JP2007243817A - Optical transmitter and optical communication system using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problems that a transmission band is narrow and a cost is high though an external modulation type optical transmitter is suitable for a large-scale multiple distribution and a long-range transmission, and there is the influence of wavelength dispersion due to a chirping phenomenon, the quality of video signals is degraded and a transmission distance is constrained while a direct luminance modulation type optical transmitter is capable of wide band transmission. <P>SOLUTION: This optical transmitter of a direct luminance modulation type using a laser diode for oscillating the narrow light beam width laser of 1.55 μm band, detects the entire power of RF signals inputted to the optical transmitter, adjusts an optical signal level to be inputted to the laser diode on the basis of the detected level, and controls the optical modulation degree of direct luminance modulation. The optical signal level is set such that the laser oscillated from the laser diode has a narrow light beam width having a chirp suitable for suppressing the generation of SBS. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical transmitter and an optical communication system using the optical transmitter, and more particularly to an FTTH (Fiber To The Home) system or an FTTO (Fiber) for transmitting an RF signal from a relay station to a home or office as a terminal subscriber using an optical fiber. It is suitable for use in the To The Office system.

(FTTH: Spread of Fiber To The Home system)
Current optical communication services have increased demand for transmission capacity, transmission speed, and multi-channel video broadcast transmission. From a conventional transmission system (HFC: Hybrid Fiber Coax) that sends a metal cable to a subscriber, as shown in FIG. 5, the FTTH (Fiber To) is used to transmit the entire area from the trunk line and the relay station to the terminal subscriber as shown in FIG. The Home) system is moving. Up to now, VHF, UHF and BS / CS-IF were transmitted separately, but in the future, BS / CS-IF transmission capable of simultaneously transmitting a wide band up to VHF, UHF, BS / CS-IF bands. Demand is expected to increase.

  Currently, the most popular optical fiber cable for optical communication is a single-mode fiber with a zero-dispersion wavelength (wavelength at which the propagation speed of light is zero and the optical transmission waveform is least degraded) in the 1.31 μm band. (SMF). Since existing SMFs are effectively used, many optical transmitters used so far for video distribution use laser diodes (LDs) having an optical oscillation wavelength in the 1.31 μm band with a dispersion value of zero. However, the 1.31 μm band has a large transmission loss compared to the 1.55 μm band, and long-distance transmission is difficult. Currently, 1.31μm band optical transmitters used in optical transmission systems have also been developed. However, the penetration rate is still low, and the number of units produced is low and expensive. Optical transmitters using wavelengths are not used much because they are judged unsuitable for FTTH systems.

  On the other hand, 1.55μm band optical transmitters are capable of long-distance transmission with less optical fiber transmission loss than 1.31μm band, and many optical amplifiers are for 1.55μm band optical signal amplification. Since then, it has been introduced into many optical communication facilities and the price is relatively low. Currently, the 1.55 μm band is mainly used for the FTTH system for the purpose of multi-distribution, long-distance distribution, and cost reduction.

  However, the optical wavelength in the 1.55 μm band deteriorates in transmission characteristics due to chromatic dispersion that occurs in a single mode fiber (SMF) (generally, chromatic dispersion occurs because optical fibers have different propagation speeds depending on the wavelength of light propagating in the fiber) Easy to do. Laser diodes used in optical transmitters have a single wavelength, but have a certain width in the spectrum of light, so there is a difference in propagation speed due to wavelength dispersion, resulting in a waveform of the optical signal after transmission. Bring change. When an optical signal whose waveform is deformed is converted into an electrical signal (O / E conversion) by an optical receiver, a signal with a second-order distortion deteriorated is extracted, and the signal quality deteriorates. The analog signal (VSB-AM) is deteriorated in video quality due to the occurrence of beat failure (diagonal stripes) in the video due to the deterioration of the secondary distortion.

  An optical transmitter using an external modulation method has been developed to reduce the influence of the chromatic dispersion characteristic and maintain the transmission characteristic, and is now used in the FTTH system. The external modulation method uses a continuous wave laser (CW laser) with a narrow optical spectrum output from the laser diode to modulate the optical signal with the intensity of the brightness. The narrow optical spectrum greatly affects the influence of chromatic dispersion. (Figs. 6 (a) and 6 (b)).

