GB2315380A

GB2315380A - Optical add/drop circuit using fibre gratings

Info

Publication number: GB2315380A
Application number: GB9625438A
Authority: GB
Inventors: Hideki Noda; Nobuhiro Fukushima
Original assignee: Fujitsu Ltd
Current assignee: Fujitsu Ltd
Priority date: 1996-07-16
Filing date: 1996-12-06
Publication date: 1998-01-28
Also published as: DE19700682C2; JPH1032562A; DE19700682A1; GB9625438D0

Abstract

An optical add/drop circuit including a first optical circulator 10 having three ports 10A, 10B, 10C, a first fibre grating 12, a second optical circulator 18 having three ports 18A, 18B, 18C, a second fibre grating 16, and an optical isolator 14 connecting the two fibre gratings 10,18. First input light having a wavelength # 10 and second input light having a wavelength # 20 are supplied to port 10A. Port 10B outputs the first input light (reflected from the first grating 12) as drop light. The second input light is output from port 10B, passes through the first grating 12, the optical isolator 14 and the second fibre grating 16 and reaches port 18A. Add light having a wavelength # 10 is supplied to port 18C. Port 18B outputs this add light (reflected from the second grating 16) and the second input light. This optical add/drop circuit is stable in operation because resonance or the like is prevented.

Description

2315380 OPTICAL A1DD/DROP CIRCUIT HAYING A FIBER GRATING The present

invention relates generally to an optical add/drop circuit, applicable to a wavelengthdivision multiplexing (WDM) system, and having a fiber grating.

In the present application, the wording "an - for optical add/drop circuit" means an optical circuit, adding and/or dropping of light.

There has been proposed a network configured by connecting a plurality of terminals through optical fiber transmission lines. It has been pointed that application of wavelength-division multiplexing (WDM) to such a network allows construction of flexible systems.

In a network employing WDM, an optical add/drop circuit is used to allow reception of an optical signal of a drop channel and/or transmission of an optical signal of an add channel at a certain terminal.

In recent years, a practical -fiber grating (Bragg arating fiber device) has been developed, and application of the fiber grating to an OpLical add/drop circuit has 1 beer- proposed (e.g., OPTR02NICS (1995) No. 11, c)c.135-i1) fic a fiber gratng reflects light having a -ransm-itts light in.av-lnc:

wavelength and I,;aveleng-,,s except --- - -- - 1 the specific waveleng h. if two or more 1.iber grat-ings for light having same,,;aveleng-t-h are provided in- a closed optical -here is a - L - bility that 10 -he opt-ical circuit - ay poss the operation o m Ll- be unstable because of resonance or he like. Further, J. a f-iber grating obtained by a general fabrication method, there occurs -increased loss of light to be transmitted at wavelengths sh-orter than -L'-Mle wavelength of light to be reflected.

In accordance With an aspect,- of the present invention, -there _15 p-rovided an opticall add/drop circuit comprising a fir-st optical circulator 'riav-'ng second, - - - - - L 2 and third ports; a first fiber grating operatively connected to the second port; a second optical circulator having fourth, fifth, and sixth ports; a second fiber grating operatively connected to the fourth port; and an optical isolator operatively connected between the first and second fiber gratings.

In this optical circuit, the optical isolator is provided between the first and second fiber gratings. This allows the possibility of instability of operation due to resonance or the like to be eliminated.

The effect obtained by providing the optical isolator is remarkable in a preferred embodiment of this aspect in which the first and second fiber gratings reflect light having the same first wavelength. Such an optical circuit may operate in the following manner, for example. First input light having a first wavelength and second input light having a second wavelengthj different from the first wavelengthjare supplied to the first port. The third port out-outs the first input light reflected by the first fiber grating as drop light. The second input light is output from the second port, next passed through the first fiber grating, the optical isolator, and the second fiber grating in this order, and next supplied to the fourth port. Add light having the 3 first wavelength is supplied to the sixth port. The fifth port outputs the add light reflected by the second fiber grating and the second input light supplied to the fourth port.

In accordance with another aspect of the present invention, there is provided an optical add/drop circuit comprising an optical circulator having first, second, and third ports; and a fiber grating operatively connected to the second port, for reflecting light having a first wavelength, said fiber grating transmitting light having a irst wavelength.

wavelength longer than the f When using an optical circuit according to this aspect, the wavelength of the light to be passed through the fiber grating is generally to be set longer than the wavelength of the light to be reflected by the fiber grating. Therefore, in the case that this optical circuit is used as an optical add circuit or an optical drop circuit, transmission loss can be reduced.

In this specification, the wording that optical components are operatively connected to each other includes the case that the optical components are directly connected together by fiber connection or spatial connection using a collimated beam and further includes the case that the optical components are connected through another optical component such as an

4 optical filter.

For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:- FIG. 1 is a block diagram of an optical add/drop circuit showing a first Preferred embodiment of the present invention; FIG. 2 is a graph showing the relation between transmittance of a fiber grating shown in FIG. 1 and wavelength; FIG. 3 is a block diagram of an optical add/drop circuit showing a second preferred embodiment of the present invention; FIG. 4 is a block diagram of an optical circuit showing a third preferred embodiment of the present invention; FIG. 5 is a diagram showing wavelength locations of four channels in FIG. 4; FIG. 6 is a block diagram of an optical drop circuit showing a fourth preferred embodiment of the present invention; FIG. 7 is a block diagram of an optical drop circuit showing a fifth preferred embodiment of the present invention; FIG. 8 is a block diagram of an optical add circuit showing a sixth preferred embodiment of the present invention; FIG. 9 is a block diagram of an optical add circuit showing a seventh preferred embodiment of the present invention; FIG. 10 is a block diagram showing a network to which the present invention is applicable; FIG. 11 is a block diagram of an optical add/drop circuit showing an eighth preferred embodiment of the present invention; and FIG. 12 is a block diagram of an optical add/drop circuit showing a ninth preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Some preferred embodiments of the present invention will now be described in detail with reference to the 6 attached drawings.

