JPH06216853A - Optical frequency stabilizing system - Google Patents

Abstract

PURPOSE:To reduce the fluctuation of optical intensity by branching the signal light undergone the optical FSK modulation and transmitted to an optical element to convert the transmitted light into an electric signal, securing the correlation between the electric signal and an original signal, and correcting the optical frequency by the error signal obtained between the electric and original signals. CONSTITUTION:A semiconductor laser 100 of a transmission light source undergoes the optical frequency shift keying FSK modulation when an injecting current is directly modulated by an original signal 101. The FSK signal light passes through an optical element 104 whose transmitted light intensity varies by the optical frequency and is mostly transmitted to an optical fiber 107 by an optical branching device 106. Thus the signal light is received by a receiver 108 through the fiber 107 and partly converted into an electric signal by a photodetector 105. The correlation is secured between the electric signal and the signal 101 by an error signal generator 102, and the optical frequency of the laser 100 is controlled by an error signal 103. So that the optical frequency of the laser 100 can be stabilized at the frequency with which the transmitted light intensity of the element 104 is maximized. Thus the residual intensity modulation component of the FSK modulated light is reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of stabilizing an optical frequency in an optical FSK communication system, and more specifically, to each optical frequency and optical multiplexer of each FSK signal in an optical FSK-FDM communication system using an optical multiplexer. Suitable for stabilizing the relative positional relationship with the optical frequency at which the transmitted or reflected light intensity is maximum, and at the same time reducing the fluctuation of the light intensity of each FSK signal light depending on the modulation code. Optical frequency stabilization method.

[0002]

2. Description of the Related Art An optical FSK communication system which modulates by shifting an optical frequency has a compact signal spectrum as compared with a conventional optical intensity modulation communication system, so that a plurality of channels can be transmitted at narrow frequency intervals. Capacity transmission is possible. Further, since the FSK signal light can be obtained by directly modulating the injection current of the semiconductor laser which is the light source, the transmitter structure is simplified. Furthermore, since the optical intensity of the optical FSK communication system is essentially constant during transmission, the transmission quality is less likely to deteriorate due to the nonlinear effect (optical Kerr effect) in the optical fiber that is the transmission path. Therefore, the optical FSK communication system has been attracting attention as a future communication system.

However, in the actual optical FSK communication system, since the injection current of the semiconductor laser is changed to generate the FSK signal light, the light intensity also changes depending on the modulation code. This phenomenon becomes more serious as the modulation bit rate of the optical FSK modulation becomes faster. This is because it is necessary to increase the shift amount of the optical frequency with the increase of the modulation bit rate, so that the change amount of the injection current to the semiconductor laser is inevitably large.

As a study on the light intensity fluctuation (residual intensity modulation component) caused by the direct modulation of the light source in this optical FSK communication system, the 1992 Spring Conference of the Institute of Electronics, Information and Communication Engineers, Volume 4, pages 433 to 433 It is described on page 434 as "Influence of residual intensity modulation component in coherent optical amplification repeater transmission system". According to this document, when transmitting 1-channel FSK signal light, unnecessary phase fluctuations in the signal light due to the nonlinear effect of the fiber,
At the same time, it has been confirmed by simulation that unnecessary frequency fluctuations occur. In this document, when the FSK signal light having a residual intensity modulation component of 4% is transmitted at 10,000 km in a multi-relay system, the repeater output is approximately +
It mentions that it needs to be set to 10 dBm or less.

[0005]

As described in the above document, F
If there is a residual intensity modulation component in the SK signal light, unnecessary phase fluctuations and unnecessary frequency fluctuations occur during optical fiber transmission. When such unnecessary phase and frequency fluctuations occur, the transmission quality is degraded. Further, in order to suppress the deterioration of the transmission quality to a certain value or less, the optical output of the transmitter and the maximum output of the repeater are limited.

