JPH11231360A - Optical pulse compression device, optical pulse transmission equipment using the same, and laser beam generator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To compress about 10pj weak optical pulse transmitted through an optical fiber or the like. SOLUTION: A saturable absorption layer 203 is arranged in a position coinciding with the peak of the intensity of light in a distributed Bragg reflector or the like where the periodic distribution of the refractive index starts with a low-refractive index layer 201. When an optical pulse having a wide pulse width is made incident on this distributed Bragg reflector, the intensity of the optical pulse is increased by the low-refractive index layer 201 and a refractive index layer 202, and a part to be the leading edge of the optical pulse with respect to the time disappears by the saturable absorption layer 203. Thus, the pulse width is compressed only with one reflection.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for compressing an optical pulse by utilizing an absorption saturation phenomenon of a saturable absorber in a semiconductor, and more particularly, to an optical pulse having an energy of about 10 pj (pico-joule). 1. Field of the Invention The present invention relates to an optical pulse compression device suitable for application to optical transmission using, and an optical pulse transmission device and a laser light generator using the same.

[0002]

2. Description of the Related Art In a conventional optical pulse compression technique, absorption saturation in a semiconductor is used for the purpose of downsizing and self-oscillation of a mode-locked solid-state laser that generates short pulse light. A device in which a single quantum well of GaAs is stacked as a saturable absorber on an AlAs / AlGaAs distributed Bragg reflector having no absorption is used as a saturable reflector, and an optical pulse having a pulse width of 100 fs (femtosecond) or less is mode-locked. : Li
Examples generated by SAF or Ti: sapphire solid-state lasers are described in T. Tsuda, et al., "Optical Letters 19
1995 Vol. 20, No. 12, 1406-1408 (Opt
ics Letters Vol. 20, No. 12, pp. 1406-14
08 (1995)) ".

Further, there is an example in which a saturable absorber is placed in a Fabry-Perot etalon under anti-resonance conditions and an optical pulse having a pulse width of 3.3 ps (picoseconds) is generated by a mode-locked Nd: YLF solid-state laser. U. Keller, et al., Optics Letters, Vol. 17, No. 7, pp. 50, pp. 50-507, Vol. 17, No. 7, 1992.
5-507 (1992)).

Further, in the conventional optical pulse compression technique, as an example of a technique which does not use saturable absorption, an optical pulse from a pulse laser is compressed using self-phase modulation and optical fiber grating by the Kerr effect of an optical fiber. J.A.R. Williams, et al.
Photonics Technology Letters 7th 1995
Vol. 5, No. 491-493 (IEEE Photonics T
echnology Letters Vol.7, No.5, pp.491-49
3 (1995)) ".

[0005] By the way, in large-capacity optical transmission using an optical fiber, the above-described optical pulse compression is used in order to compress the pulse width expanded by transmission at an arbitrary position on the optical fiber and retransmit it as a short pulse. Technology is required. However, when each of the above-described optical pulse compression techniques is used for optical pulse compression in large-capacity optical transmission, there are the following problems, respectively.

First, in the conventional short pulse light generation technique utilizing absorption saturation in a semiconductor, the purpose of its use is to reduce the size of a mode-locked solid-state laser having a high optical pulse energy in a resonator and to achieve self-oscillation. In other words, if the light pulse energy is 10 nj (nanojoule) or higher, saturable absorption readily occurs in ordinary semiconductors, which is not suitable for short pulse generation. To cope with this, a measure is taken to reduce the effective light pulse energy incident on the semiconductor by utilizing the suppression of the light intensity in a high reflection Bragg reflector or an anti-resonant Fabry-Perot etalon.

However, in optical fiber transmission, the light pulse energy is about 10 pj or less. Therefore,
The above-described light pulse compression technique of reducing the effective light pulse energy to lower the absorption saturation efficiency of the semiconductor cannot be applied. Conversely, there is a need for an optical pulse compression technique that increases the effective optical pulse energy to some extent and increases the efficiency of semiconductor absorption saturation.

In the prior art which does not rely on saturable absorption, the pulse energy of light is lower than that in the prior art using saturable absorption, but still requires about 1 nj of pulse energy. Furthermore, this technique utilizes group velocity dispersion correction by optical fiber grating. Therefore, this technique is effective only for an optical pulse having a specific intensity and pulse width, and cannot compress an optical pulse having an arbitrary pulse width at a required position on an optical fiber transmission line.

[0009]

The problem to be solved is that the conventional technology cannot perform optical pulse compression with an increased efficiency of absorption saturation of a semiconductor by increasing the effective optical pulse energy. is there. An object of the present invention is to solve the problems of the prior art, to enable compression of the width of an optical pulse transmitted by an optical fiber at an arbitrary distance, and to be used for time division multiplexing optical transmission and wavelength division multiplexing optical transmission. It is an object of the present invention to provide an optical pulse compression device capable of efficiently generating short pulse light, and an optical pulse transmission device and a laser light generation device using the same.

[0010]

In order to achieve the above object, an optical pulse compression apparatus according to the present invention comprises a light intensity peak within a distributed Bragg reflector or the like in which a periodic distribution of refractive index starts from a low refractive index layer. The structure is such that a saturable absorption layer is arranged at a coincident position. When an optical pulse whose pulse width is widened by transmission through an optical fiber or the like is incident on such a distributed Bragg reflector, the intensity of the optical pulse is enhanced, and the portion that becomes the leading edge in time of the optical pulse due to absorption saturation. Disappears. Thus, the pulse width can be compressed by only one reflection. The optical pulse transmission device and the laser light generating device of the present invention use the optical pulse compression and phase modulation by the optical pulse compression device to provide a short optical pulse suitable for time division multiplexed optical transmission or wavelength division multiplexed optical transmission. Can be generated and transmitted.

[0011]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a sectional view showing a first embodiment of the configuration according to the present invention of the optical pulse compression device of the present invention. The optical pulse compression device of this example constitutes a phase shift distribution Bragg reflector, and includes a unit cell 204 including three layers of an n (L) layer 201, an n (H) layer 202, and a saturable absorption layer 203. It is periodically laminated on a substrate 205.

