Optical: systems and elements – Light interference – Produced by coating or lamina
Patent
1994-02-03
1997-12-16
Dzierzynski, Paul M.
Optical: systems and elements
Light interference
Produced by coating or lamina
359359, 359580, 359629, G02B 110
Patent
active
056991878
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an optical coupler for dividing a signal light beam at a predetermined ratio in an optical apparatus, an optical communication system, or the like, and more particularly to an optical coupler for dividing a portion of a signal beam transmitted from a light emitting device such as a semiconductor laser or the like through an optical fiber and monitoring the divided signal beam for the optical intensity or the like of the signal light beam.
BACKGROUND ART
In recent years, research efforts have intensively been made with respect to optical amplifiers for directly amplifying light as it is. In an optical transmission path having an optical amplifier, it is necessary to keep constant the intensity of a signal light beam transmitted over the optical transmission path. To meet such a requirement, the intensity of the signal light beam has to be monitored accurately. An optical coupler is known as a device for monitoring the intensity of the signal light beam.
A general optical coupler comprises a dividing film inserted at an angle to the optical path for reflecting a portion of the signal light beam thereby to divide the light beam.
The dividing film is preferably inserted at 45 degrees to the optical path so that the divided light beam is bent 90 degrees with respect to the optical path. Such an arrangement allows the optical amplifier to be simplified in structure and reduced in size.
The divided light beam produced by the optical coupler is used to monitor the intensity of the signal light beam. It is therefore necessary that the ratio of the divided light beam to the signal light beam be constant. If the angle at which the signal light is applied to the dividing film is large (the angle of 45 degrees belongs to a large range), then the dividing ratio is unstable in time due to the polarization dependency of the dividing film. Therefore, there is a demand for an optical coupler which is less susceptible to the polarization dependency.
FIG. 1(A) is a view illustrative of a conventional optical coupler. In FIG. 1(A), a multilayer dividing film 200 is disposed on a surface of a substrate 100 of optical glass. The multilayer dividing film 200 is formed by evaporating silicon dioxide (SiO.sub.2 )or the like several times on the substrate 100 of optical glass. The substrate 100 with the multilayer dividing film 200 deposited thereon is inserted in an optical path.
A signal light beam P0 applied from a transmission path is divided by the dividing film 200 into a signal light beam (divided light beam P1) reflected by the dividing film 200 and a signal light beam (transmitted light beam P2) transmitted through the dividing film 200. The divided light beam P1 is used to monitor the intensity of the signal light beam P0.
FIG. 1(B) is a diagram showing the transmission characteristics of the conventional optical coupler shown in FIG. 1(A) at the time the signal light beam is applied at an incident angle of 45 degrees. The horizontal axis of FIG. 1(B) represents the wavelength .lambda. (nm) of the incident light beam and the vertical axis thereof represents the transmittance T (%). Tp and Ts indicate the transmittances (%) of respective P- and S-polarized light contained in the signal light beam.
If the incident angle of the signal light beam P0 is 45 degrees for the reason described above, then the polarization dependency of the dividing ratio by the dividing film 200 is large. Specifically, an incident signal light beam generally contains P- and S-polarized light at a ratio that varies with time. As shown in FIG. 1(B), the transmittances T (%) of the P-polarized light (Tp) and the S-polarized light (Ts) differ from each other. Therefore, the total dividing ratio varies as the ratio of the P- and S-polarized light varies. Consequently, the intensity of the divided light beam P1 may vary greatly even when the intensity of the incident signal light beam is constant.
If the incident angle is 5 degrees, for example, then the polarization dependency of the dividing ratio of
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Fukushima Nobuhiro
Matsuda Hiroaki
Noda Hideki
Dzierzynski Paul M.
Fujitsu Limited
Schuberg Darren E.
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