WO2008055640A1

WO2008055640A1 - Self-mixing laser sensor

Info

Publication number: WO2008055640A1
Application number: PCT/EP2007/009575
Authority: WO
Inventors: John Douglas Lambkin
Original assignee: Firecomms Ltd
Current assignee: Firecomms Ltd
Priority date: 2006-11-09
Filing date: 2007-11-05
Publication date: 2008-05-15
Anticipated expiration: 2009-05-09
Also published as: GB0622354D0; GB2443662A

Abstract

A laser sensor is used as a velocimeter using principles of self-mixing. A laser device having an optical cavity with an optical output face and a beam splitter integrated into the laser device provides at least two output beams from the optical output facet. A first one of the output beams is used as a probe beam directed to a target surface external to the sensor and a second one of the output beams is used as a monitor beam directed to an optical detector which measures the laser output power. From the measured output power, motion of the target surface relative to the sensor is determined as a function of self-mixing of a portion of the probe beam returned to the optical cavity by reflection from the target surface.

Description

SELF-MIXING LASER SENSOR

The present invention relates to laser Doppler velocimeters and position sensors using the phenomenon of self -mixing in semiconductor lasers.

The use of the self -mixing interference phenomenon within a laser for the determination of velocity and position of a target object relative to the laser was first revealed in 1968 by MJ. Rudd [1] using a HeNe gas laser. In 1986, Shinohara et al [2] demonstrated a laser Doppler velocimeter based on a semiconductor laser. Self-mixing consists of feeding back a relatively small proportion of the laser's output beam back into the laser cavity either through scattering or reflection from a remote target surface. Providing the frequency of the scattered light remains within the linewidth of the laser, then the scattered light coupled back into the laser cavity will add constructively or destructively with the light in the cavity depending upon the round-trip path length and the corresponding field phase delay incurred. _^_

For a semiconductor laser this will affect the gain threshold of the laser and give rise to a modification in a number of the laser's characteristics including operating junction voltage (and hence junction impedance), output power and spectral linewidth. If the remote target is moving, the return path length and hence the round-trip phase delay will vary as a function of time causing a periodic modulation in the junction voltage [3], output power [4] and spectral linewidth [4]. This variation in the phase delay can also be understood as the Doppler shift in frequency experienced by the back scattered light by the velocity component of the moving target along the direction of the laser beam.

For a semiconductor laser, measurement of the variation in the junction voltage via an external circuit or measurement of the laser's rear facet output power via a monitor light detector such as a γin photodiode are straightforward. A frequency spectral analysis of collected time-varying modified characteristics will determine the Doppler shift frequeiuy Δvi which can then be related to the target's velocity through the relationship Av₁ = 2vi/λ, where V₁ is the velocity component of the moving target along the direction of the laser beam and λ is the wavelength of the unperturbed laser.

A highly attractive feature of self-mixing that is not present in more conventional interferometric apparatus such as a two-beam Michelson interferometer is that the Doppler shift frequency can be determined and also whether the component of velocity along the direction of the laser beam is towards or away from the laser. In 1987 Shimizu [5] noted that as the proportion of the back scattered light (also known as the feedback strength) is increased_/ the form of the laser's modified characteristics changed from that of sinusoidal to that of an inclined 'saw-tooth' -like waveform such that the inclination of the waveform changed sign for positive and negative Doppler shifts. A detailed theoretical treatment of self-mixing and the effect upon the laser's modified characteristic waveforms has been reported in the literature [6, 7 and 8].

For a self-mixing laser sensor to be used as a range finder, the absolute distance from the laser to a fixed target must be determined. This can be achieved [9, 10] by linearly varying the output frequency of the laser at a known rate and determining the mode spacing between the resonant frequencies (modes) that are supported by the external cavity formed by the laser and the remote target by measuring the periodic fluctuations in the power of the monitor beam. If the laser frequency v can be linearly varied at η = (dv/dt) then a Fourier analysis of the power variation of the

2X monitor beam will yield a frequency component — - η which is the mode spacing set up by the external cavity. Since η is known by design and c is the speed of light, the precise length, Lj, of the external cavity, i.e. the range of the target, can be determined. The laser frequency can be linear!}_' varied by driving the laser with a current that varies linearty over time. In practice it is most convenient to drive the laser with a symmetric triangular waveform where dv/dt will vary +η and -η in alternate half cycles.

REFERENCES

[1] M. J. Rudd, "A laser Doppler velocimeter employing the laser as a mixer-oscillator," Journal of Physics E-Scientific Instruments 1, 723-726,

(1968).

[2] S. Shinohara, A. Mochizuki, H Yoshida and M. Sumi, "Laser Doppler velocimeter using the self-mixing effect of a semiconductor laser diode," Appl. Opt. 25, 1417-1419 (1996).

[3] E.A. Callan and J.G. Mclnerney, "Measurements of velocities by back scattered modulation in semiconductor injection lasers/' Proc. SPIE, vol.

1634, p. 509, 1992.

[4] W.M. Wang, W. J. O. Boyle, K.T. V. Grartan and A. W. Palmer, "SeIf- mixing interference in a diode laser: experimental observations and theoretical analysis/' Appl. Opt. 32, 1551-1558, (1993).

[5] E. T. Shimizu, "Directional discrimination in the self-mixing type laser

Doppler velocimeter," Appl. Opt., 26, 4541- 4544, (1987).

