We report relaxation oscillation free, true continuous-wave operation of a singly-resonant, intracavity optical parametric oscillator (OPO) based upon periodically-poled, MgO-doped LiNbO3 and pumped internal to the cavity of a compact, optically-excited semiconductor disk laser (or VECSEL). The very short upper-laser-state lifetime of this laser gain medium, coupled with the enhancing effect of the high-finesse pump laser cavity in which the OPO is located, enables a low threshold, high efficiency intracavity device to be operated free of relaxation oscillations in continuous-wave mode. By optimizing for low-power operation, parametric threshold was achieved at a diode-laser power of only 1.4W. At 8.5W of diode-laser power, 205mW of idler power was extracted, indicating a total down-converted power of 1.25W, and hence a down-conversion efficiency of 83%.
© 2009 Optical Society of America
Continuous-wave optical parametric oscillators (OPOs) have long held promise as highly versatile sources of high power, wavelength-flexible, narrow linewidth, near- to mid-infrared radiation; ideal sources for use in a host of high-resolution spectroscopic applications [1-4]. Their widespread implementation has, however, been hampered by operational problems inherent with the various cavity geometries deployed.
Whilst achieving threshold at very low external pump powers, the excessively high mechanical stability requirements placed on the doubly-resonant OPO (DRO) [5-8] coupled with the complexity of attaining continuous smooth tuning in such devices (requiring synchronous servo-adjustment of dual coupled-cavities [7, 8] or simultaneous tuning of the pump and cavity length control ) precludes their use outside very carefully controlled environments. Resonating only one of the down-converted waves [10, 11] removes this stability requirement and significantly simplifies tuning schemes, but in doing so the singly-resonant OPO (SRO) requires multi-watt pumping lasers in order to reach threshold. Whilst this high threshold condition can be offset by placing the SRO within a high-finesse optical cavity that is held on resonance with the (single-frequency) pump laser (the pump-enhanced SRO (PESRO)) [12, 13], the requirement for active cavity locking schemes, a single frequency pump laser, operation of the device within an environment exhibiting low mechanical perturbation, and optical isolation between the pump laser and enhancement cavity have rendered the PESRO unsuitable for most applications.
A further recognized route to realizing a low-threshold system is to place an SRO within the (high finesse) cavity of the pumping laser, thereby taking advantage of the very high circulating pump power oscillating therein. Such a device, the continuous-wave intracavity SRO (cw-ICSRO), was demonstrated firstly using titanium sapphire lasers [14, 15] and subsequently with well-established, directly diode-laser-pumped gain media; in particular crystal hosts doped with Neodymium (Nd) such as Nd:YAG and Nd:YVO4 [16, 17]. The latter approach was found to be particularly attractive in realizing a truly miniature, wavelength flexible, continuous-wave, mid-IR source. However, the utility of this device was seriously compromised by the problem of relaxation oscillations which resulted from the inclusion of the SRO within the cavity of this class of pump laser.
Placing a continuous-wave, singly-resonant OPO within the cavity of the parent pump laser can make such a laser susceptible to the onset of high-frequency and prolonged bursts of relaxation oscillations. Importantly these relaxation oscillations are different in kind to those associated with the basic pump laser. Based on our earlier theoretical modeling  of these relaxation oscillations we can deduce two useful ‘rules of thumb’ that highlight the critical differences. Firstly, the frequency of the oscillations is governed by the radiative decay times associated with the (passive) pump (τp) and signal (τs) cavities, together with the number of times above OPO oscillation threshold the device is being pumped (X), namely:
where ωosc is the angular frequency of the relaxation oscillations. This is in contrast to the case for the basic laser:
where τ is the lifetime of the upper state of the laser gain medium, and in this case X is the number of times above laser threshold the device is being pumped.
