WO2005116997A1

WO2005116997A1 - Optical scanning device

Info

Publication number: WO2005116997A1
Application number: PCT/IB2005/051625
Authority: WO
Inventors: Coen A. Verschuren
Original assignee: Koninklijke Philips Electronics N.V.,
Priority date: 2004-05-27
Filing date: 2005-05-19
Publication date: 2005-12-08
Also published as: TW200609934A

Abstract

In a particular type of high-density optical recording systems, a solid immersion lens (SIL) is used to focus a radiation beam onto an information layer of an optical record carrier. A suitable gap signal , representing the width of the gap between the exit face of the SIL and the entrance surface of the record carrier, can be obtained from the intensity of the reflected light with an appropriate polarization. The gap signal is normalized with respect to the laser power which may be derived from a power control signal for the laser. The laser power may first be adjusted at a particular gap width.

Description

Optical scanning device

FIELD OF THE INVENTION The present invention relates to an optical scanning device for scanning record carriers, in particular for scanning record carriers using evanescent coupling of radiation.

BACKGROUND OF THE INVENTION In a particular type of high-density optical scanning device, a solid immersion lens (SIL) is used to focus a radiation beam to a scanning spot on an information layer of a record carrier. A certain size of an air gap between the exit face of the SIL and the entrance face of the record carrier, for example 25 nm, is desirable to allow evanescent coupling of the radiation beam from the SIL to the record carrier. Evanescent coupling may otherwise be referred to as frustrated total internal reflection (FTIR). Recording systems using evanescent coupling are also known as near- field systems, deriving their name from the field formed by the evanescent wave at an exit face of the SIL, which is sometimes referred to as the near field. An exemplary optical scanning device may use a radiation source which is a blue laser emitting a radiation beam having a wavelength of approximately 405nm. During scanning of the record carrier the evanescent coupling between the exit face of the SIL and the outer face of the record carrier should be maintained. This involves maintaining the size of the gap at a desired, very small value during motion between the SIL and the record carrier. An efficiency of this evanescent coupling in general varies with a change in the size of the gap between the exit face and the entrance face. When the gap size becomes larger than a desired gap size the coupling efficiency tends to decrease and a quality of the scanning spot will also decrease. If the scanning procedure involves reading data from the record carrier, for example, this decrease in efficiency will result in a decrease in the quality of the data being read, possibly with the introduction of errors into the data signal. Too small a gap size may result in collision of the SIL and the record carrier. To allow control of the width of the air gap using a mechanical actuator at such small distances, a suitable control signal or gap signal is required as input for a gap servo system. As disclosed in the paper by T. Ishimoto et al. in the Proceedings of Optical Data Storage 2001 in Santa Fe, a suitable gap signal is obtained from the light reflected from the SIL with a polarization state perpendicular to that of the forward radiation beam that is focused on the SIL. A fraction of the light becomes elliptically polarized after reflection at the SIL-air- record carrier interfaces: this effect creates the well-known Maltese cross when the reflected light is observed through crossed polarizers. The gap signal is generated by integrating all the reflected light having the perpendicular polarization using polarizing optics and a radiation detector, which can be a single photodetector. The value of the gap signal is zero for zero gap width and increases with increasing gap width. The gap signal levels off at a maximum value when the gap width is approximately a tenth of the wavelength of the radiation beam. The desired gap width corresponds to a certain value of the gap signal, the set-point. The gap signal and a fixed voltage equal to the set-point are input into a comparator, e.g. a subtractor, which forms a gap error signal at its output. The gap error signal is used to control the gap servo system. During recording, very short, high-power laser pulses are emitted by the laser. These pulses dynamically change the average laser power, leading to corresponding changes in the gap signal. A gap servo system then uses this gap signal to reduce the air gap distance in order to arrive at the set-point again after the laser power fluctuations. Hence if, for example, the laser power increases suddenly, the gap signal will also increase. A similar effect occurs during playback when the laser power changes, e.g. due to temperature drift. As a solution to separate the gap signal from changes in the laser power, the article by Shinoda et al in ODS2004 proposes to use a separate light path with a different wavelength (larger to increase the focus capture range as an additional advantage) to generate the gap signal. By using suitable optics, this gap signal will be independent from changes in the read and write power of the laser, thus improving robustness during reading and making recording possible. However, this solution is relatively complex and expensive because of the additional optics and the additional laser.