Theoretically, the narrow linewidth laser is a single frequency, i.e., single transverse mode, single longitudinal mode, corresponding to a single spectral laser output in the frequency domain generated only by coherent stimulated radiation, intracavity carrier rise and fall, the optical phase and photon density are in a stable state, with low relative intensity noise and low frequency noise, etc., and at the same time, the excitation wavelength has a very high side-mode suppression ratio.
However, in practice, due to the spontaneous radiation that cannot be eliminated in the active region, the phase and intensity perturbation effects are introduced within the excited radiation mode, making the laser output signal frequency always Gaussian white noise, which leads to an intrinsic Lorentzian line-type broadening of the single laser frequency spectrum, and a certain width of the envelope on the spectrum, and the fluctuation of this quantum noise determines the lower limit of the laser linewidth. This small fluctuation is easily masked by larger fluctuations caused by mechanical/acoustic changes or thermal changes in the external environment, resulting in continued broadening of the laser linewidth, and these classical noise effects determine the upper limit of the laser linewidth. Linewidth describes the frequency or phase noise from a frequency domain perspective, and a narrower linewidth means lower laser frequency or phase noise.
The linewidth is positively correlated with the laser's spontaneous radiation coefficient and linewidth spreading factor; and negatively correlated with the laser's resonant cavity length and output power. The longer the cavity length of the laser, the smaller the intracavity loss, the higher the end reflectivity, the longer the photon lifetime; the higher the output power, the lower the proportion of spontaneous radiation. Therefore, increasing the cavity length and power is an effective way to compress the single longitudinal mode laser line width.
The basic premise of narrow linewidth laser output is to achieve a single longitudinal mode output, narrow linewidth semiconductor lasers are usually integrated in the resonant cavity frequency selection structure or coupled with the mode selection device outside the cavity, to provide optical feedback on a specific frequency, when the passband width of the frequency selection device is less than 2 times the longitudinal mode spacing, you can effectively control the gain and loss of the different longitudinal modes, to ensure that the laser gain bandwidth within the effective gain of only a single longitudinal mode The excitation is obtained only for a single longitudinal mode within the effective gain bandwidth of the laser.
According to the different frequency selection structures distributed inside and outside the active cavity, narrow linewidth semiconductor lasers are generally categorized into internal cavity feedback type and external cavity feedback type lasers.
Internal-cavity feedback narrow-linewidth semiconductor lasers usually integrate Bragg gratings or special waveguide structures inside the active cavity, such as Distributed Feedback (DFB) semiconductor lasers, Distributed Bragg Refl ector (DBR) semiconductor lasers, and coupled-cavity semiconductor lasers. The DFB, DBR, and coupled-cavity semiconductor lasers. Doping of the waveguide layer in long active cavities leads to a dramatic increase in optical loss, which limits the laser power, constrains the increase in active cavity length, and leads to limited laser linewidth compression. Typical DFB and DBR lasers usually use uniform or distributed feedback Bragg grating structures with phase shifts as resonant cavities, withsizes limited to the order of a hundred microns, small quality factors for the resonant cavities, low output powers, and laser linewidths in the range of a few MHz to tens of MHz.
External Cavity Diode Laser (ECDL) is divided into two parts, i.e., active internal cavity to provide gain and passive external cavity to provide feedback. The light emitted from the active gain medium is fed back to the gain medium after passing through the low-loss passive external medium, while the introduction of the low-loss passive external cavity increases the photon lifetime of the system, thus narrowing the linewidth. It should be noted that the narrow linewidth semiconductor laser external cavity is a broad concept, in the strict sense, only when the active cavity for the non-resonant cavity, known as the external cavity structure, such as reflective semiconductor optical amplifier (Reflective Semiconductor Optical Amplifier (RSOA) front surface made of reflective film, the rear surface of the production of high transmittance membrane (The reflectivity of the end face is generally 10-3 ~10-5).
As the rear face optical feedback is too small, the cavity can not form optical oscillation, so only outside the cavity to provide a high enough optical feedback, so that the optical gain in the laser light path round-trip process inside the cavity is higher than the optical loss, in order to form the excitation; another situation is the active cavity for independent excitation of the laser, known as the self-injection structure, the selection of a specific wavelength of the longitudinal mode injected into the laser, resulting in the longitudinal mode in the mode of competition in the priority of the resonance, priority saturation is reached, causing the gain profile in the active region to decrease. However, both are "locked" to the output frequency of the resonant cavity by lengthening the laser cavity length and injecting the selected laser frequency into the resonant cavity through a narrowband feedback element, and the core idea of obtaining a narrower linewidth is the same in both cases.
Planar Light Waveguide Chip (PLC) is an important application of photonic integration technology, which provides more diversified and flexible choices for narrowband filtering and optical feedback devices in external cavity feedback semiconductor lasers. By fabricating waveguide, grating or micro-ring structures on silicon-based materials with low optical loss, such as Silicon on Insulator (SOI), Silicon Dioxide (SiO2) or Silicon Nitride (Si3N4), and then coupling and integrating them with III-V semiconductor gain chips, RSOAs or DFBs through mode-spot transducers or microlenses, the PLCs can improve the photonic density of photon inside the cavity while taking into account the cavity density. The coupling and integration with III-V semiconductor gainer chip, RSOA or DFB through mode spot converter or microlens can improve the photon lifetime and compress the laser linewidth while taking into account the cavity photon density. In addition, a quasi-monolithic structure can be formed by fixing both of them on the same heat sink through bonding process, which helps to reduce the size and cost of the device.
Mr. Chang Lin's group at Peking University has realized an ultra-narrow linewidth hybrid integrated narrow linewidth laser, with the active part being a DFB laser, the passive filtering part being a Si3N4 micro-ring with a quality factor of 2.6 × 108, and the low-limit silicon nitride waveguide structure has reduced the optical transmission loss to 0.1 dB/m, which ultimately realizes a Hz-scale linewidth output.
