Single-mode semiconductor lasers fabricated by standard photolithography for direct modulation

Pijie Ma; Anjin Liu; Fengxin Dong; Mingjin Wang; Wanhua Zheng

doi:10.1364/OE.27.005502

1. Introduction

Single-mode semiconductor lasers are widely used in optical interconnects, optical communications, optical sensing, and so on. Distributed feedback (DFB) lasers and distributed Bragg reflector (DBR) lasers are the popular candidates for stable single-mode operation in edge-emitting semiconductor lasers [1–3]. These single-mode semiconductor lasers require high resolution processing like electron-beam lithography for patterning the gratings and complicated regrowth steps. Also, laterally coupled DFB lasers demonstrate very stable characteristics of single-mode operation [4–7]. The low-order gratings in such single-mode semiconductor lasers are always fabricated by the electron-beam lithography with a merit of high resolution. However, the electron-beam lithography is very expensive and time consuming compared with the standard photolithography, resulting in a higher cost and lower yield.

Alternatively, introducing surface higher-order gratings in the ridge waveguides of semiconductor lasers is an efficient method to achieve single-mode operation [8–11]. This is a post-growth wavelength control approach without regrowth steps, relieving the difficulties of fabrication processing. On the other hand, the feature size of the higher-order grating is about 1 μm. Therefore, the higher-order grating can be patterned by the standard photolithography, which can greatly reduce the cost and increase the yield. Therefore, such single-mode lasers with surface higher-order gratings have attracted a lot of attention.

The single-mode semiconductor laser with a surface higher-order grating is featured with a few slots in the ridge. Significant progresses of single-mode semiconductor lasers with surface higher-order gratings have been achieved. The single-mode semiconductor laser demonstrated very stable single longitudinal mode operation with a side mode suppression ratio (SMSR) larger than 50 dB [12–15], and a large temperature range from −40 °C to 95 °C operation without mode hop [16]. Also, the single-mode semiconductor laser realized a linewidth of 3 kHz [17], useful in optical coherent communications, sensors, and clocks. Previously, the surface higher-order gratings (i.e. slots) were fabricated by the electron beam lithography. The surface higher-order grating was considered as an active DBR, and commonly arranged at the portion close to the output facet as a mirror [12–15]. Recently, a surface higher-order grating laser array has been demonstrated and a channel space of 0.8 nm was achieved by the standard photolithography [18,19]. The surface higher-order gratings in each element of the laser array were introduced in the middle between the two facets of the lasers.

In this work, we designed, fabricated, and characterized single-mode semiconductor lasers with different order numbers of the surface higher-order gratings in the ridge waveguides. Then the effects of the position of the higher-order grating on the performance (including modal property and far-field profile) of this kind of single-mode semiconductor laser are presented. Also, the dynamic modulation characteristics of these single-mode semiconductor lasers are demonstrated.

2. Simulation and design

In the simulations, an InP-based epitaxy structure has been simplified to three layers as shown in Fig. 1. The core layer consists of five AlGaInAs quantum wells and two separate confinement layers. Grating parameters are calculated by $d_{s} = λ_{B} \cdot (2 p + 1) / 4 n_{s}$ and $d_{w} = λ_{B} \cdot (2 q + 1) / 4 n_{w}$ , where $d_{s}$ and $d_{w}$ are the width of slots and distance between adjacent slots, respectively. p and q are integers, and $m = p + q + 1$ is the grating order. $n_{s}$ and $n_{w}$ are the effective refractive index of the slot region and the waveguide region without slots, respectively. $λ_{B}$ is the Bragg wavelength in vacuum. The refractive index and thickness of the simplified layers are the same as those in [20].

Fig. 1 Simplified structure of the InP-based epitaxy layers. $d_{s}$ and $d_{w}$ are the width of slots and distance between adjacent slots, respectively. h is the depth of slots.

