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Optics Express

Optics Express

  • Editor: J. H. Eberly
  • Vol. 3, Iss. 8 — Oct. 12, 1998
  • pp: 298–304
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Pump-probe switching in gain-switched lasers

Richard T. White and Iain T. McKinnie  »View Author Affiliations


Optics Express, Vol. 3, Issue 8, pp. 298-304 (1998)
http://dx.doi.org/10.1364/OE.3.000298


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Abstract

We report a novel pump-probe switching phenomenon exhibited by gain-switched vibronic lasers under dual pulse excitation. A spatio-temporal rate equation model reveals the mechanism responsible, and predicts observations of pump-probe switching in a Cr:forsterite laser under pulsed 1064nm excitation. The sensitivity of this energy storage-extraction process to the pump and probe fluences and their temporal separation is investigated for switching in the nanosecond and microsecond excitation regimes.

© Optical Society of America

1. Introduction

Vibronic solid state lasers based on transition metal ions provide broad coverage of the near and mid infrared spectral regions [1–5

1. J. C. Walling, H. P. Jenssen, R. C. Morris, E. W. O’Dell, and O. G. Petersen, “Tunable laser performance in BeAl2O4:Cr3+,” Opt. Lett. 4, 182–183, (1979). [CrossRef] [PubMed]

]. Tunable lasers in these regions are important for applications in remote sensing, eyesafe ranging and communications. Upper state lifetimes of transition metal ions typically lie in the 1–100μs range, depending on the host medium. Under short pulse laser excitation (< 100ns) vibronic lasers are generally gain-switched. In this operating regime the interplay of pump and laser pulses can lead to interesting and potentially useful dynamic behaviour. This study was motivated by the recent observation of an interesting energy storage-extraction process in a vibronic Cr:forsterite laser under dual pulse excitation [6

6. Z. X. Jiang, I. T. MKinnie, L. A. W. Gloster, and T. A. King, “Temporal and kinetic studies of chromium forsterite oscillators with 1064nm laser excitation,” Pure Appl. Opt. 5, 77–88 (1997). [CrossRef]

,7

7. R. T. White, I. T. MKinnie, and N. L. Moise, “Pump-probe switching in Cr:forsterite lasers,” in Quantum Electronics and Laser Science Conference, 1997 OSA Technical Digest Series (Optical Society of America, Washington D.C., 1997), 147–148.

]. In those reports the authors observed an unusual dependence of the Cr:forsterite emission on the characteristics of the pumping pulses. For example, in some cases a weak lasing pulse followed a strong pumping pulse, and in others a strong lasing pulse followed a weak pumping pulse. We have investigated this phenomenon, which we term pump-probe switching (PPS), in a Cr:forsterite laser under longitudinal 1064nm Q-switched laser excitation. For clarity, we designate the first and second excitation pulses as the pump and probe pulses, respectively. Observations are in good agreement with predictions of a spatio-temporal rate equation model. Gain-switching simulations reveal the underlying mechanism for PPS and predict its dependence on the fluence and temporal separation of the pump and probe laser pulses. The PPS technique is generic and is expected to occur in other gain media and for shorter pulse durations. It may be of interest in optical ranging or switching applications.

2. Theory

Figure 1 shows a simplified energy level diagram of the Cr4+ ion in forsterite. In the tetrahedral field of forsterite, the free ion 3F state splits into the three states: 3A2, 3T2 and 3T1 [8

8. W. Jia, H. Liu, S. Jaffe, and W. M. Yen, “Spectroscopy of Cr3+ and Cr4+ ions in forsterite,” Phys. Rev. B 43, 5234–5242, (1991). [CrossRef]

,9

9. T. S. Rose, R. A. Fields, M. H. Whitmore, and D. J. Singel, “Optical Zeeman spectroscopy of the near-infrared lasing center in chromium:forsterite,” J. Opt. Soc. Am. B 11, 428–435, (1994). [CrossRef]

]. Lasing occurs on the 3T2-3A2 vibronic transition. The fluorescence lifetime of the 3T2 state is 2.7 μs. Excited state absorption (ESA) of pump and laser radiation occurs on the 3T2-3T1 transition, and is followed by fast relaxation back to the 3T2 state [10

10. H. R. Verdun and L. Merkle, “Evidence of excited-state absorption of pump radiation in the Cr:forsterite laser,” OSA Proceedings on Advanced Solid-State Lasers, George Dubé and Lloyd Chase, eds. (Optical Society of America, Washington D.C, 1991) 10, 35–40.

