JPH01208922A - Optical transmission system - Google Patents

Abstract

PURPOSE:To optimize separately a wavelength for the operation of a light emitting/photodetecting element and a wavelength for an optical signal propagated through a transmission line by utilizing it that a crystal having a nonlinear optical coefficient is used for an optical parametric frequency conversion element and converting the light emitting/light receiving wavelength and the wavelength in the transmission line. CONSTITUTION:A light source 11 emits light beams in frequency omega1 and a light source 12 emits light beam in frequency omega2. The light beams are summed by an optical coupler 13 and fed to a parametric optical frequency converter 14. The parametric optical frequency converter 14 outputs an optical signal having a frequency omega3 being a difference between the frequencies omega1 and omega2, and given to an optical fiber transmission line 4 through a lines system 15. Then the frequency omega3 satisfying the relation of omegas omega3=omega1-omega2 is selected as the transmission wavelength. The information to be sent is given to one or both the light sources 11, 12 to modulate the output light. As a result, the wavelength for the operation of the light emitting/light receiving element and the wavelength propagated through the transmission line are optimized independently respectively.

Description

Detailed Description of the Invention "Industrial Application Field" The present invention allows the operating wavelengths of light emitting and light receiving elements and the wavelength of optical signals propagating through a transmission path to be independently optimized using nonlinear optical effects. The present invention relates to an optical transmission system.

"Prior Art" Conventionally, optical transmission systems have basically taken the configuration shown in FIG. 6.

That is, an electrical input signal is converted into an optical signal by the light emitting element 2 in the transmitter l, and is supplied to the optical fiber transmission line 4 via the lens system 3. The optical signal transmitted through the optical fiber transmission line 4 is received by the light receiving element 6 in the receiving section 5 and extracted as an electrical output signal.

It has been found that in silica-based optical fibers, which are most widely used as this type of optical fiber transmission line 4, the lowest loss wavelength is around 1.5 μm. It is said that the theoretical limit of the lowest loss is not less than 0.1 dB/km.

In order to overcome this, attempts are being made to search for fluoride-based and chalcogenide-based materials and to fabricate them into fibers.

The lower limit of the loss on the shorter wavelength side than the minimum loss is Rayleigh scattering. This tends to increase uniformly as the wavelength becomes shorter, and the difference depending on the material is not very large. On the other hand, the lower limit of the loss on the longer wavelength side than the minimum loss uniformly increases as the wavelength increases.

This is due to the lattice vibration of the constituent molecules, so it is highly dependent on the material.

Therefore, it can be said that materials whose absorption due to lattice vibration of molecules is on the longer wavelength side have the potential for lower loss. For example, in a fluoride system, 1/
It is expected that there will be a minimum loss of l 000 dB/km. Therefore, if this type of fiber is used, considerable long-distance transmission will be possible at wavelengths longer than currently used wavelengths.

[Problems to be Solved by the Invention] However, in semiconductor devices, which currently occupy the mainstream of light-emitting and light-receiving elements in optical transmission systems, problems become more serious as the wavelengths of light emission and light reception become longer.

For example, semiconductor lasers, which are light-emitting devices, utilize stimulated emission when injected carriers (N atoms, holes) recombine, but at long wavelengths, a non-luminous phenomenon called Auger recombination occurs. The probability of recombination increases, and the luminous efficiency tends to decrease.

Furthermore, as for photodiodes, which are the mainstream of light-receiving elements, at present, ternary type ones made of InGaAs have sensitivity up to the longest wavelength, but this has a limit of 1.6 μm.

Therefore, in order to realize ultra-long distance transmission at ultra-long wavelengths, it was necessary to solve these problems with light-emitting and light-receiving elements.

The present invention was made against this background, and uses optical parametric wavelength conversion technology to separately optimize the wavelength at which the light emitting/light receiving element operates and the wavelength of the optical signal propagating through the transmission line. The aim is to provide an optical transmission system that makes it possible to

"Means for Solving the Problems" In order to solve the above problems, the present invention provides a transmitting device that manually generates an electrical signal and outputs an optical signal, and a transmitting device that transmits the output optical signal from a transmitting end to an academic terminal. In an optical transmission system that includes an optical transmission line that transmits light to a receiver, and a receiver that converts output light sent from the receiving end of the optical transmission line into an electrical signal and outputs it, the transmitter has two light sources and , an optical coupler that combines optical signals from these light sources, and an optical crystal that has optical nonlinearity, and which outputs an optical signal having a frequency corresponding to the difference or sum of the frequencies of the lights from the two light sources. It is characterized by comprising an optical frequency converter that outputs.