[6] M.H. Koelink, M.Slot, F.F. M. de MuI, J. Greve, R. Graaff, A.C. M. Dassel and J.G. Aarnoudse, " Laser Doppler velocimeter based on the self-mixing effect in a fiber-coupled semiconductor laser: theory," Appl. Opt. 31, 3401-

3408, (1992).

[7] L. Scalise, Y. Yu, G. Guido, G. Plantier and T. Bosch, "Self-mixing laser diode velocimetry: application to vibration and velocity measuremenr," IEEE Trans. Instrum. Meas., 53, 223-232, (2004). [8] W.M. Wang, K.T. V. Grattan, A.W. Palmer and W. J. O. Boyle, "Self- mixing inside a single mode diode laser for optical sensing applications/' J. Lightwave Technol. 12, 1577-1587, (1994).

[9] G. Beheim and K. Fritch, "Range finding using frequenc3<-modulated laser diode/' Appl. Opt. 25, 1439-1442, 1986.

[10] E. Gagnon and J-F Ri vest, "Laser range imaging using the self-mixing effect in a laser diode," IEEE Trans. Instrum. Meas., 48, 693-699, (1999). [11] J.P. Justice, P. Lambkin, M. Meister, R. Winfield and B. Corbett, "Monolithic integration of wavelength scale diffractive structures on red vertical cavity lasers b}' focused ion beam etching" IEEE Photonics Technol. Lett., 16, 1795-1797, (2004).

[12] H. Martinsson, J. Bengtsson, M. Ghisoni and A. Larsson, "Monolithic integration of vertical cavity surface emitting laser and diffractive optical element for advanced beam shaping", IEEE Photonics Technol. Lett., 11, 503-505, 1999.

Self-mixing in a semiconductor laser at an appropriate level of feedback strength may lead to a highly compact sensor capable of measuring position, speed, direction and distance of a remote target object. Using the self-mixing phenomenon in semiconductor lasers to create an optical input device has been revealed in US patent 6,707,027 whereby the optical input device can be used to replace a touch pad or miniature keyboard joy stick to control a screen cursor by sensing the motion of a user's finger-tip. Alternatively the optical input device can serve as the position sensor in a computer mouse. In these instances, the use of the self -mixing semiconductor lasers leads to an optical input device that is both robust in that there are no moving parts and low cost in that they avoid the need of silicon chip CMOS cameras that are commonly used in today's optical LED and laser mouse engines. However, for an optical self-mixing laser input sensor to measure for example the position of the user's finger-tip in an x-y plane it has been prior art practice [e.g. US 6,707,027] that two separate lasers are used whose beams are fixed in orthogonal directions to one another which are then used to determine the relative motion of the finger-tip or computer mouse in those orthogonal directions. In addition, if it is also desirable that the sensor should be capable of accurately detecting movement of the finger-tip in the z direction to enable the user to 'tap' or 'click' the sensor to allow added functions such as selecting items from a screen menu to be implemented, then a further third laser may be added to the sensor to measure movement in the z direction.

It would be desirable if the measurement of position, velocity and distance of a target surface can be achieved with a sensor that employs a single self- mixing semiconductor laser chip and thereby -avoids the cost of additional lasers. Another advantage would be gained by using a visible VCSEL chip in that the user can see the path of the beams and be aware that the device is operational.

The present invention provides a self -mixing laser sensor comprising: a laser device having an optical cavity with an optical output face and a beam splitter integrated into the laser device for providing at least two output beams from the optical output facet, a first one of the output beams being a first probe beam for directing to a target surface external to the sensor and a second one of the output beams being a first monitor beam for directing to an optical detector; an optical detector for receiving said first monitor beam; a chive circuit for controlling the laser output; and a control circuit configured to determine, from the first monitor beam, motion of the target surface relative to the ^'sensor as a function of • self-mixing of a portion of the first probe beam returned to the optical cavity by reflection from the target surface.

According to another aspect, the present invention provides a method of detecting movement of a target surface relative to a self-mixing laser sensor comprising the steps of: generating at least two output beams from an optical output face of a laser device having an optical cavity and a beam splitter integrated into the laser device; directing a first one of the output beams to the target surface as a first probe beam so that a portion of the first probe beam returns to the optical cavity by reflection from the target surface and causes self -mixing in the laser cavity; directing a second one of the output beams to an optical detector as a first monitor beam so as to generate, by the optical detector, an output signal indicative of the laser output power; determining, from the output of the detector, motion of the target surface relative to the sensor as a function of the self-mixing of the portion of the first probe beam returned to the optical cavity by reflection from the target surface.

Particular embodiments of the invention use a single vertical cavit5? surface emitting laser (VCSEL) chip from which a minimum of two beams are emitted from the VCSEL chip surface. In use, at least one beam (the monitor beam) will be directed towards a light detector and at least one beam (the probe beam) will be directed towards a target surface from which a small proportion of light from the probe beam will be reflected such that the reflected light couples back into the VCSEL. This gives rise to self-mixing effects which modify the behaviour of the VCSEL. TWs modified behaviour is monitored by measurement of the variation in the optical power of the VCSEL monitor beam by the light detector. From an analysis of the monitor beam's power fluctuations the relative movement of the target surface with respect to the VCSEL chip can be determined with high resolution. Such a VCSEL₇ being compact, operating with low power consumption and having low cost, can serve as an input device such as a trackball, joystick or touch pad in equipment such as computers, mobile telephones and personal organisers, or as a compact range finder. When these embodiments operate at visible wavelengths they provide enhanced user advantage by virtue of being able to see the light beams.

