We have developed a diode-laser based master oscillator power amplifier (MOPA) light source which emits high-power spectrally stabilized and nearly-diffraction limited optical pulses in the nanoseconds range as required by many applications. The MOPA consists of a distributed Bragg reflector (DBR) laser as master oscillator driven by a constant current and a ridge waveguide power amplifier (PA) which can be driven by a constant current (DC) or by rectangular current pulses with a width of 5 ns at a repetition frequency of 200 kHz. Under pulsed operation the amplifier acts as an optical gate, converting the CW input beam emitted by the DBR laser into a train of short amplified optical pulses. With this experimental MOPA arrangement no relaxation oscillations occur. A continuous wave power of 1 W under DC injection and a pulse power of 4 W under pulsed operation are reached. For both operational modes the optical spectrum of the emission of the amplifier exhibits a peak at a constant wavelength of 973.5 nm with a spectral width < 10 pm.
© 2014 Optical Society of America
There is an increasing demand for semiconductor based sources which emit high-power spectrally stable nearly diffraction-limited optical pulses in the nanosecond range. They can be used for a large variety of applications, such as free-space communications, metrology, material processing, seed lasers for fiber or solid state lasers, spectroscopy, LIDAR and frequency doubling. Gain switching, i.e. turning on and off the current injected into the active section of a semiconductor based device e.g. a laser diode, 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 emits. This can be reached by fundamental mode sources with a ridge waveguide. 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]. DBR ridge-waveguide lasers emitting at 976 nm and 1064 nm were reported in  to generate optical pulses with pulse lengths between 4 ns and 48 ns. Stable pulses with a maximum peak power of 600 mW were obtained. In  a 1.8 mm long DFB ridge-waveguide laser emitting at 976 nm having lateral Bragg gratings was used to generate 50 ns-long optical pulses with a peak power of 1.6 W. Klamkin et al.  reported a slab-coupled optical waveguide laser emitting at 965 nm spectrally stabilized with an external fiber Bragg grating. A fiber-coupled power of 1.6 W (2.4 W emitted by the laser) was obtained with 35-ns long pulses. The spectral full width at half maximum (FWHM) was 0.2 nm. We achieved peak powers of 2.6 W  and 3.8 W  from a 1 mm long DFB-RW laser emitting at 1064 nm. The pulse width was 4 ns and the repetition frequency 250 kHz.
The disadvantages of all gain-switched ridge waveguide diode lasers under operation with short current pulses are the limited output power, the change of the wavelength during the optical pulse and fluctuations in the near and far field profiles . To achieve high optical peak powers tapered devices have been developed. With a monolithic integrated source consisting of an ultra-fast semiconductor modulator as optical gate and a tapered amplifier for pulse amplification, short optical pulses were selected and amplified . By using a continuous wave (CW) DFB laser diode as master oscillator the modulator was capable of switching the optical beam within a few hundred picoseconds, generating pulses with pulse durations of 400 – 1000 ps and energy of 650 pJ . A master oscillator power amplifier (MOPA) system for the generation of ns-pulses with high peak power, stabilized wavelength and narrow spectral line width at 1064 nm was presented in . The master oscillator was a distributed feedback (DFB) ridge waveguide (RW) laser and the PA consisted of three RW sections, one of them used for pulse gating, and a flared tapered gain-guided section. For an optical pulse length of 2 ns a peak power of 16 W was obtained. Distinct disadvantages of tapered devices are the different positions of the vertical and lateral waists of the emitted beam (called astigmatism) as well as the significantly structured lateral beam profiles in beam waist and far field resulting in beam propagation ratios M2s between 2 and 10 depending on the injection current thus imposing challenges on the coupling optics .
In this paper we present detailed experimental studies of semiconductor optical amplifiers with a ridge waveguide for lateral optical confinement under DC and pulse current injection in a MOPA configuration. As a master oscillator (MO) a DBR laser is used driven by a constant current. In Section 2 the design of the laser and amplifier is presented. Section 3 describes the experimental setup. In Sections 4 and 5 we will present the results of the experimental investigations of the static and dynamic behavior of the MOPA system. In Section 6 a summary is given.
