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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 17 — Aug. 16, 2010
  • pp: 17584–17590
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Pulse width tunable passively mode-locked Nd:YVO4 laser based on hybrid laser gain medium locations

Hua Lin, Yen-Yin Lin, Yuan-Yao Lin, Jinping He, Jinfeng Li, and Xiaoyan Liang  »View Author Affiliations


Optics Express, Vol. 18, Issue 17, pp. 17584-17590 (2010)
http://dx.doi.org/10.1364/OE.18.017584


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Abstract

A pulse width tunable passively mode-locked laser with a wavelength of 1,064 nm and two Nd:YVO4 crystals located in asymmetric positions is demonstrated. By adjusting the pump power of the crystals, the pulse width can be continuously tuned from 8.8 to 20.3 ps at a stable mode-locked repetition rate of 122 MHz. A theoretical model is proposed to describe the experiment results phenomenologically. In this system, a maximum output power of 4.44 W is achieved with a pump of 13.68 W, corresponding to an optical-to-optical efficiency of 32.5%. The beam quality factor, M2, is found to be M2x = 1.15 and M2y = 1.13 in the orthogonal directions at an output power of 3.2 W.

© 2010 OSA

1. Introduction

Pulse width tunable lasers are desired in many applications, such as in ultrahigh bit rate soliton transmission and optical signal processing systems [9

9. A. Takada, K. Sato, M. Saruwatari, and M. Yamamoto, “Pulse width tunable subpicosecond pulse generation from an actively modelocked monolithic MOW laser/MQW electroabsorption modulator,” Electron. Lett. 30(11), 898–900 (1994). [CrossRef]

]. Thus, reliable and cost-effective pulse width tunable light sources are highly sought. A pulse width tunable laser can be obtained in several ways, usually by controlling the SHB effect, controlling the output spectrum by inserting an etalon into the cavity, and so on. Flood et al. demonstrated an actively mode-locked Nd:YAG laser with a pulse width tunable range from 15 to 40 ps obtained by controlling the SHB effect [7

7. C. J. Flood, D. R. Walker, and H. M. van Driel, “Role of Spatial Hole Burning in an Actively Mode-Locked Solid-State Laser, ” in Advanced Solid State Lasers, B. Chai and S. Payne, eds., Vol. 24 of OSA Proceedings Series (Optical Society of America, 1995), paper PL1 http://www.opticsinfobase.org/abstract.cfm?URI=ASSL-1995-PL1.

]. Brunner et al. reported a passively mode-locked thin-disk Yb:YAG laser obtained by inserting an etalon into the cavity [5

5. F. Brunner, R. Paschotta, J. Aus der Au, G. J. Spühler, F. Morier-Genoud, R. Hövel, M. Moser, S. Erhard, M. Karszewski, A. Giesen, and U. Keller, “Widely tunable pulse durations from a passively mode-locked thin-disk Yb:YAG laser,” Opt. Lett. 26(6), 379–381 (2001), http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-26-6-379. [CrossRef]

]. More recently, similar to Ref. 5

5. F. Brunner, R. Paschotta, J. Aus der Au, G. J. Spühler, F. Morier-Genoud, R. Hövel, M. Moser, S. Erhard, M. Karszewski, A. Giesen, and U. Keller, “Widely tunable pulse durations from a passively mode-locked thin-disk Yb:YAG laser,” Opt. Lett. 26(6), 379–381 (2001), http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-26-6-379. [CrossRef]

, Luhrmann et al. demonstrated an efficient active mode-locked Nd:YVO4 laser with a pulse duration adjustable between 34 ps and 1 ns [10

10. M. Lührmann, C. Theobald, R. Wallenstein, and J. A. L’huillier, “Efficient generation of mode-locked pulses in Nd:YVO4 with a pulse duration adjustable between 34 ps and 1 ns,” Opt. Express 17(8), 6177–6186 (2009). [CrossRef] [PubMed]

]. Although these results are impressive, realigning the laser cavity is often mandatory when the pulse width is adjusted. In this paper, we investigate a novel passively mode-locked, pulse width tunable, dual-gain medium laser system. Compared to a conventional dual-gain medium laser, the locations of laser gain medium are asymmetrically situated. One is located at the end of the laser cavity, and the other is located at the middle of the laser cavity. Therefore, the SHB effect can be directly manipulated by the pump power level into the gain media. We demonstrate what we believe to the first pulse width tunable passively mode-locked Nd:YVO4 laser obtained by combining GE and GM structures in a single cavity. The major feature of this design is that no moving components or realignment is required when varying laser pulse widths, such that the maintenance of this system will be extremely easy for a user without laser expertise.

