Karine Alain

Brian J Tindall

Laurent Intertaglia

Philippe Catala

Philippe Lebaron
email: philippe.lebaron@obs-banyuls.fr

Umr6197

a novel prosthecate alphaproteobacterium from the Mediterranean Sea

Keywords:

The GenBank/EMBL/DDBJ accession number for the 16S rRNA gene sequence of strain 26III/A02/215 T is AY576758.

) is available in IJSEM online.

A novel aerobic, heterotrophic, prosthecate bacterium designated 26III/A02/215 T , was isolated from surface waters of the north-western Mediterranean sea. Cells were Gram-negative, straight to slightly curved rods, forming red colonies on agar plates. The strain grew at 15-37°C inclusive (optimum:

30°C), and optimally at seawater salinity. Growth on organic acids, amino-acids and complex organic substrates was observed. The fatty acids (> 5%) detected in strain 26III/A02/215 T were C 17:1 ω6c, The phylum Proteobacteria is one of the 24 phyla of the domain Bacteria, described in Bergey's Manual of Systematic Bacteriology, 2 nd edn [START_REF] Garrity | The road map to the Manual[END_REF]. To date, more than 200 genera have been described, making this phylum one of the largest bacterial phyla. Representative members of this group are widely distributed in nature and are physiologically and metabolically diverse. The phylum Proteobacteria is currently divided into 5 classes, called the Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Deltaproteobacteria and Epsilonproteobacteria, all of which have been defined exclusively on the basis of 16S rRNA gene sequence analysis [START_REF] Garrity | The road map to the Manual[END_REF]. At present, the class Alphaproteobacteria Garrity et al. 2006(Validation List 107, Garrity et al., 2005a) is composed of seven orders: Caulobacterales [START_REF] Henrici | Stalked bacteria, a new order of schizomycetes[END_REF][START_REF] Henrici | Stalked bacteria, a new order of schizomycetes[END_REF], Kordiimonadales [START_REF] Kwon | Kordiimonas gwangyangensis gen. nov., sp. nov., a marine bacterium isolated from marine sediments that forms a distinct phyletic lineage (Kordiimonadales ord. nov.) in the Alphaproteobacteria[END_REF][START_REF] Kwon | Kordiimonas gwangyangensis gen. nov., sp. nov., a marine bacterium isolated from marine sediments that forms a distinct phyletic lineage (Kordiimonadales ord. nov.) in the Alphaproteobacteria[END_REF], Rhodobacterales Garrity et al. 2006(Validation List 107, Garrity et al., 2005b) Rhodospirillales [START_REF] Pfenning | Higher taxa of the phototrophic bacteria[END_REF] [START_REF] Pfenning | Higher taxa of the phototrophic bacteria[END_REF], Rickettsiales Gieszczykiewicz, 1939(Gieszczykiewicz, 1939), Rhizobiales Kuykendall 2006(Validation List 107, Kuykendall, 2005) and Sphingomonadales Yabuuchi & Kosako, 2006(Validation List 107, Yabuuchi & Kosako, 2005). Marine species make up more than half of the Alphaproteobacteria described to date. Some confusion is being caused at present by the different taxonomic placement of the "stalked" bacteria.

While [START_REF] Lee | The hierarchical system of the Alphaproteobacteria: description of Hyphomonadaceae fam. nov., Xanthobacteraceae fam. nov. and Erythrobacteraceae fam. nov[END_REF] place the members of the genera Hyphomonas, Oceanicaulis, Hirschia, and Maricaulis in a new family, the Hyphomonadaceae, within the order Caulobacterales (which includes members of the family Rhodobacteraceae), Garrity et al. (2005a) have placed members of these genera within the family Rhodobacteraceae, within the order Rhodobacterales, leaving the members of the family Caulobacteraceae within the order Caulobacterales. This situation is particularly unsatisfactory since use of the name of the order Caulobacterales alone does not give unambiguous information on which taxa are to be included within it. Furthermore, [START_REF] Lee | The hierarchical system of the Alphaproteobacteria: description of Hyphomonadaceae fam. nov., Xanthobacteraceae fam. nov. and Erythrobacteraceae fam. nov[END_REF] dealt with the taxonomy of the family Rhodobacteraceae before the name was validly published (Validation List 107). Paradoxically [START_REF] Lee | The hierarchical system of the Alphaproteobacteria: description of Hyphomonadaceae fam. nov., Xanthobacteraceae fam. nov. and Erythrobacteraceae fam. nov[END_REF] created a new family, the family Hyphomonadaceae, with the type defined as the genus Hyphomonas, a taxon specifically included in the taxon proposed by Garrity et al. (2005b) as the family Rhodobacteraceae. Based on the principle of priority, the family proposed by Garrity et al. (2005b) must be named after the earliest validly published family name, which is the family Hyphomonadaceae. The family name Rhodobacteraceae Garrity et al. 2006 may only be used if specifically defined to exclude the type genus of the family Hyphomonadaceae. It should be noted that members of the genera Woodsholea [START_REF] Abraham | Woodsholea maritima gen. nov., sp. nov., a marine bacterium with a low diversity of polar lipids[END_REF] and Robiginitomaculum [START_REF] Lee | Robiginitomaculum antarcticum gen. nov., sp. nov., a member of the family Hyphomonadaceae, from Antarctic seawater[END_REF] should be included in the family Hyphomonadaceae [START_REF] Lee | Robiginitomaculum antarcticum gen. nov., sp. nov., a member of the family Hyphomonadaceae, from Antarctic seawater[END_REF].

The members of the families Hyphomonadaceae and Caulobacteraceae contain organisms that share the particular feature of being appendaged [START_REF] Poindexter | The caulobacters: ubiquitous unusual bacteria[END_REF][START_REF] Abraham | Phylogeny and polyphasic taxonomy of Caulobacter species. Proposal of Maricaulis gen. nov. with Maricaulis maris (Poindexter) comb. nov. as the type species, and emended description of the genera Brevundimonas and Caulobacter[END_REF][START_REF] Weiner | Hyphomonas adhaerens, sp. nov., Hyphomonas johnsonii sp. nov. and Hyphomonas rosenbergii sp. nov., marine budding and prosthecate bacteria[END_REF][START_REF] Strömpl | Oceanicaulis alexandrii gen. nov., sp. nov., a novel stalked bacterium isolated from a culture of the dinoflagellate Alexandrium tamarense (Lebour) Balech[END_REF]. As indicated by their vernacular name (in Latin, caulis means stalk), these 'caulobacteria' bear one or several stalks, so-called prosthecae. These stalks are cytoplasm extrusions that undoubtedly play a role in attachment. As a result, they increase significantly the surface to volume ratio of the cells. Consequently, they have often been interpreted as an evolutionary adaptation to life in oligotrophic waters. Most genera of the families Hyphomonadaceae and Caulobacteraceae (i.e. members of the genera Hyphomonas, Caulobacter, Asticcacaulis, Phenylobacterium, Hirschia, Robiginitomaculum, Woodsholea, Maricaulis, Oceanicaulis and Brevundimonas) are widely distributed in marine environments [START_REF] Anast | Isolation and characterization of marine caulobacters and assessment of their potential for generic experimentation[END_REF], and especially (but not exclusively) in oligotrophic waters. They are believed to play an important role in the mineralization of dissolved organic matter [START_REF] Abraham | Phylogeny and polyphasic taxonomy of Caulobacter species. Proposal of Maricaulis gen. nov. with Maricaulis maris (Poindexter) comb. nov. as the type species, and emended description of the genera Brevundimonas and Caulobacter[END_REF].

