Literature Review - Constructing a Computer Interfaced Swept Wavelength Laser

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Introduction The design and construction of swept wavelength lasers depend on the areas of application and hence, the performance and operational bandwidth varies from one design to another. The current generation swept wavelength lasers are designed to provide high performance in terms of high accuracy, less noise generation, repeatability and highly flexible tuning and thus combining factors of mode hop – free, ultra wide and fast. [1] Ideally the swept wavelength lasers are suitable for applications in spectroscopy, testing based on fiber – optics, fiber sensing, metrology, laser seeding etc. One of the notable or significant features in the process of development of a swept wavelength laser is the optimization, which means that by using wide range of options, the system can be built as per requirements. [2] Most of the designed swept - wavelength lasers possess OEM laser source that is of high sweep rate for applications such as spectrum analysis development, OCT, fiber sensor etc. The rate at which the wavelength of laser can be swept is generally in the range of 16 kHz, having a spectral range of about 150 nm and with an optical power output of about 20 MW. Most of the swept - wavelength lasers include a power monitoring unit at the output in order to indicate the instantaneous output power of the laser for every wavelength. Also, the lasers consist of a digital interface through which factors such as chip temperature, driving current and laser average power etc are provided. In order to acquire stability for a longer period, an automatic polarization optimization unit is incorporated in the swept - wavelength lasers. [3] The output from swept - wavelength lasers can be obtained either in polarized or depolarized form. This feature makes the swept - wavelength lasers a flexible and reliable tool used for research in domains of OCT (optical coherent tomography) and fiber sensing etc. Also, few lasers are equipped with some additional features such as VOA (Variable Optical Attenuator) and also sweep monitor with 2 wavelength – trigger signals which identifies the precise starting and ending wavelengths of every wavelength signal. [4] The application of wavelength triggers can be combined with a lookup table of sweep profile in order to indicate the wavelength or absolute frequency of the swept - wavelength laser during wavelength sweep. This combination of characteristics makes it highly feasible in the integration of swept – wavelength laser into OCT (optical coherent tomography) equipment, measurement devices, spectrum analysis equipment, sensor interrogators etc. [5] Problem Statement The design and construction of swept wavelength tuneable lasers are mainly dependent on the areas of application and hence, it is important to understand the application – specific characteristics that must be possessed by swept wavelength tuneable lasers in order to assure high performance in the application area. Thus, the aim of this report is to discuss the basic design and construction of a wavelength - swept tuneable laser and understand clearly its features and applications. Design and Construction of computer interfaced wavelength swept laser A wide range of family of lasers were effectively designed specifically for testing of high – volume of highly efficient and reliable components of WDM and also for amplifier test, which hence give rise to high speed, low noise and high performance and high resolution measurements. The components can be characterized over the entire range of C + L, O, E + S and faster for bands of 980 nm and more specifically for an OSA. [6] The swept - wavelength lasers provide tuning of type mode – hop – free of about 100 nm / sec, open – loop accuracy of about 30 – pm (pico – meter) and rugged type of reliability which is for 24 / 7 and these are available in high – power and low – noise range. Calculations or measurements that specifically require high dynamic range, for instance characterization of gratings of fiber – Bragg, the model extension of the type – L must be chosen. [7] It provides more than 70 – dB ASE and also an integrated – type of dynamic range which basically is greater than about 60 dB. Specifically the applications that require more power, the model extension of the type –H has to be chosen in the swept - wavelength lasers. In order to simplify the operational process, swept - wavelength lasers have been exclusively designed with a language – independent and also icon – based interface for the purpose of easy global deployment. It is important to note that the controls and adjustments are very easy and can be done by any user. The total number of “clicks” has been kept to minimum, so as to help the users or operators to easily and quickly program the swept - wavelength lasers and also steps through a range of wavelengths. Customization of the swept - wavelength laser at the application side is achieved by detaching the control unit from the base unit and hence achieve optimal productivity and ease of convenience. [8] By making effective use of USB, Ethernet, RS – 232, or GBIP interface (also known as IEEE 488) and a PC, the associated parameters of application can be easily set and monitored. The detector input that is present at the back panel of the device helps the user to implement algorithms that are based on digital – control. Signals of analog type that are read through this input are digitalized and are made easily made available through the interface of computer. [9] Figure – 1 show the important characteristics of three variations – C + L, O, E + S Band Lasers.

