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We present detailed experimental investigations of the temporal, spectral and spatial behavior of a gain-switched distributed feedback (DFB) laser emitting at a wavelength of 1064 nm. Gain-switching is achieved by injecting nearly rectangular shaped current pulses having a length of 50 ns and a very high amplitude up to 2.5 A. The repetition frequency is 200 kHz. The laser has a ridge waveguide (RW) for lateral waveguiding with a ridge width of 3 µm and a cavity length of 1.5 mm. Time resolved investigations show, depending on the amplitude of the current pulses, that the optical power exhibits different types of oscillatory behavior during the pulses, accompanied by changes in the lateral near field intensity profiles and optical spectra. Three different types of instabilities can be distinguished: mode beating with frequencies between 25 GHz and 30 GHz, switching between different lateral intensity profiles with a frequency of 0.4 GHz and self-sustained oscillations with a frequency of 4 GHz. The investigations are of great relevance for the utilization of gain-switched DFB-RW lasers as seed lasers for fiber laser systems and in other applications, which require a high optical power.
©2013 Optical Society of America
1. Introduction
High-power diode lasers capable of generating spectrally stable nearly diffraction-limited optical pulses in the nanosecond range can be used in a variety of applications including free-space communications, metrology, material processing and frequency doubling. Gain switching, i.e. turning on and off the current injected into the active section of a diode laser, offers a simple, cost-effective and power-efficient possibility to generate optical pulses in the ns range.
Diffraction-limited emission is achieved by a proper design of the laser waveguide, so that only the fundamental transverse mode lases. Spectral stabilization can be realized with Bragg gratings integrated into the semiconductor chip. With gain-switched distributed feedback (DFB) as well as distributed Bragg reflector (DBR) ridge-waveguide (RW) lasers the generation of optical pulses in the ns range with peak powers of more than 1 W has been demonstrated by several groups [1–3].
The applications mentioned in the beginning require an optical power as high as possible. We achieved a peak power of 3.8 W from a 1 mm long DFB-RW laser emitting at 1064 nm [4], [5]. The pulse width was 4 ns and the repetition frequency 250 kHz. If the width of the current pulses and their amplitude is further increased, we observed different types of dynamical instabilities. A pulse break down occurring 25 ns after turn-on of a current pulse with a high amplitude was also reported in Ref [1]. Different types of spatio-temporal instabilities have been observed at Fabry-Perot RW lasers, driven by 25 ns long current pulses, too [6].
In this paper we present detailed experimental studies of the instable behavior of a gain-switched 1.5 mm long DFB-RW laser driven by current pulses with a width of 50 ns and amplitudes up to 2.5 A in the temporal, spectral and spatial domains.
2. Device structure and experimental setup
The layer structure of the DFB laser bases on a symmetric super-large cavity, where the active region consisting of a triple InGaAs/GaAsP quantum well is placed symmetrically in a 3.6 µm broad Al0.25Ga0.75As waveguide core. The uniform Bragg grating is defined by holographic photolithography and transferred by wet chemical etching into an InGaP/GaAs/InGaP layer sequence, which is located in the upper (p-doped) part of the waveguide core. To obtain a lasing wavelength of 1064 nm, the period of the second-order grating was adjusted to 320 nm. The coupling coefficient is 3 cm−1 determined by a fit of the subthreshold spectrum of the amplified spontaneous emission (ASE) to a parameterized theoretical model [7]. Lateral optical confinement and p-contacting is provided by a ridge waveguide (RW) with a ridge width of 3 µm and an effective-index step of 3 × 10−3. More details of the fabrication can be found in [8].
The cavity has a length of 1.5 mm and anti- and high-reflection coated facets with reflection coefficients of 10−4 and 0.95, respectively. The DFB laser was soldered p-side up on a C-mount and attached to test equipment suitable to transmit high frequency electric fields. For the amplification and control of the nanosecond current pulses an electronic circuit containing a high-frequency GaN transistor was developed. Details of the pulsed diode laser driver can be found in [9].
A pulse generator (Stanford Research Systems, DG 645) is used to trigger the electronics. The mount is stabilized to 25°C with a temperature controller (Newport 350). The scheme for the experimental setup is shown in Fig. 1 .
The light emitted by the DFB laser is collimated with an aspheric lens with a focal length of 8 mm, passed through a two staged optical isolator and focused into a fiber with a second 8 mm focus aspheric lens. The incoming light is split with a fiber coupler (ANGK, Fibre Optical Components) into 3 fibers with 33% intensity of the light into each. Two of them are connected with fast 45 GHz photo diodes (New Focus 1014) to measure the temporal behavior with a 33 GHz real-time oscilloscope with 80 GSsa/s (Agilent DSOX 93204A) and the power radio frequency spectrum with an electrical spectrum analyzer (Rohde & Schwarz FSUP). One fiber is connected with an optical spectrum analyzer (Advantest 8384) to measure the time-averaged optical spectrum.