In this study, a novel marine caulobacterium is described. Based on the results of a polyphasic taxonomic analysis, the strain 26III/A02/215 T represents a novel species and genus, Hellea balneolensis gen. nov., sp.

nov.

In September 2001, coastal waters were collected in the bay of Banyuls-sur-mer (42°29'N, 3°08'E), in the Mediterranean Sea, France. A sea sample from the surface microlayer was spread on marine agar 2216 (MA; Difco) plate, and then incubated at 25°C. After 2 weeks, a red-coloured colony was picked, purified by repeated streaking on MA plates, and referenced as strain 26III/A02/215 T [START_REF] Agogué | A survey on bacteria inhabiting the sea surface microlayer of coastal ecosystems[END_REF]. Stock cultures were stored at ─80°C in marine broth 2216 (MB; Difco) supplemented with 5% (v/v) DMSO or 35% (v/v) glycerol, until characterization.

Both strands of the almost complete 16S rRNA gene (1412 bp) of the strain were sequenced from one single colony, as described elsewhere [START_REF] Agogué | A survey on bacteria inhabiting the sea surface microlayer of coastal ecosystems[END_REF]. This sequence was compared to those in available databases by use of the BLAST program [START_REF] Altschul | Basic local alignment search tool[END_REF] and then aligned to its nearest neighbours using the CLUSTALX program [START_REF] Thompson | The ClustalX windows interface: flexible strategies for multiple sequence alignment aided by quality analysis tools[END_REF]. Alignments were refined manually using the SEAVIEW program [START_REF] Galtier | SEAVIEW and PHYLO_WIN: two graphic tools for sequence alignment and molecular phylogeny[END_REF]. Phylogenetic trees were constructed by the PHYLIP (PHYlogeny Inference Package) version 3.63 software (http://evolution.genetics.washington.edu/phylip/getme.html) on the basis of evolutionary distance (neighbour-joining method with Jukes and Cantor corrections) [START_REF] Saitou | The neighbour-joining method: a new method for reconstructing phylogenetic trees[END_REF] and maximum likelihood [START_REF] Felsenstein | Evolutionary trees from DNA sequences: a maximum likelihood approach[END_REF]. The robustness of the inferred topologies was assessed by bootstrap analyses based on 1000 bootstrap resamplings for the neighbour-joining and 100 replications for the maximum likelihood method [START_REF] Felsenstein | Confidence limits on phylogenies: an approach using the bootstrap[END_REF]. The 16S rRNA gene-based analysis located the strain 26III/A02/215 T within the class Alphaproteobacteria, in the bacterial domain. The results of different phylogenetic reconstructions performed with different treeing algorithms located the novel isolate within the Hyphomonas-Hirschia-Robiginitomaculum branch, amongst the marine caulobacteria of family Hyphomonadaceae [START_REF] Lee | The hierarchical system of the Alphaproteobacteria: description of Hyphomonadaceae fam. nov., Xanthobacteraceae fam. nov. and Erythrobacteraceae fam. nov[END_REF], order Caulobacterales (Fig. 1). Within this branch, the novel isolate clustered with the recently described genus Robiginitomaculum [START_REF] Lee | Robiginitomaculum antarcticum gen. nov., sp. nov., a member of the family Hyphomonadaceae, from Antarctic seawater[END_REF] sharing 93.9% 16S rDNA sequence similarity with the only species of this genus. The level of 16S rRNA gene sequence similarity between strain 26III/A02/215 T and representative of the genera Hyphomonas and Hirschia ranged from 89 to 92%.

The DNA G+C content was determined, by the Identification Service of the DSMZ (Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH, Braunschweig

Germany), by HPLC analysis of deoxyribonucleosides as described by [START_REF] Mesbah | Precise measurement of the G+C content of deoxyribonucleic acid by high performance liquid chromatography[END_REF]. The G+C content of strain 26III/A02/215 T was 46.8 mol%. Thus, it differed by more than 10 mol% from the DNA G+C content of its closest relative Robiginitomaculum antarcticum (60.3 mol%). Clearly, this large difference in the DNA base ratio, together with the 16S rDNA level of similarity, suggest that strain 26III/A02/215 T belongs to a novel genus [START_REF] Rosselló-Mora | The species concept for prokaryotes[END_REF].

Colonies on MA were circular, smooth, brilliant, convex, with an entire edge and intensely pigmented brickred. After 1 week incubation, colonies were about 1 mm in diameter. Morphological characteristics of the cells were determined by light microscopy (Olympus AX70) and by transmission electron microscopy (Hitachi H-7500) after negative staining with uranyl acetate [START_REF] Raguénès | Vibrio diabolicus sp. nov., a new polysaccharide-secreting organism isolated from a deep-sea hydrothermal vent polychaete annelid, Alvinella pompejana[END_REF]. Briefly, cells of strain 26III/A02/215 T were Gram-negative, thin straight to curved rods bearing one polar stalk (Fig. 2). Cells bearing several lateral stalks were occasionally observed. Mid-exponential phase cells were 2.70-5.60 µm in length (mean 3.58 ± 0.88 µm, n=15), 0.28-0.48 µm in width (mean 0.42 ± 0.06 µm, n=15), some of which produced a stalk(s). When present, the stalk was more generally cylindrical and extended centrally along the cell axis from one pole. This stalk showed constriction sites distributed equally all along the tube, but which not corresponded to a compartmentalization. This type of constriction has been observed previously in

Oceanicaulis alexandrii [START_REF] Strömpl | Oceanicaulis alexandrii gen. nov., sp. nov., a novel stalked bacterium isolated from a culture of the dinoflagellate Alexandrium tamarense (Lebour) Balech[END_REF]. Stalked-cells were non-motile, while non-stalked cells were motile by means of a polar flagellum. Cells divided by budding.

In order to analyse respiratory quinones and polar lipids, strain 26III/A02/215 T was grown for 3 days on MB medium at 30°C, and checked for purity. Initial analyses of the polar lipids and respiratory quinones were carried out by the Identification Service, DSMZ, Braunschweig, Germany. Ubiquinone (Q10) was determined to be the sole respiratory quinone. The thin layer chromatogram obtained with cell extracts from the novel isolate was very characteristic (Fig. 3). The polar lipid pattern showed the presence of phosphatidylglycerol (PG), monoglycosyldiglyceride (MGDG), glucuronopyranosyldiglyceride (GUDG), one unidentified glycolipid (Gl) and three phospholipids (PL1, PL2, PL3). The presence of the polar lipids monoglycosyldiglyceride (MGDG) and glucuronopyranosyldiglyceride (GUDG) appears to be a characteristic signature for other members of the families Hyphomonadaceae and Caulobacteraceae, together with the absence of phosphatidylethanolamine, phosphatidylcholine and diphosphatidylglycerol.

The presence of two unidentified phospholipids PL2 and PL3, together with an unidentified glycolipid appeared to be a characteristic feature of the lipid pattern of this taxon. The determination of the whole-cell fatty acid composition was performed on cultures grown at 30°C for 72h on marine agar 2216. The analysis was carried out at the DSMZ according to the standard protocol of the Microbial Identification System (MIDI Inc., Del. USA, 2001). Extracts were analysed using a Hewlett Packard model HP6890A gas chromatograph equipped with a flame-ionization detector as described by [START_REF] Kämpfer | Numerical analysis of fatty acid patterns of coryneform bacteria and related taxa[END_REF].

