Lasers are the core devices of optical transceivers, which injecting current into semiconductor materials and injecting laser light through the photon oscillations and gains in the resonator. At present, the most commonly used lasers are VCSEL, FP, and DFB laser. The difference between them is that semiconductor materials and resonator structures. DFB lasers are more expensive than FP lasers. The optical modules of transmission distance within 40km generally use VCSEL, FP lasers; transmission distance ≥ 40km generally use DFB lasers. Do you know all the transceiver laser types? Let us learn this knowledge.
Light-emitting diode referred to as LED. Made of a compound containing gallium (Ga), arsenic (As), phosphorus (P), nitrogen (N). Visible light is emitted when electrons recombine with holes and thus can be used to make light emitting diodes. In the circuit and equipment as a light, or composed of text or digital display. Gallium arsenide diode red, gallium phosphide diode green, silicon carbide diode yellow, gallium nitride diode blue. Due to the chemical nature of organic light-emitting diode OLED and inorganic light-emitting diode LED.
For optical fiber communication systems, LEDs are the best light source of choice if the multimode fiber is used and the bit rate is under 100-200Mb/s while only requiring input optical power of tens of microwatts. Compared with the semiconductor laser, because the LED does not need thermal stability and light stabilization circuit, so the LED drive circuit is relatively simple, its production cost is low, high yield LED emission spectrum line light, poor directivity, its own response speed Slow, so only for the lower speed communication system. The LED laser commonly used in 155M 1×9 multimode transceivers.
Vertical-Cavity Surface-Emitting Laser (VCSEL) is a type of semiconductor laser whose laser is perpendicular to the top surface It is made of a separate chip that is generally cut with a slit, and the edge-emitting laser is different from the edge-emitting laser. VCSELs typically use 850nm wavelengths for short-range transmission of Gigabit Ethernet to 10GbE SR multimode fiber.
VCSEL laser has many advantages over edge-beam lasers in the production process. Edge-beam lasers cannot be tested after production. If an edge-emitting laser does not work, it is a waste of processing time and material processing time, either because of poor contact or poor material growth. However, VCSEL can be tested its quality and troubleshoot any manufacturing process. For example, if the paths between the dielectrics are not completely and cleanly connected, the top metal layer is not in contact with the test metal layer during the pre-packaged test and the test result is incorrect. Further, since the laser light emitted from the VCSEL is perpendicular to the reaction zone, and edge emitting laser light emitted in parallel to the reaction zone contrary, there can be tens of thousands of VCSEL to be processed on a three-inch large gallium arsenide chip simultaneously. In addition, even though VCSELs require more labor and finer material in the manufacturing process, more predictable production results can be controlled.
For example, the below optical transceiver laser source is made of VCSEL laser.
- 1000BASE-SX SFP, 1000BASE-SX SFF, 1000BASE-SX 1×9, 1000BASE-SX GBIC
- 2.5G multimode SFP, 2G FC SFP multimode
- 4G FC multimode SFP
- 10GBASE-SR SFP+, 8G FC SFP+ SW
- 40GBASE-SR QSFP+
Fabry-Perot Laser is called FP laser, which is a semiconductor light-emitting device that emits multi-longitudinal-mode coherent light with an FP cavity as a resonator. The characteristics of such devices are large output power, smaller divergence angle, narrow spectrum, high modulation rate, suitable for longer distances. For the general FP laser, when the injection current is near the threshold current, multiple longitudinal modes can be observed. When the injection current is further increased, a certain wavelength at the peak is first excited and most of the carriers are consumed. Other modes of lasing, it is possible to form a single longitudinal mode of work; when the FP laser for high-speed modulation, the original lasing mode will change, the emergence of multi-mode work. This determines the FP laser cannot be applied to the high-speed optical fiber communication system. However, compared with other structures of the laser, FP laser structure, and production process is the easiest, the lowest cost, suitable for the modulation rate of less than 2.5Gbit/s optical fiber communication system. At present, the fabrication technology of FP laser used in optical fiber communication is already quite mature, and the structures of active layers with double heterojunction multiple quanta well, carriers and light respectively are generally adopted.
