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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 5 — Mar. 1, 2010
  • pp: 4980–4985
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Narrow linewidth high output-coupling dual VBG-locked Yb-doped fiber laser

Pär Jelger, Valdas Pasiskevicius, and Fredrik Laurell  »View Author Affiliations


Optics Express, Vol. 18, Issue 5, pp. 4980-4985 (2010)
http://dx.doi.org/10.1364/OE.18.004980


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Abstract

Two equal highly reflective volume Bragg gratings (VBGs) were used to lock an Yb-doped fiber laser. By heating one of the VBGs, its center wavelength was shifted and the laser was locked on the overlap between the main peak of one grating and the side-lobe of the other creating a large outcoupling with high spectral selectivity. With this simple arrangement, unidirectional output is achieved with a narrow linewidth (<2.5GHz), high efficiency (>70%) and with an output power above 7W.

© 2010 OSA

Introduction

Experiments and results

A schematic of the experimental setup is shown in Fig. 1
Fig. 1 Experimental setup. In the mirror reference setup, VBG2 was exchanged for a HR-mirror and the left fiber end was perpendicularly cleaved.
. A fiber coupled laser diode emitting at 976 nm was launched through a series of lenses and signal rejection filters into the gain fiber. The pump delivery fiber had a numerical aperture of 0.22 and a diameter of 75 μm and the pump launch efficiency into the gain fiber was estimated to be 85%. The gain fiber was an Yb-doped double-clad fiber from Liekki (YB1200-20/400DC) with a core diameter of 20 µm and a cladding diameter of 400 μm. The corresponding numerical apertures were 0.06 and 0.45, respectively. Although the peak pump absorption was 3 dB/m at 976 nm, the actual pump absorption was somewhat lower due to the broad (> 6 nm) and power dependent output spectrum of the laser diode. A fiber length of 5 m was chosen to ensure that the whole fiber was adequately pumped.

To determine where the maximum gain was located for this fiber length, a free-running reference experiment was carried out (see inset in Fig. 1). A broadband highly reflective mirror was placed on the pump side (right) and the left fiber end was perpendicularly cleaved to provide ~4% Fresnel reflection while the intra-cavity fiber end was angle-polished. In the dual-VBG experiment, both fiber-ends were manually angle-polished to 10 degrees to suppress parasitic lasing from the end faces.

The two VBGs, designated VBG1 and VBG2 in Fig. 1, were cut from the same sample to get them close to identical with respect to linewidth and peak reflectivity wavelength. This meant that the peak reflectivity of the VBGs were 99% at 1066 nm with a FWHM bandwidth of 0.22 nm. Both gratings were angle polished and AR-coated for 1 μm radiation to avoid parasitic reflections. The VBGs were placed on temperature controlled Peltier elements to enable temperature tuning and stabilization. Although a slight indirect heating from absorbed and scattered laser emission was unavoidable, care was taken to reduce the heating effects from the pump radiation. An aperture was placed between VBG1 and the collimating lens on the left side to block unabsorbed pump and VBG2 was positioned outside the pump beam path with the help of folding mirrors.

The Bragg wavelength of the VBG has a temperature dependence of roughly 8.8 pm/K [9

9. P. Jelger, P. Wang, J. K. Sahu, F. Laurell, and W. A. Clarkson, “High-power linearly-polarized operation of a cladding-pumped Yb fibre laser using a volume Bragg grating for wavelength selection,” Opt. Express 16(13), 9507–9512 (2008). [CrossRef] [PubMed]

] and could, with the available thermo-electric controllers (TECs) used, be tuned by 0.66 nm corresponding to a temperature difference of 75 °C. As can be seen in Fig. 2(a)
Fig. 2 (a) Example of spectral overlap between the VBGs. Here the temperature difference of the VBGs is 45 °C, corresponding to 0.4 nm or two-times the bandwidth. (b) Difference in output power at the two fiber ends P1 and P2 as the two VBGs are detuned from each other.
, this is large enough to completely separate the main peaks of the VBGs. When separated, lasing occurs at the wavelength where the losses (and therefore the threshold) are lowest. This will in general appear where the main peak of the one of the gratings overlaps with a side-lobe of the other grating.

The output dynamics of the system was characterized for a constant pump power of 9 W by keeping VBG1 at 15 °C while the temperature of VBG2 was increased in increments of 2 °C. In Fig. 2(b), the measured output power at both fiber ends is shown. It can be seen that the output power from VBG1 (P1) increases with increasing temperature up to ~40 °C where the direction of output power became erratic. This corresponds to a detuning of ~0.2 nm which is equivalent to the bandwidth of the VBGs. The increase in power can be understood from that the cavity configuration initially is rather inefficient with a high feedback on both sides. As the VBGs are detuned, the feedback is reduced, which increases the output coupling and thus the output power.

