1. Introduction

There is a big difference between the spot size of diode lasers and that of fibers, which leads to a high cost related to the assembling and packaging of lasers. To reduce the cost of optical laser transmitter module, it is promising to integrate monolithically a spot size converter (SSC) which expands spot size of LDs and thus leads to a high coupling efficiency and a large alignment tolerance between lasers and fibers [1,2]. SSC integrated laser structures are also useful in the fabrication of hybrid III–V/silicon lasers [3,4]. With the help of SSC, light can be coupled between III–V and silicon waveguides efficiently and smoothly, which is essential for high performance light sources or amplifiers for silicon photonics circuits [5].

Up to now, a number of different types of ridge waveguide SSCs for InP based lasers have been proposed, having either surface or buried waveguides which are either laterally or vertically tapered [6–10]. Among them, a laterally tapered surface waveguide SSC fabricated in InP cladding layer of a LD needs only a simple fabrication process, helping to lower the device cost potentially [11]. With this type of SSC, a beam divergence of 9 ° × 9 ° has been obtained for InP based SSC integrated InGaAsP multi-quantum well (MQW) lasers working at 1.5 µm wavelength [11]. However, it is difficult to obtained a narrow waveguide tip usually less than 1 µm through conventional contact photolithography with high reproductivity, which is key for high quality SSC. Though techniques such as E-beam lithography can be used for forming narrow tips of SSCs [12], the technique is expensive and the lithography needs a long time, which limits the yield and increases the cost of lasers.

In this paper, we report the fabrication of laterally tapered surface InP SSCs in a reverse mesa shape. With proper wet etching conditions, a reverse mesa ridge waveguide for which the top width is significantly larger than the bottom width can be obtained. With this process, high quality SSCs having narrow tip width can be fabricated through conventional photolithography with a high reproductivity, helping to lower the device cost. 1.3 µm InGaAlAs/InP lasers integrated with the laterally tapered SSC have been fabricated. The threshold current of the 1000 µm long device is 25 mA and 44 mW single facet optical power can be obtained at 300 mA current. The lateral and vertical divergence angles are as low as 8 ° and 11 °, respectively. The reverse mesa shape of the SSC ridge helps to ease the fabrication of the SSC and thus reduce the cost of related device greatly.

2. Device design and simulations

Figure 1(a) shows the schematic structure of the InP based surface SSC integrated laser which has an upper narrow ridge waveguide and a lower wide waveguide. A thin layer of InGaAsP is placed in the InP buffer layer below the MQW active layer. While the upper ridge is fabricated in the InP cladding layer of the laser, the lower ridge is formed by etching the semiconductor materials on both sides of the upper ridge down to the InGaAsP layer. As can be seen, the laser consists a rear laser (LD) section which has a uniform ridge width and a front tapered SSC section whose ridge width is decreased toward the output facet. In the uniform ridge section, the optical mode is confined in the upper ridge. In the tapered SSC section, as the upper ridge is narrowed, the optical mode is squeezed down and coupled gradually into the thin InGaAsP layer of the lower wide waveguide [11]. Because of the relatively lower effective refractive index and larger size of the lower waveguide, the size of the optical mode can be expended, leading to small far field divergence angles.

 

Fig. 1. (a) schematic structure of the InP based SSC (b)cross section of the SSC with the reverse mesa (c)cross section of the SSC with the normal vertical mesa.

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The effects of parameters of the laser, including the width of the SSC tip W_t and the width of lower ridge W_l, the thickness of the InGaAsP layer T_r and the separation between the MQW layer and the InGaAsP layer T_s, on the divergence angle are studied using FDTD method. The default values of W_t, W_l, T_r and T_s are 250 nm, 10 µm, 50 nm and 1.5 µm, respectively. The InGaAsP layer has a 1.05 µm bandgap wavelength. The MQW layer consists of three 5 nm thick InGaAlAs quantum wells and four 10 nm thick InGaAlAs barriers and has 1310 nm emission wavelength. The MQW layer is surrounded between two In(Ga)AlAs separate confinement heterostructure (SCH) layers, whose total thickness T_h is 200 nm by default. The bottom width of the ridge of the LD section is 2.5 µm, which is also the starting width of the SSC ridge bottom. The lengths of the SSC section and the LD section are both 500 µm. The effects of W_t on the divergence angle are shown in Fig. 2(a). As can be seen, when W_t = 2.5 µm, the vertical and lateral divergence angles are 27.8 ° and 19.7 °, respectively, which are the angles for a laser having a uniform 2.5 µm ridge width. As W_t decreases, the divergence angle decreases rapidly, resulting in 13 ° and 8 ° divergence angles when W_t is 500 nm. The simulated optical intensity distribution in waveguide cross sections having two different W_t is shown in Figs. 2(b) and 2(c). When W_t = 2.5 µm, the optical mode is mainly in the upper ridge and MQWs. In contrast, the majority of the mode is confined in the InGaAsP layer when W_t = 250 nm. The corresponding 1/e² mode diameters are 6.1 µm × 9.5 µm and 2.3 µm × 5.6 µm, respectively.

