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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 14, Iss. 16 — Aug. 7, 2006
  • pp: 7087–7092
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Single-frequency fiber oscillator with watt-level output power using photonic crystal phosphate glass fiber

A. Schülzgen, L. Li, V. L. Temyanko, S. Suzuki, J. V. Moloney, and N. Peyghambarian  »View Author Affiliations


Optics Express, Vol. 14, Issue 16, pp. 7087-7092 (2006)
http://dx.doi.org/10.1364/OE.14.007087


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Abstract

Utilizing phosphate glass fiber with photonic crystal cladding and highly doped, large area core a cladding-pumped, single-frequency fiber oscillator is demonstrated. The fiber oscillator contains only 3.8 cm of active fiber in a linear cavity and operates in the 1.5 micron region. Spectrally broad, multimode pump light from semiconductor laser diodes is converted into a single-mode, single-frequency light beam with an efficiency of about 12% and the oscillator output power reached 2.3 W.

© 2006 Optical Society of America

1. Introduction

Single-frequency lasers have a wide range of applications in optical sensor and communication systems. In particular, single-frequency oscillators that operate around 1.5 μm are crucial devices due to their compatibility with optical fiber communication lines and a wide range of existing photonics components. With sufficient output powers single-frequency lasers also find applications in nonlinear optical devices, e.g., as pump sources for optical parametric amplifiers. The fabrication of compact, single-frequency fiber lasers was enabled by the development of fiber Bragg grating (FBG) that can be directly written into single mode fiber cores. Combining narrow band FBGs with short, rare-earth doped, single-mode active fibers exclusive oscillation of just one laser mode in a linear cavity was achieved [1

1. J. M. Jauncy, L. Reekie, J. E. Townsend, and D. N. Payne, “Single-longitudinal-mode operation of an Nd3+-doped fibre laser,” Electron. Lett. 24, 24–26 (1988). [CrossRef]

, 2

2. G. A. Ball, W. W. Morey, and W. H. Glenn, “Standing-wave monomode erbium fiber laser,” IEEE Photon. Technol. Lett. 3, 613–615 (1991). [CrossRef]

]. In addition, single-frequency, traveling wave fiber lasers have been reported [3

3. K. Iwatsuki, H. Okamura, and M. Saruwatari, “Wavelength-tunable single-frequency and single-polarisation Er-doped fibre ring-laser with 1.4 kHz linewidth,” Electron. Lett. 26, 2033–2035 (1990). [CrossRef]

]. However, the increased complexity and reduced stability of fiber ring resonators give monolithic nonplanar ring resonators [4

4. T. J. Kane and R. L. Byer, “Monolithic, unidirectional, single-mode Nd:YAG ring laser,” Opt. Lett. 10, 65–67 (1985). [CrossRef] [PubMed]

] a clear edge.

A novel concept to increase the mode area while maintaining single transverse mode operation is the application of photonic crystal fibers (PCFs). PCFs consist of a regular array of air holes and a defect in its center which defines the core. The photonic cladding can be used to tailor the optical properties of these fibers and achieve, e.g. large dispersion, broadband single mode guidance, and extremely small or large mode areas [9

9. P. St. J. Russell, “Photonic crystal fibers,” Science 299, 358–362 (2003). [CrossRef] [PubMed]

]. Best studied PCFs with large mode areas are fibers where one central hole is replaced by a solid core. Experimentally, PCFs with the largest single mode cores have been realized with designs where more than one central air hole is removed and mode diameters on the order of 2000 μm2 have been demonstrated in case of seven missing central air holes [10

10. J. Limpert, A. Liem, M. Reich, T. Schreiber, S. Nolte, H. Zellmer, A. Tünnermann, J. Broeng, A. Petersson, and C. Jakobsen, “Low-nonlinearity single-transverse-mode ytterbium-doped photonic crystal fiber amplifier,” Opt. Express 12, 1313–1319 (2004). [CrossRef] [PubMed]

].

2. Active fiber and single-frequency oscillator design

The geometry of the linear-cavity single-frequency fiber laser is shown in Fig. 1. Three different fibers formed an all-fiber laser device that was held in a silicon U-groove. The silicon substrate was thermoelectrically cooled to provide effective heat dissipation during high power laser operation. 976 nm pump light from multimode semiconductor laser diodes was delivered through a multimode fiber with 105 μm core diameter, an outer diameter of 125 μm, and a numerical aperture of 0.22. The end facet of this fiber was coated with a dielectric stack of SiO2 and Ta2O5 layers that transmits the pump light but acts as a high reflector at the lasing wavelength of 1.534 μm. The pump delivery fiber is butt-coupled to the active fiber.

Fig. 1. Schematic of the single-frequency fiber laser. A microscope image of the active photonic crystal fiber with 125 microns outer diameter is shown in the lower left part.