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To characterize the performance of higher-order gratings, the power reflectivity R and the power transmission T of the higher-order grating have been calculated by the two-dimensional scattering matrix method (SMM) [21]. The loss of the higher-order grating is defined as $1 - R - T$ . Figure 2(a) shows simulation results of loss spectra of higher-order gratings with the order number from 20 to 100. For the higher-order gratings with the order number less than 20, it is very difficult to fabricate them by standard photolithography, because it is hard to open current injection windows and accurately etch graphic electrodes. In the simulations, $d_{s}$ is fixed at 1.09 μm for all the orders of gratings, and the adjacent slot distance $d_{w}$ is changed to achieve different grating orders. All of these higher-order gratings have the same structure parameters: the slot depth of $h = 1.35$ µm, and the slot number of 12. Figure 2(b) shows the loss spectra of five kinds of higher-order gratings extracted from Fig. 2(a). A minimum loss can be found at a wavelength of 1.55 µm for the 37^th-order grating. This lowest loss is beneficial to achieve the single-mode operation when the 37^th-order grating is introduced into a semiconductor laser, as previously reported in [14] and [20]. Also, we can find that the 20^th-order grating also has a loss minimum at 1.55 µm. However, the minimum loss of the 20^th-order grating is lower than that of the 37^th-order grating. Considering the different lengths of the 37^th- and 20^th-order gratings, the loss in the unit of cm⁻¹ are calculated to be 12.5 cm⁻¹ and 3.53 cm⁻¹ for the 37^th- and 20^th-order gratings, respectively, using a unified length of cavity length of 500 µm. Therefore, the semiconductor laser with the 20^th-order grating has a potential to achieve single longitudinal mode operation and is expected to realize better performance than that of the semiconductor laser with the 37^th-order grating.

Fig. 2 (a) Loss spectra of gratings with the order number from 20 to 100; (b) Loss spectra of 20^th-, 25^th-, 29^th-, 33^th-, and 37^th-order gratings.

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3. Fabrication

In the fabrication, firstly a 300-nm-thick SiO₂ mask layer was deposited by the plasma enhanced chemical vapor deposition (PECVD), and then the standard contact-type photolithography using a wavelength of 365 nm was performed to define slots. In order to transfer the slot patterns into the upper-InP layer, the inductively coupled plasma (ICP) was performed twice. The first time is etching SiO₂ and the second time is etching upper-InP. The depth of upper-InP is set to be 1.35 µm. The ridge waveguide was fabricated by the identical procedure of fabricating slots, and the height of ridge waveguide is set to be 1.65 µm. A 300-nm-thick SiO₂ was adopted as the electrical insulation layer and etched by reactive ion etching (RIE) to open the current injection window. Then Ti/Au was sputtered as p-electrode and the metal in the slots was etched off to avoid metallic absorption. After thinning the wafer to 120 µm in thickness, the n-electrode was sputtered and annealed under 390 °C.

It’s worth noting that for a good alignment between slots and ridges, slots are wider than ridges in the lateral direction. Thus, the sections of slots out of ridge waveguides were etched twice, which led to a larger etching depth for slots in out of the ridge waveguides. The schematic of the designed semiconductor laser is shown in Fig. 3(a), and the optical image of the fabricated semiconductor laser after package on a carrier is shown in Fig. 3(b). Figure 4 shows scanning electron microscope (SEM) image of the ridge waveguide with a 37^th-order grating. To investigate the influences of the position of the higher-order grating on the performance of the semiconductor lasers, the higher-order gratings have been arranged at the edge and in the middle of the ridge waveguides. Both the semiconductor lasers with 37^th- and 20^th-order gratings were fabricated. The width of ridge waveguide is 5 µm and the number of slots is 12. All of the cleaved facets were uncoated.

Fig. 3 (a) Schematic of the semiconductor laser with a surface higher-order grating; (b) Optical microscope image of the packaged semiconductor laser on a carrier. The p-type contact is connected to the signal (S) pad. G denotes the ground pad.

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Fig. 4 SEM image of the ridge waveguide with a 37^th-order grating. Insets are SEM images of the etched slots cleaved along the dashed lines.

