Study on the performance improvement of high power gas terahertz laser by optimizing L-shaped cavity

Optics and Laser Technology 109 (2019) 361–365

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Study on the performance improvement of high power gas terahertz laser by optimizing L-shaped cavity ⁎

T

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Lijie Geng , Zhifeng Zhang, Ruiliang Zhang, Yusheng Zhai , Yuan Luo, Yuling Su School of Physics & Electronic Engineering, Zhengzhou University of Light Industry, Zhengzhou 450002, Henan, China

H I GH L IG H T S

efficient, high power gas THz laser based on L-shaped cavity is present. • ATHzhighoutput was investigated both in theory and experiment. • The optimumperformance coupling is 0.8 and the optimum gas pressure is 500 Pa. • The maximumoutput THz pulse energy is 8.4 mJ under the pumping energy of 1.6 J. •

A R T I C LE I N FO

A B S T R A C T

Keywords: Terahertz laser Output performance High power L-shaped cavity Output coupler

A high power gas terahertz laser based on L-shaped cavity was demonstrated. We, for the first time, investigated the output performances of D2O gas 385 μm THz laser by optimizing the output coupler transmittance of the Lshaped cavity both in theory and experiment. Under the pump energy of 1.6 J, the optimum output coupler transmittance was about 0.78 and the optimum gas pressure was about 500 Pa. Up to 8.4 mJ pulse energy at 385 μm was achieved at output coupler transmittance of 0.8, corresponding to a photon conversion efficiency of 43.7%. Pulse width of 120 ns, and beam quality factor M2 of 1.58 were obtained at the highest output energy. In addition, the experimental results are in agreement with the theoretical simulation results.

1. Introduction

In order to achieve high efficiency, high power and stable THz sources, it is necessary to study the influence of the output coupler, pump energy, and gas pressure on the THz output performance. Unfortunately, as a note, the coating technology for THz wave band is not yet mature. Therefore, it is difficult to achieve a specific transmissivity or reflectivity at THz frequencies [12,17,18]. Furthermore, there is little work reported about THz output energy/power influenced by the transmittance of output coupler for high efficiency, high power OPGTL. Therefore, the main objective of this work was to research the output performance of OPGTL influenced by different transmittance of the output coupler. In this paper, we present a high efficiency, high power and good beam quality OPGTL operating at 385 μm based on L-shaped THz laser cavity at room temperature. The output THz energy was studied at different gas pressure and pump energy under different output couplers both in theory and experiment. The optimum output coupler transmittance and the optimum gas pressure of the THz laser were investigated at different pump energy. Three output couplers with transmittance of 0.38, 0.54 and 0.8 were used, and the maximum

Optically pumped gas THz laser (OPGTL) technology is one of promising ways to generate coherent THz radiation, and can be widely used, such as THz imaging [1–3], digital holography [4,5], THz radar [6,7], Optical measurement [8], and atmosphere remote sensing [9]. In the past decades, improving photon conversion efficiency (PCE) and thus enhancing THz output energy have attracted many scientists. Many experimental studies have been reported to improve the output performance of OPGTL with different kinds of THz cavity configurations, such as unstable cavity [10], metal mesh oscillator [11,12], holecoupled mirror oscillator [13,14] and intracavity-pumping configuration [15]. Using complicated intracavity-pumping configuration, the PCE of 47% at 151.5 μm has been obtained from NH3 medium, and this PCE is the highest for OPGTL [15]. In order to achieve high efficient, high-power THz laser output, we proposed a novel high-efficient and compact cavity oscillator [16], and the PCE of 44% at 385 μm output was the highest efficiency for D2O gas THz laser to the best of our knowledge.

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Corresponding authors. E-mail addresses: [email protected] (L. Geng), [email protected] (Y. Zhai).

https://doi.org/10.1016/j.optlastec.2018.08.026 Received 1 June 2018; Received in revised form 16 July 2018; Accepted 12 August 2018 0030-3992/ © 2018 Elsevier Ltd. All rights reserved.

