Research on Silicon-Substrate-Integrated Widely Tunable, Narrow Linewidth External Cavity Lasers

: Widely tunable, narrow linewidth external cavity lasers on silicon substrates have many important applications, such as white-light interferometry, wavelength division multiplexing systems, coherent optical communication, and optical ﬁber sensor technology. Wide tuning range, high laser output power, single mode, stable spectral output, and high side-mode suppression ratio external cavity lasers have attracted much attention for their merits. In this paper, two main device-integrated structures for achieving widely tunable, narrow linewidth external cavity lasers on silicon substrates are reviewed and compared in detail, such as MRR-integrated structure and MRR-and-MZI-integrated structure of external cavity semiconductor lasers. Then, the chip-integrated structures are brieﬂy introduced from the integration mode, such as monolithic integrated, heterogeneous integrated, and hybrid integrated. Results show that the silicon-substrate-integrated external cavity lasers are a potential way to realize a wide tuning range, high power, single mode, stable spectral output, and high side-mode suppression ratio laser output.


Introduction
Silicon-substrate-integrated narrow linewidth tunable external cavity semiconductor lasers (SINLT-ECSLs) are devices composed of the substrates Si, SiO 2 , Si 3 N 4 , or other containing Si materials and external optical feedback elements (low-loss waveguide, waveguide filter, or other elements). By adjusting the external cavity elements, such as polarizer, prism, gratings, etc., narrow linewidth and wide tuning range can be achieved. SINLT-ECSLs have the characteristics of tunable [1], narrow, or even ultra-narrow linewidth [2,3], low noise [4,5], wide application, and so on. In this paper, the device-integrated structures, chip-integrated structures of silicon-based external cavity semiconductor lasers are introduced, and, especially, the integration technology and development are introduced. Silicon-based external cavity semiconductor lasers have significant advantages over Littow and Littman configurations of the external cavity semiconductor lasers in terms of structure design, function types, and application range. In recent years, with the continuous development of optical fiber communication, coherence technology, and other fields, the silicon substrate external cavity semiconductor laser will be applied in more and more fields, with its unique characteristics, and will become the ideal light source. The role of the SOA is to provide gain amplification, while the silicon photonic chip is mainly for wavelength selection. Figure 2 shows the typical structure of a silicon-based external cavity semiconductor laser. The light wave of the SOA coupled to the silicon wire waveguide is filtered through two microring resonators (MRR). Two microring resonators with different radii are designed. According to Formula (1), the free spectral range (FSR) is also different due to the different radii of the microring resonators. FSR = λ 2 /2πrneff (1) In Formula (1), the wavelength of the light wave is λ, the radius of the microring resonator is r, and the effective refractive index of the waveguide is neff. The transmission spectra of the two microring resonators are superimposed on each other, and the wavelength of the mutually matched peak is determined by mode competition. The free spectral range of the microring resonator is changed by adjusting the heater through thermo-optic effect, and the transmission peak moves. The wavelength is tuned through the Vernier effect.  Figure 3 shows the working principle of wavelength tuning of a silicon-based external cavity semiconductor laser. When passing through two annular resonators with different radii, the wavelength difference of resonance results in a Vernier effect. The The role of the SOA is to provide gain amplification, while the silicon photonic chip is mainly for wavelength selection. Figure 2 shows the typical structure of a silicon-based external cavity semiconductor laser. The light wave of the SOA coupled to the silicon wire waveguide is filtered through two microring resonators (MRR). Two microring resonators with different radii are designed. According to Formula (1), the free spectral range (FSR) is also different due to the different radii of the microring resonators. The role of the SOA is to provide gain amplification, while the silicon photon is mainly for wavelength selection. Figure 2 shows the typical structure of a silicon external cavity semiconductor laser. The light wave of the SOA coupled to the wire waveguide is filtered through two microring resonators (MRR). Two mic resonators with different radii are designed. According to Formula (1), the free sp range (FSR) is also different due to the different radii of the microring resonators.

FSR = λ 2 /2πrneff
In Formula (1), the wavelength of the light wave is λ, the radius of the mic resonator is r, and the effective refractive index of the waveguide is neff. The transm spectra of the two microring resonators are superimposed on each other, and the length of the mutually matched peak is determined by mode competition. The free tral range of the microring resonator is changed by adjusting the heater through mo-optic effect, and the transmission peak moves. The wavelength is tuned throu Vernier effect.  Figure 3 shows the working principle of wavelength tuning of a silicon-bas ternal cavity semiconductor laser. When passing through two annular resonator different radii, the wavelength difference of resonance results in a Vernier effec In Formula (1), the wavelength of the light wave is λ, the radius of the microring resonator is r, and the effective refractive index of the waveguide is n eff . The transmission spectra of the two microring resonators are superimposed on each other, and the wavelength of the mutually matched peak is determined by mode competition. The free spectral range of the microring resonator is changed by adjusting the heater through thermo-optic effect, and the transmission peak moves. The wavelength is tuned through the Vernier effect. Figure 3 shows the working principle of wavelength tuning of a silicon-based external cavity semiconductor laser. When passing through two annular resonators with different radii, the wavelength difference of resonance results in a Vernier effect. The tuning wavelength range is determined by the radius difference between the two ring resonators. A small radius difference provides a wide wavelength-tuning range, although the transmittance difference between the main peak and the side peak adjacent to the main peak may be small. However, by heating one of the two ring resonators, the peak wavelength of the transmission spectrum of the dual-ring resonator filter changes discretely according to the resonant wavelength of the other ring resonator filter [6]. tuning wavelength range is determined by the radius difference between the two ring resonators. A small radius difference provides a wide wavelength-tuning range, although the transmittance difference between the main peak and the side peak adjacent to the main peak may be small. However, by heating one of the two ring resonators, the peak wavelength of the transmission spectrum of the dual-ring resonator filter changes discretely according to the resonant wavelength of the other ring resonator filter [6].

Research Progress of SINLT-ECSLs
SINLT-ECSLs can be mainly divided into the external cavity semiconductor lasers integrated with a microring resonator (MRR) [7][8][9], the external cavity semiconductor lasers integrated with an MRR and Mach-Zehnder interferometer (MZI), and the external cavity semiconductor lasers integrated with an MRR and others [10,11]. In order to subdivide it, MRR-integrated external cavity semiconductor lasers can be divided into double-MRR, three-MRR, and multiple-MRR integration. This paper mainly discusses the research progress of double-MRR integration and MRR-and-MZI-integrated external cavity semiconductor lasers.

