Performance enhancement of an ultrafast all-fiber laser based on an InN saturable absorber using GRIN coupling.

Indium nitride (InN)-based semiconductor saturable absorbers have previously shown advantages for application in near-IR fiber lasers due to their broad modulation depth, ultrafast nonlinear response and thermal stability. However, up to now all demonstrated saturable absorber elements based on InN (either transmissive or reflective) have shown limited performance due to poor coupling and insertion losses. We present here a simple mode-locking device based on a GRIN-rod lens in conjunction with an InN semiconductor saturable absorber mirror (SESAM) for its use in a passively mode-locked all-fiber laser system operating at telecom wavelengths. Our results demonstrate that this coupling element ensures not only a compact, turnkey and alignment-free design but also a highly-stable optical femtosecond pulse train. The reduction of insertion losses (3.5 dB) enables the generation of 90-fs ultrafast pulses with an average power of 40 mW and up to 7 nJ of pulse energy without the need for additional amplification.

Distributed detection of hydrogen and deuterium diffusion into a single-mode optical fiber with chirped-pulse phase-sensitive optical time-domain reflectometry.

For some infrastructures such as oil and gas extraction boreholes or radioactive waste repositories, where distributed optical fiber sensors are employed to grant the safety of the facilities, the presence of gas species such as hydrogen or deuterium is one of the most relevant parameters to monitor. The possibility of employing the same kind of sensors for this purpose is of special interest, reducing the cost by employing a single interrogator, able to measure multiple parameters by simply employing adequate sensing fibers. To meet this goal, we present here a chemical sensor based on chirped-pulse phase-sensitive optical time-domain reflectometry (CP-φOTDR), which is able to detect these species while they diffuse into the silica fiber. The ability of chirped-pulse φOTDR to measure a change in refractive index with sensitivity around 10-8 has allowed determining hydrogen concentration with accuracy on the order of 10-3 mol/m3 and spatial resolution ∼6 m. Another experiment provides an indirect measurement of the solubility of deuterium in a standard telecom-grade optical fiber, which is found to be around 1.47×1024 m3/bar.

Male exposure to bisphenol a impairs spermatogenesis and triggers histone hyperacetylation in zebrafish testes.

Bisphenol A (BPA) is an endocrine disruptor whose ubiquitous presence in the environment has been related with impairment of male reproduction. BPA can cause both transcriptomic and epigenetic changes during spermatogenesis. To evaluate the potential effects of male exposure to BPA, adult zebrafish males were exposed during spermatogenesis to doses of 100 and 2000 µg/L, which were reported in contaminated water bodies and higher than those allowed for human consumption. Fertilization capacity and survival at hatching were analysed after mating with untreated females. Spermatogenic progress was analysed through a morphometrical study of testes and apoptosis was evaluated by TUNEL assay. Testicular gene expression was evaluated by RT-qPCR and epigenetics by using ELISA and immunocytochemistry. In vitro studies were performed to investigate the role of Gper. Chromatin fragmentation and the presence of transcripts were also evaluated in ejaculated sperm. Results on testes from males treated with the highest dose showed a significant decrease in spermatocytes, an increase in apoptosis, a downregulation of ccnb1 and sycp3, all of which point to an alteration of spermatogenesis and to meiotic arrest and an upregulation of gper1 and esrrga receptors. Additionally, BPA at 2000 µg/L caused missregulation of epigenetic remodelling enzymes transcripts in testes and promoted DNA hypermethylation and H3K27me3 demethylation. BPA also triggered an increase in histone acetyltransferase activity, which led to hyperacetylation of histones (H3K9ac, H3K14ac, H4K12ac). In vitro reversion of histone acetylation changes using a specific GPER antagonist, G-36, suggested this receptor as mediator of histone hyperacetylation. Males treated with the lower dose only showed an increase in some histone acetylation marks (H3K14ac, H4K12ac) but their progeny displayed very limited survival at hatching, revealing the deleterious effects of unbalanced paternal epigenetic information. Furthermore, the highest dose of BPA led to chromatin fragmentation, promoting direct reproductive effects, which are incompatible with embryo development.

Distribution of DNA damage in the sperm nucleus: A study of zebrafish as a model of histone-packaged chromatin.

