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Abstract
We demonstrated a high output power distributed-Bragg-reflector (DBR) laser integrated with semiconductor optical amplifier (SOA) for the frequency-modulated continuous-wave (FMCW) light detection and ranging (LiDAR) system. In order to acquire higher output power, different from the conventional SG-DBR laser, the front mirror in this work is a section of uniform grating to get higher transmissivity. Therefore, the output power of the laser reaches 96 mW when the gain current and SOA current are 200 mA and 400 mA, respectively. Besides, we fabricated a spot size converter (SSC) at the laser output port to enhance the fiber coupling efficiency, which reached 64% coupled into the lensed fiber whose beam waist diameter is 2.5 μm. A tuning range of 2.8 nm with free spectral range (FSR) of 0.29 nm and narrow Lorentzian linewidth of 313 kHz is achieved. To realize distance and velocity measurement, we use the iterative learning pre-distortion method to linearize the frequency sweep, which is an important part of the FMCW LiDAR technology.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Light detecting and ranging (LiDAR) technology is widely used for autonomous vehicles, 3D imagining and artificial intelligent robots [1–4]. Compared with the pulsed time of flight (ToF) LiDAR, frequency-modulated continuous-wave (FMCW) technology has advantages of immunity of sunlight and other LiDAR transmitters, high resolution and sensitivity, and better signal to noise ratio (SNR), due to coherent detection [5–7]. Besides, FMCW technology is also able to acquire target position and Doppler-based velocity information simultaneously in a single measurement [8].
In a FMCW LiDAR system, it is critical to use lasers whose frequency can be modulated over some range because of the coherent detection nature of the FMCW LiDAR system. To obtain the position and velocity information, we usually combine the signal light reflected from the target with the reference signal to generate a beat signal, and extract the aimed frequency by the fast Fourier transform (FFT). Therefore, the linearity of the sweeping laser is essential to the resolution and the signal-to-noise-ratio (SNR) of the system [9].
Many types of lasers are available for linearization of the frequency sweep including: external cavity lasers (ECLs) [10,11], distributed feedback (DFB) lasers [12–18], vertical-cavity surface-emitting lasers (VCSELs) [18–21,] and sampled-grating distributed-Bragg-reflector (SG-DBR) lasers [21]. For example, Roos et al. demonstrated high linear frequency chirps of an ECL by compensating the nonlinear voltage ramps to the laser tuning element, and employed a feedback circuit to maintain a phase lock over the chirp bandwidth [10]. Zhang et al. tested a commercial VCSEL laser and a DFB laser by iterating the laser control current, realizing high linearity of the frequency sweep [18]. Isaac et al. presented a SG-DBR laser integrated with a frequency discriminator which contained a tunable asymmetric Mach-Zehnder Interferometer (a-MZI), multimode interference couplers and two PDs for balanced detection. And the external locking circuits was used between the phase section of the SG-DBR laser and the PDs to maintain the lasing frequency of the laser equal to the quadrature point of the a-MZI, and linear frequency sweep was achieved by tuning the quadrature point [22]. DBR lasers have advantage of fast tuning due to the nature of the electro-optical tuning. Besides, DBR lasers are also attractive for ease of integration and low linewidth.
In this paper, we demonstrated a high output power DBR laser integrated with an SOA. To achieve high output power, the front mirror in this work is a section of uniform grating which has higher transmissivity compared to the conventional SG-DBR laser [23]. Here, the output power reaches 96 mW when the gain and SOA currents are 200 mA and 400 mA, respectively. The Lorentzian linewidth is inversely proportional to the effective cavity length, and narrower linewidth of our laser can be achieved by using sampled grating in the back mirror. Moreover, we integrated a spot size converter (SSC) to increase the laser output coupling efficiency to the lensed fiber. The free spectral range of the laser is about 0.29 nm with side mode suppression ratio 53 dB and narrow Lorentzian linewidth of 313 kHz when the current is 100 mA in both gain and SOA regions. With regard to the linear frequency sweep, we utilize the iterative learning control (ILC) method in Ref. [23] achieving a residual nonlinearity of 0.026% and 0.02% of the frequency excursion in the up and down ramps, respectively. Finally, the measurement of the velocity and distance has been realized.
The paper is organized as follows: in section 2 we introduce the structure and fabrication of the DBR laser; in section 3 we characterize the device showing the output power, LI curve, SMSR, tuning range, and the Lorentzian linewidth; in section 4 we investigate the frequency sweep linearization, and then measure distance and velocity based on this device; in the final section we conclude this work and discuss the potential applications of our device.
