JP3524355B2 - Coherent broadband light source - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light source used for optical communication, and more particularly to a coherent broadband light source which has a very wide band and is capable of generating coherent light.

[0002]

2. Description of the Related Art As a light source for generating coherent supercontinuum light (light with a wide spectrum), WHKnox et al., "Amplified femtosecond o
pticalpulse and continuumgeneration at 5-kHz repet
ition rate ", Optics Letter vol.9, pp 552-554 (1984), there is a light source that uses a medium of normal dispersion and uses an ultrashort optical pulse of high intensity. In order to widen the band of light to be used, ultra-short optical pulses with an extremely high intensity (peak intensity of about 10 GW / cm ² ) (pulse width of 1
00 fs; 1 fs = 10 ⁻¹⁵ seconds), a large laser device is required, and it cannot be applied to a communication light source that requires a high repetition frequency of giga Hz or higher.

As a method of efficiently expanding the bandwidth, there is a method of using a medium having a slightly anomalous dispersion characteristic. In this case, the use of high-intensity input pulses to broaden the bandwidth and obtain high-power light causes a significant effect of higher-order solitons, and the interaction with the noise component causes the supercontinuum light to be emitted. It is difficult to maintain a coherent state.

There is also a method of widening the band by narrowing the pulse width by utilizing only the effect of pulse compression by a higher-order soliton in an anomalous dispersion medium, but only a bandwidth about the reciprocal of the pulse width can be obtained. Therefore, the bandwidth of the obtained light is narrower than that of the supercontinuum light.

Further, the coherent white light source disclosed in Japanese Patent Laid-Open No. 8-234249 uses a dispersion-decreasing optical fiber (DDF) as an optical waveguide for generating white light, and has a peak intensity at which the soliton order N is about 10. Incident optical pulse into the dispersion-decreasing optical fiber, and by performing dispersion compensation and chirp compensation to generate white light (broadband light) in the dispersion-decreasing optical fiber portion, a uniform and continuous spectrum is obtained. In addition, it is possible to generate a white pulse having a high coherence in an ultra-wide band with a low excitation power.

Here, the dispersion-decreasing optical fiber means that the dispersion characteristic decreases along the longitudinal direction of the optical fiber from a large value of anomalous dispersion toward zero dispersion, and in some cases,
An optical fiber that increases toward normal dispersion. The term "dispersed" optical fiber is used here because the group velocity dispersion is represented by a positive sign on the abnormal dispersion side and a negative sign on the normal dispersion side. An optical fiber that does not have wavelength dependency of dispersion is called a dispersion flat fiber. With the current optical fiber design technology, an optical fiber that does not have wavelength dependency of dispersion is manufactured over a wide wavelength band (spectral band). It's difficult.

[0007]

By the way, JP-A-8-
The coherent white light source disclosed in 234249 is
The light pulse light source that constitutes this coherent white light source,
When there is no noise in the optical system other than this optical pulse light source, white light with coherence can be generated, but in reality, since there is noise in the optical system, coherence is maintained. It is difficult to generate white light.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a coherent broadband light source which has a compact and simple structure and has an extremely wide wavelength band and is capable of generating coherent light. Has an aim. Another object of the present invention is to provide a coherent broadband light source capable of generating an optical signal with a wider bandwidth while maintaining the coherent property even when noise is present in the optical system constituting the light source. is there.

[0009]

According to a first aspect of the present invention, there is provided an optical pulse generating means for generating an optical pulse having at least a peak intensity sufficient to form an optical soliton, and an optical pulse formed by the optical pulse. Adiabatic pulse compression means for compressing the pulse width of the soliton, and white light generating means for expanding the pulse width and spectral width of the optical soliton with the pulse width compressed , the optical pulse generating means
At the incident end of the adiabatic pulse compression means, the order is 2 or less
Generate an optical pulse with an intensity that can form an optical soliton
A coherent broadband light source, characterized in that that. With this configuration, the pulse width of the optical soliton is narrowed by adiabatic compression, whereby the spectrum width is expanded while maintaining the coherence, and the spectrum width is greatly expanded in the white light generating means. In addition, coherent
The adiabatic pulse compression means
The intensity of the light is the intensity that produces a second-order or less soliton, and the wave
Minimize shape distortion. Also, the soliton order N is 2
The following signals are used, so the dispersion-decreasing optical fiber
(Adiabatic pulse compression means) still does not generate white light,
White with normal dispersion characteristic optical fiber (white light generation means)
Light is generated. For this reason, dispersion compensation and chirp compensation are performed.
Since it does not need to be used, white that maintains high coherency
It becomes a color light source.

