Patent Description:
The present disclosure relates to a laser, and in particular to a narrow linewidth laser.

Narrow linewidth laser is a high-quality light source for information communication(sensing) due to its excellent coherence, so it is of high value. The narrow linewidth laser not only can be applied to frontier science research such as high-precision spectral measurement, quantum(atom) frequency standards and the like, but also is a core component of the coherent light communication, distributed optical fiber sensing, thus can be widely applied to the fields of high-capacity laser communication, optical fiber communication and high sensitivity coherent detection and the like. The core competitiveness of the broadband tunable narrow linewidth laser as the light source can be further enhanced by its feature of broadband tuning and narrow linewidth in the application field.

Due to the wide and attractive application prospect of the narrow linewidth laser, numerous researchers and investment organizations devote to study the narrow linewidth laser. The following configurations are one after another adopted: (<NUM>) a configuration assembled by semiconductor gain chip, bulk wavelength selection unit, and linewidth narrowing unit (<CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>); (<NUM>) a configuration assembled by semiconductor gain chip and an optical fiber(<CIT>, <CIT>, <CIT>, <CIT>, <CIT>); (<NUM>) a configuration assembled by a semiconductor laser-pumped all-fiber laser(<CIT>, <CIT>, <CIT>, <CIT>, <CIT>); (<NUM>) a configuration of a waveguide external-cavity semiconductor laser (<CIT>, <CIT>, <CIT>, <CIT> and the like).

Compared with the above hybrid configurations of the narrow linewidth lasers, a monolithically integrated semiconductor laser is the favorite in various application fields due to its advantages of compact structure, small size, light weight, low energy consumption, high reliability and long lifetime. However, the traditional DFB or DBR laser has limited cavity length, although the wavelength is selected using a complex grating structure, the narrow linewidth is hard to be obtained, directly resulting in limitation on the application of the DFB or the DBR laser in the above fields.

In <NUM>, some researchers have proposed that a narrow linewidth laser can be achieved based on a weak-coupled single ring external cavity, however, due to loss in the ring is too large, a single ring external-cavity laser has never been realized (<NPL>)). Hereafter, a plurality of research organizations achieve a waveguide external-cavity narrow linewidth semiconductor laser by transforming this thought (OSA/OFC/ NFOEC2010/OThQ5, <NPL>), <NPL>), <NPL>), <NPL>), <NPL>), <NPL>)), <NPL>), <NPL>), <NPL>), <NPL>), <NPL>), <NPL>). Although the laser has achieved the chip-level narrow linewidth performance, all lasers here are based on dielectric photonic integrated circuits serving as linewidth narrowing units and wavelength tuning units, III-V SOA serving as a gain region, the dielectric photonic integrated circuits and the III-V SOA are hybrid integrated to form the narrow linewidth tunable semiconductor laser. Moreover, due to the fact that these dielectric waveguide photonic integrated circuits acting as wavelength selection and tuning, and linewidth reduction units are particularly complex, refractive indexes of a plurality of resonant cavities usually need to be adjusted by the electro-optic or thermal-optic effect to obtain the optimal performance, such as broadband tuning range, narrow linewidth, and high side-mode-suppression-ratio, which results in the complex electrothermal management and slow tuning speed.

This applicant has also previously proposed a monolithically integrated narrow linewidth laser (<CIT>). However, these monolithically integrated narrow linewidth lasers cannot be employed in a corresponding information system due to their lower output power and lower side-mode-suppression ratio. <CIT> discloses a semiconductor optical element. Two or more first reflectors are formed on a substrate. Each of the first reflectors reflects the light input to its input port and returns it there, while exhibiting a reflection spectrum featuring a peak at the target wavelength. A first optical coupler is formed on the substrate to divide the light output from an optical amplifier and output the divided lights to the input ports of the two or more first reflectors, as well as combining the reflected lights from the first reflector and re-inputting the combined light to the optical amplifier. Each of the first reflectors contains a ring resonator of the same size, and the delay for the light input to the input port of a first reflector to return there after being reflected is the same for all first reflectors.

