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A computer controlled time resolved optical intensity profile measurement system
Volume 45, number 5
OPTICS COMMUNICATIONS
1 May 1983
A COMPUTER CONTROLLED TIME RESOLVED OPTICAL INTENSITY PROFILE MEASUREMENT SYSTEM I.H. WHITE and J.E. CARROLL Cambridge University Engineering Department, Cambridge, CB2 1PZ, UK Received 28 January 1983
In this paper, a computer controlled system is descried, which provides data management and position control for time resolved measurement of near and far field, along with charge carrier concentration distr~utions of semiconductor lasers. Of particular interest is the manner in which microprocessor control of inexpensive motorised micrometers is used to provide accurate repeatable movements on submicron scales. Due to its noise reduction capability through computer averaging, this system has been operated in conjunction with a 50 ps risetime photodiode, and thus is believed to allow time resolved measurement on the shortest reported timescales. The system provides a further advantage in that experimental data may be stored to give easy access for further processing.
1. Introduction One major property of the semiconductor laser is its ability to couple light into an optical fibre which can then be used in a communication system. However for such operation, the optical filament within the laser must be stable with time and not shift in position, typically by more than a fraction of the fibre width. For a single mode fibre this condition implies stability to submicron dimensions. Consequently, it is of great interest [1,2] to measure the spatial distribution of optical intensity as light emerges from the laser, the angle at which it emerges, and the position of the waist of the beam before it starts to diverge. The optical Fdament within the laser is controlled to a great extent by the spontaneous emission intensity so that further information may be acquired about the spatial distribution of the spontaneous emission within the laser. As the light beam in a semiconductor laser is known to change with time i3,4] a full understanding of the light guiding properties of these devices really requires time resolved measurement of all these distributions. The purpose of this paper is to describe such a computer system providing automatic accurate measurement of light intensity profiles across a laser at 0 030-4018/83/0000-0000/$ 03.00 © 1983 North-Holland
required times during a generated optical pulse. It is versatile, inexpensive, and through convenient data management, provides easy access to experimental resuits. The system is particularly interesting in that it provides simultaneous measurement of a series of intensity profiles during an ultrashort lasing pulse. Van der Ziel [3] has already noticed that small changes in lasing profile may occur over very short periods of time (less than 1 ns). The system reported here has been developed to further investigate these changes by being constructed to operate to resolutions of 50 ps. However by measuring distributions effectively simultaneously, the authors have ensured that experimental shifting errors between the distributions may be eradicated. The computer system is based around a microcomputer (a Research Machines Ltd. 380Z microcomputer) linked to two inexpensive motorised micrometers which move the laser in the transverse and axial directions. Using the computer these micrometers may be used to position the laser to tolerances of less than 0.2 tam. Fig. 1 is a schematic diagram of the overall system. The microcomputer is used to drive the laser across the front o f the microscope objective lens. At the same time the computer is able to sample the output signal from a photodiode at speci289
Volume 45, number 5
OPTICS COMMUNICATIONS
1 May 1983
MOTORISED MICROMETERS 1.o rY D
rno. 8 rY Ld
no.6-
PULSE [GENERATOR[
[EE~ trigger
bJ
~)0.4
-q, I
I PLOTTER ~
III I MICRO
COMPUTER
Fig. 1. A schematic diagram of the computer system.
fied times during the optical pulse. The photodiode is positioned so that it monitors the light output from a point on the laser beam profile. By this method the light intensity across the laser beam can be measured. One major advantage of the reported system is that despite providing high precision positioning accuracy, it is based on inexpensive motorised micrometers. These consist of conventional d.c. motors linked to a fine screw thread. In normal use they are frequently driven directly by d.c. variable power supplies, or with pulsed supplies using various widths of electrical pulses to obtain varying speeds. However the author has determined that controlled movement on micron levels may best be carried out using varying lengths of bursts of short electrical pulses. These pulses are generally of the order of 1 ms long. The number of pulses in each burst may accurately control the speed of movement of the motorised micrometer (fig. 2). Often motormicrometers provide uneven movement because inconsistencies in the laser mounts cause varying drag to occur. However by driving the motors for short periods of time with high electrical current drives and hence high torques, the above problems may be overcome even when only very small movement is required. Even long movements of millimeters may be carried out very accurately by using many bursts of pulses. Here the speed of movement may be kept small even if the torque produced by the motors when driven is high. Linearity has been found to be maintained by running the micrometers with pulses of constant amplitude. As a variable power supply is not required the motormicrometers may be driven from a single bit of the 290
bJO.2 0 Z
<[ r- 0 ~n 0 2 4 6 8 10 NUMBER OF PULSES PER BURST i
Fig. 2. Plot of distance moved by a single burst of pulses versus the number of pulses in each burst. (Pulse width = 5 ms).
