JP2008300804A - Light modulation light source and light modulation method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform transmission by a direct modulation system even when the modulation band of an optical signal source is smaller than the bit rate of optical signals. <P>SOLUTION: The light modulation light source is provided with the optical signal source 102 driven by electric signals having multi-value signal strength, for generating the optical signals optical frequency modulated by multi-value frequencies, and an optical discriminator 10 for converting the optical frequency modulated optical signals to binary amplitude modulated optical signals. The light modulation light source can be further provided with a conversion means 101 for converting the electric signals having the binary signal strength to the electric signals having the multi-value signal strength and driving the optical signal source 102. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical modulation light source and an optical modulation method, and more particularly, optical modulation for converting an optical frequency modulation signal generated by modulation of current injection into an optical signal source into an intensity modulation signal and transmitting the signal to an optical fiber. The present invention relates to a light modulation light source and a light modulation method.

  Conventionally, semiconductor lasers have been widely used as light sources for optical communications, and DFB (Distributed Feed Back) lasers have been mainly used. As a modulation method, a direct modulation method is used in which the intensity of laser oscillation light is modulated by modulating the drive current of the DFB laser. The direct modulation method is widely used because the configuration of the optical transmitter can be made compact and the cost is low.

  However, one of the drawbacks of the direct modulation method is that the chirp is large. Chirp is a rapid change in optical frequency or phase associated with intensity modulation. Since the optical signal pulse having a large chirp is distorted after propagating through an optical fiber having dispersion, the transmission distance is limited. Therefore, a method for extending the transmission distance by removing the chirped frequency component using an optical filter has been proposed. For example, when a 1.55 μm band DFB laser is used in the optical signal bit rate 10 Gb / s and direct modulation, it has been reported that the transmission distance can be increased from 18.4 km to 38.5 km (for example, non-patent) Reference 1).

  Further, it has been reported that further long-distance transmission is possible by positively using frequency modulation components generated during direct modulation. This is called a chirp managed laser (CML). In CML, a partially frequency-modulated optical signal is incident on an optical filter whose transmission peak is shifted from the center frequency (or carrier frequency) of the signal light. Thus, the frequency modulation is converted into intensity (amplitude) modulation by using either the positive or negative slope of the transmittance of the optical filter using the transmission end of the transmission region of the optical filter. It has been reported that the transmission distance can be increased up to 250 km when a 1.55 μm band DFB laser is used in this method, with a bit rate of optical signal of 10 Gb / s and direct modulation (see, for example, Non-Patent Document 2).

PAMorton et al., “38.5km error free transmission at 10Gb / s in standard fiber using a low chirp, spectrally filtered, directly modulated 1.55um DFB laser”, IEE Electronics Letters, vol.33, No.4, p.310 -311, 1997 D. Mahgerefteh et al., “Error-free 250km transmission in standard fiber using compact 10Gb / s chirp-managed directly modulated lasers (CML) at 1550nm”, IEE Electronics Letters, vol.41, No.9, p.543- 544, 2005

  Since the conventional optical transmitter based on the direct modulation system has a simple configuration combining a semiconductor laser and an optical filter, long-distance transmission can be realized at low cost. However, since the modulation band of the semiconductor laser is required to be approximately the same as the bit rate of the optical signal, the upper limit of the modulation speed is limited to the laser relaxation oscillation frequency or less. That is, when 10 Gb / s modulation is performed, the modulation band of the semiconductor laser is required to be 7 to 8 GHz or more. Since the modulation band of the conventional DFB laser has an upper limit of about 12 to 13 GHz, it is impossible to cope with further high-speed modulation, for example, 40 Gb / s.

  The present invention has been made in view of such problems, and an object of the present invention is to perform transmission by a direct modulation method even when the modulation band of the optical signal source is smaller than the bit rate of the optical signal. It is an object to provide a light modulation source and a light modulation method that are possible.

  In order to achieve such an object, the light modulation light source according to claim 1 is driven by an electric signal having a multi-level signal intensity and is optical frequency-modulated with a multi-level frequency. And an optical discriminator for converting the optical frequency modulated optical signal into a binary amplitude modulated optical signal.

  The light modulation light source may further include conversion means for converting an electric signal having a binary signal intensity into an electric signal having a multilevel signal intensity and driving the optical signal source. The converting means limits a band of the electric signal having the binary signal strength.

