Self-mixing inteferometry sensor module, electronic device and method of determining an optical power ratio for a self-mixing inteferometry sensor module

Abstract

A self-mixing interferometry sensor module, comprising a light emitter (LE), a detector unit (DU) and an optical element (OE), wherein the light emitter (LE) is operable to emit coherent electromagnetic radiation towards an external object (ET) to be placed outside the sensor module and undergo self-mixing interference, SMI, caused by reflections of the emitted electromagnetic radiation from the external object back inside the sensor module. The detector unit (DU) is operable to generate output signals indicative of an optical power output of the light emitter (LE) due to the SMI. The optical element (OE) is aligned with respect to the light emitter (LE) such that a first fraction of electromagnetic radiation is directed towards the external target (ET) or the light emitter (LE) and a second fraction of electromagnetic radiation is directed towards the detector unit (DU). An optical power ratio determined by the first and second fractions meets a pre-determined value.

Claims

1. A self-mixing interferometry sensor module, comprising a light emitter, a detector unit and an optical element, wherein the light emitter is operable to: emit coherent electromagnetic radiation towards an external object to be placed outside the sensor module; and undergo self-mixing interference, SMI, caused by reflections of the emitted electromagnetic radiation from the external object back inside the sensor module; wherein the detector unit is operable to: generate output signals indicative of an optical power output of the light emitter due to the SMI; wherein the optical element is aligned with respect to the light emitter such that: a first fraction of electromagnetic radiation is directed towards the external target and/or the light emitter and a second fraction of electromagnetic radiation is directed towards the detector unit to generate the output signals, and such that an optical power ratio of the first and second fractions meets a pre-determined value, wherein the optical power ratio is further determined by shot noise associated with the detector unit and wherein a signal-to-noise ratio SNR(AC.sub.SMI,DU) of an AC component of the output signals generated by the detector unit (DU) yields
P.sub.0.Math.m.Math.T.sub.BS.Math.cos ?.Math.R.sub.BS.sup.2 wherein F accounts for a relative intensity noise of the light emitter and B is a system bandwidth, wherein T.sub.BS denotes a transmission of the optical element with respect to the first fraction of the electromagnetic radiation, and R.sub.BS denotes a reflection of the optical element with respect of the second fraction of electromagnetic radiation, wherein P.sub.0 is the optical power output, m is a modulation factor that is a function of optical losses associated with the sensor module, and ? is a function of an optical field phase.

2. The sensor module according to claim 1, wherein the optical power ratio is a function of an overall splitting ratio T.sub.BS/R.sub.BS of the optical element.

3. The sensor module according to claim 1, wherein optical power P.sub.SMI,DU associated with the second fraction directed to the detector unit is determined by
P.sub.SMI,DU=P.sub.SMI.Math.R.sub.BS wherein P.sub.SMI denotes the optical power associated with the first fraction directed to or emitted by the light emitter as a result of SMI.

4. The sensor module according to claim 2, wherein an AC component AC.sub.SMI,DU of the output signals generated by the detector unit yields
P.sub.0.Math.m.Math.T.sub.BS.Math.cos ?.Math.R.sub.BS wherein the AC component AC.sub.SMI,DU is in a local or global maximum for the overall splitting ratio T.sub.BS/R.sub.BS of the optical element.

5. The sensor module according to claim 2, wherein the reflection of the optical element is around 50%.

6. The sensor module according to claim 1, wherein the reflection of the optical element is around 33%.

7. The sensor module according to claim 1, further comprising a housing with a wall, top side and a bottom side, wherein: the light emitter and/or detector unit are arranged at the bottom side or wall, and the optical element is arranged in the housing, such as to distribute the first fraction of electromagnetic radiation towards the external target and/or the light emitter (LE) and the second fraction of electromagnetic radiation towards the detector unit to generate the output signals.

8. The sensor module according to claim 7, wherein the optical element comprises: a ball lens, a beam splitter, a dichroic beam splitter, or a combination thereof.

9. The sensor module according to one of claim 1, wherein the optical element comprises at least one coating layer.

10. The sensor module according to claim 7, wherein the optical element is tilted with respect to the wall, top side and/or bottom side.

11. The sensor module according to claim 1, wherein the light emitter comprises: a semiconductor laser diode, a resonant-cavity light emitting device, a distributed feedback laser and/or a vertical cavity surface emitting laser, VCSEL, diode.

