Amplification of spontaneous emission (ASE) in erbium-doped single-mode fiber amplifiers operating at lambda =1.53 mu m is studied theoretically and experimentally. The ASE noise spectra obtained from the theory are found to be in excellent quantitative agreement with the experimental data. The observed changes in ASE spectral shapes under different population inversion conditions are also explained. The model may be used to evaluate the performance of erbium-doped fiber lasers as well as to assess the noise characteristics of erbium-doped fiber amplifiers as applied to wavelength-division multiplexing optical communications. >

Amplification of spontaneous emission in erbium-doped single-mode fibers

The characteristics of high-sensitivity photodetectors suitable for wide bandwidth optical communication systems are summarized. Photodiodes, photomultipliers, and photoconductive detectors for wavelengths from 0.3 µm to 10.6 µm are covered. The use of internal current gain by means of avalanche and electron multiplication and by means of optical heterodyne detection to increase sensitivity of high speed photodetectors is discussed. The application to visible and infrared laser communication systems is reviewed.

Photodetectors for optical communication systems

The gain, saturation power, and noise of an erbium-doped single-mode traveling-wave fiber amplifier operating at a wavelength lambda =1.53 mu m are characterized. In continuous-wave (CW) measurements amplification at 2 Gbit/s was demonstrated with up to 17-dB gain for 1*10/sup -9/ bit error rate at 1.531 mu m and a 3-dB full bandwidth of 14 nm. From the determination of the fiber-amplifier's output signal-to-noise ratio versus input signal power during data transmission, it was concluded that, with signal levels used here, signal-spontaneous beat noise limited the receiver sensitivity improvement. With the fiber amplifier acting as an optical preamplifier of the receiver, the best sensitivity was -30 dBm, obtained after installing a polarizer at the fiber amplifier output to reject half of the applied spontaneous emission power. This sensitivity was 6 dB better than without the fiber amplifier, proving that the fiber amplifier can be used as a preamplifier. >

2-Gbit/s signal amplification at lambda =1.53 mu m in an erbium-doped single-mode fiber amplifier

Since the invention of the laser and its first application, there has been an almost continuous stream of new applications many of which require specific laser sources. These applications often require a laser source with a specific power, pulse duration, energy and wavelength. In some cases these demands are easily met, whilst in others they have proven rather more difficult to achieve. Traditionally, wavelength versatility has been limited to the regions for which rare earth or gas gain media are available. These lasers themselves can be used to generate other wavelengths through the nonlinear processes of second and third harmonic generation, as well as sum frequency generation. Despite all of this, there still exists a significant section of the visible and infrared spectrum for which no convenient sources exist. This thesis is concerned with the development of sources in these regions along with broadband sources covering significant portions of the spectrum by themselves. These new wavelengths are generated in a variety of speciality fibres using either nonlinear processes or new gain media doped into standard silica fibres. Three types of speciality fibre are used: low concentration bismuth doped fibre which provides gain in the 1.0-1.4 μm region; photonic crystal fibres; and very high (75%) concentration germanium fibres to generate a laser source at 2.1 μm based upon stimulated Raman scattering. Photonic crystal fibres provide high nonlinearities and controllable dispersion which enables the generation of broadband supercontinuum sources based upon the interaction of many nonlinear effects. Each source will be described in depth, with particular attention given to the underlying physics that gives rise to the source. Previous and current limitations will be examined and an outlook of the future development of such sources will be discussed.

Wavelength extension in speciality fibres

A system gain of 40 dB was realized with a semiconductor injection-laser oscillator operating into a neodymium-glass-fiber laser amplifier. InAsP ingots were grown by a modified Czochralski technique. The injection laser was fabricated from a mixed crystal of InAs 0.17 P 0.83 . This composition was chosen because laser emission occurs at 1.063 μ at liquid nitrogen temperatures. The p-n junction was obtained by zinc diffusion under phosphorus pressure. The method of fabricating contacts and the Fabry-Perot cavity is described, and optical and electrical characteristics of the devices are given. Coarse tuning of the lasing wavelength is based on x . Fine tuning can be accomplished by temperature variation of the laser junction in accordance with d\lambda/dT = 1.6 A/°K between 80- 140°K. The injection laser was optically mated to the samarium-clad neodymium-glass-fiber amplifier by microscope optics and a gimbal-mounted reflector. Design details of the optical amplifier are given in context with optimum low-noise-level operation. The peak optical amplifier wavelength can be varied over a 100- A interval by appropriate choice of the host-glass composition. The injection-laser pulse was delayed until the spontaneous emission had reached its peak for a maximum observed gain of approximately 5 \times 10^{4} .

G. Holst

Papers

Detection with a fiber laser preamplifier at 1.06 µ

G. Holst

Papers

Detection with a fiber laser preamplifier at 1.06 &#181;

Detection with a fiber laser preamplifier at 1.06 µ