1 Introduction

Terahertz (THz) gas lasers can provide narrow linewidth far-infrared waves for numerous of applications [1,2,3,4]. Using CO2 laser radiation exciting different gas molecules, thousands of intense THz laser lines can be generated [5,6,7]. The hole-coupling oscillator [8, 9] and the Fabry–Perot metal-mesh oscillator [10, 11] are used to obtain THz gas laser radiation, generally with low efficiency. In addition, several cavities including the zig–zag oscillator [12, 13], the oscillator–amplifier system [14, 15] and the unstable cavity [16] are designed to improve the pump-to-THz conversion efficiency. Efficient THz gas lasers can also be achieved based on a dichroic element that reflects/transmits infrared waves and simultaneously transmits/reflects THz waves [17,18,19]. For instance, L. Miao reported an NH3 gas laser based on a germanium mirror, and the energy conversion efficiency is 1.2% [17]. The germanium mirror is precisely manufactured to be 2.964-mm thick for 151.5 μm wavelength, and it should be redesigned when applied to other THz laser wavelengths. L. Geng reported a D2O gas laser based on a 45°-inclined crystal quartz dichroic mirror, which can give the transmittance of 75% at 385 μm and reflectance of 92% at 9.26 μm. The energy conversion efficiency is 0.5% [18]. However, the reflectance of the crystal quartz dichroic mirror is rather low at other infrared wavelengths (55% at 9.55 μm and 38% at 10.17 μm), so this efficient dichroic element is not available for other THz laser wavelengths.

In this paper, a germanium spectrum splitter is demonstrated to be efficient for tuning THz laser wavelengths. By changing the pump laser lines and the inclined angle of the germanium spectrum splitter, an efficient CH3F laser is presented with the wavelengths of 181 μm, 261 μm, 360 μm and 496 μm. The dichroic performance of the germanium spectrum splitter is discussed theoretically and experimentally; and then the tunable THz gas laser system is built based on the germanium spectrum splitter. Finally, the efficiency and tunability of the laser system are discussed.

2 Dichroic performance of the germanium spectrum splitter

Intrinsic germanium is well-known as the infrared window material. The typical absorption coefficient in 9–11 μm band is 0.01 cm−1 so that a germanium window can be transparent to CO2 laser radiation according to the anti-reflection coating technology. On the other hand, the typical absorption coefficients at 1.6 THz, 1.1 THz, 0.8 THz and 0.6 THz are measured to be 0.46 cm−1, 0.52 cm−1, 0.8 cm−1 and 1.05 cm−1 [20], which indicates good transmission characteristics in far-infrared band for intrinsic germanium. When a germanium plate processed to be a Fabry–Perot etalon, the transmittance T can be described according to the multi-beam interference [21]:

$$ T = \frac{{\left( {1 - r} \right)^{2} e^{ - \alpha d} }}{{\left( {1 - re^{ - \alpha d} } \right)^{2} + 4re^{ - \alpha d} \sin^{2} \left( {\frac{2\pi nd}{\lambda }\cos \theta_{r} } \right)}} $$
(1)

where r is the normally incident reflectivity, given by \(\frac{{\left( {n - 1} \right)^{2} }}{{\left( {n + 1} \right)^{2} }}\); n the refractive index; α the absorption coefficient; d the thickness of the etalon; λ the incident wavelength; θ the incident angle and θr the refraction angle. The relationship of θ and θr fits the equation:

$$ \frac{{\sin \theta_{r} }}{\sin \theta } = \frac{1}{n} $$
(2)

and the total reflectance R can be determined by R = 1–T.

The reflectance performance of the germanium etalon is investigated theoretically. The absorption coefficients of 0.46 cm−1, 0.52 cm−1, 0.8 cm−1 and 1.05 cm−1 are used for the wavelengths of 181 μm, 261 μm, 360 μm and 496 μm respectively, with the same refractive index of 4.004. The theoretical reflectances versus the incident angles are shown in Fig. 1. There exist some angles that can maximize the reflectance, which are defined as “optimal angles” in this paper.

