The present invention relates to an optical pulse generating apparatus, a terahertz spectroscopy apparatus, and a tomography apparatus.
In recent years, a non-destructive sensing technique in which terahertz waves (frequencies of 30 GHz to 30 THz) are used have been developed. As the applied fields of terahertz waves, a technique in which imaging is performed with a transparent inspection apparatus, a spectroscopic technique in which physical properties such as the binding state of molecules are checked by obtaining an absorption spectrum or a complex permittivity, a measuring technique in which physical properties such as the density or the mobility of carriers or the conductivity is checked, and an analysis technique for biomolecules have been developed.
A terahertz time-domain spectroscopy apparatus in which terahertz pulses are used, which is a representative technique, has an optical system in which femtosecond laser is divided into two types of light, which are radiated onto a terahertz generating element as pump light and onto a terahertz detecting element as probe light, respectively. By changing the difference between moments at which the pump light and the probe light are radiated, terahertz pulses are measured through sampling to analyze a change caused by interaction with an object.
As a method for adjusting the time difference, a mechanical delay stage is generally used. However, there has been a problem in that vibration acts as noise and the time taken to obtain a signal cannot be shortened because the time to be adjusted is of the order of milliseconds. Therefore, an asynchronous sampling method in which two types of fiber lasers that have been synchronized by phase lock loop (PLL) control are used as the pump light and the probe light, respectively, and the phase difference in the PLL is variable is attracting attention as a high-speed optical delay method (PTL 1).
However, in the case of the method according to PTL 1, since two lasers are used, cost is large, which has been a problem.
Therefore, the present invention provides an optical pulse generating apparatus that has a simple structure and with which the time difference between the pump light and the probe light can be changed at high speed.
According to an aspect of the present invention, an optical pulse generating apparatus that supplies pump light and probe light includes a light source and a modulation unit configured to modulate light emitted from the light source, thereby dividing the light into the pump light and the probe light. The modulation unit is configured such that a frequency for modulating the light is variable. The modulation unit changes a difference between a moment of the pump light incident on an object and a moment of the probe light incident on the object by changing the frequency.
Other aspects of the present invention will be clarified by the exemplary embodiments that will be described below.
An optical pulse generating apparatus can be provided that has a simple structure and with which the time difference between the pump light and the probe light can be changed at high speed.
An optical pulse generating apparatus that supplies pump light and probe light for asynchronous sampling according to an embodiment of the present invention will be described with reference to
The MZM generally has a structure 10 illustrated in
At this time, when a frequency (modulation frequency) fm of the external power source 3 is changed, the intervals between pulses are accordingly changed. However, since the two types of pulses that have opposite phases are output after being modulated by the same power supply, the pulses output to the optical output fibers 16 and 17 (fibers 4 and 5) still have a particular phase relationship. The mechanism will be described with reference to
Now, the description returns to
A terahertz time-domain spectroscopy apparatus for which the pump pulses and the probe pulses are used is illustrated in
A terahertz wave generated by the terahertz wave generating element 41 is converted into parallel light by a parabolic mirror 43a and reflected by a half mirror (mesh, Si, or the like) 44. The parallel light is then condensed by a parabolic mirror 43b and radiated onto a measurement sample 45. Arrows illustrated above the measurement sample 45 indicate that the measurement sample 45 is disposed on a stage capable of scanning a sample in a two-dimensional manner. The terahertz wave reflected by the measurement sample 45 is then reflected by the parabolic mirror 43b, and components that pass through the half mirror 44 is condensed by a parabolic mirror 43c and detected by the terahertz wave detecting element 42. Synchronous detection may be performed as necessary by modulating the terahertz wave generating element 41 with a modulation unit 46 and by using a lock-in amplifier in a signal obtaining unit 47, in order to observe a micro-signal at a high signal-to-noise ratio. A detected signal is amplified by an amplifier 48 and propagates through the signal obtaining unit 47. The detected signal can then be observed as the waveform of a terahertz pulse in a data processing/outputting unit 49. However, when the output power of a signal is high, this synchronous detection system (the modulation unit 46 and the lock-in amplifier) may be omitted and the output of the amplifier 48 may be obtained by the signal obtaining unit 47 as it is.
