JP3442264B2 - High-speed optical pulse train generator - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-speed optical pulse train generator for generating an optical pulse train of high repetition frequency used for high-speed optical communication and high-speed optical measurement.

[0002]

2. Description of the Related Art FIG. 8 shows a conventional high-speed optical pulse train generator.
A configuration example of is shown. Conventional high-speed optical pulse train generators
Repetition frequency f in pulse train generation means 1 ₀Optical pulse train
Is generated and is input to the optical pulse train multiplication means 2 for M multiplication.
Double (M is an integer greater than or equal to 2) and repeat frequency M · f₀of
This is a configuration for generating a high-speed optical pulse train.

FIG. 9 shows a configuration example of the conventional optical pulse train multiplication means 2. The optical pulse train multiplication means 2 shown here is an optical pulse multiplex circuit (hereinafter referred to as “PLC optical pulse multiplex circuit”) using a planar type silica optical waveguide circuit (PLC) (reference: S. Kawanishi et al.,). 100 Gbit / s,
50km and nonrepeated optical transmission employi
ng all-optical multiplexing and PLL timing extruct
ion ", Electron. Lett., vol.10, no.12, pp.1075-107
6, 1993, H. Takara et al., "100 Gbit / s optical si
gnal eye-diagram measurement with optical sampling
using organic nonlinear optical crystal ", Electro
n. Lett., vol.32, no.24, pp.2256-2257, 1996).

The PLC optical pulse multiplexing circuit of FIG. 9 (a) is formed by forming two 1 × N couplers 4 on a quartz substrate 3 and N optical waveguides 5 having different delay times, which are coupled between them. is there. The optical pulse train having the repetition frequency f ₀ is branched into N channels and then multiplexed by giving different delay times to the respective channels, whereby a high-speed optical pulse train having the repetition frequency N · f ₀ is generated.

The PLC optical pulse multiplex circuit shown in FIG. 9 (b) has a Mach-Zehnder interferometer composed of a 2 × 2 coupler 6 and an optical waveguide 5 connected on a quartz substrate 3 in n stages. In this configuration, 2 ⁿ different optical paths with different delay times are formed from the input end to the output end. The optical pulse train having the repetition frequency f ₀ is separated into 2 ⁿ , and propagated at different delay times and combined to generate a high-speed optical pulse train having the repetition frequency 2 ⁿ · f ₀ .

In the PLC optical pulse multiplexing circuit shown in FIGS. 9 (a) and 9 (b), the basic repetition frequency f ₀ of 4
FIG. 10 shows a process in which a double high speed optical pulse train is generated (N = 4 or n = 2).

[0007]

The optical pulse train multiplying means (PCL optical pulse multiplex circuit of FIG. 9) in the conventional high-speed optical pulse train generator has a limit in the accuracy of the delay time of each optical waveguide due to the limitation in circuit fabrication. There is a problem (about 0.1 ps), and there is a problem that the light pulse intervals are likely to be unequal intervals as shown in FIG.

Further, the optical couplers (4, 6) forming the PLC optical pulse multiplexing circuit have an excess loss of 3 dB or more. Further, it is difficult to make the optical pulse intensities of the N (2 ⁿ ) branched channels exactly equal, and there is a problem that the optical pulse intensities after multiplexing are likely to vary as shown in FIG.

The present invention is a high-speed optical pulse train generator capable of varying the repetitive frequency at several tens of GHz or higher and capable of easily and stably generating a high-speed optical pulse train having a constant optical pulse interval and constant optical pulse intensity. The purpose is to provide.

[0010]

The high-speed optical pulse train generating device of the present invention is characterized by using dispersion giving means instead of the optical pulse train multiplying means using the conventional PLC optical pulse multiplexing circuit shown in FIG. To do.

Focusing on the spectral component of the optical pulse train of the repetition frequency f ₀ given the dispersion by the dispersion providing means at a certain time, two or more different optical frequency components in the optical pulse are included depending on the magnitude of the dispersion. Be done. If the optical frequency difference between the different optical frequency components is M times f ₀ ,
A high-speed optical pulse train having a repetition frequency M · f ₀ can be obtained at the output of the dispersion imparting means.

[0012]

DETAILED DESCRIPTION OF THE INVENTION

(Basic Structure and Operating Principle) FIG. 1 shows the basic structure of a high-speed optical pulse train generator of the present invention.