According to a second aspect of the present invention, in the coherent broadband light source according to the first aspect, the adiabatic pulse compression means is an optical fiber having anomalous dispersion characteristics,
It is an optical fiber whose anomalous dispersion value decreases from the entrance end to the exit end, and the white light generating means is an optical fiber having a normal dispersion characteristic. That is, in order to efficiently generate high-intensity ultrashort optical pulses, the spectrum is expanded using adiabatic compression of solitons using dispersion-decreasing fiber, and the normal dispersion region of the optical fiber is used to further expand the spectrum width. To do.

According to a third aspect of the invention, in the coherent broadband light source according to the second aspect, the adiabatic pulse compression means and the white light generation means are constituted by one optical fiber. That is, since the means used for the adiabatic compression of solitons and the means for further broadening the spectrum are both optical fibers, by appropriately designing the dispersion characteristics in the longitudinal direction of the optical fiber,
It is possible to adopt a simple configuration in which light is incident on one optical fiber. Further, since there is no attenuation of light due to connection or the like, an efficient light source can be constructed.

[0012]

The invention according to claim 4 is the invention according to claim 2 or
In the coherent broadband light source described in the item 3 , the wavelength dependence of the dispersion of the optical fiber used as the adiabatic pulse compression means is less than 0.07 [ps / km / nm / nm]. That is, by using an optical fiber whose wavelength dependence of dispersion is smaller than that of a normal optical fiber (the wavelength dependence of dispersion of a normal optical fiber is about 0.07 [ps / km / nm / nm]), The waveform band can be expanded more efficiently.

The invention described in claim 5 is the invention according to claim 2 or
4. The coherent broadband light source according to any one of 4 , wherein the dispersion value at the incident end of the optical fiber used as the adiabatic pulse compression means is 5 [ps / km / nm] or more. .

The invention according to claim 6 is the invention according to claim 2 or
4. The coherent broadband light source according to any one of 4 , wherein the optical fiber used as the adiabatic pulse compression means has a reduction rate of anomalous dispersion value per soliton cycle of -25% or less. There is. As a result, efficient adiabatic compression is possible, and deterioration of efficiency due to optical loss of the optical fiber can be prevented.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of a coherent broadband light source according to the present invention will be described in detail below with reference to the drawings.

FIG. 1A is a block diagram showing the basic configuration of the coherent broadband light source in this embodiment. In this figure, reference numeral 1 denotes an optical pulse light source (optical pulse generating means) for generating an optical pulse which is a source of white light generated by this embodiment, and for example, a fiber laser, a semiconductor laser or the like can be used.

Reference numerals 2 and 2 ′ are optical amplifiers.
The (optical pulse generating means) amplifies the optical pulse emitted from the optical pulse light source 1, and the optical amplifier 2'amplifies the optical pulse emitted from the adiabatic pulse compressor 3 described below. As these optical amplifiers, for example, EDFA (erbium-doped optical fiber amplifier) can be used. The optical amplifier 2 can be omitted if the peak intensity of the optical pulse incident on the configuration of the next stage of these optical amplifiers satisfies a predetermined condition (details will be described later). Further, the optical amplifier 2 ′ is for appropriately amplifying the intensity of the finally emitted white light so as to have a desired value, and has high coherence and obtains white light in a wide band. It is not necessary for the configuration and can be omitted depending on the case.

