Research Progress on Narrow-Linewidth Broadband Tunable External Cavity Diode Lasers

Abstract

Narrow-linewidth broadband tunable external cavity diode lasers (NBTECDLs), with their broadband tuning range, narrow linewidth, high side-mode suppression ratio (SMSR), and high output power, have become important laser sources in many fields such as optical communication, spectral analysis, wavelength division multiplexing systems, coherent detection, and ultra-high-speed optical interconnection. This paper briefly describes the basic theory of NBTECDLs, introduces NBTECDLs with diffraction grating type, fiber Bragg grating (FBG) type, and waveguide type, and conducts an in-depth analysis on the working principles and performance characteristics of NBTECDLs based on different NBTECDL types. Then, it reviews the latest research progress on Littrow-type, Littman-type, FBG-type, and waveguide-type NBTECDLs in detail and compares and summarizes the characteristics of Littrow-type NBTECDLs, Littman-type NBTECDLs, FBG-type NBTECDLs, and waveguide-type NBTECDLs. Finally, it looks at the structural features, key technologies, optical performance, and application fields of the most cutting-edge research in recent years and summarizes the challenges and future development directions of NBTECDLs.

1. Introduction

In 1962, R. N. Hall et al. [1] successfully developed the GaAs diode laser (DL), marking the first experimental confirmation of DL. After over 60 years of development, the fabrication process of DLs has gradually matured, and the variety of devices has become more diverse. The NBTCDL has excellent temporal coherence and plays a crucial role in many cutting-edge scientific research fields such as coherent optical communication [2], wavelength division multiplexing systems [3], optical sensing [4], light detection, and ranging (LiDAR) [5,6] and precision spectroscopy. With the continuous improvement of laser coherence, the requirements for laser linewidth measurement technology have also increased, which has promoted the rapid development of narrow-linewidth lasers, achieving ultra-narrow-linewidth outputs as low as sub-hertz [7,8]. NBTCDL features broadband tunability, a narrow linewidth, high SMSR, and high output power [9,10,11,12]. Despite significant progress, this field still faces many technical bottlenecks and challenges. It is difficult for NBTCDL to maximize all performance indicators simultaneously. High-performance tunable lasers, especially NBTECDLs, have complex structures and high manufacturing process requirements, leading to high costs and limiting their large-scale application. Achieving fully monolithic integration of NBTECDL with performance comparable to discrete devices remains a challenge.

This paper aims to review the research progress on NBTECDLs, introducing the structures and principles of Littrow-type NBTECDLs, Littman-type NBTECDLs, FBG-type NBTECDLs, and waveguide-type NBTECDLs. Then, we summarize the performance of the four types of NBTECDL, focusing on key parameters such as tuning range, output power, linewidth, and SMSR. Finally, we summarize the advantages and disadvantages of different types of external cavities. The Littrow-type NBTECDL achieves an extremely wide tuning range of up to 1080 nm using a diffraction grating [13]. The Littman-type NBTECDL exhibits excellent spectral purity and output power, with an SMSR of 71.03 dB [14] and a power of 49.8 W [15]. The FBG-type NBTECDL has a simple structure and good wavelength stability and controllability and is easy to couple with optical fibers [16]. The waveguide-type NBTECDL can achieve a linewidth as low as 0.2998 kHz [17].

2. The Basic Theory of NBTECDLs

The basic structure of an NBTECDL consists of three parts: a semiconductor gain chip, a collimator, and an external cavity optical feedback element. The NBTECDL has developed various external cavity devices, among which the external cavity mirror represents the most fundamental structure. The basic structure is shown in Figure 1. r₁: The reflection coefficient of the left facet of the gain chip. r₂: The reflection coefficient of the right (output) facet of the gain chip. r₃: The reflection coefficient of the external cavity mirror. d: The physical length of the gain chip. L: The length of the external optical cavity. The right end face of the semiconductor laser is combined with a mirror to form a composite mirror. The semiconductor gain chip serves as the active region, providing optical amplification; the external cavity optical feedback element feeds the light back to the gain chip to achieve mode frequency selection and narrow linewidth.

The working principle of NBTECDLs is based on external cavity feedback and wavelength selection mechanisms. When a current is injected into the semiconductor gain chip, light is generated by spontaneous emission. After passing through the wavelength selection of the external cavity feedback element, only light of specific wavelengths can form effective feedback and establish stable laser oscillation within the cavity. By adjusting the wavelength tuning element, the wavelength of the feedback light can be changed, thereby achieving wavelength tuning of the laser output. This structure not only enables a broadband tuning range but also allows for a narrower linewidth and higher frequency stability compared to traditional diode lasers.

The external cavity structure of an NBTECDL has a significant impact on performance. The spectral linewidth is an important performance parameter of NBTECDLs, and the theoretical formula for the NBTECDL spectral linewidth is expressed as follows [18,19]:

$$∆\nu ={\displaystyle \frac{{\alpha }^2{\beta }^2{R}_{sp}\xi }{4\pi S}}+{\displaystyle \frac{R_{sp}\xi }{4\pi \left(S+S_e\right)}}$$

(1)

where $\alpha$ is the linewidth enhancement factor, and $\beta$ is the field amplitude, which is derived from the longitudinal mode phase condition of the entire cavity. $S$ and $S_e$ are the number of photons in the gain medium and the external cavity, respectively, while $R_{sp}$ represents the spontaneous radiation and $\xi$ is the external cavity length factor.

$R_{sp}$ can be expressed as follows:

$$R_{sp}={\displaystyle \frac{v_g^{2}hvg\left(v\right)a_{m}S}{2P_{out}}}$$

(2)

where h is Planck’s constant, ν is the frequency, $a_m$ is the threshold loss of the laser, and Pout is the output power.

Power is another important performance parameter of NBTECDLs. Power can be calculated using the carrier and photon rate equations [20]. The results show that the power is inversely proportional to the linewidth. The formula is as follows:

$$P_{out}\propto K{N}_{ph}={\displaystyle \frac{{\eta }_i\mathsf{\Gamma }{\tau }_{ph}}{qd}}\left[j-j_{th}\left(K\right)\right]{\displaystyle \frac{K}{1-K{\alpha }_m/\left({\alpha }_i+{\alpha }_m\right)}}$$

(3)

where j is the current density, ${\eta }_i$ is the internal cavity efficiency, $q$ is the elementary charge of an electron, τ is the carrier lifetime, Γ is the confinement factor, ${\tau }_{ph}$ is the photon lifetime, ${\alpha }_m$ is the mirror loss, ${\alpha }_i$ is the absorption coefficient of external cavity, $N_{ph}$ is the photon density, and K is the feedback strength.

