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Abstract
Uncooled direct modulation DFB laser offers high speed transmission rate over a wide temperature range with high reliability and low cost, making it a cost-effective light source choice for 5G fronthaul and data center applications. However, a significant 3dB bandwidth decrease can be observed in high temperature for conventional DFB lasers. We present an uncooled DFB laser operating up to 85°C with extended direct modulation bandwidth and high reliability based on a novel groove-in-trench ridge waveguide structure, where two narrow grooves penetrating the active layer are etched symmetrically in the two conventional trenches by deep wet etching, respectively. By optimizing the distance between the groove and the mesa stripe, we obtain a 3dB bandwidth of 15.3 GHz at 85°C, which is a 3.7 GHz improvement compared with the conventional ridge waveguide DFB laser. Transmissions of 25 Gb/s NRZ signal at 25°C and 85°C with clear eye openings have been demonstrated. It also achieves 25 Gb/s transmission over 10 km optical fiber with a low power penalty of 0.5 dB for a bit error rate of 10−12 at 85°C. In addition, the result of 2000-hour aging test shows that the proposed groove-in-trench structure DFB lasers have the same excellent reliability as the conventional ones.
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1. Introduction
Today’s rapid increase in amount of data traffic accelerates research and development of high-speed optical transmitters. Uncooled DFB lasers are widely used for short reached such as 5G fronthaul transmission and data center, since the DFB lasers offers low power consumption, wide temperature operation and low cost compare to external modulated lasers [1]. The development of uncooled DFB lasers for high bit-rate operation started in the mid-1990s. At first, the uncooled ridge waveguide (RWG) DFB lasers with InGaAsP multi-quantum well (MQW) active layer had been limited to at most 10-Gb/s at 85°C due to the poor temperature characteristics of the InGaAsP material system [2,3]. Alternatively, InGaAlAs material system became preferred choice in uncooled DFB lasers due to its strong electron confinement [4]. Then the operation temperature range was widened by using InGaAlAs-MQW lasers further up to 120°C at 10 Gb/s [5]. To further extend the bandwidth in high temperature, much attention has been paid to the buried-heterostructure (BH) structure and shorter cavity length. Clear 25 GB/s eye patterns were obtained at 95°C using 160-$\mu m$-long RWG DFB laser [6]. Clear 40 Gb/s eye patterns were also obtained at 85°C using a 100-$\mu m$-long active layer BH-type distributed reflector laser [7]. A 50-Gbit/s NRZ signal modulation operating up to 80°C was demonstrated by using a 120-$\mu m$-long BH-type DFB laser [8].
RWG structure and BH structure are two kinds of waveguide structures commonly used in uncooled DFB lasers. In the RWG structure, a mesa ridge is simply etched into the upper layers resulting in lateral optical confinement, and no additional epitaxial growth is needed. In the BH structure, the active mesa with a width of about 2-$\mu m$ is fully surrounded by InP thus creating a well-defined buried waveguide. This is achieved by a special selective regrowth process of InP layers at both sides of the previously etched active stripe [9]. In general, the BH structure can increase the relaxation oscillation frequency by reducing the mode volume, and can obtain a higher modulation bandwidth than the RWG structure [7,8]. However, in the BH fabrication process, the InGaAlAs-MQW layer is exposed to air prior to the regrowth process, which makes it difficult to achieve good reliability [10]. Special methods for cleaning the oxidized surface to reduce potential defects have been developed [11], but the process is usually complex and the reliability of lasers remains a challenge. Since reliability is an extremely important requirement in 5G fronthaul applications, RWG-type DFB laser with higher reliability is a more cost-effective solution for applications in 5G fronthaul transmission and data center. However, for conventional RWG DFB lasers, a significant 3dB bandwidth decrease of 6 GHz-10 GHz was observed over a wide temperature [12–14]. The decrease of relaxation oscillation frequency caused by the increase of non-radiative recombination or lateral carrier leakage may contribute to the degradation of bandwidth [15,16]. Conventional uncooled RWG DFB lasers may not be able to meet the high-quality 25Gb/s transmission requirements at high temperatures.