Figure – 1 important characteristic of three variations – C + L, O, E + S Band Lasers [10]

Figure – 2 shows the applications of laser and their wavelength operational regions.

The design and construction of a swept – wavelength laser is similar to the design of traditional step and measure system. The main components of a computer – interfaced swept wavelength laser are described as follows –

Tuneable source
Optical Photo receivers and
Data Acquisition and Display [10]

The only difference between the swept – wavelength laser and the traditional step and measure system is the particular requirements of photo receivers and source. In the laser of swept – wavelength type, there is continuous tuning of the source using the wavelength range at a desired constant speed. Hence, the data is acquired continuously through the sweep.

Tuneable source: This is the most important component in the system, the source should be able to linearly tune and also mode – hop free over the range of wavelengths. The accuracy or precision associated with the measurement is coupled directly to the laser sweep linearity.
Optical Photo receivers: the main reason behind choosing the optical photo receivers is the speed consideration when compared to the traditional power meters. The user must ensure to choose the optical photo receiver having larger bandwidth in order to handle fast sweep of the laser device [10]
Data Acquisition and Display: data that is received from the optical photo receiver is sent to the Data Acquisition and Display (DAQ) card and is displayed on the user PC (computer). Also, it is possible to directly display the outputs of photoreceivers on a multi – channel oscilloscope. [10]

Tuneable laser content detail:

The cavities of laser in the laser modules and external – cavity diode laser (ECDL) systems are the outcomes achieved after a lot of experimentation in designing and developing tuneable lasers. The benefit of approach followed in building the tuneable laser is visible in the measurements and tests conducted which show the precision and accuracy of the tuneable laser. Another noticeable benefit of tuneable laser over the regular diodes are that unlike the regular diodes tuneable lasers do not have large linewidths or low coherence. To make sure that there is no low coherence problem, there is a coating of Anti – reflection that leads diode to behave as gain elements. Further the tuneable laser can be positioned in an external cavity which has optics with wavelength select options hence resulting in single mode lases at a given point in time. [10]

Achieving wide wavelength tuneable property using robust AR (Anti – reflection) coating:

Actual single – mode tuning necessitates the external optics influences optical feedback unlike commonly followed reflection from diode facet. By the use of a very unique AR (Anti – reflection) coating the reflectivity of the residual diode can be reduced below 0,001 which in turn ensures proper single – mode operation. This approach leads to achievement of low reflectivity over wide wavelength ranges by the use of any obtained single – mode diode laser. In addition unlike regular external – cavity diode laser (ECDL) systems whose lifetime is limited by the amount AR (Anti – reflection) coating, the unique method explained above makes the coating last much longer. [11]

Role of precision mechanics in tuning of Mode – Hop free type:

By the application of tweaked Littman – Metcalf configuration on the external laser cavity, the coating of the diode is performed. The dispersion for the single – mode working is made available by introduction of tuning element and grazing - incidence diffraction grating. Also mode – hop – free tuning is enabled due to the design of the external cavity. The wavelength diffraction into the cavity can be altered by rotating tuning element, which is a modification of the Littman – Metcalf configuration. In order to avoid hopping, the length of the cavity must be maintained at a non-varying number of wavelengths as and when the laser tunes. In order to make lasers with no mode hops possible it is essential to position pivot around the tuning element. The pivot point positioning should be of the magnitude of sub – micron precision.

Control of the environment for narrow line - width:

After implementing the single – mode laser diode in the method described above by altering the optics of the external cavity, the cavity temperature and acoustic coupling can be changed to influence the line - width of the laser. The actions above help variate the electrical – noise coupling and alteration of the cavity length which in turn cause alteration in refraction index and piezo length of the diode. The design of the laser is aimed at controlling feature current source with fewer than 100-nA, and noise in the bandwidth of 1 – MHz. [11]

Design of the cavity for continuous reliability:

The design of the laser to be manufactured and mechanics behind construction of the optics with AR (Anti – reflection) coating and other cavity design considerations can sustain environmental conditions that are not smooth. The laser is designed to withstand vibration, shock that can work in conditions that are 80% higher than normal humidity. [11] IV CHARACTERISTICS OF SWEPT WAVELENGTH LASER Following are some of the important characteristics of swept - wavelength lasers:

Absolute wavelength accuracy – it is defined as the maximum difference that exists between the calculated wavelength and displayed wavelength of the system of laser
ASE – it stands for Amplified Spontaneous Emission and it is the ratio of factor of optical power at the laser line width center to the optical power that s measured from a certain distance, that is calculated by making use of an optical spectrum analyzer with a particular resolution bandwidth [12]
Coarse – tuning resolution – it is the smallest wavelength that can be modified by making use of coarse – tuning type of DC motor on the swept - wavelength laser
Current – modulation bandwidth – it is defined as the highest rate at which the current of laser diode can be changed effectively. Basically, current – modulation bandwidth is the 3 dB frequency of the input of direct modulation that is located at laser head
Modulation bandwidth of fine – frequency type – it is defined as the highest rate at which the process of fine tuning PZT in the laser cavity can modulate the frequency of laser. The particular bandwidth is for a drop of about 3 dB, from a baseline of low frequency range under the modulation of small signal [12]
Tuning range of fine – frequency type – it is defined as the range of frequency over which the tuning of laser can be carried out piezoelectrically
Integrated dynamic range – basically it is defined as the ratio of signal to emission of source, that is integrated over the range of all wavelengths. This is generally calculated by carefully observing the spectrum of cascaded type of fiber – Bragg gratings that possess a total rejection ratio that is greater than 100 dB and having a window of 0.8 nm.

Linewidth – it is defined to be the frequency stability of the laser for only a short term. It is measured by making use of a heterodyne beatnote that is recorded over the interval of 50 – ms. The variation of line width varies as a function of integration of time [13]
Maximum speed of coarse – tuning – it is defined as the highest speed at which the laser can be tuned by making use of coarse – tuning type of DC motor. There can be slight variations in the maximum coarse tuning speed
Few other important characteristics of swept - wavelength lasers include minimum power usage, power repeatability, power stability, Side – mode type of Suppression ratio, the tuning range, tuning speed typical maximum power, wavelength resolution, wavelength repeatability, wavelength stability etc.
Ultrafast tuning at about 2, 000 nm / sec – wide range of tuning of about 2000 nm / sec is provided in order to enable measurement capabilities in the linear time having excellent linearity

Ultra wide tuning of about 110 nm of Mode – Hop – Free type: the swept - wavelength lasers provides a single – mode and also mode – hop – free tuning across the complete specified wavelength range.
Low noise for measurement and high dynamic range test – measurements that specifically require high dynamic range, such as characterizing the gratings of fiber – Bragg, low – noise version must be chosen. Swept - wavelength lasers offers more than 70 dB ASE suppression and about 55 dB of integrated dynamic range

Also, various options available on multiple integration – with wide variety of integrated options, a flexible and reliable system is configured to the specific requirements of the application and its relevant domain. In order to acquire high precision and accurate absolute wavelength, there is availability of an option of precision wavelength reference, which is of about less than 1 pm (pico – meter). Thus, the choice of polarization controller can be made in order to set up about six states of polarization. Also, there is availability of an option called variable optical attenuator that is also available for about 20 dB modification in the optical output power. Specifically these options are made available for the swept - wavelength lasers of SM versions only.
Flexible and Simple operation – in order to simplify the operation of Swept - wavelength lasers, its design was modified in such a way that a simple operational touch screen is included. [14] Considering the current advances in remote interfaces, that include USB, GPIB (IEEE 488) and Ethernet, it is quite easy and flexible to interface the PC to monitor and set necessary parameters. Use of touch screen enable setting and monitoring the laser parameters that include sweep speed, tuning range, output power and sweep mode etc. By making use of control buttons on the device, one can use the option called all – button operation to set the parameters of laser and its related options as an option to touch screen. [15]

V. APPLICATIONS Following are some of the important applications of swept – wavelength lasers
- Swept – wavelength lasers finds wide range of applications in the field of Metrology, Spectroscopy etc
- Also, Fiber – Bragg sensing and Telecom test and measurement are few other application areas of swept – wavelength lasers [16]

CONCLUSION

By the alteration of the design and manufacture of the regular diodes swept - wavelength system can be built. The source is chosen in a way that it can be tuned linearly and the wavelength is mode – hop free. Power sensors and photo receivers are made use for speed. The techniques explained above help in building effective Swept-Wavelength Tunable Laser Systems.

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