Apparatus and method for spectrally shaping a laser beam

A master-oscillator power-amplifier (MOPA) laser, comprising:

• a seed laser for providing a linearly-polarized laser beam;
• a birefringent optic for receiving and transmitting the laser beam, the birefringent optic having orthogonal first and second polarization axes, the laser beam received by the birefringent optic having an electric-field vector oriented at an oblique angle ϕ to the first polarization axis;
• a linear polarizer having a polarizing direction, the polarizer arranged to receive the laser beam transmitted through the birefringent optic and to transmit a portion thereof, the polarizing direction oriented at an oblique angle θ to the first polarization axis, the portion of the laser beam transmitted through the polarizer having a more-uniform power spectral density than the laser beam provided by the seed laser;
• and
• an amplifier for receiving the portion of the laser beam transmitted through the polarizer and generating an amplified laser beam, the amplified laser beam having more power than the laser beam provided by the seed laser;
• wherein transmission through the birefringent optic induces a wavelength-dependent phase shift between a component of the laser beam parallel to the first polarization axis and a component of the laser beam parallel to the second polarization axis, the wavelength-dependent phase shift producing a wavelength-dependent polarization state of the laser beam transmitted through the birefringent optic, thereby modulating power spectral density in the portion of the laser beam transmitted through the polarizer;
• wherein angles ϕ and θ are in a range between 19° and 30°;
• and
• wherein the birefringent optic causes a phase delay between the component of the laser beam parallel to the first polarization axis and the component of the laser beam parallel to the second polarization axis, and the phase delay is in a range between 7π/8 and 9π/8 radians.

A method for spectrally shaping a laser beam, comprising the steps of:

• delivering a linearly-polarized laser beam to a birefringent optic, the linearly-polarized laser beam having an electric-field vector, the birefringent optic having orthogonal first and second polarization axes;
• dividing the linearly-polarized laser beam received by the birefringent optic into a component parallel to the first polarization axis and another component parallel to the second polarization axis by orienting the electric-field vector between the first and second polarization axes;
• transmitting the laser beam through the birefringent optic to induce a wavelength-dependent phase shift between the components parallel to the first and second polarization axes, the wavelength-dependent phase shift producing a wavelength-dependent polarization state of the laser beam transmitted through the birefringent optic;
• inducing a phase delay between the component of the laser beam parallel to the first polarization axis and the component of the laser beam parallel to the second polarization axis during transmission of the laser beam through the birefringent optic;
• delivering the laser beam transmitted through the birefringent optic to a linear polarizer, the polarizer having a polarizing direction oriented between the first and second polarization axes;
• and
• transmitting a portion of the laser beam received by the polarizer therethrough, the transmitted portion having a modulated power spectral density that is more uniform than the laser beam received by the birefringent optic;
• wherein the electric-field vector is oriented at an oblique angle ϕ to the first polarization axis, the polarizing direction is oriented at an oblique angle θ to the first polarization axis, angles ϕ and θ are in a range between 19° and 30°;
• and
• wherein the phase delay is in a range between 7π/8 and 9π/8 radians.