It seems like you are experiencing some temporally-localized interference. If you look at a short enough slice of time, the photodiode might pick up the constantly shifting course of constructive and destructive interference that is basically what its called speckle.
Laser Diode Modules have a fair amount of noise if you look on a very short time scale. The total noise is a combination from multiple sources including but not limited to power supply noise, mode hopping, thermal effects, and optical feedback. These noise sources can be severely reduced by using a low-noise power supply and a heat sink or preferably a Thermo-electrically cooled (TEC) laser. In order for a laser to have a perfectly consistent output even on that small of a time scale, the necessary controls need to be designed in from the ground up, which is generally not the case with laser diode modules.
Silicon detectors are typically used for detecting laser signals and converting the signal to a voltage differential. For an in-depth explanation on the use of our silicon detectors, as well as circuit diagram examples, view Basic Principles of Silicon Detectors.
Our TE cooled Spectrometers, like the one you have, are calibrated for wavelength and not irradiance. The curve seen in the software is basically counts versus wavelength. The y-axis (counts) ranges from 0 to 2 x 16-1M or 65535, which is dependent on integration time that the user is allowed to set via the included software. In order to calibrate the spectrometer, you need to use a calibrated light source. Out of the box, you will be able to measure transmittance/reflection measurements but, again, the units will be counts versus wavelength. If you need a spectrometer that is calibrated to provide irradiance versus wavelength readings, we offer a Compact Spectroradiometer which is designed for the 380-750nm wavelength range.
A Schlieren test is an optical system that detects changes within a test area medium (air) and records the changes in the form of an image on a screen. The image is formed by refraction and scattering from what is introduced into the test area, which are areas of varying refractive index. A source directs light onto a spherical mirror, which collimates the light and redirects it onto a second identical mirror. The light is then focused onto an included screen. The space between the mirrors is the test area, where the small particles introduced are made visible by the light source and seen as shadows on the screen. Brightness variations on the screen will occur according to changes within the test area. Spherical mirrors are used due to the slight off-axis nature of the set-up. Applications include the determination of refractive index, fluid and air current flow, and flame analysis. Film or video cameras can also be added to the system in place of the imaging screen. Two systems are available with different size F10 mirrors in order to select the test area size that best matches the application.