Source: http://www.google.com/patents/US5684899?dq=6650327
Timestamp: 2017-05-24 19:01:17
Document Index: 685190069

Matched Legal Cases: ['art 236', 'art 301', 'art 324', 'Application No. 07', 'Application No. 07', 'Application No. 07', 'Application No. 07', 'Application No. 07', 'Application No. 07']

Patent US5684899 - Optical communication network - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inPatentsIn an optical communication network in which a plural number of nodes are connected to each bidirectional broadcasting bus, and a node communicates with another using the packets, or an optical communication network in which a plural number of nodes are connected to a bidirectional broadcasting bus,...http://www.google.com/patents/US5684899?utm_source=gb-gplus-sharePatent US5684899 - Optical communication networkAdvanced Patent SearchTry the new Google Patents, with machine-classified Google Scholar results, and Japanese and South Korean patents.Publication numberUS5684899 APublication typeGrantApplication numberUS 08/766,931Publication dateNov 4, 1997Filing dateDec 16, 1996Priority dateMar 5, 1992Fee statusPaidAlso published asUS5854700, US5915054Publication number08766931, 766931, US 5684899 A, US 5684899A, US-A-5684899, US5684899 A, US5684899AInventorsTakeshi OtaOriginal AssigneeFuji Xerox Co., Ltd.Export CitationBiBTeX, EndNote, RefManPatent Citations (30), Non-Patent Citations (38), Referenced by (27), Classifications (20), Legal Events (3) External Links: USPTO, USPTO Assignment, EspacenetOptical communication network
US 5684899 AAbstract
1. A wavelength multiplexing transceiver for wavelength multiplexing in an optical communication network comprising:first and second slab waveguides; a first optical waveguide for supplying an output light from said first slab waveguide to an input/output optical fiber; a second optical waveguide for supplying an incident light from said input/output optical fiber to said second slab waveguide; a optical coupler for branching an optical path of said input/output optical fiber to said first and second optical waveguides; a laser array optically coupled to one end of said first slab waveguide, to the other end of which said optical coupler is coupled; and a light receiving element array optically couple to one end of said second slab waveguide, to the other end of which said optical coupler is coupled, said first and second slab waveguides, first optical waveguide, second optical waveguide, and optical coupler being commonly formed on a substrate. 2. A wavelength multiplexing transceiver comprising:a first group of optical waveguides for coupling a laser array and a first slab waveguide type wavelength multiplexer; a first optical coupler for coupling said first slab waveguide type wavelength multiplexer, an output waveguide, and at least one of said first group of optical waveguides; a second optical coupler for coupling said output waveguide, an input waveguide, and an input/output optical fiber, type wavelength multiplexer; and a light receiving element array optically coupled to said second slab waveguide type wavelength multiplexer. 3. The wavelength multiplexing transceiver according to claim 2 wherein said first and second slab waveguide type wavelength multiplexers and said first and second optical couplers are formed commonly on a substrate.
4. The wavelength multiplexing transceiver according to claim 3 wherein said substrate comprises first and second thin layers in which optical waveguides connected to different waveguide type wavelength multiplexers are respectively formed.
5. The wavelength multiplexing transceiver according to claim 4 wherein said laser array and light receiving element array are integrally formed on said substrate.
6. The wavelength multiplexing transceiver according to claim 2 wherein said first and second slab waveguide type wavelength multiplexers are arranged in directions opposite to each other.
7. The wavelength multiplexing transceiver according to claim 2, wherein at least one optical amplifier is provided to the optical path between said second optical coupler and said light receiving element array.
8. The wavelength multiplexing transceiver according to claim 2, further comprising a reflecting device coupled to an end of said first slab waveguide opposite said first group of optical waveguides.
9. The wavelength multiplexing transceiver according to claim 8, wherein said reflecting device is a Fresnel reflecting mirror.
10. The wavelength multiplexing transceiver according to claim 8, wherein said reflecting device includes concave gratings.
11. The wavelength multiplexing transceiver according to claim 10, wherein said concave gratings are saw-tooth gratings arranged in a concave shape.
12. The wavelength multiplexing transceiver according to claim 2, further comprising a reflecting device coupled to an end of said second slab waveguide opposite said input waveguide.
13. The wavelength multiplexing transceiver according to claim 10, wherein said reflecting device is a Fresnel reflecting mirror.
14. The wavelength multiplexing transceiver according to claim 10, wherein said reflecting device includes concave gratings.
15. The wavelength multiplexing transceiver according to claim 12, wherein said concave gratings are saw-tooth gratings arranged in a concave shape.
16. The wavelength multiplexing transceiver according to claim 4, wherein said input/output optical fiber is multimode, and said first and said second thin layers are plastic thin films.
17. The wavelength multiplexing transceiver according to claim 3, wherein said substrate is a semiconductor material.
18. The wavelength multiplexing transceiver according to claim 6, further comprising a first reflecting device coupled to an end of said first slab waveguide and a second reflecting device coupled to an end of said second slab waveguide opposite said first reflecting device.
19. The wavelength multiplexing transceiver according to claim 3, wherein a cross-section of said substrate is shaped like a trapezoid.
This application is a continuation, of application Ser. No. 08/459,327, filed Jun. 2, 1995, now abandoned, which was a divisional of application Ser. No. 08/026,054, filed Mar. 4,1993, now abandoned.
