JP2001251003A - Method and equipment for generating optical pulse - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method and equipment for generating an optical pulse which can generate many pulses per unit time and generate pulses of a uniform waveform in place of the conventional method and equipment which has restrictions on the increase of a phase modulation frequency fm or phase modulation index D when increasing the number of pulses per unit time by increasing fm or D to use frequency components of higher orders. SOLUTION: In the conventional ring laser oscillator two sets of systems, each consisting a phase modulator 6, isolator 7, and Fabry-Perot resonator 8, are connected in series, with the phase modulators in the two systems have the same degree of modulation and same modulation frequency. However, there is a phase difference properly set between these phase modulators to make a change of a combined permeability of these phase modulators constant with the passage of time with respect to both amplitude and phase.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pulse generating method and apparatus for generating a pulse signal in optical communication, and more particularly, to an optical pulse generating method using an active mode-locked laser for generating a homogeneous optical pulse and an optical pulse generating method. Related to the device.

[0002]

2. Description of the Related Art As a method of increasing the amount of transmission information in optical communication, a method of generating light modulated by a millimeter wave is required, and a method using a mode-locked laser is attracting attention as one of the methods. I have. Above all, a method of generating an optical pulse by installing an optical modulator, an isolator, and a Fabry-Perot etalon in a laser resonator is a frequency fp (fp = fp = Km times higher than the phase modulation frequency fm).
Km * fm) can be easily generated, so that it has a promising configuration. In the method of generating a light pulse by installing a phase modulator, an isolator, and a Fabry-Perot etalon (PM-I-FP) in a ring laser oscillator, a pulse wave having a frequency twice that of the above method is generated. A possible occurrence was recently shown and reported by Abedin et al. (Abedin, Onodera, Hyodo, “Higher O.
rder FM Mode Locking for Pulse-Repetition-Rate Enh
ancement in Actively Mode-Locking Lasers: Theory a
nd Experiment, ”IEEE Journal of Quantum Electroni
cs, VOL. 35. NO. 6. JUNE 1999).

The present invention is similar to the above-described method of generating a light pulse by installing a phase modulator, an isolator, and a Fabry-Perot etalon (PM-I-FP) in a ring laser oscillator. Unlike the conventional configuration, a uniform pulse wave could not be obtained, whereas the configuration of the present invention can generate a uniform pulse.

In a conventional ring laser oscillator, a phase modulator, an isolator and a Fabry-Perot etalon (PM)
-I-FP) to generate an optical pulse will be described with reference to FIG. FIG. 4 shows a phase modulator 3, an isolator 4, and a Fabry-Perot etalon 5 (collectively referred to as a ring modulator) in a ring laser 200 having an optical amplifier 1.
The optical pulse is transmitted from the optical amplifier 1 to the coupler 6 to the PM-IF.
This figure shows a configuration in which a periodic optical pulse train emerges from the coupler 6 while circulating in the direction from the P system to the optical amplifier 1.

In general, ωp = 2π * fp and time is t
Ωp * t = 0π, 2π,. . . 0 (zero) when a pulse passes through PM-I-FP at times even and multiples of π
States and ωp * t = π, 3π,. . . It is reported that there is a π (pi) -state in which a pulse passes through PM-I-FP at odd multiples of π and π. When the modulation index is low, the mode-locked laser operates in either the 0-state or the π-state, and as the modulation index increases,
These states can coexist, and a pulse that is twice as large as the case where the modulation index is low can be generated.

In particular, when Km is an even number and has a high modulation index, in the mode in which the 0-state and the π-state coexist, the transmission characteristics of the PM-I-FP system for each state are different. Therefore, it is reported in the above-mentioned literature that the shape of the light pulse changes periodically.

In particular, when Km is an odd number, PM-I
When light passes through the FP, a phase change occurs that changes periodically with time. For this reason, the 0-state and the π-state can be realized respectively, but cannot coexist, and a pulse of Km times the modulation frequency is generated.

Hereinafter, these matters will be described in detail. In general, it is known that the transmittance operator M of a phase modulator is given by the following equation using a Bessel function Jn having a modulation index Δ as a variable.

(Equation 1) In the Fabry-Perot etalon, this higher-order component is selectively transmitted. Therefore, assuming that K is the order and the angular frequency of the primary component is ωp = Kωm, a configuration including a phase modulator and a Fabry-Perot etalon is used. The transmittance operator Mcom is given by the following equation.

(Equation 2) In Equation 2, when K is an even number, Equation 2 becomes a real number, so that only the amplitude is modulated without changing the phase.
Is odd, it can be seen that Equation 2 is a complex number, and both phase and amplitude are modulated.