Equations (1)–(3) indicate that the linewidth and power of the NBTECDL are affected by multiple factors. By choosing the appropriate external cavity structure and optimizing the laser parameters, the output performance can be significantly improved.

3. The Structural Types of NBTECDLs

According to differences in the designs of the resonant cavity structure and the external cavity feedback optical components, NBTECDLs are mainly classified into diffraction grating-type, fiber grating-type, and waveguide-type NBTECDLs, etc. The mode selection devices for external cavity lasers mainly include types such as diffraction grating, FBG, and waveguide.

3.1. Diffraction-Grating-Type NBTECDL

The external cavity laser of the diffraction-grating-type NBTECDL is generally rotated and tuned through a micro-electromechanical system (MEMS). According to the rotating grating and the rotating mirror, it can be divided into two types: the Littrow type and the Littman type.

3.1.1. Littrow-Type NBTECDL

The Littrow-type NBTECDL is generally composed of a gain chip, a collimating lens and a diffraction grating. The laser beam emitted from the right end face of the gain chip is collimated by the collimating lens and then made incident to the diffraction grating. By rotating the diffraction grating and changing the diffraction angle, diffraction occurs. After frequency selection by the diffraction grating, the first-order diffraction beam is fed back along the original optical path to the active region of the gain chip to form laser oscillation, and the laser is outputted from the zero-order diffraction direction of the grating or the rear end face of the chip. The Littrow type achieves wavelength tuning by rotating the grating to change the diffraction angle. A Littrow-type NBTECDL is shown in Figure 2 [21].

3.1.2. Littman-Type NBTECDL

The Littman-type NBTECDL is obtained by adding a mirror to the Littrow-type NBTECDL. In this type, the position of the grating is in a fixed state, and tuning is achieved by rotating the mirror to change the diffraction wavelength. Compared with the Littman-type external cavity laser, the Littrow-type external cavity laser does not require a mirror and has a relatively simple structure and a small volume. However, the position of the outputted light will change with the rotation of the grating or be outputted through some reflector surfaces at the gain chip end. The Littman-type external cavity laser is relatively more complex and has a larger volume compared to the Littrow external cavity laser. However, the laser it emits undergoes two reflections, and its resolution is higher than that of the Littrow type, enabling a narrower linewidth to be obtained. Furthermore, the Littman–Metcalf structured external cavity laser does not require a rotating grating, and the output laser position is fixed, thus being more conducive to integrated packaging. A Littman-type NBTECDL is shown in Figure 3 [21].

The lasing wavelength of the diffraction-grating-type NBTECDL simultaneously satisfies the phase condition formula of the laser and the grating equation [22]:

$$\lambda =2L/q$$

(4)

$$\lambda =2d\mathit{sin}\theta$$

(5)

where $\lambda$ is the lasing wavelength, L is the cavity length of the external cavity laser, q is the number of modes, d is the grating constant, and $\theta$ is the incident angle (equal to the first-order diffraction angle).

3.2. Fiber-Grating-Type NBTECDL

An FBG structure was introduced to the NBTECDL owing to the continuous advancement of fiber grating technology. The gain chip is coupled with the FBG in the external resonant cavity through a tapered fiber lens to form an FBG-type NBTECDL. The structure is shown in Figure 4 [23].

The functional essence of FBG is a mirror with narrowband high reflection characteristics. It feeds back specific wavelengths within the bandwidth of the Bragg reflection peak into the gain chip, interacts with light in the active region, increases the photon lifetime of this oscillation mode, reduces its threshold, and obtains single-mode output through mode competition. Coupling LD and FBG with conical optical fibers can effectively reduce the reflected light at the end face of LD and suppress the F–P oscillation mode in the inner cavity. The wavelength of the FBG Prague center determines the lasing wavelength of the laser [24]:

$${\lambda }_B=2n_{eff}\mathsf{\Lambda }$$

(6)

where ${\lambda }_B$ is the Bragg center wavelength, $n_{eff}$ is the refractive index of the fiber core, and Λ is the grating period. Simultaneously changing the Bragg wavelength of the grating and the length of the outer cavity of the laser can achieve the continuous tuning of the laser. The FBG-type NBTECDL usually uses mechanical methods, temperature, current, and other methods to stretch the FBG to achieve wavelength tuning.

3.3. Waveguide-Type NBTECDL

Optical waveguides are formed by using refractive index changes or gain distributions to constrain the laser’s propagation along a specific path, thereby enhancing the stability and directionality of the mode. Feedback is provided through external mirrors or diffraction gratings to replace the traditional inner cavity cleavage surface, achieving more flexible wavelength selection and line width compression. The waveguide-type NBTECDL restricts the laser propagation path through the waveguide structure and uses the external cavity to provide optical feedback, achieving efficient and stable laser output.

Waveguide-type NBTECDLs are usually composed of a semiconductor optical amplifier (SOA) and an integrated photonic chip outer cavity coupling. The SOA provides gain amplification, and the integrated photonic chip outer cavity is responsible for wavelength selection. Figure 5 shows the typical structure of the NBTECDL with a silicon-based waveguide. The light waves coupled to the silicon baseline waveguide by SOA are filtered through two micro-ring resonators (MRRs). The principle is to set the radii of the two MRRs slightly different, and the free spectral ranges (FSRs) are also different [25]:

$$FSR={\lambda }^2/2\pi r{n}_{eff}$$

(7)

where $\lambda$ is the laser wavelength, $r$ is the radius of MRR, $n_{eff}$ is the effective refractive index of the MRR silicon waveguide. The transmitted light spectra of two MRRs are superimposed on each other, and the wavelengths of the matching peaks are determined by mode competition to determine the selectance wavelengths. Through the thermal-optical effect, by adjusting the Pt heater, the FSR of the MRR changes, causing the transport peak to shift. Then, through the cursor effect, wavelength tuning can be carried out over a wide range. The silicon-based waveguide-type NBTECDL with double MRRs is shown in Figure 5 [25].

At present, the commonly used MRR optical waveguides include Si-based, SiON-based, Si₃N₄-based, etc. waveguides. With the development of the new generation of MRR-ECDL, the linewidth can reach the order of hundreds of Hz, and the wavelength tuning range can reach more than hundreds of nm. However, the coupling loss between the waveguide and the gain chip is still relatively high, which limits the increase in the laser output power.

