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External cavity laser sensors
463
Sensors and Actuators A, 25-27 (1991) 463-466
External Cavity Laser Sensors R. THIESSEN Lehrstuhl ftirElektrische Messtechnik, Technische Universitiit MCnchen, Areissrrasse 21, 8#0 Munich 2 (F.R.G.)
Abstract The reflection of the beam back into a laser causes effects which make it possible to use the resulting laser system, consisting of the laser, the external cavity and the reflector, as a new, compact and cheap sensor for variations in length and refractive index. In addition, rotational parameters such as speed of rotation, runout and surface characteristics of a spindle can also be measured.
Introduction A great variety of external optical feedback effects, especially in semiconductor lasers, have been investigated in the last ten years and are described in the literature (for example [l-5]). Because of stability problems, mode hopping and complicated alignment, there are only a few sensor applications [6-91 for practical use. The goal of this work is the development of sensors for length or refractive index and rotation measurement on smooth spindles.
A change in L of half of the laser wavelength in either direction causes a minimummaximum transition. It can be shown that the same effect occurs on varying the refractive index of the external cavity. With this signal it is not possiblp to detect whether the length L is increasin,: >r decreasing or if the reflector is just vib! ,,;ng in a fixed position instead of moving laterally. For use as a length or refractive index sensor, it is necessary to stabilize the laser against noise and mode-hopping. For measurement purposes it is essential to detect the sign of the change of length or refractive index. Both these objectives are achieved by modulating the laser system, for example by varying the pumping current I of a semiconductor laser as shown in Fig. 3. A laser and photodiode mounted together in one housing are available as cheap laser diodes.
r-_--j L
Reflector
tie - Ne Loser
d
ExternalCavity Fig. 1. Laser with external cavity.
Measurement of Length and Refractive Index Figure 1 shows the coupling of a He-Ne laser to an external cavity formed by a reflector which reflects the beam back into the laser. If the reflector in Fig. 1 is moved along the optical axis of the laser system and the beam intensity is measured offset free, i.e., a.c.-coupled by a photodiode, it will show variations as in Fig. 2. 0924-4247/91/$3.50
’
L lncreaslng
---
L
CJecreasmg
Fig. 2. Half-wavelength resonances caused by reflector movement.
0 Elsevier Sequoia/Printed in The Netherlands
464
Pholodode
COIlimator
Loser
LOO mV
0
Fig. 3. External cavity sensor with semiconductor laser. LOO
For applications with a short distance between the laser and reflector or low accuracy requirements, the collimator is not essential.
0
100
200
300
ms
200 mV 100
Rotation Measurement
0
Using a rotating spindle or disk as the reflector and a laser diode with an integrated photodiode as the sensor, the beam intensity shows variations which correspond to the rotation of the spindle. This permits the contactless measurement of the speed of rotation in axial and radial directions using active filters, autocorrelators or FFT processors without requiring any marks and which can be integrated in a hand-held unit. In a configuration as in Fig. 4 using a lathed spindle, for example the spindle of a motor, the output x(t) of the photodiode X will show signals as in Fig. 5. They are very noisy, corresponding to the surface roughness, but periodic because of the rotation. The period of rotation can be determined by the autocorrelation function (ACF) [lo] of the signal x(t), which is defined as x(t)x(t
(1)
- z) dt
PhotodIode
Y
\/ Loser
Y
Coillmalor
j I WIPhoto diode X
COIlI motor
’
+ iii Sp,nd,e
rodlal
-100
-200
0
0.2
0.L
0.6
s
1
Fig. 5. Typical photodiode signals from reflection at a rotating spindle.
This expression applies even if the signal is digitized with only one bit, for example the slope of x(t) detected by a differentiator giving an offset-free signal i(t) and digitized with a comparator giving dig[$t)] where dig[k] = 1 when
i 3 0
dig[i] = 0
i < 0.
when
The continuous signal i(t) becomes a discrete bit stream dig[&] (N bits long) by sampling at a constant time-interval T,. The multiplication of expression (I), which has to be performed N2 times, is reduced to a simple exclusive-nor operation (XNOR). This led to the design of a cheap 1024 x 1-bit hardware correlator (see Fig. 6)
’ 1 :% ’
SpIndIe
Oxlol
Fig. 4. Rotation measurement with two laser diodes.
Fig. 6. Signal conditioning for hardware correlator.
