JP2017011177A - Optical modulation signal generation device and optical modulation signal generation method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve suppression of a code error due to pulse-to-pulse interference of a transmission optical signal with a simple and low-cost configuration.SOLUTION: The optical modulation signal generation device includes: a semiconductor laser 1 for outputting a frequency-modulated optical signal; an EA modulator 2 for intensity-modulating light output from the semiconductor laser 1; a capacitor C1 inserted between a voltage signal Vfor driving the EA modulator 2 and an electrode of the semiconductor laser 1; and a resistor R1 inserted in series to the capacitor C1 between the voltage signal Vand the electrode of the semiconductor laser 1. The optical modulation signal generation device executes frequency modulation so that a frequency deviation Δf generated when a voltage signal is branched by the capacitor C1 and the resistor R1 and is applied to the semiconductor laser 1 satisfies Δf=(B/2)×{(ER-1)/(ER+1)}, where a frequency deviation by frequency modulation is Δf, a bit rate is B and an extinction ratio is ER.SELECTED DRAWING: Figure 2

Description

  The present invention relates to an optical modulation signal generation apparatus and an optical modulation signal generation method for generating an optical modulation signal for optical communication.

  As a light source for a metro access system, there is a light source (EA-DFB) in which an electroabsorption optical modulator (EA modulator, Electro-Absorption Modulator) and a distributed feedback semiconductor laser (DFB laser, Distributed FeedBack Laser) are integrated. Widely used. The transmission distance of the intensity-modulated light having a wavelength of 1.5 μm using this light source through the optical fiber is approximately 80 km at a signal speed of 10 Gbit / s due to the dispersion limitation of the optical fiber caused by the chirp of the EA modulator.

  For future extension of the access network and integration with the metro network, a light source capable of long-distance transmission with low cost and low power consumption is important. Therefore, Patent Document 1 and Non-Patent Documents 1 and 2 propose a method that enables long-distance transmission using a light source based on EA-DFB. The methods disclosed in these documents indicate that long distance transmission exceeding 100 km is possible without using dispersion compensation technology. However, since another RF port for operating as a frequency modulator is required separately from the RF port for driving signal for driving the EA modulator, there is a problem that the configuration as an optical transmitter becomes complicated. there were.

  On the other hand, in Patent Document 2, using a light source in which a wavelength tunable laser and an EA modulator are integrated, a part of a signal for driving the EA modulator is applied to wavelength control of the wavelength tunable laser through a differentiation circuit. Thus, there has been proposed a method for suppressing the chirp by varying the wavelength of the wavelength tunable laser so as to cancel out the chirp generated by the EA modulator. In this method, unlike a normal DFB laser, since a laser having a wavelength variable mechanism and an EA modulator are integrated, the structure of the light source itself becomes complicated as compared with a general EA-DFB. Further, since an inverter circuit and a bridge circuit are used in addition to the differentiation circuit in order to generate a signal for driving the wavelength tunable laser, there is a problem in cost reduction.

JP 2011-159552 A Japanese Patent Laid-Open No. 7-38188

K. Hasebe et al., “Directly Frequency Modulated DFB Laser Integrated with EA modulator for Extended Transmission Reach”, ECOC2010, Th.9.D.5, 2010 H. Kim et al., “A Novel Way to Improve the Dispersion-Limited Transmission Distance of Electroabsorption Modulated Lasers”, IEEE Photonics Technology Letters, Vol. 18, No. 8, 2006

The main cause of transmission signal error after optical fiber transmission is erroneous determination of 0 and 1 levels due to inter-pulse interference of signals. In particular, in the case of a signal train including 1 bit of a 0 level signal between two 1 level signals such as 1, 0, 1, etc., pulses before and after the 0 level are caused by pulse spreading accompanying optical fiber transmission. As a result of receiving the interference, the 0 level is erroneously determined as the 1 level and an error occurs.
Furthermore, it is difficult to make the chirp associated with the modulation operation zero due to the operating principle of the EA modulator. Because of this chirp, the transmission distance by the optical fiber is further limited.

