US 11,175,224 B2

21

All ranges disclosed herein are inclusive of the endpoints,
and the endpoints are independently combinable with each
other. The ranges are continuous and thus contain every
value and subset thereof in the range. Unless otherwise
stated or contextually inapplicable, all percentages, when
expressing a quantity, are weight percentages. The suffix
“(s)” as used herein is intended to include both the singular
and the plural of the term that it modifies, thereby including
at least one of that term (e.g., the colorant(s) inchides at least
one colorants). “Optional” or “optionally” means that the
subsequently described event or circumstance can or cannot
occur, and that the description includes instances where the
event occurs and instances where it does not. As used herein,
“combination” is inclusive of blends, mixtures, alloys, reac-
tion products, and the like.

As used herein, “a combination thereof” refers to a
combination comprising at least one of the named constitu-
ents, components, compounds, or elements, optionally
together with one or more of the same class of constituents,
components, compounds, or elements

All references are incorporated herein by reference.

The use of the terms “a” and “an” and “the” and similar
referents in the context of describing the invention (espe-
cially in the context of the following claims) are to be
construed to cover both the singular and the plural, unless
otherwise indicated herein or clearly contradicted by con-
text. “Or” means “and/or.” It should further be noted that the
terms “first,” “second,” “primary,” “secondary,” and the like
herein do not denote any order, quantity, or importance, but

rather are used to distinguish one element from another. The 3

modifier “about” used in connection with a quantity is
inclusive of the stated value and has the meaning dictated by
the context (¢.g., it includes the degree of error associated
with measurement of the particular quantity). The conjunc-

tion “or” is used to link objects of a list or alternatives and 3

is not disjunctive; rather the elements can be used separately

or can be combined together under appropriate circum-

stances.

What is claimed is:

1. An optical refraction barometer for measuring pressure
based on refractivity changes between a fixed length optical
reference cavity and a fixed length optical sample cavity of
a dual fixed length optical cavity refractometer, the optical
refraction barometer comprising:

an optical light source that produces laser light;

an optical frequency controller in optical communication.
with the optical light source and that:
receives the laser light from the optical light source:
receives an optical feedback signal from an electronic

reference arm; and

produces primary light from the laser light based on the
optical feedback signal;

first optical phase controller in optical communication

with the optical frequency controller and that:

receives the primary light from the optical frequency
controller;

receives a reference phase control signal; and

produces an input reference light from the primary light
based on the reference phase control signal;

a first polarization controller in optical communication
with the first optical phase controller and that:
receives the input reference light from the first optical

phase controller;

communicates the input reference light to a dual fixed

length optical cavity refractometer;

receives an output reference light from the dual fixed

length optical cavity refractometer; and

2

20

2

40

45

50

35

65

22

communicates the output reference light to an elec-

tronic reference arm;

the electronic reference arm in optical communication
with the first polarization controller and that:
receives the output reference light from the first polar-

ization controller;

produces the reference phase control signal from the
output reference light and communicates the refer-
ence phase control signal to the first optical phase
controller;

produces the optical feedback signal and communicates
the optical feedback signal to the optical frequency
controller and optionally communicates the optical
feedback signal to the optical light source;

a second optical phase controller in optical communica-
tion with the optical frequency controller and that:
receives the primary light from the optical frequency

controller;

receives a sample phase control signal from an elec-
tronic sample arm; and

produces an input sample light from the primary light
based on the sample phase control signal;

a second polarization controller in optical communication
with the second optical phase controller and that:
receives the input sample light from the second optical

phase controller;

communicates the input sample light to the dual fixed
length optical cavity refractometer;

receives an output sample light from the dual fixed
length optical cavity refractometer; and

communicates the output sample light to the electronic
sample arm;

the electronic sample arm in optical communication with
the second polarization controller and in electrical
communication with the second optical phase control-
ler, the electronic sample arm comprising:

a detector that receives the output sample light from the
second polarization controller and produces a refer-
ence cavity signal from the output sample light;

a second sideband frequency generator that produces a
second sideband frequency;

a mixer in electrical communication with the detector
and the second sideband frequency generator and
that produces an error signal that comprises the
reference cavity signal and the second sideband
frequency; and

a first sideband frequency generator in electrical com-
munication with the mixer and that receives the error
signal from the mixer and produces a first sideband
frequency from the error signal from which the
sample phase control signal is produced by the
electronic sample arm for communication to the
second optical phase controller; and

the dual fixed length optical cavity refractometer com-
prising:
the fixed length optical reference cavity in optical

communication with the first polarization controller

and that:

receives the input reference light from the first
polarization controller;

propagates the input reference light through fixed
length optical reference cavity; and

produces the output reference light by reflecting
input reference light from a mirror disposed at a
terminus of the fixed length optical reference
cavity disposed optically distal to the first polar-
ization controller; and


US 11,175,224 B2

9

signal 229 that is communicated to optical frequency con-
troller 227 and optionally to optical light source 251; side-
band frequency source 225 in electrical communication with
radio frequency source 224 and PHD servo 206 and that:
receives first reference phase control signal 231.1 from radio
frequency source 224; receives optical feedback signal 229
from PID servo 206; and produces second reference phase
control signal 231.2 from optical feedback signal 229 and
first reference phase control signal 231.1; directional coupler
211 in electrical communication with sideband frequency
source 225 and that: receives second reference phase control
signal 231.2 from sideband frequency source 225; produces
reference phase control signal 231 from second reference
phase control signal 231.2; and communicate reference
phase control signal 231 to first optical phase controller
230.1.

Components of optical refraction barometer 200 can be
made from and include various materials. It should be
appreciated that refraction of gases in dual cavities (202,
203) of dual fixed length optical cavity refractometer 201
interact with light so that pressure of sample gas 332 is
determined. In an embodiment, with reference to FIG. 5,
dual fixed length optical cavity refractometer 201 includes
fixed length optical reference cavity 202 that includes refrac-
tometer body 259: first mirror 218.1 disposed at entry end
257 of refractometer body 259; and first optical cavity optic
256.1 disposed at exit end 258 of refractometer body 259.
First mirror 218.1 is in optical communication and optically
opposes first optical cavity optic 256.1. Fixed length optical

reference cavity 202 receives reference gas 334 at first 3

pressure P1, receives input reference light 232, produces
output reference light 234 from reflection of a portion of
input reference light 232 by first optical cavity optic 256.1
and that is communicated through reference gas 334 in fixed

length optical reference cavity 202 to exit fixed length 3

optical reference cavity 202 at entry end 257 as output
reference light 234, and produces reference transmitted light
249 from communication of a portion of input reference
light 232 through first optical cavity optic 256.1 after
interaction with reference gas 334. Fixed length optical
sample cavity 203 includes second mirror 218.2 disposed at
entry end 257 of refractometer body 259; and second optical
cavity optic 256.2 disposed at exit end 258 of refractometer
body 259. Second mirror 218.2 is in optical communication
and optically opposes second optical cavity optic 256.2.
Fixed length optical sample cavity 203 receives sample gas
332 at second pressure P2, receives input sample light 238
at second mirror 218.2, communicates input sample light
238 through sample gas 332 from mirror 218.2 to optical
cavity optic 256.2, produces output sample light 239 from
reflection of a portion of input sample light 238 by optical
cavity optic 256.2 and that is communicated through sample
gas 332 in fixed length optical sample cavity 203 to exit
fixed length optical reference cavity 202 via mirror 218.2 as
output sample light 239, and produces sample transmitted
light 250 from communication of a portion of input sample
light 238 through optical cavity optic 256.2 after interaction
with sample gas 332. Gas source 330 is in fluid communi-
cation with fixed length optical sample cavity 203 and
provides sample gas 332 to fixed length optical sample
cavity 203. Second pump 314.2 is in fluid communication
with fixed length optical sample cavity 203 to pump sample
gas 332 from fixed length optical sample cavity 203. First
pump 314.1 is in fluid communication with fixed length
optical reference cavity 202 to obtain a selected pressure
thereof at first pressure P1 via pump stem 350 connected to
fixed length optical reference cavity 202, wherein wall 322

S

2

40

&

g

10

of pump stem 350 bounds flow channel 324 through which
reference gas 334 flows to pump 314.1. Valves 328 throttle
and isolate gas source 330 and pump 314, and gas flows
through gas line 326.

In optical refraction barometer 200, laser 210 produces
laser light 226. Laser 210 can produce laser light that is
resonant or nearly resonant with or and can include a HeNe
laser, Ti:Sapphire laser, extenal-cavity diode laser, and the
like. Exemplary laser 210 includes an infrared external
cavity diode laser. Moreover, a laser linewidth can be
smaller than the free spectral range of cavity (202 or 203)
defined by cavity optic (218, 256). In an embodiment, laser
210 includes a HeNe laser and an infrared external cavity
diode laser.

Laser light 226 can include a single-frequency laser light
to probe resonances of cavity (202 or 203). Exemplary laser
light 226 includes light from a HeNe laser. A wavelength of
laser light 226 can be from UV to IR, specifically from 250
nm to 2 um, and more specifically can be a wavelength
defined by cavity optic (218 or 256). Moreover, a frequency
spectrum of laser light 226 can have a single frequency peak.
A power of laser light 226 can be from 1 uW to 1 W,
specifically from 10 uW to 100 mW, and more specifically
from 100 uW to 10 mW. When in optical communication

swith cavity (202 or 203), the wavelength can correspond to

a transmitted wavelength of cavity (202 or 203) and a
linewidth smaller than that of cavity (202 of 203) defined by
cavity optics (218, 256). In an embodiment, laser light 226
have a wavelength of 1542 nm, 10 mW power, 2 kHz
linewidth, and similar properties.

In dual fixed length optical cavity refractometer 201,
Faraday isolator 207 can include an optic that prevents the
transmission of laser light 226 in one direction but not the
other, wherein Faraday isolator 207 selectively propagates
laser light 226 from laser 210 and isolates back reflection of
laser light 226 from optical frequency controller 227 to laser
210. Exemplary optical isolators are Faraday effect isolators.

In some embodiments, laser 210 receives optical feedback
signal 229.2, wherein optical feedback signal 229.2 controls
a wavelength of laser 210. An analogue voltage signal of
optical feedback signal 229.2 can be from -100 volts to 100
volts, specifically from 10 volts to 10 volt sand more
specifically from -1 volt to 1 volt. Further, optical feedback
signal 229.2 includes an analogue voltage signal that is

5 output from the PIID servo, in this case it is only the I or

integral component that slowly tunes the laser wavelength
using piezoelectric methods or thermal.

In optical refraction barometer 200, radio frequency oscil-
lator 204 provides a radio frequency that is rapidly tunable
for driving acousto-optic modulator 208. Radio frequency
oscillator 204 receives a feedback signal and outputs a radio
frequency for driving acoustic optical modulator 208. Radio
frequency oscillator 204 can include a voltage controlled
oscillator, direct digital synthesizer, function generator,

5 radio frequency synthesizer, and the like. Exemplary radio

frequency source 204 includes a voltage controlled oscillator
(VCO) that is in resonance with a fiber acoustic optic
modulator AOM, free space AOM, and the like. VCO can
have a peak operating frequency and a tuning range that is
a fraction of the peak operating frequency. Optical feedback
signal 229 can be from -100 volts to 100 volts, specifically
from 10 volts to 10 volts, and more specifically from 0 volts
to 17 volts. Further, optical feedback signal 229 is the
resulting output from the PIID servo controller. In an

5 embodiment, includes high bandwidth PIID servo controller.

