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Double clad tapered fiber for high power applications

V. Filippov, Yu. Chamorovskii, J. Kerttula, K. Golant, M. Pessa, and O. G. Okhotnikov

Open Access Open Access

Abstract

We report a novel type of active fiber – tapered double clad fiber suitable for pumping by low brightness sources with large beam parameter product of 50÷300 mm×mrad. Ytterbium double clad all-silica fiber (core/1st clad/2nd clad diameters 27/834/890 µm, NAcore=0.11, NAclad=0.21), tapered down by a factor 4.8 for a length of 10.5 m was drawn from a preform fabricated by plasma chemical technologies. At a moderate Yb-ion concentration and 1:31 core/cladding ratio, the tapered double clad fiber demonstrates 0.9 dB/m pump absorption at 976 nm and excellent lasing slope efficiency. An ytterbium fiber laser with 84 W of output power and 92% slope efficiency, a 74 W superfluorescent source with 85% slope efficiency and amplifiers operating both in CW and pulsed regimes have been realized. All devices demonstrated robust single mode operation with a beam quality factor of M2=1.07.

©2008 Optical Society of America

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Figures (12)
Fig. 1.
Fig. 1. T-DCF: the optical equivalent scheme

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Fig. 2.
Fig. 2. T-DCF clad diameter and normalized frequency as function of fiber length.

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Fig. 3.
Fig. 3. Transmission characteristic of T-DCF: from wide end towards narrow end (black line) and in the opposite direction (red line).

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Fig. 4.
Fig. 4. Ray traces in a cladding of T-DCF.

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Fig. 5.
Fig. 5. Schematics of optical sources with T-DCF.

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Fig. 6.
Fig. 6. Output characteristics of T-DCF laser with R1,2=4%;4%. (a) Output power versus absorbed pump power; Inset : emission spectra for output 1 (black line) and output 2 (red line). (b): beam profile (dots) and Gaussian fit (red line) for output 1. M2=1.07.

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Fig. 7.
Fig. 7. Output characteristics of T-DCF laser with broadband HR mirror: (a) output power versus absorbed pump power (b) spectrum of output radiation.

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Fig. 8.
Fig. 8. Output characteristics of T-DCF laser with FBG : (a) output power versus absorbed pump power (b) spectrum of output radiation.

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Fig. 9.
Fig. 9. Output characteristics of T-DCF superluminescent source: (a) output power versus absorbed pump power (b) spectrum of output radiation.

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Fig. 10.
Fig. 10. Amplifier with T-DCF: experimental set up

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Fig. 11.
Fig. 11. Output characteristics of T-DCF pulsed amplifier : (a) average output power versus launched pump power (circles); inset : seed source spectrum (black line) and amplified signal spectrum (red line). (b) autocorrelation function of seed signal (black line) and amplified signal (red line).

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Fig. 12.
Fig. 12. Output characteristics of T-DCF amplifier with CW seed signal: a. output power versus launched pump power (black circles); inset: seed source spectrum (black line) and amplified signal spectrum (red line). b. back reflected light power as a function of output power.

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Equations (6)

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α DCfiber = α core · A core A clad
α DCfiber = α core · A core A clad · S
NA right = d 2 d · NA core − θ n 2 core − NA core 2 · ( d 2 d ) 2 and
NA left = d 1 d · NA core + θ n 2 core − NA core 2 · ( d 1 d ) 2 ,
NA right NA left = d 2 d 1
NA clad = D 2 D 1 · NA − Ω n 2 clad 1 − NA 2 · ( D 2 D 1 ) 2 ,
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