JPH10215226A - Light transmitting device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the junction loss of an up signal, to eliminate a difference between the transmission/reception levels in up/down directions and the realize light transmission without waste by arranging optical star couplers between a transmission part and a signal multiple means in a subscriber terminal equipment, and inputting light signals which the signal multiplex means outputs to a reception means in parallel. SOLUTION: Mi -pieces of optical network devices 1-1 to 1-T are connected to the subscriber terminal equipment 3 by plural optical fibers through mi branching optical star couplers 2-i. It is T=Σmi (i=1-n). The up signal from the optical network device 1-2 reaches the reception part 6 through the optical star coupler 2-i. A down signal from the transmission part 5 is branched into n-pieces of signal in an n-branch optical star coupler 7 and they are branched into T-pieces of signals in the branch optical start coupler 2-i through an up/ down signal multiples circuit array 4. Then, they reach the optical network devices 1-1 to 1-T. The difference of the transmission line is a branch loss, and T>mi when (n) is more than 2. Then, the loss of the up signal is smaller than that of the down signal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a passive double star (PDS) type optical transmission device in which one subscriber terminal device and a plurality of optical network devices are connected by a passive optical circuit.

[0002]

2. Description of the Related Art A passive double star, also called a passive optical network (PON), is an optical access transmission system in which an expensive station apparatus is shared by a plurality of users and the cost per user is reduced.

FIG. 10 is a block diagram showing a conventional passive double star optical transmission apparatus. In this optical transmission apparatus, a plurality of optical network units (ONUs) 10 placed at a predetermined point between a user's house or a station and the user's house.
1-1 to 101-T and one subscriber terminal unit (SLT) 103 on the station side are connected to an optical star coupler (SC) 102.
Are connected by an optical fiber. An optical signal from the optical network units 101-1 to 101-T to the subscriber terminal unit 103 is defined as an upstream signal, and an optical signal from the signal terminal unit 103 to the optical network units 101-1 to 101-T is defined as a downstream signal. I do. Optical network device 101
-1 to 101-T respectively include a transmitter 104 for transmitting an optical signal, a receiver 105 for receiving an optical signal, and an upper and lower signal multiplexing circuit 106 for multiplexing and demultiplexing signals in the vertical direction.
Is provided. Similarly, the subscriber terminal unit 103 includes an upper / lower signal multiplexing circuit 107, a transmitting unit 108, and a receiving unit 109.

In this conventional example, an optical network device 10
1-1 to 101-T and optical star coupler 102
, And between the subscriber terminal device 103 and the optical star coupler 102, only one optical fiber is provided. Therefore, the upstream signal and the downstream signal are transmitted in the same optical fiber. Transmitter 10 for transmitting an optical signal
In general, different elements are used in the optical network devices 101-1 to 101-T and the subscriber terminating device 103. Upper and lower signal multiplexing circuits 106 and 107 for multiplexing and demultiplexing are incorporated. In general, the elements used for the upper and lower signal multiplexing circuits 106 and 107 include a 1: 2 optical coupler and an optical circulator when the wavelength of the upstream signal and the downstream signal are the same, and when the wavelength of the upstream signal and the downstream signal are different. WDM filter (W
DM filter) is used.

The upstream signal is transmitted to a plurality of optical network devices 10.
Since the signals are transmitted from 1-1 to 101-T, it is necessary to multiplex control after packetizing the upstream signal so that these signals do not collide. As a method of multiplexing control of packets, time division multiplexing control in which a time slot in which an uplink signal may be transmitted from the subscriber terminal device 103 to the optical network devices 101-1 to 101-T is generally used. . However, the optical network devices 101-1 to 101-1
1-T Performance, Optical Network Devices 101-1 to 101
The amount of information transmitted by -T or the optical star coupler 102
Since there is a difference in transmission characteristics between the optical network devices 101-1 to 101-T, the uplink signal is a burst signal having an indefinite packet interval and having different light intensity and phase for each packet. Therefore, the subscriber terminating device 103 requires an additional circuit for receiving the burst signal in an electric signal demodulation unit (not shown) at the subsequent stage of the receiving unit 108, and has a higher receiving sensitivity than a case of receiving a continuous signal. to degrade.