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method and implementation for an evanescent coupling scanning device having an air gap control that is independent of laser power fluctuations without the need for an additional laser. According to a first aspect of the present invention, there is provided an optical scanning device for scanning a record carrier having an entrance face and located at a scanning position in the device, the device comprising: a radiation source for generating a forward radiation beam; an objective system having an exit face, the objective system being arranged in the path of the forward radiation beam between the radiation source and the scanning position and providing for evanescent coupling with the optical record carrier across a gap between the exit face and the entrance face; and a first radiation detector for detecting a reflected radiation beam coming from the objective system and providing a gap signal representing the width of the gap; characterized in that the device comprises a normalization circuit having as input signals the gap signal and a beam power signal representing the power of the forward radiation beam, the normalization circuit providing a normalized gap signal, being the gap signal normalized with the power signal. Hence, application of the invention requires only a relatively small modification of the light path compared to a dual wavelength approach disclosed in the prior art. If a scanning device already comprises an integrated forward sense detector, no additional optical component or detector is required. In preferred embodiments of the invention, a further radiation detector is provided for detecting radiation from the radiation source and providing at an output the beam power signal. Alternatively, a signal for controlling the power of the forward radiation beam is used to form the beam power signal. Preferably, at least one amplifier having a gain is arranged in the signal path of the gap signal and/or the signal path of the beam power signal. According to a second aspect of the present invention, there is provided a method of adjusting an optical scanning device as described above, including at least the steps of: increasing the gap width to at least 1/10 of the wavelength of the forward radiation beam if the entrance face of a record carrier is within 1/10 of the wavelength from the exit face of the objective system; adjusting the beam power of the forward radiation beam to a predetermined level; and adjusting the gain such that the value of the normalized gap signal is substantially equal to a predetermined value. According to a third aspect of the present invention, there is provided a method of operating an optical scanning device as described above, including at least the steps of: subtracting the normalized gap signal and a set-point voltage, and using the resulting gap error signal in a gap servo system to maintain the width of the gap at a predetermined value. Further features and advantages of the invention will become apparent from the following description of preferred embodiments of the invention, given by way of example only, which is made with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 a shows a lens focusing a light beam in air in accordance with the prior art, Fig. lb shows a lens focusing a light beam in a hemispherical solid immersion lens in accordance with the prior art, Fig. lc shows a lens focusing a light beam in an aplanatic super-hemispherical solid immersion lens in accordance with the prior art, Fig. 2 shows schematically a near field optical scanning device in accordance with the prior art, Fig. 3 shows schematically a near field optical scanning device with a reference detector and normalization circuit in accordance with an embodiment of the present invention, Fig. 4 shows a schematic diagram of a normalization circuit for a gap signal in accordance with an embodiment of the present invention, Fig. 5 shows, graphically, experimental values of the gap, reference and normalized gap signals at a laser power of 3mW in accordance with an embodiment of the present invention, Fig. 6 shows, graphically, experimental values of the gap, reference and normalized gap signals at a laser power of 3mW in accordance with an embodiment of the present invention, Fig. 7 shows, graphically, experimental values of the gap, reference and normalized gap signals at a laser power of lmW and an air gap distance of 20 nm (near field) in accordance with an embodiment of the present invention,