In the high-speed optical pulse train generating device of the present invention, the optical pulse train generating means 1 generates an optical pulse train having a basic repetition frequency f ₀ and inputs the optical pulse train to the dispersion imparting means 7 to generate a repetition frequency M · f ₀ . Generate a fast optical pulse train. As the optical pulse train generating means 1, a mode-locking fiber laser, a mode-locking semiconductor laser, a gain switching semiconductor laser or the like can be used. The dispersion imparting means 7 is an optical fiber or a fiber grating (reference: KOHill, "Aperiodic Distributed-Parame").
ter Waveguide for Integrated Optics ", Appl.Opt., vo
l.13, no.8, pp.1853-1856, 1974), dispersion compensator using PCL (reference: K. Takiguchi et al., "Planer Lig
htwave Circuit Dispersion Equaliser ", J. Lightwave
Technol., Vol.14, no.9, pp.2001-2003, 1996) and others can be used.

Now, the principle of operation in which the dispersion imparting means 7 converts the optical pulse train having the basic repetition frequency f ₀ into the high-speed optical pulse train having the repetition frequency M · f ₀ will be described with reference to FIG.

FIG. 2A shows an optical pulse train having a basic repetition frequency f ₀ (period T = 1 / f ₀ ) where the horizontal axis represents time and the vertical axis represents light intensity. In FIG. 2B, the horizontal axis represents time, the vertical axis represents optical frequency, and the spectrum width Δν ₀ of the optical pulse train is shown. 2C and 2D show a state in which the horizontal axis represents time and the vertical axis represents optical frequency, and the optical pulse train after dispersion is given by the dispersion giving means 7 is chirping. Figure 2 (e) shows
The horizontal axis represents time and the vertical axis represents light intensity, and shows the optical pulse train of the repetition frequency M · f ₀ output from the dispersion imparting means 7.

When the optical pulse trains shown in FIGS. 2 (a) and 2 (b) propagate through the dispersion imparting means 7, the dispersion characteristic (propagation speed varies depending on the optical frequency) of the dispersion imparting means 7 causes
As shown in (c) and (d), it becomes a chirp optical pulse (optical pulse whose optical frequency component changes with time) train. In FIG. 2C, A (t ₁ , ν _A ), B (t ₁ + T, ν _B ), and the absolute value of the slope of the straight line | dν / dt | is the chirping amount (optical frequency change amount / time). Change amount). If this chirping amount becomes sufficiently small, the spectra between adjacent optical pulses will temporally overlap. For example, in FIG. 2C, time t ₁
The spectrum at has two optical frequency components ν _A and ν _B.

Optical frequency interval of ν _A and ν _B Δν = | ν
The magnitude of _A −ν _B | depends only on the dispersion characteristic of the dispersion imparting means 7, and is the same at any time from −∞ to + ∞ as t ₁ in FIG. 2 (c). This spectrum thus represents an optical pulse train with a repetition frequency Δν. In particular, when Δν = M · f ₀ (M is an integer of 2 or more) is satisfied, a stable optical pulse train having an integral multiple of the basic repetition frequency f ₀ is obtained. At this time, the condition for satisfying | dν / dt | is such that, where N is a natural number, | dν / dt | = Δν / Δt = M · f ₀ / (N · T) = M · f ₀ ² / N (1 ). 2 (c) shows the case where N = 1, and FIG. 2 (d) shows the case of N = 1.
= 2. The same applies when N = 3 or more.

As described above, the high-speed optical pulse train generator of the present invention utilizes the temporal overlap of the optical frequency components of the optical pulse trains generated from the same light source, so that the optical pulse train generating means 1 has good coherence. By using, it is possible to obtain a high-speed optical pulse train with a constant optical pulse interval and constant optical pulse intensity, as shown in FIG. 2 (e).

FIG. 3 shows that the basic repetition frequency f ₀ = 10 GH
The theoretical calculation result of the high-speed optical pulse train generation when z, M = 5, and N = 1 is shown. In this calculation, the chirping amount is | dν / dt | = 5 × 10 GHz / (1 × 100 ps) = 0.5 Hz
/ S. As shown in Fig. 3, basic repetition frequency 10 GHz
From the optical pulse train (Fig. 3 (a)) of 5 times the repetition frequency 50
It can be seen that a high-speed optical pulse train of GHz (Fig. 3 (b)) can be obtained.

FIG. 4 shows the basic repetition frequency f ₀ = 10 GH
The experimental results when z, M = 5, and N = 1 are shown. A mode-locked fiber laser is used as the optical pulse train generation means 1,
An optical fiber was used as the dispersion imparting means 7, and the chirping amount used in the theoretical calculation was applied to the optical pulse train. Similar to the calculation result, a high-speed optical pulse train of 50 GHz was obtained, and the effectiveness of the present invention was confirmed.

