High-Resolution Laser in Widely Tunable Range (4.16–7.13 µm) Based on Mid-Infrared BaGa₄Se₇ Optical Parametric Oscillator Pumped at 1064 nm

Abstract

A widely tunable 4.16–7.31 μm high-resolution mid-infrared optical parametric oscillator with a type-I BaGa₄Se₇ (BGSe) crystal pumped by a 1064 nm laser was demonstrated for the first time. We have achieved a wavelength tuning accuracy of less than 5 nm through simultaneously performing temperature tuning and angular tuning on the BGSe (46°, 0°) Optical Parametric Oscillation (OPO). The wavelength resolution will be less than 1 nm when using commercial temperature control equipment.

1. Introduction

Widely tunable and high-resolution mid-infrared lasers have a wide range of applications in environmental monitoring, laser spectroscopy, infrared optoelectronic countermeasures, laser active imaging radar, remote sensing, and so on [1,2,3,4,5]. In laser spectroscopy, for example, high-resolution infrared lasers are more conducive to recognizing the spectroscopic features of molecules. As a nonlinear frequency conversion technique, Optical Parametric Oscillation (OPO) has the advantages of all-solid-state design, miniaturization, high efficiency, high beam quality, and widely tunable output wavelength. The nature of its core component, nonlinear crystal, determines the quality of the output laser. In 2010, a new type of infrared BGSe crystal was synthesized for the first time by Jiyong Yao [6]. It has a wide transmission range of 0.48–18 μm, a high laser damage threshold of 557 MW/cm², and a large nonlinear coefficient (d₁₁ = 24.3 pm/V), making it well suited for studies of wide-tunable-range laser generation. BGSe is also a temperature-tunable crystal with excellent performance. Simultaneous temperature tuning and angular tuning with BGSe OPO at 1.064 μm laser pumping can obtain high-resolution mid-infrared laser output over a widely tunable range.

In 2016, Kostyukova et al. [7] obtained idler optical output in the ultra-wide spectrum of 2.7–17 μm using BGSe OPO. The phase matching angle of the BGSe crystal was adjusted from 40° to 61° to obtain the average rate of change of wavelength with angle Δλ/Δθ = 680 nm/°. In 2017, Xu et al. [8] obtained tunable lasing in the 3.12–5.16 μm spectrum and achieved a high pulse energy output at 4.11 μm (2.56 mJ output when the pump light energy was 61.6 mJ), obtaining a Δλ/Δθ of about 212 nm/° (with a phase-matching angle of BGSe crystal tuned from 50.48° to 60.1°). In 2018, Zhao et al. [9] achieved a tunable laser output of 8–9 μm using a 1 KHz Acousto-Optical Q-tuned Ho: YAG laser to pump BGSe OPO. The phase-matching angle of its BGSe crystal was adjusted from 4.58° to 7.64°, yielding a Δλ/Δθ of about 327 nm/°. In 2019, Kang et al. [10] achieved a very high photo-optical conversion efficiency of 7.6% at 4.25 μm using a 1064 nm laser-pumped BGSe OPO. In 2020, Zhang et al. [11] used a high-precision motorized continuously rotating platform to change the rotation angle of a BGSe crystal to achieve the output of electrically tunable 3.0–6.7 μm lasers, obtaining a Δλ/Δθ of about 294 nm/° (with the phase-matching angle of BGSe crystal adjusted from 46.5° to 59.1°). In 2021, Xu et al. [12] realized a laser output with a wide tunable range of 8.24–13.3 μm using a low-threshold and high-output DP-SRO structure. The average Δλ/Δθ was 1581 nm/° while the angle of BGSe was adjusted from 41.4° to 44.6°. In 2022, He et al. [13] achieved the output of a 250 mW high-average-power 3.42–4.73 mm mid- and mid-infrared laser using a 1064 nm Nd: YAG to pump BGSe OPO (DP-SRO structure). The phase matching angle of its BGSe crystal was adjusted from 51.7° to 56°, yielding a Δλ/Δθ of about 305 nm/°. We can observe that the angle tuning is wide but not high resolution. Therefore, one degree of the angle change results in a spectral resolution of over 100 nm at an output wavelength above 100 nm, even close to micrometer level. In 2021, Kong et al. [14] achieved the first temperature tuning of BGSe OPO, obtaining the wavelength–temperature rate of change in the range of 3–5 μm for idler light. When the temperature of BGSe (56.3°, 0°) increases from 30 °C to 140 °C, the wavelength varies from 3637 nm to 3989 nm, and the wavelength–temperature rate of change is 3.2 nm/°C. In 2023, Yang et al. [15] performed temperature tuning of BGSe OPO under non-critical phase-matching conditions from 5 to 45 °C and wavelengths from 9393.3 to 10627.4 nm with a wavelength–temperature change rate of 30.85 nm/°C. It is shown that the output wavelength varies with the ambient temperature, but this variation is small compared to the angular tuning. For example, Refs [14,15] demonstrate cases in which the output wavelength increased by 3.2 and 30.85 nm when the ambient temperature changed by 1 °C. Therefore, temperature tuning can achieve a finer spectral resolution than that of angular tuning. The results of previous experiments are shown in

Table 1. Previous results.

