JPH08298340A - Optical pulse transmission system and optical pulse transmission element - Google Patents

Abstract

PURPOSE: To obtain time division/wavelength division multiplex optical transmis sion system by performing optical pluse compression.amplification and optical pulse signal isolation for which small-sized semiconductor elements are used, by installing optical pulse transmission elements which perform signal pluse width compression. CONSTITUTION: Optical pulse transmission elements which perform signal pulse width compression are installed. For example, in time division/wavelength division multiplex transmission system, an optical signal pulse train of a waveform 18 is generated by light emitting elements 1 and 2 for signal input of m channels wherein two wavelengths 11, 12 are multiplexed. The respective channel signals become time division multiplex optical pulse trains 4 and 6 of a waveform 19 through an optical pulse compression multiplex part 3, and propagate on a transmission line 13. In time division/wavelength division receiving system, time division multiplex optical pulse trains 7, 8 of wavelengths 11, 12 of a waveform 22 are subjected to waveform recovery and optical pulse isolation into m-number of channels by a pulse compression/isolation part 9. The signal is delivered to photodetection elements 10, 11.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission system for performing optical signal multiplexing using optical pulse compression / amplification, and an optical / electronic element constituting the optical transmission system.

[0002]

2. Description of the Related Art In a conventional optical pulse compression / amplification method for ultrashort pulse transmission, an optical fiber doped with erbium is excited by a high-power laser pulse and a cross phase modulation effect is utilized to generate a signal light. Means have been taken to compress the pulse width. The technical contents thereof are described in "Optical pulse compression / amplification method" in JP-A-3-46635.
According to this technology, in order to compress the signal light pulse whose pulse width has been expanded and whose intensity has been attenuated, it is incident on the optical fiber for compression, and then at a wavelength shorter than that of the signal light pulse By injecting the excitation light pulse of (1) later than this, a negative chirp is generated at the falling portion of the signal light pulse, and the pulse width is shortened. Further, the signal light pulse is amplified by the optical amplification effect under the erbium excitation.

The conventional optical pulse compression element expands the wavelength band of incident light by the self-phase modulation effect of the optical fiber and compresses the optical pulse width by using the group velocity dispersion of the dispersive medium or the diffraction grating pair. To do. Regarding its basic characteristics,
Applied Physics Letters 1982, 41st
Volume 1 to 3 (APPLIED PHYSICS LETTERS Vol.41,
pp. 1-3 (1982)). A pulse compression element using an optical fiber and a diffraction grating pair, and a pulse compression element using an optical fiber and a dispersion medium (atomic gas or semiconductor film) are applied Physics Letters 1
982 Volume 40, pp. 761-763 (APPLIED
PHYSICS LETTERS Vol.40, pp. 761-763 (1982)), Optics Letters 1981 Volume 6 13-1
Page 5 (OPTICS LETTERS, Vol. 6, PP. 13-15 (198
1)), or Applied Physics Letters 198.
2nd Volume 41, 4 to 6 pages (APPLIED PHYSICS LET
TERS Vol.41, pp. 4-6 (1982)).

[0004]

In the optical pulse compression / amplification method using the erbium-doped optical fiber, in order to compress the signal light pulse, the pumping light pulse is compressed at the same repetition frequency as the signal light pulse. Must be incident on. However, in order to achieve a transmission capacity of 100 Gb / s or more, a method called time division multiplexing is adopted. This is because it is difficult to modulate at such a high frequency with a single light emitting element. That is, 100 Gb
In the transmission region of / s or more, the signal light pulse cannot be compressed by the pumping light pulse from the single laser element. In order to generate a pumping light pulse, it is required to generate a time-division multiplexed pulse of light pulses from a number of lasers. This means that as the transmission capacity increases, the size of the device increases, which makes it difficult to use in a large-capacity transmission system. In addition, the length of the compression fiber, the time delay between the signal light pulse and the pumping light pulse, and the precise control of the repetition frequency are required to be restricted. In addition, since the wavelength range to be used is limited, it is not possible to cope with a further increase in capacity by combining time division multiplexing and wavelength division multiplexing. For the operation of large-capacity transmission system, instead of the optical pulse compression / amplification method using erbium-doped optical fiber, the optical pulse compression is performed by the linear effect instead of the non-linear effect, or the optical under the steady light / steady current is used. Means must be realized to enable pulse compression / amplification.

In the pulse compression device using the optical fiber,
In order to compress an optical pulse having a pulse width of 10 to 20 picoseconds of a semiconductor laser, a repetition frequency of 10 GHz and an average output of 5 mW to a pulse width of 1 picosecond or less, the optical fiber length is 100 m or more. When performing signal multiplex communication, this device is required for the number of signals to be multiplexed, and the volume problem becomes more serious as the degree of multiplexing increases. Installing this device in a large-capacity optical transmission network requires a large amount of cost and space, and has problems such as frequent inspection and adjustment for maintaining performance. Even if an earthquake occurs, it is necessary for a reliable large-capacity transmission system to realize a device that does not require inspection and adjustment. It is an object of the present invention to provide a time division / wavelength division multiplexing optical transmission system by performing optical pulse compression / amplification / optical pulse signal separation using a small semiconductor element.

Another object of the present invention is to provide a compact optical pulse compression element which can be installed at any place, does not require inspection and adjustment, and enables construction of seismic resistant high-speed optical communication. is there.

[0007]

The above object of the present invention is achieved by providing an optical pulse transmission device for performing signal pulse width compression in an optical pulse transmission system.

Further, a device for compressing the pulse width of a plurality of optical signals having two or more different wavelengths, a device for amplifying the pulse width of the optical signal, and a signal which has passed through the device are time-division multiplexed and wavelength-multiplexed. This is achieved by realizing an optical pulse transmission system including a device for division multiplexing or demultiplexing.

Further, when the input pulse width is Δt, the output pulse width is Δt ′, and the width of the element in the light transmission direction is L (mm), the light pulse width compression ratio per millimeter (Δt /
Δt ') / L (mm) is 1 or more, and the wavelength band expansion part having a structure in which a plurality of semiconductor columns are arranged on the substrate, and the group velocity modulation of incident light having a semiconductor multilayer structure. This is achieved by realizing an optical pulse transmission device configured by including at least a part.

