Bolometer with temperature compensation for internal laser power monitoring in heat-assisted magnetic recording device

A slider configured for heat-assisted magnetic recording comprises an optical sensor coupled to first and second bond pads. The optical sensor comprises a bolometer and a reference sensor. The bolometer is situated at a location of the slider that receives at least some of the light and exposed to an ambient temperature at the slider. The bolometer produces a signal in response to a change in the ambient temperature and the change in output optical power. The reference sensor is situated at a location of the slider unexposed to the light and exposed to the ambient temperature. The reference sensor is coupled to the bolometer and configured to produce a signal in response to the change in the ambient temperature. The optical sensor is configured to generate a sensor signal indicative of changes in output optical power of a laser source without contribution due to ambient temperature changes.

SUMMARY

Embodiments are directed to an apparatus comprising a slider having a plurality of electrical bond pads including a first bond pad and a second bond pad. A writer and a reader are provided on the slider. An optical waveguide is formed in the slider and configured to receive light from a laser source. A near-field transducer (NFT) is provided on the slider and optically coupled to the waveguide. An optical sensor is coupled only to the first and second bond pads and configured to generate a sensor signal indicative of changes in output optical power of the laser source. The optical sensor comprises a bolometer and a reference sensor. The bolometer is situated at a location of the slider that receives at least some of the light and exposed to an ambient temperature at the slider. The bolometer is configured to produce a signal in response to a change in the ambient temperature and a change in the output optical power. The reference sensor is situated at a location of the slider unexposed to the light and exposed to the ambient temperature. The reference sensor is coupled to the bolometer and configured to produce a signal in response to the change in the ambient temperature.

Embodiments are directed to an apparatus comprising a slider having a plurality of electrical bond pads including a first bond pad, a second bond pad, and a return bond pad. A writer and a reader are provided on the slider. An optical waveguide is formed in the slider and configured to receive light from a laser source. An NFT is provided on the slider and optically coupled to the waveguide. An optical sensor is coupled to the first and second bond pads and configured to generate a sensor signal indicative of changes in output optical power of the laser source. The optical sensor comprises a bolometer and a reference sensor. The bolometer is situated at a location of the slider that receives at least some of the light and exposed to an ambient temperature at the slider. The bolometer is configured to produce a signal in response to a change in the ambient temperature and a change in the output optical power. The reference sensor is situated at a location of the slider unexposed to the light and exposed to the ambient temperature. The reference sensor is coupled in series or parallel with the bolometer and configured to produce a signal in response to the change in the ambient temperature. A conductor is coupled to the return bond pad and a connection between the bolometer and the reference sensor.

Embodiments are directed to a method comprising transmitting light from a laser source through an optical waveguide of a slider. The method also comprises producing a first signal in response to sensing changes in an ambient temperature at the slider and changes in optical output power of the laser source using a bolometer exposed to the light. The method further comprises producing a second signal in response to sensing changes in the ambient temperature using a reference sensor at a slider location proximate the bolometer but unexposed to the light. The method also comprises combining the first and second signals to produce an output signal, the output signal indicative of changes in optical output power of the laser source without contribution due the ambient temperature changes.

DETAILED DESCRIPTION

The present disclosure generally relates to laser power monitoring in data storage devices that employ heat-assisted magnetic recording (HAMR), also referred to as energy-assisted magnetic recording (EAMR), thermally-assisted magnetic recording (TAMR), and thermally-assisted recording (TAR). This technology uses a laser source and a near-field transducer to heat a small spot on a magnetic disk during recording. The heat lowers magnetic coercivity at the spot, allowing a write transducer to change the orientation of a magnetic domain at the spot. Due to the relatively high coercivity of the medium after cooling, the data is less susceptible to paramagnetic effects that can lead to data errors.