In a second set up (see right sight of Fig. 1) time-resolved optical spectra and near field distributions are measured with a SpectraPro 2500i spectrograph and a single shot streak camera (Hamamatsu C2650). The same electrical driver is used.
3. Experimental results
The current pulses for the gain-switched operation of the DFB laser have a length of 50 ns. The repetition frequency is 200 kHz (period 5 µs, duty cycle 1:100). Figure 2 shows the dependence of the pulse power on the pulse current (blue). For comparison, also the continuous-wave (CW) characteristics up to a current of 0.3 A is plotted (red).
Fig. 2 Measured optical power against current. Red dashed: CW characteristics (up to 0.3 A); blue solid: pulsed characteristics (pulse width 50 ns, period 5 µs).
The average optical power was detected with a GENTEC detector XLP12-1S-H2. The pulse power and current are calculated from pulse width and repetition frequency. The threshold current is about 46 mA and the slope efficiency is 0.50 W/A for pulse currents up to 0.7 W. At higher current pulses the slope efficiency decreases slightly, probably caused by leakage currents over the hetero barriers. A pulse power of 1.1 W was reached at a pulse current of 2.5 A.
Optical spectra were measured with a resolution of 10 pm. Figure 3(a) shows a CW spectrum for a current of 300 mA and Fig. 3(b) time-averaged spectra for pulse currents from 250 mA to 1900 mA (40 times the threshold current) on a logarithmic power scale. The peak wavelength of the main mode under CW operation is 1059.8 nm. There is a small side mode on the longer wavelength side visible, which is 0.11 nm away from the main peak and suppressed by 45 dB. The spacing of the main and side modes corresponds to the width of the stop band visible in the subthreshold ASE spectrum.
Fig. 3 Time-averaged optical spectra. (a): CW operation at a current of 300 mA. (b): Pulsed operation for different pulse currents from 250 mA to 1900 mA.
Under pulsed operation the peak wavelength shifts to 1059.2 nm at a pulse current of 250 mA due to the strongly reduced self-heating. With increasing pulse current a broadening of the spectra at the longer wavelength side can be seen which indicates a thermal induced chirp during the pulses. At a current of 1900 mA the spectral full dB width is 0.5 nm. The spectral density at the peak wavelength decreases slightly by 1.3 dB with increasing current.
The temporal dependence of the output power of the 1.5 mm long DFB laser driven with 50 ns long current pulses are shown in Fig. 4 for a variety of pulse currents between 250 mA and 1900 mA. The measurements were done with a single sweep of the real-time oscilloscope. We should mention that the first relaxation oscillation is not visible due to the time scale chosen.
Fig. 4 Temporal behavior of the output power of a 1.5 mm long DFB laser driven by 50 ns long electrical pulses with a repetition frequency of 200 kHz (offset in baseline of different pulse currents between 250 mA and 1900 mA for clarity).
For currents less than 800 mA the amplitudes of the optical pulses are nearly constant. An increase of the current to 1000 mA results in oscillations of power at the end of the pulse, which appear at the largest current measured (1900 mA) already 2 ns after the turn-on of the pulse. This type of very fast oscillations is called “A” in what follows. For currents larger than 1200 mA another type of pulsations denoted “B” with a much longer period appears, following immediately the oscillations of type “A”. Finally, at the largest current a third type of oscillations termed “C” with an intermediate period terminates the pulse.
In Fig. 5 the temporal behavior of the power is shown for the three different types of dynamical instabilities in more detail.
Fig. 5 Temporal behavior of the optical pulse at 1900 mA. Top, middle and bottom diagrams correspond to instabilities of type “A”, “B” and “C” as marked in Fig. 4.
In the top diagram of Fig. 5, within a time interval of 400 ps 11 oscillations of type “A” are shown. The period is about 37.5 ps corresponding to an oscillation frequency of 26.7 GHz. Due to the fact, that this frequency coincides nearly with the frequency spacing of the both modes visible in Fig. 3(a) (29.5 GHz), we conclude, that this type of oscillations is caused by the beating of the two modes. This conclusion will be supported later by temporal-resolved measurements of the optical spectrum (cf. Figure 7).