2. Experimental setup

A schematic of the laser setup is depicted in Fig. 1
Fig. 1 Setup of the dual-gain-medium, pulse width tunable laser. M1, M3, plano-concave mirrors; M2, input coupler; M1, output coupler (T = 7% at 1064nm).
. The gain media were comprised of two Nd:YVO4 crystals doped with 0.5 mol% neodymium, the absorption coefficient of these crystals is about 4.6 cm−1 in our experiment. The dimensions of the first laser gain medium are 3 × 3 × 5 mm3 and those of the second are 4 × 4 × 8 mm3. The first laser gain medium was placed at the end of the laser cavity. One of its facets was high-reflection-coated at 1064 nm and anti-reflection (AR)-coated at 808 nm, while another facet was AR-coated at 808 and 1064 nm. The second laser gain medium was placed at the middle of the laser cavity, and both facets were AR-coated at the pump and output laser wavelength. Both crystals were wrapped in indium foil and mounted tightly in a water-cooled copper holder. The gain medium temperature was maintained at 15 °C. Two fiber-pigtail laser diodes at 808 nm, LD 1 and LD 2, were used as the pump sources. The maximum output power of LD 1 was 10 W, while that of LD 2 was 15 W. The pump beams were shaped and focused on the center of the gain media using two doublets, and their spot radii on the first and second laser gain media were ~180 and ~200 μm (1/e2 intensity), respectively. The pump sizes matched the designed laser cavity mode and produced good output laser beam quality under stable mode-locking operations. The total cavity length was 1230 mm. An output coupler (OC), M1, with a transmission of 7% at 1064 nm and a 300 mm radius of curvature was used, giving a total output coupling of 14%. Two mirrors, M2, which was planar, and M3, which had a 300 mm radius of curvature, exhibited high reflectivity at the output laser wavelength. To initiate the continuous mode-locked (CW-ML) operation, a semiconductor saturable absorber mirror (SESAM) [11

11. M. Haiml, R. Grange, and U. Keller, “Optical characterization of semiconductor saturable absorbers,” Appl. Phys. B 79(3), 331–339 (2004). [CrossRef]

13

13. U. Keller, “Ultrafast solid-state lasers,” Prog. Opt. 46, 1–115 (2004). [CrossRef]

] with 4% saturable absorption (Batop GmbH) was inserted. The spot size of the laser beam on the SESAM had a radius of about 70 μm.

3. Results and discussion

We investigated the properties of the laser during GE or GM operation based on the cavity in Fig. 1. When LD 2 was switched on and LD 1 was switched off, the laser worked in the GM mode; the reverse allowed the laser to work in GE mode. According to Ref. 3

3. B. Braun, K. J. Weingarten, F. X. Kärtner, and U. Keller, “Continuous-wave mode-locked solid-state lasers with enhanced spatial hole burning,” Appl. Phys. B 61(5), 429–437 (1995). [CrossRef]

, the GE cavity experiences an enhanced SHB effect, which effectively flattens the saturated gain and broadens the spectrum bandwidth. The GE cavity can generate shorter pulse widths compared to the GM cavity. In the dual-gain medium operation, the pulse duration can be tuned in the range between the GE and GM modes by controlling the pump ratio of LD 1 to LD 2.

