# Seeking recommendations for fiber laser mode-locking resources

1. Jul 3, 2013

### test1234

Hi there, I'm working on fiber lasers and I'm interested in getting up to speed with the topic of mode-locked fiber lasers. However, I can't seem to find resources suitable for me at this current stage, hope that you guys can help recommend a suitable review paper, book or anything that's relevant. Thanks in advance! =)

I guess I can say that I have a rough understanding of the basic principles of mode-locking, which is to select for the appropriate longitudinal modes such that they will interfere constructively and produce high energy pulses. This is achieved by ensuring that the longitudinal modes selected for have a fixed relative phase relationship with each other (i.e. $\phi_n=\phi_0 +n \phi$). Also, there generally 2 main methods of mode-locking, namely active and passive mode-locking. Active mode-locking requires the use of externally driven modulators to generate mode-locked pulses, whereas in passive mode-locking pulse generation is instrinsic and the absorption (or loss) characteristics are usually dependent on the laser light intensity. (Please feel free to correct me if I've misunderstood the concepts.)

I can understand and derive that mode-locked pulses have a total energy higher than that of a CW laser (with random phase longitudinal modes).
Total energy of n phase-locked longitudinal modes (mode-locked)= $n^2 E_0^{\phantom{0}2}$
Total energy of n random phase longitudinal modes (CW)= $n E_0^{\phantom{0}2}$

And I guess that's about all I know...I was looking at the journal paper by Haus, "Mode-Locking of Lasers" but it seems to be beyond what I can grasp currently and thus I'm looking for something in between to bridge the gap...

2. Jul 3, 2013

### wukunlin

have you looked at pages in rp-photonics? They got some nice info although they don't go into too much details or maths
http://www.rp-photonics.com/mode_locking.html?s=ak

It is quite difficult to give a summary of mode locking in a single post (so many different methods!) here is what I can tell you on the top of my head:

Mode locking is a way to produce extreme short pulses (what you do with these pulses is a different discussion). In a laser cavity like a fabre perot your output will consist of a lot of different frequencies at integer multiples of the free spectral range (ie FSR = 100GHz, your output will consist of light at 100GHz, 200GHz, 300GHz etc, the span of frequencies depends on your lasing medium and other conditions). Now, using the phase conditions you mentioned, all these modes will line up and created nice short pulses, hence the term mode locking.
Typically the phase of your modes are all over the place so your output never mode lock themselves. Active mode locking modulates a single selected mode to produce sidebands. The phase is controllable by the modulator so you can get mode locking that way. Since the duration of the pulse decreases with the span of your mode frequency, the shortness of the pulse generated this way is limited by the modulator.
Passive mode locking isn't limited by that problem because it uses properties of your optical components to generate short pulses. One way is to have 2 pulses collide. When you introduce phase difference to the ends of one of the pulses, the 2 pulses add interferometrically, the ends destructively interfere and the middle peaks constructively interfere. There are many other ways to passively mode lock a laser and each one of them elegant in their own way. The figure-8 laser is probably my favourite
One of the shortest pulses generated to this day uses "MAGIC mode locking" which apparently stands in its own category.

There are a number of textbooks I used during my undergrad optical courses but most of them are a chapter here and there, none of those books are solely about mode locking. Most of our materials came from past postgrad projects in the department. I can still refer you some of those books if you like.

3. Jul 3, 2013

### test1234

Thanks for your insightful summary. Yes, I understand that mode-locking usually takes up only a chapter or a few chapters at best in most textbooks, but nonetheless, I think I could benefit from them, hope you can drop me some references. =)

In addition, would you happen to know how to interpret the optical spectrums and deduce the regime of soliton operation? I gather that anomalous dispersion should give Kelly sidebands in the optical spectrum, is this always true?

4. Jul 3, 2013

### wukunlin

soliton happens when your nonlinear effects cancel out your dispersion effects so your pulse does not broaden indefinitely. Fundamental solitons' pulse and spectrum simply remains stationary. Higher order ones you should see the spectrum kind of split up, then merge, then split up again etc in a periodic manner.

Kelly sidebands shows up when you demand too much pulse energy from your fibre, ie your solition is about to fall apart.

For textbooks I will probably have to dig through my undergrad notes but one general go to book our teaching staff love to use is the optical communications book (title varies with each edition, 6th is the newest i think) by Yariv. There are some other nice stuff in that book but I personally find it difficult to self study with it.