JP5994965B2 - Optical fiber dispersion compensator and terahertz wave pulse generator / detector using the same - Google Patents

Description

  The present invention relates to a method for compensating for group velocity dispersion in an optical fiber, and particularly relates to a dispersion compensator suitable for transmitting femtosecond pulsed laser light.

  In signal transmission using an optical fiber, problems of dispersion of the optical fiber and nonlinear optical effects are widely known. These not only affect the transmission quality of signal transmission, but recently, they are also used for the transmission of pulse waves with extremely small pulse widths such as femtoseconds, so increasing the pulse width due to the refractive index dispersion of the fiber is also a problem. It has become.

  Conventionally, optical fiber type devices such as a dispersion compensating fiber (DCF) and an optical fiber Bragg grating (FBG) have been provided to compensate for the dispersion of the optical fiber. There are also provided dispersion compensation optical multilayer films called chirp mirrors, dispersion compensators using prism pairs and diffraction grating pairs, and the like.

When transmitting an optical pulse having a wavelength of 1.5 μm, a dispersion compensating fiber that compensates for positive dispersion of a normal single mode fiber is used. However, the dispersion compensating fiber has many drawbacks such as inability to maintain polarization, increase in pulse width due to high nonlinearity, and high insertion loss.
Also, when transmitting femtosecond pulse lasers, energy concentrates on ultrashort pulses, so dispersion compensation fibers with small core diameters can cause nonlinear phenomena that cause increased pulse width, such as self-phase modulation, and can be applied. Can not.

  Since the fiber Bragg grating is required to be manufactured with high accuracy, there is a problem that the device becomes expensive. In addition, there are many adjustment points and handling is difficult.

  On the other hand, in a measuring device using an optical fiber as a light transmission means, there is a case where light is coupled to the sensor unit with a fiber, and these devices are adapted to adapt to various measuring environments. It is required that the fiber length can be increased. Furthermore, when polarization information is important in optical measurement, it is required that polarization is preserved in transmission through the fiber.

  The related patent document 1 prepares a dispersion compensating fiber having positive chromatic dispersion and a dispersion compensating fiber having negative chromatic dispersion, guides a wavelength multiplexed optical signal to one of the dispersion compensating fibers, and once sets the wavelength to positive or negative. After shifting the entire band, it is proposed to fine-tune with a dispersion compensating fiber with the opposite sign.

  Further, Patent Document 2 includes a chirp unit including a normal dispersion fiber that imparts a positive chirp to an incident pulse, and a dispersion compensation unit configured to include an anomalous dispersion fiber, and nonlinearity of the anomalous dispersion fiber that forms the dispersion compensation unit. The absolute values of the coefficient and the second-order group velocity dispersion are set so that the soliton order is 1 or more. Further, a pulse compressor has been proposed in which the fiber length of the anomalous dispersion fiber is set to be equal to or shorter than the length at which the optical soliton is formed.

JP 2006-186489 A JP 2009-180812 A

Internet URL http://www.cas.usf.edu/lidarlab/hitran_pc.html Search May 31, 2011 Govind P. Agrawal, Nonlinear Optics Fiber 4th Edition p8 Handbook of Optics, 3rd edition, Vol. 4. McGraw-Hill 2009 Internet URL http://refractiveindex.info/?group=CRYSTALS&material=Si Search May 31, 2011

  The present invention has been created in view of the above-described problems of the prior art, and an object thereof is to provide a dispersion compensator capable of preserving the polarization state. In particular, a technique that can be used for transmission of femtosecond pulsed laser light is provided.

In order to solve the above problems, the present invention provides the following dispersion compensator.
That is, a dispersion compensator that compensates for group velocity dispersion in a single-mode optical fiber, and uses a dispersion compensation material that exhibits group velocity dispersion of the optical fiber corresponding to the wavelength of the propagating light and a reverse sign of the group velocity dispersion, and is connected to the optical fiber. A light input side end, a light output side end that outputs dispersion compensated light, and a light that passes through the dispersion compensation material between the light input side end and the light output side end The dispersion compensation unit.

  A first lens that diverges light input from the light input side end portion into parallel light parallel to the optical axis of the dispersion compensation unit, and parallel light that has passed through the dispersion compensation unit is output as the light output. The structure provided with the 2nd lens focused toward a side edge part may be sufficient.

