Monitoring of a laser source with front and rear output photodetectors to determine frontal laser power and power changes over laser lifetime

• US 20050084202A1
• Filed: 10/08/2004
• Published: 04/21/2005
• Est. Priority Date: 10/09/2001
• Status: Active Grant

• Current Assignee: Infinera Corp., Koninklijke Philips Electronics N.V. (Koninklijke Philips N.V.)
• Original Assignee: Infinera Corp.
• Inventors: Nilsson, Alan C., Smith, Robert W.
• Granted Patent: US 7,697,580 B2
• Time in Patent Office: Days
• US Class Current: 385/14
• CPC Class Codes: B82Y 20/00 Nanooptics, e.g. quantum optics or photonic crystals
• CPC Class Codes: G01R 31/31728 Optical aspects, e.g. opto-electronics used for testing, optical signal transmission for testing electronic circuits, electro-optic components to be tested in combination with electronic circuits, measuring light emission of digital circuits probes having electro-optic elements G01R1/071; electro-optic sampling for oscilloscopes G01R13/347; contactless testing of individual semiconductor devices by optical means G01R31/2656
• CPC Class Codes: G01R 31/318511 Wafer Test
• CPC Class Codes: G02B 6/12004 Combinations of two or more optical elements
• CPC Class Codes: G02B 6/12007 forming wavelength selective elements, e.g. multiplexer, demultiplexer
• CPC Class Codes: G02B 6/12011 characterised by the arrayed waveguides, e.g. comprising a filled groove in the array section
• CPC Class Codes: G02B 6/12019 characterised by the optical interconnection to or from the AWG devices, e.g. integration or coupling with lasers or photodiodes coupling integrated waveguide to fibre G02B6/30, to optoelectronic element G02B6/42; monolithic integration of integrated waveguides with other optical elements G02B6/12004
• CPC Class Codes: G02B 6/12021 Comprising cascaded AWG devices; AWG multipass configuration; Plural AWG devices integrated on a single chip
• CPC Class Codes: G02B 6/12023 characterised by means for reducing the polarisation dependence, e.g. reduced birefringence
• CPC Class Codes: G02B 6/12026 characterised by means for reducing the temperature dependence
• CPC Class Codes: G02B 6/12028 based on a combination of materials having a different refractive index temperature dependence, i.e. the materials are used for transmitting light
• CPC Class Codes: G02B 6/12033 characterised by means for configuring the device, e.g. moveable element for wavelength tuning switching G02B6/35; thermo-optic devices G02F1/0147
• CPC Class Codes: G02B 6/131 by using epitaxial growth epitaxial growth for semiconductors H01L21/02365
• CPC Class Codes: G02B 6/136 by etching
• CPC Class Codes: G02B 6/4204 the coupling comprising intermediate optical elements, e.g. lenses, holograms encapsulated active devices H01S5/02208, H01L33/52
• CPC Class Codes: G02F 1/01725 Non-rectangular quantum well structures, e.g. graded or stepped quantum wells
• CPC Class Codes: G02F 1/0175 with a spatially varied well profile, e.g. graded or stepped quantum wells