In this work, we present an uncooled DFB laser operating up to 85°C with extended direct modulation bandwidth and high reliability based on a novel groove-in-trench (GiT) RWG structure. In the GiT RWG structure, different from the conventional RWG structure, two narrow grooves penetrating the active layer are etched symmetrically in the two trenches by deep wet etching, respectively. By optimizing the distance between the groove and the mesa stripe, we obtain 3dB bandwidths of 28.7 $\textrm{GHz}$ and 15.3 $\textrm{GHz}$ at 25°C and 85°C, respectively, which are 3.0 GHz and 3.7 GHz higher than conventional RWG DFB lasers, respectively. The measured side-mode suppression ratios (SMSR) of the fabricated GiT RWG DFB lasers are greater than 42 dB over a wide temperature range. Transmissions of 25 $\textrm{Gb}/\textrm{s}$ NRZ signal at 25°C and 85°C with clear eye openings have been demonstrated. It also achieves 25 $\textrm{Gb}/\textrm{s}$ transmission over 10 km fiber with a low power penalty of 0.5 $\textrm{dB}$ for a bit error rate (BER) of ${10^{ - 12}}$ at 85°C. In addition, the result of 2000-hour aging test shows that the proposed GiT RWG DFB lasers have the same excellent reliability as the conventional ones. The uncooled GiT RWG DFB laser offers higher direct modulation bandwidth with simple fabrication process and high reliability, and may become a more cost-effective laser source for short distance optical transmission in 5G fronthaul and data center applications.
2. Device design and fabrication
Figure 1 shows the schematics of our proposed DFB laser. The cavity length of the DFB laser is set to 200 $\mu m$ to achieve a good balance between the high-speed and high-power operation. This structure is fabricated as follows. The InGaAlAs- InGaAlAs MQW wafer is grown on a n+-InP substrate by metal organic chemical vapor deposition (MOCVD). Eight InGaAlAs quantum wells and nine InGaAlAs barriers have thicknesses of 5.5 $\textrm{nm}$ and 8 $\textrm{nm}$, respectively. The active layer also consists of two thin AlGaInAs separate confinement heterostructure (SCH) layers with thicknesses of 100 $\textrm{nm}$. To achieve single longitudinal mode operation [9], a $\mathrm{\lambda }/4$ phase shift grating (with a measured coupling coefficient of about $100 c{m^{ - 1}}$) is defined in the top InGaAsP waveguide layer by electron beam lithography (EBL) system and dry etched by ICP etcher. The thickness of the grating layer is 40 $\textrm{nm}$. A p-InP cladding layer and a p+-InGaAs contact layer are grown on top of the grating layer by MOCVD. Two trenches with an identical width of 15.0 $\mu m$ are selectively wet etched to the etch stop layer to form the ridge. The width and height of the ridge are designed to be 2.0 $\mu m$ and 1.9 $\mu m$, respectively. The front and rear cleaved facets are anti-reflection (AR) and high-reflection (HR) coated, respectively.
Fig. 1. Schematic structure of the DFB laser with GiT RWG structure (a) structure diagram of the DFB laser, (b) cross-section view.
After the ridge is formed, two narrow grooves penetrating the active layer are etched symmetrically in the two trenches by deep wet etching, respectively, as shown in Fig. 1(b). The designed shape of the groove is rectangle and the width of the groove is set to 5.5 $\mu m$ considering the lithography constraint. Since the distance between the bottom of the MQW layer and the trench surface is 0.336 $\mu m$, the depth of the groove is set to 1 $\mu m$ in order to etch through the active layer. The simple fabrication process of the GiT RWG structure does not require additional epitaxial growth, which ensures that the proposed DFB laser is cost-effective.
We fabricated seven different types of DFB lasers for performance evaluation. One is a conventional RWG DFB laser with no grooves in the trench, and the other six are GiT RWG DFB lasers with a groove-to-mesa distance $w = 2\mu m,\; 3\mu m,$ $4 \mu m,\; 5 \mu m,\; 6 \mu m,$ and $7 \mu m$, respectively. In addition, other structural parameters of the seven types of lasers remained the same. We obtained the SEM images of all samples and measured their structural parameters. Specifically, the average width and height of the ridges are 1.86 $\mu m$ and 1.98 $\mu m$, and the deviations from the design values are less than 0.15 $\mu m$ and 0.14 $\mu m$, respectively. The average top and bottom widths of the grooves in GiT RWG lasers are 5.22 $\mu m$ and 4.34 $\mu m$, respectively. And the average groove depth is 0.98$\mu m$, which ensures that the groove can etch through the active layer. Figure 2 shows the SEM images of the cross-sections of the conventional RWG laser and two GiT RWG lasers with designed $w = 2 \mu m$ and $6 \mu m$, respectively. As shown in Figs. 2(b)–2(c), the measured w of the two GiT RWG laser samples are 2.31 $\mu m$ and 6.09 $\mu m$, respectively.
