A Narrow-Linewidth Optical Parametric Oscillator Inserted with Fabry–Perot Etalon

Abstract

Nowadays, the Fabry–Perot etalon (F–P) has been widely utilized in the optical parametric oscillator (OPO) to improve the filtering performance. In this paper, we reported an F–P etalon composed of two ultra-thin silicon wafers spaced with the air. The linewidth of the signal laser and the threshold are 0.03 nm and 0.6 W, respectively when the proposed etalon is employed to a OPO system based on the MgO-doped LiNbO₃ (MgO: PPLN). A stabilized output at 1492.4 nm is obtained, and a tunable, high-precision filtering performance can be achieved by varying the gap distance of the F–P etalon arbitrarily due to its ultra-thin thickness. In addition, the F–P etalon can work on a very wide bandwidth due to its weak absorption during the infrared and terahertz waveband. The high-precision tuning capability and wide-band function of proposed etalon may benefit many applications, including spectroscopy, filtering, and optical communication.

1. Introduction

Optical parametric oscillators (OPO) have been popular for decades due to their superior optical properties such as continuous tuning capability in a wide waveband, narrow linewidth, and high power [1,2,3,4,5,6]. The OPO has been one of the most promising techniques in producing wavelength tunable coherent radiation for various applications. However, the linewidth has become the priority to judge the properties of the OPO. The narrower the linewidth is, the higher the monochromatic energy density will be, which leads to a better coherence [7,8,9]. A narrow linewidth, widely tunable infrared source is of great appeal due to its potential use in molecular detection systems that are sensitive and specific. To work on these applications, a spectral narrowing mechanism is usually introduced to the oscillation system. Several mechanisms have been studied to achieve this purpose.

The most common methods to narrow the linewidth are to introduce the structures such as dispersion prism [10,11], diffraction grating [12], and Fabry–Perot (F–P) etalon [13,14,15] into the OPO system. Although a dispersion prism and the diffraction grating can both narrow the linewidth, the architecture is relatively complex [16,17,18,19,20]. Moreover, the techniques such as injection-seeded and dual-cavity doubly resonant can also be utilized to narrow the linewidth. The techniques of injection seed suffer from the major drawback that the available seed sources generally have limited or no tuning capability. Therefore, that effective injection seeding and narrowband operation of the OPO can be achieved only at a fixed wavelength or over a strictly confined spectral range. As for the techniques of dual-cavity doubly resonant, the singly resonant oscillator has a better beam quality, larger relative delay between the OPO output and pump, and smaller internal peak fluence [21,22]. However, the F–P etalon possesses several advantages such as simple structure, short cavity length, more flexibly tuning capability, and excellent filtering performance compared with other methods, especially in narrowing linewidth [23,24,25,26,27]. Therefore, the F–P etalon has been proposed by many experts for the purpose of intracavity filtering, but the filtering performance still has the potential to be optimized [28,29,30,31]. Thus, the research about the F–P etalon, aiming at narrowing the linewidth of output signal in the OPO cavity, is of significance [32,33].

In this paper, we report an F–P etalon composed of two ultra-thin silicon wafers spaced with the air; when it is employed to a OPO system based on MgO-doped LiNbO₃ (MgO: PPLN), the linewidth of the signal laser is merely 0.03 nm, the threshold is 0.6 W, and a stabilized output at 1492.4 nm is obtained. The conversion efficiency is about 1.9%, and it may function on a very wide bandwidth due to the weak absorption of the silicon during the infrared and terahertz waveband. In addition, a tunable, high-precision filtering performance can be achieved through varying the gap distance of the F–P etalon arbitrarily and conveniently due to its ultra-thin thickness. Compared with other methods, although the performance of proposed F–P etalon may not ideal (limited linewidth and low conversion efficiency), its high-precision tuning capability and wide-band function may benefit many applications, including spectroscopy, filtering, and optical communication.

