Signal Beam Research Articles

Dual-Wavelength Mid-Infrared Generation Using Intracavity Stimulated Raman Scattering of PPLN

We report, to the best of our knowledge, the first demonstration of a continuous-wave, dual-wavelength (DW) mid-infrared optical parametric oscillator (OPO) based on the stimulated Raman scattering (SRS) effect of periodically poled lithium niobate (PPLN). The OPO was pumped by two independent fiber lasers, one being fixed at 1018 nm and the other being tunable. The high-power 1018-nm source built parametric oscillation, induced SRS effect in PPLN and generated 1495-nm signal beam as well as 1506-nm Raman beam. The tunable fiber laser was set at 1080 nm and the phase-matched difference frequency generation (DFG) occurred between the 1506-nm Raman beam and 1080-nm pump beam. Finally, DW idler beam located at 3189 and 3819 nm was obtained whose power ranged from 8.044 to 8.115 W, in which the 3189-nm idler power made up the majority of about 8 W and the maximum 3819-nm idler power reached 0.154 W. The slope and conversion efficiency of DFG and OPO processes was also discussed separately.

Investigation of influence incoherent background illumination on the nonlinear response of a lithium niobate crystal sample at low light intensity

The total compensation of nonlinear diffraction of coherent laser beams of He-Ne laser (wavelengths 633 nm) with diameters on full width of half maximum near to 15 - 20 μm due of assistance of incoherent background have been experimentally demonstrated. Incoherent background had a shorter wavelengths (455 – 465) nm and much lower intensity compared with coherent signal beam with wavelengths 633 nm. Obtained the dependences of the refractive index change in the LiNbO3:Fe crystal under the conditions of such incoherent background.

Long-distance distribution of the telecom band intensity difference squeezing generated in a fiber optical parametric amplifier.

Using an all-fiber source of pulsed twin beams, with the record intensity difference squeezing of 6.1±0.15 dB for fiber optical parametric amplifiers, we experimentally distribute the pulsed continuous variable (CV) quantum state over a long distance. After separating the signal and idler twin beams by 26km single-mode fibers, the measured intensity difference noise is still below the shot noise limit by 2dB. The result shows that the distribution process is only affected by the vacuum noise originated from the fiber transmission loss. The extra noise contributed by the acoustic wave Brillouin scattering and Raman scattering in 26km transmission fibers has not been observed. The investigation is beneficial for studying long-distance CV quantum communication.

Multichannel Confocal Microscope Based on a Diffraction Focusing Multiplier with Automatic Synchronization of Scanning

Implementation of a laser scanning confocal microscope is described, where the specimen is scanned by an array of illuminating beams, which significantly increases the velocity of object image construction. The array formation is provided by using a diffractive optical element. Scanning by the array of laser beams over the specimen is performed by galvanometric scanners with moving refractive plane-parallel plates. Owing to application of such a scanning device, the beams in the illuminating channel and the signal beams in the receiving channel pass through one motionless array of confocal diaphragms; as a result, the scanning beams in the specimen plane and the signal beams in the plane of the photodetector matrix can be used without an additional synchronized pair of scanners. The proposed confocal microscope can be applied in problems that require a fast response.

Performance evaluation of perfect optical vortices transmission in an underwater optical communication system.

In this paper, we experimentally evaluated the performance of underwater optical communications using 1Mbit/s on-off keying signals of perfect optical vortices. Perfect optical vortices generated by Fourier transformation of the Bessel-Gaussian beam are propagated through a 2.6m underwater medium and affected by various water conditions such as stagnant water, water flow, temperature difference, and air bubbles, so beam wander and fluctuation of the signal are studied. Air bubbles have the maximum effect on signal fluctuations and beam wanders. Measurement of the system bit error rate (BER) based on the signal-to-noise ratio (SNR) for these situations matches the results of wandering and fluctuations. The system BER is studied for different values of SNR, and its value is more degraded due to the air bubbles.

