JP2015119034A - Short optical pulse generation device, terahertz wave generation device, camera, imaging device and measurement device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a short optical pulse generation device capable of generating an optical pulse having a small pulse width.SOLUTION: A short optical pulse generation device 100 includes: an optical pulse generation section 2 generating an optical pulse; a frequency chirp section 4 chirping a frequency of the optical pulse; optical fibers 31, 32, 33 and 34; and a group velocity dispersion section 6 causing the optical pulse chirped by the frequency chirp section 4 to generate a group velocity difference corresponding to a wavelength.

Description

  The present invention relates to a short light pulse generator, a terahertz wave generator, a camera, an imaging device, and a measurement device.

  In recent years, terahertz waves, which are electromagnetic waves having a frequency of 100 GHz to 30 THz, have attracted attention. The terahertz wave can be used for various measurements such as imaging and spectroscopic measurement, non-destructive inspection, and the like.

  The terahertz wave generator that generates the terahertz wave includes, for example, a short optical pulse generator that generates an optical pulse having a pulse width of about sub-picoseconds (several hundred femtoseconds), and an optical pulse generated by the short optical pulse generator. And a photoconductive antenna that generates a terahertz wave by being irradiated. In general, femtosecond fiber lasers, titanium sapphire lasers, semiconductor lasers, and the like are used as short optical pulse generators that generate optical pulses having a pulse width of about sub-picoseconds.

  For example, Patent Document 1 describes an optical pulse generator that directly modulates a semiconductor laser to chirp the frequency of an optical pulse and then compresses the pulse width by an optical pulse compression unit (group velocity dispersion unit) made of fiber. Has been.

Japanese Patent Laid-Open No. 11-40889

  However, in the optical pulse generator of Patent Document 1, since the frequency of the optical pulse is chirped by directly modulating the semiconductor laser, the amount of chirp is small, and the pulse width can be sufficiently compressed in the group velocity dispersion unit. could not.

  One of the objects according to some aspects of the present invention is to provide a short optical pulse generator capable of generating an optical pulse with a small pulse width. Another object of some aspects of the present invention is to provide a terahertz wave generation device, a camera, an imaging device, and a measurement device including the short light pulse generation device.

The short optical pulse generator according to the present invention is
An optical pulse generator for generating an optical pulse;
A frequency chirp section for chirping the frequency of the optical pulse;
A group velocity dispersion unit that includes an optical fiber and generates a group velocity difference according to wavelength in the optical pulse chirped by the frequency chirp unit;
including.

In such a short optical pulse generator, the chirp amount of the optical pulse can be increased as compared with, for example, a configuration having no frequency chirp part, and the pulse width can be sufficiently compressed in the group velocity dispersion part. it can. Therefore, such a short optical pulse generator can generate an optical pulse with a small pulse width.

In the short light pulse generator according to the present invention,
The optical pulse generator is
A light emitting element;
A driving circuit for driving the light emitting element by direct modulation;
Have
The drive circuit may be configured to change a repetition frequency of light emission of the light emitting element.

  In such a short light pulse generator, the repetition frequency of light emission of the light emitting element can be arbitrarily set regardless of the total length of the optical fiber. Therefore, in such a short optical pulse generator, the repetition frequency can be set with a high degree of freedom.

In the short light pulse generator according to the present invention,
A reflection mirror that returns the optical pulse emitted from the optical fiber to the optical fiber may be included.

  In such a short optical pulse generator, a large group velocity difference can be generated in an optical pulse propagating through an optical fiber without increasing the total length of the optical fiber. Therefore, such a short optical pulse generator can generate an optical pulse with a small pulse width while achieving miniaturization.

The terahertz wave generator according to the present invention is
A short light pulse generator according to the present invention;
A photoconductive antenna that generates a terahertz wave by being irradiated with a short light pulse generated by the short light pulse generator;
including.

  Such a terahertz wave generator can include a short optical pulse generator capable of generating an optical pulse with a small pulse width.

The camera according to the present invention is
A short light pulse generator according to the present invention;
A photoconductive antenna that generates a terahertz wave by being irradiated with a short light pulse generated by the short light pulse generator;
A terahertz wave detecting unit that detects the terahertz wave emitted from the photoconductive antenna and transmitted through the object or the terahertz wave reflected from the object;
A storage unit for storing a detection result of the terahertz wave detection unit;
including.

  Such a camera can include a short light pulse generator capable of generating a light pulse with a small pulse width.

An imaging apparatus according to the present invention includes:
A short light pulse generator according to the present invention;
A photoconductive antenna that generates a terahertz wave by being irradiated with a short light pulse generated by the short light pulse generator;
A terahertz wave detecting unit that detects the terahertz wave emitted from the photoconductive antenna and transmitted through the object or the terahertz wave reflected from the object;
An image forming unit that generates an image of the object based on a detection result of the terahertz wave detection unit;
including.

  Such an imaging apparatus can include a short optical pulse generator capable of generating an optical pulse with a small pulse width.

The measuring device according to the present invention is
A short light pulse generator according to the present invention;
A photoconductive antenna that generates a terahertz wave by being irradiated with a short light pulse generated by the short light pulse generator;
A terahertz wave detecting unit that detects the terahertz wave emitted from the photoconductive antenna and transmitted through the object or the terahertz wave reflected from the object;
Based on the detection result of the terahertz wave detection unit, a measurement unit that measures the object,
including.

  Such a measuring device can include a short optical pulse generator capable of generating an optical pulse with a small pulse width.

The functional block diagram of the short optical pulse generator which concerns on this embodiment. The figure which shows typically the short optical pulse generator which concerns on this embodiment. The perspective view which shows typically the light emitting element of the optical pulse generation part in the short optical pulse generator which concerns on this embodiment, and the optical waveguide of a frequency chirp. Sectional drawing which shows typically the light emitting element of the optical pulse generation part in the short optical pulse generator which concerns on this embodiment, and the optical waveguide of a frequency chirp. The graph which shows an example of the optical pulse produced | generated by the optical pulse production | generation part. The graph which shows an example of the chirp characteristic of a semiconductor saturable absorption mirror. The graph which shows an example of the optical pulse produced | generated by the group velocity dispersion | distribution part. The figure which shows the structure of the terahertz wave generator which concerns on this embodiment. 1 is a block diagram showing an imaging apparatus according to an embodiment. The top view which shows typically the terahertz wave detection part of the imaging device which concerns on this embodiment. The graph which shows the spectrum in the terahertz band of a target object. The figure of the image which shows distribution of the substances A, B, and C of a target object. The block diagram which shows the measuring device which concerns on this embodiment. The block diagram which shows the camera which concerns on this embodiment. The perspective view which shows typically the camera which concerns on this embodiment.

  DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. The embodiments described below do not unduly limit the contents of the present invention described in the claims. In addition, not all of the configurations described below are essential constituent requirements of the present invention.

1. Short light pulse generator 1.1. Configuration First, a short optical pulse generator according to the present embodiment will be described with reference to the drawings. FIG. 1 is a functional block diagram of a short optical pulse generator 100 according to this embodiment.

  As shown in FIG. 1, the short optical pulse generator 100 includes an optical pulse generator 2, a frequency chirp unit 4, and a group velocity dispersion unit 6.

  The optical pulse generator 2 generates an optical pulse. Here, the light pulse refers to light whose intensity changes sharply in a short time. The pulse width (full width at half maximum FWHM) of the optical pulse generated by the optical pulse generator 2 is not particularly limited, and is, for example, 1 ps (picosecond) or more and 100 ps or less. The optical pulse generator 2 is, for example, a semiconductor laser or a super luminescent diode (SLD).

  The frequency chirping unit 4 chirps the frequency of the optical pulse generated by the optical pulse generating unit 2. The frequency chirp portion 4 is made of, for example, a semiconductor material and has a quantum well structure. When the optical pulse propagates through the optical waveguide of the frequency chirped portion 4, the refractive index of the optical waveguide material changes due to the optical Kerr effect, and the phase of the electric field changes (self-phase modulation effect). This self-phase modulation effect chirps the frequency of the optical pulse. Here, the chirping of the frequency of the optical pulse means that the frequency of the optical pulse changes with time.

  Since the frequency chirp portion 4 is made of a semiconductor material, the response speed is slow with respect to an optical pulse having a pulse width of about 1 ps to 100 ps. Therefore, the frequency chirp unit 4 chirps (up chirp or down chirp) the frequency of the optical pulse in proportion to the intensity of the optical pulse (the square of the electric field amplitude). Here, up-chirp means that the frequency of the optical pulse increases with time, and down-chirp means that the frequency of the optical pulse decreases with time. In other words, up-chirp refers to the case where the wavelength of the optical pulse decreases with time, and down-chirp refers to the case where the wavelength of the optical pulse increases with time.

  The group velocity dispersion unit 6 generates a group velocity difference corresponding to the wavelength (frequency) in the optical pulse chirped in frequency by the frequency chirp unit 4. Specifically, the group velocity dispersion unit 6 can generate a group velocity difference such that the pulse width of the optical pulse is reduced with respect to the optical pulse having a chirped frequency (pulse compression). For example, the group velocity dispersion unit 6 can reduce the pulse width by generating positive group velocity dispersion in the down-chirped optical pulse. In this case, the group velocity dispersion unit 6 is a normal dispersion medium. As described above, the group velocity dispersion unit 6 performs pulse compression based on the group velocity dispersion. The group velocity dispersion is a phenomenon in which the group velocity changes depending on the frequency because the propagation speed of the optical pulse varies depending on the wavelength. Positive group velocity dispersion refers to a phenomenon in which the group velocity increases as the wavelength increases. In other words, the positive group velocity dispersion is a phenomenon in which the group velocity increases as the frequency decreases. The width of the optical pulse compressed by the group velocity dispersion unit 6 is not particularly limited, but is, for example, 1 fs (femtosecond) or more and 800 fs or less.

1.2. Structure Next, a specific structure of the short optical pulse generator 100 will be described with reference to the drawings. FIG. 2 is a diagram schematically showing the short optical pulse generator 100.

  As shown in FIG. 2, the optical pulse generator 2 includes a light emitting element 10. The frequency chirp unit 4 includes an optical waveguide 20. The group velocity dispersion unit 6 includes optical fibers 31, 32, 33, 34, a coupler 35, an optical amplification unit 36, and reflection mirrors 37, 38. Further, the short optical pulse generator 100 includes a beam splitter 40.

First, the light emitting element 10 constituting the optical pulse generator 2 and the optical waveguide 20 constituting the frequency chirp part 4 will be described. FIG. 3 is a perspective view schematically showing the light emitting element 10 constituting the optical pulse generator 2 and the optical waveguide 20 constituting the frequency chirp part 4. FIG.
2 is a cross-sectional view schematically showing the light emitting element 10 and the optical waveguide 20. FIG. 4 is a cross-sectional view taken along line IV-IV in FIG.

  The light emitting element 10 and the optical waveguide 20 are integrally provided as shown in FIGS. That is, the light emitting element 10 and the optical waveguide 20 are provided on the same substrate 102.

  The light emitting element 10 includes a substrate 102, a buffer layer 104, a first cladding layer 106, a core layer 108, a second cladding layer 110, a cap layer 112, an insulating layer 120, a first electrode 130, a first electrode 130, And two electrodes 132. Here, an example in which the light emitting element 10 is a DFB (Distributed Feedback) laser will be described.

  The optical waveguide 20 includes a first cladding layer 106, a core layer 108, and a second cladding layer 110.

  The substrate 102 is, for example, a first conductivity type (for example, n-type) GaAs substrate. The substrate 102 includes a first region 102a where the light emitting element 10 is formed and a second region 102b where the optical waveguide 20 is formed.

  The buffer layer 104 is provided on the substrate 102. The buffer layer 104 is, for example, an n-type GaAs layer. The buffer layer 104 can improve the crystallinity of the layer formed thereabove.

  The first cladding layer 106 is provided on the buffer layer 104. The first cladding layer 106 is, for example, an n-type AlGaAs layer.

  The core layer 108 includes a first guide layer 108a, an MQW layer 108b, and a second guide layer 108c.

