Claddingpumped Fiber Lasers

The output power of EDFAs has, to date, been limited only by the amount of single-mode coupled pump power that one has available for pumping into one of the many absorption bands of erbium-doped fibers. The optical powers that are currently available from single-mode fiber-pigtailed laser diodes are on the order of 100 mW. These powers are limited by intrinsic materials properties of the laser diodes themselves (e.g., facet damage) and are not likely to be significantly improved in the next 10 years. Power scaling with single-stripe diode lasers can be achieved by double pumping (co- and counterpropagating with respect to the signal), but effects such as pump laser diode cross talk have to be considered, and pump laser isolators may be required. Single-stripe laser diodes can be polarization multiplexed so that two pump diodes can be utilized from each singlemode fiber pump port. The insertion losses of the polarization combiners and the need to control each pump polarization tend to limit the usefulness of this approach. Another method of maximizing the pump power from single-stripe laser diodes is to combine both polarization and wavelength multiplexing through WDMs such that as many as four pump lasers can be combined through a single fiber port.61 Once again, the insertion losses and costs of additional components tend to make this an impractical power scaling approach. The reliability of the overall pump module is of great concern because multiple laser diodes are all being run at their maximum power ratings and there are no apparent methods for design of pump redundancy. Clearly, an approach that is capable of utilizing higher power pump laser diode arrays, is arbitrarily power scalable, and has redundancy and power derating as design parameters is highly desirable.

One method of obtaining scalable single-mode fiber-coupled power has been the diode-pumped solid-state (DPSS) Nd3+ laser.62 A high-power, nondiffraction-limited diode laser array is focused into a Nd:YAG or Nd: YLF crystal, which is surrounded by feedback mirrors that define the laser cavity. The laser cavity defines the spatial mode output of the Nd3+ laser, which can be readily made to operate in the fundamental TEMoo mode. The diffraction-limited output can then be efficiently (~80%) coupled into single-mode fiber. The second method of obtaining high fiber-

coupled powers is through the use of double-clad fiber lasers. These cladding-pumped fiber lasers are designed to have two distinct waveguiding regions: a large multimode guiding region for the diode pump light and a rare-earth-doped single-mode core from which the diffraction-limited laser output is extracted. A schematic diagram of a high-power Yb3+ cladding-pumped fiber laser is shown in Fig. 7.15. The diode laser pump is contained in a silica (n = 1.46) rectangular waveguiding region of dimensions 360 X 120 /xm, usually referred to as the pump cladding. The pump cladding region is typically surrounded by a low-index polymer (n = 1.39) giving a high NA pump region (NA = 0.48) into which to couple diode laser power. The low-index polymer is coated with a second protective polymer. The Yb3+-doped single-mode core is located at the center of the pump cladding. If the background losses of the pump cladding can be neglected, the only loss of mechanism of the pump light is when the rays occasionally cross the rare-earth-doped single-mode core and are absorbed. When feedback elements, either dielectric coating or fiber Bragg gratings, are present, all the laser power can be extracted from the single-mode core. The most important property of the cladding-pumped fiber laser is that a brightness conversion of highly nondiffraction-limited diode laser arrays is obtained. The brightness increase is approximately given by the ratio of areas of the pump cladding to the single-mode core area, a value of 1500 in this example.

Single-mode core doped with Yb'

Si02 pump cladding

Low index ploymer cladding

Pump

Fig. 7.15 Schematic of a cladding-pumped fiber laser.

Single-mode core doped with Yb'

Si02 pump cladding

Low index ploymer cladding

Single-mode laser output

Pump

Fig. 7.15 Schematic of a cladding-pumped fiber laser.