Radio frequency oscillator 204 receives optical feedback
signal 229, wherein optical feedback signal 229 controls


US 11,175,224 B2

29

receiving, by the first sideband frequency generator, the
servo control signal; and

producing, by the first sideband frequency generator, a

first sideband frequency from the servo control signal;
and

wherein the electronic sample arm further comprises a

power combiner in electrical communication with the
first sideband frequency generator and the second side-
band frequency generator, and the process further com-
prises:

receiving, by the power combiner, the first sideband

frequency from the first sideband frequency generator
and the second sideband frequency from the second
sideband frequency generator;

producing, by the power combiner, the sample phase

control signal from the first sideband frequency and the
second sideband frequency; and

communicating the sample phase control signal to the

second optical phase controller from the power com-
biner.

17. The process of claim 11, wherein the electronic
sample arm comprises a detector in optical communication
with the second polarization controller, and the process
further comprising:

receiving, by the detector, the output sample light from

the second polarization controller; and

producing, by the detector, a reference cavity signal from

the output sample light;

producing, by a second sideband frequency generator, a

second sideband frequency;

wherein the electronic sample arm further comprises a

mixer in electrical communication with the detector
and the second sideband frequency generator, and the
process further comprises:

10

20

2

receiving, by the mixer, the reference cavity signal from ©

the detector;

receiving, by the mixer, the second sideband frequency
from the second sideband frequency generator; and

producing, by the mixer, an error signal that comprises the
reference cavity signal and the second sideband fre-
quency;

wherein the electronic sample arm further comprises a
PID servo in electrical communication with the mixer,
and the process further comprises:

receiving, by the PID servo, the error signal; and

producing, by the PID servo, a servo control signal from
the error signal;

wherein the electronic sample arm further comprises a
first sideband frequency generator in electrical com-
munication with the PID servo and the second sideband
frequency generator, and the process further comprises:

45

50

30

receiving, by the first sideband frequency generator, the

servo control signal from the PID servo;

receiving, by the first sideband frequency generator, the

second sideband frequency from the second sideband
frequency generator; and

producing, by the first sideband frequency generator, a

first sideband frequency from the servo control signal
and the second sideband frequency; and

wherein the electronic sample arm further comprises a

directional coupler in electrical communication with
the first sideband frequency generator, and the process
further comprises:

receiving, by the directional coupler, the first sideband

frequency from the first sideband frequency generator;
producing, by the directional coupler, the sample phase
control signal from the first sideband frequency: and
communicating the sample phase control signal to the
second optical phase controller from the directional
coupler.

18. The process of claim 11, wherein the electronic
reference arm comprises a detector in optical communica-
tion with the first polarization controller, and the process
further comprises:

receiving, by the detector, the output reference light from

the first polarization controller; and

producing, by the detector, a reference cavity signal from

the output reference light;

wherein the electronic reference arm further comprises a

radio frequency source, and the process further com-
prises producing a reference phase control signal by the
radio frequency source;

wherein the electronic reference arm comprises a mixer in

electrical communication with the detector and the
radio frequency source, and the process further com-
prises:

receiving, by the mixer, the reference cavity signal from

the detector;

receiving, by the mixer, the reference phase control signal

from the radio frequency source; and

producing, by the mixer, a mixed reference signal that

comprises the reference cavity signal and the reference
phase control signal; and

wherein the electronic reference arm comprises a PIID

servo in electrical communication with the mixer, and
the process further comprises:

receiving, by the PIID servo, the mixed reference signal:

producing, by the PID servo, the optical feedback signal

from the mixed reference signal; and

communicating, by the PIID servo, the optical feedback

signal to the optical frequency controller and optionally
to the optical light source.

+ ke ee


U.S. Patent Nov. 16, 2021 Sheet 3 of 7 US 11,175,224 B2

200 Electronic sideband production
229.2
251 235
210 206 220 221
[+226 236-74
231 244
207 224 | 4 —+]213 Ion
L234
217 218)
f BT BIT 234 [201
230.1 g i 28
218) 215, 216) 202
232. ~«~«32SS«O HD
238 238 238
250
253 zis 21st 1216 203 |“
938 230.2
209 217 218)
237-4 |—239
240 245 244
241 213 212
245] ayy
221 L——pi221
247 246 263
4 223) 211} 243 4 205 220

FIG. 3


US 11,175,224 B2

23

the fixed length optical sample cavity in optical com-
munication with the second polarization controller
and that:
receives the input sample light from the second
polarization controller;
propagates the input sample light through the fixed
length optical sample cavity; and
produces the output sample light by reflecting the input
sample light from a second mirror disposed at a
terminus of the fixed length optical sample cavity
disposed optically distal to the second polarization
controller.

2. The optical refraction barometer of claim 1, wherein the
optical light source comprises a laser that produces the laser
light; and a Faraday isolator in optical communication with
the laser and that communicates the laser light to the optical
frequency controller.

3. The optical refraction barometer of claim 1, wherein the
optical frequency controller comprises a voltage controlled
oscillator in electrical communication with the electronic
reference arm and that receives the optical feedback signal
from the electronic reference arm and that produces a
reference modulation frequency from the optical feedback
signal;

an acousto-optic modulator in electrical communication

with the voltage controlled oscillator and in optical
communication with the optical light source and that:
receives the reference modulation frequency from the

voltage controlled oscillator and the laser light from ”

the optical light source; and

produces the primary light from modulation of the laser
light at a modulation frequency controlled by the
reference modulation frequency.

4. The optical refraction barometer of claim 3, wherein the ~

optical frequency controller further comprises a wavelength
detector in optical communication with the acousto-optic
modulator and that receives the primary light from the
acousto-optic modulator and produces an optical frequency
counter signal from the primary light.

5. The optical refraction barometer of claim 1, wherein the
first optical phase controller comprises an electro-optic
modulator in optical communication with the optical fre-
quency controller and in electrical communication with the
electronic reference arm and that:

receives the primary light from the optical frequency

controller;

receives the reference phase control signal from the

electronic reference arm; and

produces the input reference light from modulating the

primary light at a modulation frequency controlled by
the reference phase control signal

6. The optical refraction barometer of claim 1, wherein the
second optical phase controller comprises an electro-optic
modulator in optical communication with the optical fre-
quency controller and in electrical communication with the
electronic sample arm and that:

receives the primary light from the optical frequency

controller;

receives the sample phase control signal from the elec-

tronic sample arm; and

produces the input sample light from modulating the

primary light at a modulation frequency controlled by
the sample phase control signal.

7. The optical refraction barometer of claim 1, wherein the
first polarization controller comprises:

24

a polarizing beam splitter in optical communication with
the first optical phase controller and in electrical com-
munication with the electronic reference arm and that:
receives the input reference light from the first optical
phase controller;

communicates the input reference light to a waveplate;

receives the output reference light from the waveplate;
and

communicates the output reference light to the elec-
tronic reference arm; and
the waveplate in optical communication with the polar-
izing beam splitter and the dual fixed length optical
cavity refractometer and that:
receives the input reference light from the polarizing
beam splitter;

receives the output reference light from the fixed length
optical reference cavity; and

communicates the output reference light to the polar-
izing beam splitter,

8. The optical refraction barometer of claim 1, wherein the

second polarization controller comprises:
a polarizing beam splitter in optical communication with
the second optical phase controller and in electrical
communication with the electronic sample arm and
that:
receives the input sample light from the second optical
phase controller;

communicates the input sample light to a waveplate;

receives the output sample light from the waveplate;
and

communicates the output sample light to the electronic
sample arm; and
the waveplate in optical communication with the polar-
izing beam splitter and the dual fixed length optical
cavity refractometer and that:
receives the input sample light from the polarizing
beam splitter;

receives the output sample light from the fixed length
optical sample cavity; and

communicates the output sample light to the polarizing
beam splitter.

9. The optical refraction barometer of claim 1, wherein the

electronic reference arm comprises:
a detector in optical communication with the first polar-
ization controller and that receives the output reference
light from the first polarization controller; and produces
a reference cavity signal from the output reference
light;
a radio frequency source that produces a reference phase
control signal;
a mixer in electrical communication with the detector and
the radio frequency source and that:
receives the reference cavity signal from the detector;
receives the reference phase control signal from the
radio frequency source; and

produces a mixed reference signal that comprises the
reference cavity signal and the reference phase con-
trol signal; and

a PIID servo in electrical communication with the mixer
and that receives the mixed reference signal and pro-
duces the optical feedback signal that is communicated
to the optical frequency controller and optionally to the
optical light source.

10. The optical refraction barometer of claim 1, wherein

65 the electronic reference arm comprises:

a detector in optical communication with the first polar-
ization controller and that receives the output reference

S

20

2

40

50

60


US 11,175,224 B2

27

12. The process of claim 11, wherein the optical frequency
controller comprises a voltage controlled oscillator in elec-
trical communication with the electronic reference arm, and
process further comprises:
receiving, by the voltage controlled oscillator, the optical
feedback signal from the electronic reference arm; and

producing, by the voltage controlled oscillator, a refer-
ence modulation frequency from the optical feedback
signal; and

wherein the optical frequency controller further com-

prises an acousto-optic modulator in electrical commu-
nication with the voltage controlled oscillator and in
optical communication with the optical light source,
and the process further comprises:

receiving, by the acousto-optic modulator, the reference

modulation frequency from the voltage controlled
oscillator and the laser light from the optical light
source; and

producing, by the acousto-optic modulator, the primary

light from modulation of the laser light at a modulation
frequency controlled by the reference modulation fre-
quency.

13. The process of claim 11, wherein the first optical
phase controller comprises an electro-optic modulator in
optical communication with the optical frequency controller
and in electrical communication with the electronic refer-
ence arm, and the process further comprises:

receiving, by the electro-optic modulator, the primary

light from the optical frequency controller;

receiving, by the electro-optic modulator, the reference

phase control signal from the electronic reference arm;
and

producing, by the electro-optic modulator, the input ref-

erence light from modulating the primary light at a
modulation frequency controlled by the reference
phase control signal.