  The first guide layer 108 a is provided on the first cladding layer 106. The first guide layer 108a is, for example, an i-type AlGaAs layer.

  The MQW layer 108b is provided on the first guide layer 108a. The MQW layer 108b has, for example, a multiple quantum well structure in which three quantum well structures each composed of a GaAs well layer and an AlGaAs barrier layer are stacked. Here, the quantum well structure refers to a general quantum well structure in the field of semiconductor light-emitting devices, and a thin film (nm order) of a material having a small band gap using two or more materials having different band gaps. It is a structure sandwiched by thin films of materials with a large band gap. In the illustrated example, the MQW layer 108b has the same number of quantum wells (the number of stacked GaAs well layers and AlGaAs barrier layers) above the first region 102a and the second region 102b. That is, in the light emitting element 10 and the optical waveguide 20, the number of quantum wells in the MQW layer 108b is the same.

  Note that the number of quantum wells of the MQW layer 108b above the first region 102a and the number of quantum wells of the MQW layer 108b above the second region 102b may be different. That is, the number of quantum wells of the MQW layer 108b constituting the light emitting element 10 and the number of quantum wells of the MQW layer 108b constituting the optical waveguide 20 may be different.

The second guide layer 108c is provided on the MQW layer 108b. The second guide layer 108c is, for example, an i-type AlGaAs layer. The second guide layer 108c is provided with a periodic structure constituting a DFB type resonator. The periodic structure is provided above the first region 102a. The periodic structure is composed of two layers 108c and 110 having different refractive indexes.

  The first guide layer 108a, the MQW layer 108b, and the second guide layer 108c can constitute the core layer 108 that propagates light generated in the MQW layer 108b. The first guide layer 108a and the second guide layer 108c are layers that confine injected carriers (electrons and holes) in the MQW layer 108b and simultaneously confine light in the core layer 108.

  The second cladding layer 110 is provided on the core layer 108. The second cladding layer 110 is, for example, a second conductivity type (for example, p-type) AlGaAs layer. In the illustrated example, optical waveguides 118 and 20 are formed by the first cladding layer 106, the core layer 108, and the second cladding layer 110.

  In the light emitting device 10, for example, the p-type second cladding layer 110, the core layer 108 not doped with impurities, and the n-type first cladding layer 106 constitute a pin diode. Each of the first cladding layer 106 and the second cladding layer 110 is a layer having a larger band gap and a lower refractive index than the core layer 108. The core layer 108 has a function of generating light and guiding the light while amplifying the light. The first cladding layer 106 and the second cladding layer 110 have a function of confining injected carriers (electrons and holes) and light (a function of suppressing light leakage) with the core layer 108 interposed therebetween.

  In the light emitting element 10, when a forward bias voltage of a pin diode is applied between the first electrode 130 and the second electrode 132, recombination of electrons and holes occurs in the core layer 108 (MQW layer 108b). This recombination causes light emission. With this generated light as a starting point, stimulated emission occurs in a chain, and the light intensity is amplified in the optical waveguide (gain region) 118. The wavelength of light generated in the optical waveguide 118 is, for example, not less than 780 nm and not more than 860 nm.

  The cap layer 112 is provided on the second cladding layer 110. The cap layer 112 can be in ohmic contact with the second electrode 132. The cap layer 112 is, for example, a p-type GaAs layer.

  The cap layer 112 and a part of the second cladding layer 110 constitute a columnar portion 160. For example, in the light emitting element 10, the current path between the electrodes 130 and 132 is determined by the planar shape viewed from the stacking direction of each layer of the columnar portion 160.

As shown in FIG. 3, the insulating layer 120 is provided on the second cladding layer 110 and on the side of the columnar portion 160. Furthermore, the insulating layer 120 is provided on the cap layer 112 above the second region 102 b of the substrate 102. The insulating layer 120 is, for example, a SiN layer, a SiO ₂ layer, a SiON layer, an Al ₂ O ₃ layer, a polyimide layer, or the like.

  When the above material is used for the insulating layer 120, the current between the electrodes 130 and 132 can flow through the columnar portion 160 sandwiched between the insulating layers 120, avoiding the insulating layer 120. In addition, the insulating layer 120 may have a refractive index smaller than that of the second cladding layer 110. In this case, the effective refractive index of the vertical cross section of the portion where the insulating layer 120 is formed is smaller than the effective refractive index of the vertical cross section of the portion where the insulating layer 120 is not formed, that is, the portion where the columnar portion 160 is formed. Thereby, light can be efficiently confined in the optical waveguides 118 and 20 in the planar direction. Although not shown, the insulating layer 120 may be an air layer without using the above material. In this case, the air layer can function as the insulating layer 120.

  The first electrode 130 is provided on the entire lower surface of the substrate 102. The first electrode 130 is in contact with a layer (the substrate 102 in the illustrated example) that is in ohmic contact with the first electrode 130. The first electrode 130 is electrically connected to the first cladding layer 106 via the substrate 102. The first electrode 130 is one electrode for driving the light emitting element 10. As the first electrode 130, for example, a layer in which a Cr layer, an AuGe layer, a Ni layer, and an Au layer are stacked in this order from the substrate 102 side can be used. Note that the first electrode 130 may be provided only below the first region 102 a of the substrate 102.

  The second electrode 132 is provided on the upper surface of the cap layer 112 and above the first region 102a. Further, the second electrode 132 may be provided on the insulating layer 120. The second electrode 132 is electrically connected to the second cladding layer 110 through the cap layer 112. The second electrode 132 is the other electrode for driving the light emitting element 10. As the second electrode 132, for example, a layer in which a Cr layer, an AuZn layer, and an Au layer are stacked in this order from the cap layer 112 side can be used. In the illustrated example, the first electrode 130 is provided on the lower surface side of the substrate 102, and the second electrode 132 is provided on the upper surface side of the substrate 102. A single-sided electrode structure in which the electrode 132 is provided on the same surface side (for example, the upper surface side) of the substrate 102 may be used.

