JP4806559B2 - Method and apparatus for distortion control for an optical transmitter - Google Patents

Description

  The present invention relates to the manufacture of optical transmitters, and in particular to optical transmitters that minimize the temperature dependent distortion characteristics of optical signals transmitted through optical fibers.

  An alternative to modems for transmitting analog or binary digital information by voltage signals on conductors is to use optical (optical) signals on fiber optic cables. Electrical signals from analog radio frequency or digital circuits (high / low voltage) can be converted into amplitude or frequency modulated optical signals using LEDs or semiconductor lasers such as VCSELs or edge emitting lasers. Similarly, an optical signal can be transformed back into an electrical signal by using a photodiode or phototransistor to introduce the input of an amplifier, demodulator, or other type of circuit.

  There are two types of laser resonators that are completely different from each other in the horizontal and vertical directions. The transverse mode appears in the cross-sectional intensity profile of the beam. Longitudinal modes correspond to different resonances along the length of the laser cavity that occur at different wavelengths within the gain frequency bandwidth of the laser. Mode movement occurs when the relative intensity on different lines corresponding to different longitudinal modes shifts under certain circumstances. In order to utilize an optical transmitter to provide a reliable communication link, it is desirable to prevent mode movement in lasers used in such optical communication applications.

  A factor whether or not mode movement within the laser will tend to occur is the degree of laser stability. There are many stability forms, including wavelength stability, inter-pulse energy stability, repetition rate stability, thermal stability, bandwidth stability, and these are controlled in various ways. It is thought that you can try to. For example, energy stability and repetition rate stability often depend on the stability of the electrical or optical energy input to the gain medium. The degree of wavelength or bandwidth stability may depend on the quality of the resonator material and other factors. The degree of thermal stability may affect wavelength and bandwidth stability, and generally the heat capacity of the gain medium and the cooling and / or heating elements, i.e. temperature controller, heat exchanger, or others Depending on whether such a thermal monitor and heat transfer device is provided with a thermal sensor, and how much thermal control sensitivity these devices exhibit. Various developments have been made to stabilize various parameters of the laser system, including operating temperature, and to prevent mode shifting from occurring.

  Transmission of digital information in the form of light from the optical equipment side can be done in the air by simply pointing the transmitter or transceiver laser or laser array to a remote photodetector, but with the beam. It is difficult to transmit such that there is no very large distortion due to interference, beam divergence, scattering, dispersion, or the like. One way to avoid the problem of airborne optical data transmission is to send optical pulses through an optical fiber. The optical fiber will transmit a beam of light so that the copper wire will carry electrons, with the advantage of completely avoiding all the related problems of inductance, capacitance, and external interference that impair the electrical signal.

  Even in the single mode of an optical medium, light pulses emitted by LEDs, VCSELs, edge-emitting lasers, etc. that take a short path through the medium reach the detector earlier than light pulses that take a long path It will be. Depending on the material forming the optical medium, a certain degree of chromatic dispersion or variation in the group velocity at different frequencies is imparted to the detection signal. The result is an amplitude and phase distortion of the information carrier signal. This problem is exacerbated as the bandwidth or modulation frequency increases and the overall length of the medium increases. For very long distances, such as in optical communication equipment, even light pulses with a very narrow bandwidth will result in a signal that exhibits an undesirable degree of distortion.

  As briefly shown above, modulated laser transmission, even with very narrow bandwidth single-mode lasers, and with careful measures taken to stabilize the light pulses by conventional methods It is known that an optical signal transmitted from a machine on an optical medium exhibits a certain degree of distortion. Electronic predistortion circuits have been developed in an effort to reduce distortion in optical signals, for example, US Pat. Nos. 6,288,814, 5,798, incorporated herein by reference. 854, No. 5,252,930, No. 5,132,639, and No. 4,992,754. The distortion appears at the output of the external cavity laser (ECL) and is a function of the laser design shape. Strain also appears when the laser is connected to the optical fiber, but it is not as great as ECL strain and does not change.

  The predistortion circuit can be preferably configured to operate depending on the operating temperature of the non-linear laser because of the large thermal effect on the strain. A plurality of predistorter settings can be provided so that a particular setpoint, eg, voltage, for controlling the output of the predistorter can be selected according to the temperature of the monitored laser. In addition, the laser output is monitored through the fiber tap to detect any change in amplitude or phase, and in response, the laser temperature itself can affect the effects of thermal changes on strain, etc. by adjusting the pre-strain circuit, for example Some control can be achieved with temperature sensors and thermoelectric coolers (TEC). Because of its substantial impact on the quality of optical communication signals, it is desirable to further control and reduce distortion, especially in optical signals generated by external cavity lasers (including laser chips with external reflectors). Yes.