Fig. 2. SEM images of the cross-sections of (a) the conventional RWG laser and two GiT RWG lasers with (b) $w = 2\; \mu m$ and (c) $w = $6 $\mu m$.
3. Measurement and discussion
We measured the static and dynamic characteristic of all fabricated lasers. Figure 3 exhibits the static characteristics of the seven types of DFB lasers. The L-I curves at 25°C are shown in Fig. 3(a). The threshold current of the conventional DFB laser is 11.50 $\textrm{mA}$, while those of the GiT lasers with $w = 2 \mu m, 3 \mu m, 4 \mu m, 5 \mu m, 6 \mu m$ and 7 $\mu m$ are 10.08 $\textrm{mA}$, 9.04 $\textrm{mA}$, 8.87 $\textrm{mA}$, 8.29 $\textrm{mA}$, 8.02 $\textrm{mA}$, 8.08 $\textrm{mA}$, respectively. As the groove etch through the active layer, it may limit the lateral diffusion of carriers, thereby increasing carrier injection efficiency and reducing the threshold current. Figure 3(b) indicates the L-I curves of the fabricated GiT RWG laser with $w = 6 \mu m$ at 25°C, 55°C, and 85°C. Its output power is greater than 9 $\textrm{mW}$ and the threshold current is 21.5 $\textrm{mA}$ at 85°C. The L-I curve at 85°C shows an obvious rollover phenomenon, which may be due to the enhancement of non-radiative recombination or lateral carriers diffusion effect with the increase of temperature [15,16]. The optical spectra of the GiT RWG laser with $w\; $= 6 $\mu m$ at 25°C, 55°C and 85°C are shown in Fig. 3(c). The SMSR is larger than 42 $\textrm{dB}$ over a wide temperature range.
Fig. 3. the static characteristic of the DFB lasers (a) L-I curves of different lasers at 25°C, (b) L-I curves of laser with $w = 6 $µm at 25°C, 55°C and 85°C, (c) optical spectra of laser with $w = 6 $µm at 25°C, 55°C and 85°C.
Figure 4 depicts the measured small-signal modulation frequency responses, $R(f )$, of the seven types of DFB lasers with an inject current of 60 $\textrm{mA}$ at 25°C, 55°C and 85°C, respectively. The experimental setup is referred to [17]. The 3$\textrm{dB}$ bandwidths of the GiT lasers are significantly higher than that of the conventional laser at 85°C, and are also improved at 25°C and 55°C.
Fig. 4. the small-signal modulation frequency responses with an inject current of 60 $\textrm{mA}$ at different temperature: (a) 25°C (b) 55°C (c) 85°C .
For each type of DFB laser, we measured the $R(f )$ of two samples for performance evaluation of various structures. By fitting the $R(f )$ curve, we estimated the relaxation oscillation frequency (${f_r}$), parasitic RC constant (${R_{ld}}{C_{ld}}$) and damping factor ($\Gamma $) of each sample to evaluate the contribution of the above factors to the modulation bandwidth (${f_{3dB}}$) enhancement [18]. The $R(f )$ can be expressed as the well-known Eq. (1): [9]:
The 3dB bandwidth ${f_{3dB}}$ of each DFB laser is related to ${f_r}$, ${C_{ld}}{R_{ld}}$, and $\mathrm{\Gamma }$. We usually use $f_{{f_r}}^{3dB}$, $f_{RC}^{3dB}$ and $f_\mathrm{\Gamma }^{3dB}$ to describe the contributions of ${f_r}$, ${R_{ld}}{C_{ld}}$, and $\mathrm{\Gamma }$ to the modulation bandwidth, as given in Eq. (2): [9]
Each $R(f )$ curve has 400 sampling points. Based on Eq. (1), we use the least squares method for curve fitting, and set the average error $\le 0.5\textrm{dB}$ to meet the fitting requirement. In Fig. 5, we plot the measured and fitted curves of a conventional RWG laser and two GiT RWG lasers with $w = 2 \mu m$ and $6 \mu m$ at the three temperatures, respectively.