12. The sensor module according to claim 1, wherein the detector unit comprises at least one photodetector to detect the electromagnetic radiation integrated into a layer sequence of the light emitter, and/or at least one photodetector to detect the electromagnetic radiation, which is outside the light emitter.

13. An electronic device, comprising at least one self-mixing interferometry sensor module according to claim 1, and a host system, wherein: the sensor module is integrated into the host system, and the host system comprises one of: a mobile device, a smartphone, a wearable mobile device.

14. A method of determining an optical power ratio for a self-mixing interferometry sensor module, wherein the sensor module comprises: a light emitter to emit coherent electromagnetic radiation towards an external object, a detector unit to generate output signals indicative of an optical power output of the light emitter due to self-mixing interferometry and an optical element to direct a first fraction of electromagnetic radiation emitted by the light emitter towards the external target or the light emitter and to direct a second fraction of electromagnetic radiation is directed towards the detector unit, the method comprising: determining a first optical power associated with the first fraction of electromagnetic radiation as a function of the optical element, determining a second optical power associated with the second fraction of electromagnetic radiation as a function of the optical element, and determining the ratio of first and second optical power.

15. The method according to claim 14, wherein the optical power ratio is determined as a function of an overall splitting ratio T.sub.BS/R.sub.BS of the optical element, wherein T.sub.BS denotes the transmission of the optical element with respect to the first fraction and R.sub.BS denotes the reflection of the optical element with respect of the second fraction of electromagnetic radiation.

16. The method according to claim 14, wherein the first optical power and second optical power are adjusted to maximize the second optical power.

17. The method according to claim 14 comprising: determining the optical element splitting ratio based on the determined ratio of the first and second optical power.

18. A self-mixing interferometry sensor module, comprising a light emitter, a detector unit and an optical element, wherein the light emitter is operable to: emit coherent electromagnetic radiation towards an external object to be placed outside the sensor module; and undergo self-mixing interference, SMI, caused by reflections of the emitted electromagnetic radiation from the external object back inside the sensor module; wherein the detector unit is operable to: generate output signals indicative of an optical power output of the light emitter due to the SMI; wherein the optical element is aligned with respect to the light emitter such that: a first fraction of electromagnetic radiation is directed towards the external target and/or the light emitter and a second fraction of electromagnetic radiation is directed towards the detector unit to generate the output signals, and such that an optical power ratio of the first and second fractions meets a pre-determined value, wherein the optical element comprises a ball lens.

19. The sensor module according to claim 18, wherein the ball lens of the optical element is a partial ball lens, which comprises a base which is tilted with respect to a carrier, and a splitting ratio is fine-tuned by an amount of the tilt.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The following description of figures may further illustrate and explain aspects of the self-mixing interferometry sensor module, electronic device and the method of determining an optical power ratio for self-mixing interferometry.

(2) Components and parts of the self-mixing interferometry sensor that are functionally identical or have an identical effect are denoted by identical reference symbols. Identical or effectively identical components and parts might be described only with respect to the figures where they occur first. Their description is not necessarily repeated in successive figures.

(3) In the figures:

(4) FIGS. 1a to 1c show an example embodiment of self-mixing interferometry sensor module for a wearable electronic device,

(5) FIG. 2 shows example schematic of a splitting ratio,

(6) FIGS. 3A and 3B show example embodiments of a sensor module for SMI,

(7) FIGS. 4A and 4B show further example embodiments of a sensor module for SMI,

(8) FIGS. 5A to 5D show further example embodiments of a sensor module for SMI, and

(9) FIGS. 6A and 6B show further example embodiments of a sensor module for SMI.

DETAILED DESCRIPTION

(10) The following discussion relates to various embodiments of self-mixing interferometry sensor modules. The sensor modules suggested herein comprise at least a light emitter LE, a detector unit DU and an optical element OE. The light emitter emits coherent electromagnetic radiation out of the sensor module, e.g. towards an external target ET. The light emitter undergoes self-mixing interference, SMI, which is caused by reflections of the emitted electromagnetic radiation from the external object back inside the sensor module.

(11) The detector unit DU can be an internal part of or external of the light emitter LE. For example, the detector unit comprises a photodetector which generates output signals indicative of an optical power output of the light emitter as a result of SMI.