Further, although the example of the documents cited above is limited to the transmission of one-channel FSK signal light, the optical FSK-F is used.
Multiple FSK with one optical fiber like DM transmission
When transmitting the signal light, the phase and frequency of the signal light fluctuate due to the influence of the residual intensity modulation component of the other channel in addition to the residual intensity modulation component of the own channel.
This is due to the cross phase modulation effect (XPM), and the ratios of the phase fluctuation amount and the frequency fluctuation amount to the residual intensity modulation component are larger than the self phase modulation effect (SPM) in the case of one channel. Further, the phase fluctuation and the frequency fluctuation due to XPM increase as the number of channels to be transmitted increases, which is an obstacle in realizing a future super multi-channel optical FSK-FDM transmission system.

An object of the present invention is to realize a reduction in light intensity fluctuation which reduces a residual intensity modulation component which is an obstacle when transmitting by an optical FSK modulation method. At the same time, in the optical FSK-FDM transmission system using the optical multiplexer, the optical frequency of each transmission light source is stabilized so that the signal light intensity emitted from the optical multiplexer is maximized.

[0008]

In order to achieve the above object, the optical frequency stabilization system of the present invention comprises an optical element in which the intensity of light transmitted or reflected changes depending on the optical frequency.
Means for sending out SK signal light to the transmission path after transmitting or reflecting the SK signal light, a photodetector for detecting a part of the signal light transmitted or reflected by the optical element and converting it into an electric signal, The optical signal from the photodetector and the transmission light source are optical FSK
Having means for detecting a correlation signal with the original signal to be modulated, and adjusting one or both so that the optical frequency of the transmission light source and the optical frequency at which the optical intensity transmitted or reflected by the optical element becomes maximum After that, the optical frequency of the transmission light source, or the light transmitted or reflected by the optical element so that the fluctuation of the light intensity of the FSK signal light after being transmitted or reflected by the optical element using the correlation signal is reduced. It can be realized by providing a means for correcting the value of the optical frequency at which the intensity becomes maximum.

Further, by using as the optical element an optical element whose light intensity that is periodically transmitted or reflected on the optical frequency axis is changed, one optical element stabilizes the optical frequencies of a plurality of transmission light sources, In addition, fluctuations in light intensity can be reduced.
In this case, the correlation signal between the original signal for optical FSK modulating the transmission light source and the optical signal transmitted or reflected by the FSK signal light as a correction signal used for stabilizing the optical frequency and reducing the fluctuation of the light intensity. , The electric signal from one photodetector can be divided and used when each original signal is different. In the case of optical FSK-FDM transmission, each channel transmits a different signal, so the present invention can also be used in optical FSK-FDM transmission.

[0010]

The FSK signal light is transmitted or reflected by the optical element whose intensity varies depending on the optical frequency. A part of the transmitted light or the reflected light is detected by a photodetector, converted into an electric signal and output. When the correlation between the electric signal and the original signal obtained by FSK-modulating the transmission light source is taken, the optical frequency and FS at which the intensity of light transmitted or reflected by the optical element becomes maximum.
An error signal relative to the K signal light frequency is obtained. Therefore, by correcting the optical frequency of the transmission light source so that the error signal becomes 0, the optical frequency of the FSK signal light can be stabilized. When the FSK signal light has a residual intensity modulation component, a signal proportional to the residual intensity modulation component also appears in the error signal. Therefore, in this case, if the optical frequency of the transmission light source is controlled so that the error signal becomes 0, the residual intensity modulation component is reduced. That is, it is possible to reduce the fluctuation of the light intensity at the same time as stabilizing the frequency of the FSK signal light.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an optical frequency stabilization system according to the present invention.
1 shows a configuration of an embodiment when applied to a channel optical communication system.

The semiconductor laser 100 as the transmission light source has an original signal 1
By directly modulating the injection current by 01, optical FSK modulation is performed. The FSK signal light passes through the optical element 104 whose transmitted light intensity changes depending on the optical frequency, and then, most of it is sent to the optical fiber 107 by the optical branching device 106 and received by the receiver 108. A part of the light input to the optical branching device 106 is converted into an electric signal by the photodetector 105. In this example, the optical element 104 and the optical branching device 106 are shown as separate elements, but the number of optical components can be reduced by using another optical element having these two functions at the same time.

