US 11,226,534 B2

9

45 degrees, such that the pulse experiences different poling
periods as it passes through the crystal. Through these
chirped grating periods, bandwidths +1000 nm are gener-
ated. The chirp, and thus the generated bandwidth, is limited
by the crystal angle in the current implementation. A chirp
of the opposite sign results in a significantly more modulated
spectrum,

While the exemplary preferred embodiments of the pres-
ent invention are described herein with particularity, those
skilled in the art will appreciate various changes, additions,
and applications other than those specifically mentioned,
which are within the spirit of this invention. For example,
the potential for achieving still broader spectral coverage
and higher powers should be enabling for a wide range of
diagnostics of chemical and biological species, including
applications in astronomical heterodyne spectroscopy and
nonlinear spectroscopy.

What is claimed is:

1. An apparatus for generating a mid-IR frequency comb

comprising:

an oscillator configured to generate near-IR mode-locked
light pulses, the pulses having a duration under 500
fsec;

a normal dispersion broadening element for receiving the
near-IR pulses and generating nonlinearly spectral
broadened pulses:

an anomalous dispersion compression element for receiv-
ing the nonlinearly spectral broadened pulses and form-

ing few-cycle, temporally compressed, conditioned ,

pulses;

a comb generating element for receiving the conditioned
pulses and generating a first mid-IR frequency comb by
intrapulse difference frequency generation;

a reference near-IR oscillator for generating a reference ,

near-IR signal;

a filter for selecting a near-IR comb tooth from the first
mid-IR frequency comb;

a heterodyne element for heterodyning the reference
near-IR signal and the near-IR comb tooth and gener-
ating a heterodyned signal;

a detector for detecting the first mid-IR frequency comb;

wherein the detector further detects a beat note compris-
ing a carrier envelope offset frequency of the mode-
locked laser; and

a feedback mechanism for locking the first mid-IR fre-
quency comb using the beat note and the heterodyned
signal.

2. The apparatus of claim 1 wherein the oscillator is a

Er-fiber laser.

3. The apparatus of claim 1 wherein the broadening
element comprises a normal dispersion highly nonlinear
fiber.

4. The apparatus of claim 1 wherein the compression
element comprises anomalous dispersion fused silica.

5. The apparatus of claim 1 wherein the compression
element comprises anomalous dispersion fused silica
wedges.

6. The apparatus of claim 1 wherein the compression
element comprises anomalous dispersion dielectric chirped
mirrors,

7. The apparatus of claim 1 wherein the comb generating
element comprises a poled lithium niobate crystal.

0

40

45

50

60

10

8. The apparatus of claim 1 wherein the comb generating
element comprises an orientation patterned GaP crystal.

9. The apparatus of claim 1 wherein the comb generating
element comprises one of the following nonlinear crystals:
a CdSiP, crystal; an AgS crystal; or a GaSe crystal

10. The apparatus of claim 1 further comprising:

a second oscillator, a second broadening element, a sec-
ond compression element, and a second comb gener-
ating element for generating a second mid-IR fre-
quency comb;

an absorption cell containing a sample gas;

optics for directing the first mid-IR frequency comb and
the second mid-IR frequency comb into the absorption
cell in such a manner as to cause dual frequency comb
spectroscopy in the absorption cell; and

a mercury cadmium telluride (MCT) detector for detect-
ing light from the absorption cell.

11. A method of generating a mid-IR frequency comb

comprising the steps of

(a) generating near-IR mode-locked light pulses, the
pulses having a duration under 500 fsec:

(b) receiving the near-IR pulses and generating nonlin-
early spectral broadened pulses having normal disper-
sion;

(c) receiving the nonlinearly spectral broadened pulses
and forming few-cycle, temporally compressed, con-
ditioned pulses while applying anomalous dispersion to
account for the normal dispersion of step (b);

(d) receiving the conditioned pulses and generating a first
mid-IR frequency comb by intrapulse difference fre-
quency generation;

(e) generating a reference near-IR signal:

(f) selecting a near-IR comb tooth from the first mid-IR
frequency comb;

(g) heterodyning the reference near-IR signal and the
near-IR comb tooth and generating a heterodyned sig-
nal;

(h) detecting the first mid-IR frequency comb, and detect-
ing a beat note comprising a carrier envelope offset
frequency of the mode-locked laser; and

(i) locking the first mid-IR frequency comb using the beat
note and the heterodyned signal.

12. The method of claim 11 wherein step (a) is performed

by an Er-fiber laser.