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4. Experimental results

4.1 Single-mode lasers with different higher-order gratings

The semiconductor lasers with 37^th- and 20^th-order gratings are measured under continuous-wave (CW) condition at room temperature and the output power versus the current (P-I) curves are shown in Fig. 5. Also, Fabry-Pérot (FP) lasers as the control devices were fabricated in the same wafer without higher-order gratings introduced into the ridge waveguides. The higher-order grating is close to the output facet and the length of cavity is 500 µm. The slope efficiencies of the semiconductor lasers with 37^th- and 20^th-order gratings are approximately 0.22 mW/mA and 0.24 mW/mA, respectively. This higher slope efficiency of the semiconductor laser with the 20^th-order grating is helpful to achieve a higher output power. Moreover, the threshold current of the semiconductor laser with the 20^th-order grating is slightly lower than that of the semiconductor laser with the 37^th-order grating.

Fig. 5 Output power curves of semiconductor lasers with different higher-order gratings and FP semiconductor lasers under CW condition. The higher-order gratings are positioned at the edges of the ridge waveguides (close to the output facet).

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The slope efficiency $η_{s e}$ can be written as [22]

η_{s e} = \frac{h ν}{q} \cdot \frac{η_{i} α_{m}}{〈 α_{i} 〉 + α_{m}},

where h is the Planck constant,

ν

is the light frequency, q is the elementary charge,

η_{i}

is the injection efficiency,

α_{m}

is the mirror loss,

〈 α_{i} 〉

is the average internal loss.

〈 α_{i} 〉

is given as

〈 α_{i} 〉 = {〈 α_{i} 〉}_{s l o t} + {〈 α_{i} 〉}_{0},

where

{〈 α_{i} 〉}_{s l o t}

is the average loss caused by the slot scattering,

{〈 α_{i} 〉}_{0}

is the average loss caused by the ridge and other scattering centers.

{〈 α_{i} 〉}_{0}

for the 37^th-order grating and 20^th-order grating can be considered to be identical. It can be found that

η_{s e}

depends on

{〈 α_{i} 〉}_{s l o t}

. The internal loss introduced by the higher-order grating can be extracted from measured slope efficiency using Eq. (3)

{〈 α_{i} 〉}_{s l o t} = \frac{h ν \cdot η_{i} \cdot α_{m}}{η_{s e} \cdot q} - α_{m} - {〈 α_{i} 〉}_{0} .

For the uncoated facets,

α_{m} =

22.79 cm⁻¹ under a cavity length of 500 µm.

η_{i}

is approximately 1, and

{〈 α_{i} 〉}_{0} =

15 cm⁻¹, which have been obtained in our previous measurement. As a result, the average internal loss

{〈 α_{i} 〉}_{s l o t}

introduced by 20^th- and 37^th-order gratings are 38.32 cm⁻¹ and 45.24 cm⁻¹, respectively. They are much bigger than previous theoretically predicted results. This might be caused by other losses introduced during fabrication, such as uneven surface caused by etching. However, the differences between predicted results and measured results of the average internal loss

{〈 α_{i} 〉}_{s l o t}

for both 20^th- and 37^th-order gratings are similar, and this difference is approximately 35 cm⁻¹.

On the other hand, the threshold current can be expressed as [22]

I_{t h} ≅ \frac{q V B N_{t r}^{2}}{η_{i}} e^{2 (〈 α_{i} 〉 + α_{m}) / Γ g_{0 N}},

where V is the volume of the active region, B is the bimolecular recombination coefficient,

N_{t r}

is the transparency carrier density,

Γ

is the confinement factor,

g_{0 N}

is the gain coefficient. The higher-order grating is fabricated on the surface of the ridge waveguide. It is far away from the active region. The overlap between the optical field and the higher-order grating is small. Thus, the confinement factors

Γ

of lasers with 37^th- and 20^th-order gratings are considered to be the same. The threshold current of the semiconductor laser with the 20^th-order grating is lower than that of the semiconductor laser with the 37^th-order grating, thus it also can be inferred from Eq. (4) that the average internal loss caused by the 20^th-order grating is lower than that caused by the 37^th-order grating.

The spectra of the semiconductor lasers with 37^th- and 20^th-order gratings under different injection currents are shown in Fig. 6. Both kinds of semiconductor lasers with 37^th- and 20^th- order gratings can achieve single longitudinal mode operation with a SMSR of > 30 dB. The lasing wavelength is larger than the nominal wavelength of 1550 nm because of the deviation of fabrication from the design. A stronger wavelength shift of devices with 37^th-order grating appeared compared with devices with 20^th-order grating under the same injection current variation. This might be caused by the low slope efficiency of devices with 37^th-order grating, which results in more heat and higher temperature than that of lasers with 20^th-order gratings. As a result, a stronger wavelength shift of the laser with a 37^th-order grating was observed.

