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Function of Lasers in DWDM System

Function of Lasers in DWDM System : Laser, whose function is to generate laser, is an important component of the DWDM system. Currently, the lasers used in the DWDM system are semiconductor laser LD (laser diode). We all know that the operating wavelengths of DWDM systems are relatively dense. Generally, wavelength spacing ranges from several nano-meters to sub-nano-meters. Therefore, laser diodes are required to operate at standard wavelengths and have good stability.

Laser modules in the DWDM system have a wide temperature range for reliable performance in harsh node environments and narrow transmitter designs. It also features less adiabatic chirp to maximize signal quality in shorter and longer lengths of fiber. The excellent inherent linearity of the laser minimizes the degradation of the transmission amplitudes, which is caused by quadrant amplitude modulated (QAM) channels.
Function of Lasers in DWDM System

Lasers of DWDM system major features

These laser perform the non-electrical regeneration distance of DWDM systems is increased from 50~60km of single SDH system transmission to  500~600km. DWDM system lasers are required, so that lasers can be more advanced in technology and have excellent performance to increase the limited distance of dispersion of the transmitting system and remove fiber nonlinear effects (such as Brillouin scattering (SBS), excited Raman scattering (SRS), self-phase modulation) (SPM), cross-phase modulation (XPM), modulation inefficiency and four-wave mixing (FWM).

There are TWO major factors of Laser of DWDN System-
1.Relatively large dispersion tolerance
2.Standard and stable wavelength

Laser Modulation Modes

Currently, optical fiber communication system intensity modulation direct detection system to use wide fiber. There are two types of intensity modulation for lasers, namely direct modulation and indirect modulation.

Direct modulation

This is also called internal modulation,that is  directly modulating the laser and changing the launched lightwave intensity by controlling the injection current.In direct modulation LED or LD sources used in as usual traditional PDH and SDH systems of 2.5Gbit/s or below employ this modulation method.

One character of direct modulation is that the launched power is proportional to the modulation current. It has the advantages of simple structure, low losses and low cost. Since this laser changes the length of the resonant cavity, the variation of the modulation current will cause a linear shift of the emitted laser wavelength corresponding to the current. This variation, called modulation chirp, is actually a type of wavelength (frequency) jitter mandatory for direct modulation sources. Chirp broadens the bandwidth of the laser's emitting spectrum, deteriorating its spectrum characteristics and limiting the transmission rate and distance of the system. Generally, for the conventional G.652, the transmission distance is 100 km and the transmission rate is 2.5Gbit / s. In fact for the DWDM system without optical line amplifier, direct modulated lasers can be considered to save the cost.

Indirect modulation

This method is also called external modulation, i.e. in this system modulating the laser indirectly and adding an external modulator in its output path to modulate the light wave. In fact, this modulator works as a switch as shown in below image. 
Structure of external modulated laser

The constant laser is a highly stable source emitting light continuously with constant wavelength and power. It is not affected by the electric modulation signal during emission, so there is no modulating frequency chirp and the line width of its optical spectrum is minimal.

According to the electric modulation signal, the optical modulator processes highly stable light from continuous laser light in a way that either passes or is blocked. During the modulation process, the spectrum characteristics of the lightweight will not be affected. This guarantees the quality of the spectrum.

Lasers adopting indirect modulation are relatively complex with high loss and cost, but its modulate frequency is very low. It can be used in systems whose transmission rate is 2.5Gbit / s and transmission distance is more than 300km long. Therefore, in DWDM systems with optical line amplifiers, finally the lasers are indirectly modified.

What is commonly used is external modulators are photoelectric modulator, acoustooptic modulator, and waveguide modulator. The basic operating principle of photoelectric modulator is the crystal linear photoelectric effect. The photoelectric effect refers to the phenomenon that the electric field causes a variation of the refractive index of a crystal. A crystal that is capable of producing a photoelectric effect is called a photoelectric crystal.

The acoustic measure is performed using dielectric. The acoustooptic effect refers to the phenomenon that the dielectric changes under the pressure of the acoustic wave when it propagates through the dielectric. This change causes a variation of the refractive index of the dielectric and affects the transmission characteristics of the lightweight.

The waveguide modulator is constructed from titanium (Ti) diffused LiNbO2 substrate material, on which the waveguide is made by photoetching method. It has many advantages such as being small in dimensions, light in weight and new for optical integration.

According to the state of integration and separation of lasers and external modulators, external modulated lasers can be classified into two categories: integrated external modulated lasers and isolated external modulated lasers.