[0010]

In the optical pulse transmission system whose overall configuration is shown in FIG. 1, time division multiplexing and wavelength division multiplexing of optical signals can be realized simultaneously in one system. The basic element that made this possible is a transmission / relay / reception system centered on an optical pulse compression element / signal optical pulse phase modulation element capable of operating at multiple wavelengths. Detailed configurations of the system of FIG. 1 are shown in FIGS. here,
The operation principle of the multi-wavelength optical pulse compression element / multi-wavelength signal optical pulse phase modulation element will be described. In the system shown in FIGS. 1 to 9, the configuration is given as a system assuming a dual wavelength operation. However, it is self-evident that this system can be extended to more than two wavelengths, as described below.

First, the principle of optical pulse compression will be described. First, the process of compressing an ideal (non-chirped) light pulse will be described. An example of the wavelength-time characteristic of the light pulse is shown in FIG. Wavelength band expansion (self-phase modulation) and group velocity dispersion are used for ideal optical pulse compression. (I) The wavelength band of the incident light to be pulse-compressed is expanded by the self-phase modulation effect of the medium having the third-order optical nonlinearity (for example, the element of FIG. 11 is used), and (ii) the group velocity dispersion, that is, By group velocity modulation using the wavelength differential characteristic of the refractive index of the medium, the propagation time at each wavelength of the emitted light pulse is aligned (for example, the element of FIG. 12 is used), and the pulse width is compressed. That is, instead of expanding the wavelength band dl of the incident light, the pulse width dt is compressed on the time axis. This follows the law that reflects the coherence of light: (c / l ^ 2) dl * dt = 1. Here, c and l are the light velocity and the central wavelength of the incident light, respectively. Hereinafter, unless otherwise specified, *, /, ^ and d [] / dt are multiplication, division,
It is assumed to represent exponentiation and differential operations. The wavelength-time characteristic of the optical pulse corresponds to FIG. 10A when the pulse width is long, and corresponds to FIG. 10C when the pulse width is short. The optical pulse after self-phase modulation (wavelength band expansion) corresponds to the state of FIG. FIG. 10B is an intermediate state in the process of shifting from (a) to (b).

In the self-phase modulation effect by the optical Kerr effect, which is one of the third-order optical nonlinearities, when pulsed light is incident, the refractive index of the medium changes with the time evolution of the light intensity, and phase modulation occurs. . When formulated, dph (t) = (w / c) dn (t) * x, and dn (t) = n2 * I (t).

Where dph (t), w, dn (t),
x, n2, and I (t) are the phase change amount, the angular frequency, the refractive index change amount, the medium length, the nonlinear refractive index, and the light intensity, respectively. With the propagation of incident light, the phase change dph (t) causes a wavelength change through the following equation: (c / l ^ 2) dl (t) =-d [dph (t)] / dt.

From this, it is understood that the wavelength band expansion increases as the intensity of the incident light increases and the pulse waveform becomes steeper. Since the optical Kerr effect is a coherent process that does not disturb the phase, the self-phase modulation is not affected by the time constant such as the relaxation time of the optical carrier and completely follows the time change of the optical pulse. This is an advantage not found in self-phase modulation utilizing gain saturation in a semiconductor amplifier.

As the origin of the Kerr effect, the nonlinearity due to the optical transition of the electron, hole system or electron-hole system in the quantum dot structure array (FIG. 11) is used. FIG. 13 shows the energy states related to the inter-subband transition of the electron system, and FIG.
4 shows. The propagation direction kx of the incident light is 1110 in FIG.
The direction is parallel to the substrate as shown in. The polarization direction of the incident light is parallel to the substrate (p
There are two types, x) and orthogonal (pz). Generally, in the optical response of the quantum structure, polarization dependence occurs. However, by making the vertical confinement thickness and the lateral confinement thickness of each quantum dot in the semiconductor fine columnar structure 1102 in FIG. Dependency can be eliminated. this is,
This applies not only to nonlinear effects such as self-phase modulation, but also to linear effects such as refractive index dispersion.

First, the case of utilizing inter-subband transition will be described below. When subband transition is used, there is a possibility that wavelength band expansion (self-phase modulation) that is more efficient than interband transition is performed. This is because the oscillator strength of the subband transition is proportional to the size of the quantum confinement. At the band-to-band transition, the oscillator strength is proportional to the crystal lattice spacing.

Considering the efficiency as an element, it is necessary to reduce the loss of incident light. Therefore, electrons as carriers are injected (1103 and 1112 in FIG. 11) to form an inversion distribution, and the absorption amount of photons accompanying the transition from the low energy low level to the high energy high level is
It is necessary to compensate by the photon emission accompanying the transition from the upper level to the lower level. There are two methods for carrier injection, namely, the case of using the steady current shown in FIG. 13A and the case of using the steady light irradiation shown in FIG. 13B. In each case, the energy changes along the direction orthogonal to the substrate surface and the physical arrangement of quantum confinement are shown. The basic unit is a structure composed of three coupled quantum dots. The upper levels E1 and E2 are
It is a level generated by tunnel coupling between the dots 2 and 3 separated by the barrier 3, and electrons at these levels are not distributed to the dot 1 (1301). Lower level E3, E
4 is the dots 1 and 2 (130
This is a level generated by tunnel coupling between 2), and no electrons are distributed in the dot 3 (1303).

In the case of steady current injection, the current passes through the barrier 1 to form an electron distribution in the upper levels E3 and E4, and moves to E3 and E4 by radiative transition. By extracting electrons at high speed through the barrier 4, the electron distribution in the lower level is made sparse and the population inversion is achieved. The dots 1, 2, and 3 have thicknesses of 1 nm, 3 nm, and 1.5 nm, for example. The barrier thickness is about 1 to 3 nm and is determined so as to optimize the population inversion and the amount of injected current. The energy splitting in the upper and lower levels is sufficiently smaller than the energy difference dE between the upper and lower levels. When light whose central photon energy matches dE is incident as an incident pulse, the emitted pulse exhibits a wavelength band expansion due to the self-phase modulation effect. The structure in which the coupled quantum dot structures are multiplexed is shown in FIG. 11 (b). In the case of current injection, impurities are added to the layers D1 and D2 above and below the multiple quantum dots (1108).

