Like integrated circuits (ICs), photonic ICs (PICs) will play a predominant role in modern information communications technology. Among PIC devices, distributed feedback (DFB) multi-wavelength semiconductor laser arrays (MWLAs), where several lasers with different wavelengths are integrated together, has been considered one of the core components. Up to now, electron beam lithography (EBL) has been the main method to fabricate MWLAs for more than twenty years^[1]. Very fine structures can be fabricated by EBL, but it also offers low throughput because of the long writing time, resulting in a high cost^[1]. It is also well known that EBL suffers from drawbacks, such as blanking or deflection errors and shaping errors. As a result, it is difficult to achieve lasing wavelengths with high precision. According to the Bellcore report, with EBL, it has been shown that only 35% of lasers have a wavelength variation of less than $\pm 0.2\mathrm{nm}$^[1]. Reference [2] shows that the error associated with the EBL process may be as large as 3 nm. Since the wavelengths of individual lasers should meet ITU-T standards and the single longitudinal mode (SLM) operation of each laser should be ensured, realizing high wavelength precision and high SLM yield simultaneously is a crucial issue. Therefore, many efforts have been made to solve the problem. Some methods have already been developed, such as bent waveguides^[3], nano-imprint lithography^[4] and selective area growth^[5].

The reconstruction equivalent chirp (REC) technique is a promising way to fabricate MWLAs^[6,7]. It can equivalently realize complex grating structures through a micrometer-scale sampling pattern. During the grating fabrication, only two steps, holographic exposure for uniform basic (seed) grating and photolithography for sampling patterns, are required to fabricate the sampled grating. All other processes are the same as those for conventional DFB lasers.

Thus, the cost of the fabrication is low. The most important advantage of the REC technique is that it offers precision manufacturing in a periodic structure to control the wavelength precisely; its precision is 100 times higher than the common method in theory^[6]. The purpose of this Review is to review important results on REC-based laser arrays and their applications in radio-over-fiber (ROF) systems.

To verify the effectiveness of the REC technique, 60-wavelength laser arrays in two 2-inch wafers are experimentally demonstrated^[7]. It has been shown that the wavelength precision and single-mode yield are very high. From more than one thousand lasers randomly selected from the two wafers, the mean lasing wavelength residuals of 83.3% of the lasers are within $\pm 0.20\mathrm{nm}$ and 93.5% are within $\pm 0.30\mathrm{nm}$. Lasers with a wavelength deviation of $>0.50\mathrm{nm}$ are less than 1.0% of the total laser count. The average single-mode yield is about 98.6%.

After some efforts to optimize parameters, such as reducing the reflectivities of the laser facets, the wavelength accuracy of REC-MWLAs has been further improved recently^[8]. The position of the laser facets (i.e., the facet grating phase) is usually random. If there are some residual facet reflections on two facets, this random grating phase will affect the intra-cavity resonance and cause a wavelength deviation in the MWLA. Therefore, we use a tilt facet at the back end of the laser to reduce the reflection further.

The MWLA is designed around the center wavelength of 1550 nm, and the channel spacing is 200 GHz, which is about 1.62 nm. Three 12-wavelength MWLAs and three 20-wavelength MWLAs are demonstrated for the test.

The measured lasing wavelengths of the MWLAs are in the range from 1525 to 1555 nm. Typical spectra of the MWLAs are shown in Fig. 1(a). The SLM performance of the lasers is good, and from Fig. 1(b), the laser wavelength is varied in good linearity with the channel number.

Figure 2(a) shows the wavelength deviation from the linear fitting of all laser bars. In all 6 bars, 95 lasers are selected for testing because their laser wavelengths are all in the range of good coatings. Among them, 77 lasers, namely about 81%, have wavelength deviations less than $\pm 0.1\mathrm{nm}$, and the others are all less than $\pm 0.2\mathrm{nm}$. The corresponding distribution of the wavelength deviation is shown in Fig. 2(b). Obviously, compared with the results reported in Ref. [7], the wavelength accuracy is further improved.

