Optical Parametric Generation and Amplification

Jing-Yuan Zhang, Jung Y. Huang, Y.R. Shen

• Publisher: Routledge
• Year: 2019
• eBook ISBN: 9781351426824
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

This volume considers optical parametric generation and amplification (OPG/OPA), as a means for producing a tunable optical parametric device. It reviews the OPA/OPG systems using various crystals pumped by lasers at various frequencies with pulse duration ranging from picoseconds to femtoseconds. Part two covers the theoretical background for design of an OPA/OPG system, using two newly discovered nonlinear crystals. Experimental design considerations are discussed in section three, including the section of nonlinear crystals, pumping sources and optical configurations. In section four, the experimental results obtained are compared with the theoretical calculations.

1.  INTRODUCTION

High power, picosecond (or subpicosecond) coherent optical pulses, tunable over a wide spectral range from the ultraviolet (UV) to the infrared (IR), are most desirable in many applications.¹ They can be used, for example, in time-resolved spectroscopy to yield new information about fundamental properties of materials, to identify transient species, to characterize new nonlinear optical materials,² and to study the dynamics of optoelectronic systems.³ In the past several decades, mode-locked dye⁴ and solid state lasers⁵ have been the major sources to provide picosecond as well as subpicosecond pulses in the visible and near infrared region with limited tuning ranges. Tunable mid-IR laser pulses are more difficult to generate, presumably because of the lack of suitable laser media. It is well known that nonlinear optical effects can be employed for frequency conversion⁶ and that both second-order and third-order nonlinear optical processes are commonly used.⁷ While second-order processes require a medium without inversion symmetry, the third-order processes can occur in any medium including gases and liquids. Particularly notable is frequency conversion by stimulated Raman scattering in molecular gases or atomic vapor. Hydrogen and methane are widely used as Raman shifters.⁸ Stimulated electronic Raman scattering in alkali vapor has been employed to generate tunable mid-IR radiation.⁹ Several drawbacks of such techniques should however be noted. First, the tuning range is often limited. Second, the conversion efficiency tends to below and could fluctuate strongly. Finally, it is difficult to make the system compact.

Second-order nonlinear optical processes, such as sum-frequency generation (SFG), difference-frequency generation (DFG), and optical parametric oscillations¹⁰ (OPO) and generation (OPG), are more commonly adopted for frequency conversion. They can be highly efficient and the systems are simple and compact. DFG and OPG are particularly attractive because they can yield an output with a very large tuning range extending from the visible to the infrared, limited only by absorption and phase matching of waves in the nonlinear crystal employed.

OPG and DFG are both wave-mixing processes involving energy conversion from a pump beam at frequency ω₃ into a signal beam at ω₁ and an idler beam at ω₂ = ω₃ − ω₁. No clear distinction exists between the two. Usually, for DFG, one refers to a process with two intense input laser beams at ω₃ and ω₁ respectively, generating an output beam at the difference frequency ω₂ = ω₃ − ω₁. For OPG, only a single laser beam at ω₃ is used as the input, and coherent outputs at ω₁ and ω₂ are generated. Often, one also speaks of optical parametric amplification (OPA), which is really not different from DFG except that one has in mind that the input at ω₁ is weak and is to be strongly amplified.

In this volume, we shall focus our discussion on OPG/OPA. The theory of OPG, OPA, or DFG was worked out by Armstrong, Bloembergen, Ducuing and Pershan early in 1962.¹¹ Giordinaine and Miller¹² first demonstrated the operation of an optical parametric oscillator. Wang and Racette¹³ were the first to observe OPA in ammonium dihydrogen phosphate and Boyd et al.¹⁴ succeeded in showing CW OPA in a lithium niobate crystal. Baumgartner and Byer,¹⁵ later in 1979, conducted a detailed measurement of optical parametric gain in deuterated potassium dihydrogen phosphate and lithium niobate and compared the results with theory. More recently, Kaiser, Piskarskas and their co-workers, as well as others, have contributed significantly to the progress of the field.¹ However, because of difficulties originating from poor laser beam quality and unavailability of suitable nonlinear optical crystals, OPG/OPA as a viable, coherent, tunable light source was not duly recognized until recently. The situation has changed following the recent advancement in laser technology and nonlinear optical crystals. Highly stable pulsed lasers with good beam quality are now available.¹⁶ Nonlinear crystals with wide tuning range and high laser damage thresholds have been developed.¹⁷ It then becomes possible to construct a suitable OPG/OPA system for routine measurements in a laboratory. Indeed, high-energy, widely tunable, picosecond as well as subpicosecond pulsed OPG/OPA has recently attracted much attention. Picosecond and subpicosecond output pulses with an energy of ~1 mJ/pulse at a conversion efficiency as high as 30% and a tuning range more than 20,000 cm⁻¹ can be generated. To cover the same tuning range by a dye laser would require at least 20 different dyes, not counting the infrared range where laser dyes are presently not available.