[Basic principle of inter-pulse interference suppression by conventional technology]
In the conventional techniques disclosed in Patent Document 1 and Non-Patent Documents 1 and 2, in order to suppress a transmission error due to inter-pulse interference, an optical signal that is intensity-modulated by binary values of the modulated signal 0 level and 1 level is used. Thus, frequency modulation is performed so as to synchronize with this intensity modulation. When a frequency shift of Δf (GHz) is given by frequency modulation according to the 0 level 1 level, the bit rate B (Gb / s), the frequency shift Δf, and the extinction ratio ER of the intensity modulation have the following relationship: In this case, it is known that a phase difference of 180 ° can be given between pulses of around 0 level for 1 bit, and dispersion tolerance can be improved. The extinction ratio ER is defined by the ratio I1 / I0 between the 1-level light intensity I1 and the 0-level light intensity I0.

In order to realize such intensity modulation and frequency modulation, in the prior art disclosed in Patent Document 1 and Non-Patent Documents 1 and 2, as shown in FIG. 11, frequency modulation and EA by direct modulation of the semiconductor laser 1 are performed. The intensity modulation of the modulator 2 is individually performed and operated under the condition satisfying the expression (1), thereby improving the dispersion resistance and realizing the extension of the transmission distance. In FIG. 11, R ₅₀ is a termination resistor, S 1 is a signal source that outputs a voltage signal for driving the EA modulator 2, and S 2 is a signal source that outputs a voltage signal applied to the semiconductor laser 1.

The optical modulation signal generated by the optical modulation signal generation apparatus having the configuration shown in FIG. 11 is an ASK (Amplitude Shift Keying) -CPFSK (Continuous Phase Frequency Shift Keying) signal, but is received by directly detecting the intensity modulation signal. Is a possible signal.
However, in the conventional configuration shown in FIG. 11, the RF port P2 connected to the signal source S2 for frequency modulation and the RF port P1 connected to the signal source S1 for intensity modulation are in the optical modulation signal generation device. There is a problem that it is necessary and the configuration of the apparatus is complicated and expensive.

  The present invention has been made to solve the above-described problems, and an optical modulation signal capable of suppressing a code error due to inter-pulse interference of a transmission optical signal and extending a transmittable distance with a simple and low-cost configuration. An object of the present invention is to provide a generation device and an optical modulation signal generation method.

An optical modulation signal generation device of the present invention includes a frequency modulation light source that outputs a frequency modulated optical signal, a light intensity modulator that intensity modulates light output from the frequency modulation light source, and drives the light intensity modulator. A first capacitor inserted between the voltage signal and the electrode of the frequency modulation light source, and a resistor inserted in series with the first capacitor between the voltage signal and the electrode of the frequency modulation light source When the frequency shift due to frequency modulation is Δf, the bit rate is B, and the extinction ratio is ER, the voltage signal is branched by the first capacitor and resistor and applied to the frequency modulation light source. The frequency shift Δf is
Frequency modulation is performed so that the capacitance of the first capacitor is C, the resistance value is R, the resistance value of the frequency modulation light source is R _LD , and the voltage signal of the angular frequency ω is V _S. Then, the voltage V _LD applied to the frequency modulation light source is
It is characterized by being given by.

In the configuration example of the optical modulation signal generation device of the present invention, the frequency modulation light source is a semiconductor laser capable of direct frequency modulation, and is applied to the frequency shift Δf by the frequency modulation and the frequency modulation light source. The relationship with voltage V _LD is
A linear frequency shift-voltage characteristic region having a proportionality constant β expressed by the above equation is used.
Further, in one configuration example of the optical modulation signal generation device of the present invention, the value of the resistance and the driving condition of the frequency modulation light source are such that the frequency shift Δf by the frequency modulation is:
It is characterized by being set to satisfy.

