JPH1039154A - Light pulse compressing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a light pulse compressing device which is easy to handle, excellent in light loss characteristics, and inexpensive, by setting the length of first and second optical fibers at adequate values and compensating the secondary and ternary dispersion of the pulses to be compressed. SOLUTION: The light pulse compressing device 10 consists of the first optical fiber 11 having a length size Z1 and the second optical fiber 12 having a length size Z2 . The first and second optical fibers 11, 12 are single mode optical fibers respectively having cores 13 and clads 14 enclosing these cores 13 and the series coupling of both fibers 11, 12 by a coupler or by direct fusing is possible. The overall secondary dispersion quantity and ternary dispersion quantity may be respectively so set as to attain the values to compensate the secondary dispersion quantity and ternary dispersion quantity of the pulses to be compressed by adequately selecting the length sizes Z1 , Z2 of the first and second optical fibers 11, 12. The effective compression of the pulses to be compressed is thus made possible.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pulse compression device for compressing a pulse width of an optical pulse.

[0002]

2. Description of the Related Art When a semiconductor laser is used, an optical pulse can be generated by a gain switching method, a mode locking method, an external modulation method using a light intensity modulator, or the like. A technique for compressing the pulse width of an optical pulse generated by such a method is extremely important for transmitting a large amount of information in, for example, optical communication.

One of the pulse compression methods for compressing the pulse width of an optical pulse is a linear compression method. The linear compression method is a method of compensating for phase variation, that is, dispersion, in a spectral component obtained by Fourier transform of a light pulse to be compressed, using a dispersion medium. In this linear compression method, in order to obtain an ultrashort light pulse on the order of femtoseconds, a second order or third order
Compensation for higher order dispersion is important. Here, the second-order dispersion and the third-order dispersion mean the second derivative and the third derivative, respectively, of the frequency of the phase distribution in the spectrum.

[0004] One of the dispersion media used in the linear compression method is an optical fiber. An optical pulse compression apparatus using this optical fiber has a characteristic excellent in matching with a laser source such as a semiconductor laser. And excellent low light loss characteristics. However, in a conventional optical pulse compression apparatus using an optical fiber, one optical fiber is used alone, and the group velocity dispersion value (D) of this optical fiber is referred to as 2D.
The product of the secondary dispersion value and the length of the optical fiber determines the secondary dispersion amount. At the same time, the chromatic dispersion value (dD
/ Dλ) and the length of the optical fiber determine the third-order dispersion.

[0005]

Therefore, in a conventional optical pulse compression device comprising a single optical fiber, the length of the optical fiber is optimized for compensating either the secondary or tertiary dispersion. If the length of the optical fiber is set to a value, the dispersion of the other cannot be sufficiently compensated. For this reason, it is impossible to simultaneously and effectively compensate for both the second-order and third-order dispersion, and a Fourier transform limit pulse (hereinafter, referred to as a transform limit pulse), which is an optical pulse having the shortest possible pulse width. There was a difficulty in obtaining an ultrashort light pulse approximating.

[0006]

The present invention adopts the following constitution in order to solve the above points. <Structure> The optical pulse compression device according to the present invention comprises a first serially connected first group having at least one of a group velocity dispersion value (D) and a wavelength dispersion value (dD / dλ) different from each other. And the second single-mode optical fiber, and the lengths of both optical fibers are set to appropriate values to compensate for the second-order dispersion and the third-order dispersion related to the phase between the spectral components of the compressed light pulse. (Corresponding to claim 1).

In the optical pulse compression device according to the present invention, the total secondary dispersion by the first and second optical fibers connected in series is represented by D ₁ · Z ₁ + D ₂ · Z _2. Similarly, the total tertiary dispersion amount is (1/3) (dD / d
λ) ₁ · Z ₁ + (1/3) (dD / dλ) ₂ · Z ₂ Here, D ₁ and D ₂ are group velocity dispersion values of the first and second optical fibers, respectively, and (dD / d
λ) ₁ and (dD / dλ) ₂ represent chromatic dispersion values of group velocity dispersion of the first and second optical fibers, respectively. Therefore, the length dimensions Z ₁ , Z _{1 of} the first and second optical fibers, respectively.
By proper selection _2, so that the respective overall second-order dispersion amount and overall third order dispersion amount becomes an appropriate value to compensate for the second-order dispersion amount and third-order dispersion of the compressed optical pulse, set Thus, the compressed light pulse can be effectively compressed.

