We demonstrate a surface-emitting quantum cascade laser (QCL) based on second-order buried distributed feedback/distributed Bragg reflector (DFB/DBR) gratings for feedback and outcoupling. The grating fabricated beneath the waveguide was found to fundamentally favor lasing in symmetric mode either through analysis or experiment. Single-lobe far-field radiation pattern with full width at half maximum (FWHM) of 0.18° was obtained along the cavity-length direction. Besides, the buried DFB/DBR grating structure successfully provided an efficient vertical outcoupling mechanism with low optical losses, which manages to achieve a high surface outcouping efficiency of 46% in continuous-wave (CW) operation and 60% in pulsed operation at room temperature. Single-mode emission with a side-mode suppression ratio (SMSR) about 25 dB was continuously tunable by heat sink temperature or injection current. Our work contributes to the realization of high efficiency surface-emitting devices with high far-field beam quality that are significantly needed in many application fields.
© 2016 Optical Society of America
Since its first demonstration in 2000 , single mode surface-emitting quantum cascade lasers (QCLs) featuring high beam quality and easy packaging have been highly desirable for various applications, such as free-space communication, pollution monitoring and trace gas sensing [2–4]. For transverse-magnetic (TM) polarized QCLs, a popular way to realize surface emission is incorporating a second-order distributed feedback (DFB) grating with first-order diffraction perpendicular to the plane contributing to surface emission and second-order diffraction parallel to the plane providing feedback. However, most of the second-order gratings were fabricated in a metal/semiconductor interface, which is found both theoretically and experimentally to be favored to radiate in anti-symmetric near-field amplitude profile since such a mode has the least radiation loss [5–7]. In turn, the far-field beam pattern becomes double-lobed.
Consequently, several approaches have been proposed to modify far-field beam quality for surface grating lasers, such as incorporating a π-phase shift in the grating center or a chirped grating which is corresponding to a π-phase shift [8, 9]. Recently, R. Szedlak et al demonstrated ring cavity surface-emitting QCLs with central-lobe far-field beam pattern, either through utilization of π-phase shift with an effective grating chirp or a certain degree of radial polarization. But less than 10 mW pulsed output power was obtained from lasers employing aforementioned carefully designed gratings . Other approaches purposed at forcing the devices to operate in symmetric mode (i.e. single-lobe beam pattern). The concept proposed by C. Sigler et al purposed at realization of high power, single-lobe surface emission from second-order semiconductor/metal DFB gratings enabled by plasmon-enhanced absorption of anti-symmetric modes . However, the structure design, while successfully completed single-lobe beam radiation when carried out by C. Boyle et al, introduced too much of a penalty loss for the symmetric mode. As a result, the devices could only operate in pulsed mode and the threshold current is quite high . Another report by P. Jouy et al demonstrated surface-emitting lasers based on first-order distributed Bragg reflectors (DBRs) together with a second-order extractor, which operated on a unique defect mode through preferential current injection into the weak coupling extraction grating (i.e. characteristic coupling strength ~0.1), but output powers were limited to 2 mW . In our previous work, it is found that for buried semiconductor/semiconductor second-order grating, symmetric mode inherently presents least radiation loss, such that primarily to be radiated, except for some occasions where a double-lobe beam pattern was observed . Handmade processing nonuniformities or residual facet reflections may account for this phenomenon. To realize stable single-lobe far-field beam radiation, here we introduce DBR gratings at each end of the DFB grating region, which help to eliminate the impact of uncontrolled reflections from cleaved facets on the DFB operation.
In this letter, a buried second-order DFB/DBR grating surface-emitting QCL that favors symmetric mode lasing was demonstrated. Stable single-lobe far-field beam pattern with a divergence of 0.18° was obtained along the cavity-length direction. Besides, low loss coupling mechanism was achieved in accordance with simulation that the devices manage to operate in continuous-wave mode with high surface-emitting efficiency. Reliable single-mode emission with a side-mode suppression ratio (SMSR) about 25 dB is observed among the working temperature range of 15 °C–30 °C.
2. Structure and simulation
A three-dimension (3D) schematic representation of the structure is shown in Fig. 1. The second-order DFB/DBR grating was fabricated in the upper InGaAs confining layer and covered by an InP cladding layer. The active region of QCL is based on alternating strain compensated In0.669Ga0.331As/In0.362Al0.638As quantum well and barrier layers. The layer sequence and thickness are identical to that in Ref . The entire structure was as follows: 1.34 μm lower InP cladding layer (Si, 1.5 × 1017 cm−3), 0.3 μm-thick In0.53Ga0.47As layer (Si, 4 × 1016 cm−3), 1.764 µm active core, 0.3 μm-thick upper In0.53Ga0.47As (Si, 4 × 1016 cm−3), 2.6 μm InP cladding layer (Si, 3 × 1016 cm−3), 0.15 μm gradually doped InP contact layer (Si, 1 × 1017 cm−3 to 3 × 1017 cm−3), and finally a 0.45 µm highly doped InP contact layer (Si, 5 × 1018 cm−3).