(12) The optical element OE is aligned with respect to the light emitter LE. For example, the sensor module comprises a housing HS to mount and align its electronic and optical components. By way of the optical element a first fraction of electromagnetic radiation is directed towards an external object to be placed outside the sensor module and a second fraction of electromagnetic radiation is directed towards the detector unit DU. Thus, the optical element allows to steer beams inside the sensor module and determines a splitting ratio of said beams. For example, the optical element comprises a beam splitter, or a more complex optical system. The optical element implements a ratio of first and second fraction, which meets a pre-determined value. Said pre-determined value can be determined for a given optical design. A mathematical framework is discussed in the following.

(13) FIG. 1 shows a schematic overview of the method of determining an optical power ratio for self-mixing interferometry. The proposed concept analyzes the optimum power ratio between the light going from the light emitter to the external target to produce SMI and the power diverted to the detector unit DU to measure the SMI signal, i.e. the optimum splitting ratio used to detect the SMI signal in power read out configuration.

(14) The drawings show a coherent light emitter LE and an external target ET. In figure a) the light emitter LE emits light towards the external target ET, which reflects back from the target into the light emitter, e.g. laser cavity, coupling back a small portion of the light (<10%). The optical power signal with target feedback can be described as:
P.sub.SMI=P.sub.0+P.sub.0.Math.m.Math.cos ?,
where m is known as the modulation factor, P.sub.0 is the laser output optical power and ? is a function of the optical field phase. This function of time t is depicted on the right-hand side of the drawing. The modulation factor m is proportional to the square root of the total optical losses (i.e., in the simplest case, the target reflectivity): m??{square root over (R.sub.T)}.

(15) In figure b) an an optical element OE is introduced, e.g. a filter or beam splitter, in the middle of the optical path between light emitter LE and the external target. In this configuration transmission losses associated with the optical element need to be accounted for. The transmission is denoted T.sub.BS. Thus, the optical power signal with target feedback yields:
P.sub.SMI=P.sub.0+P.sub.0.Math.m.Math.T.sub.BS.Math.cos ?.

(16) The modulation factor m is modified to account for the transmission losses:
m=m.Math.T.sub.BS.

(17) The optical element transmission T.sub.BS is counted twice because of the double pass, i.e. emission towards the target and reflection on the target. The modified modulation factor m depends on the target reflectivity R.sub.T and of the light emitter R.sub.LE and yields:

(18) $m^{}=\frac{\sqrt{R_T}}{\sqrt{R_{LE}}}.Math.\left(1-R_{LE}\right).Math.?.Math.\frac{I_{op}-\frac{N_{tr}}{N_{th}}{I}_{th}}{I_{op}-I_{th}}.$

(19) The parameters of the equation are defined as follows: I.sub.op=laser drive operation current, I.sub.th=laser threshold current, N.sub.tr=carrier concentration at transparency, N.sub.th=carrier concentration at threshold, ?=optical mode coupling factor.

(20) In figure c) the optical element OE is configured to reflect some light into an optical detector unit DU. For example, the detector unit comprises a photodetector, electronics and signal processing, so a SMI signal can be measured. In the depicted embodiment, the optical element OE comprises a beam splitter so that part of the reflected light can be redirected towards the detector unit. The optical element has a reflectivity of R.sub.BS.

(21) Thus, the optical power signal measured by the optical detector unit DU is denoted P.sub.SMI,DU and yields:
P.sub.SMI,DU=P.sub.SMI.Math.R.sub.BS.

(22) This equation can be reformulated as follows:

(23) $\begin{array}{c}{P}_{SMI,\mathrm{DU}}=\left(P_0+P_0.Math.m^{}.Math.T_{BS}.Math.\cos ?\right).Math.R_{BS}\\ =P_0.Math.R_{BS}+P_0.Math.m^{}.Math.T_{BS}.Math.\cos ?.Math.R_{BS}\end{array}.$

(24) The first part of the optical power P.sub.0.Math.R.sub.BS is considered as a DC SMI signal and the second term P.sub.0.Math.m.Math.T.sub.BS.Math.cos ?.Math.R.sub.BS corresponds to an AC SMI signal that contains information of the target (distance, speed, etc.). The AC SMI signal depends on the transmission T.sub.BS and the reflection R.sub.BS of the optical element OE, or a splitting ratio T.sub.BS/R.sub.BS as R.sub.BS=1?T.sub.BS. For example, the AC SMI signal can be detected by means of the detector unit DU, e.g. by means of a photodetector (internal or external) and is denoted A.sub.CSMI,DU.

