The work described in this thesis is concerned with the development of self-modelocked Ti:sapphire lasers and femtosecond optical parametric oscillators based on periodically-poled rubidium titanyl arsenate and periodically-poled lithium niobate and operating in the near and mid-infrared. In Chapter 1 the theory of ultrashort pulse generation is explained with regard to the Ti:sapphire laser. The optical properties of Ti:sapphire are discussed along with the principles of laser oscillation and pulse generation. The techniques used to modelock the lasers used in the experimental work, which follows, are also considered. The second part of the chapter deals with typical measurement techniques for characterising femtosecond optical pulses from a laser or an OPO, including a detailed explanation of second harmonic generation autocorrelation. Chapter 1 concludes with a thorough description of frequency-resolved optical gating, the newest of these pulse characterisation techniques. In Chapter 2 the subject of nonlinear optics and the properties of nonlinear optical materials are discussed. Phasematching in nonlinear optical materials is explained along with the principle techniques for achieving this, including birefringent phasematching and quasi-phasematching. A review of techniques for periodically- poling nonlinear optical crystals is also given. The chapter concludes with a section on the optical effects of group velocity dispersion and self-phase modulation, that influence the output from an ultrashort pulse laser or OPO and describes methods for second and third-order dispersion compensation. Chapter 2 concludes the theory required to explain the experiments described in Chapters 3, 4, 5 and 6. Chapter 3 describes the operation and characterisation of two different Ti:sapphire laser systems involving different methods of dispersion compensation. The first laser produces 100 fs duration self-modelocked laser pulses and dispersion compensation is achieved by including a pair of prisms in the cavity. This laser system is discussed further in Chapter 5, where it is operated in conjunction with a Spectra Physics Millennia, as the pump source for an all-solid-state femtosecond OPO based on periodically-poled lithium niobate. A second laser system is described in Chapter 3, which produces self-modelocked pulses of ~15 fs duration and dispersion compensation is achieved by including chirped multilayer dielectric mirrors in the cavity. The subject matter that Chapter 4 is concerned with includes the operation and characterisation of a femtosecond OPO based on PPRTA. Ti:sapphire pump wavelength tuning and cavity-length tuning of the OPO are shown to produce wavelengths throughout the range 1.060 mum to 1.225 mum in the signal and 2.67 jam to 4.5 mum in the idler, with average output powers as high as 120 mW in the signal and 105 mW in the idler output. The effects of photorefractive damage are minimal and consequently this offers the possibility of room-temperature operation of the PPRTA- based OPO. Chapter 5 is concerned with the generation of longer idler wavelengths, in the region of 5 mum, from an all-solid-state OPO based on periodically-poled lithium niobate. The approach used with the PPRTA-based OPO is extended to PPLN and in Chapter 5, results are presented which show that the use of an all-solid-state Ti:sapphire pump source in combination with a PPLN-based OPO represents a robust source of high- repetition-rate femtosecond pulses in the mid-infrared at wavelengths out to ~5 mum. Significantly higher output powers in the signal and idler than previously reported are also measured. In Chapter 6 a similar PPLN-based OPO is described, with modifications to the cavity elements, to reduce the output pulse duration of the OPO. This system is pumped by a sub- 20 fs Ti:sapphire laser. A pulse duration of 175 fs is recorded for the signal at a wavelength of 1.07 mum. Output powers of 28 mW for the signal at 1.07 mum and 6.8 mW for the idler at 2.7 mum are also measured. The tuning range for the signal extends from 1.045 mum to 1.190 mum, and for the idler, extends from 2.57 mum to 3.67 mum.