Fig. 1
figure 1

Theoretical reflectance versus the incident angle for (a) 181 μm, (b) 261 μm, (c) 360 μm and (d) 496 μm wavelength. The germanium thicknesses of 1 mm, 1.1 mm and 1.2 mm are used in the simulation

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For a given thickness of the germanium etalon, the optimal angle is related to the THz laser wavelength. For instance, when the thickness is set as 1.2 mm, the optimal angle is 18° (50°) and the reflectance is 72% (74%) for 181 μm wavelength; the optimal angle is 38° (74°) and the reflectance is 75% (78%) for 261 μm wavelength; the optimal angle is 39° and the reflectance is 76% for 360 μm wavelength; the optimal angle is 50° and the reflectance is 76% for 496 μm wavelength. For a given incident wavelength, the optimal angle varies with the thickness of the germanium etalon, and the corresponding reflectance is almost constant. It is indicated from the theoretical results that the optimal angle of the germanium etalon is affected by both the incident wavelength and the thickness of the germanium etalon, while the reflectance of more than 70% can be obtained for different THz laser wavelengths.

3 Experiment and discussion

A germanium etalon with the thickness of 1.2 mm and the diameter of 50.8 mm is used in the experiment. The germanium etalon is anti-reflection coated at the central wavelength of 10.17 μm, giving the transmittance of 99%. The anti-reflection coatings are composed of Si (n = 3.4), TlBr (n = 2.3), PbF (n = 1.65) and SrF2 (n = 1.4), and the total coating thickness is in the micron order of magnitude. The germanium etalon is placed on a rotating stage so that the incident angle can be controlled. The transmittance at different infrared wavelengths is obtained using a transversely excited atmospheric (TEA) CO2 laser, from which 9.25 μm, 10.22 μm, 9.32 μm and 9.55 μm laser radiation can be emitted respectively. The output pulse energy is 700 mJ; the pulse width is 100 ns; the spot size is 24.2 × 24.6 mm and the repetition frequency is 5 Hz. The transmittances versus incident angles for different CO2 laser wavelengths are shown in Fig. 2. It is indicated that the transmittance is not sensitive to the incident angle, since the experimental values are about 95% when the incident angle varies from 5° to 75°.

Fig. 2
figure 2

Experimental transmittance versus the incident angle for different infrared wavelengths. The error bars represent the uncertainty of experimental data in five measurements (similarly for subsequent figures)

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The reflectances at different far-infrared wavelengths are obtained with a home-made THz gas laser [22], from which 181 μm, 261 μm, 360 μm and 496 μm laser radiation can be emitted. The pulse energy is measured to be 0.5 mJ with a pyroelectric detector (SPJ-A-8-OB, Genetec-EO). The reflectances versus incident angles for different far-infrared wavelengths are shown in Fig. 3. It can be indicated that the experimental results follow trends of periodical variation and there also exist “maximum reflectances” and “optimal angles”. The reflectances are measured to be 73% (75%) at the optimal angle of 15° (55°) for 181 μm wavelength, 76% at the optimal angle of 33° for 261 μm wavelength, 73% at the optimal angle of 25° for 360 μm wavelength and 72% at the optimal angle of 40° for 496 μm wavelength.

Fig. 3
figure 3

Experimental reflectance versus the incident angle for different THz laser wavelengths

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When comparing the experimental results with the theoretical ones, it can be found that there are some disagreements in optimal angles. On one hand, the actual thickness of germanium etalon is not exactly the same with the preset value due to the machining error, and this may cause the optimal angle varied. On the other hand, the multilayered coatings may also affect the results since they are not considered for theoretical simulation.

According to the theoretical and experimental results, high transmittance to infrared waves and high reflectance to THz waves can be obtained simultaneously by tuning the incident angle of the germanium etalon. It is indicated that the germanium etalon can act as a spectrum splitter in the tunable THz gas laser. The device of a tunable CH3F laser is shown in Fig. 4, and it is a follow-up of the previous CH3F laser system [23]. The main improvement is that a mirror group composed of three gold-coated mirrors (M1, M2 and M3) is used to reflect THz laser radiation in the cavity. The pump laser is a TEA CO2 laser as described in Sect. 3, and when it enters from the ZnSe window (W1) to excite the CH3F gas medium, THz laser oscillation is generated between the total-reflection mirror, the germanium spectrum splitter and the crystal quartz window (W2). The incident angle of the germanium spectrum splitter is controlled by the rotating stage, and each gold-coated mirror is served as the total-reflection mirror alternately to match different angles. The germanium spectrum splitter is close to W1 to minimize the absorption loss of the pump laser, giving the total cavity length of 1.2 m.