  External modulation optical transmitters are capable of long-distance transmission, but the transmission frequency band is as narrow as 1,000 MHz, so it is possible to simultaneously transmit wide bands up to VHF, UHF, and BS / CS-IF bands, which are expected to grow in demand. It cannot respond to possible BS / CS-IF transmissions, and is expensive and expensive to install.

  There is a conventional direct luminance modulation type optical transmitter capable of broadband transmission. The direct luminance modulation method is a method in which a laser diode is directly modulated by a transmission signal (RF signal) to be transmitted, and an electric signal is converted into an optical signal by superimposing the transmission signal on a laser driving current (bias), thereby increasing or decreasing the luminance of light. In this method, modulation has been performed, and it has become widespread because it is easy to implement (FIGS. 7A and 7B). The direct luminance modulation type optical transmitter is technically easy to broaden the band compared to the external modulation type optical transmitter, can be adapted to BS / CS-IF transmission, and is inexpensive.

  However, in the direct luminance modulation system, a phenomenon (chirping phenomenon) in which fluctuation occurs in the optical spectrum occurs by superimposing a transmission signal (RF signal) on a bias current for driving a laser (FIG. 8). When the chirping phenomenon occurs, the optical spectrum of the optical signal becomes wider, so that the optical signal having a single wavelength has a wavelength width. If there is a wavelength width, a difference occurs in the propagation speed (propagation time) of the optical signal due to the chromatic dispersion characteristics of the optical fiber, and the quality of the transmitted video signal deteriorates (FIGS. 9A and 9B). This effect is easily received in the direct luminance modulation method using the 1.55 μm band for the laser oscillation wavelength, and satisfactory system characteristics (transmission characteristics) cannot be obtained. Also, since chromatic dispersion is largely attributable to the fiber length, the only way to satisfy the system characteristics is to reduce the influence of chromatic dispersion by shortening the transmission distance. For this reason, when an inexpensive FTTH system is constructed with a direct luminance modulation type optical transmitter, the transmission range is limited in terms of maintaining signal quality. At present, the FTTH system using a direct luminance modulation type optical transmitter is used in a facility premises (distribution in a condominium, etc.) in a limited range of about several hundred meters to several kilometers.

  In the FTTH system that performs large-scale multi-distribution and long-distance transmission from the various situations described above, it is desirable to use an external modulation type optical transmitter having an oscillation wavelength of 1.55 μm band. Introducing a small-scale FTTH system with a small number of subscribers due to its narrow bandwidth and high cost is avoided from the viewpoint of cost.

  The present invention utilizes the characteristics of the direct luminance modulation method having an oscillation wavelength of 1.55 μm band, that is, it is possible to transmit over a wide band, can transmit over a long distance with little transmission loss in an optical fiber, and is relatively inexpensive. However, the problem of the modulation method, that is, the influence of the chromatic dispersion due to the chirping phenomenon, the deterioration of the quality of the video signal and the restriction of the transmission distance, has been improved, and the price is lower than the optical transmitter of the external modulation method. It is possible to support high-demand VHF, UHF, and broadband simultaneous transmission up to the BS / CS-IF band, and because the transmission distance can be extended by suppressing the influence of chromatic dispersion. FTTH systems and FTTO systems that were apt to be forgotten to be introduced, especially optical transmitters that can be installed at low cost in optical transmission systems in small and medium-sized facilities, and optical transmission constructed using them. It is intended to provide a stem.

  The inventor of the present invention has focused on narrowing the beam width (low chirp) of a laser oscillated from a laser diode in order to improve the influence of chromatic dispersion, the resulting deterioration in the quality of the video signal, and the restriction on the transmission distance. As a laser having a narrow beam width (low chirp), for example, there is an EC-LD (External Cavity LD). EC-LD is a laser that has been used as a wavelength tunable light source in an optical measuring instrument so far, but has not been used for optical transmission, particularly for communication of analog signals. The cause is that the transmission characteristics of the EC-LD itself are not particularly good, and the stimulated Brillouin scattering caused by the nonlinearity of the fiber when an optical signal having a narrow optical spectrum and a narrow light width (low chirp) is input to the optical fiber. It seems to have been influenced by (SBS: Stimulated Brilliant Scattering).

