WO2003010866A2

WO2003010866A2 - Calibration of voltage-driven current drivers for a multi-section semiconductor laser

Info

Publication number: WO2003010866A2
Application number: PCT/EP2002/005053
Authority: WO
Inventors: Neal O'gorman
Original assignee: Tsunami Photonics Limited
Priority date: 2001-07-24
Filing date: 2002-05-08
Publication date: 2003-02-06
Also published as: AU2002312884A1; WO2003010866A3

Abstract

A method and apparatus for precisely calibrating a digitally controllable current driver (10,100) is described. The current driver to be calibrated is used to drive a semiconductor laser (30,300) and an optical parameter of laser light emitted from the semiconductor laser is measured. The current driver is replaced by a calibrated current source (70, 700) to drive the laser so that the laser emits laser light with the previously measured optical parameter, and the current supplied by the calibrated current source recorded thereby providing a calibration point for the current driver. The invention has particular applicability to current drivers for driving tunable multi-section semiconductor lasers.

Description

Calibration of voltage-driven current drivers for a multi-section semiconductor laser

This invention relates to the field of calibration of voltage-driven current drivers. The invention has particular applicability to the calibration of current drivers for a multi-section semiconductor laser.

There are many applications where digital data in the form of a voltage is converted to an analogue voltage in order to interface to an electronic device that requires a drive current to make it function. One such device is a light-emitting device such as a LED; another is a semiconductor laser and there are many others. In some cases the drive current must be known accurately in milliamps with a precision of microampere order. A multi-section semiconductor laser is an example where two, three, four or more current drivers are required in parallel, one for each section, for wide tunability.

These currents may not be determined with an ammeter, because the insertion of an ammeter disrupts the circuit and changes the value of the drive current. Moreover, a calibrated current source cannot satisfactorily be used in the calibration of such lasers, as described in, for example WOOl/28052, since calibrated current sources cannot be rapidly swept through values of drive current as required in the calibration to generate a voltage look-up table for operating the laser in positions of stability away from mode boundaries.

It is an object of the invention at least partially to ameliorate the foregoing disadvantages.

According to a first aspect of the present invention there is provided a method of calibrating a digitally controllable current driver circuit comprising the steps of a) driving a semiconductor laser with the current driver circuit to be calibrated; b) measuring an optical parameter of light emitted by the semiconductor laser; c) replacing the current driver circuit with a calibrated current source; d) driving the semiconductor laser with the calibrated current source to emit light having the measured optical parameter; and e) determining a corresponding current provided by the calibrated current source.

Conveniently, step a) comprises driving a multi-section semiconductor laser with current driver circuits to be calibrated; step b) comprises measuring optical frequency of light emitted from the laser; and step d) comprises driving the multi- section semiconductor laser with the calibrated current sources to produce light of the same optical frequency.

Advantageously, step a) comprises the additional step of driving the laser with an auxiliary current source to bring the laser to threshold operation.

Conveniently, step b) comprises providing a photodiode and using the photodiode to measure optical power of light emitted by the semiconductor laser.

Advantageously, step b) comprises providing a thermal detector and using the thermal detector to measure optical power of light emitted by the semiconductor laser.

Conveniently, step b) comprises providing a wavemeter and using the wavemeter to measure the optical frequency of light emitted by the laser.

Advantageously, step b) comprises providing an interferometer and using the interferometer to measure the optical frequency of light emitted by the laser.

Preferably, the drive circuit to be calibrated is drivable by an analogue electric circuit.

According to a second aspect of the invention there is provided an apparatus for calibrating a digitally driven current driver circuit, the apparatus comprising a semiconductor laser for being driven by the circuit to be calibrated, measuring means for measuring an optical parameter of light emitted by the semiconductor laser driven by the circuit to be calibrated, and a calibrated current source for selectively driving the semiconductor laser to produce light having precisely the same optical parameter as when driven by the circuit to be calibrated and means for determining the current supplied by the calibrated current source.

Conveniently, the semiconductor laser is a multi-section semiconductor laser drivable by the circuit to be calibrated, and the measuring means is for measuring an optical frequency of light emitted from the laser.

Advantageously, the apparatus further comprises an auxiliary current source for bringing the laser to threshold operation.

Conveniently, the measuring means comprises a photodiode for measuring optical power.

Alternatively, the measuring means comprises a thermal detector for measuring optical power.

Conveniently, the measuring means comprises a wavemeter.

Advantageously, the measuring means comprises an interferometer.

Preferably, the drive circuit to be calibrated is driven by an analogue electric circuit.

Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 is a schematic diagram of apparatus according to a first embodiment of the invention;

Figure 2 is a schematic diagram of apparatus according to a second embodiment of the invention;

Figure 3 is a flowchart of a method used to determine back and front grating current calibrations in the second embodiment of the invention of Figure 2;

Figure 4 is a flowchart of a method used to determine phase section current calibrations in the second embodiment of the invention of Figure 2; Figure 5 is a diagrammatic representation of a four section semiconductor laser used in the embodiment of the invention of Figure 2; and

Figure 6 is a graphical representation, helpful in understanding the method of Figure 3, of drive currents applied to a multi-section laser, showing currents supplied to a back grating section of the laser as abscissa and currents applied to a front grating section of the laser as ordinates.

In a first embodiment of the invention, illustrated in Figure 1, a current driver 10 to be calibrated is controllable by a voltage digital/analogue converter 20. The current driver 10 is arranged to drive a semiconductor laser such that laser light emitted by the semiconductor laser 30 is incident on a photodiode 40. Preferably the semiconductor laser has a large output light cone and the photodiode is a large area photodiode for ease of coupling laser light from the semiconductor laser 30 to the photodiode 40. A power meter 50 is electrically connected to the photodiode 40 for measuring power generated by the photodiode when laser light from the semiconductor laser 30 is incident on a photoreceptor thereof. An auxiliary current source 60 may be electrically connected to the semiconductor laser 30 to supply a current insufficient to cause the semiconductor laser to emit laser light, such that only an incremental current is required from the current driver 10 in order to cause the laser to emit. A calibrated current source 70, capable of providing constant currents of various values, the instantaneous value of which may be precisely determined, is provided selectively to drive the semiconductor laser 30 in place of the current driver 10 controlled by the voltage digital/analogue converter 20.

In use, a digital voltage V_D is supplied to the voltage digital/analogue converter

20 whereby the digital voltage V_D is converted to an analogue voltage V_A- The digital voltage V_D may, for example, be supplied from a look-up table (LUT). The analogue voltage V_A is used to drive the current driver 10 to be calibrated and a resultant current U output from the current driver 10 is used to drive the semiconductor laser 30, causing the laser to emit light of a frequency and power dependent on the value of the resultant current !__. The optical power Op of the emitted laser light incident on the photodiode 40 from the semiconductor laser 30 determines an electrical power Ep generated by the photodiode 40 and measured or otherwise recorded by the power meter 50. The current driver 10 and voltage/analogue converter 20 are replaced by the calibrated current source 70 to drive the semiconductor laser 30, keeping the semiconductor laser 30 and the photodiode 40 in the same relative positions and configuration. The calibrated current source 70 is adjusted until the output optical power Op of the semiconductor laser 30 as indicated by the generated electrical power Ep measured by the power meter 50 electrically connected to the photodiode 40 is the same as that recorded using the current driver 10 to be calibrated. The current emitted by the calibrated current source is determined and recorded in order to calibrate the current U emitted by the current driver 10 as a function of the applied digital voltage VD for the combination of voltage digital/analogue converter 20 and current driver 10.

This calibration can be achieved with high accuracy and repeatability as the semiconductor laser 30 delivers microwatts of power with precision to the photodiode 40 as indicated by the power meter 50.

A second embodiment of the invention, illustrated in Figure 2, has application for tuneable multi-section semiconductor lasers. A tuneable multi-section semiconductor laser 300, which has been calibrated using, for example, the method described in WOOl/28052 to provide a digital voltage look-up table 201 of digital voltages for driving the laser 300 at different optical frequencies, is driven by a current driver 100. A separate current may be provided by the current driver 100 to each of the sections of the multi-section laser 300 to cause the multi-section semiconductor laser to emit laser light of a predetermined frequency. The current driver 100 is controlled by a voltage digital/analogue converter 200. The frequency of the laser light emitted by the multi-section semiconductor laser is measurable by a wavelength meter 500.

An auxiliary current driver 600 may be provided to supply current(s) to the semiconductor laser 300 insufficient to cause the semiconductor laser to emit laser light, such that only incremental current(s) are required from the current drivers 100 to cause the laser to emit. Use of the second embodiment is in principle similar to that of the first embodiment. Digital voltages stored in the look-up table 201 for the multi-section laser 300 are converted by the digital/analogue converter 200 into analogue voltages to drive the current drivers 100 to supply corresponding respective currents to each of the sections of the multi-section semiconductor laser 300. The frequency of laser light emitted from the multi-section semiconductor laser 300 is measured by the wavelength meter 500 and the frequency recorded. It will be appreciated that any other suitable known frequency measuring device, such as an interferometer, may be used in place of the wavelength meter 500. The current driver 100 to be calibrated is replaced by a calibrated current source 700 for each of the sections of the multi- section semiconductor laser respectively and the calibrated current sources are adjusted until the laser again emits laser light in the same mode at the recorded frequency. The corresponding respective currents supplied by the calibrated current sources are then determined and recorded to calibrate the frequency of the multi- section semiconductor laser in terms of the currents supplied to each of the sections. The multi-section semiconductor laser may then be used with a different current driver from that with which the laser was originally calibrated, provided the currents provided by the different current driver are known.

A method of using the invention with a multi-section semiconductor laser will now be described in more detail. As an example of the method, the case of a four section sampled-grating distributed Bragg reflector (SG-DBR) semiconductor laser for wide tunability will be described although the same method applies to other four section lasers such as, for example, so-called gain coupled sampled grating reflector (GCSR) or superstructure sampled grating distribution Bragg reflector (SSG-DBR) devices.

In a sampled-grating DBR semiconductor laser 300, as illustrated in Fig. 5, the four sections of the laser are the front and back diffraction gratings 510,540 with phase and gain sections 520,530 between the front and back gratings. The currents that drive these sections will be referred to as I_F(ΠIA), i_B(mA), I_P(mA) and I_G(ΠIA) respectively. It is required to find these values of current for any given digital parameter stored in a LUT used to drive the four D/A converters controlling the respective current sources. That is, the look-up table has stored, by known means, the four digital parameters that are required to tune the laser to each selected optical frequency in a frequency plan, for example, for telecommunications or other uses. For telecommuncations this may be the International Telecommunications Union standard channel plan, ITU-G692. In the description that follows lower case letters refer to digital voltage values in digital bits stored in the look up table, while upper case letters refer to corresponding current values in mA (or μA) applied to respective sections of the laser.

Referring to figure 3, to find calibration factors CF and CB (in m A/bit) which relate the stored digital voltage to the applied currents for the front and back grating sections respectively, the phase section current is set, step 301, to zero and the gain section current to some convenient value that delivers adequate laser power, for example approximately 100 mA. Figure 6 illustrates stable operational points 41 in a front and back grating section current plane of the multi-section semiconductor laser. That is, the graph plots stable lasing points of front grating section current and back grating section current between supermode boundaries 42 and longitudinal mode boundaries 43 for constant phase and gain section currents.

The so-called power and wavelength planes are measured at these settings by scanning Ip and I_B over their respective permitted ranges in a known fashion for characterizing such lasers. Stable operating points 41 which are remote from mode boundaries to avoid mode hopping are thereby identified, step 302, and a look-up table 303 is generated, step 304, giving iF(f f , Iβ(bbb), and frequency(THz) measured using the wavelength meter 500, for each stable point 41, also by a known method.

The stable point 410 nearest the origin in the mid-supermode in the Ip(fff), Iβ(bbb) plane is selected, step 305, and the emitted wavelength 307 measured; step 306, and the ratio 309 of fff/bbb is noted, step 308. The drive voltage signals are switched off and replaced, step 310, with the calibrated current source 500 for Ip and I_B, with the phase and gain section currents held at their respective values of Ip= 0 and I_G= 100 mA as before. I_F and I_B are incremented steadily, step 311, in the ratio 309 of fff/bbb along line 411 in Fig 6 to reach the same most stable point 410 and the values Ip(FFF) and Iβ(BBB) in mA determined from the calibrated current source 700 and recorded.

The required calibration factors for the front and back grating sections are determined respectively from CF= FFF/fff and CB= BBB/bbb in mA/sample, where sample is a 12 or 14 bit datum stored in the look-up table.

The steps 305, 306, 308, 310, 311 from the step of selecting the most stable point are repeated for the next most stable point (mid-supermode, next nearest origin) and new values of CF and CB obtained are noted. This is repeated for a stable point far from the origin and the values of CF and CB are again noted. Suitable mean values of the three values of CF and CB are then obtained.

Referring to Figure 4, to find CP, the calibration factor (in mA/sample) for the phase current Ip, the gain current is set, step 401, at I_G= 100 mA and stable points identified, step 402 to generate, step 403, a full look-up table, LUT 404, for the ITU plan by known means producing i_F(fff , Iβ(bbb), I_P(ppp), Frequency(THz) for each channel. The drivers are switched down. An operating point near the origin from this LUT is selected, step 405, and the corresponding frequency 406 is recorded, step 407. The phase section is then connected, step 408, to a calibrated current source. The front and back currents are set to the selected I_F, I_B values for that operating point and then Ip is incremented, step 409, from zero until the laser emits at the selected frequency 406, as determined using the wavelength meter. The Ip(PPP) value in mA generated by the calibrated current source 700 is noted.

The calibration factor is calculated from CP= PPP/ppp in mA/sample.

Thus a method has been described to calibrate current sources that uses either a semiconductor laser, advantageously one with a large output light cone for ease of coupling to a large area photodiode together with a power meter, or uses a multi- section laser that is to be driven, if that is the desired application, together with a wavelength measuring device. In particular, a precise method and apparatus are described for calibrating in milliamps with an accuracy of microamperes drive currents that are output from an electronic circuit driven by a voltage from a digital to analogue converter.

Claims

1. A method of calibrating a digitally controllable current driver circuit comprising the steps of a) driving a semiconductor laser with the current driver circuit to be calibrated; b) measuring an optical parameter of light emitted by the semiconductor laser; c) replacing the current driver circuit with a calibrated current source; d) driving the semiconductor laser with the calibrated current source to emit light having the measured optical parameter; and e) determining a corresponding current provided by the calibrated current source.

2. A method as claimed in claim 1, wherein step a) comprises driving a multi-section semiconductor laser with current driver circuits to be calibrated; step b) comprises measuring optical frequency of light emitted from the laser; and step d) comprises driving the multi-section semiconductor laser with the calibrated current sources to produce light of the same optical frequency.

3. A method as claimed in claim 1 or claim 2, wherein step a) comprises the additional step of driving the laser with an auxiliary current source to bring the laser to threshold operation.

4. A method as claimed in claim 1, wherein step b) comprises providing a photodiode and using the photodiode to measure optical power of light emitted by the semiconductor laser.

5. A method as claimed in claim 1, wherein step b) comprises providing a thermal detector and using the thermal detector to measure optical power of light emitted by the semiconductor laser.

6. A method as claimed in claim 2 in which step b) comprises providing a wavemeter and using the wavemeter to measure the optical frequency of light emitted by the laser.

7. A method as claimed in claim 2 in which step b) comprises providing an interferometer and using the interferometer to measure the optical frequency of light emitted by the laser.

8. A method as claimed in any of the preceding claims, wherein the drive circuit to be calibrated is drivable by an analogue electric circuit.

9. An apparatus for calibrating a digitally driven current driver circuit, the apparatus comprising a semiconductor laser for being driven by the circuit to be calibrated, measuring means for measuring an optical parameter of light emitted by the semiconductor laser driven by the circuit to be calibrated, and a calibrated current source for selectively driving the semiconductor laser to produce light having precisely the same optical parameter as when driven by the circuit to be calibrated and means for determining the current supplied by the calibrated current source.

10. An apparatus as claimed in claim 9, wherein the semiconductor laser is a multi-section semiconductor laser drivable by the circuit to be calibrated, and the measuring means is for measuring an optical frequency of light emitted from the laser.

11. An apparatus as claimed in claim 9 or 10, further comprising an auxiliary current source for bringing the laser to threshold operation.

12. An apparatus as claimed in claim 9, wherein the measuring means comprises a photodiode for measuring optical power.

13. An apparatus as claimed in claim 9, wherein the measuring means comprises a thermal detector for measuring optical power.

14. An apparatus as claimed in claim 10, wherein the measuring means comprises a wavemeter.

15. An apparatus as claimed in claim 10, wherein the measuring means comprises an interferometer.

16. An apparatus as claimed in any of claims 9 to 15, wherein the drive circuit to be calibrated is driven by an analogue electric circuit.