An actively modelocked erbium fiber laser with continuous wavelength tuning over the range 1528 nm - 1565 nm has been demonstrated, with wavelength selection provided by a holographically ruled diffraction grating. A gain-saturated InGaAsP NTW semiconductor optical amplifier (SOA) is used as the active modelocking element in an isolating unidirectional fiber loop mirror. The semiconductor amplifier is driven with a synthesized RF signal source at a harmonic of the passive cavity mode spacing of 1.75 MHz, together with a dc bias current. The SOA is used as an amplitude modulator which provides gain and has < 2 dB polarization dependence. Gain saturation also serves to shape the transmitted laser pulses. The fiber laser is diode pumped at 1480 nm with a threshold of 6 mW, and provides pulses of 25 ps (FWHM) with a transform product of 0.4 and > 12 dBm peak power at a repetition rate of 175 MHz. Modelocked pulse repetition rates up to several GHz should be possible using this fiber laser configuration.

A method for the coherent coupling of single-mode fiber lasers which produces a single Gaussian output beam is presented. The output and spectral properties of an experimental device composed of three coupled Nd³⁺ single-mode fiber lasers have been studied. Typically 70% of the total output power have been obtained in a single beam for different coupling conditions. The dynamic behavior of the emission spectrum is discussed.

The influence of the gain saturation of the active medium and system parameters such as loss coefficient, coupling coefficient, and waveguide geometry on the 3 dB modulation bandwidth in fiber distributed feedback lasers is discussed for various transverse laser modes. Additionally, the effect of the amplitude and phase of the end reflectivity is taken into account. The characteristics revealing competition between the Fabry-Perot resonator and the distributed feedback resonator are presented.

In this communication, analytical expressions for the oscillation threshold pump power and the lasing efficiency of an erbium-doped fiber laser are derived from well-known equations that describe the amplification process in erbium-doped fibers. These expressions show the dependence of both the oscillation threshold pump power and the lasing efficiency on the cavity length. As a design criteria, an optimum fiber length is derived from the analytical expressions by maximizing the ratio between the lasing efficiency and the oscillation threshold pump power. Finally, theoretical results are given for the case of an erbium-doped fiber ring laser together with numerical calculations obtained by means of computer simulations.

We analyze the excess-noise factor, caused by the mode nonorthogonality, in fiber distributed feedback laser. The influence of the coupling strength and the system parameters on the excess-noise factor is discussed. It is shown that, it increases with the increasing mode numbers and with decreasing radius of the fiber rod.

Excited-state absorption measurements performed at 980 nm in erbium-doped silica fibers provide evidence that Er³⁺ ions undergo clustering to varying degrees, depending on glass composition. The technique employed allows a quantitative characterization of the degree of clustering. It is found that the smallest degree of clustering occurs in silica fibers co- doped with aluminum, and those prepared by the sol-gel method.

Excited-state-absorption and stimulated-emission cross-section spectra have been measured in the 1300-nm region for a wide variety of Nd³⁺-doped glass compositions. The results indicate that fluoroberyllates are the best choice for fiber amplifiers in the 1300-nm optical communications window. Model calculations predict a gain spectrum peaked at 1314 nm with useful gain extending from 1304 to 1370 nm when amplified spontaneous emission is neglected.

To explain the sub-optimal performance of erbium-doped resonant fiber lasers and superfluorescent fiber sources observed experimentally, the effects of potential loss mechanisms are explored via computer simulations. Pump excited-state absorption (ESA) at 980 nm and 1.48 micrometers , and signal ESA are unable to explain the dependence of the observed effects on concentration. Cooperative upconversion among uniformly distributed erbium ions fails to explain the observed reduction in source slope efficiency with increasing concentration. On the other hand, rapid cross-relaxation between paired ions, which might form in high concentration fibers, can produce the observed dependences. Rate equations for paired ions are used to understand their saturation behavior and their effect on the slope and threshold of fiber sources. Methods to assess the fraction of paired ions are discussed. Measurements suggest that about 18% of the ions in an aluminum co-doped silica fiber with 5 X 10¹⁹ Er³⁺/cm³ are paired. The effects of paired ions on the gain of Er-doped fiber amplifiers are also briefly discussed.

