Tunable laser technology and Its application in optical fiber communication
Publish:Box Optronics  Time:2019-02-24  Views:122
1. Overview
In the field of optical communication, traditional light sources are based on fixed-wavelength laser modules. With the continuous development and application of optical communication systems, the disadvantages of fixed-wavelength lasers are gradually revealed. On the one hand, with the development of DWDM technology, the number of wavelength in the system has reached hundreds. In the case of protection, the backup of each laser must be made by the same wavelength. Laser supply leads to an increase in the number of backup lasers and cost; on the other hand, because fixed lasers need to distinguish wavelength, the type of lasers increases with the increase of wavelength number, which makes the management complexity and inventory level more complex; on the other hand, if we want to support dynamic wavelength allocation in optical networks and improve network flexibility, we need to equip a large number of different waves. Long fixed laser, but the utilization rate of each laser is very low, resulting in waste of resources. To overcome these shortcomings, with the development of semiconductor and related technologies, tunable lasers have been successfully developed, i.e. different wavelength within a certain bandwidth is controlled on the same laser module, and these wavelength values and spacing meet the requirements of ITU-T.
For the next generation optical network, tunable lasers are the key factor to realize intelligent optical network, which can provide operators with greater flexibility, faster wavelength supply speed and ultimately lower cost. In the future, long-distance optical networks will be the world of wavelength dynamic systems. These networks can achieve new wavelength assignment in a very short time. Because of the use of ultra-long-distance transmission technology, there is no need to use regenerator, which saves a lot of money. Tunable lasers are expected to provide new tools for future communication networks to manage wavelength, improve network efficiency and develop next generation optical networks. One of the most attractive applications is reconfigurable optical add-drop multiplexer (ROADM). Dynamic reconfigurable network systems will appear in the network market, and tunable lasers with large adjustable range will be required more.

2. Technical Principles and Characteristics
There are three kinds of control technologies for tunable lasers: current control technology, temperature control technology and mechanical control technology. Among them, the electronically controlled technology realizes wavelength tuning by changing injection current. It has ns-level tuning speed and wide tuning bandwidth, but its output power is small. The main electronically controlled technologies are SG-DBR (Sampling Grating DBR) and GCSR (Assisted Grating Directional Coupled Back Sampling Reflection) lasers. Temperature control technology changes the output wavelength of the laser by changing the refractive index of the active region of the laser. The technology is simple, but slow, narrow adjustable bandwidth, only a few nanometers. DFB (Distributed Feedback) and DBR (Distributed Bragg Reflection) lasers are the main technologies based on temperature control. Mechanical control is mainly based on the technology of micro-electro-mechanical system (MEMS) to complete the wavelength selection, with a larger adjustable bandwidth and higher output power. The main structures based on mechanical control technology are DFB (Distributed Feedback), ECL (External Cavity Laser) and VCSEL (Vertical Cavity Surface Emission Laser). The principle of tunable lasers from these aspects will be explained below. Among them, the current tunable technology, which is the most popular one, is emphasized.
2.1 Temperature Control Technology
Temperature-based control technology is mainly used in DFB structure, its principle is to adjust the temperature of laser cavity, so that it can emit different wavelength. The wavelength adjustment of an adjustable laser based on this principle is realized by controlling the variation of InGaAsP DFB laser working in a certain temperature range. The device consists of a built-in wave-locking device (a standard gauge and a monitoring detector) to lock the CW laser output onto the ITU grid at a 50 GHz interval. In general, two separate TECs are encapsulated in the device. One is to control the wavelength of the laser chip, and the other is to ensure that the lock and power detector in the device work at constant temperature.
The biggest advantage of these lasers is that their performance is similar to that of fixed-wavelength lasers. They have the characteristics of high output power, good wavelength stability, simple operation, low cost and mature technology. However, there are two main drawbacks: one is that the tuning width of a single device is narrow, usually only a few nanometers; the other is that the tuning time is long, which usually requires several seconds of tuning stability time.
2.2 Mechanical Control Technology
Mechanical control technology is generally implemented by using MEMS. A tunable laser based on mechanical control technology adopts MEMs-DFB structure.
Tunable lasers include DFB laser arrays, tiltable EMS lenses and other control and auxiliary parts.
There are several DFB laser arrays in the DFB laser array area, each of which can produce a specific wavelength with a bandwidth of about 1.0 nm and a spacing of 25 Ghz. By controlling the rotation angle of MEMs lenses, the required specific wavelength can be selected to output the required specific wavelength of light.

DFB Laser Array

Another tunable laser based on VCSEL structure is designed based on optically pumped vertical-cavity surface-emitting lasers. Semi-symmetrical cavity technology is used to achieve continuous wavelength tuning by using MEMS. It consists of a semiconductor laser and a vertical laser gain resonator which can emit light on the surface. There is a movable reflector at one end of the resonator, which can change the length of the resonator and the laser wavelength. The main advantage of VCSEL is that it can output pure and continuous beams, and can be easily and effectively coupled into optical fibers. Moreover, the cost is low because its properties can be measured on the wafer. The main disadvantage of VCSEL is its low output power, insufficient speed of adjustment, and an additional mobile reflector. If an optical pump is added to increase the output power, the overall complexity will be increased, and the power consumption and cost of the laser will be increased. The main disadvantage of the tunable laser based on this principle is that the tuning time is relatively slow, which usually requires several seconds of tuning stabilization time.
2.3 Current Control Technology
Unlike DFB, in tunable DBR lasers, the wavelength is changed by directing the exciting current to different parts of the resonator. Such lasers have at least four parts: usually two Bragg gratings, a gain module and a phase module with fine wavelength tuning. For this type of laser, there will be many Bragg gratings at each end. In other words, after a certain pitch of grating, there is a gap, then there is a different pitch of grating, then there is a gap, and so on. This produces a comb-like reflection spectrum. The Bragg gratings at both ends of the laser generate different comb-like reflectance spectra. When light reflects back and forth between them, the superposition of two different reflectance spectra results in a wider wavelength range. The excitation circuit used in this technology is quite complex, but its adjustment speed is very fast. So the general principle based on current control technology is to change the current of FBG and phase control part in different positions of tunable laser, so that the relative refractive index of FBG will change, and different spectra will be produced. By superimposing different spectra produced by FBG in different regions, the specific wavelength will be selected, so that the required specific wavelength will be generated. Laser.
A tunable laser based on current control technology adopts SGDBR (Sampled Grating Distributed Bragg Reflector) structure.

