
- Photonics Research
- Vol. 9, Issue 4, 637 (2021)
Abstract
1. INTRODUCTION
Brillouin spectroscopy based on the pump-probe technique can provide the intrinsic information of the medium, which has been widely used in sensing scenarios such as local temperature and strain monitoring [1,2], biomedical diagnostics [3,4], imaging [5], and material science [6]. The core of this technique is to determine the Brillouin spectrum with high efficiency, or rather the Stokes frequency shift of the medium, by acquiring the power gain/loss of the probe versus frequency scanning [7–9], as shown in Fig. 1(a). This pump-probe method is easier to operate in Raman spectroscopy, but it is still a challenge for present commercial photoelectric equipment for Brillouin spectroscopy [10]. More specifically, in contrast to the Raman spectrum, the frequency region of the Brillouin spectrum usually is several MHz to GHz, depending on the sample to be measured [11]. Consequently, it is difficult to separate the pump and probe using a frequency sorter (e.g., a dichroic mirror). Using a polarization sorter, such as a polarized beam splitter (PBS), is another common way to achieve a collinear pump-probe scheme, as shown in Fig. 1(b). Unfortunately, the pump stray light, comprising the elastic and inelastic scattering background, has almost the same polarization component as the output probe; thus, it can overwhelm the tiny variation of the probe power [12–14]. As a tradeoff, one has to use a weak pump power, which leads to a low signal-to-noise ratio (SNR) and long acquisition time. To overcome this issue, Remer et al. recently proposed a scheme that uses hot rubidium-85 vapor as a notch filter to highly absorb the background noise, and realized a high SNR and high-speed stimulated Brillouin scattering (SBS) spectroscopy measurements [15]. However, a pump wavelength first needs to match with an absorption line of the atom; then, the vapor can only remove the elastic scattering of the pump. The SBS noise, an amplified spontaneous Brillouin scattering of the pump, cannot be filtered out. The SBS noise arises from the interactions between the pump and incoherent phonons, and it will be the main background noise for the case of a strong pump and weak signal [16–18]. Therefore, to date, only a noncollinear pump-probe scheme can in principle realize a complete signal–noise separation because the signal and noise are naturally spatially separated [19–21]. In this scenario, however, one has to give up all the merits of a collinear and confocal system, such as high gain and simple operation.
Figure 1.Schematic diagrams. (a) Pump-probe technique. (b) Collinear polarization splitting configuration. (c) Orbital angular momentum mode division filtering. SPP, spiral phase plate; PBS, polarized beam splitter;
Alternatively, in our previous work, we demonstrated a signal–noise spatial separation for collinear Brillouin pump-probe scheme [as shown in Fig. 1(c)], where the degree of freedom of the orbital angular momentum (OAM) was used to extract probe signals from the strong SBS noise [22]. Generally speaking, the incoherence SBS noise can be considered as the mixing of various spatial modes, including lower-order OAM modes, which can also be demultiplexed into Gaussian modes and mixed into Brillouin signals. Moreover, the demultiplexing process needs a far enough propagating evolution distance and a pinhole with an appropriate aperture such as a low-pass filter, which results in a large energy loss. To address these problems, in this work, we put forward what we believe, to the best of our knowledge, is a novel signal–noise spatial separation method for a collinear pump-probe scheme. By spiral phase filtering for the precoding signal (called spiral phase precoding), we achieve on-demand tailoring spatial distribution of Brillouin signals and hence, in principle, the signal spatial separation from the background noise without loss. In the following, we first present the theoretical approach and discuss the principle of spiral phase precoding involving higher-order vortex filtering; then we introduce this scheme into the pump-probe technique based on stimulated Brillouin amplification (SBA). Finally, the proof-of-principle work is verified by experimentation.
2. THEORETICAL ANALYSIS
In a collinear Brillouin pump-probe scheme, to realize the signal–noise separation in the transverse plane at the output port, the key is reshaping the intensity distribution of the signal and noise, and the ideal situation is to use a lossless operation. The spiral phase precoding is inspired from the spiral phase contrast (SPC) imaging, which has been demonstrated as a powerful tool for edge detection in image processing by a vortex structure filter [23]. In this technique, exerting a different OAM to signal photons in the Fourier space can change the path of photon propagation, and then change the spatial position of photons, or rather the spatial structure of the image. Figure 2(a) shows the generic setup of SPC imaging with a typical Fourier spatial filter (i.e., spiral phase plate, SPP). The output of the object function
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Figure 2.Schematic diagrams. (a) Generic spiral phase imaging. (b) Proposed optical nonlinear operation for on-demand tailoring of Brillouin signal. L, lens; SPP, spiral phase plate; BA cell, Brillouin amplifier cell; and PBS, polarized beam splitter.
