U.S. patent application number 15/970530 was filed with the patent office on 2018-09-06 for method for gigahertz to terahertz frequency signal generation using opo and dfg.
The applicant listed for this patent is Lockheed Martin Corporation. Invention is credited to Angus J. Henderson, Yongdan Hu, Andrew Xing.
Application Number | 20180252984 15/970530 |
Document ID | / |
Family ID | 50184588 |
Filed Date | 2018-09-06 |
United States Patent
Application |
20180252984 |
Kind Code |
A1 |
Hu; Yongdan ; et
al. |
September 6, 2018 |
METHOD FOR GIGAHERTZ TO TERAHERTZ FREQUENCY SIGNAL GENERATION USING
OPO AND DFG
Abstract
Apparatus and method for high-power multi-function
millimeter-wavelength (THz-frequency) signal generation using OPO
and DFG in a single cavity. In some embodiments, the OPO-DFG cavity
includes an optical parametric oscillator (OPO) non-linear material
that receives pump light I.sub.P having pump-light frequency and
generates two different lower intermediate frequencies of light--an
OPO-signal beam I.sub.S and a spatially/temporally overlapping
OPO-idler beam I.sub.I. A difference-frequency generator non-linear
material then receives the two intermediate-frequency beams I.sub.I
and I.sub.S, and the DFG then generates a THz-frequency output
signal that has a frequency equal to the difference between the two
intermediate frequencies. In some embodiments, a single-piece
crystal of non-linear material is used for both OPO and DFG
functions. Some embodiments use a bow-tie ring having four mirrors
that define the optical path: an I.sub.P-beam-entry mirror, an
I.sub.P-light-extraction mirror to remove unconverted I.sub.P-beam,
an I.sub.I-beam-extraction mirror, and an I.sub.S-beam-extraction
mirror, and a fifth I.sub.THz-beam-extraction mirror.
Inventors: |
Hu; Yongdan; (Bothell,
WA) ; Xing; Andrew; (Bothell, WA) ; Henderson;
Angus J.; (Seattle, WA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Lockheed Martin Corporation |
Bethesda |
MD |
US |
|
|
Family ID: |
50184588 |
Appl. No.: |
15/970530 |
Filed: |
May 3, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15446931 |
Mar 1, 2017 |
10001695 |
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15970530 |
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14740257 |
Jun 16, 2015 |
9588398 |
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15446931 |
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13828875 |
Mar 14, 2013 |
9057927 |
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14740257 |
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61694763 |
Aug 29, 2012 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02F 1/39 20130101; G02F
1/3532 20130101; G02F 2001/3507 20130101; G02F 1/0128 20130101;
G02F 2203/13 20130101; G02F 1/3551 20130101; G02F 1/353 20130101;
H01S 3/0092 20130101; H01S 3/0078 20130101; G02F 1/3501 20130101;
G02F 2001/3503 20130101; G02F 1/3534 20130101; H01S 3/0675
20130101 |
International
Class: |
G02F 1/35 20060101
G02F001/35; G02F 1/355 20060101 G02F001/355; H01S 3/067 20060101
H01S003/067; H01S 3/00 20060101 H01S003/00; G02F 1/39 20060101
G02F001/39 |
Claims
1. A method for generating a gigahertz-terahertz-range signal
having a frequency in a gigahertz to terahertz frequency range, the
method comprising: receiving pump light having a pump frequency
into a single optical cavity having an optical path; generating
light that includes a first intermediate frequency and a second
intermediate frequency within the single optical cavity by using
energy from the pump light; spatially separating the light of the
first intermediate frequency from the light of the second
intermediate frequency such that the light of the first
intermediate frequency propagates along a first segment of the
optical path and the light of the second intermediate frequency
propagates along a second segment of the optical path, and
recombining the spatially separated light into a single beam; and
generating the gigahertz-terahertz-range signal within the single
optical cavity by using the light of the two intermediate
frequencies, wherein the frequency of the gigahertz-terahertz-range
signal is equal to a difference between the two intermediate
frequencies.
2. The method of claim 1, wherein the optical path has a bow-tie
ring topology, and wherein the method further includes: reflecting
light of the first intermediate frequency at a first
frequency-selective reflector, and removing unconverted pump light
from the single optical cavity through the first
frequency-selective reflector; reflecting light of the first
intermediate frequency at a second reflector; reflecting light of
the first intermediate frequency at a third reflector; passing
light of the first intermediate frequency through a
frequency-selective Fabry-Perot etalon located in the optical path
between the second reflector and the third reflector while blocking
light of the second intermediate frequency; reflecting light of the
first intermediate frequency at a fourth frequency-selective
reflector; introducing the pump light through the fourth
frequency-selective reflector into the single optical cavity;
converting the pump light into light of the first intermediate
frequency and light of the second intermediate frequency using
non-linear optical parametric oscillation in the optical path
between the fourth reflector and the first reflector; and
converting light of the first intermediate frequency and light of
the second intermediate frequency to electromagnetic radiation
having a gigahertz-terahertz frequency using non-linear difference
frequency generation in the optical path.
3. The method of claim 1, further comprising: removing the pump
light and the gigahertz-terahertz-range signal through a single
port; and separating the pump light and the
gigahertz-terahertz-range signal outside of the single optical
cavity.
4. The method of claim 1, further comprising: tuning the pump
frequency using a piezo-electric element in order to change the
frequency of the gigahertz-terahertz-range signal.
5. The method of claim 1, further comprising: reflecting light of
the first intermediate frequency at a first frequency-selective
reflector, and removing unconverted pump light from the single
optical cavity through the first frequency-selective reflector;
reflecting light of the first intermediate frequency at a second
reflector; reflecting light of the first intermediate frequency at
a third reflector; reflecting light of the first intermediate
frequency at a fourth reflector; passing light of the first
intermediate frequency through a frequency-selective Fabry-Perot
etalon located in the optical path between the third reflector and
the fourth reflector while blocking light of the second
intermediate frequency; reflecting light of the first intermediate
frequency at a fifth frequency-selective reflector; reflecting
light of the first intermediate frequency at a sixth
frequency-selective reflector; introducing the pump light through
the sixth frequency-selective reflector into the single optical
cavity; converting the pump light into light of the first
intermediate frequency and light of the second intermediate
frequency using non-linear optical parametric oscillation in the
optical path between the sixth reflector and the first reflector;
and converting light of the first intermediate frequency and light
of the second intermediate frequency to electromagnetic radiation
having a gigahertz-terahertz frequency using non-linear difference
frequency generation in the optical path.
6. The method of claim 1, wherein the spatially separating of the
light includes reflecting the light at a first diffraction
grating.
7. The method of claim 1, further comprising passing light of the
first intermediate frequency through a first frequency-selective
Fabry-Perot etalon located in the first segment of the optical
path.
8. The method of claim 1, further comprising: passing light of the
first intermediate frequency through a first frequency-selective
Fabry-Perot etalon located in the first segment of the optical
path; and passing light of the second intermediate frequency
through a second frequency-selective Fabry-Perot etalon located in
the second segment of the optical path.
9. The method of claim 1, wherein the generating of the light
includes passing the light through a non-linear optical crystal
that acts as an optical parametric oscillator.
10. The method of claim 1, wherein the generating of the
gigahertz-terahertz-range signal includes passing the light of the
two intermediate frequencies through a non-linear optical crystal
that acts as a difference frequency generator.
11. The method of claim 1, wherein the generating of the light
includes passing the light through a non-linear optical crystal
that acts as both an optical parametric oscillator and as a
difference frequency generator.
12. The method of claim 1, wherein the generating of the light
includes passing the light through a first non-linear optical
crystal that acts as an optical parametric oscillator, wherein the
generating of the gigahertz-terahertz-range signal includes passing
the light of the two intermediate frequencies through a non-linear
optical crystal that acts as a difference frequency generator, the
method further comprising: reflecting light at both the first
intermediate frequency and the second intermediate frequency at a
first frequency-selective reflector; removing unconverted pump
light from the single optical cavity through the first
frequency-selective reflector; and reflecting the
gigahertz-terahertz-range signal out of the single optical cavity
at a second frequency-selective reflector.
13. The method of claim 1, further comprising: tuning a resonant
cavity length of the single optical cavity using a piezo-electric
element.
14. The method of claim 1, further comprising: generating the pump
light having the pump frequency, wherein the generating of the pump
light includes: providing a distributed-feedback (DFB) fiber laser
that emits the pump light at the pump frequency, and controllably
varying the pump frequency.
15. The method of claim 1, further comprising providing a plurality
of optical elements that define the optical path, wherein the
providing of the plurality of optical elements includes arranging
the plurality of optical elements in a single plane.
16. The method of claim 1, further comprising: providing a unitary
block housing; surrounding the single optical cavity with the
unitary block housing such that the optical path is completely
within the unitary block housing.
17. The method of claim 1, further comprising: reflecting light at
both the first intermediate frequency and the second intermediate
frequency at a first frequency-selective reflector; and removing
unconverted pump light from the single optical cavity through the
first frequency-selective reflector.
18. A method for generating a gigahertz-terahertz-range signal
having a frequency in a gigahertz to terahertz frequency range, the
method comprising: receiving pump light having a pump frequency
into a single optical cavity having an optical path; generating
light that includes a first intermediate frequency and a second
intermediate frequency within the single optical cavity by using
energy from the pump light, wherein the generating includes passing
the light through a non-linear optical crystal that acts as an
optical parametric oscillator; spatially separating the light of
the first intermediate frequency from the light of the second
intermediate frequency, and recombining the spatially separated
light into a single beam; and generating the
gigahertz-terahertz-range signal within the single optical cavity
by using the light of the two intermediate frequencies, wherein the
frequency of the gigahertz-terahertz-range signal is equal to a
difference between the two intermediate frequencies.
19. The method of claim 18, further comprising: generating the pump
light having the pump frequency, wherein the generating of the pump
light includes: providing a distributed-feedback (DFB) fiber laser
that emits the pump light at the pump frequency.
20. A method for generating a gigahertz-terahertz-range signal
having a frequency in a gigahertz to terahertz frequency range, the
method comprising: generating pump light having a pump frequency
using a distributed-feedback (DFB) fiber laser that emits the pump
light at the pump frequency; receiving the pump light into a single
optical cavity having an optical path; generating light that
includes a first intermediate frequency and a second intermediate
frequency within the single optical cavity by using energy from the
pump light; spatially separating the light of the first
intermediate frequency from the light of the second intermediate
frequency, and recombining the spatially separated light into a
single beam; and generating the gigahertz-terahertz-range signal
within the single optical cavity by using the light of the two
intermediate frequencies, wherein the frequency of the
gigahertz-terahertz-range signal is equal to a difference between
the two intermediate frequencies.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 15/446,931 filed on Mar. 1, 2017 by Yongdan Hu et al.,
titled "SYSTEM FOR GIGAHERTZ TO TERAHERTZ FREQUENCY SIGNAL
GENERATION USING OPO AND DFG" (published as U.S. 2017/0176838 on
Jun. 22, 2017), which is a divisional of U.S. patent application
Ser. No. 14/740,257 filed on Jun. 16, 2015 by Yongdan Hu et al.,
titled "GIGAHERTZ TO TERAHERTZ FREQUENCY SIGNAL GENERATION USING
OPO AND DFG" (which issued as U.S. Pat. No. 9,588,398 on Mar. 7,
2017), which is a divisional of U.S. patent application Ser. No.
