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Abstract
A monolithic optical injection-locked (MOIL) DFB laser with large stable injection locking range is experimentally demonstrated using the side-mode injection locking technique. The low-frequency roll-off in the MOIL DFB laser is suppressed significantly. The relaxation oscillation frequency is measured to be 26.84 GHz and the intrinsic 3-dB response bandwidth is more than 30 GHz, which is about 20 GHz higher than that of the free running DFB laser. The nonlinear distortions, including the 1-dB compression point, second harmonic distortion (2HD) and third-order intermodulation distortion (IMD3), are also suppressed significantly. A simple radio-over-fiber system transmitting 40 Msymbol/s 32-QAM signal with 6 GHz carrier is achieved using the MOIL DFB laser. After 50 km transmission, the average error vector magnitude (EVM) of the whole link is 2.94% in injection locked state, while the EVM in free running DFB laser is 5.25% as a comparison. To our knowledge, this is the first time that the MOIL DFB laser is realized utilizing the side-mode injection locking method.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Directly modulated semiconductor lasers (DMLs) are of great interest in various analog fiber communication links, including cable television (CATV) distribution systems, antenna remoting and cellular networks [1–3]. However, the performances of them are usually limited by the low modulation bandwidth and nonlinear distortions of the semiconductor lasers. Numerous approaches have been developed to improve the dynamic characteristics of DMLs, such as active feedback lasers [4], passive feedback lasers [5], two-section DFB lasers [6] and optical injection locked (OIL) lasers [7]. Among them the optical injection locking method has been proven to be an effective method to improve the dynamic characteristics of the DMLs [8, 9].
Originally, the OIL lasers are realized by two separately packaged lasers (one master laser and one slave laser). In order to achieve a stable injection-locked state, the operating wavelength, the polarization and the output power of the two lasers should be controlled precisely and various optical elements are required in the OIL laser system (e.g., optical isolators, optical amplifiers and polarizers) [10,11]. As a consequence, the OIL laser systems are very complicated and the portability of the OIL lasers is limited. To make the OIL lasers more practicable, several varieties of monolithic optical injection-locked (MOIL) lasers based on discrete mode lasers [12], DBR lasers [13] and DFB lasers [14, 15] have been developed which can provide improved modulation characteristics without additional optical components. However, there are several difficulties in fabricating the MOIL lasers, which significantly increase the fabrication difficulties and costs. Firstly, due to the lack of isolator between the master laser and the slave laser, the nonlinear effects, including the period-one oscillation, quasi-periodicity and chaos, are very rich in the MOIL lasers, especially at large bias currents [16]. In order to achieve stable injection locking state, the slave laser is usually biased at a low current, and the injection locking range and injection ratio of the MOIL lasers are usually limited to small values [15]. Consequently, the response bandwidths of the MOIL lasers are limited to well-below their relaxation oscillation (resonance) frequency by the low-frequency roll-off effect [13–15], which is induced by the low biased current of the slave laser and the small injection ratio [11]. Secondly, both of the integrated DFB lasers should achieve stable single longitudinal mode (SLM) operation, especially the master laser. In order to obtain high SLM yield, the gratings with phase-shifts are utilized in the DFB lasers, which cannot be fabricated by conventional holographic exposure. Relatively expensive and sophisticated fabrication schemes, such as the E-beam lithography technique or s-bend waveguide, are usually required [17, 18]. Furthermore, the stable injection locking state requires the detuning frequency, which is defined as the frequency difference between the master laser and the slave laser, less than tens of gigahertz [8]. Unfortunately, the frequencies of the integrated DFB lasers cannot be tuned independently by changing their temperature separately, because they share the same heat sink. The only possible solution to change the detuning frequency is adjusting the bias currents of the integrated lasers, although it is not as efficient as tuning the temperature. Hence, it is very important to control the wavelengths of the integrated master and slave laser with high precision. To our knowledge, even the E-beam lithography technique cannot fulfill this requirement due to its mechanical error [19]. Therefore, new methods to realize MOIL lasers with reduced low-frequency roll-off effect, high wavelength accuracy and low costs are still expected.