13. The method of claim 11 wherein the step (b) is
performed with a normal dispersion highly nonlinear fiber.

14. The method of claim 11 wherein step (c) is performed
with either anomalous dispersion fused silica or dielectric
chirped mirrors.

15. The method of claim 11 wherein step (d) is performed
with one of the following: a poled lithium niobate crystal; an
orientation patterned GaP crystal; a CdSiP, crystal; or an
AgS crystal.

16. The method of claim 11, further including the steps of

(i) generating a second mid-IR frequency comb according
to steps (a)-(d);

(k) providing a sample gas in an absorption cell;

(I) directing the first mid-IR frequency comb and the
second mid-IR frequency comb into the absorption cell
in such a manner as to cause dual frequency comb
spectroscopy in the absorption cell; and

(m) detecting light from the absorption cell.

+ * RR Oe


U.S. Patent

Jan. 18, 2022

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102

Sheet 3 of 7

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LWIR Light

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110_h M414

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Figure 3

Wavenumber tem
1600 800 400

Intensity (dB)

12.5 25
Wavelength (um)

Figure 4

US 11,226,534 B2


US 11,226,534 B2

1
METHODS AND APPARATUS FOR
GENERATING MID-INFRARED
FREQUENCY COMBS

This invention was made with government support under
grant numbers 70NANB14H095 awarded by NIST and
70NANB14H095 awarded by DOD/DARPA. The govern-
ment has certain rights in the invention.

BACKGROUND OF THE INVENTION
Field of the Invention

The present invention relates to the generation of infrared
laser frequency combs. In particular, the present invention
relates to apparatus and methods for generating mid-infrared
laser frequency combs using intra-pulse difference fre-
quency generation (DFG).

Discussion of Related Art

Infrared molecular spectroscopy is a powerful and ubiq-
uitous technique for measuring the chemical makeup and
structure in almost any state of matter. In particular, the
inter-atomic degrees of freedom within a molecule or com-
pound lead to series of discrete, vibrational states whose
resonances are unique identifiers in the mid- to long-wave
infrared spectrum spanning the molecular fingerprint region

from approximately 6.7 to 20 jum (~1500-500 cm”). For the 3

past 50 years, Fourier transform infrared spectroscopy
(FTIR) using incoherent, thermal light has been a primary
tool for determining molecular structure in this spectral
region, imparting wide-ranging impact in the physical,

chemical, biological, and medical sciences. However, the ”

low brightness of FTIR thermal sources restricts focusabil-
ity, long-distance propagation, and spectroscopic sensitivity,
while the required mechanical delay of FTIR limits the
combination of spectral resolution and measurement time.
Infrared laser spectroscopy has been pursued over a similar
epoch, but simultaneous spectral coverage of the entire
fingerprint region remains limited to radiation only available
at infrared synchrotron facilities.

Optical frequency comb spectroscopy has been intro-
duced in the past decade as a compelling alternative to FTIR
by providing a unique combination of large spectral band-
width, high frequency precision, and rapid data acquisition
that can be integrated with cavity enhancement techniques
or long-distance propagation to enhance sensitivity. Signifi-
cant effort has gone into the development and extension of
laser frequency combs deeper into the infrared spectral
region beyond 2 um, with techniques including difference
frequency generation (DFG), optical parametric oscillation
(OPO), mode-locked quantum cascade lasers, super-con-
tinum generation, and Kerr micro-resonator technology.
However, to date, most frequency comb sources have been
restricted to wavelengths below 6 um or have only been able
to access discrete portions of the fingerprint region with
limited resolution and accuracy. In parallel, there has also
been ongoing research to generate broadband multi-tera-
hertz pulses extending up to the mid-IR region through
intra-pulse DFG using mode-locked oscillators based on
both Ti: sapphire and Er: fiber platforms. However, reported
infrared powers from such optically rectified pulses at >10
MHz repetition rates have been limited to the microwatt
(uW) scale.

20

2

2

A need remains in the art for apparatus and methods for
generating mid-IR laser frequency combs using intra-pulse
DFG.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide appa-
ratus and methods for generating mid-IR laser frequency
combs using intra-pulse DFG. Such systems include an
oscillator configured to generate near-IR mode-locked light
pulses, the pulses having a duration under 500 fsec, a normal
dispersion broadening element for receiving the near-IR
pulses and generating nonlinearly spectral broadened pulses,
an anomolous dispersion compression element for receiving
the nonlinearly spectral broadened pulses and forming few-
cycle, temporally compressed, conditioned pulses and a
comb generating element for receiving the conditioned
pulses and generating a mid-IR frequency comb by intra-
pulse difference frequency generation.

