JPH10170967A - Optical parametric amplifier - Google Patents

Abstract

(57) [Problem] To provide an optical parametric amplifying element capable of generating a large-amplitude optical pulse train with a small chirp, a pulse time width of several tens of picoseconds or less. SOLUTION: An optical parametric medium (medium exhibiting an optical parametric oscillation effect) 1, 3, 5, 7, 9, 11;
Optical media (light-transmitting media that do not exhibit an optical parametric oscillation effect) 2, 4, 6, 8, and 10 are alternately arranged on the substrate 12, and an optical pulse incident end 13 to an optical pulse output end 14 , A signal light pulse train and a pump light pulse train are input from the light pulse input end 13, and an amplified signal light pulse train is obtained from the light pulse output end 14. [Effect] A signal light pulse that is not synchronized can be amplified with low noise without separately measuring the arrival time.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical parametric amplifier using an optical parametric oscillation effect,
In particular, the present invention relates to an optical parametric amplifying element capable of amplifying a large-capacity optical pulse having a small chirp (change in wavelength width) and a narrow pulse width and pulse interval.

[0002]

2. Description of the Related Art In recent years, the practical use of optical communication has been remarkable. For example, in a telephone system, electric signals collected from a plurality of information sources such as homes and offices via a telephone line to a relay station for long-distance communication are provided. Is converted to an optical signal and sent to another relay station located several hundred km away via an optical fiber, where it is converted again to an electric signal and then sent to a target information receiving source.

However, with the rapid development of the multimedia society in recent years, the demand for large-capacity, high-speed optical communication capable of transmitting more information has been increasing. In addition to the increase in the number of information sources, the information to be transmitted, in addition to simple voice, is also larger, such as computer data files and images, and the transmission speed is faster. It is becoming more demanding.

[0004] For this purpose, for example, a method has been proposed in which an information transmission source side immediately converts the signal into an optical signal and transmits the signal. As a countermeasure, an optical pulse having a shorter pulse width is used. Accordingly, a method of increasing the signal amount per unit time by using the method has been attracting attention.

[0005] By the way, since optical fibers mainly made of silica glass are used for such optical signal transmission, 1.3 μm or 1.5 μm in which the optical transmission loss in the silica glass fiber is minimized. Is used as signal light.

The optical transmission loss at this wavelength is 0.5
It is on the order of dB / km, but if the wavelength is slightly different, the transmission loss immediately exceeds 1 dB / km.

Therefore, as a light source for optical communication, a laser having a good monochromaticity according to the characteristics of an optical fiber is used, and a laser beam generated from the laser is incident on an optical modulator, and the laser light is generated in accordance with an electric signal serving as information. By applying modulation, the intensity is changed and an optical pulse train is generated to form an optical signal.

By the way, even in the optical communication using such an optical pulse, when an optical pulse is transmitted over several hundred km, the optical signal is attenuated. Therefore, a method of installing a repeater on the way and amplifying the optical pulse is adopted. Have been. The currently used repeater converts the arriving optical signal into an electric signal once by a photodetector, and electrically performs amplification, reproduction, demodulation processing such as retiming, and the like, and then uses the electric signal. The laser beam is modulated and converted into an optical signal and transmitted to the next receiving base.

However, from the standpoint of enabling higher-speed operation, it is possible to directly perform processing such as amplification required for relaying as it is, not an optical signal repeater, but an optical signal. A device is desirable. Current,
As an apparatus capable of directly amplifying such an optical signal, that is, an optical amplifier, an optical fiber amplifier using an optical fiber doped with erbium (Er), an apparatus using re-oscillation of a semiconductor laser, and the like are being studied.

Of these, first, an optical fiber amplifying device uses a silica fiber doped with Er and having a length of several hundred m or more, and from one end of the fiber, a YAG laser beam having a wavelength of 1.06 μm and a wavelength of 1.06 μm. When a 1.5 μm signal light to be amplified is incident upon adjusting the time, Er ions in the fiber are first excited by the YAG laser light, and the signal light arriving later triggers the stimulated emission. ,
The amplification function is obtained by adding the emission light to the signal light.

Next, in an apparatus using re-oscillation of a semiconductor laser, a semiconductor laser having the same oscillation wavelength as the signal light to be amplified is used, and a signal is externally applied to a laser medium of the semiconductor laser excited by injection of electric charges. When light is introduced, the signal light is triggered to cause stimulated emission, and the emitted light is added to the signal light to obtain an amplification function.

As other optical amplification systems, those using a nonlinear optical material are known. This nonlinear optical element is capable of generating second- and third-order optical harmonics, optical mixing, or optical parametric oscillation. It is also attracting attention as a basic element of an optical computer, which is expected to be realized in the future, such as a wavelength conversion element using an optical switch and an optical bistable element as an optical switch.

By the way, as this nonlinear optical material,
Lithium niobate (Li Nb O _3), gallium arsenide (G _a
Inorganic materials such as A _s ) have been studied, but in recent years,
The nonlinear optical performance is much higher than those materials
(10 to 100 times), and an organic nonlinear optical material having an extremely high light response speed, which is important for an optical bistable element, has been studied.

As this organic nonlinear optical material, MNA
There are urea, 2-methyl, 4-nitroaniline, and N (4-nitrophenyl) L prolinol, which is abbreviated as NPP.
There is a proposal in Japanese Patent Application Laid-Open No. 500960, and the latter is proposed in Japanese Patent Application Laid-Open No. Sho 59-21665. It is known that these MNAs and NPPs have a second-order nonlinear optical effect that is 100 times or more that of inorganic materials.

By the way, there are several methods of the optical amplification. Recently, an optical parametric oscillation method using a second-order nonlinear optical effect has attracted attention. Here, the optical parametric oscillation is a certain frequency ω
_When light having _p passes through a second-order nonlinear optical medium, two types of light having lower frequencies ω _i and ω _s are generated. The light having frequency ω _i is called idler light, and Number ω _s
Is called signal light.