].

Fig. 1. Energy level diagram of the Cr4+ ion in forsterite indicating the important transitions. N 2 and τf are the Cr4+ population density and the fluorescence lifetime of the 3T2 state, respectively.

The model which we have developed to analyze gain-switching and pump-probe switching in longitudinally pumped lasers, is based on a set of four coupled nonlinear partial differential equations (Eqs. (1)–(4)).

1vgP(z,t)t+P(z,t)z=σa(NtN2(z,t))P(z,t),
(1)
N2(z,t)t=σsthν1I(z,t)N2(z,t)+P(z,t)(NtN2(z,t))1τfN2(z,t),
(2)
1vgI+(z,t)t+I+(z,t)z=σstI+(z,t)N2(z,t)βI+(z,t)+hνlτsN2(z,t),
(3)
1vgI(z,t)tI(z,t)z=σstI(z,t)N2(z,t)βI(z,t)+hνlτsN2(z,t).
(4)

Figure 2 shows a simulation of a single gain-switching event in which a Cr:forsterite laser is pumped by a single 11mJ Nd:YAG laser pulse of 20ns duration (FWHM). As expected, after a certain delay (98ns) determined by the upper state lifetime, gain, passive loss and cavity design, a Cr:forsterite laser pulse is emitted (16ns, 424μJ). Laser pulse energies and durations predicted by the model have been found to be in good agreement with experimental results for a pump pulse energy of up to 30mJ.

Fig. 2. Computer simulation of gain-switching in a Cr:forsterite laser. Surfaces show the spatio-temporal evolution of (a) the pump rate P, (b) the 3T2 state population density N 2, and (c) the total (left and right travelling) laser intensity I, inside the laser rod.

3. Experiment

The Cr:forsterite laser rod used during this study was cuboid in shape and measured 5 × 5 × 15mm, with the crystal a- and b-axes in the plane of the end faces. The Cr4+ concentration was 0.027at.%, and the rod end faces were anti-reflection coated over the forsterite emission band. Two flashlamp pumped, Q-switched Nd:YAG lasers (Continuum PL-7000, and Molectron MY34-20) operating at 1064nm were used as pump sources. Each pump laser produced a linearly polarised output, and extra-cavity half wave plates provided control of the polarisation orientation. The PL-7000 laser operates at a 10Hz repetition rate. Single mode operation is ensured by a MISER Nd:YAG seed laser. The MY34-20 pump laser emits Q-switched pulses at a repetition rate of 20Hz. It is not seeded and lases on several axial resonator modes simultaneously. This laser can also be operated in “free running” mode by holding the Q-switch open. In this mode of operation trains of pulses of 400–500ns duration are produced. A typical train consists of 15 pulses at a repetition rate of 150kHz.

A schematic of the experimental arrangement is shown in Figure 3. Pump and probe pulse energies were controlled by neutral density filters. A 2mm diameter aperture was placed in the path of the pump beam adjacent to the cavity high reflector. This produced an approximately supergaussian transverse pump profile in the laser crystal which greatly simplified the computer modelling.

Fig. 3. Experimental Configuration.

A longitudinally pumped linear forsterite laser resonator was constructed for the study. One plane end mirror was coated for a pump transmittance of 95% and high reflectance between 1150 and 1350nm. The plane output coupler provided 97% reflectance across the forsterite emission wavelength band. The resonator length was 70mm with the laser crystal positioned at the centre. Pump and laser pulse energies were measured by a Laser Instrumentation thermopile detector. Cr:forsterite laser pulses were detected by an InGaAs photodiode with 0.5ns risetime. Pump pulses were detected by a silicon p-i-n photodiode. Temporal pulse profiles were recorded by a 500MHz, 2GSa/s Hewlett Packard model HP54522A digital storage oscilloscope. In the initial experiments, multiple pulse excitation was provided by the MY34-20 Nd:YAG laser in free running mode. Dual pulse pumping was analyzed by examining the response of the Cr:forsterite laser to pairs of closely spaced pulses in the pump train. In subsequent experiments, both lasers were triggered externally which allowed fine control of the timing of the flashlamps and Q-switches of both pump lasers.

