Public Lab Research note

Harmonic Modes in a Red Laser Diode

by wln215 | March 12, 2019 16:02 | 76 views | 0 comments | #18522 | 76 views | 0 comments | #18522 12 Mar 16:02

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High Resolution Laser Diode Spectroscopy

Laser diodes, especially the inexpensive variety, emit multiple wavelengths centered around the nominal wavelength that are the function of harmonic modes within the laser cavity. Insight into how these harmonics makes for a good experiment in spectrometry.

Some Theory

Virtually all of the laser pointers on the market contain a laser diode that is made out of a lasing semiconductor between a P-N junction. When forward current is applied to this diode, the electrons in the semiconductor are excited into a metastable state, and when one of them emits a photon, the rest of the electrons emit a photon in an avalanche of monochromatic light. This light reflects within this semiconductor and create standing waves in the cavity (the lasing semiconductor). As long the laser continues in steady state, these standing waves emit light out of the sandwiched semiconductor. Lower quality lasers tend to have a higher spectral bandwidth of wavelengths emitted. A property this labs makes use of.

This is an inherent property of lasers and can be used to derive properties such as the length of the cavity at the semiconductor level. This lab is made to test laser diodes like the ones found in laser pointers. Higher quality lasers used in very expensive spectrometers have more narrow spectral widths.

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The Experimental Rig

This spectrometer is primarily cut from cardboard using a laser cutter. I dubbed the rig itself the "optic cube" given its cubular transmitter and detector. The Spectrometer has for main parts:

  1. The laser drivers and laser diode to be analyzed
  2. The Lens-Grating-Lens that expands the laser beam and diffracts the light
  3. The detector, which is a PiCamera without a lens
  4. The RaspberryPi, which makes sense of the detector's output

The Laser

The laser is supposed to be the variable in this experiment. This spectrometer is a testing framework for measuring laser harmonics of many types of semiconductor lasers, not just the 650nm one used in this one. This part of the spectrometer starts with driver circuitry to deliver constant current (and possibly controlled by a computer). Lasers operate best under regulated currents. The change in current can cause the cavity to flex a bit, which is reflected in the changing wavelengths. This is a variable that we want tight control over.

The diode itself is a KY-008 module for arduino. This module is one of many generic red laser pointers made to work with an Arduino. These things are practically bulletproof as long as current is regulated. They may contain dubious circuitry, but as a rule of thumb, They operate a 5V with a draw of about 25mA.

These lasers include a lens which focuses the beam, normally a flattened pyramid, into a dot. This gets cut to reveal bare semiconductor.

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The Spectrometer

The primary lens in this experiment is the most difficult thing to source. It is an aspheric lens with a very short focal length and large-ish diameter to collaminate the unfocused light into a beam about an inch (27mm) wide.

This wide beam serves to illuminate as many slits as possible in a holographic grating sourced from the amazon, or your local educational science material shop. It contains 1000 lines per mm, giving us 27,000 lines to diffract the laser's light.

The secondary lens has a focal length of 1m which will focus the grating's output onto the detector

The Detector/Pi

I used a raspberry pi and modified picamera using software uploaded for the lego spectrometer. The piCamera's factory lens is twisted off to expose the bare CCD. The pixel pitch of the sensor is crucial in the experimental calculations. This spectrometer operates at such high resolution, a lens complicates the results.


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