- Chinese Optics Letters
- Vol. 19, Issue 2, 021901 (2021)
Abstract
1. Introduction
The mid-infrared (IR) coherent sources in the 3–5 µm range have always been intensively desired for a wide range of scientific and technological applications in remote sensing, spectrum analysis, materials diagnostics, aerospace fields, etc.[
In 2010, the BGSe crystal was synthesized for the first time[
Angle tuning has many advantages, such as wide tuning range and continuously tunable wavelength. However, it is necessary to use a precise rotating table to control the crystal rotation angle, and the pump efficiency decreases as the tunable angles increase due to the crystal’s reflection. Temperature tuning uses a temperature control furnace to change the wavelength of idler light by adjusting the temperature of the crystal. Generally, the tuning range of temperature tuning is smaller than that of angle tuning. But, the mechanical structure is stable, and the idler energy is stable for its vertical incidence, so it can be used on many occasions such as vehicle, ship, and airborne applications. In 2017, Zhai et al. measured the principal axis refractive index , , of BGSe crystal at different temperatures (25°C–150°C) and gave the relationship between the principal thermal refractive index and temperature , , in the wavelength range of [
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2. Methods
2.1 Phase matching curve at room temperature
Up to now, four articles[
The light incident into the BGSe crystal can be represented by (), where is the angle between the light and the Z axis of the crystal, and is the angle between the kOZ plane and X axis of the crystal. Each light has two refractive indices. The larger one is called slow light (), and the smaller one is called fast light . The , can be derived from [
The pump light , signal light , and idler light should be in accord with Eqs. (2) and (3):
The was set to 1064 nm, and the sampling accuracy of was set to 1 nm. Since BGSe is a biaxial crystal, the refractive index is related to (). We investigated the relationship between and , when . Because BGSe has no suitable matching angles under type II-A phase matching condition, we only give the phase matching curves of type I and type II-B, which are shown in Fig. 1.
Figure 1.Phase matching curves of BGSe at room temperature under (a) type I and (b) type II-B conditions (
As shown in Fig. 1, when rises from 0° to 90°, the curves of type I move from right to left, and the upper curves of type II-B move from right to left, too. The changes of the phase matching curve are tiny when is 0°–10° and 80°–90°.
2.2 Phase matching curve at 20°C–140°C
The refractive index of BGSe at 20°C–140°C can be revised from Eqs. (4) and (5) with wavelength 0.901 µm ≤ λ ≤ 10.5910 µm[
Due to the effective nonlinear coefficient being large when under type I and under type II-B[
Figure 2.Phase matching curve at 20°C, 80°C, and 140°C under (a) type I and (b) type II-B conditions (
As shown in Fig. 2, when the temperature is raised from 20°C to 140°C, the curves of type I move from left to right, and the upper curves of type II-B move from left to right, too, so the wavelength of idler light increases as the temperature rises.
2.3 Relationship between
The varied when () changed. The relationship between and when is shown in Fig. 3, and the corresponding wavelength of idler light at is obtained.
Figure 3.Wavelength of idler light at T
As shown in Fig. 3, the are not given when , under type I because they don't meet the phase matching condition. The maximum value of is 6.175 nm/°C when under the type I condition. and decrease monotonically as increases. The maximum value of is 6.60 nm/°C when under the type II-B condition. and decrease monotonically as increases, too. If the unit of changes from nanometers (nm) to , is very close, which is from the minimum to the maximum . of type II-B is larger than that of type I when , while of type I is larger than that of type II-B at other points.
2.4 Peak of
We also give at each () and matching type when the wavelength of the idler light is fixed. The maximum value of and the corresponding (), matching type are listed in Table 1 when the wavelength of the idler light is from 3 µm to 5 µm at 20°C. In addition, the idler light of the corresponding () and matching type at 140°C is reported.