Comparison of the output characteristics of different erbium doped fiber lasers show that the threshold pump power increases (by a factor of 4.6) and the conversion efficiency decreases (by a factor of 1.5) as the erbium concentration is increased from around 150 to 1040 mole ppm Er₂O₃. We propose that these two effects are mostly due to rapid interaction (perhaps upconversion) between a subset of paired ions. This work suggests that for Al-Ge- doped silica fibers, concentrations of 150 mole ppm or less should be used for optimum output power. A fiber with this concentration produced a low-threshold laser with a total power conversion efficiency of 90.4%.

We report low loss, low pump power, optical-optical switches in rare-earth-doped fibers based on the third-order optical nonlinearity resonantly enhanced by the dopant. In a 0.95-m Er- doped two-mode fiber switch pumped with a 1.48-micrometers diode laser, the absorbed pump power required for switching a 906-nm signal was 8 mW, for a signal loss of only 0.25 dB. This is an enhancement by a factor of 6200 in power-length product over undoped silica. The phase shift was found to be due in part to a non-resonant contribution, thought to arise from a strong UV-VUV transition, and in part to a resonant term from the 980-nm transition. In a 0.98-m Nd-doped, elliptical-core, two-mode fiber switch, switching of a 632.8-nm signal was achieved with only 6.6 mW of absorbed power at 900 nm. The dynamic response of the switch was found to have two components, a slow component equal to the metastable level lifetime (approximately equals 380 microsecond(s) ) and a fast component (approximately equals 2 microsecond(s) ). The latter is believed to arise from rapid cross-relaxation between paired ions, a mechanism which shows promises for low- power, microsecond switching in fibers.

Experimental investigations of stimulated Brillouin scattering in two cascaded single mode optical fibers with different Brillouin shifts are presented. A fiber Brillouin laser consisting of these two cascaded fibers is demonstrated. The output of such a laser exhibits two frequencies corresponding to the Brillouin shifted frequencies of two individual fibers.

Resonance-enhanced anti-stokes of stimulated four photon mixing is demonstrated for the first time in an Er³⁺ doped silica fiber under the excitation of a 1064 nm laser. The enhancement of 977 nm anti-stokes of stimulated four-photon mixing by the excited state ⁴I_11/2 of Er³⁺ has been observed. After the fiber being prepared for efficient SHG, the spectra of stokes and anti-stokes stimulated four-photon mixing were measured again and found to have shifted 2 nm, respectively. We attribute this phenomenon to photo induced refractive-index changes in the fiber: one is caused by the short wavelength light absorptions of GeO, and the other is by the change of intensity-dependent Er³⁺ population in excited states.

Optimization of both fiber and device design is essential for maximum performance from erbium doped optical fibers as either line, pre- or power amplifiers, with optimum fiber composition and geometry being unique for each application. The field is extensive but selected examples are reviewed to indicate the range of device configurations and fiber structure available. Key advances include, via fiber design the development of ultra high gain efficiency for reduced pump power requirements and hence increased reliability of practical systems. Further, use of a novel device configuration allows exploitation of high gain efficiencies without compromising noise levels for a pre- or line amplifier. Power amplifiers demonstrating high output powers even at elevated signal levels are sought for many communications systems including local area networks. Both amplifier configuration and fiber composition have been investigated in the pursuit of output signals in excess of 20 dBm. The relative merits of these systems are discussed and future direction indicated. In summary, it is shown that optical fiber fabrication plays a key role in optimizing performance while the versatility of fiber circuitry allows full exploitation of the fiber potential.

High-gain (G > 20 dB) multimode erbium-doped amplifiers pumped at 980-nm using a Ti:Sapphire laser and at 1485-nm using a laser+amplifier photonic integrated circuit have been demonstrated. Low amplifier noise power is achieved by coupling pump light into a limited number of fiber modes.

