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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 13 — Jun. 20, 2011
  • pp: 12434–12439
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Novel technique for mode selection in a multimode fiber laser

J. M. O. Daniel, J. S. P. Chan, J. W. Kim, J. K. Sahu, M. Ibsen, and W. A. Clarkson  »View Author Affiliations


Optics Express, Vol. 19, Issue 13, pp. 12434-12439 (2011)
http://dx.doi.org/10.1364/OE.19.012434


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Abstract

A simple technique for transverse mode selection in a large-mode-area (multimode) fiber laser is described. The technique exploits the different spectral responses of feedback elements based on a fiber Bragg grating and a volume Bragg grating to achieve wavelength-dependent mode filtering. This approach has been applied to a cladding-pumped thulium-doped fiber laser with a multimode core to achieve a single-spatial-mode output beam with a beam propagation factor (M2) of 1.05 at 1923 nm. Without mode selection the free-running fiber laser has a multimode output beam with an M2 parameter of 3.3. Selective excitation of higher order modes is also possible via the technique and preliminary results for laser oscillation on the LP11 mode are also discussed along with the prospects for scaling to higher power levels.

© 2011 OSA

1. Introduction

2. Principle of operation

λFBG=2neffΛ1
(1)

Thus, higher order modes will experience reflection at shorter wavelengths than lower order modes. In contrast, a Bragg grating written into a bulk material (i.e. a volume Bragg grating) with period, Λ2, provides maximum reflectivity at wavelength, λVBG, given by [10

10. B. Jacobsson, M. Tiihonen, V. Pasiskevicius, and F. Laurell, “Narrowband bulk Bragg grating optical parametric oscillator,” Opt. Lett. 30(17), 2281–2283 (2005). [CrossRef] [PubMed]

]:
λVBG=2nVBGΛ2cos(θ)
(2)
where θ is the angle of incidence and nVBG is approximately equal to the refractive index of the bulk material. In this case, the wavelength at which the Bragg condition is satisfied is approximately the same for all modes. Thus, by forming a laser resonator with a multimode fiber as active medium and with feedback for laser oscillation provided by a FBG and an external cavity containing a VBG it is possible to restrict lasing to a single-spatial-mode (or reduce the number of lasing modes compared to a free running laser) by selecting the period and angle of incidence on the VBG such that
Λ2cos(θ)=neffΛFBGnVBG
(3)
where neff is the effective index of the desired lasing mode. This assumes that the fibre is of sufficient uniformity that intermodal coupling is negligible [8

8. M. E. Fermann, “Single-mode excitation of multimode fibers with ultrashort pulses,” Opt. Lett. 23(1), 52–54 (1998). [CrossRef]

].

3. Experimental set-up and results

Figure 4
Fig. 4 Laser output power versus launched pump power for LP01 and LP11 mode operation.
shows the output power for LP01 and LP11 mode operation as a function of launched pump power. At the maximum launched pump power of ~34 W the laser yielded output powers of 2.6 W for LP01 mode operation and 3.6 W for LP11 mode operation with corresponding slope efficiencies with the respect to launched pump power of 11% and 16% respectively. The higher output power and slope efficiency for LP11 mode operation is attributed to the better spatial overlap for the LP11 mode with the inversion distribution [11

11. M. Gong, Y. Yuan, C. Li, P. Yan, H. Zhang, and S. Liao, “Numerical modeling of transverse mode competition in strongly pumped multimode fiber lasers and amplifiers,” Opt. Express 15(6), 3236–3246 (2007). [CrossRef] [PubMed]

