FLASHLAMP PUMPED, ROOM TEMPERATURE Nd:YAG LASER. OPERATING AT 0.946 MICROMETERS. Norman P. Barnes. NASA Langley Research Center.
FLASHLAMP PUMPED, ROOM TEMPERATURE Nd:YAG LASER OPERATING AT 0.946 MICROMETERS
Norman P. Barnes NASA Langley Research Center Hampton, VA 23681 Brian M. Walsh Boston College Chestnut Hill, MA 02 167 Keith E. Murray NASA Langley Research Center Hampton, VA 23681
preclude the Nd:YAG laser from operating efficiently at 0.946 pm. Table 1 compares the Nd:YAG laser operathg at 0.946 pm with another quasi four level laser, Ho:Tm:Er:YLF. Using the thermal occupation factors or Boltzmann factors of the upper and lower laser levels for the two laser materiais, only 0.012 of the Nd atoms must be excited to the upper laser manifold to overcome the thermal population in the lower laser level, that is to achieve optical transparency. On the other hand, 0.25 of the Ho atoms must be excited to the upper laser manifold for optical transparency. In addition, the emission cross section of Ho:Tm:Er:YLF is a factor of two smaller than that of the 0.946 pm transition. Yet the flashlamp pumped Ho:Tm:Er:YLF laser operates reasonably efficiently at room temperature [2]. The additional effect which must be taken into account with the 0.946 pm Nd:YAG laser is the high gain on the competing 1.O64 pm transition. A gain of exp(1) at 0.946 pm implies a gain of exp(l0) at 1.064, a level of gain which is clearly unsustainable. Good agreement was obtained between a model of amplified spontaneous emission effects and experimental measurements. A model, developed here, allows amplified spontaneous emission effects and the population density in the upper laser manifold to be characterized as a function of time. Amplified spontaneous emission effects can be described by a product of an average emission cross section and an average path length in the laser material for the spontaneously emitted photons. The former is calculated using a measured emission spectrum to determine both the emission cross sections and level to level branching ratios. Conversely, the latter is nearly independent of the wavelength and may be longer than the length of the laser rod due total internal reflection. Using the model, the decay of the gain, and thus the decay of the population inversion density, is predicted. Experimentally, the decay of the gain coefficient and length product was measured as a function of time using a single mode,
Abstract
An efficient, flashlamp pumped, Nd:YAG laser operating on the 4F3,, to 41912transition at 0.946 pm was demonstrated and compared with operation on the 4F312to 41,,,2 transition at 1.064 pm. It is shown that a limiting feature of this laser is amplified spontaneous emission, especially at the strong 1.064 pm transition. A simple but physically significant model is developed which characterizes amplified spontaneous emission and energy storage efficiency well. Key Words Rare earth and transition metal solid state lasers, Laser materials, Rare earth doped materials. An efficient, flashlamp pumped, room temperature, 0.946 pm Nd:YAG laser was demonstrated by taking into account the quasi four level nature of this transition and the deleterious effects of amplified spontaneous emission. Whde the existence of this transition has been well known for some time, only a limited amount of results have been published on this device, perhaps because of its low efficiency. It has been reported that at reduced temperatures, 248 "K, a threshold of about 62 J and a slope efficiency of 0.00 14 have been demonstrated simultaneously [l]. Here, at room temperature, 290 OK,a threshold of 16 J and a slope efficiency of 0.0032 are simultaneously demonstrated. To obtain this performance, transition and the quasi four level nature of the 4F312to 41912 the deleterious effects of amplifíed spontaneous emission were mitigated in the design of the laser. Quasi four level operation and a relatively small stimulated emission cross section by themselves do not
OSA TOPS Vol. 10 Advanced Solid State Lasers, 1997 Clrflord R. Pollock and Walter R. Bosenberg (ed.) 01997 Optical Sociery of America
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Advanced Solid State Lasers
Table 1. Pamneters Nd:YAG Wavelength Material
0.946 Nd:YAG
Thermal Occupation Upper Lower Transparency Cross Section
0.60 0.0072 0.012 3.7
continuous wave Nd:YAG laser to probe the gain as a function of time. Results of the experiment and model appears in Fig. 1. It may be noted that the decay of the gain is initially sigmfkantly faster than exponential. Amplified spontaneous emission tends to limit the possible gain coefficient and length product of the laser. Measured gain coefficient and length product as a function of the electrical energy on the pulse forming network appears in Fig. 2. As the electrical energy increases, initially the gain coefficient and length product increases nearly linearly. However, as this parameter increases, the rate of increase slows noticeably due to amplified spontaneous emission. Since the gain coefficient is itself the product of the emission cross section and population inversion density, amplifred spontaneous emission tends to limit the population inversion density and length product. A short laser rod with undoped ends bonded on the laser rod was used to demonstrate an efficient 0.946 p Nd:YAG laser. For a four level laser transition, such as the 1.064 pm transition, a higher population inversion and length product can be obtained by increasing either the population inversion density or the length. However, for a
3r
Galn of Nd:YAG at 1.06 Micrometers
9 2
Ba
c
-i -c a 1
O 0.0000
0.0002
0.0004
0.0006
0.0008
Time in seconds
Figure 1. Gain coefficient and length product of Nd:YAG laser as a fünction of time. Time is measured fkom the peak of the gah.