In another aspect, the invention provides a self-mixing laser sensor comprising a single vertical cavity surface emitting laser (VCSEL) chip from which a multiplicity of beams, single or multi-spatial mode, are emitted from the VCSEL chip surface.

Embodiments of the present invention will now be described by way of example and with reference to the accompanying drawings in which:

Figure 1 is a schematic diagram of a laser sensor for determining motion of a target surface relative thereto;

Figure 2 is a perspective view of a laser in the laser sensor of figure

1;

Figure 3 is a graph showing a suitable waveform for frequency of the laser output as a function of time as used in the sensor of figure 1; Figure 4 is a graph showing a Fourier analysis of an output of the laser in the sensor of figure 1 that has been modulated by a time varying drive current and by self -mixing from a portion of a beam returned from a moving target surface;

Figure 5A is a perspective view of an alternative laser device for use in determining motion of a target surface relative thereto;

Figure 5B is a plan view of the alternative laser device of figure 5A; Figure 6 is a schematic view of a laser sensor using the device of figures 5 A and 5B;

Figure 7 is a schematic view of an alternative laser sensor; Figure 8 is a perspective view of an alternative laser device for use in determining motion of a target surface relative thereto;

Figure 9 is a schematic view of a laser sensor using the device of figure 8;

Figure 10 is a perspective view of an alternative laser device for use in determining motion of a target surface relative thereto; Figure 11 is a schematic view of a laser sensor using the device of figure 10;

Figure 12 is a graph showing a Fourier analysis of an output of the laser in the sensor of figure 11 that has been modulated by a time varying drive current and by self -mixing from portions of two beams returned from a moving target surface;

Figure 13 is a schematic cross-sectional view of an alternative laser sensor configuration; and

Figure 14 is a schematic plan view of the laser sensor incorporated into a computing device.

It has been revealed by Justice et al [11] and Martinsson et al [12] how to control the output beam of a surface emitting cavity device such as a vertical cavity surface emitting laser (VCSEL) by monolithicalty integrating a grating structure or diffractive optical element on the output surface of the device during the device's manufacturing process. The precise grating dimensions are formed through lithography and etching techniques as an integral part of the device manufacturing process. Particular grating designs can lead to complex beam shaping configurations such as the generation of 2-way or 4-way beam splitting or even conical beams. However, the grating is such that the electro-optic characteristics of the device are not substantially degraded. The present invention preferably makes use of such techniques for the formation of a grating on a VCSEL to provide beam splitting as will be described hereinafter.

Figure 1 shows a self-mixing laser sensor device 10 incorporating a multi- beam VCSEL 20 as shown in figure 2. The VCSEL 20 ma)' be of any suitable emission wavelength, although visible wavelengths may be preferred in some circumstances. The laser 20 has an optical cavit}⁷ and a top surface 22 that provides an output face or facet. The laser 20 incorporates a linear diffraction grating 21 monolithically integrated into the device. The linear grating 21 is adapted to generate two diffracted first order output beams 12, 15 from the optical output face 22 of the laser 20. The expression 'linear grating' encompasses any optical diffraction structure have a variation in optical transmissivity with periodicity in at least one dimension transverse to the beam direction.

In a preferred arrangement, the grating is formed in or on the layer or layers that make up an upper reflector of the optical cavity, such as a distributed Bragg reflector (DBR). The grating could also be formed in or on or under the laj^ers that may up a lower reflector of the optical cavity, such as a DBR. Alternatively, the grating may be formed on the top surface or output face. Most generally, the grating may be formed in any part of the monolithic structure that forms the laser device, including within intermediate layers.

With further reference to figure 1, a first one of the output beams 12, 15 serves as a probe beam 12 and is directed to a target surface 13 that may be optically rough and from which a small proportion o£ the probe beam light is returned as a feedback beam 14 along the same beam path as the probe beam 12. The feedback beam 14 couples into the VCSEL 20 leading to self- mixing interference within the laser 20. The second one of the output beams serves as a monitor beam 15 and is directed towards a light detector 16. The light detector may be of any suitable type, for example, a silicon photodiode. The VCSEL 20 is driven by electrical current from a drive circuit 17 which is preferably a current source capable of modulating the laser drive current in a controlled fashion.

A control circuit 18 is configured to receive an electrical output signal from the light detector 16 and is adapted, as described below, to compute a velocity component of the target surface 13 in the direction of the probe beam from the light detector output signal. The control circuit 18 ma}' also be adapted to determine the distance from the VCSEL to the target surface. It ma}' also be a function of the control circuit 18 to maintain a desired power level in the probe beam 12 such that appropriate laser eye safety power levels are maintained. The VCSEL 20, drive circuit 17 and control circuit 18 are preferabl)" packaged in a suitable housing 11 with an optical access such as a window 19 or aperture.

