Subthreshold Laser Research Articles

Subthreshold two-pulse time-delayed laser ionization of Cu

Two collinear frequency-doubled Nd:YAG laser pulses were used to study the temporal evolution of ion generation in Cu targets. The method is based on the decoupling of the total ionization energy using two individual laser beams operating at fluences below the ionization threshold. A maximum in the ion signal generated corresponding to a laser interpulse delay of 60 ns has been determined.

Subthreshold micropulse diode laser photocoagulation for clinically significant diabetic macular oedema: a three‐year follow up

To report the long-term visual outcome of subthreshold micropulse diode laser photocoagulation for clinically significant diffuse diabetic macular oedema (CSME). Non-comparative case-series. The main outcome measures of this 3-year follow-up study of infrared (810 nm) subthreshold diode micropulse laser photocoagulation for CSME were visual outcome and the angiographic CSME status of 19 patients (25 eyes). Visual acuity stabilized or improved in 84% of treated eyes by the end of the first year. The result was maintained in the second year and by the third year, 92% maintained vision. However, more patients needed supplementary grid laser in the third year than in the second year. CSME decreased in 92% of the eyes and resolved in 88% in the first year. By the second year, 92% showed complete resolution. However, in the third year, recurrent CSME was noted in 28% of patients. This study has the longest follow up reported so far and demonstrating that subthreshold micropulse diode laser photocoagulation has a beneficial long-term effect on visual acuity and resolution of CSME with minimal chorio-retinal damage.

Diode Laser-Induced Mitosis in the Rabbit Retinal Pigment Epithelium

The induction of retinal pigment epithelium (RPE) proliferation without damaging the inner layers of the retina might be helpful in patients with RPE atrophic changes and degeneration. This study aimed to induce mitosis in the RPE of the rabbit after subthreshold photocoagulation with the diode laser. Twenty-five male Dutch rabbits received retinal photocoagulation using an 810-nm diode laser with different power settings and exposure times. The eyes were processed for light microscopy, electron transmission microscopy, and immunohistochemistry. Neither morphological alterations nor mitotic activity was found after 5-mJ energies. Retinal layers were not affected and RPE hyperplasia appeared in the treated areas associated with mitotic activity when 10 mJ was used. Mitosis induction and retinal damage appeared with 20, 50, and 100 mJ and were associated with ophthalmoscopic damage. The use of subthreshold 810-nm diode laser treatment may induce mitosis in the RPE without causing damage to the neighboring layers.

Subthreshold Grid Laser Treatment of Macular Edema Secondary to Branch Retinal Vein Occlusion with Micropulse Infrared (810 Nanometer) Diode Laser

To compare the effectiveness of subthreshold grid laser treatment (SGLT) with an infrared micropulse diode laser with that of threshold grid laser treatment (TGLT) for macular edema secondary to branch retinal vein occlusion (BRVO). Randomized clinical trial. Thirty-six patients (36 eyes) were randomized either to infrared SGLT (17 eyes) or to krypton TGLT (19 eyes). Complete ophthalmic examinations, including determination of visual acuity (VA) with Early Treatment Diabetic Retinopathy Study charts, optical coherence tomography (OCT), and fluorescein angiography, were performed at the time of the study entry and at 6-month intervals, with a planned follow-up of 24 months. Primary: decrease in mean foveal thickness (FT) on OCT. Secondary: changes of the total macular volume (TMV) over the follow-up, proportion of eyes that gained at least 10 letters (approximately > or =2 lines of VA gain) at the 12- and 24-month examinations, and timing of macular edema resolution. Changes in mean FT and TMV from the initial values were statistically significant for TGLT from the 6-month examination (P<0.001) and for SGLT from the 12-month examination (P<0.001). After 1 year, there was no difference in mean FT and TMV between the 2 groups. At the 12-month examination, 10 patients of the SGLT group (59%) and 11 of the TGLT group (58%) gained at least 10 letters (2 lines) in VA. At the 24-month examination, this gain was achieved by 11 patients (65%) of the SGLT group and 11 (58%) of the TGLT group. Moreover, at the 24-month examination 59% and 26% gained 3 lines in the SGLT and TGLT groups, respectively. Resolution of macular edema and VA improvement are similar to those obtained with conventional TGLT, but SGLT is not associated with biomicroscopic and angiographic signs. A multicenter randomized clinical trial would be needed to ascertain the real efficacy and the most appropriate settings of SGLT for macular edema secondary to BRVO.