In this paper, we demonstrate an MOIL DFB laser utilizing the side-mode injection locking method which can weaken the nonlinear effects between the integrated lasers and widen the locking range [20]. The experimental results show that the proposed MOIL DFB laser achieves stable mutual injection locking state when the bias current of the master laser is changed from 110 mA to 170 mA, while the slave laser is biased >20 mA higher than its threshold current. The maximum injection ratio of 6.4 dB is obtained. Under the injection locked state, the resonance frequency is measured to be 26.84 GHz and the intrinsic response bandwidth is larger than 30 GHz. The results also illustrate that the low-frequency roll-off effect in MOIL lasers can be mitigated using the proposed method. The nonlinear distortions, including the 1-dB compression point, second order harmonic distortion (2HD) and third order intermodulation distortion (IMD3) are also suppressed significantly. In addition, an analog fiber communication system is demonstrated based on the MOIL DFB laser, in which 40 MSymbol/s 32-QAM signal with 6 GHz carrier is transmitted. After 50 km transmission in single-mode fiber, the average error vector magnitude (EVM) of the whole link is 2.94% in injection locked state. As a comparison, the EVM is 5.25% when the laser operates in free running state. The gratings of the integrated lasers are designed by the reconstruction-equivalent chirp (REC) technique and fabricated by conventional holographic exposure and μm-level photolithography, which provides high precision for wavelength controlling with low costs [21–24].
2. Device design and fabrication
The structure of the MOIL DFB laser is shown in Fig. 1. It consists of two back-to-back DFB laser sections. One section is the master laser (ML) and the other is the slave laser (SL). The lengths of the two sections are Lm = 450 μm and Ls = 350 μm respectively and the bias currents of the ML and SL are labeled as Im and Is. Usually, we apply the radio frequency (RF) modulation to the SL. Hence, in order to enhance the modulation bandwidth, the cavity length of SL is designed a little shorter than that of the ML. The epitaxial structure used for device fabrication is grown by a conventional two-stage metal-organic chemical vapor deposition (MOCVD). An n-InP buffer layer, an n-InAlGaAs lower optical confinement layer, an InAlGaAs multiple-quantum-well (MQW) structure, a p-InGaAsP upper optical confinement layer and a p-InGaAsP grating layer are successively grown on an n-InP substrate in the first epitaxial growth. The sampled grating is formed by a conventional holographic exposure combining with conventional photolithography. After the forming of the sampled grating, a p-InP cladding layer and a p-InGaAs contact layer are successively regrown over the entire structure. The devices are realized by processing ridge wave guides, opening p-metal contact windows, followed by metallization. In addition, in order to obtain high electrical isolation, a small area of the highly p-doped InGaAs contact layer between the two sections is removed by etching. Finally, AR coatings with reflectivity less than 1% are deposited on both facets to suppress the Fabry-Perot modes of the lasers.
Both of the gratings in the two sections are designed by the REC technique with the identical seed grating period Λ0. In order to obtain high SLM yield, equivalent phase shifts are employed in the gratings. The wavelengths of the lasers are determined by the sampling periods Pm and Ps [23]. Through tuning the sampling periods, the wavelengths of the two lasers and the detuning frequency can be controlled accurately, with the largest deviation less than 0.2 nm [24], which is sufficient for the optical injection locking process.
3. Experimental results
3.1 Static performance
Figure 2(a) plots the power-current (P-I) curves of the SL and ML when they are biased individually at 25 °C. The P-I curves of the SL and ML are measured from the front facet and back facet of MOIL DFB laser chip respectively. The thresholds of the SL and ML are 29 mA and 37 mA respectively and the output slope efficiencies are 0.13 mW/mA and 0.1 mW/mA. Due to its longer cavity length, the ML has a little higher threshold and lower slope efficiency comparing with the SL. The injection ratio is one of the most important parameters for MOIL DFB lasers, which is defined as the ratio between the power injected into the SL from the ML and the power of the SL at free running state. Because the grating structure of the ML is symmetrical, the light power injected into the SL is similar to the power measured from the back facet without thinking about the absorption loss in the narrow electrical isolation area. As shown in Fig. 2(b), when Is is fixed at 50 mA, the injection ratio increases from −3.7 dB to 6.4 dB when Im is changed from 50 mA to 170 mA.
