## On the tunability of a narrow-linewidth Yb-fiber laser from three- to four-level lasing behaviour |

Optics Express, Vol. 19, Issue 15, pp. 13940-13948 (2011)

http://dx.doi.org/10.1364/OE.19.013940

Acrobat PDF (762 KB)

### Abstract

We report on a tunable multi-watt ytterbium-doped fiber laser bridging the gap from three-level lasing around 980 nm to true four-level lasing at 1100 nm. Wavelength-locking and -tuning was achieved by using an external volume-Bragg grating(VBG) as the cavity end mirror. The results are compared with detailed numerical calculations based on a spectrally resolved rate equation analysis, taking competing emission at other wavelengths into account.

© 2011 OSA

## 1. Introduction

1. Y. Jeong, J. Sahu, D. Payne, and J. Nilsson, “Ytterbium-doped large-core fiber laser with 1.36 kW continuous-wave output power,” Opt. Express **12**, 6088–6092 (2004). [CrossRef] [PubMed]

2. P. Gross, M. Klein, T. Walde, K. Boller, M. Auerbach, P. Wessels, and C. Fallnich, “Fiber-laser-pumped continuous-wave singly resonant optical parametric oscillator,” Opt. Lett. **27**, 418–420 (2002). [CrossRef]

*μ*m, typically interesting for gas sensing.

3. H. M. Pask, R. J. Carman, D. C. Hanna, A. C. Tropper, C. J. Mackechnie, P. R. Barber, and J. M. Dawes, “Ytterbium-doped silica fiber lasers: versatile sources for the 1–1.2 m region,” IEEE J. Sel. Top. Quantum Electron. **1**(1), 2–13 (1995). [CrossRef]

4. M. Auerbach, P. Adel, D. Wandt, C. Fallnich, S. Unger, S. Jetschke, and H. Mueller, “10 W widely tunable narrow linewidth double-clad fiber ring laser,” Opt. Express **10**, 139–144 (2002). [PubMed]

*μ*m has not been shown. This contribution reports on the viability of such an unprecendented tuning range for a laser with excellent spatial and spectral beam quality.

## 2. Tuning range limitations for Yb-doped silica fiber lasers

*μ*m require a high population inversion (> 50%), which entails three challenges for achieving efficient and stable lasing: photodarkening, competing emission of quasi-three level transition above 1

*μ*m and life-time quenching.

### 2.1. Photodarkening

5. J. Koponen, M. Sderlund, H. Hoffman, D. Kliner, J. Koplow, and M. Hotoleanu, “Photodarkening rate in Yb-doped silica fibers,” Appl. Opt. **47**, 1247–1256 (2008). [CrossRef] [PubMed]

6. M. Engholm, P. Jelger, F. Laurell, and L. Norin, “Improved photodarkening resistivity in ytterbium-doped fiber lasers by cerium codoping,” Opt. Lett. **34**, 1285–1287 (2009). [CrossRef] [PubMed]

### 2.2. Competing emission

7. J. Nilsson, J. Minelly, R. Paschotta, A. Tropper, and D. Hanna, “Ring-doped cladding-pumped single-mode three-level fiber laser,” Opt. Lett. **23**, 355–357 (1998). [CrossRef]

*λ*) from [8

_{i}8. R. Paschotta, J. Nilsson, A. C. Tropper, and D. C. Hanna, “Ytterbium-doped fiber amplifiers,” IEEE J. Quantum Electron. **33**(7), 1049–1056 (1997). [CrossRef]

*G*and Γ

_{λ}*being the gain and the overlap factor between dopant distribution and normalized optical mode in the fiber at the respective wavelengths. The contribution from the gain at 980 nm is minor (<1.6 dB), as a common cavity with 96 % outcoupling loss per round trip requires 7 dB gain single pass at threshold.*

_{λ}*A*/

_{cla}*A*(typically between 50 and 100), which means that for each dB absorbed pump the second term in (1) amounts in significant parasitic gain at 1030 nm. It is therefore imperative to use fibers with low cladding/core ratio to ensure efficient lasing [7

_{cor}7. J. Nilsson, J. Minelly, R. Paschotta, A. Tropper, and D. Hanna, “Ring-doped cladding-pumped single-mode three-level fiber laser,” Opt. Lett. **23**, 355–357 (1998). [CrossRef]

*μ*m

^{2}, core area, i.e. doped area: 800

*μ*m

^{2}) with a cladding/core ratio of <90 and 2 dB/m pump absorption i.e. negative gain at 915 nm is theoretically limited to a length of 40 cm, given a parasitic gain suppression of 50 dB.