Results are summarized in Table 1. The fatty acids in strain 26III/A02/215 T comprised C 16:0 , C 17:0 , C 18:0 , C 19:0 , C 17:1 ω8c, C 17:1 ω6c, C 18:1 ω9c, C 18:1 ω7c, C 20:1 ω7c, 3-OH C 10:0 , 3-OH C 11:0 , 3-OH C 12:1 , 2-OH C 18:1 , TBSA 10-methyl C 18:0 , Summed feature 3, Summed feature 7. The presence of C 18:1 ω7c, together with Q10 is typical of the vast majority of taxa within the Alphaproteobacteria. Although the polar lipid composition of the recently described Robiginitomaculum antarcticum was not reported, there were clear differences in the fatty acid patterns, in particular the distribution of the 3-hydroxy fatty acids, which are probably derived from lipopolysaccharide. A number of recent publications are also not complete with regard to the chemotaxonomic data. In papers on the genera Oceanicaulis [START_REF] Strömpl | Oceanicaulis alexandrii gen. nov., sp. nov., a novel stalked bacterium isolated from a culture of the dinoflagellate Alexandrium tamarense (Lebour) Balech[END_REF], Woodsholea [START_REF] Abraham | Woodsholea maritima gen. nov., sp. nov., a marine bacterium with a low diversity of polar lipids[END_REF] and some Maricaulis [START_REF] Abraham | Phylogeny of Maricaulis Abraham et al. 1999 and proposal of Maricaulis virginensis sp. nov., M. parjimensis sp. nov., M. washingtonensis sp. nov. and M. salignorans sp[END_REF] species, the quinone composition has not been reported. In the case of Oceanicaulis, 3-OH fatty acids are not reported, probably because only fatty acids from extracted lipids have been reported. Reports on the polar lipid composition may be incomplete because emphasis has been placed on the presence of phosphate and sulfonic acid containing lipids [START_REF] Strömpl | Oceanicaulis alexandrii gen. nov., sp. nov., a novel stalked bacterium isolated from a culture of the dinoflagellate Alexandrium tamarense (Lebour) Balech[END_REF], see also [START_REF] Abraham | Phospho-and sulfolipids as biomarkers of Caulobacter, Brevundimonas and Hyphomonas[END_REF]. The glycolipids that are otherwise characteristic for this evolutionary group are not mentioned.

Unless stated otherwise, physiological characterization was carried out aerobically in marine broth medium (MB 2216;Difco), in triplicate, and cell suspension incubated with agitation in the dark. Growth was routinely monitored by measuring the increase in optical density at 600 nm using a spectrophotometer. Cell numbers were determined by flow cytometry [START_REF] Marie | Flow cytometry analysis of marine picoplankton[END_REF] in order to calculate calibration curves 'Cell numbers = f(OD 600 )'. Growth rates were calculated using linear regression analysis from five to nine points along the logarithmic portions of the resulting growth curves. Growth temperature was tested over the range 9-44°C (i.e. 9, 15, 20, 25, 30, 33, 37, 44°C). The novel isolate was found to be mesophilic, growing at 15-37°C; optimal growth yields occurred at 30°C (see Supplementary Fig. S1 in IJSEM online). The optimum pH for growth was tested at 30°C in buffered MB medium and was found to be around pH 6.0-8.0.

Salt tolerance was tested at 30°C in MB medium prepared with various concentrations of NaCl (0.02, 0.5, 1, 2, 3, 4, 5, 6, 7 and 9% w/v). Results indicated that the strain was a general typical marine-type halophile.

Growth was observed in media containing 0.02% (w/v) to 5% (w/v) NaCl, but it was better in media containing half-to full-strength seawater salinity. The optimal NaCl concentration for growth was around 3% (w/v) NaCl.

Strain 26III/A02/215 T was found to be aerobic. Conventional phenotypic tests including those for oxidase, catalase, tween esterase and nitrate reductase were performed according to standard methods [START_REF] Smibert | Phenotypic characterization[END_REF]. The results are given in Table 2. Biochemical tests were performed at 30°C using api®ZYM (bioMérieux) and Biolog GN2 microplates (Oxoid). These tests were inoculated with cells grown on MA plates, swabbed from the surface of the agar plates and then suspended in ASW ½ (diluted artificial seawater) to the density specified by the manufacturer. Supplementary biochemical tests were also performed using api®20NE strips (bioMérieux), following the manufacturer's instructions. The data obtained are given in Table 2. Testing for oxidation of carbon sources with Biolog GN2 plates indicated that the strain was able to oxidize a wide range of organic acids and amino acids. To confirm these results and to test for the capability of the strain to catabolize different substrates as sole carbon and energy source, with oxygen as a terminal electron acceptor, the strain was grown aerobically, in the dark, on a mineral medium supplemented with one substrate. The defined medium (modified from [START_REF] Widdel | Anaerobic biodegradation of hydrocarbons including methane[END_REF]) had the following composition (l ─1 ): phosphate buffer, 30 mM; NaCl 20 g, MgCl 2 .6H 2 O 3 g, CaCl 2 .2H 2 O 1.0 g, NH 4 Cl 0.3 g, KCl 0.5 g, Na 2 SO 4 3 g, NaNO 3 1 g; trace element solution, 1 ml; selenite-tungstate solution, 1 ml; vitamin solution, 1 ml. The strain was found to grow heterotrophically on a wide range of substrates. It catabolized organic acids, amino acids, and complex substrates for energy and growth (Table 2). The carbohydrates tested were unable to support growth when provided alone in the medium.

Antibiotic sensitivity tests were performed by using susceptibility discs (Biorad) or filter-paper discs impregnated with different antibiotics. Discs were placed on MA plates spread with a culture of the isolate and were then incubated at 30°C for one week. Susceptibility was scored as positive at zone diameters above 10 mm. The results are summarized in Table 2.

