JP2006203766A - Optical transmitter/receiver - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical transmitter/receiver where the transmitting wavelength of an optical filter can be adjusted so as to optimize transmission characteristic. <P>SOLUTION: An optical transmitting device which directly modulates laser beams digitally modulates the laser beams with bias current sufficiently higher than a threshold current. The modulated optical signal is transmitted through a transmission path using an optical fiber. After transmission just before photoelectric conversion by an optical receiver, a band-pass optical filter is inserted. After the photoelectric conversion, clock extraction is carried out by a clock extraction circuit from a part of electric output, but the transmitting wavelength of the optical filter is adjusted so as to make RF intensity maximal in a clock pulse. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical transceiver having an optical transmitter based on a direct modulation method of a semiconductor laser and an optical communication system using the same.

  Optical fiber communication has been speeding up year by year, and the transmission rate of a single channel is mainly 10 Gbit / s. Furthermore, practical application of a system having a transmission rate of 40 Gbit / s is progressing. In order to spread such an ultra-high-speed optical communication system, it is essential to make the optical transmitter / receiver compact and reduce the cost.

  For this reason, in an optical transmitter, a configuration based on a direct modulation method that modulates a driving current of a semiconductor laser is widely adopted mainly for short-range communication. The direct modulation method can be reduced in cost because the configuration is simple and an expensive external modulator is not required.

  However, in optical fiber communication, the transmission distance is severely limited by the group velocity dispersion of the optical fiber. The transmittable distance limited by the group velocity dispersion varies depending on the pulse waveform and pulse width of the optical signal, but varies greatly depending on the wavelength chirp of the modulated light.

  Wavelength chirp is a phenomenon in which the wavelength varies due to phase modulation that occurs simultaneously when intensity of laser light is modulated. In the direct modulation method that modulates the drive current of a semiconductor laser that oscillates in a single longitudinal mode, the wavelength chirp appears relatively larger than in the external modulation method that uses an optical modulator. Wavelength chirp expands the effect of waveform distortion caused by group velocity dispersion when transmitting through an optical fiber and causes waveform degradation, thus significantly limiting the transmission distance.

  Therefore, a method that suppresses wavelength chirp as much as possible is required. Further, when the optical signal power increases, waveform distortion occurs due to the nonlinearity and dispersion of the optical fiber, so a countermeasure against the nonlinear effect is required.

  FIG. 5 is a diagram for explaining a conventional optical transmitter with suppressed wavelength chirp. The optical transmitter is a direct modulation system using a DFB (Distributed Feedback) laser 1 and 10 is an optical filter including an optical filter. A circuit 3 is an optical fiber as a transmission line (see, for example, Non-Patent Document 1).

  The modulated light generated from the DFB laser 1 is guided to the optical filter and the optical circuit 10. It has been reported that the optical filter can reduce the chirp of the output light and improve the extinction ratio by cutting a part of the optical spectrum expanded by the wavelength chirp.

  At this time, in order to put the optical filtering effect in the best condition, a part of the light after passing through the optical filter is monitored to control the wavelength of the DFB laser. An optical transmitter having such a filtering function can improve transmission characteristics and extend the transmission distance.

D. Mahgerefeh, P.M. S. Cho, J. et al. Goldhar, and H.C. I. Mandelberg, “Penalty-free propagation over 600 km of non-dispersion-shifted fiber at 2.5 Gb / s using a directed laser modified trans 99”, CL. 182, 1999.

  In such a conventional optical communication system, the effect of optical filtering varies depending on the dispersion characteristics and non-linearity of the optical fiber as the transmission path, but since the optical filter is provided on the optical transmitter side, transmission There was a problem that it was difficult to optimize the transmission wavelength of the optical filter from the later characteristics.

  For example, if the optical transmitter and the optical receiver are regularly provided on a one-to-one basis, a test signal is transmitted from the optical transmitter at the initial stage of installing the transmission / reception device, and this is transmitted to the optical receiver. The effect of optical filtering is verified based on the result received in step (2), and if necessary, the transmission wavelength of the optical filter on the optical transmitter side can be set to the optimum wavelength. However, the optical transmitter and the optical receiver If the optical receiver as the transmission destination is unspecified, the transmission wavelength of the optical filter is changed for each optical receiver as the communication partner. It is difficult.

  An object of the present invention is to provide an optical transmission / reception apparatus capable of adjusting the transmission wavelength of an optical filter so as to optimize transmission characteristics.

