Optimization of the CuBr+Ne+HBr laser excitation source

doi:10.21203/rs.3.rs-4935530/v1

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Optimization of the CuBr+Ne+HBr laser excitation source

https://doi.org/10.21203/rs.3.rs-4935530/v1

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In this work, an experimental and model analysis of the copper bromide vapor active media excitation efficiency was carried out at a fixed energy input and pump power. The experimental results showed that for typical pumping parameters of CuBr+Ne+HBr active elements, the requirements for the excitation source in terms of the voltage amplitude on the storage capacitor can be reduced. It was shown that the reduce of voltage amplitude from 11.3 to 6.6 kV while maintaining the energy input at the level of 1 mJ/cm3, leaded to the generation power decreased by 15%, and the single-pass amplification coefficient remained virtually unchanged. Simulations have shown that at lower capacitance values, the lower operating level of the laser is populated a little more strongly, but pumping of the upper operating level during the existence of population inversion also proceeds somewhat faster, which ultimately gives approximately the same efficiency of pumping.

CuBr-laser

excitation

inversion population

efficiency of pumping

The power source is an important component of the laser, which, along with the active element, determines the generation parameters and the efficiency of converting pump energy into radiation energy. In the case of gas-discharge lasers, the source parameters can vary over a fairly wide range and significantly affect the lasing characteristics. For such lasers, including a metal vapor laser, the improvement of frequency-energy characteristics is still a hot issue associated with the formation of excitation pulses with the required parameters (amplitude, slew rate, energy, duration, etc.). In particular, in works [1–5], quite a lot of attention was paid to the increasing the radiation power of CuBr-lasers, which was achieved largely by increasing the laser pump power. In [5], an average laser power of 140 W was achieved, which became possible due to an increase in the pump power and the length of the active element in comparison with [6]. Large values of the average lasing power were achieved in [7] by the modification of the pumping scheme with an increase in the specific energy input. It was noted that the excitation efficiency is affected not only by the parameters of the pump pulse but also by the parameters of the discharge circuit [8].

There are results indicating the possibility of increasing the frequency-energy characteristics by reducing the duration of the voltage pulse leading edge on the active element, as well as the possibility of obtaining lasing in new laser media [9, 10]. The work [11] showed a linear increase in the generation power with an increase in the frequency for a copper vapor laser, and new lasing lines were detected for the first time [12].

To increase the pulse repetition frequency of metal vapor lasers and brightness amplifiers, various technical solutions related to changes in the parameters of pump pulses were used [13, 14]. In [15–17], an original design of a power source was proposed, which made it possible to obtain record pulse repetition rates for a copper bromide vapor laser and brightness amplifier.

An analysis of the works presented above shows that there are different requirements on excitation parameters for improving various laser radiation parameters (pulse repetition rate, radiation energy, gain [18], beam diameter, lasing pulse duration [19], spectral composition [20]). The pump pulse parameters can significantly change the spectral, generation, and amplification characteristics of active media on metal vapors and their halides. Within the framework of this work, we study the dependence of the lasing and amplification (amplified spontaneous emission (ASE), single-pass radiation) parameters of the active medium on copper bromide vapors in order to formulate the requirements for the excitation source. First of all, to the voltage amplitude at the anode of the switch. This issue is of fundamental importance when one looks for the critical parameters of the power source switch elements, which ultimately determine the optical characteristics of the laser.