Reproductive defects can occur when the integrity of the male gamete genome is affected. Sperm chromatin is not homogeneous, having relaxed regions which are more accessible to the transcription machinery in the embryo, and thought to be specially sensitive to DNA damage. The level of damage in specific genes located in these sensitive regions could represent an early biomarker of damage. Our objective is to test the hypothesis that these more relaxed regions show greater susceptibility to damage in zebrafish, a species lacking protamines and whose sperm chromatin is compacted with histones. After sperm UV irradiation, treatment with H2O2 and cryopreservation, global chromatin fragmentation was evaluated using the TUNEL assay, and the number of lesions per 10 Kb in specific genes (hoxa3a, hoxb5b, sox2, accessible for early transcription and rDNA 18S and rDNA 28S) was quantified by using a qPCR approach. Additionally, oxidative damage within the sperm nucleus and the potential colocalization of this injury with histone H3 and TOPO IIα+ß were located by using immunofluorescence. UV irradiation produced the highest degree of fragmentation (p = 0.041) and the highest number of lesions per 10 Kb in all the genes, but no differences were observed in sensitivity to damage in the studied genes (ranging from 14.93 to 8.03 lesions per 10 Kb in hoxb5b and 28S, respectively). In contrast, H2O2 and cryopreservation caused varying levels of damage in the analyzed genes which was not related to their accessibility, ranging from 0.00 to 1.65 lesions per 10 Kb in 28S and hoxb5b, respectively, after H2O2 treatment, and from 0.073 to 5.51 in 28S and sox2, respectively, after cryopreservation. Immunodetection near oxidative lesions also revealed different spatial patterns depending on the treatments used, these being mostly homogeneous with UV irradiation or cryopreservation, and peripherally located around the nucleus after H2O2 treatment. Oxidative lesions did not colocalize with histone H3 or TOPO IIα+ß, thus demonstrating that the relaxed DNA regions associated with these proteins were not more vulnerable to oxidative damage. Results suggest that accessibility of each agent to the nucleus could be the main factor responsible for the distribution of sperm DNA damage rather than the organization of the chromatin. Lesions in these genes important to early embryo development assayed in this study cannot be used as biomarkers of global DNA damage.

Tolerance to paternal genotoxic damage promotes survival during embryo development in zebrafish (Danio rerio).

Spermatozoa carry DNA damage that must be repaired by the oocyte machinery upon fertilization. Different strategies could be adopted by different vertebrates to face the paternal genotoxic damage. Mammals have strong sperm selection mechanisms and activate a zygotic DNA damage response (DDR) (including cell cycle arrest, DNA repair and alternative apoptosis) in order to guarantee the genomic conformity of the reduced progeny. However, external fertilizers, with different reproductive strategies, seem to proceed distinctively. Previous results from our group showed a downregulation of apoptotic activity in trout embryos with a defective DNA repairing ability, suggesting that mechanisms of tolerance to damaged DNA could be activated in fish to maintain cell survival and to progress with development. In this work, zebrafish embryos were obtained from control or UV-irradiated sperm (carrying more than 10% of fragmented DNA but still preserving fertilization ability). DNA repair (γH2AX and 53BP1 foci), apoptotic activity, expression of genes related to DDR and malformation rates were analyzed throughout development. Results showed in the progeny from damaged sperm, an enhanced repairing activity at the mid-blastula transition stage that returned to its basal level at later stages, rendering at hatching a very high rate of multimalformed larvae. The study of transcriptional and post-translational activity of tp53 (ZDF-GENE-990415-270) revealed the activation of an intense DDR in those progenies. However, the downstream pro-apoptotic factor noxa (ZDF-GENE-070119-3) showed a significant downregulation, whereas the anti-apoptotic gene bcl2 (ZDF-GENE-051015-1) was upregulated, triggering a repressive apoptotic scenario in spite of a clear genomic instability. This repression can be explained by the observed upregulation of p53 isoform Δ113p53, which is known to inhibit bcl2 transcription. Our results showed that tp53 is involved in DNA damage tolerance (DDT) pathways, allowing the embryo survival regardless of the paternal DNA damage. DDT could be an evolutionary mechanism in fish: tolerance to unrepaired sperm DNA could introduce new mutations, some of them potentially advantageous to face a changing environment.

Widely power-tunable polarization-independent ultrafast mode-locked fiber laser using bulk InN as saturable absorber.

