The characteristics of a high power Er3+:Yb3+ single frequency fiber laser pumped at 980nm are reported. The device gives 60mW output power with RIN 10MHz and linewidth 500kHz. At low output powers (&lt; 30mW) the slope efficiency is as high as 25%, falling to 12% at higher powers, the saturation behaviour is related to a bottleneck effect due to the finite Yb-Er transfer rate. Improved performance can be obtained using new fibers with an increased rare-earth concentration which show negligible signs of erbium clustering

Samson, B.N.

Dong, L.

Loh, W.H.

Hsu, K.

English

Southampton (e-Prints Soton)

High  Power (6OmW) Single Frequency Erbium:Ytterbium  Codoped Fiber  Laser B. N. Samson,  L. Dong, W.H.  Loh and K. Hsu* Optoelectronics  Research  Centre University of Southampton Southampton  SO17 1BJ United Kingdom Email:BNS@ORC.SOTON.AC.UK FAX:  +44(0)1703 593142 *Micron  Optics, Inc., 2801 Buford Hwy,  Suite 140, Atlanta, Georgia 30329, USA. ABSTRACT:  The characteristics  of a high power E?“:Yb3+ single  frequency fiber laser pumped at 980nm are reported.  The device gives  60mW  output  power  with  RIN <- 168dB/Hz  for  frequencies  > 1OMHz  and linewidth  5OOkHz.  At  low  output powers ( < 30mW) the slope efficiency is as  high as 25%, falling  to 12% at higher powers,  The saturation  behaviour  is related  to a bottleneck affect due to the finite Yb- Er transfer rate. Improved performance  can  be obtained using new  fibers  with  an  increased rare-earth concentration  which show negligible signs of erbium clustering. 1. INTRODUCTION Recently,  there has been sustained interest  towards the development of  high performance single frequency fiber grating- based  erbiumdoped fiber lasers’-7.  To ensure robust single frequency operation without mode-hopping, these  lasers  need  to be short, a  few  cm  in  length  at  most.  Whilst  the earlier grating-based  fiber lasers  relied on conventional erbium-doped germanosilicate fibers,  the  laser  efficiencies  and  output powers  were low - typically 0.1% and in the PW regime respectively1v2.  This is a direct consequence  of the low pump absorption due to  the short fiber  cavity  length.  Various configurations have been demonstrated in order to boost the power output, including a MOPA  configuration  and different  pump schemes4*5.  The  potential  for  efficient devices based on  Er:Yb  co-doped fibres, although  promising6,  suffered  due the lack of photosensitivity in these fibers.  However, the  recent development of  photosensitive E?:Yb3+  codoped fibers7 has dramatically improved the  efficiency  and hence output power from these  devices, primarily  due to the  increased 980nm  pump  absorption. Unfortunately, early DFB fibre lasers  made in  these fibres  were unable to perform  at high  powers (>  1OmW) due to  thermally induced changes in  the  grating.  These thermal problems are  a direct result of the high lasing intensity found at the centre of discrete quarter-wave phase shifted  DFB lasers’,  a problem  less likely  to  occur in linear cavity lasers  where the lasing intensity is more uniform. In  this  paper,  we  report  on  the characterisation  of a grating-based  fibre-laser capable  of producing 60mW output power, with single  frequency  and single polarization mode output.  The output power is limited by a bottleneck effect, primarily  due to the relatively  low  Yb to Er transfer rate in the fibre  used for  this  experiment.  Such a process  is well documented  in the literature’. The  bottleneck  occurs when the  Yb-ion pump  rate  is  greater than the  Yb  to  Er transfer rate.  This can be rectified  by  an increase  in the Yb to Er transfer rate, easily achieved through  an increase in  the rare earth  concentration.  However,  it  is well known  that  above a critical  concentration threshold erbium ion clustering takes  place, a  feature  detrimental  to  the  laser performance”12.  In  order to quantify this effect  we  have  measured the  saturation characteristics  of the erbium ions in a Er:Yb co-doped fibre,  when pumped directly  at 1535nm.  In  a  fibre  with  llOdB/m absorption  at  1535nm,  three  times  the erbium  concentration present in  the fibre used for  our laser experiment, we observe negligible  erbium  ion  clustering.  We anticipate these latest fibres will  show an increase  in output power compared  with the present  fibre laser. 2. EXPERIMENTAL  RESULTS Fig.  1 shows  the configuration of the E?:Yb3’  fiber Bragg  grating laser (FBGL), which  has  been  the  subject  of  recent wavelength  tuning experiments13.  