Search

We report on a double-clad fiber laser operating on the 3P0 → 3F4 Pr3+ transition (in the deep-red spectral range) pumped by a GaN diode laser at ∼442 nm. It employs a 0.8-mol% PrF3-doped ZBLAN double-clad fiber with a 7.5-µm core, a double D-shaped inner cladding, and a length of 3.0 m. The laser delivers a maximum output power of 0.71 W at 716.7 nm with a slope efficiency of 9.0% (versus the launched pump power) and a laser threshold of 0.90 W. The laser emission is partially polarized. The laser performance is simulated providing a guideline for watt-level deep-red fiber laser sources.

We report on a deep-red fiber laser delivering 0.71 W at 716.7 nm with a slope efficiency of 9.0%. It is based on a Pr3+-doped ZBLAN double-clad fiber pumped by a 442-nm GaN diode.

Narrow linewidth laser diodes (LDs) emitting in the near-UV (NUV) are gaining attention for applications ranging from spectroscopy to atom cooling and interferometry or other applications requiring high spectral purity. InGaN edge-emitting LDs can exhibit a power of hundreds of mW in an unstable multimode regime detrimental to aforementioned uses. In this paper we report on a compact and robust design based on a low-cost blue LD, a beam shaping optical system and a fiber Bragg grating (FBG) acting as a wavelength selective reflector. One longitudinal mode of the non-antireflection coated laser diode is selected by a close to 30 pm bandwidth FBG allowing a few mW output power around 400 nm and a sidemode- suppression-ratio approaching 50 dB exceeding our last published results. Our previous studies showed that a single-frequency regime with a sub-MHz integrated linewidth and an estimated intrinsic linewidth of 16 kHz was possible by a carefully engineered external cavity. We will study the influence of the cavity length with different fiber types (SM or PM). Assessment will focus on the linewidth and a detailed intensity and frequency noise analysis of the emission. We will also investigate for the first-time the stability of several types of UV-FBG submitted to tens of mW of 400 nm light guided into the fiber core. This work demonstrates state-of-the-art performances by connecting low-cost components and opens the way to the fabrication of highly coherent laser sources that could meet the markets for the NUV applications.

We realize a fiber Bragg grating InGaN-based laser diode emitting at 400 nm and demonstrate its high coherency. Thanks to the fabrication of a narrowband fiber Bragg grating in the near-UV, we can reach single-mode and single-frequency regimes for the self-injection locked diode. The device exhibits 44 dB side-mode suppression ratio and mW output power. Detailed frequency noise analysis reveals sub-MHz integrated linewidth and 16 kHz intrinsic linewidth. Such a narrow linewidth laser diode in the near-UV domain with a compact and low-cost design could find applications whenever coherency and interferometric resolutions are needed.

We report on a red praseodymium fiber laser delivering 1.07 W at 634.5 nm with a slope efficiency of 20.7% (versus the incident pump), a laser threshold of 0.55 W, and a single-mode output (

Modern flow cytometers require multiple laser wavelengths to excite the wide variety of fluorescent probes now available for high-dimensional analysis. Ultraviolet (UV) lasers (typically solid state 355 nm) have become a critical excitation source for the Brilliant Ultraviolet (BUV) series of polymer fluorochromes. The BUV dyes have pushed the number of fluorescent probes available for simultaneous analysis to nearly 30, allowing an unprecedented level of precision for immune cell analysis. However, immunologists are already seeking analyze more than 30 simultaneous parameters, requiring both new fluorochromes and corresponding laser wavelengths. A group of polymer dyes requiring deep ultraviolet (UV) excitation (~280-300 nm) is currently under development, allowing the expansion of high-dimensional cytometry beyond the current 30 color limit. In this study, we evaluated a newly available laser emitting at 280 nm as a possible laser source for exciting these dyes. Since deep UV polymer dyes are not yet available, we used quantum nanoparticles (Qdots) as a surrogate probe to assess the utility of this laser wavelength for flow cytometry. Deep UV laser light was found to excite Qdots as well as traditional UV sources. Deep UV 280 nm did not excite BUV dyes well, suggesting that BUV and deep UV polymers will be spectrally compatible with low crossbeam spillover issues. Deep UV excitation did excite considerable autofluorescence in the violet to blue range, a limitation that will need to guide deep UV fluorochrome development. A deep UV 280 nm laser may therefore be the next essential wavelength for high-dimensional flow cytometry. © 2018 International Society for Advancement of Cytometry.