An error signal generator 102 obtains a correlation between the electric signal and the original signal 101 to obtain an error signal 103. By adjusting the optical frequency of the laser 100 using the obtained error signal 103, the optical frequency of the laser 100 can be stabilized to a frequency at which the transmitted light intensity of the optical element 104 is maximized, and the optical frequency remains in the optical FSK modulated light. When the intensity modulation component is present, the residual intensity modulation component after passing through the optical element 104 is reduced. The correlation between the electrical signal output from the photodetector 105 and the original signal 101 can be easily obtained by using a double balanced mixer or the like and multiplying the two signals.

Next, referring to FIGS. 2 and 3, the error signal 103
A description will be given of how the control can stabilize the optical frequency and reduce the fluctuation of the light intensity.

FIG. 2 is a diagram schematically illustrating how the optical frequency is stabilized by the present invention when the FSK signal light has no residual intensity modulation component.

The uppermost part on the left side of FIG. 2 is an optical element whose transmitted light intensity changes depending on the optical frequency (corresponding to 104 in FIG. 1).
5 is an example showing the relationship between the optical frequency and the transmittance. A case where the FSK signal light is transmitted through this optical element will be described.
The FSK signal light has two peaks on the optical frequency axis corresponding to the marks and spaces of the original signal. There are three possible relative positional relationships between the FSK signal light and the transmission peak frequency of the optical element, as shown in FIGS. 2A, 2B, and 2C. That is, (a) when the optical frequency in the middle of the mark and space matches the transmission peak frequency, (b)
When the optical frequency between the marks and spaces is lower than the transmitted light peak frequency, (c) when the intermediate optical frequency between the marks and spaces is higher than the transmitted light peak frequency.
In each of the cases (a), (b), and (c), the time changes after the FSK signal light has passed through this optical element are shown correspondingly on the right side. In the case of (a), the transmitted light intensity does not change with time. In the case of (b), the transmitted light intensity changes with time in phase with the original signal. In the case of (c), the transmitted light intensity changes with time in the opposite phase to the original signal. That is, when a part of the transmitted light is detected and converted into an electric signal, the correlation with the original signal is taken. In the case of (a), the correlation is 0, in the case of (b) the correlation is positive, and in the case of (c) If, the correlation is negative. This is the error signal 103 in FIG. Therefore, by controlling the optical frequency of the laser so that this error signal becomes 0, the relative positional relationship between the transmission peak of the optical element and the signal light is always stabilized in the state of (a).

Although this explanation is for the case where the optical frequency of the laser is controlled by the error signal, it is obvious that the same result can be obtained if there is a means for controlling the transmission peak frequency of the optical element by the error signal. is there.

FIG. 3 is a diagram schematically illustrating how the present invention stabilizes the optical frequency and simultaneously reduces the fluctuation of the light intensity when the FSK signal light has a residual intensity modulation component.

Similar to FIG. 2, the uppermost part on the left side shows the relationship between the optical frequency and the transmittance of the optical element whose transmitted light intensity changes depending on the optical frequency, and the lower side thereof shows the FSK signal light and the light. 3 shows three relative positional relationships (a), (b), and (c) with the transmission peak frequency of the device. Also,
The time change of the transmitted light intensity in each of (a), (b), and (c) is shown on the right side. In this example, the light intensity corresponding to the mark is stronger than the light intensity corresponding to the space. In the case of (a), the intermediate frequency between the mark and the space coincides with the transmission peak frequency.
Since the mark strength is strong unlike the case of (a), the strength of the mark becomes strong even if the transmitted light on the right side changes with time. (B) is a case where the intermediate frequency between the mark and the space is slightly higher than the transmission peak frequency. In this case, since the light intensity of the transmitted mark is equal to that of the space, the transmitted light intensity does not change with time as shown in the right side graph of time change. (C)
Is the case where the intermediate frequency between the mark and the space is higher than the transmission peak frequency. In this case, the intensity of the transmitted light becomes stronger in the space, and therefore changes in the opposite phase with respect to the original signal as in the time change on the right. As in the case of FIG. 2, after detecting a part of the transmitted light and converting it into an electric signal,
Taking the correlation with the original signal, the correlation is positive in the case of (a),
In the case of (b), the correlation is 0, and in the case of (c), the correlation is negative. Therefore, by controlling the optical frequency of the laser so that the error signal becomes zero, the relative positional relationship between the transmission peak of the optical element and the signal light is always stabilized in the state of (b). That is, the optical frequency is stabilized and the transmitted light has no residual intensity modulation component.