As upper-state lifetimes exhibited by Neodymium-based gain media are of the order of 100µs (i.e. some 3000 times longer than typical cavity radiative decay times of the order of 30ns, as here) the frequency of the resulting oscillations (2-5MHz) is hence at least an order of magnitude greater than that exhibited by the parent pump laser operating in the absence of parametric down conversion. More seriously the effective damping constant of these relaxation oscillations in the case of the intracavity OPO is given by:
to be compared to:
in the case of the basic laser. This highlights the critical problem that leads to such prolonged bursts of oscillations, since, in the case when the upper laser state lifetime is long compared to the radiative decay times of the cavities - as is the case with Neodymium based ICSROs - the damping constant is severely reduced compared to the case of the laser operating in the absence of the parametric down conversion. This can lead to perturbation events triggering the oscillations on a timescale shorter than the damping time of the oscillations, and hence resulting in a quasi-continuous oscillatory behavior. Whilst these triggering mechanisms can be minimzed through the use of robust mechanical designs and optical elements which have a reduced susceptibility to the effects of thermal lensing [17, 19], the inherent instability and propensity for oscillations brought about by the inclusion of the OPO within a laser cavity utilizing a gain medium with a long upper-state remains. Such oscillations are unacceptable in the context of most spectroscopic applications as the deep and prolonged amplitude modulations lead to instability in wavelength due to erratic hopping of the pump and signal modes. Eliminating these oscillations and therefore realizing a robust, true continuous-wave device has many benefits in the context of a practical source.
In this paper, we report a solution to this problem by constructing, for the first time to our knowledge, a cw-ICSRO utilizing a semiconductor disk laser (SDL) gain media . Crucially, the carrier lifetime in this material is only ~1-10ns, over 5 orders of magnitude less than that of Neodymium. The gain exhibited by the material is therefore easily able to respond to fluctuations in the circulating pump field, thereby critically damping any oscillations which occur. Consequently, a marriage of the intracavity technique with such media offers the same excellent transient stability exhibited by the early Ti:sapphire systems whilst exhibiting the same compact, all-solid-state and high wall-plug efficiency operation which characterizes established diode-laser pumped Nd-based systems.
Laser gain materials fabricated from semiconductors exhibit many other potentially favorable characteristics compared to more traditional diode-laser pumped solid-state media, such as broad tunability, resistance to thermal lensing effects, very thin gain region (reducing the effects of spatial hole burning and greatly relaxing the brightness requirements of the pump laser) and the potential for operation at significantly longer wavelengths; all features which can be exploited by the intracavity technique.
2. Intracavity Optical Parametric Oscillator
The experimental set-up is shown in Fig. 1. The primary pump source is a fibre-coupled 808nm diode-laser array capable of delivering up to 8.5W output from a 100µm core. The very broad absorption properties of the SDL gain media eliminate the need for accurate wavelength and, hence, temperature control of the diode-laser and so cooling was achieved by attaching its housing to a water-cooled brass block. The fibre output is collimated and focused to a ~120µm diameter spot on the surface of the SDL active region via a pair of antireflection coated aspheric lenses FCL. The SDL structure is based on an InGaAs quantum well active region and grown to give tunable output over the range 1040-1063nm. The SDL chip is bonded to a 500µm-thick uncoated intracavity diamond heat spreader [21, 22], through which the diode-laser pump field and the circulating field propagate. The entire assembly is located in a water-cooled brass annulus connected to the same cooling circuit as the pump diode-laser. The high-finesse dog-leg laser cavity is formed between the Bragg mirror fabricated within the SDL structure and mirrors M1 (100mm ROC) and M2 (30mm ROC), all of which are highly reflecting over the range 1040-1063nm. A birefringent tuner BRF based on a 4mm thick quartz plate is placed in the cavity at Brewster’s angle to define both the polarization and operating wavelength of the laser. Its presence line narrows the laser to oscillate on a stable group of longitudinal modes located under one or two pass-bands of the 125GHz étalon formed by the diamond heat spreader. The SDL was operated at a fixed wavelength of ~1050nm, near the peak of the material gain bandwidth. Once significantly above threshold, the down-conversion of the ICSRO represents a highly-selective wavelength-dependant loss mechanism for the laser. Under such circumstances the laser can overcome the BRF selectivity and hop wavelength to another diamond heat spreader étalon mode where the OPO operation is reduced and hence the pump laser experiences less loss. Such behavior manifests itself in erratic down-converted power performance as the diode pump power is increased. In order to improve the selectivity of the BRF, and thereby ‘clamp’ the laser wavelength in the presence of the loss due to parametric down-conversion, two uncoated fused-silica Brewster plates BP were inserted into the pump-only section of the cavity in between M1 and the BRF. This aspect will be discussed more fully in Section 3.2.