Optics and Laser Technology 109 (2019) 361–365

L. Geng et al.

320 MHz (010)

Output coupler

AR coated ZnSe

M3

422 385 μm

Grating

413

M4

TEACO2 Section IW

A

M1 M2

R(22) 9.26 μm CO2 Pump

THz Gas Gain Section

FPI Detection System

DBS

THz Cavity

Oscilloscope

Fig. 2. Experimental setup of L-shaped cavity based on z-cut crystal quartz as DBS.

(000)

533

3. Experiment setup The experimental configuration of OPGTL is shown in the Fig. 2. The schematic of the experimental apparatus consists of three main parts: the TEA CO2 laser, the THz cavity, and the detection system. A tunable Multi-transverse mode TEA CO2 laser, with emission wavelength of 9.26 µm is utilized as a pump source. The output coupler with transmittance of 0.54 at 9.26 µm was a plano-concave mirror (with radius of curvature of 15 m). The pulse energy and pulse shape of the pump laser are detected by Newport 818E-20-50L and HgCdTe detector with bandwidth of 100 MHz, respectively. The L-shaped THz laser cavity consists of an input window (IW), a dichroic beam splitter (DBS), a mirror (M1), and a THz output coupling mirror (M2). The IW is a piece of AR-coated ZnSe with 99.5% transmittance at the pump wavelength of 9.26 μm. The flat 45° dichroic beam splitter (DBS) is high transmittance (about 75%) at 385 μm and high reflective (about 92.5%) in the wavelength range of 9.0 μm–11.0 μm. M1 with curvatures of 20 m is high reflective (about 99.2%) at 385 μm. The THz Fabry-Perot cavity consists of a high reflective mirror (M1) and an output coupler (M2), and the total physical length of the resonator is about 120 cm. Considering the transmittance losses of the ZnSe input window and the reflective losses of the DBS, nearly 92% of the pump power was coupled input to THz cavity. In this work, to study the THz output performances under different output couplers, a crystal quartz plate (4 mm thick), a high resistivity silicon plate (4 mm thick) and a Ge single crystal plate (3 mm thick) were used as the output coupler, respectively. And the corresponding transmittance at 385 µm THz laser are 0.8, 0.54 and 0.38, respectively. The THz energy was fully coupled to the THz energy detector (SPJA-8-OB, Spectrum Detector) by off-axis parabolic mirror (M4). A Schottky diode detector (VDI, Quasi-Optical Broadband Detector), with sub-ns response time at the frequency range from 0.1 THz to 1 THz radiation, was used to detect the profile of the THz pulse. Finally, all electrical signals were recorded by a Tektronix TDS3032C digital oscilloscope with a 300 MHz bandwidth.

Fig. 1. Schematic diagram of the D2O molecule energy diagram.

output energy of about 3.39 mJ, 5.3 mJ and 8.4 mJ were obtained, respectively. The corresponding photon conversion efficiency are about 43.7%, 27.8% and 17.3%. The theoretical simulation results are in agreement with the experimental results.

2. Theoretical model For D2O gas THz laser pumped by a TEA CO2 laser, a three-levelsystem approximation treatment is reasonable alternative, and the energy-level structure and the transition processes are given in Fig. 1. When the D2O molecules were pumped by a CO2 laser line with wavelength of 9.26 µm, the absorption from the 533 rotational level in the (0 0 0) ground vibrational state to the 422 level of the excited (0 1 0) vibrational state has been occurred. Two photon Raman process as well as the usual two-step laser process of 385 µm lasing transition occurred from level 422 to level 413 in the excited (0 1 0) vibrational state [19]. To model the THz output characteristics, a modified laser kinetics model based on semi-classical density matrix rate equation and time evolution equation of laser cavity-field intensity has been established by us [19]. There are many numerical methods for solving the laser kinetics model. A MATLAB computer program, based on the RungeKutta method, was used to solve these equations. The physical constants used in calculations are given in Table 1 [19], and some laser dimensions used in this work are also shown in Table 1.