We report on a red praseodymium fiber laser delivering 1.07 W at 634.5 nm with a slope efficiency of 20.7% (versus the incident pump), a laser threshold of 0.55 W, and a single-mode output ( M x , y 2 < 1.1 ) in the quasi-continuous-wave regime. It is based on a 0.6 mol.% P r 3 + -doped ZBLAN double-clad fiber with a 5.5 µm core, a double D-shaped (diameters, 115/125 µm) inner cladding, and a length of 5.0 m. The fiber is pumped by a multimode 443 nm GaN diode. The laser design is optimized using a numerical model. The proposed concept is suitable for the development of diode-pumped high brightness watt-level visible praseodymium fiber lasers.

We report on efficient diode-pumped mid-infrared lasers based on T m : L i Y F 4 , T m : Y 3 A l 5 O 12 , and T m : Y A l O 3 crystals. These lasers operate in the continuous-wave (CW) regime and deliver watt-level output power at the wavelengths of 2.2–2.3 µm (the 3 H 4 → 3 H 5 T m 3 + transition). In particular, a 1.8 at. % T m : Y A l O 3 laser pumped at 789 nm generates a maximum CW output power of 1.32 W at 2272–2277 nm with a slope efficiency of 33.1% (with respect to the absorbed pump power), a linear laser polarization ( E ∥ b ), and a fundamental transverse output mode (the measured M x 2 = 1.26 , M y 2 = 1.68 ). In the quasi-CW regime, the output peak power is scaled up to 2.69 W. The pump quantum efficiency and the fractional heat loading are estimated and discussed.

Visible 561 nm and 532 nm laser emissions from 14-mm long DPSS monolithic cavities are frequency converted to deep UV 280 nm and 266 nm in 16-mm long monolithic external cavities. Wavelength conversion is fully insensitive to mechanical vibrations and the whole UV laser sources fit in a miniaturized housing. More than 20 mW deep UV laser emission is demonstrated with high power stability, low noise and good beam quality. Aging tests are in progress but long lifetimes are expected thanks to the cavity design. Protein detection and deep UV resonant Raman spectroscopy are applications that could benefit from these laser sources.

Depuis sa premiere observation en 1834 dans un canal ecossais et son explication mathematique en 1895, la propagation de solitons a trouve une place de plus en plus importante dans de nombreux domaines de la physique et des mathematiques. Objet de nombreuses etudes mathematiques dans les annees 60, il faut attendre 1973 et les premieres fibres monomodes de silice a faible perte pour apparaitre dans le domaine de la transmission optique. C’est pourtant dans ce domaine que de nombreuses proprietes des solitons ont pu etre verifiees experimentalement : interaction de solitons, propagation de solitons d’ordre 1 et 2 sur de nombreuses periodes... Cela a pu etre realise grâce a un milieu tres transparent, la disponibilite d’amplificateurs optiques et le developpement de techniques de controle.

A new technique for measuring chirp evolution of Gaussian-like pulses transmitted on single-mode fiber is presented. It is used in a recirculation loop experiment to characterize propagation in anomalous-dispersion regime over megametrical distances. In-quadrature periodical variations of the two characteristic parameters of pulses (i.e., spectral width and chirp) are measured versus distance for different values of prechirp and in-line power. This constitutes the first experimental evidence of pseudoperiodical soliton propagation in dispersion-managed links. It also demonstrates that the prechirp is a key element for the control of nonlinearity as predicted by the theory.

