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Abstract
Optical time-stretch (OTS) imaging is effective for observing ultra-fast dynamic events in real time by virtue of its capability of acquiring images with high spatial resolution at high speed. In different implementations of OTS imaging, different configurations of its signal detection, i.e. fiber-coupled and free-space detection schemes, are employed. In this research, we quantitatively analyze and compare the two detection configurations of OTS imaging in terms of sensitivity and image quality with the USAF-1951 resolution chart and diamond films, respectively, providing a valuable guidance for the system design of OTS imaging in diverse fields.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Ultrafast imaging of transient non-repetitive events is critical to understand their underlying mechanisms in physics [1], chemistry [2], and biology [3–6], hence attracting attention from both the scientific and industrial sectors. For this purpose, charge-coupled device (CCD) and complementary metal oxide semiconductor (CMOS) image sensors are by far the most widely employed devices for high-speed imaging. They can achieve Mfps frame rates [7] at the cost of spatial resolution and imaging sensitivity, which are still not high enough for many applications such as microfluidics [8], flow cytometry [9,10], cell sorting [11–14], and surface inspection [15]. Moreover, the readout time between consecutive frames also limits their utility in continuous image acquisition. To further enhance the imaging speed, recently, multiple ultrafast imaging techniques have been developed, such as sequentially timed all-optical mapping photography (STAMP) [16], frequency recognition algorithm for multiple exposures (FRAME) [17], compressed ultrafast photography (CUP) [18], and high-speed sampling camera (HISAC) [19,20]. Although they support picosecond or even femtosecond temporal resolution, they intrinsically can only capture a limited number of frames at a time. Hence, they are not applicable to situations where continuous recording is required.
Over the last decade, single-pixel imaging based on optical time stretch called optical time-stretch (OTS) imaging [21–25] has emerged as an attractive approach to high-speed imaging because of its ability to achieve continuous real-time imaging with a frame rate up to Gfps and a spatial resolution of 780 nm. OTS imaging utilizes wavelength-to-space conversion to map the spatial information of a target object into the spectra of the incident probe pulses and wavelength-to-time conversion (also known as dispersive Fourier transform) to turn the temporal profiles of the probe pulses into the manifestation of their spectra [26], such that a single-pixel photodetector and a digitizer can be used to perform high-speed and continuous data acquisition. In OTS imaging, there are two configurations of the signal detection, namely fiber-coupled detection scheme and free-space detection scheme. The first scheme is used to detect the pulses with a fiber-coupled photodetector, which requires beam recombining and fiber-coupling after measurement [27–30], while the second scheme is to directly focus the pulses on the active area of a free-space photodetector without coupling them into the fiber [31–33]. Researchers use different detection schemes in different implementations of OTS imaging, however, the performance and utility of these two detection schemes for specific applications are not discussed or analyzed.
In this research, to facilitate the application of OTS imaging in real applications, we quantitatively analyze and compare the two detection configurations of OTS imaging in terms of sensitivity and image quality with the USAF-1951 resolution chart and diamond films, respectively. We experimentally show that although the free-space detection scheme limits the field of view (FOV) of the OTS imaging, it offers higher receiving sensitivity and higher robustness to beam misalignment compared with the fiber-coupled detection scheme. The experimental and analytical results provide a valuable guidance for the system design of OTS imaging in diverse fields.
2. OTS imaging systems with two detection configurations
The schematics of OTS imaging systems with fiber-coupled and free-space detection schemes are illustrated in Fig. 1(a) and Fig. 1(b) respectively. In the fiber-coupled detection configuration, as shown in Fig. 1(a), the system has a symmetrical structure. The optical pulse generated from the broadband pulse laser first enters the single-mode fiber (SMF) spool where its spectral profile is mapped into the time domain. After being amplified by the erbium doped fiber amplifier (EDFA), the temporally stretched pulse passes through the first collimator and is subsequently spatially dispersed by the first diffraction grating that turns the pulse into a 1D rainbow beam and focused on the target by the lenses. Then the pulse, carrying the information of the target, is collected and spatially recombined by following lenses and the second diffraction grating, respectively. Next, the pulse is coupled into the fiber, detected by the photodetector and sampled by the digitizer. As the target moves perpendicular to the 1D rainbow beam and the pulses keep coming, each pulse detects one cross section of the target. By digitally stacking the pulses, the 2D spatial profiles of the target can be constructed.