computer output port. Thus computer control of the positioning equipment is simplified. The microcomputer control accesses peripheral units in the overall system using a series of analogue to digital converter input and output channels, along with a parallel output port. The parallel output port consists of 8 outputs which may be set either at a high or low level. Four of the eight outputs are used so that the 2 output ADC channels may be switched into line with as many as 8 external units. A further 3 outputs control the two motorised micrometers via high current buffers. Here two bits independently access each motormicrometer and the third is used to control the direction of movement of the motors. In this way the laser may be swept across the front of the microscope objective lens to allow intensity distributions to be measured, and an axially directed motorised micrometer be used to position the laser for measurement of near and far field and waist distributions. A photodiode (either a 100 ps risetime avalanche photodiode or a 50 ps risetime silicon pin photodiode) is used for light detection. The lens/ photodiode distance is chosen so that the resolution limit due to the area of the photodiode is of the same order as that due to the microscope objective lens. In the reported system this was believed to be less than 1 /am. As the system is based on laser movement rather than photodiode movement, lens/photodiode distance does not affect calibration of the system. The output of the photodiode is fed into a sampling oscilloscope, which provides a facility whereby the sampling window of the scope can be externally set
Volume 45, number 5
OPTICS COMMUNICATIONS
SAMPLING SCOPE SCREEN \
t,r~e ps)
L~GNT J OUTPUT : ROiv' LASER
OUTPUT FROM SCOPE
TDUE TO SIGNAL A
"i-~ms)
' [ i
I
I
! NNnnn tSAMPLED OUTPUT hnnnnllHIINll |WAVEFORM READ BY : I ~ COMPUTER t(ms)
~ ! ; A~N~L~ RAOwMN COWMRI~ERlgO CONTROL
Fig. 3. A schematic diagram demonstrating the time resolved sampling operation of the computer system. to any position along the optical pulse. In the reported system, the computer is used to control the position of the sampling point along the optical pulse, by sending an electrical signal via an ADC output channel to the oscilloscope (fig. 3). The scope is set at a time determined by the computer, to output a level proportional to the sampled signal. This level may then be sampled by the computer, enabling noise reduction to be undertaken by computer averaging. Hence in fig. 3, if two points are required along the optical pulse, signal A is applied to the scope. This signal in turn sets the scope at the 2 required time positions sequentially, keeping the sampling window at a given point until the computer averages the required number of samples for the purposes of noise reduction. The sampled signals are digitally averaged by the microcomputer. A fast averaging routine whereby the averaging calculation is carried out as the next sample is fetched from the analogue to ditigal converter allows the microcomputer to sample and average a point around 100 000 times a minute. As the averaging routine is written in machine code, blocks of 255 samples are averaged. Generally 5 blocks of samples (over 1000 samples) are used to reduce the noise of the system to reasonable levels. The averaging process does not significantly slow the system down as, because of the mechanical inertia of the motormicrometers, time is required to allow the system to settle after each movement. Time is also required to allow the electrical system to stabilise. A near field distribution sweep of 100 points generally takes around 90 s to complete. The overall system then operates by moving the
1 May 1983
laser across the objective lens in 0.2/am increments. At each increment the computer will sample and average a series of points along the optical pulse in the manner described above. Once sampling of all the requested points is completed, the laser will then be moved by another 0.2/am and a further series of samples undertaken. In this way a series of near field distributions at various points along the optical pulse may be measured simultaneously. This method thus removes all errors related to the relative positions of the measured distributions. Although this system at present does not include any position transducers, the measurements have been found to be highly reliable (to less than 2% in near field width). Moreover distributions at different current levels may be compared by driving the laser with triangular waveforms or multilevel pulses. Useful information concerning laser operation may be gleaned from the charge carrier concentration in a laser. It is known that the spontaneous emission from a laser is directly proportional to the charge carrier concentration. Charge carrier concentration distributions have been determined using the system by measuring the spontaneous emission output from the laser. Here the stimulated emission is blocked using a Glan Thompson polariser in conjunction with an interference filter whose peak transmission was at a wavelength of around 20 nm below that of the stimulated emission. As Gallium Arsenide becomes transparent at longer wavelengths, shorter wavelengths are preferred for the monitoring of spontaneous emission. The computer system through averaging, so reduces the measurement noise that particularly low levels of spontaneous emission can be measured by sampling even on picosecond timescales. The improvement in signal to noise ratio can be maintained even if the avalanche photodiodes are used with modest current gains for good linearity of the photodiode output with input. Far field and waist distributions are measured by moving the laser axially and again carrying out profile measurements. If prefered the system can measure the profile width directly. All further data may be stored in the computer to allow further processing and comparison with theory.
