Cr4+:YAG vs Co:Spinel vs V:YAG: Choosing a Passive Q-Switch Crystal

A passive Q-switch (saturable absorber) generates short, high-peak-power laser pulses with no drive electronics: it stays opaque and holds off lasing until intracavity light bleaches it, then opens abruptly to dump a giant pulse. Cr⁴⁺:YAG, Co:Spinel and V:YAG are the three most common passive Q-switch crystals, and they are chosen primarily by the laser's operating wavelength. This guide compares their absorption bands, cross-sections, recovery times and damage thresholds so you can select the right saturable absorber.

Side-by-side comparison

Property	Cr4+:YAG	Co:Spinel	V:YAG
Formula	Cr4+:Y3Al5O12	Co2+:MgAl2O4	V3+:Y3Al5O12
Operating (absorption) band	0.9–1.2 µm	1.2–1.6 µm	1.0–1.4 µm
Best pump laser	Nd lasers @ 1064 nm	Er eye-safe lasers @ 1.5 µm	Nd lasers @ 1.3 µm (1342 nm)
Ground-state absorption cross-section	~3.4–7 × 10-18 cm2 @ 1064 nm	~3.5 × 10-19 cm2 @ 1.54 µm	~7 × 10-18 cm2 @ 1340 nm
Recovery (relaxation) time	~ns (hundreds ps–ns)	~1 ns	~ns (fast)
Laser damage threshold	High	High	High
Typical use	1 µm Nd:YAG / Nd:YVO4 microchip & pulsed lasers	Eye-safe 1.5 µm Er:glass rangefinders	1.3 µm Nd lasers (eye-safer than 1 µm)

Cr⁴⁺:YAG — the 1 µm standard

Cr⁴⁺:YAG (chromium-four-doped YAG) is the default saturable absorber for neodymium lasers around 1064 nm. Its broad 0.9–1.2 µm absorption band, high damage threshold and reliable, mature fabrication make it the workhorse for passively Q-switched Nd:YAG and Nd:YVO₄ microchip lasers, range finders and lidar. Because both absorber and gain medium can be YAG-based, Cr⁴⁺:YAG is also ideal for diffusion-bonded composite microchip assemblies (e.g. Nd:YAG / Cr⁴⁺:YAG / YAG). View Cr⁴⁺:YAG crystals.

Co:Spinel — eye-safe 1.5 µm

Cobalt-doped spinel (Co²⁺:MgAl₂O₄) absorbs across 1.2–1.6 µm, making it the standard passive Q-switch for eye-safe erbium lasers at 1.5 µm (Er:glass, Er:YAG-derived fiber). Eye-safe wavelengths are required for free-space applications such as rangefinders and lidar because they are far less hazardous to human eyes than 1 µm. Co:Spinel offers a high damage threshold and fast recovery. View Co:Spinel crystals.

V:YAG — the 1.3 µm option

Vanadium-doped YAG (V³⁺:YAG) covers roughly 1.0–1.4 µm and is most often used to Q-switch the 1.3 µm transitions of neodymium lasers (e.g. 1342 nm), which are themselves more eye-safe than the 1064 nm fundamental. It has a large ground-state absorption cross-section and fast recovery, suiting compact pulsed sources at 1.3 µm. View V:YAG crystals.

How to choose

• 1 µm Nd lasers (1064 nm) → Cr⁴⁺:YAG (the default; bondable into Nd:YAG microchips).
• Eye-safe 1.5 µm Er lasers → Co:Spinel.
• 1.3 µm Nd lasers (1342 nm, eye-safer) → V:YAG.

Initial transmission (small-signal absorption) of the absorber sets the Q-switch hold-off and pulse energy; AOG CRYSTAL supplies Cr⁴⁺:YAG, Co:Spinel and V:YAG in custom thicknesses and with AR coatings, mounted or unmounted, and can diffusion-bond Cr⁴⁺:YAG directly into a Nd:YAG composite crystal. Contact sales@aogcrystal.com with your gain medium, wavelength and target pulse energy.

Frequently asked questions

Which passive Q-switch for 1064 nm? Cr⁴⁺:YAG — it is the mature, high-damage-threshold standard for 1 µm Nd lasers.

Why Co:Spinel for eye-safe lasers? Its absorption band (1.2–1.6 µm) matches the 1.5 µm erbium eye-safe line used in rangefinders and lidar.

Can the absorber be bonded to the gain crystal? Yes — Cr⁴⁺:YAG diffusion-bonds to Nd:YAG into a single compact microchip. See our diffusion bonding guide.