A narrow-linewidth thermally tuned multi-channel interference (MCI) laser integrated with a semiconductor optical amplifier (SOA) and spot size converter (SSC) is demonstrated in this paper. A MCI laser integrated with SOA through chirped grating is successfully realized for the first time, which achieves a tuning range of more than 42.5 nm with side-mode suppression ratios (SMSRs) higher than 48 dB and Lorentzian linewidth below 100 kHz. InGaAlAs multiple quantum wells (MQWs) and thermal tuning are used to reduce linewidth. The integration of SSC greatly improves the coupling efficiency between the laser and a lensed fiber. The MCI laser integrated with SSC achieves more than 16 dBm output power coupled into a lensed fiber. Air layers are fabricated in the phase sections to increase the heating efficiency. The total thermal tuning power is below 20 mW across the whole tuning range.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Widely tunable lasers are indispensable for the operation of dense wavelength division multiplexed (DWDM) systems. With the improvement of communication rate and the adoption of high-order modulation format, the overall requirements for tunable lasers are more demanding. Full-C band tunability with high side-mode suppression ratio (SMSR), high output power and narrow spectral linewidth are typical application requirements . Generally, external cavity lasers can meet these requirements . However, monolithic tunable lasers are attractive for reasons of small chip size, ease of assembly and high reliability. Monolithic electro-optically tunable lasers have been researched extensively [3–7]. But current injection will produce shot noise and loss caused by free carrier absorption, which broadens the spectral linewidth significantly. Thermal tuning has been employed to solve the linewidth broadening issue. In 1995, super-structure grating distributed Bragg reflector (SSG-DBR) laser had demonstrated linewidth as low as 400 kHz using thermal tuning . Unfortunately, the thermal resistance of InP is very small, most of the heat will be diverted away through the InP substrate, the thermal power consumption is prohibitively large (>1 W). In order to reduce power consumption, air layers were fabricated between the waveguide and substrate to increase the thermal resistance. Sampled grating distributed Bragg reflector (SG-DBR) laser and modulated grating distributed Bragg reflector (MG-DBR) laser employed the air layer and reduced the total thermal power consumption down to around 100 mW and achieved linewidth less than 100 kHz [9,10]. Another way to reduce linewidth is to use material with a lower linewidth enhancement factor . In 2013, digital supermode distributed Bragg reflector (DS-DBR) laser was demonstrated with linewidth below 200 kHz with InGaAlAs multiple quantum wells (MQWs) . Compared with InGaAsP MQWs, InGaAlAs MQWs have lower linewidth enhancement factor. To meet the high output power requirement, tunable lasers are generally integrated with semiconductor optical amplifier (SOA) to boost the output power. Tunable lasers that use gratings as the front mirror can be directly integrated with SOAs such as the SSG-DBR laser , the SG-DBR laser  and the DS-DBR laser . Tunable lasers using a cleaved facet as the front mirror generally require an additional structure to integrate the SOA. An etched mirror was developed to monolithically integrate the tunable laser based on double ring resonators with the SOA . The modulated grating Y-branch (MGY) laser was integrated with the SOA through a short Bragg grating with high coupling coefficient .
A narrow-linewidth thermally tuned multi-channel interference (MCI) widely tunable laser has been reported recently . Both InGaAlAs MQWs and thermal tuning were employed to reduce the linewidth. In this paper, further efforts have been made to demonstrate a narrow-linewidth thermally tuned MCI laser integrated with SOA and spot size converter (SSC). A chirped grating is used as the front mirror in order to integrate the MCI laser with the SOA. In our previous work, two-port multi-mode interference reflector (MIR) was used to integrate the MCI laser with the SOA . The sidewall of the MIR is hard to be fabricated exactly perpendicular to the substrate, which causes additional loss. By using chirped grating, the ratio of reflection and transmission can be controlled freely and the loss can be reduced. In addition, in order to improve the coupling efficiency between the laser and fiber, the SSC has been integrated at the exit end of the laser, which increases the coupling efficiency greatly. Air layers were fabricated to reduce the power consumption for thermal tuning. The MCI laser realizes a tuning range of more than 42.5 nm with side-mode suppression ratios (SMSRs) above 48 dB and the output power coupled into a lensed fiber is above16 dBm. Lorentzian linewidth is below 100 kHz within the whole tuning range. The thermal power consumption is less than 20 mW for the whole tuning process, which is lower than the other reported thermally tuned tunable semiconductor lasers.
