Diamond-like film-encapsulated fibers enable long-length grating production

Moses M. David and James F. Brennan, III 3M

Several researchers have been perfecting the fabrication of fiber Bragg gratings that are several meters in length.1-3 The application of these devices in optical telecommunications systems is imminent. A great deal of theoretical and experimental work has been done to understand the optical performance of these devices, but until recently, little attention has been given to maintaining the mechanical integrity of the gratings. Before they are seriously considered for use in communications systems, long fiber gratings must pass rigorous mechanical reliability tests and be easily manufactured.

In standard fiber Bragg grating production, the protective coating on a fiber must be removed before a grating can be written. Afterward, the fiber is immediately recoated to preserve its mechanical strength. The process of stripping and recoating a fiber is time-consuming and tedious, requiring skilled technicians. Additionally, the bare fiber is prone to damage during grating inscription, which reduces production yields. These problems are exacerbated when long lengths of fiber are stripped, handled, and recoated.

Several solutions have been proposed to eliminate these process steps. Most of the solutions involve writing gratings through specialized, UV-transparent polymeric fiber coatings.4-6 Writing gratings through a fiber coating may be a solution for short-length (less than a few centimeters) fiber Bragg grating production, but to be used in long-length grating production, a write-through coating must meet demanding physical and optical requirements.7

A process has recently been developed to deposit a UV-transparent, diamond-like film onto fiber to maintain the mechanical integrity of the fiber close to that of pristine fiber. By depositing a thin film on the fiber (~1-5 microns thick), fiber geometry such as concentricity and diameter is preserved to the degree necessary to produce long fiber Bragg gratings. This diamond-like film can also withstand the temperatures used to anneal fiber gratings.

Carbon films contain substantially two types of carbon-carbon bonds: trigonal graphite bonds (sp2) and tetrahedral diamond bonds (sp3). Diamond is composed almost entirely of tetrahedral bonds; diamond-like film, approximately 50% to 90% tetrahedral bonds; and graphite, nearly all trigonal bonds. The crystallinity and nature of the bonding of the carbon network can be manipulated to affect the physical and chemical properties of a diamond-like film. Diamond is crystalline and essentially pure carbon, whereas diamond-like film is amorphous and can contain a substantial amount of additional components.

Figure 1. Optical transmission through 1-micron-thick layers of diamond-like films as a function of oxygen flow rate during deposition.

The composition of diamond-like films can be tailored so they have desirable diamond-like optical and mechanical properties such as extreme hardness (1,000 to 2,000 kg/mm2 Vickers). Diamond's optical bandgap is 5.56 eV (~223 nm) and is therefore transmissive in the ultraviolet, but the polycrystalline structure of diamond films causes light scattering from grain boundaries. Diamond-like films are amorphous and don't suffer from light scattering at grain boundaries. The carbon-carbon double-bonding prevalent in diamond-like film causes undesirable optical absorption below ~500 nm.

By incorporating silicon and oxygen atoms into the amorphous diamond-like carbon film during the deposition process, absorption is altered. For example, diamond-like film of 1 micron thick was deposited on quartz slides as the oxygen flow rate during deposition was varied, and the optical transmissions through these slides were measured (see Figure 1).

Figure 2. Tauc plots of diamond-like films made with varying oxygen flow rates.

In the visible wavelength region, the optical transmission is close to 100% for all diamond-like films, irrespective of the chemistry. In the UV region, diamond-like films' transmission drops dramatically when the oxygen flow falls below 10 sccm (standard cubic centimeters per minute). The optical transmission of the 10-sccm film drops below 30% at 244 nm, a commonly used wavelength for writing Bragg gratings. Under certain deposition conditions, a 1-micron-thick layer of diamond-like film is ~98% transmissive at 244 nm. The oscillations in the transmission spectra are due to thin-film interference.

The electronic-absorption edges in diamond-like films follow the Tauc relation:

where G is a constant, E0 the optical gap, E the photon energy, and a the absorption coefficient.8 The optical gap is taken from the extrapolation of the linear portion of the curves toward the abscissa. Tauc plots were calculated from data like that shown in Figure 1, and the optical gap was seen to change with oxygen content from ~4.1 to ~4.8 eV (see Figure 2). The chemical composition of the diamond-like films changed monotonically, even though abrupt changes occurred in their optical properties.