291
Volume 45, number 5
OPTICS COMMUNICATIONS
1 May 1983
2. Experimental results
l ~II~?HNTsITY This system has been primarily developed for investigation of twin stripe lasers [5]. Here asymmetric near field distributions may be well-controlled by injected stripe currents. Unfortunately single stripe lasers allow shifting of the optical filament only in an uncontrolled manner, and hence cannot be used to conclusively test some theoretical simulations. Twin stripe lasers however, allowing much greater control of the optical filament may be more readily used for the testing of theoretical simulations. Fig. 4 shows the charge carrier concentration and near field distributions of twin stripe lasers at threshold as the stripe current ratio is varied. These lasers consist of two lasing stripes each 3/am wide laid on the laser with a stripe separation of 3/am. We observe a symmetrical double peaked charge carrier concentration distribution in the case of both stripes being driven together. Here each stripe contributes equally to the charge carrier concentration. This in turn produces a narrow near field distribution guided by the self focusing effect. As the stripe current ratio increases, so the charge carrier concentration becomes progressively asymmetric until, with one stripe predominantly driven, a wide near field pattern is obtained. Such a pattern is believed to be gain guided. Fig. 5 shows both of these modes measured simultaneously. The near field distributions measured, when one stripe is driv-
CHARGECARRIER
(a)
CONCENTRATIONS n
I /&l~
I
II l
n ''
"
n
I =1^ =40mA ! '11.=90mA '\ 1 ~1'/'1 / ~I~ =56 mA
I0
0
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I0
I1
I0
0
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L I,
/
/ /
/
/ / 0
IO DISTANCE
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25 O 25 DISTANCEALONGLASERJUNCTION(#.m) Fig. 5. Near and far field distributions of a twin stripe laser with (a) both stripes driven together, and (b) only one stripe dirven. (Pulse width = 150 ns, 1.1 times threshold). en and when both stripes are driven together seem to confirm existing theory [6]. Further interesting results have been obtained with picosecond pulses generated from twin stripe semiconductor lasers by direct electrical modulation. Here the laser is biased to threshold, and radio frequency modulation is applied to the more weakly driven stripe. This modulation may generate pulses with widths dependent upon stripe current ratio. The pulse widths measured by intensity autocorrelation in type-1 second harmonic generation in LilO 3 vary between 40 ps with both stripes driven together, and 20 ps with one stripe predominantly driven (fig. 6). This effect is believed to be due to the optical filament being switched in regions of higher loss with one stripe predominantly driven, than with both stripes driven together. Hence the effective photon lifetime is shorter, and the minimum pulse width reduced. This work is consistent with the observed light/current characteristics. Both threshold current and incremental efficiency are dependent upon the optical loss "of the laser cavity. Hence any change in the threshold currents or quantum efficiencies of the lasers should lead to a change in the minimum pulse
> -100 r-4 ~90 T # -80
I0
J'!
L/\ / I. I0
=
110mA / ..,2=- OmA
~1_
//l
'
A ~ (a)
(l;m)
u3 0_ 40-r 30LG / 200_ 0
PULSE W I D
~
/
-70
;
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8
20 LOG (11/I2) Fig. 4. Near field and charge carrier concentration distributions o f twin stripe lasers under differing drive conditions. (Pulse width = 100 ns).
292
lo
-60
c:u
50
m z >
Fig. 6. Plot of minimum pulse width and total threshold current versus stripe current ratio.
