zotero/storage/63TNGM8Y/.zotero-ft-cache

US007586587B1

(12) United States Patent
Wang et al.

(10) Patent No.: US 7,586,587 B1

(45) Date of Patent:

Sep. 8, 2009

(54) STAND-ALONE SPEEDOMETER USING TWO
SPACEO LASER BEAMS

(76) Inventors: Ruyong Wang, 917 21st St. SE., St.

w

E. MN 'S E. Yes, so

rizzlya. Sarrell,

s

sley, 35 Edward Dr., St. Cloud,

(*) Notice: St. tO E. site th still
p p ls. e4i.d justed under
M YW- (b) by ayS.

(21) Appl. No.: 11/035,659

(22) Filed: Jan. 14, 2005

Related U.S. Application Data
(60) Provisional application No. 60/536,410, filed on Jan.
14, 2004.

(51) Int. Cl. GOIP3/36

(2006.01)

GOIC 9/72

(2006.01)

(52) U.S. Cl. ....................... 356/28.5: 356/460,356/482 (58) Field of Classification Search ................... 35628,
356/285.459,460. 477 482

See application file for complete search history s

s

(56)

References Cited

U.S. PATENT DOCUMENTS

3,791,738 A * 2/1974 Newburgh .................. 356,472

3,941,481 A 3/1976 Kramer ...................... 356,472
4,588,296 A * 5/1986 Cahill et al. ... ... 356,462
4,997.282 A * 3/1991 Pavlath .......... ... 356,460

S.S. A 88: Siya - - - - - - - - 33 E.

W - W

V a

- --

6,462.825 B1 * 10/2002 Wiebesick ..... ... 356/475

6,650,682 B1 * 1 1/2003 Diels et al. .................... 372,94

2006/0145063 A1* 7/2006 Steinberg et al. ....... 250,227.18

OTHER PUBLICATIONS
Ruyong Wang, “From the triangle Sagnac experiment to a practical
crucial experiment of the constancy ofteh speed of light using atomic
clocks on moving objects”, Europhysics Letters, Sep. 15, 1998; 43.
pp. 611-616.* Wang et al., “Modified Sagnac experiment for measuring travel-time difference between counter-propagating light beams in a uniformly moving fiber'. Jun. 2003, pp. 1-4.*

* cited by examiner

Primary Examiner Isam Alsomiri

(57)

ABSTRACT

A stand-alone speedometer directly measuring the transla tional speed of a moving body comprises a source emitting
two spaced light beams which interfere each other, mirrors or beam splitters changing directions of propagation of light
beams, and a detector measuring the phase difference of light beams. Compared with the phase difference when the speed ometer is stationary, the detector measures a first-order change of phase differences, which indicates the motion
speed.

2 Claims, 4 Drawing Sheets

Detecto1

U.S. Patent Sep. 8, 2009 Sheet 1 of 4

US 7,586,587 B1

Fig. 1

Detector2
Mirror
Fig. 2

Detectoil T

U.S. Patent Sep. 8, 2009 Sheet 2 of 4

US 7,586,587 B1

Light paths in the speedometer.
- - - Pathl 1, W1-M1 l-M12-BS1-Detector 1 o Path21, W2-M21-M22-BS2-Detector1

Detector2

M24

Laser

Path 12, W1-M1 1-BS1-Detector2 Path22, W2-M21-BS2-Detector2
Detector 1

Fig. 3

Standing-wave

M Detector
-

14
Y

CCCCCC

p

photodetector

photodetector

Fig. 4

U.S. Patent Sep. 8, 2009 Sheet 3 of 4

US 7,586,587 B1

17N
NC 1
Detector2 Coupler with
a mirror

Coupler with
a mirror 22

Fig. 5

-

:

La. S e

: 3

N a VN
Q4-1
Detector2 coupler with a mirror

- a 2A
N-4 1
Coupler with), Detector
a mirror

Fig. 6

U.S. Patent Sep. 8, 2009 Sheet 4 of 4

US 7,586,587 B1

41
Ring Laser
Standing-wavé"
Detector

NC 1
Detector2 Coupler with
a mirror

Coupler with
a mirror

Fig. 7

US 7,586,587 B1

1.

2

STAND-ALONE SPEEDOMETER USING TWO SPACEO LASER BEAMS
CROSS-REFERENCE TO RELATED

FIG.5 shows an alternative construction of the stand-alone
speedometer with the two spaced laser beams emitted from the front and the back ends of a laser using fiber technology.
FIG. 6 shows an alternative construction of the stand-alone

APPLICATIONS
This application claims the benefit of Provisional Patent Application No. 60/536,410, filed Jan. 14, 2004 by Ruyong Wang, Yi Zheng and Aiping Yao.

speedometer with the two spaced laser beams emitted from a wide laser cavity.
FIG.7 shows an alternative construction of the stand-alone
speedometer with the two spaced laser beams emitted from a ring laser.