For optical analysis of the composite DFB/DBR structure, coupled-mode theory for second-order grating surface-emitting structures and transfer-matrix method for multisection structures adjusted for TM-polarized light are both employed [16, 17]. Target key parameters obtained through the simulation are the threshold gain coefficient gth, and surface outcoupling efficiency ηsurf. Specifically, the coupled-mode theory for second-order DFB gratings was carried out to calculate the eigenfrequencies of the two contrarily transmitting symmetric and anti-symmetric modes for each finite section (one DFB section and two DBR sections). The imaginary parts of the calculation results were converted to loss parameters by formula 2ωneff/c, where ω is imaginary part of the eigenfrequency, neff is the effective reflective index and n is the speed of light. Particularly, the calculation for surface radiation loss αsurf is carried out when all materials were set lossless . However, for the case of composite DFB/DBR lasers, one has to take effective index difference between the DFB and DBR regions, background absorption loss in the DBR section, surface radiation loss, and current-injection induced index depression in DFB section into consideration. So next, modified transfer-matrix method is utilized to obtain the threshold gain coefficient (i.e., in the DFB section) [11, 19], which is given by
where 2α is the grating-related (intensity) loss coefficient calculated from transfer-matrix method, which includes surface radiation loss, absorption loss to the grating, and loss at the cleaved facets. αi is the internal cavity loss of the whole material system and Γlg is the percentage of field intensity residing in DFB region. Then the threshold modal gain Gth is naturally defined for the entire structure by
Since the grating is the same among the DFB and DBR regions, the surface outcoupling efficiency, which is the ratio of surface radiation loss to threshold modal gain, can be directly divided by Eqs. (1) and (2) as,
For the second-order grating with period of Λ = 1.47 µm and a fixed grating depth of 150 nm, the values of gth for the symmetric and antisymmetric mode as a function of the grating duty cycle σ (defined by the ratio of grating peak and period) are shown in Fig. 2. The simulation result reveals that the symmetric mode has lower threshold gain and is expected to be the lasing mode for this buried grating structure. Furthermore, the loss discrimination between the two modes is relatively high enough over duty cycle range of 45-55% to surstain stable symmetric mode emission. Given σ of 52% for our grating structure, significant parameter 2α of 4.6 cm−1 and Γlg of 0.7 were calculated by means of the transfer matrix method for the 6.4mm-long device, which is composed of one 4.5 mm-long DFB section and two 0.95 mm-long DBR sections. Then a high surface outcoupling efficiency of 63% was calculated for our grating design.
3. Device fabrication
To fabricate the second-order grating, top cladding layers was removed down to the upper InGaAs confining layer. The second-order grating was defined using holographic lithography technique and transferred by wet chemical etching. Following the implementation of re-growth, the wafer was etched into double-channel waveguide laser with an average core width of 13 μm. To further block current injection into the DBR regions, two 200-µm wide grooves at ends of the DFB region were etched through the upper InP high-doped layer with a depth of 700 nm via dry etching. Then insulation layer of 450 nm-thick SiO2 was deposited around the ridges, and the current injection window was opened just on top of the DFB region. Subsequently, electrical contact was complemented by a Ti/Au layer, and an additional 4.5 µm-thick gold layer was electroplated to further improve heat dissipation. After being thinned down to about 100 μm, a Ge/Au/Ni/Au metal contact layer was deposited on the substrate side of the wafer. For realizing vertical emission from the substrate, a 150 μm-wide window was left on the substrate contact layer by metal lift-off. Finally, the waveguides were cleaved into 6.4 mm-long bars, wire-bonded and mounted epilayer side-down on AlN heat sink with indium solder for effective heat dissipation.
4. Results and discussion
Devices were tested on an automatic temperature control stage. For electro-optical characterization, the emitted optical power was measured with a calibrated thermopile detector placed right in front of the laser facet without any correction. The measurement of lasing spectra was performed with a Fourier transform infrared (FTIR) spectrometer with a resolution of 0.25 cm−1 in rapid scan mode. Far-field characterization was performed by placing the laser on a stepped motor control rotational stage with a resolution of 0.005° in cavity-length direction. A nitrogen-cooled HgCdTe detector was located 30 cm away to collect the lasing light. Then the detector signal from the far-field was processed by lock-in amplification into data with angle coordination.