For example, the below optical transceiver laser source is made of VCSEL laser.
DFB laser is a distributed feedback laser. The difference lies in the built-in Bragg grating, which is a side-emitting semiconductor laser. At present, DFB lasers are mainly made of semiconductor materials and include GaSb, GaAs, InP, and ZnS. The most prominent feature of DFB lasers is their excellent monochromaticity (ie, spectral purity). Their linewidth is generally within 1MHz and very high side mode suppression ratio (SMSR), which can now be as high as 40-50dB.
Distributed Bragg Reflex Laser (DBR) is a single frequency laser diode . It features an optical cavity that consists of an electrical or optical pump gain region between two mirrors to provide feedback. One of the mirrors is a broadband mirror and the other is wavelength-selective, so the gain is advantageous over a single longitudinal mode, resulting in lasing at a single resonant frequency. Broadband mirrors are usually coated with a low-reflection coating to allow emission. Wavelength selective mirrors are periodically constructed diffraction gratings with high reflectivity. The diffraction grating is located in a non-pumping area or a passive area of the cavity. DBR laser is a monolithic single-chip device, grating etching in the semiconductor. DBR lasers can be edge emitting lasers or VCSELs. Alternative hybrid architectures that share the same topology include extended cavity diode lasers and bulk Bragg grating lasers, but these are not termed DBR lasers.
DMLs generally use a distributed feedback structure, a diffraction grating in the waveguide that can be the directly modulated stable operation, so this laser is also called “DFB” (distributed feedback laser diode). The modulation speed and transmission distance strongly depend on the spectral width of the laser. Higher modulation speeds (data rates) and longer distances require narrower line widths. Compared with the Fabry-Perot laser, DFB has a spectral line width of about 1/10, so the DFB structure is more suitable as a high data rate DML.
In DML, data by modulating an injection current as an input on/off electrical signal is placed on a light beam and applied directly to a laser diode chip to output a modulated optical signal. The DML is a single chip that provides a simple circuit configuration for operation so it can accommodate both compact design and low power consumption. Direct modulation changes the characteristics of the laser, as its refractive index leads to large dispersion. Compared to EML, DML performance decreases over longer periods (> 10km) due to larger dispersion, lower frequency response and relatively lower extinction ratio. DMLs are mainly used for relatively lower speeds (≤25Gbps), and shorter reaches (2-10km) in telecom and datacom applications.
For example, the below optical transceiver uses DML laser.
- 100G QSFP28 LR4, 100G QSFP28 CWDM4 CLR4
EML is a laser diode with an Electro-Absorption Modulator (EAM) integrated into a single chip. The laser diode portion, which typically has the same device structure as the DML, operates under continuous wave (CW) conditions and an input voltage on/off signal is applied to the EAM portion to produce a light output signal. The properties of the laser itself will not change due to the modulation process because they are in the DML. Compared with DML, EML is advantageous in applications with higher speed and longer distance transmission because of its smaller dispersion. EML is mainly used for higher speeds (> 25 Gbps) and longer distances (10-40 km) in telecommunication applications.
Compared with DML, since the injection current (input signal) to the laser part is not modulated, the EML has less wavelength dispersion and a stable wavelength at high-speed operation and therefore does not change. The frequency response of the EML depends on the capacitance of the EAM section and can achieve high operating speeds even above 40 GHz. Extinction in the EML is due to absorption due to a change in the coefficient of the modulation voltage applied to the EAM portion, and the extinction ratio becomes high at a large voltage input (on/off electric signal).
For example, the below optical transceiver uses EML laser.
- 10GBASE-ER SFP+, 10GBASE-ZR SFP+
- 10GBASE-ER XFP, 10GBASE-ZR XFP
- 8G SFP+ ER, 8G SFP+ ZR
- 8G XFP ER, 8G XFP ZR
- 100G QSFP28 ER4
- 10G CWDM SFP+ ER, 10G CWDM SFP+ ZR