Can I use negative voltage generator TC7660 as voltage doubler to drive LED of MAXIM30101 oximeter IC?
So can we safely use negative voltage generator as a voltage doubler supply power to LED driver?

No, they share the same reference (disregarding the effect of LED currents). Providing a negative voltage at PGND is not the reason the IC provides different power connections for the LEDs (also don't just short GND and PGND pins). You must provide a clean path for the regulated voltage used in the sensitive part of the circuit and another path for the higher current (and voltage) required by the LEDs. The two parts interface internally:

And there is another problem: AM1117-1.8 needs at least 3.3 V at its input:

If you need to use the TC7660 you can double the AAA voltage (minus two  diodes voltage drops) and connect this unregulated voltage to the LEDs if the ripple does not affect the measurement.

This higher voltage could be the input for the linear 1.8 V (low current) voltage regulator.
This is the voltage doubler configuration (not the &quot;Negative Converter&quot;):

Source: all images from the respective datasheets