Table 1 Physical constants used in the calculation [19]. Symbol (Parameter)

Values/Unit

λ (pump wavelength) T (operating temperature) E1 (Level 1) E2 (Level 2) E3 (Level 3) Ti (relaxation time) μ13 μ23 L (Length of THz cavity) R1 (Reflectance of M1) R2 (Reflectance of M2) f1 f2 f3

9.26 μm 273 K 267.53083 cm−1 1321.41375 cm−1 1347.39375 cm−1 8 ns·torr 4.0 × 10−31 C·m 6.1 × 10−30 C·m 120 cm 1.0 0.2/0.46/0.62 0.01791 0.00014 0.000125

4. Results and discussion 4.1. Simulation results and discussion In the simulation, the pumping geometry and THz cavity was shown in Fig. 2. The total physical length of the resonator is about 120 cm, and the reflectance of M1 is about 100%. The pump pulse with pulse width of 110 ns was simulated by a Gaussian function. The insertion loss of the dichroic beam splitter (DBS) is about 25%. The pump energy and the pump spot are about 1.6 J and 22 mm * 22 mm, respectively. For gas THz laser, the output THz energy depends on the gas pressure. As one can see from Fig. 3, when the pump energy is 1.6 J, there is obviously an optimum gas pressure at which the THz energy is maximized. When the output coupler transmittance was 0.38, 0.54 and 0.8, the maximum output pulse energy of 8.4 mJ at 490 Pa, 6.4 mJ at 600 Pa

fi is the Boltzmann occupation factor of level i. μij is the dipole moment from level i to level j. 362

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10 8

THz output energy (mJ)

output couplers with transmittance of 0.38, 0.54 and 0.8 were used, and the corresponding measured results were shown in Fig. 5(a), (b) and (c), respectively. Under different pump energy, when gas pressure increased, the THz energy increased firstly and then decreased. Thus, there existed an optimum operating gas pressure under which the maximum THz output is yielded. As shown in Fig. 5(a), for output coupling transmittance of about 0.38, with the pump energy of 0.65 J, 0.96 J, 1.29 J and 1.63 J, the corresponding maximum output THz pulse energy were about 1.67 mJ, 2.2 mJ, 2.69 mJ and 3.39 mJ at optimum gas pressure of 400 Pa, 450 Pa, 500 Pa, and 610 Pa, respectively. It is found that as the pump energy enhanced from 0.65 J to 1.63 J, the corresponding optimum gas pressure increased slightly from 400 Pa to 610 Pa. The similar trend can be found in Fig. 5(b) and (c). As shown in Fig. 5(b). For output coupling transmittance of about 0.54, the optimum gas pressure increased slightly from 450 Pa to 600 Pa with the increase of the pump energy from 0.6 J to 1.58 J, and the maximum THz energy increased from 2.46 mJ to 5.25 mJ. As shown in Fig. 5(c), when high resistivity silicon working as output coupler (T = 0.8), the optimum gas pressure increased slightly from 410 Pa to 500 Pa with increasing the pump energy, and the maximum output THz energy increased from 3.9 mJ to 8.4 mJ. It should be noted that when high resistivity silicon and Ge single crystal working as output coupler, a small part of pump energy could through it. Therefore, the ouput beam contains the THz energy and the residual pump laser energy. In this paper, in order to measure the correct element of the THz energy, we put two pieces of A4 (size) paper in front the detector. Because A4 (size) paper was opaque to the CO2 laser and transparent to the THz laser emission, which ensures that the detector can only receive the latter. It has a 40% transmittance to 385 µm THz laser according to our experiment. At the fixed pump energy of about 1.6 J, the dependence of the output THz pulse energy on the gas pressure was measured at three different output couplers, and the measured data was shown in Fig. 3. When the transmittance of the output coupler was 0.8, the maximum output pulse energy of 8.4 mJ was obtained at about 500 Pa under pump energy of 1.6 J, corresponding to an energy efficiency of about 0.525% and a PCE of 43.7%. The PCE was calculated according to the formula (PCE = ETHz (EIR λIR /2λTHz ) ) by Hodeges et al. [20]. Where λIR is the pump wavelength and λTHz is the THz wavelength, ETHz is the THz energy and EIR is the pump energy. As the output coupling transmittance were 0.54 and 0.38, THz pulse energy of about 5.3 mJ and 3.39 mJ were obtained at about 600 Pa and 610 Pa. The energy conversion efficiency were about 0.33% and 0.21% and the PCE were about 27.8% and 17.7%, respectively. It proves that an appropriate transmittance of the output coupler could improve energy conversion efficiency and the THz pulse output energy. One can see that from Fig. 6, when the transmittance is 0.8 or 0.54, the simulation results and experimental results are basically in agreement. And under the output coupler transmittance of 0.8, the theoretical results have a good agreement with the experimental results in the whole working pressure range. As shown in Fig. 7, one can see that the optimum transmittance is 0.78, and the maximum output energy is about 8.6 mJ. From the comparison between experimental and theoretical results, it is shown that they are basically in agreement. As shown in Fig. 8, with different output coupling transmittance, we measured the THz energy versus pump energy. As shown in Fig. 8(a), the slope efficiencies (the ratio of output THz energy to pump energy) of 0.51%, 0.28%, and 0.18% were obtained when the output coupling transmittance was 0.8, 0.54 and 0.38 at the gas pressure of 500 Pa. At the low gas pressure, such as 100 Pa (shown in Fig. 8(b)), THz pulse energy increased as the pump energy increasing firstly, then reached its saturation output energy. The saturation behaviors of the OPGTL were observed under different output coupling transmittance. Temporal behavior of pump pulse and output THz pulse (shown in Fig. 9) have been traced by a fast speed HgCdTe detector with

