Optics: measuring and testing – By light interference
Reexamination Certificate
1998-07-30
2001-05-01
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
Reexamination Certificate
active
06226090
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a technique of generating light timing pulses from light signals, and particularly relates to a method of generating light timing pulses from a light signal with the attribute of the signal unchanged and a circuit for implementing such method.
2. Description of the Related Art
Research attempts have been made vigorously concerning communication methods based on optical time division multiplexing (optical TDM) as future optical communicating means which could provide very high transmission rates (“Optoelectronics; Device and Technology” journal, vol. 10, No. 4, pp. 447-460, December 1995).
Because optical time division multiplexing communication methods (optical TDM methods) perform data coding according to whether or not short pulse light exists (intensity modulation), they basically use return-to-zero (RZ) type light signals. However, present mainstream optical communicating methods use non-return-to-zero (NRZ) light signals. Thus, it may be necessary for future optical time division multiplexing communication methods to have a circuit for converting currently prevailing NRZ type optical data signals into RZ type ones. In addition, a circuit for generating light timing pulses in synchronization with the NRZ type optical data signal is required to implement an optical code conversion circuit. As one of the conventional light timing pulses extracting circuits, for example, such a circuit structure is known that has been disclosed in Japanese Published Unexamined Patent Application No. Hei 5-37469. In this circuit, an incident optical signal of NRZ type is input to an optical being amplified, is and after amplified, input to an intensity modulation to frequency modulation converting circuit (IM-FM converting circuit). By means of a nonlinear optical effect, the IM-FM converting circuit converts the intensity modulated light of NRZ type optical signal into frequency modulated light with an incident ray from a semiconductor laser. By extracting this frequency modulated light through a narrow-band-pass optical filter, light timing pulses are produced.
But, the above light timing pulses extracting circuit has such a bottleneck as follows. Using the nonlinear optical effect such as optical Kerr effect, the above-mentioned IM-FM converting circuit converts the variation in the intensity of NRZ-type optical data signals into the variation in frequencies of semiconductor laser rays. In order to use such nonlinear optical effect, it is necessary to amplify an incident NRZ-type light signal up to a high power level beyond several tens of milliwats (mW) before inputting it to the IM-FM converting circuit. Thus, the optical amplifier is required that it amplifies the light signal to an extremely large extent. This causes a problem that the power consumed by the entire circuit (particularly, the power consumed by the optical amplifier) is very large. There is another problem which stable operation of the circuit is not always expected because the nonlinear optical effect itself is a very unstable phenomenon. That is, the above-mentioned light timing pulses extracting circuit is questionable in its reliability, economical feasibility and operational stability.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of generating light timing pulses from an NRZ-type light signal and a circuit for implementing such method in a compact and simple structure that can achieve economical and stable operation and high reliability.
A light timing pulses generating method according to one aspect of the present invention comprises the steps of making a light signal diverge into two route, regulating two diverged rays such that they travel through their specific optical paths with different lengths, and combining these two diverged rays and generating light timing pulses from t he composite light. Such method according to another aspect of the present invention comprises the steps of making a light signal diverge into two route, producing a given time difference between the optical paths for two diverged rays, and combining these two diverged rays and generating light timing pulses from the composite light. The incident light signal is a non-return-to-zero (NZR) type light signal. The two diverged rays are regulated such that the difference between the lengths of their optical paths is smaller than one-bit length of the incident light signal.
Then, a light timing circuit according to one aspect of the present invention comprises an optical circuit including a divergent section which makes an incident light diverge into two routes and optical waveguides which form different-length optical paths through which the rays diverged from the incident light are propagated; and a synchronous light pulses generating circuit to which the rays output from the optical circuit are input. Such circuit according to another aspect of the present invention comprises a Mach-Zehnder interferometer having two optical paths whose light signal propagation time differs with each other, a mode locking laser which generates light timing pulses in synchronization with a cyclic frequency of a light pulse signal output from the Mach-Zehnder interferometer, monitoring means for the interference condition of the Mach-Zehnder interferometer, interference condition control means for optimally controlling the condition of the Mach-Zehnder interferometer, based on the information derived from the monitoring means. The Mach-Zehnder interferometer can be set such that a relation between propagation time difference &Dgr;T produced when two rays diverged from a light data signal are propagated through their respective optical paths and bit rate B of the light data signal will be (m+0.05)/B≦&Dgr;T≦(m+0.95)/B. Besides, the propagation time difference &Dgr;T of the Mach-Zehnder interferometer can be set such that phase difference &Dgr;&phgr; between the two rays diverged from the light signal when they are combined after propagated through their respective optical paths will be integral multiples of radian &pgr;. In the latter circuit, the interference condition control means for the Mach-Zehnder interferometer is further specifically regarded as a means for setting the propagation time difference &Dgr;T by changing the physical length or refractive index of at least one of the two optical paths. According to another feature of the present invention, the interference condition control means for the Mach-Zehnder interferometer is also regarded as combined means for applying an electric field to at least one of the two optical paths and controlling the intensity of the electric field.
The present invention can achieve incomparably economical advantage, stable operation and high reliability when light timing pulses are generated from light data signals of NRZ type according to the method and circuit noted above.
REFERENCES:
patent: 5781326 (1998-07-01), Chiaroni et al.
Satoki Kawanishi, “Very High-Speed Optical Transmission Technology”, Optoelectronics—Devices and Technologies, vol. 10, No.4, Dec. 1995, pp. 447-460.
Font Frank G.
Lee Andrew H.
NEC Corporation
Young & Thompson
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