Years	Authors	Tuning Method	Tuning Range (μm)	Tuning Slope (nm/° or nm/°C)
2016	Kostyukova et al. [7]	angular tuning	2.70–17.00	680
2017	Xu et al. [8]	angular tuning	3.12–5.16	212
2018	Zhao et al. [9]	angular tuning	8.00–9.00	327
2020	Zhang et al. [11]	angular tuning	3.00–6.70	294
2021	Xu et al. [12]	angular tuning	8.24–13.30	1581
2021	Kong et al. [14]	temperature tuning	3.00–5.00	3.2
2022	He et al. [13]	angular tuning	3.42–4.73	305
2023	Yang et al. [15]	temperature tuning	9.39–10.63	31.0

.

Angular tuning enables a wide tuning range but not high resolution, and temperature tuning enables high resolution but a limited tuning range. In this manuscript, we propose to realize a stabilized laser output with high resolution in a wide band of 4.16–7.31 μm based on BGSe OPO with simultaneous temperature and angle tuning.

Naturally, to realize a wide-range and high-resolution tunable laser source simultaneously, we installed the BGSe crystal in a temperature-controlled furnace, and the furnace was placed on a motorized rotating platform, thus fulfilling temperature and angle tuning simultaneously. So, a stable laser output with widely tunable (4.16–7.31 µm) and high-resolution (less than 5 nm) spectral characteristics was realized for the first time.

2. Experimental Setup

The experimental setup is shown in Figure 1.

The pump source was a pulsed laser Nd: YAG of the SL800 series with a pulse width of 13 ns, a beam diameter of 8 mm, and a pulse repetition frequency of 1 Hz. To make the telescope system completely perpendicular to the pump light, use pinhole D1 to observe the reflected light of the telescope system (T) on the pump light. When both the incident pump light and the reflected pump light pass through D1, the T is completely perpendicular to the pump light. Pinhole D2 is designed to make the OPO system completely perpendicular to the pump light. The incident light is directed through the pinhole, and the reflected light is returned through the pinhole so that the incident light is perpendicular to the M1 and M2 mirrors. T is a telescope system that changes the beam size of the pump source from 4 mm to 2 mm. A 90° optical rotator R was placed before the OPO cavity. The initial state of the polarization direction of the pump light was horizontally polarized, which entered the OPO cavity with vertical polarization after passing through the 90° optical rotator R. The M1 and M2 cavity mirrors at both ends of the OPO cavity have high transmittance for pump light, high reflectivity for signal light and high transmittance for idler light. BGSe (46°, 0°) crystals are polished and coated for high transmittance of signal, pump, and idler light. An electric rotator (Zolix TBR100, Zolix, Beijing, China) was placed between the M1 and M2 cavity mirror, on which a customized temperature-controlled furnace (maximum temperature: 180 °C; temperature resolution: 1 °C) was placed. The temperature resolution of this furnace failed to match that of general commercial temperature-controlled furnaces (e.g., HCP TC038-PC, temperature resolution: 0.1 °C). However, it had the advantage of being shorter in the through-illumination direction (34 mm). The OPO cavity was fabricated using 3D printing technology. The final OPO cavity length was 60 mm. The BGSe crystal was placed inside the temperature-controlled furnace and fixed with a specific fixture.

F is a filter that can filter out the residual pump and signal light output from the OPO cavity mirror. G is a germanium sheet that completely filters out the residual pump and signal light and absorbs the laser light below 1 μm.

A grating spectrometer (Omni300λ, Zolix) was placed behind the germanium sheet G to spectralize the idler light, and three gratings were controlled using a computer with transmittance ranges of 1000–2400 nm, 2000–4800 nm, and 4800–16,000 nm, respectively, with a regulation accuracy of 1 nm.

The Vigo PCI-10.6 detects the energy of spectrally separated idler light. When connected to a DSOX3054 oscilloscope, the energy of idler light at a specific wavelength can be obtained. When the energy measured by the oscilloscope is at its maximum, the wavelength transmitted by the grating spectrometer approximates the peak wavelength of the idler light.

