Rare-Earth-Doped
`Fiber lasers
`and Amplifiers
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`Imperial College of Science,
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`Rare-Earth-Doped
`Fiber lasers
`and Amplifiers
`
`Second Edition, Revised and Expanded
`
`edited by
`
`Michel J. F. Digonnei
`Stanford University
`Sfanford, California
`
`MARCEL
`
`‘EDEKKER
`
`iii
`i i i
`MARCEL DEKKER, INC.
`
`
`
`NEW YORK - BASEL
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`

`
`The first edition was published as Rare Earth Doped Fiber Lasers and Amplifiers, Michel J. F.
`Digonnet, ed. (Marcel Dekker, Inc., 1993).
`
`ISBN: 0-8247-0458-4
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`Copyright © 2001 by Marcel Dekker, Inc. All Rights Reserved.
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`144
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`D;
`
`Since this work, extremely efficient 1.48—tLm-pumped Er-doped fiber laser
`in fact been reported by Wagener et al. at Stanford University [I07]. This study estat
`that a key factor that needs to be optimized to maximize the conversion efficie
`the erbium concentration. Measurements indicated that fibers with increasingly h
`concentrations have increasingly high thresholds and low slope efficiencies. The:
`effects were attributed to the presence of an increasing percentage of Er“ cluster
`to the fact that clusters dramatically reduce the excited lifetime [I33], and there
`quantum efficiency of the transition. The most efficient fiber laser reported in this
`used a low-concentration fiber (110 mol ppm Er2O3) and a correspondingly lon;
`(42.6 m) [107]. The cleaved fiber ends (~3.5% reflections) fonned the laser Fabry-
`resonator. In spite of the high cavity loss (~29 dB), the threshold was low (~4.8
`and the laser was successfully pumped with a low-power laser diode. It emitted si
`neously in the forward and backward direction, with a forward slope efficiency of
`[lO7]. This is the highest slope efficiency and the highest conversion efficiency re
`in an Er-doped fiber laser (see Table 3). The backward slope efficiency was 31.8‘.
`dichroic high reflector was placed at the pump input end, a total slope efficiency of a]
`imately the sum of these two figures, or ~90.4%, would be expected, as well as a su
`tial reduction in threshold.
`
`This study showed that Er—doped fiber lasers, when pumped near 1.48 pm, 4
`at least as efficient as 980-nm pumped lasers. The reasons for this high performancu
`the low Er concentration and the similarity between the pump and laser photon en
`[I07]. The slope efficiency of 90.4% is, in fact, very close to the quantum limit 0
`predicted for the ratio of pump to signal photon energies. It confirms that the qu
`efficiency of this transition can be within a few percent of unity. In this light, concenl
`quenching may well explain the suboptimal efficiencies and thresholds reported in
`Er-doped fiber lasers (see Table 3). It points to the importance of selecting a suffic
`low rare earth concentration to maximize the performance of fiber lasers or amp
`This requirement was confirmed in a more recent report of a ring fiber laser that u
`a very low-concentration fiber (see Table 3) [21]. After optimizing the fiber lengi
`output coupler transmission, the laser had a low threshold (6.5 mW) and a fairly
`slope efficiency of 38.8%. Tuning from 1525 to 1570 nm was achieved with an intra
`tunable filter.
`
`3.6.8 Summary
`
`In summary, Er—doped fiber lasers operating close to 1.55 pm are extremely eff
`When pumped at 1.48 um, their slope efficiency can be within a few percent of the tha
`cal limit ?\.,,/9», == 95%. Pumping at 980 nm produces a lower, although still subst
`slope efficiency (theoretical limit of ~63%). Pumping at about 800 nm is unfortu
`less efficient (~l5%) because of pump ESA, even with Yb co-doping. Er-dope:
`lasers are now almost exclusively pumped close to 980 or 1480 nm. They have alst
`operated at multiple wavelengths simultaneously. This feature, of great importan
`dense WDM systems, is reviewed in Chapter 5.
