JP4564517B2 - FM modulation type optical transmission device - Google Patents

Description

  The present invention relates to an FM modulation type optical transmission apparatus, and more particularly to an apparatus for optically transmitting an AM / QAM modulated broadband signal by converting it into a broadband FM modulated signal using a semiconductor laser device or the like.

As an FM modulation type optical transmission apparatus, for example, the one described in Non-Patent Document 1 is known. FIG. 8 is a block diagram showing a configuration of a conventional FM modulation type optical transmission apparatus. In FIG. 8, reference numeral 2 denotes an FM modulation laser (hereinafter referred to as an FM laser) whose optical frequency is changed by a bias current, 3 is a local oscillation laser, 5 is an optical multiplexer, and 6 is an optical heterodyne detector. These constitute the FM converter 1. Further, 7 is a transmission laser, and 8 is an optical fiber.

In the FM modulation type optical transmission apparatus, the multi-channel subcarrier signals input to the FM converter 1 are collectively converted into FM signals and then input to the FM laser 2, and the FM laser 2 uses the optical frequency band in the optical frequency band. Batch FM modulation is performed. The optical signal output from the FM laser 2 is combined with the optical signal output from the local oscillation laser 3 by the optical multiplexer 5. When the combined optical signal is converted from an optical signal to an electrical signal by the optical heterodyne detector 6, a beat signal corresponding to the optical frequency difference between the FM laser 2 and the local oscillation laser 3 is generated, and the FM modulation electrical A signal is generated. The collective FM modulated signal obtained by the FM converter 1 in this way is optical intensity modulated by the transmission laser 7 and optically transmitted using the optical fiber 8.
IEICE, IEICE Technical Report (OCS95-98)

  However, in the above-described conventional FM modulation type optical transmission apparatus, the optical power of the FM laser fluctuates because the bias current changes in order to modulate the optical frequency of the optical signal output from the FM laser, and the FM converter The power of the output FM modulated signal varies, and the light intensity modulation index changes when the batch FM modulated signal is optically transmitted. FIG. 9 shows the relationship between the spectrum and waveform of the FM modulation signal generated by the FM modulation circuit when the wavelength of the local oscillation laser is higher than the center wavelength of the FM laser. It is a schematic representation that the intensity of the signal changes.

  On the receiving side after transmission, since the signal power of the collective FM modulation signal regenerated by optical / electrical conversion is proportional to the square of the light intensity modulation index, a certain reception characteristic is obtained by the fluctuation of the light intensity modulation index. Had the problem of not. The present invention solves the above-described conventional problems, and provides an FM modulation type optical transmission apparatus that can stabilize the light intensity modulation index during optical transmission and obtain substantially constant reception characteristics. Objective.

As means for solving the above-mentioned problem, the first means is:
An FM modulation type optical transmission apparatus that collectively FM-modulates frequency-multiplexed multi-channel AM / QAM signals,
An FM converter for converting a multi-channel subcarrier signal into an FM signal;
A transmission laser for inputting a signal converted into an FM signal by the FM converter and outputting an optical signal that is FM-modulated in a lump;
The FM converter is
A local oscillation laser that generates an optical signal for local oscillation;
An optical multiplexer for combining the optical signal output from the local oscillation laser and the optical signal output from the FM laser;
The optical signal output from the optical multiplexer is converted into an electrical signal to generate a beat signal corresponding to the frequency difference between the optical signal output from the local oscillation laser and the optical signal output from the FM laser. An optical detector,
An optical filter in which the magnitude of output light changes depending on the frequency of incident light in the frequency modulation band of the FM laser, and the dependency between the magnitude and frequency of the output light is an optical signal output from the FM laser. An optical filter having a dependency relationship in a direction in which fluctuations depending on the frequency of the output light of the FM laser are canceled when the FM laser is passed is provided between the FM laser and the optical multiplexer. This is an FM modulation type optical transmission apparatus.
The second means is
An optical detector that receives an optical signal output from the optical filter and converts it into an electrical signal corresponding to the magnitude of the optical output, and feeds back the electrical signal output from the optical detector to the FM laser. A feedback device that changes the center wavelength of the FM laser in accordance with the magnitude of the electrical signal output from the optical detector, and responds to the shift when the center wavelength of the FM laser is shifted. Utilizing the fact that the magnitude of the optical signal output from the optical filter changes, an electrical signal corresponding to the change in the magnitude of the optical signal that changes corresponding to the deviation is fed back to the FM laser by the feedback device. Thus, the FM modulation type optical transmission apparatus according to the first means is characterized in that the center wavelength of the FM laser is maintained at a specific wavelength.
The third means is
The FM modulation type optical transmission apparatus according to the first or second means, wherein the optical filter is an etalon filter.

