1. Introduction

There is a wide range of practical applications that require efficient, compact and low-cost sources of high-power laser pulses of about 1 ns duration. Laser LIDARs and systems for accurate distance measurement basing on the TOF (time of flight) principle are the most intensively developing systems requiring such sources [1–4]. For problems of measuring distances of tens and hundreds of meters, semiconductor lasers can be considered as the main alternative to fiber and solid-state sources of laser radiation [5]. A typical solution for creating such pulsed sources is based on discrete circuits using external transistors as a current switch [6–9]. In this case, the extremely low resistance of laser crystals is a feature that must be taken into account when constructing circuits with semiconductor lasers. As a result, the problem of high residual inductances in a circuit with a low resistance arises when generating current pulses of ns-duration and high amplitude. In addition, most of the transistors available on the market are not designed for the mode of short current pulses through the low impedance load (i.e., a mode in which low operating voltages are used), which requires the use of additional ballast (matching) resistors. Another problem of standard solutions is the need to form a separate circuit for generating control pulses using a high-current transistor that provide a high-speed and efficient on-state transition. In [10], the feasibility of generating 14 W/ 1 ns laser pulses from a 30 µm aperture in a circuit based on a home-made HEM(T) GaN transistor was demonstrated. There are reports on avalanche bipolar junction transistors as a high-speed switch in a semiconductor laser pump circuit, e.g. [6]. In [6], the feasibility of generating 180 W/ 2 ns laser pulses in a circuit with a silicon avalanche bipolar junction transistor was demonstrated. However, the disadvantage of the available devices is a high residual voltage (up to 50 V), which generates high heat loss, and the need to use high operating voltages (up to 300 V). Another approach is based on the use of laser thyristor structures that combine the functions of a laser radiation source and a current switch [11–13]. For high-power laser-thyristors the feasibility of generating 7 W/ 10 ns laser pulses from a 200 µm aperture [14] and 1.5 W/ 90 ps laser pulses from a 20 µm aperture [15] at a wavelength of 905 nm was demonstrated. However, the integration of the laser heterostructure into the thyristor structure limits the through hole injection [16]. As a result, an efficient operation requires an efficient optical activation of the phototransistor part of the structure [17], which limits the available spectral range.

We believe that some of the issues described in the first paragraph can be resolved by the use of vertically integrated structures (vertical integration provides minimum dimensions for the current loop) based on low-voltage high-speed current switches and laser chips. In this paper, we study for the first time an approach to the generation of laser pulses of ns-duration, in which the discrete low-voltage AlGaAs/GaAs thyristor is used as a high-speed high-current switch. The proposed approach has the following advantages: (i) the feasibility of vertical mounting to form a “laser diode/ thyristor” stack, which provides compactness and minimizes the spurious inductance, (ii) any Fabry-Perot semiconductor lasers can be used regardless of the radiation wavelength, (iii) low-amplitude control current pulses with no requirements to the pulse edge.

2. Experimental samples and research techniques

Previous studies of high-power pulsed laser thyristors showed that the heterostructure of a thyristor with a p0/n+ design of a blocking p-n junction allows one to realize the above-mentioned advantages (low control currents, minimum residual voltage of 1.5 V in the on-state, high operating currents over 100 A) [11]. In the present study, a thyristor design is considered, in which there are no effects of photogeneration of minority carriers in the base regions, and the feedback at the moment of turn-on is realized only by the through injection of electrons and holes. In order to maintain a high transient rate (i) the layers for a hole transport of a minimum thickness are designed, (ii) there is a heterojunction at the N-AlGaAs-emitter – p0-GaAs-base interface in the N-p-n transistor to reduce the hole leakage into the n-emitter and enhance the effect of their accumulation in the p0-GaAs base, (iii) the thickness of the p0-GaAs base is chosen to be minimal to ensure the thyristor operating blocking voltages of ∼ 30 V. As a result, the thyristor structure is grown by MOCVD epitaxy on an n-GaAs substrate and includes a 0.15-µm-thick N-Al_0.15Ga_0.85As emitter (n = 10¹⁸cm⁻³), a 4-µm-thick p0-GaAs base (p = 10¹⁶cm⁻³), a 0.15-µm-thick n-GaAs collector (n = 10¹⁸cm⁻³), and a 0.15-µm-thick p-GaAs emitter (p = 10¹⁸cm⁻³) of the p-n-p transistor. Then, the thyristor crystals with Ti/Pt anode contact dimensions of 200 µm × 500 µm were fabricated. The AuGe/Au control electrodes are formed on the n-GaAs collector layer along the long sides of the anode contact (Fig. 1(a)). The AuGe/Au cathode contact is on the side of the n-GaAs substrate. The thyristor crystals are soldered through a conductive n-GaAs carrier, pre-covered with indium, onto the p-side of the laser chip, pre-soldered onto a copper heatsink n-side down. The design of the “laser diode mini bar – thyristor” vertical stack is shown in Fig. 1a. In the experiments, the laser diode mini bar (LDMB) had three emitters and an aperture (W) of each emitter was 100 µm wide, the distance (D) between adjacent emitters was 300 µm (Fig. 1(a)). The Fabry-Perot cavity was 3.5 mm long and had a reflective (95%) and an antireflective (5%) coating. The asymmetric laser heterostructure was MOCVD grown [18] and consisted of N-Al_0.3Ga_0.7As and P-Al_0.3Ga_0.7As claddings 1.5-µm-thick each, an undoped Al_0.1Ga_0.9As waveguide layer and a 10-nm-thick InGaAs active region emitting at 1060 nm, shifted by 0.2 µm from the waveguide center to the P-cladding. Studies of the radiative characteristics shows that near the threshold current the heterostructure has an internal optical loss of 0.6 cm⁻¹ and an internal quantum yield of 98%.

