JPS626375B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical fiber bidirectional transmission system, and more particularly to an optical fiber bidirectional transmission system that allows a longer transmission path distance than conventional optical fiber bidirectional transmission systems.

An optical fiber bidirectional transmission system, in which the output optical signals of optical transceivers connected to both ends of a single optical fiber transmission line are transmitted bidirectionally through the optical fiber transmission line, is capable of making the transmission line more economical, etc. Due to its features, various uses ranging from short distances to long distances are being considered. Conventional optical fiber bidirectional transmission systems mainly use light-emitting diodes or semiconductor lasers as light sources, but the maximum transmitted light level when using these light sources is approximately +5 dBm (approximately 3 dBm).
mW), the transmission line distance could not be made sufficiently long.

In addition, if a high-power Nd:YAG laser is used as a light source, approximately +30 dBm (approx.
Since it is possible to input a signal level of 1W), long-distance transmission is possible; for example, at a wavelength of 1.32μ
The possibility of unidirectional 100 km transmission of a 32 Mb/s signal using a Nd:YAG laser has been confirmed.
However, among the oscillation wavelengths of Nd:YAG laser,
The low-loss, low-dispersion wavelength range of optical fiber suitable for long-distance fiber transmission is approximately 1.32μm and approximately 1.34μm.
The wavelengths are only micrometers, and the wavelengths are close to each other.

By the way, in order to achieve long-distance transmission with an optical fiber bidirectional transmission system, it is important to reduce the loss of the optical bidirectional demultiplexer circuit and increase the near-end leakage attenuation. Two-way transmission was achieved through this method. But Nd:
YAG laser oscillation wavelength of approximately 1.32μm and approximately 1.34μm
However, because the wavelengths are too close to each other, the loss of the optical bidirectional demultiplexing circuit is large, and the near-end leakage attenuation is small, the transmission line distance cannot be made sufficiently long.

An object of the present invention is to provide an optical fiber bidirectional transmission system that allows a longer transmission line distance than conventional systems.

According to this invention, the first optical transceiver and the second optical transceiver
an optical transceiver and an optical fiber transmission line, and the signal light transmitted from the second optical transceiver is pumped using the transmitted signal light of the first optical transceiver as excitation light. An optical fiber bidirectional transmission system is obtained in which light is amplified by the optical fiber stimulated scattering amplification effect.

Regarding optical amplification using stimulated scattering in optical fibers, see, for example, Electronics Letters, Volume 16, August 14, 1980, pages 658 to 660, by the inventor of this invention. It is detailed in a paper by Washio, one of the authors, et al. In this paper, the wavelength of the excitation light is
1.319μm, Nd:YAG laser light with optical level of 30W or more, signal light with wavelength of 1.338μm or 1.335μ
Optical fiber stimulated scattering amplification is achieved by propagating the pump light and signal light in the same direction through a 30 m long single mode fiber using Nd:YAG laser light with an optical level of 0.26 mW to 6.2 mW. signal light with
Light is amplified by more than 30dB. In the above paper, the optical level of the excitation light is high at 30 W or more, but if the distance at which the optical fiber stimulated scattering amplification effect occurs (amplification distance) is long or the optical level of the amplified light is low,
A small signal optical amplification gain of 30 dB or more is easily possible with a pumping light level of about 1 W. Furthermore, in the above paper, the pumping light is propagated in the same direction as the propagation direction of the amplified light, but if the amplification distance is long, even if the pumping light is propagated in the opposite direction, the pumping light level of about 1W will be 30dB.
The above small signal optical amplification gain is possible. moreover,
In this case, where the propagation directions of the pump light and the amplified light are opposite,
If the excitation light is also modulated light, bidirectional optical fiber transmission becomes possible. In this optical fiber bidirectional transmission, a sufficient amplification distance for optical amplification can be secured because the optical fiber stimulated scattering amplification effect occurs in the optical fiber transmission line.