In the unit cell 204, the n (L) layer 201 has a lower refractive index than the n (H) layer 202 and is on the side where the light pulse is incident. Further, the thickness of the saturable absorption layer 203 is different from that of the other two.
Since it is thinner than the layer, the saturable absorption layer 203 is regarded as being integrated with the low refractive index layer. Thus, this phase shift distributed Bragg reflector starts with a low refractive index layer. Here, assuming that the refractive index of air is “1”, the refractive index of the n (L) layer 201 is “n (L)”, and the refractive index of the n (H) layer 202 is “n (H)”, “1” <N (L) <n (H) ". With such a refractive index distribution structure, the light intensity increases at the position of the saturable absorption layer 203, and the nonlinear optical effect of absorption saturation is enhanced.

It is necessary to use a medium having high optical nonlinearity for the saturable absorption layer 203. As such a medium,
There is a quantum well structure made of a semiconductor thin film. The two-dimensional electronic state in this quantum well has high optical nonlinearity and high efficiency of absorption saturation. In particular, when an exciton state near the band edge of the quantum well is used, the optical nonlinearity of the quantum well can be maximized.

The (L) layer 201 and the n (H) layer 202
Are transparent, and the n (L) layer 201 is a saturable absorbing layer 2
03 together with the central wavelength of the incident light.
/ 4). On the other hand, the n (H) layer 202 itself has a thickness of a quarter wavelength. Materials used for the n (L) layer 201, the n (H) layer 202, and the saturable absorption layer 203 for use at a wavelength of about 1550 nm will be described.

First, InP is used for the n (L) layer 201. The n (H) layer 202 includes GaInAsP (Ga: In composition ratio 0.33: 0.67, As: P composition ratio 0.71: 0.2).
9) is used. The saturable absorption layer 203 has a thickness of 1
GaInAs of 0 nm (Ga: In composition ratio 0.47: 0.53)
A quantum well is used. The composition ratio of each layer is set so as to lattice match with InP used as the substrate 205. Light pulses with a width of picoseconds (ps) or more are converted to picoseconds or less,
To compress to the femtosecond (fs) region, use unit cell 2
04 are laminated in 20 layers or more. The pulse compression ratio is obtained by measuring and comparing the pulse widths of the incident light pulse and the emitted light pulse from the left end of the figure.

Further, in this embodiment, the bias voltage source 206
And a reverse bias voltage is applied to extract optical carriers accumulated near the band gap. The arrangement for applying this reverse bias voltage is such that the photocurrent is parallel or perpendicular to the surface.
There is a street. In the former arrangement, positive and negative electrodes are formed along the surface, and light pulses are focused between the electrodes. In the latter,
Electrodes are formed on the front surface and the back surface of the substrate. In this case, it is necessary to form a ring-shaped electrode on the surface so as not to hinder the incidence of light, and to collect light pulses at the center of the ring, or to use a transparent electrode.

Hereinafter, the basic principle of absorption saturation in such an optical pulse compression device will be described. When a semiconductor is irradiated with light having a photon energy larger than the band gap energy, photocarriers such as electrons and holes are excited and light is absorbed. In particular, when the power of irradiation light is high, high-density photocarriers are excited. When a semiconductor is used for optical communication or optoelectronic devices, light incident on the semiconductor has a certain wavelength (for example, 1.55 μm).
m, 1.3 μm, etc.), that is, a laser light source whose power is concentrated in a spectral region around a certain photon energy.

The density of photocarriers that can be excited to a certain energy state in a semiconductor is limited by the density of state for that energy. That is, there is a limit to the density of photocarriers that can be excited. When the power of laser light is increased to increase the density of photocarriers, the limit is approached. Under such high-density excitation conditions, the absorbed light power tends to saturate no longer in proportion to the incident light power. This phenomenon is called absorption saturation and is a type of absorption-type nonlinear optical effect. In addition, a medium that exhibits remarkable absorption saturation is called a saturable absorber.

When light incident on the saturable absorber having been absorbed and saturated is blocked, the photocarriers gradually relax, so that the absorption saturation is attenuated and the absorption is restored. In addition, when an optical carrier is excited to a state having an energy higher than the band gap, the optical carrier is in a non-equilibrium state, so that momentum and energy are exchanged by mutual scattering, and quasi-equilibrium is obtained by applying energy to the crystal lattice. State (quasi-equi1ibriu
m) to relax.

In this quasi-equilibrium state, the optical carrier has a Maxwell-Boltzmann distribution of the temperature corresponding to the average value of the excitation light energy as the quasi-equilibrium energy.
For example, when about 10 ^ 17 photocarriers are excited per cubic centimeter, scattering between the photocarriers occurs frequently, and the relaxation due to the scattering is dominant. The time constant of this relaxation is several hundred fs or less. Also,
When the optical carrier density is about 10 ^ 14 or less, the scattering frequency between the optical carriers decreases, and instead, energy relaxation due to scattering with lattice vibration becomes dominant, and the time constant is about 1 ps.

Thus, when the light used for excitation has a pulse width of about 100 fs or less, 1 ps after excitation.
To some extent, relaxation due to inter-carrier scattering is observed, and thereafter, relaxation due to scattering with lattice vibration is observed. And
When the absorption saturation having such an ultrafast relaxation time constant is used, the time width of the optical pulse waveform can be shortened.

That is, when a light pulse is incident on the saturable absorber, the leading edge of the light pulse (the bottom of the pulse that is earlier in time) is absorbed by the saturable absorber and disappears. When the absorption saturates, the light is no longer absorbed and the rest of the light pulse is transmitted or reflected. When the leading end of the pulse disappears, the width of the light pulse is shortened, and light pulse compression is achieved.

Here, when the absorption saturation is recovered with a time constant of about 100 fs or less determined by scattering between optical carriers, 10
After about 0 fs, light pulse compression by absorption saturation becomes possible for the next light pulse. This means that pulse compression can be performed on a high repetition optical pulse train reaching 10 THz, and it can be used for an optical transmission system having a transmission rate of 1 Tb / s or more.