`
`3.7 Y1TERBlUM
`
`3.7.1 Basic Spectroscopy
`
`Ytterbium is one of the most versatile laser ions in a silica-based host. It offers s
`
`very attractive features, in particular an unusually broad absorption band that strn
`
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`£5
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`Continuous-Wave Silica Fiber Lasers
`
`145
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`from below 850 nm to above 1070 nm because of the 2F7,;._ —> 2F5,2 transition, as illustrated
`with a representative absorption spectrum in Figure 10 [I34]. Yb-doped silica fibers can
`thus be pumped with a wide selection of solid-state lasers, including A1GaAs (~800-850
`nm) and InGaAs (~98O nm) laser diodes, and Nd:YLF (1047 nm) and Nd:YAG (1064
`nm) lasers. This broad pump band also relaxes considerably both the requirements for
`pump wavelength and its stability with temperature. Just as importantly, Yb-doped silica
`fluoresces effectively over an equally impressive range, from approximately 970 to 1200
`nm (see Fig. 10). This is broader than the range available from Nd-doped fiber lasers,
`which is one of the attractions of Yb“ over Nd“. The Yb-doped fiber laser, therefore,
`can generate many wavelengths of general interest: for example, for spectroscopy or for
`pumping other fiber lasers and amplifiers.
`Another well-known advantage of Yb“ is the simplicity of its energy level diagram.
`As illustrated in the inset of Figure 10, Yb“ exhibits only a ground state (’F7,2) and a
`metastable state (’F5,2) spaced by approximately 10,000 cm“. All other levels are in the
`UV. The radiative lifetime of the 2F5,2 state is typically in the range of 700-1400 us,
`depending on the host [I35]. The absence of higher energy levels greatly reduces the
`incidence of multi-phonon relaxation and ESA and, therefore, should facilitate the devel-
`opment of high-power lasers. Yet another benefit is the abnormally high absorption and
`emission cross sections of Yb“, which are typically several times higher than in multicom-
`ponent glasses [I36]. These combined features allow for very strong pump absorption and
`very short fiber lasers.
`
`|
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`it
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`900
`
`1000
`
`1100
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`Wavelength (nm)
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`Figure 10 Ground-state absorption spectrum, emission spectrum, and energy level diagram of
`Yb“ in silica. ‘The solid lines identify the radiative transitions responsible for the two features in
`the emission spectrum. (From Ref. 134.)
`
`

`
`1 46
`
`Digonnet
`
`A Yb-doped fiber laser is typically pumped into the higher sublevels of the ‘Fm
`manifold (see inset of Fig. 10). At wavelengths below about 990 nm, it behaves as a true
`three-level system (transition A in Fig. 10), whereas at longer wavelengths, from ~1000
`to ~ 1200 nm (transition B), it behaves as a quasi-four-level system. The literature provides
`comprehensive descriptions of the spectroscopy of Yb“, including absorption and emis-
`sion spectra [134,137] and theoretical gain spectra [137].
`Because of the absence of higher energy levels, high concentrations of Yb“ are
`possible in silica-based hosts, often up to several thousand ppm (Table 5). However, recent
`studies have shown that in silica-based hosts a fraction of the Yb ions is strongly quenched,
`resulting in strong unbleachable absorption around 976 nm and the loss of laser action
`[135]. This effect was tentatively attributed to color centers. Because energy migrates
`rapidly between Yb ions, even a small percentage of quenched ions can lead to loss of
`inversion for most of the Yb population. It means that for practical lasers and amplifiers
`utilizing Yb as the laser ion or as a co-dopant, the pump must be detuned from 976 nm,
`while laser action near this wavelength may be extremely inefficient.
`
`'-4-4.Mu....-..t-.