Fig. 5. the measured and fitted curves of the small-signal response with an inject current of 60 $\textrm{mA}$ at 25°C, 55°C and 85°C for: (a) conventional RWG laser, (b) GiT RWG laser with $w = 2\mu m$ and (c) GiT RWG laser with $w = 6\mu m$.
Table 1 presents the fitted data for all seven types of lasers. We can see that $f_{{f_r}}^{3dB}$ is much smaller than $f_\mathrm{\Gamma }^{3dB}$ and $f_{RC}^{3dB}$, which means that the value of ${f_r}$ is the most important parameter to determine the ${f_{3dB}}$ of the RWG DFB laser. Therefore, increasing ${f_r}$ can effectively enhance the ${f_{3dB}}$. Meanwhile, the contribution of the changes in the ${R_{ld}}{C_{ld}}$ and $\mathrm{\Gamma }$ is almost negligible.
Table 1. Fitted $f_{{f_r}}^{3dB}$, $f_\mathrm{\Gamma }^{3dB}$ and $f_{RC}^{3dB}\; $ of DFB lasers at 25°C, 55°C and 85°C
Figure 6 shows the average ${f_{3dB}}$ and ${f_r}$ as a function of w at 25°C, 55°C and 85°C. Here, $w = 0\; \mu m$ represents the conventional RWG DFB laser. In Fig. 6(a), the ${f_{3dB}}$ at 25°C of the conventional laser and the GiT RWG lasers with $w$ = 2 $\mu m$, 3 $\mu m$, 4 $\mu m$, 5 $\mu m$, 6 $\mu m$ and 7 $\mu m$ are 25.7 $\textrm{GHz}$, 26.0 $\textrm{GHz}$, 26.1 $\textrm{GHz}$, 27.6 $\textrm{GHz}$, 28.1 $\textrm{GHz}$, 28.7 $\textrm{GHz}$ and 27.4 $\textrm{GHz}$, respectively. Those values at 85°C are 11.6 $\textrm{GHz}$, 14.0 $\textrm{GHz}$, 14.5 $\textrm{GHz}$, 14.6 $\textrm{GHz}$, 15.1 $\textrm{GHz}$, 15.3 $\textrm{GHz}$ and 15.1 $\textrm{GHz}$, respectively. We can find that the DFB lasers with GiT RWG structure have higher ${f_{3dB}}$ compared with the conventional RWG DFB laser in general. For the GiT DFB laser with $w = 6\; \mu m$, we obtain the maximum 3$\textrm{dB}$ bandwidth enhancements of 3.0 $\textrm{GHz}$ and 3.7 $\textrm{GHz}$ at 25°C and 85°C, respectively. Figure 6(b) shows that the ${f_r}$ enhancements are 0.45 $\textrm{GHz}$ and 2.92 $\textrm{GHz}$ of the GiT DFB laser with $w = 6\; \mu m$ at 25°C and 85°C. Figure 6 also indicates that ${f_r}$ and ${f_{3dB}}$ have similar trends with w changing. The enhancement of ${f_r}$ may be due to the limitation of carrier diffusion effect by the GiT RWG structure.
Fig. 6. the measured (a) ${f_{3dB}}$, (b) ${f_r}$ as a function of groove-to-mesa distance ($w$) at 25°C, 55°C, and 85°C
The experimental setup for measuring dynamic characteristic is as follows. An electrical signal is generated by pattern generator. The single-ended signal is input into an RF amplifier to modulate fabricated DFB laser. The optical signal with pseudo-random binary sequence (PRBS) 231-1 pattern is received by a bit error rate (BER) tester or a sampling oscilloscope.
