Coherent light generators – Particular beam control device – Nonlinear device
Reexamination Certificate
2002-04-15
2004-05-18
Ip, Paul (Department: 2828)
Coherent light generators
Particular beam control device
Nonlinear device
C372S028000, C359S248000
Reexamination Certificate
active
06738397
ABSTRACT:
This application is based on Application No. 2001-131953, filed in Japan on Apr. 27, 2001 and Application No. 2001-341797, filed in Japan on Nov. 7, 2001, the contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a solid-state light source apparatus, and particularly to a solid-state light source apparatus used for a terahertz band spectroscopic light source, an imaging light source, a light source for communication, and a light source for measurement.
2. Description of the Related Art
As a light source for generating a terahertz band beam, although there was a GaAs photoconductive device, a magnetic field application type semiconductor device, an optical parametric oscillator using LiNbO
3
, a difference frequency generation device using an organic nonlinear optical crystal, or the like, all of them had low efficiency and low output power.
Since a conventional semiconductor pseudo phase matching device using diffused junction has high scattering at a junction interface, it falls far short of practical use, and naturally, there was no terahertz light source using this technique.
A conventional solid-state light source apparatus will be described with reference to the drawing.
FIG. 5
is a view showing the structure of a conventional solid-state light source apparatus disclosed in, for example, “Laser Research, Vol. 26, No. 7, p. 515 to 521, July 1998”.
FIG. 5
is the structural view of an example of a photoconductive device used for terahertz wave generation.
In
FIG. 5
, reference numeral
100
designates a photoconductive device;
101
, a semiconductor substrate;
102
, a photoconductive thin film;
103
, parallel transmission lines;
104
, a dipole antenna;
105
, a gap;
106
, a direct current power source;
110
, an optical pulse; and
111
, a terahertz electromagnetic wave.,
In this photoconductive device
100
, the parallel transmission lines
103
made of transmission lines
103
a
and
103
b
are formed on the substrate
101
of a high speed response semiconductor such as GaAs and the photoconductive thin film
102
of low temperature growth GaAs or the like, and a single optical switch made of the minute dipole antenna
104
is provided at the center portion.
The minute gap
105
with several &mgr;m, for example, exists at the center of the optical switch
104
, and a suitable voltage is applied to the gap
105
by the direct current power source
106
.
When a laser beam having energy higher than the band gap of the semiconductor enters on the gap
105
as the optical pulse
110
, free carriers are generated in the semiconductor, a pulse-like current flows, and the terahertz electromagnetic wave
111
in proportion to the time differential of the pulse-like current is generated.
Thus, the terahertz electromagnetic wave
111
is generated when the pulse-like current is, for example, on a picosecond level or less, and further, it is generated when a short pulse laser beam in which the optical pulse
110
is on a picosecond level or less is irradiated.
As disclosed in “Laser Society Scientific Lecture Meeting, 17th Annual Conference, 23aII4, p. 194 to 197”, two continuous-wave laser beams are optically mixed with each other on a photoconductive device, so that a terahertz wave can be continuously generated. When two monochromatic beams with different frequencies are mixed, a resultant amplitude is modulated by a difference frequency. When the mixed wave (light beat) is irradiated to the photoconductive device, a photocurrent is modulated, and an electromagnetic wave corresponding to the difference frequency is radiated from an antenna. Thus, when the frequencies of the two continuous-wave laser beams are adopted so that the difference frequency becomes about terahertz, the terahertz wave is generated.
As disclosed in “Laser Research, Vol. 26, No. 7, p. 527 to 530, July 1998”, when a light pulse of picosecond or less as a laser beam is irradiated to a semiconductor such as InAs or GaAs put in a magnetic field, a terahertz wave can be generated.
Further, as disclosed in “Laser Research, Vol. 26, No. 7, p. 522 to 526, July 1998”, LiNbO
3
is used as a crystal having a secondary nonlinear optical effect, and light waves are caused to enter upon the crystal, and an optical parametric oscillator is constructed under phase matching conditions, so that a terahertz beam can be generated.
As disclosed in “OPTICS LETTERS, Vol. 25, No. 23, pp. 1714-1716, 2000”, an organic crystal with high nonlinearity is used as a crystal having a secondary nonlinear optical effect, two laser beams with a difference frequency of terahertz enters upon the crystal, and difference frequency generation is carried out under phase matching conditions, so that a terahertz beam can be generated.
Further, as disclosed in “61th Applied Physics Society Scientific Lecture Meeting, Collection of Lecture Preparatory Papers, No. 3, 4a-L-8, p957, 2000”, a bulk type semiconductor material is used as a material having a secondary nonlinear optical effect, two laser beams with a difference frequency of terahertz are caused to enter on the nonlinear material, and difference frequency generation is carried out under phase matching conditions, so that a terahertz beam can be generated.
However, the foregoing prior art had problems as follows:
In the generation of the terahertz beam by the photoconductive device using the excitation of the short pulse laser beam, the efficiency was low and the output power was low. Further, since the line width was wide, a light source of a single longitudinal mode did not exist as well.
In the generation of the terahertz beam by the photoconductive device using the excitation of the two continuous-wave laser beams, the efficiency was low and the output power was low.
In the generation of the terahertz beam by the semiconductor device put in the magnetic field using the excitation of the short pulse laser beam, the efficiency was low and the output power was low. Besides, since the line width is wide, a light source of a single longitudinal mode did not exist as well.
In the generation of the terahertz beam by the optical parametric oscillator using LiNbO
3
as the nonlinear optical device, the absorption of the terahertz beam in LiNbO
3
was large, the extraction efficiency of the generated terahertz beam was low, and the output power was low. Further, since the output angle of the terahertz beam was not coincident with the optical axis of the exciting beam, in the optical parametric oscillator, it was difficult to take a long interaction length between the exciting beam and the terahertz beam obtained by wavelength conversion, and the wavelength conversion had low efficiency and the output power was low.
In the generation of the terahertz beam by the difference frequency using the organic crystal as the nonlinear optical device, the efficiency was low and the output power was low.
Besides, in the generation of the terahertz beam by the difference frequency using the bulk type semiconductor material as the linear optical device, since it was difficult to take a long distance in the phase matching conditions, the efficiency was low and the output power was low.
Moreover, in the conventional semiconductor pseudo phase matching device using diffused junction, there were also problems that since scattering at the junction interface was high, it falls far short of practical use, and naturally, there was no terahertz beam source using this technique.
SUMMARY OF THE INVENTION
The present invention has been made to solve the foregoing problems, and a pseudo phase matching difference frequency generation device by diffused junction of semiconductors is used to generate a terahertz wave. Since the semiconductor such as GaP or GaAs has a large nonlinear optical constant, it is suitable for high efficiency wavelength conversion, and is transparent in a terahertz region. Further, the semiconductor has large thermal conductivity and is also suitable for high power operation. Further, when a tunable laser of a
Hirano Yoshihito
Kurimura Sunao
Shoji Ichiro
Taira Takunori
Yamamoto Shuhei
Birch & Stewart Kolasch & Birch, LLP
Ip Paul
Mitsubishi Denki & Kabushiki Kaisha
Nguyen Philip
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