Astigmatism Compensation Research Articles

Thermal distortions in high-power end-pumped Nd:YVO4 crystals

A high-resolution infrared thermal imager is employed to measure temperature and fit the thermal conductivity of Nd:YVO4 crystal. A theoretical model to analyze the thermal distortion of end-pumped Nd:YVO4 crystals is established. Both simulations and experiments reveal that with high-power pumping, the thermal effects of Nd:YVO4 crystal cause spherical aberration as well as astigmatism. It is identified that the major cause of astigmatism is the anisotropy of the Nd:YVO4 crystals’ thermal conductivity. The influence of astigmatism on the ellipticity of the output beam is investigated, and the significance of astigmatism compensation is demonstrated. The experimental results agree well with the simulation results.

Биоптический подход к эксимерлазерной хирургии при сочетанной патологии роговицы и хрусталика

Purpose. To consider a biooptical approach to excimer laser surgery for combined pathology of the cornea and lens. Material and methods. The work is based on 20-year clinical observations of the results of excimer laser refractive, opto-reconstructive and medical surgery in combined pathology of the cornea and lens based on a biooptical approach (more than 1500 operations). Particular emphasis was placed on the cycloplegic functional test and the use of stromal saturation with 0.25% chilled to +5–7 °C isotonic riboflavin solution during photoablation. The studies used modern diagnostic devices OPD-scan, RTVue XR100 and TMS-5 and excimer ophthalmic lasers from various companies. Results. In case of combined refractive pathology of the cornea and lens, the cycloplegic test made it possible to more accurately determine the degree of ametropia, its accommodative component with compensation of corneal astigmatism, and the presence of lens astigmatism. In all cases, the determination of the volume of photorefractive ablation was calculated taking into account the contribution of the cornea and lens to the formation of ametropia and aberrations of the optical system of the eye. The diameter of the optical and transition zone during photorefractive ablation was calculated depending on the state of the pupil diameter in photopic and mesopic conditions. Obtained in the course of studies on the OPD-Scan device, objective information on the difference between the pupil width in mesopic and photopic conditions made it possible to individualize the sizes of the optical and transition zones during photorefractive ablation, to avoid the «Halo» and «Gler» effects. Photoablation with saturation of tecorneal stroma with riboflavin reduced the response of aseptic inflammatory and regenerative reactions, the severity and duration of the corneal syndrome, and provided earlier stabilization of optical refractive indices. Conclusion. When performing excimer laser refractive, opto-reconstructive and therapeutic operations with combined pathology of the cornea and lens, a biooptical approach is required, taking into account the leading role of the cornea and lens in the formation of the optical system of the eye and visual acuity. Keywords: biooptics, photoablation, excimer laser corneal surgery

Subjective refraction using power vectors by updating a conventional phoropter with a Stokes lens for continuous astigmatic power generation.

To implement a pure power vector method for monocular subjective refraction using a regular phoropter with the only modification being the inclusion of a Stokes lens. The proposed methodology was tested with three different Stokes lenses, and the results were compared with conventional clinical refraction procedures. Power vector subjective refraction was performed by attaching a Stokes lens to the Risley prism holder. Stokes lenses allow for pure astigmatic compensation in the form of the J0 , J45 components while the spherical lenses in the phoropter allow determination of the spherical component in the form of M (spherical equivalent). The proposed routine is presented step-by-step using three Stokes lenses having different astigmatic powers. Monocular subjective refraction was performed on 26 healthy subjects with a mean age of 44 ± 16 years, mean spherical equivalent of -0.56 D (range -5.50 to +2.38 D) and refractive astigmatism ≤1.50 D. No differences were found between the results obtained with the conventional technique versus the vector-based procedure for the spherical equivalent (p = 0.28) or astigmatic components (p = 0.34). In addition, visual acuity (VA) was equivalent through the refractions measured with the conventional and vector procedures (p = 0.12). Repeatability coefficients for J0 and J45 with the new vector methodology were <0.38 D. The proposed routine could be helpful for cases where it is difficult to get a valid starting point for conventional refraction (e.g., irregular corneas and media opacities), for testing facilities with limited resources/equipment and/or for motivated clinicians who wish to know about alternative methods of refractive error determination.

Differential compensatory role of internal astigmatism in school children and adults: The Hong Kong Children Eye Study.