`
`3.7.2 Core-Pumped Yb-Doped Fiber Lasers
`
`Hanna, Tropper, and co-workers, at the University of Southampton, were first to study
`the performance of Yb“ in a silica fiber [137,l45,146]. In their original report, they used
`a pair of prisms as a dispersive element to tune a Yb-doped fiber laser over 152 nm, from
`1010 to 1162 nm, across the long-wavelength emission band of Yb“ [l46]. Because of
`its three—level nature, the Yb-doped fiber laser shifts to longer wavelengths when the fiber
`length is increased [137]. In one report, the wavelength shifted from 975 nm in a 1-m
`laser to nearly 1100 nm in a 100—m laser, while a range of lengths supported simultaneous
`oscillation at two wavelengths [I34]. Enough gain is available on the short-wavelength
`emission band of Yb“ that the two weak (3.5%) Fresnel reflections from cleaved fiber
`ends were sufficient to sustain efficient laser oscillation [146]. By proper selection of the
`fiber length, this fiber laser was also operated at 974 nm. Its characteristics were excellent,
`including a relatively low threshold of 11.5 mW (in spite of the high transmission of the
`reflectors), and a maximum output power of 9.3 mW (sum of the outputs from both ends)
`for 25.3 mW of pump power, or a slope efficiency of 67% (all figures against absorbed
`power). A 980—nm Yb-doped fiber laser with similar performance was reported soon after
`by British Telecom [I38]. It used a much shorter (8.6 cm), higher-concentration fiber. A
`summary of the characteristics of these and other Yb-doped silica fiber lasers can be found
`in Table 5.
`
`In another study, Mackechnie et al. used a long Yb-doped fiber (100 m) to force
`oscillation close to 1115 nm [144]. The resonator utilized a dichroic high reflector butted
`against one fiber end and the cleaved face of the other fiber end, which acted as the output
`coupler. When pumped with an Nd : YLF laser (1047 nm), this laser produced a maximum
`output power of 660 mW, the record at the time. However, the threshold was high (610
`mW) and the slope efficiency modest (28%, both figures against incident pump power),
`partly because the fiber was slightly multimoded at the laser wavelength.
`Two years later, the same research group reported a similar but improved fiber laser
`using a single-mode fiber [134]. When free running at 1102 nm, its threshold was 30 mW,
`its output power was 520 mW, and the slope efficiency was a record 90% (both against
`launched power). The performance was only slightly lower when pumping at 850 nm with
`a Ti:sapphire laser (30-mW threshold and 79% slope efficiency, except that the cavity
`
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`Continuous-Wave Silica Fiber Lasers
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`147
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`1 48
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`Digonnet
`
`was formed by the two cleaved fiber ends and the output power was split equally between
`the two ends). A maximum output of 2 W (1 W at each end) was reported when pumping
`this laser with a higher-power 1064—nrn Nd:YAG laser. The same fiber laser was con-
`strained to specific wavelengths by splicing a narrowband fiber grating at each fiber end
`(as was also done earlier [147]). The two wavelengths that were selected were 1020 nm
`(as a potential pump source for Pr-doped fluoride fiber amplifiers and up—conversion lasers)
`and 1140 nm (to pump blue up-conversion Tm-doped fluoride fiber lasers). Though not
`quite as efficient as the free-running lasers, these two fiber lasers had a low threshold and
`a high slope efficiency, as illustrated in Table 5 for the 1l40—nm laser.
`More recent work on Yb-doped fiber lasers has focused on single-frequency opera-
`tion (see Chap. 5) [134,140,l48]. As an example, Asseh et al. described a very short (10
`cm) single-frequency fiber laser with a very low threshold (<230 p.W) owing to the use
`of a low-loss DFB structure, and a good slope efficiency (44%) [140] comparable with
`those observed in free-running fiber lasers.
`
`3.7.3 Double-Clad Yb-Doped Fiber Lasers
`
`In spite of its important advantages, Yb“ attracted comparatively little attention in the
`early years of this field. This was partly because it was largely overshadowed by the more
`efficient Nd-doped laser, and partly because of the perceived handicap of its three-level
`nature. More recently, however, and perhaps as a result of the resounding success of the
`three-level EDFA, a growing number of laboratories have turned to Yb-doped fiber lasers
`as potential high-power lasers, using in particular double-clad fibers.
`Gapontsev et a1. were first to test cladding—pumped Yb-doped fiber lasers [I42].
