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At least 667 records · Page 37Linked to original sources
PubMed2003

Highly efficient direct third-harmonic generation based on control of the electro-optic effect in quasi-periodic optical superlattices.

We present a method for controlling the efficiency of direct third-harmonic generation in quasi-periodic optical superlattices by means of the electro-optic effect. The calculated results manifest that this method is extremely efficient for achieving high efficiency of direct third-harmonic generation in predesigned quasi-phase-matched coupled parametric processes and is feasible at any given fundamental intensity. In addition, we demonstrate that the electro-optic control approach is significantly better than the temperature-control method.

Journal Article↗
PubMed2003

Nonlinear distortion generated by semiconductor optical amplifier boosters in analog optical systems.

Closed-form theoretical expressions for the second- and third-order harmonic distortions generated by a semi-conductor optical amplifier (SOA) operating under slight saturation conditions are derived for what is believed to be the first time. The distortion results provided by these expressions agree very well with those obtained by means of simulation and experiments, showing that one should carefully adjust the optical power launched into the SOA to obtain both the good linearity and high dynamic range required by analog optical systems.

Journal Article↗
PubMed2004

Linear and nonlinear optical properties of Ag-As-Se chalcogenide glasses for all-optical switching.

We prepared Ag(x)(As0.4Se0.6)(100-x) chalcogenide glasses by a melt-quenching method and measured their linear and nonlinear optical properties to evaluate their potential applications to all-optical ultrafast switching devices. Their nonlinear refraction and absorption were measured by the Z-scan method at 1.05 microm. The addition of Ag to As2Se3 glass led to an increase in the nonlinear refractive index without introducing an increase in the nonlinear absorption coefficient. The glass with a Ag content of x = 20 at. % revealed high nonlinearity ranging from 2000 to 27,000 times that of fused silica, depending on the incident optical intensity.

Journal Article↗
PubMed2004

Loss-coupled distributed-feedback lasers with amplified optical feedback for optical microwave generation.

Multisection semiconductor lasers for optical microwave generation have been fabricated that consist of a loss-coupled distributed feedback (LC-DFB), a phase control, and an amplifier section. High-frequency self-pulsations are generated according to the concept of a single-mode laser with short optical feedback. The effect of the optical feedback via the phase control and the amplifier section on the self-pulsation is apparently shown as a result of the superior single-mode characteristic of the LC-DFB section. Continuous frequency tuning is achieved in the range of 17-35 GHz.

Journal Article↗
PubMed2005

Long optically controlled delays in optical fibers.

Optically controlled delay lines in optical fibers are demonstrated by use of the group-velocity control of signal pulses based on stimulated Brillouin scattering. We achieve continuous time delay within the range of 150 ns, much larger than the width of the 40 ns signal pulse, using cascaded fiber segments joined by unidirectional optical attenuators. In the meantime, we also observe a large amount of pulse broadening, which agrees well with a theoretical prediction based on linear theory.

Journal Article↗
PubMed2005

Optical circulators in two-dimensional magneto-optical photonic crystals.

We propose an optical circulator formed of a magneto-optical cavity in a 2D photonic crystal. With spatially engineered magnetic domain structures, the cavity can be designed to support a pair of counterrotating states at different frequencies. By coupling the cavity to three waveguides, and by proper matching of the frequency split of the cavity modes with the coupling strength between the cavity and the waveguide, ideal three-port circulators with complete isolation and transmission can be created. We present a guideline for domain design needed to maximize the modal coupling and the operational bandwidth for any given magneto-optical constant.

Journal Article↗
PubMed2006

Optical switching and contrast enhancement in intense laser systems by cascaded optical parametric amplification.