In particular, when K = 2, as shown in FIG. 5, the result of numerical analysis shows that when the modulation index Δ is small, ωpt = π and 3π show peaks and a sine wave It can be seen that a new peak (0-state) is exhibited at ωpt = 0, 2π as the modulation index Δ increases, although it has a characteristic of oscillating (π-state). When the modulation index Δ is large as described above, it can be seen that the oscillator oscillates at twice the frequency of the π-state, and that a pulse of 2K times the phase modulation frequency fm is generated.

When K = 3, as shown in FIG. 6, the peak positions do not differ due to the difference in the modulation index, and the peaks are obtained at ωpt = 0, 2π, and 4π ( 0-state) and also have peaks at ωpt = π, 3π, and 5π (0-state).
However, these lights undergo different phase modulation as shown in FIG. Here, the amount of phase change (θ0 in the case of the 0-state and θπ in the case of the π-state) of the phase modulator received during the time when light makes one round of the optical waveguide, and the change of the phase received from other than the phase modulator The sum with the quantity (θ1) must be approximately zero to an integral multiple of 2π for lasing to occur. In general, since θ0 is the opposite sign of θπ and is not equal, it does not occur that the 0-state and the π-state simultaneously satisfy the condition of laser oscillation except in special cases. Therefore, except for a special case, these two sets of states do not appear at the same time. Therefore, also in this case, it can be seen that a pulse K times the phase modulation frequency fm is generated.

When K = 4, as shown in FIG.
When the modulation index Δ is small, ωpt = 0, 2π, 4
The peaks are shown at π and 6π (0-state), and as the modulation index Δ increases, ωpt = π, 3π, 5π,
And a new peak at 7π (π-state). Therefore, in this case, the phase modulation frequency f
Although there is a possibility that a pulse of 2K times m is generated, it can be seen that the waveform is not uniform.

[0012]

In the conventional method and apparatus for generating an optical pulse, a pulse generated in a unit time by using a higher-order frequency component by increasing the phase modulation frequency fm or increasing the modulation index Δ. Although the number is increasing, there is a limit in increasing the phase modulation frequency fm and the modulation index Δ due to physical limitations of the phase modulator. When K is an even number, the number of pulses generated per unit time by increasing the modulation index becomes 2K times the phase modulation frequency fm, but a pulse waveform having a uniform amplitude cannot be obtained. When K is an odd number, a pulse waveform having a uniform amplitude can be obtained, but the number of pulses generated per unit time does not become 2K times the phase modulation frequency fm.

The present invention has been proposed in view of the above,
It is an object of the present invention to provide an optical pulse generating method and an apparatus for generating a pulse having a large number of pulses per unit time and a uniform waveform.

[0014]

In order to achieve the above object, the invention according to claim 1 relates to a method for generating an optical pulse, and when generating a pulse train using an active mode-locked laser, a first method is used. The light is modulated by the modulating means of
Selecting the harmonic component of the modulation signal by the selecting means, and modulating the light by the second modulating means and setting the second selecting means to select the harmonic having the same frequency as the harmonic component by the first selecting means. And the step of selecting the components is performed substantially continuously.

According to a second aspect of the present invention, in addition to the configuration of the first aspect of the present invention, the first modulating means and the second modulating means have the same frequency but are predetermined. The light is modulated by a signal having a phase difference.

Further, the invention according to claim 3 relates to an optical pulse generating device, and comprises an optical amplifier, an optical waveguide, and a first optical pulse generator.
, A first harmonic component selection means of the first modulation signal, a second modulation means, and a second modulation signal of the second modulation signal for selecting the same frequency as the first harmonic component selection means. And a harmonic component selecting means.

According to a fourth aspect of the present invention, in addition to the configuration of the third aspect of the present invention, a first signal applying means for applying to the first modulating means and a second modulating means are provided. And a means for giving a phase difference between the first modulation signal applied to the first modulation means and the modulation signal applied to the second modulation means. It is characterized by that.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, two sets of PM-I-FP systems are connected in series to make the modulation index and the modulation frequency in each phase modulator the same, but by setting the phase difference appropriately. The temporal change of their combined transmittance is made uniform in amplitude and phase. Hereinafter, the principle of the present invention will be described first, and embodiments will be described in detail with reference to the drawings. Note that in the drawings showing the configuration of the device, the same components or components having similar functions are denoted by the same reference numerals.

