>Seed pulse injection type THz wave parameter generator
Terahertz waves are located in the electromagnetic spectrum between the microwave and infrared regions. The rich physical and chemical information of substances is worth exploring. With the rapid development of terahertz generation and detection technology, many fields are realizing a leap from practical exploration to practical applications such as biomedical imaging, security inspection and terahertz communication.
This article introduces a seed pulse injection type terahertz wave parameter generator (ips-TPG), which can enhance gain over a wide tuning frequency range.
Since the second- and third-order nonlinear processes affect each other, the THz wave generation based on the terahertz parametric process can be described by a coupled wave equation under small signal approximation and slowly varying amplitude approximation.
Among them EP, ES and ET are pumping field, Stokes electromagnetic field and terahertz electromagnetic field, respectively. ε i 是斯托克斯(i=S)和太赫兹(i=T)波的波矢和介电常数。 k i and ε i are the wave vectors and dielectric constants of Stokes (i = S) and terahertz (i = T) waves. d 33 and d Q are second-order nonlinear coefficients and third-order nonlinear coefficients, respectively, and represent the contribution of differential frequency generation (DFG) and stimulated electromagnetic coupler scattering (SPS) to the terahertz generation process. β is the angle between the pump light and the terahertz beam. α T 表示为 The absorption coefficient α T is expressed as
ω 0 ，振子强度S0和阻尼常数 Γ 0 是LiNbO3晶体中最低阶A1对称模式的固有特性。 Among them, the intrinsic frequency ω 0 , the oscillator strength S0 and the damping constant Γ 0 are the inherent characteristics of the lowest-order A1 symmetrical mode in LiNbO3 crystals.
The generated THz intensity can be calculated numerically by equations (1)-(3). The initial condition of the pump energy (EP) is 150mJ, and the Stokes energy is 20mJ or 0mJ. The numerical solution was simulated by Runge-Kutta algorithm. Figure 1 shows the THz intensity calculations for the two initial conditions, which are normalized by the maximum of each curve, respectively, for visual comparison. Compared with the maximum THz intensity in each curve, in the high THz frequency range, the THz generation ability at an initial Stokes energy of 20 mJ is stronger than the THz generation ability at an initial Stokes energy of 0 mJ. The calculation results show that in the high THz frequency range, the increase of the initial Stokes energy is conducive to the generation of stronger THz. In addition, a qualitative explanation of THz generation performance improvement in the high THz frequency range is as follows. In our experiments, the matching conditions for both the SPS and DFG processes are met. Under non-collinear phase matching conditions, there are SPS and DFG processes during THz generation. The increase in the terahertz frequency is accompanied by a reduction in the volume of the three-wave interaction, which results in that the classical TPO or TPG system cannot achieve high Stokes gain. Low Stokes gain not only affects the efficiency of the SPS process, but also negatively affects the DFG process due to the low Stokes energy, which directly results in a reduction in THz energy. Therefore, compensation for low Stokes gain is the key to improving THz generation capability at high THz frequencies.
The numerical solution of the THz intensity produced in Fig. 1 is initially based on the Stokes energy of 20 mJ and 0 mJ.