Prophylactic Treatment of Age-Related Macular Degeneration Report Number 1: 810-Nanometer Laser to Eyes with Drusen. Unilaterally Eligible Patients

To determine the effects of subthreshold 810-nm-diode laser treatment on the rate of development of choroidal neovascularization (primary end point) and the effect on visual acuity (VA) in participants with multiple large drusen in one eye and a preexisting neovascular age-related macular degeneration (AMD) lesion in the other. Multicenter, prospective, randomized controlled trial. Two hundred forty-four patients > or =50 years of age and with a neovascular or advanced AMD lesion in one eye and, in the fellow "study" eye, (1) at least 5 drusen > or = 63 mum in diameter, (2) Early Treatment Diabetic Retinopathy Study best-corrected VA (BCVA) of 20/63 or better, and (3) no evidence of neovascularization at baseline. Patients were randomized to treatment or observation of their study eye at each of 22 centers. At each visit, the protocol specified that BCVA, a complete retinal examination, and fluorescein angiography be documented. Treated eyes had a grid of 48 extrafoveal, subthreshold diode (810 nm) laser spots, 125 mum in diameter, placed in an annulus outside of the foveola. Patients were seen at baseline and at 3, 6, 12, 18, 24, 30, and 36 months after randomization. No retreatments were allowed. Development of choroidal neovascularization (as confirmed by fluorescein angiography) and change in BCVA. Throughout follow-up, the rate of choroidal neovascularization events in treated eyes consistently exceeded that in observed eyes. At 1 year, the difference was 15.8% versus 1.4% (P = 0.05). Most of the intergroup differences in choroidal neovascularization events occurred during the first 2 years of follow-up. Treated eyes showed a higher rate of VA loss (> or =3 lines) at 3- and 6-month follow-ups relative to observed eyes (8.3% vs. 1% and 11.4% vs. 4%, respectively; Ps = 0.02, 0.07). After 6 months, no significant differences were observed in VA loss between groups. Prophylactic subthreshold 810-nm-diode laser treatment to an eye with multiple large drusen in a patient whose fellow eye has already suffered a neovascular event places the treated eye at higher risk of developing choroidal neovascularization. We advise against using prophylactic subthreshold diode laser treatment in these eyes.

Serial Optical Coherence Tomography of Subthreshold Diode Laser Micropulse Photocoagulation for Diabetic Macular Edema

To use serial optical coherence tomography (OCT) to evaluate low-intensity, high-density subthreshold diode laser micropulse photocoagulation treatment of clinically significant diabetic macular edema. Eighteen consecutive eyes of 14 patients with clinically significant diabetic macular edema and a minimum foveal thickness of 223 microm or greater were prospectively evaluated by OCT preoperatively and 1, 4, and 12 weeks following treatment. Overall, estimated macular edema 3 months postoperatively (minimum foveal thickness--223 microm) was reduced a mean of 24% (P = .02). Eleven eyes treated for recurrent or persistent clinically significant diabetic macular edema following prior treatment more than 3 months before study entry were most improved, with a mean reduction in estimated macular edema 3 months postoperatively of 59%. No treatment complications were observed. No patient demonstrated laser lesions following treatment. Low-intensity, high-density subthreshold diode laser micropulse photocoagulation can reduce or eliminate clinically significant diabetic macular edema measured by OCT. Further study is warranted.