Down-conversion takes place in a 30mm long and 1mm thick periodically-poled MgO:LiNbO3 (PPLN) nonlinear crystal NLC with a fanned grating design, the grating period of which varied from 29.5 to 32.8µm across the 12mm lateral dimension of the crystal. A 160µm diameter pump waist is formed in the centre of the PPLN crystal by an anti-reflection coated 38 mm focal length intracavity lens ICL and the end mirror M2. Doping the crystal with MgO removed the need to operate the PPLN crystal at elevated temperatures in order to avoid the problems associated with photo-refractive damage. In order to stabilize the frequencies of the down-converted waves to better than a few GHz, PPLN temperature stabilization is still required, but this was not implemented in our system. The fanned grating design of the periodic polling enables rapid and broad tuning of the down-converted waves by lateral translation of the crystal through the pump beam. However, as this communication is concerned primarily with the power characteristics and, particularly, the transient dynamics of the device, this tuning mechanism for the down-converted signal and idler was not implemented and the phase-matching condition was fixed such that the signal and idler wavelengths were 1.60 and 3.05µm, respectively (Implementation of the mechanical or similar tuning scheme is anticipated to result in signal and idler tuning ranges similar to those we have previously reported in devices based on Neodymium gain media , namely 1.5-1.65µm and 2.8-4µm for the signal and idler respectively). The end facets of the crystal were anti-reflection coated for the pump, signal and idler waves. The resonant signal cavity was discriminated from that of the pump by the dichroic beamsplitter BS. Both faces of this component are antireflection-coated for the p-polarised pump whilst its inner face is broad-band (R>99% 1.45-1.6µm) highly-reflecting for the signal. The signal cavity is formed by the beamsplitter and mirrors M2 and M3 (75mm ROC), the latter of which are both coated to be broad-band highly reflecting for the signal wave. Based upon the coating information available to us, we estimate that the round-trip loss for the pump and signal cavities were ~5% and ~2% respectively.
The position of the optical components within the pump cavity measured with respect to the SDL gain medium were: M1=55mm; ICL=255mm; NLC (centre)=301mm; M2=338mm. Mirror M3 and BS were placed such that the signal cavity (physical) length was 120mm. With the ICSRO components ICL, BS, NLC, M2 and M3 removed and replaced with an optimal (~90%R) plane output coupler, 2.3W of radiation at 1050nm could be output coupled from the laser with a diode pump power of 8.5W.
3.1. Transient dynamic behavior
In order to predictably induce oscillatory behavior (if any), the lasers’ steady-state was perturbed by introducing an optical chopper into the pump-only section of the system so as to periodically spoil the Q of the pump cavity. As the Q recovered, the circulating field was simultaneously observed by monitoring the leaking pumping field through mirror M1 with a fast photo-detector. In order to contrast the performance of this device with a previously reported and otherwise comparable system based upon a laser gain medium exhibiting a long upper-state lifetime, the experiment was repeated with a similar ICSRO system utilizing Nd:YVO4 . The resulting transient behavior of the circulating pump field in both cases is shown in fig. 2. We firstly consider the case of the Neodymium-based system, as depicted in fig. 2(a). The frequency and damping time of the relaxation oscillations exhibited by the parent laser, with the operation of the ICSRO suppressed, were 320kHz and ~50µs, respectively, and once in the steady-state (i.e. operating without external perturbation) remained stable with no tendency to spontaneously exhibit oscillatory behavior. Once down conversion was enabled then the frequency of the oscillations increased to 3-4MHz and the damping time to ~250µs, as shown in fig. 2(a). When unperturbed by the chopper, the system was susceptible to spontaneous and long lived bursts of oscillations which could endure for many seconds. On the macroscopic (>10s sec) timescale, the system was in a state of oscillation ~70% of the time. This behavior is in sharp contrast to that of the SDL-based ICSRO. Crucially, even in the presence of strong parametric down-conversion, the laser displayed no trace of oscillatory behavior when perturbed by the chopper, as is evident from fig. 2(b). Here, the ~20µs rise time of the circulating field was due to the finite reveal time of the rotating chopping wheel, and indicates how the system responds instantly and monotonically (on the timescales shown in fig. 2) to the changing cavity Q; a consequence of the very short carrier lifetime of the SDL. When the chopper was removed from the system, no evidence of spontaneous oscillatory behavior was observed.