  However, recent EC-LDs can be used for longer distance transmission than conventional DFB-LDs (Distributed FeedBack LDs) due to improved transmission characteristics, but in order to suppress the influence of chromatic dispersion, When the spectrum is narrowed to narrow the line width, stimulated Brillouin scattering (SBS) occurs due to the nonlinearity of the optical fiber that is the transmission medium. As a cause of the generation of stimulated Brillouin scattering, high light power is incident in a narrow linewidth optical spectrum. When stimulated Brillouin scattering occurs, the level of light reaching the receiver decreases due to the deterioration of transmission loss, and the image quality deteriorates. To prevent stimulated Brillouin scattering, design the transmission line so that high optical power is not incident on the optical fiber and the received light level on the receiver side becomes the reference value, including degradation of transmission loss. However, the transmission distance is shortened by doing so. In order to perform long-distance transmission, a system configuration in which the number of optical amplifiers is increased or the number of light distribution on the transmission side is increased and the number of light distribution on the terminal side is reduced must be increased.

Therefore, the present inventor uses a narrow beamwidth (low chirp) laser, and even if a high power light with a narrow linewidth light spectrum is incident on the optical fiber and the transmission distance is increased, the stimulated Brillouin scattered light In order to be able to suppress the influence of the optical signal, an appropriate chirp spread was given to the optical signal. As described above, in direct luminance modulation, an electric signal is converted into an optical signal by superimposing a transmission signal on a bias (E / O conversion), and therefore the degree of modulation (relation between the superimposed signal and the luminance of the laser) is changed. The chirp spread can be generated by increasing it (the degree of modulation and the chirp width are in a proportional relationship). Therefore, in order to make the chirp spread, the signal input level to the laser may be increased. However, it could not be increased simply for the following reasons.
1. There is a degree of modulation that can optimally operate the characteristics of the laser diode, and when a large number of signal waves are frequency-multiplexed as in CATV, a certain degree of modulation is determined by the relationship between the number of signal waves and the signal level.
2. The signal of the optical transmitter used in the current CATV has different input wave numbers depending on the area and the scale of equipment.
3. Even in a device once installed, the input wave number of the transmission signal is not always constant.
4). Strictly speaking, even the input signal level of CATV is not constant.

The present inventor has considered to control the modulation degree as follows in order to keep the optimum chirp at all times in the above situation.
1. The entire signal power is detected and controlled so that the signal level input to the optical transmitter has an optimum degree of modulation (FIG. 10).
2. Only an arbitrary signal (such as a pilot signal) is detected and controlled (FIG. 11).

  However, since the degree of modulation (signal level) per wave (1ch) increases in the control method, the output signal level of the optical / electrical converter on the signal receiver side varies depending on the input wave number and level of the optical transmitter. In this case, when an optical receiver is installed at the subscriber's home as in the FTTH system, it is not possible to know how the equipment is connected from the optical receiver to the TV. Therefore, there is a possibility that the quality of the signal deteriorates (FIG. 12).

  Actually, on the optical receiver side as well, the electric power output from the optical receiver is detected by detecting the total signal power or detecting a specific one-wave signal (such as a pilot signal) and controlling the output level. The signal level can be kept constant (FIG. 13). However, this requires the addition of a control function on the receiver side and a dedicated optical receiver corresponding to the used optical transmitter (all signal power control). However, the addition of a control function increases the cost and contributes to the introduction of the FTTH system. Accordingly, the present invention has developed the following method as a control method for maintaining the optimum modulation degree for the laser diode without changing the modulation degree per wave for the input signal.

