JP2013197815A - Modulation light source and method of generating modulation signal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a modulation light source that allows downsizing and a method of generating a modulation signal.SOLUTION: A modulation signal generating device 200 includes: a voltage source 221 generating a driving modulation voltage Vm that is set according to phase modulation of a signal string; and a control circuit 222 generating a modulation voltage Vp according to the driving modulation voltage Vm and applying the modulation voltage Vp to a frequency modulation region 104 of a light source part.

Description

  The present invention relates to a modulated light source for performing optical fiber transmission and a method for generating a modulated signal.

  Due to the increase in network traffic accompanying the spread of the Internet, it is desired to increase the speed and capacity in optical fiber transmission. To increase the capacity of optical fibers, increasing the multiplicity and increasing the modulation speed have played an important role so far. With regard to multiplexing of optical fibers, it is possible to transmit optical signals exceeding 1000 waves per optical fiber by a wavelength division multiplexing (WDM) technique that is a method of multiplexing optical fibers. It has become like this.

  In terms of increasing the modulation speed, 40 Gb / s-based transmission equipment has started to be introduced in the current backbone, and 10 Gb / s base has been used in access metro networks. However, although research results of modulation experiments using 100 Gb / s have been reported for further increases in modulation speed, a modulation speed exceeding 40 Gb / s has not been put to practical use. This is based on the fact that there are many technical issues for speeding up and that a technology for reducing costs has not been established. In addition, one of the reasons why it is difficult to put a modulation rate exceeding 40 Gb / s into practical use is that the binary ASK (Amplitude Shift Keying) method, which is the mainstream modulation method so far, has a wavelength of optical fiber due to the spread of spectrum. Due to the influence of dispersion, it is conceivable that the signal characteristics transmitted by high-speed modulation are significantly deteriorated.

  When applying modulation speeds exceeding 40 Gb / s to long-distance optical fiber communication systems, the problem of polarization mode dispersion (PMD) has become apparent. There is a growing expectation for the introduction of modulation schemes that can achieve dispersion tolerance.

  Under such circumstances, multi-level modulation technology with high frequency utilization efficiency is applied to optical fiber transmission. The multi-level modulation method is a method in which one modulation symbol is expressed by a plurality of bits by placing information on both the intensity and phase of an optical signal. Since the multi-level modulation scheme makes it possible to increase the transmission capacity without increasing the baud rate, the frequency utilization efficiency is improved over the binary ASK scheme described above. Examples of the phase modulation method include a PSK (Phase Shift Keying) method, a QPSK (Quadrature Phase Shift Keying) method, a DPSK (Differential Phase Shift Keying) method, and a DQPSK (Differential Quadrature Phase Shift Keying) method. For example, Non-Patent Document 1 discloses a technology that includes a light source unit and a Mach-Zehnder type modulator, and controls the intensity and phase of an optical signal using the Mach-Zehnder type modulator.

  FIG. 1 is a diagram for explaining a constellation map (complex electric field distribution) in various phase modulation schemes, where (a) is a PSK constellation map, (b) is a DPSK constellation map, c) shows a constellation map of QPSK and (d) a constellation map of DQPSK. In FIGS. 1A to 1D, Re (E) on the horizontal axis represents the real part, and Im (E) on the vertical axis represents the imaginary part.

  For example, FIG. 1A shows an example in which two codes 0 and 1 have a phase difference of π. Further, in FIG. 1B, the code 0 corresponds to the phase change = 0, and the code 1 corresponds to the phase change = π.

  FIG. 1C shows an example in which four patterns of 2-bit strings shifted by π / 2, that is, 00, 10, 11, 01 are given. In FIG. 1D, the code sequence 00 has a phase change = 0, the code sequence 10 has a phase change = π / 2, the code sequence 11 has a phase change = π, and the code sequence 01 has a phase change = −π / 2. Each is given correspondingly.

  Next, a configuration of a modulation light source including a Mach-Zehnder type modulator in a conventional modulation system will be described with reference to FIGS.

  FIG. 2 shows a configuration example of a conventional modulated light source. FIG. 2A shows a configuration when a PSK or DPSK system is applied, and FIG. 2B shows a configuration of (B) RZ (Return to Zero) -PSK or RZ (Return to Zero)-. 2 shows a configuration in the case of applying the DPSK method.

  A modulated light source 300 shown in FIG. 2A includes a light source unit 301 that outputs continuous (CW) light and a Mach-Zehnder type modulator 302. The modulator 302 has two arms for propagating continuous light from the light source unit 301 to two optical paths. In FIG. 2A, one arm has a phase adjustment region 303 for shifting light by π / 2. The two arms are formed with modulation regions 304 and 305 for applying a voltage in order to change the refractive index, respectively. The modulator 302 can provide binary phase modulation representing 0 and π by phase-modulating the two arms.

  A modulated light source 300A illustrated in FIG. 2B includes a light source unit 301 and a Mach-Zehnder type modulator 302A. The modulator 302A includes the three regions 303, 304, and 305 shown in FIG. 2A, and further performs pulse shaping, that is, converts an NRZ (Non Return to Zero) signal into RZ (Return to Zero). A set of arms having two modulation regions 310 and 311 is provided.