4. Research Progress on NBTECDL

In recent years, the NBTECDL has been extensively studied by researchers due to its broadband tuning range and narrow linewidth. This article will introduce the latest research progress on diffraction-grating-type, FBG-type, and waveguide-type NBTECDLs.

4.1. Research Progress on Diffraction-Grating-Type NBTECDL

The diffraction-grating-type NBTECDL is divided into Littrow-type NBTECDLs and Littman-type NBTECDLs based on its unique external cavity structure.

4.1.1. Research Progress on Littrow-Type NBTECDL

Littrow type NBTECDL has the advantages of a narrow linewidth, broadband tuning range, simple structure, and high output power. Researchers have conducted a large number of studies on it.

In 2016, Bayrakli et al. [26] proposed a Littrow-type NBTECDL for active frequency stabilization using edge stabilization techniques. The line width of this laser reaches 400 Hz, achieving a wavelength tuning range of 60 nm.

In 2017, Ding et al. [12] proposed a Littrow-type NBTECDL based on GaN LD. By paralleling the grating lines with the pn junction plane of the gain chip, the spectral line width was narrowed from 1 nm to 0.1 nm, and the output power of the central wavelength was as high as 1.24 W. They applied GaN material to the Littrow structure to break through the high-power output in the blue light band. This produced a tunable high-power blue laser with a tuning range of 3.6 nm (443.9–447.5 nm).

In 2017, Li Bin et al. [27] constructed a Littrow-type NBTECDL using a reflective holographic grating. The output wavelength of this laser was 410 nm, achieving a narrow linewidth of 50 pm, an SMSR greater than 20 dB, a tuning range of up to 2 nm, and a maximum output power of 500 mW. Reflected holographic gratings are used to replace traditional etched gratings, providing new ideas for subsequent “grating type optimization (improving damage resistance)”.

In 2018, Guo et al. [28] proposed an implementation scheme of a Littrow-type NBTECDL with the characteristics of a wide range without mode jump and a constant output beam direction. This addressed the pain points of “tuning mode jump” and “beam shift” in traditional Littrow structures. When the initial external cavity length was 12.82 mm, a mode-free continuous tunable laser output of 4.34 nm could be achieved in the 805 nm band, and the lateral offset of the output beam was only 0.033 mm.

In 2019, Podoskin et al. [29] investigated the Littrow structure outer cavity based on reflective diffraction gratings and high-power diode lasers based on low optical loss asymmetric heterostructures. The continuous wave light output power of the NBTECDL could reach 13 W, and the linewidth was 0.15 nm. The results show that reducing the length of the laser chip to 1500 µm could extend the tuning range of the laser spectrum to 100 nm and the SMSR to 45 dB.

In 2020, Kapasi et al. [30] reported a 2 μm band narrow-linewidth tunable laser for gravitational wave detection. The Littrow-type external cavity structure was composed of commercial SAF gain chips and diffraction gratings. PZT drive grating was used to replace traditional mechanical tuning and improve tuning accuracy; the line width reached 20 kHz within an integration time of 10 ms. The available laser output power exceeded 9 mW and could reach 15 mW at the high current of the diode. The wavelength tuning range was 120 nm.

In 2021, Wang Yan et al. [31] developed a Littrow-type NBTECDL based on InAs/InP quantum dot lasers in the 1.5 µm band, with a tuning range of 92 nm and a maximum output power of 6.5 mW at a current of 500 mA.

In 2021, Giraud et al. [32] reported an external cavity inter-band cascade broadband tunable laser based on the Littrow structure which achieved broadband tuning of 3.22–3.58 μm (360 nm) at room temperature, with a maximum output power of up to 13 mW and an SMSR better than 35 dB.

In 2022, Ismail B [33] reported a tunable dual-mode external cavity quantum cascade laser (QCL) based on dual Littrow and dual Littman configurations. In the dual Littrow setup, the tuning range from 4450 nm to 4850 nm was achieved by rotating the diffraction grating. The linewidth and maximum optical power of the dual Littrow setting were determined to be 0.08 cm⁻¹ and 40 mW, respectively.

In order to achieve a narrow linewidth while maintaining a high SMSR, in 2023, Wang Y et al. [34] proposed an ultra-narrow-linewidth NBTECDL using a wide interference filter and a diffraction grating. The Lorentz linewidth reached 13.6 kHz at a wavelength of 1555 nm. It achieved a wide adjustable range of 40 nm at an injection current of 200 mA, with a maximum output power of 25.6 mW. The maximum SMSR was 57 dB at 1555 nm.

In 2024, Niu S et al. [13] reported a long-wave infrared QCL with a peak wavelength of 10.5 μm. In the pulse mode and continuous wave mode, the maximum power of the Fabry–Perot laser at room temperature was 1.4 W and 0.32 W, respectively. In the Littrow external cavity configuration, in the pulsed mode, at fixed voltages of 12.6 V and 10.4 V, the wavelength of the laser could be tuned within the range of λ~8.59–9.67 μm and 10.20–10.68 μm, respectively.

In 2025, Singh N et al. [35] designed a compact structure for the Cesium D2 line, and a miniaturized adaptation of Littrow-type NBTECDL was realized, providing the possibility of quantum experimental equipment integration. By analyzing the power spectral density to characterize the frequency noise and determine the linewidth of the NBTECDL, the measured linewidth was 472 ± 24 kHz.

In 2025, Yao C et al. [36] proposed a NO₂ Scheimpflug system of an NBTECDL based on the Littrow structure. In this system, the laser used a piezoelectric transducer (PZT) for precise wavelength tuning, and the emission wavelengths were 448.1 nm and 449.7 nm. The output power was 2.97 W and the linewidth was 0.16 nm.

Littrow-type NBTECDLs have excellent performances.

Table 1. The latest research progress on Littrow-type NBTECDLs.