We study the lasing parameters of the active medium on copper bromide vapors with the HBr addition. The gas discharge tube (GDT) parameters are as follows: active zone length − 20 cm, active zone diameter − 1.8 cm. The active medium was excited using a traditional source with a pulsed charge of the storage capacitance and subsequent discharge through the thyratron TGI1–1000/25. The peak energy stored by the storage capacitor in the source remained equal to ~ 48 mJ. The repetition rate of excitation pulses was constant and equal to 15 kHz. The average power determined by the capacitor energy and operating frequency was 715 W. The power was selected based on the results obtained previously with the high-frequency excitation (150 kHz) of this active element [14]. The temperatures of the active volume, containers with the active substance and the container with HBr remained constant and were: 800, 580, and 110 degrees Celsius, respectively. During the experiment, the charge voltage (U_C) of the storage capacitor was adjusted in accordance with the target energy (48 mJ) and its capacitance (C) that was varied from 750 pF to 2200 pF. After reaching the steady-state operation mode, the laser power (P), single-pass radiation power (P1), and ASE power (P0) were recorded. These parameters characterize the energy and amplification properties of the medium.

The voltage on the GDT and storage capacitor was recorded using Tektronics P6015A high-voltage probes. The current through the GDT was recorded using a Pearson Current Monitor 5673. The laser pulse was recorded using a coaxial photocell (PEC-22). Lasing powers were recorded using an Ophir Vega power meter. All signals from the sensors were visualized by a Rigol MSO1104 oscilloscope.

Table 1 presents the results of the experiment.

Table 1

– The experiment results
C	2200 pF	1650 pF	1500 pF	1320 pF	1000 pF	750 pF
U_C	6580 V	7600 V	7970 V	8500 V	9760 V	11270 V
P	945 mW	960 mW	992 mW	945 mW	1025 mW	1110 mW
P1	450 mW	478 mW	505 mW	492 mW	580 mW	645 mW
P0	160 mW	167 mW	174 mW	165 mW	184 mW	189 mW

Figure 1 shows the dependence of power on the storage capacity.

With an increase in capacity, the radiation power decreases by 14.9%, and the single-pass radiation power decreases by 30.2%. ASE power is reduced by 15.3%. In general, this indicates a decrease in the efficiency of excitation of the active medium in terms of both the generation power and the brightness amplification [21] related to the single-pass gain.

Figures 2a-f show waveforms of voltage at the GDT (V), current through the GDT (I), as well as the radiation pulse (E).

It can be seen that the increase in the capacity of the storage capacitor leads to the increase in the excitation pulse duration, which is associated with an increase in the time constant of the discharge circuit. There is also a shift of the beginning of the generation pulse relative to the voltage pulse on the GDT.

Next, we evaluate the pumping parameters in more detail, primarily the time and amplitude values of the excitation pulses. A waveform of the instantaneous power pulse was obtained by multiplying the voltage pulse on the GDT and the current pulse through the GDT.. Integrating it over time, we obtain the energy of the excitation pulse. It is obvious that the ratio of the energy entering the active medium before the start of the generation pulse, before its end, and the total energy determines the excitation efficiency. The energy entering the medium after the generation pulse is clearly spent on heating the plasma.

Figure 3 shows power waveforms and calculated energy for a 750 pF storage capacitor. The total pump energy is ~ 30 mJ, with the stored energy in the capacitor being ~ 48 mJ. The efficiency of energy transfer in the GDT is 63.2%. In this case, the pump energy before the start of the lasing pulse is ~ 4 mJ, the energy until the end of the lasing pulse is ~ 22 mJ, and the energy until the end of the lasing pulse (0.1 from the maximum) is ~ 20 mJ.

Similar measurements were made for all storage capacitors. The results are shown in Table 2.

Table 2

— Calculated energies
C, pF	750	1000	1320	1500	1650	2200
Total calculated energy, mJ	~ 30	~ 31	~ 39	~ 33	~ 38	~ 42
Energy transfer efficiency, %	63,2	65,4	80,8	69,9	78,7	87,9
Energy before the start of the lasing pulse, mJ	~ 4	~ 4	~ 4	~ 3	~ 3	~ 3
Energy until the end of the lasing pulse, mJ	~ 22	~ 17	~ 20	~ 15	~ 16	~ 14
Energy until the end of the lasing pulse (0.1 from the maximum), mJ	~ 20	~ 18	~ 18	~ 15	~ 15	~ 15