To compare the prevalence of refractive (RA), corneal (CA), and internal astigmatism (IA) in Hong Kong children and adults and evaluate the role of IA in compensating for total astigmatism and its relations to myopic traits. The Hong Kong Children Eye Study is a population-based cross-sectional study. Totally 3704 school children (mean age 7.5 ± 1.0 years) and 5577 adults (mean age 41.1 ± 7.5 years), who were their parents, were recruited. Cycloplegic and non-cycloplegic refractive cylinders were obtained from children and adults, respectively. Spearman correlation was applied to detect associations between astigmatism, ocular biometrics, refraction, and lens power. Astigmatism compensation factor (CF) was derived from the power vector analysis J0 and J45. The prevalence of RA (≤-1.0 D), CA (≥+1.0 D) and IA (≥+1.0 D) was 21.9%, 63.9%, and 9.9% in children, and 30.9%, 39.5%, and 23.7% in adults respectively. The mean RA, CA and IA values in children and adults were -0.69 ± 0.66 D, +1.14 ± 0.61 D, +0.62 ± 0.32 D, and -0.80 ± 0.74 D, +0.97 ± 0.69 D, and +0.76 ± 0.43 D, respectively. In adults and children, IA was negatively correlated with axial length (p < 0.0001), but positively correlated with spherical values and equivalent (p < 0.0001), suggesting an association of astigmatism with myopic traits. A greater proportion of children exhibited compensation by IA than adults in J0 (86.6% vs. 66.0%, p < 0.0001) and J45 components (55.5% vs. 41.7%, p < 0.0001). Chinese children in Hong Kong exhibit a higher prevalence of RA and CA than in other cities. Children displayed a greater compensation by IA than adults, suggesting an age-related attenuation of IA compensation. IA is associated with myopic traits.

Astigmatic Stokes lens revisited.

Stokes lenses are variable power astigmatic lenses comprising of, in its standard version, two pure cylindrical lenses of equal but contrary power that rotate in opposite directions. Here, we present an optical device with variable and continuous astigmatic power which is based on a modified Stokes lens where two sphero-cylindrical lenses (in the form of pure astigmatic lenses) are combined in the classical way but merged with another fixed pure astigmatic lens for improving the capabilities of the resulting optical device concerning the expansion of the astigmatic range without worsening the dioptric power step resolution. The performance of this device is theoretically analyzed in virtue of the power vectors formalism including a three-dimensional (3-D) representation of the generated dioptric power as a function of both the meridian and the rotation angle between the cylinder's axes. In addition, we have assembled a lab-made prototype of the proposed modified Stokes lens and validated its theoretical behavior by dioptric power measurements with an automatic focimeter. As conventional Stokes lenses, the applications of this new optical device range from astigmatism compensation in optical instruments to measurement of refractive error in subjective routines with the previously commented improved capabilities.

Optical module for single-shot quantitative phase imaging based on the transport of intensity equation with field of view multiplexing.

We present a cost-effective, simple, and robust method that enables single-shot quantitative phase imaging (QPI) based on the transport of intensity equation (TIE) using an add-on optical module that can be assembled into the exit port of any regular microscope. The module integrates a beamsplitter (BS) cube (placed in a non-conventional way) for duplicating the output image onto the digital sensor (field of view - FOV - multiplexing), a Stokes lens (SL) for astigmatism compensation (introduced by the BS cube), and an optical quality glass plate over one of the FOV halves for defocusing generation (needed for single-shot TIE algorithm). Altogether, the system provides two laterally separated intensity images that are simultaneously recorded and slightly defocused one to each other, thus enabling accurate QPI by conventional TIE-based algorithms in a single snapshot. The proposed optical module is first calibrated for defining the configuration providing best QPI performance and, second, experimentally validated by using different phase samples (static and dynamic ones). The proposed configuration might be integrated in a compact three-dimensional (3D) printed module and coupled to any conventional microscope for QPI of dynamic transparent samples.

Simple method for high beam quality laser resonator design

We present a simple method for designing a high beam quality laser resonator, in which the astigmatism in all of the arms is facilely compensated, for the first time, to the best of our knowledge. The analytical expressions for astigmatism compensation are also derived. A folded resonator is designed using these analytical expressions, in which the astigmatic aberration of all the arms is completely eliminated. The theoretical research results show that not only the spot intensity profile’s deformation but also the phase distortion in all the arms of the laser resonator can be simultaneously and completely compensated.