The two diffracted beams 12 and 15 preferably follow off-axis beam paths, i.e. paths that are not orthogonal to the plane of the substrate of the VCSEL device 20. Each diffracted beam has a different azimuth angle relative to an axis in the plane of the VCSEL substrate, e.g. the relative to the positive x-axis as depicted in figure 2. In the example of figure 2, the azimuth angles of the two diffracted beams are 180 degrees apart at 0 degrees and 180 degrees relative to the x-axis. As shown in figure 1, each beam 12, 15 has an elevation angle (labelled θ) above the plane of the laser surface 22. Current may be supplied to the VCSEL 20 through electrical contact to a bond pad 23.

The velocity component Vi of the target surface 13 in the direction of the probe beam 12 and the distance Li from the VCSEL 20 to the target surface 13 can be derived with reference to figure 1. The simple analysis that follows is based on an interferometiϊc approach that does not attempt to account for the asymmetric nature of the self -mixing signals observed in experiment but does give insight as to how the velocity, the direction of velocity and the distance can be extracted.

Let the field Eo(t) inside the VCSEL cavity be given by

E₀ (t) - a exp(- z2τrvtf)

(1)

where, in general, it is assumed that the optical output frequency v(t) is also a function of time t and a is field amplitude.

The linear grating 21 splits the λ^CSEL's output beam into two beams. The monitor beam 15 is used to monitor the laser's output power over time and is thus directed towards the light detector 16. The probe beam 12 is directed towards the target surface 13. A unit vector si gives the direction of propagation of the probe beam 12 within a stationary rectangular coordinate frame of reference with respect to the laser 20 as shown in figures 1 and 2. The velocity vector of the moving target surface is V which can be decomposed into components V_x + V_3* + V₂. The path length Lj between the target surface 13 and the VCSEL output surface 22 typically remains constant over time in the case of an optical mouse sensor or sensor to measure the position and movement of a finger in the x-y plane. Due to specular reflection from the optically rough target surface, a small proportion of the probe beam 12 is fed back into the laser cavity as the feedback beam 14 along a beam path which is coincident with the probe beam 12. From the principle of reciprocity, the feedback beam 14 will, via the integrated grating 21, couple back into the VCSEL's cavity where it will mix with the cavity's field. The field Ei of the feedback beam 14 is given b_}r E₁ (t) = b exp(- ilπv (t - T₁ )t) (2) where the round trip time T₁ is _{τ >} = - ^Lι , c is the speed of light and Lj is the c distance travelled by the probe beam 12 from the laser 20 to the target surface. The amplitude b accounts for the transmission and reflective losses.

The feedback field Ei will experience a Doppler shift Av₁ in its frequency, given by

Δv₁ = v(r - τ_] )2-^- c (3)

The parameter Vi is the velocity component of the surface velocity V parallel to the known direction vector si of the probe beam 12, that is

(4) The feedback field (2) then becomes

(5)

The total field sustained in the laser cavity is then E_T (t) = ∑E, (/) ■ From this

1=0 we obtain the optical intensity / = \E_T\ . Ignoring terms oscillating at optical frequencies, the optical intensity as a function of time can be expressed as

l{t) = 2{a² + b²)+ ^abcos{2π(y - v_x - Av₁ )/)

(6) where V₁ =v(t-τ_i) .

For simplicity, although no loss of generality ensues, it is easiest to consider the specific case as shown in figure 1 when the probe beain 12 is confined to an x-z plane (e.g. the azimuth angle of the probe beam is 0 measured from the x direction) and is directed to the reflective target surface 13 at the elevation angle θ relative to the x-y plane. In such an event the velocity component Vj is directly proportional to the x component of the surface velocity J ' ^{= F} _'V

(7)

If the VCSEL 20 is driven with a constant current from the drive circuit 17 and in the presence of no external optical feedback, the laser 20 will oscillate at a constant frequency v. In this instance the term V₁ = V(V - T₁ ) will be zero. As a result, (6) becomes_/ l{t) = l(a² + bή+ Aabco{2π{Δv_γ )t)

(8)

That is, the laser output power, as measured by the light detector 16, will be modulated at a frequency proportional to the modulus of the Doppler shift Δvj. The velocity component V_x can be determined from (7) since the unit direction vector six of the probe beam 12 is fixed by design and is therefore known. This is because the angle θ may be fixed relative to the optical window 19 against which the target surface may move.

In this derivation, the sign of V_x remains unknown since cos(2π(Δv_])/') = cos(27r(- Δv_iy). In reality, and as noted by Shimizu [5], the form of I(t) at moderate to high levels of feedback gives rise to an asymmetric waveform function F(2π(Δvj)t) which is 'saw-tooth' -like so that as well as deriving the Doppler shift from a frequency spectrum of I(t), the asymmetric form of F(2π(Δvj)t) can also be used to discriminate the sign of the Doppler shift. These analytical tasks are accomplished using the control circuit 18.

Thus, in a general sense, the embodiment of figures 1 and 2 comprises a dual beam VCSEL 20 whereby two output beams are generated by way of an integrated linear grating on the surface of the VCSEL acting as a beam splitter such that one beam with a known direction is used to probe a moving target surface with a component velocity vector in the direction of the probe beam and the second beam is monitored for optical power fluctuations. From an anal y sis of the frequency components of the optical power fluctuations and asymmetry of the optical power waveform the velocity and direction of the target surface can be determined for the azimuthal direction of the probe beam.

