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The Working Method And Different Species Of LDs

The crystal diode is a p-n junction formed of a p-type semiconductor and an n-type semiconductor. A space charge layer is formed on both sides of the crystal diode, and a self-built electric field is built. When there is no applied voltage, the diffusion current caused by the difference of the carrier concentration on both sides of the p-n junction is equal to the drift current caused by the self-built electric field and is in the electric equilibrium state.


When the outside world is biased by the forward voltage, mutual suppression of the external electric field and the self-built electric field increases the diffusion current of the carrier and causes a forward current.

When the outside world has a reverse voltage bias, the external electric field and the self-built electric field are further strengthened to form a reverse saturation current I0 independent of the reverse bias voltage value within a certain reverse voltage range.

When the applied reverse voltage is high to a certain extent, the electric field intensity in the space charge layer of the pn junction reaches a critical value to generate a doubled charge carrier process, generating a large number of electron-hole pairs, resulting in a large reverse breakdown current This is called the diode breakdown phenomenon.

Conductive properties

The most important feature of a diode is unidirectional conductivity. In the circuit, current can only flow in from the anode of the diode, and the cathode flows out. The following is a simple experiment to illustrate the forward and reverse characteristics of the diode.

1· Forward Characteristics

In the electronic circuit, the positive electrode of the diode is connected to the high potential terminal, and the negative electrode is connected to the low potential terminal, and the diode is turned on. This connection mode is called forward bias. It must be stated that when the forward voltage applied across the diode is small, the diode still cannot conduct, and the forward current flowing through the diode is very weak. Only when the forward voltage reaches a certain value (this value is called "threshold voltage", the germanium tube is about 0.2V, and the silicon tube is about 0.6V) can the diode be turned on. After conduction, the voltage across the diode remains essentially constant (approximately 0.3V for the neodymium tube and approximately 0.7V for the silicon tube), which is referred to as the "forward voltage drop" of the diode.

2 reverse characteristics

In the electronic circuit, the positive electrode of the diode is connected to the low potential terminal, and the negative electrode is connected to the high potential terminal. At this time, almost no current flows in the diode. At this time, the diode is in an off state. This connection is called reverse bias. When the diode is in reverse bias, there will still be a weak reverse current flowing through the diode, called the leakage current. When the reverse voltage across the diode increases to a certain value, the reverse current will increase sharply, and the diode will lose its unidirectional conduction characteristics. This state is called diode breakdown. The injection current of the laser diode must be greater than the critical current density in order to satisfy the condition of reversal of the mass and emit the laser light. The critical current density is related to the junction temperature and indirectly affects the efficiency. During high temperature operation, the critical current increases, the benefit decreases, and even the components are damaged.

Features

When the laser diode injection current is below the critical current density, the light emission mechanism is mainly spontaneous emission, and the spectrum is dispersed widely, with a bandwidth of about 100 to 500 Angstroms (Angstroms=10-1 nm), and the order of the atom diameter is several Angstroms. However, when the current density exceeds the critical value, oscillation starts, and only a few modalities are left, and the bandwidth is reduced to less than 30 Angstroms. Moreover, the power consumption of the laser diode is extremely small. For heterostructure lasers, for example, the maximum rated voltage is usually less than 2 volts, the input current is between 15 and 100 milliamperes, and the consumed power is often less than one watt while the output power is several tens of milliwatts or more.

One of the characteristics of laser diodes is the ability to modulate the output light directly from the current. Because the output light power and the input current are mostly linear, the laser diode can use analog or digital current to directly modulate the intensity of the output light, eliminating expensive modulators and making the application of the diode more economical and practical.

species

DFB-LD

The F-P cavity LD has become a conventional product, and it has developed toward high reliability and low price. The lasing wavelength of the DFB-LD is mainly determined by the period of the tiny refraction gratings prepared inside the device, and it depends on the corrugated structure gratings that are reflected along the entire active layer at equal intervals. On both sides of the DFB-LD, semiconductor crystal layers of different materials or different compositions are generally fabricated in the optical waveguide region near the quantum well QW active layer. This corrugated structure causes the refractive index of the optical waveguide region to be periodically distributed, acts like a resonance control, and the wavelength selection mechanism is a grating. Using the size effect of QW material and the mode selection effect of the DFB grating, the spectral line of the emitted light is very wide, and it can dynamically output in a single longitudinal mode under high-rate modulation. The DFB-LD with built-in modulator satisfies the compact and low power requirements of the optical transmitter.

