Optics letters最新文献

Visible light optical coherence tomography (VIS-OCT) provides retinal oximetry at micro-level vessels by performing spatiospectral analysis. Typical methodology involves the short-time Fourier transform (STFT), which requires computationally intensive repetitive transforms. Here we report a depth-gated Fourier transform (DGFT) method to reduce the number of transforms (and time) for spectral extraction by windowing the depth domain. The number of transforms was decreased from 13 to 3 by DGFT, nearly 6× faster in computation time than STFT. We validated DGFT for retinal oximetry in a human eye. Oxygen saturation (sO₂) values matched well between STFT and DGFT (percent difference of 0.63% ± 1.10%), while the DGFT extracted spectra significantly faster than the STFT (0.15 ± 0.11 s vs 0.89 ± 0.48 s). The reported method shows potential for real-time oximetry calculation in the future.

We numerically design a compact nanolaser based on a topological guided-mode resonance (GMR) structure. It consists of a topological junction formed by two GMR gratings, which induces a leaky Jackiw-Rebbi (JR) edge state that confines in-plane light within a small mode volume. Using the finite-difference time-domain (FDTD) method to simulate active optical responses, we show that surface-emitting lasing is achieved with a threshold of 4.5 µJ/cm² within a cavity length of approximately 2.0 µm. In addition, by replacing the junction with an array of equally spaced ridges in a critical phase, the edge mode transitions into a bulk mode. This modification allows for controllable cavity sizes of 4.9, 7.8, and 10.7 µm, with corresponding thresholds of 6.0, 8.4, and 9.0 µJ/cm², achieved by using 5, 10, and 15 cycles of critical state grating. The topological GMR holds promise for compact coherent sources.

The demand for temperature measurement in optoelectronic integration has prompted research on advanced luminescent materials. Here, ferroelectric Bi₄Ti₃O₁₂:Yb³⁺/Er³⁺/Nd³⁺ nanosheets were designed for optical thermometry. The transition emissions of nanosheets were significantly enhanced through electric polarization engineering. By utilizing the fluorescence intensity ratio of emissions from Nd³⁺ and Er³⁺ ions, the poled system achieved notable absolute and relative sensing sensitivities of 3.03 and 2.52% K^-1, respectively. These results highlight the multi-band luminescence and optoelectronic coupling characteristics of lanthanide ion-doped ferroelectric nanosheets and suggest that electric field polarization-enhanced temperature measurement can serve a reference for enhancing optical sensing capabilities.

For traditional moiré-based lithography alignment technology, which is widely used in proximity lithography systems, complex alignment marks with larger areas are employed to achieve high-precision misalignment detection. However, every inch of space on the wafer is extremely precious in practice, leaving minimal space for alignment marks. Therefore, employing small-area alignment marks in lithography systems will be a very challenging task with considerable potential in the future. The primary challenge is that existing frequency-based analytical algorithms struggle to achieve misalignment values with high-precision from moiré fringe images generated by small-area marks. To address this challenge, a spatial and frequency information fusion neural network (SFFN) is proposed for processing the moiré fringe images. With SFFN, the area of the alignment mark can be reduced by 2/3, and the average error of SFFN is less than 1 nm on the test dataset.

We report, for the first time to our knowledge, a 2.3-µm continuous-wave (CW) laser operation of trivalent thulium ion (Tm³⁺)-doped Lu₂O₃ ceramic. In the free-running laser operation, co-lasing could be observed at the wavelengths of 2096 nm and 2325 nm. The laser operation and power performance of the ³F₄→³H₆ and ³H₄→³H₅ transitions were investigated separately with a birefringent tuning plate. By using a double-end-pumped x-cavity, 134 mW of CW output power was obtained at the wavelength of 2309 nm with 4 W of pump power and with 7% slope efficiency. By using two sets of cavity optics, we further achieved the broadest, continuous tuning range, obtained to date for a Tm³⁺ laser, extending from 1845 nm to 2328 nm. We also investigated a 2-µm laser operation, where with a 1.3% output coupler, a maximum CW output power of 917 mW was obtained at 2094 nm with 3.93 W of pump power. By using the laser threshold and power efficiency data measured during the 2.3-μm operation, the emission cross section of the Tm³⁺:Lu₂O₃ ceramic gain medium was determined to be 3.24 × 10^-21 cm² at the wavelength of 2309 nm.