465
,011
ACF
+350 mV
-650
-I
Fig. 7. Correlation functions of photodiode signals. 0
a 1024 x l-bit data stream dig[&]) to an output signal
which
transforms
L(k)
=
$ $ dW,l(XNOWM~~-
A
(2)
n-l
as shown in Fig. 7. The l-bit ACF shows 12 periods in 0.5 s, resulting in a rotational speed of ( 12/ 0.5) x 60 I/min = 1440 l/min. The direction of the rotation can be detected by using a second laser sensor Y (Fig. 4) giving y(r) and calculating the cross-correlation (CCF) of the signals i and 3: $,(k)
= $ $,
dig[%l(XNOR)dig[~, --kl. (3)
When the rotation period is determined by autocorrelation, the cross-correlation will have a maximum with a positive phase shift n/2 relative to the autocorrelation when the spindle (Fig. 4) is turning in the direction shown, and a phase shift 3~12 when the spindle is turning in the opposite direction. Thus, the rotational speed and direction measurement can be performed without requiring any marks. Two axially displaced sensors make torque measurements possible. The objection that the measurement would not work on ideal spindles in ideal bearings led to a test of the sensor with a precision-lapped spindle in a sinter air bearing. The result is shown in Fig. 8. During the period of rotation there are sharp peaks, occurring at half-wavelength
100
Fig. 8. Half-wavelength resonances runout of a rotating spindle.
rn5
caused
160
by the
resonances, caused by the runout of the spindle. In this example there are six peaks at a laser wavelength of about 800 nm, giving a runout of (800 nm/2) x 6 = 2400 nm. This and the similarity of the signals in Figs. 2 and 8 proves that the turning polished spindle and the laser diode form an external cavity. The half-wavelength resonances of Fig. 8 are not apparent in Fig. 5 because the signals of Fig. 5 are dominated by the backscattering effects caused by the surface roughness of the lower-quality spindle used there. Shiny parts, wires and droplets used instead of a spindle to complete the external cavity are also detectable.
Conclusions
Variations in length and refractive index, rotational speed, runout and surface roughness of a spindle can be measured as described above using semiconductor lasers, which have become increasingly inexpensive and available through their widespread use in many products such as compact-disc players and laser printers. High-quality mirrors, exact alignment and collimators are not essential, reflective foils or shiny surfaces being sufficient.
466
References 1 T. Kanada and K. Nawata, Injection laser characteristics due to reflected optical power, fEEE J. Quantum Electron., QE-15 (7) (1979) 559-565. 2 R. Lang and K. Kobayashi, External optical feed-
back effects on semiconductor injection laser properties, IEEE J. Quantum Electron., QE-I6(3) (1980) 347-355. 3 R.
0. Miles, A. Dandridge, A. B. Tveten, H. F. Taylor and T. G. Giallorenzi, Feedbackinduced line broadening in cw channel-substrate planar laser diodes, Appf. Phys. Lett., 37 (1980)
990-992. 4 M. Fujiwara, K. Kubota and R. Lang, Low-fre-
quency intensity fluctuation in laser diodes with external optical feedback, Appl. Phys. Lett., 38 (1981) 217-200. 5 H. Sato, T. Fujita and K. Fujito, Intensity fluctua-
tion in semiconductor lasers coupled to external cavity, IEEE .I. Quantum Electron., QE-21 (1) (1985) 46-51. 6 Y. Mitsuhashi, T. Morikawa, K. Sakurai. A. Seko
and J. Shimada, Self-coupled optical pickup, Opt. Commun., 17 (1) (1976) 95-97. 7 A. Dandridge, R. 0. Miles and T. G. Giallorenzi, Diode laser sensor, Electron. L&t., 16 (25) (1980) 948-949. 8 R. 0. Miles, A. Dandridge, A. B. Tveten and T. G.
Giallorenzi, An external cavity diode laser sensor, J. Lightwave Technol., LT-I (I) (1983) 81-93. 9 E. M. Strzelecki, D. A. Coldren and L. A. Coldren,
Investigation of tunable single frequency diode lasers for sensor applications, .I. Lightwave Technol., 6 (10) (1988) 1610-1618. IO E. Schrufer, Signalverarbeitung, Carl Hanscr Verlag,
Munich, 1990, pp. 238-246. 11 R. Thiessen, Laser-Sensor mit externem Resonator, Patent DE 3917388 (29.11.1990). 12 R. Thiesen, Laser-Sensor mit externem Resonator, Patent Applic. EP 0402 691 (28.5.1990). Anordnung zur markenlosen 13 R. Thiesen, Drchzahlmessung an glattcn Wellen, Patenf Applic. DE 3 943 469. GegenstandsNlherungsund 14 R. Thiessen, Tr(ipfchendetektor, Patent Applic. DE 3 943 470.