• CPC Class Codes: G02F 1/01758 with an asymmetric well profile, e.g. asymmetrically stepped quantum wells
• CPC Class Codes: G02F 2203/69 Arrangements or methods for testing or calibrating a device
• CPC Class Codes: H01L 31/125 Composite devices with photosensitive elements and electroluminescent elements within one single body
• CPC Class Codes: H01S 5/0014 : Measuring characteristics or properties thereof measuring techniques per se G01J, G01K, G01N, G01R
• CPC Class Codes: H01S 5/0085 : for modulating the output, i.e. the laser beam is modulated outside the laser cavity
• CPC Class Codes: H01S 5/02325 : Mechanically integrated components on mount members or optical micro-benches
• CPC Class Codes: H01S 5/026 : Monolithically integrated components, e.g. waveguides, monitoring photo-detectors, drivers stabilisation of output H01S5/06
• CPC Class Codes: H01S 5/0262 : Photo-diodes, e.g. transceiver devices, bidirectional devices H01S5/0265 takes precedence
• CPC Class Codes: H01S 5/0264 : for monitoring the laser-output
• CPC Class Codes: H01S 5/0265 : Intensity modulators intra-cavity modulators H01S5/0625
• CPC Class Codes: H01S 5/0268 : Integrated waveguide grating router, e.g. emission of a multi-wavelength laser array is combined by a "dragon router"
• CPC Class Codes: H01S 5/06256 : with DBR-structure
• CPC Class Codes: H01S 5/06258 : with DFB-structure
• CPC Class Codes: H01S 5/0683 : by monitoring the optical output parameters
• CPC Class Codes: H01S 5/06832 : Stabilising during amplitude modulation
• CPC Class Codes: H01S 5/12 : the resonator having a periodic structure, e.g. in distributed feedback [DFB] lasers comprising a photonic bandgap structure H01S5/11; surface-emitting lasers H01S5/18
• CPC Class Codes: H01S 5/1228 : DFB lasers with a complex coupled grating, e.g. gain or loss coupling
• CPC Class Codes: H01S 5/2077 : using lateral bandgap control during growth, e.g. selective growth, mask induced
• CPC Class Codes: H01S 5/22 : having a ridge or stripe structure
• CPC Class Codes: H01S 5/3408 : characterised by specially shaped wells, e.g. triangular
• CPC Class Codes: H01S 5/4031 : Edge-emitting structures
• CPC Class Codes: H01S 5/4087 : emitting more than one wavelength
• CPC Class Codes: H01S 5/50 : Amplifier structures not provided for in groups H01S5/02 - H01S5/30
• CPC Class Codes: H04B 10/2914 : using lumped semiconductor optical amplifiers [SOA]
• CPC Class Codes: H04B 10/50 : Transmitters
• CPC Class Codes: H04J 14/02 : Wavelength-division multiplex systems
• CPC Class Codes: H04J 14/0246 : using one wavelength per ONU
• CPC Class Codes: H04J 14/025 : using one wavelength per ONU, e.g. for transmissions from-ONU-to-OLT or from-ONU-to-ONU
• CPC Class Codes: H04J 14/0258 : Wavelength identification or labelling
• CPC Class Codes: H04J 14/0265 : Multiplex arrangements in bidirectional systems, e.g. interleaved allocation of wavelengths or allocation of wavelength groups
• CPC Class Codes: H04J 14/0276 : using pilot tones
• CPC Class Codes: H04J 14/0282 : WDM tree architectures