Optical parametric chirped-pulse amplification (OPCPA) can be used to improve the prepulse contrast in chirped-pulse amplification systems by amplifying the main pulse with a total saturated OPCPA gain, while not affecting the preceding prepulses of the seed oscillator mode-locked pulse train. We show that a simple modification of a multistage OPCPA system into a cascaded optical parametric amplifier (COPA) results in an optical switch and extreme contrast enhancement that can completely eliminate the preceding and trailing oscillator pulses. Instrument-limited measurement of a prepulse contrast ratio of 1.4 x 10(11) is demonstrated from COPA at a 30 mJ level.

Journal Article↗
PubMed2006

Optical ultrawideband monocycle pulse generation based on cross-gain modulation in a semiconductor optical amplifier.

A novel method for generating ultrawideband (UWB) monocycle pulses based on cross-gain modulation (XGM) in a semiconductor optical amplifier (SOA) is proposed and experimentally demonstrated. Thanks to the XGM in the SOA, a pair of polarity-reversed optical Gaussian pulses is generated at the output of the SOA, to which a Gaussian pulse pump and a continuous-wave probe are applied. The two polarity-reversed optical pulses are then time delayed by two cascaded fiber Bragg gratings to introduce a time delay difference. A UWB monocycle pulse with a full width at half-maximum of 48 ps and a fractional bandwidth of 188% is generated at the output of a high-speed photodetector.

Journal Article↗
PubMed2004

Anterior ischemic optic neuropathy in eyes with optic disc drusen.

BACKGROUND: There have been anecdotal reports of anterior ischemic optic neuropathy (AION) occurring in eyes with optic disc drusen (ODD), but the clinical features of this condition have not been well characterized. OBJECTIVES: To better describe the clinical features of AION associated with ODD and to compare the clinical features of this condition with those of "garden variety" nonarteritic AION. METHODS: We reviewed the medical records of 20 patients who experienced an episode of AION in an eye with ODD. In 4 patients, both eyes were affected; thus, 24 eyes were studied. The diagnosis of ODD was made by ophthalmoscopic identification, orbital ultrasonography, or computed tomographic scanning. We recorded age, sex, vascular risk factors, symptoms, visual acuity, visual fields, and results of the follow-up examination in all patients. These findings were compared with data from previously reported series of patients with nonarteritic AION. RESULTS: Our 20 patients included 14 men and 6 women (age range, 18-69 years; mean, 49.4 years). Vascular risk factors were identified in 10 patients (50%). Three patients reported episodes of transient visual loss before their fixed deficit. The visual acuity at the initial examination was 20/60 or better in 15 (62%) of the 24 eyes; 8 had a visual acuity of 20/20. The predominant pattern of visual field loss was an altitudinal or arcuate defect in 19 (79%) and a centrocecal scotoma in 5 (21%) of the 24 eyes. There was subjective worsening of vision before the initial neuro-ophthalmic examination in 11 eyes (46%) and objective documentation of progression in 7 eyes (29%). The final visual acuity was 20/40 or better in 13 (62%) of 21 eyes and 20/200 or worse in 3 (14%) of 21 eyes. CONCLUSIONS: Our patients were strikingly similar to those with nonarteritic AION unassociated with drusen in regard to prevalence of vascular risk factors, pattern of visual field loss, and occurrence of a subsequent similar event in the fellow eye. In contrast, however, patients with ODD-AION were younger than those with nonarteritic AION, were more likely to report preceding episodes of transient visual obscuration, and enjoyed a more favorable visual outcome.

Adolescent↗
PubMed1978

Quantitative perimetry in compressive optic neuropathy and optic neuritis.

The Goldmann perimetric defects in 20 cases of compressive optic neuropathy and 54 cases of optic neuritis were analyzed. While defects involving the papillomacular bundle were the rule in both compressive and neuritis cases, sparing of the fixational area occurred in 24% of neuritis eyes but in none of the eyes with compressive neuropathy. The most reliable differential perimetric sign was the presence of a hemianopic defect; at least one eye of 15 (75%) cases of compression showed such a defect, which was not found in any neuritis cases. The I2e was the largest kinetic isopter to demonstrate the hemianopic defect in a substantial proportion of cases. These defects were corroborated with sequential static presentation of the I2e to I4e stimuli to either side of the vertical meridan, and with similar techniques using 18/1,000 red test objects at the tangent screen.