An objective of the present disclosure is to provide a narrow linewidth laser to overcome the shortcomings in the prior art.

To achieve the objective, the present disclosure provides the following technical solutions:
A narrow linewidth laser is provided by some embodiments of the present disclosure, comprising a passive ring resonant cavity, a U-shaped FP (Fabry Perot) resonant cavity enveloping the passive ring resonant cavity and a first gain region, wherein the passive ring resonant cavity and the FP resonant cavity are combined to form an M-Z (Mach-Zehnder interference structure) compound external cavity structure such that two free ends of the U-shaped FP resonant cavity forms respectively a first end and a second end of the M-Z compound external cavity structure, the first end is provided with the first gain region, a direct and continuous transmission path around the passive ring resonant cavity is formed between the first end and the second end of the M-Z compound external cavity structure for allowing a transmission directly from the first end to the second end or from the second end to the first end, the M-Z compound external cavity structure is at least used for providing wavelength selection and narrowing laser linewidth, and the first gain region is provided on the outer side of the M-Z compound external cavity structure and is at least used for providing a gain for the whole laser.

In some embodiments, preferably, the passive ring resonant cavity and the FP resonant cavity are coupled in parallel to form the M-Z compound external cavity structure; and/or a refractive index of a waveguide of any one of the passive ring resonant cavity and the FP resonant cavity is adjustable, thus achieving a wavelength tuning function.

In some embodiments, preferably, the narrow linewidth laser further comprises a second gain region, the second gain region is provided on the outer side of the M-Z compound external cavity structure and is at least used for amplifying lasing light, improving output power, enhancing optical feedback, and further narrowing the linewidth and suppressing the noise.

In some embodiments, preferably, the laser is of a monolithically integrated configuration, wherein the first gain region is further provided with a grating structure, which forms a distributed feedback (DFB) laser or a distributed Bragg (DBR) laser together with the first gain region; the second gain region comprises a semiconductor optical amplifier; the laser configuration comprises a heterogeneously integrated configuration, a butt-joint integrated configuration or a micro-assembled integrated configuration; in all these configurations, the first gain region comprises the DFB laser or the DBR laser, and the second gain region comprises a semiconductor optical amplifier.

In some embodiments, preferably, the laser is of a monolithically integrated configuration, monolithic integration of which is achieved through any one of technologies of quantum well intermixing, butt-joint epitaxy, selective area epitaxy and vertical coupling integration.

In some embodiments, preferably, the laser further comprises an additional PN junction region or MOS junction region, wherein the additional PN junction region or MOS junction region is embedded in the M-Z compound external cavity structure; the additional PN junction region is a gain region and is at least used for compensating the loss of the M-Z compound external cavity structure, thus extending an external cavity optical path, enhancing optical negative feedback and narrowing the laser linewidth; or, the additional PN junction region or MOS junction region is transparent to the lasing wavelength, and a refractive index of the additional PN junction region or MOS junction region can be rapidly adjusted when performing electrical injection or reverse bias on the additional PN junction region or the MOS junction region, thus achieving rapid tuning of the lasing wavelength with lower power consumption.

In some embodiments, preferably, the laser is a monolithically integrated configuration, and the first gain region is further provided with a grating structure, which forms a distributed feedback (DFB) laser or a distributed Bragg (DBR) laser together with the first gain region; the second gain region comprises a semiconductor optical amplifier; the additional PN junction region comprises a semiconductor optical amplifier or a waveguide transparent to the lasing light; the additional MOS junction region comprises a waveguide transparent to the lasing wavelength; the laser configuration comprises a heterogeneously integrated configuration, a butt-joint integrated configuration or a micro-assembled integrated configuration; in all these configurations, the first gain region comprises the DFB laser or the DBR laser, the second gain region comprises the semiconductor optical amplifier, the additional PN junction region comprises the semiconductor optical amplifier or a waveguide transparent to the lasing light, or the additional MOS junction region is a waveguide transparent to the lasing light.

In some embodiments, preferably, the laser is formed by combining a semiconductor gain chip and the M-Z compound external cavity structure formed by coupling the FP resonant cavity and the passive ring resonant cavity in parallel.