However, carriers that have been relaxed to the band gap energy are relaxed through inter-band radiative recombination. The time constant reaches about 1 ns. Therefore, as the transmission rate increases, the distribution of the optical carriers accumulated near the band gap energy cannot be ignored. Therefore, for example, it is necessary to apply a reverse bias to extract the accumulated photocarriers from the semiconductor.

Further, in order to use such an optical pulse compression technique for optical transmission, pulse compression must be effective even for an optical pulse having an energy of about I0pj.
However, ordinary saturable absorbers require energy of about 100 pj or more. In order to cope with such low light pulse energy, it is necessary to increase the effective light pulse energy inside the saturable absorber. In this example, this is realized by placing a saturable absorber in an optical confinement structure such as an optical resonator. At the same time, since it is necessary to confine the light over the spectral band of the optical pulse, a distributed Bragg reflector having a pitch of a quarter wavelength is used.
g reflector, DBR).

The wavelength corresponding to the quarter wavelength is made coincident with the center wavelength of the spectrum of the optical pulse to be transmitted or generated. In this way, when light is incident on the distributed Bragg reflector of this example from the air, the refractive index distribution of the distributed Bragg reflector starts from the low refractive index layer. Light is strongly confined inside. In this phase shift distribution Bragg reflector, a saturable absorber is distributed at a position where the light intensity peaks according to the periodic light intensity distribution inside. By doing so, the level required for pulse compression can be reduced as the energy of the transmitted light pulse.

An optical pulse having a width of about 100 fs corresponds to about 50 light pulses in terms of light wavelength (around 1500 nm).
Therefore, while the light pulse is reflected once, light is reflected a plurality of times inside the phase shift distributed Bragg reflector, that is, multiple reflections occur. In this way, when the light pulse whose pulse width is expanded by the optical fiber transmission is incident on the phase shift distribution Bragg reflector provided with the saturable absorber, and the width of the light pulse reflected from the phase shift distribution Bragg reflector is measured. It is demonstrated that light pulse compression is performed only by one reflection. The configuration of the apparatus for this is shown in FIG. 1 and the following FIG.

The phase-shift distribution Bragg reflector of the example shown in FIG. 1 is formed by periodically laminating two layers having no absorption and one saturable absorption layer as a basic unit. In this structure, the thickness of the saturable absorbing layer is thinner than the other two layers (about 1).
1/0). Then, as shown in the example shown in FIG. 2 below, when the saturable absorbing layer is considered as a single body with the low refractive index layer or the high refractive index layer, each layer has a quarter wavelength pitch with two layers as a basic unit. A reflection spectrum or the like can be qualitatively evaluated as being qualitatively similar to a certain phase shift distribution Bragg reflector.

FIG. 2 is a sectional view showing a second embodiment of the configuration according to the present invention of the optical pulse compression device of the present invention. The optical pulse compression device of the present example has an n (L) layer 207 and an n (H) layer 2
08 are periodically stacked on a substrate 210 as a unit cell 209. In this example, the n (L) layer 207 also serves as a saturable absorption layer. Even with the optical pulse compression device having such a configuration, it is possible to perform optical pulse compression only by one reflection.

The optical pulse compression apparatus using saturable absorption shown in FIGS. 1 and 2 can compress optical pulses having various pulse widths, unlike the compression technique by correcting the group velocity dispersion. Therefore, it is not necessary to install at a specific place on the optical fiber transmission line, and it can be installed at any place. Hereinafter, the description of the optical pulse compression device of the present invention will be continued by taking optical fiber transmission as an example.

FIG. 3 is a block diagram showing a third embodiment of the optical pulse compression apparatus according to the present invention.
In FIG. 3, reference numeral 101 denotes an optical pulse whose pulse width is widened by optical fiber transmission, 102 denotes an optical amplifier, 103 denotes an optical pulse amplified by the optical amplifier 102, 104 denotes a lens for condensing the optical pulse 103, 105 Is incident light composed of light pulses collected by the lens 104, and 106 is incident light 1
An optical pulse compression unit having the configuration shown in FIG. 1 or 2 for shortening the pulse width of 05, 107 is light emitted from the optical pulse compression unit 106, 108 is an emitted light pulse, and 109 is a bias voltage source.

Since the intensity of the optical pulse 101 whose pulse width has been widened by the optical fiber transmission is also reduced, the intensity is amplified by the optical amplifier 102 as necessary. A light pulse 103 as a parallel light beam emitted from the light amplifying unit 102 is condensed through a condensing lens 104, and as an incident light 105, a light pulse compressing unit composed of a phase shift distribution Bragg reflector provided with a saturable absorber. It is incident on 106. in this way,
In order to make the pulse compression of the light pulse 103 with low pulse energy more efficient, the light pulse is condensed by the lens 104 and is incident on the light pulse compression unit 106.

In the process of propagating and multiple-reflecting in the optical pulse compression section 106, the optical pulse is well absorbed only by the leading edge of the optical pulse, which advances ahead in time, by the saturable absorber. As described above, when the incident light 105 is reflected, the light pulse is compressed by the absorption saturation in the light pulse compression unit 106, and the pulse width of the incident light 105 becomes shorter. The compressed light pulse passes through the condenser lens 104 as outgoing light 107 and becomes a light pulse 108 as a parallel light beam again.

As described above, the spread of the pulse width of the optical pulse to be transmitted as the signal and the decrease in the intensity can be recovered, and the transmission and the signal processing can be performed again. In particular, in optical fiber communication, the incident optical pulse 101 is a signal optical pulse transmitted over a long distance in the optical fiber, and the emitted optical pulse 108 is again incident on the optical fiber and transmitted.
Note that an optical fiber amplifier or a semiconductor optical amplifier can be used as the optical amplifier 102. The light pulse 107 thus compressed by light confinement by light confinement can be used as a short light pulse for large-capacity light transmission based on time division multiplexing. That is, the optical pulse compression device can be effectively used as an optical pulse transmission device.