Two reflectors at the front and back ends of the laser resonator have their own reflection peaks. By adjusting these two reflection peaks by injecting current, the laser can output different wavelengths.

The two reflectors on the side of the laser resonator have multiple reflection peaks. When the MGYL laser works, the injection current tunes them. The two reflected lights are superimposed by a 1*2 combiner/splitter. Optimizing the reflectivity of the front-end enables the laser to achieve high power output in the whole tuning range.
3. Industry status
Tunable lasers are at the forefront of the field of optical communication devices, and only a few large optical communication companies in the world can provide this product. Representative companies such as SANTUR based on mechanical tuning of MEMS, JDSU, Oclaro, Ignis, AOC based on SGBDR current regulation, etc., are also one of the few areas of optical devices that Chinese suppliers have fingered. Wuhan Aoxin Technologies Co., Ltd. has achieved core advantages in high-end packaging of tunable lasers. It is the only enterprise in China that can produce tunable lasers in batches. It has batched to Europe and the United States. Manufacturers supply.
JDSU uses the technology of InP monolithic integration to integrate lasers and modulators into a single platform to launch a small size XFP module with adjustable lasers. With the expansion of the tunable laser market, the key to the technological development of this product is miniaturization and low cost. In the future, more and more manufacturers will introduce XFP packaged adjustable wavelength modules.
In the next five years, tunable lasers will be a hot spot. The annual composite growth rate (CAGR) of the market will reach 37% and its scale will reach 1.2 billion US dollars in 2012, while the annual composite growth rate of other important components market in the same period is 24% for fixed-wavelength lasers, 28% for detectors and receivers, and 35% for external modulators. In 2012, the market for tunable lasers, fixed-wavelength lasers and photodetectors for optical networks will total $8 billion.

4. Specific Application of Tunable Laser in Optical Communication
Network applications of tunable lasers can be divided into two parts: static applications and dynamic applications.
In static applications, the wavelength of a tunable laser is set during use and does not change with time. The most common static application is as a substitute for source lasers, i.e. in dense wavelength division multiplexing (DWDM) transmission systems, where a tunable laser acts as a backup for multiple fixed-wavelength lasers and flexible-source lasers, reducing the number of line cards required to support all different wavelengths.
In static applications, the main requirements for tunable lasers are price, output power and spectral characteristics, that is to say, linewidth and stability are comparable to the fixed-wavelength lasers it replaces. The wider the wavelength range is, the better the performance-price ratio will be, without much faster adjustment speed. At present, the application of DWDM system with precision tunable laser is more and more.
In the future, tunable lasers used as backups will also require fast corresponding speeds. When a dense wavelength division multiplexing channel fails, an adjustable laser can be automatically enabled to resume its operation. To achieve this function, the laser must be tuned and locked at the failed wavelength in 10 milliseconds or less, so as to ensure that the whole recovery time is less than 50 milliseconds required by the synchronous optical network.
In dynamic applications, the wavelength of tunable lasers is required to change regularly in order to enhance the flexibility of optical networks. Such applications generally require the provision of dynamic wavelengths so that a wavelength can be added or proposed from a network segment to accommodate the required varying capacity. A simple and more flexible ROADMs architecture has been proposed, which is based on the use of both tunable lasers and tunable filters. Tunable lasers can add certain wavelengths to the system, and tunable filters can filter out certain wavelengths from the system. The tunable laser can also solve the problem of wavelength blocking in optical cross-connection. At present, most optical cross-links use optical-electro-optical interface at both ends of the fiber to avoid this problem. If an adjustable laser is used to input OXC at the input end, a certain wavelength can be selected to ensure that the light wave reaches the end point in a clear path.
In the future, tunable lasers can also be used in wavelength routing and optical packet switching.
Wavelength routing refers to the use of tunable lasers to completely replace complex all-optical switches with simple fixed cross-connectors, so that the routing signal of the network needs to be changed. Each wavelength channel is connected to a unique destination address, thus forming a network virtual connection. When transmitting signals, the tunable laser must adjust its frequency to the corresponding frequency of the target address.
Optical packet switching refers to the real optical packet switching that transmits signals by wavelength routing according to data packets. In order to achieve this mode of signal transmission, the tunable laser must be able to switch in such a short time as nanosecond, so as not to generate too long time delay in the network.
In these applications, tunable lasers can adjust the wavelength in real time to avoid wavelength blocking in the network. Therefore, tunable lasers must have a larger adjustable range, higher output power and millisecond reaction speed. In fact, most dynamic applications require a tunable optical multiplexer or a 1:N optical switch to work with the laser to ensure that the laser output can pass through the appropriate channel into the optical fiber.
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