Since the downstream signal is only split by the optical star coupler 102, all the optical network devices 101-
A continuous signal is broadcast to 1-101-T. In the optical network devices 101-1 to 101-T,
Once all the optical signals are received, a logic circuit (not shown) arranged downstream of the receiving unit 105 extracts only the information relevant to itself. Therefore, the optical network device 101
The receiving unit 105 of -1 to 101-T may be a circuit for receiving a continuous signal only.

[0007]

As described above, the passive double star is inferior in the receiving sensitivity of the upstream signal to that of the downstream signal. Since the system is designed in accordance with an uplink signal having low reception sensitivity, a margin is generated in the transmission / reception level difference of the downlink signal, and as a result, the downlink signal has excessive transmission quality. In order to construct a system while suppressing an increase in cost as much as possible, it is desired to reduce the difference between the transmission and reception levels of the uplink signal and match the difference between the transmission and reception levels of the downlink signal and the uplink signal.

[0008] As a method of adjusting the transmission / reception level difference between the upper and lower directions, the optical output of the transmission unit of the optical network device is increased, the reception sensitivity of the reception unit of the subscriber termination device is improved, and the transmission path loss in the upstream direction is reduced. One of them is required.

Of these methods, the former two are difficult to apply to an optical subscriber system from a cost and technical point of view. The transmission path loss in the upstream direction is roughly classified into a transmission loss of an optical fiber or an optical connector, a coupling loss caused by optical converging at an optical star coupler, and an excess loss of an optical component. The transmission loss and the excess loss are common to the transmission path loss in the downstream direction, and cannot reduce only the loss in the upstream direction.
The junction loss is equivalent to the downward branch loss as long as an optical star coupler is used, but can be reduced by using an element other than the optical star coupler for the junction element.

SUMMARY OF THE INVENTION The present invention has been made in view of the above, and an object of the present invention is to provide an optical transmission apparatus that eliminates a vertical transmission / reception level difference by suppressing a joining loss of an upstream signal and that eliminates waste in transmission quality. And

[0011]

An optical transmission device according to the present invention comprises:
A plurality of optical network devices and one subscriber terminating device, each of which transmits an upstream signal to the subscriber terminating device by a first optical fiber provided for each of the optical network devices. And a first transmission / reception multiplexing means for receiving a downlink signal from the subscriber terminating device, wherein the subscriber terminating device includes: a transmitting unit that collectively outputs downlink signals to a plurality of optical network devices; A receiving unit that collectively receives an upstream signal from an optical network device, and an upstream signal from a second optical fiber to which the first optical fiber is connected is guided to the receiving unit, and a downstream signal from the transmitting unit is transmitted to the receiving unit. In an optical transmission device including a second transmission / reception multiplexing means for guiding to a second optical fiber, a plurality of second optical fibers are provided in number equal to or less than the number of the first optical fibers, and the second transmission / reception multiplexing means is provided with a second optical fiber. A signal multiplexing means for transmitting / receiving multiplexing of a downlink signal and an uplink signal for each fiber; the subscriber terminal device splits the downlink signal output from the transmission unit and supplies the signal to the signal multiplexing means; And a means for inputting the uplink signal guided by the above in parallel to the receiving unit. The first optical fiber and the second optical fiber,
Connected directly or via an optical star coupler.

That is, according to the present invention, at least a part of the optical star coupler is arranged between the transmitting section of the subscriber terminating device and the signal multiplexing means, and the optical signals output from the signal multiplexing means are connected in parallel to the subscriber termination. Input to the receiver of the circuit. Focusing on the upstream signal, at least a part of the optical star coupler reaches the receiver of the subscriber terminal without passing through, and no loss due to merging occurs when inputting to the receiver of the subscriber terminal. For the optical star coupler that does not pass through, the branch loss is reduced.

When different wavelengths are used for the upstream signal and the downstream signal, a wavelength division multiplexing filter array or an arrayed waveguide type diffraction grating can be used as the signal multiplexing means. When the same wavelength is used, an optical circulator array can be used.

The means for parallel input includes a condensing lens for condensing a plurality of upstream signals from the signal multiplexing means and inputting the same to a receiving section, and a plane conductor provided with a plurality of cores and provided at an output end and having the cores close to each other. A wave circuit or a plurality of light receiving elements connected to a common electric circuit can be used. When a condensing lens is used, a large core fiber capable of multi-mode transmission that guides an upstream signal condensed by the condensing lens to a receiving unit can be used.

The transmitting section may include a plurality of light sources, and the light splitting means may include means for splitting the outputs of the plurality of light sources, respectively.