Further, in the optical transmission system, the receiving device includes: one light source; an optical coupler that combines light from the light source and a received optical signal; and an optical crystal having optical nonlinearity, The present invention is characterized by comprising an optical frequency converter that outputs an optical signal having a frequency corresponding to the sum or difference of the frequencies of the light from and the received optical signal.

That is, the present invention utilizes the fact that a crystal having a nonlinear optical coefficient can be used as an optical parametric frequency conversion element to convert the wavelength of light emitted and received light and the wavelength within a transmission line.

"Effect" When a photoelectric field is applied to a crystal lacking anti-optical strength, the polarization induced in the crystal is not necessarily proportional to the applied electric field and includes a distortion component. This means that harmonic components of the applied electric field frequency are generated.

For example, when ruby laser light (wavelength λ=0.694 μm) is made incident on a quartz crystal under appropriate conditions, second harmonic light (wavelength λ=0.347 μm) is generated.

This is the simplest example of nonlinear parametric interaction in the optical domain. More - For boat fishing, by mixing the signal light of frequency ω with the beam of frequency ω2, it is possible to obtain light whose frequency ω3 satisfies ωl + ω2 = ω3 (1). . This is called frequency up conversion. Similarly, it is also possible to generate light that satisfies ω, -ω, = ω3 (2), and this will be referred to as frequency downconversion.

The present invention uses such optical frequency conversion technology to solve the above problems.

"Example" Hereinafter, the present invention will be described in detail with reference to the drawings.

Example 1 Figure 1 is an example of this invention! 1 is a block diagram showing the configuration of an optical frequency conversion transmitting device according to FIG.

In the figure, a light source 11 emits light with a frequency ω1, and a light source 12 emits light with a frequency ω. These lights are combined at an optical coupler 13 and supplied to a parametric optical frequency converter 14.

The parametric optical frequency converter 14 has frequencies ω3 and ω
, and outputs an optical signal having a frequency ω3 (=ω1−ω,), which is the difference between the two, and supplies it to the optical fiber transmission line 4 through the lens system 15.

In this case, the wavelength that provides the optimum transmission distance and transmission speed is determined from the loss and dispersion of the transmission medium used (in this example, the optical fiber transmission line 4).

As mentioned above, ideal loss characteristics can be theoretically estimated to some extent, but in reality, even a small amount of impurity can cause non-negligible absorption.
It is difficult to predict the minimum loss wavelength at the stage of actual installation.

Furthermore, in consideration of dispersion characteristics, it cannot be said that the optimum transmission wavelength necessarily matches the minimum loss wavelength.

The optimal wavelength is determined by taking all of these into account.

Light source 11 (frequency ωl) and light source +2 in FIG.
The condition that (frequency ω,) should satisfy is that the difference between these frequencies (ω1−ω,) be equal to or closest to the optical frequency ω, which corresponds to the above-mentioned optimal wavelength.

In other words, ω, #ω3=ω1−ω, The frequency ω3 that satisfies (3) is selected as the transmission wavelength.

Information to be transmitted is input to one or both of these light sources 11 and 12, and their output lights are modulated.

In general, wavelength components shorter than the transmission wavelength are strongly subjected to Rayleigh scattering, so even if the output light itself from the light sources 11 and 12 is input to the transmission path, it is thought that it will not reach the receiving end and be detected. You can.

FIG. 2(A) shows the case where one side is pulse modulated. By increasing the strength of the light source 11 with the highest frequency, that is, the shortest wavelength, parametric amplification becomes possible, making the transmitted signal pulse with frequency ω3 higher than the modulated pulse output from the light source 12. Can be output. In this case, the light source 11 can be a high-output solid-state/gas laser, which is not suitable for high-speed modulation, since it can have a constant amplitude.

On the other hand, FIG. 2(b) shows a case where both the light output from the light source 11 and the light source 12 are modulated.

For example, the light source 11 outputs a clock pulse with a frequency of ω8, and the light source 12 outputs signal light modulated with data, so that the parametric optical frequency converter 14
From this, a signal light having a frequency ω3 as shown in FIG. 3(c) is obtained. In this case, the frequency converter 14 can also serve as a so-called NRZ/RZ converter. even here,
It goes without saying that parametric amplification becomes possible by increasing the strength of the light source II, as in the example shown in FIG. 2(a).

Embodiment 2 FIG. 3 is a block diagram showing the configuration of an optical frequency conversion receiver according to a second embodiment of the present invention.