In a general LAN, a plural number of nodes are connected to a bus. The nodes communicates with one another through the bus. One of the buses used in the LAB is a broadcasting bus. When using the broadcasting bus, a signal sent by a node can be simultaneously received by all of the nodes. ETHERNET (Trade Mark) is well known as one of the LAN using the broadcasting bus. The protocol used in ETHERNET is called a CSMA/CD (carrier sense multiple access/collision detection) system, prescribed in IEEE 803.3. In ETHERNET, coaxial cables are used for transmission medium. The nodes are connected by the coaxial cables. A node which will send a signal checks whether or not a signal from another node is present on the coaxial cable. If it is not present, the sending node starts the signal transmission. Actually, there is the possibility that two nodes simultaneously send signals. This state is called a collision. In ETHERNET, the collision is detected in the form of a voltage level in the coaxial cable.
After detecting the collision, the node sends a jamming signal for a preset time, and then is set to an random-time stand-by mode. The Jamming signal must be set to be longer than the maximum Maximum round trip time of the network, in order to broadcast the collision to all of the nodes connected to the network. The random-time stand-by mode is provided in order to avoid such a situation that a plural number of nodes fail to send, and if those nodes simultaneously start to send signals immediately after a communication channel becomes idle, the collision occurs again.
The optical communication system has gradually been used also in the LAN. In the LAN using the optical fiber as the transmission media, the number of nodes cannot be increased by simply increasing the number of taps, although it can be increased so in the LAN using the coaxial cable as transmission media.
To solve the problem, there is a proposal of a new optical communication network in which each node is provided with two separate ports, one for transmission and the other for reception, and the nodes are coupled through a star coupler. For the proposal, reference is made to E. G. RAWSON ET AL., "Fibernet: Multimode Optical Fibers for Local Computer Networks", IEEE Transaction on Communications, Vol., COM-26, No. 7, p 395 (1978).
The optical communication network using the star coupler is schematically illustrated in FIG. 1. In the figure, reference numerals 26a and 26b designate optical fibers 27, nodes 25, a star coupler of the mixing rod type; and 24, terminals. A signal output from each node 27 is converted into a light signal by a light emitting element 22 of the node. The light signal is supplied through the optical fiber 26a associated therewith to the star coupler 25. The light signals transmitted from the nodes are all mixed by the star coupler 25, and then are distributed to the light sensing elements 23 of the nodes, through the related optical fibers 26b. The light signal is converted into an electrical signal by the related light sensing element, and supplied to the node 27. In the communication network thus arranged, a signal transmitted from one node is transmitted to all of the nodes, viz., the network has a broadcasting function. Accordingly, the communication network, which is similar to ETHERNET, can be constructed.
In the proposal, as the number of nodes coupled with the star coupler is increased, the level of a receiving signal is decreased in each node. One of the possible ways to solve the level down problem is to extend the network by additionally using star couplers and relay amplifiers. This approach suffers from another problems, however. The star coupler, when receiving a signal from a node, sends it also to the receiving port of the same node. Accordingly, a feedback loop is formed between the star couplers interconnected. If a relay amplifier is located between the star couplers, an oscillation occurs. When the star coupler is used, the number of nodes that can be connected to the network is limited to the number of terminals of one star coupler.
To cope with the problem, a CRV (code rule violation) method was proposed for a collision detecting system, which is to be applied for the network using the passive star coupler as shown in FIG. 2. The CRV was discussed by Oguchi et al., in their paper "Study on Arranging Collision Detecting Circuits for Optical Star Networks", The Institution of Electronics and Communication Engineers, Optics/Radio Section, National Convention Record 341, 1982.
Let us consider a case where a collision signal as shown in FIG. 2(c) collides with a transmission signal as shown in FIG. 2(b). As the result of the collision, an intensity distribution of the receiving signal takes a profile as shown in FIG. 2(d). The receiving signal, when demodulated, has a bit pattern as shown in FIG. 2(e). The H level state of which the duration exceeds one period of the reference clock signal is found in the demodulated signal. Thus, when a code (code rule violation code), which should not exist, is detected, a CRV signal as shown in FIG. 2(c) is generated. The collision signal shown in FIG. 2(c) represent phase-shifted Manchester codes. Since the nodes are not synchronized, phases where the Manchester codes are added are indefinite.
Further, in the proposed network, a node can receive a signal from another node even if it is sending a signal. Thus, the node can concurrently perform the transmitting and receiving operations. In other words, the optical communication network in the Patent Application serves as a bidirectional bus.
For the operation of the multichannel LAN, the CSMA/CD system is theoretically discussed (by Ikebata and Okada "Multi-Channel CSMA/CD with Hybrid Load Distribution/Region Distribution Scheme", Trans. of IECE (in Japanese) (B), Vol., J70-B, No. 12, pp 1466-1474 (1987)).
When Λ=1000 calls/sec. (=103 calls/sec.), the probability Pτ that one call is observed during τ=50 sec is
P&#964;(3)=1.98×10-5.