It is assumed that the dispersion characteristic of the compressed light pulse is represented by -abΔλ. Here, Δλ is a deviation from the center wavelength, and -a corresponds to the secondary dispersion amount, and -b corresponds to the tertiary dispersion amount. In this case, the length Z ₂ of the first optical fiber length Z ₁ and a second optical fiber required to compensate the dispersion of the compressed optical pulse is given by the following equation (1) (wherein (Corresponds to item 2).

[0009]

(Equation 2)

As the two optical fibers, optical fibers each having a positive chromatic dispersion value (dD / dλ) can be used (corresponding to claim 3). Further, as at least one of the two optical fibers, an Ω-type optical fiber having a negative chromatic dispersion value (dD / dλ) can be used (corresponding to claim 4).

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the illustrated embodiments. FIG. 1 is an explanatory view of an optical pulse compression device according to the present invention. Optical pulse compression device 10 according to the present invention comprises a first optical fiber 11, the second optical fiber 12 and having a length dimension Z ₂ having a length dimension Z _1. Each first
And the second optical fibers 11 and 12 are single mode optical fibers each having a core 13 and a cladding 14 surrounding the core, as is well known in the related art. They can be connected in series by known couplers or by direct fusion.

On the left side of the first optical fiber 11 shown in FIG. 1, an envelope waveform 1 of an input optical pulse which is a compressed optical pulse is shown.
5 is shown in a graph with the horizontal axis as the time axis T and the vertical axis as the light intensity axis I. The graph also shows a characteristic line 16 showing a phase change within a pulse of an input optical pulse with the horizontal axis as the time axis T and the vertical axis as the phase axis Φ. The characteristic line 16 is shown by a curve, and the characteristic line 16 of this curve is shown.
Indicates that there is a phase variation between the spectral components of the pulses of the envelope waveform 15, and therefore,
A relatively strong chirp is observed in the light pulse indicated by the envelope waveform 15, and the pulse width is relatively wide.

The lower graph in the drawing of the first optical fiber 11 on which the optical pulse represented by the envelope waveform 15 is incident has a wavelength λ on the horizontal axis and a secondary dispersion of the optical fiber on the vertical axis. First optical fiber 1 having a certain group velocity dispersion value (D)
1 shows the dispersion characteristics. As shown by the characteristic line 17, the first optical fiber 11 has a dispersion wavelength value (third-order dispersion value) represented by the slope (dD / dλ) ₁ of the characteristic line 17.
And the group velocity dispersion value D (second order dispersion value) is D at the wavelength λs.
Indicates the value of ₁ . The secondary dispersion value, that is, the group velocity dispersion value D ₁ of the first optical fiber 11 is a value that changes according to the design value of the optical fiber, such as the refractive index of the core 13 or the diameter thereof. By selecting a value, an appropriate value can be set.

A graph shown in the lower part of the figure of the second optical fiber 12 connected in series to the first optical fiber 11 shows a second optical fiber 12 similar to that described for the first optical fiber 11. 4 shows the dispersion characteristics of the optical fiber 12. The second optical fiber 12 is, as shown by its characteristic line 18,
The dispersion wavelength value (third-order dispersion value) represented by the slope (dD / dλ) ₂ of the characteristic line 18 is shown, and the group velocity dispersion value D (second-order dispersion value) shows the value of D _{2 at} the wavelength λs. Group velocity dispersion value D ₂ is a second-order dispersion value, as in the first optical fiber 11, by selecting the design value of the optical fiber can be set to a proper value.

In the illustrated example, the dispersion wavelength value shows a positive and equal value ((dD / dλ) ₁ = (dD / dλ) ₂ ).
The group velocity dispersion values at the wavelength λs are different from each other (D ₁ ≠ D
₂ ) is set. Alternatively, optical fibers having different dispersion wavelength values and having the same group velocity dispersion value may be used in combination as the first and second optical fibers 11 and 12. Further, optical fibers having different dispersion wavelength values and different group velocity dispersion values can be used in combination as the first and second optical fibers 11 and 12.