A HAMR drive generally uses a laser diode to heat the recording medium to aid in the recording process. The laser diode generates heat and is also heated by other components (writer, reader, heater elements) in the magnetic slider. During write operation, for example, laser diode heating can vary the junction temperature of the laser diode, causing a shift in laser emission wavelength, leading to a change of optical feedback from optical path in slider to the cavity of the laser diode, a phenomenon that is known to lead to mode hopping and/or power instability of the laser diode. Mode hopping is particularly problematic in the context of single-frequency lasers. Under some external influences, a single-frequency laser may operate on one resonator mode (e.g., produce energy with a first wavelength) for some time, but then suddenly switch to another mode (produce energy, often with different magnitude, with a second wavelength) performing “mode hopping.” Temperature variation is known to cause mode hopping in laser diodes. Mode hopping is problematic for HAMR applications, as mode hopping leads to laser output power jumping and magnetic transition shifting from one block of data to another. Large transition shifts in a block of data may not be recoverable by channel decoding, resulting in error bits.

Monitoring of laser power is important to ensure proper operation of the laser diode and to avoid instabilities such as mode hopping. Conventional laser power monitoring involves use of an external photodiode situated on a submount that also supports the laser diode. The photodiode senses optical power output of the laser diode, and can be used to determine if the laser performance is sufficiently stable to ensure adequate writing performance. However, future integrated HAMR recording transducers will not be able to include an external photodiode due to reduced submount dimensions. For HAMR hard drives, it is critical to detect small fluctuations in laser output optical power delivered to the NFT, which requires highly accurate optical or temperature sensing. Typically, an electrical element such as an internal laser power monitor (e.g., photodiode) would require extra, dedicated electrical bond pads to provide highly accurate optical or temperature sensing. Since additional bond pads add cost and complexity to the head gimbal assembly, it is desirable to provide for sensing of laser output optical power without the need for such extra electrical bond pads.

Embodiments of the disclosure are directed to a sensor arrangement internal to the slider that can be used to facilitate monitoring of output optical power of the laser diode. Embodiments of the disclosure are directed to a sensor that monitors output optical power of the laser diode with an improved signal-to-noise ratio by compensating for thermal background conditions that can otherwise confound temperature measurements made by the sensor. Embodiments are directed to a bolometric sensor that includes at least two electrical components that share electrical bond pads of the slider, thereby obviating the need for additional bond pads to operate the sensor. Embodiments are directed to a bolometric sensor that includes at least two electrical components that share electrical bond pads with at least one other electrical component of the slider, such as a reader or contact sensor, thereby obviating the need for additional bond pads to operate the bolometric sensor.

According to various embodiments, a bolometer can be situated adjacent the core of an optical waveguide formed in the slider so that light produced by the laser diode impinges on or is harvested by the bolometer. A reference sensor, such as a resistor, can be situated proximate the bolometric sensor but away from the light path of slider, such that the bolometer and reference sensor are on the same isotherm. In other words, the reference sensor is situated close to the bolometer so that both components are exposed to substantially the same thermal background (e.g., same ambient temperature), but outside of the light path so that no light impinges on the reference sensor. Circuitry that includes the bolometer and the reference sensor is configured to effectively subtract off signal content of the bolometer representative of the thermal background experienced by the bolometer and the reference sensor. Having subtracted off the non-optical thermal component of the bolometer signal, the remaining component of the bolometer signal represents the temperature and temperature variations due predominately (e.g., solely) to output optical power and power fluctuations of the laser diode.

The bolometer can be situated in or near the optical light path of the slider so that it absorbs or harvests light communicated along the waveguide, while minimally or negligibly impacting light transmission (e.g., minimal or negligible effect on waveguide efficiency and/or the mode profile along the light delivery path). For example, the bolometer can be situated within the internal body of the slider proximate the core of the waveguide, such as within the cladding of the waveguide. In various embodiments, the bolometer comprises a thin metallic element, such as a wire, with a high thermal coefficient of resistance (TCR). The reference sensor can be a thin metallic element, such as a wire, with a high TCR or other type of resistance sensing apparatus. In some embodiments, the bolometer and reference sensor are substantially the same or similar in terms of composition and geometry. For example, the bolometer and reference sensor can have substantially the same geometry (e.g., size and shape) and substantially the same TCR.