Fig. 1. Configurations of the OTS imaging systems with fiber-coupled and free-space detection schemes. (a) OTS imaging system with fiber-coupled detection scheme. (b) OTS imaging system with free-space detection scheme. EDFA: erbium doped fiber amplifier. SMF: single-mode fiber. PD: photodetector.
In the system shown in Fig. 1(a), on condition that all the components are properly placed, the pulse can be perfectly received. However, in practice, the efficiency of the beam recombining and coupling is highly sensitive to the alignment of the beam, which can be affected by the aberration and displacement of the components, and the property, such as the fluctuation of the surface and heterogeneity of the material, of the target, significantly influenced the stability of the system. To simply the setup and make it more robust to beam misalignment, the OTS imaging system with free-space detection scheme, as shown in Fig. 1(b) is proposed, where the pulse passing through the target is not spatially recombined but directly focused on the active area of the photodetector. While, since the photodetector actually receives a line-shape spot, it is possible that a part of the pulse misses the active area of the photodetector, causing the intensity or information loss in the image.
In order to quantitatively analyze and compare the performance of the OTS imaging systems with above two detection configurations, we experimentally assemble the two systems according to the schematics shown in Fig. 1 by using following components/devices.
In the system with free-space detection scheme, to make sure the line-shape laser spot falls within the range of the active area of the photodetector, the focal lengths of the lenses in Fig. 1(b) (lens 1 to lens 5) are carefully designed. The focal lengths of lens 1 and lens 2 are both 50 mm, so that the FOV of the system, which is also the spot size on the target is 70 µm. After the second objective lens, the focal lengths of lens 3, lens 4 and lens5 are 50 mm, 100 mm, and 10 mm respectively. Hence the spot size when the pulse reaches the photodetector is 87 µm, which is close to the size of the active area of the photodetector. To compare the performance of the two systems with different detection configurations in the same condition, the focal lengths of the lenses in the system with fiber-coupled detection schemes (lens 1 to lens 4) are all 50 mm. In both systems, to control the bandwidth of the pulses and make the edges of the stretched pulses steep, we cut the bandwidth of the pulses to about 25 nm by placing a slit between lens 1 and lens 2.
3. Simulation and theoretical analysis
To make the comparison of the performance of the OTS imaging systems using these two detection configurations more convincing, we first analyze and simulate the pros and cons of the two different detection schemes in terms of FOV and robustness to the beam misalignment.
The FOV of the system is determined by the spot size of the line-shape beam between the two objective lenses. In the fiber-coupled detection scheme, the spatially dispersed light is recombined before detection. Hence, the FOV only depends on the bandwidth of the laser and the spatial dispersion before the target. In other words, we can enlarge the FOV by simply using a pulse laser with a wider bandwidth. While in the free-space detection scheme, since there is no beam recombining, the beam is actually still a line-shape rainbow spot when it arrives at the photodetector. Meanwhile, to achieve a large electrical bandwidth, the active area of the photodetector is not that large. As a result, to make sure that all of the wavelength components are properly received, the size of the beam when it arrives at the photodetector has to be within the range of the active area of the photodetector, which can be mathematically descripted as:
where f3, f4, f5 and fobj2·are the focal lengths of lens 3, lens 4, lens 5 and objective lens 2, and Sdet is the size of the active area of the photodetector. It is clear that once the parameters of the lenses are determined, the FOV is fundamentally limited by the size of the active area of the photodetector. In the experimental setup of this work, the diameter of the active area of the photodetector is 80 µm, the focal lengths of lens 3, lens 4 and lens 5 are 50 mm, 100 mm and 10 mm, so the achievable FOV is about 64 µm, which is much smaller than that fiber-coupled detection scheme can achieve [34,35].In practice, caused by the property of the target and the displacement of the components, the light can easily get misaligned when it arrives at the receiver. To investigate how this factor influences the performance of these two detection schemes, we establish two simulation models for these two detection schemes using component parameters exactly the same as those in Table 1, respectively. In the simulation, to make the trajectories of the light clear, we use a 24-wavelength laser with a center wavelength of 1555 nm and a wavelength interval of 1 nm as the laser source. Since the FOV of the system with free-space detection is about 64 µm, we use