I’ve spent decades working with lasers, amplifiers, and optical materials. One phenomenon that always fascinates me is excited - state absorption (ESA). In this article, I’ll walk you through what ESA is, how it impacts laser systems, and how we can harness it in upconversion lasers—all from my hands - on perspective.
When most people think of absorption, they imagine atoms, ions, or molecules absorbing photons from their ground state. In ESA, however, absorption happens after the particles have already been excited to an higher energy level. Here’s what needs to happen for ESA to kick in:
Higher - Order Energy Levels
Above the excited level, there must be one or more energy states spaced just right. So that the incoming photon’s energy matches the transition wavelength.
Long - Lived Starting Level
If the initial excited level is metastable—meaning it has an relatively long lifetime—the odds of ESA go way up. Short - lived excited levels rarely support much ESA.
I first ran into ESA’s downsides in solid - state laser media. In these gain materials, you pump ions up to the laser level. Expecting stimulated emission to amplify your beam. But ESA can snatch ions away to even higher levels. Wasting pump power and creating extra loss. Here’s a real - world example:
Erbium - Doped Fiber Amplifier (EDFA) at 808 nm
Early EDFAs pumped at 808 nm looked promising. But I noticed a puzzling drop in efficiency. It turned out that at 808 nm, erbium ions were not only climbing to the desired upper laser level. But also absorbing additional photons via ESA and jumping to useless higher states.
Solution? We switched to ~ 975 nm pumping. That simple change virtually eliminated the ESA pathway. And restored our amplifier’s efficiency.
Because of ESA:
The threshold pump power goes up.
The slope efficiency drops.
ESA can even occur under laser - wavelength or signal - wavelength light. Nibbling away gain in certain spectral windows or pushing the laser to awkward emission wavelengths to dodge ESA losses.
Broadband gain media—like transition - metal doped crystals—are especially prone to ESA. Rare - earth doped crystals have narrower transitions. So ESA is somewhat tamer. Though elements with many electrons (erbium, thulium) still show significant ESA. Whereas simpler ions (e.g., ytterbium) do not.
In my work with passive Q - switching, saturable absorbers such as Cr⁴⁺:YAG have been vital. These materials bleach their ground - state absorption under high - intensity light. But ESA remains active in the excited state. Practically, this means:
Even at high intensities, unsaturable losses persist. Often accounting for a significant fraction of total saturation loss.
For nanosecond pulses, this residual ESA loss can be a major performance limiter.
Not all ESA is bad. In upconversion lasers, we deliberately exploit ESA to climb ladder - like energy levels. Until we reach very high energy emissions:
Blue (480 nm) Laser in Thulium - Doped Media
By pumping a Tm³⁺ crystal at two successive wavelengths, I’ve watched ions absorb via ESA to populate a level that emits in the blue. This bright 480 nm output emerges thanks to those multi - phonon relaxations between steps. Guided by ESA.
Using ESA this way requires not only an sufficiently long - lived intermediate energy state but also accurate knowledge of the ESA cross - section for rate - equation modeling.
Sometimes ESA is easy to fold into the math. If ESA from the pump or signal simply adds an extra absorption term—and if excited ions rapidly relax back to the upper laser level—then I can tweak a standard rate - equation model by adding that term. In complex situations (like the multi - level schemes in thulium), I build an horizontal level - diagram model and solve coupled rate equations to predict performance.
Experimentally pinning down ESA is tougher than measuring ground - state absorption. My go - to method uses an modulated pump beam, a monochromator to select the probe wavelength, a photodetector, and a lock - in amplifier. By modulating the pump, I create an oscillating population in the excited state and monitor changes in transmission. The resulting spectra reveal both gain features and ESA peaks—though you must watch out for cross - talk from other nearby levels.
Excited - state absorption is a double - edged sword in laser science. Left unchecked, ESA can sap efficiency, raise thresholds, and distort emission spectra. Yet, when controlled, it powers upconversion lasers and opens doors to new wavelengths. From my years of tinkering with rare - earth and transition - metal materials, I’ve learned that an deep understanding of ESA—and careful choice of pump wavelength, cross - section data, and level lifetimes—is key to both mitigating its drawbacks and harnessing its potential.
Contact: Jason
Phone: +8613337332946
E-mail: [email protected]
Add: Hangzhou City, Zhejiang Province, China