A first optical fiber 11 having at least one of a group velocity dispersion value and a wavelength dispersion value different from each other.
And the length dimension Z ₁ of each of the second optical fibers 12
As will be described later, by setting the values of Z ₂ and Z ₂ to appropriate values, the input optical pulse shown by the envelope waveform 15 is plotted on the graph shown on the right side of the second optical fiber 12 in the envelope waveform 15 *. Can be compressed as shown by

The graph showing the envelope waveform 15 * is a graph in which the horizontal axis is the time axis T and the vertical axis is the light intensity axis I, like the graph on the left side of the figure showing the envelope waveform 15. It is shown. In addition, the graph shows a characteristic line 16 * indicating a phase change within the pulse of the output optical pulse with the horizontal axis being the time axis T and the vertical axis being the phase axis Φ. Characteristic line 16 *
Is indicated by a straight line, and the characteristic line 16 * of the straight line indicates that there is no phase variation between the spectral components of the pulses of the envelope waveform 15 *. The pulse width of the incident light pulse indicated by is narrowed and effectively compressed as indicated by the envelope waveform 15 *.

Prior to setting the lengths Z ₁ and Z ₂ of the first optical fiber 11 and the second optical fiber 12, a general phase change in the optical fiber will be considered. The change Φ _fiber is expressed by the following equation (2) when considering the third-order dispersion.

[0019]

(Equation 3) Here, Δω is a deviation from the optical carrier angular frequency ω ₀ ,
β ₀ is the propagation constant at the carrier angular frequency ω ₀ , β ₁ is the reciprocal of the group velocity of light at the carrier angular frequency ω ₀ , β ₂ and β ₃
Are constants related to the second and third order dispersions of the optical fiber, and the constants β ₂ and β ₃ are the group velocity dispersion value D and the wavelength dispersion value dD / dλ of the optical fiber and the following formula (3) and It has the relationship shown in the following equation (4).

[0020]

(Equation 4)

(Equation 5) Here, c represents the speed of light, and λ represents the wavelength of light.

As described above, the group velocity dispersion value D is a value that changes by changing the refractive index or the diameter of the core, which is the design value of the optical fiber. For example, by changing the mode field diameter of the fiber from 6 microns to 10 microns, the group velocity dispersion value D becomes 1.55 μm in wavelength.
In the vicinity of m, it changes from -18 ps / km / nm to +16 ps / km / nm. On the other hand, the chromatic dispersion value (dD / dλ) does not change much depending on the design value of a single fiber, and is about 0.07 ps / km / nm / nm for light having a wavelength of 1.55 μm.
Shows the value of

Equation (2) expressed by the angular frequency is replaced by equation (3)
By rewriting the equation in the frequency domain using Equation (4) and Equation (4), the following Equation (5) is obtained.

[0023]

(Equation 6)

The first term on the right side of the equation (5) is a constant term,
This is a term that does not affect the pulse waveform. The second term on the right side of the equation represents a change in the time position of the pulse, and does not affect the pulse waveform. Therefore, the underlined terms on the right side of the equation affect the pulse waveform.

In the optical pulse compression apparatus 10, since the first and second optical fibers 11 and 12 are arranged in series, the equation (5) which is a basic equation for the phase change of the optical fiber is expressed by the following equation. Rewritten to (6).

[0026]

(Equation 7)

Similarly to the expression (5), in the expression (6), the term which affects the pulse waveform is the underlined portion of the expression (6). Focusing on this term, the secondary dispersion in the optical pulse compression device 10 is
D ₁ · Z ₁ + D ₂ · Z ₂ , and the tertiary variance is
(1/3) (dD / dλ) ₁ · Z ₁ + (1/3) (dD
/ Dλ) ₂ · Z ₂ .