3.1 The CW mode-locking of the GM laser

When LD 2 was switched on, the laser could be considered a typical GM laser with a threshold of 330 mW. When the pump power was below 3.85 W, the laser operated at the Q-switched mode-locked (QML) state. When the pump power was above 3.85 W, the laser switched to a stable CW-ML regime. Figure 2
Fig. 2 The left figure shows output power versus pump power of GM laser. The right figure shows the measured autocorrelation trace of the mode-locked pulses (blue spots) and Gaussian fitting (red solid line).
shows a plot of the output power versus the pump power in GM operation. The maximum output power was 3.16 W at an LD 2 pump of 10.2 W, corresponding to an optical-to-optical efficiency of 30.9%. The measured pulse width was 17.5 ps at an output power of 1.85 W. An intensity autocorrelator (FR-103XL, Femotochrome. Research, Inc.) was used to confirm the temporal characteristics of the mode-locked pulses. The autocorrelation trace is shown in Fig. 2.

3.2 The CW mode-locking of the GE laser

When LD 2 was switched off and LD 1 was switched on, the laser was shifted to GE operation. The threshold for the GE laser was 468 mW. The CW-ML regime was achieved when the pump power was above 3.9 W. The maximum output power was 0.87 W at a pump power of 4.05 W, corresponding to an optical-to-optical efficiency of 21%. Figure 3
Fig. 3 The left figure shows output power versus pump power of GE laser. The right figure shows the measured autocorrelation trace of the mode-locked pulses (blue spots) and Gaussian fitting (red solid line).
plots the output power versus the pump power for GE operation. According to Refs. 3

3. B. Braun, K. J. Weingarten, F. X. Kärtner, and U. Keller, “Continuous-wave mode-locked solid-state lasers with enhanced spatial hole burning,” Appl. Phys. B 61(5), 429–437 (1995). [CrossRef]

and 4

4. F. X. Kärtner, B. Braun, and U. Keller, “Continuous-wave mode-locked solid-state lasers with enhanced spatial hole burning,” Appl. Phys. B 61(6), 569–579 (1995). [CrossRef]

, the time-bandwidth product in the GM mode is close to the ideal case; therefore, our laser cavity is optimized under the GM mode and the efficiency of the GM mode is better than that of the GE mode. The measured pulse width was 8.8 ps at an output power of 0.87 W in Fig. 3. This result confirms that shorter pulse widths could be generated in the GE cavity.

3.3 The dual-gain medium CW mode-locking laser

For our dual-gain medium cavity, the output power was mainly achieved by pumping the second laser gain medium; in contrast, the first laser gain medium was pumped to provide gain modulation and manipulate the pulse duration. When LD 2 was switched on, a stable CW-ML is obtained at a pump power of 3.85 W. At a pump power of 4.37 W, the pulse width was measured to be 20.3 ps. The pulse width slightly decreased with the increase in the pump power of LD 2 [14

14. F. X. Kurtner, J. A. der Au, and U. Keller, “Mode-locking with slow and fast saturable absorbers - What's the difference?” IEEE J. Quantum Electron. 4(2), 159–168 (1998). [CrossRef]

]. At a pump power of 7.9 W, the pulse width decreased to 17.0 ps, as shown in State 1 of Fig. 4
Fig. 4 The pulse width and the output power versus total pump power.
. When the pump power of LD 2 was increased to 9.63 W, the pulse width was maintained at almost the same value. In our cavity structure, 17.0 ps is a typical pulse width at GM operation, which is insensitive to the pump level of LD 2. Then, with 9.63 W of pump power from LD 2, we switched on LD 1 and increased the pump power slowly (State 2 in Fig. 4). The pulse duration became narrower even when the pump power of LD 1 was lower than the threshold of the GE laser. When the pump power of LD 1 was 23 mW, the pulse duration was 16.3 ps, as shown in State 2 of Fig. 4. This indicates that pulse duration is sensitive to the pump of LD 1. The effects of SHB were evident even with very low pumping in the hybrid cavity. With further increases in the pumping of LD 1, the pulse width decreased quickly due to enhanced SHB effects. At pump powers of 1.85 and 4.05 W from LD 1, corresponding to total pump powers of 11.48 and 13.68 W, respectively, the pulse width decreased to 12.3 and 10.6 ps. When we set the pump power of LD 1 at 4.05 W and turned LD 2 down slowly, the pulse width retained almost the same value as that in the large power range, as depicted in State 3 of Fig. 4. The shortest measured pulse width, which was as short as 8.8 ps, was obtained when LD 2 was completely switched off. Figure 4 shows that, for single crystal operation in the GM or GE modes, the pulse duration is not sensitive to the pump level. However, when two crystals were simultaneously pumped, the pulse duration could experience a large tuning range. Furthermore, in the dual-gain medium laser, the pump level of the medium in the middle of the cavity exerted less impacts on the pulse width than the one at the end of the cavity.