During the course of this work, we also have had cause to re-examine the taxonomy of members of the families Hyphomonadaceae and Caulobacteraceae. The placement of members of the genera Hyphomonas, Hirschia, Maricaulis, and Oceanicaulis, in the family Rhodobacteraceae (Garrity et al. 2005a) has been called into question by [START_REF] Lee | The hierarchical system of the Alphaproteobacteria: description of Hyphomonadaceae fam. nov., Xanthobacteraceae fam. nov. and Erythrobacteraceae fam. nov[END_REF]. Independent work on the genome of Hyphomonas neptunium has indicated that the 16S rDNA sequence based interpretation may be prone to error [START_REF] Badger | Genomic analysis of Hyphomonas neptunium contradicts 16S rRNA gene-based phylogenetic analysis: implications for the taxonomy of the orders 'Rhodobacterales' and Caulobacterales[END_REF][START_REF] Badger | Comparative genomic evidence for a close relationship between the dimorphic prosthecate bacteria Hyphomonas neptunium and Caulobacter crescentus[END_REF]. This conclusion is also in accord with the chemical composition reported for members of these genera, which share a number of distinctive similarities with members of the genera Caulobacter, Brevundimonas, Asticcacaulis and Phenylobacterium. Similarly, extensive chemotaxonomic work on additional taxa within the family Rhodobacteraceae, as defined by Garrity et al. (2005a) would also indicate inconsistencies (Biebl et al., 2005a(Biebl et al., , 2005b[START_REF] Biebl | Hoeflea phototrophica sp. nov., a novel marine aerobic alphaproteobacterium that forms bacteriochlorophyll a[END_REF][START_REF] Biebl | Description of Labrenzia alexandrii gen. nov., sp. nov., a novel alphaproteobacterium containing bacteriochlorophyll a, and a proposal for reclassification of Stappia aggregata as Labrenzia aggregata comb. nov., of Stappia marina as Labrenzia marina comb. nov. and of Stappia alba as Labrenzia alba comb. nov., and emended descriptions of the genera Pannonibacter, Stappia and Roseibium, and of the species Roseibium denhamense and Roseibium hamelinense[END_REF][START_REF] Martens | Reclassification of Roseobacter gallaeciensis Ruiz-Ponte et al. 1998 as Phaeobacter gallaeciensis gen. nov., comb. nov., description of Phaeobacter inhibens sp. nov., reclassification of Ruegeria algicola (Lafay et al. 1995) Uchino et al. 1999 as Marinovum algicola gen. nov., comb. nov., and emended descriptions of the genera Roseobacter, Ruegeria and Leisingeria[END_REF][START_REF] Labrenz | Roseovarius tolerans gen. nov., sp. nov., a budding bacterium with variable bacteriochlorophyll a production from hypersaline Ekho Lake (Antarctica)[END_REF][START_REF] Labrenz | Staleya guttiformis gen. nov., sp. nov. and Sulfitobacter brevis sp. nov. a budding bacterium with variable bacteriochlorophyll a production from hypersaline, heliothermal and meromictic antarctic Ekho Lake[END_REF] with the proposal of [START_REF] Lee | The hierarchical system of the Alphaproteobacteria: description of Hyphomonadaceae fam. nov., Xanthobacteraceae fam. nov. and Erythrobacteraceae fam. nov[END_REF] to unite members of the families Rhodobacteraceae (as defined by [START_REF] Lee | The hierarchical system of the Alphaproteobacteria: description of Hyphomonadaceae fam. nov., Xanthobacteraceae fam. nov. and Erythrobacteraceae fam. nov[END_REF]), Hyphomonadaceae and Caulobacteraceae (as defined by [START_REF] Lee | The hierarchical system of the Alphaproteobacteria: description of Hyphomonadaceae fam. nov., Xanthobacteraceae fam. nov. and Erythrobacteraceae fam. nov[END_REF]Garrity et al., 2005a). Clearly the family Hyphomonadaceae should comprise the genera Hirschia, Hyphomonas, Maricaulis, Oceanicaulis, Woodsholea, Robiginitomaculum, and the new taxon proposed here. It is interesting to note that this family may be subdivided into two groups, one with cells that divide by budding, the other by binary fission. In addition, there is some evidence that there may also be a correlation between the two groups and the polar lipid patterns, although additional work is needed to test this hypothesis. When such work is completed it would be appropriate to emend the description and circumscription of the family Hyphomonadaceae [START_REF] Lee | The hierarchical system of the Alphaproteobacteria: description of Hyphomonadaceae fam. nov., Xanthobacteraceae fam. nov. and Erythrobacteraceae fam. nov[END_REF] in the light of chemotaxonomic data, bringing it into line with recommendations dating back to the ad hoc committee reports of [START_REF] Wayne | Report of the ad hoc committee on the reconciliation of approaches to bacterial systematics[END_REF] and [START_REF] Murray | Report of the ad-hoc-committee on approaches to taxonomy within the Proteobacteria[END_REF]. A similar treatment of the family Caulobacteraceae would be appropriate, which comprises the genera Caulobacter, Brevundimonas, Asticcacaulis and Phenylobacterium. The order Caulobacterales should also be restricted to include only the members of the families Caulobacteraceae and Hyphomonadaceae and emended accordingly. A further consequence would be that the members of the family Rhodobacteraceae as defined by [START_REF] Lee | The hierarchical system of the Alphaproteobacteria: description of Hyphomonadaceae fam. nov., Xanthobacteraceae fam. nov. and Erythrobacteraceae fam. nov[END_REF], should be formally assigned to a family that excludes the type of the family Hyphomonadaceae. Based on published chemotaxonomic data it would also be prudent to test whether members of that taxon should be further divided into several families and all included in the order Rhodobacterales.

Briefly, the results of our genotypic, chemotaxonomic, morphological and physiological investigations, together with the phylogenetic analyses, revealed that strain 26III/A02/215 T is distinct from other members of the family Hyphomonadaceae. The main characteristics differentiating the novel isolate from its closest phylogenetic neighbours are summarized in Table 2. In brief, the novel taxon can be distinguished from all its closest relatives, with the exception of members of the genus Hirschia, by its significantly lower G+C content. The fatty acid composition and polar lipid composition represent other distinctive criteria between the new taxon and other members of the family Hyphomonadaceae. Although much emphasis is put on the "major fatty acids" in the majority of recent taxonomic papers we emphasise here, the fact that the large amounts of 18:1ω7c (together with the presence of Q10) only indicate that this genus is a member of the Alphaproteobacteria and cannot be described as "characteristic" of this, or any other genus. On the contrary the sum of chemotaxonomic data, not only clearly place it within the family Hyphomonadaceae, order

Caulobacterales, but also provides a unique signature for with taxon within these higher taxa. In terms of other phenotypic features, differences in morphological characteristics such as the fine structure of the stalk, its position, the flagellation of the cells, the colonial pigmentation and the mode of division of the cells can also be use to distinguish the novel isolate from members of the genera Robiginitomaculum, Hyphomonas, Hirschia, Woodsholea, Oceanicaulis and Maricaulis (Table 2).

In conclusion, on the basis of the phylogenetic position and of genotypic, chemotaxonomic and physiological, biochemical and morphological differences, we propose that the isolate 26III/A02/215 T should be assigned as the type strain of a novel genus and species, for which the name Hellea balneolensis gen. nov., sp. nov. is proposed. C 18:1 , TBSA 10-methyl C 18:0 , Summed feature 3, Summed feature 7 (percentage compositions are given in Table 1). The G+C content of the DNA is close to 47 mol%. The genus Hellea belongs to the class Alphaproteobacteria, order

Description of

Caulobacterales, family Hyphomonadaceae, showing a distant relatedness to prosthecate bacteria of marine origin, namely members of the genera Hyphomonas, Robiginitomaculum, Hirschia, Woodsholea, Maricaulis and Oceanicaulis.

The type species is Hellea balneolensis. This is the only species within the genus.

Description of Hellea balneolensis sp. nov.

Hellea balneolensis (bal.ne'o.len.sis. M. L. n. Balneola, the ancient name of Banyuls-sur-mer; N. L. fem. adj.

balneolensis, pertaining to Balneola from where the strain was isolated). In addition to the characters described for the genus, the species is characterised by the following properties. Colonies on MA medium are round, convex, brilliant and pigmented a brick red colour. Optimal growth occurs at 30°C, with a growth range from 15 to 37°C. pH optimum is close to neutrality. Grows at NaCl concentrations from 0.02% to 5% (w/v), with a clear optimum at 3% (w/v) NaCl.