The steady light injection shown in FIG. 13B is the same as the current injection except for the following points. Carrier injection is performed by irradiating light that resonates with the upper level and the lower level or hole level in the figure. In order to form an effective population inversion, the barrier at the left end in the figure is thick and has a thickness of about 10 nm or more, and an element serving as a recombination center that becomes a carrier trap is added to a portion outside the barrier at the right end. As a result, the distribution of upper levels can be made relatively high.

In the case of self-phase modulation using interband transition, the band structure shown in FIG. 14 is used. The resonance effect between electron holes is used. A process such as a one-photon transition or a two-photon transition can be used. The two-photon transition can reduce the loss due to light absorption. Other points are
Same as above.

Group velocity dispersion can be controlled based on the linear interaction of matter and light. As an example, the photonic band effect (FIG. 12A) and the resonance effect with the exciton level (FIG. 12B) can be used. By utilizing these effects, pulse compression can be performed using only the semiconductor structure without using the diffraction grating. In FIG. 12A, the interval lph of the semiconductor fine columnar structure is about the same as the wavelength of light, and a photonic band is formed by interference due to multiple scattering of light. This gives a large group velocity dispersion. By designing the photonic band so that this group velocity dispersion has a characteristic of canceling the group velocity dispersion of FIG. 10B, the short pulse of the characteristic shown in FIG.
Can occur. In a semiconductor having a superlattice structure and a multilayer structure as shown in FIG. 12B, group velocity dispersion due to excitons is used. A pulse compression rate of about 20 times can be achieved by the optical pulse compression element in which the wavelength band expansion section (self-phase modulation section) 1501 and group velocity modulation section 1502 as shown in FIG. 15 are combined.

When the semiconductor laser is directly modulated, the generated optical pulse is chirped, and its characteristics are shown in FIG.
The shape is similar to 0 (b). Therefore, it is not necessary to expand the wavelength band by self-phase modulation, and optical pulse compression can be achieved only by controlling the group velocity dispersion.

The resonance energy (wavelength) between the quantum levels (subband and band level) can be arbitrarily controlled by changing the thickness of the quantum dots. Therefore,
Figure 1 shows quantum dot systems with different thicknesses (resonant wavelengths).
By providing it in the semiconductor fine columnar structure 1102 of No. 1, it is possible to cause wavelength band expansion (self-phase modulation) and group velocity modulation in different wavelength regions, and to perform wavelength multiplexing by the number of combinations of quantum dot systems having different thicknesses. it can. If the number of combinations is 2, it corresponds to 2-wavelength multiplexing, and if it is 3, it corresponds to 3-wavelength multiplexing. In the configuration diagrams and examples of the present invention, the most basic two-wavelength multiplexing will be mainly described.

Next, the signal light pulse phase modulation will be described. Cross phase modulation is used as an operating principle. Cross-phase modulation is similar to self-phase modulation except that it is a non-linear effect that occurs when two optical pulse trains independent of each other overlap each other. Of the two, one is a signal light pulse train for transmission, and the other is a signal extraction local light pulse train. Phase modulation occurs only when both pulse trains overlap. Utilizing this fact, a specific clock frequency
It can be applied to an optical pulse separation unit that extracts only a phase signal light pulse train. For this purpose, it may be incorporated in an interference light waveguide line for converting a phase change into a light intensity change. The optical Kerr effect of the quantum dot system is used as a source of mutual phase modulation.
The wavelength multiplexing can be dealt with by providing quantum dot systems having different thicknesses, as described above.

[0025]

Embodiments of the present invention will be described below.

(Embodiment 1) The method of using time division / wavelength division multiplexing optical transmission is to make different kinds of information correspond to different wavelengths, perform time division multiplexing for each information type, and increase the transmission capacity. is there. In multimedia information communication,
Video information transmission at l1 = 1.55 μm, l2 = 1.3 μm
Used for voice information transmission. The number of multiple wavelengths can be prepared by the number of combinations of quantum dot systems having different thicknesses.
It is easy to extend the multiplexing to three wavelengths or more.

First, an embodiment of the time division / wavelength division multiplex transmission system shown in FIG. 1A will be described. A multiplex system consisting of m channels is prepared. Two wavelengths are multiplexed in each channel. The first channel is CH11 (wavelength l1) and CH12 (wavelength l2)
Are multiplexed. An optical signal pulse train is generated by each of the signal input light emitting elements 1 and 2. The waveform is 18, the repetition rate is 40 Gb / s, and the pulse width is 5 ps. The signal of each channel is propagated through the transmission line 13 through the optical pulse compression / multiplexing unit 3 into the time-division multiplexed optical pulse trains 4 and 5 having 19 waveforms. Two wavelengths are multiplexed on the transmission line 13.

The optical pulse compression / multiplexing unit 3 has the configuration shown in FIG. In (a), the wavelength multiplexing unit 203
After the pulse train 201 of 1 and the pulse train 202 of wavelength 12 are multiplexed, pulse compression is performed by the optical pulse compression unit 204, and time-division multiplexed pulse trains 207 and 208 are obtained.
The transmission time delay unit 209 connected to the transmission line 205 for each channel imposes a delay time that increases with the channel number so that the time slots are not overlapped with each other. The transmission line 205 has, for example, a wavelength l1.
It is a bundle of an optical fiber for use with an optical fiber for wavelength l2. In (b), wavelength multiplexing section 211 multiplexes after pulse compression section 210. In (a), one pulse compression unit for each channel is sufficient, but two sets of quantum dot systems corresponding to two wavelengths need to be included in the pulse compression unit 204.
The structure of the pulse compression element becomes complicated. On the other hand, in (b), the pulse compression unit is required twice as much as in (a), but one pulse compression unit 210 only needs to include one set of quantum dot systems, which simplifies the structure. Depending on the case, it is possible to properly use (a) when the number of channels is large and (b) when the number of channels is small.

In FIG. 1A, the optical pulse compression / multiplexing unit 3
The configuration of the signal input light emitting elements 1 and 2 connected through the connection transmission line 12 is as shown in FIG. 30
1 is a laser diode for direct modulation, 302 is an input signal source, 303 is an input signal path, 304 is an optical transmission line, 30
Reference numeral 5 is an emission pulse train, 306 is a continuous oscillation or mode-locked laser diode, and 307 is an optical modulator. (A)
Then, although the configuration is simple, the pulse waveform emitted from the laser diode may be inferior to that of (b).
On the other hand, in (b), the structure is complicated.