As a mature technology, integrated external modulation becomes the development trend of DWDM laser. A commonly used modulator is an electroserver modulator, small and compact and integrated with lasers, meeting most application requirements in performance.

Electroabsorption modulators, a type of loss modulator, operate at the boundary wavelength of the material absorption region. When the modulator is not biased, the wavelength from the laser is out of the absorption range of the modulator material. The absorption region changes and the wavelength from the laser is within this region. Thus the power launched is minimal and the modulator is closed as shown in the figure below.

Variation of the absorption wavelength of an electroabsorption modulator

Electroabsorption modulators can be manufactured using the same technical process as semiconductor lasers. Therefore, it is easy to integrate laser and modulator suitable for batch production. Hence its growth speed is high. For example, the InGaAsP optoelectronic integrated circuit monolithically integrates a laser and an electroabsorption modulator on a single chip mounted on a chip and thermoelectric cooler (TEC). 

This specific optoelectronic integrated circuit is called an electroabsorption modulated laser  (EML). It can support transmission of 2.5Gbit / s signal over 600 km, which directly crosses the transmission distance of the modulated laser. Its reliability is similar to standard DFB lasers with an average life span of 140 years.

The isolated external modulated laser usually uses a continuous output laser (CW) + LiNbO3 Mach-Zehnder external modulator. This modulator separates the light input into two identical signals that enter the two branches, respectively. These two branches employ electropotic materials, whose refractive index varies with the magnitude of the external electrical signal.

The change of the refractive index of the optical branches will cause a variation of the signal phases. Therefore, when the output end is indicated by recombination of the two branches, the combined optical signal is an interference signal with varying intensity. Through this method, information from the electrical signal is transferred to the optical signal. Thus optical intensity modulation is implemented. The frequency of individual external modulated lasers can be eternally zero. In addition, its cost is relatively low when compared to electroabsorption modulated external lasers.

Wavelength Stability and Control of Laser

In DWDM systems, wavelength stability of lasers is a significant problem. According to ITU-T G.692, the deviation of the central wavelength should not exceed one-tenth (1/5) of the optical channel spacing, that is, the deviation of the central wavelength should not exceed 20GHz in systems with channel spacing of 0.8nm.

Because the optical channel spacing is very small (can be as low as 0.8nm), DWDM systems have strict requirements for wavelength stability of lasers. For example, a 0.5nm variation of wavelength can transfer one optical channel to another. In practical systems, the change must be controlled within 0.2nm. The specific requirement is determined according to the wavelength spacing, that is, the smaller the spacing the smaller the requirement. Therefore lasers should adopt a strict wavelength stabilization technique.

Fine tuning of the wavelength of the integrated electroborption modulated laser is mainly implemented by adjusting the temperature. The temperature sensitivity of the wavelength is 0.08nm / s. The normal operating temperature is 25. By adjusting the chip temperature from 15 to 35, the EML can be set up with an adjustable range of 1.6nm at a specific wavelength. The chip temperature is adjusted by changing the drive current of the cooler and using the thermal resistance as a reaction. Thus the chip temperature is constant and remains at a constant value.

According to the characteristics corresponding to the wavelength and chip temperature, the distributed feedback laser (DFB) controls its wavelength by controlling the temperature of the laser chip to achieve wavelength stability. For a 1.5 m DFB laser, the wavelength-temperature coefficient is about 0.02nm / s and its central wavelength meets the requirement within a range of 15 -35. This temperature feedback control method depends entirely on the chip temperature of the DFB laser. Currently, the MWQ-DFB laser technical process can guarantee that the wavelength deviation meets the requirements of the DWDM system over the life span (20 years) of the laser.

Except the temperature, the laser drive current can also affect the wavelength. The sensitivity is 0.008nm / mA smaller than the effect of temperature in an order. In some cases, its effect is negligible. Additionally, the package temperature can affect the device wavelength (such as the temperature conduction brought by the wires from the system package to the laser platform and the due to the incoming radiation from the package shell will also affect the device wavelength). In a well-designed package, its effect can be controlled to a minimum.

The above methods can effectively solve the short-term wavelength stability problem. However, they are unable to cope with long-term wavelength variation due to factors such as laser aging. It is ideal to use the wavelength sensitive component directly for wavelength response control of the laser. The principle is shown in the figure below. This type of scheme and standard wavelength control of reference frequency disturbances is being developed and developed.
Theory for wavelength control

Last Word

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