Wavelength to Frequency Converter (nm to THz, eV and cm-1)
Laser datasheets quote wavelength, telecom channel plans quote frequency, and spectroscopy and detector specs use photon energy or wavenumber. This converter takes a vacuum wavelength in nanometers and returns optical frequency in THz, photon energy in eV, wavenumber in cm-1 and the wavelength inside common media.
Your numbers
Wavelength in vacuum. Laser datasheets quote this value.
Used only for the wavelength inside the medium. Frequency does not change.
Your results
Uses exact c and rounded hc. Refractive index varies with wavelength, temperature and material grade, so the medium values are indicative. Wavelength in air differs slightly from vacuum.
Optical frequency vs wavelength
Sweeps Vacuum wavelength from 400 to 1700, holding your other inputs fixed.
Test data next to your work orders
Optional: an expert review of linking spectrum, power and wavelength test data to lots and serial numbers.
Optional. The tool and every export work without this. No spam.
How to use the converter
Enter the vacuum wavelength in nanometers, for example 1064 for a Nd:YAG or fiber laser, 1550 for telecom or 532 for a green DPSS laser. Choose the medium to see the shortened wavelength inside glass, water or a silica fiber core. Frequency and photon energy do not depend on the medium, only the wavelength and the phase velocity do. The chart shows how frequency falls as the wavelength grows across the visible and near infrared, which is handy when checking whether two sources sit on the same part of the spectrum.
- Use the vacuum wavelength from the laser datasheet
- 1550 nm is about 193.4 THz
- 1064 nm is about 281.8 THz and 1.165 eV
- Photon energy in eV is useful for bandgap and detector work
The relationships
Frequency equals the speed of light divided by wavelength, using c equal to 299,792,458 meters per second. Photon energy is Planck constant times frequency, which works out to about 1239.84 eV nm divided by the wavelength in nanometers. Wavenumber is the inverse of wavelength expressed in cm-1, so 10 million divided by the wavelength in nm. The optical period is the inverse of frequency. Inside a medium the frequency stays fixed while the wavelength shrinks by the refractive index, which matters for coating thickness design and fiber Bragg grating pitch.
Worked example
A 1550 nm telecom laser has an optical frequency of about 193.4 THz, a photon energy of about 0.80 eV and a wavenumber of 6,452 cm-1. The optical period is roughly 5.2 fs. Inside a silica fiber core with an index near 1.468 the wavelength is about 1,056 nm. A DWDM channel on the 100 GHz ITU grid sits on a frequency such as 193.1 THz, which is about 1552.5 nm, so the conversion is how a channel plan expressed in THz gets mapped to the laser setpoint in nm.
Common mistakes
Mixing vacuum and air wavelengths is a frequent source of small offsets, around 0.03 percent, which matters for narrow-linewidth lasers and wavelength meters. Another is applying the refractive index to frequency: only wavelength and speed change in a medium. People also forget that the index depends on wavelength, so using a visible-light index for an infrared fiber gives the wrong wavelength inside the core. Unit slips between nm, um and cm-1 are also common in spectroscopy handoffs.
Frequently Asked Questions
What is 1550 nm in THz?
About 193.4 THz in vacuum. The ITU DWDM grid anchors at 193.1 THz, which corresponds to about 1552.52 nm, with channels spaced by 100 GHz or 50 GHz around it.
How do I convert nm to eV?
Divide 1239.84 by the wavelength in nanometers. For 532 nm that gives about 2.33 eV, and for 1064 nm about 1.165 eV. The constant is Planck constant times the speed of light, expressed in eV nm.
Does frequency change in glass?
No. The frequency of light is set by the source and stays constant across interfaces. Wavelength and phase velocity change by the refractive index, so a 1064 nm beam is about 702 nm in N-BK7 at an index near 1.52.
What is the difference between vacuum and air wavelength?
Air has an index near 1.0003, so an air wavelength is about 0.03 percent shorter than the vacuum value. Datasheets usually quote vacuum, but some wavelength meters report air. Check which one your instrument uses.
If your test stations log wavelength, power and spectrum data into files and databases, Netray can help make that data searchable next to work orders and lots, privately and on-prem.
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