  It is particularly preferable that the absolute value of the group velocity dispersion value of the dispersion compensation material is 10 to 100 times the absolute value of the group velocity dispersion value of the optical fiber.

  Silicon can be used as the dispersion compensation material.

  The light propagating through the optical fiber may be femtosecond pulsed laser light and may be configured to compensate for group velocity dispersion of the laser light.

  A wavelength filter that selects a wavelength region that can be compensated by the dispersion compensation unit may be provided at the light input side end.

  You may provide the beam diameter adjustment means which enables adjustment of the beam diameter which injects into said dispersion compensation part.

  In the above dispersion compensation unit, a plurality of dispersion compensation materials exhibiting different group velocity dispersions may be arranged in series on the optical axis.

  In the present invention, a single-mode optical fiber having a predetermined length is connected to the dispersion compensator described above for compensating for group velocity dispersion caused by an optical fiber having a predetermined length to form a pair of optical fibers. Can also provide an optical fiber combination including a connecting portion that can be connected to another optical fiber combination.

  The present invention can also provide a terahertz wave pulse generation / detection device including the dispersion compensator described above.

Furthermore, the following dispersion compensation method is provided.
That is, a dispersion compensation method for compensating for the group velocity dispersion in an optical fiber, which uses a dispersion compensation material that exhibits a group velocity dispersion opposite to the group velocity dispersion of the optical fiber according to the wavelength of the propagating light, and that is connected to the optical fiber. The light input from the input side end is diverged into parallel light by the first lens, and the parallel light is passed through the dispersion compensator made of the dispersion compensation material to compensate for the group velocity dispersion, and then the parallel light is output by the second lens. Are output from the light output side end.

The present invention has the following effects by adopting the above configuration.
Dispersion compensation can be performed with a simple configuration by using a dispersion compensation material such as silicon which exhibits group velocity dispersion of the optical fiber corresponding to the wavelength of propagating light and the group velocity dispersion of the opposite sign. Also, the polarization state can be preserved.

  Dispersion compensation is provided by including a first lens that diverges into parallel light parallel to the optical axis of the dispersion compensator and a second lens that focuses the parallel light that has passed through the dispersion compensator toward the light output side end. The energy per unit area is reduced by enlarging the large diameter of the substance. Further, by integrating the combination of each lens and the dispersion compensation material at the time of manufacture, adjustment during use can be made unnecessary.

  By selecting a dispersion compensation material whose absolute value of the group velocity dispersion value of the dispersion compensation material is 10 to 100 times the absolute value of the group velocity dispersion value of the optical fiber, the length of the dispersion compensation material with respect to the optical fiber is increased. This contributes to a reduction in the size of the dispersion compensator and a reduction in material costs.

  According to the optical fiber combination of the present invention, dispersion compensation materials exhibiting reverse-sign dispersion that compensate for dispersion of an optical fiber of a predetermined length are combined in advance, so that dispersion compensation can be performed correctly even when a plurality of the combinations are connected. An optical fiber line can be configured. Thereby, it is possible to add an optical fiber having a desired line length without adjusting dispersion compensation each time.

It is sectional drawing of the dispersion compensator which concerns on this invention. It is a graph which shows the dispersion parameter of a single mode fiber. It is a graph which shows the group velocity dispersion value at the time of connecting the group velocity dispersion value of the single mode fiber of length 3.2m, and 7cm silicon | silicone and the single mode fiber of 3.2m. It is a graph which shows the relationship between the length of an optical fiber, and a pulse width. It is a block diagram of a fiber coupling type terahertz wave pulse generator / detector. It is a graph of the terahertz pulse shape in the terahertz wave pulse generator / detector. It is a graph which shows the spectrum of a terahertz wave pulse. It is a configuration diagram in which a combination of a dispersion compensator and an optical fiber is added to the terahertz wave pulse generator / detector. It is a block diagram of a fiber coupled terahertz wave pulse generator / detector using a nonlinear optical crystal. It is sectional drawing of a beam diameter variable dispersion compensator. This is an embodiment in which the dispersion compensator is provided with a second dispersion compensation optical element.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. The present invention is not limited to the following examples and can be appropriately implemented within the scope of the claims.
Example 1
FIG. 1 is a sectional view of a dispersion compensator (1) according to the present invention. The dispersion compensator (1) of the present embodiment has a cylindrical shape with a total length of about 7 to 8 cm, and is extremely small. Connection portions connected to the optical fiber (2) are arranged at both ends, and after compensating for the group velocity dispersion of light input from one side, it is output from the other side.

