Optics letters最新文献

We report on the operation of an efficient Tm,Ho:YLF depressed cladding, channeled waveguide laser in both continuous-wave (CW) and passively Q-switched (PQS) regimes, producing laser emission at the wavelength of 2.05 µm. The 70-µm diameter depressed cladding waveguide, fabricated using femtosecond laser inscription, had a low propagation loss value of 0.14 dB/cm and a refractive index contrast of 8.3 × 10^-4. In the CW regime, the waveguide laser was excited at 780 nm, and an output power of up to 2 W was generated at the incident pump power of 4.14 W with a power slope efficiency of 50.0%. PQS operation was further realized by utilizing a Cr:ZnSe saturable absorber (SA), whereby the waveguide laser generated as short as 19.6-ns pulses with a power slope efficiency of 18.9%.

To the best of our knowledge, this is the first study to investigate the use of Tm³⁺ to sensitize the Dy³⁺ ion and enhance the ∼4.3 µm emission from Dy³⁺:⁶H_11/2 → ⁶H_13/2 in the KPb₂Br₅ (KPB) crystal. The ∼4.3 µm fluorescence emission properties and energy transfer mechanism of the Dy:KPB and Tm,Dy:KPB crystals were examined. The results show that Tm³⁺ is an excellent sensitizer to the Dy³⁺ ion and can provide an efficient excitation channel; thus, the Tm,Dy:KPB crystal can be pumped by a commercial 808 nm laser diode (LD). Compared with the Dy:KPB crystal, co-doping with the Tm³⁺ ion improves the absorption around 800 nm by an order of magnitude, and the energy transfer efficiency from Tm³⁺:³F₄ to Dy³⁺:⁶H_11/2 is as high as 76.7%, indicating that the Tm,Dy:KPB crystal has great potential application in ∼4.3 µm mid-infrared lasers under a commercial LD pump.

Polarization-insensitive photonic switches are crucial for the case with random polarization states encountered in optical systems. In this paper, we propose and demonstrate a polarization-insensitive 2 × 2 thermo-optic Mach-Zehnder switch (PIMZS) on a 340-nm silicon-on-insulator (SOI) platform by incorporating low-loss polarization-insensitive multimode interference (PIMMI) couplers whose core width is varied optimally. The fabricated 2 × 2 PIMZS exhibits a low excess loss of 0.15-0.79 dB and a high extinction ratio of >24 dB for both polarizations, and low polarization-dependent loss (PDL) of <0.47 dB is achieved across the C-band.

The spin-orbit (σ - l) interaction in a focused-reflected beam of light results in spatially nonuniform polarization in the beam cross section due to the superposition of orthogonal field components and polarization-dependent interface reflection coefficients. Polarization filtering the output beam leads to an interchangeable transformation of l=∓2 charge vortex into two (∓) unit charge vortices, for σ = ±1 circular polarization of the input Gaussian beam. This transformation follows a trajectory, named optical vortex trajectory, that depends on the input beam's σ and hence the l and reflecting surface characteristics. The vortex trajectory is used here to quantify both the sign and the magnitude of the chiral parameter of a quartz crystal. The Jones matrix-based simulation anticipates the chirality-dependent vortex trajectory that matches with experimental measurements.

The indistinguishable photon-pair sources are valuable in many quantum information applications, such as quantum microscopy, quantum synchronization, and quantum metrology. Based on cascaded sum-frequency generation (SFG) and spontaneous parametric downconversion (SPDC) processes, we propose and demonstrate a scheme for the generation of spatially separated broadband indistinguishable photon pairs in the telecom band by using only one piece of a fiber-pigtailed periodically poled lithium niobate waveguide in a modified Sagnac loop. The measured joint spectral intensity of the generated entangled photon pairs is 7.27 THz (57.6 nm) at the full width at half-maximum (FWHM). The Hong-Ou-Mandel (HOM) interference of the generated broadband photons is measured with bandwidths of 5.35 THz (∼42.8 nm) and 100 GHz (∼0.8 nm), respectively. Visibility of 94.0±1.4% is achieved with the bandwidth of 5.35 THz, demonstrating good indistinguishability of the generated two-photon states, which could benefit the development of quantum microscopy and quantum synchronization.