In some embodiments, the oscillator is a Er: fiber laser
and the broadening element is a normal dispersion highly
nonlinear fiber.

The compression element might be anomalous dispersion
fused silica, such as in wedges, or dielectric chirped mirrors.

The comb generating element is generally a {> Crystal,
such as a periodically poled lithium niobate crystal, an
orientation patterned GaP crystal, a CSP crystal, or an AgS
crystal.

‘One useful application for mid-IR frequency comb gen-
erating systems is dual comb spectroscopy. For these sys-
tems a second oscillator, broadening element, compression
element, and comb generating element generates a second
mid-IR frequency comb, and both combs are directed into an
absorption cell containing a sample gas in such a manner as
to cause dual frequency comb spectroscopy in the absorption
cell. A MCT detector detects the light from the absorption
cell.

‘Apparatus and methods are also provided for locking the
frequency combs, using a reference near-IR oscillator for
generating a reference near-IR signal, a filter for selecting a
near-IR comb tooth from the first mid-IR frequency comb,
a heterodyne element for heterodyning the reference near-IR
signal and the near-IR comb tooth and generating a hetero-
dyned signal, and a detector for detecting the mid-IR fre-

5. quency comb. The detector also detects a beat note com-

prising the carrier envelope offset frequency of the mode-
locked laser. A feedback mechanism locks the mid-IR
frequency comb using the beat note and the heterodyned
signal. This is especially useful in dual comb spectroscopy,
where both mid-IR frequency combs are locked according to
the beat note and the heterodyned signal.

The present invention enables bright, stabilized mid-IR
frequency combs based on intra-pulse DFG using a few-
cycle pulse derived from commonplace and robust pulse

5 generation devices (e.g. Er: fiber laser technology). The

parametric conversion occurs within a quadratic nonlinear
material, for example, periodically poled lithium niobate
(PPLN) crystal which employs quasi-phase matching to
enhance the light conversion efficiency into the mid-IR
regime. Other specific examples of quadratic nonlinear
materials are orientation-patterned gallium phosphide (OP-
GaP), gallium selenide (GaS), cadmium silicon phosphide
(CSP), and silver gallium sulfide (AgS). The invention
generates spectra containing up to several mW of average

5 power and spanning 3-27 um (3300-370 cm), with a

pathway to coverage across even broader spectral range. The
present super-octave bandwidth consists of 500,000 fre-


US 11,226,534 B2

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US011226534B2

US 11,226,534 B2
Jan. 18, 2022

(10) Patent No.
(45) Date of Patent:

(54) METHODS AND APPARATUS FOR
GENERATING MID-INFRARED
FREQUENCY COMBS

(71) Applicant: The Regents of the University of

Colorado, Denver, CO (US)

(72) Inventors: Scott Diddams, Louisville, CO (US);

Henry Timmers, Golden, CO (US);

Abijith J. Kowligy, Boulder, CO (US);

Alexander Jacob Lind, Boulder, CO

(US); Seott Papp, Boulder, CO (US)

(73)

Assignee: Fathom Radiant, PBC, Boulder, CO

(US)
Notice: Subject to any disclaimer, the term of this
patent is extended or adjusted under 35
U.S.C. 154(b) by 171 days

(21) Appl. No.: 16/546,890

(22) Filed: Aug, 21, 2019

Prior Publication Data
US 2020/0064708 Al Feb. 27, 2020

(65)

Related U.S. Application Data

Provisional application No. 62/720,869, filed on Aug.
21, 2018

(60)

(51) Int. Cl
G02F 1/35

GO2F 1/355

(2006.01)
(2006.01)

(Continued)

(52)

US. Cl.
CPC

G02F 1/3501 (2013.01); GO2F 1/3551
(2013.01); G02F 1/365 (2013.01);

(Continued)

106
608 108
a *

OO. :
Gr

(58) Field of Classification Search
CPC ...... GO2F 1/3501; GO2F 1/3551; GO2F 1/365;
GO2F 2001/3503; GO2F 2203/56; HO1S
3/1671

See application file for complete search history.