(25) FIG. 2 shows example schematic of a splitting ratio. The splitting ratio introduced above is a means to estimate an optimal power ratio. The power ratio determines the relative optical power of reflected light which, via the optical element OE, is coupled back into the cavity of the light emitter LE and the optical power of reflected light which is directed towards the detector unit DU. For example, the SMI signal amplitude can be measured by means of photodetector of the detector unit DU. The term optimal or optimum should not be understand in an absolute sense. Rather these terms are determined by the parameters used to calculate or estimate the power ratio as will be discussed below.

(26) For example, the graph on the left side of FIG. 2 shows normalized signal amplitudes of the SMI signal as a function of the reflectivity of the optical element OE, i.e. R=1?T. The graph assumes that optical losses of the sensor module are included in the transmission T.sub.BS and the reflection R.sub.BS of the optical element OE, as well as the modified modulation factor m. Under these circumstances the power ratio, i.e. the ratio resulting in a maximum SMI signal amplitude P.sub.0.Math.m T.sub.BS.Math.cos ?.Math.R.sub.Bs is determined by a splitting ratio T.sub.BS/R.sub.BS of 50%, i.e. R=1?T=50%, as indicated in the drawing. This splitting can be implemented by means of the 50/50 beam splitter as optical element OE, for example. The graph also shows a corresponding SMI voltage signal change with the amount of reflected light. Theoretically, the SMI signal is independent of the optical power and any power splitting is translated in a loss. Thus, for an idealized configuration which only considers the splitting ratio of the optical element OE, a splitting ratio T.sub.BS/R.sub.BS of 50% can be considered optimal for power readout. In this example, no extra optical losses are considered except for the splitting ratio, which can be reasonable concept for a sensor module which relies on an external photodetector, i.e. external with respect to the light emitter LE.

(27) In another level of accuracy, however, a system could be considered quantum noise limited, i.e. the noise floor will be the shot noise associated with the detector unit, e.g. photodiode, and the SNR of the system can be calculated as the ratio of the squares of the signal vs the noise. In such a configuration the signal-to-noise ratio, SNR, at the detector unit yields:

(28) $\mathrm{SNR}\left(A{C}_{SMI,\mathrm{DU}}\right)=\frac{{.Math.P_0.Math.m^{}.Math.T_{BS}.Math.\cos ?.Math.R_{BS}.Math.}^2}{2q.Math.R_{BS}.Math.P_0.Math.F.Math.B}$
where F accounts for the relative intensity noise (RIN) of the light emitter LE and B is the system bandwidth.

(29) The graph on the right side of FIG. 2 shows normalized signal amplitudes of the SMI signal as a function of the reflectivity of the optical element OE, i.e. R=1?T. Under these circumstances the power ratio, i.e. the ratio resulting in a maximum SMI signal amplitude P.sub.0.Math.m.Math.T.sub.BS.Math.cos ?.Math.R.sub.BS is determined by a splitting ratio T.sub.BS/R.sub.BS of around 66% or 2 over 3, i.e. R=1?T=1/3?33%, as indicated in the drawing. This splitting can be implemented by means of the 50/50 beam splitter as optical element OE or more complex optical designs, but may need different optical paths and/or use of additional coatings to adjust the splitting ratio T.sub.BS/R.sub.BS.

(30) The mathematical concept derived above provides a convenient framework to guide implementation of the optical design of self-mixing interferometry sensor modules. The framework allows to consider optical loses for a particular configuration, e.g. external photodetector vs. internal photodetector, etc. by means of optical design or hardware, e.g. optics. Further optical losses can be accounted for if deemed necessary. For example, consider a VCSEL laser, or any other semiconductor laser, as light emitter LE. Changing the mirrors reflectivity can be accounted for by adjusting the SMI modulation factor m. Substrate losses are included in the laser output optical power P.sub.0 for bottom-side emitting lasers.