At the fiber laser output side (left side in Fig. 1) the active fiber is spliced to a single mode silica fiber (Nufern PS-GDF-20/400) that has a large area photosensitive core. The nominal core diameter is 20 μm with a numerical aperture of 0.06. The original outer diameter of this fiber is 400 μm. To achieve a low loss fusion splice between the active phosphate PCF and the photosensitive silica fiber the latter has been etched to an outer diameter of 125 μm by hydrofluoric acid. In addition, a short (<300 μm) buffer of coreless phosphate fiber has been inserted between the PCF and the silica fiber to preserve the air holes in the PCF during fusion splicing. The fiber laser resonator is completed by a FBG written into the core of the silica fiber using 244 nm light and a 25 mm long phase mask. The reflection spectrum of the FBG in Fig. 2 shows a peak reflectivity of 17% at 1.534 μm and a 3 dB bandwidth of about 0.03 nm. Per roundtrip the complete fiber laser has a propagation loss of 3.8 dB due to the fusion splices and coupling losses between different fibers and the output loss is 7.7 dB. The roundtrip loss can easily be compensated by the large gain in the PCF with the highly doped, large area core.

Fig. 2. Reflection spectrum of the FBG used in the single-frequency laser. The FBG has a peak reflectivity of 17% at 1534 nm and a 3 dB bandwidth of 0.03 nm.

3. Single-frequency oscillator performance

The output versus pump power characteristics of the single-frequency PCF laser is shown in Fig. 3. Measured by an optical spectrum analyzer (not shown) the laser emission spectrum is a resolution limited narrow line centered at the 1534 nm reflection peak of the FBG. An optical-to-optical conversion efficiency of a ~12% is observed up to pump powers of 20 W. This efficiency is more than twice as high as the maximum conversion efficiency of ~5% reported for similar single-longitudinal-mode fiber laser that uses conventional step-index fiber [7

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

]. In addition, the previously reported step-index fiber lasers showed strong saturation effects at similar pump levels as illustrated by comparison of our results with data from Qiu et al. [7

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

] in Fig. 3. Due to saturation, the optical conversion efficiency of step-index fiber lasers decreases below the 5% level at higher pump powers. A maximum output power over 2.3 W at 20 W of pump power demonstrates the advantages of our single-frequency, PCF based laser device over conventional step-index fiber lasers with respect to overall power. In the latter case a saturated maximum output power of 1.6 W output was achieved at pump levels slightly above 35 W [7

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

].

Fig. 3. Signal output vs. pump power of a fiber laser with only 3.8 cm of active PCF (filled circles). For comparison the performance of a similar laser using 4 cm of active step-index fiber is also shown (open circles) [7]. The lines are for eye guidance indicating no saturation for the PCF laser and a typical saturation behavior of the step-index fiber laser.

Thermal management problems currently prevent us to pump our PCF laser at comparable power levels. Assuming that the thermal problems can be solved and the saturation effects remain small output powers beyond 4 W can be reached launching 35 W of pump power. These numbers indicate the future potential of cladding pumped single-frequency PCF lasers.

To obtain more details on the laser emission a heterodyne measurement has been performed. A single-frequency signal from a high-accuracy (wavelength resolution of 0.1 pm) tunable diode laser was coupled to our fiber laser output and the resulting beat signal was analyzed using a radio frequency (RF) electrical spectrum analyzer (ESA). A typical RF spectrum collected during laser operation is shown in Fig. 4(a). Only one frequency component is observed within the full span of 3 GHz of the ESA. This single-frequency operation indicates another advantage of using a PCF as the active fiber; the noncircular symmetry of the core inherently results in polarization mode discrimination while in step-index fibers special polarization maintaining fiber has to be used to break the polarization degeneracy. Without any advanced temperature stabilization or vibration isolation the single-frequency operation of our fiber laser is very stable and virtually free of mode hopping up to pump levels of 10 W. The emission line has a width of ~2 MHz and frequency drifts over a period of 1 minute are typically below 1 MHz. Even at the higher pump levels single-frequency operation prevails, however, the laser operates from time to time at multiple emission lines as shown in Fig. 4(b). This multi-line operation allows obtaining the free spectral range (FSR) of the fiber laser cavity. The measured FSR of 1.74 GHz corresponds to a total cavity length of 5.8 cm. In addition, another transverse mode separated by 240 MHz appears in the RF spectrum of Fig. 4(b) that we tentatively assign to the orthogonal polarization. The robustness of single-frequency and single-polarization operation can be further improved by accurate temperature control and better mechanical stability.

Fig. 4. RF beat signal between the emission of the PCF laser oscillator and a narrow linewidth single-frequency reference source measured by an electrical spectrum analyzer indicating (a) single-frequency and (b) multi-line operation of the PCF oscillator.