Growth occurs on acetate, citrate, propionate, pyruvate, succinate, aspartate, glutamate, L-alanine, L-asparagine, Lhistidine, L-proline, casamino acids, peptone, tryptone, yeast extract and D-mannitol. Substrates positive in Biolog GN2

plates are all the substrates cited above and as well as cis-aconitic acid, D-glucuronic acid, β-hydroxybutyric acid, γhydroxy butyric acid, α-ketoglutaric acid, methyl-pyruvate, quinic acid, urocanic acid, L-pyroglutamic acid, hydroxyl-L-

TABLES and FIGURES

  gene sequence analysis indicated that strain 26III/A02/215 T belonged to the Hyphomonas-Hirschia-Robiginitomaculum branch of the order Caulobacterales. This affiliation was consistent with the results of polar lipid analyses. Among this group, the novel isolate was most closely related to Robiginitomaculum antarcticum (93.9% 16S rDNA sequence similarity). On the basis of genotypic, chemotaxonomic and phenotypic distinctness, we propose a novel genus, Hellea gen. nov., with Hellea balneolensis sp. nov. as the type species. The type strain is 26III/A02/215 T (= DSM 19091 T = CIP 109500 T = OOB 269 T ).

  Hellea gen. nov. Hellea (He.lle'a. L. fem. n. Helle a sea goddess in Greek mythology; N. L. fem. n. Hellea, named after Helle in reference to the marine origin of the strain). Cells are Gram-negative, non-spore forming, rod-shaped to vibrioid, and dimorphic: usually, they possess one polar stalk (prostheca) and are non-motile or / they are non-stalked and motile by means of a polar flagellum. Aerobic and heterotrophic. Mesophilic. Neutrophilic. Grows best at salt concentrations close to marine salinity. The predominant quinone is Q10. Polar lipids comprise glucuronopyranosyldiglyceride, monoglycosyldiglyceride, phosphatidylglycerol, and unidentified glycolipid and phospholipids. Fatty acids comprise C 16:0 , C 17:0 , C 18:0 , C 19:0 , C 17:1 ω8c, C 17:1 ω6c, C 18:1 ω9c, C 18:1 ω7c, C 20:1 ω7c, 3-OH C 10:0 , 3-OH C 11:0 , 3-OH C 12:1 , 2-OH

Fig. 1 .

 1 Fig. 1. Phylogenetic tree based on 16S rRNA gene sequences showing the position of strain 26III/A02/215 T within the order Caulobacterales (as outlined in this article), class Alphaproteobacteria. The alignment was performed with 16S rDNA sequences of related species. Sequence data of reference strains were obtained from the GenBank/EMBL and/or RDP databases. Accession numbers are indicated in parentheses. The topology shown corresponds to an unrooted tree obtained by the neighbour-joining algorithm, established using the PHYLIP package. Bootstrap values (from 1000 replicates) are indicated at the branch nodes. The positioning of the novel isolate was confirmed by the maximum likelihood method. The scale bar indicates 1.0 nt substitutions per 100 nt.

Fig. 2 .

 2 Fig. 2. Transmission electron micrograph of a budding cell of strain 26III/A02/215 T negatively stained with uranyl acetate. It can be observed that the products of the cell division are unequal (simple budding). The cell bears a polar stalk (black arrow) which is an open ring regularly constricted on its length (open arrows). Bar, 0.5 µm.

Fig. 3 .

 3 Fig. 3. Polar lipids of strain 26III/A02/215 T . Legend: PG, phosphatidylglycerol; PL1, PL2, PL3, phospholipids; GL, unidentified glycolipid; MGDG, monoglycosyldiglyceride; GUDG, glucuronopyranosyldiglyceride.

  The type strain, 26III/A02/215 T (DSM 19091 T , CIP 109500 T = OOB 269 T ), was isolated from the surface microlayer of coastal waters, in the bay of Banyuls-sur-mer, north-western Mediterranean sea, France (42°29'N, 3°08'E).Epsilonproteobacteria, pp. 230-233. Edited by D. J. Brenner, N. R. Krieg, J. T. Staley & G. M. Garrity. New-York:

	Springer.
	proline, putrescine, n-acetyl glucosamine, D-arabitol, m-inositol and xylitol. Does not reduce NO 3	-. Catalase positive,
	oxidase negative. Tween 40 and tween 80 hydrolysis activities are positive.
	The G+C content is 46.8 mol%.

Table 1 . Whole cell fatty acid profile of strain 26III/A02/215 cultivated on marine agar.

 1 Values are percentages of total fatty acids. The nomenclature is as follows: the first number indicates the number of carbon atoms in the molecule. The prefixes 'iso', 'OH' and 'cyclo' indicate isobranched, hydroxy or cyclic fatty acids. The second number following the colon indicates the number of double bonds present. The position of the double bond is indicated by the carbon atom position starting from the methyl (ω) end of the molecule. The suffix c indicates the cis isomer. Summed feature contain one or more of each fatty acid. Summed features: 3, C 16:1 ω7c and/or 2-OH iso-C 15:0 ; 7, C 19:0 cyclo ω10c/C 19:1 ω6c and/or unknown ECL 18.846. ECL, equivalent chain length. TBSA, tuberculostearic acid (10-methyloctadecanoic acid). Major fatty acids are indicated by bold values. Only 62% of the fatty acid peaks could be assigned to the fatty acids listed in the peak naming table of the MIS database (MIS, Microbial identification System; MIDI, Del.USA). Unknown ECL that were detected are:16.760, 17.608, 18.116, 18.585, 18.797 and 19.347.

	Fatty acid	Strain 26III/A02/215 T grown on marine agar
	Saturated fatty acids
	C 16:0	0.98
	C 17:0	5.63
	C 18:0	1.81
	C 19:0	0.79
	Monounsaturated fatty acids
	C 17:1 ω8c	2.18
	C 17:1 ω6c	6.60
	C 18:1 ω9c	1.06
	C 18:1 ω7c	67.22
	C 20:1 ω7c	0.68
	Hydroxy fatty acids
	2-OH C 18:1	2.51
	3-OH C 10:0	2.23
	3-OH C 11:0	0.92
	3-OH C 12:1	1.23
	Methyl substituted fatty acid
	TBSA 10-methyl C 18:0	1.39
	Summed features
	Summed feature 3	0.66
	Summed feature 7	4.11

Table 1 . Phenotypic and genotypic characteristics of strain 26III/A02/215 T

 1

	L-alanine

. Legend: +, positive; ─, negative; W, weakly positive; ND, not determined; VS, very susceptible (diameter of inhibition zone > 20 mm); S, susceptible (diameter of inhibition zone: 10-20 mm).

Table 2 . Characteristics differentiating Hellea from the related genera of the family Hyphomonadaceae.