In general, tunable optical parametric devices can be divided into two groups: OPO and OPG/OPA. An OPO is composed of a nonlinear crystal situated in a resonant cavity. The case of pumping by nanosecond pulses has been analyzed in detail by Bronsnan and Byer¹⁸ and the case of synchronous pumping by picosecond or femtosecond mode-locked pulses is described in a concurrent volume by Tang and Cheng.¹⁹ An OPG/OPA device, on the other hand, involves no cavity and the tunable output is generated from noise (or parametric fluorescence) and amplified in the traveling wave form.²⁰ This latter case is what we would like to concentrate on in this volume.

An OPG/OPA system can have many advantages. It is simple and straightforward in construction. Without a cavity, the output tuning range is only limited by phase-matching and transparency of the nonlinear crystals. High energy (or intensity), widely tunable, picosecond or subpicosecond pulses are readily obtainable. Because of the absence of a resonant cavity, the single-pass gain of OPG/OPA must be high in order to yield a strong output. This calls for a pump of high intensity. In practical cases, the pump intensity needed is in the GW/cm² range or higher; so high-energy picosecond or subpicosecond pump lasers must be employed.

This volume is organized as follows: We first outline in Section 2 the theoretical background for design of an OPG/OPA system, using two newly discovered nonlinear crystals, β-barium borate (BBO) and lithium triborate (LBO) as examples. We then describe in Section 3 experimental design considerations, including the selection of the nonlinear crystals, pumping sources, and optical configurations. Section 4 reviews the experimental results and their comparison with theoretical calculations. We also discuss the scheme used to narrow the spectral width of the OPA output, and the scheme that can extend the tuning range of an OPG/OPA system from near IR to mid-IR by means of DFG and from visible to UV using SFG.

2. THEORETICAL BACKGROUND

The theory of optical parametric processes in a nonlinear medium follows the earlier work of Armstrong et al.¹¹ and has been reviewed by many authors.^7,15 Here, we focus only on the essential points.

2.1 OPTICAL PARAMETRIC AMPLIFICATION AS A THREE-WAVE MIXING PROCESS

Optical parametric amplification (OPA) is a three-wave mixing process, in which a pump beam, a signal beam, and an idler beam at frequencies ω₃, ω₁, and ω₂, respectively, with ω₃ = ω₁ + ω₂, propagate and interact in a nonlinear medium. We consider here the collinear propagation geometry. The three waves propagating along $\widehat{z}$ are coupled through the second-order nonlinear polarization ${\overrightarrow{P}}_{\text{NL}}\left(r,t\right)$ and are governed by the wave equation:

$\nabla \times \nabla \times \overrightarrow{E}\left(r,t\right)+\frac{1}{c^2}\frac{{\partial }^2}{\partial t^2}\overrightarrow{D}\left(r,t\right)=-\frac{4\pi }{c^2}\frac{{\partial }^2}{\partial t^2}{\overrightarrow{P}}_{\text{NL}}\left(r,t\right)$

(1)

Here, $\overrightarrow{E}\left(r,t\right)$ and $\overrightarrow{D}\left(r,t\right)$ are the total electric field and displacement current given by

$\overrightarrow{E}(...$

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Introduction to the Series
• 1. Introduction
• 2. Theoretical Background
• 3. Experimental Considerations
• 4. Experimental Results and Comparison with Theory
• 5. Generation of Tunable Femtosecond IR Radiation via OPG/OPA
• 6. Summary
• 7. Concluding Remarks
• 8. Acknowledgements
• 9. References
• Index