CATV Transmission Research Articles

Design of erbium doped fibre amplifierswith minimised gain tilt for CATVtransmission systems

High output power and low gain tilt of erbium doped fibre amplifiers for analogue applications are often conflicting requirements. The strong dependence of the gain tilt on the absorption and emission cross-section gradients of erbium allows 980 and 1480 nm pumped amplifiers to be designed with high output power and zero gain tilt.

AM-SCM-PCM lightwave CATV transmission systems

A new approach to a lightwave multichannel CATV transmission system using PCM is proposed and its advantages over conventional schemes are outlined. The principal system noise, i.e. clipping and quantisation noise, both produced by A/D convertors, has been analysed. For a 40 channel 10 bit system and 8 bit system, the received CNRs are 54 and 43 dB, respectively.

Progress in externally modulated AM CATV transmission systems

Recent advances in electronic linearization of AM CATV transmitters based on externally modulated diode pumped Nd:YAG lasers are presented. The theoretical principles and practical issues related to the methods of feedforward and predistortion linearization, the factors affecting system performance and the emerging devices, circuits, subsystems, and systems architectures are reviewed. In experiments based on a predistortion linearized system, transmission over two fibers of 30 km each, wherein each fiber carries 80 NTSC channels, is now routinely achieved with a CNR>50 dBc, a CTB<-65 dBc, and a CSO of typically -70 dBc. As the number of channels broadcast over each fiber is reduced from 80 to 60, CNR increases to 52 dBc. The fiber links served by the system under test each have 12-dB link budget, and include four connectors per fiber, with no degradation observed in one link when the connectors were opened on the other link.

Distortion and cnr characteristics in fm‐fdm optical catv transmission

AbstractIn the multichannel frequency division multiplex (FDM) optical transmission, the intermodulation distortion generated by the nonlinearity of the LD is a problem. In this paper, it is analyzed that for the number of transmission channel, N, and the optical modulation index per channel, m, the CSO and CTB are constant for a different value of N if the effective optical modulation index, is maintained. The results are compared with experiments. In the present analysis, two types of nonlinearity of the current‐to‐optical conversion characteristics are considered. One of them is the nonlinearity near the bias current of the LD and is predominant in the region of a small effective optical modulation index. Another is the nonlinearity at the threshold value of the LD and is dominant when the effective optical modulation index is large. It is shown by means of the probability density function that the CSO and CTB degrade suddenly if the effect of the nonlinearity of the threshold value appears. In the FM‐FDM optical transmission, many distortions in the transmission bandwidth behave as the white noise due to frequency modulation when the carrier is frequency modulated by a video signal. Hence, the CN ratio is effectively degraded. In this paper, it is further presented that the CN ratio degraded by the distortions can be derived from the CSO and CTB by taking into consideration the frequency allocation of the FM signal.

Asymptotic distortion spectrum of clipped, DC-biased, Gaussian noise (optical communication)

Consider a zero-mean, stationary Gaussian process g(t), to which a large positive constant A has been added. Define a distortion process h/sub A/(t) as equal to g(t)+A when the latter is negative and equal to zero otherwise. The author calculates the power spectrum of the process h/sub A/(t) asymptotically as A becomes large. The results have application for estimating the nonlinear-distortion power in the recovered signal when many frequency-multiplexed subcarriers collectively modulate a laser's output power, as would be the case for CATV transmission over an optical fiber. The process h/sub A/(t) then models the nonlinear distortion caused by occasional clipping of the DC-biased laser input.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

In search of a linear electrooptic amplitude modulator

An electrooptic modulator which offers high linearity and low intermodulation distortion (IMD) is proposed. The modulation characteristics, harmonic and intermodulation distortions, are calculated. In the two-tone test with an optical modulation depth of 16% for each channel, the third-order IMD is expected to be as low as -95 dB. Such a linearized electrooptic modulator used in conjunction with diode-pumped solid-state lasers can be expected to outperform directly modulated injection lasers for CATV transmission systems.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Multichannel AM-VSB television signal transmission using an erbium-doped optical fiber power amplifier

The use of an erbium-doped optical fiber power amplifier in a multichannel amplitude-modulated vestigial-sideband CATV transmission system for potential subscriber loop applications is discussed. Using a color-center laser pump at 1.48 mu m, a fiber-to-fiber gain of 11.5 dB (at 1.539 mu m), which is 10 dB compressed from the small-signal gain, and a total system power margin of 15 dB were achieved. No intermodulation distortion was introduced by the fiber amplifier. The received video SNR of 40-46 dB was limited by the linearity of the distributed-feedback laser diode used.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Lightguide Technology Review and Status

Remarkable progress has been made in the lightguide technology over the past ten years. The realization of low loss optical fibers approaching intrinsic material limit and long life semiconductor lasers has stimulated rapid deployment of lightwave communication systems. Within the past five years the world wide fiber production capacity has grown from a few thousand to over one million kilometers per year. The impetus for this phenomenal growth can also be traced to recent advances in computer technology allowing low cost processing and storage of a great deal of complex information. This combination of information processing and transmission technologies is bringing a host of new services such as voice, data and visual communications, CATV, videotex and facsimile transmission, over the same network. One could therefore expect an accelerating growth in deployment of lightwave communication systems.

Improvement of CATV Transmission Using an Optimum Coherent Carrier System

Aiming at the development of future multichannel CATV-transmission systems the utilization of phase-coherent picture carriers is discussed as a means of reducing intermodulation-and crossmodulation-distortion arising in the line repeater amplifiers. The paper presents experimental and theoretical data for comparison of transmission performance. As a basis the non-coherent operation is reviewed. In order to evaluate the improvement by a coherent system the problem of optimum carrier phase relation is investigated. Experimental results demonstrate the suppression of distortion, if the phases are adjusted according to a certain code which was found by optimization.

CATV Transmission System Design for Reliable Year-Round Operation

Television distribution by cable is rapidly coming of age as a widely accepted public service. Equipment is now becoming available which will do much to bring about necessary improvements in system performance, life, reliability, and operating costs. This paper analyzes the requirements for a fully satisfactory and reliable CATV system and sets forth objective technical specifications which should be met by the equipment manufacturer and the system operator. References for requirements based on subjective evaluation are given.