Since the discovery of semiconductor lasers in 1962, this technology has become an integral part of progress in various fields. Semiconductor lasers are widely used in optical communications, biomedicine, integrated optics, and materials science, but how do they work? Understanding their structure, key properties, and operating principles is crucial for exploring their applications and performance.
Electrical properties of semiconductor materials
Semiconductors are widely used due to their advantages such as long lifespan, small size, low power consumption, and compatibility with modern technologies. Their electrical properties can be tuned to enhance or restrict electron flow, making them crucial in electronic and laser applications. Their conductivity, falling between that of metals and insulators, allows for control over electrical behavior.
Electrical conductivity is determined by the movement of free electrons. Electrons occupy different energy levels, with the least tightly bound electrons located in the valence band. Above the valence band is the conduction band, where electrons must transition to conduct electricity. The energy difference between these bands (called the band gap) determines the conductivity of a material.
Compared to insulators, semiconductors have narrower band gaps, allowing for control over the movement of electrons. This property is crucial for the application of semiconductors in modern electronic and optoelectronic devices, including semiconductor lasers.
The structural characteristic of blue semiconductor lasers lies in the material of their active region, typically GaN or InGaN. These materials have different band gaps, allowing the lasers to emit different colors of light. Taking a typical GaN-based laser as an example, its structure, from bottom to top in the z-direction, consists of an n-electrode, a GaN substrate, an n-type AlGaN lower confinement layer, an n-type hGaN lower waveguide layer, a multiple quantum well (MQW) active region, an unintentionally doped hGaN upper waveguide layer, a p-type electron blocking layer (EBL), a p-type AlGaN upper confinement layer, a p-type GaN layer, and a p-electrode. This sophisticated structural design enables blue semiconductor lasers to exhibit excellent luminous efficiency and stability.
A semiconductor laser, also known as a laser diode, is a device that uses semiconductor materials as the active medium to generate laser light. Based on the unique electronic band structure of semiconductor materials, it achieves population inversion by injecting current, thereby generating stimulated emission and emitting a laser beam with high coherence, directionality, and monochromaticity. Common laser semiconductor materials include gallium arsenide (GaAs) and indium phosphide (InP). Through different doping and manufacturing processes, these materials can achieve laser output at different wavelengths, covering a wide spectral range from visible light to infrared light.
Ordinary LEDs emit light based on spontaneous emission. When a forward voltage is applied across the PN junction of an LED, electrons and holes recombine, releasing energy to emit light in the form of photons. The light emission process is random, and the emitted light is relatively dispersed in direction, phase, and spectrum.
Semiconductor lasers are based on stimulated emission. Under specific conditions, external energy excitation causes the number of electrons in high-energy levels to exceed the number of electrons in low-energy levels, resulting in population inversion. When a photon triggers a high-energy electron to transition to a low-energy level, it produces a photon with the same frequency, phase, direction, and polarization state as the incident photon, thus forming a high-intensity, highly coherent laser beam.
A typical GaN laser chip structure is shown. The material in the multiple quantum well active region (MQWs) has the highest refractive index, and the refractive index gradually decreases on both sides. This distribution of high refractive index in the middle and low refractive index at the top and bottom in the z-direction confines the optical field mainly between the upper and lower waveguide layers. In the y-direction, the p-type layers on both sides of the laser are etched and coated with thin layers of silicon dioxide (SiO2), forming a ridge structure. Since the refractive indices of silicon dioxide and air are lower than those of the p-type layers, the optical field is further confined to the middle of the ridge in the y-direction. Finally, the front and rear cavity surfaces are formed in the x-direction through mechanical cleavage or etching, and their reflectivity is controlled by evaporating dielectric films. Typically, the reflectivity of the front cavity surface is designed to be lower than that of the rear cavity surface to ensure that the laser can be effectively emitted from the front cavity surface.
Next, we will discuss the working principle of blue semiconductor lasers.
Like other types of semiconductor lasers, GaN lasers are based on the photoelectric effect of semiconductor materials. For a semiconductor laser to function properly and achieve stimulated emission of photons, several key conditions must be met: population inversion, resonant cavity, optical gain, and threshold conditions. Population inversion refers to the situation where, under specific conditions, the number of electrons in the conduction band exceeds the number of holes in the valence band, or vice versa. In GaN lasers, we primarily focus on the case where n > p. When an external electric field is applied to the laser, electrons are pushed from the bottom of the conduction band to a higher energy state, while holes are pushed from the top of the valence band to a lower energy state; this process is called electrical injection. Without external injection, the number of electrons and holes in the semiconductor is equal, and the system is in thermal equilibrium. However, once the condition of population inversion is met, the laser possesses the foundation for generating laser light.
