OSA's Digital Library

Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 10, Iss. 10 — May. 20, 2002
  • pp: 455–460
« Show journal navigation

Improved high-field laser characteristics of a diode-pumped Yb:LiYF4 crystal at low temperature

Junji Kawanaka, Koichi Yamakawa, Hajime Nishioka, and Ken-ichi Ueda  »View Author Affiliations


Optics Express, Vol. 10, Issue 10, pp. 455-460 (2002)
http://dx.doi.org/10.1364/OE.10.000455


View Full Text Article

Acrobat PDF (116 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

We have demonstrated a diode-pumped Yb:LiYF4 laser oscillator at liquid nitrogen temperature in free-running mode. The obtained laser gain was 21 cm-1, which was 15 times as high as that at room temperature. The effective tuning range was broadened to 35 nm due to absorption spectral narrowing.

© 2002 Optical Society of America

1. Introduction

The ultrahigh-peak-power lasers with a high repetition rate are in high demands as one of the most useful tools for the recent advanced application fields such as light-matter interaction in a relativistic regime.[1

1. M. D. Perry and G. Mourou, “Terawatt to petawatt subpicosecond lasers,” Science 264, 917–924 (1994). [CrossRef] [PubMed]

,2

2. D. Umstadter, S.-Y. Chen, A. Maksimchuk, G. Mourou, and R. Wagner, “Nonlinear optics in relativistic plasmas and laser wake field acceleration of electrons,” Science 273, 472–475 (1996). [CrossRef] [PubMed]

] High field lasers with high energy (~kJ) are demonstrated for such application fields. Compact high-field laser systems with low energy and short laser pulse duration are another approach, which have been developed successfully by using a chirped pulse amplification technique with a Ti:sapphire laser crystal.[3

3. J. D. Kmetec, J. J. Macklin, and J. F. Young, “0.5-TW, 125-fs Ti:sapphire laser,” Opt. Lett. 16, 1001–1003 (1991). [CrossRef] [PubMed]

,4

4. K. Yamakawa, M. Aoyama, S. Matsuoka, T. Kase, Y. Akahane, and H. Takuma, “100-TW sub-20-fs Ti:sapphire laser system operating at a 10-Hz repetition rate,” Opt. Lett. 23,1468–1470 (1998). [CrossRef]

] Considerable efforts have been spent in the development of high-power neodymium pump laser sources, which are used in the high-field Ti:sapphire laser systems. Such high output power neodymium system is still large in size with complicated optical scheme. High repetition rate operation has not been achieved due to flash lamps. By using laser diodes as a pump source in such high-field lasers, downsized laser systems with high repetition rate could be achieved to accelerate high-field science research, which opens up various novel application fields. The power conversion efficiency of a pump source would also be improved much more than flash-lump pumped second-harmonic laser sources.

A diode-pumped Yb:YLF oscillator has been demonstrated at room temperature in continuous-wave operation mode at very low cavity loss of < 2%.[16

16. J. Kawanaka, H. Nishioka, N. Inoue, and K. Ueda, “Tunable continuous-wave Yb:YLF laser operation with a diode-pumped chirped-pulse amplification system,” Appl. Opt. 40, 3542–3546 (2001). [CrossRef]

] Its slope efficiency and effective spectral tuning range were 50% and 25 nm, respectively. This tuning range, however, is much narrower than the emission spectral range (~100 nm) even when the pump intensity was considerably high (100 kW/cm2). This is because reabsorption of the emission by lower levels in the laser transition became significant, leading to a considerable loss of gain. Increase of the pump intensity is one of the practical ways to improve the laser gain and the gain range by bleaching the absorption. Further developments of high brightness laser diodes could improve the gain in future. Cooling of the Yb:YLF crystal at low temperature is another effective way of active population control to reduce the reabsorption. Our spectroscopic research at low temperature indicated that characteristics for the high-field laser materials would be dramatically improved even in current diode-pump; the laser gain would be increased and the effective spectral gain width would be broadened.

In the present work, a diode-pumped laser oscillator with an Yb:YLF crystal at liquid nitrogen temperature has been demonstrated for the first time to our knowledge. An estimated small signal gain was dramatically improved to 21 cm-1 and the tuning range was broadened to 35 nm due to absorption spectral narrowing. An all solid-state high-field laser will thus be achieved using a cooled Yb:YLF crystal.

2. Emission and absorption spectra at liquid nitrogen temperature

Fig. 1. Emission and absorption spectra of an Yb:YLF crystal for (a) 293 K, π-polarization, (b) 293 K, σ -polarization, (c) 77 K, π -polarization and (d) 77 K, σ -polarization.

