Re: Optical isolator question

When you are building a fiber optic circuit any flat surface such as a 
fiber end face will have some percentage of reflection.  This can cause 
several problems depending upon the circuit design and power levels you 
are dealing with.  The most common problem is power getting back to the 
laser diode.  The reflected power gets back into the gain medium and 
destabilizes the power output of the diode.  If the reflected power is 
high enough the diode can be damaged but what usually happens is the 
output power just randomly varies for no apparent reason.  If you have a 
commercial laser in a box an isolator is usually inside the box also.  
If you are building from scratch always put an isolator after the 
diode.  If it is an DFB laser you might need a double stage isolator as 
these units tend to be more sensitive to back reflections. 

Onto the second problem.  If you have two parallel surfaces in a fiber 
circuit such as two ends of a fiber that aren't connected to anything or 
have enough dirt on the endface to cause poor coupling then the optical 
power will bounce back and forth between the two surfaces just like in a 
Fabry Perot cavity.  If you are adding power to this then the power 
level in the cavity builds up until something nasty happens.  There 
might be a sudden surge of optical power in the circuit, the end face of 
the connector can blow off or the fiber itself can explode.  Again this 
all depends upon power levels and circuit design.

Isolators are critical for the protection of the laser diodes, good 
behavior of optical circuits and EDFAs and also for making fiber 
lasers.  A good reference for further reading would be Saleh and Tiech's 
Fundamentals of Photonics.  The best way to decide if you need isolators 
is to understand what each component in your circuit does and how it 
behaves.  Can it tolerate fluctuations in power?  Commonly isolators are 
used around anything that has gain of some sort.  Look at some SPIE 
journals and look at similar circuit diagrams.  Look at how others have 
built the same circuit.  Try to understand why they used the devices 
they did in that particular design.

And BTW, make sure you connectors are squeaky clean.  Many people use 
APCs to get around the reflection problem but you will still have power 
losses if the endfaces aren't clean.

Regards,
P. Danek

Jmm39531 wrote:

> I have seen a fiber optics part called an "isolator", that can be integrated
> inside a laser diode package or can be a fiber passive in-line device.  The
> device is said to allow light to pass in one direction, and block it from
> passing the other direction so reflections can't bounce back into the LD
> resulting in a noisier output.
> My question is:  How do I know if I need to use an isolator?  
> In what cases are they needed or required?  Do optical reflections damage the
> laser diode in any way or shorten its life-span?
> Thanks for any information on this.
>