As shown in Figure 1, 90° optical rotator R changes the horizontally polarized pump light to be vertically polarized. In order to satisfy the phase matching condition, the pump light should be an e₂ light, meaning that the polarization direction of the pump light is perpendicular to the XOZ plane of the crystal. To avoid BGSe slipping, as shown in Figure 2, we made some improvements, using the 90° optical rotator R to change the direction of polarization of light. At this time, the XOZ plane of the crystal is parallel to the ground, and changing the angle between the incident light and the Z-axis can be accomplished through rotating the Z-axis of the crystal only horizontally, which can achieve a wide range of tuning and keep the stability of the crystal. Without the rotator R, the Z-axis of the crystal would be perpendicular to the ground, resulting in the possibility of the crystal sliding downward during angular tuning.

3. Results and Discussion

3.1. Conversion Efficiency at 30 °C and Different Incidence Angles

The output wavelength will change with the change in ambient temperature [14]. In order to stabilize the output characteristics of BGSe OPO, we set a specific temperature with a temperature-controlled furnace. We carried out angular tuning to study the output characteristics of BGSe OPO using a temperature-controlled furnace.

The general length of the common temperature-controlled furnace is 90 mm. Therefore, the OPO cavity length is relatively long. Under these conditions, high pump energy is required to have a measurable output of idler light. The higher pump light will damage the film layer of BGSe crystals, so we customized a small temperature-controlled furnace and shortened the OPO cavity length to 60 mm.

The output energy of the BGSe (44.4–53.2°) OPO with an input pump light energy of 520 V (36 mJ), shown in Figure 3, is obtained through setting the temperature of the temperature-controlled furnace to 30 °C.

As shown in Figure 3, the black solid dots represent the experimental data, and the blue straight lines are obtained via polynomial fitting. The vertical coordinate is the energy of the filtered idler light measured with an energy meter. According to the law of refraction, when the outside of the crystal is rotated by 1°, the change in the angle of incidence inside the crystal is about 0.4° (${\mathrm{n}}_{\mathrm{B}\mathrm{G}\mathrm{S}\mathrm{e}}\approx 2.5$), and the change of the angle of incidence inside the solid crystal is 44.4–53.2°, corresponding to the angle of rotation of the outside of the crystal being −4–18°, at which the angle of the BGSe at positive incidence is 46°. At an angle of incidence of 44.4°, the energy is too small to be detected by the energy detector meter, but it does not affect the following conclusion. The input pump energy remains constant, and the output energy increases with the increase in the incidence angle outside the crystal, obtaining a higher energy conversion efficiency with a larger incidence angle outside the crystal. A possible reason is that as θ increases, the output wavelength decreases, the photon energy increases, and therefore the output energy increases. Some of the experimental data did not strictly conform to the increasing trend, probably because of the inhomogeneous mass within the crystal.

The energy of the input pump light remains constant at 36 mJ, and the energy of the output idler light increases from 37.6 µJ to 144.5 µJ with the increase in the angle, corresponding to the increase in the conversion efficiency from 0.104% to 0.401%. The slope of the fitted straight line is 14.37, which means that for every 1° decrease in angle within a certain range, the output energy decreases by 14.37 µJ, and the conversion efficiency decreases by 0.04%.

3.2. Output Spectra at 30 °C and Different Incidence Angles

The output spectrum of the BGSe (46°, 0°) OPO at 1064 nm pump, type I phase-matched conditions with the temperature-controlled furnace set at 30 °C is shown in Figure 4.

As shown in Figure 4, the solid black dots represent the average output voltage recorded by the oscilloscope, and the red curve is fitted using Gaussian fitting. The black horizontal lines above and below each experimental data point are the maximum and minimum values of the output voltage recorded by the oscilloscope, respectively. The vertical coordinates are obtained via oscilloscope. They represent the relative light intensity, expressed in mV. From Figure 4, we can see that the change in the maximum and minimum values of the output voltage maintains the same trend with the change in the average value; both of them are increasing first and then decreasing. We found in our experiments that the spectrum of idle frequency light output from the OPO is not completely symmetric, and generally there are more wavelength points on the left side and fewer wavelength points on the right side. Assuming the number of pump light photons is certain, the number of idle frequency light photons obtained is determined, and for the smaller wavelength light, the larger the photon energy is, which is more likely to make the mid-infrared detector respond. Therefore, there will be a slow rise on the left (more points) and a rapid decline on the right side (fewer points). The average voltage recorded by the oscilloscope reaches a maximum when the transmission range of the grating spectrometer reaches 6820–6821 nm, which is 232 mV and 233.86 mV, respectively. The oscilloscope can display the voltage value when the grating spectrometer’s light transmission range is 6810–6829 nm. When the transmission range of the grating spectrometer is not between 6810–6829 nm, the oscilloscope cannot display the voltage value. The maximum, minimum, and averages of the voltages at individual wavelengths were recorded. The Gaussian curve equation obtained from fitting is as follows:

$$y=209\times \mathit{exp}{\left(-\frac{x-6820}{7.578}\right)}^2$$

The output linewidth obtained is $2\sqrt{ln2}\times 7.578=12.62 \mathrm{n}\mathrm{m}$.

When controlling the motorized rotary stage for −1°, 0°, and +1° angle adjustments, respectively, external incidence angles of 89°, 90°, and 91° were obtained (the internal incidence angles of the crystal were 45.6°, 46°, and 46.4°). Their output spectrograms are shown in Figure 5.

The star-shaped dots of different colors are the experimental data measured at the three angles. In contrast, color curves 1–3 were obtained through fitting the experimental data with Gaussian fitting. Color curve 1 is the Gaussian curve fitted at an incident angle of −1° outside the crystal, and color curves 2 and 3 correspond to the case when the incident angle outside the crystal increases.

According to the formula of Gaussian curve line width,

Table 2. Output wavelength and linewidth of BGSe OPO at different angles.

No. of Colored Lines	Peak Wavelength (nm)	$\mathbf{External} \mathbf{Rotation} \mathbf{Angle} (°$)	$\mathbf{Internal} \mathbf{Variation} (°$)	Linewidth (nm)
1	6600	+1	46.4	8.37
2	6821	0	46	12.62
3	7051	−1	45.6	13.86

was obtained.

Table 2 and Figure 5 show that the peak wavelength becomes smaller and the line width becomes narrower as the angle increases. We can also notice that the output voltage at the peak wavelength becomes smaller as the angle increases. So, when the angle increases to a certain value, the linewidth and output voltage become very small, and it is more difficult to capture the spectral features at that angle. During the experiment, we obtained the spectral information at large angles through adjusting the trigger level of the oscilloscope downward. So, we can obtain as wide a tuning range as possible.

3.3. BGSe (44.4–53.2°) Output Wavelength at 30–70 °C

We rotated the angle of the crystal from 86° to 108° (corresponding to an internal incidence angle θ from 44.4° to 53.2°) and tuned the temperature from 30 °C to 70 °C. When the temperature is too high, thermal effects may damage the crystal when a large amount of heat is generated inside the crystal through laser pumping, so we only tuned the temperature to 70 °C to ensure that continuous coverage of a wide range of output wavelengths can be achieved via temperature tuning during angle tuning. The theoretical and experimental results are shown in Figure 6.

As shown in Figure 6, the red dashed line and the blue solid line are the curves at 30 °C and 70 °C obtained according to the theory of Kato [16] and H. Kong [14], with the vertical coordinate representing the wavelength of the output idler light and the horizontal coordinate representing the angle of incidence inside the crystal. The theoretical output wavelength is generally larger than the measured wavelength, but the slope trend is the same. Differences between theoretical and experimental values may be due to inaccuracies in the Sellmeier equation. We can obtain the relationship between output wavelength and crystal angle and temperature from the experimental data as shown in

Table 3. Crystal angle of BGSe crystal at 4.16–7.31µm laser output.

No. of Angle Points	$\mathbf{Angle} \mathsf{\theta }$ of BGSe (°)	Output Wavelength (nm)		Temperature Tuning Slope (nm/°C)	$\mathbf{Difference} \mathbf{between} \mathsf{\theta }$	Difference between Rotation Angle and 90° (°)
		30 °C	70 °C
1	44.4	7852	---	---	−1.6	−4
2	44.8	7572	---	---	−1.2	−3
3	45.2	7306	7703	9.93	−0.8	−2
4	45.6	7058	7425	9.18	−0.4	−1
5	46.0	6829	7165	8.40	0	0
6	46.4	6600	6926	8.15	0.4	1
7	46.8	6402	6701	7.48	0.8	2
8	47.2	6198	6493	7.38	1.2	3
9	47.6	6012	6304	7.30	1.6	4
10	48.0	5842	6116	6.85	2.0	5
11	48.4	5674	5935	6.53	2.4	6
12	48.8	5512	5776	6.60	2.8	7
13	49.2	5360	5610	6.25	3.2	8
14	49.6	5214	5456	6.05	3.6	9
15	50.0	5076	5303	5.68	4.0	10
16	50.4	4946	5160	5.35	4.4	11
17	50.8	4830	5035	5.13	4.8	12
18	51.2	4708	4908	5.00	5.2	13
19	51.6	4590	4798	5.20	5.6	14
20	52.0	4478	4675	4.93	6.0	15
21	52.4	4369	4559	4.75	6.4	16
22	52.8	4264	4451	4.68	6.8	17
23	53.2	4162	4342	4.50	7.2	18

.