The probability P τ that no call is observed during τ=50 μsec. is
Where τ is the Maximum round trip time, if one call is observed during the time period τ=50 sec, then no collision occurs. If two calls are observed, 2-node collision occurs. If three calls are observed, 3-node collisions occurs. Pτ(0) indicates that when the broadcasting bus is monitored during τ =50 sec, no call occurs, viz., the line is left idle.
ti Pτ(3)/Pτ(2)=1.60×10-2 =1.6%,
if Λ=1000 calls/sec and τ=50 sec. The probability comparison indicates that in most cases, 2-node collision occurs, and in rare case, 3-node collision occurs. It is known that as the average call-occurrence frequency Λ per unit time, the percentage of the 3-node collisions becomes larger. In the case of large Λ, e.g., Λ=104 calls/sec, Pτ(3)/Pτ(2)=about 20%. In the graph of FIG. 3, the abscissa represents Λ (call/sec) and the ordinate, Pτ(3)/Pτ(2).
The experimental results show that the average call-occurrence frequency Λ in ETHERNET is at most 30 calls/sec. It is also known that the peak occurrence of calls is 50 to 60 times as large as the average value per day. For this, reference is made to J. F. Snoch and J. A. Hupp, "Measured performance of an Ethernet Local Computer Network", Communications of A.C., Vol. 23, No. 12, pp 711 to 729 (1980). Accordingly, it is seen that λ=1000 calls/sec is approximately the instantaneous maximum value of the call occurrence frequency.
To achieve the above object, there is provided an optical communication network in which a plural number of nodes are connected to a bidirectional broadcasting bus, and a node communicates with another using the packets, wherein each node comprises: carrier detecting means for detecting a carrier on the broadcasting bus; and jamming detecting means for detecting a jamming state of received signals.
In the optical communication network thus arranged, in a case where when a node will send a signal, a carrier is detected on the broadcasting bus, if the node starts to send a signal in disregard of presence of the carrier, the signal from the node will interfere with a signal sent from another node. The jamming state of the received signal indicates that the collision of two or more nodes has occurred on the broadcasting bus. Under this condition, if the signal transmission starts, the collision of three or more nodes will occur. Thus, when a collision occurs in the network, each node can discriminate the type of the collision, the 2-node collision or the multiple-node collision, by detecting both the carrier and the jamming state of the received signal.
The communication system of the invention, when a collision occurs, discriminates the type of collision, the 2-node collision and the multiple-node collision. When the collision is of the 2-node type, which of the priority levels of the colliding nodes to use the broadcasting bus is higher is determined by using the priority codes previously assigned to the packets. With this feature, the communication system can more efficiently use the communication channels than the conventional communication system of the type in which the nodes randomly competes for seizing the lines.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrated presently preferred embodiments of the invention and, together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the objects, advantages and principles of the present invention. In the accompanying drawings,
FIGS. 16 and 16(a) are diagrams showing a first embodiment of a wavelength multiplexing transceiver for the wavelength multiplexing in an optical communication network;
FIGS. 17(a) and 17(b) are a plan view and a side view showing a second embodiment of a wavelength multiplexing transceiver for the wavelength multiplexing in an optical communication network; FIG. 17(c) is an enlarged view of gratings in FIG. 17(a)
FIGS. 20(a) through 20(c) are a diagram showing a fourth embodiment of a wavelength multiplexing transceiver according to the present invention;
FIGS. 25 and 25(a) are diagrams showing a first embodiment of an interconnectable 5-port star coupler according to the present invention;
FIGS. 27 and 27(a) are diagrams showing an incident angle ω of light from one waveguide to another;
FIGS. 30(a) and 30(b) are diagrams each showing an optical waveguide having intersecting portions;
FIGS. 38 and 38(a) are plan view showing an embodiment of an interconnectable star coupler according to the present invention;
FIGS. 39-39(b) are plan view showing another embodiment of an interconnectable star coupler according to the present invention;
FIGS. 40(a) to 40(c) are diagrams showing an enlarged Evernescent optical coupler; and
FIGS. 41 and 41(a) are diagrams showing an example of an interconnectable start coupler with four ports constructed using a 1×3 photocoupler.
As shown, a plural number of nodes 1 are interconnected through star couplers 2. In the communication network of FIG. 4, four star couplers 2 each with eight (8) terminals. Those couplers may be connected to one another. The star couplers 2 are interconnected through bidirectional optical amplifiers 3, thereby forming the network. Each node 1 is connected through a related optical fiber 4 to the related star coupler 2. The packet is used for the communication between the nodes 1. The optical amplifier 3 may be a semiconductor laser amplifier.
The first type of node shown in FIG. 5 will first be described. As shown in FIG. 5, the node 1 comprises a node body 5 and a communication interface 6. The communication interface 6 includes a light sensing element 9, such as a photo diode, and a light emitting element 8, such as a laser diode, and a optical coupler 7 for multiplexing and demultiplexing light signals of different wavelengths. The optical fiber 4 derived from the multiplexing/demultiplexing device 7 is connected to one of the terminals 14 of the star coupler 2 shown in FIG. 1. A transmission port 10 is a hardware for signal transmission, and a reception port 11 is a hardware for signal reception. The reception port 11 is coupled with a carrier sensor 12 and a code rule violation sensor 13. When sensing a carrier and a code rule violation, the sensors transfer signals representative of the carrier and the code rule violation to the node body 5, respectively. Thus, the node body 5 and the communication interface 6 are interconnected through four ways for signal flow.