The first cladding-pumped fiber laser to be demonstrated used a circular pump cladding.63 The modes in a fiber of circular cross section are unique in that only the HElm modes have intensity at the center of a multimode waveguide. In order to dramatically improve the pump absorption, the single-mode rare-earth-doped core was offset to the side of the pump cladding. A second version of a cladding-pumped fiber laser utilized a rectangular pump region in order to break the circular symmetry.64 The rectangular pump region also better matched the aspect ratio of the broad-area pump laser diode. The cladding-pumped laser was pumped by directly butting the fiber up to the pump laser facet, without the use of any pump coupling optics. Output powers of 5 W at slope efficiencies of 51% have been obtained from diode-pumped Nd3+ cladding-pumped fiber lasers.65 An output power of 9.2 W has recently been obtained from a diode-pumped Nd3+ cladding-pumped fiber laser.66 The slope efficiency was only 25%, a direct result of using a circular geometry for the cladding-pumped structure and the resultant inefficiency of pump light absorption by the single-mode core. Ytterbium-doped cladding-pumped operation has recently been demonstrated with slope efficiencies of greater than 70% and output powers of 6.8 W.67 The feedback elements were fiber Bragg gratings that were written in the innermost single-mode germanium-doped core. The wavelength of operation of the Yb3+ cladding-pumped fiber laser was 1090 nm, where the Yb3+ laser behaves primarily as a four-level laser system. In glass, Yb3+ ions exhibit such a high degree of Stark splitting that laser operation has been obtained from 975 to 1170 nm from the 2F5/2 excited electronic state. An energy-level diagram of Yb3+ is shown in Fig. 7.16. Modeling of Yb3+-doped cladding-pumped fiber lasers shows that in the cladding-pumped geometry, laser operation should be readily obtained from 1050 to 1150 nm, where the laser behaves primarily as a four-level or quasi-four-level

2Fs/2

Absorption 820-1060 nm

Emission 975-1150 nm

Yb3+

Fig. 7.16 Energy-level diagram of Yb3+.

laser system.68 Because of the indirect nature of the pumping in the cladding-pumped geometry, it has been generally believed that only four-level laser operation should be possible. A notable exception has been the demonstration of both lasing and amplification by Er3+ ions at 1.55 /¿m in cladding-pumped fibers."9 Pumping of three-level systems such as Er3* in double-clad fiber structures is difficult because the inherent ground-state absorption must be bleached before gain can be achieved. In the case of Er/Yb double-clad fibers, a higher inversion of Er3^ ions was made possibly by co-doping with Yb3" and pumping at 970 nm, where the large Yb3+ absorption cross section compensates for the reduction in pump rate. Recently, 980-nm pumped operation of Er3+-doped cladding-pumped amplifiers was demonstrated, although the degree of inversion was not apparent because the noise figure of the amplifier was not reported.70

There are numerous advantages to the use of cladding-pumped fiber lasers as a method of obtaining high single-mode fiber-coupled optical powers. The first is that the power is intrinsically single-mode fiber coupled: there is no power lost to this coupling step, nor are there any alignment tolerances associated with single-mode fiber coupling. Because the alignment of the pump diode light is into a highly multimode waveguide, alignment tolerances of tens of microns are typical. The second advantage is the high degree of efficiency with which pump diode light can be converted into single-mode fiber-coupled power: 50% in Nd3+ and more than 70% in Yb3+-doped cladding-pumped fibers have been demonstrated. Cladding-pumped fibers also appear to be a preferred method of power scaling for high-power CW lasers. Thermal effects are minimized in the cladding-pumped fiber laser as a result of the high surface-area-to-volume ratio. Given an active gain medium volume of 1 cm2, a cladding-pumped fiber laser has 40 times more surface area than that of a DPSS laser. DPSS lasers also suffer from thermal problems at CW powers of a few watts, which tends to make operation in the TEM0() mode difficult and thereby lowers the fiber-coupling efficiency. Because the output mode quality of the cladding-pumped fiber laser is defined by the single-mode waveguiding core, a diffraction-limited beam is obtained at all power levels of operation. Finally, cladding-pumped fiber lasers offer the possibility of writing integral feedback elements directly in the fiber core, through the use of fiber Bragg gratings.