A 40-Gb/s single-channel soliton transmission was performed on a dispersion-shifted fiber (DSF) embedded in a link connecting Rome to Pomezia. The soliton system was based on the transmission of pulses with orthogonal states of polarization (alternate polarizations) to limit the impairments due to the soliton interaction. The performance of the system operating in the field has been compared with results obtained in the laboratory. While in the laboratory the system showed a nearly-stable error-free transmission over 700 km, strong temporal fluctuations of the performance have been observed in the field trial. Numerical simulations have shown that such a degradation, and in particular the fluctuation of the Q factor, is mainly due to a polarization mode dispersion of the installed cables larger than that of the fibers used in the laboratory. However, 500 km error-free transmission was observed in the field for time interval longer than one hour, demonstrating the potentiality of a 40 Gb/s soliton system with alternate polarizations operating with long amplifier spacing (100 km) without in-line control.

We present a simple and efficient model which takes gain stabilization and internal filtering for gain equalization into account. This model allowed the optimization of the filter position in the doped fiber to obtain the best gain stabilization and lowest noise figure. Finally, we demonstrate that equalized and stabilized gain amplifiers are efficient devices for wavelength division multiplexing (WDM) transmissions.

It is shown that the power mismatch range over which the influence of continuum on soliton interaction could be neglected can be as wide as 4.5 dB (−1 to 3.5 dB) provided that the effective normalized amplifier spacing remains below 1.5. Within this limit, a simple analytical formula can describe the soliton interaction. Finally, soliton interaction is shown to be very robust against single amplifier failure.

High bit rate long-haul transmission systems on optical fibres are mainly limited by the Kerr effect. Solitons, that balance exactly the Kerr effect by the chromatic dispersion are thus the optimal pulses for fibre transmission. However, the chromatic dispersion associated to the amplifier noise is responsible for a large jitter at receiver end. A calculation of the jitter variance and statistics is presented in this paper. Soliton control techniques can reduce efficiently the jitter variance but they are shown to strongly affect the statistics. It is finally demonstrated that the whole jitter statistics is needed to correctly characterize the soliton transmission system performance.

A method allowing the evaluation of the jitter degradation induced by a filter frequency dispersion (FFD) on a soliton transmission system is presented. The method is valid up to standard deviation of the order of the soliton linewidth. It is shown that systems with sliding-frequency filtering are more robust to FFD than systems with fixed-frequency filtering. In the first ones, standard deviations of the order of the quarter of the soliton linewidth can be tolerated without significant jitter degradation. Experimental data confirm this result.

An accurate formula for the soliton arrival time probability density is derived. It can be applied to any transmission system (whatever the soliton control) and for any number of interacting solitons. The case of two interacting solitons is first studied and validated by a Monte Carlo method. Finally, the case of filtering (one and two solitons) is analysed.

Since the discovery of soliton control techniques, the highest bit rates transmitted over transoceanic distances have been obtained using soliton transmission systems. As linear and nonlinear effects are mixed, the physical understanding of nonlinear interaction of noise with the soliton or of filters or modulator effects on a soliton may be somewhat difficult. A perturbation theory in which the soliton is considered as a particle characterized by four parameters is presented. For most perturbations, meaningful mechanical equivalents can be found. This theory is applied to the most important perturbations of the soliton and it is shown that it not only provides many tools to make a qualitative study of the soliton propagation, but also yields quantitative results. Control techniques enable noise, jitter, and soliton interaction to be reduced. The efficiency of each technique is discussed. These techniques change the relative impact of the different constraints on system performance.

