JPH07104223A - Optical signal modulation method - Google Patents

Abstract

PURPOSE:To modulate the frequency of ultra high speed repetitive optical pulses without being limited by the modulation frequency by making the DC bias voltage to be an integer multiple of a half wavelength bias voltage. CONSTITUTION:Make the relationship between a DC bias voltage Vb and a half wavelength voltage Vpi to be Vb=mXVpi (where m is zero or a positive or a negative integer). When m is set to an integer, the voltage Vb applied to a branch type or a directional coupling type modulator becomes an integer multiple voltage of the voltage Vpi and this makes it F' coincidence with the peak of a relative light intensity (Pout/Pin) curve. Thus, the amplitude of the modulation signals are made coincident with the peak of the relative light intensity of a reference level corresponding to zero. When the optical input signals, which are made incident to a modulator from a semiconductor laser, are modulated by the modulation signals using the relative light intensity under this condition, the signals are modulated to optical output signals of high speed repetitive pulses.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical signal modulation method, and more particularly to an optical signal modulation method by an optical integrated device which generates ultra-high speed repetitive pulses.

[0002]

2. Description of the Related Art As a conventional device for obtaining an ultra-high speed modulated optical modulation signal, reference I ("40 GHz active m
ode locking in a 1.5 μm mo
nolithic extended-cavity l
aser ", Electronics Letters
, Vol. 10, 1989, PP 621-622).

FIG. 8 shows an ultrahigh-speed repetitive pulse generator (hereinafter referred to as an active mode-locking semiconductor laser) using the conventional active mode-locking method disclosed in this document.

First, the structure of an active mode-locked semiconductor laser will be briefly described with reference to FIG. This active mode-locked semiconductor laser is roughly classified into a gain region 114.
And a passive optical waveguide region 116. The gain region 114 is formed by providing a 1.5 μm active layer 102 on a 1.3 μm optical waveguide layer 100. Further, a p-InP upper clad layer 104 is provided on the upper surface of the active layer 102, and an n-InP lower clad layer 106 is provided on the lower surface of the optical waveguide layer 100. A structure in which the optical waveguide layer 100 and the active layer 102 are sandwiched between the upper clad layer 104 and the lower clad layer 106 in this way is called a double hetero structure.

A semi-insulating layer 107 is provided on both sides of the optical waveguide layer 100 and the active layer 102, and an n-InP substrate 108 is provided on the lower cladding layer 106.

Further, p-Ga is formed on the upper cladding layer 104.
The InAs layer 109 and the upper electrode 110 are sequentially stacked, while the lower electrode 112 is formed on the lower surface of the substrate 108.
Is provided. Further, the end portion of the active layer 102 is indicated by 103.

On the other hand, the portion forming the passive optical waveguide region 116 is provided on the same substrate 108 with the lower clad layer 106, the optical waveguide layer 100, the semi-insulating layer 107 and the upper clad layer 10.
4 are sequentially laminated and configured. The lower surface of the substrate 108 is provided with a lower electrode 112 which is common to the gain region. Thus, the gain region 114 and the passive optical waveguide region 1 are
Since the lateral portion of 16 is filled with the semi-insulating layer 107, current confinement and reduction of electric capacity are realized.

Next, a method of obtaining an optical pulse using the above-mentioned conventional active mode-locked semiconductor laser device will be briefly described.

In the active mode-locking method, an alternating voltage having a frequency that is an integral multiple of the longitudinal mode interval (Δν) is applied to the gain region 114, so that repetitive optical pulses equal to that frequency can be obtained. That is, the relationship between the repetition frequency f of the optical pulse and the mode interval is given by the following equation.

F = mΔν = mc ₀ / (2nL) (1) where m = 1, 2, 3, ..., C ₀ : speed of light in vacuum, n: refractive index of medium L : Resonator length (mm).

In the device disclosed in Document I, the resonator length is 2.07 mm, and the frequency of the longitudinal mode interval corresponding to this is about 20 GHz. Actually, an AC voltage of 40 GHz, which is twice the frequency of the longitudinal mode interval, is applied to obtain an optical pulse having a repetition frequency of 40 GHz.

[0012]

However, the above-mentioned conventional active mode-locked semiconductor laser has the following limitations when an optical pulse having an ultrafast repetition frequency is generated. (1) The repetition frequency of the light pulse is limited by the modulation band of the semiconductor laser device. (2) The repetition frequency of the light pulse is limited to the value of the maximum modulation frequency of the device that generates the high frequency modulation voltage. (3) Further, when it is attempted to perform mode-locking with a laser, the repetitive frequency is uniquely determined by the length of the resonance length (L) if the refractive index is constant from the above-mentioned equation (1). However, there are various problems such that the repetition frequency cannot be changed arbitrarily.