DETAILED DESCRIPTION OF THE INVENTION The maximum information density that can be recorded on an optical record carrier in an optical recording system scales inversely with the size of the laser spot that is focused on a scanning position on the information layer. The spot size is determined by the ratio of two optical parameters: the wavelength λ of the radiation beam forming the spot and the numerical aperture (NA) of the objective lens focusing the radiation beam. The NA of an objective lens is defined as NA= n sin (θ), where n is the refractive index of the medium in which the light is focused and θ is the half angle of the focused cone of light in that medium. It is evident that the upper limit for the NA of an objective lens that focuses in air or via air in a plane parallel plate (like a flat disk) is in general unity. Fig. la shows an example of a lens 2 focusing a light beam 4 in air, where the half angle of the focused cone of light is θ, shown as item 8, and the optical axis is shown as a dashed-dotted line 6. The NA of a lens can exceed unity if the light is focused in a high index medium without refraction at the air-medium interface, for example by focusing in the center of a hemispherical SIL. Fig. lb shows an example of a lens 12 focusing a light beam 14 through such a hemispherical SIL 16 with a radius R, shown as arrow 18. In this case the effective NA is

n NAo with n the refractive index of the hemispherical lens and NAo the NA in air of the focusing lens. A possibility to further increase the NA is by use of an aplanatic super- hemispherical SIL. Fig. lc shows an example of a lens 22 focusing a light beam 24 through such an aplanatic super-hemispherical SIL 26 of radius R, indicated by arrow 28, in which the super-hemispherical SIL 26 refracts the beam 30 towards the optical axis 36 and focuses it below the center of the super-hemisphere. In this case the effective NA is NA_eff=n NA₀. For an aplanatic super-hemispherical SIL of height R(l+l/n) along the optical axis, indicated by arrow 32, the beam can be focused at a distance nR, indicated by arrow 34, closer to lens 2 than the case in Fig. la with no SIL. Importantly, an effective NA_etτ larger than unity is only present within an extremely short distance (also called the near-field) from the exit surface of the SIL, where an evanescent wave exists. The exit surface of the objective system is the last refractive surface of the objective system before the radiation impinges on the record carrier. The short distance is typically smaller than one tenth of the wavelength of the radiation. When an entrance face of an optical record carrier is arranged within this short distance, radiation is transmitted from the SIL to the record carrier by evanescent coupling. This means that during writing or reading of an optical record carrier, the distance between the SIL and record carrier, or the gap width, should be smaller than a few tens of nanometers, for example about 25 nm for a system using a blue laser as radiation source and an NA of the objective system of 1.9. In a so-called air-incident optical record carrier, one side of the information layer is in contact with a substrate and the other side is exposed to the environment. The entrance face of such a record carrier is the interface between the information layer and the environment. Alternatively, the information layer is protected from the environment by a thin transparent layer, the outer surface of which forms the entrance face of the record carrier. In the latter case the thickness of the SIL must be corrected for the thickness of the transparent layer. Fig. 2 shows schematically a near field optical scanning device for scanning a record carrier in accordance with the prior art. The optical scanning device comprises a radiation source system which is arranged to generate radiation. In this embodiment the radiation source is a laser 60 emitting a radiation beam 62 having a predetermined wavelength λ, for example approximately 405nm. During both a start-up procedure and a record carrier scanning procedure of the optical scanning device, the radiation beam 62 passes along an optical axis (not indicated) of the optical scanning device and is collimated by a collimator lens 64 and its cross-sectional intensity distribution shaped by a beam shaper 66. The radiation beam 62 then passes through a non-polarizing beam splitter 68, followed by a polarizing beam splitter 70 and has a focus introduced between a first focus adjustment lens 72 and a second focus adjustment lens 76. An optimal adjustment of a focus position of the radiation beam 62 on the record carrier is achieved by moving the first focus adjustment lens 72 in a focus adjustment direction 74. An objective system of the optical scanning device comprises an objective lens 78 which introduces a focusing wavefront into the radiation beam 62. The objective system further comprises a solid immersion lens (SIL) 80. In this embodiment the SIL 80 has a conical super-hemispherical shape as in Fig. lc, which in this example has a NA of 1.9. The planar side of the SIL forms an exit face facing a record carrier 82. A supporting frame (not shown) ensures that an alignment and a separation distance of the objective lens 78 with the SIL 80 are maintained. The supporting frame is kept at the