Far-field Interference Pattern Research Articles

Single-mode lasing in an AlGaInAs/InP dual-port square microresonator.

Mode selection is crucial to achieving stable single-mode lasing in microlasers. Here, we demonstrate experimentally a dual-port square microresonator for single-mode lasing with a side-mode-suppression ratio (SMSR) exceeding 40 dB. By connecting waveguides at two opposite vertices, the quality factor for the antisymmetric mode (ASM) is much higher than that of the symmetric mode (SM), enabling single-mode lasing. Furthermore, far-field interference patterns similar to Young's two-slit interference are observed. This microlaser is capable of providing two optical sources simultaneously for optical signal processing in high-density integrated photonic circuits.

Statistical properties of double-slit interference of partially coherent polarization singular vector beams

The Young’s double-slit interference of partially coherent polarization singular vector beams (PC-PSVBs) is investigated both theoretically and experimentally. Unlike the case of scalar or linearly polarized light beams, the far-field interference pattern of PC-PSVB’s have spatial dependence both in horizontal and vertical directions. The visibility of these patterns is governed by the input spatial correlation length. The number of minima along the vertical direction (y-axis) depicts the magnitude of Poincaré-Hopf index (PHI) of the incident beam. Our results reveal that the far-field statistical properties namely, intensity pattern, degree of polarization (DOP) distribution and Stokes-parameters distributions of the PC-PSVBs diffracted through a double-slit are strongly influenced by the spatial correlation length of the incident beam. It is observed that the DOP distribution of the diffracted PC-PSVB does not follow the azimuthally symmetric DOP profile of the input PC-PSVB. Interestingly, the DOP distributions contain the information of the magnitude of PHI of the incident PC-PSVB even for a low value of spatial correlation length. However, the maximum value of DOP deteriorates with the decrease in the input spatial correlation length. Furthermore, the Stokes field (S12(ρ,z)) of the diffracted PC-PSVB contains the information of both the magnitude and polarity of the PHI of the input beam. The experimental results are in good agreement with the theoretical predictions, and may find applications in the areas where the statistical properties of a light field are important.

Compact internal sensing phase locking system for coherent combining of fiber laser array

A compact internal sensing phase locking system for coherent combining of large aperture fiber laser beams is demonstrated in this paper. Herein, the phase noises in the laser channels are detected and compensated before the lasers form a large cross-section laser array. Therefore, the large aperture beam splitters and lens are no longer needed for phase locking, and the phase-locking system can be more compact, especially for the coherent laser array with large cross-section. We built a 7-channel fiber laser array to experimentally verify the feasibility of this technique. The phase deviation was less than λ/31 RMS, and the visibility of far-field interference pattern of the combined beam was ∼96.3%. This technique can be used in high-power coherent beam combining system with the large aperture laser beams.

Young's double-slit experiment with vector vortex beams.

In this Letter, Young's double-slit experiment with vector vortex beams is investigated. We present the results for various Poincaré-Hopf index beams of this class considering all four major types. Polarization associated morphological changes in the far-field interference pattern are studied both theoretically and experimentally. The Fraunhofer pattern consists of lattices of polarization singularities of the generic type, located on a line, in a direction perpendicular to the slit. The number of linear lattices varies as a function of Poincaré-Hopf index η of the beam that is diffracted, and the number of intensity nulls occurring along the vertical line is equal to |η|.

W-Band Photonic Pulse Compression Radar With Dual Transmission Mode Beamforming

Millimeter wave (MMW) sensing systems have potential for enhanced range and angular resolution in comparison to lower radio frequencies. This article employs photonic-assisted radio-frequency arbitrary waveform generation to implement a high-bandwidth multi-target MMW radar. The presented sensing system operates in the W-band using chirped RF signals, with spectra between 80 and 95 GHz at the 10 dB points. We implement sum- and difference-mode beamforming using a two element transmitter array to perform azimuth angle sensing in addition to target ranging. Photonics-based waveform generation with programmable optical pulse shapers provides for incorporation of approximately uniform phase shifts across the entire bandwidth, which in turn enables control of the far-field interference patterns of the broadband waveforms. Our approach has analogies with monopulse tracking radar techniques, in which target angle information is revealed through sum- and difference-mode processing of the receiving antennae, but with the dual-mode processing implemented at the transmitter side via photonic radio-frequency arbitrary waveform generation. Experiments with a single target demonstrate unambiguous sensing of target angle with an estimated root-mean-square error of 0.18 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> over a ±4 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> angular range. We also demonstrate independent sensing of the angles of two targets that are resolved in range.