14. The process of claim 11, wherein the second optical
phase controller comprises an electro-optic modulator in
optical communication with the optical frequency controller
and in electrical communication with the electronic sample
arm, and the process further comprises:

receiving, by the electro-optic modulator, the primary

light from the optical frequency controller;

receiving, by the electro-optic modulator, the sample

phase control signal from the electronic sample arm;
and

producing, by the electro-optic modulator, the input

sample light from modulating the primary light at a
modulation frequency controlled by the sample phase
control signal

15. The process of claim 11, wherein the first polarization
controller comprises a polarizing beam splitter in optical
communication with the first optical phase controller and in
electrical communication with the electronic reference arm,
and the process further comprises:

receiving, by the first polarization controller, the input

reference light from the first optical phase controller:
communicating, by the first polarization controller, the
input reference light to a waveplate;
receiving, by the first polarization controller, the output
reference light from the waveplate; and

communicating, by the first polarization controller, the
output reference light to the electronic reference arm;
and

wherein the first polarization controller further comprises

the waveplate in optical communication with the polar-

35

40

60

28

izing beam splitter and the dual fixed length optical

cavity refractometer, and the process further comprises:

receiving, by the waveplate, the input reference light
from the polarizing beam splitter;

receiving, by the waveplate, the output reference light
from the fixed length optical reference cavity; and

communicating, by the waveplate, the output reference
light to the polarizing beam splitter; and

wherein the second polarization controller comprises a

second polarizing beam splitter in optical communica-

tion with the second optical phase controller and in

electrical communication with the electronic sample

arm, and the process further comprises:

receiving, by the second polarizing beam splitter, the
input sample light from the second optical phase
controller;

communicating the input sample light to a second
waveplate from the second polarizing beam splitter;

receiving, by the second polarizing beam splitter, the
output sample light from the second waveplate; and

communicating the output sample light to the electronic
sample arm from the second polarizing beam splitter;
and

wherein the second polarization controller further com-

prises the second waveplate in optical communication
with the second polarizing beam splitter and the fixed
length optical sample cavity, and the process further
comprising:
receiving, by the second waveplate, the input sample
light from the second polarizing beam splitter;
receiving, by the second waveplate, the output sample
light from the fixed length optical sample cavity; and
communicating the output sample light to the polariz-
ing beam splitter from the second waveplate.

16. The process of claim 11, wherein the electronic
sample arm comprises a detector in optical communication
with the second polarization controller, and the process
further comprises:

receiving, by the detector, the output sample light from

the second polarization controller; and

producing, by the detector, a reference cavity signal from

the output sample light; and

wherein the electronic sample arm further comprises a

second sideband frequency generator, and the process
further comprises producing, by the second sideband
frequency generator, a second sideband frequency;

wherein the electronic sample arm further comprises a

mixer in electrical communication with the detector
and the second sideband frequency generator, and the
process further comprising:

receiving, by the mixer, the reference cavity signal from

the detector;
receiving, by the mixer, the second sideband frequency
from the second sideband frequency generator; and

producing, by the mixer, an error signal that comprises the
reference cavity signal and the second sideband fre-
quency;

wherein the electronic sample arm further comprises a

PID servo in electrical communication with the mixer,
and the process further comprises:

receiving, by the PID servo, the error signal; and

producing, by the PID servo, a servo control signal from

the error signal;

wherein the electronic sample arm further comprises a

first sideband frequency generator in electrical com-
munication with the PID servo, and the process further
comprises:


US 11,175,224 B2

17

further includes, wherein first polarization controller 233.1
includes polarizing beam splitter 215 in optical communi-
cation with first optical phase controller 230.1 and in elec-
trical communication with electronic reference arm 235:
receiving, by first polarization controller 233.1, input refer-
ence light 232 from first optical phase controller 230.1;
communicating, by first polarization controller 233.1, input
reference light 232 to a waveplate 216; receiving, by first
polarization controller 233.1, output reference light 234
from waveplate 216; and communicating, by first polariza-
tion controller 233.1, output reference light 234 to electronic
reference arm 235; and wherein first polarization controller
233.1 further includes waveplate 216 in optical communi-
cation with polarizing beam splitter 215 and dual fixed
length optical cavity refractometer 201, the process further
includes: receiving, by waveplate 216, input reference light
232 from polarizing beam splitter 215; receiving, by wave-
plate 216, output reference light 234 from fixed length
optical reference cavity 202; and communicating, by wave-
plate 216, output reference light 234 to polarizing beam
splitter 215; and wherein second polarization controller
233.2 includes second polarizing beam splitter 215 in optical
communication with second optical phase controller 230.2
and in electrical communication with electronic sample arm
240, the process further includes: receiving, by second
polarizing beam splitter 215, input sample light 238 from
second optical phase controller 230.2; communicating input
sample light 238 to second waveplate 216 from second
polarizing beam splitter 215; receiving, by second polarizing

beam splitter 215, output reference light 239 from second 3

waveplate 216; and communicating output reference light
239 to electronic sample arm 240 from second polarizing
beam splitter 215; and wherein second polarization control-
ler 233.2 further includes second waveplate 216 in optical

communication with second polarizing beam splitter 215 3

and fixed length optical sample cavity 203, the process
further includes: receiving, by second waveplate 216, input
sample light 238 from second polarizing beam splitter 215;
receiving, by second waveplate 216, output reference light
239 from fixed length optical sample cavity 203; and com-
municating output reference light 239 to polarizing beam
splitter 215 from second waveplate 216.

In an embodiment, the process for measuring pressure
based on refractivity changes between fixed length optical
reference cavity 202 and fixed length optical sample cavity
203 of dual fixed length optical cavity refractometer 201
further includes, wherein electronic sample arm 240
includes detector 212 in optical communication with second
polarization controller 233.2: receiving, by detector 212,
output reference light 239 from second polarization control-
ler 233.2; and producing, by detector 212, a reference cavity
signal 244 from output reference light 239; and wherein
electronic sample arm 240 further includes second sideband
frequency generator 241, the process further includes pro-
ducing, by second sideband frequency generator 241, a
second sideband frequency 245; wherein electronic sample
arm 240 further includes mixer 213 in electrical communi-
cation with detector 212 and second sideband frequency
generator 241, the process further includes: receiving, by
mixer 213, reference cavity signal 244 from detector 212;
receiving, by mixer 213, second sideband frequency 245
from second sideband frequency generator 241; and produc-
ing, by mixer 213, error signal 242 that includes reference
cavity signal 244 and second sideband frequency 245;
wherein electronic sample arm 240 further includes PID
servo 205 in electrical communication with mixer 213, the
process further includes: receiving, by PID servo 205, error

S

40

&

18

signal 242; and producing, by PID servo 205, a servo control
signal 263 from error signal 242; wherein electronic sample
arm 240 further includes first sideband frequency generator
243 in electrical communication with PID servo 205, the
process further includes: receiving, by first sideband fre-
quency generator 243, servo control signal 263; and pro-
ducing, by first sideband frequency generator 243, first
sideband frequency 246 from servo control signal 263; and
wherein electronic sample aim 240 further includes power
combiner 222 in electrical communication with first side-
band frequency generator 243 and second sideband fre-
quency generator 241, the process further includes: receiv-
ing, by power combiner 222, first sideband frequency 246
from first sideband frequency generator 243 and second
sideband frequency 245 from second sideband frequency
generator 241; producing, by power combiner 222, sample
phase control signal 237 from first sideband frequency 246
and second sideband frequency 245; and communicating
sample phase control signal 237 to second optical phase
controller 230.2 from power combiner 222.

In an embodiment, the process for measuring pressure
based on refractivity changes between fixed length optical
reference cavity 202 and fixed length optical sample cavity
203 of dual fixed length optical cavity refractometer 201
further includes, wherein electronic sample arm 240
includes detector 212 in optical communication with second
polarization controller 233.2: receiving, by detector 212,
output reference light 239 from second polarization control-
ler 233.2; and producing, by detector 212, reference cavity
signal 244 from output reference light 239; producing, by
second sideband frequency generator 241, second sideband
frequency 245; wherein electronic sample arm 240 further
includes mixer 213 in electrical communication with detec-
tor 212 and second sideband frequency generator 241, the
process further includes: receiving, by mixer 213, reference
cavity signal 244 from detector 212; receiving, by mixer
213, second sideband frequency 245 from second sideband
frequency generator 241; and producing, by mixer 213, error
signal 242 that includes reference cavity signal 244 and
second sideband frequency 245; wherein electronic sample
arm 240 further includes PID servo 205 in electrical com-
munication with mixer 213, the process further includes:
receiving, by PID servo 205, error signal 242; and produc-
ing, by PID servo 205, servo control signal 263 from error
signal 242; wherein electronic sample arm 240 further
includes first sideband frequency generator 243 in electrical
communication with PID servo 205 and second sideband
frequency generator 241, the process further includes:
receiving, by first sideband frequency generator 243, servo
control signal 263 from PID servo 205; receiving, by first
sideband frequency generator 243, second sideband fre-
quency 245 from second sideband frequency generator 241;
and producing, by first sideband frequency generator 243,
first sideband frequency 246 from servo control signal 263

sand second sideband frequency 245; and wherein electronic

sample arm 240 further includes directional coupler 211 in
electrical communication with first sideband frequency gen-
erator 243, the process further includes: receiving, by direc-
tional coupler 211, first sideband frequency 246 from first
sideband frequency generator 243; producing, by directional
coupler 211, sample phase control signal 237 from first
sideband frequency 246; and communicating sample phase
control signal 237 to second optical phase controller 230.2
from directional coupler 211

In an embodiment, the process for measuring pressure
based on refractivity changes between fixed length optical
reference cavity 202 and fixed length optical sample cavity


US 11,175,224 B2

1
OPTICAL REFRACTION BAROMETER

CROSS REFERENCE TO RELATED
APPLICATIONS

This application is a continuation-in-part of U.S. patent
application Ser. No. 16/520,668, filed Jul. 24, 2019, which
claims priority to U.S. Provisional Patent Application No.
62/714,953, filed Aug. 6, 2018; U.S. patent application Ser.
No. 16/520,672, filed Jul. 24, 2019, which claims priority to
US. Provisional Patent Application No. 62/714,953, filed
Aug. 6, 2018; and U.S. patent application Ser. No. 16/520,
687, filed Jul. 24, 2019, which claims priority to U.S.
Provisional Patent Application No. 62/714,953, filed Aug. 6,
2018, the disclosures of each of which are incorporated
herein by reference in its entirety. This application also
claims priority to U.S. Provisional Patent Application No.
62/877,926, filed Jul. 24, 2019, the disclosure of which is
incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY
SPONSORED RESEARCH

This invention was made with United States Government
support from the National Institute of Standards and Tech-
nology (NIST), an agency of the United States Department
of Commerce. The Government has certain rights in the
invention. Licensing inquiries may be directed to the Tech-
nology Partnerships Office, NIST, Gaithersburg, Md.,

20899; voice (301) 301-975-2573: email tpo@nist.gov; ref- 3

erence NIST Docket Number 19-048CIP1.
BRIEF DESCRIPTION

Disclosed is an optical refraction barometer for measuring 3

pressure based on refractivity changes between a fixed
length optical reference cavity and a fixed length optical
sample cavity of a dual fixed length optical cavity refracto-
meter, the optical refraction barometer comprising: an opti-
cal light source that produces laser light; an optical fre-
quency controller in optical communication with the optical
light source and that: receives the laser light from the optical
light source; receives an optical feedback signal from an
electronic reference arm; and produces primary light from
the laser light based on the optical feedback signal; a first
optical phase controller in optical communication with the
optical frequency controller and that: receives the primary
light from the optical frequency controller; receives a ref-
erence phase control signal; and produces an input reference
light from the primary light based on the reference phase
control signal; a first polarization controller in optical com-
munication with the first optical phase controller and that:
receives the input reference light from the first optical phase
controller; communicates the input reference light to a dual
fixed length optical cavity refractometer; receives a output
reference light from the dual fixed length optical cavity
refractometer; and communicates the output reference light
to an electronic reference arm; the electronic reference arm
in optical communication with the first polarization control-
ler and that: receives the output reference light from the first
polarization controller; produces the reference phase control
signal from the output reference light and communicates the
reference phase control signal to the first optical phase
controller; produces the optical feedback signal and com-
municates the optical feedback signal to the optical fre-
quency controller and optionally communicates the optical
feedback signal to the optical light source; a second optical

