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.

A frequency stabilizer includes: a delay line interferometer that receives an optical signal corresponding to one frequency mode of a pulsed laser, divides and transmits the received optical signal to a reference arm and a delay arm including an optical fiber delay line, and then outputs an interference signal between signals passing through the reference arm and the delay arm; a photoelectric converter that converts the interference signal into an electrical signal; a mixer that generates a baseband signal of the electrical signal by mixing a carrier frequency signal; and a feedback controller that transmits a control signal generated based on the baseband signal to the pulsed laser. The optical signal passing through the delay arm is weighted with a delay time caused by the optical fiber delay line compared to the optical signal passing through the reference arm, and the optical signal passing through the delay arm is frequency shifted to a carrier frequency of an oscillator. A carrier-envelope offset frequency of the pulsed laser is stabilized by an offset frequency stabilizer.

A frequency stabilizer includes: a delay line interferometer that receives an optical signal corresponding to one frequency mode of a pulsed laser, divides and transmits the received optical signal to a reference arm and a delay arm including an optical fiber delay line, and then outputs an interference signal between signals passing through the reference arm and the delay arm; a photoelectric converter that converts the interference signal into an electrical signal; a mixer that generates a baseband signal of the electrical signal by mixing a carrier frequency signal; and a feedback controller that transmits a control signal generated based on the baseband signal to the pulsed laser. The optical signal passing through the delay arm is weighted with a delay time caused by the optical fiber delay line compared to the optical signal passing through the reference arm, and the optical signal passing through the delay arm is frequency shifted to a carrier frequency of an oscillator. A carrier-envelope offset frequency of the pulsed laser is stabilized by an offset frequency stabilizer.

A wavelength sensor for wavelength stabilization of a laser beam includes an etalon placed in the laser beam and tilted with respect to the laser beam. Reflected beams from the etalon form an interference pattern on a segmented photodetector having two detector segments. Output signals from the two detector segments are used to derive an error signal for a closed control loop to effect the wavelength stabilization.

A wavelength tunable laser includes a filter region having a wavelength selection function on light from a gain region, wherein the filter region is a Sagnac interferometer and includes two ring resonators. The ring resonator has two optical couplers, and first and second curved waveguides connecting the two optical couplers, each of the two optical couplers is configured to receive input of the light from the gain region through an input-output port, to couple light of a resonant peak to a bar port of the input-output port, and to couple light except light at a resonant peak wavelength to a cross port of the input-output port, and each of radiation waveguides connected to the cross ports of the input-output ports of the two optical couplers has a structure that radiates the light except the light at the resonant peak wavelength to a front surface or a back surface of a substrate.

A wavelength tunable laser includes a filter region having a wavelength selection function on light from a gain region, wherein the filter region is a Sagnac interferometer and includes two ring resonators. The ring resonator has two optical couplers, and first and second curved waveguides connecting the two optical couplers, each of the two optical couplers is configured to receive input of the light from the gain region through an input-output port, to couple light of a resonant peak to a bar port of the input-output port, and to couple light except light at a resonant peak wavelength to a cross port of the input-output port, and each of radiation waveguides connected to the cross ports of the input-output ports of the two optical couplers has a structure that radiates the light except the light at the resonant peak wavelength to a front surface or a back surface of a substrate.

A wavelength tunable laser includes a filter region having a wavelength selection function on light from a gain region, wherein the filter region is a Sagnac interferometer and includes two ring resonators. The ring resonator has two optical couplers, and first and second curved waveguides connecting the two optical couplers, each of the two optical couplers is configured to receive input of the light from the gain region through an input-output port, to couple light of a resonant peak to a bar port of the input-output port, and to couple light except light at a resonant peak wavelength to a cross port of the input-output port, and the first curved waveguide connects the bar ports of the input-output ports of the two optical couplers, and the second curved waveguide connects the cross ports of ports, of two optical couplers, that the first curved waveguide is connected to.

A wavelength tunable laser includes a filter region having a wavelength selection function on light from a gain region, wherein the filter region is a Sagnac interferometer and includes two ring resonators. The ring resonator has two optical couplers, and first and second curved waveguides connecting the two optical couplers, each of the two optical couplers is configured to receive input of the light from the gain region through an input-output port, to couple light of a resonant peak to a bar port of the input-output port, and to couple light except light at a resonant peak wavelength to a cross port of the input-output port, and the first curved waveguide connects the bar ports of the input-output ports of the two optical couplers, and the second curved waveguide connects the cross ports of ports, of two optical couplers, that the first curved waveguide is connected to.

An excimer laser apparatus according to the present disclosure includes a chamber configured to accommodate a laser gas and a pair of electrodes and generate pulse-oscillating laser light when the gas pressure of the laser gas is controlled in accordance with voltage applied between the pair of electrodes, a power supply configured to apply the voltage between the pair of electrodes, and a controller to which a target value of the spectral linewidth of the laser light is inputted, the controller configured to correct the voltage used to control the gas pressure, when the target value changes from a first target value to a second target value, based on a first function having the second target value as a parameter and control the gas pressure in accordance with the corrected voltage.

A wavelength tunable laser includes a filter region having a wavelength selection function on light from a gain region, wherein the filter region is a Sagnac interferometer and includes two ring resonators. The ring resonator has two optical couplers, and first and second curved waveguides that connect the two optical couplers and lengths of which equal to each other, each of the two optical couplers is configured to receive input of the light from the gain region through the input-output port, to couple light of a resonant peak to a bar port of the input-output port, and to couple light except light at a resonant peak wavelength to a cross port of the input-output port, and the first curved waveguide connects the bar ports of the input-output ports of the two optical couplers, and the second curved waveguide connects the cross ports of ports, of the two optical couplers, that the first curved waveguide is connected to.