13/828,875 filed on Mar. 14, 2013 by Yongdan Hu et al., titled
"HIGH-POWER MULTI-FUNCTION MILLIMETER-WAVE SIGNAL GENERATION USING
OPO AND DFG" (which issued as U.S. Pat. No. 9,057,927 on Jun. 16,
2015), which claims priority benefit under 35 U.S.C. .sctn. 119(e)
of U.S. Provisional Patent Application No. 61/694,763 filed Aug.
29, 2012 by Yongdan Hu et al., titled "HIGH-POWER MULTI-FUNCTION
MILLIMETER-WAVE SIGNAL GENERATION USING OPO AND DFG," each of which
is incorporated herein by reference in its entirety.
[0002] This invention is related to: [0003] U.S. Pat. No. 7,620,077
to Angus J. Henderson, which issued on Nov. 17, 2009, titled
"Apparatus and method for pumping and operating optical parametric
oscillators using DFB fiber lasers," and which claimed priority to
U.S. Provisional Patent Application No. 60/697,787 filed Jul. 8,
2005, [0004] U.S. Pat. No. 6,940,877 to Yongdan Hu, et al., which
issued on Sep. 6, 2005, titled "High-power narrow-linewidth
single-frequency laser," each of which is incorporated herein by
reference in its entirety.
FIELD OF THE INVENTION
[0005] The invention relates generally to terahertz and gigahertz
sources of electromagnetic radiation, and more particularly to
apparatus and methods for generating of two intermediate-frequency
light beams from a single laser input using an optical-parametric
oscillator having a piece of non-linear conversion material (e.g.,
a periodically poled lithium niobate (PPLN) crystal or other
suitable material), and then obtaining a terahertz or gigahertz
signal from the two intermediate-frequency light beams using a
non-linear optical difference-frequency generator, which, in some
embodiments, uses the same piece of non-linear conversion material
as the OPO; or in other embodiments, uses a separate DFG piece of
non-linear conversion material in the same cavity as the piece of
non-linear conversion material used for the OPO.
BACKGROUND OF THE INVENTION
[0006] At present, there are no millimeter-wavelength
(terahertz-frequency (THz)) sources that are compact, light-weight,
narrow-linewidth, tunable, and high-power sources. Referred to
hereafter as THz sources, these sources output electromagnetic
radiation at a frequency in the range of at least about 0.1-10
THz.
[0007] Conventional sources in the terahertz (THz) range (i.e.,
0.1-10 THz) have either too-low power or are only available in
limited wavelengths and frequency bands. FIG. 1A shows the power-
and frequency-capability characteristics of various conventional
THz sources. 1--PCA (photoconductive antenna; e.g., interdigitated
PCA); 2--OR (optical rectification); 3--CO.sub.2 laser frequency
mixing; 4--DFG (difference frequency generation); 5--optically
pumped laser; 6--QCL (quantum-cascade laser); 7--p-Ge-laser.
[0008] Overview of Existing Solutions-Existing solutions have
limited applications due to their corresponding significant issues:
[0009] PCA and OR: low power; [0010] CO.sub.2 laser frequency
mixing: small spectra range/tunability and low efficiency; [0011]
Traditional DFG: relatively low power, low efficiency and complex;
[0012] Optically pumped laser: no tunability, gas laser--high
maintenance and bulky; [0013] QCL: narrow available spectra range,
requires cryogenic cooling; and [0014] p-Ge-laser: narrow available
spectra range, operational nightmare--requires a cooling Dewar, and
high magnetic and pulsed electric fields.
[0015] U.S. Pat. No. 7,054,339 issued to Yongdan Hu et al. on May
30, 2006, titled "Fiber-laser-based Terahertz sources through
difference frequency generation (DFG) by nonlinear optical (NLO)
crystals," is incorporated herein by reference. In the U.S. Pat.
No. 7,054,339, Yongdan Hu (one of the inventors of the present
invention) and his co-inventors described a fiber-laser-based
implementation of a terahertz source through difference frequency
generation (DFG) by nonlinear optical (NLO) crystals is compact,
tunable and scalable. A pair of fiber lasers (Q-switched, CW
(continuous wave) or mode-locked) generate single-frequency outputs
at frequencies .omega.1 and .omega.2. A fiber beam combiner
combines the laser outputs and routes the combined output to a THz
generator head where a nonlinear interaction process in the NLO
crystal generates THz radiation.
[0016] U.S. Pat. No. 7,539,221 issued May 26, 2009 to Jiang, et
al., titled "Fiber-laser-based gigahertz sources through difference
frequency generation (DFG) by nonlinear optical (NLO) materials,"
is incorporated herein by reference. The U.S. Pat. No. 7,539,221
described a fiber-laser-based implementation of a gigahertz source
through difference frequency generation (DFG) by nonlinear optical
(NLO) materials is compact, tunable and scalable. A pair of pulsed
fiber lasers, preferably single-frequency lasers, generate output
pulses at frequencies .omega.1 and .omega.2 that overlap
temporally. A beam combiner combines the laser outputs and routes
the combined output to a GHz generator head where a nonlinear
interaction process in the NLO material generates GHz
radiation.
[0017] Optical parametric oscillators (OPOs) provide an efficient
way of converting short-wavelength electromagnetic radiation from
coherent-light sources to long wavelengths, while also adding the
capability to broadly tune the output wavelength. In general, an
OPO system principally includes a short-wavelength laser source and
an optical resonator (resonant optical cavity) containing a
nonlinear crystal. In some embodiments, additional components
include mode-matching optics and an optical isolator.
[0018] In general, the OPO operates with three overlapping light
beams--an input pump beam having the shortest wavelength, and thus
highest frequency (typically, this is coherent light from a laser),
and two longer-wavelength, lower-frequency beams generated in the
OPO called the signal beam (this is usually called the "OPO-signal"
beam herein to distinguish from other signals) and the idler beam
(this is usually called the "OPO-idler" beam herein). By
convention, the shorter-wavelength beam is called the OPO-signal
beam, and the longer-wavelength beam is called the OPO-idler beam.
The energy of photons in the pump beam (proportional to
1/wavelength) will equal the sum of the energy of photons in the
OPO-signal beam plus the energy of photons in the OPO-idler beam.
The pump beam (i.e., excitation light from the short-wavelength
laser source) is focused, using the mode-matching optics, through
the optical isolator and into the resonant optical cavity, passing
through the nonlinear crystal(s). Parametric fluorescence generated
within the nonlinear material(s) is circulated within the resonant
cavity and experiences optical gain. When the OPO is excited by a
pump-power-per-unit-area above a certain threshold, oscillation
occurs, and efficient conversion of pump photons to OPO-signal and
OPO-idler photons occurs. Different configurations of OPOs are
possible. Variables include the wavelengths that are resonant
within the optical cavity (pump and/or OPO-signal and/or OPO-idler)
and the type of resonator (ring versus linear). In a conventional
OPO, depending on the application, either the OPO-signal beam or
the OPO-idler beam, or both, will be the output light utilized by
other components.
[0019] U.S. Pat. No. 7,620,077 issued Nov. 17, 2009 to Angus J.
Henderson (one of the inventors of the present invention), titled
"Apparatus and method for pumping and operating optical parametric
oscillators using DFB fiber lasers," is incorporated herein by
reference. In the U.S. Pat. No. 7,620,077, Henderson described an
optical parametric oscillator (OPO) that efficiently converts a
near-infrared laser beam to tunable mid-infrared wavelength output.
In some embodiments, the OPO includes an optical resonator
containing a nonlinear crystal, such as periodically-poled lithium
niobate. The OPO is pumped by a continuous-wave fiber-laser source
having a low-power oscillator and a high-power amplifier, or using
just a power oscillator). The fiber oscillator produces a
single-frequency output defined by a distributed-feedback (DFB)
structure of the fiber. The DFB-fiber-laser output is amplified to
a pump level consistent with exceeding an oscillation threshold in
the OPO in which only one of two generated waves ("OPO-signal" and
"OPO-idler") is resonant within the optical cavity. This pump
source provides the capability to tune the DFB fiber laser by
straining the fiber (using an attached piezoelectric element or by
other means) that allows the OPO to be continuously tuned over
substantial ranges, enabling rapid, wide continuous tuning of the
OPO output frequency or frequencies.
[0020] U.S. Pat. No. 6,654,392 issued Nov. 25, 2003 to Arbore et
al. entitled "Quasi-monolithic tunable optical resonator," which is
hereby incorporated herein by reference, describes an optical
resonator having a piezoelectric element attached to a
quasi-monolithic structure that defines an optical path. Mirrors
attached to the structure deflect light along the optical path. The
piezoelectric element controllably strains the quasi-monolithic
structure to change a length of the optical path by about 1 micron.
A first feedback loop coupled to the piezoelectric element provides
fine control over the cavity length. The resonator may include a
thermally actuated spacer attached to the cavity and a mirror
attached to the spacer. The thermally actuated spacer adjusts the
cavity length by up to about 20 microns.
[0021] A monolithic resonator typically includes a single block of
transparent material having reflecting facets that serve as the
mirrors. Usually, the material is strained by changing its
temperature. U.S. Pat. No. 4,829,532 issued May 9, 1989 to Kane,
which is hereby incorporated herein by reference, describes an
alternative where the optical path length of a monolithic
oscillator can be adjusted by a piezoelectric element mounted to
uniformly strain the entire block in a plane parallel to the plane
of the optical path.
[0022] U.S. Pat. No. 8,035,083 issued Oct. 11, 2011 to Kozlov et
al., titled "Terahertz tunable sources, spectrometers, and imaging
systems," is incorporated herein by reference. Kozlov et al.
describe a source of terahertz radiation at a fundamental terahertz
frequency that is tunable over a fundamental terahertz-frequency
range, and is coupled into a first waveguide. The first waveguide
supports only a single transverse spatial mode within the
fundamental terahertz frequency range. A solid-state frequency
multiplier receives from the first waveguide the terahertz
radiation and produces terahertz radiation at a harmonic terahertz
frequency. A second waveguide receives the harmonic terahertz
radiation. The tunable terahertz source can include a backward-wave
oscillator with output tunable over about 0.10-0.18 THz, 0.18-0.26
THz, or 0.2-0.37 THz. The frequency multiplier can include at least
one varistor or Schottky diode, and can include a doubler, tripler,
pair of doublers, doubler and tripler, or pair of triplers. The
terahertz source can be incorporated into a terahertz spectrometer
or a terahertz imaging system.
