A fiber laser tutorial


  John Hansknecht, Jefferson Lab





The system begins with a laser “seed”.  The seed is a low power laser diode that is typically used in the communications industry for cable television.



        Figure 1    Laser Seed






The laser seed is biased with a dc current until it just begins to lase.  An rf sine wave is then applied to this laser through a bias tee network.  The internal capacitance and structure of these small lasers then does something rather amazing.  Instead of slowly turning on and off with the application of the rf, the diode is driven far below threshold during one part of the sine wave and then begins to store energy on the opposite swing of the wave.  When it has reached a certain level of stored energy (gain), it “snaps” on and releases all of this energy as laser light.  The laser then turns off because all of the gain was extracted.  The phenomenon is called “gain-switching”.


Tuning the seed:

The balance of rf and dc power are crucial to get the right pulse.

If dc is too high, the seed may produce light all the time.

If the rf is too high, the seed may produce an “after pulse”.

If either are too low, the seed won’t produce the desired amplitude

If the seed temperature is not maintained at a proper setpoint, the wavelength produced will not match the desired wavelength of the second harmonic generator.



      Figure 2    Laser Seed with bias tee, rf, dc, and temperature control inputs                            figure 3    Schematic representation of a gain-switched seed




Figure 4  Optical pulses of light produced from a gain-switched seed laser

The laser seed is light is fiber coupled into a fiber laser amplifer.  The amplifier can take the 1mW average power pulse structure and amplify it to over 5 Watts.  The amplification is very clean and it would be difficult to discern any differences between the scope trace of figure 4 and an output pulse trace.


figure 5    A fiber laser amplifer

The seed and amplifer configuration is what we call "laser Class 1".  This means that the light is totally contained within the apparatus (fibers) and there are no safety concerns associated with exposure (so long as the fiber and connections remain intact.)  This changes on the output fiber of the amplifier when the light is launched to free space.

The fiber launch to free space must occur in a Class 4 Laser Area.  The beam is invisible and quite dangerous if proper precautions are not taken.

Figure 6 below shows the yellow output fiber entering an optical assembly on the laser table.  This assembly is called the Second Harmonic Generator (SHG).  Its purpose is to take the 1560nm light from the fiber laser and double its frequency.  The resulting 780nm light is useful for driving polarized electrons the CEBAF accelerator photo-emission gun. 



figure 6    A Second Harmonic Generator Assembly




figure 7    A Schematic representation of the Second Harmonic Generator Assembly


Figure 7 above shows the second harmonic schematic.  Light launches from the fiber laser amplifier output (F1) and is immediately collimated by lens (L1).  The light passes through a 1/2 waveplate (W1) to allow its linear polarization state to be rotated to match the correct polarization axis of the doubling crystal.  Lens (L2) focuses the beam to a waist in the SHG crystal.  The crystal is made of periodically polled lithium niobate (PPLN).  When properly aligned and running at the correct temperature, the phase of the crystal is matched to the phase of the incoming 1560nm light.  Harmonic mixing will then convert some of the 1560nm beam to 780nm light.   Lens (L3) captures the diverging 1560nm and 780nm light and brings it back to collimation.  The combined wavelengths reach mirror (M3).  This is a dichroic mirror that has been coated to pass the 1560nm light and reflect the 780nm.  The 1560nm is dumped in a safe beamdump, and the 780nm light is reflected off mirror (M4).  The light passes through the other laser table components and eventually stikes the photo-cathode to produce our electron beam.


As figure 8 demonstrates below, the process of second harmonic generation is non-linear. 
The PPLN crystal has higher efficiency with higher peak power.  Since our laser pulses are only 30ps wide at a repitition rate of 499Mhz, the 5 watt pulsed light has a peak power of over 300 watts.  In contrast, the peak power of the 5 watt DC light is still 5 watts.

This works to our advantage.

This is ideal for the CEBAF accelerator because only the desired peak pulse will create the 780nm light to drive the electron gun.  The small level of DC light between pulses will produce little, if any, 780nm light.

figure 8    Output power vs Fiber amplifier settings