Variable-optical-delay apparatus

#1. Field of the Invention

#The present invention relates to a variable-optical-delay apparatus for delaying an optical signal by a delay time determined in accordance with an input optical signal.

#2. Description of the Prior Art

#FIG. 1 is a block diagram of a variable-optical-delay apparatus that uses an optical path loop, according to a first embodiment of the present invention.

#FIG. 2 is a block diagram of a second embodiment in which the variable-optical-delay apparatus comprises a plurality of apparatuses in a series configuration.

#FIG. 3 is a block diagram of a third embodiment that enables output of an optical signal having the same frequency as an input optical signal.

#The embodiments of the present invention are described below, with reference to the drawings. Except where stated otherwise, parts having the same functions have been given the same reference symbols.

#FIG. 1 shows an example of a first embodiment of the variable-optical-delay apparatus of the invention, which uses an optical path in the form of a loop having a circumference of 20 meters. An input optical signal (wavelength: 1549.8 nm) passes along optical path 1 and is directed onto optical path 3 by a circulator 2-1. Provided on the optical path 3 is a fiber Bragg grating (FBG) 4 used as a filter. The FBG 4 has a reflection band with a bandwidth of 250 GHz (1550-1551 nm). The input optical signal has to be at a frequency that is slightly lower (or higher) than the frequencies reflected by the FBG 4. Signals transmitted by the FBG 4 are directed onto the loop path 5 by a circulator 2-2. The loop path 5 is provided with an optical single-sideband (SSB) modulator 6. To reduce the amount of space occupied by the apparatus, a resonant type optical SSB modulator can be used, which provides high efficiency. Details of such a resonant type optical SSB modulator are described in Reference 1 (Shimotsu et al., “Optical SSB modulator using integrated type LN modulator,” IEICE Technical Report, OEIC OPE 2000-37, LQE 2000-31 (2000-07), 29-34, 2000).

#A high-frequency oscillator 9 supplies a high-frequency electric signal to the SSB modulator 6. The frequency of the high-frequency electric signal is determined in accordance with an electric signal corresponding to the optical signal supplied to the high-frequency oscillator 9 via wiring line 20. By means of this modulator, the frequency of an input optical signal is increased (or decreased) by the oscillation frequency of the high-frequency oscillator 9, which is 50 GHz (0.40 nm), as shown in FIG. 8. In this way, the optical SSB modulator functions as a frequency shifter. The frequency-shifted optical signal is amplified by an erbium-doped fiber amplifier (EDPA) 7 and returns to the circulator 2-2, whereby it is directed to the FBG 4, frequency-shifted by reflection by the FBG 4, and thereby is returned to the loop path 5, where it undergoes further a further shift in frequency. The signal thus undergoes six frequency shifts until it can pass through the FBG 4. After passing through the FBG 4, the signal is returned to the optical path 1 by the circulator 2-1 and output. This enables a signal delay of 1200 ns to be achieved. By using a high-frequency oscillator 9 having an oscillation frequency of 40 GHz, a delay of 1400 ns can be achieved by circulating the signal through the loop seven times. As can be understood from this example, each apparatus has its own delay increments,