Single longitude mode semiconductor lasers have many applications such as optical communications, laser sensing, laser radar, and light sources for silicon photonic chips. There are many types of single-mode semiconductor lasers, including distributed feedback (DFB) lasers [1,2], distributed Bragg reflector (DBR) lasers [3–5], integrated-coupled-cavity lasers [6], and cleaved-coupled-cavity lasers [7]. Nowadays, the most widely used types are DFB and DBR lasers among them, in which either buried [8] or surface gratings [9] are fabricated. When low-order gratings are used, which is the usual case for these lasers, either the holographic exposure technique [10] or the e-beam lithography technique [11] is needed, which, however, complicates the device fabrication process, leading to an increase of the device cost.

High-order slotted surface gratings have then been used for the fabrication of single-mode semiconductor lasers [12]. Because the slot width can be set as 1 µm or larger, high-order gratings can be fabricated by conventional contact photolithography, which simplifies the device fabrication. Up to now, a number of single-mode lasers based on the high-order slotted surface gratings have been demonstrated, such as lasers having a wide range of operation temperature [13], multi-wavelength laser array [14,15], narrow linewidth single-mode laser [16], high-speed directly modulated lasers [17], high-power lasers [18], and widely tunable DBR lasers having a wide wavelength tuning range [19,20]. Besides the above lasers which operate at the 1.3 and 1.5 µm low loss optical communication wavelengths, lasers with slotted high-order gratings have also been fabricated for working at other wavelengths in recent years such as 850 [21] and 956 nm [22] quantum well lasers, 3.5 µm interband cascade lasers [23], and 7.3 µm quantum cascade lasers [24]. However, all the above lasers have a surface ridge waveguide structure.

In this paper, we report the first single-mode InGaAsP/InP buried heterostructure (BH) lasers based on high-order slotted surface gratings to the best of our knowledge. Compared with ridge waveguide lasers, BH lasers have several advantages [25–28]. In BH lasers, current blocking InP layers are placed at each side of the active material, forming a current flowing channel, helping to reduce the threshold current by reducing the current lateral diffusion. Because the active material is surrounded by an InP material, the laser has a real index guiding waveguide in contrast to the quasi-index guiding for a surface ridge waveguide laser. In addition, the InP material surrounding the active material has a large thermal conductivity, helping to lower the thermal resistance of the device. Because of the adoption of the slotted high-order surface gratings, no holographic exposure or e-beam lithography technique is needed for grating formation. In addition, compared with the BH laser having buried gratings [29], the use of the slotted gratings reduces one step of the metal organic vapor deposition (MOCVD) process which is needed either before the grating fabrication when the grating is placed under the active layer or after the grating etching when the grating is above the active layer. The fabrication procedure of single-mode BH lasers can thus be simplified notably, helping to lower the device cost. The fabricated BH laser has larger than 30 dB side-mode suppression ratio (SMSR), which indicates a good single-mode emission performance. Calculations show that the gain coupling mechanism plays a key role for the slot grating to select the work wavelength.

A schematic fabrication process of the device is shown in Figs. 1(a)–1(e). The device is fabricated by a three-step MOCVD process. In the first step, an n-InP buffer layer, a compressively strained InGaAsP multi-quantum well (MQW) layer having 1.53 µm photoluminescence wavelength, and a 400 nm p-InP cap layer are grown successively on an n-InP substrate (a). Then, stripe structures having a 4 µm width are formed by wet etching the MQWs down to the buffer layer using SiO₂ stripes as masks (b). After careful cleaning, a 1 µm thick p-InP layer doped with Zn and a 1 µm thick n-InP layer doped with Si are successively grown on each side of the MQW layer in the second MOCVD run (c). To finish the material structure, a 2000 nm p-InP cladding layer and a 200 nm p+ nGaAs contact layer are grown over the entire wafer in the third MOCVD run after the SiO₂ masks are removed (d). Then, slotted gratings over the MQWs are formed by dry etching in the InGaAs contact layer and the InP cladding layer (e). A 12 µm wide mesa structure containing the active region and the current blocking structures is formed by wet etching the materials down to the InP buffer layer to reduce lateral current spreading (f).

 

Fig. 1. (a)–(f) Fabrication process of the device. (g) Cross-sectional slot structure, (h) cross-sectional SEM image of the slots.

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The designed wavelength of the device is 1560 nm, which can be calculated by

The current window of the laser which covers the slots’ regions is opened in a layer of SiO₂ grown by plasma-enhanced chemical vapor deposition. A photoresist layer is coated on the wafer first. The resist in the window area is then exposed for a certain time, during which only the window area outside the slot regions is well exposed. The resist in the slot regions has a larger thickness than in the rest window area and thus is less exposed. In the developing process, only the well-exposed resist in the window area can be removed by the developer and the resist in the slot regions stays. During the following wet etching process, only the exposed SiO₂ layer is removed, and the SiO₂ in the slot region is left because of the protection of the resist. Thus, though the metal contact covers all the window region, the current is injected into the MQWs through the unslotted regions only. After cleaving, one facet of the laser is coated with an HR film having 95% reflectivity. The other facet is left as cleaved and has about 30% reflectivity. The length of the laser is 1000 µm, with 33 slots formed at the HR end. The device is soldered onto the AlN heat sink and tested at 20°C.