The optical pulse compression unit can have the configuration shown in FIGS. 4 to 6. In FIG. 4, (a) a group velocity modulation unit only, (b) a group velocity modulation unit having an optical amplification function, (c) (d) a group velocity modulation unit and an optical amplification unit are connected. The thing is illustrated. In the present invention, for example, a traveling wave type semiconductor optical amplifier is used as the optical amplification section. 401
Is an incident light pulse train, 402 is an outgoing light pulse train, 403 is an optical transmission line, 404 is a steady current or a steady light injection source, 4
Reference numeral 05 is an injection route. The device of FIG. 4 is effective when the characteristics of the optical pulse generated by directly modulating the laser diode as shown in FIG. 3A are chirped as shown in FIG. 10B. If you need an optical amplification function and want to miniaturize (b), if you want to simplify the device fabrication,
It suffices to select (c) or (d).

The optical pulse compressor having the configuration shown in FIGS. 5 and 6 is effective for the light emitting device having the configuration shown in FIG. 3B. Figure 5
Here, 501, 502, 503, 5055, and 506 are respectively an incident light pulse train, an outgoing light pulse train, an optical transmission line,
A constant current or constant light injection source and injection path. In FIG. 5, the group velocity modulation unit or the self-phase modulation unit can have an optical amplification function. FIGS. 6A to 6D are examples of the case where the optical amplification unit is externally attached. 601, 6
Reference numerals 02 and 603 denote an incident light pulse train, an outgoing light pulse train, and an optical transmission line, respectively. When miniaturization of the element is important, any one of the configurations of FIG. 5 may be selected, and when priority is given to simplification of the element production, one of the configurations of FIG. 6 may be selected.
Any of the elements shown in FIGS. 4 to 6 can support two wavelengths (11, 12).

The wavelength band expanding section (self-phase modulating section) is prepared by fabricating a semiconductor fine columnar structure 1102 including multiple quantum dots 1108 on a substrate 1109 and forming an array in an optical waveguide 1101 as shown in FIG. To use. With the structure including two types of quantum dot system thicknesses, it is possible to generate emission light pulse trains 1106 and 1107 in which the wavelength bands of incident light pulse trains 1104 and 1107 of wavelengths 11 and 12 are expanded. Here, 1103 (1111)
Is injected from above.

In the following, GaAs / AlGa will be mainly used.
A structure using an As heterojunction as a basic material and utilizing subband transition of electrons in a GaAs quantum dot will be described. The semiconductor microcolumnar structure including quantum dots has a growth position controlled organometallic vapor phase epitaxy method (OMV).
PE). First, the production of the fine columnar structure will be described. A window having a diameter of about 100 nm is provided by depositing a SiO 2 film on the substrate, exposing the substrate with an electron beam, and then etching with an HF solution. Au is vapor-deposited to a thickness of about 0.1 to 1 nm. S by HF solution etching
By removing iO2, a substrate having a diameter of about 100 nm and having Au island-shaped growth nuclei distributed is obtained. By controlling the electron beam irradiation position, the growth position of the columnar structure can be controlled. The width of the pillar, that is, the width of the quantum dot is determined by the substrate temperature. OMVPE at a total gas pressure of about 0.1 atm by flowing trimethylgallium and trimethylaluminum in hydrogen gas at 1 cc / min and arsine at 10 cc / min.
Grow. As the substrate, GaAs having a (111) crystal plane is used. The growth rate in the direction perpendicular to the substrate is 5 to 10 nm per second. The substrate temperature is 400 degrees Celsius, and the width of the dot is 20 nm. By opening and closing the shutter for supplying trimethylaluminum along with the growth, it is possible to form a multiple quantum dot column having the layer structure shown in FIG. 3 and the energy level structure shown in FIG. The Al content of the AlGaAs layer is 40%. In this embodiment, carrier injection by current is performed. The upper and lower layers D1 and D2 are made of about 10 ^ 19 / cm ^ 3 Si
It is an n-type layer doped with a donor. Top layer D1
Connect with a lead wire for supplying an electric current. The lead wire is produced by electron beam or optical lithography. The conductivity of the substrate is n-type, and Si is 10
A device doped with about 19 / cm ^ 3 is used. If necessary, the entire top of the wavelength band expansion section is covered with a clad layer having a thickness of about 1 micron. The clad layer is, for example, Si
Is about 10 ^ 19 / cm ^ 3 of n-type GaAs. The length of the wavelength band expansion element is 5 mm. As the thickness of the quantum dot system, the quantum dot 1 (130 in FIG.
A structure including two types of 1) having a thickness of 1 nm and 1.5 nm was manufactured. By making the thickness in the horizontal direction closer to the thickness in the vertical direction, it is possible to eliminate the polarization dependence.

As the group velocity modulator, the one having the structure shown in FIG. 12 (a) was used. The semiconductor fine columnar structures are arranged in an array at about the wavelength of light, and the manufacturing method is the same as that of the wavelength band expanding section. The length of the group velocity modulator is 5 mm. The width of the optical waveguide 1101 is 10 μm.

The time division / wavelength division multiplex reception system shown in FIG. 1B will be described. This system uses waveform 22
The waveforms of the time-division multiplexed optical pulse trains 7 and 8 of the wavelengths 11 and 12 are restored and the optical pulses are separated into m channels. The main part is the optical pulse compression / separation unit 9, and the signal is sent to the light receiving elements 10 and 11. The pulse waveform immediately before the light receiving element is 23. FIG. 7 shows an example of the configuration of the optical pulse compression / separation unit. (A) is a nonlinear optical loop mirror 70
No. 6 is used, and (b) is one using the Machtzender type interference light transmission line 715. The former is easy to manufacture, and the latter is excellent in miniaturization. 701
Is an optical pulse compression / branching circuit, which has the configuration of FIG. In FIG. 8A, the time-division multiplexed signals 801 and 802 having the wavelengths 11 and 12 are waveform-reconstructed from the optical transmission line 808 through the pulse compression unit 803, and distributed to each channel by the optical branching unit 804. Wavelengths l1 and l2 pass through the optical transmission line after branching
Waveform-restored time division multiplexed signals 806 and 807 are
It is incident on the circuit that separates the light pulse.

