Research » Large-mode-area Index-Antiguided Lasers

Large-mode-area Index-Antiguided Lasers

Index-antiguided Waveguides vs. Unstable Resonators

Just like large electrical current needs large diameter copper wires to carry, high power laser needs large mode to avoid optical damage and, even more, unwanted optical nonlinearities. Many approaches have been taken and one of them is index-antiguided (IAG) fibers, first proposed by Siegman (Siegman, JOSAA 20, pp. 1617-1628, 2003). In IAG waveguides, the core has a negative index step with respect to the cladding such that all propagation modes are leaky with larger loss for higher-order modes. The idea of using IAG waveguides bears close analogy to conventional unstable resonator (Siegman, Proc. IEEE 53, pp. 277-287, 1965). Despite their diverging ray trajectories (e.g., rays 2 and 3 in Figure 1(a)), unstable resonators are known to support very stable transverse modes with large modal volume and good discrimination against high-order modes. The output modes of unstable resonators, however, have a dip at the center and suffer aperture-generated Fresnel fringes that rise from diffraction caused by the mirrors. In 1975, Dr. Lee Casperson, our co-PI on this project, has shown that unstable resonators with Gaussian-graded-reflectivity mirrors offer better mode quality than their hard-aperture counterparts (Figure 1(b)) by removing Fresnel fringes (Casperson and Lunnam, Applied Optics 14, pp. 1193-1199, 1975). He further showed that an unfolded discrete Gaussian-graded unstable resonator (Figure 1(c)), under the limit of weak lenses and closer separation between lenses, is equivalent to a waveguide with a positive quadratic gain profile and a negative quadratic index profile (Figure 1(d)). He experimentally demonstrated such a gain-guided index-antiguided (GG-IAG) waveguide laser in a high-gain Xe laser (Casperson and Aariv, Applied Physics Letters12, pp. 355-357, 1968), and confirmed that the mode size is larger than that of a non-antiguided resonator (Casperson and Aariv,  Appl. Opt. 11, pp. 462-466, 1972). The output power of the GG-IAG Xe laser, however, was very low ~ 10 uW due to verty small saturation intensity except its very high gain. It is eveident that, however, such IAG waveguide lasers can combine STM and LMA of discrete unstable resonators and superior Gaussian mode quality and ease of integration of optical waveguides.

Figure 1: Analogy among a conventional unstable resonator (a), an unstable resonator with Gaussian graded reflectivity mirror (b), an unfolded unstable resonator (c), and a gain-guided index-antiguided waveguide (d). 1, 2, and 3 are light rays with different propagation angles.
 

IAG in Photonic BandGap Waveguides (IAG-PBG)

Despite its promise, IAG waveguide lasers were found difficult to end-pumped because pump radiation is purely antiguided so it is completely trapped in the cladding (Sudesh et al., Applied Physics B 90, pp. 369-372, 2008). To mitigate this problem, we have proposed index-antiguiding in photonic bandgap (IAG-PBG) waveguides. The principle of IAG-PBG waveguides is outlined in our 1st publication (Her, Optics Express 16 (10) 7197-7202, 2008): the pump radiation falls in the stopband of the PBG and is confined in the core, even the core index is lower than the cladding; laser radiation, on the other hand, falls in the passband of the PBG such that it is leaky as if it propagates in pure index-antiguided waveguides (Figure 2(a)). A generic comparison of pump and laser radiation confinement among conventional LMA fibers, IAG fibers, and IAG-Bragg fibers is shown in Figure 2(b). IAG-PBG can be adopted in many laser platforms: it can be implemented in nearly any high-gain media (solid states, gases, or liquids), in any forms of photonic bandgap (one dimensional Bragg or two dimensional arrays), and in planar or cylindrical geometries.

Figure 2: (a) Schematic of a one-dimensional gain-guided transverse grating waveguide. The pump (blue) is confined via Bragg resonance and the signal (red) is confined by gain guiding (GG). Dashed and solid lines indicate leaky and bound rays, respectively. (b) Schematic comparison of a conventional LMA fiber, a IAG- fiber, and IAG-PBG fibers. Also shown are their schematic transverse profiles of complex refractive indexes, pump, and signal radiations.
 

Current Accomplishments

Demonstrate the principle of IAG-PBG in transverse grating waveguide slab using coupled mode theory.

  1. Tsinghua Her, “Gain-guiding in transverse grating waveguides for large modal area laser amplifiers,” Optics Express 16 (10) 7197-7202 (2008).

Rigorous and comprehensive study of gain saturation in IAG-Bragg slab waveguide amplifiers.

2.     Tsing-Hua Her, Xianyu Ao, and Lee W. Casperson, “Gain saturation in gain-guided slab waveguides with large-index antiguiding,” Optics Letters 34 (16) 2411-2413 (2009).

3.     Chaofan Wang, Tsing-Hua Her, Lei Zhao, Xianyu Ao, Lee Casperson, Chih-Hsien Lai, Hung-Chun Chang, “Gain Saturation and Output Characteristics of Index-Antiguided Planar Waveguide Amplifiers with Homogeneous Broadening,” Journal of Lightwave Technology 29, p. 1958 (2011).

Design of IAG-Bragg fibers.

  1. Xianyu Ao, Tsing-Hua Her, and Lee W. Casperson, “Gain guiding in large-core Bragg fibers,” Optics Express 17 (25) 22666-22672 (2009).

Semi-analytical model of IAG-Bragg slab laser oscillators.

  1. Chaofan Wang, Tsing-Hua Her, Lee Casperson, “Power characteristics of homogeneously broadened index-antiguided waveguide lasers,” submitted to  Optics Letters (2011).

Funding acknowledgements

This project gratefully acknowledges the financial support from ARO (W911NF-05-1-0517), DRAPA Young Faculty Award (HR0011-08-1-0065), and NSF (0925992).