KR20060051503A - Method and apparatus for distortion control for optical transmitters - Google Patents

Abstract

The method of manufacturing the optical transmitter includes assembling a laser to a module comprising a temperature sensor and a temperature controller. The optimum operating temperature of the laser for minimizing optical distortion is determined at a temperature within a predetermined range, and the distortion is determined at a plurality of temperatures between the predetermined ranges. The temperature controller is selectively adjusted to operate the laser at the optimum temperature. In addition, distortion can be monitored while the laser is actively operating. The temperature controller can optionally be adjusted based on monitoring for distortion to operate the laser at a newly controlled temperature that produces a temperature-dependent minimum distortion in its vicinity.

Optical communications, fiber optics, transmission systems, distortion, laser

Description

Method and apparatus for controlling distortion of an optical transmitter {METHOD AND APPARATUS FOR DISTORTION CONTROL FOR OPTICAL TRANSMITTERS}

1 is a block diagram illustrating a configuration for determining a temperature at which minimum distortion exists in a specific laser transmitter system.

2 is a flow chart for performing a technique of determining a minimum distortion temperature of the laser transmitter system of FIG.

3 illustrates a typical distortion versus temperature curve in a laser transmitter system that can be determined according to a preferred embodiment using the technique of FIG.

4 is a block diagram illustrating a feedback configuration for on-line monitoring of laser distortion to selectively adjust the temperature at which minimal distortion exists.

* Explanation of symbols for the main parts of the drawings

12: light source 14: electro-optical converter

16: temperature sensor 18: temperature controller

20: Distortion Analyzer

FIELD OF THE INVENTION The present invention relates to optical transmitters, and more particularly, to a method of manufacturing an optical transmitter to minimize temperature-dependent distortion characteristics of an optical signal transmitted through an optical fiber.

An alternative to modems that transmit analog or binary digital information over conductors using voltage signals is to use optical (optical) signals over fiber optic cables. Electrical signals (high / low voltages) from analog radio frequency circuits or digital circuits are converted to amplitude modulated or frequency modulated optical signals using LEDs or solid state lasers such as VCSELs or edge-emitting lasers. Can be. Likewise, the optical signal can be converted back to electrical form using a photodiode or phototransistor for input into an amplifier, demodulator, or other form of circuit.

Laser resonators have two distinct types, transverse and longitudinal modes. The transverse mode appears in the cross-sectional intensity profile of the beam. Longitudinal mode corresponds to different resonances along the length of the laser cavity, which occur at different wavelengths within the gain bandwidth of the laser. Mode hopping occurs when the relative intensity at different lines corresponding to different longitudinal modes is shifted under certain circumstances. In order to provide a reliable communication link using an optical transmitter, it is desired to avoid mode hopping in lasers used in such optical communication applications.

One factor that causes mode hopping in a laser is the degree of stability of the laser. There are many forms of stability, including wavelength stability, pulse-pulse energy stability, repetition rate stability, thermal stability, bandwidth stability, and the like, which can be attempted to be controlled in a variety of ways. For example, energy stability and repetition rate stability often depend on the stability of electrical or light energy entering 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 can affect wavelength or bandwidth stability and is typically the heat capacity of the gain medium, and cooling or (and) heating elements, ie temperature controllers, heat exchangers or other thermal monitoring and transfer. It depends on whether the device or the like is provided with a thermal sensor and the thermal control sensitivity these devices exhibit. Various developments have been made to stabilize various parameters of the laser system, including the operating temperature, and to prevent the occurrence of mode hopping.

Transmitting digital information in the form of light from the optical fiber ends can be performed open-air by simply aiming the laser or laser array of the transmitter or transceiver towards a far-field light detector, but beam interference, beam divergence, scattering This makes it difficult to transmit the beam without very significant distortion. One way to avoid the problem of optical data transmission outdoors is to transmit light pulses through the optical fiber. Fiber optics have the advantage of completely avoiding all the problems associated with inductance, capacitance and external interference that interfere with electrical signals, and transmit light beams as copper wires conduct electrons.

Even in a single mode of optical medium, light pulses emitted by LEDs, VCSELs, edge-emitting lasers, etc., which travel short paths through the optical medium, reach the detector faster than light pulses that travel long paths. The material forming the optical medium adds some variation or chromatic dispersion of the group velocity of the different frequencies of the detected signal, resulting in distortion of the amplitude and phase of the signal carrying the information. This happens. This problem is exacerbated as the bandwidth or modulation frequency is widened, and as the total medium length is increased. At long distances, such as in optical communication equipment, even light pulses with very narrow bandwidths result in signals that exhibit undesirable amounts of distortion.

