1. DFB-Laser Diode
F-P (Fabry-Perot) cavity laser diode has become a conventional product, and it is developing towards higher reliability and lower price. The lasing wavelength of DFB-laser diode is mainly determined by the gate cycle of the micro-refraction light prepared inside the device, and it relies on the wrinkle corrugated gate reflected at equal intervals along with the entire active layer.
The two sides of DFB-laser diode are semiconductor crystal layers of different materials or different components and are generally fabricated in the optical waveguide region near the QW (quantum well) active layer. This corrugated structure makes the refractive index of the optical waveguide region periodically distributed, which acts as a resonant control, and the wavelength selection mechanism is a grating.
Utilizing the size effect of the QW material and the mode selection effect of the DFB grating, the spectral line of the emitted light is very wide, and a single longitudinal mode can be dynamically output under high-rate modulation. The DFB-laser diode with a built-in modulator meets the requirements of small and low power consumption of optical transmitters.
DFB-laser diode mostly uses ternary compounds and quaternary compounds composed of III and V elements. In the 1550nm band, the most mature material is InGaAsP/InP. The research and development of new AIGaInAs/InP materials are becoming increasingly mature, and only a few manufacturers in the worlaser diode can provide products for use.
The device structure is further optimized, and the active region is a strained superlattice QW. The periphery of the active area is generally a double trench buried or ridge waveguide structure. The optical waveguide area near the active region is a DFB grating, and some special designs such as adjustable distribution coupling, complex coupling, absorption coupling, gain coupling, and composite discontinuous phase shift are adopted to improve device performance.
In production technology, MOCVD(metal organic chemical vapor deposition) and grating etching are the key processes. MOCVD can accurately control the composition, doping concentration, and thickness of several atomic layers of the epitaxial growth layer, which has high growth efficiency and is suitable for mass production.
RIBE(reactive ion beam etching) can ensure the uniformity of the grating geometric figure. Phase mask etching generated by the electron beams can complete the fabrication of array gratings in one step. 1550nm DFB-laser diode began to be widely used in 622Mb/s and 2.5Gb/s optical transmission system equipment, and becomes the main light source in large-capacity and long-distance optical fiber communications.
The integrating multi-wavelength DFB-laser diode and external cavity electro-absorption modulator on the same chip are also being developed. In the successfully developed integrated light source for the electro-absorption modulator, the active layer and the modulator absorption layer share a multi-QW structure.
The modulator acts as a high-speed switch, transforming the laser diode output into binary 0 and 1. 40 different refractive gratings, the integrated light source of 40-channel modulator with a wavelength of 1530-1590nmon are formed in one chip, in which the channel spacing is 200GHz. Its development goal is to integrate a laser diode array with 100 emission wavelengths for 9.5 THz communication of ultra-large capacity.
2. VCSEL
The characteristics of VCSEL (Vertical Cavity Surface Emitting Laser) diodes are as follows:
- A cylindrical beam is emitted from the top, and the beam can be modulated into annular beams without asymmetric or astigmatism correction, which is easy to couple with optical fibers;
- High conversion efficiency. The power consumption is only a fraction of the edge-emitting laser diode;
- Fast modulation speed, which is above 1GHz;
- Low thresholds Diode and low noise;
- Small cavity surface, easy for high-density large-scale production and the detection, packaging, packaging of the diode, which is also cost-effective.
VCSEL adopts a sandwich structure, with only 1--3QW gain region layers of 20nm in between, and the upper and lower layers are Bragg reflectors with 100% reflectivity formed by multilayer epitaxially growing thin films, thus forming a resonance cavity. A highly coherent laser beam is finally irradiated from its top.
Many manufacturers have a demonstration of tunable VCSEL sample of 1550nm low loss window and low dispersion. A typical tunable device combines a common 980nm VCSEL and a reflective cavity of the micro-optical electromechanical system together. This structure consists of a curved top mirror, a gain layer, a reflective bottom mirror, which can generate a center wavelength of 1550 nm.
The electrostatic control voltage positions the top mirror located on the supporting film, and the gap size of the resonant cavity can be adjusted by changing the control voltage, thereby adjusting the output wavelength. In the range of 1528-1560nm, it can continuously tune 43nm, and it has no error code after 500km transmission at 2.5Gb/s. The side mode rejection is better than 50dB. If VCSELs with emission wavelengths between 1310 and 1550 nm can be commercialized, it will further promote the development of optical communications.
3. DBR-Laser Diode
The superstructure grating SSG structure is the most representative in DBR-laser diode (distributed Bragg reflector laser diode). An active layer is in the center of the device, and SSG regions formed by refraction gratings are on both sides, which are periodically space modulated. Their reflection spectra become comb-like peaks. The wavelengths where the comb-like spectra overlaps are changed discontinuously, enabling a wide range of wavelengths tuning.
The DBR-laser diode is used in the wavelength converter, which is monolithically integrated with the modulator. The left side of the chip is a bistable laser part, which has two active areas and an isolation area for saturation and absorption. The right side is the wavelength control area, which consists of a phase shift area and DBR.
1550nm tunable DBR-laser diode with multi-redundancy function can obtain 16 wavelengths with a frequency interval of 100GHz or 32 wavelengths with a frequency interval of 50GHz. With mode hopping at approximately 10nm intervals, approximately 100nm wavelength tuning can be obtained. In addition to retaining the existing processing and packaging technology, switches with nano-second wavelength have been added to expand the tuning range.
4. FG-Laser Diode
FG-laser diode (Fiber Grating laser diode) uses mature packaging technology to couple an optical fiber containing FG with an FP cavity laser diode coated with an antireflection film on the end face to form a tunable laser of external cavity structure, which is composed of a laser diode chip, an air gap, and the optical fibers, and the optical resonant cavity is between the grating and the outer end face of the laser diode.
The inner end surface of the laser diode is coated with an anti-reflection film to reduce its FP mode. FG is used to feed back mode selection. Due to its extremely narrow filtering characteristics, the laser diode operating wavelength will be controlled within the Bragg emission peak bandwidth of the grating. The Bragg wavelength of the FG can be adjusted by changing the temperature, thus we can get a laser output with a controllable wavelength.
It’s simple to make and assemble FG-laser diode, and its performance is comparable to DFB-laser diode. The lasing wavelength is determined by the Bragg wavelength of FG, so it can be precisely controlled. The single-mode output power can reach more than 10mW, and the line width is less than 2.5kHz. And its low relative intensity noise and wide tuning range (50nm) make it possible to replace DFB-laser diode in some areas of optical communication.
5. GCSR-Laser Diode
GCSR-laser diode (Grating Coupling Sampling Reflective laser diode) is a laser diode with wavelengths that can be tuned in a wide range. It consists of a gain, coupler, phase, and reflector regions. By changing The injection current of each part, the emission wavelength can be adjusted. This laser diode has an adjustable wavelength range of about 80nm, and can provide 322 wavelengths in the wavelength table recommended by the ITU-T of the International Telecommunication Union.
6. MOEMS-Laser Diode
MOEMS-laser diode (Micro-Optical Electro-Mechanical System laser diode) controls the movable surface setting or adjust the physical size in the optical system with static electricity to tune the light wave in the horizontal direction. The free-space micro-optical platform technology is used to control the position of the cavity mirror to realize the change of the cavity length of the F-P cavity, bringing a tunable range of 60nm. This structure can be used in tunable optical devices and semiconductor lasers to form tunable lasers.
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