(56) References Cited
U.S. PATENT DOCUMENTS

8,861,555 B2* 10/2014 Fermann G02B 6/02038

372/4

9,627,843 B2* 4/2017 Zhang HOIS 3/1095

(Continued)

OTHER PUBLICATIONS

‘Timmers, Henry, et al. “Molecular fingerprinting with bright, broad-
band infared frequency combs.” Optica 5.6 (2018): 727-732

(Continued)

Primary Examiner — Daniel Petkovsek
(74) Attorney, Agent, or Firm — Macheledt Bales LLP;
Jennifer L. Bales

(57) ABSTRACT

Apparatus and methods for generating mid-IR frequency
combs using intra-pulse DFG. A mode-locked pulse genera-
tion laser generates near-IR pulses which are amplified. The
amplified pulses are spectrally broadened by a nonlinear
element, for example a normal dispersion highly nonlinear
fiber (ND-HNLF) to generate broadened pulses. The non-
linear spectral broadening element is a transparent dielectric
material having a cubic nonlinear response. Broadened
pulses are temporally compressed to generate short, high-
power pulses which few-cycle conditioned pulses which are
ready for the intrapulse DEG process. The DFG block
generates a mid-IR comb by difference frequency genera-
tion. It might comprise an orientation patterned GaP (OP-
GaP) crystal or a poled lithium niobate (PPLN) crystal

16 Claims, 7 Drawing Sheets

MIR Comb 2
M10) fag + Map

MIR Comb 1
fe UL

0 Wirt

164

612 114

aa a6
Frequency (MHz)

48


US 11,226,534 B2

5
PPLN crystal, or GaSe. FIG. 4 shows resulting mid-IR
frequency combs obtained using each of these elements.
FIG, 3—OP-GaP. FIG. 7A—PPLN.

FIG. 3 is a more detailed block diagram of a specific
example of a mid-IR frequency comb generation system.
Here, pulse generation laser 103 comprises an 100 MHz Er:
fiber mode-locked laser generating femtosecond pulses.
Amplifier 106 is an Er-doped fiber amplifier (EDFA) which
amplifies the pulses to a pulse energy of 3.5 nJ, correspond-
ing to an average power of 350 mW.

The compressed output of amplifier 106 is contained in
optical fiber which is spliced directly to nonlinear spectral
broadening block 110, in this case 4 cm of normal dispersion
highly nonlinear fiber (ND-HNLF). In this embodiment,
broadening block 110 generates pulses having a bandwidth
of ~600 nm. The positive chirp accumulated within ND-
HNLF 110 is compensated for using a pair of anomalous
dispersion fused silica wedges for temporal compression
114, resulting in a few-cycle pump pulses 116. Frequency
resolved optical gating may be used to measure the duration
of the pulses. Pulses 116 have a temporal duration of 10.6 fs,
corresponding to a two-cycle pulse.

Intrapulse difference frequency generation 120 is accom-
plished via OP-GaP in this embodiment. After compression
114, the few-cycle pulse 116 is focused into an OP-GaP
crystal with an orientation patterning period of A=61.1 jum
and a thickness of | mm. The OP-GaP crystal exhibits a high
nonlinear coefficient (d,,=27 um/V at 1.3 um) and broad
transparency across the pump and mid-IR wavelength

regime, making it ideally suited for intra-pulse down-con- 3

version 120.

Few-cycle pulse 116 is focused into the nonlinear crystal
120, resulting in a nonlinear polarization and forward emis-
sion of mid-IR light 122 having an optical period on the

order of the temporal duration of the pump pulse. In the 3

frequency domain this corresponds to DFG between the
spectral components within the few-cycle, near-infrared
pump spectrum. Since the few-cycle pulses occur as a
coherent pulse train, the pump spectrum consists of a comb
of frequency modes given by v,=f,+nx!,,. where fy is the
carrier-envelope offset frequency and f,,,. is the repetition
rate of the pulse train. The pairwise difference between the
nth and mth pump modes yields a mid-IR comb with
frequencies v=(n-m)xf,,.. A critical and advantageous
aspect of the intra-pulse difference frequency is that fy is
subtracted out from the pump field, providing a phase-stable,
offset-free mid-IR comb consisting of exact harmonics of

‘FIG. 4 shows experiment results using several x mate-
rials 120 for intrapulse difference frequency generation in a
system such as the system shown in FIG. 3. The generated
mid-IR light 122 is collected after the x crystal 120 and
long-pass filtered at 3.6 um (not shown). This setup gener-
ated mid-IR spectra 122 spanning across both infrared
atmospheric transmission bands (3-5 jum and 8-12 um) and
containing up to 0.25 mW of power.

Atypical spectrum optimized for bandwidth exhibits over
1.5 octaves of bandwidth from 4 to 12 um. Results are
shown for three types of x crystal 120: PPLN, GaSe, and
OP-GaP. Similar results are obtained with cadmium silicon
phosphide (CSP), and silver gallium sulfide (AgS).