S

20

3

40

2

phase controller in optical communication with the optical
frequency controller and that: receives the primary light
from the optical frequency controller; receives a sample
phase control signal from an electronic sample arm; and
produces an input sample light from the primary light based
on the sample phase control signal; a second polarization
controller in optical communication with the second optical
phase controller and that: receives the input sample light
from the second optical phase controller; communicates the
input sample light to the dual fixed length optical cavity
refractometer; receives a output reference light from the dual
fixed length optical cavity refractometer; and communicates
the output reference light to the electronic sample arm; the
electronic sample arm in optical communication with the

s_ second polarization controller and in electrical communica-

tion with the second optical phase controller, the electronic
sample arm comprising: a detector that receives the output
reference light from the second polarization controller and
produces a reference cavity signal from the output reference
light; a second sideband frequency generator that produces
a second sideband frequency; a mixer in electrical commu-
nication with the detector and the second sideband fre-
quency generator and that produces an error signal that
comprises the reference cavity signal and the second side-

s band frequency; and a first sideband frequency generator in

electrical communication with the mixer and that receives
the error signal from the mixer and produces a first sideband
frequency from the error signal from which the sample
phase control signal is produced by the electronic sample
arm for communication to the second optical phase control-
ler; and the dual fixed length optical cavity refractometer
comprising: the fixed length optical reference cavity in
optical communication with the first polarization controller
and that: receives the input reference light from the first
polarization controller; propagates the input reference light
through fixed length optical reference cavity; and produces
the output reference light by reflecting input reference light
from a mirror disposed at a terminus of the fixed length
optical reference cavity disposed optically distal to the first
polarization controller; and the fixed length optical sample
cavity in optical communication with the second polariza-
tion controller and that: receives the input sample light from
the second polarization controller; propagates the input
sample light through the fixed length optical sample cavity;
and produces the output reference light by reflecting the
input sample light from a second mirror disposed at a
terminus of the fixed length optical sample cavity disposed
optically distal to the second polarization controller.
Disclosed is a process for measuring pressure based on
refractivity changes between the fixed length optical refer-
ence cavity and the fixed length optical sample cavity of the
dual fixed length optical cavity refractometer, the process
comprising: producing laser light by the optical light source;
receiving, by the optical frequency controller, the laser light

5 from the optical light source; receiving, by the optical

frequency controller, the optical feedback signal from the
electronic reference arm; producing, by the optical fre-
quency controller, the primary light from the laser light
based on the optical feedback signal; receiving, by the first
optical phase controller, the primary light from the optical
frequency controller; receiving, by the first optical phase
controller, the reference phase control signal; producing, by
the first optical phase controller, the input reference light
from the primary light based on the reference phase control

5 signal; receiving, by the first polarization controller, the

input reference light from the first optical phase controller;
communicating, by the first polarization controller, the input


U.S. Patent Nov. 16, 2021 Sheet 5 of 7 US 11,175,224 B2

201
P2332 203 B00 Va
218.2 ( (2 ( an
239 250 260.2
(Cpe at
« rs a eens reer > Ap. >
238 257 Kr
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( (
234
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T y 239 344 212
218.1 202, 259 Pl 256.1
326 350 nN 322
326328
v 324
er 3141
3142

330

332.1 332.2

FIG. 5


U.S. Patent Nov. 16, 2021 Sheet 4 of 7 US 11,175,224 B2

200 206 Je —{220 221 235
231.1 236-7] 244
p24 | A —+f213 2 Io12
2311-4 %
225
F2312 254
2u V 223 4
Bi 31 (a3
209 217 218)
232 230.1 234 233.1; 234 | 201
2301 y ma 249
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238 238 238
218) 215 1 216) \ 203 Yo
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209 217 218
Ly
237-4 1-239
240 245 244 _y
241 213. 212
245] ayy
221 221
247 246 263 |
223) 211 243 4 205, 220

FIG. 4


US 11,175,224 B2

19

203 of dual fixed length optical cavity refractometer 201
further includes, wherein electronic reference arm 235
includes detector 212 in optical communication with first
polarization controller 233.1: receiving, by detector 212,
output reference light 234 from first polarization controller
233.1; and producing, by detector 212. reference cavity
signal 244 from output reference light 234; wherein elec-
tronic reference arm 235 further includes radio frequency
source 224, the process further includes producing reference
phase control signal 231 by radio frequency source 224;
wherein electronic reference arm 235 includes mixer 213 in
electrical communication with detector 212 and radio fre-
quency source 224, the process further includes: receiving,
by mixer 213, reference cavity signal 244 from detector 212;
receiving, by mixer 213, reference phase control signal 231
from radio frequency source 224; and producing, by mixer
213, mixed reference signal 236 that includes reference
cavity signal 244 and reference phase control signal 231;
and wherein electronic reference arm 235 includes PIID
servo 206 in electrical communication with mixer 213, the
process further includes: receiving, by PIID servo 206,
mixed reference signal 236; producing, by PIID servo 206,
optical feedback signal 229 from mixed reference signal
236; and communicating, by PIID servo 206, optical feed-
back signal 229 to optical frequency controller 227 and
optionally to optical light source 251.

It should be appreciated that, with reference to FIG. 2, the
process for measuring pressure with dual sideband produc-
tion and based on refractivity changes between fixed length

optical reference cavity 202 and fixed length optical sample 3

cavity 203 of dual fixed length optical cavity refractometer
201 includes measuring pressure with a single laser source
and does not require the measurement of the optical beat
frequency of two lasers sources. The exact frequency dif-

ference between both arms is tuned with a source, and a 3

position of this source is measured with a frequency counter
directly. The dual sideband or sideband on sideband involves
combining both modulation and tuning frequencies.

With reference to FIG. 3, the process for measuring
pressure with electronic sideband production and based on
refractivity changes between fixed length optical reference
cavity 202 and fixed length optical sample cavity 203 of dual
fixed length optical cavity refractometer 201 includes lock-
ing the laser to a reference cavity and splitting the laser light
and frequency modulating a tuning sideband frequency that
can be independently locked to the sample cavity to provide
more efficient use of laser light with no light wasted in
sidebands around the carrier frequency.

With reference to FIG. 4, the process for measuring
pressure based on refractivity changes between fixed length
optical reference cavity 202 and fixed length optical sample
cavity 203 of dual fixed length optical cavity refractometer
201 includes splitting light from a stabilized laser source
into two arms (reference and sample) of the cavity that are
locked independently. Here, both arms can use the dual side
band or electronic sideband process for offset tuning from
the stabilized laser and locking to the respective cavity. The
process operates in an absence of an external determination
of the wavelength of the laser locked to the reference cavity.
That is, the wavelength is known exactly, and the offset
frequency from the known frequency of the laser source is
measured directly for each arm with the directional coupler
and frequency counter. The pressure is determined by mea-
suring the difference in resonant frequency between the two
cavities at vacuum and at pressure and is realized by the sign
of the offset (+/—from the carrier) and summing the frequen-
cies from the two counters.

2

40

&

3

20
Optical refraction barometer 200 and processes disclosed
herein have numerous beneficial uses, including absence of
more than one laser source, fast photodetector to determine
a beat note from transmission of light through sample and
reference cavities, and external determination of the wave-
length. Advantageously, optical refraction barometer 200
overcomes limitations or technical deficiencies of conver
tional devices such as noise and electro-optic complexity
from heterodyning two independent laser systems to mea-
sure the difference frequency. In an aspect, optical refraction
barometer 200 has sensitivity and resolution of 4 mPa for
gases such as N3.
Optical refraction barometer 200 and processes herein

s unexpectedly determine pressure of a sample gas in the

sample cavity referenced to the reference cavity via optical
methods and sideband production in an absence of a fast
photodetector, fiber splitters, and combiners for measuring a
difference frequency. In optical refraction barometer 200,
the frequency difference is measured exactly from an
extremely low noise high stability frequency source. The
stability of the tight reference lock is transferred to the
sample cavity and thus makes the laser locking to the sample
cavity more robust and easier to implement. Drifts in the

S laser itself would appear in both arms and have greater

common mode noise reduction providing long term stability
and accurate pressure measurement. Optical refraction
barometer is less costly than other devices that have multiple
lasers and a fast photodetector. Moreover, optical refraction
barometer 200 provides a method to realize pressure that can
be made as a primary standard with much improved robust-
ness and streamlined setup.

The articles and processes herein are illustrated further by
the following Example, which is non-limiting,

EXAMPLE

Measurements were performed in an optical refraction
barometer configured according to FIG. 3. The system
included vacuum pressure in the reference cavity for the
measurement. The measurement had a noise floor for the
pressure measurement. A narrow linewidth fiber laser was
modulated with an EOM and PDH-locked to the reference
cavity using fast feedback to an AOM and slow feedback to
the laser piezo transducer. A portion of the locked laser light
was split and sent into a second EOM. This EOM was driven
by a microwave synthesizer that was frequency modulated at
10 MHz. The PDH error signal was digitized and PID servo
in software was used to control the frequency of the micro-
wave synthesizer to maintain lock on the sample cavity.

Results are shown in FIG. 7. In the upper panel of FIG.
7, the frequency stability of the lock signal on the sample
cavity is shown. The lower panel of FIG. 7 shows the Allan
deviation. The Allan deviation was about 2 kHz over the
measurement period of 4 minutes sets a minimum detectable
pressure and pressure resolution for this particular configu-
ration. For the 15 cm long sample cavity, the frequency shift
at 1542 nm was about 510 kHz/Pa for N5. An Allan deviation
of 2 kHz corresponded to 4 mPa N, as measured by the
single near IR laser with offset-locking

While one or more embodiments have been shown and
described, modifications and substitutions may be made
thereto without departing from the spirit and scope of the
invention. Accordingly, it is to be understood that the present
invention has been described by way of illustrations and not
limitation. Embodiments herein can be used independently
or can be combined.


U.S. Patent Nov. 16, 2021 Sheet 7 of 7 US 11,175,224 B2

Frequency (Hz)
i er
3S S383
oo eo mo
SoS SS
Sonera
me
te
eT ap
ee es
re
Pon
mosrccisgs ae
pec

1052

so we To?
Time {s}

FIG. 7


a2 United States Patent

Douglass et al.

US011175224B2

US 11,175,224 B2
Nov. 16, 2021

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

(54)
(71)

(72)

(73)

(*)

(21)

(22)
(65)

(63)

(51)

(52)

OPTICAL REFRACTION BAROMETER

Applicant:

Inventors:

Assignee:

Notice:

Appl. No.:
Filed:

US 2020/0355606 Al

Government of the United States of
America, as represented by the
Secretary of Commerce, Gaithersburg,
MD (US)

Kevin O’Connell Douglass, Ellicott
City, MD (US); Stephen Paul Eckel,
Gaithersburg, MD (US); Jacob
Edmond Ricker, Gaithersburg, MD
(US); Jay Howard Hendricks,
Clarksburg, MD (US)

GOVERNMENT OF THE UNITED
STATES OF AMERICA, AS
REPRESENTED BY THE
SECRETARY OF COMMERCE,
Gaithersburg, MD (US)

Subject to any disclaimer, the term of this
patent is extended or adjusted under 35
U.S.C. 154(b) by 0 days.