  The optical discriminator has a transmission or reflection characteristic in which both a region having a positive slope and a region having a negative slope exist between a minimum frequency and a maximum frequency of the optical frequency modulated optical signal. And

  The invention according to claim 13 is an optical modulation method of an optical modulation light source for inputting an electric signal having a binary signal intensity and outputting an optical signal subjected to binary amplitude modulation, wherein the binary signal A first step of converting an electric signal having an intensity into an electric signal having a multi-level signal intensity; and converting the electric signal having the multi-level signal intensity into an optical signal modulated by an optical frequency at a multi-level frequency. And a third step of converting the optical frequency modulated optical signal into a binary amplitude modulated optical signal.

  The first step is characterized in that a band of the electric signal having the binary signal intensity is limited. The third step has an input / output characteristic in which both a region having a positive slope and a region having a negative slope exist between the minimum frequency and the maximum frequency of the optical signal modulated by the optical frequency. To do.

  As described above, according to the present invention, an optical signal that is optical frequency modulated at a multi-value frequency is generated by an electrical signal having multi-value signal strength, and then converted into a binary amplitude-modulated optical signal. Since the conversion is performed, even if the modulation band of the optical signal source is smaller than the bit rate of the optical signal, transmission can be performed by the direct modulation method.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(First embodiment)
(Light modulation light source)
FIG. 1 shows a configuration of a light modulation light source according to the first embodiment of the present invention. The optical modulation light source includes a band limiting filter 101 that limits a high frequency band of an input electric signal, an optical signal source 102 that generates a frequency modulation signal by inputting the band limited signal, and an optical discriminator 103. It is connected to the. As the band limiting filter 101, for example, a low-pass filter such as a Bessel filter can be used. As the optical signal source 102, for example, a direct modulation semiconductor laser can be used. As the optical discriminator 103, for example, a cavity type interference filter called an etalon can be used.

  The operation principle of the light modulation light source will be described with reference to FIGS. FIG. 2A shows a signal intensity waveform of the electric signal S1 input to the band limiting filter 101 shown in FIG. 1, and represents a binary digital signal. The vertical axis is voltage, and the horizontal axis is time. The bit rate can be 40 Gb / s, for example. The high frequency 3 dB cutoff frequency fc of the band limiting filter 101 can be set to 12 GHz, for example. Details will be described later.

  FIG. 2B shows the waveform of the band-limited electric signal S2 output from the band-limiting filter 101. The vertical axis is voltage, and the horizontal axis is time. The signal level is converted into three values of H, M, and L. The band-limited electric signal S <b> 2 is directly input to the optical signal source 102. As a driving condition of the optical signal source 102 which is a semiconductor laser, a DC bias current higher than a threshold value is applied, and the band-limited electric signal S2 is superimposed thereon as a high frequency signal. The optical signal source 102 generates an optical frequency modulation signal synchronized with the high frequency signal. The modulation frequency band required for this semiconductor laser may be about 12 GHz which is equivalent to the electric signal because the electric signal is band-limited to 12 GHz.

For the optical signal S3 output from the optical signal source 102, the vertical axis in FIG. 2B may be read as the optical frequency. Optical signal S3, the electric signal S2 is band-limited are also modulated into three values, and has a frequency f ₃ at the frequency f _2, H-level at a frequency f _1, M-level at the L level. In the case of a direct modulation type semiconductor laser, the frequency becomes higher as the current value is larger, so that f ₁ <f ₂ <f ₃ . Here, for convenience, the frequency f ₁ corresponding to the L level is referred to as the minimum frequency, and the frequency f ₃ corresponding to the H level is referred to as the maximum frequency.

FIG. 2C shows an optical signal output from the optical signal source 102 when the semiconductor laser driving conditions are set so that the modulation factor of frequency modulation (Δf = f ₃ −f ₁ ) is, for example, 20 GHz. It is an optical spectrum of S3. The horizontal axis, as the center frequency f _3, represents the deviation from the frequency f _3, the vertical axis represents the logarithm of the light intensity.

FIG. 3A shows the transmission or reflection characteristics of the light discriminator 103. For example, an etalon in which a cavity is configured by a mirror pair having a reflectance R = 0.3 to 0.7 is used as the light discriminator 103. Here, between the minimum frequency f ₁ and the maximum frequency f ₃ of the frequency-modulated optical signal, both a region having a positive slope and a region having a negative slope are set. The output level for the frequency f ₁ is P ₁ , the output level for the frequency f ₂ is P ₂ , and the output level for the frequency f ₃ is P ₁ . The optical signal S3, which is a ternary frequency modulation signal, is converted into an optical signal S4 by the optical discriminator 103.