  For example, in an optical transmitter that directly modulates laser light emitted from a DFB laser that oscillates in a single longitudinal mode, digital modulation is performed at a bias current sufficiently higher than a threshold current. In this way, modulation with a bias current higher than the laser threshold improves the modulation characteristics but decreases the extinction ratio. The modulated optical signal is transmitted through a transmission line using an optical fiber. After transmission, a bandpass optical filter is inserted immediately before photoelectric conversion by the optical receiver.

  After photoelectric conversion, a clock extraction circuit extracts a clock from a part of the electrical output, and the transmission wavelength of the optical filter is adjusted so that the RF (Radio Frequency) intensity of the clock is maximized.

  That is, although waveform distortion due to dispersion in the transmission path occurs, adjusting the transmission wavelength of the optical filter increases the strength of the extracted clock if the distortion due to dispersion is reduced. Further, even if the extinction ratio of the optical signal is improved by the optical filter, the strength of the extracted clock increases. Therefore, stable communication with a low error rate (bit error rate) can be obtained by adjusting the transmission wavelength of the optical filter so that the strength of the clock is maximized.

  A Mach-Zehnder type optical filter can be used as the optical filter. In this case, the output of the optical filter has two ports whose transmission wavelength band is shifted, and each optical output intensity is monitored, and the transmission wavelength of the Mach-Zehnder type optical filter is adjusted so that the optical output intensity becomes an optimum ratio. . The optimum ratio is a value when the RF intensity of the clock becomes maximum when the clock signal is extracted from a part of the output by the clock extraction circuit after photoelectric conversion of the optical signal.

  In operation, the present invention directly modulates with a bias higher than the laser threshold current and transmits an optical signal with a low extinction ratio. As described above, when modulation is performed with a high bias, the modulation band of the laser is expanded, and a high-speed signal can be stably generated. Also, the lower extinction ratio is less affected by the nonlinear effect of the optical fiber. This effect is prominent when the extinction ratio is 3 dB or less.

  Waveform degradation occurs depending on the dispersion and nonlinear characteristics of the optical fiber, but the extinction ratio is improved by filtering the optical signal directly modulated using an optical filter on the optical receiver side. Chirp to be reduced.

  The condition for optimum transmission characteristics is determined by the fact that the RF intensity of the clock is maximized when a clock extraction circuit extracts a part of the output from the optical signal after photoelectric conversion.

  By adjusting the pass wavelength of the optical filter so as to be such an optimum point, the transmission characteristics can be stabilized and the quality can be improved.

  That is, a first aspect of the present invention is an optical transmitter including an optical modulator that directly modulates laser light, an optical fiber that transmits an optical signal transmitted from the optical transmitter, and the optical signal that is electrically An optical transceiver including an optical receiver including a photodetector that converts a signal.

  Here, a feature of the present invention is that the optical receiver includes a band-pass optical filter between the optical detector and the optical fiber, and means for extracting a clock from the electrical signal; And means for adjusting the transmission wavelength of the bandpass optical filter so that the RF of the clock extracted by the extracting means is maximized.

  In addition, a Mach-Zehnder type optical filter having two output ports whose transmission wavelength bands are shifted instead of the bandpass type optical filter is provided, and a photodetector for detecting the intensity of the optical output is provided in each of the two output ports. Each of the adjusting means is provided with means for adjusting the ratio of the two light output intensities output from the two output ports so that the RF intensity of the clock extracted by the extracting means is maximized. be able to.

  At this time, the ratio of the two optical output intensities when the error rate in the optical receiver is the lowest is measured in advance, and the adjusting means determines the ratio of the two optical output intensities to the ratio measured in advance. It is also possible to provide means for adjusting so that According to this, since the time for measuring the RF intensity after receiving the optical signal can be omitted, the optical filter can be controlled at high speed.

  Further, the two photodetectors provided at the two output ports can be configured as one balanced photodetector. According to this, frequency shift keying (FSK) generated during direct modulation of the laser can be effectively utilized.

  The optical modulator includes, for example, an optical modulation unit that directly modulates laser light emitted from a laser that oscillates in a single longitudinal mode, and the optical modulation unit has a bias current sufficiently higher than a threshold current of the laser. Means for digitally modulating the light intensity so that the extinction ratio of the light intensity after laser emission is 3 dB or less.

