JP3478985B2 - Optical pulse compressor - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultrashort optical pulse generation technique used for optical communication or an optical measuring instrument.
The present invention relates to an optical pulse compressor that converts an input optical pulse having a wide time width into an optical pulse having an extremely narrow time width.

[0002]

2. Description of the Related Art There are various optical pulse compression techniques. Here, a method using a dispersion-decreasing fiber whose anomalous dispersion value decreases from the entrance end to the exit end will be described. A pulse compressor using dispersion-decreasing fiber
It has the advantage of being able to efficiently generate a transform-limited optical pulse with an extremely narrow pulse width (Reference 1: S.
V. Chernikov et al., JOSA B, vol.8, pp.1633-1641, A
ug.1991, Reference 2: M. Nakazawa et al., Electron.
Lett., Vol.30, pp.2038-2040, Nov.1994).

The principle of optical pulse compression by dispersion-decreasing fiber is based on adiabatic optical soliton propagation, which is explained below. An optical soliton is a special optical pulse existing in an optical fiber, and a so-called N = 1 optical soliton is defined as follows. The amplitude a (t) of the N = 1 optical soliton having the pulse width t _s and the energy E _s is a (t) = (E _s / (2t _s )) ^1/2 sech (t / t _s ) exp (iz / (2L _D )) is expressed as (1). Where t is time, z is distance, and L _D = (2
πct _s ² ) / (Dλ ² ) is the dispersion distance, λ is the wavelength of light, c is the speed of light, and D is the dispersion value of the fiber. The characteristics of this optical soliton, the dispersion value D of the fiber, and the nonlinear refractive index constant n
₂ has a relationship represented by t _s = (λ ³ AD) / (8π ² cn ₂ E _s ) (2). Where A is the effective area of the fiber core.

A normal light pulse, when propagating through a fiber,
Waveforms are degraded by dispersion and nonlinear effects. But,
The N = 1 optical soliton propagates while maintaining the waveform expressed by the equation (1), and has an excellent feature that waveform deterioration does not occur. There are optical solitons other than the N = 1 optical soliton (for example, N = 2, 3, ...), and the waveform of such an optical soliton changes periodically during propagation. In this description, the optical pulse is limited to the N = 1 optical soliton or the optical pulse closest to the N = 1 optical soliton, and such an optical pulse is hereinafter referred to as an “optical soliton”.

The soliton period Z ₀ of the optical soliton is defined as Z ₀ = (π ² t _s ² c) / (λ ² D) (3). The soliton period has a special meaning when the properties (eg, dispersion) of the fiber through which the optical soliton propagates vary slightly with the length of the fiber. If this slight change occurs at a distance almost equal to the soliton period and the energy of the optical soliton is constant, the pulse width of the optical soliton is automatically adjusted so that equation (1) is always satisfied. Change. Such propagation is called adiabatic soliton propagation, and the occurrence of completely adiabatic propagation means that the waveform change occurs while the energy of the optical soliton is preserved.

The principle of the optical pulse compressor using adiabatic soliton propagation will be described. When an optical soliton propagates adiabatically in a dispersion-decreasing fiber in which the anomalous dispersion value decreases from the input end to the output end, the pulse width of the optical soliton is proportional to the dispersion value according to Eq. (1). Thins. If more complete adiabatic compression occurs,
All the energy of the input optical soliton is absorbed by the compressed optical soliton, and an ideal ultrashort optical soliton with extremely narrow pulse width is generated.

Reference 2 above uses a dispersion-decreasing fiber having a length of 1 km, a dispersion value of +10 ps / nm / km on the input side, and a dispersion value of almost 0 on the output side, and a repetition frequency of 10 G.
An example in which an optical pulse train of Hz, pulse width 3 ps, wavelength 1550 nm is compressed has been reported. Compressed pulse width is 170f
s, and the spectrum width is 13.8 nm. The product of pulse width and bandwidth of 0.30 indicates that transform-limited optical solitons are generated.

However, the conventional dispersion-decreasing fiber has some problems. First, it is difficult to do complete adiabatic pulse compression. Non-adiabatic compression occurs due to dispersion or effective area non-uniformity, loss, higher order dispersion effects in dispersion-decreasing fibers, or the distance over which dispersion value D changes is too short. When non-adiabatic compression occurs, light components not included in solitons are generated. Further, since it is difficult to make the input optical pulse an ideal optical soliton, there are optical components that are not included in the optical soliton compressed in the input optical pulse. An optical component that is not included in the optical soliton is called a linear dispersion wave, and appears as a pedestal component that forms the bottom of the compressed optical soliton waveform. This pedestal component appears as a narrow-band spectral spike component existing at the same wavelength as the wavelength of the input optical pulse in the spectral region.