Geometric-phase-induced arbitrary polarization and orbital angular momentum generation in helically twisted birefringent photonic crystal fiber

The evolutions of polarization and orbital angular momentum (OAM) states of light in helically twisted birefringent photonic crystal fibers (TB-PCFs) are analyzed. It is shown that a circular polarization (CP) component ( S 3 of a Stokes parameter) is periodically excited when usual linearly polarized (LP) modes of PCF are launched. The excitation originates from a geometric phase in TB-PCFs. The S 3 excitation is larger for larger linear birefringence for a fixed twisting rate. If the linear birefringence is large enough, a CP filtering behavior can be seen in addition to the S 3 excitation. From the analytical consideration of the sign of the geometric phase, the TB-PCF with periodical inversion of twisting is proposed to generate arbitrary polarization state on the Poincaré sphere. Next, an OAM state generation in multimode TB-PCFs is shown for higher-order LP mode input. By observing a far-field interference pattern from TB-PCF mixed with LP 01 mode, a vortex associated with the OAM state can be seen. Similar to the single-mode case, by using periodical twisting inversion, efficient OAM generation is possible. These results indicate that by simply launching fiber’s LP mode into TB-PCF, arbitrary polarization and OAM states can be generated, leading to a novel mechanism for the manipulation of the spatial state of light.

Small footprint cholesteric liquid crystal laser.

We demonstrate a small optical bench footprint laser assembly based on the small pulsed Nd:YAG laser head SSY-1 for pumping cholesteric liquid crystal (CLC) lasers and illustrate its performance using low molecular weight CLC samples doped with the fluorescent dye PM597. A low lasing threshold, narrow laser line, and far-field interference patterns of the CLC laser were observed using the SSY-1-based laser assembly as the pump. The emission characteristics of the CLC laser are similar to those observed with comparable CLC materials pumped by an order of magnitude physically larger and many orders of magnitude more expensive commercial Nd:YAG laser systems. The small footprint CLC laser demonstrated in this work provides an opportunity for significant size and cost reduction of CLC lasers and fostering their practical applications.

Probing the fractional topological charge of a vortex light beam by using dynamic angular double slits

Vortex beams with fractional topological charge (FTC) have many special characteristics and novel applications. However, one of the obstacles for their application is the difficulty of precisely determining the FTC of fractional vortex beams. We find that when a vortex beam with an FTC illuminates a dynamic angular double slit (ADS), the far-field interference patterns that include the information of the FTC of the beam at the angular bisector direction of the ADS vary periodically. Based on this property, a simple dynamic ADS device and data fitting method can be used to precisely measure the FTC of a vortex light beam with an error of less than 5%.

Dynamic interferometry measurement of orbital angular momentum of light.

We present a dynamic interferometry to measure the orbital angular momentum (OAM) of beams. An opaque screen with two nonparallel air slits is employed that can be regarded as the Young's double-pinhole interference. When the OAM beams with an annular intensity distribution vertically incident, the far-field interference patterns depend on the phase difference of the light in the two pinholes. We scan the angle between the two slits, and the output intensity at center changes alternatively between darkness and brightness. Utilizing this characteristic, we can measure the OAM of light. This scheme is very simple and low-cost. In addition, it can measure very large topological charge of OAM beams due to continuous scanning.

Passive Phase Locking of Three Nanosecond Fiber Amplifiers Using a Dammann Grating Spatial Filter

A passive coherent beam combination of three nanosecond Yb-doped fiber amplifiers by an all-optical feedback loop is realized by a Dammann grating intracavity spatial filter. By using this diffractive-optics-based spatial filtering technique, three tile-aperture laser beams are phase-locked with a peak power of 1.02 kW. The width of the combined pulses is 9.6 ns, and the repetition frequency is 2.208 MHz. The visibility of the far-field interference pattern is up to 82.9%. The results show that this approach can scale to larger arrays and higher powers.

Dammann-grating-based passive phase locking by an all-optical feedback loop

A Dammann grating is used as a spatial filter for a passive coherent beam combination (CBC) of three Yb-doped fiber amplifiers with an all-optical feedback loop. Using this diffractive-optics-based spatial filtering technique, we demonstrate CBC with 20 W output power, and the visibility of the far-field interference pattern is up to 88.7%. Measurements suggest that this approach is robust with respect to laboratory environment perturbations, and it can scale to high powers and large arrays.

Superimposed surface-relief diffraction grating holographic lenses on azo-polymer films

Various superimposed chirped relief gratings, acting as diffracting holographic lenses, were photo-inscribed on azo-polymer films upon exposure to the interference pattern of a plane and a curved laser light wavefronts. Depending on the configuration used, this resulted in incident light being focused independently of polarization along the 0th or 1st diffracted order of the grating. The focal point and focalization angle of the resulting holographic lenses were easily tuned during the fabrication process. Furthermore, a dual-focus chirped holographic lens grating was fabricated and shown to exhibit a far-field interference pattern.

Selective reflection of Airy beam at an interface between dielectric and homogeneous atomic medium

We examine selective reflection (SR) spectrum of an Airy beam at an interface between a dielectric and a homogeneous atomic medium. It is shown that both the general reflection (GR) and the SR of Airy beams exhibit accelerating dynamics with a parabolic trajectory, however, the accelerating rate for the SR is slightly greater than that for the GR. Due to interaction of atoms and the Airy beams at the interface between dielectric and resonant atoms, the SR beams can create far-field interference patterns. We also show that the amplitude of the SR can be dramatically modified by the detuning of the incident fields, the reduced x-coordinate and the distance from the interface. The SR at the resonant medium interface of Airy beams can probably be a powerful tool in the use of optical power delivering, resonant particle manipulation and spatial spectrum detection of a resonant medium at an interface.