16/938,300

Jul. 24, 2020

Prior Publication Data
Nov. 12, 2020

Related U.S. Application Data

Continuation-in-part of application No. 16/520,672,
filed on Jul. 24, 2019, now Pat. No. 10,816,325, and

(Continued)
Int. CL
GOIN 21/41 (2006.01)
GOIB 9/02 (2006.01)
US. Cl.
CPC weve GOIN 21/41 (2013.01); GOIB 9/02011

(2013.01); GOIN 2021/4106 (2013.01), GOIN
2021/4153 (2013.01)

(58) Field of Classification Search
CPC .. GOIB 9/02011; GOIN 21/41; GOIN
2021/4106; GOIN 2021/4153; GOIL
11/02
See application file for complete search history.

(56) References Cited
U.S. PATENT DOCUMENTS
4,070,683 A 1/1978 Altschuler et al.
5,528,040 A 6/1996 Lehmann
(Continued)
FOREIGN PATENT DOCUMENTS
RU 2089848 C1 9/1997

OTHER PUBLICATIONS

Thorpe, JL, et al. “Laser frequency stabilization and control
through offset sideband locking to optical cavities”, Optics Express,
2008, p. 15980-15990, vol. 16 No. 20.

(Continued)

Primary Examiner — Michael A Lyons
(74) Attorney, Agent, or Firm — Otlice of Chief Counsel
for National Institute of Standards and Technology

(67) ABSTRACT

An optical refraction barometer measures pressure based on
refractivity changes and includes: an optical light source; an
optical frequency controller; a first optical phase controller;
a first polarization controller; an electronic reference arm in
optical communication with the first polarization controller;
a second optical phase controller in optical communication
with the optical frequency controller; a second polarization
controller in optical communication with the second optical
phase controller; an electronic sample arm in optical com-
munication with the second polarization controller and in
electrical communication with the second optical phase
controller; a second sideband frequency generator; a mixer
in electrical communication with the detector and the second
sideband frequency generator; and a first sideband frequency

(Continued)
234 20
234
\ nt
y i
22 23232
238
iy nN
a | Y
239
L239



US 11,175,224 B2

5

cavity and sample cavity is not used since the optical
refraction barometer directly determines the difference in
frequency with a counter. That is, instead of determining
pressure in the sample cavity from the beat note or frequency
difference between the reference cavity and the sample
cavity, the optical refraction barometer directly measures the
frequency difference by the tuning frequency input to the
EOM. It is contemplated that a high-performance micro-
wave synthesizer can be used to tune the EOM, or a
direct-digital-synthesis (DDS) device with fast analog con-
trol can be used to tune the KOM. A low bandwidth
correction signal maintains lock to the sample cavity
because the laser is pre-stabilized to the reference cavity. An
error signal from the sample cavity is digitized, and a low
bandwidth proportional-integral-derivative (PID) servo
directly feedbacks to the microwave synthesizer.

Optical refraction barometer 200 measures pressure based
on refractivity changes between fixed length optical refer-
ence cavity 202 and fixed length optical sample cavity 203
of dual fixed length optical cavity refractometer 201. In an
embodiment, with reference to FIG. 1, FIG. 2, FIG. 3, FIG.
4, and FIG. 5, optical refraction barometer 200 includes:
optical refraction barometer 200 for measuring pressure
based on refractivity changes between fixed length optical
reference cavity 202 and fixed length optical sample cavity 2s
203 of dual fixed length optical cavity refractometer 201,
optical refraction barometer 200 including: optical light
source 251 that produces laser light 226; optical frequency
controller 227 in optical communication with optical light
source 251 and that: receives laser light 226 from optical 3
light source 251; receives optical feedback signal 229 from
electronic reference arm 235; and produces primary light
228 from laser light 226 based on optical feedback signal
229; first optical phase controller 230.1 in optical commu-
nication with optical frequency controller 227 and that:
receives primary light 228 from optical frequency controller
227; receives reference phase control signal 231; and pro-
duces input reference light 232 from primary light 228 based
on reference phase control signal 231; first polarization
controller 233.1 in optical communication with first optical
phase controller 230.1 and that: receives input reference
light 232 from first optical phase controller 230.1; commu-
nicates input reference light 232 to dual fixed length optical
cavity refractometer 201; receives output reference light 234
from dual fixed length optical cavity refractometer 201; and 45
communicates output reference light 234 to electronic ref-
erence arm 235; electronic reference arm 235 in optical
communication with first polarization controller 233.1 and
that: receives output reference light 234 from first polariza-
tion controller 233.1; produces reference phase control sig-
nal 231 from output reference light 234 and communicates
reference phase control signal 231 to first optical phase
controller 230.1; produces optical feedback signal 229 and
communicates optical feedback signal 229 to optical fre-
quency controller 227 and optionally communicates optical 55
feedback signal 229 to optical light source 251; second
optical phase controller 230.2 in optical communication
with optical frequency controller 227 and that: receives
primary light 228 from optical frequency controller 227;
receives sample phase control signal 237 from electronic
sample arm 240; and produces input sample light 238 from
primary light 228 based on sample phase control signal 23
second polarization controller 233.2 in optical communic:
tion with second optical phase controller 230.2 and that:
receives input sample light 238 from second optical phase 65
controller 230.2; communicates input sample light 238 to
dual fixed length optical cavity refractometer 201; receives

3

40

6

output reference light 239 from dual fixed length optical
cavity refractometer 201; and communicates output refer-
ence light 239 to electronic sample arm 240; electronic
sample arm 240 in optical communication with second
polarization controller 233.2 and in electrical communica-
tion with second optical phase controller 230.2, electronic
sample arm 240 including: detector 212 that receives output
reference light 239 from second polarization controller
233.2 and produces reference cavity signal 244 from output
reference light 239; second sideband frequency generator
241 that produces second sideband frequency 245; mixer
213 in electrical communication with detector 212 and
second sideband frequency generator 241 and that produces
error signal 242 that includes reference cavity signal 244 and
second sideband frequency 245; and first sideband fre-
quency generator 243 in electrical communication with
mixer 213 and that receives error signal 242 from mixer 213
and produces first sideband frequency 246 from error signal
242 from which sample phase control signal 237 is produced
by electronic sample arm 240 for communication to second
optical phase controller 230.2; and dual fixed length optical
cavity refractometer 201 including: fixed length optical
reference cavity 202 in optical communication with first
polarization controller 233.1 and that: receives input refer-
ence light 232 from first polarization controller 233.1;
propagates input reference light 232 through fixed length
optical reference cavity 202; and produces output reference
light 234 by reflecting input reference light 232 from a
mirror disposed at a terminus of fixed length optical refer-
ence cavity 202 disposed optically distal to first polarization
controller 233.1; and fixed length optical sample cavity 203
in optical communication with second polarization control-
ler 233.2 and that: receives input sample light 238 from
second polarization controller 233.2; propagates input
sample light 238 through fixed length optical sample cavity
203; and produces output reference light 239 by reflecting
input sample light 238 from a second mirror disposed at a
terminus of fixed length optical sample cavity 203 disposed
optically distal to second polarization controller 233.2.

In an embodiment, optical light source 251 includes laser
210 that produces laser light 226; and Faraday isolator 207
in optical communication with laser 210 and that commu-
nicates laser light 226 to optical frequency controller 227

In an embodiment, optical frequency controller 227
includes voltage controlled oscillator 204 in electrical com-
munication with electronic reference arm 235 and that
receives optical feedback signal 229 from electronic refer-
ence arm 235 and that produces reference modulation fre-
quency 252 from optical feedback signal 229; acousto-optic
modulator 208 in electrical communication with voltage
controlled oscillator 204 and in optical communication with
optical light source 251 and that: receives reference modu-
lation frequency 252 from voltage controlled oscillator 204
and laser light 226 from optical light source 251; and
produces primary light 228 from modulation of laser light
226 at a modulation frequency controlled by reference
modulation frequency 252.

In an embodiment, optical frequency controller 227 fur
ther includes wavelength detector 214 in optical communi-
cation with acousto-optic modulator 208 and that receives
primary light 228 from acousto-optic modulator 208 and
produces optical frequency counter signal 253 from primary
light 228.

In an embodiment, first optical phase controller 230.1
includes electro-optic modulator 209 in optical communi-
cation with optical frequency controller 227 and in electrical
communication with electronic reference arm 235 and that:


US 11,175,224 B2

3

reference light to the dual fixed length optical cavity refrac-
tometer; receiving, by the first polarization controller, the
output reference light from the dual fixed length optical
cavity refractometer; communicating, by the first polariza-
tion controller, the output reference light to the electronic
reference arm; receiving, by the electronic reference arm,
the output reference light from the first polarization control-
ler; producing, by the electronic reference arm, the reference
phase control signal from the output reference light; com-
municating, by the electronic reference arm, the reference
phase control signal to the first optical phase controller;
producing, by the electronic reference arm, the optical
feedback signal; communicating, by the electronic reference
arm, the optical feedback signal to the optical frequency
controller; optionally communicating the optical feedback
signal to the optical light source from the electronic refer-
ence arm; receiving, by the second optical phase controller,
the primary light from the optical frequency controller;
receiving, by the second optical phase controller, the sample
phase control signal from the electronic sample arm; pro-
ducing, by the second optical phase controller, the input
sample light from the primary light based on the sample
phase control signal; receiving, by the second polarization
controller, the input sample light from the second optical
phase controller; communicating, by the second polarization
controller, the input sample light to the dual fixed length
optical cavity refractometer; receiving, by the second polar-
ization controller, an output reference light from the dual
fixed length optical cavity refractometer; communicating, by

the second polarization controller, the output reference light 3

to the electronic sample arm; receiving, by the detector of
the electronic sample arm, the output reference light from
the second polarization controller and producing the refer-
ence cavity signal from the output reference light; produc-
ing, by the second sideband frequency generator of the
electronic sample arm, the second sideband frequency: pro-
ducing, by the mixer of the electronic sample arm, the error
signal; receiving, by the first sideband frequency generator
of the electronic sample arm, the error signal and producing
the first sideband frequency 246 from the error signal;
producing, by the electronic sample arm, the sample phase
control signal from the first sideband frequency 246; com-
municating the sample phase control signal to the second
optical phase controller from the electronic sample arm;
receiving, by the fixed length optical reference cavity, the
input reference light from the first polarization controller;
propagating the input reference light through fixed length
optical reference cavity; producing, by the fixed length
optical reference cavity, the output reference light by reflect-
ing the input reference light from the mirror disposed at the
terminus of the fixed length optical reference cavity; receiv-
ing, by the fixed length optical sample cavity, the input
sample light from the second polarization controller; propa-
gating the input sample light through the fixed length optical
sample cavity; and producing, by the fixed length optical
sample cavity, the output reference light by reflecting the
input sample light from the second mirror disposed at the
terminus of the fixed length optical sample cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description should not be considered lim-
iting in any way. With reference to the accompanying
drawings, like elements are numbered alike.

