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Abstract
Optically pumped gas molecular terahertz (THz) lasers are promising for generating high-power and high-beam-quality coherent THz radiation. However, for pulsed gas THz lasers, the temporal behavior of the output THz pulse has rarely been investigated. In this study, the temporal behavior of a pulsed gas THz pumped by a fundamental-mode TEA CO2 laser has been presented for the first time both in simulation and experiment. A modified laser kinetics model based on the density matrix rate equation was used to simulate the temporal behavior and output pulse energy of a pulsed gas THz laser at different gas pressures. The results clearly show that the working gas pressure and pump pulse energy have critical influences on the output THz pulse shape. Three typical pulse shapes were obtained, and the THz pulse splitting caused by gain switching was quantitatively simulated and explained based on the laser dynamic process. Besides, with an incident pump pulse energy of 342 mJ, the maximum output THz pulse energy of 2.31 mJ was obtained at 385 µm, which corresponds to a photon conversion efficiency of approximately 56.1%, and to our knowledge, this is the highest efficiency for D2O gas THz laser. The experimental results agreed well with those of the numerical simulation for the entire working gas pressure range, indicating that our model is a powerful tool and paves the way for designing and optimizing high-power pulsed gas lasers.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Terahertz (THz) waves with frequency range 0.1–10 THz is a unique electromagnetic wave region which lies between microwave and infrared light. Owing to their unique properties, especially low photon energy, ability to penetrate many visually opaque materials, and ability to be absorbed by water, THz waves have various applications. Coherent THz radiation sources are needed for many applications, such as digital holography [1–3], non-destructive and security imaging [4,5], THz radar [6,7], and remote sensing of the atmosphere [8,9]. Among these methods, an optically pumped gas THz laser (OPGTL) is one of the most powerful and efficient methods for generating high-beam-quality coherent THz radiation, and it was first reported by Chang and Bridges [10].
OPGTL can be divided into two types: continuous-wave (CW) output lasers and pulsed output lasers [11]. A typical CW OPGTL usually uses a line-tunable CW CO2 laser as the pump source to pump gas molecules and operates at low pressures. The gain is generally obtained by employing population inversions excited in molecule-specific rotational-vibrational transitions, which are almost empty at room temperature. A Gas THz laser pumped by a tunable CW CO2 laser is perhaps one of the most commercially used THz sources in THz applications [1,2,7] because it offers several benefits, such as frequency tunability, high power, and room temperature operation. Several new experimental and theoretical studies have recently been reported to improve the output performance of CW OPGTL [12–18]. Among these new types of laser configurations, the Quantum Cascade Laser (QCL) pumped gas molecular THz laser is a new class of highly efficient, continuously tunable, and compact gas THz lasers. The advantage of the QCL-pumped gas molecular laser concept is that it uses a compact, continuously tunable mid-IR QCL as a pumping source instead of a relatively bulky, line-tunable mid-IR gas laser.
However, a pulsed laser source is often required for some applications. For example, in remote detection or penetration imaging applications, high-energy THz pulsed lasers are generally regarded as more useful tools than CW sources because of their high peak power. It is well known that a gas THz laser pumped by a tunable TEA CO2 laser is an important technical approach for developing high-peak-power nanosecond pulsed THz lasers. Compared with CW THz lasers, high-energy pulsed gas terahertz lasers have several important differences, such as high pump power density, nonlinear processes, high working gas pressure, and the addition of a new important indicator (THz pulse shape). Several different types of cavity configurations have been used for this type of pulsed gas terahertz laser, such as the Zig-Zag pumping cavity, metal mesh oscillator, and L-shaped cavity [19,20]. Using a complicated intracavity-pumping configuration, a photon conversion efficiency (PCE) of 47% at 151.5 µm was obtained from NH3 medium, and this PCE was the highest for pulsed OPGTL [21]. Unfortunately, the temporal behavior of the output pulse for pulsed THz lasers has rarely been investigated. To date, this type of high-power pulsed gas terahertz laser has not been commercially available.
