An external-cavity semiconductor laser includes a semiconductor laser element containing a gallium nitride material, a first lens disposed in an optical path of light emitted from the semiconductor laser element, a wavelength selective element disposed in an optical path of light transmitted through the first lens and configured to selectively transmit light having a predetermined wavelength, a second lens disposed in an optical path of light transmitted through the wavelength selective element, an output coupler disposed in an optical path of light condensed through the second lens, and a light-transmissive protective member bonded to at least one surface of the output coupler. The second lens is configured to cause light transmitted through the second lens and incident on the output coupler to form an image on a surface of the output coupler. The protective member covers the surface of the output coupler on which the image is formed.

A mode-locked solid state laser apparatus including an optical film, a gain medium crystal, a Fabry-Perot element, a first mirror, a second mirror, a third mirror and an output coupler is disclosed. The optical film is configured to receive a pumping light having a first wavelength incident in a first direction. The gain medium crystal receives the pumping light passing the optical film, and generates an initial laser beam having a second wavelength, wherein the initial laser beam forms a first optical path starting at one end thereof from the gain medium crystal. The Fabry-Perot element is disposed on the other end of the first optical path opposite to the one end, and reflects the initial laser beam along a second optical path having one end thereof starting from the Fabry-Perot element. The first mirror is disposed on the other end of the second optical path opposite to the one end of the second optical path, and reflects the initial laser beam along a third optical path having one end thereof starting from the first mirror.

Systems and methods for a self-injection locked SBS laser are provided herein. In certain embodiments, a system includes a pump laser source providing a pump laser; an SBS resonator receiving the pump laser through a first port and scattering some of the pump laser to provide an SBS laser through the first port, wherein a frequency shift of Brillouin scattering within the SBS resonator is an integer multiple of a free-spectral range for the SBS resonator; a filter receiving the pump laser on a first filter port and the SBS laser on a second filter port, wherein the pump laser is output through the second filter port and the SBS laser is output through a drop port; and a pump laser path coupling the output pump laser into the pump laser source, wherein a frequency of the pump laser becomes locked to a resonance frequency of the SBS resonator.

Systems and methods for a self-injection locked SBS laser are provided herein. In certain embodiments, a system includes a pump laser source providing a pump laser; an SBS resonator receiving the pump laser through a first port and scattering some of the pump laser to provide an SBS laser through the first port, wherein a frequency shift of Brillouin scattering within the SBS resonator is an integer multiple of a free-spectral range for the SBS resonator; a filter receiving the pump laser on a first filter port and the SBS laser on a second filter port, wherein the pump laser is output through the second filter port and the SBS laser is output through a drop port; and a pump laser path coupling the output pump laser into the pump laser source, wherein a frequency of the pump laser becomes locked to a resonance frequency of the SBS resonator.

A wavelength-tunable, polarization-maintaining (PM) fiber laser for use in the two micron wavelength region is based upon a ring laser geometry and includes sections of polarization-maintaining (PM) optical fiber for supporting propagation of the circulating laser radiation around the ring. At least one gain module is included in the ring and is formed of polarization-maintaining active optical fiber including a core region that is doped with either Thulium (Tm) or Holmium (Ho), or co-doped with both of these rare earth materials. In the presence of a pump beam operating at a suitable wavelength, the gain module(s) generate laser radiation at a wavelength within the two micron region. A PM-based tunable bandpass filter (BPF) is included within the ring and used to control/adjust the wavelength of the output beam provided by the fiber laser.

An optical system can lock a wavelength of a tunable laser to a specified wavelength of a temperature-insensitive spectral profile of a spectral filter. In some examples, the spectral filter, such as a Fabry-Perot filter, can have a temperature-insensitive peak wavelength and increasing attenuation at wavelengths away from the peak wavelength. The spectral filter can spectrally filter the laser light to form filtered laser light. A detector can detect at least a fraction of the filtered laser light. Circuitry coupled to the detector and the laser can tune the tunable laser to set a signal from the detector to a specified value corresponding to a specified wavelength in the spectral profile, and thereby adjust the selectable wavelength of the tunable laser to match the specified wavelength. In some examples, the optical system can include a polarization rotator, and can use polarization to separate incident light from return light.

Provided are a silicon-based tunable filter, laser and an optical module. The tunable laser comprises a semiconductor optical amplifier and a silicon photonic integrated chip, wherein a first coupler, a phase regulator and a tunable filter are provided on the silicon photonic integrated chip; the tunable filter comprises a flat-top band-pass filter structure, a Mach-Zehnder interferometry (MZI) structure and a micro ring resonation (MRR) structure, which are cascaded; gain light emitted by the semiconductor optical amplifier is coupled to the silicon photonic integrated chip by means of the first coupler, and a narrowband filtered optical signal is output by means of the tunable filter; and the phase of the gain light is regulated by means of the phase regulator so as to output single-peak narrowband laser light with a tunable target wavelength.

A Holmium-doped fiber amplifier (HDFA) with cascaded pumping is disclosed. The cascaded pumping has at least two pumping stages arranged so that an emission spectrum of a preceding pumping stage at least partly corresponds to an absorption spectrum of the succeeding pumping stage, and the pumping stages are staggered so that an emission spectrum of the last pumping stage at least partly corresponds to an absorption spectrum of the Holmium-doped fiber.

A low-noise amplifier includes a gain medium and two or more amplifier stages. Each amplifier stage includes an optical filter to pass all wavelengths of a respective input optical signal in a given propagation direction over the gain medium and reflect wavelengths above a respective threshold wavelength received in the opposite direction, and a respective Raman pump to inject a pump light centered at a wavelength lower than the threshold wavelength onto the gain medium for transmission in the given direction. A first amplifier stage outputs a first combined optical signal including all wavelengths of the respective input optical signal and a pump light injected by the respective Raman pump. The second amplifier stage receives the first combined optical signal as its input and outputs a second combined optical signal including all wavelengths of the first combined optical signal and a pump light injected by the respective Raman pump.

A CO.sub.2 laser that generates laser-radiation in just one emission band of a CO.sub.2 gas-mixture has resonator mirrors that form an unstable resonator and at least one spectrally-selective element located on the optical axis of the resonator. The spectrally-selective element may be in the form of one or more protruding or recessed surfaces. Spectral-selectivity is enhanced by forming a stable resonator along the optical axis that includes the spectrally-selective element. The CO.sub.2 laser is tunable between emission bands by translating the spectrally-selective element along the optical axis.