The all-solid-state (or "holosteric") optical parametric oscillator has resulted from the recent development of diode-laser-pumped solid-state lasers and from recent advancements in new optically nonlinear materials. As a result, all-solid-state sources of widely tunable (ultraviolet - visible - near infrared) coherent radiation are now possible by using the radiation from diode-laser-pumped solid-state lasers, either directly or after frequency conversion, to pump optical parametric oscillators. Such devices can be made compact, efficient and reliable. The work described in this thesis explores the feasibility of obtaining widely tunable radiation from such devices, with particular reference to low threshold, high efficiency operation, so requiring only modest energies (1 mJ in ultraviolet) from the pump source. In particular, a frequency tripled or frequency quadruped Nd:YAG laser pumped by pulsed, GaAlAs diode laser bars has been used as the pump source, and lithium triborate has been used as the nonlinear medium in the optical parametric oscillator. Two geometries of lithium triborate crystals have been investigated as the nonlinear medium. One was cut for a type II non-critical phase matching geometry, while the other was cut for a type I critical phase matching geometry. The oscillator cavities were designed for optimum focusing and mode matching aiming for operation with a low pump energy through the use of tightly focused pump radiation. The ultraviolet pump source was based on a Q-switched diode-laser-pumped Nd:YAG laser which generated pulses at 1.064 mum with energy 10 mJ and duration around 10 ns. These were then frequency up-converted to the UV at 355 nm or 266 nm, so as to be suitable for pumping the parametric oscillators. Generally, an overall conversion efficiency from 1.064 mum to 355 nm of >30% was obtained using the nonlinear materials potassium titanyl phosphate and lithium triborate for second harmonic generation and sum-frequency mixing respectively. For conversion to 266 nm, an overall efficiency of > 18 % was obtained using the nonlinear materials KTP and BBO for two step second harmonic generation. In the experimental investigations of the all-solid-state lithium triborate optical parametric oscillator pumped at 355 nm a low oscillation threshold was obtained in the type II non-critical phase matching geometry (around 0.2 mJ) and pump depletions of 50 % were obtained at around six times threshold. This device could be temperature tuned (20 - 200 °C) from 457 to 481 nm (signal wave) and 1.6 to 1.35 mum (idler wave). Optimised focusing conditions corresponding to the theory of Guha et al were approached in the type I phase matching geometry, and despite the existence of a 1° walkoff angle, the minimum oscillation threshold was measured to be around 0.3 mJ. Generally, pump depletions of about 35 % were obtained, at around four times threshold. These devices could be angle tuned (through crystal internal angle 14°) from 457 to 666 nm (signal wave) and 1.6 mum to 768 nm (idler wave). (The whole of the range 420 nm to 2.3 mum could be covered with such a device given additional mirror sets). The all-solid-state type II geometry lithium triborate optical parametric oscillator was also pumped at 266 nm, when it was temperature tunable (20 - 200 °C) from 306 to 314 nm (signal wave) and 2.03 to 1.75 mum (idler wave). Pump depletions of 25 % were demonstrated with this device at pump energies of four times threshold. In addition to the above experimental investigations, the thesis addresses the issues of (i) choice of nonlinear material for optical parametric oscillators in terms of appropriate figures of merit, and (ii) optimisation of pump and resonated wave focusing parameters. Reviews of the appropriate theoretical background to phase matching geometries and optical parametric interaction are included.

The work described herein concerns the characterisation and development of optical parametric oscillators (OPOs) tunable in the ultraviolet, visible and near infrared regions. These devices were pumped by the 308nm output from line-narrowed Xenon Chloride excimer lasers of pulse energy up to 150mJ. The behaviour of Type 2 phase-matched Urea, and Type 1 phase-matched Barium Borate OPOs in terms of oscillation threshold and conversion efficiency, has been explored. The detrimental effects of pump beam walkoff on the threshold of the critically phase-matched Barium Borate OPO have been quantified. It was found that minimum 17ns pulse energies of 5mJ were required to reach threshold in a device based on a crystal of 20mm length. By contrast, noncritically phase-matched Urea OPOs using crystal lengths of 25mm were operated with as little as 0.6mJ pump energy. A deterioration in performance was observed in both cases with decreasing pump beam waist. Maximum pump depletions of 72% and 64% were observed in Urea and BBO respectively. The useful output from the urea device reached 65%, while higher absorption/scattering losses meant that the useful fraction in BBO was very much lower. Two different types of noncollinear phase-matching were studied in the BBO-OPO. The first recorded observation of operation of a Type 1 OPO at crystal angles beyond the degenerate wavelength point was made. The output took the form of two concentric rings and was attributed to simultaneous singly and doubly resonant operation. Finally, single longitudinal mode operation of the BBO-OPO was demonstrated using a dispersive cavity arrangement. The widely varying inherent linewidth of the device required that different strategies be adopted over different wavelength ranges. Encouraging performance in terms of threshold was observed using the dispersive cavity, and the feasibility of using this device as a low-power first stage for an oscillator/amplifier set-up was studied.

