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Abstract
We present an 8-µm-wide 800-µm-long high-power, single-mode and low RIN DFB laser using a dual-waveguide structure. The introduced passive lower waveguide has weakenes the lateral optical confinement for the ridge waveguide, and thus reduces losses caused by the p-doped layers and maintains single mode stability of the laser. The fabricated laser exhibited an output power higher than 170 mW and a relative intensity noise (RIN) below −157 dB/Hz along with a side-mode suppression-ratio (SMSR) over 55 dB. The temperature tuning from −10°C to 60°C allows an 8.6 nm wavelength tunability with a variation coefficient of 0.12 nm/K. The relaxation oscillation frequency is around 8 GHz, and the linewidth is about 250 kHz at 100 mW output power for the fabricated laser. The characteristics of the proposed high-power laser, including high slope efficiency, single mode stability and low noise, make it a suitable candidate for optical communication.
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1. Introduction
High power semiconductor lasers working around 1550 nm have broad range applications, including free space optical (FSO) communication, silicon photonics (SiPh) co-packaged optics, coherent fiber optical communication, light detection and ranging (LiDAR) systems, etc., due to the low loss in optical fibers and atmosphere as well as 1-W level at “eye-safe” wavelengths near 1550 nm compared to 980 nm [1–5]. In addition, a low RIN level in semiconductor lasers is a key parameter for numerous applications such as high-resolution spectroscopy, fiber-optic sensors, signal distribution in broadband analog communications as CATV, and more generally for microwave photonics systems [6].
A lot of work has been done in the past on high power, single mode, low RIN and narrow linewidth semiconductor lasers for optical communication [6–14] as seen in Table 1. To keep single mode operation a narrow ridge waveguide structure is preferred for these lasers. One impressive work has achieved 440 mW output, 30 dB SMSR and 1.6 MHz linewidth at 18 °C by using 3-µm-wide dual-channel ridge waveguide (RWG) structure with 2 mm cavity length [8]. Jia-Sheng Huang et al. reported a 1-mm-long laser with the optical power of 140 mW, the RIN <−161 dB/Hz, the linewidth of 115kHz and the SMSR over 55 dB [9]. At the same time, J.-R. Burie et al. have obtained 1.5-mm-long lasers with the output power of 130 mW and the RIN <−165 dB/Hz [6]. Adopting slab-coupled optical waveguide external cavity laser (SCOW-ECL) with an area of 16×6 in2, William Loh et al. have achieved 370 mW output power, 1 kHz linewidth and RIN <−160 dB/Hz at 16 °C [10]. Michaël Faugeron et al. reported the directly modulated distributed feedback lasers with output power of 146 mW, the SMSR > 55 dB, and the RIN <−157 dB/Hz [11]. In order to improve the thermal behavior, by introducing dilute waveguide below the active region of 1-mm-long DFB lasers, 180 mW and 160 mW CW output power at 25°C were demonstrated based on InGaAsP multiple quantum wells (MQWs) and InGaAlAs MQWs with the ridge widths of 3 µm and 2.3 µm, respectively [2,12]. Freedom Photonics has developed the DFB lasers based on InGaAsP quantum well and the peak power in excess of 300 mW and the RIN <−160 dB/Hz have been achieved [13]. Moreover, to further increase the output power of semiconductor lasers, DFB lasers with CW output power of 850 mW and SMSR over 50 dB were realized by using 4-mm-long and 5-µm-wide SCOW [3]. Shoko Yokokawa et al. have achieved 1-mm-long lasers with kink-free stable longitudinal single-mode operation (SMSR > 40 dB) with the output power over 200 mW at 20°C (100 mW at 80°C), the RIN <−150 dB/Hz and the linewidth <500 kHz [14]. The widths of ridge waveguides of these lasers are usually less than 6 µm to maintain good single lateral mode stability, which limits the maximum output power. To improve the output power, the lengths of these lasers are generally greater than 1 mm.
Table 1. Parameters of 1550 nm high power lasers reported in recent years
In this study, we present the design and fabrication of a high power, low RIN, and single mode 1550 nm DFB laser with a dual-waveguide structure. The introduced lower passive waveguide has weakened the lateral optical confinement for the ridge waveguide structure, therefore single lateral mode operation has been achieved even for an 8-µm-wide ridge waveguide. The 800-µm-long fabricated laser shows a stable single mode operation around 1550 nm with the SMSR over 55 dB within the chip temperature range from −10 °C to 60 °C. The output power of about 177 mW has been achieved with the relative intensity noise (RIN) below −157 dB/Hz. The relaxation oscillation frequency reaches 8 GHz and the linewidth is about 250 kHz at 100 mW output power for the fabricated laser.