The construction of an interconnectable star coupler with eight (8) terminals, which is used in the communication network of FIG. 4, is illustrated in FIG. 6. The star coupler illustrated may be connected to one or more other star couplers. A substrate 18 is made of glass or polycarbonate. Disposed on the substrate 18 are optical couplers 15 each having a branching ratio of 7:1, optical couplers 16 each having a branching ratio of 2:1, and a optical coupler 17 having a branching ratio of 1:1. Those devices being connected by optical wave guides, thereby forming an integrated optical circuit. In the integrated optical circuit thus formed, desired distribution ratios of light powers are obtained. The multiplexing/demultiplexing device 15 having a branching ratio of 7:1 functions to distribute the light power of 7 to the multiplexing/demultiplexing device 16, and distributes the light power of 1 to other multiplexing/demultiplexing devices. The multiplexing/demultiplexing device 16 having a branching ratio of 2:1 distributes the light power of 2 to the multiplexing/demultiplexing device 17, and the light power of 1 to the multiplexing/demultiplexing device 16. Eight number of optical fibers 4 are connected to the substrate 18. In FIG. 6, Rc represents the radius of curvature of the optical waveguide. For more details of the interconnectable star coupler, reference is made to the specification of the co-pending U.S. patent application Ser. No. 07/813,443, filed by the Applicant of the present patent application.
In the communication network thus constructed, when only one node sends a signal, all other nodes connected to the network can receive the signal. When two nodes simultaneously send signals, one of the sending nodes can correctly receive the signal from the other, and vice versa. The other nodes than the two sending nodes can receive the signals as a jamming signal. In other words, the communication network has the secure function. This arises from the fact that the network is bidirectional. When three or more nodes simultaneously send signals, any node can receive a jamming signal alone. Thus, in the communication network, the secret of communication contents can be observed in a state that when two nodes send signals, one of them sends a signal to the other and vice versa. The details of this is described in Published Unexamined Japanese Patent Application No. Hei. 3-270432.
The construction of another type of node, that may be used for the optical communication network of FIG. 4, will be described. When the nodes to be discussed are applied for the FIG. 4 network, an optical communication network of the multichannel type, which is based on the wavelength multiplexing, can be realized.
As shown, the node includes a node body 5, four communication interfaces 6a to 6c, each having the same construction as the communication interface 6 in FIG. 2, and a wavelength multiplexer 19. In the communication interfaces 6a to 6c, laser diodes as light emitting elements emit light signals of different wavelengths λa to λd. The wavelength multiplexer 19 multiplexes those light signals of the wavelengths λa to λd.
The bidirectional, optical amplifiers 3 in the optical communication network shown in FIG. 4, as recalled, are semiconductor laser amplifiers. In each amplifier, the range of wavelengths that can be amplified is broad, 50 to 70 nm. If the wavelengths λa to λd are selected to be different by 10 nm from one another, one semiconductor laser amplifier is capable of amplifying all of the light signals of the wavelengths λa to λd.
Some specific examples of communication methods, or protocols, according to the present invention will be described. FIG. 8 is a diagram showing a status transition of a sending node in a first protocol according to the invention. The protocol of FIG. 8, applied for a single transmission channel (broadcasting bus), is designed to avoid the 2-node collision. Variables in FIG. 8 have the following meanings. A variable CS represents presence or absence of a carrier sense. When CS is "1", the carrier is present. When it is "0", no carrier is present. A variable CRV indicates occurrence of the code rule violation. When CRY is "1", the CRV occurs. When it is "0", no CRV occurs. Pi is a priority level of the packet sent by a sending node in the form of a variable. Po is a priority level of the packet sent by a responding node in the form of a variable. Signs ">", "<", and "=" indicate "higher" "lower", and "equal" in the priority level. For example, if Pi >P0, the sending node is higher in priority level than the responding node.
When receiving a send request from the higher layer protocol, the node senses a carrier on the transmission channel. When the carrier is sensed (CS=1), the node is placed to the random-time stand-by mode, and then senses a carrier on the transmission channel since presence of the carrier indicates that another node uses the transmission channel. If the carrier is not sensed (CS=0), the node starts to send a signal. When the header of the packet has been sent, the node senses the carrier again while continuing the signal transmission. The packet format is shown in FIG. 9. The carrier sensing operation continues till the signal transmission has normally ended. If carrier is not sensed before the transmission ends (CS=0), the signal transmission has normally ended. If any carrier is sensed before the transmission ends (CS=1), a collision has occurred.
Upon sensing of the carrier (CS=1), the sending node switches the contents of the sending signal to a jamming signal and fetches the sensed signal. The fetched signal contains the header of a packet that is sent from another node.
The sending node compares the code train indicative of the fetched-packet priority with the code train of the packet that has sent by the sending node per se. If the priority level of the packet sent by the sending node is higher than that of the fetched packet (pi>Po), the node continues to sense the carrier for a fixed time period while sending the jamming signal. After confirming that the carrier of the ompeted node disappears from the transmission channel (CS=0), the node sends the packet again. When the any carrier does not appear from the transmission channel (CS=1), the nodes is placed to a random-time stand-by mode. If the priority level of the competed station is higher than that of the sending node (Pi<Po), the sending node immediately stops the sending operation and is placed to the random-time stand-by mode. If the priority levels of both stations or nodes are equal to each other (Pi=Po), the node is set to the random-time stand-by mode after the jamming signal is sent for a fixed time period. In the status transition diagram of FIG. 8, the carrier sensing is limited within a fixed period of time as "Carrier sense (jamming signal) within a fixed time period". The reason for this follows. When by some error, two nodes collide with each other, one of the nodes decides that the priority level of the node per se is higher than that of the other, and the other also makes the same decision. As a result, the competition to seize the channel continues on the transmission channel in an endless manner. It is for this reason that the carrier sensing time is limited.