Accordingly, each dispersion amount is determined by the length Z ₁ of the _first optical fiber 11 and the length of the second optical fiber 12.
Depending on the length Z ₂ of the second-order dispersion amount and third-order dispersion amount is determined in accordance with the value. That is, when the amount of dispersion necessary for dispersion compensation of the incident light pulse is a + bΔλ (a is 2
Represents the third-order dispersion amount and b represents the third-order dispersion amount), and the length dimension Z _{1 of} each of the optical fibers 11 and 12 required for the dispersion compensation.
And Z ₂ are given by equation (1) above. The condition under which the solution of equation (1) exists is given by the following equation (7).

[0029]

(Equation 8)

In order to satisfy the expression (7), at least one of the group velocity dispersion value D and the chromatic dispersion value (dD / dλ) of the first and second first optical fibers 11 and 12 is required. Are derived from each other.

FIG. 2 shows an optical pulse compression apparatus 1 according to the present invention.
6 is a graph showing dispersion characteristics of 0. The vertical axis of the graph is 2
The total variance Σ ｛D _i, which is the sum of the variances of the second and third order
Z _i + (1/3) · (dD / dλ) _i ΔλZ _i }, and the horizontal axis represents the wavelength λ. In the graph of FIG. 2, each length dimension Z _{1 of} the first optical fiber 11 and the second optical fiber 12 is shown.
Using Z and Z ₂ as parameters, the respective calculation results are shown as characteristic lines 19, 20, 21 and 22.

In this graph, 16 ps / km / nm is adopted as a secondary dispersion value, that is, a group velocity dispersion value D ₁ of the first optical fiber 11 at a wavelength of 1552 nm, which is indicated by reference numeral SMF. 0.07 ps / km / nm was adopted as the secondary dispersion value, that is, the wavelength dispersion value (dD / dλ) ₁ . On the other hand, the second optical fiber 12
0.24 ps / km / nm is adopted as the second-order dispersion value at the wavelength of 1552 nm, that is, the group velocity dispersion value D ₂ , and 0.07 ps as the third-order dispersion value, that is, the chromatic dispersion value (dD / dλ) ₂ . / km / nm was adopted.

The first and second optical fibers 11 and 12 are so adjusted that the total dispersion of the first and second optical fibers 11 and 12 is 1.04 ps / nm at a wavelength of 1522 nm.
The length dimensions Z ₁ and Z 2 of each of the combinations ₂ were adjusted. Characteristic line 19 shown in FIG. 2, the length dimension Z ₁ is 20
The total dispersion characteristic when the first optical fiber 11 having a length of m and the second optical fiber 12 having a length Z ₂ of 3 km is shown. Similarly, the characteristic line 20 has a length dimension Z ₁
Is the first optical fiber 11 having a length of 35 m and the length Z ₂ is 2 k.
10 shows the total dispersion characteristics when the second optical fiber 12 is combined with m. Further, the characteristic line 21, likewise, shows the total dispersion characteristic when the length Z ₁ is the first optical fiber 11 and the length dimension Z ₂ of 50m a combination of the second optical fiber 12 of 1km . Further, the characteristic line 22 indicates the total dispersion characteristic when the length Z ₁ as a comparative example was used only alone optical fiber 65 m.

As is clear from the characteristic lines 19 to 22 shown in FIG. 2, for example, even if the total amount of dispersion at the wavelength of 1552 nm is the same value, it corresponds to the inclination degree of the characteristic lines 19 to 22. The secondary dispersion amount can be changed. Therefore, by selecting the length dimension of the first optical fiber 11 and the length dimension of the second optical fiber 12, a desired secondary dispersion amount and tertiary dispersion having a value necessary for dispersion compensation of an input pulse are selected. An optical pulse compression device 10 indicating the quantity can be configured.

FIG. 3 is a graph showing an example of the spectrum distribution of the compressed pulse. The horizontal axis represents the wavelength (nm).
The vertical axis indicates the spectrum intensity (arbitrary unit). As a pulse light source represented by the spectrum distribution characteristic line 23 shown in FIG. 3, a semiconductor mode-locked laser having a wavelength of 1.55 μm was used. The pulse width of the output pulse from this laser is about 9.5 ps, and its spectral width is about 5 nm. Its time-bandwidth product is about 6, which corresponds to about 19 times considering that the time-bandwidth product of the shortest pulse having zero phase difference in the spectrum, that is, the conversion limit pulse is 0.315. . This means that there is a large dispersion in the laser output pulse.