FIG. 5 is a schematic block diagram illustrating a dual
comb spectroscopy system based on two comb sources 502,
504 similar to the one shown in FIG. 3. Comb source blocks
502 and 504 each include a bandpass filter 530 before
amplifier 106, for example a 10 nm bandpass filter to filter
the oscillator 102 spectrum bandwidth of Ad. 40 nm centered

3

40

&

6
around 1570 nm to a bandwidth of AA~10 nm. The filtered
spectra 108 are then stretched temporally in 1 m of PANDA
1550 fiber (not shown) and amplified using a quad-pumped
erbium doped fiber amplifier 106 (EDFA). A normal disper-
sion Er gain fiber is used to enable non-linear amplification
and pulse compression. The output of amplifier 106 yields
pulses 108 having powers ranging between 300-400 mW
depending on the quality of the fiber components and
splices. The output of amplifier 106 is spliced to nonlinear
spectral broadening block 110, here a normal dispersion,
highly non-linear fiber (ND-HNLF) for controlled spectral
broadening. The parameters of the HNLF in this embodi-
ment are as follows: the dispersion is D=-1.0 ps/nm km, the
dispersion slope is D,=0.006 ps/nm? km, and the non-

5 linearity is y~10.8 (W km)". The fiber 106 length that gave

the best compression (typically 30-50 fs) was spliced
directly to the ND-HNLF.

The ND-HNLF 110 was connectorized with a FC/APC
connector (not shown) and connected to an off-axis para-
bolic collimator (not shown) to output-couple the light into
free space without imparting unwanted dispersion onto the
broadened pulses 112.

The pulses 112 were then compressed 114 with either a
bulk fused silica window (5 mm) or a pair of fused silica

s wedges. The anomalous dispersion from the fused silica

material compensated for the normal dispersion and self-
phase modulation in the ND-HNLF 110. The compressed
pulses 116 were characterized using second harmonic gen-
eration FROG. Experimentally, the duration of compressed
pulses 116 was 10.6 fs.

After pulse compression, the few-cycle pulses 116 were
focused into an orientation patterned GaP (OP-GaP) crystal
120 with a length of 1 mm and an orientation patterning
period of A=61.1 um. The pulses were focused using an
off-axis parabolic mirror to avoid imparting unwanted dis-
persion onto the few-cycle pulses 116. The generated mid-IR
light 122 was collected with a ZnSe lens (not shown) and
filtered using either a 3600 nm or 4500 nm long-pass filter
(not shown) to remove the pump spectrum.

Block 502 generates first mid-IR frequency comb 1224
and block 504 generates second mid-IR frequency comb
122B. For dual-comb spectroscopy (DCS), the two combs
122A and 122B were combined together using a mid-IR,
50-50 beamsplitter. One optical path from the beamsplitter

5 travelled through a 15 cm absorption cell 506 containing

some quantity of a sample gas. After absorption cell 506, the
light was collected onto a liquid nitrogen cooled HgCdTe
(MCT) detector 512. This is used as the main photo-detector
for the DCS experiment. The second path from the beam-
splitter travels to either a Fourier transform spectrometer
(ETS) 508, where a low-resolution spectrum of the mid-IR
light can be measured, or to a second MCT photo-detector
510, where a DCS reference spectrum can be acquired for
the absorbance measurement. Note that the two mid-IR

5 frequency combs 122A, 122B are mutually phase coherent,

for example by locking their seed Er: fiber mode-locked
lasers to a common 1550 nm continuous wave laser. Due to
the nature of intra-pulse difference frequency generation
according to the present invention, the mid-IR combs are
offset free. However, the pump combs still exhibit an offset
frequency. Therefore, the carrier-envelope offset frequencies
of both combs were stabilized, for example with two inline
-2f interferometers (not shown).

The embodiment of FIG. 5 was used to perform high-

5 resolution spectroscopy in ethanol. In addition to simulta-

neously measuring the four broadband absorption features in
ethanol at atmospheric pressure, it also recorded a highly


US 11,226,534 B2

3

quency comb modes, providing a spectral resolution down
to 100 MHz (0.0033 cm='). The present invention produces
coherent, broadband infrared light exhibiting comparable
brightness to an infrared beamline at synchrotron user
facilities.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a high-level schematic block diagram of an
infrared comb source according to the present invention.

FIG. 2 is a schematic diagram illustrating the process of
conditioning pulses in the comb source of FIG. 1

FIG. 3 is a detailed block diagram of a first embodiment
of the comb source of FIG. 1.

FIG. 4 is a plot showing the performance of the embodi-
ment of FIG. 3 using various ~° materials for intrapulse
difference frequency generation.