The growing demand of ultrafast mode-locked fiber lasers in the near infrared has boosted the research activity in this area. One of the most convenient ways to achieve passive mode locking consists of inserting a semiconductor saturable absorber in the laser cavity to modulate the losses. However, in such a configuration, the limited power range of operation is still an unsolved issue. Here we report the fabrication of an ultrafast, high-power, widely power-tunable and non-polarization-dependent mode-locked fiber laser operating at 1.55 µm, using an InN layer as saturable absorber. With post-amplification, this laser delivers 55-fs pulses with a repetition rate of 4.84 MHz and peak power in the range of 1 MW in an all-fiber arrangement.

Probiotic administration improves sperm quality in asthenozoospermic human donors.

The objective of this study is to analyse the effect of the ingestion of two selected antioxidant probiotics strains (Lactobacillus rhamnosus CECT8361 and Bifidobacterium longum CECT7347) on sperm quality parameters in asthenozoospermic males after three and six weeks of administration. Nine asthenozoospermic men without any medical treatment under similar diet conditions participated in the study. The quality of individual sperm samples was evaluated before (previous to ingestion), during (after 3 and 6 weeks of ingestion) and after probiotic administration (3 and 6 weeks after finishing the treatment). Sperm motility was evaluated by computer-assisted sperm analysis system, DNA fragmentation by sperm chromatin structure assay, cell viability by flow cytometry and measurement of intracellular H2O2 (reactive oxygen species; ROS) by flow cytometry using dichloro-dihydrofluorescein diacetate. Sperm motility was drastically improved after the treatment (approximately 6 fold change), DNA fragmentation was statistically reduced after probiotic administration from (approximately 1.2 fold change) and intracellular H2O2 level was decreased (approximately 3.5 fold change). Cell viability was not affected by the treatment. The significant improvement in sperm motility and the decrease in DNA fragmentation reported in this study provide preliminary evidence that probiotics could be administrated to improve motility and decrease DNA fragmentation and ROS levels in asthenozoospermic human males.

Myogenic temporomandibular disorders: Clinical systemic comorbidities in a female population sample.

BACKGROUND: Myogenic temporomandibular disorders (MTMD) frequently coexist with other clinical conditions in the same individual. In the last decades, several authors have analyzed these comorbidities looking for the origin of this overlapping. Objetives: The aim of this study was to perform a comparative anaylisis between a group of patients with MTMD and a control group of dental patients without dysfunctional pathology to assess whether there are significant differences in the presence of systemic medical comorbidities between the two groups. MATERIAL AND METHODS: Restrospective epidemiological analysis, based on medical questionnaires in a group of 31 patients, women, aged from 24 to 58 (average 39.96 years), diagnosed with MTMD (Masticatory Myofascial Pain), with a control group with the same number of individuals, gender and age range to evaluate if there is a significant statistical difference in the presence of medical comorbidities in this group of patients with MTMD and if they are in a higher risk of suffering different pathological conditions. RESULTS: It was found that the group affected by MTMD presented many more associated medical conditions than the control group: health changes during the last year, medical evaluations and treatments, presence of pain, sinus disease, tinnitus, headache, joint pain, ocular disorders, fatigue, dizziness, genitourinary disorders and xerostomia among others; and they were also in a higher risk to suffer other pathological entities as headaches and articular pain. CONCLUSIONS: These results reinforce our hypothesis that MTMD belong to a group of medical conditions triggered by a loss of equilibrium of the individual's Psycho-Neuro-Endocrine-Immune (PNEI) Axis that produces alterations in the response against external stimuli in some genetically predisposed individuals. It is, therefore, necessary to change the way of diagnosing and managing these individual's medical conditions, being mandatory to look from a more multidisciplinary perspective than the one we are currently offering.

Real time dynamic strain monitoring of optical links using the backreflection of live PSK data.

A major cause of faults in optical communication links is related to unintentional third party intrusions (normally related to civil/agricultural works) causing fiber breaks or cable damage. These intrusions could be anticipated and avoided by monitoring the dynamic strain recorded along the cable. In this work, a novel technique is proposed to implement real-time distributed strain sensing in parallel with an operating optical communication channel. The technique relies on monitoring the Rayleigh backscattered light from optical communication data transmitted using standard modulation formats. The system is treated as a phase-sensitive OTDR (ΦOTDR) using random and non-periodical non-return-to-zero (NRZ) phase-shift keying (PSK) pulse coding. An I/Q detection unit allows for a full (amplitude, phase and polarization) characterization of the backscattered optical signal, thus achieving a fully linear system in terms of ΦOTDR trace coding/decoding. The technique can be used with different modulation formats, and operation using 4 Gbaud single-polarization dual PSK and 4 Gbaud dual-polarization quadrature PSK is demonstrated. As a proof of concept, distributed sensing of dynamic strain with a sampling of 125 kHz and a spatial resolution of 2.5 cm (set by the bit size) over 500 m is demonstrated for applied sinusoidal strain signals of 500 Hz. The limitations and possibilities for improvement of the technique are also discussed.

