FORMING MEASUREMENT OPTICAL SIGNALS WITH THE AID OF INJECTION LASERS S. V. Tikhomlrov and A. A. Chernoyarskii

UDC 621.375.826:53.087.92

Studies of the dynamic characteristics of the measurement parameters of laser radiation are complicated by the absence of optical-radiation measurement generators suitable for generating standard test signals of various types: slnusoldal, single pulse, single gradient, white noise, Furthermore, when adjusting, testing, and operating instruments for measuring the parameters of laser radiation, we need to simulate optical signals of given waveform and duration. This has also led to the creation and investigation of stable sources of optical radiation with extensive possibilities of adjusting the parameters and characteristics of the radiation itself. An analysis of the present state of development of pulsed sources of radiation [I, 2] indicates the promise of employing injection lasers as the basic component of measurement generators of optical pulses. These lasers directly convert electrical energy into optical energy, unlike gas and solld-state lasers, which have a number of intermediate conversion stages. This feature enables us to control in a flexible manner the output parameters of the radiation in injection lasers due to the definite llnk that exists with the parameters of the injection current; it also facilitates stabilization of these parameters. The most widespread method of controlling the parameters of the output radiation from injection lasers is the direct modulation of the injection current [3, 4]. However, in the case of measurement generators, hlgh-frequency amplitude modulation is recommended, based on supplementing the mean pump current by a supplementary generator of signals of the required frequency that are variable with respect to time. Two modes of operation of these lasers are possible with amplitude modulation by hf currents. In the first case, the mean pump current pumps the laser well away from threshold, where the relationship of the output radiant power to the pump current is close to being linear. In such a regime, the depth of radiation modulation will be determined by the ratio of the amplitude of the hf current to the bias current. Measurement generators of optical signals have been developed on the basis of radiation sources operating in this regime, for investigating the frequency response of photomultlpliers up to a frequency of 1.5 GHz [5]. However, the absence of any method of directly checking the depth of modulation of the output optical radiation and the level of its hf components can result in considerable measurement errors due to nonlinearity of the amplltude-frequency characteristics of such sources. Besides the effect due to reduction in the depth of modulation when the period of the modulation is comparable with the spontaneous recombination time [4], the Inadequate bandwidth of the laser design and the systems of current-carrylng electrodes, and also the resonance and noise characteristics of injection laser, can lead to nonlinear distortion. An additional requirement for these lasers is that there should be no spontaneous pulsation of intensity at frequencies close to or at multiples of the hf current-modulatlon frequency. If this requirement is not met, there will be a considerable increase in the depth of modulation of the output radiation over a certain range of frequencies, due to synchronization of the spontaneous pulsation [6]. The other mode of operation of injection lasers as sources of trains of optical pulses is the region that involves switching the laser by means of the hf component of the injection current. For this purpose, square wave pulses of injection current are applied to the laser, causing prethreshold excitation. The positive half-cycle of the hf injection current switches on the laser while the negative half-cycle extinguishes it. The operation of this scheme relies on the small time lag that exists in injection lasers. In the preexcltation regime, the delay of the radiation pulse relative to the switching current pulse (this role being played in the present instance by the hf current) comprises I0-9-10-*~ see [6]. A serious drawback to this method of modulation is the low power of the output pulses (10-2i0-* W) since the laser is operating close to the threshold of generation. To increase the Translated from Izmerltel'naya Tekhnika, No. ii, pp. 25-28, November, 1978.

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0543-1972/78/2111-1510507.50