  FIG. 3B is a light spectrum of the optical signal S4 output from the optical discriminator 103, and FIG. 3C is an intensity waveform of the optical signal S4 output from the optical discriminator 103. As shown in FIG. 3B, the light spectrum of the optical signal S4 has a concave shape at the center. The time waveform of the optical signal S4 is a binary amplitude modulation signal of 40 Gb / s.

  In this way, the step of multi-leveling the input electrical signal, the step of generating a multi-level optical frequency modulation signal from the multi-level electrical signal, and the multi-level optical frequency modulation signal of binary The step of converting into an amplitude modulation signal, a 40 Gb / s modulation signal can be generated using the optical signal source 102 having only a 12 GHz modulation band.

  Here, an example in which an electrical signal having a ternary signal level is converted into a ternary frequency-modulated optical signal is shown.

(Band limiting filter)
As described above, for example, a low-pass filter represented by a Bessel filter can be used as the band limiting filter 101. At this time, from the viewpoint of converting a binary signal into a ternary signal, the value of the high frequency 3 dB cutoff frequency fc is preferably set to about 20% to 35% of the bit rate of the input electric signal. For example, when the bit rate is 40 Gb / s, the value of the high frequency 3 dB cutoff frequency fc is desirably set to about 8 to 14 GHz.

(Optical signal source-1)
The modulation degree of optical frequency modulation (Δf = f ₃ −f ₁ ) is not particularly limited. However, from the viewpoint of maximizing the transmission distance, it is desirable that the value of the modulation degree is set so that the modulation degree of the optical frequency modulation is half the bit rate. For example, when the bit rate is 40 Gb / s, it is most suitable for long-distance transmission that the optical frequency modulation degree is Δf = f ₃ −f ₁ = 20 GHz. In general, a state in which the modulation degree of optical frequency modulation is half of the bit rate is called minimum shift keying (MSK), and it is known that the width of the modulation spectrum is minimized. Also in this embodiment, by using the MSK condition, the influence of optical fiber dispersion can be minimized and the transmission distance can be increased.

  Even if the modulation band of the semiconductor laser is smaller than the bit rate of the optical signal, the degree of modulation of the optical frequency modulation is not limited as long as the objective effect that transmission can be performed by the direct modulation method is achieved. By selecting an appropriate optical frequency response of the optical filter used as the optical discriminator according to the modulation degree of the optical frequency modulation, the above-described objective effect can be obtained.

  As described above, the value of the high frequency 3 dB cutoff frequency fc of the band limiting filter is desirably set to about 20% to 35% of the bit rate of the input electric signal. It is sufficient that the modulation band required for the optical signal source driven by the band-limited electric signal is the same as that of the electric signal to be driven, and about 20% to 35% of the bit rate is required. Conversely, if the modulation band of the optical signal source is known, the bit rate that can be realized by the light modulation light source of this embodiment can be derived from the reciprocal of the high frequency 3 dB cutoff frequency fc to be about 3 to 5 times the modulation band. .

(Light discriminator-1)
As an optical discriminator, it is necessary that both a region having a positive slope and a region having a negative slope exist between the minimum frequency f ₁ and the maximum frequency f ₃ of the frequency-modulated optical signal. For example, if the optical discriminator shown in FIG. 3 (a), a negative slope at the frequency f _1, takes a minimum value at a frequency f _2, the slope in the frequency f ₃ is positive. Thus, any filter may be used as long as it has an extreme value near the center frequency f ₂ of the ternary signal and has an optical frequency response in which the sign of the slope is different before and after that.

FIG. 4 shows the configuration of the optical discriminator according to the first embodiment. This is a single cavity type interference filter (etalon). The optical discriminator is composed of dielectric multilayer mirrors 301 and 302 and a spacer layer 305 having a thickness L sandwiched between them. In the dielectric multilayer mirrors 301 and 302, a high refractive index layer 303 made of Ta ₂ O ₅ and a low refractive index layer 304 made of SiO ₂ are alternately laminated.

  The thickness L of the spacer layer 305 is determined by the interval between the transmission peak or reflection peak of the etalon on the optical frequency axis, and when the peak optical frequency interval (or FSR: Free Spectral Range) is to be set to Δf, L = C / 2 / n / Δf may be set. Here, c is the speed of light in vacuum, and n is the refractive index of the spacer layer. For example, when the refractive index of the spacer layer is n = 1.5, if it is desired to set the FSR to 100 GHz, L = 1 mm may be set. FIG. 3A shows an optical frequency response when an etalon having a reflectivity R = 0.7 and FSR = 200 GHz of a dielectric multilayer mirror is used in a reflection mode.