3 | 3.2 | 3.4 | 3.6 | 3.8 | 4 | 4.2 | 4.4 | 4.6 | 4.8 | 5 | ||
---|---|---|---|---|---|---|---|---|---|---|---|---|
I | 59.5 | 58.4 | 57.3 | 56.4 | 55.4 | 54.6 | 53.6 | 52.9 | 52.2 | 51.5 | 50.8 | |
4.1 | 4.5 | 5.3 | 3.4 | 4.3 | 1.8 | 5 | 3.3 | 2 | 1.9 | 3 | ||
3.28 | 3.48 | 3.69 | 3.90 | 4.11 | 4.32 | 4.53 | 4.75 | 4.96 | 5.18 | 5.40 | ||
2.36 | 2.36 | 2.39 | 2.47 | 2.55 | 2.66 | 2.77 | 2.90 | 3.03 | 3.18 | 3.33 | ||
II-B | 89.3 | 77.6 | 72.6 | 68.8 | 66.1 | 63.9 | 61.9 | 60.2 | 58.7 | 57.4 | 56.2 | |
8.5 | 3.4 | 1.9 | 5.9 | 4.4 | 1.3 | 2.6 | 2.4 | 2.5 | 1 | 1.4 | ||
3.12 | 3.33 | 3.55 | 3.76 | 3.97 | 4.19 | 4.40 | 4.62 | 4.84 | 5.05 | 5.27 | ||
0.99 | 1.10 | 1.21 | 1.33 | 1.44 | 1.57 | 1.70 | 1.83 | 1.97 | 2.12 | 2.27 |
Table 1. Peak of
As shown in Table 1, increases monotonically with the increase of . The of type I is larger than that of type II-B at 3–5 µm, and the corresponding to the maximum is near 0°. For example, when is 3.6 µm, the and corresponding (), matching type are shown in Table 2 when are close to 0°.
56.3 | 56.4 | 56.5 | |
5.2 | 3.4 | 0 | |
3895 | 3896 | 3892 | |
2.458 | 2.467 | 2.433 |
Table 2.
As shown in Table 2, of is slightly lower compared to the maximum when . For the convenience of crystal cutting, it is recommended to cut according to . The following is the experimental verification of the peak wavelength of BGSe (56.3°, 0°) at different temperatures under the type I phase matching condition.
3. Experimental Setup
The experimental setup is shown in Fig. 4. The BGSe OPO is pumped by an SL800 Series pulsed Nd:YAG along with 13 ns pulse width (FWHM), 8 mm beam diameter, and 1 Hz pulse repetition frequency. A pinhole is placed behind the Nd:YAG for adjusting the light path, and the beam diameter is compressed to 4 mm through a telescope system to improve the energy density of the pump light.
Figure 4.Schematic diagram of the experimental setup.
Due to M2 being highly reflective of the pump light, in order to prevent feedback to the Nd:YAG laser, a polarizer and a Faraday rotator were placed after the telescope system to form an optical isolator. Since the polarization direction of the laser output was horizontal, the polarization direction of the polarizer was also adjusted to the horizontal direction. The polarization direction of the pump light rotated 45° to the right after passing the isolator. The BGSe crystal was a cuboid with a cutting angle of (56.3°, 0°). Phase matching is satisfied when the pump light is . The polarization direction of is perpendicular to the XOZ plane, that is, parallel to the long side of the crystal at 8 mm. In order to make the BGSe crystal horizontally placed on the temperature control furnace and improve the stability of the equipment, it is necessary to use a 45° phase delay plate to rotate the polarization direction 45° to the left again to make the polarization direction return to the horizontal direction.
M1 is highly transmissive (HT) for the pump (, ) and highly reflective (HR) for the signal (, 1.4–1.6 µm, ). M2 is HR for the pump (, ) and the signal (, 1.4–1.6 µm, ) and HT for the idler (, ). The BGSe crystal was polished but not coated. A copper gripper holds the crystals in a temperature control furnace (HCP TC038-PC) that can tune the temperature up to 200°C, and tunable accuracy is 0.1°C.
A filter and a Ge plate are placed behind M2. The transmittance of the filter is about 1% at 1064 nm and 95%–99% at 3–5 µm. The transmittance of Ge is zero at 1064 nm and about 80% at 3–5 µm.