For lensless ghost imaging (GI) with thermal light, the axially relative motion constrained in the range of the system's depth of focus (DOF) can still cause image blurring because of a variable magnification. We propose a motion-deblurring GI system with pseudo-thermal light, which can overcome the resolution degradation caused by the axial motion. Both the analytical and experimental results demonstrate that high-resolution GI can be always obtained as long as the target's random motion range is smaller than the system's DOF, without using the prior information of motion estimation. We also show that the system's DOF can be extended by optimizing the geometrical shape of the laser spot on the rotating ground glass disk (RGGD). The imaging performance comparison between the proposed GI system and the corresponding lensless GI system is also discussed. This technique can promote the practical application of GI in the field of moving-target detection and recognition.

Organic-crystal-based optical terahertz (THz) sources and detectors are powerful tools for THz spectroscopy, owing to the wide frequency tunability. A drawback of this technique lies in the inherent absorption peaks of nonlinear crystals, leaving several gaps in the spectral coverage. As an alternative type of organic crystal, hydrogen-bonded OH1 is promising to complement the existing gaps. To demonstrate the potential of OH1, we set up an active and coherent THz frequency-domain system and investigate its performance in difference-frequency generation (DFG) and upconversion (UC) detection. Efficient frequency response extending from 1.58 to 33.38 THz is achieved at room temperature. A strong peak at 31.86 THz is observed for the first time, to the best of our knowledge. Compared with a commercial thermal detector Golay cell, the sensitivity of the OH1-based upconversion detection is 67.8 dB better. These results make it possible to provide a flat and wide THz spectral coverage with a high signal-to-noise ratio by the combination of different types of organic nonlinear crystals.

We describe improved methods for locating the fixed point of an optical frequency comb. Two continuous-wave lasers are locked to a reference frequency comb and track the optical phase of a second comb-under-test (CUT) at two points separated by approximately 1.6 THz. Carrier-envelope and optical phase tracking (OPT) yields a precise fixed point measurement across a range of pump modulation frequencies (400 Hz-250 kHz). Sub-nanometer shifts of the fixed point are observed. The fixed point is also determined with high precision using dual-comb interferometry (DCI), and the value closely matches the calculation from the dual-point tracking method.

Control of transverse modes in the oxide-confined vertical-cavity surface-emitting laser (VCSEL) is a key issue. We demonstrate a VCSEL with a double heterostructure high-contrast grating (HCG) as the top mirror to control the transverse mode characteristics. Compared with the common HCG-VCSEL with a uniform HCG, theoretically the proposed double heterostructure HCG-VCSEL can suppress the higher-order transverse modes. Experimentally, the double heterostructure HCG-VCSEL has a reduced spectral width and red-shifted fundamental mode compared with a common HCG-VCSEL with a uniform HCG, which is consistent well with the theoretical results. This work provides a new, to the best of our knowledge, approach to control the transverse modes of the VCSEL.

A heterodyne laser Doppler vibrometer (LDV) with a Bragg cell has a stationary signal carrier at a frequency of at least 35 MHz. The expensive Bragg cell with the restricted shift frequency is not an optimal solution to meet the requirements for many measurement scenarios. For vibrations with low frequencies and small amplitudes, a tens-of-megahertz carrier frequency not only wastes bandwidth at the photodetector but also requires a fast and expensive analog-to-digital converter (ADC). Compared to the Bragg-cell-based LDVs, LDVs with an optical phase-locked loop (OPLL) enable a selectable carrier and thus can provide a low carrier frequency. However, problems arise when the carrier frequency is smaller than the laser linewidth and the OPLL bandwidth. We accidentally were able to offset-lock the OPLL on the harmonic component of the heterodyne oscillator signal and explored this phenomenon, which enables carrier frequencies much smaller than the laser linewidth of the locked lasers at a smaller noise level. In this paper, we demonstrate a carrier frequency down to 3 MHz with a 3.3 dB better signal-to-noise-ratio (SNR) compared to the traditional locking technology. Our findings may enable very cost-efficient LDVs.