Further, in one configuration example of the optical modulation signal generating device of the present invention, the chirp generated by the optical intensity modulator is a positive chirp or a negative chirp and is smaller than a frequency shift Δf by the frequency modulation. It is a feature.
In addition, one configuration example of the optical modulation signal generation device of the present invention further includes a second capacitor inserted between the electrode of the frequency modulation light source and the ground, and the capacitance of the second capacitor is represented by C _d. The voltage V _LDD applied to the frequency modulation light source is
It is characterized by being given by.

Further, the light modulation signal generation method of the present invention applies a voltage signal to the light intensity modulator, and applies a voltage signal to the electrode of the frequency modulation light source through the first capacitor and resistor, A frequency modulation step of generating an optical signal frequency-modulated by an operation of the frequency-modulated light source according to a voltage applied via the first capacitor and a resistor, and a light intensity modulator according to the voltage signal. An intensity modulation step of intensity-modulating the output light of the frequency-modulated light source by operation, when the frequency shift due to frequency modulation is Δf, the bit rate is B, and the extinction ratio is ER, the first capacitor and the resistor The frequency shift Δf when the voltage signal is branched and applied to the frequency-modulated light source,
Frequency modulation is performed so that the capacitance of the first capacitor is C, the resistance value is R, the resistance value of the frequency modulation light source is R _LD , and the voltage signal of the angular frequency ω is V _S. Then, the voltage V _LD applied to the frequency modulation light source is
It is characterized by being given by.

  According to the present invention, by providing the first capacitor and the resistor between the voltage signal for driving the light intensity modulator and the electrode of the frequency modulation light source, it is possible to provide a simple and low-cost configuration between the pulses of the transmitted optical signal. It is possible to suppress a code error due to interference and extend a transmittable distance.

It is a block diagram which shows the structure of the conventional optical modulation signal generation apparatus. It is a block diagram which shows the structure of the optical modulation signal generation apparatus which concerns on the 1st Embodiment of this invention. 1 is an equivalent circuit diagram of an optical modulation signal generation device according to a first embodiment of the present invention. It is a figure which shows one example of the time change of the voltage signal which drives an EA modulator, and the branch voltage applied to a semiconductor laser in the 1st Embodiment of this invention. It is a figure explaining the optical modulation signal in the 1st Embodiment of this invention. It is a figure which shows an example of the relationship between the resistance inserted between the voltage signal which drives an EA modulator, and a semiconductor laser in the 1st Embodiment of this invention, and the branch voltage applied to a semiconductor laser. . It is a figure which shows one example of the relationship between the amplitude of the branch voltage applied to a semiconductor laser, and the frequency shift of the optical modulation signal by frequency modulation in the 1st Embodiment of this invention. It is a schematic diagram of the EA modulator integrated light source which concerns on the 1st Embodiment of this invention. It is a figure which shows the eye pattern of the signal waveform after 100-km transmission of the optical modulation signal output from the conventional optical modulation signal generation apparatus and the optical modulation signal generation apparatus which concerns on the 1st Embodiment of this invention. It is a block diagram which shows the structure of the optical modulation signal generation apparatus which concerns on the 2nd Embodiment of this invention. It is a block diagram which shows the structure of the conventional optical modulation signal generation apparatus.

[First Embodiment]
Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the electrical wiring of a normal EA modulator integrated light source that performs only intensity modulation, wiring is performed so that a high-frequency voltage signal is applied only to the EA modulator 2 using a single wiring board as shown in FIG. . The reason why the voltage signal is applied only to the EA modulator 2 is to avoid the occurrence of another chirp from the EA modulator 2 by applying the voltage signal to the semiconductor laser 1.

  On the other hand, in the optical modulation signal generation apparatus disclosed in Patent Document 1 and Non-Patent Documents 1 and 2, frequency modulation by direct modulation of the semiconductor laser 1 and intensity modulation by the EA modulator 2 are performed under the condition that satisfies Expression (1). However, there is a problem in that the configuration of the light source is complicated and expensive as described above.

