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`IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 1, NO. I. APRIL 1995
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`Ytterbium-Doped Silica Fiber Lasers: Versatile
`Sources for the 1-1.2 ,u.m Region
`
`H. M. Pask, Robert J. Carman, David C. Hanna, Anne C. Tropper,
`Colin J. Mackechnie, Paul R. Barber, and Judith M. Dawes
`
`Invited Paper
`
`Abstract— Ytterbium-doped silica fibers exhibit very broad
`absorption and emission bands, from ~800 nm to ~1064 nm
`for absorption and ~970 nm to ~l200 nm for emission. The
`simplicity of the level structure provides freedom from un-
`wanted processes such as excited state absorption, multiphonon
`nonradiative decay, and concentration quenching. These fiber
`lasers therefore offer a very efficient and convenient means
`of wavelength conversion from a wide variety of pump lasers,
`including AlGaAs and InGaAs diodes and Nd:YAG lasers. Effi-
`cient operation with narrow linewidth at any wavelength in the
`emission range can be conveniently achieved using fiber gratings.
`A wide range of application for these sources can be anticipated.
`In this paper, the capabilities of this versatile source are reviewed.
`Analytical procedures and numerical data are presented to enable
`design choices to be made for the wide range of operating
`conditions.
`
`I.
`
`INTRODUCTION
`
`FTER the first report in 1962 of laser action in Yb3+-
`doped silicate glass [1]. Yb“ has, until recently, at-
`tracted relatively little interest as a laser-active ion. It has been
`overshadowed by the Nd“ ion with its important advantage of
`a four level transition, whereas Yb3" has only three level and
`quasi—three level transitions. In fact. the most important role
`of the Yb“ ion has so far been as a sensitizer ion, absorbing
`pump photons over a wide spectral range and then transferring
`the excitation to an acceptor ion, such as Er3+, which then acts
`as the laser-active ion [2]. [3].
`More recently, interest has been shown in Yb3+ as a laser
`ion, in the form of Yb3+-doped silica and fluoride fiber lasers
`[4]—[8], and Yb3+-doped YAG [9], [10]. There are several
`reasons for this growth of interest. As shown in Fig. 1(a), the
`Yb3+ energy level structure is a simple one, consisting of two
`manifolds; the ground manifold ("F7/2 (with four Stark levels
`labeled (a)—(d)
`in the figure) and a well-separated excited
`manifold 2F,-,/2 (with three Stark levels labeled (e)—(g) in the
`figure), ~lO000 cm‘1 above the ground level. Thus there is no
`excited state absorption at either pump or laser wavelengths.
`The large energy gap between 2F,-,/2 and 2F,—/2 precludes
`Manuscript received August 5. 1994: revised October 10. I994. This work
`was supported by the Science and Engineering Research Council (SERC) and
`by the RACE II GAIN program.
`H. M. Pask, D. C. Hanna A. C. Tropper, C. J. Mackechnie, P. R. Barber,
`and J. M. Dawes are with the Optoelectronics Research Centre, University of
`Southampton, HANTS SO17 IBJ, U.K.
`R. J. Carman is with the Centre for Lasers and Applications, Macquarie
`University. N.S.W. 2109, Australia.
`IEEE Log Number 9409720.
`
`nonradiative decay via multiphonon emission from ZF5/2,
`even in a host of high phonon energy such as silica, and also
`precludes concentration quenching. These features contribute
`to the high efficiency of operation that can be achieved in
`Yb3+ lasers, as does the closeness of the pump and laser
`wavelengths. In fact, this energy defect, which leads to heating
`of the host, is a factor of ~3 smaller for Yb:YAG compared
`to Nd:YAG (pumped at 800 nm and lasing at 1064 nm). This
`reduced thermal burden is a motivating interest for Yb:YAG.
`Finally, the Yb3+ spectrum is rather broad both in absorption
`and emission, and particularly so in a gerrnanosilicate host, as
`shown in Fig.
`l(b). The broad absorption spectrum allows a
`wide choice of pump wavelengths. In the form of a fiber, which
`allows even very weak absorption to be exploited, pumping
`can extend from 800 nm out to 1064 nm. Similarly, for a fiber,
`laser operation can be made to extend well into the weak wings
`of the emission, provided sufficient frequency discrimination
`can be introduced to suppress lasing at
`the peaks of the
`emission profile. The impressive progress in development
`of fiber gratings [1 I] has made this matter of frequency
`discrimination very straightforward and practical, so one can
`now contemplate the prospect of narrow linewidth operation at
`any discrete wavelength between ~97S and ~l200 nm, with
`some degree of tunability (~l)% available by stretching or
`temperature tuning the grating. This range covers a number
`of wavelengths needed for specific applications, and these
`can now be generated very conveniently from an Yb3+ -doped
`silica fiber equipped with appropriate gratings. Examples in-
`clude l02O nm for pumping 1300-nm fiber amplifiers [12] and
`upconversion lasers based on Pr3+—doped ZBLAN [13]—[l5],
`1140 nm for pumping Tm3+—doped ZBLAN upconversion
`lasers [16], [17], and 1083 nm for optical pumping of He [18].