The dispersion compensator (1) has a rod lens (12) on the left and right sides of the dispersion compensator (10) centered on the dispersion compensator (10) made of a bulk-form dispersion compensator which is a feature of the present invention. ) (14).
The light input end (e.g., left side) dispersion compensator incident light from the optical fiber (2) in input to the (10), parallel to parallel to the optical axis of the dispersion compensation unit (10) by the rod lens (12) Diverge into light (20). The parallel light (20) passes through the entire length of the dispersion compensator (10), and then enters the right rod lens (14) and is focused again. The converged light beam is output from the light output side end portion to the optical fiber (2).

In this dispersion compensator (1), the rod lenses (12) (14) and the end face of the optical fiber (2) are fused, and the rod lenses (12) (14) are fixed by the lens fixing portions (13) (15). By fixing the dispersion compensator (10) in an adjusted state by the dispersion compensator fixing part (11), adjustment during use can be made unnecessary.
In order to improve the robustness as described above, the use of the rod lens and the lens fixing portion is particularly suitable, but it is not always necessary to use the rod lens in the implementation. A lens system such as a known aspherical lens or a beam expander can be arbitrarily used.

  Furthermore, in the present invention, the first lens and the second lens are not essential. That is, when the diameter of the beam transmitted by the optical fiber is large, or when the pulse energy is small, the output light from the optical fiber (2) may be input to the dispersion compensator (1) as it is.

Next, the principle of the present invention will be described.
In brief, the present invention compensates for the group velocity dispersion that occurs when a single-mode optical fiber is transmitted by a dispersion compensation material having a group velocity dispersion opposite in sign to the dispersion.
Since dispersion in an optical fiber is a reversible phenomenon, it is obvious to try to compensate by dispersion of the opposite sign. For example, a dispersion compensation fiber (DCF: Dispersion Compensating Fiber), which is a typical device for performing dispersion compensation, In contrast to the material dispersion (usually positive dispersion) that occurs in a single mode fiber, the inside of the fiber cross-section is designed to have a special refractive index distribution, and the structure is dispersed (negative dispersion).

On the other hand, the present invention is not based on such structural dispersion, but realizes dispersion compensation by a method using a material having a dispersion characteristic opposite to that of a single mode fiber. In other words, unlike the conventional dispersion compensation by structural dispersion, the dispersion compensator is configured by selecting a substance having a necessary material dispersion characteristic.
By using a new dispersion compensation method, the present invention can simultaneously solve the problems in the conventional dispersion compensation fiber, such as the fact that the polarization state cannot be maintained and the nonlinearity is high.

  In this embodiment, silicon (Si) is used as an example of the dispersion compensation material. Hereinafter, a method for selecting the dispersion compensation material of the present invention will be described while showing the dispersion characteristics of silicon. Therefore, if a substance satisfying the conditions shown here is selected, the present invention can be implemented not only with silicon but also with any substance.

  First, the single mode fiber has a characteristic that the sign of group velocity dispersion is reversed at the zero dispersion wavelength (about 1300 nm) as a boundary. Light having a wavelength of 1300 nm or more has positive dispersion. A single mode fiber made of quartz has a group velocity delay for light other than 1300 nm on the wavelength axis.

As a compensation method for the group velocity dispersion of a single mode fiber, it is only necessary to connect a fiber having an absolute value of the group velocity dispersion (GVD) equal to that of the single mode fiber and a sign opposite to that of the single mode fiber. That is, the following conditions may be satisfied.
(Equation 1)
GVD _Si + GVD _SMF = D _Si L _Si + D _SMF L _SMF = 0

Here, GVD _Si is silicon and GVD _SMF is the group velocity dispersion of a single mode fiber. L is the length of each device, D is a dispersion parameter, the subscript Si is silicon, and SMF is a single mode fiber value.

In the dispersion compensation method according to the present invention, a material having a dispersion parameter having an opposite sign to the dispersion parameter D of the single mode fiber is used. Silicon can be used as one of them. By multiplying the dispersion parameter D by the distance of the medium, the group velocity dispersion (GVD) of the device is obtained.
The dispersion parameter D is a function of the refractive index and the wavelength, and is given by the following equation (Equation 2). (See Non-Patent Document 2)

ここで、λは波長、cは光速度、nは屈折率である。

Here, λ is the wavelength, c is the speed of light, and n is the refractive index.