The experimental setup of seed pulse injection terahertz parameter generator (ips-TPG) is shown in Figure 2. A Nd: YAG laser with a repetition frequency of 10 Hz and a pulse width of 10 ns was used as the pump source. The 1064 nm laser beam is divided into two equal parts by a mirror M1, and the mirror M1 is coated with a transmittance of 50% in the infrared range at an incidence angle of 45 °. The two separated 1064nm laser beams are shaped and collimated by the telescope lenses T1 and T2 respectively, which reduces the spot size of the two laser beams to 5mm. Mirrors M2 and M3 are coated with a highly reflective (HR) layer in the infrared range with an incident angle of 45 °. A laser beam converted to S polarization by a half-wave plate (HWP1) and a Brewster polarizer (BP1) was used as the pump beam of the ips-TPG and named Ep. φ = 23.5°）中二次谐波产生（SHG）的相位匹配条件。 The polarization of another laser beam was adjusted by HWP2 to meet the phase matching conditions of the second harmonic generation (SHG) in a potassium potassium phosphate (KTP1) crystal (7 × 7 × 10mm3, θ = 90 °, φ = 23.5 °). The fundamental (1064.4nm) and second harmonic (532.2nm) waves are separated by M4 and are coated with a 532nm high reflection (HR) layer and a 1064nm high transmission (HT) layer. The 532nm laser beam is used as the pump beam of the single-resonance optical parametric oscillator (SR-OPO) system and named E0, which is converted into P polarization by HWP3 and BP2. φ = 24.5°）和格兰棱镜（GP）。 SR-OPO consists of two flat mirrors M5 and M6, coated with HT at 532 nm and HR in the infrared range, and titanium potassium phosphate (KTP2) crystals (10 × 8 × 20 mm3, θ = 90 °, φ = 24.5 °) And Gran Prism (GP). An output idler wave tuned from 1068.08 nm to 1084.76 nm was used as a seed pulse laser. The mirror M7 coated with the HR layer in the infrared range is mounted on a rotating table, which can change the phase matching angle between the pump wave and the seed pulse wave in a 1 mol.% Magnesium oxide-doped lithium niobate (MgO: SLN) crystal. . The MgO: SLN crystal was cut into an isosceles trapezoidal structure, and the crystal surface was optically polished without a coating. In addition, the energy of the infrared and THz waves were measured by an energy meter (Newport, 1919-R) and a calibrated Golay cell detector (TYDEX, GC-1P), respectively. The manufacturer (Tydex, Inc.) specifies that the calibration of the Golay cell detector is 86.95kV / W at a repetition rate of 10Hz. A 1 mm thick black polyethylene sheet was used as a THz filter, which was covered on the detector to prevent the injection of scattered infrared waves. The wavelength in the infrared range was measured by a spectrum analyzer (Agilent, 86142B).
Figure 2 Schematic diagram of the structure of ips-TPG. Inset: Geometric parameters of the crystal
In our experiments, the seed pulse was generated by the above-mentioned SR-OPO. SR-OPO operates near the degenerate point and has type II phase matching. When the P-polarized signal wave transmits GP and oscillates in the cavity, the S-polarized idle wave reflected by the GP is used as the seed pulse wave of the ips-TPG system. By controlling the optical path of the pump beam, a complete overlap of the pump and seed pulse beams in the time domain is achieved. The time-domain distribution of the two beams was measured by a fast-response InGaAs detector (Thorlabs DET08C) after M7, and is shown in the inset of Figure 3. The tuning characteristics of the idle wave from SR-OPO are shown in Figure 3. KTP2's tuning angle (Φ angle tuning) varies from 0 ° to 10.5 °, measuring idle wavelengths from 1068.08nm to 1084.76nm. When the pump energy (E0) is 110mJ, the idle wave energy is about 23mJ, which is relatively stable over the entire tuning wavelength range. In addition, as the pump energy (E0) increases, the maximum idle wave energy reaches 37.5 mJ. In addition, in order to correctly calculate the THz frequency, the generated Stokes wavelength is monitored by a spectrum analyzer. The measured Stokes wavelength is also shown in Figure 3. Obviously, the seed pulse and Stokes wavelengths are almost the same, and no other wavelength is found at the same time. Based on the law of conservation of energy, the corresponding THz frequency is calculated from the wavelength difference between the pump and seed pulse waves.
Figure 3 Measured Stokes wavelength and output characteristics of idler waves from SR-OPO. Inset: Time-domain curves of pump and seed pulses.
Figure 4 shows the THz adjustable output characteristics of ips-TPG when the pump energy (EP) is 150 mJ and the seed pulse energy is 20 mJ. As the seed pulse wavelength changes from 1068.34nm to 1084.20nm, the corresponding THz output frequency is adjusted from 1.04THz to 5.15THz. The maximum and minimum THz output energy is 4.98 μJ at 1.79 THz and 115.01 nJ at 5.15 THz, which correspond to the detection voltages of 4333 mV and 100 mV, respectively. Considering the wide linewidth operation and low THz energy in a TPG system without seed laser injection, the THz adjustable output characteristics of the traditional LiNbO3 terahertz parametric oscillator (SLN-TPO) that we previously proposed were measured to make Convincing comparison. Under the same pumping conditions, a tunable range of 1.25 THz to 3.72 THz is achieved, and the maximum THz output energy is 3.120 mV at 1.74 THz in SLN-TPO. In addition, the THz output enhancement ratio between ips-TPG and SLN TPO was calculated. We can clearly see that in the high THz frequency range, the THz output energy has improved significantly. When the THz frequency was increased from 1.9 THz to 3.6 THz, the enhancement ratio increased significantly from 1.6 times to 34.7 times. At the same time, the enhancement ratio of 1.24 THz is 17.5 times. In general, although the THz output peak of ips-TPG and SLN-TPO is located at about 1.8 THz and the output energy is similar, the performance of ips-TPG far exceeds the adjustable range of SLN-TPO and the THz output energy at high frequencies > 2.5 THz). The significant enhancement of the terahertz output can be attributed to the increase in initial Stokes energy, which results from a tunable seed pulse laser injection. The seed pulse injection not only provides a frequency selection mechanism, concentrates the Stokes gain in a relatively narrow Stokes frequency band, and participates in the feedback amplification of the SPS process to enhance the generation of terahertz, while also increasing the DFG process Interaction of Medium Pump Wave and Stokes Wave.