[0023] U.S. Pat. No. 7,421,171 issued Sep. 2, 2008 to Ibanescu et
al., titled "Efficient terahertz sources by optical rectification
in photonic crystals and meta-materials exploiting tailored
transverse dispersion relations," is incorporated herein by
reference. Ibanescu et al. describe generating terahertz (THz)
radiation. Their system includes a photonic-crystal structure
including at least one nonlinear material that enables optical
rectification. The photonic-crystal structure is configured to have
the suitable transverse dispersion relations and enhanced density
photonic states so as to allow THz radiation to be emitted
efficiently when an optical or near-infrared pulse travels through
the nonlinear part of the photonic crystal.
[0024] U.S. Pat. No. 7,473,898 issued Jan. 6, 2009 to Holly et al.,
titled "Cryogenic terahertz spectroscopy," is incorporated herein
by reference. Holly et al. describe a terahertz spectroscopy system
that includes a source of terahertz radiation, a detector of
terahertz radiation, a source of sample gas, and a sample cell that
can be cooled to cryogenic temperatures. The sample cell may be
configured to receive the sample gas, received terahertz radiation
from the source of terahertz radiation, provide the terahertz
radiation to the detector after the terahertz radiation has passed
through the sample gas, and facilitate cryogenic cooling thereof.
The sample cell may be cryogenically cooled to freeze the sample
gas and subsequently warmed either continuously or in steps in
temperature so that individual components or groups of components
of the sample gas may evaporate and thus have absorption spectra
formed therefor.
[0025] U.S. Patent Application Publication US 2005/0018298 of Trotz
et al., published Jan. 27, 2005 and titled "Method and apparatus
for generating terahertz radiation," is incorporated herein by
reference. Trotz et al. describe generating terahertz radiation.
Their terahertz source is described as a versatile terahertz device
that can be configured to transmit a plurality of wavelengths,
thereby facilitating the detection of multiple contaminants using a
single source device. In one embodiment, the Smith-Purcell
radiation effect is exploited by passing an electron beam over a
modulated conducting surface, wherein the spacing of the periods of
the modulated surface is varied. The variations in the modulated
surface enable the source to produce light of varying
wavelengths.
[0026] U.S. Pat. No. 7,781,737 issued Aug. 24, 2010 to Zhdaneev,
titled "Apparatus and methods for oil-water-gas analysis using
terahertz radiation," is incorporated herein by reference. Zhdaneev
describes analyzing gas-oil-water compounds in oilfield and other
applications using terahertz radiation. A sample analyzer includes
a sample chamber having a fluid communication port configured to
receive the sample. The analyzer also includes a filter to filter
samples and selectively remove oil, water or gas from reservoir
mixture received by the sample chamber. A terahertz (THz) radiation
detector is provided in electromagnetic communication with the
sample. The terahertz detector provides a detected output signal
indicative of the terahertz electromagnetic radiation detected from
the sample. In some embodiments, the device also includes a
terahertz source illuminating the sample, the terahertz detector
detecting a portion of the terahertz source illumination as
modified by the sample. The detected portion of the spectrum of
terahertz radiation can be processed to analyze the composition of
the sample.
[0027] U.S. Pat. No. 7,995,628 issued Aug. 9, 2011 to Wu, titled
"Recycling pump-beam method and system for a high-power terahertz
parametric source," is incorporated herein by reference. Wu
describes the fabrication of a portable high-power terahertz beam
source that can produce what Wu calls a tunable, high-power
terahertz beam over the frequency from 0.1 THz to 2.5 THz. Wu's
terahertz source employs a recycling pump beam method and a beam
quality-control device. The beam quality-control device may or may
not be required for a high-power terahertz beam generation. In
exemplary embodiments, a lithium niobate (LiNbO.sub.3) crystal or a
lithium niobate crystal doped with 5% magnesium oxide
(LiNbO.sub.3:MgO) can be used. Other nonlinear optical crystals,
including GaSe can be used in place of the LiNbO.sub.3 crystal.
Through proper alignment of a pump beam, along with recycling a
pump beam, high conversion efficiency is achieved, and a high
output power beam is produced at terahertz frequencies.
[0028] U.S. Pat. No. 7,391,561 (Attorney Docket 5032.008US1) titled
"Fiber- or rod-based optical source featuring a large-core,
rare-earth-doped photonic-crystal device for generation of
high-power pulsed radiation and method," which issued to Di
Teodoro, et al. on Jun. 24, 2008, is incorporated herein by
reference. Di Teodoro, et al. describe a photonic-crystal fiber
having a very large core while maintaining a single transverse
mode. The typical problems of multiple-modes and mode hopping,
which result from the use of large-diameter waveguides, are
addressed by the invention. By using multiple small waveguides in
parallel, large amounts of energy can be passed through a laser,
but with better control such that the aforementioned problems can
be reduced. An additional advantage is that the polarization of the
light can be better maintained as compared to using a single fiber
core.
[0029] There is still a heretofore unmet need in the art for an
improved method and apparatus for high-power fiber-laser-based
gigahertz-to-terahertz, millimeter-wave, signal sources for
advanced sensors, photonics, and optical computing.
BRIEF SUMMARY OF THE INVENTION
[0030] Terahertz technology is a very promising technology for both
defense and commercial applications. In some embodiments, the
present invention provides a high-power fiber-laser-based
gigahertz-to-terahertz, millimeter-wave, signal-generation
apparatus that provides key wanted features in a compact and
light-weight package. In some embodiments, the present invention
offers at least ten times the output power as previous conventional
sources, while having multiple functions incorporated in one
system.
[0031] In some embodiments, the present invention uses a high-power
infrared (IR) laser (e.g., an IR fiber laser provides an
advantageous solutions in some environments) as a source of
pump-laser energy to pump a tunable optical parametric oscillator
(OPO) crystal plus a difference-frequency-generation (DFG) crystal
to generate narrow-linewidth, tunable and high-power signals in the
GHz to THz range a compact and light-weight package while having an
option to use residual high-power IR laser beam (i.e., the pump,
OPO-idler and/or OPO-signal wavelengths) for other useful
applications in the same instrument. In some embodiments, a single
crystal is used for both the OPO and the DFG. The OPO function is
used to receive a pump frequency and use that energy to generate an
OPO-idler frequency and an OPO-signal frequency that differ from
one another by the desired THz or GHz frequency that is to be
output. For example, in some embodiments, an infrared (IR) pump
wavelength of 1060 nm has a frequency of 283.0188679 THz, and is
used to generate OPO-signal and OPO-idler wavelengths of, e.g.,
about 2112.5357072 nm and about 2127.517228 nm (corresponding to
OPO-signal and OPO-idler frequencies of about 142.009434 THz and
about 141.009434 THz), which are then used as
electromagnetic-radiation inputs to a DFG that outputs the
frequency difference of about 1.00000002 THz.
[0032] In some embodiments, the OPO and the DFG are located in the
same cavity, in some embodiments, the OPO and the DFG are
implemented using the same type of non-linear optical crystal, and
in some embodiments, the OPO and the DFG are implemented using the
same non-linear optical crystal.
[0033] Some embodiments provide an apparatus and a related method
for high-power multi-function millimeter-wavelength (THz-frequency)
signal generation using an OPO and a DFG in a single cavity. In
some embodiments, the OPO-DFG cavity includes an optical parametric
oscillator (OPO) non-linear material that receives pump light
I.sub.P having pump-light frequency and generates two lower
intermediate frequencies of light--an OPO-signal beam I.sub.S and a
spatially overlapping OPO-idler beam I.sub.I. A
difference-frequency generator non-linear material then receives
the two intermediate-frequency beams I.sub.I and I.sub.S, and the
DFG then generates a THz-frequency output signal that has a
frequency equal to the difference between the two intermediate
frequencies. In some embodiments, a single-piece crystal of
non-linear material is used for both OPO and DFG functions. Some
embodiments use a bow-tie ring having four mirrors that define four
corners of the bow-tie-shaped optical path: a frequency-selective
I.sub.P-beam-entry mirror (in some embodiments, this first mirror
is highly transmissive to I.sub.P and highly reflective to I.sub.S
and/or I.sub.I), a frequency-selective I.sub.P-light-extraction
mirror (in some embodiments, this second mirror is highly
transmissive to I.sub.P and highly reflective to I.sub.S and/or
I.sub.I) to remove unconverted I.sub.P-beam, a frequency-selective
or partially transmissive I.sub.I-beam-extraction mirror (in some
embodiments, this third mirror is partially transmissive to I.sub.I
and highly reflective to I.sub.S), and a frequency-selective or
partially transmissive I.sub.S-beam-extraction mirror (in some
embodiments, this fourth mirror is partially transmissive and
partially reflective to I.sub.S), and optionally a fifth
I.sub.THz-beam-extraction mirror within the bow-tie optical path.
Alternatively, the I.sub.P-light-extraction mirror is highly
transmissive to both I.sub.THz and I.sub.P, and is used to extract
both the unconverted I.sub.P light and the I.sub.THz output beam,
which are then separated by a beam-separation mirror external to
the bow-tie path. Other embodiments use two diffraction gratings to
split and recombine the two intermediate frequencies (or to select
one of the intermediate-frequency beams (either the OPO-signal beam
I.sub.S or the OPO-idler beam I.sub.I) as the
fixed-intermediate-frequency circulating beam and to dump the other
one of the intermediate-frequency beams, and two corner mirrors,
which together define the bow-tie optical path.
[0034] Other embodiments use two diffraction gratings, one used to
split the two intermediate frequencies into separate beams, and the
other grating used to recombine the two intermediate-frequency
beams, and two (or more, if separate mirrors are used for the two
beams) corner mirrors. In some such embodiments, one or more
etalons are used to frequency-filter each of the two
intermediate-frequency beams independently to their respective
frequencies, and two cylindrical mirrors are used to re-converge
each of the two separated beams toward the second diffraction
grating. In some such embodiments, a different piezo-electric
actuator is attached to each of the two mirrors that are used to
re-converge each of the two separated beams toward the second
diffraction grating, wherein the two piezo-electric actuators allow
independent adjustment of the lengths of the cavity as seen by each
intermediate beam, for tuning purposes.
[0035] One advantage of this solution is that it provides a
light-weight system that outputs a tunable, high-power, THz and/or
GHz signal that, in some embodiments, has a narrow linewidth.
BRIEF DESCRIPTION OF THE FIGURES
[0036] Each of the items shown in the following brief description
of the drawings represents some embodiments of the present
invention.
[0037] FIG. 1A is a graphical representation of the power ranges
(vertical axis) and frequency ranges (horizontal axis) available
from each of a plurality of conventional THz sources.
[0038] FIG. 1B is a graphical representation of the power range
(vertical axis) and frequency range (horizontal axis) available
from embodiments of the present invention, as compared to each of a
plurality of conventional THz sources as shown in FIG. 1A.
[0039] FIG. 1C is a block diagram of a distributed-feedback fiber
laser 100 having a tuning mechanism.
[0040] FIG. 2 is a block diagram of a system 200 having a
two-mirror combined OPO and DFG pumped by a fiber laser.
[0041] FIG. 3A is a block diagram of an alternative four-mirror
bow-tie combined OPO and DFG cavity device 301.
[0042] FIG. 3B is a block diagram of another four-mirror bow-tie
combined OPO and DFG cavity device 302.
