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A 1x2 multi-mode-interferometer (MMI) laser diode was successfully designed and fabricated, which demonstrated three coherent outputs of tunable single frequency emission with more than 30dB side mode suppression ratio (SMSR), a tuning range of 25nm in C and L band, as well as 750 kHz linewidth. This 1x2 MMI laser could be expanded to more advanced configurations such as 1xN or MxN (M≥1, N>2) MMI lasers to achieve a multiple coherent output source. In addition, these lasers do not require material regrowth and high resolution gratings which can significantly increase the yield and reduce the cost.
© 2016 Optical Society of America
1. Introduction
Tunable single frequency semiconductor laser diodes with a narrow linewidth in the C and L bands have diverse applications in optical communication systems, DWDM systems, remote sensing and spectroscopy [1, 2]. Adding multiple coherent outputs to these sources is of increasing importance and interest in applications such as coherent optical communication systems, advanced modulation formats, interferometry, holography, sensors, beam combining as well as Lidar [3,4]. Distributed feedback (DFB) lasers and distributed Bragg reflector (DBR) lasers have gained wide recognition as reliable tunable single longitudinal mode laser sources, yet the required fabrication complexity of nanometer dimensional gratings and material regrowth limits the yield and increases the cost. In addition, the typical free-running linewidth of 1-5 MHz restricts their applications in coherent optical communications systems and ultra-high resolution spectroscopy [5, 6]. An alternative laser source: the regrowth and grating-free slotted Fabry-Perot (SFP) laser diode, has been demonstrated with excellent tunable single longitudinal mode performance and narrow linewidth (around 100 kHz). These are fabricated by etching single or multiple micron dimension reflective slots into the ridge waveguide of FP lasers, which eases the fabrication significantly and reduces the cost. However, these etched slots result in additional mode loss and the required high optical quality of the slots remains a challenge for the photolithography and etching processes [7–9]. Coherent optical sources have been reported using injection locking techniques where an array of slave lasers are injection locked to a tunable single mode master laser that is monolithically integrated on the same wafer [10–12]. This kind of module requires a tunable single mode laser such as a DFB, DBR or slotted FP laser, an MMI or AWG to split the light from the master laser into multi-channels, as well as integration which increases the complexity resulting in lower yield and higher cost.
Optical devices and PICs based on the multimode interference (MMI) self-imaging effect have attained broad interest as passive couplers and splitters, and are used in balanced detectors and high speed modulators [13]. Active MMI lasers have also been reported with different configurations and functionality. For example, symmetric active 1x1 MMI lasers were demonstrated with single lateral mode emission and multiple FP longitudinal modes. These achieved high output power due to the large gain area [14, 15]. An asymmetric 1x1 MMI laser and a 1x3 MMI laser with only the middle arm on the output side have shown single lateral and longitudinal mode operation due to the reflection at the MMI which results in the coupled cavity effect [16, 17]. These regrowth and grating free active MMI lasers simplify the fabrication process thus becomes a promising candidate for low cost single mode laser sources. In this paper, we present a novel configuration of a low cost 1x2 MMI composite cavity laser which gives three coherent tunable low linewidth single mode outputs with two of them on same facet.
2. Design and fabrication
The three dimensional configuration of the 1 × 2 MMI laser is shown in Fig. 1. An active 1x2 MMI is connected to symmetric twin angled waveguides with spacing of 250µm at the end on the left and a straight waveguide on the right. Each of the twin angled waveguides has a metal contact that is electrically isolated using 7° angled etched slots (1.0 µm gap). The MMI and the straight waveguide to the right share another common metal contact. In principle, each angled waveguide together with the 1x2 MMI and straight waveguide forms a FP cavity and the two cavities overlap via the MMI and straight waveguide. This leads to mode beating between these two cavity modes. When proper bias is applied to the three contacts, the induced index changes between the two FP cavities results in single longitudinal mode emission from the composite cavity. The emission wavelength is then tuned by altering the bias conditions. Thus three tunable single mode coherent outputs are obtained, two on the left side and one on the right.
In this design, the 1x2 MMI plays an important role in joining the two FP cavities for splitting and combing the light and it also provides gain due to the relatively large area compared with the narrow etched waveguides. Thus, the MMI section contributes toward a high power output of the laser diode while benefiting from a lower contact resistance and thus better thermal stability. In our design, the 1x2 MMI is 12.5µm wide and 195µm long. The center to center separation between the output twin waveguides is 7µm, where the outputs are separated at the cleaved facet by using two π/6 curves with a radius of 450µm. The waveguides are nominally 2.5µm wide, but are expanded to 3.5µm before entering the MMI by using a 100µm linear taper. Expanding the waveguide width provides a more robust design, which can lead to better MMI performance [18]. The MMI parameters were determined using a 1D effective index simulation. After calibration of the material index profile, the fabricated MMI was re-simulated using the commercial software BeamPROP [19]. The optical transmission, as well as the calculated modes, at the beginning and end of the device is shown in Fig. 2 for the TE mode. From the updated simulation results, it was found that the fabricated MMI dimensions were non-optimized, which results in excess optical loss of approximately ~45% through the coupler. This could affect the MMI laser’s output power and leads to optical reflections at the joint of MMI and input/output waveguides.