In the example shown in FIG. 3, the incident light 105 is arranged to be obliquely incident on the light pulse compression unit 106. However, in order to avoid the polarization dependence, it is necessary to be as close to normal incidence as possible. As a guide, the angle of incidence is 1 from the direction perpendicular to the surface of the light pulse compression unit 106.
Within 0 degrees. In the structure of FIG. 3, the intensity of the optical pulse is amplified by the optical amplifier 102. Such optical amplification is performed when a decrease in the optical intensity due to transmission or signal processing becomes a problem. If decline is not a problem,
The following structure in FIG. 4 can be used.

FIG. 4 is a block diagram showing a fourth embodiment of the configuration according to the present invention of the optical pulse compression apparatus according to the present invention.
The optical pulse compression apparatus of the present embodiment is obtained by removing the optical amplification unit 102 from the optical pulse compression apparatus having the configuration shown in FIG. 3, and the intensity of the optical pulse transmitted from the optical fiber is absorbed by the optical pulse compression unit 106. Applies when the value is sufficient for saturation.

Next, an embodiment in which the optical pulse compression device of the present invention is used as an optical pulse transmission device for optical fiber communication will be described with reference to FIG. FIG. 5 is a block diagram showing a fifth embodiment of the configuration according to the present invention of the optical pulse compression device of the present invention. Light pulse compression device 31 of this example
3 is packaged as an element and has a function as an optical pulse transmission device in optical fiber communication.

The signal light pulse transmitted through the incident side optical fiber transmission line 301 is amplified in intensity by the optical amplifier 302. If the decrease in the light pulse intensity is not a problem, the light amplifying unit 302 may be omitted. The incident-side optical fiber transmission line 301 and the optical amplifier 302 are coupled to an optical pulse compression device 313 via an incident optical pulse introducing unit 303.

The light pulse introduced from the incident light pulse introducing unit 303 to the optical pulse compression device 313 passes through the incident light pulse transmission optical fiber 304, is converted into a parallel light beam by the incident light coupling unit 305, and then is condensed by the condenser lens 306. Incident light 307
The light is focused on the optical pulse compression unit 308 having the configuration shown in FIG. 1 or FIG. Then, the signal light pulse is pulse-compressed by absorption saturation in the light pulse compression unit 308,
The emitted light 309 is reflected by the light pulse compression unit 308.

The outgoing light 309 becomes a parallel light beam after passing through the condenser lens 306, and is introduced into the outgoing light pulse transmission optical fiber 311 by the outgoing light coupling section 310. The outgoing light pulse transmission optical fiber 311 is connected to the outgoing light pulse deriving unit 3.
12 and these components are contained in a storage package. The signal light pulse thus pulse-compressed is transmitted through the emission-side optical fiber transmission line 314 connected to the emission light pulse deriving unit 312.

In this embodiment, the incident light coupling section 305 or the outgoing light coupling section 310 and the condenser lens 306 are provided between the incident light pulse transmitting optical fiber 304 or the outgoing light pulse transmitting optical fiber 311 and the light pulse compressing section 308. And different optical coupling systems. The reason is that, in order to minimize the insertion loss between the optical fiber and the parallel light beam, and to maximize the optical pulse compression ratio, the numerical aperture of the incident light pulse transmitting optical fiber 304 and the outgoing light pulse transmitting optical fiber 311 is changed. And the incident light 307 to the optical pulse compression unit 308
This is because it is necessary to independently minimize the diameter of the focused beam.

The bias voltage source 315 supplies the optical carrier generated for each optical pulse incident word to the optical pulse compression unit 3.
It is used for pulling out by applying a reverse bias so as not to be accumulated at 08. As described above, the optical pulse compression device 313 of the present example, that is, the optical pulse transmission device is different from the conventional optical pulse compression device based on group velocity dispersion correction, and is installed at an arbitrary point in the optical fiber transmission path. It can be used as an optical pulse compression repeater that is not restricted by the installation location.

Next, with reference to FIG. 6, another configuration for performing integration in which the optical amplifying unit is housed in a package will be described. FIG. 6 is a block diagram showing a sixth embodiment of the configuration according to the present invention of the optical pulse compression device of the present invention. This example shows another configuration example of an optical pulse compression device as an optical pulse transmission device used for optical fiber communication. The optical pulse compression device 413 of this example is also the same as the optical pulse compression device 3 in FIG.
13 and packaged in the same manner as in FIG.
01 and the output light pulse deriving unit 402 are connected to the input side optical fiber transmission line and the output side optical fiber transmission line.

The signal light pulse is sent to the optical amplifier 404 through the incident light pulse transmission optical fiber 403. Here, the incident light pulse is amplified in intensity, and then becomes a parallel light beam via the incident light coupling section 406 composed of the incident light pulse transmission optical fiber 405 and the lens. Conversely, the outgoing light coupling unit 407 for condensing the parallel light flux and entering the fiber is
It is provided on the emission side.

The incident light converted into a parallel light beam by the incident light coupling unit 406 is incident on the light pulse compression unit 410 as the incident light 409 condensed by the condenser lens 408, and after being pulse-compressed, the outgoing light 41l And is led out through the condenser lens 408, the outgoing light coupling unit 407, and the outgoing light pulse transmission optical fiber 412. All of these components are contained in the storage package. Further, the bias voltage source 414 is used for extracting optical carriers so as not to be accumulated in the optical pulse compression unit 410.

The light pulse compression section 410 has the sectional structure described with reference to FIG. 1 or FIG. By installing the optical pulse compression device packaged in the form of an element of this example as an optical pulse transmission device having a pulse compression function at an arbitrary position on an optical fiber transmission line, a transmission signal is transmitted as an optical pulse compression repeater. It is possible to efficiently compress the width of the light pulse carried.

FIG. 7 is a block diagram showing a seventh embodiment of the configuration according to the present invention of the optical pulse compression apparatus of the present invention. In this example, the optical pulse compression device of the present invention is used as an optical pulse transmission device for transmission using wavelength division multiplexing. In a transmission system using wavelength multiplexing, it may be necessary to use an optical pulse compression / transmission device that is optimized for an optical pulse having a central wavelength corresponding to each channel. This optimization refers to changing the center wavelength of the Bragg reflection and the absorption peak wavelength according to the center wavelength of the light pulse so that the compression rate of the light pulse of each channel is maximized.