One cavity end  reflector  consists  of  a  dielectric (TiO,/SiO,) high reflecting (99.86% at 1.55 pm)  mirror  epoxied onto one end of  the polished E?:Yb3+  fiber laser cavity, while the  other  end is a centimetre long  Bragg grating (98% reflectivity at 1536 nm) written directly  into the Er:Yb  fiber, and situated  a few  mm  away from  the dielectric mirror. The entire laser is thus less than 1.5 cm in length.  An  advantage of  this  compact configuration is that the 980 nm pump can be efficiently and directly coupled into the laser through  the  dielectric  mirror,  which transmits well  at 1 pm. Fig. 2 shows  the lasing characteristic of the FBGL, which is pumped by a 980 nm Ti:Sapphire laser. A maximum output power of  58 mW  (measured at  the output  of  a pigtailed isolator) is obtained, for a launched pump  power  of  500 mW.  Although  this laser is operating at 1535nm, lasing around 155Onm has been demonstrated in  these lasers13.  A  scanning  Fabry-Perot interferometer  confirmed  both  single frequency  as well as single polarisation state operation.  In figure 3, the measured BIN  is <  -168 dB/Hz  for  frequencies beyond 10 MHz,  with  a peak BIN  value of just  -133 dB/Hz at the relaxation oscillation frequency of 2.8 MHz.  We have measured  the optical linewidth using the delayed self-heterodyne technique to be 500 kHz when operating at high power. Although  this value is greater than that obtained in fibre DFB’s,  albeit at considerably  lower  powers,  it  is  still considerably less than that  obtained from semiconductor  devices. To determine  the degree  to which the laser  is  indeed  operating  in  a  single polarisation  state,  the  laser  output  was connected  to a polarisation  controller, and on to  a  (pigtailed)  polarising  isolator.  By adjusting  the  polarisation  controller,  the output from the polarising isolator could be reduced  by -50 dB, probably limited by the extinction of the polariser used. The second orthogonally polarised mode of the laser is thus  shown  to be effectively nonexistent, and the laser output is established  to be that of a single  polarisation state, with a purity better than 50 dB at high output power. It  should be mentioned that single frequency  operation  in  this  laser  is  not always maintained  over  the  entire  pump power  range.  As  the  pump  power  is increased,  the  lasing  wavelength correspondingly increases,  and eventually a mode change  occurs. It  is worth noting that when the pump power is quickly turned on, from  zero  to  full  power,  the  fiber  laser correspondingly turns  on,  but  the  lasing wavelength is  observed to  gradually  shift longer by 0.3 m-n  over the course of several minutes before finally  stabilising. We thus infer  that the associated  temperature rise in the fiber  laser is ‘30°C  for  the maximum operating power. The 60 mW FBGL has a reasonable net  efficiency  of  12%  with  respect  to launched pump power, however, it  is clear from  Fig.  2  that  the  efficiency  is considerably better at lower pump powers. For this laser the measured  slope is around 25 % at output powers less than 30mW and clearly the output starts  to saturate  at higher powers. Unlike the recent measurements  on Er:Yb  DFB’?,  we believe the origin of this is not thermal but rather a consequence  of the bottleneck effect, arising from the finite energy transfer rate from  excited Yb3+ to ground state  E?’  ions.  Recent  modelling of single  frequency  Yb:Er  fibre  laser.?, has pointed  out  that  fibres  with  a  high concentration  of Er and Yb ions should show reduced bottlenecking due to the increased Yb  to  Er  transfer  rate  in  these fibres. However,  the  theoretically  improved performance, of these  devices  is based  upon the assumption that erbium ion clustering is not  occurring  in  heavily  co-doped Yb:Er fibres.  This assumption is based  upon the belief  that  the  Ytterbium  ions  actually disperse  the erbium clusters, thus aiding the homogenous distribution  of  erbium  ions within  the glass. In  order to validate this assumption we have studied  the saturation  characteristics of  a  heavily  doped  Yb:Er  fibre.  The saturation curve for  a fibre  with  llOdB/m absorption at 1535nm, a value 3 times that measured  in the FBGL laser fibre,  is shown in figure  4,  Despite the high concentration of erbium ions in this fibre, the absorption is almost completely bleachable  indicating the very  low  level of clustered erbium ions in this  fibre.  Based on  the  pair  induced quenching  model*,  the  measured unbleachable  loss corresponds  to a clustered ion fraction of less  than 3 %  .  We anticipate that  single  frequency  fibre-grating  based lasers made from this fibre will  give output powers  greater  than  the  60mW  already achieved. 