Volume 45, number 5
OPTICS COMMUNICATIONS
width which may be generated by a semiconductor laser. Fig. 6 also shows a plot of total threshold current versus stripe current ratio. We can clearly see that the threshold current with one stripe predominantly driven is approximately twice that with both stripes driven together. Moreover the quantum efficiency with both stripes driven together is 1.6 times that with a high stripe current ratio. Thus from the light/current characteristics we would expect to find the greater optical loss with one stripe predominantly driven leading to a lower photon lifetime and hence smaller minimum pulse width. Near field and far field distributions measured at specific times along the optical pulse (to resolutions of 100 ps) also agree with this theory. Near field distributions have been carried out observing both the peak and trough of the picosecond pulse. We can see (fig. 7) that as the stripe current ratio is altered, so the near field distribution changes. For the case of both stripes being driven together, the near field mode builds up in its normal
(a)
L ~',~
PEAK OF THE / I \ 40 ps PULSE"--.~/ I / TROUGH Or T.E , o .s PU
/ /
I I
/ /
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5
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PEAKOF 20ps s PULSE
I
zero order self focused mode in a similar manner to normal biasing operations. Here the asymmetric application of the radio frequency modulation does not significantly affect the waveguiding of the filament. This is probably due to the near field distribution being strongly self focused and rather less perceptable to movement on picosecond timescale. With a high stripe current ratio, the near field pattern is remarkably different. The near field distribution at the trough of the pulse is asymmetric, and similar to that obtained under normal bias with high stripe current ratios. However at the peak of the pulse, the near field distribution (fig. 7(b)) shows a second peak on the opposite side of the laser from the original one. This second peak is likely to be directly due to the radio frequency modulation being supplied to the more weakly driven stripe. It is believed that as the optical pulse width (~20 ps) is so much shorter than the charge carrier diffusion time (~3 ns), time is not available for diffusion of carriers to allow a mode similar to that generated under d.c. conditions to be generated. On the contrary the radio frequency modulated stripe is more likely to act in a relatively independent manner. Hence a separate peak in the near field distribution is produced in a high loss region. As the pulse is generated in a region of higher optical loss, we expect a decrease in pulse width to be observed. Thus near field measurements agree well with direct measurements of pulse width.
5 (pm)
L (b)
1 May 1983
I
5 0 5 (pm) DISTANCE ALONG THE LASER JUNCTION Fig. 7. Near field distributions of a twin stripe laser during the generation of picosecond pulses (a) with both stripes driven together, and (b) with one stripe predominately driven. (These measurements were made using a 100 ps risetime avalanche photodiode.)
In conclusion, an automatic computer system providing the measurement of the evolution of near and far field, and charge carrier concentration distributions through time has been reported. This system may be operated using low sensitivity ultrafast photodiodes as detectors, and hence is believed to be the fastest such system. It is extremely versatile through not requiring recalibration when used for different applications. Moreover through efficient computer management of experimental data, further processing of results may be easily carried out. Novel effects have already been observed in the measurement of twin stripe lasers, and the system has already allowed checking of theoretical work. 293
Volume 45, number 5
OPTICS COMMUNICATIONS
1 May 1983
Acknowledgement
References
The authors would like to thank Mr. B Herscher for many helpful discussions. They would also like to thank Dr. R.G. Plumb o f Standard Telecommunication Laboratories Ltd., Harlow, England for the supply o f the twin stripe lasers used in this work. One o f the authors (I.H. White) would like to thank the Department o f education for Northern Ireland for a personal maintenance grant. The UK Science and Engineering Research Council are thanked for supporting this work with the award o f an equipment grant.
[1] G.H.B. Thompson, D.F. Lovelace and S.E.H. Turley, IEEE. J. Solid State and Electron Devices 2 (1978) 12. [2] R.P. Brouwer, C.H.F. Velzel, and B.S. Yeh, IEEE. J. Quantum Electron. 17 (1981) 694. [3] J.P. van der Ziel, IEEE J. Quantum Electron. 17 (1981) 60. .[4] J.P. Goure and J.N. Massot, Optical and Quantum Electronics 14 (1982) 445. [5] I.H. White, J.E. Carroll and R.G. Plumb, lEE Proc.-l, Solid State and Electron Devices 129 (1982) 216. [6] P.M. Asbeck, D.A. Cammack, J.J. Daniele and Y. Klebanoff, IEEE J. Quantum Electron. 15 (1979) 727.