Method of the Cyclotorsion Error Compensation for Myopic Astigmatism during SMILE Surgery

Purpose: to develop an effective and safe method of cyclotorsion compensation, to improve the clinical and functional results of the of myopic astigmatism correction with SMILE surgery.Patients and methods: Two equivalent groups of 30 people (30 eyes) were formed with cyclotorsion from ±5 degrees («Standard» and «Verion»). The correction for cyclotorsion was carried out only in the «Verion» group. Before the laser-refraction surgery, all eyes were examined using «Verion» image guided system (Alcon, USA) to determine cyclotorsion. In the «Verion» group, the obtained cyclotorsion data was used to aligement the astigmatism axis by Introducing into the «Visumax» femtolaser system. Further, the operation was carried out according to the standard protocol. At 3 months after SMILE surgery, all patients were examined by a UDVA, CDVA, objective refraction. Furthermore, efficiency and safety, as well as vector analysis of corneal astigmatism according to Alpins were calculated indices at 3 months after surgery.Results: The mean and standard deviation of cyclotorsion of the «Standard» and «Verion» groups was 6.16 ± 1.31 D and 6.30 ± 1.36 degrees, respectively (p &gt; 0.05). The increasement of 1 or more UDVA lines was noted at 20 % and 7 %, respectively after 3 months in the «Standard» and «Verion» groups. The safety index (postoperative UDVA / preoperative UDVA) in both groups was not statistically significantly different (p &gt; 0.05). The predictability of the spherical equivalent within ±0.5 D relative to the target refraction (emmetropia) in the «Standard» and «Verion» groups was 70 % and 100 %, respectively. The predictability of the cylindrical component of refraction within ±0.5 D relative to the target refraction (emmetropia) was 40 % and 90 % for the «Standard» and «Verion» groups, respectively (p &lt; 0.05). Difference vector was 1.01 ± 0.59 D in the group «Standard» and 0.64 ± 0.33 D in the «Verion» group (p &lt; 0.05). Correction index was 0.80 ± 0.43 in the «Standard» group and 0.99 ± 0.04 in the «Verion» group (p &lt; 0.05).Conclusions: Сyclotorsion compensation using the «Verion» image guided system is a safe method. It improves the efficiency and predictability of correction myopic astigmatism in patient undergoing to SMILE surgery. This method recommended for patients with cyclotorsion from ±5 degrees and myopic astigmatism from –0.75 D.

Nonastigmatic alexandrite ring laser design with wavelength-tunable single-longitudinal-mode operation

This work presents a study of a fully nonastigmatic design of a single-longitudinal-mode, wavelength-tunable, unidirectional alexandrite ring laser cavity and assessment of its performance compared to more complex laser design requiring astigmatism compensation. A “displaced mode” nonastigmatic laser cavity design eliminating astigmatic cavity elements is developed around an alexandrite crystal end-pumped by a low-brightness, high-power red diode laser pump system. Single-longitudinal-mode, continuous-wave operation is demonstrated with output power of 700 mW with an excellent T E M 00 mode ( M 2 &lt; 1.1 ) across a wide pump power range. Wavelength tuning from 748–773 nm is produced using a birefringent filter plate. The nonastigmatic alexandrite laser design achieves better spatial quality and resilience to maintain T E M 00 operation across wide variation in pump-induced lensing compared to the astigmatic design. To the best of our knowledge, this is the first wavelength-tunable, single-longitudinal-mode operation of a unidirectional alexandrite ring system in a fully nonastigmatic cavity regime.

Characterization of a compact low-cost Stokes lens for astigmatism compensation in optical instruments.

Variable power cross-cylinder lenses (or Stokes lenses) have been widely known in the literature for decades. In this paper, we describe how to build a low-cost Stokes lens and discuss its calibration and its application to two significant cases. The construction is in virtue of a phoropter's Risley prism mount for assembling a couple of equal but opposite sign cylindrical lenses (we have selected $\,\pm 1.50$±1.50 D). Thus, variable astigmatic power is achieved by relative rotation of the lenses in opposite directions, and the resulting astigmatic axis is defined by the global rotation of the device. Calibration measurements are performed using an automatic lensmeter (Topcon CL-300) and an aberrometer (Zeiss iProfiler plus) for low and high order, respectively, aberration characterization. The proposed device has been adapted to a manual Topcon LM-8 lensmeter and to a regular Olympus BX-60 upright microscope for experimental validation concerning astigmatism compensation in a digital microscope and astigmatism cancellation in ophthalmic lenses, respectively. The device can be easily adapted to any ophthalmic/optic instrument for the compensation and/or measurement of astigmatism up to a maximum range of $|3|$|3| D of astigmatism.