  The optical transmitter according to the present invention is a direct luminance modulation type optical transmitter according to claim 1, wherein a laser diode that oscillates a narrow beam width laser having an oscillation wavelength of 1.55 μm band is used as the laser diode. The total power of the RF signal input to the laser diode is detected, and based on the detection, the level of the optical signal input to the laser diode is adjusted to control the light modulation degree of direct luminance modulation, and the light modulation degree is obtained from the laser diode. The laser to be oscillated was set to have a narrow beam width with a chirp suitable for suppressing the generation of SBS.

  The optical transmitter according to the present invention is a direct luminance modulation type optical transmitter according to claim 2, wherein a laser diode that oscillates a narrow beam width laser having an oscillation wavelength of 1.55 μm band is used as the laser diode. The RF signal input to the optical signal and the internal signal generated in the optical transmitter or the external signal input from the outside of the optical transmitter are combined to form an input signal to the laser, and the RF signal level is detected. Then, based on the detection, the level of the internal signal or external signal is adjusted, the level of the optical signal input to the laser diode is adjusted, and the light modulation degree of direct luminance modulation is controlled. The laser oscillated from the laser diode was set to have a narrow beam width with a chirp suitable for suppressing the generation of SBS. In this case, the frequency of the internal transmission signal or the external input signal can be set to a frequency that does not affect the level of the RF signal.

  The optical transmitter according to the present invention is a direct luminance modulation type optical transmitter as described in claim 4, wherein a laser diode that oscillates a narrow beam width laser having an oscillation wavelength of 1.55 μm band is used as the laser diode. When the level of the RF signal input to the head-end signal processor is lowered or cut off, a pseudo signal is sent to the signal processor and the signal processor operates normally, even when the input level is lowered or cut off. The signal level input to the signal processor is maintained in the same manner as normal, and the light modulation degree of direct luminance modulation is controlled by the input signal, and the laser oscillated from the laser diode suppresses SBS by the input signal level. It was set to have a narrow beam width with a suitable chirp. In this case, as described in claim 5, the pseudo signal can have the same frequency and the same level as the RF signal whose input level is reduced or cut off.

  As described in claim 6, the optical transmission system of the present invention uses any one of the above optical transmitters as an optical transmitter such as a center or a relay station in an optical communication system such as an FTTH system or an FTTO system. It is.

The optical transmitter according to claims 1 to 5 of the present application has the following effects.
1. Since it is a direct luminance modulation type and the laser diode oscillates a narrow beam width laser having an oscillation wavelength band of 1.55 μm, the following effects are obtained.
(1) Wide band transmission is possible, and collective transmission in the BS / CS-IF band, which has been increasing in demand in recent years, is possible.
(2) Long-distance transmission is possible with little transmission loss in the optical fiber, and multiple distribution becomes possible.
(3) It is less expensive than an external modulation type optical transmitter, and the introduction cost to the FTTH system or the FTTO system can be reduced.
(4) Since a narrow beam width laser is oscillated, it is possible to prevent secondary distortion due to the influence of chromatic dispersion, which is a problem in direct luminance modulation in the 1.55 μm band, and it is also possible to prevent image quality deterioration due to secondary distortion.
2. The total power of the RF signal input to the optical transmitter is detected, and based on the detected RF signal power, the input signal level to the laser diode is increased or decreased so that the laser diode is suitable to have an appropriate chirp. Since the modulation degree is controlled, the following effects are obtained.
(1) Since a narrow beam width laser has an appropriate chirp, it is not easily affected by stimulated Brillouin scattering, and the quality of an image is unlikely to deteriorate.
(2) Since the modulation degree (signal level) per wave (1ch) does not increase, the output signal level of the optical / electrical converter on the signal receiver side varies depending on the input wave number and level of the optical transmitter. There is nothing to do. For this reason, even if an optical receiver is installed at the subscriber's home as in the FTTH system or the FTTO system, and it is not possible to grasp how the equipment is connected from the optical receiver to the TV. The signal does not deteriorate in quality until it is finally transmitted to the TV. Further, there is no need to add a control function on the receiver side or to provide a dedicated optical receiver corresponding to the optical transmitter (all signal power control) to be used. And easy to install in FTTO system.
(4) By generating an appropriate chirp, even if a high-power optical signal is incident, it is not easily affected by Brillouin scattering, and a stable optical transmission service is possible.