`They reported a very efficient laser at ~109O nm pumped with an 875-nm laser diode
`(5-mW threshold, 69% slope efficiency), but unfortunately their laser diode power was
`low and the fiber laser output was limited to 50 mW. It was nevertheless a landmark, and
`the first 1aser—diode pumped laser of this kind. Pask et al. improved on this result with
`a 1040-nm, free-running, double-clad Yb-doped fiber laser pumped at 974 nm with a
`Ti:sapphire laser [134,l39]. The maximum output power was 470 mW at an incident
`pump power of 750 mW (see Table 5).
`Since the mid-1990s, high-power Yb-doped fiber lasers have progressed rapidly,
`from 2 W in 1995 [134], to 20 W [141] and 35 W [143] in 1997, and 110 W in 1999
`[61], the published record at the time of this writing. The 20-W fiber laser, reported by
`Inniss et al., used a Fabry—Perot cavity made with photoinduced gratings formed in a
`deuterium-loaded fiber [141]. The pump source was a high-power 915-nrn InGaAs/Al-
`GaAs laser diode bar. A special optical beam shaper was designed to couple over 50 W
`into the Yb-doped fiber cladding [I49]. The fiber laser emitted 16.4 W at 1065 nm, and
`20.4 W at 1101 nm, in both cases for 32.5 W of launched pump power [I41]. Both lasers
`had slope efficiencies in excess of 50%, and a fairly narrow spectrum (0.24 nm and 0.37
`nm, respectively). The threshold of the 1.1-um laser was less than 100 mW. Its output
`was used to pump a cascaded Raman fiber laser and was efficiently converted to 8.5 W
`of 1472-nm light [l41].
`The 35-W fiber laser reported at the same time by Muendel and co-workers of Polar-
`oid Corporation improved on this result in two ways. First, it achieved an even greater
`slope efficiency of ~65 % against incident power. Second, it was pumped bidirectionally
`with two laser diodes in each direction [I43]. The cavity was made of a high reflector at
`one end and the 3.6% Fresnel reflection from the other, polished fiber end. This laser
`
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`Continuous-Wave Silica Fiber Lasers
`
`149
`
`(FWHM), perhaps as a result of self phase modulation. At high power the output was
`very quiet (0.2% ms noise) and almost fuHy unpolarized.
`Dominic and co-workers, of SDL, Inc., boosted this record by roughly a factor of
`three by using a similar design; namely, a highly efficient laser (58.3% slope efficiency
`against incident pump power) made with a high reflector and a 3.6% Fresnel reflector,
`and pumped with four laser diode bars [61]. The main improvement stems from increasing
`the incident pump power to 180 W. The Yb-doped fiber was spooled and cooled with a
`fan. The maximum output power in the recollimated output beam exceeded 110 W. Al-
`though the average optical density in the fiber core was ~30O MW/cm2, no catastrophic
`fiber failure was observed. The output did, however, exhibit some measurable beam steer-
`ing and a slight degradation ofbeam quality, worsening with increasing power [61]. These
`to 190 W of total cw power. Such issues need to be addressed in the future to achieve
`higher output beam quality, although the double-clad Yb-doped fiber laser has already
`emerged as a viable alternative to commercial cw Nd—doped crystal lasers.
`
`3.8 THULIUM
`
`3.8.1 Basic Spectroscopy
`Energy Level Diagram
`A partial energy level diagram of Tm“ in silica is shown in Figure 11. Thulium presents
`three major absorption bands in the IR; namely, from the ground state 3H5 to the 3F4 level
`(~1630 nm), to the 3H5 level (~121O nm), and to the 3H4 level (~79O nm). Note that,
`
`at most ~12% in MCVD fibers [I53]. This is in sharp contrast with fluoride fibers, in
`which this quantum efficiency is close to unity.
`In silica, the 3H4 level is nonradiatively coupled to the underlying 3H5 level, and it
`is only weakly metastable. One reference cites its lifetime as $10 us [150]. The H, level
`has a short lifetime because it is strongly coupled nonradiatively to the nearby 3F4 level.