Type	Wavelength	Tunable Range	Output Power	Linewidth	SMSR	Year
Littrow	-	60 nm	-	400 Hz	30 dB	2016 [26]
Littrow	410 nm	2 nm	500 mW	50 pm	20 dB	2017 [27]
Littrow	1035	100 nm	13 W	0.15 nm	45 dB	2019 [29]
Littrow	2 μm	120 nm	15 mW	20 kHz	-	2020 [30]
Littrow	1.5 μm	92 nm	6.5 mW	-	-	2021 [31]
Littrow	-	360 nm	13 mW	-	35 dB	2021 [32]
Littrow	4620 nm	400 nm	40 mW	0.08 cm−1	40 dB	2022 [33]
Littrow	1555 nm	40 nm	25.6 mW	13.6 kHz	57 dB	2023 [34]
Littrow	10.5 μm	1080 nm	1.4 W	-	-	2024 [13]
Littrow	448.1 nm	-	2.97 W	0.16 nm	-	2025 [36]

summarizes research progress on Littrow-type NBTECDLs in recent years, highlighting the significant advancements in this field.

From 2016 to 2025, research on Littrow-type NBTECDLs has achieved continuous breakthroughs across performance dimensions, though trade-offs between parameters and application-specific limitations remain evident. In terms of strengths, early progress (2016–2019) focused on foundational performance optimization: Bayrakli et al. [26] realized a narrow 400 Hz linewidth with a 60 nm tuning range via edge stabilization. Later studies (2020–2025) expanded into specialized bands and ultra-precision: Kapasi et al. [30] developed a 2 μm band version for gravitational wave detection (20 kHz linewidth, 120 nm tuning, 15 mW high-current power); compactness and practicality also advanced, such as Singh N et al.’s [35] Cesium D2 line compact laser design and Yao C et al.’s [36] PZT-tuned system for NO₂ detection (2.97 W power, dual emission wavelengths). However, limitations persist: early low-power models [27] sacrificed range for linewidth (50 pm); some broadband designs [12] have narrow ranges despite high power (1.24 W), etc. Overall, the research trend is clear: Littrow-type NBTECDLs are evolving toward wider tuning ranges (from 2 nm to 360 nm), narrower linewidths (from 0.15 nm to 400 Hz), higher-power outputs (from 500 mW to 13 W), and specialized band adaptation (visible, near-infrared, mid-infrared, long-wave infrared), while integrating practical features (compactness, PZT precision tuning) for fields like gravitational wave detection and gas sensing.

4.1.2. Research Progress on Littman-Type NBTECDL

With the development of semiconductor fabrication processes and optical coating technologies, the Littman-type NBTECDL can achieve a narrow linewidth and broadband tuning range.

In 2016, Luo W et al. [37] reported a 6.9 µm band external cavity QCL based on the Littman–Metcalf structure. By rotating the mirror, continuous tuning within the tuning range of 1340–1640 cm⁻¹ was achieved, with a range exceeding 300 cm⁻¹. The linewidth was less than 0.14 cm⁻¹.

In 2017, Jimenez A et al. [38] proposed a small NBTECDL based on three structures: the micro-Littman structure, bulk holographic grating, and micro-transmission grating. From the traditional Littman–Metcalf NBTECDL to the combination of micro-Littman and compound-grating NBTECDLs, the volume was reduced, the integration degree was enhanced, and features were included such as high output power, a narrow linewidth, broadband tunability, and high SMSR, enabling portability and handheld device operation. The power of this laser exceeded 50 mW, the SMSR reached 60 dB, the line width was less than 100 kHz, and the tuning range reached the GHz level.

In 2018, Shirazi et al. [39] developed an NBTECDL using the transmission mode to select components. The Littman free space configuration was used to demonstrate laser operation. The line- and bandwidths of this source were 0.27 nm (~110 GHz) and 43 nm, respectively, at a center wavelength of 860 nm. The output power was 20 mW at an operating current of 150 mA. The output laser beam obtained from the transmitted light of the diffraction grating achieved a tuning range of 52 nm.

In 2018, Chichkov et al. [40] conducted research on a 3.2 µm GasB-based cascade Type I quantum well tunable laser. This laser adopted the improved Littman–Metcalf structure. A grating with a line density of 450 lines/mm was selected as the tuning element of the external resonant cavity. The gain chip was a cascade-pumped GaSb chip with a narrow ridge waveguide structure. The continuous wave output power of this laser reached 8 mW at room temperature. The wavelength tuning range exceeded 300 nm (in the 3 µm band).

In 2019, Hoppe et al. [41] proposed a novel Littman-type NBTECDL based on MEMS. Replacing traditional mechanical components with MEMS addressed the issues of slow tuning speed and mode hopping. After the laser formed oscillations in the cavity, it was coupled and output from the back of the semiconductor gain chip. The MEMS rotating mirror could simultaneously change the cavity length and angle and reduce the harmonic length without mode skipping, and the tuning speed could reach the kHz level.

In 2020, Morten Hoppe et al. [42] designed a GaSb NBTECDL based on the Littman structure. The Littman-type NBTECDL comprised a collimated diode laser chip, a MEMS mirror, and a reflection grating. Replacing traditional mechanical components with MEMS addressed the issues of slow tuning speed and mode hopping. It could be continuously tuned at 120 nm (1480 nm to 1600 nm), with an SMSR greater than 50 dB.

In 2021, Morten H et al. [43] reported on an MEMS-type NBTECDL with wavelengths covering 1980–2090 nm. A GaSb-based laser diode with a curved waveguide and an MEMS driver driven by resonance were adopted to achieve the NBTECDL with fast tuning and no mode jump. At different central wavelengths, it had a mode-free frequency range from 8.4 to 34 GHz.

In 2022, Ismail B et al. [33] reported on a tunable dual-mode external cavity QCL based on dual Littrow and dual Littman configurations. In the dual Littman configuration, by rotating the reflector, the spectral range was between 4460 and 4790 nm, achieving a heading dual-mode tuning range of 330 nm. By modulating the injection current within a few milliseconds without moving any components, a fine continuous tuning range of over 10 nm could be achieved. The linewidth and maximum optical power of the dual Littman configuration were 0.01 cm⁻¹ and 35 mW, respectively.

In 2023, Sheng L et al. [44] reported a narrow-linewidth NBTECDL without mode skipping and experimentally studied its tuning characteristics. The NBTECDL achieved a continuous wavelength tuning range of 100 nm from 1480 nm to 1580 nm without mode hopping. The SMSR exceeded 65.54 dB and the linewidth was less than 98.27 kHz.