Our devices realized CW operation with high surface-emitting efficiency at room temperature. Figure 3 shows the typical light-current-voltage (L-I-V) curves of the uncoated 13 μm-wide and 6.4 mm-long DFB/DBR laser at heat sink temperature of 25 °C. CW power of 24.4 mW was collected from the substrate with a low threshold current density of 1.624 kA/cm2. In contrast, the edge-emitting output power of the laser was also measured. But only 12.7 mW was obtained from front facet, and 25.4mW total edge-emitting output power was derived for the uncoated laser. Without consideration of the 100 µm-thick, n-InP doped substrate absorption, the ratio between surface emission and edge emission for this uncoated device is derived to be about 0.96, which depicts a high extraction efficiency of 45% in substrate direction. The improved surface emission performance is attributed to the appropriate coupling strength |κ|L of ~1.8 and low optical losses achieved in the buried DFB/DBR grating design, in comparison with the second-order surface InP/Au grating structure adopted in . The inset shows the light-current density (L-J) curves of the same device in pulsed mode with a repetition frequency of 10 kHz and pulsed width of 5µs. The maximum peak surface emitted power (average power to duty cycle) was 66.4 mW, while the edge emitted power was 22.1 mW. Consequently, a higher surface outcoupling efficiency of 60% is observed in pulsed operation. By the way, the characteristic of a 4 mm-long FP counterpart in the same pulsed condition was also illustrated in the inset. The maximum peak power of 245.3 mW is comparative to the total power of the uncoated DFB/DBR laser taking the substrate absorption into account.
The CW surface-emitrting spectra of the same device at different heat sink temperatures from 15 °C to 30 °C is shown in Fig. 4. Single-mode emission with a SMSR about 25 dB is observed among the entire investigated temperature range at injection currents of 1.1 Ith. The upper inset shows the lasing spectra of QCL at different injection currents in CW operation at 20 °C. As the driving current increases from 1.1 A to 1.3 A with a step of 0.1 A, the peak emission spectrum shifts from 2121.19 cm−1 to 2015.63 cm−1. A current tunability of − 55.6 cm−1A−1 was revealed. As is shown in the lower inset of Fig. 4, the peak emission spectrum was observed to shift from 2118.06 cm−1 at 15 °C to 2114.81 cm−1 at 30 °C, corresponding to a temperature tuning coefficient Δν/ΔT = − 0.217 cm−1K−1. In fact, second-order DFB lasers are inherently complex-coupled with first-order diffraction contributing to surface emission and second-order diffraction providing feedback. The present of loss-coupling would prevent the device from oscillation at both edges of the stop-band . Besides, the introduction of two DBR regions on each side of the DFB region helps to eliminate uncontrolled cleaved facets reflections which can bring about mode oscillation. Just as expected, reliable single-mode emission is able to be continuously tuned by injection current or heat sink temperature. This conversely convinces the fact experimentally that we do not observe any instability between the two gap modes or spectral sensitivity to additional random phase introduced by uncontrolled cleaved facet during the analysis.
Of particular interest on the surface emitting DFB/DBR laser is the far-field radiation pattern along cavity-length direction. In this longitudinal direction, the buried second-order grating design generates a single-lobe far-field distribution at different driving currents, as shown in Fig. 5. The measured full width at half maximum (FWHM) of the far-field pattern is 0.18° at a driving current of 1.0 A at 20 °C, which is slightly larger than the diffraction limited angle (i.e., 0.07° for the 4.5 mm-wide longitudinal aperture). The deviation might be attributed to substrate-air interface reflections or substrate surface roughness. Regardless of the increase of driving current, stable single-lobe far-field radiation is always observed. This further confirms that the symmetric mode with lower threshold gain is the lasing mode, irrespective of any phase shifts brought by the cleaved facets.
In conclusion, we have demonstrated surface-emitting DFB/DBR QCLs with a buried second-order grating that fundamentally favors to lase in symmetric mode. Reliable single-lobe far-field beam pattern was obtained in longitudinal direction, even at high driving current. The grating design contributes to a low radiation loss and highly efficient surface outcoupling mechanism that more light was collected in substrate direction than edge in both CW and pulsed operation for the uncoated lasers. Continuously tunable single-mode operation further confirms the stable emission of symmetric mode, as a sign of independence from the phase of the facet reflection. If the lasers are fabricated on epitaxial wafer with higher power conversion efficiency, high power surface emission with good far-field distribution might be achieved based on our buried grating design, and that is worthy of future study.
This work was supported by the National Basic Research Program of China (Grant Nos. 2013CB632801); National Natural Science Foundation of China (Grant Nos.61435014, 61574136, 61306058); Key projects of Chinese Academy of Sciences (Grant No.ZDRW-XH-2016-4, 2016YFB0402303); and Beijing Natural Science Foundation (Grant No. 4162060).
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