[0043] FIG. 4A is a block diagram of another four-mirror bow-tie
combined OPO and DFG cavity device 401.
[0044] FIG. 4B is a block diagram of another four-mirror bow-tie
combined OPO and DFG cavity device 402.
[0045] FIG. 5A is an elevation-view block diagram of another
four-mirror bow-tie combined OPO and DFG cavity device 501.
[0046] FIG. 5B is a plan-view block diagram of the four-mirror
bow-tie combined OPO and DFG cavity device 501 shown in FIG.
5A.
DETAILED DESCRIPTION OF THE INVENTION
[0047] Although the following detailed description contains many
specifics for the purpose of illustration, a person of ordinary
skill in the art will appreciate that many variations and
alterations to the following details are within the scope of the
invention. Very narrow and specific examples are used to illustrate
particular embodiments; however, the invention described in the
claims is not intended to be limited to only these examples, but
rather includes the full scope of the attached claims. Accordingly,
the following preferred embodiments of the invention are set forth
without any loss of generality to, and without imposing limitations
upon the claimed invention. Further, in the following detailed
description of the preferred embodiments, reference is made to the
accompanying drawings that form a part hereof, and in which are
shown by way of illustration specific embodiments in which the
invention may be practiced. It is understood that other embodiments
may be utilized and structural changes may be made without
departing from the scope of the present invention.
[0048] The embodiments shown in the Figures and described here may
include features that are not included in all specific embodiments.
A particular embodiment may include only a subset of all of the
features described, or a particular embodiment may include all of
the features described.
[0049] The leading digit(s) of reference numbers appearing in the
Figures generally corresponds to the Figure number in which that
component is first introduced, such that the same reference number
is used throughout to refer to an identical component which appears
in multiple Figures. Signals and connections may be referred to by
the same reference number or label, and the actual meaning will be
clear from its use in the context of the description.
[0050] FIG. 1A is a graphical representation of the approximate
power ranges (vertical axis) and frequency ranges (horizontal axis)
available from each of a plurality of conventional THz sources.
Conventional OR THz sources (with power/frequency ranges of oval
111) appear to have a power range of approximately
5.times.10.sup.-8 watts (5.times.10.sup.-5 mW) to approximately
1.times.10.sup.-5 watts (1.times.10.sup.-2 mW), and a frequency
range of approximately 100 GHz (0.1 THz) to approximately 30 THz.
Conventional PCA THz sources (with power/frequency ranges of oval
112) appear to have a power range of approximately
5.times.10.sup.-8 watts (5.times.10.sup.-5 mW) to approximately
2.times.10.sup.-4 watts (2.times.10.sup.-1 mW), and a frequency
range of approximately 100 GHz (0.1 THz) to approximately 5 THz.
Conventional CO.sub.2-based laser-frequency-mixing THz sources
(with power/frequency ranges of oval 113) appear to have a power
range of approximately 5.times.10.sup.-6 watts (5.times.10.sup.-3
mW) to approximately 5.times.10.sup.-4 watts (2.times.10.sup.-1 mW)
and a frequency range of approximately 100 GHz (0.1 THz) to
approximately 50 THz, and a frequency range of approximately 200
GHz (0.2 THz) to approximately 5 THz. Conventional traditional DFG
THz sources (with power/frequency ranges of oval 114) appear to
have a power range of approximately 1.times.10.sup.-4 watts (0.1
mW) to approximately 0.1 watts (100 mW), and a frequency range of
approximately 200 GHz (0.2 THz) to approximately 5 THz.
Conventional optically pumped laser THz sources (with
power/frequency ranges of oval 115) appear to have a power range of
approximately 5.times.10.sup.-2 watts (50 mW) to approximately 0.5
watts (500 mW), and a frequency range of approximately 100 GHz (0.1
THz) to approximately 5 THz. Conventional quantum-cascade-laser THz
sources (with power/frequency ranges of oval 116) appear to have a
power range of approximately 0.5 watts (500 mW) to approximately 5
watts (5,000 mW), and a frequency range of approximately 800 GHz
(0.8 THz) to approximately 2 THz. Conventional p-Ge-laser THz
sources (with power/frequency ranges of oval 117) appear to have a
power range of approximately 5 watts to approximately 100 W, and a
frequency range of approximately 800 GHz (0.8 THz) to approximately
2 THz.
[0051] FIG. 1B is a graphical representation of the power range
(vertical axis) and frequency range (horizontal axis) available
from embodiments of the present invention, as compared to each of a
plurality of conventional THz sources as shown in FIG. 1A. In some
embodiments, the OPO-DFG THz sources (with power/frequency ranges
of oval 118) typically have a power range of approximately 0.01
watts to 100 W or more, and a frequency range of 30 GHz (0.03 THz)
or less, to 50 THz or more.
[0052] In some embodiments, the present invention uses a high-power
infrared (IR) laser--in some embodiments, a fiber laser--to pump a
unit including tunable OPO plus DFG
(difference-frequency-generation) crystals to generate
narrow-linewidth, tunable and high-power THz sources in a compact
and light-weight package, while having an option to use residual
high-power IR laser beam for other useful applications.
[0053] FIG. 1C is a block diagram of a distributed-feedback fiber
laser 100 having a tuning mechanism, which, in some embodiments, is
used as the pump source or as the master-oscillator seed pump
signal to a power amplified (MOPA) pump-light source for the
present invention. In some embodiments, the present invention
provides an optical-parametric-oscillator
difference-frequency-generator (OPO-DFG) THz source having
continuous tuning that is enabled by using a distributed-feedback
fiber laser 100 such as shown in FIG. 1C (and described in U.S.
Pat. No. 7,620,077 titled "Apparatus and method for pumping and
operating optical parametric oscillators using DFB fiber lasers,"
which is incorporated herein by reference in its entirety) as its
pump source. One embodiment of such a distributed-feedback fiber
laser 100 includes an optical fiber 120 having a core 122 and one
or more cladding layers 121. In some embodiments, a portion of
fiber 120 includes a distributed-feedback grating 123 having a gap
124 that has a length .delta.l.sub.GAP=.lamda..sub.B/4, or
.lamda..sub.B(N+0.25) where N is an integer. In some embodiments, a
tuning mechanism (such as, for example, a PZT (piezo-electric
element) and/or a heating element used to stretch the length of
grating 123 and/or gap 124) is used to change the wavelength of the
laser light output of DFB fiber laser pump source 100. In some
embodiments, DFB-pump light 125 of a suitable wavelength (e.g., in
some embodiments, 980 nm wavelength is used for the optical
excitation 125 to pump the DFB laser 100) is input to the DFB
laser, and laser light 126 and 127 is output having a wavelength
useful for pumping any of the OPO-DFG devices described below. In
some embodiments, pump laser 100 is used for master oscillator 211
described below.
[0054] FIG. 2 is a block diagram of a system 200 having a
two-mirror combined OPO and DFG pumped by a fiber laser. In some
embodiments, system 200 includes a pump laser 210 and an OPO-DFG
220. In some embodiments, pump laser 210 includes a master
oscillator 211 (e.g., a DFB tunable fiber laser such as described
above in FIG. 1C, in some embodiments) and an optical power
amplifier 212. In other embodiments, only a power master oscillator
211 (e.g., a powerful DFB tunable fiber laser) is used, when that
provides sufficient laser pump light power by itself. In some
embodiments, fiber connector 213 connects pump laser 210 to OPO-DFG
220. In some embodiments, OPO-DFG 220 includes a first collimating
lens 221, a Faraday isolator 222, and a second collimating lens 223
that together condition the pump light for injection into OPO-DFG
resonator 230, which, in some embodiments, is tuned (e.g., by PZT
227 moving mirror 226 to adjust the length of the OPO-DFG cavity
between input mirror 224 and output mirror 226) to resonate at the
OPO-signal wavelength (and/or (in some embodiments) at the
OPO-idler wavelength and/or (in some embodiments) at the pump
wavelength). In some embodiments, the OPO-signal frequency is kept
at a fixed frequency, such that as the pump wavelength is changed,
the OPO-idler frequency is also changed in order to tune the
frequency of the THz output signal, such that the DFG non-linear
crystal generates a THz signal having a frequency equal to the
difference between the OPO-signal frequency and the OPO-idler
frequency. In some embodiments, a non-linear crystal 225 (such as,
e.g., periodically poled lithium niobate (PPLN)) is used to convert
the pump-wavelength light (having the shortest wavelength) into
OPO-signal-wavelength light (having a wavelength between that of
the pump wavelength and that of the OPO-idler wavelength) and
OPO-idler-wavelength light (having the longest wavelength). In some
embodiments, M2 mirror 227 is wavelength-selective such that it
passes the THz output signal and perhaps some pump, OPO-signal
and/or OPO-idler light. In some embodiments, lens 229 collimates,
and wavelength-selective mirror 231 removes, the pump light
(directed downward) and they pass THz output signal 250 as output
to the right. The OPO-DFG resonator cavity 230 is generally desired
to resonate at only a single wavelength (e.g., the OPO-signal
wavelength if OPO-idler-wavelength light is the tunable
variable-wavelength input to the DFG 226 (for example, the longer
wavelength is the tunable wavelength that is input to the DFG 226),
or the OPO-idler wavelength if OPO-signal-wavelength light is the
variable-wavelength input to the DFG 226 (for example, the shorter
wavelength is the tunable wavelength that is input to the DFG
226)). In some embodiments, mirror 227 (which, in some embodiments,
is adjusted using piezo element 228 to tune the fixed resonant
wavelength (either OPO-signal or OPO-idler wavelength)) is made
substantially transparent or non-reflective at the pump wavelength
and at the wavelength of the variable-wavelength input to the DFG
226, in order that those wavelengths do not become resonant in the
cavity (in some embodiments, if two or more wavelengths (of the
pump, OPO-signal, and OPO-idler wavelengths) become resonant, then
wavelength and/or amplitude instabilities ensue since power may
shift between the resonant wavelengths). To avoid such
instabilities, some (or most) embodiments of the present invention
are designed to resonate at a single wavelength (of the pump,
OPO-signal, and OPO-idler wavelengths), and some or all of the
mirrors are designed to transmit other wavelengths that may
arise.