]. The difference in slope efficiency for LP01 and LP11 operation should be less pronounced at higher power levels (i.e. far above threshold) when the intensity in the wings of the LP01 mode is well above the saturation intensity. The relatively low slope efficiencies for both LP01 and LP11 operation can be attributed to hydrogen loading, which leads to a large increase in core propagation loss. The Tm fiber (with and without hydrogen loading) was tested in simple free-running laser configurations. The slope efficiency for the TDF laser without hydrogen loading was approximately 40% and roughly a factor-of-two higher than for the TDF laser with hydrogen loading. The core propagation loss in the hydrogen-loaded TDF was estimated to be ~1 dB/m. Hence, by hydrogen loading only the section of TDF where the FBG is to be written it should be possible to achieve a substantial increase in output power and efficiency. A modified hydrogen loading rig to allow selective loading of short sections of fiber is currently under development. It should also be noted that the TDF used in our experiments had a rather low Tm concentration and thus was not designed to enhance the efficiency via promoting the ‘two-for-one’ cross-relaxation process (3H4 + 3H63F4 + 3F4). Hence, further optimization of the core design with high Tm concentration should yield a further improvement in efficiency commensurate with conventional Tm fiber laser architectures. The extent by which the core diameter can be increased using this approach, whilst retaining robust single-mode operation and avoiding mode distortion, is the subject of ongoing studies. Preliminary calculations suggest that core diameters around 70 µm should be feasible for two-micron operation.

5. Conclusion

We have demonstrated a simple and effective method for mode selection in a fiber laser oscillator with a multimode fiber. The technique exploits the different spectral responses of fiber Bragg grating and free-space wavelength-selective elements (e.g. VBGs) to provide selective feedback for a particular mode (e.g. the fundamental mode) and thereby suppress lasing on other modes. This approach can be applied to simple multimode (or large-mode-area) core designs and avoids the need for complicated and costly fibers with special core geometries designed to provide built-in suppression of higher-order modes. In this preliminary proof-of-concept study applied to a Tm fiber laser, the output power and efficiency were limited by high background core propagation loss due to hydrogen loading. Further optimization of the fiber design in combination with the use of selective hydrogen loading should yield a dramatic improvement in performance. Preliminary calculations suggest that it should be possible to scale the core diameter to ~70 µm whilst maintaining robust single-mode operation at ~2 µm. This would pave the way for a simple Q-switched Tm fiber laser oscillator with pulse energies in the multi-millijoule regime and beyond. Moreover, the ability to selectively excite higher order modes with larger transverse dimensions than the fundamental mode should help to improve extraction efficiency and raise energy damage thresholds opening up the prospect of even higher pulse energies.

References and links

1.

S. Wielandy, A. D. Yablon, S. Ramachandran, and S. Ghalmi, “Implications of higher-order mode content in large mode area fibers with good beam quality,” Opt. Express 15(23), 15402–15409 (2007). [CrossRef] [PubMed]

2.

K. Tankala, B. Samson, A. Carter, J. Farroni, D. Machewirth, N. Jacobson, U. Manyam, A. Sanchez, M.-Y. Chen, A. Galvanauskas, W. Torruellas, and Y. Chen, “New developments in high power eye-safe LMA fibers,” Proc. SPIE 6102, 610206, 610206-9 (2006). [CrossRef]

3.

J. C. Knight, T. A. Birks, R. F. Cregan, and P. St. J. Russell, “Large mode area photonic crystal fiber,” Opt. Photon. News 9(12), 34–35 (1998). [CrossRef]

4.

J. R. Marciante, “Gain filtering for single-spatial-mode operation of large-mode-area fiber amplifiers,” IEEE J. Sel. Top. Quantum Electron. 15(1), 30–36 (2009). [CrossRef]

5.

J. P. Koplow, D. A. Kliner, and L. Goldberg, “Single-mode operation of a coiled multimode fiber amplifier,” Opt. Lett. 25(7), 442–444 (2000). [CrossRef]

6.

L. Dong, X. Peng, and J. Li, “Leakage channel optical fibers with large effective area,” J. Opt. Soc. Am. 24(8), 1689–1690 (2007). [CrossRef]

7.

C. Liu, G. Chang, N. Litchinitser, A. Galvanauskas, D. Guertin, N. Jabobson, and K. Tankala, “Effectively single-mode chirally-coupled core fiber,” ASSP 2007, 1–3 (2007).

8.