Another important parameter for the MOIL DFB laser is the detuning frequency. Here we define the detuning frequency as the difference between the frequency of the ML and the frequency of the side-mode of the SL. Because the ML and SL are fabricated on the same chip, the detuning frequency cannot be changed by tuning the temperature of the two lasers separately. Hence the bias current of the ML (Im) is the only tuning parameter for changing the detuning frequency. The measured spectra of the MOIL DFB laser with Is fixed at 50 mA and different Im are shown in Fig. 3. It can be seen that when Is is fixed at 50 mA and the ML is biased with different currents, the MOIL DFB laser exhibits various characteristics. As shown in Fig. 3(a), when the SL and ML are biased with the same current of 50 mA, the SL and ML operate independently and there is no interaction between them. The wavelength of the ML locates between the main mode and the side mode of the SL. As Im is increased from 50 mA to 175 mA, the wavelength of the ML has red shift due to the heating effect. Because of the thermal crosstalk between the ML and SL, the wavelength of the SL also drifts to the long wavelength when Im is increased. As shown in the Fig. 4(a), the wavelength shift rate of the ML is slightly larger than the SL. Hence, the ML wavelength shifts towards the side-mode of the SL and the higher Im is set, the smaller detuning frequency is achieved. As a consequence, the main mode of the SL is suppressed and the side-mode is enhanced. When Im is large enough, both modes of the SL are suppressed and SL is locked to the output of the ML section. It can be seen from Fig. 3(b)-(e) that the SL section is locked to the output of the ML at the bias current Im in the range of 110 mA to 170 mA. As shown in Fig. 4(b), the SMSR of the MOIL DFB laser which is defined as the suppression ratio between the ML mode and the SL mode increases from 33.8 dB to 45 dB and the detuning frequency reduces from 26.8 GHz to 20.9 GHz, when Im rises from 110 mA to 170 mA. However, as shown in Fig. 3(f), when Im is higher than 175 mA, the main mode of SL lases again, and the MOIL DFB laser is unlocked.
Fig. 3 Measured spectra of the MOIL DFB laser with Is = 50 mA and different Im (a) Im = 50 mA, (b) Im = 110 mA, (c) Im = 130 mA, (d) Im = 150 mA, (e) Im = 170 mA, (f) Im = 175 mA.
Fig. 4 (a) Frequency of the ML and SL side-mode and (b) detuning frequency and SMSR of the MOIL DFB laser against Im with Is = 50 mA.
It is noted that the temperature of the MOIL DFB laser is controlled at 25°C by a thermoelectric cooler (TEC) during the measurement. Since the ML and SL are fabricated on the same chip, the wavelengths and output powers of the ML and SL vary with the ambient temperature simultaneously. So the detuning frequency and injection ratio of the MOIL DFB laser are stable when the ambient temperature is changed. Consequently, the temperature stability of the MOIL DFB laser is better than the OIL laser constituted by separately packaged DFB lasers. However, the bias currents of ML and SL are so large that the TEC is employed to avoid the damage of the laser induced by the heating effect.
3.2 Enhancement of resonance frequency and intrinsic response bandwidth
The experimental setup for measuring the frequency response, nonlinear distortions of the MOIL DFB laser is shown in Fig. 5. In order to investigate the dynamic properties of the MOIL DFB laser, we packaged the laser utilizing industry-standard 7-pin butterfly style with GPO connector as RF input which is shown in the inset of Fig. 5. Unfortunately, the packaging design was not optimized for large bandwidth (tens of GHz) performance and its modulation bandwidth was about a dozen gigahertz.
Fig. 5 The experimental setup for frequency response and nonlinear distortions measurement. The inset is the photograph of the packaged laser. PD: photodetector, ESA: electrical signal analyzer, OSA: optical spectrum analyzer.