In some embodiments, preferably, the M-Z compound external cavity structure is a low-loss waveguide structure, the low-loss waveguide structure comprises a waveguide made of the same system of materials which is transparent with respect to a gain material, or a waveguide composed of a material system different from the gain region, or a waveguide made of a mixture of the same system of materials and the different material system, preferably an InP, GaAs-based In(Ga)As(P) waveguide, an InGa(Al)As(P) waveguide, an SOI-based Si waveguide, an SiN waveguide, a lithium niobate waveguide, an SiNO waveguide, an SiO<NUM> waveguide, an In(Ga)As(P)/Si or Si/SiO<NUM>/InGaAsP hybrid waveguide.

Compared with the prior art, the narrow linewidth laser provided by the embodiments of the present disclosure has a simple structure and can achieve broadband tunability, and high side-mode suppression ratio, narrow linewidth, high output power. Moreover, such narrow linewidth laser is simple in tuning mode, rapid in tuning speed, and low in tuning energy consumption.

To describe the technical solutions in the embodiments of the present disclosure or the prior art more clearly, the following briefly introduces the accompanying drawings required for describing the embodiments or the prior art. Apparently, the accompanying drawings in the following description show merely some embodiments in the present disclosure.

The following describes the technical solutions in the embodiments of the present disclosure with reference to the accompanying drawings in the embodiments of the present disclosure. Optional features are defined by the appended dependent claims.

As shown in <FIG>, in a first embodiment of the present disclosure, a narrow linewidth laser comprises a first gain region SOA1, a passive ring resonant cavity, and an FP resonant cavity,.

wherein the first gain region is used for providing a gain for the whole laser, and the passive ring resonant cavity and the FP resonant cavity forms an M-Z compound external cavity structure. Further, the M-Z compound external cavity structure may serve as linewidth narrowing, wavelength tuning and frequency selecting units of the laser.

With respect to DFB or DBR lasers, the narrow linewidth laser belongs to external-cavity semiconductor lasers; compared with other external cavity structures, the principle of the narrow linewidth laser is as follows: the passive ring resonant cavity and the FP resonant cavity can provide resonant peaks with different free spectral ranges, thus achieving wavelength selection using vernier effect of the two resonant cavities, and then a wavelength tuning function can be achieved by changing a refractive index of a waveguide of any of the resonant cavities; and an M-Z interferometer structure formed by coupling the passive ring resonant cavity and the FP resonant cavity in parallel can further achieve wavelength selection. These effects are combined to form a wavelength selective reflection mirror, thus acquiring stable lasing output with the improved side-mode-suppression-ratio. Moreover, the ring resonant cavity may also extend an optical path to effectively narrow the linewidth; in addition, the compound waveguide external cavity can further narrow the linewidth by utilizing optical negative feedback effect.

In accordance with the present disclosure, the vernier effect refers to a method to achieve intensity modulated resonance peak of the resonance frequency spectrum in the whole compound cavity by connecting two resonant cavities with different optical lengths in series or in parallel, and further to achieve broadband tuning by changing the refractive index of any of the resonant cavities.

Further, as shown in <FIG>, to further improve the device performance, in a second embodiment of the present disclosure, a second gain region SOA2 can also be further integrated at the outer side of the M-Z compound external cavity structure; on one hand, the output power can be improved, and the linewidth can be narrowed and the noise can be suppressed; on the other hand, the feedback strength of the gain region in the laser can be adjusted to achieve narrow linewidth stable output for the laser under strong feedback.

Further, as shown in <FIG>, in a third embodiment of the present disclosure, an additional PN junction region can be further embedded in the passive ring resonant cavity; the additional PN junction region is also a gain region, and can be named a third gain region SOA3 for compensating external cavity loss, thus extending an external cavity optical path and enhancing optical negative feedback to effectively narrow the laser linewidth. More specifically, the third gain region is embedded in the passive ring resonant cavity, on one hand, the loss in the ring can be compensated, on the other hand, the refractive index change is introduced through current injection to achieve phase matching of the lasing wavelength of the laser, thus acquiring a high side-mode suppression ratio and narrow linewidth; and the vernier effect caused by the refractive index change can make the lasing wavelength of the laser be rapidly adjustable in a large range.