We measured the dynamic characteristic of the GiT RWG lasers with $w$ = 6 $\mu m$. Figures 7(a)–7(b) exhibit the 25 $\textrm{Gb}/\textrm{s}$ eye patterns at 25°C and 85°C, respectively. To obtain a dynamic extinction ratio (ER) of 4.5 $\textrm{dB}$, we set the bias current to 40.0 $\textrm{mA}$ at 25°C and 73.0 $\textrm{mA}$ at 85°C, respectively. Clear eye openings were observed at each temperature. Figure 7(c) indicates the BER curves as function of received optical power for 25 $\textrm{Gb}/\textrm{s}$ NRZ signal with a low bit-error operation for back-to-back (BTB) and 10-km transmission. Error-free operation was obtained up to 85°C, and the maximum power penalty at BER = ${10^{ - 12}}$ after a 10-km transmission was 0.5$\textrm{dB}$. These results demonstrate that the uncooled fabricated high speed directly modulated DFB laser with GiT RWG structure can meet 25 $\textrm{Gb}/\textrm{s}$ transmission requirements in 5G forward transmission and data center.
Fig. 7. 25 $\textrm{Gb}/\textrm{s}$ eye diagrams at (a) 25°C, (b) 85°C, and (c) BER curves for back-to-back (BTB) and 10-km single mode optical fiber transmission at 25°C, 55°C and 85°C
We also calculate the mode field of the DFB laser with GiT RWG waveguide structure. The radius of the mode field is 0.98 $\mu m$ and it is at the center of the ridge. Meanwhile, the distance between the mesa stripe and the groove w is larger than 2.0 $\mu m$. The possible defects caused by etching grooves would not affect the reliability of the fabricated laser because the groove-to-mesa distance is long enough so that the overlap between the etching interface and the laser mode field is negligible. Actually, we have done the reliability aging test of 20 conventional RWG lasers and 20 GiT RWG DFB lasers for 2000 hours at 85°C. The results are shown in Fig. 8. During the 2000-hour aging, the optical power change of all samples is less than 0.5%, and no samples fail, which shows that the proposed GiT RWG laser has the same excellent reliability as the conventional RWG laser.
Fig. 8. Reliability aging test over 2000 hours for (a) 20 conventional RWG DFB samples and (b) 20GiT RWG DFB samples at 85°C
There are some reports which find that the DFB laser with the ridge buried in benzocyclobutene can realize low parasitic capacitance and high ${f_r}$ [19,20]. If we fill the two grooves with benzocyclobutene, we believe that it may further increase the 3$\textrm{dB}$ bandwidth of the GiT DFB laser.
4. Conclusion
We propose an uncooled 1.3 $\mu m$ DFB laser with extended direct modulation bandwidth and high reliability based on GiT RWG structure. After the two common trenches are selectively wet etched to form the ridge, two grooves are deeply wet etched through the active layer in the trenches. Because the distance between the bottom of the active layer and the trench is 0.336 $\mu m$, the groove with a depth of 0.98 $\mu m$ etches through the active layer. Since the groove etches through the active layer, it may limit the lateral diffusion of carriers, thereby increasing carrier injection efficiency. Actually, we can find that the threshold current of each GiT RWG laser is smaller than that of the conventional laser. The SMSR of the all fabricated DFB lasers are greater than 42 $\textrm{dB}$ over a wide temperature range (25°C–85°C). Compared with the conventional RWG DFB laser, the 3$\textrm{dB}$ bandwidth increases in the fabricated lasers with GiT RWG structure. For the GiT DFB laser with a 6-$\mu m$ groove-to-mesa distance, we obtain 3dB bandwidth of 28.7 $\textrm{GHz}$ and 15.3 $\textrm{GHz}$ at 25°C and 85°C, respectively, with 3dB bandwidth enhancements of 3.0 $\textrm{GHz}$ and 3.7 $\textrm{GHz}$ compared with the conventional RWG-type DFB laser. The possible defects caused by etching grooves would not affect the reliability of the fabricated laser because the groove-to-mesa distance is long enough so that the overlap between the etching interface and the laser mode field is negligible. The result of 2000-hour aging test confirms that the proposed GiT RWG DFB laser has the same excellent reliability as the conventional RWG one. Due to the improved performance and low cost of GiT RWG structure, the GiT RWG DFB laser may be highly competitive for high-speed optical transmission in 5G fronthaul and data center applications.
Funding
Key Technologies Research and Development Program (2018YFB2201500).
Disclosures
The authors declare no conflict of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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