2. Design of the devices
2.1 Master oscillator
The DBR laser used as master oscillator was grown on n-type GaAs via low pressure metal-organic vapour phase epitaxy (MOVPE). The structure consists of an n- Al0.4Ga0.6As cladding layer, a 1.6 µm thick n-Al0.3Ga0.7As waveguide layer, an undoped InGaAs single quantum well (SQW), a 900 nm thick p- Al0.3Ga0.7As waveguide layer, a p-Al0.4Ga0.6As cladding layer and a highly doped GaAs contact layer. The structure provides a vertical far field angle of 26° full width at half maximum (FWHM).
Lateral optical confinement and p-contacting is realized by a 4 µm wide ridge-waveguide (RW) fabricated by reactive ion etching and deposition of SiNx which is opened on top of the RW before the p-metallization is performed. The height of the ridge is 1.4 µm corresponding to an effective index step of 0.003.
The device has a 2 mm long gain section, a 1 mm long absorber or phase section and 1 mm long DBR section with a 7th-order surface Bragg grating (period 1016.7 nm) defined by I-line wafer stepper lithography and dry etching. As outlined in , reflection coefficients higher than 80% can be obtained with 1 mm long gratings if a high duty cycle of more than 90% and an optimum (not to large) etch depth is chosen. Such a high duty cycle can be realized effectively by etching V-shaped grooves into the surface. In the device under investigation, period and etch depth of the grating were 1 µm and 1.5 µm, respectively, and the angle of the slopes of the grooves were tilted by about 10° with respect to the normal of the surface.
The front and rear facets are coated to obtain reflection coefficients of 30% and <0.1%, respectively. The devices are mounted p-up on a C-mount heat sink. In the experiments the gain- and absorber sections are interconnected.
2.2. Power amplifier
The vertical laser structure of the RW power amplifier was also grown by MOVPE. The structure consists of an n-Al0.45Ga0.55As cladding layer, a 1.8 µm thick n-Al0.35Ga0.65As waveguide, an active InGaAs double quantum well (DQW) embedded in GaAsP barrier and spacer layers, a 600 nm thick p-Al0.35Ga0.65As waveguide, a p-Al0.85Ga0.15As cladding layer and a highly p-doped GaAs contact layer. The amplifier emission has a vertical far field angle of 22° (FWHM). The ridge height is 1 µm corresponding to an effective index step of 0.003.
The RW designed to guide only the fundamental lateral mode is tilted with respect to the normal of the facets in order to minimize their reflectivity. In order to determine the optimum tilt angle, the reflectivity of the facets was calculated in dependence on the ridge width (w) using a commercial two-dimensional mode matching tool. The optical field on either side of the facet is expanded in terms of local waveguide modes, taking into account the tilted propagation. The reflection and transmission matrices are obtained by matching the tangential components of the fields at the facets. The simulation area is laterally 5 µm + w + 5 µm and vertically 4 µm wide. It is bounded by magnetic and electric walls. A total number of 50 two-dimensional modes (guided and discretized radiation modes) were found to yield the correct reflectivity (≈0.3) of an untilted facet.
Figure 1(a) shows the power reflectivity of the fundamental predominantly transverse-electric polarized mode for different tilt angles and ridge widths. It can be seen, that for the range of tilt angles investigated the reflectivity drops the stronger, the larger the ridge width is. For a tilt angle of only 3° and a ridge width of 4 µm a reflectivity as small as 10−4 can be reached for an as-cleaved facet.
Next the impact of an additional anti-reflection (AR) coating consisting of one ZnSe and one Al2O3 layer was investigated. Figure 1(b) shows a pseudo-color mapping of the modal reflection coefficient for a ridge width of 4 µm in dependence on tilt angle and thickness of Al2O3 layer. The Al2O3 layer thickness for minimum reflectivity is independent of the tilt angle. With increasing tilt angle, the reflectivity minimum broadens and reaches values down to 10−6. In what follows, an AR coated amplifier having a width of the ridge of 4 µm and a tilt angle of 3° is used.