  The buffer layer 104, the first cladding layer 106, the core layer 108, the second cladding layer 110, and the cap layer 112 are provided over the first region 102a and the second region 102b of the substrate 102. That is, these layers 104, 106, 108, 110, and 112 are layers common to the light emitting element 10 and the optical waveguide 20 and are continuous layers. The first cladding layer 106, the core layer 108, and the second cladding layer 110 that are continuous in the first region 102a and the second region 102b constitute the optical waveguide 118 and the optical waveguide 20. The optical waveguide 118 is provided above the first region 102a, and the optical waveguide 20 is provided above the second region 102b.

  The case where an AlGaAs-based semiconductor material is used as an example of the light-emitting element 10 and the optical waveguide 20 has been described above. However, the present invention is not limited to this. Other semiconductor materials such as ZnSd and ZnCdSe may be used.

  Moreover, although the example in which the light emitting element 10 is a DFB laser has been described, the present invention is not limited thereto, and may be a semiconductor laser such as a DBR laser or a mode-locked laser. The light emitting element 10 may be a super luminescent diode (SLD).

  Although not shown, an electrode for applying a reverse bias to the optical waveguide 20 may be provided. Thereby, the absorption characteristic of the optical waveguide 20 can be controlled, and the frequency chirp amount can be adjusted.

  Although the case where the light emitting element 10 and the optical waveguide 20 are provided on the same substrate has been described here, the light emitting element 10 and the optical waveguide 20 may be provided on different substrates.

  The optical pulse generator 2 further includes a drive circuit 12 as shown in FIG. The drive circuit 12 drives the light emitting element 10 by direct modulation. Here, the direct modulation means that a modulation signal is used as a drive current for generating a light pulse in the light emitting element 10. The light emitting element 10, the drive circuit 12, and the optical waveguide 20 are accommodated in the package 14. In FIG. 2, the package 14 is shown through.

Next, members constituting the group velocity dispersion unit 6 (optical fibers 31, 32, 33, 34, coupler 35, optical amplification unit 36, reflection mirrors 37, 38) will be described with reference to FIG.

  One end of the first optical fiber 31 is connected to the package 14. The other end of the first optical fiber 31 is connected to the coupler 35. The light pulse emitted from the frequency chirp unit 4 is incident on the first optical fiber 31.

  One end of the second optical fiber 32 is connected to the coupler 35. Light propagated through the second optical fiber 32 is emitted from the other end 32 a of the second optical fiber 32. In the illustrated example, the other end 32 a of the second optical fiber 32 faces the first reflection mirror 37 via the beam splitter 40.

  One end of the third optical fiber 33 is connected to the coupler 35. The other end of the third optical fiber 33 is connected to the optical amplifier 36.

  One end of the fourth optical fiber 34 is connected to the optical amplifier 36. Light propagated through the fourth optical fiber 34 is emitted from the other end 34 a of the fourth optical fiber 34. In the illustrated example, the other end 34 a of the fourth optical fiber 34 faces the second reflecting mirror 38.

  Although not shown, the optical fibers 31, 32, 33, and 34 are configured to include a core portion, a cladding portion outside the core portion, and a covering portion that covers the cladding portion. The refractive index of the core part is higher than the refractive index of the cladding part, and the light pulse propagates through the core part. The material of the core part and the clad part is, for example, quartz glass or plastic having a high transmittance with respect to the light pulse.

  In the optical fibers 31, 32, 33, and 34, a group velocity dispersion value corresponding to the distance that the optical pulse propagates through the optical fibers 31, 32, 33, and 34 can be given. That is, as the distance that the light pulse propagates through the optical fibers 31, 32, 33, and 34 is longer, a larger group velocity difference can be generated in the light pulse. Therefore, by increasing the number of times the light pulse is reflected by the two reflecting mirrors 37 and 38, a large group velocity difference can be generated in the light pulse. The cores of the optical fibers 31, 32, 33, and 34 are positive group velocity dispersion media. Therefore, the down-chirped light pulse is pulse-compressed while propagating through the optical fibers 31, 32, 33 and 34.

Here, when the material of the core portion of the optical fibers 31, 32, 33, and 34 is quartz glass (SiO ₂ ), the group velocity dispersion value per 1 mm of the optical fibers 31, 32, 33, and 34 is an optical pulse with a wavelength of 850 nm. On the other hand, it is 3.2 × 10 ⁻²⁹ s ² / mm. Assuming that the desired group velocity dispersion value is 1 × 10 ⁻²⁴ s ² and that the group velocity dispersion value in the optical fibers 31, 32, 33, and 34 is determined only by the group velocity dispersion value of the core portion, the necessary optical fibers 31 and 32 are used. , 33, 34 in total (full length) L is as follows.

L = 1 × 10 ⁻²⁴ /3.2×10 ⁻²⁹ = 31250 mm≈31 m

  In order to reduce the size of the short optical pulse generator 100, it is preferable to reduce the total length L as much as possible and increase the number of reflections at the reflection mirrors 37 and 38. However, the coupler 35, the optical amplification unit 36, and the beam splitter 40 are preferred. In consideration of the arrangement, the total length L is preferably about 1 m or less.

  The coupler 35 connects the first optical fiber 31, the second optical fiber 32, and the third optical fiber 33. The optical pulse generated by the optical pulse generator 2 propagates through the first optical fiber 31 and then propagates through the optical fibers 32 and 33 via the coupler 35.

  The light amplifying unit 36 amplifies the incident light pulse by stimulated emission and emits it. As the optical amplifying unit 36, for example, a semiconductor optical amplifier (SOA) or an optical fiber amplifier is used. A semiconductor optical amplifier is an optical amplifier that uses a semiconductor (for example, an AlGaAs layer) for amplification. An antireflection film may be provided on the entrance surface and the exit surface of the semiconductor optical amplifier. The optical fiber amplifier is an amplifier that uses an optical fiber to which impurities are added. Specifically, the optical fiber amplifier is an optical fiber (EDFA) including a core portion made of a fluoride doped with Er.

  The first reflection mirror 37 is a mirror for returning the light pulse emitted from the other end 32 a of the second optical fiber 32 to the second optical fiber 32. That is, at least a part of the light pulse emitted from the second optical fiber 32 is reflected by the first reflecting mirror 37 and enters the second optical fiber 32. The material of the first reflection mirror 37 is, for example, a metal such as Au.