The conventional dispersion-decreasing fiber has a problem that the input wavelength range is limited. Since the dispersion-decreasing fiber is usually composed of dispersion-shifted fiber, the wavelength dependence of dispersion (dispersion slope) is about +0.07.
It is ps / nm ² / km. Therefore, the dispersion value on the output side of the dispersion-decreasing fiber is optimum only at the wavelength used. When the input wavelength deviates from the optimum wavelength, a phenomenon occurs in which the spectrum of the compressed optical pulse spreads asymmetrically, and the wavelength at the peak position of the spectrum occurs in a region different from the input wavelength.

The above-mentioned problems cause many problems in application. For example, when a compressed optical pulse is used in ultra-high-speed time-division multiplexed optical communication, the pedestal component causes noise and intersymbol interference, which deteriorates the transmission error rate of the system. Further, in the case where an optical pulse train of multiple wavelengths and a narrow band is simultaneously generated from the wide spectrum of the compressed optical pulse, it is necessary to avoid the wavelength region where the spectrum spike exists. Furthermore, it is difficult to generate a wavelength-tunable ultrashort optical pulse.

[0011]

In the conventional optical pulse compressor using the dispersion-decreasing fiber, it is difficult to perform optical pulse compression so that the pedestal component and the spectrum spike component are not generated. Further, since the dispersion-decreasing fiber composed of the ordinary dispersion-shifted fiber (DSF) has a large dispersion slope, the waveform of the compressed optical pulse changes when the input wavelength is varied. Further, there is a problem that the peak wavelength of the spectrum of the compressed optical pulse occurs in a wavelength region different from the central wavelength of the input optical pulse.

An object of the present invention is to provide an optical pulse compressor having a small size, a low cost, a simple structure, an extremely small pedestal component, and an ultrashort optical pulse generated in a wide input wavelength range.

[0013]

An optical pulse compressor according to claim 1, wherein a dispersion-decreasing fiber having a small anomalous dispersion value from the input side to the output side and a small dispersion slope on the output side, and an optical directional coupling. Loop compensator, an optical phase compensator, a dispersive medium, and a soliton propagation fiber having an anomalous dispersion value and a small dispersion slope, and a dispersion compensating medium for compensating for dispersion in the nonlinear loop mirror. The dispersion-decreasing fiber compresses the pulse width of the input optical pulse, and the nonlinear loop mirror and the dispersion compensating medium generate an ultrashort optical pulse with the pedestal component removed. Further, by reducing the dispersion slope of the output side of the dispersion-decreasing fiber and the soliton propagation fiber, optical pulse compression can be performed in a wide wavelength range.

An optical pulse compressor according to a second aspect is the optical pulse compressor according to the first aspect, wherein an optical directional coupler forming a nonlinear loop mirror, a dispersive medium, and a soliton propagation fiber are polarized. It is a conservative type and does not have an optical phase compensator. Since the polarization-preserving type automatically optimizes the operating point of the nonlinear loop mirror, stable operation is possible without using an optical phase compensator. Also, since the optical phase compensator is unnecessary, the number of parts is reduced,
It is possible to realize an optical pulse compressor that is cheaper and has a simple structure. Further, the polarization direction of the nonlinear loop mirror is fixed, so that the stability is improved.

An optical pulse compressor according to a third aspect is the optical pulse compressor according to the first and second aspects, wherein the dispersive medium and the dispersion compensating medium are optical fibers. This allows
Since the main part of the optical pulse compressor is an optical fiber, it is possible to realize an optical pulse compressor of low cost, small size, and simple structure.

The optical pulse compressor according to claim 4 or 5,
In the optical pulse compressor according to any one of claims 1 to 3, the dispersion reducing fiber and the dispersion value and dispersion slope of the soliton propagation fiber are specified. First, the abnormal variance value on the input side is +5
Abnormal dispersion value on the output side is +0.2 p or more when ps / nm / km or more
Designing a dispersion-reducing fiber with s / nm / km or less is
The pulse width is suitable for compressing the light pulse in the picosecond region, and the femtosecond light pulse can be efficiently generated. A soliton propagation fiber with an anomalous dispersion value of +1.0 ps / nm / km or less is also suitable for propagating femtosecond optical solitons, and is a nonlinear that efficiently removes pedestal components from femtosecond optical pulses. A loop mirror can be realized. Also, by setting the absolute value of the dispersion slope of the output side of the dispersion-decreasing fiber and the soliton propagation fiber to 0.02 ps / nm ² / km or less, optimum operation is possible in a wide wavelength range, and ultrashort light with a small pedestal component is possible. Pulses can be generated.