 

Fig. 1. Connection layout (a) of the LDMB-1×T vertical stack and the LDMB-2×T vertical stack, where the current control resistor is R1 = 0.22 Ohm, the separating resistor is R2 = 50 Ohm, the charging capacitor is C = 300 pF, W is the width of the emitter aperture in the LDMB, D is the distance between the emitters in the LDMB. A and B are the points for monitoring current pulses: A1 and B1 – for the T1 thyristor, A2 and B2 – for thyristor T2. (b) Duty cycle diagrams for the LDMB-T vertical stack: I – the stage of charging the storage capacitor C when the thyristor is off, II – the stage of discharging the storage capacitor C when the thyristor is on. U_C is the voltage across the storage capacitor C, I_CONT is the control current pulse, I_THYR is the current through the thyristor, PLDMB is the optical power emitted by LDMB.

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Since the study deals with laser pulses of ns-duration, then an electric circuit with a storage capacitor С = 300 pF is considered as a basic one. In order to control the current a low-ohmic resistor R1 = 0.22 Ohm was placed in the low impedance circuit (Fig. 1(a)). An active probe (Agilent 1158A, 4 GHz bandwidth) and an oscilloscope (Agilent 54855, 6 GHz bandwidth) were used to control the current. To measure the shape of the optical signal, a high-speed photodetector (New Focus 1444, 18 ps rise time), which has a fiber output (50 µm), and an oscilloscope (Agilent 86100) with a module (86117A, 50 GHz bandwidth) were used. A thermal detector (Ophir 3A-P-FS-12) was used to measure the average optical power of the LDMB. To calculate the peak power, the near field (magnification factor is about 10) of each emitter from the LDMB was formed and the average optical pulse shape was measured by point-by-point scanning along all the emitters with a step of 50 µm in the image space.

The cycle of laser pulse generation has two stages in the proposed circuit. At the first stage, the storage capacitor C is charged from the voltage source U, while the thyristor is in the off state of high resistance (high enough resistance so that the influence of leakage currents can be neglected) (Fig. 1(b)). At the second stage, a control pulse I_CONT is applied from an external pulse generator to the control electrode (Fig. 1(b)). The control pulse launches the process of turning on the thyristor, the transition of which to the on-state of low resistance ensures the discharge of the storage capacitor C through the LDMB and the generation of a laser pulse (Fig. 1(b)). After the capacitor is completely discharged, the current in the circuit drops, and the thyristor goes into the off-state of high resistance (Fig. 1(b)), after which the cycle of laser pulse generation can be repeated. We used control current pulses of 1 µs duration with on and off fronts of ∼ 100 ns typically in the experiments, the control current pulse frequency determined the frequency of the laser pulses and was equal to 12.6 kHz. No additional matching resistors were used to generate control pulses; therefore, the consumed energy was determined by the current amplitude, pulse duration, and voltage drop across the GaAs p-n-junction, which was up to 1.6 V according to our estimates (Fig. 1(a)). Thyristor control efficiency is described in more detail in [19,20].