Therefore, in order to calculate a dispersion value necessary for compensating the dispersion by using the laser pulse light as a pulse to be compressed, a value of 2 to the frequency of the phase distribution in the spectrum is calculated.
The value corresponding to the differential is plotted based on the characteristic line 23 shown in FIG. 3, and the following equation, + 1.0571 + 0.051386Δλ, was derived from a straight line 24 connecting the plots. Here, Δλ is a deviation from the wavelength of 1552 nm.
Here, +1.0571 and 0.051386 correspond to the values of a and b in the above equation (1), respectively.

Accordingly, each of these values is substituted into equation (1), and the group velocity dispersion value D ₁ described with reference to FIG.
nm @ 1552nm, as D ₂ 0.24ps/kn/nm@155
2 nm, and chromatic dispersion value (dD / dλ) ₁ as 0.0
7ps / km / nm, 0.07ps / km / nm as (dD / dλ) ₂
When the lengths of the first optical fiber 11 and the second optical fiber 12 are obtained using the values of the above, the length Z ₁ of the first optical fiber 11 is 33.5 m, and the length of the second optical fiber 11 is 23.5 m. 2.17 km was determined as the length Z ₂ of the optical fiber 12.

FIG. 4 shows a case where the pulse light shown in FIG. 3 is used as a pulse light to be compressed by a compression device comprising the first optical fiber 11 and the second optical fiber 12 and each fiber set shown in FIG. 7 is a graph showing the result of observing the pulse waveform after the SHG correlation waveform. The vertical axis of the graph in FIG. 4 indicates the delay time (ps), and the vertical axis indicates the SHG intensity (arbitrary unit).

The characteristic line 25 shown in the graph of FIG. 4 is a characteristic line representing the calculated value of the SHG correlation waveform of the conversion limit pulse. Further, the reference numerals attached to the other characteristic lines correspond to the example of the combination of the first optical fiber 11 and the second optical fiber 12 shown in FIG. As is clear from the comparison of the characteristics 19 to 22 and 25 shown in the graph of FIG. 4, the characteristic line indicated by reference numeral 20 is most similar to the characteristic line 25 of the conversion limit pulse. The length dimensions Z ₁ and Z ₂ of the first optical fiber 11 and the second optical fiber 12 that provide the following are 35 m and 2 km, respectively, as described along FIG.

The length of the first optical fiber 11 and the length of the second optical fiber 12, which have been experimentally confirmed to be closest to the conversion limit pulse, are 35 m and 2 km, respectively.
The value of the length Z ₁ of the first optical fiber 11 calculated from the equation (1) based on the characteristic line 24 shown in FIG. 3 is 33.5 m, and the value of the second optical fiber 12 the value of the length dimension Z ₂ is very similar to the combination of 2.17Km. From this, it can be said that the combination of the first optical fiber 11 and the second optical fiber 12 having the length dimension derived from the equation (1) can effectively compensate the secondary and tertiary dispersion. .

Accordingly, the optical pulse compression device 1 according to the present invention
According to 0, the second-order dispersion and the third-order dispersion of the light pulse can be simultaneously and effectively compensated, and the light pulse can be effectively compressed. Moreover, the optical pulse compression device 1
0 is an optical fiber whose optical loss shows a smaller value than other dispersion media such as a diffraction grating pair and a prism pair, so that the optical loss accompanying compensation is as low as about 9%. Value. In addition, the optical pulse compression device 10 composed of an optical fiber does not require complicated optical adjustment and is not affected by chromatic aberration, so that it is relatively easy to handle and has a relatively low cost. The pulse can be effectively compressed.

As described above, the first optical fiber 1
Although an example using an optical fiber exhibiting a positive chromatic dispersion value (dD / dλ) for both the first and second optical fibers 12 has been described, at least the first optical fiber 11 or the second optical fiber 12 On the other hand, an optical fiber exhibiting a negative chromatic dispersion value can be used.

