Design constraints of optical parametric chirped pulse amplification based on chirped quasi-phase-matching gratings

Abstract

Chirped quasi-phase-matching (QPM) gratings offer efficient, ultra-broadband optical parametric chirped pulse amplification (OPCPA) in the mid-infrared as well as other spectral regions. Only recently, however, has this potential begun to be realized [1]. In this paper, we study the design of chirped QPM-based OPCPA in detail, revealing several important constraints which must be accounted for in order to obtain broad-band, high-quality amplification. We determine these constraints in terms of the underlying saturated nonlinear processes, and explain how they were met when designing our mid-IR OPCPA system. The issues considered include gain and saturation based on the basic three-wave mixing equations; suppression of unwanted non-collinear gain-guided modes; minimizing and characterizing nonlinear losses associated with random duty cycle errors in the QPM grating; avoiding coincidentally-phase-matched nonlinear processes; and controlling the temporal/spectral characteristics of the saturated nonlinear interaction in order to maintain the chirped-pulse structure required for OPCPA. The issues considered place constraints both on the QPM devices as well as the OPCPA system. The resulting experimental guidelines are detailed. Our results represent the first comprehensive discussion of chirped QPM devices operated in strongly nonlinear regimes, and provide a roadmap for advancing and experimentally implementing OPCPA systems based on these devices.

Mid-infrared pulse generation via achromatic quasi-phase-matched OPCPA

“…In section 6 of [20], we considered the possibility of phase-matching additional sum and different frequency mixing processes in chirped QPM devices. If the quasi-phase-matching period for some unwanted processes (such as idler SHG) overlaps with the QPM periods required for the nominal OPA process, then this unwanted process may be efficient, potentially distorting the OPCPA output.…”

“…If the quasi-phase-matching period for some unwanted processes (such as idler SHG) overlaps with the QPM periods required for the nominal OPA process, then this unwanted process may be efficient, potentially distorting the OPCPA output. Experimentally, we avoided these effects by the choice of QPM grating chirp rate [18,20], but as the range of QPM periods is increased (to amplify a broader bandwidth), it becomes more difficult to avoid such processes in a collinear geometry.…”

“…(a) θp=0° and λp=1.064 nm, i.e. collinear beams, as currently used in our APPLN devices and discussed in [20]. (b) Non-collinear configuration with θp=7.5°and λp=1030 nm.…”

“…However, due to a number of design constraints that were explained recently [20], achieving broadband OPCPA supporting high-quality pulses is more challenging than obtaining a sufficient phase-matching bandwidth. In particular, two of the constraints discussed in [20] motivated the present work: First, unwanted nonlinear processes involving sum-and difference-frequency mixing between the nominal pump, signal and idler pulses can be phase-matched due to the broad range of QPM periods involved.…”

“…In particular, two of the constraints discussed in [20] motivated the present work: First, unwanted nonlinear processes involving sum-and difference-frequency mixing between the nominal pump, signal and idler pulses can be phase-matched due to the broad range of QPM periods involved. Second, when large intensity-times-length products are involved, the device becomes more sensitive to random duty cycle (RDC) errors in the QPM grating [21], which can lead to an unwanted enhancement of pump second-harmonic generation (SHG), even if this process is not phasematched by the nominal grating design.…”