294
OPTICS COMMUNICATIONS
1 May 1983
A COMPUTER CONTROLLED TIME RESOLVED OPTICAL INTENSITY PROFILE MEASUREMENT SYSTEM I.H. WHITE and J.E. CARROLL Cambridge University Engineering Department, Cambridge, CB2 1PZ, UK Received 28 January 1983
In this paper, a computer controlled system is descried, which provides data management and position control for time resolved measurement of near and far field, along with charge carrier concentration distr~utions of semiconductor lasers. Of particular interest is the manner in which microprocessor control of inexpensive motorised micrometers is used to provide accurate repeatable movements on submicron scales. Due to its noise reduction capability through computer averaging, this system has been operated in conjunction with a 50 ps risetime photodiode, and thus is believed to allow time resolved measurement on the shortest reported timescales. The system provides a further advantage in that experimental data may be stored to give easy access for further processing.
1. Introduction One major property of the semiconductor laser is its ability to couple light into an optical fibre which can then be used in a communication system. However for such operation, the optical filament within the laser must be stable with time and not shift in position, typically by more than a fraction of the fibre width. For a single mode fibre this condition implies stability to submicron dimensions. Consequently, it is of great interest [1,2] to measure the spatial distribution of optical intensity as light emerges from the laser, the angle at which it emerges, and the position of the waist of the beam before it starts to diverge. The optical Fdament within the laser is controlled to a great extent by the spontaneous emission intensity so that further information may be acquired about the spatial distribution of the spontaneous emission within the laser. As the light beam in a semiconductor laser is known to change with time i3,4] a full understanding of the light guiding properties of these devices really requires time resolved measurement of all these distributions. The purpose of this paper is to describe such a computer system providing automatic accurate measurement of light intensity profiles across a laser at 0 030-4018/83/0000-0000/$ 03.00 © 1983 North-Holland
required times during a generated optical pulse. It is versatile, inexpensive, and through convenient data management, provides easy access to experimental resuits. The system is particularly interesting in that it provides simultaneous measurement of a series of intensity profiles during an ultrashort lasing pulse. Van der Ziel [3] has already noticed that small changes in lasing profile may occur over very short periods of time (less than 1 ns). The system reported here has been developed to further investigate these changes by being constructed to operate to resolutions of 50 ps. However by measuring distributions effectively simultaneously, the authors have ensured that experimental shifting errors between the distributions may be eradicated. The computer system is based around a microcomputer (a Research Machines Ltd. 380Z microcomputer) linked to two inexpensive motorised micrometers which move the laser in the transverse and axial directions. Using the computer these micrometers may be used to position the laser to tolerances of less than 0.2 tam. Fig. 1 is a schematic diagram of the overall system. The microcomputer is used to drive the laser across the front o f the microscope objective lens. At the same time the computer is able to sample the output signal from a photodiode at speci289
Volume 45, number 5
OPTICS COMMUNICATIONS
1 May 1983
MOTORISED MICROMETERS 1.o rY D
rno. 8 rY Ld
no.6-
PULSE [GENERATOR[
[EE~ trigger
bJ
~)0.4
-q, I
I PLOTTER ~
III I MICRO
COMPUTER
Fig. 1. A schematic diagram of the computer system.
fied times during the optical pulse. The photodiode is positioned so that it monitors the light output from a point on the laser beam profile. By this method the light intensity across the laser beam can be measured. One major advantage of the reported system is that despite providing high precision positioning accuracy, it is based on inexpensive motorised micrometers. These consist of conventional d.c. motors linked to a fine screw thread. In normal use they are frequently driven directly by d.c. variable power supplies, or with pulsed supplies using various widths of electrical pulses to obtain varying speeds. However the author has determined that controlled movement on micron levels may best be carried out using varying lengths of bursts of short electrical pulses. These pulses are generally of the order of 1 ms long. The number of pulses in each burst may accurately control the speed of movement of the motorised micrometer (fig. 2). Often motormicrometers provide uneven movement because inconsistencies in the laser mounts cause varying drag to occur. However by driving the motors for short periods of time with high electrical current drives and hence high torques, the above problems may be overcome even when only very small movement is required. Even long movements of millimeters may be carried out very accurately by using many bursts of pulses. Here the speed of movement may be kept small even if the torque produced by the motors when driven is high. Linearity has been found to be maintained by running the micrometers with pulses of constant amplitude. As a variable power supply is not required the motormicrometers may be driven from a single bit of the 290
bJO.2 0 Z
<[ r- 0 ~n 0 2 4 6 8 10 NUMBER OF PULSES PER BURST i
Fig. 2. Plot of distance moved by a single burst of pulses versus the number of pulses in each burst. (Pulse width = 5 ms).