The crystal was cooled at liquid nitrogen temperature of 77 K to reduce the absorption of lower laser levels. At 77 K, the spectral overlap was almost vanished for both polarizations, as shown in Figs. 1 (c) and (d). This leads to dramatic increase of the laser gain at any emission spectral range. The peak emission cross sections for π-polarization were 1.8 × 10-20 cm2 and 1.7 × 10-20cm2at 995 nm and 1017 nm, respectively. Both of them are one order of magnitude higher compared to the room temperature value of 1.7 × 10-21 cm2 at 1040 nm, resulting in the laser gain more than one order of magnitude under the similar pump power. Since the peak emission cross section was 1.6 × 10-20 cm2 for σ-polarization, a comparable laser gain is expected. In addition, the laser gain spectral range would be broadened to the whole emission spectral range of ~35 nm for both polarizations, which enables the laser amplification of 40-fs laser pulses under the assumption of the sech2 field envelop.

3. Cooled Yb:YLF oscillator

Fig. 2 shows a schematic diagram of our diode-pumped oscillator with a cooled Yb:YLF. The cavity was x-type configuration with a flat high reflector, a flat output coupler and two dichroic mirrors with 100-mm radius of curvature. The distance between the output coupler and the facing dichroic mirror was 700 mm, which was the same as that between the high reflector and the other dichroic mirror. The distance between two dichroic mirrors was ~100 mm. The angle of off-axis reflection of a lasing beam on dichroic mirrors was set at 9 degrees to minimize its astigmatism. An YLF crystal with 60 at. % Yb3+ dope with 0.3 mm thickness was used. The cross section of the Yb:YLF crystal was 5 mm × 5 mm. The crystal was sandwiched between two thermally conductive 1.5-mm-thick copper plates with a 2-mm-diameter hole, which attached to a 40-mm-diameter copper sample stage of a He cryostat with an electrical heating unit (CRT-M310-OP, Iwatani Int. Co.). Thin indium foils with a 100-□m thickness were used between copper and crystal contact surfaces to obtain a high thermal conductivity. The copper plate temperature Tc was measured as an index of the crystal temperature with an Au-Fe thermocouple, and was kept at liquid nitrogen temperature Tc = 77 K. The temperature fluctuation was stabilized within 0.5 K during the laser operation. The inside of the cryostat was evacuated below 3 × 10-4 torr. The crystal and two cryostat windows were set at the Brewster angle for both the lasing and pump beams. A high brightness single-emitter cw laser diode (OptPower Corporation, H01-A001-940-CT) was used as a pump source at 936 nm. The diode laser beam was tightly focused upon the laser crystal by two lenses and a telescope with a cylindrical lens pair. The observed focusing spot size with a knife edge was 40 × 100 μm. Pump intensity could thus be changed to as much as 40 kW/cm2, which was more than the saturation intensity of 28 kW/cm2 of the crystal at liquid nitrogen temperature. The pump laser beam was linearly polarized along p-polarization for the crystal and windows, which was parallel to either c- (π-polarization) or a-axis (σ-polarization) by rotating the crystal.

Fig. 2. Schematic diagram of a diode-pumped Yb:YLF laser oscillator at liquid nitrogen temperature.

4. Output characteristics

Free-running laser oscillation could not be observed at room temperature due to a low pump intensity and an increased cavity loss of two additional cryostat windows. The laser oscillation was, however, observed at Tc = 77 K for both π- and σ-polarizations. The oscillation wavelength was 1017 nm for π -polarization where the emission cross section peak was shown in Fig. 1(c). The other emission cross section peak at 995 nm was not the oscillation wavelength in free-running operation mode due to the absorption. The laser-beam profile was Gaussian with a diameter of 2 mm at the output coupler. Output power was measured as a function of an absorbed pump power by use of two output couplers with transmittances of 1.6% and 8.4%.

Fig. 3. Laser output power for π -polarization as a function of an absorbed pump power.
Fig. 4. Tuning range of a Yb:YLF laser oscillator at 77 K for π - and σ -polarization.

For σ-polarization, the oscillation wavelength was 1017 nm. The small signal gain was 15 cm-1 at the absorbed pump power of 500 mW. That was reasonable due to the comparable emission cross section for σ-polarization with that for π-polarization.