Note that the amplified optical frequency ν at which a high amplification gain can be obtained is
Frequency difference between s and excitation light frequency νp (νp−ν
The maximum value of s) is when the optical fiber has a zero dispersion wavelength of approximately
Approximately 2000cm ^-1 when using a 1.3μm single mode fiber and a light with a wavelength of approximately 1.32μm as the excitation light
(equivalent to a wavelength difference of approximately 0.27 μm), which is relatively large. This value can be varied from around 100 cm ^-1 by changing the dispersion characteristics of the optical fiber and the wavelength of the excitation light.
It can be freely changed over a relatively wide range up to about 3000cm ^-1 . Therefore, a system capable of optical amplification by optical fiber stimulated scattering amplification and bidirectional optical fiber transmission by wavelength multiplexing can be constructed.

In the optical fiber bidirectional transmission system of the present invention, a light source with a high output level of about +30 dBm or more is used as the light source of the first optical transceiver, and this light is modulated at high speed by an external optical modulator to become a transmitted optical signal. I can do it. Therefore, the transmitted light level can be increased by about 30 dB or more compared to conventional systems. On the other hand, a semiconductor laser can be used as the light source of the second optical transceiver as in the conventional case. The signal light, which is the output light of the semiconductor laser, is amplified by about 30 dB in the optical fiber transmission line by the optical fiber stimulated scattering amplification effect excited by the high-level light output from the first optical transceiver. 1 reaches the optical receiver of the optical transceiver.

Therefore, in the optical fiber bidirectional transmission system of the present invention, the permissible transmission line loss in both directions can be increased by about 26 dB or more compared to the conventional system, and the transmission line distance can be increased by that much. For example, if the loss is about
If a 0.6 dB/Km optical fiber is used for the transmission line and the transmission line distance is within the loss limit of the optical fiber, the transmission line distance can be increased by 40 km or more.

Furthermore, in the optical fiber bidirectional transmission system of the present invention, a semiconductor laser can be used as a light source in one of the two optical transceivers, and semiconductor lasers with various wavelengths have been developed.
The wavelength is in the low-loss wavelength range of optical fiber transmission lines,
Furthermore, the wavelength difference (approximately
A light source for a second optical transceiver having a diameter of 0.05 μm or more is readily available. In addition, since the wavelength difference between the light sources of the first and second optical transceivers can be large, it is easy to obtain high-performance optical bidirectional demultiplexing circuits with high near-end leakage attenuation, and the permissible transmission associated with bidirectional transmission can be easily obtained. The reduction in path loss can be made very small.

Next, the present invention will be described by way of examples with reference to the drawings.

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention. The first optical transceiver 1 is provided with a first optical transmitter 2 , a first optical receiver 3 , and a first optical bidirectional branching circuit 4 . The first electrical signal input terminal 5 has a bit rate of 100 Mb/s, DMI
(Differential Mark Inversion) A binary pulse signal modulated with a code pulse is input. This pulse signal is sent to the drive circuit 7 of the first optical transmitter 2.
After being amplified to a 10Vp-p signal, the optical modulator 8
is guided by. This optical modulator contains Nd:
The wavelength of the output light of YAG laser 9 is 1.32μm,
A laser beam with an output light level of +35 dBm is guided, modulated by the pulse signal, and then sent to the first optical bidirectional branching circuit 4 through an optical fiber. This first optical bidirectional demultiplexing circuit 4 is composed of an interference film filter and a lens, and sends the light with a wavelength of 1.32 μm from the optical modulator 8 to the optical fiber transmission line 10 with a low loss of 1 dB. There is. The pulse peak level of the optical pulse signal sent to the optical fiber transmission line 10 is +30 dBm. The first bidirectional demultiplexing circuit 4 guides the light with a wavelength of 1.32 μm from the first optical transmitter 2 to the optical fiber transmission line 10 with low loss. 3, it will suffer a loss of 90dB or more.