The maximum output power of the laser was 4.44 W when pumped optically with 13.68 W, in which 4.05 W was from LD 1 and 9.63 W was from LD 2. This resulted in an optical-to-optical efficiency of 32.5%. The repetition rate of the ML pulse train at State 2 (hybrid state) is 122 MHz, which was read from the radio frequency (RF) spectrum of the mode-locked laser, as shown in Fig. 7
Fig. 7 RF spectrum of the mode-locked laser measured at state 2 (hybrid state) with maximum output power.
. The RF spectrum in Fig. 7 reveals that the side band was suppressed; this phenomenon indicates that the CW-ML operation of this laser is stable.

In addition to the variable pulse duration, the laser emits a near TEM00 Gaussian beam, the detailed characteristics of which were measured by a charge coupled device-based beam profiler. When the output power was 3.2 W and the total pump power was 11.4 W (consisting of 3.5 W of LD 1 and 7.9 W of LD 2), the M2 values of the laser were measured in the two transverse directions. Standard position-beam-radius measurements showed these to be 1.15 and 1.13 in the x- and y-directions, respectively, as shown in Fig. 8
Fig. 8 Beam-quality measurements at output power of 3.2 W.
. During the CW-ML regime, the output beam is a near diffraction limited Gaussian beam because of good cavity mode-matching in a two-gain medium. If the pump beam does not match the cavity mode well in one of the gain media, the laser tends to operate in QML instead of CW-ML. Thus, precise alignment is more critical for a dual-gain media laser than for a single-gain media laser.

4. Conclusion

We demonstrated for the first time a pulse width tunable mode-locked Nd:YVO4 laser containing a dual-gain-medium. The laser takes advantage of the controllable spatial hole burning effect to change the pulse width. The pulse width was tuned from 8.8 to 20.3 ps, and is supported by a simple theoretical model. The maximum output power was 4.44 W at a total pump power of 13.68 W, giving an optical-to-optical efficiency of 32.5%. The beam quality factor was measured to be M2 x = 1.15 and M2 y = 1.13 in the orthogonal directions near the diffraction limit. By optimizing the cavity and improving the pump level, further improvements in output power scaling are feasible. One of the advantages of the laser just demonstrated is that it has a cavity that does not need to be realigned and lasing does not break during pulse width tuning. This kind of laser could be used in research on transient dynamics and nonlinear optical effects.

Acknowledgements

The authors acknowledge support from the National Science Foundation of China (Grant No. 60578052) and the National Basic Research Program of China (Grant No. 2006CB806000).

References and links

1.

E. Gratton and M. J. van de Ven, “Laser sources for confocal microscopy,” in Handbook of Biological confocal microscopy, James B. Pawley, ed. (Springer Science + Business Media, LLC, 2006).

2.

K. Weingarten, “High Energy Picosecond Lasers: Ready for Prime Time,” Laser Technik Journal 6(3), 51–54 (2009). [CrossRef]

3.

B. Braun, K. J. Weingarten, F. X. Kärtner, and U. Keller, “Continuous-wave mode-locked solid-state lasers with enhanced spatial hole burning,” Appl. Phys. B 61(5), 429–437 (1995). [CrossRef]

4.

F. X. Kärtner, B. Braun, and U. Keller, “Continuous-wave mode-locked solid-state lasers with enhanced spatial hole burning,” Appl. Phys. B 61(6), 569–579 (1995). [CrossRef]

5.