In this example, the light intensity of the mark is stronger than the light intensity of the space. Conversely, even when the light intensity of the space is stronger than the light intensity of the mark, the optical frequency is similarly stabilized and the light intensity fluctuation is caused. Can be reduced. Furthermore, even when the positional relationship between the mark and the space is reversed on the optical frequency axis (that is, when the optical frequency of the mark is lower than the optical frequency of the space), the optical frequency is stabilized and the fluctuation of light intensity is reduced. Is possible.

FIG. 4 shows an optical FSK having a plurality of transmission light sources.
FIG. 7 is a configuration diagram of a second embodiment when the optical frequency stabilization system according to the present invention is applied to the FDM transmission system.

The lasers 400, 410, 420 emit light at different optical frequencies, and the FSK signal lights from the respective lasers are combined by the optical multiplexer 404 to become an optical frequency division multiplexed (optical FDM) signal. This optical FDM signal is sent to the optical splitter 40.
It is branched into two by 6 and most of them are sent to the optical fiber 407 and received by the receiver 408.

On the other hand, a part of the optical FDM signal is detected by the photodetector 40.
5, and converted into an electric signal. This electrical signal is 3
It is distributed to two error signal generators 402, 412, 422. The optical frequency of the lasers 400, 410, 420 is adjusted by adjusting the bias current and the temperature of each of the optical multiplexers 404.
The respective transmitted light intensities are substantially matched to the optical frequency at which the transmitted light intensity becomes maximum. As a result, the electric signal output from the photodetector 405 includes the information on the optical frequency fluctuation of each of the three lasers and the information on the light intensity fluctuation. In each error signal generator 402, 412, 422, this electrical signal and the original signal 401, 411, 42 corresponding to each laser
By taking the correlation with 1, the frequency fluctuation information and the light intensity fluctuation information corresponding to each laser are separated, and each error signal 403, 413, 423 is generated. Using the error signals 403, 413, 423, the lasers 400, 41
By controlling the optical frequencies of 0 and 420, it is possible to stabilize the optical frequency of each laser and reduce the fluctuation of the light intensity.

Although FIG. 4 shows the case where three lasers are used as the transmission light source, the present invention can be similarly applied even when there are more lasers.

As the optical multiplexer 404, a Mach-Zehnder type optical multiplexer in which Mach-Zehnder type interferometers are connected in multiple stages, an arrayed waveguide type optical multiplexer, or the like can be used.

[0026]

As described above, according to the present invention, it is possible to reduce the fluctuation of the light intensity, that is, the residual intensity modulation component that leads to the deterioration of the quality during transmission in the optical FSK communication system, so that the transmission quality is improved. In addition, since the rate of deterioration of transmission quality depends only on the magnitude of the residual intensity modulation component, the reduction of the residual intensity modulation component in the overall optical intensity of the signal light means that the signal light intensity that can be input to the optical fiber is increased. It also means that you can. Therefore, there is also an advantage that the transmission distance is extended. This also means that in a long-distance transmission system using optical repeaters, the repeater interval is expanded, so the number of optical repeaters to be used is reduced, the cost of the entire system is reduced, and the maintainability is also easy. Occurs.

Further, when the present invention is applied to the optical FSK-FDM communication system, the optical frequency of each transmitter can be stabilized near the peak of the transmitted light or the reflected light of the optical multiplexer for multiplexing the signal light of the transmitter. Therefore, there is also an advantage that the signal light can be stably sent to the optical fiber with low loss.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an embodiment when a frequency stabilization system according to the present invention is applied to an optical FSK communication system.

FIG. 2 is a diagram for explaining how the optical frequency of the FSK signal light is stabilized by the present invention.

