Main Page/PHYS 4210/He-Ne Lasers

He-Ne Lasers

In this experiment we first align an open-ended laser. Then we set up some transverse mode patterns, and perform further exercises and experiments to understand how a laser works.

Reading and Exercises

• Read pages 94 to 105 from Preston-Dietz. Carry out Exercise 1 (pg. 100), Exercise 2 (pg. 103) and Exercise 3 (pg. 104), but do not submit them as part of your report.
• Continue with exercise 4 (pg. 104).
• Read pages 100 to 112, on laser cavity modes.
• Do exercise 1 (pp 111-112). Do not forget to answer the last question of the exercise: Calculate the frequency difference between two adjacent axial modes TEM_oom & TEM_oo(m+1).

Experiments

Aligning the laser

Align the laser until it begins lasing using either, the instruction sheets "Aligning the Laser" by Preston or the Alternative method. Both methods are in a handbook (Red Binder) in the laboratory.

When you are successful with the aligning process, and lasing is achieved, try varying the distances between the mirrors (using the adjusting screws on the laser mount) that still supports lasing. Record, in your lab book, the aligning process used, maximum, minimum and 'best' distances between the mirrors, radius of curvature of the mirrors etc.

Use the polarizers to determine the polarization of the 'open' and 'aligning' lasers. Explain the difference(s), if any, between the two lasers.

Assume that the He-Ne produces 3 mW of laser output power and that the electrical data given applies to your gas discharge tube. Compute the efficiency, in percent, for converting electrical energy to red laser light energy with this He-Ne laser. Discuss your results.

Brewster's Angle

Using the etalon, find an approximate value for the Brewster's angle. You can determine this by rotating the etalon until lasing stops. Only at the Brewster's angle does lasing resume.

Knowing that the tangent of the Brewster's angle is the ratio of the refractive indices of the lasing medium to air, determine the refractive index of the glass medium.

Every resonant laser cavity has a characteristic quality factor or Q that measures the internal losses. The higher the Q, the lower the losses.

A Q-switch pulse can be made by blocking one end of the mirror, then exciting the medium and then quickly unblocking the mirror. Stimulated emission will quickly drain the stored laser energy from the cavity in a short pulse with peak power much higher than the laser can produce. One can think of a Q-switch as a device that quickly switches from absorbing to transmitting, suddenly reducing cavity losses.

The Q-switch pulse length is given by

180px

where t is the round trip time (back and forth in the cavity), and R is the output mirror reflectivity ( >98% ).

Therefore

where L is the distance between the mirrors, n₁ is the refractive index of the medium, and c is the speed of light. Pulse length can then be written as

Using the data from your laser, what is the theoretical value for the pulse length?

TEM Modes

Set up a camera with the screen at about 1 to 2 meters from the output mirror. Adjust the output coupler screws (or any other adjustments) to produce the TEM_00m, TEM_10m, TEM_01m, .... modes. Photograph or sketch a few of them.