Ultracold Rydberg atoms for efficient storage of terahertz frequency signals using electromagnetically induced transparency

Quantum communication with terahertz (THz) frequency signals has many advantages like reduced attenuation and scintillation effects in certain atmospheric conditions along with very high level of data security. In this work, we propose a scheme to realize Quantum Memory (QM) for efficient storage of terahertz (THz) frequency signals using Electromagnetically Induced Transparency (EIT) in an ultracold atomic medium of 87Rb Rydberg atoms prepared in a Two Dimensional Magneto Optical Trap (2D-MOT). The uniqueness of our scheme lies in the choice of the energy levels involved in the EIT process, all three of which have been chosen to be the Rydberg levels (enabling signal beam to be in THz) in a lambda type arrangement. This first of its kind proposal reveals that atomic media are a potential candidate for devising QMs which can store THz frequency signals. We have estimated that the Optical Depth (OD) in our scheme can reach a very high value of 690, very high maximum obtainable storage efficiency (η) of ∼99%, the group velocity (vg) can be as low as 5.07 × 103 m/s, and the Delay Bandwidth Product (DBP) can be as high as 9.5. All of these estimates emphasize the feasibility of our scheme as a QM device for efficient storage of THz pulses.

Imaging with ghostly electrons

Electron Imaging Ghost imaging is a computational imaging approach that uses correlations between a signal beam that interacts with a sample and a reference beam that doesn't. The correlations are then used to reconstruct an image of the sample and can, in principle, be used to image samples with very weak signal beams that might otherwise be damaged with a typical probe beam. Already demonstrated for optics and atoms, Li et al. now extend the method of ghost imaging to electrons. To circumvent the difficulty of splitting an electron beam, they instead used a structured electron beam to probe the sample. They show that a reconstructed image of a sample with a lower electron dose than that used in direct imaging methods can be faithfully produced. This technique could reduce acquisition time and avoid damaging sensitive samples in electron imaging applications. Phys. Rev. Lett. 121 , 114801 (2018).

Frequency Down-Conversion of Dual-Wavelength Raman Fiber Laser in PPLN-Based Optical Parametric Oscillator

We report on frequency down-conversion of dual-wavelength (DW) Raman fiber laser in a periodically poled lithium niobate-based optical parametric oscillator. The DW pump source was fixed at 1060 and 1111 nm that was obtained based on stimulated Raman scattering effect by combining a home-made linearly polarized 1060-nm fiber laser and 137-m-long polarization-maintaining passive fiber. The total pump power went through three stages, in the latter two of which the 1111-nm wave appeared. In the entire experiment, the 1060-nm pump beam achieved parametric oscillation and generated 1626-nm signal beam and 3056-nm idler beam. The 1111-nm pump beam generated 3503-nm idler beam based on difference frequency generation (DFG) between itself and 1626-nm signal beam in the second stage. With power enhancement, it built independent parametric oscillation in the third stage and generated 1621-nm signal beam as well as 3530-nm idler beam. The DW idler power ranged from 3.94 to 7.78 W in the latter two stages. The power and efficiency characteristics in frequency conversion processes of 1060- and 1111-nm pump beams were also analyzed separately.

Numerical Simulations of Transfer of Spatial Beam Aberrations in Optical Parametric Chirped-Pulse Amplification

In this paper, the spatial characteristics of the optical parametric chirped-pulse amplification (OPCPA) process were numerically studied when initial pump beam was aberrated. Numerical results showed that the spatial walk-off effect transferred phase modulation partly to the signal beam as the pump phase was modulated. Moreover, the modulation amplitude became increasingly severe as the nonlinear length extended. In the absence of phase aberration in the initial input signal, the induced phase aberration in the output signal was assumed as the differential form of the pump beam phase. As the pump beam intensity was modulated, the spatial walk-off effect reduced the influence of pump beam noise on beam quality and the angular spectrum but reduced signal gain simultaneously; thus, it may do more harm than good in the OPCPA process. In the case of a non-diffraction-limited pump beam, the greater the beam quality factor Mp2, the lower the conversion efficiency of the output signal in the OPCPA process. These results have important guiding significance for optimized design of an OPCPA system for high power laser.

High power picosecond parametric mid-IR source tunable between 17 and 26 μm

A high-average-power wavelength-tunable picosecond mid-IR source based on parametric downconversion has been developed. The conversion system consists of two stages, optical parametric generator and optical parametric amplifier (OPA), which are pumped by an Yb:YAG thin-disk laser operated at 77 kHz repetition rate, 1030 nm wavelength, and pulse duration down to 1.3 ps. The signal beam is amplified up to 9.2 W and the idler up to 5.4 W at OPA pumping of 43 W. Tunability between 1.70 and 1.95 μm for the signal and between 2.2 and 2.6 μm for the idler has been achieved. The system is rather simple and power scalable.