FIG. 3 is a diagram showing an FS having a residual intensity modulation component according to the present invention.
It is a figure explaining a mode that the optical frequency of K signal light is stabilized and a residual intensity modulation component is reduced at the same time.

FIG. 4 shows an optical FSK-F based frequency stabilization system according to the present invention.
It is a figure which shows the structure of one Example at the time of applying to a DM communication system.

[Explanation of symbols]

100,400,410,420… Transmission light source, laser, 101,401,411,421
... optical FSK modulated original signal, 102,402,412,422 ... error signal generator, 103,403,413,423 ... error signal, 104,404 ... optical element,
Or multiplexer, 105,405 ... Photodetector, 106,406 ... Optical brancher,
107,407 ... optical fiber, 108,408 ... receiver.

Claims (7)

[Claims] 1. Optical frequency shift keying modulation (optical FS
In the optical communication system using the K) method, the FSK signal light as a transmission signal is transmitted to an optical element whose transmitted light intensity or reflected light intensity changes according to the optical frequency,
Or means for reflecting, a means for sending the transmitted light or the reflected light to a transmission line, a light detecting means for detecting a part of the transmitted light or the reflected light and converting it into an electric signal, and the light detecting means. The FSK is calculated from the received electric signal.
A means for correlating the electric signal with an original signal for optical FSK-modulating the light source, in order to generate an error signal containing information on the optical frequency fluctuation of the signal light and information on the light intensity fluctuation of the FSK signal light; Adjusting the optical frequency of the light source using the error signal obtained from the correlation between the electric signal and the original signal to obtain the optical frequency value of the FSK signal light and the transmitted light intensity of the optical element, Or a means for adjusting the relative positional relationship with the optical frequency value that maximizes the reflected light intensity, and the adjusting means adjusts the optical frequency of the FSK signal light to the transmitted light intensity of the optical element or the reflected light. Optical frequency stabilization method characterized by stabilizing near the optical frequency where the intensity is maximum. 2. Optical frequency shift keying modulation (optical FS
In the optical communication system using the K) method, the FSK signal light as a transmission signal is transmitted to an optical element whose transmitted light intensity or reflected light intensity changes according to the optical frequency,
Or means for reflecting, a means for sending the transmitted light or the reflected light to a transmission line, a light detecting means for detecting a part of the transmitted light or the reflected light and converting it into an electric signal, and the light detecting means. The FSK is calculated from the received electric signal.
A means for correlating the electric signal with an original signal for optical FSK-modulating the light source, in order to generate an error signal containing information on the optical frequency fluctuation of the signal light and information on the light intensity fluctuation of the FSK signal light; The FSK signal light is adjusted by adjusting the optical frequency at which the transmitted light intensity or the reflected light intensity of the optical element is maximized by using the error signal obtained from the correlation between the electric signal and the original signal. Means for adjusting the relative positional relationship between the optical frequency value of the optical element and the optical frequency value at which the transmitted light intensity or the reflected light intensity of the optical element is maximum, and the adjusting means controls the light of the FSK signal light. An optical frequency stabilization system, characterized in that the frequency is stabilized in the vicinity of an optical frequency where the transmitted light intensity or the reflected light intensity of the optical element is maximized. 3. The optical frequency stabilization system according to claim 1 or 2, wherein the optical frequency fluctuations of the plurality of light sources are commonly controlled by one or more optical elements, so that the optical frequency variation is controlled according to the optical frequency. An optical frequency stabilizing method using an optical element in which transmitted light intensity or reflected light intensity changes periodically. 4. An optical frequency stabilization system, wherein the optical frequency stabilization system according to claim 1, 2 or 3 is applied to an optical FSK and optical frequency division multiplexing (optical FDM) transmission system. 5. The optical frequency stabilizing system according to claim 3, wherein an optical multiplexer is used as the optical element. 6. The optical frequency stabilizing system according to claim 5, wherein a Mach-Zehnder type optical multiplexer is used as the optical multiplexer. 7. The optical frequency stabilizing system according to claim 5, wherein an arrayed waveguide type optical multiplexer is used as the optical multiplexer.