In order to compare them with the DFB lasers fabricated by the conventional method, the linewidths of the DFB lasers designed by the REC technique are also measured utilizing the delayed self-heterodyne method. The linewidth of the REC-DFB lasers is the similar to that of the commercial DFB lasers, which is about several megahertz. The details can be found in Ref. [9].

Thus, the REC technique provide a simple and good solution for precision manufacturing in wavelength structure control and can be used in high-end and complex optical components, for example, DFB lasers for analog applications.

To obtain analog directly modulated (DM) DFB lasers with high linearity, a two-section (TS) DFB laser with an integrated reflector section was also developed^[10]. As show in Fig. 3, one section is a DFB laser based on the REC technique, and the other section acts as a reflector.

In order to study the dynamic characteristics of the two-section DFB lasers, we packaged the laser using a commercial 10 GHz butterfly housing, which contained 7 pins and a GPO connector. Compared with conventional one-section (OS) DFB lasers fabricated with the same wafer, the 3 dB modulation bandwidth of the TS-DFB lasers was 1.9 GHz larger than that of the OS-DFB laser, as shown in Fig. 4. Furthermore, the small-signal-frequency response curves of the TS-DFB lasers were much flatter and the relaxation resonance peaks were suppressed significantly, which was of benefit to ROF systems.

The relative intensity noise (RIN) is a key parameter for DFB lasers used in communication systems that can limit the signal-to noise ratio. The RIN spectrum of the TS-DFB lasers and OS-DFB lasers is shown in Fig. 5. The RIN of the TS-DFB lasers is $-146.5\mathrm{dB}/\mathrm{Hz}$ with $I_1=90\mathrm{mA}$ and $I_2=0\mathrm{mA}$, about 10 dB/Hz lower than that of the OS-DFB lasers under the same injection current. From the RIN spectrum, the relaxation resonance frequency can also be evaluated. For instance, the orange line demonstrates that the relaxation resonance frequency of the OS-DFB laser is around 7 GHz, which agrees well with the results shown in Fig. 5.

The spurious-free dynamic range (SFDR) was one of the most important figures of merit for a DFB laser DM by analog signals, which determines the linearity of the device. Figures 6(a) and 6(b) show the SFDRs of the TS-DFB laser and a conventional DFB laser under the same injection current of 90 mA, respectively. The lasers were DM by two-tone RF signals with frequencies of 10 and 10.02 GHz. The figures clearly show that the SFDR of the TS-DFB laser is $92\mathrm{dB}{\mathrm{Hz}}^{2/3}$ and that of the conventional DFB laser is $87\mathrm{dB}{\mathrm{Hz}}^{2/3}$. That is to say, the TS-DFB laser has a better performance than the conventional DFB laser in terms of suppressing the nonlinear distortion.

As shown in Fig. 7, a simple ROF link was set up to test the analog performance of the TS-DFB laser. The laser was DM by a 50 MSymbol/s 64-QAM signal, and the carrier frequency is 10 GHz. The 64-QAM signal was generated by a PSG Signal Generator, and the output power is 0 dBm. After the transmission over a 40 km single-mode fiber, the signal was detected by a PIN detector integrated with a transimpedance amplifier. Then, the electrical signal from the PIN detector was transmitted to the signal analyzer for demodulation and analysis. Figure 8 shows the analysis results of the received signal under the bias currents of $I_1=90$ and $I_2=0\mathrm{mA}$. The results clearly indicate that the received constellation diagram is good and the average error vector magnitude is 2.97%. The presented TS-DFB lasers have good characteristics in direct modulation and are promising in a wide number of applications in the ROF system, which is our research direction in the future. More details will be presented in the future.

In the present TS-DFB lasers, the fabricated grating periods of section 1 and section 2 are the same. However, because of the different injection currents, the grating pitches should be different when working. The difference between the two grating pitches may affect the analog performance, which will be studied in the future. Moreover, complex grating structures can be introduced to change the analog characteristics of the TS-DFB lasers. Thus, the precise control of the grating structure along the cavity should be important for the optimization of the laser performance.