2. Device design and fabrication
2.1 Device structure and design
The fabricated high output power DBR laser integrated with SOA and SSC is shown in Fig. 1. The device, which has a total size of 3.65 mm × 0.25 mm, including: an SOA region for optical amplification, a front mirror region and a back mirror region for mode selection, a gain region for optical gain, a phase region for frequency sweeping, and a photodetector (PD) region for monitoring the power. A square waveguide is at the end of the laser to eliminate any reflection. The length of the SOA and gain section are 1000 μm and 600 μm, respectively. To obtain higher output power, we tapered 50 μm gain waveguide from 3 μm to 4.5 μm at each side (totally 100 μm). Besides, the SOA section is titled by 7° and the exit cleaved facet is antireflection (AR) coated to reduce the reflection. The SSC is 210 μm long including 60 μm for cleaving process. The phase section with the length of 150 μm is utilized to tune the laser finely by injecting the current. Moreover, by simulation, the front mirror is a section of 38 μm long uniform grating whereas the back mirror is sampled gratings which is about 980 μm long with 31 burst periods.
The schematic wafer structure of the high output power DBR laser is shown in Fig. 2. The wafer is based on the n-doped InP substrate. To reduce the linewidth, we employ InGaAlAs MQWs consisting of 5 compressively strained quantum wells and 6 tensile strained barriers. Offset quantum-well structure is used for active-passive integration [24]. The grating is fabricated on the 1.3Q passive core waveguide, which is under the MQWs. Moreover, the p-doped InP upper cladding and high doped InGaAs ohmic contact layer are re-grown after the grating fabrication process.
Mode selection of the DBR laser is realized by aligning the reflectivity spectrum of the front mirror and back mirror. Reflectivity and transmissivity can be designed by changing the etched depth or the length of the uniform and sampled grating. Supermode spacing can be changed by adjusting the burst period of the sampled grating. The simulation result is shown in Fig. 3(a). The numerical calculation results reveal that the peak reflectivity of the front mirror and back mirror are 0.27 and 0.8, respectively. The etching depth of the gratings is 40 nm. The free spectral range is 0.3 nm with 10.5 nm supermode spacing. As shown in Fig. 3(b), the passive waveguide core is the 1.3Q InGaAsP layer, which is 300 nm thick. As shown in Fig. 3(c), the far field divergence angles of the output waveguide are 26° (lateral) and 44° (vertical), respectively. By thinning the thickness of the core layer, the vertical divergence angle can be reduced [25]. Figure 3(d) shows that the far field divergence angles are 22° (lateral) and 22° (vertical) when the thickness of the output core layer is reduced to 100 nm. In this way, the coupling efficiency to fiber can be improved. Moreover, the loss can be less than 0.2 dB when the length of the taper is longer than 150 μm and the ending width of the taper is no more than 200 nm [25]. The total length of the SSC is 210 μm including 60 μm for the cleavage process.
Fig. 3. (a) Reflectivity of the front mirror and back mirror; (b) schematic structure of the SSC (c) the input mode of the SSC; (d) the output mode of the SSC.
2.2 Device fabrication
At first, the quantum wells are selectively removed from the passive sections. Then, the SSC and gratings are pattered by electron-beam lithography (EBL) and the 1.3Q InGaAsP passive waveguide layer is etched by reactive ion etching (RIE). The etching depth of the grating and the SSC are 40 nm and 200 nm, respectively. Scanning electron microscope (SEM) imagines of the SSC, gratings and waveguide are shown in Fig. 4(a), (b), and (c), respectively. As shown in Fig. 4(b), the duty cycle of the grating is 0.5. Subsequently, a p-InP cladding layer and heavily doped p-InGaAs ohmic contact layer are re-grown by the MOCVD process. After regrowth, P-metal contacts were fabricated on the top of the InGaAs ohmic contact layer. And then the ridge waveguide is fabricated by combination of coupled plasma reactive ion etching (ICP-RIE) dry etching and wet chemical etching. Moreover, electrical isolation between different regions is achieved by etching of the highly p doped InGaAs layer. P-electrodes are then fabricated through a lift-off process. After thinning down the substrate, N-electrode is deposited on the backside of the wafer. At last, the lasers are cleaved and then AR coated.