The power scaling limitations of cladding-pumped fiber lasers have not yet become apparent. A power limit of several tens of watts has been estimated.71 Nonlinear effects rather than thermal effects will probably be the ultimate limiting mechanism. Stimulated Brillouin gain will probably not be the limiting nonlinearity because of the large number of longitudinal laser modes that are operational. It is much more likely that stimulated Raman scattering will be the ultimate limiting nonlinearity. The cladding-pumped fiber laser power will most likely not be significantly decreased but will be frequency converted by approximately 450 cm '. The future of cladding-pumped lasers will depend on new methods of efficient coupling of high-power diode laser power into cladding-pumped fibers. The development of higher brightness laser sources will also be important and will increase the efficiency and wavelength range over which cladding-pumped fiber laser operation is possible.

B. Er/Yb AMPLIFIERS AND LASERS

The absorption spectrum of Yb3+ in silica fibers consists of an intense, broad peak centered at 975 nm. The absorption spectra of both Er3+ and Er3+/Yb3+ co-doped fibers are shown in Fig. 7.17. Co-doping with Yb3+ provides a much greater spectral region into which to pump these fibers, from approximately 800-1070 nm. A diagram illustrating the Yb —* Er energy transfer process is shown in Fig. 7.18. Absorption of a pump photon by Yb3+ ions promotes an electron from the 2F7/2 ground-state level to the 2F5/2 manifold, which is followed by efficient energy transfer from this level to the 4Iu/2 level of Er3+ and nonradiative decay to the 4I13/2 amplifying level. This energy transfer can be up to 85% efficient provided that the energy is efficiently funneled from the Yb3+ sensitizer network (the Yb3+ concentration is usually 10 times the concentration of Er3+ ions), and that

Wavelength (nm)

Fig. 7.17 Absorption spectra of Er3+ and (Er3+/Yb3+) co-doped silica fibers.

Wavelength (nm)

Fig. 7.17 Absorption spectra of Er3+ and (Er3+/Yb3+) co-doped silica fibers.

Fig. 7.18 Er/Yb energy-transfer diagram

Fig. 7.18 Er/Yb energy-transfer diagram the transferred energy remains in the Er'~ ion (i.e.. there is no significant amount of back transfer of energy from the 4JM,<2 level of Er1' to the 2F3/2 level of Yb3+). The host glass composition has been found to be critically important in controlling the rate of back transfer of energy.72'73 High phonon energy phosphate glasses and subsequently phosphosilicate fibers have been found to be necessary in order to increase the 4In/2 —» 41]3/2 nonradiative relaxation rate compared with the 4IM/2 —» 2F5/2 back-transfer process.

The sensitization of Er3+-doped silica fiber by YbiT presents several advantages. Ytterbium sensitization was initially proposed for more efficient pumping of the 800-nm band with AlGaAs diode lasers. Early results with 807-nm pumping of Er3+-doped fibers were disappointing: there was a strong ESA band in this region that degraded the amplifier performance.74 There is a strong decrease in ESA in the 820- to 830-nm spectral region. Co-doping with Yb3+ would allow one to pump in this region with AlGaAs diode lasers without the deleterious effects of ESA. However, the results obtained with pumping of Er/Yb co-doped fibers in the 800-nm band have never appeared attractive enough to compete with 980- or 1480-nm pumping of Er3*-doped fibers.75 By far, the biggest advantage of co-doped fibers has been the pumping with high-power DPSS or cladding-pumped lasers at wavelengths near 1060 nm. These results, which are summarized next, have allowed almost infinite power scaling of 1.5-/xm fiber amplifiers with diode laser-based sources. Another advantage of Er/Yb co-doped fibers has been for 980-nm pumping of short laser and amplifier devices. Because of the large oscillator strength of the Yb3+ transition and the high Yb3t concentrations necessary to be in the fast donor diffusion limit, 980-nm pump absorption occurs within 1 cm of these co-doped fibers. This is a tremendous advantage for short-cavity, single-frequency, fiber Bragg grating lasers. An output power of 19 mW was obtained at a slope efficiency of 55% in a 2-cm cavity.76 This laser had the advantage of a high-output power without using a fiber master oscillator power amplifier (MOPA) structure, which adds significant noise to the source. The short absorption length also has a tremendous advantage in 1.55-/Am planar amplifiers, where high gains need to be achieved in short devices. Internal gains in excess of 30 dB have been demonstrated in Er/Yb planar waveguide amplifiers.77 Furthermore, an analysis shows that for amplifier lengths of 1 m or less, Er/Yb co-doped amplifiers will in all cases exhibit superior performance to Er3+-doped waveguides.78