[0055] FIG. 3A is a block diagram of an alternative four-mirror
bow-tie combined OPO and DFG cavity device 301. In some
embodiments, OPO-DFG 320 accepts input pump light from fiber 310
(in some embodiments, coming from a tunable DFB fiber laser, not
shown), and includes a fiber-pigtail-input Faraday isolator 322,
and a collimating lens 323 that together condition the pump light
for injection into OPO-DFG resonator 330, which, in some
embodiments, is tuned (e.g., by PZT 327 moving mirror 331 to adjust
the length of the OPO cavity between input mirror 324 and itself
(i.e., the path that returns to mirror 324) by way of output mirror
326, movable mirror 331, and mirror 332) to resonate at the
OPO-signal wavelength (and/or the OPO-idler wavelength and/or the
pump wavelength). In some embodiments, moving mirror 331 is highly
reflective at the frequency of OPO-signal I.sub.S, and transmissive
at the frequency of OPO-idler I.sub.I, such that OPO-idler I.sub.I
leaves the bow-tie ring 391 as I.sub.I output beam 351. In some
embodiments, an etalon 333 that passes just the OPO-signal
wavelength is also used (in addition to movable mirror 331), or
alternatively used, to tune the resonant OPO-signal wavelength Xs
to match the path length of bow-tie ring 391. In some embodiments,
mirror 332 is partially reflective and partially transmissive at
the frequency of OPO-signal I.sub.S, such that some of OPO-signal
I.sub.S leaves the bow-tie ring 391 as I.sub.S output beam 352. In
some embodiments, the OPO-signal frequency (of light I.sub.S) is
kept at a fixed frequency, such that as the pump frequency of pump
light I.sub.P is changed, the OPO-idler frequency (of light
I.sub.I) is also changed, and thus the THz output-signal frequency
is changed by the same amount. In other embodiments, a
fixed-frequency pump light I.sub.P is used, a fixed-frequency light
I.sub.I is used, a fixed-frequency light I.sub.S is used, and a
fixed-frequency THz output signal is generated.
[0056] In some embodiments, a non-linear crystal 325 (such as,
e.g., PPLN) is used to convert the pump-wavelength light I.sub.P
(the photons having the highest frequency) into
OPO-signal-wavelength light I.sub.S (photons having a frequency
between the pump frequency and the OPO-idler frequency, which enter
the DFG crystal 335) and OPO-idler-wavelength light I.sub.I
(photons having the lowest intermediate frequency entering the DFG
crystal 335). In some embodiments, non-linear crystal 325 and/or
non-linear crystal 335 are/is heated and kept at a constant
temperature in an oven. In some embodiments, non-linear crystal 325
is touching (as shown in FIG. 3A) or closely adjacent to non-linear
crystal 335, while in other embodiments (not shown here), there is
a spatial separation between non-linear crystal 325 and non-linear
crystal 335. In some other embodiments, the cavity 330 is made to
be resonant at the OPO-idler frequency, and in this case, the
OPO-signal frequency (i.e., a frequency higher than the OPO-idler
frequency) is the variable-frequency and/or non-resonant light
entering DFG 335 to produce the THz output signal electromagnetic
radiation 350. In some embodiments, M2 mirror 326 is
wavelength-selective such that it reflects some or all of the
resonant and/or non-resonant intermediate-frequency light (i.e.,
the OPO-idler light I.sub.I and/or OPO-signal light I.sub.S, as the
case may be, is reflected (directed leftward and slightly
downward)) and but passes the unconverted pump light and the THz
output signal I.sub.THz. In some embodiments, wavelength-selective
mirror 353 passes the pump light I.sub.P 329 and any leaked
OPO-idler light I.sub.I and/or OPO-signal light I.sub.S (directed
rightward), but reflects THz output-signal electromagnetic
radiation I.sub.THz 350 as output (upward in FIG. 3A). In some
other embodiments (not shown here), wavelength-selective mirror 353
reflects the pump light I.sub.P, and any leaked OPO-idler light
I.sub.I and/or OPO-signal light I.sub.S (directed upward) and
passes THz output-signal electromagnetic radiation I.sub.THz 350 as
output (which would be rightward if depicted in FIG. 3A).
[0057] In some embodiments, non-linear crystal 325 and/or
non-linear crystal 335 have/has periodically alternating
ferroelectric domain structures that vary in period (the poling
period) across the width of the crystal (such crystals are called
periodically poled; e.g., PPLN is periodically poled lithium
niobate). In some embodiments, the sideways positioning of
non-linear crystal 325 and/or non-linear crystal 335 can be varied,
in order to vary the poling period encountered by the light
propagating through the crystal(s). The OPO-DFG resonator 320
includes the bow-tie ring path 391 (i.e., the bow-tie ring 391
being the optical path from mirror 324, through OPO 325 and DFG 335
to mirror 326, then to mirror 331, then through etalon 333 to
mirror 332, and finally back to mirror 324), configured to avoid
unwanted double or triple resonances by circulating only one of the
two intermediate-frequency beams (e.g., OPO-signal light I.sub.S)
and to maintain a sufficient amount of pump light I.sub.P and a
sufficient amount of OPO-signal light I.sub.S such that OPO 325 is
above threshold in order to generate the OPO-idler light I.sub.I,
such that amounts of the OPO-signal light I.sub.S and OPO-idler
light I.sub.I needed by DFG 335 to generate the THz output light
are maintained. When OPO-signal light I.sub.S is the frequency that
circulates around bow-tie ring path 391, the amount of OPO-signal
light I.sub.S exceeds the amount of OPO-idler light I.sub.I, since
one photon of OPO-pump light I.sub.P and one photon of OPO-signal
light I.sub.S will cause the loss of the one OPO-pump light I.sub.P
photon and the emission of one photon of OPO-idler light I.sub.I
and one photon of OPO-signal light I.sub.S in addition to the
starting one photon of OPO-signal light I.sub.S. In other
embodiments, it is the OPO-idler light I.sub.I (the lower-frequency
of the two intermediate beams) that is circulated around bow-tie
ring path 391 (including through etalon 333), and the OPO-signal
light I.sub.S is removed by mirror 331 and/or etalon 333.
[0058] FIG. 3B is a block diagram of another four-mirror bow-tie
combined OPO and DFG cavity device 302 having certain parts
corresponding to those of FIG. 3A, and showing a MOPA with
master-oscillator (the DFB fiber laser 305) and power-amplifier
(the fiber amplifier 308) shown. In contrast to the device 301
embodiment shown in FIG. 3A, device 302 separates OPO crystal 325
from DFG crystal 335 so that each is in a separate respective leg
of the bow-tie ring 392 (i.e., bow-tie ring 392 being the optical
path from mirror 324, through OPO 325 to mirror 346, then through
DFG 335 and mirror 353 to mirror 331, then through etalon 333 to
mirror 332, and finally back to mirror 324), which allows removal
of the pump light I.sub.P through wavelength-selective mirror 346
before the OPO-idler light I.sub.I and OPO-signal light I.sub.S
enter the DFG crystal 335 to be converted to electromagnetic
radiation of the THz output-signal frequency.
[0059] In some embodiments, fiber laser 309 (e.g., in some
embodiments, seeded by a DFB fiber laser such as shown in FIG. 1C;
or in other embodiments, seeded by a Q-switched fiber laser 305) is
optically pumped from one or more laser diodes, is configured in a
MOPA arrangement, which optionally uses one or more high-power
photonic-crystal amplifiers as fiber amplifier 308 (e.g., shown
here as a loop of fiber, but alternatively using a laser and/or
power amplifier such as those described in commonly-assigned U.S.
Pat. No. 7,391,561 titled "Fiber- or rod-based optical source
featuring a large-core, rare-earth-doped photonic-crystal device
for generation of high-power pulsed radiation and method," which is
incorporated herein by reference). In some embodiments, laser 309
includes a fiber isolator 306, fiber connector 307 (to fiber
amplifier 308) and output fiber 310 (the diode pump laser(s), not
shown, is/are coupled using methods well known to the art, such as
shown in U.S. Pat. No. 7,391,561).
[0060] In some embodiments, the output beam of pump laser 309 is
collimated using lens 341, reflected by mirror 342, passed through
one-way bulk isolator 322, and reflected by mirror 343 through
focusing lens 323 that acts to focus (taking into account the
focusing effects of mirror 324) the pump beam at the center of the
PPLN 325. Pump-extraction mirror 346 is transparent to the pump
frequency (which is output through mirror 326) (in order to prevent
a doubly or triply resonant cavity), while being highly reflective
to light at the OPO-signal frequency and the OPO-idler frequency.
The coincident beams I.sub.S and I.sub.I containing the
intermediate OPO-signal frequency and the OPO-idler frequency are
used by DFG 335 to generate the THz output that is reflected off
frequency-selective mirror 353 to become THz output 350. In some
embodiments, frequency-selective mirror 353 passes the unconverted
portions of intermediate OPO-signal I.sub.S and OPO-idler I.sub.I.
In some embodiments, the mirror 331 is highly reflective at the
resonant frequency (e.g., the OPO-signal frequency), and highly (or
at least partially) transmissive at the non-resonant intermediate
(e.g., OPO-idler) frequency, the etalon 333 is tuned to be
transparent only at the resonant (e.g., OPO-signal) frequency
(blocking or reflecting any of the other intermediate frequency
that may have been reflected by mirror 331), and mirror 332 is
highly reflective at the resonant (e.g., OPO-signal) frequency,
such that only that frequency returns to input mirror 324. In some
other embodiments, the mirror 331 need not be transmissive at the
non-resonant intermediate (e.g., OPO-idler) frequency since that
frequency should be blocked by etalon 333.
[0061] In some embodiments, as described further below, one or more
of the mirrors 324, 326, 331, and/or 332 is partially transparent
to the OPO-signal frequency and/or OPO-idler frequency (e.g., one-
to five-percent transparent in total), in order to prevent
excessive buildup of light at that/those frequency(ies) in the
cavity, which would tend to overheat the crystal(s) 325 and/or 335.
In some embodiments (as shown in FIG. 3B), the THz output signal
350 is extracted by reflection at frequency-selective mirror 353
within the bow-tie ring. In some other embodiments (not shown),
mirror 353 is omitted and the THz output signal 350 is extracted by
transmission through a THz-transmissive, I.sub.S-reflective and/or
I.sub.I-reflective, frequency-selective mirror that replaces mirror
331. Thus, in some embodiments, the unconverted pump light I.sub.P
is removed from the bow-tie ring of resonator 330 once it leaves
OPO 325 and before the remaining OPO-signal light I.sub.S and
OPO-idler light I.sub.I enter the DFG 335 (e.g., in the embodiment
shown, unconverted pump light I.sub.P passes through
frequency-selective mirror 346, while the remaining OPO-signal
light I.sub.S and OPO-idler light I.sub.I are reflected toward DFG
335). Further, in some embodiments, the THz output electromagnetic
radiation I.sub.THz is removed from the bow-tie ring of resonator
330 once it leaves DFG 335 and before the remaining OPO-signal
light I.sub.S and OPO-idler light I.sub.I impinge on mirror 331
and/or etalon 333, which allows one of those to pass (e.g., in some
embodiments, remaining OPO-signal light I.sub.S), while the other
OPO-idler light I.sub.I is blocked and/or removed from the bow-tie
ring).