  The second reflecting mirror 38 is a mirror for returning the light pulse emitted from the other end 34 a of the fourth optical fiber 34 to the fourth optical fiber 34. That is, at least a part of the light pulse emitted from the fourth optical fiber 34 is reflected by the second reflecting mirror 38 and enters the fourth optical fiber 34. The material of the second reflection mirror 38 is the same as the material of the first reflection mirror 37, for example.

  The beam splitter 40 is provided between the other end 32 a of the second optical fiber 32 and the first reflection mirror 37. The beam splitter 40 reflects a part of the light pulse emitted from the second optical fiber 32 and transmits a part of the light pulse emitted from the second optical fiber 32. The light pulse reflected by the beam splitter 40 (light pulse P3 in the example shown in FIG. 2) is emitted to the outside. The light pulse that has passed through the beam splitter 40 is reflected by the first reflecting mirror 37, passes through the beam splitter 40 again, and enters the second optical fiber 32.

1.3. Operation Next, the operation of the short optical pulse generator 100 will be described. FIG. 5 is a graph showing an example of the optical pulse P1 generated by the optical pulse generator 2. The horizontal axis t of the graph shown in FIG. 5 is time, and the vertical axis I is the light intensity (the square of the electric field amplitude). FIG. 6 is a graph showing an example of the chirp characteristic of the frequency chirp unit 4. In the graph shown in FIG. 6, the horizontal axis t is time, and the vertical axis Δω is the chirp amount (frequency change amount). In FIG. 6, the optical pulse P1 is indicated by a one-dot chain line, and the chirp amount Δω corresponding to the optical pulse P1 is indicated by a solid line. FIG. 7 is a graph showing an example of the optical pulse P3 generated by the group velocity dispersion unit 6. As shown in FIG. In the graph shown in FIG. 7, the horizontal axis t is time, and the vertical axis I is light intensity.

  The optical pulse generator 2 generates, for example, an optical pulse P1 shown in FIG. The optical pulse generator 2 generates an optical pulse P1 by being driven by the drive circuit 12. The optical pulse P1 is a Gaussian waveform in the illustrated example. The pulse width (full width at half maximum FWHM) t of the optical pulse P1 is 10 ps (picosecond) in the illustrated example. The optical pulse P1 propagates through the optical waveguide 118 and enters the optical waveguide 20 of the frequency chirp unit 4.

  The frequency chirp unit 4 has a chirp characteristic proportional to the light intensity. The following formula (1) is a formula showing the effect of frequency chirp.

Here, Δω is the chirp amount (frequency change amount), c is the speed of light, τ _r is the response time of the nonlinear refractive index effect, n ₂ is the nonlinear refractive index, l is the waveguide length, and ω ₀ is the center frequency of the optical pulse. , E is the amplitude of the electric field.

  The frequency chirp unit 4 imparts the frequency chirp shown in Expression (1) to the optical pulse P1 propagating through the optical waveguide 20. Specifically, as shown in FIG. 6, the frequency chirping unit 4 decreases the frequency with time in the front part of the optical pulse P1 and increases the frequency with time in the rear part of the optical pulse P1 with respect to the optical pulse P1. Let That is, the frequency chirp unit 4 down-chirps the front part of the optical pulse P1 and up-chirps the rear part of the optical pulse P1. Accordingly, the optical pulse P1 generated by the optical pulse generation unit 2 passes through the frequency chirping unit 4, and thus the front part is down-chirped and the rear part is up-chirped (hereinafter referred to as “optical pulse P2”). Become. The chirped light pulse P <b> 2 (not shown) is incident on the first optical fiber 31.

  As shown in FIG. 2, the light pulse P <b> 2 that has entered the first optical fiber 31 of the group velocity dispersion unit 6 propagates through the first optical fiber 31 and enters, for example, the second optical fiber 32 through the coupler 35. The light pulse incident on the second optical fiber 32 propagates through the second optical fiber 32, exits from the second optical fiber 32, is reflected by the first reflecting mirror 37, and then enters the second optical fiber 32 again. The light pulse incident on the second optical fiber again is emitted from the fourth optical fiber 34 via the coupler 35, the third optical fiber 33, and the optical amplifying unit 36, then reflected by the second reflecting mirror 38, and again, The light enters the fourth optical fiber 34. The light pulse incident on the fourth optical fiber 34 again is emitted from the second optical fiber 32 via the optical amplifier 36, the third optical fiber 33, and the coupler 35, and then reflected by the first reflecting mirror 37. Thus, the light pulse P2 incident on the group velocity dispersion unit 6 is multiple-reflected by the reflection mirrors 37 and 38.

  When the chirped light pulse P2 propagates through the optical fibers 31, 32, 33, and 34, the group velocity dispersion unit 6 generates a group velocity difference corresponding to the wavelength (frequency) (group velocity dispersion) and performs pulse compression. Do. Specifically, as shown in FIG. 7, the optical fibers 31, 32, 33, and 34 of the group velocity dispersion unit 6 cause a positive group velocity dispersion in the optical pulse P2, and before the down-chirped optical pulse P2. Compress the part. Thereby, the optical pulse P3 is generated. In the illustrated example, the pulse width t of the optical pulse P3 is 0.33 ps.

  As described above, the light pulse P2 that has entered the group velocity dispersion unit 6 is subjected to multiple reflections by the reflection mirrors 37 and 38 and pulse compression, and then reflected by the beam splitter 40 as shown in FIG. The light is emitted from the short light pulse generator 100 as a pulse P3.

  The number of reflections of the light pulse P2 on the reflection mirrors 37 and 38 is not particularly limited, but a larger group velocity difference can be generated in the light pulse as the number of reflections increases. For example, the light pulse P <b> 2 may be reflected by the beam splitter 40 and emitted from the short light pulse generator 100 without being reflected by the reflection mirrors 37 and 38.

  The short light pulse generator 100 has the following features, for example.