`
`bandwidth of this transition: the lasers that have been demonstrated in various composi-
`tlons range in wavelength from ~1.7 to ~2.l um. Such bandwidth makes Tm-doped silica
`3 great source of coherent radiation at mid-IR wavelengths not available from other rare
`Earths. Because of the strong Stark splitting, this laser is quasi—tl1ree-level near 1.9 pm,
`moving toward a four-level laser at longer wavelengths. The wavelength range of Tm-
`
`
`
`

`
`1630nm
`
`transition
`
`Figure 11 Energy level diagram of Tm“ in silica. Solid lines: radiative transitions. Dashed lines:
`nonradiative transitions.
`
`doped fiber lasers includes the strong absorption overtone of water around 1.98 pm, a
`wavelength now used in various developing microsurgical procedures, such as laser angio-
`plasty, blood coagulation, and microsurgery. This laser is also expected to find applications
`in eye-safe LIDAR and for atmospheric sensing, in particular the detection of CO2 and
`methane, which exhibit absorption lines in this range. It may also become an important
`laser source in ultra~low-loss fiber communication.
`Stimulated emission arising from up-conversion has also been observed in the blue
`and UV in Tm—doped fibers pumped in the IR [154]. However, as described in the follow-
`ing these have not yet produced laser oscillation.
`
`Pump Wavelengths
`
`The 3H6 —> 3F4 absorption band of Tm-doped silica possesses an extremely broad line-
`width, close to 130 nm. In fact, it is one of the broadest in any of the trivalent rare earths.
`This material can thus be pumped in the short-wavelength wing of this transition at 1064
`nm. However, as shown in Figure 11 this wavelength suffers from ESA to the 3F2,3F-3
`levels [153,155,l56]. Another possible pump band is at about 670 rim, on the 3H6 —)
`3F2,3F3 transition, although performance is again limited by pump ESA [l5l,l57]. Photo-
`chromic effects in Tm—doped fibers pumped with an Ar-ion laser may preclude the use
`of short pump Wavelengths [I58].
`The pump band most commonly used is the 3H6 —) 3H4 transition at about 800 nm,
`which fortunately exhibits no significant ESA. This transition is also very broad, and it
`allows pumping at the strong absorption peak near 790 nm with either a AlGaAs laser
`
`

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`diode or a Tirsapphire laser. Another useful feature is cross-relaxation between Tm“
`pairs, which takes place at higher Tm concentrations [I53]. This process leads to energy
`transfer from a Tm“ ion in the 3H4 level (the donor) to a neighboring Tm“ ion in the
`ground state (the acceptor). The latter is thus excited to the 3F4 level, whereas the donor
`drops to the 3F4 level, yielding two excited ions for one pump photon, or a quantum
`efficiency of 2. This effect has been used effectively to pump a fiber laser with a high
`Tm concentration (7 X 10” cm”) [l53]. It is thought to be responsible for the near-unity
`quantum efficiency measured in Tm-doped fiber lasers pumped at about 800 nm [151,152].
`Tm-doped silica fibers have also been pumped at 1570 nm, which corresponds to
`the absorption peak of the 3H,, ——> 3F., transition. Because it excites the Tm“ ions directly
`into the 3F4 level, it produces a higher efficiency [20]. The energy level diagram of Tm“
`(see Fig. 11) suggests ESA at this wavelength, although no mention of it is made in the
`literature.
`
`3.8.2 Thulium-Doped Fiber Lasers
`
`General Properties
`
`Table 6 summarizes the properties of representative cw Tm-doped silica fiber lasers. Again
`most of the early work was carried out at the University of Southampton by Hanna, Trop-
`per, and co-workers, who coauthored nearly two-thirds of all publications on this subject.
`The threshold of Tm-doped fiber lasers is typically tens of milliwatts, mostly as a result
`of the low quantum efficiency of the transition. However, by increasing the metastable
`lifetime with Al co-doping and reducing the cavity loss, a launched pump power threshold
`of only 4.4 mW (with a moderate slope efficiency of 17%) was reported, which allowed
`pumping with a laser diode [13]. In contrast, the slope efficiencies are typically fairly
`high, in the 30% range or higher. The highest reported value, 66% against launched pump
`power, was obtained with 1.57-um pumping. Fiber lengths are typically fairly short for
`a rare earth doped silica fiber laser, from as low as 22 cm to 4 rn.