(31) The following figures show various implementations of sensor modules. These examples implement the splitting ratio of first and second fractions of emitted electromagnetic radiation. The splitting ratio is considered a pre-determined value and can be calculated along the lines introduced above. For example, the splitting ratio is given by T.sub.BS/R.sub.BS of the optical element. In general, said ratio can be determined by a single optical element OE, such as a beam splitter, and denotes the ratio of transmission towards/from the light emitter and reflection to the detector unit DU of the single optical element. In case of more complex optical elements comprising a combination of several single elements, splitting ratio is considered an overall ratio of the complex optical element. The overall optical losses and optical absorption may be accounted for in the formula but the principle is the same. The sensor modules comprise a package with a housing HS, e.g. a molded housing. The housing encloses the at least one light emitter LE, detector unit DU and optical element OE. The housing comprises walls WL to optically separate the sensor module from the ambient. Furthermore, the housing provides means to mount and/or align the optical and electronic components, including the at least one light emitter LE, detector unit DU and optical element OE. Furthermore, the sensor modules comprise a carrier CR, which may be part of the housing and/or a substrate or integrated circuit. The housing is arranged on the carrier, for example. The housing provides a hollow space, or chamber, which encloses the at least one light emitter LE and the detector unit DU, which are arranged in or on the carrier or the carrier, i.e. substrate and/or integrated circuit. Furthermore, the housing provides one or more apertures for light to be emitted out of the sensor module and for light to be reflected into the sensor module.

(32) The package may for example be implemented as a TO can, i.e.

(33) a common transistor-outline-can (TO-can) package for optical components. A TO package comprises two components: a TO header (carrier) and a TO cap (housing). While the TO header ensures that the encapsulated components are provided with power, the cap ensures the transmission of optical signals.

(34) The arrows in the drawing indicate how light emitted by the light emitter LE travels through the sensor module and towards an external target ET. It should be noted that self-mixing interference builds up almost the moment the sensor module starts emitting light towards the target. The drawings implicitly assume that SMI is already established by means of reflections at the target. In this sense in SMI the signal is everywhere when the feedback from the target is present. Thus, hereinafter not all light beams are indicated in the graphs. The arrows shown in the figures relate to transmission and reflection on the optical element.

(35) FIGS. 3A and 3B show example embodiments of a sensor module for SMI. In FIG. 3A the optical element is arranged on top TP of the housing TP, opposite to the carrier CR at the bottom of the housing. The optical element is essentially parallel with respect to the carrier. In this embodiment the detector unit DU comprises two photodetectors PD, which flank the light emitter LE in the middle. The optical element OE can be implemented as a transparent cover plate, for example. As an alternative, the optical element OE can be implemented as an optical lens. In FIG. 3B the optical element is arranged on top of the housing, opposite to the carrier at the bottom of the housing. However, the optical element is tilted with respect to the carrier. In this embodiment the detector unit comprises one photodetector, which is arranged on the carrier next to the light emitter LE. The optical element OE can be implemented as a transparent cover plate CP, for example.

(36) For example, TO cans and other packaging configurations use a cover glass as optical element to reflect some light to the photodetector(s) or power monitor of the detector unit DU to control the power variations of the light emitter LE. In this case, to reduce optical losses, the photodetectors can be oversized (FIG. 3B) or use several photodetectors PD can be implemented (FIG. 3B). Both configurations may, however, increase costs.

(37) In both embodiments, the optical element OE splits the emitted light beam or received light beam according to a splitting ratio given by T.sub.BS/R.sub.BS. The transmission and reflection are set by design to split corresponding fractions of electromagnetic radiation according to a predetermined ratio. For example, with a desired ratio T.sub.BS/R.sub.BS of around 50% the splitting can be implemented by means of a 50/50 beam splitter as optical element as shown in FIG. 3A. A different desired ratio T.sub.BS/R.sub.BS, e.g. of around 66% can be implemented by adjusting the transmission and reflection. In other words, a first fraction of electromagnetic radiation is directed towards the external object and a second fraction of electromagnetic radiation is directed towards the detector unit DU in order to detect a SMI signal.

(38) This can be done by means of a coating layer (not shown), such as an anti-reflection layer. With respect to FIG. 3A the so coated cover can act as a beam splitter with different ratio, e.g. a 60/40 beam splitter. Alternatively, or in addition, the optical element can be tilted, which essentially alters the transmission and reflection due to different angles of incidence.