Figure 4 THz adjustable output characteristics measured with ips-TPG and SLN-TPO.
In order to further quantitatively evaluate the performance of ips-TPG, a thz energy attenuation factor was defined.
Among them, E Max is the maximum THz output energy, and ET is the THz output energy of each frequency point. It is worth mentioning that the smaller the AT value, the stronger the THz generation capability at this frequency.
Figure 5 shows the THz energy attenuation factors for ips-TPG and SLN-TPO. For ips-TPG, the THz energy attenuation coefficients of 4.41 dB, 0.3 dB, 0.65 dB, and 3.27 dB are achieved at 1.24 THz, 2.16 THz, 2.61 THz, and 3.58 THz, respectively. In comparison, the THz energy attenuation factors of SLN-TPO are 15.39 dB, 3.01 dB, 6.37 dB, and 16.88 dB at 1.24 THz, 2.24 THz, 2.53, and 3.53 THz, respectively. In addition, a 3dB bandwidth is introduced to characterize THz output performance over the entire tuning frequency range. Within a 3dB bandwidth, the THz output energy at each frequency point exceeds half of the maximum THz output energy. As shown in Figure 5, the 3dB bandwidth of ips-TPG and SLN TPO are 2.1 THz (1.3 THz to 3.5 THz) and 0.8 THz (1.4 THz to 2.2 THz). The 3dB bandwidth is increased by about 2.6 times, which proves that ips-TPG has a larger dynamic range and a relatively flat output curve in a wide frequency range. From the above experimental results, ips-TPG is more conducive to practical applications in the high THz frequency range.
Figure 5 THz energy attenuation factor for ips-TPG and SLN-TPO.
Figure 6 shows the effect of pump energy and seed pulse energy on THz output characteristics. With a fixed seed pulse energy of 20 mJ, the terahertz output energy of ips-TPG at 1.79 THz was measured at different pumping energies, as shown in Figure 6 (a). Obviously, the terahertz output energy increases approximately linearly. When the pump energy is 160 mJ, the pump energy increases, and the maximum THz output energy is 5.46 μJ. Meanwhile, when the pump energy is fixed at 100mJ, the THz output energy is measured between the seed pulse energy from 0.42mJ to 37.5mJ, as shown in Figure 6 (b). Obviously, the THz output curve can be divided into two parts. First, the THz output energy increases approximately linearly, and the seed pulse energy increases from 0.42 mJ to 8.17 mJ. After that, as the seed pulse energy increases, the THz output gradually increases and gradually saturates. At the same time, the greater the pump energy, the greater the seed pulse energy required for saturation. For a certain pump energy, as the THz frequency increases, the less seed pulse energy is needed for saturation. The results show that the inherent THz generating capacity is mainly limited by the pump energy. However, the seed pulse laser injection can compensate the low Stokes gain to some extent, which is conducive to the inherent THz generation. In our experiments, the phase matching conditions of stimulated electromagnetic coupler scattering (SPS) and differential frequency generation (DFG) are both satisfied. As shown in Figure 6 (b), considering that the energy ratio between the pump and the seed pulse is about 8: 1 when saturation occurs, it is much higher than the energy ratio when saturation occurs in a pure DFG process. Therefore, we conclude that there are SPS and DFG processes during THz generation. At a fixed pump energy, the SPS and DFG processes coexist. When the seed pulse energy is small, THz generation is mainly controlled by the SPS process. With the increase of seed pulse energy, the generation of THz is gradually dominated by the DFG process. Due to insufficient utilization of pump energy in the crystal, the SPS process still consumes a portion of the pump energy. With the full use of pump energy, THz output energy is higher. Before the seed energy reaches a certain value, the saturation of the terahertz energy is caused by the saturation of the DFG process. This is due to the fact that the conversion efficiency of the DFG process may be the greatest when the energy of two interacting waves is close. In addition, Figure 6 (b) shows the threshold energy of ips-TPG at different seed pulse energies. When the detected THz signal voltage is slightly higher than the noise voltage of the Golay battery (about 10mV), the pump energy is defined as the threshold energy. A significant reduction in threshold caused by seed pulse laser injection was observed. When the seed pulse energy is 37.3 mJ, the minimum threshold energy is 11.9 mJ. Compared with the TPG system without seed laser injection, the threshold energy of ips-TPG is reduced by 9.66 times.
