Mid-infrared lasers: Challenges imposed by the population dynamics of the gain system

This paper discusses the influence of the population dynamics on mid-infrared lasers. Specifically, the typically longer lifetime of the lower compared to the upper laser level has to be addressed in order to achieve highly efficient laser operation in this wavelength range. Examples of different operational regimes of the erbium-doped ZBLAN 3-μm laser are presented

Heat generation and thermal lensing in 2.8-µm Er3+:LiYF4 lasers

Finite-element calculations of the population mechanisms in diode-end-pumped 3-µm Er:YLF lasers demonstrate that interionic-upconversion-induced multiphonon relaxations significantly increase heat generation, temperature gradients, and thermal lensing. This explains rod fracture above 1 W of output power

The route toward a diode-pumped 1-W erbium 3-µm fiber laser

A rate-equation analysis of the erbium 3-um ZBLAN fiber laser is performed. The computer calculation includes the longitudinal spatial resolution of the host material. It considers ground-state bleaching, excited-state absorption (ESA), interionic processes, lifetime quenching by co-doping, and stimulated emission at 2.7 um and 850 nm. State-of-the-art technology including double-clad diode pumping is assumed in the calculation. Pump ESA is identified as the major problem of this laser. With high Er3+ concentration, suitable Pr3+ co-doping, and low pump density, ESA is avoided and a diode-pumped erbium 3-um ZBLAN laser is predicted which is capable of emitting a transversely single-mode output power of 1.0 W when pumped with 7-W incident power at 800 nm. The corresponding output intensity which is relevant for surgical applications will be in the range of 1.8 MW/cm2. Compared to Ti:sapphire-pumped cascade-lasing regimes, the proposed approach represents a strong decrease of the requirements on mirror coatings, cavity alignment, and especially pump intensity. Of the possible drawbacks investigated in the simulation, only insufficient lifetime quenching is found to have a significant influence on laser performance

Analysis of heat generation and thermal lensing in erbium 3-µm lasers

The influence of energy-transfer upconversion (ETU) between neighboring ions in the upper and lower laser levels of erbium 3-µm continuous-wave lasers on heat generation and thermal lensing is investigated. It is shown that the multiphonon relaxations following each ETU process generate significant heat dissipation in the crystal. This undesired effect is an unavoidable consequence of the efficient energy recycling by ETU in erbium 3-µm crystal lasers, but is further enhanced under nonlasing conditions. Similar mechanisms may affect future erbium 3-µm fiber lasers. In a threedimensional finite-element calculation, excitation densities, upconversion rates, heat generation, temperature profiles, and thermal lensing are calculated for a LiYF4:Er3+ 3-µm laser. In the chosen example, the fraction of the absorbed pump power converted to heat is 40% under lasing and 72% under nonlasing conditions. The heat generation in a LiYF4:Er3+ 3-um laser is 1.7 and the thermal-lens power up to 2.2 times larger than in a LiYF4:Nd3+ 1-um laser under equivalent pump conditions, thus, also putting a higher risk of rod fracture on the erbium system

Homoepitaxial growth of high-quality BaSO4:Mn6+ using low-temperature liquid phase epitaxy

Single-crystalline host materials doped with 3d1 transition-metal ions are of high interest for applications as tunable lasers. The Mn6+ ion exhibits broadband luminescence, however, Mn6+-doped crystals or waveguide structures could as yet not be grown with sufficient optical quality. The active material has to be free of defects and inclusions larger than 1/20 of the optical wavelength. In addition, the surface of the active layer and its interface to the substrate must be optically flat to receive low-loss guiding properties.\ud The structure of the barite-phase BaSO4 contains tetrahedra which are replaced partly by the dopant complexes. The BaSO4:Mn6+ growth temperature is limited by the phase transition above 1010°C and especially the noticeable reduction of Mn6+ to Mn5+/Mn4+ above 600°C. Therefore, the growth of BaSO4:Mn6+ from a solution at lower temperatures is the most suitable method. Liquid-phase growth is close to the thermodynamic equilibrium and has enabled us to grow high-quality layers.\ud First, we grew undoped BaSO4 substrate crystals of 10 x 5 x 4 mm3 in a, b, and c-direction, respectively, using the flux method with LiCl as the solvent. Subsequently, the growth of high-quality undoped BaSO4 was performed by liquid phase epitaxy (LPE), using the additive ternary CsCl-KCl-NaCl solution. We obtained flat layers free of inclusions with step heights of 1.4 nm, equal to 2 unit cells, and step distances of about 200 nm. Finally, layers of BaSO4:Mn6+ were grown on c-oriented faces with thicknesses up to 150 μm, at growth rates of 3 μm/h and growth temperatures of 500–550°C. The Mn6+ concentration in the doped layer was up to 1 mol% with respect to S6+.\ud Absorption and emission spectra were measured, which confirmed that the manganese ion was incorporated in the layer solely in its hexavalent oxidation state. Room-temperature broadband luminescence in the wavelength range 850-1600 nm was observed