A modulator and an external power supply illustrated in
In this embodiment, as described above, the time difference between optical pulses to be radiated onto the terahertz wave generating element 41 and the terahertz wave detecting element 42 can be adjusted by changing the modulation frequency of the MZM. Therefore, a terahertz waveform can be obtained at high speed through asynchronous sampling of light. Since a mechanical delay stage is not necessary, noise that would otherwise be caused by vibration is not generated.
It is to be noted that, although an example in which the MZM having a Y-branch structure is used has been described, an EO modulator having two outputs realized by a directional coupler or the like may be used. In addition, although an embodiment in which the pump light and the probe light according to the embodiment of the present invention are used for a terahertz time-domain spectroscopy apparatus has been described, the pump light and the probe light may be used in a pump-probe method by which the physical properties of an object in a relatively high-speed phenomenon (for example, the carrier lifetime in a semiconductor) are measured. In that case, the pump light and the probe light are radiated onto the same region or close regions of an object, with a time difference provided therebetween.
Example 1, which is a specific example of the first embodiment, will be described.
As the light source 1, a distributed feedback laser diode (DFB-LD) that oscillates at 1.53 μm in the single mode is used, and a continuous-wave (CW) operation is performed at 10 mW. The MZM is modulated by a known technique with an initial frequency of 10 GHz. At this time, because wavelength chirping is caused, the SMFs 6a and 6b in the subsequent stages shape pulses such that the wavelength chirping is compensated, thereby providing a pulse width of, for example, several ps. The pulses are then amplified by the first and second optical amplifiers 7a and 7b that include Er-doped fibers and compressed by the first and second dispersion compensation units 8a and 8b that include dispersion-flattened dispersion-decreasing fibers (DF-DDF). The output power and the pulse width of the optical output of the first dispersion compensation unit 8a are adjusted to be 30 mW on average and 150 fs, respectively, and the output power and the pulse width of the optical output of the second dispersion compensation unit 8b are adjusted to be 5 mW on average and 200 fs, respectively.
The pump light and the probe light generated in such a manner are guided to the terahertz wave generating element 41 and the terahertz wave detecting element 42, respectively, illustrated in
It is to be noted that, because the speed at which the modulation frequency is changed is sufficiently slow (for example, MHz order) relative to the modulation frequency fm of light, the period does not change for every pulse, but changes at, for example, every 1000th pulse as described above.
By analyzing a terahertz pulse reflected from the measurement sample 45 in the system illustrated in
In Example 2, which is another specific example of the first embodiment, a second harmonic wave generating (SHG) element (not illustrated) composed of periodically poled lithium niobate (PPLN) or the like is inserted between the fiber output and the terahertz wave generating element 41, in order to improve the signal-to-noise ratio of the terahertz spectroscopy apparatus or the tomography apparatus. In doing so, the output power of optical pulses can be improved and a photoconductive element containing low-temperature-growth GaAs can be used as the terahertz wave detecting element 42.
Because the output power cannot be largely increased with the DF-DDF used in Example 1, a combination between a photonic crystal fiber and a highly nonlinear fiber is used instead. In addition, in order to decrease the pulse width, the Er-doped fiber is designed such that the wavelength bandwidth is increased through linear chirping caused by self-phase modulation. In the output of the SMFs 6a and 6b in the previous stage, not only dispersion compensation but also inverse chirping is performed, so that the amount of chirp is adjusted when the output is amplified by the Er-doped fiber and the wavelength at which self-phase modulation is conspicuously caused. In such a configuration, the pulse width and the output power of the first dispersion compensation unit 8a are controlled in such a way as to be 30 fs and 60 mW, respectively, and those of the second dispersion compensation unit 8b are controlled in such a way as to be 30 fs and 120 mW, respectively. As described above, since the probe light passes through the SHG element, the pulse width and the output power become about 60 fs and 10 mW, respectively, when the probe light reaches the terahertz wave detecting element 42.
In such a system, the pulse width of a terahertz wave decreases to about 300 fs, and the signal strength of the terahertz wave increases. Therefore, the measurement bandwidth extends to about 7 THz, and the time taken for a measurement can be further reduced compared to Example 1.