[0016]

FIG. 1 is a block diagram showing an embodiment of the present invention.
1 to 1-T and one subscriber terminal device 3. The configuration of the optical network devices 1-1 to 1-T is the same as that of the conventional example shown in FIG.
The transmission of the upstream signal to the subscriber terminal device 3 and the reception of the downstream signal from the subscriber terminal device 3 are performed by the optical fibers provided one for each -T. The subscriber terminating device 3 includes a transmitting unit 5 that collectively outputs downlink signals to a plurality of optical network devices 1-1 to 1-T, and a plurality of optical network devices 1
A receiving unit 6 for collectively receiving upstream signals from -1 to 1-T; an upstream signal from an optical fiber connected to the subscriber terminal device 3 to the receiving unit 6; An upper / lower signal multiplexing circuit array 4 for guiding signals to the optical fiber is provided. The subscriber terminating device 3 is provided with a plurality of n optical fibers, and the plurality of n optical fibers are respectively provided with:
Of the optical network unit 1-1 to 1-T m _i number (i =
1 to n) each are connected via the m _i branched optical star coupler 2-i. Here, T = Σm _i (i = 1 to n), m _i is a natural number, and m _i = 1 means directly connected without an optical star coupler. Since there are a plurality of optical fibers provided in the subscriber terminal unit 3, the upper and lower signal multiplexing circuit array 4 includes n upper and lower signal multiplexing circuits for transmitting and receiving multiplexing of a downlink signal and an uplink signal for each optical fiber. They are provided in an array. The subscriber terminating device 3 further includes an n-branch optical star coupler 7 for branching the downstream signal output from the transmitting unit 5 and supplying the branched signal to the upper / lower signal multiplexing circuit array 4;
There is provided a parallel input section 8 for inputting the upstream signals guided by the upper / lower signal multiplexing circuit array 4 to the receiving section in parallel.

[0017] m _i uplink signal outputted from the optical network units connected to the branch optical star coupler 2-i leads to the subscriber termination device 3 through the optical star coupler 2-i. In the subscriber termination device 3, the upper and lower signal multiplexing circuit array 4
And is input to one of the upper and lower signal multiplexing circuits, is separated from the upstream signal, is input to the optical receiving unit 6 in parallel with the downstream signal output from the other upper and lower signal multiplexing circuit by the parallel input unit 8, and Converted to a signal. If excess loss in each circuit of the subscriber terminal unit 3 and the optical star coupler is neglected, the upstream transmission path loss is only the fiber loss and the branch loss for the _mi branch.

The downstream signal is transmitted to the transmitting unit 5 of the subscriber terminal device 3.
Is output to the n-branch optical star coupler 7, and is input to the upper / lower signal multiplexing circuit array 4. Vertical signal circuit further downstream signals output from the array 4, the subscriber termination device is branched by m _i branched optical star coupler 2-i after being outputted from the 3, eventually branched into T book optical network The input is made to each of the devices 1-1 to 1-T. If excess loss is ignored as in the case of the upstream signal, the downstream transmission path loss is only the fiber loss and the branch loss for the T branch.

When n is 2 or more, T> _mi , so that
The loss of the upstream signal is smaller than that of the downstream signal,
Deterioration of reception sensitivity due to burst reception can be compensated.
For example, if the bit rate is 155.52 Mb / s
In the case of (1), it is experimentally required that the reception sensitivity of the burst signal deteriorates by about 6 dB as compared with that of the continuous signal. Assuming that the number of branches n in the subscriber termination unit 3 is 4, the loss of the upstream signal can be reduced by 10 log (4) = 6 dB as compared with the downstream signal. Assuming that the output light intensities of the transmitter 5 are equal, the difference between the transmission and reception levels in both the upper and lower directions is equal.

In this example, when n> 4, the downlink transmission path loss exceeds the uplink transmission path loss, and this time, the transmission / reception level difference of the downlink signal determines the transmission / reception level difference of the entire system. For example, when n = 8, the transmission / reception level difference of the uplink signal is 10 log compared to the conventional passive double star.
(8) = 9 dB improvement. This is 3 dB better than the transmission / reception level difference of the downlink signal. However, by arranging two semiconductor lasers as the light source of the transmitting unit in the subscriber terminal device and connecting a four-branch optical star coupler to each semiconductor laser, the light intensity of the downstream signal input to the upper and lower signal multiplexing circuit can be reduced.
The output light from one semiconductor laser can be improved by 3 dB as compared with the case where the output light is distributed by an eight-branch optical star coupler. Also at this time, the difference between the transmission and reception levels in the upper and lower directions becomes equal.