  According to a second aspect of the present invention, an optical transmitter including an optical modulator that directly modulates laser light, an optical fiber that transmits an optical signal transmitted from the optical transmitter, and the optical signal as an electrical signal. It is the said optical receiver applied to the optical transmission / reception apparatus containing the optical receiver provided with the photodetector to convert.

  Here, a feature of the present invention is that a band-pass optical filter is provided between the photodetector and the optical fiber, and a clock is extracted from the electrical signal, and is extracted by the extracting means. And means for adjusting the transmission wavelength of the bandpass optical filter so that the RF intensity of the clock is maximized.

  In addition, a Mach-Zehnder type optical filter having two output ports whose transmission wavelength bands are shifted instead of the bandpass type optical filter is provided, and a photodetector for detecting the intensity of the optical output is provided in each of the two output ports. Each of the adjusting means is provided with means for adjusting the ratio of the two light output intensities output from the two output ports so that the RF intensity of the clock extracted by the extracting means is maximized. be able to.

  Alternatively, the ratio of the two light output intensities when the error rate in the optical receiver is the lowest is measured in advance, and the adjusting means sets the ratio of the two light output intensities to the pre-measured ratio. Means for adjusting can be provided.

  Further, the two photodetectors provided in the two output ports can be configured as one balanced photodetector.

  According to the present invention, the directly modulated optical signal is transmitted through the optical fiber and then passed through the optical filter, so that the extinction ratio is improved and the chirp generated in the laser is reduced. By adjusting the transmission wavelength of the optical filter so as to be optimal, stable and high-quality communication becomes possible. The laser is digitally modulated with a high bias current and has a low extinction ratio, so that it is difficult to receive the nonlinear effect of the optical fiber.

(First Example)
A first embodiment will be described with reference to FIGS. FIG. 1 is a block diagram of the optical communication system of the first embodiment. An optical transmitter composed of a 1.55 μm band distributed feedback (DFB) semiconductor laser 1 that oscillates in a single mode and a transmission circuit 2 is directly modulated by an NRZ electric signal of 10 Gbit / s or 40 Gbit / s. .

  The laser threshold is about 10 mA, but the laser is directly modulated with an amplitude of 40 mA at 90 times the bias current of 90 mA. A high bias current is used to improve the modulation characteristics and obtain a high light output. At this time, the extinction ratio is 2.2 dB.

  FIG. 2 is a diagram for explaining an outline of a modulated waveform and an extinction ratio. Time is plotted on the horizontal axis and light intensity is plotted on the vertical axis. The modulated light is coupled to the 40 km optical fiber 3 and transmitted. On the receiving side, the received signal passes through the band-pass optical filter 4 using the dielectric multilayer film, is photoelectrically converted by the photodetector 5 and amplified by the amplifier 6.

  A part of the electrical signal is led to the clock extraction circuit 7 to extract the clock. A signal is regenerated by the receiving circuit 8 using this clock. At this time, the transmission wavelength of the optical filter 4 is adjusted using the control circuit 9 so that the RF intensity becomes maximum. As a result, the extinction ratio is improved, chirp generated in the laser is reduced, and stable transmission characteristics with a low error rate can be obtained.

(Second embodiment)
A second embodiment will be described with reference to FIG. FIG. 3 is a block diagram of the optical communication system of the second embodiment. The configuration on the transmitter side is the same as in FIG. 1, but a Mach-Zehnder type optical filter 11 is used as the optical filter. In this case, the output of the optical filter has two ports (1) and (2) whose transmission wavelength band is shifted, and one optical output is monitored by the photodetector 12.

  The remaining light output is guided to the photoelectric conversion circuit 13 including the photodetector 5. The transmission wavelength of the Mach-Zehnder optical filter 11 is adjusted so that the two light output intensities have an optimum ratio. The optimum ratio is a value at which the RF intensity of the clock becomes maximum when the optical signal is photoelectrically converted by the photodetector 5 and then extracted from a part of the output by the clock extraction circuit 7.

  In setting the optimum point of the optical filter, when the transmission wavelength of the Mach-Zehnder type optical filter is adjusted in advance, the optical output of the two ports of the optical filter when the bit error rate is the lowest in the optical receiver consisting of the photodetector 5 The intensity ratio may be evaluated and controlled by the control circuit 9 so that the light output intensity ratio is obtained.

(Third embodiment)
A third embodiment will be described with reference to FIG. FIG. 4 is a block diagram of the optical communication system of the third embodiment. In the third embodiment, the receiver has a balance detection configuration. In direct laser modulation, chirp is generated at the same time as intensity modulation, and frequency modulation occurs, which means that frequency shift keying (FSK) is mixed.

