JP3486612B2 - Ultrashort pulse fiber laser light source - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention improves a mode-locking ring laser used as a light source for optical communication, which generates an optical pulse having a narrow pulse width and a uniform waveform, and an ultrashort optical pulse having a pulse width of sub-picosecond. The present invention relates to an ultra-short pulse fiber laser light source for generating light.

The present invention also provides a femtosecond light source,
The present invention relates to an ultrashort pulse fiber laser light source used as a light source for optical sampling, time-resolved spectroscopy, and evaluation of optical elements.

[0003]

2. Description of the Related Art As a laser for generating an ultrashort light pulse (sub-picosecond light pulse) having a pulse width of less than 1 picosecond (1 × 10 ^-12 second), a dye laser centered at a wavelength of 600 nm has been conventionally used. Has been used. A dye laser requires a liquid circulation device because it uses an organic dye dissolved in a solvent as a laser medium, and since the organic dye is chemically unstable, it deteriorates quickly and its solution is frequently replaced. Handling was complicated, such as when it was necessary. Furthermore, a large-sized and high-output YAG laser or gas laser is required as an excitation light source for supplying energy to the dye laser, and the repetition frequency thereof is several tens of megahertz or less.

There is a titanium sapphire laser having a wavelength of 700 nm to 1 μm that overcomes the drawbacks of complicated handling, but the repetition frequency of the optical pulse is several tens to several hundreds of megahertz, and optical communication is performed. Needed in
It has been difficult to achieve above 10 GHz.

On the other hand, a mode-locking ring laser in which a ring resonator is constructed by a single-mode optical fiber has been known as a stable pulse light source, and has been used in optical communication experiments and parts / components.
It is used as a light source for material evaluation. At present, the minimum pulse width of the optical pulse output from these mode-locked ring lasers is about 1 picosecond, and it is necessary to perform pulse compression using an optical pulse compressor or the like in order to generate an even narrower optical pulse. there were.

[0006]

By the way, when the optical pulse compressor is used, there are drawbacks such that the tail of the optical pulse remains uncompressed and a small parasitic pulse is generated in addition to the main pulse. However, it was difficult to generate a light pulse having a well-shaped waveform.

The present invention does not use an optical pulse compressor, has a sub-picosecond pulse width from a mode-locked ring laser,
Another object of the present invention is to provide an ultrashort pulse fiber laser light source which can directly generate a light pulse having a well-shaped waveform.

[0008]

[Means for Solving the Problems] An optical fiber whose main component is silica glass has an anomalous dispersion characteristic in which the propagation speed becomes slower as the wavelength becomes longer in the wavelength band of 1.55 μm. On the other hand, the optical signal propagating in the optical fiber undergoes self-phase modulation due to the optical Kerr effect. From these events, the optical signal propagating in the optical fiber has a nonlinear Schrodinger equation i (∂u / ∂z) + (1/2) (∂ ² u / ∂t ² ) + | u | ² u = 0 … Represented by (1).

The first term of the equation (1) represents the propagation of light, the second term represents the group velocity dispersion effect of the optical fiber, and the third term represents the optical Kerr effect. The lowest-order solution among the stable solutions of equation (1) is represented by the hyperbolic secant function (sech function) u (z, t) = sech (t) exp (iz / 2) (2) It is an "optical soliton pulse". The optical soliton pulse is a pulse spread due to the group velocity dispersion of the optical fiber and the effect of narrowing the optical pulse due to the optical Kerr effect, so that the waveform does not change even when propagating in the optical fiber, and the phase in the pulse does not change. Is always constant (Reference 1: GP Non-linear Fiber Optics by Aglawar, published by Yoshioka Shoten, 1997).

Therefore, if the sub-picosecond pulse can be generated from the mode-locking ring laser that directly generates the optical soliton pulse, the above problem can be solved.