Abstract

A power monitoring and correction to a desired power level of a laser or group of lasers utilizes two photodetectors which are employed to accurately determine the amount of output power from the front end or “customer” end of a laser or a plurality of such lasers. During power detection, which may be accomplished intermittently or continuously, the laser is modulated with a tone of low frequency modulation. One photodetector at the rear of the laser is employed to detect the DC value of the frequency tone, i.e., a value or number representative of the AC peak-to-peak swing, amplitude or modulation depth of the tone. Also, the rear photodetector may be employed to determine the optical modulation index (OMI). In either case, these values may be employed in a closed loop feedback system to adjust or otherwise calibrate the value of the low tone frequency relative to the total desired bias current applied to the laser. A front photodetector is employed to receive a portion of the total output of the laser, or of each laser, and the average output power of the laser, or of each laser, is determined from already knowing the optical modulation index (OMI) via the rear photodetector. Thus, by measuring and/or calibrating the laser OMI with the use of a rear photodetector, the average output power from the front end output can be unambiguously determined from detection of the AC peak-to-peak swing or amplitude of the low frequency tone received via the front photodetector.

Claims

1. A method of determining the output power of a laser particularly where the output power from a front end of the laser is not the same as the output power from a rear end of the laser, comprising the steps of:

modulating the laser with a frequency tone superimposed on a bias current to the laser;

detecting the output power with a first photodetector providing s first detection signal;

determining a first optical attribute from the first detected signal;

detecting the output power from the laser front end with a second photodetector providing a second detection signal;

determining a second optical attribute from the second detection signal; and

employing the first and second optical attributes to determine the average output power from the front end of the laser.

2. The method of claim 1 wherein the first and second optical attributes are the same optical attribute.

3. The method of claim 2 wherein the optical attribute is the modulation depth of the frequency tone.

4. The method of claim 3 wherein the modulation depth of the superimposed frequency tone is a predetermined percentage of the bias current applied to the laser.

5. The method of claim 4 wherein the predetermined percentage is in the range of about 0.1% to about 10%.

6. The method of claim 1 comprising the further step of employing the first optical attribute to calibrate the frequency tone modulation depth as a predetermined percentage of the bias current applied to the laser.

7. The method of claim 1 wherein said first optical attribute is the modulation depth of the frequency tone determined from the first detection signal and the second optical attribute is the modulation depth determined from the second detection signal.

8. The method of claim 1 wherein said first optical attribute is the optical modulation index (OMI) of the frequency tone determined from the first detection signal and the second optical attribute is the modulation depth determined from the second detection signal.

9. The method of claim 1 comprising the further steps of:

providing an array of lasers having different operational wavelengths;

modulating each laser with a tone of different frequency;

providing a first photodetector for each of the lasers to provide a first detection signal from each of the lasers; and

following the steps of determining, detecting and employing set forth in claim 1 to determine the average output from the front end of each of the lasers.

10. The method of claim 9 wherein said array of lasers and said first and second photodetectors are discrete electro-optic components.

11. The method of claim 9 wherein said array of lasers and said first and second photodetectors are integrated in a photonic integrated circuit (PIC).

12. The method of claim 9 comprising the further steps of:

providing a second photodetector for each of the lasers to respectively detect their front end output;

following the steps of determining, detecting and employing set forth in claim 1 to determine the average output from the front end of each of the lasers.

13. The method of claim 9 wherein said array of lasers and said first and second photodetectors are discrete electro-optic components.

14. The method of claim 9 wherein said array of lasers and said first and second photodetectors are integrated in a photonic integrated circuit (PIC).

15. The method of claim 9 comprising the further steps of:

providing a plurality of electro-optic modulators to each receive a respective front end output of one of the lasers and together provide a plurality of modulated channel signals; and

positioning the second photodetector to receive a portion of all of the modulated channel signals for determination of the second optical attribute of each of the lasers.

16. The method of claim 15 wherein said arrays of lasers and electro-optic modulators as well as said first and second photodetectors are discrete electro-optic components.

17. The method of claim 9 wherein said arrays of lasers and electro-optic modulators as well as said first and second photodetectors are integrated in a photonic integrated circuit (PIC).

18. The method of claim 17 wherein said photonic integrated circuit (PIC) includes additional electro-optic components in said circuit.

36. The method of claim 19, comprising the further step of providing an array of lasers and consecutively modulating each laser with the frequency tone to determine the respective output power of each laser according to the steps of claim 1.

37. The method of claim 19, comprising the further step of providing an array of lasers and modulating each laser with a different frequency tone to determine the output power of each laser according to the steps of claim 1.

38. The method of claim 19, comprising the further step of concurrently modulating each laser with a different frequency tone to determine the output power of each laser according to the steps of claim 1.

19. A method of achieving a measurement of the output power provided from a front end of a laser, comprising the steps of:

modulating the laser with a frequency tone at a desired laser DC bias level providing a predetermined output power from a laser front end output where the DC value of the frequency tone is a predetermined percentage of the desired laser DC bias level;

measuring the output power from a back end of the laser and providing feedback indicative of the accuracy of the predetermined percentage;

extracting the frequency tone from at least a portion of the laser front end output; and

converting the extracted frequency tone to a value indicative of the tone modulation depth from which the laser front end output power can be determined.