The amplifier having a length of 6 mm is soldered p-side up on a CuW heat spreader which is mounted on a C-mount, see Fig. 2. To minimize the inductivity of the mounting for pulse applications a large number of bond wires are used.
3. Experimental set-up
Figure 3 shows the experimental set-up. The interconnected gain- and absorber sections of the DBR laser acting as master oscillator (MO) are biased and temperature stabilized to 25 °C with the laser driver LDC 2724B. The emitted light was collimated with an AR coated aspheric lens L1 having a focal length of 3.1 mm and a numerical aperture NA = 0.68 . The light passes a two staged optical isolator (60 dB isolation) and is coupled with a second lens L2 into the ridge of the amplifier at an angle of 10° with respect to the facet normal. The amplifier was biased with a laser driver for DC measurements. For measurements under pulsed operation an in-house developed electrical driver circuit was used, as described by Liero et al , which was triggered with the delay generator DG 645. The amplifier was temperature stabilized.. For the measurement of the CW power-current characteristics of MO and amplifier a Gentec SOLO 2 with a detector head HLP12-3S-H2 was used.
The light emitted by the amplifier was focused with two lenses L3 and L4 into the input of a fiber coupler which is split into 3 fibers with 33% intensity of the light into each fiber. One of them was connected with a fast 25 GHz photo diode to measure the temporal behavior with a 33 GHz real-time oscilloscope with 80 Gsa/s. One fiber is connected with an optical spectrum analyzer (, resolution 10 pm) to measure the time-averaged optical spectrum and one is used for power control with a lightwave multimeter. The resolution of the temporal measurements was limited by the photo diode to 40 ps.
For investigations of the beam profile of the amplifier output the mirror M1 was folded into the light pass. The beam profile was measured with a CCD camera according to ISO 11146.
4. Experimental results
4.1 CW operation
The dependence of the continuous-wave (CW) optical power of the DBR master oscillator on the injection current before and behind the isolator at a heat sink temperature of T = 25 °C is shown in Fig. 4(a). The threshold current is about 40 mA. The slope efficiency η before the isolator is only 0.25 W/A due to the high reflectivity of the front facet and is further reduced behind the isolator to 0.17 W/A due to the optical losses of the isolator. At a current of 100 mA output powers of 16 mW (before isolator) and 10 mW (behind) are achieved.
An optical spectrum of the MO measured with an optical spectrum analyzer Q8384 with a resolution of 10 pm at a current of IMO = 70 mA is shown in Fig. 4(b). The emission wavelength of the laser is about 973.5 nm. The MO operates in a single longitudinal mode with a side mode suppression ratio of 50 dB.
The dependence of the optical power measured at the output facet of the RW amplifier on the current Iamp,dc injected into amplifier without (red curve) and with (blue curve) seed power from the MO are shown in Fig. 5(a). Without the input from the MO the amplifier shows weak amplified spontaneous emission (ASE) below a current of 100 mA. Above 100 mA the ASE power increases almost linearly with current. The slope efficiency is 0.42 W/A per facet. At Iamp,dc = 1.5 A an ASE power of about 0.6 W per facet is reached.
With an input power from the MO of about PMO = 5 mW (IMO = 70 mA), see Fig. 5(a), the output power of the amplifier increases. A slope efficiency of 0.74 W/A at the output facet is achieved. At Iamp,dc = 1.5 A an output power for more than 1 W is reached, which corresponds to an amplification factor of 200 (amplifier gain 23 dB).
In Fig. 5(b) the dependence of the output power of the amplifier on the input power at Iamp,dc = 1.5 A is shown. At PMO = 0.2 mW (near MO threshold) an abrupt increase of the amplifier output power from 0.60 W to 0.86 W is observed. A further increase of the seed power causes only a small increase of the amplifier power (from Pamp,cw = 0.94 W at PMO = 1.45 mW to Pamp,cw = 1.01 W at PMO = 10 mW).