 2

	479
	480
	Characteristic				Genus
		Hellea	Robiginitomaculum ¶	Hyphomonas*	Hirschia †	Oceanicaulis ‡	Maricaulis §	Woodsholea #
	Colony colour	Brick red	Rusty-orange	Grey or	Yellow	colourless	colourless	colourless
				colourless may
				produce a water
				soluble
				brown/red-brown
				pigment
	Prostheca(e)	One, polar	One, polar	One to two, polar One to two, polar	One, polar	One, polar	One, polar
	Stalk cross	+	-	─	─	+	─	+
	wall
	Mode of	Budding	Binary fission	Budding	Budding	Binary fission	Binary fission	Binary fission
	division
	Flagellation	Monotrichous,	Absent	One to three,	Monotrichous,	Monotrichous,	Monotrichous,	Monotrichous, polar
		polar		polar	polar	polar	polar
	Nitrate	─	+	+	─	+	±	-
	reduction
	Growth at 6%	-	-	V	ND	+	±	+
	NaCl
	Polar lipid(s) ¦#	PG, MGDG,	ND	MGDG, GUDG,	MGDG, GUDG,	(PG), SQDG	PG, SQDG, Tau	SQDG, Tau, MGDG,
		PL 2 , PL 3 GUDG, GL, PL 1 ,		PG, Tau	PG, GL 5	GUDG 1 ] [MGDG,	MGDG, GUDG	GUDG
	Major fatty acids	3-OH C 10:0 , 3-OH C 12:1 ,	3-OH C 9:0 , 3-OH C 10:0 ,	3-OH C 12:0 , 3-OH C 12:1 ,	3-OH C 12:0 , 3-OH C 14:1	3-OH FAME 2 *, C 16:0 , C 17:1 ω6 4 ,	3-OH iso C 11:0 , C 16:0 , C 17:0 ,	3-OH C 12:0 , C 16:0 , C 17:0 , C 18:0 , C 18:1 ω7c
		3-OH C 11:0 , C 17:1 ω6c,	3-OH C 11:0 , C 15:1 ω8c, C 15:1 ω6c,	(C 15:0 ) C 16:0 , C 17:1 ω8 3 , (C 17:1	C 16:1 ω11c, C 16:1 ω7c , C 16:0 ,	C 17:0 , C 18:1 ω7, C 18:0 , 7-Me-	C 16:1 ω7c*, C 17:0 , iso C 17:0 , iso
		C 17:1 ω8c,	C 15:0 , C 16:0 , C 16:1 ω9c,	ω6), C 17:0 ,	C 18:0 , C 18:1 ω7c,	C 18:1 ω6, C 19:0	C 17:1 ω9c,
		C 18:1 ω7c, C 17:0	C 16:1 ω7c, C 17:1 ω8c,	C 18:1 ω7,	C 18:2 ω7		C 17:1 ω6c,
			C 17:0 , C 17:1 ω6c, C 18:0 ,	11-Me-C 18:1 ω6,			C 17:1 ω8c,
			C 18:1 ω7c, C 18:1 ω9c	C 19:1 ω8			C 18:1 ω7c*,
							C 18:1 ω9c
	Quinones	Q10	ND	Q10 or Q11	Q10	ND	Q10	ND
	DNA G+C	47	60	57-64	45-47	61-62	62-65	65
	content (mol%)

ACKNOWLEDGEMENTS

We thank Marie-Line Escande for assistance with the transmission electron microscopy and Hélène Agogué for her contribution to the initial sequencing work. We acknowledge Prof. J. P. Euzéby for support in the Latin etymologies of the genus and species names. This work was financially supported by the Equipe Mixte de Recherche linking the University Pierre et Marie Curie and the Centre National de la Recherche Scientifique to the Pierre Fabre Laboratories. The project was also carried out in the framework of the MarBEF Network of Excellence 'Marine Biodiversity and Ecosystem Functioning' which is funded by the Sustainable Development, Global Change and Ecosystems Program of the European Community's Sixth Framework Program (contract no. GOCE-CT-2003-505446). This publication is contribution number MPS-07059 of MarBEF. It was also partly funded by the French program 'Bio-diversité et Changement Globalproject: development of a coastal microbial observatory' from the 'Institut Français de la Biodiversité'.

 [START_REF] Weiner | Hyphomonas oceanitis sp. nov., Hyphomonas hirschiana sp. nov., and Hyphomonas jannaschiana sp. nov[END_REF][START_REF] Weiner | Hyphomonas adhaerens, sp. nov., Hyphomonas johnsonii sp. nov. and Hyphomonas rosenbergii sp. nov., marine budding and prosthecate bacteria[END_REF][START_REF] Moore | Genus Hyphomonas Pongratz 1957 nom. rev. emend., Hyphomonas polymorpha Pongratz 1957 nom. rev. emend., and Hyphomonas neptunium (Leifson 1964) comb. nov. emend. (Hyphomicrobium neptunium)[END_REF]The lipids MGDG and GUDG are not specifically mentioned, but the lipid fraction containing them was not investigated. 2 Hydroxylated fatty acids not mentioned, but only the fatty acids from a polar lipid fraction was examined 3 Fatty acid nomenclature used in the original paper was the ∆ nomenclature -this has been converted to the ω nomenclature in this table. Cis-or trans-isomers not specified 4 Cis-or trans-isomers not specified 5 Tindall, unpublished Hydoxy fatty acids (which probably originate for the lipopolysaccharide are underlined ¦ PG, Phosphatidylglycerol; MGDG, monoglycosyldiglyceride; SQDG, 1,2-diacyl-3-O-sulfoquinovosylglycerol; GUDG, glucuronopyranosyldiglyceride; SQDG, sulfo-quinovosyl diacylglycerol; Tau, 1,2-diacyl-3-α-D-glucuropyranosyl-sn-glycerol taurineamide; PL, unidentified phospholipid; GL, unidentified glycolipids. Legend: V, variable; ND Not determined Fig. S1. Effect of temperature on the maximum growth rate of strain 26III/A02/215 T . The strain was grown in MB medium. Growth rates were calculated by performing linear regression analysis along the logarithmic part of the growth curves.


Alexandre Baron

Aleksandr Ryasnyanskiy

Quyn Vy

Sylvain Combrié

Nicolas Dubreuil
email: nicolas.dubreuil@institutoptique.fr

Chad Husko

Philippe Delaye

Robert Frey

Gérald Roosen

Simone Cassette

Alfredo De Rossi

REPONSE NONLINEAIRE RAPIDE A L'ECHELLE PICOSECONDE DANS UN GUIDE A CRISTAL PHOTONIQUE DE GAAS

Keywords: guide optique, cristal photonique, optique nonlinéaire

RESUME

L'étude de la propagation d'impulsions courtes dans des guides membranaires à cristal photonique de GaAs, révèle la présence d'effets nonlinéaires très rapides avec une constante de temps estimée inférieure à 10 ps. Ces travaux illustrent la potentialité de réaliser des composants en GaAs très rapides pour le traitement de l'information.

INTRODUCTION

Les guides à cristal photonique présentent des propriétés de propagation linéaire et nonlinéaire originales qui permettent d'entrevoir le développement de composants compacts pour le traitement de l'information. En particulier, la lumière se propage dans un mode à très fort confinement transversal, de l'ordre de 10 -9 cm 2 , ce qui tend à exalter les nonlinéarités. Des réalisations de commutateurs optiques à faible puissance de commande ont été démontrées dans des microcavités à cristal photoniques en silicium [1], AlGaAs [2] et GaAs [3]. Les composants à base de semi-conducteurs III-V offrent l'avantage d'une ingénierie du gap électronique en vue d'accéder à des effets Kerr d'origine électronique afin de minimiser les temps de commutation. Bien que ce dernier argument soit à l'avantage des structures en AlGaAs, l'effet des porteurs générés par absorption à deux photons peut donner lieu à des phénomènes similaires au Kerr optique (de type "Kerr-like") très rapides, grâce à la diminution du temps de recombinaison des porteurs dans ce type de structures.

Dans ce travail, nous nous intéressons à l'étude d'effets de type Kerr-like dans des guides à cristal photonique en GaAs, matériau pour lequel le coefficient d'absorption à deux photons est dix fois supérieur à celui du silicium [4]. Les guides se présentent sous la forme d'une fine membrane suspendue de GaAs d'épaisseur 250 nm. La période a du cristal photonique, formé de trous d'air de rayon r=0,3a, est de 410 nm. Les guides de type W1 sont obtenus en ne gravant pas une rangée de trous dans la direction ΓK du cristal photonique [5].

