All fiber lasers to date emit radiation only along the fiber axis. Here a fiber that exhibits laser emission that is radially directed from its circumferential surface is demonstrated. A unique and controlled azimuthally anisotropic optical wave front results from the interplay between a cylindrical photonic bandgap fiber resonator, anisotropic organic dye gain, and a linearly polarized axial pump. Low threshold (86nJ) lasing at nine different wavelengths is demonstrated throughout the visible and near-infrared spectra. We also report the experimental realization of unprecedented layer thicknesses of 29.5 nm maintained throughout meter-long fibers. Such a device may have interesting medical applications ranging from photodynamic therapy to in vivo molecular imaging, as well as textile fabric displays.
© 2006 Optical Society of America
The great variety of optical cavities and their coupling to gain media has made lasers pervasive in science and technology. In particular, fiber lasers have recently been the focus of much interest [1–3]. A common feature of all fiber lasers is the emission along the fiber axis from an area that is small when compared to the total cavity surface and with nearly planar wave front. Here we report on the design, fabrication and characterization of a photonic band gap fiber laser that emits radiation in the plane transverse to the fiber axis from an extended surface area. Furthermore, the unique wave front is azimuthally anisotropic with a radiation pattern resembles an optical dipole. This is due to the interaction between a cylindrical resonator, anisotropic gain, and a linearly polarized axial pump beam. The cylindrical fiber-cavity structure consists of a hollow-core multilayer  photonic band gap fiber, and an organic dye gain medium that is introduced into the core. The complete photonic band gap provides the longitudinal confinement of the higher-frequency optical pump and at the same allows light to travel along the fiber at steep angles (with respect to the fiber axis) not afforded by conventional index-guiding fibers. Lasing at 9 different wavelengths is demonstrated throughout the visible and near-infrared spectra. We also report the experimental realization of unprecedented layer thicknesses of 29.5 nm maintained throughout meter-long fibers. Such a device may have interesting medical applications ranging from photodynamic therapy  to in vivo molecular imaging , as well as textile fabric displays .
2. Fiber laser structure
The surface-emitting fiber laser structure and pumping arrangement are shown schematically in Fig. 1. The structure comprises a gain medium in the core surrounded by a photonic band gap (PBG) structure [8–11] made of 58 layers of a wide mobility gap amorphous semiconductor, As2S3, alternating with a high glass-transition temperature polymer, poly(etherimide) (PEI). A scanning electron microscope micrograph of the multilayer structure [Fig. 2(a)] demonstrates the uniformity of the layer thicknesses throughout the fiber. The individual layer thicknesses of As2S3 and PEI are 59 nm and 89 nm, respectively, and the structure is terminated by a 29.5-nm thick layer of As2S3 to eliminate surface modes. The gain medium is pumped axially while the resonant cavity provided by the PBG ensures laser emission in the radial direction. The PBG structure performs a dual role enabled by the characteristic shift of the band edges to higher frequencies with increase in wave vector as depicted in Fig. 2(b). The normal-incidence band gap, defined for axial wave vector k=0 (region A), provides the optical feedback necessary for emitting laser light from the whole surface area in the radial direction. Concurrently, the blue-shifted band gap having axial wave vectors near the light line (region B) is responsible for guiding the pump frequency.
While different types of gain media may be used in conjunction with the fiber, for convenience we chose an organic laser dye  incorporated into a copolymer matrix. The upper inset in Fig. 2(a) is a fluorescence micrograph of the fiber cross section (see Appendix A). An organic dye, LDS698, having a fluorescence peak at 645 nm was dispersed in a copolymer and inserted into the otherwise hollow core of a PBG fiber. Since the normal-incidence PBG is 26% of its centre-frequency, it encompasses the entire fluorescence spectrum as shown in Fig. 2(c) (see Appendix B). This same fiber supports the propagation of a pulsed optical pump at 532 nm traveling through the fiber core.
3. Optical properties
We observe broad fluorescence emission from the above described fiber laser at pump-pulse energies lower than the 86 nJ threshold, while radially directed lasing occurs with sharp peaks at 652 nm above threshold [Fig. 3(a)]. To confirm that the emitted radiation is indeed laser light and not amplified spontaneous emission, we show in Fig. 3a the emission spectra of the fiber for three different pump energies: below (A), near (B) and above (C) threshold. The lasing threshold occurs at pump energies of 86 nJ and 110 nJ for dye concentration of 500 ppm and 50ppm, respectively. The dependence of the emission bandwidth (Fig. 3a inset) and energy [Fig. 3(b)] on the pump energy for both 500 ppm and 50 ppm dye concentrations are shown. Both clearly demonstrate laser thresholds. The slope efficiencies are 37.5% and 16.5% for the 500 ppm and 50 ppm concentrations, respectively.