In some embodiments, the TCRs of the bolometer and reference sensor have the same sign (e.g., both positive or both negative). In other embodiments, the TCRs of the bolometer and reference sensor have different signs (e.g., one positive, the other negative). According to some embodiments, the circuitry that includes the bolometer and reference sensor is coupled only to two electrical bond pads each serving as a bias source. In other embodiments, the circuitry that includes the bolometer and reference sensor is coupled only to two electrical bond pads each serving as a bias source and one return bond pad (e.g., a bond pad tied to ground, and not a bias source).

When a small bias current is applied across the bolometer, any change in bolometer temperature will create a corresponding change in measured voltage. This change in bolometer temperature results from changes in output optical power of the laser diode and from changes in ambient temperature. At the same time, any changes in ambient temperature are also sensed by the reference sensor. The common ambient temperature component of the bolometer signal is subtracted off, leaving only the temperature and temperature changes due to output optical power of the laser diode. As a result, the bolometer can be used to monitor fluctuations in laser output optical power that cause fluctuations in absorption and temperature in the bolometric sensor with high fidelity. In general, a bolometer-based internal power monitor according to embodiments of the disclosure does not appreciably decrease light path efficiency yet still absorbs enough light to create a sufficiently large signal for detection. Moreover, embodiments of a bolometric sensor arrangement do not require any additional bond pads for temperature compensation, and can also be wired in series or parallel with an existing electrical component of the slider.

FIG. 1shows a side view of a slider102configured for heat-assisted magnetic recording in accordance with a representative embodiment. The slider102may be used in a magnetic data storage device, e.g., a hard disk drive. The slider102may also be referred to herein as a recording head, read/write transducer, etc. The slider102is coupled to an arm104by way of a suspension106that allows some relative motion between the slider102and arm104. The slider102includes read/write transducers108at a trailing edge that are held proximate to a surface110of a magnetic recording medium111, e.g., magnetic disk. The slider102further supports a laser120and incorporates an optical waveguide122. The waveguide122delivers light from the laser120to components (e.g., a near-field transducer) near the read/write transducers108.

When the slider102is located over surface110of recording medium111, a flying height112is maintained between the slider102and the surface110by a downward force of arm104. This downward force is counterbalanced by an air cushion that exists between the surface110and an air bearing surface103(also referred to herein as a “media-facing surface”) of the slider102when the recording medium111is rotating. It is desirable to maintain a predetermined slider flying height112over a range of disk rotational speeds during both reading and writing operations to ensure consistent performance. Region114is a “close point” of the slider102, which is generally understood to be the closest spacing between the read/write transducers108and the magnetic recording medium111, and generally defines the head-to-medium spacing113.

To account for both static and dynamic variations that may affect slider flying height112, the slider102may be configured such that a region114of the slider102can be configurably adjusted during operation in order to finely adjust the head-to-medium spacing113. This is shown inFIG. 1by a dotted line that represents a change in geometry of the region114. In this example, the geometry change may be induced, in whole or in part, by an increase or decrease in temperature of the region114via a heater116. A thermal sensor115is shown situated at or near the close point114(e.g., adjacent the read/write transducers108, such as near the near-field transducer) or can be positioned at other locations of the ABS103.

FIG. 2shows a HAMR head arrangement200in accordance with various embodiments. The HAMR head arrangement200includes a slider202positioned proximate a rotating magnetic medium211. The slider202includes a reader204and a writer206proximate the ABS215for respectively reading and writing data from/to the magnetic medium211. The writer206is located adjacent a near-field transducer (NFT)210which is optically coupled to a light source220(e.g., laser diode) via a waveguide222. The light source220can be mounted external, or integral, to the slider202. The light source220energizes the NFT210via the waveguide222. The writer206includes a corresponding heater207, and the reader204includes a corresponding heater205according to various embodiments. The writer heater207can be powered to cause protrusion of the ABS215predominately in the ABS region at or proximate the writer206, and the reader heater205can be powered to cause protrusion of the ABS215predominately in the ABS region at or proximate the reader204. Power can be controllably delivered independently to the heaters207and205to adjust the fly height (e.g., clearance) of the slider202relative to the surface of the recording medium211. One or more thermal sensors212a,212bcan be situated at various locations on the slider202at or near the ABS215for purposes of monitoring temperature, head-medium spacing changes, and head-medium contact.