  In order to make effective use of this, balance detection is effective. The light output is monitored from the direct currents of the two photodetectors 5 and 12, and the transmission wavelength of the Mach-Zehnder type optical filter is controlled by the control circuit 9 so that the ratio of the two light output intensities becomes the optimum point.

  In the setting of the optimum point of the optical filter, when the transmission wavelength of the Mach-Zehnder optical filter is adjusted in advance, the two-port light of the optical filter when the bit error rate is the lowest in the photoelectric conversion circuit 13 composed of the photodetector 5 The output intensity ratio is evaluated, and control is performed so that this light output intensity ratio is obtained.

  According to the present invention, stable and high-quality communication is possible by adjusting the transmission wavelength of the optical filter so that the transmission characteristics are optimized, and therefore, it can be used for high-speed optical communication.

The block diagram of the optical communication system of a 1st Example. The figure for demonstrating the outline and extinction ratio of the modulated waveform. The block diagram of the optical communication system of a 2nd Example. The block diagram of the optical communication system of a 3rd Example. The figure for demonstrating the optical transmitter which suppressed the conventional wavelength chirp.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 DFB laser 2 Transmission circuit 3 Optical fiber 4 Band pass type optical filter 5, 12 Photo detector 6 Amplifier 7 Clock extraction circuit 8 Reception circuit 9 Control circuit 10 Optical filter and optical circuit 11 Mach-Zehnder type optical filter 13 Photoelectric conversion circuit

Claims (9)

An optical transmitter including an optical modulator that directly modulates laser light;
An optical fiber for transmitting an optical signal transmitted from the optical transmitter;
In an optical transceiver including an optical receiver including a photodetector that converts the optical signal into an electrical signal,
The optical receiver is:
A band pass optical filter is provided between the photodetector and the optical fiber,
Means for extracting a clock from the electrical signal;
Means for adjusting the transmission wavelength of the band-pass optical filter so that the RF (Radio Frequency) intensity of the clock extracted by the extracting means is maximized. A Mach-Zehnder type optical filter having two output ports whose transmission wavelength bands are shifted instead of the bandpass type optical filter,
Each of these two output ports is provided with a photodetector for detecting the intensity of the light output,
The means for adjusting comprises means for adjusting a ratio of two light output intensities output from the two output ports so that the RF intensity of the clock extracted by the extracting means is maximized. The optical transmitter-receiver described. The ratio of the two optical output intensities when the error rate in the optical receiver is lowest is measured in advance.
The optical transmission / reception apparatus according to claim 2, wherein the adjusting means includes means for adjusting a ratio of the two optical output intensities to be a ratio measured in advance. The optical transmission / reception apparatus according to claim 2 or 3, wherein the two photodetectors respectively provided in the two output ports are configured as one balanced photodetector. The optical modulator includes an optical modulation unit that directly modulates laser light emitted from a laser that oscillates in a single longitudinal mode,
The light modulation means includes means for performing digital modulation with a bias current sufficiently higher than a threshold current of the laser and performing light modulation so that an extinction ratio of light intensity after laser emission is 3 dB or less. Optical transceiver. An optical transmitter including an optical modulator that directly modulates laser light;
An optical fiber for transmitting an optical signal transmitted from the optical transmitter;
In the optical receiver applied to the optical transceiver including the optical receiver including the photodetector for converting the optical signal into an electrical signal,
A band pass optical filter is provided between the photodetector and the optical fiber,
Means for extracting a clock from the electrical signal;
An optical receiver comprising: means for adjusting a transmission wavelength of the bandpass optical filter so that an RF (Radio Frequency) intensity of the clock extracted by the extracting means is maximized. A Mach-Zehnder type optical filter having two output ports whose transmission wavelength bands are shifted instead of the bandpass type optical filter,
Each of these two output ports is provided with a photodetector for detecting the intensity of the light output,
The means for adjusting includes means for adjusting a ratio of two light output intensities output from the two output ports so that an RF intensity of the clock extracted by the extracting means is maximized. The optical receiver described. The ratio of the two optical output intensities when the error rate in the optical receiver is lowest is measured in advance.
The optical receiver according to claim 7, wherein the adjusting means includes means for adjusting a ratio of the two light output intensities to be a ratio measured in advance. The optical receiver according to claim 7, wherein the two photodetectors provided in the two output ports are configured as one balanced photodetector.

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