Reviewed by Lexie CornerAug 27 2024
An international group of scientists from the Sichuan University in China and Tampere University in Finland have developed a frequency downconversion temporal ghost imaging (TGI) scheme that extends the operation regime to arbitrary wavelength regions where fast modulators and detectors are not available. The study was published in the journal Light: Science & Application.
Experimental setup for computational temporal ghost imaging based on frequency downconversion. Preprogrammed probing temporal patterns generated from the arbitrary waveform generator (AWG) are applied to an acousto-optic modulator operating in the 1.5 μm range (AOM1) to modulate the 1542 nm laser. The probing patterns are transferred to an idler wavelength at 3.4 μm using downconversion in a periodically poled lithium niobate crystal (PPLN) with a strong 1060 nm CW pump. The 3.4 μm idler light is transmitted through the temporal object generated by an acoustic-optic modulator operating in the 3 μm range (AOM2) and detected by a mid-infrared slow detector (MIR slow detector). The temporal intensity from the slow detector and the preprogrammed temporal probing patterns are recorded by the oscilloscope (OSC), and the temporal object is retrieved from their correlation. FC, fiber collimator; DM, dichroic mirror (HR@1135‒1600 nm, HT@1060 nm). Image Credit: Han Wu, Bo Hu, Lu Chen, Fei Peng, Zinan Wang, Goëry Genty, Houkun Liang
Ghost imaging in the time domain allows for the reconstruction of fast temporal objects using a slow photodetector. The technique relies on correlating random or pre-programmed probing temporal intensity patterns with the integrated signal measured after the temporal object's modulation.
However, applying temporal ghost imaging typically requires ultrafast detectors or modulators, which are particularly scarce in the mid-infrared region, making it challenging to detect or pre-program the probing intensity patterns.
To address this, the method involves modulating a near-infrared signal with temporal intensity patterns and then using difference-frequency generation in a nonlinear crystal to transfer these patterns to an idler at a wavelength suitable for retrieving the temporal object.
Researchers have demonstrated a computational proof-of-concept for temporal ghost imaging in the mid-infrared range, with an adjustable working wavelength between 3.2 and 4.3 μm. This approach is flexible and can be extended to other spectral regimes.
These findings open up new possibilities for studying ultrafast dynamics and performing scan-free pump-probe imaging in spectral regions such as the mid-infrared and THz, where ultrafast modulation or detection is challenging. The researchers have summarized the working principle of temporal ghost imaging using mid-infrared technology.
They described, “The absence of suitable instrumentation, such as ultrafast mid-infrared electro-optic modulators for pre-programming temporal patterns at mid-infrared light sources, has been a bottleneck in the direct implementation of computational TGI in the mid-infrared. In the proposed new scheme, instead of directly pre-programming temporal patterns at mid-infrared wavelengths.”
“We modulate pre-programmed temporal patterns at near-infrared wavelengths using a conventional telecom modulator and, subsequently, these modulated patterns are transferred to a mid-infrared idler via difference-frequency generation using a temporally stable continuous-wave near-infrared pump light source,” the scientists noted.
“The wide availability of tunable lasers in the near-infrared allows for flexible and versatile operation of the downconversion TGI scheme, enabling to extend TGI to wavelength regimes where there is a lack of fast detectors and modulators. Using this approach, we have experimentally demonstrated computational TGI in the wavelength range from 3.2 to 4.3 μm. Besides, orthogonal temporal patterns can be used in computational downconversion TGI to reduce the number of distinct probing measurements,” they added.
The researchers added, “The presented technique can provide new possibilities for the study of ultrafast dynamics in the mid-infrared spectral region and high-speed free-space optical communications in atmospheric transmission windows even in the presence of atmospheric turbulence. We emphasize that the concept of frequency downconversion ghost imaging is generic, and it can also be applied in the spatial and spectral domains, which could open a new venue for single-pixel imaging and spectroscopy in the mid-infrared and THz regions.”
Journal Reference:
Wu, H., et al. (2024) Mid-infrared computational temporal ghost imaging. Light Science & Applications. doi.org/10.1038/s41377-024-01426-0.