2. Device design and fabrication
2.1 Device structure and design
The epitaxial structure of our DFB laser is shown in Fig. 1. The metalorganic chemical vapor deposition (MOCVD) grows the structure on n-type InP substrate.
The active layer consists of five compressively strained InAlGaAs quantum wells (MQWs) sandwiched with six tensile strained barriers, and the upper and lower InGaAlAs separate-confinement heterostructure (SCH) layers. The InGaAlAs MQWs have low temperature sensitivity and larger differential gain compared to the InGaAsP MQWs. The composition of the InGaAlAs wells is designed to get the photoluminescence emission peak near 1.53 µm. The lower optical confinement InP layer and the 300-nm-thick lightly doped n-type InGaAsP passive waveguide layer are introduced below the active layer to form a dual-waveguide structure which reduces the overlap between the optical mode and the p-type doped layers. At 1.55 µm the internal loss is mainly caused by the p-type doped layers of the structure [15]. By adjusting the thickness of both the passive waveguide and the InP layer, the optical confinement factor is designed of about 4.6% in p-InP layers and 3.8% in MQWs, respectively, which helps to realize a low threshold current, a high slope efficiency and a high saturation power for the laser [16,17]. The grating layer locates above the active layer and the grating is patterned by electronic-beam lithography (EBL). To realize the single-longitudinal mode operation, a quarter wavelength (λ/4) phase shift is inserted into the grating. The coupling coefficient of grating is designed with a κ·L value of around 1 to reduce spatial hole burning effect while increases the maximum output power [18].
Lateral mode confinement is offered by a double trench ridge waveguide structure. Figure 2 shows the simulated mode distribution of the fundamental mode and the first-order mode for the ridge waveguide with the ridge width of 8 µm. It is obviously that, due to the dual waveguide structure, the mode field is mainly distributed in the lower passive waveguide layer instead of the active layer, which reduces optical confinement of ridge waveguide and thus maintains single mode behavior in a relative wide ridge waveguide.
Fig. 2. Simulated mode distribution of the fundamental mode (a) and the first-order mode (b) for the ridge waveguide with the ridge width of 8 µm
Figure 3 shows the effective indices and confinement factors of the fundamental mode and the first-order mode versus waveguide width. A wider waveguide width allows higher saturation output power of the laser. On the other hand, it decreases the difference of both the effective index and confinement factor between the fundamental mode and the first-order mode, which finally influences the single lateral mode stability. Due to the non-uniform current distribution in the active layer and higher sidewall scattering loss in the first-order mode, a relative larger threshold gain difference will be obtained between the fundamental mode and the first-order mode, which helps the fundamental mode get sufficient gain to lase with the higher-order modes filtered. Therefore, the waveguide width has been optimized to be about 8µm to achieve sufficiently high saturation output power and maintain single lateral mode stability at the same time.
Fig. 3. Calculated effective index (a) and confinement factor (b) of the fundamental mode and the first-order mode for the ridge waveguide with different ridge width
2.2 Device fabrication
The laser has been fabricated with the similar fabrication process in [19]. The grating has been patterned with a 50% duty cycle on the grating layer by electron-beam lithography (EBL), and etched by RIE using CH4/H2/Ar. The grating takes a floating grating layer structure. InP layers are arranged above and beneath the grating layer. During dry etching, the grating layer has been etched through. Therefore, the thickness of the grating is about 30 nm, which is determined by the MOCVD growth and is not influenced by the dry etching. Scanned electron microscope (SEM) images of the etched grating and the λ/4 phase shift before the epitaxial regrowth are shown in Fig. 4(a) and (b), respectively. After cleaning the wafer by buffered oxide etch (BOE), the p-InP upper cladding layer and the p-InGaAs contact layer are re-grown by MOCVD.
Fig. 4. (a) SEM image of grating; (b) SEM image of the λ/4 phase shift; (c) SEM image of the ridge waveguide; (d) Microscope image of the fabricated high-power lasers
The shallow ridge waveguide is etched by ICP-RIE followed by wet etching after the regrowth. Figure 4(c) shows the SEM images of the etched ridge waveguide. Subsequently a SiNx insulation layer is deposited by plasma enhanced chemical vaper deposition (PECVD) and a window is etched through the SiNx layer to expose P-contact metal. After the P-electrode is deposited, the n-InP substrate is thinned down to 100 µm and the N-electrode is deposited. Finally, the lasers are cleaved into lengths of about 800 µm with facets anti reflection (AR) and high reflection (HR) coated for test. Figure 4(d) shows the microscope image of the fabricated high-power lasers.