Regarding theory, different laser kinetic models, such as rate equations [16,18,22–24] and semi-classical [25–29] laser theory, are used to describe the kinetic processes of gas THz lasers. For CW gas THz lasers, the rate equation model can be used for the theoretical analysis and numerical calculations. Chua et al. reported a theoretical analysis of the lasing action in optically-pumped methyl fluoride gas THz lasers [22]. Yan et al. reported a numerical study of a CH3OH gas THz laser based on a photonic crystal fiber cavity [18]. Wang et al. reported a high-efficiency regime for 13CH3F gas-phase waveguide cavity terahertz lasers [23]. Recently, an improved three-level model was reported to maximize the performance of QCL-pumped N2O molecular lasers [16]. For high-power pulsed gas THz lasers, such as the D2O gas THz laser, two-photon stimulated Raman emission becomes more important than the laser process when the pump laser is detuned from the absorbing levels. To describe this process, one needs to use a semi-classical treatment that can describe the main interactions and questions associated with electromagnetic field statistics. Recently, we reported a laser kinetics model for a pulsed D2O gas THz laser based on density matrix rate equations and performed a preliminary theoretical simulation [28].
In this study, we introduce a modified laser kinetics model that generalizes previous studies to allow for a realistic description of the pulse temporal behavior of an actual pulsed gas THz laser. We experimentally and numerically report, for the first time, the output THz pulse shapes and their variation with the gas pressure and pump pulse energy for an L-shaped gas THz laser. Their typical THz pulse shapes were obtained, and the simulated results were in good agreement with the experimental results. In addition, we demonstrate the physical mechanism for understanding THz pulse splitting and pulse evolution with the variation of gas pressure and pump pulse energy. Furthermore, when the length of THz cavity was 140 cm, pulse energy of 2.31 mJ was achieved at wavelength of 385 µm. The photon conversion efficiency (PCE) was approximately 56.1%, and to our knowledge, this is the highest efficiency ever reported in D2O gas 385 µm THz cavity laser systems.
2. Theoretical model
For the D2O gas THz laser pumped by 9.26 µm high power pulsed TEA CO2 laser, the two-photon Raman process and the usual two-step laser process play an important role. A three-level system approximation treatment is a reasonable alternative; the energy-level structure and transition processes are shown in Fig. 1(a). Intense pumping absorption occurred. The THz radiation transitions of the 385 µm stimulated Raman emission were then generated between rotational levels 422–413 in the first-excited vibration state of the D2O gas molecule [25,28,29]. To model the laser kinetic processes, a laser kinetics model based on the semi-classical density matrix rate equation and the time evolution equation of the laser cavity field intensity was established. The following equations describe the population density variation as a function of time in a three-level-system and the variation of the pump and THz intensity in the cavity [28,29].
Fig. 1. Schematic diagram of the energy-level diagram and the pump pulses used in this work. (a) Schematic diagram of the D2O gas molecule energy diagram and the transition processes. (b) Pump pulses used in simulation and in experiment.
The form of ${G_s}$ in Eq. (7) shows that, in contrast to the usual laser, there are two contributions to the THz gain: a laser-like term proportional to $({\rho _{33}} - {\rho _{22}})$ and a Raman-like term proportional to $({\rho _{11}} - {\rho _{22}})$. Where ${\sigma _{32}}$ and ${\sigma _{13}}$ are the homogeneously broadened line center cross sections for the 3→2 and 1→3 transitions, and they are defined as: ${\sigma _{32}} = {{ - {\omega _S}\mu _{32}^2{T_2}} / {(\hbar {\varepsilon _0}\textrm{c})}}$, and ${\sigma _{13}} = {{ - {\omega _P}\mu _{13}^2{T_2}} / {(\hbar {\varepsilon _0}\textrm{c})}}$. The ${\mu _{\textrm{13}}}$ and ${\mu _{\textrm{32}}}$ are the dipole moment from level 3 to level 2 and the dipole moment from level 1to level 3, respectively. The cross section multipliers ${S_i}$ are complicated functions of pump and THz intensities as well as pump and THz offsets from the line center, and have been given in Ref. [29].