F. Brunner, R. Paschotta, J. Aus der Au, G. J. Spühler, F. Morier-Genoud, R. Hövel, M. Moser, S. Erhard, M. Karszewski, A. Giesen, and U. Keller, “Widely tunable pulse durations from a passively mode-locked thin-disk Yb:YAG laser,” Opt. Lett. 26(6), 379–381 (2001), http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-26-6-379. [CrossRef]

6.

C. J. Flood, D. R. Walker, and H. M. van Driel, “Effect of spatial hole burning in a mode-locked diode end-pumped Nd:YAG laser,” Opt. Lett. 20(1), 58–60 (1995), http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-20-1-58. [CrossRef] [PubMed]

7.

C. J. Flood, D. R. Walker, and H. M. van Driel, “Role of Spatial Hole Burning in an Actively Mode-Locked Solid-State Laser, ” in Advanced Solid State Lasers, B. Chai and S. Payne, eds., Vol. 24 of OSA Proceedings Series (Optical Society of America, 1995), paper PL1 http://www.opticsinfobase.org/abstract.cfm?URI=ASSL-1995-PL1.

8.

A. Agnesi, A. Lucca, G. Reali, and A. Tomaselli, “All-solid-state high-repetition-rate optical source tunable in wavelength and in pulse duration,” J. Opt. Soc. Am. B 18(3), 286–290 (2001), http://www.opticsinfobase.org/josab/abstract.cfm?URI=josab-18-3-286. [CrossRef]

9.

A. Takada, K. Sato, M. Saruwatari, and M. Yamamoto, “Pulse width tunable subpicosecond pulse generation from an actively modelocked monolithic MOW laser/MQW electroabsorption modulator,” Electron. Lett. 30(11), 898–900 (1994). [CrossRef]

10.

M. Lührmann, C. Theobald, R. Wallenstein, and J. A. L’huillier, “Efficient generation of mode-locked pulses in Nd:YVO4 with a pulse duration adjustable between 34 ps and 1 ns,” Opt. Express 17(8), 6177–6186 (2009). [CrossRef] [PubMed]

11.

M. Haiml, R. Grange, and U. Keller, “Optical characterization of semiconductor saturable absorbers,” Appl. Phys. B 79(3), 331–339 (2004). [CrossRef]

12.

U. Keller, “Recent developments in compact ultrafast lasers,” Nature 424(6950), 831–838 (2003). [CrossRef] [PubMed]

13.

U. Keller, “Ultrafast solid-state lasers,” Prog. Opt. 46, 1–115 (2004). [CrossRef]

14.

F. X. Kurtner, J. A. der Au, and U. Keller, “Mode-locking with slow and fast saturable absorbers - What's the difference?” IEEE J. Quantum Electron. 4(2), 159–168 (1998). [CrossRef]

15.

B. A. Malomed, A. G. Vladimirov, and V. Galina, “Stable autosolitons in dispersive media with saturable gain and absorption,” Phys. Lett. A 274(3-4), 111–116 (2000). [CrossRef]

16.

A. G. Vladimirov, N. N. Rozanov, S. V. Fedorov, and G. V. Khodova, “Bifurcation analysis of laser autosolitons,” Quantum Electron. 27(11), 949–952 (1997). [CrossRef]

OCIS Codes
(140.3480) Lasers and laser optics : Lasers, diode-pumped
(140.3580) Lasers and laser optics : Lasers, solid-state
(140.4050) Lasers and laser optics : Mode-locked lasers

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: March 8, 2010
Revised Manuscript: July 9, 2010
Manuscript Accepted: July 23, 2010
Published: August 2, 2010

Citation
Hua Lin, Yen-Yin Lin, Yuan-Yao Lin, Jinping He, Jinfeng Li, and Xiaoyan Liang, "Pulse width tunable passively mode-locked Nd:YVO4 laser based on hybrid laser gain medium locations," Opt. Express 18, 17584-17590 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-17-17584