9 1979 Plenum Publishing Corporation

power of the output radiation pulses it is advisable to use bistable laser diodes [6~, a feature of which is the high rise in power close to the threshold of generation with a small increase in the injection current. The diode is not switched on smoothly, but by a sudden jump into the generating state with a relatively high output radiation power (up to 5"10-* W). Under these circumstances, there is a significant reduction in the rise time of the optical pulses (up to %10-*~ sec). The radiation pulses will also be shorter, which is connected with the delay between the injection current and the coherent radiation inherent in all injection lasers. By combining the waveform of the pulse of injection current and its amplitude, and by selecting lasers with a definite delay on radiation, we are able to obtain sufficiently powerful pulses of radiation with durations of ~i0-*~ sec from current pulses of nanosecond duration [4]. By increasing the duration of the current pulsesup to l0-e10-7 , we are able to form optical pulses of somewhat shorter durations with rise times of ~i0- * ~ At this point, we should note the following circumstance. As was shown in [7], when a semiconductor laser is being excited by pulses of pump current with sufficiently steep leading edges (with durations comparable to the spontaneous recombination time of the minority carriers in the active region of the diode), we observe a transition process of steady-state generation into the form of damped pulsations of radiant intensity with large depths of modulation. The time needed to establish the generation process and also the depth of modulation of the intensity pulsations depends upon the internal properties of the specific laser diode and its conditions of operation. Thus, the delay on coherent radiation relative to the pulse of injection current and the process of establishing generation can lead to a serious distortion of the waveform of the optical pulse compared to the injection pulse, which makes it difficult to formulate optical pulses of specified waveform by means of injection lasers in the nanosecond range of durations and to control their parameters. The first effect leads to a maintenance of the duration of the radiation pulse relative to the current pulse, together with a reduction in the rise time. The second effect prevents us obtaining an optical pulse without overshoots of intensity at the beginning of the flat top. The effect of these factors on the distortion of the injection current waveform when it is converted into radiation can be reduced by means of an injection laser that operates in the diode preexcitation regime, whereby a number of the basic factors can be reduced (removal of the barrier potential, compensation of traps in the active region, creation of an inverse population). If a basic current pulse of the required waveform is applied to the laser diode in this state, then the delay time comprises ~i0 -I~ sec, while the waveform of the radiation pulse will repeat that of the current pulse, with linear conversion of electrical current into optical radiation. This condition of operation of an injection laser is free from the effect of increased threshold current with reduced duration of injection current pulse that exists when the laser is excited directly. The most powerful of these relationships, taking on an exponential character, exists for lasers with two heterojunctions, the lesser for diffusion diodes. To obtain optical pulses of nanosecond duration for investigating the pulse characteristics of high-speed photomultipliers, [8] uses a laser excited by a pulse of injection current of a duration and amplitude which ensure the appearance of only the first peak of the transmission characteristic. The parameters of the optical pulse are determined by the inherent properties of the laser resonator and are therefore little affected by variations of the amplitude and duration of the injection current pulse over a specific range. This range is bounded on one side by failure to lase and on the other by the appearance of a second peak in the transmission characteristic. A study of the dynamics of injection laser generation shows that the radiation is stationary in time only near the threshold. When the pump current is increased above the threshold by 10-20%, a spontaneous undamped pulsation of the radiation of random characteristic usually arises [4-6]. The average period and duration of these pulsations is determined both by the parameters of the laser itself and also by its conditions of operation. As the length of the resonator is increased, the period and duration of the pulsations also increase. As the injection current is increased, the period and duration of the pulsations fall, while the depth of modulation increases (it can reach 80%). The average recurrence rate of the peaks for various lengths of resonator and various levels of pumping usually lies in the range 0.5-5 GHz.