The available THz output power and the output pulse energy are given by the following equations [26]:
where ${v_S}$ is the THz photon frequency and A is the spot radius size of the THz beam on the reflector.To verify the accuracy of the theoretical model, the shape of the pump pulse used in the simulation must be close to the actual pulse waveform. As shown in Fig. 1(b), the temporal pulse shape of the measured pump pulse was close to that of the simulated pump pulse. The pump pulse used in the simulation was fitted using Eq. (11), which comprise four Gaussian functions: Both the measured and simulated pulses of the TEA CO2 laser consisted of a main pulse and tail pulse. The full width at half maximum (FWHM) of the simulated and measured pump pulses is approximately 80 ns and 75 ns, respectively, with a delay of approximately 55 ns between the peak of the simulated and measured pump pulses.
3. Experimental configuration
A schematic of the experimental setup is shown in Fig. 2. A tunable TEA CO2 laser with TEM00 mode output was used as the pump laser. The resonant cavity of the TEA CO2 laser was formed with a 55% reflective ZnSe output coupler and a 150 line/mm grating, which was placed at a Littrow angle and used to tune the laser on a specific 9R (22) line. An aperture approximately 11 mm in diameter was employed to yield the fundamental mode of the CO2 laser, which had a relatively smooth pulse waveform. In this study, the tunable TEA CO2 laser produced an output energy of up to approximately 360 mJ with a repetition rate of 10 Hz and pulse width of approximately 75-80 ns (FWHM) with a fundamental mode output. Two beam splitters (BS1 and BS2) with a transmission/reflection ratio of 95/5 (%) were used to divide the CO2 laser beam into a pump beam and two reference beams for pump pulse energy and pulse shape detection. Approximately 95% of the pump light was coupled into the THz laser cavity through an input window (IW). The IW was a piece of AR-coated ZnSe with approximately 99% transmittance at 9.26 µm. A piece of z-cut crystal quartz (1 mm thick) was used as a dichroic beam splitter (DBS) and was placed at 45° inside the THz cavity. The DBS is highly reflective of the pump radiation (approximately 92.5% at 9.26 µm) and highly transparent to THz radiation (approximately 75% at 385 µm). M1 and M2 constituted the Fabry-Perot cavity of the THz laser, and the transmittance of M2 in the 385 µm THz laser was approximately 0.8. M1 is a movable reflective mirror with curvatures of 15 m, M2 is a piece of crystal quartz plate with a thickness of approximately 4 mm. The distances between M1 and M2, M1 and DBS, and IW and DBS were 140, 6 and 6 cm, respectively.
A1 is an attenuator for the pump reference light and A2 is an attenuator for THz light. BS3 is a high-resistivity float zone silicon (HRFZ-Si) beam splitter with a transmittance/reflectance ratio of approximately 55/45 (%). The THz laser beam was compressed by an off-axis parabolic mirror and then incident on the THz energy detector. The pulse energy and pulse shape of the pump light were detected using an energy detector (Newport 818E-20-50 L) and HgCdTe detector with a bandwidth of 100 MHz, respectively. The THz pulse energy and pulse shape were detected by a THz energy detector (SPJ-A-8-OB, Spectrum Detector) and Schottky diode detector (VDI, Quasi-Optical Broadband Detector), respectively. Finally, all signals were recorded using a Tektronix TDS3032C digital oscilloscope with a 300 MHz bandwidth. A vacuum pump, a pressure sensor, and a gas cell were used to set the pressure in the THz laser cavity. To accurately measure the gas pressure, we calibrated the pressure sensor using the saturation vapor pressure equation of D2O gas [30], and the measurement error of the pressure values was within approximately 5%.
4. Results and discussions
The output THz pulse waveform is one of the most important indicators for pulse lasers. To simulate the actual pumping conditions, a temporally varying pump field, as shown in Fig. 1(b), was used. The physical constants used in the simulations are listed in Table 1.
When the incident pump pulse energy was 342 mJ, the simulated variation curve of the THz output power as a function of time at different operating gas pressures was obtained; the simulation results are shown in Fig. 3.
Fig. 3. Simulation results for temporal variation of the output THz pulse at different gas pressure.
It can be observed that the output laser pulse shape varies significantly at different gas pressures. Figure 3 shows how the output THz pulse is expected to distort with gas pressure. The output THz pulse shapes can be divided into three types, which correspond to higher, medium, and lower gas pressure regions. In the medium gas pressure region (approximately 250 Pa-700 Pa), the output THz pulse was similar to the pump pulse, which was composed of a strong main pulse and a relatively weak tail pulse, as described in the following section. When the THz laser operate at high pressure, the trail pulse disappears, only the main pulse exists, and the pulse width of the main pulse becomes significantly narrower. However, the main pulse splits into two pulses in the lower pressure region (lower than 200 Pa), and the lower the gas pressure, the more severe the splitting. Instantaneous gain switching causes splitting of the THz main pulse.
