The overall goal of this procedure is to demonstrate the proper assembly and characterization of external cavity diode lasers. This is accomplished by first finding the proper orientation of optic elements and achieving feedback lazing. The second step is to set up a saturated absorption system for tuning the laser frequency.
Next, tune the laser on resonance and obtain a Doppler free absorption signal. The final step is to interfere the beam with that of a second tuned laser to measure the line width. Ultimately, an external cavity DDE laser on residents with the desired atomic transition is built and its line width is measured.
Visual demonstration of this method's useful because the procedural steps are difficult to learn. This video will start with the assembly of the external cavity diode laser. After the selection of the laser diode lens, grating and electronics wear a grounding strap as a precaution against damaging the diode through static discharge.
Here the mechanical system, except for the diode lens and grating, is mounted on a thermal electric cooler continuous assembly of the laser By placing the laser diode in its mounting hole and securing it using its mounting ring, the mounting ring should be snug but not type the DDE can and ground pins should be permanently grounded. Mount the lens in front of the diode and mount the lens tube assembly. After checking the pin assignments, connect the laser diode to a protection circuit and the current supply.
Remove the grounding strap and set the proper operating conditions for the diode and thermoelectric cooler by setting the diode temperature and current to the suggested value. For the wavelength of interest, turn the temperature controller on and allow the temperature to stabilize. Next, take proper safety precautions for working with lasers, including the use of goggles.
Turn on the diode and place an infrared viewing card in front of it. Increase the current so that the output beam is clearly observed with the diode and lens set up. Turn attention to the diffraction grading.
First, check the orientation of the grading lines. The diffraction plane is usually labeled with an arrow that is perpendicular to the grading lines and in the direction of the blazed reflection. Double check the labeling by working under a light bulb and viewing the grading from the direction pointed to by the arrow.
The light reflected from the broadband source should change color as the angle is varied. Prepare to mount the grading by orienting it on the tuning arm of the external cavity diode laser for maximum feedback power. Ensure that the arrow points back towards the dde.
Then use a fast setting glue to mount the grading. Now prepare to collate the beam with an aspheric collating lens. Mount the lens in front of the diode.
The distance between the diode and the lens can be adjusted. Once the lens is mounted, use the beam card to check that the beam diameter is constant over at least three meters. Adjust the diode lens separation if necessary.
Next place a rotatable polarizer in the beam path to check that the polarization is in the desired plane for the diffraction grading. This completes the construction of the external cavity diode laser. Start alignment by placing a viewing card in the external cavity diode laser beam.
Next for the diode. In this experiment, adjust the set current on the diode control box to just below threshold. Then begin working with the adjustment screws of the system.
Use the screws to alter the angle of the grading arm until an external feedback cavity is achieved. As the adjustments are made, observe the viewing card. One sign of a feedback cavity is an increase in brightness or a flash on the viewing card.
The next step is to prevent instability in the laser through back reflection. Do this by adding an optical isolator immediately after the laser. Now to help with laser frequency tuning, prepare to make a course measurement of the absolute wavelength at less than one nanometer precision.
To do this, use a half wave plate and a polarizing beam splitter to pick off a secondary beam from the main beam and input it into a wave meter. Adjust the external cavity diode laser until the desired output wavelength is obtained about 780 nanometers for this rubidium diode. Now prepare the system for saturated absorption.
Spectroscopy direct some of the laser beam through a polarizing beam splitter and a quarter wave plate. After the quarter wave plate, place a reference vapor cell surrounded by a solenoid. Follow the solenoid with a mirror light reflected from the mirror is directed by the beam splitter to a photo detector.
Attach the photo detector to an oscilloscope. Use the DDE controller to scan the wavelength until an absorption signal can be seen. For a rubidium cell at the 780 nanometer transition, there is a doppler broadened absorption signal of width, about five gigahertz with several sharp 10 megahertz transitions also present.
Also, when the laser scans over the rubidium 780 nanometer atomic transition, the laser beam should be visible in the vapor cell to create an error signal for locking. Use a function generator to modulate the magnetic field of the solenoid at around 250 kilohertz with a magnitude of one gause. Mix the signal from the absorption photodetector output with the modulation signal from the function generator to get an error signal on the oscilloscope.
Similar to this here, each hyperfine F two F prime transition is labeled. Control the magnitude of the error signal by adjusting the relative phase with the quarter wave plate before the vapor cell At this point, center the scan over the transition of interest. Then progressively reduce the scan range until no other transitions are present.
Employ a proportional integral derivative circuit to lock the laser wavelength using the error signal. To make an accurate line width measurement, use two external cavity diode lasers. Each laser should follow the schematic shown here.
Direct the beam from each laser by adding a half wave plate and a polarizing beam splitter. After the course wavelength measurement apparatus begin by locking the two lasers to different hyperfine transitions about 100 megahertz apart and matching their modes, power and polarization. Once this is done, use a 50 50 non-polar beam splitter to cause the two beams to interfere.
Direct the resulting beam to a photo detector. Check the signal output from the photo detector on an oscilloscope. The signal should be a sine wave with a frequency equal to the difference between the frequencies of the two lasers.
Use a spectrum analyzer for the best resolution of the frequency fluctuations. As in this example, there will be a void profile centered on the beat frequency, which can be approximated by a Gaussian. Here the beat has a frequency of about 206.24 megahertz and aligned with of 0.3 megahertz.
After watching this video, you should have a good understanding of how to construct and characterize the common external cavity dial laser.
