LASER (Light Amplification by Stimulated Emission of Radiation) is a type of coherent light, i.e., having the same frequency, the same direction of vibration, and the same phase (or the phase difference remains constant). Optical communication systems rely heavily on high-quality laser light sources. Laser is the heart of optical module, and its cost accounts for about 50% of the total cost of optical module. This paper will introduce what lasers are commonly used in optical module and their characteristics.
The optical module consists of optoelectronic devices, functional circuits and optical interfaces, etc. The optoelectronic devices include two parts: transmitting (TOSA) and receiving (ROSA). The transmitter converts the electrical signal into an optical signal, which is transmitted through optical fiber, and then the receiver converts the optical signal into an electrical signal. The main components inside the optical module are optical devices, detectors, lasers, amplifiers, clock data recovery, driver chips and MUX & DeMUX. Among them, Transmitter Optical Sub Assembly (TOSA) is the component used to realize the electric/optical conversion in the optical module, including laser, MPD, TEC, isolator, Mux, coupling lens, etc. TOSA has TO-CAN, Gold-BOX, COC (chip on chip), COB (chip on board) and other package forms. For the optical modules used in the data center, TEC, MPD and isolator can be omitted in order to save cost. MUX is also only used in the optical modules requiring wavelength division multiplexing. In addition, the LDD of some optical modules is also packaged in TOSA. And the core of TOSA is the laser.
What types of lasers are available for optical modules?
1）VCSEL (Vertical-cavity Surface-emitting Laser)
Semiconductor lasers, according to their laser chip structure, are divided into VCSEL and EEL lasers.
VCSEL refers to a vertical surface cavity laser with a wavelength of 850nm. Many short-range multimode optical modules use this laser, such as QSFP28 SR4 optical module. VCSEL is a kind of semiconductor laser. Semiconductor lasers, that is, lasers manufactured by applying semiconductor manufacturing technology, like all semiconductor devices, have small size, light weight, low power consumption, high reliability and long life compared to solid, gas and liquid lasers. VCSEL emits laser light from the face of a vertical substrate, so it is easy to achieve a large-scale light-emitting array by a planar process. And the beam dispersion angle of the emitted laser is small, and the spot is approximately circular. In addition, the requirements for optical systems are low, and the process is compatible with LED, which is conducive to large-scale manufacturing and cost reduction. But the main drawback of VCSEL is the complexity of the process and low power.
EEL refers to a vertical surface cavity laser with a wavelength of 850nm, which is used in many single-mode optical modules. The spot of EEL is elliptical, which is characterized by high power, high photoelectric efficiency and high illumination (peak power 125W), but the laser spectrum is slightly wider. EEL also includes DFB, FP, DBR, etc.
·FP (Fabry-Perot Edge Emitting Laser): it is mainly used for short distance transmission (generally within 20km). The FP laser has the advantages of simple structure and low cost. The FP has two wavelengths, 1310nm and 1550nm. For example, the 10G SFP+ IR optical module with FP laser has a transmission wavelength of 1310nm and a transmission distance of 2km.
·DFB (Distributed Feedback Laser Diodes): it is based on the FP laser, the Bragg grating is integrated into the active layer inside the laser (i.e. gain medium), and a mode selection structure is formed in the resonant cavity to realize single-mode operation. It belongs to the side emission type and uses the horizontal cavity structure. The DFB generally uses 1310nm and 1550nm wavelengths, which are divided into refrigeration and non refrigeration. It is mainly used for medium and long-distance transmission, such as 40G QSFP LR4 optical module.
·DBR (Distributed Bragg Reflector Laser): it is a new type of laser, which is very similar to DFB laser, but has better performance. It has the advantages of narrow line width, good wavelength tuning performance and long locking time. A grating is built in the DFB laser to stabilize the wavelength, and the whole resonator is composed of a periodic structure. The built-in grating of DBR laser is used to stabilize the wavelength. It is a multi-stage laser with gain, phase and DFB cross section.
- EML (Electlro-absorption Modulated Laser）
EML is an integrated device of electric absorption modulator (EAM) and DFB laser, which is used as a high performance optical communication light source. EML has a lot of applications in access network, such as signal optical emission source for optical fiber transmission in metropolitan area network and LAN, light source for electro-optical signal conversion and remote transmission in phased array radar base station, etc. Compared with directly modulated DFB laser, EML has better transmission characteristics and effect, especially in high frequency modulation or long-distance transmission. EML products are divided into chip products, component products and module products. The module product contains the component product, and the component product contains the chip product. So the key core of EML is the EML chip, which is the core of the electro-absorption modulated laser.
- DML (Directly Modulated Laser)
The DML features a single chip with simple circuitry, making it ideal for circuit designs that require small size and low power consumption. It puts information on the beam by modulating an on/off electrical input generated by the driver IC. This, in turn, is applied directly to the laser diode to produce a modulated optical signal output. Compared to EML, DML is used for low data rate applications and shorter distances, while EML handles higher data rates over longer distances. For example, in the 25G optical module, the SFP28 ER with a transmission distance of 40km uses an EML laser, while the SFP28 LR with a transmission distance of 10km uses a DML laser.
The selection and use of optical modules are completely different according to different scenarios. The most important thing is to select the laser type and modulation mode according to the transmission rate, transmission distance and different wavelength. In the high-speed 100G optical module, VCSEL laser is used within 100m, DFB laser is used from 500m to 10km, and EML laser is used for 40km. In the 10G Optical module, 1310nm wavelength belongs to the zero dispersion region. DML laser can be selected for 20km transmission, and EML laser needs to be selected for 1550nm wavelength.
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