computer output port. Thus computer control of the positioning equipment is simplified. The microcomputer control accesses peripheral units in the overall system using a series of analogue to digital converter input and output channels, along with a parallel output port. The parallel output port consists of 8 outputs which may be set either at a high or low level. Four of the eight outputs are used so that the 2 output ADC channels may be switched into line with as many as 8 external units. A further 3 outputs control the two motorised micrometers via high current buffers. Here two bits independently access each motormicrometer and the third is used to control the direction of movement of the motors. In this way the laser may be swept across the front of the microscope objective lens to allow intensity distributions to be measured, and an axially directed motorised micrometer be used to position the laser for measurement of near and far field and waist distributions. A photodiode (either a 100 ps risetime avalanche photodiode or a 50 ps risetime silicon pin photodiode) is used for light detection. The lens/ photodiode distance is chosen so that the resolution limit due to the area of the photodiode is of the same order as that due to the microscope objective lens. In the reported system this was believed to be less than 1 /am. As the system is based on laser movement rather than photodiode movement, lens/photodiode distance does not affect calibration of the system. The output of the photodiode is fed into a sampling oscilloscope, which provides a facility whereby the sampling window of the scope can be externally set
Volume 45, number 5
OPTICS COMMUNICATIONS
SAMPLING SCOPE SCREEN \
t,r~e ps)
L~GNT J OUTPUT : ROiv' LASER
OUTPUT FROM SCOPE
TDUE TO SIGNAL A
"i-~ms)
' [ i
I
I
! NNnnn tSAMPLED OUTPUT hnnnnllHIINll |WAVEFORM READ BY : I ~ COMPUTER t(ms)
~ ! ; A~N~L~ RAOwMN COWMRI~ERlgO CONTROL
Fig. 3. A schematic diagram demonstrating the time resolved sampling operation of the computer system. to any position along the optical pulse. In the reported system, the computer is used to control the position of the sampling point along the optical pulse, by sending an electrical signal via an ADC output channel to the oscilloscope (fig. 3). The scope is set at a time determined by the computer, to output a level proportional to the sampled signal. This level may then be sampled by the computer, enabling noise reduction to be undertaken by computer averaging. Hence in fig. 3, if two points are required along the optical pulse, signal A is applied to the scope. This signal in turn sets the scope at the 2 required time positions sequentially, keeping the sampling window at a given point until the computer averages the required number of samples for the purposes of noise reduction. The sampled signals are digitally averaged by the microcomputer. A fast averaging routine whereby the averaging calculation is carried out as the next sample is fetched from the analogue to ditigal converter allows the microcomputer to sample and average a point around 100 000 times a minute. As the averaging routine is written in machine code, blocks of 255 samples are averaged. Generally 5 blocks of samples (over 1000 samples) are used to reduce the noise of the system to reasonable levels. The averaging process does not significantly slow the system down as, because of the mechanical inertia of the motormicrometers, time is required to allow the system to settle after each movement. Time is also required to allow the electrical system to stabilise. A near field distribution sweep of 100 points generally takes around 90 s to complete. The overall system then operates by moving the
1 May 1983
laser across the objective lens in 0.2/am increments. At each increment the computer will sample and average a series of points along the optical pulse in the manner described above. Once sampling of all the requested points is completed, the laser will then be moved by another 0.2/am and a further series of samples undertaken. In this way a series of near field distributions at various points along the optical pulse may be measured simultaneously. This method thus removes all errors related to the relative positions of the measured distributions. Although this system at present does not include any position transducers, the measurements have been found to be highly reliable (to less than 2% in near field width). Moreover distributions at different current levels may be compared by driving the laser with triangular waveforms or multilevel pulses. Useful information concerning laser operation may be gleaned from the charge carrier concentration in a laser. It is known that the spontaneous emission from a laser is directly proportional to the charge carrier concentration. Charge carrier concentration distributions have been determined using the system by measuring the spontaneous emission output from the laser. Here the stimulated emission is blocked using a Glan Thompson polariser in conjunction with an interference filter whose peak transmission was at a wavelength of around 20 nm below that of the stimulated emission. As Gallium Arsenide becomes transparent at longer wavelengths, shorter wavelengths are preferred for the monitoring of spontaneous emission. The computer system through averaging, so reduces the measurement noise that particularly low levels of spontaneous emission can be measured by sampling even on picosecond timescales. The improvement in signal to noise ratio can be maintained even if the avalanche photodiodes are used with modest current gains for good linearity of the photodiode output with input. Far field and waist distributions are measured by moving the laser axially and again carrying out profile measurements. If prefered the system can measure the profile width directly. All further data may be stored in the computer to allow further processing and comparison with theory.