  The wavelength dependence n (λ) of the refractive index of silicon can be derived from the Selmeier formula. The Cermeier equation and its parameters for deriving the refractive index of silicon are shown in Equation 3 and Table 1, respectively. (See Non-Patent Documents 3 and 4)

The dispersion parameter of silicon is obtained from Equations 2 and 3.
FIG. 2 shows the dispersion parameter D of silicon. For comparison, an example of a dispersion parameter of a single mode fiber is shown. In the present application, the unit of the dispersion parameter D is fs / nm-m, the product of this multiplied by the length L of the medium is group velocity dispersion (GVD), and the unit is fs / nm.

  As is apparent from the figure, the dispersion parameter D of the single mode fiber indicates positive dispersion. On the other hand, the dispersion of silicon is negative. Taking the wavelength 1560 nm as an example, the dispersion parameter of the single mode fiber is about 18 fs / nm-m. On the other hand, it can be seen that the dispersion of silicon is negative. It can be seen that the value of the dispersion parameter D is -855 fs / nm-m, which is larger than the group velocity dispersion of the fiber.

In order to transmit the femtosecond pulse through the optical fiber, the relationship between the dispersion parameter and the length of the single mode fiber and silicon at the wavelength constituting the femtosecond pulse may be configured so as to satisfy the equation (1). From Equation 1, if the length of the single mode fiber to which the femtosecond laser is to be transmitted is determined, the silicon length to be used for dispersion compensation can be determined.
In other words, if the dispersion parameter of the single mode fiber is D _SMF and the dispersion parameter of silicon is D _Si , the optimum silicon length for the length of the single mode fiber can be determined from Equation 4.

As an example, an attempt was made to transmit a femtosecond pulse having a center wavelength of 1560 nm and a pulse width of 100 fsec or less through a single mode fiber of 3.2 m. The dispersion parameters of single mode fiber and silicon at 1560 nm are about 18 fs / nm-m and -855 fs / nm-m, respectively.
From Equation 4, the length of silicon for the purpose of dispersion compensation is required to be about 7 cm because the ratio with the length of the single mode fiber should be 18: 855≈1: 47.

  FIG. 3 shows the wavelength dependence of the group velocity dispersion value of a 3.2 m long single mode fiber and the group velocity dispersion value when 7 cm silicon and a 3.2 m single mode fiber are connected. It can be seen that the group velocity dispersion is almost zero at 1560 nm.

Next, in order to demonstrate that pulse transmission is possible with the above-mentioned combination of silicon and single mode fiber, 7 cm long silicon was used as a dispersion compensation material, and the pulse width was measured by changing the length of the single mode fiber. .
The length of the fiber was changed in units of 1 m. FIG. 4 shows the femtosecond pulse width after fiber transmission. It can be seen from FIG. 4 that the pulse width decreases as the fiber length increases, shows a minimum at 3.2 m, and increases again. The observed pulse width was about 100 fsec. The pulse width was measured with an autocorrelator. The pulse width was FWHM (full width at half maximum).

As described above, since the group velocity dispersion of light including various wavelengths occurs during the transmission of a single mode fiber, even if it is output from a fiber laser device, even if it is an ultrashort pulse, light from a wavelength having a large group velocity delay is generated. However, there is a problem that waveform distortion occurs to light having a wavelength with less delay, and the pulse width is widened. An increase in the width of the optical pulse may cause an increase in error rate and decrease in transmission efficiency in signal transmission, and a decrease in accuracy when terahertz waves are used for analysis, inspection, measurement, and the like.
Therefore, there is an increasing demand for a pulse width that can be output in a femtosecond pulse generator, but the required pulse width is about 150 fsec. In an example device. In this case, the pulse obtained in the experiment of FIG. A width of 100 fsec. Will fully satisfy this requirement.

  In the experiment of FIG. 4, 3.2 m, which has the shortest pulse width, is the design value derived from Equation 4 above when the silicon length is 7 cm. This also confirms that the design method of the present invention shown above is correct.

  As is apparent from FIG. 2, the length of the optical fiber and the length of the dispersion compensation material depend on the wavelength of the transmitted light. For example, in the case of 1500 nm, the dispersion value of the single mode fiber is 15 fsec./nm-m, and the dispersion value of silicon is -980 fsec./nm-m. In this case, the length ratio is 980: 15≈65: 1. This ratio increases gradually toward the zero dispersion wavelength (1300 nm). Conversely, this ratio decreases gradually toward longer wavelengths.