  The optical transmitter according to claim 2 of the present invention combines an RF signal input to the optical transmitter and an internal signal generated in the optical transmitter or an external signal input from the outside of the optical transmitter. By controlling the level of the combined signal as an input signal to the laser, the light modulation degree of direct luminance modulation is controlled, and the combined signal level suppresses SBS by the laser oscillated from the laser diode. In addition to the above effects, the chirp is always suitable for suppressing SBS regardless of the fluctuation in the level of the RF signal input to the optical transmitter. The narrow beam width laser is oscillated, and the image quality is hardly deteriorated due to being hardly affected by SBS.

  In the optical transmitter according to claim 3 of the present invention, the frequency of the internal signal or the external signal in the optical transmitter according to claim 2 is set to a frequency that does not affect the level of the RF signal. There is no adverse effect on the RF signal even if the signal is combined with an external signal or an internal signal input from the outside of the laser and input to the laser.

  In the optical transmitter according to claim 4 of the present invention, when the input level of the RF signal to the SP at the head end of the optical transmission system is lowered or cut off, the unmodulated carrier wave or other signal is affected to the SP. A pseudo signal of a modulated carrier wave that is not applied is transmitted and the SP is operated normally, and the signal level input from the SP to the optical transmitter is maintained as in the normal state even when the input level is lowered or cut off. The light modulation degree of direct luminance modulation is controlled by the signal, and the input signal level is set so that the laser oscillated from the laser diode has a narrow beam width with a chirp suitable for suppressing SBS. In addition to the effects, there are the following effects. That is, even if the level of the stop signal or the transmitted signal decreases after the broadcast ends, the signal level input to the optical transmitter will be in the same state as normal, and the modulation factor setting of the optical transmitter will be in proportion to the analog signal. There is also an effect that the stimulated Brillouin scattering can be easily suppressed when the number is large. Incidentally, the SP has a squelch function, and when the wave stops or the level of the transmission signal decreases after the broadcast ends, the squelch function may be turned on and the output from the SP may be turned off. In this case, the input of the optical transmitter is reduced, the modulation degree of the laser diode is lowered, stimulated Brillouin scattering occurs, and transmission characteristics deteriorate.

  In the optical transmitter according to claim 5 of the present invention, since the pseudo signal has the same frequency and the same level as the RF signal whose input level is reduced or cut off, the distortion characteristic does not change even after switching to the pseudo signal. The modulation degree can be controlled in the same state as when the modulation degree is controlled based on the RF signal.

  In the optical transmission system according to claim 6 of the present invention, the optical transmitter is used in an optical communication system by an optical transmitter such as a center or a relay station in an optical communication system such as an FTTH system or an FTTO system. An optical transmission system having the characteristics of a transmitter is obtained.

(Embodiment 1)
A first embodiment of the optical transmitter of the present invention will be described with reference to FIG. The optical transmitter of this embodiment uses a laser diode that oscillates a narrow-light-width laser with an oscillation wavelength of 1.55 μm as the laser diode 1, and transmits an RF signal input to the input terminal (RF input) 3 of the optical transmitter 2. This is a method in which the luminance of the laser diode 1 is directly modulated with all the electric power. In this case, a part of the input RF signal is branched by the branching device 4 and detected by the detector 5, and the amplification degree of the amplifier 6 is increased / decreased (controlled) by the detected power, thereby the oscillator 7 in the optical transmitter 2. The level of the internal signal oscillated at is always controlled to be constant, and the controlled internal signal is combined with the RF signal (the RF signal amplified by the amplifiers 9 and 10) by the multiplexer 8 so that the laser diode 1 As an input signal, the laser diode 1 directly modulates the luminance. The level control of the internal signal is such that a narrow beam width laser having a chirp suitable for suppressing SBS has a modulation degree oscillated from the laser diode 1. An output signal including an RF signal directly luminance-modulated by the laser diode 1 is transmitted to the outside from an output terminal (RFoutput) 11 of the optical transmitter 2.

