JP2011159752A - Optical modulation signal generation device and optical modulation signal generation method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical modulation signal generation method for executing high-speed transmission having a high wavelength dispersion resistance. <P>SOLUTION: The optical modulation signal generation method generates an optical modulation signal by utilizing an optical modulation signal generation device. The optical modulation signal generation device comprises a frequency modulation light source for generating and outputting a frequency modulation signal, and an intensity modulator that generates an intensity modulation signal and receives the frequency modulation signal as an input so as to add intensity modulation to the frequency modulation signal by the intensity modulation signal. The optical modulation signal generation device drives the frequency modulation light source and the intensity modulator by using the same bit pattern. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an optical modulation signal generation apparatus and an optical modulation signal generation method, and more particularly, an optical modulation signal generation apparatus and an optical modulation signal generation method for performing high-speed and high chromatic dispersion tolerance transmission using an optical fiber. About.

  Due to the explosive increase in network traffic accompanying the spread of the Internet in recent years, high-speed and large-capacity optical fiber transmission is being promoted. Up to now, in the backbone network, the introduction of wavelength division multiplexing (WDM) technology that places optical signals of different wavelengths on a single optical fiber cable will increase the capacity of point-to-point transmission and increase the speed of transmission equipment. Has been planned. Conventionally, transmission technology based on 10 Gb / s has been applied mainly to backbone networks. However, access networks and metro area networks have already been promoted to gigabits, and this trend toward higher speeds will continue. It is done.

  Conventionally, transmission between nodes is a point-to-point method, and optical-electrical-optical (OEO) conversion has been performed in the nodes. Currently, application of optical signal processing in nodes is progressing, and construction of a photonic network using optical routing processing based on WDM technology is being promoted centering on ROADM (Reconfigurable Optical Add / Drop Multiplexing). In the future, with the introduction of optical MPLS (Multi-Protocol Label Switching) technology, etc., it will be possible to cut through nodes without changing the light, and it will be more important to increase the distance of repeaterless transmission. Thus, there is a great expectation for an optical transmitter that can realize high-speed modulation and long-distance transmission.

  FIG. 1 is a diagram showing a method of an optical modulation signal generation apparatus widely used at present. FIG. 1A shows a direct modulation method for directly modulating a laser, and FIG. 1B shows an external modulation method for externally modulating continuous light (CW light) output from a laser light source by an intensity modulation element.

  In the direct modulation method, current modulation driving of a distributed feedback laser diode (DFB-LD) that can obtain light with a narrow spectral width and a uniform wavelength is generally used.

  In the external modulation system, the types of intensity modulation elements can be classified into electroabsorption (EA) type modulators and phase intensity modulation type modulators.

  The EA modulator modulates the intensity of light transmitted to the waveguide using a change in electroabsorption accompanying application of an electric field in the semiconductor waveguide. In the case of a quantum well structure, the quantum confined stark effect (QCSE) is generally used, and in the case of a bulk structure, the Franz-Keldysh (FK) effect is generally used. This structure is widely used as an electroabsorption modulator integrated DFB-LD because it can be easily integrated with the DFB-LD.

  As the phase intensity modulation type modulator, a Mach-Zehnder type modulator is mainly used. In this modulator, after the input light is branched into two optical paths by a coupler, a phase difference is generated between the branched lights in the two optical paths by a change in refractive index accompanying application of an electric field. When two branched lights having a phase difference are multiplexed, intensity modulation is performed using an intensity change due to the phase difference. For the refractive index change, the electro-optic effect is used in addition to the quantum confined Stark effect and the Franz-Keldish effect.

  Further, as a signal transmission / reception method, amplitude modulation (OOK) representing a bit string of “1” and “0” is used according to the modulation amplitude of the signal, direct detection is performed with a photodiode, and bit determination is performed based on the amplitude of the received light current. The detection method is currently the mainstream technology.

PA Morton et al., ''38 .5km error free transmission at 10 Gbit / s in standard fiber using a low chirp, spectrally filtered, directly modulated 1.55 mm DFB laser,' 'Electronics Letters vol. 33, no. 4, pp. 310-311, 1997)

  However, in the intensity-modulated signal using the conventional light source, the spectrum is broadened during the modulation, which causes the transmission distance and the transmission speed to be limited due to the chromatic dispersion of the optical fiber. Further, in the driving of a conventional light source, frequency variation (chirping) occurs with carrier variation, so the transmission distance is further limited.

  FIG. 2 shows an optical fiber transmission waveform (eye pattern) of a NRZ (Non-Return-to-Zero) signal with a bit rate of 10 Gb / s generated by a direct modulation type light source. The chromatic dispersion of the optical fiber is 16.3 ps / nm / km at a used wavelength of 1.55 μm. FIG. 2A shows the waveform of the initial data of the signal, FIG. 2B shows the waveform of the signal after 20 km transmission, and FIG. 2C shows the waveform of the signal after transmission of 100 km. As the transmission distance increases, the waveform greatly collapses, and waveform distortion is observed even after 20 km transmission as shown in FIG. As shown in FIG. 2C, it is impossible to discriminate the signal at 100 km.

  In general, the transmission distance is shortened in inverse proportion to the square of the bit rate, so that the distance that can be transmitted is abruptly shortened as the modulation speed is increased. The direct modulation laser described above has a transmission distance of about 10 km at 10 Gb / s, which is very short. In the EA type modulator, it is necessary to improve the characteristic by chirp control of the modulation layer. In the Mach-Zehnder type modulator, zero chirp driving is possible. However, in the normal amplitude modulation, the EA type modulator is 80 km, Mach. A zender type modulator has a general limit value of about 100 km.

  In order to solve this deterioration of transmission characteristics, for example, as in Non-Patent Document 1, a method of suppressing frequency fluctuation by performing mutual conversion between frequency amplitude and intensity amplitude using a direct modulation laser and a frequency filter. Has been proposed. However, the technique of Non-Patent Document 1 has a problem that the operating speed is limited because the modulation band is limited by the relaxation oscillation of the laser during high-speed operation by directly modulating the laser. Currently commercialized lasers have a relaxation oscillation frequency of about 10 GHz, and it is difficult to cope with high-speed modulation beyond that. Further, since an optical filter is required, strict control of the oscillation wavelength is required.

  The present invention has been made in view of such problems, and the object of the present invention is to obtain a narrow-band modulation spectrum in an optical modulation signal, thereby having high chromatic dispersion tolerance, high-speed modulation and long-distance transmission. An object of the present invention is to provide an optical modulation signal generation apparatus and an optical modulation signal generation method that are realized.