The optical signal transmitted through the optical medium from the modulated laser transmitter is to some extent a single mode laser with a very narrow bandwidth, and even if steps for stabilizing the optical pulse by the conventional method are briefly described as described above. It is known to indicate distortion of. In an effort to reduce distortion in optical signals, electronic pre-distortion circuits have been developed (US Pat. Nos. 6,228,814, 5,798,854, 5,252,930, 5,132,639, incorporated herein by reference). And 4,992,754). Distortion appears at the output of an external cavity laser (ECL), which is an influence of the laser design. In addition, distortion appears even when the laser is linked to the optical fiber, but this is not as noticeable or change much as ECL distortion.

Because of the relative influence of heat on distortion, it may be desirable for the predistortion circuit to be configured to operate depending on the operating temperature of the nonlinear laser. Multiple pre-distorter settings may be provided such that a particular setting, such as, for example, a voltage for controlling the predistorter output, may be selected depending on the monitored temperature of the laser. And, the laser output can be monitored through an optical fiber tap to detect a change in amplitude or phase, and in response, for example, using a temperature sensor and a thermal electric cooler (TEC). The laser temperature itself can be controlled and stabilized to some extent, thus, by adjusting the predistortion circuit, the effect of temperature changes on distortion can be reduced. Because of the substantial effect of distortion on the quality of an optical communication signal, it is necessary, in particular, to control and reduce distortion in optical signals produced by external cavity lasers (including laser chips with external reflectors).

It is known that the degree of distortion of the optical signal transmitted from an external cavity laser depends on the temperature of the laser transmitter. It is also known that distortion typically has a minimum discernible value at a given temperature in a single mode of operation of the laser transmitter, which varies from laser to laser. It is therefore an object of the present invention to provide a construction and manufacturing method for a laser transmitter system for optical communication that is thermally stabilized near the distortion minimum of the laser used.

Although there is undesirable optical signal distortion in a laser transmitter system for communication, such distortion can be reduced or minimized under certain circumstances, in particular by controlling the laser transmitter temperature. The present invention contemplates that the laser light source of the optical transmitter produces a modulated optical signal having optical signal distortion characteristics that depend on the laser operating temperature when the signal is transmitted through a dispersion medium such as air, water, glass, plastic, or the like. . It is also recognized that there is a minimum in the distortion versus laser operating temperature curve of the optical signal transmitter system in the single operating portrait mode of the laser. Then, it is preferable to operate the laser at a temperature at or near the minimum distortion.

Although the distortion curve of the laser transmitter system has a constant temperature dependent minimum distortion in the operating mode, the minimum values of each laser transmitter system do not all appear at the same temperature. That is, the temperature at which the minimum distortion exists in each laser transmitter system is different for each laser system. Once the temperature of the minimum distortion in each laser transmitter system is determined, the system can be operated operatively at that temperature, i. Thermally-induced mode hopping is less likely to occur when the temperature is maintained near the temperature that produces the least distortion, and minimized distortion is also generally recognized as beneficial in laser transmitter systems.

Thus, a method of manufacturing an optical transmitter system is provided. The optical transmitter system includes a laser or laser array for generating a modulated optical signal coupled to and distributed through a distributed optical medium. The laser has a temperature dependent optical signal distortion characteristic at its output. The distortion characteristic has a minimum value in the operating temperature range between the first predetermined temperature and the second predetermined temperature. The first and second temperatures are defined by the optical radiation generated by the laser in different modes when operated at temperatures lower than the first temperature and higher than the second temperature.

The manufacturing method includes assembling the laser device into a module comprising a temperature sensor and a temperature controller. The optimum operating temperature of the laser is determined to minimize distortion at a temperature between the first and second temperatures. The temperature controller is selectively adjusted to operate the laser near the optimum temperature.

The optimum temperature can be determined by initially setting the temperature to a first temperature, a second temperature or another temperature away from the minimum temperature and measuring the distortion. The temperature is then incremented toward another temperature that is not set during the first and second temperatures in the minimum temperature direction, and again the distortion is measured. This increase and measurement is repeated until the temperature range between the first and second temperatures is fully scanned or at least until the minimum value is scanned and determined. The temperature may be stepped or alternatively scanned through a temperature range or at least a temperature at which minimal distortion exists.

