# What's the difference between pulsed lasers and frequency combs?

Can you please explain the difference between high-energy pulsed lasers and frequency combs?

• What "difference" are you interested in? – Solar Mike May 6 '19 at 8:51
• The way they are made, maybe their spectrum and their applications. I looked up some information and I find them to be very similar.. – y1004 May 6 '19 at 9:18

There is not necessarily any relationship between the two. In theory, a pulsed laser could exist without exhibiting a frequency comb spectrum, and a frequency comb could exist without operating in pulsed mode. Your confusion probably arises because one has a discrete set of pulses in the time domain while the other has a discrete set of 'pulses' in the frequency domain.

Pulsed Lasers: Pulsed lasers emit very short bursts of laser energy. The width of the pulses can be as short as ~10s of femtoseconds with no upper limit on the pulse length. Common pulsed lasers used in industrial processing applications operate with pulse lengths of ~10s of nanoseconds. Most pulsed lasers operate in a repetitive pulse mode with pulse repetition frequencies (PRF) ranging from Hertz up to Megahertz.

Frequency Combs: Frequency combs may or may not be pulsed (although the most common type is a pulsed laser). The requirement to be a frequency comb is that the frequency spectrum (or equivalently wavelength spectrum) exhibit discrete and evenly-spaced frequencies. This may come as a surprise because most lasers are designed to operate at a single well-defined frequency, but that is not a requirement to be a laser.

• I get it now! thank you so much for this detailed explanation – y1004 May 7 '19 at 9:04

A pulsed laser is defined via the difference between phase and group velocity - the frequency $$f_{ceo}$$ - and the repetition frequency of the pulses - $$f_{rep}$$. In the frequency domain (because the pulses are following each other in time with frequency $$f_{rep}$$), the spectrum is a line spectrum since it is the Fourier transform of arbitrarily thin pulses in the time domain AND vice versa. $$f_{ceo}$$ is the offset of the line spectrum against frequency $$0$$. This is always a comb spectrum/frequency comb.

• Thank you for your response :) – y1004 May 7 '19 at 9:04
• I do not have the right to fully deny the other answer. But since the relation between the pulsed laser and the comb in my explanation is the Fourier transform, it must be invertible so that any kind of comb spectrum refers to a pulsed laser in time. – Kutsubato May 7 '19 at 9:09
• according the articles I've read you can make a frequency comb out of a pulsed laser but under special conditions. – y1004 May 7 '19 at 9:12
• I think we must differentiate between an ideal comb and what physicists are talking about in reality. It is definitely true that a frequency comb in frequency is the equivalent to a pulse comb in time and vice versa. If you want your pulses to have another shape or limit your comb to a certain spectrum, the respectively other domain can behave unexpectedly. Than you might not want to speak about a comb in this case. – Kutsubato May 7 '19 at 9:15
• Well I guess I found the difference! they say that the first time a frequency comb was made it was by using an electrooptique modulator in a resonant cavity with minimized dispertion. The electrooptique modulator imposes sidebands on one frequency continuous laser (so the difference is that a frequency comb carries many frequencies while a pulsed laser is centered around 1 frequency) – y1004 May 7 '19 at 9:18