The details of the optical parametric oscillation are disclosed in the following publication. A.Yariv, "Quantum Electronics" 3rd Edition John Wiley & Sons Publishing 1998

Next, what kind of light can be generated according to the optical parametric oscillation will be described below. First, from the law of conservation of energy,
The following equation holds, ie, ω _p = ω _i + ω _s . Then, from the law of conservation of momentum, according to the phase matching condition, the following equation holds: ω _p × n (ω _p ) = ω _i × n (ω _p ) + Ω _s × n (ω _p ). Here, n (ω _p ), n (ω _p ), n (ω _p ) are
Each frequency [ω _p, ω _p, represents the refractive index of the nonlinear optical medium in omega _p.

Such a condition cannot be satisfied with an isotropic medium, and an azimuth satisfying this condition is determined by changing the incident angle of the pump light using an anisotropic medium.
This effect is used as a wavelength-variable laser, and light of a target wavelength can be extracted with high intensity.

For example, when it is desired to use idler light having a frequency of ω _i , a technique of using a resonator mirror capable of selectively confining light of that frequency and sandwiching a non-linear optical medium is used. Use it.

Note that this technique is described in the following publication. Takashi Ukaji, New Organic Nonlinear Optical Materials II, 1st Edition,
1991, CMC Publishing "Optical parametric oscillation using β-BaB204 crystal"

When pump light is introduced into a nonlinear optical medium from an azimuth satisfying the energy conservation law and the phase matching condition, idler light and signal light are generated. At this time, idler light or signal light having the same wavelength as the signal light is generated. Is simultaneously input with the pump light, the intensity of the signal light is added, and an amplification effect is exhibited. This is the principle of optical parametric amplification.

Among such optical parametric amplifiers, an optical parametric amplifier using a compound semiconductor is disclosed in JP-A-56-164588, JP-A-61-172392 and JP-A-6-75526.
JP-A-6-503892 discloses an optical parametric amplifier using an organic compound, and JP-A-7-254758 discloses an optical parametric amplifier using a semiconductor inverted polarization structure. The publications each disclose.

[0023]

The prior art described above does not consider the amplification of an optical signal having a narrow pulse width and pulse interval, and has a problem in application to large-capacity optical communication. As described above, long-distance optical communication requires an optical amplifier to recover a transmission loss. At this time, in a large-capacity optical communication that transmits more light pulses per unit time, a shorter time is required. Light pulses need to be sent at short time intervals. However, as explained below,
In the above-mentioned conventional technology, this requirement cannot be satisfied, and therefore, a problem arises in application to large-capacity optical communication.

First, in the prior art, an apparatus which once converts an optical signal into an electric signal, electrically amplifies it, and returns it to an optical pulse cannot follow an optical pulse train having an extremely short time interval of several tens of GHz or more.

For this reason, it is necessary to perform optical amplification so that efficient amplification can be obtained as it is, but the stimulated emission of the laser medium by the conventional optical fiber amplifier or re-oscillation of the semiconductor laser is used. In the apparatus of the system, it takes time to excite the laser medium, so that it is not possible to amplify an optical pulse that arrives at a high repetition rate higher than the laser oscillation characteristic.

In particular, in an optical amplification system using an Er-doped fiber, chirping occurs due to group velocity dispersion in the fiber.
(Change in wavelength width), and could not be applied to ultrafast light pulses of the order of fs (= 10 ⁻¹⁵ seconds).

In order to directly amplify such an optical pulse train having an extremely short time interval of several tens of GHz or more, when optical amplification using the optical parametric effect is used, the signal light pulse and the pump light are simultaneously emitted. Since the light pulse must be introduced into the optical parametric medium, it becomes more difficult to synchronize the timings of the two light pulse trains, and the desired performance cannot be obtained.

As a method using the optical parametric effect that has been proposed so far, for example, there is a method described in Japanese Patent Application Laid-Open No. 1-231033. In this method, a signal light pulse is transmitted. Electrical detection is performed, and thereby tuning is performed with the pump light. As a result, tuning can be performed only within the range of the time accuracy and the time constant obtained by the electrical control system.

For this reason, in the conventional optical parametric amplifier, it is necessary to keep pump light incident at all times, and there is a problem of noise due to loss of pump light and spontaneous generation of parametric light by the pump light itself.

An object of the present invention is to provide an optical parametric amplifier capable of generating a large amplitude optical pulse train with a small chirp, a pulse time width of several tens of picoseconds or less.

[0031]

The object of the present invention is to firstly input signal light and pump light together from one end of an optical path made of a medium exhibiting an optical parametric effect, and to amplify the amplified signal from the other end. In an optical parametric amplification element configured to obtain light, a medium exhibiting an optical parametric effect is divided into a predetermined length along a light passing direction in the optical path, and a wavelength of the signal light is set to λ. _1, when the wavelength of the pump light was set to lambda _2, 1/2 times the wavelength lambda ₁ is accomplished as equal to the wavelength lambda _2.

The object is that the signal light and the pump light are optical pulse trains having a pulse time width of t ₁ and a time interval of T ₁ , and the interval between the media exhibiting the optical parametric effect is set to be smaller than the interval between the media. Light propagation speed and the pulse time width t ₁
This is achieved even if the value is set to a value equal to or less than the product of.

[0033] The object is quite common, when the length of the medium of an optical parametric effect, the wavelength lambda ₁ is the medium, was the refractive index each n _1, n ₂ shown in lambda _2, lambda ₁ /
2 (n ₁ -n ₂ ). The above object is further achieved when the medium exhibiting the optical parametric effect is formed in a periodic structure in units of its length.

[0034] The above object is further achieved when the medium exhibiting the optical parametric effect is filled with another medium different from the medium. The purpose is
Further, between the medium exhibiting the optical parametric effect,
This is achieved even if the medium is filled with a medium having a different orientation from the medium. The above object is further achieved even when an electrode for applying an electric field to the medium exhibiting the optical parametric effect is provided.

The above object can be attained even when the medium exhibiting the optical parametric effect is made of an organic material in which a low molecular organic material is dispersed in a high molecular organic material. The purpose is to set the wavelength λ ₁ from 1.2 μm to 1.
This is achieved even if it is in the range of 7 μm.

