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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 16 — Aug. 6, 2007
  • pp: 10340–10345
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Birefringent in-phase supermode operation of a multicore microstructured fiber laser

X. Zhu, A. Schülzgen, L. Li, H. Li, V. L. Temyanko, J. V. Moloney, and N. Peyghambarian  »View Author Affiliations


Optics Express, Vol. 15, Issue 16, pp. 10340-10345 (2007)
http://dx.doi.org/10.1364/OE.15.010340


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Abstract

We report the first observation of birefringent in-phase supermode operation of a phase-locked multicore fiber laser. The in-phase mode operation of our 12-core rectangular-array microstructured fiber laser was confirmed by the near-field distribution, the far-field diffraction pattern, and the optical spectrum. The birefringence of the in-phase mode in propagation constant Δγ was measured as ~4×10-6 1/µm. The break of the polarization degeneracy indicates the possibility of single polarization operation of phase-locked multicore fiber lasers and amplifiers.

© 2007 Optical Society of America

1. Introduction

Phased laser arrays are often put forward as a means for achieving diffraction-limited output beams from a number of spatially separated coherent optical sources for applications including laser pumps, space communications, industrial welding and cutting, directed energy weapons, and laser induced nuclear fusion. These arrays are valuable alternatives wherever the required output beam power, brightness, and mode quality cannot be easily obtained from individual lasers. Agile beam steering and shaping are also potential benefits of coherent arrays, in analogy to phase-array radar antenna technology. Phase-locked diode lasers [1

1. D. R. Scifres, R. D. Burnham, and W. Streifer, “Phase-locked semiconductor laser array,” Appl. Phys. Lett. 32, 1015–1017 (1978). [CrossRef]

], CO2 waveguide lasers [2

2. M. Oka, H. Masuda, Y. Kaneda, and S. Kubota, “Laser-diode-pumped phase-locked Nd:YAG laser arrays,” IEEE J. Quantum Electron. 28, 1142–1147 (1992). [CrossRef]

], solid-state lasers [3

3. D. G. Youmans, “Phase locking of adjacent channel leaky waveguide CO2 lasers,” Appl. Phys. Lett. 44, 365–367 (1984). [CrossRef]

], and fiber lasers [4

4. J. Morel, A. Woodtli, and R. Dandliker, “Coherent coupling of an array of Nd3+-doped single-mode fiber lasers by use of an intracavity phase grating,” Opt. Lett. 18, 1520–1522 (1993). [CrossRef] [PubMed]

] have been demonstrated, usually shortly after their corresponding individual lasers have been reported.

2. Multicore microstructured active fiber and its lasing characteristics

For pump powers below the lasing threshold individual cores emit only spontaneous emission and operate to a large extend separately, resulting in nearly identical intensities of individual cores as shown in Fig. 1(b). In contrast, when pumping the MCF above the lasing threshold, individual cores do not operate separately any more. In this case, the cores interact due to evanescent coupling between neighboring cores, and the intensity of individual cores has an uneven distribution which reflects a combined near-field pattern of several lasing supermodes, shown in Fig. 1(c).

Fig. 1. Microscopic images of the output facet of a 12-core microstructured Er/Yb co-doped phosphate fiber laser when (a) without pumping, (b) spontaneous emission, and (c) stimulated emission.
Fig. 2. Far-field intensity distribution of a free-running 12-core microstructured Er/Yb co-doped phosphate fiber laser. (a) Pattern image, (b) 3-D description of (a).

3. Experiments and analyses of in-phase supermode operation

Fig. 3. Experimental setup for the phase-locked 12-core microstructured Er/Yb co-doped phosphate fiber laser.

E11X,Y(x,y,z)=l=13m=14sin(lπ4)sin(mπ5)E00X,Y(xxlm,yylm)eiγ11X,Yz,
(1)

where X and Y represent the two orthogonal polarization fields, l and m are the core unit indices, E00 X,Y(x-xlm, y-ylm) is the field distribution of individual cores, and γX,Y 11 is the polarization dependent propagation constant of the fundamental supermode, which can be written as

γ11X,Y=βX,Y+κxX,Ycos(π4)+κyX,Ycos(π5),
(2)

where βX,Y is the propagation constant of the individual core; κX,Y x and κX,Y y are the coupling constants between two neighboring cores in x and y directions, respectively. The far-field intensity distribution of the fundamental supermode can be written as