FIG. 2(a) shows the configuration of a receiving device that performs up-conversion of optical frequency and receives the signal. In the figure, the received optical signal of frequency ω3 transmitted through the optical fiber transmission line 4 is combined with the pump light of frequency ω output from the pump light source 21 at the optical coupler 22, and then sent to the parametric optical frequency converter 23. Supplied. Parametric optical frequency converter 2
3 outputs signal light having a frequency of the sum of these frequencies ω, (−ω3+ω4). This signal light is sent to the light receiving element 26 of the receiving section 25 through the optical filter 24.

In such a configuration, if the optical frequency at which the light-receiving element 26 exhibits the best characteristics, i.e., high-speed response, low noise, quantum efficiency, multiplication factor, etc. is ω, then this is, as described above,
In general, it is likely to be on the shorter wavelength side than the transmission wavelength. The sum of the transmission frequency ω and the pump light frequency ω4 is required to be equal to or as close as possible to ω. In other words, it is necessary to select the pump light frequency ω such that ωrLrω,=ω3+ω4 (4).

At this time, if necessary, use the optical filter 24 to set the frequency ω3,
The light receiving element 26 blocks the light of frequency ω4 and only receives the light of frequency ω5.
Let it guide you. Such a receiver configuration can be applied to both direct detection and coherent detection.

Furthermore, as shown in FIG. 3 (b), pump light of frequency ω4 (=ω, +Δω) having a frequency difference Δω of about the microwave frequency with respect to the transmission frequency ω3 is transmitted to the pump light source 3.
1, the received optical signal from the optical fiber transmission line 4 is combined with the optical coupler 32, and then guided to the parametric optical frequency converter 33 for frequency downconversion, thereby directly obtaining a microwave signal. It is also possible to configure

The microwave signal output from the frequency converter 33 is detected by the receiving element 35 in the receiving section 34. in this case,
The receiving element 35 may be one that receives microwaves, and does not need to be sensitive to optical wavelengths.

Embodiment 3 FIG. 4 is a block diagram showing the configuration of an intermediate repeater in an optical transmission system according to Embodiment 3 of the present invention.

Here, we consider the case of pulse modulation, and for simplicity, we consider the case of underwater u? Assume that the pump frequencies on both the receiving and transmitting sides of +Z are the same frequency ω.

The received optical signal Sa (frequency ω3) is combined with the pump light sb from the pump light source 41 in the multiplexing filter 42, and in the optical parameter increase converter 43, the frequency ω1 (=ω
+ω) into signal light Sc.

The pump light sb from this pump light source 41 is as shown in FIG.
), the intensity is slightly modulated by a sine wave from a voltage controlled oscillator (VCO) 44, and the average optical power is stabilized by an automatic power controller (APC) 45. Furthermore, the monitor 46 monitors the average received power of the received optical signal Sa. The frequency of the sine wave output from the VCO 44 is adjusted so that it is equal to the modulation frequency of the received optical signal Sa, as shown in FIGS. 5(a) and 5(b).
Further, control is performed so that the phases of the received optical signal Sa and the modulated components of the pump light sb are phase-locked when they are shifted by 90 degrees.

The output signal Sc from the optical parametric increase converter 43 (
5(C)) is divided into two by an optical branch 47, and the negative half is identified as an optical signal by an optical discriminator 48 and outputted as an optical signal Se (FIG. 5(e)).

The other signal is converted into an electrical signal by a monitor 49 and then passed through an amplifier 50 and a low-pass filter (LPF) 51 to become a correlation signal Sd (FIG. 5(d)). The difference between the correlation signal Sd and the signal from the monitor 46 is calculated using the differential amplifier 51, and then input as a control signal to the VCO 44. This is a measure to prevent the VCO 44 from inadvertently changing its oscillation frequency due to fluctuations in received optical power.

Here, the phase and the gain of the amplifier 50 are adjusted so that the correlation signal Sd becomes zero when the phases of the modulation components of the received optical signal Sa and the pump light sb are shifted by 90 degrees from each other. By doing this, the phase of the received optical signal Sa becomes the fifth
As shown in the figure, when there is a delay, the correlation signal Sd increases,
When proceeding in the opposite direction, the correlation signal Sd can be made to decrease, so the phase fluctuation of the received optical signal Sa can be detected, and the VCO4
It is possible to make the oscillation frequency of 4 coincide with the modulation frequency of the received optical signal Sa, and to perform phase locking at a point where their phases are shifted by 90 degrees.

The signal from the VCO 44 is amplified by a power amplifier 52 and then supplied to a pulse generator 54 to generate clock pulses. Furthermore, this clock pulse is converted into an optical clock pulse Sr (FIG. 5(f)) by the transmitting side pump light source 55, and is combined with the output Se of the optical discriminator 48 by the multiplexing filter 56 to perform optical parametric downconversion. 57, and by the operation shown in FIG. 2(B), the transmission signal Sg (FIG. g)) is obtained.