The development of all-solid-state, diode-laser pumped neodymium (Nd) lasers and optical parametric oscillators (OPOs) is described, which realise practical sources of coherent radiation with a high degree of frequency agility, are efficient, reliable and potentially compact. A comparison of various neodymium doped host materials reveals yttrium lithium fluoride (YLF) to be an appropriate replacement for the more widely known host yttrium aluminium garnet (YAG) in diode-laser pumped devices. The development of an end-pumped Nd:YLF laser that utilises a 12-mJ, 60W, quasi-CW diode-laser bar is initially described. Multilongitudinal-mode, TEM00 pulse energies of greater than 2 mJ have been observed, with corresponding peak output powers in excess of 118 kW. The incorporation of a novel pre-lase Q-switching technique has realised single-longitudinal-mode peak powers in excess of 90 kW continuing to be achieved. Further, the development of a more powerful end- pumped Nd:YLF laser utilising 2, 3-bar diode-laser arrays, each providing 72-mJ of pump energy is described. In this case, Q-switched, multilongitudinal-mode, TEM00 pulse energies of greater than 11 mJ are reported, with the clear potential for increasing this to greater than 20 mJ, based on measured fixed-Q pulse energies of greater than 30 mJ. Complementing the development of these diode-laser pumped solid-state lasers is the development of optical parametric oscillators based on the nonlinear materials lithium triborate (LBO) and beta-barium borate (?-BBO). Pumped by the frequency up-converted (third harmonic) output of the mid laser, such optical parametric oscillators introduce extensive frequency agility spanning a spectral range from the deep blue (0.4 mum) to the mid-infrared (2.5 mum). Initially, the development of an LBO based device is reported, which in a type I critical phase- match (CPM) geometry has a measured oscillation threshold of < 0.3 mJ, when pumped by the frequency tripled output of the 144-mJ diode-laser pumped Nd:YLF laser at 0.349 mum. Observed pump depletions are as high as 35%. A similar CPM geometry is reported in beta-BBO, in this case pumped by the frequency tripled and amplified output of a diode-laser pumped Nd:YAG laser at 0.355 mum. This is a more energetic device with thresholds of >5 mJ, but through the introduction of interferometric, dispersive and injection seeding techniques made to operate on a single axial mode. Near transform limited linewidths are reported in devices which continue to have modest pump thresholds and broad tunability. The parametric generation of broad spectral bandwidths (polychromatic) by the use of suitable phase-matching geometries is also reported. Greater than 100 nm simultaneous bandwidth in the visible spectrum is generated in a collimated signal-wave from a novel, noncollinear phase-matching geometry in a beta-BBO optical parametric oscillator, which is pumped by the collimated output of frequency tripled diode-laser pumped Nd:YAG laser. The device is demonstrated to be efficient, having a similar pump threshold and efficiency to that of the well known collinear phase-matched tunable device, and to continue to encompass a degree of tunability allowing the large simultaneous bandwidth to be tuned across the entire visible spectrum. Dispersive cavity tuning of the optical parametric oscillator by the use of a Littrow-mounted grating or acousto-optic tuning filter, with a static crystal and pump configuration, is also described.

The work presented in this thesis describes the design, configuration and operation of femtosecond optical parametric oscillators based on the materials KTiOPO4 (KTP) and RbTiOAsO4 (RTA) and pumped by a self- modelocked Ti:sapphire laser. The alignment of the pump laser is detailed and thermal effects in the Ti:sapphire rod are examined in the context of a general technique which optimises modelocked performance at any pump power. A KTP-based femtosecond parametric oscillator is described which produces 400-fs-duration signal pulses at an average output power of 150 mW when operated in the absence of group-velocity dispersion- compensation. With intracavity dispersion-compensation, the oscillator produces 40-fs-duration pulses with an average power of 50 mW. Tuning is demonstrated from 1.12 - 1.25 mum in the signal wave and from 2.5 - 3.0 mum in the idler wave by changing only the pump-laser wavelength. Using a novel idler-feedback arrangement, reductions in the oscillation threshold and increases in the signal output power of 10 % are described. Soliton generation in the oscillator is achieved when the net cavity dispersion is positive and results show good agreement with theory. An oscillator using RTA is demonstrated which achieves conversion efficiencies exceeding 30 % and has an operating threshold of only 50 mW. Average signal powers of 100 mW and 185 mW are extracted from the oscillators with and without dispersion-compensation respectively. The corresponding pulse durations are 67 fs and 980 fs and tunability in the signal and idler waves from 1.23 - 1.34 mum and 2.10 - 2.43 mum is demonstrated. Visible output from 620 - 660 nm is obtained by intracavity- doubling and powers of up to 170 mW are measured. These results suggest that RTA has a higher nonlinear coefficient than KTP.