The adiabatic pulse compressor 3 (adiabatic pulse compression means) forms an optical soliton from the optical pulse emitted from the optical amplifier 2 and adiabatically compresses the formed optical soliton. Here, for adiabatic compression of coherent solitons,
Already, JP-A-7-318988, "Optical pulse compression device", and US Pat. No. 5,583,959,
The "Optical Pulse Compression Device" disclosed using a dispersion-reducing optical fiber is disclosed.

That is, when an optical soliton of order N = 1 is formed at the incident end of the dispersion-reducing optical fiber and the formed optical soliton is incident on the dispersion-reducing optical fiber, the optical soliton passes through the dispersion-reducing optical fiber. While doing so, the pulse width becomes thinner while keeping the product of the pulse width and the intensity constant. This phenomenon is called adiabatic compression of optical solitons.

Reference numeral 4 denotes a white light generator (white light generating means) that changes the light pulse compressed by the adiabatic pulse compressor 3 into white light that is broadband and coherent.

Next, FIG. 1B shows a structure in which optical fibers are used as the adiabatic pulse compressor 3 and the white light generator 4 in the coherent broadband light source having the structure shown in FIG. 1A. It is a block diagram shown. In this figure, F ₁ is a dispersion-decreasing optical fiber corresponding to the adiabatic pulse compressor 3, and F ₂ is an optical fiber having a normal dispersion characteristic corresponding to the white light generator 4. The entrance end of the dispersion-decreasing optical fiber F ₁ has an anomalous dispersion characteristic. Further, in this figure, f is an optical pulse light source 1 and an optical amplifier 2.
An optical fiber for connection for connecting with.

Next, in the coherent broadband light source having the configuration shown in FIG. 1B, a method of determining various parameters in each part will be described. First, various parameters of the dispersion reducing optical fiber F ₁ will be described. The peak intensity P [W] of the optical soliton of the fundamental order is expressed as P = 0.776 × (λ ³ D) / (π ² cn ₂ t ² ) × A. In the above equation, D is the absolute value of the group velocity dispersion value at wavelength λ, c is the speed of light in a vacuum, n ₂ is the nonlinear refractive index of the optical fiber, t is the full width at half maximum of the pulse, and A is the effective core of the optical fiber. The cross-sectional area.

When a silica-based optical fiber is used as the dispersion-reducing optical fiber F ₁ , in a typical silica-based optical fiber, the effective area A of the core is usually 50 [μ
m ² ], and the nonlinear refractive index n ₂ is about 2.24 ×
It is 10 ⁻²⁰ [m ² / W]. Further, the full width at half maximum t of the light pulse generated by the light pulse light source 1 is set to 1.5 [ps], the wavelength λ is set to 1.55 [μm], and the target is to obtain a peak intensity P of about 5 to 10 W (watt). Assuming that the absolute value D of the group velocity dispersion value of the dispersion-reducing optical fiber F ₁ is defined as, the absolute value D of the group velocity dispersion value is 5 [ps / km / nm] (5 × 10 ⁻⁶ s /
m ² ), the peak intensity P of the optical soliton of the fundamental order is 4.8.
[W] and 10 [ps / km / nm] (10 × 10 ^-6 s / m ² )
At that time, the peak intensity P is 9.7 [W]. here,
The absolute value D of the group velocity dispersion value is set to 8 [ps / km / nm] (the peak intensity P is 7.8 [W]).

Next, the normalized amplitude U of the optical soliton is given by U = (input peak power / P) ^1/2 , and the order N of the soliton is an integer closest to U. Here, in the configuration shown in FIG. 1B, when a soliton having a high order of solitons, that is, a large order N is used, white light having a wider frequency spectrum can be obtained.
However, if a soliton having a large order N is used, the waveform is likely to be disturbed due to the influence of optical noise, and it is difficult to maintain coherency. Therefore, in the present embodiment, the optical pulse light source 1 generates an optical pulse having a peak intensity capable of forming an optical soliton of order 2 or less, and uses it as the input light of the dispersion reducing optical fiber F ₁ . In the following explanation,
In particular, it is assumed that the optical pulse light source 1 generates an optical soliton having an intensity near the order N = 1 with a small change in waveform.