Transpupillary Thermotherapy Alone and Enhanced by Standard Doses of Indocyanine Green in Pigmented Rabbits

To evaluate the chorioretinal effects of transpupillary thermotherapy (TTT) and TTT enhanced by standard doses of indocyanine green (ICG) in pigmented rabbits. In 25 eyes, two retinal areas were irradiated using identical subthreshold diode laser irradiation. The subthreshold diode laser irradiation level was first determined using the other eye. ICG (0.5 mg/kg) was injected before irradiation of the second area. Red-free photographs, fluorescein angiography, and ICG videoangiography were performed after TTT. Specimens were prepared for light microscopy. In 35% of the cases, ICG videoangiography revealed a hypofluorescent spot in the area irradiated after injection of ICG that disappeared after ICG re-injection. None of the areas irradiated before ICG injection were hypofluorescent. Red-free photographs and fluorescein angiography were normal in most cases. Light microscopy demonstrated patent choriocapillaris and choroid in most areas. Subthreshold TTT, whether enhanced by ICG or not, did not cause significant choroidal or choriocapillaris vascular occlusion. Standard doses of ICG had no significant effect on TTT.

EFFECT OF SUBTHRESHOLD INFRARED LASER TREATMENT FOR DRUSEN REGRESSION ON MACULAR AUTOFLUORESCENCE IN PATIENTS WITH AGE-RELATED MACULAR DEGENERATION

Subthreshold laser therapy has been shown to cause drusen reduction. Using this method, visible laser burns are not created in the retina; presumably, the energy is absorbed by the retinal pigment epithelium (RPE) and stimulates reabsorption of drusen material, sparing photoreceptors from destruction. We hypothesized that autofluorescence (AF) changes would be visible after such treatment and might be sensitive to quantify RPE changes. Twenty eyes of 13 patients with non-exudative age-related macular degeneration were studied. Forty-eight subthreshold infrared diode laser spots were applied as treatment to cause drusen regression. Before, immediately after, and 3 months after treatment, color fundus photography, fluorescein angiography (FA), and autofluorescence (AF) imaging were performed. Treated lesions were identified on overlay images of all three imaging methods. : The averaged sensitivity of AF imaging compared with FA was 29.6+/-28.7% versus 10.2+/-12.2% (P=0.008) at the immediate posttreatment time point and 43.5+/-28.7% versus 33.8+/-26.5% (P=0.043) at the 3-month posttreatment time point, respectively. Reduction of drusen at 3 months correlated with the detection sensitivity of AF imaging at the immediate posttreatment time point (P=0.022). Immediately after subthreshold laser treatment, AF imaging was more sensitive to detect RPE changes than FA. This suggests that noninvasive AF imaging may allow prediction of the effect of subthreshold laser treatment and might be used to titrate treatment dose.

Measurement of reflectivity of butt-joint active-passive interfaces in integrated extended cavity lasers

A method and measurement results are presented for the determination of the reflectivity of butt-joint active-passive interfaces in a series of extended cavity Fabry-Pe/spl acute/rot lasers. The method is based on the analysis of subthreshold laser spectra. The small reflections at the two intracavity active-passive interfaces modify the mode structure of the laser. By fitting the calculated subthreshold mode structure to the recorded data, values of the reflectivities are extracted. An average value of 9.10/sup -5/ has been determined. The value of the reflectivity of those interfaces is relevant for photonic integrated circuits and particularly integrated mode-locked lasers.

Laser driver with subthreshold laser bias and turn-on-delay compensation by prejittering of the modulating current

In the paper the solution of burst mode capable laser driver with high extinction ratio is presented. The concept is to bias the laser in subthreshold regime and to introduce the prejittering of modulating current. It is shown that even quite small bias gives significant reduction of laser turn-on delay when compared with the zero bias scheme, still allowing high extinction ratio, in range of 35–40 dB. The proposed prejittering circuit performs pattern dependent compensation of turn-on delay induced distortions of optical signal, making the driver suitable for high bit rate data transmission. The experimental driver design is described, and measurement results are reported.

