US20110116521A1 - Tracking injection seeding power based on back facet monitoring (bfm) of an injection seeded laser - Google Patents
Tracking injection seeding power based on back facet monitoring (bfm) of an injection seeded laser Download PDFInfo
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- US20110116521A1 US20110116521A1 US12/620,745 US62074509A US2011116521A1 US 20110116521 A1 US20110116521 A1 US 20110116521A1 US 62074509 A US62074509 A US 62074509A US 2011116521 A1 US2011116521 A1 US 2011116521A1
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- 238000002347 injection Methods 0.000 title claims abstract description 73
- 239000007924 injection Substances 0.000 title claims abstract description 73
- 238000012544 monitoring process Methods 0.000 title claims abstract description 12
- 238000010899 nucleation Methods 0.000 title description 3
- 230000004044 response Effects 0.000 claims abstract description 74
- 238000000034 method Methods 0.000 claims abstract description 35
- 230000006870 function Effects 0.000 claims description 9
- 230000003287 optical effect Effects 0.000 description 16
- 239000004065 semiconductor Substances 0.000 description 7
- 238000012886 linear function Methods 0.000 description 4
- 230000005540 biological transmission Effects 0.000 description 2
- 238000004364 calculation method Methods 0.000 description 2
- 238000007689 inspection Methods 0.000 description 2
- 230000008569 process Effects 0.000 description 2
- 230000008859 change Effects 0.000 description 1
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
- H01S5/068—Stabilisation of laser output parameters
- H01S5/0683—Stabilisation of laser output parameters by monitoring the optical output parameters
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
- H01S5/068—Stabilisation of laser output parameters
- H01S5/06804—Stabilisation of laser output parameters by monitoring an external parameter, e.g. temperature
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- Condensed Matter Physics & Semiconductors (AREA)
- General Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Optics & Photonics (AREA)
- Semiconductor Lasers (AREA)
Abstract
Description
- This is the first application filed in respect of the present invention
- The present application relates generally to controlling injection seeded lasers and, more specifically, to tracking injection seeding power based on Back Facet Monitoring (BFM) of an injection seeded transmitter.
- In the field of optical communications, it is well known to use semi-conductor laser diodes to generate a narrowband optical signal onto which data is modulated for transmission through an optical medium such as an optical fibre link. In order to obtain desired characteristics of the optical signal (such as center wavelength, line width, signal reach, for example) the output power of the laser diode must be maintained within narrow tolerances. Because different laser diodes have different output power characteristics in response to a given driving current, it is desirable to monitor the output power from each laser diode, and adjust the driving current as needed to maintain the output power at a desired level.
FIG. 1 schematically illustrates afeedback control loop 2 for this purpose. - In the
feedback control loop 2 ofFIG. 1 , asemiconductor laser diode 4 is typically constructed as a multi-layer doped semiconductor structure defining a laser cavity in which light is generated in response to adrive current 6 supplied by acontroller 8. In the case of a direct modulation transmitter, thedrive current 6 will comprise a bias current and a modulation current derived from data being transmitted. In the case of an external modulation transmitter, thedrive current 6 will typically comprise only the bias current. Reflective front andback facets front facet 10 is designed to be only partially reflective. Light emitted through the front facet forms theoptical signal 14 onto which data is modulated for transmission. The optical power level of theoptical signal 14 emitted by thefront facet 10 is considered to be the output power of thelaser diode 4. - By contrast, the
back facet 12 is normally designed to be a highly reflective surface, so as to minimize “leakage” of light through theback facet 12, and thereby maximize the output power of thelaser diode 4. However, the leakage of light through theback facet 12 is not zero, so thatback facet light 16 leaks through theback facet 12 of thediode 4. The power level of theback facet light 16 is known to be proportional to the power level of theoptical signal 14 emitted by thefront facet 10. This relationship between laser output andback facet light 16 affords the opportunity to monitor the output power from thefront facet 10 by detecting theback facet light 16. - Typically, a
photodetector 18 is placed proximal theback facet 12 of thelaser diode 4 to detect theback facet light 16 emitted through theback facet 12. Theoutput current 20 of thephotodetector 18 is proportional to the power level of theback facet light 16, and thus is also proportional to the output power of theoptical signal 14 emitted through thefront facet 10 of thelaser diode 4. Thecontroller 8 can then use various techniques known in the art, to control the output power of thelaser 4 by adjusting thelaser drive current 6 based on the monitoredphotodetector current 20. For this reason, the photodetector current 20 may conveniently be referred to as Back Facet Monitoring (BFM) current IBFM. - Back Facet Monitoring is commonly used for controlling non-injection seeded lasers, as described above with reference to
FIG. 1 . It would be desirable to also utilize Back Facet Monitoring to control injection seeded transmitters, including injection seeded lasers and reflective semiconductor optical amplifiers (RSOAs). As may be seen inFIG. 1 b, an injection seededlaser 22 receives aseed light 24, which is used in combination with thedrive current 6 to generate the outputoptical signal 14. However, in this case, theback facet light 16 emitted from theback facet 12 includes a firstoptical component 26 due to the drive current, and a secondoptical component 28 due to theseed light 24. Consequently, theBFM current 20 is highly dependent on the injection seed light power. This raises a difficulty in that the power level of theinjection seed light 24 is unknown, and may change rapidly with time. As a result, conventional BFM techniques cannot be used to control injection seeded lasers. RSOAs suffer the same limitation, and thus cannot be controlled using conventional BFM techniques. - Techniques that overcome the above-noted limitations in the prior art remain highly desirable.