On this basis, we can obtain the output after filtering with any higher order (
Here, we choose a phase pattern that consists of a pentagram, square, and circle, as shown in Fig. 3(b0), to demonstrate selective highlighting of higher-order vortex filtering and spatial reshaping of Brillouin signals. The phase pattern contains rich information such as curves, straight lines, and corners so that we can clearly observe the highlighting and tailoring of the spatial structure of the light field. More generally, another phase pattern can be used according to different scenarios. The phase jump on the edge of each pattern is set to
Figure 3.(a0) Input Gaussian beam. (b0) Phase object image. (a1)–(a6) Simulated spatial profiles after spiral phase filtering of
The SPC imaging has been explored in second-order nonlinear optical processes (e.g., second harmonic generation) to achieve a visible edge enhancement with invisible illumination [31–33]. Different from previous studies, we show, for the first time, to the best of our knowledge, a new mechanism of the SPC technique in an SBA-based pump-probe scheme. The nonlinear interaction occurs between the pump and precoding probe signal. This probe beam refers to the angular spectrum of Brillouin signal loading a specific image, with synchronous processing of spiral phase filtering in a Fourier space, as shown in Fig. 2(b). Behind the SPP a further set of two relay lenses is arranged such that the signal angular spectrum is imaged in the Brillouin amplifier cell (BA cell). Hence, the process of nonlinear energy transfer for weak signals can be simplified with the dot product operation as
3. EXPERIMENTAL RESULTS AND DISSCUSSION
Figure 4 shows the schematic illustration of this proof-of-principle configuration. The pump beam is produced by a pulsed Nd:YAG laser, which has an output at 532 nm after frequency doubling, a pulse width of 7.8 ns, a repetition rate of 1 Hz, and a linear polarization Gaussian profile. A 6.9 ns Gaussian-pulse beam with a Stokes frequency shift is used as the signal beam, which is generated by the SBS in the same nonlinear medium as the BA cell (the generation setup of the Stokes signal is omitted in Fig. 4). Thus, the phase-matching condition can be satisfied in the SBA process to ensure high gain. We choose
Figure 4.Schematic of the experimental setup. Key components include the half-wave plate (HWP), quarter-wave plate (
In our experiment, we first investigated the intensity distribution of the amplified output and compared it with the corresponding input precoding signal profile. The average energies of the pump and input signal are set to 0.5 mJ and 10 nJ, with the same diameter of 1.5 mm, respectively. The CCD is set to a fixed exposure. Figures 5(a1)–5(a3) show the intensity profiles of the signal without vortex filtering [Fig. 5(a1)] and with vortex filtering of
Figure 5.Experimental results. (a1) Observation of the input precoding signals without spiral phase filtering and (a2), (a3) with spiral phase filtering of
Then, to demonstrate the signal–noise separation method, we need to increase the pump energy to generate a more noticeable noise. Figure 6 shows the output mixed beam of the amplified signal and noise when the pump and input signal are set to 5 mJ and 1 μJ, respectively. In this case, the SBS noise is the main component of the background noise. We can see that the noise can be focused to a smaller speckle spot after passing through lens
Figure 6.Experimental results of the output intensity distribution with the spiral phase precoding of (a)
Finally, we experimentally study the energy transfer between the pump and signal angular spectrum during nonlinear amplification. Figure 7(a) shows the dependence of the energy conversion efficiency [
Figure 7.Experimental results. (a) Energy conversion efficiency versus the pump energy
We can also see from Fig. 7 that
4. CONCLUSION
In summary, we have demonstrated on-demand tailoring spatial distribution of Brillouin spectrum signals by introducing the spiral phase precoding into the pump-probe scheme. We take a complex phase pattern as an example to study the selective highlighting via higher-vortex spiral phase filtering and spatial reshaping of Brillouin signals. The proof-of-principle experiments show that the intensity distribution of the Brillouin signal can be tailored and separated from the background noise by the ingenious Fourier transform. Furthermore, this method offers the advantages of high efficiency and high gain in Brillouin amplification. We believe our work provides a practical way toward quasi-noise-free nonlinear interactions involving pump-probe scanning detection.
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