MRR-Integrated External Cavity Semiconductor Laser
In 2006, Masahige Ishizaka et al. [12] reported an external cavity semiconductor laser integrated with SiO2 dual MRR and SOA. The structure is shown in Figure 4, and the wavelength-tuning range is 45 nm, which can completely cover the C-band and L-band in a wavelength division multiplexing (WDM) optical communication system. In 2009, Takeshi Takeuchi et al. [13] reported the use of silicon waveguide (core material is SiON) three-MRR-and-SOA-integrated external cavity semiconductor laser, coupling SOA with silicon substrate through passive alignment technology, where the coupling mode is direct coupling. A waveguide reflector is used instead of high-reflection (HR) mirror to reduce the manufacturing cost. In the design of the mi-

Research Progress of SINLT-ECSLs
SINLT-ECSLs can be mainly divided into the external cavity semiconductor lasers integrated with a microring resonator (MRR) [7][8][9], the external cavity semiconductor lasers integrated with an MRR and Mach-Zehnder interferometer (MZI), and the external cavity semiconductor lasers integrated with an MRR and others [10,11]. In order to subdivide it, MRR-integrated external cavity semiconductor lasers can be divided into double-MRR, three-MRR, and multiple-MRR integration. This paper mainly discusses the research progress of double-MRR integration and MRR-and-MZI-integrated external cavity semiconductor lasers.

MRR-Integrated External Cavity Semiconductor Laser
In 2006, Masahige Ishizaka et al. [12] reported an external cavity semiconductor laser integrated with SiO 2 dual MRR and SOA. The structure is shown in Figure 4, and the wavelength-tuning range is 45 nm, which can completely cover the C-band and L-band in a wavelength division multiplexing (WDM) optical communication system. tuning wavelength range is determined by the radius difference between the two ring resonators. A small radius difference provides a wide wavelength-tuning range, although the transmittance difference between the main peak and the side peak adjacent to the main peak may be small. However, by heating one of the two ring resonators, the peak wavelength of the transmission spectrum of the dual-ring resonator filter changes discretely according to the resonant wavelength of the other ring resonator filter [6].

Research Progress of SINLT-ECSLs
SINLT-ECSLs can be mainly divided into the external cavity semiconductor lasers integrated with a microring resonator (MRR) [7][8][9], the external cavity semiconductor lasers integrated with an MRR and Mach-Zehnder interferometer (MZI), and the external cavity semiconductor lasers integrated with an MRR and others [10,11]. In order to subdivide it, MRR-integrated external cavity semiconductor lasers can be divided into double-MRR, three-MRR, and multiple-MRR integration. This paper mainly discusses the research progress of double-MRR integration and MRR-and-MZI-integrated external cavity semiconductor lasers.