Single-shot distributed temperature and strain tracking using direct detection phase-sensitive OTDR with chirped pulses.

So far, the optical pulses used in phase-sensitive OTDR (ΦOTDR) were typically engineered so as to have a constant phase along the pulse. In this work, it is demonstrated that by acting on the phase profile of the optical pulses, it is possible to introduce important conceptual and practical changes to the traditional ΦOTDR operation, thus opening a door for new possibilities which are yet to be explored. Using a ΦOTDR with linearly chirped pulses and direct detection, the distributed measurement of temperature/strain changes from trace to trace, with 1mK/4nε resolution, is theoreticaly and experimentaly demonstrated. The measurand resolution and sensitivity can be tuned by acting on the pulse chirp profile. The technique does not require a frequency sweep, thus greatly decreasing the measurement time and complexity of the system, while maintaining the potential for metric spatial resolutions over tens of kilometers as in conventional ΦOTDR. The technique allows for measurements at kHz rates, while maintaining reliability over several hours.

Strength and toughness of structural fibres for composite material reinforcement.

The characterization of the strength and fracture toughness of three common structural fibres, E-glass, AS4 carbon and Kevlar KM2, is presented in this work. The notched specimens were prepared by means of selective carving of individual fibres by means of the focused ion beam. A straight-fronted edge notch was introduced in a plane perpendicular to the fibre axis, with the relative notch depth being a0/D≈0.1 and the notch radius at the tip approximately 50 nm. The selection of the appropriate beam current during milling operations was performed to avoid to as much as possible any microstructural changes owing to ion impingement. Both notched and un-notched fibres were submitted to uniaxial tensile tests up to failure. The strength of the un-notched fibres was characterized in terms of the Weibull statistics, whereas the residual strength of the notched fibres was used to determine their apparent toughness. To this end, the stress intensity factor of a fronted edge crack was computed by means of the finite-element method for different crack lengths. The experimental results agreed with those reported in the literature for polyacrylonitrile-based carbon fibres obtained by using similar techniques. After mechanical testing, the fracture surface of the fibres was analysed to ascertain the failure mechanisms. It was found that AS4 carbon and E-glass fibres presented the lower toughness with fracture surfaces perpendicular to the fibre axis, emanating from the notch tip. The fractured region of Kevlar KM2 fibres extended along the fibre and showed large permanent deformation, which explains their higher degree of toughness when compared with carbon and glass fibres. This article is part of the themed issue 'Multiscale modelling of the structural integrity of composite materials'.

Selection of nonapoptotic sperm by magnetic-activated cell sorting in Senegalese sole (Solea senegalensis).

Senegalese sole (Solea senegalensis) is a promising species in aquaculture. However, owing to decreased sperm quality in F1 generations and the absence of courtship in those individuals born in captivity, artificial fertilization is being used to generate new progenies. The objective of this study was to implement a sperm selection method for nonapoptotic sperm subpopulation recovery before sperm cryopreservation. In particular, magnetic-activated cell sorting is used to eliminate apoptotic spermatozoa. This study represents the proof-of-concept for magnetic-activated cell sorting applicability in teleost species relevant in aquaculture. Apoptotic cell population was studied by flow cytometry using YO-PRO-1 and a caspase detection kit. Also, reactive oxygen species were measured in sperm samples. Our data demonstrated that caspase detection is more specific than YO-PRO-1 in the identification of apoptotic cells in S senegalensis seminal samples. The results showed that the percentage of apoptotic cells (caspase positive) was significantly higher (P = 0.04) in seminal samples from F1 than that from wild individuals. Magnetic-activated cell sorting removed a significant number of apoptotic cells from the samples (54% and 75% in wild and F1 individuals, respectively), decreasing the level of cells positive for reactive oxygen species (P = 0.17). In conclusion, this technique reduces the percentage of nonfunctional spermatozoa in a seminal sample before cryopreservation. This novel technique can be applied directly in the aquaculture industry.