First, the principle of the present invention will be described.
FIG. 1 is a block diagram showing a preferred configuration of the present invention. FIG. 1 shows a first phase modulator 3, a first isolator 4, and a first Fabry-Perot etalon 5 (hereinafter collectively referred to as a first PM-I) in a ring laser 100 having an optical amplifier 1. -FP system), a second phase modulator 6, a second isolator 7, and a second Fabry-Perot etalon 8 (hereinafter collectively referred to as a second PM-IF).
P system) is inserted, and the optical pulse is transmitted from the optical amplifier 1 to the first PM-I-FP system to the second PM.
−I-FP system → A configuration in which a periodic optical pulse train emerges from the coupler 2 while circulating in the direction of the optical amplifier 1. Here, the first PM-I-FP system and the second PM-I-FP
The systems are of approximately the same performance.

Next, the method proposed by the present invention will be described with reference to FIG. From the first PM-I-FP system to the second PM-IF
The time required for the light pulse to propagate to the P system is defined as Δt. Further, the first PM-I-FP system modulated signal is

(Equation 3) And the modulation signal of the second PM-I-FP system is

(Equation 4) And here

(Equation 5) And n is an integer. Further, the angular frequency of the Fabry-Perot etalon with respect to the free spectral interval fp is ω
When p (= 2πfp), fp is made equal to an integral multiple of the modulation frequency, so K is a natural number,

(Equation 6) And

At this time, as described above, the first
Of the PM-I-FP system

(Equation 7) And the second PM-I-FP at time t + Δt
The state of the system is

(Equation 8) Is determined by Here, the difference between Equation 8 and Equation 7 is obtained by modulo 2π.

(Equation 9) It is. Therefore, a certain light pulse is generated by the first PM-I-
When passing through the FP system in the 0-state (π-state), the second
In the π-state (0-state). Further, each PM-I-FP system has π / ωp
Since the 0-state and the π-state are alternately changed at time intervals, if an optical pulse having a time interval of an optical pulse train of π / ωp is incident on the first PM-I-FP system, each of the light pulses will be changed. The light pulse is modulated once in the 0-state (π-state) in the first PM-I-FP system, and is further modulated once in the π-state (0-state) in the second PM-I-FP system. Therefore, when these modulations are combined, every light pulse receives the same amount of modulation. This means that according to the method of FIG. 1, an optical pulse train having a repetition period of ωp / π can be generated, and the waveform is uniform. Also,

(Equation 10) Therefore, the generated optical pulse train repetition period is 2 Kf
At p, this is twice the conventional method.

In order to explain the above description in FIG. 2, K = 2,
An example of a signal and transmittance is shown with Δ = 3, n = 0, and ωmΔt = 0.6. The horizontal axis is ωm × t. FIG. 2A shows a modulated signal, in which a solid line indicates a first PM-I-FP type modulated signal and a broken line indicates a second PM-I-FP type modulated signal. FIG. 2B shows the change in transmittance with time, and the solid line indicates the first P
The time change of the transmittance of the MI-FP system, the broken line indicates the second PM
-It shows the time change of the transmittance of the I-FP system. FIG. 2C shows a change over time in the combined transmittance of the first PM-I-FP system and the second PM-I-FP system.

Therefore, the modulation signal from the signal generator 10 shown in FIG. 1 is applied to the phase modulator 3, and the modulation signal from the signal generator 10 is given a phase difference shown in Expression 5. Thereafter, by applying the pulse to the phase modulator 6, a uniform pulse is obtained.

Next, the result of a simulation by numerical calculation is shown in FIG. FIG. 3A shows that K = 2 and fm = 10.
GHz, D = 3, Fabry-Perot etalon FSR
(FreeSpectral Range) = 1st when 20GHz
The solid line shows the transmittance of the PM-I-FP system of FIG.
A second PM-IF configured similarly to the MI-FP system
The transmittance of the P system is indicated by a dashed line. FIG. 3 (b)
Shows the combined transmittances of them, and FIG. 3C shows the output from the coupler. Here, the phase difference between the phase modulation signals applied to the two sets of phase modulators is in accordance with Equation 4. As a result, their combined transmittances are as shown in FIG.
Since the transmittance curves of the 0-state and the π-state have the same shape, and the modulation of the transmitted light pulse in both states is equal, a uniform light pulse is generated as shown in FIG.