High-Power Single-Frequency Source in the Mid-Infrared Using a Singly Resonant Optical Parametric Oscillator Pumped by Yb-Fiber Laser

We present a high-power single-frequency continuously tunable mid-infrared (MIR) singly resonant optical parametric oscillator (SRO) using $\text{80-mm}$ -long MgO-doped PPLN (MgO:PPLN) crystal pumped by a continuous-wave Yb-fiber laser. The SRO having four-mirror ring-cavity configuration is tunable in the MIR band of $\text{3761--3970 nm}$ yielding a maximum power of $\text{2W}$ at $\text{3895 nm}$ when incident pump power is $\approx \text{16 W}$ . Due to strong phonon–polariton coupling in MgO:PPLN crystal, the intracavity signal-beam spectrum exhibits multiple peaks depicting stimulated polariton scattering. By replacing one of the cavity mirrors with a $\text{5}\%$ outcoupler, the stimulated polariton modes are suppressed as a consequence of additional cavity losses. Along with a tunable signal extending from $\text{1453}$ to $\text{1484 nm}$ , we obtained the maximum signal power of $\approx \text{4W}$ at $\text{1464 nm}$ at the maximum pump power of $\approx \text{16W}$ . In addition, we have measured the frequency and power stability of the outcoupled signal and idler beams over long as well as short durations. The improvement in beam quality of idler was discernible when the intracavity signal was outcoupled, which is essentially a consequence of reduced thermal load on MgO:PPLN crystal.

Potential applications of high-index acoustic metamaterials

In general, sound propagates in solid materials with a speed that is larger than that of air. Here, we discuss and demonstrate a simple metamaterial design that provides a sound speed slower than that of air. We highlight the potential of high-index acoustic metamaterials for open waveguiding, real-time analog acoustic signal processing, acoustic birefringence, and beam splitting.

Tunable Dual-Wavelength, Continuous-Wave, Mid-Infrared Generation Using Intracavity Difference Frequency Mixing in PPLN-Based Optical Parametric Oscillator

We report on a widely tunable, dual-wavelength, continuous-wave, mid-infrared optical parametric oscillator (OPO), pumped by a dual-wavelength fiber source that was realized by combining two fiber lasers in series. The OPO was designed as a four-mirror singly resonant cavity, in which a periodically poled lithium niobate (PPLN) crystal was used as nonlinear crystal. Two idler emissions with different wavelengths were observed, one being basically fixed at 3140 nm and the other being tunable from 3227 to 3413 nm. Under the maximum pump power, the total idler power was over 6 W and the corresponding pump-to-idler slope efficiency reached 11.4%. The generated signal beam had just one central wavelength around 1600 nm. Through theoretical analysis and experimental verification, it was analyzed that two nonlinear processes, OPO and intracavity difference frequency generation (DFG), simultaneously occurred in the resonant cavity. The experimental phenomena revealed large widened DFG gain spectrum, which was in absolute contrast to simulation results based on classical DFG theory. It indicated great potential in practical application such ultra-wide nonlinear frequency conversion.

Analysis of interference fringes based on three circularly polarized beams targeted for birefringence distribution measurements.

As a potential means of measuring birefringence distribution, we analyzed the interference fringes based on three circularly polarized beams: a right-handed signal beam, a left-handed reference beam, and a right-handed reference beam. All beams were crossed at the same angle on the interfering plane, creating a two-dimensional interference fringe with three grating vectors. We proposed that by analyzing the interference fringes, we can measure the anisotropic phase shift in the signal beam. The obtained features of the anisotropic phase shift can be extended to the measurement of two-dimensional birefringence distributions without rotational manipulations of the objectives or polarizers. The fringes were generated by monolithic gratings, which can generate three-beam interfering fields precisely and easily. Finally, we confirmed the feasibility of a birefringence measurement system without any rotational manipulations of optics.

Experimental implementation of multiplexing/demultiplexing in digital images using virtual phase conjugation for holographic data storage

We experimentally implemented and proved the concept of multiplexing and demultiplexing in digital images using virtual phase conjugation, which was proposed in our previous study. In the experiment, we concluded that two digital images multiplexed in a single signal beam are recorded in a holographic medium, and these images are independently and successfully reproduced. In this method, the digital images are multiplexed by superimposing them on a complex amplitude, and not using volume hologram’s multiplexing. Thus, the exposure amount in the holographic medium is constant regardless of the number of multiplexing of digital images, and the method has great potential for achieving high recording density.