a cylinder with a diameter of 50 µm attached to a transparent glass plate as the imaging target. The misalignment of the light caused by the components can be presented as the tilting of the light and the displacement of the focusing point when the light arrives at the receiver. To show how those factors influence the signal detection, in the simulation, we observed the light trajectories right at the surface of the receiver under different conditions. Specifically, we set the thickness of the target to be 0, 2 µm and 20 µm, the tilting angle of the light to be 0° and 5°, and the displacement of the focusing point to be 0 and 10 µm. By combining these parameters, we calculate and draw the trajectories of the light at the end surface of the fiber (for the fiber-coupled detection scheme) and the surface of the photodetector (for the free-space detection scheme) in Fig. 2. Here, we consider both single-mode fiber (core diameter: 9 µm, Fig. 2(a)) coupling and multi-mode fiber (core diameter: 50 µm, Fig. 2(b)) coupling for the fiber-coupled detection scheme. In Fig. 2(a) and Fig. 2(b), the black circles are the core areas of the fibers, while in Fig. 2(c), the black lines are the active areas of the photodetector. It can be seen from the figures that as the thickness of the target increases, the tilting angle of the light increases, and the displacement of the focusing point increases, more and more light goes beyond the bounds of the receiver in both schemes, meaning the performance of the signal detection decreases. Nevertheless, under the same conditions, for the fiber-coupled detection scheme, the multi-mode fiber helps significantly receive more light than the single-mode fiber. While generally speaking, the free-space detection scheme can receive more light than the fiber-coupled detection scheme, meaning that the free-space detection scheme is more robust to the beam misalignment.
Fig. 2. Simulation of the signal receiving under different conditions. (a) Light trajectories at the end surface of the single-mode fiber in the fiber-coupled detection scheme; (b) Light trajectories at the end surface of the multimode fiber in the fiber-coupled detection scheme; (c) Light trajectories at the surface of the photodetector in the free-space detection scheme.
Table 1. Types and Specifications of the Components/Devices in the Experimental Systems
4. Experimental results
To experimentally show the performance of the two signal detection configurations in OTS imaging, we perform imaging of a standard USAF-1951 resolution chart and diamond films grown by a home-made MPCVD with the systems shown in Fig. 1.
4.1 Imaging the standard USAF-1951 resolution chart
We first image an USAF-1951 chart using the two systems with different incident power (Pi) levels to evaluate their imaging performance for an ideal target. Here Pi means the optical power incident to the first diffraction grating, that is, we measure the input power right before the first diffraction grating, ranging from 120 mW to 1 mW. The images of the USAF-1951 chart obtained by the two systems at different Pis are shown in Fig. 3(a) and Fig. 3(b), respectively. The line pairs of the group 7 element 6, whose width is 2.19 µm in the resolution chart, can be clearly resolved by both systems when Pi is 120 mw, indicating that both systems can achieve a spatial resolution of 2 µm, which is consistent with the theoretical calculation. Figure 3 also shows that with the same Pi, the images acquired by the free-space detection scheme are brighter and less noisy than those acquired by the fiber-coupled detection scheme. Quantitatively, we calculate the signal to noise ratio (SNR) of all the images shown in Fig. 3(a) and Fig. 3(b) and depict the curves in Fig. 3(c). On average, under the same Pi, the images acquired by free-space detection scheme have an about 3-dB higher SNR than the images acquired by fiber-coupled detection scheme. This trend can also be seen in the figures that the quality of the images acquired by free-space detection scheme is almost the same as the images acquired by fiber-coupled detection scheme with double Pi. The experimental results show that OTS imaging with free-space detection scheme has higher sensitivity than the one with fiber-coupled detection scheme, which also provides the system design with larger power budget. In addition to SNR, we also quantitatively compare the quality of the images acquired by these two systems using the parameters named structural similarity index (SSIM) [36] and visual information fidelity (VIF) [37]. Specifically, we use the image obtained by the OTS imaging system with free-space detection scheme at the Pi of 120 mW as the reference and calculate the SSIM and VIF values of all the other images in Fig. 3(a) and Fig. 3(b), and show the results in Fig. 3(d) and Fig. 3(e), respectively. In general, the images have larger SSIM and VIF values at the same Pi levels using the OTS imaging system with free-space detection scheme, indicating that the free-space detection configuration can achieve a better imaging quality with a given input power.