A HAMR device utilizes the types of optical devices described above to heat a magnetic recording media (e.g., hard disk) in order to overcome superparamagnetic effects that limit the areal data density of typical magnetic media. When writing with a HAMR device, the electromagnetic energy (e.g., laser or light) is concentrated onto a small hot spot213over the track of the magnetic medium211where writing takes place, as shown inFIG. 2. The light from the source220propagates to the NFT210, e.g., either directly from the source220or through the mode converter or by way of a focusing element. Other optical elements, such as couplers, mirrors, prisms, etc., may also be formed integral to the slider. As a result of what is known as the diffraction limit, optical components cannot be used to focus light to a dimension that is less than about half the wavelength of the light. The lasers used in some HAMR designs produce light with wavelengths on the order of 700-1550 nm, yet the desired hot spot213is on the order of 50 nm or less. Thus, the desired hot spot size is well below half the wavelength of the light. Optical focusers cannot be used to obtain the desired hot spot size, being diffraction limited at this scale. As a result, the NFT210is employed to create a hot spot on the media.

The NFT210is a near-field optics device configured to generate local surface plasmon resonance at a designated (e.g., design) wavelength. The NFT210is generally formed from a thin film of plasmonic material (e.g., gold, silver, copper) on a substrate. In a HAMR slider202, the NFT210is positioned proximate the write pole of the writer206. The NFT210is aligned with the plane of the ABS215parallel to the read/write surface of the magnetic medium211. The NFT210achieves surface plasmon resonance in response to the incident electromagnetic energy. The plasmons generated by this resonance are emitted from the NFT210towards the magnetic medium211where they are absorbed to create the hot spot213. At resonance, a high electric field surrounds the NFT210due to the collective oscillations of electrons at the metal surface (e.g., substrate) of the magnetic medium211. At least a portion of the electric field surrounding the NFT210tunnels into, and gets absorbed by, the magnetic medium211, thereby raising the temperature of the spot213on the medium211as data is being recorded.

Turning now toFIG. 3A, there is illustrated an optical sensor configured for implementation on a slider in accordance with various embodiments. The slider302shown inFIG. 3Asupports a multiplicity of electrical components coupled to a set305of bond pads (P1-P9). The set305of bond pads includes eight electrical bond pads (P2-P9) and one ground pad (P9), also referred to as a shared return path. The term “electrical bond pad” refers to a bond pad that is coupled to a detector or a bias source, such as a voltage or current source (AC or DC) that provides power for an electrical component or components. It is noted that the polarity of the electrical bond pads can change during operation, such that a given pad can be at a positive potential at one time and a negative potential at another time. The slider302shown inFIG. 3Autilizes eight electrical bond pads (P2-P9), it being understood that the number of electrical bond pads can vary depending on the particular slider design. For simplicity of explanation, only three electrical bond pads (P7-P9) and one ground pad (P1) are illustrated inFIG. 3Aand other figures.

FIG. 3Ashows an optical sensor308provided on a slider configured for heat-assisted magnetic recording in accordance with various embodiments. The optical sensor308includes a bolometer310and a reference sensor312. The bolometer310and the reference sensor312are coupled in series. One end of the optical sensor308is coupled to a first bond pad, P7, and the other end of the optical sensor308is coupled to a second bond pad, P8. In the embodiment shown inFIG. 3A, the optical sensor308is coupled only to first and second bond pads P7and P8, and to no other electrical bond pad or ground pad. The optical sensor308shown inFIG. 3Acan be considered a dual-ended component which requires two electrical bond pads for proper operation. In contrast to a dual-ended component, a single-ended component is one which is coupled between a single electrical bond pad and a ground pad.