  For the laser diode 1, for example, an EC-LD can be used. The light width of the EC-LD is about 400 kHz, and an optical signal having a narrow light width that can be put to practical use in the present invention can be obtained. The beam width of DFB-LD is as wide as several MHz (about 3 MHz). However, since the line width of DFB-LD has been recently reduced, DFB-LD can be used in the present invention in the near future.

  In the present invention, an external signal such as a pilot signal can be used instead of the internal oscillation signal. In this case, the RF signal level input to the optical transmitter 2 is detected, and a laser with a narrow beam width having a chirp suitable for suppressing SBS from the external signal level is oscillated from the laser diode 1 based on the detected level. It controls so that it may become a modulation degree. In this case, the modulation degree can be controlled without changing the level of the RF signal used on the receiver side.

(Embodiment 2)
A second embodiment of the optical transmitter of the present invention will be described with reference to FIGS. This optical transmitter is also a direct luminance modulation method, and a laser diode that oscillates a narrow beam width laser with an oscillation wavelength of 1.55 μm band is used as the laser diode. In recent FTTH system transmission methods, both analog signals and digital signals are transmitted. In this case, since the input level of the analog signal is higher than the input level of the digital signal, the analog signal has a higher ratio related to the modulation degree. In the CATV system, when an analog signal is transmitted, a signal processor (SP: FIG. 3) with automatic gain control (AGC) is installed at the head end, so that the input level to the optical transmitter is almost constant during the daytime program broadcast. It is. However, at midnight, a program using an analog signal may stop the wave or lower the level of the transmission signal after the broadcast ends depending on the broadcasting station. Normally, the SP has a squelch function that determines that there is no input signal when the input level falls below a predetermined level and turns off the output. For this reason, as described above, when the level of the wave stop or the transmission signal is reduced and the input of the optical transmitter is reduced, the modulation degree of the laser diode is reduced, SBS is generated, and transmission characteristics are deteriorated (FIG. 3). The optical transmitter of the second embodiment shown in FIG. 2 is configured such that even in such a state, the modulation degree of the laser diode does not decrease, SBS is unlikely to occur, and transmission characteristics are unlikely to deteriorate.

  In the embodiment of FIG. 3, a pseudo signal is sent to the SP at the time of a wave stop or a level drop so that the SP squelch function does not operate and the SP operates in the same manner as when it is normal. Therefore, for example, a switch or the like is provided in the squelch circuit portion of the SP, and this switch is automatically switched to the pseudo signal generator when the wave is stopped or the level is lowered to generate a pseudo signal from the pseudo signal generator. , So that it is automatically sent to the SP. It is desirable to use an unmodulated carrier wave or a modulated carrier wave having the same frequency and the same level as the RF signal (operation signal) input to the optical transmitter for this pseudo signal (FIG. 2). An optical signal with an appropriate chirp is oscillated from the diode 1 in the same state as when there is an RF signal. By the way, the spread of the chirp during normal broadcasting without stopping or lowering the level was about 1.2 GHz, whereas it was 700 MHz when 10 VHF waves (1 to 12 ch) stopped. Based on this embodiment, when a pseudo signal is input when the wave is stopped, the chirp width becomes 1.2 GHz as in the normal case.

(Embodiment of optical transmission system)
The optical transmission system according to the present invention is obtained by introducing the optical transmitter of the above-described embodiment into an optical transmitter such as a center of an optical transmission system such as an FTTH system or an FTTO system or a relay station.

(Experimental example of the present invention)
Table 1 shows the experimental results of transmitting a multi-channel signal using the transmission characteristics of the optical transmitter actually developed by the present inventors. In Table 1, the horizontal axis indicates the distance of the fiber that is the transmission line, and the vertical axis indicates the degree of deterioration of the secondary distortion (CSO). Deterioration due to increasing the fiber distance was confirmed by setting the state of no fiber to zero. When affected by the chromatic dispersion, the second order distortion is mainly deteriorated. Therefore, Table 1 shows the measurement result of the composite second order (CSO). From Table 1, although EC-LD is a direct modulation type, the secondary distortion is less deteriorated than DFB-LD, which is the same direct modulation type, and the external modulator (EXT-MOD) is much affected by the distance of the fiber. You can see that it is not.