Adolescent↗
PubMed1995

Leber's hereditary optic neuropathy (LHON)-related mitochondrial DNA sequence changes in italian patients presenting with sporadic bilateral optic neuritis.

Eighteen Italian patients presenting with sporadic, bilateral, simultaneous, or sequential optic neuritis (ON) were evaluated for 14 base changes in mitochondrial DNA (mtDNA) previously found associated with Leber's hereditary optic neuropathy (LHON), aiming to identify at a molecular level LHON cases with nontypical phenotypes. During a 36-month follow-up, 11 ON patients developed clinical or laboratory features allowing diagnosis of clinically definite multiple sclerosis (MS). None was positive for any of the "primary" LHON-associated mutations. However, single or multiple "secondary" LHON-associated sequence changes at 4216/ND1, 4917/ND2, and 13708/ND5 were detected in ON and ON-MS patients. MS controls without visual failure as well as healthy control subjects harbored the same base changes at similar frequencies. In addition, coexistence of three sequence changes was found in two cases (1 ON-MS patient and 1 MS control patient). We also report finding two new neutral sequence base changes in the ND-4 gene which were identified by SSCP and confirmed by automated DNA sequence analysis. The results suggests that these secondary mutations do not contribute to MS susceptibility in these patients, but rather represent neutral mitochondrial DNA polymorphisms. In addition, whether there are biochemical abnormalities related to single and multiple secondary mtDNA sequence changes remain to be demonstrated.

Adult↗
PubMed2004

Optical coherence tomography in optic pit maculopathy managed with vitrectomy-laser-gas.

BACKGROUND: Optic disc pit (ODP) maculopathy has a poor visual prognosis if left to its natural course. Several therapeutic approaches have been attempted. The cases of 11 patients evaluated with optical coherence tomography (OCT) and treated with vitrectomy-laser-gas and their functional and anatomical outcomes are presented. METHODS: Retrospective interventional consecutive case series, including 11 eyes with ODP maculopathy. Pre- and postoperative best-corrected visual acuity (BCVA), OCT and angiography were recorded. All patients underwent pars plana vitrectomy, posterior hyaloid dissection peripapillary diode laser prior to retinal reapplication and C(3)F(8) 15% injection. RESULTS: Mean preoperative BCVA was 20/126. Median preoperative BCVA was 1.0 LogMAR (range 1.3-0.4) . Eighty-two per cent of patients gained 2 or more Snellen lines of vision (mean 4.4 lines gained). Mean final BCVA was 20/32, and median final BCVA was 20/30 in Snellen VA and 0.2 in LogMAR (range 0.7-0) Preoperative OCT in all but one case confirmed the bilaminar structure of the macular detachment. Postoperative OCT helped in monitoring reabsorption of the macular detachment, which was achieved in all cases after an average of 6.5 months post-surgery. BCVA increased progressively as the subretinal fluid was reabsorbed (P=0.006). Mean duration of postoperative follow-up was 15 months. Recurrence was observed in two cases. CONCLUSIONS: In our series, the vitrectomy-laser-gas procedure for ODP maculopathy improved vision and achieved satisfactory anatomic results in all 11 cases. OCT was useful in the diagnosis and follow-up of this pathology. However, the low incidence of this entity makes it difficult to obtain series large enough to determine the efficacy of the vitrectomy-laser-gas procedure and other treatment modalities and be able to suggest a procedure of choice.

Adult↗
PubMed2007

Optic disc edema in non-arteritic anterior ischemic optic neuropathy.