  FIG. 3 shows a configuration example of a conventional modulated light source, where (a) shows a configuration in the case of applying the QPSK or DQPSK system, and (b) RZ (Return to Zero) -QPSK or RZ (Return to Zero)- 2 shows a configuration in the case of applying the DQPSK method.

  3A includes a light source unit 301 and a Mach-Zehnder type modulator 302B. The modulator 302B includes two Mach-Zehnder type modulators. This is because QPSK or DQPSK is represented by a 2-bit string of four patterns. On the four arms of the modulator 302B, modulation regions 304, 305, 320, and 321 to which a voltage is applied in order to change the refractive index are formed. In this case, QPSK can give four-level phase modulation representing 0, π / 2, −π / 2, and π, and DQPSK can give a phase change of 0, ± π / 2, π.

  A modulated light source 300C illustrated in FIG. 3B includes a light source unit 301 and a Mach-Zehnder modulator 302C. The modulator 302C includes the modulator 302B shown in FIG. 2A, but further includes two modulation regions 310 for pulse shaping, that is, converting an NRZ (Non Return to Zero) signal into RZ (Return to Zero). , 311 with a set of arms.

P. J. Winzer, “Advanced Optical Modulation Format,” IEEE, vol. 94, no. 5, pp. 952-985.

  As described above, the conventional multi-level modulated light source has a configuration in which the CW light source and the Mach-Zehnder type modulator are combined. However, since the Mach-Zehnder type modulator is large, propagation loss and power consumption are likely to increase. For the future large-scale integration, the realization of a phase-modulated light source in which a light source and a modulator are integrated in a small size is an issue.

  For example, in the case of an absorption type (EA) modulator integrated distributed reflection type laser that is a modulated light source for ASK, the combined chip size of the light source and the modulator is about 1 mm. On the other hand, in the case of a Mach-Zehnder modulator using a compound semiconductor material, an element length of about 1 mm is required only in the modulation region. For example, when integrating a CW light source and an RZ modulator, modulation for ASK is required. Compared to the light source, it is inevitable that the size is twice or more. An increase in element size causes an increase in power dissipation such as an increase in propagation loss and an increase in temperature control power, and is not suitable for large-scale integration.

  Therefore, an object of the present invention is to provide a modulated light source that can be miniaturized and a method for generating a modulated signal.

  A modulation light source for achieving the above object includes a voltage source that generates a drive voltage that is set according to phase modulation of a signal sequence, and a control circuit that generates a modulation voltage according to the drive voltage. A signal generation unit and a light source unit that generates a frequency modulation signal corresponding to the modulation voltage are provided.

  Here, when the amount of change in frequency of the frequency modulation signal is Δf, the phase difference between the signals is ΔΦ, and the time is t, the light source unit is ΔΦ (t) when the phase modulation is PSK or DPSK. The frequency modulation signal may be generated so as to satisfy = −2π∫Δf (t) dt and ΔΦ (t) = (2m + 1) × π (m: integer).

  Alternatively, if the amount of change in the frequency of the frequency modulation signal is Δf, the phase difference between the signals is ΔΦ, and the time is t, the light source unit has ΔΦ (t) = when the phase modulation is QPSK or DQPSK. The frequency modulation signal may be generated so as to satisfy −2π∫Δf (t) dt, ΔΦ (t) = (m / 2) × π (m: integer).

  A modulation signal generation method for achieving the above object includes a step of generating a drive voltage set according to phase modulation of a signal sequence, a step of generating a modulation voltage according to the drive voltage, and the modulation Generating a frequency modulation signal corresponding to the voltage.

  Here, when the amount of change in frequency of the frequency modulation signal is Δf, the phase difference between the signals is ΔΦ, and time is t, the step of generating the frequency modulation signal is performed when the phase modulation is PSK or DPSK. ΔΦ (t) = − 2π∫Δf (t) dt, ΔΦ (t) = (2m + 1) × π (m: integer).

  Alternatively, when the amount of change in frequency of the frequency modulation signal is Δf, the phase difference between the signals is ΔΦ, and time is t, the step of generating the frequency modulation signal is performed when the phase modulation is QPSK or DQPSK. ΔΦ (t) = − 2π∫Δf (t) dt, ΔΦ (t) = (m / 2) × π (m: integer) may be satisfied.

  According to the present invention, the modulation light source can be reduced in size. Alternatively, a method for generating a modulation signal can be provided.