Referring to FIG. 1, there is shown a block diagram of an optical add/drop circuit showing a first preferred embodiment of the present invention. This optical circuit has ports 2, 4, 6, and 8, and an optical circuit including an optical circulator 10, a fiber granting 12, an optical isolator 14, a fiber grating 16, and an optical circulator 18. The optical circulator 10 has a port 10A connected to the port 2, a port 10B connected to a first end of the fiber grating 12, and a port 10C connected to the port 4. A second end of the fiber grating 12 is connected to an input port of the optical isolator 14. An output port of the optical isolator 14 is connected to a first end of the fiber grating 16. The optical isolator 14 transmits light only in a direction from its input port toward its output port. The optical circulator 18 has a port 18A connected to a second end of the fiber grating 16, a port 18B connected to the port 6, and a port 18C connected to the port 8.

In the drawings, the direction of an arrow shown inside a circle representing each optical circulator defines a direction of circulation of light in each optical circulator. For example, the optical circulator 10 7 outputs light supplied to the port 101A. from the port 10B, outputs light supplied to the port 10B from the port 10C, or outputs light supplied to the port 10C from the port 10A.

In the case that the refractive index of an opticall medium (e.g., glass) is permanently changed by exposure to light, it is generally said that the optical medium is photosensitive. By utilizing this property, a fiber grating can be fabricated in the core of an optical fiber. Such a fiber grating has a characteristic that it can Bragg-reflect light in an narrow band near a resonance wavelength determined by the pitch of gratings and the effective refractive index of a fiber mode. The fiber grating can be fabricated, for example, by directing excimer laser having an oscillation wavelength of 248 nm or 193 nm to a fiber by using a phase mask (K. 0. Hill, B. Malo, F. Bilodeau, D. C. Johnson, and J. Albert, "Bragg gratings fabricated in monomode photosensitive optical fiber by UV exposure through a phase mask", Applied Physics Letters, Vol. 62, No. 10, pp. 1035- 1037, March 8, 1993).

The photosensitivity inherent in an optical fiber can be enhanced by H2 loading (P. J. Lemaire, R. M. Atkins, V. Mizrahi, and W. A. Read, "High pressure H2 loading as a 8 technique for achieving ultrahigh UV photosensitivity and thermal sensitivity in Ge02 doped optical fibres", Electronics Letters, Vol. 29, No. 13, pp. 1191-1193, June 24, 1993), by frame brushing (F. Bilodeau, B. Malo, J. Albert, D. C. Johnson, K. 0. Hill, Y. Hibino, M. Abe, and M. Kawachi, "Photosensitization of optical fiber and silica-on- silicon/silica waveguides", Optics Letters, Vol. 18, No. 12, pp. 953-955, June 15, 1993), or by high exposure to ultraviolet light (B. Malo, J. Albert, K. 0. Hill, F. Bilodeau, D. C. Johnson, and S. Theriault, "Enhanced photosensitivity in lightly doped standard telecommunication fibre exposed to high fluence ArF excimer laser light", Electronics Letters, Vol. 31, No. 11, pp. 879-880, May 25, 1995).

In this preferred embodiment, the fiber gratings 12 and 16 reflect light having a wavelength A10 and t ransmi t 1 i ght having a wave 1 ength A 20 ( /1 10 ' /120) The operation of the optical circuit shown in FIG. 1 is summarized by the feature that when WDM (wavelengthdivision multiplexed) optical signals having wavelengths A,, and /120 are supplied, an exchange between drop light having the wavelength A10 and add light having the wavelength /110 is carried out. The operation will now be described more specifically. Optical signals having 9 wavelengths A,O and /120 are input from the port 2 to the port 10A of the optical circulator 10. The input optical signals are supplied from the port 10B to the fiber grating 12 The optical signal having the wavelength A., is reflected by the fiber grating 12, next passed through the port 10B and the port 10C in this order, and next output as drop light from the port 10C. The optical signal having the wavelength 112, is transmitted by the fiber grating 12, next passed through the optical isolator 14 and the fiber grating 16 in this order, and next supplied to the port 18A of the optical circulator 18. Add light having the wavelength A 10 is input to the port B. The add light is next passed through the port 18C and the port 18A in this order, and next supplied to the fiber grating 16. The add light is reflected by the fiber grating 16, and next supplied again to the port 18A. The optical signal having the wavelength A20 and the add light having the wavelength A 10 both supplied to the port 18A are passed through the port 18B, and next output from the port 6.

In this preferred embodiment, the fiber gratings 12 and 16 reflecting light having the same wavelength (/110) are provided between the optical circulators 10 and 18. Since the reflectivity of each of the fiber gratings 12 and 16 for the light having the wavelength Al. is less than 100 10 % in general, the light having the wavelength A10 is partially transmitted by each fiber grating. Accordingly, if the optical isolator 14 is not provided, there is a possibility of resonance of the light having the wavelength A iC/ depending on the relation between an optical distance between the fiber gratings 12 and 16 and the wavelength /110, The occurrence of such resonance causes instability of the operation of the optical add/drop circuit. Also in the case that the wavelengths of light to be reflected by two adjacent fiber gratings are different, the instability of the operation may be caused by interference or the like between harmonics if an optical isolator is not provided.