As shown in Table 3, the maximum output wavelength at each angular point is greater than the minimum output wavelength at the previous angular point, and thus, continuous 4.16–7.31 µm wavelength tuning can be realized. This is the first time we have used detailed data to show that it is feasible to utilize both temperature tuning and angular tuning to obtain continuously tunable (4.16–7.31 µm) and higher resolution (less than 5 nm) laser output. Since the output idle light energy becomes smaller with wavelength longer, it is also groundbreaking to be able to validate and realize continuous tunability with high resolution in the mid-infrared wavelengths. The output wavelengths at 44.4° and 44.8° at 70 °C could not be measured experimentally. The possible reasons are as follows: (1) The relatively long output wavelength caused the PCI-10.6 detector to fail to detect the weak idler light signal (below 40 µJ). (2) The output wavelength at this time has a lower transmittance in the atmosphere. Owing to the pumping energy at this time, already up to 36 mJ, in order to protect the BGSe crystals, we did not continue to increase the voltage. In addition, we can see from the table that the slope of the wavelength with temperature tuning is 4.50–9.93 nm/°C and increases with the increase in the output idler light wavelength. When the temperature tuning accuracy is 0.1 °C, the output wavelength resolution will be 0.45–0.99 nm.

3.4. Temperature Tuning at a Fixed Angle (θ = 46°)

As shown in Figure 7, in order to verify the resolution of the output wavelength with the temperature given in Table 3, we measured the output wavelength for positive incidence conditions (θ = 46°), controlling the crystal temperature from 30 °C to 35 °C and with a step of 1 °C in the temperature rise.

The experimental data indicate that when the temperature increases from 30 °C to 35 °C in steps of 1 °C, the output wavelength of the idler light increases from 4202 nm to 4217 nm. The output wavelength increases by 2–4 nm for every 1 °C of tuning, meaning that the actual resolution of wavelength tuning reaches 2–4 nm. Moreover, the temperature tuning slope increases with increasing temperature. For example, when the temperature increases from 30 °C to 31 °C, the output wavelength increases by 2 nm, and when the temperature increases from 34 °C to 35 °C, the output wavelength increases by 4 nm.

From Refs [7,8,9,10,11,12,13], we can see that the wavelength resolution of angular tuning is above 100 nm when the angle is changed by 1°, while the wavelength resolution of temperature tuning is around 3 nm when the temperature is changed by 1 °C. The resolution of temperature tuning is two orders of magnitude higher than that of angle tuning.

The wavelength resolution can be increased even more dramatically when using a more accurate commercial temperature-controlled furnace. When the temperature tuning accuracy is 0.1 °C, the resolution of the temperature tuning is theoretically increased to within 1 nm, which is the highest resolution in the mid-infrared range to the best of our knowledge.

4. Conclusions

Through simultaneously tuning the BGSe OPO for temperature and angle, we have obtained, for the first time, a wavelength output in the mid-infrared spectrum with a wide tunable range of 4.16–7.31 µm and high precision with a wavelength resolution of better than 5 nm (the resolution of a temperature-controlled oven is 1 °C), which will be better than 1 nm when using a commercially available temperature-controlled device. We also analyzed the spectral characteristics of the output wavelength at specific temperatures. When the output wavelength is tuned from 6600 nm to 7051 nm, the conversion efficiency decreases from 0.231% to 0.144% and the output linewidth increases from 8.37 nm to 13.86 nm. Adding a temperature-controlled furnace improves the tuning accuracy and stabilizes the temperature of the BGSe crystal to ensure the stability of the BGSe OPO output wavelength.

The range and step of angular tuning (from 44.4° to 53.2° in 0.4° steps) and temperature tuning (from 30 °C to 35 °C in 1 °C steps) have been demonstrated in the experimental data. We have successfully realized the wavelength output with high precision (better than 5 nm) in the continuous tunable range of 4.16–7.31 µm, which is a good reference for BGSe OPO technology and is beneficial to the development of mid-infrared laser generation.