Optical fibers are used in many fields including telecommunications, laser machining and welding, laser beam and power delivery, fiber lasers, sensors and medical diagnostics and surgery. They are typically made entirely from solid transparent materials such as glass and each fiber typically has the same cross-sectional structure along its length. The transparent material in one part (usually the middle) of the cross-section has a higher refractive index than the rest and forms an optical core within which light is guided by total internal reflection. We refer to such a fiber as a standard fiber.
Although the light is confined to the core in a standard fiber, the cladding plays an active part in the wave-guiding process because a guided mode will extend some distance into the cladding. The cladding is also important for a relatively new class of fiber devices, know as cladding-pumped fiber lasers and amplifiers. The fibers used in such devices have an inner core, in which signal light propagates single-mode, and which is doped with some active material, typically a rare earth element. The inner core is nested in a larger outer core, which is multimode at both signal and pump wavelengths. Typically, the inner core is nested off-center within the outer core, to improve the overlap between the core mode and the modes of the cladding. High-power multi-mode pump light can be introduced into the outer core with a high efficiency, and propagates down the fiber, being gradually absorbed by the rare earth element present in the inner core. The signal in the inner core is then amplified, forming an optical amplifier or, with appropriate feedback, a laser.
As most optical signals propagating through an optical fiber based network have an arbitrary polarization state, it is often preferred that optical components integrated into the optical network be substantially polarization insensitive. Planar waveguides usually have different propagation constants for TE (transverse electric) and TM (transverse magnetic) waveguide modes that are known to be polarization sensitive, that is the response of the waveguide differs for orthogonally polarized light beams. The difference in refractive index of the waveguide seen by the different polarizations of the optical signal result in a wavelength splitting of the signal. This wavelength splitting is defined as the birefringence of the waveguide.
There are a number of applications where a given amount of birefringence in an optical fiber or waveguide is desirable, among the most important being the single polarization fiber lasers as described by Mizrahi et al. in J. Lightwave Technol. 11 (12), pp. 2021-2025 (1993), where higher output powers can be realized if the gain in the resonator cavity of the laser couples into a single polarization state rather than into two states of polarization. The short cavity fiber laser as described by Mizrahi et al. is obtained by inscribing Bragg gratings directly into the core of the Erbium and Germanium-doped active fiber, to act as the laser cavity mirrors. Typically single polarization lasers require complicated design incorporating polarization maintaining fiber and bulk optics such as Faraday rotators as taught for example by MacCormack et al. in U.S. Pat. No. 6,282,016.
Another application where a given amount of waveguide birefringence is desirable is for sensing using fiber Bragg gratings as described by Kreger et al. in the Proceedings of the Optical Fiber Sensors Conference (OFS 15) 2002, pg 355-358 and taught by Schulz et al. in U.S. Pat. No. 6,600,149 where a grating written onto birefringent optical fiber generates two spectral peaks that are reflected in the absence of a load. As the grating written onto birefringent fiber is transversely loaded, the spacing between the two spectral peaks will change. This variation in spacing can be used to monitor pressure, while the simultaneous wavelength shift of the two spectral peaks can be used to monitor temperature. In this fashion, an intrinsic fiber grating sensor can be created which can simultaneously monitor temperature and pressure.
Typically, the designs of the highly birefringent fibers used in the above applications are complex, often requiring the incorporation of rods of dissimilar materials along the fiber axis in order to generate internal stresses that are local to the guiding core region. It is well know that such localized internal stress in glass fiber optics leads to optical birefringence in and around the stressed region. Birefringence in optical fiber can also be photoinduced by locally processing the optical substrate with high energy laser pulses. Small amounts of birefringence (4×10−5) can be created by UV exposure of UV photosensitive fibers cores as described by Erdogan et al. in J. Opt. Soc. Am. B, 11 (10), pp. 2100-2105, 1994 when the polarization of the UV source is normal to the waveguide axis. The birefringence can be minimized if the induced index change is symmetric about the waveguide core as described by Vengsarkar et al. in Opt. Lett. 19 (16), pp. 1260-1262, 1994.