Wavelength selection in an integrated multiwavelength ring laser

The wavelength selection mechanism of a compact integrated multiwavelength ring laser is demonstrated. The device contains four semiconductor optical amplifiers, a compact arrayed waveguide grating and passive waveguides integrated on a single InP wafer. The device can produce seven different wavelengths through biasing one or two out of the four amplifiers. Comparison of calculated and measured subthreshold laser spectra demonstrates the role of crosstalk in the arrayed waveguide grating in the laser and allows the crosstalk to be quantified. A rate-equation model of the laser and measurements are presented that describe the switching between wavelengths of the laser as a function of bias currents. A comparison between the measured data and the model is made.

Échec de la photocoagulation au cours des œdèmes maculaires diabétiques sévères

To analyse clinical and tomographic features of diabetic macular edemas (DME) unresponsive to laser photocoagulation. Retrospective study of 43 consecutive eyes presenting with severe DME defined by a macular thickening 450 microm or more, involving the fovea, and associated with either cystoid spaces and/or hard exudates threatening the center of the macula. All 43 eyes underwent subthreshold grid laser therapy (a mean of 2.3 sessions). Mean follow-up was 21 months. Photocoagulation was beneficial in 39% of the eyes (n=17) and failed in 60% (n=26). Comparing these two groups, we found that some tomographic and clinical signs were strikingly more frequent in the unresponsive eyes than in the responsive ones. OCT findings reported foveal thickening greater than 700 microm (38%), subclinical serous foveal detachment (65%), subclinical vitreomacular interface abnormalities (19%) and as clinical signs, 44% of the unresponsive eyes exhibited rapid progression of retinal ischemia preceding ME constitution. Long-standing diffuse DME carries an unfavorable prognosis. In cases of unresponsive DME exhibiting these tomographic and clinical signs, alternative therapies such as intravitreal injections of steroids and/or vitrectomy with posterior hyaloid removal should be considered, instead of iatrogenic and useless iterative grid laser treatments.

Subthreshold Transpupillary Thermotherapy Reduces Experimental Choroidal Neovascularization in the Mouse without Collateral Damage to the Neural Retina

Transpupillary thermotherapy (TTT) is currently being evaluated for treatment of choroidal neovascularization (CNV) in age-related macular degeneration. To optimize TTT for CNV, the effect was analyzed of invisible (subthreshold) or visible (threshold) doses of TTT on the normal mouse retina and on experimental CNV. TTT was delivered to the normal retina of 42 mice with a diode laser at increasing power settings (50, 60, 70, or 80 mW), to obtain thermal lesions ranging from invisible (subthreshold) to visible (threshold) burns. CNV was induced in 53 mice by krypton laser photocoagulation of the fundus, after which the CNV lesions were treated with TTT (50, 60, or 80 mW). Eyes were enucleated 7 days after TTT and prepared for histology, and the CNV complex was evaluated on hematoxylin-eosin stained serial sections by measuring the maximum height of the CNV lesions. Ultrastructural changes were examined by transmission electron microscopy. Increasing the TTT laser power yielded gradually more visible effects. At 50 mW, which induced subthreshold burns, no damage was seen in the neural retina, retinal pigment epithelium (RPE), or choroid at any time point. By contrast, eyes treated with higher power exhibited progressively more damage to the neural retina, including a complete disruption of the outer nuclear layer. When TTT was applied to the laser-induced CNV lesions, the height of lesions was significantly reduced (P < 0.001) in response to all three power settings at 7 days after treatment. The mean relative thickness of the CNV lesion was 3.29 +/- 0.89 in untreated mice, whereas in TTT-treated mice it was 1.69 +/- 0.35, 1.69 +/- 0.41 and 1.70 +/- 0.17 at power settings of 50, 60, and 80 mW, respectively. The overlying neural retina showed no apparent damage with the 50- or 60-mW settings, whereas outer nuclear layer disruption occurred with a power of 80 mW. Electron microscopy confirmed the presence of vascular occlusion at 1 day and a fibrotic scar at 7 days after TTT. Subthreshold TTT can effectively occlude newly formed vessels and cause regression of the experimental CNV complex without damaging the neural retina. The results demonstrate the importance of using subthreshold laser power in experimental and clinical evaluation of TTT.