- An aspect of the present invention provides a method of estimating an injection power of seed light injected into an injection-seeded transmitter. A back face monitoring (BFM) response of the injection-seeded transmitter is determined, and data representative of the BFM response stored in a memory. During run-time, a controller of the injection-seeded transmitter detects a temperature of the injection-seeded laser and an instantaneous BFM current. BFM response data is obtained from the memory based on the detected temperature, and the seed light injection power estimated based on the obtained data and the detected instantaneous BFM current.
- Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which:
-
FIGS. 1 a and 1 b schematically illustrate conventional laser feedback control loops known in the prior art; -
FIG. 2 illustrates a representative response surface of an injection seeded laser; -
FIG. 3 illustrates representative BFM response slope curves for an injection seeded laser at a set of different temperatures, in accordance with a first embodiment of the present invention; -
FIG. 4 schematically illustrates principal elements and operations of a feedback control loop for an injection seeded laser, in accordance with an embodiment of the present invention; and -
FIG. 5 illustrates a representative set of normalized BFM response curves for an injection seeded laser for a set of different bias current values, in accordance with a second embodiment of the present invention. - It will be noted that throughout the appended drawings, like features are identified by like reference numerals.
- The present invention provides techniques for tracking injection seeding power based on Back Facet Monitoring (BFM) of an injection seeded transmitter. A representative embodiment is described below with reference to
FIGS. 2-5 . - In very general terms, the injection seed light power by can be tracked (monitored) by first characterising the BFM response of the injection seeded transmitter. This information can be used, during run-time, to determine the respective instantaneous power levels of the injection seed light and the laser output light. This, in turn enables the laser output to be controlled by adjusting the laser bias current. In the following description, two alternative techniques are described. In the first technique described below, the slope of the BFM response is used to estimate the injection seed power level. In the second technique described below, the injection seed power level is estimated from the difference between the BFM response curve for the case of zero seed injection power and the corresponding response curve for the case of a non-zero seed injection power.
- As noted above, in the first technique, the injection seed power level is estimated using the slope of the BFM response curve.
-
FIG. 2 is a chart showing the BFM current as a function of seed injection power and bias current, for a typical injection seeded laser. As may be seen inFIG. 2 a, the BFM current is an approximately linear function of bias current, and a non-linear function of seed light injection power. In addition, the BFM current is a non-linear function of laser temperature, so that the total BFM response of any given laser may be characterised by determining the respective response surface (FIG. 2 ) at each one of a set of laser temperature values. - In accordance with the first technique, the BFM response surface (
FIG. 2 ) is analysed to determine the slope of the BFM response, relative to bias current, as a function of seed injection power. One method for performing this computation is to calculate the partial derivative -
- (where IBFM is the BFM current and IBIAS is the bias current) at each one of a selected set of seed injection power levels. The computed BFM slope values for a desired range of seed light injection power values yields a response slope curve defining the slope of the BFM response as a function of seed light injection power.