  The optical transmitter and optical transmission system of the present invention can also be used for optical communication systems other than optical transmission systems such as FTTH systems and FTTO systems other than CATV.

Explanatory drawing which shows 1st Embodiment of the optical transmitter of this invention. Explanatory drawing which shows 2nd Embodiment of the optical transmitter of this invention. Operation | movement explanatory drawing of a normal signal processor. Explanatory drawing of the conventional HFC transmission system. Explanatory drawing of the conventional FTTH transmission system. (A) is explanatory drawing of an external modulation system, (b) is explanatory drawing which shows the mode of the time difference after transmission by an external modulation system and wavelength dispersion. (A), (b) is explanatory drawing of a direct luminance modulation system. Explanatory drawing which shows the mode of the time difference after transmission by a chirping phenomenon and wavelength dispersion. (A) is explanatory drawing which shows the mode of the time difference after transmission by the wavelength dispersion at the time of 1.31 micrometer band laser transmission, (b) is explanatory drawing which shows the mode of the time difference after transmission by the wavelength dispersion at the time of 1.55 micrometer band laser transmission. Block explanatory diagram of the all signal power control method. The block diagram of the arbitrary signal (pilot signal) detection type control block in the input signal to an optical transmitter. FIG. 3 is a connection state explanatory diagram on the subscriber side. The block diagram of the arbitrary signal (pilot signal) detection type | mold control block in the optical receiver side.

Explanation of symbols

1 Laser diode 2 Optical transmitter 3 Optical transmitter input terminal (RF input)
4 Branching device 5 Detector 6 Amplifier 7 Oscillator 8 Mux 9, 10 RF signal amplifier 11 Transmitter output (RF output)

Claims (6)

  In a direct luminance modulation type optical transmitter, a laser diode that oscillates a narrow beam width laser with an oscillation wavelength of 1.55 μm is used as the laser diode, and the total power of the RF signal input to the optical transmitter is detected. Based on this, the optical signal level input to the laser diode is adjusted to control the light modulation degree of direct luminance modulation, and the light modulation degree is a chirp suitable for the laser emitted from the laser diode to suppress the occurrence of SBS. An optical transmitter characterized in that it is set so as to have a narrow light beam width.   In a direct luminance modulation type optical transmitter, a laser diode that oscillates a narrow beam width laser having an oscillation wavelength of 1.55 μm band is used as a laser diode, and an RF signal input to the optical transmitter and an optical transmitter are generated. An internal signal or an external signal input from the outside of the optical transmitter is combined to be an input signal to the laser, and the RF signal level is detected. Based on the detection, the internal signal or the external signal The level of the optical signal input to the laser diode is adjusted to adjust the optical modulation degree of direct luminance modulation, and the optical modulation degree is controlled by the laser oscillated from the laser diode to suppress the generation of SBS. An optical transmitter set to have a narrow beam width with a chirp suitable for the above.   3. The optical transmitter according to claim 2, wherein the frequency of the internal signal or the external signal is a frequency that does not affect the level of the RF signal.   In a direct luminance modulation type optical transmitter, a laser diode that oscillates a narrow beam width laser with an oscillation wavelength of 1.55 μm is used as the laser diode, and the level of the RF signal input to the signal processor at the head end of the optical transmission system is A pseudo signal is sent to the signal processor when it is lowered or shut down, and the signal processor operates normally. Even when the input level drops or shuts down, the signal level input to the signal processor remains the same as when it is normal. The light modulation degree of direct luminance modulation is controlled by the input signal, and the input signal level is set so that the laser oscillated from the laser diode has a narrow beam width with a chirp suitable for suppressing SBS. An optical transmitter.   5. The optical transmitter according to claim 4, wherein the pseudo signal has the same frequency and the same level as the RF signal whose input level is reduced or cut off. An optical communication system, wherein an optical transmitter such as a center or a relay station in an optical communication system such as an FTTH system or an FTTO system is the optical transmitter according to any one of claims 1 to 5.

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