To achieve fiber optic communication, the first problem to solve is how to load the electrical signal onto the light beam emitted by the light source, which requires optical modulation. Based on the relationship between the modulation and the light source, optical modulation can be divided into two main categories: direct modulation (internal modulation) and indirect modulation (external modulation).
Direct modulation of the light source
Direct modulation involves directly injecting an electrical signal into the light source, converting the information to be transmitted into a power signal and injecting it into a laser diode (LD) or light-emitting diode (LED) to obtain the corresponding optical signal. This causes the intensity of the output optical carrier signal to vary with the modulation signal, and is also known as internal modulation. This method actually modulates the luminous intensity of the light source, so it is a type of optical intensity modulation (IM). The diagram illustrates the principle of direct light intensity digital modulation.. Although direct modulation suffers from wavelength (frequency) jitter, it has advantages such as simplicity, low loss, and low cost, making it a widely used modulation method in fiber optic communication systems.
Indirect modulation of the light source
The advantage of internal modulation of the light source is that the circuit is simple and easy to implement. However, using this modulation method at high data rates will degrade the performance of the light source, such as broadening the dynamic spectral lines, increasing dispersion during transmission, and thus broadening the pulse waveform transmitted in the optical fiber, which ultimately limits the transmission capacity of the optical fiber. Therefore, in high-speed intensity-modulated direct-detection fiber optic communication systems or heterodyne fiber optic communication systems, indirect modulation of the light source can be used.
Indirect modulation does not directly modulate the light source, but instead utilizes the electro-optic, magneto-optic, and acousto-optic properties of a crystal to modulate the optical carrier emitted by the laser diode (LD). This means that the modulation voltage is applied after the light is emitted, causing the optical carrier to be modulated by the modulator. This modulation method is also known as external modulation. The structure of an indirectly modulated laser is shown in the figure.
Currently available external modulation methods include electro-optic modulation, acousto-optic modulation, and magneto-optic modulation.
- (1) Electro-optic Modulation: The basic working principle of electro-optic modulation is the linear electro-optic effect of crystals. The electro-optic effect refers to the phenomenon that causes a change in the refractive index of a crystal. Crystals that can produce the electro-optic effect are called electro-optic crystals. Electro-optic modulators can be electro-optic intensity modulators, electro-optic frequency modulators, or electro-optic phase modulators (i.e., electro-optic phase modulation).
- (2) Acousto-optic Modulation: Acousto-optic modulators are made using the acousto-optic effect of a medium. Their working principle is as follows: when the modulating electrical signal changes, the piezoelectric crystal generates mechanical vibrations due to the piezoelectric effect, forming an ultrasonic wave. This sound wave causes a change in the density of the medium, which in turn changes the refractive index, thus forming a changing grating. Due to the change in the grating, the light intensity changes accordingly, resulting in the modulation of the light wave.
- (3) Magneto-optic Modulation: Magneto-optic modulation is a type of external optical modulation obtained using the Faraday effect. The incident light signal passes through a polarizer, making the incident light polarized. When this polarized light passes through a YIG (yttrium iron garnet) magnetic rod, its polarization direction changes with the modulating signal applied to the coil wound around it. When the polarization direction is the same as that of the subsequent analyzer, the output light intensity is quite large; when the polarization direction is perpendicular to the direction of the analyzer, the output light intensity is minimal. This causes the output light intensity to change with the modulating signal, thus achieving external modulation of the light.
External modulation systems are relatively complex, have a high extinction ratio (greater than 13), high insertion loss (typically 5-6 dB), high driving voltage (5V), are difficult to integrate with light sources, are polarization-sensitive, and have high losses and high costs; however, they have a narrow spectral linewidth and can be used in high-speed, high-capacity transmission systems at or above 2.5 Gbit/s, with transmission distances exceeding 300 km.
Modulation characteristics
(1) Electro-optic Delay and Relaxation Oscillation Phenomena: Under high-speed pulse modulation, the transient response waveform of the output optical pulse of a laser is shown in the figure. There is an initial delay time between the output optical pulse and the injected current pulse, called the electro-optic delay time (td), which is generally on the order of nanoseconds. After the current pulse is injected into the laser, the output optical pulse will exhibit oscillations with gradually decreasing amplitude, called relaxation oscillations. The consequence of relaxation oscillations and electro-optic delay is to limit the modulation rate.
(2) Code Pattern Effect: To produce a code pattern effect, as shown in the figure, when the electro-optic delay time is of the same order of magnitude as the symbol duration T/2 of the digital modulation, it will cause the pulse width of the first "1" bit after a sequence of "0" bits to narrow and its amplitude to decrease. In severe cases, a single "1" bit may be lost. This phenomenon is called the code pattern effect, as shown in Figures a and b. In two consecutive "1" bits, before the arrival of the first pulse, there is a long sequence of "0" bits. Due to the long electro-optic delay time and the influence of the optical pulse rise time, the pulse becomes smaller. When the second pulse arrives, because the electron recombination of the first pulse has not completely disappeared, the electron density in the active region is higher, so the electro-optic delay time is shorter, and the pulse is larger. The code pattern effect can be eliminated by using an appropriate "over-modulation" compensation method, as shown in Figure c.
Self-pulsation phenomenon
In some lasers, under pulsed modulation or even DC driving, when the injection current reaches a certain range, the output light pulse exhibits sustained, constant-amplitude high-frequency oscillations. This phenomenon is called self-pulsation, as shown in the figure. The self-pulsation frequency can reach 2 GHz, which seriously affects the high-speed modulation characteristics of the laser diode (LD).
