Small Aperture Laser Research Articles

Variable-inclination Mach-Zehnder interferometer for testing laser robs

In order to measure the transmission wavefront of laser rods and to improve the edge diffraction effect of small-aperture laser rods measured by Tayman or Fizeau interferometer, a variable-inclination Mach-Zehnder interferometer was proposed. The incident angle was changed by adjusting the tilting attitude of the phase shifting reflector, then the optical path difference was changed that the phase shift was introduced to the coherent light and the phase shifting interferometry was realized. The transmission wavefront of a laser rod (Nd:YAG) with the diameter of 6 mm and the length of 60 mm was measured by this interferometer, the peak-valley (PV) and root mean square (RMS) of the wavefront were 0.391λ and 0.056λ. The same laser rod was measured by ZYGO GPI XP interferometer, the peak-valley (PV) and root mean square (RMS) were 0.370λ and 0.064λ. The comparison results show that the interferometer can achieve high-precision detection of transmission wavefront of laser robs. The variable-inclination Mach-Zehnder interferometer has high phase-shifting precision and wide phase-shifting range, and the beam in the system can pass through the laser rod only once, which can suppress the multi-beam interference and improve the edge diffraction effect of the small-aperture laser rods.

Effect of Coherent Structures on Aero-Optic Distortion in a Turbulent Boundary Layer

The deflection of a small-aperture laser beam was studied as it passed through an incompressible turbulent boundary layer that was heated at the wall. The heating at the wall was sufficiently mild that the temperature and density fields acted as passive scalars with a Prandtl number of 0.71. Simultaneous particle image velocimetry and Malley probe laser deflection measurements were performed in overlapping regions of the boundary layer to identify correlations between coherent velocity structures, passive scalar transport, and optical beam deflection. Streamwise gradients in the streamwise and wall-normal velocity fields were observed to be correlated to the deflection of the optical beam and to streamwise density gradients. The passage of a large-scale motion through the beam path was shown to affect the statistics of the optical beam deflection as well as the local distribution of small-scale velocity features. The wall-normal small-scale velocity features were consistently correlated to the beam deflection, throughout different phases of the large-scale motion convection. The observations motivated a hypothesis that views the large scales as heat carriers, whereas the small scales modify the local sense of a velocity and density gradient toward a streamwise gradient that directly affects the optical beam deflection.

Laser-induced damage of a large-aperture potassium dihydrogen phosphate crystal due to transverse stimulated Raman scattering

A unique ‘double-lung’ laser-induced damage pattern of a large-aperture potassium dihydrogen phosphate (KDP) crystal is observed during a damage experiment carried out on a large-aperture laser system, which has not been predicted by damage tests on small-aperture laser systems. Investigations show that ‘double-lung’ damage results from transverse stimulated Raman scattering (SRS) and the non-uniform fluence and intensity distribution of transverse SRS leads to the ‘double-lung’ damage pattern.

The effect of laser beam size on laser-induced damage performance

The influence of laser beam size on laser-induced damage performance, especially damage probability and the laser-induced damage threshold (LIDT), is investigated. It is found that damage probability is dependent on beam size when various damage precursors with different potential behaviors are involved. This causes the damage probability and the LIDT to be different between cases under a large-aperture beam and a small-aperture beam. Moreover, the fluence fluctuation of the large-aperture laser beam brings out hot spots, which move randomly across the beam from shot to shot. Thus this leads the most probable maximum fluence after many shots at any location on the optical component to be several times the average beam fluence. These two effects result in the difference in the damage performance of the optical component between the cases under a large-aperture and small-aperture laser.

Enhancement of the far-field output power and the beam properties of plasmonic very small aperture lasers

The properties of plasmonic very small aperture lasers are shown: these integrate surface plasmon structures with very small aperture lasers. The transmission field can be confined to a spot of subwavelength width in the far field, and according to the finite difference time domain simulation results the focal length of the spot can be modulated using different ring periods. Scanning of the subwavelength gating in the far field has been realized numerically. Such a device can be used with a high-resolution far-field scanning optical microscope.

Observation of geometrical resonance in optical throughput of very small aperture lasers associated with surface plasmons

Very small aperture lasers utilizing single rectangular apertures with geometrically resonant surface plasmons are experimentally characterized. Bimetal layers of tungsten and silver were employed to realize narrow apertures in a focused ion beam etching process. The tungsten overlayer acts as a mask suppressing aperture opening induced by the tail of a gallium ion beam. We observed strong enhancements in the optical output of narrow apertures (<45 nm) fabricated by single-pass mode, which we attribute to the geometrical resonance of surface plasmons within the apertures. Wider apertures (160 nm) fabricated in raster mode did not exhibit this enhancement, consistent with the explanation of geometrical resonance.