I11X,Y(θ,ϕ)={sin[3(Sx+π4)2]sin[(Sx+π4)2]+sin[3(Sxπ4)2]sin[(Sxπ4)2]}2,
×{sin[4(Sy+π5)2]sin[(Sy+π5)2]+sin[4(Syπ5)2]sin[(Syπ5)2]}2I00X,Y(θ,ϕ)
(3)

where Sx=k0px sin θ cosϕ, Sy=k0py, sinθ sinϕ px, and py are the periods of the core array in x and y directions, respectively, and I X,Y 00(θ,ϕ) is the far-field intensity distribution of an individual core.

The near-field distribution of the phased MCF laser, shown in Fig. 4(b), was recorded by imaging the output facet of the MCF to the CCD camera with a magnifying lens. The distributions of two selected cross-sections along x and y direction [along the lines in Fig. 4(b)] is shown in Figs. 4(a) and (c), respectively. The experimentally obtained distributions are in good agreement with theoretical results for the in-phase supermode obtained from Eq. (1).

The far-field pattern was recorded on a screen 11 cm away from the output facet and its distributions are shown in Figs. 5(a)–(c). The angle spreads, which are taken the full width at half maximum (FWHW), along x and y directions are 36 and 27 mrad, respectively. Optical spectra were also measured to further characterize free-running vs. in-phase supermode operation. As shown in Fig. 6, in-phase supermode operation gives a narrower emission spectrum than free-running operation does. The output power during in-phase supermode operation was also measured as a function of pump power. The slope efficiency was about 10% and the maximum output was 2.6 W.

Fig. 4. Near-field distribution of the phase-locked 12-core microstructured Er/Yb co-doped phosphate fiber laser. (a) Profile along x direction, (b) near-field image, (c) profile along y direction.
Fig. 5. Far-field intensity distribution of the phase-locked 12-core microstructured Er/Yb co-doped phosphate fiber laser. (a) Far-field pattern, (b) diffraction profile along x direction, (c) diffraction profile along y direction.

To evaluate the polarization mode splitting, the radio frequency (RF) spectrum of the output, i.e., beating signals generated by interference between various longitudinal modes, has been measured. Longitudinal mode beating (LMB) signals, generated by interference between modes of the same polarization field, tell us the free spectral range ΔνLMB=c/(2neffL), while polarization mode beating (PMB) signals, generated by interference between modes of two polarization fields, can tell us the polarization mode splitting in frequency ΔνPMB≈cΔneff/(λneff). All RF signals can be recorded, as shown in Fig. 7, with a RF spectrum analyzer. To note, the 744 MHz signal, paired with the zero frequency component, corresponds to ΔνLMB; and the signals at 120, 624, and 864 MHz are generated by PMB. This indicates that the in-phase mode is split into two non-degenerate polarization states and ΔνPMB is 120 MHz. The corresponding birefringence Δneff can thus be obtained by Δneff=ΔνPMBλ/(ΔνLMBL). The effective group index birefringence is ~10-6 and the corresponding propagation constant birefringence, namely, Δγ=2πΔneff/λ, is ~4×10-6 1/µm. However, according to Eq. 2, the polarization mode discrimination can be caused by polarization dependence of either the propagation constant β of individual cores or the coupling coefficient κx and κy. Although a birefringence value is measured here, it remains to be identified whether it is induced by the birefringence in individual cores or by the asymmetry of the rectangular array and the microstructured fiber cross-section.

Fig. 6. Optical spectra of free-running (dashed line) and in-phase operation (solid line) of the 12-core microstructured Er/Yb co-doped phosphate fiber laser.
Fig. 7. RF spectrum of the LMB and PMB signals showing two non-degenerated, polarized in-phase supermodes emitted by the 12-core microstructured Er/Yb phosphate fiber laser.

4. Conclusion

We have analyzed free-running and in-phase operation of a 12-core rectangular-array microstructured fiber laser. The agreement between experimental results and theoretical calculations confirmed the in-phase operation of the MCF laser. Through fiber and cavity design we can tailor the beam profile and emission pattern of the MCF laser. Our observed birefringence of the in-phase supermode indicates the possibility of polarization maintaining MCF and single polarization, high-power MCF lasers and amplifiers.

