Mid-Infrared Fiber Lasers

Transcription

1 Mid-Infrared Fiber Lasers Markus Pollnau 1 and Stuart D. Jackson 2 1 Advanced Photonics Laboratory, Institute for Biomedical Imaging, Optics and Engineering, Swiss Federal Institute of Technology 1015 Lausanne, Switzerland markus.pollnau@epfl.ch 2 Optical Fibre Technology Centre, Australian Photonics CRC. The University of Sydney 206 National Innovation Centre, Australian Technology Park Eveleigh NSW 1430, Australia s.jackson@oftc.usyd.edu.au Abstract. The current state of the art in mid-infrared fiber lasers is reviewed in this chapter. The relevant fiber-host materials such as silicates, fluorides, chalcogenides, and ceramics, the fiber, pump, and resonator geometries, and the spectroscopic properties of rare-earth ions are introduced. Lasers at transitions ranging from 1.9 to 4 µm occurring in the rare-earth ions Tm 3+,Ho 3+,andEr 3+ and their population mechanisms are discussed on the basis of the fundamental spectroscopic properties of these ions. Continuous-wave, fundamental-mode power levels ranging from a few mw near 4 µm upto 10 W near 2 µm have been demonstrated in recent years. Power-scaling methods and their limitations, the possibilities to optimize the population mechanisms and increase the efficiencies of these lasers, as well as the prospects of future mid-infrared fiber lasers in a number of rare-earth ions at transitions in the wavelength range beyond 3 µm and extending to 5 µm are described. 1 Introduction Since the introduction of the double-clad fiber more than a decade ago and with the recent technological advances in the fields of fiber fabrication and beam-shaped high-power diode lasers, the performance of diode-pumped fiber lasers has steadily improved. Today, fiber lasers can compete with their corresponding bulk crystalline systems in certain applications, especially when transverse-fundamental-mode, continuous-wave (CW) laser operation at output powers in the milliwatt to multiwatt range is required. The increased recent interest in fiber lasers emitting at mid-infrared wavelengths between 2 and 3µm primarily relates to the high potential of these wavelengths for applications in laser microsurgery. Due to the high absorption of water in the spectral region at µm, high-quality laser cutting or ablation has been demonstrated in biological tissues. In addition, laser wavelengths near 2 µm could be suitable for tissue welding. A number of other potential laser applications in the mid-infrared spectral region, e.g. environmental trace-gas I. T. Sorokina, K. L. Vodopyanov (Eds.): Solid-State Mid-Infrared Laser Sources, Topics Appl. Phys. 89, (2003) c Springer-Verlag Berlin Heidelberg 2003

2 220 Markus Pollnau and Stuart D. Jackson detection, are currently becoming increasingly important. In all these applications fiber lasers may find their niches. The high development costs of fabricating fibers with sufficiently low losses in the mid-infrared spectral region has impeded the necessary research efforts in the field of mid-infrared fiber lasers. The currently available fiber materials that are suitable as host materials for specific rare-earth-doped fiber lasers in the spectral region 2 5 µm will be introduced in Sect. 2. More than any other idea, the invention of the double-clad fiber geometry has accelerated the output-power scaling and hence the success of fiber lasers. The various aspects of the fiber, pump, and resonator geometries will be describedinsect.3. A significant number of spectroscopic investigations has led to a better understanding of the population mechanisms of rare-earth-doped laser systems. The fundamental spectroscopic properties of rare-earth ions in solid-state host materials will be reviewed in Sect. 4. Equipped with this general information, the performance of the most important mid-infrared fiber laser transitions in the wavelength range 2 3 µm can be understood in detail. Sect. 5 will be devoted to the Tm 3+ fiber lasers at 1.9 and 2.3 µm, whereas the Ho 3+ fiber lasers at 2.1 and 2.9 µm will be discussed in Sect. 6. An impressive example of the variety of population mechanisms and operational regimes in a single system is the Er µm fiber laser transition that will be investigated in Sect. 7. At wavelengths beyond 3 µm, it becomes increasingly difficult to find suitable host materials for actively doped laser systems. This statement holds true for glass fibers in the same way as for crystalline materials. The prospects of future midinfrared fiber lasers in this wavelength range will be discussed in Sect. 8. Besides general introductions to the different topics of lasers [1,2] that include many aspects relevant also to mid-infrared fiber lasers, a comprehensive introduction to the field of rare-earth-doped fiber lasers can be found in [3]. 2 Fiber Materials The choice of the fiber material involves a number of considerations: the maximum phonon energy, the environmental durability, the draw ability, the rare-earth solubility, and the purity of the starting materials. The maximum phonon energy of the glass sets the overall infrared transparency range of the fiber and the multiphonon relaxation rates which influence the quantum efficiency. The multiphonon relaxation rates for the common fiber glasses as a function of the energy gap between energy levels are shown in Fig. 1. The optical transparency range relates to both the size of the band gap and also the infrared absorption cut-off, hence to the vibrational frequency ν of the anion cation bonds of the glass. For an ordered structure, ν =(1/2π) k/m, (1)

3 Mid-Infrared Fiber Lasers 221 Multiphonon Relaxation Rate (s -1 ) GLS (350 cm -1 ) Borate (1400 cm -1 ) Phosphate (1200 cm -1 ) Silicate (1100 cm -1 ) Germanate (900 cm -1 ) Tellurite (700 cm -1 ) ZBLA (500 cm -1 ) Energy Gap (cm -1 ) Fig. 1. Calculated and measured multiphonon relaxation rates as a function of the energy gap between energy levels for glasses with different maximum phonon energies. (Data taken from [4,5]) where M = m 1 m 2 /(m 1 + m 2 ) is the reduced mass for two bodies m 1,m 2 vibrating with an elastic restoring force k. While for disordered structures like glass, this is not an accurate expression, nevertheless, it does highlight the important contributions to the glass transparency. The relative cation anion bond strength is intimated by the field strength Z/r 2,whereZ is the valence state of the cation or anion and r is the ionic radius. Generally, glasses composed of large anions and cations with low field strengths display high transparency in the mid-infrared spectral region. The important physical properties of the popular glasses used for optical fibers are shown in Table 1. Table 1. Properties of popular fiber materials Fiber material Max. phonon energy (cm 1 ) Infrared transparency ( µm) Propagation losses (λ at minimum) (db/km) Thermal conductivity (W/K m) Silica 1100 [4] < (1.55 µm) 1.38 [6] ZBLAN 550 [7] < (2.55 µm) [8] GLS 425 [5] < (3.50 µm) [9] 2.1 Silicates This glass is perhaps the most important material used for optical fiber production [3,10], however, the maximum phonon energy is high ( 1100 cm 1 ) and has so far limited the emission wavelength of mid-infrared fiber lasers using this material to 2.2 µm [11]. Silica is robust and involves the very effecwww.cadfamily.com cadserv21@hotmail.com