Diode lasers have been demonstrated to operate over a great part of the visible spectrum: InGaN diodes cover the violet-blue- green part ( 635 nm). Some fluorophorus in biotechnology applications are excited by intermediate wavelengths, from 540 to 630 nm. Optically pumped InGaAs lasers were demonstrated from 460 nm up to 580 nm. Standard frequency doubled diode pumped solid state (DPSS) lasers lack of suitable transition to cover the 565-650nm region. It is possible to modify the semiconductor composition to extend the frequency range or to frequency mix DPSS laser wavelengths, but it comes either with a significant R&D effort or with a complexity in the design. Raman scattering can red-shift the strong transitions of Nd or Yb lasers so that many wavelengths lying in the 1080-1300 nm range can be achieved. Recently several CW diode pumped Raman lasers were demonstrated, some of them including intra-cavity frequency doubling or mixing. The problems with these Raman lasers are the high pump threshold and the high noise. Based on monolithic cavities, we have built several visible Raman lasers with a reduced loss presenting a low pump threshold (

International audience; This paper proposes an analysis of a Yb:KYW laser emitting at 981 nm intracavity pumped by a Nd : YVO4 laser operating at 914 nm. It gives some guidelines to optimize the laser performance. An output power of 1 W has been obtained at 981 nm for a pump power of 23 W at 808 nm. It presents a simple and original model to deal with the line competition between 981 nm and the other lines at 1000 and 1023 nm and explains how a temperature increase can promote the 981 nm laser emission. This approach could be useful for other lasers that are subject to line competition

Second order diffraction has been observed from Bragg gratings written within germanosilicate glass fibres. Experimental results are reported on the evolution of the first and the second order Bragg grating reflectivities as function of the uv laser shot number used for the inscription. Second order diffraction can be detected once the first order grating reflectivity has begun to saturate. Possible explanations of the second order grating formation are discussed. Our experimental results are consistent with a saturation in the photo-induced change in refractive index.

Single longitudinal mode visible DPSS are more and more used in devices where performance can be affected by short term (minutes) frequency drifts and hops. Long exposure holography and Raman spectrometry are applications requiring high frequency stability. Resonant external cavity frequency doubling (to generate CW deep UV) and pumping doubly resonant OPOs may be even more demanding applications in terms of frequency stability. Mechanical vibrations and thermal fluctuations are usual sources of short term frequency variations or instabilities. Monolithic ring cavities (such as NPRO) are known to solve this problem but they are quite expensive to manufacture. We will show that much simpler linear monolithic cavities used in our standard product line (SLIM-532, SLIM-550, SLIM-561 and SLIM-660) present best of class frequency stabilities compatible with the most demanding applications. Frequency tuning capabilities will be discussed and could be used in an active stabilization of the laser. Some applications can benefit from long term wavelength stability as well. Raman spectra can be monitored without control of the pump wavelength if the long term stability is good. In addition, narrow filters can be used to measure small Stokes shifts. We are monitoring several monolithic laser sources. After more than 6000 hours of operations, the wavelength shift is within 1 pm. The laser source has been restarted more than 1000 times without any change of the operating wavelength. Finally, thermal cycles do not impact the wavelength. In conclusion, we demonstrate that monolithic linear cavities are best suited for all applications requiring wavelength stability.

Since the first introduction of DPSS lasers at 561 nm in 2004, the power level required by some biotechnical applications has always increased. Oxxius has contributed to fulfill the demand thanks to the introduction of the SLIM-561 100mW in 2008 and of the SLIM-561 200mW and 300mW in 2009. More recently, new dyes or nano-dots have required shorter wavelengths (such as 553 nm) and new applications such as Laser Doppler Velocimetry requiring both high power and single frequency operation have appeared. In this presentation, we demonstrate how to further increase the power. 553nm and 561 nm emission are obtained by frequency doubling the 1106 nm and 1123 nm lines of Nd:YAG. The latter transitions are significantly weaker than the 1064 nm line. As a consequence, any loss in the cavity significantly increases the laser threshold. Because of the perfect alignment of the crystal interfaces and the low divergence of the intracavity beam, monolithic cavities demonstrate significantly reduced round-trip losses compared to standard cavities. Consequently, laser threshold can be dramatically reduced and the nonlinear loss, responsible for the visible emission, can easily dominate the linear losses. We have taken standard monolithic cavities of our commercial SLIM-561 products and have increased the pumping power up to 2.8 W. Yellow and Yellow-green powers have not shown any sign of saturation and 0.5 W could be achieved at both wavelengths. This results in a record 18% pump to signal optical efficiency. We have checked that the emission remained single frequency whatever the pumping power.