Therefore, in order to generate an optical soliton of order N = 1, an amplitude of ± 50% with respect to the peak intensity P of the optical soliton of the base order (in terms of intensity, 0.25 times to 2.25).
It is sufficient to make the optical pulse of (2 times) incident on the dispersion-decreasing optical fiber F ₁ . The intensity of the light incident on the dispersion-reducing optical fiber F ₁ can be changed by changing the amplification degree of the optical amplifier 2, but the intensity of the optical pulse emitted from the optical pulse light source 1 is
When the above condition is satisfied with respect to the peak intensity P of the optical soliton of the fundamental order, the optical amplifier 2 becomes unnecessary.

Further, in order to perform adiabatic compression as disclosed in the above-mentioned Japanese Patent Laid-Open No. 7-318988, the dispersion-decreasing optical fiber F ₁ first has an anomalous dispersion characteristic,
The soliton period Z whose rate of decrease of the dispersion value is defined by the following equation.
Need to be gentle against. Z = 0.322 × (π ² ct ² ) / (λ ² D)

In the ideal adiabatic soliton compression, the pulse width becomes thinner in proportion to the decrease of the dispersion value, and the spectrum broadens in proportion to the pulse width. However, when the reduction rate of the dispersion value is large, the phase matching condition is broken due to the dispersion value of each wavelength being different, and thus the pulse is not easily thin. Therefore, adiabatic compression can be efficiently performed by using an optical fiber with a small reduction rate of the dispersion value. Further, by using an optical fiber having a small reduction rate of the dispersion value, phase matching can be caused in a wide wavelength region even when white light is generated, and thus white light with a wide band is easily generated.

On the other hand, when the length of the optical fiber becomes long, the efficiency becomes poor due to the optical loss of the optical fiber. Taking these facts into consideration, when the reduction rate of the dispersion value in the dispersion-reducing optical fiber F ₁ is determined, the amount by which the dispersion value decreases per soliton period Z is ¼ (25) of the original dispersion value.
%) Or less (that is, the reduction rate of the dispersion value per soliton period Z is −25% or less).

The pulse width of the dispersion values and compression, since differs in passing course of dispersion decreasing optical fiber F _1, when calculating the soliton period Z based on the values at the input of the dispersion decreasing optical fiber F ₁ 110 It becomes [m]. Therefore, at a length of 800 m, the dispersion value changes from 8 [ps / km / nm] to 0 (the amount by which the dispersion value decreases per soliton cycle Z is 13.75% of the original dispersion value). An optical fiber will be used.

Next, the normal dispersion characteristic optical fiber F ₂ is not particularly limited except that it has a normal dispersion characteristic, but when the group velocity dispersion value is about −2 [ps / km / nm], A wide band of white light is obtained. Here, a dispersion-decreasing optical fiber whose dispersion value changes from 8 [ps / km / nm] to 0 at a length of 800 m described above is
An optical fiber manufactured by km will be used. In this case, the output has a normal dispersion value of -2 [ps / km / nm], and the part from 800 m to 1 km is from zero dispersion to -2.
The optical fiber has a normal dispersion characteristic in which the normal dispersion value changes up to [ps / km / nm] (see FIG. 1 (d)).

That is, 0 to 800 m is 8 [ps
/ km / nm] to 0, the dispersion-reducing optical fiber F ₁ changes its anomalous dispersion value, and the normal dispersion value changes from zero dispersion to -2 [ps / km / nm] in the 800 m to 1 km area. The optical fiber F, which is the characteristic optical fiber F ₂ portion, is used. As a result, the dispersion reducing optical fiber F ₁ and the normal dispersion characteristic optical fiber F ₂ can be covered by one optical fiber F. Thus, the optical amplifier 2 'is not necessary, the optical loss occurring in both connection points when the a dispersion decreasing optical fiber F ₁ and the normal dispersion characteristics optical fiber F ₂ prepared separately can be eliminated. Of course, it is also possible to separately prepare the optical fibers F ₁ and F ₂ and use them by connecting them.