correct distance from the record carrier by a gap servo system (not shown) which is described in more detail below. After the introduction of the focusing wavefront by the objective system, the radiation beam forms a radiation beam spot on the record carrier 82. The radiation beam which falls onto the record carrier 82 has a linear polarization. The record carrier 82 has an entrance face 120 which faces the SIL 80 exit face 122. The objective system is arranged between the radiation source 60 and the record carrier 82 and a gap between the exit face 122 and the entrance face 120 has a gap size which is the distance between the exit face 122 and the entrance face 122 along the optical axis. The optical scanning device includes a plurality of optical detection paths. In a first optical detection path there is arranged a polarizer 110, a half- wave plate 112, a polarizing beam splitter 104, a folding mirror 114, a first condenser lens 106 for focusing a detection radiation beam onto a first detector 108 and a second condenser lens 116 for focusing a detection radiation beam onto a second detector 118. The polarizer 110, half-wave plate 112, folding mirror 114, second condenser lens 116 and second detector 118 are optional components for experimental research purposes. The second detector could for example be a CCD type detector. Radiation passing through the polarizing beam splitter 104 is reflected by the folding mirror 114 and focused by the condensing lens 116 onto the second detector 118. If these optional components are not used, the polarizing beam splitter 104 could be replaced with a folding mirror in order to guide a portion of the detection radiation beam onto the first detector. In a second, different, detection path there is arranged a half-wave plate 84, a polarizing beam splitter 86, a non-polarizing beam splitter 92, a third condenser lens 90 for focusing a detection radiation beam onto a third detector 88, a fourth condenser lens 96 for focusing a detection radiation beam onto a fourth detector 94, a folding mirror 98 and a fifth condenser lens 102 for focusing a detection radiation beam onto a fifth detector 100. Similarly to the first detection path, the half- wave plate 84, folding mirror 98, fifth condenser lens 102 and fifth detector 100 are optional components for experimental research purposes. The fifth detector could be for example a CCD type detector. Radiation passing through the non-polarizing beam splitter 92 is reflected by the folding mirror 98 and focused by the condensing lens 102 onto the fifth detector 100. If these optional components are not used, the non-polarizing beam splitter 92 could be replaced with a folding mirror in order to guide a portion of the detection radiation beam onto the fourth detector. The first, third and fourth detectors, shown by items 108, 88 and 94 respectively, constitute a radiation detector arrangement for generating detector signals representing information detected in the radiation after interaction with the record carrier 82. The first detection path is used for detection of radiation reflected from the SIL 80 and polarized perpendicular to the forward radiation beam that is focused on the record carrier. The perpendicularly polarized radiation is referred to as the RF JL pol. signal. The gap signal 152 is derived from the low-frequency part (e.g. DC to 30 kHz) of the RF J_ pol. signal. The second detection path is used for detection of radiation that is polarized parallel to the forward radiation beam that is focused on the record carrier and is modulated by the information read from the information layer. The portion of light in the second detection path that is detected by the third detector is referred to as the RF // pol. signal, the function of which is described in more detail later. The portion of light in the second detection path that is detected by the fourth detector is referred to as the push-pull signal and is used to generate a signal representing the transverse distance between the spot and the center of the data track of the record carrier 82 to be followed. The signal is used to maintain a radial tracking of the scanning radiation spot on the data track. The radiation passing along the first detection path and the radiation passing along the second detection path are orthogonally polarized with respect to each other. The method is based on a normalization of the gap signal with a reference signal representing the radiation power in the forward radiation beam, i.e. the radiation beam emitted by the laser and impinging on the SIL. The reference signal should not depend on the gap width and on the properties of the optical record carrier. The invention is based on the insight that the gap signal (RF J_ pol.) is relatively insensitive to the properties of the optical record carrier, such as its reflection. In contrast, the RF // pol. signal depends strongly on the properties of the record carrier. As a consequence, the gap signal cannot be normalized with RF // pol. or the sum of RF // pol. and RF _L pol. Hence, it is not possible to employ the usual method of normalization in optical recording, i.e. normalizing a signal generated by a detector arranged in a radiation beam with another signal derived from another part of the same radiation beam or from the overall power