MRR-Integrated External Cavity Semiconductor Laser
In 2006, Masahige Ishizaka et al. [12] reported an external cavity semiconductor laser integrated with SiO2 dual MRR and SOA. The structure is shown in Figure 4, and the wavelength-tuning range is 45 nm, which can completely cover the C-band and L-band in a wavelength division multiplexing (WDM) optical communication system. In 2009, Takeshi Takeuchi et al. [13] reported the use of silicon waveguide (core material is SiON) three-MRR-and-SOA-integrated external cavity semiconductor laser, coupling SOA with silicon substrate through passive alignment technology, where the coupling mode is direct coupling. A waveguide reflector is used instead of high-reflection (HR) mirror to reduce the manufacturing cost. In the design of the mi- In 2009, Takeshi Takeuchi et al. [13] reported the use of silicon waveguide (core material is SiON) three-MRR-and-SOA-integrated external cavity semiconductor laser, coupling SOA with silicon substrate through passive alignment technology, where the coupling mode is direct coupling. A waveguide reflector is used instead of high-reflection (HR) mirror to reduce the manufacturing cost. In the design of the microring structure, the threshold difference is fully considered, and the three-MRR structure is adopted. Compared with the two-MRR structure, the three-MRR structure has a larger threshold gain difference and can provide a more stable laser in a larger tuning range. The silicon-based external cavity semiconductor laser has a simple structure and is suitable for mass production. It has a high fiber output power of more than 15 dBm, is able to tune wavelengths in the 60 nm range in the L-band, and contains 147 ITU-T channels with channel spacing of 50 GHz.
In the same year, Tao Chu et al. [14] proposed a silica-based external cavity tunable laser, which is mainly integrated through dual MRR and SOA. The structure is compact, and the size of the external cavity is only 0.7 × 0.45 mm 2 , about 1/25 of that of the traditional tunable laser. There is a wide tuning range, covering the optical communication C-band (1530-1565 nm) or L-band (1565-1610 nm), at the power of 26 mW, obtaining the maximum wavelength-tuning range of 38 nm.
Its structure is shown in Figure 5, consisting of an SOA and an external resonator. The resonator is made of silicon photonic line waveguide, and it is a double MRR. It is the first external cavity semiconductor laser made by silicon photonic technology. The ring resonator has a wide FSR due to its short cavity length. In addition, compared with the ring resonators made of SiON material, the ring resonators made of silicon photonic line waveguides have wider FSR due to their smaller bending radius of several microns. Therefore, the larger gain difference and wavelength-tuning range required for single-mode laser oscillation can be obtained more easily using a silicon ring resonator.
croring structure, the threshold difference is fully considered, and the three-MRR structure is adopted. Compared with the two-MRR structure, the three-MRR structure has a larger threshold gain difference and can provide a more stable laser in a larger tuning range. The silicon-based external cavity semiconductor laser has a simple structure and is suitable for mass production. It has a high fiber output power of more than 15 dBm, is able to tune wavelengths in the 60 nm range in the L-band, and contains 147 ITU-T channels with channel spacing of 50 GHz.
In the same year, Tao Chu et al. [14] proposed a silica-based external cavity tunable laser, which is mainly integrated through dual MRR and SOA. The structure is compact, and the size of the external cavity is only 0.7 × 0.45 mm 2 , about 1/25 of that of the traditional tunable laser. There is a wide tuning range, covering the optical communication C-band (1530-1565 nm) or L-band (1565-1610 nm), at the power of 26 mW, obtaining the maximum wavelength-tuning range of 38 nm.
Its structure is shown in Figure 5, consisting of an SOA and an external resonator. The resonator is made of silicon photonic line waveguide, and it is a double MRR. It is the first external cavity semiconductor laser made by silicon photonic technology. The ring resonator has a wide FSR due to its short cavity length. In addition, compared with the ring resonators made of SiON material, the ring resonators made of silicon photonic line waveguides have wider FSR due to their smaller bending radius of several microns. Therefore, the larger gain difference and wavelength-tuning range required for single-mode laser oscillation can be obtained more easily using a silicon ring resonator. In 2012, Keita Nemoto et al. [15] optimized the design of silicon substrate outer cavity semiconductor laser, using the ring resonator of silicon optical wire as the outer cavity, and produced a semiconductor laser with adjustable wavelength. The size of the ring resonator wavelength filter with outer cavity length of 6.0 mm is 1.78 × 0.52 mm 2 , which is about 1/8 of that of silicon (SiON) material. The maximum laser output power is 18.9 MW, using heating power of 115.7 mW and tuning operation of wavelength above 45.1 nm. The spectral linewidth of the whole L-band is less than 100 kHz, which is suitable for being used as the light source of the digital coherent light transmission system. The structure is shown in Figure 6. In 2012, Keita Nemoto et al. [15] optimized the design of silicon substrate outer cavity semiconductor laser, using the ring resonator of silicon optical wire as the outer cavity, and produced a semiconductor laser with adjustable wavelength. The size of the ring resonator wavelength filter with outer cavity length of 6.0 mm is 1.78 × 0.52 mm 2 , which is about 1/8 of that of silicon (SiON) material. The maximum laser output power is 18.9 MW, using heating power of 115.7 mW and tuning operation of wavelength above 45.1 nm. The spectral linewidth of the whole L-band is less than 100 kHz, which is suitable for being used as the light source of the digital coherent light transmission system. The structure is shown in Figure 6. In 2013, Tomohiro Kita et al. [16] fabricated a tunable semiconductor laser with a maximum output power of 25.1 mW using a silicon photonic line waveguide ring resonator as an external optical cavity. The micro-heater can be continuously tuned to wavelengths above 50 nm, with a linewidth less than 100 kHz and a smaller size. While im- In 2013, Tomohiro Kita et al. [16] fabricated a tunable semiconductor laser with a maximum output power of 25.1 mW using a silicon photonic line waveguide ring resonator as an external optical cavity. The micro-heater can be continuously tuned to wavelengths above 50 nm, with a linewidth less than 100 kHz and a smaller size. While improving laser stability, it can be used in the actual digital coherent transmission system, as shown in Figure 7. In 2013, Tomohiro Kita et al. [16] fabricated a tunable semiconductor laser with maximum output power of 25.1 mW using a silicon photonic line waveguide ring re nator as an external optical cavity. The micro-heater can be continuously tuned to wa lengths above 50 nm, with a linewidth less than 100 kHz and a smaller size. While i proving laser stability, it can be used in the actual digital coherent transmission syste as shown in Figure 7. In 2014, Sato et al. [17] integrated a silicon-based tunable filter, gain chip, and bo SOA, as shown in Figure 8. The tunable filter consists of two ring resonators, and waveguide core of the gain part is composed of InGaAsP/InGaAsP base multiple qua tum well. The laser side of the gain chip is coated with low-reflection (LR) coating a the output side of the boost SOA is coated with anti-reflection (AR) coating. The opti fiber coupling output power is greater than 100 mW, the linewidth is less than 15 kH the side-mode suppression ratio (SMSR) is greater than 45 dB, and the wa length-tunable range is about 65 nm, enough to cover the entire C-band. In 2014, Sato et al. [17] integrated a silicon-based tunable filter, gain chip, and boost SOA, as shown in Figure 8. The tunable filter consists of two ring resonators, and the waveguide core of the gain part is composed of InGaAsP/InGaAsP base multiple quantum well. The laser side of the gain chip is coated with low-reflection (LR) coating and the output side of the boost SOA is coated with anti-reflection (AR) coating. The optical fiber coupling output power is greater than 100 mW, the linewidth is less than 15 kHz, the side-mode suppression ratio (SMSR) is greater than 45 dB, and the wavelength-tunable range is about 65 nm, enough to cover the entire C-band. In 2015, Tin Komljenovic et al. [18] demonstrated a widely tunable external cavit semiconductor laser with an external cavity length of 4 cm through monolithic integra tion. The laser works in O-band and can be tuned in the range of 1237.7-1292.4 nm, with tuning range of about 54 nm. Over the entire tuning range, SMSR is greater than 45 dB output power is more than 10 mW, linewidth is less than 100 kHz, and the best singl mode linewidth is 50 kHz.