4. Results

Two measurements of the temporal response of the dual pulse pumped Cr:forsterite laser to b-axis polarised excitation are displayed in Figure 4. The pump and probe pulses were both provided by the MY34-20 Nd:YAG laser in free-running mode. In both cases the pump pulse is much stronger than the probe pulse. Figure 4(a) shows the observation we might intuitively expect, where the strong-weak pump-probe pulse pair produces a strong-weak Cr:forsterite laser pulse pair. This may be considered an unswitched case. In Figure 4(b), however, a strong-weak pump-probe pair produces a weak-strong laser pulse pair. This second observation is an example of PPS.

Fig. 4. Observations of dual long pulse (420ns) gain-switching in a Cr:forsterite laser. The strong-weak pump pulse pair (upper trace) can produce either (a) a strong-weak, or (b) a weak-strong Cr:forsterite laser pulse pair (lower trace). The latter is an example of pump-probe switching.

Switched and unswitched cases are also predicted by the spatio-temporal model. Figure 5 shows two simulations of dual 420ns pulse, b-axis polarised excitation. In Figure 5(a) the pump and probe Nd:YAG pulse energies are 1.6 and 0.8 times the threshold level, respectively. In response to the pump pulse, a 20ns Cr:forsterite laser pulse is emitted. Although the probe pulse is below threshold, a second 20ns Cr:forsterite laser pulse is produced as a result of the residual inversion following the first Cr:forsterite pulse. This is an unswitched case since the first Cr:forsterite laser pulse is more energetic than the second. Another simulation of the same system with different pump and probe pulse energies is displayed in Figure 5(b). In this case the pump and probe pulse energies are 1.1 and 0.5 times threshold respectively, and switching occurs. These simulations may be compared with the observations in Figure 4.

Fig. 5. Computer simulations of dual long (420ns) pulse excitation of a Cr:forsterite laser. The strong-weak pump pulse pair (upper trace) can produce either a (a) strong-weak or (b) weak-strong laser pulse pair (lower trace), depending on the intensity and duration of the two pump pulses.

Since the probe pulse energy is sub-threshold in all cases shown, PPS clearly involves some form of coupling between the two gain-switching events. Figure 6(a) shows a computer simulation which illustrates this coupling. It shows the temporal dependence of the Cr4+ population in the 3T2 state, and the output laser intensity for a single gain-switched Cr:forsterite laser pulse under near threshold pumping. Almost half of the initial 3T2 population remains after the termination of the Cr:forsterite laser pulse. This residual population decays exponentially with a time constant equal to the 3T2 state lifetime of 2.7μs. Figure 6(b) shows the predicted relationship between the pump pulse energy and the magnitude of the residual 3T2 state population remaining after the pulse is emitted by the same laser. The residual 3T2 population peaks at threshold and decreases with increasing pump pulse energy.

If a second Nd: YAG probe pulse is incident on the laser crystal during the fluorescence decay period, the 3T2 state population density will increase. Consequently, a sub-threshold probe pulse may still induce a Cr:forsterite laser pulse. PPS occurs when this second Cr:forsterite pulse is stronger than the first. It is observed in laser systems operating close to threshold, since only in this region is the residual excited state population significant. The relatively high passive loss inherent in Cr:forsterite makes this region readily accessible.

These results confirm that pump-probe switching observations with 500ns pump pulses are predictable and reproducible. However, the characteristics of the Nd:YAG pulse train in free-running mode are somewhat erratic and difficult to control. Dual Q-switched Nd:YAG laser pumping of Cr:forsterite utilizing the external triggering of the pump lasers provides a much more controlled experimental environment and allows an investigation of switching with nanosecond pulses. In this case, individual pump and probe pulse energies are easily measured before they are combined at the beamsplitter. The combined Cr:forsterite output energy can be measured but individual pulses cannot be distinguished by the thermal power meter.

Fig. 6. (a) Computer simulation of single pulse gain-switching of a Cr:forsterite laser showing the 7mJ, b-axis polarised pump pulse (upper), 3T2 state population (center), and Cr:forsterite laser emission (lower). In the case shown, 43% of the initial 3T2 population remains after the Cr:forsterite laser pulse. (b) Predicted dependence of the residual 3T2 population (normalised to the total number of Cr4+ ions) on pump pulse energy for excitation with 50ns, b-axis polarised pump pulses.