FIG. 5 is a schematic block diagram of a dual comb
spectroscopy system based on two comb sources such as
those shown in FIG. 3.

FIG. 6 is a detailed block diagram of the portion of the
invention which generates higher power, few-cycle condi-
tioned pulses.

FIG. 7A is a schematic block diagram of an infrared comb
source implementing a beat note as a lock parameter for
comb offset frequency. FIG. 7B is a plot showing the results
of the embodiment of FIG. 7A. FIG. 7C is a plot showing
three carrier envelope offset frequencies from the embodi-
ment of FIG. 7A. FIG. 7D is a schematic block diagram of

a dual comb spectroscopy systems based upon two infrared >

comb sources of FIG. 7A.

DETAILED DESCRIPTION OF THE
INVENTION

TABLE 1

Ref. no. Element

100, 300, 502, 504, 800 Mid-IR comb generating systems
2 Pulse generating laser (near IR)

104/204/304

Near-IR pulses
106 Light amplifier
108/208/308 Amplified near-IR pulses
110 Nonlinear spectral broadening,
112/212/312 Broadened pulses
4 ‘Temporal compression
116/216/316 Few-cyele conditioned pulses
17 Off-axis parabolic mirror

Intrapulse difference frequency generation
Mid-IR frequeney comb
First mid-IR comb source
Second mid-IR comb source
Absorption gas cell

PIS

MCT, reference spectrum
MCT detector

Bandpass filter

Optical isolator
Preamplifier

Amplifier

ND-HNLF

Chirped misrors

Fused silica wedges

Mid-IR comb generating system
Lowpass filter

Comb

MCT Detector

Des

Offset frequency beat

Fo lock parameter
Heterodyned lock parameter

10

3

45

50

3!

60

6:

TABLE 1-continued
Ref. no. Element
814 Comb tooth
816 Stable laser signal
818 Photodetector
820 Heterodyned signal
850 Lock dual comb spectroscopy system

Table 1 provides reference numbers and associated ele-
ments of the invention.

FIG. 1 is a schematic block diagram of an infrared comb
source 100 according to the present invention. Mode-locked
pulse generation laser 102 generates near-IR pulses 104
Amplifier 106 amplifies pulses 104 to generate higher-
energy pulses 108. In general amplifier 106 is utilized after
laser 102 because commercially available lasers do not
produce high enough energy pulses. As an alternative,
amplifier 106 could incorporated into block 102.

Next, amplified pulses 108 are spectrally broadened by a
nonlinear element 110, for example a normal dispersion
highly nonlinear fiber (ND-HNLF) to generate broadened
pulses 112. Nonlinear element 110 is a transparent, dielectric
material having a cubic nonlinear response. Next, pulses 112
are temporally compressed by block 114 to generate short,
high-power pulses 116. Pulses 116 are thus few-cycle con-
ditioned pulses which are ready for the intrapulse DFG
process (see FIG. 2). Intrapulse DFG block 120 receives
few-cycle conditioned pulses 116 and generates a mid-IR
comb 122 by difference frequency generation. Intrapulse
DFG block 120 might comprise an orientation patterned
GaP (OP-GaP) crystal or a poled lithium niobate (PPLN)
crystal.

FIG. 2 is a schematic diagram showing the process of
generating mid-IR frequency comb 122 according to the
embodiment of FIG. 1. The top row shows time domain
pulses 204-222, while the bottom row shows frequency
domain spectra 304-322. Thus, for example pulses 104 have
a time domain plot 204 and a frequency domain plot 304
The rest of the reference numbers follow the same pattern.

In one embodiment, the pulse 104 wavelength is around
800 nm-2000 nm. Pulses 104 might be around 100-500 fs.
A commercial Er: fiber mode-locked laser generating 1550
nm pulses works well. Generally, the pulses are then ampli-
fied by amplifier 106 to generate pulses 108 of around 0.3-10
Watts in energy. Pulses 108 will be about the same length
and have about the same spectrum as pulses 104, just higher
energy.

Nonlinear spectral broadening block 110 broadens pulses
112 with normal dispersion as shown in plot 212, wherein
the higher frequency portions of the spectrum are separated
from the lower-frequency portions in time. Plot 312 shows
that pulses 312 are broader in frequency as well. Nonlinear
spectral broadening block 110 is a transparent, dielectric
material having a cubic nonlinear response, for example a
normal dispersion highly nonlinear fiber (ND-HNLF)

Temporal compression block 114 compresses pulses 112
to form pulses 116, so that the pulses are short (plot 216) but
still broad in frequency (plot 316). Temporal compression
block 114 is anomalously dispersive and compensates for
the normal dispersion and self-phase modulation imparted
by nonlinear element 110. Pulses 116 are thus few-cycle
conditioned pulses 116 which are ready for the intrapulse
DFG process 120.