A nonlinear optical medium having a period width of λ is λ / 2
When light is introduced, light of this wavelength is stored in the medium,
When the signal light is input to the light of λ, the energy of the light of λ / 2 is transferred to the light of λ due to the parametric oscillation effect. In the present invention, signal light can be amplified without chirp by linking a large number of units with this configuration as a basic unit.

In particular, at this time, by taking an arrangement set by the time width of the light pulse, it is not necessary to electrically measure the synchronization signal between the signal light and the pump light, and the signal light can be amplified at high speed. it can. Such an optical parametric amplifying element uses an ultrashort light pulse of picoseconds or less such that the light travels for a time equal to or shorter than the coherence length of the medium exhibiting the parametric effect. It can be used particularly effectively as an optical signal amplifying element for communication.

According to the present invention, an optical signal repeater can be realized completely optically without requiring a series of operations of optical / electrical conversion, electrical amplification, and electric / optical conversion. According to the present invention,
Higher speed, compared to methods such as semiconductors and fiber amplifiers
An optical amplifier that can be used for high-repetition, large-capacity optical communication can be realized.

[0039]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an optical parametric amplifier according to the present invention will be described in detail with reference to the illustrated embodiments. First, the basic structure of the optical parametric amplifier according to the present invention will be described with reference to FIG.

In FIG. 1, 1, 3, 5, 7, 9,
11 is an optical parametric medium (medium exhibiting an optical parametric oscillation effect), 2, 4, 6, 8, and 10 are optical media (a light-transmitting medium exhibiting no optical parametric oscillation effect), 12 is a substrate, and 13 is an optical medium. Pulse input end, and 14
Is an emission end of the light pulse.

The optical parametric media 1 to 11 and the optical media 2 to 10 are provided on one surface of the planar substrate 12 along the propagation direction of the light pulse from the input end 13 to the output end 14 as shown in FIG. Are arranged alternately in a line, thereby forming an optical path from the incident end 13 to the emission end 14.

Here, first, it is assumed that the wavelength of the signal light pulse to be subjected to optical parametric amplification is λ ₁ , the pulse time width is t ₁ , and the pulse interval is T ₁ . Then, this signal light pulse is applied to the same pulse time width t ₁ and pulse interval T
In ₁ the case, it is assumed that wavelength is amplified by pump light pulse train lambda _2. At this time, λ ₁ > λ ₂ .

Next, the refractive indices of the optical parametric media 1 to 11 at the wavelengths λ ₁ and λ ₂ are n ₁ and n ₂ , respectively. Therefore, first, each of the optical path direction of the length of these optical parametric medium 1-11, i.e. optical parametric length L ₁ is made equal to the coherence length of the optical parametric oscillation given by the following equation.

[0044] _{L 1 = λ 1/2 (} n 1 -n 2) Next, the optical path direction of the length L ₂ of the optical medium 2-10 is selected so as to satisfy the following equation.

C ₁ = c ₀ / n ₁ L ₅ = c ₁ × t ₁ c ₃ = c ₀ / n ₃ t ₃ = L ₂ / c ₃ N = T ₁ / t ₃ where c ₀ is in vacuum , C ₁ is the speed of light when the refractive index is n _{1 in} the optical parametric media 1 to 11, and L ₅ is the distance traveled by the light of speed c ₁ during the time width t _1. ,
n ₃ is the refractive index of the optical mediums 2 to 10 (the minimum refractive index at the wavelengths λ1 and λ2), and c ₃ is the optical medium 2
The speed of light within 10, and t ₃ is the time that light travels through the optical media 2〜１０10.

Here, N represents the number of sets through which the signal light passes when the optical parametric medium and the subsequent optical medium are made into one set. For example, in the case of FIG. 1, a coefficient indicating the degree of overlap of such pulses is an integer greater than 1, and in the case of FIG. 1, the set of the optical parametric medium and the optical medium is 5
N = 5 because there is a group.

In the case of the present invention, the more light passes through many optical parametric media, the more light amplification is obtained.
As the value of N is increased, the amplification factor can be increased.

Next, in FIG. 1, the dimension L ₃ is the height (dimension in the direction perpendicular to the surface of the substrate 12) of the optical parametric media 1 to 11 and the optical media 2 to 10, and the dimension L ₄ is the same. The width (dimension in a direction parallel to the surface of the substrate 12) is the cross-sectional dimension of the optical path.

At this time, the optical parametric media 1 to 11
And the optical media 2 to 10 have the same cross-sectional dimension, so that alignment of the optical path is maintained.

Here, these dimensions L ₃ and L ₄ are not particularly limited, but in the amplification of the signal light pulse transmitted in the optical communication, in order to amplify the signal light more efficiently, an optical waveguide is used. Since light is confined and transmitted inside, it is desirable that the size of the cross section of the optical path determined by these dimensions L ₃ and L ₄ be the same.

At this time, the polarization directions of the signal light pulse and the pump light pulse differ depending on the type of the optical parametric medium used in the optical parametric media 1 to 11. For example, in the quasi-phase matching type optical parametric medium, the medium It is desirable to set the polarization directions of the two types of light pulses of the signal light and the pump light in parallel in the direction that gives the second-order nonlinear optical constant d33.

Next, in such an optical parametric amplifying element, the difference in noise generation due to pump light between the optical parametric amplifying element according to the present invention and the conventional optical parametric amplifying element will be described with reference to FIG. In FIG. 2, for ease of explanation,
A comparison will be made in the case of quasi-phase matching in which three optical parametric media P1, P2, and P3 are connected.

In the case of the prior art shown in FIG.
Of the optical parametric medium P1 (x1-x0), the length of the second optical parametric medium P2 (x3-x2), and the length of the third optical parametric medium P2 (x5-x
4) is the coherence length, which is the length that works as the most efficient quasi-phase-matched optical parametric amplification element.
The distance (x2-x1) between P1 and P2 and the distance (x4-x3) between the optical parametric media P2 and P3 are also the same length, so that optical amplification is obtained.

This is because, in an optical parametric medium other than the phase matching type, signal light is amplified depending on the distance traveled in the parametric medium, but also up to the first coherence length (here, x2−x1). If the optical parametric medium continues beyond that, the light will be attenuated until the next second coherent length.