Impact of sperm DNA damage and oocyte-repairing capacity on trout development.

Zygotic repair of paternal DNA is essential during embryo development. In spite of the interest devoted to sperm DNA damage, its combined effect with defect-repairing oocytes has not been analyzed. Modification of the breeding season is a common practice in aquaculture. This practice reduces developmental success and could affect the both factors: sperm DNA integrity and oocyte repair capacity. To evaluate the maternal role, we analyzed the progeny outcome after fertilizing in-season trout oocytes with untreated and with UV-irradiated sperm. We also analyzed the offspring obtained out of season with untreated sperm. The analysis of the number of lesions in 4 sperm nuclear genes revealed an increase of 1.22-11.18 lesions/10 kb in out-of-season sperm, similar to that obtained after sperm UV irradiation (400 µW/cm(2)5 min). Gene expression showed in out-of-season oocytes the overexpression of repair genes (ogg1, ung, lig3, rad1) and downregulation of tp53, indicating an enhanced repairing activity and reduced capacity to arrest development upon damage. The analysis of the progeny in out-of-season embryos revealed a similar profile tolerant to DNA damage, leading to a much lower apoptotic activity at organogenesis, lower hatching rates and increased rate of malformations. The effects were milder in descendants from in-season-irradiated sperm, showing an enhanced repairing activity at epibolia. Results point out the importance of the repairing machinery provided by the oocyte and show how susceptible it is to environmental changes. Transcripts related to DNA damage signalization and repair could be used as markers of oocyte quality.

Exploiting nonreciprocity in BOTDA systems.

In this Letter, we present and demonstrate a novel technique for distributed measurements in Brillouin optical time-domain analysis based on the use of the nonlinear phase-shift induced by stimulated Brillouin scattering. Employing a Sagnac interferometer (SI), the position-resolved Brillouin phase-shift spectrum (BPS) along the fiber can be obtained, benefiting from the sensitivity to nonreciprocal phase-shifts of the SI scheme. This proposal simplifies the existing methods to retrieve the BPS distribution along an optical fiber since phase modulation, filtering, and high-bandwidth detectors are not required. The fundamentals of the technique are described theoretically and validated through numerical simulations and experimental measurements.

Improved time-resolved acousto-optic technique for optical fiber analysis of axial non-uniformities by using edge interrogation.

The time-resolved acousto-optic technique demonstrated recently to be a very useful method for the analysis of fiber axial non-uniformities, able to detect variations of fiber diameter in the nanometric scale with a spatial resolution of few cm. An edge interrogation approach is proposed to improve further the performance of this technique. The detection of subnanometer fiber diameter changes or sub-ppm changes of the core refractive index is demonstrated.

Inhibition of zygotic DNA repair: transcriptome analysis of the offspring in trout (Oncorhynchus mykiss).

Zygotic repair of the paternal genome is a key event after fertilization. Spermatozoa accumulate DNA strand breaks during spermatogenesis and can suffer additional damage by different factors, including cryopreservation. Fertilization with DNA-damaged spermatozoa (DDS) is considered to promote implantation failures and abortions, but also long-term effects on the progeny that could be related with a defective repair. Base excision repair (BER) pathway is considered the most active in zygotic DNA repair, but healthy oocytes contain enzymes for all repairing pathways. In this study, the effects of the inhibition of the BER pathway in the zygote were analyzed on the progeny obtained after fertilization with differentially DDS. Massive gene expression (GE; 61 657 unique probes) was analyzed after hatching using microarrays. Trout oocytes are easily fertilized with DDS and the high prolificacy allows live progeny to be obtained even with a high rate of abortions. Nevertheless, the zygotic inhibition of Poly (ADP-ribose) polymerase, upstream of BER pathway, resulted in 810 differentially expressed genes (DEGs) after hatching. DEGs are related with DNA repair, apoptosis, telomere maintenance, or growth and development, revealing a scenario of impaired DNA damage signalization and repair. Downregulation of the apoptotic cascade was noticed, suggesting a selection of embryos tolerant to residual DNA damage during embryo development. Our results reveal changes in the progeny from defective repairing zygotes including higher malformations rate, weight gain, longer telomeres, and lower caspase 3/7 activity, whose long-term consequences should be analyzed in depth.