Fig. 2 Simulated optical transmission (a) and waveguide modes (b), (c) of the 1X2 MMI. In this simulation, the structure does not include the SiO2 passivation layer and metal layer.
The designed 1X2 MMI lasers were fabricated using 5 pairs of compressively strained AlInGaAs/AlInGaAs quantum wells ( + 1.2% strain, 6nm thick quantum well and 10nm thick barrier, λPL = 1.55 μm, where PL stands for photoluminescence) on n-doped InP. The processing is the same as our typical processing for a Fabry-Perot semiconductor laser with common photolithography and etching techniques. First 500nm thick SiO2 was deposited on top of the wafer by sputtering, followed by standard photolithography which was performed to define the MMI and waveguides. Inductively coupled plasma (ICP) dry etching of the SiO2 with a CF4/CHF3 was then done to transfer the pattern into the SiO2 mask. After removing the photoresist, room temperature ICP dry etching with Cl2/CH4/H2 was used to etch around 2.5μm deep through the multiple quantum wells. Following a wafer passivation using 300 nm of SiO2, a window opening was made in the SiO2 by ICP dry etching which was followed by a p metal (TiAu) deposition and annealing at 420 degree for 5 minutes under an N2/H2 atmosphere. Finally, the samples were thinned to 100 μm and AuGeAuNiAu was deposited on the back of the wafer, followed by annealing at 380 degree for 5 minutes in a nitrogen furnace.
3. Characterization
The fabricated 1x2 MMI lasers were cleaved into multiple devices: device A with dimensions (L1 = 490μm, L2 = 100μm, L3 = 195μm, L4 = 66μm) as shown in Fig. 1 and device B (L1 = 524μm, L2 = 100μm, L3 = 195μm, L4 = 180μm). The microscopic picture of a gold-wire bonded device is shown in the inset of Fig. 2(b). All the measurements of the devices were carried out with DC biasing and room temperature control.
3.1 Light-current-voltage (LIV) test
The light-current-voltage characteristics of the fabricated 1x2 MMI laser (device A) were tested. In the measurements, when only a single pad was biased, no lasing was observed. When the MMI and one arm were biased above a certain value with the second arm unbiased, lasing was observed from the output on the MMI side on the right and that of the biased arm but no light was detected from the output of the unbiased arm. This is because the unbiased arm has the same material bandgap and therefore absorbs the light. When all the three pads are biased properly above a certain current value, lasing was observed from all the three outputs. Figure 3(a) shows the light output power of device A collected from the MMI side when the bias current on MMI (I1) is constant and the bias current on the arm 1 (I2) changes, while Fig. 3(b) shows the output power when I2 is constant while I1 varies. In both cases, the bias current on the second arm of the twin waveguide (I3) is zero. The output power increases with the bias current I1 and I2, and around 10mW was detected in the measurement range where the power roll-off appears. This power level is lower than the expected high-power output from the larger gain area of MMI, which attributes to the excess loss of the non-optimized MMI described in the above design part. Figure 3 also gives the plot of voltage Vs. current which shows the lower contact resistance on the MMI region.
Fig. 3 Measured optical power and voltage vs. biasing current. Inset of (b) is the microscopic picture of a wired bonded 1x2 MMI laser.
3.2 Lasing spectra and cavity analysis
The lasing spectra of the 1X2 MMI lasers were measured with an optical spectrum analyzer and fiber coupling under different bias conditions. Figure 4(a), 4(b) and 4(c) plot the spectra from the MMI (right) side of device A under different bias conditions as shown in each plot. The measured spectra demonstrate that the 1 × 2 MMI laser lases in a single longitudinal mode with a SMSR between 30 to 36dB under the specific bias conditions. The lasing wavelength shifts to longer wavelength from 1559.08nm to 1571.7nm when the biasing current on the MMI or the arms increases.
Fig. 4 (a), (b), (c) are the measured lasing spectra of the 1x2 MMI laser under different biasing conditions shown in each plots; (d) is the Fourier analysis of the spectrum shown in Fig. 4(a).
The Fourier Transform of the wavelength data was taken to examine the resonant cavities present in the lasing spectra of Fig. 4(a) and the result is shown in Fig. 4(d) where two cavity lengths of 805.7μm and 872.3μm are observed. These are compared to the dimensions of device A shown in Fig. 1. The length of 872.3μm corresponds to the total length of the device (including the curved waveguides) while the second cavity length of 805.7μm is around 66.6μm shorter than the total length of the device. This value is close to the length of the waveguide (L4) to the right of the MMI. These two cavity lengths indicate that when the MMI and only one arm are biased (the second arm is off), the first cavity (C1) is between the two cleaved facets of the biased angled waveguide and the straight waveguide to the right of the MMI, while the second cavity (C2) is between the left cleaved facet of the biased angled waveguide and the right etched facet of MMI, instead of the cavity (C3) as designed between the cleaved facet of the unbiased angled waveguide and the straight waveguide to the right of MMI. This is due to the light absorption of the unbiased angled waveguide and thus there is no light reflection from the cleaved facet of this arm. However, this extra sub-cavity C2 enables more flexibility of tuning and optimizing the single mode performance. In this case, the laser works with two FP cavities C1 and C2 and the two modes beats together to achieve the single longitudinal mode from two outputs.