DFB-LD mostly uses ternary compounds and quaternary compounds composed of III and V elements. In the 1550 nm band, the most mature material is InGaAsP/InP. The research and development of the new AIGaInAs/InP materials are maturing, and only a few manufacturers in the world can provide commercial products. The device structure is optimized and the active region is a strained superlattice QW. The periphery of the active area is generally a double-buried or ridged waveguide structure. The optical waveguide area near the active area is a DFB grating, and adopts some special designs, such as adjustable distributed coupling, multiple coupling, absorption coupling, gain coupling, composite discontinuous phase shift, etc., to improve the performance of the device. In the production technology, metal organic chemical vapor deposition MOCVD and grating etching are key processes. MOCVD can accurately control the composition, doping concentration, and thickness of thin layers to several atomic layers of the epitaxially grown layer. It has high growth efficiency and is suitable for mass production. Reactive ion beam etching can ensure the uniformity of the grating geometry, and electron beam generation. The phase mask etching can complete the fabrication of the array grating in one step. The 1550nm DFB-LD has been widely used in 622Mb/s and 2.5Gb/s optical transmission system equipment. The wavelength selection makes DFB-LD the main light source in large-capacity, long-distance optical fiber communications.

Single-chip light sources with integrated multi-wavelength DFB-LD and external cavity electro-absorption modulators on the same chip are also under development. The developed integrated electro-absorption modulator integrated light source uses a multi-QW structure that uses an active layer and a modulator absorber layer. The modulator acts like a high-speed switch that converts the LD output to binary 0s and 1s. 40 different refractive gratings are formed on a single chip, and a 40-channel modulator integrated light source with a wavelength of 1530-1590 nm has a channel spacing of 200 GHz. Its development goal is to integrate 100 emission wavelength LD arrays for 9.5 THz large capacity communications.

VCSEL

The characteristics of the VCSEL (Vertical Cavity Surface Emitting Laser) diode are as follows: A cylindrical beam is emitted from the top of the VCSEL, and the beam can be modulated into a widely used annular beam without the need for asymmetric correction or astigmatism correction, and is easily coupled with the optical fiber; conversion High efficiency, power consumption is only a fraction of that of edge-emitting LD; modulation speed is fast, above 1GHz; threshold is low, noise is small; rehash cavity surface is small, easy to high-density large-scale production and pre-tube Chip detection, packaging, assembly, low cost.

The VCSEL adopts a sandwich structure with only a 20nm, 1- to 3-layer QW gain region in between, and the upper and lower layers are a Bragg reflection layer with a high reflectance of 100% formed by a plurality of epitaxially grown thin films, thereby forming a resonance. Cavity. The coherent laser beam is finally ejected from its top. Many manufacturers have 1550 nm low-loss windows and low dispersion tunable VCSEL sample displays. The 1310nm product is expected to be available in the next 1-2 years. A typical tunable device is a combination of a common 980 nm VCSEL and the reflective cavity of a micro-opto-electromechanical system. A curved top mirror, a gain layer, a reflective mirror, etc., can be used to generate a tunable structure with a center wavelength of 1550 nm, with an electrostatic charge. The control voltage locates the top mirror located on the supporting film. The change of the control voltage can adjust the gap size of the resonant cavity to achieve the purpose of adjusting the output wavelength. Continuously tunable 43nm in the 1528-1560nm range, after 500km transmission through 2.5Gb/s experiments without error, side mode suppression is better than 50dB. If VCSELs with emission wavelengths between 1310-1550 nm can be commercialized, it will further promote the development of optical communications and bring optical networks closer to home. Many companies have published some technical data on the VCSEL prototype at this wavelength.

DBR-LD

The most representative DBR-LD (distributed Bragg reflector laser diode) is a superstructure grating SSG structure. In the center of the device is an active layer with SSG regions formed by refractive gratings on both sides. The periodical interval modulation modulates the reflected spectrum into a comb-like peak. The wavelength of the comb-like spectrum overlaps with a large discontinuous change and can achieve a wide range of wavelengths. Tuning. The wavelength converter is composed of DBR-LD and monolithically integrated with the modulator. The left part of the chip is a bistable laser part with two active areas and one isolation area for saturation absorption; the right side is the wavelength control area. Phase shift area and DBR composition.

The 1550nm multi-redundant function tunable DBR-LD can obtain 16 wavelengths with a frequency of 100GHz or a frequency interval of 50GHz with a frequency of 50GHz, and can obtain a wavelength tuning of about 100nm with a mode hop of about 10nm. In addition to retaining existing processing and packaging processes, nanosecond-level wavelength switches have been added to extend the tuning range.

FG-LD

FG-LD (Fiber-Grating Laser Diode) uses a well-established packaging technology to couple an FG-containing optical fiber with an FP-cavity-coated FP cavity LD to form a tunable external-cavity structure laser, which consists of an LD chip, an air gap, The fiber part of the front end of the optical fiber is composed of an optical resonant cavity between the grating and the outer end face of the LD. The inner surface of the LD is coated with an anti-reflective coating to reduce its FP mode. The FG is used for feedback mode selection. Due to its extremely narrow filter characteristics, the LD operating wavelength will be controlled within the Bragg emission peak bandwidth of the grating, and the strain will be increased by the pressure. Or change the temperature method, tune FG's Bragg wavelength, you can get a wavelength-controlled laser output. FG-LD fabrication and assembly is relatively simple, performance can be compared with DFB-LD, lasing wavelength is determined by the FG Bragg wavelength, it can be fine control, single-mode output power up to 10mW above, less than 2.5kHz line width, more Low relative intensity noise and wide tuning range (50nm) may replace DFB-LD in some areas of optical communication. Experiments have been conducted for signal transmission of 2.5Gb/sx64 channels, and the results are very good.