In 2016, Zhao et al. [19] fabricated a low loss (0.1 dB/cm), high Q factor microrin resonator based on double fringe SiN/SiO and developed a tunable InP/SiN mixed ex ternal cavity semiconductor laser, and the waveguide has good performance. The wave In 2015, Tin Komljenovic et al. [18] demonstrated a widely tunable external cavity semiconductor laser with an external cavity length of 4 cm through monolithic integration. The laser works in O-band and can be tuned in the range of 1237.7-1292.4 nm, with a tuning range of about 54 nm. Over the entire tuning range, SMSR is greater than 45 dB, output power is more than 10 mW, linewidth is less than 100 kHz, and the best single mode linewidth is 50 kHz. In 2016, Zhao et al. [19] fabricated a low loss (0.1 dB/cm), high Q factor microring resonator based on double fringe SiN/SiO and developed a tunable InP/SiN mixed external cavity semiconductor laser, and the waveguide has good performance. The wavelengthtuning range of the laser is about 1530-1580 nm, the output power is 16 mW, the SMSR is more than 45 dB, and the linewidth is 65 kHz. It has broad application prospects in coherent transmission systems.
The schematic diagram of the laser is shown in Figure 9. It consists of a high-power InP/n-GaAsP SOA gain chip and two microring resonators. The front and back of the SOA are coated with highly reflective and AR coatings. Using the cursor effect, two MRR with slightly different radii are used to increase the wavelength-tuning range. Phase and power tuning sections are for fine-tuning longitudinal mode and output power, respectively. output power is more than 10 mW, linewidth is less than 100 kHz, and the best mode linewidth is 50 kHz.
In 2016, Zhao et al. [19] fabricated a low loss (0.1 dB/cm), high Q factor mic resonator based on double fringe SiN/SiO and developed a tunable InP/SiN mix ternal cavity semiconductor laser, and the waveguide has good performance. The length-tuning range of the laser is about 1530-1580 nm, the output power is 16 mW SMSR is more than 45 dB, and the linewidth is 65 kHz. It has broad application pro in coherent transmission systems.
The schematic diagram of the laser is shown in Figure 9. It consists of a high-InP/n-GaAsP SOA gain chip and two microring resonators. The front and back SOA are coated with highly reflective and AR coatings. Using the cursor effect, two with slightly different radii are used to increase the wavelength-tuning range. Pha power tuning sections are for fine-tuning longitudinal mode and output power, r tively. In 2017, Jing Zhang et al. [20] proposed a non-uniformly integrated, wideunidirectional III-V ring laser on silicon. The wavelength-tuning range of the la 1560-1600 nm, and the ring radius of the ring resonator structure is 25 μm and 2 respectively. The FSR in the wavelength range of 1550 nm is 4.1 nm and 3.7 nm, r tively. Using the Vernier effect of two ring resonators, a wide tuning range of 40 obtained. Unidirectional operation is achieved throughout the tuning range, w clockwise SMSR of about 10 dB. The linewidth is less than 1 MHz throughout the t range and can be reduced to 550 kHz at the optimal operating point. In 2017, Jing Zhang et al. [20] proposed a non-uniformly integrated, wide-tuned, unidirectional III-V ring laser on silicon. The wavelength-tuning range of the laser is 1560-1600 nm, and the ring radius of the ring resonator structure is 25 µm and 27 µm, respectively. The FSR in the wavelength range of 1550 nm is 4.1 nm and 3.7 nm, respectively. Using the Vernier effect of two ring resonators, a wide tuning range of 40 nm is obtained. Unidirectional operation is achieved throughout the tuning range, with a clockwise SMSR of about 10 dB. The linewidth is less than 1 MHz throughout the tuning range and can be reduced to 550 kHz at the optimal operating point.
In 2018, Hang Guan et al. [21] demonstrated a III-V/Si mixed external cavity laser with a tuning range greater than 60 nm, a maximum output power of 11 mW, a minimum linewidth of 37 kHz, a C-band that is always less than 80 kHz, a maximum SMSR of 55 dB, and a C-band that is always greater than 46 dB. It consists of a reflective external cavity constructed by RSOA, SSC, and a ring resonator.
In the same year, Minha Tran et al. [22] designed and manufactured a narrow-linewidth tuned laser with multi-ring mirrors. The structure is shown in Figure 10, including two-ring mirrors and three-ring mirrors. In heterogeneous silicon photons, a laser using a three-ring mirror was implemented, with an average SMSR of 55 dB in the 30 nm tuning range and a linewidth reduced to 17.5 kHz. In 2018, Hang Guan et al. [21] demonstrated a III-V/Si mixed external cavity laser with a tuning range greater than 60 nm, a maximum output power of 11 mW, a minimum linewidth of 37 kHz, a C-band that is always less than 80 kHz, a maximum SMSR of 55 dB, and a C-band that is always greater than 46 dB. It consists of a reflective external cavity constructed by RSOA, SSC, and a ring resonator.
In the same year, Minha Tran et al. [22] designed and manufactured a narrow-linewidth tuned laser with multi-ring mirrors. The structure is shown in Figure 10, including two-ring mirrors and three-ring mirrors. In heterogeneous silicon photons, a laser using a three-ring mirror was implemented, with an average SMSR of 55 dB in the 30 nm tuning range and a linewidth reduced to 17.5 kHz. In 2019, Yongkang Gao et al. [23] demonstrated a miniaturized packaged hybrid integrated silicon-photon (SiPh) tunable laser for coherent modules of small size. By integrating an internally designed high-power SOA, the SiPh laser developed achieved a record 21.5 dBm C-band output power with a linewidth of 60 kHz, an SMSR greater than 50 dB, a relative intensity noise less than 150 dB/Hz, and a tuning range of 65 nm. In addition, frequency stability of the SiPh tunable laser at 1 GHz was achieved over the package temperature range from 10 °C to 80 °C and over SOA current variations of more than 200 mA, as shown in Figure 11. In 2019, Yongkang Gao et al. [23] demonstrated a miniaturized packaged hybrid integrated silicon-photon (SiPh) tunable laser for coherent modules of small size. By integrating an internally designed high-power SOA, the SiPh laser developed achieved a record 21.5 dBm C-band output power with a linewidth of 60 kHz, an SMSR greater than 50 dB, a relative intensity noise less than 150 dB/Hz, and a tuning range of 65 nm. In addition, frequency stability of the SiPh tunable laser at 1 GHz was achieved over the package temperature range from 10 • C to 80 • C and over SOA current variations of more than 200 mA, as shown in Figure 11.
In 2019, Yongkang Gao et al. [23] demonstrated a miniaturized p integrated silicon-photon (SiPh) tunable laser for coherent modules of s tegrating an internally designed high-power SOA, the SiPh laser develo record 21.5 dBm C-band output power with a linewidth of 60 kHz, an SM 50 dB, a relative intensity noise less than 150 dB/Hz, and a tuning range dition, frequency stability of the SiPh tunable laser at 1 GHz was ac package temperature range from 10 °C to 80 °C and over SOA current va than 200 mA, as shown in Figure 11. In 2021, Ruiling Zhao et al. [26] demonstrated a dual-gain InP-Si3N cavity laser, whose structure is shown in Figure 12. The working wavele In 2021, Ruiling Zhao et al. [26] demonstrated a dual-gain InP-Si 3 N 4 mixed external cavity laser, whose structure is shown in Figure 12. The working wavelength of the laser is 1550 nm, the tuning range is 44 nm of the working wavelength, the linewidth is 6.6 kHz, SMSR is greater than 67 dB, and the two gain parts work in parallel, thus providing high output power; the maximum power is about 23.5 mW.
ystals 2022, 12, x FOR PEER REVIEW is 1550 nm, the tuning range is 44 nm of the working wavelength, the lin kHz, SMSR is greater than 67 dB, and the two gain parts work in parallel, th high output power; the maximum power is about 23.5 mW. In the same year, Yuyao Guo et al. [27] introduced a III-V /Si3N4 hybr laser with faster switching time, and its structure is shown in Figure 13. wavelength of the laser is 1516.5-1575 nm, the tuning range is 58.5 nm, the 2.5 kHz, and the SMSR is greater than 70 dB. The maximum output power 500 mA injection current. In the same year, Yuyao Guo et al. [27] introduced a III-V /Si 3 N 4 hybrid integrated laser with faster switching time, and its structure is shown in Figure 13. The working wavelength of the laser is 1516.5-1575 nm, the tuning range is 58.5 nm, the linewidth is 2.5 kHz, and the SMSR is greater than 70 dB. The maximum output power is 34 mW at 500 mA injection current. In the same year, Yuyao Guo et al. [27] introduced a III-V /Si3N4 hybrid laser with faster switching time, and its structure is shown in Figure 13. T wavelength of the laser is 1516.5-1575 nm, the tuning range is 58.5 nm, the l 2.5 kHz, and the SMSR is greater than 70 dB. The maximum output power i 500 mA injection current.