The bolometer310is disposed on the slider302at a location that receives at least some of the light produced by a laser source that propagates through an optical waveguide (see waveguide222inFIG. 2) of the slider302. The propagating light impinges on an NFT210which is shown situated proximate to a writer206. The slider302also includes a reader, which is not shown inFIG. 3Afor simplicity of explanation. The bolometer310can be situated within cladding of the optical waveguide, for example, and preferably has negligible impact on optical efficiency (e.g., reduces optical efficiency by less than 5%). For example, the bolometer310can be situated within about 5 μm from a centerline of the core of the optical waveguide. The response of the bolometer310is based primarily on the light impinging on it from the optical waveguide and the ambient temperature at the location where the bolometer310is situated on the slider302. The reference sensor312is situated at a location of the slider302unexposed to the light but exposed to the ambient temperature. The reference sensor312is situated in proximity to the bolometer310, such that the ambient temperature of the reference sensor312is substantially the same as the ambient temperature of the bolometer310(e.g., to within 1-2%). For example, the reference sensor312can be situated to within about 10 to 15 μm from the bolometer310. In this regard, the bolometer310and the reference sensor312are situated on the same isotherm314.

The bolometer310and the reference sensor312are configured or otherwise operated so that a response of these two components to a change in ambient temperature is effectively canceled. For example, the bolometer310can be configured to have a positive change in signal output in response to a change in ambient temperature, and the reference sensor312can be configured to have a negative change in signal output in response to the change in ambient temperature. It is understood that either of the bolometer310and the reference sensor312can be configured to have a positive or negative change in signal output in response to a change in ambient temperature, as long as the two components produce output signals that can be combined in a manner that allows cancellation of the effect of temperature variation.

The bolometer310and the reference sensor312are preferably constructed so that they produce output signals of substantially the same magnitude. Because the bolometer310and the reference sensor312are subject to the same changes in ambient temperature, the response of the reference sensor312to a change in ambient temperature can be effectively subtracted from the response of the bolometer310to the same change in ambient temperature. Having canceled the common change to ambient temperature in the bolometer310and the reference sensor312, the resulting output signal produced by the optical sensor308is representative predominantly of a change in output optical power of the laser source as sensed by the bolometer310.

For purposes of explanation, the bolometer310and the reference sensor312can be modeled (and implemented) as resistors each having a temperature coefficient of resistance. For example, the bolometer310and the reference sensor312can each be a metal wire having a TCR fabricated into the slider302. Suitable materials for fabricating the bolometer310and the reference sensor312include Cr, NiFe, Ni, and other materials that have high TCR. In some implementations, it may be desirable to fabricate the electrical conductors (e.g., traces or leads) that connect the bolometer310to the reference sensor312and the optical sensor308to the bond pads P7and P8from a material having a relatively low, or near zero, TCR. Suitable materials include NiCu alloys. Use of low or near zero TCR materials for the electrical conductors insures that nearly all temperature-related changes in the optical sensor308are due to temperature changes experienced by the bolometer310and the reference sensor312, rather than by the electrical conductors coupled thereto.

A DC bias can be supplied to the optical sensor308to establish a potential difference between electrical bond pads P7and P8. A constant current can be supplied to the optical sensor308via bond pads P7and P8. In the following illustrative example, it is assumed that the bolometer310(RB) and the reference sensor312(RA) have the same TCR (a) but of opposite sign. For example, RBhas one of a positive and negative TCR, and RAhas the other of the positive and negative TCR. For this representative example, the resistance versus temperature relationship can be modeled using the following equation:
R(T1)=R(T0)[1+α(T1−T0)]  (1)
where T1and T0are temperatures, R(T) is the resistance at a given temperature T, and α is the temperature coefficient of resistance. It is noted that this equation is an approximation and is most accurate when α(T1−T0)<<1.