A code system to determine the packet priority in a circulating manner is employed for the code train indicative of packet priority. JANKEN, the game of "scissors, "paper", and "stone", will assist your understanding of the code train. (See Appendix A of the following paper: Yasumoto et al., "PROSPEX: A Graphical LOTOS Simulator for Protocol Specification with N Nodes" IEICE Trans. Commun. Vol. E 75∝8, No. 10, pp 1015-1023 (1992)) Two (2) bits provide four combinations of "00", "01", "10", and "11". Of the four combinations, three combinations "00", "01", and "10" are assigned to "stone", "paper", and "scissors", respectively. "01" is prior to "00"; "10", to "01"; and "00", to "10". In this way, the priority level is determined in a circulating manner. In other words, the packet priority is relatively determined.
In the embodiment of the invention, 24 number of 2-bits codes (totally 48 bits) are arranged. The priority is successively determined by comparing firstly the first code with the second code, secondly the second code with the third code, and so on. The priority is given to the code having first won. The probability that 24 code trains are all at the same priority level is (1/3)24 =3.5×10-12. Practically, the event of the probability will little occur. The priority determining method can give impartially the rights to use the channel to the nodes.
JANKEN, which consists of only three combinations, "stone", "scissors", and "paper", may be modified such that "11" is additionally applied as the fourth hand to JANKEN, and "11" is prior to the remaining combinations, "00", "01", and "10". From the top of the code trains indicative of priority, "11" is arranged succeeding to them. The number of successions of "11" indicates the absolute priority of the packet. The absolute priority is given to the packet under a predetermined rule, in consideration of the nature or the contents of the packet.
In the description thus far made, there is no guarantee of successful communication that when three or more nodes collid. Accordingly, when the multiple-collision occurs, it is necessary to take such a control procedure that the random-time stand-by mode follows the transmission of a jamming signal for a preset time period (as in the normal protocol of CSMA/CD). The multiple-collision can be detected by sensing occurrence of the code rule violation.
In the status transition diagram of FIG. 8, when a code rule violation occurs (CRV=1) in the status of "Sense the carrier while sending a signal", "Make the priority comparison while sending a jamming signal" or "Carrier (jamming signal) sense for a preset time", such a control procedure is taken.
The responding node constantly monitors the transmission channel. When recognizing a packet directed to the receiving mode per se, it receives the packet and sends it to the higher layer protocol. A status transition of the responding node in the first protocol is simple, and hence the status transition diagram is omitted here.
The status transition diagram shown in FIG. 11 is based on that of FIG. 8. The control procedure for the normal packet after transmission of the header has been completed is different from that for the secure communication. The control procedure for the normal packet (PT=00) resembles the procedure as described in the status transition diagram of FIG. 8. In the control procedure for the secure communication (PT=01), the sending node sends a jamming signal immediately after the completion of the header transmission. In this mode, the transmission is permitted only when the address of the sender is coincident with that of the responder (R=1) and PT (packet type)=10. When the address of the sender, from which the packet is received, is not coincident with the address of the responder (R=0), the random-time stand-by mode is set up after the jamming signal is sent for a preset time. Also when CRV=1 in the processings of "Sense a carrier while sending a jamming signal" and "Sending" (FIG. 11), the random-time stand-by mode is set up after the jamming signal is sent for a preset time.
The integrated optical circuit substrate 101 is a glass substrate in this embodiment. A first slab waveguide 102 for transmission, a second slab waveguide 103, a first optical coupler 107, a second optical coupler 108, and wiring optical waveguides 109a to 109k are formed on the integrated optical circuit substrate 101 by metal ion diffusion process. For the waveguide formation by the metal ion diffusion process, reference is made to E. Okuda, I. Tanaka, and T. Yamasaki: "Planar gradient--index glass waveguide and its applications to a 4-port branched circuit and star coupler", Appl. Opt. 23, p 1745 (1984). The wiring optical waveguides 109a to 109k have each 10 μm in diameter, and its waveguide mode is a single mode. The first slab waveguides 102 and 103 are each 10 μm thick. The size of the integrated optical circuit substrate 101 is: L1=50 mm and L2=40 mm.
The first slab waveguide 102 and the laser array 106 with the anti-reflection coat are interconnected with a group of wiring optical waveguides 109a to 109e, of which end face pitch is 100 μm. The first slab waveguide 102 and the Fresnel reflecting mirror 104a cooperate to form a polychrometer. The four wiring optical waveguides 109a to 109d, arrayed at equal pitches, are connected to the first slab waveguide 102, thereby to form a polychrometer output 114.