DESCRIPTION DE LA SOURCE OPO ET DU BANC D'INJECTION

Notre objectif est d'étudier la dynamique des interactions nonlinéaires dans des structures à cristal photonique. Nous avons développé une source laser impulsionnelle spécifique, largement accordable autour de 1550 nm. Notre choix s'est porté sur la réalisation d'un oscillateur paramétrique optique (OPO) pompé par un laser Ti :Saphir émettant des impulsions de 10 ps à une cadence de 80 MHz et avec une puissance moyenne maximale de 1,7 W. La cavité de l'OPO est en anneau et seul le faisceau signal est résonant dans la cavité. La longueur de la cavité est ajustée de sorte que les impulsions signal qui résonnent dans la cavité soient amplifiées de manière synchrone par les impulsion délivrées par le laser Ti :Sa. Le cristal nonlinéaire à quasi-accord de phase est un PP-SLT de 20 mm de long. En utilisant un coupleur de sortie pour le signal de 90%, l'OPO présente un seuil d'oscillation autour de 400 mW et une puissance nominale de 1W pour une puissance de pompe de 1,7 W. Les impulsions délivrées pour le signal ont des durées égales à celle du laser de pompe et présentent des largeurs spectrales proches de la limite de Fourier.

Le faisceau signal délivré par l'OPO est relié au banc d'injection des guides par l'intermédiaire d'une fibre optique monomode de type SMF28, comme illustré Fig. 1. La longueur de la fibre est suffisamment courte, de l'ordre de 20 cm, afin de s'affranchir de l'automodulation de phase. Par ailleurs, en conservant la fibre dans une position fixe, un taux de polarisation supérieur à 95% a été mesuré. En sortie du coupleur à fibre, une lame demi-onde (notée λ/2) est insérée pour ajuster la direction de polarisation selon la direction de polarisation TM du mode du guide. L'injection dans le guide est réalisée grâce un objectif de microscope de grossissement 60 et d'ouverture numérique 0,85. Un second objectif, similaire au précédent, réalise la collimation du faisceau transmis en sortie de guide qui est alors injecté dans une seconde fibre monomode reliée à un analyseur de spectre optique (ASO). Les deux objectifs de microscope sont montés sur deux systèmes de translation XYZ disposant de trois actuateurs piézoélectriques. Afin de s'assurer que le flux analysé provient bien du guide, une image de sa face de sortie est projetée sur une caméra CCD en InGaAs au travers d'une lame séparatrice 50/50.

ABSORPTION A DEUX PHOTONS ET EFFET KERR

Une première étape consiste à mesurer la transmission d'un des guides en fonction de la puissance du faisceau signal de l'OPO injecté dans le guide. La puissance moyenne incidente est mesurée avant le premier objectif de microscope grâce à un puissancemètre calibré. La puissance moyenne transmise est mesurée par un second puissancemètre calibré, en sortie de la seconde fibre. Tout au long de la mesure, le contrôle des conditions d'injection dans le guide est assuré grâce à l'image prise par la caméra CCD. La variation de la puissance transmise en fonction de la puissance incidente est présentée Fig. 2. Son allure s'écarte très nettement d'une droite. La courbe expérimentale s'ajuste parfaitement par une tendance caractéristique du phénomène d'absorption à deux photons (TPA). Cette courbe est de la forme P out = K 2 P in /(1+ K " TPA L P in / A eff ) , où P in et P out représentent respectivement les puissances crêtes incidentes et de sortie du guide, A eff l'aire effective du mode, L la longueur du guide, K l'efficacité de couplage dans le guide supposée identique en entrée et sortie du guide et enfin " TPA est le coefficient d'absorption par TPA. En tenant compte des valeurs A eff =10 "9 cm 2 , L =1mm, des pertes de couplage dans la seconde fibre, et des pertes de la lame séparatrice, la valeur du coefficient " TPA , après ajustement de la courbe expérimentale, est égale à 3 10 -8 cm/W. Cette valeur reste indicative et devra être déterminée avec plus de précision, en tenant compte notamment de l'indice de groupe du guide. Son ordre de grandeur est néanmoins compatible avec les valeurs de la littérature (10 -8 cm/W [6]).

L'évolution de la puissance transmise démontre la présence de TPA. La brusque variation de la densité de porteurs alors générés doit induire une variation temporelle de la phase et donc un élargissement du spectre des impulsions transmises. C'est effectivement ce que nous avons observé expérimentalement à l'aide de l'analyseur de spectre optique réglé sur une résolution de 0,05 nm. Les spectres des impulsions transmises pour une puissance moyenne incidente de 8 mW et 80 mW sont représentés dans la Fig. 3. Pour une faible puissance injectée, le spectre est comparable à celui des impulsions incidentes. La largeur spectrale de 0,3 nm est compatible avec la durée des impulsions. Pour une puissance moyenne incidente de 80 mW (équivalent à une puissance crête de 80 W), le spectre est nettement élargi. La courbe présente une modulation qui provient de l'effet

CONCLUSION

Au travers de mesures de transmission en régime picoseconde dans des guides à cristal photonique de GaAs, nous avons observé des effets nonlinéaires équivalents à de l'effet Kerr avec une constante de temps très rapide, inférieure à 10 ps, la durée des impulsions de l'OPO. Bien qu'une étude plus poussée devra déterminer les contributions respectives des effets d'absorption à deux photons et Kerr optique, ces travaux démontrent néanmoins la potentialité de la réalisation de composants optiques très rapides dans la filière GaAs.

A. Ryasnyanskiy bénéficie du soutien du RTRA le Triangle de la Physique.

FIG. 1 :

 1 FIG. 1 : Banc d'injection du faisceau signal issu de l'OPO dans les guides à cristal photonique.

  FIG. 2 : Puissance transmise à travers le guide en fonction de la puissance incidente.


A Ryasnyanskiy
email: aleksandr.ryasnyanskiy@institutoptique.fr

Ph N Dubreuil

R Delaye

G Frey

Roosen

Fourier transformed picosecond OPO based on PPSLT without spectral filtering element

We demonstrate tunable synchronously pumped OPO system delivering picosecond Fourier-transformed limited pulses in near infrared spectral region. The developed OPO source offers a good compromise with Fourier transformed pulses around 10-15 ps duration and tunable in the whole telecommunication window from 1300 nm to 1600nm.

Introduction

Synchronously pumped optical parametric oscillators (OPO) are widely used sources of tunable ultrashort pulses, which can be applied for the measurements of optical nonlinearities of variety of materials. In this paper we demonstrate a tunable OPO based on a periodically poled stoichiometric lithium tantalate (PPSLT) crystal delivering picosecond Fourier-transformed pulses in near infrared spectral region. To the best of our knowledge, this is the first demonstration of a near infrared picosecond OPO that uses PPSLT. It has been developed for the characterization of nonlinear photonic crystals. The short pulses give high peak power required to easily generate the nonlinearities under studies, and also give access to time resolution in pump probe configuration.