US Pat. No. 6,288,814 US Pat. No. 5,798,854 US Pat. No. 5,252,930 US Pat. No. 5,132,639 US Pat. No. 4,992,754

  It is well known from other investigators that the degree of distortion of the transmitted optical signal in the external cavity laser depends on the temperature of the laser transmitter. It is further known from such studies that strain generally has a minimum value that can be discerned at a certain temperature for a single mode of operation of the laser transmitter that varies with each laser. Accordingly, it is an object of the present invention to provide a method and system configuration for manufacturing a laser transmitter system for optically stable optical communications in the vicinity of its distortion minimum value of the laser being utilized.

  There is undesirable optical signal distortion in the laser transmitter system for communication, which can be reduced and minimized under certain circumstances, especially by controlling the laser transmitter temperature. . The present invention generates a modulated optical signal with optical signal distortion characteristics that depend on the laser operating temperature when the optical transmitter laser source is sent through a dispersive medium such as air, water, glass, plastic, etc. Is to be considered. Furthermore, it has been recognized that there is a minimum in the distortion versus laser operating temperature curve of these optical signal transmitter systems within a single operating longitudinal mode of the laser. It is therefore desirable to operate the laser at or near the temperature at which there is a strain minimum.

  Although the distortion curves of these laser transmitter systems have uniform temperature-dependent distortion minimum values within the operating mode, it is recognized that the minimum values for each laser transmitter system are not all at the same temperature. ing. That is, the temperature at which there is a minimum distortion value for each laser transmitter varies with each laser system. If the temperature of the minimum strain for each laser transmitter system is determined, the system can be kept operative at about that temperature and thus at that minimum strain. When the temperature is maintained at a temperature that results in approximately the distortion minimum, mode transfer caused by heat is less likely to occur, and minimized distortion is also generally recognized as beneficial in laser transmitter systems.

Accordingly, a method for manufacturing an optical transmitter system is provided. An optical transmitter system includes a laser or laser array for generating a modulated optical signal coupled to and transmitted through a dispersive optical medium. The laser has a temperature-dependent optical signal distortion characteristic at its output. The strain characteristic has a minimum value in the operating temperature range between the first predetermined temperature and a second predetermined temperature higher than the first predetermined temperature. The first and second temperatures are formed by generating light radiation in different modes when the laser is operating at a temperature lower than the first temperature and higher than the second temperature.
The manufacturing method includes assembling a laser device on a module including a temperature sensor and a temperature controller. In order to minimize distortion at a temperature between the first and second temperatures, the optimum operating temperature of the laser is determined. A temperature controller is selectively adjusted to operate the laser at approximately the optimum temperature.

  The optimum temperature can be determined by measuring the strain by initializing the temperature to a first temperature or a second temperature or another temperature away from the strain minimum. Next, the temperature is incremented in the direction of the strain minimum towards the other of the first and second temperatures and the strain is measured again. The increment and measurement are repeated until the entire temperature range between the first and second temperatures is scanned, or at least the minimum value is scanned and determined. The temperature is preferably done stepwise through a temperature range or at least through a temperature where there is a strain minimum, and can alternatively be scanned.

  Furthermore, it has been recognized that the on-line method can be advantageously used alone or in conjunction with the manufacturing method of the present invention. The method may involve measuring the distortion of a separate part of the active laser transmission, or some or all of the temporary offline transmission. To the extent that the strain minimum is determined to have changed to the new temperature, the temperature controller is adjusted to control the temperature of the laser to or near the new temperature. A feedback loop can be placed, and the strain is measured continuously or periodically, and the temperature of the laser is adjusted based on the strain measurement. The feedback loop can be set by separating the beam part that is in operation for laser transmission or by inserting a strain detector in the laser transmission path for a predetermined time and then removing it from the path.

  An optical signal transmission system is also provided. The optical module includes a light source that generates a light pulse that propagates from the module through the light guide as a modulated optical signal. The light source has an optical signal distortion characteristic that is temperature dependent when the optical signal is transmitted through the light guide, and such characteristic has a minimum value at an optimum temperature within the operating temperature range. The distortion analysis detector performs an analysis of the distortion of the optical signal, including measuring distortion of at least a portion of the optical signal or another parameter representative thereof, and generating a diagnostic signal based thereon. The control module determines the temperature at which this module is adjusted following analysis. The temperature controller receives control signals from the control module and maintains the module at or near a predetermined temperature. According to a plurality of strain measurements, an initial optimum temperature is approximately determined and set as the operating temperature maintained by the temperature controller.