It is a figure for demonstrating the constellation map in various phase modulation systems. It is a figure which shows the structural example of the conventional modulation light source. It is a figure which shows the structural example of the conventional modulation light source. It is a figure for demonstrating an example of the cross-sectional structure of the optical semiconductor device in embodiment of this invention. It is a figure for demonstrating an example of the cross-section of a light source part and a modulator along the waveguide direction of light. It is a figure for demonstrating the structural example of a modulation signal generation apparatus. It is a figure for demonstrating the operation principle of the modulator of an optical semiconductor device. It is a figure for demonstrating the frequency change and phase characteristic in the case of PSK modulation. It is a figure for demonstrating the operation principle of the modulation signal generation apparatus in the case of PSK modulation. It is a figure for demonstrating the other principle of operation in the case of PSK modulation. It is a figure for demonstrating the operation principle of the modulation signal generation apparatus in the case of DPSK modulation. It is a figure for demonstrating the principle of operation of the modulation signal generation apparatus in the case of QPSK modulation. In the case of QPSK modulation, it is a figure for demonstrating an example of the drive modulation voltage and modulation voltage which are produced | generated by the modulation signal generation apparatus. It is a figure for demonstrating the principle of operation of a modulation light source in the case of DQPSK modulation.

  Hereinafter, the optical semiconductor device according to the present embodiment includes a light source unit, a modulation signal generation device, and a modulator, and performs phase modulation of a signal sequence using the modulation signal generated by the modulation signal generation device. It is a device for generating a frequency modulation signal according to the response.

  The optical semiconductor device in this embodiment will be described with reference to FIGS.

[Configuration of optical semiconductor device]
FIG. 4 is a diagram for explaining an example of a cross-sectional structure of the optical semiconductor device 10 in the present embodiment.

  As shown in FIG. 4, the optical semiconductor device 10 includes a light source unit 101 and a modulator 102. In this embodiment, as an example, the light source unit 101 is a distributed Bragg reflector (DBR) type frequency modulation light source, and the modulator 102 is an electroabsorption modulator. The light source unit 101 and the modulator 102 shown in FIG. 4 are monolithically integrated.

  The optical semiconductor device 10 also includes a modulation signal generation device (modulation signal generation unit) 200 shown in FIG. 6 to be described later. In FIG. 4, only the light source unit 101 and the modulator 102 are displayed. In the present embodiment, the light source unit 101 and the modulation signal generation device 200 constitute a modulation light source.

  In FIG. 4, the light source unit 101 includes a first DBR reflector 103, a frequency modulation region (phase adjustment region) 104, an active layer region 105, and a second DBR reflector 106.

  An n-type InP cladding layer 108 is formed in the light source unit 101, and the DBR reflectors 103 and 106 and the InGaAsP layer 109 in the frequency modulation region 104 and the active layer region 105 are formed on the cladding layer 108. An eight-layer InGaAsP quantum well layer 110 is formed. A diffraction grating 112 is formed in the waveguide layer of each DBR reflector 103, 106.

  A contact layer 114 and a p-type electrode 115 are stacked on the p-type InP cladding layer 113 formed on the core layer (InGaAsP layer 109 and 8-layer InGaAsP quantum well layer 110) of the light source unit 101.

  The n-type electrode 116 is formed under the cladding layer 108.

The frequency modulation region 104 is connected to the modulation voltage source 118 and is supplied with the voltage Vp. The active layer region 105 is connected to a direct current source 119 and is supplied with a current _ID from the direct current source 119. Direct current sources 117 and 120 are connected to the DBR reflectors 103 and 106, respectively, and wavelength control currents I _R and I _F are given thereto.

  In this embodiment, as an example, the length of the active layer region 105 is 300 μm, and the length of the frequency modulation region 104 is 200 μm. Further, the length of the first DBR reflector 103 is set to 300 μm, for example, and the length of the second DBR reflector 106 is set to 600 μm, for example.

The doping concentration of the clad layer 108 is, for example, 10 ¹⁸ cm ⁻³ . In the InGaAsP layer 109, the band gap wavelength is 1.3 μm, for example, and the thickness is 300 nm, for example.

In the quantum well layer 110, the band gap wavelength is, for example, 1.55 μm, and the thickness is, for example, 200 nm. The diffraction grating 112 is formed so that the Bragg wavelength is, for example, 1.55 μm. The coupling constant of the diffraction grating 112 is, for example, 30 cm ⁻¹ .

In the p-type InP cladding layer 113, the doping concentration is, for example, 10 ¹⁸ cm ⁻³ and the thickness is, for example, 1.5 μm. In the contact layer 114, the band gap wavelength is, for example, 1.5 μm, the doping concentration is, for example, 10 ¹⁹ cm ⁻³ , and the thickness is, for example, 300 nm. In this embodiment, the regions 103, 104, 105, 106, and 107 are formed, for example, 10 μm apart from each other, so that the contact layer 114 is not formed therebetween.

  The modulator 102 includes a modulator region 107 connected to a voltage source 121 that provides an intensity modulation voltage VI. In the modulator region 107, an eight-layer InGaAsP quantum well layer 111, a p-type InP cladding layer 113, a contact layer 114, and a p-type electrode 115 are stacked on the n-type InP cladding layer 108 described above. An n-type electrode 116 is formed under the cladding layer 108. In this embodiment, the n-type electrode, the n-type InP clad layer 108, the p-type InP clad layer 113, the contact layer 114, and the p-type electrode 115 extend in the light guiding direction over the region of the light source unit 101 and the modulator 102. Uniformly formed.