  In order to achieve such an object, an optical modulation signal generation method according to claim 1 generates a frequency modulation light source that generates and outputs a frequency modulation signal, generates an intensity modulation signal, and inputs the frequency modulation signal. An optical modulation signal generation method for generating an optical modulation signal using an optical modulation signal generation device configured to include an intensity modulator that adds intensity modulation to a frequency modulation signal by the intensity modulation signal, The modulation signal generation device is characterized in that the frequency modulation light source and the intensity modulator are driven using the same bit pattern.

  The optical modulation signal generation method according to claim 2 is the optical modulation signal generation method according to claim 1, wherein the intensity modulator has a positive chirp parameter. It is.

  The optical modulation signal generation method according to claim 3 is the optical modulation signal generation method according to claim 1, wherein the intensity modulator has a negative chirp parameter. It is.

  The optical modulation signal generation method according to claim 4 is the optical modulation signal generation method according to any one of claims 1 to 3, wherein the optical modulation signal includes a frequency modulation component of the frequency modulation signal and the intensity. An optical modulation signal generating method, wherein the intensity modulation component of the modulation is modulated so as to be synchronized.

  The optical modulation signal generation method according to claim 5 is the optical modulation signal generation method according to any one of claims 1 to 4, wherein the intensity modulator is an electroabsorption modulator. This is an optical modulation signal generation method.

  The optical modulation signal generation method according to claim 6 is the optical modulation signal generation method according to claims 1 to 5, wherein the frequency modulation light source is a distributed Bragg reflection laser. This is a signal generation method.

  The optical modulation signal generation method according to claim 7 is the optical modulation signal generation method according to claim 6, wherein the frequency modulation light source bias-modulates a phase adjustment region of the distributed Bragg reflection laser. An optical modulation signal generation method characterized by generating a frequency modulation signal.

  An optical modulation signal generation method according to an eighth aspect is the optical modulation signal generation method according to any one of the first to fifth aspects, wherein the frequency modulation light source is a distributed feedback laser. This is an optical modulation signal generation method.

  The optical modulation signal generation method according to claim 9 is the optical modulation signal generation method according to any one of claims 1 to 8, wherein the optical modulation signal generation device includes the frequency modulation light source and the intensity modulator. A method for generating an optical modulation signal, comprising a semiconductor optical amplifier between the two.

  The optical modulation signal generation method according to claim 10 is the optical modulation signal generation method according to any one of claims 4 to 9, wherein the optical modulation signal generation device includes a frequency modulation amplitude Δf of the optical modulation signal. , Bit rate B and extinction ratio ER,

The optical modulation signal generation method is characterized in that the optical modulation signal is generated so as to satisfy the relationship.

  The optical modulation signal generation method according to claim 11 is the optical modulation signal generation method according to claim 2 or 3, wherein the optical modulation signal generation device generates a frequency chirp generated by the intensity modulator at the frequency. An optical modulation signal generation method comprising canceling out by frequency modulation of a modulated light source.

  The optical modulation signal generation method according to claim 12 is the optical modulation signal generation method according to claim 2, wherein the optical modulation signal generation device generates a positive frequency chirp generated by the intensity modulator at the frequency. An optical modulation signal generation method comprising: inverting by frequency modulation of a modulation light source and converting to negative frequency chirp.

  According to the present invention, an optical modulation signal having an intensity modulation component and a frequency modulation component can be transmitted at high speed, and the spectrum width can be narrowed by adjusting the amplitude of the optical modulation signal. Long distance transmission is possible. Furthermore, the transmission distance is dramatically increased by applying intensity modulation having a positive chirp parameter. Further, in the frequency modulation region, it is possible to improve the propagation characteristics by compensating the frequency chirp component generated in the intensity modulation region or by inverting the chirp code.

It is explanatory drawing of the conventional optical modulation signal generation apparatus. It is a figure which shows the transmission characteristic of the conventional direct modulation laser. It is explanatory drawing of the optical modulation signal generation apparatus by this invention. It is explanatory drawing of the cross-section of the optical modulation signal production | generation apparatus in Example 1 of this invention. It is explanatory drawing of the cross-section of the optical modulation signal generation apparatus in this invention. It is explanatory drawing of the principle of operation of an electroabsorption type modulator. It is explanatory drawing of the optical modulation signal in this invention. It is a figure which shows the waveform after optical fiber transmission of the signal in this invention. It is explanatory drawing of the chirp characteristic of an electro-absorption type modulator. It is explanatory drawing of the modulation method using a frequency chirp characteristic. It is a figure which shows the transmission characteristic of the signal output from the optical modulation signal generation apparatus of this invention in the modulation method using a frequency chirp characteristic. It is a figure which shows the operation example of the drive of the frequency modulation light source for compensating the frequency chirp produced by an EA type | mold modulator. It is a figure which shows the transmission characteristic of the signal output from the optical modulation signal production | generation apparatus by the drive operation shown by FIG. It is a figure explaining the production | generation method of the signal for a drive of the DBR type frequency modulation light source 101. FIG. It is a figure which shows the operation example of the frequency modulation light source and EA type | mold modulator in the case of inverting the frequency chirp produced by the modulator. It is a figure which shows the transmission characteristic of the signal output from the optical modulation signal generation apparatus in the case of inverting the frequency chirp produced by the modulator. It is explanatory drawing of the cross-section of the optical modulation signal production | generation apparatus in Example 2 of this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. An optical modulation signal generation apparatus according to the present invention includes a frequency modulation light source that generates a frequency modulation signal and an intensity modulator that adds intensity modulation to the frequency modulation signal. A configuration using a reflection (DBR) type frequency modulation light source and an electroabsorption (EA) type modulator as an intensity modulator will be described. However, what is used for the frequency modulation light source and the intensity modulator in the configuration of the present invention is not limited to the DBR type frequency modulation light source and the EA type modulator.

  FIG. 3 is an explanatory diagram showing a simplified example of the embodiment of the optical modulation signal generating apparatus of the present invention. The optical modulation signal generation device of this embodiment includes a DBR type frequency modulation light source 101 that generates a frequency modulation signal and an EA type modulator 102 that generates an intensity modulation signal. The frequency modulation signal generated from the DBR type frequency modulation light source 101 is guided to the EA type modulator 102. This direction is defined as a light guiding direction. The frequency modulation light source 101 and the EA modulator 102 are driven using the same bit pattern, and the EA modulator 102 is driven so that the frequency modulation signal and the intensity modulation signal are synchronized. In the EA type modulator 102, the input frequency modulation signal is intensity modulated by the intensity modulation signal. Further, by adjusting the modulation bit rate, the extinction ratio, and the frequency modulation amplitude, a narrow band light modulation spectrum can be obtained. Therefore, optical fiber transmission with high chromatic dispersion tolerance is realized. The above will be described in detail below.