  In the present invention, since it is desirable to provide a sufficiently small dispersion compensator for an optical fiber, it is necessary to select a dispersion compensation material and design its length in accordance with the wavelength to be transmitted. In the case of transmission of femtosecond pulsed laser light in this embodiment, an optical fiber of several meters to several tens of meters is usually used, unlike a communication transmission cable. For such applications, silicon with a ratio of 47: 1 at the center frequency is optimal and a dispersion compensator of several centimeters is realized.

  In general, from the above viewpoint, the absolute value of the group velocity dispersion value of the dispersion compensation material is preferably 10 to 100 times the absolute value of the group velocity dispersion value of the optical fiber. In this case, the length of the dispersion compensation material is 1/10 to 1/100 of the length of the optical fiber.

  In addition, since the present invention uses a dispersion compensation material having a dispersion characteristic opposite to the dispersion generated in the optical fiber, for example, when transmitting light in a wavelength range shorter than the zero dispersion wavelength, the dispersion compensation material is A material having a positive dispersion may be used. Even in this case, the length can be appropriately designed according to the ratio of the absolute values of the dispersion values.

(Example 2)
Next, as an example of incorporation into the apparatus of the present invention, a fiber-coupled terahertz wave pulse generator / detector was configured using a silicon dispersion compensator (1). FIG. 5 is a configuration diagram thereof.
The fiber-coupled terahertz wave pulse generator / detector includes a femtosecond fiber laser (50), a dispersion compensator (1), a beam splitter (53), an optical delay mechanism (54), and a terahertz wave pulse transmitter (57). It consists of a photoelectric conducting antenna of the receiver (59). A terahertz pulse wave (58) is generated between the photoconductive antennas.

The femtosecond fiber laser has a center wavelength of 1560 nm, a maximum output of 130 mW, and a pulse width of <100 fsec. The femtosecond pulse has a femtosecond pulse width at the output fiber end face of the femtosecond fiber laser.
The optical pulse is introduced into the dispersion compensator (1) according to the invention. The dispersion compensator (1) used here is designed to minimize the pulse width at the end face when connected to a 1.7 m optical fiber (52) and (55) or (52) and (56). ing.

The light pulse is branched by a beam splitter (53), and one is transmitted to a transmitter (57) via a 1.1 m single mode fiber (55). The other is transmitted to the receiver (59) via the optical delay mechanism (54) and the single mode fiber (56).
The fiber length from the dispersion compensator (1) to the transmitter (57) and the receiver (59) is 1.7 m, respectively.
When the pulse width at the fiber exit end face was measured with an autocorrelator, the pulse width was about 100 fsec. In FWHM (full width at half maximum).

For the transmitter (57) and the receiver (59) of the terahertz wave pulse, a photoconductive antenna capable of operating in the 1560 nm band was used. A rectangular wave voltage is applied to the transmitter (57) side at 1 kHz for bias voltage and intensity modulation. The intensity-modulated terahertz wave pulse (58) enters the receiver (59) and is detected by the lock-in amplifier (61) via the current amplifier (60).

  FIG. 6 shows the detected terahertz wave pulse shape, and it can be seen that it is a monocycle. This device can obtain spectrum information in the terahertz frequency band. FIG. 7 shows a spectrum obtained by Fourier-transforming the detected terahertz wave pulse. The figure shows a water vapor absorption line at room temperature determined by HITRAN-PC (see Non-Patent Document 1). In the spectrum of the terahertz wave pulse, the absorption line of water vapor in the vicinity of 3 THz is detected, so that it can be seen that it has been generated up to around 3 THz.

(Example 3)
FIG. 8 shows an embodiment in which the dispersion compensator (1) according to the present invention is added to the apparatus configuration of FIG. 5 as an extension module (80) (81) configured in combination with an optical fiber (2) having a predetermined length. It is.
That is, the pulse output from the femtosecond pulse generator including the output from the beam splitter (53) and the input to the optical delay mechanism (54) is connected to the extension module (81) and extended by a predetermined length. Above, a terahertz pulse is generated from the transmitter (57). Similarly, femtosecond pulses are input to a receiver (59) that detects terahertz waves.