1.O64 Nd:YAG
2.051 Ho:Tm:Er:YLF
0.40
0.0874 0.0286 0.25 1.8
0.0 0.0
34.0
w
.10-24
m2
Galn Coefflclent Length Product Versus Electrlcal Energy 5.0 by 38 mm NdYAG iaser rod
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10
20
30
I 40
Electrical energy in Joulss
Figure 2. Gain coefficient and length product of NdYAG laser as a function of electrical energy. quasi four level laser transition, such as the 0.946 pm transition, the population inversion density is negative at low pump energies because of the thermal population in the lower laser level. As a consequence, it is more productive to increase the population inversion density rather than increasing the length. Increasing the length increases the gain coefficient and length product for amplified spontaneous emission. However, an increase in the length increases the total lower laser level population for the 0.946 pm transition as well. In addition, because of mechanical mounting requirements, most laser rods are longer than the arc length of the flashlamp. For four level laser transitions, this extra unpumped length has little effect. However, for quasi four level lasers, this extra unpumped length adds to the total lower laser population. Consequently, a bonded laser rod was fabricated with a doped section as long as the arc length of the flashlamp and two undoped sections, one on either end, for mechanical mounting. Lasing on the 0.946 pm transition was achieved by using dichroic mirror~in a folded resonator to discriminate against the stronger 1.064 pm transition. Dichroic mirrors were obtained which are highly transmitting at 1.064 pm
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Advanced Solid State Lasers Threshold and Slope Efficiency
and highly reflecting at 0.946 pm. However, after aging, a single dichroic mirror in a linear resonator was unable to discriminate against the stronger 1.064 pm transition. With a folded resonator, where the evolving laser radiation impinged on a dichroic three times in a round trip, the stronger 1.064 p m transition was no longer able to reach threshold. All experimental results were obtained using the folded resonator arrangement. Results at 1.064 pm were obtained by replacing the dichroic mirrors with mirrors which are highly reflecting at 1.O64 ym. Laser operation at 0.946 pm was obtained for a variety of output mirrors and operating temperatures. Laser output energy under normal mode operation on the 4F3,2to 419,, transition was obtained and the resulting data fit to obtain a threshold and a slope efficiency. Typical results of laser output energy versus electrical energy appear in Fig. 3 for three dlfferent output mirror reflectivities. Threshold and slope efficiency obtained for each of these c w e s appear in Fig. 4 as a function of the negative logarithm of the output mirror reflectivity. Due to the aging of the coatings, a sigruficant amount of energy escaped through the dichroics which are nominally high reflectivity at 0.946 p.If this is counted as output, the slope efiiciency increases by a factor of nearly 1.2 over that shown in Fig. 4. Similar threshold and slope efficiency curves for operation on the 1.064 pn transition appear in Fig. 5 . While the measured slope efficiency at 0.946 pm is a factor of 2.0 hgher than that previously reported, it is still well below that of the 1.O64 ym transition. Preliminary experimental results indicate that the slope efficiency at 0.946 p m can be increased more than a factor of 2.0 if the proper multilayer coatings are used. Laser operation on the 0.946 pm transition was evaluated as a function of temperature of the coolant bath.
NdYAG at 0.946 pm
2o 18
7 0.004
r
-
c
-
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c .
i
Threshold
,.e'
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-
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o
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0.04
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0.08
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Figure 4. Threshold and slope efficiency of Nd:YAG at 0.946 pm as a functionof the negative logarithm of the mirror reflectivity.
Laser output energy as a function of the electrical energy is obtained at three different temperatures. Data are curve fit to obtain the threshold and slope efficiency. Results are shown in Fig. 6 for different temperatures of the coolant bath. Threshold results can be compared with a simple model for the threshold as a function of temperature. Comparing the rate of change of the threshold with temperature with the rate of change calculated using the thermal occupation factors, the actual rate of change is si@icantly less than predicted. This indicates that the actual temperature of the laser rod is sigmfkantly higher than the temperature of the coolant bath. A higher laser rod temperature is a realistic implication since the coolant lines are severa1 meters in length and the flow is relatively slow.
Performance of Nd:YAG at 0.946 pm 10
E
r 0.993 relectivity 0.968 reflectivity
1.6
A 0.871 relectlvitv
1.4
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1 4 1.0 f
Threshold and Slope Efficiency
.
0.8
-
0.6
.
-
Nd:YAG at 1.O64pm
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-
-
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Slope efficiency
01 O
I
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5
10
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25
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Electrlcal energy in Joules
Figure 3. Performance of Nd:YAG at 0.946 pm versus electrical energy for various output mirror reflectivities.
' 0.00
0.05
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0.10 0.15 0.20 0.25 Negative logarithm ot mirror refleaivity
0.020
0.015 0.010
1;
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Figure 5. Threshold and slope efficiency of Nd:YAG at 1.O64 pm as a function of the negative logarithm of the mirror reflectivity.
Advanced Solid State Lasers
118 Threshold and Slope Efficiency
References and Notes
NdYAG et 0.946 pm
25
0.005
20
0.004
The authors would like to thank Dr. Helmut Meisner of Onyx Optical for fabricationof the bonded laser rods. i?
0.005
0.m
Q t8
0.001
-5
-3
-1
1
7 9 Tempemture in C 3
5
O.OO0
1 1 1 3 1 5 1 7
Figure 6. Threshold and slope eficiency of Nd:YAG at 0.946 pm as a function of the temperam of the coolant bath.
1. S. Dimov, E. Pelik, and H. Walther, "A Flashlamp Pumped 946 nm Nd:YAG Laser," Bppl. Phvs. B Z 6-10, (1991). 2. E. P. Chicklis, C. S . Naiman, R. C. Folweiler, and J. C. Doherty, "Stimulated Emission In Multiply Doped Ho:YLF and YAG, A Comparison," IEEE J, mt.Elect Q&& 225-230 (1972) A threshold of about 62 J and a slope efficiency of 0.0075 was reported.