FIG. 3 is a chart showing a family of response slope curves generating by repeating the above computation for each one of a set of laser temperature values. - As may be seen in
FIG. 3 , for all temperature values, the response slope increases with increasing injection power, and this effect becomes more pronounced with increasing laser temperature. It is therefore possible to estimate the seed injection power, during run-time, by determining the instantaneous slope of the BFM response, relative to the bias current.FIG. 4 schematically illustrates a representative laser control system implementing this approach. - In the embodiment of
FIG. 4 , the injection seededlaser 22 is constructed and operates in the same manner as described above with reference toFIG. 1 . Accordingly,semiconductor laser 22 is typically constructed as a multi-layer doped semiconductor structure defining a laser cavity in which light is generated in response to a drive current 6 supplied by acontroller 30. Reflective front andback facets front facet 10 is partially reflective and thus emitsoptical signal 14. Theback facet 12 is highly reflective, so as to minimize “leakage” of light through theback facet 12, and thereby maximize the output power of theoptical signal 14. The power level of theback facet light 16 emitted through theback facet 12 is detected by aphotodetector 18 is placed proximal theback facet 12 of thelaser 22. The back facet monitoring (BFM) current 20 output from thephotodetector 18 is supplied to thecontroller 30. In addition, a temperature sensor 32 (such as, for example, a thermocouple) detects the temperature of thelaser 22, and supplies acorresponding temperature signal 34 to thecontroller 30. - As an initial step, the BFM response of the
laser 22 is determined, as described above with reference toFIGS. 2 and 3 , and data representative of a set of BFM slope curves (FIG. 3 ) stored in amemory 36 of thecontroller 30. Since thelaser 22 and its feedback control loop will typically be manufactured and sold as an integrated package, this operation can conveniently be performed by the manufacturer of the laser integrated package, for example as part of product inspection and calibration processes. - During operation of the
laser 22, thecontroller 30 can use the detected temperature of thelaser 22 to select the appropriate BFM slope curve stored in thememory 36. In some embodiments, thecontroller 30 may use the data stored in thememory 36 to compute (e.g. by interpolation) a set of BFM slope values for the detected laser temperature. This approach can be used to obtain BFM slope values for laser temperatures lying between the specific temperature values for which data is stored in thememory 36. - In addition, the
controller 30 can estimate the instantaneous slope of the BFM response, by alternately offsetting the bias current a predetermined amount above and below its present value, and detecting the corresponding changes in the BFM current. Once the instantaneous slope of the BFM response has been estimated in this manner, thecontroller 30 can use the selected (or computed) BFM slope data to estimate the seed injection power. - As may be appreciated, the technique described above enables the seed injection power to be estimated during run time. However, this technique requires that the bias current be repeatedly offset (or dithered), in order to monitor the instantaneous BFM response slope. In some cases, dithering the bias current in this manner may be undesirable. The second technique, which avoids this difficulty, is described below.
- In the second technique, the injection seed power level is estimated from the difference between the BFM response for the case of zero seed injection power and the corresponding response for the case of a non-zero seed injection power. Referring back to
FIG. 2 , it may be seen that, for the case of zero seed injection power, the BFM response is an approximately linear function of bias current. For any given (fixed) value of bias current, increasing the seed injection power from zero produces a corresponding increase in the BFM current. - In accordance with the second technique, the BFM response surface (
FIG. 2 ) is analysed to determine a normalized BFM response to seed injection power, as a function of bias current. One method for performing this computation is to calculate the difference ΔIBFM=IBFM(x)−IBFM(0), where IBFM(x) is the BFM current for a given non-zero seed injection power level and IBFM(0) is the BFM current for zero seed injection power level, for each one of a selected set of bias current levels. Repeating this calculation for each value of bias current yields a family of BFM response difference curves that define the normalized BFM response as a function of seed light injection power.FIG. 5 is a chart showing a normalized BFM response a given laser temperature. The total normalized BFM response can be derived by repeating the above calculations for each one of a set of laser temperature values. - As may be seen in
FIG. 5 , the BFM current increases with increasing seed injection power, and this trend hold true for all values of the bias current. It is therefore possible to estimate the seed injection power, during run-time, by correlating the instantaneous BFM current with the bias current. - As an initial step, the BFM response of the