On the other hand, there is an optical modulator using the interference or coupling of a hybrid type semiconductor laser. As an example of this modulator, a branching interferometric modulator or a directional coupling modulator is disclosed in Document II (Document II: "Optical integrated circuit",
Ohmsha, 1985, PP300-312).

A conventional method for obtaining an optical output signal modulated by using the semiconductor laser light and the branching interferometric modulator disclosed in Document II will be described with reference to FIG.

FIG. 9 shows the power I ₁ (t) of the input signal at a constant DC level, the relative light intensity distribution curve F, the modulation signal A ₁ (t), and the power of the optical output signal I _2. , And the modulation signal A ₁ (t) is A ₁ (t) = sin (2π
ft) (where f is frequency and t is time).

In the method of Document II, the bias voltage V _{b is set} to the relative light intensity (P _out / P _in : where P _in is the power of the optical input signal to the modulator and P _out is the power of the optical output signal from the modulator. ) Around 1/2, that is, relative light intensity P _out
When the voltage (also referred to as a half-wavelength voltage) that minimizes / P _in is V _pi , V _{b is} near V _pi / 2, and the maximum value of the modulation signal A (t) is V _pi / 2. The following settings are set and adjusted. A detailed description of the relative light intensity distribution curve and the bias voltage application method will be given later. Therefore, in this conventional method, since the highest priority is to make adjustments so that distortion does not occur, modulation is performed using the linear segment of the relative light intensity distribution curve F as much as possible. , The input signal I ₁ (t) is amplitude-modulated by the modulating signal A ₁ (t) and the curve F, and the frequency is the same as the optical output signal I ₂ (t) having the same frequency as the modulating signal A ₁ (t). Become. However, conventional literature II
Does not mention any example of generating an optical output signal having a frequency higher than the frequency of the modulation signal, that is, generating a high-speed repetitive optical pulse as the optical output signal.

Further, the conventional hybrid semiconductor laser is similar to the active mode-locked semiconductor laser described above.
There is also a problem that the above-mentioned restrictions (1) and (2) are applied to the frequency modulation.

The present invention has been made in view of the above-mentioned problems, that is, an object of the present invention is to provide a modulator of a hybrid semiconductor laser in which an ultrahigh-speed repetitive optical pulse is not limited by a modulation frequency. An object of the present invention is to provide an optical signal modulation method capable of modulating the frequency of.

[0019]

To achieve this object, according to the optical signal modulation method of the present invention, the power of the optical input signal to the branching interference type or directional coupling type modulator is set to P _in , Let P _out be the power of the optical output signal from the modulator,
A modulation input voltage V externally applied to the modulator is a superposed voltage of a DC bias voltage _Vb and a modulation signal A (t), and further, the power P _{in of the} optical input signal and the power P of the optical output signal. relative light intensity given by the ratio of the _out (P _out /
In the optical modulation method, the voltage that minimizes P _in ) is a half-wave voltage V _pi , and the optical output signal is modulated by the modulation input voltage V using the relative light intensity to generate an optical output signal. , The DC bias voltage V _b and the half-wave voltage V
The relation with _pi is V _b = m × V _pi (where m is 0 or a positive or negative integer).

[0020]

According to this optical signal modulation method described above, the relationship between the DC bias voltage V _b and the half-wave voltage V _pi is V _b = m ×
It is as V _pi . By doing so, when m is set to a certain integer, the DC bias voltage V _b applied to the branched or directional coupling modulator becomes a voltage that is an integral multiple of the half-wave voltage V _pi , which is just Relative light intensity (P _out / P _in )
It coincides with the peak of the curve. Therefore, the reference level corresponding to the amplitude of the modulation signal of zero coincides with the peak of the relative light intensity. In such a state, when the optical input signal incident on the modulator from the semiconductor laser is modulated by the modulating signal by using this relative light intensity, as is clear from the experimental results described later, the Can be modulated into output signal.

Further, the pulsed optical output signal is converted into a pulse signal by coding the optical output signal of the ultra-high speed repetitive pulse modulated by the branching interferometric modulator or the directional coupling modulator by using the intensity modulator. can do.

[0022]

Embodiments of the present invention will be described below with reference to the drawings. In addition, each drawing used for explaining the configuration of the present invention, each constituent component, to the extent that the present invention can be understood,
The size and the positional relationship are only schematically shown.

FIG. 1 is a block diagram for explaining the configuration of the optical signal generator according to the first embodiment of the present invention.