  Generally, when filtering the optical spectrum of signal light, the FSR value of the optical discriminator is about 5 times the bit rate of signal light. This is because if the FSR is narrow, the transmission band of the signal light is also narrowed in proportion to the FSR, and the spectrum of the signal light is filtered more than necessary. At this time, the time waveform of the signal light is significantly distorted. According to this embodiment, since the modulation band of the semiconductor laser is 1/3 or less compared to the bit rate of the optical signal, the spectral width of the signal light input to the optical discriminator is the case of the conventional NRZ modulation. Narrow compared to. Therefore, the required FSR can be narrower than the conventional NRZ modulation. Further, since the spectrum width of the signal light is narrow, the transmission characteristics at the time of optical fiber transmission are superior to the conventional NRZ modulation, and it is possible to transmit a longer distance.

  In the above-described embodiment, an etalon having an FSR = 200 GHz that is five times as high as the optical signal bit rate of 40 Gb / s is shown. However, an etalon having an FSR = 50 GHz that is 1.25 times may be used. Even when an etalon having a reflectance R = 0.5 and FSR = 100 GHz and an etalon having a reflectance R = 0.3 and FSR = 50 GHz are used, the optical frequency response in the reflection mode has the characteristics shown in FIG. Obtainable. Conventionally, since an etalon filter of FSR = 50 GHz widely used for 10 Gb / s NRZ modulation can be used, the cost of the system can be reduced. The relationship between the reflectance R and the FSR is not limited to the above combination, and an appropriate FSR is set for a value of about 0.3 to 0.95 as the range of the reflectance R. Thus, the present invention can be applied to the light modulation light source of the present embodiment.

  FIG. 5 shows an arrangement method of the light discriminator. FIG. 5A shows a configuration in which the etalon shown in FIG. 4 is used in the reflection mode. The optical discriminator input 402a is the optical signal S3 shown in FIG. 1, and is incident on the optical discriminator 401a at an angle. The optical discriminator output 403a is the optical signal S4 shown in FIG. 1, and is configured so that the optical paths of incident light and reflected light do not overlap.

  FIG. 5B also shows a configuration in which the etalon is used in the reflection mode. The light discriminator input 402b is reflected by the light discriminator 401b and output by the optical circulator 404 as a light discriminator output 403b. As shown in FIG. 5C, the reflected light can be extracted using a beam splitter 405 instead of the optical circulator 404. The beam splitter 405 may be a bulk type or a fiber coupler.

  In FIG. 5D, the reflected light is extracted using the polarizing beam splitter 406 and the quarter wavelength plate 407. A polarization state that travels straight through the polarization beam splitter 406 is referred to as a state A. When the optical discriminator input 402d is incident on the polarization beam splitter 406 in the polarization state of the state A, the light discriminator 401d is transmitted through the polarization beam splitter 406 as it is and the polarization state is rotated 45 degrees by the quarter wavelength plate 407. Is incident and reflected. The reflected light is rotated 45 degrees again by the quarter-wave plate 407 and is incident on the polarization beam splitter 406 in a state that is 90 degrees different from the state A. Since the polarization state is 90 degrees different from the state A, it is reflected by the polarization beam splitter 407 and output as an optical discriminator output 403d.

  As described above, in the configuration in which the polarization beam splitter 406 and the quarter wavelength plate 407 are combined, the optical path conversion operation is performed depending on the polarization state, and therefore, compared with the configuration using the beam splitter 405 in FIG. Thus, there is an advantage that there is no branching loss.

(Light discriminator-2)
FIG. 6 shows the configuration of the optical discriminator according to the second embodiment. This is a double cavity type interference filter (etalon). The optical discriminator is composed of dielectric multilayer mirrors 306, 307, 308 and spacer layers 311a, 311b having a thickness L sandwiched between them. The dielectric multilayer mirrors 306, 307, and 308 are formed by alternately stacking a high refractive index layer 309 made of Ta ₂ O ₅ and a low refractive index layer 310 made of SiO ₂ .

For example, the reflectance of the reflecting mirrors 306 and 307 is set to about 0.2 to 0.4, and the reflectance of the reflecting mirror 308 is set to about 0.5 to 0.9. By setting the thicknesses of the two spacer layers 311a and 311b to about 0.5 to 4 mm, the characteristics shown in FIG. 3A can be obtained. That is, it is possible to configure a filter in which both a region having a positive slope and a region having a negative slope exist between the minimum frequency f ₁ and the maximum frequency f ₃ of the frequency-modulated optical signal.

  Although the double cavity filter having two spacers has been described in the second embodiment, it goes without saying that the same effect can be expected for a multicavity filter having a plurality of spacers (three or more). The arrangement of the light discriminator used in the reflection type is the same as that of the first embodiment described with reference to FIGS.