In general, when the transmission line loss in the upstream direction is reduced and the transmission / reception level difference in the upstream direction is better than that in the downstream direction, the optical output at the transmission unit in the subscriber termination unit may be increased. . In order to increase the light output, a plurality of semiconductor lasers may be used as described above, or a semiconductor laser having a high light output intensity may be used. Such a measure leads to an increase in the cost of the subscriber terminating equipment, but since the subscriber terminating equipment is shared by T users, the cost increase per user is multiplied by 1 / T. Rather, it is expected that the number T of users that can be accommodated by one subscriber terminal device will increase due to the increase in optical output, so that the load per user may even be reduced.

2 to 4 show the constituent columns of the upper / lower signal multiplexing circuit array 4. FIG. For each of the upper and lower signal multiplexing circuits in the array, it is preferable to use an element that does not cause loss when multiplexing and demultiplexing the upper and lower signals. 2 shows a WDM filter array in which n 1: 2 WDM filters are arranged in parallel, FIG. 3 shows an arrayed waveguide grating (AWG), and FIG. 4 shows an optical circulator array in which n optical circulators are arranged in parallel. An example of the configuration used is shown.

The upper / lower signal multiplexing circuit array shown in FIG.
It is composed of 1: 2 WDM filters 11-1 to 11-n. This configuration is used when the wavelengths of the upstream signal and the downstream signal are different, and here, the wavelength of the upstream signal is 1.3 μm.
m, the wavelength of the downstream signal is 1.55 μm. 1: 2
Each of the WDM filters 11-1 to 11-n allows a wavelength of 1.55 μm to pass between the first port 111 and the second port 112 (common port).
A configuration in which a wavelength of 1.3 μm passes between the second port 113 and the third port 113 is provided. The transmitting section 5 is connected to the first port 111 via the n-branch optical star coupler 7, and the second port 113 is connected to the second port 113. One of the optical fibers on the optical network device side is connected to the port 112, and the parallel input unit 8 is connected to the third port 113.
The receiving unit 6 is connected via the. As the 1: 2 WDM filters 11-1 to 11-n, a dielectric multilayer film, an optical directional coupler, a Mach-Zehnder interferometer, or the like can be used.

FIG. 3 shows an example in which one 8-port AWG is used and n =
4 shows an example in which four upper and lower signal multiplexing circuit arrays are configured. This AWG has a structure in which two slab waveguides 12 and 13 are connected by an arrayed waveguide 14. Generally, AWG
Wavelength λ ₀ input to port number A of one slab waveguide
Is output to the port number A of the opposing slab waveguide,
Light having a wavelength of λ ₀ + (B−A) × Δλ input to the same port is output to the port number B of the opposing slab waveguide. However, λ ₀ and Δλ are determined by the structure of the AWG.

In the example of FIG. 3, the wavelength λ _d =
λ ₀ , the wavelength of the upstream signal is λ _u = λ _d + Δλ, and the odd-numbered port of the slab waveguide 12 passes through the n-branch optical star coupler 7 to the transmitting section 5, and the even-numbered port passes through the parallel input section 8. The optical fiber on the optical network device side is connected to the odd-numbered port of the slab waveguide 13. The downstream signal input to the odd-numbered port #K of the slab waveguide 12 is output to the same-numbered port #K of the slab waveguide 13 and transmitted to the optical network device. The upstream signal input to the odd-numbered port #K of the slab waveguide 13 is transmitted to the even-numbered port #K of the slab waveguide 12.
(K + 1).

When the wavelengths of the upper and lower signals are the same, the optical circulators 15-1 arranged in an array as shown in FIG.
~ 15-n can be used. Each of the optical circulators 15-1 to 15-n outputs the signal light input to the first port 151 to the second port 152, and outputs the signal light input to the second port 152 to the third port. 153, the first port 151 is connected to the transmitting unit 5 via the n-branch optical star coupler 7, the second port 152 is connected to the optical fiber of the optical network device side, The receiving unit 6 is connected to the third port 153 via the parallel input unit 8.