3. Device measurement and discussion
The laser is mounted onto an AlN carrier and then is placed on a copper heat sink for characterization. A thermoelectric cooler (TEC) is used to keep the chip temperature at 25 °C constantly. Figure 5(a) shows the LIV curve of the device. The laser biasing current varies form 0 mA up to 100 mA, and the LI curve is measured by reverse biasing the SOA as a photodetector. During the test, all the other regions of the laser are kept floated. The result shows that the threshold current of the laser is 23.5 mA with 6 Ω series resistance. Then the output power is detected by a large area Ge detector with the current of the gain section fixed at 200 mA. Figure 5(b) shows the output power versus the SOA current. The output power achieves 96 mW when the gain current and SOA current are 200 mA and 400 mA, respectively. In order to evaluate the effect of the integrated SSC, two lasers with and without SSC are measured for comparison. The output powers of the two lasers are detected by a large area Ge detector. Then the output powers coupled into lensed fibers with beam waist diameters of 2.5 μm and 5 μm are measured, respectively. Coupling efficiency is calculated when the gain and SOA current are 120 mA and 200 mA, respectively. As shown in Fig. 5(c) and (d), the output power of the laser without SSC is smaller than that of the laser with SSC. The reason is that the divergence angle of the laser without SSC is relatively large, which is 26° (lateral) and 44° (vertical) as simulation shows, and the area of the Ge detector is limited and it is not large enough to receive all the power. Besides, by using the SSC structure, the output power coupled into a 2.5 μm lensed fiber reaches 64%, while the coupling efficiency to the 5 um lensed fiber is 44%.
Fig. 5. (a) The LIV curve of the DBR laser measured by reverse biasing the SOA; (b) the LI curve when the gain current is 200 mA constantly and the SOA current varying; (c)∼(d) output power received by a Ge large area detector and coupled into lensed fibers whose beam waist diameter are 2.5 and 5μm, respectively
The tuning range, optical spectrum and linewidth of the DBR laser are measured when the gain and SOA regions are both biased at 100 mA. By sweeping the currents injected into the back mirror and phase regions separately, the tuning ability of the laser can be demonstrated. As shown in Fig. 6(a), the tuning range is about 2.8 nm when the current injected into the back mirror section varies from 0 mA to 100 mA with a step of 1 mA. Moreover, Fig. 6(b) reveals that the free spectral range (FSR) is 0.29 nm which could satisfy the need for frequency sweeping of 30 GHz.
Fig. 6. Wavelength tuning characteristics of the DBR laser when current injected into the (a) back mirror section (b) phase section.
Figure 7(a) shows the optical spectrum coupled by a lensed fiber from the DBR laser. The supermode spacing is 10.5 nm with a 53 dB side mode suppression ratio (SMSR) which agrees with our design. Besides, the sensing range of the FMCW LiDAR system is directly related to the linewidth of the laser. So it is necessary to measure the linewidth of the laser. The linewidth is characterized by using the self-homodyne coherent receiver method [26]. The FM noise spectrum is shown in Fig. 7(b) from which the Lorentzian linewidth could be calculated from frequency noise averaged between 800 and 900 MHz, corresponding to the white noise. The result shows that Lorentzian linewidth is 313 kHz. The rising edge towards high frequencies is caused by the measurement technique due to differentiated additive white Gaussian noise (AWGN) from the receiver [27].
4. Frequency sweep linearization, distance and velocity measurement
4.1 Principle of distance and velocity measurement
Figure 8(a) schematically shows the FMCW-based LiDAR system. The light from the laser is divided into two paths. One path is used for signal light, the other path is used for reference light. The signal light reflected from the target is combined with the reference light and generates a beat signal. Then the beat signal is detected by a detector. The frequency of the beating signal can be extracted by the fast Fourier transform (FFT) processing. Since the frequency of the laser is linearly modulated, the value of the beating frequency is proportional to the target distance. The distance R of the target and distance resolution δR can be written as:
where fb is the beat signal frequency, γ is the frequency sweep rate, c is the light speed, Δt is the time delay between the signal light and reference light and B is the bandwidth of the frequency sweep.Fig. 8. (a) Principle schematic of the FMCW LiDAR; (b) triangular modulation frequency signal for moving target.
As shown in Fig. 8(b), the triangular chirping waveform in a single period consists of two ramps: up ramp and down ramp. If the target is not still, the Doppler frequency shift fD, which is proportional to the velocity of the moving target. When the target is approaching, the beat frequency of the down ramp is increased by fD and decreased by the same value for up ramp. The distance D and velocity v of the moving target can be expressed as [28]:
where λ is the wavelength of the laser, and fb,down and fb,up are the beat frequencies of the down ramp and up ramp, respectively.
4.2 Frequency sweep linearization
To realize the linearity of the frequency sweep, we use the method which is proposed in Ref. [18]. To reduce the influence of the noise and environmental interference, the 14-pin high output power DBR laser in the butterfly package, which contains a TEC to keep the temperature constant during the test, is characterized. The constant injection currents in gain and SOA regions are 100 mA and 150 mA, respectively. The modulated signal is injected into the phase region. The original input and required output signal are both triangular waveforms with 10 kHz frequency (both up and down ramps are 50 μs). Moreover, a 40 μs region of interest (ROI) (the central 80% in each ramp) is used for calculating the frequency sweep linearity. Figure 9(a) shows the value of the frequency sweep nonlinear root mean square (RMS) versus the times of iteration. As shown in Fig. 9(b)∼(c), the original RMS of the up and down ramps are 931.84 MHz and 853.02 MHz, respectively. After iterating 139 times, the RMS of the up ramp reduces to 6.17 MHz, leaving the residual nonlinearity of 0.026%, and the RMS of the down ramp reduces to 4.88 MHz, leaving the residual nonlinearity of 0.02%. The results are shown in Fig. 9(d)∼(e).