Bragg grating inscription in germanosilicate fibres by transversally exposing the fibre to a UV fringe pattern was recently reported, for the first time, by G. Meltz et al. (Opt. Lett., vol.14, p.823 (1989)). The authors use this holographic method to write various Bragg filters within optical fibres. Starting from the results given by a coupled mode theoretical model, they recall some practical rules related to Bragg grating inscription within optical fibres. Then, they describe the interferometric set up and the method they use to write the filters. They analyse the results obtained in the course of the experiments. In order to emphasize the advantages of the writing method they detail the way used to write two particular filters, namely a low reflectivity grating and a Fabry-Perot cavity used as a reflector.

We demonstrated for the first time three-level operation at 981 nm with a Yb:KYW crystal inserted into the cavity of a diode pumped Nd:YVO 4 laser operating at 914 nm. We achieved an output power up to 1.4 W at 981 nm. Moreover we demonstrated that crystal heating favored laser emission at 981 nm rather than emission at higher wavelength.

We present the first demonstration of the three-level-laser transition at 985 nm in an Yb:S-FAP crystal intracavity pumped at 914 nm. We obtained 940 mW output power at 985 nm for 20 W incident pump power at 808 nm.

International audience

We present what is, to the best of our knowledge, the first experiment of intracavity pumping at 914 nm of an Yb:SFAP crystal emitting at 985 nm on the three-level laser transition. This configuration enabled us to indirectly diode-pump this ytterbium doped crystal, and to obtain 1.4 W output power at 985 nm for 20 W of incident pump power at 808 nm. Intracavity second harmonic generation has also been demonstrated in a KNbO3 crystal with a total of 120 mW linearly polarized output power at 492.5 nm on two output beams.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We present the first experiment of intracavity pumping at 914nm of an Yb:S-FAP crystal emitting at 985nm on the three-level-laser transition. We obtained 250mW output power at 985nm for 9.7W incident pump power at 808nm.

A numerical analysis of soliton propagation in a dispersion-compensated transmission system reveals the presence of a small dispersive-wave contribution to the propagating soliton that increases dramatically whenever the input soliton energy is detuned from its optimal value. A straightforward consequence of the dispersive-wave emission is an unexpectedly strong contribution to the long-range soliton–soliton interaction. We show that the combination of long- and short-range interactions induces soliton timing jitter that grows larger than that which arises from the Gordon–Haus effect.

The power control and the laser noise of a diode-pumped solid-state laser with increased intracavity power is investigated. An electrical spectrum analyser is employed to monitor the laser output of the system; wherein two kinds of dynamics are observed: (i) a stable operation when the laser emits more than 15 mW of UV power and (ii) a "green noise" like operation when many oscillating modes appear. Results also show that the oscillating modes can be controlled through the amplifier and the nonlinear crystal temperature.

conference 6451 « Solid State Lasers XVI: Technology and Devices », session 1 « UV Lasers - CW » [6451-04]; International audience; Low noise CW UV lasers are needed for applications in bioinstrumentation (cell sorting, cytometry...) and semiconductors (wafer inspection, micromachining ...). We have recently demonstrated that such laser sources can be obtained with diode pumped solid state (DPSS) architectures. One key to success was the quasimonolithic structure of the laser. The advantages of quasi-monolithic DPSS lasers for UV generation are simplicity of design, compactness, efficiency and thus low power requirements and limited heating. In this paper, we present for the first time a long term characterization of the diode pumped CW 355 nm laser. The interplay between pump absorption, cavity length, 1064 mode size, walk-off angle, acceptance angle has been optimized. In our experiments, the temperature of each element of the laser was controlled and UV power, noise and spectra were monitored versus these temperatures. At 2.5 W pump power, low noise UV power beyond 30 mW was measured on most samples built. At a reduced pump power of 1.65 W, all lasers were operating at 10 to 15 mW and could be maintained at 10 mW over days. The noise level remained below the 1% rms level. More long-term measurements will be presented at the conference.