`As in other quasi—four-level lasers, the laser wavelength can be adjusted by changing
`the fiber length or the reflectivity of the mirrors [20,70]. Jackson and King reported a
`detailed experimental analysis of the dependence of the threshold, slope efficiency, and
`laser wavelength on the fiber length and mirror reflectivity [70]. By varying these two
`parameters, they could adjust the laser wavelength from 1877 to 2033 nm. Using an intra-
`cavity three-plate birefringent filter as a tuning element, a Tm-doped fiber laser was also
`tuned over 237 nm for a given fiber length, and over 276 nm (1780 to 2056 nm) with
`two different fiber lengths [151]. The tuning curve is reproduced in Figure 12 [15 1,153].
`In a similar study, a Tm-doped alurninosilicate fiber laser was tuned from 1.71 to 2.0 tun,
`or a range of nearly 300 nm [13]. This is the widest tuning range achieved in a silica fiber
`laser. A second laser made with a gennanosilicate fiber was tuned over a slightly shorter
`range (210 nm) shifted 60-80 nm toward shorter wavelengths, showing the considerable
`influence of the host on the laser wavelength [13].
`For reference, Tm-doped fibers have also been used in a ring laser configuration
`[20] and in a photoinduced Bragg grating cavity [152]. They have been pumped with laser
`diodes [13,70,152], Q-switched [159], gain switched [I60], and mode—locked [l6l]. The
`energy budget of cw Tm-doped fiber lasers has been the object of several theoretical
`Studies [l62,l63] that tend to be rather complex owing to the number of cross-relaxation
`and BSA processes involved.
`
`

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`152
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`Digannet
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`l
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`153
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` J
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`Continuous-Wave Silica Fiber Lasers
`
`1.0
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`1700 1750 1800 1850 1900 1950 2000 200 2100
`
`Wavelength (nm)
`
`Figure 12 Experimental tuning range of two Tm-doped silica fiber lasers of different fiber
`lengths. (From Ref. 151.)
`
`High-Power Lasers
`
`The Tm-doped silica laser system is efficient enough that core-pumped fiber lasers with
`output powers in excess of 1.3 W were achieved very early on [156]. Higher powers, in
`the 5-W range, were later observed in double-clad fibers [70]. Although many of the lasers
`listed in Table 6 had a cutoff wavelength below 1.9 pm, they were probably oscillating
`in the fundamental mode [l5l].
`In spite of ESA at 1064 nm, Tm-doped fiber lasers have been essentially as efficient
`when pumped at this wavelength [l55,l56]. For 1064-mn pumping to be efficient, it is
`essential to maintain the cavity loss as low as possible to minimize the population in the
`3F4 level, and thus minimize the loss of pump photons through ESA. In spite of BSA, the
`Output power grows linearly with absorbed pump power, because above threshold the laser
`population is clamped [155], and the relative pump power lost to ESA does not change.
`Tm-doped fiber lasers pumped at 1064 nm have produced up to 1.35 W of output power
`for 4.5 W of absorbed pump power [156].
`
`Up-Conversion
`
`In fluorozirconate fibers, Tm“ has produced efficient up-conversion fiber lasers (see Chap.
`4). In silica fibers, however, up-conversion is expected to be hindered by short nonradiative
`lifetimes. In spite of this shortcoming, fluorescence caused by frequency up-conversion
`has been observed near 370 and 460 nm in Tm-doped silica fibers excited at 660 nm
`[154]- The 460-nm fluorescence is thought to arise from two-photon absorption, a first
`Photon from the 3H6 level to the 3F;/3F3 levels, which rapidly decay to the underlying 3H4
`level, followed by absorption of a second photon to the ‘D2 level (see Fig. 11). Decay
`fF0H1 this level to the ground state produces fluorescence at 460 nm. Fluorescence was
`also observed at 370 and 467 run under 1064-nm pumping [I54]. The latter was attributed
`I0 three—photon absorption to the ‘G4 level followed by fluorescence to the ground state
`(S36 Fig. 11). Yb-sensitized Tm-doped silica fibers pumped at 1064 nm also produced
`blue fluorescence by multiple energy transfers from the excited Yb ions to the Tm ions
`
`

`
`154
`
`Digonnet
`
`[154]. These up-conversion processes were too inefficient (only ~ 105 of the ground state
`was depleted) to produce a useful laser, largely because of the short nonradiative lifetime
`of the levels involved. The observation of photodarkening in Tm-doped silica fibers ex-
`posed to strong 475-nm light, attributed to the formation of color centers [I58], points to
`another difficulty with the operation of this host as a blue laser.