In 2023, Naoaki K et al. [45] proposed a dynamic spectral-width-tunable Littman-type NBTECDL in this study, which replaced the flat end mirror of the external cavity with a curved one with a tunable radius of curvature (RoC). The simulation revealed the tuning range with a single-mode spectral width of 250 kHz, multi-mode spectral width of 1.2–47 GHz, and ASE of 50 GHz–3.9 THz. The dynamic spectral width adjustable Littman-type NBTECDL is shown in Figure 6.

In 2025, Liwen Sheng et al. [14] developed a wide mode jump and narrow-linewidth tunable diode laser tube source, which adopted a Littman–Metcalf configuration with a diffraction grating. A continuous and mode-free wavelength tuning range of approximately 180 nm was achieved within the S+C+L band, with a spectral line width of less than 32.95 kHz. Furthermore, the device maintained an optical signal-to-noise ratio of over 62.60 dB and an output power higher than 10.11 dBm throughout the entire tuning range.

In 2025, Wang X et al. [15] constructed a dual-mode combined Littman external cavity using a transmission grating. At a driving current of 2.4 A and a cooling temperature of 20 °C, the external cavity of the blue diode laser achieved an output power of 49.8 W, a linewidth of 0.321 nm, and a tuning range of 0.817 nm, and the spectral line locking efficiency of the external cavity was approximately 80.6%.

In 2025, Wang X et al. [46] constructed a dual Littman structure by achieving a narrow linewidth and dual-wavelength output through a diode laser split along the slow axis. When one wavelength was locked at 447.23 nm and the driving current was 1.5 A, the tuning range of the other wavelength was from 443.92 nm to 450.61 nm. By adjusting the angle of the reflector and changing the energy ratio of the two feedback beams, it can be observed that the longest interval between the locked peaks was 4.02 nm (6.06 THz frequency interval). When the wavelength interval was fixed at 1.4 nm, the output power was 1.12 W and the light-to-light efficiency was 51.32%.

In 2025, Wang X et al. [47] proposed a spectral beam combination configuration based on a locked wavelength array, with a blue diode array as the basic unit and combined along the slow axis. The Littman external cavity structure could adjust the wavelength of each array to optimize the beam quality. At an injection current of 2.0 A, the spectral width of the output beam was 2.718 nm, the power was 40.1 W, and the SBC efficiency was 86.05%.

Littman-type NBTECDLs have excellent performances such as a narrow linewidth and broadband tuning range.

Table 2. The latest research progress on Littman-type NBTECDLs.

Type	Wavelength	Tunable Range	Output Power	Linewidth	SMSR	Year
Littman	1.4 μm		50 mW	100 kHz	60 dB	2017 [38]
Littman	3.2 μm	300 nm	8 mW	-		2018 [40]
Littman	1550 nm	120 nm	-	-	50 dB	2020 [42]
Littman	2020 nm	110 nm	7.1 mW	100 kHz	53 dB	2021 [43]
Littman	4620 nm	330 nm	35 mW	0.01 cm−1	40 dB	2022 [33]
Littman	1550 nm	100 nm	16.95 dBm	98.27 kHz	65.54 dB	2023 [44]
Littman	1590 nm	180 nm	10.11 dBm	32.95 kHz	71.03 dB	2025 [14]
Littman	443.265 nm	0.817 nm	49.8 W	0.321 nm	-	2025 [15]
Littman	447.23 nm	6.69 nm	1.12 W	0.93 nm	-	2025 [46]

summarizes research progress on Littman-type NBTECDLs in recent years.

Research on Littman-type NBTECDLs has achieved continuous advancements in tuning range, linewidth, integration, and power. Early breakthroughs focused on expanding mid-infrared coverage [37] and miniaturization [38]. Subsequent studies optimized core performance: Hoppe et al. [41,42] introduced MEMS rotating mirrors, enabling kHz-level tuning speed, 120 nm mode-free tuning (1480–1600 nm), and SMSR > 50 dB, resolving traditional mechanical tuning inefficiencies. Later innovations pushed precision and multi-functionality [33,44]. High-power and array applications also emerged in 2025 [28,47]. Limitations persist, however: early mid-infrared models [40] had low power (8 mW); high-power designs [15] sacrificed tuning range (only 0.817 nm); MEMS-based systems [41] may face long-term stability issues from micro-mirror wear; and dual-mode/array structures [33,47] increase complexity and cost. Overall, the research trend is clear: Littman-type NBTECDLs are evolving toward broadband tunable, narrower-linewidth, higher integration, multi-mode/dual-wavelength functionality, and high-power industrial adaptation.

4.1.3. Comparative Analysis of Diffraction-Grating-Type NBTECDL

Table 3. Comparison of characteristics between Littrow-type NBTECDL and Littman-type NBTECDL.

Type	Littrow	Littman
Maximum Tuning (nm)	1080	330
Minimum Linewidth (kHz)	13.6	32.95
Maximum Power (W)	13	49.8
Maximum SMSR (dB)	57	71.03
Volume	small	big
Structure	simple	complex

shows a comparison of the characteristics of NBTECDLs with different structures. It is not difficult to find that the Littrow-type NBTECDL has a wide wavelength tuning range, up to 1080 nm, can achieve fine tuning, and has a simple structure. Based on the Littrow-type NBTECDL, the Littman-type NBTECDL introduces a mirror, enabling the laser to select the wavelength after being diffracted twice by the grating. Compared with the Littrow-type NBTECDL, the Littman-type NBTECDL has better model selectivity. It can achieve a narrow linewidth while obtaining a high laser output power and retaining its wavelength tuning ability. However, due to the introduction of additional optical components, the complexity of the system has been further increased, and the loss of the entire system has become higher.

4.2. Research Progress on FBG-Type NBTECDLs

Research on FBG-type NBTECDLs abroad started earlier. Initially, the F–P cavity coupled with the outer cavity of the FBG studied by the British Telecom laboratory achieved a linewidth output of 50 kHz. Subsequently, Bell Laboratories in the United States and NTT Corporation in Japan also carried out research on the outer cavity lasers of the FBG. In recent years, domestic scientific research institutions such as the Institute of Semiconductors of the Chinese Academy of Sciences and the Shanghai Institute of Optics and Fine Mechanics have also conducted research on FBG-type NBTECDLs.

In 2017, Zhang et al. [48] designed a mode-free NBTECDL using glued and filled V-slot packaging technology, achieving a continuous mode-free tuning range of up to 0.5 nm (5 times the cavity pitch), a narrow linewidth of 35 kHz, and a high linear thermal tuning speed of 65 pm/°C (8.125 GHz/°C).