conference 6190 « Solid State Lasers and Amplifiers II », session 3 « UV and Visible Lasers » [6190-14]; International audience; Low noise lasers in the UV spectrum (355 nm) are important for many applications like spectroscopy, confocal microscopy, flow cytometry, cell sorting, CD mastering, semiconductor inspection and reprographics. The UV CW laser sources currently commercially available, are restricted to excimers lasers, gas ions lasers which suffer from low power efficiency (0.01% or less), high electrical consumption and bulky dimensions. These products, which are expensive, are still waiting for practical replacements. To our best knowledge, we report in this paper the first efficient low noise diode-pumped neodymium doped solid-state laser operating at 355 nm by intra-cavity third harmonic generation (THG). The fundamental infrared laser light is generated by a diode-pumped Nd:YVO4 crystal optically contacted with others components of the laser cavity. Intra-cavity SHG and THG are achieved with a KTP and LBO respectively. Several configurations have led to low noise 355 nm single-frequency operation at a power exceeding 10 mW. We believe that this power can still be improved.

We present for the first time a Nd:YVO 4 laser emitting at 1064-nm intracavity pumped by a 914-nm diode-pumped Nd:YVO 4 laser. Intracavity sum-frequency operation is realized to reach blue range. We obtained 67-mW at 492-nm.

conference 6100 « Solid State Lasers XV: Technology and Devices », session 2 « Compact and Microchip Lasers II » [6100-08]; International audience; Low noise CW milliWatt scale UV lasers are needed for many analysis applications in the semiconductor and the biological fields. Intracavity tripling has been widely used to improve the UV output power of Q-switched or modelocked lasers, but no efficient diode-pumped CW UV laser was ever reported. One of the key to success is the use of a monolithic laser structure which both eliminates the birefringence interference issue and facilitates the single frequency operation. The monolithic structure is obtained by optically contacting crystals. It does not require any alignment, reduces the manufacturing cost and improves reliability. The optimization of the amplifying medium and doubling and tripling crystals involves as many parameters as pump absorption, thermal lens, cavity length, 1064 nm mode size, walk-off, acceptance angles, polarizations, phases... The interplay between these parameters will be discussed. Finally, several amplifying media (Nd:YAG and Nd:YVO4), doubling crystals (KTP, KNbO3, BBO, BiBO and LBO) and tripling crystals (BBO, BiBO, LBO) were tested. With a 2.4W 808 nm diode pump, several configurations have led to low noise 355 nm single frequency operation exceeding 5 mW. We believe that this power can still be improved.

Diode pumped frequency doubled Nd :YAG microchip lasers should become an alternative to air-cooled argon ion lasers. The main issues to be solved are the long-term stabilization of the single frequency operation and the power control of the multi-frequency operation. These questions are mostly related to the laser dynamics. In this paper, we present an accurate modelling of the laser dynamics, including quenching processes, non uniform pumping, partial overlap of optical signals and excited-state populations, hole burning and type-I frequency doubling. We theoretically predict that even in multimode operation, only one mode is oscillating at a time, with a mode hoping at a rate of about 50 kHz. This behavior, quite different from the well known dynamics of intra-cavity type-II frequency doubled lasers (green noise) is experimentally confirmed. Diode pumped frequency doubled Nd:YAG 473 nm lasers based on a simple linear cavity are built and exhibit high output power (90+ mW) and record slope efficiency (45% with respect to absorbed power). The understanding of its 2-mode operation allowed us to stabilize the average output power. A similar laser was operated on a single frequency. No wavelength drift could be measured and no mode hopping was observed over 24 hours.