[0062] FIG. 4A is a block diagram of another four-mirror bow-tie
combined OPO and DFG cavity device 401. In some embodiments, pump
source 411 supplies pump light I.sub.P that passes through
frequency-selective mirror 324 that is highly transmissive to the
frequency of pump light I.sub.P and highly reflective to the
frequency of at least one of the intermediate-frequency beams of
light I.sub.S and/or I.sub.I. In some embodiments, the pump light
I.sub.P and OPO-signal light I.sub.S enter OPO crystal 325, and
stimulate absorption of pump light I.sub.P and emission of
OPO-idler light I.sub.I and additional OPO-signal light I.sub.S
(denoted as I.sub.P+I.sub.I+I.sub.S propagating rightward in the
FIG. 4A). In some embodiments, pump-extraction mirror 346 is highly
transmissive to the frequency of pump light I.sub.P, and highly
reflective to the frequency of OPO-idler light I.sub.I and to the
frequency OPO-signal light I.sub.S (denoted as I.sub.I+I.sub.S
propagating leftward and downward in FIG. 4A), that enter DFG
crystal 335, which generates the difference-frequency signal
I.sub.THz that is output, by reflection by frequency-selective
mirror 353 (which transmits the frequency of OPO-idler light
I.sub.I and to the frequency OPO-signal light I.sub.S but reflects
THz signals), as THz output signal 350. The unconverted OPO-idler
light I.sub.I and OPO-signal light I.sub.S are transmitted through
frequency-selective mirror 353 and are diffracted back (upward and
rightward) by beam-separating diffraction grating 412 (in some
embodiments, the beam of unconverted OPO-idler light I.sub.I and
OPO-signal light I.sub.S impinges on beam-separating diffraction
grating 412 at or very near to the Littrow angle (wherein the
outgoing OPO-idler light beam I.sub.I will diffract back to a small
angle to one side of the incoming beam to grating 412, and the
outgoing OPO-signal light beam I.sub.S will diffract back to a
small angle to the opposite side of the incoming beam to grating
412) in order to obtain the most light in the two primary
diffracted beams OPO-idler light I.sub.I and OPO-signal light
I.sub.S, which diffract back toward mirror 431 at slightly
different angles due to their slightly different frequencies. In
some embodiments, mirror 431 is again mounted to a piezo-electric
(PZT) actuator 327 used to tune the resonant cavity length, but
unlike mirror 331 of FIG. 3A, in some embodiments, mirror 431 is
highly reflective to both I.sub.I and I.sub.S. In some embodiments,
an additional PZT actuator (not shown here) is coupled to one or
the other of mirrors 448 or 449, so that PZT actuator 327 is used
to tune the length of cavity for the intermediate-frequency beam
that reflects from the fixed one of mirrors 448 or 449, and then
the additional PZT actuator is used to move the other one of
mirrors 448 or 449 to tune the length of the cavity seen by the
other intermediate-frequency beam. Because of the angular
separation (which is caused by beam-separating diffraction grating
412) between OPO-idler light I.sub.I and OPO-signal light I.sub.S,
separate etalons 439 and 438 are used to individually and
independently tune the respective frequencies of OPO-idler light
I.sub.I and OPO-signal light I.sub.S to obtain the desired output
frequency of THz output signal 350. In some embodiments,
cylindrical mirrors 449 and 448, respectively, are used to reverse
the divergence of the separate beams of OPO-idler light I.sub.I and
OPO-signal light I.sub.S (which divergence is caused by slightly
different angles of diffraction for frequencies at opposite edges
of the linewidth of each beam) in the plane of ring 393 and have
them converge to beam-combining diffraction grating 413, which, in
some embodiments, is also at a near-Littrow angle (wherein the
incoming OPO-idler light beam I.sub.I will impinge at a small angle
to one side of the outgoing beam from grating 413, and the incoming
OPO-signal light beam I.sub.S will impinge at a small angle to the
opposite side of the outgoing beam to grating 412, and both
frequencies will thus recombine into a single collimated beam) to
obtain maximum efficiency in diffracting and recombining the two
separate beams of OPO-idler light I.sub.I and OPO-signal light
I.sub.S into a single beam directed at mirror 324. In some
embodiments, beam-combining diffraction grating 413 (which, by
itself, cannot compensate for the spatial-divergence widening
within each beam, and thus would produce a wider beam propagating
toward OPO 325 than desired) and cylindrical mirrors 448 and 449
(which, by themselves, cannot compensate for the angular difference
between the two beams, and thus could not produce a single
collimated beam as desired) together provide the requisite
compensation to both reverse the chromatic dispersion or divergence
within each beam and the angular separation between OPO-idler light
I.sub.I and OPO-signal light I.sub.S, both of which were introduced
by beam-separating diffraction grating 412 earlier in the ring.
[0063] In some such embodiments, one etalon is used to
frequency-filter both of the two intermediate-frequency beams
(because the two beams traverse the etalon at different angles, a
single etalon (such as 537 of FIG. 5A) can be used to tune both of
two different frequencies) or in other embodiments, one or more
etalons 438 and 439 are used to frequency-filter each of the two
intermediate-frequency beams independently to their respective
frequencies, for tuning purposes. In some embodiments, two
cylindrical mirrors 448 and 449 are used to re-converge each of the
two separated beams toward the second diffraction grating. In some
such embodiments, a different piezo-electric actuator is attached
to each of the two mirrors 448 and 449 (i.e., rather than using the
single piezo-electric actuator 327 on mirror 431) that are used to
re-converge each of the two separated beams toward the second
diffraction grating 413, wherein the two piezo-electric actuators
allow independent adjustment of the lengths of the cavity 393 as
seen by each intermediate beam, for tuning purposes. In other
embodiments, single piezo-electric actuator 327 on mirror 431 is
used in conjunction with an additional single piezo-electric
actuator one or the other of mirrors 448 or 449, in order to
independently tune the lengths of the cavity seen by the two
intermediate-frequency beams. In these manners, in some
embodiments, the cavity 393 can be independently frequency tuned
(using the etalons 438-439 and/or diffraction grating 412 used in
conjunction with a mask having two slits (one for each of the two
intermediate-frequency beams) and length tuned (using the two
piezo-electric actuators) to each of the two intermediate frequency
beams, if desired.
[0064] In contrast to the device 301 embodiment shown in FIG. 3A,
device 401 of FIG. 4A separates OPO crystal 325 from DFG crystal
335 so that each is in a separate respective leg of the bow-tie
ring 393 (i.e., bow-tie ring 393 being the optical path from mirror
324, through OPO 325 to mirror 346, then through DFG 335 and mirror
353 to diffraction grating 412 and back (via slightly separate
paths for the I.sub.S and I.sub.I beams) to mirror 431, then the
I.sub.S beam goes through etalon 439 to mirror 449 and then to
grating 413, while the I.sub.I beam goes through etalon 438 to
mirror 448 and then to grating 413, where the I.sub.S and I.sub.I
beams are combined into a single beam, and finally back to mirror
324), which allows removal of the pump light I.sub.P through
wavelength-selective mirror 346 before the OPO-idler light I.sub.I
and OPO-signal light I.sub.S enter the DFG crystal 335 to be
converted to electromagnetic radiation of the THz output-signal
frequency. This bow-tie ring path 393, which separates the I.sub.S
and I.sub.I beams from one another by diffraction, allows each of
the intermediate-frequency I.sub.S and I.sub.I beams to be filtered
by an etalon specifically tuned for the two respective
frequencies.
[0065] In other embodiments (not shown), a device that is the same
as device 401 but omitting etalon 438 and mirror 448 is used, in
order that the ring 393 is resonant only to the frequency of the
I.sub.S beam (the I.sub.I beam being dumped), and only circulates
the I.sub.S beam completely around the optical path of ring 393. In
yet other embodiments (not shown), a device that is the same as
device 401 but omitting etalon 439 and mirror 449 is used, in order
that the ring 393 is resonant only to the frequency of the I.sub.I
beam (the I.sub.S beam being dumped), and only circulates the
I.sub.I beam completely around the optical path of ring 393. In
these cases, the diffraction gratings 412 and 413 and the one
etalon allow improved tuning and stability of the frequencies used
to generate the THz output signal 350. In some embodiments, some of
the optical elements that define the optical path of ring 393 are
slightly out of the plane and tilted such that the optical elements
do not interfere with the optical path 393.
[0066] FIG. 4B is a block diagram of another four-mirror bow-tie
combined OPO and DFG cavity device 402. The topology and operation
of device 402 are substantially the same as for device 401
described above. In other embodiments (not shown), a device that is
the same as device 402 but omitting etalon 438 and mirror 448 is
used, in order that the ring 394 is resonant only to the frequency
of the I.sub.S beam, and only circulates the I.sub.S beam
completely around the optical path of ring 394. In yet other
embodiments (not shown), a device that is the same as device 402
but omitting etalon 439 and mirror 449 is used, in order that the
ring 394 is resonant only to the frequency of the I.sub.I beam, and
only circulates the I.sub.I beam completely around the optical path
of ring 394. In these cases, the diffraction gratings and etalon
allow improved tuning of the frequencies used to generate the THz
output signal 350. In some embodiments of device 402 (in contrast
to some embodiment of device 401 described above), all of the
optical elements that define the optical path of ring 394 are in a
single plane and need not be tilted or out of the plane in order to
not interfere with the optical path 394. In some embodiments, this
causes the angles between segments of the optical path of ring 394
to be larger (less acute) than is the case for FIG. 5A described
below.
[0067] FIG. 5A is an elevation-view block diagram of another
four-mirror bow-tie combined OPO and DFG cavity device 501 having a
bow-tie-ring optical path 395. The topology and operation of device
501 are substantially the same as for device 401 described above,
except that device 501 uses a single etalon 537 (optionally having
a mask 536 that has one opening for the I.sub.I beam and one
opening for the I.sub.S beam) in place of the two etalons 438 and
439 of device 401, and device 501 uses a single cylindrical mirror
545 (optionally having a mask not shown here) in place of the two
cylindrical mirrors 448 and 449 of device 401. In other
embodiments, etalon 537 is omitted and mask 536, that has one slit
opening for the I.sub.I beam and one slit opening for the I.sub.S
beam, is used to select the two intermediate frequencies (in some
such embodiments, mask 536 is configured to have one fixed slit for
the fixed-frequency intermediate beam and one movable or
adjustable-position slit for the adjustable-frequency intermediate
beam). In contrast to beam-splitting diffractive grating 412 of
FIG. 4A (which diffracts the I.sub.S beam on one side of the
combined input beam that passes through output mirror 353 and
diffracts the I.sub.I beam on the opposite side), beam-splitting
diffractive grating 334 is configured to diffract the I.sub.S beam
and the I.sub.I beam both along respective angled paths that are
both above the combined input beam that passes through output
mirror 353. Also in contrast to beam-combining diffractive grating
413 of FIG. 4A (which diffracts the I.sub.S beam from one side of
the combined output beam that propagates toward mirror 324, and
diffracts the I.sub.I beam from the opposite side), beam-combining
diffractive grating 339 is configured to diffract the I.sub.S beam
and the I.sub.I beam from their respective angled paths that are
both above the combined output beam that propagates toward mirror
324.
[0068] The centers of the angle-separated I.sub.S beam and the
I.sub.I beam each impinge on the single etalon 537 at slightly
different angles, which allows the single etalon 537 to be used to
simultaneously select (i.e., provide a narrow-linewidth optical
filter function) for the two different frequencies desired for the
I.sub.S beam and the I.sub.I beam (since the different angles of
incidence provide a different spacing between the two faces of the
etalon). Similarly, the two etalon-filtered beams can both be
focussed by a single cylindrical mirror 545 (which is configured to
retro-reflect each of the two beams) to a single spot on the
recombining grating 339 in order to be spectral-beam combined into
a single beam directed toward M1 mirror 324. In some embodiments,
the angles of divergence of the I.sub.S beam and the I.sub.I beam
as they leave beam-splitting diffractive grating 334 match the
respective angles of convergence of the I.sub.S beam and the
I.sub.I beam as they impinge toward beam-combining diffractive
grating 339.