The optical soliton pulse is characterized in that when the waveform is changed adiabatically, the pulse width changes while maintaining the waveform. In order to cause this adiabatic change, an optical soliton pulse may be input to an optical fiber whose group velocity dispersion changes in the traveling direction. For example, the pulse width is compressed when input to an optical fiber whose group velocity dispersion decreases in the traveling direction, and the pulse width is expanded when input to an optical fiber whose group velocity dispersion increases in the traveling direction (Reference 2: S.
V. Chernikov et al., Electron. Lett., Vol. 28, p. 1842, 19
92).

Therefore, an ultrashort pulse fiber laser light source having a sub-picosecond pulse width and generating an optical pulse with a well-shaped waveform can be constructed by the following configuration. First, a mode-locked ring laser in which a ring resonator is formed by a single-mode optical fiber having an anomalous dispersion of the average group velocity dispersion is constructed and stably oscillated with a pulse width of about 1 picosecond. Here, a dispersion-reduced portion that adiabatically compresses the pulse width is provided in the ring resonator, and a thinned optical pulse is extracted. When this thin optical pulse is circulated in the ring resonator as it is, it is further compressed when passing through the dispersion reduced portion again, and the optical pulse is finally destroyed.
Therefore, by providing a dispersion increasing part that adiabatically expands the pulse width in the ring resonator and restoring the pulse width of the optical pulse compressed by the dispersion decreasing part, both stability and thinness of the optical pulse are achieved. Let

As described above, since the basic configuration of the present invention is a mode-locking ring laser which stably generates an optical soliton pulse with a pulse width of about 1 picosecond, an optical pulse having a stable and clean waveform can be obtained. A dispersion-reducing portion for performing adiabatic pulse compression as a part or all of the single-mode optical fiber of this mode-locked ring laser (dispersion-reducing optical fiber in which the magnitude of the anomalous dispersion decreases with respect to the traveling direction of the optical pulse). By incorporating a dispersion-increasing portion (a dispersion-increasing optical fiber in which the magnitude of the anomalous dispersion increases with respect to the traveling direction of the optical pulse) that performs adiabatic pulse expansion, and configuring it so that the waveform does not change before and after that, It is possible to obtain a thin and stable optical pulse at the exit of the dispersion-decreasing optical fiber (pulse compression unit) while preventing the optical pulse from collapsing.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION (First Embodiment) FIG. 1 shows a first embodiment of the present invention. In the figure, an optical amplifier 1,
The band-pass type optical filter 2, the optical modulator 3, the single-mode optical fiber 4, the output coupler 5 that extracts a part of the power of the optical pulse as output light, and the optical isolator 6 that determines the circulation direction of the optical pulse are formed in a ring shape. Combined to form a normal mode-locked ring laser. The optical amplifier 1 has a wavelength of 1.55
In the μm band, an erbium-doped optical fiber amplifier or a semiconductor optical amplifier can be used. The light modulator 3 can use an intensity modulator or a phase modulator.

In the structure of the mode-locked ring laser described above, a stable optical soliton pulse having a pulse width of about 1 ps is obtained by using the single mode optical fiber 4 with anomalous dispersion. In this laser, the waveform in the single mode optical fiber 4 can be regarded as approximately constant.

Therefore, in the present embodiment, the single-mode optical fiber 4 is divided into two, and the size of the anomalous dispersion decreases in the meantime with respect to the traveling direction of the optical pulse. 7) and a single mode optical fiber (hereinafter referred to as “dispersion-increasing optical fiber”) 8 in which the magnitude of the anomalous dispersion increases with respect to the traveling direction of the optical pulse, according to the traveling direction of the optical pulse, Further, the output coupler 5 is arranged between them. That is,
The optical pulse is taken out from the output side of the dispersion reducing optical fiber 7.