4 222 Markus Pollnau and Stuart D. Jackson tive modified chemical vapor deposition (MCVD) technique for fiber fabrication. Reducing the OH content in the glass, which has two main absorption peaks in the range µm [12], improves the near-to-mid-infrared utility. Rare-earth ions such as Nd 3+ and Er 3+ whichhavehighfieldstrengthshave low solubility in silicate glass which can lead to clustering and micro-scale phase separation. 2.2 Fluorides The use of fluoride glasses, especially the heavy-metal fluorides [13,14], as host materials for mid-infrared fiber lasers has found wide acceptance. The most common form of heavy-metal fluoride glass is the fluorozirconate (ZrF 4 ) composition and the most widespread fluoride fiber material is ZBLAN [15], a mixture of 53 mol.% ZrF 4, 20 mol.% BaF 2, 4 mol.% LaF 3, 3 mol.% AlF 3, and 20 mol.% NaF. Since it can be readily drawn into single-mode optical fiber [16] it is particularly important to mid-infrared fiber lasers [17]. The large atomic weight of the zirconium atom combined with relatively weak bonding provides a maximum phonon energy for ZBLAN of 550 cm 1 and allows for high infrared transparency up to 6 µm. Multiphonon relaxation, however, becomes significant for transitions at wavelengths longer than 3 µm. Compared to silica, ZBLAN has a lower damage threshold and a lower level of inhomogeneous spectral-line broadening (Sect. 4.1) because the rareearth ion is placed in sites of a less perturbed network. The crystal-field strength is also comparatively weaker [18]. An overview of the spectroscopic properties of rare-earth ions doped into ZBLAN has been given in [7]. 2.3 Chalcogenides Chalcogenides are composed of the chalcogen elements S, Se and Te [19,20,21]. They are environmentally durable, have a low toxicity and have reasonably large glass forming regions. When the rare-earth ions are doped into these glasses [22], the radiative transition probabilities and, therefore, the absorption and emission cross-sections are high as a result of the high refractive index ( 2.6) of the glass and the high degree of covalency of the rare-earth ion with the surrounding medium. Maximum phonon energies of cm 1 produce low rates of multiphonon relaxation, see Fig. 1, and therefore high quantum efficiencies. The low thermal conductivity, see Table 1, is howeveran important factor to be considered in the design of chalcogenide-based lasers. Of the large number of rare-earth chalcogenides studied for luminescent emission, the most important glasses are the sulfide glasses GaLaS (GLS) [23] and GeGaS [24] because of the reasonably high rare-earth solubility. 2.4 Ceramics Studies into the use of ceramics as host materials for the rare earths have recently made a lot of progress [25]. These ceramics are composed of nanowww.cadfamily.com cadserv21@hotmail.com

5 Mid-Infrared Fiber Lasers 223 crystallites of materials such as Y 3 Al 5 O 12 (YAG) and can be produced in a simple and cost-efficient process at relatively low temperatures. This allows the fabrication of materials with very high melting points [26] that are difficult to grow by other techniques such as the Czochralski method [27]. This class of materials is also available in fiber geometry [28]. Ceramic fibers combine the characteristics of crystalline materials such as high absorption and emission cross-sections, large thermal conductivity, and even the possibility of doping with transition-metal ions [28] with the convenience of guiding the pump and signal light in a fiber. Currently, the losses of these fibers are comparatively high, but further improvement can be expected. 3 Fiber, Pump, and Resonator Geometries The light oscillating in a fiber-laser resonator can be either free running or deliberately modulated depending on whether CW or pulsed output, respectively, is desired. Consequently, a large number of techniques for pulsed operation including Q-switching and mode locking of fiber lasers have been explored. These techniques have been investigated intensively for the common laser transitions at 1 µm innd 3+ and Yb 3+ and at 1.5 µm iner 3+,andare usually described in combination with these lasers. The small fiber size limits the peak power through the damage-threshold intensity (propagating power per core area) and, hence, crystalline lasers in bulk geometries or optical parametric processes are often preferred when high-energy short pulses are needed. This argument accounts especially for mid-infrared ZBLAN-based fiber lasers, because these fibers possess a lower damage threshold compared to silica fibers. The description of mid-infrared fiber lasers is, therefore, confined to CW operation and specific techniques for pulsed operation of fiber lasers are not discussed in this chapter. In an analogous way to the optical excitation of bulk gain media, doped optical fibers can be either end pumped (core pumped) or side pumped (cladding pumped). The former method is less scalable since it relies on the use of expensive high-beam-quality pump sources because core areas are usually < 100 µm 2. On the other hand, the larger cladding area (> 10 4 µm 2 ) allows for high-power diode-array pumping [29,30,31,32,33]. The obvious simplicity of the core-pumping method negates further explanation and we will concentrate on the cladding-pumping technique: one of the most important developments in fiber-laser technology. 3.1 Fiber Designs for Cladding Pumping In the design of fibers for cladding pumping, the core of the fiber is generally made to guide a single-transverse LP 01 mode. The shape of the multimode pump cladding, see Fig. 2, however, remains somewhat flexible and can be shaped with a number of considerations in mind. The pump cladding,