A laser gain spectral range was measured by tuning an oscillation wavelength with a birefringence filter inside a cavity. The result is shown in Fig. 4. The red and blue circles represent the laser output power for π- and σ-polarization at TC = 77 K, respectively, when the pump intensity was 30 kW/cm2, which corresponded to the absorbed pump powers of 480 mW and 720 mW in π- and σ-polarization, respectively. The tuning range was limited to 25 nm for π-polarization at room temperature even at 100 kW/cm2 pump intensity without additional windows.[16

16. J. Kawanaka, H. Nishioka, N. Inoue, and K. Ueda, “Tunable continuous-wave Yb:YLF laser operation with a diode-pumped chirped-pulse amplification system,” Appl. Opt. 40, 3542–3546 (2001). [CrossRef]

] At Tc = 77 K, the tuning ranges for both polarizations were obtained to be 35 nm. The broadened spectral gain range enables an amplification of ultrashort laser pulses below 40 fs duration. In such amplification, active spectral controls should be necessary to prevent a spectral gain narrowing. When using the spectral shaping with etalons [15

15. C. P. J. Barty, G. Korn, F. Raksi, C. Rose-Petruck, J. Squier, A.-C. Tien, K. R. Wilson, V. V. Yakovlev, and K. Yamakawa, “Regenerative pulse shaping and amplification of ultrabroadband optical pulses,” Opt. Lett. 21, 219–221 (1996). [CrossRef] [PubMed]

], which is one of the effective controls, σ-polarization will be preferred to π-polarization due to its smoother emission spectral shape.

The emission wavelength of laser diodes should be matched to the absorption spectra of a laser material. A peak absorption cross section was increased at 77 K for both polarizations. Pumping at 960 nm for π-polarization might be preferable because the absorption cross section was the highest. The emission wavelength of laser diodes, however, must be controlled within ±1 nm, that implies the diode temperature stabilization within ±3 °C. This means the use of an expensive electrically controlled chiller. Pumping at 940 nm for σ-polarization permits the wavelength drift of ±10 nm. The corresponding temperature drift is ±30 °C and this requires less stringent temperature control of laser diodes.

The measured absorption of the pump diode laser was increased at low temperature for both polarizations; the absorption coefficient at 936 nm pump wavelength was changed to 14 cm-1 and 26 cm-1 at 77 K from 11 cm-1 and 17 cm-1 at room temperature for π- and σ-polarizations, respectively. The increased absorption leads to a thinner laser crystal, which reduces a B-integral to improve the spatial beam quality, which is especially required in a regenerative amplifier. The thinner materials also lead to higher thermal conductivity by using a proper scheme such like a thin disk laser [8

8. U. Brauch, A. Giesen, M. Karszewski, Chr. Stewen, and A. Voss, “Multiwatt diode-pumped Yb:YAG thin disk laser continuously tunable between 1018 and 1053 nm,” Opt. Lett. 20, 713–715 (1995). [CrossRef] [PubMed]

]. The thermal effect for a beam profile, such as thermal lensing and thermal birefringence, must be considered in high-field lasers with an average power of hundreds of watts. The measured thermal conductivity of Yb:YLF was improved to 23 W/mK at 77 K from 4 W/mK at room temperature. The thermal effects would thus be suppressed at low temperature.

5. Conclusions

Acknowledgment

We wish to acknowledge Y. Kubota in Chemical Research Center of Central Glass Co., Ltd. for Yb:YLF sample crystal growth and helpful suggestions.

References and links

1.

M. D. Perry and G. Mourou, “Terawatt to petawatt subpicosecond lasers,” Science 264, 917–924 (1994). [CrossRef] [PubMed]

2.

D. Umstadter, S.-Y. Chen, A. Maksimchuk, G. Mourou, and R. Wagner, “Nonlinear optics in relativistic plasmas and laser wake field acceleration of electrons,” Science 273, 472–475 (1996). [CrossRef] [PubMed]

3.

J. D. Kmetec, J. J. Macklin, and J. F. Young, “0.5-TW, 125-fs Ti:sapphire laser,” Opt. Lett. 16, 1001–1003 (1991). [CrossRef] [PubMed]

4.

K. Yamakawa, M. Aoyama, S. Matsuoka, T. Kase, Y. Akahane, and H. Takuma, “100-TW sub-20-fs Ti:sapphire laser system operating at a 10-Hz repetition rate,” Opt. Lett. 23,1468–1470 (1998). [CrossRef]

5.

P. Lacovara, H. K. Choi, C. A. Wang, R. L. Aggarwal, and T. Y. Fan, “Room-temperature diode-pumped Yb:YAG laser,” Opt. Lett. 16, 1089–1091 (1991). [CrossRef] [PubMed]

6.