In contrast, in the present embodiment, a high-frequency voltage signal is intentionally applied to the semiconductor laser 1 side using a wiring board for differential driving or the like that applies a high-frequency voltage signal only to the EA modulator 2. Wiring or mounting as shown. FIG. 2 is a block diagram showing a configuration of the optical modulation signal generating apparatus according to the present embodiment. In the present embodiment, the direct frequency modulation of the semiconductor laser 1 can be performed only by the voltage signal V _S for driving the EA modulator 2 as described below. Similar to the configuration, the RF port required for the modulation drive can be only the RF port P1 connected to the signal source S1.

As in the prior art, the voltage signal V _S is applied from the signal source S1 to the driving electrode of the EA modulator 2 via the RF port P1.
On the other hand, between the RF port P1 and the current injection electrode of the semiconductor laser 1, the capacitor C1 and the EA modulator 2 for driving the bias current for the semiconductor laser from flowing into the EA modulator 2 are driven. A resistor R1 for dividing the voltage signal V _S is inserted in series. By adding the capacitor C1 and the resistor R1, the voltage signal V _S is branched and applied to the semiconductor laser 1, thereby realizing direct frequency modulation of the semiconductor laser 1.

FIG. 3 shows an equivalent circuit of the optical modulation signal generating apparatus of this embodiment. In FIG. 3, I is a bias current to the semiconductor laser 1, and L is an inductance of a bias tee used for applying the bias current I to the semiconductor laser 1. If the capacitance of the capacitor C1 is C, the value of the resistor R1 is R, and the resistance value of the semiconductor laser 1 is R _LD , the semiconductor laser is obtained from the capacitance C, the resistance values R and R _LD and the voltage signal V _S of the angular frequency ω. The magnitude V _{LD of the} voltage applied to 1 can be expressed as the following formula (2).

As described above, L represents the inductance of the bias tee, but it can be ignored for the high-frequency voltage signal V _S for driving the EA modulator 2 because it is open.
FIG. 4 is a diagram showing an example of a time change of the voltage signal V _S obtained by numerical calculation based on the equivalent circuit shown in FIG. 3 and the branch voltage V _LD applied to the semiconductor laser 1. As apparent from FIG. 4, since the branch voltage V _LD applied to the semiconductor laser 1 and the voltage signal V _S applied to the EA modulator 2 are synchronized, frequency modulation is performed using these signals. By performing the intensity modulation simultaneously, an optical modulation signal as shown in FIGS. 5A and 5B can be obtained. FIG. 5A shows the light intensity-time characteristic of the optical modulation signal output from the optical modulation signal generation device of this embodiment, and FIG. 5B shows the optical modulation signal output from the optical modulation signal generation device. The frequency-time characteristic is shown.

FIG. 6 is a diagram showing an example in which the relationship between the value R of the resistor R1 and the branch voltage V _LD is calculated. Here, the calculation is performed assuming that the resistance value R _LD of the semiconductor laser 1 is 5Ω, the voltage signal V _S = 2V, the capacitance C = 1 pF, and the frequency f = 10 GHz. As can be seen from FIG. 6, the magnitude of the branch voltage V _LD can be adjusted by the value R of the resistor R1.
The capacitance C of the capacitor C1 only needs to be able to prevent the bias current of the semiconductor laser 1 from flowing to the EA modulator 2, and the size in a wide range of several pico F to several hundred pico F is sufficient. It is.

FIG. 7 is a diagram showing an example of the relationship between the amplitude of the branch voltage V _LD applied to the semiconductor laser 1 and the frequency shift Δf of the optical modulation signal by frequency modulation. The relationship between the branch voltage V _LD and the frequency shift Δf depends on the bias current and temperature that are the driving conditions of the semiconductor laser 1, but in the case of standard driving conditions, for example, the relationship between current and optical output is linear. Is used, the frequency shift Δf is also linear, and therefore can be expressed as in Equation (3).

In Expression (3), β is a proportionality constant and is a parameter that varies depending on the driving conditions (bias current and temperature) of the semiconductor laser 1.
In the present embodiment, the EA modulator is simultaneously performed while performing frequency modulation by direct modulation of the semiconductor laser 1 so that the frequency shift Δf defined by the equations (2) and (3) satisfies the equation (1). 2. Intensity modulation by 2. In order for the frequency shift Δf of the optical modulation signal by frequency modulation to satisfy the equation (1), the constant β is adjusted by setting the driving conditions (bias current and temperature) of the semiconductor laser 1, or the resistance R1 The branch voltage V _LD may be adjusted by setting the value R.