[0069] In other embodiments (not shown), one or the other of
OPO-signal beam I.sub.S and OPO-idler beam I.sub.I are blocked,
masked, or dumped such that only a single intermediate frequency
(i.e., either OPO-signal beam I.sub.S or OPO-idler beam I.sub.I)
circulates completely around ring 395.
[0070] FIG. 5B is a plan-view block diagram of the four-mirror
bow-tie combined OPO and DFG cavity device 501 shown in FIG. 5A.
This is simply a view from a different angle of the device 501
shown in FIG. 5A.
[0071] In the drawings herein, a dashed-line arrow is sometimes
used to indicate the normal vector relative to the center of the
mirror face.
[0072] In some embodiments, the present invention is mounted and
sealed in a unitary housing having at least one removable cover and
a plurality of optical ports for launching pump light I.sub.P into
the OPO-DFG and removing I.sub.THz, I.sub.S and I.sub.I light that
result from the non-linear OPO conversion and difference frequency
generation. In some embodiments, the housing is similar to that
shown and described in co-owned U.S. Pat. No. 7,620,077, which is
incorporated herein by reference.
[0073] In some embodiments, a set of up to four different-frequency
output beams (i.e., I.sub.P, I.sub.S, I.sub.I and/or I.sub.THz)
exit from the device 301, 302, 401, 402 and/or 501, each beam of
which can be used by itself, or in combination with other beams of
this set, for various purposes such as spectroscopy, LIDAR, LADAR,
materials engineering (such as heat treating and the like),
chemical processing, imaging (such as airport security searches for
hidden weapons, or locating firefighters or the like, who would
have THz reflectors or resonators on their person that would be
imaged once the THz output beam hit such a device, through smoke or
in the dark), non-lethal or lethal weapons, surgical coagulation,
cutting, and the like.
[0074] In some embodiments, the OPO and DFG are made into one
crystal with different sections having different functions.
[0075] Novel attributes of some embodiments of the present
invention include: (a.) high power, (b.) wide available spectral
range, (c.) narrow-linewidth, (d.) widely tunable, (e.) compact,
(f.) light-weight, and/or (g.) maintenance-free since they are
fiber-laser based.
[0076] One purpose of this invention is to create a high-power,
tunable, and narrow-linewidth millimeter-wave or
terahertz-frequency systems that can use converted millimeter or
terahertz waves for one set of functions, such as imaging,
spectroscopy, non-lethal weapon and the like, while using its
fundamental wavelength(s) that were not converted as output for
another set of functions, such as coagulation, cutting, lethal
weapons, and the like.
[0077] In some embodiments, the present invention uses two
narrow-linewidth OPO-generated laser seed signals (called the
OPO-signal and the OPO-idler, these are sometimes simply referred
to as "seeds"), at least one of which is tunable in some
embodiments, wherein in some embodiments, the OPO pump source uses
a DFB seed laser that in turn is pumped by one or more
semiconductor lasers. In some embodiments, these beams are operated
in pulsed modes. In some embodiments, the seed signals are
amplified in fiber amplifiers. Pulses from the two amplified seed
signals are combined and sent through nonlinear
difference-frequency-generation (DFG) optics. Electronics and/or
algorithms and beam-shaping optics are used to synchronize and
overlap precisely two pulses from the two fiber amplifiers' output
through the DFG optics. Tunability of the seed lasers is achieved
through drive conditions such as current or temperature, or
acoustic optics and the like, which are applying to key
laser-cavity elements such as grating or feedback mirrors. In some
embodiments, an external enhancement cavity is used to improve DFG
efficiency. In some embodiments, thulium-doped fiber amplifiers are
used to amplify the two 2-.mu.m (two-micron wavelength) signal
lights for both power scalability and DEC (direct evaporative
cooling) efficiency improvement. Also non-converted .about.2-.mu.m
output power can be used for a number of other applications.
Beam-shaping and -directing optics are employed to manipulate
different output beams for different sets of applications. In some
embodiments, DFG-conversion optics are controlled so that
millimeter-wavelength terahertz output power is controlled.
[0078] In contrast to the present invention, electromagnetic
signals in conventionally available commercial terahertz sources
are typically generated through either a free-electron laser or a
waveguide filled with gaseous organic molecules and generated
through high-voltage discharge through the gaseous organic
molecules. These sources are neither narrow-linewidth nor widely
tunable. Further, they typically are not available in
high-frequency ranges, and they are usually very large in size. A
quantum cascade laser (QCL) has limited output power and can only
produce certain frequencies and requires cryogenic cooling.
Terahertz generation through femtosecond lasers are usually low
power and not tunable. So, in summary, the problem posed by the
combination of requirements has not been solved before.
[0079] In some embodiments, the present invention provides an
apparatus for generating a gigahertz-terahertz-range signal having
a frequency in a gigahertz to terahertz frequency range. This
apparatus includes a pump laser that outputs pump light having a
pump frequency; and a single cavity, operably coupled to the pump
laser to receive the pump light, the single cavity having
non-linear material in an optical path in the cavity that receives
the pump light and generates light of two intermediate frequencies,
and that uses the light of the two intermediate frequencies to
generate the gigahertz-terahertz-range signal, wherein the
gigahertz-terahertz-range signal has a frequency that is equal to a
difference between the two intermediate frequencies.
[0080] In some embodiments, the optical path has a bow-tie ring
topology, and wherein the single cavity further includes: a first
frequency-selective mirror that is highly transparent to the
frequency of the pump light and through which unconverted pump
light is removed from the cavity, a second mirror that is highly
reflective to at least a fixed frequency of the two intermediate
frequencies, such that between 1% and 5% of the other of the two
intermediate frequencies is transmitted through the second mirror,
a third mirror that is highly reflective to the fixed frequency of
the two intermediate frequencies, a frequency-selective Fabry-Perot
etalon that is located in the optical path between the second
mirror and the third mirror and that is configured to pass the
fixed frequency of the two intermediate frequencies, a fourth
frequency-selective mirror that is highly reflective to the fixed
frequency of the two intermediate frequencies and highly
transparent to the frequency of the pump light and through which
the pump light is introduced into the cavity. In some such
embodiments, the non-linear material in the single cavity includes:
a first non-linear optical (NLO) crystal of that acts as an optical
parametric oscillator, and a second non-linear optical (NLO)
crystal that acts as a difference frequency generator, wherein the
first NLO crystal and the second NLO crystal are located in the
optical path between the fourth mirror and the first mirror.
[0081] In some embodiments, the optical path has a linear non-ring
topology, and the non-linear material in the single cavity includes
a single non-linear optical (NLO) crystal of that acts both as an
optical parametric oscillator and as a difference frequency
generator.
[0082] In some embodiments, the optical path has a ring topology,
and the non-linear material in the single cavity includes a single
non-linear optical (NLO) crystal of that acts both as an optical
parametric oscillator and as a difference frequency generator.
[0083] In some embodiments, the optical path has a ring topology,
and the non-linear material in the single cavity includes: a first
non-linear optical (NLO) crystal of that acts as an optical
parametric oscillator, a second non-linear optical (NLO) crystal
that acts as a difference frequency generator, and a first mirror
located in the optical path between the first NLO crystal and the
second NLO crystal, wherein the first mirror is highly reflective
at both of the two intermediate frequencies.
[0084] In some embodiments, the optical path has a ring topology,
and wherein the non-linear material in the single cavity includes:
a first non-linear optical (NLO) crystal located in the optical
path that acts as an optical parametric oscillator, a second
non-linear optical (NLO) crystal located in the optical path that
acts as a difference frequency generator, and a first mirror
located in the optical path between the first NLO crystal and the
second NLO crystal, wherein the first mirror is highly reflective
at both of the two intermediate frequencies and highly transmissive
at the pump frequency.
[0085] In some embodiments, the optical path has a bow-tie ring
topology, and the non-linear material in the single cavity
includes: a first non-linear optical (NLO) crystal of that acts as
an optical parametric oscillator, a second non-linear optical (NLO)
crystal that acts as a difference frequency generator and that has
a first face that receives light of the two intermediate
frequencies and a second face that emits the
gigahertz-terahertz-range signal, a first mirror located in the
optical path between the first NLO crystal and the second NLO
crystal, wherein the first mirror is highly reflective at both of
the two intermediate frequencies and highly transmissive at the
pump frequency, and wherein light of the two intermediate
frequencies reflected by the first mirror enters the first face of
the second NLO crystal, and a second reflector located in the
optical path facing the second face of the second NLO crystal,
wherein the second reflector is highly reflective at the
gigahertz-terahertz-frequency range, such that the
gigahertz-terahertz-range signal is reflected by the second
reflector and exits the cavity.
[0086] In some embodiments, the optical path in the single cavity
is configured to have a bow-tie ring topology.
[0087] In some embodiments, the optical path has a bow-tie ring
topology, and the apparatus further includes a unitary block
housing surrounding the single cavity, wherein the optical path is
completely within the housing, and wherein the housing has at least
one cover and a plurality of optical ports that are coupled to the
optical path.
[0088] In some embodiments, the housing also holds the pump laser
in the housing.
[0089] In some embodiments, the pump laser is configured to
controllably vary the pump frequency, and the cavity is tuned to
resonate at a fixed one of the two intermediate frequencies, such
that the other of the two intermediate frequencies varies based on
the varied pump frequency, and such that the frequency of the
terahertz-range signal is controllably varied based on the varied
pump frequency.
[0090] In some embodiments, the single cavity is arranged in a ring
topology and further includes: a first frequency-selective mirror
that is highly transparent to the frequency of the pump light and
through which unconverted pump light is removed from the cavity,
and highly transparent to the frequency of the
gigahertz-terahertz-range signal and through which the
gigahertz-terahertz-range signal is removed from the cavity, a
second mirror that is highly reflective to at least a fixed
frequency of the two intermediate frequencies, such that between 1%
and 5% of the other of the two intermediate frequencies is
transmitted through the second mirror, a third mirror that is
highly reflective to the fixed frequency of the two intermediate
frequencies, a frequency-selective Fabry-Perot etalon located in
the optical path between the second mirror and the third mirror,
and that is configured to pass the fixed frequency of the two
intermediate frequencies, a fourth frequency-selective mirror that
is highly reflective to at least the fixed frequency of the two
intermediate frequencies and highly transparent to the frequency of
the pump light and through which the pump light is introduced into
the cavity. In some such embodiments, the non-linear material in
the single cavity includes a single non-linear optical (NLO)
crystal of that acts both as an optical parametric oscillator, and
as a difference frequency generator, wherein the single NLO crystal
is located in the optical path between the fourth mirror and the
first mirror.