Passive coherent beam combining of four Yb-doped fiber amplifier chains with injection-locked seed source

An injection-locked fiber laser is introduced to the passive fiber laser coherent beam combination with all-optical feedback loop. A coherent beam combining system with two-dimensional four Yb-doped fiber amplifier chains is established, and the injection-locked fiber laser works as a switchable seed source. The 1064 nm output laser of the injection-locked fiber laser is extinguished automatically as the feedback injection power is high enough, and the injection-locked fiber laser acts as an amplifier for the feedback laser with 7.4 dB gains. We find that the phase-locked far-field interference pattern of our system with seed laser extinguished is stable, and the visibility is up to 91.5%, which is slightly higher than the prevalent method with auxiliary seed laser (88.2%).

Instability of higher-order optical vortices analyzed with a multi-pinhole interferometer

Higher-order optical vortices are inherently unstable in the sense that they tend to split up in a series of vortices with unity charge. We demonstrate this vortex-splitting phenomenon in beams produced with holograms and spatial light modulators and discuss its generic and practically unavoidable nature. To analyze the splitting phenomena in detail, we use a multi-pinhole interferometer to map the combined amplitude and phase profile of the optical field. This technique, which is based on the analysis of the far-field interference pattern observed behind an opaque screen perforated with multiple pinholes, turns out to be very robust and can among others be used to study very 'dark' regions of electromagnetic fields. Furthermore, the vortex splitting provides an ultra-sensitive measurement method of unwanted scattering from holograms and other phase-changing optical elements.

Phase Locking of Laser Diode Array by Using an Off-Axis External Talbot Cavity

Phase locking of a laser diode array is demonstrated experimentally by using an off-axis external Talbot cavity with a feedback plane mirror. Due to good spatial mode discrimination, the cavity does not need a spatial filter. By employing the cavity, a clear and stable far-field interference pattern can be observed when the driver current is less than 14 A. In addition, the spectral line width can be reduced to 0.8 nm. The slope efficiency of the phase-locked laser diode array is about 0.62 W/A.

The Improved Power of the Central Lobe in the Beam Combination and High Power Output

In order to increase the power fraction of the central lobe in the coherent beam combination of lasers in an array, the effects of the distance factor of near-field distribution on far-field interference patterns are calculated and demonstrated experimentally. An improved beam array of interwoven distribution is demonstrated to enable the power in the central lobe to reach 41%. An optimized mirror array is carefully designed to obtain a high duty ratio, which is up to 53.3% at a high power level. By using these optimized methods and designs, the passive phase locking of eight Yb-doped fiber amplifiers with ring cavities are obtained, and a pleasing interference pattern with 87% visibility is observed. The maximum coherent output power of the system is up to 1066 W.

Coherent beam combination of broad-area laser diode array using of f-axis external cavity with double feedback

In this letter, we demonstrate coherent beam combination of laser diode array using the external Talbot cavity with double feedback. The double feedback elements consist of grating and high-reflection plane mirror. Compared with single high-reflection plane mirror feedback, the external Talbot cavity with double feedback reduce the number of interference strips in the far-field pattern and narrow spectral line-width of the laser diode array. The results indicate that the application of the external Talbot cavity with double feedback produces a clear far-field interference pattern. In addition, line-width is reduced to 0.15 nm full-width at half-maximum (FWHM).

Modeling and experimental observation of an on-chip two-dimensional far-field interference pattern

In this paper, we model and experimentally observe the far-field radiation produced by interfering beams propagating in two-dimensional (2D) slab waveguides. Using a transmission-line analogy, we compare the 2D propagation with standard three-dimensional (3D) far-field representations and derive the 2D conditions for using standard far-field approximations. Then we test our theoretical results by experimentally observing the 2D far-field pattern produced by a 1×3 multimode interference (MMI) coupler on a silicon nanomembrane. The MMI outputs are connected to a slab silicon waveguide, and the far field is observed at the edge of the silicon slab. This represents the observation of 2D far-field pattern produced by an array of on-chip radiators.

Coherent combination of two ytterbium fiber amplifier based on an active segmented mirror

To control the phase noise of two ytterbium fiber amplifiers, a coherent combination system based on an active segmented mirror (ASM) has been established in our laboratory. The ASM is controlled by a feed back control loop on the basis of a Peak Rate (PR) algorithm which is realized on a DSP + FPGA hardware control board. Experimental results indicate that when the control loop is off, the far-field interference pattern is blurred and dynamic, while when it is on, the far-field beams interference pattern achieves clear and stable. At two different output powers, the contrasts of the interference stripes are improved from 7% to 19% and 8% to 28% respectively.