of the same radiation beam. According to the invention the gap signal, derived from a radiation beam reflected from the objective system, is normalized with a signal derived from a different radiation beam, i.e. the forward radiation beam impinging on the objective system. More specifically, the gap signal is normalized with a reference signal depending on the radiation power incident on the SIL. This normalization is possible because both the gap signal and the reference signal are independent of the record carrier properties and the reference signal is independent of the gap width. Some prior art scanning devices comprise a detection system for measuring the power of the forward radiation beam, the output signal of which is used to control the radiation power emitted by the laser. The same output signal can, according to the invention, be used to normalize the gap signal. Further, a procedure is described for adjustment of the circuit forming the gap signal and a method for operating the scanning device. A detailed description is given below. For the present invention, a modification to the prior art device set-up of Fig. 2 is the addition of a forward sense detector, which detects part of the forward radiation beam emitted by the laser. An example of such a set-up is shown schematically in Fig. 3, where only the additional reference detector and normalization circuit components over and above those shown in Fig. 2 are labeled and described. A portion of the forward radiation beam is directed by the non-polarizing beam splitter 68 and is focused onto a sixth detector 142 by a sixth condenser lens 140. In the embodiment shown in Fig. 3 the radiation source is a laser, preferably a semi-conductor laser. The sixth detector 142 generates a beam power signal representing the power in the radiation beam emitted by the laser that can be used to monitor dynamically the laser power. The beam power signal is used as a reference signal REF 156, which is passed through a low-pass filter (LPF) 144 and through an amplifier 146. Similarly, the gap signal (GS) 152 is passed through an LPF 148 and an amplifier 150. The REF signal (REF) 156 is then used to normalize GS 152 by use of a divider 158. The divider and amplifier components are discussed in more detail below, along with possible additional pre-amplifier components. The output signal of the divider is used to control the air gap actuator. As an alternative to the set-up shown in Fig. 3, the radiation detector can be included in a laser module. Commercial laser modules often have such a built-in detector for output power feedback of the laser. Since the power depends on the current through the laser, the beam power signal can further alternatively be derived from a measurement of that current and a relationship between the current and the radiation output of the laser. In that case the relation between the beam power signal and the current is preferably made dependent on the temperature of the laser using the current-power-temperature dependence of the semiconductor laser. During recording, very short, high-power laser pulses are used with rise and fall times in the order of Ins for modern recording systems. These fast changes cannot be tracked fast enough by the air gap actuator. Therefore, it is sufficient for both the GS and REF detectors that the detection bandwidth is at least larger than the bandwidth of the air gap servo system. Currently, the latter bandwidth is from zero to around 3 kHz and up to 10 kHz for a state-of-the-art system as disclosed in the article by Ishimoto et al in the ODS 2004. For digital servo implementations the signals are sampled, and the detector signals should be low- pass filtered at a frequency less than half the sampling frequency to avoid aliasing. Preferably, this low-pass frequency should be higher than the servo bandwidth. For example, a sampling frequency of 75 kHz and low-pass filtering at 30 kHz with a slope of 6 dB is a good combination for current systems. Detector bandwidths far exceeding the low-pass frequency are possible but not required. It is desirable that the GS and REF detectors should not saturate at the highest average laser power during reading or writing; for example, an appropriate neutral density filter in front of the detector can be used if needed. Preferably, the detector response is linear over the range of laser powers to be used, without significant offset. For most practical detectors, this is indeed the case. Otherwise, the characteristic should be known or calibrated in the drive in order to compensate, e.g. in the normalization circuit. The detectors, any pre-amplifiers of the detectors and any amplifiers before the subtractor preferably have a linear response. A non- linear response can be compensated for with additional circuitry (not shown). The gain before and/or after a low-pass filter should be smaller than or equal to the gain that gives the maximum allowed input to the normalization circuit (for example an ADC) at the highest average laser power, i.e. at the highest laser power, clipping should not occur in the normalization circuit. The input signal accuracy (i.e. for the ADC) should be sufficient in that the maximum average write level is normally about a factor of 5 higher than the read level. Even for an 8 bit ADC resolution (256 levels), this would give an accuracy of approximately 2% around the read level. For an air gap control range of 50 nm and a typical work point of approximately 25 nm, this corresponds to an air gap accuracy of approximately 1 nm, which is sufficient for a practical system. The circuit forming the gap signal may comprise a pre-amplifier near the radiation detector(s), an ADC for a digital version of the circuit, an amplifier after the ADC, a divider (for dividing A by B, see below) and an amplifier after the subtractor. Fig. 4 shows a schematic diagram of a normalization circuit whereby GS 152 is normalized by REF signal 156 to produce a normalized gap signal (GSn) 206. The detectors may have built-in pre-amplifiers having gains G_I,_GS and G_I,_REF as indicated in Fig. 4 by items 192 and 190 respectively. If the circuit is digital, an ADC 194 converts the analogue signals to digital signals. The analogue and digital domains are indicated by items 180 and 182 respectively. If required, the output signals of the ADC are amplified by two amplifiers having gains G_2,GS and G₂,_REpas indicated by items 196 and 198 respectively. The signals A and B, shown by items 200 and 203 respectively, are GS and REF, respectively, after amplification and as input in the normalization circuit. The output signal of the divider 208 is input to an amplifier 204 having an amplification F. The output of the amplifier 204 is the normalized gap-signal GSn 206, which is input in the subtractor (not shown) for comparison with the set-point. The signal at the output of the circuit is GSn and the value of the set-point should be in concordance to obtain the desired gap width when the gap servo system is in operation. The following method according to the invention provides a relatively simple adjustment of the gain of the amplifiers to achieve the desired concordance. The adjustment of the gain should be carried out in a situation where the gap signal is not affected by changes in the width of the gap. The preferred situation is when the record carrier is still far from the exit face of the objective system. A gap width larger than one tenth of the laser wavelength is preferred. The record carrier does not even need to be in the device. A position of the record carrier within the near field distance, between one tenth of the wavelength and 0 (i.e. contact) is not a good alternative. This is because the servo is turned off during calibration and any vibration will cause a change in gap width, and there is a significant risk that the exit face of the objective system will hit the record carrier, causing damage to the objective system. The gain of the amplifiers must be adjusted to obtain a predetermined value of GSn. Each of the gains affects the slope of the gap signal versus the gap width. The G₂,_Gs and G_2,REF amplifiers arc optional, but may be used as digitally adjustable gain(s) to make the signals A and B substantially equal. These amplifiers are also useful in case the preamplifiers G_I,G_S and G_I._REF are fixed in hardware, or when their gain adjustment is insufficiently accurate. It is advantageous to select the 'worst case' power, especially when G₂,_GS and G₂,_REF are absent and G_I,G_S and G_I,_REF are accurately adjustable, because in this case, the maximum gains for the ADC, for example (as explained above), can be checked at the same time as the fine-tuning of the gains for the calibration. When the gap width is large, the gap signal has a maximum value, independent of the gap width. This far field level of the gap signal basically defines the slope of the gap signal vs. air gap distance. The value of GSn can be adjusted by setting the gains of the various amplifiers in the circuit. The value of GSn should be adjusted to a predetermined value. Since the behavior of the gap signal as a function of the gap width is known, apart from an arbitrary gain factor, the fraction of the maximum value of the gap signal can be determined where the gap has the desired width. The value of the set-point is made equal to the said fraction of the predetermined value of GSn. For example, the maximum value of GSn is adjusted to a predetermined value of 1 V. From the behavior of the gap signal for the embodiment of the device it is known that at a fraction of 0.625 of the maximum signal, the gap width is equal to λ/16. When the wavelength of the radiation source in the device is 400 nm, a set-point of 0.625 V will result in a gap width of 25 nm. This adjustment ensures that during operation the normalized gap signal has the value of the set-point when the gap width has the desired value. During scanning the gap servo system will keep the normalized gap signal at the value of the set-point. The normalized gap signal remains constant when the laser power changes. If necessary during calibration, the record carrier is moved away from the objective lens such that the gap is significantly wider than 1/10 of laser wavelength. It is also possible that no optical disc is in the device and this is in fact preferable. The calibration can be carried out before or even during insertion of the record carrier. If necessary, all amplifier offsets are zeroed with a blocked radiation beam or switched-off laser. A fixed laser power level or pulse sequence is applied, e.g. a level or pulse sequence