  In another embodiment, the optical signal transmission system again includes a light source that generates a light pulse that is transmitted from the module through the light guide as a modulated optical signal. The light source has a temperature dependent optical signal distortion characteristic when the optical signal is transmitted through the light guide, and such characteristics are optimal within the operating temperature range including the light source and light guide for transmitting the optical signal. Has a minimum at temperature. The distortion analysis detector of this embodiment includes a step of measuring a distortion of a portion of the optical signal or another parameter representative thereof and generating a diagnostic signal based thereon during active operation of the optical signal transmission module. Analyze online optical signal distortion. The control module determines the temperature at which this module is maintained or adjusted as is according to the analysis. The temperature controller receives control signals from the control module and maintains the module at or near a predetermined temperature. According to the strain measurement, one or more optimum temperatures are approximately determined and set as the operating temperature maintained by the temperature controller from time to time during active operation of the module.

  An exemplary apparatus for determining the temperature at which a distortion minimum exists for an optical transmitter is illustrated in block form in FIG. Module 2 is shown having an electrical terminal 4 and an optical terminal 6. The electrical cable 8 is coupled to the module 2 at the electrical terminal 4, while the optical fiber cable 10 is coupled to the module 2 at the optical terminal 6. The module includes a light source 12 that emits an optical signal based on an electrical input signal. The emitted optical signal is like an optical fiber in a building by placing a light source in the immediate vicinity of the optical terminal 6 and / or by placing a light guide between the light source 12 and the optical terminal 6 of the module 2. To the right optical medium. An electro-optic transducer module 14 is also shown schematically in FIG.

  The optical transmitter module 2 of FIG. 1 is also described to include a temperature sensor (TS) 16 and a temperature controller (TC) 18. The temperature sensor 16 may be part of the temperature controller 18 or may be a separate component. The temperature controller 18 includes heating and / or cooling elements such as a thermo-electric cooler (TEC), water flow, or another heat flow mechanism, as will be appreciated by those skilled in the art. The strain analyzer 20 is also illustrated in FIG. 1 coupled to another location on the optical medium 10. The strain analyzer 20 receives an optical signal transmitted from the light source 12 of the module 2 into and through the optical medium 10. The strain analyzer 20 analyzes the amount or parameter of the optical signal that can be based on a wattmeter, spectrum analyzer, or distortion characteristics for each point or by plotting multiple data points or transforms thereof. Other measuring devices to measure can be included. The temperature controller is selectively set to different temperatures within a temperature range around the strain minimum. As the module is operating at or being scanned through each of the plurality of temperatures, the strain analyzer measures the optical signal it receives so that the strain at each temperature is determined. From the multiple data points of strain versus temperature, the temperature at which the minimum strain occurs can be determined.

  FIG. 2 is a flow diagram illustrating the steps or operations for performing a technique for determining the temperature at which minimum distortion occurs for the laser transmitter system of FIG. In S1, the temperature controller 18 of the assembled system of FIG. 1 is set to a first temperature T1 that is at the boundary of the temperature range that includes the temperature at which the minimum distortion occurs. This temperature range preferably corresponds to the emission of light in a single mode. The signal is received at the distortion analyzer and measured, and at S2, distortion is determined for the optical signal transmitted by the system of FIG. 1 when the module is operating at the first temperature T1. .

  In S3, the temperature is then stepped or scanned through to the second temperature T2 in the direction of the strain minimum temperature. In S4, the distortion at the second temperature T2 is determined. The stage or operation shown as S5 in FIG. 2 is a staged or scanned stage, and the stage of determining strain at third, fourth, and other temperatures is incremented by the temperature range. Is repeated until the data of the strain data point is taken for a temperature in the range including the temperature at which the minimum value of occurs.

  In S6, the plurality of data points are analyzed so that the temperature at which the strain minimum occurs is determined. Since this is a manufacturing method, the temperature controller is set to maintain the temperature of the module 2 at a substantially predetermined temperature. In this way, the temperature of the module is controlled to vary around a central temperature, which is the temperature determined to provide the minimum value in the strain vs. temperature curve for this particular laser or LED transmitter module 2. It will be.