In the following illustrative example, a special case is considered in which the TCR of the two resistors, RAand RB, in the circuit shown inFIG. 3Aare identical in magnitude but of opposite sign, i.e. αA=(−)αB, and the initial resistance, R, of the two resistors is identical, i.e. RB(T0)=RA(T0). Inserting these assumptions into Equation (1) above results in the following expressions for resistance:
RA(T1)=RA(T0)[1+αA(T1−T0)]
RB(T1)=RB(T0)[1−αA(T1−T0)]  (2)&(3)

It can be seen that in the series combination, the total resistance does not change with temperature, RA(T1)+RB(T1)=RA(T0)+RB(T0). In the context of the circuit shown inFIG. 3A, the signals produced by the bolometer310and the reference sensor312in response to a common change in ambient temperature have substantially the same magnitude but are opposite in polarity, and thus cancel out. Any changes in resistance of the bolometer310relative to the reference sensor312are due to changes in output optical power of the laser source, and not due to changes in ambient temperature.

It is understood that implementing the bolometer310and the reference sensor312using materials having substantially the same values of TCR but of opposite sign simplifies the signal processing and circuitry. Moreover, implementing the bolometer310and the referent sensor312to have substantially the same size and shape (i.e., the same geometry), in addition to the same values of TCR (but opposite in sign), serves to produce components that generate output signals of equal magnitude in response to the same changes in temperature.

In some embodiments, some degree of imbalance between the bolometer310and the reference sensor312can exist, as long as the resistance and TCR magnitudes of the bolometer310and the reference sensor312are similar. The following illustrative example considers the effect of imbalance in one or both of initial resistance and TCR magnitude between the series-connected bolometer310and the reference sensor312. In this scenario, the following equations apply:
αB=(−)βαA(4)
RB(T0)=δA(T0)  (5)
Equations (4) and (5) are correct in general, but it is assumed in this illustrative example that both β and δ are nearly equal to unity. Inserting these assumptions for Equations (4) and (5) into Equation (1) above results in the following expressions for resistance:
RA(T1)=RA(T0)[1+αA(T1−T0)]
RB(T1)=δRA(T0)[1−βαA(T1−T0)]  (6)&(7)
RA(T1)+RB(T1)=RA(T0)[1+δ]+RA(T0)αA(T1−T0)[1−β]  (8)

For the case of perfect matching of the reference and bolometer resistors (i.e. β=δ=1), the previous results of complete insensitivity to temperature are obtained. On the other hand, when β and/or δ differ from unity, errors are incurred. However, as long as β and δ are both somewhat close to unity (e.g., between 0.8 and 1.2), much of the thermal background variation is canceled even if not perfectly so.

According to some embodiments, the optical sensor308is situated on the slider302away from ABS215. More particularly, the optical sensor308is situated at a distance away from the heat-producing components at the ABS215, including the NFT210and the writer206. Situating the optical sensor308away from the heat-producing components at the ABS215reduces the number of heat sources that can impact the thermal environment surrounding the bolometer310and the reference sensor312. However, it may be necessary to locate the optical sensor308at less than preferred locations of the slider302due to design layout constraints. As long as the bolometer310and the reference sensor312are located on the same isotherm, the optical sensor308may be implemented at any location of the slider302.

FIG. 3Billustrates an optical sensor for incorporation in a slider configured for heat-assisted magnetic recording accordance with various embodiments. The optical sensor308shown inFIG. 3Bincludes a bolometer310and a reference sensor312as described in the embodiment ofFIG. 3A. InFIG. 3B, the bolometer310and the reference sensor312are coupled in parallel between bond pads P7and P8. Although the circuit configuration shown inFIG. 3Bis somewhat more complex than that shown inFIG. 3A, the parallel-connected bolometer310and reference sensor312operate to produce a signal indicative of output optical power of a laser source with high fidelity (e.g., a signal not confounded by extraneous heat sources).