The dynamical instability of type “B” (middle diagram of Fig. 5) is characterized by longer pulses with a period of about 2.24 ns and small rise and fall times. Between the power drops, the pulses are characterized by plateau-like features. The pulses of type “C” (bottom diagram) have a period of 250 ps corresponding to a pulsation frequency of 4 GHz. A fast rise time of about ~40 ps is followed by a slow fall time. Due to the fact that the pulsation frequency is of the order of the relaxation oscillation frequency of the device, we conclude that this type of pulsation is undamped relaxation oscillations caused by absorptive self-Q-switching as described in [10] and the references therein. In what follows the instabilities of types ”A” and “B” are investigated in more detail.
The dependence of the period and frequency of the oscillations of type “A” on the pulse time for different currents (marked by A, A`, A`` in Fig. 4) is shown in Fig. 6 . It can be seen, that the period decreases with advancing pulse time and that the shift of the frequency with time becomes faster with increasing current. The frequency increases from 25.8 GHz to 28.6 GHz within 28 ns at a current for 1265 mA and from 26.4 GHz to 29.8 GHz within 11.5 ns at 1900 mA.
Fig. 6 Dependence of the period and the frequency of oscillations of type “A” on the pulse time for different pulse currents (marked A, A`, A`` in Fig. 4)
Figure 7 shows the result of a measurement of the time-resolved optical spectrum at a current of 1900 mA with the streak camera as a color-scaled mapping. The ranges denoted by “A”, “B” and “C” are the same as in Fig. 4. It can be seen that lasing starts in a single longitudinal mode at about 1059.5 nm. During the first 2 ns this mode is shifted by 0.1 nm towards a shorter wavelength. Then two modes start to oscillate simultaneously. With increasing time these modes shift to longer wavelengths. The spacing of the wavelengths of the two modes is about 100 pm, which corresponds exactly to the frequency of the oscillations shown in the upper plot of Fig. 5. Furthermore, in the spectrum the four-wave mixing wavelengths below and above the wavelengths of the two modes are clearly visible. Thus the oscillations of type “A” are caused by the beating of the two modes on either side of the stop band, as already supposed.
Fig. 7 Color-scale mapping of the optical spectrum versus time obtained with a streak camera for a pulse current of 1900 mA. The ranges denoted by “A”, “B” and “C” are the same as in Fig. 4. Red (blue) denotes high (low) spectral density.
In the ranges “B” and “C” marked like the corresponding types of oscillations only the mode on the short-wavelength side of the stop band lases. It is the mode which is lasing under CW operation (cf. Fig. 3(a)).
The result of a measurement of the dynamics of the lateral profile of the near-field intensity with the streak camera is shown in Fig. 8 as a color-scaled mapping.
Fig. 8 Color-scale mapping of the lateral profile of the near field intensity versus time obtained with a streak camera for a pulse current of 1900 mA. The ranges denoted by “A”, “B” and “C” are the same as in Fig. 4. Red (blue)
The lateral width of 4 µm shown in Fig. (8) is estimated from the ridge width of 3 µm and the measured average near field profile under the same pulsed injection conditions.
Clearly the same three different dynamic ranges denoted by “A”, “B” and “C” as in Figs. 4 and 7 can be distinguished. The oscillations of type “A” are characterized by a quasi-stationary lateral intensity profile. The maximum shifts, however, sidewards with increasing pulse time.
After about 15 ns the maximum jumps back to the other side of the ridge and back again and so on for the next 25 ns (range “B”). The switching time between the two spatial profiles is very fast (< 50 ps). The duration of lasing with one spatial state is half the period of the pulsations of type “B”, i.e. about 1.1 ns - 1.2 ns. This explains the plateau-like features of the pulsations of type “B” visible in the middle diagram of Fig. 5. A power drop occurs only during a jump of the intensity maximum into one direction.
Finally, the oscillations of type “C” are again characterized by a quasi-stationary spatial profile, located at the opposite side of the ridge compared to the beginning of the pulse. A slight shift of the maximum of the intensity profile can be observed
4. Discussion
As shown above, a gain-switched DFB laser driven by current pulses with a high amplitude can exhibit a complicated temporal, spectral and spatial behavior, the origin of which is not completely understood yet. In what follows we discuss some of the experimental results for the pulse current 1900 mA.
The decrease of the emission wavelength during the first 2 ns by 0.1 nm can only be caused by an increase of carrier density in the waveguide including the active quantum wells, which reduce the effective index and hence the Bragg wavelength. The following increase of the emission wavelengths by about 0.65 nm can be attributed to an increase of the temperature. Assuming a wavelength shift of 70 pm/K [11] the temperature increases by about 9 K during the 50 ns pulse. This value is reasonable with regard to the dissipated power of 5W, a volume of about 10−16 m−3, and the heat capacity of GaAs of 1.8 × 10−6 K−1m−3. Thus the observed broadening of the time-averaged optical spectra, see Fig. 3(b), is caused by the self-heating of the waveguide.