In this example, n channels (wavelengths W1 to W1) are used.
Wn), one optical pulse compression device for each (in the figure,
"W1 to Wn") 501-1 to 501-n. Each optical pulse compression device 501-1
501-n is obtained by performing optimization using the configuration shown in FIG. 5 or FIG.

The wavelength-multiplexed optical pulse is converted to a wavelength demultiplexing part 50.
Divide into each channel by 2. The optical pulse divided into each channel is supplied to the optical pulse compression device 501 of each channel.
-1 to 501-n, and after being pulse-compressed by each of the optical pulse compression devices 501-1 to 501-n, the wavelength is multiplexed again by the wavelength multiplexing unit 503, and the signal is transmitted to a subsequent path. Transmitted.

As described above, in the examples shown in FIGS. 1 to 7, the optical pulse compression device and the optical pulse transmission device having the surface type structure called the phase shift distribution Bragg reflector have been described.
A phase shift distributed feedback waveguide can also be used as an optical pulse compression / transmission device. In this case, a phase shift (for a quarter wavelength) is provided in the periodic diffraction grating in the waveguide, and a confinement effect due to localization of light in the phase shift unit is used.

In the reflection spectrum of this optical waveguide, there is a wavelength region where the reflectance is low and the light is strongly confined inside. The wavelength region is made coincident with the center wavelength of the spectrum of the optical pulse to be transmitted or generated. Further, in this optical waveguide, distributed feedback type optical feedback by a diffraction grating formed inside and feedback modulation in a phase shift unit are used. Therefore, the end face must be in a non-reflective condition. Hereinafter, an example in which a phase shift distributed feedback waveguide is used as such an optical pulse compression device will be described.

FIG. 8 is a sectional view showing an eighth embodiment of the configuration according to the present invention of the optical pulse compression apparatus of the present invention. In this example, an optical waveguide type optical pulse compressor is used instead of the surface type optical pulse compressor used in FIGS. 1 to 7 to compress an optical pulse. Is shown.

In the optical waveguide type optical pulse compression apparatus of this embodiment, the lower cladding 601 is formed with a thickness of 1 μm, and then a high refractive index n (H) layer 602 is laminated with a thickness of 0.5 μm. After stacking a quantum well layer to be a saturable absorption layer 603, an n (H) layer 60 is further added.
2 are laminated. Thereafter, the cross-sectional structure of the square-wave diffraction grating shown in this figure is processed by electron beam drawing and dry etching, and is embedded in the n (L) layer 604 having a low refractive index. Then, an upper cladding layer 605 is laminated thereon by 1 μm.
Further, a bias voltage source 6 for applying a reverse bias voltage and extracting optical carriers accumulated near the band gap.
08 is connected to the upper surface of the optical waveguide. The voltage runs from the top surface of the substrate to the back surface of the substrate. Also, a grounding electrode (GN
D) is provided.

The n (H) layer 602, the saturable absorption layer 603, and the n (L) layer 604 form the core of the optical waveguide. AlInAs (Al: In composition ratio 0.47: 0.53) for the cladding layer,
The n (H) layer 602 contains GaInAsP (Ga: In composition ratio 0.3).
3: 0.67, As: P composition ratio 0.71: 0.29)
The saturable absorption layer 603 has a thickness of 10 nm of GaInAs (G
a: In composition ratio 0.47: 0.53), and InP is used for the n (L) layer 604. The substrate is InP, and each layer is set to lattice match with InP.

In this optical waveguide, the periodicity of the diffraction grating changes discontinuously in the phase shift section 606. The phase shift unit 606 is a phase boundary surface that exists in the vertical direction along the arrow in the figure. In this phase shift unit 606,
Interference conditions due to multiple reflection are modulated, and high-intensity light is localized. Further, a non-reflection coating layer 607 is formed on the end face to remove reflection on the incident surface of the optical waveguide. In this example, the incident light and the outgoing light propagate along the same axis. Therefore, in order to configure an element device using an optical waveguide type optical pulse compression apparatus, a light condensing system different from a plane type is required. The configuration for this will be described with reference to FIG.

FIG. 9 is a block diagram showing a ninth embodiment of the configuration according to the present invention of the optical pulse compression apparatus of the present invention. This example shows the configuration of an optical pulse compression device 712 applied as an optical pulse transmission device for optical fiber communication, and includes an incident light pulse introduction unit 701, an emission light pulse derivation unit 702, an incident light pulse transmission optical fiber 703, and an optical amplifier. A section 704, an amplified incident light pulse transmission optical fiber 705, an optical isolator 706 for removing return light, a coupling optical fiber 707 in which an incident light optical fiber and an outgoing light optical fiber are integrated into one, Light unit 708, incident light and outgoing light 70
9, the optical waveguide type optical pulse compression unit 71 having the structure shown in FIG.
0, and an output light pulse transmission optical fiber 711.

These elements have a device configuration housed in a storage package. Note that a reverse bias voltage is applied to the optical pulse compression unit 710 by the bias voltage source 713. In this example, since the incident light and the outgoing light 709 are transmitted on the same axis, the fiber for the incident light and the fiber for the outgoing light are fused by the coupling optical fiber 707. In this case, there is a possibility that the outgoing light enters the path of the incident light and becomes return light, thereby making the entire transmission system unstable. An optical isolator 706 is provided to remove this return light.

The light collecting section 708 has a configuration in which two lenses are arranged in series. First, after the light incident on the optical pulse compression unit 710 is converted into parallel light by the former lens (on the side of the coupling optical fiber 707), the latter lens (the optical pulse compression unit 71).
(0 side), the light is focused on the end face of the optical waveguide. Optical pulse compression unit 7
Follow the reverse path of the light coming out of 10. With such a two-piece configuration, the numerical aperture of the condensing unit 708 can be reduced by the coupling optical fiber 707.
And the respective values of the optical pulse compression unit 710 can be matched, and the insertion loss can be minimized.

