Table 1.

The comparison table between the proposed LDPA techniques with the existing dual-pulse nonlinear PA technique.

	Dual-pulse nonlinear PA	Proposed LDPA by CW laser excitation	Proposed LDPA by Quasi-CW laser excitation
Physical mechanism
Nonlinear origin	Impulse temperature rise then heat relaxation during laser interval	Continuous temperature rise and heat accumulation	Quasi-continuous temperature rise and heat accumulation
Physical conditions	Thermal and stress confinements satisfied for two PA generations	First PA signal: satisfied Second PA signal: unsatisfied	Thermal and stress confinements satisfied for all PA signals
PA generation mechanism	Two laser pulse induce two PA signals	One laser pulse induce two PA signals	N consecutive laser pulses induce N PA signals
System performance
Heating efficiency	×Low (single short laser pulse heating)	√ High (continuous laser heating)	√ High (Quasi-continuous laser heating)
PA signal strength	√ High (high peak power pulsed laser)	×Low (low-power CW laser)	√ High (high peak power pulsed laser)
System cost	×Very high (two high-power laser systems required)	√ Very low (single CW laser diode)	√ Low (single high-rep-rate pulsed laser diode)
Applications	×Need tight focusing to improve the heating efficiency and nonlinearity (e.g. time-reversal optical focusing in scattering medium)	×Need strong absorber and focusing to improve the PA SNR (e.g. contrast-enhanced PA microscopy)	√ Generally applicable to most PA embodiments (e.g. PA tomography, microscopy, endoscopy, etc.)