NFT back reflection monitor for heat-assisted magnetic recording head
Disclosed are methods and configurations for monitoring reflections in heat-assisted magnetic recording (HAMR) heads that may lead to laser instability and power variation. HAMR heads direct light emitted by a laser toward a near field transducer (NFT) for the production of plasmons that assist in data recording. A certain amount of the laser light may be reflected at or near the NFT back toward the laser, and this reflected light can cause laser instabilities over time. As a screening measure during manufacturing or assembly, or as a laser health monitoring measure during drive operation, a back reflection monitor is used to detect the intensity of a portion of the reflected light. Back reflection monitoring capability may be provided by a bolometer integrated into HAMR recording head assemblies, and optionally along with optical components and electrical circuitry that provides laser light transmission monitoring capability.
1 . A heat-assisted magnetic recording head comprising:
a laser;
a near field transducer positioned at an air bearing surface of the recording head and configured to receive light emitted by the laser;
a primary waveguide coupled between the laser and the near field transducer and configured to deliver light emitted by the laser to the near field transducer;
a back reflection monitor configured to detect light reflected at or near the near field transducer back toward the laser along the primary waveguide and that is coupled into the back reflection monitor; and
a transmission monitor separate from the back reflection monitor, the transmission monitor configured to detect light transmitted by the primary waveguide from the laser toward the near field transducer and that is coupled into the transmission monitor.
2 . The heat-assisted magnetic recording head of claim 1 , wherein the back reflection monitor is a bolometer.
3 . The heat-assisted magnetic recording head of claim 1 , wherein the back reflection monitor is a photodiode.
4 . The heat-assisted magnetic recording head of claim 1 , further comprising a secondary waveguide, wherein the back reflection monitor is coupled to the secondary waveguide.
5 . The heat-assisted magnetic recording head of claim 4 , wherein the transmission monitor is coupled to the secondary waveguide.
6 . The heat-assisted magnetic recording head of claim 5 , wherein the secondary waveguide comprises a reflection branch, a transmission branch, and a coupler disposed between the reflection branch and the transmission branch, and wherein the coupler is positioned proximate to the primary waveguide to thereby allow coupling of both transmitted light and reflected light propagating along the primary waveguide such that transmitted light coupled into the secondary waveguide is propagated along the transmission branch and reflected light coupled into the secondary waveguide is propagated along the reflection branch.
7 . The heat-assisted magnetic recording head of claim 6 , wherein the back reflection monitor is coupled along the reflection branch of the secondary waveguide and the transmission monitor is coupled along the transmission branch of the secondary waveguide.
8 . The heat-assisted magnetic recording head of claim 1 , wherein the transmission monitor is disposed within a cladding layer of the primary waveguide.
9 . The heat-assisted magnetic recording head of claim 1 , further comprising a reference sensor positioned to detect background temperatures affecting the back reflection monitor.
10 . A heat-assisted magnetic recording hard drive incorporating the heat-assisted magnetic recording head of claim 1 .
11 . A method for testing a heat-assisted magnetic recording head that includes a primary waveguide configured to receive light emitted by a laser and to direct the received light toward a near field transducer, the method comprising:
emitting light from the laser into the primary waveguide such that emitted light is delivered to the near field transducer; and
monitoring an intensity of at least a portion of any light that is reflected by the near field transducer back toward the laser along the primary waveguide, wherein the intensity is used to determine a health metric of the near field transducer that is correlated to a breakpoint of a peg of the near field transducer.
12 . The method of claim 11 , further comprising determining whether the intensity exceeds a threshold amount.
13 . The method of claim 12 , wherein the threshold amount is correlated to a back reflection intensity that is determined to cause instability in the laser.
14 . The method of claim 12 , further comprising determining whether the heat-assisted magnetic recording head is suitable for use in a hard disk drive based on whether the intensity exceeds the threshold amount.
15 . The method of claim 11 , wherein monitoring the intensity is performed using a bolometer.
16 . The method of claim 15 , further comprising compensating for a background temperature near the bolometer when monitoring the intensity.