In Fig. 6 the CW spectral characteristics of the amplifier in dependence on the MO current for an amplifier current of Iamp,dc = 1.5 A are presented. Figure 6(a) shows a pseudo-color contour plot of the optical spectral density in dependence on wavelength and MO current between 30 mA and 100 mA. Above the threshold of the MO (~40 mA) the amplified laser line (strong yellow line) can be seen. The ASE background is much weaker on the shorter wavelength side of the laser line than on the longer wavelength side. This indicates a stronger depletion of the carriers on the short wavelength side in comparison to the long wavelength side by the seed laser.
Figure 6(b) shows optical spectra at different MO currents near the seed laser emission line at 973.5 nm. Below the threshold of the MO (~40 mA) the ASE spectrum of the amplifier is only weakly influenced by the injected emission of the DBR laser. Above the threshold of the MO the DBR laser line is amplified resulting in a decrease of the ASE level by about 20 dB. In the optical spectra the side modes of the DBR laser are clearly visible. The side mode suppression ratio (SMSR) increases slightly from 45 dB at IMO = 50 mA to 50 dB at IMO = 100 mA. The laser line shifts to longer wavelengths (~1.5 pm/mA) with the increase of the MO current due to self-heating of the DBR laser.
In Fig. 7 the CW spectral characteristics of the amplifier in dependence on the amplifier current for a MO current of IMO = 70 mA corresponding to a seed power of about 5 mW are presented. Figure 7(a) shows a pseudo-color contour plot of the optical spectral density in dependence on the wavelength over a range of 50 nm and the amplifier current between 0.05 A and 1.5 A. The wavelength of the seed laser is located on the short wavelength side of the amplification spectrum. With increasing amplifier current the gain maximum shifts away from the laser line due to the self-heating. The amplified laser line is independent of the amplifier current over the whole current range investigated.
Figure 7(b) shows optical spectra for amplifier currents of 1.0 A (black), 1.5 A (red) and 2.0 A (blue) without (dashed lines) and with (solid lines) input from the MO. For vanishing seed power the broad ASE spectra show no signs of lasing for Iamp,dc = 1.0 A and Iamp,dc = 1.5 A. However, at Iamp,dc = 2.0 A self-lasing of the amplifier starts due to feedback effects of the coupling lens (AR coating ~0.25%), probably. For a seed power of about 5 mW the amplified laser line of the MO can be seen and the ASE is strongly reduced, in particular at the short wavelength side of the laser line caused by the fact that the amplification spectrum shifts with increasing DC amplifier current to longer wavelengths. A higher CW output power could be expected if the composition or thickness of the active QW is optimized for a better match of the peak wavelength of the amplification spectrum and the laser line. By integrating the areas under the spectra shown in Fig. 7 (b), at an amplifier current of Iamp,dc = 1.5 A a reduction of the ASE power at the output facet from 0.6 W without input from the MO to 0.012 W for a MO power of 5 mW can be estimated.
4.2 Pulsed Operation
For pulsed operation the RW amplifier was driven with rectangular current pulses with a length of 5 ns and a repetition frequency of 200 kHz . The DBR laser was operated in CW mode. The amplifier acts as an optical gate. If no current is injected, the beam emitted by the DBR laser is absorbed. During the injection of a current pulse the beam can propagate along the amplifier. Thus the CW input beam from the MO is converted into a train of amplified short optical pulses behind the amplifier.
From the averaged output power and the knowledge of pulse width and repetition frequency the pulse power can be estimated. Figure 8a shows the pulse power (Ppulse) in dependence on the amplitude of the current pulses injected into the amplifier. The CW input power is about 5 mW. The power-current characteristic is linear up to a current of 4.5 A corresponding to a power of 2.5 W and starts to saturate slightly above. A pulse power of 4.1 W corresponding to an amplification factor of 820 (amplifier gain 29 dB) is reached at a pulse current of 9 A.
Figure 8(b) shows the measured temporal behavior of the amplifier output power for different pulse currents. With increasing amplitude of the current, the widths of the optical pulses approache the width of the current pulse (5 ns): At Iamp,pulse = 0.5 A the optical pulse width is only 3.6 ns (FWHM), but at Iamp,pulse = 7.7 A it is 4.9 ns. This could be explained by the fact that the time needed for carrier accumulation in the amplifier decreases with increasing injection current. The rise and fall times are about 1.1 ns and 2.0 ns, respectively (10% to 90% of pulse amplitude). The form of the pulses changes also with increasing current amplitude. At amplitudes lower than 4 A a near rectangular shape of the pulses was found. At higher current amplitudes the output power reached the maximum value after 2 ns followed by a slight decrease. This can be explained by the limited storage capacity of the capacitors used in the electronic circuit.