The short optical pulse generator 100 includes a frequency chirp unit 4 for chirping the frequency of the optical pulse and optical fibers 31, 32, 33, 34, and a group of optical pulses chirped by the frequency chirp unit 4 according to the wavelength. And a group velocity dispersion unit 6 that produces a velocity difference. Therefore, the short optical pulse generator 100 can increase the chirp amount of the optical pulse as compared with, for example, a configuration without the frequency chirp unit 4, and the group velocity dispersion unit 6 sufficiently compresses the pulse width. be able to. Therefore, the short optical pulse generator 100 can generate an optical pulse with a small pulse width.

  In the short optical pulse generator 100, the optical pulse generator 2 has a drive circuit 12 that drives the light emitting element 10 by direct modulation. And the drive circuit 12 is comprised so that the repetition frequency of light emission of the light emitting element 10 can be changed. Specifically, the frequency of the modulation signal of the drive current can be changed. Therefore, in the short optical pulse generator 100, the repetition frequency of light emission of the light emitting element 10 can be arbitrarily set regardless of the total length of the optical fibers 31, 32, 33, 34. Therefore, the short optical pulse generator 100 can set the repetition frequency with a high degree of freedom. For example, in a form that does not have a drive circuit that drives a light emitting element by direct modulation, the repetition frequency of the light emitting element is determined by the total length of the optical fiber, and the repetition frequency cannot be set with a high degree of freedom. Note that the repetition frequency of light emission is the number of light pulses generated in one second (unit time) in the light emitting element.

  In the short light pulse generator 100, the first reflection mirror 37 that returns the light pulse emitted from the second optical fiber 32 to the second optical fiber 32 and the light pulse emitted from the fourth optical fiber 34 are applied to the fourth optical fiber 34. And a second reflecting mirror 38 to be returned. Therefore, in the short optical pulse generator 100, even if the total length of the optical fibers 31, 32, 33, 34 is not increased, a large group velocity difference is generated in the optical pulses propagating through the optical fibers 31, 32, 33, 34. Can do. Therefore, the short optical pulse generator 100 can generate an optical pulse with a small pulse width while reducing the size.

2. Next, a terahertz wave generation apparatus 1000 according to the present embodiment will be described with reference to the drawings. FIG. 8 is a diagram illustrating a configuration of the terahertz wave generation apparatus 1000 according to the present embodiment.

  As shown in FIG. 8, the terahertz wave generation device 1000 includes a short light pulse generation device according to the present invention and a photoconductive antenna 1010. Here, the case where the short optical pulse generator 100 is used as the short optical pulse generator according to the present invention will be described.

  The short light pulse generator 100 generates a short light pulse (for example, a light pulse P3 shown in FIG. 7) that is excitation light. The pulse width of the short light pulse generated by the short light pulse generator 100 is, for example, not less than 1 fs and not more than 800 fs.

  The photoconductive antenna 1010 generates a terahertz wave when irradiated with the short light pulse generated by the short light pulse generator 100. The terahertz wave means an electromagnetic wave having a frequency of 100 GHz to 30 THz, particularly an electromagnetic wave of 300 GHz to 3 THz.

  In the illustrated example, the photoconductive antenna 1010 is a dipole photoconductive antenna (PCA). The photoconductive antenna 1010 includes a substrate 1012 that is a semiconductor substrate, and a pair of electrodes 1014 that are provided on the substrate 1012 and arranged to face each other with a gap 1016 interposed therebetween. When a light pulse is irradiated between the electrodes 1014, the photoconductive antenna 1010 generates a terahertz wave.

  The substrate 1012 includes, for example, a semi-insulating GaAs (SI-GaAs) substrate and a low temperature growth GaAs (LT-GaAs) layer provided on the SI-GaAs substrate. The material of the electrode 1014 is, for example, Au. The distance between the pair of electrodes 1014 is not particularly limited, and is appropriately set according to conditions. The distance between the pair of electrodes 1014 is, for example, not less than 1 μm and not more than 10 μm.

  In the terahertz wave generation device 1000, first, the short light pulse generation device 100 generates a short light pulse and emits it toward the gap 1016 of the photoconductive antenna 1010. The short light pulse emitted from the short light pulse generator 100 irradiates the gap 1016 of the photoconductive antenna 1010. In the photoconductive antenna 1010, free electrons are excited by irradiating the gap 1016 with a short light pulse. The free electrons are accelerated by applying a voltage between the electrodes 1014. Thereby, a terahertz wave is generated.

3. Imaging Device Next, an imaging device 1100 according to the present embodiment will be described with reference to the drawings. FIG. 9 is a block diagram showing an imaging apparatus 1100 according to this embodiment. FIG. 10 is a plan view schematically showing the terahertz wave detection unit 1120 of the imaging apparatus 1100 according to the present embodiment. FIG. 11 is a graph showing the spectrum of the target in the terahertz band. FIG. 12 is a diagram of an image showing the distribution of the substances A, B and C of the object.

  As illustrated in FIG. 9, the imaging apparatus 1100 includes a terahertz wave generation unit 1110 that generates a terahertz wave, and a terahertz wave that is emitted from the terahertz wave generation unit 1110 and is transmitted through the object O or is reflected by the object O. A terahertz wave detection unit 1120 that detects a wave and an image forming unit 1130 that generates an image of the object O, that is, image data based on the detection result of the terahertz wave detection unit 1120 are provided.

  As the terahertz wave generation unit 1110, the terahertz wave generation device according to the present invention can be used. Here, the case where the terahertz wave generation device 1000 is used as the terahertz wave generation device according to the present invention will be described.

  As shown in FIG. 10, the terahertz wave detection unit 1120 includes a filter 80 that transmits a terahertz wave having a target wavelength, and a detection unit 84 that detects the terahertz wave having the target wavelength that has passed through the filter 80. Use things. Further, as the detection unit 84, for example, a detection unit that converts a terahertz wave into heat and detects it, that is, a unit that can convert a terahertz wave into heat and detect the energy (intensity) of the terahertz wave is used. Examples of such a detection unit include a pyroelectric sensor and a bolometer. Note that the configuration of the terahertz wave detection unit 1120 is not limited to the above configuration.

  The filter 80 includes a plurality of pixels (unit filter units) 82 that are two-dimensionally arranged. That is, the pixels 82 are arranged in a matrix.

