Characterizing a Violet Diode Laser

Lindsey Garay, Stony Brook Laser Teaching Center

**Please note this page will be in a constant state of construction until the completion of this project.**

My Project

The overall goal of this project is to characterize a violet diode laser (nominal wavelength λ = 404 nm). This characterization will include careful measurements of the laser wavelength and wavelength changes with changing temperature, the output power and its stability over time, the degree of polarization, and the shape and size, quality, and divergence of the elliptical beam.

I picked this project for two main reasons: first, the laser had only recently been donated to the lab and not much was known about it; second, I had developed an interest in spectroscopy and measuring wavelengths as precisely as possible using diffraction gratings and wanted to do more challenging measurements in this area. In the course of the project I not only learned a great deal about this laser and diode lasers in general, I also got a lot of experience with optical and electronic instruments and devices such a linear-diode-array detector, oscilloscopes, etc.

Diode lasers are most commonly found is laser pointers. Originally in red, and then green, in color, their wavelengths have dropped into the violet range within the past couple of years. Diode lasers, unlike other types, do not have fixed wavelengths. Their wavelengths can be varied with temperature, currect etc.

So far the tools I've used have included a linear-diode-array detector, an oscilloscope, a diffraction grating, a lens, a volt meter, and a power meter. I've used the linear-diode-array detector and the oscilloscope to see the light after it passes through the lens and the diffraction grating. The volt meter was used to keep track of the voltage and temperature, and the power meter was used to measure the output power.

The wavelength was measured using a diffraction grating and measuring the movement of the diffraction pattern on an oscilloscope. This method, along with altering the temperature, was also used to find the change in wavelength when the temperature was changed.

The next steps in the characterization process will be to look at the transverse properties of the beam such as the polarization, size, shape and divergence. It is expected that the beam of a violet laser will diverge less rapidly than a that of a red laser beam of the same initial size.

My experiments have shown that it is possible to measure very small wavelength changes (less than 0.1 nm) with a very simple setup using equipment that was available in the lab. This technique could easily be applied to other lasers. In the future I hope to make even more precise wavelength measurements using a Fabry-Perot spectrum analyzer.

The Equation

In order to find the wavelength of the laser using a diffraction grating the following equation was used:

    m λ = d sin θ

where :

    m = order (integer)

    λ = wavelength

    d = distance between grooves on the diffraction grating

    θ = angle between 0 order and the order measured

The HeNe and the Diffraction Grating

In order to eventually find the wavelength, a measurement of 'd' had to be found. To do this a laser with a known wavelength had to be used. A HeNe laser was used because it has a known wavelength of 632.8 nm. The laser was projected through the diffraction grating at various distances away. The distance from the diffraction grating to the 0 order (L) and the distance from the 0 order to the 1st order (x) (in order to find θ) were measured. This was done at six different distances and, after plugging the numbers into the equation, the six different measurements for 'd' were averaged together.

Trial L (cm) x (cm) d (µm)
1 50.0 26.7 1.345
2 45.0 24.0 1.345
3 40.0 21.2 1.351
4 35.0 18.5 1.351
5 30.0 .159 1.353
6 25.0 13.2 1.360

Average 'd'

d = 1.351 µm

Standard Deviation


The Wavelength

In order to find the temperature the HeNe laser was replaced with the violet diode laser. 'L' and 'x' were measured to find θ and plugged it into the equation. Then, instead of filling in a wavelength, d = 1.351 was filled in. This was done at seven different distances and the seven answers were averaged together.

Trial L (cm) x (cm) λ (nm)
1 55.0 17.5 410.25
2 50.0 15.9 410.12
3 45.0 14.4 410.42
4 40 12.6 406.91
5 35.0 11.1 406.65
6 30.0 09.4 403.81
7 25.0 07.9 405.80

Average λ

λ = 407.71 nm

Standard Deviation


Changing the Temperature

Part of the beauty of this laser is that is comes with a built in device to change the temperature. In order to change the temperature what needs to be done is to change the voltage, which can be done by adjusting a screw in the laser. Using two wires to connect the laser to a volt meter, the change in voltage, and therefore the change in temperature, could be changed and detected. There is also a table with the laser that shows the voltage at each different degree of temperature. Using this, it could be determined what voltage to set the laser at to reach a certain temperature and the screw could be adjusted accordingly.

Temp (°C) Voltage (V)
20 1.112
21 1.139
22 1.167
23 1.195
24 1.223
25 1.250
26 1.277
27 1.304
28 1.331
29 1.358
30 1.385

The Set-Up

In order to be able to observe such a small movement in a spot of the diffraction pattern (which would be caused by a change in wavelength) this set-up needed to include a linear-array photo detector and an oscilloscope. The photo detector was first set up at the 1st order, but was later switched to the 2nd when it was realized that 2nd order would create a greater dispersion, therefore giving more accuracy. The photo detector was connected to the oscilloscope, which gave the readings of the individual pixels of the light and their movements. Another component of the set-up was a lens. Because the detector was so narrow, the spot on it had to be very small. The lens was able to focus the light. Although conventionally a lens should go after the diffraction grating so that as much of the diffraction grating is covered with light as possible, therefore giving more accuracy, because of limitations of the table the set-up was on, the lens was put right before the diffraction grating. However, because the focal length of the lens was relatively large, 0.5 meters, the amount of diffraction grating covered was not affected greatly. The photo detector was 0.5 meters away from the lens and was able to detect a much smaller spot with more accuracy.

The Data - So Far

These pictures were taken as the temperature was being slightly changed. They can be used to measure an overall change in the peak by measuring the centriod of the peak. This would allow you to see the shift of the peak over a large change in temperature.

Data Pictures

Picture Centriod Temperature (°C)
575 23.35 20.19
576 23.02 20.66
577 22.45 21.23
578 21.24 21.88
581 20.54 22.35
582 19.95 22.95
583 19.70 23.33
584 19.65 23.62
585 19.55 24.05
586 19.51 24.52
587 19.22 24.99
640 18.90 25.13
589 18.77 25.67
639 18.80 25.67
590 18.54 26.00
638 18.53 26.11
592 18.28 26.43
637 18.38 26.61
636 18.27 26.97
635 18.00 27.44
634 17.88 27.73
633 17.47 28.31
632 17.18 29.07
631 16.91 29.54
630 16.73 29.86

In order to find the wavelength change for the change in centroid the derivative dλ / dθ = L m / dcosθ was used. It was found that dλ / dθ = 817 µm/nm. Since the centroid told how far x had moved, it could be figured out how far λ had moved. When this was done a goodness-of-fit line was put over the points and the mode hop at around 21°C became obvious.

Wavelength vs. Temperature

Beam-spot Size & Shape versus Distance Data

Multiple Modes

There were certain temperatures, such as between 20°C and 22°C were the oscilloscope didn't always show a reading of a single line of pixels, but instead showed two. This happened at points when the laser was switching between modes. Because of this, although the temperature doesn't change drastically, the wavelength could. This explains data such as why the movement from 22°C to 20°C caused double the movement of the spot as the movement from 25°C to 23°C. Because of this the final graph is expected to resemble steps, instead of a straight line.

Mode Hops

The OutPut Power

The process of measuring the output power consisted only of a power meter. When the laser and focused into the meter at different areas it read different powers, but when it was measured through the lens, which absorbed some of the power, it read at 3.25 mWatts. This reading stayed constant as the power meter was moved closer and farther form the laser.