[Device configuration]
FIG. 4 shows the configuration of the optical modulation signal generating apparatus according to the first embodiment of the present invention. This apparatus has a configuration in which a DBR type frequency modulation light source 101 and an EA type modulator 102 are monolithically integrated. The DBR type frequency modulation light source 101 is composed of a plurality of regions. The DBR type frequency modulation light source 101 includes a first super periodic structure diffraction grating (SSG) -DBR type reflector region 103 having a length of 600 μm, a frequency modulation region 104 having a length of 200 μm, and an active region 105 having a length of 300 μm. And a second SSG-DBR type reflector region 106 having a length of 300 μm. A DC current source 118 is connected to the first SSG-DBR reflector region 103, a modulation voltage source 119 is connected to the frequency modulation region 104, a DC current source 120 is connected to the active region 105, and a second A direct current source 121 is connected to the SSG-DBR type reflector region 106. The first SSG-DBR reflector region 103 and the second SSG-DBR reflector region 106 are processed with a concavo-convex superperiod diffraction grating, each having a comb-like reflection spectrum.

  The EA modulator 102 includes an EA modulator region 107 having a length of 200 μm. A modulation voltage source 122 is connected to the EA type modulator region 107.

  An optical modulation signal generating apparatus according to Example 1 of the present invention includes a first SSG-DBR reflector region 103, a frequency modulation region 104, an active region 105, a second SSG-DBR reflector region 106, The EA type modulator region 107 is configured in this order.

  The first SSG-DBR reflector region 103 includes an n-type electrode 116, an n-type InP cladding layer 108 formed on the n-type electrode 116, and an InGaAsP layer 109 formed on the n-type InP cladding layer 108. A first diffraction grating 112 on the InGaAsP layer 109, a p-type InP cladding layer 114 formed on the first diffraction grating 112, a contact layer 115 formed on the p-type InP cladding layer 114, The p-type electrode 117 is formed on the contact layer 115.

  The frequency modulation region 104 includes an n-type electrode 116, an n-type InP clad layer 108 formed on the n-type electrode 116, an InGaAsP layer 109 formed on the n-type InP clad layer 108, and the InGaAsP layer 109. The p-type InP clad layer 114 is formed, the contact layer 115 is formed on the p-type InP clad layer 114, and the p-type electrode 117 is formed on the contact layer 115.

  The active region 105 includes an n-type electrode 116, an n-type InP cladding layer 108 formed on the n-type electrode 116, and an eight-layer InGaAsP quantum well layer 110 for active layer formed on the n-type InP cladding layer 108. The p-type InP clad layer 114 formed on the eight active layer InGaAsP quantum well layers 110, the contact layer 115 formed on the p-type InP clad layer 114, and the p-type formed on the contact layer 115. And electrode 117.

  The second SSG-DBR type reflector region 106 includes an n-type electrode 116, an n-type InP cladding layer 108 formed on the n-type electrode 116, and an InGaAsP layer 109 formed on the n-type InP cladding layer 108. A second diffraction grating 113 on the InGaAsP layer 109, a p-type InP cladding layer 114 formed on the second diffraction grating 112, a contact layer 115 formed on the p-type InP cladding layer 114, The p-type electrode 117 is formed on the contact layer 115.

  The EA-type modulator region 107 includes an n-type electrode 116, an n-type InP clad layer 108 formed on the n-type electrode 116, and an eight-layer InGaAsP quantum for an intensity modulation layer formed on the n-type InP clad layer 108. On the well layer 111, the p-type InP cladding layer 114 formed on the eight-layer InGaAsP quantum well layer 111 for the intensity modulation layer, the contact layer 115 formed on the p-type InP cladding layer 114, and the contact layer 115 The p-type electrode 117 is formed.

The n-type electrode 116 is grounded, and the n-type InP cladding layer 108 has a doping concentration of 10 ¹⁸ cm ⁻³ . The InGaAsP layer 109 has a band gap wavelength of 1.3 μm and a thickness of 300 nm. The InGaAsP layer 109 in the first SSG-DBR type reflector region 103 is used as a DBR layer, and the InGaAsP layer 109 in the frequency modulation region 104 is used as a frequency modulation layer. The active layer eight-layer InGaAsP quantum well layer 110 has a band gap wavelength of 1.55 μm and a thickness of 200 nm. The eight-layer InGaAsP quantum well layer 111 for intensity modulation layer has a band gap wavelength of 1.5 μm and a thickness of 200 nm. The first diffraction grating 112 and the second diffraction grating 113 have a Bragg wavelength of 1.55 μm, an effective coupling constant of 30 cm ⁻¹ , a reflection peak interval of 5 nm, and nine reflection peaks. The first diffraction grating 112 and the second diffraction grating 113 have different free spectral regions (FSR) of 60 GHz. The InGaAsP layer 109, the active layer eight-layer InGaAsP quantum well layer 110, and the intensity modulation layer eight-layer InGaAsP quantum well layer 111 are each formed as a waveguide core layer (corresponding to the core layer 123 in FIG. 5). . The p-type InP cladding layer 114 has a doping concentration of 10 ¹⁸ cm ⁻³ and a thickness of 1.5 μm. The contact layer 115 has a band gap wavelength of 1.5 μm, a doping concentration of 10 ¹⁹ cm ⁻³ , and a thickness of 300 nm. Contact layers 115 between the regions are formed at intervals of 100 μm. Further, a non-reflective coating is applied to the faces of the DBR type frequency modulation light source 101 and the EA type modulator 102 that face each other in the light guiding direction. As shown in FIG. 4, the n-type electrode 116, the n-type InP clad layer 108, and the p-type InP clad layer 114 extend over the regions of the DBR type frequency modulation light source 101 and the EA type modulator 102, and the light guide direction Are uniformly formed.

FIG. 5 shows a cross section of the DBR type frequency modulation light source 101 and the EA type modulator 102 in a direction perpendicular to the light guiding direction. The DBR type frequency modulation light source 101 and the EA type modulator 102 have a ridge structure with a waveguide width of 1.5 μm. As shown in FIG. 4, an n-type InP cladding layer 108 is formed on the n-type electrode 116. The core layer 123 is formed on the n-type InP cladding layer 108 and the contact layer 115 is formed on the p-type InP cladding layer 114. A SiO ₂ insulating film 124 is formed on both sides of the ridge, and a benzocyclobutene (BCB) 125 is formed on the SiO ₂ insulating film 124. A p-type electrode 117 is formed on the planarized contact layer 115, SiO ₂ insulating film 124, and BCB 125.