Novel crystalline-waveguide broadband light sources for interferometry

In recent years, broadband fiber interferometers have become very popular as basic instruments used in optical low-coherence reflectometry for diagnostics of fiber and integrated optics devices or in optical coherence tomography (OCT) for imaging applications in the biomedical field. The longitudinal resolution of such instruments is inversely proportional to the optical bandwidth of the light source. Broadband luminescence from transition-metal-ion doped materials can significantly improve the longitudinal resolution compared to superluminescent diodes, but the low brightness of its luminescence typically leads to a low dynamic range in OCT. Femtosecond lasers based on, e.g., Ti:sapphire have, therefore, been used as large-bandwidth high-brightness light sources, and subcellular imaging has been demonstrated in this way. However, since current femtosecond light sources do not necessarily meet the requirements of compactness, ease of use, and low cost, a suitable light source for OCT is still not available. Within the past two years, we have persued a novel approach toward compact broadband light sources for interferometry. Recently, the suitability of a superluminescent Ti:sapphire crystal as a light source in the spectral region 700-1000 nm for ultrahigh-resolution OCT with ~2 µm axial resolution has been demonstrated [1]. Guiding of the fluorescence in waveguide geometry can further increase the single-mode fluorescence output powers [2]. We have successfully created rib and ridge channel-waveguide structures from Ti:sapphire planar waveguides grown by pulsed laser deposition [3] by several different methods: ion-beam implantation and subsequent wet chemical etching [4], reactive ion etching [5], Ar-ion milling [6], and refractive-index variation by proton implantation employing either direct channel writing or in combination with polyimide strip-loading [7]. The maximum fluorescence output power of ~300 µW currently obtained from such channel-waveguide structures is sufficient for ultrahigh resolution OCT imaging at low speed for many applications. With further improvements in waveguide quality and coupling efficiency to a single-mode fiber, the usable fluorescence power is expected to further increase to the mW level. The significantly improved sensitivity that will result at this fluorescence power will allow for rapid in vivo ultrahigh-resolution OCT with a simple broadband light source. Currently, we are investigating a novel approach using parallel channel waveguides as a light source for parallel OCT [8]. First results will be reported at the conference. Since our structuring methods allow to produce a channel waveguide in Ti:sapphire with a loss value comparable to its planar-waveguide counterpart, this structure may also lend itself to channel-waveguide lasing in Ti:sapphire. Furthermore, we have investigated BaSO4:Mn6+ as a novel broadband emitting material with fluorescence emission obtained in the spectral region 900-1600 nm. BaSO4:Mn6+ layers were grown by liquid phase epitaxy on undoped flux-grown BaSO4 [9]. Optical waveguide experiments are currently in progress. This material also shows a potential as a broadly tunable laser material in this wavelength range if excited by <2 µs pulses at 532 nm [10]. [1] A.M. Kowalevicz, T. Ko, I. Hartl, J.G. Fujimoto, M. Pollnau, R.P. Salathé, Opt. Express 10, 349 (2002). [2] M. Pollnau, R.P. Salathé, T. Bhutta, D.P. Shepherd, R.W. Eason, Opt. Lett. 26, 283 (2001). [3] A. A. Anderson, R. W. Eason, M. Jelinek, C. Grivas, D. Lane, K. Rogers, L. M. B. Hickey, C. Fotakis, Thin Solid Films 300, 68 (1997). [4] A. Crunteanu, G. Jänchen, P. Hoffmann, M. Pollnau, Ch. Buchal, A. Petraru, R.W. Eason, D.P. Shepherd, "Three-dimensional structuring of sapphire by sequential He+ ion-beam implantation and wet chemical etching", Appl. Phys. A, accepted. [5] A. Crunteanu, M. Pollnau, G. Jänchen, C. Hibert, P. Hoffmann, R.P. Salathé, R.W. Eason, C. Grivas, D.P. Shepherd, Appl. Phys. B 75, 15 (2002). [6] C. Grivas, D.P. Shepherd, T.C. May-Smith, R.W. Eason, M. Pollnau, A. Crunteanu, M. Jelinek, "Three-dimensional structuring of sapphire by sequential He+ ion-beam implantation and wet chemical etching", IEEE J. Quantum Electron., accepted. [7] L. Laversenne, A. Crunteanu, P. Hoffmann, M. Pollnau, P. Moretti, J. Mugnier, "Proton implanted sapphire planar and channel waveguides", Conference on Lasers and Electro-Optics Europe, Munich, Germany, 2003, accepted. [8] L. Laversenne, S. Rivier, A. Crunteanu, M. Pollnau, C. Grivas, D.P. Shepherd, R.W. Eason, to be published. [9] D. Ehrentraut, M. Pollnau, S. Kück, Appl. Phys. B 75, 59 (2002). [10] D. Ehrentraut M. Pollnau, "On the potential of BaSO4:Mn6+ for broadly tunable laser emission in the near infrared spectral region", Conference on Lasers and Electro-Optics Europe, Munich, Germany, 2003, accepted