4. Conclusion

We have demonstrated a fiber oscillator that applies the concept of large core PCF to achieve high-power, single-frequency operation around 1.5 μm. Combining extremely high doping in phosphate glass and large core area sufficient multimode absorption has been achieved over only 3.8 cm active fiber length even in a cladding pumping scheme. This new concept enables utilizing readily available, high-power, multimode laser diodes to pump single-frequency fiber lasers and boost their output power to the level of several Watts.

Acknowledgments

The authors thank NP Photonics Inc. for providing the phosphate glasses and Y. Merzlyak and E. Temyanko for technical assistance. This work is supported by the Air Force Office of Scientific Research through a MRI program No. F49620-02-1-0380. We also acknowledge support from the TRIF Photonics program of the State of Arizona.

References and links

1.

J. M. Jauncy, L. Reekie, J. E. Townsend, and D. N. Payne, “Single-longitudinal-mode operation of an Nd3+-doped fibre laser,” Electron. Lett. 24, 24–26 (1988). [CrossRef]

2.

G. A. Ball, W. W. Morey, and W. H. Glenn, “Standing-wave monomode erbium fiber laser,” IEEE Photon. Technol. Lett. 3, 613–615 (1991). [CrossRef]

3.

K. Iwatsuki, H. Okamura, and M. Saruwatari, “Wavelength-tunable single-frequency and single-polarisation Er-doped fibre ring-laser with 1.4 kHz linewidth,” Electron. Lett. 26, 2033–2035 (1990). [CrossRef]

4.

T. J. Kane and R. L. Byer, “Monolithic, unidirectional, single-mode Nd:YAG ring laser,” Opt. Lett. 10, 65–67 (1985). [CrossRef] [PubMed]

5.

M. Seijka, P. Varming, J. Hübner, and M. Kristensen, “Distributed feedback Er3+-doped fibre laser,” Electron. Lett. 31, 1445–1446 (1995). [CrossRef]

6.

Ch. Spiegelberg, J. Geng, Y. Hu, Y. Kaneda, S. Jiang, and N. Peyghambarian, “Low-noise narrow-linewidth fiber laser at 1550 nm,” J. Lightwave Technol. 22, 57–62 (2004). [CrossRef]

7.

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

8.

P. Polynkin, A. Polynkin, M. Mansuripur, J. Moloney, and N. Peyghamabrian, “Single-frequency laser oscillator with watts-level output power at 1.5 μm by use of a twisted-mode technique,” Opt. Lett. 30, 2745–2747 (2005). [CrossRef] [PubMed]

9.

P. St. J. Russell, “Photonic crystal fibers,” Science 299, 358–362 (2003). [CrossRef] [PubMed]

10.

J. Limpert, A. Liem, M. Reich, T. Schreiber, S. Nolte, H. Zellmer, A. Tünnermann, J. Broeng, A. Petersson, and C. Jakobsen, “Low-nonlinearity single-transverse-mode ytterbium-doped photonic crystal fiber amplifier,” Opt. Express 12, 1313–1319 (2004). [CrossRef] [PubMed]

11.

L. Li, A. Schülzgen, V. L. Temyanko, T. Qiu, M. M. Morrell, Q. Wang, A. Mafi, J. V. Moloney, and N. Peyghambarian, “Short-length microstructured phosphate glass fiber lasers with large mode areas,” Opt. Lett. 30, 1141–1143 (2005). [CrossRef] [PubMed]

12.

L. Li, A. Schülzgen, V. L. Temyanko, S. Sabet, M. M. Morrell, H. Li, A. Mafi, J. V. Moloney, and N. Peyghambarian, “Investigation of modal properties of microstructured optical fibers with large depressed-index cores,” Opt. Lett. 30, 3275 (2005). [CrossRef]

13.

J. K. Sahu, C. C. Renaud, K. Furusawa, R Selvas, J. A. Alvarez-Chavez, D. J. Richardson, and J. Nilsson, “Jacketed air clad cladding pumped ytterbium doped fibre laser with wide tuning range,” Electron. Lett. 38, 1116–1117 (2001). [CrossRef]

14.

W. J. Wadsworth, J. C. Knight, and P. St. J. Russell, “Large mode area photonic crystal fibre laser,” in OSA Trends in Optics and Photonics 56, Conference on Lasers and Electro-Optics, Technical Digest, Postconference Edition (Optical Society of America, Washington DC, 2001), pp. 319.

15.