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To obtain regular (periodic) pulsations of radiation intensity, a study [9] was carried out of the synchronization of pulsations by means of an external inducing force. For this purpose, an hf current at a frequency close to the recurrence rate of the spontaneous pulsations was applied to the diode concurrently with the pulse current. This resulted in periodic modulation of the loss or amplification in the laser, which determines not just the intensity but also the duration and depth of modulation of the optical pulses from the diode, regardless of the instants at which they would otherwise arise; i.e., it synchronizes the peak conditions of the laser radiation. A more promising source of radiation pulse trains with known recurrence rates is the double laser diode [6], which comprises a combination of two electrically insulated lasers which are accommodated within the one resonator [8, i0]. By varying the ratio of the currents injected into the various parts of the diode, we can alter the gains of these parts over wide ranges and obtain various dynamic conditions of operation. The condition of regular pulsation of radiation from these lasers with large depths of modulation (*i00%) is only observed wlth sharply nonuniform injection currents, when a current is injected into one part with a density very much smaller than that of the current injected into the other part. The factor that influences the appearance of pulsations of radiation is the nonlinear loss along the optical axis of the resonator. With sharply nonuniform excitation, one of the parts of the diode will operate as an amplifier while the other functions as an easily saturated nonlinear absorber. Since there exists a clearly defined region of absorption under these conditions (the part of the diode with low injection density), the pulses of radiation assume a regular character with highly repeatable time parameters. As the injection current rises, the period and duration of the pulsations fall off in all parts of the diode, the relationships of these parameters to the injection current in the passive part of the diode being considerably stronger than the analogous relationships to the current in the amplifying part. By using a double diode with various lengths of resonator, and by varying the level of injection, we can obtain regular pulses of radiation with recurrence rates of 3"i0 a to i0 *~ Hz and durations of i0-9-I0-*~ sec. The depth of modulation of these intensity pulsations is usually close to 100%. The advantages of the double diode in comparison with a single diode are the regularity of the spontaneous pulsations of radiation and the large depth of modulation, and also the possibility of controlling its time parameters. Furthermore, the presence of two insulated parts of the diodes enables us to avoid interaction between the hf oscillator and the square wave pulse generator in the modulation of injection current regime. By applying a subthreshold injection current to one part and the hf current to the other part, we are able to induce a switching mode in the laser by the hf current. Under these circumstances, we arrive at the condition of loss modulation and the pulses of radiation we obtain are of higher power than under similar conditions for a single laser with modulation close to 100%. In [i0], the duration of the optical pulses obtained by this method comprises ~2'i0- * ~ which does not vary with variations in the frequency of the external oscillator over the range 150-1000 MHz, provided the level of the hf current is maintained constant. This is linked with the fact that the pulses obtained in this manner correspond to the first peak of the transmission characteristic of the laser diode, the parameters of which are determined 5y the degree of nonuniformity of the excitation. In the case we are now considering, the nonuniformity of the excitation is high and is determined by the injection current in the active region, which also determines the duration of the optical pulses. An increase in the amplitude of the hf current leads to the appearance of not just the first, but also successive peaks of the transmission characteristic. Under these circumstances, as was the case with the single laser, we have a region in which the amplitude and duration of the injection current corresponds to the generation of only one peak of the transmission characteristic of the laser. This feature of the process of establishing generation in semiconductor lasers enables us on one hand to control the maintenance of the required conditions of supply and on the other hand to reduce the accuracy requirements as regards maintaining the level of the hf current with changes in the frequency of the master oscillator. When the recurrence period and the duration of the spontaneous regular pulsations with sharply nonuniform excitation of a double laser diode are compared at the beginning and end of a current pulse of duration %2"I0 ~ sec, an increase in these parameters was observed by [ii] at the end of the injection pulse. This can be explained by a temperature r~se in the

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active region of the laser diode, and also by a droop in the flat top of the injection current pulse, which according to [6] leads to an increase in the duration and period of recurrence of the pulsations. The relative instability of the period of the pulsation of any particular part of the flat top of the current pulse comprises ~20%. This can be explained by the amplitude of the instability in the current injected into the passive part, which greatly influences the parameters of the radiation pulses. To stabilize the recurrence rate of the pulsations in single lasers, [ii] synchronized them by the external inducing-force method used in [9]. The amplitude of the hf signal does not exceed the injection current in the absorber part and comprises a few percent of the summated injection current. The duration of the pulsations measured at the half-amplitude level of maximum intensity is practically independent of the frequency of the synchronizing current within the limits of the clamp bandwidth of the external force. When the amplitude of the injection current is varied over a specified range and with fixed frequency and hf signal level, the duration of the synchronizing pulses varies, but the recurrence rate of the peaks remains equal to the frequency of the master oscillator. The clamp region of the pulsations with variation of the external oscillator frequency comprises ~50-I00 MHz, after which the peak recurrence rate returns to its spontaneous frequency. The upper limit of the synchronization region is determined by the limiting frequency of the external oscillator (i GHz). In principle, this could be raised significantly, since the highest recurrence rate of the regular spontaneous pulsations observed for the lasers in use comprised i0 GHz with pulse durations of ~3"i0-I* sec, this being limited by the maximum time resolution of the photodetector employed. When the two regimes we are now considering are compared, we should bear in mind that the method of turning on thelaser by hf current is a particularly simple way of obtaining optical pulses with recurrence periods of (1-5)'10-9 sec at mark space ratios of ~i0. The advantage of this method of excitation lies in the lower dispersion of the radiation pulse parameters from diode to diode than is the case with the regular spontaneous-pulsations synchronization regime. In the latter, we must select our laser diodes according to their radiation pulse parameters (recurrence rate and mark space ratio), and also monitor the injection current to ensure that the specified generation conditions are satisfied. However, it is possible to obtain greater powers (which calculations indicate to be of the order of tens of watts), and also a wider range of spontaneous-pulsation recurrence rates (up to i0 GHz), so that this regime holds promise for use in power photometer. We should stress that both methods can Be realized with the aid of quantity-produced apparatus without requiring the high-power microwave sources necessary for obtaining similar signals from semiconductor lasers with electrical excitation [12]. In contrast to the direct modulation method, the conversion efficiency does not depend upon the duration of the pulses obtained. Investigations of double-lnjection lasers during transition from the steady-state generation mode to the spontaneous pulsation mode do not show a sudden variation in the power averaged over the radiation pulse, monitored with the aid of a type FD-TK photodiode. Calculations show that the maximum value of the power in the radiation pulse train from double-injection lasers lies in the range 0.5-20 W in the synchronization mode and 0.I-i W in the hf current-switching mode. Another possible mode of the double laser diode is normal modulation by injection current in the passive part of the laser. When the diode is operating under conditions of sharply nonuniform excitation, small variations in the current in the passive part lead to changes in the operating condition of the laser and a corresponding change in the output radiation power. When modulating by an hf signal current injected into the passive part of the diode, the output radiation can have a larger depth of modulation than the ratio of the amplitude of the hf current to the uniform injection current, thereby ensuring the same output radiation power as would be obtained from single lasers. The amplitude distortion that arises in this case can be considered and taken into account, provided we know the relationship of the output power to the current in the passive part of the diode with a fixed current in the active part corresponding to the conditions of operation of a double laser diode. An analysis of the basic conditions of operation of injection lasers based on GaAs indicates that we can obtain pulses of radiation with durations of 10-5-10-11 sec at recurrence rates of up to i0 *~ Hz by means of these devices. By controlling the supply to the