6 224 Markus Pollnau and Stuart D. Jackson Jacket Outer cladding Pump cladding Core (a) (b) (c) (d) Fig. 2. Principal double-clad fiber geometries which include (a) circular shaped pump cladding with axially positioned core, (b) circular shaped pump cladding with off-axially positioned core, (c) rectangular shaped pump cladding and (d) D-shaped pump cladding which in turn is surrounded by a low-refractive-index transparent polymer or glass, provides a high numerical aperture (NA) of for the pump cladding. There are three main double-clad-fiber layouts: circular, circular with offset core, and rectangular as shown schematically in Fig. 2. Maximum pump-light absorption sees the core near the outer edge of the circular pump cladding [34] because a portion of the launched light is skew to the fiber axis and produces an inner caustic and never crosses the central region of the pump cladding. Scrambling these skew rays by bending [35] orby using a graded and slightly elliptical pump cladding [36] increases the pumpabsorption efficiency as does spatially varying refractive-index fluctuations in inhomogeneous pump claddings [37]. Inner caustics can be avoided by rectilinearly shaping the pump cladding [38] which has the ancillary advantage of matching the shape of diode-array output. The overall absorption coefficient of the fiber is reduced by the ratio of the core area to the area of the pump cladding [34]. The propagation losses for the rectangular-shaped pump cladding are higher and the effective numerical aperture lower as compared to the circular shape [39]; however, in certain cases higher dopant concentrations can provide shorter fiber lengths that also lead to reduced nonlinear effects. A D-shaped or truncated circular pump cladding [40], see Fig. 2d, is also effective while being easier to make than rectangular preforms. The circular-multimode pump cladding may also have the gain medium distributed in a ring around the edge of the pump cladding either discretely or continuously in multi-core [41] and M-profile [42] arrangements, respectively. The effective absorption coefficient is now further increased while maintaining high-beam-quality output. A large-mode-area core [43] can also increase the effective absorption coefficient of the fiber. Recently, double-clad pump schemes have been demonstrated also with holey fibers [44]. These structures offer the additional advantage of singlemode guiding over a broad spectral range [45].

7 Mid-Infrared Fiber Lasers Fiber-Laser Resonators Typical free-running fiber-laser resonators are shown schematically in Fig. 3. In the simplest resonator,see Fig.3a, the pump light passes through a dichroic mirror that is highly reflective for the oscillating laser light. Fresnel reflection at the cleaved output end facet of the fiber can provide sufficient feedback for laser oscillation; however, with an output-coupler mirror and pump retroreflector placed at the output end of the fiber the optical efficiency can be maximized. In an alternative arrangement, the pump light can be launched into the output end of the fiber, see Fig. 3b. A dichroic mirror oriented at 45 to the fiber axis extracts the laser output and a broadband highly reflecting mirror is placed at the rear fiber end. To scale the output power, each end of the fiber can be pumped, see Fig. 3c. Periodic V-grooves [46] orprism coupling [47] along the fiber to distribute the pump access allow one to further scale the output power and are useful for pumping fiber ring resonators. Spectrally combining the output from a number of separate fiber lasers is also a promising power-scaling technique [48,49,50]. The highest reported fiberlaser output powers of 110 W in a singly Yb 3+ -doped fiber [51] and 150 W in and 3+,Yb 3+ -codoped fiber [52] have been obtained using arrangements as shown schematically in Fig. 3c. Bragg gratings can substitute the fiber-butted mirror if spectrally well-defined output is required. (a) Pump M Fiber (b) Output Pump M Output Fiber (c) M Pump M Output Fiber M Pump Fig. 3. Schematic diagram of resonators used for free-running fiber lasers with (a) a single-end co-propagating pump, (b) a single-end counter-propagating pump and (c) dual end pumps. M represents the mirror

8 226 Markus Pollnau and Stuart D. Jackson 3.3 Thermal Issues As higher pump powers become available from laser-diode systems, it is generally recognized that thermal and thermo-optical issues set limitations to the power scalability of end-pumped bulk-laser systems. Owing to the unfavorable temperature dependence of thermal and thermo-optical parameters [53], the large heat load in the crystal leads, firstly, to a significant temperature increase in the rod, secondly, to strong thermal lensing with pronounced spherical aberrations, and ultimately, to rod fracture in a high-average-power end-pumped system. Due to its geometry, the fiber provides potentially high pump- and signalbeam intensities without the drawbacks of significant thermal and thermooptical effects. Its large surface-area-to-volume ratio means that the heat generated from multiphonon relaxation in the core is dissipated effectively by radiation and convection from the outer surface of the fiber. This is especially true for single-clad, core-pumped single-mode fibers where this ratio is highest [54]. Double-clad fibers have a relatively smaller surface-area-tovolume ratio and thermal issues need to be taken into account [6,55,56]. Thermal management will be required when very high output powers are desired. In particular, for high-power mid-infrared operation, thermal management may be very important because of the decreased quantum efficiency and the consequently higher amount of heat dissipation. 4 Spectroscopic and Laser Properties of Rare-Earth Ions The structure of a glass is less well defined as compared to a crystalline material. The local variation of the chemical environment of active ions in a glass has a number of consequences. Most important, the active ions may undergo chemical reactions during the fabrication process and be incorporated in the host in several oxidation states with different spectroscopic properties. Oxidation states other than the desired one may act as impurities that introduce undesired optical effects such as parasitic pump absorption, the reabsorption of oscillating laser light, the lifetime quenching of the laser ion, and the trapping of the excitation energy. A stable oxidation state of the optically active ion is thus highly desirable. The necessity of a stable oxidation state excludes a number of transition-metal ions from the list of suitable dopants in glass environments. This is one of the possible reasons why examples of transitionmetal-ion-doped lasers in glass hosts are rare. On the other hand, most of the rare-earth ions prefer to stabilize in the trivalent oxidation state and are, therefore, suitable candidates as glass and fiber dopants. This chapter will, therefore, concentrate on the rare-earth ions as active dopants of fiber lasers.