### 2.3. Life-time quenching

9. R. Paschotta, J. Nilsson, P. R. Barber, J. E. Caplen, A. C. Tropper, and D. C. Hanna, “Lifetime quenching in Yb-doped fibres,” Opt. Commun. **136**(5–6), 375–378 (1997). [CrossRef]

8. R. Paschotta, J. Nilsson, A. C. Tropper, and D. C. Hanna, “Ytterbium-doped fiber amplifiers,” IEEE J. Quantum Electron. **33**(7), 1049–1056 (1997). [CrossRef]

*ξ*being the fraction of ions that have a shorter excited state life-time. Figure 1 displays the spectral dependence of

## 3. Modelling of the laser process - rate equation analysis

10. A. Cucinotta, S. Selleri, L. Vincetti, and M. Zoboli, “Numerical and experimental analysis of erbium-doped fiber linear cavity lasers,” Opt. Commun. **156**(4–6), 264–270 (1998). [CrossRef]

*N*slots with bandwidth Δ

*λ*, rate equations [Eq. (3)] governing the evolution of pump and signal powers along the fiber can be expressed.

*P*is the power at wavelength

_{i}*λ*,

_{i}*n*

_{1}(

*n*

_{2}) and

*n*the coordinate dependent lower(upper) state population densities and total Yb-ion concentration with

*n*=

_{tot}*n*

_{1}+

*n*

_{2}.

*A*denotes the effective transversal mode area, which can be approximated with core area and cladding area for signal and pump radiation, respectively.

_{i}*A*is the doped area of the fiber and

*τ*the excited-state lifetime. The term

*α*accounts for losses and the signs ± refer to either forward or backward propagation of the beams. The described system of non-linear coupled rate with its corresponding set of boundary conditions [Eq. (4)] for a linear cavity (

_{i}*R*and

_{m,i}*η*mirror reflectivity and coupling loss,

_{m,i}*λ*and

_{p}*P*pump wavelength and launched pump power) can be numerically solved with the Runge-Kutta method making use of a shooting method which effectively reduces the solution of a boundary value problem to the solution of an initial value problem [11]. In this configuration the developed numerical tool allows to simulate spectral particularities of the laser in detail, as all variables (transmission losses, coupling losses, mirror reflectivities) can be spectrally resolved, it does however not regard other transversal modes than the fundamental mode and is not sensitive to longitudinal mode selection of the linear cavity, which proved to be a valid simplification as experimental and theoretical results are in good agreement.

_{p}## 4. Competing emission in laser cavity

## 5. Experimental setup - cavity configuration

### 5.1. Wavelength selection

12. B. Jacobsson, M. Tiihonen, V. Pasiskevicius, and F. Laurell, “Narrowband bulk Bragg grating optical parametric oscillator,” Opt. Lett. **30**, 2281–2283 (2005). [CrossRef] [PubMed]

13. B. Jacobsson, J. Hellstrom, V. Pasiskevicius, and F. Laurell, “Widely tunable Yb:KYW laser with a volume Bragg grating,” Opt. Express **15**, 1003–1010 (2007). [CrossRef] [PubMed]

14. P. Jelger and F. Laurell, “Efficient narrow-linewidth volume-Bragg grating-locked Nd:fiber laser,” Opt. Express **15**, 11336–11340 (2007). [CrossRef] [PubMed]

15. J. E. Hellstrom, B. Jacobsson, V. Pasiskevicius, and F. Laurell, “Finite beams in reflective volume Bragg gratings: theory and experiments,” IEEE J. Quantum Electron. **44**(1), 81–89 (2008). [CrossRef]

*w*

_{e−2}the Gaussian beam width, Δ

*λ*the gratings bandwidth and

_{B}*θ*the incident angle with respect to the grating normal (compare Fig. 3). For oblique incidence the Bragg condition

