Photoacoustic (PA) wavefront shaping (WS; PAWS) could allow focusing light deep in biological tissue. This could enable increasing the penetration depth of biomedical optical techniques including PA imaging. However, focussing at depth requires a light source of long coherence length (CL), presenting a challenge because the CLs of typical PA excitation lasers are short. To address this challenge, we developed a PAWS system based on an externally modulated external cavity laser with a long CL. The system was demonstrated by focussing light through rigid scattering media using both PAWS and optical WS. PAWS enabled focussing through diffusers with 8 × enhancements, while all-optical WS enabled focussing through various scattering media including a 5.8 mm thick tissue phantom. By enabling PAWS with increased coherence, the system could facilitate exploring the practical depth limits of PAWS, paving the way to focussing light deep in tissue