Further, in the optical pulse compressor according to the fifth aspect, the dispersion value of the dispersive medium is +10 ps / nm / km or more. Such a dispersion value can be realized by, for example, a 1.3 μm single mode fiber, and the pulse width of the ultrashort optical pulse can be widened by using a short optical fiber.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the basic configuration of the optical pulse compressor of the present invention. In FIG. 1, 10 is an input terminal, 1
1 is an optical amplifier, 12 is a dispersion-reducing fiber whose anomalous dispersion value decreases from the input side to the output side, and whose dispersion slope on the output side is small, 13 is an optical directional coupler with 2 inputs and 2 outputs and a coupling ratio of 50%, 14 is an optical phase compensator, 15 is a dispersive medium,
Reference numeral 16 is a soliton propagation fiber having an anomalous dispersion value and a small dispersion slope, 17 is a dispersion compensation medium, and 18 is an output terminal.

An input port of an optical directional coupler 13 is connected to the input terminal 10 via an optical amplifier 11 and a dispersion reducing fiber 12. The output port of the optical directional coupler 13 is connected in a loop through the optical phase compensator 14, the dispersive medium 15, the soliton propagation fiber 16, and the optical fiber to form a nonlinear loop mirror (reference document 3). : WSWong et al., Opt. Lett., Vol.22, pp.1150
-1152, Aug. 1997, Reference 4: M. Matsumoto et al.,
(Electron. Lett., Vol.34, pp.1140-1141, May 1998)
. An output terminal 18 is connected to an input port of the optical directional coupler 13 via a dispersion compensation medium 17. When the optical pulse input to the input terminal 10 is an optical soliton, the optical amplifier 11 is unnecessary.

Next, each part of the optical pulse compressor shown in FIG. 1 will be described in detail. When an optical soliton is incident on the dispersion reducing fiber 12, the pulse width of the optical soliton is compressed as described above, and an ultrashort optical soliton is generated. However,
In the case of the present invention, since the dispersion slope on the output side of the dispersion reducing fiber 12 is small, even if the input wavelength is varied in a wide wavelength range, the spectrum of the compressed optical soliton spreads symmetrically and a pulse is generated in a wide wavelength range. It becomes possible to compress. However, due to the above-mentioned causes, the pedestal component is still present in the compressed optical soliton.

Now, a method of removing the pedestal component in the structure of the present invention will be described. The input light of the nonlinear loop mirror composed of the optical directional coupler 13, the optical phase compensator 14, the dispersive medium 15, and the soliton propagation fiber 16 is distributed to the components propagating in the clockwise direction and the counterclockwise direction, and propagates in the loop. However, the optical directional coupler 13 causes interference. The interfering light is reflected or transmitted depending on the phase difference, but here, the optical phase compensator 14 is used to set the light linearly propagated through the loop to be reflected.

When the respective parts constituting the non-linear loop mirror are of polarization-maintaining type, the light propagating linearly in the loop is automatically reflected and therefore the optical phase compensator 14 is not necessary. Also, the dispersion reducing fiber 12 and the dispersion compensating medium 1
The entire optical pulse compressor including 7 may be a polarization-maintaining type. In this case, it is not necessary to adjust the polarization direction, and a stable optical pulse compressor whose polarization direction is fixed can be realized.

When the ultrashort optical pulse output from the dispersion reducing fiber 12 is input to the non-linear loop mirror, the optical pulse propagating in the clockwise direction has its pulse width widened by the dispersive medium 15, and propagates through the soliton propagating fiber 16 with low intensity. To do. On the other hand, the optical pulse propagating in the counterclockwise direction propagates soliton-like in the soliton propagation fiber 16 and becomes a dispersive medium 15.
This widens the pulse width. Although the pulse widths that spread in the clockwise direction and the counterclockwise direction are almost equal, a nonlinear phase rotation occurs due to soliton-like propagation in the optical pulse that propagates in the counterclockwise direction, and thus a phase difference occurs with the clockwise optical pulse. Due to this phase difference, the optical pulse passes through the nonlinear loop mirror and is output from the input port. The transmittance changes sinusoidally with respect to the phase difference, and is approximately 100 when the phase difference is π.
%To Penetrate. The ultrashort optical pulse output is a dispersive medium 15
Therefore, the dispersion compensation medium 17 for compensating the chirp is necessary to obtain the ultrashort optical pulse. The optimum dispersion amount of the dispersion compensation medium 17 is a value that compensates for dispersion in the nonlinear loop mirror.