In another embodiment, the velocity and direction of the target object 13 ma}' be accurately determined when the laser is driven with a modulated current source. A characteristic of a semiconductor laser is that the output frequency varies linearly with drive current at approximately the rate of - 100 GHz/mA for a VCSEL. Hence, by modulating the laser current source over time, the output frequency of the laser is also modulated. Consider the special case when the frequency difference term (V -V₁) is proportional to the round trip time τi. v(t)-v{t -T_x) ^t₁

(9) This is only true when the optical frequency is a linear function of time,

(10) where η = (dv/dt) is the rate at which the frequency changes with time. To maintain a linear variation of frequency over an extended period of time the most convenient solution is to arrange for the drive circuit 17 to drive the VCSEL 20 with a symmetric 'saw-tooth' drive current whereby in one half cycle the value of dv/dt will be +η and in the other half cycle dv/dt will be -η. The resulting laser output frequency as a function of time is shown iii figure 3. In the cycles when η is +ve the intensity (8) then becomes lit) = ip- ÷b²)+ 4abco{2π(ητ_λ -Δυ,)f)

(11) and in the cycles when η is -ve the intensity becomes l{t) = l{a² +bή÷ Aabco{2π(ητ_λ +Au₁ » (12)

The frequency spectrum of the output power of the laser therefore comprises a frequency component whose value is dictated b}? the time of flight from the laser to the reflective surface _τ, = ' and the additional c

Doppler shift Av₁, acquired hy virtue of reflecting from a moving surface. This situation is illustrated in figure 4 which is a schematic representation of the Fourier analysis of I(t).

In the event of the target surface being stationary, the Fourier analysis of

I(t) will yield a frequency component of ητ_] = ^:^^Lr\ (which is the mode c spacing set up by the external cavity). Since η is known hy design, the precise length Lj of the external cavity can be determined. If the target surface 13 has a velocity component in the direction of the probe beam 12, then the direction in which the stationary frequency T)X₁ will shift on the frequency axis as a consequence of the incurred Doppler shift will depend on the sign of the velocity and which half cycle of the laser frequenc)' modulation the measurement is being made. Hence both the magnitude and sign of Av₁ can be determined.

The use of the 'saw-tooth' current therefore enables an unambiguous measurement in a single defined direction of the magnitude and sign of the velocity component of the target surface as well as the length Li of the external cavity. It will be recognised that other drive current waveforms, periodic or otherwise, could be provided by the drive circuit that are suitable for analysing the velocity and direction of the target surface 13 relative to the laser sensor, even though these ma)⁷ make the analysis computationally intensive. Still further, it will be understood that other techniques for modulating the output frequency of the laser as a function of time may be used in addition or instead of modulating drive current, such as modulating the laser's temperature.

It is advantageous if the velocity of the target surface 13 can also be known in orthogonal x and y directions. Another embodiment shown in figures 5 and 6 enables this. As shown in figure 5, a multi-beam VCSEL 50 provides two laser elements 5OA, 5OB each having a diffraction grating to provide at least two beam outputs each. The two lasers 5OA, 5OB between them thereby provide first and second probe beams 54, 56 and first and second monitor beams 53, 55. Preferably, the two lasers 5OA, 5OB are fabricated on a single semiconductor chip e.g. in a 1x2 array of dual-beam VCSEL elements connected with a ^"common anode bond pad contact 57. Each laser 5OA, 5OB has a linear diffraction grating 51, 52 monolithically integrated onto the respective laser emission aperture. The diffraction gratings 51, 52 are disposed in different directions, preferably orthogonal to one another. In this way, a pair of first order diffracted beams 53 and 54 from VCSEL 5OA and a pair of first order diffracted beams 55 and 56 from VCSEL 5OB comprise beam pairs with azimuthal directions that are different to one another, and preferably orthogonal to one another.

The VCSEL chip 50 therefore emits a total of four beams. As shown in figure 6, it can be seen that the four beam VCSEL chip can be configured so that probe beams 54 and 56 impinge upon the target surface 13 maintained at a fixed distance from the VCSEL chip surface and that light detectors such as silicon photodiodes 61 and 62 can be used to measure the power fluctuations of the monitor beams 53 and 55 respectively from which measurements of the x and y velocity components of the target surface can be independently derived.

For example, VCSEL 5OA can be used to measure the x velocity component and VCSEL 5OB can be used to measure the y velocity component of the target surface 13. VCSEL elements 5OA, 5OB are preferably independent with identical device characteristics such as threshold current and thus only a single laser drive circuit 17 either supplying a constant current or a modulating the current with a 'saw-tooth' waveform is required to drive the lasers. However, with the provision of a separate bond pad for each VCSEL element, a separate laser drive circuit could be used for each VCSEL 5OA, 5OB if so desired. Advantage may also be gained by alternately driving each laser element with a periodic drive current such that when one element is 'on' the other is 'off and visa versa. In such a manner the velocity components in the x and y directions can be sequentially determined without mutual interference arising from between the laser elements.

The frequency spectrum analysis of the monitor beams 53 and 55 is performed by the control circuit 18 which is also responsible for controlling the power in the probe beams such that appropriate laser eye safety power levels are maintained in the probe beams while still achieving the necessary level of feedback into the laser cavity, as previously described.