9 4.1 Spectra of Rare-Earth Ions in Glasses Mid-Infrared Fiber Lasers 227 The optical transitions of lanthanide (rare-earth) ions in the visible and infrared spectral region occur within the 4f subshell. This subshell is shielded by the outer 5s and 5p subshells and the influence of the host material is relatively small compared to, e.g., the 3d transitions in transition-metal ions. The electronic structure of trivalent rare-earthions derivesfrom the perturbation of the 4f energy level in the central-field approximation by the noncentrosymmetric electron electron interaction, the spin orbit interaction, and the crystal-field splitting (Stark effect); see the example of the energy-level scheme of Er 3+ in Fig. 4. The spin orbit multiplets are commonly denoted by their 2S+1 L J terms in Russell Saunders coupling, although the 4f electrons of lanthanide ions exhibit intermediate coupling and the total angular momenta J of the spin orbit multiplets are linear combinations of the total orbital angular momenta L and total spins S. Single crystal-field (Stark) transitions between two spin orbit multiplets cannot be distinguished in glasses at ambient temperature, because inhomogeneous spectral-line broadening occurs due to the local variation of the ligand electric field. Also homogeneous (lifetime) broadening mechanisms are relevant in a number of glasses. This spectral-line broadening makes glasses the preferred hosts when broadband, 4 F 3/2 4f 11 Er 3+ 2 H 4 S 4 F 4 F 4 F 5/2 7/2 2 H 11/2 4 S 3/2 4 F 9/2 4 I 9/2 4 I 4 I 11/2 Central- Field Approx. Non- Centrosym. Splitting 4 I 13/2 Spin- Orbit Splitting 4 I 15/2 Crystal- Field Splitting Fig. 4. Energy-level scheme of trivalent erbium indicating the splitting of the 4f 11 configuration in the centralfield approximation by the noncentrosymmetric electron electron interaction, the spin orbit interaction, and the Stark splitting by the local electric field of the host material (indicated only for selected spin orbit multiplets)

10 228 Markus Pollnau and Stuart D. Jackson continuous tunability of lasers is desired. On the other hand, the spectralline broadening leads to lower absorption and emission cross-sections for the same transition in glasses compared to single-crystalline hosts. The reduced cross-sections lead to generally higher pump threshold of laser transitions in glasses, a fact that is compensated in fiber geometry because a high pump confinement is achieved over the whole fiber length. 4.2 Intraionic Processes Generally, the probability of an allowed electric-dipole transition is seven orders of magnitude larger than that of an allowed magnetic-dipole transition. Since electric-dipole transitions within the 4f subshell are parity forbidden, the intensities of radiative transitions in rare-earth ions are weak and the radiative lifetimes of the emitting states are long, typically in the ms range. Mixing of the 4f states with higher-lying (typically 5d) electronic states of opposite parity at ion sites without inversion symmetry, however, means that electric-dipole transitions become partially allowed and are usually the dominant transitions between 4f electronic states. The oscillator strengths f and integrated absorption and emission cross-sections σ of these spin orbit multiplet-to-multiplet transitions can be calculated with the help of the semiempirical Judd Ofelt theory [57,58]. If the degree of inhomogeneous spectralline broadening is relatively small and the absorption and emission spectra remain structured, as is the case for ZBLAN, the cross-sections σ(λ) atindividual wavelengths that are relevant to pump absorption and stimulated emission of narrow laser lines must be determined experimentally. Besides ground-state absorption (GSA), excited-state absorption (ESA) of pump photons, see Fig. 5a, can play a significant role in fiber lasers, specifically in the case of high-intensity core pumping. An experimental example will be given later in Sect Since the absorption increases exponentially with the absorption coefficient α(λ P )=Nσ(λ P ), ESA becomes relevant for the population dynamics of a laser when (a) the ESA and GSA cross-sections σ(λ P ) are comparable at the pump wavelength λ P and (b) the population density N of the excited state in which the second pump-absorption step originates becomes a significant fraction of the density of ions in the ground state, i.e., a large degree of ground-state bleaching must be present for ESA to play a significant role. A radiative transition from an excited state i toalower-lyingstatej is characterized by the radiative rate constant A ij. If the decay occurs to several lower-lying states, the overall radiative rate constant A i is the sum of all individual rate constants. The branching ratio of each radiative transition is defined as β ij = A ij /A i. Radiative decay of excited states is in competition with nonradiative decay by interaction with vibrations of the host material, called multiphonon relaxation. The rate constant of a multiphonon relaxation process decreases exponentially with the energy gap to the next lower-lying state and with the order of the process, i.e., the number of phonons required

11 Mid-Infrared Fiber Lasers 229 (a) (b) 2 2 Pump (ESA) 1 1 Pump (GSA) Pump 0 Ion 0 Donor Ion Acceptor Ion Acceptor Ion (c) (d) 2 2 Laser Laser 1 1 Pump Pump 0 Sensitizing Ion A Laser Ion B 0 Laser Ion A Quenching Ion B (e) 2 (f) Pump Pump Pump 0 Donor Ion Acceptor Ion 0 Donor Ion Acceptor Ion Fig. 5. Intra- and interionic processes in fiber lasers: (a) excited-state absorption (ESA); (b) energy migration; (c) sensitization and (d) quenching of a laser ion by an ion of a different type; (e) cross-relaxation and (f) energy-transfer upconversion