To study the dynamics of lasing in a circuit with a low-voltage thyristor, two types of designs of vertical stacks “LDMB – thyristor” were used. The first type of vertical stack design included one LDMB and one thyristor (LDMB-1×T), and the second included one LDMB and two thyristors (LDMB-2×T) (Fig. 1(a)). As shown in Fig. 1(a), for a design of the second type, two individual thyristors were placed on a common n-GaAs conductive carrier. An individual storage capacitor of 300 pF nominal was connected in parallel with each thyristor (Fig. 1(a)). Moreover, to reduce the mutual coupling, the anode contacts were separated by a resistor R2 = 50 Ohm (Fig. (1a)). The current was monitored at points A1 and B1, as well as at points A2 and B2 for each thyristor (Fig. (1a)). To calculate a current pulse, a voltage pulse measured between points A and B was divided by the load resistor nominal R1 = 0.22 Ohm. For the LDMB-2×T case, the total current is as a sum of current pulses calculated for T1 and T2 thyristors.

3. Results and discussions

The first part of the experimental studies was the measurement of the electro-optical characteristics of the LDMB-1×T vertical stacks. Figure 2 shows a typical lasing dynamics and a calculated dynamics of a current pulse for an operating voltage of 28 V, measured in accordance with the procedures described in the previous section. For clarity, when interpreting the results, the optical pulses were artificially aligned with each other's leading edge, and the current pulses – with a trailing edge (Fig. 2(a)). In fact, the delay time of the laser pulse relative to the thyristor control current pulse is different for different voltages and has two components. The first component of the laser pulse turn-on delay is related to the thyristor turn-on delay relative to the start of the control current pulse (Fig. 1(b)). The second component is due to the delay in turning on the laser when it is pumped by short current pulses, and is governed by the rate of accumulation of the threshold e-h plasma density in the active region and the photon density in the cavity, which is necessary to exceed the threshold conditions. It is seen that lasing is shifted to the second half of the current pulse. This demonstrates the fact of insufficient current pulse duration to provide a peak power, which is in the case of long laser pulses. Nevertheless, in the simplest case of the LDMB-1×T vertical stack with only one thyristor switch the peak power of 5.5 W with a pulse width of 950 ps is demonstrated (Fig. 2(a)). In what follows, we do not consider the peak power of the first oscillation peak, the value of which reaches 12 W.

 

Fig. 2. Lasing dynamics of a LDMB-1×T vertical stack for various values of the operating voltage (solid lines) and a current pulse at an operating voltage of 28 V (dashed line) (a) and current dynamics for various operating voltages (b). Control current I_CONT = 64 mA, storage capacitor C = 300 pF.

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Figure 2(b) shows the current pulses generated in the LDMB-1×T circuit at various operating voltages, and the inset of the Fig. 2(a) shows the peak current depending on the operating voltage. It is seen that the operating voltage range can be divided into two regions with a boundary of about 21 V. Below 21 V, the leading edge of the current pulse reaches 3 ns (10% to 90%). Above 21 V, a significant current pulse sharpening is observed, and the leading edge decreases to 240 ps. The effect of pulse sharpening was previously observed both in optothyristors [21] and in laser thyristors [15] and is due to the turning the impact ionization on. As was shown in [22] the impact ionization turn-on allows the generation of an excess e-h plasma in a low-doped p0-base with a high speed and, thus, a significant increase in the thyristor turn-on speed.