Characterisation of internal, refractive, and corneal astigmatism in a UK university student population

PurposeThere is a clear benefit in defining internal (IA) and corneal astigmatic error (CA) prior to surgical and other refractive interventions, such as orthokeratology, to minimise risk of unsatisfactory refractive outcomes. Such data would also be of relevance to broader areas of ophthalmic care such as spectacle dispensing and other types of rigid lens fitting. This study offers a detailed characterisation of astigmatic error in a group of university students and specifically investigates compensation of corneal astigmatism by the eye’s internal optics. MethodsFor 176 young-adult participants, objective measurements of refractive error were obtained using the open-view Grand Seiko WAM-5500 autorefractor; corneal curvature and axial length were measured using the Aladdin biometer. Clinical measurements of corneal and refractive astigmatism were converted into vector components J0 and J45; followed by an assessment of corneal astigmatism compensation. ResultsMean total refractive astigmatism (RA), CA, and IA were 0.24 ± 0.32D, 0.46 ± 0.27D and -0.21 ± 0.25D respectively for J0 and -0.05 ± 0.20D, 0.01 ± 0.16D, and -0.06 ± 0.18D for J45. Significant linear correlations were noted between RA, CA, and IA for both J0 and J45 (P < 0.01). A significant linear regression was also noted between axial length and J45 RA and IA, but not CA. Levels of full compensation were low, 7% and 9% for J0 and J45 respectively, however, a complete absence of compensation was also uncommon particularly for J45 (2%). ConclusionsIn general, partial compensation for corneal astigmatism by the eye’s internal optics is noted, but it is unclear whether this is an active compensatory mechanism. Further, larger scale, studies would be required to characterise differences in corneal astigmatic compensation with respect to ethnicity.

Tunable polymer dye laser pumped by two 513 nm diodes

We demonstrate a tunable solid-state polymer dye laser with quasi-longitudinal pumping by two green (λ = 513 nm) laser diodes. Tuning curves with a maximum efficiency of 13% were attained, using a plate of crystalline quartz (Lyot filter) for wavelength selection. In a three-mirror cavity with partial compensation of astigmatism, the total tuning range was 62 nm (from 545 nm to 607 nm) for pyrromethene dyes: PM567, PM580 and PM597. The best results were achieved with active elements based on Pyrromethene 580 laser dye in a mixture of polyvinyl butyral or a copolymer of butylmethacrylate and methacrylic acid (ratio 9:1) with the addition of 30% dibutylsebacinate as a plasticizer.

Polymer dye laser pumped with green semiconductor lasers

We report the results of experiments on quasi-longitudinal pumping of a polymer dye laser by two green ( λ = 513 nm) pulsed laser diodes. The lasing thresholds, efficiency, and photostability of pyrromethenes in various polymers are studied. The efficiency of dye lasers based on pyrromethene 567 and pyrromethene 580 with a three-mirror cavity with partial astigmatism compensation exceeds 20 %.

Development of non-axisymmetric aspheric ultraprecision machining using FPGA-based piezoelectric FTS

With increasing demands in photoelectric industry, the non-axisymmetric aspherical machining for manufacturing specific optical components such as head up display for car with very short projection distance and astigmatic compensation lens for the laser diode is more and more essential. In this study, a field programmable gate array (FPGA)-based fast tool servo (FTS) using piezoelectric (PZT) tool holder was implemented for performing non-axisymmetric aspherical machining. Detail system implementation was described, experimental examinations for static and dynamic characteristics of the tool holder were presented, dry run test for the FTS function was conducted, and finally, the system was successfully verified by means of practical machining experiment, and measurement for 3D form accuracy based on the Levenberg-Marquardt algorithm (LMA). Future works are to configure a compensation methodology according to the evaluated form error.

Single-shot slightly off-axis digital holographic microscopy with add-on module based on beamsplitter cube.

Slightly off-axis digital holographic microscopy (SO-DHM) has recently emerged as a novel experimental arrangement for quantitative phase imaging (QPI). It offers improved capabilities in conventional on-axis and off-axis interferometric configurations. In this contribution, we report on a single-shot SO-DHM approach based on an add-on module adapted to the exit port of a regular microscope. The module employs a beamsplitter (BS) cube interferometer and includes, in addition, a Stokes lens (SL) for astigmatism compensation. Each recorded frame contains two fields of view (FOVs) of the sample, where each FOV is a hologram which is phase shifted by π rads with respect to the other. These two simultaneously recorded holograms are numerically processed, in order to retrieve complex amplitude distribution with enhanced quality. The tradeoff is done in the FOV which becomes penalized as a consequence of the simultaneous recording of the two holograms in a single snapshot. Experimental validation is presented for a wide variety of samples using a regular Olympus BX-60 upright microscope. The proposed approach provides an optimized use of the imaging system, in terms of the space-bandwidth product, in comparison with off-axis configuration; allows the analysis of fast-dynamic events, owing to its single-shot capability when compared with on-axis arrangement; and becomes easily implementable in conventional white-light microscopes for upgrading them into holographic microscopes for QPI.