The optical signal generator according to the first embodiment of the present invention is roughly composed of a semiconductor laser 15 part for oscillating a single mode and a modulator 18 part having two optical waveguides capable of refractive index modulation. Has been done. In the embodiment of the present invention, the semiconductor laser 15 and the modulator 18 are hybrid.

The semiconductor laser 15 comprises a single mode semiconductor laser device 10, a DC power supply 12 and a ground 1.
It has 4. Also, a single mode semiconductor laser 1
The laser light obtained from 0 is used as an optical input signal (also referred to as incident light) 16, and the light modulated and extracted by the modulator 18 is used as an optical output signal (also referred to as emitted light) 20.

FIG. 2 shows a cross section of the main part of a branching interferometric modulator as an example of the modulator 18 used in the first embodiment of the present invention.

This branch interferometric modulator uses Z-cut LiNbO ₃ as the substrate 22. And this substrate 22
The optical waveguide provided above is provided with one optical waveguide along the horizontal axis direction, and branches into two optical waveguides on the way to extend, and further matches one optical waveguide. Here, the first branch point of the optical waveguide is referred to as the first branch point 24, and
One optical waveguide branched from the branch point 24 is referred to as a first optical waveguide 27. The other optical waveguide branched from the first branch point 24 is referred to as a second optical waveguide 28. The matching branch point is referred to as a second branch point 26.

The two waveguides 27 and 28 are covered with the first and second electrodes 30 and 32 so as to cover the respective waveguides.
Is provided. Then, the first electrode 30 and the ground 36
Are connected via a power supply 34. On the other hand, the second electrode 32 is connected to the earth 38.

2 and 3, the power (P _in ) of the optical input signal 16 and the optical output signal 20 of the semiconductor laser will be described.
The relationship with the power (P _out ) will be described. Figure 3
As in the conventional method with reference to FIG. 9, the power I ₁ (t) of the optical input signal 16 input to the modulator, the relative light intensity distribution curve F of the optical modulator, and the modulation signal A (t) FIG. 3 is an explanatory diagram of the first embodiment of the present invention showing the power I ₂ of the optical output signal 20 from the optical modulator and the modulator, respectively. The optical input signal 16
Power (P _in ) and the power of the optical output signal 20 (P _out )
Since the relationship between and is disclosed in detail in Document II, detailed description thereof will be omitted here, and only the main points will be briefly described.

When the optical input signal power 16 is guided in the TM mode, the guided light branched and propagated in the first waveguide 27 undergoes a phase change due to the voltage applied by the external power supply 34. On the other hand, the guided light branched and propagated in the second waveguide 28 undergoes an opposite phase (for example, negative phase) change. Therefore, by combining and interfering the guided light propagating through the first and second waveguides at the branch point 26 on the output side, the light intensity of the optical output signal 20 changes corresponding to the phase difference between the respective guided lights. . This relationship will be described below.

Light intensity (power) of the optical input signal 16
Although (P _in ) causes scattering loss at the branch point 24, this scattering loss is ignored here. Further, the power division ratio of the branch point 24 on the input side (r _p = {[E _A ] /
Let [E _B ]} ² ) be 1: 1. In this case, the light intensity of the optical input signal 16 (P _in ) and the light intensity of the optical output signal 20 (P _in )
_out ) relation is given by the following equation (P301 of Document II)
reference).

P _out = P _in cos ² {(π / 2) (V / V _pi )} (2) where P _in : optical intensity of optical input signal P _out : optical intensity of optical output signal V : Voltage applied from power source 34 (V) V _pi : Half wavelength voltage (V). The half-wave voltage _Vpi is a voltage determined by the element of the modulator. A modulation input voltage (also referred to as an applied voltage) V that is generally applied to the first and second waveguides 27 and 28 is given by the following equation.

V = V _b + A (t) (3) where V _{b is a} DC bias voltage (V) A (t) is a modulation signal (V).

Using the relational expression (3), the relative light when the modulation signal is A (t) = sin (2πft) and the DC bias voltage V _b is V _b = 0 (V) The relationship between the intensities (P _out / P _in ) is shown in FIG. In the figure, the horizontal axis represents voltage (V) and the vertical axis represents relative light intensity (P _out / P _in ). In addition, the DC bias voltage V _b is a half-wave voltage V
_It represents an optical output signal when the modulation signal A (t) is a sine wave and is an integral multiple of _pi . Here, the relationship between the DC bias voltage V _b and the half-wave voltage V _pi of the present invention, however _{V b = mV pi, m =} 0, ± 1, ± 2, ± 3 ·
・ ・

The modulation signal is A (t) = V _rf si
n (2πft).