`Given the wide range of different operating characteristics
`that might be required for various applications, and the variety
`of ways (e.g., different pump wavelengths)
`that could be
`used to achieve these characteristics, there is a need for a
`comprehensive discussion of the capabilities of Yb3+ fiber
`lasers, giving quantitative design procedures. Such has been
`the aim of this paper. A general discussion of Yb laser char-
`acteristics is presented in Section II. In Section III, analytical
`procedures and numerical data are presented to enable design
`choices to be made. Such calculations depend heavily on the
`availability of accurate absorption and emission cross section
`data, and the relevant spectroscopy is described in Section IV.
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`l()77—260X/95$()4.00 © I995 IEEE
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`PASK er al.: YTTERBIUM-DOPED SILICA FIBER LASERS
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`300
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`250
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`200
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`150
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`100
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`(mW)
`1020nmoutputpower
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`50
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`O
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`100
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`200
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`300
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`400
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`500
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`600
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`launched pump power (mW)
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`Fig. 8. Laser performance at 1020 nm achieved using fiber gratings. Feed-
`back provided by gratings with refiectivitics of ~95% and ~30% at 1020 nm
`spliced to the input and output ends of the Yb3+-doped fiber respectively
`(fiber length l0m, diameter 3.0 um, NA~0.17, [Yb3+]~550 ppm. pump
`wavelength 840 nm).
`
`has been developed using a 5 m length of 3.0-um diameter
`fiber, which was chosen to give equal amounts of unabsorbed
`pump (840 nm) and laser output (1020 nm). This output was
`used to pump a Pr3+—doped ZBLAN upconversion fiber laser
`from which, when provided with mirrors for blue. green, or
`red lasing, 6.5 mW at 491 nm, 18 mW at 520. and 55 mW at
`635 nm were generated for a combined incident pump power
`of 380 mW at the two wavelengths [15].
`
`B. Laser Action at 1140 nm
`
`Another wavelength of particular interest is 1140 nm, re-
`quired for pumping the efficient blue laser source based
`on upconversion in Tm3+—doped fluoride fiber. Laser action
`at 1140 nm has therefore been investigated in Yb“-doped
`fiber using fiber gratings, and in this case using a diode-
`pumped Nd:YLF laser operating at 1047 nm as the pump
`source [17]. The computer—modeling in Fig. 9 shows gain
`spectra calculated for various launched pump powers. For each
`spectrum, the fiber length was optimized to maximize the gain
`at 1140 nm. As described in previous sections, the long fiber
`lengths (~l00 m) required, are a consequence of the very
`weak absorption of the pump.
`Laser action has been demonstrated at 1140 nm in a laser
`
`cavity consisting of a highly—reflecting dielectric-coated mirror
`butted to the input end of a 90 m length of 3.75-Mm diameter
`doped fiber and a grating (~50% reflectivity at 1140 nm)
`written into a short
`length of the doped fiber and fusion
`spliced at
`the output end. The amount of discrimination
`provided against the free—running wavelength (1106 nm) was
`therefore ~4 dB per pass, and it was found that this was
`sufficient to constrain laser oscillation to 1140 nm. As shown
`
`in Fig. 10, the laser threshold was reached with only 6 mW
`of launched pump power, and 330 mW of output power was
`achieved for an launched pump power of 500 mW. The slope
`efficiency of the laser, with respect to launched pump power
`was ~66%; the difference between this and the limiting value
`of the ratio of laser to pump photon energies (92)%, probably
`
`Gain
`
`(:11!) 1 100
`
`Wavelength (nm)
`
`Fig. 9. Gain spectra calculated for various launched pump powers, with the
`fiber length chosen to maximize gain at 1140 nm as shown in the inset (fiber
`diameter 3.75 pm, NA~0. 17, lYb3+ ]~550 ppm, pump wavelength 1047 nm).
`
`400
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`‘|140nmoutputpower(mW)
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`350
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`300
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`250
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`200
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`150
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`100
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`50
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`O
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`100
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`200
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`300
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`400
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`500
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`600
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`launched pump power (mW)
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`ll40 nm achieved using a fiber grating.
`Fig. 10. Laser performance at
`Feedback provided by a dielectric-coated mirror
`(99% reflectivity at
`1100-1200 nm) and a grating (50% reflectivity at 1140 nm) written into
`the doped fiber at the output end (fiber length 100 m, diameter 3.75 pm,
`NA~O.17, [Yb3+]~550 ppm, pump wavelength 1047 nm).
`
`being due to the fiber background loss, mirror butt loss, and
`splice loss. This 1140 nm source has been used to pump a
`Tm3+:ZBLAN upconversion laser, producing 30—mW output
`power at 480 nm [17].
`
`VII. CLADDING—PUMPED OPERATION OF YB3+ LASERS
`
`In previous sections we have presented results which il-
`lustrate that Yb3+-doped fiber lasers are very efficient with
`respect to absorbed pump power. The overall conversion of
`incident pump light to laser output, however, is typically only
`~40—60%, limited largely by the efficiency with which a near-
`diffraction—limited pump can be launched into single—mode
`fiber. In this section we consider the uses of cladding—pumping,
`which relaxes the requirements on pump beam quality and
`launch alignment, and which may find wider use, not just
`for Yb3+—doped fibers,
`to get the highest launch efficiency
`for near—diffraction-limited pump sources. Another possible
`use for cladding—pumping would be to allow a much higher
`incident power than could be tolerated, for damage reasons,