2. The Fabrication and Optical Properties of the Ultra-Thin Silicon Wafers

Nowadays, the F–P etalon has been widely utilized in the OPO system to improve the quality of the output signal laser; there are many optical materials which can be used such as MgF₂, CaF₂, and Quartz, but silicon holds the advantages of weak absorption in a wide-band and easiness to be fabricated into a thin thickness, which contributes to a high-precision tuning capability. Herein, we designed an F–P etalon consisting of two ultra-thin silicon wafers to improve the performance of OPO. The process of fabricating the ultra-thin silicon wafer contains two main steps: (1) The underlying silicon and the oxide layer of the silicon-on-insulate (SOI) wafer is etched by the inductively coupled plasma (ICP) method; (2) The remaining oxide layer is removed using the wet etched technology to ensure the fabricated silicon wafer uniformly and smoothly. The micrograph of the fabricated ultra-thin wafer is depicted in Figure 1a; it shows that the wafer is smooth and ultra-thin. Figure 1b shows the thickness of the wafer to be about 15.8 μm. The quality of silicon wafers can be optimized in fabrication processes by selecting a proper parameter in experiments, which will influence the transmission spectrum of an F–P etalon and ultimately the quality of output signal beam.

The transmission spectra of two ultra-thin silicon wafers are recorded in Figure 2 separately using an optical testing system (Agilent 8614A, point for the experiment and solid line for the fitting curve). The full width at half maximum (FWHM) of the reflection peak is about 9.8 nm at the wavelength of 1.55 μm, and the ratio of the free spectral range (FSR) is about 22.5 nm. The narrow width of FWHM laid a foundation for the fabricated silicon wafer to improve the filtering performance in OPO system.

An F–P etalon was designed using above fabricated silicon wafers spaced with the air. The schematic diagram of the F–P etalon is showed in Figure 3; n is the refractive index of the air, L represents the gap between two dual-thin silicon wafers, and R denotes the reflective of the wafer. The silicon can be adjusted arbitrarily due to its ultra-thin width, which will provide a high-precision tuning capability. In addition, the distance of the gap can be conveniently controlled. Thus, the quality of the output signal laser can be improved greatly by selecting a proper parameter.

The formula of the transmission can be expressed as:

$$T=\frac{1}{1+\frac{4R}{{(1-R)}^2}{\sin }^2\frac{2\pi nL}{\lambda }}$$

(1)

Assuming the output signal intensity without the F–P etalon is I₀, the light intensity of each point on the spectrum is I = I₀ T(λ) after inserting the F–P etalon, it can be inferred from Equation (1) that the transmission spectrum of the proposed F–P etalon depends on the air gap between two wafers to a great extent. Thus, different filtering performances can be realized by varying the distance of the gap. However, more transmittance at the peak is desirable, while less transmittance near the peak is needed in order to narrow the linewidth effectively. In other words, the F–P etalon with a narrower FWHM will have a better filtering performance. Furthermore, the proposed F–P etalon may function on a very wide bandwidth due to the weak absorption of the silicon during the infrared and terahertz waveband.

The transmission spectra of the F–P etalon with a gap distance of 174.95 μm, 295.95 μm, and 584.38 μm are shown in Figure 4a–c, respectively (point for the experiment and solid line for the fitting curve). Comparing Figure 2 with Figure 4, it can be inferred that the resolution of the F–P etalon is better than the monolithic silicon within the interference reflection peak. The maximum ratio of the free spectral range is increased to 92 nm, leading to a higher quality of intracavity filtering performance than monolithic silicon, which will improve the quality of signal laser and have a potential application in the field of filtering, spectroscopy, and optical communication.

3. Experiment and Discussion

The schematic diagram of the OPO system is presented in Figure 5. The crystal we used is a periodic polarized lithium niobate crystal doped with magnesium oxide (MgO: PPLN, procured from CRYSTECH, China). The polarization period, the aperture, and the crystal length of the PPLN crystal are 29.64 μm, 3 mm × 5 mm, and 50 mm, respectively. It is a type of single-resonant parametric oscillator. The pump source is a Q-switched Nd: YAG laser with a wavelength, pulse duration, and repetition rate of 1064 nm, 36 ns, and 5 kHz, respectively. To optimize the quality of the pump beam and improve the efficiency, the pump pulse is converged into the MgO: PPLN through a lens with a focal length of 20 cm. The temperature is set by an oven (CORP.TC038-PC, HC PHOTONICS); it can realize a range of 30 to 200 °C, and the corresponding output signal laser is 1483 to 1527 nm. A length of 8 cm OPO cavity is employed, which consists of a back-cavity mirror, OM, and an output mirror, OC. The reflector, DM, has a reflectance larger than 99.5% at the wavelength of 1.5 μm while a transmittance larger than 95% at the wavelength of 1064 nm, which can separate the pump laser from the signal laser effectively.