FIG. 1 shows an optical refraction barometer;

FIG. 2 shows an optical refraction barometer to produce
dual sidebands;

S

2

40

&

6:

4

FIG. 3 shows an optical refraction barometer to produce
electronic sidebands;

FIG. 4 shows an optical refraction barometer;

FIG. 5 shows a dual fixed length optical cavity refracto-
meter;

FIG. 6 shows: in panel A, a graph of relative intensity
versus frequency for laser light transmission through refer-
ence cavity 202 with laser light 228 locked to a peak of the
transmission; and in panel B, transmission of light though
the sample cavity, wherein offset tuning frequency 243 is
applied to EOM 209 to tune a sideband into resonance with
the sample cavity. Some dashed lines show sidebands output
from EOM 209. Frequency 241 creates an error signal for
locking the laser to the offset sideband. Panel C shows a
dashed lines around laser light 228 that indicate sidebands
from frequency 241 applied to EOM 209 for dual sideband
production. For the electronic sideband production, these
sidebands are absent; and

FIG. 7 shows a graph of frequency versus time and a
graph of Allan deviation versus time.

DETAILED DESCRIPTION

A detailed description of one or more embodiments is

s presented herein by way of exemplification and not limita-

tion.

It has been discovered that an optical refraction barometer
described herein measures refractivity-based pressure
changes using dual Fabry-Perot cavities with a single laser
and off-set sideband locking to one of the cavities. The
optical refraction barometer provides refractivity-based
pressure measurements performed to very high precision in
an absence of being tied to a mercury manometer. As a
result, the optical refraction barometer moves away from
traceability to the International System of Units (SI, abbre-
viated from the French Systéme international (d’unités)) via
mechanical measurements of pressure, wherein conven-
tional traceability to the SI involves a manometer based on
fluid density, local gravity, and column height or piston
gauge force per unit area. Advantageously, the optical
refraction barometer achieves traceability to the SI through
quantum mechanical aspects of photons interacting with a
gas. Quantum chemical determinate of the refractive index
of helium provides traceability to the SI sot that the optical
refraction barometer can be a quantum-based pressure stan-
dard with a dual fixed length optical cavity refractometer
that receives light from a single laser producing light that
can have a telecom wavelength. Beneficially, inclusion of
the single laser lowers cost compared to conventional
devices,

In an embodiment, the optical refraction barometer
includes offset sideband locking to the dual fixed length
optical cavity refractometer, wherein the laser light from the
single laser is split, and a portion of the laser light power is

‘5 used to stabilize the laser to a fixed length optical reference

cavity and another portion of the laser light power is coupled
to a broad bandwidth electro-optic modulator (EOM) that is
separately locked to a fixed length optical sample cavity of
the dual fixed length optical cavity refractometer. The EOM
in optical communication with the fixed length optical
sample cavity can be driven by an RF source at a frequency,
e.g. on the order of a gigahertz and a second frequency used
for an error signal such as a Pound-Drever-Hall (PDH) lock.
Dual-side band and electronic side-band production are
provided by the optical refraction barometer.

With the optical refraction barometer, a high-speed pho-
todiode measuring the beat frequency between the reference


U.S. Patent Nov. 16, 2021 Sheet 6 of 7 US 11,175,224 B2

L (A)
LLight in Reference Cavity 202 HN 228
. iia
Ee t 249
2 . 3 /
5 3 x
= "i " rt a A " " x x.
— i
2 4® 1 7778243 203 |
= 1 2
@ fightin * —>\
Sample Cavity i i
203 {242 '
paral
ri
" et z ime z cae " bedked "

Sxeneesenerer

Frequency

FIG. 6


US 11,175,224 B2

25

light from the first polarization controller; and produces
a reference cavity signal from the output reference
light;
a radio frequency source that produces a first reference
phase control signal;
a mixer in electrical communication with the detector and
the radio frequency source and that:
receives the reference cavity signal from the detector;
receives the first reference phase control signal from
the radio frequency source; and

produces a mixed reference signal that comprises the
reference cavity signal and the first reference phase
control signal;
a PIID servo in electrical communication with the mixer
and that receives the mixed reference signal and pro-
duces the optical feedback signal that is communicated
to the optical frequency controller and optionally to the
optical light source;
a sideband frequency source in electrical communication
with the radio frequency source and the PIID servo and
that:
receives the first reference phase control signal from
the radio frequency source;

receives the optical feedback signal from the PIID
servo; and

produces a second reference phase control signal from
the optical feedback signal and the first reference
phase control signal;

a directional coupler in electrical communication with the 3

sideband frequency source and that:

receives the second reference phase control signal from
the sideband frequency source:

produces the reference phase control signal from the
second reference phase control signal; and

communicate the reference phase control signal to the
first optical phase controller.

11. A process for measuring pressure based on refractivity
changes between the fixed length optical reference cavity
and the fixed length optical sample cavity of the dual fixed
length optical cavity refractometer of the optical refraction
barometer of claim 1, the process comprising:

producing laser light by the optical light source:

receiving, by the optical frequency controller, the laser

light from the optical light source;
receiving, by the optical frequency controller, the optical
feedback signal from the electronic reference arm;

producing, by the optical frequency controller, the pri-
mary light from the laser light based on the optical
feedback signal;

receiving, by the first optical phase controller, the primary

light from the optical frequency controller;

receiving, by the first optical phase controller, the refer-

ence phase control signal;

producing, by the first optical phase controller, the input

reference light from the primary light based on the
reference phase control signal;
receiving, by the first polarization controller, the input
reference light from the first optical phase controller;

communicating, by the first polarization controller, the
input reference light to the dual fixed length optical
cavity refractometer;

receiving, by the first polarization controller, the output

reference light from the dual fixed length optical cavity
refractometer;

communicating, by the first polarization controller, the

output reference light to the electronic reference arm:

10

0

2

45

50

0

65

26

receiving, by the electronic reference arm, the output
reference light from the first polarization controller;

producing, by the electronic reference arm, the reference
phase control signal from the output reference light;

communicating, by the electronic reference arm, the ref-
erence phase control signal to the first optical phase
controller;

producing, by the electronic reference arm, the optical
feedback signal:

communicating, by the electronic reference arm, the opti-
cal feedback signal to the optical frequency controller;

optionally communicating the optical feedback signal to
the optical light source from the electronic reference
arm;

receiving, by the second optical phase controller, the
primary light from the optical frequency controller;

receiving, by the second optical phase controller, the
sample phase control signal from the electronic sample
arm;

producing, by the second optical phase controller, the
input sample light from the primary light based on the
sample phase control signal;

receiving, by the second polarization controller, the input
sample light from the second optical phase controller;

communicating, by the second polarization controller, the
input sample light to the dual fixed length optical cavity
refractometer;

receiving, by the second polarization controller, an output
sample light from the dual fixed length optical cavity
refractometer;

communicating, by the second polarization controller, the
output sample light to the electronic sample arm;

receiving, by the detector of the electronic sample arm,
the output sample light from the second polarization
controller and producing the reference cavity signal
from the output sample light;

producing, by the second sideband frequency generator of
the electronic sample arm, the second sideband fre-
quency;

producing, by the mixer of the electronic sample arm, the
error signal;

receiving, by the first sideband frequency generator of the
electronic sample arm, the error signal and producing
the first sideband frequency from the error signal;

producing, by the electronic sample arm, the sample
phase control signal from the first sideband frequency;

communicating the sample phase control signal to the
second optical phase controller from the electronic
sample arm;

receiving, by the fixed length optical reference cavity, the
input reference light from the first polarization control-
ler;

propagating the input reference light through fixed length
optical reference cavity;

producing, by the fixed length optical reference cavity, the
output reference light by reflecting the input reference
light from the mirror disposed at the terminus of the
fixed length optical reference cavity;

receiving, by the fixed length optical sample cavity, the
input sample light from the second polarization con-
troller;

propagating the input sample light through the fixed
length optical sample cavity; and

producing, by the fixed length optical sample cavity, the
output sample light by reflecting the input sample light
from the second mirror disposed at the terminus of the
fixed length optical sample cavity.



U.S. Patent Nov. 16, 2021 Sheet 2 of 7 US 11,175,224 B2

200 Dual sideband production
229.2
251 235
10 206 220 221
[+226 236-4
231 244
207 24 |_ 2 >]213 2212
: |_—234
218)
234 233.1) 234 | 201
4 4 249
¥ S
218 215 iN 21 202
232 232
238 238
z
5 \ 250
215 216 203] | 4
214 y
238 230.2 239 233.2) 239
209 217 218
237-7] 239
240 245 245 244
222 241 213, 212
246-4 r—-4
221 221
247 246 263
223) 211 1243 205, 220

FIG. 2


US 11,175,224 B2

11

output of a frequency of radio frequency oscillator 204. A
radio frequency oscillator with output at a given peak
frequency near the resonance of the AOM+/-20% of AOM
bandwidth. The radio frequency oscillator has an output
from 1 MHz to 3 GHZ, and more specifically 50 MHz+/-20
MHz.

Acousto-optic modulator 208 receives laser light 226
from laser 210, a control signal from radio frequency
oscillator 204 that controls a modulation frequency of
acousto-optic modulator 208, and produces primary light
228 that is modulated at the modulation frequency. The
modulation frequency can be from 50 MHz to 5 GHz,
specifically from 50 MHz to 1 GHz, and more specifically
from 40 MHz to 60 MHz. Primary light 228 has a wave-
length that can be from 600 nm to 3000 nm, specifically
from 1000 nm to 1600 nm, and more specifically from 1540
nm to 1545 nm and can have an optically modulation
frequency provided by that of acousto-optic modulator 208.
Other properties of primary light 228 are provided by laser
light 226.

It is contemplated that laser light 226 and primary light
228 as well as other laser light used in optical refraction
barometer 200 can propagate through free space or fiber
optic cable. For Fiber optic propagation of laser light, optical
splitter 255 receives primary light 228 from acousto-optic
modulator 208, wherein optical splitter 255 is a condensed-
matter medium that splits light. Exemplary fiber opti
include single-mode fiber optical cable, polarization-maii
taining single-mode fiber optical cable, and the like.

Wavelength detector 214 receives primary light 228 from
optical splitter 255, detects the wavelength or power of
primary light 228, and outputs optical frequency counter
signal 253 from primary light 228 that indicates the wave-
length of primary light 228.

Electro-optic modulator 209 can include a fiber-based
electrooptic crystal driven by reference phase control signal
231 or sample phase control signal 237 to phase modulate
laser light traveling through optical fiber. Phase modulation
depths generated by electro-optic modulator 209 can be
from 0.01 to 0.1. Electro-optic modulation of primary light
228 by electro-optic modulator 209 in optical phase con-
troller 230.1 produces input reference light 232, and electro-
optic modulation of primary light 228 in optical phase
controller 230.2 produces input sample light 238. Input
reference light 232 and input sample light 238 independently
have a wavelength that can be from 600 nm to 3000 nm,
specifically from 1000 nm to 1600 nm, and more specifically
from 1540 nm to 1545 nm provided by electro-optic modu-
lator 209. Other properties of input reference light 232 and
input sample light 238 are respectively provided by primary
light 228.

In optical refraction barometer 200, fiber port 217 can
include an optic that couples light from air to a condensed
matter medium such as fiber optic cable. The efficiency of
the coupling can be from 0.1 to 1, specifically from 0.5 to
1.0. Exemplary fiber ports 217 include fiber-to-free space
couplers. It is contemplated that light upstream of fiber port
217 propagates in fiber optic cable while light downstream
of fiber port 217 propagates in free space.