Results and Discussion

The OPO is pumped by a mode-locked Ti:Sapphire laser with a pulse duration of 10 ps, a central wavelength set at 725 nm. It delivers an average output power of 1.7 W with a repetition rate of 80 MHz. A stable, bow tie, singly resonant cavity has been designed to minimize the round-trip loss. An output coupler with 80% reflectivity in the range 1530-1650 nm (and antireflective at the idler wavelength) is used. The PPSLT crystal is 20-mm long and has been selected for its high photorefractive damage resistance [1]. In order to achieve a signal radiation around 1550 nm, the crystal period has been set to 18µm and the temperature of the crystal is kept around 125 0 C.

The cavity round trip has been set first to optimize the output power. With the maximal pump power of 1.7 W we obtained up to 450 mW of signal and 450 mW of idler radiation. The threshold of oscillation was found to be 750 mW. In that regime the spectrum for both signal and idler is considerably broadened with a full bandwidth of 10-15 nm at 1550nm in comparison with the spectral width of a Fourier transformed pulse of 10 ps.

It is well established that the cavity length detuning between a synchronously pumped OPO and the pump laser strongly influences the OPO output characteristics [2,3]. Fig. 1 presents the dependence of the output power with the cavity length detuning. Although the same behaviour for both signal and idler radiations is observed, the dependence looks like asymmetric with respect to the zero detuning. It is worth noting that both spectral and temporal behaviour were dramatically changed by varying the cavity length and that several regimes of generation have been observed with two main regions I and II (Fig. 1). In Fig. 2, the autocorrelation traces and spectral profiles of the signal pulses are shown for these two regions. The differences in the pulses behaviour come from the strong group velocity mismatch between pump and signal pulses. The group velocity for the pump is lower than that of the signal. In the case of region I, corresponding to a positive cavity length detuning, the round trip time of the signal pulse is longer than the time separation between two pump pulses. Short pulses with duration between 1 and 2 ps (Fig. 2, a1, b1) are generated through a compression effect [3]. At the crystal input, only the leading part of the signal pulse interacts with the trailing part of the pump pulse, thus leading to its amplification and depletion of the corresponding part of the pump pulse. Because the group velocity mismatch is of the order of the pump pulse duration, at the crystal output, the short signal pulse is now amplified by the unsaturated leading part of the pump pulse. This process allows a compression of the signal pulse to be 5-10 times in comparison with the pump pulse duration.

In the situation of a negative cavity length detuning (Region II, Fig. 1), the signal pulse duration is of the same order of that of the pump pulse. For a given cavity length detuning, a spectral filtering effect occurs towards the wavelength that minimised the group velocity mismatch between the pump pulse and enables a Fourier transform pulse (Fig. 2, a2, b2).

Large tuning of the OPO is obtained using temperature control of the PPSLT crystal. Wavelength tuning within the region of 1530-1640 nm (limited only by a output coupler transmission range) and idler wavelength in the region of 1300-1375 nm has been achieved. A minimum tuning step of 0.5 nm was obtained by the change of the crystal temperature by 0.1 0 C.

Fig. 1 .

 1 Fig. 1. OPO output power measurement of signal (filled circles) and idler (open circles) with the cavity length detuning.

Fig. 2 .

 2 Fig. 2. Autocorrelations and spectra of signal pulses for the regions I (a1, b1) and II (a2, b2).


F Kroeger

A Baron

A Ryasnyanskiy

P Delaye

R Frey

N Dubreuil
email: nicolas.dubreuil@institutoptique.fr

Saturation of the Raman amplification by self-phase modulation in silicon nanowaveguides

We experimentally show that the self-phase modulation of picosecond pump pulses, induced by both the optical Kerr effect and free-carrier refraction, has a detrimental effect on the maximum on-off Raman gain achievable in silicon on insulator nanowaveguides, causing it to saturate, as we confirm with a simple calculation of the Raman gain.

Introduction

Silicon-on-insulator (SOI) photonics has attracted a great deal of attention due to its potential solutions for on-chip optical data processing. The high refractive index combined with the large third-order nonlinearity exhibited by silicon [1], enable tight optical confinement in sub-micron waveguides and efficient nonlinear functionality, making SOI a promising platform for ultra-compact devices with low-command powers [2]. Stimulated Raman scattering is of particular interest for the amplification of optical signals.

Experimental results

We have carried out pump-probe experiments with a 11-mm long ridge-type silicon nanowaveguide along the [110] crystallographic axis of silicon, which has an effective mode area of only 0.17 µm 2 . It was fabricated on silicon-on-insulator (SOI) wafers with a 2-µm-thick oxide layer. The pump pulses are τ =15 ps in duration with a repetition rate of F = 80 MHz, and they are delivered by an optical parametric oscillator (OPO). A synchronous pulsed probe of 150 ps is used to measure the Raman gain. Both pump and probe pulses are injected into the nanowaveguide with a microscope. The light is collected behind the waveguide with a similar microscope objective and then injected into a singlemode fiber that is connected to an optical spectrum analyzer (OSA). Fig. 1 shows the output spectra of both the pump (a) and probe pulses (b) for increasing input pump powers.

Discussion

Fig. 1(a) shows the pump spectra, which are broadened due to self-phase modulation (SPM) that is induced by the Kerr effect and the free-carrier refraction (FCR). The probe spectrum, shown in Fig. 1(b), consists of a narrow peak (corresponding to the spectrum of the injected probe pulses) and a broader, blueshifted component which is generated by Raman amplification. The total optical power P S of the Raman amplified part can be determined by applying a numerical low pass filter to the measured probe spectra. A simple analytical model that we have developed allows for the definition of a parameter X, which is proportional to the peak intensity of the pump pulses in the waveguide, so that the output probe power P S in dBm is expected to be a linear function of (1+X) in dB, as is illustrated by the straight line in Fig. 2. However, as shown by the experimental data (filled circles in Fig. 2), the Raman amplification of the probe pulses saturates for high pump intensities [3]. By taking into account that the effective Raman gain experienced by the probe beam depends on the spectrum of the pump beam, we have extended our simple model to calculate the expected effective Raman gain based on the measured pump spectra shown in Fig. 1(a). The resulting values are depicted as the open squares in Fig. 2. They show an excellent agreement with the experimental data, and they allow to determine a value of the CW Raman gain coefficient of silicon to be γ R = 8.9 cm/GW [3], which is also in good agreement with previously published values.

Conclusion

In conclusion, we have experimentally shown a saturation of the Raman gain in a SOI nanowaveguide in the ps regime which is caused by a nonlinear spectral broadening of the pump beam, governed by both the optical Kerr and free carrier effects. With a simple model, using the lineshapes of the outgoing pump spectra, we reproduce the saturated behavior of the on-off Raman gain along with its maximum value (28 dB).

Fig. 2 .

 2 Fig. 2. Output spectra of (a) the pump pulses and (b) the amplified probe pulses measured for input average pump powers varying from 10 mW to 120 mW (the signal spectra are magnified by a factor 20). The spectral resolution is 0.01 nm.

Fig. 1 .

 1 Fig. 1. Output spectra of (a) the pump pulses and (b) the amplified probe pulses measured for input average pump powers varying from 10 mW to 120 mW (the probe spectra are magnified by a factor 20). The spectral resolution is 0.01 nm.

Fig. 2 .

 2 Fig. 2. Average power of the Raman amplified output signal in dBm with the corresponding on-off Raman gain (right axis) versus the parameter 1+X in dB (filled circles). The values directly calculated from the measured lineshape of the pump spectra are plotted with open squares. The solid line corresponds to the un-broadened pump case.