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References

  1. E. Gratton and M. J. van de Ven, “Laser sources for confocal microscopy,” in Handbook of Biological confocal microscopy, James B. Pawley, ed. (Springer Science + Business Media, LLC, 2006).
  2. K. Weingarten, “High Energy Picosecond Lasers: Ready for Prime Time,” Laser Technik Journal 6(3), 51–54 (2009). [CrossRef]
  3. B. Braun, K. J. Weingarten, F. X. Kärtner, and U. Keller, “Continuous-wave mode-locked solid-state lasers with enhanced spatial hole burning,” Appl. Phys. B 61(5), 429–437 (1995). [CrossRef]
  4. F. X. Kärtner, B. Braun, and U. Keller, “Continuous-wave mode-locked solid-state lasers with enhanced spatial hole burning,” Appl. Phys. B 61(6), 569–579 (1995). [CrossRef]
  5. F. Brunner, R. Paschotta, J. Aus der Au, G. J. Spühler, F. Morier-Genoud, R. Hövel, M. Moser, S. Erhard, M. Karszewski, A. Giesen, and U. Keller, “Widely tunable pulse durations from a passively mode-locked thin-disk Yb:YAG laser,” Opt. Lett. 26(6), 379–381 (2001), http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-26-6-379 . [CrossRef]
  6. C. J. Flood, D. R. Walker, and H. M. van Driel, “Effect of spatial hole burning in a mode-locked diode end-pumped Nd:YAG laser,” Opt. Lett. 20(1), 58–60 (1995), http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-20-1-58 . [CrossRef] [PubMed]
  7. C. J. Flood, D. R. Walker, and H. M. van Driel, “Role of Spatial Hole Burning in an Actively Mode-Locked Solid-State Laser, ” in Advanced Solid State Lasers, B. Chai and S. Payne, eds., Vol. 24 of OSA Proceedings Series (Optical Society of America, 1995), paper PL1 http://www.opticsinfobase.org/abstract.cfm?URI=ASSL-1995-PL1 .
  8. A. Agnesi, A. Lucca, G. Reali, and A. Tomaselli, “All-solid-state high-repetition-rate optical source tunable in wavelength and in pulse duration,” J. Opt. Soc. Am. B 18(3), 286–290 (2001), http://www.opticsinfobase.org/josab/abstract.cfm?URI=josab-18-3-286 . [CrossRef]
  9. A. Takada, K. Sato, M. Saruwatari, and M. Yamamoto, “Pulse width tunable subpicosecond pulse generation from an actively modelocked monolithic MOW laser/MQW electroabsorption modulator,” Electron. Lett. 30(11), 898–900 (1994). [CrossRef]
  10. M. Lührmann, C. Theobald, R. Wallenstein, and J. A. L’huillier, “Efficient generation of mode-locked pulses in Nd:YVO4 with a pulse duration adjustable between 34 ps and 1 ns,” Opt. Express 17(8), 6177–6186 (2009). [CrossRef] [PubMed]
  11. M. Haiml, R. Grange, and U. Keller, “Optical characterization of semiconductor saturable absorbers,” Appl. Phys. B 79(3), 331–339 (2004). [CrossRef]
  12. U. Keller, “Recent developments in compact ultrafast lasers,” Nature 424(6950), 831–838 (2003). [CrossRef] [PubMed]
  13. U. Keller, “Ultrafast solid-state lasers,” Prog. Opt. 46, 1–115 (2004). [CrossRef]
  14. F. X. Kurtner, J. A. der Au, and U. Keller, “Mode-locking with slow and fast saturable absorbers - What's the difference?” IEEE J. Quantum Electron. 4(2), 159–168 (1998). [CrossRef]
  15. B. A. Malomed, A. G. Vladimirov, and V. Galina, “Stable autosolitons in dispersive media with saturable gain and absorption,” Phys. Lett. A 274(3-4), 111–116 (2000). [CrossRef]
  16. A. G. Vladimirov, N. N. Rozanov, S. V. Fedorov, and G. V. Khodova, “Bifurcation analysis of laser autosolitons,” Quantum Electron. 27(11), 949–952 (1997). [CrossRef]

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