To quantify the stability of each device, the amplitude spectra of their respective circulating pump-fields were monitored using a 3GHz RF spectrum analyzer. These traces are shown in fig. 3, where the respective systems were allowed to free-run without perturbations introduced by the optical chopper. The trace from the Nd-based system was recorded during a single spontaneous oscillation event. The fundamental frequency and the higher harmonic components associated with the relaxation oscillations, which are present in the case of the Nd-based system, are clearly visible in fig. 3(a). As the frequency of the oscillations is determined in part by the gain experienced by the pump and signal fields, parameters which are dynamic during an oscillation event, the frequency of the relaxation oscillations was constantly changing over the range ~3-4MHz. Therefore the frequency comb depicted in fig. 3(a) (and the temporal fluctuations in amplitude shown inset), while being characteristic of the relaxation oscillations in general, relate to a representative but particular and selected time interval only. The non-sinusoidal nature of the oscillations gives rise to the higher-order spectral components. In contrast, the excellent amplitude stability of the SDL-based ICSRO is evident from fig. 3(b). Here, the power spectrum exhibits none of the high frequency components associated with oscillatory behavior. The rise of the curve at very low frequencies is a consequence of amplitude fluctuations on the kHz timescale caused by acoustic noise perturbations within the laboratory environment (and possibly noise associated with mode hopping events). Such low frequency noise could be further reduced by implementing improving the robustness of the mechanical design and utilizing a nonlinear crystal with a reduced propensity for thermal lensing, such as PPRTA.
3.2. Operation without Additional Brewster Plates
Initial power performance characteristics of the device, taken before the inclusion of the additional Brewster plates BP described in Section 2 above, are shown in Fig. 4(a). Here, the evolution of the extracted idler is shown as the diode-laser pump power is increased. The erratic growth in idler output is due to hopping of the pump-laser wavelength in order to avoid the loss associated with parametric output coupling of the ICSRO; an effect that can occur when the effective bandwidth of the gain medium exceeds the phase-matching-bandwidth of the nonlinear crystal. The spectrum of the SDL laser both free-running and in the presence of down-conversion is shown in Fig. 4(b).
It is clear from Fig. 4(b) that when parametric oscillation is suppressed, the selectivity of the Brewster-angled BRF is sufficient to maintain laser oscillation on one (or two) modes of the étalon formed by the intracavity diamond heat spreader (blue trace). This selectivity is however insufficient to overcome the effects of the frequency-dependent loss due to down-conversion when the OPO is operating substantially above its threshold. This results in the pump laser then operating away from the wavelength selected by the BRF, a wavelength corresponding to optimum phase-matching, in an attempt to minimize such loss (red trace). These jumps in pump laser wavelength coincide with the periodic dips in power shown in fig 4(a). Such erratic hopping of the pump wavelength is undesirable as it results in associated changes in the idler wavelength, reducing the utility of the device in spectroscopic applications. In order to eliminate this effect, the additional Brewster plates discussed previously were included within the pump-only section of the cavity. The resulting increased loss experienced by wavelengths other than those selected by the BRF was then sufficient to dominate over the loss introduced by the operation of the ICSRO, even in the presence of strong down-conversion, so maintaining the operation of the pump-laser at the optimum wavelength for phase-matching and hence down-conversion, as may be seen from the high depletion of the pump-wave under these conditions apparent in fig. 4(b) (black trace). The results presented elsewhere within this communication were obtained in the presence of the additional Brewster surfaces.