Now, the influence of the nonlinear loop mirror on the pedestal component will be described. Since the pulse width of the pedestal component is wide, the pulse broadening due to the dispersive medium 15 hardly occurs. The pedestal component propagating in the clockwise direction and the counterclockwise direction propagates in the soliton propagation fiber 16 with almost the same intensity, so that there is no phase difference due to the nonlinear effect. Therefore, when interference is caused by the optical directional coupler 13, the pedestal component returns to the input port and is removed from the ultrashort optical pulse output from the input port, similarly to the light propagating linearly in the loop.

Since the soliton propagation fiber 16 of the nonlinear loop mirror has a small dispersion slope, it can operate in a wide wavelength range. Further, since soliton-like transmission is used, waveform deterioration due to self-phase modulation effect and dispersion does not occur.

FIG. 2 shows an experimental system in which the optical pulse compressor of the present invention is realized by using an optical fiber. The dispersion reducing fiber 12 has a length of 1 km and a dispersion value on the input side of +10 ps / n.
m / km, the dispersion value on the output side is about 0, and the dispersion slope on the output side is +0.005 ps / nm ² / km. The dispersive medium 15 is a single mode fiber (SMF) having a length of 1 m and a dispersion value of +17 ps / nm / km. The soliton propagation fiber 16 has a length of 100 m and a dispersion value of +0.04 ps.
/ Nm / km, dispersion slope is +0.005 ps / nm ² /
It is a dispersion flat fiber (DFF) of km. The dispersion compensation medium 17 has a length of 1.1 m and a dispersion value of −50 ps / nm.
/ Km dispersion compensation fiber (DCF). The input light pulse was obtained from a reproduction mode-locked fiber laser having a repetition frequency of 10 GHz and a pulse width of 3 ps. The tunable range of the oscillation wavelength of the fiber laser was 1533 to 1565 nm. An optical amplifier 11 having a maximum output of 140 mW was used on the input side of the dispersion-reducing fiber 12, and an input optical pulse was amplified to an average power of 50 mW to perform a compression experiment.

Here, in the case of the conventional optical pulse compressor using only the dispersion reducing fiber composed of the dispersion shifted fiber, in the case of the optical pulse compressor using only the dispersion reducing fiber composed of the dispersion flat fiber, The results of comparison between the dispersion-decreasing fiber 12 of the present invention and the optical pulse compressor using the nonlinear loop mirror will be described with reference to FIG. In FIGS. 3 (a), 3 (b) and 3 (c), the four waveforms are obtained by changing the input light wavelength.

FIG. 3 (a) shows the output spectrum of the conventional optical pulse compressor using only the dispersion-decreasing fiber composed of the dispersion-shifting fiber. The spectrum spreads asymmetrically, and the peak wavelength of the spectrum changes greatly toward the long wavelength side of the input wavelength. Further, the shape of the spectrum changes greatly depending on the input wavelength, and a spike component exists at the input wavelength. Therefore, it is considered that optical pulse compression cannot be realized in a wide wavelength range.

FIG. 3B shows the output spectrum of the optical pulse compressor using only the dispersion-decreasing fiber composed of the dispersion flat fiber. In this case, the spectrum is spread symmetrically, and the shape of the spectrum is almost independent of the wavelength of the input. Therefore, it is considered that optical pulse compression can be realized in a wide wavelength range. However, there is a spectrum spike component, and a large pedestal component is present in the autocorrelation waveform as shown in FIG.

FIG. 3 (c) shows a dispersion-decreasing fiber 1 of the present invention.
2 shows the output spectrum of the optical pulse compressor using 2 and a nonlinear loop mirror. In this case, there is no spike component. FIG. 4 shows the autocorrelation waveform of the compressed optical pulse. It can be seen that the pedestal component is reduced to -40 dB or less. The pulse width is 160 fs and the spectral width is approximately 14 nm. The product of pulse width and bandwidth is 0.30
Indicates that a transform-limited optical pulse has been generated. Furthermore, it can be seen that pulse compression was realized in a wide input wavelength range.