A second embodiment of the present invention is illustrated in
In this embodiment, in order to output two types of optical pulses having a certain phase difference between each other, the polarization direction of the laser light 57 emitted from the polarization modulation laser 50 is switched by a signal transmitted from the external power supply (modulation unit) 51. The external power supply 51 is configured such that the modulation frequency thereof is variable. Therefore, if the modulation frequency is changed by the external power supply 51, the intervals of optical pulses generated by switching are changed. If lights that are differently polarized from each other are divided by the PBS 52, two types of optical pulses that have a particular phase relationship are generated. As in the first embodiment, the two types of optical pulses divided by the PBS 52 are guided to an object such as a photoconductive element by SMFs 54a and 54b, optical amplifiers 55a and 55b, and dispersion compensation units 56a and 56b, respectively. By changing the modulation frequency of the external power supply 51, a difference between a moment of the pump light incident on the object and a moment of the probe light incident on the object changes.
In this embodiment, since the two types of optical pulse strings that have a certain phase difference therebetween are generated by modulating the light source 50, the PBS 52 as a dividing unit is a passive component. Therefore, a driving system can be simplified, which is advantageous. In this embodiment, the polarization direction of light emitted from the light source 50 is modulated as the oscillation state of the light source 50. However, the wavelength of the light emitted from the light source 50 may be modulated instead. In that case, a laser that can change the wavelength thereof may be used as the light source 50 and a dichroic mirror may be used instead of the PBS.
A third embodiment of the present invention is illustrated in
The AOM 61 is a modulator that generates a surface acoustic wave on an acousto-optic element when the RF signal 62 is applied thereto and that outputs incident light that has been deflected from the travel direction due to diffraction. The direction of deflection depends on the frequency of the RF signal 62. Zero-order light when the RF signal 62 is not applied is used as the pump light, and first-order diffracted light that has been deflected upon application of the RF signal 62 is used the probe light. The pump light and the probe light are used as two types of optical pulse signal strings that pass through SMFs 67a and 67b. At this time, turning on and off of the RF signal 62 is controlled by the digital signal source 63 that outputs digital signals and the mixer modulator 64.
Therefore, when the seed laser 60 is continuous light, pulses that reflect the waveform of the digital signal source 63 appear as two types of optical outputs of the AOM 61. After that, through waveform shaping performed by the SMFs 67a and 67b, optical amplification performed by optical amplifiers 68a and 68b, and dispersion compensation performed by dispersion compensation units 69a and 69b, the pump light and the probe light can be generated as the two types of optical pulse signal strings that have a certain phase difference therebetween.
Typically, the frequency of the RF signal 62 is about 2 GHz and the repeated modulation frequency of the digital signal source 63 is 250 Mhz during operation, but the modulation may be performed at higher frequencies.
If the modulation frequency is gradually changed, the pulse intervals of the pump light and the probe light also gradually change, thereby changing the time difference between the two types of pulse strings in the same principle as in the first embodiment.
In a fourth embodiment of the present invention, a ring laser is used as a light source having optical output that has been modulated and divided. In this embodiment, a ring-type fiber laser 70 illustrated in
The direction switching isolators 78 are two isolators in different directions, and the oscillation/circulation direction (oscillation state), which is a direction in which laser oscillates, can be selected by selecting either isolator through switching of the optical path. In the case of sinusoidal modulation, for example, by selecting the clockwise circulation with a positive amplitude or the counterclockwise circulation with a negative amplitude while the switching is synchronized with the external power supply 79, output a) or b) of the coupler 76 that are inverse to each other can be obtained as illustrated in
Amplification and dispersion compensation of pulses in the subsequent stages may be performed as necessary as in the above-described embodiments. In addition, the method of asynchronous sampling in which the time difference between the pump light and the probe light is changed by changing the period of optical pulses is the same as in the above-described embodiments.
By using the ring-type fiber laser 70, optical pulses that generate smaller timing jitter therebetween can be provided. It is to be noted that, although the coupler 76 is used as a dividing unit in this embodiment, micro-electro-mechanical systems (MEMS) may be used as a dividing unit that divides the light propagation direction.
It is to be understood that, although the exemplary embodiments of the present invention have been described above, the present invention is not limited by these embodiments and may be modified or altered in various ways within the scope thereof. For example, the optical pulse generating apparatus in the present invention may be used as a light source of a pump-probe measuring apparatus. In the pump-probe measuring apparatus, the optical pulse generating apparatus in the present invention changes the difference between a moment at which pump light of the pump light incident on an object and a moment of the probe light incident on the object to be measured.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2010-191321, filed Aug. 27, 2010, which is hereby incorporated by reference herein in its entirety.
Number | Date | Country | Kind |
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2010-191321 | Aug 2010 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP2011/067576 | 7/26/2011 | WO | 00 | 2/22/2013 |