Subpopulation pattern of eel spermatozoa is affected by post-activation time, hormonal treatment and the thermal regimen.

There has been a marked reduction in natural stocks of eels (genus Anguilla) over the past 60 years, and the culture of eels is still based on the capture of very large quantities of juveniles. It is necessary to close the life cycle in captivity in order to ease the pressure on wild populations. The aims of the present study were to evaluate sperm subpopulations (through cluster analysis of computer-aided sperm analysis data) in the European eel (Anguilla anguilla) and to assess the effects of motility acquisition time after activation (i.e. at 30, 60 and 90s), the thermal regimen (i.e. 10°C (T10) or 15°C (T15) and up to 20°C, or constant at 20°C (T20)) and hormonal treatments (i.e. human chorionic gonadotropin (hCG), recombinant (r) hCG or pregnant mare serum gonadotropin (PMSG)) on these subpopulations. In all cases, we obtained three subpopulations of spermatozoa: low velocity and linear (S1); high velocity with low linearity (S2); and high velocity and linear (S3; considered high quality). Total motility and S1 were affected by acquisition time; thus, 30s is recommended as the standard time for motility acquisition. When eels were kept at 20°C (T20), motility data fitted quadratic models, with the highest motility and proportion of S3 between Weeks 8 and 12 after the first injection. Lower temperatures (T10, T15) delayed spermiation and the obtaining of high-quality spermatozoa (S3), but did not seem to alter the spermiation process (similar subpopulation pattern). Conversely, the hormonal treatments altered both the dynamics of the subpopulation pattern and the onset of spermiation (with PMSG delaying it). Total motility and the yield of S3 with the widely used hCG treatment varied throughout the spermiation period. However, using rhCG allowed us to obtain high-quality and constant motility for most of the study (Weeks 7-20), and the S3 yield was also higher overall (61.8±1.3%; mean ± s.e.m.) and more stable over time than the other hormonal treatments (averaging 53.0±1.4%). Using T20 and rhCG would be more economical and practical, allowing us to obtain a higher number of S3 spermatozoa over an extended time.

In vitro generation of zebrafish PGC-like cells.

The possibility of generating primordial germ cells (PGCs) in vitro from noncommitted embryonic cells represents an extremely useful tool in current research. Primordial germ cell in vitro differentiation has been successfully reported in mammals. However, contrary to fish, PGC specification in mammals is an inductive mechanism. This study is the first to date to describe a rapid method for PGC in vitro differentiation in teleosts. Primordial germ cell-like cells were characterized by several lines of evidence, including gene expression, cell complexity, size, and image analysis for the quantification of fluorescence under vasa promoter. Moreover, differentiated cells were able to colonize the genital ridge after transplantation. Differentiation treatments increased the number of PGCs in culture, causing differentiation of cells rather than inducing their proliferation. These results open up the possibility of differentiating genetically modified embryonic cells to PGC-like cells to ensure their transmission to the progeny and could be crucial for an in-depth understanding of germline differentiation in teleosts.

High-sensitivity dispersive Mach-Zehnder interferometer based on a dissimilar-doping dual-core fiber for sensing applications.

A dual-core fiber in which one of the cores is doped with germanium and the other with phosphorus is used as an in-line Mach-Zehnder dispersive interferometer. By ensuring an equal length but with different dispersion dependencies in the interferometer arms (the two cores), high-sensitivity strain and temperature sensing are achieved. Opposite sensitivities for high and low wavelength peaks were also demonstrated when strain and temperature was applied. To our knowledge this is the first time that such behavior is demonstrated using this type of in-line interferometer based on a dual-core fiber. A sensitivity of (0.102±0.002) nm/µÎµ, between 0 and 800 µÎµ and (-4.2±0.2) nm/°C between 47°C and 62°C is demonstrated.

Time-resolved acousto-optic interaction in single-mode optical fibers: characterization of axial nonuniformities at the nanometer scale.

We report on a time-resolved acousto-optic interaction technique for the detection of axial nonuniformities in single-mode fibers. It is based on the propagation of short packets of flexural acoustic waves. Small axial nonuniformities (of the order of nanometers) are detected by measuring the transmittance of the fundamental mode as a function of time. It is shown that the technique allows the detection of axial nonuniformities along sections of single-mode fiber exceeding 1 m long with spatial resolution of the order of a few centimeters.