In 2018, Yan Z et al. [49] modeled the theoretical model of FBG using the transfer matrix method (TMM). The influences of the length of the Bragg grating and the modulation depth on the reflectance, the spectral width of the grating, and the wavelength were studied. Then 976 nm laser modules with different grating reflectances (i.e., 4%, 6%, 8% and 10%) were fabricated. Optimizing the reflectivity of FBG via TMM simulation, the SMSR was optimized to above 40 dB, and the threshold current was decreased.

In 2019, Yang et al. [50] reported a 1550 nm continuously tunable laser using chirped FBG and SOA. The line width of the laser was less than 0.03 nm and the SMSR was greater than 25 dB. It could be continuously tuned to approximately 48 nm (1526.02–1573.91 nm), and the power variation within the tuning range was only 1.46 dB.

In 2020, Song-Sui L et al. [51] conducted a numerical study on the periodic 1 (P1) oscillation of diode lasers under the optical feedback of narrowband FBG. The FBG had a narrow bandwidth less than the laser relaxation oscillation frequency. FBG feedback could reduce microwave linewidth and phase noise by maintaining a stronger feedback power, and improved the side peak rejection ratio (SPSR) by filtering the external cavity mode. The FBG feedback reduced the microwave linewidth by more than an order of magnitude and increased the SPSR by more than two orders of magnitude compared to the mirror feedback using the same delay time.

In 2021, Antoine C et al. [52] achieved an InGaN-based diode laser tube with an emission wavelength of 400 nm based on an FBG. Research has expanded the short wavelength band. This device had an SMSR of 44 dB and an output power of mW. Detailed frequency noise analysis revealed the sub-MHz integral line width and the 16 kHz natural linewidth.

In 2022, Su et al. [16] reported on a frequency-stabilized narrow linewidth FBG-type NBTECDL for lidar used in carbon dioxide detection. The FBG-type NBTECDL module was integrated in a compact butterfly package, with a transmission wavelength of 1572.02 nm, a continuous tuning frequency of 22 GHz (approximately 0.173 nm), and an SMSR greater than 50 dB. At an injection current of 340 mA, the output power was 30 mW. The linewidth of the NBTECDL was 15 kHz.

In 2022, Jingjing Y et al. [53] developed a novel single-chip dual-wavelength conical semiconductor laser tube based on a composite distributed Bragg reflection (DBR) grating, leading to a significant improvement in integration density. At a ridge current of 190–300 mA and a heat sink temperature of 20 °C, the conical diode laser tube achieved dual-wavelength emissions at wavelengths of 1025.4 nm and 1036.2 nm, respectively. The line widths of both wavelengths were less than 36 pm, and the minimum power difference between the two wavelengths was 1.2 dB. By adjusting the heat sink temperature of the conical semiconductor laser tube from 13.8 °C to 20.7 °C, the spectral distance between the two wavelengths could be modulated to 10.4 nm to 10.9 nm.

In 2022, Ye L et al. [54] introduced a 405 nm external cavity semiconductor laser using a volumetric Bragg grating (VBG) as the feedback element. The emission wavelength of the semiconductor laser was stably locked at 405.1 nm, and the spectral line width was 0.08 nm. The output power reached 292 mW, and the wavelength was reduced to 0.0006 nm/°C with temperature drift.

In 2024, Jiaqi C et al. [55] demonstrated the NBTECDL with linear polarization and a narrow linewidth. In polarization-maintaining fibers (PMF), birefringent Bragg gratings fabricated by femtosecond laser point-by-point technology were used to provide external cavity feedback. The NBTECDL achieved an output power greater than 60 mW and a polarization–extinction ratio (PER) greater than 30 dB, addressing the issue of no polarization control in traditional FBG. The Lorentz line width of 2.58 kHz was obtained based on the delay self-heterodyne beat frequency measurement.

In 2024, Duraev V et al. [56] reported the manufacturing results of a single-frequency tunable semiconductor laser module with a wavelength of 1550 nm, whose outer cavity was based on an FBG formed in a single-mode optical fiber. The laser module could generate dynamic and stable single-frequency radiation. The side mode suppression exceeded 40 dB, the laser linewidth was 100 kHz, and the output optical power was higher than 50 mW. The wavelength of the emission spectrum of the laser module was tuned to 2 nm.

In 2025, Huang Z et al. [57] proposed a dual-wavelength output composite grating DFB laser, which achieved the selection of two longitudinal modes through transversely coupled gratings and ridge gratings of different periods. At a current of 0.4 A and a voltage of 2.83 V, the device achieved an output power of 78.28 mW, with a threshold current of approximately 0.13 A. When the working current was between 0.28 A and 0.35 A, the device maintained a stable dual-wavelength output with a wavelength separation of approximately 0.75 nm. The maximum measured dual-wavelength output power was 71.95 mW.

The FBG-type NBTECDL has the advantages of simple structure, low noise output, small size, low cost, good stability and low cost. The line width of this laser can reach several kHz, or even below 1 kHz, which is its most prominent feature. However, due to the limited tuning method, the wavelength tuning range is very small.

Table 4. The latest research progress on FBG-type NBTECDLs.

Type	Wavelength	Tunable Range	Output Power	SMSR	Linewidth	Year
FBG	1550.8 nm	0.5 nm	1 dBm	50 dB	35 kHz	2017 [48]
FBG	976 nm	-	41.5 mW	44.6 dB	-	2018 [49]
FBG	1550 nm	48 nm	-	25 dB	0.03 nm	2019 [50]
FBG	400 nm	-	1 mW	44 dB	16 kHz	2021 [52]
FBG	1572.02 nm	0.173 nm	30 mW	50 dB	15 kHz	2022 [16]
VBG	405 nm	-	292 mW	-	0.08 nm	2022 [54]
FBG	-	-	60 mW	30 dB	2.58 kHz	2024 [55]
FBG	1550 nm	2 nm	50 mW	40 dB	100 kHz	2024 [56]
DFB	785 nm	0.75 nm	71.95 mW	-	-	2025 [57]

summarizes the research progress on FBG-type NBTECDLs in recent years. At present, the preparation process of FBG-type NBTECDLs is relatively mature and can be widely applied in the fields of distributed fiber optic sensing, coherent spectral analysis, synthetic aperture radar, etc.