Mirror 218 can include a concave, convex, or flat mirror
to reflect and direct light. Exemplary mirror 218 includes a
planar dielectric mirror that is anti-reflective coated on the
input and highly reflective on the inside. The reflectivity of
mirror 218 can be from 0.1 to 1.0, specifically from 0.9 to
1.0, and more specifically from 0.999 to 1.0 ata wavelength
from the ultraviolet (UV) to infrared (IR).

40

60

12

Polarizing beam splitter 218 can include an optic that
splits the incoming beam power according to its polarization
state, with orthogonal linear polarizations traveling orthogo-
nally to each other. In this manner, polarizing beam splitter
215, can communicate light to dual fixed length optical
cavity refractometer 201 and receive reflected light from
dual fixed length optical cavity refractometer 201 for com-
munication, e.g., to electronic reference arm 235 or elec-
tronic sample arm 240. Exemplary polarizing beam splitter
are polarizing beam splitting cubes and Glan-Laser calcite
polarizers.

Waveplate 216 can include an optic to manipulate a
polarization of light (e.g., 232, 238, 234, 239) propagating
through waveplate 216. Exemplary waveplates and polariz-

5 ers include birefringent crystals and calcite polarizers.

Detector 212 can transduce the power of light (e.g., 234,
239) into reference cavity signal 244. A bandwidth of
detector 212 can be from 0 Hz and 10 GHz, specifically from
0 and the frequency of the free spectral range define by
cavity (202 or 203). Exemplary detector 212 includes a
power meter, photodiode, photoreceiver, and the like. Cavity
signal 260 can include an electrical signal, e.g., voltage or
current, that provides the frequency, power, or intensity, of
light received by detector 212

Radio frequency source 224 produces reference phase
control signal 231 that is an oscillating electrical signal for
electro-optic modulator 209 and mixer 213. The frequency
and power of reference phase control signal 231 are in a
range of operation of electro-optic modulator 209.

Mixer 213 receives multiple electrical inputs (e.g., 231,
244, 245), combines the inputs, and produces an electrical
output (e.g., 236, 242) that includes both of the electrical
signals in a combined electrical waveform. Amplifier 221
Receives such electrical signals and amplifies an amplitude
of the signal. Further, bandpass filter 220 filters frequency
electrical signals to eliminate frequency components outside
a selected range. The filter bandwidth can be 30% or less
than the modulation frequency from 5 MHz to 12 MHz,
more specifically from 3 MHz and 6 MHz.

PIID servo 206 provides low-noise servo control of laser
210 and radio frequency oscillator 204, PIID servo 206 can
include integral feedback and locking to cavities (202, 203),
e.g., with atomic or molecular transitions. Further, loop-filter
parameters provide servo-loop optimization and can detect
when the servo loop filter has gone out of lock and with
recapture of lock, wherein PIID servo 206 can monitor
optical feedback signal 229 for detecting unlocking that
produces a disengagement of electro-optic modulator 209
with return of optical feedback signal 229 to the last known
valid value until electro-optic modulator 209 re-engages the
lock.

In some embodiments, sideband, frequency source 225
receives reference phase control signal 231.1 from radio
frequency source 224 to produce reference phase control

5. signal 231.2 that is communicated to directional coupler 211

for communication as reference phase control signal 231 to
electro-optic modulator 209 of optical phase controller
230.1 and also to frequency counter 223 that produces
reference arm counter signal 254 from reference phase
control signal 231. In this manner, an embodiment config-
ured as shown in FIG. 4 can be different than embodiments
configured according to FIG. 2 and FIG. 3, wherein an
alternate method of measuring the laser wavelength is
eliminated. The exact wavelength is known and uses radio

5 frequency tuning of modulation frequencies to lock the two

arms (reference and sample) of dual fixed length optical
cavity refractometer 201. Frequency tuning is measured


US 11,175,224 B2

15

changes between fixed length optical reference cavity 202
and fixed length optical sample cavity 203 of dual fixed
length optical cavity refractometer 201 includes: producing
laser light 226 by optical light source 251; receiving, by
optical frequency controller 227, laser light 226 from optical
light source 251; receiving, by optical frequency controller
227, optical feedback signal 229 from electronic reference
arm 235; producing, by optical frequency controller 227,
primary light 228 from laser light 226 based on optical
feedback signal 229; receiving, by first optical phase con-
troller 230.1, primary light 228 from optical frequency
controller 227; receiving, by first optical phase controller
230.1, reference phase control signal 231; producing, by first
optical phase controller 230.1, input reference light 232
from primary light 228 based on reference phase control
signal 231; receiving, by first polarization controller 233.1,
input reference light 232 from first optical phase controller
230.1; communicating, by first polarization controller 233.1,
input reference light 232 to dual fixed length optical cavity
refractometer 201; receiving, by first polarization controller
233.1, output reference light 234 from dual fixed length
optical cavity refractometer 201; communicating, by first
polarization controller 233.1, output reference light 234 to
electronic reference arm 235; receiving, by electronic ref-
erence arm 235, output reference light 234 from first polar-
ization controller 233.1; producing, by electronic reference
arm 235, reference phase control signal 231 from output
reference light 234; communicating, by electronic reference
arm 235, reference phase control signal 231 to first optical

phase controller 230.1; producing, by electronic reference 3

arm 235, optical feedback signal 229; communicating, by
electronic reference arm 235, optical feedback signal 229 to
optical frequency controller 227; optionally communicating
optical feedback signal 229 to optical light source 251 from

electronic reference arm 235; receiving, by second optical 3

phase controller 230.2, primary light 228 from optical
frequency controller 227; receiving, by second optical phase
controller 230.2, sample phase control signal 237 from
electronic sample arm 240; producing, by second optical
phase controller 230.2, input sample light 238 from primary
light 228 based on sample phase control signal 237; receiv-
ing, by second polarization controller 233.2, input sample
light 238 from second optical phase controller 230.2; com-
municating, by second polarization controller 233.2, input
sample light 238 to dual fixed length optical cavity refrac-
tometer 201; receiving, by second polarization controller
233.2, an output reference light 239 from dual fixed length
optical cavity refractometer 201; communicating, by second
polarization controller 233.2, output reference light 239 to
electronic sample arm 240; receiving, by detector 212 of
electronic sample arm 240, output reference light 239 from
second polarization controller 233.2 and producing refer-
ence cavity signal 244 from output reference light 239;
producing, by second sideband frequency generator 241 of
electronic sample arm 240, second sideband frequency 245;
producing, by mixer 213 of electronic sample arm 240, error
signal 242; receiving, by first sideband frequency generator
243 of electronic sample arm 240, error signal 242 and
producing first sideband frequency 246 from error signal
242; producing, by electronic sample arm 240, sample phase
control signal 237 from first sideband frequency 246; com-
municating sample phase control signal 237 to second
optical phase controller 230.2 from electronic sample arm
240; receiving, by fixed length optical reference cavity 202,
input reference light 232 from first polarization controller
233.1; propagating input reference light 232 through fixed
length optical reference cavity 202; producing, by fixed

S

40

&

16

length optical reference cavity 202, output reference light
234 by reflecting input reference light 232 from mirror
disposed at terminus of fixed length optical reference cavity
202; receiving, by fixed length optical sample cavity 203,
input sample light 238 from second polarization controller
233.2; propagating input sample light 238 through fixed
length optical sample cavity 203; and producing, by fixed
length optical sample cavity 203, output reference light 239
by reflecting input sample light 238 from second mirror
disposed at terminus of fixed length optical sample cavity
203.

In an embodiment, the process for measuring pressure
based on refractivity changes between fixed length optical
reference cavity 202 and fixed length optical sample cavity
203 of dual fixed length optical cavity refractometer 201
thither includes, wherein optical frequency controller 227
includes voltage controlled oscillator 204 in electrical com-
munication with electronic reference arm 235: receiving, by
voltage controlled oscillator 204, optical feedback signal
229 from electronic reference arm 235; and producing, by
voltage controlled oscillator 204, a reference modulation
frequency 252 from optical feedback signal 229; and
wherein optical frequency controller 227 further includes
acousto-optic modulator 208 in electrical communication
with voltage controlled oscillator 204 and in optical com-
munication with optical light source 251, the process further
includes: receiving, by acousto-optic modulator 208, refer-
ence modulation frequency 252 from voltage controlled
oscillator 204 and laser light 226 from optical light source
251; and producing, by acousto-optic modulator 208, pri-
mary light 228 from modulation of laser light 226 at a
modulation frequency controlled by reference modulation
frequency 252.

In an embodiment, the process for measuring pressure
based on refractivity changes between fixed length optical
reference cavity 202 and fixed length optical sample cavity
203 of dual fixed length optical cavity refractometer 201
further includes, wherein first optical phase controller 230.1
includes electro-optic modulator 209 in optical communi-
cation with optical frequency controller 227 and in electrical
communication with electronic reference arm 235: receiv-
ing, by electro-optic modulator 209, primary light 228 from
optical frequency controller 227; receiving, by electro-optic
modulator 209, reference phase control signal 231 from
electronic reference arm 235; and producing, by electro-
optic modulator 209, input reference light 232 from modu-
lating primary light 228 at a modulation frequency con-
trolled by reference phase control signal 231.

In an embodiment, the process for measuring pressure
based on refractivity changes between fixed length optical
reference cavity 202 and fixed length optical sample cavity
203 of dual fixed length optical cavity refractometer 201
further includes, wherein second optical phase controller
230.2 includes electro-optic modulator 209 in optical com-

5 munication with optical frequency controller 227 and in

electrical communication with electronic sample arm 240:
receiving, by electro-optic modulator 209, primary light 228
from optical frequency controller 227; receiving, by electro-
optic modulator 209, sample phase control signal 237 from
electronic sample arm 240; and producing, by electro-optic
modulator 209, input sample light 238 from modulating
primary light 228 at a modulation frequency controlled by
sample phase control signal 237

In an embodiment, the process for measuring pressure

5 based on refractivity changes between fixed length optical

reference cavity 202 and fixed length optical sample cavity
203 of dual fixed length optical cavity refractometer 201


U.S. Patent Nov. 16, 2021 Sheet 1 of 7 US 11,175,224 B2

200
229 234
rl -------— 235 ___\
1
| 229_| 231 234 20
234 4
H 20
I 228 232 \
f \__,f5301-1. 1
nH 227 — 38
238
) 233.2] 238 \
226 \ 1
sg
i i | 203
239 —
239
L239

FIG. 1


US 11,175,224 B2
Page 2

generator in electrical communication with the mixer; and a
dual fixed length optical cavity refractometer.

18 Claims, 7 Drawing Sheets

Related U.S. Application Data

a continuation-in-part of application No. 16/520,668,
filed on Jul. 24, 2019, now Pat. No. 10,935,370, and
a continuation-in-part of application No. 16/520,687,
filed on Jul. 24, 2019, now Pat. No. 10,942,089.

(60) Provisional application No. 62/87,926, filed on Jul
24, 2019, provisional application No. 62/714,953,
filed on Aug, 6, 2018.

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* cited by examiner


US 11,175,224 B2

7

receives primary light 228 from optical frequency controller
227, receives reference phase control signal 231 from elec-
tronic reference arm 235; and produces input reference light
232 from modulating primary light 228 at a modulation
frequency controlled by reference phase control signal 231.