With further temperature increase the output power still increases but the main output switches between the two fiber ends. This continues up to a temperature of 70 °C where the output again stabilizes but at a constant power level for increasing temperature. The emission wavelength was continuously monitored by coupling part of the output from VBG2 (P2) into an optical spectrum analyzer (OSA) with a resolution limit of 0.05 nm. It was found that the laser always emitted on a single wavelength, but that this wavelength changed each time the output power switched between the two fiber ends, indicating that the VBG acting as the HR-mirror changed. It was also verified that lasing indeed took place in the cavity formed by the two VBGs by misaligning each VBG by a small amount while the other remained aligned. As expected, when either VBG was misaligned, the lasing stopped and output power at both fiber ends dropped to zero.

The two VBGs were maximally detuned by heating VBG2 to 75 °C to measure the output power and slope efficiency and the results are presented in Fig. 3
Fig. 3 Slope measurement for mirror reference configuration and dual VBG configuration with VBG2 heated to 75 °C.
. Plotted in the same image is the free-running reference slope measurement carried out using the highly reflective mirror, as mentioned earlier. It was found that the slope was ~70% with respect to launched power in both cases with a maximum output power of 7.15 W.

The spectral output from the fiber laser with the dual-VBG configuration and the reference mirror configuration was measured and the result is shown in Fig. 4(a)
Fig. 4 (a) Emission spectrum for the two configurations. (b) Fabry Perot measurement of emission from Dual-VBG configuration.
. For the free-running laser, the emission linewidth was as expected several nm broad, while it was below the resolution limit of the OSA in the dual-VBG configuration. A scanning Fabry-Perot interferometer was instead used to determine the linewidth for the dual VBG configuration and it was found to be below 2.5 GHz, as seen in the oscilloscope trace of the measurement shown in Fig. 4(b).

Discussion and outlook

In our experiments, the maximum relative detuning of the gratings was limited by the temperature controllers used. A further relative tuning would reduce the reflectivity even more, creating a higher outcoupling for the oscillating wavelength. In high power fiber lasers, a certain amount of losses is often unavoidable as bulk components such as polarizer’s, AO-modulators and even the fiber end-faces will affect the oscillating radiation. Since the slope efficiency of a laser scales linearly with the ratio of useful outcoupling to the total round-trip loss, the ability to dynamically control the outcoupling could indeed be useful, especially for fiber lasers operating far above threshold.

As the cavity is formed by two nearly identical gratings, it is not evident which of the VBGs that will act as the output coupler. Throughout these experiments, the forward direction (signal output co-propagating with the pump) was favored for large enough detuning. This was regardless of which VBG that was heated. We attribute this effect to a slight difference in peak reflectivity for the two VBGs, likely coming from an uneven exposure of the original grating they were cut from. The details of this behavior are subject to further study. For future work, by using an apodized VBG as one of the cavity delimiters, this ambiguity could be avoided.

A drawback of high output coupling is the risk of a reduced temporal stability due to self-pulsing. This has been attributed to interaction between the signal and the population inversion [11

11. F. Brunet, Y. Taillon, P. Galarneau, and S. LaRochelle, “A Simple Model Describing Both Self-Mode Locking and Sustained Self-Pulsing in Ytterbium-Doped Ring Fiber Lasers,” J. Lightwave Technol. 23(6), 2131–2138 (2005). [CrossRef]

] as well as reabsorption in the more weakly pumped part of the fiber. These instabilities can be avoided by making sure the whole fiber is adequately pumped, by cooling the fiber [12

12. P. Jelger, K. Seger, V. Pasiskevicius, and F. Laurell, “Highly efficient temporally stable narrow linewidth cryogenically cooled Yb-fiber laser,” Opt. Express 17(10), 8433–8438 (2009). [CrossRef] [PubMed]

] or by increasing the cavity length [13

13. W. Guan and J. R. Marciante, “Complete elimination of self-pulsations in dual-clad ytterbium-doped fiber lasers at all pumping levels,” Opt. Lett. 34(6), 815–817 (2009). [CrossRef] [PubMed]

]. It has also been shown that given the right conditions, the instabilities are reduced with increasing pump power [14

14. S. Fu, X. Feng, SL.. Si, Z. Guo, X.. Jia, Y. Zhao, S. Yuan, and X. Dong, “Self-pulsing dynamics of high-power Yb-doped fiber lasers,” Microw. Opt. Technol. Lett. 48(11), 2282–2285 (2006). [CrossRef]

]. In these experiments, it was found that the temporal stability was poor for low pump powers but became quasi-continuous as the pump power increased.