2. Device design and fabrication
2.1 Device structure and design
The microscope image of the fabricated narrow-linewidth thermally tuned MCI laser integrated with SOA and SSC is showed in Fig. 1. The MCI laser consists of gain section, common phase section and multi-channel interference (MCI) section. The MCI section comprises a 1 × 8 multi-mode interferometer (MMI) and eight arms with unequal length difference. There is a MIR at the rear of each arm. A thermally tuned arm phase section is included in each arm to tune the phase of each arm independently. The MCI laser is integrated with the SOA through a chirped grating. In order to improve the coupling efficiency, the SSC is integrated at the exit end of the laser. The SOA section is tilted by 7° and the front facet of laser is antireflection coated. The total chip size is 3.2 mm × 0.5 mm, including the bonding pads. The gain section and SOA are 400 µm and 600 µm long, respectively. The thermally tuned phase section is 150 µm long. The 1 × 8 MMI is 24 μm wide and 148 μm long. The one-port MIR is 6 μm wide and 37.5 μm long. Chirped grating and SSC are 200 μm and 210 μm long, respectively. Mode selection is achieved by the constructive interference of the eight arms. The interference of eight arms of different lengths produces a reflection spectrum dominated by a single reflection peak. Wavelength tuning is achieved by adjusting the phase section to change the position of the main reflection peak .
The wafer structure of the thermally tuned MCI laser is shown schematically in Fig. 2. The SOA and gain sections contain five compressively strained InGaAlAs quantum wells providing optical gain. The core of the passive section is bulk 1.3Q InGaAsP. Offset quantum-well scheme is used for the active-passive integration . All sections have used the same n-doped InP lower cladding and p-doped upper cladding. InGaAs ohmic contact layer is made atop waveguide of the SOA and gain sections. Thermal-optic effect is employed to change the phase of the phase sections. Air layers are added between the waveguide and the substrate to increase the heating efficiency, which is realized by selectively wet etching the InGaAs sacrificial layer. The chirped grating is placed between the gain section and the SOA. Thus, the reflection and transmission of the chirped grating provides optical feedback to the laser cavity and access to the SOA for amplification and output, respectively. Chirped grating is fabricated on the 1.3Q InGaAsP waveguide layer. The grating period of the chirped grating gradually increases from left to right, and the gratings of different periods reflect different wavelengths so as to achieve broad spectrum reflection. The ratio of reflection and transmission can be adjusted by changing the etching depth or length of the grating.
Schematic structure of the integrated SSC is shown in Fig. 3(a). The core of the input waveguide is a 300-nm-thick 1.3Q InGaAsP layer, whose far field divergence angles (FWHM) are 25° (lateral) and 44° (vertical), respectively. It is clearly seen that the far-field divergence angle in the vertical direction is much larger than the lateral direction. The divergence angle in the vertical direction can be reduced by thinning the core layer thickness. Through numerical simulation, the core thickness of the output waveguide is determined to be 100 nm, whose far field divergence angles (FWHM) are 22° (lateral) and 22° (vertical), respectively. The far field of the output waveguide is therefore circular, which is critical to improve the coupling efficiency with a fiber. In addition, this thickness has good fabrication tolerance to still keep the effective vertical optical confinement. A taper structure is used to reduce the width of the top 200-nm-thick InGaAsP gently. We use beam propagation method to simulate loss of the tapered structure versus the length for different ending width of the taper. As can be seen from Fig. 3, the wider the ending of the taper is, the shorter the taper length is needed before the loss stabilizes. When the ending width of the taper is less than 200 nm and the length of the taper is more than 150 μm, the loss can be below 0.2 dB. In order to facilitate the cleavage process, a 60-μm-long waveguide with a core thickness of 100 nm is left in front of the SSC. Finally, the total length of the SSC is designed to be 210 μm in our final fabrication device.
