Differences between optical laser and amplifier I am preparing for an applied optoelectronics exam and I am having some trouble with telling the differences between optical fiber laser and optical amplifier. For now I only came up with following differences:  


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*No Bragg reflectors and optical resonators in amplifiers  

*No signal conversion in optical amplifiers


I'm pretty sure there's more to it but I couldn't find anything that would answer this question fully. 
 A: The main difference between a fiber laser and a fiber amplifier is the cavity.
A laser (fiber laser or solid state laser) is an gain media, a pump and a cavity whereas a amplifier has no cavity. The cavity allow selecting the oscillating mode so there is no need to seed with a signal to generate a single frequency. Because it's initiated by noise. Whereas an amplifier need a signal seed.
You can make an analogy with Larcen effect. An audio amplifier acts just as its optical counterpart : it needs an input signal to amplify it. Now, if you get a microphone closer of the speaker plugged to the amplifier you form a loop and if unlucky you get that really annoying noise at a selected frequency.
For fiber laser, the cavity can be a ring cavity, a bragg cavity, or mirrors.
A: An optical laser without a feedback mechanism is essentially an optical amplifier. The feedback mechanism can be made using 2 reflectors, mirrors, or gratings that creates an optical resonator cavity.
Another difference between optical amplifier and laser is that amplifier integrates isolators to avoid feedback, backreflection or lasing effects. The tunability of laser comes from bandwidth of its amplifier (active gain medium)


