Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Patent
1987-11-12
1989-07-11
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
372 29, G01J 120
Patent
active
048474770
DESCRIPTION:
BRIEF SUMMARY
The invention relates to methods and apparatus for reducing a fluctuation in a characteristic of a beam of radiation.
The invention is concerned, for example, with the reduction of noise in a characteristic such as the phase, frequency, or amplitude of a beam of radiation.
In the field of optical communication networks it has been determined that for high performance PSK and DPSK schemes ratios of line width to bit rate of between 0.05% to 0.3% are required. For the lower performance ASK and FSK schemes, featuring non-synchronous detection, laser line width/bit rate ratios of between 10% and 30% may be acceptable although it should be noted that practical considerations may dictate narrower line widths if good performance is to be achieved.
At moderate bit rates it is expected that DFB lasers will be suitable for FSK and ASK providing sufficient receiver bandwidth is available to operate at a suitably high IF frequency to enable simple envelope detection schemes to be used. Alternatively the more complex I & Q strategy may be necessary, as described in "Demodulation of optical DPSK using in-phase and quadrature detection" by T. G. Hodgekinson, R. A. Harman and D. W. Smith, Elec. Lett. Vol 21 (1985) pp 867-868. However, using currently available DFB lasers, say 15 MHz linewidth, it is unlikely that they could be used in DPSK/PSK schemes at bit rates less than 5 to 15 GBit/s unless line narrowing techniques are employed.
So far only two techniques have been truly successful, injection locking using a gas laser and the application of external cavities. Both techniques, however, are not ideal for widespread low cost high volume application.
Wideband opto-electronic feedback in the form of frequency control systems or phase locked loops has been proposed to reduce laser phase noise. Both suffer from the serious practical difficulty, common to all wideband feedback control system design, of closed loop stability. In practice, it is necessary to take great care in reducing loop time constants and pure time delay around the loop. Furthermore, loop parameters will be a function of several device features, i.e. laser output power, laser FM characteristics, signal polarisation etc. and it is expected that loop stability could be unpredictable and re-optimisation will often be necessary.
In accordance with one aspect of the present invention, a method of reducing a fluctuation in a characteristic of a coherent beam of radiation comprises sensing the fluctuation at a first postion to generate a compensating signal related to the sensed fluctuation; and modifying the characteristic of the beam at a second position downstream of the first position under control of the compensating signal to reduce the random fluctuation.
In accordance with a second aspect of the present invention, apparatus for reducing a fluctuation in a characteristic of a coherent beam of radiation comprises sensing means for sensing the fluctuation at a first position; signal generating means responsive to the sensing means to generate a compensating signal related to the sensed fluctuation; and radiation beam modifying means for modifying the characteristic of the beam at a second position downstream of the first position under control of the compensating signal to reduce the random fluctuation.
The invention is based on a feed foward technique which should be contrasted with the feedback methods described above. The feed forward technique should not suffer from problems of instability.
The invention is particularly applicable to the reduction or cancellation of random fluctuations such as phase noise and this could enable presently available single mode semiconductor lasers, such as DFB lasers, to be used in the most demanding PSK transmission systems, even at moderate (<1 Gbits/s) data rates.
The invention could also be used to remove a non-random fluctuation such as a low frequency identification signal.
The invention is particularly applicable to beams of coherent optical radiation although the principles may be applied to radiation outside t
REFERENCES:
patent: 3699463 (1972-10-01), Stone
patent: 4221960 (1980-09-01), Maeda et al.
patent: 4287606 (1981-09-01), Lutes, Jr. et al.
Patents Abstracts of Japan, vol. 7, No. 119 (E-177) [1264], 24th May 1983; & JPA-58-39-084 (Nippon Denki K.K.), 07-03-1983.
English Translation of Japanese Patent Application No. 58-39084-(2 sheets).
Patent Abstracts of Japan, vol. 6, No. 38 (E-144) [1116], 26th Nov. 1982; & JP-A-57 139 979 (Nippon Denki K.K.), 30-08-1982.
Demodulation of Optical DPSK Using In-Phase and Quadrature Detection by T. G. Hodgkinson et al., 31 Jul. 1985-pp. 867-868.
British Telecommunications public limited company
Nelms David C.
Shami Khaled
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