12 230 Markus Pollnau and Stuart D. Jackson to bridge the energy gap [59,60]. This fact is illustrated in Fig. 1 for different glasses. The rate constant of multiphonon relaxation increases with host temperature. The measurable luminescence lifetime τ i of an excited state i is the inverse of the sum of the overall radiative rate constant A i and the rate constant of multiphonon relaxation, W i. The radiative quantum efficiency is defined as η = A i /(A i + W i ). The influence of multiphonon relaxations is stronger in oxides as compared to fluorides because of the smaller atomic mass m 2 of the anion and the larger elastic restoring force k, see(1), due to stronger covalent bonds in oxides [3], both resulting in larger maximum phonon energies in oxides. A brief example: The luminescence lifetime of the 4 I 11/2 upper laser level of the erbium 3 µm laser (Sect. 7) is partly quenched by multiphonon relaxation. Typically, nonradiative decay becomes dominant if five or less phonons are required to bridge the energy gap. With an energy gap between the 4 I 11/2 and the next lower lying 4 I 13/2 levels of cm 1, radiative decay prevails for phonon energies below 600 cm 1, roughly the maximum phonon energy of ZBLAN, see Table 1. Fluorides are, therefore, preferred over oxides as host materials for most of the mid-infrared laser transitions. Like absorption, the strength of a stimulated-emission process is characterized by the emission cross-section σ(λ L ) of the laser transition. From a simple analysis, for one resonator round-trip of oscillating laser photons, the product τσ(λ L )withτ the luminescence lifetime of the upper laser level, is identified as a figure of merit for a possible laser transition. The larger this product, the lower is the expected pump threshold of the laser transition. This figure of merit, however, does not take into account the numerous parasitic effects that can occur in the population dynamics of a laser system, such as pump ESA, reabsorption of laser photons, and energy-transfer processes. It is often these parasitic processes that lead to surprising performance characteristics as likely in the negative as in the positive sense and make the interpretation of rare-earth-doped solid-state lasers challenging. Examples will be discussed in Sects Interionic Processes In addition to intraionic excitation and decay mechanisms, radiative energy transfer due to reabsorption of emitted photons by other active ions in the sample and nonradiative energy-transfer processes due to multipole multipole or exchange interactions between neighboring active ions can occur. Radiative energy transfer leads to an increase in the luminescence lifetime. Among the nonradiative energy-transfer processes, most common is the electric dipole dipole interaction, which can occur as a direct [61] or phononassisted [62] energy transfer. A direct energy transfer requires spectral resonance between the involved emission and absorption transitions whereas an indirect transfer can also be nonresonant, i.e., an existing energy gap between the emission and absorption transitions involved in the transfer is bridged by

13 Mid-Infrared Fiber Lasers 231 one or several phonons. A process that leads to phonon emission has typically a higher probability than a process requiring phonon absorption. Since the electrostatic field of an electric dipole decreases with distance r as r 3,the probability of an energy transfer between two such dipoles exhibits a strong distance sensitivity of r 6. Therefore, nonradiative energy transfer occurs predominantly between neighboring active ions. An obvious possibility of an energy-transfer process is shown in Fig. 5b. An excited ion transfers its excitation to a nearby ion of the same type. If this process occurs consecutively between a number of similar ions and the energy is thus transferred over a larger distance, it is called energy migration. Quenching of the luminescence lifetime of an excited state by energy transfer to impurities is often accelerated by energy migration among the excited donor ions [63]. Figure 5 displays further energy-transfer processes that typically occur in rare-earth-doped solid-state lasers. Rare-earth ions of a different type can be deliberately co-doped into the host material in order to influence the laser properties of the lasing ions. Efficient excitation by means of absorption and energy transfer of pump light from sensitizing ions to the upper laser level of the lasing ions, see Fig. 5c, can be exploited when the lasing ions do not sufficiently absorb the pump light at the desired pump wavelength or the dopant concentration of the lasing ions is limited because, e.g., the laser transition terminates in the ground-state multiplet. Similarly, the transfer of excitation from the lower laser level of the lasing ions to nearby quenching ions, see Fig. 5d, is desirable when the lifetime of the lower laser level is extremely long. The low relaxation rate from the lower laser level would otherwise lead to accumulation of excitation in this level, which can result in self-terminating laser behavior and/or bleaching of the ground-state population density and, consequently, decreased GSA. Energy-transfer processes that have both ions in excited states before or after the energy transfer are shown in Fig. 5e,f. In the former case, an excited ion transfers part of its excitation to a nearby ion in its ground state. This process is called cross-relaxation. Its rate increases with the average number of non-excited neighboring ions, i.e., with dopant concentration. Therefore, cross-relaxation leads to concentration quenching of the measured luminescent decay time of the initial excited state involved in the transfer process. In the inverse process called energy-transfer upconversion (ETU) excitation is transferred from one to another excited ion, see Fig. 5f. After the absorption of two low-energy pump photons, ETU leads to a single excitation of higher energy and a single high-energy photon may be emitted from the second excited state. In the presence of fast energy migration among the active ions, the excitation is spatially diffused and all these energy-transfer processes can be described by rate-equation analysis using a rate term WN d N a that comprises a macroscopic energy transfer probability W and the population densities N d

14 232 Markus Pollnau and Stuart D. Jackson and N a of the initial states of the donor and acceptor ions, respectively [64]. This model, however, is usually not applicable at low dopant concentrations where energy migration is weak. In addition, a number of authors reported on active-ion clusters in fiber materials, see, e.g., [65,66,67,68]. This or any other non-uniform distribution of active ions complicates the analysis of the influence of energy-transfer processes on the performance of rare-earth-doped fiber lasers, as ions within such clusters are more susceptible to interionic processes than isolated ions. In the simplest approach, this distinction defines two different classes of ions that exhibit different population dynamics [69]. 4.4 Overview of Mid-Infrared Fiber Lasers The relevant parameters of the fiber lasers with the highest output powers reported as of the end of 2001 for each mid-infrared transition are summarized in Table 2. As can be seen, most of the realized mid-infrared fiber lasers used ZBLAN as the host material, because silica glasses, on the one hand, have maximum phonon energies that are too high and hence a transparency that is too low in the wavelength region beyond 2.2 µm and chalcogenide glasses, on the other hand, are only at the beginning of their career as fiber-laser host materials. The transitions included in Table 2 will be discussed in detail in the following sections of this chapter. Table 2. Mid-infrared fiber lasers Ion Fiber host λ Pump (nm) λ Laser ( µm) Transition Output power Slope Eff. a) Ref. Tm 3+ Silica Ho 3+ ZBLAN Tm 3+ ZBLAN Er 3+ ZBLAN Ho 3+ ZBLAN Ho 3+ ZBLAN Er 3+ ZBLAN Ho 3+ ZBLAN F 4 3 H 6 14 W 46 % [70] 5 I 7 5 I 8 8.8W 36% [71] 3 H 4 3 H 5 22 mw 7 % [72] 4 I 11/2 4 I 13/2 1.7W 17% [73] 5 I 6 5 I 7 1.3W 30% [74] 5 S 2 5 F 5 11 mw 2.8 % [75] 4 F 9/2 4 I 9/2 8mW b) 3% [76] 5 I 5 5 I 6 11 mw b) 3.7 % [77] a) The values of the slope efficiency are given versus incident, launched, or absorbed pump power as stated by the authors and are not necessarily comparable to each other b) Operation with the fiber cooled below ambient temperature