*λ*= 2

_{B}*n*

_{0}Λ cos

*θ*relates grating period Λ and Bragg wavelength

*λ*. Angle-tuning a grating with design wavelength 1100 nm to 980 nm requires a tuning angle of 27 degree. With a typical grating bandwidth of 0.4 nm this requires a beam diameter of at least 5 mm. As the fiber was slightly multi-mode with a

_{B}*M*

^{2}-value of <3.5 within the tuning range the minimally required beam diameter increases to 8.5 mm, while the available VBGs featured clear apertures of just 4 mm x 4 mm. So instead of using one VBG four VBGs with comparable bandwidth (0.4 nm) and reflectivity (99 %) but different design wavelengths (997 nm, 1030 nm, 1066 nm and 1100 nm) were employed in the experiments. To guarantee that the spectral selectivity of the VBGs suffices to suppress end-face parasitic lasing, the grating planes were tilted with respect to the bulk glass surface by an angle of 2 degrees.

### 5.2. Choice of outcoupling mirror

## 6. Results and discussion

## 7. Conclusion and outlook

## Acknowledgments

## References and links

1. | Y. Jeong, J. Sahu, D. Payne, and J. Nilsson, “Ytterbium-doped large-core fiber laser with 1.36 kW continuous-wave output power,” Opt. Express |

2. | P. Gross, M. Klein, T. Walde, K. Boller, M. Auerbach, P. Wessels, and C. Fallnich, “Fiber-laser-pumped continuous-wave singly resonant optical parametric oscillator,” Opt. Lett. |

3. | H. M. Pask, R. J. Carman, D. C. Hanna, A. C. Tropper, C. J. Mackechnie, P. R. Barber, and J. M. Dawes, “Ytterbium-doped silica fiber lasers: versatile sources for the 1–1.2 m region,” IEEE J. Sel. Top. Quantum Electron. |

4. | M. Auerbach, P. Adel, D. Wandt, C. Fallnich, S. Unger, S. Jetschke, and H. Mueller, “10 W widely tunable narrow linewidth double-clad fiber ring laser,” Opt. Express |

5. | J. Koponen, M. Sderlund, H. Hoffman, D. Kliner, J. Koplow, and M. Hotoleanu, “Photodarkening rate in Yb-doped silica fibers,” Appl. Opt. |

6. | M. Engholm, P. Jelger, F. Laurell, and L. Norin, “Improved photodarkening resistivity in ytterbium-doped fiber lasers by cerium codoping,” Opt. Lett. |

7. | J. Nilsson, J. Minelly, R. Paschotta, A. Tropper, and D. Hanna, “Ring-doped cladding-pumped single-mode three-level fiber laser,” Opt. Lett. |

8. | R. Paschotta, J. Nilsson, A. C. Tropper, and D. C. Hanna, “Ytterbium-doped fiber amplifiers,” IEEE J. Quantum Electron. |

9. | R. Paschotta, J. Nilsson, P. R. Barber, J. E. Caplen, A. C. Tropper, and D. C. Hanna, “Lifetime quenching in Yb-doped fibres,” Opt. Commun. |

10. | A. Cucinotta, S. Selleri, L. Vincetti, and M. Zoboli, “Numerical and experimental analysis of erbium-doped fiber linear cavity lasers,” Opt. Commun. |

11. | Michael T. Heath, |

12. | B. Jacobsson, M. Tiihonen, V. Pasiskevicius, and F. Laurell, “Narrowband bulk Bragg grating optical parametric oscillator,” Opt. Lett. |

13. | B. Jacobsson, J. Hellstrom, V. Pasiskevicius, and F. Laurell, “Widely tunable Yb:KYW laser with a volume Bragg grating,” Opt. Express |

14. | P. Jelger and F. Laurell, “Efficient narrow-linewidth volume-Bragg grating-locked Nd:fiber laser,” Opt. Express |

15. | J. E. Hellstrom, B. Jacobsson, V. Pasiskevicius, and F. Laurell, “Finite beams in reflective volume Bragg gratings: theory and experiments,” IEEE J. Quantum Electron. |