Optical parametric oscillator based on LBO crystal at degeneracy

A pulsed doubly resonant LiB3O5 (LBO) optical parametric oscillator design (OPO) at degenerate case pumped with a frequency doubled Q-switched Nd: YAG laser was built-up. The LBO crystal was designed for collinear phasematching. The threshold power density at degenerate case was 141.4 MW/cm2. The linewidth of the amplified output at degenerate case due to both signal and idler beams was found to be 10 ns. The effect of crystal rotation in the horizontal plane on the output energy per pulse of both signal and idler were studied. The OPO maximum output energy per pulse at degeneracy was assessed, it was 54 μJ for 20 mJ pumping energy at optimum phasematching angle θpm=11.2°. The theoretical calculations that describe the output energy per pulse at different input pumping energy for the LBO-OPO linear cavity are numerically simulated and compared with the experimental results as well as conversion efficiency calculations at optimum phasematching.

Investigation of residual amplitude modulation in squeezed state generation system.

We present an analysis on how the optical parametric oscillator (OPO) detuning and the relative phase drift deteriorate the stability of the squeezed states, including the output power and the squeezed degree, and investigate the influence of RAM on the cavity detuning and the relative phase drift under different cases. Subsequently, the RAM is experimentally measured. In term of the measurement results, we perform a comparative study about RAM's influence on the cavity and phase locking in two cases. As a result, with the error signal extracted from the transmission of the OPO, the output power stability of the squeezed light is greatly improved. With the phase modulation imposed on the signal beam, the long-term stability of the squeezed degree is significantly enhanced.

Point-ahead demonstration of a transmitting antenna for satellite quantum communication.

A low-divergence beam is an essential prerequisite for a high-efficiency long-distance optical link, particularly for satellite-based quantum communication. A point-ahead angle, caused by satellite motion, is always several times larger than the divergence angle of the signal beam. We design a novel transmitting antenna with a point-ahead function, and provide an easy-to-perform calibration method with an accuracy better than 0.2 μrad. Subsequently, our antenna establishes an uplink to the quantum satellite, Micius, with a link loss of 41-52 dB over a distance of 500-1,400 km. The results clearly confirm the validity of our model, and provide the ability to conduct quantum communications. Our approach can be adopted in various free space optical communication systems between moving platforms.

Fiber-laser-pumped, continuous-wave, dual-wavelength, mid-infrared optical parametric oscillator

We demonstrated a new kind of dual-wavelength mid-infrared optical parametric oscillator (OPO), synchronously pumped by two independent fiber lasers, which were combined in parallel by a wavelength division multiplexer and had central wavelengths located at 1018 nm and 1064 nm separately. With reasonable cavity design and under the maximum pump power of 55.5 W, two idler emissions fixed at 3200 nm and 3704 nm were obtained and the total idler output power reached 2.6 W, indicating a 4.7% pump-to-idler conversion efficiency. Just one signal beam was observed in the experiment, which was around 1493 nm. It was analyzed that two OPO processes existed in the resonant cavity and shared the same signal beam, both which simultaneously met the phase-matching conditions.

Flexoelectro-optic effect and two-beam energy exchange in a hybrid photorefractive cholesteric cell with a short-pitch horizontal helix.

We develop a theoretical model to describe two-beam energy exchange in a hybrid photorefractive cholesteric cell with a short-pitch helix oriented parallel to the cell substrates (so-called uniformly lying helix configuration). Weak and strong light beams incident on the hybrid cell interfere and induce a periodic space-charge field in the photorefractive substrate of the cell, which penetrates into the cholesteric liquid crystal (LC). Due to the flexoelectro-optic effect an interaction of the photorefractive field with the LC flexopolarization causes the spatially periodic modulation of the helix axis in the plane parallel to the cell substrates. Coupling of a weak signal beam with a strong pump beam at the LC permittivity grating, induced by the periodically tilted helix axis, leads to the energy gain of the weak signal beam. Dependence of the signal beam gain coefficient on the parameters of the short-pitch cholesteric LC is studied.