The PBG fiber (core diameter dc = 70 μm) supports many longitudinal cavity modes in the transverse plane having a free spectral range Δλ ≈ λ0/2ndc, where n is the core refractive index and λ0 is the lasing centre wavelength. The lasing spectrum was resolved into its modes, as shown in Fig. 3c, using an optical spectrum analyzer (ANDO AQ6317), and the 2-nm mode spacing (corresponding to a 68-μm core diameter) is in good agreement with the expected value. The measured quality factor Q = λ0/δλ (δλ is the spectral width of one mode) of 640 is lower than theoretically expected. Possible reasons for this discrepancy are the losses arising from an imperfect cavity structure and the limited spectral resolution of the measurement setup.
The optical wave front emanating from the fiber laser has several unique characteristics that stem from the combination of the emission properties of the dye and the resonant cavity design. First, the emitted laser wave front has a dipole-like radiation pattern, shown in Fig. 4a. This result was obtained in two different ways. First the orientation of the pump polarization at the input to the fiber was rotated while a probe recorded the emitted intensity in the x-polarization at a fixed location along the y-axis. The measurement was then corroborated by physically rotating the probe around the surface of the fiber laser while keeping the pump polarization fixed in the x-direction. Upon comparing the radiation pattern to that of a bulk dye-doped copolymer excited with the same pump, we find the dipole-like radiation pattern is not as pronounced as in the fiber laser. These results can be understood by noting that the polarization of the dye fluorescence is determined mainly by the pump polarization and the relative orientation of the transition moments in the dye molecule for the absorption and emission transitions . The polarized dye molecules that are aligned with the pump polarization contribute the most to the fluorescence. This may be confirmed by analyzing the dye emission with a linear polarizer rotated in the x-z plane, normal to the direction of maximum emission (y-axis), with a fixed x-polarized pump, as shown in Fig. 4(b) (dotted line). The dipole radiation patterns of the dye molecules combine to result in the radiation pattern shown in Fig. 4(a) where the strongest radiation is in the direction orthogonal to the pump polarization and the fiber axis. Since fluorescence polarized parallel to the pump is stronger, cavity modes with this polarization have lower thresholds. Consequently, the fiber laser has an enhanced polarization component parallel to the pump as compared to that of the bulk dye emission (Fig. 4(b), solid line) and a more prominent dipole-like radiation pattern.
These interesting results suggests that the direction of the laser beam can be controlled remotely just by rotating the pump polarization.
A second unique feature of this laser is that emission occurs over a spatially extended region by virtue of the extended surface area of the fiber resonator walls. This is in contrast to semiconductor [13, 14] and polymer  planar annular resonators in which the resonator thickness is on the order of the emission wavelength. Figure 4(c) shows the emission spectrum as a function of position along a large-core (dc = 200 μm) PBG fiber. By moving a probe along its axis we observed laser radiation extending along ~ 5 mm of the fiber (measured at full-width-half-maximum). The upper panel of Fig. 4c shows a photograph of this operating laser. One may further increase the surface area from which laser light is emitted by optimizing dye concentration, core size, and PBG structure.
The dual action of the PBG structure as both a transverse laser cavity and a transmission waveguide is highlighted in Fig. 5(a). In this specific fiber, a short segment of Rhodamine 590 doped copolymer was introduced into a PBG fiber, leaving the rest of the core hollow. The photograph of the bent fiber displays both features: the hollow-core portion of the fiber transmits the pump light (green, λ = 532 nm, top of the photograph), and the dye-doped portion emits orange-colored laser light (λ = 576 nm).
Furthermore, the placement of dye-doped segments along a fiber can be carefully controlled. A demonstration that highlights the ability to finely tune segment size, location, and composition is shown in Fig. 5(b). A lasing display projects the letters “MIT” in two colors. All the lasing fibers contain copolymer segments doped with DCM (orange). Additionally, the “i”-fiber contains both DCM and LDS698 (red) demonstrating that more than one gain medium can be precisely placed in the same fiber. This specific display contains 12 dye-filled fibers that are pumped from both ends. The large segments contain less than 0.5 μl of dye-doped polymer.