15 Mid-Infrared Fiber Lasers Thulium-Doped Fiber Lasers at µm and µm TheuseoftheTm 3+ ion for mid-infrared fiber laser applications has been widespread mainly as a result of the convenient absorption band near 0.79 µm which allows for AlGaAs diode laser pumping. The primary luminescent transitions of Tm 3+ relevant to mid-infrared laser emission are the 3 F 4 3 H 5 transition at 2.3 µm andthe 3 H 4 3 H 6 ground-state transition at 1.9 µm, see the energy-level scheme in Fig. 6. The 3 F 4 level is excited by the 0.79 µm pump wavelength. 5.1 Three-Level Lasers at µm The first explorations into fiber lasers utilizing the 1.9 µm ground-state transition related to the dye-laser pumping at 797 nm of a Tm 3+ -doped silica fiber laser [79]. Overlap of the main absorption band with the emission wavelength of AlGaAs diode lasers quickly resulted in diode-laser pumping of these fiber lasers based on either silica [80] or fluoride [81] glass hosts. A useful phononassisted cross-relaxation process, ( 3 F 4, 3 H 6 ) ( 3 H 4, 3 H 4 ), can transform one absorbed pump photon into two excitations in the 3 H 4 upper laser level of the 2µm transition [82], see Fig. 6. The efficient room-temperature CW operation of Tm 3+ -doped diode-pumped bulk crystalline lasers [83] is largely due to this self-quenching process. This process is highly dependent upon the overall concentration of Tm 3+ ions and competition by multiphonon relaxation from the 3 F 4 level. High concentrations of Tm 3+ in low-phonon-energy glasses enable full exploitation of this beneficial phenomenon. The significantly stronger multiphonon relaxation and corresponding shorter lifetime of the 3 F 4 level in silica ( 20 µs) means that the cross-relaxation process is significantly weaker in silica as compared to fluoride glass. The large degree of Stark splitting of the 3 H 6 ground state provides the 3 H 4 3 H 6 transition with a very broad emission spanning 400 nm in 3 F 4 Laser 2.3 µm NR CR τ 3 = 1.5 ms 3 H 5 3 H 4 NR τ 1 = 6.8 ms GSA 3 H 6 Laser 2.0 µm Tm 3+ Fig. 6. Partial energy-level scheme of Tm 3+ displaying the measured lifetimes when doped into fluoride glass [78]. NR and CR represent nonradiative decay and cross-relaxation, respectively

16 234 Markus Pollnau and Stuart D. Jackson many hosts and represents one of the broadest luminescent transitions available from any rare-earth ion. The broad emission spectrum produces a large degree of wavelength tunability [84]. Since the Tm µm transition can be favorably operated in silica fiber (with its higher peak-power damage threshold compared to ZBLAN fiber), pulses in the range of fs have been obtained in additive-pulse [85] or passive [86] mode-locking from this broad emission spectrum. The smaller emission cross-section and the three-level nature of the laser transition usually resulted in relatively higher pump thresholds as compared to the standard Nd 3+ -doped fiber lasers. Reabsorption from the ground state of the Tm 3+ ions has to be overcome because the groundstate multiplet is the lower laser level. Reducing the population of the higher Stark levels of the ground state by way of cooling the fiber causes emission at shorter wavelengths. Tunability to longer wavelengths can be obtained by variation of the fiber length because of the increased level of reabsorption by the ground state with longer lengths of fiber [87]. Early power-scaling experiments involved the use of the convenient µm Nd 3+ :YAG laser which pumped the short wavelength side of the 3 H 5 level [88]. Pumping the long wavelength side of the 3 H 5 level with ahigh-power1.319 µm Nd 3+ :YAG laser also yields efficient output [89]. Inband pumping of the transition at 1.57 µm in silica [90] and at µm in fluoride glass [91,92] has also been demonstrated. Whilst theoretical modeling of Tm 3+ -doped silica fiber lasers [93] confirms that inband pumping is the most efficient pump method for silica based fiber lasers because of the high Stokes efficiency, nevertheless, the wide availability of high-power AlGaAS diode lasers means that diode-cladding-pumped systems in both standing-wave [87,70] and ring-resonator [94] arrangements are perhaps the most practical ways of producing high output power, see Fig. 7. Currently, the Tm 3+ -doped silica fiber laser is probably the most mature of the midinfrared fiber-laser systems primarily because of the robustness and convenience offered by the silica glass host. While less efficient than Tm 3+ -doped fluoride fiber lasers, the comparatively higher pump threshold relevant to Tm 3+ -doped silica fiber lasers is easily provided for by currently available high-power diode-laser pump sources. The maximum output power from highpower Tm 3+ -doped fiber lasers is still currently an order of magnitude lower than comparable diode-pumped crystalline systems [95]. 5.2 Four-Level Lasers at µm The increased quantum efficiency of the 3 F 4 level when Tm 3+ ions are doped into a ZBLAN host offers a greater range of emission wavelengths. Besides lasers at shorter wavelengths such as 1.47 µm ( 3 F 4 3 H 4 )and 0.8 µm ( 3 F 4 3 H 6 ), the mid-infrared four-level CW laser at 2.3 µm ( 3 F 4 3 H 5 )[96,97,72] is of interest, see Fig. 6. Deliberately designing the fiber to have a relatively low Tm 3+ -ion concentration reduces cross-relaxation and hence severe lifetime quenching of the 3 F 4 level. The lifetime of the lower