MRR-and-MZI-Integrated External Cavity Semiconductor Lasers
The principle of how MZI works is shown in Figure 14. D1 and D2 are de and BS2 are beam splitters, and M1 and M2 are mirrors. A beam splitter, BS incoming monochromatic light beam from source S into two beams, which, tion by mirrors M1 and M2, recombine and interfere at BS2 to result in tw beams (collected by detectors D1 and D2). When the phase along one of the p signals in both D1 and D2 oscillate out of phase, and as no photons are bei sum of both signals stays always equal to the input, S.

MRR-and-MZI-Integrated External Cavity Semiconductor Lasers
The principle of how MZI works is shown in Figure 14. D1 and D2 are detectors, BS1 and BS2 are beam splitters, and M1 and M2 are mirrors. A beam splitter, BS1, splits an incoming monochromatic light beam from source S into two beams, which, after reflection by mirrors M1 and M2, recombine and interfere at BS2 to result in two outgoing beams (collected by detectors D1 and D2). When the phase along one of the paths varies, signals in both D1 and D2 oscillate out of phase, and as no photons are being lost, the sum of both signals stays always equal to the input, S.  In 2014, Debregeas et al. [28] proposed an integrated tunable laser th reflective SOA (RSOA) with a silicon ring resonator-based outer cavity structure is shown in Figure 15. The external cavity of the laser is com MRRs. The first ring is set to 25 GHz and the second ring is integrated with In 2014, Debregeas et al. [28] proposed an integrated tunable laser that combines a reflective SOA (RSOA) with a silicon ring resonator-based outer cavity and MZI. The structure is shown in Figure 15. The external cavity of the laser is composed of two MRRs. The first ring is set to 25 GHz and the second ring is integrated with MZI.
In 2014, Debregeas et al. [28] proposed an integrated tunable laser t reflective SOA (RSOA) with a silicon ring resonator-based outer cavity structure is shown in Figure 15. The external cavity of the laser is co MRRs. The first ring is set to 25 GHz and the second ring is integrated wit In the same year, Tomohiro Kita et al. [29] fabricated a wavelengt using a silicon photonic wavelength filter consisting of a ring resonato metric MZI (A-MZI), as shown in Figure 16. The size, including the SOA only 2.6 × 0.5 mm 2 , about 1/9 of the size of the silicon nitrous tunable la length-tuning range is more than 61.7 ± 0.2 nm, covering the whole optic tion L-band, and SMSR is more than 38 dB. When the SOA injection cur the maximum optical output power is 42.2 mW, achieving a stable sin output. By optimizing the outer cavity design, a spectral linewidth of less obtained. In the same year, Tomohiro Kita et al. [29] fabricated a wavelength-tunable laser using a silicon photonic wavelength filter consisting of a ring resonator and an asymmetric MZI (A-MZI), as shown in Figure 16. The size, including the SOA, is very small, only 2.6 × 0.5 mm 2 , about 1/9 of the size of the silicon nitrous tunable laser. The wavelengthtuning range is more than 61.7 ± 0.2 nm, covering the whole optical communication L-band, and SMSR is more than 38 dB. When the SOA injection current is 300 mA, the maximum optical output power is 42.2 mW, achieving a stable single-mode laser output. By optimizing the outer cavity design, a spectral linewidth of less than 100 kHz is obtained. In 2014, Debregeas et al. [28] proposed an integrated tunable laser that combines a reflective SOA (RSOA) with a silicon ring resonator-based outer cavity and MZI. The structure is shown in Figure 15. The external cavity of the laser is composed of two MRRs. The first ring is set to 25 GHz and the second ring is integrated with MZI. In the same year, Tomohiro Kita et al. [29] fabricated a wavelength-tunable laser using a silicon photonic wavelength filter consisting of a ring resonator and an asymmetric MZI (A-MZI), as shown in Figure 16. The size, including the SOA, is very small, only 2.6 × 0.5 mm 2 , about 1/9 of the size of the silicon nitrous tunable laser. The wavelength-tuning range is more than 61.7 ± 0.2 nm, covering the whole optical communication L-band, and SMSR is more than 38 dB. When the SOA injection current is 300 mA, the maximum optical output power is 42.2 mW, achieving a stable single-mode laser output. By optimizing the outer cavity design, a spectral linewidth of less than 100 kHz is obtained. In 2015, Tomohiro Kita et al. [30] proposed a wavelength-tunable laser using silicon photonics to create a compact wavelength-tunable filter with high wavelength selectivity. Two ring resonators and A-MZI are used to realize a silicon photonic wavelength-tunable filter with a wide wavelength-tuning range. A wavelength-tunable laser made by docking a silicon photonic filter and an SOA achieved stable single-mode operation in a wide wavelength range. The size of the chip is 2.5 × 0.6 mm 2 , the laser threshold is 25 mA, the maximum fiber coupling output power is 8.9 mW, and the maximum output power is estimated to be 35 mW. The tuning wavelength range is 99.2 nm (1527.9-1627.1 nm), covering both C-band and L-band. Through the fine control of heating power, the side mode rejection ratio is greater than 29 dB. Figure 17 is a schematic diagram of a silicon photonic tunable laser. The light from the SOA is filtered using two MRRs with slightly different FSRs and an A-MZI, and the FSR is about twice as large as the ring resonator FSRs. Laser wavelength is determined by the Vernier effect between two ring resonators. Wavelength selectivity is defined as the transmittance difference between the main mode and the nearest mode. A large transmittance difference can achieve a stable single-mode laser. Figure 17 is a schematic diagram of a silicon photonic tunable lase the SOA is filtered using two MRRs with slightly different FSRs and an FSR is about twice as large as the ring resonator FSRs. Laser wavelength the Vernier effect between two ring resonators. Wavelength selectivity transmittance difference between the main mode and the nearest mod mittance difference can achieve a stable single-mode laser. In the same year, Rui Tang et al. [31] proposed a narrow line-width tunable laser with a high A-MZI. The laser consists of two silicon ring different perimeters and a highly asymmetric MZI, with significantly path lengths. The calculation and experimental results show that the creases the gain difference between the longitudinal modes. The resu gle-mode oscillation with a narrow band width of 12 kHz, which can wavelength range of 42.7 nm. The surface structure can also be appl resonator filters, regardless of the waveguide type.
The basic structure of the laser is shown in Figure 18. Both structu SOA and an external wavelength-tunable filter. SOA is a gain medium in filter consists of two ring resonators with different perimeters. The Ver ring resonators is used to roughly select the oscillation wavelength. In the same year, Rui Tang et al. [31] proposed a narrow line-width silicon photonic tunable laser with a high A-MZI. The laser consists of two silicon ring resonators with different perimeters and a highly asymmetric MZI, with significantly different optical path lengths. The calculation and experimental results show that the high A-MZI increases the gain difference between the longitudinal modes. The result is a stable single-mode oscillation with a narrow band width of 12 kHz, which can be tuned in the wavelength range of 42.7 nm. The surface structure can also be applied to other ring resonator filters, regardless of the waveguide type.
The basic structure of the laser is shown in Figure 18. Both structures consist of an SOA and an external wavelength-tunable filter. SOA is a gain medium in C-band, and the filter consists of two ring resonators with different perimeters. The Vernier effect of two ring resonators is used to roughly select the oscillation wavelength. Figure 17. Schematic diagram of a silicon photonic tunable laser.
In the same year, Rui Tang et al. [31] proposed a narrow line tunable laser with a high A-MZI. The laser consists of two silico different perimeters and a highly asymmetric MZI, with signifi path lengths. The calculation and experimental results show th creases the gain difference between the longitudinal modes. Th gle-mode oscillation with a narrow band width of 12 kHz, whi wavelength range of 42.7 nm. The surface structure can also b resonator filters, regardless of the waveguide type.
The basic structure of the laser is shown in Figure 18. Both SOA and an external wavelength-tunable filter. SOA is a gain med filter consists of two ring resonators with different perimeters. T ring resonators is used to roughly select the oscillation wavelengt In 2020, Aditya Malik et al. [32] proposed a widely tunable quantum dot laser heterointegrated on a silicon insulator substrate, and its structure is shown in Figure 19. The tuning mechanism is based on the Vernier double-ring geometry, and the tuning range is 47 nm at 52 dB SMSR. When the wavelength filter in the form of MZI is added to the cavity, the SMSR is increased to 58 dB, the tuning range is increased to 52 nm, and the linewidth is as low as 5.3 kHz. linewidth is as low as 5.3 kHz.
For Vernier ring lasers, the linewidth is in the range of 10-20 kHz, but due to poor SMSR at the edge of the gain spectrum, the linewidth is as high as 50 kHz when output wavelength is close to 1290 nm. When MZI is used, better SMSR can be obtain so the linewidth is always less than 10 kHz in the total tuning range. The research development on performance of SINLT-ECSLs in recent years is lis in Table 1. Compared with MRR-integrated external cavity semiconductor las MRR-and-MZI-integrated external cavity semiconductor lasers have the characteristic narrow-band filtering, which can minimize the transmittance of adjacent wavelength the maximum transmittance and improve the wavelength selectivity in the wavegui By changing the temperature of the material through the micro-heater, the refractive dex of the waveguide is changed, and wider wavelength-tuning range is realiz Therefore, MRR-and-MZI-integrated external cavity semiconductor lasers can make lasers obtain narrower linewidth, wider wavelength-tuning range, and higher SMSR.  For Vernier ring lasers, the linewidth is in the range of 10-20 kHz, but due to the poor SMSR at the edge of the gain spectrum, the linewidth is as high as 50 kHz when the output wavelength is close to 1290 nm. When MZI is used, better SMSR can be obtained, so the linewidth is always less than 10 kHz in the total tuning range.
The research development on performance of SINLT-ECSLs in recent years is listed in Table 1. Compared with MRR-integrated external cavity semiconductor lasers, MRR-and-MZI-integrated external cavity semiconductor lasers have the characteristics of narrowband filtering, which can minimize the transmittance of adjacent wavelengths at the maximum transmittance and improve the wavelength selectivity in the waveguide. By changing the temperature of the material through the micro-heater, the refractive index of the waveguide is changed, and wider wavelength-tuning range is realized. Therefore, MRR-and-MZI-integrated external cavity semiconductor lasers can make the lasers obtain narrower linewidth, wider wavelength-tuning range, and higher SMSR. Legend: *-Calculated.