Similarly, optical amplification is repeated until the next third coherence length, and optical attenuation and optical attenuation are repeated periodically until the fourth coherence length. Therefore, in the quasi-phase matching type, the even-numbered optical medium in which this optical attenuation occurs is changed from an optical parametric medium to a normal optical medium so that signal attenuation does not occur,
By leaving only the odd-numbered optical parametric medium, only the amplifying function occurs.

Therefore, when a pump light pulse train is incident on the optical parametric amplifier according to the prior art as shown in the figure, when the incident pump light pulse train passes through the optical parametric amplifier, a signal coming from outside is output. In addition to the light, all the light that can be parametrically oscillated is generated by this pump light pulse train, while being weak.

Then, the wavelength component of the light, which is set to be selectively amplified by the structure of the element, is amplified in the element and is generated as a spontaneous parametric pulse shown in the figure. This light is generated independently of the signal light by the pump light pulse train alone, and thus becomes noise of the amplification element.

Therefore, the optical parametric amplifier according to the present invention employs the configuration shown in FIG.
As shown in (b), the interval between adjacent optical parametric media is set to be shorter than the coherence length. As a result, as shown in FIG. Parametric amplification can be prevented from occurring continuously, and the generation of noise light by pump light alone can be suppressed, thereby suppressing noise.

Next, a method of amplifying a signal light pulse by the optical parametric amplifier of the present invention will be described with reference to FIG. For simplicity of description, a case of quasi-phase matching in which two optical parametric media are connected will be described here.

First, a description will be given of an amplification system based on the operation principle 1 shown in FIG. 3A. In this system, an optical pulse train having the same repetition time as a signal light pulse train is used as a pump light pulse train A, which is used in the present invention. Optical parametric media P1, P constituting the optical parametric amplifying device of FIG.
2 is incident.

Then, the pump light pulse A
Is made to pass through the optical parametric element composed of the optical parametric media P1 and P2 at a constant repetition time, and then the signal light pulse train is made to be incident thereon. It is assumed that a signal light pulse train C whose arrival time is synchronized with the light pulse train A is incident and a signal light pulse train D which is not synchronized arrives.

In the case of the signal light pulse train C, that is, when synchronized, the pump light pulse train A and the signal light pulse train C
Means that the light passes through the optical parametric media P1 and P2 almost simultaneously, so that the energy of the pump light pulse is propagated to the signal light pulse, and parametric amplification occurs.

In the case of the signal light pulse train D, that is, when the signal light pulse train D is not synchronized, the pump light pulse train A and the signal light pulse train D only partially overlap with each other.
As shown in the figure, if a partially distorted light pulse is amplified and only incomplete amplification is obtained, or the arrival time is shifted by a half cycle, it is not shown. Since they do not overlap, no amplification action can be obtained at all. Therefore, when the method shown in FIG. 3A is adopted, it is necessary to separately measure the arrival time of the signal light pulse and synchronize the pump light pulse.

Then, next, an amplification method for evenly amplifying the signal light pulse train D even when synchronization is not achieved will be described with reference to FIG. 3 (b).
In the amplification method based on the operation principle 2 shown in FIG.
The point that the pump light pulse train A is used is the same as that of the operation principle 1. However, a pump light pulse train B whose repetition time is shifted from the pump light pulse train A by half is generated by another optical system. The pump light pulse train B and the pump light pulse train A are incident on the optical parametric media P1 and P2 constituting the optical parametric amplification element of the present invention.

Then, as shown, the portion where the signal light pulse train D shown in FIG. 3A is shifted from the pump light pulse train A overlaps with the pump light pulse train B. When introduced into the optical parametric amplifier of the invention, the remaining part of the signal light pulse train D, which was only partially amplified by the pump light pulse train A, is amplified by the pump light pulse train B. As shown, full amplification can always be obtained.

It should be noted that this looks similar to the prior art, which allows the use of continuous pump light to amplify similarly unsynchronized signals, but is quite different.

This is because when continuous light is incident as in the prior art, one optical parametric medium has:
The noise light due to the pump light alone is constantly generated, and the amplification thereof is caused, so that the noise is remarkably increased. However, the operation principle 2 shown in FIG. In this case, the respective pump light pulse trains A and B are shown in FIG.
This is because the operation of suppressing the spontaneous parametric pulse shown in (1) is obtained, and there is almost no fear of noise.

Next, as one embodiment of the present invention, an optical amplifier for an optical communication repeater using the optical parametric amplifier according to the present invention will be described with reference to FIG. In FIG. 4, reference numeral 15 denotes a high repetition laser light source;
Is a light mixing element, 17 and 21 are first and second optical parametric amplification elements, 18 and 22 are wavelength division elements, 19 is an optical delay element, 23 is a light absorber, 24 is an optical input terminal, and 25 is an optical output. The optical path shown by a thick black line in the figure is formed between these terminals by an optical fiber or a mirror.

First and second optical parametric amplification elements 1
Reference numerals 7 and 21 are the optical parametric amplification elements according to the present invention shown in FIG. The optical input end 24 is an entrance of a signal light pulse train to be amplified transmitted through an optical fiber or the like. The wavelength of the signal light pulse train supplied here is λ ₁ as described above. The high repetition rate laser light source 15 functions to generate a pump light pulse train and supply it to one input of the light mixing element 16. The wavelength of this pump light pulse train is λ ₂ as described above.

The light mixing elements 16 and 20 mix and output the light of wavelength λ ₁ supplied to one input and the light of wavelength λ ₂ supplied to the other input. The wavelength division elements 18 and 22 divide the input light having the wavelengths λ ₁ and λ ₂ , and output only the light having the wavelength λ _{1 to one} output, and
The other output functions to extract only light of wavelength λ ₂ .

The optical delay element 19 functions to delay the light of wavelength λ ₂ for a predetermined time. The predetermined delay time will be described later. The light absorber 23 mainly has a wavelength λ ₂
Works to efficiently absorb the light of

The optical output terminal 15 is an output port of the amplified signal light pulse train, is connected to an optical fiber transmission line or the like, and functions to send out the amplified signal light pulse train.