Conclusions

A narrow linewidth (<2.5 GHz), spectrally locked, highly efficient Yb-doped fiber laser with an output power exceeding 7 W was built using two approximately identical highly reflective VBGs as cavity delimiters. The VBGs were detuned with respect to each other by heating and the output characteristics, spectrum and efficiency were evaluated. It was found that the output was switching erratically between the output ports when the one VBG was changed in temperature relative to the other by 20 - 50°C, corresponding to a detuning of the reflection peaks by 0.2 nm to 0.5 nm. For temperatures above this range, the output was emitted in the forward direction, i.e. co-propagating with the pump. This shows that the fiber laser could successfully be locked on the main peak of one VBG together with a side-lobe of the other. This simple arrangement also makes it possible to dynamically change the reflectivity of the outcoupler.

References and links

1.

Y. Wang, H. Bartelt, S. Brueckner, J. Kobelke, M. Rothhardt, K. Mörl, W. Ecke, and R. Willsch, “Splicing Ge-doped photonic crystal fibers using commercial fusion splicer with default discharge parameters,” Opt. Express 16(10), 7258–7263 (2008). [CrossRef] [PubMed]

2.

O. M. Efimov, L. B. Glebov, L. N. Glebova, K. C. Richardson, and V. I. Smirnov, “High-efficiency bragg gratings in photothermorefractive glass,” Appl. Opt. 38(4), 619–627 (1999). [CrossRef]

3.

B. G. Leonid, “Volume Bragg Gratings in PTR Glass–New Optical Elements for Laser Design,” in Advanced Solid-State Photonics, OSA Technical Digest Series (CD) (Optical Society of America, 2008), MD1.

4.

L. B. Glebov, L. N. Glebova, V. I. Smirnov, M. Dubinskii, L. D. Merkle, S. Papernov, and A. W. Schmid, “Laser damage resistance of photo-thermo-refractive glass Bragg gratings,” Proceedings of Solid State and Diode Lasers Technical Review. Albuquerque (2004).

5.

B. L. Volodin, S. V. Dolgy, E. D. Melnik, E. Downs, J. Shaw, and V. S. Ban, “Wavelength stabilization and spectrum narrowing of high-power multimode laser diodes and arrays by use of volume Bragg gratings,” Opt. Lett. 29(16), 1891–1893 (2004). [CrossRef] [PubMed]

6.

B. Jacobsson, V. Pasiskevicius, and F. Laurell, “Single-longitudinal-mode Nd-laser with a Bragg-grating Fabry-Perot cavity,” Opt. Express 14(20), 9284–9292 (2006). [CrossRef] [PubMed]

7.

M. Henriksson, L. Sjöqvist, V. Pasiskevicius, and F. Laurell, “Narrow linewidth 2 µm optical parametric oscillation in periodically poled LiNbO 3 with volume Bragg grating outcoupler,” Appl. Phys. B 86(3), 497–501 (2007). [CrossRef]

8.

P. Jelger and F. Laurell, “Efficient skew-angle cladding-pumped tunable narrow-linewidth Yb-doped fiber laser,” Opt. Lett. 32(24), 3501–3503 (2007). [CrossRef] [PubMed]

9.

P. Jelger, P. Wang, J. K. Sahu, F. Laurell, and W. A. Clarkson, “High-power linearly-polarized operation of a cladding-pumped Yb fibre laser using a volume Bragg grating for wavelength selection,” Opt. Express 16(13), 9507–9512 (2008). [CrossRef] [PubMed]

10.

H. Shu, S. Mokhov, B. Y. Zeldovich, and M. Bass, “More on analyzing the reflection of a laser beam by a deformed highly reflective volume Bragg grating using iteration of the beam propagation method,” Appl. Opt. 48(1), 22–27 (2009). [CrossRef]

11.

F. Brunet, Y. Taillon, P. Galarneau, and S. LaRochelle, “A Simple Model Describing Both Self-Mode Locking and Sustained Self-Pulsing in Ytterbium-Doped Ring Fiber Lasers,” J. Lightwave Technol. 23(6), 2131–2138 (2005). [CrossRef]

12.

P. Jelger, K. Seger, V. Pasiskevicius, and F. Laurell, “Highly efficient temporally stable narrow linewidth cryogenically cooled Yb-fiber laser,” Opt. Express 17(10), 8433–8438 (2009). [CrossRef] [PubMed]

13.

W. Guan and J. R. Marciante, “Complete elimination of self-pulsations in dual-clad ytterbium-doped fiber lasers at all pumping levels,” Opt. Lett. 34(6), 815–817 (2009). [CrossRef] [PubMed]

14.