Fig. 7. Animation showing the dependence of PPS on the pump-probe temporal separation. The separation of the 15ns, b-axis polarised pump and 40ns, a-axis polarised probe ranges from 640ns to 2.6μs. [Media 1]

We have also studied the sensitivity of PPS to the pump pulse energy. With a probe pulse energy of 0.95 times the threshold level, PPS has been observed for pump pulse energies in the range 0.6–1.2 times threshold. Although it is enhanced in a high passive loss gain medium such as Cr:forsterite, PPS is a generic technique. Other solid state laser media with longer upper state lifetimes such as Nd:YAG may be more suitable for PPS, especially in applications requiring all-solid-state devices.

5. Summary

We have investigated novel pump-probe switching (PPS) behaviour in a gain-switched Cr:forsterite laser under near threshold dual pulse 1064nm Q-switched excitation. The underlying mechanism for PPS involves the interaction of a second (probe) laser pulse with the residual 3T2 state population following the emission of a first near threshold pulse. The subsequent 3T2 population is greater than in the absence of the first pump pulse, and a sub-threshold probe pulse may still induce a laser pulse. Beyond a certain pump-probe temporal separation determined by the 3T2 state lifetime, the residual 3T2 population is insufficient to produce PPS. The dependence of PPS on the pump-probe temporal separation and on the pump fluence has been investigated. Switching has been observed with nanosecond and microsecond pump and probe pulses. In the nanosecond regime with a probe pulse energy of 0.95 times the threshold level, PPS has been observed for pump-probe temporal separations less than 1.7μs, and for pump pulse energies in the range 0.6–1.2 times threshold. We anticipate that the PPS technique will be applicable to other gain media, and to shorter pulses. Specific cases are currently under investigation. PPS may be of interest for optical ranging or optical switching where the pump and probe pulses would be used to switch consecutive signal pulses high or low. In this case a single crystal without a resonator could be used. In considering switching, it is also significant that Cr4+ lasers operate in the 1300nm–1550nm spectral region, which contains both optical fibre communication windows.

Acknowledgments

This work was supported by the New Zealand Foundation for Research Science and Technology and Hughes Aircraft. We thank Don Warrington, Peter Manson and Wes Sandle at the University of Otago for technical assistance.

References

1.

J. C. Walling, H. P. Jenssen, R. C. Morris, E. W. O’Dell, and O. G. Petersen, “Tunable laser performance in BeAl2O4:Cr3+,” Opt. Lett. 4, 182–183, (1979). [CrossRef] [PubMed]

2.

P. F. Moulton, “Spectroscopic and laser characteristics of Ti:Al2O3,” J. Opt. Soc. Am. B 3, 125–132, (1986). [CrossRef]

3.

V. Petricevic, S. K. Gayen, R. R. Alfano, K. Yamagishi, H. Anzai, and Y. Yamaguchi, “Laser action in chromium doped forsterite,” Appl. Phys. Lett. 52, 1040–1042, (1988). [CrossRef]

4.

N. B. Angert, N. I. Borodin, V. M. Garmash, V. A. Zhitnyuk, A. G. Okhrimchuk, O. G. Siyuchenko, and A. V. Shestakov, “Lasing due to impurity colour centres in yttrium aluminium garnet crystals at wavelengths in the range 1.35–1.45μm,” Sov. J. Quantum Electron. 18, 73–74, (1988). [CrossRef]

5.

P. F. Moulton, “Tunable solid-state lasers,” Proceedings of the IEEE 80, 348–364 (1992). [CrossRef]

6.

Z. X. Jiang, I. T. MKinnie, L. A. W. Gloster, and T. A. King, “Temporal and kinetic studies of chromium forsterite oscillators with 1064nm laser excitation,” Pure Appl. Opt. 5, 77–88 (1997). [CrossRef]

7.

R. T. White, I. T. MKinnie, and N. L. Moise, “Pump-probe switching in Cr:forsterite lasers,” in Quantum Electronics and Laser Science Conference, 1997 OSA Technical Digest Series (Optical Society of America, Washington D.C., 1997), 147–148.

8.