  FIG. 8 is a schematic diagram of an EA modulator integrated light source in which the semiconductor laser 1 and the EA modulator 2 used in the present embodiment are integrated. The EA modulator integrated light source includes a semiconductor substrate 100, a core layer (active layer) 101 of the semiconductor laser 1 formed on the semiconductor substrate 100, and a diffraction grating layer 102 of the semiconductor laser 1 formed on the core layer 101. The passive layer 103 for propagating light emitted from the semiconductor laser 1 to the EA modulator 2, the core layer (absorption layer) 104 of the EA modulator 2 formed on the semiconductor substrate 100, the diffraction grating layer 102, and the passive layer 103 A cladding layer 105 formed on the layer 103 and the core layer 104; a current blocking layer 106 made of semi-insulating semiconductor or polyimide; a contact layer 107 formed on the cladding layer 105; and a current blocking layer 106 The passivation film 108 formed on the passivation film 108, the electrode 109 of the semiconductor laser 1 formed on the passivation film 108, and the passivation film 108. Composed of the EA modulator 2 of electrode 110. formed on the film 108.

  The EA modulator integrated light source may be configured such that the semiconductor laser 1 and the EA modulator 2 are formed by burying regrowth of a semi-insulating semiconductor, or a ridge structure and a burying structure made of a polyimide resin. I do not care.

  9A is a diagram showing an eye pattern of a signal waveform after 100 km transmission of an optical modulation signal output from the conventional optical modulation signal generation device shown in FIG. 1, and FIG. 9B is an embodiment of the present embodiment. It is a figure which shows the eye pattern of the signal waveform after 100-km transmission of the optical modulation signal output from the optical modulation signal generation apparatus. Here, the bit rate is 10 Gbit / s. According to the present embodiment, it can be confirmed that a favorable eye opening can be obtained as compared with the conventional optical modulation signal generation device.

  As described above, in this embodiment, a simple and low-cost configuration with only one RF port can suppress a code error due to interpulse interference of a transmission optical signal, and can extend a transmission distance. .

  In the present embodiment and the second embodiment described below, since the frequency modulation is additionally applied to the inherent chirp of the EA modulator 2, the driving conditions for the EA modulator 2 are the same. The sign of the chirp that changes depending on whether it is positive or negative. However, this chirp is preferably smaller than the frequency shift Δf of the optical modulation signal by frequency modulation.

[Second Embodiment]
Next, a second embodiment of the present invention will be described. FIG. 10 is a block diagram showing the configuration of the optical modulation signal generating apparatus according to the second embodiment of the present invention. The same components as those in FIG. 2 are denoted by the same reference numerals. The optical modulation signal generation apparatus according to the present embodiment is different from the optical modulation signal generation apparatus according to the first embodiment in that a semiconductor is used for timing adjustment of frequency modulation by the semiconductor laser 1 and intensity modulation by the EA modulator 2. A capacitor C2 for delay adjustment is added in parallel with the laser 1.

Specifically, the capacitor C <b> 2 may be inserted between the electrode 109 in FIG. 8 and a ground (GND) side electrode (not shown) formed on the lower surface of the semiconductor substrate 100. As the capacitor C2, a capacitor having an order of capacitance of about several pico F may be used. When the capacitance of the capacitor C2 and C _d, with respect to the applied voltage V _LD of the semiconductor laser 1 when the capacitor C2 is no applied voltage V _LDD to the semiconductor laser 1 in the case of adding the capacitor C2 is as follows Indicated.

Thus, in this embodiment, in order to adjust the delay time of the intensity modulation for frequency modulation, it is possible to adjust the charge time of the capacitor C2 by setting the capacitance C _d of the capacitor C2, to the semiconductor laser 1 Since the timing of the applied voltage V _LDD can be adjusted, the frequency modulation by the semiconductor laser 1 and the intensity modulation by the EA modulator 2 can be accurately synchronized.