In the structure of FIG. 1B, a schematic waveform of the optical pulse at each part is shown in FIG. here,
(A) shows the waveform of the optical pulse at the emitting end of the pulse light source 1, (B) shows the waveform of the optical pulse at the emitting end of the optical amplifier 2, and (C) shows the waveform of the optical pulse after passing through the dispersion optical fiber F _1. (D) represents the waveform of the optical pulse after passing through the optical fiber F ₂ having the normal dispersion characteristic.

The output light pulse from the pulse light source 1 is amplified by the optical amplifier 2 to a light intensity corresponding to the fundamental soliton. This optical soliton pulse propagates through the dispersion-decreasing fiber F ₁ and undergoes adiabatic compression to narrow the pulse width, widen the spectrum width, and increase the peak intensity. Then, the thin and high-intensity optical pulse after passing through the dispersion-reducing optical fiber F ₁ has a normal dispersion.
Propagating ₂ spreads the pulse waveform and the band of light rapidly due to the effects of self-phase modulation and four-wave mixing.

FIG. 2 shows the characteristics of the coherent broadband light source having the structure shown in FIG. 1 (b), obtained when various parameters are set as described above, by numerical calculation. Here, the optical pulse made incident on the optical fiber F from the optical pulse light source 1 has an input intensity of 8 [W] and a pulse repetition frequency of 10 [GHz]. The wavelength dependence of the dispersion in the optical fiber F is set to 0.07 [ps / km / nm / nm], which is the value of a general silica optical fiber.

As for the white light pulse obtained under such conditions, as shown in FIG. 2 (a), wide-band output light having a bandwidth of 80 [nm] is obtained. In addition, as shown in FIG.
(C) is an enlarged view of a lower limit wavelength portion and an upper limit wavelength portion of a white light band generated respectively.
The comb-shaped diagrams as shown in these figures show that the obtained optical signal is coherent and the information of the original pulse train, namely,
This shows that the repetition frequency of the light pulse emitted from the light pulse light source 1 is held.

FIG. 3 shows the case of using a dispersion-flat optical fiber in which the wavelength dependence of dispersion is small, and the value was 0.01 [ps / km / nm / nm]. The other conditions are the same as those when the characteristics shown in FIG. 2 are obtained. In this case, as shown in (a), the band 225 [n
m] has been obtained with a broadband output light. 3 (b) and 3 (c) are enlarged views of the lower limit wavelength portion and the upper limit wavelength portion of the generated white light band, respectively, and use an optical fiber with little wavelength dependence of dispersion. 3B and 3C, the obtained optical signal is coherent and the original pulse train information, that is, the repetition of the optical pulse emitted from the optical pulse light source 1 is repeated. It indicates that the frequency is retained.

Regarding the wavelength dependency of the dispersion in the optical fiber F, the less the dependency is, the wider the band of white light finally obtained tends to be, and the wavelength band of the white light is intentionally wider than that of a normal optical fiber. Optical fiber with little wavelength dependence (that is, the wavelength dependence of dispersion is 0.07 [ps / km / nm
/ nm] optical fiber), it is possible to obtain white light with a wider band.

Here, the coherence of the coherent broadband light source in the present embodiment will be described with reference to the noise levels shown in FIGS. 2 and 3 which are spectra of the light source obtained in the present invention and each figure. . Noise exists everywhere, such as the optical pulse light source 1 and the optical amplifier 2, and has various adverse effects when configuring such a white light source. In the light source as disclosed in the above-mentioned JP-A-8-234249, in order to use a strong nonlinear effect, that is, to expand the bandwidth by using an optical soliton whose order N is about 10, noise is generated. The signal and the signal are mixed, and the noise increases. One of such effects is "deterioration of coherence".