K. Furusawa, A. Malinowski, J. Price, T. Monro, J. Sahu, J. Nilsson, and D. Richardson, “Cladding pumped Ytterbium-doped fiber laser with holey inner and outer cladding,” Opt. Express 9, 714–720 (2001). [CrossRef] [PubMed]

OCIS Codes
(060.2410) Fiber optics and optical communications : Fibers, erbium
(140.3510) Lasers and laser optics : Lasers, fiber

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: May 24, 2006
Revised Manuscript: July 25, 2006
Manuscript Accepted: July 26, 2006
Published: August 7, 2006

Citation
A. Schülzgen, L. Li, V. L. Temyanko, S. Suzuki, J. V. Moloney, and N. Peyghambarian, "Single-frequency fiber oscillator with watt-level output power using photonic crystal phosphate glass fiber," Opt. Express 14, 7087-7092 (2006)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-14-16-7087


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References

  1. J. M. Jauncy, L. Reekie, J. E. Townsend, and D. N. Payne, "Single-longitudinal-mode operation of an Nd3+-doped fibre laser," Electron. Lett. 24, 24-26 (1988). [CrossRef]
  2. G. A. Ball, W. W. Morey, and W. H. Glenn, "Standing-wave monomode erbium fiber laser," IEEE Photon. Technol. Lett. 3, 613-615 (1991). [CrossRef]
  3. K. Iwatsuki, H. Okamura, and M. Saruwatari, "Wavelength-tunable single-frequency and single-polarisation Er-doped fibre ring-laser with 1.4 kHz linewidth," Electron. Lett. 26, 2033-2035 (1990). [CrossRef]
  4. T. J. Kane and R. L. Byer, "Monolithic, unidirectional, single-mode Nd:YAG ring laser," Opt. Lett. 10, 65-67 (1985). [CrossRef] [PubMed]
  5. M. Seijka, P. Varming, J. Hübner, and M. Kristensen, "Distributed feedback Er3+-doped fibre laser," Electron. Lett. 31, 1445-1446 (1995). [CrossRef]
  6. Ch. Spiegelberg, J. Geng, Y. Hu, Y. Kaneda, S. Jiang, and N. Peyghambarian, "Low-noise narrow-linewidth fiber laser at 1550 nm," J. Lightwave Technol. 22, 57-62 (2004). [CrossRef]
  7. T. Qiu, A. Schülzgen, L. Li, A. Polynkin, V. L. Temyanko, J. V. Moloney, and N. Peyghambarian, "Generation of watt-level single longitudinal mode output from cladding pumped short fiber lasers," Opt. Lett. 30, 2748-2750 (2005). [CrossRef] [PubMed]
  8. P. Polynkin, A. Polynkin, M. Mansuripur, J. Moloney, and N. Peyghamabrian, "Single-frequency laser oscillator with watts-level output power at 1.5 µm by use of a twisted-mode technique," Opt. Lett. 30, 2745-2747 (2005). [CrossRef] [PubMed]
  9. P. St. J. Russell, "Photonic crystal fibers," Science 299, 358-362 (2003). [CrossRef] [PubMed]
  10. J. Limpert, A. Liem, M. Reich, T. Schreiber, S. Nolte, H. Zellmer, A. Tünnermann, J. Broeng, A. Petersson, and C. Jakobsen, "Low-nonlinearity single-transverse-mode ytterbium-doped photonic crystal fiber amplifier," Opt. Express 12, 1313-1319 (2004). [CrossRef] [PubMed]
  11. L. Li, A. Schülzgen, V. L. Temyanko, T. Qiu, M. M. Morrell, Q. Wang, A. Mafi, J. V. Moloney, and N. Peyghambarian, "Short-length microstructured phosphate glass fiber lasers with large mode areas," Opt. Lett. 30, 1141-1143 (2005). [CrossRef] [PubMed]
  12. L. Li, A. Schülzgen, V. L. Temyanko, S. Sabet, M. M. Morrell, H. Li, A. Mafi, J. V. Moloney, and N. Peyghambarian, "Investigation of modal properties of microstructured optical fibers with large depressed-index cores," Opt. Lett. 30, 3275 (2005). [CrossRef]
  13. J. K. Sahu, C. C. Renaud, K. Furusawa, R Selvas, J. A. Alvarez-Chavez, D. J. Richardson, and J. Nilsson, "Jacketed air clad cladding pumped ytterbium doped fibre laser with wide tuning range," Electron. Lett. 38, 1116-1117 (2001). [CrossRef]
  14. W. J. Wadsworth, J. C. Knight, and P. St. J. Russell, "Large mode area photonic crystal fibre laser," in OSA Trends in Optics and Photonics 56, Conference on Lasers and Electro-Optics, Technical Digest, Postconference Edition (Optical Society of America, Washington DC, 2001), pp. 319.
  15. K. Furusawa, A. Malinowski, J. Price, T. Monro, J. Sahu, J. Nilsson, and D. Richardson, "Cladding pumped Ytterbium-doped fiber laser with holey inner and outer cladding," Opt. Express 9, 714-720 (2001). [CrossRef] [PubMed]

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