A low threshold, efficient optical parametric oscillator (OPO) based on the material Potassium Titanyl Phosphate (KTP) and pumped by a diode-laser-pumped, Q-switched Nd:YLF laser has been demonstrated and investigated. This all-solid-state device was operated in a non-critical phase match (NCPM) geometry converting the 1 mum pump light to output wavelengths of 1.54 and 3.28 mum, and has potential as an 'eyesafe' laser source with scaling to higher powers. A major contributing factor to the success of this work was the extension of the steady state theory of the singly resonant OPO to include the build-up time effects that are dominant in the pulsed regime. A number of diode pumped lasers were constructed, allowing a comparison to be made between side- and end-pumping geometries, and also between the materials Nd:YAG and Nd:YLF. The end-pumping geometry in conjunction with the higher absorption and longer upper state lifetime in Nd:YLF made it the design of choice for the case of low pump pulse energies (~ 12 mJ at 797 nm). Anamorphic expansion of the laser mode in the plane parallel to the diode laser junction was employed to achieve TEM00 operation of this laser. Subsequent Q-switching with a polariser and LiNbO3 Pockels cell combination produced 2.2 mJ at 1.047 mum in an 18 ns pulse. Investigation of the dynamic loss of the Q-switch (which is due to the elasto-optic effect) allowed improvement of laser performance. The established model for a pulsed singly resonant OPO which describes the case for a plane-plane resonator was inappropriate in this work and so the steady state focused beam theory was extended to include time dependence. Fair agreement was found between the computer model and the experimental results, where the effects of pump and signal focusing, and output coupling were investigated. The high conversion efficiency of 30% for converting the 1 mum pump light to the eyesafe wavelength of 1.54 mum is superior to the present alternative source of the Er:glass laser. Pump energy thresholds of less than 0.5 mJ were obtained, along with internal conversions approaching 50 %. An empirical relation describing pump depletion was derived which showed good agreement with experiment. A high resolution investigation of the spectral properties of the OPO identified the roles of resonant reflection and doubly resonant behaviour on the mode structure of the output. The former suggests a way in which single mode operation could be achieved without the use of additional intracavity elements, or a seeding source.

The research presented in this thesis is concerned with the generation and characterisation of femtosecond pulses in the near and mid-infrared spectral regions. The three optical parametric oscillators which were constructed were synchronously- pumped by a self-modelocked femtosecond Ti:sapphire laser. Noncollinear critical birefringent phasematching was used in an oscillator based on KTiOAsO4, which was tunable from 1.03 to 1.2 mum and 2.51 to 4.1 mum by varying the crystal angle. The mid-infrared pulses were sub-100 fs, and essentially free from frequency chirp. With appropriate dispersion compensation the near-infrared signal pulses were temporally compressed to 69 fs. Theoretical models of noncollinear phasematching were derived and the results agreed closely with experiment. A novel optical parametric oscillator design based on a semi-monolithic noncritically phasematched RbTiOAsO4 crystal was implemented. This unique cavity configuration allowed independent focussing of the pump and signal beams within the crystal. It facilitated a reduction in cavity length to bring the signal pulse repetition rate into synchronism with the second (172 MHz) and fourth (344 MHz) harmonics of the pump pulse repetition frequency. Extraction efficiencies as high as 55% were observed. Quasi-phasematched femtosecond optical parametric oscillation was demonstrated in periodically poled lithium niobate. This device offered extensive tunability, covering 0.975 to 1.54 mum in the signal branch and 1.67 to 4.55 mum in the idler branch, from a combination of grating, pump wavelength and cavity length tuning. A theoretical model indicated that a very broad gain bandwidth allowed the wide tuning range. An attractively low oscillation threshold of 45 mW was recorded, and a visible output of 70 mW at 540 nm was observed, caused by simultaneously phasematched frequency-doubling of the signal output. The pulses from the Ti:sapphire laser and from the optical parametric oscillators were characterised by autocorrelation and frequency-resolved optical gating techniques. A highly advantageous autocorrelator arrangement based on quadratic nonlinearity in light-emitting diodes and photodiodes was demonstrated, and a novel second harmonic generation frequency-resolved optical gating system allowed real-time monitoring of pulsed outputs and complete characterisation of the intensity and phase of pulses.