M. E. Fermann, “Single-mode excitation of multimode fibers with ultrashort pulses,” Opt. Lett. 23(1), 52–54 (1998). [CrossRef]

9.

A. Othonos, “Fiber Bragg gratings,” Rev. Sci. Instrum. 68(12), 4309 (1997). [CrossRef]

10.

B. Jacobsson, M. Tiihonen, V. Pasiskevicius, and F. Laurell, “Narrowband bulk Bragg grating optical parametric oscillator,” Opt. Lett. 30(17), 2281–2283 (2005). [CrossRef] [PubMed]

11.

M. Gong, Y. Yuan, C. Li, P. Yan, H. Zhang, and S. Liao, “Numerical modeling of transverse mode competition in strongly pumped multimode fiber lasers and amplifiers,” Opt. Express 15(6), 3236–3246 (2007). [CrossRef] [PubMed]

OCIS Codes
(060.2430) Fiber optics and optical communications : Fibers, single-mode
(140.3510) Lasers and laser optics : Lasers, fiber

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: March 9, 2011
Revised Manuscript: April 5, 2011
Manuscript Accepted: April 6, 2011
Published: June 13, 2011

Citation
J. M. O. Daniel, J. S. P. Chan, J. W. Kim, J. K. Sahu, M. Ibsen, and W. A. Clarkson, "Novel technique for mode selection in a multimode fiber laser," Opt. Express 19, 12434-12439 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-13-12434


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References

  1. S. Wielandy, A. D. Yablon, S. Ramachandran, and S. Ghalmi, “Implications of higher-order mode content in large mode area fibers with good beam quality,” Opt. Express 15(23), 15402–15409 (2007). [CrossRef] [PubMed]
  2. K. Tankala, B. Samson, A. Carter, J. Farroni, D. Machewirth, N. Jacobson, U. Manyam, A. Sanchez, M.-Y. Chen, A. Galvanauskas, W. Torruellas, and Y. Chen, “New developments in high power eye-safe LMA fibers,” Proc. SPIE 6102, 610206, 610206-9 (2006). [CrossRef]
  3. J. C. Knight, T. A. Birks, R. F. Cregan, and P. St. J. Russell, “Large mode area photonic crystal fiber,” Opt. Photon. News 9(12), 34–35 (1998). [CrossRef]
  4. J. R. Marciante, “Gain filtering for single-spatial-mode operation of large-mode-area fiber amplifiers,” IEEE J. Sel. Top. Quantum Electron. 15(1), 30–36 (2009). [CrossRef]
  5. J. P. Koplow, D. A. Kliner, and L. Goldberg, “Single-mode operation of a coiled multimode fiber amplifier,” Opt. Lett. 25(7), 442–444 (2000). [CrossRef]
  6. L. Dong, X. Peng, and J. Li, “Leakage channel optical fibers with large effective area,” J. Opt. Soc. Am. 24(8), 1689–1690 (2007). [CrossRef]
  7. C. Liu, G. Chang, N. Litchinitser, A. Galvanauskas, D. Guertin, N. Jabobson, and K. Tankala, “Effectively single-mode chirally-coupled core fiber,” ASSP 2007, 1–3 (2007).
  8. M. E. Fermann, “Single-mode excitation of multimode fibers with ultrashort pulses,” Opt. Lett. 23(1), 52–54 (1998). [CrossRef]
  9. A. Othonos, “Fiber Bragg gratings,” Rev. Sci. Instrum. 68(12), 4309 (1997). [CrossRef]
  10. B. Jacobsson, M. Tiihonen, V. Pasiskevicius, and F. Laurell, “Narrowband bulk Bragg grating optical parametric oscillator,” Opt. Lett. 30(17), 2281–2283 (2005). [CrossRef] [PubMed]
  11. M. Gong, Y. Yuan, C. Li, P. Yan, H. Zhang, and S. Liao, “Numerical modeling of transverse mode competition in strongly pumped multimode fiber lasers and amplifiers,” Opt. Express 15(6), 3236–3246 (2007). [CrossRef] [PubMed]

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