[Method for generating optical modulation signal]
The operation principle of this apparatus will be described. Spontaneous emission light is generated by injecting current into the active region 105 by the direct current source 120. The spontaneous emission light coupled to the waveguide mode is reflected by the InGaAsP layer 109 in the first SSG-DBR type reflector region 103 and the second SSG-DBR type reflector region 106, and is used as an eight-layer InGaAsP quantum well for the active layer. Stimulated emission by the layer 110 occurs. Both the first diffraction grating 112 and the second diffraction grating 113 have nine reflection peaks, but the FSR of the reflection spectrum is slightly different, so that only a single wavelength can be selectively reflected. It is. When the optical gain in stimulated emission exceeds the loss due to light absorption of the DBR type frequency modulation light source 101, laser oscillation occurs. The oscillation frequency at this time is given as follows.

L is the effective resonator length of the DBR type frequency modulation light source 101, the effective length L _{DBR of} the first SSG-DBR type reflector region 103 and the second SSG-DBR type reflector region 106, and the frequency modulation region 104. the length L _p of the sum of the length L _a of the active layer region 105. n _eq is the effective refractive index and is assumed to be all uniform. c is the speed of light in vacuum. m is an integer indicating the longitudinal oscillation mode.

  In the present invention, the region corresponding to the frequency modulation region 104 is used as a region in which phase adjustment is performed by current-modulating the region in a general DBR type frequency modulation light source, as described below. In the present invention, the frequency modulation region 104 is used as a region for frequency modulation by bias modulation.

From the equation (1), it can be seen that when the effective refractive index of the frequency modulation region 104 changes by Δn _eq , the oscillation frequency changes as follows.

  Specifically, the oscillation frequency of the DBR type frequency modulation light source 101 can be changed by applying a voltage to the frequency modulation region 104 by the modulation voltage source 119 and changing the effective refractive index of the frequency modulation region 104. Therefore, the frequency modulation operation of the DBR frequency modulation light source 101 can be performed by superimposing the modulation signal on the applied voltage. With respect to the applied voltage, an electro-optic effect in which the refractive index changes as the voltage is applied may be used in the reverse bias direction, or a carrier effect due to current may be used in the forward direction. In terms of modulation efficiency, current driving by forward bias is advantageous, but in terms of high-speed modulation, reverse bias driving is more advantageous.

  In general, the change in the refractive index of a compound semiconductor due to the application of a voltage is linked by the relationship between the change in light absorption and Kramers-Kronig, and the change in refractive index is accompanied by a change in light absorption. This change in light absorption causes a change in the threshold current value of the laser and causes unnecessary intensity modulation. When an intensity modulation operation is applied, relaxation oscillation of the laser is generated, which causes a deterioration in device response.

  However, for example, in the refractive index modulation by reverse bias, if the band gap wavelength of the frequency modulation region 104 is set to a wavelength shorter by 100 nm or more than the use region, the propagation loss due to light absorption due to voltage application can be almost ignored. it can. Therefore, in this configuration, an ideal frequency modulation signal without intensity modulation can be generated by setting the band gap wavelength of the frequency modulation region 104 to a wavelength shorter than the use region by 100 nm or more.

  On the other hand, by modulating the current in the active region 105, a frequency modulation signal can be generated by the intensity modulation component and the adiabatic chirp component. However, in the direct modulation operation, when the current of the active region 105 is modulated, the modulation band is limited by the relaxation oscillation of the laser during high-speed operation, and unnecessary frequency chirp is added during modulation. Therefore, in the present invention, bias modulation of the frequency modulation region 104 is more effective in terms of high-speed operation than current modulation of the active region 105. Therefore, the frequency modulation region 104 is bias-modulated.

  Intensity modulation is added in the EA type modulator region 107 to the frequency modulation signal without intensity modulation generated by bias modulation of the frequency modulation region 104 as described above. FIG. 6 shows a modulation operation of the EA type modulator region 107 by voltage application. In the EA type modulator 102, since the band edge wavelength of the core layer 123 is set to the short wavelength side with respect to the laser driving wavelength of 1.55 μm, the signal transmission loss is small when the bias voltage is low. On the other hand, when reverse bias is applied, the overlap of electron and hole envelope functions is reduced due to the quantum confined Stark effect, the exciton absorption peak is reduced, and the light absorption wavelength is shifted to the longer wavelength side due to the inclination of the band. , Propagation loss increases. Using this operation, an intensity modulation signal can be generated by superimposing the modulation signal on the reverse bias voltage.

  In general, in the EA type modulator, the refractive index change due to the change of the absorption coefficient due to the reverse bias occurs due to the above-mentioned Kramers-Kronig relationship, which causes the frequency chirp, but depending on the design of the quantum well, zero chirp or negative chirp It is also possible to realize the operation. By combining the above characteristics, an optical modulation signal combining the functions of frequency modulation and intensity modulation can be generated.

  Further, this configuration employs an SSG-DBR structure for the DBR type frequency modulation light source 101. For this reason, by injecting current into the first SSG-DBR reflector region 103 and the second SSG-DBR reflector region 106, a minute wavelength change is expanded using the vernier effect, and the wavelength variable region 40 nm. Wide wavelength switching operation can be realized. When the drive wavelength is changed, the wavelength dependency becomes a problem, and there is a concern about the change of the drive condition at the time of modulation. However, since the DBR type frequency modulation light source 101 of this configuration has a small wavelength dependency, the drive condition is not changed. Equivalent modulation characteristics can be obtained. The EA type modulator 102 has a problem of wavelength dependence of band edge absorption, but it is possible to ensure a high extinction ratio over a wide band by adjusting the bias voltage and the modulation amplitude.

[Optical fiber transmission method]
A method of performing optical fiber transmission using this apparatus will be described. In addition to performing current injection into the active region 105, current injection is performed into the first SSG-DBR reflector region 103 and the second SSG-DBR reflector region 106, and laser oscillation at an oscillation wavelength of 1.55 μm is performed. I do. In addition, a reverse bias in which an NRZ modulation signal having a bit rate of 10 Gb / s is superimposed on the frequency modulation region 104 and the EA type modulator region 107 is applied. The chirp of the EA type modulator region 107 is zero. The frequency modulation region 104 and the EA type modulator 102 are driven using the same bit pattern, and the frequency modulation signal input to the EA type modulator 102 and the intensity modulation signal generated in the EA type modulator 102 are between In order to synchronize, the following method can be used.

  For example, when the intensity modulation response is delayed with respect to the frequency modulation signal, the cable length for transmitting the electric signal connecting the modulation voltage source 119 and the electrodes of the frequency modulation region 104 to delay the input signal of the frequency modulation signal. Alternatively, the wiring length may be made longer than the cable length or wiring length for transmitting an electrical signal that connects between the modulation voltage source 122 and the electrodes of the EA type modulator region 107. When the intensity modulation response is fast with respect to the frequency modulation signal, the cable length or wiring length for transmitting the electrical signal connecting the modulation voltage source 119 and the electrodes of the frequency modulation region 104 is used to delay the intensity modulation signal. May be shortened with respect to the cable length or wiring length for transmitting an electrical signal for connecting between the modulation voltage source 122 and the electrodes of the EA type modulator region 107. The delay time can be arbitrarily adjusted by adjusting the cable or wiring length. Similar functions can also be realized using a memory or the like. By the above method, the frequency modulation signal input to the EA type modulator 102 and the intensity modulation signal generated in the EA type modulator 102 can be synchronized.