Next, the operation of the embodiment shown in FIG. 4 will be described. First, a pump light pulse train output from the laser light source 15 is referred to as a pump light pulse train A shown in FIG. Since the laser light source 15 operates independently of the signal light pulse train input from the optical input terminal 24 as shown in the figure, the pump light pulse train A output from the laser light source 15 There is no synchronism with the signal light pulse train input from the terminal 24, and therefore, the same as FIG.
The signal light pulse train C or D shown in FIG.

Therefore, first, these pump light pulse trains A
And the signal light pulse train C or D are mixed by the light mixing device 16 and introduced into the first optical parametric amplification device 17, where they are partially amplified as described with reference to FIG. Output from

Next, the light output from the first optical parametric amplifying element 17 is input to a wavelength division element 18, where the light pulse train of wavelength λ ₁ , that is, the partially amplified signal light pulse train C or D and an optical pulse train of wavelength λ ₂ , that is, a pump optical pulse train A.

The partially amplified signal light pulse train C or D is supplied to the optical mixing device 20 as it is, but the pump light pulse train A of wavelength λ ₂ is first input to the optical delay device 19. Here, after being optically delayed by a half cycle of the pulse train, it is supplied to the light mixing element 20.

Considering the pump light pulse train output from the optical delay element 19, the pump light pulse train A is optically delayed by a half cycle of the pulse train. The phase relationship between the light pulse train A and the signal light pulse train C or D is the same as that of the pump light pulse train B shown in FIG.

Then, these are recombined by the optical mixing element 20 and introduced into the second optical parametric amplification element 21, so that the amplification effect by the pump light pulse train B shown in FIG. As a result, the portion of the signal light pulse train D that has not been amplified by the first optical parametric amplifier 17 is amplified, and the signal light pulse train E that has been totally amplified is output from the second optical parametric amplifier 21. Will be done.

This second optical parametric amplifier 21
Is input to the wavelength division element 22, where it is again divided into an optical pulse train of wavelength λ _{1 and} an optical pulse train of wavelength λ _{2, and then} an optical pulse train of wavelength λ ₂ , that is, a pump light pulse train B is removed is guided to the light-absorbing element 23, the wavelength lambda ₁ of the optical pulse train, that only the signal optical pulse train is to be transmitted to the optical output terminal 25, as a result, an optical output terminal of the amplified signal light pulse train 25.

Therefore, according to the embodiment shown in FIG. 4, a signal light pulse train can be obtained from the light output terminal 25 by inputting a signal light pulse train from the optical input terminal 24. By installing the optical amplifier on a road, an optical amplifier that operates as a repeater for optical communication can be obtained.

According to the embodiment shown in FIG. 4, without arranging the synchronism between the signal light pulse train and the pump light pulse train, first, the arrived light is amplified as it is, followed by processing such as waveform shaping and signal division. Therefore, the optical signal can be processed without a reading error.

Next, the manufacturing procedure of an embodiment of the optical parametric amplifier according to the present invention will be described. However,
The target material and the manufacturing procedure are not limited to those described in this embodiment.

First, in the following embodiment, the configuration of an element for amplifying light having a wavelength of 1.6 μm as a wavelength λ ₁ of signal light, that is, an optical parametric amplifier having a wavelength of λ ₁ = 1.6 μm will be described with reference to FIG. First, a substrate 26 made of a fused silica plate material having an area of 1 cm square, a thickness of 1 mm, and a surface optically polished to an accuracy of 1/20 lambda (λ) is prepared (FIG. 5A). Next, an ITO transparent electrode 27 having a thickness of 10 nm is deposited on one surface of the substrate 26 by a sputtering method (FIG. 5B).

Next, a photoresist is spin-coated on the upper surface of the transparent electrode 27, and a pattern forming photopolymerization, removal of unpolymerized resist, etching, and removal of the cured resist are performed on the surface by using a photomask. , FIG.
Is formed into a transparent electrode 27 'having a comb-shaped pattern with an electrode width of 6 μm and an inter-electrode width of 50 μm (FIG. 5C). Next, the whole was treated with 4- (N, N-dimethyl) amino-4-nitrotolan ((H ₃ C) ₂ —N—C ₆ H ₄ —C≡C—C ₆ H ₄ —N
A chloroform solution of polymethyl methacrylate mixed with 1% by weight of O ₂ ) is immersed and pulled up, and the solvent is gently evaporated to form a 1.6 μm thick polymer layer 28 on the transparent electrode 27 ′. (FIG. 5D).

Next, about 20 nm of gold is deposited on this polymer layer 28 by vacuum deposition to form a gold electrode 29.
(FIG. 5 (E)). Next, the sample is subjected to a poling process (FIG. 5F). That is, while keeping this sample at 95 ° C,
A DC voltage is applied between the ITO transparent electrode 27 and the gold electrode 29 to maintain the electric field strength (10 V / μm). After maintaining this state for 3 hours, the temperature is lowered at a rate of 10 ° C. per hour while the voltage is being applied, the sample is brought to room temperature, and the polymer layer 28 ′ which has been polled is formed.

Finally, reactive ion etching of oxygen radicals is performed using a mask pattern to remove a part of the polymer layer 28 ′ which has been polled with the gold electrode 29 by a width of 1.6 μm.
An optical waveguide structure was formed while leaving m and a length of 1 cm to obtain an optical parametric amplifier (FIG. 5 (G)).

In the optical parametric amplifying device thus obtained, the substrate 26 corresponds to the substrate 12 in the embodiment of FIG.
And the gold electrode 2 of the polled polymer layer 28 ′
The part where 9 is present corresponds to 1 in the embodiment of FIG.
.., And the portion where the gold electrode 29 does not exist constitutes 2, 4,... In the embodiment of FIG.

Here, the poling process will be described. Generally, in order to exhibit a second-order nonlinear optical effect such as an optical parametric function, the arrangement of atoms and molecules constituting a medium becomes non-centrosymmetric. Need to be, that is, not isotropic.

Therefore, the polymer film immediately after the application is formed of molecules (here, 4- (N, N
Since (dimethyl) amino-4-nitrotolan) is randomly arranged, non-centrosymmetricity can be obtained by aligning molecules in the direction of the electric field by poling.