(Condition of Spectral Width Δν ₀ of Optical Pulse Train) By the way, when the chirping amount is large or the spectral width Δν ₀ of the optical pulse train is small, the density of the optical frequency component varies depending on the time. For example, Figure 5 (a)
In the example shown in (2), it cannot be said that the optical frequency components have two or more at all times. Therefore, as shown in FIG. 5B, the optical pulse intensity of the high-speed optical pulse train having the repetition frequency M · f ₀ fluctuates at the repetition frequency of f ₀ . In addition,
Even in this case, as long as the magnitude of the fluctuation of the optical pulse intensity does not matter, it can be applied to applications such as optical measurement.

However, in the field of optical communication, a high-speed optical pulse train without fluctuations in optical pulse intensity is desirable. In order to obtain this high-speed optical pulse train without fluctuations in optical pulse intensity, the chirping amount should be sufficiently small so that the optical pulse train has two or more optical frequency components at all times, or the spectral width Δν ₀ of the optical pulse train should be It should be set large enough.
FIG. 5C shows the case of the maximum chirping amount having two or more optical frequency components at all times. In the case of this maximum chirping amount, the condition for obtaining a high-speed optical pulse train without optical pulse intensity fluctuation in the case of N = 1 in the equation (1) is Δν ₀ / (2 · T) = Δν ₀ · f ₀ / 2 ≧ | dν / dt | =
M · f ₀ ² / N. From this, Δν ₀ ≧ 2M · f ₀ (2) is obtained as the condition for the spectral width Δν ₀ of the optical pulse train. This equation (2) means that the maximum repetition frequency of a high-speed optical pulse train without fluctuations in optical pulse intensity is limited by the spectral width Δν ₀ of the basic optical pulse train.

Therefore, the expressions (1) and (2) are the conditions for obtaining a high-speed optical pulse train without fluctuations in the optical pulse intensity. For example, if an optical pulse train with a spectral width Δν ₀ = 1 THz and a basic repetition frequency f ₀ = 10 GHz is used,
In the range of 500 Gbit / s, it is possible to convert into a high-speed optical pulse train with no fluctuation of optical pulse intensity at any repetition frequency that is an integral multiple of 10 GHz.

(Allowable range of chirping amount) From the optical pulse train having the basic repetition frequency f ₀ , the repetition frequency M · f ₀
The chirping amount to obtain the ideal high-speed optical pulse train of
As shown in the equation (1), | dν / dt | = M · f ₀ ² / N. The chirping amount, as shown in FIG. 6, is represented by the time width Delta] t ₀ and spectral width .DELTA..nu ₀ of the light pulses chirped, | dν / dt | = Δν 0 / Δt 0 becomes, Δt ₀ = _{Δν 0}・ N / M · f ₀ ² (3)

On the other hand, when variations in the optical pulse intensity and the optical pulse interval of the high-speed optical pulse train are allowed to some extent,
The amount of chirping to be given may be adjusted within an allowable range.
The time width Δt of the optical pulse corresponding to this allowable range can be expressed as Δt = Δν ₀ / | dν / dt | (4). As the allowable range of Δt, Δt
_If it is possible to shift from ₀ by a time corresponding to α times the period 1 / (M · f ₀ ) of the high-speed optical pulse train having the repetition frequency M · f ₀ , Δt ₀ −α / (M · f ₀ ² ) ≦ Δt ≦ Δt ₀ + α / (M ・
f ₀ ² ). Therefore, according to the equations (3) and (4), Δν ₀ · N / M · f ₀ ² −α / (M · f ₀ ) ≦ Δν ₀ / | dν / dt | ≦ Δν ₀ · N / M · f ₀ ^{If a} chirping amount | dν / dt | that satisfies ² + α / (M · f ₀ ) ... (5) is given, a high-speed optical pulse train with a repetition frequency M · f ₀ can be obtained.

Here, when a simulation was performed to specify the lower limit and the upper limit of the allowable range of the chirping amount, the output waveform of the optical pulse train shown in FIG. 7 was obtained, and the upper limit of α was 0.7 (70%). I got the result. That is, in order to suppress the variations in the light pulse intensity and the light pulse interval to be small,
It is necessary to set the amount of chirping to be given based on Eq. (1), but if the conditions for changes in optical pulse intensity and optical pulse interval are loose, set the amount of chirping within the allowable range based on Eq. (5). Just set it.

Further, by maintaining the polarization of the optical pulse train generating means 1 and the dispersion imparting means 7, it is possible to generate a stable high-speed optical pulse train independent of polarization.

