Layer for inducing varying delays in ultrasonic signals propagating in ultrasonic sensor

A method for operating an ultrasonic sensor, the method comprising:

• transmitting, by an ultrasonic transmitter of the ultrasonic sensor, an ultrasonic signal toward a platen interface of the ultrasonic sensor along an acoustic path that includes an acoustic delay gradient layer;
• receiving a reflection of the transmitted ultrasonic signal reflected from the platen interface at a first receiver region of an ultrasonic receiver of the ultrasonic sensor at a first time;
• and
• receiving the reflection of the transmitted ultrasonic signal reflected from the platen interface at a second receiver region of the ultrasonic receiver at a second time different from the first time.

A mobile device comprising:

• means for transmitting an ultrasonic signal toward a platen interface of an ultrasonic sensor along an acoustic path that includes means for introducing an acoustic delay gradient;
• means for receiving a reflection of the transmitted ultrasonic signal reflected from the platen interface at a first receiver region of an ultrasonic receiver of the ultrasonic sensor at a first time;
• and
• means for receiving the reflection of the transmitted ultrasonic signal reflected from the platen interface at a second receiver region of the ultrasonic receiver at a second time different from the first time.

A non-transitory computer readable medium storing instructions for execution by one or more processors of a mobile device to perform operations comprising:

• controlling an ultrasonic transmitter of an ultrasonic sensor to transmit an ultrasonic signal toward a platen interface of the ultrasonic sensor along an acoustic path that includes an acoustic delay gradient layer;
• enabling a first region of an ultrasonic receiver of the ultrasonic sensor to measure a reflection of the transmitted ultrasonic signal during a first window beginning at a first window begin time;
• and
• enabling a second region of the ultrasonic receiver of the ultrasonic sensor to measure the reflection of the transmitted ultrasonic signal during a second window beginning at a second window begin time different from the first window begin time.