Thus, with an increase in capacitance from 750 pF to 2200 pF, the pump energy (directly on the active element) increased from ~ 30 mJ to ~ 42 mJ, i.e. energy transfer efficiency increased from 63.2–87.9%. In our opinion, this is due to the matching of the active element, discharge circuit, and power source. In this case, a decrease in the pump energy entering the medium before the end of the generation pulse is observed, which has a greater effect on the population inversion and, accordingly, the excitation efficiency. The energy until the end of the lasing pulse, when determining the end of the pulse at a level of 0.1 from the maximum, decreases from ~ 20 mJ to ~ 15 mJ with an increase in the storage capacitance from 750 pF to 2200 pF, respectively. We can conclude that, for a storage capacitance of 750 pF, ~ 33% of the energy is spent on heating the active medium, while for a storage capacitance of 2200 pF this value is 64.2%, which significantly reduces the excitation efficiency. It can be seen that the ratio of the energy before the start of the excitation pulse to the total energy decreases from 13.7–7.7%. The nonlinear change is most likely associated with the permissible deviation of the KVI-3 capacitance from the nominal value as the voltage increases, as well as with some resonant processes in the discharge circuit. Note that in the absence of voltage, the capacitance value measured using the device coincided with the nominal value.

Modeling of the active medium kinetic processes

The experimental configuration was also studied using a mathematical model. For this purpose, the kinetic model presented in [22] was used. The model analysis will make it possible to evaluate in more details the influence of pump parameters on the plasma medium features, such as electron temperature, concentration of the atoms in various energy states, etc. To simulate pumping, a simplified circuit of the discharge circuit was used, taking into account the inductive and capacitive components of the GDT resistance. The waveforms obtained in the model for three different capacities are presented in Fig. 4. The active resistance of the active medium is a function of time, determined by the spatiotemporal distribution of the concentration and temperature of the electrons of the active medium [22]. Figure 5 shows the changes of the active part of the plasma resistance during the time. The pre-pulse resistance of the active medium was the same for all capacitance values due to the fact that the pre-pulse distribution of electron concentration and their temperature were almost identical. This is a consequence of the fact that concentration and temperature are sufficiently inertial so that their prepulse values were determined by the pump energy rather than the shape of the pump pulse. The same is true for the Cu atom population in the lower operating level (metastable). It is worth noting that the peak resistance value is almost identical for all capacitances, i.e. the GDT is the similar type of load for the power source, even with a multiple change in the capacitance of the storage capacitor.

The values of ASE and radiation power were calculated in the model. For all configurations considered (the same as in the experiment), the laser power was 0.94–0.99 W, with the maximum being achieved for 1320 and 1500 pF. This spread is insignificant and fits into the model error, which can be seen, in particular, when comparing the model values with the experimental ones. The ASE power in the model also varies slightly for different capacities and is equal to ~ 0.19 W. The total energy input per pulse directly into the active medium was 30–33 mJ for all capacities, i.e. energy losses in the electrical circuit were weakly dependent on the pumping mode. Some difference in the behavior of the energy transfer efficiency in the model from the experiment is mainly due to the fact that the model did not take into account the dependence of the characteristics of the components of the pump circuit on voltage, as well as the simplicity of the model of components of the discharge circuit, primarily the switch element. For capacities of 1320–2200 pF, the behavior of the radiation power coincides with the model results, however, for capacitors of 750 and 1000 pF, a noticeable increase in the radiation power was observed in the experiment, which was not in the model results. This can be explained by a probable decrease in the loss of electrical energy for heating cold zones in the GDT at low capacitance values (sufficiently fast discharge), which results in a slight change in pumping conditions and an increase in its real efficiency. However, such an increase in radiation power is not due to fundamental factors associated with the kinetics of heated zones of the active medium, judging by the simulation results.