Low noise erbium-doped fiber amplifiers find widespread use in telecommunication systems. Different design ideas have been presented for such amplifiers, including various pump configurations, the use of filters, and advanced active fiber design. In this paper we present an experimental confirmation of a fiber design based on an Er/Al doped untapered fiber. As recently predicted theoretically, a continuous uptapering of the radius of the core of the active fiber from the signal input end to the signal output end improves the population inversion of the erbium in the signal input end of the fiber. This ensures that the generated amplifier spontaneous emission is reduced while the high gain properties are retained. The result is a significant reduction of the noise figure. The active fiber is pumped at 980 nm with the pump power copropagating with the signal. In this way we have demonstrated noise figure improvements in excess of 1.5 dB compared with similar conventional fibers, while still obtaining high gain. To our knowledge, this represents the first experimental verification that tapered fiber amplifiers have considerably improved noise properties. This communication will include a short description of the method used to fabricate the tapered fiber.

A composite-EDFA configuration which incorporates an optical isolator has been investigated theoretically and experimentally. The isolator prevents the build-up of the backward-ASE and results in an amplifier with high gain and near-quantum-limited noise figure (NF). The optimum position of the isolator has been calculated as a function of the pump power so that minimum NF and maximum gain are achieved simultaneously. It is shown that under practical pump powers, the optimized composite EDFA exhibits a gain improvement of about 5 dB and a NF reduction in excess of 1.5 dB when compared with an optimized conventional EDFA. Finally, a high-gain composite EDFA has been experimentally demonstrated which exhibits a gain of 51 dB and NF of 3.1 dB for only 45 mW of pump power.

Using a temporal model we have shown the effect of pulse amplification of sub-micro second pulses in an Erbium doped fiber amplifier (EDFA). We have considered the effect of amplification of rectangular and triangular pulses of different powers. The amplification of large signal pulses causes distortion of the pulses because the leading edge of the pulse has larger gain as compared to the rest of the pulse. It has also been shown that the population at the pump state is negligible for typical signal and pump powers. The amount of energy that can be extracted from the EDFA can be increased by increasing the input pulse energy. Increasing the power of the input signal pulse also results in an increase in the peak output signal power and a decrease in the pulse width. There exists an optimum length for pulsed EDFA which is the same as that of small signal EDFA. The energy of the pulse increases until the optimum length is reached after which it starts to decrease. The pulse width of the output signal increases beyond the optimum length.

This paper describes recent progress at BTLabs in high gain high output power praseodymium doped fluoride fiber amplifiers around 1.3 micrometers . Firstly, results using Ti:sapphire pumping will be described; using a high numerical aperture fiber, a net gain of 15 dB was achieved for 200 mW pump power, with a maximum small signal net gain of 29.5 dB. When operated as a power amplifier > 200 mW output power was obtained, with a 30% slope efficiency. This represents the first achievement of efficient operation in both small and large signal regimes. In a separate experiment, a diode-pumped Nd:YLF laser was used to pump a medium numerical aperture fiber. 29.5 dB small signal gain was again achieved, with a saturated output of 250 mW when operated as a power amplifier. We believe that this represents the highest amplified output from such an amplifier, irrespective of pump power. Additional details of the spectral performance of the two amplifiers will be given, for both small signal and large signal operation.

We report on optimized performance of Pr³⁺ doped fluoride fiber amplifiers (PDFA) using a comprehensive spatial and spectral computer model. We show the correspondence between results that were experimentally determined and those obtained using our model. Waveguide parameters for these amplifiers have been optimized for low and moderate values of relative refractive index difference. A cut-off wavelength of 750 nm gives maximum gain for NA equals 0.1 and 0.2. Particular emphasis is given to a comparison of the PDFA's performance with and without a Bragg reflecting pump mirror etched at the output end of the fiber. A pump reflecting mirror gives higher gain at smaller amplifier lengths (approximately equals 1/3) in both, the small and large signal regimes. We also report on the quantum conversion efficiencies (QCE) of PDFA. For the same length of fiber and a 0 dBm input signal, PDFA with a pump reflecting mirror has a QCE about twice that of a PDFA without a pump reflecting mirror at its output end.