To explain the gain-switched splitting phenomenon of the main pulse at a lower gas pressure, splitting disappears at a higher gas pressure. We quantitatively simulated the laser dynamics process, and the simulation results for several of the main parameters are shown in Fig. 4. The results for gas pressures of 100 and 400 Pa are shown in Fig. 4(a) and Fig. 4(b), respectively. At the beginning of the pump pulse, the pump power is small, the population density of ${\rho _{33}} - {\rho _{22}}$ is approximately zero, and the THz gain coefficient is negative, considering the loss in the cavity. Subsequently, ${\rho _{33}} - {\rho _{22}}$ increased slowly as the pump power increased, leading to an increase in the THz gain coefficient. When the time is approximately 235 ns, there is a sharp instantaneous increase in both the pump absorption coefficient and the THz gain coefficient, and the spike is labeled as “A” in Fig. 4(a). This is because the pump field intensity and ${\rho _{11}} - {\rho _{22}}$ are both sufficiently large to generate a two-photon enhancement process [31,32]. Pump absorption and THz photon emission occur simultaneously in the enhanced two-photons process, causing the absorption of the pump field and rapid amplification of the THz field. Subsequently, the stimulated two-photons emission becomes the main THz radiation generating process, and the THz output power is further increased to the first peak value. Meanwhile, both ${\rho _{33}} - {\rho _{22}}$ and ${\rho _{11}} - {\rho _{22}}$ decrease to minimum values that are smaller than zero. However, the incident pump power was continuously increased to the saturation intensity of the enhanced two-photon emission. The enhanced two-photon emission is thus self-quenched [31], and the output THz power rapidly declines to approximately zero. Under these highly saturated conditions, the increased pumping reduces the THz gain coefficient, and a gain-switched phenomenon appears. The two-photon enhancement process starts again until the pump power returns to the light intensity condition for two-photon enhancement at approximately 420 ns. Pump absorption and THz gain coefficient spikes reappear, which is labeled as “B” in Fig. 4(a). Thus, the THz field is rapidly amplified again, which causes the output THz main pulse to split into two parts at a lower gas pressure of 100 Pa. Under the action of the tail pump pulse, stimulated Raman radiation dominates the THz light amplification, which produces the tail pulse of the output THz pulse.
Fig. 4. Temporal variation of the main parameters in the THz laser kinetic processes. (a) gas pressure of 100 Pa. (b) gas pressure of 400 Pa. Several parameters were given including the pump pulse in THz cavity, the output THz pulse, the pump and THz gain coefficients, and the population density of ${\rho _{33}} - {\rho _{22}}$ and ${\rho _{11}} - {\rho _{22}}$.
The corresponding simulation results when the gas pressure is increased to approximately 400 Pa are shown in Fig. 4(b). As the pump laser was increased, the THz gain coefficient gradually increased, which was attributed to the laser-like term of ${\rho _{33}} - {\rho _{22}}$. The THz gain coefficient reached its maximum and is labeled as “A” in Fig. 4(b) at approximately 250 ns. Meanwhile, the THz laser pulse increased sharply and the laser-like gain disappeared. Subsequently, the stimulated two-photon emission becomes the main THz radiation-generating process, and the THz output power is further increased to its peak of the main pulse.