291
Volume 45, number 5
OPTICS COMMUNICATIONS
1 May 1983
2. Experimental results
l ~II~?HNTsITY This system has been primarily developed for investigation of twin stripe lasers [5]. Here asymmetric near field distributions may be well-controlled by injected stripe currents. Unfortunately single stripe lasers allow shifting of the optical filament only in an uncontrolled manner, and hence cannot be used to conclusively test some theoretical simulations. Twin stripe lasers however, allowing much greater control of the optical filament may be more readily used for the testing of theoretical simulations. Fig. 4 shows the charge carrier concentration and near field distributions of twin stripe lasers at threshold as the stripe current ratio is varied. These lasers consist of two lasing stripes each 3/am wide laid on the laser with a stripe separation of 3/am. We observe a symmetrical double peaked charge carrier concentration distribution in the case of both stripes being driven together. Here each stripe contributes equally to the charge carrier concentration. This in turn produces a narrow near field distribution guided by the self focusing effect. As the stripe current ratio increases, so the charge carrier concentration becomes progressively asymmetric until, with one stripe predominantly driven, a wide near field pattern is obtained. Such a pattern is believed to be gain guided. Fig. 5 shows both of these modes measured simultaneously. The near field distributions measured, when one stripe is driv-
CHARGECARRIER
(a)
CONCENTRATIONS n
I /&l~
I
II l
n ''
"
n
I =1^ =40mA ! '11.=90mA '\ 1 ~1'/'1 / ~I~ =56 mA
I0
0
I0
0
I0
(b) NEAR FIELD DISTRIBUTIONS
I0
I1
I0
0
Lj
L I,
/
/ /
/
/ / 0
IO DISTANCE
IC) ALONG
\
'\
\ i'o
THE
LASER
JUNCTION
25 O 25 DISTANCEALONGLASERJUNCTION(#.m) Fig. 5. Near and far field distributions of a twin stripe laser with (a) both stripes driven together, and (b) only one stripe dirven. (Pulse width = 150 ns, 1.1 times threshold). en and when both stripes are driven together seem to confirm existing theory [6]. Further interesting results have been obtained with picosecond pulses generated from twin stripe semiconductor lasers by direct electrical modulation. Here the laser is biased to threshold, and radio frequency modulation is applied to the more weakly driven stripe. This modulation may generate pulses with widths dependent upon stripe current ratio. The pulse widths measured by intensity autocorrelation in type-1 second harmonic generation in LilO 3 vary between 40 ps with both stripes driven together, and 20 ps with one stripe predominantly driven (fig. 6). This effect is believed to be due to the optical filament being switched in regions of higher loss with one stripe predominantly driven, than with both stripes driven together. Hence the effective photon lifetime is shorter, and the minimum pulse width reduced. This work is consistent with the observed light/current characteristics. Both threshold current and incremental efficiency are dependent upon the optical loss "of the laser cavity. Hence any change in the threshold currents or quantum efficiencies of the lasers should lead to a change in the minimum pulse
> -100 r-4 ~90 T # -80
I0
J'!
L/\ / I. I0
=
110mA / ..,2=- OmA
~1_
//l
'
A ~ (a)
(l;m)
u3 0_ 40-r 30LG / 200_ 0
PULSE W I D
~
/
-70
;
6
8
20 LOG (11/I2) Fig. 4. Near field and charge carrier concentration distributions o f twin stripe lasers under differing drive conditions. (Pulse width = 100 ns).
292
lo
-60
c:u
50
m z >
Fig. 6. Plot of minimum pulse width and total threshold current versus stripe current ratio.