Fig. 6 Spectra of semiconductor lasers with (a) the 37^th-order grating and (b) the 20^th-order grating under different injection currents. The higher-order gratings are positioned at the edges of the ridge waveguides (close to the output facet).

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4.2 Small-signal modulation of single-mode semiconductor lasers

This single-mode semiconductor laser is suitable for photonic integration. To characterize the dynamic modulation responses, the single-mode semiconductor lasers were p-side up mounted on an AlN submount and the small signal responses were measured with a probe (Cascade ACP65-Am-GSG), a photodetector (Finisar XPDV3120), and a vector network analyzer (Keysight N5247B) under 25 °C. The cavity lengths of the single-mode semiconductor lasers are 400 µm, and the higher-order gratings are positioned at the edge of the ridge waveguides. Four devices of each type of single-mode semiconductor lasers (total eight devices) were measured. Figure 7 shows the measured small signal response of the single-mode semiconductor lasers with 37^th- and 20^th-order gratings, respectively. The −3 dB bandwidths reach 9 GHz at the current of 100 mA for the semiconductor lasers with 37^th- and 20^th-order gratings. We find that there is no obvious difference in the small signal response between single-mode semiconductor lasers with 37^th- and 20^th-order gratings. However, the single-mode semiconductor laser with 20^th-order grating is preferred in respect to reducing the cavity length to achieve a higher −3 dB bandwidth. The measured −3 dB bandwidths of the single-mode semiconductor lasers are not as high as those of the DFB lasers lasing at 1550 nm wavelength range [3,23]. However, to the best of our knowledge, here the reported −3 dB bandwidth of this kind of the single-mode semiconductor laser is the highest one [13,24,25]. In such a method of fabricating single-mode semiconductor lasers without regrowth processes, we adopted the most common and cost-effective fabrication technologies, such as the standard photolithography, not the e-beam lithography. To further enhance the −3 dB bandwidth, the feasible approaches include planarization with benzocyclobutene as the electrical insulation layer, and reducing the cavity length and the ridge width.

Fig. 7 Typical small signal responses of (a) single-mode semiconductor laser with the 37^th-order grating and (b) single-mode semiconductor laser with the 20^th-order grating measured at 25 $℃$ , respectively. The higher-order gratings are positioned at the edges of the ridge waveguides (close to the output facet).

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4.3 Influence of the position of higher-order grating

Arguments exist that whether the higher-order grating in such a single-mode semiconductor laser works like a DFB laser, a DBR laser, or neither [26–28]. We defined the higher-order gratings in the different positions along the ridge waveguide when the single-mode semiconductor lasers were fabricated. The 37^th-order gratings were placed in two ways: one is that the grating is in the middle along the ridge waveguide as shown in Fig. 3(a), and the other is that the grating is at the edge (close to the output facet) of the ridge waveguide. The output powers from both the front facet and the rear facet of the same laser were measured for both kinds of laser structures mentioned above. However, we find that there are no distinct differences for the output power from the front facet and the rear facet of the same laser. The spectra were measured under the injected currents from the threshold current to 100 mA, and the SMSR is larger than 30 dB for the semiconductor lasers with 37^th-order gratings arranged both in the middle and at the edge of the ridge waveguides.

In previous report by other research groups [12], parameters of the higher-order grating have been optimized to obtain a high reflectivity for the aimed wavelength. The front cleaved facet was AR coated, and longitudinal mode is selected by the reflection mechanism of the higher-order grating. There should be no FP modes under their design. However, in our work, because both cleaved facets were uncoated and the reflection is provided by the cleaved facets, there are many FP modes in the optical spectrum. The surface higher-order grating fabricated in the ridge waveguide introduces different losses to different FP modes and modulates their threshold gains. It can be described that this surface higher-order grating selects a single longitudinal mode by the loss mechanism, even though it can indeed provide a weak reflectivity.