To further examine the working resonant cavities of these lasers, device B with different dimensions (L1 = 524μm, L2 = 100μm, L3 = 195μm, L4 = 180μm, radius: 250 μm, angle: π/4) was tested and analyzed with the same method. The measured spectra under two bias conditions are plotted in Fig. 5(a) where I1 = 30.5mA, I2 = 110.5mA and I3 = 0mA and Fig. 5(b) where I1 = 82mA, I2 = 51mA and I3 = 29mA. The Fourier transform analysis of these two spectra is plotted in Fig. 5(c) for comparison. It is observed that when the MMI and one arm is biased with the second arm off, the dotted line shows that two cavities exist with lengths of 852.7μm and 1032.5μm respectively, the length difference of 179.8μm is close to the length (L4) of the waveguide right of the MMI which is measured to be 180μm m. This is consistent with that result of device A, that is, when one arm is off, the lasing mode is defined by cavities. When both arms are biased, the solid line in Fig. 5(c) shows that three cavities exist and two of them have very similar length as each other, corresponding to cavity C1 and C3 as designed. At same time, the effect of the extra cavity C2 is still apparent. This extra cavity C2 results from the light reflection at the joint of MMI and the waveguide where not 100% is coupled into the waveguide.
Fig. 5 (a) and (b): measured spectra of device B, and (c): analysis of spectra (a) and (b) with the Fourier transform method, where the dotted line corresponds to (a) and solid line corresponds to (b).
3.3 Tuning features and coherent outputs
The tuning features of the 1x2 MMI laser were further investigated and the tuning wavelength mapping of device A with bias current is illustrated in Fig. 6. It is observed that the lasing wavelength is tunable by changing the bias current either on the MMI or on the arms. Three isolated contacts provide sufficient flexibility to tune the laser to different wavelengths. Figure 6(a) presents the tuning effect due to changing the bias on one waveguide arm while the MMI bias is constant at 150mA and the bias on second arm is off. The wavelength shifts to longer wavelength continuously with biasing current I2 in a range then it jumps to another range by around 5nm, and it covers from 1558nm to 1585nm with average SMSR around 30dBm. Figure 6(b) shows the tuning effect by changing the bias to the MMI when both arms are biased at a constant of 50mA and 60mA respectively. Similar tuning features as Fig. 6(a) are observed while the single mode lasing wavelength range is different. This indicates that more wavelengths are selectable with the different tuning combination on the three contacts. In Fig. 6(a) and 6(b), the light was collected from the output of the waveguide to the right of MMI.
In addition, the tuning mapping of the light output from the twin waveguides were measured under the same condition as that in Fig. 6(b) and are shown in Fig. 6(c) and 6(d). The three outputs show the identical tuning behavior as expected although the measured output power of arm1 is higher that of arm2 due to the non-stable manual fiber coupling condition in the measurements. Figure 7 gives the spectra of the three outputs under same bias condition where I1 = 100mA, I2 = 50mA and I3 = 60mA. The light from the right waveguide was collected by coupling to an angled fiber while those from the left waveguide arms were coupled to a cleaved fiber array with 250µm fiber to fiber spacing. The three outputs show coherent spectra with the same lasing peak wavelength. The higher power from the right side waveguide is attributed to better coupling as the fiber array used simple cleaved facets, while the angled fiber was lensed.
3.4 Linewidth
The spectral linewidth of the 1x2 MMI lasers (device A) was measured by using the delayed self-heterodyne method and the result is shown in Fig. 8.
In the measurement, the delay length is 25 km, which corresponds to a minimum measureable linewidth of 2.5 kHz using a Lorentzian line shape. An acoustic-optic modulator (AOM) with a 70-MHz frequency shift was used in the experiment. Beat signals were detected by a photodetector (PD) and then measured by an electrical spectrum analyzer (Agilent PXA-N9030A). Figure 8 shows the measured normalized spectrum and the Lorentzian fit demonstrating a 750 kHz linewidth when the 1X2 MMI laser is biased with I1 = 62.16mA, I2 = 60mA and I3 = 0mA and the SMSR is 30dB.
4. Conclusion
In summary, we present a novel 1x2 MMI composite cavity laser which demonstrates three coherent tunable single frequency outputs in a range of 25nm in the C and L band with a narrow linewidth of 750 kHz. Two coherent outputs with 250µm spacing on are the same facet. This 1x2 MMI laser provides an example of multiple tunable coherent single frequency sources with the more advanced NxM configuration MMI lasers. The regrowth free and simple FP laser processing enables high yield and low cost.
Acknowledgment
This work was supported by Science Foundation Ireland: the Irish Photonic Integration Centre (IPIC) under the grant SFI 12/RC/2276, and the SFI Investigators Programme: SFI 13/IA/1960.
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