At this time, the transmitted data signals themselves are all processed by optical elements, so high-speed operation can be expected. In addition, high-speed operation is required for the VCO 44 and the circuit portion that generates the pump lights sb and sr using its output, but it is not necessary to have flat gain characteristics over a wide band.

In addition, if an error occurs in the output Se of the optical discriminator 48 due to the correlation signal Sd being superimposed on the frequency-converted light Sc, an optical limit amplifier is placed before the optical discriminator 48, and Correlation signal S lower in level than Sc
What is necessary is to remove the influence of d.

"Effects of the Invention" As explained above, according to the present invention, it is possible to independently optimize the wavelength at which the light emitting/light receiving element operates and the wavelength at which the transmission path propagates. Parametric interactions also have the advantage of being accompanied by amplification.

Furthermore, the optical transmitter/receiver described above can be applied not only to wired transmission but also to wireless transmission.

The transmittance characteristics for atmospheric light have a peak around 108 m in wavelength. If this wavelength band is used as a channel and a semiconductor light-emitting/light-receiving element is to be used, there is currently no means other than the present invention.

When considering satellite-to-ground communications, semiconductor devices are suitable for on-board equipment from the standpoint of reducing the size and weight of the equipment.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of the optical frequency conversion transmitting device according to the first embodiment of the present invention, and FIG. 2 is a waveform diagram showing the operation of the optical frequency converting transmitting device according to the first embodiment of the present invention. (a) is the case where one side is modulated, and (b) of the same figure is the case where both are modulated. FIG. 3 is a block diagram showing the configuration of an optical frequency conversion receiving device according to a second embodiment of the present invention, and FIG. B) is a block diagram showing the configuration of a receiving device that directly converts into microwaves, FIG. 4 is a block diagram showing the configuration of an intermediate repeater in the optical transmission system according to Embodiment 3 of the present invention, and FIG. FIG. 6 is a block diagram showing the basic configuration of a conventional optical transmission system. l...Transmission unit, 2...Light emitting element, 3.
... Lens system, 4 ... Optical fiber transmission line, 5 ... Receiving section, 6 ... Light receiving element, 1
1...Light source, 12...Light source, 13...
...Optical coupler, 14...Parametric optical frequency converter, 15...Lens system, 21...
... Pump light source, 22 ... Optical coupler, 23.
...Parametric optical frequency converter, 24...
... Optical filter, 25 ... Receiving section, 26 ...
. . . Light receiving element, 31 . . . Pump light source, 32
...... Optical coupler, 33... Parametric optical frequency converter, 34... Receiving unit, 35...
...Reception element, 41 ...Pump light source, 42
...Mixing filter, 43 ... Optical parameter increase converter, 44 ... VCo, 45.
...APC (automatic power controller), 46...
・Monitor, 47... Optical branch, 48...
Optical identifier, 49...Monitor, 50...
Amplifier, 51...LPF, 52...Differential amplifier, 53...Power amplifier, 54...
... Pulse generator, 55 ... Pump light source, 56
. . . Combined filter, 57 . . . Optical parametric down converter. Applicant Nippon Telegraph and Telephone Corporation

Claims (2)

[Claims] (1) A transmitting device that inputs an electrical signal and outputs an optical signal, an optical transmission line that transmits the output optical signal of this transmitting device from the transmitting end to the receiving end, and the optical signal transmitted from the receiving end of this optical transmission path. In an optical transmission system that includes a receiving device that converts output light into an electrical signal and outputs it, the transmitting device includes two light sources, an optical coupler that combines optical signals from these light sources, and an optical coupler that combines optical signals from these light sources. An optical transmission system comprising: an optical frequency converter comprising an optical crystal having nonlinearity and outputting an optical signal having a frequency corresponding to the difference or sum of frequencies of light from the two light sources. (2) A transmitting device that inputs an electrical signal and outputs an optical signal, an optical transmission line that transmits the output optical signal of this transmitting device from the transmitting end to the receiving end, and the optical signal transmitted from the receiving end of this optical transmission path. In an optical transmission system that includes a receiving device that converts output light into an electrical signal and outputs it, the receiving device includes one light source, an optical coupler that multiplexes the light from this light source and the received optical signal, and , an optical frequency converter comprising an optical crystal having optical nonlinearity and outputting an optical signal having a frequency corresponding to the sum or difference of the frequencies of the light from the light source and the received optical signal. Optical transmission method.