This thesis describes the design, configuration and operation of picosecond optical parametric oscillators (OPOs) tunable from the visible to mid infrared. These systems were based on the materials LiB3O5 (LBO) and KTiOAsO4 (KTA), and were pumped by a self-mode-locked Ti:sapphire laser at a repetition rate of 81 MHz. The initial design of the picosecond parametric oscillator was based on a 16 mm long crystal of LBO. This system produces transform-limited signal pulses with durations of -720 fs. Total average output powers of up to 90 mW over a signal (idler) tuning range of 1.374-1.530 mum (1.676-1.828 mum) have been generated at 1.3 times the 900 mW threshold. The system performance was improved by the use of a new LBO crystal of length 30 mm. This system was continuously tunable from 1.160 to 2.185 mum. Up to 690 mW of output power has been generated for 2 W of input pump power at 5 times threshold. For this output power a depletion of 52 % was achieved with a corresponding external extraction efficiency of 34.5 %. Picosecond pulse generation in the visible by external single-pass frequency- doubling of the LBO OPO to provide picosecond pulses in the 584-771 nm range has been demonstrated. Conversion efficiencies as high as 18 % have been demonstrated, with output powers in excess of 65 mW being measured, when utilising a combination of type I and type II temperature-tuned non-critical phase-matching in LBO. The pulse width of the second harmonic was in the region of 840-880 fs. A further new source of tunable high-repetition-rate picosecond pulses for the visible has also been demonstrated, which is based on an internally-doubled, Ti:sapphire-pumped OPO that uses temperature-tuned LBO both as the OPO and SHG crystal. Oscillation has been obtained for an input pump power of 700 mW with output powers in excess of 320 mW being generated, representing conversion efficiencies of as much as 16 %. The system is continuously tunable from 584 to 771 nm and can provide transform-limited visible pulses with durations of 840-880 fs across the available range. The ability to tune beyond wavelengths of 2.5 mum was also required. To this end a new source of tunable picosecond pulses for the near - to mid - infrared has been developed which is based on the material KTA. Oscillation has been obtained for input pump powers as low as 230 mW. The system produces total output powers in excess of 403 mW with conversion efficiencies of 31 % at 5.2 times threshold. Transform-limited signal (idler) pulses of 1.02 (2.9) ps have been generated over the tuning range 1.139-1.281 (2.377-3.160) mum.

This thesis describes the development of singly-resonant optical parametric oscillators (OPOs) based on the nonlinear material KTP (potassium titanyl phosphate), and used to provide tunable light in the infrared, with low oscillation threshold and high efficiency. Further, the generation of tunable red light by the frequency mixing of the signal wave from the OPO with the pump wave in a non-critical temperature phase-matched lithium triborate crystal (LBO) is reported. We believe this is the first demonstration of such an application of LBO. Two diode-pumped solid-state lasers were used as the pump sources. One was an electro-optically Q-switched Nd:YLF laser which provided high peak power (~600 kW) pulses; and the other one was an acousto-optically Q-switched slab-geometry Nd:YLF laser which provided high repetition rate (1~10 kHz) a.nd low peak power(< 30 kW) pulses. A second version of the acousto-optically Q-switched slab-geometry Nd:YLF laser was designed and constructed with improvements in the pump module and cooling system so as to be much more compact and easier to control. In the first stage of this work, two theoretical models were constructed. One was a model for pump threshold of singly-resonant OPOs for the case of focused Gaussian beams, and was based on Guha's theory. The second one was a model for conversion efficiency of singly-resonant OPOs, for the case of plane waves with pump depletion, and was derived from the coupled wave equations. In the second stage of this work, the effects of beam focusing and Poynting vector walk-off on pump threshold and conversion efficiency for OPOs were extensively studied theoretically and experimentally. Experimental results were found to be in good agreement with theory. The high pump threshold of the critically phase-matched KTP OPO led to several other pump configurations being considered, including intracavity OPOs, cylindrical focusing, and donble-pass of the pump. As a result of the KTP OPO study, very low pump thresholds were achieved in both non-critical phase-matched (NCPM) and critical phase-matched (CPM) KTP OPOs by using long crystal in both intracavity OPOs and the double-pass-pump configuration. Maximum external conversion efficiency from pump to signal was demonstrated to be 37% for the NCPM OPO and 40% for the CPM OPO. The signal wavelength tuning ranges were observed to be 1.54-1.56 mum from the NCPM KTP OPO, and 1.58-1.8 mum from the CPM KTP OPO. In the final stage of this work, the temperature phase-matching properties of LBO were investigated with the use of our measured thermo-optical coefficients of LBO. A particularly interesting result of the investigation is the possibility of sum-frequency generation in non-critically phase-matched LBO with temperature tuning giving considerable wavelength ranges for both the type I and type II geometries. Experimentally, we demonstrated tunalile red light generation by sum-frequency mixing of the 1 mum pump wave and the signal wave of the KTP OPO with an over all conversion efficiency of more than 13%. The wavelength tuning range was observed to 0.62-0.65 mum. The effects of beam focusing on the conversion efficiency for sum-frequency generation were analysed theoretically, and several opportunities for further improvement were shown clearly from this analysis.