The embodiment of the optical parametric amplifying device according to the present invention can be formed by subjecting lithium niobate single crystal to fine processing in addition to the one shown in FIG. The form is not limited to the material and the method of preparation as long as the size and arrangement of the optical parametric medium can be processed so as to satisfy the same conditions as in the embodiment of FIG.

Next, for comparison, in the same processing procedure, by omitting the polling processing shown in FIG. 5F, elements having an optical waveguide structure showing no optical parametric effect were produced in parallel. Using this, the result of confirming the effect of amplifying the optical pulse train by the optical parametric amplifying element created by the processing procedure of FIG.

First, the generation system of the signal light pulse train and the pump light pulse train will be described. First, a titanium sapphire laser 30 excited by an argon gas laser was used, whereby a pulse time width was 80 fs, and a repetition frequency was 82 MHz.
z, oscillation wavelength 0.8 μm, energy 1p per pulse
An optical pulse of J (10 ⁻¹² joules) is generated, and this is regenerated by the regenerative amplifier 31 into light having a pulse time width of 80 fs, a repetition frequency of 1 kHz, an oscillation wavelength of 0.8 μm, and an energy per pulse of 30 nJ (10 ⁻⁹ joules). Convert to pulse.

The converted light pulse is first applied to the splitter 3
3, the system is divided into two systems for pump light and signal light pulse.
The light pulse for the pump light is introduced into the demultiplexer 34, and the light pulse for the signal light pulse is made incident on the wavelength converter 32.

First, the wavelength 0.8 incident on the demultiplexer 34
The light pulse for the pump light of μm (= λ ₂ ) is further divided into two here, one is passed through the optical delay line 44 and the other is sent as it is, and is again adjusted to the same path by the multiplexer 35. At this point, it has been converted into two types of pump light pulse trains having different arrival times by the optical delay path 44.

The same processing is performed by the demultiplexer 36 and the optical delay path 4
5. Performed by the multiplexer 37, the two types of pump light pulse trains are converted into four types of pump light pulse trains, and the same processing is performed by the demultiplexer 38, the optical delay path 46, and the multiplexer 39. The kind of pump light pulse trains is converted into eight kinds of pump light pulse trains.

In this system, the optical delay paths 44, 45, and 46 are set so that a time difference of 2.5 ps (10 ^-12 seconds) is obtained between each pump light pulse train.
The wavelength pump light pulse train having eight kinds of time differences generated in this way is configured to be sent to the multiplexer 40.

It is to be noted that, since the amplification effect can be obtained without separately measuring the synchronization signal of the signal light and the pump light by an electric means, a fixed distance is provided between the pump light train and the arrangement of the parametric medium. This is the result of satisfying the following relationship. That is, when the pulse interval T ₁ is obtained from the above equation, the following equation is obtained: T ₁ = L ₂ × (n ₃ / c ₀ ) × N where L ₂ = 50 μm, n ₃ = 1.5, c ₀ = 10 ⁸ m
/ S, N = 1000, T ₁ = 2.5 ps is obtained.

Next, the pulse train for signal light incident on the wavelength converter 32 is converted into an optical pulse having a pulse time width of 100 fs, a repetition frequency of 1 kHz, an oscillation wavelength of 1.6 μm, and an energy of 0.6 nJ per pulse. Is converted.

The signal light pulse train having a wavelength of 1.6 μm (= λ ₁ ) output from the wavelength converter 32 is then introduced into the demultiplexer 43, where it is divided into two. Then, first, one of the optical pulse trains is guided to a light meter 56, where it is converted into an electric signal, and then a computer 6 for controlling the measurement system is used.
0 is used as a trigger signal for detecting a signal light pulse train.

Next, the other optical pulse train is supplied to the optical delay path 47.
Then, the light is introduced into the duplexer 41 via the mirror 49, where it is further branched into two systems. Then, one of the branched optical pulse trains is used as reference light, so that the mirror 50 and the lens 5 are used.
3, the other optical pulse train is used as a signal light pulse train, and the mirror 4
It is guided to the multiplexer 40 via 8.

Next, the wavelength λ ₂ (= 0.
8 μm) and a wavelength λ ₁ (= 1.6 μm)
An optical system for introducing the signal light pulse train m) into the optical parametric amplifier according to the present invention and measuring the amplification effect will be described.

As described above, the pump light pulse train and the signal light pulse train are matched to the same optical path by the multiplexer 40.
The light is incident on the optical parametric amplification element 54 via the lens 51. This optical parametric amplification element 54 has the configuration shown in FIG.
Is created by the method described in FIG.

After passing through the optical parametric amplification element 54,
The output pump light pulse train and signal light pulse train are taken out through a lens 52 and then output to a wavelength division type demultiplexer 4.
2 where it is first split into a pump light pulse train and a signal light pulse train, whereby the pump light pulse train of wavelength λ ₂ is guided and absorbed by the light absorbing medium 55 and the signal light pulse train of wavelength λ ₁ Only through the lens 53 to the second harmonic generation crystal 58.

The second harmonic generation crystal 58 has a splitter 43
Since the reference light pulse train branched by the above is input, the signal light pulse train and the reference light pulse train are combined here. At this time, the optical delay path 47 is adjusted so that these two types of pulse trains have the same arrival time.

As a result, the second harmonic of both the signal light pulse train and the reference light pulse train is generated in the second harmonic generation crystal 58, and this is incident on the light meter 57 via the infrared filter 59, and the light quantity of these Is generated and supplied to the computer 60, whereby the computer 60 calculates and outputs the signal light intensity measurement value based on the electric signal from the light meter 57.

Next, the measurement processing will be described. As described above, the pump light pulse train and the signal light pulse train are respectively introduced into the optical parametric amplification element 54. At this time, the shutter 6 is provided in the path of the pump light pulse train.
An optical shutter 62 is provided in the path of the signal light pulse train, and the incidence and cutoff of the pump light pulse train and the signal light pulse train with respect to the optical parametric amplification element 54 can be arbitrarily controlled. .