laser 22 is determined, as described above with reference toFIGS. 2 and 3 , and data representative of a set of BFM difference curves (FIG. 5 ) stored in thememory 36 of thecontroller 30. Since thelaser 22 and its feedback control loop will typically be manufactured and sold as an integrated package, this operation can conveniently be performed by the manufacturer of the laser integrated package, for example as part of product inspection and calibration processes. - During operation of the
laser 22, thecontroller 30 can use the detected temperature of thelaser 22 and the known bias current value the appropriate BFM difference curve stored in thememory 36. In some embodiments, thecontroller 30 may use the data stored in thememory 36 to compute (e.g. by interpolation) a set of BFM difference values for the detected laser temperature and instantaneous bias current. This approach can be used to obtain BFM difference values for laser temperature and/or bias current values lying between the specific values for which data is stored in thememory 36. - The
controller 30 can use the selected (or computed) BFM difference data and the instantaneous BFM current to estimate the seed injection power. For example, for the known bias current value, the BFM current IBFM(0) for zero seed injection power level can be retrieved from memory and subtracted from the instantaneous value of the BFM current 20. The resulting BFM difference value can then be compared to the selected (or computed) BFM difference data from thememory 36 to estimate the seed injection power level. - In the foregoing description, the invention is described by way of example embodiments in which the injection seeded transmitter is a laser. However, the person of ordinary skill in the art will recognise that the same techniques may equally be used to control an injection seeded Reflective Semiconductor Optical Amplifier (RSOA). Thus it will be appreciated that the present invention is not limited to injection seeded lasers.
- The embodiments of the invention described above are intended to be illustrative only. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.
Claims (12)
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US12/620,745 US7940822B1 (en) | 2009-11-18 | 2009-11-18 | Tracking injection seeding power based on back facet monitoring (BFM) of an injection seeded laser |
US13/082,690 US8170074B2 (en) | 2009-11-18 | 2011-04-08 | Tracking injection seeding power based on back facet monitoring (BFM) of an injection seeded laser |
Applications Claiming Priority (1)
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US12/620,745 US7940822B1 (en) | 2009-11-18 | 2009-11-18 | Tracking injection seeding power based on back facet monitoring (BFM) of an injection seeded laser |
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US13/082,690 Continuation US8170074B2 (en) | 2009-11-18 | 2011-04-08 | Tracking injection seeding power based on back facet monitoring (BFM) of an injection seeded laser |
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US7940822B1 US7940822B1 (en) | 2011-05-10 |
US20110116521A1 true US20110116521A1 (en) | 2011-05-19 |
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US12/620,745 Expired - Fee Related US7940822B1 (en) | 2009-11-18 | 2009-11-18 | Tracking injection seeding power based on back facet monitoring (BFM) of an injection seeded laser |
US13/082,690 Expired - Fee Related US8170074B2 (en) | 2009-11-18 | 2011-04-08 | Tracking injection seeding power based on back facet monitoring (BFM) of an injection seeded laser |
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US20170034602A1 (en) * | 2015-07-28 | 2017-02-02 | Fujitsu Optical Components Limited | Optical transmitter and control method |
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CN104717018B (en) * | 2015-03-25 | 2017-07-11 | 青岛海信宽带多媒体技术有限公司 | A kind of optical module |
US10594281B1 (en) | 2019-03-04 | 2020-03-17 | Ciena Corporation | Receiver automatic gain control systems and methods for asymmetrical or unbalanced constellations |
US11552703B2 (en) | 2020-12-09 | 2023-01-10 | Ciena Corporation | Detecting power of low-bandwidth and broad-bandwidth optical signals |
US11404596B1 (en) | 2021-04-20 | 2022-08-02 | Ciena Corporation | Balancing a pair of avalanche photodiodes in a coherent receiver |
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US7991031B2 (en) * | 2009-08-21 | 2011-08-02 | Lg-Ericsson Co., Ltd. | Injection seeded laser ratio loop control |
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Cited By (3)
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US20170034602A1 (en) * | 2015-07-28 | 2017-02-02 | Fujitsu Optical Components Limited | Optical transmitter and control method |
JP2017034310A (en) * | 2015-07-28 | 2017-02-09 | 富士通オプティカルコンポーネンツ株式会社 | Optical transmitter, and control method |
US9941972B2 (en) * | 2015-07-28 | 2018-04-10 | Fujitsu Optical Components Limited | Optical transmitter and control method |
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US20110182307A1 (en) | 2011-07-28 |
US8170074B2 (en) | 2012-05-01 |
US7940822B1 (en) | 2011-05-10 |
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