It is also recognized that the on-line method can be used alone or in combination with the manufacturing method of the present invention. The method may involve measuring the distortion of the split-off portion of the active laser transmission signal or the distortion of all or part of the temporary off-line transmission signal. To the extent that it is determined that the distortion minimum can be shifted to a new temperature, the temperature controller is adjusted to control the laser temperature at or near the new temperature. A feedback loop can be constructed in which the distortion is measured constantly or periodically and the temperature of the laser is adjusted based on the distortion measurement. The feedback loop may be configured by splitting the beam portion of the laser active transmission signal or inserting a distortion detector in the laser transmission light path for a predetermined time, and then removing the detector from the light path.

Also provided is an optical signal transmission system. The optical module includes a light source that generates light pulses that propagate from the module as modulated optical signals through a light guide. The light source has a temperature dependent optical signal distortion characteristic when the optical signal is transmitted through the light guide, and this characteristic has a minimum value at an optimum temperature within the operating temperature range. The distortion analysis detector analyzes the distortion of the optical signal, including measuring the distortion of at least a portion of the optical signal or other parameter representing it and generating a diagnostic signal based on the measurement. The control module determines the temperature at which the module will be adjusted after analysis. The temperature controller receives a control signal from the control module to maintain the module at or near the determined temperature. According to the plurality of distortion measurements, the initial optimum temperature is approximately determined and set as the operating temperature to be maintained by the temperature controller.

In another embodiment, an optical signal transmission system includes a light source for generating light pulses that propagate as modulated optical signals from a module through a light guide. The light source has a temperature dependent optical signal distortion characteristic when the optical signal is transmitted through the light guide, and this characteristic has a minimum value at an optimum temperature within the operating temperature range. The distortion analysis detector of this embodiment measures the distortion of a portion of the optical signal or other parameter representing it and generates a diagnostic signal based on the measurement, the optical signal during active operation of the optical signal transmission module. Analyze the distortion online. The control module determines the temperature at which the module will be maintained or adjusted according to the analysis. The temperature controller receives a control signal from the control module to maintain the module at or near the determined temperature. According to the distortion measurement, one or more optimum temperatures are determined approximately and set as the operating temperature to be maintained by the temperature controller from time to time during the active operation of the module.

An exemplary configuration for determining the temperature at which minimal distortion exists in an optical transmitter is shown in the block diagram of FIG. The module 2 is shown to have an electrical end 4 and an optical end 6. The electrical cable 8 is coupled to the module 2 at the electrical end 4, and the fiber optic cable 10 is coupled to the module 2 at the optical end 6. The module includes a light source 12 that emits an optical signal based on the electrical input signal. The emitted optical signal is housed by placing the light source very close to the optical end 6 and / or by providing a light guide between the light source 12 and the optical end 6 of the module 2. Guided to optical media such as optical fiber cables. In addition, an electro-optical converter module 14 is shown in FIG.

The optical transmitter module 2 of FIG. 1 is also shown to include a temperature sensor (TS) 16 and a temperature controller (TC) 18. The temperature sensor 16 may be part or a separate component of the temperature controller 18. The temperature controller 18 includes heating and / or cooling elements, such as thermoelectric coolers (TEC), water flow, or other heat flow mechanisms that are understood by one of ordinary skill in the art. The distortion analyzer 20 is also shown in FIG. 1 as being coupled to the other position of the optical medium 10. The distortion analyzer 20 receives an optical signal transmitted through the optical medium 10 from the light source 12 of the module 2. The distortion analyzer 20 may include a power meter, spectrum analyzer, or other measuring device that measures the amount or parameter of an optical signal whose distortion characteristics may be determined by analyzing a plurality of data points or variations thereof. The temperature controller is selectively set to a different temperature within the temperature range near the minimum distortion. When the module is operated or scanned at each of a plurality of temperatures, the distortion analyzer measures the optical signal it receives so that the distortion at each temperature is determined. From the plurality of distortion versus temperature data, the temperature at which the minimum distortion occurs can be determined.

FIG. 2 is a flowchart illustrating steps or operations of performing a technique for determining a temperature at which minimum distortion occurs in the laser transmitter system of FIG. In step S1, the temperature controller 18 of the assembled system of FIG. 1 is set to the first temperature T1 of the boundary of the temperature range including the temperature at which the minimum distortion occurs. This range of temperatures preferably corresponds to the generation of optical radiation in a single mode. When the module is operating at the first temperature T1, a signal is received at the distortion analyzer, a measurement is made, and a determination of distortion is made at step S2 for the optical signal transmitted by the system of FIG.