Where 0 and 0 represent the equilibrium carrier concentrations in the conduction and valence bands, respectively, while and correspond to the energies at the bottom of the conduction band and the top of the valence band, respectively. Furthermore, represents the Fermi level, is the Boltzmann constant, and represents the temperature. Under the influence of an external electric field, electrons and holes are injected into the active layer, with their concentrations becoming n and p, respectively. When the injected current reaches a certain intensity, the following condition will be satisfied:
Here, and represent the effective densities of states in the conduction band and valence band, respectively. To achieve population inversion, the injected current must reach a certain intensity, causing the Fermi level to shift towards the conduction band, thus making the electron concentration in the conduction band greater than the hole concentration in the valence band. This process can be specifically described by the following conditions:
The threshold current ℎ of a laser can be calculated using the following formula:
Here, represents the electron charge, and represents the cross-sectional area of the active region. In practical applications, the threshold current is usually expressed in the form of current density ℎ.
Taking all the above factors into account, we can further derive the expression for the threshold current density.
Therefore, we can derive the specific expression for the threshold current density ℎ. To ensure that the laser gain exceeds the loss, we must satisfy the following condition:
Where represents the intracavity loss, and represents the output coupling loss.
Next, we will explore the resonant cavity theory of GaN lasers.
This theory is based on the principle of optical resonators, and its core lies in how to construct and maintain optical oscillations within a laser. As an optical structure, the laser resonator amplifies the optical signal through multiple round trips between two or more reflecting surfaces. This structure typically consists of partially transmissive mirrors, providing the necessary conditions for stable oscillation of light waves. To satisfy this condition, the wave vector of the light wave must conform to a specific relationship.
Here, the wavelength representing the light wave is the optical length of the resonant cavity, while the integer part symbolizes the order of the mode. Within the resonant cavity, the oscillation mode of the light wave can be a transverse mode (such as the TEM mode), a longitudinal mode (such as the TE or TM mode), or a combination thereof. Transverse modes are particularly noteworthy in semiconductor lasers because they exhibit stable electric and magnetic field distributions perpendicular to the laser waveguide.
The working principle of a semiconductor laser is based on excitation. It uses the transition of electrons between energy bands in a semiconductor material to emit light. Two parallel reflective surfaces formed by the cleavage planes of the semiconductor crystal are used as reflectors to form a resonant cavity, causing the light to oscillate and feedback, generating radiation amplification of the light, and outputting laser light.
Semiconductor lasers operate by injecting charge carriers, and three basic conditions must be met to emit laser light:
1. To produce a sufficient number inversion distribution of particles, that is, the number of particles in the high-energy state must be greater than the number of particles in the low-energy state.
2. A suitable resonant cavity can act as a feedback mechanism, causing stimulated emission of photons to increase, thereby generating laser oscillation.
3. A certain threshold condition must be met so that the photon gain is equal to or greater than the photon loss.
II. Are semiconductor lasers and fiber lasers the same?
Semiconductor lasers and fiber lasers are different.
1. Different media materials
The difference between fiber lasers and semiconductor lasers lies in the material they use to emit laser light. Fiber lasers use optical fibers as the gain medium, while semiconductor lasers use semiconductor materials, typically arsenide, styrene, etc.
2. Different light-emitting mechanisms
The light-emitting mechanism of semiconductor lasers is that particles transition between the conduction band and the valence band to generate photons. Because they are semiconductors, they can be directly electro-optically converted by electrical excitation. However, optical fibers cannot directly achieve electro-optical conversion and require light to pump the gain medium (usually using laser diodes). They achieve optical-optical conversion.
3. Different heat dissipation performance
Fiber lasers have good heat dissipation and can generally be cooled by air. Semiconductor lasers, on the other hand, are greatly affected by temperature and require water cooling when the power is high.
4. Different main characteristics
The main characteristics of fiber lasers are small size and flexibility. They offer a wide range of output spectral lines, excellent monochromaticity, and a wide tuning range. Furthermore, their performance is independent of the light polarization direction, resulting in low coupling loss between the device and the fiber. They also boast high conversion efficiency and low laser threshold. The geometry of the fiber provides very low volume and surface area, and in single-mode, the laser and pump can be fully coupled.
Semiconductor lasers are easily integrated with other semiconductor devices. Their characteristics include direct electrical modulation, ease of optoelectronic integration with various optoelectronic devices, small size and light weight, low drive power and current, high efficiency, long lifespan, compatibility with semiconductor manufacturing technology, and mass production capability.
High brightness and high energy density: Due to their high directionality and coherence, semiconductor lasers can concentrate extremely high energy in a small spot area, with brightness several orders of magnitude higher than that of ordinary LEDs. This gives them an irreplaceable advantage in applications requiring high energy density illumination, such as laser cutting and laser welding in industrial processing.
Long-distance transmission characteristics: The extremely small divergence angle allows semiconductor lasers to transmit laser light with minimal energy loss over long distances, enabling applications such as long-distance optical communication and laser ranging. For example, in optical communication, semiconductor lasers, as a light source, can achieve high-speed, long-distance data transmission in optical fibers, while ordinary LEDs, due to severe light divergence, cannot meet such requirements.
High coherence: The high coherence of semiconductor lasers makes them play an important role in optical precision measurement and imaging fields such as interferometry and holographic imaging. Lasers with good coherence can produce clear and stable interference fringes, which can be used for high-precision length measurement, surface topography detection, etc.