This thesis gives a description of studies relating to the development of continuous-wave (cw) optical parametric oscillators (OPOs) and their application to schemes that require high-precision, narrow-linewidth, and frequency-tunable radiation. There are three separate aims to the work presented within this thesis. First, the requirements on pump sources, nonlinear materials, OPO cavity resonances and phase-matching geometries are analysed with a view to operating cw OPOs with stability above threshold. Second, the results of four distinct experiments are outlined, compared to theory, and discussed within the general context of cw OPO development. Third, this thesis is the first comprehensive review of the above-mentioned performance characteristics of cw OPOs, and focuses on their role as optical frequency dividers within frequency synthesis chains spanning the optical spectrum. The modelling sections highlight the importance of cavity resonances in cw OPOs when evaluating pump power thresholds, conversion efficiencies, and mode-selection properties. Simultaneous signal and idler cavity resonances are shown to be critical when relying upon cw laser sources to reach OPO threshold powers. Such arrangements require the use of stable pump lasers and servo-locked OPO cavity lengths to maintain this doubleresonance condition. There is an in-depth analysis of OPO cavity geometries that can generate frequency-stable and continuously-tunable outputs. The selection of nonlinear materials for cw OPOs is also considered with regard to providing signal and idler frequencies in integral-related frequency ratios, thereby satisfying an important requirement within optical frequency division techniques. Four specific experiments were designed to address many of the issues raised within the modelling sections. These experiments realized the following novel arrangements; the use of lithium triborate as a gain material within cw OPOs; a cw OPO which used a pump source operating in the ultraviolet spectral region; the highest frequency output from a cw OPO; a cw OPO which used a tunable pump source operating in the near infra-red spectral region; multiple parameter pump / OPO coarse frequency tuning; a non- degenerate type II cw OPO phase-matching geometry; and a dual-cavity doubly-resonant cw OPO. Other notable features of the experimental designs included stabilized single-frequency output from a single-cavity cw OPO geometry, continuous frequency tuning from a dual-cavity, doubly-resonant OPO resonator, and the general characteristics of low pump power thresholds and moderate conversion efficiencies. An important feature discussed in detail throughout the thesis is the comparison between type I and type II phase-matching geometries. These two cases give rise to different polarization states for the signal and idler fields within OPO cavities. Type II phase-matching geometries are shown, both in theory and experiments, to be preferable to equivalent type I geometries, when considering stable OPO operation, fine frequency tuning, and multiple cavity oscillators. This is so because type II phase-matching geometries, in general, provide significantly different signal and idler refractive indices which in turn yield a considerable mis-match in the signal and idler free spectral ranges. Subsequently this relaxes the stability requirements within single-cavity doubly-resonant OPOs, and allows for polarization separation to form dual-cavity resonators which are vital to the effective operation of cw OPOs within metrology and spectroscopy. The work contained in this thesis forms an integral part of current research in cw OPOs, a field presently enjoying its most productive and prosperous period. The potential incorporation of cw OPOs within frequency synthesis chains is shown to be dependent on the further development of pump lasers and nonlinear materials. In the short term, the actual use of cw OPOs is assessed in relation to more convenient and widespread techniques for converting, comparing, and measuring absolute frequencies.