Other diodes: Diode types
When using a laser diode it is essential to know its performance characteristics. Accordingly laser diode specifications are required when designing equipment using laser diodes or for maintenance using near equivalents.
Like any electronics components, many of the specifications are relatively generic, but other parameters will tend to be more focussed on the particular component. This is true for laser diode specifications and characteristics.
There are a number of laser diode specifications, or laser diode characteristics that are key to the overall performance and these are outlined.
Laser diode L/I characteristic
One of the most commonly used and important laser diode specifications or characteristics is the L/I curve. It plots the drive current supplied against the light output.
This laser diode specification is used to determine the current required to obtain a particular level of light output at a given current. It can also be seen that the light output is also very dependent upon the temperature.
From this characteristic, it can be seen that there is a threshold current below which the laser action does not take place. The laser diode should be operated clear of this point to ensure reliable operation over the full operating temperature range as the threshold current rises with increasing temperature. It is typically found that the laser threshold current rises exponentially with temperature.
Laser diode efficiency characteristic
It is possible to deduce the laser diode efficiency parameter from the L/I curve. However it is easier to visualise when plotted separately. In view of the importance of the laser diode efficiency this is often usefully plotted.
The plot of the laser diode efficiency characteristic will show that the efficiency falls with increasing temperature. A typical laser diode specification for efficiency will be around 0.3 mW per mA at around 25°C and will fall by about 0.01 for each 10°C increase.
Laser diode tracking ratio characteristic
Many laser diode packages include a second photo diode to monitor the output of the laser. In this way the output power of the laser can be controlled and stabilised - the output from the monitor diode is fed back into the laser diode control and drive circuitry. Normally the unwanted light exiting from the back face of the laser diode is used for the measurement as this light cannot be used elsewhere.
In order for the laser output to be accurately controlled it is necessary for the monitor diode to accurately track the laser diode output. The monitor photodiode current is directly proportional to the light output from the laser and therefore a figure known as the tracing ratio is used. Measured in mA/mW it is the ratio of the photodiode current in milliamps compared to the laser diode light output in milliwatts.
Laser diode specification for V/I
The laser diode specification for the forward voltage across the diode is required in a number of areas of the design. Often laser diode manufacturers prefer to place the voltage on the vertical axis.
From the diagram it can be seen that the voltage across the laser diode is typically around 1.5 volts, although it is necessary to check for the particular laser diode in question. The forward voltage specification will vary according to the materials used in the diode, current, etc.
Although the forward voltage does vary with temperature, this is not normally a major consideration.
Reverse voltage specification
Laser diodes are easily damaged by reverse voltages. It is therefore unwise to allow the laser diode to be reverse biased.
Maximum reverse currents of 10µA are typically the maximum reverse current levels tolerated.
Laser diode far-field beam pattern
The pattern for the beam of light emitted is an important laser diode specification from an optical viewpoint. The light emanating from the diode itself is not collimated, but it is typically in the form of an oval cone of light. The ellipticity occurs because the emission area or aperture of the diode occurs as a slit in the plane parallel to the junction.
The divergence of the light beam are measured at the half maximum light power angles in the axes perpendicular and parallel to the active region of the laser diode.
Typical values are 30° and 12° for the two angles of the elliptical cone.
Laser diode wavelength specification
The laser diode specification for wavelength is one of the key parameters within the datasheet. It will determine many of the applications for which the laser diode can be used.
The wavelength is normally specified in nm - nanometres.
While other forms of laser may be able to provide a stable signal in terms of the wavelength, laser diodes are notoriously poor in this respect. They are affected by both the drive current and the temperature. Changes in temperature affect the bandgap, and hence the gain frequency profile of the junction.
Typical figures for wavelength variations with respect to voltage may be around 0.1 to 0.5 nm/°C, but this is very dependent upon the device, its frequency and a number of other considerations.
Laser diodes single / multimode specification
Laser diodes may be specified as being either single or multimode. These two types of laser diode are generally used for different applications.
Whether a laser diode is single mode or multimode is governed by the geometry of the laser diode itself. In the vertical direction, the light is contained in a very thin layer, and the structure supports only a single optical mode of operation in the direction perpendicular to the layers. However if the waveguide is wide compared to the wavelength of light in the lateral direction, then the waveguide can support multiple lateral optical modes, and the laser is known as a "multi-mode" laser diode.
Multimode laser diodes tend to be used where high power is required and a larger laser diode is required to accommodate the higher power levels.
In applications where a small focused beam is needed, the waveguide must be made narrow when compared to the wavelength of the light being generated. As a result the laser diode can only support a single lateral mode. The beam created is then diffraction-limited in terms of its dispersion. These single mode laser diodes are used for optical storage, laser pointers, and fiber optics. Note that these lasers may still support multiple longitudinal modes, and thus can lase at multiple wavelengths simultaneously.
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