20. The method of claim 19 comprising the further steps of:

determining an optical attribute from the measured output power from the laser back end based upon the frequency tone;

determining the average output power of the laser front end output from the tone modulation depth value determined from the laser front end output and the optical attribute determined from the laser read end output; and

correcting the laser front end output power based upon the determined average output power.

21. The method of claim 20 wherein the optical attribute is the tone modulation depth or the tone optical modulation index (OMI).

22. The method of claim 19, in the step of measuring of laser back end power further comprising the step of providing feedback to a tone modulator to adjust the modulation depth of the frequency tone to be the predetermined percentage of the desired laser DC bias level.

23. The method of claim 19 comprising the further step of providing a first photodetector to receive at least a portion of the laser front end output and thereafter performing the step of frequency tone extracting.

24. The method of claim 23 wherein said photodetector is an avalanche photodiode or a PIN photodiode.

25. The method of claim 23 wherein said photodetector is integrated on a substrate with said laser.

26. The method of claim 23 comprising the further step of providing an electro-optic modulator coupled to receive the laser front end output and coupling output from the modulator for reception by the first photodetector.

27. The method of claim 26 wherein said electro-optic modulator is an electro-absorption modulator or a Mach-Zehnder modulator.

28. The method of claim 26 wherein said electro-optic modulator and said first photodetector are integrated on the same substrate with said laser.

29. The method of claim 23 comprising the further step of providing said first photodetector to receive the laser front end with the photodetector output coupled to an electro-optic modulator.

30. The method of claim 29 wherein said electro-optic modulator is an electro-absorption modulator or a Mach-Zehnder modulator.

31. The method of claim 29 wherein said electro-optic modulator and said first photodetector are integrated on the same substrate with said laser.

32. The method of claim 19, in the step of extracting the frequency tone comprises the step of passing the laser front facet output through a frequency tone passband filter.

33. The method of claim 19, in the step of converting to the frequency tone to a DC value comprises the step of integrating the extracted frequency tone over a period of time to achieve a representative value of the modulation depth of the extracted frequency tone.

34. The method of claim 19 wherein the predetermined percentage of the frequency tone is in the range of about 0.1% to about 10%.

35. The method of claim 19, comprising the further step of utilizing the feedback from the laser back end to adjust the tone frequency modulation depth to the predetermined percentage of the desired DC bias level.

39. The method of claim 19, in the step of converting the extracted frequency tone, the value is indicative of the ratio of the modulation depth over the desired laser DC bias level which is equal to the optical modulation index of the frequency tone.

40. The method of claim 39, in the step of converting the extracted frequency tone to a value indicative of the tone modulation depth, the laser output power is determined from the measured optical modulation index and the converted tone modulation depth value from the extracted from the laser front end output.

41. A system for accurately measuring the power output level from a laser comprising:

a laser having a front end output and a back end output;

means to modulate said laser with a frequency tone;

a first photodetector optically coupled to receive at least a portion of the laser output from said front end output;

a second photodetector optically coupled to receive at least a portion of the laser output from said back end output;

means to employ photodetector output from said second photodetector to calculate a value representative of the optical modulation index (OMI) of the frequency tone; and

means to employ photodetector output from said first photodetector to calculate a value representative of the frequency tone modulation depth in said laser front end output which is employed in conjunction with the optical modulation index (OMI) to determine the front end output power of said laser.

42. The system of claim 41 wherein said first photodetector output means comprises a tone passband filter to isolate said frequency tone and a tone detector to provide an integrated value of the tone over a period of time indicative of tone modulation depth.

43. The system of claim 41 wherein said laser front end output power is determinative from a combination of said fist and second photodetector calculated values.