Next, regarding the third harmonic of the modulation frequency,
A case using the configuration of FIG. 1 will be described. First, FIG.
In the conventional pulse generator shown in FIG. 7, the pulse amplitude is uniform as shown in FIG. 6, but the pulse phase is not uniform as shown in FIG. Next, in the case of the configuration of FIG. 1 of the present invention, using the phase difference φ shown in Expression 3,
By modulating the first and second PM-I-FP systems, the pulsed light undergoes phase modulation as a combination of these. Since this is a modulation different from that of the conventional pulse generator shown in FIG. 4, it is necessary to readjust the waveguide to adjust the phase so as to satisfy the above condition. Laser oscillation can be easily performed. Therefore, in the combined transmittance of the two sets of PM-I-FP systems, the 0-state and the π-state coexist, and the 2K of the phase modulation frequency fm having a uniform amplitude is obtained.
It can be seen that double pulses are generated.

Although not shown in the figure, it has been found that the present invention is effective even when the pulse width is reduced and a short pulse is obtained at the following modulation index. In a simulation by numerical calculation under the same conditions as those for obtaining FIG. 3, the modulation index Δ is 1.6 <Δ <when K = 2.
3.1, when K = 3, 1.8 <Δ, when K = 4,
It has been found that short pulses are obtained for 2.3 <Δ <2.5 and 3.5 <Δ <5.8.

Although K = 2, 3, and 4 have been described above, it can be easily understood that similar results can be obtained for higher-order K.

[0028]

Since the present invention has the above-described configuration,
The following effects can be obtained. According to the first and second aspects of the present invention, since the method is disclosed, it is possible to generate a large number of pulses per unit time and to generate a pulse having a uniform waveform in amplitude and phase.

In the inventions according to the third and fourth aspects, since the configuration of the device is disclosed, it is possible to generate a large number of pulses per unit time and to generate a pulse having a uniform waveform in amplitude and phase. Now you can.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a preferred configuration of the present invention.

FIGS. 2A and 2B are diagrams showing transmittance, wherein FIG. 2A is a modulation signal, a solid line is a modulation signal of a first PM-I-FP system, and a broken line is a second P-I-FP system;
3 shows a modulation signal of an MI-FP system. FIG. 2 (b) shows a time change of the transmittance, a solid line shows a time change of the transmittance of the first PM-I-FP system, and a broken line shows a time change of the transmittance of the second PM-I-FP system. FIG. 2C shows a change over time in the combined transmittance of the first PM-I-FP system and the second PM-I-FP system.

3A and 3B are diagrams showing the results of a simulation by numerical calculation, in which FIG. 3A shows the transmittance of a first PM-I-FP system as a solid line and the transmittance of a second PM-I-FP system as a wavy line. ,
(B) is a diagram showing the combined transmittance, and (c) is a diagram showing the output from the coupler.

FIG. 4 is a diagram illustrating a configuration of a device that generates an optical pulse by installing an isolator, a Fabry-Perot etalon (PM-I-FP), and a phase modulator in a conventional ring laser oscillator.

FIG. 5 is a diagram illustrating transmission characteristics of a second harmonic of a modulation frequency by numerical analysis.

FIG. 6 is a diagram showing transmission characteristics of a third harmonic of a modulation frequency by numerical analysis.

FIG. 7 is a diagram illustrating a phase characteristic of a third harmonic of a modulation frequency by numerical analysis.

FIG. 8 is a diagram illustrating transmission characteristics of a fourth harmonic of a modulation frequency by numerical analysis.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Optical amplifier 2 Coupler 3, 6 Phase modulator 4, 7 Isolator 5, 8 Fabry-Perot etalon 10 Signal generator 11 Phase shifter 20 Optical waveguide 100 Ring laser of the present invention 200 Conventional ring laser

Claims (4)

[Claims] When generating a pulse train using an active mode-locked laser, a procedure of modulating light by a first modulation means and selecting a harmonic component of a modulation signal by a first selection means is provided. Modulating the light with the modulating means and selecting the harmonic component having the same frequency as the harmonic component by the first selecting means by the second selecting means is performed substantially continuously. A light pulse generation method according to 2. The method according to claim 1, wherein the first and second modulating means modulate the light with a signal having the same frequency but having a predetermined phase difference. An optical pulse generation method according to the above. 3. An optical amplifier, an optical waveguide, a first modulator, a harmonic component selector of the first modulation signal, a second modulator, and a harmonic of the first modulation signal. An optical pulse generator comprising: a harmonic component selection unit for a second modulation signal that selects the same frequency as the component selection unit. 4. The optical pulse generating apparatus according to claim 3, further comprising: a first signal applying means for applying to the first modulating means; and a second signal applying means for applying to the second modulating means. , A first modulation signal applied to the first modulation means and a second modulation signal.
Means for providing a phase difference between the modulation signal applied to the modulation means and the optical pulse generation apparatus.