Fig. 9. (a) RMS of the up and down ramps versus the times of iteration; the up- and down- ramp laser frequency sweeps and residual error (b)∼(c): the initial; (d)∼(e): after 139 times of iteration.
4.2 Distance and velocity measurement
Figure 10 shows the experimental setups of the FMCW-based LiDAR for distance and velocity measurement. The required waveform, which is generated from the arbitrary wave generator after iteration, is applied to the phase section of the DBR laser. Then, the output light is divided into 2 paths by a 90:10 coupler. And 10% of the light is used as reference light, and 90% as the signal light. The signal light enters a collimator through an optical circulator and the light is collected by the collimator again after propagating to the target and reflected back. Finally, the light reflected from the target is mixed with the reference light, then the beating signal is acquired by a balanced photodetector and an oscilloscope. And the value of the beating frequency of the up and down ramps can be obtained by FFT processing, respectively. The value of the distance and velocity can be calculated by the equations above. In order to synchronize the arbitrary waveform generator with the oscilloscope, the oscilloscope uses a sync trigger signal from the arbitrary waveform generator as the trigger.
Fig. 10. The schematic of the FMCW-based LiDAR for distance and velocity measurement. AWG: arbitrary waveform generator; PC: polarization controller; BPD: balanced photodetector; OSC: oscilloscope.
The result of the distance measurement of the still target is shown in Fig. 11(a). The collimator is mounted on an adjustable stage and the target is fixed on a movable stage. The target is placed 155cm away from the collimator. As shown in Fig. 11(a), the FFT spectra of the up and down ramps are almost consonant. Moreover, three peaks of the FFT are seen because the transmitter and receiver share the same light path by using an optical circulator and the test arm is longer than the reference arm. The first and second obvious peaks are caused by the reflection of the connection point between the two fibers and the end facet of the collimator, respectively. The distance can be extracted by calculating the frequency difference between the third and the second peak of the FFT spectra and then bringing the frequency difference as fb to the Eq. (1). The result shows that the target is 155 cm away from the collimator, which agrees with the real distance. And Fig. 11(b) shows that the resolution of the distance is 9.24 mm which is larger than the theoretical resolution 6.25 mm. It is the system noise during linearizing the frequency sweep and low resolution ADC of the oscilloscope which is only 8 bits that caused the excursion. The resolution of the distance can be improved by using a high resolution data acquisition card which has 14 bits resolution and the influence of the system noise can be reduced by averaging the data with more periods.
Fig. 11. (a) Frequency spectra of the beat signal after FFT where the target is still; (b) close up view of the reflected peak.
Figure 12(a) and (b) show the FFT spectra when the target is approaching and moving away from the collimator with the same value of velocity, respectively. The FFT spectra peak of the up and down ramps are separate due to the Doppler effect. When the target is moving away, the beat frequency difference of the up and down ramps is equal to that of approaching, which is 0.65 MHz. And the velocity of the moving target is 253.3 mm/s by using Eq. (3).
Fig. 12. Frequency spectra of the beat signal after FFT where the target is (a) approaching; (b) moving away.
The initial results show that the DBR laser has been successfully used in the FMCW LiDAR system for distance and velocity measurement. Further refinement of the measurement and improvement of the DBR laser will be carried out in the near future.
5. Conclusion
We demonstrate a high-power DBR laser integrated with SOA for the FMCW LiDAR system. By using the uniform grating in the front mirror and tapered waveguide in SOA and gain regions, the output power of the laser has reached 96 mW when the gain current and SOA current are 200 mA and 400 mA, respectively. To increase the coupling efficiency to the fiber, SSC is fabricated at the output end of the laser. The output power coupled into a 2.5 μm beam waist diameter lensed fiber increases to 64% by using the SSC structure. The SMSR of the laser has realized 53 dB when the SOA and gain sections are both forward biased at 100 mA. The tuning range of the laser is 2.8 nm with a free spectral range of 0.29 nm and narrow Lorentzian linewidth of 313 kHz. Moreover, we have achieved a residual nonlinearity of 0.026% and 0.02% of the 24 GHz frequency excursion in the up and down ramps after linearizing the frequency sweep, respectively. Besides, the distance and velocity measurement has been successfully realized. The high output power and narrow linewidth DBR laser is an attractive candidate for linear frequency sweeping source for the FMCW LiDAR system.
Funding
National Key Research and Development Program of China (2018YFB2201701).
Disclosures
The authors declare no conflicts 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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