The laser array 106, the first slab waveguide 102, nd the Fresnel reflecting mirror 104a make up a wavelength multiplexing resonance optical system, which generates wavelength-multiplexed laser light. In the optical system, the common laser element 106e is coupled, with difference wavelengths, with the remaining laser elements 106a to 106d through the Fresnel reflecting mirror 104a, so that laser oscillation occurs at the respective wavelengths. In more particular, the common laser element 106e and the laser element 106a are coupled with each other, with a wavelength λ1, through an optical path including the wiring optical waveguide 109e, the first slab waveguide 102, the Fresnel reflecting mirror 104a, the first slab waveguide 102, and the wiring optical waveguide 109a. The common laser element 106e and the laser element 106b are coupled with each other, with a wavelength λ2, through an optical path including the wiring optical waveguide 109e, the first slab waveguide 102, the Fresnel reflecting mirror 104a, the first slab waveguide 102, and the wiring optical waveguide 109b. The wavelength is determined depending on the positional relationship of the common laser element 6e and the remaining laser elements 106a to 106d to the Fresnel reflecting mirror 104a. For the details of this, reference is made to Japanese Patent Application No. Hei. 3-251677.
To cope with the above problem, in the third embodiment shown in FIGS. 18 and 19, the substrate 101 has a structure consisting of two plastic layers layered one over the other. The wavelength multiplexing transceiver when seen in the direction of an arrow B in FIG. 18 is perspectively illustrated in FIG. 19. For a better illustration of the structure of the substrate 101, the photo diode array 105, the semiconductor laser array 106 and the optical fiber 110 are contoured by dotted lines. As shown, the substrate 101 is structured such that a thin plastic thin film 117 is layered on a thick plastic thin film 118. An optical waveguide circuit including the optical waveguides 109a to 109f, which connects to the laser array 106, is formed in the thin plastic thin film 117. The optical waveguides 109h to 109k connecting to the photo diode array 105 and the optical waveguide 109g connecting to the optical fiber 110 are formed in the thick plastic thin film 118. The thickness d1 of the thin plastic thin film 117 is 10 μm, and the thickness d2 of the thick plastic thin film 118 is 30 μm. The second optical coupler 108 is formed by laying the optical waveguide 109f of the thin plastic thin film 117 on the optical waveguide 109g of the thick plastic thin film 118. The optical waveguide 109f, which couples the second optical coupler 108 with the first slab waveguide 102, is shaped such that a portion 109f of he waveguide closer to the second optical coupler 108 is broad, 40 μm, and a portion 109f2 closer to the first slab waveguide 102 is narrow, 10 μm. Accordingly, the optical waveguide at the location coupled with the optical fiber 110 is shaped in square, 40 μm×40 μm. The known selective photopolymerization is used for forming the optical waveguides in the plastic thin films 117 and 118. The selective photopolymerization is discussed by T. Kurosawa, N. Takato, S. Okikawa and T. Okada in their paper "Fiber optic sheet formation by selective photopolymerization, Appl. Opt. 17, p 646 (1978). The substrate 101 was formed by laminating two plastic thin films having optical waveguides already formed therein. The thin films may be another other material than plastic, if it allows optical waveguides to be formed therein.
FIG. 20 shows a fourth embodiment of the present invention. In the fourth embodiment, a wavelength multiplexing transceiver for optical communication is formed as an integrated optical circuit on a semiconductor substrate 131. FIG. 20(a) is a plan view of the wavelength multiplexing transceiver; FIG. 20(b) is a cross sectional view taken along a line of X--X in FIG. 20(a); and FIG. 20(c) is a side view of the same. The wavelength multiplexing transceivers of the fourth embodiment, and fifth to seventh embodiments to be given later are three-wavelengths multiplexing transceivers.
In FIG. 20, reference numerals 134a to 134c, and 135 designate semiconductor laser elements. The semiconductor laser elements 134a to 134c correspond to the semiconductor laser elements 106a to 106c. The semiconductor laser element 135 corresponds to the semiconductor laser element 106e. Photo diodes 136a to 136c correspond to the photo diodes (not shown) of the photo diode array 105 shown in FIG. 16. The structure of the photo diodes 136a to 136c is substantially the same as that of the semiconductor laser elements 134a to 134c. When it is fed with current, it serves as a laser diode. When it receives light, it generates photo current. The laser elements 134a to 134c are different in element length from the photo diodes 136a to 136c. The length of the laser elements 134a to 134c is 250 μm, and the length β is 10 μm (the illustration of FIG. 20 roughly shows a layout of elements, and the layout is not exact in the reduced scale). Such a figure of the photo diode length is selected because the photo diode of 10 μm long can satisfactorily absorb light. If the element length is selected to be long, the stray capacitance of the element is increased, and cannot handle the received light signals, which are modulated at high speed. The pitch of the laser elements 134a to 134c is 10 μm, and the pitch of the photo diodes 136a to 136c is also 10 μm. The width S of the optical waveguide is 3 μm. The substrate 31 is: L3×L4=10 mm×10 mm.
The first slab waveguide 102, the second slab waveguide 103, the first optical coupler 107, the second optical coupler 108, and the optical waveguides for wiring are semiconductor optical waveguides. The portion of-the substrate connecting to the optical fiber 110 and the end face thereof on which the photo diodes 136a to 136c are covered with a anti-reflection coat 137. The anti-reflection coat 137 is not formed on the portion of the substrate where the laser elements 134a to 134c and 135. The substrate 131 and the optical fiber 110 are optically coupled with each other by a coupling lens 138.