In an embodiment, second optical phase controller 230.2
includes electro-optic modulator 209 in optical communi-
cation with optical frequency controller 227 and in electrical
communication with electronic sample arm 240 and that:
receives primary light 228 from optical frequency controller
227; receives sample phase control signal 237 from elec-
tronic sample arm 240; and produces input sample light 238
from modulating primary light 228 at a modulation fre-
quency controlled by sample phase control signal 237.

In an embodiment, first polarization controller 233.1
includes: polarizing beam splitter 215 in optical communi-
cation with first optical phase controller 230.1 and in elec-
trical communication with electronic reference arm 235 and
that: receives input reference light 232 from first optical
phase controller 230.1; communicates input reference light
232 to waveplate 216; receives output reference light 234
from waveplate 216; and communicates output reference
light 234 to electronic reference arm 235; and waveplate 216
in optical communication with polarizing beam splitter 215
and dual fixed length optical cavity refractometer 201 and
that: receives input reference light 232 from polarizing beam
splitter 215; receives output reference light 234 from fixed
length optical reference cavity 202; and communicates out-
put reference light 234 to polarizing beam splitter 215.

In an embodiment, second polarization controller 233.2 3

includes: polarizing beam splitter 215 in optical communi-
cation with second optical phase controller 230.2 and in
electrical communication with electronic sample arm 240
and that: receives input sample light 238 from second optical

phase controller 230.2; communicates input sample light 3

238 to waveplate 216; receives output reference light 239
from waveplate 216; and communicates output reference
light 239 to electronic sample arm 240; and waveplate 216
in optical communication with polarizing beam splitter 215
and dual fixed length optical cavity refractometer 201 and
that: receives input sample light 238 from polarizing beam
splitter 215; receives output reference light 239 from fixed
length optical sample cavity 203; and communicates output
reference light 239 to polarizing beam splitter 215.

In an embodiment, electronic sample arm 240 includes:
detector 212 in optical communication with second polar-
ization controller 233.2 and that: receives output reference
light 239 from second polarization controller 233.2; and
produces reference cavity signal 244 from output reference
light 239; second sideband frequency generator 241 that
produces second sideband frequency 245; mixer 213 in
electrical communication with detector 212 and second
sideband frequency generator 241 and that: receives refer-
ence cavity signal 244 from detector 212; receives second
sideband frequency 245 from second sideband frequency
generator 241; and produces error signal 242 that includes
reference cavity signal 244 and second sideband frequency
245; PID servo 205 in electrical communication with mixer
213 and that receives error signal 242 and produces servo
control signal 263 from error signal 242; first sideband
frequency generator 243 in electrical communication with
PID servo 205 and that receives servo control signal 263 and
produces first sideband frequency 246 from servo control
signal 263; and power combiner 222 in electrical commu-
nication with first sideband frequency generator 243 and
second sideband frequency generator 241 and that: receives
first sideband frequency 246 from first sideband frequency

S

40

8

generator 243 and second sideband frequency 245 from
second sideband frequency generator 241; produces sample
phase control signal 237 from first sideband frequency 246
and second sideband frequency 245; and communicates
sample phase control signal 237 to second optical phase
controller 230.2.

In an embodiment, electronic sample arm 240 includes:
detector 212 in optical communication with second polar-
ization controller 233.2 and that: receives output reference
light 239 from second polarization controller 233.2; and
produces reference cavity signal 244 from output reference
light 239; second sideband frequency generator 241 that
produces second sideband frequency 245; mixer 213 in
electrical communication with detector 212 and second
sideband frequency generator 241 and that: receives refer-
ence cavity signal 244 from detector 212; receives second
sideband frequency 245 from second sideband frequency
generator 241; and produces error signal 242 that includes
reference cavity signal 244 and second sideband frequency
245; PID servo 205 in electrical communication with mixer
213 and that receives error signal 242 and produces servo
control signal 263 from error signal 242; first sideband
frequency generator 243 in electrical communication with
PID servo 205 and second sideband frequency generator 241
and that: receives servo control signal 263 from PID servo
205; receives second sideband frequency 245 from second
sideband frequency generator 241; and produces first side-
band frequency 246 from servo control signal 263 and
second sideband frequency 245; and directional coupler 211
in electrical communication with first sideband frequency
generator 243 and that: receives first sideband frequency 246
from first sideband frequency generator 243; produces
sample phase control signal 237 from first sideband fre-
quency 246; and communicates sample phase contro! signal
237 to second optical phase controller 230.2.

In an embodiment, electronic reference arm 235 includes:
detector 212 in optical communication with first polarization
controller 233.1 and that receives output reference light 234
from first polarization controller 233.1; and produces refer-
ence cavity signal 244 from output reference light 234; radio
frequency source 224 that produces reference phase control
signal 231; mixer 213 in electrical communication with
detector 212 and radio frequency source 224 and that:
receives reference cavity signal 244 from detector 212;
receives reference phase control signal 231 from radio
frequency source 224; and produces mixed reference signal
236 that includes reference cavity signal 244 and reference
phase control signal 231; and PIID servo 206 in electrical
communication with mixer 213 and that receives mixed
reference signal 236 and produces optical feedback signal
229 that is communicated to optical frequency controller
227 and optionally to optical light source 251.

In an embodiment, electronic reference arm 235 includes:
detector 212 in optical communication with first polarization

5 controller 233.1 and that receives output reference light 234

from first polarization controller 233.1; and produces refer-
ence cavity signal 244 from output reference light 234; radio
frequency source 224 that produces first reference phase
control signal 231.1; mixer 213 in electrical communication.
with detector 212 and radio frequency source 224 and that:
receives reference cavity signal 244 from detector 212;
receives first reference phase control signal 231.1 from radio
frequency source 224; and produces mixed reference signal
236 that includes reference cavity signal 244 and first

5 reference phase control signal 231.1; PID servo 206 in

electrical communication with mixer 213 and that receives
mixed reference signal 236 and produces optical feedback


US 11,175,224 B2

13

directly, eliminating measurement of a beat note signal
between a source of unknown wavelength against a source
of known wavelength.

PID servo 205 receives an electric signal such as error
signal 242 and provides output servo control signal 263 that
can control a frequency of first sideband frequency 246
output by first sideband frequency generator 243. Here, the
frequency of first sideband frequency generator 243 can be
from 1 MHz to 150 GHz, specifically from 50 MHz to 50
GHz, and more specifically from 500 MHz to 10 GHz, and
first sideband frequency generator 243 can be, e.g. a voltage
controlled oscillator, microwave synthesizer, direct digital
synthesizer, arbitrary waveform generator, and the like.

Directional coupler 211 directs a portion of the electro-
magnetic power in first sideband frequency 246 to amplifier
221 and a portion of first sideband frequency 246 to fre-
quency counter 223. Further, directional coupler 211 directs
the power so that the power flows in one direction. Fre-
quency counter 223 receives first sideband frequency 246
and produces offset frequency 247 for first sideband fre-
quency 246.

Power combiner 222 combines first sideband frequency
246 with first sideband frequency 246 from second sideband
frequency generator 241 to produce sample phase control
signal 237 that is communicated to electro-optic modulator
209 of optical phase controller 230.2. Exemplary second
sideband frequency generator 241 includes function genera-
tor, direct digital synthesizer, radio frequency source,
wherein second sideband frequency 245 can have a fre-

quency from 1 MHz to 100 MHz, specifically from 5 MHz, 3

to 50 MHz, and more specifically from 9 MHz to 15 MHz.

It should be appreciated that alternative configurations of
hardware for is contemplated as shown in FIG. 2 and FIG.
3. With reference to FIG. 2, optical refraction barometer 200
uses electronic sample arm 240 to produce dual sidebands
As shown FIG. 2 and in the upper panel of FIG. 6, dual
sideband production is accomplished by combining the radio
frequency modulation with an offset tuning frequency
amplify this signal and applying to the EOM. The resulting
laser frequency has modulation sidebands around the laser
carrier frequency and has sidebands at the offset modulation
frequency which also have sidebands from the radio fre-
quency modulation. This configuration provides locking the
laser to the offset frequency for servo control to the offset
frequency. With reference to FIG. 3, optical refraction
barometer 200 uses electronic sample arm 240 to produce
electronic sidebands. As shown FIG. 3 and in the bottom
panel of FIG. 6, electronic sideband production is accom-
plished by frequency modulation of the offset frequency. As
a result, the laser carrier frequency has sidebands at the
offset tuning frequency and those sidebands have the radio
frequency sidebands on them, which are used for servo
control. Advantageously, there are no radio frequency modu-
lation sidebands around the carrier and less light is wasted,
which is more efficient for the modulator.

Optical refraction barometer 200 can be made in various
ways. In an embodiment, a process for making optical
refraction barometer 200 includes arranged optical compo-
nents to be in optical communication so that light propagates
either through optical fiber or free space and arranging
electrical components to communicate electrical signals
amongst the various electrical components.

The process for making optical refraction barometer 200
also can include locking the laser using AOM and internal
slow laser controller that can be piezo or thermal. With the
laser locked, the offset sideband can be tuned into resonance
and locked onto a nearest cavity resonance. The sign of the

0

40

35

3

14

sideband (carrier plus or carrier minus) will be used for
calculation of pressure of sample gas in the sample cavity.
This is performed initially at vacuum, f;,, and then at
pressure, fz fz is the wavelength of the laser locked to the
reference cavity, according to

(fai ~ fas) w
arto"

fag + Sap

(4) = FSRU +¢9)
ke

such that, from these measurements, the known virial refrac-
tivity and density coefficients and known prior determina-
tion of the distortion constants. The absolute pressure of a
gas can be determined using the following equation

malt), ea), *
cr = Crd)? = 6311 = dn = 4) ASP
nana (Te

a)

34
wherein c, = *

RF

—4AgB, + 4BR)
8KT)?

oF

BISAR —4ARB, + 164 QB} +
4AnBa-16BpBr —8ARCp +8Cr]
oF Tour)

with d,, being a bulk distortion term; d, being a mirror
bending distortion term; Ar, Br, Cr being virial refractivity
coefficients: and Bp Cp being density virial coefficients with
Boltzmann constant k and temperature T.

With respect to a process for measuring, pressure based on
refractivity changes with dual fixed length optical cavities of
acousto-optica modulator 208 with optical refraction barom-
eter 200, index of refraction involves a photon with a fixed
wavelength that has a different frequency in the presence of
gas than in a vacuum. Laser light can propagate in the two
cavities, one filled with gas and the other evacuated to
measure the frequency change. A laser is wavelength-locked
in resonance to a Fabry-Perot cavity (reference cavity), if
gas density (i.e., pressure) changes in the sample cavity, the
servo adjusts the frequency f to maintain resonance with the
cavity. Changes in f then give the index of refraction
according to:

~Af + Amico /2L)
f

n-1

where Af=ff, (f, is the laser frequency in vacuum, and fis
the frequency in the gas medium, Am is the change in mode
order, and Lis the length of the cavity. It is contemplated that
the laser frequency can be determined by measuring the
difference in frequency between the measurement laser and
a reference laser locked to the vacuum channel. Alterna-
tively, the difference frequency can be measured directly on
the frequency counter that measures the output frequency of
the offset tuning sideband.

Optical refraction barometer 200 has numerous advanta-
geous and unexpected benefits and uses. In an embodiment,
a process for measuring pressure based on refractivity