L. D. DeLoach, S. A. Payne, L. L. Chase, L. K. Smith, W. L. Kway, and W. F. Krupke, “Evaluation of absorption and emission properties of Yb3+ doped crystals for laser applications,” IEEE J. Quantum Electron 29, 1179–1191 (1993). [CrossRef]

7.

F. D. Patel, E. C. Honea, J. Speth, S.A. Payne, R. Hutcheson, and R. Equall, “Laser demonstration of Yb3Al5O12 (YbAG) and materials properties of highly doped Yb:YAG,” IEEE J. Quantum Electron 37, 135–144 (2001). [CrossRef]

8.

U. Brauch, A. Giesen, M. Karszewski, Chr. Stewen, and A. Voss, “Multiwatt diode-pumped Yb:YAG thin disk laser continuously tunable between 1018 and 1053 nm,” Opt. Lett. 20, 713–715 (1995). [CrossRef] [PubMed]

9.

H. Bruesselbach and D. S. Sumida, “69-W-average-power Yb:YAG laser,” Opt. Lett. 21, 480–482 (1996). [CrossRef] [PubMed]

10.

H. Liu, S. Biswal, J. Paye, J. Nees, and G. Mourou, “Directly diode-pumped millijoule subpicosecond Yb:glass regenerative amplifier,” Opt. Lett. 24, 917–919 (1999). [CrossRef]

11.

N. Uehara, K. Ueda, and Y. Kubota, “Spectroscopic measurement of a high-concentration Yb3+:LiYF4 crystal,” Jpn. J. Appl. Phys. Part 2 35, L499–501 (1996). [CrossRef]

12.

N. V. Kuleshov, A. A. Lagatsky, A. V. Podlipensky, V. P. Mikhailov, and G. huber, “Pulsed laser operation of Yb-doped KY(WO4)2 and KGd(WO4)2,” Opt. Lett. 22, 1317–1319 (1997). [CrossRef]

13.

F. Brunner, G. J. Spühler, J. Aus der Au, L. Krainer, F. Morier-Genoud, R. Paschotta, N. Lichtenstein, S. Weiss, C. Harder, A. A. Lagatsky, A. Abdolvand, N. V. Kuleshov, and U. Keller, “Diode-pumped femtosecond Yb:KGd(WO4)2 laser with 1.1-W average power,” Opt. Lett. 25, 1119–1121 (2000). [CrossRef]

14.

L. Shah, Q. Ye, J. M. Eichenholz, D. A. Hammonus, M. Richardson, B. H. T. Chai, and R. E. Peale, “Laser tenability in Yb3+:YCa4O(BO3)3{Yb:YCOB},” Opt. Comm. 167, 149–153 (1999). [CrossRef]

15.

C. P. J. Barty, G. Korn, F. Raksi, C. Rose-Petruck, J. Squier, A.-C. Tien, K. R. Wilson, V. V. Yakovlev, and K. Yamakawa, “Regenerative pulse shaping and amplification of ultrabroadband optical pulses,” Opt. Lett. 21, 219–221 (1996). [CrossRef] [PubMed]

16.

J. Kawanaka, H. Nishioka, N. Inoue, and K. Ueda, “Tunable continuous-wave Yb:YLF laser operation with a diode-pumped chirped-pulse amplification system,” Appl. Opt. 40, 3542–3546 (2001). [CrossRef]

17.

W. Koechner, Solid-State Laser Engineering, 4th ed. (Springer-Verlag, Berlin,1996), 82.

OCIS Codes
(140.3480) Lasers and laser optics : Lasers, diode-pumped
(140.3580) Lasers and laser optics : Lasers, solid-state
(140.5680) Lasers and laser optics : Rare earth and transition metal solid-state lasers

ToC Category:
Research Papers

History
Original Manuscript: March 26, 2002
Revised Manuscript: May 14, 2002
Published: May 20, 2002

Citation
Junji Kawanaka, Koichi Yamakawa, Hajime Nishioka, and Ken-ichi Ueda, "Improved high-field laser characteristics of a diode-pumped Yb:LiYF4 crystal at low temperature," Opt. Express 10, 455-460 (2002)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-10-10-455