The optical power of the slot laser collected by an integrating sphere as a function of the injected current (PI) is shown in Fig. 2. The threshold current is 91 mA. The peak output power is 10.2 mW at 290 mA injected current, after which the light power starts to decrease because of current self-heating effect. The PI curve of a Fabry–Perot (FP) laser with the same structure but without slot gratings is also shown in Fig. 2 for comparison. As can be seen, the threshold current and the peak optical power of the reference laser are 75 and 22.6 mW, respectively. The larger threshold current and the lower optical power of the slotted laser indicate that the slots introduce an extra optical loss into the device [30].

 

Fig. 2. PI curve for the slot grating BH laser and normal FP BH laser.

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A single-mode fiber is used to couple the light of the slotted laser out for optical spectra measurements. The optical spectrum analyzer for measurements is set at a 0.02 nm resolution. Figure 3(a) shows the obtained optical spectra measured at different injected current. The lasing wavelength is around 1563 nm. The difference between the designed and measured wavelengths can be attributed to the fabrication errors during the device fabrication process. The side-mode suppression ratio (SMSR) of the spectra as a function of the current is shown in Fig. 3(b). The SMSR increases continuously from 24.7 dB at 170 mA to 39.3 dB at 350 mA and then decreases to 36.8 dB at 400 mA. The SMSR is larger than 30 dB when the current is between 200 and 400 mA, indicating a good single-mode emission performance. The lasing wavelength as a function of the current is shown in Fig. 3(c). The wavelength is redshifted by the current heating effect from 1563.5 at 170 mA to 1566 at 400 mA, which is a result of the refractive index change with temperature. At the same time, the peak wavelength of FP modes around the lasing mode is increased from 1558.3 to 1568.7, which reflects the variation of the MQW bandgap with temperature. The wavelength variation rate is 0.02 nm/mA for the lasing wavelength, which is notably smaller than the 0.08 nm/mA for the peak of the FP mode wavelength. This trend is similar to the case of a normal DFB laser.

 

Fig. 3. Laser spectra at different current (a), SMSR (b), and lasing wavelength (c) as a function of the current.

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The coupling strength of the slot gratings is estimated numerically. The slotted gratings introduce both index coupled and gain coupled mechanisms into the device. The refractive indices of the slotted and unslotted regions calculated by Lumerical are 3.1938 and 3.1941, respectively, for the 1.4 µm slot depth. In the slotted regions, there is no current injected into the MQW layer vertically, and there are carriers defused laterally from the unslotted regions, leading to a carrier density difference in the MQW layer between the slotted and unslotted regions. A gain contrast then exists between the two regions, resulting in a gain coupled mechanism. The coupling strength κ of the slotted gratings can be calculated by [22]

 

Fig. 4. Calculated real (a) and imaginary (b) parts of κ as a function of the slot depth.

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The linewidth of the laser emission is measured by the delayed self-heterodyne method with 25 km fiber length [31]. Figure 5(a) shows the measurement result at 200 mA injected current. Both Gaussian and Lorentzian profiles are used to fit the experimental data. The obtained Gaussian and Lorentzian linewidths are 1500 and 550 kHz, respectively. Figure 5(b) shows the far-field patterns in the vertical and horizontal directions measured at 200 mA. The corresponding far-field divergence angles are 23.8 and 30.8°, respectively.

 

Fig. 5. (a) Linewidth measurement results and (b) far-field patterns of the device.

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As a prototype device of its kind, the linewidth and SMSR of the laser emission are comparable to some single-mode lasers that are reported earlier in Refs. [2,16]. The threshold current and output light power, however, are relatively lower. The device performance can be improved by optimizing both the BH material parameters and the slotted grating parameters. As shown in Fig. 2, even for the reference FP BH laser without slot gratings, the threshold current is large and the output power is low. To enhance the device performance, the etching profile of the MQW stripe and the doping level of the p and n current blocking InP layers can be optimized to get better current confinement. For the grating parameters, while a large m of the grating used in our device helps to ease the device fabrication, a high optical loss can be induced [12]. In our future work, both d_w and d_s will be optimized. To suppress the intensity of the FP modes as shown in Fig. 3(a), an anti-reflection coating having low reflectivity can be applied. As a result, both the SMSR and light output power can be improved. The linewidth performance can also be further improved by lowering the laser internal loss, which can be realized by decreasing the optical confinement factor of the active layer, increasing the cavity length, and increasing the grating coupling strength [32].

We have fabricated an InGaAsP/InP BH laser based on high-order slotted surface gratings. The introduction of the surface slotted grating can notably simplify the fabrication process of single-mode BH lasers. The laser shows a good single-mode emission performance, with larger than 30 dB SMSR when the current is between 200 and 400 mA. The linewidth of the laser is measured. The fitted Gaussian and Lorentzian line widths are 1500 and 550 kHz, respectively.