During the measurement of frequency response of the MOIL DFB laser, the modulation signal from Port 1 of the vector network analyzer (VNA: R&S®ZVA67) is fed to the SL via a Bias Tee, while the ML is DC biased. The modulated light coming out of the MOIL DFB laser is converted into electrical signal by the high-speed photodetector (PD: U2T XPDV 2120R) and sent back to Port 2 of the VNA. The optical spectrum analyzer (OSA: YOKOGAWA AQ6370) is used to monitor the optical spectrum simultaneously. The modulation frequency response is shown in Fig. 6. The resonance frequency of the SL is measured to be 6.80 GHz in free running state (Is = 50 mA, Im = 0 mA). In injection locked state, the resonance frequency is increased to 26.84 GHz, when Is = 50 mA and Im = 110 mA. Commonly, the direct modulation on the laser will create symmetric sidebands on the longer and shorter wavelength sides of the laser mode. According to Ref [11], in injection locked state, the modulated optical sideband of the locked mode is resonantly enhanced by the intrinsic cavity mode of the SL. Here, the intrinsic cavity mode is the side-mode of the SL, because the frequency difference between the main mode of the SL and ML is much larger than the modulation frequency. As a consequence, there is a resonance peak at the frequency which equals the frequency difference between the ML and the side-mode of the SL. That is to say, it is the detuning frequency of the MOIL DFB laser that determines the resonance frequency enhancement. Hence, the resonance frequencies obtained from the measured response curves shown in Fig. 6 are reduced from 26.84 GHz to 22.96 GHz when Im is changed from 110 mA to 150 mA, which agrees well with the detuning frequency shown in Fig. 4(b).
It is obvious that there are notches in the frequency response curves near 15 GHz. The notches are induced by the package process due to the RC time-constant-limited frequency bandwidth. Due to the effects of the notches, it is difficult to obtain the response properties from the measured curves in Fig. 6 distinctly. Fortunately, we can extract the intrinsic frequency response of the MOIL DFB laser chip from the measured frequency response curves using the reported methods [25, 26].
According to Ref [25], the frequency response (in decibels) of a packaged DFB laser can be expressed as
The term denotes the intrinsic frequency response, while expresses the extrinsic effects of the parasitic network mainly caused by the package. More importantly, is independent from current. So, we can remove the effects of package by subtracting the frequency response from two different bias levels.
Unlike conventional one-section DFB lasers, the intrinsic frequency response of a MOIL DFB laser can be expressed as [8]
where is the resonance frequency, is the first order pole frequency that factors greatly into the 3-dB bandwidth of the MOIL response, Γ is the damping factor. The parameter T equals towhere , g, z and denote the steady-state photon number, gain, injection rate and optical field phase respectively. Hence, the subtracted frequency response between two different bias levels can be expressed aswhere , ,and are the parameters at bias level x (x = 1, 2), respectively. After performing the subtraction between two different bias levels resulting in Eq. (4), the external effects can be removed, leaving the bias-dependent intrinsic effects. Then, the parameters (, , and ) can be extracted by fitting the subtracted frequency response curve. Therefore, the intrinsic frequency response can be obtained by substituting the extracted parameters into Eq. (2).For instance, the subtracted frequency response achieved by subtracting the measured frequency response with Is = 50 mA, Im = 140 mA from the response with Is = 50 mA, Im = 150 mA is shown in Fig. 7(a). Then, by substituting the parameters obtained through fitting the subtracted response curve shown in Fig. 7(a) into Eq. (2), we can obtain the intrinsic frequency responses of the MOIL DFB laser with Im = 140 mA and Im = 150 mA respectively. Figure 7(b) shows the normalized intrinsic response of the MOIL DFB at different bias levels and it is simple to calculate the intrinsic response bandwidth utilizing the curves. It can be seen that the intrinsic response bandwidth of the MOIL DFB laser is about 10 GHz at free running state (Im = 0 mA). For comparison, the maximum intrinsic response bandwidth of the MOIL DFB laser in injection locked state increases to 31.6 GHz, which is about 20 GHz larger than that in free running state. By substituting the intrinsic frequency responses and the measured frequency responses into Eq. (1) we can obtain the extrinsic responses of the parasitic network in the package, which are shown in Fig. 8. It is obvious that the extrinsic response of the parasitic network is independent from current and the notches in the measured response curves are generated by the parasitic network of the package. The results also show that the low-frequency roll-off effect in the proposed MOIL DFB laser is suppressed.
Fig. 7 (a) Subtracted frequency response obtained by subtracting the frequency response with Im = 140 mA from Im = 150 mA. The red line is the fitting curve, (b) normalized intrinsic frequency response of the MOIL DFB laser at different bias levels.