An optical amplifier 9 may be inserted between the output coupler 5 and the dispersion increasing optical fiber 8 if necessary. Further, the single mode optical fiber 4 does not necessarily have to be divided into two, and may be omitted when the dispersion decreasing optical fiber 7 and the dispersion increasing optical fiber 8 have their functions. That is, to operate this laser as a mode-locked ring laser that generates optical soliton pulses,
The average dispersion of the optical fibers (4, 7, 8) used may be anomalous dispersion.

If the group velocity dispersion of the single mode optical fiber 4 is 4 ps / km / nm, for example, the soliton period (see Reference 1) is about 100 m. It is preferable that the dispersion-reducing optical fiber 7 has substantially the same group velocity dispersion on the entrance side in order to suppress the disturbance of the optical pulse due to the connection with the single-mode optical fiber 4. In this example, one having a group velocity dispersion of 4 ps / km / nm at the input end is used. In the adiabatic compression process, the pulse width and group velocity dispersion are inversely proportional, so in order to obtain a double compression rate, the group velocity dispersion of the exit side and half of the inlet side, that is, 2ps
A dispersion reducing optical fiber 7 having a group velocity dispersion of / km / nm is used. In order to obtain a clean adiabatic compression, the length at which the magnitude of group velocity dispersion is halved may be set to about 4 times the soliton period or more. The optical pulse compressed in this way is output through the output coupler 5 and also input to the dispersion increasing optical fiber 8 and returned to the original pulse width by the adiabatic expansion process, and the other single mode optical fiber 4 Entered in.

The dispersion-decreasing optical fiber 7 and the dispersion-increasing optical fiber 8 may have the same characteristics in opposite directions or may have different characteristics. At this time, the optical amplifier 9 is inserted in the subsequent stage of the output coupler 5 to compensate for the decrease in energy in the ring resonator caused by taking out the output,
The change in the waveform may be symmetrical. Further, by adjusting the gain (amplification factor) of the optical amplifier 9, it is possible to compensate the imbalance of the characteristics of the dispersion decreasing optical fiber 7 and the dispersion increasing optical fiber 8. That is, the dispersion decreasing optical fiber 7 is adjusted so that the optical pulses at the entrance of the dispersion decreasing optical fiber 7 and the exit of the dispersion increasing optical fiber 8 become equal.
The characteristics of the dispersion increasing optical fiber 8 or the gain of the optical amplifier 9 may be designed.

Here, the decrease / increase of group velocity dispersion in the dispersion decreasing optical fiber 7 and the dispersion increasing optical fiber 8 is as follows.
It need not be monotonic and need not be smooth. This is because when the group velocity dispersion value of the optical fiber is changed by a sufficiently short distance as compared with the dispersion distance determined by the signal light (see Reference 1), the behavior of the signal light is the local group velocity. This is because it is not determined by the dispersion value, but by the average of the group velocity dispersion values.
Therefore, even if a large number of optical fibers having different group velocity dispersions are connected, the group velocity dispersion can be set to increase / decrease as an "average of a certain distance" (reference document 3: Masataka Nakazawa et al., Application Journal of the Physical Society of Japan 34
681, 1995, Ref. 4: SVChernikov et al., Elec
tron.Lett., vol.29, p.1788, 1993).

FIG. 2 shows an example of group velocity dispersion characteristics of the dispersion-decreasing optical fiber 7 (change in group velocity dispersion value in the traveling direction of the optical pulse). The same applies to the dispersion-increasing optical fiber 8, but the characteristics of the both need not necessarily be symmetrical as described above.

FIG. 2 (1) shows an example in which the group velocity dispersion value changes smoothly and linearly along the traveling direction of the optical pulse, FIG. 2 (2)
Shows an example in which the group velocity dispersion value changes smoothly and non-linearly along the traveling direction of the light pulse, and FIG. 2 (3) shows an example in which the group velocity dispersion value changes stepwise along the traveling direction of the light pulse, Figure 2
(4) and (5) are examples in which the group velocity dispersion value changes in a comb shape along the traveling direction of the optical pulse.