`
`3.9 HOLMIUM
`
`Figure 13 shows a pa.rtial energy level diagram of Ho”. This ion exhibits strong GSA
`bands centered around 450, 540, and 650 nm. Laser emission has been observed at about
`2 um from the 5I7 level to the ground state 513. It is typically centered between 1.9 and
`2.0 um and exhibits a FWHM bandwidth of about 200 nm [1 1,137,164,165]. Therefore,
`this laser is potentially useful for some of the same applications as the Tm-doped laser,
`in particular for medical applications. When pumped on any of the foregoing pump bands,
`it is a quasi—three-level transition. The lifetime of the 517 level in silica has been measured
`to be 600 us [164]. A quantum efficiency was inferred [165] for the 517 —> 51,; transition
`of 0.11, which is lower than in fluoride fibers, perhaps as a result of nonradiative relaxation
`from the 517 level.
`The earliest Ho-doped silica fiber laser was pumped at 457.9 nm with an Ar-ion
`laser, and it had a relatively high threshold of 46 mW and a low slope efficiency of 1.7%
`[ 137,164] (Table 7). Most of the subsequent work used H05"-doped silica fibers sensitized
`with Tm“ [1 1,56,162,l65,l66] which present the advantage that they can be pumped be-
`tween 800 and 830 nm and at 1064 nm. As illustrated in Figure 13 for 0.8—tLm pumping,
`a pump photon excites a Tm“ ion to the 5H4 level. This level decays nonradiatively to
`the metastable level (5F4) of Tm“, and its energy is transferred to an Ho“ ion in the ground
`state, which is thus excited to the metastable level 517. To achieve high pump absorption,
`especially when pumping in the wings of the Tm“ absorption band, the Tm concentration
`is generally chosen to be very high, typically 5-40 times larger than the Ho concentration
`
`Energy (cm‘1)
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`Figure 13 Energy level diagram of H0“ in silica, showing sensitization with Tm“. Solid lines:
`radiative transitions. Dashed lines: nonradiative transitions.
`
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`1 56
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`Digonne;
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`(see Table 7). High energy transfers have been reported [11]. For a given fiber length,
`the pump wavelength must be carefully selected to optimize the fiber laser output power,
`If the pump wavelength is such that the pump is absorbed too strongly, more power is
`absorbed per unit time by the Tm ions than can be transferred to the Ho ions. If the pump
`is absorbed too weakly, the inversion drops and the output power is reduced [11].
`As in any quasi-three-level laser, the wavelength can be adjusted by changing the
`fiber length. In Ho/Tm-doped silica fiber lasers, oscillation was thus demonstrated from
`2037 to 2096 nm (length change from 13 to 35 cm) [l65], from 1960 to 2032 nm (1-35
`cm) [162], and from 1960 to 2170 nm (23—l24 cm) [I66]. Several laser peaks have been
`observed between 1840 and 2260 nm in a free-running Ho/Tm-doped silica fiber laser,
`indicating a potentially much broader tuning range [1 1]. This last wavelength is the longest
`reported for a doped silica fiber laser.
`Ho/Tm-doped fiber lasers have been pumped with Ti : sapphire lasers [56,l62,165],
`dye lasers [1 1], Nd:YAG lasers [166], and AlGaAs laser diodes [1 1]. As can be seen in
`Table 7, all pump bands have produced relatively high thresholds and low to modest slope
`efficiencies. Pumping