Next, an example in which a laser light source for generating short pulse light is constructed by using the optical pulse compression device having the structure shown in FIGS. The apparatus will be described with reference to FIG.

FIG. 10 is a block diagram showing a first embodiment of the configuration according to the present invention of the laser beam generator of the present invention. As shown in FIG. 10A, the laser light generating device of the present example includes an optical pulse compression unit 801, a laser activation unit 802, a condensing unit 803, an outgoing light 804, and a propagation light 805 in a resonator.
It is composed of

The internal configuration of the optical pulse compression unit 801 is shown in FIG.
0 (b) or FIG. 10 (c). The laser active section 802 is obtained by applying an anti-reflection coating to the end face of the semiconductor laser diode chip on the resonator side (the right end face of the laser active section 802 in FIG. 10A). Considering use in a wavelength band of 1300 nm or 1550 nm used for optical communication, a laser chip uses a Fabry-Perot laser or a DFB laser having InGaAsP as an active layer.

In the laser resonator, the light pulse intensity is strong,
The optical pulse compression principle used in the first to ninth embodiments does not function, but rather the phase modulation function becomes stronger. In other words, most of the pulse is transmitted because the pulse intensity is strong,
The frequency shift due to the self-phase modulation becomes remarkable at the rising and falling portions of the pulse. When an optical pulse that has undergone a frequency shift at both ends of this pulse waveform and an optical pulse that has not been frequency-shifted and have the original waveform are superimposed,
Both ends of the pulse disappear due to the interference, and the vicinity of the peak of the pulse reinforces. This superposition effect enables pulse compression instead of absorption saturation.

FIGS. 10 (b) and 10 (c) each show a configuration of an optical pulse compression unit based on a Michelson interferometer. FIG. 10B shows an example using a planar phase modulation unit, and FIG. 10C shows an optical waveguide type.
First, FIG. 10B will be described. The intra-cavity propagation light 805 is transmitted from the laser activation unit 802 to the light pulse compression unit 8.
An optical pulse component propagating in the direction toward 01 and an optical pulse component propagating in the direction from the optical pulse compression unit 801 to the laser active unit 802 are included.

The light pulse component coming from the laser activating unit 802 is split by the beam splitter 806 in two directions. One of the split beams 807 is condensed by a condensing unit 808 and is incident on a plane-type phase modulation unit 809. The optical pulse that has undergone self-phase modulation in the phase modulation unit 809 is reflected and returned to the beam splitter 806. Also, the other split light 81
0 is incident on the high reflection mirror 811 and is reflected in the original direction without being subjected to phase modulation.

These two-component light pulses (branch light 80
7, 810) are superimposed by the beam splitter 806 and undergo light pulse compression due to interference. Then, the light pulse thus compressed is returned to the laser active part 802, where it is amplified, and the pulse waveform becomes sharper. By repeating this process, the laser light having a short pulse width is emitted as the emission light 804 at the laser end face (FIG. 10).
(Left side of (a)).

FIG. 10C shows the configuration of an optical pulse compression unit using an optical waveguide. Here, the intra-cavity propagation light 805 is incident on the optical waveguide portion through the condensing portion 812. One of them propagates through the branch optical waveguide 813, and
4, the light enters the optical waveguide type phase modulation unit 815,
The self-phase modulated light pulse is reflected. The other light propagates through the branching optical waveguide 816, enters the optical waveguide mirror 818 through the condensing unit 817, and is reflected without undergoing phase modulation.

Then, the optical pulses reflected from the two directions are superimposed, subjected to pulse compression, and returned from the end of the optical waveguide to the laser active section 802. As a result, laser light having a short pulse width is emitted as emission light 804.
If necessary, an electric signal having a frequency that resonates with the reciprocation cycle of the optical resonator is added to the laser activation unit 802 to perform forced mode locking. 10 (b) and 10 (c), a bias voltage source 8 is also provided so that a reverse bias voltage can be supplied.
19 are provided.

When a surface-type or optical-waveguide-type optical pulse compression unit is used, the optical pulse width can be shortened. Therefore, when the pulse compression ratio is high, the optical pulse compressed by the compression unit without using phase modulation is used. By returning the laser light directly to the resonator, a laser light source that generates a short pulse can be configured. Such a configuration will be described with reference to FIG.

FIG. 11 is a block diagram showing a second embodiment of the configuration according to the present invention of the laser beam generator of the present invention. In this example, a surface-type or optical-waveguide-type optical pulse compression unit 901 is placed at the end of the resonator and used as a reflection-type optical pulse compression unit. Other components are the laser activation unit 9
02, condensing units 903 and 904, outgoing light 905 and light propagating in the resonator 906.

The same laser activating section 902 as that of the first embodiment in FIG. 10 is used. Laser active part 9
The light emitted from the light source 02 is converted into a parallel light beam by the light collector 903, and is collected by the light pulse compressor 901 by the light collector 904. The light returned from the optical pulse compression unit 901 travels in the opposite direction to the light incident on the same optical path. These incident light and return light constitute the propagation light 906 in the resonator.

As the light reciprocates in this manner, the light is pulse-compressed by the light pulse compression unit 901 to be emitted as short pulses and emitted out of the resonator. At this time, if necessary, an electric signal having a frequency that resonates with the reciprocating period of the optical resonator is added to the laser active unit 902 to perform forced mode synchronization. The bias voltage source 907 is used to supply a reverse bias voltage.

FIG. 12 is a block diagram showing one embodiment of the configuration according to the present invention of the optical pulse transmission device of the present invention.
In this example, a transmitter for a short-pulse optical signal was configured using a light source that generates an optical pulse serving as a signal for transmission and the optical pulse compression unit described in the first to ninth embodiments. Things.

As the signal generating light source 1001, a Fabry-Perot laser or a DFB laser using InGaAsP as an active layer is used as in the above-described embodiments. In this embodiment, since the laser itself forms a resonator, no reflection is applied to the end face. Further, an electric input signal 1002 is transmitted through an input signal transmission line 1003 to a signal generation light source 1001.
To generate an optical pulse train to be a transmission signal. In this method, a laser is directly modulated to generate an optical signal. Further, an external modulation technique of generating an optical signal by inputting an input signal 1002 to an optical modulator optically connected to the laser without modulating the laser can also be used. In this technique, the signal generation light source 1001 in this figure is schematically equivalent only by replacing it with a continuous wave laser and an external modulator.