T=0.8, calculated curve T=0.54, calculated curve T=0.38, calculated curve Pump energy = 1.6 J

6 4 2 0

0

400

800

1200

1600

2000

Gas pressure (Pa) Fig. 3. THz output energy as a function of the gas pressure when the output coupler transmittance of the THz laser is 0.8, 0.54 and 0.38, respectively.

and 4.3 mJ at 670 Pa were obtained, respectively. And the optimum gas pressure is increased with the decreasing of output coupler transmittance. To design and obtain a high efficiency and high energy Gas THz laser, the cavity of THz laser, especially the output coupler transmittance should be optimized. We simulated the THz output energy at different output coupler transmittance, and the simulated curve was shown in Fig. 4. The output THz energy reaches the maximum value of about 8.6 mJ under the transmittance of about 0.78. There is an optimum output coupler transmittance for which the output THz energy is maximized.

4.2. Experimental results and discussion Multi-transverse mode TEA CO2 laser, with the maximum pulse energy of about 1.6 J output at 9.26 µm, served as the pump source. We measured the pump spot by the knife-edge technique at the input window of the THz cavity, which was approximately a square of 22 mm * 22 mm, and the diameter of pump beam was nearly unchanged over the length of the THz laser. The pump pulse was detected by HgCdTe detector and the pulse width was about 110 ns. The output pulse energy of THz laser at wavelength of 385 μm as a function of the gas pressure is illustrated in Fig. 5. Three different

10

THz output energy (mJ)

8

Pump energy = 1.6 J Gas pressure = 500 Pa

6

4

2 0.2

0.4

0.6

0.8

1.0

Transmission of output mirror Fig. 4. THz output energy as a function of the output coupler transmittance. 363

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6

Pump energy=1.63J Pump energy=1.29J Pump energy=0.96J Pump energy=0.65J

(a)

3

THz output energy (mJ)

THz output energy (mJ)

4

T=0.38 2

1

0

Pump energy=1.58J Pump energy=1.25J Pump energy=0.9J Pump energy=0.6J

(b)