2.2 Device fabrication
The first step of the device fabrication is to define the active section, which is realized by etching the quantum wells off in the passive sections. Subsequently, the SSC and chirped grating pattern is formed by electron-beam lithography and reactive ion etching (RIE) dry etching the 1.3Q InGaAsP waveguide layer. The etching depth of the SSC and grating are 200 nm and 75 nm, respectively. Scanned electron microscope (SEM) images of the SSC and grating before epitaxial regrowth are shown in Fig. 4(a) and (b), respectively. And then, the p-InP upper cladding layer and p-InGaAs contact layer are re-grown by MOCVD over the entire wafer.
After regrowth, the fabrication of waveguides is an important step. The SOA and gain section are shallow ridge waveguides, which have better thermal dissipation, lower sidewall scattering loss and surface recombination. The passive sections are deep ridge waveguides. The shallow ridge waveguides are etched by inductively coupled plasma reactive ion etching (ICP-RIE) followed by wet etching, while the deep ridge waveguides are completely etched by ICP-RIE dry etching. To reduce the loss resulted from mode mismatch between the shallow ridge and deep ridge waveguides, a tapered shallow-deep transition structure is fabricated. After the fabrication of the deep ridge waveguides, air layers are formed between the waveguide and the substrate in the phase sections. Two grooves are etched through the InGaAs sacrificial layer on both sides of the waveguide, and then the InGaAs sacrificial layer under the waveguide is removed by selective wet etching to form the air layer. A thick SiO2 is deposited to protect the suspended waveguide. SEM images of the suspended thermal tuning waveguide is shown in Fig. 4(c). It can be seen that the InGaAs sacrificial layer is well removed and the waveguide has no great deformation. In addition, P-electrodes and heating electrodes are fabricated by lift-off process. After thinning down the substrate, N-electrode is deposited on the back. Finally, the lasers are cleaved and antireflection coated for test.
3. Device measurement and discussion
The device is soldered onto an AlN carrier and then placed onto a copper heat sink for testing. A thermoelectric cooler (TEC) controls the chip temperature at 20 °C. We characterized the thermally tuned MCI laser with a wavelength spacing of 2.5 nm . During the characterization process, the SOA and the gain section are biased at a constant current of 100 mA and 120 mA, respectively. Figure 5(a) shows the superimposed lasing spectra of 18 lasing wavelengths from 1525 nm to 1567.5 nm. The corresponding SMSRs are shown in Fig. 5(b). The thermally tuned MCI laser achieved a tuning range of more than 42.5 nm with SMSRs above 48 dB across the tuning range, which verifies that the chirped grating can provide broad spectrum reflection. Figure 5(c) shows LI curves of the corresponding lasing wavelengths, where the threshold current is between 15.5 and 19.5 mA. LI curves are measured by reversely biasing the SOA as a detector to detect the output power. The power difference between different wavelengths is caused by two aspects. Firstly, the gain is different throughout the tuning range. Secondly, the reflection spectrum of grating is not smooth enough, which requires further optimization of the design and process of the grating. Simultaneously, in the next round of fabrication, the length of the grating can be shortened appropriately to reduce reflection and increase transmission to increase the output power.
The phase tuning capability of a single thermally tuned arm phase section is measured. Figure 6(a) shows the output power versus different micro-heater power of an arm phase section. The output power changes with the micro-heater power of one arm phase section periodically. Only about 2.9 mW is required for thermally tuned arm phase section to achieve 2π round-trip phase change. Total tuning powers at different lasing wavelength are shown in Fig. 6(b). The total thermal tuning power of 8 phase sections is less than 20 mW across the tuning range. The thermal power consumption is relatively low, which shows that the air layer has a good thermal insulation effect.
The linewidth of the laser is measured using the test system based on coherent optical receiver . To prevent the light being reflected back into the cavity by the ending face of fiber, the output light is coupled into a single-mode fiber through two collimating lenses, and a 60 dB free space isolator is inserted between the two lenses. The light from the laser enters into the single-mode fiber and is divided into two channels, one of which is delayed through 20-km fiber, and then two channels enter into the coherence receiver. The detector converts the interference optical signal into electrical signal, and the data is read through a real-time oscilloscope from which the FM noise spectrum is calculated. The Lorentzian linewidth is estimated from the frequency noise averaged between 200 and 400 MHz, the frequency range corresponding to white noise. The result is shown in Fig. 6(c), the linewidth below 100 kHz across the tuning range has been achieved.