FIELD OF INVENTION

10
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

This invention is generally related to instruments of navi gation and more specifically related to a speedometer.

15

The Sagnac effect shows that there is a travel-time differ ence between two counter-propagating light beams traveling

BACKGROUND OF THE INVENTION

along a rotating path. In the recent generalized Sagnac experi ments conducted by Wang et al. (Ruyong Wang, Yi Zheng,

Most speedometers measuring translational speed of a
moving body either are not stand-alone or do not directly measure the speed. A speedometer that is not stand-alone uses contact information from outside of the moving body. For example, the speedometerina car only works when the wheel
of the car contacts with the ground, the Pitot tube of an airplane only works when the Pitot tube probes the surround ingair, theSonarofaSubmarineonly works when Sound wave reflects from some reference objects, and the GPS receiver

25

Aiping Yao, “Generalized Sagnac Effect”, Physical Review Letters 93 (2004) 143901), which publication is hereby incor
porated by reference, the travel-time difference of two counter-propagating light beams has been observed in a uni formly and translationally moving glass or air-core fiber
while the light source and detector are moving together with fiber in a fiber optic conveyor. The stand-alone speedometer and method directly measuring the translational speed
according to this invention utilize the discoveries of the experiments.

only works when it receives the signal from the GPS satel lites. A speedometer that is not directly measuring the moving speed calculates the speed based on other measurable infor mation. For example, an accelerometerina navigation system
measures the translational acceleration and the translational
speed is determined by integrating the accelerometer output with an initial speed.
A patent (U.S. Pat. No. 6,813,006) has invented a stand alone speedometer directly measuring the translational

30 35

The new speedometer is also related to the following gen
eral calculation.
In an apparatus, we have two spaced laser beams which interfere with each other. Light beam 1 starts from A and goes through path I to B, and light beam 2 starts from A and goes through path II to B (FIG. 1).
Neglecting all terms of higher than the first-order, we have the speed of light u,

speed. The present invention provides a new method and a

new speedometer that utilizes two spaced laser beams emitted

from a laser and measures their travel-time difference.

where v is the velocity of the apparatus relative to the pre

SUMMARY OF THE INVENTION

ferred frame, c is the constant speed of light in the preferred 40 frame, n is the refractive index and e' is a unit vector in the

direction of the path.

In accordance with one aspect of the present invention, a Hence,

stand-alone speedometer directly measuring the translational

speed of a moving body comprises a source emitting two 45

spaced interfering light beams, mirrors or beam splitters changing directions of propagation of said light beams, and

We have the travel-time of light beam 1 in path I,

detectors measuring the phase difference of said light beams.

According to another aspect of the invention, a method directly measuring the translational speed of a moving body 50

comprises selecting a light Source emitting two spaced light

beams which interfere each other, changing light beam direc

tions, and measuring the phase difference between the two where ds=e'ds.

beams, converting the phase difference to the translational

speed of the moving body.

55 The travel-time of light beam 2 in path II,

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the propagation of two spaced light beams in

a medium.

60

FIG. 2 shows an example of the basic construction of the The travel-time difference between two beams is: stand-alone speedometer with the two spaced laser beams
emitted from the front and the back ends of a laser.

FIG.3 shows the paths of light beams in the basic construc tion of the stand-alone speedometer.

65

12 - i. = (1/c?rds

(1/e (Ids)-

FIG. 4 shows standing wave detectors in a laser cavity.

US 7,586,587 B1
3

Because

-continued
(1fc) nds +(1/e (as 5

At2 = t2 - 122 = (1?o nds-f nds) vLic

2

22

where the projection of path W1-W2 on the direction of the velocity v is -L. Thus,

10
which can be measured by detecting a fringe shift Af-vL/ c. on Detector2. Comparing D1 with D2, we have

where III is a virtual and arbitrary path from A to A, we have 15 It indicates that the difference between the fringe shift on

Detector1 and the fringe shift on Detector2. Af-Af, is 2vL/

cw when the speedometer moves at a speed V.

In this configuration, each fringe shift on Detector 1 or

Detector2 is sensitive to the stability of the optical path having

the length of L, but the value of the difference between them, It indicates that the travel-time difference between two (Af-Af.), is not sensitive to the stability of the optical path beams is related to V, the speed relative to the preferred frame. and it is optically stable.

Therefore, the change of travel-time difference between two If the air can be removed from the apparatus, the fringe

beams from stationary to moving is:

25 shifts on two detectors will be more stable.
As an example, if we can detect the difference between the
fringeshifts, Af-Af, with a sensitivity of10'fringe, wecan

(-)-(1-t) = (1/onds - (1/e) nds: (1/e ("I ds)-

detect a speed or a speed difference as small as 10'c/ 2L=310 m/s with L=0.3 m and 0.6 um.

(1 fo)

inds - (1 fo) nds

(1/e (e. ds) = vLf c.