  Each pixel 82 has a plurality of regions that transmit terahertz waves having different wavelengths, that is, a plurality of regions that have different wavelengths of terahertz waves that pass (hereinafter also referred to as “passing wavelengths”). In the illustrated configuration, each pixel 82 includes a first region 821, a second region 822, a third region 823, and a fourth region 824.

The detection unit 84 is a first unit provided corresponding to the first region 821, the second region 822, the third region 823, and the fourth region 824 of each pixel 82 of the filter 80. A detection unit 841, a second unit detection unit 842, a third unit detection unit 843, and a fourth unit detection unit 844 are included. Each first unit detector 841, each second unit detector 842,
Each of the third unit detection units 843 and each of the fourth unit detection units 844 includes a first region 821, a second region 822, a third region 823, and a fourth region 824 of each pixel 82, respectively. The terahertz wave that has passed through is converted into heat and detected. Thereby, in each of the pixels 82, terahertz waves having four target wavelengths can be reliably detected.

  Next, a usage example of the imaging apparatus 1100 will be described.

  First, it is assumed that the object O to be subjected to spectral imaging is composed of three substances A, B, and C. The imaging apparatus 1100 performs spectral imaging of the object O. Here, as an example, the terahertz wave detection unit 1120 detects the terahertz wave reflected by the object O.

  In each pixel 82 of the filter 80 of the terahertz wave detection unit 1120, the first region 821 and the second region 822 are used. The transmission wavelength of the first region 821 is λ1, the transmission wavelength of the second region 822 is λ2, the intensity of the component of wavelength λ1 of the terahertz wave reflected by the object O is α1, and the intensity of the component of wavelength λ2 is α2. , The difference (α2−α1) between the intensity α2 and the intensity α1 can be distinguished from each other among the substance A, the substance B, and the substance C, so that the transmission wavelength λ1 and the second of the first region 821 The pass wavelength λ2 of the region 822 is set.

  As shown in FIG. 11, in the substance A, the difference (α2−α1) between the intensity α2 of the component of wavelength λ2 and the intensity α1 of the component of wavelength λ1 of the terahertz wave reflected by the object O is a positive value. . In the substance B, the difference (α2−α1) between the strength α2 and the strength α1 is zero. In the substance C, the difference (α2−α1) between the strength α2 and the strength α1 is a negative value.

  When spectral imaging of the object O is performed by the imaging apparatus 1100, first, a terahertz wave is generated by the terahertz wave generation unit 1110, and the object O is irradiated with the terahertz wave. Then, the terahertz wave reflected by the object O is detected by the terahertz wave detection unit 1120 as α1 and α2. This detection result is sent to the image forming unit 1130. Note that the irradiation of the terahertz wave to the object O and the detection of the terahertz wave reflected by the object O are performed on the entire object O.

  In the image forming unit 1130, based on the detection result, the intensity α2 of the component of the wavelength λ2 of the terahertz wave that has passed through the second region 822 of the filter 80 and the wavelength λ1 of the terahertz wave that has passed through the first region 821 The difference (α2−α1) from the intensity α1 of the component is obtained. Of the object O, the part where the difference is a positive value is determined as the substance A, the part where the difference is zero is determined as the substance B, and the part where the difference is a negative value is determined as the substance C.

  In addition, the image forming unit 1130 creates image data of an image indicating the distribution of the substances A, B, and C of the object O as shown in FIG. This image data is sent from the image forming unit 1130 to a monitor (not shown), and an image showing the distribution of the substances A, B and C of the object O is displayed on the monitor. In this case, for example, the area where the substance A of the object O is distributed is displayed in black, the area where the substance B is distributed is gray, and the area where the substance C is distributed is displayed in white. In this imaging apparatus 1100, as described above, identification of each substance constituting the object O and distribution measurement of each property can be performed simultaneously.

  Note that the use of the imaging apparatus 1100 is not limited to that described above. For example, a terahertz wave is irradiated on a person, the terahertz wave transmitted or reflected by the person is detected, and the image forming unit 1130 performs processing. Thus, it is possible to determine whether or not the person has a handgun, a knife, an illegal drug, or the like.

4). Measurement Device Next, a measurement device 1200 according to the present embodiment will be described with reference to the drawings. FIG. 13 is a block diagram showing a measuring apparatus 1200 according to this embodiment. In the measurement apparatus 1200 according to the present embodiment described below, members having the same functions as those of the components of the imaging apparatus 1100 described above are denoted by the same reference numerals, and detailed description thereof is omitted.

  As illustrated in FIG. 13, the measurement device 1200 includes a terahertz wave generation unit 1110 that generates a terahertz wave, and a terahertz wave that is emitted from the terahertz wave generation unit 1110 and is transmitted through the object O or is reflected by the object O. A terahertz wave detection unit 1120 that detects a wave; and a measurement unit 1210 that measures an object O based on the detection result of the terahertz wave detection unit 1120.

  Next, a usage example of the measuring apparatus 1200 will be described. When spectroscopic measurement of the object O is performed by the measuring device 1200, first, a terahertz wave is generated by the terahertz wave generation unit 1110, and the object O is irradiated with the terahertz wave. Then, the terahertz wave transmitted through the object O or the terahertz wave reflected by the object O is detected by the terahertz wave detection unit 1120. The detection result is sent to the measurement unit 1210. The irradiation of the terahertz wave to the object O and the detection of the terahertz wave transmitted through the object O or the terahertz wave reflected by the object O are performed on the entire object O.

  In the measurement unit 1210, from the detection result, each of the terahertz waves that have passed through the first region 821, the second region 822, the third region 823, and the fourth region 824 of each pixel 82 of the filter 80 is measured. The strength is grasped, and the components of the object O and the distribution thereof are analyzed.

5. Camera Next, a camera 1300 according to the present embodiment will be described with reference to the drawings. FIG. 14 is a block diagram showing a camera 1300 according to this embodiment. FIG. 15 is a perspective view schematically showing a camera 1300 according to the present embodiment. In the camera 1300 according to the present embodiment described below, members having the same functions as those of the components of the imaging apparatus 1100 described above are denoted by the same reference numerals, and detailed description thereof is omitted.