  FIG. 3 illustrates a strain versus temperature plot for a particular transmitter module. It will be appreciated that the transmitter module generally exhibits a curve similar to that shown in FIG. However, as shown, the one or more minimum values of the transmitter module will be different. The two temperatures T1 and T2 are shown as temperature range boundaries. Strain is determined by a preferred method at a temperature between T1 and T2. The plot shown can be determined from the data by connecting the data points with smooth connecting lines based on adjacent data points. As long as enough data points are taken, the minimum distortion can be discerned from the data and the temperature at which it occurs can be determined. Finally, the temperature controller (TC) 18 (see FIG. 1) of the transmitter module 2 sets the predetermined temperature Tmin at which the strain minimum is measured and / or otherwise determined to occur. Can do.

In another embodiment of the present invention, the apparatus schematically described in FIG. 1 can be used during the online strain monitoring process. The active communication process may be performed periodically or temporarily at selected times, such as when signal quality deteriorates beyond a certain acceptable limit, so that the distortion analyzer 20 can be inserted into the optical path of the laser transmission. Can be offline. The process of stepping through or scanning through the temperature range that includes the strain minimum can be repeated as described above or similarly. To the extent that the optimum temperature at which the strain minimum exists is changed, the temperature controller can selectively adjust to that new temperature. After monitoring, the strain analyzer is removed and the communication process is continued back online.
Although not shown in FIG. 1, the apparatus can be configured so that the strain analyzer communicates directly with the temperature controller or with the control module. The results of strain monitoring are preferably processed in this control module. An optimum temperature is determined and if the optimum temperature is different from the current setting, a signal is sent to adjust the temperature controller to the first or new optimum temperature.

  FIG. 4 schematically illustrates an alternative embodiment of the embodiment of FIG. 1 and the feedback configuration described above. FIG. 4 illustrates a feedback configuration that can be used to implement an online method of monitoring strain and adjusting and controlling the optimum temperature at or near the strain minimum. The configuration of FIG. 4 includes a transmission module 2 including an electrical input terminal 4 and an optical output terminal 6 for receiving an electrical input transmission 8 and transmitting an optical signal 10 respectively. Module 2 preferably includes a laser, LED or other optical light source 12, electro-optic conversion module 14, temperature sensor 16, and temperature controller 18. The input signal 8 can be wireless, optical, or otherwise, as will be appreciated by those skilled in the art, and the power 8 can be provided in an alternative manner. The laser 12 can be pumped optically, electrically, or otherwise, as will be appreciated by those skilled in the art.

  The beam splitter 22 is arranged in the optical path of laser transmission to divide the signal into two components. One of these components will be or will be directed to the receiver in the communication process. The other component will be transmitted to the strain analyzer 20. In this way, the communication process can continue to actively transmit signals from the optical communication module 2 while online while analyzing distortion of the laser transmission signal. The signal is preferably sent from the strain analyzer to the control module (CM) 24. The control module 24 can be separated from or incorporated in the optical communication module 2. The control module 24 preferably determines whether the temperature is an optimal temperature based on one or more signals received from the strain analyzer 20. The control module 24 communicates a signal to the temperature controller 18 to adjust the temperature of the laser module 2 when it is determined that the operating and / or set temperature is different from the optimum temperature.

  There are many alternative configurations that are possible and advantageous. For example, strain can be measured periodically at one or more temperature points on either side of the set temperature. As long as the strain at these temperatures remains above that at the optimum temperature, no adjustment is made. If the amount of strain is measured at one of these “external” temperatures, which is less than that measured at the optimum temperature, is another measurement then taken in the process to determine the new optimum temperature? Or the new optimum temperature can be changed to the temperature at which less strain was measured, and then the same process of measuring strain at points around the new optimum temperature is continued. In another embodiment, during the pause or stop of the optical communication process, which may be scheduled or otherwise occur for other purposes, so that irregular optimal temperature determinations can be made at a convenient time. The extended temperature range can be scanned or stepped in.

  While illustrative drawings and specific embodiments of the present invention have been described and illustrated, it is to be understood that the scope of the invention is not limited to the specific embodiments described. That is, the embodiments should be considered as illustrative rather than limiting, and these embodiments depart from the scope of the present invention as set forth in the claims and the structural and functional equivalents thereof. It should be understood that variations can be made by those skilled in the art.

  Furthermore, in the methods described above or in the claims provided that can be carried out in accordance with the present invention and / or preferred embodiments thereof, its operation has been described in or selected from a selected printing order. Provided by other means. However, the order is chosen for printing convenience and so arranged, where a special order is explicitly indicated or a person skilled in the art would need a special order. Is not intended to mean any special order of operation.