Then, in step S3, the temperature is stepped or scanned to the second temperature T2 in the direction towards the minimum distortion temperature. In step S4, the distortion is determined at this second temperature T2. The operation indicated in step S5 of FIG. 2 is stepped to a third, fourth, or the like temperature until a distortion data pointer is obtained at a temperature within a range including a temperature where the temperature range is incremented and the minimum distortion occurs. Scanning and judging distortion are repeated.

In step S6, a plurality of data points are analyzed, and as a result, the temperature at which the minimum distortion occurs is determined. This is a manufacturing method, in which the temperature controller is set to maintain the temperature of the module 2 near the determined temperature. In this way, the temperature of the module is controlled such that it changes near the center temperature at which the determination is made to produce a minimum value in the distortion versus temperature curve for the particular laser or LED transmitter module 2.

3 shows the distortion versus temperature curve in a particular transmitter module. It will be appreciated that the transmitter module typically exhibits a curve similar to the curve shown in FIG. 3. However, as shown, the minimum value of the transmitter module will be changed. Two temperatures T1 and T2 are shown at the boundary of the temperature range. Distortion is judged at a temperature between T1 and T2 in a preferred manner. The picture shown can be determined from the data by connecting the data points with a smooth line based on adjacent data points. As long as enough data points are obtained, the minimum distortion can be identified from the data and the temperature at which the minimum distortion occurs. As a result, the temperature controller TC 18 of the transmitter module 2 (FIG. 1) can be set to a temperature T _{min at} which the minimum distortion has been measured and / or determined to occur.

In another embodiment of the present invention, the configuration shown in Figure 1 may be used during the online distortion monitoring process. An active communication process can be done temporarily off-line, for example when the signal quality degrades above a certain tolerance, at a selected time or periodically so that the distortion analyzer 20 can be inserted into the optical path of the laser transmission. Can be done offline. The process of stepping or scanning through a temperature range that includes minimal distortion can be repeated as described above or similarly. To the extent that the optimum temperature at which there is minimal distortion has changed, the temperature controller can be selectively adjusted to that new temperature. The distortion analyzer is removed after monitoring, and the communication process comes back online and continues.

Although not shown in FIG. 1, the distortion analyzer may be configured to communicate directly with the temperature controller or with the control module. The result of the distortion monitoring is preferably processed in this control module. An optimum temperature is determined and a signal is sent to adjust the temperature controller to the original or new optimal temperature if the optimum temperature is different from the current setting.

4 illustrates an alternative to the embodiment of FIG. 1 and the feedback configuration just described. 4 shows a feedback configuration that can be used to monitor distortion and perform an online method of adjusting and controlling the optimum temperature at or near the distortion minimum. The arrangement of FIG. 4 comprises a transmission module 2 comprising an electrical input end 4 and an optical output end 6 for receiving an electrical input signal 8 and for transmitting an optical signal 10, respectively. Module 2 includes a laser, LED, or other light source 12, an electro-optical converter module 14, a temperature sensor 16, and a temperature controller 18. The input signal 8 may be a radio signal, an optical signal, or other signal understood by one of ordinary skill in the art, and power may be supplied in other ways. The laser 12 can be pumped optically, electrically, or in other ways as understood by one of ordinary skill in the art.

A beam splitter 22 is placed in the optical path of the laser transmission to split 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 is to be sent to the distortion analyzer 20. In this way, while the distortion of the laser transmission signal is analyzed, the communication process is still online and can continue to maintain the signal actively transmitted from the optical communication module 2. The signal is preferably transmitted from the distortion analyzer to the control module (CM) 24. The control module 24 may be separate from the optical communication module 2 or may be embedded in the optical communication module 2. The control module 24 determines whether the temperature is an optimal temperature based on the signal received from the distortion analyzer 20. If it is determined that the operating temperature and / or the set temperature is different from the optimum temperature, the control module 24 sends a signal to the temperature controller 18 to adjust the temperature of the laser module 2.

There are many alternative configurations that may be possible and advantageous. For example, distortion may be measured periodically at one or more temperatures on both sides of the set temperature. If the distortion at these temperatures is kept above the distortion at the optimum temperature, no adjustment is made. If the degree of distortion measured at one of these external temperatures is less than the distortion measured at the optimum temperature, then further measurements are taken as a process to determine a new optimum temperature, or a lower optimal distortion is measured at the measured temperature. The temperature may change, and the same process of measuring distortion at points near the new optimal temperature may continue. In another example, an extended range of temperatures may be scanned or stepped while the optical communication process is briefly interrupted for a predetermined or other purpose, and as a result, determination of the optimal temperature may be made irregularly at the appropriate time.