3.3 Suppression of the nonlinear distortions
The nonlinear distortions are the main factors that limit the application of DMLs in analog systems. The nonlinear distortions of the MOIL DFB laser, including the 1-dB compression point, 2HD and IMD3, are investigated experimentally.
In the measurement of the 1-dB compression point, the SL is modulated by a single tone 6 GHz RF signal and then the output signal from the MOIL DFB laser is detected by a PD. The signal from the PD is linked to an electrical signal analyzer (ESA: Anritsu MS2668C) to measure the output RF power. Commonly, the output RF power has a linear relation to the input RF power. However, when the input RF power is increased large enough, the output RF power deviates from the linear curve due to the RF link gain compression caused by the MOIL DFB laser. We identify the point where the output RF power has dropped by 1 dB from the linear curve as the 1-dB compression point. Figure 9 shows the 1-dB compression point of the MOIL DFB laser in different states. When the MOIL DFB laser is in free running state (i.e., Is = 50 mA, Im = 0 mA), the 1-dB compression point is 15.2 dBm. However, the 1-dB compression point occurs at 18.2 dBm when the current of the ML is increased to 150 mA. It should be noted the output RF power of the injection locked laser in Fig. 9(b) is smaller than that of the free running laser in Fig. 9(a) at the same input RF power. It corresponds with Fig. 6 where the intensity of frequency response at 6 GHz in injection locking state is lower than that in free running state. It is caused by the optical injection locking process, which produces a different modulation depth and different RF power at the receiver.
Fig. 9 The measured 1-dB compression point of the MOIL DFB laser at different bias levels, (a) Is = 50 mA, Im = 0 mA, (b) Is = 50 mA, Im = 150 mA.
Figure 10(a) and (b) plot the spectra of the second harmonic signal in different bias states. During the measurement of the 2HD, the SL is also modulated by a single tone 6 GHz RF signal. Since the frequency of the RF signal is 6 GHz, the second harmonic product is at 12 GHz. When the SL is biased at 50 mA, the 2HD is measured to be −16.98 dBc and −30.64 dBc with the ML biased at 0 mA and 150 mA respectively. When the SL is in free running state, the input RF power is 0 dBm. Since the response intensity in injection locking state is different from that in free running state (see Fig. 6), it is necessary to adjust the input RF power to keep the received RF power at the fundamental frequency equal to that of the free running DFB laser [27]. To investigate the 2HD further, we fix Is at 50 mA and change Im from 0 mA to 170 mA. During the process, the input RF power is also adjusted to keep the received RF power at the fundamental frequency constant. The measured 2HD versus Im is shown in Fig. 11. In the injection locked state, the second harmonic signal power is reduced by 13.96 dB comparing with the free running state.
Fig. 10 (a) Measured spectrum of the second harmonic signal with Is = 50 mA, Im = 0 mA, (b) measured spectrum of the second harmonic signal with Is = 50 mA, Im = 150 mA.
The IMD3 is one of the most important figure-of-merits for DMLs, and it determines the linearity of the device. The spectra of the IMD3 signal in free running and injection locked state are shown in Fig. 12(a) and (b) respectively. The SL is directly modulated by a two-tone RF signal with frequencies of 5.99 GHz and 6.01 GHz. As mentioned above, during the measurement of the IMD3 spectra, the power of the two-tone RF signal is also tuned to make the received RF power at fundamental frequency the same with that in free running state. The figures clearly show that the IMD3 is suppressed by 24.54 dB utilizing the side-mode OIL method. The spurious free dynamic ranges (SFDRs) of the MOIL DFB laser with different states are also measured, which are shown in Fig. 13. It is obvious that, under injection locked state (Is = 50 mA, Im = 150 mA), the SFDR increases from 80.7 dB·Hz2/3 to 89.8 dB·Hz2/3.
Fig. 12 (a) Spectrum of IMD3 signal with Is = 50 mA, Im = 0 mA, (b) spectrum of IMD3 signal with Is = 50 mA, Im = 150 m.
Fig. 13 SFDR of the MOIL DFB laser at different bias state. In free running state, Is = 50 mA, Im = 0 mA, while in injection locked state, Is = 50 mA, Im = 150 mA.