In FIG. 7, reference numerals 702 and 703 denote time-division multiplexed signals whose waveforms have wavelengths l1 and l2, respectively.
The waveform is 713. The signal propagates through the optical transmission line 704 and enters the loop mirror in (a) and the interference optical transmission line in (b). Nonlinear optical loop mirror 706
Is composed of a 3 dB coupling section 705, a ring-shaped optical fiber, and a signal pulse phase modulation section 707. The Mach-Zehnder interferometer optical transmission line 715 includes a signal pulse phase modulator 70.
7, a constant phase delay unit 716, and an optical amplification unit 717. The signal pulse phase modulator 707 has the configuration shown in FIG. 901 is a signal pulse train of wavelengths l1 and l2, 90
Reference numeral 2 is a phase-modulated outgoing light, and 903 is an optical transmission line. Reference numeral 904 represents a main body. 905 is a local optical pulse train, 9
Reference numeral 06 is a current or light injection source, and 907 is an injection path.
Local optical pulse trains 708 and 7 for extracting signals of wavelengths 11 and 12
09 enters through the optical transmission line 704 and induces mutual phase modulation to perform optical pulse separation in synchronization with the local optical pulse train.

The time division / wavelength division multiplex relay system of FIG. 1C will be described. At the relay point, the pulse widths 14 and 15 of the wavelengths 11 and 12 that have been expanded in pulse width and have been attenuated
Optical pulse train 1 in which the waveform is restored to 21 waveforms by the waveform restoration unit 6
6 and 17 are injected.

The transmission capacity of this transmission system will be described. 25 or more channels having a transmission capacity of 40 Gb / s can be multiplexed. Since we are multiplexing two wavelengths,
A transmission capacity of 2 Tb / s or more is achieved.

(Embodiment 2) An embodiment of the optical pulse compression element shown in FIG. 15 will be described. In this device, a wavelength band expansion part 1501 made of a semiconductor fine columnar structure including multiple quantum dots (1108 in FIG. 11) and a group velocity modulation part 1502 made of a semiconductor multilayer structure in FIG. 12 (b) are provided on a substrate 1503. It has a formed structure. As shown in Figure 15,
Incident light 1504 with a picosecond pulse width is input to the wavelength band expansion unit 1.
Emitted light 1506 (wavelengths l1 and l2), passing through the group velocity modulation unit and pulse-compressed to femtoseconds 1506,
1507 (wavelengths 11 and 12) is obtained. This element is
It is also a basic constituent part of the time division signal multiplexing element shown in FIG. The device is manufactured in the same manner as in Example 1.

This device has a wavelength of 10 μm and a pulse width of 2
It can be shown that the pulse width is 1 picosecond when the optical pulse of 0 picosecond is incident and the pulse width of the output pulse is measured, and the pulse compression rate is 20 times. Further, when an optical pulse having a wavelength of 760 nm and a pulse width of 1 picosecond is incident on this element, pulse compression of 10 times can be performed by transition between bands. The pulse compression rate per unit length is 2 / mm.

This element is an element for pulse-compressing a signal for optical communication in the 1.55 or 1.3 micron band by using sub-band transition by changing the materials of the quantum dots and the group velocity modulation section. Available. For example, Ga
A structure based on an As / ZnSe-based material corresponds to this. Further, in this element, the group velocity modulation portion is crystal-grown directly on the substrate, but it is also possible to join the wavelength band widening portion by adhering after growing on another substrate.

By connecting the element of this embodiment to the emission end of the laser device, it is possible to shorten the pulse of the laser for optical communication or for physicochemical measurement. It is a small device made on a single substrate and can be packaged and firmly connected to an optical fiber. It is highly earthquake resistant and does not require regular adjustment. It can be installed in any place,
It can be installed in underground seismic trenches. Therefore, this element can be useful for constructing a large-capacity transmission network that can withstand earthquakes. (Embodiment 3) This embodiment will be described with reference to FIG. A large number of the pulse compression elements described in the second embodiment are integrated as a pulse compression unit 1601 to form a connection unit 1602.
Thus, an element connected to the substrate 1603 was manufactured. This element functions as an optical signal multiplexing element. FIG. 16 illustrates an example in which two pieces are integrated. Explaining based on the element in the case of using the GaAs / AlGaAs based material in the first embodiment, AlGaAs can be used as the material of the connecting portion. First, the optical pulse compression units 1601 are manufactured according to the procedure of the first embodiment, and then the connection units 1602 are manufactured by optical lithography. The total length of the element is 15 mm, and the width of the portion where the incident light propagates is 10 μm.

A method of performing time division multiplexing of a transmission signal using this element will be described. A case where signals of two systems are multiplexed will be described with reference to FIG. As an incident light on each pulse compression unit 1601, an optical pulse train 1604 (wavelength l1) having a pulse width of 10 picoseconds and a repetition frequency of 20 GHz,
1605 (wavelength 12) is used. A relative delay time of 25 picoseconds is added to the pulse train of one signal system with respect to the other. Since the pulse width is compressed to 0.5 picoseconds, the application of this delay time eliminates the possibility that the signals of both systems overlap each other, and the signals of the two systems can be time-division multiplexed. Emitted light 1607 (wavelength l1), 1608 (wavelength l1
In 2), the repetition frequency is 40 GHz as a multiplexed signal.

The incident signal pulse width is 10 picoseconds,
When compressed to 0.5 picoseconds, the emitted light 1607, 16
The 08 repetition frequency can be raised to a minimum of 500 GHz. If the original incident signal is 20 GHz, signals of 25 or more systems can be multiplexed. The pulse compression rate per unit length is 1.3 / mm.

The element of this embodiment can be used as a signal multiplexing element in optical communication. Moreover, the point that it is manufactured on a single substrate is the same as that of the first embodiment, and it can be installed in any place and used for large-capacity transmission that withstands an earthquake.

(Embodiment 4) As shown in FIG. 12, a fine columnar structure 1202 including quantum dots is formed at a wavelength lph of incident light.
Two-dimensional regular arrangement is performed in the optical waveguide 1201 at regular intervals. The fine pillar structure 1202 is shown in FIG. 11. Due to the regular arrangement, a photonic band is formed by multiple scattering of light, and the refractive index dispersion can be artificially controlled. Thereby, group velocity modulation can be performed. The micro pillar structure 1202 includes quantum dots and has a self-phase modulation effect. With this array structure alone, wavelength band expansion and group velocity modulation can be performed simultaneously and pulse compression can be performed. The pulse compression rate is 20 times or more as in the second embodiment. The total length of the element is 5 mm, and the width of the portion where the incident light propagates is 10 μm. The pulse compression rate per unit length is 4 / mm.