According to this preferred embodiment, the optical isolator 14 is interposed between the fiber gratings 12 and 16. Therefore, there is no possibility of resonance of light between the fiber gratings 12 and 16, so that the operation of the optical add/drop circuit can be stabilized.

Referring to FIG. 2, there is shown the relationship between transmittance of the fiber grating 12 (16) and wavelength. The transmittance is minimum at the wavelength /110 (i.e., the reflectivity is maximum at the wavelength /1,0), and the transmittance is high in all 11 bands except a narrow band including the wavelength A,O The point to be noted herein is that the transmittance in a band including wavelengths shorter than the wavelength Al. is lower than the transmittance in a band including wavelengths longer than the wavelength A,.. That is, the loss is higher in the shorter-wavelength band than in the longer-wavelength band. This fact such that higher loss occurs in the band including wavelengths shorter than the Bragg reflection wavelength can be explained by a radiation mode to a clad in a fiber grating.

To avoid the increase in loss mentioned above, thE wavelength /120 of the optical signal to be transmitted by(passed through) the fiber gratings 12 and 16 is set longer than the wavelength A,, of the optical signal to be reflected by the fiber gratings 12 and 16 according to the first preferred embodiment shown in FIG. 1. By such relative setting of the wavelengths, the increase in loss described with reference to FIG. 2 can be avoided.

Referring to FIG. 3, there is shown a block diagram of an optical add/drop circuit showing a second preferred embodiment of the present invention. This preferred embodiment is characterized in that the drop light is composed of a plurality of drop channels and that the add light is composed of a plurality of add channels.

12 The drop channels have wavelengths A,, and 11,2, and the add channels also have wavelengths and A, in the case of 12 FIG.3.

WDM optical signals of three channels are in-out from the port 2 to the port 10A of the optical circulator 10. The three channels have wavelengths A 11, J. 2. and To obtain the drop channels having the wavelengths and A,2, fiber gratings 20 and 22 are provided between the port IOB of the optical circulator 10 and the optical isolator 14 in such a manner that the fiber grating 20 is located on the port 10B side and the fiber grating 22 is located on the optical isolator 14 side. That is, the fiber gratings 20 and 22 are cascaded. The fiber gratings 20 and 22 reflect light having the wavelengths 1, and A12, respectively, and transmit light having the wavelength A 20 ' To provide the add channels having the wavelengths A,, and A12, fiber gratings 24 and 26 are cascaded between the optical isolator 14 and the port 18A of the optical circulator 18 in such a manner that the fiber grating 24 is located on the optical isolator 14 side and the fiber grating 26 is located on the port 18A side. The fiber gratings 24 and 26 reflect light having the wavelengths A12 and A,,, respectively, and transmit light having the 13 wavelength A 20 The optical signals having Lhe wavelengths A,,, and A20 supplied to the port 2 are output from the port 10B of the optical circulator 10. The optical signal having the wavelength A,, is reflected by the fiber grating 20, and next returned to the port 10B. The optical signal having the wavelength A,2 is reflected by the fiber grating 22, and next returned to the port 10B. The optical signals having the wavelengths All and A,, returned to the port 10B are output as the drop light from the port loc.

The optical signal having the wavelength '12, transmitted by the fiber gratings 20 and 22 are passed through the optical isolator 14 and the fiber gratings 24 and 26 in this order, and next supplied to the port 18A of the Optical circulator 18. The add light having the wavelengths A,, and supplied from the port 8 to the port 18C is output from the port 18A. The add light having the wavelength /1,1 is reflected by the fiber grating 26, and next returned to the port 18A. The add light having the wavelength A12 is reflected by the fiber grating 24, and next returned to the port 18A. The optical signal having the wavelength A2. and the add light having the wavelengths All and A2 all supplied to the port 18A are output from the port 18B to the port 6. The 14 relation between the wavelengths of the three channels is as follows:

1111 < A 12 < A 20 The reason whv the wavelength of the channel of transmission from the port 2 to the port 6 is set longer than each of the wavelengths A, of the drop , and 'k 12 channels and the add channels is to reduce the loss during transmission of the optical signal having the wavelength /120 through the fiber gratings 20, 22, 24, and 26. Further, the reason why the wavelengths Of 'the drop channels are set as A,, < A12 is to reduce the loss when the drop light having the wavelength A,. to be reflected by the fiber grating 22 is transmitted forward and backward through the fiber grating 20. For the same reason, the wavelengths of the add channels are set so that the wavelength (A,,) of the add light to be reflected by the fiber grating 26 located nearer to the optical circulator 18 is shorter than the wavelength (A,2) of the add light to be reflected by the fiber grating 24 located farther from the optical circulator 18.

According to this preferred embodiment, in the case that each of the drop light and the add light is composed of a plurality of channels, the loss in the optical add/drop circuit can be reduced. Further, since the optical isolator 14 is interposed between the fiber gratings 22 and 24, the instability of the operation of the optical circuit due to resonance can be prevented.

Referring to FIG. 4, there is shown a block diagram of an optical circuit showing a third preferred embodiment of the present invention. In the optical circuit shown in FIG. 4, an optical add/drop circuit 28 similar to that shown in FIG. 3 is used. While the optical signal to be transmitted from the port 2 to the port 6 in the second preferred embodiment shown in FIG. 3 is of one channel (wavelength A20)1 optical signals of two channels having wavelengths A,, and A22 are transmitted from the port 2 to the port 6 in the fourth preferred embodiment shown in FIG. 4. In this preferred embodiment, optical signals of totally four channels having wavelengths All, A 12 1 " 2 11 and A,2 are supplied to the port 2, and the relation between the wavelengths 11,,, A121 1121, and A22 is expressed by the following inequality and shown in FIG. S.