Finally, block 120 generates the mid-IR frequency comb
122 by intrapulse difference frequency generation. This
process may be accomplished with an OP-GaP block, a


U.S. Patent Jan. 18, 2022 Sheet 6 of 7 US 11,226,534 B2

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Figure 7B

(i) A=600 nm (i) A=900 nm (i) A=3500 am
8 50 B50 so
& 25 & 25 & 5
8 5 3
zg & x

~50 8 56 ~58 8 50 58 9 50
Detuning (kHz) Detuning (kHz) Detuning (kHz)

Figure 7C


US 11,226,534 B2

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Page 2

(51) Int. CL
G02F 1/365 (2006.01)
HOIS 3/16 (2006.01)
(52) US. CL
CPC veces HOTS 3/1671 (2013.01); GO2F 1/3503
(2021.01); GO2F 2203/56 (2013.01)

(56) References Cited
U.S. PATENT DOCUMENTS

10,042,231 B2* 8/2018 Apolonskiy GO2F 1/3501
2018/0003623 Al* 1/2018 Apolonskiy GOIN 21/35

OTHER PUBLICATIONS

‘Timmers, Henry, et al. “Molecular fingerprinting with bright, broad-
band infared frequency combs: supplementary material.” Optica 5.6
(2018),

* cited by examiner


US 11,226,534 B2

7

resolved ethanol spectrum at a lower backing pressure of 10
torr in order to discern the individual ro-vibrational lines.
This is a better result than was found in the literature.

FIG. 6 is a detailed block diagram of the portion of the
invention which generates higher-power (1-10 W) few-cycle
conditioned pulses 116 which are configured to be ready for
the intrapulse DFG process 120 (see FIG. 2). In this embodi-
ment, pulse generating laser 102 is an Er: Fiber Mode-
locked laser generating a near-IR pulse train. An optical
isolator 602 is shown to prevent light from leaking back into
laser 102. Light amplifier 106 includes both a lower-power
pre-amplifier 604 that employs core-pumped Er: fiber, as
well as a higher-power amplifier 606 that employs cladding-
pumped Er/Yb: fiber. Nonlinear spectral broadening block
110 comprises temporal compression of the amplified pulses
using gratings followed by nonlinear spectral broadening in
normal dispersion highly nonlinear fiber (ND-HNLF) 608.
Final temporal compression to few-cycle duration occurs in
block 114 which comprises chirped dielectric mirrors 610
and fused-silica wedges 612.

FIG. 7A is a schematic block diagram of a mid-IR infrared
comb source 800 implementing a beat note 808 as a lock
parameter for comb offset frequency. While frequency sta-
bilization using beat note 808 is not required for MIR light
generation, it is useful for applications such as dual-comb
spectroscopy. A long-pass filter (LPF) 122 allows the mid-IR
light 122 to pass and focus onto a MCT detector 804.
Additionally, the offset frequency (f,) is detected here. The
light 803 reflected from LPF 122 is coupled into optical fiber

814 to heterodyne with a cavity-stabilized 1.55 wm laser 3

output 816, which is used to fully stabilize the comb.
FIG. 7B is a plot showing results obtained with the

implementation of FIG. 7A. FIG. 7B shows the near infrared

spectrum of the few-cycle pulse extending between 1 and 2

microns, as well as a mid-IR spectrum between 3 and 4 3

microns and visible spectrum in the region of 0.5-1 microns.

FIG. 7C shows three replicas of the carrier envelope offset
frequency of the mode-locked laser 102 which are detected
directly in the output spectra 122 at 600 nm, 900 nm and
3500 nm.

Comb source 800 generates a mid-IR frequency comb 122
by driving few-cycle pulses 116 within a y® material 120
(see FIG. 2). Frequency combs 122 span 3 jum to 5.5 um. In
addition to generating mid-IR frequency combs, the system
also provides the carrier-envelope offset frequency of the
driving pulse train 116 directly, which is used to fully
stabilize the mode-locked laser 102. Elements 102-114
generate near-IR pulses 116 as shown in FIGS. 1-6. The
offset frequency beat 808, measured directly from the MCT
detector 804 (along with dual comb spectroscopy 806), as a
consequence of cascaded non-linear processes, allows sta-
bilization of the combs to an in-loop timing jitter <100 as.

