Optogenetics lets you control genetically defined neurons with light — switching them on or off on millisecond timescales while an animal behaves, a slice is recorded, or a circuit is mapped. But the technique only works if the light source is right. Pick the wrong wavelength and your opsin barely responds. Pick the wrong power and you either fail to drive the neurons or cook the tissue. Pick a laser that can't switch fast enough and your temporal precision evaporates.
This guide walks through how to choose a laser for optogenetics: matching wavelength to your opsin, setting the right power, getting the modulation you need, and delivering the light to the tissue.
Start With Your Opsin: Wavelength Is Everything
Every opsin has an action spectrum — a curve describing how strongly it responds to each wavelength of light. The peak of that curve is where activation is most efficient. Position your laser at or near that peak and you get maximal photocurrent for the least light. Drift away from it and you need far more power to achieve the same effect, which wastes photons and heats tissue.
Opsins fall into two broad functional classes. Excitatory opsins (like channelrhodopsin-2) open cation channels and depolarise neurons to drive spiking. Inhibitory opsins (like halorhodopsin and archaerhodopsin) hyperpolarise neurons to silence them. Critically, the two classes often respond to different wavelengths — which is what makes bidirectional control with two lasers possible in a single experiment.
Wavelength-to-Opsin Reference
Here are the most widely used opsins and the laser lines that match them:
| Opsin | Function | Peak | Recommended Laser |
|---|---|---|---|
| Channelrhodopsin-2 (ChR2) | Excitatory | ~470 nm | 473 nm DPSS or 488 nm diode |
| iC++ (chloride channel) | Inhibitory | ~470 nm | 473 nm DPSS or 488 nm diode |
| ChRmine | Excitatory | ~530 nm | 520 nm diode or 532 nm DPSS |
| C1V1 | Excitatory | ~540 nm | 532 nm DPSS |
| Archaerhodopsin (Arch) | Inhibitory | ~560 nm | 561 nm DPSS |
| Halorhodopsin (NpHR) | Inhibitory | ~580 nm | 561 nm DPSS |
| ReaChR | Excitatory | ~590–630 nm | 589 nm DPSS or 633 nm diode |
| Jaws | Inhibitory | ~630 nm | 633 nm or 640 nm diode |
| Red-shifted ChRmine | Excitatory | ~600–650 nm | 633 nm or 640 nm diode |
A common bidirectional setup pairs a 473 nm laser for ChR2 excitation with a 561 nm laser for NpHR or Arch inhibition — driving and silencing the same population on demand. For a fuller list of opsins and delivery configurations, see our lasers for optogenetics page.
How Much Power Do You Need?
In optogenetics, what matters is not the laser's total output but the irradiance — the optical power per unit area — that actually reaches the opsin-expressing tissue. This is usually expressed in mW/mm². Most opsins require somewhere in the range of 1–20 mW/mm² at the tissue to reliably drive or silence neurons, depending on the opsin's sensitivity and the expression level.
The catch is that light is lost at every step between the laser and the neuron. Coupling into the fiber, transmission through the patch cable and rotary joint, the fiber tip itself, and scattering and absorption in tissue all reduce the delivered power. As a rule of thumb, only a fraction of the laser's aperture power reaches the target — so you specify the laser with that budget in mind.
Three practical points:
- Specify power at the fiber tip, not the aperture. What reaches the animal is what counts. A laser rated at the cannula output removes the guesswork.
- Account for tissue penetration. Blue light (473 nm) scatters and is absorbed quickly, illuminating only tissue near the fiber tip. Red and far-red light (633 nm+) penetrates deeper — one reason red-shifted opsins are valuable for reaching larger or deeper volumes.
- Don't over-power. Excess light causes heating, which itself alters neural activity and confounds your results. Use the minimum irradiance that gives reliable activation.
Temporal Precision: Modulation Matters
One of the defining strengths of optogenetics is temporal control — the ability to deliver single millisecond pulses, sustained trains at 20–40 Hz or higher, or arbitrary patterns synchronised to behaviour or electrophysiology. Your laser has to keep up.
This is where diode lasers have a decisive advantage. Because their output is controlled directly by drive current, diode lasers can be switched on and off in microseconds via a simple TTL or analog input — no external shutter or acousto-optic modulator required. For the blue and red opsins served by diode wavelengths (488 nm, 633 nm, 640 nm), this makes pulsing trivial and tightly synchronisable with acquisition systems.
DPSS lasers (473 nm, 532 nm, 561 nm) are slower to modulate directly because of the thermal dynamics of the pump-and-convert process. For continuous illumination or slow gating this is a non-issue, but for fast pulse trains a DPSS line typically needs an external modulator. If your protocol depends on crisp, high-frequency pulsing at a DPSS wavelength, factor that into your system design. (For a full comparison of the two technologies, see our guide on diode laser vs. DPSS laser.)
Delivering the Light to the Tissue
How you get light from the laser to the neurons shapes your laser choice as much as wavelength and power do. The common delivery methods:
Fiber-coupled, freely moving. The workhorse of in vivo behaviour. The laser couples into an optical fiber, which runs through a patch cable and rotary joint to a cannula implanted over the target region. This demands stable output even as the fiber moves, and power specified at the cannula tip.
Free-space through an objective. Used in head-fixed and in vitro slice work, where light is delivered through the microscope objective onto opsin-expressing neurons during patch-clamp recording. Beam quality and clean alignment matter here.
Combined with imaging. Single-photon optogenetic activation alongside two-photon imaging requires careful wavelength separation and precise TTL synchronisation so stimulation and recording don't interfere.
A Practical Selection Checklist
- Identify your opsin and find its action-spectrum peak — this fixes the wavelength.
- Set your irradiance target at the tissue (typically 1–20 mW/mm²) and work backward through your delivery losses to the laser power you need.
- Define your temporal protocol — continuous, pulsed trains, or patterned — and confirm the laser can modulate fast enough (diode for fast pulsing).
- Choose your delivery method — fiber-coupled for freely moving work, free-space for slice and head-fixed — and specify connectors and power at the tip.
- Confirm stability and synchronisation — low noise for consistent dosing, TTL/analog inputs matched to your acquisition or behaviour control system.
How Aimpico Can Help
Aimpico manufactures both diode and DPSS lasers across all the wavelengths used in optogenetics — from 473 nm for ChR2 through 561 nm for inhibitory opsins to 633 nm and 640 nm for red-shifted variants. We offer fiber-coupled configurations with power measured at the cannula tip, fast direct modulation for tight synchronisation with electrophysiology, and the low-noise stability that reproducible dosing demands.
Tell us your opsin, target region, and protocol, and our applications team will recommend a wavelength, power, and delivery configuration matched to your experiment. Learn more about our lasers for optogenetics.
Need help selecting a laser for your optogenetics experiment?