Therefore, first, in this optical parametric amplifying element 54, in order to confirm whether or not spontaneous parametric oscillation occurs, the shutter 61 is closed and the shutter 62 is closed.
Is opened, and only the pump light pulse train is introduced into the optical parametric amplifying element 54. As shown in Table 1, the signal light intensity at this time is 0, so that spontaneous parametric oscillation does not occur. It was confirmed that.

Next, conversely, when the shutter 61 is opened, the shutter 62 is closed, and only the signal light pulse train is introduced into the optical parametric amplification element 54, the signal light is detected. , As shown in Table 1,
1.0. Then, finally, when the shutters 61 and 62 were both opened and the measurement was performed, the signal light intensity at this time was 4.2 as shown in Table 1.

[0109]

[Table 1]

Therefore, the method described with reference to FIG.
According to the optical parametric amplification device according to the embodiment of the present invention described in the above, it has been found that signal light can be reliably amplified while sufficiently suppressing noise due to spontaneous parametric oscillation.

On the other hand, an element manufactured for comparison and not subjected to the poling process was arranged in the optical system shown in FIG. 5, and the same evaluation was performed. The results are as follows. That is, first, when the shutter 61 is opened, the shutter 62 is closed and only the signal light pulse train is introduced, only the signal light is detected, and the measured value is set to 1.0.

Next, when both shutters 61 and 62 were opened and the same measurement was performed, the measured value was 1.0, and it was confirmed that no light amplification was performed in this case.

Next, various requirements necessary for implementing the present invention will be described. First, as the medium of an optical parametric effect for use in an embodiment of the present invention, inorganic crystals such as lithium niobate (Li Nb O ₃₎ or lithium iodate (Li I O _3), gallium arsenide (Ga As) or Bulk and low-dimensional crystals of compound semiconductors such as gallium arsenide (GaAs) or gallium nitride (GaN), zinc selenide (ZnSe), 2-methyl-4-nitroaniline (H ₂ N—C ₆
H ₃ (—CH ₃ ) —NO ₂ ) or 4-methyl-4-tolan (H ₃
An organic compound such as C—C ₆ H ₄ —C≡C—C ₆ H ₄ —CN), or a polymer compound obtained by dispersing or bonding them;
Various second-order nonlinear optical materials such as the following can be used.

These media exhibiting the optical parametric effect can be used alone, in a crystalline or non-crystalline state, in the form of a lump, a plate, a fiber, a powder, or a thin film. Further, in these shapes, different materials or different materials having different structures according to the present invention can be used together with or mixed with each other. In particular, specific examples of these applications include optical waveguides for communication, optical cables, optical integrated circuits, and two-dimensional logic elements.

In the embodiment of the present invention, when a medium exhibiting an optical parametric effect is used in a crystalline state, the crystal may be formed by, for example, a Bridgman method, a temperature drop method, a solvent evaporation method, or a solvent composition change method. There are various known methods. When used in an amorphous state, it can be dispersed in a polymer material such as polymethyl methacrylate or polyimide.

In addition, these media can be subjected to a poling process using an electric or magnetic field for producing an optical parametric effect. Alternatively, a phase inversion distribution structure can be obtained by a partial ion exchange method. Further, various thin film forming techniques, for example, a spin coating method, a sputtering method, a vacuum evaporation method, a molecular beam evaporation method, a liquid phase epitaxy method, an atomic layer epitaxy method, and the like can also be used.

Next, in order to form the optical parametric amplifier according to the present invention, various precision processing techniques can be used according to the required characteristics of the light pulse.
For example, precision diamond cutting, laser processing, etching, photolithography, reactive ion etching, focused ion beam etching and the like can be mentioned. In addition, a medium that exhibits an optical parametric effect that has been processed in advance can be multi-layered, arranged at regular intervals, or sealed in that state.

Next, as other materials which can coexist and coexist, there are inorganic substances and organic substances. First, as the inorganic substance, for example, glass, quartz, diamond, silicon dioxide, mica, marble, calcite, single crystal silicon, amorphous silicon, GaP, CdS, potassium dihydrogen phosphate, potassium trihydrogen phosphate, bromide Potassium, Rochelle salt, copper sulfate, calcium fluoride, graphite, tin dioxide, barium titanate, red blood salt, porcelain, ceramics, bentonite, cement, or a metal or alloy can be used.

Examples of the organic substance include polycarbonate, polysulfone, polyarylate, polyester, polyamide, polyimide, polysiloxane, polyethylene terephthalate, polyvinyl acetate, polyethylene, polypropylene, acrylic resin, polybutadiene, and the like.
Polyvinyl chloride, polyvinylidene chloride, petroleum resin, melamine resin, epoxy resin, phenol resin, isoprene rubber, ethylene propylene rubber, norbonene resin, cyanoacrylate resin, styrene resin, and copolymers of these resins, or cellulose , Starch, chitin,
Agar, silk thread, nylon thread, albumin, globulin, other proteins, wood, bone meal, etc .; low molecular organic substances such as condensed aromatic compounds such as naphthalene and anthracene, dyes, pigments, urea, tartaric acid, optically active amino acids, etc. Is mentioned.

The optical parametric amplification device of the present invention may be subjected to a process for improving the appearance and characteristics and extending the life of the product after the product is formed. Examples of such post-processing include thermal annealing, radiation irradiation, electron beam irradiation, light irradiation, radio wave irradiation, magnetic field line irradiation, and ultrasonic irradiation. Furthermore, the element is made into various composites, for example, adhesion, fusion, electrodeposition, vapor deposition, pressure bonding,
Compounding can be carried out by means such as dyeing, melt molding, kneading, press molding, coating, etc., depending on the use or purpose.

In the optical parametric amplifying device according to the present invention, in particular, by filling the space to be the core surrounded by the cladding layer with a medium exhibiting the optical parametric effect, the optical amplifying effect can be effectively achieved even with a smaller incident light intensity. Can be pulled out.

Further, in particular, a wavelength of 1.3 μm to 1.5 μm
When the optical parametric amplification device according to the present invention is applied as an optical parametric amplification device for a signal light pulse for optical communication in a band, a wavelength of 0.65 μm to 0.8 μm is used.
A μm GaAIAs semiconductor laser, a titanium-doped sapphire solid-state laser, or the like can be used as a pump light pulse light source. Further, at this time, these lasers can be formed as an integrated device incorporating an optical waveguide or an optical fiber.