To demonstrate and evaluate the laser kinetics model of the high-power pulse optically pumped gas THz laser, we targeted the output THz pulse performance and compared the simulation results with the corresponding experimental results. To show how the output THz pulses are expected to distort with pressure, their effects were compared by measuring the pump pulse and output THz pulse at various gas pressures. The experiment was conducted by setting the pump pulse energy to a fixed value of 342 mJ and slowly adjusting the gas pressure. The simulated results for several typical output THz pulse shapes are shown in Fig. 5 and the corresponding experimental results are shown in Fig. 6. Because there aren't many molecules, the instantaneous gain switching causes the THz main pulse to split into two separate pulses at a low gas pressure of 100 Pa, as shown in Fig. 5(a) and Fig. 6(a). When the gas pressure was increased to 140, 170, and 200 Pa, the pulse splitting gradually decreased until the two sub pulses were completely synthesized into one pulse at 300 Pa. This is because the pump power cannot reach the gain switching condition when the number of gas molecules increase to approximately 300 Pa. The pulse shapes of the THz pulse and pump pulses were similar, both containing a main pulse and a trailing pulse. As the gas pressure increases further, as shown in Fig. 5(b) and Fig. 6(b), the trailing pulse gradually decreases until it disappears, and eventually only the main pulse contributes. The pump threshold power increased with the gas pressure. Raising the pressure caused collisional depopulation to both narrow and reduce the energy of the pulse, and the pump trailing pulse no longer exceeded the threshold condition. On the other hand, the pulse delay between the pump pulse and the output THz pulse was observed both theoretically and experimentally, and the pulse delay increased with increasing gas pressure. The experimental and simulation results clearly show that the working gas pressure has a critical influence on the output THz pulse shape, and the experimental results agree well with those of the numerical simulation for the entire working gas pressure range. In addition, three typical pulse shapes were obtained, and the THz pulse splitting caused by gain switching was quantitatively simulated and explained based on the laser dynamics process.
Fig. 5. Simulation results of the temporal variation of the pump pulse in cavity (black curve) and the corresponding output THz pulse (red curve) at different gas pressure. (a) gas pressure is about 100 Pa, 140 Pa, 170 Pa, 200 Pa and 300 Pa. (b) gas pressure is about 400 Pa, 700 Pa, 1000 Pa, 1200 Pa and 1500 Pa. And the corresponding experimental results are shown in Fig. 6.
Fig. 6. Experimental results of the temporal variation of the pump pulse in cavity (black curve) and the corresponding output THz pulse (red curve) at different gas pressure. (a) gas pressure is about 100 Pa, 140 Pa, 170 Pa, 200 Pa and 300 Pa. (b) gas pressure is about 400 Pa, 700 Pa, 1000 Pa, 1200 Pa and 1500 Pa.
At a fixed incident pump pulse energy, the influence of gas pressure on the output THz pulse energy was simulated, and the corresponding measured data is shown in Fig. 7. The pump pulse energy and output THz pulse energy were measured using an energy detector (Newport 818E-20-50 L) and a THz energy detector (SPJ-A-8-OB, Spectrum Detector), respectively. As shown in Fig. 7, when the incident pump pulse energy is 280 mJ or 150 mJ, the simulation results and experimental results are in agreement. Under an incident pump pulse energy of 342 mJ, the theoretical results are in good agreement with the experimental results for the entire working pressure range. With incident pump pulse energy of 342 mJ, 280 mJ, and 150 mJ, the simulated maximum output THz pulse energy was approximately 2.36 mJ, 1.96 mJ, and 1.2 mJ at optimal gas pressures of 460 Pa, 430 Pa, and 380 Pa, respectively. The corresponding experimental maximum values were about 2.31 mJ, 1.92 mJ, and 1.16 mJ at optimal gas pressures of approximately 410 Pa, 390 pa, and 300 Pa, respectively. The optimal gas pressure increased slightly with an increase in pump pulse energy. The ratio of the main pulse energy to the trailing pulse energy was approximately 2.2:1, main pulse width was approximately 90 ns, and peak power of the main THz pulse was approximately 18.8 kW. When the gas pressure is 800 Pa, the output pulse energy was approximately 1.56 mJ, which only contained the main pulse with an FWHM of 75 ns, and the corresponding power was approximately 20.8 kW. This peak power was most probably the maximum peak power. Under the maximum THz output energy of 2.31 mJ, the energy conversion efficiency was approximately 0.675%, which corresponds to a photon conversion efficiency of approximately 56.1% [33]. To the best of our knowledge, this is the highest efficiency for a D2O gas THz laser. Moreover, the M2 factor of approximately 1.32 was measured using a 90/10 knife-edge technique at a THz output energy of approximately 2.31 mJ.
Fig. 7. Simulation results and experimental results of the output THz pulse energy as a function of the gas pressure when the incident pump pulse energy is 342 mJ, 280 mJ, and 150 mJ, respectively.