GCSR-LD

The GCSR-LD (Grating Coupled Reflected Laser Diode) is a wavelength-tunable LD whose structure is from left to right for gain, coupler, phase, and reflector regions, changing their gain, coupling, phase, and reflection. The injected current of each part of the device can change its emission wavelength. The LD wavelength is adjustable in the range of about 80nm, and it can provide wavelengths in 322 wavelength tables recommended by the International Telecommunication Union (ITU-T). Lifetime tests have been conducted.

MOEMS-LD

MOEMS-LD (Micro-Electro-Mechanical-Electro-Mechanical-System Laser Diode) controls the movable surface to set or adjust the physical size in the optical system by means of electrostatic control, and performs horizontal tuning of the light wave. Using the free-space micro-optics platform technology, the cavity position of the F-P cavity is controlled by controlling the position of the cavity mirror to bring about a tunable range of 60 nm. This structure can be used as a tunable optical device as well as semiconductor laser integration to form a tunable laser.

Other types of LD

Optical module Laser diode Built-in MQWF-P cavity LD or DFB-LD, control circuit, drive circuit, output optical signal. Its small size, high reliability, easy to use, in the metropolitan area network, synchronous transmission system, synchronous optical fiber networks are widely used 2.5Gb / s optical transmission module, 10Gb / s, 40Gb / s in the initial phase of the trial to the high-speed , low cost, miniaturized development. Using polymer polymer refractive index changes with temperature characteristics, the heater changes the temperature of the polymer material grating, causing its refractive index and grating pitch changes, so that the reflection wavelength changes. A Polymer-AWG wavelength-tunable integrated module has been developed with 16 wavelength channels, a 200GHz wavelength interval, an insertion loss of 8-9dB, and crosstalk of -25dB. Multi-wavelength LDs that time-modulate each wavelength with a high-speed modulator are in the development stage. This is a brand new multi-wavelength and wavelength programmable light source.

Structural performance

Physical structure

A layer of photoactive semiconductor is disposed between the junctions of the light emitting diodes, and its end surface is polished to have a partial reflection function, thereby forming an optical resonant cavity. In the forward-biased case, the LED junction emits light and interacts with the optical resonator to further excite a single wavelength of light emitted from the junction. The physical properties of the light are related to the material.

In the VCD machine, the semiconductor laser diode is one of the core components of the laser head. It is mostly composed of a double heterostructure, AsALGA ternary compound, and is a near-infrared semiconductor device with a wavelength of 780-. 820 nm, rated power 3 ~ 5 mw. In addition, there is a visible light (eg, red) semiconductor laser diode that is also widely used in VCD machines and bar code readers.

The laser diode includes two parts: the first part is the laser emitting part (which can be represented by LD), its role is to emit the laser, as shown in the electrode (2); the second part is the laser receiving part (which can be represented by PD), its The role is to receive and monitor the laser light emitted by the JD (of course, if you do not need to monitor the output of the LD, PD part can not be used), as shown in the electrode (3); these two parts share the common electrode (1), therefore, the laser The diode has three electrodes.

Laser diodes have the advantages of small size, light weight, low power consumption, simple drive circuit, convenient modulation, resistance to mechanical shock, and shock resistance, but they are extremely sensitive to over-current, over-voltage, and static interference. Therefore, when used, Pay special attention not to make its operating parameters exceed its maximum allowable value. The following methods can be used:

(1) Drive the laser diode with a DC constant current source.

(2) Connect the current limiting resistor in series with the laser diode circuit, and connect the bypass capacitor in parallel.

(3) Since the temperature of the laser diode will increase the current flowing through it, necessary heat removal measures must be taken to ensure that the device operates within a certain temperature range.

(4) In order to avoid breakdown damage due to excessive reverse voltage from the laser diode, fast diodes can be connected in anti-parallel to both ends.

Detection method

(1) Resistance measurement method: Remove the laser diode and measure the positive and negative resistance values with a multimeter R×1k or R×10k. Normally, the forward resistance value is between 20~40kΩ and the reverse resistance value is ∞ (infinity). If the measured forward resistance exceeds 50kΩ, the performance of the laser diode has dropped. If the measured forward resistance value is greater than 90kΩ, it indicates that the diode has been severely deteriorated and can no longer be used.

(2) Current measurement method: Use a multimeter to measure the voltage drop across the load resistor in the laser diode drive circuit, and then estimate the current value flowing through the tube according to Ohm's law. If the current exceeds 100mA, adjust the laser power potential.


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