Fig. 4
figure 4

Schematic diagram of the tunable THz gas laser based on the germanium spectrum splitter

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When 9.25 μm, 10.22 μm, 9.32 μm and 9.55 μm laser radiations are emitted for pumping, 181 μm, 261 μm, 360 μm and 496 μm laser radiations can be obtained [5, 24]. When the angles between the pump CO2 laser and the normal of the germanium etalon are tuned to be 55°, 33°, 25° and 40° respectively, and the THz laser energies versus the operation pressure are measured, as shown in Fig. 5. For 181 μm wavelength, laser energy of 1.1 mJ is obtained at 1600 Pa; for 261 μm wavelength, laser energy of 2 mJ is obtained at 1000 Pa; for 360 μm wavelength, laser energy of 1.55 mJ is obtained at 1300 Pa; for 496 μm wavelength, laser energy of 1.2 mJ is obtained at 700 Pa.

Fig. 5
figure 5

THz laser energy versus the operation pressure for different THz laser wavelengths

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THz laser energy versus the pump energy is shown in Fig. 6. The energy conversion efficiency is calculated from the slope of the fitting curves, when the operation pressure is optimal. It is indicated that the energy conversion efficiency is 0.15% for 181 μm wavelength, 0.28% for 261 μm wavelength, 0.22% for 360 μm wavelength, and 0.17% for 496 μm wavelength.

Fig. 6
figure 6

THz laser energy versus the pump energy for different THz laser wavelengths. The measured data are obtained when the operation pressure is optimal

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Pulse shapes of the tunable THz gas laser are obtained with a Schottky diode (the Quasi-Optical Broadband Detector, Virginia Diodes Inc.). As shown in Fig. 7, the pulse width is measured to be 103 ns for 181 μm wavelength at the pressure of 1600 Pa, 85 ns for 261 μm wavelength at the pressure of 1000 Pa, 98 ns for 360 μm wavelength at the pressure of 1300 Pa and 90 ns for 496 μm wavelength at the pressure of 700 Pa.

Fig. 7
figure 7

Pulse shapes of the tunable THz gas laser for different THz laser wavelengths. The measured data are obtained when the operation pressure is optimal

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The beam quality factor of the tunable THz gas laser can be determined based on the knife-edge technology [25]. The beam widths at different positions are measured using a 50-mm focal-length TPX lens. As shown in Fig. 8, the beam quality factor M2 are measured to be 1.61, 1.52, 1.57 and 1.55 for the wavelengths of 181 μm, 261 μm, 360 μm and 496 μm, respectively.

Fig. 8
figure 8

M2 factor measurement of the tunable THz gas laser. The measured data are obtained when the operation pressure is optimal

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The output performance of the laser system is summarized in Table 1. With the pump energy of 700 mJ, four THz laser lines are obtained with the energy conversion efficiency between 10–3 and 10–2. The efficiency is comparable to the Raman CH3F laser with a long cavity of 15 m [24], and an improvement in efficiency can be anticipated since the crystal quartz window W2 (with the transmittance of 78%) is used as the output coupler, which may be not optimal for the output energy.

Table 1 The output performance of the tunable THz gas laser
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On the other hand, the generated THz laser wavelength is not related to the pump wavelength. For instance, a long pump wavelength of 10.22 μm corresponds to a relatively short infrared wavelength of 261 μm. Limited by the tuning ability of the pump CO2 laser and the dynamic range of the THz energy meter, other CH3F laser wavelengths are not presented in this paper. However, it can be indicated that efficient THz gas lasers with wavelengths from 180 to 500 μm can be obtained by tuning the pump wavelengths together with the incident angle of the pump laser.

4 Conclusion

In summary, an efficient tunable THz gas laser is demonstrated when an anti-reflection-coated germanium etalon used as the spectrum splitter. Compared to other dichroic elements, the germanium spectrum splitter is more applicable for tuning THz laser wavelengths, due to the good dichroic performance in broad band. With the anticipated reflectance of 73% at 385 μm and transmittance of 95% at 9.26 μm, the germanium spectrum splitter can give comparable insertion loss with the crystal quartz dichroic mirror (transmittance of 75% at 385 μm and reflectance of 92% at 9.26 μm [18]), and this indicates an efficient D2O laser with the wavelength of 385 μm can be obtained. In addition, the germanium spectrum splitter can also replace the germanium mirror in the 151.5 μm NH3 gas laser [17], and the difficulty to precisely manufacture the germanium mirror can be eliminated correspondingly.