From 2017 to 2025, research on FBG-type NBTECDLs has yielded substantial progress, with core strengths centered on targeted performance optimization, scenario-specific adaptation, and functional diversification. The study [48] leveraged glued-and-filled V-slot packaging to develop a mode-free NBTECDL, achieving a 0.5 nm continuous mode-free tuning range, 35 kHz narrow linewidth, and 65 pm/°C high linear thermal tuning speed. Subsequent breakthroughs expanded key metrics: Yang et al. [50] utilized chirped FBGs and semiconductor optical amplifiers (SOAs) for a 1550 nm laser with a 48 nm tuning range and only 1.46 dB power fluctuation, and Jiaqi C et al. [55] integrated femtosecond-laser-written birefringent FBGs in polarization-maintaining fibers (PMFs) to achieve an NBTECDL with a 2.58 kHz ultra-narrow Lorentz linewidth and 60 mW output power. Huang Z et al.’s [57] composite-grating DFB laser (78.28 mW output, stable 0.75 nm separated dual wavelengths). Scenario-specific designs were refined too, such as Su et al.’s [16] butterfly-packaged FBG-based NBTECDL for CO₂ detection lidar.

However, there are limitations: some studies exhibit a narrow tuning range [16] (0.173 nm), certain devices still have significant room for linewidth optimization (100 kHz) [56], and dual-wavelength devices feature a small wavelength spacing [57], which makes it difficult to meet the requirements of some wide-spacing applications. FBG-type NBTECDLs are advancing toward the simultaneous realization of a narrow linewidth, high power, a wide tuning range, a high SMSR/PER, and low wavelength drift. Meanwhile, they are developing in the direction of being tailored for scenarios such as lidar and gas detection, supporting characteristic outputs including dual-wavelength and linear polarization, and achieving integrated miniaturization.

4.3. Research Progress on Waveguide-Type NBTECDL

The waveguide-type NBTECDL has the characteristics of a narrow linewidth and broadband tuning range of the outer cavity, and at the same time possesses the features of low loss, high integration and high reliability of single-chip integrated lasers. Due to its excellent characteristics, it has been favored by researchers.

In 2016, Youwen Fan et al. [58] demonstrated the first integrated InP-Si₃N₄ hybrid laser. The outer cavity of the exciter was composed of two cascaded MRRs, which were used to perform single-frequency operation under a narrow-band spectral bandwidth, achieving a tuning range of over 43 nm, with an SMSR of 35 dB and a minimum linewidth of 90 kHz.

In 2017, H. Ishii et al. [59] used a DFB array structure as the light source and mixed and integrated the lens group with a 129 mm long waveguide on a SiO₂/Si platform; ultimately narrow linewidths of less than 10 kHz were obtained over the entire C-band. However, the 129 mm long waveguide resulted in an excessively large overall size of the device.

In 2018, Yu Li et al. [60] coupled commercial FP diode lasers with external high-Q value micro-resonators. The laser achieved a wavelength tuning range of 17 nm, a narrow linewidth of 8 kHz, an output power of 7 dBm (50 mW), and an SMSR of 45 dB. The narrow tuning range fails to meet the requirements of wide-wavelength-band applications.

In 2019, Zhu et al. [61] demonstrated a hybrid integrated dual-band laser of InP/GaAs RSOA and SiN. The laser provides tunable external cavities for both InP and GaAs gain chips simultaneously. The spectral linewidth-divisions are 18 kHz and 70 kHz, and the SMSR divisions are 52 dB and 46 dB. The tuning ranges of 46 nm and 38 nm were achieved, respectively, at 1.55 μm and 1 μm.

In 2020, Fan et al. [62] fabricated a hybrid integrated InP-Si₃N₄ NBTECDL with an intrinsic linewidth of 40 Hz, a maximum output power of 23 mW (fiber coupling), spectral coverage greater than 70 nm (C-band) at 3 mW, and SMSR > 60 dB. The study achieved a narrow linewidth and high SMSR, yet suffered from insufficient output power and tuning flexibility.

In 2021, Xiang et al. [63] integrated a laser on a SiN waveguide, achieving a sub-khz linewidth and an output power of tens of milliwatts. In the same year, the traditional DFB laser was self-injected and locked to the Si₃N₄ micro-ring resonator. The noise was reduced by five orders of magnitude, achieving a frequency noise of 0.2 Hz²/Hz and a short-term linewidth of 1.2 Hz.

In 2021, Dass et al. [64] demonstrated a new type of InP-Si₃N₄ dual laser module. Each one was fabricated using a hybrid coupling of InP-based SOA and low-loss Si₃N₄ feedback circuits. The work reduced coupling loss and improved integration density. This work proposed the NBTECDL can be tuned over 100 nm while maintaining a SMSR greater than 50 dB and RIN approximately −160 dB/Hz.

In 2022, Mateus Z C et al. [65] achieved coarse tuning up to 12.5 nm and mode-free fine-tuning up to 33.9 GHz using micron-scale Si₃N₄ resonators and commercial Fabry–Perot semiconductor laser tubes, with an inherent linewidth as low as several thousand Hertz. Furthermore, we also demonstrated a fine-tuning speed of up to 267 GHz μs⁻¹, a fiber-coupled power of up to 10 mW, and a typical side-mode rejection ratio higher than 35 dB.

In 2023, Chen et al. [66] reported a hybrid integrated Si₃N₄ NBTECDL with full C-band wavelength stability and narrow-linewidth output. The InP gain chip was coupled with the Si₃N₄ dual micro-ring and then integrated with the AlGaInAs quantum well rib waveguide SOA through dual collimating lens coupling. A wavelength tuning range of 55 nm, an SMSR exceeding 50 dB, a high output power of 220 mW, and a linewidth less than 8 kHz were obtained.

In 2023, Nouman Z et al. [67] first demonstrated a widely tunable GaSb/Si₃N₄ hybrid laser with a wavelength of approximately 2 μm. The study expanded the long-wavelength band. The hybrid laser demonstrated a relatively high continuous output power of 15 mW, a low threshold current of 100 mA, and a broadband tuning range of approximately 90 nm, covering wavelengths between 1937 nm and 2026 nm.

In 2025, Li S et al. [17] introduced the application of a four-layer Nb₂O₅/SiO₂ thin film system in RSOA, which showed a 3 dB spectral width of 79.4 nm in its spontaneous emission spectrum, covering the wavelength range from 1497.2 nm to 1576.6 nm. Within this range, the ripple remained below 1 dB, and the ripple near 1550 nm was as low as 0.5 dB. Under the test conditions of 25 °C and 180 mA, the line width of the external cavity laser was 299.8 Hz, and the output power was 12.6 mW. A four-layer Nb₂O₅/SiO₂ thin film system was introduced to optimize the spectral ripple of the RSOA, continuously approaching the “quantum-limited narrow linewidth”.