Volume 45, number 5
OPTICS COMMUNICATIONS
width which may be generated by a semiconductor laser. Fig. 6 also shows a plot of total threshold current versus stripe current ratio. We can clearly see that the threshold current with one stripe predominantly driven is approximately twice that with both stripes driven together. Moreover the quantum efficiency with both stripes driven together is 1.6 times that with a high stripe current ratio. Thus from the light/current characteristics we would expect to find the greater optical loss with one stripe predominantly driven leading to a lower photon lifetime and hence smaller minimum pulse width. Near field and far field distributions measured at specific times along the optical pulse (to resolutions of 100 ps) also agree with this theory. Near field distributions have been carried out observing both the peak and trough of the picosecond pulse. We can see (fig. 7) that as the stripe current ratio is altered, so the near field distribution changes. For the case of both stripes being driven together, the near field mode builds up in its normal
(a)
L ~',~
PEAK OF THE / I \ 40 ps PULSE"--.~/ I / TROUGH Or T.E , o .s PU
/ /
I I
/ /
0
5
3. Conclusions
PEAKOF 20ps s PULSE
I
zero order self focused mode in a similar manner to normal biasing operations. Here the asymmetric application of the radio frequency modulation does not significantly affect the waveguiding of the filament. This is probably due to the near field distribution being strongly self focused and rather less perceptable to movement on picosecond timescale. With a high stripe current ratio, the near field pattern is remarkably different. The near field distribution at the trough of the pulse is asymmetric, and similar to that obtained under normal bias with high stripe current ratios. However at the peak of the pulse, the near field distribution (fig. 7(b)) shows a second peak on the opposite side of the laser from the original one. This second peak is likely to be directly due to the radio frequency modulation being supplied to the more weakly driven stripe. It is believed that as the optical pulse width (~20 ps) is so much shorter than the charge carrier diffusion time (~3 ns), time is not available for diffusion of carriers to allow a mode similar to that generated under d.c. conditions to be generated. On the contrary the radio frequency modulated stripe is more likely to act in a relatively independent manner. Hence a separate peak in the near field distribution is produced in a high loss region. As the pulse is generated in a region of higher optical loss, we expect a decrease in pulse width to be observed. Thus near field measurements agree well with direct measurements of pulse width.
5 (pm)
L (b)
1 May 1983
I
5 0 5 (pm) DISTANCE ALONG THE LASER JUNCTION Fig. 7. Near field distributions of a twin stripe laser during the generation of picosecond pulses (a) with both stripes driven together, and (b) with one stripe predominately driven. (These measurements were made using a 100 ps risetime avalanche photodiode.)
In conclusion, an automatic computer system providing the measurement of the evolution of near and far field, and charge carrier concentration distributions through time has been reported. This system may be operated using low sensitivity ultrafast photodiodes as detectors, and hence is believed to be the fastest such system. It is extremely versatile through not requiring recalibration when used for different applications. Moreover through efficient computer management of experimental data, further processing of results may be easily carried out. Novel effects have already been observed in the measurement of twin stripe lasers, and the system has already allowed checking of theoretical work. 293
Volume 45, number 5
OPTICS COMMUNICATIONS
1 May 1983
Acknowledgement
References
The authors would like to thank Mr. B Herscher for many helpful discussions. They would also like to thank Dr. R.G. Plumb o f Standard Telecommunication Laboratories Ltd., Harlow, England for the supply o f the twin stripe lasers used in this work. One o f the authors (I.H. White) would like to thank the Department o f education for Northern Ireland for a personal maintenance grant. The UK Science and Engineering Research Council are thanked for supporting this work with the award o f an equipment grant.
[1] G.H.B. Thompson, D.F. Lovelace and S.E.H. Turley, IEEE. J. Solid State and Electron Devices 2 (1978) 12. [2] R.P. Brouwer, C.H.F. Velzel, and B.S. Yeh, IEEE. J. Quantum Electron. 17 (1981) 694. [3] J.P. van der Ziel, IEEE J. Quantum Electron. 17 (1981) 60. .[4] J.P. Goure and J.N. Massot, Optical and Quantum Electronics 14 (1982) 445. [5] I.H. White, J.E. Carroll and R.G. Plumb, lEE Proc.-l, Solid State and Electron Devices 129 (1982) 216. [6] P.M. Asbeck, D.A. Cammack, J.J. Daniele and Y. Klebanoff, IEEE J. Quantum Electron. 15 (1979) 727.
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