The dispersion compensator (1) of the present invention is dispersion-compensated by combining a dispersion compensation material having a length determined with respect to the length of the optical fiber as described repeatedly. Therefore, for example, by providing a module in which a 1 m optical fiber and a dispersion compensator (1) having a length corresponding to the 1 m optical fiber are provided in advance, the connecting portions provided at the end portions of the module are connected as necessary. Only with this, dispersion-compensated optical fiber transmission can be realized.
In particular, if it is provided as a product adjusted by fusing a rod lens and an optical fiber, adjustment when assembling the optical system is unnecessary, and an extension module that is extremely easy to handle can be provided.

Example 4
FIG. 9 shows another embodiment of the fiber-coupled terahertz wave pulse generator / detector shown in FIG. 5, which generates a terahertz wave using a nonlinear optical crystal (93). Note that a non-linear optical effect electro-optic modulation technique may be used as the detector method.
As is well known, the nonlinear optical crystal (93) can shift the frequency of the femtosecond pulsed laser light to the terahertz region by utilizing the optical rectification effect due to the nonlinear optical effect. At this time, it is necessary to match the incident beam, the optical axis, and the axial angle of the crystal.

The conventional dispersion compensating fiber has a problem that polarization cannot be maintained. On the other hand, since the polarization compensator (1) of the present invention maintains the polarization, the polarization state can be maintained in the entire optical system by using a polarization preserving fiber for the optical fiber.
That is, if all of the optical fibers (51), (52), (91), and (92) in FIG. 9 are polarization-preserving fibers and the incident light is linearly polarized, it is possible to transmit femtosecond pulses while maintaining linearly polarized light. It is. Therefore, by rotating the nonlinear optical crystal (93) and adjusting the crystal axis and the polarization of the femtosecond pulse to an optimum angle, frequency conversion to a terahertz wave can be easily realized.

  As the dispersion compensation material used in the present invention, it is preferable to select a material that does not have a birefringence and can maintain the polarization of incident light. The silicon used in this example satisfies this condition and is suitable.

(Example 5)
FIG. 10 proposes a configuration with a variable beam diameter in the dispersion compensator (1 ′) of the present invention. In this embodiment, the beam diameter of the parallel light diverged by the rod lens (12) is changed using the beam diameter variable telescope (3) which is a beam diameter adjusting means.

In the case of a femtosecond pulse, energy concentrates on an ultrashort pulse, so that the energy peak increases even if the laser output itself is not so strong. Application to processing technology is also performed using such characteristics, and dispersion compensation devices are also required to cope with ultrashort pulses.
In addition, when the energy peak increases, nonlinear phenomena occur in the material and loss increases. Therefore, it is necessary to suppress energy per unit area.

  In the present invention, since the dispersion compensation material having a diameter much larger than the core diameter of the optical fiber is passed, it is very advantageous in that the energy density per unit area is lowered. The beam diameter variable telescope (3) is particularly suitable in combination with a femtosecond terahertz pulse generator because the beam diameter can be expanded when energy is large.

(Example 6)
FIG. 11 shows an embodiment provided with a second dispersion compensation optical element in addition to the dispersion compensation material.
Incident light from the optical fiber (111) is diverged by the rod lens (112), passes through the optical prism (113), passes through the dispersion compensator (114) of the present invention, and enters the dispersion compensating optical element (115).

In this embodiment, a chirp mirror is used as the dispersion compensation optical element (115). Since the optical fiber and silicon are not completely opposite characteristics, residual dispersion that cannot be eliminated by a combination of these is intended to be compensated by a dispersion compensating optical element such as a chirp mirror.
As a specific example, the total dispersion value shown in FIG. 3 is slightly inclined depending on the wavelength, but by combining different dispersion compensation optical elements such as a chirp mirror,
Higher order dispersion compensation is possible.

The light reflected by the chirp mirror (115) again passes through the dispersion compensator (114), is reflected upward by the optical prism (113), is focused by the rod lens (116), and then converged by the optical fiber (117). Emanates from.
Since the chirp mirror (115) passes through the dispersion compensator (114) twice, the total length of the dispersion compensator (114) in this embodiment is half that of the embodiment of FIG.

  As the second dispersion compensation optical element, a known dispersion compensation mechanism can be appropriately selected. In addition to the chirped mirror, a dispersion compensating fiber, an optical fiber Bragg grating (FBG), a dispersion prism, or the like may be used. Moreover, you may combine the other device provided as a dispersion compensator. Thereby, compensation for higher-order dispersion can be realized.