  In this embodiment, both end surfaces of the optical semiconductor device 10 are provided with, for example, a non-reflective coating.

  FIG. 5 is a diagram for explaining an example of a cross-sectional structure of the light source unit 101 and the modulator 102 along the light guiding direction.

  FIG. 5 shows an example in which the light source unit 101 and the modulator 102 have a ridge structure with a waveguide width of 1.5 μm. An n-type InP clad layer 108 is formed on the electrode 116, and a core layer 122 is formed on the InP clad layer 108. The core layer 122 corresponds to the InGaAsP layer 109 and the eight-layer InGaAsP quantum well layer 110 shown in FIG.

The SiO ₂ insulating film 123 is formed on both sides of the ridge, and benzocyclobutene (BCB) 124 is formed on the SiO ₂ insulating film 123. A p-type electrode 115 is formed on the planarized contact layer 114, SiO ₂ insulating film 123, and BCB 124.

  FIG. 6 is a diagram for explaining a configuration example of the modulation signal generating apparatus 200 for applying the modulation voltage Vp118 in FIG.

  In FIG. 6, the modulation signal generating apparatus 200 includes a voltage source 221 that generates a drive modulation voltage (drive voltage) Vm, and a control circuit 222 that generates a modulation voltage (modulation signal) Vp corresponding to the drive modulation voltage Vm. . The control circuit 222 is a differentiation circuit composed of two resistors 223 and 224 and a capacitor 225. One end of the frequency modulation region 104 of the light source unit 101 is connected to one end of the capacitor 225, and the other end is grounded.

  In the example of FIG. 6, the capacitance of the capacitor 225 is C, the resistance value of the resistor 223 is R, and the resistance value of the resistor 224 is 50Ω, for example. In this case, since the control circuit 222 is a differentiation circuit, the drive modulation voltage Vm indicated by the signal waveform d1 in FIG. 6 is a modulation voltage Vp having a differential component of the drive modulation voltage Vm as indicated by the signal waveform d2. Become. This modulation voltage Vp is applied to the frequency modulation region 104.

  In this embodiment, the drive modulation voltage Vm generated by the voltage source 221 has a value corresponding to the phase modulation of the signal train. That is, the drive modulation voltage Vm is adjusted so that, for example, a phase change between signals corresponding to PSK, DPSK, QPSK, or DQPSK can be expressed. This will be described in detail later.

[Operation Principle of Light Source Unit 101]
Next, the operation principle of the light source unit 101 will be described with reference to FIG. Spontaneously emitted light is generated in the active layer region 105 by injecting a current into the active layer 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 reflector region 103 and the second reflector region 106, and stimulated emission is generated by the InGaAsP quantum well layer 110. The light source unit 101 can selectively reflect only a single wavelength. When the optical gain in stimulated emission exceeds the loss due to light absorption of the light source unit 101, laser oscillation occurs. The oscillation frequency f _{0 at} this time is given as follows.
f ₀ = 2 cLm / neq (9)

Here, L is the resonator length of the light source unit 101, neq is the effective refractive index, c is the speed of light in vacuum, and m is an integer indicating the oscillation longitudinal mode. L represents the sum (L _DBR + Lp + La) of the effective length L _DBR of the two DBR regions 103 and 106, the length Lp of the frequency modulation region 104, and the length La of the active layer region 105. Note that neq has a uniform effective refractive index.

  In general, it is known that the region corresponding to the frequency modulation region 104 is phase-adjusted by current injection, but the frequency modulation region 104 of this embodiment performs frequency modulation by voltage application. This is because an ideal frequency modulation signal without intensity modulation is generated as will be described later.

From the equation (9), when the effective refractive index of the frequency modulation region 104 changes by Δneq, the oscillation frequency changes as follows.
Δf to f ₀ × (Δneq × Lp) / (neq × L) (10)

  Specifically, when the voltage source 118 applies the voltage Vp to the frequency modulation region 104, the oscillation frequency of the light source unit 101 changes as the effective refractive index of the frequency modulation region 104 changes. Therefore, frequency modulation of the light source unit 101 can be performed by superimposing the modulation signal on the applied voltage. In this case, the applied voltage described above may be applied in the reverse bias direction to obtain an electro-optic effect, that is, an effect in which the refractive index changes as the voltage is applied, or the applied voltage is applied in the forward direction. Thus, the carrier effect by the current may be obtained. From the viewpoint of modulation efficiency, current driving by forward bias is effective, but from the viewpoint of high-speed modulation, current driving by reverse bias is more effective.

  In general, the change in the refractive index of a compound semiconductor due to voltage application is based on the relationship between the change in light absorption and the Kramers-Kronig. That is, the change in refractive index is accompanied by a change in light absorption. This change in light absorption changes the threshold current value at which the light source unit 101 outputs laser light, which causes unnecessary intensity modulation. When intensity modulation is performed, relaxation oscillation of the laser light occurs, so that the influence on the response of the light source unit 101 increases.

