Abstract:
A system and method for correcting non-linearities in the output of a tunable laser over a sweep range. Electromagnetic radiation is directed over a range of wavelengths to a measurement system from the tunable laser source, wherein the measurement system collects data over the range of wavelengths. The electromagnetic radiation emitted over the range of wavelengths is monitored. A non-linearity in one or more wavelengths over the range of wavelengths is determined. A signal is transmitted to the measurement system to cease collecting data when the one or more wavelengths having the non-linearity is output from the tunable laser source or the data is ignored.

Description:
RELATED APPLICATION DATA 
     This application claims the benefit of U.S. Provisional Application No. 61/510,769, filed Jul. 22, 2011, which is hereby incorporated in by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to a system and a method to utilize time-slice-eradication to eliminate the effects of wavelength non-linearities from swept-wavelength testing of optical components for laser sources with non-linearities. 
     BACKGROUND 
     Vernier-tuned Sampled Grating Distributed Bragg Reflector (VTDBR) monolithic tunable lasers are now entering the production phase in telecommunications applications. These tunable lasers are unique in that they offer wide wavelength tuning (e.g., 1525 to 1565 nm) and fast wavelength tuning (e.g., full sweep in microseconds) all on the same monolithic chip. Such a tunable laser will be referred to herein as a Semiconductor Monolithic Tunable Laser Source (SMTLS). SMTLS have a characteristic sweep that is very short. However, the sweep may include potentially impactful non-linearities in the wavelength sweep. These non-linearities could be as short as 100 s of ns, or longer. 
     A typical measurement system begins measurement at the beginning of a wavelength sweep by the SMTLS, and uses the sweep rate to determine the wavelength at subsequent times during the sweep using the formula
 
 WL   a   =WL   start +time a *sweep rate
 
     For this formula to be accurate, the sweep must be linear with time. 
     Alternatively, a swept wavelength meter can be used to determine the wavelength at each point during the sweep. An exemplary wavelength meter may be a model 410 Wavelength Meter manufactured by dBm Optics. 
     There are two effects from short-period non-linearities. First, the measurements made during the period of non-linearity are not representative of the actual results at those predicted wavelengths. Second, the potentially large amplitude variations that can occur during these non-linearities in wavelength (due to relatively high change in loss versus wavelength of a measured device in the vicinity of the non-linearity) can cause the measurements immediately following the non-linearity to be incorrect, due to explicit or implicit filtering of the signal. For example, the signal may jump to a much larger signal during a non-linearity, and even after the non-linearity, the signal may not have settled back to the nominal level due to filter settling time. 
     SUMMARY 
     Due to the unique nature of the non-linearities in an SMTLS, it is possible to have exact information about where these non-linearities will occur during the sweep. This information may be used to change the way the measurement system utilizes its response to avoid the non-linearity errors. 
     One aspect of the invention relates to a method of correcting a non-linearity in a tunable laser source, the method includes: directing electromagnetic radiation over a range of wavelengths to a measurement system from the tunable laser source, wherein the measurement system collects data over the range of wavelengths; monitoring the electromagnetic radiation emitted over the range of wavelengths; determining a non-linearity in one or more wavelengths over the range of wavelengths; and transmitting a signal to the measurement system to cease collecting data when the one or more wavelengths having the non-linearity is output from the tunable laser source. 
     Another aspect of the invention relates to a method for collecting data from a tunable laser source having one or more nonlinearities occurring when the tunable laser source is swept over a range of wavelengths, the method includes: directing electromagnetic radiation from the tunable laser source over the range of wavelengths to a measurement system, wherein the measurement system collects data over the range of wavelengths; determining one or more non-nonlinearities in the collected data; and storing the one or more non-linearities in an electronic storage device communicatively coupled to the measurement system. 
     Another aspect of the invention relates to a system for correcting data based on a non-linearity in a tunable laser source, the system includes: a tunable laser source for directing incident electromagnetic energy over a range of wavelengths; a measurement system for detecting electromagnetic radiation from the tunable laser source; and a storage device communicatively coupled to the measurement system for storing data representative of one or more optical characteristics associated with wavelength of the incident electromagnetic energy detected by the measurement system; and a processor communicatively coupled to the storage device, wherein the processor executes an algorithm for correcting data associated with non-linearities from the tunable laser source. 
     A number of features are described herein with respect to embodiments of the invention. It will be appreciated that features described with respect to a given embodiment also may be employed in connection with other embodiments. 
     The invention comprises the features described herein, including the description, the annexed drawings, and, if appended, the claims, which set forth in detail certain illustrative embodiments. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the annexed drawings: 
         FIG. 1  illustrates an exemplary non-linearity signal effect of a SMTLS. 
         FIG. 2  illustrates an exemplary system in accordance with aspects of the present invention. 
         FIG. 3  illustrates an exemplary analog measurement circuit in accordance with aspects of the present invention. 
     