Figure 6 illustrates the relationship between input pump power and output signal power with the F–P etalon inserted and not inserted, respectively (the air gap distance is about 2 mm). The oscillation threshold is about 0.60 W when the F–P etalon is inserted, and the conversion efficiency is decreased due to the extra intracavity loss of the inserted F–P etalon (such as the surface reflection of the silicon for the signal laser). For example, its conversion efficiency has decreased from 7.4% to 1.9% at the pump power of 1.83 W. However, it is notable if we can obtain an ultra-narrow linewidth and a better beam quality at the cost of conversion efficiency as these properties are the primal purposes of designing this type of F–P etalon. The conversion efficiency can be further optimized by improving the quality of the silicon wafers during fabrication processes and selecting a proper parameter in experiment.

Simultaneously, the output spectrum of the signal laser with the pump power of 1.83 W is plotted in Figure 7 when the proposed F–P etalon is absent. The spectrum is analyzed by the spectrograph (bought from Shenzhen Zeruiguang company of China, Anristu MS9740A) with a resolution of 0.03 nm, and the spectrum width of the signal laser is about 0.5 nm. It can also be seen that the beam quality has great potential to be improved.

The output spectrum of the signal laser with the F–P etalon inserted and the simulated transmission spectrum of the F–P etalon are illustrated in Figure 8. It shows that the output of the signal laser has a good beam quality and stability; more importantly, the peak fits well with the simulated results. The spectrum is presented in Figure 8a when the air gap is 4 μm. The output signal peaks are located at the wavelength of 1491.8 nm, 1492.4 nm, and 1492.5 nm, and it is oscillating at the wavelengths of the interfering peak of the F–P etalon. However, when the gap distance of the etalon is decreased to 2 mm, as shown in Figure 8b, the signal laser only appeared at the wavelength of 1492.4 nm, and the linewidth is 0.03 nm. As the 0.03 nm is the maximum resolution of the spectrum analyzer, the actual linewidth of the output signal may be less than 0.03 nm. Compared with Figure 8a,b, it can be seen that the filtering performance can be conveniently controlled by the distance of two wafers, and it will have a high-precision tuning capacity due to the ultra-thin width of the wafers. When comparing Figure 7 with Figure 8, the characteristic of proposed F–P etalon in narrowing linewidth is obvious, and the linewidth has been narrowed from 0.5 nm to less than 0.03 nm. It can be inferred from the output signal that the inserted F–P etalon can achieve a strong linewidth narrowing effect due to the wavelength selection of multiple roundtrips in the oscillation. While forming the F–P etalon using two silicon wafers, this architecture is forming three etalons, two thin wafer etalons, and one air-spaced etalon. As the air-spaced etalon has a larger thickness compared to the wafer etalons, the FSR of the whole system will be the FSR of the thin etalon while placing the F–P etalon inside the cavity. In fact, the OPO cavity has a particular FSR based on its cavity length. For the single-frequency operation of the OPO, a thin etalon with a large FSR, is used to eliminate the mode hopping. In this case, the FSR of the OPO is determined by the FSR of the etalon. Due to the larger FSR, the etalon can suppress the closely spaced wavelengths of the high gain-bandwidth OPO, which leads to a good filtering performance. In addition, a tunable, high-precision signal laser output can be achieved through varying the gap distance of the F–P etalon arbitrarily and conveniently due to its ultra-thin width. Thus, the proposed F–P etalon may have a promising application in the intracavity filtering of OPO cavity.

4. Conclusions

In conclusion, we have reported a Fabry–Perot etalon composed of two ultra-thin silicon wafers spaced with air. When it is employed to a OPO system based on MgO-doped LiNbO₃ (MgO: PPLN), the linewidth of the signal laser, threshold, and conversion efficiency are 0.03 nm, 0.6 W, and the 1.9%, respectively. It may function on a very wide bandwidth due to the weak absorption of the silicon during the infrared and terahertz waveband. In addition, a tunable, high-precision signal laser output can be achieved through varying the gap distance of the F–P etalon arbitrarily and conveniently, which may have promising applications in the field of spectroscopy, filtering, and optical communication.