  However, for example, in refractive index modulation by reverse bias, by making the band gap wavelength of the frequency modulation region 104 shorter than the use region by 100 nm or more, propagation loss due to light absorption due to voltage application is reduced. The effect of propagation loss can be ignored.

  From this point of view, the light source unit 101 of the present embodiment generates an ideal frequency modulation signal without intensity modulation 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. I have to. Note that the modulator 102 performs intensity modulation of the frequency modulation signal.

  In the light source unit 101, it is also possible to generate a frequency modulation signal by performing current modulation of the active layer region 105. For example, when a modulation signal is superimposed on the active layer region 105, the frequency modulation signal is generated so as to have an intensity modulation component and an adiabatic chirp component. However, in the case of direct modulation operation, the modulation band due to relaxation oscillation of the laser beam is limited during high-speed operation in current modulation of the active layer region 105, and unnecessary frequency chirp is generated during current modulation. Therefore, from the viewpoint of the high-speed operation, the case where the frequency modulation region 104 is bias-modulated is more effective than the case where the active layer region 105 is current-modulated.

[Operation Principle of Modulation Unit 102]
Next, the operation principle of the modulation unit 102 will be described with reference to FIGS. FIG. 7 is a diagram for explaining the operation principle of the modulator 102.

  As shown in FIG. 7, the drive wavelength of the laser light in the modulator 102 is 1.55 μm. In this case, in the modulator region 107 of the modulator 102, since the band edge wavelength of the core layer 122 is set to be shorter than 1.55 μm, when applied voltage = 0 (in FIG. 7, reference numeral In A), the signal propagation loss is small. On the other hand, when a reverse bias is applied (indicated by symbol B in FIG. 7), the overlap of the envelope function of electrons and holes becomes small due to the quantum confined Stark effect, so that the exciton absorption peak has an applied voltage = 0. Smaller than the case. Further, the optical absorption wavelength is shifted so as to be longer than 1.55 μm due to the inclination of the band, and the propagation loss increases. As a result, the modulation unit 102 generates an intensity modulation signal by superimposing the modulation signal on the reverse bias voltage.

  In general, in the modulator 102, a change in refractive index due to a change in absorption coefficient due to a reverse bias occurs due to the above-described relationship between the light absorption change and Kramers-Kronig, and frequency chirp is likely to occur. However, zero chirp or negative chirp operation can be realized by setting the quantum well.

  The optical semiconductor device 10 can be configured to include the light source unit 101 and the modulator 102 to generate an optical modulation signal including the components generated by the above-described frequency modulation and intensity modulation.

[Operation Principle of Modulation Signal Generation Device 200]
Next, the operation principle of the modulation signal generation device 200 will be described with reference to FIGS. In this modulation signal generation device 200, by performing frequency modulation having a positive or negative frequency change Δf according to a change in phase Φ between signals, the phase Φ between signals is changed to generate a phase modulation signal. .

  8A and 8B are diagrams for explaining the frequency change and the phase characteristic in the case of PSK modulation, where FIG. 8A is a frequency change characteristic when the signal frequency change is positive, and FIG. 8B is a signal frequency change. (C) shows the frequency change characteristic when the signal frequency change is negative, and (d) shows the signal phase characteristic when the signal frequency change is negative. .

  For example, when the frequency change between signals included in the signal sequence changes from “0” to “+ Δf” (FIG. 8A), the signal phase Φ changes from “π” to “0” (FIG. 8). (B)). On the other hand, when the frequency change between signals included in the signal sequence changes from “0” to “−Δf” (FIG. 8C), the signal phase Φ changes from “0” to “π” (FIG. 8 ( d)).

In this case, if the frequency modulation component of the signal is expressed as f ₀ + Δf (t) as a function of time t, the phase change (phase difference) ΔΦ (t) between the signals is given by the following equation. In this case, f ₀ is the oscillation frequency of the light source unit 101 when no modulation is performed.
ΔΦ (t) = − 2π∫Δf (t) dt (11)

  From the equation (11), the phase difference ΔΦ is expressed by an integral value of the frequency change amount. Therefore, when the frequency change is positive, it is positive or negative, and when the frequency change is negative, it is positive or negative. It becomes possible to represent a negative phase change.

  From this point of view, in the modulation signal generating apparatus 200 of the present embodiment, the frequency modulation signal is expressed by the equation (11) so as to represent a phase change according to various modulation methods (for example, PSK, DPSK, QPSK, DQPSK) between signals. Is adjusted. Specifically, ΔΦ = (2m + 1) × π (m: integer) in the case of PSK or DPSK, and ΔΦ = (m / 2) × π (m: integer) in the case of QPSK or DQPSK. . As a result, it becomes possible to generate frequency modulation signals corresponding to various modulation methods.

  Next, a method for generating a frequency modulation signal corresponding to various modulation schemes will be described.