FIG. 3 shows the result of computer analysis of the optical pulse waveform of each part of the first embodiment. This is an example in which the change in group velocity dispersion value is increased from 12 ps / km / nm to 3 ps / km / nm by 4 times, and the pulse width compression ratio is increased by 4 times.

FIG. 3A shows an input side A of the dispersion decreasing optical fiber 7, an output side of the dispersion decreasing optical fiber 7 (an input side of the dispersion increasing optical fiber 8), and an output side C of the dispersion increasing optical fiber 8.
The optical pulse waveform of is shown. It can be seen that the pulse width gradually decreases from the point A to the point B, and gradually increases from the point B to the point C. FIG. 3 (2) is the optical pulse waveform at point A, and FIG. 3 (3) is the optical pulse waveform at point B. The pulse widths are 0.83ps and 0.21ps, respectively, and it can be seen that a compression ratio of 4 times is obtained almost as designed.

(Second Embodiment) FIG. 4 shows a second embodiment of the present invention.
2 shows an embodiment of the present invention. In the figure, an optical amplifier 1, a bandpass optical filter 2, an optical modulator 3, and a single mode optical fiber 4 are shown.
Form a mode-locked ring laser by ring-shaped coupling as in the first embodiment. In this embodiment, the optical circulator 11 is placed at the position of the output coupler 5 of the first embodiment.
And connect its first and third ports to the ring. To the second port of the optical circulator 11, a dispersion-reducing optical fiber 7, a bidirectional optical amplifier 12, and an output coupler (for example, a half mirror) 13 for extracting a part of an optical pulse as output light and reflecting the rest are sequentially connected, and reciprocated. Form a path. The optical circulator 11 is configured to output the optical pulse input from the first port to the second port and output the optical pulse input from the second port to the third port, and block the light input from the third port ( By terminating the fourth port), the function of the optical isolator 6 in the first embodiment can be provided.

The optical pulse that circulates in the mode-locking ring laser passes through the optical circulator 11 from the first port to the second port, is input to the dispersion-reducing optical fiber 7, and is subjected to pulse compression. The compressed optical pulse is transmitted to the bidirectional optical amplifier 1
2 and output via the output coupler 13,
A part of it is reflected by the output coupler 13, and the bidirectional optical amplifier 1
The signal is amplified by 2 and input to the dispersion reducing optical fiber 7 from the opposite direction. As a result, the dispersion-decreasing optical fiber 7 functions as a dispersion-increasing optical fiber, the optical pulse is returned to the original pulse width, and is input into the ring via the optical circulator 11. That is, the configuration in which the optical pulse reciprocates in the dispersion-decreasing optical fiber 7 enables the dispersion-decreasing optical fiber 7 to compress and expand the optical pulse.

As described above, in this embodiment, by using one dispersion decreasing optical fiber 7, the dispersion decreasing optical fiber 7 and the dispersion increasing optical fiber 8 as in the first embodiment.
It is not necessary to consider the difference in the characteristics of. Bidirectional optical amplifier 1
2 is used as necessary in order to compensate for the decrease in energy in the ring resonator due to the output being taken out by the output coupler 13 and to make the optical pulses the same at the first port and the third port of the optical circulator 11. To be

Further, as shown in FIG. 5, when the optical coupler 14 is used in place of the optical circulator 11, it is necessary to insert the optical isolator 6 at the first port side of the optical coupler 14, for example. In FIG. 5, the port number of the optical coupler 14 is shown in correspondence with the port number of the optical circulator 11 shown in FIG.

[0029]

As described above, in the ultrashort pulse fiber laser light source of the present invention, the output of the dispersion-decreasing optical fiber is obtained by inserting the dispersion-decreasing optical fiber and the dispersion-increasing optical fiber in combination in the mode-locked ring laser. An optical pulse having a pulse width of sub-picosecond can be stably extracted from the side.