    
    
     DESCRIPTION 
     Referring to  FIG. 2 , an optical testing system  20  (also referred to as optical measurement system) is illustrated. The optical testing system  20  includes an electromagnetic energy source  21 , a measurement system  22 , a computer  23  and a system output  24 . A device under test (DUT)  25  is placed in the optical testing system  20 , and in operation of the optical testing system the DUT  25  may be characterized. In characterizing the DUT  25  incident electromagnetic energy  20  is provided to the DUT  25 , and output electromagnetic energy  31  from the DUT  25  is detected by the measurement system  22  and may be used to obtain characteristics of the DUT  25 , as will be described further below. 
     If the non-linearities of source  21  is detected, corrections, adjustments, etc., in the data for characterizing the DUT  15  can be made to account for the non-linearities. This capability to the non-linearities and to accommodate errors in the measurement data allows the possibility to use in the measurement system  20  less expensive electromagnetic energy sources. 
     The electromagnetic energy source  21  provides electromagnetic energy over a range of wavelengths. The range may be a continuous range, for example, as in an analog signal that changes continuously from one wavelength to another without discrete steps in between the wavelengths. Alternatively or additionally, the range may include a number of distinct wavelengths that are between respective wavelength values at respective opposite ends of the range. The wavelength range may be the visible range of wavelengths or part of the visible range, may be in the ultraviolet, infrared, or some other range, or may be a combination or part or all of the mentioned and/or other ranges. The range may be continuous or may include discontinuous portions. An exemplary wavelength range is from about 1520 nm to about 1620 nm. One of ordinary skill in the art will readily appreciate that the invention may be used with other wavelengths and wavelength ranges. 
     An example of such an electromagnetic energy source  21  is a SMTLS, and the invention will be described below with respect to a SMTLS that provides a laser beam (sometimes referred to as light but may be other electromagnetic energy)  30  as incident electromagnetic energy to the DUT  25 . It will be appreciated that other types of electromagnetic sources may be used and fall within the spirit and scope of the present invention. For convenience and brevity the electromagnetic energy from the electromagnetic energy source will be referred to as laser beam  30  or simply as light or light beam. The sweep of the SMTLS may be very fast, thus allowing the measurement system  20  to fully examine the power spectrum of light from a DUT in a time frame that is in the second to microsecond (μs) range. 
     The term “swept”, as used herein, means that the SMTLS  21  provides its light output at a number of wavelengths over a range of wavelengths. That the wavelengths are “over” a range or “in” a range does not require that all wavelengths are used in the range, although use of all may be possible, for example, if the optical testing system  20  can use, e.g., produce and detect, all wavelengths in the range. A TLS  21  may have a sweep cycle; it may have periodic operation whereby the light output periodically and repeatedly is produced as light  30  that is swept over such range. 
     The light  30  from the TLS  21  is incident on the DUT  25 , and light  31  from the DUT, e.g., transmitted or reflected thereby, is sensed or detected by the measurement system  22  to provide an output signal on line  32  that is representative of the sensed or detected light  31 . The signal on line  32  may be provided the computer  23  (or to another computer or computational device), and/or to an amplification or other signal conditioning circuit (not shown); and the resulting amplified, conditioned or otherwise adjusted signal on line  33  is provided via the system output  24 . The system output  24  may include the mentioned and/or other amplification, signal conditioning, computational, control, etc. circuitry or may include other circuitry for the purpose of obtaining useful system output information and/or signal for characterizing the DUT  25 . 
     In an exemplary optical testing system  20 , the computer  23  coordinates operation of the SMTLS  21  with the signal on line  33  to the system output  24 . Therefore, as the SMTLS moves, scans, sweeps, etc. through the wavelengths of light it produces, the system output information is representative of characteristics of the DUT  25  as the DUT receives incident light  30  at respective wavelengths. As one example, such coordination between the SMTLS  21  operation and the measurement system  22  output signal on line  32  may be achieved by a signal from the SMTLS provided on line  34  to the computer  23  so the computer is “aware” of the wavelength or other characteristic of the light produced by the SMTLS. As another example, the computer  23  may provide a control signal on line  34   a  to the SMTLS to “direct” or to instruct the SMTLS to produce a given light output. Coordination between the SMTLS  21  and computer  23  also may be based on time, whereby a timing signal may be used to indicate that the SMTLS is starting (or is elsewhere in) its scan cycle or period; and based on what are supposed to be known characteristics of the SMTLS, the wavelength of its output at subsequent times would be expected. Other possibilities for such coordination also are possible. 
     In a first embodiment of a system  20 , the SMTLS  21  is configured to communicate with the measurement system  22 . For example, the SMTLS  22  may communicate with the measurement system  22  to identify when the non-linearities occur in the SMTLS may occur. Analog signals that correspond with the non-linearites may then be interrupted from the measurement system  22 . The interrupted data may be ignored such that it does not impact the analog measurement circuitry, for example. 
     In another embodiment, the tunable laser source (e.g., an SMTLS) can provide a gating signal to the measurement trigger, which may stop monitoring the signal during the occurrence of the non-linearities to avoid having the circuitry negatively affected by the potentially large amplitude changes during the non-linearity. 
     In another embodiment, the settling time of the measurement analog circuitry may be addressed by adding a switched sample and hold circuit to the front end, as illustrated in  FIG. 3 . Referring to  FIG. 3 , a time slice eradication signal is received from the SMTLS at a switch control  42 , which controls operation of switch  44 . When the switch is closed, the measurement system  22  collects data. When the switch is open, for example, when a non-linearity is imminent, data is not collected by the measurement system  22 . Such a control mechanism allows the system to replace the signal input to the measurement circuit  22  with a sampled version prior to the non-linearity and held at that value until the end of the non-linearity. Once the non-linearity is over, then the switch  44  may be closed and the signal is once again connected to the measurement system  22 , for example. Since the variation in the nominal value of the signal would typically be much smaller than the variation during the non-linearity, the resulting settling time would be much smaller. In fact, by controlling the SMTLS appropriately, the value of the wavelength immediately after the eradicated time-slice can be made to be identical to that just prior to it. 
     Although the invention is shown and described with respect to illustrative embodiments, it is evident that equivalents and modifications will occur to those persons skilled in the art upon the reading and understanding hereof. The present invention includes all such equivalents and modifications and is limited only by the scope of the claims if appended hereto.