(Light discriminator-3)
FIG. 7 shows a configuration of the optical discriminator according to the third embodiment. FIG. 7A is a diagram showing a ring filter constituted by an optical waveguide. The ring filter is an interference type filter having a transmission mode, and is classified as a cavity type because it constitutes a ring cavity. However, since it is constituted by a waveguide, it is also classified as a waveguide type. The ring filter includes a coupler 314 that connects the input waveguide 312 to one surface and the output waveguide 313 to the other surface, and the ring waveguide 315 is connected from one surface of the coupler 314 to the other surface. . The transmission characteristics of the ring filter can be obtained as shown in FIG. That is, it is possible to configure a filter in which both a region having a positive slope and a region having a negative slope exist between the minimum frequency f ₁ and the maximum frequency f ₃ of the frequency-modulated optical signal.

  The ring filter is a transmission mode filter and has the arrangement shown in FIG. The optical discriminator input 402e is the optical signal S3 shown in FIG. 1, and is incident on the input waveguide 312 of the ring filter that is the optical discriminator 401e. The optical discriminator output 403a is the optical signal S4 shown in FIG. 1, and is emitted from the output waveguide 313 of the ring filter. Since the optical discriminator is used in a transmissive state, the arrangement of the optical discriminator is simplified, and it is only necessary to insert the optical discriminator in the optical path.

(Light discriminator-4)
FIG. 8 shows a configuration of the optical discriminator according to the fourth embodiment. It is an interference type filter having a transmission mode, and is a waveguide type Mach-Zehnder type filter. The Mach-Zehnder type filter includes a coupler 314 a connected to the input waveguide 312, a coupler 314 b connected to the output waveguide 313, and two arm waveguides 318 a provided with an optical path length difference between the couplers 314 a and 314 b. , 318b. The transmission characteristic of the Mach-Zehnder type filter can be obtained as shown in FIG. That is, it is possible to configure a filter in which both a region having a positive slope and a region having a negative slope exist between the minimum frequency f ₁ and the maximum frequency f ₃ of the frequency-modulated optical signal. In the fourth embodiment, the transmission type filter is used. As shown in FIG. 7B, it is only necessary to insert the optical discriminator in the optical path, and the arrangement of the optical discriminator is simplified.

(Light discriminator-5)
FIG. 9 shows the configuration of the optical discriminator according to the fifth embodiment. This is a fiber grating filter in which a periodic perturbation 317 of the refractive index is formed in the optical fiber 316. The transmission characteristics of the fiber grating filter can be obtained as shown in FIG. That is, it is possible to configure a filter in which both a region having a positive slope and a region having a negative slope exist between the minimum frequency f ₁ and the maximum frequency f ₃ of the frequency-modulated optical signal.

  In the fifth embodiment, the fiber grating has been described. However, the grating reflector may be a grating formed in a quartz waveguide, and the same operation is performed even with a grating formed in a semiconductor. It goes without saying that it can be realized. There are no particular restrictions on the construction method of the grating as long as the refractive index of the medium itself changes periodically. Further, a structure in which the structure such as the thickness of the core layer or the cladding layer of the waveguide is periodically changed may be used. In the fifth embodiment, the transmission type filter is used. As shown in FIG. 7B, it is only necessary to insert the optical discriminator in the optical path, and the arrangement of the optical discriminator is simplified.

(Second Embodiment)
(Light modulation light source)
In 1st Embodiment, the optical frequency response of the optical filter which is an optical discriminator demonstrated the case where Fig.3 (a) shows. That is, both a region having a positive slope and a region having a negative slope exist between the minimum frequency f ₁ and the maximum frequency f ₃ of the frequency-modulated optical signal, and negative in the vicinity of the minimum frequency f ₁ . The slope is positive in the vicinity of the maximum frequency f ₃ . As the frequency response of the optical discriminator that converts the multilevel optical frequency modulation signal into the binary amplitude modulation signal, it is also possible to use a characteristic in which the characteristic curve of FIG. That is, both a region having a positive slope and a region having a negative slope exist between the minimum frequency f ₁ and the maximum frequency f ₃ of the frequency-modulated optical signal, and are positive in the vicinity of the minimum frequency f ₁ . It is a light discriminator having a negative inclination in the vicinity of the inclination and the maximum frequency f ₃ .