Field localization in very small aperture lasers studied by apertureless near-field microscopy

Localized surface plasmon polaritons (SSPs) have been observed on very small aperture lasers using apertureless near-field microscopy. Fields around multiple apertures are shown to result from interferences of SPP point sources at each aperture and optical fields. The near-field optical pattern around a single aperture indicates the interference of SPPs with their scattered counterparts. Near-field measurements also confirmed a preferred orientation of the rectangular aperture waveguide for the signal localization in very small aperture lasers.

Rigorous transmittance analysis of a sub-wavelength Sb thin film lens

We quantitatively analysed the factors contributing to the optical transmission enhancement of a sub-wavelength Sb thin film lens, using the finite-difference time-domain (FDTD) method. The results show that the transmission enhancement of the dielectric with a Gaussian distributed refractive index loaded in a sub-wavelength circular hole is not only due to the high refractive index dielectric, but also due to the specific distributions of refractive index. It is the first study about the effects of the refractive index distribution on the transmission of a sub-wavelength aperture. This kind of lens has practical applications in the very small aperture lasers and for near-field optical storage and lithography.

Study of the parameters of a difference image in the presence of a linear phase shift

The spatial filtering in the localization range of the holographic lateral shearing interferograms of a solid object shows that the effect of the subtraction of the compared original data arrays is realized in the presence of a filtering aperture in the center of a dark fringe [1, 2]. Methods for the holographic subtraction of the images involving the detection and spatial filtering in the Fourier plane of the optical system are proposed and realized in [3–5]. Thus, the difference data can be reconstructed with a narrow (unexpanded) laser beam. However, a relatively high level of speckle noise in the difference image is caused by the spatial filtering with a small aperture (laser beam) in the minimum of the superposition field in the Fourier hologram plane. The detection of double-exposed Fourier holograms with a linear phase shift and the effect of the size of the filtering reconstructing beam on the resolution of the difference image are studied. In addition, the application of the multiple exposure for the broadening of the interference maxima is analyzed.

Design, Fabrication and Characterization of Very Small Aperture Lasers

An L-shaped aperture with 15nm ultra-high optical resolution and strong field enhancement is designed. The very small aperture lasers (VSAL) with L- and rectangular aperture are fabricated, and their confined optical fields are detected using an apertured scanning near-field optical microscope (SNOM). For L-aperture VSAL, when the drive current is 28mA, the power output can reach 1.049�W , while the power output of the rectangular aperture VSAL at most reaches 0.078�W . In the experiment based on the apertures in Au film on glass substrate, the peak intensity of C aperture is about 10 times over the square aperture. The experimental results indicate that the unconventional L- and C-aperture have stronger field enhancement than rectangular and square aperture.

Design tips of nanoapertures with strong field enhancement and proposal of novel L-shaped aperture

We summarize some design tips and design procedures of nanoapertures with strong field enhancement, based on the theoretical calculations and analyses of near-field distributions of unconventional C apertures and bow-tie apertures by a finite-difference time-domain method. A novel L-shaped aperture is proposed and further validates the proposed design rule. As a result, a 15×15-nm full width half maximum spot forms in the light exit plane of the L aperture. Its intensity maximum enhancement reaches a factor of about 568 and the power throughput is bigger than unity. The mechanisms of electromagnetic field enhancement are preliminarily analyzed. This unconventional nanoaperture is likely to be directly used as an exit aperture of very small aperture lasers, or as an unattached shield, to produce a subwavelength light source with high transmission.

Aperture shape effect on the performance of very small aperture lasers

We report on the effect of different aperture shapes on the power output of a very small aperture laser (VSAL) fabricated from commercial edge-emitting laser diodes and correlate the results to the layer structure and polarization of the optical field. A waveguide theory is used to explain the experimental observations. We show that the shape of the aperture has a significant effect on the VSAL output power. In particular shapes exploiting the asymmetry of the laser can achieve much higher throughput over square apertures while keeping the aperture area constant. This work also indicates the validity of analyzing near-field small apertures as cutoff waveguide structures.

Fabrication of Very Small Aperture Laser (VSAL) from a Commercial Edge Emitting Laser

In this paper we report a method for producing very small aperture lasers (VSALs) from low-cost, commercial index guided edge emitting diode lasers. The detailed fabrication procedure is provided and the VSALs are characterized. Both high power and high aperture power to background noise power ratio are obtained even with a small aperture. This fabrication method can be applied to most commercial diode lasers without knowledge of the detailed geometry and is highly reproducible.

Small solid-state switched pulser-sustainer TEA CO2 laser

A corona preionized pulser-sustainer, transverse, electric, atmospheric CO2 laser has been developed. This small-aperture laser can produce 180 mJ optical pulses of 5 mu s duration from a discharge volume of 15.6 cm2 with an efficiency of 12.4%. The laser was driven by a simple inexpensive solid-state exciter.

Wide-aperture hybrid TEA CO2 lasers

The single mode performance of a wide aperture CW-TEA CO2 hybrid laser is reported, single longitudinal mode (SLM) is obtained from a wide aperture laser. The SLM from small aperture laser is given also for comparison.