  FIG. 7 shows the waveform of an optical modulation signal when this apparatus is driven under the condition that the chirp of the EA type modulator region 107 is zero. FIG. 7A shows the optical intensity-time characteristic of the optical modulation signal output from the optical modulation signal generation device, and FIG. 7B shows the frequency of the optical modulation signal output from the optical modulation signal generation device. The time characteristic is shown. As shown in FIGS. 7A and 7B, the optical modulation signal output from the optical modulation signal generation device has a waveform to which frequency modulation in phase with the optical intensity is added. An optical modulation signal having frequency modulation in phase with intensity modulation has a narrower spectral width than that of intensity modulation alone, and improves chromatic dispersion resistance during optical fiber transmission. Hereinafter, modulation conditions that have a particularly narrow spectral width and a high chromatic dispersion tolerance will be described.

In the optical modulation signal output from the optical modulation signal generator, the extinction ratio ER is defined by the ratio I ₁ / I ₀ of the light intensity I ₁ of 1 level and the light intensity I ₀ of 0 level, and the frequency modulation amplitude Δf (GHz) ), The bit rate B (Gb / s), and the extinction ratio ER are adjusted so that the optical modulation signal generation device is adjusted as follows.

  This driving condition is a condition in which the sideband of the modulation signal disappears, and the chromatic dispersion tolerance during transmission is drastically improved by narrowing the spectrum width. In the configuration of the present embodiment, a driving condition that satisfies the expression (3) is shown using a reverse bias operation. Here, the optical modulation signal generating device was driven so that the extinction ratio was 10 dB and Δf was 4 GHz. In the apparatus configuration of the present embodiment, the frequency change amount according to the equation (1) is 2 GHz / V. Therefore, a bias voltage of −2 V and a modulation amplitude voltage of 2 V are applied to the frequency modulation region 104 in order to obtain a frequency modulation amount of 4 GHz. The current in the active region 105 is 30 mA. A bias voltage of −1 V and a modulation amplitude voltage of 2 V were applied to the EA type modulator region 107 in order to obtain an extinction ratio ER = 10, that is, 10 dB. By adjusting the modulation bit rate, extinction ratio, and frequency modulation amplitude as described above, a narrow band light modulation spectrum can be obtained. This realizes optical fiber transmission that is highly resistant to chromatic dispersion.

  FIG. 8 shows the transmission waveform of the NRZ signal in the optical modulation signal generating apparatus according to the present invention when the chromatic dispersion of the optical fiber is 16.3 ps / nm / km. FIG. 8A shows the waveform of the initial data of the signal, FIG. 8B shows the waveform of the signal after 100 km transmission, and FIG. 8C shows the waveform of the signal after transmission of 200 km. As shown in FIGS. 8B and 8C, a clear eye opening was observed at 100 km transmission, and the eye opening could be maintained even at 200 km transmission. The same effect can be obtained even if the relationship between the modulation direction of the frequency modulation signal and the modulation direction of the intensity modulation signal is opposite to that of the present embodiment.

  In addition, the transmission characteristics can be further improved by using the frequency chirp accompanying the driving of the EA type modulator 102. FIG. 9 shows frequency chirp characteristics when the EA modulator 102 drives the NRZ signal. FIG. 9A shows the light intensity-time characteristic of the intensity modulation signal of the EA modulator 102 having a positive chirp parameter. FIG. 9B shows the frequency-time characteristic of the intensity modulation signal of the EA modulator 102 having a positive chirp parameter. As shown in FIGS. 9A and 9B, the frequency of the intensity modulation signal increases at the rising edge of the light intensity pulse and decreases at the falling edge. On the other hand, FIG. 9C shows the light intensity-time characteristic of the intensity modulation signal of the EA modulator 102 having a negative chirp parameter. FIG. 9D shows the frequency-time characteristic of the intensity modulation signal of the EA modulator 102 having a negative chirp parameter. As shown in FIGS. 9C and 9D, the frequency of the intensity modulation signal decreases at the rising edge of the light intensity pulse and increases at the falling edge.

  By using this frequency chirp characteristic, the transmission characteristic can be further improved. FIG. 10 shows a modulation method using frequency chirp characteristics. 10A and 10B show frequency modulation signal characteristics without intensity modulation generated by the DBR type frequency modulation light source 101. FIG. 10A shows the light intensity-time characteristic of the frequency modulation signal of the DBR type frequency modulation light source 101, and FIG. 10B shows the frequency-time characteristic of the frequency modulation signal of the DBR type frequency modulation light source 101. . FIGS. 10C and 10D show the intensity modulation signal characteristics of the EA type modulator 102 having a positive chirp parameter. FIG. 10C shows the light intensity-time characteristic of the intensity modulation signal of the EA type modulator 102, and FIG. 10D shows the frequency-time characteristic of the intensity modulation signal of the EA type modulator 102. FIGS. 10 (e) and 10 (f) show the optical modulation signal generation apparatus in which the frequency modulation signals shown in FIGS. 10 (a) and 10 (b) are subjected to the intensity modulation shown in FIGS. 10 (c) and 10 (d). The optical modulation signal characteristic of the optical modulation signal generation device in the case of the above will be shown. FIG. 10E shows the light intensity-time characteristic of the frequency modulation signal of the optical modulation signal generation device, and FIG. 10F shows the frequency-time characteristic of the frequency modulation signal of the optical modulation signal generation device.

  The EA type modulator 102 having the modulation characteristics shown in FIGS. 10C and 10D is used for the frequency modulation signal shown in FIGS. 10A and 10B generated by the DBR type frequency modulation light source 101. Apply intensity modulation. As shown in FIGS. 10E and 10F, in the optical modulation signal generation device, the optical intensity is modulated, and at the same time, an optical modulation signal whose frequency chirp characteristic is changed is generated. Specifically, as shown in FIG. 10 (f), the rising edge of the pulse becomes steep, while a sharp chirp change occurs at the falling edge of the pulse, and a characteristic greatly biased in the negative direction is obtained. . Due to this effect, the phase change in the signal transition region becomes steep, and unnecessary frequency components are removed, thereby improving the chromatic dispersion resistance and further realizing optical fiber transmission that is strong in chromatic dispersion resistance.