FIG. 2 schematically illustrates in block form an apparatus for determining the temperature at which a minimum strain value exists for a particular laser transmitter system. 2 is a flowchart for executing a technique for determining a minimum strain temperature of the laser communication system of FIG. 1. FIG. 3 illustrates a typical strain versus temperature curve for a laser transmitter system that can be determined using, for example, the technique of FIG. 2 in accordance with a preferred embodiment. FIG. 6 diagrammatically illustrates, in block form, a feedback configuration for online monitoring of laser distortion to selectively adjust the temperature at which a distortion minimum exists.

Explanation of symbols

2 Optical transmitter module 4 Electrical terminal 6 Optical terminal 8 Electrical cable 10 Optical fiber cable 12 Light source 20 Strain analyzer

Claims (9)

Coupled to an optical fiber, and a method of manufacturing an optical transmitter including a laser resonator for emitting a laser for generating a modulated optical signal transmitted through it, the laser light of the temperature dependence at its output Having a signal distortion characteristic, said characteristic having a minimum value in an operating temperature range between a first predetermined temperature and a second predetermined temperature higher than said first predetermined temperature, said temperature being said laser resonator There is defined by generating optical radiation in different modes when operating at a temperature higher than the second temperature when operating at the first temperature lower than the temperature,
Comprising the steps of assembling the laser resonator on a module including a temperature sensor and a temperature controller,
Determining an optimal operating temperature of the laser resonator to the distortion to a minimum at a temperature between the first and second temperature,
The method comprising the optimum the laser resonator at the operating temperature to adjust the temperature controller selectively to operate,
A method comprising:
The assembling includes coupling an initial position of the optical fiber to a fiber end of the module to couple the laser emitted by the laser resonator into the optical fiber;
Said determining comprises disposing a strain analysis detector for analyzing the strain of the detected light at a second position of said optical fiber;
Operating the laser resonator at a controlled temperature between the first and second temperatures that is close to a temperature that produces a temperature dependent strain minimum;
Monitoring distortion of a laser transmission signal of the laser;
Selectively adjusting the temperature controller based on monitoring of the strain to operate the laser resonator at a new control temperature that is closer to the temperature that produces the temperature-dependent strain minimum;
The method of further comprising . The determining step includes adjusting the temperature controller to set the temperature to a plurality of selected temperatures between the first and second temperatures, and determining strain at each temperature. the method according to claim 1, characterized in that it comprises a step of determining a minimum value of the distortion based on the data of the strain versus temperature. And a step of adjusting the temperature of the laser resonator in order to substantially minimize the distortion and determining repeatedly the distortion, the distortion in the active transmission of the laser - further the step of operating a temperature feedback loop The method of claim 1, comprising: An optical signal transmission module including a laser resonator ;
An optical fiber in which an optical pulse generated by the laser resonator propagates as a modulated optical signal from the module;
Including
The laser resonator is configured to generate a modulated optical signal that is coupled to and transmitted through the optical fiber , the laser resonator also having a minimum temperature at an optimum temperature within an operating temperature range. Optical signal distortion characteristics
And distortion analyzer detector for analyzing the distortion of the optical signal and at least a portion of the measurement of the strain diagnostic signal based thereon generating the optical signal,
A control module for determining a temperature at which the optical signal transmission module will be adjusted following the analysis;
A temperature controller for maintaining said modules predetermined temperature or near the receiving control signals from said control module,
Further including
An optical signal transmission system, wherein an initial optimum temperature is approximately determined according to a plurality of strain measurements and set as an operating temperature maintained by the temperature controller. The distortion analysis detector is signal coupled to the control module and configured to analyze optical signal distortion online during operation of the optical signal transmission module;
Based on the signal from the strain analysis detector, the control module maintains the temperature of the optical signal transmission module at the initial optimum temperature or brings it to a current optimum temperature or a new temperature closer thereto. Decide whether to adjust,
The system according to claim 4 . 6. The system of claim 5 , wherein the analysis and determination is repeated and the temperature of the optical signal transmission module is maintained at or adjusted to a predetermined temperature. The system of claim 6, wherein the optical signal transmission module further includes an electrical signal connector and an electro-optic signal conversion module. One or more optimum temperatures are approximately determined according to the strain measurement and configured to be set as an operating temperature maintained by the temperature controller during the active operation of the optical signal transmission module. The system according to claim 4 . The temperature is automatically incremented and / or decremented during operation by the temperature controller , the strain is measured at each temperature by a strain analysis detector, and the optical signal transmission is performed by a control module. 9. The system of claim 8 , further configured to make a determination as to whether to maintain or adjust the temperature of the module.

Images