According to [28], the nonlinear distortion is influenced by several nonlinearities, such as spatial hole burning (SHB) effect, relaxation oscillation and so on. The SHB effect dominates the nonlinear distortion at low frequency (i.e., usually less than 1 GHz), whereas the relaxation oscillation influences the nonlinear distortion at high frequency. It is well known that the nonlinear distortion become more severe as the modulating frequency approaches the resonance frequency due to the nonlinear coupling between electrons and photons [29]. Hence, if the resonance frequency of the laser is increased, the nonlinear distortions can be reduced. Utilizing the proposed method, the resonance frequency is increased from 6.80 GHz to 26.84 GHz. Therefore, the nonlinear distortions are suppressed distinctly by injection locking method due to the enhanced resonance frequency.
3.4 Radio over fiber link based on the MOIL DFB laser
We also test the modulation properties of the MOIL DFB laser using a simple Radio-Over-Fiber (ROF) link. The schematic of the ROF link is shown in Fig. 14. The SL is directly modulated by a 40 MSymbol/s 32-QAM signal with 6 GHz carrier. The 32-QAM signal is generated by a vector signal generator (VSG: Keysight N5172B). The light signal is transmitted through a 50 km single mode fiber and then amplified by an Erbium doped fiber amplifier (EDFA). The light signal is finally converted into electrical signal by a PD and then transmitted to the electrical signal analyzer for demodulation and analysis.
Figure 15(a) and (b) show the analysis results of the received signal when the MOIL DFB laser is in free running and injection locking state (Is = 50 mA, Im = 150 mA) respectively. The figures clearly indicate that the received constellation diagram of the injection locked state is better than that of the free running state. The EVM of the injection locked state is reduced from 5.25% to 2.94% comparing with the free running state. The EVM is also related to the received optical power of the PD. The measured EVMs with different received optical powers at different states are plotted in Fig. 16. The received optical power of the PD is adjusted by changing the current of the EDFA. It can be seen from Fig. 16 that the EVM is reduced with the increasing of the received optical power. When the received optical power is changed from 0.56 dBm to 5.87 dBm, the EVM of injection locked state is reduced from 5.62% to 2.94%. As a comparison, the EVM of free running state is reduced from 7.48% to 5.25%. The received 32-QAM signal is improved significantly utilizing the proposed method.
Fig. 15 Analysis of the received signal with Is = 50 mA and (a) Im = 0 mA, (b) Im = 150 mA. The received optical power is fixed at 5.87 dBm in both states.
4. Conclusion
A MOIL DFB laser based on the side-mode injection locking method is demonstrated. It has large injection locking range with a maximum injection ratio of 6.4 dB. Under the injection locked state, the resonance frequency is 26.84 GHz, which is 20 GHz larger than that in free running state. Using the proposed method, the nonlinear distortions of the laser (e.g., the 1-dB compression point, 2HD and IMD3) are suppressed significantly. In addition, a simple ROF link base on the MOIL DFB laser is also demonstrated. The EVM of the system after 50 km transmission is reduced from 5.25% to 2.94% when the MOIL DFB laser is switched from free running state to injection locked state. The intrinsic response bandwidth of the MOIL DFB laser is also investigated, which is 31.4 GHz and 10 GHz under injection locked state and free running state respectively. It should be noted that the characteristics of the MOIL DFB laser can be further improved by optimizing the epitaxial materials, the cavity length, the grating structures, the waveguide structure and the packaging process. Hence, this work provides a promising low-cost method for realizing high-performance DMLs.
Funding
National Natural Science Foundation of China for the Youth (61504058, 61504170, 61405096 and 61640419); Natural Science Foundation of Jiangsu Province for the Youth (BK20140414, BK20160907 and BK20141168); National Natural Science Foundation of China (61435014, 61575186, 61635001, and 11574141); National “863” project of China (2015AA016902)
Scientific Research Foundation of Nanjing University of Posts and Telecommunications (NY215041); Open Foundation of Research Center of Optical Communications Engineering & Technology, Jiangsu Province (ZXF20170302).
Acknowledgments
The authors would like to thank Z.W. Liao and W.C. Zhang with Dalian Canglong Opto-electronic Technology Co. Ltd., for their help in laser package.
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