[0028]

As described above, the high-speed optical pulse train generating device of the present invention has the basic repetition frequency f ₀ and the spectral width Δ of the optical pulse train for generating the optical pulse train generating means.
ν ₀ , chirping amount given by the dispersion imparting means | dν / d
According to t |, it is possible to easily generate a stable high-speed optical pulse train with little variation in optical pulse intensity and optical pulse interval at an arbitrary repetition frequency.

[Brief description of drawings]

FIG. 1 is a block diagram showing the basic configuration of a high-speed optical pulse train generator of the present invention.

FIG. 2 is a diagram for explaining the operating principle of the dispersion imparting means 7.

FIG. 3 is a diagram showing a theoretical calculation result of high-speed optical pulse train generation.

FIG. 4 is a diagram showing experimental results.

FIG. 5 is a diagram illustrating a condition of a spectral width Δν ₀ of an optical pulse train.

FIG. 6 is a diagram illustrating a relationship between a time width Δt ₀ of a chirped optical pulse train and a spectral width Δν ₀ .

FIG. 7 is a diagram showing an output waveform of an optical pulse train corresponding to an allowable range of a chirping amount.

FIG. 8 is a block diagram showing a configuration example of a conventional high-speed optical pulse train generator.

FIG. 9 is a diagram showing a configuration example of a conventional optical pulse train multiplier 2;

FIG. 10 is a diagram showing an operation example of a PLC optical pulse multiplexing circuit.

[Explanation of symbols]

1 Optical pulse train generation means 2 Optical pulse train multiplication means 3 Quartz substrate 4 1 × N coupler 5 Optical waveguide 6 2x2 coupler 7 Dispersion giving means

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-9-236834 (JP, A) JP-A-8-29814 (JP, A) JP-A-11-72757 (JP, A) JP-A-8- 171103 (JP, A) JP 7-193534 (JP, A) JP 63-249827 (JP, A) JP 8-279646 (JP, A) JP 8-307391 (JP, A) JP-A-10-229364 (JP, A) JP-A-8-328052 (JP, A) JP-A-64-67043 (JP, A) JP-A-8-234249 (JP, A) JP-A-7-281217 (JP, A) (58) Fields surveyed (Int.Cl. ⁷ , DB name) G02B 6/00-6/43 H04B 10/00-10/28 H04J 14/00-14/08 G02B 27/00- 27/64

Claims (4)

(57) [Claims] And 1. A basic repetitive optical pulse train generating means for generating an optical pulse train of frequency f _0, enter the optical pulse train of the fundamental repetition frequency f _0, the repetition frequency M · f ₀ of the (M is an integer of 2 or more) In a high-speed optical pulse train generation device including an optical pulse train multiplier for generating a high-speed optical pulse train, ν is an optical frequency, t is time, T is a light pulse repetition period, and N is a natural number, as the optical pulse train multiplier. At this time, the chirping amount | dν / dt | given to the optical pulse train of the basic repetition frequency f ₀ is | dν / dt | = M · f ₀ / (N · T) = M · f ₀ ² / N A high-speed optical pulse train generator characterized by using dispersion imparting means. 2. The high-speed optical pulse train generating device according to claim 1, wherein the spectral width Δν ₀ of the optical pulse train generated by the optical pulse train generating means satisfies Δν ₀ ≧ 2M · f _0. Pulse train generator. 3. An optical pulse train of basic repetition frequency f ₀
The optical pulse train generating means to be generated and the optical pulse train having the basic repetition frequency f ₀ are input and repeated.
High-speed optical power of return frequency M · f ₀ (M is an integer of 2 or more)
High-speed light having optical pulse train multiplication means for generating a loose train
In the pulse train generator , ν is an optical frequency, and t is an optical pulse train multiplying means.
, T is the light pulse repetition period, and N is a natural number.
The optical pulse train of the basic repetition frequency f ₀
The chirping amount | dν / dt | is defined as Δν ₀ is the spectrum width of the optical pulse train, and the coefficient α is in the range of 0 <α ≦ 0.7.
When set, Δν ₀ · N / M · f ₀ ² −α / (M · f ₀ ) ≦ Δν ₀ / | dν
A high-speed optical pulse train generator characterized by using a dispersion imparting means such that / dt | ≦ Δν ₀ · N / M · f ₀ ² + α / (M · f ₀ ). In high-speed optical pulse train generator according to any one of claims 4] claims 1 to 3, the optical pulse train generating means and dispersion providing means high-speed optical pulse train generator, characterized in that a polarization maintaining.