44. The system of claim 41 wherein said laser is a DFB laser or a DBR laser.

45. The system of claim 41 further comprising an electro-optic component optically coupled to receive the front end output from said laser.

46. The system of claim 45 wherein said first photodetector is positioned in an optical path relative to said laser front end output between said laser and said electro-optic component.

47. The system of claim 45 wherein said first photodetector is positioned in an optical path relative to said laser front end output after said electro-optic component.

48. The system of claim 45 wherein said optical component is an electro-optic modulator, a photodetector, a VOA, a SOA, or a VOA/SOA.

49. The system of claim 45 wherein said laser, said first and second photodetectors and said electro-optic component are integrated on a single substrate.

50. The system of claim 41 wherein said laser and said first and second photodetectors are integrated on a single substrate.

51. The system of claim 41 wherein said first and second photodetectors are avalanche photodiodes or PIN photodiodes.

52. The system of claim 41 wherein said second photodetector is also operated as an attenuator to adjust the output power of said laser front end output.

53. The system of claim 41 further comprising an SOA/VOA optically coupled to receive the front facet output from said laser via said second photodetector;

to SOA/VOA to adjust the power of said laser front facet output.

54. A monolithic transmitter photonic integrated circuit (TxPIC) chip comprising:

an array of N semiconductor lasers integrated on said chip that are each operating at a different wavelength on a standardized wavelength grid and providing a respective front end output;

together which form a plurality of N transmitter signal channels.

an optical combiner integrated on said chip that is optically coupled to receive said channel signals from said electro-optic modulators and combined them into a combined channel output signal on an optical waveguide output from said chip;

an array of N first photodetectors, one for each of said lasers, and optically coupled to receive a rear end output from a corresponding laser;

a least one second photodetector optically coupled to receive a portion of said chip combined channel output signal;

a frequency tone source that sequentially modulates each of said lasers with a frequency tone as superimposed on a bias current applied to each respective laser;

first circuit means that receives photocurrent output from each of said first photodetectors in the presence of the frequency tone modulation of its respective laser to determine a value representative of an optical attribute of said frequency tone; and

second circuit means that receives photocurrent output from said second photodetector to determine a value representative of a modulation depth of said frequency tone so that said optical attribute and said modulation depth value employed to accurately determine a magnitude of front end output power for each of said lasers.

55. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 54 wherein said optical attribute is the modulation depth or the optical modulation index (OMI) of said frequency tone.

56. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 54 wherein said second circuit means comprises a tone passband filter to isolate said frequency tone from said chip combined channel output signal portion and a detector that receives said isolated tone and produces a value representative of the modulation depth of said frequency tone.

57. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 56 wherein each of said first photodetector outputs are respectively employed to calibrate said frequency tone to be a predetermined percentage of total bias current applied to each of said lasers.

58. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 54 wherein each of said first photodetector outputs are respectively employed to calibrate said frequency tone to be a predetermined percentage of total bias current applied to each of said lasers.

59. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 54 wherein said lasers are a DFB laser or a DBR laser.

60. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 54 further comprising an array of N semiconductor electro-optic modulators integrated on said chip that are respectively optically coupled to receive the front end output of a laser to provide a modulated output signal in each signal channel, all of said channel signals together representative of a plurality of wavelengths on the standardized wavelength grid.

61. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 60 wherein a second photodetector is positioned in an optical signal channel relative to said laser front end output between each of said lasers and its corresponding electro-optic modulator.

62. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 60 wherein a second photodetector is positioned in an optical signal channel relative to said laser front end output after each of said electro-optic modulators.

63. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 60 wherein said laser, said first photodetectors and said electro-optic modulators are integrated on a single substrate.

64. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 60 wherein said laser, said first and second photodetectors and said electro-optic modulators are integrated on a single substrate.

65. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 54 wherein said lasers and said first and second photodetectors are integrated on a single substrate.

66. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 54 wherein said first and second photodetectors comprise avalanche photodiodes or PIN photodiodes.

67. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 54 further comprising a plurality of second photodetectors that are integrated on said chip and each respectively positioned in a signal channel to receive the front end output power from a respective laser and that are each further operated as an attenuator to adjust the laser front end to a desired output power level.

68. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 54 further comprising an SOA or VOA or combined SOA/VOA integrated in each of said signal channels and optically coupled to respectively receive the front end output from one of said lasers, said second circuit means providing an output signal to adjust the bias of said SOA or VOA or combined SOA/VOA to provide a desired output power level based on any discrepancy between the determined magnitude of the front end output power for each of said lasers and a predetermined desired magnitude of front end output power.

69. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 54 wherein said frequency tone is employed, in addition to power monitoring and correction with respect to each of said lasers, to also monitor and correct the operational wavelength of each of said lasers via said second circuit means.

70. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 54 further comprising:

a frequency tone source that provides multiple, different frequency tones, one each to modulate a respective laser as superimposed on a bias current applied to each respective laser;

each of said frequency tones representing an identification of each of said lasers.

71. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 70 wherein said optical attribute is the modulation depth or the optical modulation index (OMI) of each of said frequency tone.

72. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 70 wherein said second circuit means comprises a tone passband filter to isolate said frequency tones from said chip combined channel output signal portion and a detector that receives each of said isolated tones and produces a value representative of the modulation depth of each of said frequency tones.

73. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 72 wherein each of said first photodetector outputs are respectively employed to calibrate each of said frequency tones to be a predetermined percentage of total bias current applied to each of said lasers.

74. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 70 wherein each of said first photodetector outputs are respectively employed to calibrate each of said frequency tones to be a predetermined percentage of total bias current applied to each of said lasers.

75. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 70 wherein said lasers are a DFB laser or a DBR laser.

76. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 70 further comprising an array of electro-optical modulators integrated and each optically coupled to receive the front end output from one of said lasers.

77. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 76 further comprising an array of N semiconductor electro-optic modulators integrated on said chip that are respectively optically coupled to receive the front end output of a laser to provide a modulated output signal in each signal channel, all of said channel signals together representative of a plurality of wavelengths on the standardized wavelength grid.

78. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 76 wherein a second photodetector is positioned in an optical signal channel relative to said laser front end output after each of said electro-optic modulators.

79. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 76 wherein said laser, said first photodetectors and said electro-optic modulators are integrated on a single substrate.

80. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 76 wherein said laser, said first and second photodetectors and said electro-optic modulators are integrated on a single substrate.

81. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 70 wherein said laser and said first and second photodetectors are integrated on a single substrate.

82. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 70 wherein said first and second photodetectors comprise avalanche photodiodes or PIN photodiodes.

83. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 70 further comprising a plurality of second photodetectors that are integrated on said chip and each respectively positioned in a signal channel to receive the front end output power from a respective laser and that are each further operated as an attenuator to adjust the laser front end to a desired output power level.

84. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 70 further comprising an SOA or VOA or combined SOA/VOA integrated in each of said signal channels and optically coupled to respectively receive the front end output from one of said lasers, said second circuit means providing an output signal to adjust the bias of said SOA or VOA or combined SOA/VOA to provide a desired output power level based on any discrepancy between the determined magnitude of the front end output power for each of said lasers and a predetermined desired magnitude of front end output power.

85. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 70 wherein said frequency tones are employed, in addition to power monitoring and correction with respect to each of said lasers, to also monitor and correct the operational wavelength of each of said lasers via said second circuit means.

86. The monolithic transmitter photonic integrated circuit (TxPIC) chip of claim 85 further comprising a first set of frequency tones for tagging each of said channel signals relative to power level monitoring and correction of their respective power levels and a second set of frequency tones for tagging each of said channel signals relative to wavelength monitoring and correction of their respective operational wavelengths.