  As shown in FIGS. 10 (e) and 10 (f), the optimum condition is obtained by applying the same frequency modulation amplitude as that of the DBR type frequency modulation light source 101 to the EA type modulator 102, and a one-level bit of the optical modulation signal. The frequency modulation amount with respect to is flat, and only the frequency modulation amount for the 0-level bit is greatly shifted to the negative chirp side. In order to obtain such a condition, the chirp parameter may be designed so as to obtain the frequency modulation amount when a desired extinction ratio is obtained.

  FIG. 11 shows a transmission result of an optical modulation signal output from the optical modulation signal generation apparatus of the present invention by a modulation technique using the positive frequency chirp characteristic of the EA type modulator. 11A shows the waveform of the initial data of the signal, FIG. 11B shows the waveform of the signal after 100 km transmission, and FIG. 11C shows the waveform of the signal after transmission of 200 km. The driving conditions are the same as in the above-described configuration, the bit rate is 10 Gb / s, the operating wavelength is 1.5 μm, the frequency modulation amplitude is 4 GHz, and the chirp parameter of the EA modulator 102 is 0.6. It is.

  As shown in FIG. 11C, the eye opening is confirmed over 200 km, and the EA type modulator 102 is more positive as shown in FIG. 10 than in the case of FIG. 8 in which the chirp parameter of the EA type modulator 102 is set to 0. The transmission characteristics can be further improved when the chirp parameter is used. The same effect can be obtained even if the relationship between the modulation direction of the frequency modulation signal and the modulation direction of the intensity modulation signal is opposite to that of the present embodiment. Normally, in the transmission using the EA type modulator 102, the chromatic dispersion tolerance is improved by using a negative chirp parameter. However, in the present invention, the use of the EA type modulator 102 having a positive chirp parameter further increases the chromatic dispersion resistance. The main feature is that the transmission characteristics can be improved. In a general EA type modulator, when the reverse bias applied voltage is small, the chirp parameter becomes positive, and as the applied voltage is increased, the chirp parameter becomes negative. Therefore, the driving voltage can be suppressed as compared with a method of operating a normal EA type modulator in the negative chirp region. Further, as described in the quantum confined Stark effect section, in the EA type modulator, since the propagation loss is small when the driving voltage is low, the low bias driving is advantageous also in the laser output intensity.

  In the present embodiment, the effect of dramatically increasing the transmission distance when the frequency chirp associated with the driving of the EA type modulator 102 is positive is shown. However, when the frequency chirp of the EA type modulator 102 is negative, However, the phase change in the signal transition region becomes steep, and the effect of removing unnecessary frequency components is the same. When the frequency chirp of the EA type modulator 102 is negative, the degree of effect is smaller than that of the positive chirp, but optical fiber transmission strong against chromatic dispersion resistance is realized, and the frequency chirp is zero. Thus, the chromatic dispersion tolerance is improved, and the transmission characteristics can be improved.

  Further, the frequency chirp generated by the EA type modulator 102 can be compensated by driving the DBR type frequency modulation light source 101. FIG. 12 shows an operation example of driving the frequency modulation light source 101 for compensating the frequency chirp generated by the EA type modulator 102. 12A and 12B are diagrams showing the frequency modulation signal characteristics of the DBR type frequency modulation light source 101. FIG. 12A shows the light intensity-time characteristic of the frequency modulation signal of the DBR type frequency modulation light source 101, and FIG. 12B shows the frequency-time characteristic of the frequency modulation signal of the DBR type frequency modulation light source 101. . FIGS. 12C and 12D show intensity modulation signal characteristics of the EA modulator 102 having a positive chirp parameter. FIG. 12C shows the light intensity-time characteristic of the intensity modulation signal of the EA type modulator 102, and FIG. 12D shows the frequency-time characteristic of the intensity modulation signal of the EA type modulator 102. 12 (e) and 12 (f) show the optical modulation signal generation apparatus in which the intensity modulation shown in FIGS. 10 (c) and 10 (d) is applied to the frequency modulation signal shown in FIGS. 10 (a) and 10 (b). The optical modulation signal characteristic of the optical modulation signal generation device in the case of the above will be shown. FIG. 12E shows the light intensity-time characteristic of the frequency modulation signal of the optical modulation signal generation device, and FIG. 12F shows the frequency-time characteristic of the frequency modulation signal of the optical modulation signal generation device.

  As shown in FIGS. 12B and 12D, the frequency modulation component of the frequency modulation signal and the frequency modulation component generated by intensity modulation are in opposite phases to each other. Further, the frequency modulation amplitude of the DBR type frequency modulation light source 101 is adjusted so that the magnitude of the amplitude of the frequency modulation component of the frequency modulation signal is equal to the magnitude of the frequency modulation amplitude generated by intensity modulation. By applying a pulse-like frequency modulation signal to the frequency modulation region in the signal transition region, the frequency chirp on the EA modulator 102 side can be canceled. At this time, as shown in FIGS. 12E and 12F, it is possible to generate an NRZ signal having only the intensity modulation component without the frequency modulation component.

  FIG. 13 shows the transmission characteristics of the optical modulation signal output from the optical modulation signal generator by the driving operation shown in FIG. 13A shows the waveform of the initial data of the signal, FIG. 13B shows the waveform of the signal after 50 km transmission, and FIG. 13C shows the waveform of the signal after 100 km transmission. By canceling the frequency chirp on the EA type modulator 102 side, as shown in FIG. 13, it was possible to obtain a clearer eye opening over 100 km as compared with FIG. 11 (c).

  FIG. 14 is a diagram for explaining a method of generating a driving signal for the DBR type frequency modulation light source 101. FIG. 14A shows an electric waveform of the original signal of the DBR type frequency modulation light source 101 and an electric waveform of a delayed signal obtained by delaying the original signal. FIG. 14B shows a waveform when the difference between the electrical waveform of the original signal and the electrical waveform of the delayed signal is taken. In FIG. 14A, the electric waveform of the original signal is indicated by a solid line, and the delayed signal is indicated by a dotted line. For example, as shown in FIG. 14A, a bit difference between a signal sequence and a signal sequence provided with a delay shorter than the bit interval is obtained. The generated pulse is shown in FIG. 14B, which is a modulation pulse corresponding to the signal transition region, which is equivalent to FIGS. 10D and 12D. In the case of an NRZ signal, the original signal is delayed by 1 bit to obtain an amplitude difference. In the case of the RZ signal, a difference in amplitude may be obtained by giving a delay time shorter than 1 bit. Alternatively, a method using a transient response of a transistor can be applied.