  According to the above-described means, the optical power input to the optical multiplexer can be obtained even when the optical power of the FM laser fluctuates due to the change in the bias current of the FM laser by arranging the optical filter at the subsequent stage of the FM laser. It can be almost constant. As a result, the light intensity modulation index when optically transmitting the collective FM modulation signal can be kept substantially constant, so that stable reception characteristics can be easily obtained. Further, by branching and monitoring a part of the output from the optical filter, the center wavelength of the FM laser can be stabilized. In addition, in this case, even if compensation is performed to keep the light intensity modulation index constant, there is almost no increase in noise or deterioration of other characteristics due to the compensation. As a result, it was found that the performance can be dramatically improved. This is because by adopting an optical filter, so-called passive type elements can be adopted, so that the light intensity modulation index is kept constant while suppressing noise generation and signal degradation as compared with the case of active elements. This is probably because the influence on the characteristics of the entire FM modulation type optical transmission apparatus was far beyond expectations.

(First embodiment)
FIG. 1 is a block diagram showing a first embodiment of an FM modulation type optical transmission apparatus according to the present invention. In FIG. 1, a multi-channel subcarrier signal S1 is added to the FM converter 1 to be converted into a batch FM modulation signal, and the batch FM modulation signal output from the FM converter 1 is added to the transmission laser 7. The light intensity is modulated and then transmitted by the optical fiber 8. The FM converter 1 includes an FM laser 2, a local oscillation laser 3, an optical filter 4, an optical multiplexer 5, and an optical detector 6. The FM laser 2 FM-modulates the input multichannel subcarrier signal S1 in the optical frequency band.

The optical filter 4 inputs an optical signal output from the FM laser 2 whose magnitude changes depending on the optical frequency, and cancels the change in the magnitude of the input optical signal. Output. The optical multiplexer 5 combines the FM modulated light output from the optical filter and the local oscillation optical signal output from the local oscillation laser 3. The optical detector 6 detects the optical signal from the optical multiplexer 5 and converts it into an electrical signal, thereby generating a beat signal corresponding to the difference in optical frequency between the FM laser 2 and the local oscillation laser 3 to collect FM. A modulated electric signal is generated.

  Here, the function of the optical filter 4 disposed between the FM laser 2 and the optical multiplexer 5 is as follows. When the bias current is changed in order to change the optical frequency of the optical signal output from the FM laser 2, the optical output (magnitude) of the FM laser 2 also varies. The optical filter 4 has a frequency transmission characteristic that offsets the fluctuation of the optical power <magnitude> accompanying the change of the optical frequency. As a result, even when the optical power of the FM laser 2 fluctuates, the power of the optical signal transmitted through the optical filter 4 becomes substantially constant, and stable reception sensitivity can be obtained.

The optical filter 4 needs to have a function of controlling the transmittance of an optical signal in a narrow frequency band. As a specific configuration, for example, an etalon filter can be considered. The relationship between the optical frequency of the etalon filter and the transmittance is given by the following equation.

As a specific example of the etalon filter used in this embodiment, for example, an etalon filter having an end face reflectance of 60%, a material refractive index of 1.46, and a thickness of 1 mm can be cited. FIG.
These are graphs showing the relationship between optical frequency and transmittance when light is incident from the normal direction for an etalon filter having an end face reflectance of 60%, a material refractive index of 1.46, and a thickness of 1 mm. FIG. 3 is an enlarged view of the characteristics of the 193,302 to 193,342 GHz band in the relationship between the optical frequency and transmittance of the etalon filter shown in FIG. 4 further shows 193, 317 to 19 in the relationship between the optical frequency and the transmittance of the etalon filter shown in FIGS.
The characteristics of the 3,322 GHz band are expanded.

As a specific example of the FM laser 2 used in this embodiment, the slope efficiency is 0.0.
35mW / mA, FM modulation efficiency is 35MHz / mA, and optical frequency is 193,322GH
A laser having a characteristic of outputting an optical power of 12.5 mW when outputting an optical signal of z can be mentioned. FIG. 5 shows a slope efficiency of 0.035 mW / mA and an FM modulation efficiency of 35 MH.
z / mA, and 12.5 mW when outputting an optical signal with an optical frequency of 193,322 GHz
It is a graph which shows the relationship between the optical frequency and optical power at the time of changing the bias current of the laser with the characteristic which outputs the optical power of.

  The etalon filter has a characteristic that substantially offsets the optical frequency characteristic of the optical power of the laser. FIG. 6 is a graph showing the optical power of the optical signal of the FM laser transmitted through the etalon filter according to the embodiment. As shown in FIG. 6, the optical power of the optical signal of the FM laser that has passed through the etalon filter is substantially constant. Thereby, reception characteristics can be stabilized.

  Each parameter of the FM laser and the etalon filter is given as an example. If the relationship between the optical frequency of the FM laser and the optical power and the relationship between the optical frequency of the optical filter and the transmittance are offset from each other, Different parameters may be used. Further, a filter other than an etalon may be used as long as it has similar characteristics as an optical filter.