17 Mid-Infrared Fiber Lasers 235 Output Power (W) wt.% Tm mol.% Tm 3+, 0.4 mol.% Ho wt.% Tm Incident Pump Power (W) Fig. 7. Measured output powers from diode-cladding-pumped fiber lasers using 1.8 wt.% Tm 3+ -doped silica [86], 2.2 wt.% Tm 3+ -doped silica [70], and 3.6 mol.% Tm 3+, 0.4 mol.% Ho 3+ -doped fluoride glass [71] laser level of the 3 F 4 3 H 5 transition is quite short and leads to a low pump threshold and broad tunability which can extend from 2.25µm to 2.5 µm [78]. Simultaneous lasing on the 3 H 4 3 H 6 transition at 1.9 µm produces a two-color fiber laser [98]. Applications requiring highly efficient output or multi-mid-infrared-wavelength output will benefit from the use of Tm 3+ -doped ZBLAN fibers. 6 Holmium-Doped Fiber Lasers at 2.1 µm and 2.9 µm The use of the Ho 3+ ion as the active dopant for fiber lasers opens up a number of very useful mid-infrared transitions. In this section, we will concentrate on the 5 I 7 5 I 8 ground-state transition at 2.1 µm andthe 5 I 6 5 I 7 transition at 2.9 µm, see the energy-level scheme in Fig. 8. One of the significant shortcomings of Ho 3+, however, is the lack of GSA transitions [99] thatoverlap with convenient high-power pump sources. As a result, many of the early demonstrations of Ho 3+ -doped room-temperature crystalline CW lasers [82] involved sensitizing with Tm 3+ in order to access the convenient absorption bands and the practical cross-relaxation process Tm 3+ provides. Energy migration amongst the Tm 3+ ions and a suitable Tm 3+ :Ho 3+ concentration ratio ensures that efficient energy transfer to Ho 3+ takes place [100,101], see Fig. 8. The results of a recent spectroscopic study of Ho:YAG [102] suggest that the cross-relaxation process ( 5 I 5, 5 I 8 ) ( 5 I 7, 5 I 7 ) and the related ETU process will probably be important in some highly Ho 3+ -doped fibers; however, the cross-relaxation process ( 5 I 6, 5 I 8 ) ( 5 I 7, 5 I 7 ) and its related ETU process seem to have a negligible effect on the overall population dynamics.

18 236 Markus Pollnau and Stuart D. Jackson Laser 3.2 µm 5 S 2 5 F 5 3 F 4 5 I 4 3 H 5 3 H 4 CR Laser 3.9 µm ET Laser 2.9µm 5 I 5 5 I 6 5 I 7 GSA Laser 2.1 µm 3 H 6 Tm 3+ Ho 3+ 5 I 8 Fig. 8. Partial energy-level scheme of Ho 3+ with Tm 3+ sensitizer. ET represents energy transfer 6.1 Three-Level Lasers at 2.1 µm As is the case with the ground-state transition of the Tm 3+ ion discussed above, the 5 I 7 5 I 8 ground-state transition has been the most widely explored laser transition of Ho 3+. The first fiber laser configuration making use of this transition employed ZBLANP glass (a variant of ZBLAN) and argonion pumping [103]. A year later, this was followed by a demonstration of an argon-ion-pumped Ho 3+ -doped silica fiber laser [104]. In both cases, the fiber was singly doped with Ho 3+, the output power < 1 mw, and each needed a relatively high pump power to reach laser threshold. Improvements in the output power and efficiency have been made recently with Yb 3+ -doped silica fiber laser pumping of the 5 I 6 level [105]; however, the output power has only increased to 280 mw. As mentioned above, a practical method of efficiently generating laser emission on the 5 I 7 5 I 8 transition is to co-dope Ho 3+ laser ions with Tm 3+ sensitizer ions. The first demonstration of a fiber laser operating with the Tm 3+,Ho 3+ system occurred in 1991 [106] when 250 mw was generated at a slope efficiency of 52 % from a Ti:sapphire-pumped fluoride fiber laser. A year later [107], this work was followed by an increase in the Tm 3+ concentration to improve the cross-relaxation and resulted in a higher slope efficiency being obtained. Demonstration of a Tm 3+,Ho 3+ -doped silica fiber laser soon followed [108,109]; however, owing to lower Tm 3+ concentrations and strong multiphonon relaxation of the 3 F 4 level which forces weaker cross-relaxation, significantly lower slope efficiencies were measured, especially when pumped at 1.064µm [11]. To date, the highest output power from a fiber laser operating on the 5 I 7 5 I 8 transition has been produced by a diode-cladding-pumped Tm 3+,Ho 3+ -doped fluoride fiber laser [71], see

19 Mid-Infrared Fiber Lasers 237 Fig. 7. In an analogous way to recent demonstrations in bulk laser systems [110,111], tandem-pumping Ho 3+ with a separate Tm 3+ laser operating at 1.9 µm may also prove effective in fibers, because it similarly exploits the cross-relaxation process between Tm 3+ ions but avoids any ETU between Ho 3+ ions in the 5 I 7 upper laser level and excited Tm 3+ ions [112]. 6.2 Four-Level Lasers at 2.9 µm As a result of the infrared absorption cut-off of silica and the strong multiphonon relaxation quenching of mid-infrared transitions of rare-earth ions in this host, four-level fiber lasers operating on the 5 I 6 5 I 7 transition at 2.9 µm have to date only involved fluoride glass as the host material. The lifetime of the 5 I 7 level is longer than the lifetime of the 5 I 6 level and hence the 2.9 µm transition can be self-terminating. The first demonstration of a fiber laser using this transition [113] produced only 13 mw when pumped at a wavelength of 640 nm. High-power cascade lasing at 2.9 µm and2.1 µm has been employed recently to extend the output power and to remove bottlenecking at the 5 I 7 level [114]. With direct pumping of the upper laser level, 1.3 W of output power has been measured from this laser scheme [74]. Sensitizing Ho 3+ with Yb 3+ ions, see the energy-level scheme in Fig. 9, in order to exploit the more favorable absorption features of Yb 3+ has been used in diode-pumped crystalline lasers for the generation of both 2.1 µm[115] and 2.9 µm [116] output. When sensitizing with Yb 3+ ions, Ho 3+ -doped silica fiber lasers at 2.1 µm may produce significantly more output because Yb 3+ can be doped to quite high levels in silica thus ensuring strong absorption and sufficient energy transfer to Ho 3+. Similarly, a Yb 3+ -sensitized Ho 3+ -doped ZBLAN fiber will also be diode-laser pumpable and may efficiently provide high-power 2.9 µm output without the costly requirement of an intermediate laser system. Initial spectroscopic results look encouraging [117]. ET 5 I 5 2 F 5/2 GSA 2 F 7/2 τ 2 = 3.5 ms τ 1 = 12 ms Yb 3+ Laser 2.9 µm Ho 3+ 5 I 6 5 I 7 Laser 2.1 µm 5 I 8 Fig. 9. Partial energy-level scheme of Ho 3+ with Yb 3+ sensitizer displaying the measured lifetimes of Ho 3+ when doped into fluoride glass host [114]