Reflectivity Measurements of Intracavity Defects in Laser Diodes

A method for measuring the complex reflectivity associated with a localized defect existing inside a laser diode cavity is presented. It relies on analyzing the magnitudes of resonant peaks in the Fourier transform of a subthreshold laser spectrum. Reflectivities between 0.01 and 0.02 with zero phase have been measured in a laser with a deliberately induced scattering center produced by standard lithographic techniques.

Mechanism of unidirectional motions of chiral molecular motors driven by linearly polarized pulses

The mechanism of the unidirectional rotational motion of a chiral molecular motor driven by linearly polarized laser pulses was theoretically studied. A simple aldehyde molecule was adopted as a chiral molecular motor, in which a formyl group (–CHO) was the rotating part of the motor. Temporal evolutions of the instantaneous angular momentum averaged over an ensemble of randomly oriented motors were taken as a measure of the unidirectional motion. The contour plots of the averaged instantaneous angular momentum were obtained by using a quantum master equation approach that took into account relaxation effects and a classical trajectory approach. Two regimes are found in the contour plots. One is an intense laser field regime in which the laser–motor interaction energy exceeds the asymmetric potential barrier. In this regime, the motors are unidirectionally driven in the intuitive direction, i.e., the gentle slope of the potential. The other regime is a subthreshold laser intensity regime in which unintuitive rotational motions also occur. This unintuitive rotation is found to be a quantum effect, as indicated by contour plots calculated by taking into account temperature effects.

Subthreshold and micropulse diode laser photocoagulation

Subthreshold and micropulse diode laser photocoagulation

Subthreshold and micropulse diode laser photocoagulation

Retinal laser photocoagulation is a proven, effective treatment for various retinal disorders. Common clinical protocols use intra-operatively visible endpoints that cause iatrogenic chorioretinal damage. For this reason, laser therapy is normally limited to levels of disease severity for which the benefit-to-risk ratio justifies its application. The use of 810 nm diode lasers in the MicroPulse mode offers the surgeon the possibility to minimize iatrogenic retinal damage. A less destructive laser therapy with a more favorable benefit-to-risk ratio could justify treatment earlier in the course of the disease, allowing for stabilization or improvement of less compromised visual functions.

Diabetic macular edema: classification, medical and laser therapy

Purpose. to propose and describe a new classification of diabetic macular edema (DME) based on its etiopathogenetic features and to present efficacy and safety data on medical and laser treatments currently used and under investigation. Methods. available literature on DME has been reviewed and information provided by contact lens biomicroscopy, fluorescein angiography and optical coherence tomomography has been combined; published data from trials and reports investigating effectiveness of medical and laser treatments were also analyzed. Results. DME was classified according to its main etiopathogenetic components: prevalently retinovascular, tractional and with taut attached posterior hyaloid, each of them having peculiar aspects on fundus, angiographic and tomographic examination. Focal laser treatment remains the only effective intervention, as shown by a large randomized controlled trial, but new less-invasive photocoagulation strategies (i.e., light laser photocoagulation and subthreshold micropulse diode laser) as well as pharmacological approaches (i.e., antioxidants, PKC inhibitors, aldose-reductase and AGE inhibitors, VEGF inhibitors, intravitreal triamcinolone) are emerging. Conclusions. based on our review a classification of DME was developed that may prove useful in making the appropriate treatment decision; the new laser and pharmacological treatments currently investigated are promising and need to be confirmed by large, controlled clinical trials.