In an alternative arrangement, a two dimensional diffraction grating (i.e. one with a periodic variation in optical transmissivity in two different directions) might be used to generate two or more probe beams and two or more monitor beams each in two or more azimuthal directions instead of using two lasers each with a linear diffraction grating. For the embodiments described thus far, the distance between the target surface 13 and the VCSEL output surface 22 is maintained at a constant value. In the event that this distance varies with time, the x and y components of velocity can no longer be uniquely determined as the total Doppler shift will now have a component derived from the target motion in the z direction. This is not a concern where a multi-beam VCSEL sensor is used in an application such as a computer mouse and where the separation between the sensor and target surface remains constant by design. However for an application such as a finger controlled optical joystick that includes a finger 'click' or 'tap' functionality, it can be desirable to provide a detection of target motion in the z direction. In practice, this can be implemented by monitoring for large changes or 'swings' in the Doppler Shift frequency Δv as a consequence of the large and sudden increase in the length of the external cavity as the finger is lifted and returned to the sensor.

Therefore in another embodiment, a 'click' or 'tap' functionality is implemented by detecting a large variation in the measured Doppler frequency Δv. For example, the control circuit 18 may be adapted to detect when a rate, or a rate of change, of motion of the target surface relative to the sensor exceeds a predetermined threshold, and to indicate such motion as motion in a different axial direction (e.g. z direction) that that of the azimuthal direction, e.g. x and / or y.

In particular for a two element multi-beam VCSEL such as that described in connection with figures 5 and 6, a finger 'tap' or 'click' can be detected with greater confidence when a large and similar variation in the Doppler shift is simultaneously detected for both VCSEL elements 5OA and 5OB. This simultaneous detection will occur in the event that the external path lengths of both probe beams 54 and 56 sudden!)⁷ change b}? a large amount as would be the case for a finger tap.

In another embodiment of the invention as shown in figure 7, a range finder 70 comprises a dual beam VCSEL 20 packaged such that for maximum ease of use the probe beam 12 exits the sensor housing 11 nominally normal to the optical window 19 and is allowed to impinge upon a distant stationarjr target surface 74. It can be readily appreciated from the previous description that the external cavity length formed by the distant target surface 74 and the surface of the VCSEL 20 can be measured using a modulated laser drive current supplied by the laser driver circuit

17 and analysed by the control circuit 18.

For an environment in which it is known that the target surface 74 has no velocity components orthogonal to the probe beam direction, then any velocity of the target surface in the direction of the probe beam 12 can also be determined. For the particular embodiment shown in figure 1 , the dual beam VCSEL 20 is mounted on an angled sub-mount 71 such that it is disposed at an oblique angle relative to the optical window 19 which may be regarded as a reference surface of the sensor, relative to which the target surface moves. The angled sub-mount 71 compensates for the angle at which the probe beam 12 exits the surface of the VCSEL 20. It is noted that the use of mirror surfaces or waveguides as alternative methods to steer the probe beam 12 can also be employed.

In another embodiment as shown in figure 9, a range finder 90, comprises a multi-beam VCSEL 80 that emits 3 beams. Figure 8 shows the VCSEL 80 which is a triple-beam VCSEL device. The triple beam VCSEL 80 comprises a linear grating 82 monolithically integrated on the surface of the emission aperture of the VCSEL chip surface 81 such that the emission output of the VCSEL 80 is split into three beams. The two first order diffracted beams 83 and 84 follow predominantly off-axis beam paths, i.e. oblique to the surface of the emission aperture. In addition however, the grating 82 is designed such that zero order beam 85 of the VCSEL is not fully suppressed which can be engineered by design through careful control of the grating etch-depth. The zero order beam 85 is preferably normal to the plane of the emission aperture. The ratio of power distribution between the first order diffracted beams 83 and 84 and the zero order beam 85 can be determined by detailed design of the grating.

For the embodiment shown in figure 9, the zero order beam 85 is used as a probe beam while one of the first order diffraction beams, for example beam 84, is used as the monitor beam. The remaining diffracted beam S3 is preferabl}⁷ interrupted using a beam stop 91. Current is supplied to the VCSEL 80 through electrical contact to a bond-pad 86.

It can be appreciated from the previous description that the external cavity length formed by the distant target surface 74 and the surface of the VCSEL 80 can be measured using a modulated laser drive current supplied by the laser driver circuit 17 and analysed by the circuit 18. For an environment in which it is known that the target surface 74 has no velocity components orthogonal to the probe beam direction, then any velocity of the target surface in the direction of the probe beam 85 can also be determined. This embodiment has the advantage that the probe beam 85 lies normal to the chip surface and hence the need for a mount such as 71 shown in figure 7 is avoided.

Another embodiment of a laser velocimeter sensor 110 is shown in figure 11. This comprises a single element, triple-beam VCSEL 100 (shown in figure 10) and a single light detector 16 such that both the x and y velocity components of a target surface 13 can be measured when the distance between the surface of the VCSEL 100 and the target surface 13 is kept constant. One of the beams 84 is used as the monitor beam while the remaining two beams S3 and 85 are used as probe beams to probe the target surface. The VCSEL 80 as shown in figure 8 shows the three beams 83, 84 and 85 all lying in the z-x plane. However in the embodiment of figures 10 and 11, VCSEL 100 is tilted as shown in figure 11 so that probe beams 83 and 85 have unit direction vectors Si and Sa with non-zero components in the y direction.