3. CONCLUSION We have demonstrated  a high output power  (60mW)  stable  single  frequency E?:Yb3+  fiber  laser pumped at  980nm. The laser  characteristics,  RIN <  -168 dB/Hz for frequencies  greater than lOMHz,  optical SNR  >  65 dB and single polarisation state purity  >  50  dB,  make these lasers highly attractive  for  CATV  applications.  The measured slope efficiency  at  low  output power (25% at 30mW) decreases  at higher powers due to a bottleneck in the Yb to Er transfer  rate. Improved fibre, with  increased rare  earth concentration, show  very  little sign of erbium ion clustering, indicating that further  increases in  the  available  output power may be possible. 4. ACKNOWLEDGEMENTS The  authors  are  grateful  to  J.  E. Caplen for  preform  fabrication  and R.I. Laming  for  useful discussions. This  work was supported in  part  by  the  EC  ACTS project  PHOTOS.  The  Optoelectronics Research  Centre  is  an  EPSRC-funded Interdisciplinary Research  Centre. REFERENCES 1.J.  L.  Zyskind,  V.  Mizrahi,  D.  J. DiGiovanni and  J. W. Sulhoff,  ‘Short single frequency  erbium-doped  fiber  laser’, Electron.  Lett.,  vol.  28,  pp.  1385-1387, 1992. 2. W,H, Lob and  R.1,  Laming,  “13  pm phase-shifted  E?-doped  fiber  laser”, Electron. L&t.,  Vol31,  ~~1440-1442, 1995. 3.G. A. Ball, C. E. Holton,  G. Hull-Allen and W . W. Morey , ’ 60 mW 1.5 pm single frequency low  noise fiber  laser MOPA’, IEEE  Photon.  Technol.  Lett.,  vol.  6, pp. 192-194, 1994. 4. W.H.  Loh, S.D. Butterworth and W.A. Clarkson,“Efficient  distributed  feedback erbium-doped  germanosilicate fiber pumped in the 52Onm  band”, Electron. Iett.,  vol.  32, ~2088-2089, 1996. .5.W.H.  Loh,  B.N.  Samson,  Z.E. Hartutjunian  and R.  I.  Laming,  “Intracavity pumping for increased  output power from a distributed  feedback erbium  fibre  laser”, Electron. Lett., vol. 32, pp1204-1205, 1996. 6.5. T. Kringlebotn,  J.-L.  Archambault, L. Reekie, J. E. Townsend, G. G. Vienne and D.  N.  Payne, ‘Highly  efficient,  low  noise grating feedback E?+:Yb3+  codoped fiber laser’, Electron. Lett., vol. 30, pp. 972-973, 1994. 7.L. Dong, W. H. Loh, J. E. Caplen, J. D. Minelly,  K.  Hsu and L.  Reekie, ‘Efficient single-frequency  fiber  lasers with  novel photosensitive Er/Yb  optical  fibers’ ,  Opt. L&t.,  vol. 22, pp. 694-696, 1997. 8. W.H.  Loh,  B.N.  Samson and J.P. de Sandro,  “Intensity  profile  in  distributed feedback  fiber laser characterised  by a green fluorescence  scanning  technique”,  App. Phys. Lett.,  Vol69,  pp. 3773-3775, 1996. 9.  F.  Di  Pasquale, “Modeling  of  highly- efficient  grating-feedback and  fabry-Perot E?-Yb3’  co-doped fiber  lasers”, IEEE  J. @ant. Electron., vol. 32, ~~326-332, 1996. 10.J. L. Wagener,  P. F. Wysocki, M.  J. F. Digonnet, H. J Shaw and D. J. DiGiovanni, ‘Effects  of  concentration and  clusters in erbium-doped fiber lasers’, Opt. Lett.,  vol. 18, pp. 2014-2016, 1993. 11.E. Delevaque,  T. Georges,  M.  Monerie, P. Lamouler and J.-F. Bayon, ‘Modeling  of pair-induced  quenching  in  erbium-doped silicate  fibers’ ,  IEEE  Photon.  Technol. La.,  vol. 5, pp. 73-75, 1993. 12.F.  Sanchez, P.  Le  Boudec,  P.-L. Francois and G.  Stephan, ‘Effects  of  ion pairs  on the dynamics of erbium-doped fiber lasers’, Phys. Rev. A.,  vol.  48, pp. 2220- 2229, 1993. 13-K. Hsu, W. H. Loh, L. Dong and C. M. Miller,  ‘Wavelength  tuning  in  efficient Er/Yb  fiber  grating  lasers’,  submitted to 23rd Europ. Conf. on Opt. Commun. 1997. dielectric  high reflector f- output  isolator  E;:Ybr  980nm  pump Figure  1 Er:Yb  co-doped  FBGL  laser 

13 pm phase-shifted E?-doped fiber laser”,

Clarkson,“Efficient distributed feedback erbium-doped germanosilicate fiber pumped in the 52Onm band”,

Effects of concentration and clusters in erbium-doped fiber lasers’,

Effects of ion pairs on the dynamics of erbium-doped fiber lasers’,

Efficient single-frequency fiber lasers with novel photosensitive Er/Yb optical fibers’

Highly efficient, low noise grating feedback E?+:Yb3+ codoped fiber laser’,

Intensity profile in distributed feedback fiber laser characterised by a green fluorescence scanning technique”,

Modeling of highlyefficient grating-feedback and fabry-Perot E?-Yb3’ co-doped fiber lasers”,

Modeling of pair-induced quenching in erbium-doped silicate fibers’

pm single frequency low noise fiber laser MOPA’,

Short single frequency erbium-doped fiber laser’,

High power (60mW) single frequency erbium:ytterbium codoped fiber laser

High power (60mW) single frequency erbium:ytterbium codoped fiber laser

Abstract

Similar works

Full text

Available Versions

Southampton (e-Prints Soton)