Linewidth
The names of the line width and bandwidth of the laser are very similar, but the meanings are very different. First, let's look at the line width. The line width is relatively easy to understand, which is the half-peak full width of the laser spectrum.
Bandwidth
The laser bandwidth is not a unit of length of a spectrum. Its full name should be called laser modulation bandwidth.
The modulation bandwidth of a semiconductor laser refers to the maximum signal rate that can be output or loaded (for digital signals), or the maximum bandwidth of the output (or loaded) analog signal.
Therefore, if you want to understand the bandwidth, you must first understand the modulation of the laser, the modulation mode, and the definition. Bandwidth is the limit that appears in the modulation.
The principle of laser communication is actually a binary mode, coded modulation of 1 and 0.
For example, the laser light intensity under high-level drive is large, representing 1, and the laser light power under low-level drive is weak, representing 0.
Information can be transmitted by quickly switching between different powers.
This fast switching can artificially add a predetermined signal and output it to the laser power curve, which will form an "eye diagram".
Eye diagram formation
For digital signals, the changes of high and low levels can have multiple sequence combinations. Taking 3 bits as an example, there can be 8 combinations of 000-111. In the time domain, enough of the above sequences are aligned according to a certain reference point, and then the waveforms are superimposed to form an eye diagram. As shown in Figure 1. For the test instrument, the clock signal of the signal is first recovered from the signal to be tested, and then the eye diagram is superimposed according to the clock reference, and finally displayed.
For a real eye diagram, as shown in Figure 2, we can first see the basic level conversion parameters of the digital waveform, such as the average rise time (Rise Time), fall time (Fall Time), overshoot (Overshoot), undershoot (Undershoot), threshold level (Threshold/CrossingPercent).
It is impossible for the voltage values of the high and low levels of the signal to remain completely consistent every time, and it is also impossible to ensure that the rising edge and falling edge of each high and low level are at the same time. As shown in Figure 3, due to the superposition of multiple signals, the signal line of the eye diagram becomes thicker and blurry (Blur) occurs. Therefore, the eye diagram also reflects the noise and jitter of the signal: on the vertical voltage axis, it is reflected as voltage noise (VoltageNoise); on the horizontal time axis, it is reflected as time domain jitter (Jitter).
This is a bit far-fetched. The eye diagram is not a patent of laser transmission. It is used in other communication fields.
Let's go back to the bandwidth of the laser.
Inside the laser chip, the bandwidth should be limited by the recombination time constant of electron holes.
In fact, it is the rate of conversion of electricity into light. Whether it is fast or not, because the injected current must quickly switch the voltage size according to the signal. During this switching time, it is required that the electricity is converted into light as soon as possible and emitted, so as not to affect the next signal of the electricity. However, electrons and holes do not recombine immediately after entering. Under a certain voltage, they will choose to run slowly. Occasionally, there is a shortcut and they still want to go directly through the recombination zone. Defects, resistance, capacitance, etc. in the material will have an impact. Therefore, there is a bandwidth limit.
In practice, there are many limiting factors for bandwidth.
If the modulation bandwidth of the laser is to be improved, the key is to reduce the influence of the electrical parasitic factors of the laser, especially the parasitic capacitance and the transport process of carriers in the quantum well structure.
When manufacturing high-speed lasers, the following measures can be taken to improve the 3dB bandwidth of the device:
① The active area adopts a strain (compensation) multi-quantum well structure-the quantum well laser well material is subjected to biaxial compressive strain in the direction parallel to the well surface and tensile strain in the direction perpendicular to the well surface, and the heavy hole energy level at the top of the valence band rises, and this valence band is degenerate, making the probability of electron transition from the spin-orbit splitting band to the heavy hole band approximately equal to zero, reducing the Auger recombination probability at room temperature, resulting in a decrease in the threshold current of this quantum well laser, a decrease in the line width enhancement factor, and a significant increase in the relaxation oscillation frequency, modulation bandwidth, and differential gain coefficient.
② P-type doping in the active region - p-type doping can reduce hole transport when passing through the SCH region, which is the main limitation for high-speed quantum well devices; p-type doping can obtain very high differential gain and make the distribution of carriers in the quantum well more uniform.
If the Zn doping concentration in the active region is close to 1018 cm-3, its 3dB bandwidth can reach 25GHz, and doping can also increase the oscillation frequency of the device to 30GHz (cavity length is 300μm). In addition, heavy doping is also beneficial to reduce the line width enhancement factor and further improve the differential gain, which are all beneficial to improve the modulation characteristics of the device.
③ Reduce electrical parasitic parameters - In order to reduce the electrical parasitic parameters of high-speed lasers, especially parasitic capacitance, semi-insulating Fe-InP regrowth burial technology can be used, and the electrode area needs to be reduced at the same time; self-aligned narrow mesa structure (SA-CM) is used to reduce the parasitic capacitance of the device. People also often use the method of filling polyimide to reduce parasitic capacitance.
④ Increase the photon concentration and differential gain inside the laser - Increasing the photon concentration in the laser cavity can increase the intrinsic resonant frequency. Using the DFB structure to make the lasing wavelength and the gain peak wavelength negatively detuned (-10nm) can increase the differential gain, which can increase the -3dB modulation bandwidth.
The above analysis of the factors that limit the high-speed modulation characteristics of semiconductor lasers and the ways to increase the modulation bandwidth of lasers, these factors and their static characteristics are mutually influential, so when designing high-speed lasers, other characteristics, such as thresholds, temperature characteristics, etc., need to be considered.
Our address
B-1507 Ruiding Mansion,No.200 Zhenhua Rd,Xihu District
Phone Number
0086 181 5840 0345
info@brandnew-china.com