20 238 Markus Pollnau and Stuart D. Jackson 7 Erbium-Doped Fiber Lasers at µm The first observation of coherent emission near 3 µm fromerbiumionswas reported in 1967 [118]. In 1983, the first CW lasing near 3 µm was obtained [119]. The first erbium 3 µm fiber laser was demonstrated in 1988 [120]. Although the lifetime of the 4 I 13/2 lower laser level exceeds that of the 4 I 11/2 upper laser level, CW lasing can be obtained on this four-level-laser transition in ZBLAN without employing special techniques to depopulate the 4 I 13/2 lower laser level, because the lower laser level is not fed significantly by luminescent decay or multiphonon relaxation from the upper laser level [121]. In addition, the Stark splitting of the laser levels contributes to population inversion, because the laser transition occurs between a low-lying Stark component of the upper and a high-lying Stark component of the lower laser level [122]. During the relaxation oscillations at the onset of lasing, a red-shift of the lasing wavelength is often observed in erbium 3 µm laser systems [123,124,125], because the excitation energy is accumulated in the long-lived 4 I 13/2 lower laser level and the character of the lasing process changes from four-level to three-level lasing [122]. For the same reason, the tunability range of a 3 µm CW laser [126] is narrowed and red-shifted with increasing pump power. Depending on the erbium concentration and fiber geometry, the 3 µm fiber laser has been operated in a number of different regimes that will be discussed in the following sections. 7.1 Excited-State Absorption and Cascade Lasing Pump ESA has a major influence on the performance of low-doped, corepumped erbium 2.7 µm ZBLAN fiber lasers. Pumping at 980 nm directly into the upper laser level provides the highest Stokes efficiency of η St = λ p /λ l = 35 % [127]. However, ESA at 980 nm from the 4 I 11/2 upper laser level [128] is detrimental to lasing and must be avoided. Experimentally, the best pump wavelength [129] is near 792 nm. This wavelength is at the peak of ESA from the 4 I 13/2 lower laser level [130], as can be seen from the measured GSA and ESA cross-sections in Fig. 10a. The reason for the strong influence of this ESA is that depletion of the lower laser level by ESA favorably results in a redistribution of its population density and overcomes the bottleneck that results from the long lower-level lifetime. Slope efficiencies obtained in this way were < 15 %. Moreover, a saturation of the output power at 2.7 µm was observed, regardless of the pump wavelength, and the highest reported output powers were in the 20 mw region [131,132]. The output power saturated, because the excitation of the metastable 4 S 3/2 level (lifetime 580 µs [133]) led to inversion with respect to the 4 I 13/2 level. A second laser transition at 850 nm repopulated the 4 I 13/2 lower laser level of the 2.7 µm transition, see Fig. 10b, causing the 2.7 µm laser to saturate at low output powers [129].

21 Mid-Infrared Fiber Lasers 239 (a) σesa [10-22 cm 2 ] I15/2 4I13/2 4I11/2 (b) 4 F 7/2 2 H 11/2 4 S 3/2 4 F 9/2 4 I 9/2 ESA Laser 1.7µm τ 5 = 580 µs Laser 850nm Wavelength [nm] 4 I 11/2 4 I 13/2 Laser 2.7µm τ 2 = 6.9 ms τ 1 = 9.0 ms GSA 4 I 15/2 Fig. 10. (a) Absorption cross-sections in ZBLAN:Er 3+ near 800 nm: GSA 4 I 15/2 4 I 9/2 and ESA 4 I 13/2 2 H 11/2, 4 I 11/2 4 F 3/2,and 4 I 11/2 4 F 5/2. (Data taken from [130].) (b) Partial energy-level scheme of erbium indicating the processes relevant to the ZBLAN:Er 3+ cascade laser: Lower loop with GSA to 4 I 9/2, multiphonon relaxation, laser transition at 2.7 µm, luminescent decay, and upper loop with ESA to 2 H 11/2, thermal relaxation, laser transition at 1.7 µm, multiphonon relaxation, laser transition at 2.7 µm. Competitive lasing at 850 nm is suppressed in the cascade regime Er 3+ Significant improvement in the performance of this laser system was obtained by deliberately operating a third laser transition 4 S 3/2 4 I 9/2 at 1.7 µm, thereby suppressing the competitive laser at 850 nm and recycling the excitation energy accumulated in the 4 S 3/2 level into the upper laser level, see the energy-level scheme in Fig. 10b. The slope efficiency of the 2.7 µm transition increased significantly to 23 % [134], close to the Stokes-efficiency limit of 29 % under 800-nm pumping. An output power of 150 mw was demonstrated experimentally [134]. This cascade lasing regime represents the best option for dopant concentrations of typically 0.1 mol% ( cm 3 )at which ESA is important. Also a three-transition-cascade lasing regime with additional lasing at the transition 4 I 13/2 4 I 15/2 near 1.6 µm was demonstrated [135], but no further improvement was obtained. 7.2 Lifetime Quenching by Pr 3+ Co-Doping In ZBLAN fibers with higher dopant concentrations of typically 1 5 mol% ( cm 3 ) and with the double-clad geometry, ESA is much less