  The operation principle of the light modulation light source according to the second embodiment of the present invention will be described with reference to FIGS. FIG. 10A shows an intensity waveform of the electric signal S1 input to the band limiting filter 101 shown in FIG. 1, and represents a binary digital signal. The vertical axis is voltage, and the horizontal axis is time. The waveform is the same as in FIG.

FIG. 10B shows the waveform of the band-limited electric signal S2 output from the band-limiting filter 101. The vertical axis is voltage, and the horizontal axis is time. The signal level is converted into three values of H, M, and L. The waveform is the same as in FIG. For the optical signal S3 output from the optical signal source 102, the vertical axis in FIG. 10B may be read as the optical frequency. Optical signal S3, the electric signal S2 is band-limited are also modulated into three values, and has a frequency f ₃ at the frequency f _2, H-level at a frequency f _1, M-level at the L level. In the case of a direct modulation type semiconductor laser, the frequency becomes higher as the current value is larger, so that f ₁ <f ₂ <f ₃ .

FIG. 10C shows an optical signal output from the optical signal source 102 when the driving conditions of the semiconductor laser are set so that the modulation factor (Δf = f ₃ −f ₁ ) of frequency modulation is, for example, 40 GHz. It is an optical spectrum of S3. The horizontal axis, as the center frequency f _3, represents the deviation from the frequency f _3, the vertical axis represents the logarithm of the light intensity.

As described in the first embodiment, from the viewpoint of maximizing the transmission distance, it is desirable to set the modulation factor of frequency modulation to be half the bit rate. For example, when the bit rate is 40 Gb / s, it is most suitable for long-distance transmission that the frequency modulation degree is Δf = f ₃ −f ₁ = 20 GHz. However, the degree of modulation of frequency modulation (Δf = f ₃ −f ₁ ) need not be particularly limited, and the second embodiment shows that it can operate at 40 GHz, which is twice the optimum value.

When the modulation factor of the frequency modulation shown in FIG. 10C is 40 GHz, the width of the optical spectrum becomes wider than when the modulation factor shown in FIG. 2C is 20 GHz. In addition, a line spectrum appears at frequencies corresponding to the minimum frequency f ₁ and the maximum frequency f ₃ . The optical signal S3 modulated into three values is incident on the optical filter 103.

FIG. 11A shows the transmission or reflection characteristics of the light discriminator 103. For example, an etalon in which a cavity is configured by a mirror pair having a reflectivity R = 0.3 to 0.7, and a single cavity Fabry-Perot filter shown in FIG. This is applied to the discriminator 103. Here, an etalon filter with reflectivity R = 0.3 and FSR = 100 GHz was used. There are both a positive slope area and a negative slope area between the minimum frequency f ₁ and the maximum frequency f ₃ of the frequency-modulated optical signal, and a positive slope in the vicinity of the minimum frequency f ₁ . In the vicinity of the maximum frequency f ₃ , the slope is negative. The output level for frequency f ₁ is P ₂ , the output level for frequency f ₂ is P ₁ , and the output level for frequency f ₃ is P ₂ . The optical signal S3, which is a ternary frequency modulation signal, is converted into an optical signal S4 by the optical discriminator 103.

  FIG. 11B shows the light spectrum of the optical signal S4 output from the optical discriminator 103, and FIG. 11C shows the intensity waveform of the optical signal S4 output from the optical discriminator 103. The spectrum of the light of the optical signal S4 is narrower than the spectrum of the light of the optical signal S3 shown in FIG. The time waveform of the optical signal S4 is a binary amplitude modulation signal of 40 Gb / s. This time waveform is a waveform in which the logic level is inverted as compared with FIG.

  In this way, the step of multi-leveling the input electrical signal, the step of generating a multi-level optical frequency modulation signal from the multi-level electrical signal, and the binary of the multi-level optical frequency modulation signal The step of converting into an amplitude modulation signal can generate a 40 Gb / s modulation signal using the optical signal source 102 having only a 12 GHz modulation band.

  In the second embodiment, an etalon with reflectivity R = 0.3 and FSR = 100 GHz is used as an optical discriminator. However, for example, an etalon with reflectivity R = 0.6 and FSR = 200 GHz can also be used. Similarly to the first embodiment, the relationship between the reflectance R and the FSR is not limited to the above combination, and the reflectance R ranges from about 0.3 to 0.95. On the other hand, it can be applied to the light modulation light source of the present embodiment by setting an appropriate FSR.