Sort:  Journal  |  Reset  

References

  1. M. D. Perry and G. Mourou, �??Terawatt to petawatt subpicosecond lasers,�?? Science 264, 917-924 (1994). [CrossRef] [PubMed]
  2. D. Umstadter, S.-Y. Chen, A. Maksimchuk, G. Mourou, and R. Wagner, �??Nonlinear optics in relativistic plasmas and laser wake field acceleration of electrons,�?? Science 273, 472-475 (1996). [CrossRef] [PubMed]
  3. J. D. Kmetec, J. J. Macklin and J. F. Young, �??0.5-TW, 125-fs Ti:sapphire laser,�?? Opt. Lett. 16, 1001-1003 (1991). [CrossRef] [PubMed]
  4. K. Yamakawa, M. Aoyama, S. Matsuoka, T. Kase, Y. Akahane and H. Takuma, �??100-TW sub-20-fs Ti:sapphire laser system operating at a 10-Hz repetition rate,�?? Opt. Lett. 23, 1468-1470 (1998). [CrossRef]
  5. P. Lacovara, H. K. Choi, C. A. Wang, R. L. Aggarwal, and T. Y. Fan, �??Room-temperature diode-pumped Yb:YAG laser,�?? Opt. Lett. 16, 1089-1091 (1991). [CrossRef] [PubMed]
  6. L. D. DeLoach, S. A. Payne, L. L. Chase, L. K. Smith, W. L. Kway, andW. F. Krupke, �??Evaluation of absorption and emission properties of Yb3+ doped crystals for laser applications,�?? IEEE J. Quantum Electron. 29, 1179-1191 (1993). [CrossRef]
  7. F. D. Patel, E. C. Honea, J. Speth, S.A. Payne, R. Hutcheson, and R. Equall, �??Laser demonstration of Yb3Al5O12 (YbAG) and materials properties of highly doped Yb:YAG,�?? IEEE J. Quantum Electron. 37, 135-144 (2001). [CrossRef]
  8. U. Brauch, A. Giesen, M. Karszewski, Chr. Stewen and A. Voss, �??Multiwatt diode-pumped Yb:YAG thin disk laser continuously tunable between 1018 and 1053 nm,�?? Opt. Lett. 20, 713-715 (1995). [CrossRef] [PubMed]
  9. H. Bruesselbach and D. S. Sumida, �??69-W-average-power Yb:YAG laser,�?? Opt. Lett. 21, 480-482 (1996). [CrossRef] [PubMed]
  10. H. Liu, S. Biswal, J. Paye, J. Nees and G. Mourou, �??Directly diode-pumped millijoule subpicosecond Yb:glass regenerative amplifier,�?? Opt. Lett. 24, 917-919 (1999). [CrossRef]
  11. N. Uehara, K. Ueda and Y. Kubota, �??Spectroscopic measurement of a high-concentration Yb3+:LiYF4 crystal,�?? Jpn. J. Appl. Phys. Part 2 35, L499-501 (1996). [CrossRef]
  12. N. V. Kuleshov, A. A. Lagatsky, A. V. Podlipensky, V. P. Mikhailov and G. huber, �??Pulsed laser operation of Yb-doped KY(WO4)2 and KGd(WO4)2,�?? Opt. Lett. 22, 1317-1319 (1997). [CrossRef]
  13. F. Brunner, G. J. Spühler, J. Aus der Au, L. Krainer, F. Morier-Genoud, R. Paschotta, N. Lichtenstein, S. Weiss, C. Harder, A. A. Lagatsky, A. Abdolvand, N. V. Kuleshov and U. Keller, �??Diode-pumped femtosecond Yb:KGd(WO4)2 laser with 1.1-W average power,�?? Opt. Lett. 25, 1119-1121 (2000). [CrossRef]
  14. L.Shah, Q. Ye, J. M. Eichenholz, D. A. Hammonus, M. Richardson, B. H. T. Chai, R. E. Peale, �??Laser tenability in Yb3+:YCa4O(BO3)3{Yb:YCOB},�?? Opt. Commun. 167, 149-153 (1999). [CrossRef]
  15. C. P. J. Barty, G. Korn, F. Raksi, C. Rose-Petruck, J. Squier, A.-C. Tien, K. R. Wilson, V. V. Yakovlev and K. Yamakawa, �??Regenerative pulse shaping and amplification of ultrabroadband optical pulses,�?? Opt. Lett. 21, 219-221 (1996). [CrossRef] [PubMed]
  16. J. Kawanaka, H. Nishioka, N. Inoue and K. Ueda, �??Tunable continuous-wave Yb:YLF laser operation with a diode-pumped chirped-pulse amplification system,�?? Appl. Opt. 40, 3542-3546 (2001). [CrossRef]
  17. W. Koechner, Solid-State Laser Engineering, 4th ed. (Springer-Verlag, Berlin, 1996), 82.

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.

Figures

Fig. 1. Fig. 2. Fig. 3.
 
Fig. 4.
 

« Previous Article

OSA is a member of CrossRef.

CrossCheck Deposited