Acknowledgments

The authors would like to thank Y. Merzlyak for technical support. This work is supported by the U. S. Air Force Office of Scientific Research through MRI program, F49620-02-1-0380, National Science Foundation grants 0335101 and 0725479, and the State of Arizona TRIF Photonics Initiative.

References and links

1.

D. R. Scifres, R. D. Burnham, and W. Streifer, “Phase-locked semiconductor laser array,” Appl. Phys. Lett. 32, 1015–1017 (1978). [CrossRef]

2.

M. Oka, H. Masuda, Y. Kaneda, and S. Kubota, “Laser-diode-pumped phase-locked Nd:YAG laser arrays,” IEEE J. Quantum Electron. 28, 1142–1147 (1992). [CrossRef]

3.

D. G. Youmans, “Phase locking of adjacent channel leaky waveguide CO2 lasers,” Appl. Phys. Lett. 44, 365–367 (1984). [CrossRef]

4.

J. Morel, A. Woodtli, and R. Dandliker, “Coherent coupling of an array of Nd3+-doped single-mode fiber lasers by use of an intracavity phase grating,” Opt. Lett. 18, 1520–1522 (1993). [CrossRef] [PubMed]

5.

Y. Jeong, J. K. 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]

6.

T. Qiu, L. Li, A. Schülzgen, V. Temyanko, T. Luo, S. Jiang, A. Mafi, J. Moloney, and N. Peyghambarian, “Generation of 6.6 W multimode and 4 W single mode output from 7 cm short fiber lasers,” IEEE Photon. Technol. Lett. 16, 2592–2594 (2004). [CrossRef]

7.

T. Qiu, S. Suzuki, A. Schülzgen, L. Li, A. Polynkin, V. Temyanko, J. Moloney, and N. Peyghambarian, “Generation of watts-level single-longitudinal-mode output from cladding-pumped short fiber lasers,” Opt. Lett. 30, 2748–2750 (2005). [CrossRef] [PubMed]

8.

X. Zhu and R. K. Jain, “10-W-level diode-pumped compact 2.78 µm ZBLAN fiber laser,” Opt. Lett. 32, 26–28 (2007). [CrossRef]

9.

D. R. Scifres, “Multiple core fiber lasers and optical amplifiers,” US Patent #5,566,196 (1996).

10.

M. Wrage, P. Glas, D. Fischer, M. Leitner, D. V. Vysotsky, and A. P. Napartovich, “Phase locking in a multicore fiber laser by means of a Talbot resonator,” Opt. Lett. 25, 1436–1438 (2000). [CrossRef]

11.

M. Wrage, P. Glas, and M. Leitner, “Combined phase locking and beam shaping of a multicore fiber laser by structured mirrors,” Opt. Lett. 26, 980–982 (2001). [CrossRef]

12.

M. Wrage, P. Glas, D. Fischer, M. Leitner, N. N. Elkin, D. V. Vysotsky, A. P. Napartovich, and V. N. Troshchieva, “Phase-locking of a multicore fiber laser by wave propagating through an annular waveguide,” Opt. Commun. 205, 367–375 (2002). [CrossRef]

13.

L. Michaille, C. R. Bennett, D. M. Taylor, T. J. Shepherd, J. Broeng, H. R. Simonsen, and A. Peterson, “Phase locking and supermode selection in multicore photonic crystal fiber lasers with a large doped area,” Opt. Lett. 30, 1668–1670 (2005). [CrossRef] [PubMed]

14.

P. K. Cheo, A. Liu, and G. G. King, “A high-brightness laser beam from a phase-locked multicore Yb-doped fiber laser array,” IEEE Photon. Technol. Lett. 13, 439–441 (2001). [CrossRef]

15.

L. Li, A. Schülzgen, S. Chen, V. L. Temyanko, J. V. Moloney, and N. Peyghambarian, “Phase locking and in-phase supermode selection in monolithic multicore fiber lasers,” Opt. Lett. 31, 2577–2579 (2006). [CrossRef] [PubMed]

16.

R. J. Beach, M. D. Feit, S. C. Mitchell, K. P. Cutter, S. A. Payne, R. W. Mead, J. S. Hayden, D. Krashkevich, and D. A. Alumni, “Phase-locked antiguided multiple-core ribbon fiber,” IEEE Photon. Technol. Lett. 15, 670–672 (2003). [CrossRef]

17.