Further, the optical pulse train generated from the signal generating light source 1001 is passed through an incident optical transmission line 1005 as an incident optical signal 1004, and is guided to an optical pulse compression unit 1006. The incident optical signal 1004 is pulse-compressed by the optical pulse compression unit 1006, and transmitted through the output optical transmission line 1008 as an output optical signal 1007 having a short pulse width. In this way, an optical pulse transmission device that transmits a short pulse optical signal can be configured.

It is to be noted that a plurality of short pulse optical signal transmitters described in this example can be used for time division multiplex transmission. In this case, a short pulse optical signal transmitter for a transmission channel to be multiplexed is used. Then, each short-pulse optical signal transmitter is driven at an equal pulse repetition period (bit rate). Further, a time delay is added so that the optical pulse trains corresponding to the signals of the respective channels do not overlap on the time axis and are arranged in a time series on the time axis at equal time intervals. Thereafter, by multiplexing the optical pulse trains of the respective channels into one optical fiber, an optical pulse transmission system for time division multiplexing can be configured.

As described above with reference to FIGS. 1 to 12, in the optical pulse compression device and the optical pulse transmission device according to the present embodiment, the phase shift distribution in which the periodic distribution of the refractive index starts from the low refractive index layer is the Bragg reflection. The device has a configuration in which a saturable absorption layer is arranged at a position that coincides with the light intensity peak inside the container. With this configuration, when a pulse having a widened pulse width is incident, a portion that becomes a temporally leading edge of the optical pulse disappears due to absorption saturation, and the pulse width is reduced as a light pulse by only one reflection. Can be transmitted.

Further, the laser light generator of the present embodiment can generate a short light pulse by utilizing the light pulse compression by saturable absorption or the phase modulation accompanying the saturable absorption in the light pulse compressor of the present embodiment. The short optical pulse generated by the laser light generator can be used for time division multiplexed optical transmission and wavelength division multiplexed optical transmission in the optical pulse transmission device of the present embodiment. The present invention is not limited to the embodiment described with reference to FIGS. 1 to 12, and can be variously modified without departing from the gist thereof.

[0078]

According to the present invention, it is possible to increase the effective light pulse energy to perform light pulse compression with increased absorption saturation efficiency of a semiconductor, and to transmit 10 pj ( It is possible to reduce the width of an optical pulse having an energy of about pico-joule (short pulse), and it is possible to efficiently generate short pulse light used for time division multiplexing optical transmission and wavelength division multiplexing optical transmission.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a first embodiment of a configuration according to the present invention of an optical pulse compression device according to the present invention.

FIG. 2 is a sectional view showing a second embodiment of the configuration according to the present invention of the optical pulse compression device of the present invention.

FIG. 3 is a block diagram showing a third embodiment of the configuration according to the present invention of the optical pulse compression device of the present invention.

FIG. 4 is a block diagram showing a fourth embodiment of the configuration according to the present invention of the optical pulse compression device of the present invention.

FIG. 5 is a block diagram showing a fifth embodiment of the configuration according to the present invention of the optical pulse compression device of the present invention.

FIG. 6 is a block diagram showing a sixth embodiment of the configuration according to the present invention of the optical pulse compression device of the present invention.

FIG. 7 is a block diagram showing a seventh embodiment of the configuration according to the present invention of the optical pulse compression device of the present invention.

FIG. 8 is a sectional view showing an optical pulse compression apparatus according to an eighth embodiment of the present invention;

FIG. 9 is a block diagram showing a ninth embodiment of the configuration according to the present invention of the optical pulse compression device of the present invention.

FIG. 10 is a block diagram showing a first embodiment of the configuration according to the present invention of the laser light generator of the present invention.

FIG. 11 is a block diagram showing a second embodiment of the configuration according to the present invention of the laser beam generator of the present invention.

FIG. 12 is a block diagram showing one embodiment of a configuration according to the present invention of the optical pulse transmission device of the present invention.

[Explanation of symbols]

101: optical pulse (incident), 102: optical amplifier, 10
3: light pulse (after amplification), 104: condenser lens, 10
5: incident light, 106: light pulse compression unit, 107: outgoing light, 108: light pulse (emission), 109: bias voltage source, 201: n (L) layer, 202: n (H) layer, 203: acceptable Saturated absorption layer, 204: unit cell, 205: substrate, 20
6: bias voltage source, 207: n (L) layer, 208: n
(H) layer, 209: unit cell, 210: substrate, 301: incident side optical fiber transmission line, 302: optical amplification section, 303: incident light pulse introduction section, 304: incident light pulse transmission optical fiber, 305: incident light Coupling part, 306: condenser lens, 30
7: incident light, 308: light pulse compression unit, 309: emission light, 310: emission light coupling unit, 311: emission light pulse transmission optical fiber, 312: emission light pulse derivation unit, 313: light pulse compression device, 314: Outgoing optical fiber transmission line, 3
15: bias voltage source, 401: incident light pulse introduction unit,
402: outgoing light pulse deriving unit, 403: incident light pulse transmitting optical fiber, 404: light amplifying unit, 405: incident light pulse transmitting optical fiber (after amplification), 406: incident light coupling unit,
407: outgoing light coupling section, 408: condenser lens, 409:
Incident light, 410: light pulse compression unit, 411: outgoing light, 4
12: outgoing light pulse transmission optical fiber, 413: light pulse compression device, 414: bias voltage source, 501-1 to 50
1-n: optical pulse compression device, 502: wavelength demultiplexing unit, 50
3: wavelength multiplexing part, 601: lower cladding, 602: n
(H) layer, 603: saturable absorption layer, 604: n (L) layer, 6
05: upper cladding, 606: phase shift portion, 607:
Anti-reflection coating layer, 608: bias voltage source, 701: incident light pulse introduction unit, 702: emission light pulse derivation unit, 70
3: incident optical pulse transmission optical fiber, 704: optical amplifier,
705: incident optical pulse transmission optical fiber (after amplification), 70
6: optical isolator, 707: coupling optical fiber, 70
8: condensing unit, 709: incident and outgoing light, 710: optical pulse compressing unit (optical waveguide type), 711: outgoing optical pulse transmission optical fiber, 712: optical pulse compressing device, 713: bias voltage source, 801: light Pulse compression section, 802: laser active section, 803: condensing section, 804: outgoing light, 805: light propagating in the resonator, 806: beam splitter, 807: branch light, 808: condensing section, 809: phase modulation section , 810:
Branch light, 811: High reflection mirror, 812: Condenser, 81
3: branch optical waveguide, 814: condensing section, 815: phase modulation section (optical waveguide type), 816: branch optical waveguide, 817: condensing section, 818: waveguide mirror, 819: bias voltage source, 901: Light pulse compression unit, 902: laser active unit,
903, 904: condensing unit, 905: outgoing light, 906: light propagating in the resonator, 907: bias voltage source, 1001: signal generation light source, 1002: input signal, 1003: input signal transmission path, 1004: incident light signal , 1005: incident light transmission path, 1006: light pulse compression section, 1007: output light signal, 1008: output light transmission path.