Further, as shown in <FIG>, in a fourth embodiment of the present disclosure, a third gain region SOA3 can also be embedded in the M-Z compound external cavity structure. On one hand, the third gain region SOA3 is used for compensating the external cavity loss, extending an external cavity optical path, enhancing optical negative feedback and narrowing the laser linewidth; on the other hand, as the third gain region is located on a coherent arm of the M-Z compound external cavity structure, the refractive index of the third gain region can be changed by changing the current injection or reverse bias of the third gain region, thus making the laser achieve rapid tuning based on the vernier effect and the M-Z interference effect, but low-power consumption.

Further, as shown in <FIG>, in a fifth embodiment of the present disclosure, the first gain region SOA1 and the second gain region SOA2 can be simultaneously integrated on both sides of the FP resonant cavity respectively, and the third gain region SOA3 can be embedded in the passive ring resonant cavity, thus making the laser have the advantages of the first embodiment to the third embodiment.

Further, as shown in <FIG>, in a sixth embodiment of the present disclosure, the first gain region SOA1 and the second gain region SOA2 can be simultaneously integrated on both sides of the FP resonant cavity respectively, and the third gain region SOA3 can be embedded in the M-Z compound external cavity structure, thus making the laser have the advantages of the first embodiment to the third embodiment.

Further, as shown in <FIG>, in a seventh embodiment of the present disclosure, the first gain region SOA1 and the second gain region SOA2 can be simultaneously integrated on both sides of the FP resonant cavity respectively, and another additional PN junction region SOA3' or MOS junction region can be embedded in the M-Z compound external cavity structure; the additional PN junction region or MOS junction region SOA3' can be formed by performing modification processing or alteration on the third gain region SOA3, or can be formed by depositing a dielectric film and a semiconductor film after removing the third gain region part; the additional PN junction region or MOS junction region SOA3' is transparent to the lasing wavelength of the whole laser; and by performing current injection or reverse bias on the additional PN junction region or MOS junction region SOA3', a laser with broadband rapid tuning, narrow linewidth, and low-power consumption can be achieved, and the tuning quality is improved.

Moreover, for the devices of the fifth embodiment, the sixth embodiment and the seventh embodiment, the laser with narrow-linewidth, broadband tuning, and high-power output can be obtained under electric management.

Further, as shown in <FIG> and <FIG>, in an eighth embodiment of the present disclosure, the DFB or DBR laser and the second gain region SOA2 can be simultaneously integrated on both sides of the FP resonant cavity respectively, and an additional SOA3 (as shown in <FIG>) or SOA3' (as shown in <FIG>) is embedded in the M-Z compound external cavity structure; the SOA3' is a PN junction region or MOS junction region; the additional PN junction region or MOS junction region SOA3' may be formed by performing modification processing or alteration treatment on the third gain region SOA3, or can be formed by depositing a dielectric film and a semiconductor material after removing the third gain region part; the additional PN junction region or MOS junction region SOA3' is transparent to the lasing wavelength; by performing electric injection or reverse bias on the additional PN junction region or MOS junction region SOA3', rapidly frequency-modulated and narrow linewidth laser with low power consumption can be achieved.

Further, as shown in <FIG>, in the eighth embodiment of the present disclosure, the DFB or DBR laser and the second gain region SOA2 can be integrated on the same side of the FP resonant cavity respectively, and an additional SOA3 (as shown in <FIG>) or SOA3' (as shown in <FIG>) can be embedded in the M-Z compound external cavity structure with a view of achieving high-power narrow linewidth frequency-modulated laser.

The devices of the first embodiment to the eighth embodiment can be achieved based on a semiconductor optoelectronic device substrate and a semiconductor monolithic integration technology or a hybrid integration technology thereon. For example, in some embodiments, as shown in <FIG>, the method for fabricating any of the above-mentioned narrow linewidth lasers by the monolithic integration technology may comprises the following steps:.