S. Fu, X. Feng, SL.. Si, Z. Guo, X.. Jia, Y. Zhao, S. Yuan, and X. Dong, “Self-pulsing dynamics of high-power Yb-doped fiber lasers,” Microw. Opt. Technol. Lett. 48(11), 2282–2285 (2006). [CrossRef]

OCIS Codes
(050.7330) Diffraction and gratings : Volume gratings
(140.3510) Lasers and laser optics : Lasers, fiber
(140.3615) Lasers and laser optics : Lasers, ytterbium

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: February 1, 2010
Revised Manuscript: February 17, 2010
Manuscript Accepted: February 17, 2010
Published: February 24, 2010

Citation
Pär Jelger, Valdas Pasiskevicius, and Fredrik Laurell, "Narrow linewidth high output-coupling dual VBG-locked Yb-doped fiber laser," Opt. Express 18, 4980-4985 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-5-4980


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References

  1. Y. Wang, H. Bartelt, S. Brueckner, J. Kobelke, M. Rothhardt, K. Mörl, W. Ecke, and R. Willsch, “Splicing Ge-doped photonic crystal fibers using commercial fusion splicer with default discharge parameters,” Opt. Express 16(10), 7258–7263 (2008). [CrossRef] [PubMed]
  2. O. M. Efimov, L. B. Glebov, L. N. Glebova, K. C. Richardson, and V. I. Smirnov, “High-efficiency bragg gratings in photothermorefractive glass,” Appl. Opt. 38(4), 619–627 (1999). [CrossRef]
  3. B. G. Leonid, “Volume Bragg Gratings in PTR Glass–New Optical Elements for Laser Design,” in Advanced Solid-State Photonics, OSA Technical Digest Series (CD) (Optical Society of America, 2008), MD1.
  4. L. B. Glebov, L. N. Glebova, V. I. Smirnov, M. Dubinskii, L. D. Merkle, S. Papernov, and A. W. Schmid, “Laser damage resistance of photo-thermo-refractive glass Bragg gratings,” Proceedings of Solid State and Diode Lasers Technical Review. Albuquerque (2004).
  5. B. L. Volodin, S. V. Dolgy, E. D. Melnik, E. Downs, J. Shaw, and V. S. Ban, “Wavelength stabilization and spectrum narrowing of high-power multimode laser diodes and arrays by use of volume Bragg gratings,” Opt. Lett. 29(16), 1891–1893 (2004). [CrossRef] [PubMed]
  6. B. Jacobsson, V. Pasiskevicius, and F. Laurell, “Single-longitudinal-mode Nd-laser with a Bragg-grating Fabry-Perot cavity,” Opt. Express 14(20), 9284–9292 (2006). [CrossRef] [PubMed]
  7. M. Henriksson, L. Sjöqvist, V. Pasiskevicius, and F. Laurell, “Narrow linewidth 2 µm optical parametric oscillation in periodically poled LiNbO 3 with volume Bragg grating outcoupler,” Appl. Phys. B 86(3), 497–501 (2007). [CrossRef]
  8. P. Jelger and F. Laurell, “Efficient skew-angle cladding-pumped tunable narrow-linewidth Yb-doped fiber laser,” Opt. Lett. 32(24), 3501–3503 (2007). [CrossRef] [PubMed]
  9. P. Jelger, P. Wang, J. K. Sahu, F. Laurell, and W. A. Clarkson, “High-power linearly-polarized operation of a cladding-pumped Yb fibre laser using a volume Bragg grating for wavelength selection,” Opt. Express 16(13), 9507–9512 (2008). [CrossRef] [PubMed]
  10. H. Shu, S. Mokhov, B. Y. Zeldovich, and M. Bass, “More on analyzing the reflection of a laser beam by a deformed highly reflective volume Bragg grating using iteration of the beam propagation method,” Appl. Opt. 48(1), 22–27 (2009). [CrossRef]
  11. F. Brunet, Y. Taillon, P. Galarneau, and S. LaRochelle, “A Simple Model Describing Both Self-Mode Locking and Sustained Self-Pulsing in Ytterbium-Doped Ring Fiber Lasers,” J. Lightwave Technol. 23(6), 2131–2138 (2005). [CrossRef]
  12. P. Jelger, K. Seger, V. Pasiskevicius, and F. Laurell, “Highly efficient temporally stable narrow linewidth cryogenically cooled Yb-fiber laser,” Opt. Express 17(10), 8433–8438 (2009). [CrossRef] [PubMed]
  13. W. Guan and J. R. Marciante, “Complete elimination of self-pulsations in dual-clad ytterbium-doped fiber lasers at all pumping levels,” Opt. Lett. 34(6), 815–817 (2009). [CrossRef] [PubMed]
  14. S. Fu, X. Feng, SL.. Si, Z. Guo, X.. Jia, Y. Zhao, S. Yuan, and X. Dong, “Self-pulsing dynamics of high-power Yb-doped fiber lasers,” Microw. Opt. Technol. Lett. 48(11), 2282–2285 (2006). [CrossRef]

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