Claims (18)

[Claims] 1. An optical pulse compression apparatus for compressing the width of a pulse (optical pulse) of incident light (reflected by incident light) due to absorption saturation of a saturable absorber in a semiconductor. A low-refractive-index layer and a high-refractive-index layer are stacked so as to form a periodic distribution of the refractive index starting from the low refractive index, and a thin-film layer for strongly confining the incident light, and a position where the light intensity distribution peaks in the thin-film layer A saturable absorber layer disposed in the uppermost layer, the light intensity of the light pulse incident on the low refractive index layer of the uppermost layer is increased by the thin film layer, and the absorption saturation of the saturable absorber is enhanced. Characteristic light pulse compression device. 2. The optical pulse compression device according to claim 1, wherein a sum of an optical thickness of the saturable absorber layer and an optical thickness of the low refractive index layer and an optical thickness of the high refractive index layer. Each, or the sum of the optical thickness of the saturable absorber layer and the optical thickness of the high refractive index layer, and each of the optical thickness of the low refractive index layer, 1/1 of the center wavelength of the incident light 4. An optical pulse compression apparatus, wherein 3. The optical pulse compression device according to claim 1, wherein the thin film layer comprises:
A low refractive index layer made of InP and a high refractive index layer made of GaInAsP are used as unit cells, and a plurality of the unit cells are stacked. The saturable absorber layer is arranged at the top of the unit cells and between the unit cells. An optical pulse compression device. 4. The optical pulse compression device according to claim 1, wherein the saturable absorber layer is formed of the low refractive index layer or the high refractive index layer. Optical pulse compression device. 5. The optical pulse compression device according to claim 1, wherein the saturable absorber is formed of a semiconductor thin film having a quantum well structure. 6. The optical pulse compression apparatus according to claim 1, further comprising means for applying a reverse bias voltage for extracting optical carriers accumulated near a band gap. Light pulse compression device. 7. An optical pulse compression apparatus according to claim 1, further comprising means for amplifying a light intensity of a light pulse incident on the low refractive index layer as the uppermost layer. An optical pulse compression device characterized by the above-mentioned. 8. An optical pulse compression apparatus according to claim 1, further comprising a light condensing means for converging said incident light on said thin film layer. apparatus. 9. The optical pulse compression apparatus according to claim 1, further comprising an optical fiber for transmitting the incident light and the reflected light, wherein said optical fiber compresses the incident light spread by said optical fiber. An optical pulse compression device for compressing a pulse width. 10. The optical pulse compression device according to claim 1, wherein said thin film layer and said saturable absorber are provided in a distributed Bragg reflector. . 11. The optical pulse compression apparatus according to claim 1, wherein an optical waveguide is formed by said low refractive index layer and said high refractive index layer, and said periodic waveguide in said waveguide is formed. An optical pulse compression device, wherein a diffraction grating is provided with a phase shift unit for localizing light. 12. The optical pulse compression device according to claim 11, wherein a non-reflection coating layer is provided on the uppermost low refractive index layer. 13. The optical pulse compression device according to claim 11, wherein the wavelength region where the reflectance in the reflection spectrum of the optical waveguide is low is set to the center wavelength of the spectrum of the incident light. An optical pulse compression device characterized by: 14. An optical pulse transmission device comprising the optical pulse compression device according to claim 1, wherein the optical pulse transmission device performs pulse compression on an optical signal and transmits the optical signal. 15. A light pulse compression device according to claim 1, wherein a plurality of light pulse compression devices are provided for each light pulse having a different center wavelength, and each light beam having a different center wavelength is multiplexed and transmitted. An optical pulse transmission device, wherein a pulse is branched and incident on each of the optical pulse compression devices associated with the center wavelength, the pulse width is compressed by each of the optical pulse compression devices, and then combined and transmitted. 16. A means for driving a plurality of optical pulse compression devices according to any one of claims 1 to 13 at an equal pulse repetition period, and adding a time delay to optical pulses from each optical pulse compression device. Given means having time-series arrangement on the time axis at uniform time intervals, and means for multiplexing and transmitting each optical pulse arranged at the uniform time intervals on one transmission path. Characteristic optical pulse transmission device. 17. A laser light generator using the optical pulse compression device according to claim 1 as a reflection surface. 18. An optical pulse compression device according to claim 1, laser activation means for generating laser light, and said laser light in two paths on the reflection surface side of said laser activation means. The laser beam is branched, one of the laser beams is made incident on the optical pulse compression device, the other laser beam is made incident on the reflecting mirror, and the reflected light from the optical pulse compression device is superimposed on the reflected light from the reflecting mirror. A pulse combining unit that overlaps the reflected light pulse whose frequency has been shifted by the optical pulse compression device and the reflected light pulse that has not been frequency shifted from the reflecting mirror and causes both ends of the pulse to disappear. A laser light generating device for emitting the laser light.