FIG. 22 shows a sixth embodiment of the invention. In this embodiment, an optical amplifier 139, such as a semiconductor laser amplifier, is formed in the optical waveguide, which connects the second optical coupler 108 to the second slab waveguide 103. The structure of the optical amplifier 139 is substantially the same as that of the laser elements 134a to 134c and 135. The element length of γ the optical amplifier 139 is 500 μm, in this embodiment. The optical amplifier 139, which amplifies the received light signal, compensates for the loss of the light signal when it passes through the waveguide. The loss by the semiconductor waveguide is approximately 10 dB/cm, and this figure is much larger than 0.1 dB/cm of the glass waveguide. Particularly, in a case where the wavelength multiplexing transceiver is integrated on the semiconductor substrate, the semiconductor laser amplifier can compensate for the waveguide loss, which results from the use of the semiconductor slab waveguides. In this respect, the sixth embodiment can prevent the performance deterioration of the wavelength multiplexing transceiver when it is integrated.
FIG. 24 shows an eighth embodiment of the invention. The eighth embodiment, like the first to third embodiments, is of the four-wavelengths multiplexing type. Like reference numerals are used for designating like or equivalent portions in the first to third embodiments. In the eighth embodiment, the substrate 101a is shaped like a trapezoid as viewed in cross section. The upper end faces of the slab waveguides 102 and 103 are exposed at the stepped portion of the substrate 101a. The narrow part of the substrate 101a, which is located closer to the slab waveguide 102, is inwardly cut away to form a cut-away portion 101b. A semiconductor laser array 106 is disposed in the cut-away portion 101b. The laser array 106 contains five semiconductor laser elements 106a to 106e. Both end faces of the laser element 106e is covered with anti-reflection coats. In the case of the remaining laser elements 106a to 106d, only the end faces thereof closer to the first slab waveguide 102 are covered with the anti-reflection coat. The laser element 106e of the laser array 106 is connected to the second optical coupler 108 by means of the wiring optical waveguide 109f. The wavelength multiplexing operation in the eighth embodiment is similar to that of each of the embodiments as mentioned above. In the present embodiment, the light signal is output from the end of the laser element 106e of the laser array 106, which is far from the first slab waveguide 102, while in the above-mentioned embodiments, it is output from the first slab waveguide 102.
FIG. 25 shows a first embodiment of an interconnectable 5-port star coupler according to of the present invention. Three 1×2 equal branching circuits 205 are combined in a tree fashion, thereby forming an equal branching circuit unit 203 with four ports. As shown, five light-equal branching circuit units 203 are arrayed on a substrate 201 in a star fashion, thereby forming the star coupler. A light signal emanating from an optical fiber 202 is equally divided into four light signals, by the branching circuit unit 203. Those divided light are distributed to the remaining optical fibers 242, through optical waveguides 201a formed on the same plane of the substrate 201. The integrated optical circuit contains five intersecting portions 204 where the optical waveguides 201a intersect.
In the 1×2 equal branching circuit 205, as shown in FIG. 26(a), a light signal enters an optical waveguide 231, passes through a mixing part 236, and is branched into two optical waveguides 233 and 234. The equal branching circuit 205 is frequently called a Y branching circuit since it is shaped like letter Y. In the case of the equal branching circuit 205, two optical waveguides 233 and 234 are directly coupled together. Because of this, its junction loss is small. However, in some light propagation modes, its branching ratio is often limited. Specifically, in a single mode, it can branch the light signal at only 1:1 of the branching ratio, basically.
A 1×2 Evanescent optical coupler (in other words, an optical coupler based on coupled mode theory), as shown in FIG. 26(b), includes a portion where two optical waveguides 235a and 235b are closely located. This optical coupler functions to transfer a light signal from one waveguide to another via by the Evanescent coupling. Structurally, an extremely thin medium (clad) of low refractive index is located between two optical waveguides (core), made of medium of high refractive index. Energy is transferred from one waveguide to another via Evanescent wave. In other words, this optical coupler is based on coupled mode theory. This optical coupler is advantageous in that it can take a desired branching ratio, but is disadvantageous in that the junction loss is large as described above.
where n1 is refractive index of the core, and n2 is refractive index of the clad (n1>n2). Where the refractive index of the core and clad is 2%, θ is approximately 3° C. Accordingly, in this case, δ must be larger than 6°.
Where the intersection angle δ is large, the light which does not couple the waveguides at the intersecting portion 204 increases relative to the other light. This results in increase of loss (transmission loss). The fact that the transmission loss abruptly increases when the intersection angle δ decreases below 20°, has been numerically calculated (seen Takahashi and Inagaki "Analysis of the transmission loss in matrix optical waveguide", The 1992 IEICE (institute of electronics/information/communication Engineers) spring conference record, C-192 (1992)). For this reason, δ is preferably lager than 20°.
FIG. 28 is a diagram showing an interconnectable star coupler with a pair of 5-port groups according to a second embodiment of the invention. As shown, optical waveguides, a 1×2 equal branching circuit 205, and a 2×2 equal branching circuit 206 are formed on the substrate 201. Optical fibers 202 are derived from the substrate 1, corresponding to the respective ports.