Finally, the use of dyes with fluorescence spectra that extend over the visible and near-IR wavelengths is made possible by simply scaling the PBG structure and hence shifting the band gap. Nine different dyes were inserted into separate fibers having band gaps matched to their emission peaks. The lasing spectra of these fibers are displayed in Fig. 5(c), and photographs of three lasers are shown depicting bright blue, green and red laser light. Moreover, the fiber’s photonic band gap is readily scalable from the UV  to the IR [8, 10].
This new polymer surface-emitting fiber laser offers unique control over the direction and polarization of the lasing wave front, is inherently wavelength scalable, and can be used for the remote delivery of radial laser emission. The ability to control the gain medium location, spatial extent, and concentration coupled with the fiber’s mechanical flexibility paves the way for lasing textile fabrics or even 3D laser-light-emitting structures.
Appendix A: Fiber Preparation
The hollow-core PBG fiber preform was fabricated by thermal evaporation of an As2S3 layer (5 μm) on both sides of a free-standing 8-μm-thick PEI film and the subsequent rolling of the coated film into a hollow multilayer tube. This hollow macroscopic preform with a thick protective outer layer of PEI was consolidated by heating under vacuum at ~ 260 °C and was then drawn in a fiber draw tower into hundreds of meters of fiber at ~ 305 °C. The SEM samples were prepared with JEOL Cross Section Polisher (CP) SM-09010 and the micrographs produced with JEOL JSM 7000F.
Mixed solutions of methyl methacrylate (MMA) and 2-hydroxyethyl methacrylate (HEMA) monomers containing t-butyl peroxide (tBP) or azobisisobutyronitrile (AIBN), n-butyl mercaptan, and organic dyes (0.05-0.5 wt%) were prepared and inserted into the hollow core PBG fibers. The fibers were placed in an oven at either 90 °C (tBP) or 60 oC (AIBN) for 20 hours for polymerization. All dyes were obtained from Exciton, Inc. The dyes used to produce the laser emissions shown in Fig. 5(a) are as follows: (1), 0.5 wt% coumarin 503; (2), 0.5 wt% coumarin 500; (3), 0.5 wt% coumarin 540A; (4), 0.1 wt% rhodamine 590; (5), 0.1 wt% DCM; (6), 0.1 wt% LD698; (7), oxazine 720; (8), 0.1 wt% LD700; (9), 0.1 wt% oxazine 725.
Appendix B: Optical Measurements
The reflection spectra of the hollow-core fibers were measured using a xenon lamp (Oriel 68840-M). The light was collimated using a 5-cm focal-length lens, directed through a beam splitter, and focused onto the fiber outer surface using a ×10 microscope objective lens. The light reflected back from the multilayer structure was collimated by the objective lens and directed by the beam-splitter to a spectrometer (Ocean Optics HR2000CG-UV-NI) and then normalized with respect to the spectrum of the lamp.
The optical pump for the fiber lasers was a linearly polarized, pulsed Nd:YAG laser (Continuum Minilite II) with nominal pulse durations of 9 ns and repetition rate of 10 Hz. Both the second (532nm) and third (355nm) harmonics were utilized as pumps in accordance with the dye’s fluorescence. The pump beam was spatially filtered by a 500-μm pinhole, a small percentage of the energy was directed away by a beam splitter to monitor the pump energy, a half-wavelength plate controlled the pump polarization, and a one-inch focal-length lens coupled the pump into the fiber core. The pump input energy was measured using an energy meter (Coherent PM1000, J4-09 and J3S-10). The energy of the resulting laser light emitted from the fiber laser was collected by an integrating sphere (Sphere Optics) and measured using the same energy meter with a high-pass filter mounted in front to eliminate any pump signal. The pump energy was adjusted using a variable optical attenuator. The emission spectra of the generated laser light were measured with the spectrometer after being collected by a 600-μm-diameter multimode fiber probe. The polarization measurements of both the bulk dye and the fiber were made using a linear sheet polarizer placed between the sample and the spectrometer.
We thank S. D. Hart, Y. Kuriki, R. Dasari, M. Feld, J. Gardecki, F. Sorin, P. Bermel and JEOL Ltd. Japan for assistance. N. Orf gratefully acknowledges support though the National Defense Science and Engineering Graduate Fellowship program. This work was supported by US DOE DE-FG02-99ER45778 , DARPA/Christodoulou, the ARO through the ISN (contract number DAAD-19-02-D-0002), the HEL-MURI through AFOSR (contract number Y77011) and the NSF ECS - 0123460. This work was also supported in part by the MRSEC Program of the National Science Foundation under award number DMR 02-13282. This work made use of the G.R. Harrison Spectroscopy Laboratory’s laser facilities supported by the National Science Foundation (0111370-CHE).
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