While illustrative drawings and specific embodiments of the invention have been shown and described, it will be understood that the scope of the invention is not limited to the specific embodiments described above. Accordingly, the examples are to be regarded as illustrative rather than restrictive, and should be understood by those of ordinary skill in the art without departing from the scope of the invention and its structural and functional equivalents as set forth in the appended claims. It will be appreciated that variations in can be made.

And, it may be performed according to the present invention and / or embodiments thereof, and in the method described above, the operation is described as an optional sequence for print editing, and this sequence is selected for the convenience of the selected print editing. It is not intended to mean a particular order for carrying out an operation, except as specifically stated or that a person having ordinary skill in the art considers that a particular order be required.

As described above, according to the present invention, a method and a configuration thereof for manufacturing a laser transmitter system for optical communication that is thermally stabilized near the minimum distortion of a laser to be used are provided.

Claims (11)

A method of manufacturing an optical transmitter comprising a laser for generating a modulated optical signal coupled to and transmitted through an optical fiber, wherein the laser has a temperature dependent optical signal distortion characteristic at its output, the characteristic being a first Has a minimum in an operating temperature range between a predetermined temperature and a second predetermined temperature higher, wherein the temperatures are generated in different modes when operated at a temperature lower than the first temperature and higher than the second temperature. Defined by optical radiation, Assembling the laser device to a module comprising a temperature sensor and a temperature controller; Determining an optimum operating temperature of the laser to minimize distortion at a temperature between the first temperature and the second temperature; And Selectively adjusting the temperature controller to operate the laser at the optimum temperature How to include. The method of claim 1, The determining step, Setting the temperature to a plurality of temperatures selected between the first temperature and the second temperature by adjusting the temperature controller; Determining the distortion at each temperature; And Determining a minimum distortion based on the distortion versus temperature data Way. The method of claim 1, The assembling includes coupling a first position of the optical fiber cable to the optical end of the module to connect the light emitted by the laser to the optical fiber cable, The determining includes disposing a light meter for analyzing the distortion of light detected at the second position of the optical fiber cable. Way. The method of claim 1, Operating a distortion-temperature feedback loop during active transmission of the laser, including repeatedly determining the distortion and adjusting the temperature of the laser to substantially minimize the distortion. How to include more. The method of claim 1, Operating the laser at a controlled temperature between a first temperature and a second temperature near a temperature that produces a temperature dependent minimum distortion; Monitoring for distortion of a laser transmission signal of the laser; And Selectively adjusting the temperature controller based on monitoring for the distortion to operate the laser at a newly controlled temperature closer to the temperature that produces the temperature dependent minimum distortion How to include more. In an optical signal transmission system, An optical signal transmission module including a light source; A light guide wherein a light pulse generated by the light source propagates as a modulated optical signal from the module, wherein the light source is configured to generate a modulated optical signal coupled to the light guide and transmitted through the light guide, The light source has a temperature dependent optical signal distortion characteristic having a minimum at an optimum temperature within an operating temperature range; A distortion analysis detector for analyzing the distortion of the optical signal, including measuring a distortion of at least a portion of the optical signal or other parameter indicative thereof and generating a diagnostic signal based on the measurement; A control module for determining a temperature at which the module is to be adjusted after analysis; And A temperature controller for receiving a control signal from the control module to hold the module at or near the determined temperature, wherein, according to the plurality of distortion measurements, the first optimal temperature is approximately determined, Set as the operating temperature to be maintained by the temperature controller; System comprising a. The method of claim 6, The distortion analysis detector is coupled to the control module and is configured to analyze optical signal distortion online while the optical signal transmission module is in operation, the control module based on a signal from the distortion analysis detector, To determine whether to maintain the temperature at the first optimal temperature or to adjust to a new temperature at or near the current optimal temperature system. The method of claim 7, wherein The analysis and judgment are repeated from time to time, and the temperature of the module is maintained at or judged to the determined temperature. system. The method of claim 6, The module further includes an electrical signal connector and an electro-optic signal converter module. system. The method of claim 6, According to the distortion measurement, the system is configured such that one or more optimum temperatures are determined approximately and set as an operating temperature to be maintained by the temperature controller from time to time during active operation of the module. system. The method of claim 10, If the system is configured to automatically increase or decrease the temperature during operation based on a programmed event, the distortion is measured at each temperature and a determination is made as to whether to maintain or adjust the module temperature. system.

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