3.3. Power Characteristics
Unlike in the case of a laser, minimizing the point at which the ICSRO comes to threshold (in terms of the primary pump power from the laser-diode) does not necessarily bring about the highest output (or efficiency) at maximum available pump power. For a particular value of laser threshold and maximum available pumping power, optimum down-conversion efficiency occurs when the condition
is met , where PLth and PSROth are the primary pump powers at which the laser and OPO, respectively, reach threshold, and Pin is the power at which the device is pumped. When operated in this regime, the ICSRO acts as an optimum output coupler to the parent pump laser and maximum conversion of primary pump to down-converted power is achieved (this power being equal to that extractable from the pump laser under optimal output-coupling conditions with the OPO components accommodated within the pump cavity but with down-conversion suppressed). Whilst a very low value of PSROth is highly desirable when the available primary pumping power is limited, reduced down-conversion powers are experienced when higher power pump sources are available and the system is operated many times above threshold. This is a result of the ICSRO ‘over coupling’ power from the pump into the signal and idler fields. Due to the high pumping fields available when using the intracavity technique, coupled with the low parametric thresholds enabled by high-nonlinearity periodically-poled crystals, a choice can therefore be made when optimizing the performance of the device either for maximum down-converted power or minimizing parametric threshold in terms of primary pump power. Both of these cases are considered below.
Figure 5 shows the down-conversion power characteristics of the ICSRO operating in both the low threshold and high down-conversion regimes. It should be noted that the periodic dips in down-converted power discussed previously are now absent, indicating the efficacy of the additional Brewster plates in holding the pump wavelength at a value corresponding to optimum phase-matching. Also in both cases it may be seen that once the OPO comes above threshold the intensity/power in the intracavity pump field clamps at its OPO threshold value - a classic feature of the intracavity configuration, indicative of a robust optical geometry for the pump-wave and signal-wave cavities.
Figure 5(a) indicates the performance of the ICSRO when optimized for low parametric threshold. Here, laser and SRO threshold are achieved at diode pump powers of 1W and 1.4W (corresponding to a circulating pump field of 3.1W), respectively. The threshold pumping level of the ICSRO in this case indicates the utility of the intracavity approach in that the 3.1W circulating field required to bring the SRO to threshold can be achieved for a primary diode-laser pump power of only 1.4W. Were the SRO to be located outside of the pump-laser cavity, then >3.1W output coupled power would be required from the pump laser. In the current setup, SRO threshold would therefore be unobtainable as only 2.3W pump power could be extracted when the ICSRO components were removed and when the pump-laser was operated as a simple, optimally output-coupled laser pumped at (the maximum) diode-laser pump power available; in the present case 8.5W. In comparison, 2.3W of circulating pump power was obtained within the high-finesse pump cavity when implementing the ICSRO for a diode pump power of only ~1.2W, a factor of 8 less. It is clear that in mobile, perhaps battery operated applications where diode-laser primary pump power might be limited to ~3W, the intracavity SRO would operate at >2 times threshold delivering an idler power of 30mW, a level ample for many spectroscopic applications. When pumped at the maximum diode-laser pump power of 8.5W, the extracted idler power was 95mW. Hence the calculated total down-converted power, after the combined coating loss experienced by the idler at mirror M2 and the nonlinear crystal NLC (~5%), the dual-direction generation of the idler radiation, and the signal/idler quantum defect have been taken into account, was 581mW. It will be appreciated that given the use of a high finesse signal wave cavity in the current configuration, no useful output power at the signal wavelength is presently being coupled out. The optimization of signal wave output coupling and its impact upon idler wave extraction will be reported in a future publication. At this pumping level, the SRO was operating 6 times threshold and 4.3 times greater than the optimum (in terms of down-conversion efficiency) pumping level of 1.96W, as set by the threshold levels of the pump and SRO.