FIG. 5 shows the difference between the anomalous dispersion fiber (dispersion flat fiber) used as the soliton propagation fiber 16 of the nonlinear loop mirror and the normal dispersion fiber. In FIG. 5, spectrum (a) is the case of using an abnormal dispersion fiber, and spectrum (b) is the case of using a normal dispersion fiber. Since the spectrum (b) spreads widely, it is difficult to generate a transform-limited optical pulse. However, when an anomalous dispersion fiber is used as the soliton propagation fiber 16 as in the spectrum (a), a soliton-like spectrum is retained and a transform-limited optical pulse can be generated. This result shows that it is important to use soliton propagation in the nonlinear loop mirror.

[0032]

As described above, the optical pulse compressor according to the present invention can generate an ultrashort optical pulse with a small pedestal component in a wide wavelength range with a small size, a low cost, and a simple structure. Such an ultrashort optical pulse having a small pedestal component is extremely useful in realizing ultrafast time-division multiplexed optical communication. It can also be used as a sampling pulse for an E / O sampling measuring instrument.
Furthermore, by combining with a multi-wavelength optical filter,
It can be used as a multi-wavelength light source.

[Brief description of drawings]

FIG. 1 is a block diagram showing the basic configuration of an optical pulse compressor according to the present invention.

FIG. 2 is a diagram showing an experimental system in which the optical pulse compressor of the present invention is realized by using an optical fiber.

FIG. 3 is a diagram showing experimental results. (a) is the output spectrum of a conventional optical pulse compressor using only dispersion-decreasing fiber composed of dispersion-shifted fiber, (b) is the optical pulse compressor using only dispersion-decreasing fiber composed of dispersion-flat fiber (C) is the output spectrum of the optical pulse compressor using the dispersion-decreasing fiber and the nonlinear loop mirror of the present invention.

FIG. 4 is a diagram showing an autocorrelation waveform of a compressed optical pulse.

FIG. 5 is a diagram showing a difference between an anomalous dispersion fiber (dispersion flat fiber) used as a soliton propagation fiber and a normal dispersion fiber.

[Explanation of symbols]

10 input terminals 11 Optical amplifier 12 Dispersion reducing fiber 13 Optical directional coupler 14 Optical phase compensator 15 Dispersive medium 16 Soliton propagation fiber 17 Dispersion compensation medium 18 output terminals

─────────────────────────────────────────────────── ─── Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) G02F 1/35 H01S 3/10 JISC file (JOIS) IEL

Claims (5)

(57) [Claims] 1. An optical pulse compressor for inputting an optical pulse into a dispersion-decreasing fiber whose anomalous dispersion value decreases from the input side to the output side and outputting an ultrashort optical pulse whose pulse width is compressed, wherein the dispersion-decreasing fiber is A two-input two-output optical directional coupler having a configuration in which at least an output-side dispersion slope is small, and an ultrashort optical pulse output from the dispersion-reducing fiber is input to one input port, and between the two output ports An optical phase compensator connected to, a non-linear loop mirror composed of a soliton propagation fiber having a dispersion medium and an anomalous dispersion value and a small dispersion slope, and a dispersion value for compensating for dispersion in the nonlinear loop mirror, and the optical direction And a dispersion compensating medium for compensating and outputting the chirp of the ultrashort optical pulse output from the other input port of the static coupler. Optical pulse compressor. 2. The non-linear loop mirror includes a polarization-maintaining optical directional coupler, a dispersive medium, and a soliton propagation fiber, and is configured without an optical phase compensator. 1. The optical pulse compressor according to 1. 3. The optical pulse compressor according to claim 1, wherein the dispersive medium and the dispersion compensation medium are optical fibers. 4. The anomalous dispersion value on the input side of the dispersion reducing fiber is +5 ps / nm / km or more, and the anomalous dispersion value on the output side of the dispersion reducing fiber is +0.2 ps / nm / k.
m or less, the anomalous dispersion value of the soliton propagation fiber is +1.0 ps / nm / km or less, and the absolute value of the dispersion slope of the output side of the dispersion reducing fiber and the soliton propagation fiber is 0.02 ps / nm ² / Km or less, the optical pulse compressor according to claim 1 or claim 2, characterized in that. 5. The anomalous dispersion value on the input side of the dispersion reducing fiber is +5 ps / nm / km or more, and the anomalous dispersion value on the output side of the dispersion reducing fiber is +0.2 ps / nm / k.
m or less, the anomalous dispersion value of the soliton propagation fiber is +1.0 ps / nm / km or less, and the absolute value of the dispersion slope of the output side of the dispersion reducing fiber and the soliton propagation fiber is 0.02 ps / nm ² 4. The optical pulse compressor according to claim 3, wherein the dispersion value of the dispersive medium is +10 ps / nm / km or more.