It will also be realised from figures 10 and 11 that, lyy virtue of this geometrical arrangement, the path lengths Lj and L: of the beams 83 and 85 to the moving target surface in the x-y plane will be different. As previously stated the initial field in the VCSEL cavity is given by,

E₀ (/) = a exp(- ilπvi)

(13) As a result of specular reflections two additional fields will also be present in the VCSEL cavity that have been fed back from the two probe beams 83 and 85 having acquired Doppler shifts from the moving target surface which has a velocity vector V,

E₁ (t) = bexp(-i2π[v(t -T₁)+ Av₁)) (14)

E₂ (t) = c exp(- ilπ [v (t - τ₂ ) + Δ V₂ } )

(15)

, . 2X₁ . 2L₁ where, τj is r, = — - , Ώ IS r₇ = — -,

Av₁ = v(r -τ, )2- F K^,₁ V and Av2 = v(t -τ2)2- K₁ ÷,

The velocity vectors in the direction of the probe beams 83 and 85 are given by

Vi = VSi and V₂ = V. S₂. The total field driving the laser cavity is then E_τ(r) = ∑E_ι (t) from which the ι=0 intensity, I = \E_r ^'is obtained. Ignoring terms oscillating at optical frequencies the following expression for the intensity is therefore given by

J(ή = 2{o² +b² + c² )+

Aabcos(lπ(υ - υ, - Au₁ Jt)+ 4αccos(2π(υ - iΛ - Δυ₂ )r) +

4Z)C COS(2TΓ(L)₁ - υ, + Δv, - Δv₂)/)

(16)

As previously described the laser is current driven with a 'saw-tooth' waveform which leads to a linear variation of optical frequency with time v (/J = V₀ ± ηt . By way of example, the intensity for the half cycle when η is positive then becomes

I(ή = 2{a² + b³ + c² )+

/) + 4accos(2π(rιτ₂ - Aυ₂ )t)+

Abe cos(2π{η(τ₂ -T₁)^ Av₁ - Δv₂)r) (17)

The Fourier spectrum of the output of the laser is shown in figure 12 and comprises two 'high' frequency components whose values are dictated primarily by the round trip times X₁ and τ_% of each probe beam which are different due to the different path lengths Li and L2 of the probe beams of the triple beam VCSEL 110. The exact frequency values depend on the Doppler shifts each beam acquires. Low frequency terms correspond to the differences in optical path lengths and Doppler shifts. It is the case therefore that from a frequency analysis made by control circuit 18 of the intensity measured by the optical light detector 17, the x and y components of the target surface 13 can be deduced. This embodiment has the advantage of lowered cost as only a single laser, single laser driver and single detector axe required to make a measurement of the x and 3_' components of the target surface velocity in the x-}' plane. It is also noted that this embodiment of the invention can also be realised using a single element VCSEL with an integrated grating such that the single element YCSEL produces four beams and in which case one beam is used as a monitor beam, two probe beams are allowed to intersect with the target surface and the fourth beam is stopped by a beam block.

It is noted that the precise packaging arrangements of the embodiments may differ from those indicated in the figures. By way of example, figure 13 shows how the embodiment of figure 1 may be alternatively packaged for greater ease of use and low cost using surface mount packaging. This embodiment generally indicated by 130 comprises clear plastic 131 moulded around a lead frame 133 upon which the VCSEL 20, drive circuit IC 17, control circuit IC 18 and light detector 16 are mounted using a sub- mount 134. As an integral part of the package moulding a lens 132 can be formed to collimate or focus the probe beam 12 onto the target surface 13. Either through total internal reflection or through the surface 135 being so treated so as to be reflective, the monitor beam 15 may be steered to intersect the light detector 16. Alternative techniques such as the use of fibres or waveguides rn^ be use to steer and direct the light beams as and when required.

Shown in figure 14 by way of example is how the embodiment 130 may be used in a handheld computer or telephone generally shown as 140 to control the vertical motion on a screen 141 of a cursor 142.

Although the preferred embodiments have been described using one or more VCSELs as the laser device in the laser sensor, it will be understood that some principles as described here can be implemented with other semiconductor lasers, e.g. edge emitters, providing that it is possible to integrate a beam splitter such as a diffraction grating into the laser.

Furthermore, the expression 'laser' as used here is intended to encompass any other optoelectronic device having an optical cavity in which the characteristics of the device can be modified by feeding light from outside the cavity back into the cavity to constructively or destructivel}⁷ interfere with the light in the cavity, providing the self-mixing phenomenon described earlier.

Other embodiments are intentionally within the scope of the accompanying claims.

Claims

1. A self -mixing laser sensor comprising: a laser device having an optical cavity with an optical output face and a beam splitter integrated into the laser device for providing at least two output beams from the optical output facet, a first one of the output beams being a first probe beam for directing to a target surface external to the sensor and a second one of the output beams being a first monitor beam for directing to an optical detector; an optical detector for receiving said first monitor beam; a drive circuit for controlling the laser output; and a control circuit configured to determine, from the first monitor beam, motion of the target surface relative to the sensor as a function of self-mixing of a portion of the first probe beam returned to the optical cavity by reflection from the target surface.

2. The sensor of claim 1 in which the laser device is a VCSEL.

3. The sensor of claim 1 or claim 2 in which the beam splitter is a diffraction grating integrated into the laser device.