  The dispersion compensation unit is not limited to a configuration using one dispersion compensation material, and may be a configuration in which a plurality of dispersion compensation materials exhibiting different group velocity dispersions are arranged in series on the optical axis. By appropriately combining the lengths of the respective dispersion compensation substances, as described above, it contributes to compensation of higher-order dispersion.

  It is also possible to provide a dispersion compensator that allows the incident light of the dispersion compensator to pass through a wavelength filter. That is, dispersion compensation can be performed only for the designed wavelength by using a wavelength filter to adjust to a wavelength region that can be compensated by the dispersion compensation material and cutting other wavelength regions.

  The change in the pulse width in the fiber is not only due to group velocity dispersion but also due to self-phase modulation, which is a kind of nonlinear optical effect. The change in the pulse width varies depending on the intensity of the optical pulse transmitted through the fiber. For this reason, the mismatch between the dispersion values of the dispersion compensator and the optical fiber according to the present invention may be designed such that the light intensity propagating through the fiber is adjusted so that the pulse width becomes the shortest at the fiber exit end.

DESCRIPTION OF SYMBOLS 1 Dispersion compensator 2 Optical fiber 10 Dispersion compensation part 11 Dispersion compensation part fixing | fixed part 12 Rod lens 13 Lens fixing | fixed part 14 Rod lens 15 Lens fixing | fixed part 20 Parallel light

Claims (10)

A dispersion compensator for compensating for group velocity dispersion in a single mode optical fiber,
Using a dispersion compensation material that exhibits a group velocity dispersion opposite to the group velocity dispersion of the optical fiber according to the wavelength of the propagating light,
An optical input side end connected to the optical fiber;
A light output side end portion for outputting dispersion-compensated light;
Between the light input side end portion and the light output side end portion, a light is allowed to pass through the dispersion compensation material that is bulky, and a dispersion compensation unit that performs dispersion compensation only by material dispersion. ,
A first lens that diverges light input from the light input side end portion into parallel light parallel to the optical axis of the dispersion compensation unit;
A second lens that focuses the parallel light that has passed through the dispersion compensation unit toward the light output side end;
Dispersion compensator, characterized in that it comprises a. The absolute value of the group velocity dispersion of the dispersion compensating material, the dispersion compensator of claim 1 wherein the optical fiber group velocity dispersion value of the absolute value of more than 10 times is 100 times or less. The dispersion compensator according to claim 1 or 2 , wherein the dispersion compensation material is silicon. The light propagating through the optical fiber is femtosecond pulsed laser light,
Dispersion compensator according to any one of 3 claims 1 used for compensation of the group velocity dispersion of the laser beam. At the light input side end,
Dispersion compensator according to any one of 4 to claims 1 comprising a wavelength filter for selecting a wavelength region capable compensated by the dispersion compensator. The dispersion compensator according to any one of claims 1 to 5 , further comprising a beam diameter adjusting unit that enables adjustment of a beam diameter incident on the dispersion compensating unit. The dispersion compensator according to claim 1 or 2 , wherein a plurality of dispersion compensation substances exhibiting different group velocity dispersions are arranged in series on the optical axis in the dispersion compensation unit. A single mode optical fiber of a predetermined length;
By connecting the dispersion compensator of the claims 1 to 7 for compensating the group velocity dispersion by the optical fiber of the predetermined length to constitute a pair of optical fibers combinations thereof,
An optical fiber combination characterized in that both ends of the optical fiber combination are provided with connecting portions that can be connected to other optical fiber combinations. A terahertz wave pulse generating / detecting device comprising the dispersion compensator according to any one of claims 1 to 7 . A dispersion compensation method for compensating for group velocity dispersion in an optical fiber,
Using a dispersion compensation material that exhibits a group velocity dispersion opposite to the group velocity dispersion of the optical fiber according to the wavelength of the propagating light,
The light input from the light input side end connected to the optical fiber is diverged into parallel light by the first lens,
After compensating the group velocity dispersion only by material dispersion by allowing the parallel light to pass through the dispersion compensation portion made of the dispersion compensation material that is bulky ,
A dispersion compensation method, characterized in that the parallel light is converged by a second lens and output from an end portion on the light output side.

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