Monolithic Integrated
Monolithic integration mainly refers to the direct epitaxial growth of group III-V compound semiconductor materials on the silicon substrate and synchronous device fabrication process. Due to the high-density thread dislocation in heteroepitaxy, the laser device performance and reliability will be poor due to the direct growth on silicon [33]. However, the gain characteristics can be fine-tuned by changing the growth conditions, so that the device has a long life even when it is epitaxial grown on silicon with high dislocation density [34]. For example, Chen et al. realized high-performance quantum dot lasers on silicon by combining the nucleation layer and dislocation filter layer with in situ thermal annealing and adopting the molecular beam epitaxy (MBE) epitaxial growth method to achieve high-quality GaAS-on-Si layer with low defects. The large lattice mismatch between III-V materials and silicon is no longer an obstacle to the single epitaxial growth of III-V photonic devices on silicon substrates, demonstrating the ability to grow uniformly high-quality III-V materials on the entire Si substrate, which is a significant advance in silicon-based photonics and optoelectronics integration [35].
In 2020, Bahawal Haq et al. [36] produced a C-band monolithic integration laser; its structure is shown in Figure 20. At 20 • C, the threshold current of 80 mA and the maximum single-waveguide coupled output power exceeding 6.9 mW are obtained, with the slope efficiency of 0.27 W/A and the SMSR greater than 33 dB.