[Brief description of drawings]

FIG. 1 is a block diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram showing an example of group velocity dispersion characteristics of a dispersion decreasing optical fiber 7.

FIG. 3 is a view showing a result of analyzing a light pulse waveform of each part of the first embodiment by a computer.

FIG. 4 is a block diagram showing a second embodiment of the present invention.

FIG. 5 is a block diagram showing a modification of the second embodiment of the present invention.

[Explanation of symbols]

1 Optical amplifier 2 Bandpass optical filter 3 Optical modulator 4 Single mode optical fiber (optional) 5 output coupler 6 Optical isolator 7 Dispersion reduced optical fiber 8 Dispersion-increasing optical fiber 9 Optical amplifier (optional) 11 Optical circulator 12 Bidirectional optical amplifier (optional) 13 Output coupler 14 Optical coupler

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Claims (4)

(57) [Claims] 1. An optical amplifier, a bandpass optical filter,
An output that extracts an optical soliton pulse generated in the ring resonator to the outside by combining an optical modulator and a single-mode optical fiber in which the average value of group velocity dispersion is anomalous dispersion to form a ring resonator. A mode-locking ring laser provided with a coupler, as a part or all of the single-mode optical fiber, a dispersion-decreasing optical fiber whose magnitude of anomalous dispersion decreases with respect to the traveling direction of an optical soliton pulse, and anomalous dispersion Size is light
Using a dispersion increasing optical fiber that increases with respect to the traveling direction of the soliton pulse, a part of the power of the optical soliton pulse whose pulse width is adiabatically compressed by the dispersion decreasing optical fiber is extracted from the output coupler, and the optical soliton pulse is extracted. configuration der to circulate the ring resonator and the remaining power of extended pulse width input to the dispersion increases optical fiber adiabatically is, photosensitizers between said output coupler the dispersion increases optical fiber
Insert a width device and insert an optical source on the input side of the dispersion-reducing optical fiber.
Reton pulse and optical fiber at the output side of the dispersion-increasing optical fiber
The gain is adjusted so that the reton pulses are equal.
Ultrashort pulse fiber laser light source, which is a formed. 2. An optical amplifier, a bandpass optical filter,
An output coupler that forms a ring resonator by combining an optical modulator and a single-mode optical fiber whose group velocity dispersion average is anomalous to form a ring resonator, and extracts the optical pulse generated in the ring resonator to the outside. A mode-locking ring laser comprising: a ring resonator, wherein a first port and a third port of an optical circulator are connected to each other, the input light from the first port is output, and the output light to the third port is output. The output coupler is connected to a second port for inputting to form a round-trip path, and the optical coupler is provided between the second port of the optical circulator and the output coupler as a part or all of the single mode optical fiber. A dispersion-decreasing optical fiber whose anomalous dispersion magnitude decreases with respect to the traveling direction of the optical pulse viewed from the circulator is inserted, and the power of the optical pulse whose pulse width is compressed by the dispersion-decreasing optical fiber is inserted. Is extracted from the output coupler, the remaining power of the optical pulse is reflected by the output coupler and input to the dispersion-reducing optical fiber from the opposite direction, and the pulse width is extended to circulate in the ring resonator. An ultra-short pulse fiber laser light source having a configuration. 3. The ultrashort pulse fiber laser light source according to claim 2, wherein an optical amplifier is inserted between the output coupler and the dispersion-reducing optical fiber, and an optical pulse is input to the first port of the optical circulator. , An ultrashort pulse fiber laser light source having a configuration in which the gain is adjusted so that the output light pulses of the third port become equal. 4. The ultrashort pulse fiber laser light source according to claim 2 or 3, wherein an optical coupler is used instead of the optical circulator, and a direction of an optical pulse circulating in the ring resonator is made constant. An ultrashort pulse fiber laser light source having a structure in which an optical isolator is inserted.