**OCIS Codes**

(140.3615) Lasers and laser optics : Lasers, ytterbium

(060.3510) Fiber optics and optical communications : Lasers, fiber

**ToC Category:**

Lasers and Laser Optics

**History**

Original Manuscript: May 23, 2011

Revised Manuscript: June 17, 2011

Manuscript Accepted: June 17, 2011

Published: July 6, 2011

**Citation**

Peter Zeil and Fredrik Laurell, "On the tunability of a narrow-linewidth Yb-fiber laser from three- to four-level lasing behaviour," Opt. Express **19**, 13940-13948 (2011)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-15-13940

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### References

- Y. Jeong, J. Sahu, D. Payne, and J. Nilsson, “Ytterbium-doped large-core fiber laser with 1.36 kW continuous-wave output power,” Opt. Express 12, 6088–6092 (2004). [CrossRef] [PubMed]
- P. Gross, M. Klein, T. Walde, K. Boller, M. Auerbach, P. Wessels, and C. Fallnich, “Fiber-laser-pumped continuous-wave singly resonant optical parametric oscillator,” Opt. Lett. 27, 418–420 (2002). [CrossRef]
- H. M. Pask, R. J. Carman, D. C. Hanna, A. C. Tropper, C. J. Mackechnie, P. R. Barber, and J. M. Dawes, “Ytterbium-doped silica fiber lasers: versatile sources for the 1–1.2 m region,” IEEE J. Sel. Top. Quantum Electron. 1(1), 2–13 (1995). [CrossRef]
- M. Auerbach, P. Adel, D. Wandt, C. Fallnich, S. Unger, S. Jetschke, and H. Mueller, “10 W widely tunable narrow linewidth double-clad fiber ring laser,” Opt. Express 10, 139–144 (2002). [PubMed]
- J. Koponen, M. Sderlund, H. Hoffman, D. Kliner, J. Koplow, and M. Hotoleanu, “Photodarkening rate in Yb-doped silica fibers,” Appl. Opt. 47, 1247–1256 (2008). [CrossRef] [PubMed]
- M. Engholm, P. Jelger, F. Laurell, and L. Norin, “Improved photodarkening resistivity in ytterbium-doped fiber lasers by cerium codoping,” Opt. Lett. 34, 1285–1287 (2009). [CrossRef] [PubMed]
- J. Nilsson, J. Minelly, R. Paschotta, A. Tropper, and D. Hanna, “Ring-doped cladding-pumped single-mode three-level fiber laser,” Opt. Lett. 23, 355–357 (1998). [CrossRef]
- R. Paschotta, J. Nilsson, A. C. Tropper, and D. C. Hanna, “Ytterbium-doped fiber amplifiers,” IEEE J. Quantum Electron. 33(7), 1049–1056 (1997). [CrossRef]
- R. Paschotta, J. Nilsson, P. R. Barber, J. E. Caplen, A. C. Tropper, and D. C. Hanna, “Lifetime quenching in Yb-doped fibres,” Opt. Commun. 136(5–6), 375–378 (1997). [CrossRef]
- A. Cucinotta, S. Selleri, L. Vincetti, and M. Zoboli, “Numerical and experimental analysis of erbium-doped fiber linear cavity lasers,” Opt. Commun. 156(4–6), 264–270 (1998). [CrossRef]
- Michael T. Heath, Scientific Computing , 2nd ed. (McGraw-Hill, 2002).
- B. Jacobsson, M. Tiihonen, V. Pasiskevicius, and F. Laurell, “Narrowband bulk Bragg grating optical parametric oscillator,” Opt. Lett. 30, 2281–2283 (2005). [CrossRef] [PubMed]
- B. Jacobsson, J. Hellstrom, V. Pasiskevicius, and F. Laurell, “Widely tunable Yb:KYW laser with a volume Bragg grating,” Opt. Express 15, 1003–1010 (2007). [CrossRef] [PubMed]
- P. Jelger and F. Laurell, “Efficient narrow-linewidth volume-Bragg grating-locked Nd:fiber laser,” Opt. Express 15, 11336–11340 (2007). [CrossRef] [PubMed]
- J. E. Hellstrom, B. Jacobsson, V. Pasiskevicius, and F. Laurell, “Finite beams in reflective volume Bragg gratings: theory and experiments,” IEEE J. Quantum Electron. 44(1), 81–89 (2008). [CrossRef]

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