Monolithic Integrated
Monolithic integration mainly refers to the direct epitaxial growth of gr compound semiconductor materials on the silicon substrate and synchrono fabrication process. Due to the high-density thread dislocation in heteroepitaxy device performance and reliability will be poor due to the direct growth on s However, the gain characteristics can be fine-tuned by changing the growth c so that the device has a long life even when it is epitaxial grown on silicon dislocation density [34]. For example, Chen et al. realized high-performance qu lasers on silicon by combining the nucleation layer and dislocation filter layer w thermal annealing and adopting the molecular beam epitaxy (MBE) epitaxi method to achieve high-quality GaAS-on-Si layer with low defects. The la mismatch between III-V materials and silicon is no longer an obstacle to the taxial growth of III-V photonic devices on silicon substrates, demonstrating th grow uniformly high-quality III-V materials on the entire Si substrate, which icant advance in silicon-based photonics and optoelectronics integration [35].
In 2020, Bahawal Haq et al. [36] produced a C-band monolithic integratio structure is shown in Figure 20. At 20 °C, the threshold current of 80 mA and mum single-waveguide coupled output power exceeding 6.9 mW are obtained slope efficiency of 0.27 W/A and the SMSR greater than 33 dB.

Heterogeneous Integration
Heterointegration [37] refers to the integration of an epitaxial growth group III-V compound semiconductor with silicon substrate through bonding technology, and then the device manufacturing process. Bonding can be divided into direct bonding, adhesive bonding, and metal bonding. Similar to hybrid integration, heterogeneous integration has the advantage of selecting the best materials for each function (i.e., lasers, low-loss waveguides, detectors), resulting in highly complex photonic integrated circuits (PIC). Thus, heterogeneous integration has all the scaling advantages of monolithic integration, while gaining greater flexibility in material selection, resulting in superior performance. However, because the output power of heterogeneous integration is relatively low, it cannot be well applied in multi-channel communication, and the whole processing process is complicated, the bonding repetition rate is not high, and it is difficult to carry out large-scale mass production.
In 2018, Sulakshna Kumari et al. [38] designed a heterogeneously integrated continuouswave electrically-pumped vertical-cavity Si-integrated laser (VCSIL). Its structure is shown in Figure 21. The VCSIL structure consists of two distinct parts, called the upper half and the lower half. The upper part of the structure is a semi-vertical cavity surface-emitting laser based on GaAs, oxide layer, DBR, and gold film. The lower part of the structure is a SiN waveguide/dielectric DBR combination on a silicon substrate. SiO 2 cladding at the top and bottom prevents the waveguide mode from leaking into the high-refractive-index GaAs semi-vertical cavity surface-emitting laser and the high-refractive-index dielectric DBR and Si substrates. A VCSIL with a 5 µm oxide aperture diameter has a threshold current of 1.13 mA and produces a maximum single-sided waveguide coupled output power of 73 µW at 856 nm. The slope efficiency and the thermal impedance of the corresponding device is 0.085 W/A and 11.8 K/mW, respectively. The SMSR is 29 dB at a bias current of 2.5 mA.
waveguides, detectors), resulting in highly complex photonic Thus, heterogeneous integration has all the scaling advantages o while gaining greater flexibility in material selection, resulting However, because the output power of heterogeneous integra cannot be well applied in multi-channel communication, and th cess is complicated, the bonding repetition rate is not high, and large-scale mass production.
In 2018, Sulakshna Kumari et al. [38] designed a heterog tinuous-wave electrically-pumped vertical-cavity Si-integrated ture is shown in Figure 21. The VCSIL structure consists of two upper half and the lower half. The upper part of the structure surface-emitting laser based on GaAs, oxide layer, DBR, and go the structure is a SiN waveguide/dielectric DBR combination on cladding at the top and bottom prevents the waveguide mod high-refractive-index GaAs semi-vertical cavity surface-em high-refractive-index dielectric DBR and Si substrates. A VCSI erture diameter has a threshold current of 1.13 mA and pro gle-sided waveguide coupled output power of 73 μW at 856 nm the thermal impedance of the corresponding device is 0.085 W spectively. The SMSR is 29 dB at a bias current of 2.5 mA. In 2020, Chao Xiang et al. [39] designed a multilayer hetero laser structure, which can achieve a high-efficiency electro-pum grated Si3N4-based outer cavity. The structure is shown in Figur laser is 6 kHz, and it has good temperature stability and low pha grating provides a narrowband filter together with high extincti In 2020, Chao Xiang et al. [39] designed a multilayer heterointegrated III-V/Si/Si 3 N 4 laser structure, which can achieve a high-efficiency electro-pumped laser in a fully integrated Si 3 N 4 -based outer cavity. The structure is shown in Figure 22. The linewidth of the laser is 6 kHz, and it has good temperature stability and low phase noise. The Si 3 N 4 spiral grating provides a narrowband filter together with high extinction ratio. This results in a large lasing SMSR of over 58 dB. ls 2022, 12, x FOR PEER REVIEW Figure 22. Schematic structure of hetero integrated laser.

