CA1122262A - Stabilization of output characteristics of injection lasers - Google Patents
Stabilization of output characteristics of injection lasersInfo
- Publication number
- CA1122262A CA1122262A CA000299740A CA299740A CA1122262A CA 1122262 A CA1122262 A CA 1122262A CA 000299740 A CA000299740 A CA 000299740A CA 299740 A CA299740 A CA 299740A CA 1122262 A CA1122262 A CA 1122262A
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- laser
- level
- electrical signal
- signal
- bias
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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
- H01S5/06835—Stabilising during pulse modulation or generation
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- Physics & Mathematics (AREA)
- Condensed Matter Physics & Semiconductors (AREA)
- General Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Optics & Photonics (AREA)
- Semiconductor Lasers (AREA)
- Led Devices (AREA)
- Lasers (AREA)
Abstract
ABSTRACT
Apparatus for stabilizing output characteristicss of an injection laser by monitoring the output level of the laser corresponding to the d.c. bias level and maintaining this level constant at a value which corresponds substantially to the laser being biased at or very close to threshold. The apparatus has a light sensitive device for producing an electrical signal corresponding to the d.c. bias level of the laser. The electrical signal is compared with a reference signal to produce a further signal which is applied to d.c. bias control means for the laser.
The d.c. bias control means responds to the further signal to adjust the d.c. bias level such that the d.c. light output level remains substantially at a predetermined level. The mean or peak output of the laser can be controlled in a similar manner.
Apparatus for stabilizing output characteristicss of an injection laser by monitoring the output level of the laser corresponding to the d.c. bias level and maintaining this level constant at a value which corresponds substantially to the laser being biased at or very close to threshold. The apparatus has a light sensitive device for producing an electrical signal corresponding to the d.c. bias level of the laser. The electrical signal is compared with a reference signal to produce a further signal which is applied to d.c. bias control means for the laser.
The d.c. bias control means responds to the further signal to adjust the d.c. bias level such that the d.c. light output level remains substantially at a predetermined level. The mean or peak output of the laser can be controlled in a similar manner.
Description
l~ZZ2~iZ
This invention relates to the stabilisatiorl of the optical output characteristics of an injection laser.
Semiconductor injection lasers (e.g. gallium arsenide lasers) have been proposed for use in optical communication systems to convert either digital or analo~ue electrical signals into optical signals.
A problem in such applications is that the output characteristics of a ~emiconductor laser change with temperature and passage of time. The output characteristics which can vary are-Iq the laser threshold current, /dP~
~ dI/ ~ ~ Iq, the laser efficiency ~ when thecurrent applied to the laser is greater than the threshold current, and ( ~ I < Iq, the spontaneous e~ficiency ~ when the current applied to the laser is les3 than the threshold current where P is the optical output power of the laser and I 'he current applied to the laser.
In addition there is an unwanted switch-on time delay for lasing action between the application of the input modulating current &nd the appearance o~
ll'~Z2~;~
the optical output pulse when the bias current Ib applied to the laser is below the threshold current Iq. l'his is usually of importance 2S it is preferable to operate the laser with its bias current Ib below Iq for two reasons in most digital systems:
(i) to optimise the rat:io bet~een the 'on' and 'off' pulses within the limits permitted by the unwanted switch-on delay and the maximum permitted peak optical output power. For low speed digital systems e.g. 8 Mbits/s, the d.c. bias current may be zero.
' (li) as ~ < ~ , a less sensiti~e control of the bias current is required in order to maintain the optical output power in the 'o~f' state at or substantially at a predetermined level~
For intensity modulated, fre~uency modulated and very high ,speed di~ital syst,ems it may be preferable to apply a bias current at a value that is greater than the thre~hold current.
'L'he switch-on delay (td) can be approximately represente-l by the equation td - ~ ln Im Im + Ib Iq where Im is the modulating current Ib is the bias current I~ is the threshold current ll'~Z26Z
and ~ is the carrier lifetime (typically of the order of 1 ns to 4 ns).
Thus it can be seen that if the threshold current varies the switch-on delay td will also vary unless an adjustment is rnade to the d.c. bias current Ib. A change in the switch-on delay td c~n be important~
in an optical communication system particularly if td increases to such an extent that it becomes significant compared to the shortest desired input pulse length.
On the assumption that the light output of the laser at t~eshold remains substantially constant irrespective of temperature changes or passage of time we are proposing to monitor the d.c. light level of the laser and t~ adjust the d.c. bias current (Ib~
applied to the laser so as to maintain the monitored d.c. light level of the laser at or substantially at a predetermined value which, for digital systerns, is at or below the monitored d.c. light output at threshold. ~or intensity and frequency rno~ulated communication systems it may be desirable to malntain the d.c. bias current (Ib) at a value greater than the thresho d curre~t. In thls way it is hoped to maintain acceptable values of optical output power and switch-on delay td throughout the working life of the laser given that the slope efficiency may decrease with time and any increase in ternperature.
_ 4 _ ll;~Z2~Z
According to one aspect of the present in~ention there is provided apparatus for stab~lising output characteristics of an injection laser comprising first mean~ for producing an electrical signal indicative of the d.c. optical output level of the laser, second means for producing a reference electrical signal which is indicative of a predetermined d.c. output light level, third means for comparing the electrical signal with the reference signal to produce a difference signal, and means for feeding the difference signal to d.c. bias control means of the laser, the d. 5 . bias control means being responsive to said difference signal to adjust the d.c. bia~ level such that the d.c. light output ievel remains substantially at the predeterrnined level.
The predetermined level may be the substantially constant threshold light output level of the laser. It has been found that the light output of given lasers, operating at thresho~d current, may remain approximately con~tant, and independent of variations in temperature or degradation resulting from the passage of time.
~ he first means may comprise a pho-todiode and means for sensing the lowest level of the signal produced by the photodiode. The photodiode rnay be arranged either to sample a proportion of the light emitted from the front face of the laser or to rnonitor the light emitted from the back face of the la~er.
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In view of the variation of slope efficiencies and/3 with both temperature and passage of time it is also proposed to monitor the mean or peak or other level of optical output powe~ o~ the laser and to maintain this at a predetermined substantially co~stant value.
According to another aspect of the present invention there is provided apparatus. for stabilising output characteristics of an injection laser compris ng first means for producing an electrical signal indicative of a preselected optical output level of the laser, second means for producing a reference electrical signal indicative of a predetermined value of ~aid optical output level, third me~ns f~,~ cornparing the electrical signal with the reference ~ignal to produce a dif~erence signal, and means for feeding the difference signal to modulating current drive means ~or the laser, said drive means bein~ responsive to said difference qignal to adjust the modulating drive current applied to the laser such that the preselected optical output le,vel remains substantially at said pred.etermine~ level. l~r~ferably the preselected level is the mean opti.cal output level or the peak optical output level, 'l~e first means may comprise a photodiode and me~ns for sensing the mean or peak level of the signal produced by the photodiode. 'l~e photodiode may be arrangeA either to sample a proportion of the light emitted from the front face of the laser or to monitor the light emitted from the back face of the laser.
A preferred embo~iment of the inven-tion includes both of said aspects which are arranged with a common photodiode.
The,invention will be de~cribed now by way of example only with particular reference to the accompanying drawings. In the drawings:
Figure 1 is a graph illustrating the variation of optical power output with current ~or an injection laser;
E`igure 2 shows how the characteristics of Figure 1 may vary with temperature;
E'igure 3 is a block schematic diagram of one embodiment of apparatus in accor~ance with the present in~ention;
F'i~ure 4 is a block schernatis diagram of another embodiment of apparatus in accordance with the present invention, and ll;~Z2~2 Figure 5 is a block schernatic diagram of a further embodiment of apparatus in accordarlce with the present invention.
me characteristics of an injection laser which can vary with te~perature ~nd time are the laser threshold current I , the lasing efficiency (dI) I ~ Iq, and the spontaneous efficiency (dI)I < Iq.
These characteris-tics are illustrated in Figure 1 of the drawings. In addition there is a switch-on time c;elay for lasing action between the application of an input modulating current to the laser and the appearance of the output of light from the laser when the bias current applied to the laser is below the threshold current Iq. The switch-on delay td can be approximately represented by the following equation td = ~ ln Im Im + Ib Iq where Im is the modulating current Ib is the bias current Iq is the thre,shold current and ~ is the carrier lifetime (typica]ly in the order of 1 nS to 4 nS).
Thus the switch-on delay will vary with ~ariation in the threshold current of the laser.
This can be import~nt since, although it may be ZZ~2 desirable to bias the laser below the threshcld current (for digital systellls), the switch-on delay may represent a limitation in 'he speed of operation of the laser. The switch-on delay is of importance when the laser is used in optical col~nunication systems to transmit information in digital form. me switch-on delay can be important in such systems when it becomes significar.t compared to the shortest desired input pulse duration. For intensity and frequency modulated optical communication systems, the variation of threshold current and slope efficiency ~ with temperature and the passage of time is important.
We propose to control the d.c. bias by monitoring the d.c. light output level of t~e laser and adjusting the d.c. bias to maintain the light output level at a substantially constant level. The control means relies for its operation on the assumption that the light output at threshold remains substantially constant irrespective of temperature changes or ageing of the laser. Th~s is illustrated in F~ure 2 for three temperatures T1, T2 and T~.
Referring now to E'igure ~ there is shown an apparatus for controlling the d.c. bias by monitorin~
the light output of the laser. '~he apparatus can also control the peak output light level of the laser. The apparatus of Figure 3 has a modulating drive current .~ _ g _ ZZ~i2 circui-c 15 which receives either pul~e or analogue data and produces a drive current in response thereto for o~erating a semiconductor ~aser 20. Li~ht from the laser is transmitted towards an optical waveguide (not sho~) via a semi-reflecting member 21 which deflects a portion of the light towards a photodiode 23. rme output of the photodiode is connected to the input of à minimum level monitoring circuit 24 and also to the input of a peak monitoring circuit 25.
me minimu~ level monitoring circuit 24 is arranged to produce an output signal which represents the d.c. light output level of the laser and the peak monitorin~ circuit 25 produces an output signal which represents t~,e peak optical power output of th~
laser 20. The output of the minimum level monitoring circuit 24 is connected to a comp~rator 27 and the output of the peak monitoring circuit 25 is connected to a comparator 28. ~ach colnparator 27, 28 recelves an input from a reference circuit 30 the output of the reference circult 30 representing predetermined ]ight output levels. The reference signal app~ied to the cor~parator 27 is indicative of a predetermined d.c.
light output level and the refererloe sil~al applied to the comparator 28 is indicative of a prede-termined peak light output. The output of the comparator 27 is cor~ected to a d.c. bias circuit 32 which provides the d.c. bias signal for the laser 20, and the output of the il'~2Z~2 comparator 28 is connected to the modulatin~ drive current circuit 15. An input signal level monitoring circuit 35 is connected to the modulatin~; circuit 15 and to the bias circuit 32.
In the circuit of Figure 3 the modulating drive current circuit 15 can comprise a bipolar transistor which is arranged to vary the current through the laser 20 according to the ~ifference signal received from the comparator 25. Alternatively the circuit 15 can be an E`ET which i~ connected in parallel with the laser, the difference signal being applied to the gate of the FET.
The minimum level and peak level monitoring circuits can each comprise a conventional diode and capacitance detector. The comparators 27, 28 each comprise a low off,set operational amplifier. The output of the photodiode 23 is oonnected to the clrcuits 24, 25 by a d.c. coupled, ternperature compensateA amplifier.
'~he d.c. bias circuit 32 ~an be a ramp circuit which controls the d.c. bias current according to the difference si~nal ~rom the comparator 27.
The input signal level monitoring circuit 35 can be a diode-capacitance detection circuit lor detecting data pulses at the inpu' to the drive cir~uit 15~ The circuit 35 is provide~ to ensure that the drive -- 1~ --ll;~;~Z~2 circuit 15 hnd '~ias circuit 32 are oll]-r operational when data pulses are fed to the laser.
In use if the d.c. light output level of the laser varies the outpu-t of the minimum level monitoring circuit 24 will vary and a difference signal will be produced by the comparator 27 which difference signal is fed to the d.c. bias circuit 32.
1`he d.c. bias circuit operates to vary -the output level of the laser 20 so that the d.c. light output level is returned to the value represente~ by the reference circuit 30. In this way the d.c. light output o~ the laser 20 is maintained at a constant level. I'he peak monit,oring circuit 25 i~ provided to monitor peak optical output power of the laser and mai~tain this at a constant level in view of the possible variation of slope elf.ciency of the laser with ternperature and time. If the peak output value varies this is sensed by the comparator 28 which provides an appropriate signal for the rnodulating drive ourrent circuit. me modulating current provided by the modulating drive current circuit is then varied appropriately to return the peal~ output to its predetermined va~ue.
I~lus it will be seen that the present arrangement provides a means for controlling the d.c. bias current in order that it may be maintained ~lZZ2~Z
within certain limitations. Ln the case of digital systems the arrangement maintains the switch-on delay to within certain limitations. The arrangement also monitors th~ peak output light level of the la,ser and maintain~s this at a constant level.
The e~nbodiment described with reference to E'igure 3 monitors the peak outp~lt iight level of a laser. An alternative to this arral~ement is one which monitors the mean op-ti.cal output level of the laser.
As explained above laser switch-on delay occurs when the d.c. bias current app].i.ed to the laser is below the threshold current. The main effects of the switch-on delay in the performance of an optical co~nunications system using optical fibres are:
(a) a reduction in the power of the pul.se.s transmitted down the optical fibre.
(b) Mis-equalisation in the deGision circuitry because the received pulses are of shorter duration than an equaliser of` the circuitry is designed to accommodate.
The relative importance of these two effect.s depends upon the method by which the laser is driven i.e. whether by l~RZ (non return to zero) or by RTZ
(return to zero) pulses. We have found that a reduction in pulse power signif'icantly ~ffects the system particularly when reduced width RTZ pulses are used.
il'~ZZti2 We h.ave ~evised an arrange!-1ent which monitors the mean optical output level of a laser and maintain~
the opti.cal output -,oulse power of the laser con~tant by varying thc amplitude of the pulse. The arrangement is shown in E'igure 4. This arrangement has a modulati.ng ~rive current circuit 50 which receives either pulse or analoKue data and produces a drive current in respon~.e thereto for operating a semiconductor laser 51. Light from the laser is transmitted towa-rds an optical wave~uide (not shown) via a semi~reflecting member 52 which deflects a portion of the light towards a photodiode 54, The output of the photodiode is connected, to the input of a mean power detector 56 whi.ch produces an output signal indicative of the mean optical power output level of the laser 51, The OUtpl,lt o~ the detector 56 is connected to a comparator 57 which also receives a signal from a reference circuit 58, the signal from the reference circuit 58 being indicative of a predetermined rne~n optica.l power output.
The output of the comparator 57 is connectçd to the circuit 50~ An input signal level monitoring circuit 60 is connected between the in~ut of the dri~e current circuit 50 an~ the output from the cornr)arator ~7.
In the arrangement o~ E`igure 4 the block~
can be constituted by similar componen,ts to those described with reference to Fi~,ure 3 with the e~ception _ A~ 4 _ ~zz~z that th~ pllû~,odiGde 54 is conllecte~ to ~he co~iparator 5'7 by a large value resistor which effectively forms the block 56. The volta~e si~nal across this resistor is applied to the comparator 57.
In use the output signal o~ the comparator 5'7 is used to control the amplitude o~ the input current drive pulses to rrlaintain the optical power outpu-t constant in a manner similar to that described with reference to Figure ~. For example, consider an increase in switch-on delay of the laser. In the case o~ ~TZ pu'.se operation the resu~t i,s bhat the pulse duration is reduced and the amplitude of the pulses increased to maintain const~nt optical power output. In the case of i~Z pulse operation the first pulse in a level O to level 1 transition is o~ shûrter cluration and the ampli'ude of the pulses is increased to maintain const~nt optical power output.
The arran~ement shown in Figure 4 is suitable for relatively lo~; speed systems (e.g. ~ Mbits/sec).
E'or higher speecl systems (e.g. 140 }~its/sec) the switch-on delay when pulsin~ ~rorn zero bias ~,ay be too large and a d.c. bias current, contro:l led relative to threshold, may have to be employed in a-3dition to the mean power output control. Such an arrangement is shown in h`i~e 5. The d.c. bias current control operates in a manner similar to that describe~l for 11222~2 E'igure 3 ~hilst the Mean power out~ju-t control is similar to that described with reference to ~ ure 4 with the exception that it i.s desi~ned to respon~
to the mean of pulse component of the optical output o~ the laser rather than the rnean of total opti.cal power output.
This invention relates to the stabilisatiorl of the optical output characteristics of an injection laser.
Semiconductor injection lasers (e.g. gallium arsenide lasers) have been proposed for use in optical communication systems to convert either digital or analo~ue electrical signals into optical signals.
A problem in such applications is that the output characteristics of a ~emiconductor laser change with temperature and passage of time. The output characteristics which can vary are-Iq the laser threshold current, /dP~
~ dI/ ~ ~ Iq, the laser efficiency ~ when thecurrent applied to the laser is greater than the threshold current, and ( ~ I < Iq, the spontaneous e~ficiency ~ when the current applied to the laser is les3 than the threshold current where P is the optical output power of the laser and I 'he current applied to the laser.
In addition there is an unwanted switch-on time delay for lasing action between the application of the input modulating current &nd the appearance o~
ll'~Z2~;~
the optical output pulse when the bias current Ib applied to the laser is below the threshold current Iq. l'his is usually of importance 2S it is preferable to operate the laser with its bias current Ib below Iq for two reasons in most digital systems:
(i) to optimise the rat:io bet~een the 'on' and 'off' pulses within the limits permitted by the unwanted switch-on delay and the maximum permitted peak optical output power. For low speed digital systems e.g. 8 Mbits/s, the d.c. bias current may be zero.
' (li) as ~ < ~ , a less sensiti~e control of the bias current is required in order to maintain the optical output power in the 'o~f' state at or substantially at a predetermined level~
For intensity modulated, fre~uency modulated and very high ,speed di~ital syst,ems it may be preferable to apply a bias current at a value that is greater than the thre~hold current.
'L'he switch-on delay (td) can be approximately represente-l by the equation td - ~ ln Im Im + Ib Iq where Im is the modulating current Ib is the bias current I~ is the threshold current ll'~Z26Z
and ~ is the carrier lifetime (typically of the order of 1 ns to 4 ns).
Thus it can be seen that if the threshold current varies the switch-on delay td will also vary unless an adjustment is rnade to the d.c. bias current Ib. A change in the switch-on delay td c~n be important~
in an optical communication system particularly if td increases to such an extent that it becomes significant compared to the shortest desired input pulse length.
On the assumption that the light output of the laser at t~eshold remains substantially constant irrespective of temperature changes or passage of time we are proposing to monitor the d.c. light level of the laser and t~ adjust the d.c. bias current (Ib~
applied to the laser so as to maintain the monitored d.c. light level of the laser at or substantially at a predetermined value which, for digital systerns, is at or below the monitored d.c. light output at threshold. ~or intensity and frequency rno~ulated communication systems it may be desirable to malntain the d.c. bias current (Ib) at a value greater than the thresho d curre~t. In thls way it is hoped to maintain acceptable values of optical output power and switch-on delay td throughout the working life of the laser given that the slope efficiency may decrease with time and any increase in ternperature.
_ 4 _ ll;~Z2~Z
According to one aspect of the present in~ention there is provided apparatus for stab~lising output characteristics of an injection laser comprising first mean~ for producing an electrical signal indicative of the d.c. optical output level of the laser, second means for producing a reference electrical signal which is indicative of a predetermined d.c. output light level, third means for comparing the electrical signal with the reference signal to produce a difference signal, and means for feeding the difference signal to d.c. bias control means of the laser, the d. 5 . bias control means being responsive to said difference signal to adjust the d.c. bia~ level such that the d.c. light output ievel remains substantially at the predeterrnined level.
The predetermined level may be the substantially constant threshold light output level of the laser. It has been found that the light output of given lasers, operating at thresho~d current, may remain approximately con~tant, and independent of variations in temperature or degradation resulting from the passage of time.
~ he first means may comprise a pho-todiode and means for sensing the lowest level of the signal produced by the photodiode. The photodiode rnay be arranged either to sample a proportion of the light emitted from the front face of the laser or to rnonitor the light emitted from the back face of the la~er.
ZZ~Z
In view of the variation of slope efficiencies and/3 with both temperature and passage of time it is also proposed to monitor the mean or peak or other level of optical output powe~ o~ the laser and to maintain this at a predetermined substantially co~stant value.
According to another aspect of the present invention there is provided apparatus. for stabilising output characteristics of an injection laser compris ng first means for producing an electrical signal indicative of a preselected optical output level of the laser, second means for producing a reference electrical signal indicative of a predetermined value of ~aid optical output level, third me~ns f~,~ cornparing the electrical signal with the reference ~ignal to produce a dif~erence signal, and means for feeding the difference signal to modulating current drive means ~or the laser, said drive means bein~ responsive to said difference qignal to adjust the modulating drive current applied to the laser such that the preselected optical output le,vel remains substantially at said pred.etermine~ level. l~r~ferably the preselected level is the mean opti.cal output level or the peak optical output level, 'l~e first means may comprise a photodiode and me~ns for sensing the mean or peak level of the signal produced by the photodiode. 'l~e photodiode may be arrangeA either to sample a proportion of the light emitted from the front face of the laser or to monitor the light emitted from the back face of the laser.
A preferred embo~iment of the inven-tion includes both of said aspects which are arranged with a common photodiode.
The,invention will be de~cribed now by way of example only with particular reference to the accompanying drawings. In the drawings:
Figure 1 is a graph illustrating the variation of optical power output with current ~or an injection laser;
E`igure 2 shows how the characteristics of Figure 1 may vary with temperature;
E'igure 3 is a block schematic diagram of one embodiment of apparatus in accor~ance with the present in~ention;
F'i~ure 4 is a block schernatis diagram of another embodiment of apparatus in accordance with the present invention, and ll;~Z2~2 Figure 5 is a block schernatic diagram of a further embodiment of apparatus in accordarlce with the present invention.
me characteristics of an injection laser which can vary with te~perature ~nd time are the laser threshold current I , the lasing efficiency (dI) I ~ Iq, and the spontaneous efficiency (dI)I < Iq.
These characteris-tics are illustrated in Figure 1 of the drawings. In addition there is a switch-on time c;elay for lasing action between the application of an input modulating current to the laser and the appearance of the output of light from the laser when the bias current applied to the laser is below the threshold current Iq. The switch-on delay td can be approximately represented by the following equation td = ~ ln Im Im + Ib Iq where Im is the modulating current Ib is the bias current Iq is the thre,shold current and ~ is the carrier lifetime (typica]ly in the order of 1 nS to 4 nS).
Thus the switch-on delay will vary with ~ariation in the threshold current of the laser.
This can be import~nt since, although it may be ZZ~2 desirable to bias the laser below the threshcld current (for digital systellls), the switch-on delay may represent a limitation in 'he speed of operation of the laser. The switch-on delay is of importance when the laser is used in optical col~nunication systems to transmit information in digital form. me switch-on delay can be important in such systems when it becomes significar.t compared to the shortest desired input pulse duration. For intensity and frequency modulated optical communication systems, the variation of threshold current and slope efficiency ~ with temperature and the passage of time is important.
We propose to control the d.c. bias by monitoring the d.c. light output level of t~e laser and adjusting the d.c. bias to maintain the light output level at a substantially constant level. The control means relies for its operation on the assumption that the light output at threshold remains substantially constant irrespective of temperature changes or ageing of the laser. Th~s is illustrated in F~ure 2 for three temperatures T1, T2 and T~.
Referring now to E'igure ~ there is shown an apparatus for controlling the d.c. bias by monitorin~
the light output of the laser. '~he apparatus can also control the peak output light level of the laser. The apparatus of Figure 3 has a modulating drive current .~ _ g _ ZZ~i2 circui-c 15 which receives either pul~e or analogue data and produces a drive current in response thereto for o~erating a semiconductor ~aser 20. Li~ht from the laser is transmitted towards an optical waveguide (not sho~) via a semi-reflecting member 21 which deflects a portion of the light towards a photodiode 23. rme output of the photodiode is connected to the input of à minimum level monitoring circuit 24 and also to the input of a peak monitoring circuit 25.
me minimu~ level monitoring circuit 24 is arranged to produce an output signal which represents the d.c. light output level of the laser and the peak monitorin~ circuit 25 produces an output signal which represents t~,e peak optical power output of th~
laser 20. The output of the minimum level monitoring circuit 24 is connected to a comp~rator 27 and the output of the peak monitoring circuit 25 is connected to a comparator 28. ~ach colnparator 27, 28 recelves an input from a reference circuit 30 the output of the reference circult 30 representing predetermined ]ight output levels. The reference signal app~ied to the cor~parator 27 is indicative of a predetermined d.c.
light output level and the refererloe sil~al applied to the comparator 28 is indicative of a prede-termined peak light output. The output of the comparator 27 is cor~ected to a d.c. bias circuit 32 which provides the d.c. bias signal for the laser 20, and the output of the il'~2Z~2 comparator 28 is connected to the modulatin~ drive current circuit 15. An input signal level monitoring circuit 35 is connected to the modulatin~; circuit 15 and to the bias circuit 32.
In the circuit of Figure 3 the modulating drive current circuit 15 can comprise a bipolar transistor which is arranged to vary the current through the laser 20 according to the ~ifference signal received from the comparator 25. Alternatively the circuit 15 can be an E`ET which i~ connected in parallel with the laser, the difference signal being applied to the gate of the FET.
The minimum level and peak level monitoring circuits can each comprise a conventional diode and capacitance detector. The comparators 27, 28 each comprise a low off,set operational amplifier. The output of the photodiode 23 is oonnected to the clrcuits 24, 25 by a d.c. coupled, ternperature compensateA amplifier.
'~he d.c. bias circuit 32 ~an be a ramp circuit which controls the d.c. bias current according to the difference si~nal ~rom the comparator 27.
The input signal level monitoring circuit 35 can be a diode-capacitance detection circuit lor detecting data pulses at the inpu' to the drive cir~uit 15~ The circuit 35 is provide~ to ensure that the drive -- 1~ --ll;~;~Z~2 circuit 15 hnd '~ias circuit 32 are oll]-r operational when data pulses are fed to the laser.
In use if the d.c. light output level of the laser varies the outpu-t of the minimum level monitoring circuit 24 will vary and a difference signal will be produced by the comparator 27 which difference signal is fed to the d.c. bias circuit 32.
1`he d.c. bias circuit operates to vary -the output level of the laser 20 so that the d.c. light output level is returned to the value represente~ by the reference circuit 30. In this way the d.c. light output o~ the laser 20 is maintained at a constant level. I'he peak monit,oring circuit 25 i~ provided to monitor peak optical output power of the laser and mai~tain this at a constant level in view of the possible variation of slope elf.ciency of the laser with ternperature and time. If the peak output value varies this is sensed by the comparator 28 which provides an appropriate signal for the rnodulating drive ourrent circuit. me modulating current provided by the modulating drive current circuit is then varied appropriately to return the peal~ output to its predetermined va~ue.
I~lus it will be seen that the present arrangement provides a means for controlling the d.c. bias current in order that it may be maintained ~lZZ2~Z
within certain limitations. Ln the case of digital systems the arrangement maintains the switch-on delay to within certain limitations. The arrangement also monitors th~ peak output light level of the la,ser and maintain~s this at a constant level.
The e~nbodiment described with reference to E'igure 3 monitors the peak outp~lt iight level of a laser. An alternative to this arral~ement is one which monitors the mean op-ti.cal output level of the laser.
As explained above laser switch-on delay occurs when the d.c. bias current app].i.ed to the laser is below the threshold current. The main effects of the switch-on delay in the performance of an optical co~nunications system using optical fibres are:
(a) a reduction in the power of the pul.se.s transmitted down the optical fibre.
(b) Mis-equalisation in the deGision circuitry because the received pulses are of shorter duration than an equaliser of` the circuitry is designed to accommodate.
The relative importance of these two effect.s depends upon the method by which the laser is driven i.e. whether by l~RZ (non return to zero) or by RTZ
(return to zero) pulses. We have found that a reduction in pulse power signif'icantly ~ffects the system particularly when reduced width RTZ pulses are used.
il'~ZZti2 We h.ave ~evised an arrange!-1ent which monitors the mean optical output level of a laser and maintain~
the opti.cal output -,oulse power of the laser con~tant by varying thc amplitude of the pulse. The arrangement is shown in E'igure 4. This arrangement has a modulati.ng ~rive current circuit 50 which receives either pulse or analoKue data and produces a drive current in respon~.e thereto for operating a semiconductor laser 51. Light from the laser is transmitted towa-rds an optical wave~uide (not shown) via a semi~reflecting member 52 which deflects a portion of the light towards a photodiode 54, The output of the photodiode is connected, to the input of a mean power detector 56 whi.ch produces an output signal indicative of the mean optical power output level of the laser 51, The OUtpl,lt o~ the detector 56 is connected to a comparator 57 which also receives a signal from a reference circuit 58, the signal from the reference circuit 58 being indicative of a predetermined rne~n optica.l power output.
The output of the comparator 57 is connectçd to the circuit 50~ An input signal level monitoring circuit 60 is connected between the in~ut of the dri~e current circuit 50 an~ the output from the cornr)arator ~7.
In the arrangement o~ E`igure 4 the block~
can be constituted by similar componen,ts to those described with reference to Fi~,ure 3 with the e~ception _ A~ 4 _ ~zz~z that th~ pllû~,odiGde 54 is conllecte~ to ~he co~iparator 5'7 by a large value resistor which effectively forms the block 56. The volta~e si~nal across this resistor is applied to the comparator 57.
In use the output signal o~ the comparator 5'7 is used to control the amplitude o~ the input current drive pulses to rrlaintain the optical power outpu-t constant in a manner similar to that described with reference to Figure ~. For example, consider an increase in switch-on delay of the laser. In the case o~ ~TZ pu'.se operation the resu~t i,s bhat the pulse duration is reduced and the amplitude of the pulses increased to maintain const~nt optical power output. In the case of i~Z pulse operation the first pulse in a level O to level 1 transition is o~ shûrter cluration and the ampli'ude of the pulses is increased to maintain const~nt optical power output.
The arran~ement shown in Figure 4 is suitable for relatively lo~; speed systems (e.g. ~ Mbits/sec).
E'or higher speecl systems (e.g. 140 }~its/sec) the switch-on delay when pulsin~ ~rorn zero bias ~,ay be too large and a d.c. bias current, contro:l led relative to threshold, may have to be employed in a-3dition to the mean power output control. Such an arrangement is shown in h`i~e 5. The d.c. bias current control operates in a manner similar to that describe~l for 11222~2 E'igure 3 ~hilst the Mean power out~ju-t control is similar to that described with reference to ~ ure 4 with the exception that it i.s desi~ned to respon~
to the mean of pulse component of the optical output o~ the laser rather than the rnean of total opti.cal power output.
Claims (9)
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. Apparatus for stabilising output characteristics of an injection laser which has current supply means for supplying a d.c. bias current to the laser and drive circuitry for supplying current pulse to the laser, comprising first means for producing an electrical signal indicative of the optical output level of the laser at said d.c. bias current, second means for producing a reference electrical signal which is indicative of a predetermined d.c. bias current output light level, third means for comparing the electrical signal with the reference signal to produce a difference signal, and means for feeding the difference signal to said d.c. bias current supply means for the laser, the d.c. bias current supply means being responsive to said difference signal to adjust the d.c. bias level such that the d.c. light output level remains substantially at the predetermined level.
2. Apparatus as claimed in claim 1 wherein the predetermined level is the substantially constant threshold light output level of the laser.
3. Apparatus as claimed in claim 1 wherein said first means comprises a photodiode and means for sensing the lowest level of the signal produced by the photodiode.
4. Apparatus as claimed in claim 3 wherein the photodiode is arranged to receive light from the front face of the laser which has been reflected from a semi-reflecting surface.
5. Apparatus as claimed in claim 3 wherein the photodiode is arranged to receive light from the back face of the laser.
6. Apparatus for stabilising output characteristics of an injection laser which has current supply means for supplying a d.c. bias current to the laser and drive circuitry for supplying current pulses to the laser comprising light sensitive means for producing a first electrical signal indicative of the optical output level of the laser, means for sensing the lowest level of said electrical signal to produce a second electrical signal indicative of the optical output level of the laser at said d.c. bias current, second means for producing a first reference electrical signal which is indicative of a predetermined d.c. bias current light output level, third means for comparing the second electrical signal with the first reference signal to produce a first difference signal, and means for feeding the first difference signal to the d.c.
bias current supply means of the laser, the d.c.
bias current supply means being responsive to said first difference signal to adjust the d.c. bias level such that the d.c. light output level remains substantially at the predetermined level, fourth means for sensing a preselected level of the first electrical signal to produce a third electrical signal indicative of a preselected light output level of the laser, fifth means for producing a second reference electrical signal indicative of a predetermined light output level, sixth means for comparing the third electrical signal with the second reference signal to produce a second difference signal, and means for feeding the second difference signal to modulating current drive means for the laser, said drive means being responsive to said second difference signal to adjust the modulating drive current applied to the laser such that the optical output level remains substantially at the predetermined level.
bias current supply means of the laser, the d.c.
bias current supply means being responsive to said first difference signal to adjust the d.c. bias level such that the d.c. light output level remains substantially at the predetermined level, fourth means for sensing a preselected level of the first electrical signal to produce a third electrical signal indicative of a preselected light output level of the laser, fifth means for producing a second reference electrical signal indicative of a predetermined light output level, sixth means for comparing the third electrical signal with the second reference signal to produce a second difference signal, and means for feeding the second difference signal to modulating current drive means for the laser, said drive means being responsive to said second difference signal to adjust the modulating drive current applied to the laser such that the optical output level remains substantially at the predetermined level.
7. Apparatus as claimed in claim 6 wherein the preselected level is the peak optical output level.
8. Apparatus as claimed in claim 6 wherein the preselected level is the mean optical output level.
9. Apparatus as claimed in claim 6 wherein said light sensitive means is a photodiode.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB13090/77 | 1977-03-29 | ||
| GB13090/77A GB1563944A (en) | 1977-03-29 | 1977-03-29 | Imjection lasers |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1122262A true CA1122262A (en) | 1982-04-20 |
Family
ID=10016661
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000299740A Expired CA1122262A (en) | 1977-03-29 | 1978-03-23 | Stabilization of output characteristics of injection lasers |
Country Status (8)
| Country | Link |
|---|---|
| JP (1) | JPS53121590A (en) |
| AU (1) | AU512696B2 (en) |
| CA (1) | CA1122262A (en) |
| DE (1) | DE2813513A1 (en) |
| FR (1) | FR2386165B1 (en) |
| GB (1) | GB1563944A (en) |
| NL (1) | NL7803125A (en) |
| SE (1) | SE446044B (en) |
Families Citing this family (25)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2415332A1 (en) * | 1978-01-20 | 1979-08-17 | Thomson Csf | SEMICONDUCTOR LIGHT SOURCE POWER SUPPLY |
| IT1160592B (en) * | 1978-08-16 | 1987-03-11 | Cselt Centro Studi Lab Telecom | PROCEDURE AND CONTROL SYSTEM OF THE EFFICIENCY OF AN ELECTRONIC DEVICE |
| DE2841433C2 (en) * | 1978-09-22 | 1983-08-25 | Siemens Ag, 1000 Berlin Und 8000 Muenchen | Bias current regulation of laser diodes |
| IT1108119B (en) * | 1978-10-05 | 1985-12-02 | Cselt Centro Studi Lab Telecom | PROCEDURE AND PILOTING CIRCUIT FOR THRESHOLD NON-LINEAR DEVICES |
| FR2448274A1 (en) * | 1979-02-02 | 1980-08-29 | Thomson Csf | Supply circuit for semiconductor laser or LED - protects against overload using photocurrent to control series transistor |
| US4375067A (en) * | 1979-05-08 | 1983-02-22 | Canon Kabushiki Kaisha | Semiconductor laser device having a stabilized output beam |
| NL7907683A (en) * | 1979-10-18 | 1981-04-22 | Philips Nv | CONTROL CHAIN FOR A LASER RATING CURRENT. |
| DE3153710C2 (en) * | 1980-01-25 | 1994-06-23 | Canon Kk | Light output monitor from semiconductor laser |
| US4443695A (en) * | 1980-01-25 | 1984-04-17 | Canon Kabushiki Kaisha | Apparatus for controlling the quantity of light |
| DE3045545C2 (en) * | 1980-12-03 | 1984-04-26 | ANT Nachrichtentechnik GmbH, 7150 Backnang | Method for regulating the output signal of a semiconductor laser and circuit arrangement for carrying out this method |
| DE3045511C2 (en) * | 1980-12-03 | 1984-04-26 | ANT Nachrichtentechnik GmbH, 7150 Backnang | Method for regulating the output signal of a semiconductor laser and circuit arrangement for carrying out this method |
| JPS5840878A (en) * | 1981-09-04 | 1983-03-09 | Hitachi Ltd | Driving method for semiconductor laser |
| DE3207741A1 (en) * | 1982-03-04 | 1984-02-02 | ANT Nachrichtentechnik GmbH, 7150 Backnang | Method for controlling the output signal of a semiconductor laser and a circuit arrangement for this purpose |
| JPS5914143A (en) * | 1982-07-13 | 1984-01-25 | Victor Co Of Japan Ltd | Modulating system of semiconductor laser |
| JPS59146457A (en) * | 1983-02-10 | 1984-08-22 | Olympus Optical Co Ltd | Automatic controller for optical output |
| DE3416525A1 (en) * | 1983-05-06 | 1984-11-08 | Coherent Inc., Palo Alto, Calif. | LASER |
| US4689795A (en) * | 1984-04-27 | 1987-08-25 | Mitsubishi Denki Kabushiki Kaisha | Semiconductor laser controller |
| JPS6116044A (en) * | 1984-07-02 | 1986-01-24 | Pioneer Electronic Corp | Semiconductor laser drive circuit |
| JPH0821173B2 (en) * | 1984-07-27 | 1996-03-04 | 松下電器産業株式会社 | Semiconductor laser drive circuit |
| JPS63193582A (en) * | 1987-02-06 | 1988-08-10 | Ando Electric Co Ltd | Laser-diode driving circuit |
| JPH0614570B2 (en) * | 1987-04-28 | 1994-02-23 | シャープ株式会社 | Optical output stabilizing method and photodetector used in the method |
| JPS63313119A (en) * | 1987-06-16 | 1988-12-21 | Nec Corp | Light quantity control system |
| US4995045A (en) * | 1990-02-01 | 1991-02-19 | Northern Telecom Limited | Laser control circuit |
| CN106340805B (en) * | 2016-07-18 | 2023-08-29 | 厦门优迅高速芯片有限公司 | Double closed-loop control circuit of laser driver |
| US11600966B2 (en) * | 2020-02-03 | 2023-03-07 | Analog Devices International Unlimited Company | Light source system |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4009385A (en) * | 1976-03-22 | 1977-02-22 | Bell Telephone Laboratories, Incorporated | Laser control circuit |
-
1977
- 1977-03-29 GB GB13090/77A patent/GB1563944A/en not_active Expired
-
1978
- 1978-03-23 NL NL7803125A patent/NL7803125A/en not_active Application Discontinuation
- 1978-03-23 CA CA000299740A patent/CA1122262A/en not_active Expired
- 1978-03-28 SE SE7803499A patent/SE446044B/en unknown
- 1978-03-29 AU AU34551/78A patent/AU512696B2/en not_active Expired
- 1978-03-29 DE DE19782813513 patent/DE2813513A1/en not_active Withdrawn
- 1978-03-29 FR FR7809085A patent/FR2386165B1/en not_active Expired
- 1978-03-29 JP JP3651378A patent/JPS53121590A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| AU3455178A (en) | 1979-10-04 |
| JPS53121590A (en) | 1978-10-24 |
| FR2386165B1 (en) | 1985-09-06 |
| SE446044B (en) | 1986-08-04 |
| AU512696B2 (en) | 1980-10-23 |
| SE7803499L (en) | 1978-09-30 |
| NL7803125A (en) | 1978-10-03 |
| FR2386165A1 (en) | 1978-10-27 |
| GB1563944A (en) | 1980-04-02 |
| DE2813513A1 (en) | 1978-10-05 |
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