The second part of the experimental studies was the measurement of the electro-optical characteristics of the LDMB-2×T vertical stacks based on two thyristors connected in parallel (Fig. 1(a)). The aim was to assess the possibility of increasing the peak power by increasing the maximum peak current when using a 1D array of parallel-connected thyristors. Figure 3(a) shows a typical lasing dynamics and a calculated dynamics of a current pulse for an operating voltage of 28 V, measured in accordance with the procedures described in the previous section. The maximum peak power for LDMB-2×T reached 8 W with a pulse duration of 1.75 ns. We did not study here the effects determining the maximum repetition rate, as was done for thyristor lasers [20]. Therefore, the repetition rate was 12.6 kHz to simplify measurements, which gave an average power of 160 µW. Based on the results in [20], one can expect the pulse duty cycle close to 10, the verification of which requires further study in the future. Spectra as a function of the current amplitude show that the redshift of the laser line is less than 2 nm. This may be due to the broadening of the lasing spectrum with increasing pump current, which is typical for high-power multimode lasers operating in the pulse mode [23]. In our opinion, this indicates that the thermal heating of the laser chip, as well as the contribution from the thyristor, is not significant. However, thermal heating at higher pulse repetition rates or when using thyristor arrays of a larger number of elements may need to be taken into account. These effects require further studies. It can be seen that, compared with LDMB-1×T, the use of two thyristor switches led both to an increase in peak power and an increase in pulse duration (from the comparison of Fig. 2(a) and Fig. 3(a)). However, a multiple (double) increase in peak power did not occur. To analyze the reasons for the slight increase in optical power, we consider the current dynamics measured for each thyristor. Since the control current pulse for both thyristors had the same amplitude and was applied simultaneously (Fig. 1(a)), in the ideal case, the time moments of turning both thyristors on should coincide. In fact, the delay between turning T1 and T2 thyristors on was found to be ∼ 1.2 ns in the whole range of operating voltages (12-28 V) (Fig. 3). If we compare the shapes of current pulses generated by individual thyristors in the LDMB-1×T and LDMB-2×T circuits, then the use of two thyristors led to an increase in duration from 1.4 ns for LDMB-1×T to 2 ns for LDMB-2×T and a decrease in current pulse amplitude from 12 A for LDMB-1×T to 9 A for LDMB-2×T. As a result, the main reason explaining the insufficiently high pump efficiency is a turn-on time mismatch between thyristors in the 1D array. Figure 3(b) shows the calculated current pulse shape through the LDMB for a LDMB-2×T. It is seen that the peak value of the current reaches 15 A, while the maximum current is in the second half of the pulse due to the time mismatch in the turn-on. A decrease in operating voltage leads to both a decrease in peak power and pulse duration. It can be seen from Fig. 4 that, at an operating voltage of 16 V, the peak power and duration of the laser pulse were 3 W and 1.3 ns, respectively. The calculated current pulse shape for LDMB-2×T at an operating voltage of 16 V is shown in Fig. 4(b). It should be noted that lasing was not observed for the LDMB-1×T stack at an operating voltage of 16 V (Fig. 2(a)). This means that, despite the nonzero current, the injected number of charge carriers is not enough to fill the LDMB QW to a level, at which the gain compensates for the total loss.

 

Fig. 3. Lasing dynamics of a LDMB-2×T vertical stack at an operating voltage 28 V for different values of the control current: 2.8 mA (blue solid line), 6.5 mA (green solid line) and 322 mA (black solid line) (a); current dynamics of the T1 thyristor (red dashed line), the T2 thyristor (black dashed line) and T1 + T2 (blue solid line) for the operating voltage 28 V and the control current 322 mA (b). Storage capacitor C = 300 pF.

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Fig. 4. Lasing dynamics of a LDMB-2×T vertical stack at an operating voltage 16 V for different values of the control current: 6.5 mA (green solid line), 22 mA (pink solid line), 63 mA (blue solid line), 140 mA (red solid line) and 322 mA (black solid line) (a); and current dynamics of the T1 thyristor (red dashed line), the T2 thyristor (black dashed line) and T1 + T2 (blue solid line) for the operating voltage 16 V and the control current 322 mA (b). Storage capacitor C = 300 pF.

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As has been shown previously [19] one of the advantages of thyristor swithes generating long pulses (tens or hundreds of nanoseconds) is the absence of rigorous requirements for the shape and amplitude of the control current pulse required to turn on the thyristor. Consider the validity of this statement in the case of the thyristor as a switch for short current pulses. Figure 3(a) shows a set of laser pulses obtained for the LDMB-2×T at an operating voltage of 28 V and various values of the control current. It is seen that a decrease in the amplitude of the control current leads to a decrease in the peak optical power. Moreover, the decrease does not exceed 10% in the range of control currents 3-320 mA. It is important to note that hereinafter the values of the total control current for the stack of two thyristor switches are indicated. Noticeable changes occur for an operating voltage of 16 V. A drop in peak optical power by 10% is observed when the control current decreases from 320 mA to 140 mA, and there is no lasing at control current less than 6 mA.