The idler light was detected by a grating spectrometer (Omni-300λ, Zolix). The peak wavelength of the blazed grating is 3000 nm, the grating line is 300 g/mm, and the minimum resolution is 1 nm. The computer controls the rotation of the grating to make its transmission wavelength tunable from 2000 nm to 6000 nm, and the adjustment accuracy is 1 nm. The DEC-M204-InSb detector and ZAMP amplifier from Zolix detected and amplified the idler light transmitted from the grating spectrometer. And the idler light energy from ZAMP amplifier was measured by a DSOX3054 oscilloscope. When the maximum energy emerges in the oscilloscope, the wavelength set by the grating spectrometer is the peak wavelength of the idler light.
4. Results and Discussion
Before measuring the idler light wavelength of the BGSe OPO, we used the same device that measured the idler light wavelength of (KTA) (90°, 0°) produced from CRYSTECH. The wavelength of the idler light was 3464 nm. In Refs. [17,18], KTA (90°, 0°) comes from the same company. Their measured data were 3467 nm and 3473 nm, respectively, and the deviation was 3 nm and 9 nm, respectively. The results showed the reliability and accuracy of the measuring devices and methods applied in this manuscript.
Then, we used BGSe to replace the KTA. When the pump energy reached 20.62 mJ, the crystal surface was slightly damaged, and the idler energy was 1.07 mJ. So, the peak wavelengths at different temperatures were measured under the pump energy of 15.74 mJ, corresponding to the idler energy of 0.62 mJ.
First, we searched and measured the output of the BGSe (56.3°, 0°) OPO in the range of 3600–3700 nm at room temperature. The oscilloscope triggered at 3611–3619 nm and failed to trigger at other wavelengths. In the experiment, the noise of the InSb detector passing through the ZAMP amplifier was slightly less than the signal of the idler light, and the oscilloscope had no data when the wavelength set by the grating spectrometer was far from the peak wavelength. So, the precise idler light output spectrum cannot be obtained, but the peak wavelength can still be measured.
As shown in Fig. 5, the output energy of BGSe (56.3°, 0°) at 3614 nm is the largest, so the peak wavelength of BGSe (56.3°, 0°) at room temperature is located at 3614 nm, which is 23 nm smaller than the theoretical value of 3637 nm. Then, we used the temperature control furnace to adjust the temperature of the BGSe crystal, recorded the peak wavelength of its idler light output at 30°C–140°C, and compared it with the theoretical value given in Refs. [8,9].
Figure 5.Output at 3611–3619 nm of BGSe (56.3°, 0°).
Theoretical and experimental values are presented in Fig. 6. When the temperature of BGSe (56.3°, 0°) increased from 30°C to 140°C, the wavelength of idler frequency light under type I phase matching increased from 3637 nm to 3989 nm. The experimental values of the peak wavelength are close to the theoretical value from Ref. [9], while is close to the theoretical value from Ref. [8]. is 2.49 nm/°C from Ref. [9], 3.36 nm/°C from Ref. [8], and 3.20 nm/°C from the experiment. It might be due to the experimental wavelength in Ref. [9] being wider than in Ref. [8], so the prediction of the idler wavelength is more precise. While are measured directly in Ref. [8] rather than changed by SHG in Ref. [9], from Ref. [8] is closer to the experimental value from this manuscript.
Figure 6.Idler light’s peak wavelength of BGSe (56.3°, 0°) at 30°C–140°C.
5. Conclusions
BGSe possesses a wide temperature tuning range. corresponding to the maximum is close to 0° when , and of type I is larger than that of type II-B. The increases as increases. When , the maximum ; while , the maximum . According to our experimental results, the wavelength of the idler light derived from Ref. [9] is more precise, while derived from Ref. [8] is more precise. To the best of our knowledge, the temperature tuning of the BGSe OPO was demonstrated for the first time. The peak wavelength of the idler light is 3637 nm at 30°C and 3989 nm at 140°C, corresponding to the of 3.20 nm/°C. Our results indicate that BGSe possesses advantages for application in wide band temperature tuning.
References
[15] W.-Q. Zhang. General ray-tracing formulas for crystal. Appl. Opt., 31, 7328(1992).
[17] F. Bai. The studies of novel solid-state lasers based on optical parametric oscillation and stimulated Raman scattering(2013).
[18] X. Dong. All-solid-state KTA optical parametric oscillation(2009).
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