[PSK modulation]
First, a method of generating a frequency modulation signal in the case of PSK modulation will be described with reference to FIG. 4, FIG. 6, and FIG. FIG. 9 is a diagram for explaining a frequency generation method in the case of PSK modulation, where (a) is a signal sequence, (b) is a signal phase, (c) is a modulation voltage, and (d) is a signal sequence. Intensity, (e) indicates a frequency modulation signal.

  As shown in FIG. 9 (a), for example, when a signal sequence (code sequence) composed of NRZ signals of 010100110010 ... is given, the phase modulation waveform corresponding to this signal sequence is shown in FIG. 9 (b). As shown in FIG. 4, when the signal is “0”, the phase is “0”, and when the signal is “1”, the waveform is phase = “π”.

  In this case, the voltage source 221 of the modulation signal generation device 200 generates the drive modulation voltage Vm corresponding to the phase modulation waveform shown in FIG. In the example of FIG. 6, this drive modulation voltage Vm is represented by a signal waveform d1. Then, the control circuit 222 of the modulation signal generation device 200 generates a modulation voltage Vp (FIG. 9C) having a differential component of the drive modulation voltage Vm. In the example of FIG. 6, the modulation voltage Vp shown in FIG. 9C is represented by the signal waveform d <b> 2 of FIG. 6 and is given to the frequency modulation region 104 of the light source unit 101.

  In the frequency modulation region 104, the modulation voltage Vp shown in FIG. 9C is applied to generate the intensity modulation signal shown in FIG. 9D, and the frequency modulation signal shown in FIG. Generate. In this embodiment, the bandgap wavelength of the frequency modulation region 104 is set to a wavelength shorter than the use region by 100 nm or more, and propagation loss due to light absorption due to voltage application is reduced. The intensity-modulated signal shown in FIG. On the other hand, the frequency modulation signal shown in FIG. 9E has a waveform similar to that of the modulation voltage Vp shown in FIG. Accordingly, the light source unit 101 can generate a frequency modulation signal corresponding to a phase change between signals of a signal sequence corresponding to PSK modulation.

  Here, in the example of FIG. 9C, the modulation voltage Vp is a pulse that takes a positive or negative value with the bias voltage Vb as a reference, but a pulse that takes only a positive or negative value with the bias voltage Vb as a reference. It is good. In this case, the control circuit 222 of the modulation signal generation device 200 applies a circuit configuration that generates a modulation voltage having only a positive or negative value. A method for generating a frequency modulation signal when a modulation voltage Vp having only a positive value with reference to the bias voltage Vb will be described with reference to FIG.

  FIG. 10 is a diagram for explaining another method of generating a frequency modulation signal in the case of PSK modulation, where (a) is a signal sequence, (b) is a phase of a signal, (c) is a modulation voltage, d) shows the intensity of the signal, and (e) shows the frequency modulation signal. Note that FIG. 10 differs from FIG. 9 only in (c) and (e), and therefore, description will be made focusing on FIG. 10 (c) and (e).

  In FIG. 10A, for example, when a signal sequence similar to that in FIG. 9A is given, the modulation voltage Vp is only a positive value with reference to the bias voltage Vb as shown in FIG. 10C. Therefore, the frequency modulation signal shown in FIG. 10E also has a waveform having only a positive frequency change Δf. Even in this case, the frequency modulation signal represents a phase change between signals based on PSK modulation.

[DPSK modulation]
Next, a method of generating a frequency modulation signal in the case of DPSK modulation will be described with reference to FIG. 4, FIG. 6, and FIG. FIG. 11 is a diagram for explaining a frequency generation method in the case of DPSK modulation, where (a) is a signal sequence, (b) is a signal phase, (c) is a modulation voltage, and (d) is a signal sequence. Intensity, (e) indicates a frequency modulation signal.

  As shown in FIG. 11 (a), for example, when a signal sequence consisting of NRZ signals of 010100110010 ... is given, the phase modulation waveform corresponding to this signal sequence is as shown in FIG. 11 (b). When the signal is “0”, the phase change = “0”, or when the signal is “1”, the phase change = “π” (absolute value).

  In this case, the modulation voltage Vp generated by the control circuit 222 of the modulation signal generation device 200 has a value as shown in FIG. That is, the modulation voltage Vp has a differential component of the drive modulation voltage Vm representing a signal sequence (for example, (011000100010...) Corresponding to the phase modulation waveform shown in FIG. Vm is generated by the voltage source 221 of the modulation signal generation apparatus 200.

  In the frequency modulation region 104 of the light source unit 101, the modulation voltage Vp is applied to generate the intensity modulation signal shown in FIG. 11D and the frequency modulation signal shown in FIG. In this embodiment, the band gap wavelength of the frequency modulation region 104 is set to a wavelength that is shorter than the use region by 100 nm or more, and propagation loss due to light absorption due to voltage application is reduced. The intensity-modulated signal shown in FIG. On the other hand, the frequency modulation signal shown in FIG. 11E has a waveform similar to that of the modulation voltage Vp shown in FIG. Thereby, the light source unit 101 can generate a frequency modulation signal corresponding to a phase change between signals of a signal sequence according to DPSK modulation.