Besides the chip, the packaging is also a sophisticated problem and has a great effect on the performance of the DM laser module. In order to improve the response bandwidth and linearity of the DM DFB laser for analog links, the packaging of the DFB laser was also investigated, and a 24 GHz DM DFB laser module with an in-band flatness of $\pm 3\mathrm{dB}$ was achieved^[11]. The laser module was packaged in butterfly housing with a K connector, as shown in Fig. 9. The laser chip in the module was an InAlGaAs DFB laser with a ridge-waveguide structure, and the length of the laser chip was 220 μm. For the packaged wide-bandwidth laser module, the design of a high-frequency circuit is a key point. It should be particularly designed to supply enough bandwidth for a laser chip. In the laser module, the high-frequency circuit is comprised of a K connector, a transition line, and a grounded coplanar waveguide (CPW) shown in Fig. 9(b). The transition line is designed just like a short CPW with an ${\mathrm{Al}}_2{\mathrm{O}}_3$ substrate. A gap is introduced to match the electro-magnetic field and reduce the reflection from the electro-magnetic field mismatch. Two vias are made to achieve better grounding. The grounded CPW is made on an AlN substrate with a gold trace on it. In order to solve the finite ground problem, the side walls of the CPW are all metalized, and the grounding vias can be omitted. A curved path is also included to change the direction of the microwave propagation and supply enough space for the optical coupling components. All the high-frequency circuits are designed with 50-ohm impedance matching. Due to the fact that the AC impendence of the laser chip is about only 5 Ω, a matching resistor of about 45 Ω is series with the laser chip.

The laser module showed a high linearity. Figure 10 shows the small-signal modulation response of the laser module, and the bandwidth of module was 24 GHz with a 100 mA injected current. The third intercept of the laser module at a different frequency is shown in Fig. 11, and the third-order intercept of the module was measured to be 32.2 dBm at 24 GHz.

The packaging for the MWLA is even more complicated. To achieve a good performance at a high frequency, the electrical circuit should be specially designed. One key difficulty of high-frequency circuit design in this laser array module package is the limited available space. Due to the limited space, the usual coaxial connectors such as K or V connectors are not suitable for RF signal feeding in the MWLA module. In our module, the high-frequency circuit is composed with a feed-through structure and a grounded coplanar waveguide array.

Based on the design indicated in the above, an 8-channel DM MWLA module based on the REC technique has been first packaged with a bandwidth of 10 GHz^[12].

The measured small-signal modulation response of the DM MWLA module is shown in Fig. 12.

The biasing currents for all DM lasers are set to be 55 mA, and the RF signal fed into the laser chip is 1 mW. The 3 dB bandwidth obtained from S21 is 10 GHz for all eight lasers, which meets the demand of an $8\times 12.5\mathrm{Gb}/\mathrm{s}$ operation. The detailed measurement parameters, such as bit error rate (BER) curves at 12.5 Gb/s, can be found in Ref. [11]. In addition, Table 1 lists the progress of the MWLA used for digital signal transmission systems in recent years.

In conclusion, the REC technique is a promising method for low-cost and precision manufacturing in grating structures/wavelength control. Such a technique is thus a good solution for the fabrication of MWLAs. Wavelength deviation of 83.3% lasers in a 60-wavelegnth MWLA is demonstrated with a wavelength liner deviation less than $\pm 0.2\mathrm{nm}$. After improvements, the wavelength deviation can be reduced to a value of $\pm 0.1\mathrm{nm}$. Based on the REC technique, a TS-DFB laser with an integrated reflector is also proposed and successfully applied to an ROF link with a 50 MSymbol/s 64-QAM signal and a 10 GHz carrier. The analog performance can be improved greatly using an optimized package. With the improved package, a 24 GHz analog DFB laser module with an in-band flatness of $\pm 3\mathrm{dB}$ is achieved. Furthermore, the packaging technique for the MWLA was also investigated, and an $8\times 12.5\mathrm{Gb}/\mathrm{s}$ MWLA module is presented.