  If a similar method is used, the frequency chirp generated by the EA modulator 102 can be inverted. FIG. 15 shows an operation example of the DBR type frequency modulation light source 101 and the EA type modulator 102 when the frequency chirp generated by the EA type modulator 102 is inverted. FIGS. 15A and 15B are diagrams showing frequency modulation signal characteristics of the DBR type frequency modulation light source 101. FIG. 15A shows the light intensity-time characteristic of the frequency modulation signal of the DBR type frequency modulation light source 101, and FIG. 15B shows the frequency-time characteristic of the frequency modulation signal of the DBR type frequency modulation light source 101. FIG. FIGS. 15C and 15D show the intensity modulation signal characteristics of the EA type modulator 102 having a positive chirp parameter. 15C shows the light intensity-time characteristic of the intensity modulation signal of the EA type modulator 102, and FIG. 15D shows the frequency-time characteristic of the intensity modulation signal of the EA type modulator 102. FIG. FIGS. 15 (e) and 15 (f) show the optical modulation signal generation apparatus in which the frequency modulation signals shown in FIGS. 15 (a) and 15 (b) are subjected to the intensity modulation shown in FIGS. The optical modulation signal characteristic of the optical modulation signal generation device in the case of the above will be shown. FIG. 15E shows the light intensity-time characteristic of the frequency modulation signal of the optical modulation signal generation apparatus, and FIG. 15F shows the frequency-time characteristic of the frequency modulation signal of the optical modulation signal generation apparatus.

  As shown in FIGS. 15B and 15D, the frequency modulation amplitude of the frequency modulation signal of the frequency modulation light source 101 is larger than the frequency modulation amplitude of the intensity modulation signal of the EA type modulator 102 whose chirp parameter is positive. In this case, by applying a pulse-like frequency modulation signal to the frequency modulation region in the signal transition region, the frequency chirp on the EA modulator 102 side as shown in FIG. it can. At this time, an NRZ signal equivalent to the modulation signal of the EA modulator having a negative chirp parameter as shown in FIGS. 9A and 9B can be generated.

  FIG. 16 shows the transmission characteristics of the optical modulation signal generating apparatus when the frequency chirp generated by the EA modulator 102 is inverted. FIG. 16A shows the waveform of the initial data of the signal, FIG. 16B shows the waveform of the signal after 50 km transmission, and FIG. 16C shows the waveform of the signal after 100 km transmission. By reversing the frequency chirp generated by the EA type modulator 102, as shown in FIG. 16, it was possible to obtain a clearer eye opening over 100 km as compared with FIG.

  As described above, the frequency chirp generated in the intensity modulation signal is compensated by the frequency modulation signal or converted into a negative chirp, so that even when the EA modulator 102 has a positive chirp, light with high chromatic dispersion tolerance is obtained. Fiber transmission can be realized.

[Device configuration]
2 shows a configuration of an optical modulation signal generating apparatus according to Embodiment 2 of the present invention. In the EA type modulator 102, it is necessary to set an optimum driving bias. However, since the band gap wavelength and the driving wavelength are close to each other, a propagation loss occurs when a bias voltage is applied as shown in FIG. Inevitable. FIG. 17 shows a configuration in which a semiconductor optical amplifier (SOA) is integrated for the loss compensation.

  The optical modulation signal generating apparatus according to the second embodiment of the present invention has a configuration in which a DBR type frequency modulation light source 101, an SOA 126, and an EA type modulator 102 are monolithically integrated. An SOA 126 is disposed between the DBR type frequency modulation light source 101 and the EA type modulator 102. The DBR type frequency modulation light source 101 and the EA type modulator 102 have the same configuration as in the first embodiment.

  The optical modulation signal generating apparatus according to Example 2 of the present invention includes a first SSG-DBR reflector region 103, a frequency modulation region 104, an active region 105, a second SSG-DBR reflector region 106, The SOA area 127 and the EA type modulator area 107 are configured in this order.

  The SOA region 127 has a length of 400 μm and is connected with a direct current source 129 for optical amplification. The SOA region 127 includes an n-type electrode 116, an n-type InP cladding layer 108 formed on the n-type electrode 116, an 8-layer InGaAsP quantum well layer 128 for SOA formed on the n-type InP cladding layer 108, A p-type InP cladding layer 114 formed on the 8-layer InGaAsP quantum well layer 128 for SOA, a contact layer 115 formed on the p-type InP cladding layer 114, and a p-type electrode 117 formed on the contact layer 115. It consists of. The 8-layer InGaAsP quantum well layer 128 for SOA is formed with the same structure as the 8-layer InGaAsP quantum well layer 110 for active layer, but it may have a different configuration.

The n-type electrode 116 is grounded, and the n-type InP cladding layer 108 has a doping concentration of 10 ¹⁸ cm ⁻³ . The 8-layer InGaAsP quantum well layer 128 for SOA has a band gap wavelength of 1.55 μm and a thickness of 200 nm. The p-type InP cladding layer 114 has a doping concentration of 10 ¹⁸ cm ⁻³ and a thickness of 1.5 μm. The contact layer 115 has a band gap wavelength of 1.5 μm, a doping concentration of 10 ¹⁹ cm ⁻³ , and a thickness of 300 nm. Contact layers 115 between the regions are formed at intervals of 100 μm. Further, a non-reflective coating is applied to the faces of the DBR type frequency modulation light source 101 and the EA type modulator 102 that face each other in the light guiding direction. As shown in FIG. 17, the n-type electrode 116, the n-type InP cladding layer 108, and the p-type InP cladding layer 114 are configured to guide light over the regions of the DBR type frequency modulation light source 101, the EA type modulator 102, and the SOA 126. It is uniformly formed in the wave direction. The InGaAsP layer 109, the eight-layer InGaAsP quantum well layer 110 for the active layer, the eight-layer InGaAsP quantum well layer 111 for the intensity modulation layer, and the eight-layer InGaAsP quantum well layer 128 for SOA are the core layers of the waveguide (the core of FIG. 5). Corresponding to the layer 123).

  A cross-sectional view of the DBR type frequency modulation light source 101 or the EA type modulator 102 in the direction perpendicular to the light guiding direction is the same as that shown in FIG.

[Generation method of optical modulation signal and optical fiber transmission method]
In this apparatus, in addition to the modulation operation combining the frequency modulation and the intensity modulation as in the first embodiment, the optical amplification operation by the stimulated emission of the eight-layer InGaAsP quantum well layer 128 for SOA by the current injection into the SOA region 128. Can be realized. As a result, high-power operation of the laser can be realized. The feature of the second embodiment is that the SOA 123 is arranged between the DBR type frequency modulation light source 101 and the EA type modulator 102.