For example, in some more specific embodiments, as shown in <FIG>, a method for fabricating any of the previous narrow linewidth lasers by the monolithic integration technology may comprises the following steps:.

Further, in the above fabrication method, the procedure of modifying the third gain region SOA3 can also be added to convert the third gain region to be transparent to the lasing light of the device, by which, the electrically injected or reverse biased PN junction or MOS junction can be achieved, and the device configurations shown in <FIG> or <FIG> should be obtained.

Further, on basis of a primary test, an anti-reflection film can be deposited at a light-emitting facet of a fabricated chip to avoid residual reflection, and a high-reflection film can be deposited on one side of a non-light-emitting facet to enhance light feedback, thus narrowing the linewidth and suppressing the noise.

Compared with other chip-level narrow linewidth semiconductor lasers, the narrow linewidth laser provided by the embodiments of the present disclosure is simple in waveguide structure, simple in tuning management, and it is expected to obtain broadband rapidly tuning with low power consumption, high side-mode suppression ratio, and narrow linewidth.

Compared with other technological processes (such as hybrid integration of a semiconductor active region defined on a semiconductor substrate and an M-Z compound external cavity defined on an SOI substrate through a butt-joint coupling), the monolithically integrated narrow linewidth laser formed by the embodiments of the present disclosure at least has the following advantages: ① coupling loss of the external cavity and the gain region can be greatly eliminated, thus effectively reducing a threshold value and narrowing linewidth; ② the same material has similar thermal and mechanical properties, thus the device is good in stability and reliability and has the increased susceptibility to severe environment; ③ the devices are uniform and low cost due to their batch design and fabrication; ④ it is expected to achieve electric tuning, high tuning speed, good tuning linearity, and low tuning power consumption.

<FIG> is a spectrum of a laser in an embodiment as shown in <FIG>. It can be known that the laser has a high side-mode suppression ratio and broadband tuning function. <FIG> is a frequency noise spectrum of the laser, and based on the noise level at <NUM> frequency section of this frequency noise spectrum, it can be extracted that the laser has a Lorentzian linewidth of the order of kHz level.

In some other embodiments, for the lasers of the first and second embodiments, the device can also be achieved through butt-joint integration, optical micro-assembling and other technical solutions, while the basic configuration of the device is to be presented as a semiconductor gain chip and an M-Z compound external cavity structure formed by coupling the FP resonant cavity and the ring resonant cavity, wherein the gain chip of the device can be any gain unit with required wavelengths, which comprises but is not limited to semiconductor gain chips with various substrates and wavelength ranges, an optical fiber gain unit, a solid (gas) gain unit and the like; the M-Z compound external cavity can be In(AlGa)As(P) waveguide, SOI waveguide, Si waveguide, SiN waveguide, SiNO waveguide and any other waveguide structure which is transparent and low loss with respect to the gain units, thus guaranteeing that the broadband tunable narrow linewidth laser can be achieved; wherein the gain region SOA1 part may select a semiconductor optical amplifier, a DFB laser or a DBR laser; and SOA2 and SOA3 generally take the gain region as the semiconductor optical amplifier. In some cases, the SOA3 can also be replaced with the previous SOA3', i.e., the PN junction or MOS junction.

Claim 1:
A narrow linewidth laser, comprising a passive ring resonant cavity, a U-shaped Fabry Perot (FP) resonant cavity enveloping the passive ring resonant cavity, and a first gain region (SOA1), wherein the passive ring resonant cavity and the FP resonant cavity are combined to form a Mach-Zehnder (M-Z) compound external cavity structure such that two free ends of the U-shaped FP resonant cavity forms respectively a first end and a second end of the M-Z compound external cavity structure, the first end is provided in the first gain region (SOA1), a direct and continuous transmission path around the passive ring resonant cavity is formed between the first end and the second end of the M-Z compound external cavity structure for allowing a transmission directly from the first end to the second end or from the second end to the first end; the M-Z compound external cavity structure is at least used for providing wavelength selection and narrowing laser linewidth; and the first gain region (SOA1) is provided on the outer side of the M-Z compound external cavity structure and is at least used for providing a gain for the whole laser.