  In the second embodiment, the modulation has been described under the condition where the frequency modulation degree is 40 GHz and the spectrum is wider than the MSK condition. According to the present embodiment, since the frequency-modulated optical signal S3 from the optical signal source is filtered by the optical discriminator, the spectrum of the optical signal S4 after being converted to amplitude modulation becomes narrower than before the conversion. Yes. Therefore, it is possible to reduce waveform deterioration due to dispersion of the optical fiber, and the transmission characteristics at the time of optical fiber transmission are the same as those of the conventional NRZ modulation even in the driving condition where the spectrum is wider than the MSK condition. Better.

(Light discriminator-6)
The double cavity type interference filter (etalon) shown in FIG. 6 can be used as a transmission type optical discriminator. The reflectance of the reflection mirrors 306 and 307 is set to about 0.2 to 0.4, and the reflectance of the reflection mirror 308 is set to about 0.5 to 0.9. By setting the thicknesses of the two spacer layers 311a and 311b to about 0.5 to 4 mm, the characteristics shown in FIG. 11A can be obtained. That is, both a region having a positive slope and a region having a negative slope exist between the minimum frequency f ₁ and the maximum frequency f ₃ of the frequency-modulated optical signal, and are positive in the vicinity of the minimum frequency f ₁ . In the vicinity of the slope and the maximum frequency f ₃ , the slope is negative. In this way, the modulation operation can be performed even for a bit rate exceeding the modulation band of the optical signal source.

As described in the first embodiment, the optical discriminator has both a region having a positive slope and a region having a negative slope between the minimum frequency f ₁ and the maximum frequency f ₃ of the frequency-modulated optical signal. It is necessary to exist. For example, not only an etalon but also a Mach-Zehnder interference filter, a ring filter, or the like can be used. In this way, any filter may be used as long as it has an extreme value near the center frequency f ₂ of the ternary signal of the optical frequency and has an optical frequency response in which the sign of the slope is different before and after that.

(Light discriminator-7)
The fiber grating filter shown in FIG. 9 can also be used as a reflection type optical discriminator. The characteristic shown in FIG. 11 (a), that is, there are both a region having a positive slope and a region having a negative slope between the minimum frequency f ₁ and the maximum frequency f ₃ of the frequency-modulated optical signal, An optical frequency response having a positive slope near the minimum frequency f _{1 and} a negative slope near the maximum frequency f ₃ can be realized.

(Other embodiments)
(Band limiting filter)
As described above, a low-pass filter can be used as the band limiting filter. A low-pass filter can also be used when converting to a 4-value or multi-value, but if a signal processing circuit using an addition circuit is used, the conversion can be performed with high accuracy. For example, if two series of binary electrical signals are weighted and voltage-added, they can be converted into quaternary electrical signals.

(Optical signal source)
A direct modulation type semiconductor laser was used as an optical signal source for generating a multilevel optical frequency modulation signal. The structure of the semiconductor laser is not particularly limited, and may be any structure as long as it is a DFB laser, DBR laser, ring laser, or other laser capable of modulating the optical frequency. In general, when a DFB laser is directly modulated to generate a frequency modulation signal, an intensity modulation component having the same time waveform as the optical frequency modulation component is often superimposed along with the frequency modulation. Although the modulation operation can be performed even for a bit rate exceeding the modulation band of the optical signal source, in the present embodiment, only the frequency modulation component is used, so that the intensity modulation component is preferably small.

  FIG. 12 is a diagram for comparing the intensity waveforms of the optical signals output from the optical discriminator. The waveform in FIG. 12A is the same as that in FIG. 3C in the first embodiment. A case where a multi-value frequency modulation signal generated from a direct modulation type semiconductor laser (optical signal source 102) does not include an intensity modulation component is shown. FIG. 12B shows a case where an intensity modulation component is included and the intensity of the multi-level frequency modulation signal changes by 20% in the same phase as the optical frequency. It can be seen that the modulation operation can be realized even for a bit rate exceeding the modulation band of the optical signal source.

(Light discriminator)
An optical discriminator that converts a frequency modulation signal converted into a four-value or higher multi-value into a binary amplitude modulation signal will be described. FIG. 13 shows the transmission or reflection characteristics of an optical discriminator that converts a quaternary frequency modulation signal into a binary amplitude modulation signal. When dealing with a four-level or more multi-level frequency modulation signal, a filter having a spectrum including a plurality of peaks and a plurality of valleys in the output level is used. Among the multi-level frequency modulation signals, for example, odd number frequency modulation signal is set to peak (output level is P ₁ ), and even number frequency modulation signal is set to valley (output level is P ₂ ) or vice versa. To do. Thereby, the optical signal which is a quaternary frequency modulation signal is converted into a binary optical signal by the optical discriminator.