 

Fig. 2. (a) The effects of W_t on the divergence angle, (b) the cross section optical intensity distribution when W_t = 2.5 µm (b) and 250 nm (c).

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The effects of T_r on the divergence angle are shown in Fig. 3(a). As can be seen, while the vertical divergence angle increases rapidly from 13 ° to 18 ° when T_r is increased from 50 nm to 150 nm, the lateral divergence angle remains below 8.5 °. A larger T_r leads to better optical mode confinement in the InGaAsP layer of the lower ridge and thus a smaller mode size when the optical mode is confined in the lower ridge. Figure 3(b) shows the divergence angle as a function of T_s. For the lateral direction, the variation of the divergence angle is less than 0.5 ° when T_s is increased from 0.5 µm to 3 µm. In contrast, the vertical divergence angle decreases notably from 17.2 ° to 10 °. The dependence of divergence angle on W_l is shown in Fig. 3(c). As W_l is increased from 6 to 16 µm, while the vertical divergence angle increases from 11.8 ° to 13.5 °, the lateral divergence angle decreases from 10.6 ° to 5.4 °. An increasing W_l increases the lateral mode size effectively, but decreases the vertical mode size at the same time. The effects of the total thickness of the SCH layer T_h for two different T_s of 1.5 and 2.0 µm are shown in Fig. 3(d). The divergence angles in both the two directions increase with T_h gradually. A larger T_h makes it more difficult for the optical mode to be coupled from the upper ridge into the InGaAsP layer. As can be seen from Fig. 3(d), when T_s and T_h are 2.0 µm and 160 nm, divergence angles of 11.4 ° and 7.5 ° can be obtained for the vertical and lateral directions, respectively.

 

Fig. 3. The effects of T_r (a), T_s (b), W_l (c), and T_h (d) on the divergence angle. The default values of T_r, T_s, W_l, and T_h are 50 nm, 1.5 µm, 10 µm, 200 nm, respectively.

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3. Device fabrication and characterizations

It can be seen from the above simulation results that a smaller than 1 µm W_t is necessary to obtain less than 15 degrees divergence angles. What is more, for practical device fabrication, a smaller W_t is needed to get a uniform distribution of divergence angle among different devices. As shown Fig. 2(a), there is only a slow variation of the divergence angle with W_t, when W_t is smaller than 0.5 µm. When W_t is in this range, the fluctuation of W_t of actual devices resulted from fabrication errors will lead to only a small divergence angle distribution. However, it is difficult to obtained waveguide having less than 1 µm width with a high reproductivity by conventional photolithography. Though E-beam lithography can be used, the expensive facility and slow lithography process will increase the device cost notably. Here, we demonstrate SSCs having a reverse mesa shape as shown in Fig. 1, which eases the fabrication of SSC integrated lasers greatly.

For InP laser structure to get InP reverse mesa waveguide [13], an InGaAs stripe in the [011] direction is fabricated in the InGaAs contact layer of the laser first. Then, HBr and H₃PO₄ solution (1:1) is used for InP ridge etching with the InGaAs stripe as a mask. The side walls of the reverse mesa ridge form an about 60 degrees angle with the substrate surface as shown mathematically in Fig. 1(b) in contrast to the nearly 90 degrees angle for vertical ridge structure as shown in Fig. 1(c). With this shape, the top width W_p is significantly larger than the bottom width of the ridge. When this reverse mesa ridge is used for the fabrication of SSCs, a small W_t can be obtained with a large W_p, which can be defined with conventional photolithography easily. To study the fabrication of reverse mesa SSCs experimentally, an InP based laser structures as shown in Fig. 1 are grown with a metal organic chemical vapor deposition (MOCVD) system. The thickness of p-InP cladding layer and InGaAs contact layer of the structure are 1.5 µm and 200 nm, respectively. A SEM picture of the cross section of a reverse mesa ridge obtained by etching with the HBr and H₃PO₄ solution at 25 °C is shown in Fig. 4(a). As can be seen, while the width of the ridge bottom is as small as about 270 nm, the top width of the ridge is over 2 µm, which is well within the fabrication ability of conventional contact photolithography. We have conducted ridge wet etching experiments at different temperatures ranging from 10 to 45 °C and found that there is no apparent temperature dependence of the side wall angles. The other samples as followings are etched at 10 °C. A lower temperature helps to maintain a relatively constant acid concentration of the etching solution for a longer time.