Hybrid Integrated
Hybrid integration refers to the integrated assembly of the IIIcon substrate. These can be achieved with prior technology, but ther two devices of different sizes or materials must be aligned to subeffective coupling [40]. However, it can also be improved in certain w as much as possible. For example, Alexander W. Fang et al. [41 structure that can be performed at the wafer, partial wafer, or wafer lasers do not require any critical alignment of the silicon waveguide In addition, this highly scalable structure can be extended to other icon, such as optical amplifiers, modulators, and optical detectors, b ing the III-V structure through processes such as quantum well m wafer bonding.
In 2020, Yeyu Zhu et al. [42] designed the hybrid integration Si3N4 outer cavity and 1.3 μm quantum dot RSOA, and its structure is Chip-scale, tunable, narrow-linewidth hybrid-integrated diode las tum-dot RSOAs at 1.3 μm are demonstrated through butt-coupling photonic integrated circuit. The hybrid laser linewidth is around 85 range is around 47 nm.

Hybrid Integrated
Hybrid integration refers to the integrated assembly of the III-V laser chip and silicon substrate. These can be achieved with prior technology, but there is a drawback that two devices of different sizes or materials must be aligned to sub-micron accuracy for effective coupling [40]. However, it can also be improved in certain ways to avoid defects as much as possible. For example, Alexander W. Fang et al. [41] designed a special structure that can be performed at the wafer, partial wafer, or wafer level so that multiple lasers do not require any critical alignment of the silicon waveguide with III-V materials. In addition, this highly scalable structure can be extended to other active devices on silicon, such as optical amplifiers, modulators, and optical detectors, by selectively changing the III-V structure through processes such as quantum well mixing or non-planar wafer bonding.
In 2020, Yeyu Zhu et al. [42] designed the hybrid integration of low-loss passive Si 3 N 4 outer cavity and 1.3 µm quantum dot RSOA, and its structure is shown in Figure 23. Chipscale, tunable, narrow-linewidth hybrid-integrated diode lasers based on quantum-dot RSOAs at 1.3 µm are demonstrated through butt-coupling to a silicon-nitride photonic integrated circuit. The hybrid laser linewidth is around 85 kHz, and the tuning range is around 47 nm.

Hybrid Integrated
Hybrid integration refers to the integrated assembly of the II con substrate. These can be achieved with prior technology, but th two devices of different sizes or materials must be aligned to su effective coupling [40]. However, it can also be improved in certain as much as possible. For example, Alexander W. Fang et al. [ structure that can be performed at the wafer, partial wafer, or wafe lasers do not require any critical alignment of the silicon waveguid In addition, this highly scalable structure can be extended to othe icon, such as optical amplifiers, modulators, and optical detectors ing the III-V structure through processes such as quantum well wafer bonding.
In 2020, Yeyu Zhu et al. [42] designed the hybrid integrati Si3N4 outer cavity and 1.3 μm quantum dot RSOA, and its structure Chip-scale, tunable, narrow-linewidth hybrid-integrated diode tum-dot RSOAs at 1.3 μm are demonstrated through butt-coupl photonic integrated circuit. The hybrid laser linewidth is around range is around 47 nm. In 2021, Yilin Xu et al. [43] designed a new hybrid-integrated laser, with the structure shown in Figure 24. The device consists of an InP-based RSOA that is connected to a thermally tunable feedback circuit on a silicon photonic (SiP) chip. A photonic wire bond connects the facet of the RSOA to the SiP external-cavity feedback circuit. The assembly is built on a metal submount that simultaneously acts as an efficient heat sink. The photonic wire bonding can be written in situ in a fully automated process and is shaped to fit the size of the mode field and the positions of the chips at both ends, thus providing low loss coupling even with limited placement accuracy. It demonstrates a tuning range from 1515 to 1565 nm along with side-mode suppression ratios above 40 dB and intrinsic linewidths down to 105 kHz. The approach combines the scalability advantages of monolithic integration with the performance and flexibility of hybrid multi-chip assemblies and may thus open a path towards integrated external cavity semiconductor lasers on a wide variety of integration platforms.

Conclusions
SINLT-ECSLs are developing towards wider tuning range, narrower line higher side-mode rejection ratio. Through the selection of gain media, mat grated devices, etc., and the design of new silicon-based outer cavity structu taxial design of SOA is improved, the loss of silicon waveguide is reduced [4 pling efficiency is increased, the reflectivity is reduced, and the stability is en as to meet the application requirements in various fields. High-perform con-based external cavity semiconductor lasers with narrow linewidth o tra-narrow linewidth, wide tuning range, stable output, low noise, small vo low cost are realized. With the development of the information age, silicon-b nal cavity semiconductor lasers will have a broader application market in o munication, coherent detection, and other fields [45]. How to realize wide tun high power, single mode, stable spectral output, and high SMSR laser outpu research direction for the future development of external cavity semiconducto In summary, the advantages of the SINLT-ECSLs over solitary diode la important that their future looks encouraging. This conclusion becomes still ported if we consider multichannel external cavity lasers that are particularly for optical interconnection applications. New materials and configurations a make these lasers still more attractive due to extension of the operation spe and high power. Last, but not least, the SINLT-ECSLs have the potential to be cheaply enough to promote more applications.

Conclusions
SINLT-ECSLs are developing towards wider tuning range, narrower linewidth, and higher side-mode rejection ratio. Through the selection of gain media, materials, integrated devices, etc., and the design of new silicon-based outer cavity structure, the epitaxial design of SOA is improved, the loss of silicon waveguide is reduced [44], the coupling efficiency is increased, the reflectivity is reduced, and the stability is enhanced, so as to meet the application requirements in various fields. High-performance silicon-based external cavity semiconductor lasers with narrow linewidth or even ultra-narrow linewidth, wide tuning range, stable output, low noise, small volume, and low cost are realized. With the development of the information age, silicon-based external cavity semiconductor lasers will have a broader application market in optical communication, coherent detection, and other fields [45]. How to realize wide tuning range, high power, single mode, stable spectral output, and high SMSR laser output is a main research direction for the future development of external cavity semiconductor lasers.
In summary, the advantages of the SINLT-ECSLs over solitary diode lasers are so important that their future looks encouraging. This conclusion becomes still better supported if we consider multichannel external cavity lasers that are particularly interesting for optical interconnection applications. New materials and configurations appear that make these lasers still more attractive due to extension of the operation spectral range and high power. Last, but not least, the SINLT-ECSLs have the potential to be fabricated cheaply enough to promote more applications.