In the second embodiment of the FIG. 28, the 1×2 equal branching circuit 205 is used in place of the 1×2 fiber coupler 231, and the 2×2 equal branching circuit 206 is used in place of the 2×2 fiber coupler 232. In a conventional fiber coupler, only the Evanescent optical coupler can be manufactured with some restrictions on the manufacturing. On the other hand, use of the equal branching circuit 205 is allowed in the integrated optical circuit of the second embodiment shown in FIG. 28. In the fiber coupler, it is difficult to directly couple two optical fibers shaped circular in cross section. For this season, two optical fibers shaped circular in cross section are located closely, and it is filled with medium of low refractive index in a manner that the medium surrounds the optical fibers. Therefore, only the optical coupler by the Evanescent wave coupling can be formed. On the other hand, in the integrated optical circuits, optical waveguides are formed in or on the substrate by the photolithographic technique. Accordingly, it is very easy to manufacture two optical waveguides directly coupled, thereby eliminating the junction loss. Further, in the instant embodiment, the angle δ at the intersecting portions in the integrated optical circuit is larger than the critical angle θ of the optical waveguide. This eliminates the interference between the optical waveguides.
The 2×2 equal branching circuit 206 may be either of the junction type as shown in FIG. 26(c) or the Evanescent optical coupler type. In this case, the Y equal branching circuits 205 must be provided at the ports connecting to the optical fibers 202. In FIGS. 26(a) and 26(c), the optical waveguides 231 and 232 or 233 and 234 intersect at an angle, which is much smaller than the critical angle θ.
FIG. 29 shows an interconnectable star coupler with a pair of 9-port groups according to a third embodiment of the invention. The third embodiment is different from the second embodiment in that the number of ports is increased from five to nine. The increase of the number of ports makes the integrated optical circuit complicated. Accordingly, intersecting portions 207 where three optical waveguides intersect are present, as shown in FIG. 29. Also in this case, angles α, β and γ formed by the waveguides 221, 222, and 223, shown in FIG. 30(a), are selected to be larger than a value two times of the critical angle θ of the optical waveguide. When the number of optical waveguides is increased, scattered light will increase. To avoid this, it is suggestible to slightly change the paths of the waveguides so as not to cause the intersecting portions of three or more waveguides.
According to the invention, the single mode, interconnectable multi-port start coupler can be constructed not using the 1×2 Evanescent optical coupler. Therefore, the star coupler is free from the junction loss caused by the 1×2 Evanescent optical coupler. The circuit can be constructed by using merely the combination of the 1×2 equal branching circuit and the 2×2 equal branching circuit. Therefore, also in the multi-mode, interconnectable star coupler, its manufacturing is easy.
In the single mode optical waveguide, as shown in FIG. 32, most of light from the common part 301 goes to the waveguide 302b, and the light goes little or no to the waveguide 302a. This property of single mode asymmetric y-branching circuit is described in: Bures et al. "Mode Conversion In Planar-Dielectric Separating Waveguide", IEEE J. Qauntum Electron., vol. QE-11, No. 1, pp 32-39 (1975). In the Evanescent optical coupler 304, light propagating through the waveguide 304b is branched to the waveguide 304a by the mode coupling. Accordingly, the optical coupler of FIG. 31 functions as an unequal optical coupler.
The embodiment of FIG. 36 includes an asymmetrical Y branching circuit 36 in which optical waveguides having different sectional areas are substantially symmetrically branched at angles ω1 and ω2. The sectional area of the waveguide 306a is smaller than that of the waveguide 306b in FIG. 36. Further, the waveguide is branched at an angle ω1 -ω2 with respect to the original waveguide 301. The waveguide of the smaller sectional area has a smaller refractive index. Accordingly, the light propagating speed is larger. If ω1=ω2, the phase matching condition is not satisfied. For this reason, ω1 is set to be slightly larger than ω2 so that the optical path difference δ is 0.
An enlarge view of the Evanescent optical coupler 312 is shown in FIG. 40(a). An enlarged view of the bent portion 314 is shown in FIG. 40(c).
In FIG. 40(a), a light signal coming in through the optical waveguide 321a is equally distributed into optical waveguides 322a and 322b, through the mode coupling action in a coupling part 324 where two optical waveguides are closely arrayed side by side. A light signal coming in through another optical waveguide 321b is also equally distributed into optical waveguides 322a and 322b in a similar way.
In FIG. 40(b), a light signal coming in through the optical waveguide 321a is equally distributed into optical waveguides 322a and 322b, through a mixing optical waveguide 324. A light signal coming in through another optical waveguide 321b is also equally distributed into optical waveguides 322a and 322b in a similar way.
In FIG. 40(c), a light signal coming in through the optical waveguide 321a is reflected by the reflecting means 325 to be bent toward the optical waveguide 322. The reflecting means 325 is a total reflection mirror fabricated on the integrated optical circuit by dry etching process, in this instance of the embodiment. Such a technique is known (see Shibata, Okuda, Ikeda, and Monda, "Branching characteristic of multi-stage connected asymetric y-branch using total reflection", The 1992 IEICE (institute of electronics/information/communication engineering) Spring Conference, C-198 (1992)).
The present invention is also applicable for the 1×3 optical coupler. An interconnectable star coupler with four ports constructed using the 1×3 optical coupler 315 of the invention is illustrated in FIG. 41. The structure of a 1×3 branching circuit 317 is basically asymmetrical. Accordingly, if it is connected in series with an Evanescent optical coupler 318 (three optical waveguides are arrayed in parallel), the 1×3 optical coupler 315 free from the junction loss can be formed. Accordingly, an interconnectable star coupler with four ports can be formed by combining four number of such 1×3 optical couplers 315. Because of the rectilinear propagation of light, light signals propagating through the waveguides crossing at right angles do not interfere with each other in the intersecting portion 316 of the waveguides.
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