(39) FIGS. 4A and 4B show further example embodiments of a sensor module for SMI. In these examples, the optical element OE comprises a partial ball lens, which is arranged on top TP of the housing HS, opposite to the carrier CR at the bottom of the housing. The ball lens can be implemented as a solid immersion lens (SIL), e.g. a hemispherical SIL or a Weierstrass SIL. There are two types of SIL. A hemispherical SIL which can increase the numerical aperture up to n, n being the index of refraction of the material of the lens. Another type is denoted Weierstrass SIL (or super hemispherical SIL). Such a SIL comprises a truncated sphere which can increase the numerical aperture up to n.sup.2. Both types of SIL are effective means to increase the numerical aperture, and, thus effects also the splitting ratio T.sub.BS/R.sub.BS. Numerical aperture indicates the range of angles over which the system can accept or emit light. This effectively increases numerical apertures and, thus, enhance detection efficiency.

(40) The ball lenses in FIGS. 4A and 4B have a base, which is tilted with respect to the carrier. The tilt is chosen such that the emitted light beam or received light beam are split according to a splitting ratio given by T.sub.BS/R.sub.BS. The amount of tilt allows to fine tune the splitting ratio to a desired value, e.g. 50% or 66%, or any other value. As a consequence, of relative amounts of transmission and reflection corresponding fractions of electromagnetic radiation are splitted according to the predetermined ratio.

(41) FIG. 4A shows a sensor module for SMI with a tilted partial ball lens. FIG. 4B shows a modified sensor module, where the tilted partial ball lens is arranged on a transparent cover plate CP. The transparent cover plate is partly covered with a light blocking structure LB, which blocks stray light from reaching the photodetector. The tilted partial ball lens and transparent cover plate together form the optical element OE, which effectively leads to an overall splitting ratio with a desired value, e.g. 50% or 66%, or any other value.

(42) FIGS. 5A to 5D show further example embodiments of a sensor module for SMI. These examples are modifications of the embodiments shown in FIGS. 3A, 3B and 4A, 4B.

(43) The embodiment in FIG. 5A is based on the corresponding example in FIG. 3B. The optical element comprises the tilted cover plate and, additionally, a partial ball lens arranged on the detector unit DU, e.g. the photodetector.

(44) The embodiment in FIG. 5B is based on the corresponding example in FIG. 4A. The optical element comprises the tilted ball lens and, additionally, a partial ball lens arranged on the detector unit DU, e.g. the photodetector.

(45) The embodiment in FIG. 5C is based on the corresponding example in FIG. 4B. The optical element comprises the tilted ball lens and, additionally, a partial ball lens arranged on the detector unit DU, e.g. the photodetector. The tilted partial ball lens is arranged on a transparent cover plate. The transparent cover plate is partly covered with a light blocking structure, which blocks stray light from reaching the photodetector.

(46) The embodiment in FIG. 5D can be considered a mixture of several examples. With respect to FIG. 3A a partial ball lens is arranged on the transparent cover plate, i.e. the base of the partial ball lens is parallel with the carrier. Furthermore, the transparent cover plate is partly covered with a light blocking structure, which blocks stray light from reaching the photodetector. For example, the light blocking structure is arranged on the transparent cover plate opposite of photodetectors of the detector unit.

(47) FIGS. 6A and 6B show further example embodiments of a sensor module for SMI. These examples are based on the embodiments with partial ball lenses and are complemented with a beam splitter. The optical element is a hybrid system and comprises both a partial ball lens and the beam splitter. The light emitter is arranged on the carrier CR while one or two photodetectors PD of the detector unit DU are arranged on the walls WL of the housing HS.

(48) While this specification contains many specifics, these should not be construed as limitations on the scope of the invention or of what may be claimed, but rather as descriptions of features specific to particular embodiments of the invention. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable sub-combination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination.

(49) Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous.

(50) Furthermore, as used herein, the term comprising does not exclude other elements. In addition, as used herein, the article a is intended to include one or more than one component or element, and is not limited to be construed as meaning only one.

REFERENCES

(51) CP cover plate CR carrier DU detector unit ET external target HS housing LB light blocking structure LE light emitter OE optical element PD photodetector TP top of the housing WL wall