The optical fiber transmission line 10 is composed of a single mode fiber made of silica and has a core diameter of 10 μm, a fiber outer diameter of 125 μm, and a zero dispersion wavelength of about 1.3 μm. The optical pulse signal having a wavelength of 1.32 μm propagates through the optical fiber transmission line 10, enters the second optical bidirectional demultiplexing circuit 12 of the second optical transceiver 11, and is transmitted to the second optical receiver via the optical fiber. I am guided by 13. The second optical receiver 13 detects and amplifies the received optical pulse signal, demodulates it to the original electrical signal, and then sends it out from the second electrical signal output terminal 16.

On the other hand, a second electrical signal input terminal 15 provided in the second optical transceiver 11 receives a pulse code modulated binary pulse signal with a bit rate of 97 Mb/s and a duty factor of approximately 50%. is inputting. This pulse signal is converted into an optical pulse signal with a wavelength of 1.55 μm by an InGaAsP semiconductor laser provided in the second optical transmitter 14, and then guided to the second optical bidirectional demultiplexing circuit 12 through an optical fiber. After that, the optical pulse is transmitted to the optical fiber transmission line 10 at a peak optical pulse level of 0 dBm. The second optical bidirectional demultiplexing circuit 12 is connected to the first optical bidirectional demultiplexing circuit 4.
Similarly, it is composed of an interference film filter and a lens, and guides the light with a wavelength of 1.55 μm from the second optical transmitter 14 to the optical fiber transmission line 10 with a low loss of 1 dB. When the light leaks into the optical receiver 13 of No. 2, it suffers a loss of 60 dB or more.

From the second optical transceiver 11 to the optical fiber transmission line 1
The optical pulse signal with a wavelength of 1.55 μm transmitted through the optical fiber transmission line 10 reaches the first optical transceiver 1, passes through the first optical bidirectional demultiplexing circuit 4, and is transmitted through the optical fiber to the first optical transceiver 1. is guided to an optical receiver 3.
This first optical receiver 3 detects and amplifies the received optical pulse signal, demodulates it to the original pulse signal, and then sends it out from the first electrical signal output terminal 6.

In the first and second optical receivers 3 and 13, the minimum value of the optical reception level at which the bit error rate is 10 ^-9 or less is -37 dBm or less as a pulse peak value. Note that when this value is converted into a value at the output end of the optical fiber transmission line 10, it becomes -36 dBm or less. By the way, 1.32 transmitted from the first optical transceiver 1
Since the pulse peak level of an optical pulse signal with a wavelength of μm is +30 dBm, the allowable loss between the transmitting and receiving sections for this optical pulse signal is a large value of 66 dB.

On the other hand, while the optical pulse signal with a wavelength of 1.55 μm transmitted from the second optical transceiver 11 is propagating through the optical fiber transmission line 10, the signal with a wavelength of 1.32 μm transmitted from the first optical transceiver 1 Due to the optical fiber stimulated scattering amplification effect using light as excitation light, when this optical pulse signal reaches the first optical transceiver 1, it undergoes optical amplification of about 30 dB. Therefore, the pulse peak value of the optical pulse signal with a wavelength of 1.55 μm transmitted from the second optical transceiver 11 is 0 dBm,
Optical amplification gain by optical fiber stimulated scattering amplification: approx.
Considering 30 dB, the allowable loss between the transmitter and receiver for this optical pulse signal is approximately 66 dB, which is equivalent to that for an optical pulse signal with a wavelength of 1.32 μm.

Note that the level at which the optical pulse signal output from the first optical transmitter 2 leaks into the first optical receiver 3 and the level at which the optical pulse signal output from the second optical transmitter 14 leaks to the second optical receiver 13 The level leaking into the optical pulse is -60 dBm or less at the peak value of the optical pulse, which is more than 20 dB lower than the signal light level, so it hardly deteriorates the code error rate.

In addition, the light with a wavelength of 1.32 μm that acts as excitation light is propagating in the opposite direction to the optical pulse signal with a wavelength of 1.55 μm, which is the light to be amplified, and since it is an optical pulse signal modulated with a DMI code, it is pulsed. Since the continuous time of "1" or "0" is as short as about 10 ms or less, and the mark rate is constant, optical fiber stimulated scattering amplification occurs almost on average over a period of several tens of ms or more.