The optical intensity distribution of the emitted light from the MCI laser without SSC and with SSC measured by a camera is shown in Fig. 7. After the MCI laser integrated with SSC, the optical intensity distribution of the emitted light is close to be circular, which is beneficial for coupling to a fiber. In order to evaluate the effect of reduced divergence angle and increased coupling efficiency with fiber, two devices with SSC and without SSC are compared. Firstly, a large-area Ge detector is used to detect the output power of two devices. Subsequently, the power coupled into a lensed fiber with a beam waist diameter of 2.5 μm is detected. During the measurement, the gain section current is held constant at 160 mA and the output power versus SOA current is measured. Figure 8 shows the comparison of the two devices with and without SSC. The laser with SSC shows slightly higher output power. The reason is that the divergence angle of the laser without SSC is relatively large, and the limited area of the Ge detector is not large enough to receive all the power. Figure 8(b) shows the output power coupled into a lensed fiber versus SOA current at different lasing wavelength. The integration of SSC greatly improves the coupling efficiency between the laser and the lensed fiber. The output power coupled into a lensed fiber increases by 70% at a gain current of 160 mA and SOA current of 280 mA. The output power coupled into a lensed fiber of the thermally tuned MCI laser integrated with SSC can be higher than 16 dBm.
The test is a simple chip-on-carrier test where the carrier is simply placed on a copper heat sink which is temperature controlled through TEC. The heat dissipation is thus not good enough. When high current is injected into the SOA and the gain section together, the temperature of the chip rises up which therefore shifts the longitudinal mode and causes the mode hops as observed in the LI curves of Fig. 8(a) and (b). This influence should be less when the device is properly packaged. Also if the longitudinal mode position can be placed at more suitable positions under the reflection peak, the tolerance to longitudinal mode shift can also be made larger, which therefore helps to avoid mode hops during the LI curve scan.
By still keeping the chip-on-carrier test, we compare the situations for higher or lower currents injected into the gain and SOA sections. The current injections into all the phase sections keep the same. Five wavelengths are compared when the SOA and the gain section are biased at currents of 100 mA and 120 mA, respectively, or at 280 mA and 160 mA, respectively. Figure 9(a) shows the lasing spectra when the gain and SOA currents are relatively high. The wavelengths have slightly red shifted due to the thermal effect as mentioned above. The SMSRs and linewidths for the two groups of current are compared. It can be seen from Fig. 9(b) and Fig. 9(c) that SMSRs and linewidths are slightly worse. This is mainly due to the fact that the position of the longitudinal mode relative to the reflection peak has changed a little bit. In the future work, a precise placement of the longitudinal mode under the reflection peak will be carried out. However this will involve a new way to characterize the laser instead of the blind optimization method based on the particle swarm optimization (PSO) algorithm as implemented in this work.
We demonstrate a narrow-linewidth thermally tuned MCI laser integrated with SOA and SSC. MCI laser integrated with SOA through chirped grating is successfully realized for the first time, which realizes a tuning range of more than 42.5 nm with SMSRs above 48 dB. The scheme has been initially verified, and further optimization of the design and process of the grating to increase the flatness of the reflection spectrum is necessary. In order to reduce the linewidth, InGaAlAs MQWs with lower linewidth enhancement factor are used. Simultaneously, thermal tuning is used to avoid shot noise and loss caused by free carrier absorption. Lorentzian linewidth of thermally tuned MCI laser is below 100 kHz in the whole tuning range. In order to increase the output power coupled into fiber, SSC is integrated at the MCI laser exit end, which has the effect of significantly increasing the coupling efficiency with the lensed fiber. The output power coupled into a lensed fiber of the MCI laser integrated with SSC can be above 16 dBm. Air layers are fabricated between the waveguide and the substrate in the phase sections, which provided a good thermal insulation effect. Only 2.9 mW is needed for 2π round-trip phase shift, the total thermal tuning power needed is less than 20 mW across the tuning range.
National Natural Science Foundation of China (61675077, 61861136001, 61904064); National Key Research and Development Program of China (2018YFB2201201).
The authors declare no conflicts of interest.
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