A 11 < A 12 < A 21 < A 22 FIG. 4 shows a network configured by connecting terminals 30, 32, and 34 through optical fiber transmission lines. The terminal 30 outputs WDM optical signals of four channels; the terminal 32 receives WDM Optical signals of four channels; and the terminal 34 performs an exchange 16 between two drop channels and two add channels. The configuration will now be described more specifically.

The terminal 30 has optical transmitters 36 (#1, #2, T#3, and #4) for outputting optical signals having wavelengths 11,1, 1121 A 21f and A -92, respectively. These optical signals are wavelength-division multiplexed by an optical multiplexer (MUX) 38, and next output to an optical fiber transmission line 40. The optical fiber transmission line 40 connects the optical multiplexer 38 to the port 2 of the optical add/drop circuit 28. A plurality of optical amplifiers 42 are provided in the optical fiber transmission line 40.

For example, each optical amplifier used in the preferred embodiments has an optical amplifying medium and means for pumping the optical amplifying medium so that the optical amplifying medium has a gain band. As the optical amplifying medium, a doped fiber doped with a rare earth element such as Er (erbium) may be used. Pumping of the doped fiber may be performed by inputting pump light having a predetermined wavelength into the doped fiber. In the case that the wavelengths of the optical signals to be amplified fall in a 1.55,Um band and that the dopant in the doped fiber is Er, the wavelength of the pump light is set promisingly to a 0. 98gm band or 1.48,um 17 band.

Drop light output from the porL- 4 of the optical add/drop circuit 28 is sent through an optical fiber transmission line 44 to the terminal 34. A plurality of optical amplifiers 46 are provided in the optical fiber transmission line 44. The drop light sent by the optical fiber transmission line 44 is separated into two channels having the wavelengths A,, and A 12 by an optical demultiplexer (DEMUX) 48. The drop light of two channels is next separately supplied to optical receivers 50 (#1 and #2). The terminal 34 further includes optical transmitters 52 (#1 and #2) for outputting add light of two channels having the wavelengths and 2 The add light of two channels output from the optical transmitters S2 (#1 and #2) are wavelength-division multiplexed by an optical multiplexer 54, and next sent through an optical fiber transmission line 56 to the port 8 of the optical add/drop circuit 23. A plurality of optical amplifiers 58 are provided in the optical fiber transmission line 56.

The add light having the wavelengths and 12 and the optical signals having the wavelengths 21 and A..2 all output from the port 6 are sent through an optical fiber transmission line 60 to the terminal 32. A plurality 18 of optical amplifiers 62 are provided in the optical fiber transmission line 60. The terminal 32 has an optical demulti-plexer 64 connected to the optical fiber transmission line 60. The optical demultiplexer 64 supplies the add light having the wavelengths All and A,, respectively to optical receivers 66 (#1 and #2), and supplies the optical signals having the wavelengths A 21 and A 22 respectively to optical receivers 66 (#3 and #4).

According to the network shown in FIG. 4, for example, in the case that the terminals 30 and 32 are located on different continents and that the optical fiber transmission lines 40 and 60, the optical amplifiers 42 and 62, and the optical add/drop circuit 28 are laid on the bottom of a sea between the different continents, an exchange between the drop light of two channels and the add light of two channels can be performed at the terminal 34 located on an island. As described above, it is possible to realize the stability of the operation and the reduction in loss in the optical add/drop circuit 28.

Referring to FIG. 6, there is shown a block diagram of an optical drop circuit showing a fourth preferred embodiment of the present invention. This optical drop circuit employs the optical circulator 10 and the fiber grating 12 as a part of the first preferred 19 embodiment shown in FIG. 1. The fourth preferred embodiment shown in FIG. 6 is characterized in that the second end of the fiber grating 12 is terminated by a port 68. This characteristic allows the following operation.

Of the optical signals having the wavelengths and 'k2, supplied to the port 2, the optical signal having the wavelength A20 is output from the port 68, and the optical signal having the wavelength /110 is output as drop light from the port 4. The wavelength /120 of the optical signal to be transmitted by the fiber grating 12 is longer than the wavelength A,. of the optical signal (drop light) to be reflected by the fiber grating 12. Therefore, in the case that the fiber grating 12 has such a characteristic as shown in FIG. 2, the loss of the optical signal having the wavelength A., to be transmitted from the port 2 to the port 68 can be reduced.

Referring to FIG. 7, there is shown a block diagram of an optical drop circuit showing a fifth preferred embodiment of the present invention. This optical drop circuit employs the optical circulator 10 and the fiber gratings 20 and 22 as a part of the second preferred embodiment shown in FIG. 3. The fifth preferred embodiment shown in FIG. 7 is characterized in that the fiber grating 22 is terminated by a port 70.

The wavelength /1,0 of the optical signal to be transmitted by the fiber gratings 20 and 22 is longer than each of the wavelengths All and A,, of the optical signals (drop light) to be respectively reflected by the fiber gratings 20 and 22. Therefore, the loss of the optical signal having the wavelength A 2. to be transmitted from the port 2 to the port 70 can be reduced. Further, the wavelength All of the light to be reflected by the fiber grating 20 located nearer to the optical circulator 10 is set shorter than the wavelength /1.2 of the light to be reflected by the fiber grating 22 located farther from the optical circulator 10. Therefore, it is possible to reduce the loss of the optical signal (drop light) having the wavelength /1.2 Lo be reflected by the fiber grating 22 when transmitted forward and backward by the fiber grating 20.

Referring to FIG. 8, there is shown a block diagram of an optical add circuit showing a sixth preferred embodiment of the present invention. This optical add circuit employs the fiber grating 16 and the optical circulator 18 as a part of -the first preferred embodiment shown in FIG. 1. The sixth preferred embodiment shown in FIG. 8 is characterized in that the fiber grating 16 is 21 terminated by a port 72.