Since the amplification distance at which the optical fiber stimulated scattering enhancement effect occurs is sufficiently longer than the distance (approximately 5 m) corresponding to a propagation time of several tens of ms, the optical amplification gain is equal to the sign of the signal input to the first electrical signal input terminal 5. It rarely changes with change. Furthermore, in a long-distance optical fiber transmission system, the signal light propagated through the optical fiber transmission line 10 is reduced to a weak level near its output end. Therefore, even if approximately 30 dB of optical amplification is performed near the output end of the optical fiber due to the optical fiber stimulated scattering amplification effect, the excitation light with a wavelength of 1.32 μm will be replaced by the amplified light with a wavelength of 1.55 μm. The energy transfer to is small, and the resulting drop in the level of the excitation light with a wavelength of 1.32 μm is hardly a problem. Note that as the signal light with a wavelength of 1.32 μm, which acts as excitation light, propagates through the optical fiber transmission line 10, its level decreases due to loss in the optical fiber, so the optical fiber stimulated scattering amplification effect is the same as that of the first optical transceiver. 1, but not near the second optical transceiver 11.

In the conventional optical fiber bidirectional transmission system, a semiconductor laser or a light emitting diode is used as the light source of the first and second optical transmitters 2 and 14, but when the bit rate is 100 Mb/s, which is the same as in the above embodiment, The allowable loss between the transmitting and receiving sections was approximately 40 dB or less. Therefore, in the embodiment, the permissible transmission line loss can be improved by about 26 dB compared to the conventional example. If an optical fiber transmission line 10 with a loss of 0.6 dB/Km or less is used, the transmission line distance in the above embodiment is 40
You can go longer than Km.

Further, in the above embodiment, since the signal light having a wavelength of 1.32 μm is also used as the excitation light for causing optical fiber stimulated scattering amplification, there is also the advantage that there is no need to use a special light source for excitation.

Note that in the above embodiment, the first electrical signal input terminal 5
The bias level of the optical modulator 8 is set so that when no signal is input to the optical modulator 8, the output light of the Nd:YAG laser 9 passes through the optical modulator 8 with low loss without being modulated by the optical modulator 8. You can leave it there. In this way, even when no signal is input to the first electrical signal input terminal 5, the first optical transceiver 1
Since light is transmitted from the optical fiber and can be amplified by stimulated scattering through optical fibers, time-division bidirectional transmission becomes possible.

Further, in the above embodiment, Nd of the first optical transmitter 2:
The wavelength of the YAG laser 9 is 1.32 μm, and the wavelength of the output light of the second optical transmitter 14 is 1.55 μm. The optical signal that has been transmitted is amplified by fiber-stimulated scattering in the optical fiber transmission line 10, and as long as wavelength multiplexing bidirectional transmission is possible,
Other wavelengths may also be used.

Although the signal input to the first electrical signal input terminal 5 is a binary pulse signal modulated with DMI code pulses, it may also be a general 1B2B type code such as a CMI code or biphase code. It may also be an mBnB type code in general.
The signal input to the second electrical signal input terminal 15 may be a signal such as that described above, a multivalued pulse signal, or an analog signal.

According to the present invention, as described above, it is possible to obtain an optical fiber bidirectional transmission system in which the transmission path distance can be longer than that of the conventional system.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention. 1, 11... Optical transceiver, 2, 14... Optical transmitter, 3, 13... Optical receiver, 4, 12... Optical bidirectional branching circuit, 5, 15... Electric signal input terminal,
6, 16... Electric signal output terminal, 7... Drive circuit, 8... Optical modulator, 9... Nd:YAG laser,
10...Optical fiber transmission line.

Claims (1)

[Claims] 1 a first optical transceiver, a second optical transceiver,
an optical fiber transmission line, the signal light transmitted from the second optical transceiver using the transmission signal light of the first optical transceiver as excitation light is optical fiber stimulated scattering amplification in the optical fiber transmission line. An optical fiber bidirectional transmission system characterized by optical amplification through action.