3. Device measurement and discussion
3.1 Extracting parameters from uniform grating DFB lasers
In order to extract the grating coupling coefficient, we fabricated the DFB laser with uniform gratings and measured its amplified spontaneous emission (ASE) spectrum. The output light of the lasers is coupled to a single-mode fiber and input to the optical spectrum analyzer (OSA). Figure 5 shows the measured ASE spectrum for a 500 µm long DFB laser with uniform gratings. It is seen that the stop band width Δλs is about 0.73 nm and the Bragg wavelength is about 1546 nm as shown in the figure. The grating coupling coefficient can be estimated from Δλs using the following formula [20].
where κ is the grating coupling coefficient; neff is the effective index; λB is the Bragg wavelength of the grating. Therefore, the coupling coefficient of the fabricated grating is estimated to be about 10 cm−1 as calculated from the formula (1), which agrees well with the design.3.2 Characteristics of the high-power DFB laser
The fabricated device with n-side down is soldered onto an AlN carrier and then placed onto a copper heat sink for testing. A thermoelectric cooler (TEC) controls the chip temperature. We have measured light intensity versus continuous-wave current (L-I) for a temperature range from −10 °C to 60 °C. The results are shown in Fig. 6(a). Single mode operation is observed with the injection current up to 800 mA without any mode hops. The device obtained a maximum output power of 177 mW at −10 °C, 134 mW at 20 °C, and 86 mW at 50 °C, respectively, with the corresponding threshold currents of about 46 mA, 54 mA and 88 mA. The maximum slope efficiency is higher than 0.36 mW/mA, and the output power shows a good linearity before the thermal rollover. In order to improve the maximum output power, we need to reduce the temperature in the MQWs as much as possible, therefore a low series resistance is essential. The series resistance is about 1.3 Ω for an 800 µm long laser as shown in Fig. 6(b). Through the measured threshold currents at different temperatures, we have calculated the characteristic temperature T0 to be about 78 K in the range of 10 °C to 60 °C. Heat dissipation and laser performance will be further improved if the device is mounted p-side down on the AlN carrier.
Fig. 6. (a) Measured light-current (L-I) curves for the fabricated 800 µm-long proposed DFB laser at different temperature; (b) Power and voltage versus injected current at 20 °C
Figure 7(a) plots the optical spectra of the DFB laser at 300 mA and various temperatures. It is seen that the SMSR of the fabricated high power DFB lasers is higher than 55 dB with the temperature range from −10 °C to 60 °C and the temperature shift permits a tunability of about 8.6 nm with a variation coefficient of 0.12 nm/K as shown in Fig. 7(b). The thermal resistance is measured of about 27 K/W based on the temperature shifts at different heat power which is equal to the difference between the total power and the output power. Figure 8(a) shows the optical spectra of the fabricated laser at −10 °C with various injection currents. It is seen that the laser behavioured the stable single mode operation with the SMSR higher than 55 dB at the injected current from 200 mA to 900 mA. The good single mode stability of the fabricated laser thanks to the suitable position of the λ/4 phase shift and the dual-waveguide structure. We made a control sample of the DFB laser without the dual waveguide structure, which is fabricated with the same processing steps. Figure 8(b) shows the measured output spectrum for the control sample laser at 300 mA and 20 °C. The multi-transverse-mode operation was observed even when the ridge width of the laser is only of 4 µm.
Fig. 7. Measured laser performances for the proposed DFB laser within the temperature range from −10 °C to 60 °C at 300 mA: (a) Optical spetrum; (b) Corresponding SMSRs and lasing wavelengths
Fig. 8. (a) Measured optical spetra for the proposed DFB laser within the current range from 200 mA to 900 mA at −10 °C; (b) Measued optical spetrum for the control sample DFB laser without dual waveguide at 300 mA and 20 °C with the ridge width of 4-µm
There is a negative detuning in order to increase differential gain and therefore decrease optical linewidth and RIN due to the lower Henry factor and higher relaxation frequency [21]. In addition, we can further reduce series resistance by optimizing the fabrication process and the laser epitaxial structure.