  As a specific configuration of the optical discriminator, as described above, a cavity type or waveguide type filter can be used. In addition to the above-described filter, an arrayed waveguide grating (AWG) filter can also be applied. Any filter may be used as long as the transmission or reflection characteristics shown in FIG. 3A, FIG. 11A, or FIG. 13 can be realized.

It is a figure which shows the structure of the light modulation light source concerning the 1st Embodiment of this invention. It is a figure for demonstrating the principle of operation of a light modulation light source. It is a figure for demonstrating the principle of operation of a light modulation light source. It is a figure which shows the structure of the optical discriminator concerning Example 1. FIG. It is a figure which shows the arrangement | positioning method of a light discriminator. It is a figure which shows the structure of the optical discriminator concerning Example 2. FIG. It is a figure which shows the structure of the optical discriminator concerning Example 3. FIG. It is a figure which shows the structure of the optical discriminator concerning Example 4. FIG. It is a figure which shows the structure of the optical discriminator concerning Example 5. FIG. It is a figure for demonstrating the principle of operation of the light modulation light source concerning the 2nd Embodiment of this invention. It is a figure for demonstrating the principle of operation of the light modulation light source concerning the 2nd Embodiment of this invention. It is a figure for comparing the intensity waveform of the optical signal output from an optical discriminator. It is a figure which shows the permeation | transmission or reflection characteristic of the optical discriminator which converts a quaternary frequency modulation signal into a binary amplitude modulation signal.

Explanation of symbols

DESCRIPTION OF SYMBOLS 101 Band-limiting filter 102 Optical signal source 103 Optical discriminator 301,302,306,307,308 Dielectric multilayer mirror 303,309 High refractive index layer 304,310 Low refractive index layer 305,311 Spacer layer 312 Input waveguide 313 Output waveguide 314 Coupler 315 Ring waveguide 316 Optical fiber 317 Periodic perturbation of refractive index 318 Arm waveguide 401 Optical discriminator 402 Optical discriminator input 403 Optical discriminator output 404 Optical circulator 405 Beam splitter 406 Polarizing beam splitter 407 1 / 4 wavelength plate

Claims (15)

An optical signal source that is driven by an electrical signal having multi-level signal strength and generates an optical signal that is optical frequency modulated at a multi-level frequency;
An optical discriminator comprising: an optical discriminator that converts the optical signal that has been optical frequency modulated into a binary amplitude-modulated optical signal.   2. The optical modulation according to claim 1, further comprising conversion means for converting an electric signal having a binary signal intensity into an electric signal having a multi-level signal intensity and driving the optical signal source. light source.   The said conversion means is a low-pass filter which restrict | limits the zone | band of the electric signal which has the said binary signal strength, and converts it into the electrical signal which has a ternary signal strength. Light modulation light source.   4. The light modulation light source according to claim 3, wherein a value of a high-frequency 3 dB cutoff frequency of the low-pass filter is 20% to 35% of a bit rate of the electric signal having the binary signal intensity. .   5. The light modulation light source according to claim 1, wherein the optical signal source is a semiconductor laser.   6. The optical modulation light source according to claim 5, wherein the value of the optical frequency modulation degree of the optical signal source is half of the bit rate of the electric signal having the binary signal intensity.   The optical discriminator has a transmission or reflection characteristic in which both a region having a positive slope and a region having a negative slope exist between a minimum frequency and a maximum frequency of the optical frequency modulated optical signal. The light modulation light source according to any one of claims 1 to 6.   The light modulation light source according to claim 7, wherein the light discriminator is an interference filter having a transmission mode or a reflection mode.   The light modulation light source according to claim 8, wherein the interference filter is a cavity type.   9. The light modulation light source according to claim 8, wherein the interference filter is a waveguide type.   The light modulation light source according to claim 7, wherein the light discriminator is a diffraction grating.   The light modulation light source according to claim 11, wherein the diffraction grating is a fiber grating. A light modulation method of a light modulation light source that inputs an electrical signal having a binary signal intensity and outputs a binary amplitude modulated optical signal,
A first step of converting the electrical signal having a binary signal strength into an electrical signal having a multilevel signal strength;
A second step of converting the electrical signal having the multi-level signal strength into an optical signal that is optical frequency modulated at a multi-level frequency;
And a third step of converting the optical frequency modulated optical signal into a binary amplitude modulated optical signal.   The optical modulation method according to claim 13, wherein the first step limits a band of the electric signal having the binary signal intensity.   The third step has an input / output characteristic in which both a region having a positive slope and a region having a negative slope exist between a minimum frequency and a maximum frequency of the optical frequency modulated optical signal. The light modulation light source according to claim 13 or 14.

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