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Passively Q-switched microchip laser with SAM

  Contents  
  microchip on 12.7 mm copper heat sink  
Microchip setup  
  microchip setup

The passively Q-switched monolithic microchip laser shown in the figure left consits of a Nd doped YVO4 laser crystal and a saturable absorber mirror (SAM) , which are bonded together. The laser cavity with the length L in the region of 100 µm .. 300 µm is determined mainly by the laser crystal thickness. Typical Nd doping concentratios are 2 % or 3%. The linear polarized 808 nm pump light from a cw diode laser is parallel oriented to the c-axis af the laser crystal. A typical pump spot diameter in the laser crystal is in the region of 50 µm - 100 µm. The pulsed single mode output laser light with 1064 nm wavelength is reflected from a dichroic mirror.

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  The microchip laser is mainly characterized by the following parameters:  
 
Working principle up
 

The SAM serves as passively Q-switch. The low intensity non-saturated SAM reflectance is in the region of 0.75 - 0.9 for the laser wavelength of 1064 nm. During the continuous pumping of the laser crystal the upper states will be filled until the gain is large enough for spontaneous emission. Then the SAM is saturated and a powerful short pulse leave the laser. During continuous pumping this development repeats with a repetition frequency frep inceasing nearly linear with the pump power density. Because a part of the optical energy is dissipated in the absorber layer of the SAM, arrangements have to be done to remove this heat from the microchip. If the laser cavity length L is short and the pump spot diameter small the laser works in single mode. See Applied Physics, Vol. B97, p.317, 2009 (pdf)

 
 
pulse energy Ep  
 

According to the well established formalism (Journal of the Optical Society of America B, Vol. 16, Issue 3, pp. 376-388, 1999) of passive Q-switched microchip laser the pulse energy Ep can be estimated as follows:

formula pulse energy Ep

 
  with  
  Fsat,L saturation fluence of the laser material  
  A pump spot area in the resonator  
  ΔR modulation depth of the SAM  
  T transmission of the output coupler  
  Ans non-saturable losses, mainly from SAM.  
 
 

With typical values Fsat,L = 37.3 mJ/cm2, A = 1.3x10-5cm2 ( 40 µm spot diameter), ΔR = 0.1, T = 0.1, and loss = 0.05 the pulse energy can be estimated to Ep ~ 32 nJ.

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repetition rate frep  
 

The average output power Pav is proportional to the pump power. Therefore the repetition rate shows a linear behavior by changing the pump power according to:

formula repetition rate

microchip repetition rate
  with
  ηs slope efficiency
  PP pump power
  PP,th pump power threshold
 
 

By changing the pump power PP repetition rates between 100 kHz and 2 MHz can be realized with a nearly constant pulse energy EP. Because the pulses start with spontaneous emission, the repetition rate shows a time jitter of about 1 %, increasing with decreasing pump power and rate.

 
 
 
pulse duration tP  
 

The pulse duration tP in passive Q-switched lasers can be estimated by:

formula pulse duration

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  with  
  TR resonator round trip time  
  ΔR modulation depth of the SAM  
  n refractive index of the resonator material  
  L resonator length  
  c speed of light in vacuum.  
 
  With a short laser crystal length L = 0.2 mm, refractive index n ~ 1.95, and a SAM modulation depth of ΔR = 0.1 the pulse duration is about 91 ps.  
 
 
 
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