The first long-wavelength pumped (1.06-/im) Er/Yb co-doped fiber laser was reported in 1988 by a group from the University of Southampton.79 The fiber host was an aluminosilicate glass with a relatively low concentration of Yb3+ — 5000 ppm. The optical conversion efficiency from 1.06 —» 1.55 /¿m was reported to be only 4% in this case. The first optical fiber amplifier based on Er/Yb co-doped fibers and a diode-pumped Nd: YAG pump laser at 1064 nm was reported in 1991; output powers of +13 dBm and gains of 35 dB were reported.80 The fiber was based on a bulk phosphate glass composition and fabricated by the rod-in-tube method. Both the host glass composition effects and the high concentration of Yb3+ ions necessary to be in the fast donor diffusion limit were recognized. A major disadvantage of this approach, however, was that the use of the low-melting-point phosphate glass fiber prohibited direct fusion splicing to silica fibers. However, the efficiency of the pure phosphate glass host was soon reproduced in a co-doped phosphosilicate fiber.81 Output powers of +24 dBm and small-signal gains of 50 dB were reported using a diode-pumped NdrYLF laser at 1053 nm as the pumping source.82 The optical conversion efficiency from 1.06 —» 1.55 /xrn was reported to be approaching 40% in these fibers. Further power scaling of Er/Yb co-doped fiber amplifiers came with the use of cladding-pumped fiber lasers as pumping sources. The first report was of a 3-W multistripe AlGaAs diode laser at 805 nm that gave a gain of 45 dB and an output power in excess of +20 dBm.83 An amplifier based on cladding pumping of Er/Yb fibers with an output power of +17 dBm was demonstrated.84 A 1-W broad-stripe diode laser at 962 nm was the pump source. Cladding pumping of the Er/Yb core with a pump wavelength of 962 nm gave approximately the same pump absorption per unit length as 1060-nm pumping of the co-doped single-mode core. A Nd3+-doped cladding-pumped fiber laser with an output of 4.2 W was used to demonstrate a 1.5-W (+32.6-dBm) Er/Yb power amplifier.85 A 1-W Er34 fiber amplifier was demonstrated by pumping with four 980-nm semiconductor MOPA devices.86 Three cladding-pumped fiber lasers and a three-stage Er/Yb amplifier produced an output power of 4.3 W ( + 36.2 dBm).87 The output power of this amplifier was more than 4 W over a 30-nm spectral

The noise figure of Er/Yb amplifiers, which is determined by the inversion of Er3 ions at the input end of the amplifier, is highly dependent on the Yb/Er dopant ratio, pump intensity, and wavelength.88 Er/Yb power amplifiers with noise figures of 4 dB have been reported.89 Generally, the noise figure of Er/Yb amplifiers is somewhat worse than that of 980-nm pumped Eru amplifiers but better than that of 1480-nm pumped amplifiers. In the ideal case, the noise figures of Er/Yb amplifiers can approach quantum-limited values within a few tenths of a decibel. Further studies of the host glass compositional effects, pump wavelength dependence, and dopant ratios are needed to fully optimize both the conversion efficiency and the noise figure of Er/Yb co-doped fiber amplifiers.

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