In this chapter, parameters of laser diodes are explained such as monitor current, slope efficiency, oscillation threshold and so on. Understandings of parameters will help you to select an adequate laser diode.
TO-CAN and 14 pin butterfly type package has a photo diode incorporated. The photo diode generates a current in accordance with the intensity of the laser emitted from laser diode. Meaning, by monitoring the current generated by the photo diode, it is possible to monitor the output power of the laser diode. By feeding back the situation to the temperature control element (because laser diodes are temperature-dependent), it is possible to control the laser output power. Amount of current generated at a standard output is on the laser diode’s data sheet.
In the specification documents of a laser diode, reverse voltage and forward voltage is described. This is the reverse voltage that must not be applied to the laser diode. If applied the laser diode will be destroyed. In order to prevent the laser diode being destroyed, it must have a protection circuit. In reverse voltages, there are PD reverse voltage and laser diode reverse voltage.
Laser diodes must be used in a temperature controlled state. Operating temperatures are described in the data sheet of the laser diode. Operating temperature of a typical laser diode is -20 ~ 50 °C. The various parameters on the data sheet is when operated at 25 °C.
Storage temperature is the temperature that should be kept when a laser diode is not operated. It is usually about -40 ~ 80 °C. The range is wider than the operating temperature.
External quantum efficiency and internal quantum efficiency
Quantum efficiency is the probability of a photon being obtained from a transition of an electron. The internal quantum efficiency is quantum efficiency considering the loss within the resonator. The external quantum efficiency is quantum efficiency that takes into account the loss caused by the outside parts of the laser diode body.
Differential quantum efficiency
Differential quantum efficiency is the quantum efficiency obtained from the change in the number of photons (ΔPop) within the changes in the current flowing in the semiconductor (ΔI). Differential quantum efficiency is constant at the threshold current and over. Differential quantum efficiency can be determined from the slope efficiency as introduced below.
Slope efficiency is the increases percentage of the light output proportional to the current, when a threshold current is injected. That is to say, slope efficiency is the slope of output light – injected current curve (LI curve) of the laser diode. The unit is W / A or mW / mA, it is sometimes written in the specifications of the laser diode.
Fig. 4.4.1. Output light – injected current curve.
Unlike the LED, a laser diode is a laser with a resonator, so it has a resonating threshold current. Figure 4.4.1. is a diagram showing the output light – injected current characteristic. When current is between 0 ~ IthA spontaneous emission light is emitted from the laser diode. The spectrum at this time is broad, like a LED. When the current exceeds the Ith, laser oscillation occurs. At this time, the spectrum becomes narrow, almost one wavelength. When it is current > Ith, output light increases in proportion to the current. The video below illustrates how the laser oscillates.
Unit of light output of the laser diode is watt, the same as other lasers. Units of the brightness of LED, which is often compared to laser diode, is also watts. However, LED’s watt shows the power consumption of the LED. It is a display for inferring the brightness by the power consumed. In laser diode, the electrical energy is converted to light, because of this luminous efficiency of the laser diode can be easily obtained. Efficiency(%) = Optical output power / ( Operating voltage x Operating current )
The method of modulating light intensity by modulating the injection current of a laser diode is called direct modulation. With this frequency chirping occurs. Because of this, the spectrum gets wider than the spectral broadening due to the modulation, strongly affects the wavelength dispersion of the optical fiber. With the use of an external modulator, it is possible to avoid this effect, because the laser diode light source can be driven with a steady current.
The types of external modulators are, Electro-optic effect type (EO), Photo acoustic effect type (AO), and the Electro-absorption type(EA) that uses the absorption effect.
Among the EO modulator, the lithium niobate (LiNbO3, LN) waveguide types are called LN modulators.
The laser beam of laser diode spreads in an elliptical cone shape. The shape of the beam of a laser diode near the output end and a few cm from the output end is different. The pattern of the beam near the output end is called Near Field Pattern (NFP). The beam pattern a few cm away from the output end is called Far Field Pattern (FFP).
The figure below shows NFP and FFP.
Fig. 4.7.1. NFP and FFP
Quote from: SPIE Digital Library:Modeling the near-field and far-field modes of single spatical mode laser diode
Near the laser diode facet, the laser is a horizontal oval shape in the x-axis direction, but in FFP, the ellipse is horizontal in the y-axis direction. This is, because the outlet is narrower on the y-axis direction, compared to the light that comes out of the x-axis direction, thus causing it to diffract largely.
In the data sheet of the laser diode, FFP is represented by θ// and θ⊥. θ// indicates the angle at which the laser diode spreads laterally, and θ⊥ indicates the angle that extends in the vertical direction. The angle is defined in FWHM, which is when the intensity of the laser beam is 1/2 of the central portion of the laser beam. Usually, θ// is 8 ° and θ⊥ is 30 °, so the laser extends in the vertical direction.