Figure 6 (a) THz output energy of ips-TPG at different pump energies. (B) THz output energy and threshold energy at different seed pulse energies.
Figure 7 shows the reception angle generated by THz in ips-TPG. The receiving angle is defined as the output energy dropping to 40.5% of the maximum value at a certain frequency, which is caused by the angle mismatch. 0 ° is defined as the angle between the pump and the seed pulse beam, with a maximum THz output energy. When the pump and seed pulse wavelengths are fixed at 1064.4 nm and 1070.8 nm, respectively, the THz energy is measured when the angle between the pump and seed pulse beams changes from -0.6 ° to 1.0 °. At the same time, the Stokes wavelength generated is monitored by a spectrum analyzer. Obviously, the receiving angle of ips-TPG is estimated to be about 0.42 °. Within the reception angle, the drift of the Stokes center wavelength is less than 0.3 nm, and the corresponding THz center frequency drift is less than 87 GHz. Considering the line width of the seed pulse is about 0.33nm, the THz center frequency drift can be further reduced by reducing the line width of the seed pulse. In addition, the reception angles measured at different frequencies are shown in the inset of FIG. 7. As the THz frequency increases, the reception angle gradually decreases. Because the traditional TPO system is angle sensitive and easy to detune, such a large receiving angle makes the performance of ips-TPG insensitive to angle, which is more suitable for practical applications.
Figure 7 Acceptance angle generated by THz in ips-TPG. Inset: Reception angle at different THz frequencies.
In addition, the THz output energy of ips-TPG with a pump energy of 100 mJ and a seed pulse energy of 20 mJ was measured at 1.79 THz, as shown in Figure 8. Obviously, the stable operation of ips-TPG is more than an hour. As shown in the inset of Figure 8, the THz output fluctuation is measured at different seed pulse energies. As the seed pulse energy increases, the THz output fluctuates slowly and becomes constant. With a pulse energy of 35mJ, the minimum THz output fluctuation is 5.62%. Considering that the large fluctuations of the pump and seed pulse energy are 1.82% and 2.66%, respectively, it is inferred that the THz output stability can be further improved by improving the stability of the pump and seed pulse laser.
Figure 8 THz output stability of ips-TPG in one hour. Inset: THz output fluctuations at different seed pulse energies.
In summary, this article shows an ips-TPG with gain enhancement over a wide tuning frequency range. At 1.79 THz, the maximum THz output energy of ips-TPG is 5.46 μJ, and the THz frequency range is continuously tuned from 1.04 THz to 5.15 THz. Seed pulse laser injection is used to compensate the initial Stokes energy, and a significant increase in THz output energy is achieved in the high THz frequency range (> 2.5 THz). As the THz frequency increases from 1.9 THz to 3.6 THz, the ratio between THz output enhancement ips-TPG and SLN-TPO is 1.6 times to 34.7 times. In addition, the 3dB bandwidth of ips-TPG is 2.1 THz, which is about 2.6 times that of SLN-TPO. The terahertz energy enhancement mechanism of ips-TPG can be attributed to the increased interaction between SPS and DFG processes. In addition, the large receiving angle and relatively stable THz output energy of ips-TPG can enhance its practical prospects.
From <Injection pulse-seeded terahertz-wave parametric generator with gain enhancement in wide frequency range>