  As a conventional example, for example, a case where SOA is integrated in the subsequent stage of an electroabsorption modulator integrated DFB-LD is considered. At this time, the intensity-modulated signal is amplified in the SOA, but a response deterioration due to a change in the number of carriers accompanying a change in the light intensity, that is, a so-called pattern effect occurs in the SOA. On the other hand, as described in the first embodiment, a frequency modulation signal without an intensity modulation component is generated from the DBR type frequency modulation light source 101 of this configuration, so that no pattern effect occurs in the optical amplification in the SOA 123. Therefore, it is possible to increase the output without performing signal degradation of frequency modulation and intensity modulation. By using the configuration according to the second embodiment and performing optical fiber transmission under the driving conditions shown in the first embodiment, further long-distance transmission can be achieved. In the apparatus configuration according to the second embodiment, the optical modulation signal generation apparatus includes the SOA 123 between the DBR type frequency modulation light source 101 and the EA type modulator 102, so that high-speed and high-output modulation operation strong against pattern effects can be performed. It can be realized.

[Other configurations]
In the above embodiments, the structures of the DBR type frequency modulation light source 101 and the EA type modulator 102 in the InGaAsP material system have been shown. However, this configuration is not limited to the material, and other compound semiconductors such as an InAlGaAs system can be used. Obviously it can be applied to devices. This configuration is realized by combining a frequency modulation light source and an intensity modulator. Therefore, the frequency modulation light source is not only the SSG-DBR laser exemplified in this embodiment, but also a normal DBR laser, SG-DBR laser, double resonator ring laser, DFB (Distributed Feedback) laser, etc. The present invention can be applied to the configuration, and as the intensity modulator, a Mach-Zehnder type modulator can be applied.

  In particular, when direct modulation of the DFB laser is applied as a frequency modulation light source, the cavity length of the DFB laser is made shorter than a general one, and SOA is added between the DFB laser and the modulator. It is good to do. This is because when the present invention is applied to a DFB laser, the resonator length of the DFB laser needs to be shorter than a general length (typically 450 μm or more), thereby reducing the output light intensity of the laser. This is because it is necessary to make up for what is done with SOA.

  It is known that when a DFB laser is used as a frequency modulation light source, intensity modulation occurs simultaneously with frequency modulation. This is because the modulation intensity of frequency modulation is determined by Δn / n (where n is the refractive index of the active layer of the modulator, Δn is the change in refractive index due to the modulation current), and Δn is proportional to the square root of the current density of the modulation current. This is because the current value required for modulation increases as the laser resonator lengthens, but this current causes unintentional intensity modulation to be added to the output light intensity. Even when the laser resonator has a general length, the intensity modulation accompanying the frequency modulation appears in a non-negligible size, the chirp becomes large, and the transmission distance does not become as theoretically long. In addition, even when an SOA is provided to supplement the output of the DFB laser, as described above, the SOA generally has a problem that when the light with intensity modulation is input, a pattern effect appears and the output light intensity becomes unstable. For this reason, we want to avoid unintentional intensity modulation. In order to avoid such unintended intensity modulation when frequency modulation is performed with the DFB laser, it is preferable to shorten the resonator length to 300 μm or less and to reduce the amount of current necessary for frequency modulation. The shorter it is, the more effective frequency modulation can be obtained where the intensity modulation is not sufficiently affected by the chirp, and since the intensity modulation is small, there is no effect even if SOA is integrated, and the resonator length is shortened. It is possible to compensate for the decrease in the output light intensity caused by the SOA. However, if the resonator length is made too short, the output light intensity becomes too small, making it easier to ride noise during amplification with the SOA and making the laser oscillation itself unstable. In order not to cause such a problem, the resonator length is desirably 30 μm or more. For the above reasons, it is desirable that the DFB laser has a resonator length shorter than that of a general one and that SOA is added between the DFB laser and the modulator.

  Further, the relationship between the frequency modulation and the intensity modulation under the driving conditions of the DBR type frequency modulation light source 101 and the EA type modulator 102 is not strictly limited by the equation (3). If frequency modulation can be applied to the intensity-modulated signal, chromatic dispersion tolerance can be improved, although the degree of effect is different. Also, the driving conditions may be set for different bit rates using the formula (3) as a guide. For example, in the case of 40 Gb / s operation, the same effect can be obtained by setting the frequency modulation amount at an extinction ratio of 10 dB to 16 GHz.

101 SSG-DBR type frequency modulation light source 102 EA type modulator 103 first SSG-DBR type reflector region 104 frequency modulation region 105 active region 106 second SSG-DBR type reflector region 107 EA type modulator region 108 n Type InP cladding layer 109 InGaAsP layer 110 InGaAsP quantum well layer 111 InGaAsP quantum well layer 112 first diffraction grating 113 second diffraction grating 114 p-type InP cladding layer 115 contact layer 116 n-type electrode 117 p-type electrode 118 DC current source 119 Modulation voltage source 120 DC current source 121 DC current source 122 Modulation voltage source 123 Core layer 124 SiO ₂ insulating film 125 BCB
126 SOA
127 SOA region 128 InGaAsP quantum well layer 129 DC current source

Claims (12)

A frequency modulation light source that generates and outputs a frequency modulation signal, and an intensity modulator that generates an intensity modulation signal, inputs the frequency modulation signal, and adds intensity modulation to the frequency modulation signal by the intensity modulation signal An optical modulation signal generation method for generating an optical modulation signal using an optical modulation signal generation device to be used,
The optical modulation signal generation device drives the frequency modulation light source and the intensity modulator using the same bit pattern.   The method of claim 1, wherein the intensity modulator has a positive chirp parameter.   The method of claim 1, wherein the intensity modulator has a negative chirp parameter.   4. The optical modulation according to claim 1, wherein the optical modulation signal is modulated so that a frequency modulation component of the frequency modulation signal and an intensity modulation component of the intensity modulation are synchronized. Signal generation method.   5. The optical modulation signal generation method according to claim 1, wherein the intensity modulator is an electroabsorption modulator.   6. The optical modulation signal generation method according to claim 1, wherein the frequency modulation light source is a distributed Bragg reflection laser.   7. The optical modulation signal generation method according to claim 6, wherein the frequency modulation light source generates a frequency modulation signal by bias-modulating a phase adjustment region of the distributed Bragg reflection laser.   6. The optical modulation signal generation method according to claim 1, wherein the frequency modulation light source is a distributed feedback laser.   9. The optical modulation signal generation method according to claim 1, wherein the optical modulation signal generation device includes a semiconductor optical amplifier between the frequency modulation light source and the intensity modulator. When the optical modulation signal generation device has a frequency modulation amplitude Δf, a bit rate B, and an extinction ratio ER of the optical modulation signal,

The optical modulation signal generation method according to claim 4, wherein the optical modulation signal is generated so as to satisfy the relationship.   4. The optical modulation signal generation method according to claim 2, wherein the optical modulation signal generation device cancels the frequency chirp generated by the intensity modulator by the frequency modulation of the frequency modulation light source. 5.   3. The light according to claim 2, wherein the optical modulation signal generation device converts the positive frequency chirp generated by the intensity modulator into a negative frequency chirp by inverting the frequency modulation with the frequency modulation light source. Modulation signal generation method.

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