Sensors and Actuators A, 25-27 (1991) 463-466
External Cavity Laser Sensors R. THIESSEN Lehrstuhl ftirElektrische Messtechnik, Technische Universitiit MCnchen, Areissrrasse 21, 8#0 Munich 2 (F.R.G.)
Abstract The reflection of the beam back into a laser causes effects which make it possible to use the resulting laser system, consisting of the laser, the external cavity and the reflector, as a new, compact and cheap sensor for variations in length and refractive index. In addition, rotational parameters such as speed of rotation, runout and surface characteristics of a spindle can also be measured.
Introduction A great variety of external optical feedback effects, especially in semiconductor lasers, have been investigated in the last ten years and are described in the literature (for example [l-5]). Because of stability problems, mode hopping and complicated alignment, there are only a few sensor applications [6-91 for practical use. The goal of this work is the development of sensors for length or refractive index and rotation measurement on smooth spindles.
A change in L of half of the laser wavelength in either direction causes a minimummaximum transition. It can be shown that the same effect occurs on varying the refractive index of the external cavity. With this signal it is not possiblp to detect whether the length L is increasin,: >r decreasing or if the reflector is just vib! ,,;ng in a fixed position instead of moving laterally. For use as a length or refractive index sensor, it is necessary to stabilize the laser against noise and mode-hopping. For measurement purposes it is essential to detect the sign of the change of length or refractive index. Both these objectives are achieved by modulating the laser system, for example by varying the pumping current I of a semiconductor laser as shown in Fig. 3. A laser and photodiode mounted together in one housing are available as cheap laser diodes.
r-_--j L
Reflector
tie - Ne Loser
d
ExternalCavity Fig. 1. Laser with external cavity.
Measurement of Length and Refractive Index Figure 1 shows the coupling of a He-Ne laser to an external cavity formed by a reflector which reflects the beam back into the laser. If the reflector in Fig. 1 is moved along the optical axis of the laser system and the beam intensity is measured offset free, i.e., a.c.-coupled by a photodiode, it will show variations as in Fig. 2. 0924-4247/91/$3.50
’
L lncreaslng
---
L
CJecreasmg
Fig. 2. Half-wavelength resonances caused by reflector movement.
0 Elsevier Sequoia/Printed in The Netherlands
464
Pholodode
COIlimator
Loser
LOO mV
0
Fig. 3. External cavity sensor with semiconductor laser. LOO
For applications with a short distance between the laser and reflector or low accuracy requirements, the collimator is not essential.
0
100
200
300
ms
200 mV 100
Rotation Measurement
0
Using a rotating spindle or disk as the reflector and a laser diode with an integrated photodiode as the sensor, the beam intensity shows variations which correspond to the rotation of the spindle. This permits the contactless measurement of the speed of rotation in axial and radial directions using active filters, autocorrelators or FFT processors without requiring any marks and which can be integrated in a hand-held unit. In a configuration as in Fig. 4 using a lathed spindle, for example the spindle of a motor, the output x(t) of the photodiode X will show signals as in Fig. 5. They are very noisy, corresponding to the surface roughness, but periodic because of the rotation. The period of rotation can be determined by the autocorrelation function (ACF) [lo] of the signal x(t), which is defined as x(t)x(t
(1)
- z) dt
PhotodIode
Y
\/ Loser
Y
Coillmalor
j I WIPhoto diode X
COIlI motor
’
+ iii Sp,nd,e
rodlal
-100
-200
0
0.2
0.L
0.6
s
1
Fig. 5. Typical photodiode signals from reflection at a rotating spindle.
This expression applies even if the signal is digitized with only one bit, for example the slope of x(t) detected by a differentiator giving an offset-free signal i(t) and digitized with a comparator giving dig[$t)] where dig[k] = 1 when
i 3 0
dig[i] = 0
i < 0.
when
The continuous signal i(t) becomes a discrete bit stream dig[&] (N bits long) by sampling at a constant time-interval T,. The multiplication of expression (I), which has to be performed N2 times, is reduced to a simple exclusive-nor operation (XNOR). This led to the design of a cheap 1024 x 1-bit hardware correlator (see Fig. 6)
’ 1 :% ’
SpIndIe
Oxlol
Fig. 4. Rotation measurement with two laser diodes.
Fig. 6. Signal conditioning for hardware correlator.