Further, an optical signal multiplex device having a total length of 1 cm can be manufactured by integrating two or more of the above devices on a single substrate and optically connecting them by a connecting portion as in the third embodiment. .

(Embodiment 5) This embodiment will be described with reference to FIG. FIG. 17 relates to an electro-optical conversion device that generates a short pulse light train. It can be used as a portable and movable short pulse light generator. First, (a) will be described. Input electric pulse train 1701 to transmission line 1
The light is input to the light emitting element 1704 through the connection unit 702 and the connection unit 1703, and the light pulse compression unit 1705 shortens the pulse. Output light pulse train 1 to the outside through the optical transmission line 1705
711 is released. 1710 is an optical window or an optical connection part. This device includes a current source 1707 and a constant current circuit 1708 so as to be suitable for carrying and moving.
It is built in 12. 1709 is a current supply path. (B) is an apparatus capable of wavelength division multiplexing operation. Reference numeral 1713 is an electric pulse train containing more information amount or information type than 1701. Also, the transmission line 1
714 and the connecting portion 1715 are designed to be suitable for a large capacity input. Reference numerals 1718 and 1719 are light emitting elements having wavelengths 11 and 12 and reference numerals 1720 and 1721 are optical pulse compression units at the respective wavelengths. After being multiplexed by the wavelength multiplexing unit 1722, the output pulse train 1717 is transmitted through the lossless optical window or the optical connection unit 1716 at the wavelengths 11 and 12.

(Embodiment 6) In this embodiment, the transmission and reception systems of FIGS. 1 (a) and 1 (b) are integrated, and examples are given in FIGS. 18 and 19. The functions are the same as those described in FIG. However, in order to improve compatibility in connection with peripheral digital devices, an electric amplification unit or a D / A conversion unit 1801
Also, the electric amplification unit or the A / D conversion unit 1911 is included. In FIG. 18, reference numeral 1802 is a signal input light emitting element (wavelength l1 or 12), and 1803 is an optical pulse compression unit. A transmission time delay unit 1806 is installed on the optical transmission line 1804 for multiplexing, and output optical pulse trains 1807 and 1808 are emitted through the optical pulse multiplexing unit 1805.
Reference numerals 1809, 1810, and 1811 denote power supply paths to the electric amplification unit, the light emitting element, and the pulse compression unit, respectively.
These elements and parts are a single support (eg circuit board)
Integrated on 1812.

FIG. 19 shows an example of integration on the receiving side. 19
01 and 1902 are time-division multiplexed optical pulse trains having wavelengths l1 and l2. Reference numeral 1903 denotes an optical pulse waveform restoration unit, 1904.
Is a local optical pulse train generator of wavelengths 11 and 12;
Reference numerals 05 and 1906 are local light pulse trains having wavelengths 11 and 12. The waveform-restored time-division multiplexed optical pulse train propagates through the optical transmission line on each branch and propagates through the optical pulse demultiplexing unit 19
It is incident on 09. The local optical pulse train is the optical transmission line 1908.
The signal pulse component that is incident on the optical pulse separation unit 1909 through the signal pulse component is synchronized with the signal pulse component 1910.
Detected in. The electric signal generated by the light receiving element is amplified by the electric amplification section 1911. These elements and components are integrated on a single support 1916. 1912, 191
Reference numerals 3, 1914 and 1915 denote power supply paths, respectively. If the integrated support of FIGS. 18 and 19 is a circuit board, it can be easily connected to a computer terminal, an exchange, or the like. Alternatively, it can be used as an interface device for a large-capacity multimedia communication network.

In each of the above embodiments, the semiconductor material is mainly used.
GaAs / AlGaAs is used, but InP, InGaAs, InGaAsP, III-V other than the above materials are used.
Similar effects can be obtained by using Group II-VI semiconductor materials.

[0052]

As described above, the optical pulse transmission system according to the present invention can be used for large capacity information transmission by time division / wavelength division multiplexing.

[0053]

[Brief description of drawings]

FIG. 1 is a configuration diagram of a time division / wavelength division multiplexing optical transmission system and a transmission pulse waveform.

FIG. 2 is a configuration diagram of an optical pulse compression / multiplexing unit in FIG.

FIG. 3 is a configuration diagram of a signal input light emitting element.

FIG. 4 is a configuration diagram of an optical pulse compression element using a group velocity modulation unit and an optical amplification function.

FIG. 5 is a configuration diagram of an optical pulse compression element using a self-phase modulation unit (wavelength band expansion unit) and a group velocity modulation unit.

FIG. 6 is a configuration diagram of an optical pulse compression element using a self-phase modulation unit (wavelength band expansion unit), a group velocity modulation unit, and an optical amplification unit.

7 is a configuration diagram of an optical pulse compression / separation unit in FIG.

8 is a configuration diagram of the optical pulse compression / branching circuit in FIG.

9 is a configuration diagram of a signal light pulse phase modulator in FIG. 7.

FIG. 10 is a schematic diagram of wavelength-time characteristics of long pulse light, wavelength-time characteristics of chirped pulse light, and wavelength-time characteristics of short pulse light.

FIG. 11 is a structural perspective view of a wavelength band expansion section (self-phase modulation section), a perspective view of a semiconductor microcolumnar structure including a multiple quantum dot structure, and an arrangement view of polarization directions of incident light.

FIG. 12 is a group velocity modulation section utilizing refractive index dispersion in a columnar structure array or wavelength band expansion (self-phase modulation).
FIG. 3 is a perspective view of an optical pulse compression element that simultaneously utilizes the above, and a perspective view of a group velocity modulation unit that utilizes refractive index dispersion in a multilayer structure.

FIG. 13 is an energy diagram of a subband level of a multiple quantum dot structure for carrier injection by current and light.

FIG. 14 is an energy level diagram of conduction and valence bands of a quantum dot structure and a schematic diagram of transition between one-photon and two-photon bands.

FIG. 15 is a perspective view of an optical pulse compression element including a wavelength band expansion section and a group velocity modulation section.

FIG. 16: Optical pulse compression and time division multiplexing,
The perspective view of an optical pulse transmission element.

FIG. 17 is a configuration diagram of an electro-optical conversion device that generates a short pulse light train.

FIG. 18 is a configuration diagram of an integrated circuit for time division / wavelength division multiplexing optical transmission.

