Periodically poled nonlinear devices are now widely established as potential flexible and efficient sources of coherent, tunable near-to-mid infrared radiation [1–5]. Central to the implementation of such sources is the high effective nonlinearity and grating-engineered phase-matching accessible through the periodic poling of established nonlinear materials. The enhanced effective nonlinearity of these materials, most notably that of periodically poled LiNbO₃ (PPLN), has placed once prohibitively-high oscillation thresholds of externally-pumped continuous-wave (cw) SROs within the domain of diode-pumped solid-state lasers [3]. Practical utilisation of more recently developed poled nonlinear materials such as periodically poled RbTiOAsO₄ (PPRTA) and periodically poled KTiOPO₄ (PPKTP), with shorter interaction lengths and effective nonlinearities around half that of PPLN places a more rigorous power demand on the pump laser source. We have recently demonstrated an intracavity pumping regime to address the higher SRO oscillation thresholds of PPKTP, PPRTA and conventional birefringent material based devices [4–6]. The static pump-tuned geometry of the cw intracavity SRO approach offers a versatile technique for generating tunable near to mid infrared cw laser radiation from a single-grating crystal. This paper details the room-temperature operation of cw intracavity SRO based on PPKTP. Significantly, practical temperature tunability of the device is demonstrated outlining the potential of PPKTP for generating tunable output from fixed-frequency pump laser sources. Configuration of the device in a ring-laser geometry has enabled the production of 115 mW of single frequency idler field at 2.35 µm.

The KTP sample was electric-field poled [7] using a 28.5 µm grating to phase-match the room-temperature parametric interaction for pump wavelengths around 800 nm. Overall dimensions were 1×3×20 mm along the x, y and z axes, respectively. Using this periodically poled sample an intracavity SRO was configured as shown in figure 1.

 

Fig. 1. Experimental PPKTP SRO configuration.

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The OPO cavity is formed at a secondary intracavity focus of a standing-wave Ti:sapphire (TIS) laser, itself pumped by an argon-ion laser. Components within this cavity consist of a brewster-cut uncoated TIS rhomb (c-cut; 7.6mm), a birefringent tuner, a dichroic-coated beamsplitter (BS) and a 20 mm PPKTP crystal with anti-reflective coatings for resonant pump and signal fields. The signal cavity is discriminated from that of the pump by the BS which is specified to be highly transmitting for the p-polarised pump field and highly reflecting at 45 degrees for the p-polarised signal field. The SRO is configured in a three-mirror cavity with mirrors specified to be highly reflecting (R>99.7%) over the range 1.1 to 1.3 µm, corresponding to type I non-critical quasi-phase-matched signal wavelengths. The mirrors also had a transmission T>85% over the idler wavelength range of 2.0 to 3.5 µm. The pump beam waist at the centre of the PPKTP crystal is astigmatic measuring 35×34 µm, an astigmatic signal waist of 51×52 µm is also formed at the centre of the PPKTP crystal. A further astigmatic pump waist measuring 19×20 µm is formed at the centre of the TIS crystal.

Intracavity SRO operation is illustrated in figure 2. The SRO operates with an input argon-ion primary pump threshold of 3W corresponding to a circulating TIS field input of 6±1 W.

 

Fig. 2. SRO down-conversion performance.

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Pumping above the 3 W argon-ion threshold results in the clamping of the TIS pump field at this threshold value and is characterised by the increase in down-converted power delivered to the signal and idler fields [5]. The TIS pump laser was configured with all cavity mirrors specified as highly reflecting and operated with an argon-ion pump threshold of 1.4 W, resulting in a theoretical output power, (P^Lout)_max, of 1.9 W at 10 W of argon-ion pump power under optimum output coupling conditions. Theoretical SRO down-conversion is represented by the line, P_DC , shown in figure 2. Above threshold, the SRO down-conversion rapidly approaches 100% of the optimum TIS output power at an argon-ion pump power of 6.4 W, corresponding to the condition [5]

which represents the optimum choice of TIS and SRO thresholds, where P_in is the argon-ion input power, $P_{\mathit{\text{th}}}^{\mathit{\text{TIS}}}$ and $P_{\mathit{\text{th}}}^{\mathit{\text{SRO}}}$ are the argon-ion power thresholds for the TIS laser and SRO respectively. The total down-converted power, P_DC , in signal and idler photons is inferred from measurements of the single-pass idler power, P_i , coupled out of the SRO cavity through mirror M₅ using the relation P_DC =[2P_i /η_i ][1+λ_i /λ_s ], where η_i is the idler output coupling efficiency from the cavity taking into account the crystal and mirror coating losses at the idler wavelength (measured as η_i =0.91). The factor of 2 in the expression accounts for the two-way idler output. The maximum measured two-way idler output power was 455 mW at 2.47 µm. This corresponds to a total down-converted power, P_DC , of 1.51 W at 10 W of argon-ion input power. It can be seen from figure 2 that for 10 W argon-ion input power, the SRO is operating beyond the optimum threshold conditions as specified by (1). Raising the SRO threshold to 3.7 W of argon-ion pump power through the choice of a suitable signal field output-coupler would result in condition (1) being satisfied at 10 W of argon-ion power. This would also result in useful output at the signal. For a measured signal round-trip loss of 2.8±0.6%, the appropriate choice of output coupling would be 0.6 % to satisfy (1). The maximum efficiency of the device was 87 % at an argon-ion input power of 7.8 W, with a predicted efficiency at this power of 96 %.

