limit is around 250nm. According to Bachelot, the simplest method, known as the shadow effect, is to metallize a rotating tip from the side. Using this approach, the French scientists have succeeded in creating a 100nm-wide optical aperture at the extremity of the tip.
Typically, near-field probes are made by tapering optical fibres. Because the taper can be very small, around the cut-off diameter, these probes often act as poor lightguides. "If they launch 1mW at the extremity of the fibre, at the other extremity where the hole is, they get only 1?W," said Bachelot. He added: "If they want to increase the launch power, they can destroy the tip end because light is absorbed by the metal film [destroying the aperture]. In our case, there is no taper as the light is guided. For a 100nm aperture, we observe a transmission in the range of 5-10%. This is huge."
The microlens used in far-field microscopy also functions as a very effective tool for coupling optical components to fibres. Recently, Bachelot and his co-workers reported that they had used their tip technology to couple 70% of the output from a 9.5mW laser diode (1310nm) into an optical fibre (Optics Letters 29 1971). The maximum coupled output between the 15?m long tip at the end of a 9?m core-diameter fibre was found to be 6.7mW for an optimal tip-laser distance of 4?m. By comparison, when the same experiment was carried out with a bare cleaved fibre, the coupled power was less than 1.5mW. Bachelot believes that it would also be possible to couple tips to other optical components such as photonic crystal structures and integrated waveguides.
Although the team has concentrated on making single-peaked tips, it is now considering other options. It has recently managed to produce multi-peaked tips on multimode fibre by applying mechanical strain to the fibre during photopolymerization. This selectively excites linearly polarized modes within the fibre. The multi-peaked tip is a three-dimensional mould of the intensity distribution within the fibre. This could potentially allow a new format of optical communication in which distinct modes carry information rather than wavelengths.
Initially, Royer and Bachelot preferred to explore their ideas from behind the university's closed doors. Now that several patent applications have been filed, the team is keen to test its discovery in the marketplace. French law sets strict limits on the commercial activities of university staff and so Royer and Bachelot have hired a full-time director to lead LovaLite. They appointed Brahim Dahmani, a former CNRS scientist who has spent the past 15 years working for Corning, along with an engineer and a technician. The company is located near to the university in Technopole de l'Aube, a science park funded by the Champagne-Ardenne region that specializes in incubating hi-tech start-ups.
LovaLite licence
The Universit? de Technologie de Troyes has granted LovaLite an exclusive licence to commercialize the polymer tip technology. "The university will still pursue development, improving the technology under contract, and will get funding from LovaLite for that," Dahmani told OLE.
Together with private investors, LovaLite has so far managed to raise €400,000 in funds. This includes a €230,000 development prize awarded to LovaLite at France's National Contest for Business Creation in Innovative Technologies in July. The result is a good omen, as an impressive 94% of the 600 companies funded by the contest over the past six years are still in business.
LovaLite has already caught the attention of big names like Veeco. "In the microscopy market, we are currently in talks with most of the near-field companies," confirmed Dahmani. "In the photonic segment, we are beginning to identify potential partners."
Dahmani is confident that LovaLite's tips offer a significant performance benefit to customers. "The main advantage of our technology is that it is reproducible, because it depends on light coupled to the components at the end of the fibre," he said. Also, as Bachelot points out, the technology is very cheap - an important selling point in terms of commercializing the technology.
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