We investigated the clinical characteristics, time to resolution and the factors that influence it, and evolutionary pattern of optic disc edema (ODE) in non-arteritic anterior ischemic optic neuropathy (NA-AION). Our study was conducted in 591 consecutive patients (749 eyes) with NA-AION who fulfilled our inclusion criteria. On their first visit to our clinic, all patients had a detailed ophthalmic and medical history, a comprehensive ophthalmic evaluation, and stereoscopic color fundus photography and fluorescein fundus angiography. On each follow-up visit, the same ophthalmic evaluation was performed, except for fluorescein fundus angiography. The effect of steroid therapy on ODE was evaluated in a "patient choice study" in 723 eyes, i.e., patients who voluntarily opted to have (343 eyes) or not have (380 eyes) this therapy. To identify the factors that influence time to ODE resolution, parametric regression models for interval-censored data were fitted by maximum likelihood estimation using an SAS procedure. Our results indicate that the overall median time (25-75th percentile) to spontaneous resolution of ODE from the onset of visual loss was 7.9 (5.8-11.4) weeks. The ODE resolution time was longer in diabetics than in non-diabetics (p = 0.003) in the single factor model. Multi-factor analysis showed that worse initial visual field defects (p < 0.0001) and worse visual acuity (p = 0.04) were associated with a faster resolution of ODE. Those treated with steroid therapy within 2 weeks after onset of NA-AION had significantly (p = 0.0006) faster ODE resolution than untreated cases. Severity of initial visual loss and systemic diseases were identical in steroid treated and untreated patients. A characteristic evolutionary pattern of ODE in NA-AION was observed. In conclusion, our study showed that in NA-AION the time course for resolution of ODE is shorter with greater severity of initial visual field and visual acuity loss, which may relate to the number of axons permanently damaged during the acute stage. Steroid therapy was associated with shorter time to resolution of ODE. Resolution of ODE goes through a characteristic evolutionary process.

Aged↗
PubMed1981

Electrophysiological study of the nucleus of the optic tract that transfers optic signals to the nucleus reticularis tegmenti pontis - the visual mossy fiber pathway to the cerebellar flocculus.

Neuronal activities (n = 43) in the pretectal region in rabbits were recorded. They were orthodromically activated from the optic chiasm (latency, 1.86 +/- 0.35 msec) and antidromically from the ipsilateral nucleus reticularis tegmenti pontis (Nrt) (latency, 0.97 +/- 0.22 msec). Thirty-one (72%) neurons were in the nucleus of the optic tract (NOT), four (9%) in the anterior pretectal nucleus (PA) and seven (16%) in the border between NOT and PA. These findings demonstrate that the NOT is involved in the visual mossy fiber pathways to the flocculus and may contribute to optokinetic eye movements.

Animals↗
PubMed1995

Antibodies to human optic nerve in Leber's hereditary optic neuropathy.

Mutations of mitochondrial DNA have now been identified in the majority of patients with Leber's hereditary optic neuropathy (LHON). However, these mutations do not explain all the clinical features of LHON, and other pathogenetic factors are likely to be operating. We have analysed serum from 69 LHON patients and their relatives, 58 controls and 14 patients with ischemic or compressive optic neuropathy. A significant proportion of LHON patients had circulating antibodies to tubulin protein. This finding supports the theory that autoimmunity may play some role in the pathogenesis of LHON.

Adult↗
PubMedDate not supplied

Anatomy of the optic nerve head and glaucomatous optic neuropathy.

The mechanism of axon damage in eyes with glaucomatous optic neuropathy remains undefined. Interestingly, it has been observed that, although the entire nerve cross-section may be involved by the nerve damage, in many instances, the superior and inferior axon bundles are preferentially affected by the pressure insult. Thus, recent studies by many investigators have stressed a re-examination of the optic nerve head anatomy, including the nerve head microcirculation, the glial and connective tissue elements within the nerve head, and the morphology of the axons themselves. Any correlation between regional differences in this anatomy and the preferential involvement by specific axon bundles within the nerve head by the pressure insult may suggest some further insight into the mechanisms underlying the pressure-induced axon loss in glaucomatous eyes.

Axons↗