Impurity-doped micro-lasers

Recently rare-earth-ion-doped dielectric channel waveguides have proven their ability to generate highly efficient laser output in the fundamental mode. Here we review our recent achievements obtained in crystalline potassium double tungstates and amorphous aluminum oxide

Sapphire and Ti:sapphire buried waveguide structures

Due to its excellent thermal, mechanical, and optical properties, sapphire is one of the most suitable material for integrated optical devices. Although this hard crystalline material is particularly difficult to process, fabrication of Ti3+-doped sapphire surface channel waveguides by surface structuring [1,2] of planar waveguides or ion in-diffusion [3] has been demonstrated. Generally, device performance can be considerably improved by burying the guiding structure into the bulk of the sample. Advantages of buried waveguides derive not only from surface scattering losses being avoided, but also from a reduction in mode asymmetry compared to surface waveguides, thus providing higher efficiency for mode coupling to optical fibers. We reported the fabrication of complex waveguiding structures such as buried and stacked planar as well as buried single and parallel channel waveguides in sapphire by high-energy proton implantation [4]. Compared to the significant damage created in sapphire by He+ implantation, which resulted in poor waveguiding quality, protons create less damage in the guiding region, thus assuring better waveguiding quality. Moreover, deeper damage profiles are obtained with protons, for the same incident energy, thus providing larger design flexibility. Pure c-cut, optically polished sapphire substrates of dimensions 8 mm x 8 mm x 1 mm were irradiated by use of a Van de Graaf accelerator with incident ion energies of 0.4-1.5 MeV and doses of 1015-1016 ions/cm2. Good control of the implantation parameters enables writing of precisely localized optical barriers with well-defined decrease of refractive index, resulting in excellent confinement of the propagating light in each structure, with significantly reduced vertical mode asymmetry in the case of buried waveguides. Different mode shapes can be obtained by adjusting the implantation parameters, which demonstrates the versatility of the fabrication method. Fundamental-mode, buried channel waveguides with propagation losses <2 dB/cm are obtained without post-implantation annealing. Horizontal and vertical parallelization is demonstrated for the design of one- or two-dimensional waveguide arrays in hard crystalline materials. Transfer of the method to the fabrication of Ti3+:sapphire active waveguides and fluorescence guiding after excitation by an Ar-ion laser has been demonstrated. This work was performed in a collaboration with the University of Lyon, France [4]. We also employed femtosecond laser writing in order to induce refractive-index changes and waveguides in Ti3+-doped sapphire [5]. The femtosecond writing system was a Ti3+:sapphire laser at a repetition rate of 1 kHz, with a center pulse wavelength of 790 nm and a pulse energy between 0.5 and 6 µJ. Doping the sapphire crystal with an appropriate ion significantly reduces the threshold for creating structural changes, thus enabling the writing of waveguide structures. Possible sensitization mechanisms are firstly, exploitation of two-photon absorption into the Ti3+ absorption band as an intermediate level and secondly, initial changes in the crystalline structure of sapphire by replacing the Al3+ ion with the larger Ti3+ ion. Passive and active buried channel waveguiding is demonstrated by end-coupling a HeNe laser at 633 nm and exciting the Ti3+ fluorescence centered at 760 nm by a laser, respectively. Comparison of measured fluorescence spectra in the waveguiding and bulk regions of the sample exhibit the same shape and input-output curves after excitation by an Ar-ion laser provide an efficiency of several 10-5, which is as high as in investigations of surface channel waveguides produced by other methods [1,2]. Negative refractive-index changes in the laser-damaged region are measured by digital holography. The guiding area of typically 20-µm diameter is located around the laser-damaged region, indicating that the guiding effect is stress-induced. Waveguide losses of typically 2.5-4 dB/cm have been detected without optimization of the irradiation parameters. Proper active doping should enable femtosecond processing and waveguide writing in various crystalline materials. This work was performed in a collaboration with the Politecnico di Milano, Italy [5]. [1] A. Crunteanu, M. Pollnau, G. Jänchen, C. Hibert, P. Hoffmann, R.P. Salathé, R.W. Eason, C. Grivas, and D.P. Shepherd, Appl. Phys. B 75, 15 (2002). [2] C. Grivas, D.P. Shepherd, T.C. May-Smith, R.W. Eason, M. Pollnau, A. Crunteanu, and M. Jelinek, IEEE J. Quantum Electron. 39, 501 (2003). [3] V. Apostolopoulos, L.M.B. Hickey, D.A. Sager, and J.S. Wilkinson, Opt. Lett. 26, 1586 (2001). [4] L. Laversenne, P. Hoffmann, M. Pollnau, and P. Moretti, Conference on Lasers and Electro-Optics, San Francisco, California, Technical Digest (Optical Society of America, Washington, DC 2004), paper CTuU5. [5] V. Apostolopoulos, L. Laversenne, T. Colomb, C. Depeursinge, R.P. Salathé, M. Pollnau, R. Osellame, G. Cerullo, and P. Laporta, Conference on Lasers and Electro-Optics, San Francisco, California, Technical Digest (Optical Society of America, Washington, DC 2004), paper CMY4