FIG. 19 is a configuration diagram of an integrated circuit for time division / wavelength division multiplexing optical reception.

[Explanation of symbols]

1 ... Signal input light emitting element (wavelength l1), 2 ... Signal input light emitting element (wavelength l2), 3 ... Optical pulse compression / multiplexer, 4 ... Transmission side time division multiplexed optical pulse train (wavelength l)
1), 5 ... Time-division multiplexed optical pulse train on transmission side (wavelength l
2), 6 ... Optical pulse waveform restoration unit, 7 ... Receiving side time division multiplexed optical pulse train (wavelength l1), 8 ... Receiving side time division multiplexed optical pulse train (wavelength 12), 9 ... Optical pulse compression / separation , 10 ... Signal receiving element (wavelength 11), 11 ... Signal receiving element (wavelength 12), 12 ... Connection optical transmission line,
13 ... Time-division / wavelength-division multiplexed signal transmission line, 14 ...
・ Time-division multiplexed optical pulse train (wavelength l1) before waveform restoration,
15 ··· Time-division multiplexed optical pulse train (wavelength l before waveform restoration)
2), 16 ... Time-division multiplexed optical pulse train (wavelength l1) after waveform restoration, 17 ... Time-division multiplexed optical pulse train (wavelength l2) after waveform restoration, 18 ... Emission of light-emitting elements 1 and 2 for signal input Pulse waveforms, 19 ... Pulse waveforms of optical pulse trains 4 and 5, 20 ... Pulse waveforms of optical pulse trains 14 and 15, 21 ... Pulse waveforms of optical pulse trains 16 and 17, 22 ... Pulse waveforms of optical pulse trains 7 and 8 ,
23 .. Signal light pulse waveforms immediately before the signal light-receiving elements 10 and 11, 201 .. Output pulse train of the signal input light emitting element 1 (wavelength l1, channel 1-1), 202 .. Output of the signal input light emitting element 1 Pulse train (wavelength l2, channel 1
-2), 203 ... Wavelength multiplexer, 204 ... Optical pulse compressor operating at wavelengths 11 and 12, 205 ... Optical transmission line, 206 ... Optical pulse multiplexer, 207 ... Transmission side time division multiplexed light Pulse train (wavelength l1, equivalent to 4 in FIG. 1),
208 .. Transmitting side time division multiplexed optical pulse train (wavelength l2,
(Equivalent to 5 in FIG. 1), 209 ... Transmission time delay unit, 210
..Optical pulse compressors operating at wavelengths 11 or 12, 2
11..Compressed optical pulse multiplexing unit, 301..Direct modulation laser diode, 302..Input electric signal source, 303 ..
・ Input signal path, 304 ・ ・ Optical transmission line (equivalent to 12 in FIG. 1), 305 ・ ・ Ejection pulse train (corresponding waveform is 18 in FIG. 1), 306 ・ ・ Continuous oscillation or mode-locked laser diode, 307 ..Optical modulator, 401..incident light pulse train, 402..outgoing light pulse train, 403..optical transmission line, 404..steady current or steady light injection source, 4
05 .. Current or light injection path, 501 .. Incident light pulse train, 502 .. Outgoing light pulse train, 503 .. Optical transmission line, 505 .. Steady current or steady light injection source, 506.
.Current or light injection path 601, Incident light pulse train, 6
02 ... Emitted light pulse train, 603 ... Optical transmission line, 70
1 ... Optical pulse compression / branching circuit, 702 ... Receiving side time division multiplexed optical pulse train (wavelength l1) after waveform restoration and signal branching, 703 ... Receiving side time division multiplexed optical pulse train after waveform restoration and signal branching ( Wavelength 12), 704 ... Optical transmission line, 705 ... 3 dB coupling section, 706 ... Non-linear optical loop mirror, 707 ... Signal light pulse phase modulation section, 7
08 .. Signal extraction local optical pulse train (wavelength l1), 70
9 ... Local optical pulse train for signal extraction (wavelength 12), 710
..Wavelength separation unit 711..optical pulse train to signal light receiving element (wavelength 11), 712..optical pulse train to signal light receiving element (wavelength 12), 713..optical pulse trains 702 and 7
03 waveform, 714 ... Optical pulse trains 711 and 712
, 715 ··· Mahhertzender type interference optical transmission line, 716 ··· constant phase delay section, 717 · · optical amplification section, 8
01 ... Receiving side time division multiplexed optical pulse train (wavelength l1, equivalent to 7 in FIG. 1), 802 ... Receiving side time division multiplexed optical pulse train (wavelength l2, equivalent to 8 in FIG. 1), 803 ... Optical pulse compression , 804 ... Optical branching unit, 805 .. Branching optical transmission line, 806 .. Time division multiplexed optical pulse train after waveform restoration and signal branching (wavelength l1, equivalent to 702 in FIG. 7), 8
07 .. Time division multiplexed optical pulse train after waveform restoration and signal branching (wavelength 12; equivalent to 703 in FIG. 7), 808 ...
Optical transmission line, 901 ... Incident light pulse train (wavelengths 11 and 12), 902 .. Outgoing light pulse train, 903 .. Optical transmission line, 904 .. Signal light pulse phase modulator main body, 90
5 ... Local light pulse train, 906 ... Steady current or steady light injection source, 907 ... Current or light injection path, 1101
..Optical waveguide, 1102..Semiconductor fine columnar structure, 11
03 .. steady injection current or steady injection light, 1104 ..
Incident light pulse train (wavelength l1), 1105 ... Incident light pulse train (wavelength l2), 1106 ... Emission light pulse train (wavelength l1), 1107 ... Emission light pulse train (wavelength l2),
1108 ... Multiple quantum dots, 1109 ... Substrate, 11
10 ··· Wavenumber vector ky of incoming signal light in traveling direction, polarization vectors px, py, 1111 ··· Injection light (incident direction x, y or z) 1112 · · Steady injection current, 12
01..Optical waveguide part 1202..Semiconductor micro pillars, 12
03 .. Incident light (wavelength 11) 1204 .. Incident light (wavelength 12) 1205 .. Outgoing light (wavelength 11) 1206
..Emitted light (wavelength 12), 1207..steady injection current or steady injection light, 1208..multilayer structure (group velocity modulation part), 1301..dot 1, 1302..dot 2,
1303 ... Dots 3 1304 Barriers 1 130
5 ... Barrier 2, 1306 ... Barrier 3, 1307 ...
Barrier 4, 1308 .. Steady injection current, 1309 .. Incident pulse, 1310 .. Outgoing pulse, 1312 .. Injection light, 1313 .. Recombination center, 1401 .. Conduction band, 1402 .. Valence band, 1403.・ One-photon transition process, 1404 ・ ・ Two-photon transition process, 1501 ・ ・ Waveband expansion part (self-phase modulation part), 1502 ・ ・ Group velocity modulation part, 1503 ・ ・ Substrate, 1504 ・ ・ Incoming light (wavelength l
1), 1505 ... Incident light (wavelength 12), 1506 ...
Output light (wavelength l1), 1507 ... Output light (wavelength l1)
2), 1601 ... Optical pulse compression section, 1602 ... Connection section, 1603 ... Substrate, 1604 ... Incident light (wavelength l
1), 1605 ... Incident light (wavelength 12), 1606 ...
Pulse waveform of incident light, 1607 ... Emitted light (wavelength l
1), 1608 ... Emitted light (wavelength 12), 1609 ...
Output light pulse waveform, 1701 ... Input electric pulse train,
1702..Electric pulse transmission line, 1703..Connection part, 1704..Light emitting element for signal input, 1705..Optical pulse compression unit, 1706..Optical transmission line, 1707 ..
Stationary current source, 1708 ... Constant current circuit, 1709 ...
Current supply path, 1710..Light emission window or connection part, 1
711 ... Emitted light pulse train, 1712 ... Main body, 171
3 ... Input electric pulse train, 1714 .. Electric pulse transmission line, 1715 .. Connection part, 1716 .. Light emission window or connection part, 1717 .. Output light pulse train (l1 and l
2), 1718 ... Light emitting element for signal input (wavelength l1),
1719..Light-emitting element for signal input (wavelength 12), 172
0 ... optical pulse compression unit (wavelength l1), 1721 ... optical pulse compression unit (wavelength l2), 1722 ... wavelength multiplexing unit, 18
01 ... Electric amplifier or D / A converter, 1802 ...
Light emitting element for signal input (wavelength l1 or l2), 1803
..Optical pulse compression unit, 1804 .. ・ Optical transmission line, 180
5 .. Optical pulse multiplexing unit, 1806 .. Transmission time delay unit,
1807 ... Output optical pulse train (wavelength 11), 1808
Output light pulse train (wavelength 12), 1809 ... Power supply path, 1810 ... Power supply path, 1811 ... Power supply path, 1812 ... Support for integration, 1901 .. Input light pulse train (wavelength 11), 1902 ... Input optical pulse train (wavelength 12), 1903 ... Optical pulse waveform restoration unit, 1
904 ... Local optical pulse generator (wavelengths l1 and l
2), 1905 ... Local optical pulse train (wavelength l1), 19
06..local optical pulse train (wavelength 12), 1907..branching optical transmission line, 1908..local optical pulse transmission line, 1
909 .. Optical pulse separation section, 1910 .. Signal light receiving element, 1911 .. Electric amplification section or A / D conversion section, 19
12 ... Power supply path, 1913 ... Power supply path, 1
914 ... Power supply path, 1915 ... Power supply path,
1916 ... Support for integration.