  As shown in FIGS. 14 and 15, the camera 1300 emits a terahertz wave that is generated from the terahertz wave and the terahertz wave that is emitted from the terahertz wave generation unit 1110 and reflected by the object O or transmitted through the object O. The terahertz wave detecting unit 1120 for detecting the terahertz wave thus generated and the storage unit 1301 are provided. These units 1110, 1120, and 1301 are housed in a housing 1310 of the camera 1300. The camera 1300 includes a lens (optical system) 1320 that converges (images) the terahertz wave reflected by the object O on the terahertz wave detection unit 1120, and the terahertz wave generated by the terahertz wave generation unit 1110. A window portion 1330 for injecting the outside is provided. The lens 1320 and the window 1330 are made of a member such as silicon, quartz, or polyethylene that transmits and refracts terahertz waves. Note that the window portion 1330 may have a configuration in which an opening is simply provided like a slit.

Next, a usage example of the camera 1300 will be described. When imaging the object O with the camera 1300, first, a terahertz wave is generated by the terahertz wave generation unit 1110, and the object O is irradiated with the terahertz wave. Then, the terahertz wave reflected by the object O is converged (imaged) on the terahertz wave detection unit 1120 by the lens 1320 and detected. This detection result is sent to and stored in the storage unit 1301. The irradiation of the terahertz wave to the object O and the detection of the terahertz wave reflected by the object O are performed on the entire object O. The detection result can be transmitted to an external device such as a personal computer. In the personal computer, each process can be performed based on the detection result.

  The above-described embodiments and modifications are merely examples, and the present invention is not limited to these. For example, it is possible to appropriately combine each embodiment and each modification.

  The present invention includes configurations that are substantially the same as the configurations described in the embodiments (for example, configurations that have the same functions, methods, and results, or configurations that have the same objects and effects). In addition, the invention includes a configuration in which a non-essential part of the configuration described in the embodiment is replaced. In addition, the present invention includes a configuration that exhibits the same operational effects as the configuration described in the embodiment or a configuration that can achieve the same object. Further, the invention includes a configuration in which a known technique is added to the configuration described in the embodiment.

DESCRIPTION OF SYMBOLS 2 ... Optical pulse production | generation part, 4 ... Frequency chirp part, 6 ... Group velocity dispersion | distribution part, 10 ... Light emitting element, 12 ... Drive circuit, 14 ... Package, 20 ... Optical waveguide, 31 ... 1st optical fiber, 32 ... 2nd optical fiber 32a ... the other end, 33 ... the third optical fiber, 34 ... the fourth optical fiber, 34a ... the other end, 35 ... the coupler, 36 ... the optical amplifying unit, 37 ... the first reflecting mirror, 38 ... the second reflecting mirror, 40 ... the beam Splitter, 80 ... filter, 82 ... pixel, 84 ... detection unit, 100 ... short light pulse generator, 102 ... substrate, 102a ... first region, 102b ... second region, 104 ... buffer layer, 106 ... first cladding layer 108 ... Core layer 108a ... First guide layer 108b ... MQW layer 108c ... Second guide layer 110 ... Second clad layer 112 ... Cap layer 114 ... Second core Layer 118, optical waveguide, 120 insulating layer, 130 first electrode, 132 second electrode, 160 columnar portion, 821 first region, 822 second region, 823 third region, 824 ... 4th area, 841 ... 1st unit detection part, 842 ... 2nd unit detection part, 843 ... 3rd unit detection part, 844 ... 4th unit detection part, 1000 ... terahertz wave generator, 1010 DESCRIPTION OF SYMBOLS ... Photoconductive antenna, 1012 ... Substrate, 1014 ... Electrode, 1016 ... Gap, 1100 ... Imaging device, 1110 ... Terahertz wave generation unit, 1120 ... Terahertz wave detection unit, 1130 ... Image forming unit, 1200 ... Measurement device, 1210 ... Measurement Part, 1300 ... camera, 1301 ... storage part, 1310 ... housing, 1320 ... lens, 1330 ... window part

Claims (7)

An optical pulse generator for generating an optical pulse;
A frequency chirp section for chirping the frequency of the optical pulse;
A group velocity dispersion unit that includes an optical fiber and generates a group velocity difference according to wavelength in the optical pulse chirped by the frequency chirp unit;
A short light pulse generator comprising: The optical pulse generator is
A light emitting element;
A driving circuit for driving the light emitting element by direct modulation;
Have
2. The short light pulse generator according to claim 1, wherein the drive circuit is configured to be able to change a repetition frequency of light emission of the light emitting element.   The short optical pulse generator according to claim 1, further comprising a reflection mirror that returns the optical pulse emitted from the optical fiber to the optical fiber. The short light pulse generator according to any one of claims 1 to 3,
A photoconductive antenna that generates a terahertz wave by being irradiated with a short light pulse generated by the short light pulse generator;
The terahertz wave generator characterized by including. The short light pulse generator according to any one of claims 1 to 3,
A photoconductive antenna that generates a terahertz wave by being irradiated with a short light pulse generated by the short light pulse generator;
A terahertz wave detecting unit that detects the terahertz wave emitted from the photoconductive antenna and transmitted through the object or the terahertz wave reflected from the object;
A storage unit for storing a detection result of the terahertz wave detection unit;
Including a camera. The short light pulse generator according to any one of claims 1 to 3,
A photoconductive antenna that generates a terahertz wave by being irradiated with a short light pulse generated by the short light pulse generator;
A terahertz wave detecting unit that detects the terahertz wave emitted from the photoconductive antenna and transmitted through the object or the terahertz wave reflected from the object;
An image forming unit that generates an image of the object based on a detection result of the terahertz wave detection unit;
An imaging apparatus comprising: The short light pulse generator according to any one of claims 1 to 3,
A photoconductive antenna that generates a terahertz wave by being irradiated with a short light pulse generated by the short light pulse generator;
A terahertz wave detecting unit that detects the terahertz wave emitted from the photoconductive antenna and transmitted through the object or the terahertz wave reflected from the object;
Based on the detection result of the terahertz wave detection unit, a measurement unit that measures the object,
A measuring device comprising:

Images