DBR laser: Experimental set-up and result

This section describes the realisation of a distributed Bragg reflector (DBR) for increasing the pump absorption by using a longer cavity length. In [11, 12], the DBR fibre laser was formed with two Bragg gratings that were written into the core of the Er3+-doped fibre by using a two beam interference pattern. The two reflector gratings were written on opposite ends of the fibre. In this work, a DBR Ho3+-doped fibre

length (L) of Ho3+-doped fibre, to form a cavity. By splicing the gratings on both ends of the fibre additional splice loss will be introduced; however, it was the initial trial for this set-up. The gratings were cut from both ends of the DFB laser and spliced to both ends of the Ho3+-doped fibre. The gratings were 4 cm long so that the Bragg wavelength of the two reflectors was the same. The reflectivities of the 4 cm long gratings were 96.4%, i.e. 0.16 dB loss at the reflectivity of the gratings. The cavity lengths of the laser used were 31 and 85 cm long. The laser was end pumped with the Yb3+-doped fibre laser with 230 mW at 1119 nm. Figure 6.10 shows the schematic diagram of the DBR fibre laser configuration together with the pump source.

Figure 6.10:Ho3+-doped DBR fibre laser configuration.

2000 2050 2100 2150 2200 Wavelength (nm) In te n si ty (a .u ) 31 cm 85 cm Without gratings

Figure 6.11:Spectrum of DBR for a cavity length of 31 and 85 cm; and without reflector gratings for

3+ HR Grating @1119 nm OC Grating @1119 nm Monochro- mator/ Thermal powermeter L 977 nm MM laser diode module GTWave

The power measured at the output of the DBR laser was 50 and 2 mW for a cavity length of 31 and 85 cm respectively. The output of the laser was investigated with the scanning monochromator and the spectrum is shown in Figure 6.11. With the 31 cm cavity length, peaks were observed at ~2010 and 2175 nm which is corresponded to the emission spectrum of Ho3+. However, neither of these wavelengths was corresponded to the designed operating wavelength. Accordingly, the gratings at the ends of the fibre were then un-spliced and the 31 cm long Ho3+-doped fibre was spliced directly to the output of the Yb3+-doped fibre laser. Again, the peaks coincided exactly with the DBR laser wavelength and lasing occurred as explained in section 6.4. For a cavity length of 85 cm, only the ASE was observed and no peak. This might be because the cavity length was too long and signal reabsorption can occur.

6.6 Conclusion

This chapter describes the preliminary work involved to implement a Ho3+-doped DFB fibre laser and assess the performance of the laser. The DFB laser was designed to operate at 2140 nm, the peak emission of Ho3+. It was pumped at 1119 nm by an Yb3+-doped fibre laser and at 1836 nm by a Tm3+DFB laser. The quantum efficiency limit at pump wavelengths 1119 nm and 1836 nm was 52% and 86% respectively. However, neither pump wavelength managed to get the DFB laser to lase. This could be because the losses in the cavity were high and the gain of the fibre was insufficient to overcome it. The other possibility was due to the pump absorption for a 12 cm Ho3+-doped fibre, that was low at both pump wavelengths ~0.2 - 0.3 dB/cm. Since the absorption of the DFB is low, a pump source with much higher power, perhaps, could reach the threshold of the laser. Then, a DBR fibre laser was constructed to increase the pump absorption by using a much longer cavity length. The cavity lengths of 31 and 85 cm were used, but still no lasing was observed. Concentration quenching effects could be one of the problems as our fibre was doped with a huge Ho3+ concentration. Other possible problems were due to the dominant non-radiative transition in the 2 μm region and the large intrinsic losses of the silica

6.7 References

[1] D. C. Hanna, R. M. Percival, R. G. Smart, J. E. Townsend, and A. C. Tropper, "Continuous- wave oscillation of holmium-doped silica fibre laser,"Electronics Letters, vol. 25, pp. 593- 594, 1989.

[2] K. Oh, T.F. Morse, A. Kilian, and L. Reinhart, "Continuous-wave oscillation of thulium- sensitized holmium-doped silica fiber laser,"Optics Letters, vol. 19, pp. 278-280, 1994. [3] A. S. Kurkov, E. M. Dianov, O. I. Medvedkov, G. A. Ivanov, V. A. Aksenov, V. M.

Paramonov, S. A. Vasiliev, and E. V. Pershina, "Efficient silica-based Ho3+ fibre laser for 2

m spectral region pumped at 1.15m,"Electronics Letters, vol. 36, pp. 1015-1016, 2000. [4] S. D. Jackson and S. Mossman, "Diode-cladding-pumped Yb3+, Ho3+-doped silica fiber laser

operating at 2.1-m,"Applied Optics, vol. 42, pp. 3546-3549, 2003.

[5] S. D. Jackson, "2.7-W Ho3+_{-doped silica fibre laser pumped at 1100 nm and operating at 2.1} m,"Applied Physics B: Lasers and Optics, vol. 76, pp. 793-795, 2003.

[6] A. Taniguchi, T. Kuwayama, A. Shirakawa, M. Musha, and K. Ueda, "1212 nm pumping of 2 m Tm-Ho-codoped silica fiber laser," Applied Physics Letters, vol. 81, pp. 3723-3725, 2002.

[7] S. D. Jackson and A. A. King, "High-power diode-cladding-pumped Tm-doped silica fiber laser,"Optics Letters, vol. 23, pp. 1462-1464, 1998.

[8] J. Y. Allain, M. Monerie, and H. Poignant, "High-efficiency CW thulium-sensitised holmium-doped fluoride fibre laser operating at 2.04μm," Electronics Letters, vol. 27, pp. 1513-1515, 1991.

[9] R. M. Percival, D. Szebesta, S. T. Davey, N. A. Swain, and T. A. King, "Thulium sensitised holmium-doped CW fluoride fibre laser of high efficiency,"Electronics Letters, vol. 28, pp. 2231-2232, 1992.

[10] S.D. Jackson and Y. Li, "High-power broadly tunable Ho3+-doped silica fibre laser,"

Electronics Letters, vol. 40, pp. 1474-1475, 2004.

[11] G. A. Ball, W. W. Morey, and W. H. Glenn, "Standing-wave monomode erbium fiber laser,"

IEEE Photonics Technology Letters, vol. 3, pp. 613-615, 1991.

[12] J. L. Zyskind, V. Mizrahi, D. J. DiGiovanni, and J. W. Sulhoff, "Short single frequency erbium-doped fibre laser,"Electronics Letters, vol. 28, pp. 1385-1387, 1992.

Chapter 7