  In the first and second embodiments, the semiconductor laser 1, specifically the DFB laser capable of direct frequency modulation, is used as the frequency modulation light source for obtaining the frequency shift Δf by frequency modulation. The present invention is not limited to this, and a variable mechanism of a wavelength tunable laser such as a DBR (Distributed Bragg Reflector) laser, a double-resonance ring laser, an SSG (Super Structure Grating) -DBR laser, or the like is controlled by voltage or current. As long as the operation satisfying the expressions (1) and (4) is possible, it can be used as the frequency modulation light source of the present invention.

  In the first and second embodiments, the EA modulator is used as the light intensity modulator, but a Mach-Zehnder type optical modulator may be used.

  The present invention can be applied to a technique for generating an optical modulation signal for optical communication.

1 ... semiconductor laser, 2 ... EA modulator, R ₅₀ ... terminating resistor, R1 ... resistor, C1, C2 ... capacitor, P1 ... RF port, S1 ... signal source.

Claims (6)

A frequency-modulated light source that outputs a frequency-modulated optical signal;
A light intensity modulator for intensity modulating light output from the frequency modulation light source;
A first capacitor inserted between a voltage signal for driving the light intensity modulator and an electrode of the frequency modulation light source;
A resistor inserted in series with the first capacitor between the voltage signal and the electrode of the frequency modulation light source;
When the frequency shift due to frequency modulation is Δf, the bit rate is B, and the extinction ratio is ER, the frequency shift when the voltage signal is branched by the first capacitor and resistor and applied to the frequency modulation light source. Δf is
Perform frequency modulation to satisfy
When the capacitance of the first capacitor is C, the resistance value is R, the resistance value of the frequency modulation light source is R _LD , and the voltage signal of the angular frequency ω is V _S , the capacitance is applied to the frequency modulation light source. The voltage V _LD is
An optical modulation signal generating device provided by: The optical modulation signal generating device according to claim 1,
The frequency modulation light source is a semiconductor laser capable of direct frequency modulation,
The relationship between the frequency shift Δf due to the frequency modulation and the voltage V _LD applied to the frequency modulation light source is:
An optical modulation signal generating device using a linear frequency shift-voltage characteristic region having a proportionality constant β expressed by: In the optical modulation signal generation device according to claim 1 or 2,
The value of the resistance and the driving condition of the frequency modulation light source are the frequency shift Δf due to the frequency modulation,
An optical modulation signal generation device characterized by being set to satisfy In the optical modulation signal generation device according to any one of claims 1 to 3,
The chirp generated by the light intensity modulator is a positive chirp or a negative chirp, and is smaller than the frequency shift Δf by the frequency modulation. In the optical modulation signal generation device according to any one of claims 1 to 4,
A second capacitor inserted between the electrode of the frequency modulation light source and the ground;
When the capacitance of the second capacitor is C _d , the voltage V _LDD applied to the frequency modulation light source is
An optical modulation signal generating device provided by: Applying a voltage signal to the light intensity modulator and applying the voltage signal to the electrode of the frequency modulation light source via a first capacitor and a resistor;
A frequency modulation step of generating an optical signal frequency-modulated by an operation of the frequency-modulated light source according to a voltage applied via the first capacitor and a resistor;
An intensity modulation step of intensity-modulating the output light of the frequency modulation light source by the operation of the light intensity modulator according to the voltage signal,
When the frequency shift due to frequency modulation is Δf, the bit rate is B, and the extinction ratio is ER, the frequency shift when the voltage signal is branched by the first capacitor and resistor and applied to the frequency modulation light source. Δf is
Perform frequency modulation to satisfy
When the capacitance of the first capacitor is C, the resistance value is R, the resistance value of the frequency modulation light source is R _LD , and the voltage signal of the angular frequency ω is V _S , the capacitance is applied to the frequency modulation light source. The voltage V _LD is
A method for generating an optical modulation signal, comprising: