CA1277701C - Method and apparatus for stabilizing the spectral characteristics of a semiconductor laser diode - Google Patents
Method and apparatus for stabilizing the spectral characteristics of a semiconductor laser diodeInfo
- Publication number
- CA1277701C CA1277701C CA000576356A CA576356A CA1277701C CA 1277701 C CA1277701 C CA 1277701C CA 000576356 A CA000576356 A CA 000576356A CA 576356 A CA576356 A CA 576356A CA 1277701 C CA1277701 C CA 1277701C
- Authority
- CA
- Canada
- Prior art keywords
- phase
- laser diode
- external cavity
- dither
- operating point
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
Links
Classifications
-
- 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
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/13—Stabilisation of laser output parameters, e.g. frequency or amplitude
- H01S3/139—Stabilisation of laser output parameters, e.g. frequency or amplitude by controlling the mutual position or the reflecting properties of the reflectors of the cavity, e.g. by controlling the cavity length
-
- 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
- H01S5/065—Mode locking; Mode suppression; Mode selection ; Self pulsating
-
- 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
- H01S5/0014—Measuring characteristics or properties thereof
- H01S5/0028—Laser diodes used as detectors
-
- 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
- H01S5/0687—Stabilising the frequency of the laser
-
- 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
- H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
- H01S5/14—External cavity lasers
Abstract
METHOD AND APPARATUS FOR STABILIZING THE SPECTRAL
CHARACTERISTICS OF A SEMICONDUCTOR LASER DIODE
Abstract of the Disclosure In a method and apparatus for stabilizing the emission spectrum of a semiconductor laser diode, an external optical cavity is coupled to the laser diode to couple light from an active region of the laser diode to the external cavity and from the external cavity to the active region of the laser diode. The phase of light coupled from the external cavity to the active region of the laser diode is dithered about a phase operating point at a dither frequency.
A component of the forward voltage across the laser at the dither frequency is sensed and a feedback signal indicative of the amplitude of said component of the forward voltage and the phase of said component of the forward voltage relative to the phase of the dither is generated. The phase operating point is changed in response to the feedback signal to minimize the amplitude of said component of the forward voltage. The method and apparatus are useful for stabilization of laser diodes used in high performance optical communications systems.
CHARACTERISTICS OF A SEMICONDUCTOR LASER DIODE
Abstract of the Disclosure In a method and apparatus for stabilizing the emission spectrum of a semiconductor laser diode, an external optical cavity is coupled to the laser diode to couple light from an active region of the laser diode to the external cavity and from the external cavity to the active region of the laser diode. The phase of light coupled from the external cavity to the active region of the laser diode is dithered about a phase operating point at a dither frequency.
A component of the forward voltage across the laser at the dither frequency is sensed and a feedback signal indicative of the amplitude of said component of the forward voltage and the phase of said component of the forward voltage relative to the phase of the dither is generated. The phase operating point is changed in response to the feedback signal to minimize the amplitude of said component of the forward voltage. The method and apparatus are useful for stabilization of laser diodes used in high performance optical communications systems.
Description
~'~77~0~L
ME~HOD ~ND APPARATUS FOR STABILIZING THE SPECTRAL
CHARACTERISTICS OF A SEMICONDUCTOR LASER DIODE
Eield o~ the Invention The present invention relates to methods and apparatus for stabilizing the spectral characteristics of a semiconductor laser diode.
Backqround of the Inventio~
Semiconductor laser diodes having stable emission spectra are desirable ~or use in high performance optical communications systems.
External optical cavities have been optically coupled to semiconductor laser diodes to modify the emission spectra of the semiconductor laser diodes (Goldberg et al, IEEE J. Quantum Electron., 19~2, QE-18, pp.555-563; Acket et al, IEEE J. Quantum Electron., 1984, QE-20, pp.1163-116g:
Tkach et al, IEEE J. Lightwave Technology, 1986, LT-4, pp.1655-1661). The modified spectra are sensitive to the phase o~ external optical feedbacX from the external optical cavity. Hence, the phase of the external optical feedback must be controlled in order to stabilize the modified spectra.
Several schemes for controlling the phase of the external optical feedback have been discussad in the literature ~Favre et al, Electron. ~ett., 1983, Val. 19, Z5 pp.l046-1048, Ho et al, Third International Conference on Optical Fiber Sensors, San Diego, 13-14 February 19~5;
Neilsen et al, J. Opt. Commun., 19~, pp. 41; Jain et al, Electron. Lett., Vol. 21. pp.957-958)~ Each of the e schemes requires one or more photodetectors which add to the cost and compl~xity of optical transmitters employing such schemes.
Summary of the Invention The present invention provides a novel method and apparatus for stabilizing the emission spectrum of a semiconductor diode laser which avoid the requirement for photodetectors.
One aspect o the present invention provides a .
t~70~L
method for stabilizing the emission spectrum of a semiconductor laser diode, the method comprising:
optically coupling an external optical cavity to the laser diode to couple light from an active region of the laser diode to the external cavity and from the external cavity to the active region of the laser diode;
dithering the phaæe of light coupled from the external cavity to the active region of the laser diode about a phase operating point at a dither frequency;
sensing a component of the forward voltage across the laser at the dither fre~uency;
generating a feedback signal indicative of the amplitude of said component of the forward voltage and the phase of said component of the forward voltage relative to the phase of the dither; and changing the phase operating point in response to the feedback signal to minimize the amplitude of said component of the forward voltage.
Another aspect of the invention provides apparatus for stabilizing the emission ~pectrum of a semiconductor laser diode, the apparatus comprising:
an external optical cavity optically coupled to the laser diode to couple light from an active region of the laser diode to the external cavity and from the external cavity to the active region of the laser diode;
phase control means responsive to a phase control signal for controlling the phase of light coupled from the ~xternal cavity to the active region o~ the laser diode;
dither means for dithering the phase control ~ignal to dither the phase of light coupled ~rom the external cavity to the active region of the laser diode about a phase operating point at a dither frequency;
feedback means for sensing a component of the forward voltage across the laser at the dither frequency, generating a feedback si~nal in~icative of the amplitude of said component of the forward voltage and the phase of said component of the forward voltage relative to the phase of the dither, and changing the phase operating point in response to 7t7~) 3L
the feedback ignal to minimize the amplitude of said component of the forward voltage.
The method and apparatus according to the invention rely on detection of the first derivative of the forward ~oltage across the laser diode with respect to the phase of the optical feedback to sense the phase of the optical feedback from the external cavity into the active region of the laser diode. Consequently no photodetectors are required in the feedback loop used to stabilize the phase of the optical feedback.
Brief Description of the Drawinqs Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which:
Figure 1 is a schematic diagram of apparatus according to a first embodiment;
Figure 2 is a plot of forward voltage across a laser diode o~ the apparatus of Fiyure 1 versus the position o~ a mirror of the apparatus of Figure 1;
Figure 3 is a plot of forward voltage versus time for different positions of the ~irror;
Figure 4 is a schematic cliagram of apparatus according to a second embodiment;
Figure 5 is a schematic diagram of apparatus according to a third embodiment;
Figure 6 is a schematic diagram of apparatus according to a fourth embodiment; and ~ igure 7 is a schematic diagram of one implementation of a phase sensitive detector of the apparatus of Figures 1, 4, 5 and 6.
Figure 1 illustrates apparatus 100 according to a ~irst embodiment for stabilizing the emission spectrum of a semiconductor laser diode 110. The apparatus 100 comprises drive means in the form of a modulated current source 120 for driving the laser diode 110.
~ he apparatus 100 further comprises an external optical cavity 130 which is optically coupled to the laser ~'Z,~7~7V~L
diode 110 to couple light from an active region of the laser diode 110 to the external cavity 130 and from the external cavity 130 to the active region of the laser diode 110.
An axial dimension of the external cavity 130 is defined by a reflecting means in th~ form of a mirror 132.
The mirror 132 is mounted to moving means in the form of a piezoelectric crystal 134 for axial movement toward and away from the laser diode 110 as indicated by the arrow A.
Movement o~ the mirror 132 toward or away from the laser diode 110 changes the phase of light coupled from the external cavity 130 to the active region of the laser diode 110, thereby changing the centre frequency and ~idth of the emission spectrum of the laser diode 110. Thus, the mirror 132 and the piezoelectric crystal 134 together comprise phase control means responsive to a phase control signal applied to the piezoelectric crystal 134 to control the phase of light coupled from the external cavity 130 to the active region o~
the laser diode 110.
The apparatus 100 further comprises dither means in the form of a dither signal generator 140 which generates a dither signal at a particular dither frequency. The dither frequency is much lower than the frequency of the modulated current source 120. The dither signal drives the piezoelectric crystal 134 through summation means comprising a summer 150 and a drive amplifier 152 to dither the phase o~
light coupled from the external cavity 130 to the active region of the laser diode 110.
Figure 2 is a plot of the ~orward volta~e across the laser diode 110 versus the position of the mirror 132.
3G The position o~ the mirror 132 determines the phase of optical feedback from the external cavity 130 to the active region of the laser diode 110. The phase of the optical feedback affects the centre frequency o~ the laser diode 110.
Any change in the centre ~requency of the laser diode 110 is accompanied by a corresponding change in the forward voltage across the laser diode 11~.
The dither signal drives the mirror 132 into periodic motion, causing the phase o~ light coupled ~rom the 7~7~)~
external cavity 130 to the active region of the laser diode 110 to dithar about a phase operating point. The phase dither causes the forward voltage across the laser diode 110 to vary periodically at the dither frequency. As shown in Figure 2, the amplitude A of the periodic variation of the forward voltage for a given amplitude B of mirror motion depends on the phase operating point. In particular, the amplitude A of tha periodic variation of the forward vol~age is minimized when the phase operating point corresponds to a local maximum C or minimum D in the forward voltage characteristis shown in Figure 2.
Moreover, as shown in Figure 3, the periodic variation of the forward voltage is in phase with the dither signal for phase operating points on one side of a local maximum C or minimum D and out of phase with the dither signal on an opposite sides of the local maximum C or minimum D.
The apparatus 100 ~urther comprises feedback means in the form of a phase sensitive detector 160 which is AC
- 20 coupled to the laser diode 110 via a high pass filter 162 and a low frequency amplifier 164. The phase sensitive detector 160 is tuned to the dither frequency to sense a component of the forward voltage across the laser diode 110 at the dither frequency. The phase sensitive detector 160 generates a feedback signal having a magnitude corresponding to the amplitude of the component of the forward voltage at the dither frequency and having a polarity corresponding to the phase of that component relative to the phase of the dither signal.
The phase sensitive detector 160 is connected to the summer 150 to add the feedback signal to the dither signal for application to the piezoelectric crystal 134 via the drive amplifier 152. The feedback signal adds a component to the dither signal which shifts the position about which the mirror 132 is dithered toward or away from the laser diode 110. This shl~t in the mirror dither changes the phase operating point about which the phase of light coupled from the external cavity 130 to the active region of ,?,7~7t7~3~
the laser diode 110 is dithered so as to reduce the amplitude of the sensed component of the forward voltage. Continued feedback stabilizes the phase operating point at a point corresponding to a local maximum C or minimum D of the forward voltage characteris~ic where the amplitude of the sensed component of the forward voltage is minimized, thereby stabiliziny the emission spectrum of the laser diode 110. Any small perturbation of the phase operating point away from the local maximum C or minimum D gives rise to a feedback signal which drives the phase operating point back toward the local maximum C or minimum D.
The time constant of the phase sensitive detector 160 is made large enough to ensure stability. For example, a phase sensitive detector 160 having a time constant of three seconds is suitable for a modulation rate of 300 Mbps and a reference signal frequency of 400 Hertz.
In order to keep the periodic variation of the centre frequency and spectral width within acceptable bounds, the dither signal amplitude and the gain of the piezoelectric crystal drive amplifier 152 should be adjusted for very slight oscillatory movement of the mirror 132, for example 0.05 microns of peak to peak displacement.
Figure 4 shows apparatus 200 according to a second embodiment which differs from th~ apparatus 100 according to the first embodiment in that variable DC offset means in the form of a variable DC voltage source 170 and a summer 172 are connected between the output of the summer 150 and the input of the piezoelectric crystal drive amplifier 152. The DC
of~set may bs varied by means of the variable DC voltage source 170 to select a different peak or trough of the forward voltage versus mirror position characteristic shown in Figure 2 as the point of stabilization. Alternatively, the DC offset means may be built into the phase sensitive detector ~60 or the summer 150.
Figure S shows apparatus 300 according to a third embodiment which differs ~rom the apparatuæ 200 according to the second embodiment in that the means for adjusting the phase operating point in the form of a variable DC voltage ~v~ 7~70~L
source ~70 and a summing amplifier 172 are replaced with mechanical mirror position adjustment means in the form of a micrometer 174 to which the piezoelectric crystal 134 i8 mounted. The mirror position may be varied by means of the micrometer 174 to select a different peak or trough of the forward voltage versus mirror position characteristic shown in Figure 2 as the point of stabilization. Alternatively, the micrometer 174 may be mechanically connected to the laser diode 110 to adjust the position of the laser diode 110.
Figure 6 shows apparatus 400 according to a fourth embodiment which differs from the apparatus 100 according to the first embodiment in that the mirror 132 is fixed, and an electro-optic crystal 136 is disposed within the external cavity 130. The output of the amplifier 152 is applied to the electro-optic crystal 136 to vary the refractive index of the crystal 136, thereby varying the effective optical length of the external cavity 130 and hence the phase of optical feedback from the external cavity 130 to the active region of the laser diode 110.
The DC offset means or mechanical mirror position adjustment means of the apparatus 200, 300 according to the second and third embodiments may be added to the apparatus 400 according to the fourth embodiment to provide some selectivity of the point of stabilization.
The phase sensitive detector 160 of the apparatus 100, 200, 300, 400 according to the first, second, third and four~h embodiments may be implemented as a synchronous rectifier 166 connected in series with a low pass filter 168 as shown in Figure 7.
The summer 150 or drive amplifier 152 may be provided with capacitive feedback so as to integrate the feedback signal. Integration of the feedback signal should improve system stability for high D.C. loop gains.
ME~HOD ~ND APPARATUS FOR STABILIZING THE SPECTRAL
CHARACTERISTICS OF A SEMICONDUCTOR LASER DIODE
Eield o~ the Invention The present invention relates to methods and apparatus for stabilizing the spectral characteristics of a semiconductor laser diode.
Backqround of the Inventio~
Semiconductor laser diodes having stable emission spectra are desirable ~or use in high performance optical communications systems.
External optical cavities have been optically coupled to semiconductor laser diodes to modify the emission spectra of the semiconductor laser diodes (Goldberg et al, IEEE J. Quantum Electron., 19~2, QE-18, pp.555-563; Acket et al, IEEE J. Quantum Electron., 1984, QE-20, pp.1163-116g:
Tkach et al, IEEE J. Lightwave Technology, 1986, LT-4, pp.1655-1661). The modified spectra are sensitive to the phase o~ external optical feedbacX from the external optical cavity. Hence, the phase of the external optical feedback must be controlled in order to stabilize the modified spectra.
Several schemes for controlling the phase of the external optical feedback have been discussad in the literature ~Favre et al, Electron. ~ett., 1983, Val. 19, Z5 pp.l046-1048, Ho et al, Third International Conference on Optical Fiber Sensors, San Diego, 13-14 February 19~5;
Neilsen et al, J. Opt. Commun., 19~, pp. 41; Jain et al, Electron. Lett., Vol. 21. pp.957-958)~ Each of the e schemes requires one or more photodetectors which add to the cost and compl~xity of optical transmitters employing such schemes.
Summary of the Invention The present invention provides a novel method and apparatus for stabilizing the emission spectrum of a semiconductor diode laser which avoid the requirement for photodetectors.
One aspect o the present invention provides a .
t~70~L
method for stabilizing the emission spectrum of a semiconductor laser diode, the method comprising:
optically coupling an external optical cavity to the laser diode to couple light from an active region of the laser diode to the external cavity and from the external cavity to the active region of the laser diode;
dithering the phaæe of light coupled from the external cavity to the active region of the laser diode about a phase operating point at a dither frequency;
sensing a component of the forward voltage across the laser at the dither fre~uency;
generating a feedback signal indicative of the amplitude of said component of the forward voltage and the phase of said component of the forward voltage relative to the phase of the dither; and changing the phase operating point in response to the feedback signal to minimize the amplitude of said component of the forward voltage.
Another aspect of the invention provides apparatus for stabilizing the emission ~pectrum of a semiconductor laser diode, the apparatus comprising:
an external optical cavity optically coupled to the laser diode to couple light from an active region of the laser diode to the external cavity and from the external cavity to the active region of the laser diode;
phase control means responsive to a phase control signal for controlling the phase of light coupled from the ~xternal cavity to the active region o~ the laser diode;
dither means for dithering the phase control ~ignal to dither the phase of light coupled ~rom the external cavity to the active region of the laser diode about a phase operating point at a dither frequency;
feedback means for sensing a component of the forward voltage across the laser at the dither frequency, generating a feedback si~nal in~icative of the amplitude of said component of the forward voltage and the phase of said component of the forward voltage relative to the phase of the dither, and changing the phase operating point in response to 7t7~) 3L
the feedback ignal to minimize the amplitude of said component of the forward voltage.
The method and apparatus according to the invention rely on detection of the first derivative of the forward ~oltage across the laser diode with respect to the phase of the optical feedback to sense the phase of the optical feedback from the external cavity into the active region of the laser diode. Consequently no photodetectors are required in the feedback loop used to stabilize the phase of the optical feedback.
Brief Description of the Drawinqs Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which:
Figure 1 is a schematic diagram of apparatus according to a first embodiment;
Figure 2 is a plot of forward voltage across a laser diode o~ the apparatus of Fiyure 1 versus the position o~ a mirror of the apparatus of Figure 1;
Figure 3 is a plot of forward voltage versus time for different positions of the ~irror;
Figure 4 is a schematic cliagram of apparatus according to a second embodiment;
Figure 5 is a schematic diagram of apparatus according to a third embodiment;
Figure 6 is a schematic diagram of apparatus according to a fourth embodiment; and ~ igure 7 is a schematic diagram of one implementation of a phase sensitive detector of the apparatus of Figures 1, 4, 5 and 6.
Figure 1 illustrates apparatus 100 according to a ~irst embodiment for stabilizing the emission spectrum of a semiconductor laser diode 110. The apparatus 100 comprises drive means in the form of a modulated current source 120 for driving the laser diode 110.
~ he apparatus 100 further comprises an external optical cavity 130 which is optically coupled to the laser ~'Z,~7~7V~L
diode 110 to couple light from an active region of the laser diode 110 to the external cavity 130 and from the external cavity 130 to the active region of the laser diode 110.
An axial dimension of the external cavity 130 is defined by a reflecting means in th~ form of a mirror 132.
The mirror 132 is mounted to moving means in the form of a piezoelectric crystal 134 for axial movement toward and away from the laser diode 110 as indicated by the arrow A.
Movement o~ the mirror 132 toward or away from the laser diode 110 changes the phase of light coupled from the external cavity 130 to the active region of the laser diode 110, thereby changing the centre frequency and ~idth of the emission spectrum of the laser diode 110. Thus, the mirror 132 and the piezoelectric crystal 134 together comprise phase control means responsive to a phase control signal applied to the piezoelectric crystal 134 to control the phase of light coupled from the external cavity 130 to the active region o~
the laser diode 110.
The apparatus 100 further comprises dither means in the form of a dither signal generator 140 which generates a dither signal at a particular dither frequency. The dither frequency is much lower than the frequency of the modulated current source 120. The dither signal drives the piezoelectric crystal 134 through summation means comprising a summer 150 and a drive amplifier 152 to dither the phase o~
light coupled from the external cavity 130 to the active region of the laser diode 110.
Figure 2 is a plot of the ~orward volta~e across the laser diode 110 versus the position of the mirror 132.
3G The position o~ the mirror 132 determines the phase of optical feedback from the external cavity 130 to the active region of the laser diode 110. The phase of the optical feedback affects the centre frequency o~ the laser diode 110.
Any change in the centre ~requency of the laser diode 110 is accompanied by a corresponding change in the forward voltage across the laser diode 11~.
The dither signal drives the mirror 132 into periodic motion, causing the phase o~ light coupled ~rom the 7~7~)~
external cavity 130 to the active region of the laser diode 110 to dithar about a phase operating point. The phase dither causes the forward voltage across the laser diode 110 to vary periodically at the dither frequency. As shown in Figure 2, the amplitude A of the periodic variation of the forward voltage for a given amplitude B of mirror motion depends on the phase operating point. In particular, the amplitude A of tha periodic variation of the forward vol~age is minimized when the phase operating point corresponds to a local maximum C or minimum D in the forward voltage characteristis shown in Figure 2.
Moreover, as shown in Figure 3, the periodic variation of the forward voltage is in phase with the dither signal for phase operating points on one side of a local maximum C or minimum D and out of phase with the dither signal on an opposite sides of the local maximum C or minimum D.
The apparatus 100 ~urther comprises feedback means in the form of a phase sensitive detector 160 which is AC
- 20 coupled to the laser diode 110 via a high pass filter 162 and a low frequency amplifier 164. The phase sensitive detector 160 is tuned to the dither frequency to sense a component of the forward voltage across the laser diode 110 at the dither frequency. The phase sensitive detector 160 generates a feedback signal having a magnitude corresponding to the amplitude of the component of the forward voltage at the dither frequency and having a polarity corresponding to the phase of that component relative to the phase of the dither signal.
The phase sensitive detector 160 is connected to the summer 150 to add the feedback signal to the dither signal for application to the piezoelectric crystal 134 via the drive amplifier 152. The feedback signal adds a component to the dither signal which shifts the position about which the mirror 132 is dithered toward or away from the laser diode 110. This shl~t in the mirror dither changes the phase operating point about which the phase of light coupled from the external cavity 130 to the active region of ,?,7~7t7~3~
the laser diode 110 is dithered so as to reduce the amplitude of the sensed component of the forward voltage. Continued feedback stabilizes the phase operating point at a point corresponding to a local maximum C or minimum D of the forward voltage characteris~ic where the amplitude of the sensed component of the forward voltage is minimized, thereby stabiliziny the emission spectrum of the laser diode 110. Any small perturbation of the phase operating point away from the local maximum C or minimum D gives rise to a feedback signal which drives the phase operating point back toward the local maximum C or minimum D.
The time constant of the phase sensitive detector 160 is made large enough to ensure stability. For example, a phase sensitive detector 160 having a time constant of three seconds is suitable for a modulation rate of 300 Mbps and a reference signal frequency of 400 Hertz.
In order to keep the periodic variation of the centre frequency and spectral width within acceptable bounds, the dither signal amplitude and the gain of the piezoelectric crystal drive amplifier 152 should be adjusted for very slight oscillatory movement of the mirror 132, for example 0.05 microns of peak to peak displacement.
Figure 4 shows apparatus 200 according to a second embodiment which differs from th~ apparatus 100 according to the first embodiment in that variable DC offset means in the form of a variable DC voltage source 170 and a summer 172 are connected between the output of the summer 150 and the input of the piezoelectric crystal drive amplifier 152. The DC
of~set may bs varied by means of the variable DC voltage source 170 to select a different peak or trough of the forward voltage versus mirror position characteristic shown in Figure 2 as the point of stabilization. Alternatively, the DC offset means may be built into the phase sensitive detector ~60 or the summer 150.
Figure S shows apparatus 300 according to a third embodiment which differs ~rom the apparatuæ 200 according to the second embodiment in that the means for adjusting the phase operating point in the form of a variable DC voltage ~v~ 7~70~L
source ~70 and a summing amplifier 172 are replaced with mechanical mirror position adjustment means in the form of a micrometer 174 to which the piezoelectric crystal 134 i8 mounted. The mirror position may be varied by means of the micrometer 174 to select a different peak or trough of the forward voltage versus mirror position characteristic shown in Figure 2 as the point of stabilization. Alternatively, the micrometer 174 may be mechanically connected to the laser diode 110 to adjust the position of the laser diode 110.
Figure 6 shows apparatus 400 according to a fourth embodiment which differs from the apparatus 100 according to the first embodiment in that the mirror 132 is fixed, and an electro-optic crystal 136 is disposed within the external cavity 130. The output of the amplifier 152 is applied to the electro-optic crystal 136 to vary the refractive index of the crystal 136, thereby varying the effective optical length of the external cavity 130 and hence the phase of optical feedback from the external cavity 130 to the active region of the laser diode 110.
The DC offset means or mechanical mirror position adjustment means of the apparatus 200, 300 according to the second and third embodiments may be added to the apparatus 400 according to the fourth embodiment to provide some selectivity of the point of stabilization.
The phase sensitive detector 160 of the apparatus 100, 200, 300, 400 according to the first, second, third and four~h embodiments may be implemented as a synchronous rectifier 166 connected in series with a low pass filter 168 as shown in Figure 7.
The summer 150 or drive amplifier 152 may be provided with capacitive feedback so as to integrate the feedback signal. Integration of the feedback signal should improve system stability for high D.C. loop gains.
Claims (20)
1. A method for stabilizing the emission spectrum of a semiconductor laser diode, the method comprising:
optically coupling an external optical cavity to the laser diode to couple light from an active region of the laser diode to the external cavity and from the external cavity to the active region of the laser diode;
dithering the phase of light coupled from the external cavity to the active region of the laser diode about a phase operating point at a dither frequency;
sensing a component of the forward voltage across the laser at the dither frequency;
generating a feedback signal indicative of the amplitude of said component of the forward voltage and the phase of said component of the forward voltage relative to the phase of the dither: and changing the phase operating point in response to the feedback signal to minimize the amplitude of said component of the forward voltage.
optically coupling an external optical cavity to the laser diode to couple light from an active region of the laser diode to the external cavity and from the external cavity to the active region of the laser diode;
dithering the phase of light coupled from the external cavity to the active region of the laser diode about a phase operating point at a dither frequency;
sensing a component of the forward voltage across the laser at the dither frequency;
generating a feedback signal indicative of the amplitude of said component of the forward voltage and the phase of said component of the forward voltage relative to the phase of the dither: and changing the phase operating point in response to the feedback signal to minimize the amplitude of said component of the forward voltage.
2. A method as defined in claim 1, comprising dithering the phase of light coupled from the external cavity to the active region of the laser diode and changing the phase operating point by moving reflecting means defining a dimension of the external cavity to change said dimension of the external cavity.
3. A method as defined in claim 1, comprising dithering the phase of light coupled from the external cavity to the active region of the laser diode and changing the phase operating point by changing the refractive index of at least a portion of the external cavity.
4. A method as defined in claim 1, further comprising adjusting the phase operating point to adjust the spectral characteristics of the laser diode.
5. A method as defined in claim 4, comprising dithering the phase of light coupled from the external cavity to the active region of the laser diode, changing the phase operating point and adjusting the phase operating point by moving reflecting means defining a dimension of the external cavity to change said dimension of the external cavity.
6. A method as defined in claim 4, comprising dithering the phase of light coupled from the external cavity to the active region of the laser diode, changing the phase operating point and adjusting the phase operating point by changing the refractive index of at least a portion of the external cavity.
7. A method as defined in claim 1, further comprisingdriving the laser diode with a modulated current at a frequency much greater than the dither frequency.
8. Apparatus for stabilizing the emission spectrum of a semiconductor laser diode, the apparatus comprising:
an external optical cavity optically coupled to the laser diode to couple light from an active region of the laser diode to the external cavity and from the external cavity to the active region of the laser diode:
phase control means responsive to a phase control signal for controlling the phase of light coupled from the external cavity to the active region of the laser diode;
dither means for dithering the phase control signal to dither the phase of light coupled from the external cavity to the active region of the laser diode about a phase operating point at a dither frequency;
feedback means for sensing a component of the forward voltage across the laser at the dither frequency, generating a feedback signal indicative of the amplitude of said component of the forward voltage and the phase of said component of the forward voltage relative to the phase of the dither, and changing the phase operating point in response to the feedback signal to minimize the amplitude of said component of the forward voltage.
an external optical cavity optically coupled to the laser diode to couple light from an active region of the laser diode to the external cavity and from the external cavity to the active region of the laser diode:
phase control means responsive to a phase control signal for controlling the phase of light coupled from the external cavity to the active region of the laser diode;
dither means for dithering the phase control signal to dither the phase of light coupled from the external cavity to the active region of the laser diode about a phase operating point at a dither frequency;
feedback means for sensing a component of the forward voltage across the laser at the dither frequency, generating a feedback signal indicative of the amplitude of said component of the forward voltage and the phase of said component of the forward voltage relative to the phase of the dither, and changing the phase operating point in response to the feedback signal to minimize the amplitude of said component of the forward voltage.
9. Apparatus as defined in claim 8, wherein the dither means comprises:
a dither signal generator for generating a dither signal at the dither frequency;
and the feedback means comprises:
a phase sensitive detector tuned to the dither frequency, said phase sensitive detector being AC coupled to the laser diode to sense said component of the forward voltage across the laser diode; and summation means coupled to the dither signal generator and the phase sensitive detector for summing the dither signal and the feedback signal and applying the resulting phase control signal to the phase control means.
a dither signal generator for generating a dither signal at the dither frequency;
and the feedback means comprises:
a phase sensitive detector tuned to the dither frequency, said phase sensitive detector being AC coupled to the laser diode to sense said component of the forward voltage across the laser diode; and summation means coupled to the dither signal generator and the phase sensitive detector for summing the dither signal and the feedback signal and applying the resulting phase control signal to the phase control means.
10. Apparatus as defined in claim 9, wherein the phase sensitive detector comprises a synchronous rectifier and a low pass filter connected in series.
11. Apparatus as defined in claim 9, wherein the summation means comprises amplification means.
12. Apparatus as defined in claim 9, wherein the summation means integrates the feedback signal for application to the phase control means.
13. Apparatus as defined in claim 8, wherein the phase control means comprises movably mounted reflecting means defining a dimension of the external cavity and moving means for moving the reflecting means to control said dimension of the external cavity.
14. Apparatus as defined in claim 13, wherein the moving means comprises a piezoelectric crystal and the reflecting means comprises a reflecting surface carried on the crystal.
15. Apparatus as defined in claim 8, wherein the phase control means comprises refractive index changing means for changing the refractive index of at least a portion of the external cavity in response to an electrical signal.
16. Apparatus as defined in claim 15, wherein the refractive index changing means comprises an electro-optic crystal.
17. Apparatus as defined in claim 8, further comprising means for adjusting the phase operating point to adjust the spectral characteristics of the laser diode.
18. Apparatus as defined in claim 9, further comprising means for adjusting the phase operating point to adjust the spectral characteristics of the laser diode, said means comprising variable DC offset means connected to the summation means for applying an adjustable DC offset to the phase control signal.
19. Apparatus as defined in claim 13, further comprising means for adjusting the phase operating point to adjust the spectral characteristics of the laser diode, said means comprising mechanical adjustment means for adjusting the relative positions of the reflecting means and the laser diode.
20. Apparatus as defined in claim 8, further comprising means for driving the laser diode with a modulated current at a frequency much greater than the dither frequency.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000576356A CA1277701C (en) | 1988-09-01 | 1988-09-01 | Method and apparatus for stabilizing the spectral characteristics of a semiconductor laser diode |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000576356A CA1277701C (en) | 1988-09-01 | 1988-09-01 | Method and apparatus for stabilizing the spectral characteristics of a semiconductor laser diode |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1277701C true CA1277701C (en) | 1990-12-11 |
Family
ID=4138663
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000576356A Expired - Lifetime CA1277701C (en) | 1988-09-01 | 1988-09-01 | Method and apparatus for stabilizing the spectral characteristics of a semiconductor laser diode |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1277701C (en) |
-
1988
- 1988-09-01 CA CA000576356A patent/CA1277701C/en not_active Expired - Lifetime
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| US4856010A (en) | Laser frequency control | |
| US5579328A (en) | Digital control of laser diode power levels | |
| US7313156B2 (en) | Control method of distributed Bragg reflection semiconductor laser, and image projecting apparatus | |
| EP0240512B1 (en) | Frequency referencing system and method | |
| JPS59205789A (en) | Semiconductor device | |
| JP2942619B2 (en) | Harmonic generator | |
| EP0229825B1 (en) | Apparatus for stabilizing a laser beam | |
| MX9300337A (en) | FREQUENTLY OPTICAL SOURCE ASSURED TO A GRID RESONATOR IN FIBER. | |
| CA2132043A1 (en) | Method and Apparatus for Frequency Modulating a Semiconductor Laser, and an Optical Communication System Using the Same | |
| JPH04157780A (en) | Frequency stabilizer for semiconductor laser | |
| CA1261046A (en) | Dual-wavelength laser apparatus | |
| US4592058A (en) | Frequency stabilized laser | |
| US7209498B1 (en) | Method and apparatus for tuning a laser | |
| CA1277701C (en) | Method and apparatus for stabilizing the spectral characteristics of a semiconductor laser diode | |
| US4872173A (en) | Method and apparatus for stabilizing the spectral characteristics of a semiconductor laser diode | |
| US5442326A (en) | Atomic time standard with piezoelectric stabilization of diode laser light source | |
| US6744792B1 (en) | Wavelength stabilization of tunable lasers by current modulation | |
| JPH0783154B2 (en) | Wavelength switching light source | |
| US5305333A (en) | Method and apparatus for amplitude modulation of laser light | |
| JPS62230073A (en) | Variable wavelength light source | |
| JP2541095B2 (en) | Laser wavelength stabilization method | |
| WO2002075874A1 (en) | Method and apparatus for tuning a laser | |
| JPS6344783A (en) | Frequency stabilizer of laser light source | |
| JP3479599B2 (en) | Method for stabilizing light source wavelength of semiconductor laser and optical module | |
| JP3411832B2 (en) | Optical phase-locked light source and optical frequency-locked light source |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKLA | Lapsed |