MadSci Network: Physics

Re: Why does a laser only work upto a certain input f

Date: Mon May 3 16:47:17 2004
Posted By: Kenneth Beck, Staff, Chemistry and Physics of Complex Systems (C&PCS), Pacific Northwest National Laboratory
Area of science: Physics
ID: 1079954934.Ph

Dear No name entered,

The 2700 Hz. frequency modulation limitation you are experiencing is a 
characteristic of the laser diode and laser driver cicuitry specific to 
your application. From your question, I cannot acertain your application, 
but I can tell you that laser diodes can be routinely modulated above 2 
GHz (gigahertz).  Such systems are commercially avaliable for a number of 
different applications and fields of research.

But let me begin at the beginning...

What is a semiconductor diode laser?

In its simplest form, the semiconductor laser is a p-n junction of a 
single crystal of semiconductor material arranged in a Fabry-Perot 
cavity. The type and configuration of the material used determines the 
optical characteristics of the laser diode emission. A wide range of 
Fabry-Perot laser diode wavelengths and powers for many different 
applications are available. For example, ther are laser diodes 
specifically for communications applications, as well as, high power 
laser diodes and arrays for pumping solid state lasers.

A word about wavelength and wavelength tuning…

The center wavelength of a Fabry-Perot laser diode is given as a nominal 
value at 25°C and is subject to a tolerance of ±10nm typically. The 
wavelength varies with temperature by approximately 0.2nm-0.5nm/°C, 
depending on the diode. Laser diodes do not exhibit continuous wavelength 
tuning with temperature, but instead exhibit a behavior known as mode 
hopping where the wavelength changes by a discrete value. This can be a 
disadvantage for some applications. Using a Peltier effect bipolar 
thermoelectric controller, the temperature can be stabilized and even 
used to tune the emission to a specific wavelength. 

What is a laser diode module?

Laser diode modules incorporate a semiconductor laser diode circuit, 
collimating lens and laser diode drive circuit, packaged in a rugged 
housing to provide continuous or pulsed laser output.  Most laser diodes 
operate in closed loop mode, where the power output is monitored and the 
drive current adjusted automatically to keep the power constant.  
Operating the device in constant current, however, avoids mode hopping in 
wavelength critical applications, and prevent damage to the diode at 
elevated temperatures (thermal runaway). 

A modulated diode laser driver can be designed to produce laser light up 
to about 2 GHz modulation frequency with interchangeable laser heads. 
These device provides an ideal ultra fast excitation source for phase- or 
frequency- modulation fluorescence lifetime measurements. The laser 
driver consists of a controlling generator box and separate laser heads. 
The laser heads are usually available from 400 nm to 1500 nm and offer a 
modulation bandwidth of 500 MHz to 2 GHz dependent on the wavelength.

Hope this helps in you exploration of laser diode technology!

---* Dr. Ken Beck

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