Table 5. The latest research progress on waveguide-type NBTECDLs.

Type	Wavelength	Tunable Range	Output Power	SMSR	Linewidth	Year
waveguide	1550 nm	45 nm	1.7 mW	35 dB	90 kHz	2016 [58]
waveguide	1550 nm	35 nm	1.5 mW	45 dB	10 kHz	2017 [59]
waveguide	1555 nm	17 nm	50 mW	45 dB	8 kHz	2018 [60]
waveguide	1.55 μm	46 nm	-	52 dB	18 kHz	2019 [61]
waveguide	1.55 μm	70 nm	23 mW	60 dB	40 Hz	2020 [62]
waveguide	1553 nm	100 nm	10 dBm	50 dB	-	2021 [64]
waveguide	488 nm	12.5 nm	10 mW	35 dB	7 kHz	2022 [65]
waveguide	1550 nm	55 nm	220 mW	50 dB	8 kHz	2023 [66]
waveguide	2 μm	89 nm	15 mW	25 dB	-	2023 [67]
waveguide	1550 nm	79.4 nm	12.6 mW	3 dB	299.8 Hz	2025 [17]

summarizes the performance of the waveguide-type NBTECDL. For the waveguide-type NBTECDL, its greatest advantage lies in its high integration, flexible selection of frequency selection components, and the ability to achieve a long effective cavity length. It has a narrow linewidth, broadband tuning range and noise performance, and has a very good commercial preparation and application prospect.

From 2016 to 2025, research on waveguide-type NBTECDLs has achieved continuous breakthroughs, with core advantages focusing on the progressive optimization of key performance indicators and the expansion of application scenarios. The study [58] pioneered InP-Si₃N₄ hybrid lasers with a tuning range of over 43 nm and SMSR of 35 dB. Subsequent works continuously narrowed the linewidth from 90 kHz (2016) to sub-kHz, even 1.2 Hz [63] and 299.8 Hz [17], while expanding the tuning range to 100 nm [64] and extending the wavelength to the 2 μm band [67]. Additionally, the output power was significantly improved (up to 220 mW) [66] and the SMSR was enhanced to over 60 dB [62], with material systems evolving from InP-Si₃N₄/GaAs to GaSb-Si₃N₄ and four-layer Nb₂O₅/SiO₂ [17], and integration structures advancing from cascaded MRRs and DFB arrays to dual-band lasers and NBTECDLs. However, there are still limitations: some early studies had narrow tuning ranges (17 nm) [60], high-power designs occasionally sacrificed linewidth performance [66], and non-C-band lasers (2 μm) [67] had lower output power (15 mW) than C-band counterparts, with partial reliance on commercial devices restricting full integration. The overall trend is clear: hybrid integrated lasers are moving toward diversified material systems, broader wavelength coverage, synergistic improvement of multi-performance metrics (narrower linewidth, broadband tunable, higher power, higher SMSR), and higher integration density, gradually meeting the demands of high-performance photonic applications.

4.4. Comparative Analysis of Different Types of NBTECDLs

NBTECDLs have received extensive attention due to their superior characteristics such as their broadband tuning range and narrow linewidth. This paper mainly introduces research progress on diffraction-grating-type NBTECDLs, FBG-type NBTECDLs, and waveguide-type NBTECDLs.

Table 6. Comparison of performance parameters of different types of NBTECDLs.

Type	Littrow	Littman	FBG	Waveguide
Tunable Range (nm)	1080	330	48	100
Linewidth (kHz)	13.6	32.95	2.58	0.2998
Output Power (mW)	13,000	49,800	292	220
SMSR (dB)	57	71.03	50	60
Cost	low	high	low	low
Structure	simple	complex	simple	small

compares the performance parameters of different types of NBTECDLs. NBTECDLs adopt external cavity feedback technology to greatly narrow linewidth, generally reaching the kilohertz level. Even the waveguide-type NBTECDL has reached hundreds of Hertz. The diffraction-grating-type NBTECDL directly achieved a broadband tuning range by changing its grating angle, resulting in a high output optical power. However, mechanical tuning is generally adopted, and the tuning speed is relatively slow. The FBG-type NBTECDL features a simple structure, good wavelength stability and controllability, and easy coupling with optical fibers. It is widely applied in many optical communication and sensing systems. However, the tuning range of the laser is only tens of nanometers, which limits its application in other areas. Thanks to the development of silicon-based photonics technology, waveguide-type NBTECDLs have received widespread attention in recent years. The waveguide-type NBTECDL has excellent characteristics such as a compact structure, low cost, large-scale production, integrated packaging, and small size. It can achieve a narrow linewidth while having a broadband tuning range.

Table 6 shows the performance of different types of NBTECDLs. In comparison, it is not difficult to find that the tuning range of the Littrow-type NBTECDL is the widest, reaching 1080 nm. The waveguide-type NBTECDL achieves a narrow linewidth of 0.2998 kHz. NBTECDLs are developing towards a narrow linewidth, broadband tuning range, high output power, and other directions.

5. Conclusions and Prospects

NBTECDLs, as an important optical device, have made remarkable progress in the past few decades. By continuously optimizing the diode gain chip, the external cavity feedback element, and the wavelength tuning mechanism, the performance of the laser has been significantly improved and its application scope has also been continuously expanded. In the future, with the introduction of new materials and new structures, as well as the development of integrated and intelligent technologies, NBTECDLs will play an important role in a wider range of fields.

The application scope of NBTECDLs is mostly used in the communication band and the visible light band, and the exploration of applications in the mid-infrared band is very limited. The mid-infrared band encompasses several transparent windows of the atmosphere and many fingerprint absorption lines of molecules, making it highly suitable for various photon sensors or devices with high social impact. The future development of NBTECDLs will focus on performance improvement, integration, and the expansion of new applications. NBTECDLs have demonstrated irreplaceability in traditional fields, while emerging technologies such as integrated photonics, intelligent control, and new materials are driving them towards greater compactness, stability, and multi-functionality. In the future, NBTECDL may become a core component of disruptive technologies such as quantum Internet and on-chip spectrometers and further penetrate the industrial market.