Fig. 3. Imaging the USAF-1951 resolution chart by OTS imaging with fiber-coupled and free-space detection schemes. (a), (b) Images of the group 7 on the USAF-1951 resolution chart at different Pis. (c), (d), (e) SNR, SSIM, VIF of the USAF-1951 resolution chart images at different Pis. Scale bar: 10 µm.
4.2 Imaging the single-crystal diamond films
Similar as above, by using single-crystal diamond films we further evaluate the performance of the two systems for common targets. The images of the films acquired by the two systems under different Pis are shown in Fig. 4(a) to Fig. 4(d). Figure 4(a) and Fig. 4(b) show the images of the film No.1 with pyramidal hillocks, which indicate the formation of defects and overgrowth in the corresponding areas. While Fig. 4(c) and Fig. 4(d) show the images of the film No.2 with growth steps, which indicate the step-flow growth mode, meaning fewer defects and better crystal quality. Figure 4(e) and Fig. 4(f) show the SNR curves of the images of film No.1 and film No.2 captured by the two systems, respectively. It is clear in both the images and the SNR curves that for both film No.1 and film No.2, the images acquired by free-space detection scheme are less noisy than those acquired by fiber-coupled detection scheme at the same Pi, which is consistent with the trend when imaging the USAF-1951 resolution chart shown in Fig. 3(c) above. By using the images of the films captured by free-space detection scheme at the Pi of 120 mW as the references, the SSIM and VIF curves of the images of the two films are shown in Fig. 4(g) to Fig. 4(j). It is clear that for both diamond films, the images acquired by the OTS imaging system with free-space detection scheme can achieve the same image quality in terms of SSIM and VIF as the system using fiber-coupled detection scheme with much lower Pi. These results, again, show that the free-space detection configuration offers a better sensitivity and imaging quality.
Fig. 4. Imaging the single-crystal diamond films by OTS imaging with fiber-coupled and free-space detection schemes. (a), (b) Images of the film No. 1 at different Pis. (c), (d) Images of the film No. 2 at different Pis. (e), (g), (i) SNR, SSIM, VIF of the film No. 1 at different Pis. (f), (h), (j) SNR, SSIM, VIF of the film No. 2 at different Pis. Scale bar: 10 µm.
4.3 Imaging the diamond film with polycrystalline grains
A sample with a highly fluctuating surface can significantly scatter the incident beam and hence cause beam misalignment afterwards, which can severely degrade the efficiency of the signal detection. To evaluate how the systems with two different detection configurations perform for the samples with complex structures, we use them to image a diamond film with large-size polycrystalline grains on the surface, which are usually considered as defects during CVD growth. Figure 5(a) shows the image of the film acquired by the system with SMF-coupled detection scheme. It is clear that the brightness of the image varies significantly as the pulses scan along the film. This is because that the core diameter of the SMF is very small (only 9 µm), the beam misalignment caused by the highly fluctuating surface of the sample significantly influences the coupling efficiency of the light, hence dramatically degrades the image quality. To compare the performance of these two signal detection configurations in a more equitable condition. We replace the SMF patch cable in the fiber-coupled detection scheme with a multi-mode one, which has a core diameter of 50 µm, closing to the size of the active area of the free-space. The images of the diamond film acquired with these two systems at different Pis and corresponding curves of SNR, SSIM and VIF are shown in Fig. 5(b) to Fig. 5(f), where the image obtained by the OTS imaging system with free-space detection scheme at the Pi of 120 mW is used as the reference. It is clear that using multi-mode patch cable notably improves the image quality of the system with fiber-coupled detection scheme. While the system with free-space detection scheme still has a better sensitivity and imaging quality than the one with fiber-coupled detection scheme at a given input power. It should be noted that for the calculation of SNR, SSIM and VIF in Fig. 3, Fig. 4 and Fig. 5, the reason why we choose the images acquired with the free-space detection scheme at the Pi of 120 mW as the references is simply because the image quality under this condition is the best. Changing the references to be the images acquired with the fiber-coupled detection scheme will not change the trend of the curves and the conclusion.