J. Yoo, J. R. Hayes, E. G. Paek, A. Scherer, and Y. Kwon, “Array mode analysis of two-dimensional phased arrays of vertical cavity surface emitting lasers,” IEEE J. Quantum. Electron. 26, 1039–1051 (1990). [CrossRef]

18.

J. C. Knight, T. A. Birks, P. St. J. Russell, and D. M. Atkin, “All-silica single-mode optical fiber with photonic crystal cladding,” Opt. Lett. 21, 1547–1549 (1996). [CrossRef] [PubMed]

19.

R. F. Cregan, B. J. Mangan, J. C. Knight, T. A. Birks, P. St. J. Russell, P. J. Roberts, and D. C. Allan, “Single-mode photonic band gap guidance of light in air,” Science 285, 1537–1539 (1999). [CrossRef] [PubMed]

20.

J. K. Ranka, R. S. Windeler, and A. J. Stentz, “Visible continuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nm,” Opt. Lett. 25, 25–27 (2000). [CrossRef]

21.

H. Kubota, S. Kawanishi, S. Koyanagi, M. Tanaka, and S. Yamaguchi, “Absolutely single polarization photonic crystal fiber,” IEEE Photon. Technol. Lett. 16, 182–184 (2004). [CrossRef]

22.

A. Mafi and J. V. Moloney, “Shaping modes in multicore photonic crystal fibers,” IEEE Photon. Technol. Lett. 17, 348–350 (2005). [CrossRef]

23.

J. K. Butler, D. E. Ackley, and D. Botez, “Coupled-mode analysis of phase-locked injection laser arrays,” Appl. Phys. Lett. 44, 293–295 (1984). [CrossRef]

24.

E. Kapon, J. Katz, and A. Yariv, “Supermode analysis of phase-locked arrays of semiconductor lasers,” Opt. Lett. 10, 125–127 (1984). [CrossRef]

OCIS Codes
(060.2280) Fiber optics and optical communications : Fiber design and fabrication
(140.3290) Lasers and laser optics : Laser arrays
(140.3510) Lasers and laser optics : Lasers, fiber
(260.1440) Physical optics : Birefringence

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: May 16, 2007
Revised Manuscript: July 27, 2007
Manuscript Accepted: July 30, 2007
Published: August 1, 2007

Citation
X. Zhu, A. Schülzgen, L. Li, H. Li, V. L. Temyanko, J. V. Moloney, and N. Peyghambarian, "Birefringent in-phase supermode operation of a multicore microstructured fiber laser," Opt. Express 15, 10340-10345 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-16-10340