To widen the band of light wavelength is disclosed in Japanese Patent Laid-Open No. 8-
This can be easily done by using the technique disclosed in No. 234249, but it becomes difficult to maintain sufficient coherence. On the other hand, in the coherent broadband light source according to the present embodiment, the bandwidth is widened by using an optical soliton having a low order of about 1 or 2, so that the waveform of the optical soliton due to the influence of optical noise is generated. White light with little change and high coherence can be obtained.

That is, FIGS. 2B, 2C and 3
The "comb-shaped" spectrum remaining in the frequency spectra shown in (b) and (c) has a repetition frequency of 10 [GH.
z] is a sideband for the pulse train, and this “comb-shaped” spectrum indicates that the generated light maintains sufficient coherence.

[0042]

As described above, according to the present invention,
With a compact and simple structure, coherent light having an extremely wide wavelength band can be generated. A wavelength variable light source can be obtained by selecting such a light source with a variable bandpass filter or the like.

Also, by combining a large number of comb filters, it can be used as a light source for wavelength division multiplexing communication using more wavelengths than ever before. In multi-long wave multiplex communication, the degree of multiplicity can be dramatically increased from the conventional time multiplex communication to the same fiber, and a large amount of data can be easily transmitted.

Furthermore, since coherent light having a wide band wavelength is important for fundamentally evaluating various optical parts, it may be widely used as such a light source for evaluation.

[Brief description of drawings]

FIG. 1A is a block diagram showing a configuration example of a coherent broadband light source according to an embodiment of the present invention, and FIG. 1B is a configuration diagram of FIG. 1A in which an adiabatic pulse compressor 3 and a white light generator 4 are connected to an optical fiber. Block diagram when configured with,
(C) is a waveform diagram of an optical signal output from each unit shown in (b), and (d) is a diagram showing a change in dispersion value with respect to the length of the optical fiber F.

FIG. 2 is a waveform diagram obtained by numerical calculation in the coherent broadband light source having the configuration shown in FIG. 1 (b).

3 is a waveform diagram obtained by numerical calculation under a condition different from the condition when the waveform diagram shown in FIG. 2 is obtained in the coherent broadband light source having the configuration shown in FIG. 1 (b).

[Explanation of symbols]

1 light pulse light source 2,2 'optical amplifier 3 Adiabatic pulse compressor 4 White light generator

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References Proceedings of the 1997 IEICE General Conference, March 6, 1997, Communications 2, 851-852 (58) Fields investigated (Int.Cl. ⁷ , DB name) G02F 1/35 JISST file (JOIS)

Claims (6)

(57) [Claims] 1. An optical pulse generating means for generating an optical pulse having a peak intensity sufficient to form at least an optical soliton, and an adiabatic pulse for compressing a pulse width of the optical soliton formed by the optical pulse. comprising a compression means, and a white light generating means for expanding the pulse width and <br/> spectral width of the compressed soliton of the pulse width, the incident the light pulse generating means of the adiabatic pulse compression means
At the edges, strong solitons that can form optical solitons of order 2 or less
A coherent broadband light source characterized by generating a light pulse having a degree . 2. The adiabatic pulse compression means is an optical fiber having an anomalous dispersion characteristic, and is a dispersion-decreasing optical fiber whose anomalous dispersion value decreases from the entrance end to the exit end, and the white light generating means is The coherent broadband light source according to claim 1, wherein the coherent broadband light source is an optical fiber having a normal dispersion characteristic. 3. The coherent broadband light source according to claim 2, wherein the adiabatic pulse compression means and the white light generation means are constituted by one optical fiber. 4. The wavelength dependence of dispersion of an optical fiber used as the adiabatic pulse compression means is 0.07 [ps / km / nm / n
m], the coherent broadband light source according to claim 2 or 3 . 5. The dispersion value at the entrance end of the optical fiber used as the adiabatic pulse compression means is 5 [ps / km / n
m] or more, The coherent broadband light source according to any one of claims 2 to 4 . 6. The optical fiber used as the adiabatic pulse compression means, the reduction rate of anomalous dispersion value per soliton period, claim 2, characterized in that not more than -25%
5. The coherent broadband light source according to any one of items 1 to 4 .