We have also measured the relative intensity noise of the fabricated DFB laser. Figure 9 shows the measurement setup [22]. The output light from a TO packaged laser was attenuated before incident onto a 40 GHz photo detector, and AC output from the detector was monitored using an electrical spectrum analyzer. The DC current I0 is monitored by a source meter (KEITHLEY2400). The thermal noise was measured with the light source off, and at least two or more RF spectra were recorded with different DC current by varying the attenuation. The data was taken by personal computer through GPIB connection and processed using MATLAB.
The RF spectrum analyzer setup including RBW = 3 MHz and VBW = 300 KHz. Figure 10(a) shows the measured RIN with the laser biased from 200 mA to 800 mA. It is seen that the RIN of the fabricated laser is about −148 dB/Hz at 200 mA and the minimum value is below −157 dB/Hz at 500 mA over the complete frequency range from 0.1 GHz up to 20 GHz. Figure 10(b) shows the measured RIN and the corresponding relaxation oscillation frequency versus the injected current. It is seen that, when the current is larger than 400 mA, RIN /relaxation oscillation frequency tends to be satuated due to the slope efficiency saturation and and the noise floor for the measurement setup. The further decreasing of the RIN level with noise figure is limited by shot noise [23]. The maximum relaxation oscillation frequency obtained is around 8 GHz at 700 mA.
Fig. 10. (a) Measured RIN spectra; (b) Measured RIN peak and relaxation oscillation frequency for the proposed DFB laser versus the injection current at 20 °C
We measued the linewidth of the fabricated DFB laser based on the time-delay homodyne method [24]. The output light is coupled into a fiber by a coupling system, which consists of a 11.00 mm focal length collimation lens, a two-stage optical isolator with ∼60 dB isolation and a 18.75 mm focal length focusing lens. Then, the coupled output light is divided into two channels througth a 10:90 coupler and one channel passes through a 20-km fiber. The two channals go into the coherent optical receiver with 90-degree hybrids inside to mix them. Then the sine and cosine data is collected by a real-time oscilloscope. The FM noise spetrum can be obtained by the differention of the phase noise extracted, and the Lorentzian linewidth can be extracted from the white noise region of the measured FM noise spectrum. Figure 11(a) shows the measured FM noise spectrum. By averaging the frequency range from 100 MHz to 1 GHz, the Lorentzian linewidth is extracted of about 394.4 kHz. The measured linewidths versus output power and current are shown in Fig. 11(b) and Fig. 11(c), respectively. With the output power increasing from 30 mW to 90 mW, the linewidth of the laser is below 400 kHz. The linewidth in semiconductor lasers can be expressed with the modified Schawlow-Townes expression, which is inversely proportional to the output power P as shown in Fig. 11(b). It is seen that the linewidth decreases from 900 kHz to 250 kHz with relatively good linearity. The linwidth is within 300 kHz when the injection current is higher than 300 mA as shown in Fig. 11(c). Considering that Quantum dot (QD)s gain material has advantages such as nearly zero linewidth enhancement factor, low transparency current density, and excellent optical gain thermal stability, therefore, we could further decrease the linewidth by adpoting quantum dots (QDs) as the active region as in [25].
Fig. 11. (a) Measured FM noise spectrum curve at 200 mA; measured Optical linewidth with different output power (b) and different current (c) for the proposed DFB laser
4. Conclusion
We have demonstrated an 800-µm-long high power single mode DFB laser with an 8-µm-wide ridge waveguide. By using a dual-waveguide structure, the mode field is mainly distributed in the passive waveguide layer instead of the active layer, which reduces confinement of ridge waveguides as well as the optical overlap between the mode and the p-doped layers, and thus decreases loss and maintains single mode operation in 8-µm-wide ridge waveguide. The fabricated DFB laser exhibited an output power over 175 mW, slope efficiency larger than 0.36 W/A and SMSR larger than 55 dB from −10 °C to 60 °C, without sacrificing dynamic performances that RIN less than −157 dB/Hz. The passive waveguide is possible to make gratings on it in the future to replace the HR coating. The non-absorbing region is created using an etching process near the output facet that is both optically non-absorbing and electrically nonconductive to improve the catastrophic optical damage (COD) power and the reliability of the device. Therefore, the proposed lasers can be suitable candidates for optical communication.
Funding
Opened Fund of the State Key Laboratory of Integrated Optoelectronics (IOSKL2020KF18); National Natural Science Foundation of China (61875066); National Key Research and Development Program of China (2018YFB2201701).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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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