Unidirectional single-frequency operation of a continuous-wave Alexandrite ring laser with wavelength tunability.

High resolution spectroscopy, metrology and quantum technologies (e.g. trapping and cooling) require precision laser sources with narrow linewidth and wavelength tunability. The widespread use of these lasers will be promoted if they are cost-effective, compact and efficient. Alexandrite lasers with a broad tuning band pumped efficiently by low-cost red diodes are a potential candidate, but full performance as a precision light source has not been fully achieved. We present in this work the first continuous-wave (CW) and single-frequency operation of a unidirectional diode-end-pumped Alexandrite ring laser with wavelength tunability. An ultra-compact bow-tie ring cavity is developed with astigmatic compensation and a novel 'displaced mode' design producing CW output power > 1 W in excellent TEM00 mode (M2 < 1.2) when using a low brightness pump (M2 ≥ 30). Wavelength tuning from 727 - 792 nm is demonstrated using a birefringent filter plate. This successful operation opens the prospects of precision light source applications.

Normal-Incidence Imaging Spectrograph Based on an Aperiodic Spherical Grating for the Vacuum Spectral Region

The possibility of astigmatism compensation in a wide wavelength range in normal incidence schemes based on a spherical diffraction grating with variable groove density has been considered. Two schemes imaging spectrographs with dimensions of about 1 and 5 m for the wavelength ranges 820–1690 and 980–1520 A that operate in the first external and first internal diffraction orders, respectively, have been calculated. It has been shown that the advantage of the external order of diffraction is a wide (more than an octave) spectral range at relatively compact sizes of the device. Schemes operating in internal orders of diffraction have better imaging characteristics, but large dimensions of the device are required to maintain the width of the wavelength range at the octave level. A comparison of the above schemes with schemes based on gratings with curvilinear grooves has been carried out. It has been shown that the variable line-spaced gratings yield a gain in the limiting spatial resolution, while the gratings with curvilinear grooves allow the achievement of a better limiting spectral resolution. The schemes of the considered type can be applied in the soft X-ray range when using diffraction gratings with a multilayer coating.

Large astigmatic laser cavity modes and astigmatic compensation

In this paper, it was experimentally verified that anisotropic (astigmatic) Hermite–Gaussian modes can be generated from a hemi-cylindrical cavity and can be transformed into Laguerre–Gaussian modes by using an extra-cavity plano-convex cylindrical lens which can modify different beam divergence angle in the x–z and y–z planes. By controlling the off-axis pumping and the angle of the extra-cavity cylindrical lens, a variation in laser patterns was observed. Moreover, several additional relationships under different conditions have been analyzed. All experimental results corresponding to the theoretical analysis, and the role of the hemi-cylindrical cavity can be completely understood by comparing the theoretical and experimental results.

Design and experimental study on high quality beam and high stability power of laser

High-performance lasers with high-quality beam laser and high-stability power are widely used for laser machining, laser precision measuring, etc. Reflection curved mirrors are widely used in lasers to provide several small intracavity focal spots and reduce the dispersion and volume of the laser. The primary disadvantage of using reflection curved mirrors in folded resonators is that relatively large angles of incidence deform the circular transverse pattern of the output beam and limit laser performance. In addition, in high power lasers or ultra short pulse lasers, the gain medium thermal lens focal length fluctuation is the primary cause of the instability of laser output power. This paper focuses on beam quality and power stability of laser, and an effective method of solving the two problems, i.e., astigmatism and instability power of laser, is presented. The laser resonator with high-quality laser beam and high-stability power is very easy and intuitive to design by this method, in which the resonator transform circle graphic theory is used and the thermal lens and astigmatism compensation is taken into account. The theoretical investigation shows that the astigmatism in two terminal arms of the folded laser resonator can be successfully eliminated by using this method, and the experimental measurements of the pattern of the laser output beam show that the deformations of spot intensity profiles in the two terminal arms can be simultaneously compensated for completely in the cavity, which is in good agreement with the analytical prediction. When the focal length of the laser crystal thermal lens varies, the variations of radii of spots not only at some key position, but also at all locations of the laser resonance designed by this method, are overtly smaller than the variation of the normal resonant cavity. The stability of the output laser power of the laser is better than that of a universal laser resonator under the same external conditions.

飞秒脉冲同步泵浦光学参量振荡器中的像散补偿

飞秒脉冲同步泵浦光学参量振荡器中的像散补偿