FIGS. 5 and 6 show examples of the configuration of the parallel input unit 8. FIG. Here, the upstream signals output from the upper / lower signal multiplexing circuit array 4 are transmitted to the single mode fibers 21-1 to 21-1.
−n, and those signals are
2 shows an example of a configuration for coupling to a light receiving element in the inside.

In the configuration example shown in FIG. 5, a condensing lens 22 and a large core fiber 23 are used as the parallel input section 8.
The upstream signals output from the single mode fibers 21-1 to 21-n are coupled to the large core fiber 23 through the condenser lens 22 and input to the light receiving element in the receiving unit 6. As the large core fiber 23, a fiber capable of multi-mode transmission such as a GI (graded index) fiber is used, and single mode fibers 21-1 to 21-n are used.
The light emitted into the space is coupled through the condenser lens 22 without loss. As the light receiving element in the receiving unit 6, a light receiving surface having a sufficient area larger than the core diameter of the large core diameter fiber 23 and capable of being coupled to the large core diameter fiber 23 without loss is used. The large-core diameter fiber 23 can be omitted, and the light collected by the condenser lens 22 can be directly input to the light receiving element.

FIG. 6 shows a planar waveguide circuit 25 in which a plurality of cores are provided as the parallel input section 8 and approach each other at the output end until the maximum distance between the cores becomes equal to or less than the diameter of the light receiving element.
Here is an example using. The input end of the planar waveguide circuit 25 is coupled to the single mode fibers 21-1 to 21-n through, for example, a self-fox lens 24, and the distance between the cores is gradually reduced to reach the output end. At the output end or in the vicinity thereof, light leaks into another core or leaks into the clad. However, if the light receiving diameter of the light receiving element is sufficiently larger than the maximum distance between the cores, light can be received without loss.

FIGS. 7 and 8 show modified examples of the parallel input unit 8 shown in FIG. In the modification shown in FIG. 7, a slab waveguide 26 is arranged between the output end of the planar waveguide circuit 25 and the light receiving element. In the modification shown in FIG. 8, a waveguide 27 having a core width of at least the output end equal to or smaller than the diameter of the light receiving element is arranged between the output end of the planar waveguide circuit 25 and the light receiving element.

FIG. 9 shows a configuration in which signals are aggregated not at the optical stage but at the electrical stage. In this case, it is not necessary to provide the parallel input section 8, and the light receiving element array 28 is provided in the receiving section 6, and the outputs from the n single mode fibers 21-1 to 21-n are separated from each other in the light receiving element array 28. And their outputs are combined into one line by an electric stage. The loss generated in the electric stage is amplified by an amplifier and compensated.

[0032]

As described above, the optical transmission device of the present invention reduces the number of optical star couplers by reducing at least a part of the optical star couplers from the upstream signal transmission path in the passive double star. The upstream transmission path loss is reduced by the loss. As a result, it is possible to balance the difference between the transmission and reception levels in the upper and lower directions, and there is an effect that optical transmission can be performed without waste, and as a result, at low cost.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a diagram showing a configuration column of an upper / lower signal multiplexing circuit array using a WDM filter array.

FIG. 3 is a diagram showing a configuration column of an upper / lower signal multiplexing circuit array using an arrayed waveguide type diffraction grating.

FIG. 4 is a diagram showing a configuration column of an upper / lower signal multiplexing circuit array using an optical circulator array.

FIG. 5 is a diagram showing a configuration example of a parallel input unit using a condenser lens and a large core diameter fiber.

FIG. 6 is a diagram showing a configuration example of a parallel input unit using a planar waveguide circuit.

FIG. 7 is a diagram showing a modification of the parallel input unit shown in FIG. 6;

FIG. 8 is a diagram showing another modification of the parallel input unit shown in FIG. 6;

FIG. 9 is a diagram illustrating a configuration example in which signals are aggregated in an electric stage.

FIG. 10 is a block diagram showing a conventional passive double-star optical transmission device.