5 4

T=0.54

3 2 1 0

0

400

800

1200

1600

2000

0

400

Gas pressure (Pa)

800

1200

1600

2000

Gas pressure (Pa)

10 Pump energy=1.6J Pump energy=1.24J Pump energy=0.9J Pump energy=0.61J

THz output energy (mJ)

(c) 8 6

T=0.8

4 2 0 0

400

800

1200

1600

2000

Gas pressure (Pa) Fig. 5. THz output energy as a function of the gas pressure when the pump energy is different. (a) Germanium single crystal plate as output coupler (T = 0.38); (b) High resistivity silicon plate as output coupler (T = 0.54); (c) Crystal quartz plate as output coupler (T = 0.8).

10

6

Output THz pulse energy (mJ)

8

THz output energy (mJ)

10

T=0.8, calculated curve T=0.54, calculated curve T=0.38, calculated curve T=0.8, measured data T=0.54, measured data T=0.38, measured data

4 2 0

0

400

800

1200

1600

2000

8

Pump energy = 1.6 J Gas pressure = 500 Pa Calculated curve Measured data

6 4 2 0

0.2

0.4

0.6

0.8

1.0

Transmission of output mirror

Gas pressure (Pa)

Fig. 7. Simulated and experimental results of THz output energy versus output coupler transmittance.

Fig. 6. Simulated and experimental results of THz output energy versus the gas pressure under different output coupler transmittance.

the pulse repetition frequency was 6 Hz, and pump pulse laser energy fluctuation was less than 3%. The transverse beam quality was measured by the 90/10 knife-edge method at the maximum THz output energy of 8.4 mJ, and the beam quality factor M2 was calculated to be about 1.58.

broadband of 250 MHz and a Schottky diode detector (VDI, Quasi-Optical Broadband Detector) with sub-ns response time, respectively. The FWHM of pump pulse and THz pulse is about 110 ns and 120 ns, about 10 ns THz pulse broadening and 45 ns pulse delay are observed. Considering the THz pulse energy was about 8.4 mJ, the peak power was about 70 kW. In addition, the THz pulse laser energy fluctuation was within 6%, 364

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Fig. 8. THz output energy as a function of the pump pulse energy when the output coupling transmittance is different. (a) Gas pressure is about 500 Pa; (b) Gas pressure is about 100 Pa.

THz pulse 120 ns

Gas pressure= 500 Pa

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0.4 110 ns

Pump pulse

-0.2

0.0 0.0

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5.0x10

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1.0x10

1.5x10

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THz power intensity (a.u)

Pump power intensity (a.u)

References

0.4

1.2

-6

2.0x10

Time (s) Fig. 9. The experimental temporal behavior of the pump power and output THz power.

5. Conclusions In summary, we have demonstrated a room temperature high efficiency, high power and good beam quality 385 μm THz laser both in theory and experiment. Under incident pulse energy of about 1.6 J, the theoretical and experimental results present that the optimum output coupler transmittance was about 0.78 and the optimum gas pressure was about 500 Pa. The maximum pulsed output energy of 8.4 mJ with pulse width of about 120 ns and beam quality factor M2 of about 1.58 was achieved at pulse repetition frequency of 6 Hz. The corresponding energy conversion efficiency and PCE were about 0.53% and 43.7%, respectively. The optimum gas pressure is increased with the decreasing of output coupler transmittance. The saturation behaviors of the gas THz laser were observed with different output coupler transmittance at low gas pressure of 100 Pa. The experimental results are in agreement with the theoretical simulation results. In addition, the simulation of the THz laser helps the understanding of the laser physics and the prediction of output power trace guides the experimental research. Acknowledgments This work was supported by National Natural Science Foundation of China (No. 61571403), Henan science and technology development plan project (Nos. 182102310032, 172102210552), Foundation of Henan Educational Committee (No. 16A140020) and Doctor’s Innovation Fund of Zhengzhou University of Light Industry. 365