The relatively new field of femtosecond laser processing is potentially better suited to induce birefringence in optical materials. There are a number of reports that suggest that the femtosecond induced refractive index in dielectric materials such as fused silica is intrinsically birefringent where the refractive index change along the axis of the polarization of the writing beam is larger than along the orthogonal direction, an effect associated with the creation of a periodic nanostructure as described by Bricchi et al. in Opt. Lett. 29 (1), pp. 119-121 (2004). At higher intensities, Glezer et al. in Appl. Phys. Lett. 71 (24), pp. 882-884 (1997) describe that multiphoton absorption of femtosecond pulse duration IR radiation within the glass forms a hot high-density electron plasma during the duration of the laser pulse generating temperatures up to ˜106 Kelvin, however the optical excitation of the plasma ends before the surrounding lattice structure of the glass is disturbed. A micro-explosion occurs within the glass due to of the super heated plasma that forms a void surrounded by densified material. Dürr et al. in Appl. Phys. Lett. 84 (24), pp. 4983-4985 (2004) presented tomographic measurements of Ge-doped SMF-28 telecommunication fiber cores that were exposed to femtosecond pulse duration infrared laser pulses below intensity levels needed to induce multiphoton ionization of the material. They showed that in the regions of the optical fiber exposed to the laser, increased levels of induced stress were observed. Birefringent long period gratings were also written in single mode fiber by placing the grating off center of the core of the fiber as was described by M. Dubov et al. in paper OWI50 of the Proceedings of the Optical Fiber Communications Conference, (2006). The levels of birefringence produced were ˜2×10−5.
Fiber Bragg gratings (FBG) written uniformly across the core of single mode fibers with femtosecond infrared (IR) radiation and a phase mask as taught by Mihailov et al. in U.S. Pat. Nos. 6,993,221 and 7,031,571 typically produce low levels of birefringence (˜10−5) that do not generate large polarization dependent wavelength shifts (PDW) or polarization dependent loss (PDL). FBGs written in the cores of Er:Yb-codoped phosphosilicate fiber with femtosecond lasers and the point-by-point technique were used to create a fiber laser as described by Lai et al in Opt. Lett. 31 (11), pp. 1672-1674, 2006. The FBGs were birefringent as a result of the index modulation being highly localized to one side of the fiber core. The levels of birefringence produced were ˜4×10−5.
Femtosecond exposure of the waveguide cladding region in proximity of the waveguide core could potentially generate enough birefringence to allow a photo-induced Bragg grating written in the core region to have a birefringent response. For applications such as high power, short cavity lasers, which utilize complex active fiber design architectures, the capability of locally inducing large birefringence would relax design requirements of the active fiber allowing a non-birefringent fiber design to be used instead of a specialty high birefringent fiber design. Some applications not only require the separation of orthogonal polarization but the cancellation or reduction of one of the polarization states as well. This function is usually performed by single polarization fibers that require a relatively large propagation length before a substantial cancellation of one polarization is obtained. In U.S. Pat. No. 5,511,083 for example, D'Amato et al. teach a technique for producing a single polarization fiber laser source by inducing a blazed or tapped grating within the resonator cavity of a fiber laser. The tapped grating out couples one state of polarization within the cavity through radiation mode coupling thus reducing one of the states of polarization within the laser cavity.
In U.S. Pat. No. 7,095,931 Sezerman et al. teach a technique for inducing a birefringence in an optical fiber cladding with a femtosecond laser by applying a transverse stress to the fiber during the exposure of the cladding to high intensity femtosecond pulse duration infrared radiation, the intensity being sufficient enough to induce multiphoton ionization and melting of the glass. Upon release of the stress, the memory of stress remains in the exposed region of the cladding producing a birefringence. It is our understanding that these techniques taught by Sezerman et al. have not been reduced to practice so the amounts of induced birefringence possible with this technique are unknown.
The prior art techniques of inducing birefringence in an optical fiber or waveguide serve a useful function, but in terms of UV-induced birefringence, the levels are very low unless specialty high germania doped optical fibers are used. For femtosecond IR induced birefringence by exposing a portion of the core or cladding, very high intensities are required, often by focusing regeneratively amplified femtosecond laser pulses with high numerical aperture (NA) microscope objectives to near diffraction limited focal spot sizes (a few microns in diameter) in order to produce multiphotonic absorption and multiphotonic ionization and disruption to the glass structure. The resultant levels of induced birefringence are also low and on the same order of UV laser induced birefringence (˜10−5). The induction of birefringence through exposure of the core to high intensity femtosecond infrared laser pulses that result in multiphotonic ionization of the exposed region, also generate significant loss in the waveguide due to scattering. For fiber laser applications especially those requiring short fiber laser cavity designs, this scattering loss limits the output power and performance of the fiber laser.