(Second Embodiment)
FIG. 7 is a block diagram showing a second embodiment of the FM modulation type optical transmission apparatus of the present invention. As shown in FIG. 7, in this embodiment, a part of the optical output from the optical filter 4 is branched, detected by the photodetector 9, and fed back to the FM laser 2 by the feedback circuit 10. is there. When the FM laser 2 has a center wavelength of 193, 319.5 GHz, and the etalon filter having the transmission characteristics shown in FIG. 4 is used as the optical filter 4, the center wavelength of the FM laser 2 is higher on the frequency side. Drifting increases the transmittance of the filter. If it does so, the optical power detected with the photodetector 9 will become high.

  Further, when the center wavelength of the FM laser 2 drifts to a lower frequency side, the transmittance of the filter decreases. If it does so, the optical power detected with the photodetector 9 will become low. Therefore, the bias current of the FM laser 2 is changed by disposing the optical filter 4 having the above-described characteristics between the FM laser 2 and the optical multiplexer 5 and monitoring the optical power detected by the photodetector 9. At the same time, the optical power input to the optical multiplexer can be stabilized, and at the same time, the wavelength drift of the FM laser can be detected, and the wavelength of the FM laser can be stabilized by applying feedback.

  INDUSTRIAL APPLICABILITY The present invention can be widely used when an AM / QAM modulated wideband signal is converted into a wideband FM modulated signal for optical transmission. For example, the present invention can be used as an optical signal transmission device for cable television. Can do.

1 is a block diagram showing a first embodiment of an FM modulation type optical transmission apparatus of the present invention. It is a graph which shows the relationship between the optical frequency at the time of entering light from a normal line direction, and the transmittance | permeability about the etalon filter whose reflectance of an end surface is 60%, the refractive index of material is 1.46, and thickness is 1 mm. In the relationship between the optical frequency and transmittance of the etalon filter shown in FIG. 2, the characteristics of the 193,302 to 193,342 GHz band are expanded. In the relationship between the optical frequency and transmittance of the etalon filter shown in FIGS. 2 and 3, the characteristics of the 193,317 to 193,322 GHz band are further expanded. A bias current of a laser having a characteristic of outputting an optical power of 12.5 mW when outputting an optical signal with a slope efficiency of 0.035 mW / mA, an FM modulation efficiency of 35 MHz / mA, and an optical frequency of 193,322 GHz. It is a graph which shows the relationship between the optical frequency at the time of changing, and optical power. It is a graph which shows the optical power of the optical signal of FM laser which permeate | transmitted the etalon filter concerning embodiment. It is a block diagram which shows 2nd Embodiment of the FM modulation type | mold optical transmission apparatus of this invention. It is a block diagram which shows an example of the conventional broadband FM modulation type | mold optical transmission apparatus. It is a figure which shows the spectrum and waveform relationship of FM modulation signal.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... FM converter 2 ... FM laser 3 ... Local oscillation laser 4 ... Optical filter 5 ... Optical multiplexer 6 ... Optical detector 7 ... Transmission laser 8 ... Transmission fiber 9 ... Optical detector 10 ... Feedback circuit

Claims (2)

An FM modulation type optical transmission apparatus that collectively FM-modulates frequency-multiplexed multi-channel AM / QAM signals,
An FM converter for converting a multi-channel subcarrier signal into an FM signal;
Enter the signal converted into the FM signal by the FM converter, and transmitting laser for outputting an optical signal which is FM-modulated at once,
Have
The FM converter is
A local oscillation laser that generates an optical signal for local oscillation;
A laser for FM modulation, wherein the frequency of the output light is changed by a bias current;
An optical multiplexer for multiplexing the optical signals output optical signal output from the local oscillator laser and from the FM laser,
Generating a beat signal corresponding to the frequency difference between the light output signal from the optical signal and the FM laser output light signal after converting into an electric signal the local oscillation laser output from the optical multiplexer An optical detector,
An etalon filter in which the magnitude of output light changes depending on the frequency of incident light in the frequency modulation band of the FM laser, and the magnitude of output light when the optical power fluctuates when the frequency is modulated in the FM laser. is depend on it the relationship between the frequency is, the optical signal output from the FM laser when Ru is passed through the etalon filter, in the direction of dependency to offset the variation of the optical power which depends on the frequency of the output light of the FM laser An FM modulation type optical transmission apparatus, wherein only one etalon filter is provided between the FM laser and the optical multiplexer. An optical detector that inputs an optical signal output from the etalon filter and converts it into an electrical signal corresponding to the magnitude of the optical output, and an electrical signal output from the optical detector is fed back to the FM laser. A feedback device that changes the center wavelength of the FM laser in accordance with the magnitude of the electrical signal output from the optical detector;
Utilizing the fact that when the center wavelength of the FM laser shifts, the magnitude of the optical signal output from the etalon filter changes corresponding to the shift,
An electrical signal corresponding to a change in the magnitude of an optical signal that changes corresponding to the shift is fed back to the FM laser by the feedback device, and the center wavelength of the FM laser is maintained at a specific wavelength. The FM modulation type optical transmission apparatus according to claim 1.

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