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References

  1. D. R. Scifres, R. D. Burnham, and W. Streifer, "Phase-locked semiconductor laser array," Appl. Phys. Lett. 32, 1015-1017 (1978). [CrossRef]
  2. M. Oka, H. Masuda, Y. Kaneda, and S. Kubota, "Laser-diode-pumped phase-locked Nd:YAG laser arrays," IEEE J. Quantum Electron. 28, 1142-1147 (1992). [CrossRef]
  3. D. G. Youmans, "Phase locking of adjacent channel leaky waveguide CO2 lasers," Appl. Phys. Lett. 44, 365-367 (1984). [CrossRef]
  4. J. Morel, A. Woodtli, and R. Dandliker, "Coherent coupling of an array of Nd3+-doped single-mode fiber lasers by use of an intracavity phase grating," Opt. Lett. 18, 1520-1522 (1993). [CrossRef] [PubMed]
  5. Y. Jeong, J. K. 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]
  6. T. Qiu, L. Li, A. Schülzgen, V. Temyanko, T. Luo, S. Jiang, A. Mafi, J. Moloney, N. Peyghambarian, "Generation of 6.6 W multimode and 4 W single mode output from 7 cm short fiber lasers," IEEE Photon. Technol. Lett. 16, 2592-2594 (2004). [CrossRef]
  7. T. Qiu, S. Suzuki, A. Schülzgen, L. Li, A. Polynkin, V. Temyanko, J. Moloney, and N. Peyghambarian, "Generation of watts-level single-longitudinal-mode output from cladding-pumped short fiber lasers," Opt. Lett. 30, 2748-2750 (2005). [CrossRef] [PubMed]
  8. X. Zhu and R. K. Jain, "10-W-level diode-pumped compact 2.78 μm ZBLAN fiber laser," Opt. Lett. 32, 26-28 (2007). [CrossRef]
  9. D. R. Scifres, "Multiple core fiber lasers and optical amplifiers," US Patent #5,566,196 (1996).
  10. M. Wrage, P. Glas, D. Fischer, M. Leitner, D. V. Vysotsky, and A. P. Napartovich, "Phase locking in a multicore fiber laser by means of a Talbot resonator," Opt. Lett. 25, 1436-1438 (2000). [CrossRef]
  11. M. Wrage, P. Glas, and M. Leitner, "Combined phase locking and beam shaping of a multicore fiber laser by structured mirrors," Opt. Lett. 26, 980-982 (2001). [CrossRef]
  12. M. Wrage, P. Glas, D. Fischer, M. Leitner, N. N. Elkin, D. V. Vysotsky, A. P. Napartovich, and V. N. Troshchieva, "Phase-locking of a multicore fiber laser by wave propagating through an annular waveguide," Opt. Commun. 205, 367-375 (2002). [CrossRef]
  13. L. Michaille, C. R. Bennett, D. M. Taylor, T. J. Shepherd, J. Broeng, H. R. Simonsen, and A. Peterson, "Phase locking and supermode selection in multicore photonic crystal fiber lasers with a large doped area," Opt. Lett. 30, 1668-1670 (2005). [CrossRef] [PubMed]
  14. P. K. Cheo, A. Liu, and G. G. King, "A high-brightness laser beam from a phase-locked multicore Yb-doped fiber laser array," IEEE Photon. Technol. Lett. 13, 439-441 (2001). [CrossRef]
  15. L. Li, A. Schülzgen, S. Chen, V. L. Temyanko, J. V. Moloney, and N. Peyghambarian, "Phase locking and in-phase supermode selection in monolithic multicore fiber lasers," Opt. Lett. 31, 2577-2579 (2006). [CrossRef] [PubMed]
  16. R. J. Beach, M. D. Feit, S. C. Mitchell, K. P. Cutter, S. A. Payne, R. W. Mead, J. S. Hayden, D. Krashkevich, and D. A. Alumni, "Phase-locked antiguided multiple-core ribbon fiber," IEEE Photon. Technol. Lett. 15, 670-672 (2003). [CrossRef]
  17. J. Yoo, J. R. Hayes, E. G. Paek, A. Scherer, and Y. Kwon, "Array mode analysis of two-dimensional phased arrays of vertical cavity surface emitting lasers," IEEE J. Quantum. Electron. 26, 1039-1051 (1990). [CrossRef]
  18. J. C. Knight, T. A. Birks, P. St. J. Russell, and D. M. Atkin, "All-silica single-mode optical fiber with photonic crystal cladding," Opt. Lett. 21, 1547-1549 (1996). [CrossRef] [PubMed]
  19. R. F. Cregan, B. J. Mangan, J. C. Knight, T. A. Birks, P. St. J. Russell, P. J. Roberts, D. C. Allan, "Single-mode photonic band gap guidance of light in air," Science 285, 1537-1539 (1999). [CrossRef] [PubMed]
  20. J. K. Ranka, R. S. Windeler, and A. J. Stentz, "Visible continuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nm," Opt. Lett. 25, 25-27 (2000). [CrossRef]
  21. H. Kubota, S. Kawanishi, S. Koyanagi, M. Tanaka, and S. Yamaguchi, "Absolutely single polarization photonic crystal fiber," IEEE Photon. Technol. Lett. 16, 182-184 (2004). [CrossRef]
  22. A. Mafi and J. V. Moloney, "Shaping modes in multicore photonic crystal fibers," IEEE Photon. Technol. Lett. 17, 348-350 (2005). [CrossRef]
  23. J. K. Butler, D. E. Ackley, and D. Botez, "Coupled-mode analysis of phase-locked injection laser arrays," Appl. Phys. Lett. 44, 293- 295 (1984). [CrossRef]
  24. E. Kapon, J. Katz, and A. Yariv, "Supermode analysis of phase-locked arrays of semiconductor lasers," Opt. Lett. 10, 125-127 (1984). [CrossRef]

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