Predictors of drusen reduction after subthreshold infrared (810 nm) diode laser macular grid photocoagulation for nonexudative age-related macular degeneration

PURPOSE: To determine the predictors of drusen reduction in eyes with nonexudative age-related macular degeneration (ARMD) treated with subthreshold infrared (810 nm) diode laser macular grid photocoagulation. Additionally, to determine the relationship of laser-induced drusen reduction and best-corrected visual acuity (BCVA) 18 months after laser treatment. DESIGN: Randomized controlled clinical trial. METHODS: Fifty patients (100 eyes) with bilateral nonexudative ARMD were enrolled at two centers. One eye of each patient was randomized to the observation; the other eye was treated with 48 subthreshold (invisible end point) applications of infrared (810 nm) diode laser in a macular grid pattern. The eyes that received subthreshold laser treatment were compared with the eyes that received no treatment. The baseline fundus characteristics (number, size, and distribution of drusen, as well as focal hyperpigmentation) from two macula areas (central 1500 μ diameter, pericentral 1500 μ ring area) on stereo color photographs, the number of laser-induced lesions, and the area of laser induced retinal pigment epithelial (RPE) lesions on fluorescein angiography 3 months after treatment were studied as predictors of major drusen reduction (≥ 50% drusen reduction from baseline) 18 months after laser treatment. BCVA at baseline and 18 months later was compared in observation eyes and in laser-treated eyes. RESULTS: Eighteen months after randomization, 24 (48%) of 50 eyes treated with subthreshold laser had major drusen reduction compared with three (6%) of 50 observation eyes ( P = .00001). At 3 months post-treatment in laser-treated eyes with major drusen reduction, the mean number of laser-induced lesions on fluorescein angiography was 30.7 and the mean area of RPE change was 0.81 mm 2 compared with 14.8 laser-induced lesions and 0.35 mm 2 area of RPE change in eyes without major drusen reduction ( P = .0001 and P = .0003, respectively). At baseline, fundus characteristics were not significantly different between observation eyes and laser-treated eyes or between the major drusen reduction group and the nonmajor drusen reduction group. At 18 months after treatment, BCVA was not significantly different in laser-treated eyes and in observation eyes. CONCLUSIONS: Subthreshold infrared (810 nm) diode laser macular grid photocoagulation in eyes with nonexudative ARMD significantly reduced drusen 18 months after laser treatment. Both the number of subthreshold laser lesions and the area of RPE changes visible on fluorescein angiography 3 months after treatment appeared to be predictors for major drusen reduction 18 months after treatment. However, it remains to be determined whether laser-induced drusen reduction is beneficial for visual acuity or reduces the incidence of choroidal neovascularization (CNV) in eyes with nonexudative ARMD.

Ultrashort laser ablation of metals: pump–probe experiments, the role of ballistic electrons and the two-temperature model

The dynamics of laser ablation from metallic surfaces (Ag, Al, Fe and Ni) induced by the combined effect of two 30 fs sub-threshold laser pulses has been examined. In a pump–probe setup the yield of the emitted secondary ions and neutrals has been determined as a function of the delay between the two laser pulses. The instantaneous generation of highly excited (ballistic) electrons by the laser pulse and the thermal properties of the metal, which have been modified to be valid into the regime of high electron temperatures have been found to be determining factors for the ablation process. Unexpectedly, two distinct maxima for particle emission have been observed as a function of the time separation of the pump and the probe pulse. The energy relaxation is discussed within the frame of the two-temperature model (TTM) and it is shown that the measured behavior (in the time domain) of ablated particles can only be explained by taking into account a general expression for the thermal conductivity, valid for a wide range of electron temperatures and in addition a substantial role of hot, ballistic electrons.