4. The sensor of claim 3 in which the diffraction grating is a one dimensional grating generating at least two divergent output beams.

5. The sensor of claim 1 in which the drive circuit is adapted to modulate the output frequency of the laser as a function of time and the control circuit is adapted to determine a direction of movement of the target surface relative to the sensor thereby.

6. The sensor of claim 5 in which the drive circuit is adapted to modulate the output frequency of the laser as a linear function of time.

7. The sensor of claim 6 in which the drive circuit is adapted to drive the laser with a current modulated with a saw tooth wave.

8. The sensor of claim 5 in which the control circuit is adapted to determine a first modulation in the monitor beam arising from the frequency modulation of the laser output by the drive circuit and a second modulation in the monitor beam arising from self -mixing of the portion of the probe beam returned to the optical cavity by reflection from the target surface.

9. The sensor of claim 1 in which the diffraction grating is a two dimensional grating generating at least four divergent output beams, the third one of the output beams being a second probe beam for directing to the target surface and the fourth one of the output beams being a second monitor beam for directing to a second optical detector, the first and second probe beams having different azimuth angles.

10. The sensor of claim 9 in which the first and second probe beams have orthogonal azimuth angles.

11. The sensor of claim 1 further including a second laser device having an optical cavity with an optical output facet and a beam splitter integrated into the laser device for providing at least two output beams from the optical output facet, a first one of the output beams being a second probe beam for directing to the target surface and a second one of the output beams being a second monitor beam for directing to a second optical detector; a second optical detector for receiving said second monitor beam; a drive circuit for controlling the second laser output; and a control circuit configured to determine, from the second monitor beam, motion of the target surface relative to the sensor as a function of self-mixing of a portion of the second probe beam returned to the optical cavity by reflection from the target surface.

12. The sensor of claim 11 in which the first and second probe beams have different azimuth angles.

13. The sensor of claim 12 in which the first and second probe beams have orthogonal azimuth angles.

14. The sensor of claim 11 in which the first and second lasers are monolithically integrated on a single substrate.

15. The sensor of claim 11 in which the drive circuit for controlling the second laser output is the same as the drive circuit for controlling the first laser output.

16. The sensor of claim 11 in which the drive circuits are adapted to drive the first and second laser outputs at different, alternating times.

17. The sensor of claim 1 or claim 11 in which the control circuit(s) is or are adapted to detect when a rate, or a rate of change, of motion of the target surface relative to the sensor exceeds a predetermined threshold and to indicate such motion as motion in a different axial direction than relative motion below the predetermined threshold.

18. The sensor of claim 1 in which the laser device is mounted at an oblique angle relative to an optical aperture formed in or as a reference surface of the sensor relative to which a target surface moves.

19. The sensor of claim 3 in which the laser device is adapted to emit a zero order beam and two first order diffraction beams, the zero order beam providing said first probe beam and one of said first order diffraction beams providing said first monitor beam.

20. The sensor of claim 1 in which the laser device is adapted to provide at least three output beams, the third of said output beams being a second probe beam for directing to the target surface at a different azimuthal angle than the first probe beam, the control circuit configured to determine, from the monitor beam, motion of the target surface relative to the sensor as a function of self-mixing of a portion of each of the first and second probe beams returned to the optical cavity by reflection from the target surface.

21. The sensor of claim 20 in which the laser device is positioned at an oblique angle relative to an optical aperture or window formed in or as a reference surface of the sensor relative to which a target surface moves.

22. The sensor of any preceding claim in which the laser device(s), the optical detector (s), the drive circuit(s) and the control circuit(s) are monolithicaUy integrated onto a single substrate.

23. The sensor of claim 1 in which the sensor comprises an input device for a computing apparatus.

24. A method of detecting movement of a target surface relative to a self-mixing laser sensor comprising the steps of: generating at least two output beams from an optical output face of a laser device having an optical cavity and a beam splitter integrated into the laser device; directing a first one of the output beams to the target surface as a first probe beam so that a portion of the first probe beam returns to the optical cavity by reflection from the target surface and causes self -mixing in the laser cavity; directing a second one of the output beams to an optical detector as a first monitor beam so as to generate, by the optical detector, an output signal indicative of the laser output power; determining, from the output of the detector, motion of the target surface relative to the sensor as a function of the self-mixing of the portion of the first probe beam returned to the optical cavity by reflection from the target surface.

25. The method of claim 24 further including the step of using a drive circuit to modulate the output frequency of the laser as a function of time, and determining a direction of movement of the target surface relative to the sensor thereby.

26. The method of claim 25 in which the step of modulating comprises modulating the output frequency of the laser as a linear function of time.

27. The method of claim 26 in which the step of modulating comprises modulating the output frequency of the laser with a saw tooth "wave.

28. The method of claim 24 further including generating a third output beam and directing the third output beam to the target surface as a second probe beam so that a portion of the second probe beam returns to the optical cavity by reflection from the target surface and causes self -mixing in the laser cavity, the first and second probe beams being directed so that they have different optical path lengths to the target surface, and wherein the determining step further includes determining motion of the target surface relative to the sensor as a function of the self-mixing of the portions of the first and second probe beams returned to the optical cavity from the target surface.

29. A self-mixing laser sensor substantially as described herein with reference to the accompanying drawings.