[Explanation of symbols]

1-1~1-T, 101-1~101-T optical network device 2-1 to 2-n m _i branched optical star coupler 3,103 subscriber termination device 4 vertical signal circuit array 5,104,108 transmission Unit 6, 105, 109 Receiver 7 n-branch optical star coupler 8 Parallel input unit 11-1 to 11-n 1: 2 WDM filter 111 First port 112 Second port 113 Third port 12, 13 Slab waveguide 14 Array Waveguide 15-1 to 15-n Optical Circulator 151 First Port 151 Second Port 153 Third Port 21-1 to 21-n Single Mode Fiber 22 Condenser Lens 23 Large Core Diameter Fiber 24 Selfoc Lens 25 Planar waveguide circuit 26 Slab waveguide 27 Waveguide 28 Light receiving element array 102 Optical star coupler 106, 107 Lower signal circuit

Claims (11)

[Claims] 1. An optical network device comprising: a plurality of optical network devices; and one subscriber terminating device, wherein each of the plurality of optical network devices is connected by a first optical fiber provided for each of the optical network devices. A first transmission / reception multiplexing means for transmitting an upstream signal to the subscriber terminal device and receiving a downstream signal from the subscriber terminal device, wherein the subscriber terminal device includes a downstream signal to the plurality of optical network devices. And a receiving unit that collectively receives uplink signals from the plurality of optical network devices, and an uplink signal from a second optical fiber connected to the first optical fiber. In an optical transmission device including a second transmission / reception multiplexing unit that guides the downstream signal from the transmission unit to the second optical fiber while guiding the signal to the reception unit, the second optical fiber includes the first optical fiber. The second transmission / reception multiplexing means includes signal multiplexing means for transmitting / receiving and multiplexing a downlink signal and an uplink signal for each of the second optical fibers. An optical branching unit that branches a downstream signal output from the transmission unit and supplies the branched signal to the signal multiplexing unit;
Means for inputting the upstream signal guided by said signal multiplexing means to said receiving section in parallel. 2. A different wavelength is used for an upstream signal and a downstream signal, wherein the signal multiplexing means passes a first wavelength between a first port and a second port, and outputs a second port. A plurality of wavelength division multiplexing filters through which a second wavelength passes between the third port and the third port. 2. The optical transmission device according to claim 1, wherein the optical transmission device is connected to a second port, and one of the second optical fibers is connected to a second port. 3. An upstream signal and a downstream signal having different wavelengths are used. The second transmission / reception multiplexing means includes two slab waveguides connected by an array waveguide, and one port of one slab waveguide. The first wavelength light input to the other slab waveguide is output to a corresponding port of the other slab waveguide, and the second wavelength light input to the same port is output to another port of the other slab waveguide. The optical waveguide branching means and the parallel inputting means are connected to a port of one slab waveguide of the arrayed waveguide type diffraction grating, and a part of the other slab waveguide is connected. 2. The optical transmission device according to claim 1, wherein the second optical fiber is connected to a port. 4. The signal multiplexing means outputs a signal light input to a first port to a second port, and outputs a signal light input to the second port to a third port. A plurality of circulators, wherein the optical branching means is connected to a first port of each optical circulator, one of the second optical fibers is connected to a second port, and the means for parallel input is provided to a third port. The optical transmission device according to claim 1 connected. 5. The optical transmission device according to claim 1, wherein said means for parallel input includes a condenser lens for condensing a plurality of upstream signals from said signal multiplexing means and inputting them to said receiver. 6. The optical transmission device according to claim 5, wherein the means for parallel input includes a large-core fiber capable of multi-mode transmission for guiding an upstream signal collected by the condenser lens to the reception unit. 7. The parallel input means includes a plurality of planar waveguide circuits provided with a plurality of cores and arranged close to each other at an output end until a maximum distance between the cores is equal to or less than a diameter of a light receiving element in the receiving unit. 2. The optical transmission device according to 1. 8. The parallel input means includes: a planar waveguide circuit provided with a plurality of cores, the cores being close to each other at an output end; and an output end of the planar waveguide circuit and a light receiving element in the receiving unit. 2. An optically connected slab waveguide.
An optical transmission device according to claim 1. 9. The parallel input means includes: a planar waveguide circuit provided with a plurality of cores, the cores being close to each other at an output end; and an output end of the planar waveguide circuit and a light receiving element in the receiving unit. The optical transmission device according to claim 1, further comprising: a waveguide that is optically connected and has at least a core width at an output end equal to or smaller than a diameter of a light receiving element in the receiving unit. 10. The optical transmission device according to claim 1, wherein said means for parallel input includes a plurality of light receiving elements connected to a common electric circuit. 11. The optical transmission device according to claim 1, wherein said transmitting section includes a plurality of light sources, and said optical branching means includes means for branching outputs of said plurality of light sources, respectively.