It is important to note, that with this special MOPA arrangement (CW laser, pulsed amplifier) the optical pulses are free from relaxation oscillations. The pulse form is mainly determined by the driver used, the high-frequency properties of the amplifier and the bonding scheme. This should be interesting for a number of applications e.g. for seeding of fiber lasers.
Figure 9(a) shows time-averaged optical spectra measured over a wide spectral range of 50 nm at 7.7 A pulse amplifier current without (red line) and with (blue line) input from the MO. Without seed power a broad ASE spectrum is observed. The maximum wavelength of the ASE of about 973 nm is shorter than under CW operation and coincides with the wavelength of the MO. The seed power of 5 mW leads to a strong decrease of the ASE, in particular on the short wavelength side of the laser line. By integrating the areas under the spectra shown in Fig. 9 (a), an ASE pulse power of 0.006 W at the output facet can be estimated assuming an ASE power of 2.2 W without input from the MO.
Results of measurements in a smaller wavelength range of 4 nm with a resolution of 10 pm are presented in Fig. 9(b). The spectral density o the ASE without input from the MO (red line) decreases with input from the MO by about 30 dB on the short wavelength side of the laser line and by 25 dB on the longer wavelength side. The ratio of laser and ASE levels of the spectral density was estimated to 57 dB and is higher than for CW operation of the amplifier. This can be understood, because the coincidence of ASE maximum and MO wavelength under pulsed operation results in stronger carrier depletion than for CW operation of the amplifier and hence to reduced spontaneous emission.
The spatial beam characteristics of the collimated beam of the pulsed amplifier was measured at Iamp,pulse = 6.3A by recording beam profiles with a CCD camera along a caustics behind a spherical lens with a focal length of 250 mm. The vertical (blue line) and lateral (green line) diameters of the beam determined from the second moments are plotted in Fig. 10(a) as a function of the distance from a reference plane. The lateral and vertical beam propagation ratios calculated from least squares fits of the caustics are M2lat = 1.1 and M2ver = 1.2, respectively, at a pulse current of 6.3 A corresponding to a pulse power of 3.2 W. The beam propagation ratios are almost constant over the whole amplifier current range investigated.
An example of the intensity distribution and the beam profiles is shown in Fig. 10(b). These measurements show that the pulses emitted by the amplifier have a nearly Gaussian intensity distribution, suitable for an efficient coupling into a single-mode fiber.
We have developed an all-semiconductor based master oscillator power amplifier (MOPA) light source emitting high-power spectrally stabilized and nearly-diffraction limited optical pulses as required by many applications. The MOPA consists of a distributed Bragg reflector (DBR) lasers as master oscillator driven with a constant current and at ridge-waveguide power amplifier (PA) which can be driven DC or by pulse currents. In pulse regime the amplifier modulated with rectangular current pulses of a length of 5 ns and a repetition frequency of 200 kHz acts as an optical gate, converting the CW input beam emitted by the DBR laser into a train of short optical pulses which are amplified. With this experimental MOPA arrangement no relaxation oscillations occur. With a seed power of about 5 mW output powers behind the amplifier of about 1 W under DC injection and 4 W under pulsed operation, corresponding to amplification factors of 200 (amplifier gain 23 dB) and 800 (gain 29 dB), respectively, are reached. For both operational modes the optical spectrum of the emission of the amplifier exhibits a peak at a constant wavelength of 973.5 nm with a spectral width < 10 pm independent on the output power. The output beam is nearly diffraction limited with beam propagation ratios M2lat ~1.1 and M2ver ~1.2 up to 4 W pulse power.
The authors are grateful to A. Ginolas for the development of adequate mounting techniques for the sophisticated devices. A. Liero and Th. Hoffmann for the development of the short pulse electronics.
References and links
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