In Fig. 6 shows the active medium inversion formation for the green line in the center of the GDT. It can be seen that the pumping efficiency of the upper operating level is almost the same at different capacities. At the same time, for smaller capacitances (faster discharge), the lower operating level is populated somewhat more strongly (the peak population value of the lower operating level increases by approximately 8% when the capacitance decreases from 2200 pF to 1320 pF and from 1320 pF to 750 pF in Fig. 6). At relatively low frequencies, which were used in this work, this difference has little effect on the pre-pulse concentration of copper atoms in the metastable state (lower operating level), which at the center of the GDT was ~ 6∙10¹¹ cm^− 3 for the green line and ~ 6∙10¹⁰ cm^− 3 for the yellow line. Since the active medium is pumped by Joule heating of the electron gas, it is also of interest to consider the change in electron temperature during the discharge, which is shown in Fig. 7.

Figure 7 shows that the electron temperature is weakly inertial and begins to increase almost simultaneously with an increase in the pumping voltage and current. Since the discharge had excess power to form lasing, a population inversion is formed at the beginning of pumping (from the point of view of the deposited energy), which is confirmed by Fig. 6 and calculation of the energy input before the generation pulse in the experiment (see Table 2). The main “useful” pump power occurs during the period of time when a population inversion has already been formed and is maintained by maintaining the electron temperature above 2 eV, necessary to create a population inversion (see [23] and references therein). In this case, maintaining such a high electron temperature leads to active ionization of the active medium, which is why the active resistance of the plasma quickly decreases (Fig. 5). As a result, due to the low active resistance of the GDT, ensuring a pump power high enough to maintain the electron temperature requires a further increase in the current. Thus, in practice, the duration of the existence of a population inversion can be limited not only by the growth of the population of the lower operating level [24], but also by the ionization of the active medium.

Thus, the work carried out an experimental and model analysis of the conditions for excitation of the CuBr + Ne + HBr active medium in a circuit with direct discharge of a storage capacitor through a thyratron under the most typical operating conditions. Based on the experimental results, the following conclusions can be drawn. For a fixed energy input, excitation with a lower capacitance and higher voltage is more efficient. This is due to the fact that in this case the energy before the end of the radiation pulse is higher. When the capacitance increases by a factor of 3, the pump energy before the end of the lasing pulse decreases by a factor of 1.3. The generation power decreases from 1.1 to 0.95 W, i.e. at 1.16 times. This reduction is not critical. In terms of power supply requirements, the parameters have been changed significantly: the voltage has been reduced from 11.3 kV to 6.6 kV, which provides a distinct advantage in power supply design. Thus, in practice, a compromise can be found in which the requirements for the power source will be significantly reduced, with a slight reduction in the generation power or amplification of the active medium.

Mechanisms leading to a decrease in generation power were also established. Simulations have shown that at lower capacitance values, the lower operating level of the laser is populated a little more strongly, but pumping of the upper operating level during the existence of population inversion also proceeds somewhat faster, which ultimately gives approximately the same efficiency of pumping the active medium in terms of radiation power for both high voltages and low ones (small and large capacities, respectively) without taking into account electrical energy losses. The time dependence of the resistance of the active medium during pumping also does not fundamentally change for different values of capacitance/voltage at a fixed energy input. Note that these conclusions are valid for near optimal pumping, but the situation can be radically different when the active medium is operating on the verge of lasing failure, for example, when operating at record pulse repetition rates [15, 16].

Author Contribution

M.T. - Conceptualization, Supervision, Writing – Review & EditingP.G. - Investigation, Writing – Original Draft PreparationA.K. - Software, Writing – Original Draft Preparation

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No competing interests reported.

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Optimization of the CuBr+Ne+HBr laser excitation source

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Abstract

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Introduction

Experimental Setup

Experimental results and discussion

Modeling of the active medium kinetic processes

Conclusion

Declarations

Author Contribution

References

Additional Declarations

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