 

Fig. 4. SEM cross-section images of a reverse mesa ridge having 270 nm bottom width and 2.1 µm top width (a), SEM cross-section images of the laser side (b) and SSC side (c) of a SSC integrated laser.

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SSC integrated lasers with reverse mesa ridge having different T_r and T_h are then fabricated with conventional photolithography. The lengths of both the SSC section and laser section are 500 µm with both laser facets left uncoated. For all the lasers, W_l and T_s are 8 µm and 2000 nm, respectively. The top and bottom widths of the ridge of the laser section are 3.7 and 2.0 µm, respectively, as can be seen form the cross-section SEM image shown in Fig. 4(b). For the SSC section, the top width of the ridge is decreased linearly from 3.7 µm at the laser side to 2.2 µm at the output facet side, with the bottom ridge width decreasing correspondingly from 2 to 0.5 µm. A cross section SEM image measured at the end of the SSC is shown in Fig. 4(c). The difference between the top and bottom ridge width is about 1.7 µm, which is smaller than that shown in Fig. 4(a). This can be attributed to the fluctuation of acid concentration of the etching solution used in out experiments. By forming SSCs having about 500 nm W_t, the effect of fabrication errors including the acid concentration variation on the far field angles can be alleviated because the angles vary slowly with W_t at this W_t value.

Figure 5(a) shows the far field patterns measured from the SSC side of a device whose T_r and T_h are 50 nm and 150 nm, respectively. The lateral and vertical divergence angles are 8 ° and 11 °, respectively, which are significantly smaller than the 20 ° and 25 ° measured from the laser side facet of the device as shown in Fig. 5(b), helping to ease the coupling and increase the coupling coefficient between the lasers and fibers. The far field patterns measured from the SSC side of two devices whose T_r and T_h are 80 nm and 150 nm, and 80 nm and 180 nm, respectively are shown in Figs. 5(c) and 5(d), respectively. The corresponding lateral and vertical divergence angles are 8.5 ° and 13 °, and 9 ° and 15 °, respectively. As can be seen, a larger T_r or T_h enlarges the divergence angles which agrees with the calculation results.

 

Fig. 5. Far field patterns measured from the SSC facet (a) and the LD facet (b) of a device with W_l = 8 µm, T_s = 2 µm, T_h = 150 nm, T_r = 50 nm, from the SSC facets of devices with W_l = 8 µm, T_s = 2 µm, T_h = 150 nm, T_r = 80 nm (c) and with W_l = 8 µm, T_s = 2 µm, T_h = 180 nm, T_r = 80 nm (d).

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The light power current (LI) curve of a SSC integrated laser whose T_r and T_h are 50 nm and 150 nm, respectively, is shown in Fig. 6. For comparison the LI curve of a normal laser having 1000 µm uniform ridge with all the parameters the same as the LD section of the SSC integrated laser is also shown. The threshold current of the SSC integrated device is 25 mA and a 44 mW optical power can be obtained at 300 mA inject current. As can be seen, compared to the normal laser, there is only less than 5 mA increase of threshold current for the SSC integrated device. The two lasers have nearly the same external efficiency within the current range. It is finally worth noting that besides easing the fabrication of high quality SSCs, the reverse mesa shape ridge can also provide other potential advantages including lower threshold current and lower electrical and thermal resistances when compared to vertical ridge laser structures as shown in Ref. [13].

 

Fig. 6. LI characteristics of a 1000 µm SSC integrated device and a 1000 µm long normal LD.

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4. Conclusion

We report surface InP SSCs in a reverse mesa shape. With this shape, high quality SSCs having narrow tip width can be fabricated through conventional photolithography with a high reproductivity, helping to lower the device cost. A 1.3 µm InGaAlAs/InP laser integrated with the laterally tapered SSC has been fabricated. The Threshold current of the 1000 µm long device is 25 mA and 44 mW single facet optical power can be obtained at 300 mA current. The lateral and vertical divergence angles are as low as 8 ° and 11 °, respectively.