"Photonic lantern" spectral filters in multi-core Fiber.

Fiber Bragg gratings are written across all 120 single-mode cores of a multi-core optical Fiber. The Fiber is interfaced to multimode ports by tapering it within a depressed-index glass jacket. The result is a compact multimode "photonic lantern" filter with astrophotonic applications. The tapered structure is also an effective mode scrambler.

Delivery of high energy Er:YAG pulsed laser light at 2.94 µm through a silica hollow core photonic crystal fibre.

In this paper the delivery of high power Er:YAG laser pulses through a silica hollow core photonic crystal fibre is demonstrated. The Er:YAG wavelength of 2.94 µm is well beyond the normal transmittance of bulk silica but the unique hollow core guidance allows silica to guide in this regime. We have demonstrated for the first time the ability to deliver high energy pulses through an all-silica fibre at 2.94 µm. These silica fibres are mechanically and chemically robust, biocompatible and have low sensitivity to bending. A maximum pulse energy of 14 mJ at 2.94 µm was delivered through the fibre. This, to our knowledge, is the first time a silica hollow core photonic crystal fibre has been shown to transmit 2.94 µm laser light at a fluence exceeding the thresholds required for modification (e.g. cutting and drilling) of hard biological tissue. Consequently, laser delivery systems based on these fibres have the potential for the realization of novel, minimally-invasive surgical procedures.

Stress induced birefringence in hybrid TIR/PBG guiding solid photonic crystal fibers.

We report on two types of polarization maintaining solid photonic crystal fibers that guide light by a combination of a photonic bandgap and total internal reflection. Group and phase birefringence are studied experimentally and numerically for stress-applying parts made from B-doped and F-doped silica. The stress field originating from Ge-doped cladding rods is shown to interfere with the stress field from the B-doped and F-doped rods. Since the differential expansion coefficients of B-doped and F-doped silica have opposite signs this interference is either destructive or constructive. Consequently, we found that the fiber with F-doped stress applying parts has the highest modal phase birefringence, and polarization cross talk is characterized by an h-parameter below 310(-5) m(-1).

Compact and portable multiline UV and visible Raman lasers in hydrogen-filled HC-PCF.

We report on the realization of compact UV visible multiline Raman lasers based on two types of hydrogen-filled hollow-core photonic crystal fiber. The first, with a large pitch Kagome lattice structure, offers a broad spectral coverage from near IR through to the much sought after yellow, deep-blue and UV, whereas the other, based on photonic bandgap guidance, presents a pump conversion concentrated in the visible region. The high Raman efficiency achieved through these fibers allows for compact, portable diode-pumped solid-state lasers to be used as pumps. Each discrete component of this laser system exhibits a spectral density several orders of magnitude larger than what is achieved with supercontinuum sources and a narrow linewidth, making it an ideal candidate for forensics and biomedical applications.

Ultrashort pulse compression and delivery in a hollow-core photonic crystal fiber at 540 nm wavelength.

We have fabricated a bandgap-guiding hollow-core photonic crystal fiber (PCF) capable of transmitting and compressing ultrashort pulses in the green spectral region around 532 nm. When propagating subpicosecond pulses through 1 m of this fiber, we have observed soliton-effect temporal compression by up to a factor of 3 to around 100 fs. This reduces the wavelength at which soliton effects have been observed in hollow-core PCF by over 200 nm. We have used the pulses delivered at the output of the fiber to machine micrometer-scale features in copper.

7-cell core hollow-core photonic crystal fibers with low loss in the spectral region around 2 microm.

Several 7 cell core hollow-core photonic crystal fibers with bandgaps in the spectral range of 1.4 microm to 2.3 microm have been fabricated. The transmission loss follows the approximately lambda(-3) dependency previously reported, with a minimum measured loss of 9.5 dB/km at 1.99 microm. One fiber with a transmission loss of 26 dB/km at 2.3 microm is reported, which is significantly lower than the transmission loss of solid silica fibers at this wavelength.

Design of low-loss and highly birefringent hollow-core photonic crystal fiber.

A practical hollow-core photonic crystal fiber design suitable for attaining low-loss propagation is analyzed. The geometry involves a number of localized elliptical features positioned on the glass ring that surrounds the air core and separates the core and cladding regions. The size of each feature is tuned so that the composite core-surround geometry is antiresonant within the cladding band gap, thus minimizing the guided mode field intensity both within the fiber material and at material/air interfaces. A birefringent design, which involves a 2-fold symmetric arrangement of the features on the core-surround ring, gives rise to wavelength ranges where the effective index difference between the polarization modes is larger than 10(-4). At such high birefringence levels, one of the polarization modes retains favorable field exclusion characteristics, thus enabling low-loss propagation of this polarization channel.

Excitation of cladding modes in photonic crystal fibers by flexural acoustic waves.

We report the excitation by flexural acoustic waves of an individual cladding mode in a single-mode photonic crystal fiber. The propagation constant and the field distributions of the mode have been investigated by use of this technique. The results give the basis for developing a family of acousto-optic devices based on photonic crystal fibers.

Fundamental-mode cutoff in a photonic crystal fiber with a depressed-index core.

We report a photonic crystal fiber with a depressed-index core doped with fluorine. The effective index of the cladding matches that of the core at an antiguiding wavelength, below which the fiber does not guide light at all.

Structural rocking filters in highly birefringent photonic crystal fiber.

We report what we believe is the first example of efficient rocking filter formation in polarization-maintaining photonic crystal fiber. Very high coupling efficiencies (as much as -23.5-dB suppression of the input polarization) and loss of < 0.02 dB were achieved for fibers as short as 11 mm. The filters, which we prepared by periodic mechanical twisting and heating with a scanned CO2 laser beam, are highly compact, and they are expected to be temperature stable.

Highly birefringent photonic crystal fibers.

We report a strongly anisotropic photonic crystal fiber. Twofold rotational symmetry was introduced into a single-mode fiber structure by creation of a regular array of airholes of two sizes disposed about a pure-silica core. Based on spectral measurements of the polarization mode beating, we estimate that the fiber has a beat length of approximately 0.4 mm at a wavelength of 1540 nm, in good agreement with the results of modeling.