Emission wavelength
The emission wavelength of a laser depends on the energy released when an electron transitions from the conduction band to the valence band, which is approximately equal to the band gap E (eV).
hf=Eg--(3.4)
Since c = f, where f and λ are the frequency and wavelength of the emitted light, respectively, c = 3 × 10⁻³ m/s, h = 6.628 × 10⁻³ J·s, and 1 eV = 1.60 × 10⁻¹ J, substituting into equation (3-4) yields：
Since the bandgap is related to the composition and content of semiconductor materials, lasers with different emission wavelengths can be made based on this principle.
Threshold characteristics (P-I characteristics)
For lasers, when the applied forward current reaches a certain value, the output optical power increases sharply, resulting in laser oscillation. This current is called the threshold current, denoted by ε. The output characteristic curve of a typical semiconductor laser is shown in Figure 3-6. For stable and reliable operation, the lower the threshold current, the better.
Figure 3-6 Output characteristic curves of a typical laser
Spectral characteristics
The spectral characteristics of a laser are primarily determined by its longitudinal modes. Typical spectral curves for multi-mode and single-mode lasers are shown in Figures 3-7a and 3-7b. Here, λ0 represents the wavelength corresponding to the peak of the longitudinal mode with maximum radiant power, called the peak wavelength, typically 850 nm, 1310 nm, and 1550 nm; ΔλA is the spectral width of the laser, defined as the wavelength width corresponding to the longitudinal mode envelope decreasing to half its maximum value, also known as the full width at half maximum (FWHM) spectral width. The spectral width of a single-mode laser is also called the linewidth. The spectral envelope of a multi-mode laser generally contains 3-5 longitudinal modes, with an Δλ value of approximately 3-5 mm; a good single-mode laser has a Δλ value of approximately 0.1 nm, or even smaller. Δλ is the wavelength interval between two points on the spectral line where the spectral radiant power of a longitudinal mode is half its maximum value.
Figure 3-7 Spectral characteristics of the laser
For a single-longitudinal-mode laser, the side-mode suppression ratio (MSR) is defined as the ratio of the principal mode power P0 to the secondary side-mode power P0, and it is a measure of the laser's harmonic purity.
MSR = 10lg(3-6) The laser's emission spectrum changes with operating conditions. When the injection current is below the threshold current, the laser emits fluorescence with a broad spectrum; when the current increases to the threshold current, the spectrum suddenly narrows, the intensity increases, and lasing occurs; when the injection current further increases, the gain of the principal mode increases, while the gain of the side-modes decreases, the number of oscillation modes decreases, and finally, a single-longitudinal-mode laser appears. The relationship between the laser's output spectrum and the injection current is shown in Figure 3-8.
Figure 3-8 Relationship between laser output spectrum and injection current
Spectral width can also be represented by frequency. Based on the relationship between frequency and wavelength, we can obtain：
Photoelectric efficiency
Photoelectric efficiency is the ratio of electrical power to optical power. It can be expressed in several ways:
(1) Internal quantum efficiency Lasers emit light through the recombination of electrons and holes injected into the active layer, but not all injected electrons and holes can undergo radiative recombination. The internal quantum efficiency represents the ratio of the number of photons generated in the active layer to the number of injected electron-hole pairs, i.e., the number of photons generated per unit time - the number of injected electron-hole pairs per unit time.
(2) External quantum efficiency The internal quantum efficiency of lasers can be made very high, some even approaching 100%, but the actual number of photons emitted by a laser is much lower than the number of photons generated in the active layer. This is partly because photons generated in the emitting region are absorbed by other materials, and partly because the waveguide effect of the PN junction greatly reduces the number of photons that can escape from the interface. Therefore, the external quantum efficiency, i.e. the total efficiency, is defined as: (3-8) the number of emitted photons r - the number of injected electron-hole pairs per unit time. (3-9)
Temperature characteristics
The characteristics of a laser's threshold current and output optical power as a function of temperature are known as temperature characteristics. The curve showing the laser's threshold current versus temperature is shown in Figure 3-9. As can be seen from the figure, the threshold current increases with increasing temperature.
To address the laser's temperature sensitivity, temperature compensation can be implemented in the drive circuit, or a cooler can be used to maintain the device's temperature stability. Typically, the laser is packaged together with a thermistor, a semiconductor cooler, etc., to form a component.
The thermistor is used to detect the device temperature and control the cooler, achieving closed-loop negative feedback automatic temperature control.
Distributed feedback laser
Distributed feedback lasers (DFB-LDs) are a type of laser capable of generating dynamically controlled single-mode lasers, also known as dynamic single-mode lasers, meaning they are semiconductor lasers that can still operate in a single mode under high-speed modulation. They are constructed by etching a corrugated periodic grating near the active layer, which provides optical amplification, in a heterojunction laser. A schematic diagram of a distributed feedback laser structure is shown in Figure 3-10.
Figure 3-10 Schematic diagram of distributed anti-laser structure
