Lasers for Photoacoustic Microscopy

Photoacoustic signal generation depends on stress and thermal confinement: the optical pulse must deposit its energy faster than the heated region can expand or diffuse. For the micrometre-scale absorbers imaged in PAM, that means the pulse should be in the nanosecond or sub-ns range. Picosecond pulses satisfy stress confinement comfortably, producing efficient, high-amplitude photoacoustic signals.

Wavelength sets the contrast. At 532 nm the excitation sits close to the peak optical absorption of haemoglobin, making it the workhorse wavelength for label-free imaging of blood vessels and microvasculature. Moving to longer wavelengths trades some haemoglobin contrast for depth: 767 nm already penetrates further into tissue while remaining absorbed by whole blood, and 1064 nm penetrates deeper still and adds sensitivity to lipids and other absorbers, extending PAM toward greater depth. Having several wavelengths available — ideally from one platform — is what lets a system match its contrast and depth to the target, and what turns a structural imager into a functional one.

In point-scanning optical-resolution PAM (OR-PAM), one laser pulse typically generates one image pixel (one A-line). Frame rate is therefore set directly by pulse repetition rate: at a few kHz, imaging a wide field of view is slow; at 100–200 kHz, the same field is covered one to two orders of magnitude faster. For live-sample, functional or in-vivo imaging — where motion and physiology don't wait — this is often the single most limiting laser specification.

Picophotonics microchip lasers are tunable from single pulse to 200 kHz, so the same platform supports both careful high-SNR imaging and fast wide-field scanning.

Oxygenated and deoxygenated haemoglobin absorb light differently. By exciting the same region at two wavelengths and comparing the photoacoustic amplitudes, PAM can map blood oxygen saturation (sO₂) — a functional readout used in vascular, tumour and neuro imaging research. This requires two excitation wavelengths with a stable, known timing relationship between them.

Our CP2L delivers synchronized 532 nm and 1064 nm output from a single microchip source, and the CP32/CP64 platforms cover the individual wavelengths where a single band is enough. Because the two channels share one timing reference, the wavelength-to-wavelength jitter that would otherwise corrupt an sO₂ measurement is designed out.

PAM places several demands on the excitation source at once:

Our picosecond microchip lasers are built around proprietary SESAM-based passive Q-switching, generating stable single-frequency picosecond pulses from a compact, monolithic architecture — without the cost or complexity of a mode-locked system. Three platforms cover most photoacoustic setups:

Picophotonics microchip lasers have been used as the excitation source in peer-reviewed photoacoustic imaging research, including enhanced-resolution optoacoustic (photoacoustic) microscopy work at the University of Kent.