conference 5332 « Solid State Lasers XIII: Technology and Devices », session 5 « Compact Laser Sources » [5332-21]; International audience; Although powerful (2.8 W) blue emission at 473 nm was recently reported, the design of low-cost compact and efficient 473 nm source failed so far. We have theoretically and experimentally analyzed the operation of a simple 2.1 mm long microchip assembly pumped by a 100x1 μm laser diode and found several ways to improve the efficiency of such a laser. As a first result, we achieved high beam quality (M2

1.02 Tbit/s (51-wavelength each at 20 Gbit/s) dense WDM (0.4 nm channel-spacing) soliton transmission over 1000 km of standard step-index fiber with 100 lan (21 dB loss) amplifier spans was successfully achieved for the first time.

We achieved 640 Gbit/s (32/spl times/20 Gbit/s) soliton dense-WDM (0.4 nm channel-spacing) transmission over 1200 km with 100 km dispersion-compensated spans of standard step-index fibre.

In conclusion, we have resented a new type of soliton propagation. It requires both a dispersion compensation and an initial pre-chirp. Among its advantages, it is firstly compatible with the large chromatic dispersion of nondispersion shifted fibre (NDSF). Secondly, jitter and SNR performances are improved with respect to those of systems with constant chromatic dispersion due to the large value of the excess power coefficient k. Only soliton interaction is increased. Finally, in a 2500-km 20-Gbit/s transmission, it operates over a large power range: from

International audience; In this communication, we report a straight-line transmission experiment at 40 Gbit/s over 800 km without any in-line soliton control nor local dispersion management and with enhanced amplifier span to 100 km. Q factors >6.5 were measured at 800 km. Two fiber configurations were investigated for which the chromatic dispersion was respectively decreasing and increasing over the amplification span. The two dispersion maps, which correspond to two extreme cases in local fluctuations

A high-density dense wavelength multiplexing (DWDM) dispersion-managed soliton loop experiment at 10 Gbit/s per channel with 25 GHz spacing have been conducted over 100 km/ 25 dB spans of SMF. The 1500 km error-free transmission using EDFA was improved to 2100 km using hybrid Raman/EDFA amplification.

Spectral efficiency will be discussed as a key issue for long reach systems. Ready to deploy, 0.4 bit/s/Hz, N/spl times/10 Gbit/s experiments will be highlighted, as well as next generation 40 Gbit/s systems. Following already installed WDM systems at 2.5 Gbit/s, the deployment of the next generations at 10 Gbit/s and later at 40 Gbit/s will improve spectral efficiency over most of fibre types at the price of finding a compromise between reach, span loss and capacity. The key techniques to be controlled are the dispersion managed solitons, Raman amplification and foward error correction methods.

Dispersion-managed solitons exhibit superior transmission quality as soon as Kerr nonlinearity in non negligible. Long transmission distances, large span loss and high spectral efficiency are expected for the next generation of transmission systems based on 20 Gbit/s and 40 Gbit/s data rate.

WDM dispersion-managed soliton transmission on standard fibre based on 20 Gbit/s channels appears to be the most efficient solution to transmit high capacities over long distances.

In the scope of high rate submarine long haul transmissions, different kind of in line control systems are investigated.

The transmission of 20 Gbit/s path-averaged solitons with in-line control is possible over unlimited distances [1]. In such experiments, Kerr nonlinearity is not a limitation but is used to separate the signal from the noise. Unfortunately, this transmission technique does not allow the increase of the bit rate of a single channel to much more than 40 Gbit/s nor the increase of the overall bit rate with help of Wavelength Division Multiplexing (WDM) because of the Four Wave Mixing (FWM) [2]. The other drawback is the incompatibility to the Non Dispersion Shifted Fibre (NDSF) or standard fibre in the terrestrial networks.