In the applications like optical interconnects, far-field profile of the single-mode semiconductor laser is critical for the coupling efficiency between the laser and the waveguide. The typical far-field profiles of the single-mode semiconductor lasers with the 37^th-order gratings placed in the middle and at the edge along the ridge waveguides are shown in Fig. 8. Also, the far-field profile of the FP semiconductor laser was measured for comparison. Compared with the far-field profile of the FP semiconductor laser shown in Fig. 8(a), there are many ripples appeared in the vertical far-field curve of the single-mode semiconductor laser with the 37^th-order gratings placed at the edge of the ridge waveguide, as shown in Fig. 8(c). The ripples are caused by higher-order grating scattering [20]. However, the far-field profile of the single-mode semiconductor laser with the 37^th-order grating placed in the middle of the ridge waveguide is smoother than that in Fig. 8(c), because the scattered field emits toward the submount and is reflected to other directions. On the other hand, the horizontal far-field divergency angle (full width at half maximum) of the single-mode semiconductor laser with the 37^th-order grating placed at the edge is slightly higher than that of the FP semiconductor laser and that of the single-mode semiconductor laser with the 37^th-order grating placed in the middle of the ridge waveguide. This is because that the deeper slots at the regions out of the ridge waveguide reduce the effective refractive index in the regions. Therefore, the deeper slots realize a larger refractive index difference between the ridge waveguide and the region out of the ridge waveguide in the lateral direction, which enhances the optical confinement in the lateral direction. Thus, the horizontal divergency angle is increased for the single-mode semiconductor laser with the 37^th-order grating placed at the edge of the ridge waveguide.

Fig. 8 Vertical and horizontal far-field profiles of (a) the FP semiconductor laser, (b) the semiconductor laser with the higher-order grating placed in the middle of the ridge waveguide, and (c) the semiconductor laser with the higher-order grating placed at the edge of the ridge waveguide (close to the output facet).

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5. Conclusions

Single-mode semiconductor lasers with surface higher-order gratings have been fabricated by the standard photolithography, and their output characteristics, modal properties, far-field profiles, and dynamic modulation responses are presented. The semiconductor lasers with 20^th-order gratings have a lower threshold current and a higher slope efficiency compared to those with 37^th-order gratings, due to the lower loss introduced by the 20^th-order gratings in the ridge waveguides. The semiconductor lasers with both 20^th- and 37^th-order gratings can achieve single-mode operation with a SMSR of larger than 30 dB. The higher-order grating at the edge of the ridge waveguide (close to the output facet) can deteriorate the vertical far-field profiles of the single-mode semiconductor laser. The semiconductor lasers with higher-order gratings both in the middle of the ridge waveguide and at the edge of the ridge waveguide can achieve single-mode operation with a SMSR of larger than 30 dB. The −3 dB bandwidth of these single-mode semiconductor lasers achieves 9 GHz at 100 mA, and a higher −3 dB bandwidth can be expected. The results presented here shows that the single-mode semiconductor lasers with surface higher-order gratings fabricated by the standard photolithography provides a simple and cost-effective approach to realize single-mode, high-speed modulation semiconductor lasers and semiconductor laser arrays for applications, such as photonic integrated circuits.

Funding

Chinese National Key Basic Research Special Fund (2017YFA0206401); National Natural Science Foundation of China (61675193); Strategic Priority Research Program of the Chinese Academy of Sciences (XDB24010100, XDB24010200, XDB24020100, XDB24030100); Chinese Academy of Sciences (CAS) Pioneer Hundred Talents Program.

Acknowledgments

Prof. Liang Xie is gratefully acknowledged for the COC package of the single-mode semiconductor lasers.

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Single-mode semiconductor lasers fabricated by standard photolithography for direct modulation

Abstract

1. Introduction

2. Simulation and design

3. Fabrication

4. Experimental results

4.1 Single-mode lasers with different higher-order gratings

4.2 Small-signal modulation of single-mode semiconductor lasers

4.3 Influence of the position of higher-order grating

5. Conclusions

Funding

Acknowledgments

References

Cited By

Figures (8)

Equations (4)

Optics Express