To explain the observed effect, the dynamics of the voltage across the storage capacitor was measured. Figure 5 shows a set of tracks of the dynamics of the voltage across the storage capacitor for the operating voltages of 16 V and 28 V, obtained at various control currents. It is seen that a decrease in the control current leads to the appearance of a low-speed stage of the thyristor transition to the on-state. The transition between the low-speed and high-speed stages is determined by the magnitude of a critical current through the thyristor and the voltage that is blocked by the device at this current. In this case, the voltage value is critical, because it determines the electric field of the reverse biased p0-n+ junction. Simulations showed that an important source for the excess e-h plasma generation in the low-doped p0-base layer (namely, the accumulation rate of e-h plasma in the p0-base governs the thyristor switching rate [22]) is an impact ionization in the region of the p0-n+ junction. In this case, the impact ionization rate is determined by two quantities: the impact ionization coefficient, which is specified by the magnitude of the electric field, and the carrier flux. Thus, at high values of the operating voltage (in our case, more than 20 V), the impact ionization in the region of the reverse biased p0-n+ junction is quite efficient and a high e-h plasma generation rate can be initiated by a small through current through the device. A decrease in the operating voltage leads to an increase in the critical current at which the impact ionization is efficient. On the other hand, the through current discharges the storage capacitor at the low-speed stage of the thyristor turn-on, which reduces even further the electric field in the p0-n+ junction due to a decrease in voltage, which is clearly seen in Fig. 5(b). As a result, at low operating voltages, the high-speed stage of the turn on of the thyristor begins with a residual voltage that is significantly less than the initial operating one. As a result, there is a nonlinear increase in the peak current from the operating voltage, which was previously observed in LDMB-1×T (an inset in Fig. 2(b)). In other words, under a low control current (2.8 mA) and a low operating voltage (16 V), the delay time is long enough, which leads to the discharge of the storage capacitor, therefore, the switching starts with a sufficiently lower voltage than the initial one, as can be seen in Fig. 5(b).

 

Fig. 5. Tracks of the dynamics of the voltage across the storage capacitor for the operating voltages of 28 V (a) and 16 V (b), obtained at various control currents: 2.8 mA (pink solid line), 6.4 mA (cayn solid line), 22 mA (blue solid line), 63 mA (green solid line), 140 mA (red solid line) and 322 mA (black solid line)

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An analysis of laser dynamics deserves special attention. It can be seen that for all variants of turning the thyristor on, the laser pulse starts with a clearly pronounced intense peak (Fig. 2(a), Fig. 3(a), Fig. 4(a)), which is much faster and shorter than the pump current pulse. The presence of the first relaxation peak is a unique property of a semiconductor laser operating in the gain switching mode. This feature also experimentally confirms the fact of a generation of pump current pulses with front edges of 1 ns and less, as well as the insignificance of the contribution of the thyristor switch control current pulse. In addition, it can be seen that, depending on the amplitude of the current pulse generated in the LDMB-T circuit, the laser pulse may contain, in addition to the first high-speed relaxation peak, a second part characterized by slower dynamics and related to the dynamics of the current pulse generated in the LDMB-T circuit. It can be seen that the contribution of the second part of the laser pulse decreases with a decrease in the amplitude of the current pulse generated in the LDMB-T circuit, and the operating modes with almost the entire laser output in the first relaxation peak can be determined (Fig. 2(a)), Fig. 4(a)). However, the laser structures used in the present study are not optimal for the gain switching, when there is only the first relaxation peak, which is manifested in a significant decrease in peak and average optical power (Fig. 2(a), Fig. 4(a)). It should be noted that the optimization of the heterostructure and the design of the laser chip can significantly increase the efficiency in the operating mode of gain switching. For example, in [24] a laser based on a structure contained a relatively thick active layer (80 nm), for which, due to asymmetry, the optical confinement factor of the active region had a typical value, as for a conventional laser structure with QWs, as well as a saturating absorber section, was shown to operate in the gain switching mode with a peak power of 35 W (pulse energy of ∼ 3 nJ) and a duration of 80 ps, under pulsed current pump with a pulse amplitude of 17 A and a duration of 1.3 ns. Thus, the results demonstrate the potential of the proposed approach for generating of sub-ns laser pulses in the gain switching mode.

4. Conclusion

The studies showed that the proposed approach is an efficient way of generating high-power ns-duration laser pulses. It is clearly seen from the results on the thyristors in the stack that a turn-on delay time mismatch of about 1 ns allows the use of 1D and 2D thyristor arrays to generate high-power laser pulses of several nanoseconds in laser bars and also in stacks of high-power semiconductor lasers. However, to reach a duration of 1 ns or less requires additional studies aimed to reduce the mismatch of the turn-on delay time between the thyristors. Then one can use the individual thyristors as efficient switches for generating ns and sub-ns laser pulses in single-mode or multimode individual semiconductor lasers that have significantly lower threshold currents than the laser arrays, which will significantly increase the pump efficiency. Another important experimental fact is that the turn-on of the thyristors does not require the use of high control currents, which also allows one to use efficiently 1D and 2D arrays of individual thyristors for generating ns pulses with an amplitude of hundreds of amperes.