FIG. 5 is a dispersion characteristic graph of a so-called Ω-type optical fiber showing a negative wavelength dispersion characteristic. The vertical axis of the graph in FIG. 5 indicates the group velocity dispersion value D, and the horizontal axis indicates the wavelength λ. In this graph, a wavelength dispersion characteristic line 28 is shown as the sum of the waveguide dispersion characteristic line indicated by reference numeral 26 and the material dispersion characteristic line indicated by reference numeral 27. The dispersion characteristic of the Ω-type optical fiber shows a high-wavelength band low-dispersion single mode characteristic similar to that of a so-called W-type fiber, and in the example shown in the graph of FIG. 5, the dispersion characteristic line 28 has a wavelength of about 1.5 μm or more. The band shows a negative slope (corresponding to a negative chromatic dispersion value, ie, a negative third-order dispersion).

When the chromatic dispersion values of both the first optical fiber 11 and the second optical fiber 12 are positive, the optical pulse compression device 10 composed of a combination of the two fibers has a negative third-order dispersion. The compressed light pulse shown is effectively compressed,
It is not possible to compensate for the third-order dispersion of the compressed light pulse whose third-order dispersion value is a positive value, and it is not possible to effectively compress this compressed light pulse. On the other hand, an optical fiber having a negative chromatic dispersion value, such as an Ω-type optical fiber, is applied to at least one of the first optical fiber 11 and the second optical fiber 12, and based on the above formula (1), As described above, by setting the lengths Z ₁ and Z ₂ of the fibers 11 and 12 to appropriate values, it is possible to effectively compress the compressed light pulse whose tertiary dispersion value is a positive value. it can.

[0045]

According to the present invention, as described above, the lengths of the first and second optical fibers exhibit excellent characteristics in matching with a laser source and exhibit excellent optical loss characteristics. With this setting, the second-order dispersion and the third-order dispersion of the compressed light pulse can be effectively compensated, so that the compressed light pulse can be effectively compressed. An inexpensive optical pulse compression device having excellent loss characteristics can be provided.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of an optical pulse compression device according to the present invention.

FIG. 2 is a graph showing dispersion characteristics of the optical pulse compression device according to the present invention.

FIG. 3 is a graph showing an example of a spectral distribution of a compressed pulse.

FIG. 4 is an SHG correlation waveform diagram after compression of an optical pulse.

FIG. 5 is a graph showing dispersion characteristics of an Ω-type optical fiber.

[Explanation of symbols]

 Reference Signs List 10 optical pulse compression device 11 first optical fiber 12 second optical fiber

Continuation of the front page (51) Int.Cl. ⁶ Identification number Agency reference number FI Technical display location H04B 10/06 10/02 10/18 10/18

Claims (4)

[Claims] 1. A first and second single-mode light serially connected to each other and having at least one of a group velocity dispersion value (D) and a wavelength dispersion value (dD / dλ) different from each other. An optical pulse compression device comprising a fiber,
An optical pulse compression apparatus, wherein the length dimension of both optical fibers is set to an appropriate value so as to compensate for second-order dispersion and third-order dispersion related to the phase between the spectral components of the compressed light pulse. 2. The amount of secondary dispersion of a compressed light pulse is -a, 3
When the following dispersion amount and -b, optical pulses according to claim 1, wherein the length dimension Z ₂ of the first optical fiber length Z ₁ and the second optical fiber is characterized in that given the number 1 Compression device (where D ₁ and D ₂ represent group velocity dispersion values of the first and second optical fibers, respectively, and (dD / dλ)
₁ and (dD / dλ) ₂ represent the chromatic dispersion values of the first and second optical fibers, respectively. ). (Equation 1) 3. The optical pulse compression device according to claim 1, wherein the two optical fibers are optical fibers each having a positive chromatic dispersion value (dD / dλ). 4. At least one of the two optical fibers,
The optical pulse compression device according to claim 1, wherein the optical pulse compression device is an Ω-type optical fiber whose chromatic dispersion value (dD / dλ) shows a negative value.