When the laser is operated at a low gas pressure (lower than 200 Pa), the output THz pulse energy is low, the THz pulse waveform is complex, the main THz pulse is split and broadened, and the trail pulse accounts for a larger proportion of the total pulse energy. As the working gas pressure increased to the medium-pressure zone (250 Pa to 700 Pa), the output THz pulse temporal had the same structure as the pump pulse, both of which were composed of an unsplit main pulse and a trail pulse, and the main pulse broadening effect still existed. The optimal gas pressure was in the medium-pressure zone, and at this gas pressure, the output THz pulse energy reached its maximum energy value. By further increasing the working pressure in the high gas pressure zone (higher than 800 Pa), the output THz pulse energy decreases, the trail pulse disappears, and only the main pulse exists. Although the pulse energy decreases significantly relative to the maximum output pulse energy, the peak power of the output THz pulse reaches a maximum value at approximately 800 Pa. Therefore, it can be concluded that when a high pulse energy value is pursued for THz lasers in some applications, such as direct detection system, the THz laser is required to work at the optimal gas pressure. However, when a high peak power or good pulse shape is pursued for the THz laser, the laser should operate at a higher pressure of approximately 800 Pa.
The main pulse width of the pump pulse used in the simulation is approximately 80 ns, whereas the experimental pump pulse width is approximately 75 ns. Compared to the pump pulse, THz pulse broadening at low gas pressure and pulse compression at high power were observed both in the simulation and experimental results. The FWHM of the main pulse for the THz laser as a function of gas pressure is shown in Fig. 8. For the simulated results, the FWHM of the main pulse was approximately 160 ns at a gas pressure of 140 Pa, decreased to approximately 80 ns as the gas pressure increased to approximately 960 Pa, and then further compressed to approximately 30 ns when the gas pressure increased to 1500 Pa. From the experimental results, the pulse width was approximately 120 ns at a gas pressure of approximately 140 Pa, which decreased to approximately 75 ns as the gas pressure increased to approximately 800 Pa, and then further compressed to approximately 26 ns when the gas pressure increased to 1500 Pa.
Fig. 8. Simulation results and experimental results of the THz pulse width as a function of the gas pressure.
As discussed above, for the temporal variation of the output THz pulse, there was good agreement between the simulation and experimental results at different gas pressures. It should be noted that the experimentally measured THz pulse waveform is a normalized waveform and not the absolute true value of the THz power.
However, as demonstrated by the results shown in Fig. 7, both the theoretical and experimental THz pulse energies were in good agreement at different D2O gas pressures. Therefore, it can be concluded that the simulations and the actual output THz power of the THz laser versus time are not only in agreement in waveform but also in absolute value.
Considering that the actual pump pulse is not exactly the same and the pulse width is approximately 75–80 ns, the pulse width used in our theoretical simulation is 80 ns, which is slightly larger than the actual pulse width. We summarize the ratio of the experimentally measured THz pulse width to the simulated output THz pulse at different gas pressure in Fig. 9. It can be seen that, among the entire working pressures, the ratio value was between 0.75 and 0.9. In the high-pressure region, the pulse width value was closer than that in the low-pressure region.
However, at a fixed gas pressure, the temporal behavior of the pulsed THz laser was simulated as the incident pump pulse energy increased. At working gas pressures of 100 and 300 Pa, several typical output THz pulse shapes are shown in Fig. 10(a) and Fig. 10(b), respectively. One can see that, when the working gas pressure is fixed, the output THz pulse waveform varies greatly under different incident pump pulse energy. With an increase in the incident pump pulse energy, the change in the output THz pulse shape was similar to that with a decrease in the working gas pressure. When the pump pulse energy is moderate, the waveform structure of the output THz pulse is similar to that of the pump pulse, which consists of a main pulse and trailing pulse. In the case of low pump pulse energy, the THz trail pulse disappears, and only the main pulse exists. However, when the pump pulse energy increases to a high value, the main pulse of the output THz pulse waveform appears split, and the trail pulse increases accordingly. As the pump pulse energy further increases, the output THz main pulse is split into two independent sub-pulses, and the sub pulse interval increases with an increase in the pump energy.