The increase of the frequency of the oscillations of type “A” with time is more difficult to understand. Basically it can be explained by a decrease of the modal group index or an increase of the coupling coefficient of the Bragg grating. A decrease of the modal group index can be excluded, because this should be also associated with a decrease of the modal index, leading to a shift of the wavelength of the modes to shorter values. However, the opposite is observed here. On the other hand, an increase of the coupling coefficient by 1 cm−1 could explain the broadening of the stop band by 13 pm corresponding to the frequency shift of 3.8 GHz.
The increase of the coupling coefficient is probably caused by an improved lateral optical confinement. The built-in effective-index step of about 3 × 10−3 can be easily compensated through the high carrier density injected, which leads to an decrease of the effective index in the ridge region as discussed above. Due to the fact, that the Bragg grating is removed in the trenches etched to define the RW, the lateral overlap of the optical field with the Bragg grating is diminished. The depletion of the carrier density and the increase of the temperature in the waveguide by 3K in region A (0.2nm wavelength shift), the latter leading to an rise of the effective index by ~10−3 [12], increases the lateral optical confinement factor as revealed by simulations and thus causes the observed rise of the coupling coefficient .The type “B” of pulsations could be explained by assuming, that the switching is due to a coexistence and interaction of the fundamental and first order lateral modes as proposed in [6]. Due to the fact that different lateral modes have different modal indices, they should lase with different wavelengths. In our case, the difference of the modal indices of fundamental and first order modes is about 2 × 10−3 which leads to a spacing of the corresponding Bragg wavelengths of about 0.6 nm. However, in range “B” of Fig. 7 there is only one mode visible. In Ref [6] a similar observation was made and the authors argued that the switching behavior results from beating between lateral modes that belong to different longitudinal modes. The kinks in the CW power-current characteristics of RW lasers have been explained similarly [13]. However, in contrast to a FP laser, in a DFB the longitudinal modes away from the stop band have rising cavity losses and hence the argument of Ref [6] cannot be applied here.
Due to the fact that both lateral modes lase at the same wavelength, both modes must have the same modal index. In waveguides with asymmetric gain/loss profiles, a degeneracy of the modes has been numerically observed [14]. Degeneracy means, that for a certain set of parameters (denoted degeneracy point), both the index and the gain of both modes and hence the mode profiles coincide, i.e. the mode is orthogonal to itself. It is well known, that in the vicinity of a degeneracy point different spatio-temporal instabilities can occur [15]. In Ref [16] it has been also shown, that weakly index guided lasers are prone for a breaking of the lateral symmetry, leading to oscillations between mirror-image lateral field profiles.
5. Summary
We presented detailed experimental investigations of the temporal, spectral and spatial behavior of a gain-switched DFB-RW laser. The optical pulses were generated by injecting current pulses with a width of 50 ns and a very high amplitude up to 40 times the threshold current. Time resolved investigations showed, depending on the amplitude of the current pulses, that the optical power exhibits different types of oscillatory behavior during the pulses, accompanied by changes in the lateral near field intensity profiles and optical spectra. Three different types of instabilities could be distinguished: Mode beating with frequencies between 25 GHz and 30 GHz, switching between different lateral intensity profiles with a frequency of 0.4 GHz and self-sustained oscillations with a frequency of 4 GHz.
The emission wavelength decreases within the first nanoseconds of the pulse caused probably by an increase of the carrier density in the waveguide, followed by a steady increase of the wavelength caused by the starting self-heating. For a current of 1900 mA a temperature increase of about 9 K during the 50 ns long pulse can be estimated. The thermal-induced chirp of the pulse is the reason of the broadening of the time-averaged optical spectra.
Our results are relevant for the utilization of gain-switched DFB-RW lasers as seed lasers for fiber laser systems and in other applications, which require high optical power. We investigated DFB-RW lasers with different ridge widths and layer structures and found always a combination of maximum pulse length and peak power, from which one the gain-switched lasers exhibit unstable behavior. With increasing duration of the pulse the maximal obtainable power without pulsations decreases. In order to rise the output power, one has to amplify the pulses, for example in a tapered semiconductor amplifier, which should be operated with ns current pulses, too, in order to diminish self-heating effects and the generation of amplified spontaneous emission as demonstrated in [5] and [17].
Acknowledgment
The authors are grateful to J. Hopp and R. Olschewski for technical assistance. This project was supported by the German Ministry for Education and Research (BMBF) within the project “FAZIT” (contract FKZ 13N9670).
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