However, f: modulation frequency (Hz) t: time (second) V _rf : amplitude of modulation signal (V).

In the embodiment shown in FIG. 3, m = 0. Further, the amplitude V _rf of the modulation signal is set to an integral multiple of the half-wave voltage V _pi , but the relationship between V _rf and V _{pi at} this time is expressed by the following equation.

V _rf = kV _pi , where k> 0, and
When k> 1, it is an integer. That is, this is an example in which the amplitude V _rf of the modulation signal A (t) is made smaller than twice the half-wave voltage V _pi . In this case, as can be understood from the experimental result of FIG. 3, the modulation signal A (t) changes within the half-wave voltage V _{pi on} both sides of the maximum peak value of the relative light intensity distribution curve F as a center. Therefore, it can be seen that the power P _out of the optical output signal 20 is output at a frequency twice that of the modulation signal A (t).

FIG. 4 is a waveform characteristic diagram in which the modulated light signal and the optical output signal of FIG. 3 are superimposed and plotted. In the figure, the horizontal axis represents time (ps) and the vertical axis represents relative light intensity and voltage (V).

FIG. 4 shows m in the above equations (2) and (3).
= 0, V _pi = 1V, V _rf = 1V, f = 20 GHz, the modulation signal A (t) (curve I corresponds).
And the optical output signal 20 (curve II corresponds). As can be understood from FIG. 4, the frequency of the optical output signal 20 is twice that of the modulation signal A (t).

From the above experimental results, the power (P _in ) of the optical input signal 16 is modulated by the modulator 18 and the power (P _in ) of the optical output signal 20 having a frequency twice that of the modulation signal A (t). it can be seen _{that out)} and made to output. Therefore, according to the embodiment of the present invention, as can be understood from FIGS. 3 and 4, the frequency of 20 GHz is used as the modulation signal A (t).
When z is given, a modulation frequency of 40 GHz can be obtained as the optical output signal 20.

Next, the optical output signal 20 when a triangular waveform is given as the modulation signal A (t) according to the second embodiment of the present invention will be described with reference to FIG.

In the second embodiment, V _b = mV _pi as in the case of the first embodiment, and in addition to this, for example, a triangular waveform is given as the above-mentioned modulation signal A (t).

FIG. 5 is an explanatory diagram similar to FIG. 3, and shows how the power (P _out ) of the optical output signal 20 is modulated when the modulation signal A (t) having a triangular waveform is used. Shows. In the second embodiment, the modulation signal A (t) and the optical output signal I ₂ (t) when m = 0 and k = 3 are schematically shown.

As can be seen from FIG. 5, the half-wave voltage V
The power (P _out ) of the optical output signal 20 having a modulation frequency of 6 times can be obtained by applying the modulation signal A (t) having an amplitude (V _rf ) three times _pi . Thus, according to the optical signal modulation method of the present invention, as is clear from the experimental results of the first and second embodiments, the frequency of the optical output signal 20 is generally the modulation signal A (t). 2k times the frequency of.

Further, in the first and second embodiments of the present invention, the example of the sine wave and the triangular wave is described, but for example, a rectangular wave may be given as the modulation signal A (t). next,
An application example of the present invention will be described as a third embodiment. In the third embodiment, a method will be described in which the optical output signal (I ₂ ) 20 from the modulator 18 of the first embodiment is again guided to the intensity modulator 40 to be coded and taken out as an optical signal.

FIG. 6 is an explanatory diagram of the third embodiment of the present invention and is a block diagram of the optical signal generator.

Intensity modulator 40 used in the third embodiment
Is an electric field absorption effect modulator using a conventionally known semiconductor, and an example of this device is disclosed in Document III (Document III:
"Optical properties and applications of superlattice structure", Corona Publishing Co., 1988
, P159).

According to the modulation method of the third embodiment of the present invention, the optical input signal (incident light) 16 oscillated from the semiconductor laser that oscillates in single mode is modulated through the branch interferometric modulator 18. It becomes an optical output signal (emitted light) 20, and is further extracted as a signal encoded by the intensity modulator 40 using this emitted light 20. At this time, the signal extracted from the intensity modulator 40 is used as outgoing light 42.

Next, a method of encoding the optical output signal of the intensity modulator 40 will be described with reference to FIGS. 7A, 7B and 7C.