The optical signal having the wavelength A,O supplied to the port 72 is transmitted by the -fiber grating 16, next passed through the ports 18A and 18B in this order, and next supplied to the port 6. The add light having the wavelength A10 supplied to the port 8 is passed through the ports 18C and 18A in this order, and next supplied to the fiber grating 16. The add light having the wavelength /110 is reflected by the fiber grating 16, next passed through the ports 18A and 18B in this order, and next supplied to the port 6. The wavelength A., of the light to be transmitted by the fiber grating 16 is set longer than the wavelength Al. of the liglit to be reflected by the fiber grating 16. Therefore, the loss of the light to be transmitted from the port 72 to the port 6 can be reduced.

Referring to FIG. 9, there is shown a block diagram of an optical add circuit showing a seventh preferred embodiment of the present invention. This optical add circuit employs the optical circulator 18 and the fiber gratings 24 and 26 as a part of the second preferred embodiment shown in FIG. 3. The seventh preferred embodiment shown in FIG. 9 is characterized in that the fiber grating 24 is terminated by a port 74.

22 The optical signal having the wavelength A,, supplied to IC-he port 74 is transmitted by the fiber gratings 24 and 26, next passed through the ports 18A and 18B in this order, and next supplied to the port 6. The add light of two channels having the wavelengths A,, and A' 12 supplied to the port 8 is passed through the port 18C and next output from the port 18A. The add light having the wavelength A,, is reflected by the fiber grating 26 and next returned to the port 18A. The add light having the wavelength Al, is reflected by the fiber grating 24 and next returned to the port 18A. The add light having the wavelengths A,, and A, 2 returned to the port 18A is passed through the port 18B and next supplied to the port 6.

The wavelength /120 of the light to be transmitted by the fiber gratings 24 and 26 is set longer than each of the wavelengths Al, and /1,1 of the light to be respectively reflected by the fiber gratings 24 and 26. Therefore, the loss of the light having the wavelength /120 to be transmitted from the port 74 to the port 6 can be reduced. Further, the wavelength A ii of the light to be reflected by the fiber grating 26 located nearer to the optical circulator 18 is set shorter than the wavelength A. of the light to be reflected by the fiber grating 24 2 23 located farther from the optical circulator 18. Therefore, it is possible to reduce the loss of the light having the wavelength A,, to be reflected by the fiber grating 24 when transmitted forward and backward by the fiber grating 26.

Referring to FIG. 10, there is shown a network to which the present invention is applicable. This network is configured by connecting terminals 76, 78, and 80 through optical fiber transmission lines. To allow bidirectional transmission, an optical add/drop circuit 82 for bidirectional transmission is used. The optical add/drop circuit 82 hasports 84 and 86 operatively connected to the terminal 76, ports 88 and 90 operatively connected to the terminal 80, and ports 92 and 94 operatively connected to the terminal 78. In this network, four channels of wavelengths A If A 12 " A 2 1 f and A12 are defined. These channels satisfy the relation shown in FIG. S. In the following description, "four channels" mean the wavelengths /1,1, A121 A,,, and 1122, and "two channels" mean the wavelengths All and A, or the wavelengths A21 and /122. In particular, "two channels of shorter wavelengths" mean the wavelengths A,,I and 11,2, and "two channels of longer wavelengths" mean the wavelengths

A 2, and A...

24 In the Optical add/drop circu-it 82, optical signals of two channels of longer wavelengths of the four channels supplied to the port 84 are transmitted to the port 92, and optical signals of the remaining two channels of shorter wavelengths are output as drop light from the port 88. Add light of two channels of shorter wavelengths of the four channels supplied to the port 90 is output from the port 92. Further, in the optical add/drop circuit 82, optical signals of two channels of shorter wavelengths of the four channels supplied to the port 94 are transmitted to the port 86, and optical signals of the remaining two channels of longer wavelengths are output as drop light from the port 88. Add light of the remaining two channels of longer wavelengths supplied to the port 90 is output from the port 86.

The terminal 76 has optical transmitters 96 (#l, #2, #3, and #4) and an optical multiplexer 98 for wavelength-division multiplexing optical signals of four channels supplied from the optical transmitters 96 (#l, 142, 1"3, and #4). The WDM optical signals of ' Tr Lour channels are supplied through an optical fiber transmission line 100 to the port 84 of the optical add/drop circuit 82. A plurality of optical amplifiers 102 are provided in the optical fiber transmission line 100. The optical signals of two channels of longer wavelengths and the add light of two channels of shorter wavelengths all output from the port 92 are transmitted through an optical fiber transmission line 104 to the terminal 78. A plurality of optical amplifiers 106 are provided in the optical fiber transmission line 104.

The terminal 78 has an optical demultiplexer 108 for separating the light received from the optical fiber transmission line 104 into the optical signals of two channels of longer wavelengths and the add light of two channels of shorter wavelengths. The add light of two channels of shorter wavelengths is separately supplied to optical receivers 110 (#1 and rlr'2), and the optical signals of two channels of longer wavelengths are separately supplied to optical receivers 110 (#3 and #4). The terminal 78 further has optical transmitters 112 (#1, #2, #3, and #4) and an optical multiplexer 114 for wavelength-division multiplexing optical signals of four channels supplied from the optical transmitters 112 (#l, #2, #3, and #4).

The WDM optical signals of four channels are supplied through an optical fiber transmission line 116 to the port 94 of the optical add/drop circuit 82. A plurality of optical amplifiers 118 are provided in the 26 optical fiber transmission line 116. The optical signals of two channels of shorter wavelengths are transmitted from the port 94 to the port 86, and the optical signals of two channels of longer wavelengths are output as drop light from the port 88. The optical signals of two channels of shorter wavelengths and the add light of two channels of longer wavelengths all output from the port 86 are transmitted through an optical fiber transmission line 120 to the terminal 76. A plurality of optical amplifiers 122 are provided in the optical fiber transmission line 120.

The terminal 76 has an optical demultiplexer 124 for separating the light of four channels received from the optical fiber transmission line 120 into the optical signals of two channels of shorter wavelengths and the add light of two channels of longer wavelengths. The optical signals of two channels of shorter wavelengths are separately supplied to optical receivers 126 (#1 and #2), and the add light of two channels of longer wavelengths is separately supplied to optical receivers (#3 and #4).

The drop light of four channels output from the port 88 is transmitted through an optical fiber transmission line 128 to the terminal 80. A plurality of optical amplifiers 130 are provided in the optical fiber 27 transmission line 128. The drop light of four channels input into the terminal 80 is separated by an optical demultiplexer 132, and next separately supplied to optical receivers 134 (#1, #2, #3, and 1#4). The terminal 80 has optical transmitters 136 (#1, 412, #3, and #4) for outputting add light of four channels. The add light of four channels is wavelength-division multiplexed by an optical multiplexer 138, and next supplied through an optical fiber transmission line 140 to the port 90 of the optical add/drop circuit 82. A plurality of optical amplifiers 142 are provided in the optical fiber transmission line 140. Of the add light of four channels supplied to the port 90, the add light of two channels of shorter wavelengths is output from the port 92, and the add light of two channels of longer wavelengths is output from the port 86.

Referring to FIG. 11, there is shown a block diagram of an optical add/drop circuit showing an eighth preferred embodiment of the present invention. This optical add/drop circuit may be used as the optical add/drop circuit 82 shown in FIG. 10. To allow add/drop in bidirectional transmission, six optical circulators 144, 146, 148, 150, 152, and 154 are used.

A port 144A of the optical circulator 144 is 28 Y.W connected to the port 84; a port 144B of the optical circulator 144 is connected to a first end of a -fiber grating 156; and a port 144C of the optical circulator 144 is connected to a port 148A of the optical circulator 148. A second end of the fiber grating 156 is connected to a first end of a fiber grating 158. A second end of the fiber grating 158 is connected to an input port of an optical isolator 160. An output port of the optical isolator 160 is connected to a first end of a fiber grating 162. A second end of the fiber grating 162 is connected to a first end of a fiber grating 164. k -iber grating 164 is connected to a second end of the f port 146A of the optical circulator 146. A port 146B of the optical circulator 146 is connected to the port 92, and a port 146C of the optical circulator 146 is connected to a port 150A of the optical circulator 150. A port 1503 of the optical circulator 150 is connected to a first end of a fiber grating 166, and a port 150C of the optical circulator 150 is connected to the port 90. A second end of the fiber grating 166 is connected to a first end of a fiber grating 168. A second end of the fiber grating 168 is connected to a port 152A of the optical circulator 152. A port 152B of the optical circulator 152 is connected to the pOr-L 86, and a port 29 152C of the optical circulator 152 is connected to a first end of a fiber grating 170. A second end of the fiber grating 170 is connected to a first end of a fiber grating 172, and a second end of the fiber grating 172 is connected to an output port of an optical isolator 174. An input port of the optical isolator 174 is connected to a first end of a fiber grating 176, and a second end of the fiber grating 176 is connected to a first end of a fiber grating 178. A second end of the fiber grating 178 is connected to a port 154A of the optical circulator 154. A port 154B of the optical circulator 154 is connected to a first end of a fiber grating 180, and a port 154C of the optical circulator 154 is connected to the port 94. A second end of the fiber grating 180 is connected to a first end of a fiber grating 182, and a second end of the fiber grating 182 is connected to a port 148B of the optical circulator 148. A port 148C of the optical circulator 148 is connected to the port 88.

The Bragg reflection wavelength of each of the fiber gratings 156, 164, 166, and 182 is /111; the Bragg reflection wavelength of each of the fiber gratings 158, 162, 168, and 180 is A,2; the Bragg reflection wavelength of each of the fiber gratings 170 and 178 is ' 2,; and the Bragg reflection wavelength of each of the fiber gratings 172 and 176 is 11.12.

The operation of this optical add/drop circuit will now be described by presenting the following path of light of each channel supplied to the ports 84, 90, and 94.

(1) The propagation path of an optical signal having the wavelength A., supplied to the port 84 flows in fiber the order of the port 84, port 144A, port 144B, Igrating 156, port 144B, port 144C, port 148A, port 148B, fiber grating 182, port 148B, port 148C, and port 88.

(2) The propagation path of an optical signal having the wavelength A 12 supplied to the port 84 flows in the order of the port 84, port 144A, port 144B, fiber grating 156, fiber grating 158, fiber grating 156, port 144B, port 144C, port 148A, fiber grating 182, fiber grating 180, fiber grating 182, port 148B, port 148C, and port 88.

(3) The propagation path of an optical signal having the wavelength A,, supplied to the port 84 flows in the order of the port 84, port 144A, port 144B, fiber grating 156, fiber grating 158, optical isolator 160, fiber grating 162, fiber grating 164, port 146A, port 146B, and port 92.

(4) The propagation path of an optical signal having the wavelength A, supplied to the port 84 is the 31 same as the above path (3).

(5) The propagation path of add light having the wavelength Ali supplied to the port 90 flows in the order of the port 90, port ISOC, port 150B, fiber grating 166, port 150B, port 150A, port 146C, port 146A, fiber grating 164, port 146A, port 146B, and port 92.

(6) The propagation path of add light having the wavelength /1,, supplied to the port 90 flows in the order of the port 90, port 150C, port 150B, fiber grating 166, fiber grating 168, fiber grating 166, port 150B, port 150A, port 146C, port 146A, fiber grating 164, fiber grating 162, fiber grating 164, port 146A, port 146B, and port 92.

(7) The propagation path of add light having the wavelength /12, supplied to the port 90 flows in the order of the port 90, port ISOC, port 150B, fiber grating 166, fiber grating 168, port 152A, port 152C, fiber grating 170, port 152C, port 152B, and port 86.

(8) The propagation path of add light having the wavelength A.2 supplied to the port 90 flows in the order of the port 90, port 150C, port 150B, fiber grating 166, fiber grating 168, port 152A, port 152C, fiber grating 170, fiber grating 172, fiber grating 170, port 152C, port 152B, and port 86.

(9) The propagation path of an optical signal having the wavelength A,, supplied to the port 94 flows in fiber the order of the Dort 94, port 154C, port 154A, I grating 178, fiber grating 176, optical isolator 174, fiber grating 172, fiber grating 170, port 152C, port 152B, and port 86.

(10) The propagation path of an optical signal having the wavelength /1,2 supplied to the port 94 is the same as the above path (9).

(11) The propagation path of an optical signal having the wavelength A2. supplied to the port 94 flows in the order of the port 94, port 154C, port 154A, fiber grating 178, port IS4A, port 154B, fiber grating 180, fiber grating 182, port 148B, port 148C, and port 88.

(12) The propagation path of an optical signal having the wavelength A,2 supplied to the port 94 flows in the order of the port 94, port 154C, port 154A, fiber grating 178, fiber grating 176, fiber grating 178, port 154A, port 154B, fiber grating 180, fiber grating 182, port 148B, port 148C, and port 88.

In this preferred embodiment, the optical isolators 160 and 174 are provided at proper positions in accordance with the present invention. Accordingly, there is no possibility of instability oJL the operation of the optical add/drop circuit due to resonance. In this 33 preferred embodiment, however, the light of two channels of longer wavelengths is passed through the fiber gratings 178, 176, 172, and 170 in this order. Accordingly, there is a possibility that the loss of the light of two channels of longer wavelengths to be transmitted from the port 94 to the port 86 may be increased. An improved embodiment to avoid this possibility will now be described.

Referring to FIG. 12, there is shown a block diagram of an optical add/drop circuit showing a ninth preferred embodiment of the present invention. This optical add/drop circuit may be used as the optical add/drop circuit 82 shown in FIG. 10. In this p-referred embodiment, optical circulators 152' and 1S4' are used in place of the optical circulators 152 and 154 shown in FIG. 11, respectively. The direction of circulation of light in the optical circulator 1521 is opposite to that in the optical circulator 152, and the direction of circulation of light in the optical circulator 154' is opposite to that in the optical circulator 154. This change accompanies the following changes.

The second end of the fiber grating 168 is connected to an input port of an optical isolator 184, and an output port of the optical isolatcr 184 is connected to 34 a first end of a fiber grating 186. A second end of the fiber grating 186 is connected to a first end of a fiber grating 188. A second end of the fiber grating 188 is connected to a port lS2A' of the optical circulator 1S21. A port 152B' of the optical circulator 152' is connected to the port 86, and a port 152C' of the optical circulator 1521 is connected to a port 154A' of the optical circulator 1541. A port 154B' of the optical circulator 1541 is connected to a first end of a fiber grating 190, and a port 154C' of the optical circulator 1S4' is connected to the port 94. A second end of the fiber grating 190 is connected to a first end of a fiber grating 192, and a second end of the f -iber grating 192 is connected to an input port of an optical isolator 194. An output port of the optical isolator 194 is connected to the first end of the fiber grating 180.

The Bragg reflection wavelength of each of the fiber gratings 188 and 190 is A,,. and the Bragg reflection wavelength of each of the fiber gratings 186 and 192 is 12 The operation of the optical add/drop circuit shown in FIG. 12 will now be described by presenting a difference in propagation path of light of each channel from the eighth preferred embodiment shown in FIG. 11.

While the propagation paths (1) to (6) described with reference to FIG. 11 are applied to FIG. 12 without any changes, the propagation paths (7) to (12) are changed to the following paths (7') to (121), respectively.

(7') The propagation path of add light having flows in the the wavelength A,, supplied to the port 90 1 order of the port 90, port ISOC, port 150B, fiber grating 166, fiber grating 168, optical isolator 184, fiber grating 186, fiber grating 188, port 152A', port 152B', and port 86.

(8') The propagation path of add light having the wavelength /1,, supplied to the port 90 is the same as the above path (7').

(9') The propagation path of an optical signal having the wavelength Al, supplied to the port 94 flows in the order of the port 94, port 154C', port 154B', fiber grating 190, port 154B', port 154A', port 152C', port 152A', fiber grating 188, port 152A', port 152B', and port 86.

(10') The propagation path of an optical signal having the wavelength A. 12 supplied to the port 94 flows in the order of the port 94, port 154C', port 154B1, fiber grating 190, fiber grating 192, fiber grating 190, port 154B', port 154A', port 152C', port 152A1, fiber grating 36 188, fiber grating 186, fiber grating 188, port 152A', port 152BI, and port 86.

(11') The propagation path ofL an optical signal having the wavelength /12, supplied to the port 94 flows in the order of the port 94, port IS4C', port 154B', fiber grating 190, fiber grating 192, optical isolator 194, fiber grating 180, fiber grating 182, port 148B, port 148C, and port 88.