30

There is an implied assumption in the calculations above that the phase difference between two spaced beams is con

stant when they leave the laser cavity. This assumption is true

because only standing waves are built in the laser resonant

which means that the travel time difference of the two spaced
laser beams is the first-order effect of V, VL/c, where the
speed is relative to the preferred frame and L is the projection

35

cavity and the phase difference of two ends of a standing wave is nat. To check this, standing-wave detectors 14, transparent
photodetectors, can be located in the resonant cavity (FIG. 4). Otherwise, the changes of measurements of the standing

of the path III on the direction of the velocity v. The projection wave detectors measure the motion.

could be positive or negative, depending on the angle between This speedometer has several features:

the path and the velocity. It is positive in FIG. 1.

40 1) Two beams in the main part of the speedometer propa

The basic construction of the stand-alone speedometer is gate in the same path but in opposite directions for the two

described hereunder with reference to FIG. 2. There are two detectors. Therefore, this speedometer is optically stable if

spaced light beams emitted from a laser 11. Both light beams are changed the directions on the mirrors and beam splitters
12, and the interference fringes and travel-time differences are detected at two detectors 13. The light paths are shown in

45

the difference of the two detectors is used. It is very important for a speedometer mounted on a moving object.
2) When the speedometer is moving at a speed V, the speed can be measured by comparing travel-time differences before and after turning the speedometer 90 degrees because we

FIG. 3. The propagation time intervals via different paths in the speedometer can be specifically calculated below.

have (Af-Af.)oo-0 after turning 90 degrees. Therefore, the measurement is V-(Af-Af.)-(Af-Af,)oolch/2L.

The travel-time difference between path21 and path11 is: 50 3) After turning the speedometer 180 degrees, the speed V

becomes speed -v and we have (A?-Af.), so -2vL/c.
Therefore, the measurement will be doubled by comparing

the fringes before and after turning 180 degrees, it is (Af

Af,)-(Af-Af,) so-4VL/cv.
55 This speedometer can be also constructed with fiber tech

nology. An alternative construction of the stand-alone speed

where the projection of path W2-W1 on the direction of the ometer is described hereunder with reference to FIG.5. There

velocity v is L. Thus,

are two spaced light beams emitted from the front and back ends of a laser 21. Both light beams are changed the directions

(At), -(At)o (ti-tii), (ti-tii)o-vL/c

60 on the couplers with mirrors 22, and the interference phase

and travel-time differences are detected at two detectors 23. A

which can be measured by detecting a fringe shift Af=VL/cw modulator 24 can be utilized to increase the sensitivity of the

on Detector1 when the speedometer has a speed V relative to speedometer if it is necessary. This fiber speedometer is simi

the preferred frame.

lar to the fiber optic conveyor. Therefore the difference

65 between the phase shift on Detector1 and the phase shift on

Similarly, the travel-time difference between path12 and Detector2. Acp-A?p is 4CVL/cw when the speedometer

path22 is:

moves at a speed V. If we can detect the difference between the

US 7,586,587 B1

5

6

phase shifts, A?p-A?p2, with a sensitivity of 107 radians, we variations are considered within the scope and spirit of the

can detect a speed or a speed difference as small as 10'c/ invention. Accordingly, the protection sought herein is as set

47t|L=8 um/s with L=0.3 m and -1 um. The fiber can be glass, forth in the claims below.

air-core or vacuum-core fiber.
Another alternative construction of the stand-alone speed
ometeris described hereunder with reference to FIG. 6. There
are two spaced light beams emitted from the same end of a

What is claimed is:
1. A speedometer, comprising: a. a laser cavity directly and immediately emitting two
spaced light beams which interfere each other, and hav ing an internal standing-wave detector to measure the

wide laser cavity 31. Both light beams are changed the direc

phase difference between said two spaced light beams,

tions on the couplers with mirrors 32. The interference fringes 10 b. mirrors and/or beam splitters changing the directions of

and travel-time differences are detected at two detectors 33.

said two beams,

Another alternative construction of the stand-alone speed c. a detector for measuring the phase difference of said two

ometeris described hereunder with reference to FIG. 7. There

beams,

are two spaced light beams emitted from the same ring laser 41. Both light beams are changed the directions on the cou plers with mirrors 42. The interference fringes and travel-time differences are detected at two detectors 43. A standing-wave

15

whereby said speedometer will be a stand-alone speedom eter for measuring the translational speed of a moving body.
2. A method of directly measuring the translational speed ofa moving body comprising,

detector 44 can be located in the ring laser cavity.

a. Selecting a laser cavity directly and immediately emit

An apparatus using three stand-alone speedometers can

ting two spaced light beams which interfere each other,

measure motion in three-dimensional space.

and having an internal standing-wave detector to mea

The particular embodiments disclosed above are illustra

sure the phase difference between said two spaced light

tive only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Further 25
more, no limitations are intended to the details ofconstruction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments

beams, b. changing the light beams directions, c. measuring the phase difference between said two light
beams, d. converting said phase difference to the translational
speed of the moving body.

disclosed above may be altered or modified and all such

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