FIG. 7A shows an example of the optical output signal 20 emitted from the branch interferometric modulator 18. Then, the output optical signal 20 is made incident on the intensity modulator 40,
This signal 20 can be converted into a pulse signal encoded as the optical output signal I ₃ (t) of the modulator 40.
At this time, 0, -1V as an external voltage to the intensity modulator 40
Pulse voltage is applied. At this time, the 0V level voltage is "1" and the -1V level voltage is "0". At this time, for example, "0", "1", "1", "0", "1",
It becomes a sequential pulse signal of "0" ((B) of FIG. 7).

At this time, the emitted optical output signal 42 extracted from the intensity modulator 40 is modulated by the intensity modulator 40. Therefore, this optical output signal (I ₃ ) 42
Are code signal pulses “0”, “1”, “1”,
The voltage level is "1" corresponding to "0", "1", and "0"
When the voltage level is "0", the light intensity is practically zero, and when the voltage level is "0", the output light signals are "0", "1", "1", "0", "1". , And "0" signals ((C) of FIG. 7).

In each of the above-described embodiments, an example using the branch interferometric modulator has been described, but the present invention is not limited to this, and for example, a directional coupling modulator may be used.

Although the optical signal generators described in the first and second embodiments are hybrid, they may be monolithically integrated on the same substrate.

Li is used as the substrate material of the branch interferometric modulator.
Although N _b O ₃ is used, the material is not limited to this material and, for example, an InP-based or GaAs-based semiconductor material may be used.

[0056]

As is apparent from the above description, according to the optical signal modulation method of the present invention, the DC bias voltage V
_b and the half-wave voltage V _pi were set to V _b = m × V _pi . Thus, by setting the DC bias voltage to be an integral multiple of the half-wave voltage, it is possible to arbitrarily obtain ultrahigh-speed repetitive modulation pulses that cannot be realized by the conventional laser or the active mode-locked semiconductor laser. Therefore, when the ultra-high speed repetitive pulse is generated as in the conventional case, it is not necessary to apply the high frequency modulation voltage to the semiconductor laser element, and the load on the power source is reduced. In addition, since the burden on the high frequency mounting of the semiconductor laser element is also reduced, the optical signal generator can be manufactured with an inexpensive power supply.

Further, by modulating the optical output signal of the branch interferometric modulator into a pulsed optical output signal coded by using an intensity modulator, high speed optical transmission becomes possible.

[Brief description of drawings]

FIG. 1 is a configuration diagram for explaining an optical signal generator according to an embodiment of the present invention.

FIG. 2 is a branching interferometric modulator used in an embodiment of the present invention.

FIG. 3 is an explanatory diagram of the first embodiment of the present invention and is a diagram mainly for explaining a modulated light output signal waveform when a sine wave is given as a modulation signal A (t).

FIG. 4 is a diagram for explaining a relationship between an optical input signal waveform and an optical output signal waveform.

FIG. 5 is an explanatory diagram of the second embodiment of the present invention and is a diagram for explaining a modulated light output signal waveform when a triangular wave is used as a modulation signal.

FIG. 6 is a diagram for explaining an application example of the present invention as a third embodiment, and is a configuration diagram for explaining an optical signal generator according to the third embodiment.

FIG. 7 is a diagram for explaining a coded output signal waveform for explaining a method for coding an output waveform in the third embodiment of the present invention.

FIG. 8 is a schematic perspective view for explaining the structure of a conventional active mode-locked semiconductor laser.

FIG. 9 is a diagram for explaining an optical output waveform modulated by using a conventional hybrid type interferometric modulator.

[Explanation of symbols]

10: Single-mode semiconductor laser device 12: DC power supply 14: Ground 15: Semiconductor laser 16: Incident light 18: Branching interferometric modulator 20: Emitting light 22: Z-cut LiN _b O ₃ substrate 24, 26: Branching point 27: First waveguide 28: Second waveguide 30: First electrode 32: Second electrode 34: Voltage source 36, 38: Ground 40: Intensity modulator 42: Emitted light

Claims (1)

[Claims] 1. A power of an optical input signal to a branch interference type or directional coupling type modulator is P _in , a power of an optical output signal from the modulator is P _out, and the power is externally applied to the modulator. The modulation input voltage V is a superimposed voltage of the DC bias voltage _Vb and the modulation signal A (t), and the relative value is given by the ratio of the power P _in of the optical input signal and the power P _out of the optical output signal. The voltage at which the light intensity (P _out / P _in ) is minimized is defined as the half-wave voltage V _pi , and the light output signal is modulated by the modulation input voltage V using the relative light intensity to obtain the light output signal. In the generated optical modulation method, the relationship between the DC bias voltage V _b and the half-wave voltage V _pi is V _b = m × V _pi (where m is 0 or a positive or negative integer). An optical signal modulation method characterized by the above.