Figure 3. Schematic of chamber for depositing diamond-like film onto optical fiber. After it is evacuated with the pumps (RP1, RP2), power is applied at radio frequencies to the electrodes.

To deposit diamond-like film onto fibers, a capacitively coupled reactor system is used with two electrodes in a reaction chamber (see Figure 3). During deposition, the chamber is partially evacuated, and power at radio frequencies is applied to the electrodes. A carbon-containing source is introduced between the electrodes to form plasma comprising reactive species in proximity to the electrodes as well as form an ion sheath proximate to the electrodes. A fiber-handling apparatus is also used, where a fiber is stripped of its acrylate coating with a continuous-flow acid bath before it enters the reaction chamber and is drawn through the reactive ion sheath at a controlled speed. A uniform thickness film of ~5 microns was deposited onto the fiber, as shown in the micrograph in Figure 4. The variations in film thickness along the fiber were immeasurable but are estimated to be <10 nm.

Figure 4. Micrograph of optical fiber protected with ~5-micron-thick diamond-like film.

The mechanical properties of diamond-like film-encapsulated fibers were compared with those of acrylate-coated fibers and uncoated fibers. The diamond-like film-encapsulated fibers were insensitive to handling, such as by wiping the fibers between fingers or wrapping them on mandrels. In contrast, stripped fibers easily broke under such handling. As shown in Figure 5, the probability of fracture for both the acrylate-coated and diamond-like-film fibers were similar and occurred at a much higher tensile force than that for uncoated fibers.

Figure 5. Breaking strength after handling optical fibers with diamond-like-film protection, acrylate coatings, and no coating.

Gratings were fabricated in the diamond-like film-encapsulated fiber with a frequency-doubled Ar+ laser (244 nm) and compared to gratings written in standard fibers under identical conditions. As shown in Figure 6, there was no significant difference in the quality or reflectivity between the gratings. Visual inspection under a microscope discerned no signs of physical damage to the fiber surface. Long-length gratings (>10 cm) were fabricated in these fibers with a previously detailed method2, and there was no noticeable degradation in grating quality compared to gratings written in bare fiber. Any deviations that may have been caused by the diamond-like film are within the current limitations of the long-length fiber Bragg grating fabrication system. The mechanical strengths of these long-length gratings were then tested. Results similar to those in Figure 4 were obtained. These fibers were annealed at 100°C for 18 hours, and no mechanical strength degradation was observed.

Figure 6. Transmission spectra through fiber Bragg gratings written in uncoated and diamond-like film-protected fibers.

These findings present significant opportunity for the fabrication of long-length fiber Bragg gratings that lend themselves to high-yield manufacturing processes. Critical to the process is this UV-transparent film, which protects the fiber during fiber grating inscription and maintains the geometry of the fiber during fabrication.

By developing a process to deposit a uniform layer of diamond-like film onto optical fibers, many of the major process and handling obstacles have been overcome. This significant new process opens up opportunity for further applications of longer-length fiber Bragg gratings throughout the optical network.

H. Rourke, et al., BGPP '99, ThD1.
J. Brennan and D. LaBrake, BGPP '99, ThD2.
L. Garrett, et al., OFC '99, PD15.
R. Espindola, et al., OFC '97, PD4.
K. Imamura, et al., Electron. Lett., 1998, 34 (10), 1016-7.
L. Chao and L. Reekie, OFC '99, ThD5.
M. Ibsen and R Laming, OFC '99, FA1.
J. Mort and F. Jansen, "Plasma Deposited Thin Films" (CRC Press, Boca Raton, 1986), 111-4.

Moses David, Ph.D., is an advanced research specialist at 3M (St. Paul, MN), where he specializes in plasma processing, diamond-like films, and thin-film optics. James F. Brennan, III, Ph.D., is a research specialist for 3M Telecom Systems Division (Austin, TX), where he works on the fabrication of long-length gratings for dispersion compensation.