In order to optimize the down-conversion efficiency of the device when operating at maximum available diode-laser primary pumping power, and hence maximize the extracted idler power, it was necessary to increase the threshold of the SRO. This was achieved by slight misalignment of the signal mirror M3 whilst pumping the system at full power and monitoring the idler power transmitted through M2. The output power of the device when optimised for maximum down-converted power is shown in Fig 5(b). The threshold of the SRO, in terms of diode pump power, increased from 1.4W (in the case of the low-threshold device) to 3.6W, at which point the circulating pump field was 13.2W. (Note that the SRO threshold of 3.6W is some 700mW greater than the calculated threshold of 2.9W for optimal down-conversion at 8.5W diode pumping). The extracted idler power at this pumping level was 205mW, corresponding to a total calculated down-converted power, as previously described, of 1.25W. When mirror M2 is replaced with an optimal output coupler for the circulating pump, keeping the beamsplitter and nonlinear crystal in place within the cavity, but not now down-converting, 1.5W power at the pump wavelength was obtained. This implies that the 1.25W converted through the parametric process from pump into signal and idler power equates to a down-conversion efficiency of 83.3%.
We tabulate the various efficiency metrics below.
3.4. Spectral Characteristics
As the primary focus during this investigation was the power and, in particular, the temporal characteristics of this device, the broad tuning behavior of the down-converted signal- and idler-waves by either variation of the quasi phase-matching condition through lateral adjustment of the nonlinear crystal (and hence grating period) or tuning of the pump (or both) was not fully explored, and will be discussed in a future communication.
Some preliminary measurements of linewidths were made, however. This was facilitated by measuring the linewidth of the sum-frequency light generated parasitically within the MgO:PPLN crystal, as a consequence of the mixing of the (high intensity) pump- and signal-waves. With a free running (resonant) signal-wave, this was found to be 3.1GHz (FWHM). Although single-frequency (single transverse mode) oscillation of the signal-wave is anticipated under these circumstances (nonlinear materials do not exhibit spatial hole-burning), the frequency of the signal-wave can still, to some extent, mode hop within the broad gain bandwidth (~1THz) associated with the phase-matching. To prevent this, an 1mm thick uncoated fused silica etalon was placed within the resonant cavity of the signal-wave. With the etalon in place, the linewidth of the signal-wave was directly measured as <100MHz (FWHM), the instrumental limit of our wavemeter, and the linewidth of the sum-frequency radiation was found to reduce to 1.5GHz. This implies that the effective linewidth of the pump-wave is ~1.5GHz and that this linewidth is transfered to the (non-resonant) idler-wave when the frequency of the signal-wave is clamped by the inclusion of the etalon. (The idler line-width was not measured directly as our available instrumentation did not operate in such a long wavelength spectral band.) Since the free spectral range (~1.2GHz) associated with the signal-wave cavity is >100MHz, we can also conclude that the above etalon is effective in restraining the signal-wave to single-frequency (single longitudinal mode) operation at least over the timescale of the measurement. In order to fully realize the potential of this device as a high resolution spectroscopic source, single-frequency operation of the pump-wave and smooth-tuning either the pump- or signal-waves in order to yield single frequency, mode-hop-free tuning of the idler is highly desirable. It is anticipated that these issues will be readily addressed by the application of standard techniques (e.g. the use of frequency selective elements such as Fabry-Perot étalons within the cavities of the pump and signal waves, cavity length control and ring-resonator designs), and an investigation of this will be the subject of a future communication.
We have demonstrated a true continuous-wave, singly-resonant optical parametric oscillator pumped internal to a semiconductor disk laser. The system, characterized by high down-conversion efficiency, displays both narrow linewidth and high output power operation and requires low primary pump power (in the present case provided by a diode-laser) to reach oscillation threshold, compared to conventional extra-cavity OPOs. Crucially, the synergy of the intracavity technique with an SDL as pump-laser eliminates the problem of relaxation oscillations which plague similar systems based upon Neodymium and similar solid-state, lasers. Therefore, for the first time to our knowledge, the potential of the intracavity technique in an all solid-state, diode-laser-pumped system can be fully realised. Implementing frequency selection techniques in order to realize single-frequency operation of the pump laser, and smooth tuning capabilities, will result in a wavelength-flexible, single frequency mid-infrared source ideal for high-resolution spectroscopic applications over a spectrally important range. These improvements and the extending of the generation of down-converted radiation to new spectral ranges via the use of alternative SDL gain-media will be the focus of our research in the near future.
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