Pr³⁺- and Nd³⁺-doped glasses are the most promising candidates for fiber amplifiers in the 1300-nm telecommunication window, but each has serious drawbacks. Using a quantitative numerical model and measured cross sections, we have examined the practical limits of performance for Pr³⁺-doped fiber amplifiers (PDFAs) in small-signal and power amplifier applications. A comparison has been made between power PDFAs and Nd³⁺-doped fiber amplifiers (NDFAs). In general, only PDFAs provide gain over the complete wavelength range of interest. NDFAs are expected to have higher efficiencies if the short wavelength amplified spontaneous emission is suppressed and operation is limited to the long wavelength side of the 1300-nm window. Even when potential improvements are considered, the performance of PDFAs and NDFAs does not approach that of Er³⁺- doped fiber amplifiers.

The erbium-doped fiber amplifier (EDFA) operating at 1.5 micrometers wavelength and the praseodymium-doped fiber amplifier (PDFA) at 1.3 micrometers wavelength are promising components for optical fiber communications due to their high gain, high saturation powers, low noise and low crosstalk. In this paper, using a comprehensive model which takes into account the spectroscopic properties of the fiber amplifiers, we have analyzed the noise performance in EDFAs and PDFAs under different operating conditions. We found that the noise figures (NF) is strongly signal wavelength dependent due to lower population inversion in EDFAs for 1.48 micrometers pumping and due to ground state absorption (GSA) in PDFAs. Outside of the GSA wavelengths, the noise performance of the PDFA is comparable with that of EDFA pumped in 0.98 micrometers band. We have confirmed that the NF is independent of numerical aperture (NA) for both amplifiers. The improvement in noise performance due to the addition of an inline isolator and an optical fiber filter has also been studied.

A technique for significantly increasing the pump power in an erbium doped fiber amplifier by multiplexing pump lasers at different wavelengths is demonstrated. Two pump lasers with a wavelength separation of at least 10 nm are combined with an insertion loss of less than 0.5 dB. The performance tradeoffs as a result of multiwavelength pumping are discussed.

We report the results of an investigation of the noise figure and conversion efficiency of erbium-doped fiber amplifiers for unsaturated, moderately saturated and heavily saturated operation. Quantum-limited 3 dB noise figures result only for small signal operation of low gain amplifiers. For high gain, partially saturated amplifiers, useful as in-line repeaters, bi- directional pumping results in the best combination of noise performance and conversion efficiency while co-propagation of signal and pump produces the best noise performance.

The behavior of alumino-phosphate-silicate (APS) and fluorophosphate (FP) erbium doped fiber power amplifiers (EDFA) for the various pump wavelengths, in the small signal and large signal regime, with 800 nm band pumping has been well studied. It has already been shown that bi-directional pumping is more efficient especially at shorter wavelengths due to the presence of large excited state absorption (ESA). However bi-directional pumping requires more components which can increase the complexity and cost. In this paper we have compared, using a computer model, the performance of power EDFAs with a Bragg reflecting pump mirror etched at the output end of the fiber. For small pump power (P_p approximately equals 10 mW) this configuration is better than bi-directional pumping, both in terms of large signal gain and noise figure (NF) for APS-EDFA and FP-EDFA. At large pump powers (P_p approximately equals 200 mW) the output signal powers are comparable and the NF is better for the configuration with the reflecting mirror for APS-EDFA and better in the case of FP-EDFA.

The gain efficiency of a fully optimized EDFA is calculated as a function of the fiber NA and dopant confinement in the core and is shown to agree well with experimental data. A gain efficiency of 8.9 dB/mW is demonstrated which is the best reported value to date for MCVD fibers. In addition, the detrimental effect of pump and signal background losses on the optimum gain efficiency is considered in detail.

Erbium doped optical fibers represent an elegant class of on-line optical amplifier working in the low loss window at 1.5 microns. This paper reports on an experimental study made of an Erbium doped optical fiber amplifier pumped at various wavelengths. The fiber was pumped at different wavelengths using an Argon ion laser, a dye laser and a He-Ne laser. Both contra and codirectional pumping geometries were investigated and further, simultaneous pumping at two different wavelengths with codirectional and contradirectional geometry was studied. The gain was measured as a function of fiber length, input signal strength and input pump powers. These experimental gain characteristics have been studied and analyzed.