In one embodiment, driving pulses 116 in the 1.55 um
region pump periodically poled lithium niobate (PPLN). The
system amplifies 106 and spectrally broadens 110 the output
104 of a 100 MHz Er: fiber oscillator 102 to generate 3 nJ
few-cycle pulses 116 (10.6 fs, 2.1 cycles) centered at 1.55
jum. The few-cycle pulses 116 are focused with a 25 mm
focal length silver-coated off-axis parabolic (OAP) mirror
117 into 1 mm-long PPLN crystal 120, achieving a mini-
mum beam diameter of 16 tm. The broadband output 120 of
the PPLN crystal is collected with another OAP mirror 122,
optically filtered 812, and measured on a Fourier transform
spectrometer (see FIG. 5) or sent to a liquid nitrogen-cooled
mercury cadmium telluride (MCT) detector 804.

The modes of a frequency comb are defined through two
radio frequency parameters, the repetition rate (f,,,,) and the

20

2

&

g

0

8

offset frequency (f,), such that a given mode, n, has a
well-defined optical frequency, v,=nf,,,,+f). For DFG that
occurs strictly within the original input pulse, the resulting
mid-IR light will be “offset-free”, meaning the offset fre-
quency of the comb will subtract out in the DFG process
However, due to the high peak intensities within the crystal,
cascaded x nonlinear processes give rise to additional
comb modes throughout the spectrum with which we
observe f,, in heterodyne beats at multiple wavelengths, as
shown in FIG. 7C. For example, at 600 nm, we observe the
2£-3f interference, from cascaded QPM 1.8 um tripling and
1.2 jm (third-order phase-matched) doubling at a beat
frequency, fb,

Fe“3X rap tfo)-2XB 2p he)“ 2o fo a)

At 900 nm, a f-2f interference results from QPM 1.8 um
frequency doubling beating with slightly broadened comb
light. At 3500 nm, we observe an “f-0” beat note between the
“offset-free” DFG comb, and DFG between doubled light
from 2 um (with a factor of 2f,) and the original 1.5 um
comb (containing a single f,). The two mid-IR combs
heterodyne together on the MCT photo detector to generate
an offset frequency tone in the RF spectrum.

System 850 implements optical frequency control at 1.55
tum, which ensures a high degree of mutual coherence
necessary for applications like DCS. A near-IR comb tooth
814 heterodyned against a stable 1.55 jum laser 816 signal
using photodetector 818 provides one lock parameter f,, 808
as feedback 812, and an offset frequency beat 808 provides
the second feedback, 810. An additional external f-2f inter-
ferometer branch is usually required to detect an offset
frequency beat note; however, in this configuration, the need
for such a branch is bypassed by utilizing the offset fre-
quencies directly detected throughout the spectrum. The
beat note 808 from MCT detector 804 is used for mid-IR
spectroscopy to lock the offset frequency of combs 502, 504,
as shown in FIG. 7D. The integrated phase noise on the
offset frequency is 115 mrad, integrated from 50 Hz to 2
MHz, and comparable to other methods of offset frequency
stabilization. This demonstrates that using the f, in the
mid-IR results in similar performance to a traditional f-2f
lock, allowing for simultaneous comb stabilization and
mid-IR spectroscopy from the same detector.

The PPLN crystal 120 contains a set of discrete poling
periods, ranging from /\=24.1-35.6 um in steps of approxi-
mately 0.6 jum. The longer (shorter) poling periods provide
QPM for DFG into shorter (longer) wave-lengths in the
mid-IR. By tuning across these poling periods, mid-IR light
spans 3-5.5 um (FIG. 2(a)). The maximum power measured
in the mid-IR spectra is found to be 1.3 mW. However, on
average these spectra yield approximately 500 pW, which is
suflicient to saturate high-speed, low-noise MCT detectors
in lab-scale linear spectroscopy measurements.

FIG. 7B shows the full optical spectrum after propagation

‘5 through one PPLN poling period. The spectrum of the input

few-cycle pulse spans 1-2 um (dashed curve). Below 1 um,
the sharp spectral features come from higher-order quasi
phase-matched (QPM) harmonic generation from the input
spectrum. The mid-IR light arises from difference-frequency
mixing within the input spectrum.

The bandwidth generated (300-500 nm) in the mid-IR is
limited by group velocity dispersion elongating the pulse
(112 f3?/mm at 1.55 pm) and the phasematching bandwidth
for a single poling period. Instead, by using crystals with a

5 chirped poling period (aperiodic poling), we significantly

increase the QPM bandwidth. To implement a chirped
poling period, we use a fanout PPLN tilted at approximately