An optical parametric amplifier according to the present invention,
Or various elements incorporating the same include, for example, various light sources such as an optical wavelength conversion element, an optical modulator, an optical switch, an optical memory, an optical mixer, an optical phase discriminator, an optical phase conjugate mirror, an image display element, and an image printing element. It can be applied to functional elements.

In particular, by superimposing other external signal information, for example, image information in the visible region, on the pump light pulse, it is also used as an element for writing and transmitting a signal to the optical pulse train in the communication wavelength region. It is possible.

[0125]

According to the present invention, generation of noise light due to pump light can be sufficiently suppressed without synchronizing the signal light pulse and the pump light pulse. In addition, a signal light pulse that is not always synchronized can be amplified without separately measuring its arrival time. As a result, according to the present invention, the terabit pulse that cannot perform high-speed electrical signal processing can be obtained. It is possible to easily realize an optical signal repeater that is extremely effective in the above-mentioned ultra-high-speed optical communication.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a basic structure of an embodiment of an optical parametric amplifier according to the present invention.

FIG. 2 is an explanatory diagram showing an operation of the optical parametric amplification element according to the present invention in comparison with a conventional technique.

FIG. 3 is an explanatory diagram showing an operation of the optical parametric amplifier according to the present invention.

FIG. 4 is a block diagram showing an embodiment of an optical repeater using an optical parametric amplifier according to the present invention.

FIG. 5 is an explanatory diagram showing one embodiment of a procedure for producing an optical parametric amplifier according to the present invention.

FIG. 6 is an electrode pattern diagram in one embodiment of the optical parametric amplification element according to the present invention.

FIG. 7 is a configuration diagram of an optical system for confirming an optical parametric amplification effect of the optical parametric amplification element according to the present invention.

[Explanation of symbols]

1, 3, 5, 7, 9, 11 Optical parametric medium (medium exhibiting optical parametric oscillation effect) 2, 4, 6, 8, 10 Optical medium (light transmitting medium exhibiting no optical parametric oscillation effect) 12 Substrate Reference Signs List 13 incident end of light pulse 14 emission end of light pulse 15 high repetition laser for pump light 16, 20 light mixing element 17 first optical parametric amplification element 18, 22 wavelength division element 19 optical delay element 21 second optical parametric amplification Element 23 Light absorber 24 Light input terminal 25 Light output terminal 26 Substrate 27 Transparent electrode 27 'Patterned transparent electrode 28 Polymer layer with non-linear optical molecules dispersed 28' Non-linear optical molecules after poling treatment are dispersed Polymer layer 29 gold electrode 30 titanium sapphire laser excited by argon gas laser 31 regenerative amplifier 32 wavelength converter 3 3, 34, 36, 38, 41, 42, 43 Optical demultiplexer 35, 37, 39, 40 Optical multiplexer 44, 45, 46, 47 Optical delay path 48, 49, 50 Mirror 51, 52, 53 Lens 54 Optical parametric amplifying element 55 Optical absorption medium 56, 57 Light meter 58 Infrared filter 60 Computer for measuring system control 61, 62 Shutter L1 Length of optical pulse traveling direction of optical parametric medium L2 Length of optical pulse traveling direction of optical medium Length L3 Height of the optical waveguide through which the light pulse passes L4 Width of the optical waveguide through which the light pulse passes P1 First optical parametric medium P2 Second optical parametric medium P3 Third optical parametric medium x0 Light pulse progression in the optical waveguide Incident position of the first optical parametric medium in the first direction x1 first optical parameter in the light pulse traveling direction in the optical waveguide Emission position of the medium x2 Incident position of the first optical parametric medium in the optical pulse traveling direction in the optical waveguide x3 Emission position of the first optical parametric medium in the optical pulse traveling direction in the optical waveguide x4 Optical pulse traveling in the optical waveguide The incident position of the first optical parametric medium in the direction x5 The emission position of the first optical parametric medium in the light pulse traveling direction in the optical waveguide

Claims (9)

[Claims] 1. An optical parametric amplifier in which signal light and pump light are incident together from one end of an optical path made of a medium exhibiting an optical parametric effect, and amplified signal light is obtained from the other end. In the element, the medium exhibiting the optical parametric effect is divided into predetermined lengths along the light passing direction in the optical path, and the wavelength of the signal light is λ ₁ and the wavelength of the pump light is λ. when a _2, optical parametric amplification element characterized in that half of the wavelength lambda ₁ is configured to be equal to the wavelength lambda _2. 2. The invention according to claim 1, wherein the signal light and the pump light are optical pulse trains having a pulse time width of t ₁ and a time interval of T ₁ , and the distance between the media exhibiting the optical parametric effect is set to optical parametric amplification element characterized in that it is set to the product the following values of the propagation velocity and the pulse time width t ₁ of light in the medium. 3. The invention according to claim 1, wherein the length of the medium exhibiting the optical parametric effect is n ₁ , the refractive index of the medium at wavelengths λ ₁ and λ ₂ , respectively.
when the n _2, λ _1/2 optical parametric amplification element characterized in that it is set to a value equal to (n ₁ -n _2). 4. The optical parametric amplification device according to claim 3, wherein the medium exhibiting the optical parametric effect is formed in a periodic structure whose length is a unit. 5. The optical parametric amplifier according to claim 4, wherein a space between the media exhibiting the optical parametric effect is filled with another medium different from the medium. 6. The optical parametric amplifier according to claim 4, wherein a space between the media exhibiting the optical parametric effect is filled with a medium having a different orientation from the medium. 7. The optical parametric amplification device according to claim 1, wherein an electrode for applying an electric field is provided to the medium exhibiting the optical parametric effect. 8. The medium according to claim 1, wherein the medium exhibiting the optical parametric effect is made of an organic material in which a low molecular weight organic material is dispersed in a high molecular weight organic material. An optical parametric amplifier element characterized by the above-mentioned. 9. The optical parametric amplification device according to claim 1, wherein the wavelength λ ₁ is in a range of 1.2 μm to 1.7 μm.