While low loss and large bandwidth have allowed optical fibers to become essential in a large amount of telecommunication systems, this has not yet been true for video distribution systems where the cost per subscriber must be as low as possible. First optical video distribution systems have been developped using 1.3 pm distributed-feedback (DFB) lasers. Up to now, the modulation format used in such systems is analog amplitude-modulation vestigial-sideband (AM-VSB) scheme since it provides direct compatibility with cable TV industry and present television sets. However, this modulation scheme requires a very high signal-to-noise ratio on the receiver side so that it is necessary to detect large optical powers (above -10 dBm) to ensure good picture quality. This means that the power budget (budget one can afford for optical fibers and splitters losses) for 1.3 im systems, even with powerful optical sources, is not very important; in turn, this means that it seems difficult to cut the cost per subscriber with this technology. With optical Erbium-Doped Fiber Amplifier (EDFA) breakthrough, many suppliers of cable TV transmission systems moved to 1.5 .tm technology. Whereas this new component is revolutionizing most of the telecommunication systems as optical fiber did 20 years ago, the use of optical amplifiers for analog systems is not so impressive as for robust digital transmission systems. This is due to the fragility of analog signal in respect with noise and distortion. Because of the noise generated by an optical amplifier, it is not possible to cascade a large number of them while reaching a realistic signalto-noise ratio at the network termination. Optical amplifiers may also distort the analog signal when the transmitter consists of a directly-modulated semiconductor laser, suffering from chirp (unwanted wavelength modulation when power modulation is achieved through current modulation). Moreover, shifting from 1 .3 .tm to 1 .5 pm wavelength brings new problems : 1 .5 tm DFB lasers are not yet as linear as 1.3 im DFB lasers and analog system designers have to cope with chromatic dispersion at 1 .5 .tm in standard fibers (i.e. non dispersion-shifted fibers). It is clear that many of the technical issues for 1.5 im analog systems are far from being resolved. In this paper, we investigate the way, and to what extent, erbium-doped fiber amplifiers may improve analog systems power budget. The organization of the paper is as follows. In Section 2, we discuss what is the relevant noise parameter for optical amplifiers to predict the signal-to-noise degradation they cause. We also show that the same overall noise performance can be reached with amplifiers featuring different noise parameter and output power combinations. This section is completed with experimental measurements. Linearity definitions and requirements for AM-VSB systems are presented in Section 3. Included is a discussion of some specific factors that degrade the signal linearity within an optical amplifier in respect with optical source characteristics. Finally, the still opened technical matters and potential evolution for 1.5 jim analog systems are briefly summarized in Section 4

Theoretical limits in noise figures for a long haul transmission line based on lumped amplification are compared with distributed amplification. A reduction of approximately 60% of the required number of pump power stations is achieved. The distributed optical amplification is provided by an erbium doped fiber and comparisons of aluminum and germanium as codopant materials are shown. The pump power consumption and noise figure are analyzed with respect to the background loss.

In this paper, modelling results are presented for a passively mode-locked F8L supporting a linear polarization state. Starting from A.S.E., the model produces soliton-like, chirped, red shifted, ultra-short pulses. The self-starting conditions of the laser are investigated, and the EDFA gain threshold to passively mode-lock the laser is also determined as a function of the center coupler splitting ratio (C_r). In addition, the influence of C_r to the F8L operating parameters and output pulse characteristics such as width, peak power, chirp, Self- Frequency Shift and envelope distortion is reported.

We present an efficient and compact (5 X 10 X 1.8 cm³) Er:doped fiber amplifier (EDFA) module pumped with a 980 nm laser diode. We showed that this EDFA module is capable of operating with optimum gain and noise performance over a 30 nm bandwidth at 1550 nm, and with low electrical consumption (less than 100 mW). Using the EDFA as a rack mounted preamplifier module in a 2.5 Gbit/s direct detection system experiment, we achieved a -38.5 dBm receiver sensitivity at 10^-10 bit error rate, and demonstrated long term operation (BER of 6.10^-14) of the overall system.

We investigated the gain and noise figure dependence on signal wavelength for erbium doped fiber amplifiers (EDFA) counter-pumped at 980 nm. Using a two level modeling technique, we found a good agreement between our experimental measurements and the corresponding theoretical predictions for EDFAs operating under or far from full population inversion. Finally, we describe design trade-offs between small signal gain and noise figure for a range of active fiber length EDFAs at wavelengths of 1531 nm and 1548 nm.