In this Letter, we develop a temporal multiplexing complex amplitude holography for a three-dimensional (3D) display with natural depth perception. A series of band-limited random phases is optimized through the designed pseudo incoherent wave propagation model, which can be used as the initialization condition for arbitrary target images. The temporal multiplexing holograms (TMHs) are calculated according to the optical field distribution in the image plane and encoded through the complex amplitude converting method. Based on temporal multiplexing, our method overcomes the drawbacks brought by random phase and constant phase on 3D display performance and enables 3D reconstruction with high image quality as well as natural depth cues. It is expected that our method offers an efficient route toward the future high-performance holographic 3D display.

Fourier-inspired single-pixel holography (FISH) is an effective digital holography (DH) approach that utilizes a single-pixel detector instead of a conventional camera to capture light field information. FISH combines the Fourier single-pixel imaging and off-axis holography technique, allowing one to acquire useful information directly, rather than recording the hologram in the spatial domain and filtering unwanted terms in the Fourier domain. Furthermore, we employ a deep learning technique to jointly optimize the sampling mask and the imaging enhancement model, to achieve high-quality results at a low sampling ratio. Both simulations and experimental results demonstrate the effectiveness of FISH in single-pixel phase imaging. FISH combines the strengths of single-pixel imaging (SPI) and DH, potentially expanding DH's applications to specialized spectral bands and low-light environments while equipping SPI with capabilities for phase detection and coherent gating.

The rapid development of wavelength-division multiplexing (WDM) systems has underscored the critical requirement for effective link monitoring to ensure system reliability and performance. Traditional approaches often rely on separate devices for communication and sensing, which can compromise spectral efficiency and increase system complexity. This work presents an innovative method for integrating communication and sensing within a conventional optical supervisory channel. Four QPSK data streams with different duty ratios enable robust communication and precise sensing with a 125-MBaud transmitter. The forward transmission of communication signals is demonstrated with impeccable accuracy, delivering bit-error-free performance over two fiber links. Concurrently, sensing data is extracted through polarization-diversity reception of the backscattering signal. The distributed acoustic sensing sensitivity achieves 0.50 nε/Hz in 10.2 km and 0.65 nε/Hz in 40.0 km at a spatial resolution of 10 m by employing a matched filter. This approach effectively adopts the defined forwarded transmitted control signals or channel information signals, such as channel power, optical signal-to-noise ratio, and Q-factor, simultaneously achieving sensing applications without any dedicated channel or resource.

We demonstrated optical amplification ranging from 1600 to over 1680 nm in Tm³⁺-doped fluorotellurite fibers (FTFs) with a Tb³⁺-doped cladding by using a 1212 nm fiber laser as the pump source. The FTFs based on TeO₂-BaF₂- Y₂O₃ glasses were fabricated by using a rod-in-tube method. The doping concentration of Tm³⁺ ions in the core was about 4000 ppm and that of Tb³⁺ ions in the fiber cladding was about 10,000 ppm. By introducing the Tb³⁺ ions into the cladding, the emission in the long-wavelength region (>1750 nm) originating from the transition ³F₄→³H₆ of Tm³⁺ ions was suppressed efficiently by broadband absorption (1700-2100 nm) originating from the transitions ⁷F₆→⁷F_0,1,2 of the Tb³⁺ ions in the cladding. As a result, a positive net gain ranging from 1600 to over 1680 nm was achieved in a 1.5 m long Tm³⁺-doped FTF as the launched power of the 1212 nm laser was about 3 W. The gain value at 1675 nm was about 19.7 dB for an input signal power of ∼0 dBm (or 1 mW). The gain was gradually reduced for shorter wavelengths, but it was still above 10 dB at ∼1644 nm. Our results show that Tm³⁺-doped FTFs with a Tb³⁺-doped cladding are promising gain media for constructing fiber amplifiers and lasers in the wavelength region of 1600-1700 nm.

An analytical model of an electromagnetically induced transparency (EIT) and Autler-Townes (AT) splitting spectrum with a four-level Rydberg atom was presented, and equations were derived to explain the dependence of the transient absorption for a probe laser on light and microwave (MW) fields. The analytical solution of absorption for the probe laser Im[χ(t)] shows that it depends on the spontaneous decay rate from level |2〉 to level |1〉, Rabi frequencies of the control and MW fields. For ⁸⁷Rb atoms, a stronger control laser shortens the steady-state time window and a stronger MW field will lead to a higher oscillation frequency shown in analytical and numerical results. The time-dependent EIT-AT splitting spectrum is also investigated, and the stable splitting distance shows a linear dependence on the continuous MW E-field strength.