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lasers we are able to vary the parameters of the radiation pulses smoothly, and to a large extent independently, as regards duration, recurrence rate, and output power [13], which is an important advantage of these lasers when used in power-englneerlng photometry over other sources of monochromatic radiation. The creation of various structures of injection laser with specific properties and ease of control of radiation kinetics shows promise, w~ile other methods of obtaining pulses of radiation of various waveforms in the nanosecond range are possible with their aid, among which are the optical interaction of lasers [4, 6] and optical [3], electrical, and optoelectrical [14] feedback. Injection lasers are therefore a promising basis for the development of measurement generators of optical pulses, and experience in their use in power-englneerlng photometry as a means of studying the dynamics [8, I0] and calibration [15] characteristics of instruments for measuring the instantaneous power of pulsed lasers tends to confirm this. LITERATURE CITED I.

2.

A. F. Kotyuk et al., in: Metrological Service to the Measurement of the Time Characteristics of Lasers, Series Metrological Service to Measurements [in Russian], AllUnion Scientiflc-Research Institute of Technical Information, Classification, and Coding (VNIIKI), Moscow (1975). B.M. Stepanov et al., in: The Present State of Work on Standard Semiconductor Sources of Radiation, Series Metrology and Measurement Technology [in Russian], VNIIKI, Moscow

(1974). 3. 4. 5. 6. 7. 8. 9. i0. ii. 12. 13.

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T. Pauli and J. Ripper, Proc. IEEE, 58, No. i0 (1970). L. A. Rivlin, The Dynamics of the Radiation of Semiconductor Lasers [in Russian], Sovetskoe Radio, Moscow (1976). L. I. Andreeva et al., Prib. Tekh. ~sp., No. 4 (1970). N. G. Basov et al., Usp. Fiz. Nauk, 97 (1969). L. N. Kurbatov, Kvantovaya Elektron.-~Moscow), No. 6 (1971). V. F. Kabanov et al., Kvantovaya Elektron. (Moscow), l, No. 2 (1974). Yu. P. Zakharov et al., Zh. Eksp. Teor. Fiz., 53, No. 5 (1967). V. F. Kabanov et al., Izmer. Tekh., No. 12 (1974). V. F. Kabanov et al., in: Physical Electronics [in Russian], Nauka, Moscow (1976). S. V. Korolev, Elektron. Prom-st, No. 2 (16) (1973). S. V. Tikhomirov and N. P. Khatyrev, Thesis Papers of the Third All-Union Seminar--Conference on Metrology in Radio Engineering [in Russian], All-Union Scientific-Research Institute of Physicotechnical and Radiotechnical Measurements (VNIIFTRI), Moscow (1975). T. Pauli and J. Ripper, IEEE J. Quant. Electron., QE-6, No. 6 (1970). A. F. Kostyuk et al., Thesis Papers of the Third All-Union Seminar-Conference on "Metrology in Radio Engineering" [in Russian], VNIIFTRI, Moscow (1975).