[QPSK modulation]
A method of generating a frequency modulation signal in the case of QPSK modulation will be described with reference to FIG. 4, FIG. 12, and FIG.

  FIG. 12 is a diagram for explaining a frequency generation method in the case of QPSK modulation, where (a) is a signal sequence, (b) is a phase of a signal, (c) is a modulation voltage, and (d) is a signal voltage. Intensity, (e) indicates a frequency modulation signal.

  As shown in FIG. 12A, for example, when a signal sequence composed of NRZ signals of 010100110010... Is given, the phase modulation waveform corresponding to this signal sequence is as shown in FIG. , Phase = “0” when the signal sequence is “00”, phase = “π / 2” when the signal sequence is “10”, phase = “π” when the signal sequence is “11”, or When the signal sequence is “01”, it represents a phase of a waveform representing phase = “− π / 2” (for example, −π / 2, −π / 2, 0, π, 0, π / 2...). )

  In this case, the drive modulation voltage Vm corresponding to the phase modulation waveform shown in FIG. 11B is generated by the voltage source 221 of the modulation signal generation device 200. This drive modulation voltage Vm is shown in FIG. FIG. 13 is a diagram for explaining an example of the drive modulation voltage Vm and the modulation voltage Vp generated by the modulation signal generation device 200. Note that the configuration of the modulation signal generating apparatus 200 shown in FIG. 13 is the same as that of FIG.

  In FIG. 13, the voltage source 221 of the modulation signal generation device 200 generates a drive modulation voltage Vm having a voltage value of −V / 2 to V as shown by a signal waveform d11. In the signal waveform d11, the voltage values −V / 2, 0, V / 2, and V of the drive modulation voltage Vm have the phases −π / 2, 0, π / 2, and π shown in FIG. Correspondingly set.

  In this case, the control circuit 222 of the modulation signal generation device 200 generates a modulation voltage Vp having a differential waveform of the drive modulation voltage Vm as shown by the signal waveform d11 and applies it to the frequency modulation region 104 of the light source unit 101. . The signal waveform d12 in FIG. 13 is the same as the modulation voltage Vm in FIG.

  In the frequency modulation region 104, by applying the modulation voltage Vp, the intensity modulation signal shown in FIG. 12D is generated and the frequency modulation signal shown in FIG. 12E is generated. In this embodiment, the band gap wavelength of the frequency modulation region 104 is set to a wavelength that is shorter than the use region by 100 nm or more, and propagation loss due to light absorption accompanying voltage application is reduced. The intensity-modulated signal shown in FIG. On the other hand, the frequency modulation signal shown in FIG. 12E has a waveform similar to that of the modulation voltage Vp shown in FIG. Thereby, the light source unit 101 can generate a frequency modulation signal corresponding to a phase change between signals of a signal sequence corresponding to QPSK modulation.

[DQPSK modulation]
Next, a method of generating a frequency modulation signal in the case of DQPSK modulation will be described with reference to FIG. 4, FIG. 13, and FIG.

  FIG. 14 is a diagram for explaining a frequency generation method in the case of DQPSK modulation, where (a) is a signal sequence, (b) is a phase of a signal, (c) is a modulation voltage, and (d) is a signal voltage. Intensity, (e) indicates a frequency modulation signal.

  As shown in FIG. 14 (a), for example, when a signal sequence composed of NRZ signals of 010100110010 ... is given, the phase modulation waveform corresponding to this signal sequence is as shown in FIG. 14 (b). When the signal sequence is “00”, the phase change = 0, when the signal sequence is “10”, the phase change = π / 2, when the signal sequence is “01”, the phase = −π / 2, or the signal When the column is “11”, a waveform indicating phase change = π (for example, a phase of −π / 2, −π / 2, −π, −π, −π, −π, 0...). It becomes.

  In this case, in the control circuit 222 of the modulation signal generation device 200, the voltage source 221 generates the drive modulation voltage Vm corresponding to the phase modulation waveform shown in FIG. A modulation voltage Vp (FIG. 14C) is generated.

  In the frequency modulation region 104 of the light source unit 101, the modulation voltage Vp shown in FIG. 14C is applied to generate the intensity modulation signal shown in FIG. 14D, and also shown in FIG. A frequency modulation signal is generated. In this embodiment, the band gap wavelength of the frequency modulation region 104 is set to a wavelength shorter than the use region by 100 nm or more, and propagation loss due to light absorption due to voltage application is reduced. The intensity-modulated signal shown in FIG. On the other hand, the frequency modulation signal shown in FIG. 14E has a waveform similar to that of the modulation voltage Vp shown in FIG. Thereby, the light source unit 101 can generate a frequency modulation signal corresponding to a phase change between signals of a signal sequence according to DQPSK modulation.