(12') The propagation path of an optical signal having the wavelength /122 supplied to the port 94 is the same as the above path (11').

In this preferred embodiment, the optical isolators 160, 184, and 194 are provided at proper positions in accordance with the present invention, there is no possibility of instability of the operation of the optical add/drop circuit due to resonance. Further, the wavelength of the light to be transmitted by each fiber grating is set longer than the Bragg reflection wavelength. Therefore, the loss can be reduced.

As described above, according to an aspect of the present invention, it is possible to provide an optical add/drop circuit which can eliminate the possibility of instability of the operation due to resonance or the like. According to another aspect of the present 37 invention, i- is possible to provide ari optical- add/drop circuit which. can reduce the loss.

The present invention is not Iiimited to the details off the above described preferred embodi-ments.

38

Claims

WHXT IS CLAIMED IS:

1. An optical circuit comprising: a first optical circulator having first, second, and third ports; a first fiber grating operatively connected to said second port; a second optical circulator having fourth, fifth, and sixth ports; a second fiber grating operatively connected to said fourth port; and an optical isolator operatively connected between said first and second fiber gratings. 2. An optical circuit according to claim 1, wherein said first and second fiber gratings reflect light having a first wavelength. 3. An optical circuit according to claim 2, wherein: said first port is supplied with first input light having said first wavelength and second input light having a second wavelength different from said first wavelength; said third port Outputs said first input light reflected by said first fiber grating as drop light; said second input light is output from said second port, next passed through said first fiber grating, said 39 optical isolator, and said second fiber grating in this order, and next supplied to said fourth port; said sixth port is supplied with add light having said first wavelength; and said fifth port outputs said add light reflected by said second fiber grating and said second input light supplied to said fourth port. 4. An optical circuit according to claim 3, wherein said second wavelength is longer than said first wavelength. 5. An optical circuit according to claim 3, wherein: said first input light comprises a plurality of input channels; said plurality of input channels have different wavelengths; said first fiber grating comprises a plurality of first fiber gratings corresponding to said plurality of input channels; said add light comprises a plurality of add channels; said plurality of add channels have different wavelengths; and said second fiber grating comprises a plurality of second fiber gratings corresponding to said plurality of add channels. 6. An optical circuit according to claim 5, wherein: said plurality of first fiber gratings are cascaded between said first optical circulator and said optical isolator; said different wavelengths of said input channels of said first input light to be reflected by said plurality of first fiber gratings are in increasing order from said first optical circulator toward said optical isolator; said plurality of second fiber gratings are cascaded between said second optical circulator and said optical isolator; and said different wavelengths of said add channels of said add light to be reflected by said plurality of second fiber gratings are in increasing order from said second optical circulator toward said optical isolator. 7. An optical circuit according to claim 3, wherein said second input light comprises a plurality of input channels. 8. An optical circuit according to claim 3, further comprising: a first optical transmitter operatively connected LO said first port, for outputting said first input light and said second input light; 41 a first optical receiver operatively connected to said third port, for receiving said drop light; a second optical transmitter operatively connected to said sixth port, for outputting said add light; and a second optical receiver operatively connected to said fifth port, for receiving said second input light and said add light.

9. An optical circuit comprising:

an optical circulator having first, second, and third ports; and a fiber grating operatively connected to said second port, for reflecting light having a first wavelength, said fiber grating transmitting light having a wavelength longer than said first wavelength.

10. Axi optical circuit according to claim 9, wherein said light having said first wavelength is passed through said first and second ports in this order, and next supplied to said fiber grating.

11. An optical circuit according to claim 10, wherein:

said first port is supplied with first input light having said first wavelength and second input light having a second wavelength longer than said first wavelength; said third port outputs said first input light 42 reflected by said fiber grating as drop light; and said second input light is passed through said fiber grating. 12. An optical circuit according to claim 11, wherein said second input light comprises a plurality of input channels. 13. An optical circuit according to claim 11, wherein: said first input light comprises a plurality of input channels; said plurality of input channels have different wavelengths; and said fiber grating comprises a plurality of fiber gratings corresponding to said plurality Of input channels. 14. An optical circuit according to claim 13, wherein: said plurality of fiber gratings are cascaded; and said different wavelengths of said input channels of said first input light to be reflected by said plurality of fiber gratings are in increasing order from said optical circulator. is. An optical circuit according to claim 11, further comprising: an optical transmitter operatively connected to said first port, for Outputting said first input light and said second input light; 43 a first optical receiver operatively connected to said fiber grating, for receiving said second input light; and a second optical receiver operatively connected to said third port, for receiving said drop light.

16. An optical circuit according to claim 10, wherein:

said first port is supplied with add light having said first wavelength; said fiber granting is supplied with input light having a wavelength longer than said first wavelength; and said third port outputs said add light reflected by said fiber grating and said input 1-ight passed through said fiber grating.

17. An optical circuit according to claim 16, wherein said input light comprises a plurality of input channels.

18. An optical circuit according to claim 16, wherein:

said add light comprises a plurality of add channels; said plurality of add channels have different wavelengths; and said fiber grating comprises a plurality of fiber gratings corresponding to said plurality of add channels.

19. An optical circuit according to claim 18, wherein:

44 said plurality of fiber gratings are cascaded; and 4erent wavelengths of said add channels of said dill said add light to be reflected by said plurality of fiber gratings are in increasing order from said optical circulator.

20. An optical circuit according to claim 16, further comprising:

first ontical transmitter operatively connected to said first port, for outputting said add light; second optical transmitter operatively connected grating, for outputting said input light; and to said fL an optical receiver operatively connected to said third port, for receiving said input light and said add light.

21. An optical circuit substantially as hereinbefore described with reference to the accompanying drawings.