Fig. 5. Imaging the polycrystalline diamond grains by OTS imaging with fiber-coupled and free-space detection schemes. (a) Image of the film captured by fiber-coupled detection scheme with the single-mode fiber at the Pi of 120 mW. (b) Images of the film captured by fiber-coupled detection scheme with the multi-mode fiber at different Pis. (c) Images of the film captured by free-space detection scheme at different Pis. (d), (e), (f) SNR, SSIM and VIF of the film images captured by multi-mode fiber-coupled detection and free-space detection schemes at different Pis. Scale bar: 10 µm.
4.4 Power loss of the fiber-coupled and free-space receivers
In an active optical imaging system, such as an OTS imaging system, lower power loss can relieve the requirement on the illumination power and minimize the possible photic damage or phototoxicity on the samples, meaning a broader application field of the technique. In the experimental system, the sources of the power losses mainly include the transmission through the lenses (about 0.2 dB each), the efficiency of the diffraction grating (about 2.6 dB each), the transmission through the objective lenses (about 4.0 dB each), the absorption or scattering of the target (about 0.7 dB - 6 dB depending on the type of the target), and the efficiency of the receiver. Here we show the power loss of the systems with fiber-coupled and free-space receivers by measuring the optical power right before the photodetector when the Pi is 120 mW for different kinds of samples, including the USAF-1951 resolution chart and the diamond films. For the fiber-coupled detection scheme, we measure the power loss for the receivers with both SMF and multi-mode patch cables. The results are shown in Fig. 6, where it can be seen that, for all types of samples, the received power in the system with free-space detection scheme is obviously higher than those in the systems with fiber-coupled detection scheme, meaning that the OTS imaging system with free-space detection scheme has lower power loss than that with fiber-coupled detection scheme.
Fig. 6. Comparison of the power loss of the OTS systems with fiber-coupled detection and free-space detection schemes. Single-crystal diamond film1: pyramidal hillocks on the surface of the single-crystal diamond film. Single-crystal diamond film2: growth steps on the surface of the single crystal diamond film.
5. Conclusion and discussion
In this research, we quantitatively analyze and compare the performance of the OTS imaging systems with fiber-coupled and free-space detection configurations by experimentally imaging the USAF-1951 resolution chart and diamond films. The experimental results show that the system with free-space detection scheme is more robust to the beam misalignment and has higher sensitivity and imaging quality than the system with fiber-coupled detection scheme. The performance of the system with fiber-coupled detection scheme can be improved by using multi-mode patch cable in the receiver. However, compared with fiber-coupled detection scheme, in the system with free-space detection scheme, to make sure that the size of the beam spot is within the range of the active area of the photodetector, the FOV is usually limited. These conclusions are extremely helpful for the design of an OTS imaging system for specific applications. Generally, free-space detection is preferable for the applications that do not need large FOV or/and with highly irregular or heterogenous targets, such as crystal surface. Typical applications include the online monitoring of the characteristics of the molten pool and the surface of the manufactured parts in the additive manufacturing process, which is of great significance to understand the mechanism of the manufacturing process and improving the quality of the products. Due to the fluctuations of the molten pool and the surface of the manufactured parts, the optical beam easily gets misaligned. Therefore, free-space detection is suitable for this kind of application, offering good and stable imaging quality. While fiber-coupled detection is more suitable for the applications that require large or flexible FOV and with relatively regular targets. Typical applications include the screening of cells, barcodes, etc. It should be mentioned that the optical fiber in the fiber-coupled detection scheme can also perform as a pin hole, which can be used to improve the contrast of the image by asymmetric detection [30]. Therefore, it is important to properly choose the detection scheme for different applications. It is our hope that this research can provide a valuable guidance for the system design and optimization of OTS imaging in diverse fields.
Funding
National Natural Science Foundation of China (51727901, 61905182); The Hubei Provincial Major Program of Technological Innovation (2017AAA121); Fundamental Research Funds for the Central Universities; Wuhan Research Program of Application Foundation and Advanced Technology; Japan Society for the Promotion of Science Core-to-Core Program; White Rock Foundation.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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