  As described above, according to the present embodiment, the modulation signal generation device 200 is based on the drive modulation voltage Vm generated according to the signal sequence corresponding to various phase modulations (PSK, DPSK, QPSK, DQPSK). A modulation voltage Vp is generated, and the light source unit 101 generates a frequency modulation signal corresponding to the modulation voltage Vp in the frequency modulation region 104. In this case, the frequency modulation signal represents a phase change between signals of a signal sequence corresponding to various phase modulations (PSK, DPSK, QPSK, DQPSK). Therefore, the light source unit 101 and the modulation signal generation device 200 can obtain frequency modulation signals corresponding to various phase modulations (PSK, DPSK, QPSK, DQPSK) of a signal sequence without using a Mach-Zehnder type modulator. Therefore, the light source unit 101 (modulated light source) can be reduced in size. Further, the light source unit 101 and the modulation signal generation device 200 (modulation light source) can generate a frequency modulation signal relatively easily compared to the case where a Mach-Zehnder type modulator is used.

Next, a modification of this embodiment will be described.
[RZ modulation]
In the modulator 102 shown in FIG. 4, a periodic modulation signal is given and reverse bias modulation is performed, whereby the NRZ signal of the signal sequence is extracted and converted into an RZ signal. In this case, the modulator 102 performs repetitive modulation so that the extinction ratio of the signal component in the rising or falling area of the bit becomes small. Thereby, since the frequency modulation component used when generating the phase change can be cut, the dispersion tolerance can be further improved. Although the electroabsorption type (EA) modulator has been described as an example of the modulator 102, for example, a Mach-Zehnder modulator may be applied.

  In the light source unit 101 shown in FIG. 4, the phase adjustment region modulation of the DBR laser is performed, but other methods suitable for the frequency modulation light source may be applied. For example, a frequency modulation pulse is generated using a frequency change that occurs during direct current modulation of a distributed reflection laser.

  In the modulation signal generation device 200, the control circuit 222 is not limited to the example shown in FIG. 6 as long as the pulse voltage composed of the differential waveform of the drive signal of the NRZ signal can be applied to the frequency modulation region 104. It is clear that the configuration can be applied.

  In the embodiment, an InGaAsP-based material has been described as an example of a material applied to the optical semiconductor device 10, but another material such as an InAlGaAs-based material may be applied.

  Although the waveguide structure has been described as a ridge waveguide structure, it may be a high mesa structure or a buried structure.

DESCRIPTION OF SYMBOLS 10 Optical semiconductor device 101 Light source part 102 Modulator 103 1st DBR reflector 104 Frequency modulation area | region 105 Active layer area | region 106 2nd DBR reflector 200 Modulation signal generation apparatus 221 Voltage source 222 Control circuit

Claims (8)

A modulation signal generator including a voltage source that generates a drive voltage set according to the phase modulation of the signal sequence, and a control circuit that generates a modulation voltage according to the drive voltage;
A light source unit for generating a frequency modulation signal corresponding to the modulation voltage;
A modulated light source comprising: When the amount of change in the frequency of the frequency modulation signal is Δf, the phase difference between the signals is ΔΦ, and the time is t, the light source unit is configured so that the phase modulation is PSK or DPSK.
ΔΦ (t) = − 2π∫Δf (t) dt (1)
ΔΦ (t) = (2m + 1) × π (m: integer) (2)
The modulated light source according to claim 1, wherein the frequency modulation signal is generated so as to satisfy. When the amount of change in frequency of the frequency modulation signal is Δf, the phase difference between the signals is ΔΦ, and the time is t, the light source unit is configured such that when the phase modulation is QPSK or DQPSK,
ΔΦ (t) = − 2π∫Δf (t) dt (3)
ΔΦ (t) = (m / 2) × π (m: integer) (4)
The modulated light source according to claim 1, wherein the frequency modulation signal is generated so as to satisfy.   4. The modulation according to claim 1, wherein the control circuit is configured to apply a modulation voltage composed of a differential waveform of a driving voltage in accordance with an NRZ signal to a frequency modulation region of the light source unit. 5. light source. Generating a drive voltage set according to the phase modulation of the signal sequence;
Generating a modulation voltage according to the drive voltage;
Generating a frequency modulation signal in accordance with the modulation voltage;
A modulation signal generating method comprising: When the amount of change in frequency of the frequency modulation signal is Δf, the phase difference between the signals is ΔΦ, and time is t, the step of generating the frequency modulation signal is performed when the phase modulation is PSK or DPSK.
ΔΦ (t) = − 2π∫Δf (t) dt (5)
ΔΦ (t) = (2m + 1) × π (m: integer) (6)
The modulation signal generation method according to claim 5, wherein: When the amount of change in frequency of the frequency modulation signal is Δf, the phase difference between the signals is ΔΦ, and time is t, the step of generating the frequency modulation signal is performed when the phase modulation is QPSK or DQPSK.
ΔΦ (t) = − 2π∫Δf (t) dt (7)
ΔΦ (t) = (m / 2) × π (m: integer) (8)
The modulation signal generation method according to claim 5, wherein:   The step of generating the modulation voltage includes a step of applying a modulation voltage comprising a differential waveform of the drive voltage in accordance with the NRZ signal to the frequency modulation region of the light source unit. The modulation signal generating method according to the item.

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