[0091] In some embodiments, the single cavity further includes: a
first frequency-selective mirror that is highly transparent to the
frequency of the pump light and through which unconverted pump
light is removed from the cavity, a second mirror that is highly
reflective to at least a fixed frequency of the two intermediate
frequencies, such that between 1% and 5% of the other of the two
intermediate frequencies is transmitted through the second mirror,
a third mirror that is highly reflective to the fixed frequency of
the two intermediate frequencies, a frequency-selective Fabry-Perot
etalon located in the optical path between the second mirror and
the third mirror, and that is configured to pass the fixed
frequency of the two intermediate frequencies, a fourth
frequency-selective mirror that is highly reflective to at least
the fixed frequency of the two intermediate frequencies and highly
transparent to the frequency of the pump light and through which
the pump light is introduced into the cavity. In some such
embodiments, the non-linear material in the single cavity includes:
a first non-linear optical (NLO) crystal of that acts as an optical
parametric oscillator located in the optical path between the
fourth mirror and the first mirror, and a second non-linear optical
(NLO) crystal that acts as a difference frequency generator,
wherein the first NLO crystal and the second NLO crystal are
located in the optical path between the fourth mirror and the first
mirror, and a frequency-selective reflector located in the optical
path between the second NLO crystal and the third mirror, wherein
the frequency-selective reflector is configured to pass the two
intermediate frequencies, and to reflect the
gigahertz-terahertz-range signal out of the cavity.
[0092] In some embodiments, the optical path has a ring topology,
and wherein the non-linear material in the single cavity includes
material that acts as an optical parametric oscillator and material
that acts as a difference frequency generator.
[0093] In some embodiments, the present invention provides a method
for generating a gigahertz-terahertz-range signal having a
frequency in a gigahertz to terahertz frequency range. This method
includes: receiving pump light having a pump frequency into a
single optical cavity; generating light of two intermediate
frequencies within the single cavity by using energy from the pump
light, and generating the gigahertz-terahertz-range signal within
the single cavity by using the light of the two intermediate
frequencies, wherein the gigahertz-terahertz-range signal has a
frequency that is equal to a difference between the two
intermediate frequencies.
[0094] In some embodiments of the method, the optical cavity has an
optical path that has a bow-tie ring topology, and the method
further includes: reflecting light of at least one of the two
intermediate frequencies at a first frequency-selective mirror, and
removing unconverted pump light from the cavity through the first
frequency-selective mirror, reflecting light of a fixed frequency
of the two intermediate frequencies at a second mirror, reflecting
light of the fixed frequency of the two intermediate frequencies at
a third mirror, passing light of the fixed frequency of the two
intermediate frequencies through a frequency-selective Fabry-Perot
etalon located in the optical path between the second mirror and
the third mirror, reflecting light of the fixed frequency of the
two intermediate frequencies at a fourth frequency-selective mirror
introducing the pump light through fourth frequency-selective
mirror into the cavity, converting pump light into light of the two
intermediate frequencies using non-linear optical parametric
oscillation in the optical path between the fourth mirror and the
first mirror, and converting light of the two intermediate
frequencies to electromagnetic radiation having a
gigahertz-terahertz frequency using non-linear difference frequency
generation, in the optical path between the fourth mirror and the
first mirror.
[0095] In some embodiments of the method, the optical cavity has an
optical path that has a bow-tie ring topology, and the method
further includes: reflecting light of at least one of the two
intermediate frequencies at a first frequency-selective mirror, and
removing unconverted pump light from the cavity through the first
frequency-selective mirror, reflecting light of a fixed frequency
of the two intermediate frequencies at a second mirror, reflecting
light of the fixed frequency of the two intermediate frequencies at
a third mirror, passing light of the fixed frequency of the two
intermediate frequencies through a frequency-selective Fabry-Perot
etalon located in the optical path between the second mirror and
the third mirror, reflecting light of the fixed frequency of the
two intermediate frequencies at a fourth frequency-selective mirror
introducing the pump light through fourth frequency-selective
mirror into the cavity, converting pump light into light of the two
intermediate frequencies using non-linear optical parametric
oscillation in the optical path between the fourth mirror and the
first mirror, and converting light of the two intermediate
frequencies to electromagnetic radiation having a
gigahertz-terahertz frequency using non-linear difference frequency
generation, in the optical path between the first mirror and the
second mirror.
[0096] In some embodiments, the present invention provides an
apparatus for generating a gigahertz-terahertz-range signal having
a frequency in a gigahertz to terahertz frequency range. This
apparatus includes means for receiving pump light having a pump
frequency into a single optical cavity; and means within the single
cavity for generating light of two intermediate frequencies by
using energy from the pump light, and for generating the
gigahertz-terahertz-range signal by using the light of the two
intermediate frequencies, wherein the gigahertz-terahertz-range
signal has a frequency that is equal to a difference between the
two intermediate frequencies.
[0097] In some embodiments, the optical cavity has an optical path
that has a bow-tie ring topology, and the apparatus further
includes: means for reflecting light of at least one of the two
intermediate frequencies at a first frequency-selective mirror, and
removing unconverted pump light from the cavity through the first
frequency-selective mirror, means for reflecting light of a fixed
frequency of the two intermediate frequencies at a second mirror,
and transmitting between 1% and 5% of the other of the two
intermediate frequencies through the second mirror, means for
reflecting light of the fixed frequency of the two intermediate
frequencies at a third mirror, means for passing light of the fixed
frequency of the two intermediate frequencies through a
frequency-selective Fabry-Perot etalon located in the optical path
between the second mirror and the third mirror, means for
reflecting light of the fixed frequency of the two intermediate
frequencies at a fourth frequency-selective mirror introducing the
pump light through fourth frequency-selective mirror into the
cavity, means for converting pump light into light of the two
intermediate frequencies using non-linear optical parametric
oscillation in the optical path between the fourth mirror and the
first mirror, and means for converting light of the two
intermediate frequencies to electromagnetic radiation having a
gigahertz-terahertz frequency using non-linear difference frequency
generation, in the optical path between the fourth mirror and the
first mirror.
[0098] In some embodiments, the optical cavity has an optical path
that has a bow-tie ring topology, and the apparatus further
includes: means for reflecting light of at least one of the two
intermediate frequencies at a first frequency-selective mirror, and
removing unconverted pump light from the cavity through the first
frequency-selective mirror, means for reflecting light of a fixed
frequency of the two intermediate frequencies at a second mirror,
means for reflecting light of the fixed frequency of the two
intermediate frequencies at a third mirror, means for passing light
of the fixed frequency of the two intermediate frequencies through
a frequency-selective Fabry-Perot etalon located in the optical
path between the second mirror and the third mirror, means for
reflecting light of the fixed frequency of the two intermediate
frequencies at a fourth frequency-selective mirror introducing the
pump light through fourth frequency-selective mirror into the
cavity, means for converting pump light into light of the two
intermediate frequencies using non-linear optical parametric
oscillation in the optical path between the fourth mirror and the
first mirror, and means for converting light of the two
intermediate frequencies to electromagnetic radiation having a
gigahertz-terahertz frequency using non-linear difference frequency
generation, in the optical path between the first mirror and the
second mirror.
[0099] It is specifically contemplated that the present invention
includes embodiments having combinations and subcombinations of the
various embodiments and features that are individually described
herein (i.e., rather than listing every combinatorial of the
elements, this specification includes descriptions of
representative embodiments and contemplates embodiments that
include some of the features from one embodiment combined with some
of the features of another embodiment). Further, some embodiments
include fewer than all the components described as part of any one
of the embodiments described herein.
[0100] All publications patents and patent applications mentioned
in this specification are herein incorporated in their entirety by
reference into the specification, to the same extent as if each
individual publication, patent or patent application was
specifically and individually indicated to be incorporated herein
by reference. In addition, citation or identification of any
reference in this application shall not be construed as an
admission that such reference is available as prior art to the
present invention. To the extent that section headings are used,
they should not be construed as necessarily limiting. Some
embodiments of the present invention can be used as laboratory
equipment.
[0101] As used herein the term "about" refers to .+-.10% inclusive.
As used herein the term "most" refers to more than 50%.
[0102] The word "exemplary" is used herein to mean "serving as an
example, instance or illustration." Any embodiment described as
"exemplary" is not necessarily to be construed as preferred or
advantageous over other embodiments and/or to exclude the
incorporation of features from other embodiments.
[0103] The term "in some embodiments" and the word "optionally" are
used herein to mean "is provided in some embodiments and not
provided in other embodiments." Any particular embodiment of the
invention may include a plurality of "optional" features unless
such features conflict.
[0104] The terms "comprises", "comprising", "includes",
"including", "having" and their conjugates mean "including but not
limited to".
[0105] The term "consisting of" means "including and limited
to".
[0106] The term "consisting essentially of" means that the
composition, method or structure may include additional
ingredients, steps and/or parts, but only if the additional
ingredients, steps and/or parts do not materially alter the basic
and novel characteristics of the claimed composition, method or
structure.
[0107] As used herein, the singular form "a", "an" and "the"
include plural versions unless the context clearly dictates
otherwise. For example, the term "a compound" or "at least one
compound" may include a plurality of compounds, including mixtures
thereof.
[0108] Throughout this application, various embodiments of this
invention may be presented in a range format. It is to be
understood that the description in range format is merely for
convenience and brevity and should not be construed as an
inflexible limitation on the scope of the invention. Accordingly,
the description of a range should be considered to have
specifically disclosed all the possible sub-ranges as well as
individual numerical values within that range. For example,
description of a range such as from 1 to 6 should be considered to
have specifically disclosed sub-ranges such as from 1 to 3, from 1
to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as
well as individual numbers within that range, for example, 1, 2, 3,
4, 5, and 6, as well as fractions for those cases not requiring an
integer number. This applies regardless of the breadth of the
range.
[0109] Whenever a numerical range is indicated herein, it is meant
to include any cited numeral (fractional or integer) within the
indicated range. The phrases "ranging/ranges between" a first
indicated number and a second indicated number and "ranging/ranges
from" a first indicated number "to" a second indicated number are
used herein interchangeably and are meant to include the first and
second indicated numbers and all the fractional and integer numbers
there between.
[0110] As used herein the term "method" refers to processes,
manners, means, techniques and procedures for accomplishing a given
task including those manners, means, techniques and procedures
either known to, or readily developed from known processes,
manners, means, techniques and procedures by practitioners of the
optical, electrical, semiconductor, mechanical, chemical,
pharmacological, biological, biochemical and medical arts.
[0111] It is to be understood that the above description is
intended to be illustrative, and not restrictive. Although numerous
characteristics and advantages of various embodiments as described
herein have been set forth in the foregoing description, together
with details of the structure and function of various embodiments,
many other embodiments and changes to details will be apparent to
those of skill in the art upon reviewing the above description. The
scope of the invention should be, therefore, determined with
reference to the appended claims, along with the full scope of
equivalents to which such claims are entitled. In the appended
claims, the terms "including" and "in which" are used as the
plain-English equivalents of the respective terms "comprising" and
"wherein," respectively. Moreover, the terms "first," "second," and
"third," etc., are used merely as labels, and are not intended to
impose numerical requirements on their objects.
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