465
,011
ACF
+350 mV
-650
-I
Fig. 7. Correlation functions of photodiode signals. 0
a 1024 x l-bit data stream dig[&]) to an output signal
which
transforms
L(k)
=
$ $ dW,l(XNOWM~~-
A
(2)
n-l
as shown in Fig. 7. The l-bit ACF shows 12 periods in 0.5 s, resulting in a rotational speed of ( 12/ 0.5) x 60 I/min = 1440 l/min. The direction of the rotation can be detected by using a second laser sensor Y (Fig. 4) giving y(r) and calculating the cross-correlation (CCF) of the signals i and 3: $,(k)
= $ $,
dig[%l(XNOR)dig[~, --kl. (3)
When the rotation period is determined by autocorrelation, the cross-correlation will have a maximum with a positive phase shift n/2 relative to the autocorrelation when the spindle (Fig. 4) is turning in the direction shown, and a phase shift 3~12 when the spindle is turning in the opposite direction. Thus, the rotational speed and direction measurement can be performed without requiring any marks. Two axially displaced sensors make torque measurements possible. The objection that the measurement would not work on ideal spindles in ideal bearings led to a test of the sensor with a precision-lapped spindle in a sinter air bearing. The result is shown in Fig. 8. During the period of rotation there are sharp peaks, occurring at half-wavelength
100
Fig. 8. Half-wavelength resonances runout of a rotating spindle.
rn5
caused
160
by the
resonances, caused by the runout of the spindle. In this example there are six peaks at a laser wavelength of about 800 nm, giving a runout of (800 nm/2) x 6 = 2400 nm. This and the similarity of the signals in Figs. 2 and 8 proves that the turning polished spindle and the laser diode form an external cavity. The half-wavelength resonances of Fig. 8 are not apparent in Fig. 5 because the signals of Fig. 5 are dominated by the backscattering effects caused by the surface roughness of the lower-quality spindle used there. Shiny parts, wires and droplets used instead of a spindle to complete the external cavity are also detectable.
Conclusions
Variations in length and refractive index, rotational speed, runout and surface roughness of a spindle can be measured as described above using semiconductor lasers, which have become increasingly inexpensive and available through their widespread use in many products such as compact-disc players and laser printers. High-quality mirrors, exact alignment and collimators are not essential, reflective foils or shiny surfaces being sufficient.
466
References 1 T. Kanada and K. Nawata, Injection laser characteristics due to reflected optical power, fEEE J. Quantum Electron., QE-15 (7) (1979) 559-565. 2 R. Lang and K. Kobayashi, External optical feed-
back effects on semiconductor injection laser properties, IEEE J. Quantum Electron., QE-I6(3) (1980) 347-355. 3 R.
0. Miles, A. Dandridge, A. B. Tveten, H. F. Taylor and T. G. Giallorenzi, Feedbackinduced line broadening in cw channel-substrate planar laser diodes, Appf. Phys. Lett., 37 (1980)
990-992. 4 M. Fujiwara, K. Kubota and R. Lang, Low-fre-
quency intensity fluctuation in laser diodes with external optical feedback, Appl. Phys. Lett., 38 (1981) 217-200. 5 H. Sato, T. Fujita and K. Fujito, Intensity fluctua-
tion in semiconductor lasers coupled to external cavity, IEEE .I. Quantum Electron., QE-21 (1) (1985) 46-51. 6 Y. Mitsuhashi, T. Morikawa, K. Sakurai. A. Seko
and J. Shimada, Self-coupled optical pickup, Opt. Commun., 17 (1) (1976) 95-97. 7 A. Dandridge, R. 0. Miles and T. G. Giallorenzi, Diode laser sensor, Electron. L&t., 16 (25) (1980) 948-949. 8 R. 0. Miles, A. Dandridge, A. B. Tveten and T. G.
Giallorenzi, An external cavity diode laser sensor, J. Lightwave Technol., LT-I (I) (1983) 81-93. 9 E. M. Strzelecki, D. A. Coldren and L. A. Coldren,
Investigation of tunable single frequency diode lasers for sensor applications, .I. Lightwave Technol., 6 (10) (1988) 1610-1618. IO E. Schrufer, Signalverarbeitung, Carl Hanscr Verlag,
Munich, 1990, pp. 238-246. 11 R. Thiessen, Laser-Sensor mit externem Resonator, Patent DE 3917388 (29.11.1990). 12 R. Thiesen, Laser-Sensor mit externem Resonator, Patent Applic. EP 0402 691 (28.5.1990). Anordnung zur markenlosen 13 R. Thiesen, Drchzahlmessung an glattcn Wellen, Patenf Applic. DE 3 943 469. GegenstandsNlherungsund 14 R. Thiessen, Tr(ipfchendetektor, Patent Applic. DE 3 943 470.