W. Jia, H. Liu, S. Jaffe, and W. M. Yen, “Spectroscopy of Cr3+ and Cr4+ ions in forsterite,” Phys. Rev. B 43, 5234–5242, (1991). [CrossRef]

9.

T. S. Rose, R. A. Fields, M. H. Whitmore, and D. J. Singel, “Optical Zeeman spectroscopy of the near-infrared lasing center in chromium:forsterite,” J. Opt. Soc. Am. B 11, 428–435, (1994). [CrossRef]

10.

H. R. Verdun and L. Merkle, “Evidence of excited-state absorption of pump radiation in the Cr:forsterite laser,” OSA Proceedings on Advanced Solid-State Lasers, George Dubé and Lloyd Chase, eds. (Optical Society of America, Washington D.C, 1991) 10, 35–40.

11.

R. T. White, I. T. MKinnie, and N. L. Moise, “Dynamics of gain-switched Cr4+ lasers,” in preparation.

OCIS Codes
(140.3070) Lasers and laser optics : Infrared and far-infrared lasers
(140.3580) Lasers and laser optics : Lasers, solid-state
(140.3600) Lasers and laser optics : Lasers, tunable
(140.5680) Lasers and laser optics : Rare earth and transition metal solid-state lasers

ToC Category:
Research Papers

History
Original Manuscript: August 25, 1998
Revised Manuscript: August 10, 1998
Published: October 12, 1998

Citation
Richard White and Iain McKinnie, "Pump-probe switching in gain-switched lasers," Opt. Express 3, 298-304 (1998)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-3-8-298


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References

  1. J. C. Walling, H. P. Jenssen, R. C. Morris, E. W. O Dell, and O. G. Petersen, "Tunable laser performance in BeAl2O4:Cr 3+ ," Opt. Lett. 4, 182-183 (1979). [CrossRef] [PubMed]
  2. P. F. Moulton, "Spectroscopic and laser characteristics of Ti:Al2O3," J. Opt. Soc. Am. B 3, 125-132 (1986). [CrossRef]
  3. V. Petricevic, S. K. Gayen, R. R. Alfano, K. Yamagishi, H. Anzai, and Y. Yamaguchi, "Laser action in chromium doped forsterite," Appl. Phys. Lett. 52, 1040-1042 (1988). [CrossRef]
  4. N. B. Angert, N. I. Borodin, V. M. Garmash, V. A. Zhitnyuk, A. G. Okhrimchuk, O. G. Siyuchenko, and A. V. Shestakov, "Lasing due to impurity colour centres in yttrium aluminium garnet crystals at wavelengths in the range 1.35-1.45um," Sov. J. Quantum Electron. 18, 73-74 (1988). [CrossRef]
  5. P. F. Moulton, "Tunable solid-state lasers," Proceedings of the IEEE 80, 348-364 (1992). [CrossRef]
  6. Z. X. Jiang, I. T. M c Kinnie, L. A. W. Gloster, and T. A. King, "Temporal and kinetic studies of chromium forsterite oscillators with 1064nm laser excitation," Pure Appl. Opt. 5, 77-88 (1997). [CrossRef]
  7. R. T. White, I. T. M c Kinnie, and N. L. Moise, "Pump-probe switching in Cr:forsterite lasers," in Quantum Electronics and Laser Science Conference, 1997 OSA Technical Digest Series (Optical Society of America, Washington D.C., 1997), 147-148.
  8. W. Jia, H. Liu, S. Jaffe, and W. M. Yen, "Spectroscopy of Cr 3+ and Cr 4+ ions in forsterite," Phys. Rev. B 43, 5234-5242 (1991). [CrossRef]
  9. T. S. Rose, R. A. Fields, M. H. Whitmore, and D. J. Singel, "Optical Zeeman spectroscopy of the near-infrared lasing center in chromium:for-sterite," J. Opt. Soc. Am. B 11, 428-435 (1994). [CrossRef]
  10. H. R. Verdun, and L. Merkle, "Evidence of excited-state absorption of pump radiation in the Cr:forsterite laser," OSA Proceedings on Advanced Solid-State Lasers, George Dube and Lloyd Chase, eds. (Optical Society of America, Washington D.C., 1991) 10, 35-40.
  11. R. T. White, I. T. Mc Kinnie, and N. L. Moise, "Dynamics of gain-switched Cr 4+ lasers," in preparation.

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