Fig. 10. Simulation results of the temporal variation of the pump pulse in cavity (black curve) and the corresponding output THz pulse (red curve) at different incident pump pulse energy. (a) At gas pressure of approximately 100 Pa, different pulse shape was given as the incident pump pulse energy at about 90 mJ, 210 mJ, 360 mJ, 660 mJ, and 1550 mJ. (b) At gas pressure of approximately 300 Pa, different pulse shape was given as the incident pump pulse energy at about 250 mJ, 740 mJ, 1220 mJ, 2050 mJ, and 3400 mJ.
When splitting of the THz main pulse occurs, as the pump pulse energy further increases, the peak power of the first sub-pulse hardly increases. It can be seen from Fig. 10(a) that at a gas pressure of 100 Pa, when the pump energy is 210 mJ, 360 mJ, and 660 mJ, the corresponding peak powers of the first THz sub-pulse are 0.97 kW, 0.99 kW, and 1.0 kW, respectively. Similarly, when the gas pressure was maintained at 300 Pa and the pump pulse energy was 1220 mJ, 2050 mJ, and 3400 mJ, the corresponding peak powers of the first THz sub-pulse were approximately 6.18 kW, 6.19 kW, and 6.20 kW, respectively. As discussed above, the splitting phenomenon of the THz main pulse was caused by the gain-switching effect, which can be explained by the enhanced two-photon process. In the case of saturated pumping, increasing the pump pulse energy results in a decrease in the gain coefficient, and a gain-switching phenomenon appears. When the gas pressure is fixed, the saturated conditions remain almost unchanged; thus, increasing the pump pulse energy cannot effectively increase the first sub pulse peak power. Compared with the gas pressure of 100 Pa, the gas molecules increased and the saturated pump power increased at a working gas pressure of 300 Pa; thus, gain switching and THz pulse splitting occurred at higher pump pulse energies.
5. Conclusion
This work demonstrates the pulse temporal behavior of a high-efficiency L-shaped 385 µm D2O gas terahertz laser pumped by a fundamental mode TEA CO2 laser in both simulation and experiment. A modified laser kinetics model was used to simulate the temporal pulse behavior and pulse energy of the gas THz laser at different gas pressures and pump pulse energies. The results clearly show that the working gas pressure and pump pulse energy have critical influences on the output THz pulse shape. Three typical pulse shapes can be obtained by changing the pump pulse energy or working gas pressure, and the THz pulse splitting caused by gain switching is quantitatively simulated and explained. The results clearly show that the working gas pressure and pump pulse energy have critical influences on the output THz pulse shape. Three typical pulse shapes can be obtained by changing the pump pulse energy or gas pressure, and the THz pulse splitting caused by gain switching is quantitatively simulated and explained. To our knowledge, this is the first report of a comprehensive theoretical and experimental analysis of the pulse temporal behavior, gain-switched splitting, and their variation with respect to gas pressure. When the pump pulse energy was approximately 342 mJ, in a medium-gas pressure region, the output THz pulse was similar to that of the pump pulse, which was composed of a strong main pulse and a weak tail pulse. In the high-pressure region, the trailing pulse disappears and only the main pulse exists.
However, at low gas pressures, the instantaneous gain switching causes the splitting phenomenon of the THz main pulse, and the lower the gas pressure, the more severe the splitting. The pulse width of the THz main pulse gradually decreases with the increase in gas pressure, from 120 ns at 140 Pa to 75 ns at 800 Pa, and further compressed to approximately 26 ns at 1500 Pa. A similar change in the output THz pulse shape was observed with an increase in pump pulse energy at a fixed gas pressure.
With respect to the output pulse energy and pulse temporal behavior, the experimental results agree well with those of the numerical simulation for the entire working gas pressure range, indicating that our model is an effective tool for designing and optimizing high-power pulsed gas THz lasers, especially for obtaining the desired specific pulse shape output. Furthermore, with pump pulse energy of about 342 mJ, THz output pulse energy of up to 2.31 mJ was obtained at the optimal gas pressure of about 410 Pa, which corresponds to a photon conversion efficiency of about 56.1%, and to our knowledge, this is the highest efficiency for D2O gas THz laser.
Funding
National Natural Science Foundation of China (11904327, 61905223, 62105296); Henan Provincial Science and Technology Research Project (202102210318, 212102210619, 222102110279, 222102210085).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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