which corresponds to the highest average laser power that will be used in the drive. The amplifier gains (GI,GS, G^REF, G₂,GS, G₂,REF and/or F) are adjusted such that the normalized gap signal has a predetermined value. Gains should be sufficiently low to prevent clipping or saturation at the highest average laser power. The value of the set-point of the subtractor is a predetermined fraction of the predetermined value of the normalized gap signal. The procedure has been tested experimentally in an optical scanning device. The device comprises an REF detector branch as shown in Fig. 3. The pre-amplifiers and low-pass filters were EG&G Model 5113 adjustable pre-amps with low-pass filtering at 30 kHz, 6 dB slope. The normalization circuit of Fig. 4 has been digitally implemented in the same environment as the air gap servo. The results of a simple proof-of-principle experiment are shown in Figs. 5, 6 and 7. The G_2JGS and G_2;RE_F amplifiers were not used in the experiment. The three displays in Figs. 5, 6 and 7 show the value of signals as a function of time. The display in Fig. 5 shows the value of the gap signal as the trace indicated by item 230, the value of REF as the trace indicated by item 232 (coincident with trace 230) and the value of GSn as the trace indicated by item 234. These values are obtained after calibration at a laser power P of 3 W, as indicated by text box 236. Both the gap signal and the REF signal are amplified such that they are equal (approximately 5V in this example). There is no record carrier near the lens (i.e. the 'far field' situation), as indicated by item 238. Fig. 6 shows the values of these signals when the laser power is reduced to 1 mW, as indicated by text box 246. The REF and gap signal signals are seen to drop to about IV, as shown by the coincident traces indicated by items 242 and 244 respectively. The normalized gap signal shown as the trace indicated by item 240, however, remains at its previous level, showing that this signal is indeed independent of the laser power. Fig. 7 shows the values of the signals when a record carrier is very close to the lens (at an example air gap distance of 20 nm, as shown by item 256), similar to a situation for actual reading or writing. The laser power is still at 1 mW, as indicated by text box 258. The reference signal REF, as indicated by item 250 is indeed at the same level of IV as in the display of Fig. 6. Both the gap signal, as indicated by item 254 and the normalized gap signal, as indicated by item 252 (coincident with the trace of item 250) are reduced to the expected level for this air gap, demonstrating that the normalized gap signal works perfectly. Dynamic experiments with laser pulse sequences have been performed as well, and confirm correct operation. The above embodiments are to be understood as illustrative examples of the invention. Further embodiments of the invention can be envisaged. In further embodiments of the present invention, the objective system comprises a different SIL. Such a different SIL may have a shape different to that described previously, for example a non-conical super- hemispherical shape, or a mesa super-hemispherical shape where the exit face is a protrusion of the SIL, or a hemispherical shape. In the described embodiments of the present invention, the record carrier has an information layer and the outer face is a surface of this information layer. It is alternatively envisaged that the record carrier has an information layer and a cover layer. One surface of the cover layer is the entrance face whereas the information layer is arranged on the other surface of the cover layer. In this alternative embodiment the optical scanning device is adapted so that during the scanning procedure the radiation beam is focused through the cover layer to a spot on the information layer. One such adaptation is a change in a thickness of the SIL along the optical axis. The above-disclosed embodiment of the invention concerns a scanning device suitable for scanning an optical record carrier. The invention is applicable to both reading from and writing to such record carriers. The invention is not limited to the field of optical recording, and can be advantageously used in other related fields such as scanning microscopy. The described embodiments of the present invention detail the radiation beam having a certain wavelength. It is envisaged that the radiation beam has a different certain wavelength and the optical scanning device and the record carrier are suitably arranged to operate at this different certain wavelength. The record carrier in the described embodiment of the invention is an optical record carrier, however it is envisaged in further embodiments that the optical scanning device is adapted to scan different types of record carrier including for example a disc employing hybrid recording such as heat assisted magnetic recording (HAMR) or a disc of a hard disc drive (HDD). The sixth detector may also be arranged to capture radiation emitted by the rear facet of the semi-conductor laser instead of radiation from the forward radiation beam emitted by the front facet of the laser. The reference signal may also be formed using a signal for controlling the radiation power of the laser. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims

CLAIMS:

1. An optical scanning device for scanning a record carrier (82) having an entrance face (120) and located at a scanning position in the device, the device comprising:

- a radiation source (60) for generating a forward radiation beam (62);

- an objective system having an exit face (122), the objective system being arranged in the path of the forward radiation beam between the radiation source and the scanning position and providing for evanescent coupling with the optical record carrier across a gap between the exit face and the entrance face; and

- a first radiation detector (108) for detecting a reflected radiation beam coming from the objective system and providing a gap signal representing the width of the gap; characterized in that the device comprises a normalization circuit having as input signals the gap signal (152) and a beam power signal (156) representing the power of the forward radiation beam, the normalization circuit providing a normalized gap signal, being the gap signal normalized with the power signal.

2. An optical scanning device according to claim 1, comprising a further radiation detector (142) for detecting radiation from the radiation source and providing at an output the beam power signal.

3. An optical scanning device according to claim 2, wherein the further radiation detector has a range of linear operation commensurate with the highest power of the forward radiation beam providable by the radiation source.

4. An optical scanning device according to claim 1 , wherein a signal for controlling the power of the forward radiation beam is used to form the beam power signal.

5. An optical scanning device according to any preceding claim, wherein the first radiation detector has a range of linear operation commensurate with the highest power of the forward radiation beam providable by the radiation source.

6. An optical scanning device according to any preceding claim, wherein at least one amplifier having a gain is arranged in the signal path of the gap signal and/or the signal path of the beam power signal.

7. A method of adjusting an optical scanning device arranged according to claim

6, including at least the steps of:

- increasing the gap width to at least 1/10 of the wavelength of the forward radiation beam if the entrance face of a record carrier is within 1/10 of the wavelength from the exit face of the objective system; - adjusting the beam power of the forward radiation beam to a predetermined level; and

- adjusting the gain such that the value of the normalized gap signal is substantially equal to a predetermined value.

8. A method of operating an optical scanning device arranged according to any of claims 1 to 6, including at least the steps of:

- subtracting the normalized gap signal and a set-point voltage, and using the resulting gap error signal in a gap servo system to maintain the width of the gap at a predetermined value.