In the following simplified example, it is assumed that the bolometer310(RB) and the reference sensor312(RA) have the same TCR, a, but are opposite in sign, i.e. αA=(−)αBand the initial resistance, R, of the two resistors is identical, i.e. RB(T0)=RA(T0). For the parallel combination shown inFIG. 3B, similar temperature insensitivity is found as demonstrated in the following equations:

FIG. 4illustrates an optical sensor for incorporation in a slider configured for heat-assisted magnetic recording in accordance with various embodiments. The optical sensor308shown inFIG. 4includes a bolometer310coupled in series to a reference sensor312. The bolometer310and the reference sensor312are situated on the slider302at locations that share substantially the same ambient temperature (e.g., located on the same isotherm). One end of the optical sensor308is coupled to a first bond pad, P7, while the other end of the optical sensor308is coupled to a second bond pad, P8. In the embodiment shown inFIG. 4, a conductor309is coupled to the return bond pad, P1, and to a connection between the bolometer310and the reference sensor312. Although the optical sensor308shown inFIG. 4requires only two electrical bond pads (e.g., P7and P8), the optical sensor308ofFIG. 4is also tied to a return or ground bond pad (e.g., P1).

According to various embodiments, the bolometer310and the reference signal312are constructed from the same material and have substantially the same geometry (i.e., size and shape). In the embodiment shown inFIG. 4, the bolometer310and the reference sensor312are constructed from a material having the same TCR sign (i.e., both positive or both negative). Constant current is applied to each of the bolometer310and the reference sensor312via bond pads P8and P7, respectively. In the case of the bolometer310and reference sensor312being constructed of the same material and having the same geometry, it can be seen that the bias currents IA(through the bolometer310) and IB(through the reference sensor312) are equal. It is readily apparent that common changes in temperature of the bolometer310and the reference sensor312produce a negligible signal, Vsig, between the bond pads P7and P8. As such, the signal, Vsig, detected at the bond pads P7and P8is predominately (e.g., solely) representative of changes in temperature experienced by the bolometer310due to changes in output optical power of the laser source, and not due to changes in ambient temperature.

It is noted that a differential amplifier can be coupled to bond pads P7and P8for purposes of detecting signals produced by the optical sensor308. A logic device or processor, for example, can be coupled to the differential amplifier and configured to determine the magnitude of the detected changes in output optical power of the laser source. The logic device or processor can further be configured to adjust current supplied to the laser source in response to the detected changes in output optical power sensed by the optical sensor308(e.g., to avoid mode hopping).

FIGS. 5-8illustrate various circuits involving bond pad sharing between an optical sensor308and one or more other electrical components of the slider302in accordance with various embodiments. As was discussed previously, the additional electrical and optical components of a HAMR slider place a greater demand on the available electrical bond pads of the slider. Although appearing to be a simple solution, adding additional bond pads to the slider is both costly and complex, requiring significant modifications to the wafer design and fabrication processes.FIG. 5illustrates an optical sensor308of a type previously described coupled to a pair of bond pads, P7and P8, in parallel with a reader514in accordance with some embodiments.

During a read operation, the reader514is active and the optical sensor308is generally inactive. Because the laser diode is generally inactive during a read operation, the reference sensor312and the bolometer310are relatively quiescent. A readback signal produced by the reader514during read operations can be sensed across bond pads P7and P8with little to no signal contribution from the optical sensor308. During a write operation, the reader514is inactive and the optical sensor308is active. The signals produced by the optical sensor308during write operations can be sensed across bond pads P7and P8with little to no signal contribution from the reader514, which remains relatively quiescent during write operations.

FIG. 6illustrates an optical sensor308of a type previously described coupled to a pair of bond pads, P7and P8, in parallel with a contact sensor614in accordance with various embodiments. In some embodiments, the contact sensor614is implemented as a wire having a high TCR. The contact sensor614may alternatively be implemented using a thermocouple, for example. The contact sensor614can be used for setting clearance, determining fly height, and detecting asperities, voids, and other topographical features of a magnetic recording medium. The contact sensor614can be implemented at a close point of the slider at or near the ABS, such as proximate the writer or the reader, for example.