Front page continuation (72) Inventor Naoki Kayane 1-280 Higashi Koikeku, Kokubunji, Tokyo Inside Central Research Laboratory, Hitachi, Ltd. (72) Nobuhiko Kikuchi 1-280 Higashi Koikeku, Kokubunji, Tokyo Inside Central Research Center, Hitachi Ltd.

Claims (13)

[Claims] 1. An optical pulse transmission system comprising an optical pulse transmission element for performing signal pulse width compression. 2. The optical pulse transmission system according to claim 1, wherein the transmitted optical signal is light having two or more different wavelengths. 3. The optical pulse transmission system according to claim 1, wherein the optical pulse transmission element includes a wavelength band expansion section and a group velocity modulation section. 4. The optical pulse transmission system according to claim 3, wherein the wavelength band expanding section is composed of a plurality of semiconductor fine columnar structures. 5. The optical pulse transmission system according to claim 3, wherein the group velocity modulation section has a semiconductor multilayer structure. 6. An apparatus for compressing the pulse width of a plurality of optical signals having two or more different wavelengths, an apparatus for amplifying the pulse width of the optical signal, a signal that has passed through the apparatus by time division multiplexing and wavelength. An optical pulse transmission system consisting of equipment for division multiplexing or demultiplexing. 7. An element for processing an electric signal on a support plate,
An optical pulse transmission system comprising: an element for generating an optical signal, an element for compressing the pulse width of the optical signal, and a portion for multiplexing the optical signal with the compressed pulse width. 8. An element for compressing a pulse width on a support plate, a portion for branching an optical signal, an element for demultiplexing the optical signal, an element for detecting the optical signal, and an electrical element for processing the detected electrical signal. An optical pulse transmission system comprising: 9. A portion for inputting an electric signal from the outside, a portion for converting the electric signal into light, a portion for compressing the pulse width of the converted light, and a power supply portion for driving the compressed portion. An optical transmission system having a portion for emitting the compressed optical signal. 10. The input pulse width is Δt and the output pulse width is Δt.
t ′, where L (mm) is the width of the element in the light transmission direction, the light pulse width compression ratio per millimeter (Δt / Δ
t ') / L (mm) is 1 or more, an optical pulse transmission device characterized in that 11. An optical pulse transmission device including at least a wavelength band expanding part having a structure in which a plurality of semiconductor columns are arranged on a substrate and a group velocity modulating part for incident light having a semiconductor multilayer structure. 12. An optical pulse transmission device including at least a wavelength band expansion part having a structure in which a plurality of semiconductor columns are arranged on a substrate and a group velocity modulation part of incident light. 13. A substrate including a portion in which a plurality of semiconductor pillars are arranged, said semiconductor pillars being regularly arranged at intervals of a wavelength of incident light, and expanding a wavelength band of incident light based on a nonlinear effect of said semiconductor pillar. , The pulse width of the incident light is compressed by group velocity dispersion by a regular array, and the energy of the incident light is reduced or eliminated on the time axis by electrically or optically injecting energy into the semiconductor. Or, an optical pulse transmission element having a function of amplifying.