Coarse tuning of the device was achieved through tuning the TIS laser by means of a birefringent tuner. The room-temperature tuning ranges are shown in figure 3 with the solid line representing theoretical tuning as calculated from Sellmeier equation [8].

 

Fig. 3. Pump tuning of SRO.

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The SRO tunes from 1.14–1.27 µm (signal) and 2.23–2.73 µm (idler) limited by optical coating bandwidths and corresponding to a pump tuning range of 805.5–811.2 nm. Unlike birefringently phase-matched KTP, PPKTP shows significant temperature tunability. Since the polarisations for all coupled fields of the nonlinear interaction are along the z-axis of the poled crystal, it is the change in refractive index with temperature and not the change in birefringence that governs the phase-matched condition within the sample. Through the insertion of a temperature-controlled oven around the PPKTP sample we were able to measure the temperature tunability of the device at a fixed pump wavelength of 810 nm.

 

Fig. 4. Temperature tuning of SRO.

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Figure 4 shows the corresponding tuning data together with the predicted tuning as predicted by Sellmeier equation [8] modified with temperature gradient measurements of references [9,10]. The device tunes from 1.18–1.26 µm and 2.29–2.57 µm over the signal and idler wavelengths, respectively, corresponding to an average tuning rate of 1.2 nm/degree (signal) and 4.5 nm/degree (idler). Deviation from the tuning predicted by references [9,10] is attributed to a bias in the calculated dn/dt towards visible and near-infrared data. This result is of particular significance for devices pumped by fixed frequency laser sources.

Amplitude stability of the device is typically ±5% over a 50 second time scale for pumping levels of 3.3 times above threshold. This is achieved under free running laboratory conditions in the absence of active stability control, demonstrating the practical nature of intracavity SROs as stable sources of cw infrared radiation. The SRO operates over a time scale of several hours limited only by argon-ion beam and TIS laser component misalignment.

Generation of single-frequency idler field has been observed by configuring the pump laser and SRO in ring-cavity geometries as shown in figure 5.

 

Fig. 5. Ring-cavity configuration of PPKTP SRO.

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The utilisation of uncoated etalons in both the resonant signal and pump fields resulted in the single-frequency generation of 115 mW of idler field at 2.35 µm. Figure 6. shows simultaneous Ti:Sapphire-pump and SRO-signal output spectrums. The traces are obtained by scanning two separate 2GHz interferometers (one for the pump field and one for the signal field) with a common, high-voltage ramp. The trace is taken from a dual-channel oscilloscope triggered by a reference voltage derived from the applied ramp voltage. Both pump and signal fields are operating on single longitudinal modes (SLM) with mode-hop interval times typically of the order of 10 seconds under passive, free-running, laboratory conditions. The SLM characteristic of the idler was not directly measured as an appropriate interferometer was unavailable. Due to the SLM nature of the pump and signal fields, the idler field generated through the nonlinear interaction must also operate on a single mode. The linewidth measurements shown, <15 MHz (signal) and <25 MHz (pump), are limited by the bandwidth of the interferometers used. An important factor in the implementation of such a ring-based device is the differential loss presented to the pump laser field by SRO operation. In order to maintain uni-directional operation of the Ti:Sapphire laser any uni-directional device, such as a faraday rotator/waveplate combination must present a differential loss greater than the nonlinear loss presented by the SRO itself. Full characterization of this SRO configuration is the subject of further investigation. Fine tuning of the output frequency through the implementation of a servo-locked active stabilisation scheme is very promising in view of the passive stability of the device.

 

Fig. 6. Simultaneous pump and signal output spectrum from which SLM operation of the idler can be inferred.

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In summary, we have demonstrated the room-temperature operation of a cw SRO based on PPKTP. Using a 28.5 µm grating sample measuring 20 mm long we have utilised the intracavity approach to implement a practical, tunable infrared source delivering a maximum of 455 mW of amplitude stable cw radiation with a down-conversion efficiency of up to 87%. Significant temperature tunability of the device has been achieved with important implication for the operation of fixed-frequency laser pumped SROs. The static pump-tuned geometry of the Ti:sapphire PPKTP SRO is uniquely placed to take advantage of this nonlinear material with the ability to pump the device several times above threshold. Work towards optimisation of ring-cavity geometries has been shown to produce in excess of 100 mW of single-frequency idler output with promising stability characteristics for the implementation of active servo frequency tuning. It is predicted that such a device will operate comfortably within the pumping range of commercially available diode-pumped frequency-doubled Nd:YVO₄ lasers, making it a practical solution for the generation of tunable cw infrared radiation.