According to some embodiments, during a contact detection operation (e.g., clearance testing), DC bias power is supplied to contact sensor614via bond pads P7and P8and to a heater (via another bond pad) in proximity to the contact sensor614to raise the surface temperature of the contact sensor614to be substantially higher than the temperature of the magnetic recording medium. As such, the recording medium acts as a heat sink in this scenario. When the slider contacts the medium, the slider surface temperature will drop due to a change in heat transfer rate resulting from the contact. The slider surface temperature will then increase due to heating from the heater and frictional heating. The abrupt drop in temperature or excursion in temperature trajectory of the contact sensor signal can be used to detect head-to-medium contact.

Operations involving the contact sensor614can be implemented at times when the optical sensor308is not active (e.g., at times other than during write operations). During active use of the contact sensor614, the optical sensor308remains quiescent, contributing little or no signal to the output signal sensed at the bond pads P7and P8. During use of the optical sensor308(e.g., during a write operation), the contact sensor614is inactive and remains quiescent. The signals produced by the optical sensor308during inactivity of the contact sensor614can be sensed across bond pads P7and P8with little to no signal contribution from the contact sensor614.

FIG. 7shows the embodiment ofFIG. 4configured for bond pad sharing between an optical sensor308and a reader714. The description of bond pad sharing provided hereinabove with reference toFIG. 5is applicable to the embodiment shown inFIG. 7.FIG. 8shows the embodiment ofFIG. 6configured for bond pad sharing between an optical sensor308and a contact sensor814. The description of bond pad sharing provided hereinabove with reference toFIG. 6is applicable to the embodiment shown inFIG. 8.

FIG. 9is a wafer view of an optical sensor308implemented in a heat-assisted magnetic recording slider302in accordance with various embodiments. The wafer view ofFIG. 9shows different levels of the slider302using transparent elements (a first wafer level) and solid elements (a second wafer level). A set305of the bond pads and an optical waveguide222are shown on the first wafer level. At the ABS215, an NFT210and a writer206are situated (occupying several wafer layers). A bolometer310and a reference sensor312of the optical sensor308are shown on the second wafer level, as is a conductor L2which connects the bolometer310and the reference sensor312in series. The bolometer310is coupled to a terminal305b(Vb) which, together with terminal305a(Va), biases the optical sensor308via conductor L1. The terminals305aand305bare contact points for two of the electrical bond pads shown on the first wafer level but expressed on the second wafer level.

The reference sensor312is coupled to terminal305avia a conductor L3. A conductor L4is coupled between a ground terminal305cand a connection point between the bolometer310and the reference sensor312. The wafer view of the slider302shown inFIG. 9is representative of the optical sensor implementation shown inFIG. 4. It is understood that a similar wafer level schematic can be developed for the optical sensor implementation shown inFIG. 3.

FIG. 10is a flow chart showing various processes for sensing changes in optical output power of the laser source in accordance with various embodiments. The method shown inFIG. 10involves transmitting1002light from a laser source through an optical waveguide of a slider. The method also involves producing1004a first signal in response to sensing changes in an ambient temperature at the slider and changes in optical output power of the laser source using a bolometer exposed to the light. The method further involves producing1006a second signal in response to sensing changes in the ambient temperature using a reference sensor at a slider location proximate the bolometer but unexposed to the light. The method also involves combining1008the first and second signals to produce an output signal. The output signal is indicative of changes in optical output power of the laser source without contribution due the ambient temperature changes.

Systems, devices or methods disclosed herein may include one or more of the features structures, methods, or combination thereof described herein. For example, a device or method may be implemented to include one or more of the features and/or processes above. It is intended that such device or method need not include all of the features and/or processes described herein, but may be implemented to include selected features and/or processes that provide useful structures and/or functionality. Various modifications and additions can be made to the disclosed embodiments discussed above. Accordingly, the scope of the present disclosure should not be limited by the particular embodiments described above, but should be defined only by the claims set forth below and equivalents thereof.